Abstract

TiNi based shape memory alloy (SMA) thin films including TiNi, TiNiPd and TiNiCu have been used to develop diaphragm microactuators. The TiNi film is a standard material and the ternary TiNiPd and TiNiCu alloy films have their own attractive characteristics when compared with TiNi films. The TiNiPd alloy is characterized by high transformation temperatures so that it is expected to show quick response due to a higher cooling rate: the cooling rate increases with increasing the temperature difference between the transformation temperature and room temperature which is the minimum temperature in conventional circumstances. The martensitic transformation of the TiNiCu and the R-phase transformation of the TiNi are characterized by narrow transformation hystereses which are one-fourth and one-tenth of the hysteresis of the martensitic transformation in the TiNi film. Thus, these transformations with a narrow hysteresis are also attractive for high response microactuators. The working frequencies of two types of microactuators utilizing the TiNiPd thin film and the TiNiCu film reached 100Hz, while the working frequency of the microactuator using the R-phase transformation reached 125Hz.

Introduction

The demand for the development of powerful microactuators has stimulated the research to develop sputter-deposited TiNi shape memory alloy (SMA) thin films, because they possess attractive characteristics useful for microactuators. The most prominent characteristics of SMA thin films are small dimensions and high response speed in addition to conventional ones, such as a large recoverable strain of 6–8% and recoverable stress up to 600MPa [1, 2, 3]. These values are extremely large when compared with other actuator materials or mechanisms, such as piezoelectric materials and the electrostatic force.