On the carbothermal vapour-liquid-solid (VLS) mechanism for TaC, TiC, and TaxTi1-xC whisker growth

The growth of TaC, TiC and TaxTi1-xC whiskers has been studied in some deta il. The whiskers were synthesised via a vapour-liquid-solid (VLS) growth me chanism in the temperature range 1220-1400 degreesC. The starting materials were Ta2O5, TiO2, C, NaCl, and a catalyst metal (Ni, Co, Fe, and Cu were t ested). The main reaction during synthesis was a carbothermal reduction of Ta2O5 and TiO2, and NaCl was added to form the oxochlorides and chlorides o f Ta and Ti that account for the transport to the catalyst metal. The synth eses were made in a protecting Ar atmosphere. From experiments interrupted after different times at the synthesis temperature it is clear that sodium tantalates form as intermediate products, whereas sodium titanates cannot b e identified. Only metals that are able to dissolve the elements building u p the whiskers work as catalysts. Whisker growth starts either from a catal yst droplet in contact with carbon or from an oxide particle in contact wit h both catalyst metal and carbon. For TaC and TaxTi1-xC the only growth dir ection observed is [100], while TiC may grow either along [100] or along [1 11]. (C) 2000 Elsevier Science Ltd. All rights reserved.