Mechanisms for the Variation of Electrical Resistivity of Palladium Films in Hydridation-dehydridation Processes
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Mechanisms for the Variation of Electrical Resistivity of Palladium Films in Hydridationdehydridation Processes Yu Ming Tang, Yiu Bun Chan, and Chung Wo Ong Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China, People's Republic of ABSTRACT Hydridation of palladium (Pd) is accompanied by a substantial volume expansion. The electrical resistivity (ρ) would rise because the volume fraction of the hydride phase increases. However, if the material structure is porous/defective, hydridation induced volume expansion may lead to the closing of some pores/defects to result in a drop of ρ. We verified that a magnetron sputtered Pd film deposited at a higher argon ambient pressure (φ) was more defective, such that the contribution from the latter mechanism was stronger with increasing φ, and reached a maximum level for a film deposited at a φ of 6 mTorr. However, the film structure was weaker and unstable during the switching cycles, such that the magnitude of the change of ρ was found to increase successively with increasing number of switching cycles. The performances of ρ during initial soaking in H2 and subsequent hydridation and dehydridation switching cycles were different. The observed results are presented and discussed in this paper. INTRODUCTION Palladium (Pd) can absorb conspicuous amount of hydrogen (H2), accompanied by a substantial expansion of volume [1-6] --- hydrogen-induced volume expansion (HIVE) effect. The electrical resistivity (ρ) would be changed, mainly due to two mechanisms. The first is primarily because of the emergence of the hydride phase, which is more resistive such that the value of ρ increases as a consequence. For example, a study on pulsed laser deposited Pd films illustrated that this mechanism dominated the change of ρ when reacting with H2 [7]. Typically, when reacting with a gas containing a few percent of H2, the change of ρ shows a response time of a few ten seconds to a few minutes, and the magnitude of the change is in the order of 10%. On the other hand, if the structure of Pd is made to be very porous, the HIVE effect may result in the closure of some pores/defects in the structure, such that the ρ value would drop as a consequence. This phenomenon has attracted more interests because it gives a much stronger and faster response and so is considered to have great application potential in H-sensing technology. For example, Pd mesowires made on a graphite surface by using electrodeposition [8], nano-sized Pd clusters deposited on a siloxane-coated Si substrate [9], and thin Pd film deposited on a porous Si substrate [10] illustrate HIVE-induced response of ρ when reacting with H2. Response time as short as 70 ms and a 100-% change of ρ were recorded. In this study, we investigated the response of ρ of Pd films prepared by magnetron sputtering at different ambient pressures of argon (φ) when reacting with H2. The study is based on an assumption that the film structure produced at a higher φ
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