N. Ahlen et al. Journal of the European Ceramic Society 20 (2000)2607-2618 33. Growth directions and with smooth surfaces. Also in the Tao.s TiosC e The growth directions of the whiskers were identified by sample(1250 C), only the [100] growth direction was ectron diffraction In TaC whisker samples(synthesised found, but the surface shape of these whiskers was quite at 1250C)only the growth direction [100 was found. different. If they are studied at fairly low resolution, or The Tac whiskers are generally well shaped-straight if they are not properly oriented, they may appear to have smooth surfaces, but if the crystals are investi- gated carefully(aligned perpendicular to the whisker direction) it is clear that the surfaces are wavy. This appearance can be compatible with a screw-like three- dimensional shape of the whisker that was also found for TiC (1275C)(see Fig. Ila and b). The screw-like shape can be more or less pronounced than in this example Studying the appearance of the TiC whiskers as a function of temperature, we found that at low tempera tures(around 1150-1200oC) the whiskers were fairly straight. At about 1250C the screw-like shape were most prominent(see Fig. 12). Samples at moderately high temperatures (1275C) had screw-like shape similar to the TaosTio.sC(1250C)sample(see Fig 13) At 1300C, the crystals are again straighter, but still not as perfectly shaped as the whiskers grown at lower temperatures (see Fig. 13). At even higher tempera tures (1400oC) the shape of the whiskers is again smooth The majority (roughly three quarters) of the TiC whiskers obtained at 1250C were screw-shaped. The growth direction of these"screws"is a crystallographic direction which would be either [100]or [lll]. On many occasions there was a sharp bend in the crystals, where 500nm the growth changed to another direction, either crystal lographically equivalent or the other one possible. For instance, the whisker in Fig. 12b changes growth direc- tion from [100] to [010]. In addition to these screwed whiskers in the TiC (1250C) sample, slightly thinner, straight whiskers could also be found [00 TaC and Ta.Til-C whiskers cannot be prepared in a high yield above about 1250C. At higher temperatures TaxTil-C particles are instead formed by a direct car 100 bothermal reduction reaction between the oxide and 111 minor fraction(<5%)of the Tax Til-C whiskers (250oC)show a secondary growth of carbide phase out the whiskers and the particle constitute he i>w from t Fig 9. (a)Whisker that may have started to gro an oxide particle; (b) TiC particle terminati 200nm particle may be the result of a direct carbothermal Fig. 10. Narrowing Taos TiosC whisker. At least two mechanisms for the formation of this type of whiskers are possible(see text).3.3.1. Growth directions The growth directions of the whiskers were identi®ed by electron di€raction. In TaC whisker samples (synthesised at 1250C) only the growth direction [100] was found. The TaC whiskers are generally well shaped Ð straight and with smooth surfaces. Also in the Ta0:5Ti0:5C sample (1250C), only the [100] growth direction was found, but the surface shape of these whiskers was quite di€erent. If they are studied at fairly low resolution, or if they are not properly oriented, they may appear to have smooth surfaces, but if the crystals are investi￾gated carefully (aligned perpendicular to the whisker direction) it is clear that the surfaces are wavy. This appearance can be compatible with a screw-like three￾dimensional shape of the whisker that was also found for TiC (1275C) (see Fig. 11a and b). The screw-like shape can be more or less pronounced than in this example. Studying the appearance of the TiC whiskers as a function of temperature, we found that at low tempera￾tures (around 1150±1200C) the whiskers were fairly straight. At about 1250C the screw-like shape were most prominent (see Fig. 12). Samples at moderately high temperatures (1275C) had screw-like shapes similar to the Ta0:5Ti0:5C (1250C) sample (see Fig. 13). At 1300C, the crystals are again straighter, but still not as perfectly shaped as the whiskers grown at lower temperatures (see Fig. 13). At even higher tempera￾tures (1400C) the shape of the whiskers is again smooth. The majority (roughly three quarters) of the TiC whiskers obtained at 1250C were screw-shaped. The growth direction of these ``screws'' is a crystallographic direction which would be either [100] or [111]. On many occasions there was a sharp bend in the crystals, where the growth changed to another direction, either crystal￾lographically equivalent or the other one possible. For instance, the whisker in Fig. 12b changes growth direc￾tion from [100] to [010]. In addition to these screwed whiskers in the TiC (1250C) sample, slightly thinner, straight whiskers could also be found. TaC and TaxTi1ÿxC whiskers cannot be prepared in a high yield above about 1250C. At higher temperatures TaxTi1ÿxC particles are instead formed by a direct car￾bothermal reduction reaction between the oxide and carbon. A minor fraction (<5%) of the TaxTi1ÿxC whiskers (1250C) show a secondary growth of carbide phase out Fig. 9. (a) Whisker that may have started to grow from the surface of an oxide particle; (b) TiC particle terminating TiC whiskers (1300C) the whiskers and the particle constitute the same crystal. The carbide particle may be the result of a direct carbothermal reduction of oxide. Fig. 10. Narrowing Ta0:5Ti0:5C whisker. At least two mechanisms for the formation of this type of whiskers are possible (see text). N. AhleÂn et al. / Journal of the European Ceramic Society 20 (2000) 2607±2618 2615