Synthesis of boride, carbide, and carbonitride whiskers Mats Johnsson * Department of Inorganic Chemistry, Stockholm University, Stockholm S-106 91, Sweden Received 9 November 2003; accepted 4 January 2004 Abstract A chemical route for synthesizing whiskers of refractory metal borides, carbides, and carbonitrides will be presented. Examples of whiskers are TiB 2 ,B 4 C, TiC, TaC, NbC, and Ti x Ta 1  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 – 1800 jC in argon or nitrogen. Synthesis of e.g. TaC whiskers was made with Ta 2 O 5 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 NiCl 2 (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 Am and a length of 10–30 Am. The intended use for the whiskers is as reinforcing materials in ceramic composites. Application areas are e.g. ceramic cutting tools. D 2004 Elsevier B.V. All rights reserved. PACS: 68.70.+w; 81.10 Bk Keywords: Ceramic whiskers; VLS growth; Synthesis; Reaction mechanism 1. Introduction Whiskers are elongated single crystals, typically having cross-sectional diameters of 0.1–10 Am and lengths of 10–1000 Am. Because they are single crystals and in the ideal case contain very few dislocations, their strength and Young’s modulus are expected to be very high. Due to the high hardness and strength of refractory metal carbides and borides, these whiskers are highly interesting as reinforcing materials in ceramics. SiC whiskers are commercially available on a large scale and are mainly used as a reinforcing material, e.g. in high performance ceramics like those used in cutting tools for metal machining and other wear-resistant applications that need improved toughness and resistance to thermal crack formation. The whiskers reinforce the material by forming crack-deflecting interfaces [1]. A large number of new whisker phases have been produced by various synthesis methods. Three types of reaction mechanism can be pointed out: (i) precipitation from a melt, named the liquid–solid (LS) method; (ii) vapor-phase transport of one or more of the whisker constituents to a solid surface from which the whiskers grow, named the vapor –solid (VS) method; (iii) vapor- phase transport of one or more of the whisker constitu- ents to a metal catalyst droplet where the components decompose and dissolve, supersaturating the droplet, whereupon a whisker grows out from the surface—this is named the vapor–liquid–solid (VLS) method. The chemical vapor deposition (CVD) process can certainly be a VLS process, and it is frequently used for exploring new whisker phases, whereas another mechanism, namely carbothermal reduction (CTR) in combination with the VLS process, is most commonly employed for large-scale production of SiC whiskers. The starting material in this case is often rice husks, which naturally contain SiO 2 , organic material as precursor for carbon and small amounts of iron as catalyst [2]. The milled husks are stepwise heated to 1500–1600 jC, below 1000 jC in a CO atmosphere and above 1000 jC in nitrogen and/or ammonia. The overall reaction is a carbothermal reduc- tion process in which both whiskers and particles are 0167-2738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2004.01.045 * Tel.: +46-8-16-21-69; fax: +46-8-15-21-87. E-mail address: matsj@inorg.su.se (M. Johnsson). www.elsevier.com/locate/ssi Solid State Ionics 172 (2004) 365 – 368