Experimental Methods for Investigation of Shape Memory Based Elastocaloric Cooling Processes and Model Validation

Summary

Experimental methods for investigation of solid state cooling processes and characterization of elastocaloric material properties of Shape Memory Alloys (SMA) are presented. A custom-built test rig has been designed for controlling and comprehensive monitoring of elastocaloric cooling processes. Furthermore, it provides a validation platform for thermomechanically coupled modeling approaches.

Abstract

Shape Memory Alloys (SMA) using elastocaloric cooling processes have the potential to be an environmentally friendly alternative to the conventional vapor compression based cooling process. Nickel-Titanium (Ni-Ti) based alloy systems, especially, show large elastocaloric effects. Furthermore, exhibit large latent heats which is a necessary material property for the development of an efficient solid-state based cooling process. A scientific test rig has been designed to investigate these processes and the elastocaloric effects in SMAs. The realized test rig enables independent control of an SMA's mechanical loading and unloading cycles, as well as conductive heat transfer between SMA cooling elements and a heat source/sink. The test rig is equipped with a comprehensive monitoring system capable of synchronized measurements of mechanical and thermal parameters. In addition to determining the process-dependent mechanical work, the system also enables measurement of thermal caloric aspects of the elastocaloric cooling effect through use of a high-performance infrared camera. This combination is of particular interest, because it allows illustrations of localization and rate effects — both important for efficient heat transfer from the medium to be cooled.

The work presented describes an experimental method to identify elastocaloric material properties in different materials and sample geometries. Furthermore, the test rig is used to investigate different cooling process variations. The introduced analysis methods enable a differentiated consideration of material, process and related boundary condition influences on the process efficiency. The comparison of the experimental data with the simulation results (of a thermomechanically coupled finite element model) allows for better understanding of the underlying physics of the elastocaloric effect. In addition, the experimental results, as well as the findings based on the simulation results, are used to improve the material properties.

Introduction

Solid state cooling processes based on ferroic materials have potential to be environmentally friendly alternatives to the conventional vapor compression based process. Ferroic materials may exhibit magnetocaloric, electrocaloric and elastocaloric effects ^1,2, as well as combinations of these effects, which are described as multicaloric material behavior ³. The different caloric effects in ferroic materials are currently being investigated as part of the German Science Foundation (DFG) Priority program SPP 1599 "Caloric Effects in Ferroic Materials: New Concepts for Cooling" ⁴. Shape Memory Alloys (SMA) which are investigate....

Protocol

1. Sample Preparation

1. Measure the SMA ribbon with calipers and determine the cross section of the sample.
2. Prepare the sample for IR measurements by coating the ribbon with a thin layer of high emissivity (ε=0.96) paint.
   Caution: The paint is classified as an irritant. Gloves, safety glasses and mouth protection must be worn during the processing of the paint.

2. Material Stabilization (Training)

Note: Initial mechanical cycling leads to a mechanical and thermal material stabilization. The investigation of the stabilization effect, and the training procedure itself, requires....

Results

Material stabilization (Training):

Figure 9 shows a stress/strain diagram of 50 training cycles. The investigated sample is a Ni-Ti ribbon with a cross section of A = 1.45 mm². The applied strain rate of 1 x 10^-3 sec^-1 leads to a mean temperature increase of ΔT = 12.2 K. The temperature increase has a significant influence on the stabilization effect ^12-

Discussion

The presented scientific test rig enables comprehensive investigation of elastocaloric materials and cooling processes by performing the experiments described in the protocol section. Precise alignment of the sample before clamping is crucial for all the experiments. Bad alignment can potentially lead to early material failure. Furthermore, the maximum applied strain has significant influence on the material lifetime, whereas the required strain to reach a complete phase transformation depends on the alloy composition. T.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Linear direct drives	ESR-Pollmeier	ML 1418-U5-W1	SMA loading/unloading; heat transfer
Pneumatic cylinder	Festo	ADNGF-40 574031	Contact between heat source/sink and SMA
Inductive position measurement system	AMO	LMKA-1101.1NN-1.0-0
Tension and compression load cell	Futek	LCF451; FSH02241	SMA force
Compression load cell	Futek	LTH300; FSH00297	Contact force
IR camera	Infra Tec	Image IR 9360; M91129	1,280 x 1,024 pixels; Maximum frame rate 3,200 Hz
Real-Time Controller	National Instruments	NI CompactRIO-9074	Data acquisiton and control system
Camera varnish	Tetenal	105202