Carbothermal synthesis of TaC whiskers via a vapor-liquid-solid growth mechanism

Tantalum carbide whiskers have been synthesized via a vapor-liquid-solid (VLS) growth mechanism in the temperature region 1200-1300 degrees C in nitrogen or argon. The starting materials consisted of Ta2O5, C, Ni, and NaCl. Carbon was added to reduce tantalum pentoxide, via a carbothermal reduction process, and Ni was used to catalyze the whisker growth. Thermodynamic calculations showed that tantalum is transported in the vapor phase as an oxochloride rather than as a chloride. An alkali metal chloride such as NaCl can be used as a source of Cl. The formation of TaC whiskers was found to be strongly dependent on the processing conditions used, on the choice of precursor materials, e.g., their particle sizes, and on the mixing procedure. So far we have obtained TaC whisker in a yield of 75-90 vol%. These whiskers are 0.1-0.6 mu m in diameter and 10-30 mu m in length, and they are straight and exhibit smooth surfaces. The main impurities are TaC particles, minor amounts of unreacted carbon, and remnants of the Ni catalyst.