Synthesis of boride, carbide, and carbonitride whiskers

A chemical route for synthesizing whiskers of refractory metal borides, carbides, and carbonitrides will be presented. Examples of whiskers are TiB2, B4C, TiC, TaC, NbC, and TixTa1 - (x)(C,N). The synthesis technique used for the preparation is carbothermal reduction (CTR) of a starting oxide in combination with a vapor- liquid- solid growth mechanism (VLS) in the temperature range 1150-1800degreesC in argon or nitrogen. Synthesis of e.g. TaC whiskers was made with Ta2O5 as a source for Ta. Carbon black was used to reduce the oxide, Ni to catalyse the whisker growth. NaCl was used as a source of Cl for vapor phase transportation of Ta-oxochlorides to the catalyst. The catalyst metal was recycled several times during the synthesis and was transported as NiCl2(g) according to thermodynamic calculations. The rate of whisker formation was found to depend on many parameters e.g. the synthesis temperature, the choice of catalyst, and the atmosphere. Many of the different whisker types were obtained in a yield of 70-90 vol.% with a diameter of 0.5-1 mum and a length of 10-30 mum. The intended use for the whiskers is as reinforcing materials in ceramic composites. Application areas are e.g. ceramic cutting tools. (C) 2004 Elsevier B.V. All rights reserved.