E驅≈3S Journal of the European Ceramic Society 20(2000)2607-2618 On the carbothermal vapour-liquid-solid (Vls) mechanism for Tac, TiC, and Ta_c whisker growth Niklas ahlen. Mats Johnsson a, Ann-Kristin larsson a. Bo Sundman b a Department of Inorganic Chemistry, Stockholm University, S-10691 Stockholm. Swede dEpartment of Material Science and Engineering, Royal Institute of Technology, S-100 44 Stockholm, Sweden Received 9 December 1999: received in revised form 10 April 2000; accepted 13 April 2000 Abstract The growth of TaC, TiC and TaxTil-C whiskers has been studied in some detail. The whiskers were synthesised via a vapour- liquid-solid (VLS)growth mechanism in the temperature range 1220-1400oC. The starting materials were Ta2Os, TiO2, C, NaCl and a catalyst metal (Ni, Co, Fe, and Cu were tested). The main reaction during synthesis was a carbothermal reduction of Ta O and TiO2, and NaCl was added to form the oxochlorides and chlorides of Ta and Ti that account for the transport to the catalyst metal. The syntheses 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 be identified. Only metals that are able to dissolve the elements building up the whiskers work as catalysts. Whisker growth starts either from a catalys droplet in contact with carbon or from an oxide particle in contact with both catalyst metal and carbon. For Tac and TaxTil-xC the only growth direction observed is [100], while TiC may grow either along [100] or along [lll]. 2000 Elsevier Science Ltd. All rights reserved Keywords: Carbides; TaC; TiC; VLS process; Whiskers 1. ntroduction can be favoured by optimising the molar ratios, the particle sizes of the precursor materials and the reaction This article presents a study of the carbothermal temperature. The temperature should be raised as higl vapour-liquid-solid (VLS) mechanism for whisker as possible in order to reduce the residual amount of owth. The whisker materials we have studied are TaC, oxygen. There is a compromise to be achieved, however TiC and Ta Ti-C. The Vls growth mechanism because at a too high synthesis temperature the solid involves all three aggregation states: a vapour phase state reaction between the oxide precursor and carbon transport of one or more of the whisker components to may dominate over the VLs growth process. Further- a melted catalyst droplet where the desired whisker more, at high temperatures the gaseous chloride and grows. The catalyst must be able to dissolve the whisker oxochloride species that are responsible for transport of components and the vls mechanism is only operative Ta and/ or Ti show a tendency to escape from the reac- at temperatures above the melting point of the catalyst tion chamber before reacting at the catalyst. Ar nother (i.e. the eutectic temperature of the multi-component problem at too high synthesis temperatures is that system catalyst-whisker), which therefore constitutes the whiskers may start to sinter together, forming agglom lower temperature limit for whisker synthesis via this erates growth mechanism. The catalyst metal must also be able wo main types growth mechanisms can reactions takia Components. There are two defined in general: (i)all mass transport takes place in perature: the VLs growth of whiskers and the direct acteristic for growth of whiskers via chemical vapour reaction between carbon and the transition metal oxides deposition, CVD), and(i) as in the present case solid resulting in carbide particles. The VLS growth mechanism carbon dissolves into the catalyst at the droplet inter face, while the other whisker component are transported in the vapour phase to the catalysts(characteristic for the carbothermal VLS mechanism).2,3 0955-2219/00/S-see front matter C 2000 Elsevier Science Ltd. All rights reserved PII:S0955-2219(00)00121-7On the carbothermal vapour±liquid±solid (VLS) mechanism for TaC, TiC, and TaxTi1ÿxC whisker growth Niklas AhleÂn a , Mats Johnsson a,*, Ann-Kristin Larsson a , Bo Sundman b a Department of Inorganic Chemistry, Stockholm University, S-106 91 Stockholm, Sweden bDepartment of Material Science and Engineering, Royal Institute of Technology, S-100 44 Stockholm, Sweden Received 9 December 1999; received in revised form 10 April 2000; accepted 13 April 2000 Abstract The growth of TaC, TiC and TaxTi1ÿxC whiskers has been studied in some detail. The whiskers were synthesised via a vapour± liquid±solid (VLS) growth mechanism in the temperature range 1220±1400C. The starting materials were Ta2O5, TiO2, C, NaCl, and a catalyst metal (Ni, Co, Fe, and Cu were tested). The main reaction during synthesis was a carbothermal reduction of Ta2O5 and TiO2, and NaCl was added to form the oxochlorides and chlorides of Ta and Ti that account for the transport to the catalyst metal. The syntheses were made in a protecting Ar atmosphere. From experiments interrupted after di€erent times at the synthesis temperature it is clear that sodium tantalates form as intermediate products, whereas sodium titanates cannot be identi®ed. Only metals that are able to dissolve the elements building up the whiskers work as catalysts. Whisker growth starts either from a catalyst droplet in contact with carbon or from an oxide particle in contact with both catalyst metal and carbon. For TaC and TaxTi1ÿxC the only growth direction observed is [100], while TiC may grow either along [100] or along [111]. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Carbides; TaC; TiC; VLS process; Whiskers 1. Introduction This article presents a study of the carbothermal vapour±liquid±solid (VLS) mechanism for whisker growth. The whisker materials we have studied are TaC, TiC and TaxTi1ÿxC. The VLS growth mechanism involves all three aggregation states: a vapour phase transport of one or more of the whisker components to a melted catalyst droplet where the desired whisker grows. The catalyst must be able to dissolve the whisker components and the VLS mechanism is only operative at temperatures above the melting point of the catalyst (i.e. the eutectic temperature of the multi-component system catalyst-whisker), which therefore constitutes the lower temperature limit for whisker synthesis via this growth mechanism. The catalyst metal must also be able to dissolve the whisker components. There are two competing reactions taking place at the synthesis tem￾perature: the VLS growth of whiskers and the direct reaction between carbon and the transition metal oxides resulting in carbide particles. The VLS growth mechanism can be favoured by optimising the molar ratios, the particle sizes of the precursor materials and the reaction temperature. The temperature should be raised as high as possible in order to reduce the residual amount of oxygen. There is a compromise to be achieved, however, because at a too high synthesis temperature the solid￾state reaction between the oxide precursor and carbon may dominate over the VLS growth process. Further￾more, at high temperatures the gaseous chloride and oxochloride species that are responsible for transport of Ta and/or Ti show a tendency to escape from the reac￾tion chamber before reacting at the catalyst. Another problem at too high synthesis temperatures is that whiskers may start to sinter together, forming agglom￾erates. Two main types of VLS growth mechanisms can be de®ned in general: (i) all mass transport takes place in the vapour phase to the vapour±liquid interface (char￾acteristic for growth of whiskers via chemical vapour deposition, CVD),1 and (ii) as in the present case solid carbon dissolves into the catalyst at the droplet inter￾face, while the other whisker component are transported in the vapour phase to the catalysts (characteristic for the carbothermal VLS mechanism).2,3 0955-2219/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0955-2219(00)00121-7 Journal of the European Ceramic Society 20 (2000) 2607±2618 * Corresponding author. E-mail address: matsj@inorg.su.se (M. Johnsson)