Multicalorics

Magnetocaloric, electrocaloric, and mechanocaloric effects are nominally reversible thermal changes that occur in magnetically, electrically, and mechanically responsive materials when subjected to changes in applied magnetic, electric, and mechanical field, respectively.¹ These caloric effects are typically large near phase transitions and analogous to the pressure-induced thermal changes in fluids that have been exploited for many decades in refrigeration and air-conditioning systems. However, caloric effects promise high energy efficiencies without greenhouse gases.² 

The growing research area of multicalorics involves more than one type of caloric effect (Fig. 1) and could bring further improvements in terms of energy efficiency and operating temperature span. The “Multicalorics” Special Topic in Journal of Applied Physics is divided into magnetocaloric, electrocaloric, and mechanocaloric effects, and the papers that we include describe fundamental aspects all the way through to applications in heat pumps.

A wide range of magnetocaloric materials is featured in the “Multicalorics” Special Topic, namely, Heusler alloys,^3–8 MnNiGe-based alloys,^9,10 Fe-Rh alloys,¹¹ Nd-Tb-Co alloys,¹² Laves-phase alloys,¹³ and rare-earth alloys.^14,15 The effects of kinetics and inhomogeneity are investigated in Refs. 16–18, and direct measurements of temperature change are discussed in Ref. 19. The growing body of work on magnetocaloric films is represented here by the growth of thick La-Fe-Si films with and without post-hydriding,²⁰ the enhancement of the operating-temperature range in La_0.8Sr_0.2MnO₃/La_0.7Ca_0.3MnO₃ bilayers,²¹ and the report of magnetocaloric effects in Co-Fe-B/Ni-Cu/Co-Fe trilayers.²² Modeling papers describe magnetocaloric effects in Heusler alloys,²³ Fe-Rh alloys, heavy rare earths, and antiperovskite nitrides.²⁴ 

The electrocaloric materials featured in the “Multicalorics” Special Topic comprise a number of bulk ceramics^25–27 and a thick polymer film.²⁸ The electrocaloric response of bulk ceramics^29,30 and ceramic nanowires with graded composition³¹ are modeled and discussed. Mechanocaloric materials are also featured via the observation of large elastocaloric effects driven by uniaxial stress in a Ti_52.8Ni_22.2Cu_22.5Co_2.5 alloy with ultra-low fatigue,³² and the prediction of large barocaloric effects driven by uniaxial pressure in a Pd-In-Fe alloy.³³ 

Applications of caloric materials are presented in papers that model hysteresis in cooling cycles,³⁴ that model active caloric regenerators,³⁵ and that model thermal diodes.³⁶ There is also an overview of actuation technologies,³⁷ and a report on a prototype that is based on elastocaloric effects in natural rubber.³⁸ 

Research on calorics is timely because it can lead to environmentally friendly solutions for heating and cooling. The collection of works described in the “Multicalorics” Special Topic provides a glimpse of the growing activity on caloric materials and prototypes (Fig. 2), and we hope that the featured works will stimulate further research and development on this area.