Carbide synthesis

In the second part of this study the carbon-coated materials were used as supports for Ni-Mo and Ni-W carbides synthesis. 

The advantage of metal carbide synthesis via reaction of solid metal oxides with gaseons hydrocarbons is because one reactant is in the gas phase. In contrast to (7-92), a solid oxide reacts in this method with a gas-phase hydrocarbon. Consider, as an example, the synthesis of titanium carbide from titanium oxide (Hancock, 1973 Walder Hancock, 1973) ... 

Conversion of Solid Oxides into Carbides Using Gaseous Hydrocarbons inside of RF-ICP Thermal Plasma Discharge and Some Other Plasma Technologies for Carbide Synthesis... 

Unfortunately in real situation the reactions are not carried to completion. This is why one has to solve the problem of chemical reactivity before determining the appropriate quenching rate. These remarks also apply to carbide synthesis. 

This technique has been used by Stokes and co-workers among others for tantalum and tungsten carbide synthesis and the authors have been concerned with a detailed analysis of the influence of the quenching rate on the form of the final product. This work has been extensively reported in Ref. Perhaps it would now be possible to analyse the experimental data taking into account thermodynamic and transport considerations. 

An interesting n thod similar to Matsumoto s has been proposed by Wickens for boron carbide synthesis. The experimental arrangement is shown Fig. 81 a hi intensity arc is struck between a graphite cathode and a composite consumable anode made of boron oxide or sodium borate which are more readily available and cheaper than boron trichloride. B4C was obtained by this method, but the author indicates that the power comsumption (kWh/kg B4C) was high owing to insufficient erosion of the consumable anode. 

Fine carbon (black), which we used to synthesise carbides, is poorly wetted with LiCl and CaCl2 melts and therefore it comes to the surface when the salt melts at the beginning of the experiment, whereas the metal powders are left on the tube bottom. The density of the molten lithium and calcium is lower than the density of their chloride melts and therefore they come to the melt surface too. Thus, at the very beginning of the process the components are spatially separated and the question arises as to the mechanism of the carbide synthesis under these conditions. 

The X-ray phase analysis confirmed that the synthesis products are TiC, ZrC, NbC and TaC carbides, which do not contain free carbon. Fineness of the carbides is determined by the grain size of the metal powder. The synthesis time depends on the diffusion rate of carbon into the volume of the grain or the crystallite. We used oxides of refractory metals as the starting materials for production of fine (1-5 pm) carbides. In this case the mass of the metallic calcium was increased taking into account its consumption for the calcium thermal reduction of the oxides. The presence of a higher amount of the calcium oxide in the salt melt did not incur large difficulties in the synthesis of the carbides and their washing with water to remove salts, because the calcium oxide easily dissolves in acidified water. This method of the carbide synthesis has been covered by a Russian Federation patent [6]. 

Zirconium silicide synthesis Zirconium diboride Zirconium carbide synthesis... 

The nanorod represents an important building block for nanostructures. Nanorods would be useful as reinforcements in metal and ceramic matrix composites as well as ideal structures with which to pin vortices in high-temperature superconductors (119). Whiskers represent attractive reinforcing additives for metal and ceramic matrix compositions to impart more strength to the ceramic object. The superior performance of titanium carbide reinforcements is found in two different forms, discontinuous fiber and hollow microspheres, by controlled morphology carbide synthesis where titanium and carbon precursors combine in specially designed graphite... 

In carbide synthesis, the 20 % CH4/H2 mixture similarly reduces and carburizes the oxides, and the H2O, CO and CO2 signals provide the information on the progress of the reaction. Again, no features are observed in the CH4 and H2 signals due to their presence in large excess. In all cases the reaction intermediates are quenched at intermediate points to identify the solid phases by X-ray diffraction (XRD). 

This observation may be related to topotactic transformations that commonly occur in nitride synthesis, but are absent in carbide synthesis. For example, in VN synthesis, when a sample of V2O5 crystals with preferential orientation along [001] planes is nitrided, the product conserves the oriented nature, with a preferential orientation along [200] planes (5). However, when the same V2O5 crystals are carburized, the preferential orientation is lost, and the powder x-ray diffraction shows a randomly oriented sample (22). 

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