M. Nygren, Z Shen/Solid State Sciences 5(2003)125-131 13 sible to adjust the microstructures of the produced SiN Acknowledgements based ceramics by manipulating the grain growth kinetics and accordingly to tailor their mechanical properties Te wish to thank our colleagues dr. Zhe Zhao Dr. Mats Johnsson, Dr Jekabs Grins and Ms. Hong Peng for their contributions to the work presented in this article 6. Concluding remarks References Using conventional sintering processes, densification is ways accompanied by grain growth. We have demon [K Inoue, US Patent No. 3, 241, 956(1966) strated that the sPs process provides us with a unique op 22] M. Tokita, J. Soc. Powder Technol, Jpn. 30(1993)790 portunity to separate grain growth from densification. One []K. Yamazaki, S.H. Risbud, H. Aoyama, K Shoda, J. Mater. Proc outstanding feature of the SPS process is the short sintering ech65(1996955 times and low sintering temperatures needed to obtain fully [4 W.M. Goldberger, Mater. Tech. 10(1995)48. dense compacts. The use of extremely high heating and cool 5]ZA Munir, H. Schmalzried, J of Mater. Synth. Proc 1(1993)3 [6].S. Mishra, S.H. Risbud, A K. Mukherjee, J. Mater, Res. 13(1998) ing rates in conjunction with high pressures is certainly of importance in this connection. As discussed above, the oc- [7N Murayama, Bull. Ceram Soc. Jpn. 32(1997)445 currence of a plasma discharge is still debated, but it seems [8] M. Yoshimura, T. Ohji, M. Sando, K. Niihara, J. Mater. Sci. Lett. 17 to be widely accepted that an electric discharge process takes (1998)1389 9Y. Zhou, K. Hirao, M. Toriyama, H. Tanaka, J. Am. Ceram Soc. 83 place on a microscopic level. Since no current or only a very 000)654 weak one will pass through non-conducting samples, the dis- 10]RS Mishra, A K. Mukherjee, Mater. Sci. Eng. A 287(2000)178. charge ought to originate from the electric field set up by the [II]L Gao, Z Shen, H. Miyamoto, M. Nygren, J. Am. Ceram Soc. 82 pulsed direct current used. The intensity of this discharge is 9)1061 presumably not only dependent on the intensity of the ap- [12] Z Shen, M. Johnsson, Z Zhao, M. Nygren, J. Am. Ceram Soc. 85 02)1921 plied pulses but also on factors such as particle size, pore [13]J. Grins, S, Esmaeilzadeh, G, Svensson, Z. Shen, J. Euro. Ceram. size, and relative density of the compact. Anyhow, it seems Soc.19(1999)2723 plausible that it is only during the initial part of a sinter- 14Z Shen, M. Nygren, J. Euro Ceram Soc. 21(2001)611 ing process, i.e., up to the point where the system reaches 5]J Grins, Z Shen, S. Esmaeilzadeh, P. Berastegui, J. Mater. Chem. II a"closed porosity"state, that such a discharge process can 2001)238 make a major contribution to the densification. From that [16]J. Grins, Z Shen, in manuscript. [17Z Shen, E. Adolfsson, M. Nygren, L Gao, H Kawaoka, K Nihar, stage on, grain boundary diffusion and grain boundary mi- Adv. Mater.13(2001)214 gration ought to be the rate determining processes. An elec- [18T Hirai, L Chen, Mater. Interg. 12(1999)51 tric discharge process would certainly clean the surfaces of 19]J. Zhang, Z. Huang, D Jiang, S. Tan, Z. Shen, M. Nygren, J.Am. he powders from adsorbed species and create various types Ceram Soc. 85(2002) of surface defects that will enhance the grain boundary dif- 20]Z Shen, N. Nygren, Key Eng Mater. 206-213(2002)2155 21]Z Shen, M. Nygren, J Mater. Chem. 11(2001)20 fusion [22 Z Shen, Z Zhao, H Peng, M. Nygren, Nature 417(2002)266M. Nygren, Z. Shen / Solid State Sciences 5 (2003) 125–131 131 sible to adjust the microstructures of the produced Si3N4 based ceramics by manipulating the grain growth kinetics and accordingly to tailor their mechanical properties. 6. Concluding remarks Using conventional sintering processes, densification is always accompanied by grain growth. We have demon￾strated that the SPS process provides us with a unique op￾portunity to separate grain growth from densification. One outstanding feature of the SPS process is the short sintering times and low sintering temperatures needed to obtain fully dense compacts. The use of extremely high heating and cool￾ing rates in conjunction with high pressures is certainly of importance in this connection. As discussed above, the oc￾currence of a plasma discharge is still debated, but it seems to be widely accepted that an electric discharge process takes place on a microscopic level. Since no current or only a very weak one will pass through non-conducting samples, the dis￾charge ought to originate from the electric field set up by the pulsed direct current used. The intensity of this discharge is presumably not only dependent on the intensity of the ap￾plied pulses but also on factors such as particle size, pore size, and relative density of the compact. Anyhow, it seems plausible that it is only during the initial part of a sinter￾ing process, i.e., up to the point where the system reaches a “closed porosity” state, that such a discharge process can make a major contribution to the densification. From that stage on, grain boundary diffusion and grain boundary mi￾gration ought to be the rate determining processes. An elec￾tric discharge process would certainly clean the surfaces of the powders from adsorbed species and create various types of surface defects that will enhance the grain boundary dif￾fusion. Acknowledgements We wish to thank our colleagues Dr. Zhe Zhao, Dr. Mats Johnsson, Dr. Jekabs Grins and Ms. Hong Peng for their contributions to the work presented in this article. References [1] K. Inoue, US Patent No. 3,241,956 (1966). [2] M. Tokita, J. Soc. Powder Technol., Jpn. 30 (1993) 790. [3] K. Yamazaki, S.H. Risbud, H. Aoyama, K. Shoda, J. Mater. Proc. Tech. 65 (1996) 955. [4] W.M. Goldberger, Mater. Tech. 10 (1995) 48. [5] Z.A. Munir, H. Schmalzried, J. of Mater. Synth. & Proc. 1 (1993) 3. [6] R.S. Mishra, S.H. Risbud, A.K. Mukherjee, J. Mater. Res. 13 (1998) 86. [7] N. Murayama, Bull. Ceram. Soc. Jpn. 32 (1997) 445. [8] M. Yoshimura, T. Ohji, M. Sando, K. Niihara, J. Mater. Sci. Lett. 17 (1998) 1389. [9] Y. Zhou, K. Hirao, M. Toriyama, H. Tanaka, J. Am. Ceram. Soc. 83 (2000) 654. [10] R.S. Mishra, A.K. Mukherjee, Mater. Sci. Eng. A 287 (2000) 178. [11] L. Gao, Z. Shen, H. Miyamoto, M. Nygren, J. Am. Ceram. Soc. 82 (1999) 1061. [12] Z. Shen, M. Johnsson, Z. Zhao, M. Nygren, J. Am. Ceram. Soc. 85 (2002) 1921. [13] J. Grins, S. Esmaeilzadeh, G. Svensson, Z. Shen, J. Euro. Ceram. Soc. 19 (1999) 2723. [14] Z. Shen, M. Nygren, J. Euro. Ceram. Soc. 21 (2001) 611. [15] J. Grins, Z. Shen, S. Esmaeilzadeh, P. Berastegui, J. Mater. Chem. 11 (2001) 2358. [16] J. Grins, Z. Shen, in manuscript. [17] Z. Shen, E. Adolfsson, M. Nygren, L. Gao, H. Kawaoka, K. Niihar, Adv. Mater. 13 (2001) 214. [18] T. Hirai, L. Chen, Mater. Interg. 12 (1999) 51. [19] J. Zhang, Z. Huang, D. Jiang, S. Tan, Z. Shen, M. Nygren, J. Am. Ceram. Soc. 85 (2002). [20] Z. Shen, N. Nygren, Key Eng. Mater. 206–213 (2002) 2155. [21] Z. Shen, M. Nygren, J. Mater. Chem. 11 (2001) 204. [22] Z. Shen, Z. Zhao, H. Peng, M. Nygren, Nature 417 (2002) 266