1. Department of Materials Science and Engineering, Yonsei University, 134 Shinchon-dong, Seoul 120-749, Republic of Korea
2. First R&D Centre, Agency for Defence Development, P.O. Box 35-1, Yuseong, Daejeon 305-600, Republic of Korea
3. Department of Ceramics Engineering, Kangnung National University, 120 Gangneung Daehangno, Kangnung, Kangwon-do 210-702, Republic of Korea