L.C. Let et al. /Materials Chemistry and Physics 56(1998)256-261 ence of crystalline defects on the substrates to be deposited 1250 1190 11361087 could effectively reduce the activation barrier required for nucleation and growth of crystalline deposits. Moreover, discussed by Kitamura et al. [19], the activation energy for the incorporation of growth units into the potential growth sites demonstrates an increasing order of kink, step, re-entrant 3 comer and surface. Fig. 5(a) shows a transmission electron Activation Energy 180KJ/mole pic image in which contrast bands due to stacking a 100p [20,21] are clearly observed. The presence of such packed stacking faults with random widths is believed to form microtwins in the order of a few lattice parameters thick [20]. These microtwins are expected to develop microfacets on the whisker surface. The points of deflection in microfaceted surface profile were found to coin- cide with planar defects intersecting the whisker surface 16 nd have been shown in lattice images by Nutt [16] and Wang et al. [22]. A schematic illustration of the zig-zag surface as a result of the intersection of defect planes with the whisker surface is depicted in Fig. 5(b). Closely distributed Reciprocal Temp. (x10-4 steps, kinks and re-entrant comers are thus believed to prevall Fig. 6. Arrhenius plot for the thickening kinetics of CVD SiC whiskers for on the surface of the whiskers these low -barrier sites for an MTS concentration of 0. 25% and pressure of 100 torr. deposition will effectively help the process of nucleation and growth of Sic on the lateral surfaces to proceed at the rather which is somewhat smaller than the values of about 200-400 ow reactant concentration employed in the present study as kJ/mol reported in the literature for CVD SiC coatings under the whiskers are nucleated from the liquid droplet by the VLs similar processing conditions [23 ] This discrepancy may be mechanism. The temperature dependence of the average attributed to the abovementioned kinetic assistance of the radial growth rate as measured for whiskers at the steady- microfaceted surface state growth stage is plotted in Fig. 6. An activation energy In summary, as whiskers are grown by the VLS mecha of about 180 kJ/mol is obtained from this Arrhenius plot, nism, the determining factors for whisker diameter should be Fig. 5.(a) TEM micrograph and (b) a schematic drawing of microfacets caused by stacking faults intersecting the whisker surface.260 1-C, Leu et aL /Materials Chemistry and Physics 56 (t998) 256-261 ence of crystalline defects on the substrates to be deposited could effectively reduce the activation barrier required for nucleation and growth of crystalline deposits. Moreover, as discussed by Kitamura et at. [ 19], the activation energy for the incorporation of growth units into the potential growth sites demonstrates an increasing order of kink, step, re-entrant corner and surface. Fig. 5 (a) shows a transmission electron microscopic image in which contrast bands due to stacking faults [20,21] are clearly observed. The presence of such densely packed stacking faults with random widths is believed to form microtwins in the order of a few lattice parameters thick [20]. These microtwins are expected to develop microfacets on the whisker surface. The points of deflection in microfaceted surface profile were found to coin￾cide with planar defects intersecting the whisker surface [ 16] and have been shown in lattice images by Nutt [16] and Wang et al. [22]. A schematic illustration of the zig-zag surface as a result of the intersection of defect planes with the whisker surface is depicted in Fig. 5 (b). Closely distributed steps, kinks and re-entrant comers are thus believed to prevail on the surface of the whiskers. These low-barrier sites for deposition will effectively help the process of nucleation and growth of SiC on the lateral surfaces to proceed at the rather low reactant concentration employed in the present study as the whiskers are nucleated from the liquid droplet by the VLS mechanism. The temperature dependence of the average radial growth rate as measured for whiskers at the steady￾state growth stage is plotted in Fig. 6. An activation energy of about 180 kJ/mol is obtained from this Arrhenius plot, E e¢ e- (-9 N "o e¢ 1316 looo I I I 10 1250 1190 1136 1087 1 , I , I , I 7.6 8,0 8.4 8.8 9.2 Reciprocal Temp. (xl 0 -4) Fig. 6. Arrhenius piot for the thickening kinetics of CVD SiC whiskers for an MTS concentration of 0.25% and pressure of 100 torr. which is somewhat smaller than the values of about 200-400 kJ/mol reported in the literature for CVD SiC coatings under similar processing conditions [23 ]. This discrepancy may be attributed to the abovementioned kinetic assistance of the microfaceted surface. In summary, as whiskers are gown by the VLS mecha￾nism, the determining factors for whisker diameter should be (a) (b) Fig. 5. (a) TEM micrograph and (b) a schematic drawing of microfacets caused by stacking faults intersecting the whisker surface. =