Powder carbides

Obviously, it is important to extend the fundamental information, obtained on the single crystal model surfaces, to the more realistic powder materials. In principle, the comparison of the electronic properties of these two systems can be achieved by using the near-edge X-ray absorption fine structure (NEXAFS) technique, which can be applied to investigate the properties of both single crystals and powder materials. The catalytic properties of the model and powder carbide materials can also be compared if the relevant reactions are carried out for both systems. 

The electronic properties of powder carbide materials have been the subject of many investigations.1,2 Among the early transition metal... 

The catalyst samples were prepared by pelletizing mixtures of powdered carbides and inert materials (for instance, BaS04). Oxygen or nitrous oxide were used as oxidants. Experiments were run in a quartz flow reactor at atmospheric pressure at 973-1023 K utilizing 0.2-0.5 g of carbide at flow rate of 30-100 cm3/min. The reactants and reaction products were separated on CaA molecular sieves and l,2,3-tn. v-/ -cyanoethoxypropane/ polysorb A columns. 

High-purity UC and UCj can be prepared by arc melting the elements. Annealing below 1700°C, preferably under pressure, is required - to form UjCj. Because of the high reactivity of the powdered carbides, all studies should be done with material that is as coarse as possible. The finely divided carbides are pyrophoric and aquire any oxygen in an otherwise inert atmosphere at RT. 

Both carbides are prepared as crystals from fused-sait electrolysis of carbonate-borate-fluoride-metal oxide salt baths. Large crystals of MojC are grown in a process d where the powdered carbide is added to a molten region on a growing solid rod. Zone refining also gives single crystals. 

Here the powdered carbide is placed in horizontal retorts similar to the retorts used in making coal gas these are heated externally to 8oo°-iooo° C. by being placed in a gas-fired furnace, while a stream of nitrogen is forced into the retorts for absorption by the carbide. 

The Gesellschaft fiir Stickstoffdiinger, at Westerregeln, add anything up to 10 per cent. CaCU to the powdered carbide, the absorption temperature of the N being only 700° C. The product, containing 20 per cent. N, 45 per cent. Ca, 19.5 per cent. C, 6.5 per cent. Cl, and 9 per cent, impurities, is put on the market under the name Stickstoffkalk. ... 

Oxide powders Pure metal powders Carbide powders Nitride powders... 

CHEMICAL PROPERTIES strong oxidizer reacts with water to produce heat corrosive in presence of traces of oxides attacks all base metals except aluminum and special chromium steels reacts vigorously with combustibles, organic solvents, metallic powders, carbides, cyanides, sulfides, or readily oxidizable materials reacts with alkalies vigorous reaction caused by addition of water to concentrated nitric acid FP (NA) LFL/UFL (not combustible) AT (NA) HC (NA) HF (-174.1 kJ/mol liquid at 25°C) Hf (2.503 kcal/mol) HID (-7971 cal/mol at 25°C). 

In some forms of generators for the gas, which are used commercially, the finely powdered carbide runs slowly into a reservoir of water. A better regulation of the gas supply is effected by using this method, and overheating, which may lead to an explosion, is avoided. Acetylene prepared from commercial calcium carbide contains ammonia, hydrogen sulphide, and phosphine. These substances must be removed when the gas is to be used for indoor illumination. The purification is effected by washing the gas with water and a mixture of slaked lime and bleaching powder, or with a solution of chromic acid in hydrochloric acid. In the laboratory a solution of mercuric chloride in dilute hydrochloric acid can be conveniently used for the purpose. 

It is shown experimentally that powdered carbides, borides and silicides of Ti, Zr, Nb and Ta can be synthesised in the Li-LiCl and Ca-CaCl2 melts. 

The powdered carbides are very reactive to water vapor, to a greater extent than UC, and are pyrophoric in some cases (Ogard et al, 1962 Drummond et al, 1957 Palfreyman and Russell, 1962 Kruger, 1962). Kruger (1962) noted that PU2C3 is more reactive than PuC and a dry-box atmosphere of KXlOppm (O2 -I- H2O) produced excessive surface corrosion within a few minutes. 

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