JOURNALOF CRYST GROW ELSEVIER Journal of Crystal Growth 236(2002)171-175 www.elsevier.com/locate/jcrysgro Nucleation behavior of silicon carbide whiskers grown by chemical vapor deposition Ing-Chi Leu", Min-Hsiung Ho Department of Materials Science and Engineering, National Cheng Kung Unicersity, 1, Ta-Shueh Rd, Tainan 701, Ta Received 16 October 2001; accepted 28 November 2001 Communicated by T hibiya Silicon carbide (SiC) whiskers as a kind of high strength fibrous material are commonly used as an effective reinforcing element for composite materials. The preparation and characterization of such high aspect ratio material are of great importance to the understanding of fundamental properties and potential industrial applications of whisker materials. In this study the growth of Sic whiskers by chemical vapor deposition(CVD)from the thermal decomposition of methyltrichlorosilane was performed on Ni-coated graphite substrates in a hot wall reactor with an emphasis on the study of the fundamental nucleation and growth characteristics of the whiskers. Since vapor-liquid- solid mechanism is found to be responsible for the growth of whiskers, the formation of the Ni catalyst and its subsequent evolution on whisker nucleation and growth is a subject of extensive research. Scanning electron microscope is employed to characterize the nucleation and growth of SiC whiskers. It is found that the incubation period fc whisker nucleation and growth to noticeable dimension depends on the size of the Ni particles. It takes about 3 min the first whisker to appear and about 8 min to complete the whole nucleation stage for the case of Ni-coating thickness of about 2.5 um under the CVd parameters employed in the present study. Besides, due to the balance between the volume and the curvature effect of the liquid droplet of Ni catalyst formed on the graphite substrate during CVD process, the shortest incubation time for SiC whisker nucleation was found for droplets of 2 um in diameter, instead of ith larger or smaller dimensions. 2002 Elsevier Science B v. All rights reserved. PACS:68.70;81.10.a:52.75r;81.10.Bk:8L.15Gh;81.10Aj Keywords: Al. Growth models: Al. Nucleation; A3. Chemical vapor deposition processes 1. Introduction tional applications, whisker and other fibrous forms of elemental and compound semiconductors With the increasing interest in the fabrication and ceramics have received considerable attention and characterization of one-dimensional crystal- [1-3]. Silicon carbide(SiC) whiskers as a kind of line materials for structural and potentially func- high strength fibrous material are commonly used as an effective reinforcing element for composite Corresponding author. Tel :+886-6-2757575: fax:+886-6 materials. As a response to the request from the 2380208 high-performance metallic and ceramic compo- E-mail address. icleu mail ncku.edu. tw (I.-C. Leu) sites, SiC whiskers as a reinforcing material have 0022-0248/02/S-see front matter c 2002 Elsevier Science B V. All rights reserved. PI:S0022-0248(01)02274-6
Journal of Crystal Growth 236 (2002) 171–175 Nucleation behavior of silicon carbide whiskers grown by chemical vapor deposition Ing-Chi Leu*, Min-Hsiung Hon Department of Materials Science and Engineering, National Cheng Kung University, 1, Ta-Shueh Rd., Tainan 701, Taiwan Received16 October 2001; accepted28 November 2001 Communicatedby T. Hibiya Abstract Silicon carbide (SiC) whiskers as a kind of high strength fibrous material are commonly used as an effective reinforcing element for composite materials. The preparation andcharacterization of such high aspect ratio materials are of great importance to the understanding of fundamental properties and potential industrial applications of whisker materials. In this study the growth of SiC whiskers by chemical vapor deposition (CVD) from the thermal decomposition of methyltrichlorosilane was performed on Ni-coated graphite substrates in a hot wall reactor with an emphasis on the study of the fundamental nucleation and growth characteristics of the whiskers. Since vapor–liquid– solidmechanism is foundto be responsible for the growth of whiskers, the formation of the Ni catalyst andits subsequent evolution on whisker nucleation andgrowth is a subject of extensive research. Scanning electron microscope is employedto characterize the nucleation andgrowth of SiC whiskers. It is foundthat the incubation periodfor whisker nucleation and growth to noticeable dimension depends on the size of the Ni particles. It takes about 3 min for the first whisker to appear andabout 8 min to complete the whole nucleation stage for the case of Ni-coating thickness of about 2.5 mm under the CVD parameters employed in the present study. Besides, due to the balance between the volume and the curvature effect of the liquid droplet of Ni catalyst formed on the graphite substrate during CVD process, the shortest incubation time for SiC whisker nucleation was foundfor droplets of 2 mm in diameter, instead of those with larger or smaller dimensions. r 2002 Elsevier Science B.V. All rights reserved. PACS: 68.70; 81.10.a; 52.75.r; 81.10.Bk; 81.15.Gh; 81.10.Aj Keywords: A1. Growth models; A1. Nucleation; A3. Chemical vapor deposition processes 1. Introduction With the increasing interest in the fabrication andcharacterization of one-dimensional crystalline materials for structural andpotentially functional applications, whisker andother fibrous forms of elemental andcompoundsemiconductors andceramics have receivedconsiderable attention [1–3]. Silicon carbide (SiC) whiskers as a kind of high strength fibrous material are commonly used as an effective reinforcing element for composite materials. As a response to the request from the high-performance metallic andceramic composites, SiC whiskers as a reinforcing material have *Corresponding author. Tel.: +886-6-2757575; fax: +886-6- 2380208. E-mail address: icleu@mail.ncku.edu.tw (I.-C. Leu). 0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0022-0248(01)02274-6
L-C. Leu, M-H. Hon /Journal of Crystal Growth 236(2002)171-175 been the subject of extensive research and devel 2. Experimental procedure opment since the 1970s of last century. Many methods suitable for the preparation of SiC whis- The preparation of Sic whiskers was carried out kers have been developed, among which the carbo- using a gas mixture of methyltrichlorosilane thermal reduction of silica-containing materials (MTS)and purified H2, where H2 acted both as and the chemical vapor deposition(CvD)are con- a reducing agent and a carrier gas for MTS vapor sidered to be the most commonly employed ones. MTS vapor was obtained by bubbling H, through Moreover, since the discovery of carbon nano- an MTS saturator maintained at constant tem ube by Iijima [4 the exploitation of such perature (in this study 0C was employed) nanometer-scale materials for fundamental re- Isotropic graphite plates electroplated with ele- search and possible application have gained great mental Ni were used as substrates for the present attention. Following the development of carbon study. The thickness of electroplated Ni coating nanotubes, similar forms such as nanowhiskers, was measured to be 2.5 um. The substrates were nanofibers, nanorods, and nanowires are currently placed on a graphite susceptor located within a studied by a great number of researchers through- graphite tube, which was then inserted into the out the world. Though the dimension of nanofi- mullite tube of the CVD reactor. The deposition brous materials differs from that of the traditional was conducted in an externally heated hot wall whiskers by about one to two orders of magnitude CVD reactor with SiC heaters smaller, both these fibrous materials are generally The flow rates of H2 and MTS for the present prepared by the well-known vapor-liquid-solid study were fixed at 1360 and 3. 4 sccm(standard LS) mechanism cubic centimeter per minute), respectively. Total The essential features of the VLS mechanism gas pressure in the reaction chamber was main- can be expressed as the growth of whiskers via the tained at 100 Torr. A heating rate of 10.C/min to ingredient of the whiskers to be grown. The tion times of appropriate duration were close assistance of liquid solution containing the desired the presetting temperature of 1300.C and depos processes are complex and the fundamental issues The nucleation behavior of sic whiskers. after remain to be ascertained. In general, the prepara- appropriate duration of deposition time, was tion and characterization of such high aspect ratio characterized by measuring the nucleation density fibrous materials are of great importance to the using SEM, i.e., number of Sic whiskers nucleated understanding of fundamental properties and per unit substrate area. SEM was also employed potential industrial applications of whisker mate- for the study of the dependence of Ni droplet size rials of micrometer and nanometer dimensions on the nucleation characteristics of sic whiskers The growth of whisker involves the dissolution Especially, the relationship between the incubation of solute at the vapor/liquid interface and its time and the Ni droplet size, in terms of the time subsequent precipitation at the liquid /solid inter- required for the Sic whisker to nucleate from face during the VLs growth process. Although respective Ni droplets, was summarized many studies have been conducted for the past decades, the nucleation and initial stages of whisker growth behavior have never been exam- 3. Results and discussion ined. The understanding and the capability of control over the nucleation and growth of It is commonly accepted for the operation of the whiskers will be of great help to tailor the VLS mechanism [5], that the formation of catalyst properties of the whiskers and their subsequent droplet, dissolution of solute, mass transfer of processing steps. In this study the preparation of solute to the growing liquid /solid interface and the Sic whiskers through the addition of nickel as a precipitation of solute are essential factors for the catalyst for whisker growth by the VLS mechan- successful preparation and effective control of the ism will be performed, especially their nucleation VLs process for whisker growth. Since VLS behavior will be focused proceeds by the precipitation of solute from the
been the subject of extensive research anddevelopment since the 1970s of last century. Many methods suitable for the preparation of SiC whiskers have been developed, among which the carbothermal reduction of silica-containing materials andthe chemical vapor deposition (CVD) are considered to be the most commonly employed ones. Moreover, since the discovery of carbon nanotube by Iijima [4] the exploitation of such nanometer-scale materials for fundamental research andpossible application have gainedgreat attention. Following the development of carbon nanotubes, similar forms such as nanowhiskers, nanofibers, nanorods, and nanowires are currently studied by a great number of researchers throughout the world. Though the dimension of nanofi- brous materials differs from that of the traditional whiskers by about one to two orders of magnitude smaller, both these fibrous materials are generally preparedby the well-known vapor–liquid–solid (VLS) mechanism. The essential features of the VLS mechanism can be expressedas the growth of whiskers via the assistance of liquidsolution containing the desired ingredient of the whiskers to be grown. The processes are complex andthe fundamental issues remain to be ascertained. In general, the preparation andcharacterization of such high aspect ratio fibrous materials are of great importance to the understanding of fundamental properties and potential industrial applications of whisker materials of micrometer andnanometer dimensions. The growth of whisker involves the dissolution of solute at the vapor/liquidinterface andits subsequent precipitation at the liquid/solid interface during the VLS growth process. Although many studies have been conducted for the past decades, the nucleation and initial stages of whisker growth behavior have never been examined. The understanding and the capability of control over the nucleation andgrowth of whiskers will be of great help to tailor the properties of the whiskers andtheir subsequent processing steps. In this study the preparation of SiC whiskers through the addition of nickel as a catalyst for whisker growth by the VLS mechanism will be performed, especially their nucleation behavior will be focused. 2. Experimental procedure The preparation of SiC whiskers was carriedout using a gas mixture of methyltrichlorosilane (MTS) andpurifiedH2, where H2 actedboth as a reducing agent and a carrier gas for MTS vapor. MTS vapor was obtainedby bubbling H2 through an MTS saturator maintainedat constant temperature (in this study 01C was employed). Isotropic graphite plates electroplatedwith elemental Ni were usedas substrates for the present study. The thickness of electroplated Ni coating was measuredto be 2.5 mm. The substrates were placedon a graphite susceptor locatedwithin a graphite tube, which was then insertedinto the mullite tube of the CVD reactor. The deposition was conducted in an externally heated hot wall CVD reactor with SiC heaters. The flow rates of H2 andMTS for the present study were fixed at 1360 and 3.4 sccm (standard cubic centimeter per minute), respectively. Total gas pressure in the reaction chamber was maintainedat 100 Torr. A heating rate of 101C/min to the presetting temperature of 13001C anddeposition times of appropriate duration were chosen. The nucleation behavior of SiC whiskers, after appropriate duration of deposition time, was characterizedby measuring the nucleation density using SEM, i.e., number of SiC whiskers nucleated per unit substrate area. SEM was also employed for the study of the dependence of Ni droplet size on the nucleation characteristics of SiC whiskers. Especially, the relationship between the incubation time andthe Ni droplet size, in terms of the time requiredfor the SiC whisker to nucleate from respective Ni droplets, was summarized. 3. Results and discussion It is commonly acceptedfor the operation of the VLS mechanism [5], that the formation of catalyst droplet, dissolution of solute, mass transfer of solute to the growing liquid/solid interface and the precipitation of solute are essential factors for the successful preparation andeffective control of the VLS process for whisker growth. Since VLS proceeds by the precipitation of solute from the 172 I.-C. Leu, M.-H. Hon / Journal of Crystal Growth 236 (2002) 171–175
L-C. Leu, M-H. Hon /Journal of Crystal Growth 236(2002)171-175 supersaturated liquid solution, which is similar to solute precipitation, but this incubation perio a phase transformation involving nucleation and may change as a result of adapting different CVD then followed by growth, the degree of super- conditions. The incubation period, in fact, is saturation and the evolution of solution character determined by the time required by the catalyst istics of the liquid solution would influence the Ni droplet to reach a sufficient degree of super nucleation behavior. In addition to the metallur- saturation as a result of the arrival flux of gical properties of the solution itself, concentration components coming from the vapor-phase deposi- gaseous reactants and tion at the vapor/liquid interface. The MTS temperature may also have significant effects. concentration within the gaseous reactants and The incubation periods ranging widely from nearl ly the metallurgical properties of the catalyst char- zero to larger than I h have been reported in the acteristics are both important parameters for literature [6-8] as a result of the complexity of the determining the value of the incubation period VLs process In the next section, we will further study the The dependence of the nucleation density of Sic relationship between the individual droplet char whiskers nucleated from Ni catalyst on graphite acteristics and the incubation behavior for N substrate is illustrated in Fig. 1. It took about droplet with specific dimension 3 min for the sic whiskers to first appear under the As discussed before [9]. the catalyst Ni is first in CVd parameters mentioned above on the graphite the form of a continuous coating on the graphite substrate electroplated with 2.5 um Ni. The value substrate after electroplating, it undergoes an of nucleation density increases rapidly with agglomeration process during the heating process deposition time in the initial stage of deposition, to produce Ni particles or droplets suitable for then up to about 8 min, the nucleation density whisker nucleation and growth by the Vs reaches its saturation value of about 10m. The mechanism. The size of the particles thus produced above result is reasonable since it is necessary for is indeed within a specific range instead of being the droplet to take an appropriate incubation monodispersed due to the nature of the agglom period for solute absorption, mass transfer, and eration process. The incubation behavior of whisker nucleation has been studied to some extent; however, the relationship between whisk incubation period and the characteristics of catalyst ha the dependence of incubation time on the size of he catalyst droplet during CVD of SiC whiskers The experiments were conducted on Ni-coated graphite substrates for appropriate duration of deposition and then cooled to rod temperature to examine their nucleation behavior by SEM observation. In order to distinguish between the nucleation stage and the growth stage for incubation period determination, we define that a whisker with its length just more than that of its diameter as the one that finishes the nucleation stages. The data are obtained on graphite substrates for Cvd deposition from 3 to 8 min to facilitate whisker nucleation The action Time (min) incubation period of the whisker from the above Fig 1. The dependence of the nucleation density of Sic experiments was determined as a function of whiskers grown from Ni catalyst on graphite substrate as a droplet size. It can be seen that the value of function of reaction time incubation period varies with the droplet size and
supersaturatedliquidsolution, which is similar to a phase transformation involving nucleation and then followedby growth, the degree of supersaturation andthe evolution of solution characteristics of the liquidsolution wouldinfluence the nucleation behavior. In addition to the metallurgical properties of the solution itself, concentration of the gaseous reactants andthe deposition temperature may also have significant effects. The incubation periods ranging widely from nearly zero to larger than 1 h have been reportedin the literature [6–8] as a result of the complexity of the VLS process. The dependence of the nucleation density of SiC whiskers nucleatedfrom Ni catalyst on graphite substrate is illustratedin Fig. 1. It took about 3 min for the SiC whiskers to first appear under the CVD parameters mentionedabove on the graphite substrate electroplatedwith 2.5 mm Ni. The value of nucleation density increases rapidly with deposition time in the initial stage of deposition, then up to about 8 min, the nucleation density reaches its saturation value of about 109 m2 . The above result is reasonable since it is necessary for the droplet to take an appropriate incubation periodfor solute absorption, mass transfer, and solute precipitation, but this incubation period may change as a result of adapting different CVD conditions. The incubation period, in fact, is determined by the time required by the catalyst Ni droplet to reach a sufficient degree of supersaturation as a result of the arrival flux of components coming from the vapor-phase deposition at the vapor/liquidinterface. The MTS concentration within the gaseous reactants and the metallurgical properties of the catalyst characteristics are both important parameters for determining the value of the incubation period. In the next section, we will further study the relationship between the individual droplet characteristics andthe incubation behavior for Ni droplet with specific dimension. As discussed before [9], the catalyst Ni is first in the form of a continuous coating on the graphite substrate after electroplating, it undergoes an agglomeration process during the heating process to produce Ni particles or droplets suitable for whisker nucleation andgrowth by the VLS mechanism. The size of the particles thus produced is indeed within a specific range instead of being monodispersed due to the nature of the agglomeration process. The incubation behavior of whisker nucleation has been studied to some extent; however, the relationship between whisker incubation periodandthe characteristics of catalyst has never been constructed. Fig. 2 depicts the dependence of incubation time on the size of the catalyst droplet during CVD of SiC whiskers. The experiments were conducted on Ni-coated graphite substrates for appropriate duration of vapor deposition and then cooled to room temperature to examine their nucleation behavior by SEM observation. In order to distinguish between the nucleation stage andthe growth stage for incubation period determination, we define that a whisker with its length just more than that of its diameter as the one that finishes the nucleation stages. The data are obtained on graphite substrates for CVD deposition from 3 to 8 min to facilitate whisker nucleation. The incubation periodof the whisker from the above experiments was determined as a function of droplet size. It can be seen that the value of incubation periodvaries with the droplet size and Fig. 1. The dependence of the nucleation density of SiC whiskers grown from Ni catalyst on graphite substrate as a function of reaction time. I.-C. Leu, M.-H. Hon / Journal of Crystal Growth 236 (2002) 171–175 173
L-C. Leu, M-H. Hon /Journal of Crystal Growth 236(2002)171-175 G G1 Fig. 3. The effect of droplet size on the liquid/solid equilibrium omposition(X, vs. Xo)[10]. (The size of droplet designated by r is smaller than the one designated by oo.) Droplet Sie(um) Fig. 2. The dependence of incubation time on the size of the liquid droplet during CVD of Sic whiskers. the shortest incubation period of about 3 min was found for the whiskers with a droplet size of about 2 um. The values of incubation periods will ncrease for whiskers with either larger or smaller droplet size. The tendency of incubation behavior can be explained as follows, where the balance 000648 between the solute supersaturation and the droplet volume size effect is believed to be the determining Fig 4. SEM micrograph shows the nucleation result of droplet are expected to take more time to accumulate ion different dimensions in an intermediate stage of incuba- factors. It is intuitive that droplets with larger size solute from the vapor supply to reach the desired degree of supersaturation. On the other hand, due Gibbs-Thomson effect can lead to different to the presence of the Gibbs-Tompson effect (or equilibrium compositions, Xr and Xo, at the curvature effect)the droplets with smaller dimen- liquid /solid interface for droplets with different sions are expected to have higher equilibrium sizes, i.e., X, is larger than Xo in the present case composition at the solid/liquid interface. The In brief, the smaller the dimensions of the droplets, relation is schematically illustrated in Fig 3 [10 the higher the equilibrium compositions at the for the free energy curve of droplets with different liquid/ solid interface are. Consequently, the higher sizes. In Fig 3, the dimension for droplet r is the required degree of supersaturation will be smaller than that for droplet oo, and their free needed in the liquid droplet to facilitate whisker energies are designated as Gr and Go, respectively nucleation. The shortest incubation period of (the superscript B in Fig 3 represents the phase of about 3 min for droplets with 2 um in diameter is nterest). As a result of the presence of the Gibbbs- believed to be the balanced effect of these two Thomson effect, the value of Gr is expected to be competing parameters, i. e, the curvature effect higher than Go. From the common tangent and the volume effect. Fig 4 is a typical SEM construction in Fig 3 [10], the presence of the micrograph showing the nucleation
the shortest incubation periodof about 3 min was foundfor the whiskers with a droplet size of about 2 mm. The values of incubation periods will increase for whiskers with either larger or smaller droplet size. The tendency of incubation behavior can be explainedas follows, where the balance between the solute supersaturation andthe droplet volume size effect is believedto be the determining factors. It is intuitive that droplets with larger size are expectedto take more time to accumulate solute from the vapor supply to reach the desired degree of supersaturation. On the other hand, due to the presence of the Gibbs–Tompson effect (or curvature effect) the droplets with smaller dimensions are expectedto have higher equilibrium composition at the solid/liquid interface. The relation is schematically illustratedin Fig. 3 [10] for the free energy curve of droplets with different sizes. In Fig. 3, the dimension for droplet r is smaller than that for droplet N, andtheir free energies are designated as Gr and GN; respectively (the superscript b in Fig. 3 represents the phase of interest). As a result of the presence of the Gibbbs– Thomson effect, the value of Gr is expectedto be higher than GN: From the common tangent construction in Fig. 3 [10], the presence of the Gibbs–Thomson effect can leadto different equilibrium compositions, Xr and XN; at the liquid/solid interface for droplets with different sizes, i.e., Xr is larger than XN in the present case. In brief, the smaller the dimensions of the droplets, the higher the equilibrium compositions at the liquid/solid interface are. Consequently, the higher the requireddegree of supersaturation will be needed in the liquid droplet to facilitate whisker nucleation. The shortest incubation periodof about 3 min for droplets with 2 mm in diameter is believedto be the balancedeffect of these two competing parameters, i.e., the curvature effect andthe volume effect. Fig. 4 is a typical SEM micrograph showing the nucleation result of 0 1 2 3 4 Droplet Size (um ) 2 3 4 5 6 7 8 9 1 0 Incubation Period (min) Fig. 2. The dependence of incubation time on the size of the liquid droplet during CVD of SiC whiskers. Fig. 3. The effect of droplet size on the liquid/solid equilibrium composition (Xr vs. XN) [10]. (The size of droplet designated by r is smaller than the one designated by N:) Fig. 4. SEM micrograph shows the nucleation result of droplet with different dimensions in an intermediate stage of incubation. 174 I.-C. Leu, M.-H. Hon / Journal of Crystal Growth 236 (2002) 171–175
L-C. Leu, M-H. Hon /Journal of Crystal Growth 236(2002)171-175 droplet with different dimensions in an intermedi shortest incubation time for Sic whisker nuclea ate stage of incubation where the smaller whiskers tion was found for droplets of 2 um in diameter have already nucleated from their respective instead of those with larger or smaller dimensions droplets, while larger ones are still under the process of nucleation 4. Conclusion The financial support from the National Science The nucleation behavior of Sic whiskers pre- Council of the Republic of China, TAIWAN ared by the thermal decomposition of MTS on under contract No. NSC89-2216-E-006-072 is Ni-coated graphite substrates was studied. The greatly appreciated effect of Ni droplet size on the incubation period of Sic whiskers was determined. According to the above experimentation, results and discussion the ollowing conclusion can be drawn. It is found that References the incubation period for whiskers to nucleate to noticeable dimension depends on the size of the Ni [s. Motojima, M. Hasegawa, T. Hattori, J. Crystal Growth 87(1988)311 particles. It takes about 3 min for the whiskers to [2]K.Hiruma, et al., J. Appl. Phys. 77 (2)(1995)447 appear and about 8 min to complete the whole nucleation stage for the case of Ni coating Mimo orales, C M. Lieber, Science 279(1998)208 hickness of 2.5 um and the Cvd parameters [R.S. Wagner, w.C. Ellis, Appl. Phys. Lett. 4(5)(1964)85 employed. The nucleation density took about [6]K. Sugiyama, et al., J. Crystal Growth 44(1978)499 8 min to achieve a value of 10% P.D. Shalek, W.J. Parkinson. Mat. Res. Soc. Symp Proc remains at about this value through the whole [82 Wokulski, J. Crystal Growth 82(1987)427 period of CVD deposition. Besides, due to the 9I.C. Leu, Y M. Lu, M.H. Hon, J. Crystal Growth 167 balance between the volume and the curvature (1996)607 effect of the liquid droplet of Ni catalyst formed [10] D.A. Porter, K.E. Eastering, Phase Transformation in Metals and Alloys, Van Nostrand Reinhold Co., New on the graphite substrate during CVD process, the York,1981,p.45
droplet with different dimensions in an intermediate stage of incubation where the smaller whiskers have already nucleated from their respective droplets, while larger ones are still under the process of nucleation. 4. Conclusion The nucleation behavior of SiC whiskers preparedby the thermal decomposition of MTS on Ni-coated graphite substrates was studied. The effect of Ni droplet size on the incubation period of SiC whiskers was determined. According to the above experimentation, results anddiscussion the following conclusion can be drawn. It is found that the incubation periodfor whiskers to nucleate to noticeable dimension depends on the size of the Ni particles. It takes about 3 min for the whiskers to appear andabout 8 min to complete the whole nucleation stage for the case of Ni coating thickness of 2.5 mm andthe CVD parameters employed. The nucleation density took about 8 min to achieve a value of 109 m2 , andthen remains at about this value through the whole period of CVD deposition. Besides, due to the balance between the volume andthe curvature effect of the liquiddroplet of Ni catalyst formed on the graphite substrate during CVD process, the shortest incubation time for SiC whisker nucleation was foundfor droplets of 2 mm in diameter insteadof those with larger or smaller dimensions. Acknowledgements The financial support from the National Science Council of the Republic of China, TAIWAN under contract No. NSC89-2216-E-006-072 is greatly appreciated. References [1] S. Motojima, M. Hasegawa, T. Hattori, J. Crystal Growth 87 (1988) 311. [2] K. Hiruma, et al., J. Appl. Phys. 77 (2) (1995) 447. [3] A.M. Morales, C.M. Lieber, Science 279 (1998) 208. [4] S. Iijima, Nature 354 (1991) 56. [5] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (5) (1964) 89. [6] K. Sugiyama, et al., J. Crystal Growth 44 (1978) 499. [7] P.D. Shalek, W.J. Parkinson, Mat. Res. Soc. Symp. Proc. 168 (1990) 255. [8] Z. Wokulski, J. Crystal Growth 82 (1987) 427. [9] I.C. Leu, Y.M. Lu, M.H. Hon, J. Crystal Growth 167 (1996) 607. [10] D.A. Porter, K.E. Eastering, Phase Transformation in Metals andAlloys, Van NostrandReinholdCo., New York, 1981, p. 45. I.-C. Leu, M.-H. Hon / Journal of Crystal Growth 236 (2002) 171–175 175
