Dynamic thermo-mechanical properties of evaporated TiNi shape memory thin film

Abstract

The shape recovery and re-deflection responses of shape memory alloy (SMA) thin film to thermal cycles were investigated using the bulge method. It was deposited by flash evaporation and had a nominal composition of 50 at.% Ti–50 at.% Ni and a thickness of about 6 μm. After being released from silicon substrates and undergoing vacuum-annealing to obtain memorisation of an initial flat shape, it was deformed into a cap shape of 5 mm in diameter by pressurisation at 400 kPa. Then, by applying 100 ms voltage pulses, its initial shape was recovered by resistive heating at various energies. During these shape recovery and re-deflection cycles, change in displacement with time was measured continuously using a laser displacement meter. The thin film exhibited shape recovery at energies for heating of more than 1 J due to reverse martensitic transformation. Displacement due to shape recovery increased with increasing energy for heating, reaching saturation at around 100 μm at energies of more than 2 J. After heating was completed, the thin film deflected again due to martensitic transformation under pressure. The period for each shape recovery and re-deflection cycle was about 600 ms at an energy of 2.1 J. It exhibited stable shape recovery and re-deflection properties at up to 1000 cycles, which was the maximum number of thermal cycles tested. Finally, the pumping pressures and flow rates which might be expected with such an SMA micropump were also roughly estimated.