N. Ahlen et al. Journal of the European Ceramic Society 20(2000)2607-2618 3. Results and Discussion it grows to full length very rapidly. The rate-determining step should then be the formation of supersaturated 3.1. Interrupted experiments catalyst droplets from which whiskers can grow. The time dependence of the TaC formation is extracted from 3.. TaC the data given in Tables 3 and 4. The fraction of expec- We followed the reaction forming TaC whiskers at the ted weight loss is thus plotted versus reaction time(see optimum synthesis temperature, 1220C. TaC whiskers Fig. 4a). These data show that the reaction is complete were found as soon as the synthesis temperature had after 210 min and that Tac is then the only phase that been reached, with the same length and diameter as can be identified. The change in chemical composition hose present after completed carbothermal Vls during synthesis can be followed in a graph showing the growth. This indicates that, once a whisker has nucleated, observed weight change as a fraction of expected vs. reac- tion time for the different elements, assuming that the starting mixture will transform to Tac during the reaction (see Fig. 4b). The sodium level rapidly dropped to about 10% of the initial amount, but stayed at this level during the remaining tac whisker growth period, just until the reaction came to completion after 210 min. According to XRD analysis, this is due to that sodium tantalates (NaTaO3 and Na Ta4O11 are formed as intermediate phases during synthesis(see Table 4). As the last reduc tion step those sodium tantalates react. The main impurity in the synthesis product, found from chemical analysis, is remnants of the Ni catalyst that, however, is too low to be detected by use of XRD 100nm 3.1.2.TiC Two series of experiments interrupted after different reaction times were performed, one at 1250C and one at 1400 C. At 1400C the TiC whisker yield is slightl higher and the residual level of oxygen is lower than at 1250C. Primary chemical analysis data are given 200nm Tables 5 and 6 The reaction time required to form TiC whiskers at b) 1250C is about the same as for TaC. deduced both from weight loss curves and from chemical analysi data(Fig. 5a-c). Just as for TaC synthesis, a few TiC whiskers had already formed when the synthesis tem perature was reached. No sodium titanates were found as intermediates during the reaction, according to the XRD studies, and neither did chemical analysis show plateau in the ent(see Fig 5b and c), wh was the case for TaC synthesis. Traces of reduced tita- nium oxides(e.g. Ti3Os and Ti,O3)were, however, found in some of the interrupted experiments(see Table 7) 3. 2. The catalyst metal Ni has proved to be the best catalyst metal for synthesis of TiC and TaC whiskers. 3 After completed whisker 200nm 100nm growth, however, only a minor fraction of the whiskers terminate with a Ni droplet (<5%), and even those droplets are found at different levels of erosion(see Fig Fig. 6. (a)TEM micrograph of a Tic whisker terminated by a N 6). An attempt was made to study the catalyst droplets sharp, indicating that the whisker has nucleated on the surface of the that terminate some of the whiskers. Phosphorous additions to the reaction mixture were found to increase the possibility for finding whiskers terminated with3. Results and Discussion 3.1. Interrupted experiments 3.1.1. TaC We followed the reaction forming TaC whiskers at the optimum synthesis temperature, 1220C. TaC whiskers were found as soon as the synthesis temperature had been reached, with the same length and diameter as those present after completed carbothermal VLS growth. This indicates that, once a whisker has nucleated, it grows to full length very rapidly. The rate-determining step should then be the formation of supersaturated catalyst droplets from which whiskers can grow. The time dependence of the TaC formation is extracted from the data given in Tables 3 and 4. The fraction of expec￾ted weight loss is thus plotted versus reaction time (see Fig. 4a). These data show that the reaction is complete after 210 min and that TaC is then the only phase that can be identi®ed. The change in chemical composition during synthesis can be followed in a graph showing the observed weight change as a fraction of expected vs. reac￾tion time for the di€erent elements, assuming that the starting mixture will transform to TaC during the reaction (see Fig. 4b). The sodium level rapidly dropped to about 10% of the initial amount, but stayed at this level during the remaining TaC whisker growth period, just until the reaction came to completion after 210 min. According to XRD analysis, this is due to that sodium tantalates (NaTaO3 and Na2Ta4O11) are formed as intermediate phases during synthesis (see Table 4). As the last reduc￾tion step those sodium tantalates react. The main impurity in the synthesis product, found from chemical analysis, is remnants of the Ni catalyst (see Table 3) that, however, is too low to be detected by use of XRD. 3.1.2. TiC Two series of experiments interrupted after di€erent reaction times were performed, one at 1250C and one at 1400C. At 1400C the TiC whisker yield is slightly higher and the residual level of oxygen is lower than at 1250C. Primary chemical analysis data are given in Tables 5 and 6. The reaction time required to form TiC whiskers at 1250C is about the same as for TaC, deduced both from weight loss curves and from chemical analysis data (Fig. 5a±c). Just as for TaC synthesis, a few TiC whiskers had already formed when the synthesis tem￾perature was reached. No sodium titanates were found as intermediates during the reaction, according to the XRD studies, and neither did chemical analysis show any plateau in the Na content (see Fig. 5b and c), which was the case for TaC synthesis. Traces of reduced tita￾nium oxides (e.g. Ti3O5 and Ti2O3) were, however, found in some of the interrupted experiments (see Table 7). 3.2. The catalyst metal Ni has proved to be the best catalyst metal for synthesis of TiC and TaC whiskers.2,3 After completed whisker growth, however, only a minor fraction of the whiskers terminate with a Ni droplet (<5%), and even those droplets are found at di€erent levels of erosion (see Fig. 6). An attempt was made to study the catalyst droplets that terminate some of the whiskers. Phosphorous additions to the reaction mixture were found to increase the possibility for ®nding whiskers terminated with a Fig. 6. (a) TEM micrograph of a TiC whisker terminated by a Ni droplet. The interface between the whisker and the droplet is very sharp, indicating that the whisker has nucleated on the surface of the droplet; (b±d) droplets terminating whiskers in di€erent degrees of erosion. 2612 N. AhleÂn et al. / Journal of the European Ceramic Society 20 (2000) 2607±2618