A research group from Kyoto University, AIST and others discovers new oxide energy material to power highly efficient cooling system

We may be on the cusp of developing a cooler that runs on pressure. A research group led by doctorate student Yoshihisa Kosugi, Assistant Professor Masato Goto, doctorate student Zhenhong Tan, Associate Professor Daisuke Kan and Professor Yuichi Shimakawa of the Institute for Chemical Research, Kyoto University, research team leader Asaya Fujita of the Magnetic Powder Metallurgy Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Associate Professor Takashi Saito and Professor Takashi Kamiyama from the Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), in collaboration with research teams from Taiwan National University and the Taiwan National Synchrotron Radiation Research Center (NSRRC), have proven the potential for highly efficient thermal control, demonstrating the colossal barocaloric effect of the A-site-ordered perovskite-structure oxide, NdCu₃Fe₄O₁₂. The results were published in Advanced Functional Materials.

Some 25-30% of the world's electricity is used for cooling to counter the effects of global warming, including large-scale refrigeration or freezing in food storage, or to keep servers and computers cool. With this in mind, the calorific effect of heat storage and heat release of materials through external field change is gaining much attention.

Reduction of entropy releases heat, while increasing entropy absorbs heat. Therefore, if it were possible to bring about solid-state barocaloric effect heat storage or heat release with pressure, it would have the major benefits of alleviating the need for compressors and allowing equipment to be made smaller, as well as being much more efficient than gas compression cooling. To date, however, basically no calorific effect materials have been commercialized.

The research group turned its attention to the A-site-ordered perovskite-structure oxide NdCu₃Fe₄O₁₂, as a candidate material demonstrating significant caloric effects and that is tightly bound by spin (magnetism), electrical charge and latticing. NdCu₃Fe₄O₁₂ is a material in which Neodymium (Nd) can be used as a replacement for Lanthanum (La) in La Cu₃Fe₄O₁₂. La Cu₃Fe₄O₁₂ was found in 2009, when a research group from the Kyoto University Institute for Chemical Research discovered the new phenomenon of intersite charge transfer accompanied with its negative thermal expansion behavior. The intersite charge transfer of NdCu₃Fe₄O₁₂ is triggered near room temperature.

This study found an enormous release of latent heat in the first-order intersite-charge-transfer transition (transfer of ion charge between copper and steel): 25.5 kilojoules per cubic centimeter. The entropic change of this latent heat release is a significant 84.2 J kg−1 K−1 (joules per kilogram per kelvin), comparable to the largest changes reported in inorganic solid materials, making it the highest ever reported near room temperature.

What is the reason for such large entropy change? The research group made a detailed analysis using the Special Environment Neutron Powder Diffractometer (SPICA) at J-PARC. They found it was caused by a change in magnetic entropy as a result of the spin alignment of the anomalous ferrous ion.

By further adding pressure to NdCu₃Fe₄O₁₂, the group proved that it is possible to efficiently use almost all of the enormous latent heat. As a result of this barocaloric effect, applying about 510 MPa pressure achieved significant adiabatic temperature change of around 13.7℃ near room temperature.

Shimakawa states that, "Swapping materials - such as neodymium for gadolinium or samarium - allowed us to adjust the temperature of interstate charge transfer. This means we can build cooling systems that are able to operate in a wide temperature range. In addition, pressure can be applied using screws. In future, we plan to design a cooling system using these materials and develop it for the commercial cooler demonstration stage."