Caloric materials are a class of materials that demonstrate interesting thermodynamic behaviour - when they are exposed to an external field they exchange heat with the environment. Whether they respond to a magnetic, electrical, or mechanical field determines the type of caloric material - with each displaying its own unique behaviour. These materials show great promise for use in next-generation cooling devices.
AZoM spoke to Dr Xavier Moya, of the University of Cambridge, about his team research into caloric materials, how they measure the changes in temperature of the material, and the benefits thermal imaging has delivered to his research.
What are magnetocaloric, electrocaloric and mechanocaloric effects?
These three different effects are thermal changes that occur in materials when an external field is applied. Each effect is named after the type of field that is involved.
Magnetocaloric effects are thermal changes that occur in a material when a magnetic field is applied, electrocaloric effects occur when an electrical field is applied, and mechanocaloric effects are driven by the application of mechanical stresses. These mechanical stresses can either be uniaxial stress or hydrostatic pressure from a gas or liquid.
Magnetocaloric materials undergo thermal changes when under the influence of a magnetic field. Shutterstock | Quality Stock Arts
Other than the field used to induce the effect are there any other significant differences between magnetocaloric, electrocaloric and mechanocaloric materials?
The different materials also differ in the strength of their response to the applied field.
For example, magnetocaloric materials work well and they have been studied extensively over the past 20 years, however for them to show a large temperature change a very large magnetic field must be applied. This makes it expensive and challenging to use magnetocaloric materials in real world applications.
As a result research into electrocaloric and mechanocaloric materials is blossoming and we’re still finding out a lot about them.
How and why do these effects occur?
Application of an external field can be used to drive a phase transition in a material. A phase transition results in a change in the internal structure of the material which, in turn, alters its thermodynamic properties. This change in thermodynamic properties means that the temperature of the system changes and heat can be exchanged.
A simple example of this would be the thermal changes that occur in the natural rubber found in party balloons. In the balloons resting state the rubber polymer chains are disordered and scrambled together. However, when the balloon is stretched the polymer chains are all pulled tightly and straighten up along the direction of the stretching force.This represents a change from a disordered system to an ordered system and, as the order of the system is changed there is a change in entropy which results in a heat transfer.
The process is similar for an electrocaloric material – a phase change (for example, paraelectric to ferroelectric) occurs when applying an electric field, and there is a transfer of heat.
The application of an electric field to an electrocaloric material results in a phase change, which is associated with a transfer of heat. Shutterstock | pixelparticle
What practical applications can caloric effects be used in?
The traditional application that has been generating a lot of interest is for use in solid state cooling applications.
Current cooling technology, such as that found in fridges and air conditioning systems, exploits the changes in temperature when gases are compressed and expanded. This technology often uses environmentally damaging greenhouse gases. By developing solid state cooling systems, which use solid materials rather than greenhouse gases to achieve cooling, the environmental impact can be reduced.
Most of the current research being undertaken is on the development of new materials for use in solid-state cooling systems. There are also groups, including ours, that are developing prototype and proof-of-principle coolers based on these solid-state materials.
Solid-state cooling technology will still take some time to develop to a point that may compete with fridges and air-conditioners that use vapour compression.
How method do you use to measure caloric effects?
There are three quantities which are important when characterizing caloric materials.
The first of these is the temperature change that can be induced in the material by rapidly applying the external field in adiabatic conditions. The remaining two, which are directly related to each other, are the entropy change that results from the field application in isothermal conditions, and the heat that is exchanged.
These three quantities are all connected and all of them can be calculated if just one value is known. However, for the sake of accuracy, it is best practice to measure the temperature and the heat independently as the conversion can sometimes introduce error; especially for materials that have abrupt phase transitions.
To measure heat, we use calorimeters that use heat-flux sensors to determine the heat transfer that occurs when our material is exposed to an external field.
Accurately measuring the temperature is a little harder. Typically, traditional thermometers are used to measure temperature however this requires physical contact with the material. The thermometer then acts as a heat sink – drawing heat away from the material and reducing the temperature change – which introduces error into our measurements.
This effect is not a big problem when dealing with large samples, but when the material and the thermometer have similar sizes the effect can significantly affect the measurements accuracy. This is why non-contact thermometry techniques are so useful. There are two main non-contact thermometry techniques that we can use in the lab. One uses infrared imaging and the other, scanning thermal microscopy, uses a very small thermometer at the tip of a scanning probe.
We prefer to use infrared imaging. Whilst scanning thermal microscopy is a useful measurement there is a lack of spatial resolution. Essentially you can only measure the temperature change of a single point in the material. This is in stark contrast to infrared cameras which allow the simultaneous measurement of an entire sample, i.e. complete spatial resolution. In addition to this, as video recording is possible, you can also have full temporal (time) resolution.
Infrared imaging is more effective than thermal microscopy as it provides spatial data. Shutterstock | vrx
To avoid the problem of significant heat loss upon thermometer contact why don’t you use a larger sample of your material?
Due to the nature of electrocaloric materials this is not an option for us. In electrocaloric materials the change in temperature is directly proportional to the strength of the electric field applied to the material. The electric field is related to both the voltage applied and the materials thickness; meaning that to have a very large electric field you typically need a very thin material.
We’re confident that infra-red imaging is the best method of determining temperature changes in electrocaloric materials. It avoids any problems involving physical contact between the measurement device and the sample and, additionally, it also allows us to visualise the flow of heat in the material which is useful in itself.
Why did you choose the FLIR SC7200 over other IR cameras that are available?
The main reason we chose this model was its ability to rapidly capture images due to its fast frequency of operation.
As part of our research we have to measure a range of different sample sizes – from large and bulky samples to very thin samples. With the very small samples the transfer of heat is extremely rapid so a fast camera is required to accurately capture the change in temperature.
The FLIR SC7200 has exceeded expectations in this regard. We’ve ran tests using thin film samples which are as thin as one micron and we’ve still been able to accurately measure the temperature change.
The FLIR SC7200
Have you found the camera easy to use? How simple is the processing of data from the camera?
We’ve found the camera really easy to use. We frequently have visiting researchers in our lab, and they would pick up on how to use the camera really quickly. This was great as it meant it wasn’t long before they were gathering good results.
As for the data – the quality is great and it is simple to extract and analyse.
What developments can see your research leading to?
At the moment we’re currently extending our work on mechanocaloric materials as we believe they will help us overcome the limitations on magnetocaloric and electrocaloric materials, which are limited by the requirement of either a large and expensive magnetic field, or an extremely thin sample, both of which are impractical for a cooling device.
Mechanocaloric materials have a few other advantages the most obvious being that only a mechanical force is required to drive the effect, which is straightforward to generate. I believe that mechanocaloric materials could play a part in some important technologies in the future.
In terms of our developments we’re currently looking at some mechanocaloric materials and the testing is looking very promising. We’re going to continue to explore these materials further to come up with an engineered and energy-efficient cooling solution.
Where could our readers find out more about your research?
You can find out more about my work on my Cambridge University research page.
About Dr. Xavier Moya
Xavier Moya is a Royal Society University Research Fellow at the Department of Materials Science & Metallurgy, University of Cambridge. He received a BSc in Physics in 2003, and a PhD in Physics in 2008 from the University of Barcelona and has been a Fellow of Churchill College since 2014.
Dr. Moya was the recipient of the Ramon Margalef Prize 2009, and the Young Researcher in Experimental Physics Prize 2015.
His interests are in the phase transitions in functional materials whose structural, magnetic, electrical and thermal properties display strong coupling with his research focusing specifically on caloric materials for cooling applications and magnetoelectric materials for data storage.
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