Ni-rich Thin Films Research Articles

Electrosynthesis-Induced Pt Skin Effect in Mesoporous Ni-Rich Ni-Pt Thin Films for Hydrogen Evolution Reaction.

A Pt skin effect, i.e., an enrichment of Pt within the first 1-2 nm from the surface, is observed in as-prepared electrodeposited Ni-rich Ni-Pt thin films. This effect, revealed by Rutherford backscattering (RBS), is present for both dense thin films and mesoporous thin films synthesized by micelle-assisted electrodeposition from a chloride-based electrolyte. Due to the Pt skin effect, the Ni-rich thin films show excellent stability at the hydrogen evolution reaction (HER) in acidic media, during which a gradient in the Pt/Ni ratio is established along the thickness of the thin films, while the activity at the HER remains unaffected by this structural change. Further characterization by elastic recoil detection with He ions analysis shows that hydrogen profiles are similar to those of Pt: a surface hydrogen peak coincides with the Pt skin, and a gradient in hydrogen concentration is established during HER in acidic media, together with a considerable uptake in hydrogen. A comparative study shows that in alkaline media, hydrogen evolution has little to no effect on the structural properties of the thin films, even for much longer times of exposure. The mesoporous thin films, in addition to their higher efficiency at HER compared to dense thin films, also show lower internal stress, as determined by Rietveld refinement of grazing incidence X-ray diffraction patterns. The latter also reveal a fully single-phase and nanocrystalline structure for all thin films with varying Ni contents.

Estimation of Thickness Ratio of Bi-Layer TiNi to Enhance Shape Memory Behavior

Shape memory thin films deposited by sputtering are attractive candidates for micro-electro-mechanical-system (MEMS) because of their large deformation and strong recovery force. In the present study Ni-Ti thin films have been deposited on NaCl substrates by DC magnetron sputtering source fitted with an 80mm diameter alloy target. In order to obtain a variety of film compositions, several discs of alloy target, which prepared in vacuum arc remelting (VAR), were used. Three types of thin films have been deposited; Ti and Ni-rich thin films were separately deposited on NaCl substrate and also a composite layer of Ni45Ti50Cu5 and Ni-rich. The as deposited Ni-Ti thin films were crystallized to change the amorphous structure to a nanostructured material to characterize shape memory and superelastic behaviors. The effect of composition on film structure and mechanical behavior was studied by using X-ray diffraction (XRD) and nanoindentation. The results of thin films behavior were used to calculate the thickness ratio of be-layer composite NiTi to obtain enhanced shape memory behavior.

「スマートマテリアル」 低温結晶化させた非晶質Ｔｉ‐Ｎｉ薄膜の形状記憶特性と内部組織

The shape memory properties of sputter-deposited Ti-Ni thin films crystallized from amorphous at low-temperatures, 60-90 K lower than the crystallization peak temperature measured by differential scanning calorimetry (DSC), were investigated. The Ti-47.7 at%Ni (Ti-rich) and Ti-51.6 at%Ni (Ni-rich) thin films were fabricated by a r.f. magnetron sputter-deposition method. Heat-treatment temperature was varied from 643 to 693 K and heat-treatment time was varied from 3.6 to 360 ks. Thin plate precipitates were formed on {100}B2 in the Ti-rich thin films heat-treated at 653 K. Maximum recovery strain (εAmax) of the Ti-rich thin films increased up to 5.4% with increasing heat-treatment time. But it started to decrease after reaching the maximum value. The decrease in εAmax was caused by the decrease of the critical stress for slip (σs) with increase of heat-treatment time because of the growth of thin plate precipitates. Ti3Ni4 precipitates were formed in the Ni-rich thin films heat-treated at 673-693 K. εAmax of the Ni-rich thin films increased with increasing heat-treatment time. But, the εAmax reached only 2.9%, since the martensitic transformation and the growth of martensites were strongly suppressed by Ti3Ni4 precipitates. The martensitic transformation temperature (Ms) of the Ti-rich and Ni-rich thin films increased with increasing heat-treatment time.

Shape Memory Effects Associated with the Martensitic and R-Phase Transformations in Sputter-Deposited Ti-Ni Thin Films

Ti-Ni shape memory alloy thin films were deposited using RF magnetron sputtering. The lattice constants, the transformation temperatures and shape memory behavior were measured by the X-ray diffractometry, DSC and mechanical tests, respectively. These films showed a perfect shape memory effect which is the same as that of bulk alloys. The shape recovery stress and strain measured in the thin films were large enough for fabricating microactuators. In an age-treated Ni-rich Ti-Ni thin films, the shape recovery stress was more than 600MPa, while the shape memory strain amounted up to 2.6%. The thin films showed both the martensitic and R-phase transformations. Accordingly, they showed a two-stage deformation upon cooling ; the strain induced in each stage recovered perfectly upon heating. The transformation temperatures and shape memory characteristics were strongly affected by age-treatment in Ni-rich thin films, while they were not in Ni-poor thin films.