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摘要
Thin films of Pd-Cu-Si metallic glassy alloys of varying composition were prepared by simultaneous three sources (Pd, Cu and Si) sputtering method using a rotating mechanism of substrates. Their H-2 responses were observed by measuring the electric resistance changes of them exposed in N-2 and H-2. In addition, their linear expansion coefficients (LECs) induced by absorbed hydrogen were also measured.
Contrary to a normal H-2 response transient with a rapid increase in electric resistance of a thin film, several thin films indicated abnormal H-2 response transients consisting of complex changes, an increase and a decrease, in electric resistance when the thin films were exposed in H-2. These thin films have higher Pd/Si atomic ratios than those indicated normal H-2 response transients. Additionally, by the characterization of the thin films, the existence of Pd-nanocrystals of about 2 nm in diameter was observed in the amorphous matrix which possibly includes Pd-clusters as well as Pd atoms.
The mechanism of the abnormal H-2 response transient can be explained by two conflicting behaviors in electric resistance of the thin films when they are exposed in H-2: a decrease by the formation of electrical contacts of Pd-nanocrystals connected with volume expanded Pd-clusters by hydrogen absorption and a following increase by hydrogenation of Pd-nanocrystals. Observed time lags between two conflicting behaviors can be explained by the different transfer speeds of hydrogen relating to two kinds of pathways: inside of Pd-nanocrystals and the amorphous matrix along which hydrogen atoms transfer in the structure.
The Pd-nanocrystals also affected on LEC and the H-2 response significantly. The thin films with them indicated much higher LEC than the thin films without them. However, the thin films with them did not indicate the high H-2 response expected from their high LEC. The result of LEC suggests that Pd-nanocrystals can absorb much more hydrogen atoms than Pd in an amorphous matrix. The Pd in an amorphous matrix is supposed to be Pd-clusters, randomly distributed Pd atoms and Pd atoms forming a trigonal prism that is a structural unit of the Pd-Cu-Si alloys. On the other hand, the high H-2 response according to the high amount of absorbed hydrogen can not be expected in the thin films with Pd-nanocrystals, due to a decrease in electric resistance by forming electrical contacts of the Pd-nanocrystals. [doi:10.2320/matertrans.M2010356]
- 出版日期2011-6
