Y. Yuan, J. Pan/Journal of Crystal Growth 193(1998)585-591 Table 1 The characteristics of the TiC whiskers(deposition time: 75 min) Deposition temperature (C) Diameter(um) Aspect ratio(L/D) Yield (% growth direction b 1125 005-1.5 100-1000 71.2 [ooJ[11] aSEM the tic whiskers were observed with a scanning electron microscope (SEM). The contents of the components and impurities of the TiC whisker were examined by the energy-dispersive spectro- scopic(EDS)analysis. The structure and growth direction of the TiC whiskers were determined by a transmission electron microscope (TEM) 3. Results and discussions Table 1 shows the characteristics of the typical iC whiskers prepared by the modified CVD method. Here the yield of whiskers is defined as the t3 um ratio of the titanium in the tic whiskers obtained to that in the reactant TiCl4. The typical morpho- Fig. 2. Typical scanning electron micrograph of TiC whiskers logy of TiC whiskers is shown in Fig. 2. It shows (1125.C. 75 min). that the TiC whiskers obtained in the experimental are smooth, straight. The whisker geometric factors such as shape, diameter, length and aspect ratio, are esirable whisker diameter of the tiC whiskers shows a max It is found that the flow rates of vapors have an imum at this flow rate(Fig. 3b). Although the tic important effect on the morphologies. In order to whiskers can be obtained in a wider flow rate range investigate the gas flow rate dependence, the depe of methane, no whisker deposits at the flow rate sition temperature is fixed at 1125.C When the gas below 20 ml min. The optimum flow rate o flow rate of hydrogen is below 165 ml minI or methane for whisker growth is about 85 ml min above 750 ml min, the Tic whiskers cannot be as shown in Fig 3c obtained and the TiC deposits are in the polyhedral Besides the vapor flow rate, the flow ratio, espe microcrystals and in the black polyhedral coatings, cially the ratio of the flow rate of TiCl. to that of respectively. The effect of hydrogen flow rate on the methane( C/Ti, plays a key role in determining the whisker diameter is shown in Fig 3a, and the whisker morphology of TiC whiskers. When the whisker diameter shows a maximum at the hydr C/Ti ratio is more than 2.0, the TiC whiskers spo- gen flow rate of 350 ml min-1. Compared to the radically scatter in the substrate. Wokulski et al. [6] effect of hydrogen flow rate on the growth of Tic explained that it was difficult for the Tic whisker to whiskers, it is also disadvantageous to the growth grow at a high C/Ti ratio due to sedimenting car- of the TiC whiskers if the flow rate of the TiCl, bon hindering its growth. But our experimental vapor is excessively high or low. The optimum flow result is in contrast to this conclusion. In a specially rate of the TiCl4 is about 50 ml min, and the designed experiment, we intentionally depositedTable 1 The characteristics of the TiC whiskers (deposition time: 75 min) Deposition temperature (°C) Diameter! (lm) Aspect ratio (L/D) Yield (%) Growth direction" 1125 0.05—1.5 100—1000 71.2 [1 0 0][1 1 1] !SEM. "TEM. Fig. 2. Typical scanning electron micrograph of TiC whiskers (1125°C, 75 min). the TiC whiskers were observed with a scanning electron microscope (SEM). The contents of the components and impurities of the TiC whisker were examined by the energy-dispersive spectro￾scopic (EDS) analysis. The structure and growth direction of the TiC whiskers were determined by a transmission electron microscope (TEM). 3. Results and discussions Table 1 shows the characteristics of the typical TiC whiskers prepared by the modified CVD method. Here the yield of whiskers is defined as the ratio of the titanium in the TiC whiskers obtained to that in the reactant TiCl4 . The typical morpho￾logy of TiC whiskers is shown in Fig. 2. It shows that the TiC whiskers obtained in the experimental are smooth, straight. The whisker geometric factors such as shape, diameter, length and aspect ratio, are desirable. It is found that the flow rates of vapors have an important effect on the morphologies. In order to investigate the gas flow rate dependence, the depo￾sition temperature is fixed at 1125°C. When the gas flow rate of hydrogen is below 165 ml min~1 or above 750 ml min~1, the TiC whiskers cannot be obtained and the TiC deposits are in the polyhedral microcrystals and in the black polyhedral coatings, respectively. The effect of hydrogen flow rate on the whisker diameter is shown in Fig. 3a, and the whisker diameter shows a maximum at the hydro￾gen flow rate of 350 ml min~1. Compared to the effect of hydrogen flow rate on the growth of TiC whiskers, it is also disadvantageous to the growth of the TiC whiskers if the flow rate of the TiCl4 vapor is excessively high or low. The optimum flow rate of the TiCl4 is about 50 ml min~1, and the whisker diameter of the TiC whiskers shows a max￾imum at this flow rate (Fig. 3b). Although the TiC whiskers can be obtained in a wider flow rate range of methane, no whisker deposits at the flow rate below 20 ml min~1. The optimum flow rate of methane for whisker growth is about 85 ml min~1, as shown in Fig. 3c. Besides the vapor flow rate, the flow ratio, espe￾cially the ratio of the flow rate of TiCl4 to that of methane (C/Ti), plays a key role in determining the whisker morphology of TiC whiskers. When the C/Ti ratio is more than 2.0, the TiC whiskers spo￾radically scatter in the substrate. Wokulski et al. [6] explained that it was difficult for the TiC whisker to grow at a high C/Ti ratio due to sedimenting car￾bon hindering its growth. But our experimental result is in contrast to this conclusion. In a specially designed experiment, we intentionally deposited Y. Yuan, J. Pan / Journal of Crystal Growth 193 (1998) 585–591 587