Criticality: Concept to Enhance the Piezoelectric and Electrocaloric Properties of Ferroelectrics

Abstract

Compositional engineering with a focus on structural phase transitions has been considered as the most important approach for enhancement of the functional properties of ferroelectric materials due to the critical fluctuation of physical properties. Of special interest are electric‐field‐induced phase transitions, which can terminate in a liquid–vapor‐type critical point with a strong enhancement of functional properties. Whereas the critical point in liquid–vapor space considers changes in temperature and pressure, the critical point in this study is placed in electric field–temperature diagrams. In single crystals, temperature and electric field of a critical point are sharply defined and therefore not appealing for practical applications. However, in ceramics, it is demonstrated that the orientational dependence of the critical point leads to a broadened temperature and electric field range. The presence of a diffuse critical point in ceramics provides a conceptually novel approach for the enhancement of functional properties, such as piezoelectric and electrocaloric (EC) responses, as validated here on the example of the 0.75Bi1/2Na1/2TiO3‐0.25SrTiO3 lead‐free relaxor ferroelectric ceramics. The realization of a broad criticality range will further facilitate the development of the piezoelectric and EC materials and provide an alternative concept to manipulate the functional properties by application of an electric field.

Large strain response with low driving field in Bi1/2Na1/2TiO3–Bi1/2K1/2TiO3–Bi(Mg2/3Nb1/3)O3 ceramics

“…Among the different lead‐free counterparts, (Bi 1/2 Na 1/2 )TiO 3 (BNT) is a promising candidate for its excellent piezoelectric properties and has attracted considerable attention . In previous, enhanced piezoelectric and electromechanical properties are mostly achieved in modified binary or ternary BNT‐based compositions, such as BaTiO 3 (BT), Bi 1/2 K 1/2 TiO 3 (BKT), BaZrO 3 (BZ), and K 1/2 Na 1/2 Nb 1/2 O 3 (KNN).…”

Multilayer Ceramics for Electrocaloric Cooling Applications

“…Their low surface to volume ratio accompanied with a low thermal conductivity leads to insufficient heat transfer . Because of their high thickness, bulk ceramics require large operation voltages or they have to be ground or polished to lower thicknesses in an additional and laborious processing step …”

Electric-field-induced phase transition and electrocaloric effect in PMN-PT

“…However, T c in many ferroelectric materials is considerably higher than room temperature, which substantially limits their potential application in solid-state cooling devices. In the past decade, considerable efforts have been exerted to achieve high EC effects at room temperature, [13,[18][19][20][21][22][23][24][25][26] such as applying a stress field, [20,21,27] doping, [22] introducing defects, [18,23,27] using the tetragonal-cubic phase transition, [10,12] and taking advantage of the morphotropic phase boundary (MPB) region in solid solutions. [13,24,28] PMN-PT is a solid solution of the relaxor PMN and the ferroelectric PbTiO 3.…”