No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Molecular dynamics simulation of the interfacial thermal resistance between phosphorene and silicon substrate
Abstract
Abstract Phosphorene is a recently discovered member of the two-dimensional (2D) monolayer materials, which has been reported to exhibit unique characteristics on mechanical and thermal properties. This study is the first time to show the exceptional thermal conductance between phosphorene and crystalline silicon substrate through classical molecular dynamics (MD) simulations. MD simulations revealed that under conventional conditions, the interfacial thermal resistance (R) between phosphorene and silicon is very low and independent on the thickness (h) of silicon substrate when h is larger than 3.12 nm. It was also found that R decreases remarkably with the increases in system temperature (Tie) and contact strength (χ). To further explicitly display the superiority of phosphorene on interfacial heat transfer, R of other two popular 2D monolayer materials, i.e., graphene and silicene, were calculated for comparison. The comparisons revealed that R of phosphorene shows two distinct advantages over graphene and silicene. On one hand, within the studied ranges of Tie and χ, R between phosphorene and silicon substrate is about quarter of that between graphene and silicon substrate, which proves that phosphorene is really a high-performance 2D monolayer material for interfacial heat transfer. On the other hand, with the increases in Tie and χ, R between phosphorene and silicon substrate decreases more sharply than that between silicene and silicon substrate, indicating that phosphorene is more sensitive to environmental variations.
... The MoS2-a-SiO2 was firstly equilibrated at 300 K with a time step of 0.1 fs in a constant pressure condition (NPT ensemble) for 1 ns, followed by a constant volume condition (NVT ensemble) for 1 ns. Then, a 200 K temperature rise was created between the MoS2 monolayer and the a-SiO2 substrate by rescaling the velocities of MoS2 atoms [32,33] and then the heterostructure was switched to constant energy and constant volume condition (NVE ensemble) with a time step of 0.05 fs to capture phonon dynamics at high frequencies [21]. The value of G can be obtained by fitting the temperature decay with time through a thermal RC circuit with the known mass and heat capacity of MoS2 ( 2 , 2 ) and a-SiO2 ( 2 , 2 ) as the inputs, ...
... Although this high thermal resistance has been confirmed by recent experimental characterizations [59][60][61] and numerical simulations [32,62], strategies to enhance heat transfer through 2D-3D ...
... A block of crystalline SiO2 was generated first and then heated up to 6000 K, which was suddenly cooled down to 300 K later to form amorphous structures. The interaction between the MoS2 monolayer and the a-SiO2 substrate is assumed to be of van der Waals (vdW) type, which can be modeled by the Lennard-Jones (LJ) potential [21,32,73]. The results of calculated energy ( ) and distance parameters ( ) of LJ potential from the universal force field (UFF) potential [23] to describe the interactions between the MoS2 monolayer and the a-SiO2 substrate are listed in the table below. ...
With novel electronic and optical properties, two-dimensional (2D) materials and their heterogeneous integration have enabled promising electronic and photonic applications. However, significant thermal challenges arise due to numerous van der Waals (vdW) interfaces limiting dissipation of heat generated in the device, induces significant temperature rise, and creates large thermal mismatch, resulting in the degradation of device performance and even failure of the device. The highly localized heat generation during device operation thus becomes a major bottleneck of 2D nanodevice performance. Nevertheless, classical descriptions of heat transfer, i.e., Fourier’s Law, become invalid from the microscopic view. Furthermore, it remains challenging to measure heat transport precisely. Advances in the characterization and understanding of heat transfer at the nanoscale are thus needed for practical thermal management of nanoelectronics. Recent theoretical and experimental progress promises more effective nanoelectronics thermal management. On the one hand, atomistic simulation provides great opportunities to investigate fundamental thermal transport processes under ideal conditions by tracking the motion of all atoms. Raman spectroscopy, on the other hand, has been widely applied to detect lattice or molecule vibration on small scales owing to its superior spatial resolution. In this thesis, we leverage the power of atomistic simulation and Raman spectroscopy to understand and characterize thermophysical and thermal transport properties for engineering thermal transport in 2D vdW nanoelectronics. The thesis presents a method of characterizing thermal expansion coefficients for 2D transitional metal dichalcogenide monolayers experimentally and theoretically, and an atomistic simulation framework to predict thermal transport properties, which is used to study vdW binding effects on anisotropic heat transfer and phonon transport through an MoS2-amorphous silica heterostructure toward optimal 2D device heat dissipation. With combined efforts of experiments and simulation, this thesis opens up new avenues to understand, characterize, and engineer thermal transport in 2D vdW nanoelectronics.
... The universal force field [49] A for all vdW interactions. A large-scale atomic/molecular massively parallel simulator [50] was employed to accomplish all the simulations in this work which has been broadly utilized to characterize the thermal transportation of diverse 2D nanostructures [51][52][53]. ...
... The transient pump-probe technique is an extensively applied technique to characterize R. Figure 1(c) portrays the arrangement for implementing the transient pump-probe scheme in the stanene/2D-SiC heterostructure. This method has previously been used to characterize the flexural thermal transportation in graphene/silicon [60], graphene/h-BN [61], silicene/SiO2 [62], phosphorene/silicon [53], graphene/phosphorene [63] and graphene/copper [64]. To measure the R between stanene and 2D-SiC, the structure was relaxed at a particular temperature by following the same steps which have been described at the in-plane thermal conductivity calculation section. ...
... This occurs because the Sn atoms of the stanene monolayer are now occupied as a scattering center of 2D-SiC which indirectly smooths the thermal conductivity through the interface. The trend of the decrement of thermal resistance is incongruous to the previous studies of MoSe 2 /MoSe 2 [78], graphene/phosphorene [63], phosphorene/silicon [53] and graphene/C 3 N [43]. ...
Thermal management is one of the key challenges in nanoelectronic and optoelectronic devices. The development of a van der Waals heterostructure (vdWH) using the vertical positioning of different two-dimensional (2D) materials has recently appeared as a promising way of attaining desirable electrical, optical, and thermal properties. Here, we explore the lateral and flexural thermal conductivity of stanene/2D-SiC vdWH utilizing the reverse non-equilibrium molecular dynamics simulation and transient pump-probe technique. The effects of length, area, coupling strength and temperature on the thermal transport are studied systematically. The projected lateral thermal conductivity of a stanene/2D-SiC hetero-bilayer is found to be 66.67 [Formula: see text], which is greater than stanene, silicene, germanene, MoSe2 and even higher than some hetero-bilayers, including MoS2/MoSe2 and stanene/silicene. The lateral thermal conductivity increases as the length increases, while it tends to decrease with increasing temperature. The computed flexural interfacial thermal resistance between stanene and 2D-SiC is 3.0622 [Formula: see text] [Formula: see text] K.m2 W-1, which is close to other 2D hetero-bilayers. The interfacial resistance between stanene and 2D-SiC is reduced by 70.49% and 50.118% as the temperature increases from 100 K to 600 K and the coupling factor increases from [Formula: see text] to [Formula: see text], respectively. In addition, various phonon modes are evaluated to disclose the fundamental mechanisms of thermal transport in the lateral and flexural direction of the hetero-bilayer. These results are important in order to understand the heat transport phenomena of stanene/2D-SiC vdWH, which could be useful for enhancing their promising applications.
... Zhang and Hong et al. established the MD contact model of phosphorene (two-dimensional monolayer material) on a silicon substrate to acquire the exceptional interfacial thermal conductance. 6 Kim et al. verified the thermal resistance length empirical model at the liquid-solid interface using the MD model to understand the complex interfacial thermal resistance phenomena. 7 In general, Newton's law of cooling can describe the convective heat transfer ability between two media. ...
... Assume that G between the solid surface of a particular material and water is a constant value under certain conditions. 6,7,12 The water-solid interfacial thermal conductance formula can then be calculated. ...
With the integrated high-power device packaging structure rapidly developing, the embedded heat dissipation architectures are challenged by the local micro-/nanoscale massive heat flux. The slip flow molecular dynamics models were established to explore the liquid–solid interfacial thermal conductance. With stepwise declining shear forces (0.032 pN/200, 0.024 pN/200, and 0.016 pN/200 ps, respectively), the slip flow [the slip shear velocity is Si: (125.43 ± 0.92 m/s), graphite: (142.43 ± 1.92 m/s), and Cu: (180.93 ± 3.42 m/s), respectively] water–solid interfacial thermal conductance of different materials [Si: (8.11 ± 0.1) × 10⁷ W/m² K, graphite: (10.18 ± 0.1) × 10⁷ W/m² K, and Cu: (17.97 ± 0.1) × 10⁷ W/m² K] can be calculated. The rationality of the calculated values can be verified in the literature. The slip flow water–solid interfacial thermal conductance values are about 0.5 times higher than the static ones. It can be significantly affected by the slip shear velocity. The slip shear velocity increasing about five times can enhance the interfacial thermal conductance two times. From the water layer density distribution, it is found that the dependence of interfacial thermal conductance on velocity slip relies more on the dynamical properties than on the fluid structure. This molecular dynamics model provides an operative methodology to investigate the slip flow liquid–solid interfacial heat transfer for the various embedded cooling surfaces.
... Nowadays, molecular dynamics (MD) simulation [1][2][3] has become an important way to investigate the micro scale physical and chemical reactions [4][5][6]. In many research fields, charge can apparently influence the physical and chemical processes. ...
... 2. Solve the EEM equation set in the sub-system. 3. Obtain the charge of the specified atom i. 4. Repeat steps 1-3 for next atom, until the charges of all atoms are calculated. ...
Nowadays, molecular dynamics (MD) using force fields has become an effective tool for physical and chemical problems. In the MD method, the classical Electronegativity Equalization Method (EEM) is widely accepted for atom charge computing, which may result in unreasonable predictions for large scale non-uniform systems. In order to improve the accuracy of charge computing in MD simulations, two versions of improved EEM for charge distribution calculation are proposed in the present work, without introducing new empirical parameters, namely the locally equilibrated EEM and the sub-zone EEM. The aim of the proposed methods is to eliminate the unreasonable interaction of the long-range charge in large scale non-uniform systems. Comparison between the two improved EEMs and the original EEM in both uniform and non-uniform systems shows that both the improved EEMs successfully eliminate the effect of the unreasonable long-range charge interaction which is inevitably encountered in the original EEM.
... For example, Witherspoon and Saraf [12] used the capillary-cell method to measure diffusion coefficient of methane, ethane, propane and n-butane in water from 24.8 to 42.6 • C. Ferrell and Himmelblau [13] did measurements of laminar dispersion in a capillary to determine diffusion coefficient of hydrogen and helium dissolved in water over the temperature range of 10 to 55 • C. A statistical analysis of experiments indicated that diffusion coefficient could be related to the absolute temperature by a semiempirical correlation. With the development of computer, MD simulation is an alternative way to investigate physical and chemical properties in recent years [14,15]. Martins et al. [16] predicted diffusion coefficient of chlorophenols in water by MD simulation. ...
... Thus, the velocity Verlet algorithm uses both Eqs. (12) and (14) to evolve the positions and velocities simultaneously. For the calculation of diffusion coefficient, the mean-squared-displacement (MSD) method are used which is based on the Einstein relation that MSD is linear with diffusion time. ...
Diffusion coefficient of H2, CH4, CO, O2 and CO2 in water near the critical point (600-670K, 250atm) is numerically investigated using Molecular Dynamics (MD) simulation. Main factors determining diffusion coefficient are discussed. Arrhenius behavior of temperature can be divided into two separate parts which are subcritical region and supercritical region. The activation energy has a huge difference between two regions. Diffusion coefficient has a negative power relation with density of water through logarithmic plot. Viscosity of water has effects on diffusion coefficient by a combination with temperature that term 1/Tη has a quadratic relation with diffusion coefficient. A new empirical equation to predict diffusion coefficient in water near the critical point is developed in which the effect of solute gas and solvent water is separated to the pre-factor A0 and the second part Fw. A0 is a unique constant for different solutes and Fw considers temperature, density and viscosity of water. It successfully predicts diffusion coefficient near the critical point for all solute gases and average absolute relative deviation is only 7.65%. Compared to other equations, our equation shows the best accuracy and simplicity for extension and modification.
... The chemical stability of black phosphorus is better than that of red and white ones [1]. Phosphorene is a direct bandgap semiconductor (>1.5 eV) with high carrier mobility [8,[13][14][15][16]. The lattice thermal conductivity for phosphorene in the zigzag direction is higher than it is in the armchair direction [15,17]. ...
The Seebeck coefficient is an important quantity in determining the thermoelectric efficiency of a material. Phosphorene is a two-dimensional material with a puckered structure, which makes its properties anisotropic. In this work, a phosphorene nanodisk (PDisk) with a radius of 3.1 nm connected to two zigzag phosphorene nanoribbons is studied, numerically, by the tight-binding and non-equilibrium Green's function (NEGF) methods in the presence of transverse and perpendicular electric fields. Our results show that the change in structure from a zigzag ribbon to a disk form creates an energy gap in the structure, such that for a typical nanodisk with a radius of 3.1 nm, the size of the energy gap is 3.88 eV. Besides, with this change, the maximum Seebeck coefficient increases from 1.54 to 2.03 mV/K. Furthermore, we can control the electron transmission and Seebeck coefficients with the help of the electric fields. The numerical results show that with the increase of the electric field, the transmission coefficient decreases and the Seebeck coefficient changes. The effect of a perpendicular electric field on the Seebeck coefficient is weaker than a transverse electric field. For an applied transverse electric field of 0.3 V/nm, the maximum Seebeck coefficient enhances to 2.09 mV/K.
... The chemical stability of black phosphorus is better than that of red and white ones [1]. Phosphorene is a direct bandgap semiconductor (>1.5 eV) with high carrier mobility [8,[13][14][15][16]. Lattice thermal conductivity in the zigzag direction is higher than armchair direction for phosphorene [15,17]. ...
The Seebeck coefficient is an important quantity in determining the thermoelectric efficiency of a material. Phosphorene is a two-dimensional material with a puckered structure, which makes its properties anisotropic. In this work, a phosphorene nanodisk (PDisk) with a radius of 3.1 nm connected to two zigzag phosphorene nanoribbons is studied, numerically, by the tight-binding (TB) and non-equilibrium Greens function (NEGF) methods in the presence of transverse and perpendicular electric fields. Our results show that the change of the structure from a zigzag ribbon form to a disk one creates an energy gap in the structure, so that for a typical nanodisk with a radius of 3.1 nm, the size of the energy gap is 3.88 eV. Besides, with this change, the maximum Seebeck coefficient increases from 1.54 to 2.03 mV/K. Furthermore, we can control the electron transmission and Seebeck coefficients with the help of the electric fields. The numerical results show that with the increase of the electric field, the transmission coefficient decreases, and the Seebeck coefficient changes. The effect of a perpendicular electric field on the Seebeck coefficient is weaker than a transverse electric field. For an applied transverse electric field of 0.3 V/nm, the maximum Seebeck coefficient enhances to 2.09 mV/K.
... Such surface electromagnetic waves are generated by the rational interaction of free electrons and light waves on metal surfaces. e exponential drop indicates that the surface plasmons are oriented perpendicular to the surface and propagate along the surface of the metallic medium [12]. Surface plasmons have two modes. ...
With the rapid development of nanotechnology, surface plasmon (SP) of metal nanostructures has become one of the research hotspots in the research of information science, physics, biology, materials science, chemistry, and other interdisciplinary fields. Surface plasmon research is an emerging discipline. In this paper, based on the CPU algorithm and the GPU algorithm for the extraction of pressure and single-atom average potential energy, time tests and error analysis of different molecular scales are carried out. The analytical results show that the absolute and relative errors of pressure and single-atom average potential energy extraction increase with the molecular scale. When the molecular scale reaches 100k, the errors of pressure and single-atom average potential energy reach 0.32 and 0.052, respectively. However, compared with the pressure and single-atom average potential energy extracted by the CPU algorithm, the error of the results extracted by the GPU algorithm is only 1.55% and −1.45%.
... 450,451 Prediction models for thermoelectric interfaces in devices have also been explored recently. [452][453][454][455][456] The currently available analytical, highthroughput and machine learning models demonstrate advantages such as reasonable accuracy in prediction for a wide temperature range and several interface types; however, they are mainly limited by the physics of scattering mechanisms in various external conditions, and the availability of experimental or computational data. Therefore, there is room for developing approaches through the underlying theory or considerably accurate prediction by extensive data mining. ...
The rising demand for energy has accelerated the search for clean and renewable sources and newer approaches towards efficient energy management. One of the most promising approaches is the conversion of the waste heat into electrical energy by using thermoelectric materials. Such conversion approaches utilize and harvest the waste heat, thereby mitigating the environmental concerns. High-performance thermoelectric materials simultaneously require excellent electronic transport and favorable thermal transport. The performance of any thermoelectric material is determined by the thermoelectric figure of merit (ZT), where the governing parameters such as thermopower and electrical and thermal conductivity are material dependent. Given the interdependence of various transport parameters, designing thermoelectric materials with desirable efficiency is highly challenging. This has fueled the interest in developing new strategies and the search for potential materials, which can unfold broader aspects and provide a larger search space for thermoelectric research. The concept of tuning crystal structures by varying dimensionality, controlling defect chemistry, and modifying atomic order in compounds represents an emerging breakthrough in designing next-generation thermoelectric materials. With the surge in advancements in this emerging field, another step forward is exploring the physical and chemical property-based concepts such as spin driven transport, unusual transport in organic thermoelectric materials, non-trivial bonding and orbital chemistry and many more. This review provides a broad overview of these advanced approaches for improving thermoelectric efficiency with a detailed discussion of the current investigations in this area. Finally, a discussion has been provided highlighting the existing shortcomings and potential prospects.
... Optimization of structure or molecule is important to obtained maximum performance during simulation by external effects forms in the structure. The MD simulations normally will be equipped with LAMMPS software and applied for thermal properties of materials such as DNA [5], phosphorene [6], molten salt [7], argentum [8] and others. The analysis of the interaction between atoms in a molecule in a smaller dimension, such as graphene and CNT, uses the empirical bond order of reactive intermolecular (AIREBO) potential. ...
The heat transfer performance in materials to remove heat is attained in various designs according to the devices’ design. Simulation studies are comprising of heat transfer knowledge in detail suit the theories and applications. This review provides an understanding of the simulation work focusing on the heat transfer in the various design of electronic devices. This discussion begins with a briefing on the simulation principle and current focus. Then, the review continues by explaining various simulation methods that exhibit recent heat transfer analysis. The properties of simulation studies are also summarized in detail to understand the significant properties that impact analysis. The application of simulation in thermal model is looked forward to obtain significant heat transfer improvement and impactful research direction. This review also provides insights into challenges in simulation work with available opportunities to solve the heat transfer issue by understanding fundamental knowledge.
... In general, the thermal transport including out-of-plane and in-plane properties can both be regulated by altering the strain resulting from the variation of coupling strength between graphene and substrate in theory and in experiments, which is highly affected by the residual H 2 O molecules at the graphene-substrate interface. The phonon dispersion can be modified by altering the coupling strength between graphene and substrate, especially for flexural acoustic (ZA) phonons, which are the main carriers of interfacial thermal transport (out-ofplane) in supported graphene [18]. In addition, the impact of coupling strength on in-plane thermal transport of graphene supported on an SiO 2 /Si substrate with an ultra-thin sild oxide layer (8 nm and 10 nm) has been experimentally characterized via an optothermal Raman technique [15]. ...
The coupling strength between two-dimensional (2D) materials and substrate plays a vital role on thermal transport properties of 2D materials. Here we systematically investigate the influence of vacuum thermal annealing on the temperature-dependence of in-plane Raman phonon modes in monolayer graphene supported on silicon dioxide substrate via Raman spectroscopy. Intriguingly, raising the thermal annealing temperature can significantly enlarge the temperature coefficient of supported monolayer graphene. The derived temperature coefficient of G band remains mostly unchanged with thermal annealing temperature below 473 K, while it increases from −0.030 cm−1/K to −0.0602 cm−1/K with thermal annealing temperature ranging from 473 K to 773 K, suggesting the great impact of thermal annealing on thermal transport in supported monolayer graphene. Such an impact might reveal the vital role of coupling strength on phonon scattering and on the thermal transport property of supported monolayer graphene. To further interpret the thermal annealing mechanism, the compressive stress in supported monolayer graphene, which is closely related to coupling strength and is studied through the temperature-dependent Raman spectra. It is found that the variation tendency for compressive stress induced by thermal annealing is the same as that for temperature coefficient, implying the intense connection between compressive stress and thermal transport. Actually, 773 K thermal annealing can result in 2.02 GPa compressive stress on supported monolayer graphene due to the lattice mismatch of graphene and substrate. This study proposes thermal annealing as a feasible path to modulate the thermal transport in supported graphene and to design future graphene-based devices.
... Finally, the density of defects in the workpiece is conducive to the machining direction along the plane with high shear strength, and fewer defects result in higher cutting energy [20,24]. [26]. There are three kinds of boundary conditions in molecular dynamics: periodic boundary, nonperiodic boundary, and fixed boundary. ...
Mechanical tribology is the key research object in the process of materials machining. The machining process will produce inevitable surface and surface damage, which is directly related to the relative motion between tool and workpiece. Moreover, in case of precision machining, there are limitations of existing measurement technique on observation of nanoscale materials removal mechanism. Therefore, a numerical simulation method is needed to observe various physical phenomena at the nanoscale and carry out microscopic interpretation. Microcosmic interpretation can guide the manufacturing process and predict possible problems. Molecular dynamics (MD) is considered to be one of the most suitable numerical simulation methods. MD method was used to simulate the whole machining process and analyze the changes of various machining parameters in the process, so as to predict and guide the machining process. In this chapter, the application of MD in tribology is systematically described by describing the important parameters in modeling and machining. Potential energy function, model size effect, boundary condition, machining parameters, and tool geometry parameters in MD machining model are discussed. The materials removal is related to the cutting force, energy, and frictional coefficient in MD model. Residual stress, surface and subsurface damage can also be evaluated by MD machining process.
... Experimental 105 measurement of Kapitza resistance is often challenging since it requires precise measurement of the temperature gradient within a distance of a few nanometers or below. On the other hand, molecular dynamics simulation provides an important tool for probing the interfacial heat transfer behavior at nano or sub-nano scale, in particular on 2D nanomaterials [44,45]. In this study, the relationship between Kapitza resistance and evaporative heat transfer is investigated for an argon nanofilm 110 evaporating on solid surface covered by different layers of nanocoatings. ...
Two-phase liquid cooling on nanoengineered surfaces has shown great promise for tackling overheating in high performance microelectronics. At the nanoscale, phase change heat transfer can be affected substantially by the thermal resistance at the solid–liquid interface (also known as the Kapitza resistance). For example, surface modification by nanocoatings has been shown to efficiently enhance boiling heat transfer. However, there have been few studies on the effect of the Kapitza resistance on thin-film evaporative heat transfer. Here, the transport behavior of a liquid argon nanofilm evaporating on silicon (100) surfaces coated with different layers of graphene is analyzed using non-equilibrium molecular dynamics (MD) simulations. The results show that increasing layers of graphene coatings lead to a 136% increase in the Kapitza resistance and a 62% reduction in the evaporation rate. The large increase in Kapitza resistance is attributed to the strong repellence between graphene and argon molecules, manifested by a higher contact angle. These findings demonstrate the dominant role of interfacial thermal transport in the evaporation behavior of thin liquid films, and provide guidelines for selecting new materials when designing nanoengineered surfaces to realize improved evaporative cooling performance.
... The nonbonded van der Waals interactions between atoms were reproduced by the Lennard-Jones potential, while the Coulomb interactions were calculated by the PPPM method with a root mean of 0.0001. 30 The system contained 33,600 atoms in total, allowing for statistical conclusions to be drawn by looking at the average behavior of the ensemble. ...
Evaporation-based thin-film coating processes are ubiquitous in fields such as organic electronics, dye coatings, and the pharmaceutical industry. Based on experimental observations of enhanced thin-film alignment during meniscus-guided coating (MGC), a molecular dynamics simulation was developed to describe the thin-film formation that occurs due to the interplay between evaporation and fluid dynamics. The process of solvent evaporation from solution is first examined, which leads to the formation of a solid thin film after evaporation is complete. Next, an MGC process was simulated by introducing a flow profile coupled with the evaporative system. We gain insights into how molecules respond and reorient in the presence of a flow profile. These results indicate that both the evaporation and the flow profile have an impact on orienting small molecules in a thin film during the MCG process. Combining experimental techniques with simulations helps enhance the understanding of the forces that are significant in directing molecular aggregation during coating processes.
... Concerning the variables that can be used for tuning nanoscale thermal transport in phononic devices, there are two major parameters that have turned out to be crucial: (i) structural asymmetry, either at the interface to the thermal baths or intrinsic to the materials used as thermal devices [33][34][35][36][37][38][39]; and (ii) substrate deposition [40][41][42][43][44][45]. For instance, the outer and inner atomic conformations of two-dimensional (2D) materials have been engineered in order to control the heat-flux intensity and direction, improving their performance as thermal rectifiers [36,37,46]. ...
Thermal management is a current global challenge that must be exhaustively addressed. We propose the design of a nanoscale phononic analogue of the Ranque-Hilsch vortex tube in which heat flowing at a given temperature is split into two different streams going to the two ends of the device inducing a temperature asymmetry. Our nanoscale prototype consists of two carbon nanotubes (capped and open) connected by molecular chains. The results show that the structural asymmetry in the contact regions is the key factor for producing the flux asymmetry and, hence, the induced temperature bias effect. The effect can be controlled by tuning the , number of chains, and chain length. Deposition on a substrate gives another variable to the manipulation of the flux asymmetry, but the effect vanishes at very large substrate temperatures. Our study yields new insights into the thermal management in nanoscale materials, specially the crucial issue of whether the thermal asymmetry can survive phonon scattering over relatively long distances, and thus provides a starting point for designing a nanoscale phononic analogue of the Ranque-Hilsch vortex tube.
... Because the phonon density of states (PDOS) could provide the spectral description of the atomic vibration and motion in the interface system, an analysis for the PDOS is of great significance for the understanding of interfacial thermal conductance. In this case, the interfacial phonon thermal transport is strongly dependent on the matching degree of the PDOS between the two materials, the higher PDOS matching degree indicates that phonons are more likely to transfer heat through the interface [46]. The concept of PDOS matching degree has been used effectively to address the phonon thermal transport in many other interface systems [25,45,47,48]. ...
In the present work, the effects of vacancy in graphene on the bonding strength, electronic characteristics and thermal conductance of Cu/graphene/Al interface are deeply studied by first-principles calculation. It is found that the introduction of appropriate vacancies could simultaneously enhance the Cu/graphene/Al interfacial bonding strength and thermal conductance. The Cu(111)/perfect graphene/Al(111) interfacial bonding strength is very low because of the limited interface charge transfer. However, when the vacancy is introduced in the graphene, the Cu(111)/graphene/Al(111) interfacial bonding strength could enhance several times, the enhancement effect increases with the vacancy concentration, and the increasement of vacancy area has a better effect for the enhancement of Cu(111)/graphene/Al(111) interfacial bonding strength at the same vacancy concentration. The reason why the vacancy in graphene could significantly improve the interfacial bonding strength is that the Cu-p and Al-p state of interfacial Cu and Al atom are more delocalized and could effectively hybridize with the p states of C atom near the vacancy in graphene. In addition, the introduction of vacancy could effectively increase the phonon density of states matching degree between the interfacial Cu, Al atoms and C atoms in graphene, and thus could effectively enhance the thermal conductance across the Cu(111)/graphene/Al(111) interface.
Graphical abstract
... In this way, molecular dynamics simulation has the advantage of analyzing macroscopic mechanisms at the molecular level. At present, molecular dynamics simulation is widely used in the study of flow behavior [19][20][21], interfacial heat transfer [22][23][24] and interfacial adhesion [25][26][27]. ...
Injection molding is an economical and effective method for manufacturing polymer parts with nanostructures and residual stress in the parts is an important factor affecting the quality of molding. In this paper, taking the injection molding of polymethyl methacrylate (PMMA) polymer in a nano-cavity with an aspect ratio of 2.0 as an example, the formation mechanism of residual stresses in the injection molding process was studied, using a molecular dynamics simulation. The changes in dynamic stress in the process were compared and analyzed, and the morphological and structural evolution of molecular chains in the process of flow were observed and explained. The effects of different aspect ratios of nano-cavities on the stress distribution and deformation in the nanostructures were studied. The potential energy, radius of gyration and elastic recovery percentage of the polymer was calculated. The results showed that the essence of stress formation was that the molecular chains compressed and entangled under the flow pressure and the restriction of the cavity wall. In addition, the orientation of molecular chains changed from isotropic to anisotropic, resulting in the stress concentration. At the same time, with the increase in aspect ratio, the overall stress and deformation of the nanostructures after demolding also increased.
... The cost associated with such an effective amount (10 g.L −1 ) is approximately 0.14 $. 10 kg of tested zeolite could be utilized in the treatment of 1 ton of groundwater or even of wastewater by this economically viable and regenerative zeolite for a cost of 140 $/ton. This will aid and facilitate the circular economy of synthesizing highly active adsorbents and value-added products for various industrial applications, however, keeping the cost and related emissions into consideration [83][84][85][86][87][88][89]. ...
... Among the water treatment technologies, adsorption is considered as an effective approach for eliminating several kinds of pollutants in water [19][20][21]. Recently, carbon-rich materials have been used as adsorbents in water purification such as activated carbons [22][23][24][25], carbon nanotubes [20,[26][27][28][29][30], biochar [31][32][33][34], carbon fibers [35], and graphene oxides [36][37][38]. Furthermore, zeolites [39][40][41], silica [42][43][44], and clay-based nanocomposites [45][46][47] have also been used in water treatment [48][49][50][51]. ...
Purification of waste and contaminated water using safe and cost-effective methods is a global and local endeavor. The present work investigates the capability of zinc ferrite (ZFO) nanoparticles (NPs) as superior absorbents to eliminate the radionuclides from radioactive waste. The facile and eco-friendly sol–gel technique was utilized to synthesize ZFO NPs. The ZFO NPs are characterized via energy-dispersive X-ray (EDX), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscope (SEM), high-resolution transmission electron microscope (HRTEM), diffuse reflectance spectroscopy (DRS), and vibrating sample magnetometer (VSM). The EDX and FTIR analyses confirm the chemical composition and modes of the cubic ZFO phase. The FullProf Suite software is employed to analyze the XRD data via Rietveld refinement. The Williamson–Hall (W–H) method is used to determine the average crystallite size of ZFO NPs which found around 40.7 nm. SEM micrograph illustrates that ZFO NPs have a porous nature. Also, the TEM image exhibits that the ZFO NPs hold particles in the nanoscale range with a spherical form. Furthermore, the ZFO NPs show a superparamagnetic nature and have a semiconductor bandgap. Sorption behavior of 134Cs and 152+154Eu radionuclides in HNO3 acid medium was investigated using the batch technique. The obtained results indicated that the selectivity of 152+154Eu radionuclides is higher than 134Cs at acidic medium. The sorption kinetics results follow the pseudo-second-order model. The results obtained show that the adsorbent, ZFO, is an effective adsorbent for the removal of 134Cs and 152+154Eu radionuclides from the nitric acid medium.
... Conventional methods for R calculations include nonequilibrium molecular dynamics (NEMD) and transient pump-probe methods. [144][145][146] Zhan et al. 147 collected the thermal boundary resistance data for various materials from 62 published journal papers. A series of impact factors, such as measurement temperature, film thickness, and heat capacity, has been considered. ...
Artificial intelligence (AI) has been referred to as the “fourth paradigm of science,” and as part of a coherent toolbox of data‐driven approaches, machine learning (ML) dramatically accelerates the computational discoveries. As the machinery for ML algorithms matures, significant advances have been made not only by the mainstream AI researchers, but also those work in computational materials science. The number of ML and artificial neural network (ANN) applications in the computational materials science is growing at an astounding rate. This perspective briefly reviews the state‐of‐the‐art progress in some supervised and unsupervised methods with their respective applications. The characteristics of primary ML and ANN algorithms are first described. Then, the most critical applications of AI in computational materials science such as empirical interatomic potential development, ML‐based potential, property predictions, and molecular discoveries using generative adversarial networks (GAN) are comprehensively reviewed. The central ideas underlying these ML applications are discussed, and future directions for integrating ML with computational materials science are given. Finally, a discussion on the applicability and limitations of current ML techniques and the remaining challenges are summarized. This article is categorized under: Computer and Information Science > Chemoinformatics. Structure and Mechanism > Computational Materials Science. Computer and Information Science > Computer Algorithms and Programming. Software > Molecular Modeling. Accelerated discoveries in computational materials science enabled by supervised machine learning and generative adversarial network.
... While in EMD, the transport process could be quantified by the linear response. The crucial difference in calculation thermophysical properties between NEMD and EMD arises from finitesize effects [44,45]. For instance, the disparity of thermal conductivity between RNEMD calculation and experiments could have resulted from the finite simulation cell. ...
Molecular dynamics (MD) simulations were conducted to evaluate paraffin/EVA/graphene nanocomposites as phase change materials (PCM) so as to study the thermophysical properties. Along with experimental fabricated paraffin PCMs in EVA matrix with or without graphene, five molecular models with graphene content ranging from 0 to 7.0 wt% were constructed. The results demonstrated microscopic distribution and structure of the three components, which therefore would affect corresponding thermal and mechanical properties under different temperatures. Non-equilibrium MD simulations showed that thermal conduction would be promoted with the introduction of graphene. However, complex interactions between EVA and graphene would restrain further improvement due to the disordered PCM crystal structures as graphene content reached 7.0 wt%. Equilibrium MD simulations exhibited agreement that the diffusion coefficient of molecules increased at first and decreased later with graphene content, consequently influencing the mobility of molecules and phonon transport.
... Up to now, available simulations and experiments have indicated that several factors such as nanoparticle size, nanoparticle material, volume fraction, and temperature [65,66] have a great impact on the improvement of the thermal transport properties.. Besides, before a solar thermal power generation system is developed, several thermal characteristics must be obtained to make the system work properly, including density, thermal conductivity, shear viscosity and self-diffusion coefficient in a whole operating temperature range. However, it is quite difficult to obtain these characteristics from experimental measurements owing to extreme high temperature conditions and high chemical activity of molten salts [54]. ...
It is urgently needed to improve the thermal properties of molten salt based phase change materials used for effective storage and utilization of solar energy. In this paper, the physical model of NaCl-SiO2 composite phase change materials (CPCM) was established. An effective method based on molecular dynamics (MD) simulation was proposed and validated to predict the thermal properties of CPCM. The structural deformation during phase transition process of CPCM system was observed and the radial distribution function (RDF) was calculated to analyze the local structure. The results indicate that the thermal conductivity of NaCl is enhanced remarkably with a maximum increase of 44.2% by adding 2.4% volume fraction of SiO2 nanoparticles and the mechanism of the thermal conductivity enhancement was discussed at the atomic level. The shear viscosity increases with the increase of the volume fraction of nanoparticles, with a maximum average increase of 23.6%. The relationship between self-diffusion coefficient and temperature is approximate to predict melting point. The force field and simulation methods adopted in this paper are desired to be useful for the prediction of thermal properties and further investigation into molten salts based thermal energy storage systems.
... The formation of Carbon Nanotubes (CNTs) is tubular structures made of carbon atoms, having a diameter in nanometres but a length in micrometres. CNTs possess many properties that make them highly valued and sought after; such as high strength comparable to steel, electrical and thermal conductivity, stiffness and toughness and they have a wide range of applications in different industries [20][21][22][23][24] . There are different methods to produce CNTs including Chemical Vapour Deposition (CVD), Arc Discharge, Laser Ablation and the other less explored methods such as Pyrolysis 25 . ...
In this study activated carbon (AC) and carbon nanotubes (CNTs) were synthesised from Brewer's Spent Grain (BSG); a form of lignocellulosic biomass, more commonly known as barley waste. A novel approach involving two activation steps; firstly, with phosphoric acid (designated BAC‐P) and then using potassium hydroxide (designated BAC‐K) was proposed for the production of AC and CNTs from BSG. The AC produced showed a surface area as high as 692.3 m2.g−1 with a pore volume of 0.44 cm3.g−1. This can help aid and facilitate the circular economy by effectively up‐cycling and valorising waste lignocellulosic biomass to high surface area AC and subsequently, multi‐walled carbon nanotubes (MWCNTs). Consequently, MWCNTs were prepared from the produced AC by mixing it with the nitrogen‐based material melamine and iron precursor, iron (III) oxalate hexahydrate, where it produced a hydrophilic multi‐wall carbon nanotubes (MWCNTs). Both AC and CNTs materials were used in heavy metal removal (HMR), where the maximum lead absorption was observed for sample BAC‐K with 77% removal capacity after the first hour of testing. This result signifies that the synthesis of these up‐cycled materials can have application in the areas of wastewater treatment or other AC/CNT end uses with a rapid cycle time. This article is protected by copyright. All rights reserved.
... Overall, manipulation of surface wetting was conceived as the most effective way to control ITR. Multiple studies were dedicated to characterizing the effect of various forms of surface modifications, such as surface patterning [35,36], surface conditions [37], coating material [38][39][40], and thickness of solid coating [41], which provide a passive ITR control. Instead, coupling at the liquid-solid interface can be actively controlled using an electric field. ...
Nanoscale heat transfer between two parallel silicon slabs filled with deionized water was studied under varying electric field in heat transfer direction. Two oppositely charged electrodes were embedded into the silicon walls to create a uniform electric field perpendicular to the surface, similar to electrowetting-on-dielectric technologies. Through the electrostatic interactions, (i) surface charge altered the silicon/water interface energy and (ii) electric field created orientation polarization of water by aligning dipoles to the direction of the electric field. We found that the first mechanism can manipulate the interface thermal resistance and the later can change the thermal conductivity of water. By increasing electric field, Kapitza length substantially decreased to 1/5 of its original value due to enhanced water layering, but also the water thermal conductivity lessened slightly since water dynamics were restricted; in this range of electric field, heat transfer was doubled. With a further increase of the electric field, electro-freezing (EF) developed as the aligned water dipoles formed a crystalline structure. During EF (0.53 V/nm), water thermal conductivity increased to 1.5 times of its thermodynamic value while Kapitza did not change; but once the EF is formed, both Kapitza and conductivity remained constant with increasing electric field. Overall, the heat transfer rate increased 2.25 times at 0.53 V/nm after which it remains constant with further increase of the electric field.
It is proven that introduction of graphene into typical heterostructures can effectively reduce the high interfacial thermal resistance in semiconductor chips. The crystallinity of graphene varies greatly; thus, we have investigated the effects of single-crystal and polycrystalline graphene on the thermal transport of AlN/graphene/3C-SiC heterostructures by molecular dynamics. The results show that polycrystalline graphene contributes more to the interfacial thermal conductance (ITC) inside the chip with a maximum increase of 75.09%, which is further confirmed by the energy transport and thermal relaxation time. Multiple analyses indicate that grain boundaries lead to the increase in C−Si covalent bonds, and thus, strong interactions improve the ITC. However, covalent bonding further causes local tensile strain and wrinkles in graphene. The former decreases the ITC, and the latter leads to the fluctuation of the van der Waals interaction at the interface. The combined effect of various influential factors results in the increase in the ITC, which are confirmed by phonon transmission with 0−18 THz. In addition, wrinkles and covalent bonding lead to increased stress concentration in polycrystalline graphene. This leads to a maximum reduction of 19.23% in the in-plane thermal conductivity, which is not conducive to the lateral diffusion of hot spots within the chip. The research results would provide important guidance in designing for high thermal transport performance high-power chips.
The rapid urbanization and industrialization is causing worldwide water pollution, calling for advanced cleaning methods.
For instance, pollutant adsorption on magnetic oxides is efcient and very practical due to the easy separation from solutions
by an magnetic feld. Here we review the synthesis and performance of magnetic oxides such as iron oxides, spinel ferrites,
and perovskite oxides for water remediation. We present structural, optical, and magnetic properties. Magnetic oxides are
also promising photocatalysts for the degradation of organic pollutants. Antimicrobial activities and adsorption of heavy
metals and radionucleides are also discussed.
Graphene/β-Ga2O3 heterojunctions are widely used in high-power and high-frequency devices, for which thermal management is vital to the device operation and life. Here we apply molecular dynamics simulation to calculate the interfacial thermal resistance (ITR) between graphene and β-Ga2O3. Based on the rigid ion model, a self-consistent interatomic potential with a set of parameters that can well reproduce the basic physical properties of crystal β-Ga2O3 is fitted. Using this potential, the effects of model size, interface type, temperature, vacancy defects and graphene hydrogenation on the ITR of graphene/β-Ga2O3 heterojunctions are evaluated. The results show that there is no obvious dependence of ITR on the size of graphene and β-Ga2O3. It is reported that the ITR values of the (100), (010) and (001) interfaces are 7.28 ± 0.35 × 10-8 K m2 W-1, 6.69 ± 0.44 × 10-8 K m2 W-1 and 5.22 ± 0.35 × 10-8 K m2 W-1 at 300 K, respectively. Both temperature increase and vacancy defect increase can prompt the energy propagation across graphene/β-Ga2O3 interfaces due to the enhancement of phonon coupling. In addition, graphene hydrogenation provides new channels for in-plane and out-of-plane phonon coupling, and thus reduces the ITR between graphene and β-Ga2O3. This study provides basic strategies for the thermal design and management of graphene/β-Ga2O3 based photoelectric devices.
Similar to graphene, diamane is a single layer of diamond that has been investigated in recent years due to its peculiar mechanical, thermal, and electronic properties. Motivated by earlier work that showed an exceptionally high intra-plane thermal conductivity in diamane, in this work, we investigate the interfacial thermal resistance ( R) between graphene and diamane using non-equilibrium classic molecular dynamics (MD) simulations. The calculated R for a pristine graphene and AB-stacked diamane at room temperature is 1.89 ×10 ⁻⁷ K×m ² /W, which is comparable to other common graphene/semi-conductor bilayers. These results are understood in terms of the overlap of the phonon density of states between the graphene and diamane layers. We further explore the impact of stacking pattern, system temperature, coupling strength, in-plane tensile strain, and hydrogenation ratio on R. Intriguingly, we find that unlike single layer diamane, where the intra-plane thermal conductively is reduced by ~50% under 5% strain, the inter-plane thermal conductance of the graphene-diamane bilayer is enhanced by ~50% under 8% strain. The difference is caused by the opposite behavior between the inter- and intra-layer conductance as phonon relaxation time is decreased. The high intra-plane thermal conductivity and low inter-plane thermal resistance shows the high potential of using graphene-diamane heterostructures in electronic applications.
In this paper, the effect of hydrostatic pressure on the thermal transport properties of silicon carbide (SiC) and germanium carbide (GeC) with zinc blende (ZB) and rock salt (RS) phases at room temperature is investigated by solving the phonon Boltzmann heat transport equation. Studies show that the thermal conductivities of the two materials increase with the raising pressure before the phase transition, however, the enhancement of GeC (40%) is much smaller than that of SiC (100%). Combined with phonon analysis, it is found that this phenomenon originates from the abnormal enhanced phonon scattering rate under high pressure, which partly offsets the increase of phonon group velocity and further suppresses the increase of thermal conductivity. Next, when the phase transition occurs, the thermal conductivities of SiC and GeC decrease sharply by 76% and 86%, respectively, which is mainly caused by the increase of the anharmonic characteristic related to phonon scattering. Finally, after the phase transition, the thermal conductivities of the two materials still show increasing trends with the raising pressure, however, the phonon group velocity of GeC does not change significantly and the variation of its thermal conductivity is mainly determined by the phonon lifetime, which is opposite to the case before phase transition, whose enhancement of thermal conductivity is mainly caused by the increase of phonon group velocity. The results indicate that GeC has a more complex phonon transport mechanism compared with SiC, i.e., its thermal conductivity before phase transition is determined by harmonic characteristic and that after phase transition shows a closer relationship with the anharmonic characteristic. This study provides data and mechanism support for the regulation of thermal conductivity from the perspective of pressure-induced phase transition, and as well as supplies some guiding suggestions for engineering applications in the field of thermal transport regulation.
In the field of layered two-dimensional functional materials, black phosphorus has attracted considerable attention in many applications due to its outstanding electrical properties. It has experimentally shown superior chemical sensing performance for the room temperature detection of NO2, highlighting high sensitivity at a ppb level. Unfortunately, pristine black phosphorus demonstrated an unstable functionality due to the fast degradation of the material when exposed to the ambient atmosphere. In the present work, a deepened investigation by density functional theory was carried out to study how nickel decoration of phosphorene can improve the stability of the material. Further, an insight into the sensing mechanism of nickel-loaded phosphorene toward NO2 was given and compared to pristine phosphorene. This first-principles study proved that, by introducing nickel adatoms, the band gap of the material decreases and the positions of the conduction band minimum and the valence band maximum move toward each other, resulting in a drop in the conduction band minimum under the redox potential of O2/O2 -, which may result in a more stable material. Studying the adsorption of O2 molecules on pristine phosphorene, we also proved that all oxygen molecules coming from the surrounding atmosphere react with phosphorus atoms in the layer, resulting in the oxidation of the material forming oxidized phosphorus species (PO x ). Instead, by introducing nickel adatoms, part of the oxygen from the surrounding atmosphere reacts with nickel atoms, resulting in a decrease of the oxidation rate of the material and in subsequent long-term stability of the device. Finally, possible reaction paths for the detection of NO2 are given by charge transfer analyses, occurring at the surface during the adsorption of oxygen molecules and the interaction with the target gas.
Ball end magnetorheological finishing (BEMRF) is a nanofinishing process for finishing 3D surfaces of a large variety of materials such as glass, steel, copper, polycarbonates, silicon, etc. Under the influence of magnetic field, abrasive-laden ball of magnetorheological polishing fluid present at the tip of the tool removes material from the workpiece surface. The knowledge of forces associated with the process aids in understanding the material removal mechanism and the process physics. Also, the prediction of finishing spot plays a vital role in increasing the process capabilities of BEMRF process in the area of localized/selective finishing. In this work, a theoretical model of finishing forces is presented that helps in improving the in-depth understanding of the nanofinishing process. In addition to it, a theoretical model of finishing spot size is also proposed. Depending upon the area of workpiece to be finished locally/selectively, the finishing spot model provides a deterministic way to alter the size of finishing spot of BEMRF process by changing the finishing parameters.
The tribological properties and behaviors in the nanometric machining play a critical role in the high surface quality and low subsurface damage for the machined materials. However, in situ TEM experiments have the limitation of length and time scales to investigate the dynamic nanomachining process. Molecular dynamics (MD) simulation is widely employed to describe the nanomachining at atomic scale, and provides some dynamic deformation details which can be hardly revealed by the experiment. In this chapter, we review recent works on the nanometric machining to understand the tribological behaviour. The fundamentals of nanometric machining in term of the friction, material removal, tool wear, and lubrication, are discussed for deeply understanding of the physical mechanisms of nanomachining induced tribological behaviour.
The rapid urbanization and industrialization is causing worldwide water pollution, calling for advanced cleaning methods. For instance, pollutant adsorption on magnetic oxides is efficient and very practical due to the easy separation from solutions by an magnetic field. Here we review the synthesis and performance of magnetic oxides such as iron oxides, spinel ferrites, and perovskite oxides for water remediation. We present structural, optical, and magnetic properties. Magnetic oxides are also promising photocatalysts for the degradation of organic pollutants. Antimicrobial activities and adsorption of heavy metals and radionucleides are also discussed.
The Kapitza resistance is of fundamental importance for the thermal stability of the interface between the ceramic top coat and the thermal growth oxide layer in the thermal barrier coating structure, which is widely used to protect high-temperature components in current gas turbine engines. The top coat typically consists of the ZrO\(_{\mathrm {2}}\) partially stabilized by 8% Y\(_{\mathrm {2}}\)O\(_{\mathrm {3}}\) (YSZ), and the main component of the thermal growth oxide is \(\alpha \)-Al\(_{\mathrm {2}}\)O\(_{\mathrm {3}}\). In this work, the Kapitza resistance is found to be a small value of 0.69 \({\hbox {m}^{\mathrm {2}}}\cdot \)K/GW for the YSZ/\(\alpha \)-Al\(_{\mathrm {2}}\)O\(_{\mathrm {3}}\) interface based on the heat dissipation simulation method. It indicates that the localization of thermal energy is rather weak, which is beneficial for the thermal stability of the YSZ/\(\alpha \)-Al\(_{\mathrm {2}}\)O\(_{\mathrm {3}}\) interface. This Kapitza resistance can be further reduced to 0.50 \({\hbox {m}^{\mathrm {2}}}\cdot \)K/GW by a mechanical or thermal compressive strain of 8%. To explore the underlying mechanism for this strain effect, we analyze the phonon vibration and the microscopic deformation in the interface region. It is revealed that the interface becomes denser through the compression-induced twisting of some Al-O\(_{\mathrm {Zr}}\) and Al-O\(_{\mathrm {Al}}\) chemical bonds in the interface region, which is responsible for the reduction in the Kapitza resistance. The temperature effect and crystal size effect on the Kapitza resistance of the YSZ/\(\alpha \)-Al\(_{\mathrm {2}}\)O\(_{\mathrm {3}}\) interface are also systematically studied. These findings shall provide valuable information for further understanding of the thermal conductivity and thermal stability of the thermal barrier coating structures.
The supercritical water oxidation (SCWO) is a technology with very low environmental impact and significant economic benefits. In present paper, the micro-scale dynamics and the kinetic reaction mechanism of the SCWO of a coal particle is investigated by the reactive force-field (ReaxFF) method for the first time. Comparisons between experimental and analytical results on the activation energy show the coal particle model and the ReaxFF MD can successfully reproduce the SCWO process. Results on the atomistic scale conversion ways of O2, CO2, H or H2 and H2O show the SCWO is not simply the gasification plus the combustion. In SCWO, the direct combustion of the coal is weakened apparently due to the existence of the water, the oxidation reaction between the water or H radicals and the oxygen to generate OH plays an important role in coal conversion. The oxidation between carbon structures and OH to generate CO2 and H2O has substituted for the direct combustion reaction with the oxygen to be the dominant way in coal conversion. This process contains the characteristics of both combustion and gasification process. The role of water molecules is something similar to the catalysis. It is converted to OH radicals first in a series of gasification and oxidation processes, then the OH radicals turn into H2O in the following oxidation reactions. After that, the effect of reaction conditions and the transition of N on the SCWO process are studied. Comparing to the single supercritical water gasification, N2 and NO are found to be the extra products in SCWO. CN, CHN and CHON are the dominant N-containing products before they are converted to N2 and NO. The results in present work give insight into the oxidation mechanism of the coal particle, and the SCWO can effectively reduce the pollution emission in a much cleaner way compared with the conventional coal-fired mode.
Interfacial thermal resistance (ITR) plays a critical role in the thermal properties of a variety of material systems. Accurate and reliable ITR prediction is vital in the structure design and thermal management of nanodevices, aircraft, buildings, etc. However, because ITR is affected by dozens of factors, traditional models have difficulty predicting it. To address this high-dimensional problem, we employ machine learning and deep learning algorithms in this work. First, exploratory data analysis and data visualization were performed on the raw data to obtain a comprehensive picture of the objects. Second, XGBoost was chosen to demonstrate the significance of various descriptors in ITR prediction. Following that, the top 20 descriptors with the highest importance scores were chosen except for fdensity, fmass, and smass, to build concise models based on XGBoost, Kernel Ridge Regression, and deep neural network algorithms. Finally, ensemble learning was used to combine all three models and predict high melting points, high ITR material systems for spacecraft, automotive, building insulation, etc. The predicted ITR of the Pb/diamond high melting point material system was consistent with the experimental value reported in the literature, while the other predicted material systems provide valuable guidelines for experimentalists and engineers searching for high melting point, high ITR material systems.
In recent years, interest in the thermal properties of graphene constituents has seen rapid growth in the fields of science and engineering. The removal of heat in the continuous processes in the electronics industry has had major issues in thermal transmission in lower-dimensional assemblies. It has also shown fascinating topographies as the carbon allotropes and their derivative compounds expel heat. Numerous research articles reported within the past 15 years have demonstrated enhanced electron flexibility, exceptional thermal conductivity and mechanical behaviour, as well as excellent optical properties of graphene as a single atomic layer. This review article tries to provide a detailed summary of the heat exchange properties of graphene structures and graphene-based materials such as nanoribbons with few-layered graphene. Thermal and energy storage management systems have played a major role in the increase in marketable products in recent times. The purpose of this review is to summarize the current research on thermal properties with regard to the management and energy storage of graphene materials, focusing on characteristic properties, industrialization, modelling and simulation, and their applications in specific thermal storage systems.
Here we use first-principles calculations and phonon interface transport modeling to calculate the thermal boundary conductance (TBC) in single layers of beyond-graphene 2D materials, including silicene, hBN, BAs, and blue and black phosphorene, on amorphous SiO2and crystalline GaN substrates. Our results show that for 2D/3D systems, the room temperature TBC can span a wide range from ~7-70 W.m-2.K-1with the lowest being for BP and highest for hBN. We also show that 2D/3D TBC has a strong temperature dependence that can be alleviated by encapsulating the 2D/3D stack. Upon encapsulation with AlOx, the TBC of several beyond-graphene 2D materials can match or exceed reported values for graphene and numerous TMDs which are in the range of 15-40 W.m-2.K-1. We compute the room temperature TBC as a function of van der Waals spring coupling and show that the TBC falls in the range of 50-150 W.m-2.K-1at coupling strengths of Ka=2-4 N.m-1for silicene, BAs, and blue phosphorene. We also identify group III-V materials with ultra-soft flexural branches as being promising 2D materials for thermal isolation and energy scavenging applications when matched with crystalline materials.
Computational materials science based on data-driven approach has gained increasing interest in recent years. The capability of trained machine learning (ML) models, such as an artificial neural network (ANN), to predict the material properties without repetitive calculations is an appealing idea to save computational time. Thermal conductivity in single or multilayer structure is a quintessential property that plays a pivotal role in electronic applications. In this work, we exemplified a data-driven approach based on ML and high-throughput computation (HTC) to investigate the cross-plane thermal transport in multilayer stanene. Stanene has attracted considerable attention due to its novel electronic properties such as topological insulating features with a wide bandgap, making it an appealing candidate to ferry current in electronic devices. Classical molecular dynamics simulations are performed to extract the lattice thermal conductivities (κL). The calculated cross-plane κL is orders of magnitude lower than its lateral counterparts. Impact factors such as layer number, system temperature, interlayer coupling strength, and compressive/tensile strains are explored. It is found that κL of multilayer stanene in the cross-plane direction can be diminished by 86.7% with weakened coupling strength, or 66.6% with tensile strains. A total of 2700 κL data are generated using HTC, which are fed into 9 different ANN models for training and testing. The best prediction performance is given by the 2-layer ANN with 30 neurons in each layer.
As the machinery of artificial intelligence matures in recent years, there has been a surge in applying machine learning techniques for material property predictions. Artificial neural network (ANN) is a branch of machine learning and has gained increasing popularity due to its capabilities of modeling complex correlations among large datasets. The interfacial thermal transport plays a significant role in the thermal management of graphene-pentacene based organic electronics. In this work, the thermal boundary resistance (TBR) between graphene and pentacene is comprehensively investigated by classical molecular dynamics simulations combined with the machine learning technique. The TBR values along the a, b and c directions of pentacene at 300 K are 5.19±0.18×108 m2 K W1, 3.66±0.36×108 m2 K W1 and 5.03±0.14×108 m2 K W1, respectively. Different architectures of ANN models are trained to predict the TBR between graphene and pentacene. Two important hyperparameters, i.e. network layer and the number of neurons are explored to achieve the best prediction results. It is reported that the 2-layer ANN with 40 neurons each layer provides the optimal model performance with a normalized mean square error loss of 7.04 104. Our results provide reasonable guidelines for the thermal design and development of graphene-pentacene electronic devices.
Micro-injection molding has attracted a wide range of research interests to fabricate polymer products with nanostructures for its advantages of cheap and fast production. The heat transfer between the polymer and the mold insert is important to the performance of products. In this study, the interface thermal resistance (ITR) between the polypropylene (PP) layer and the nickel (Ni) mold insert layer in micro-injection molding was studied by using the method of non-equilibrium molecular dynamics (NEMD) simulation. The relationships among the ITR, the temperature, the packing pressure, the interface morphology, and the interface interaction were investigated. The simulation results showed that the ITR decreased obviously with the increase of the temperature, the packing pressure and the interface interaction. Both rectangle and triangle interface morphologies could enhance the heat transfer compared with the smooth interface. Moreover, the ITR of triangle interface was higher than that of rectangle interface. Based on the analysis of phonon density of states (DOS) for PP-Ni system, it was found that the mismatch between the phonon DOS of the PP atoms and Ni atoms was the main cause of the interface resistance. The frequency distribution of phonon DOS also affected the interface resistance.
A novel photothermoelectric resistance effect of the Ti/SiO 2 /Si films induced by 10.6 [Formula: see text]m CO 2 laser is discovered and investigated. The transient response of the resistance is observed and analyzed in this work. Under the continuous irradiation of the laser, the thermal resistance value changes with the irradiating time and gradually reaches a stable saturation. The results indicate that the rise time of thermal resistance is shortened and its change rate increased as laser power gets higher. The inner battery of the ohmmeter exerts the positive or negative bias voltage, causing the diffusion motion direction of the hot electrons to be opposite or the same direction with the drift motion, which can increase or decrease the thermal resistance value. Those experimental phenomena are explained by the drift and diffusion motion of the electrons. Based on the results, the Ti/SiO 2 /Si structure is an attractive candidate for thermal effect devices.
In-plane heterojunctions, obtained by seamless joining two or more nanoribbon edges of isolated two-dimensional atomic crystals such as C3N and graphene, are emerging nanomaterials for the development of future multifunctional devices. The thermal transport behavior at the interface of these heterojunctions plays a pivotal role in determining their thermal conductivity and functional performance. Using molecular dynamics simulations, the interfacial thermal conductance G and effective thermal conductivity keff of C3N/graphene in-plane heterojunctions are investigated. The value of G for the C3N/graphene heterojunction at room temperature is 31.49 GWm⁻²K⁻¹ when heat transfers from C3N to graphene, which is larger than the value of 28.62 GWm⁻²K⁻¹in the reverse direction, indicating that thermal rectification exists at the interface. The keff values of the C3N/graphene nanoribbon along the direction from C3N to graphene and the reverse direction are 1183.60 Wm⁻¹K⁻¹ and 1346.51 Wm⁻¹K⁻¹, respectively. In addition, the G and keff of heterojunctions are effectively manipulated by changing the temperature, doping with nitrogen, applying strain and employing a substrate. A vibrational spectral analysis is performed to explore the thermal transport mechanism. The thermal energy transport across C3N/graphene interfaces is enhanced by increasing the size, temperature, nitrogen doping concentration, and compressive strain perpendicular to the heat flux direction or by depositing the materials on an amorphous silicon dioxide substrate. Furthermore, increasing the temperature and compressive strain are efficient methods to increase keff. The results provide valuable insights into the design and application of C3N/graphene-based electronic devices.
Novel hetero-structured silicon carbide-boron nitride nanosheets (SiC-BNNS) by sol-gel and in-situ growth method were performed as thermally conductive & insulating fillers, and the SiC-BNNS/epoxy thermally conductive nanocomposites were then prepared by blending-casting approach. Synthesized hetero-structured SiC-BNNS fillers have synergistic improvement effects on the thermal conductivities of the SiC-BNNS/epoxy nanocomposites. When the amount of hetero-structured SiC-BNNS fillers is 20 wt% (SiC-BNNS, 1/1, w/w), the thermal conductivity coefficient (λ) value of the SiC-BNNS/epoxy nanocomposites (0.89 W/mK) is 4.1 times that of pure epoxy resin (0.22 W/mK), and 2.1, 1.4, and 1.7 times of SiC/epoxy (0.43 W/mK), BNNS/epoxy (0.62 W/mK), and (SiC/BNNS)/epoxy thermally conductive nanocomposites (0.52 W/mK) with the same amount of fillers (20 wt% single BNNS, SiC, or SiC/BNNS hybrid fillers), respectively. Meantime, the obtained (SiC-BNNS)/epoxy thermally conductive nanocomposites also demonstrate favorable electrical insulating properties, and the breakdown strength, volume resistivity as well as surface resistivity is 22.1 kV/mm, 2.32 × 10¹⁵ Ω cm, and 1.26 × 10¹⁵ Ω cm, respectively.
ZnO is a widely used semiconductor material due to its excellent physical and mechanical properties. The thermal transport behavior across a single grain boundary (GB) of bicrystal ZnO with varying tilt angles from 5.45° to 67.38° was investigated using a nonequilibrium molecular dynamics simulation. The GB energy and Kapitza resistance as a function of tilt angle were determined and parameters of the extended Read-Shockley model were calculated. The Kapitza resistance varied monotonically with a GB angle <36° and was nearly constant when the angle was >36°. Furthermore, effective thermal conductivity and Kapitza resistance were found to depend strongly on the sample length and temperature. Finally, we compared the phonon density of states of the two types of GBs and found a mismatch in the low frequency that might explain the effect of the GBs structures on heat conduction.
Supercritical water fluidized bed is a novel gasification reactor which can achieve efficient and clean utilization of coal. The rough surface of particle produced during grinding and thermochemical conversion processing will deeply affect supercritical water-particle two-phase flow and heat transfer characteristics. In this paper, fully resolved numerical simulation of supercritical water flow past single rough sphere particle with the Reynolds number ranging from 10 to 200 was carried out to investigate the effect of surface roughness. The simulation results show that as roughness increases, the separation bubbles generated in the dimple enhance the flow separation but has no significant effect on the drag coefficient. Particle surface-average Nusselt number decreases with an increase of roughness and surface enlargement coefficient due to the isolation effect at low Re and local separation bubbles in the dimple at high Re. Furthermore, the effect of surface enlargement coefficient on heat transfer efficiency factor for supercritical water near the critical point is greater than that under constant property condition and has a higher dependence on Re.
During the last 30 years, microelectronic devices have been continuously designed and developed with smaller size and yet more functionalities. Today, hundreds of millions of transistors and complementary metal-oxide-semiconductor cells can be designed and integrated on a single microchip through 3D packaging and chip stacking technology. A large amount of heat will be generated in a limited space during the operation of microchips. Moreover, there is a high possibility of hot spots due to non-uniform integrated circuit design patterns as some core parts of a microchip work harder than other memory parts. This issue becomes acute as stacked microchips get thinner. In other applications, laser devices can generate heat fluxes up to 1000 W/cm2 in less than 0.5 mm2 areas. Light-emitting diodes also entail high heat intensities between 300 and 600 W/cm2 due to extremely high power density. Therefore, it is of technological importance that heat dissipation can be well managed and controlled in microelectronics devices. This thesis is mainly focused on the micro/nanoscale thermal conductivity and interfacial thermal resistance characterization and optimization in two-dimensional (2D) nanostructures, such as graphene, C2N, C3N, phosphorene, stanene, molybdenum disulfide, and molybdenum diselenide. Various approaches including non-equilibrium molecular dynamics (NEMD) simulation, equilibrium molecular dynamics (EMD) simulation, and transient pump-probe approaches have been utilized to explore the thermal properties. Phonon behaviors have also been studied to explain the mechanism of heat transfer. Then various machine learning (ML) models such as linear regression, polynomial regression, decision tree, random forest, and artificial neural network have been employed to predict the thermal properties of 2D materials. In a different area of research, the water desalination performance of carbon nanotube with rim functionalization has been systematically investigated using molecular dynamics (MD) simulations. Advisor: Xiao Cheng Zeng
Despite the spurring interests in two-dimensional transition metal dichalcogenides (TMDCs) materials, knowledge on the mechanical properties of its important member, i.e., molybdenum diselenide (MoSe2), is scarce and remains an open topic. In this work, the mechanical properties of h-MoSe2 and t-MoSe2 are systematically investigated using classical molecular dynamics (MD) simulations combined with machine learning (ML) techniques. Effects of chirality, temperature and strain rate on fracture strain, fracture strength and Young’s modulus are characterized in both armchair and zigzag directions. For h-MoSe2, the fracture strengths equal 13.6 and 13.0 GPa for armchair and zigzag chirality, respectively, at 1 K and strain rate of 5×10⁻⁴ ps⁻¹. The corresponding fracture strains equal 0.23 and 0.27. The Young’s moduli in armchair and zigzag directions have similar values of 100.9 and 99.5 GPa separately. For t-MoSe2, much lower fracture strengths of 6.1 and 6.3 GPa, fracture strains of 0.13 and 0.15, and Young’s moduli of 83.7 and 83.0 GPa are predicted under the same conditions. A total of 700 MD simulation cases are calculated under different impact factors and initial conditions, which are subsequently fed into the support vector machine (SVM) algorithm for ML modeling. After training, the ML model can predict the mechanical properties of both MoSe2 types given only the input features such as chirality, temperature and strain rate.
It has been a long-standing challenge to produce air-stable few- or monolayer samples of phosphorene because thin phosphorene films degrade rapidly in ambient conditions. Here we demonstrate a new highly controllable method for fabricating high quality, air-stable phosphorene films with a designated number of layers ranging from a few down to monolayer. Our approach involves the use of oxygen plasma dry etching to thin down thick-exfoliated phosphorene flakes, layer by layer with atomic precision. Moreover, in a stabilized phosphorene monolayer, we were able to precisely engineer defects for the first time, which led to efficient emission of photons at new frequencies in the near infrared at room temperature. In addition, we demonstrate the use of an electrostatic gate to tune the photon emission from the defects in a monolayer phosphorene. This could lead to new electronic and optoelectronic devices, such as electrically tunable, broadband near infrared lighting devices operating at room temperature.
It is generally expected that the interface coupling leads to the suppression of thermal transport through coupled nanostructures due to the additional interface phonon-phonon scattering. However, recent experiments demonstrated that the interface van der Waals interactions can significantly enhance the thermal transfer of bonding boron nanoribbons compared to a single freestanding nanoribbon. To obtain a more in-depth understanding on the important role of the nonlinear interface coupling in the heat transports, in the present paper, we explore the effect of nonlinearity in the interface interaction on the phonon transport by studying the coupled one-dimensional (1D) Frenkel-Kontorova lattices. It is found that the thermal conductivity increases with increasing interface nonlinear intensity for weak inter-chain nonlinearity. By developing the effective phonon theory of coupled systems, we calculate the dependence of heat conductivity on interfacial nonlinearity in weak inter-chain couplings regime which is qualitatively in good agreement with the result obtained from molecular dynamics simulations. Moreover, we demonstrate that, with increasing interface nonlinear intensity, the system dimensionless nonlinearity strength is reduced, which in turn gives rise to the enhancement of thermal conductivity. Our results pave the way for manipulating the energy transport through coupled nanostructures for future emerging applications.
The effects of size, strain, and vacancies on thermal properties of armchair
phosphorene nanotubes are investigated through molecular dynamics simulations.
It is found that the thermal conductivity has a remarkable size effect because
of the restricted paths for phonon transport, strongly depending on the
diameter and length of nanotube. Owing to the intensified low-frequency
phonons, axial tensile strain can facilitate thermal transport. On the
contrary, compressive strain weakens thermal transport due to the enhanced
phonon scattering around the buckling of nanotube. In addition, the thermal
conductivity is dramatically reduced by single vacancies, especially upon high
defect concentrations.
As a new two-dimensional (2D) material, phosphorene has drawn growing attention owing to its novel electronic properties, such as layer-dependent direct bandgaps and high carrier mobility. Herein we investigate the in-plane and cross-plane thermal conductivities of single- and multi-layer phosphorene, focusing on geometrical (sample size, orientation and layer number) and strain (compression and tension) effects. A strong anisotropy is found in the in-plane thermal conductivity with its value along the zigzag direction being much higher than that along the armchair direction. Interestingly, the in-plane thermal conductivity of multi-layer phosphorene is insensitive to the layer number, which is in strong contrast to that of graphene where the interlayer interactions strongly influence the thermal transport. Surprisingly, tensile strain leads to an anomalous increase in the in-plane thermal conductivity of phosphorene, in particular in the armchair direction. Both the in-plane and cross-plane thermal conductivities can be modulated by external strain; however, the strain modulation along the cross-plane direction is more effective and thus more tunable than that along the in-plane direction. Our findings here are of great importance for the thermal management in phosphorene-based nanoelectronic devices and for thermoelectric applications of phosphorene.
Phosphorene, a new two-dimensional (2D) material beyond graphene, has attracted great
attention in recent years due to its superior physical and electrical properties. However,
compared to graphene and other 2D materials, phosphorene has a relatively low Young’s
modulus and fracture strength, which may limit its applications due to possible structure
failures. For the mechanical reliability of future phosphorene-based nanodevices, it is
necessary to have a deep understanding of the mechanical properties and fracture behaviors
of phosphorene. Previous studies on the mechanical properties of phosphorene were
based on first principles calculations at 0 K. In this work, we employ molecular dynamics
simulations to explore the mechanical properties and fracture behaviors of phosphorene at
finite temperatures. It is found that temperature has a significant effect on the mechanical
properties of phosphorene. The fracture strength and strain reduce by more than 65% when
the temperature increases from 0 K to 450 K. Moreover, the fracture strength and strain
in the zigzag direction is more sensitive to the temperature rise than that in the armchair
direction. More interestingly, the failure crack propagates preferably along the groove in the
puckered structure when uniaxial tension is applied in the armchair direction. In contrast,
when the uniaxial tension is applied in the zigzag direction, multiple cracks are observed with
rough fracture surfaces. Our present work provides useful information about the mechanical
properties and failure behaviors of phosphorene at finite temperatures.
We investigate the electromechanical coupling in 2d materials. For non-Bravais lattices, we find important corrections to the standard macroscopic strain - microscopic atomic-displacement theory. We put forward a general and systematic approach to calculate strain-displacement relations for several classes of 2d materials. We apply our findings to graphene as a study case, by combining a tight binding and a valence force-field model to calculate electronic and mechanical properties of graphene nanoribbons under strain. The results show good agreement with the predictions of the Dirac equation coupled to continuum mechanics. For this long wave-limit effective theory, we find that the strain-displacement relations lead to a renormalization correction to the strain-induced pseudo-magnetic fields. Implications for nanomechanical properties and electromechanical coupling in 2d materials are discussed.
Emergence of two-dimensional (2D) materials with atomic-layer structures such as graphene and MoS2, which have excellent physical properties, provide the opportunity of substituting silicon-based micro/nano-electronics. An important issue before large-scale applications is the heat dissipation performance of these materials, especially when they are supported on a substrate, as in most scenarios. Thermal transport across the atomic-layer interface is essential to the heat dissipation of 2D materials due to the extremely large contact area with the substrate, when compared with their atomic-scale cross-sections. Therefore, the understanding of the interfacial thermal transport is important, but the characterization is very challenging due to the limitations for temperature/thermal probing of these atomic-layer structures. In this review, widely used characterization techniques for experimental characterization as well as their results are presented. Emphasis is placed on the Raman-based technology for nm and sub-nm temperature differential characterization. Then we present physical understanding through theoretical analysis and molecular dynamics. A few representative works about the molecular dynamics studies, including our studies on the size effect and rectification phenomenon of the graphene-Si interfaces are presented. Challenges as well as opportunities in the thermal transport study of atomic-layer structures are discussed. Though many works have been reported, there is still much room in both the development of experimental techniques as well as atomic-scale simulations for a clearer understanding of the physical fundamentals of thermal transport across the atomic-layer interfaces, considering the remarkable complexity of physical/chemical conditions at the interface.
At the nanoscale, differently to what happens at the macroscale, friction even without an applied normal pressure and spontaneous adhesion take place. In particular, the nanotribology between two layers of graphene, or other two-dimensional nanomaterials (even curved, such as nanotube walls), remains controversial. It is sufficient to say that friction between two graphene layers or nanotube walls is described in the current literature giving as ”material property” a constant friction force or a constant friction shear strength, even if such views are obviously mutually exclusive. Is friction dominated by a strength, by a force or by an energy? Coupling elasticity and energy balance we solve this paradox deriving a generalization of the celebrated Coulomb’s friction law, reconciling the two current views. Molecular dynamics simulations on graphene are conducted to verify its validity at the nanoscale whereas statistical simulations confirm its validity even at the macroscale.
Low thermal resistance GaN-on-diamond wafers offer enhanced thermal management with respect to GaN-on-SiC devices. The GaN/diamond interfacial thermal resistance can contribute significantly to the total device thermal resistance and must therefore be minimized to gain the maximum benefit from GaN-on-diamond. A contactless thermoreflectance measurement technique has been developed, which can be used after wafer growth and before device fabrication, enabling rapid feedback about the influence of growth parameters on interfacial thermal resistance. A measured (2times ) reduction in the GaN/diamond interfacial resistance is achieved by reducing the dielectric thickness between the GaN and diamond from 90 to 50 nm, enabling a potential 25% increase in transistor power dissipation for GaN-on-diamond.
The translational symmetry breaking of a crystal at its surface may form two-dimensional (2D) electronic states. We observed
one-dimensional nonlinear optical edge states of a single atomic membrane of molybdenum disulfide (MoS2), a transition metal dichalcogenide. The electronic structure changes at the edges of the 2D crystal result in strong resonant
nonlinear optical susceptibilities, allowing direct optical imaging of the atomic edges and boundaries of a 2D material. Using
the symmetry of the nonlinear optical responses, we developed a nonlinear optical imaging technique that allows rapid and
all-optical determination of the crystal orientations of the 2D material at a large scale. Our technique provides a route
toward understanding and making use of the emerging 2D materials and devices.
Using equilibrium molecular dynamic simulations, we calculate the phonon thermal conductivity of a graphene-like silicon nanosheet called silicene at room temperature. We find that the in-plane thermal conductivity of silicene sheets is about one order of magnitude lower than that of bulk silicon. We further investigate the effects of vacancy defects on thermal conductivity and observe its significant diminution owing to the effect of phonon-defect scattering. Our results show that phonon transport in a silicene sheet is strongly affected by vacancy concentration, vacancy size, and vacancy boundary shape; this could be used to guide defects engineering of the thermal properties of low-dimensional silicon materials.
Interfacial thermal resistance is an important factor that has a considerable effect on the thermal conductivity of composites. especially nanocomposites. and must therefore be considered when developing new composites for various structural and nonstructural applications. However, reported data on interfacial thermal resistance arc sparse as a result of a lack of efficient measurement methods. We developed a new analytical and measurement method for the determination of the interfacial thermal resistance between a metal and a dielectric material by using a technique involving periodic Joule (ohmic) heating and thermo-reflectance. The principle is based on a one-dimensional model of heat conduction in a two-layered system, taking into account the interfacial thermal resistance. By using this method, the interfacial thermal resistances between An films and substrates of SiO2, glass or sapphire single crystal were measured. The results were compared with values calculated by the diffusion mismatch model, and the experimental factors that might affect the interfacial thermal resistance are discussed.
Using nonequilibrium molecular dynamics simulations in which a temperature gradient is imposed, we determine the thermal resistance of a model liquid–solid interface. Our simulations reveal that the strength of the bonding between liquid and solid atoms plays a key role in determining interfacial thermal resistance. Moreover, we find that the functional dependence of the thermal resistance on the strength of the liquid–solid interactions exhibits two distinct regimes: (i) exponential dependence for weak bonding (nonwetting liquid) and (ii) power law dependence for strong bonding (wetting liquid). The identification of the two regimes of the Kapitza resistance has profound implications for understanding and designing the thermal properties of nanocomposite materials. © 2003 American Institute of Physics.
The observation of massless Dirac fermions in monolayer graphene has generated a new area of science and technology seeking to harness charge carriers that behave relativistically within solid-state materials. Both massless and massive Dirac fermions have been studied and proposed in a growing class of Dirac materials that includes bilayer graphene, surface states of topological insulators and iron-based high-temperature superconductors. Because the accessibility of this physics is predicated on the synthesis of new materials, the quest for Dirac quasi-particles has expanded to artificial systems such as lattices comprising ultracold atoms. Here we report the emergence of Dirac fermions in a fully tunable condensed-matter system-molecular graphene-assembled by atomic manipulation of carbon monoxide molecules over a conventional two-dimensional electron system at a copper surface. Using low-temperature scanning tunnelling microscopy and spectroscopy, we embed the symmetries underlying the two-dimensional Dirac equation into electron lattices, and then visualize and shape the resulting ground states. These experiments show the existence within the system of linearly dispersing, massless quasi-particles accompanied by a density of states characteristic of graphene. We then tune the quantum tunnelling between lattice sites locally to adjust the phase accrual of propagating electrons. Spatial texturing of lattice distortions produces atomically sharp p-n and p-n-p junction devices with two-dimensional control of Dirac fermion density and the power to endow Dirac particles with mass. Moreover, we apply scalar and vector potentials locally and globally to engender topologically distinct ground states and, ultimately, embedded gauge fields, wherein Dirac electrons react to 'pseudo' electric and magnetic fields present in their reference frame but absent from the laboratory frame. We demonstrate that Landau levels created by these gauge fields can be taken to the relativistic magnetic quantum limit, which has so far been inaccessible in natural graphene. Molecular graphene provides a versatile means of synthesizing exotic topological electronic phases in condensed matter using tailored nanostructures.
Using equilibrium and non-equilibrium molecular dynamics simulations, we determine the Kapitza resistance (or thermal contact resistance) at a model liquid-solid interface. The Kapitza resistance (or the associated Kapitza length) can reach appreciable values when the liquid does not wet the solid. The analogy with the hydrodynamic slip length is discussed.
A non-equilibrium molecular dynamics model is developed to investigate how a thin film confined between two dissimilar solids affects the thermal transport across the material interface. For two highly dissimilar (phonon frequency mismatched) solids, it is found that the insertion of a thin film between them can greatly enhance thermal transport across the material interface by a factor of 2.3 if the thin film has one of the following characteristics: (1) a multi-atom-thick thin film of which the phonon density of states (DOS) bridges the two different phonon DOSs for the solid on each side of the thin film; (2) a single-atom-thick film which is weakly bonded to the solid on both sides of the thin film. The enhanced thermal transport in the single-atom-thick film case is found mainly due to the increased inelastic scattering of phonons by the atoms in the film. However, for solid-solid interfaces with a relatively small difference in the phonon DOS, it is found that the insertion of a thin film may decrease the thermal transport.
On-line monitoring of the unit’s primary frequency regulation performance can be realized on the basis of measured data from WAMS (Wide Area Measurement System). Based on measured data and according to the index importance, this paper establishes the unit primary frequency regulation performance evaluation index system, including the percentage of pass of the primary frequency regulation response lag time and stabilization time, the effect of primary frequency regulation, and percentage of pass of maximum regulation amplitude of load. Then, it employs close value methods to achieve the comprehensive performance evaluation of the primary frequency. The proposed method can provide some reference for comprehensive evaluations of the actual performance of a unit’s primary frequency regulation.
The performance of electronic and optoelectronic devices based on two-dimensional layered crystals, including graphene, semiconductors of the transition metal dichalcogenide family such as molybdenum disulphide (MoS2) and tungsten diselenide (WSe2), as well as other emerging two-dimensional semiconductors such as atomically thin black phosphorus, is significantly affected by the electrical contacts that connect these materials with external circuitry. Here, we present a comprehensive treatment of the physics of such interfaces at the contact region and discuss recent progress towards realizing optimal contacts for two-dimensional materials. We also discuss the requirements that must be fulfilled to realize efficient spin injection in transition metal dichalcogenides.
A recently discovered two-dimensional (2D) layered material phosphorene has attracted considerable interest as a promising p-type semiconducting material. In this work, thermal conductivity ([small kappa]) of monolayer phosphorene is calculated using large-scale classical non-equilibrium molecular dynamics (NEMD) simulations. The predicted thermal conductivities for infinite length armchair and zigzag phosphorene sheets are 63.6+3.9-3.9 and 110.7+1.75-1.75 W m-1 K-1 respectively. The strong anisotropic thermal transport is attributed to the distinct atomic structures at altered chiral directions and direction-dependent group velocities. Thermal conductivities of 2D graphene sheets with the same dimensions are also computed for comparison. The extrapolated [small kappa] of the 2D graphene sheet are 1008.5+37.6-37.6 and 1086.9+59.1-59.1 W m-1 K-1 in the armchair and zigzag directions, respectively, which are an order of magnitude higher than those of phosphorene. The overall and decomposed phonon density of states (PDOS) are calculated in both structures to elucidate their thermal conductivity differences. In comparison with graphene, the vibrational frequencies that can be excited in phosphorene are severely limited. The temperature effect on the thermal conductivity of phosphorene and graphene sheets is investigated, which reveals a monotonic decreasing trend for both structures.
This review summarizes the state-of-the-art progress in the molecular dynamics (MD) simulation of graphene's novel thermal properties. Graphene's novel thermal properties, including anisotropic thermal conductivity, decoupled phonon thermal transport, thermal rectification, and tunable interfacial thermal conductance, have attracted enormous interests in the development of next-generation nano-devices. Molecular dynamics simulation is one of the main approaches in numerical simulation of graphene's novel thermal properties. In this paper, the widely used potentials in MD for modeling graphene's novel thermal properties were first described. Then the activities on MD simulation of anisotropic thermal conductivity, decoupled phonon thermal transport, thermal rectification, and tunable interfacial thermal conductance, were discussed, respectively. Finally, the paper was concluded with highlights on both the current status and future directions of the MD simulation of graphene's novel thermal properties.
In this work, the interfacial thermal transport across silicene and various substrates, i.e., crystalline silicon (c-Si), amorphous silicon (a-Si), crystalline silica (c-SiO2) and amorphous silica (a-SiO2) are explored by classical molecular dynamics (MD) simulations. A transient pulsed heating technique is applied in this work to characterize the interfacial thermal resistance in all hybrid systems. It is reported that the interfacial thermal resistances between silicene and all substrates decrease nearly 40% with temperature from 100 K to 400 K, which is due to the enhanced phonon couplings from anharmonicity effect. The phonon power spectra analysis for all systems is performed to interpret simulation results. Contradictory to the traditional thought that amorphous structures intend to have poor thermal transport capabilities due to the disordered atomic configurations, it is calculated that amorphous silicon and silica substrates facilitate the interfacial thermal transport compared with their crystalline structures. Besides, the coupling effect from substrate can improve the interface thermal transport up to 43.5 % for coupling strengths from 1.0 to 2.0. Our results provide fundamental knowledge and rational guidelines for the design and development of the next-generation silicene-based nanoelectronics and thermal interface materials.
Amorphous black phosphorus (a-BP) ultrathin films are deposited by pulsed laser deposition. a-BP field-effect trans-istors, exhibiting high carrier mobility and moderate on/off current ratio are demonstrated. Thickness dependence of band gap, mobility, and on/off ratio are observed. These results offer not only a new nanoscale member in the BP family, but also a new opportunity to develop nano-electronic devices.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Enhancing thermal transport mechanisms in nanostructures and nanomaterials are important factors for their use in green renewable energy applications. The behaviors and reliability of nanoscale devices strongly depend on the way the systems dissipate heat. Therefore, using non-equilibrium molecular dynamics (MD) simulations, we investigated the interface thermal resistance between liquid water and various metallic surfaces in nanochannels. Solid–liquid interface thermal resistance is well known as the Kapitza length. In this study, we model heat transfer through two parallel solid walls separated by liquid water, holding each solid wall at a different temperature to impose a temperature gradient. Silicon with a diamond crystal lattice structure and copper, silver, gold, and platinum with face-centered cubic (FCC) crystal lattice structure were chosen as the solid materials due to their extensive applications in nanotechnology. Temperature jumps at such solid–liquid interfaces are due to thermal transport between the dissimilar materials, resulting in an interface thermal resistance. We observed the behavior of liquid water molecules in the vicinity of the metallic surfaces, revealing that the Kapitza length varies as a function of solid–liquid interaction strength, and confirming the effect of Lennard–Jones (LJ) interaction added to long-range Coulombic interaction in the liquid model, and using liquid water instead of simple LJ liquid.
We propose to parametrize the Stillinger-Weber potential for covalent
materials starting from the valence force field model. All geometrical
parameters in the Stillinger-Weber potential are determined analytically
according to the equilibrium condition for each individual potential term,
while the energy parameters are derived from the valence force field model.
There are several advanced features for the Stillinger-Weber potentials
parametrized by this approach. First, this potential inherits the accuracy in
the description of linear properties from the valence force field model.
Second, this potential supports for stable molecular dynamics simulations, as
each potential term is at energy minimum state separately at the equilibrium
configuration. We employ this procedure to parametrize Stillinger-Weber
potentials for the single-layer MoS2 and black phosphorous. The obtained
Stillinger-Weber potentials predict accurate phonon spectrum and mechanical
behaviors. We also provide input scripts of these Stillinger-Weber potentials
used by publicly available simulation packages including GULP and LAMMPS.
As the dimensions of nanocircuits and nanoelectronics shrink, thermal energies are being generated in more confined spaces, making it extremely important and urgent to explore for efficient heat dissipation pathways. In this work, the phonon energy transport across graphene and hexagonal boron-nitride (h-BN) interface is studied using classic molecular dynamics simulations. Effects of temperature, interatomic bond strength, heat flux direction, and functionalization on interfacial thermal transport are investigated. It is found out that by hydrogenating graphene in the hybrid structure, the interfacial thermal resistance (R) between graphene and h-BN can be reduced by 76.3%, indicating an effective approach to manipulate the interfacial thermal transport. Improved in-plane/out-of-plane phonon couplings and broadened phonon channels are observed in the hydrogenated graphene system by analyzing its phonon power spectra. The reported R results monotonically decrease with temperature and interatomic bond strengths. No thermal rectification phenomenon is observed in this interfacial thermal transport. Results reported in this work give the fundamental knowledge on graphene and h-BN thermal transport and provide rational guidelines for next generation thermal interface
material designs.
We show that black phosphorus has room-temperature charge mobilities on the
order of 10$^4$ cm$^2$V$^{-1}$s$^{-1}$, which are about one order of magnitude
larger than silicon. We also demonstrate strong anisotropic transport in black
phosphorus, where the mobilities along the armchair direction are about one
order of magnitude larger than zigzag direction. A photocarrier lifetime as
long as 100 ps is also determined. These results illustrate that black
phosphorus is a promising candidate for future electronic and optoelectronic
applications.
Due to rapidly increasing power densities in nanoelectronics, efficient heat removal has become one of the most critical issues in thermal management and nanocircuit design. In this study, we report a surface nanoengineering design that can reduce the interfacial thermal resistance between graphene and copper substrate by 17%. Contrary to the conventional view that a rough surface tends to give higher thermal contact resistances, we find that by engraving the copper substrate with nanopillared patterns, an optimized hybrid structure can effectively facilitate the thermal transport across the graphene–copper interface. This counterintuitive behavior is due to the enhanced phonon interactions with the optimal nanopillared pattern. For pliable 2D materials like graphene, the structures can be easily bent to fit the surface formations of the substrate. The suspended areas of graphene are pulled towards the substrate via an attractive interatomic force, causing high local pressures ([similar]2.9 MPa) on the top region of nanopillars. The high local pressures can greatly enhance the thermal energy coupling between graphene and copper, thereby lowering the thermal contact resistances. Our study provides a practical way to manipulate the thermal contact resistance between graphene and copper for the improvement of nano-device performance through engineering optimal nanoscale contact.
The effect of interatomic interaction between graphene and 4H-SiC on their interfacial thermal transport is investigated by empirical molecular dynamics simulation. Two magnitudes of interfacial thermal conductance (ITC) improvement are observed for graphene/4H-SiC interface interacting through covalent bonds than through van der Waals interaction, which can be explained by the bond strength and the number of covalent bonds. Besides, it is found that the ITC of covalent graphene/C-terminated SiC is larger than that Si-terminated SiC, which is due to the stronger bond strength of C–C than that of C–Si. The effect of crystallinity of the substrate is studied, and the result shows that the ITC of graphene/a-SiC is higher than that of graphene/c-SiC. These results are crucial to the understanding of thermal transport across graphene interfaces, which are useful for thermal design in graphene-based transistors.
Due to the high surface–to–volume ratio in nanostructured components and devices, thermal transport across the solid–solid interface strongly affects the overall thermal behavior. Materials such as Si, Ge, SiO2 and GaAs are widely used in advanced semiconductor devices. These materials may have differences in both crystal structure and Debye temperature. We have shown that the thermal transport across such interfaces can be improved by inserting an interlayer between the two confining solids.
If the two confining solids are similar in crystal structure and lattice constant but different in Debye temperature, it is predicted from the molecular dynamics modeling that an over 50% reduction of the thermal boundary resistance can be achieved by inserting a 1– to 2–nm–thick interlayer which has similar crystal structure and lattice constant as the two solids. In this case, the Debye temperature of the optimized interlayer is approximately the square root of the product of the Debye temperatures of the two solids. However, if the interlayer has large lattice mismatches with the two confining solids, a thin disordered layer is formed in the solid and in the interlayer adjacent to their interface. Such a disordered layer can distort the phonon density of states at the interface and strongly affects the interfacial phonon transport. In this case, it is found that a 70% reduction of the thermal boundary resistance can be achieved if the lattice constant of the interlayer is smaller than that of the two solids and the Debye temperature of the interlayer is approximately the average of the Debye temperatures of the two solids.
On the other hand, if the two solids have a large difference in both lattice constant and Debye temperature, the optimized interlayer should have a lattice constant near the average of the lattice constants of the two solids. For this case, an over 60% reduction of the thermal boundary resistance can be achieved if the Debye temperature of the interlayer is equal to or slightly higher than the square root of the product of the Debye temperatures of the two solids. The calculated phonon density of states shows that the distorted phonon spectra induced by large lattice mismatches are generally broader than the phonon spectra of the corresponding undistorted case. The broader interfacial phonon spectra increase the overlap between the phonon spectra of the two solids at the interface which leads to improved thermal boundary transport.
Few-layer and thin film forms of layered black phosphorus (BP) have recently
emerged as a promising material for applications in high performance
nanoelectronics and infrared optoelectronics. Layered BP thin film offers a
moderate bandgap of around 0.3 eV and high carrier mobility, leading to
transistors with decent on-off ratio and high on-state current density. Here,
we demonstrate the gigahertz frequency operation of black phosphorus
field-effect transistors for the first time. The BP transistors demonstrated
here show excellent current saturation with an on-off ratio exceeding 2000. We
achieved a current density in excess of 270 mA/mm and DC transconductance above
180 mS/mm for hole conduction. Using standard high frequency characterization
techniques, we measured a short-circuit current-gain cut-off frequency fT of 12
GHz and a maximum oscillation frequency fmax of 20 GHz in 300 nm channel length
devices. BP devices may offer advantages over graphene transistors for high
frequency electronics in terms of voltage and power gain due to the good
current saturation properties arising from their finite bandgap, thus enabling
the future ubiquitous transistor technology that can operate in the multi-GHz
frequency range and beyond.