Vithal Revankar

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.

Preparation of tungsten and tungsten carbide submicron powders in a chlorine-hydrogen flame by the chemical vapor phase reaction

Abstract A hydrogen-chlorine flame chemical vapor deposition reactor has been developed to synthesize ultrafine powders of refractory compounds (e.g. carbides and nitrides). At the laboratory scale, synthesis of tungsten and tungsten carbides (both (WC)1−itx and α-W2C) gave encouraging results. The collected refractory powders do not have internal porosity, they exhibit spherical shape and have a very narrow size distribution. The size of the particles and the crystalline structure depends on the flame temperature, flow rate of the reactants and residence time of particles. In short, they depend on the flame characteristics. Thermochemical calculations were carried out to obtain the optimum conditions for different carbide powder synthesis. The flame is completely characterized and the temperature distribution within the reactor is obtained.

Design and development of the d.c. plasma reactor for the synthesis of ultrafine powders

After analysing different plasma processes, we have designed and developed a 14kW d.c. plasma unit for generating ultrafine powders. Experiments to characterize the torch demonstrated that it is possible to operate the plasma unit over a wide range of conditions without losing its discharge stability. Temperature distribution profiles were determined radially as well as axially in the reaction chamber, and the efficiency of the torch was calculated for a fixed power input. Ultrafine powders of Ta, Mo and W were synthesized, and size and size distribution obtained. The particle size was found to be 0.03–0.1 μm, with a perfect spherical shape. The analysis of the samples is discussed.

A chemical vapor deposition method for the manufacture of silicon carbide tubes

Abstract An investigation on the deposition of silicon carbide to manufacture silicon carbide tubes from thermal decomposition of CH3SiCl3 in a hydrogen atmosphere is presented. A thermodynamic analysis is carried out to obtain the optimum conditions for this operation. A detailed design of the reactors is described. Two critical problems in the operation of the deposition and the solutions are given. A method of carbon coating is used to eliminate the crack induced by the different expansion coefficients of the β-SiC coating and the graphite substrate. The problem of scaling up is discussed.

HIGH PERFORMANCE CERAMIC FIBERS BY CVD TECHNOLOGY

Abstract An analysis of the momentum, energy and mass transport phenomena occurring in a cylindrical CVD reactor used for the coating of fibrous substrates is presented. The reactor is analyzed for the batch mode of operation. The reaction system leading to TiB2 fibers is investigated in detail. Experimental results for B2C, TiC, and B fibers are presented as well. Numerical results are reported in the range of actual experimental conditions for vertical and horizontal arrangements of the reaction system, These results serve not only to illustrate the complex interaction between the flow field and the different transport mechanisms but also constitute a valuable help in the development of design and scale-up criteria.

Multifilament silicon carbide fibers by chemical vapor deposition (CVD)

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CVD reactors for the synthesis of inorganic fibers. Modeling and experimental evaluation

Abstract In this work the application of CVD technologies for coating of fibrous substrates is addressed. The transport phenomena occurring in the reactor chamber are modeled in terms of conservation laws. The resulting mathematical model is solved by means of the GFEM. Two- and preliminary three-dimensional results are reported in the actual range of experimental conditions. Based on boundary layer assumptions for the gas phase, a one-dimensional “moving boundary” model for the deposition surface is formulated. Characteristics of fibers manufactured in the laboratory are presented and model predictions are analyzed. Several design and operation problems are pinpointed and further modeling studies are outlined.