Preparation of TiFe thin Films by Pulsed Ion Beam Evaporation
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PREPARATION OF TiFe THIN FILMS BY PULSED ION BEAM EVAPORATION Hisayuki Suematsu, Tsuyoshi Saikusa, Tsuneo Suzuki, Weihua Jiang and Kiyoshi Yatsui Extreme Energy-Density Research Institute, Nagaoka University of Technology, Nagaoka 940-2188, Japan
ABSTRACT Thin films of titanium iron (TiFe) were prepared by a pulsed ion-beam evaporation (IBE) method. A pulsed ion beam of proton accelerated at 1 MV (peak) with a pulse width of 50 ns and a current of 70 kA was focused on TiFe alloy targets. Soda lime glass substrates were placed in front of the targets. Phases in the thin films were identified by X-ray diffraction (XRD). XRD results revealed that the thin films deposited on the glass substrates consist of a TiFe phase. Crystallized Ti-Fe thin films without oxides were successfully obtained. Surface roughness of the thin film was 0.16 µm. INTRODUCTION Titanium iron (TiFe) is known as a hydrogen absorbing alloy. When hydrogen atoms absorb and dissolve in the alloy, metal hydrides are formed. TiFe absorbs 1.8 wt.% of hydrogen[1] which is higher than those of some hydrogen absorbing alloys. Furthermore, TiFe is comparatively cheaper than other hydrogen absorbing alloys containing rare earth metals, such as LaNi5[1,2]. For applications in secondary batteries[3] and hydrogen separation membranes[4,5], hydrogen absorbing alloy thin films is being developed. Preparation of Ti-Fe thin films has also been attempted by a sputtering method. However, the thin films consisted of an amorphous Ti-Fe phase[1,2,6]. Since hydrogen absorption weight in amorphous Ti-Fe is less than that of crystallized TiFe[2], crystallization of TiFe thin films is required. However, crystallized TiFe thin films have not been obtained, to the best of the authors’ knowledge. Substrate heating or sample annealing may enhance the crystallization of the TiFe phase. However, oxidation of Ti must accompany because Ti is highly reactive. This would form secondary phases in the thin film. Thus, other methods, which enable us to prepare thin films at low temperature, have to be developed to obtain single phase, crystallized TiFe thin films. Another problem in hydrogen absorbing alloy thin films is also known. During the hydrogenation and dehydrogenation cycles, the lattice repeatedly expands and contracts. The stress caused by the hydrogenation tends to pulverize the hydrogen absorbing alloy samples. If the porous thin films are prepared, it is expected that the pulverization can be suppressed. Furthermore, the increase in the specific surface area by the decrease in the packing density would enhance speed of hydrogenation and dehydrogenation. A novel thin film preparation method of pulsed ion-beam evaporation (IBE) was developed[7]. Various thin films including crystallized (Ba,Sr)TiO3[8], SrAl2O4[9] and B4C[10] have been prepared. The thin films were deposited at room temperature, which would be the ideal condition for the preparation of crystallized TiFe thin films. Furthermore, porous thin films were easily obtained by IBE. In the present study, thin films of
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