JOURNALOF CRYSTAL GROWTH ELSEVIER Journal of Crystal Growth 193(1998)585-591 The effect of vapor phase on the growth of TiC whiskers prepared by chemical vapor deposition Yongwen Yuan" D, * Jinsheng Pan Received 15 April 1998; accepted 15 May 1998 Abstract Quality TiC whiskers with high yield are prepared by a modified chemical vapor deposition(CVD) method. The The effect of vapor phase on the whisker morphology at fixed temperature s studied in detail. It is found that the morphology and size of Tic whiskers are significantly affected by both the flow rate of vapor phase and the C/Tiratio. The C/Tieffect is supposed to be related to the formation of Ni-Ti eutectic liquid phase according to the vapor-liquid-solid (vls)mechanism which is confirmed in this experiment. It is also found that the morphologies of Tic deposits are different at different locations of the substrate depending on the variation of the ontacting time and axial effective concentration of vapor phase along the vertical reaction tube. The high whisker yield is closely related to the movement of the vapor phase. A fast vapor fluid spouting upwards and diverging into the annulus makes the radial concentration of the vapor phase more homogeneous, and it also increases the collision of the vapor species on to the growing fronts and accelerates the mass transition of the Cvd process. c 1998 Elsevier Science B v. All rights reserved PACS:68.70.+w;81.15Gh;81.20.Lb Keywords: TiC whiskers; Chemical vapor deposition; VLS mechanism 1. Introduction is to toughen the brittle ceramic matrix. the re- quired properties of the whiskers for use in CMC Ceramic whiskers are now being used in the are their size and morphology, the compatibility composites for reinforcing and toughening the with matrix materials and the interfacial bonding metal matrix composites(MMC) and ceramic between matrix and whiskers. So far, the cerami matrix composites(CMC)[1, 2]. In ceramic matrix whiskers in use are mainly Sic and Si3N4. How- composites( CMC), the main effect of the whiskers ever, the newly developed Tic whisker has much higher strength, especially high-temperature strength. and corrosion resistance thermal and Corresponding author. Fax: +86 10 62771160 electrical properties, as well as better compatibility 0022-0248/98/-see front matter@ 1998 Elsevier Science B V. All rights reserved PI:S0022-0248(98)00484-9
Journal of Crystal Growth 193 (1998) 585—591 The effect of vapor phase on the growth of TiC whiskers prepared by chemical vapor deposition Yongwen Yuan!,",*, Jinsheng Pan!," ! Laboratory of Advanced Materials, Tsinghua University, Beijing 100084, People+s Republic of China " Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People+s Republic of China Received 15 April 1998; accepted 15 May 1998 Abstract Quality TiC whiskers with high yield are prepared by a modified chemical vapor deposition (CVD) method. The appropriate deposition parameters are found. The effect of vapor phase on the whisker morphology at fixed temperature is studied in detail. It is found that the morphology and size of TiC whiskers are significantly affected by both the flow rate of vapor phase and the C/Ti ratio. The C/Ti effect is supposed to be related to the formation of Ni—Ti eutectic liquid phase according to the vapor—liquid—solid (VLS) mechanism which is confirmed in this experiment. It is also found that the morphologies of TiC deposits are different at different locations of the substrate depending on the variation of the contacting time and axial effective concentration of vapor phase along the vertical reaction tube. The high whisker yield is closely related to the movement of the vapor phase. A fast vapor fluid spouting upwards and diverging into the annulus makes the radial concentration of the vapor phase more homogeneous, and it also increases the collision of the vapor species on to the growing fronts and accelerates the mass transition of the CVD process. ( 1998 Elsevier Science B.V. All rights reserved. PACS: 68.70.#w; 81.15.Gh; 81.20.Lb Keywords: TiC whiskers; Chemical vapor deposition; VLS mechanism 1. Introduction Ceramic whiskers are now being used in the composites for reinforcing and toughening the metal matrix composites (MMC) and ceramic matrix composites (CMC) [1,2]. In ceramic matrix composites (CMC), the main effect of the whiskers * Corresponding author. Fax: #86 10 62771160. is to toughen the brittle ceramic matrix. The required properties of the whiskers for use in CMC are their size and morphology, the compatibility with matrix materials and the interfacial bonding between matrix and whiskers. So far, the ceramic whiskers in use are mainly SiC and Si3 N4 . However, the newly developed TiC whisker has much higher strength, especially high-temperature strength, and corrosion resistance, thermal and electrical properties, as well as better compatibility 0022-0248/98/$ — see front matter ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 4 8 4 - 9
Y. Yuan, J. Pan/Journal of Crystal Growth 193(1998)585-591 with alumina matrix [3-5]. Therefore, in It describes this modified CVD method with empha years, many experiments have been made on the sis on the effect of vapor phase on the growth of preparation and properties of the TiC whiskers TiC whiskers [6-8] iC whiskers are usually prepared by the CVD method in a horizontal reaction tube in which the 2. Experimental procedure iCl-CH4-H2-and / or Ar)gas mixture was used as reactant and pure nickel as substrate. In the The schematic diagram of the experimental ap- early 1990s, our group began to prepare TiC paratus is shown in Fig. 1. It is composed of two whiskers and develop their applications. As the parts, a gas-flow system and a reactor system. In conventional CVD method, an apparatus made of stead of the horizontal reaction tube used in our a horizontal alumina reaction tube was used to previous works [9, 10], the modified CVD method prepare TiC whiskers in our previous work [9, 10]. uses a vertical graphite tube of 30 mm in inner The TiC whiskers prepared by the CVD method diameter heated by a nickel-chromium alloy coil usually have high purity and fewer defects resulting The inner diameter of the gas inlet is reduced great to better mechanical properties as well as desirable ly, so that the flow rate increases significantly. The geometric characteristics such as shape, size, aspect TiCl4-CH4-H2-Ar gas mixture is used as the reac- ratio, surface smoothness which are closely related tant and a pure hollow nickel cylinder as the sub- drawback associated with the conventional CVD is inner reaction tube wall the gas inlet around the to the toughening of CMCs. However, a serious strate which is placed near the gas inlet around the the low yield and, consequently, the high cost of The temperature is measured with a Pt/Pt-Ph whiskers. To overcome this drawback, a modified thermocouple. The temperature of the TiCla evap- CVD method is presented, by which quality Tic orator is ma aintained at 60C. The gas flow rate whiskers with high yield are obtained. This pap measured with the flow meter. The morphologies of alumina tube nickel subtrate- Fig. 1. Schematic diagram of the whisker deposition apparatus
Fig. 1. Schematic diagram of the whisker deposition apparatus. with alumina matrix [3—5]. Therefore, in recent years, many experiments have been made on the preparation and properties of the TiC whiskers [6—8] TiC whiskers are usually prepared by the CVD method in a horizontal reaction tube in which the TiCl4 —CH4 —H2 —(and/or Ar) gas mixture was used as reactant and pure nickel as substrate. In the early 1990s, our group began to prepare TiC whiskers and develop their applications. As the conventional CVD method, an apparatus made of a horizontal alumina reaction tube was used to prepare TiC whiskers in our previous work [9,10]. The TiC whiskers prepared by the CVD method usually have high purity and fewer defects resulting to better mechanical properties as well as desirable geometric characteristics such as shape, size, aspect ratio, surface smoothness which are closely related to the toughening of CMCs. However, a serious drawback associated with the conventional CVD is the low yield and, consequently, the high cost of whiskers. To overcome this drawback, a modified CVD method is presented, by which quality TiC whiskers with high yield are obtained. This paper describes this modified CVD method with emphasis on the effect of vapor phase on the growth of TiC whiskers. 2. Experimental procedure The schematic diagram of the experimental apparatus is shown in Fig. 1. It is composed of two parts, a gas-flow system and a reactor system. Instead of the horizontal reaction tube used in our previous works [9,10], the modified CVD method uses a vertical graphite tube of 30 mm in inner diameter heated by a nickel—chromium alloy coil. The inner diameter of the gas inlet is reduced greatly, so that the flow rate increases significantly. The TiCl4 —CH4 —H2 —Ar gas mixture is used as the reactant and a pure hollow nickel cylinder as the substrate which is placed near the gas inlet around the inner reaction tube wall. The temperature is measured with a Pt/Pt—Ph thermocouple. The temperature of the TiCl4 evaporator is maintained at 60°C. The gas flow rate is measured with the flow meter. The morphologies of 586 Y. Yuan, J. Pan / Journal of Crystal Growth 193 (1998) 585–591
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 deposited
Table 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 spectroscopic (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 morphology 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 deposition 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 hydrogen 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 maximum 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, especially 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 sporadically 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 carbon 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
Y. Yuan, J. Pan/Journal of Crystal Growth 193(1998)585-591 Flow of hydrogen(mWm Fig 4. Growth of hedgehog-like whiskers in the carbon rich regons. Flow rate of titanium tetrachloride (m Um in) Flow rate of methane( mWmin) Fig 3. The effects of vapor flow rates on the diameters of the TiC whiskers obtained at 1125C, 75 min. (a)The effects of hydrogen flow rate on the diameters, TiCl4 flow 50 ml min-I, methane flow rate=85 ml min-I.(b)The 500nm effects of TiCl+ flow rate on the diameters, hydrogen flow rate= 350 ml min-1. methane flow 85 ml min low ha e 5 o m mia ne Tic fe w te d 50 me m:in rogen the i scmacchnag ismtron micrograph of Tic whiskers grown by some carbon on nickel substrate prior to the forma- of the TiC whiskers have spherical liquid droplets tion reaction of the Tic whisker, and found that the the inherent characteristic of the vls mechanism TiC whiskers with a high yield grew hedgehog-like on their tips(see Fig. 5). The EDS analysis shows in the carbon rich regions, as shown in Fig 4. The that apart from titanium, nickel is also found in TiC whiskers grown by the VLs mechanism are these spherical tips, but not in whisker roots. Ac- experimentally verified in this experiment as most cording to the vLs mechanism, a binary Ni-Ti
Fig. 3. The effects of vapor flow rates on the diameters of the TiC whiskers obtained at 1125°C, 75 min. (a) The effects of hydrogen flow rate on the diameters, TiCl4 flow rate"50 ml min~1, methane flow rate"85 ml min~1. (b) The effects of TiCl4 flow rate on the diameters, hydrogen flow rate"350 ml min~1, methane flow rate"85 ml min~1. (c) The effects of methane flow rate on the diameters, hydrogen flow rate"350 ml min~1, TiCl4 flow rate"50 ml min~1. Fig. 4. Growth of hedgehog-like whiskers in the carbon rich regions. Fig. 5. Scanning electron micrograph of TiC whiskers grown by the VLS mechanism. some carbon on nickel substrate prior to the formation reaction of the TiC whisker, and found that the TiC whiskers with a high yield grew hedgehog-like in the carbon rich regions, as shown in Fig. 4. The TiC whiskers grown by the VLS mechanism are experimentally verified in this experiment as most of the TiC whiskers have spherical liquid droplets — the inherent characteristic of the VLS mechanism — on their tips (see Fig. 5). The EDS analysis shows that apart from titanium, nickel is also found in these spherical tips, but not in whisker roots. According to the VLS mechanism, a binary Ni—Ti 588 Y. Yuan, J. Pan / Journal of Crystal Growth 193 (1998) 585–591
Y. Yuan, J. Pan/Journal of Crystal Growth 193(1998)585-591 eutectic liquid phase will be formed to initiate the As shown in Fig. 6, the morphologies of the TiC growth of TiC whiskers [10]. Therefore, a little deposits are often different in different locations of amount of whiskers deposited at the high C/Ti may the nickel substrate. According to the morphology be due to the deficiency of titanium, resulting from variation of whiskers deposited on nickel, from the reduction of TiCl4 with H,, rather than excess- bottom to top the inner surface of the nickel sub- ive carbon to form this Ni-Ti eutectic liquid phase. strate cylinder is divided into three sections, I, Il, When the C/Ti ratio is approximate to or less than Ill, respectively. Three distinctive morphologies are 1, TiC whiskers will almost not form due to insuffi- observed: (1)Type A: Whiskers from location I of cient amount of carbon. Instead, titanium will react the substrate are generally thick(0. 1-1.0 um), long with the nickel substrate to form microcrystals of (0.05-2.0 mm), straight and smooth, as shown in Ni-Ti binary eutectic alloy with low melting point. Fig 6a. The quality TiC whiskers with a high yield The optimum value of the C/Ti is about 1.7 in this are obtained in this location.(2)Type B: Whiskers experiment from location II are thin (0.02-0.5 um), short m 2μm ig. 6. Three types of TiC whiskers obtained at different locations of the nickel substrate: (a)Type A, (b) Type B, (c) Type C
Fig. 6. Three types of TiC whiskers obtained at different locations of the nickel substrate: (a) Type A, (b) Type B, (c) Type C. eutectic liquid phase will be formed to initiate the growth of TiC whiskers [10]. Therefore, a little amount of whiskers deposited at the high C/Ti may be due to the deficiency of titanium, resulting from the reduction of TiCl4 with H2 , rather than excessive carbon to form this Ni—Ti eutectic liquid phase. When the C/Ti ratio is approximate to or less than 1, TiC whiskers will almost not form due to insuffi- cient amount of carbon. Instead, titanium will react with the nickel substrate to form microcrystals of Ni—Ti binary eutectic alloy with low melting point. The optimum value of the C/Ti is about 1.7 in this experiment. As shown in Fig. 6, the morphologies of the TiC deposits are often different in different locations of the nickel substrate. According to the morphology variation of whiskers deposited on nickel, from bottom to top, the inner surface of the nickel substrate cylinder is divided into three sections, I, II, III, respectively. Three distinctive morphologies are observed: (1) Type A: Whiskers from location I of the substrate are generally thick (0.1—1.0 lm), long (0.05—2.0 mm), straight and smooth, as shown in Fig. 6a. The quality TiC whiskers with a high yield are obtained in this location. (2) Type B: Whiskers from location II are thin (0.02—0.5 lm), short Y. Yuan, J. Pan / Journal of Crystal Growth 193 (1998) 585–591 589
590 Y. Yuan, J. Pan/Journal of Crystal Growth 193(1998)585-591 (5-500 um), as shown in Fig. 6b. (3) Type C the whisker growth on the upper side of the nickel Whiskers from location III have similar size as substrate cylinder. However, in a vertical reactor shown in Fig. 6c. The difference of whisker mor- blows vertically upwards, a spout form resull o s Type B, but they grew as a dot-distribution, as along with the greatly reduced gas inlet, a fluid phology at different locations may be due to the produce a fast fluid along the annulus where the difference of contacting time and axial effective formation reaction of TiC whiskers occurs, the re linder.According to the ng the nickel substrate cy- action gas velocity in the vertical tube increases vapor concentration al gas movement with jet inlet This not only eliminates the radial laminar, and gas [11-13], which is similar to the movement of makes the radial concentration distribution of va vapor phase in the present experiment, from the por phase more homogeneous, but also accelerates bottom of the annulus in the vertical reaction tube the CVd process of mass transition limit On the to its top, the velocity of the vapor phase increases, other hand, apart from the fast gas blowing verti and the residence time of the vapor phase decreases. cally upwards, part of the vapor phases spouting Therefore, the effective vapor concentration and from the gas inlet diverges into the annulus. Com- the time of flowing vapor phase contacting the pared to the vapors directly flowing through the growing whisker fronts decrease from the inlet to substrate in the horizontal reaction tube, a higher the top. In location L, the effective vapor concentra- percentage of the vapor phase is taken to form the tion and the contacting time is the biggest, so the deposits in the vertical reaction tube due to the whiskers grow thick and long In location Il, the divergence of the vapor jet. This increases the col vapor concentration of the reactant is enough to lision of the vapor species on to the growing fronts form Ni-Ti binary eutectic liquid droplets as nu- and accelerates the mass transition of the CVD cleaton sites of TiC whiskers, while the flowing process. Therefore, the yield of the TiC whisker vapor phase cannot completely contact the whisker prepared by the modified CVD method increases tips. Thus in this location, the whiskers are thin and significantly short(Fig. 6b). In location Ill, the contacting time and the effective concentration of vapor phase are the least, the whiskers grow as a thin, short, dot- 4. Conclusions distribution due to incomplete growth and few nu- cleaton sites Quality TiC whiskers with a high yield are pre- From Table 1, it shows that the whisker yield in pared by a modified CVD method. The major fac the modified CVD method increases several times tors affecting the whisker morphology and size are than that in our previous experiment [10]. The the flow rate of the vapor phase and the CTiratio macrographs of whiskers clearly show a much The morphology and size of TiC whiskers are affec thicker TiC whisker layer to be growing on the ted by the flow rate of the vapor phase and the cti nickel substrate by the modified CVD method, ratio, Our experiment has confirmed that the effect while the morphologies of TiC whiskers prepared of the C/Ti on the whisker morphology is related to by the conventional method can only be observed the formation of Ni-Ti eutectic liquid phase with the aid of the microscope. The high yield and, cording to the VLS mechanism. Different whisker consequently, low cost of the whiskers prepared by morphologies are found at different locations of the this new method may be related to the movement of nickel substrate, this may be ascribed to the vari- the vapor phase in the vertical reaction tube, which ation of the contacting time and axial effective is different from that in the horizontal reactor. concentration of vapor phase along the vertical When the reactant vapor phases flow through reaction tube. The high yield of whiskers is closely a horizontal reaction tube, a laminar flow usually related to the movement of the vapor phase. A fast exists due to the density difference of different va- gas fluid spouting upwards accelerates the mass por components. This makes the radial reactant transition of the CVd process, and makes the concentration distribution inhomogeneous. It is radial concentration of the vapor phase more ho- difficult to obtain the methane and TiCl4 vapor for mogeneous. Furthermore, part of the vapor phases
(5—500 lm), as shown in Fig. 6b. (3) Type C: Whiskers from location III have similar size as Type B, but they grew as a dot-distribution, as shown in Fig. 6c. The difference of whisker morphology at different locations may be due to the difference of contacting time and axial effective vapor concentration along the nickel substrate cylinder. According to the gas movement with jet inlet gas [11—13], which is similar to the movement of vapor phase in the present experiment, from the bottom of the annulus in the vertical reaction tube to its top, the velocity of the vapor phase increases, and the residence time of the vapor phase decreases. Therefore, the effective vapor concentration and the time of flowing vapor phase contacting the growing whisker fronts decrease from the inlet to the top. In location I, the effective vapor concentration and the contacting time is the biggest, so the whiskers grow thick and long. In location II, the vapor concentration of the reactant is enough to form Ni—Ti binary eutectic liquid droplets as nucleation sites of TiC whiskers, while the flowing vapor phase cannot completely contact the whisker tips. Thus in this location, the whiskers are thin and short (Fig. 6b). In location III, the contacting time and the effective concentration of vapor phase are the least, the whiskers grow as a thin, short, dotdistribution due to incomplete growth and few nucleation sites. From Table 1, it shows that the whisker yield in the modified CVD method increases several times than that in our previous experiment [10]. The macrographs of whiskers clearly show a much thicker TiC whisker layer to be growing on the nickel substrate by the modified CVD method, while the morphologies of TiC whiskers prepared by the conventional method can only be observed with the aid of the microscope. The high yield and, consequently, low cost of the whiskers prepared by this new method may be related to the movement of the vapor phase in the vertical reaction tube, which is different from that in the horizontal reactor. When the reactant vapor phases flow through a horizontal reaction tube, a laminar flow usually exists due to the density difference of different vapor components. This makes the radial reactant concentration distribution inhomogeneous. It is difficult to obtain the methane and TiCl4 vapor for the whisker growth on the upper side of the nickel substrate cylinder. However, in a vertical reactor along with the greatly reduced gas inlet, a fluid jet blows vertically upwards, a spout form results to produce a fast fluid along the annulus where the formation reaction of TiC whiskers occurs, the reaction gas velocity in the vertical tube increases. This not only eliminates the radial laminar, and makes the radial concentration distribution of vapor phase more homogeneous, but also accelerates the CVD process of mass transition limit. On the other hand, apart from the fast gas blowing vertically upwards, part of the vapor phases spouting from the gas inlet diverges into the annulus. Compared to the vapors directly flowing through the substrate in the horizontal reaction tube, a higher percentage of the vapor phase is taken to form the deposits in the vertical reaction tube due to the divergence of the vapor jet. This increases the collision of the vapor species on to the growing fronts and accelerates the mass transition of the CVD process. Therefore, the yield of the TiC whiskers prepared by the modified CVD method increases significantly. 4. Conclusions Quality TiC whiskers with a high yield are prepared by a modified CVD method. The major factors affecting the whisker morphology and size are the flow rate of the vapor phase and the C/Ti ratio. The morphology and size of TiC whiskers are affected by the flow rate of the vapor phase and the C/Ti ratio, Our experiment has confirmed that the effect of the C/Ti on the whisker morphology is related to the formation of Ni—Ti eutectic liquid phase according to the VLS mechanism. Different whisker morphologies are found at different locations of the nickel substrate, this may be ascribed to the variation of the contacting time and axial effective concentration of vapor phase along the vertical reaction tube. The high yield of whiskers is closely related to the movement of the vapor phase. A fast gas fluid spouting upwards accelerates the mass transition of the CVD process, and makes the radial concentration of the vapor phase more homogeneous. Furthermore, part of the vapor phases 590 Y. Yuan, J. Pan / Journal of Crystal Growth 193 (1998) 585–591
Y. Yuan, J. Pan/Journal of Crystal Growth 193(1998)585-591 spouting from the gas inlet diverges into the an- [4]T Takahashi,KSugiyama, H Itoh,JElectrochem. Soc ulus, a bigger percentage of the vapor phase is 11701970)541. taken to form the Tic whiskers. This increases the [5] D.T. Morelli, Phys. Rev. B 44(1991)5453. collision of the vapor species on to the growing [6Z. Wokulski, K. Wokulska, J Crystal Growth 62(1983) fronts and also accelerates the mass transition of [7 N. Tamari, A Kato, J. Crystal Growth 46(1979)221. the Cvd process [8Z. Wokulski, J. Crystal Growth 82(1987)427. [9]JS. Pan, Y.H. Chen, Acta Mater. Composite Sin. 12 (1995)1. References [10JY H. Chen, J.S. Pan, X.Y. Huang, J. Crystal Growth 172 (197)171 para, Y Nishida, M. Yamada, I Shirayanagi, T. [11] CJ. Lim, K B Mathur, A I.Ch.E.J. 22(1976)674. later. Sci. Lett. 6( 1987)1313 [12] H. Littman, M.H. Morgan Ill, P.V. Narayanan, SJ. K P F. Becher, Am. Ceram Soc. Bull. 64(1985) Can. J. Chem. Eng. 63(1985)188. [13]JY. Day, H. Littman, M.H. Morgan Ill, Z.B. Grbavcic, B3L.E. Toth, Transition Metal Carbides and Nitrides, Aca D E. Hadzismajlovic, D.V. Vukovic, Chem. Eng. Sci. 46 demic Press. New York, 1971 (1991)773
spouting from the gas inlet diverges into the annulus, a bigger percentage of the vapor phase is taken to form the TiC whiskers. This increases the collision of the vapor species on to the growing fronts and also accelerates the mass transition of the CVD process. References [1] H. Matsubara, Y. Nishida, M. Yamada, I. Shirayanagi, T. Imai, J. Mater. Sci. Lett. 6 (1987) 1313. [2] G.C. Wei, P.F. Becher, Am. Ceram. Soc. Bull. 64 (1985) 298. [3] L.E. Toth, Transition Metal Carbides and Nitrides, Academic Press, New York, 1971. [4] T. Takahashi, K. Sugiyama, H. Itoh, J. Electrochem. Soc. 117 (1970) 541. [5] D.T. Morelli, Phys. Rev. B 44 (1991) 5453. [6] Z. Wokulski, K. Wokulska, J. Crystal Growth 62 (1983) 439. [7] N. Tamari, A. Kato, J. Crystal Growth 46 (1979) 221. [8] Z. Wokulski, J. Crystal Growth 82 (1987) 427. [9] J.S. Pan, Y.H. Chen, Acta Mater. Composite Sin. 12 (1995) 1. [10] Y.H. Chen, J.S. Pan, X.Y. Huang, J. Crystal Growth 172 (1997) 171. [11] C.J. Lim, K.B. Mathur, A.I.Ch.E. J. 22 (1976) 674. [12] H. Littman, M.H. Morgan III, P.V. Narayanan, S.J. Kim, Can. J. Chem. Eng. 63 (1985) 188. [13] J.Y. Day, H. Littman, M.H. Morgan III, Z.B. Grbavcic, D.E. Hadzismajlovic, D.V. Vukovic, Chem. Eng. Sci. 46 (1991) 773. Y. Yuan, J. Pan / Journal of Crystal Growth 193 (1998) 585–591 591
