Sputtering pressure dependence of hydrogen-sensing effect of palladium films
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The electrical resistivity r of palladium (Pd) films prepared by using magnetron sputtering at different pressures f ranging from 2 to 15 mTorr showed very different hydrogen (H)-induced response. This is because the mean free path of the particles in vacuum changes substantially with f, such that the structure of the deposits is altered accordingly. A film prepared at a moderate f value of 6 mTorr has a moderate strength. After a few hydrogenation-dehydrogenation cycles, some cracks are generated because of the great difference in the specific volumes of the metal and hydride phases. Breathing of the cracks in subsequent switching cycles occurred, which led to the response gain of r, defined as the resistivity ratio of the dehydrogenated-to-hydrogenated states during a cycle, to increase to 17. This result demonstrates the attractiveness of using the Pd films in H2 detection application. The H-induced resistive response of the films prepared at higher or lower f values was found to be much smaller.
I. INTRODUCTION
Palladium (Pd) can dissociate hydrogen molecules (H2) and absorb H atoms at room temperature to result in a remarkable change in electrical resistivity (r).1 This effect becomes more attractive due to a great variety of the demands on H2 detection.2–4 When a small amount of H atoms is incorporated into Pd, a metal-hydride a-phase is first produced. The resistivity r would increase to approximately 1.05 times that of the pure metal phase ro. A further increase in H content leads to nucleation of a metal-hydride b-phase.5 The value of r would increase further to 1.6 to 1.8 times of ro when the H content is increased to 65 to 75 at.%.6 This effect is primarily attributed to the difference of the r values between the metal phase and the hydride phase. The effect has been used for making H2 sensors, in which the sensing elements are in the form of Pd wires or thin films.7,8 Typical resistive change based on this mechanism is approximately 0.5 to 8% in an environment containing 1 to 5% of H2. In general, the response time is in the order of 10 to 20 s. More recently, Pd made to have a defective structure containing pores/defects was found to be able to produce a much higher and faster resistive response to H2 based on a different mechanism. One distinctive feature of a response based on such a mechanism is that the direction of the change of r is opposite to that of the foresaid one. This is because the lattice constants of pure Pd metal and a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0238 J. Mater. Res., Vol. 24, No. 6, Jun 2009
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the metal-hydride b-phase are in the ranges of 0.3889– 0.3890 and 0.4013–0.4025 nm, respectively,9 i.e., a mean difference of 3.3%, such that full hydrogenation of pure metal Pd is estimated to give a volume increase by 10%. As a consequence, some pores/defects in the structure could be closed to result in a rapid drop of r.10–12 The opposite occurs during dehydrogenati
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