Investigations on design and development of copper based shape memory alloy bimorph and their applicatoin towards micro-robotics and energy systems

Abstract

The research work focuses on developing CuAlNi shape memory alloys thin films on pre-strained Kapton polyimide substrates through physical vapour deposition. Flexible substrate such as polyimide, is suitable for achieving two-way shape memory effect without any post processing and training. Investigations on the influence of substrate thickness and deposition temperature of polyimide were probed in detail. A substrate thickness of 75 μm was found to be suitable for potential applications, exhibiting large recovery ratio and stable life for 500 cycles. Growth substrate temperature of 150 °C and 100 °C was appropriate for developing the bimorphs, as martensitic textures were large in number with low anisotropic ratio. Further, the samples developed at 150 °C displayed larger displacement than rest of the bimorphs. A custom-made thermomechanical setup using Joule heating was utilized to analyse the thermomechanical and life cycle behaviour of the developed bimorphs. Various actuation properties such as voltage, frequency and load were varied to realize the bimorph for potential applications. Further, enhancement of life cycle from 14,000 cycles to 25,000cycles was observed when manganese was added as the quaternary element. Improvement in ductility with suppression of precipitates, displayed excellent shape memory properties by displacing up to 8 mm with 1 V. However, Mn addition led to loss in load bearing capacity, as the bimorphs failed to actuate beyond 60 mg loads. Further investigations with improving the force generated beyond 1.5 mN at varying frequencies were performed. The experimental results were compared with theoretical simulation on the structural and thermal behaviour of the bimorphs using COMSOL Multiphysics software. The developed bimorphs were implemented in MEMS applications such as micro-flapper for aerial robots at frequencies beyond 5 Hz. The utilization of bimorph as temperature sensor in transformer oils wasprobed, and a sensitivity of 0.2 °C /min was observed. Further, a bimorph belt was developed using customized flash evaporation system for developing SMA Johnson’s heat engine for energy harvesting applications.