LATERIAL CHEMISTRYAND LSEVIER Materials Chemistry and Physics 56(1998)256-261 Factors determining the diameter of silicon carbide whiskers prepared by chemical vapor deposition Ing-Chi Leu", Yang-Ming Lu 1. Min-Hsiung Department of Materials Science and Engineering, National Cheng-Kung University, Tainan, Taiwan Abstract Since the whisker diameter is one of the important parameters for determining the characteristics of whisker-related systems, an understand ing of the factors that affect its size is of great value for whisker preparation. In this study, chemical vapor deposition(CVD)of silicon carbide( Sic)whiskers using a gas mixture of methyltrichlorosilane and hydrogen has been conducted in a hot-wall reactor on graphite plates coated with Ni as a liquid-forming agent, The deposited Sic whiskers are then characterized by scanning electron microscopy(SEM)to determine their nucleation and growth behavior. Experimental results show that the diameter of Sic whiskers is determined by both the vapor liquid-solid (VLS)mechanism and vapor-solid( VS)radial deposition, where the former is affected by the area of the solid-liquid interface from which the crystal precipitates and the latter by the thickening kinetics of vapor-deposited SiC on the lateral face. However, a comparison of the two factors indicates that an appropriate choice of the diameter of liquid droplets for VLS whisker growth is more effective than radial s deposition for obtaining whiskers of desired diame D 1998 Elsevier Science S.A. all rights reserved Keywords: Silicon carbides; Chemical vapor deposition: Whiskers 1. Introduction common method for the preparation of almost any kind of elemental and compound whisker with high quality and high The vapor-liquid-Solid (VLS) mechanism as one of the most commonly accepted mechanisms for whisker growth Because of their unique combinations of physical and during vapor-phase reaction was demonstrated by Wagner chemical properties, whiskers are expected to be useful in a and Ellis [1] in the 1960s. For its academic and practical variety of engineering fields. The application of silicon car- importance, various aspects of the VLS mechanismhave been bide(Sic) and other kinds of strong and lightweight whisk extensively studied ever since. The presence of a liquid layer ers as a second-phase reinforcement into metal and ceramic situated between the vapor and the growing crystal is what matrix composites is one of the most well-known examples distinguishes this mechanism from previous theories [2]. There also exist other kinds of applications which are less That is, with the appropriate addition or the accidental pres- established and still under development. As far as their ence of a liquid-forming agent, a liquid solution containing mechanical and functional applications are concerned, the the crystalline material to be grown and fed from the vapor characteristics of whiskers are of great importance in deter- through the liquid-vapor interface will be formed [3]. As mining the properties of whisker-related devices and articles he constituents for whiskers become supersaturated within For example, the diameter, strength, surface chemistry and the liquid solution, crystal growth proceeds by precipitation morphology of the reinforcing whiskers can definitely influ of crystalline materials from the solid-liquid interface. The ence the strengthening and toughening effects obtainable in liquid solution is consequently lifted upward at the same time reinforced composites [4]. In the case of studying the pho have the precipitation process operating continuously toluminescence properties of ultrathin GaAs whiskers grown With the advance of fundamental is by metal-organic vapor-phase epitaxy on GaAs substrate ndustrial practice, the VLS-based process is now the most Hiruma et al. [5] observed a shift of the luminescence peak energy to higher values with decreasing whisker diameter Corresponding anthor. Accordingly, the control of whisker characteristics is an Currently with the China College of Medical Technology, Tainan lmportant ste for optimizing the performance of whisker related applications. In general, the control of the character- 0254-0584/98/s- see front matter e 1998 Elsevier Science S.A. All rights reserved. PHS0254-0584(98)001898ELSEVIER Materials Chemistry and Physics 56 (1998) 256--261 MATERIALS CHEMISTRYAND PHYSICS Factors determining the diameter of silicon carbide whiskers prepared by chemical vapor deposition Ing-Chi Leu *, Yang-Ming Lu 1, Min-Hsiung Hon Department of Materials Science and Engineering, National Cheng-Kung University, Tainan, Taiwan Received 26 February I998; received in revised form 20 June 1998; accepted 22 June 1998 Abstract Since the whisker diameter is one of the important parameters for deternffning the characteristics of whisker-related systems, an understand￾ing of the factors that affect its size is of great value for whisker preparation. In this study, chemical vapor deposition (CVD) of silicon carbide (SIC) whiskers using a gas mixture of methyltrichlorosilane and hydrogen has been conducted in a hot-wall reactor on graphite plates coated with Ni as a liquid-forming agent. The deposited SiC whiskers are then characterized by scanning electron microscopy (SEM) to determine their nucleation and growth behavior. Experimental results show that the diameter of SiC whiskers is determined by both the vapor￾liquid-solid (VLS) mechanism and vapor-solid (VS) radial deposition, where the former is affected by the area of the solid-liquid interface from which the crystal precipitates and the latter by the thickening kinetics of vapor-deposited SiC on the lateral face. However, a comparison of the two factors indicates that an appropriate choice of the diameter of liquid droplets for VLS whisker growth is more effective than radial VS deposition for obtaining whiskers of desired diameters. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Silicon carbides; Chemical vapor deposition; Whiskers 1. Introduction The vapor-liquid-solid (VLS) mechanism as one of the most commonly accepted mechanisms for whisker growth during vapor-phase reaction was demonstrated by Wagner and Ellis [1] in the 1960s. For its academic and practical importance, various aspects of the VLS mechanism have been extensively studied ever since. The presence of a liquid layer situated between the vapor and the growing crystal is what distinguishes this mechanism from previous theories [2]. That is, with the appropriate addition or the accidental pres￾ence of a liquid-fornfing agent, a liquid solution containing the crystalline material to be grown and fed from the vapor through the liquid-vapor interface will be formed [3]. As the constituents for whiskers become supersaturated within the liquid solution, crystal growth proceeds by precipitation of crystalline materials from the solid-liquid interface. The liquid solution is consequently lifted upward at the same time to have the precipitation process operating continuously. With the advance of fundamental issues and the maturing of industrial practice, the VLS-based process is now the most * Corresponding author. L Currently with the China College of Medical Technology, Tainan, Taiwan. common method for the preparation of almost any kind of elemental and compound whisker with high quality and high efficiency. Because of their unique combinations of physical and chemical properties, whiskers are expected to be useful in a variety of engineering fields. The application of silicon car￾bide (SIC) and other kinds of strong and lightweight whisk￾ers as a second-phase reinforcement into metal and ceramic matrix composites is one of the most well-known examples. There also exist other kinds of applications which are less established and still under development. As far as their mechanical and functional applications are concerned, the characteristics of whiskers are of great importance in deter￾mining the properties of whisker-related devices and articles. For example, the diameter, strength, surface chemistry and morphology of the reinforcing whiskers can definitely influ￾ence the strengthening and toughening effects obtainable in reinforced composites [4]. In the case of studying the pho￾toluminescence properties of uttrathin GaAs whiskers grown by metal-organic vapor-phase epitaxy on GaAs substrates, Hiruma et al. [5] observed a shift of the luminescence peak energy to higher values with decreasing whisker diameter. Accordingly, the control of whisker characteristics is an important step for optimizing the performance of whisker￾related applications. In general, the control of the character- 0254-0584/98/$ - see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S0254-0584(98) 00 i89-8