Ferroelectric/antiferroelectric thin/thick films with large positive or negative electrocaloric (EC) effects could be very useful in designing commercial refrigeration devices. Here, a giant negative EC effect (maximum ΔT ∼ −42.5 K with ΔS ∼ −29.3 J K−1 kg−1) comparable to the best positive EC effects reported so far is demonstrated for 0.5(Ba0.8Ca0.2)TiO3–0.5Bi(Mg0.5Ti0.5)O3 (BCT–BMT) lead-free relaxor ferroelectric thin films prepared on Pt(111)/TiOx/SiO2/Si substrates using a sol–gel method. An electric-field induced structural phase transition (nanoscale tetragonal and orthorhombic to rhombohedral) along the out-of-plane [111] direction plays a very key role in developing the giant negative EC effect. This breakthrough will pave the way for practical applications of next-generation refrigeration devices with high cooling efficiency in one cycle by ingeniously utilizing and combining both the giant negative and positive EC effects. Moreover, a large energy density of 51.7 J cm−3 with a high power density of 1.15 × 1010 W kg−1 at room temperature is also achieved in the thin film, indicating that it is also an attractive multifunctional material for energy storage.

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Phase-transition induced giant negative electrocaloric effect in a lead-free relaxor ferroelectric thin film

Study of the effect of La3+ doping on the properties of ceramics based on BaTiOx

Enhanced energy storage density and efficiency in lead-free Bi(Mg1/2Hf1/2)O3-modified BaTiO3 ceramics

The electrocaloric effect (ECE), which refers to changes in the temperature of a material when an electric field is applied to or removed from the material, is one of the key phenomena of future highly efficient solid-state refrigeration devices. This field was not explored extensively owing to the deficiencies in usable and sufficient cooling capacity; however, a boom occurred approximately 15 years ago with the discovery of giant ECE in ferroelectric thin films. Since then, many reports have been published proposing different approaches to achieve giant ECE in different compositions or forms of ceramics, which can be broadly categorized as lead-based or lead-free, bulk or thin/thick, and multilayered or heterostructures. Various ECE-based cooling devices were also developed and their cooling performance was demonstrated. Two important aspects in this field are (1) to achieve giant ECE in materials by changing different parameters such as composition, geometry, and type, and (2) successful use of giant ECE in practical EC-based cooling devices. This review provides a focused view of the above two aspects, which will assist not only in the selection of materials for future cooling applications, but also in the realization of solid-state EC-based refrigeration.

Ming-Ding Li

Papers

Giant electrocaloric effect in BaTiO3–Bi(Mg1/2Ti1/2)O3 lead-free ferroelectric ceramics

Pyroelectric energy harvesting capabilities and electrocaloric effect in lead-free SrxBa1-xNb2O6 ferroelectric ceramics