N. Ahlen et al. Journal of the European Ceramic Society 20 (2000)2607-2618 (see Fig. 8), most likely due to that e presenc of 3. 3. Whisker growth nitrogen suppresses the formation of Fe3c at the synthesis temperature. The whisker diameter is gen Most observations indicate that whisker growth starts erally larger with Fe than with Ni catalysts, indicating a from where nickel is in contact with carbon. 2 Both SEM difference in the surface tension between the droplet and and TEM micrographs of interrupted experiments show the solid whisker phase. EDS analysis of Fe droplets that whisker growth occasionally also may start on also gives very low contents of Ti(<I at. %) Copper oxide particles that are in contact with both carbon and does not work as catalyst metal. It has no solubility for nickel, giving very short transport distances(see Fig carbon at the synthesis temperature 9a). Such oxide particles were found in interrupted experiments for all types of whiskers studied. They were detected by giving a distinct EDS peak for oxygen in the SEM. For TaosTio.sC the oxide phase found contained both Ta and Ti, which showed it to be a secondary pre- cipitate. Whiskers also frequently terminate in a carbide particle that is actually a part of the same crystal as the whisker itself(see Fig. 9b). Such a carbide particle can be the result of direct carbothermal reduction of oxide remnants Some whiskers are thick at one end and much thinner at the other (see Fig. 10). Some narrowing whiskers terminated in a small Ni droplet at the thinnest end This narrowing may be explained by that the catalyst droplet has been eroded during whisker growth by Distance reacting with chlorine gas, giving a progressively smaller diameter. and since the surface tension balance with the solid whisker must be retained, this also leads to a pro gressively thinner whisker Fig. 7.(a) SEM microgel f a TiC whisker terminated by a Ni droplet; (b) linescan of the whisker in(a)showing that no Ni is incor- porated in its structural framework and that very little Ti remains dissolved in the Ni droplet after solidification. The Ti level is the same Table 9 the solubility of the whisker constituents in some System Eutectic temperature(C) at the eut = Ni-Ta-C Ni6.7 at.%o Ta-78 at.%C Ni-TI-C Ni9.0 at.%o T1-5.6 at.%oC Co-Ta-c 1311 Co-0.8 at.% Ta-ll 8 at. %C 200nm Co-TIC Co-3.9at.%T}-15.5at.%C Fe-Ti-C 1151 Fe-0.2 at. %T-17.5 at. %C sker terminated by a Fe droplet. Those The eutectic temperature given is the lowest temperature were generally have larger diameters than those grown with Ni as catalyst liquid is in contact with carbide phase, e.g. TiC or TaC.(see Fig. 8), most likely due to that the presence of nitrogen suppresses the formation of Fe3C at the synthesis temperature. The whisker diameter is gen￾erally larger with Fe than with Ni catalysts, indicating a di€erence in the surface tension between the droplet and the solid whisker phase. EDS analysis of Fe droplets also gives very low contents of Ti (<1 at.%). Copper does not work as catalyst metal. It has no solubility for carbon at the synthesis temperature. 3.3. Whisker growth Most observations indicate that whisker growth starts from where nickel is in contact with carbon.2 Both SEM and TEM micrographs of interrupted experiments show that whisker growth occasionally also may start on oxide particles that are in contact with both carbon and nickel, giving very short transport distances (see Fig. 9a). Such oxide particles were found in interrupted experiments for all types of whiskers studied. They were detected by giving a distinct EDS peak for oxygen in the SEM. For Ta0:5Ti0:5C the oxide phase found contained both Ta and Ti, which showed it to be a secondary pre￾cipitate. Whiskers also frequently terminate in a carbide particle that is actually a part of the same crystal as the whisker itself (see Fig. 9b). Such a carbide particle can be the result of direct carbothermal reduction of oxide remnants. Some whiskers are thick at one end and much thinner at the other (see Fig. 10). Some narrowing whiskers terminated in a small Ni droplet at the thinnest end. This narrowing may be explained by that the catalyst droplet has been eroded during whisker growth by reacting with chlorine gas, giving a progressively smaller diameter, and since the surface tension balance with the solid whisker must be retained, this also leads to a pro￾gressively thinner whisker. Fig. 7. (a) SEM micrograph of a TiC whisker terminated by a Ni droplet; (b) linescan of the whisker in (a) showing that no Ni is incor￾porated in its structural framework and that very little Ti remains dissolved in the Ni droplet after solidi®cation. The Ti level is the same as the background level. Table 9 Composition of the liquid phase at the eutectic temperature to indicate the solubility of the whisker constituents in some catalyst metals testeda System Eutectic temperature (C) Composition of the liquid phase at the eutectic temperature Ni±Ta±C 1347 Ni±6.7 at.% Ta±7.8 at.%C Ni±Ti±C 1257 Ni±9.0 at.% Ti±5.6 at.%C Co±Ta±C 1311 Co±0.8 at.% Ta±11.8 at.%C Co±Ti±C 1284 Co±3.9 at.% Ti±15.5 at.%C Fe±Ti±C 1151 Fe±0.2 at.% T±17.5 at.%C a The eutectic temperature given is the lowest temperature were liquid is in contact with carbide phase, e.g. TiC or TaC. Fig. 8. TiCyN1ÿy whisker terminated by a Fe droplet. Those whiskers generally have larger diameters than those grown with Ni as catalyst metal. 2614 N. AhleÂn et al. / Journal of the European Ceramic Society 20 (2000) 2607±2618