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Materials to cool the future

Posted on 08/11/2016

A great deal of effort has gone into developing vapour-compression alternative technology to reduce artificial cooling environmental impact and expand its benefits.

Cooling and heating of buildings is responsible for 36% of global warming emissions

“Clearly, progress in the field of electrocaloric coolers is determined not only by development of new working media, but also by the ability to traceably and accurately measure electrocaloric properties in materials.”

Cooling and heating of buildings represent almost 40% of total EU27 energy consumption and are responsible for 36% of global warming emissions. A great deal of effort has gone into developing vapour-compression alternative technology to reduce artificial cooling environmental impact and expand its benefits. On the other hand, industries, such as electronics, automotive, healthcare and oil-gas, recognise the development of innovative cooling as a crucial part of their systems design. A good example is the ever-expanding microelectronic industry. The demand for higher levels of integration of electronic circuits urgently requires easily integrated thermal management solutions in order to improve their reliability and avert premature failure.

Thermoelectric, magnetic and electrocaloric cooling

Solid-state refrigeration, e.g. thermoelectric and magnetic cooling, can play a transformative role in these billion dollar industries by offering a “clean” and effective cooling solution. But despite the unquestionable and impressive development over the last six decades, solid-state refrigerants have yet a long way to go before reaching the wider refrigeration market. Their still very limited commercial viability is due to the fact that they provide low levels of efficiency (maximum 13% for thermoelectrics) and/or expensive cooling, as magnetic refrigeration requires powerful and expensive magnets. The electric analogue of magnetic cooling – electrocaloric cooling – which capitalises on the ability of some materials to adiabatically change their temperature when an electric field is varied, appears as a very promising alternative to “cool the future”, as it were. The underlying physical phenomenon, electrocaloric effect, has been known for more than a century.

The “Giant Electrocaloric Effect”

Nonetheless, electrocaloric cooling has as yet never been achieved owing to the fact that only a small temperature change has ever been measured in bulk materials. In 2006, a team of UK researchers discovered giant electrocaloric effect in ferroic thin films, translated as temperature variation up to 10 times higher than that ever measured in bulk materials. This breakthrough inspired researchers to focus on the development of new electrocaloric materials with enhanced cooling performance, thus potentially creating significant new markets for solid-state coolers.

However, the exploitation of electrocaloric effect in refrigeration applications is still in its early stages, and a number of fundamental and technical features have yet to be resolved. Thus advances in materials, which support the absorption of large amounts of heat from a cold reservoir, have been established as being a clear priority. As such, we have witnessed impressive progress over the last seven years, with improvement of electrocaloric temperature change from 2.5 K (in ceramics) to 40 K (in thin films). Unfortunately, discrepancies are all too often reported in electrocaloric effect measured on account of different measuring methods and/or experiments being carried out in different labs. The lack of traceability in existing techniques and procedures does not make for a great measure of certainty regarding the electrocaloric cooling performance of materials, and may have led to unquantified parasitic contributions in the measurement of electrocaloric heat/temperature change. Clearly, progress in the field of electrocaloric coolers is determined not only by development of new working media, but also by the ability to traceably and accurately measure electrocaloric properties in materials.

Defying thermodynamics…

Most measurements of electrocaloric effect are indirectly derived from measurement of pyroelectric effect rather than from directly measured heat or temperature change. This method, which assumes thermodynamic Maxwell relation, is relatively straightforward and has provided accurate predictions of electrocaloric response for a number of materials. But its universality and validity has been recently brought into question following reports revealing discrepancies in electrocaloric data measured directly and indirectly in relaxor materials. It may well be that indirect measurements introduce spurious caloric effects such as hysteresis with the result that irreversible processes are not taken into account in Maxwell relation. Relaxor materials, and other materials with coexisting phases (e.g. materials in the vicinity of morphotropic phase boundaries), are not in equilibrium, which is not consentaneous with the ground assumptions of Maxwell relations.

Measuring electrocaloric effect in thin films

Thin films appear today as promising candidates for cooling applications, as large electrocaloric effect can be exploited in these materials. Yet, their characteristic small thermal mass and the inexistence of reliable scale-up deposition methods are currently derailing their commercial exploitation. A no less important limiting factor is the lack of adequate metrological capabilities to provide accurate data of cooling performance of electrocaloric thin films. Direct measurement of electrocaloric heat or temperature change in thin films is, though, extremely challenging due to their small heat capacity, deviations from adiabatic conditions and heat transfer to temperature measurement devices, substrates and connections. For this reason, electrocaloric effect in thin films has been mostly indirectly measured using Maxwell relations, which do not constitute a traceable and accurate approach, as previously discussed.

A few attempts have been made to directly measure electrocaloric heat generated in thin films constrained to a thick substrate using high-resolution calorimetry. This method relies on modelling and geometrical assumptions to subtract the thermal contribution of the thick substrate, introducing significant uncertainties in the resulting measurement. Fast thermometry techniques, like infrared thermometry and thermocouples, have also been used to measure transient electrocaloric effect so as to mitigate the characteristic fast heat transfer relaxation to the substrate. However, traceability and uncertainty budgets for these metrological capabilities have thus far never been set.

Benchmarking electrocaloric materials

The assessment of electrocaloric materials for cooling purposes has usually been carried out taking only into account electrocaloric heat or temperature variation, either through direct or indirect methods. But a more comprehensive analysis of electrocaloric materials should be undertaken, wherein crucial criteria, such as electrocaloric effect, are further complemented by other relevant parameters, namely, operating temperature span, hysteresis, heat flow timescale, cost and environmental issues, with view to developing a powerful, cost-efficient and environmentally friendly cooling device. Hence, it is of the utmost urgency to identify these critical parameters and summarise them in a Figure-of-Merit for electrocaloric materials.

The Knowledge Transfer Network recognises the importance of materials to the new generation of coolers. We would like to know your views about hosting an event aiming at bringing together end-users, materials developers, system integrators and metrologists to discuss opportunities and challenges of solid-state cooling.

 

Dr Tatiana Correia – Knowledge Transfer Manager, Nanotechnology