1. Department of Materials Science & Engineering, Seoul National University, Seoul 151‐744, South Korea, Fax: +82‐2‐886‐4156
2. Ceramic Research & Development division, Dongil Technology Ltd, Hwasung 445‐854, Korea
3. Department of Materials Science & Engineering, Ajou University, Suwon 443‐749, South Korea, Fax: +82‐31‐219‐1612