Conclusions (1)The mean positron lifetime has been found to decrease with an increase in the size of the grains. At large grain size,comparable to the diffusion length,annihilation occurs in the bulk state. (2)The diffusion coefficient D+could be described by the relation D.DoT-12.The variation of the S- parameter in nanocrystalline samples suggests that the material undergoes significant changes at low grain size. PDF文件使用"pdfFactory”试用版本创建ww.fineprint.cnConclusions (1)The mean positron lifetime has been found to decrease with an increase in the size of the grains. At large grain size, comparable to the diffusion length, annihilation occurs in the bulk state. (2) The diffusion coefficient D+ could be described by the relation D+ = D0T -1/2 . The variation of the S￾parameter in nanocrystalline samples suggests that the material undergoes significant changes at low grain size. PDF 文件使用 "pdfFactory" 试用版本创建 ÿwww.fineprint.cn