I.-C. Leu et al. Materials Chemistry and Physics 56(1998)256-261 the size of the solid-liquid interface where crystallization growth rate of Sic whiskers leads to an apparent activation ccurs as well as the thickening rate in the radial direction of energy of about 180 kJ/mol. However, a comparison of the the whisker during vapor-phase deposition. The size of the two factors using the parameters employed in the present solid-liquid interface is determined by the volume of the study indicates that an appropriate choice of the volume of liquid droplet and the equilit liquid droplets during VlS whisker growth is more effective tensions among the phases involved. Proper selection of the than radial vs deposition for obtaining whiskers of desired type and volume of liquid droplets for a specific whisker- diameters substrate system together with a careful choice of processing parameters can facilitate the tailoring of wetting characteris ics of liquid droplets. The controllability of the diameter of Acknowledgements whiskers grown by the VLS-based process will in turn be chanced. Nevertheless the effect of the substrate on the The financial support of this study by the National Science interfacial energy balance can be negligible as the liquid Council of the Republic of China under grant contractnumber droplets are lifted from the substrate [24]. A constant whisker NSC84-2216-E006-012 is greatly appre diameter is obtained at the steady-state growth stage provided nat the interfacial energy balance has not been disturbed during processing. The above reasoning can simplify further References discussion and a certain dimensional relationship between [1]RS. Wagner, w.C. Ellis, Appl Let.4(1964)89 the liquid droplets and the whiskers would exist for a specific [2]RS Wagner, W.C. Ellis, Trans E233(1965)1053 growth stage, i.e, the steady-state growth stage, as was com- [3] J.V. Milewski, F D. Gac, J.J. Petrovic, S.R. Skaggs, J. Mater. Sci. 20 monly reported by others. In the present study, the difference (1985)1160 in the effectiveness of the two factors in determining the [4] P F. Becher, T N. Tiegs, P.A. Angelini, in K.S. Mazdiyasni(Ed whisker diameter is significant. The diameter of Sic whiskers Fiber reinforced Ceramic Composites- Materials, Processing and Technology, ch Il, Noyes Publications, Park Ridge, NJ, USA, 1990 grown by the addition of a 2.5 um thick Ni coating as the [5] K. Hiruma, M. Yaeawa, K. Haraguchi, K. Ogawa, J. Appl. Phys. 74 liquid-forming agent is about 2-3 um due to the disintegrated metal particles being about twice as large, whereas an average 6]R De Jong, R.A. McCauley, J Am Ceram Soc. 70(1987)C radial growth rate of Sic whiskers of about 0. 2 um/h is [7] G F. Hurley, J.J. Petrovic, Advanced Composites Conf. Proc Materials Park, OH, USA, 1985, p. 20 obtained under the conditions mentioned above except for a J P.D. Shalek, W.J. Parkinson, Mater. Res, Soc. Symp Proc., vol. 168, deposition temperature of 1300C instead. Consequently, the Mater Res. Soc., Pittsburgh, PA, USA, 1990, p. 255 appropriate choice of the size of activating metal particles fo [9] M. Yazawa, M. Koguchi, A. Muto, M. Ozawa, K. Hiruma, Appl whisker growth, if no further breakup of the liquid droplet Phys.Let.61(1992)2051 takes place during deposition, is more effective than the thick [10] J. Westwater, D. Pal Gosain, S. Usui, Jpn J. Appl. Phys. 36(1997) ening deposition in the radial direction in controlling the [11] DJ.Cheng, WJ. Shyy, D HKuo, MHHon,JElectrochem. Soc diameter of Sic whiskers (1987)3145 [13] G.A. Bootsma, H.J. Gassen, J. Crystal Growth 10(1971)223 [14] G.A. Bootsma, W.F. Knippenberg, G. Verspui, J Crystal Growth 11 4. Conclusions M H. Hon, Y M. Lu, to be published he factors that control the diameter of sic whiskers are 16]SR Nutt, J Am Ceram Soc. 71(1988)149. summarized as the result of a combined operation of the VlS 17] E.I. Givargizov, J. Crystal Growth 20(1973)217. mechanism and VS radial deposition. The former is deter [18] JANAF Thermochemical Tables, 2nd edn., NatL. Stand. Ref, Data Ser 37.1971 mined by the area of the solid-liquid interface and the latter [19] M Kitamura, S Hosoya, I Sunagawa, J, Crystal Growth 47(1979 by the thickening kinetics of vapor-deposited SiC on the face. The solid-liquid contact area is determined by [20] L.I. Van Torne, J Appl. Phys. 37(1966)1849 he volume of the liquid droplets and the equilibrium condi [21] J.J. Comer, Mater Res. Bull. 4(1969)279. [22] L. Wang, H Wada, L.F. Allard, J Mater, Res. 7(1992)148. tion at the vapor-liquid-solid triple-phase junction. An [231 J Schlichting, Powder Metallurgy Intemational 12(1980)141 Arrhenius plot of the temperature dependence of radial 24]Z Wokulski, J, Crystal Growth 82(1987)427.L-C. Leu et al./ Materials Chemistry and Physics 56 (1998) 256-261 261 the size of the solid-liquid interface where crystallization occurs as well as the thickening rate in the radial direction of the whisker during vapor-phase deposition. The size of the solid-liquid interface is determined by the volume of the liquid droplet and the equilibrium condition of interfacial tensions among the phases involved. Proper selection of the type and volume of liquid droplets for a specific whisker￾substrate system together with a careful choice of processing parameters can facilitate the tailoring of wetting characteris￾tics of liquid droplets. The controllability of the diameter of whiskers grown by the VLS-based process will in turn be enhanced. Nevertheless, the effect of the substrate on the interfacial energy balance can be negligible as the liquid droplets are lifted from the substrate [ 24 ]. A constant whisker diameter is obtained at the steady-state growth stage provided that the interfacial energy balance has not been disturbed during processing. The above reasoning can simplify further discussion and a certain dimensional relationship between the liquid droplets and the whiskers would exist for a specific growth stage, i.e., the steady-state growth stage, as was com￾monly reported by others. In the present study, the difference in the effectiveness of the two factors in determining the whisker diameter is significant. The diameter of SiC whiskers grown by the addition of a 2.5 b~m thick Ni coating as the liquid-forming agent is about 2-3 bun due to the disintegrated metal particles being about twice as large, whereas an average radial growth rate of SiC whiskers of about 0.2 /xm/h is obtained under the conditions mentioned above except for a deposition temperature of 1300°C instead. Consequently, the appropriate choice of the size of activating metal particles for whisker growth, if no further breakup of the liquid droplet takes place during deposition, is more effective than the thick￾ening deposition in the radial direction in controlling the diameter of SiC whiskers. 4. Conclusions The factors that control the diameter of SiC whiskers are summarized as the result of a combined operation of the VLS mechanism and VS radial deposition. The former is deter￾mined by the area of the solid-liquid interface and the latter by the thickening kinetics of vapor-deposited SiC on the lateral face. The solid-liquid contact area is determined by the volume of the liquid droplets and the equilibrium condi￾tion at the vapor-liquid-solid triple-phase junction. An Arrhenius plot of the temperature dependence of radial growth rate of SiC whiskers leads to an apparent activation energy of about 180 kJ/mol. However, a comparison of the two factors using the parameters employed in the present study indicates that an appropriate choice of the volume of liquid droplets during VLS whisker growth is more effective than radial VS deposition for obtaining whiskers of desired diameters. Acknowledgements The financial support of this study by the National Science Council of the Republic of China under grant contract number NSC84-2216-E006-012 is greatly appreciated. References [ 1 ] R.S. Wagner, W.C. Ellis, Appt. Phys. Lett. 4 (I964) 89. [2l R.S. Wagner, W.C. Ellis, Trans. AIME 233 (1965) 1053. [3] J.V. Milewski, F.D. Gac, J.J. Petrovic, S.R. Skaggs, J. Mater. Sci. 20 (1985) i160. [4] P.F. Becher, T.N. Tiegs, P.A. Angelini, in K.S. Mazdiyasni (Ed.), Fiber Reinforced Ceramic Composites -- Materials, Processing and Technology, ch. i 1, Noyes Publications, Park Ridge, NJ, USA, 1990. [5] K. Hiruma, M. Yaeawa, K. Haraguchi, K. Ogawa, J. Appi. Phys. 74 (1993) 3162. [6] R. De Jong, R.A. McCauley, J. Am. Ceram. Soc. 70 (1987) C-338. [7] G.F. Hurley, J.J. Petrovic, Advanced Composites Conf. Proc., ASM, Materials Park, OH, USA, 1985, p. 207. [8] P.D. Shalek, W.J. Parkinson, Mater. Res. Soc. Symp. Proc., vot. 168, Mater. Res. Soc., Pittsburgh, PA, USA, 1990, p. 255. [9] M. Yazawa, M. Koguchi, A. Muto, M. Ozawa, K. Hiruma, Appl. Phys. Lett. 61 (1992) 2051. [10] J. Westwater, D. Pal Gosain, S. Usui, Jpn. J. Appl. Phys. 36 (1997) 62O4. [ 11 ] D.J. Cheng, W.J. Shyy, D.H. Kuo, M.H. Hon, J. Electrochem. Soc. i34 (1987) 3145. [ 12] I.C. Leu, Y.M. Lu, M.H. Hon, J. Crystal Growth I67 (1996) 607. [ 13] G.A. Bootsma, HJ. Gassen, J. Crystal Growth 10 ( 1971 ) 223. [ 14] G.A. Bootsma, W.F. Knippenberg, O. Verspui, J. Crystal Growth I 1 (I971) 297. [ 15] I.C. Leu, M.H. Hon, Y.M. Lu, to be published. [ 16] S.R. Nutt, J. Am. Ceram. Soc. 7i (1988) 149. [ 17] E.I. Givargizov, J. Crystal Growth 20 (1973) 217. [18] JANAF Thermochemical Tables, 2nd edn., NatI. Stand. Ref. Data Set. (US Natl. Bur. Stand.) No. 37, 1971. [ 19] M. Kitamura, S. Hosoya, I. Sunagawa, J. Crystal Growth 47 (1979) 93. [20] L.I. Van Tome, J. Appl. Phys. 37 (i966) I849. [21] JJ. Comer, Mater. Res. Bull. 4 (1969) 279. ~22] L. Wang, H. Wada, LF. Allard, J. Mater. Res. 7 (1992) 148. [23] J. Schlichting, Powder Metallurgy International 12 (1980) 141. [24] Z. Wokulski, J. Crystal Growth 82 (1987) 427