Caloric effects in ferroic materials
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duction The worldwide energy use for cooling by refrigerators and air conditioners is substantial. For example, in the United States, households used about 20% of all the generated electricity in 2016.1 This usage is projected to rise at least through 2050, despite steady improvements in the efficiencies of conventional cooling systems.2 Today’s mainstream cooling technology, in development since the early 20th century, is based on vaporcompression cycles, and has reached about 30% of Carnot efficiency (representing the thermodynamic limit). Some alternatives, such as Peltier-based cooling using thermoelectric materials, which are a modern staple for solid-state refrigeration, realize an efficiency of only about 10% of the Carnot efficiency, and therefore have little but niche use. Ferroic cooling already exceeds the efficiency of thermoelectric systems, and has the potential to overtake that of vapor compression.3 However, this new technology is in the early stages, and will need some time to become cost competitive with conventional systems because of the large investment over the long research and development time older technologies have enjoyed. This issue of MRS Bulletin introduces the state-of-theart and ongoing developments in ferroic-caloric materials and their applications to cooling devices. In this article, we first explain a ferroic cooling cycle and describe similarities and differences between magnetocaloric, electrocaloric, and elastocaloric cooling. We then summarize the current status
of materials and device development, as well as the challenges that lie ahead. We end with an outlook on possible novel applications beyond refrigeration enabled by ferroiccaloric materials.
How can ferroic materials be used in a cooling cycle? Ferroic materials used as refrigerants in the core of ferroic cooling cycles earn the name ferroics due to peculiar transitions that alter symmetry at certain transition temperatures, TC.4 (For ferromagnetic and ferroelectric materials, TC is the Curie temperature; for ferroelastic materials it is the martensitic transition temperature.) As a result, a particular kind of ferroic order, described in a later section, is established when cooling, which can be either ferromagnetic, ferroelectric, or ferroelastic (as a starting point, one may just think ferromagnetic instead of ferroic in the rest of this section). In addition to temperature, these phase transitions can be driven by applying and removing external fields, namely magnetic, electric, or elastic fields. To understand what makes a ferroic material suitable for a caloric application, a typical ferroic cooling cycle is sketched in Figure 1.5–8 Consider a case where the ferroic material undergoes a first-order phase transition, which is generally accompanied by latent heat, ΔL. Around TC, this phase transition can also be induced by external fields. Initially, both the material and its surroundings are at the same temperature, T.
Sebastian Fähler, IFW Dresden, Germany; [email protected] Vitalij K. Pecharsky, Departm
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