Enhanced Cooling of Shape Memory Alloy Wires Using Semiconductor "Heat Pump" Modules

Abstract

A novel concept of using semiconductor "heat pump" modules to heat and cool Nitinol shape memory alloy (SMA) wires under a zero-stress condition is analytically investigated in an effort to examine the feasibility of reducing the response time for complete thermal cycling. A heat transfer model is developed to provide both a qualitative and quantitative comparison between the effectiveness of semiconductor modules and other conventional modes of heating and cooling SMA wires, specifically, resistive heating/free convection cooling and resistive heating/forced con vection cooling. In the model, a complete thermal cycle is examined which includes both the forward and reverse phase transformations between martensite and austenite. The rate of temperature change as a function of energy transfer can be estimated during each stage of thermal cycling. A numerical integration program was developed to include both the energy transfers associated with the latent heat of transformation and the internal energy changes. Temperature profiles are presented for each of the heating/cooling methods as well as estimates of the total response time needed to complete an entire thermal cycle. Results of the heat transfer modeling indicate that the time required to cool the SMA represents the dominant part of the cycle time in the conventional methods of cooling. Both phase transformations (martensite-to-austenite and austenite-to-martensite) retard heating and cool ing rates). Compared to other existing methods of thermal cycling, semiconductor "heat pump" modules can potentially reduce heating/cooling cycle times by up to an order of magnitude.