Functionalized Thin-Film Shape Memory Alloys for Novel MEMS Applications
Shape memory alloys (SMAs) are a type of smart material that can undergo a stress or temperature-induced solid-to-solid phase transformation between two different crystalline phases. In nickel-titanium (NiTi) SMAs, the two phases are known as martensite with a monoclinic crystalline structure and austenite with a cubic crystal structure. The stress and temperature induced phase transformations can be used to switch between the martensite and austenite phases, and thus switch between two sets of material properties in the SMA. For example, in NiTi SMAs the Young’s modulus and coefficient of thermal expansion (CTE) of the austenite phase are typically 2X larger than that of the martensite phase. Thin-film NiTi SMAs are known to have a plethora of advantageous material properties associated with the phase transformation including recovery of large intrinsic strains, large work densities, and a high strength-to-weight ratio. These properties are leveraged for the development of micro and nano technologies such as sensors, actuators, and implantable medical devices. The thermal-induced phase transformation is often used for sensors and actuators based on sputtered bimorph NiTi/Si film composites. The stress-induced transformation allows for the crimping and deployment of self-expanding stents with sputtered freestanding, biocompatible, NiTi thin-films. The transformation temperatures, recovery strains, enthalpy of transformation, and fatigue properties of NiTi SMAs can be tuned by alloying NiTi with other elements like copper (Cu), cobalt (Co), and hafnium (Hf). For example, certain compositions of sputtered TiNiCu and TiNiCuCo are known to be ultra-low fatigue SMAs, able to reversibly undergo the phase transformation for 10+ million cycles without degradation in the mechanical or thermal properties. Even though thin-film NiTi-based SMAs are known to have large intrinsic strains of up to 8%, and low electrical resistivities, surprisingly, they have not yet been...
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