Researchers in China have shown that applying strain to a composite material using an electric field induces a large and reversible caloric effect. This novel way of enhancing the caloric effect without a magnetic field could open new avenues of solid-state cooling and lead to more energy efficient and lighter refrigerators.

The International Institute of Refrigeration estimates that 20% of all electricity used globally is expended on vapour-compression refrigeration – which is the technology used in conventional refrigerators and air conditioners. What is more, the refrigerants used in these systems are powerful greenhouse gases that contribute significantly to global warming. As a result, scientists are trying to develop more environmentally friendly refrigeration systems.

Cooling systems can also be made from completely solid-state systems, but these cannot currently compete with vapour compression for most mainstream applications. Today, most commercial solid-state cooling systems use the Peltier effect, which is a thermoelectric process that suffers from high cost and low efficiency.

External fields

Solid-state cooling systems based on caloric materials offer both high refrigeration efficiency and zero greenhouse emissions and are emerging as promising candidates to replace vapour compression technology. These systems employ a solid material as a refrigerant, which when subjected to an external field (electric, magnetic, strain or pressure) undergoes a change in temperature – a phenomenon called the caloric effect.

So far, most research into solid-state caloric cooling systems have focussed on magnetic refrigerants. However, practical refrigerants must exhibit a significant caloric effect near room temperature, and such materials are generally hard to find. One potential material is Mn₃SnC, which displays a significant caloric effect when exposed to magnetic fields greater than 2 T. But employing such a high magnetic field necessitates the use of expensive and bulky magnets, which is not practical.

Now, Peng Wu and colleagues at ShanghaiTech University, Shanghai Institute of Microsystem and Information Technology, University of Chinese Academy of Sciences and Beijing Jiaotong University, have eliminated the need for magnets by combining a Mn₃SnC layer with a piezoelectric layer of lead zirconate titanate (PZT).

Doing away with the magnets

In a series of experiments described in Acta Materialia, the team observed a reversible caloric effect without the need for a magnetic field. The adiabatic temperature change achieved was around double that measured for Mn₃SnC in the presence of a 3 T magnetic field.

The caloric effect was observed by applying an electric field to the material, which induces strain in the PZT via the reverse piezoelectric effect. The strain is transferred from the PZT layer to the Mn₃SnC layer, which results in a change in magnetic ordering of the Mn₃SnC. This causes a temperature drop of up to 0.57 K in the material. When the electric field is removed, the temperature increases by the same value.

Wu tells Physics World, that he got this idea from  microelectromechanical systems (MEMS), which often use piezoelectrical materials for actuation. According to Wu, using electric-field mediated strain could help eliminate the need for costly and large magnets, creating a more efficient and sustainable refrigeration system.

Challenging measurement

The caloric effect is measured either by estimating the adiabatic change in temperature or the isothermal entropy change. In both industry and research, temperature change, is the preferred method. While this is a straightforward experiment for pure bulk materials, it is extremely difficult to do for a device-based composite material that is subject to an electric field.

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To make the measurement, Wu and colleagues used a system equipped with a thermocouple probe attached to the Mn₃SnC surface in an adiabatic environment with precisely controlled magnetic field and temperature.

To assess the accuracy of their measurement system, the researchers carried out several magnetocaloric effect measurements in the temperature range of 275–290 K. They were able to monitor temperature changes down to 0.03 K, hence verifying the system’s high resolution temperature capacity.

Wu believes the team’s work is a breakthrough in measuring temperature change directly, given the challenge of making an adiabatic temperature measurement while applying a voltage to the PZT. He adds, “This approach of temperature measurement could be useful for other thermal electronic devices”. However, Wu stresses “the system is not completely adiabatic; it may cause heat loss, hence further improvement is necessary for any heat measurements”.

Interesting and unexplained

The team also observed some very interesting and unexpected phenomena during the temperature measurement. “No matter whether one applies a positive or negative electric field, the surface temperature of Mn₃SnC always decreases”, Wu says. They also found that by applying a magnetic field to the composite, the surface temperature of Mn₃SnC rises, whereas applying an electric field does the opposite and causes a reduction in the temperature. Wu says that the team does not yet understand these observations.

The researchers now aim to study the underlying physics behind the contrasting behaviour of Mn₃SnC/PZT under magnetic and electric fields. To further improve the temperature measurement system, they are also trying to solve the problem of heat loss.