Understanding the Thermodynamic Properties of the Elastocaloric Effect Through Experimentation and Modelling

Lluis Mañosa,Jaka Tušek,Nini Pryds,Eduard Vives,Kurt Engelbrecht

doi:10.1007/s40830-016-0094-8

Abstract

This paper presents direct and indirect methods for studying the elastocaloric effect (eCE) in shape memory materials and its comparison. The eCE can be characterized by the adiabatic temperature change or the isothermal entropy change (both as a function of applied stress/strain). To get these quantities, the evaluation of the eCE can be done using either direct methods, where one measures (adiabatic) temperature changes or indirect methods where one can measure the stress–strain–temperature characteristics of the materials and from these deduce the adiabatic temperature and isothermal entropy changes. The former can be done using the basic thermodynamic relations, i.e. Maxwell relation and Clausius–Clapeyron equation. This paper further presents basic thermodynamic properties of shape memory materials, such as the adiabatic temperature change, isothermal entropy change and total entropy–temperature diagrams (all as a function of temperature and applied stress/strain) of two groups of materials (Ni–Ti and Cu–Zn–Al alloys) obtained using indirect methods through phenomenological modelling and Maxwell relation. In the last part of the paper, the basic definition of the efficiency of the elastocaloric thermodynamic cycle (coefficient of performance) is defined and discussed.

Highlights

The elastocaloric effect is associated with superelasticity of shape memory alloys (SMAs)
This paper presents direct and indirect methods for studying the elastocaloric effect in shape memory materials and its comparison
This paper further presents basic thermodynamic properties of shape memory materials, such as the adiabatic temperature change, isothermal entropy change and total entropy–temperature diagrams of two groups of materials (Ni–Ti and Cu–Zn–Al alloys) obtained using indirect methods through phenomenological modelling and Maxwell relation

Summary

Introduction

The elastocaloric effect (eCE) is associated with superelasticity of shape memory alloys (SMAs). A reverse process, namely, the endothermic martensitic–austenitic transformation occurs when the stress is released, which causes heat to be absorbed from the surroundings (for an isothermal process) or a temperature decrease of the material (for an adiabatic process). These thermal effects are related to the martensitic transformation and were detected already in the 1980s in single-crystal Cu-based alloys (Cu–Al–Ni and Cu–Zn–Sn) [1,2,3] and in 1990s in the poly-crystal Ni–Ti alloys [4].

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Methods

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Conclusion