1. School of Materials Science and Engineering, Seoul National University, San 56-1, Shillim-dong, Kwanak-gu, Seoul 151-744, South Korea, School of Advanced Materials Engineering, Kookmin University, Jeongneung-dong, Seongbuk-gu, Seoul 136-702, South Korea, Materials Science & Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and Department of Ceramic Engineering, Kangnung National University, Kangnung, 210-702, South Korea