Cryogenic shape memory alloy
Shape memory alloy (SMA) with thermoelastic martensitic transformation (MT) has a much larger output strain and stress compared to piezoelectric materials or magnetostrictive materials. Such properties of the SMA makes it suitable as actuator materials for areas that require a large motion and high energy density. Another advantage of SMA is that it can be driven simply by heating the actuator elements. Because of these two characteristics, simplification and miniaturization of an actuation system can be easily achieved. Therefore the SMA actuators are suitable for those used under various extreme conditions. Cryogenic shape memory actuators can be used in gas liquefiers (H2, He, N2, O2, etc. with boiling points summarized in Fig.)，gas separation devices，compressors，cryostats，temperature controllers，liquid pumps, and so on. Hence they are useful for the coming hydrogen society. Astonishingly, there are very few studies regarding the cryogenic shape memory alloy. Since the underlying mechanism of shape memory effect is shape change by MT, the information on MT temperature is very important. However, only limited data is available for MT temperatures below the Liquid N2 temperature (~77 K). Prado et al. reported that the Cu-Al-Mn alloy exhibits MT down to 17K [1]. The focus of the present study is to develop a SMA actuator for cryogenic conditions. Figure 2 shows the temperature dependence of electrical resistivity of Cu-Al-Mn alloy. This sample exhibits MT around 100K. We are currently trying to control the transformation temperature based on metallurgical viewpoints.
[1] Prado, Scripta Mater., 44(2001), 2431.
Fig. Temperature dependence of electrical resistivity in Cu-Al-Mn alloy.