Carbothermal synthesis of TaC whiskers via a vapor-liquid-solid growth mechanism
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Carbothermal synthesis of TaC whiskers via a vapor-liquid-solid growth mechanism M. Johnsson and M. Nygren Department of Inorganic Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden (Received 24 January 1997; accepted 15 April 1997)
Tantalum carbide whiskers have been synthesized via a vapor-liquid-solid (VLS) growth mechanism in the temperature region 1200 –1300 ±C in nitrogen or argon. The starting materials consisted of Ta2 O5 , 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 mm in diameter and 10 –30 mm 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.
I. INTRODUCTION
Besides SiC whiskers there are very few other types of whiskers produced on a large scale. The SiC whisker has a low thermal expansion coefficient and can withstand oxidative environments at elevated temperatures, and it has therefore found use as a reinforcing material, e.g., in ceramic cutting tools. SiC whiskers have limited chemical stability, however, and in many ceramic applications one wants to have a reinforcing component with a thermal expansion coefficient different from that of the matrix material by a given percentage. During recent years we have investigated the possibility of preparing whiskers of different types, e.g., with different thermal expansion coefficients. In this article we report on the preparation of TaC whiskers via a vapor-liquid-solid (VLS) growth mechanism. There are no articles dealing with a carbothermal VLS synthesis of TaC whiskers, but there is a recent paper on the preparation of TiC whiskers.1 In addition there are some patent applications concerning preparation of transition metal carbides via a VLS mechanism.2–6 Typically, the following precursor materials have been used: (i) Ni, which acts as a catalyst and is able to dissolve both C and Ta; (ii) NaCl, which is partly decomposed to Na(g) and Cl(g) at the reaction temperature (1200–1300 ±C); (iii) Ta2 O5 : it should, however, be noted that, according to the thermodynamic calculations described below, Ta is transported in the vapor phase as an oxochloride rather than as a chloride in this VLS process; (iv) C: TaC is formed via a carbothermal reduction process. J. Mater. Res., Vol. 12, No. 9, Sep 1997
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