Sputtering growth of Ni–Ti shape memory alloy nano fibers on porous alumina membrane

A novel method of preparing a large number of fiber-shaped Ni–Ti nano crystals with medium aspect ratio in the order of 10 was examined. A target of nickel–titanium shape memory alloy was sputter-deposited on a porous alumina membrane having holes with 200 nm diameter in average. The porous geometry of the substrate caused the growth of columnar structure under the sputtering conditions, the argon gas pressure of 0.2 Pa and the substrate temperature of 250 °C, which would provide homogeneous alloy films, if the surface of substrate was flat. The columnar structure thus obtained was the assemble of isolated columns which were weakly adhered with each others, and could be separated into individual ones by applying ultrasonic vibration. Each column had a fibrous shape of diameter about 200 nm and 2.5 μm long (the aspect ratio was 12.5). The crystals can exhibit both martensitic transformation and the R-phase change with the total amount of latent heat 18 J g−1, indicating that 75% of the nano crystals was active to these transformations.

► A novel fabrication route of nickel–titanium alloy nano fibrous crystals was proposed.
► The column crystals about 200 nm in diameter and 2.5 μm long were prepared.
► The columnar structure was grown by the sputtering on porous alumina membrane.
► The column crystals were separated from the columnar structure of sputter-deposited film.