Abnormal Behavior of Hydrogen Response and Hydrogen Induced Linear Expansion Coefficient of Pd-Cu-Si Metallic Glassy Alloys for Thin Film Hydrogen Sensor

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 H2 responses were observed by measuring the electric resistance changes of them exposed in N2 and H2. In addition, their linear expansion coefficients (LECs) induced by absorbed hydrogen were also measured. Contrary to a normal H2 response transient with a rapid increase in electric resistance of a thin film, several thin films indicated abnormal H2 response transients consisting of complex changes, an increase and a decrease, in electric resistance when the thin films were exposed in H2. These thin films have higher Pd/Si atomic ratios than those indicated normal H2 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 H2 response transient can be explained by two conflicting behaviors in electric resistance of the thin films when they are exposed in H2: 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 H2 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 H2 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 H2 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.