N. Ahlen et al. Journal of the European Ceramic Society 20(2000)2607-2618 8 EDS-TEM analyses of Ni droplets terminating TaxTiI-C whiskers and Fe-droplets terminating TiCx Ni- whiskers Whisker Droplet Ni T1 Na composition at (at.%) (at%) (at % (at.%) (at.%) TaxTik-C 123456667888 0024 7873 04000 26.5 ccccc TIC NE 6062 TIC NE 78.3 TIC NE 85.8 TIC NE 8.6 TIC NE 001222222333344444 97.2 1.7 TIC NE 82.0 TIC NE 84.6 TIC NE 86.6 .2.0 124 TIC NE 1.1 TIC NE 990 0.1 99.5 84.2 15.6 5.5 Red phosphorus was added to the starting mixtures in order to enhance the possibility of finding droplets. Different spot analysis on the same us content due to Ni-droplet in the product. TEM-EDS point analyses of elements also being present, e.g. O and Na, are lowering Ni droplets terminating whiskers show very little Ti the eutectic temperature and or Ta left in the Ni catalyst after solidification(see Cobalt also works as catalyst metal for whisker Table 8). A SEM-EDS line scan over a Ni droplet ter- growth, but the whisker yield is not as good as when N minating a TiC whisker shows that there is no Ni dis- is used. According to calculations with the computer solved into the structural framework of the carbid program ThermoCalc, cobalt behaves similarly to nickel whisker (see Fig. 7). The experimental results do not and can dissolve both Ti and Ta, but in much lower contradict that Ti and/ or Ta dissolve in the droplet at concentrations (Table 9). The reason why cobalt is not the synthesis temperature as efficient a catalyst as nickel is unclear but may be due Equilibrium calculations with the program Thermo- to the lower solubility of Ta (a factor of 8), especiall Calcusing data from "the Hard Materials Thermo- considering that both Ni and Co can form gaseous dynamic Database gives for hand that both C and oxochlorides and chlorides at the synthesis temperature Ti/Ta are soluble in liquid Ni (see Table 9). The carbide which may account for transport during the synthesis phase thus seems to have precipitated from a super In our studies we have found that iron does not saturated Ni droplet. The eutectic temperatures found function as a catalyst for Ta Ti-C whiskers, which is from the equilibrium calculations are, however, a bit most likely due to poisoning of the catalytic effect by higher than the temperatures that are working for the formation of Fe3C as observed by Wokulski and synthesis. The fact that the catalyst metal forms a melt Wokulska, 2 in a CVd study of TiC whisker synthesis at the synthesis temperature may be due to that other However, iron catalyses growth of TiC,Ni-y whiskersNi-droplet in the product. TEM-EDS point analyses of Ni droplets terminating whiskers show very little Ti and/or Ta left in the Ni catalyst after solidi®cation (see Table 8). A SEM-EDS line scan over a Ni droplet ter￾minating a TiC whisker shows that there is no Ni dis￾solved into the structural framework of the carbide whisker (see Fig. 7). The experimental results do not contradict that Ti and/or Ta dissolve in the droplet at the synthesis temperature. Equilibrium calculations with the program Thermo￾Calc9 using data from ``the Hard Materials Thermo￾dynamic Database''10 gives for hand that both C and Ti/Ta are soluble in liquid Ni (see Table 9). The carbide phase thus seems to have precipitated from a super￾saturated Ni droplet. The eutectic temperatures found from the equilibrium calculations are, however, a bit higher than the temperatures that are working for the synthesis. The fact that the catalyst metal forms a melt at the synthesis temperature may be due to that other elements also being present, e.g. O and Na, are lowering the eutectic temperature. Cobalt also works as catalyst metal for whisker growth, but the whisker yield is not as good as when Ni is used. According to calculations with the computer program ThermoCalc, cobalt behaves similarly to nickel and can dissolve both Ti and Ta, but in much lower concentrations (Table 9). The reason why cobalt is not as ecient a catalyst as nickel is unclear but may be due to the lower solubility of Ta (a factor of 8), especially considering that both Ni and Co can form gaseous oxochlorides and chlorides at the synthesis temperature, which may account for transport during the synthesis. In our studies we have found that iron does not function as a catalyst for TaxTi1ÿxC whiskers, which is most likely due to poisoning of the catalytic e€ect by formation of Fe3C as observed by Wokulski and Wokulska11,12 in a CVD study of TiC whisker synthesis. However, iron catalyses growth of TiCyN1ÿy whiskers Table 8 EDS±TEM analyses of Ni droplets terminating TaxTi1ÿxC whiskers and Fe±droplets terminating TiCxN1ÿx whiskersa Whisker composition Droplet no. Ni (at.%) Fe (at.%) Ti (at.%) Ta (at.%) Na (at.%) Cl (at.%) P (at.%) TaxTi1ÿxC 1 99.1 ± 0.1 0 0.5 0.3 0 TaxTi1ÿxC 2 97.5 ± 0.6 0.4 1.5 0 0 TaxTi1ÿxC 3 92.7 ± 0.3 0.4 2.8 1.4 2.4 TaxTi1ÿxC 4 94.3 ± 0.3 0.2 1.0 0 4.2 TaxTi1ÿxC 5 86.1 ± 0.4 0 0.4 0 13.1 TaxTi1ÿxC 6 96.4 ± 1.0 0.5 0.3 0.5 1.3 TaxTi1ÿxC 6 77.9 ± 0.2 0 0.3 0 21.6 TaxTi1ÿxC 6 78.8 ± 0.2 0 0 0 21.0 TiC 7 77.7 ± 1.0 ± ± ± 21.3 TiC 8 73.7 ± 0 ± ± ± 26.3 TiC 8 73.1 ± 0.4 ± ± ± 26.5 TiC 8 73.8 ± 1.0 ± ± ± 25.2 TiC 9 70.6 ± 1.0 ± ± ± 28.4 TiC 9 71.9 ± 1.0 ± ± ± 27.1 TiC 10 69.4 ± 0.6 ± ± ± 30.0 TiC 10 73.9 ± 2.6 ± ± ± 23.5 TiC 11 77.6 ± 1.0 ± ± ± 21.4 TiCxN1ÿx 12 ± 98.0 0.6 ± ± ± 1.4 TiCxN1ÿx 12 ± 97.4 0.2 ± ± ± 2.4 TiCxN1ÿx 12 ± 78.3 0.8 ± ± ± 20.9 TiCxN1ÿx 12 ± 85.8 0 ± ± ± 14.2 TiCxN1ÿx 12 ± 91.1 0.3 ± ± ± 8.6 TiCxN1ÿx 12 ± 97.2 1.1 ± ± ± 1.7 TiCxN1ÿx 13 ± 82.0 1.1 ± ± ± 16.9 TiCxN1ÿx 13 ± 84.6 1.2 ± ± ± 14.2 TiCxN1ÿx 13 ± 86.6 1.0 ± ± ± 12.4 TiCxN1ÿx 13 ± 97.9 2.0 ± ± ± 1.1 TiCxN1ÿx 14 ± 99.0 0.9 ± ± ± 0.1 TiCxN1ÿx 14 ± 81.5 0.6 ± ± ± 17.9 TiCxN1ÿx 14 ± 99.5 0.5 ± ± ± 0 TiCxN1ÿx 14 ± 84.2 0.2 ± ± ± 15.6 TiCxN1ÿx 14 ± 99.5 0.5 ± ± ± 0 TiCxN1ÿx 14 ± 93.8 0.7 ± ± ± 5.5 a Red phosphorus was added to the starting mixtures in order to enhance the possibility of ®nding droplets. Di€erent spot analysis on the same droplet may give a wide spread in the phosphorous content due to that it is present in small discrete Ni± or Fe±phosphide particles. N. AhleÂn et al. / Journal of the European Ceramic Society 20 (2000) 2607±2618 2613