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

The overall goal of this experiment is to investigate elastocaloric materials and elastocaloric cooling processes. To this end, a scientific test rig has been developed to perform basic material characterizations and advanced elastocaloric process controls. Our method describes the influence of material properties and process control on the cooling power and efficiency of the cooling process.

The findings enable the development of optimized elastocaloric cooling processes, which are the basis for the design of efficient cooling devices. During fast adiabatic loading, the latent heats of the shape memory alloy lead to a temperature increase during loading, and a temperature decrease during subsequent unloading. The investigation of the elastocaloric cooling process is a collaboration between the material science group at Ruhr University in Bochum, and two groups from mechatronics engineering at Saarland University, Saarbrucken, Germany.

It involves the optimization of the materials as well as the processes, and also the development of a simulation tool. The developed scientific test rig enables the study of the elastocaloric effect for solid state heat transfer between the shape memory alloy and a heat source and the shape memory alloy and a heat sink. The main advantage of this technique is to the independent investigation of the influence of each control parameter on process values like work and heat.

Furthermore, this system is equipped into comprehensive system to measure mechanical and terminal quantities at every process step. To begin, use calipers to measure the shape memory alloy ribbon, and determine the cross section of the sample. Then, coat the sample with a thin layer of a high emissivity plate.

Next, set the target position in the motor controller program to zero micrometers, and click on the operation enabled button. At this position, the distance between the clamps is 90 millimeters. Place the sample between clamps of the experimental setup, and use a special designed alignment tool to align the sample.

Then, use a mounting aid to tighten the clamps, and a torque wrench to tighten the screws to a force of 20 newton-meters. The lineup of the semper is very critical. The pulley on semper will fail after just a few cycles.

Start the IR camera software, and load the calibration for a 50 millimeter lens combined with a closeup lens. Choose an image size of 1, 280 by 100 pixels, and a temperature range of minus 20 to 50 degrees celsius, and then use the motor focus unit to position the camera. Open the control program for training and material characterization.

Next, set the start position to zero micrometers and choose a target position of 4, 500 micrometers, so the material undergoes a complete phase transformation. Set the linear direct drive velocity to 45 microns per second, which is equivalent to a strain rate of five times 10 to the negative four per second. Next, set the holding time to zero seconds, the number of cycles to one, the IR camera acquisition rate to 50 milliseconds per frame, and then click on the start button to load the settings.

Now, open the IR camera software, choose a file name, and allocate 5, 000 frames. Switch from an internal to an external trigger source, and start the data acquisition mode. Then, open the control program and press the start experiment button to run the experiment.

To begin characterizing the material, open the control program for training and material. Then, set the start position so that the sample is under zero load to begin with, and set the target position equivalent to the target position of the training, which was 4, 500 micrometers. Next, set the strain rate as desired, and choose a linear direct drive velocity of 9, 000 micrometers per second, which leads to an adiabatic phase transformation for samples with a cross sectional area that is 0.75 millimeters by 1.4 millimeters or larger.

Set the holding time to 180 seconds to allow enough time for the sample to reach the desired initial temperature prior to the experiment. Then, set the number of cycles to one, the IR camera acquisition rate to 5 milliseconds per frame, and click on the start button to load the settings. Next, open the IR camera software, choose a file name, and allocate 80, 000 frames for the experiment.

Switch from an internal to an external trigger source, and start the data acquisition mode. In the control program, press the start experiment button to start the experiment. In order to investigate local temperature peaks, first switch off the light.

Then, remove all heat sources from the IR camera's field of view, and change the lens to a microscope lens. Next, change the camera calibration settings, load a microscope lens, and calibrate an image size of 500 by 250 pixels within the range of 20 to 50 degrees celsius. Use the motor focus unit to focus the sample.

Then, perform a standard tensile test at a linear direct drive velocity of 900 microns per second, as previously described. With the sample still in place start the IR camera software and load the calibration for the 50 millimeter lens with closeup lens. Choose an image size of 1, 280 by 1, 024 pixels, and a temperature range of negative 20 to 50 degrees celsius.

Open the control program and set the control parameters. Set the start position of the linear direct drive for the shape memory alloys so that the sample is under zero load. Also, set the target position equivalent to the target position of the training.

Set the velocity of the linear direct drive for loading and unloading of the shape memory alloy to 9, 000 micrometers per second. Then, set the velocity of the linear direct drive in the lower level of the setup to 100 millimeters per second. Next, set the contact time to six seconds.

Choose the contact after the loading and unloading mode and set the number of cycles to 40. Choose an IR camera acquisition rate of 20 milliseconds per frame, and then click on the start button to load the settings. In the IR camera software, choose a file name, and allocate 50, 000 frames for the experiment.

Switch from an internal to an external trigger source and start the data acquisition mode. Finally, open the control program and press the start experiment button. This will begin the elastocaloric cooling cycle.

In this movie clip, a nickel titanium ribbon is being strained during training. The controlled stretching leads to a mean temperature increase of 12.2 degrees Kelvin. The material follows a typical hysteresis curve and eventually settles into a response like that shown in red.

Shown here with a nickel titanium copper vanadium ribbon, the hysteresis width increases with increased strain rates. This is a result of the temperature change during the phase transformation. The diagram shows that after a certain point, there is no further increase of the temperature change in response to increasing strain rate.

This infrared video shows that by increasing the number of cooling cycles, the temperature differences between heat sink and heat source increase, and results in a decreasing minimum and maximum temperature change of the material. After the first cycle, an inhomogeneous temperature profile arises because the heat sink and heat source do not contact the entire ribbon. Here you can see the comparison between experiment and simulation of a tensile test.

The underlying model of the simulation is a modification of the thermomechanically coupled Mueller Achenbach Selleck Model. This shows that the model is able to reproduce the mechanical as well as the thermal behavior of the material. So while attempting the procedure, it's important to remember that the design requirements are monitoring all the cooling steps, and also a straight forward control of the variation parameters.

Thermal encapsulation will increase the efficiency of the process, however the observability would be greatly reduced. For the development of a real device afterwards, you would of course take it into consideration. Following this procedure, other process variations besides the adiabatic process control, like unadiabatic adiabatic combined processes can be performed in order to answer additional questions like the influence of the contact phase and on the process efficiency and cooling power.

After watching this video, you should have a good understanding of the elastocaloric cooling effect, and how materials optimization and process control influence the cooling power and efficiency of the process.