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Study on nanoparticle production process characteristics in transferred arc plasma system using heat and mass balance
Abstract
The nanoparticle production process in a transferred arc plasma system was studied. The plasma temperature, particle heating time, and particle residence time in plasma were calculated using heat and mass balance with a lumped capacitance method. We analyzed the nanoparticle production characteristics based on different operating conditions by comparing the particle vaporization time with the particle residence time in the plasma. The limit size for particle vaporization was derived. With higher plasma power, the nanoparticle production rate increased and the energy consumption rate decreased. It was confirmed that the energy consumption rate reaches an optimal point according to the plasma power. Experiments to determine the nanoparticle production rate according to plasma power were also conducted and the experimental data were compared with numerical values. The results show that the error rate between the numerical values and experimental data was approximately ±18%. Therefore, the developed model which was studied could be useful for designing nanoparticle production process using a transferred arc plasma system because of its simple approach.
... However, with dramatic expanding of the thermal plasma application fields, specific requirements on the plasma parameters, including the maximum values and spatial distributions of the gas temperatures, electron temperatures, and species number densities, become more and more important. For example, in the nanoparticle syntheses using thermal plasmas, on the one hand, a high-temperature plasma region is necessary to heat and evaporate the precursors quickly; while on the other hand, a steep heavy-particle temperature gradient is required to ensure that the evaporated materials enter the low-temperature plasma region rapidly, thus, the time to form nanoparticles through nucleation and condensation is shortened, and the excessive particle agglomeration is avoided accompanied by the increase of the production rate and the particle size uniformity [1,[6][7][8]. This means that the coexistence of the regions with high and low gas temperatures, steep gas temperature gradients and high chemical reactivity is indispensable for balancing the production rate, particle size and its distribution during the synthesis of nanoparticles. ...
In this letter, an annular anode is designed for producing arc plasmas with a large non-equilibrium region by using a counterflow cold gas through the annular anode. The coupled mass-momentum-energy exchange processes in an argon arc plasma are studied numerically and experimentally. The counter-injection of the cold argon gas from the center of the anode leads to a steep gradient of the heavy-particle temperature due to the formation of a thin stagnation layer resulting from the interaction of the high temperature plasma with the cold gas; and in particular, a large volume non-equilibrium “dark” plasma region is obtained above the anode surface. The results show that, with the enhancement of the convective heat transfer process in the plasma core region, the fraction of the non-equilibrium region to the whole arc plasma region reaches 92.2% where the heavy-particle temperature can be reduced significantly, e.g., ~ 2300 K, while simultaneously, the electron temperature and number density are remained at high levels greater than 8000 K and 2.4×1020 m-3, respectively, under the operating condition studied in this letter. This research not only deepens the understanding to the non-equilibrium synergistic transport mechanisms of arc plasmas, but also provides a method for producing a large volume non-equilibrium plasma region so as to promote various existing applications, or even creating new applications in the future.
... However, in order for the waste neutralization process to be fully realized using a plasma environment, it is necessary that waste and other materials remain in the plasma environment for at least 1-2 s. To meet this condition, plasma-chemical reactors of various constructions are designed and developed and the waste particles inside them are retained for the required time and neutralized [13,14]. ...
A transferred arc plasma torch chemical rector was used to process waste formed from mixtures of dry clay powder and hydroquinone. Such reactors are best suited for the treatment of electrically conductive waste. In these types of reactors, the electric arc moves chaotically throughout the entire reactor volume, making it possible to ensure an even temperature distribution in the reaction zones. An analysis of the literature has shown that there are not many study results related to this type of reactor. The novelty of the work is that the behavior of the operating electric arc inside the reactor was recorded by using a high-speed camera. The distribution of the temperature profile at the cooled reactor wall was investigated. The electrical potential difference inside the reactor was also investigated. To better understand the behavioral properties of the electric arc when the reactor is filled with treated material, hydroquinone-contaminated clay was used. In this case, the movement of the electric arc, as well as the probability of its formation, is the greatest at the location where the thinnest layer of the material to be processed is located. In addition, it has been observed that the use of a graphite anode poses problems because, over time, the anode of such a design deforms due to interactions with the electric arc. While analyzing research results, it can be observed that these types of reactors are very suitable for the treatment of electrically conductive materials and for the treatment of small amounts of nonconductive materials when the material occupies a relatively small part of the reactor. A further development of these studies in the future is planned in order to make the reactors as versatile as possible and as suitable as possible for handling the widest range of materials possible.
... Nowadays, the methods of producing tungsten NPs mainly include high energy ball milling [5], reduction of tungsten oxides by hydrogen [6,7], electrical explosion of wire [8], and arc plasma [9,10]. Compared with these methods, in spite of the arc plasma method requires high-level technical equipment, this method with extremely high quenching rate, controllable reaction gas, high spheroidization rate, and high efficiency is widely used in the production of spherical metal NPs with uniform particle size, high purity, and good particle dispersion [11,12]. Although the cost of preparing tungsten NPs by arc plasma method is rather high, its product quality is irreplaceable by other methods. ...
As an important refractory metal, tungsten (W) has unique physical-chemical properties, and the applications of high-quality tungsten NPs with uniform particle size, high purity, and good particle dispersion in industrial fields are highly desirable. Herein, the integration of tungsten metallurgical purification and nanoparticle preparation is achieved by the hydrogen arc plasma method. In the process of purification, on the condition of arc current of 250 A, hydrogen concentrations of 30 vol%, purification time of 40 min, and gas pressure of 60 kPa, the highest impurity removal rate is achieved. The active H⁺ is able to react with impurities to generate metal hydride (MH, M = Al, Cr, Mg, and Fe), which promotes the removal rate of impurity elements. During the preparation of nanoparticles (NPs), the as-prepared tungsten NPs with an average particle size of 44.1 nm have a regular spherical structure, high purity, and a relatively uniform particle size distribution. When the hydrogen concentration is controlled at 50 vol%, the extremely high temperature promotes the evaporation of tungsten. Meanwhile, the active H⁺ reacts with tungsten to form tungsten hydride (WH), further increasing the evaporation rate of tungsten. According to the above results, the hydrogen arc plasma promotes the simplification of the production process steps of high purity tungsten NPs and reduces the industrialized production cost of high-quality refractory metal NPs.
... This goal was achieved by using the finite element method. This method has positively proved itself in the simulating of such processes [10][11][12][13]. ...
A computer model of the plasmatron was created, which adequately describes the plasma flow. The effect of changing the length of the anode on the speed and temperature at characteristic points of the flow is investigated. To confirm the results of the numerical experiment, a field experiment was performed. The results of the comparative analysis of the data of the computer and field experiments are in agreement. The presented results are of practical value for manufacturers and users of technological plasma installation for improving the equipment used in the processes of obtaining metal powders and applying protective coatings and can be used to predict the parameters of a plasma flow. Recommendations for choosing the length of the anode are presented.
... To generate alumina nanoparticles with a small GSD, we have employed the aerosol-through-plasma (ATP) method [12,13]. It is known that nanoparticles can be produced continuously from various precursors [14][15][16]. ...
Control of the particle size distribution of fabricated alumina nanoparticles from general alumina powder with a large geometric standard deviation (GSD) was studied. A thermophoretic separator was used to control the GSD of the nanoparticles, and unevaporated and primary particles were separated to yield a small GSD. The fabricated nanoparticles were characterized by field emission scanning electron microscopy (FESEM) and a scanning mobility particle sizer (SMPS). We confirmed that the GSD of the nanoparticles was controlled by the thermophoretic separator. A temperature difference between 79 K and 151 K was applied to the thermophoretic separator for control of the nanoparticle GSD. The GSD of the fabricated alumina nanoparticles was improved from 1.74 to 1.44.
High temperature synthesis process of microcrystalline and nanocrystalline (LaCe)B6 using Self-propagating High temperature Synthesis (SHS) and arc plasma gas phase condensation methods, respectively have been investigated. These methods are rapid and economically viable processes used for the synthesis of technologically important refractory hexaborides. Microcrystalline powders of (LaCe)B6 were synthesized using the SHS process, starting from oxide precursors of lanthanum, cerium, and boron. The powders obtained using SHS process were used as a precursor to getting nanocrystalline (LaCe)B6 using the thermal plasma route. The thermal plasma synthesis was carried out using nitrogen and argon plasmas, respectively. In-situ plasma diagnostics were used to identify evaporated species and determine plasma temperature during the formation of nanocrystalline (LaCe)B6 using optical emission spectroscopy. Further, an effect of plasma input parameters on the structural and optical properties of as synthesized nanocrystalline (LaCe)6 were investigated using XRD analysis, Raman spectroscopy, and X-ray photoelectron spectroscopy. A thorough investigation of morphological and structural properties of synthesized nanocrystalline (LaCe)6 was carried out using transmission electron microscopy. Finally, field effect electron emission properties of the microcrystalline and nanocrystalline product were investigated and current density, turn on field, stability of emission performance were determined.
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Aerosol processing is gaining much interest in fabricating particles for various applications due to its easy application and good control of particle characteristics. In this study, spherical alumina particles were prepared using a transferred arc plasma aerosol system with argon shield gas and nitrogen carrier gas. The operating conditions for spheroidization were controlled by comparing the particle heating and particle residence times in plasma to obtain the calculated critical particle size. Nitrogen fractions (10%–63%) and plasma powers (1–2.5 kW) were used as control parameters to verify the process design for the average injected particle sizes of 10 μm and 100 μm. The spherical particle sizes were examined by using field-emission scanning electron microscopy. Results showed that the particle size limits for melting(dP,LM) and vaporization(dP,LV) increased with increasing plasma power. Higher plasma power is required to fabricate spherical shapes from larger injected particles at nitrogen fractions of 10% and 63% because of the longer residence time needed to reach the melting state of the raw materials.
Spherical Ti-6Al-4V powders were prepared using radio-frequency plasma spheroidization. A laser particle size analyser, a scanning electron microscope, an X-ray diffractometer and a Freeman FT4 powder rheometer were used to analyse the granulometric parameters, micro-morphologies, phase constitutions and flow properties of the raw and the spheroidized powders, respectively. The spheroidized powders exhibited an almost 100% degree of sphericity, smooth surfaces, favourable dispersion and narrow particle size distribution under appropriate plasma technological parameters. The average particle size of the spheroidized powders increased slightly as compared with that of the raw powders. In addition, the spheroidized powders exhibited higher conditioned bulk density and improved flow properties (including the dynamic flow properties, aeration, compressibility, permeability and shear properties) as compared with those of the raw powders.
Liquid atomization is applied in nanostructure dense coating technology to inject suspended nano-size powder materials into a suspension plasma spray (SPS) torch. This paper presents the effects of the atomization parameters on the nanoparticle processing. A numerical model was developed to simulate the dynamic behaviors of the suspension droplets, the solid nanoparticles or agglomerates, as well as the interactions between them and the plasma gas. The plasma gas was calculated as compressible, multi-component, turbulent jet flow in Eulerian scheme. The droplets and the solid particles were calculated as discrete Lagrangian entities, being tracked through the spray process. The motion and thermal histories of the particles were given in this paper and their release and melting status were observed. The key parameters of atomization, including droplet size, injection angle and velocity were also analyzed. The study revealed that the nanoparticle processing in SPS preferred small droplets with better atomization and less aggregation from suspension preparation. The injection angle and velocity influenced the nanoparticle release percentage. Small angle and low initial velocity might have more nanoparticles released. Besides, the melting percentage of nanoparticles and agglomerates were studied, and the critical droplet diameter to ensure solid melting was drawn. Results showed that most released nanoparticles were well melted, but the agglomerates might be totally melted, partially melted, or even not melted at all, mainly depending on the agglomerate size. For better coating quality, the suspension droplet size should be limited to a critical droplet diameter, which was inversely proportional to the cubic root of weight content, for given critical agglomerate diameter of being totally melted.
The size of the primary particles in aerosol agglomerates is determined in part by the interplay of surface growth, coagulation, and sintering. These processes are modelled by a one-dimensional (1-D) discrete-sectional model, DISGLOM2, which predicts the evolution of agglomerate and primary particle size distributions. DISGLOM2 is an extended version of DISGLOM (Rogak 1997), in which particles smaller than the "melting diameter" were assumed to sinter instantly while bigger particles did not sinter at all. Gradual sintering, "condensational obliteration" (whereby primary particles are lost during heavy surface growth), and diffusional wall deposition have been incorporated into DISGLOM2. Results from DISGLOM2 were comparable with those from 2-D sectional models, but DISGLOM2 was much faster. In addition, DISGLOM2 includes the effects of "condensation" of small spherical particles on large agglomerates, which were not modelled previously. The effect of condensation was shown to be significant at low temperature. DISGLOM2 was used to predict the primary particle diameter of titania particles generated by precursor reaction. By adding gradual sintering, the growth rate of agglomerate particles by coagulation was slightly decreased and the primary particle size considerably increased compared with the results given by DISGLOM. Although DISGLOM2 is an efficient model of the relevant physical processes, the predictions are sensitive to the kinetics of precursor reactions and particle sintering, which can be difficult to characterize in real experimental systems.
The effect of the expansion of hollow micro-spherical droplets due to their heating at their motion in a plasma jet is considered by the example of ZrO2. A fairly simple model is proposed, which accounts for the variation of the droplets size and shell thickness because of the thermal expansion of the gas cavity as well as their possible evaporation. The conducted computations have enabled the assessment of the scale of the variation of diameter (10–20 %) and shell thickness (up to 50 %) of ZrO2 particles under the conditions typical of the plasma treatment of powder materials and producing the coatings. The influence of the given effect on the dynamics of particles heating and acceleration is investigated, and a comparative analysis of the behavior of hollow and dense particles in plasma jet is done.
An atmospheric pressure DC plasma torch reactor was employed using Hexamethyldisiloxane (HMDSO) and oxygen reactants in nitrogen carrier gas to produce silicon oxide nanoparticles and nanoparticle agglomerates with the diameters in the ranges of 8-14 nm and 130-260 nm, respectively. The mean sizes of primary nanoparticles and nanoparticle agglomerates as well as their surface properties are studied by varying the process conditions. In general the mean sizes of primary nanoparticles and nanoparticle agglomerates decrease with the increase of the plasma torch power, the increase of the oxygen concentration, and the decrease of the HMDSO concentration. Under those conditions, the more completely-oxidized silicon oxide clusters are formed and aggregate into the compact, densely-packed, and small-size primary silicon oxide nanoparticles, which further agglomerate into the densely-packed, small nanoparticle agglomerates. The hydrophilic and white nanoparticle powder with a low BET surface area can be thus obtained. Under the opposite processing conditions, the loosely-packed, low density and large-size carbon-containing silicon oxide nanoparticles and nanoparticle agglomerates can be obtained with hydrophobic surface, high BET surface areas, and gray color.
Iron oxide-silica composite was synthesized using atmospheric microwave plasma and DC thermal plasma. There has recently been increasing interest in predicting the final product during vapor phase synthesis using plasma because of difficulty obtaining desirable product. In this study, vapor phase synthesis of iron oxide-silica composite from iron pentacarbonyl (Fe(CO)5) and tetraethyl orthosilicate (SiC8H20O4, TEOS) was conducted using various Fe/Si ratios and different types of plasma to identify the formation mechanism in the Fe-Si-O multi-component system. The morphologies and phase compositions of the synthesized particles were analyzed and compared. The results showed that the Fe/Si ratio and the type of plasma influenced the morphologies and the phase composition. A thermodynamic consideration was introduced to investigate the particle formation phenomena, which could explain the differences induced by varying the Fe/Si ratio and type of plasma. The particle formation mechanism was divided into a condensation step and a diffusion step. At the condensation step, the Fe/Si ratio determined the condensation temperature, which is related to the morphology. At the diffusion step, the quenching rate of the plasma determined the degree of diffusion, which was related to the phase composition and formation of the external layer.
Fine-grained spherical tungsten powder with particle size from 6. μm to 11. μm has been fabricated by a well-designed method with two simple steps. A pretreatment of tungsten powder was carried out in the first step. Complete dispersion, effective classification and favorable shape modification have been achieved by jet milling process. Accordingly, monodisperse tungsten powders with narrow particle size distribution were obtained. On the basis of the existing calculation and simulation results, plasma spheroidization parameters have been optimized. Thus, fine-grained spherical tungsten powder with narrow particle size distribution has been prepared by subsequent radio frequency (RF) inductively coupled plasma spheroidization. Furthermore, the presented method can be useful in fabricating spherical tungsten powder of different particle size with narrow particle size distribution as well as for process control in large-scale continuous production.
Particle size of coarse silicon is a very important parameter in plasma-assisted productions of nano-silicon, because an appropriate size cannot only improve nano-silicon's quality, but also reduce the cost of raw materials. In this work, simulations are founded to investigate the effect of particles' size on silicon's vaporization process inside radio-frequency (RF) thermal plasma reactor. By counting particles' residence time between high temperature region and fully-gasified time inside the reactor, particles' motion and vaporization process can be obtained in statistics. After comparing these two parameters, the gasification ratio of coarse silicon with different size can be finally captured. It is shown that particles below 30 μm all have the gasification ratio above 99.0%, which are suggested as the upper limit of coarse silicon in future experimental works. Moreover, it is also shown that gasification ratios of particles above 30 μm are greatly reduced, illustrating that sacrificing gasification ratio to apply relative bigger particles to prepare silicon nanopowder is inappropriate and uneconomic.
The spark plasma sintering (SPS) of silicon nitride (Si3N4) was investigated using nanocomposite particles composed of submicron-size α-Si3N4 and nano-size sintering aids of 5wt% Y2O3 and 2wt% MgO prepared through a mechanical treatment. As a result of the SPS, Si3N4 ceramics with a higher density were obtained using the nanocomposite particles compared with a powder mixture prepared using conventional wet ball-milling. The shrinkage curve of the powder compact prepared using the mechanical treatment was also different from that prepared using the ball-milling, because the formation of the secondary phase identified by the X-ray diffraction (XRD) method and liquid phase was influenced by the presence of the sintering aids in the powder compact. Scanning electron microscopy (SEM) observations showed that elongated grain structure in the Si3N4 ceramics with the nanocomposite particles was more developed than that using the powder mixture and ball-milling because of the enhancement of the densification and α-β phase transformation. The fracture toughness was improved by the development of the microstructure using the nanocomposite particles as the raw material. Consequently, it was shown that the powder design of the Si3N4 and sintering aids is important to fabricate denser Si3N4 ceramics with better mechanical properties using SPS.
Using a non-transferred DC plasma spray torch, nanosized alumina particles were synthesized from micron-sized aluminium particles. Irregular-shaped micron-sized alumina particles were spheroidized using the plasma spray torch. The synthesized powder particles were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX) analysis, Fourier transform infra-red (FT-IR) spectroscopy and photoluminescence (PL) spectroscopy. The synthesized alumina nanoparticles appear spherical in shape, but mostly agglomerated in the size range 30–75 nm. It was identified that the main phase is γ-alumina with a small amount of δ-alumina. In the spheroidized alumina particles, the main phase is α-alumina with a small amount of γ-alumina. The mechanism of formation of alumina and its phase change are explained in detail.
A brief review on the thermal plasma synthesis of nano-sized powders is presented according to the application materials, such as, metals, ceramics, glasses, carbonaceous materials and other functional composites, such as, supported metal catalyst and core-shell structured nano materials. As widely adopted plasma sources available for thermal plasma synthesis of nanosized powders, three kinds of plasma torches, such as transferred and non-transferred DC and RF plasma torches, are introduced with the main features of each torch system. In the basis of the described torch features and the properties of suggested materials, application results including synthesis mechanism are reviewed in this paper.
We have developed a 1-D numerical model of an RF argon-silane plasma in
which dust particles nucleate and grow. This model self-consistently
couples a plasma module, a chemistry module and an aerosol module. The
plasma module solves population balance equations for electrons and
ions, the electron energy equation under the assumption of a Maxwellian
velocity distribution, and Poisson's equation for the electric field.
The chemistry module treats silane dissociation and reactions of silicon
hydrides containing up to two silicon atoms. The aerosol module uses a
sectional method to model particle size and charge distributions. The
nucleation rate is equated to the rates of formation of anions
containing two Si atoms, and a heterogeneous reaction model is used to
model particle surface growth. Aerosol effects considered include
particle charging, coagulation, and particle transport by neutral drag,
ion drag, electric force, gravity and Brownian diffusion. Simulation
results are shown for the case of a 13.56 MHz plasma at a pressure of 13
Pa and applied RF voltage of 100 V (amplitude), with flow through a
showerhead electrode. These results show the strong coupling between the
plasma and the spatiotemporal evolution of the nanoparticle cloud.
The process control for reproducibility, uniformity, and achievement of desired structures for carbon black generated in thermal plasma devices is studied in this paper through modeling, and correlated with experimental results. A numerical simulation of the flow and energy fields, stream function lines and the quench rates of the plasma gas in a conical shape reactor at different pressures was made. An argon plasma is used with highly diluted methane (0.6–7%) as the carbon precursor. The quench rates were studied in order to observe the flow development and hence the thermal history of particle nucleation. Three pressure cases of 20.7, 55.2 and 101.3 kPa and two plasma powers cases of 10 and 20 kW were studied. The modeling results enabled carbon nanoflakes production in the experimental tests performed on an inductively coupled thermal plasma system. Results indicate a robust process control enabling very little particle morphology variation over this wide range of reactor pressure values and varying plasma power, and a very high reproducibility of the particle morphologies obtained.
The present study addresses the feasibility to synthesize aluminium nanoparticles (NPs) from micron-sized aluminium powder with the use of a customized atmospheric plasma spraying (APS) technique. Using APS, nanoparticle synthesis can be achieved via rapid melting and vaporization of the initial micrometric particles and their subsequent re-nucleation. A custom mantle system was designed and developed with the aid of relevant simplified CFD simulations. The mantle provided the necessary inert environment (argon), at ambient pressure, in order to avoid any oxidation of the metal during plasma spraying while promoted rapid quenching of the gasified metal. The particles formed were collected with the aid of a quartz filter downstream of the plasma flame and the production rate achieved was 2 g min−1. Ex situ post-characterization of the particles via X-ray diffraction, specific surface area measurement (BET), transmission electron microscopy (TEM) coupled with energy dispersive spectrometry (EDS) and thermogravimetric analysis (TGA) under air revealed that the powders obtained primarily comprised of monocrystalline metallic aluminium nanoparticles of almost spherical shape. The NPs possessed a 2–5 nm oxide coating layer. By regulating the conditions inside the mantle, a variety of different size distributions were obtained.
Synthesis of alumina nanoparticles in an Ar–O2 inductively coupled radio frequency (rf) plasma controlled by axial quench gas injection was investigated. The flow and temperature fields for various quench gas flow rates in the plasma reactor, as well as the corresponding quench rates, were predicted using a renormalization group (RNG) k–ε turbulence model. Nanosize alumina particles were synthesized from the vapor phase by oxidation of aluminum in the plasma. The collected products were characterized by means of field emission scanning electron microscopy (FE-SEM), dynamic light scattering (DLS), and BET surface area analysis. The dependences of particle size and shape on the injection position and the flow rate of the quench gas were discussed.
Plasma technology has given a new direction and impetus to many industrial operations by opening up a new range of mechanical, chemical and metallur- gical processing techniques. The high temperatures together with the high reactivity due to the presence of free ions and radicals, make the plasma a powerful medium to promote high heat transfer rates and chemical reactions. This paper focuses on atmospheric plasmas known as thermal-plasmas produced by plasma torches. Conventionally, as in plasma cutting, welding, melting or spraying operations, the plasma torch acts as a source of highly concentrated thermal energy and the plasma-forming gas provides an inert atmosphere and helps to prevent undesirable reac- tions, thus ensuring the purity of the operation and the product. In contrast, the emerging trend is to use reactions, where the plasma gas enters the reaction scheme. Some of the above issues are reviewed and illustrated with examples of plasma torches developed and experiments carried out at Bhabha Atomic Research Centre (BARC), Mumbai.
The structure of fractal-like agglomerates (physically-bonded) and aggregates (chemically- or sinter-bonded) is important in aerosol synthesis of nanoparticles, and in monitoring combustion emissions and atmospheric particles. It influences also particle mobility, scattering, and eventually performance of nanocomposites, suspensions and devices made with such particles. Here, aggregate sintering by viscous flow of amorphous materials (silica, polymers) and grain boundary diffusion of crystalline ceramics (titania, alumina) or metals (Ni, Fe, Ag etc.) is investigated. A scaling law is found between average aggregate projected area and equivalent number of constituent primary particles during sintering: from fractal-like agglomerates to aggregates and eventually compact particles (e.g. spheres). This is essentially a relation independent of time, material properties and sintering mechanisms. It is used to estimate the equivalent primary particle diameter and number in aggregates. The evolution of aggregate morphology or structure is quantified by the effective fractal dimension (Df ) and mass-mobility exponent (Dfm ) and the corresponding prefactors. The Dfm increases monotonically during sintering converging to 3 for a compact particle. Therefore Dfm and its prefactor could be used to gauge the degree or extent of sintering of agglomerates made by a known collision mechanism. This analysis is exemplified by comparison to experiments of silver nanoparticle aggregates sintered at different temperatures in an electric tube furnace.
Nanoparticles of aluminum metal were generated by passing an aerosol of micrometer-scale (mean 50 μm) particles in argon through an atmospheric pressure plasma torch operated at less than 1000 W. A designed experiment was conducted to investigate the effects of plasma gas flow rate, aerosol gas flow rate, and applied power on the shape, size, and size distribution of the final particles. The size and shape of the metal particles were dramatically impacted by the operating parameters employed. At relatively low powers or at high powers and short residence times, virtually all the particles are spherical. Under other conditions, the particles had spherical heads, and virtually all had tails, some quite long. The particle size distributions also were influenced by the operating conditions. Under most conditions the size distributions were log-normal, consistent with growth by agglomeration. However, under some conditions, the population of particles above or below the mode was far too great to be consistent with a log-normal distribution. For example, the particle distributions tend to show an unusual concentration of very small particles at relatively short residence times and low aluminum feed rates. The distributions tend to show an unusual concentration of large particles at relatively long residence times and high aluminum feed rates. On the basis of the data collected, some simple models of the mechanism of nanoparticle formation were postulated which should be of value in future studies of the process.
A thermal plasma process was used to synthesize nanosized zirconia and yttria-stabilized zirconia powders. This paper presents a new method on the synthesis of nanosized zirconia and yttria-stabilized zirconia powders from the carbonates (zirconium carbonate and/or yttrium carbonate). The products from this process using zirconium carbonate were mixtures of monoclinic and tetragonal phases of zirconia. The amount of the tetragonal phase in the mixture increased as the plasma torch power or plasma gas flow rate increased. This is attributed to the formation of metastable tetragonal phase by rapid quenching. With the addition of yttria precursor, two combinations of zirconia phases were formed depending on the yttria contents. Pure tetragonal phase was obtained at 3.8 mol% yttria, while a mixture of tetragonal and cubic phases was produced at 7.7 mol% yttria. The yttria-doped tetragonal phase was stable at high temperatures without the tetragonal–monoclinic phase transition. The grain size of each phase, the particle size, and the BET are also discussed in this paper.
We describe a process in which nanosize particles with u narrow size distribution are generated by expanding a thermal plasma carrying vapor-phase precursors through a nozzle. The plasma temperature and velocity profiles are characterized by enthalpy probe measurements. by calorimetric energy balances. and by a model of the nozzle flow. Aerosol samples are extracted from the flow downstream of the nozzle by means of a capillary probe interfaced to a two-stage ejection diluter. The diluted aerosol is directed to a scanning electrical mobility spectrometer (SEMS) which provides on-line size distributions down to particle diameters of 4 nmt. We have generated silicon, carbon, and silicon carbide particles with number mean diameters of about 10 not or less, and we have obtained some correlations between the product and the operating conditions. Inspection of the size distributions obtained in the experiments, together with the modeling results, suggests that under our conditions silicon carbide formation is initiated by nucleation of extremely small silicon particles from supersaturated silicon vapor, followed by chemical reactions at the particle surfaces involving carbon-containing species from the gas phase.
For decades, plasma processing of materials on the nanoscale has been an underlying enabling technology for many ldquoplanarrdquo technologies, particularly virtually every aspect of modern electronics from integrated-circuit fabrication with nanoscale elements to the newest generation of photovoltaics. However, it is only recent developments that suggest that plasma processing can be used to make ldquoparticulaterdquo structures of value in fields, including catalysis, drug delivery, imaging, higher energy density batteries, and other forms of energy storage. In this paper, the development of the science and technology of one class of plasma production of particulates, namely, aerosol-through-plasma (A-T-P), is reviewed. Various plasma systems, particularly RF and microwave, have been used to create nanoparticles of metals and ceramics, as well as supported metal catalysts. Gradually, the complexity of the nanoparticles, and concomitantly their potential value, has increased. First, unique two-layer particles were generated. These were postprocessed to create unique three-layer nanoscale particles. Also, the technique has been successfully employed to make other high-value materials, including carbon nanotubes, unsupported graphene, and spherical boron nitride. Some interesting plasma science has also emerged from efforts to characterize and map aerosol-containing plasmas. For example, it is clear that even a very low concentration of particles dramatically changes plasma characteristics. Some have also argued that the local-thermodynamic-equilibrium approach is inappropriate to these systems. Instead, it has been suggested that charged- and neutral-species models must be independently developed and allowed to ldquointeractrdquo only in generation terms.
Aerosol synthesis of materials is a vibrant field of particle technology and chemical reaction engineering. Examples include the manufacture of carbon blacks, fumed SiO(2), pigmentary TiO(2), ZnO vulcanizing catalysts, filamentary Ni, and optical fibers, materials that impact transportation, construction, pharmaceuticals, energy, and communications. Parallel to this, development of novel, scalable aerosol processes has enabled synthesis of new functional nanomaterials (e.g., catalysts, biomaterials, electroceramics) and devices (e.g., gas sensors). This review provides an access point for engineers to the multiscale design of aerosol reactors for the synthesis of nanomaterials using continuum, mesoscale, molecular dynamics, and quantum mechanics models spanning 10 and 15 orders of magnitude in length and time, respectively. Key design features are the rapid chemistry; the high particle concentrations but low volume fractions; the attainment of a self-preserving particle size distribution by coagulation; the ratio of the characteristic times of coagulation and sintering, which controls the extent of particle aggregation; and the narrowing of the aggregate primary particle size distribution by sintering.
A study on DC thermal plasma generation and its characteristics
- W K Kim
- K W Whang
W.K. Kim, K.W. Whang, A study on DC thermal plasma generation and its
characteristics, Trans. Korean Inst. Electr. Eng. 39 (1990) 1219-1226.
- W C Hinds
W.C. Hinds, Aerosol Technology, Wiley, New York, 1999.