Heat-engines using shape memory alloy (SMA) were expected to be employed as heat-engines driven by low-temperature thermal energy. Therefore, some trial products of heat-engines using SMA were produced and studied. However, since the inhomogeneous temperatures of the SMA element result during heating / cooling of the SMA element, the product-life cycle of conventional heat-engines using SMA becomes short. Moreover, if large power outputs are to be achieved, these engines would become large in size. Therefore, conventional SMA heat-engines have not been put into practical use. We propose a new actuator using a SMA spiral spring. By using this system, the uniform heating / cooling of the SMA element (improvement of the product life) and downsizing of the equipment are expected. Therefore, we manufactured a prototype of a heat-engine using a SMA spiral spring actuator, and the output characteristics of this heat-engine were investigated.

A heat-engine using Ti-Ni shape memory alloy (SMA) driven by low-temperature waste heat energy has been investigated for waste heat recovery potential system. We had proposed the use of the tape-shaped SMA element because of the cooling efficiency improvement of the SMA element. However, the manufacturing cost of the tape-shaped SMA fabricated by arc melting method is higher than that of a wire or coil shaped SMA. To reduce the manufacturing cost, we fabricated a tape-shaped SMA element by the centrifugal casting method using an alloy fabricated by the powder metallurgy method, and investigated the effect of heat-treatment temperature on shape memory characteristics of this tape-shaped Ti-Ni SMA specimen fabricated by the centrifugal casting method. The mechanical and shape memory properties of tape-shaped SMA specimen show a minimum at 803 K, and it increases as temperature rises up to 953 K, above which it decreases. The fluctuation of mechanical and shape memory properties are caused by the variation of dislocation density, secondary phase and precipitated phase. From these results, it was found that the optimum heat-treatment temperature of the tape-shaped Ti-Ni SMA specimen is 953 K. Moreover, the output properties of SMA heat-engine driven by an SMA element fabricated by the centrifugal casting method, is superior to the output properties of the heat-engine using an SMA element fabricated by arc-melting method.

   301Trans. Mat. Res. Soc. Japan 40[3] 301-304 (2015)

For the purpose of developing an actuator for wearable-assistive devices, we have devised and produced a weft yarn-type shape memory alloy (SMA) actuator using the resistance feedback control method. This actuator consists of a polyethylene terephthalate (PET) plate and SMA wire woven into the weft pattern. Since the PET plate and SMA wire are constrained in the lateral direction, this actuator can do the bending operation by the actuation of the SMA wire. In this research, the effects of constraint conditions of the PET plate and SMA wire (i.e. the distance between PET plate and SMA wire (D)), and the number of constraining points (N) were investigated with respect to the bending operating characteristics (bending deflection and operating velocity) of the weft yarn-type SMA actuator. The bending deflection of the actuator was seen to increases with the increase of N. However, the bending deflection of the actuator was seen to decreases with the increase of N beyond the maximum value. The variation of deflection is caused by the variation of frictional force due to the variation of N. Furthermore, the bending deflection of the actuator and operating velocity decrease as D increases. This tendency is caused by the increase of the curvature radius of actuator with the increase of D.

In this research, the effects of heating/cooling water temperatures of a heat engine on the output characteristics of the forced cooling pulley-type SMA heat-engine is investigated for the purpose of operating temperature optimization of the engine. The engine output increases almost linearly as the heating water temperature (TH) rises. On the other hand, the engine output shows a gradual decrease as the temperature of SMA element during cooling (TSMA) rises to 321.4 K and then an abrupt decrease when the TSMA = 322.7 K. Thermal cycling tests under constrained strain conditions for the SMA element were carried out to investigate the effect of heating/cooling temperatures to generation stress () and reaction velocity (VR) of the SMA element. In result,  increases gradually with the increase of TH. Besides, VR increases almost linearly as TH increases. This result suggests that the increase of engine output as TH increases is mainly due to the increase of the VR. Meanwhile, VR increases gradually with the increase of TSMA. Besides,  increases as TSMA rise up to 318 K, above which it decreases rapidly. From these results, it may be concluded that the contributing factor to the engine output is the variation of  and VR with the variation of TSMA.