Synthesis of ultra-fine SiC powders in a d.c. plasma reactor

Synthesis of SiC powder in a 15 kW d.c. plasma reactor by using the reaction systems of SiCl₄+CH₄ and CH₃SiCl₃+CH₄ was studied. The powder produced was characterized by X-ray diffraction and electron microscopy (SEM and TEM). Specific surface area was determined by the BET method using a sorptograph. The oxygen content was analysed by thermogravimetric method. Powder characteristics with respect to plasma conditions were analysed. The SiC powder generated was sintered in the presence of boron and carbide.     

Production and characterization of nanosized Cu/O/SiC composite particles in a thermal r.f. plasma reactor

An inductively coupled thermal plasma process was used to produce nanosized Cu/SiC composite particles. The powders were characterized by means of chemical analysis, energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), specific surface measurements (BET), X-ray powder diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). X-ray absorption fine structure analysis (EXAFS) was performed to determine the near range order structure of the nanosized particles.     

The formation of hollow spherical ceramic oxide particles in a d.c. plasma

Investigations have been carried out to determine the conditions that lead to the production of spherical hollow ceramic oxide particles during melting in a d.c. plasma jet. Reports in the literature indicated that such ceramic particles were formed by plasma spraying spray dried agglomerates, but precise details of the conditions necessary for their formation were not stated. In this study it is shown that for hollow particles to be formed several conditions had to be met. Spherical spray dried agglomerates had to be used as starting materials, the material being sprayed had to melt over a narrow temperature range and the size of the particles had to exceed a certain diameter. Experiments, using yttria, showed that the relative size of the pore was dependent on particle diameter, and it has been proposed that the major controlling factor that influences this dependence is the escape of gas trapped in the spray dried agglomerate during melting rather than surface tension or undercooling which were shown to produce only minor effects. In addition, the results also showed that the nature of porosity within the hollow particles as well as the surface morphology was dependent on the material being sprayed.     

Synthesis and characterization of silicon nitride powders produced in a d.c. thermal plasma reactor

Ultra-fine silicon nitride powder was synthesized from the SiCl₄-NH₃-H₂-Ar system using a d.c. plasma torch reactor (production rate 150–400 g h⁻¹). The powder produced is pure white, fluffy and amorphous. The particles are spheroidal in shape with a mean diameter between 30–60 nm forming aggregates of 0.1–0.4 μm depending on the operational conditions. Chemical analysis on the crude powder handled at ambient atmosphere revealed: N(−NH₄Cl):37–39%, O:3–5% and Cl:2–3%. The amorphous powder can be crystallized around 1500 °C under nitrogen to give an α-phase content in excess of 90%. Infrared spectra can be used to semi-quantitatively determine the NH₄Cl content of the crude powder. That proportion is between 2.5 and 4%. The influence of some process parameters e.g. (N/Si and H₂/N molar ratios, internal pressure) on powder properties was also investigated. The N/Si molar ratio was found to be the most important parameter for the powder composition whereas the internal pressure plays a major role on the powder morphology.     

High-rate deposition of nanostructured SiC films by thermal plasma PVD

With ultrafine SiC powder as starting material, thermal plasma physical vapor deposition has been applied successfully to the deposition of SiC films on Si substrates. The control of processing parameters such as substrate temperature, powder feeding rate and composition of plasma gases, permits the deposition of SiC films on a wide area of around 400 cm² with a variety of microstructures from amorphous to nanostructured and with various morphologies from dense to columnar. For the nanostructured case, the crystallite size was between 3 and 15 nm and the maximum deposition rate calculated based on the actual deposition duty time reached 200 nm/s. The deposition mechanism is discussed briefly.     

High-rate deposition of nanostructured SiC films by thermal plasma PVD

With ultrafine SiC powder as starting material, thermal plasma physical vapor deposition has been applied successfully to the deposition of SiC films on Si substrates. The control of processing parameters such as substrate temperature, powder feeding rate and composition of plasma gases, permits the deposition of SiCfilms on a wide area of around 400 cm² with a variety of microstructures fromamorphous to nanostructured and with various morphologies from dense to columnar. For the nanostructured case, the crystallite size was between 3 and 15 nm and the maximum deposition rate calculated based on the actual deposition duty time reached 200 nm/s. The deposition mechanism is discussed briefly.     

Production of nanosized carbon black from hydrocarbon by a thermal plasma

Thermal plasma conditions (optimal heat and radical sources for the thermal decomposition) can be used to accelerate thermodynamically favorable chemical reactions or provide the energy required for endothermic reforming processes. Direct thermal decomposition of hydrocarbon (methane, acetylene, and propane) was carried out using a thermal plasma system which is an environmentally favorable process. In case of thermal decomposition, high purity of the hydrogen and solidified nano-sized carbon can be achieved without any contaminant. The main product carbon produced by thermal decomposition can be either sequestered or used as a raw material and it can be applied for the varieties of industry fields. The morphology of the carbon was characterized by SEM and the particle size was determined by a particle size analyzer. It was observed that the carbon black particles were sphere particles with mainly several tens of nano-sized diameters, those are about 10-80 nm. It can be expected to be used as a raw material of laser printer toner which requires small sized carbon black particles; An average primary particle size of PRINTEX L (Degussa Fillers & Pigment) used in a part of printing inks is 23 nm. In case of the XRD pattern of the produced carbon black from acetylene is of higher crystalline than the commercialized carbon black used for fuel cells. Also carbon species produced were characterized by EA and TGA.     

Preparation and characterization of ultra-fine diamond powders obtained by using a d.c. arc plasma jet

Ultra-fine diamond powders were prepared from C₂H₂ and H₂ by quenching a d.c arc plasma jet on a metal disc cooled with water. The powders obtained were agglomerated with very small particles about 20 nm in diameter, with an average size of about 80 nm. Raman spectra, X-ray diffraction, and transmission electron micrographs of them showed that each particle consisted of very small diamond crystals and a small amount of amorphous carbon, and that a single crystal particle had a twin plane. The formation mechanism of the powders has been briefly discussed from the viewpoint of the supersaturation ratio based on the classical theory of homogeneous nucleation.     

Production of solar absorbing cermet films by dual cathode d.c. magnetron sputtering

A technique using d.c. reactive sputtering from two cathodes for the production of graded-index solar selective cermet films is described. Two variations are presented, one in which the grading is carried out by changing the electrical inputs alone, and the other in which the substrate is translated through a flux of sputtered material that is constant in time. The second variation employs a magnetron source. The coatings produced are iron-silicon oxide and copper-aluminium oxide. The annealing behaviour of the films was studied and the iron-silicon oxide film was found to be stable in vacuum up to 400°C.     

The dependence of diamond growth rate on hydrogen dissociation in a d.c. arcjet plasma

A calorimetric technique was used to measure the plasma enthalpy in a d.c. arcjet diamond deposition system. Using these measurements, and a model based on the assumption of local thermal equilibrium (LTE), the temperature of the plasma emerging from the torch nozzle could be calculated. By controlling the electrical power into the torch, the plasma temperature could be varied from 2900 to 4500 K. This range of plasma temperatures corresponded to a fraction of dissociated H₂ () which ranged from 0.19 to 0.98. Surprisingly, this variation in the concentration of atomic hydrogen had no effect on the diamond growth rate.     

The synthesis of carbides by carbothermic reduction of mineral oxides in a d.c. transferred arc plasma

The carbothermic reduction of commercially available mineral concentrates of magnetite and chromite has been investigated using a 70kW d.c. transferred arc plasma system. The degree of metallization achieved is discussed with respect to the presence of slag forming constituents present in the mineral, particle size, thermodynamics and kinetics. The products were characterized using optical, electron and Auger microscopy techniques and the degree of metastability evaluated.     

Effect of submicron size SiC particles on microstructure and mechanical properties of AZ31B magnesium matrix composites

The magnesium matrix composites reinforced with three volume fractions (3, 5 and 10 vol.%) of submicron-SiC particles (∼0.5 μm) were fabricated by semisolid stirring assisted ultrasonic vibration method. With increasing the volume fraction of the submicron SiC particles (SiCp), the grain size of matrix in the SiCp/AZ31B composites was gradually decreased. Most of the submicron SiC particles exhibited homogeneous distribution in the SiCp/AZ31B composites. The ultimate tensile strength and yield strength of the 10 vol.% SiCp/AZ31B composites were simultaneously improved. The study of interface between the submicron SiCp and the matrix in the SiCp/AZ31B composite suggested that submicron SiCp bonded well with the matrix without interfacial activity.     

Influence of operating parameters on the retention of chromatographic particles by thermal field-flow fractionation

We have investigated the retention behavior of chromatographic particles in thermal field-flow fractionation (FFF). Retention time is found to increase with increasing temperature drop across the channel thickness, as expected for species exhibiting a thermophoretic mobility. Experiments have been performed with a vertically oriented channel rather than by using the classical horizontal configuration as this leads to much more reproducible retention data. In acetonitrile, silica-based particles are more retained than octadecyl-bonded silica particles, which confirms our previous finding, by means of a different method, that the thermophoretic mobility of the latter is smaller than that of the former. Whatever the type of particles and the nature of the carrier liquid, the relative retention time is observed to decrease with increasing carrier flow rate. This indicates that a hydrodynamic lift force acts on particles to move them away from the accumulation wall, as is usually observed in all FFF experiments with micrometer-sized particles. However, upward and downward flow directions in the vertical channel lead to similar retention data, indicating that inertial lift forces have a minor influence on retention. In addition, the relative retention time steadily decreases with increasing sample concentration, suggesting that the hydrodynamic lift force increases significantly with sample concentration. Accordingly, we speculate that a new transport phenomenon, called shear-induced hydrodynamic diffusion, not previously accounted for in the modeling of retention in FFF, is controlling the migration of the particles in the FFF channel. Implications of the influence of this phenomenon in other FFF experiments are discussed.     

Thermoelectric properties of SiC thick films deposited by thermal plasma physical vapor deposition

SiC thick films of about 300 µm could be prepared with a deposition rate above 300 nm/s by thermal plasma physical vapor deposition (TPPVD) using ultrafine SiC powder as a starting material. The thermoelectric properties were investigated as a function of composition and doping content. The nondoped films showed n-type conduction. Although the Seebeck coefficient reached as high as -480 µV/K, the power factor was only around 1.6 × 10^-4 Wm^-1 K^-2 at 973 K due to the relatively high electrical resistivity. In order to reduce the electrical resistivity and to deposit layers with n-type and p-type conduction, N₂, B and B₄C were selected as the dopants. Nitrogen-doped samples exhibit n-type characterization, B and B₄C-doped samples exhibit p-type characterization, and the electrical resistivity decreased from 10^-2–10^-3 to 10^-4–10^-5 Ωm after doping. The maximum power factor of the nitrogen-doped SiC and the thick films deposited with B₄C powder reached 1.0 × 10^-3 and 6.4 × 10^-4 Wm^-1 K^-2 at 973 K, respectively.

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Influence of carbon precursors on thermal plasma assisted synthesis of SiC nanoparticles

Nanosized SiC was synthesized by solid state method using silicon and carbon powders followed by non-transferred arc thermal plasma processing. X-ray diffraction (XRD) analysis revealed that activated carbon has highest reactivity while graphite has lowest activity in the crystallization of SiC through solid state method. The reactivity was dependent on surface area of carbon source and activated carbon with highest surface area (590.18 m² g⁻¹) showed highest reactivity, whereas graphite with least surface area (15.69 m² g⁻¹) showed lowest reactivity. The free silicon content was decreased with increasing reaction time as well as carbon mole ratio. Scanning electron microscope (SEM) study showed that the shape and size of synthesized SiC depends on the shape and size of carbon source. SiC nanoparticles within 500 nm were formed for carbon black while bigger particles (∼5 μm) were formed for activated carbon and graphite. Plasma processing of these solid–solid synthesized SiC resulted into the formation of well dispersed, ultrafine SiC nanoparticles (30–40 nm) without any structural modification. Thermal plasma processing resulted into the increase in crystallite size of SiC.     

Influence of SiC particles on mechanical properties of Mg based composite

AZ91 magnesium alloy reinforced with different sizes of SiC particulates has been fabricated using powder metallurgy route. Mechanical properties of the specimens have been studied. Yield and ultimate tensile stresses show a decrease with the increase in the size of SiC particulates. The influence of thermal shock between 400°C and 30°C on the mechanical properties was also investigated. The results show a decrease in yield stress and elongation to fracture with the number of thermal shock cycles.     

Diamond deposition using anX-Y stage in a d.c. plasma jet chemical vapour deposition

Diamond was deposited using the substrate scanning mode in a d.c. plasma jet of the Ar-H₂-CH₄ system. Film thickness profiles, SEM and Raman spectra of the diamonds obtained revealed that uniformity of film thickness, crystal size and crystal quality was improved, but the deposition rate decreased with the scanning speed and the crystal quality was worse than that made in the centre of the flame without scanning.