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 thebeen the subject of extensive research anddevel￾opment since the 1970s of last century. Many methods suitable for the preparation of SiC whis￾kers have been developed, among which the carbo￾thermal reduction of silica-containing materials andthe chemical vapor deposition (CVD) are con￾sidered to be the most commonly employed ones. Moreover, since the discovery of carbon nano￾tube by Iijima [4] the exploitation of such nanometer-scale materials for fundamental re￾search 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 through￾out 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 prepara￾tion andcharacterization of such high aspect ratio fibrous materials are of great importance to the understanding of fundamental properties and potential industrial applications of whisker mate￾rials of micrometer andnanometer dimensions. The growth of whisker involves the dissolution of solute at the vapor/liquidinterface andits subsequent precipitation at the liquid/solid inter￾face 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 exam￾ined. 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 mechan￾ism 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 tem￾perature (in this study 01C was employed). Isotropic graphite plates electroplatedwith ele￾mental 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 main￾tainedat 100 Torr. A heating rate of 101C/min to the presetting temperature of 13001C anddeposi￾tion 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