10 N. Ahlen et al. Journal of the European Ceramic Society 20 (2000)2607-2618 source contained 21 wt. volatile components that nace(Thermal Technology) at a rate of 1000C/h to were burned off during the heating-up period. The choice the synthesis temperature in an Ar atmosphere. All of carbon source has a strong influence on the whisker experiments were conducted at atmospheric pressure ield. It has been shown in earlier studies that carbon and at temperatures in the range 1220-1400C Previous black with a volatile part retains its fluffy consistency after studies have revealed the optimum temperature for tac heat treatment, in contrast to carbon powder without and Tax Til- C whisker growth to be 1220-1250C3, 5 volatiles, and we believe that this is essential in improv- and 1400C for TiC whiskers. The whis sers o ng the ease with which the volatile Ta and Ti species were 0. 2-0. 6 um in diameter and 5-30 um in length. The reach the catalyst.2,3 The weighed-in TiO2/Ta2Os molar ratio can be varied in order to yield whiskers with dif- ferent x-values. The molar ratios of the different pre- Table 5 cursor materials used in this study for preparation of TaC, Chemical analysis data for the series of interrupted Tic synthesis TiC and Tao. Tios C are listed in Table 2 experiments The starting materials were mixed to homogeneity in Total o c Na Ni a blender. Portions of about 10-20 g were placed in weight (wt %)(wt %)(wt %)(wt %)(wt %)(wt % graphite crucible covered with a lid with a number of holes to allow controlled gas exchange between the (wt%) reactor chamber and the surrounding atmosphere(see LECO LECO AAs AAS Fig. 3). The mixture was then heated in a graphite fur 597.52.9 12.519.324.77.63.2 333.1 23418.32506.23.69938.1 34.216.72534.14.06944. 43.114.225.82.24.8 48.112.226.00.45.3 9725.30.085.30.0158.9 52.58.224.60.055.5 G0.4 a The experiments were quenched after different holding times, from 0 to 960 min, at the synthesis temperature 1250 C. The compo- sition of the starting mixture is not analysed it is calculated from the 总02 weight in amounts of the different starting materials. Chemical analysis data for the series of interrupted TiC synthesis Total o C Ni weight (wt %)(wt %)(wt %)(wt %)(wt %)(wt % 08 (wt.%) :2 LECO LECO AAS AAS Balance 0.925.97.2 11.631.3 谷04 23.717.723.86.53.510.937.5 47.31102501.7 51.39.224.80.4 53.27624.30.085.6 968 58.8 61.2 00250 3.86.623.80.026.2 1062.5 55.1 5.3 .700164 1.164.0 time(min) 14.623.60.016.0 240 3 0.0096.2 0.01658 Fig 4.(a) Observed weight change as a fraction of the expected vs. 960 57.5 2.1 23. 1 0.006 6.2 0 reaction time for the series of interrupted Tac whisker synthesis experiments at 1220.C;(b)observed weight change of the different The experiments were quenched after different holding times, Na,Cl, and Ni disappear from the reactor during synthesi that all O, sition of the starting mixture is not analysed it is calculated from o- elements as a fraction of the expected vs reaction time. Based on the from 0 to 960 min, at the synthesis temperature 1400.C. The com assumption that the starting mixture transforms to Tac and weight in amounts of the different starting materials.source contained 21 wt.% volatile components that were burned o€ during the heating-up period. The choice of carbon source has a strong in¯uence on the whisker yield. It has been shown in earlier studies that carbon black with a volatile part retains its ¯u€y consistency after heat treatment, in contrast to carbon powder without volatiles, and we believe that this is essential in improv￾ing the ease with which the volatile Ta and Ti species reach the catalyst.2,3 The weighed-in TiO2=Ta2O5 molar ratio can be varied in order to yield whiskers with dif￾ferent x-values. The molar ratios of the di€erent pre￾cursor materials used in this study for preparation of TaC, TiC and Ta0:5Ti0:5C are listed in Table 2. The starting materials were mixed to homogeneity in a blender. Portions of about 10±20 g were placed in a graphite crucible covered with a lid with a number of holes to allow controlled gas exchange between the reactor chamber and the surrounding atmosphere (see Fig. 3). The mixture was then heated in a graphite fur￾nace (Thermal Technology) at a rate of 1000C/h to the synthesis temperature in an Ar atmosphere. All experiments were conducted at atmospheric pressure and at temperatures in the range 1220±1400C. Previous studies have revealed the optimum temperature for TaC and TaxTi1ÿxC whisker growth to be 1220±1250C3,5 and 1400C for TiC whiskers.2 The whiskers obtained were 0.2±0.6 mm in diameter and 5±30 mm in length. The Fig. 4. (a) Observed weight change as a fraction of the expected vs. reaction time for the series of interrupted TaC whisker synthesis experiments at 1220C; (b) observed weight change of the di€erent elements as a fraction of the expected vs. reaction time. Based on the assumption that the starting mixture transforms to TaC and that all O, Na, Cl, and Ni disappear from the reactor during synthesis. Table 5 Chemical analysis data for the series of interrupted TiC synthesis experimentsa Total weight loss (wt.%) O (wt.%) C (wt.%) Na (wt.%) Ni (wt.%) Cl (wt.%) Ti (wt.%) Analysis method LECO LECO AAS AAS Balance Starting mixture 20.9 25.9 7.5 2.9 11.6 31.3 0 12.5 19.3 24.7 7.6 3.2 12.3 33.1 30 23.4 18.3 25.0 6.2 3.6 9.9 38.1 60 34.2 16.7 25.3 4.1 4.0 6.9 44.3 90 43.1 14.2 25.8 2.2 4.8 3.8 50.8 120 48.1 12.2 26.0 0.4 5.3 2.2 55.0 180 50.8 9.7 25.3 0.08 5.3 0.01 58.9 240 52.5 8.2 24.6 0.05 5.5 0 60.7 960 55.7 5.0 22.8 0 6.5 0 64.4 a The experiments were quenched after di€erent holding times, from 0 to 960 min, at the synthesis temperature 1250C. The compo￾sition of the starting mixture is not analysed it is calculated from the weight in amounts of the di€erent starting materials. Table 6 Chemical analysis data for the series of interrupted TiC synthesis experimentsa Total weight loss (wt.%) O (wt.%) C (wt.%) Na (wt.%) Ni (wt.%) Cl (wt.%) Ti (wt.%) Analysis method LECO LECO AAS AAS Balance Starting mixture ± 20.9 25.9 7.5 2.9 11.6 31.3 0 23.7 17.7 23.8 6.5 3.5 10.9 37.5 10 39.5 13.8 24.5 4.3 4.5 8.7 47.0 20 47.3 11.0 25.0 1.7 5.2 3.9 55.0 30 51.3 9.2 24.8 0.4 5.5 1.6 58.8 40 53.2 7.6 24.3 0.08 5.6 1.8 61.2 60 53.8 6.6 23.8 0.02 6.2 1.0 62.5 120 55.1 5.3 23.7 0.01 6.4 1.1 64.0 180 56.1 4.6 23.6 0.01 6.0 0.02 65.2 240 56.3 3.7 23.1 0.009 6.2 0.01 65.8 960 57.5 2.1 23.1 0.006 6.2 0 68.3 a The experiments were quenched after di€erent holding times, from 0 to 960 min, at the synthesis temperature 1400C. The compo￾sition of the starting mixture is not analysed it is calculated from the weight in amounts of the di€erent starting materials. 2610 N. AhleÂn et al. / Journal of the European Ceramic Society 20 (2000) 2607±2618