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 andsupersaturatedliquidsolution, which is similar to a phase transformation involving nucleation and then followedby growth, the degree of super￾saturation andthe evolution of solution character￾istics of the liquidsolution wouldinfluence the nucleation behavior. In addition to the metallur￾gical 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 m
2 . 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 super￾saturation as a result of the arrival flux of components coming from the vapor-phase deposi￾tion at the vapor/liquidinterface. The MTS concentration within the gaseous reactants and the metallurgical properties of the catalyst char￾acteristics 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 char￾acteristics 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 agglom￾eration 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