Shape memory alloys (SMA) have been an extensively used material for actuators in micro-electromechanical systems (MEMS) because actuation force and displacement are greatest in SMA amongst many actuator materials [1]. Of the alloys currently available for SMA actuators, the most popular system is Nitinol (or NiTi) due to its good oxidation resistance, reversible martensitic transformation, broad range of transformation temperatures (from -100 - 100°C), and specific power density [2]. Current commercially available SMA wire has easily achieved no-load strain of 5% with medium gage SMA wires demonstrating an axial force capacity of 2 Newtons or more. While the potential use of SMA materials in a thermal-electric motor has been documented beginning in the 1980's, there are a number of new allows and fatigue-resistant materials that may lead to more general designs with a wide range of motions and applications. Shape memory alloys are a special type of material that exhibit two unique properties, pseudo-elasticity and shape memory effect (SME). SMA undergoes SME because of martensitic or diffusionless transformation where each atom has a slight displacement, creating observable changes throughout the structure as the allow changes states. This alloy has the ability, once heated, to return to its parent austenite phase where it exists at higher symmetry. Upon cooling, the material returns to one of many lower symmetry martensitic phases. This thermal cycle is shown in Figure 1. [3,4]. It is even possible for many variants of martensite to be present in the same material. Pseudo- elasticity is a rubber-like flexibility that allows the SMA to be contorted for a variety of purposes. Once contorted, the application of heat will cause the alloy to undergo martensitic transformation. Upon completion of the cycle, the alloy will have returned to its original shape.
展开▼
