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The second law of thermodynamics dictates that energy is required to pump heat, which is problematic because human-engineered heating and cooling now represent the largest global source of carbon emissions [
1]. Heating and cooling are primarily achieved via the combustion of natural gas and the compression of harmful gases, respectively [
2]. Alternative heating and cooling methods are highly desirable, and it has been suggested that caloric heat-pump devices could be energy efficient, environmentally friendly, or useful in other ways by being small and quiet. Progress on caloric materials, measurements, and devices (Fig.1) was discussed at a recent conference, which is intended to run biennially (Calorics 2022, Cambridge, UK, 12–14 September 2022).
Caloric effects are nominally reversible field-driven thermal changes that may be parameterized in the isothermal limit (as entropy change Δ
S or heat
Q) or the adiabatic limit (as temperature change Δ
T), with
Q =
TΔ
S ~ -
cΔ
T (
c denotes specific heat capacity). Caloric effects are sub-divided into magnetocaloric, electrocaloric, and mechanocaloric effects that arise in magnetically, electrically, and mechanically responsive materials owing to changes in the magnetic, electric, and mechanical fields, respectively [
3–
15]. Therefore, three main types of caloric effects can be identified. Most of the mechanocaloric effects that have been studied to date are sub-divided into those driven by uniaxial stress (elastocaloric effects) and those driven by hydrostatic pressure (barocaloric effects). Caloric effects can be driven simultaneously or sequentially by more than one type of field (multicaloric effects) [
7–
9,
16,
17], and can be large at finite temperatures if they are driven near phase transitions, implying the possibility of heating and cooling applications. However, further studies on both materials and devices are required to improve device temperature span, cooling power density, and energy efficiency.
The majority of materials that show giant magnetocaloric effects exploit first-order magnetostructural phase transitions. Magnetic hysteresis can be transmuted into mechanical hysteresis (Vitalij Pecharsky, Ames Laboratory), and electron correlations and spin fluctuations influence latent heat (Asaya Fujita, National Institute of Advanced Industrial Science and Technology). Analytical tools can be used to determine the order of magnetocaloric phase transitions, identify critical exponents, and evaluate tricritical compositions (Victorino Franco, University of Seville). Magnetocaloric effects in Ni-Mn-based thin films and disks appear to be promising (Simone Fabbrici, Institute of Materials for Electronics and Magnetism, Parma).
In recent decades, magnetocaloric materials have been used to pump heat near room temperature, e.g., 293 K. For example, a cooling power of 1 kW can be achieved in a magnetocaloric heat pump that is based on either the second-order phase transition in gadolinium or the first-order phase transition in La-Fe-Si-Mn-H (Christian Bahl, Technical University of Denmark). Thermal diodes, thermal switches, and thermal modeling with new open-source software could improve heat-pump performance (Andrej Kitanovski, University of Ljubljana). Encouragingly, the development of a magnetocaloric wine cooler (Max Fries, MagnoTherm Solutions, Darmstadt) has overcome various challenges involving heat transfer, corrosion, and mechanical stability.
Many of the electrocaloric materials under study are perovskite oxides. Multilayer capacitors of these materials combine multiple thin layers to which large electric fields can be applied without breakdown. In what follows, we consider multilayer capacitors to be ‘materials’, and we reserve ‘devices’ to describe heat pumps. At higher temperatures, electrocaloric multilayer capacitors of ferroelectric PbSc0.5Ta0.5O3 and antiferroelectric PbMg0.5W0.5O3 show conventional electrocaloric effects (field application increases the temperature); and at lower starting temperatures, the multilayer capacitors of PbMg0.5W0.5O3 exhibit inverse electrocaloric effects (field application decreases the temperature) (Sakyo Hirose, Murata Manufacturing, Kyoto). Ceramic samples of ferroelectric 0.9PbMg1/3Nb2/3O3-0.1PbTiO3 were prepared via mechanochemical synthesis, and a mixture of ferroelectric PbFe0.5Nb0.5O3 and antiferromagnetic BiFeO3 yielded samples that display both electrocaloric and magnetocaloric effects (Hana Uršič, Jožef Stefan Institute). Electrocaloric materials have recently been investigated via various theoretical methods; for example, molecular dynamic simulations have been used to optimize microstructure (Anna Grünebohm, Ruhr-University Bochum), and local properties have been elucidated using phase-field modeling (Jiamian Hu, University of Wisconsin-Madison).
An increasing number of electrocaloric heat pumps are currently being developed. Multilayer capacitors of PbSc0.5Ta0.5O3 (Sakyo Hirose, Murata Manufacturing, Kyoto) were used in electrocaloric devices that exploit regenerative fluid (Alvar Torelló, Luxembourg Institute of Science and Technology) or evaporative fluid (Kilian Bartholomé, Fraunhofer Institute for Physical Measurement Techniques IPM, Freiburg im Breisgau). The same multilayer capacitors have also been used to achieve improved energy-harvesting performance (Alvar Torelló, Luxembourg Institute of Science and Technology). Good efficiency in any electrocaloric device requires as much as possible of the electrical work done to be recovered, and 88% recovery was achieved using electrocaloric multilayer capacitors (Morgan Almanza, University of Paris-Saclay).
Many studies have been conducted on elastocaloric polycrystals and single crystals. Novel dynamic methods can probe low-temperature elastocaloric effects in various quantum materials: nematic fluctuations were observed in TmVO4 (Ian Fisher, Stanford University); and detailed entropy-temperature-strain maps were constructed for superconducting Sr2RuO4 (Andreas Rost, University of St Andrews). For elastocaloric single crystals that operate near room temperature, oriented precipitates in Ni-Ti and tweed textures in Fe-Pd influence the caloric response (Antoni Planes, University of Barcelona). Latent heat measurements of elastocaloric materials at finite stress will be useful for the evaluation of caloric properties (Stefan Seelecke, Saarland University).
Elastocaloric heat pumps typically exploit Ni-Ti under compressive or tensile stress, but copper-based alloys are also promising (Jun Cui, Ames Laboratory/Iowa State University). For example, tension can be used to drive elastocaloric effects in Ni-Ti wires (Stefan Seelecke, Saarland University), and compression can be used to drive elastocaloric effects in Ni-Ti tubes that exist either in bundles (Jun Cui, Ames Laboratory/Iowa State University) or in a shell-and-tube-like configuration (Jaka Tušek, University of Ljubljana). If compressive stress can be applied without buckling then it is possible to achieve lifespans (105 cycles) that greatly exceed the lifespans associated with tensile stress (Kurt Engelbrecht, Technical University of Denmark). Interestingly, an elastocaloric heat pump based on Ni-Ti alloys has been developed outside academia (Mike Langan, Exergyn Ltd., Dublin).
Barocaloric effects occur in various types of material. For example, hybrid organic–inorganic perovskites show colossal barocaloric effects near room temperature (Pol Lloveras, Polytechnic University of Catalonia); plastic crystals exhibit colossal barocaloric effects that can be modeled using Landau theory (Gian Guzmán-Verri, University of Costa Rica); and manganese antiperovskites can be alloyed to reduce hysteresis without compromising barocaloric performance (David Boldrin, University of Glasgow). However, barocaloric devices are in their infancy, and heat transfer is a key issue in their development (Enric Stern-Taulats, University of Barcelona).
The desire to address multicaloric materials has inspired advanced experimental setups that permit the application of more than one type of driving field. For example, uniaxial stress was applied while simultaneously applying a steady magnetic field during calorimetry (Lluís Mañosa, University of Barcelona) or a pulsed magnetic field during measurements of temperature change (Tino Gottschall, Helmholtz-Zentrum Dresden-Rossendorf). Multicaloric materials include magnetic shape-memory alloys with various microstructures, chemical compositions, and fabrication protocols (Franziska Scheibel, Technical University Darmstadt; Lluís Mañosa, University of Barcelona). Multicaloric materials with strong magnetostructural coupling have also been studied using ab initio models (Eduardo Mendive-Tapia, Forschungszentrum Jülich).
The different caloric materials discussed above have various merits and demerits, and thus it is too early to determine which type of caloric material is best for specific applications. If any caloric technology is successfully launched then it would provide a huge stimulus for the field of calorics. If a magnetocaloric wine cooler (Max Fries, MagnoTherm Solutions, Darmstadt) can be available in the next two years then we will drink to that at Calorics 2024.
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