1. a Dept. of Materials Sci. & Eng. , Sangju National University , 386, Gajang-dong, Sangju , Kyungbuk , 742-711 , Korea
2. b Sehyang Ind. Co. 9-2 Horim-dong , Daegu , 704-240 , Korea
3. c Dept. of Materials Sci. & Eng , University of Virginia , Charlottesville , VA , 22904-4745 , USA