Tantalum carbides

Separation of tantalum from niobium requires several complicated steps. Several methods are used to commercially produce the element, including electrolysis of molten potassium fluorotantalate, reduction of potassium fluorotantalate with sodium, or reacting tantalum carbide with tantalum oxide. Twenty five isotopes of tantalum are known to exist. Natural tantalum contains two isotopes. 

Niobium carbide is used as a component of hard metals, eg, mixtures of metal carbides that are cemented with cobalt, iron, and nickel. Along with tantalum carbide, niobium carbide is added to impart toughness and shock and erosion resistance. The spiraling rise in the price of tantalum has spurred the development of a hafnium carbide—niobium carbide substitute for tantalum carbide (68). These cemented carbides are used for tool bits, drill bits, shovel teeth, and other wear-resistant components turbine blades and as dies in high pressure apparatus (see Carbides). 

Straight WC—Co tools are not suitable for machining steels that produce long chips because straight grades undergo crater wear from diffusion of WC into the steel chip surface. However, soHd solutions of WC—TiC, WC—TiC—TaC, etc, resist this type of chemical attack. In addition, tantalum carbide can improve thermal-shock resistance. Steel cutting compositions thus typically contain WC—TiC—(Ta,Nb)C—Co. Tantalum carbide is often added as (Ta,Nb)C because the chemical similarity between TaC and NbC makes their separation expensive. 

The four most important carbides for the production of hard metals are tungsten carbide [12070-12-17, WC, titanium carbide [12070-08-5] TiC, tantalum carbide [12070-06-3J, TaC, and niobium carbide [12069-94-2] NbC. The binary and ternary soHd solutions of these carbides such as WC—TiC and WC—TiC—TaC (NbC) are also of great importance. Chromium carbide (3 2) [12012-39-0], molybdenum carbide [12011-97-1], MoC, and... 

Tantalum carbide is produced by carburization of the element or the oxide with carbon, ia a manner similar to the preparation of WC or TiC. Final carburization in a vacuum gives a golden yellow carbide, free of oxygen and nitrogen, that contains 6.1—6.3 wt % C and 0—0.2 wt % graphite. 

Hafnium carbide (HfC) is an interstitial carbide which, with tantalum carbide, is the most refractory compound known. Its characteristics and properties are summarized in Table 9.4. 

Tantalum carbide (TaC) is arefractory interstitial carbide with a high melting point. It is structurally and chemically similar to niobium carbide. It has two phases Ta and the monocarbide TaC. Thelatteris the only phase of industrial importance and the only one described here. The characteristics and properties of TaC are summarized in Table 9.7. 

The industrial practice for the production of tantalum consists of two steps. In the first, the carbide is made by charging a graphite crucible with an intimate, pelletized mixture of lamp black and tantalum pentoxide and heating it in a high-frequency furnace under a dynamic vacuum (10 torr). In the next step, the ground carbide and the requisite amount of tantalum pentoxide are mixed, palletized, and fed to a reduction furnace where the reduction to the metal occurs. The formation of tantalum carbide as well as the reduction to the metal occur at about 2000 °C. The product leaving the reduction furnace is in the form of pellets or roundels (small cylinders) of porous metal, usually sintered together. 

McKenna, P. M. Tantalum Carbide and its Relation to Other Hard Refrac-... 

Synroc Synroc. tantalum Kroll (1). tantalum carbide Menstruum, tar acids Phenolsolvan. 

Industrial hard carbides, 4 674-695. See also Tantalum carbide Tungsten carbide... 

Main uses of the metal. Applications of Ta in medicine (in prosthetic implants) are also well known because of its lack of toxicity and excellent compatibility with tissue. It is used for producing capacitors. Tantalum carbide is used in cutting tools. 

Tantalum carbide (TaG) is one of the hardest substances known. This compound represents its oxidation state of +4 for tantalum. 

The free energies of formation of the transition metal carbides are somewhat more negative than the free energies of formation of the actinide carbides. To facilitate separation of the actinide metal from the reaction products and excess transition metal reductant, a transition metal with the lowest possible vapor pressure is chosen as the reductant. Tantalum metal and tantalum carbide have vapor pressures which are low enough (at the necessary reaction temperature) to avoid contamination of the actinide metal by co-evaporation. 

Thermal reduction processes have been apphed successfully in making the metal from salts. In one such process, potassium fluotantalate is reduced with sodium metal at high temperatures to form tantalum powder of high purity and small particle size. Also, tantalum oxide can be reduced at high temperatures in vacuum with aluminum, silicon, or tantalum carbide. When the oxide is reduced by tantalum carbide, a metal sponge is obtained which can be embrittled with hydrogen to form powder metal. 

CVD of SiC normally uses silane and a hydrocarbon as the precursor gases and a hydrogen carrier gas. The gases pass over a heated graphite susceptor that is coated by SiC or tantalum carbide (TaC). 

Tantalum Carbide, TaC, has been prepared by heating a mixture of tantalum pentoxide and carbon in a molybdenum boat at 1260° C. in a stream of hydrogen,2 or by the action of hydrogen and carbon monoxide on tantalum pentachloride. It is a dark grey or black substance which is insoluble in all acids, and bums to the pentoxide when powdered and heated in air. Density—13 96. It melts with decomposition at 4100° abs., which is probably the highest melting-point yet recorded for a chemical compound. Its hardness coefficient fees between 9 and 10. It is a good conductor of electricity.4 For its crystal structure see reference cited.5... 

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