[Show abstract][Hide abstract] ABSTRACT: Thermal management to prevent extreme heat surge in integrated electronic systems and nuclear reactors is a critical issue. To delay the thermal surge on the heater effectively, we report the benefit of a three dimensional nanotubular porous layer via noncovalent interactions (hydrophobic forces and hydrogen bonds). To observe the contribution of individual noncovalent interactions in a porous network formation, pristine carbon nanotubes (PCNTs) and oxidatively functionalized carbon nanotubes (FCNTs) were compared. Hydrogen-bonded interwoven nanotubular porous layer showed approximately two times critical heat flux (CHF) increase compared to that of a plain surface. It is assumed that the hydrophilic group-tethered nanotubular porous wicks and enhanced fluidity are the main causes for promoting the CHF increase. Reinforced hydrophilicity assists liquid spreading and capillarity-induced liquid pumping, which are estimated by using Electrochemical Impedance Spectroscopy. Also, shear induced thermal conduction, thermal boundary reduction, and rheology of nanoparticles could attribute to CHF enhancement phenomena.
[Show abstract][Hide abstract] ABSTRACT: Interfacial shear strength (IFSS) between particle and matrix in particulate polymer composites is a critical property in determining the mechanical behaviors since it is directly related to not only their Young’s modulus or specific strength, but also energy absorbing capability. However, the conventional techniques often present a technical challenge to accurately measure the IFSS between fillers and matrix in the composites. This is more apparent in graphene particulate composites due to their nano-scale dimensions as well as the platelet-shaped geometry. Here, the focus of this study is to use a semi-empirical approach to determine the IFSS of graphene particulate composites by combining experiments with finite element (FE) modeling. The materials of interest are reduced graphene oxide (RGO) and polycarbonate (PC). The tensile testing was performed to characterize the mechanical properties, while simultaneously monitoring the acoustic emission events in order to measure the global debonding stress (GDS) in the composites. By coupling thermal stress analysis and deformation analysis with the GDS as input to a FE model, the IFSS of the RGO particulate PC composites was successfully estimated by about 136 MPa, avoiding unnecessary assumptions and uncertainties which are seem to be inevitable with the conventional techniques for the IFSS measurement.
[Show abstract][Hide abstract] ABSTRACT: Due to their exceptional flexibility and transparency, CVD graphene films have been regarded as an ideal replacement of indium tin oxide for transparent electrodes, especially in applications where electronic devices may be subjected to large tensile strain. However, the search for a desirable combination of stretchability and electrochemical performance of such devices remains a huge challenge. Here, we demonstrate the implementation of a laminated ultrathin CVD graphene film as a stretchable and transparent electrode for supercapacitors. Transferred and buckled on PDMS substrates by a prestraininig-then-buckling strategy, the 4-layer graphene films maintained its outstanding quality, as evidenced by Raman spectra. Optical transmittance of up to 72.9% at a wavelength of 550 nm and stretchability of 40% were achieved. As the tensile strain increased up to 40%, the specific capacitance showed no degradation and even increased slightly. Furthermore, the supercapacitor demonstrated excellent frequency capability with small time constants under stretching.
[Show abstract][Hide abstract] ABSTRACT: Graphene oxide (GO) has recently become an attractive building block for fabricating graphene-based functional materials. GO films and fibers have been prepared mainly by vacuum filtration and wet spinning. These materials exhibit relatively high Young's moduli but low toughness and a high tendency to tear or break. Here, we report an alternative method, using bar coating and drying of water/GO dispersions, for preparing large area GO thin films (e.g. 800-1200 cm(2) or larger) with an outstanding mechanical behavior and excellent tear resistance. These dried films were subsequently scrolled to prepare GO fibers with extremely large elongation to fracture (up to 76 %), high toughness (up to 17 J/m(3)) and attractive macroscopic properties, such as uniform circular cross section, smooth surface, and great knotability. This method is simple and after thermal reduction of the GO material, it can render highly electrically conducting graphene-based fibers with values up to 416 S/cm at room temperature. In this context, GO fibers annealed at 2000 °C were also successfully used as electron field emitters operating at low turn on voltages of ca. 0.48 V/μm and high current densities (5.3 A/cm(2)). Robust GO fibers and large-area films with fascinating architectures and outstanding mechanical and electrical properties were prepared bar coating followed by dry film scrolling.
[Show abstract][Hide abstract] ABSTRACT: The high Young's modulus and tensile strength of carbon nanotubes has attracted great attention from the research community given the potential for developing super-strong, super-stiff composites with carbon nanotube reinforcements. Over the decades, the strength and stiffness of carbon nanotube-reinforced polymer nanocomposites have been researched extensively. However, unfortunately, such strong composite materials have not been developed yet. It has been reported that the efficiency of load transfer in such systems is critically dependent on the quality of adhesion between the nanotubes and the polymer chains. In addition, the waviness and orientation of the nanotubes embedded in a matrix reduce the reinforcement effectiveness. In this study, we carried out performed micromechanics-based numerical modeling and analysis by varying the geometry of carbon nanotubes including their aspect ratio, orientation, and waviness. The results of this analysis allow for a better understanding of the load transfer capabilities of carbon nanotube-reinforced polymer composites.
Transactions of the Korean Society of Mechanical Engineers B 01/2014; 38(1).
[Show abstract][Hide abstract] ABSTRACT: A three-dimensional (3D) nitrogen-doped multiwall carbon nanotube (N-MWCNT) sponge possessing junctions induced by both nitrogen and sulfur was synthesized by chemical vapor deposition (CVD). The formation of "elbow" junctions as well as "welded" junctions, which are attributed to the synergistic effect of the nitrogen dopant and the sulfur promoter, plays a critically important role in the formation of 3D nanotube sponges. To the best of our knowledge, this is the first report showing the synthesis of macroscale 3D N-MWCNT sponges. Most importantly, the diameter of N-MWCNT can be simply controlled by varying the concentration of sulfur, which in turn controls both the sponge's mechanical and its electrical properties. It was experimentally shown that, with increasing diameter of N-MWCNT, the elastic modulus of the sponge increased while the electrical conductivity decreased. The mechanical behaviors of the sponges have also been quantitatively analyzed by employing strain energy function modeling.
[Show abstract][Hide abstract] ABSTRACT: The focus of this study is to experimentally investigate the effect of
debonding stress, the interface between the fibers and the polymer
matrix, on the damping properties of the short fiberglass reinforced
polymer composites. In this study, short fiberglass reinforced
polycarbonate composite materials were fabricated and characterized for
their tensile properties by varying the fiberglass loading fraction. The
debonding stress was evaluated by coupling the acoustic emission
technique with the tensile testing. After the determination of the
debonding stress was completed, dynamic cyclic testing was performed in
order to investigate the effect of debonding on the damping properties
of the polymer composites. It was experimentally observed in this study
that the debonding can facilitate the stick-slip friction under cyclic
loadings, which then gives rise to better damping performance in the
Modern Physics Letters B 06/2013; 27(15):50108-. · 0.69 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recently, considerable attention has been paid on the utilization of natural materials in structures. Utilizing natural materials over traditional, synthetic structures results in a reduction of carbon emissions from material synthesis; such a source of materials could also be renewable and recyclable. Currently, few reports exist on sound and vibrational properties in sandwich composites with the use of natural materials. Sandwich composites are commonly used in structures for their superior strength and stiffness-to-weight ratios, but from these same properties, they radiate noise efficiently. Here, in this study, the sound and vibration damping properties of natural material based sandwich composites were explored and characterized. It was experimentally observed that utilizing a balsa wood core with a natural fiber based face sheet has a 100% improvement in coincidence frequency, a metric of acoustic performance, and the combination of a natural fiber based face sheet with a Rohacell 51 WF synthetic core exhibits a 233% increase over a fully synthetic sandwich composite. As these improvements in acoustic performance are achieved with only small sacrifices in bending stiffness, these results suggest that, if optimized, natural material based sandwich composites could be an environmentally friendly solution to the sandwich structure-noise radiation problem.
[Show abstract][Hide abstract] ABSTRACT: An ever increasing demand for material performance coupled with recent advances in the production and availability of nanoscale materials has led to a significant interest in the use of nanoscale fillers to augment and tailor material performance in nanocomposites. Specifically, the use of high aspect ratio fillers, such as carbon nanotubes and carbon nanofibers (CNF) to augment the viscoelastic performance of nanocomposites has been the focus of many studies. Previous study has shown the use of high aspect ratio fillers to significantly enhance the damping capacity at low frequencies by more than 100 %, relative to the neat epoxy. In light of the promise, this technology holds for use in engineered applications, requiring specific damping performance, there remains a fundamental lack in understanding of the precise mechanisms and thereby a lack of ability to accurately predict material performance, which is limiting application of the technology. This study looks at both the effect of the random filler orientation and the effect of filler waviness in examining the viscoelastic response of CNF-reinforced nanocomposites. Using a fundamental approach, this study employs experimental, analytical, and numerical modeling techniques to characterize the amount of strain energy transferred to the filler and the matrix, and to indirectly estimate the effective loss factor of the filler. Utilizing experimental investigation coupled with parametric inquiries using strain energy methods relative to both filler orientation and waviness, this study provides fundamental insight into the effect of imperfect geometries and random filler distributions seen in nanocomposites utilizing high aspect ratio fillers, such as CNF.
Journal of Materials Science 01/2013; 48(2):832-840. · 2.31 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This study investigates strain dependent energy dissipation characteristics in carbon nanofiber (CNF) reinforced carbon fiber epoxy composites (multi-scale composites) by characterizing their viscoelastic properties and vibrational damping response. The air damping effect on the energy dissipation characteristics is also examined. The viscoelastic properties of epoxy containing two weight fractions (3 and 5 wt%) of added CNFs were characterized using dynamic mechanical analysis. Carbon fiber layers were then infiltrated with the two epoxy resins containing the CNFs to form multi-scale composites. A strain dependent loss factor behavior of the multi-scale composites was observed in the dynamic cyclic testing due to CNF’s stick–slip friction, showing a 53% increase in loss factor for the composites containing 5 wt% CNFs. The beam vibration test results also indicated an improvement in loss factor for the multi-scale composite beams relative to those without the CNF addition in the first two resonant frequencies. The multi-scale composite beams exhibit an increase in loss factor, up to 43%, at high amplitude excitation, while a reduction in loss factor was seen at low amplitude. These observed strain dependent damping characteristics seem to result from both the stick–slip friction and the air damping effect.
[Show abstract][Hide abstract] ABSTRACT: A sandwich composite is typically designed to possess high bending stiffness and low density and consists of two thin and stiff skin sheets and a lightweight core. Due to the high stiffness-to-weight and strength-to-weight ratios, sandwich composite materials are widely used in various structural applications including aircraft, spacecraft, automotive, wind-turbine blades and so on. However, sandwich composite structures used in such applications often suffer from poor acoustic performance. Ironically, these superior mechanical properties make the sandwich composites “excellent” noise radiators. There is a growing interest in optimizing and developing a new sandwich composite which will meet the high stiffness-to-weight ratio and offer improved acoustic performance. The focus of this study is to investigate the structural–vibrational performance of carbon-fiber face sheet sandwich composite beams with varying core materials and properties. Core materials utilized in this study included Nomex and Kevlar Honeycomb cores, and Rohacell foam cores with different densities and shear moduli. The structural–vibrational performance including acoustic and vibrational damping properties was experimentally characterized by analyzing the wave number response, and structural damping loss factor (η) from the frequency response functions, respectively. It was observed that the relationship between the slopes of the wave number data for frequencies above 1000 Hz is inversely proportional to the core material’s specific modulus (G/ρ). The analysis also showed the importance of using a honeycomb core’s effective properties for equal comparison to foam-cored sandwich structures. Utilizing analytical modeling, the loss factors of the core materials (β) was determined based upon the measured structural loss factors (η) for a frequency range up to 4000 Hz. It was determined that low shear modulus cores have similar material damping values to structural damping values. However as the core’s shear modulus increases, the percent difference between these values is found to increase linearly. It was also observed that high structural damping values correlated to low wave number amplitudes, which correspond to reductions in the level of noise radiation from the structure.
Composites Science and Technology 08/2012; 72(13):1493–1499. · 3.63 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Carbon fiber-synthetic foam core sandwich composites are widely used for many structural applications due to their superior mechanical performance and low weight. Unfortunately these structures typically have very poor acoustic performance. There is increasingly growing demand in mitigating this noise issue in sandwich composite structures. This study shows that marrying carbon fiber composites with natural cork in a sandwich structure provides a synergistic effect yielding a noise-free sandwich composite structure without the sacrifice of mechanical performance or weight. Moreover the cork-core sandwich composites boast a 250% improvement in damping performance, providing increased durability and lifetime operation. Additionally as the world seeks environmentally friendly materials, the harvesting of cork is a natural, renewable process which reduces subsequent carbon footprints. Such a transition from synthetic foam cores to natural cork cores could provide unprecedented improvements in acoustic and vibrational performance in applications such as aircraft cabins or wind turbine blades.
[Show abstract][Hide abstract] ABSTRACT: Due to their higher strength-to-weight and stiffness-to-weight ratios compared to metals, fiber reinforced composite materials are a great alternative for use in many structural applications. However these properties lead to poor acoustic performance as composite materials are excellent noise radiators. This is particularly true for sandwich composite structures. Therefore the focus of this study is to investigate the effect of a core thickness change on the vibrational properties of Rohacell foam/carbon-fiber face sheet sandwich composite beams. Four different foam core thicknesses were explored, using a combination of experimental and analytical methods to characterize sound and vibrational properties of the sandwich beams. First, the wave number responses of the beams were obtained, from which coincidence frequencies were identified. Second, from the frequency response functions the structural damping loss factor, η, was determined using the half-power bandwidth method. Experimental and analytical results show that the relationship between core thickness and coincidence frequency is non-linear. A drastic increase in coincidence frequency was observed for the sandwich beam with the thinnest core thickness due to the low bending stiffness. Moreover this low bending stiffness results in low damping values, and consequently high wave number amplitude responses at low frequency ranges (<1000 Hz).
Composites Science and Technology 03/2012; 72(6):724–730. · 3.63 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Sandwich composite structures have highly desirable properties including
superior stiffness and strength-to-weight ratios. Such properties arise
from combining thin, stiff materials called face sheets with a soft,
thick core. Unfortunately these properties give rise to poor acoustic
performance, as sandwich structures efficiently radiate noise at low
vibrational frequencies. Therefore much consideration has been given to
improve acoustic performance with small sacrifices in key mechanical
performances, such as bending stiffness and weight. This study focuses
on sandwich composite structures with both high noise mitigation and
passive structural dampening. Specifically, it is sought to understand
how the vibrational responses of carbon-fiber face sheet sandwich
composite beams are affected by the core's thickness, as well as its
properties. Here, it is shown that the relationship between bending
stiffness and coincidence frequency (a metric of acoustic performance)
is non-linear. By reducing the core thickness from 10.7mm to 8.4mm,
approximately 120% improvement is seen in acoustic performance. Also,
the core materials' specific shear modulus is inversely proportional to
acoustic performance. Finally, superior damping performance can lead to
substantial noise mitigation in low vibrational frequencies. Thus
coupling these concepts will provide vastly improved acoustic
performance with minimal sacrifices in mechanical performance or weight.
[Show abstract][Hide abstract] ABSTRACT: Toughness of a polymer is a key material property for energy absorbing
capability for various engineering applications. Significant effort has
been made to improve toughness of a polymer and hence increase the
energy absorbing capability; typically rigid-particles in thermoplastics
or rubbery modifiers in a brittle polymer matrix. The focus of this
study is to investigate toughening mechanisms of a thermoplastic polymer
composite. Micron-size thermoplastic particle reinforced polycarbonate
(PC) composite materials was fabricated via a solution mixing method.
The mechanical properties of the polymer composites were characterized
in tensile testing while the acoustic emission was monitored to assess
the material failure modes during the tensile test. Substantial
improvement in tensile toughness was observed for the polymer composites
and the toughening mechanisms responsible for the improvement were
identified and quantified for each contribution to the observation.
[Show abstract][Hide abstract] ABSTRACT: This study involves the investigation of spherically shaped filler
diameter and interphase effects on the Young's modulus of micro and nano
size silicon dioxide (SiO2) particle reinforced epoxy composite
materials. Specifically, 10μm and 80nm size SiO2 particles and Epon
862 epoxy are chosen as fillers and a matrix material, respectively.
While 10μm and 80nm SiO2 particles are dispersed in the epoxy through
a direct shear mixing method, nano-composites are fabricated with
hardener at desirable ratios. Both micro- and nano-composites are
prepared at 2 different particle loading fractions for tensile testing.
It is observed that the nano-composites show significant increase in
Young's modulus over micro-composites, showing a linear increase as
particle volume fraction increases. This could indicate that for
nano-composites, the interphase region between the particle and matrix
can considerably affect their mechanical properties. Here, we develop a
finite element analysis (FEA) model to investigate the interphase effect
on the Young's modulus of both micro- and nano-composites. This model
demonstrates how to estimate the effective volume fraction of a particle
as filler using a combined experimental/numerical approach. The
effective volume fraction is shown to be important in predicting the
mechanical response of nano-scale particles reinforced composite
[Show abstract][Hide abstract] ABSTRACT: Recent studies show that continuously reinforced multi-walled carbon
nanotubes (MWCNT) composite can have extraordinary mechanical
properties. It was observed that the continuous MWCNT polymer composites
exhibit both significant reinforcement and large damping capability in
compressive loadings, which typically remain compromised. The damping
property might result from buckling behavior of the MWCNT in composites.
Here, this paper is to study the buckling response of carbon nanotubes
(CNT) within a polymer matrix by using analytical models including
Euler, Timoshenko and shell buckling models. Also, the modeling results
are analyzed and compared to better understand the bucking behavior of
the CNT in the composite and also investigate the effect of their aspect
ratio (L/D) on buckling behavior. This study provides us with insight to
better understand the structure-property relation for such continuous
CNT polymer composites.