The University of Tokyo, AISIN and other organizations jointly developed a solid refrigerant with the world's highest cooling performance

A research team led by Professor Shin-ichi Ohkoshi from the School of Science at the University of Tokyo (also affiliated with the Cryogenic Research Center) and AISIN CORPORATION, in collaboration with the University of Tsukuba, Osaka University, and MOLSIS Inc., has successfully developed a solid refrigerant featuring the world's highest adiabatic cooling and heating temperatures due to applied pressure. This new inorganic solid refrigerant is an inorganic compound called rubidium cyanide-bridged manganese-iron-cobalt (RbMnFeCo), which belongs to the Prussian blue material family. The team found that this material exhibits a barocaloric effect: applying pressure releases heat and raises the temperature, while releasing pressure absorbs heat and lowers the temperature.

The change from one phase of a material to another is called a phase transition. The material undergoes a rapid transition from a high-temperature phase to a low-temperature phase while maintaining its solid state when pressure is applied. Conversely, it returns from a low-temperature phase to a high-temperature phase when pressure is released. For example, the team found that this material exhibits large reversible adiabatic temperature changes of 74 K (from 57 ℃ to -17 ℃) at 340 MPa and 85 K (from 88 ℃ to 3 ℃) at 560 MPa.

Ohkoshi explained that these large reversible adiabatic temperature changes are the largest reported in the world in terms of the calorific effect of solid refrigerants (specifically solid-solid phase transition refrigerants). This material exhibits barocaloric effects even at low pressure; for example, it achieves a large value of 21 K for reversible adiabatic temperature change at 90 MPa.

Furthermore, using a thermocouple made of this material in a measurement device, a large temperature increase of 44 K (440 MPa) was observed as a result of the applied pressure. Even after repeating this process more than 100 times, no degradation in performance was observed.

Currently, 20% of the electricity generated at power plants is used for cooling air conditioners and refrigerators. Most of these cooling technologies use the expansion and compression of gas refrigerants. However, gas refrigerants have detrimental effects on the environment, particularly by contributing to global warming. Therefore, solid refrigerants that demonstrate a calorific effect are highly promising alternatives to gaseous refrigerants.

Among such refrigerants, pressure calorimetric (barolithic) effect materials, which exhibit a calorific effect when pressure is applied or released, have attracted much attention. In this study, the team developed RbMnFeCo Prussian blue by using RbMnFe Prussian blue (initially reported by Ohkoshi et al. in 2002) as the base material and substituting a certain percentage of Fe with Co. They discovered that this material exhibits a large reversible barocaloric effect, surpassing typical values by a huge margin.

RbMnFeCo Prussian blue can be mass-produced, which makes the material affordable, and it exhibits a wide temperature range of 142 K. Therefore, cooling and heating can generate a significant temperature difference in a single step, eliminating the need for a complex system in which refrigerants are combined in multiple steps. The group expects that this material can be used to realize a high-efficiency solid refrigerant.

For example, instant cooling from 100 ℃ to 25 ℃ and instant freezing from 25 ℃ to -50 ℃ are possible. In a new application, attaching this material to a piezoelectric substrate enables the realization of a compact solid-state refrigerant, which can be utilized for preventing device overheating.

Director Yoshihisa Yamamoto of AISIN, a company involved in the electrification of cars, mentioned that its application to electric vehicles will be considered, with plans for commercialization anticipated around 2030.

Journal Information
Publication: Nature Communications
Title: Giant adiabatic temperature change and its direct measurement of a barocaloric effect in a charge-transfer solid
DOI: 10.1038/s41467-023-44350-4