N. Ahlen et al. / Journal of the European Ceramic Society 20(2000)2607-2618 2Ta2O5(s)+6CI(g)+3C(s)-2TaOCl3(g)+3CO(g) (2) TaOCl3 (g)+C(s)+ Ni-Ti-C()- Ni-Ta-Ti-CO +Co(g)+3cl(g) TiO2(s)+3Cl(g)+2C(s)- TiCl3 (g)+ 2C0(g) (3) Ni-Ta-Ti-C()- TaxTil-C(s)+NiO) Other gas-phase species that also form by similar A simplified sequence for the carbothermal VLS eactions but in lower concentrations are TaO, CI(g), growth mechanism is outlined in Fig. I TaCls(g), and TiCl(g), and these may also to some For some years, we have been studying growth of extent be responsible for the transport of Ta and Ti different transition-metal carbide and carbonitride The melting temperature of the catalyst metal must whiskers 2.3, 5-7 We have achieved control of the synth match the temperature range where TiCl3(g) and esis so as to gain a high whisker yield(see Fig. 2), but TaOCl3(g)form in high concentrations. If a catalyst we have not obtained a detailed understanding of the metal with a too high melting point is chosen, then growth mechanism. In the present article we have per direct carbothermal reaction, resulting in particles, may formed a detailed study of the growth mechanisms of dominate over the whisker formation by the VLS Tax Til-xC whiskers with 0<x<I designed to explore mechanism. The following reactions are expected to some essential features of the VLs mechanism occur at the catalyst(Ni-C, etc, denote that C, Ta, and Ti are dissolved in the Ni catalyst) 2. Experimental C(s)+Ni()→Ni-C() 2.1. Synthesis The starting materials used in the preparation of TiCl3(g)+Ni-CO->Ni-Ti +3cl(g) (5) Ta Ti-C whiskers are listed in Table 1. The carbon Table 3 Chemical analysis data and measured weight loss for the series with interrupted TaC synthesis experiments" eight loss(wt % O(wt % C(wt % Na(wt % Ni(wt % N(wt % Ta+Cl (wt % nalys sis method LECO LECO ng mixture 6.8 23.5 30.6 00000 33 33.0 6.3 33.3 6.4 Table 4 Phases present ding to powder X-ray diffraction data(XRD) after different reaction times for the series of interrupted Tac-whisker syntheses Starting mixture 15 90 20 240 a Na]Ta,O 5xxxx 4xxxx not the experiments were interrupted after different holding times, from 0 to 240 min, at the synthesis temperature 1220oC.NaCl,Ni,and Ccould2Ta2O5… †‡s 6Cl g… †‡3C s…†! 2TaOCl3… †‡g 3CO g…† …2† TiO2… †‡ s 3Cl g… †‡ 2C s…†! TiCl3… †‡ g 2CO g…† …3† Other gas-phase species that also form by similar reactions but in lower concentrations are TaO2Cl(g), TaCl5(g), and TiCl4(g), and these may also to some extent be responsible for the transport of Ta and Ti. The melting temperature of the catalyst metal must match the temperature range where TiCl3(g) and TaOCl3(g) form in high concentrations. If a catalyst metal with a too high melting point is chosen, then a direct carbothermal reaction, resulting in particles, may dominate over the whisker formation by the VLS mechanism. The following reactions are expected to occur at the catalyst (Ni±C, etc., denote that C, Ta, and Ti are dissolved in the Ni catalyst). C s… †‡ Ni l…† ! NiÿC l…† …4† TiCl3… †‡ g NiÿC l…† ! NiÿTiÿC I… †‡ 3Cl g…† …5† TaOCl3… †‡ g C s… †‡ NiÿTiÿC l…† ! NiÿTaÿTiÿC l… † ‡ CO g… †‡ 3Cl g…† …6† NiÿTaÿTiÿC l…† ! TaxTi1ÿxC s… †‡ Ni l…† …7† A simpli®ed sequence for the carbothermal VLS growth mechanism is outlined in Fig. 1. For some years, we have been studying growth of di€erent transition-metal carbide and carbonitride whiskers.2,3,5ÿ7 We have achieved control of the synth￾esis so as to gain a high whisker yield (see Fig. 2), but we have not obtained a detailed understanding of the growth mechanism. In the present article we have per￾formed a detailed study of the growth mechanisms of TaxTi1ÿxC whiskers with 04x41 designed to explore some essential features of the VLS mechanism. 2. Experimental 2.1. Synthesis The starting materials used in the preparation of TaxTi1ÿxC whiskers are listed in Table 1. The carbon Table 3 Chemical analysis data and measured weight loss for the series with interrupted TaC synthesis experimentsa Weight loss (wt.%) O (wt.%) C (wt.%) Na (wt.%) Ni (wt.%) N (wt.%) Ta+Cl (wt.%) Analysis method LECO LECO AAS AAS LECO Balance Starting mixture 0 16.2 15.0 2.4 0.8 0.2 65.1 0 6.8 13.7 14.3 1.9 0.8 0.2 69.1 5 8.6 13.2 14.0 1.6 0.9 0.1 70.2 15 12.2 12.3 13.4 1.3 0.9 0.1 72.0 25 16.7 10.8 12.6 0.8 0.7 0.1 74.9 45 21.2 8.4 11.3 0.3 1.3 0.2 78.5 60 23.5 7.3 10.7 0.2 1.1 0.1 80.6 90 25.8 5.5 9.7 0.2 1.0 0.2 83.5 120 28.1 3.7 8.6 0.2 1.1 0.3 86.2 150 30.6 2.0 8.6 0.2 1.1 0.2 88.0 180 33.0 0.4 6.5 0.2 1.1 0.2 91.6 210 33.0 0.09 6.3 0.02 1.2 0.2 92.2 240 33.3 0.1 6.4 0.01 1.2 0.2 92.1 a The experiments were quenched after di€erent holding times, from 0 to 240 min, at the synthesis temperature 1220C. Ta and Cl were not analysed. No information on the number of vacancies is available. Table 4 Phases present according to powder X-ray di€raction data (XRD) after di€erent reaction times for the series of interrupted TaC-whisker synthesesa Starting mixture 0 5 15 25 45 60 90 120 150 180 210 240 Ta2O5 ± ± Na2Ta4O11 ± ±± ±±± NaTaO3 ± ±±± ± ± TaC ± a The experiments were interrupted after di€erent holding times, from 0 to 240 min, at the synthesis temperature 1220C. NaCl, Ni, and C could not be detected. N. AhleÂn et al. / Journal of the European Ceramic Society 20 (2000) 2607±2618 2609