Large room-temperature electrocaloric effect in lead-free BaHf Ti O3 ceramics under low electric field

Abstract

Ferroelectrics are promising candidate materials for electrocaloric refrigeration. Materials with a large electrocaloric effect (ECE) near room temperature and a broad working temperature range are getting closer to practical applications. However, the enhanced ECE is always achieved under high electric field, which limits their wide cooling applications. In this paper, the phase diagram of lead-free BaHf x Ti 1-x O 3 (BHT) ferroelectric ceramics was established. A large ECE under relatively low electric field (ΔE=10 kV/cm) is firstly reported in BHT ferroelectric ceramics. The direct temperature change (ΔT=0.35 °C under 10 kV/cm) in BHT ceramics is comparable with those reported in the literature under high electric fields. Meanwhile, the electrocaloric efficiency (ΔT/ΔE=0.35 K mm kV-1 under 10 kV/cm) is thirteen percent higher than the best value reported previously under high electric field (ΔE=145 kV/cm). We demonstrate that the ECE can be greatly enhanced by tuning the composition of the lead-free BHT ceramics to its first-order phase transition (FPT), invariant critical point (ICP) or diffuse phase transition (DPT). It is shown that the enhancement in ECE is strongly dependent on the nature of structural phase transition and electric field coupling effect, which has been confirmed by both the indirect and direct ECE measurements. A phenomenological explanation based on Landau model was also proposed to understand this phenomenon. Our findings in this work may provide a better understanding and design methodology for developing more practically useful electrocaloric materials.

Electrocaloric effect and pyroelectric performance in (K,Na)NbO3‐based lead‐free ceramics

“…1 Thereafter, some works on large ECE have been reported in lead-based ceramics and films. 8,10,11,13 However, T C is often higher than room temperature for the current reported ferroelectric materials, which is not conducive to the application of electronic device cooling at room temperature. Therefore, lead-free ferroelectric materials have been considered as better substitute for electrocaloric refrigeration, such as BaTiO 3 (BT)-based, Na 0.5 Ba 0.5 TiO 3 (NBT)-based, and K 0.5 Na 0.5 NbO 3 (KNN)-based materials.…”

“…Many efforts have been devoted to the ECE in the last few decades, especially after the giant ECE was demonstrated in PbZr 0.95 Ti 0.05 O 3 (PZT) ferroelectric thin-films. [8][9][10][11][12] Normally, the high ECE can be achieved in the vicinity of the Curie temperature (T C ) or the polymorphic phase transition (PPT) region with different ferroelectric phases in ferroelectric materials due to the large entropy change. [2][3][4][5][6][7] Unfortunately, the PZT-based materials contain a large portion of hazardous lead substance, confronting with global restrictions.…”

“…[2][3][4][5][6][7] Unfortunately, the PZT-based materials contain a large portion of hazardous lead substance, confronting with global restrictions. Li et al 11 reported that a ΔT of 0.42 K at 35°C can be obtained in BaHf 0.11 Ti 0.89 O 3 ceramic through the orthorhombic and tetragonal (O-T) phase transition. [8][9][10][11][12] Normally, the high ECE can be achieved in the vicinity of the Curie temperature (T C ) or the polymorphic phase transition (PPT) region with different ferroelectric phases in ferroelectric materials due to the large entropy change.…”

Local phenomena in bismuth sodium titanate perovskite studied by Raman spectroscopy

“…To understand the physical mechanism of the thermal evolution in local structure, we adopt a phenomenological Landau model, which can predict the states of a system in terms of the average order parameter . Consequently, the locally ordered polarization in microstates can be taken into consideration.…”

“…Consequently, the locally ordered polarization in microstates can be taken into consideration. Based on the Landau‐Devonshire theory, the free energy ( F ) of the material system without and with external applied field can be expressed by Landau polynomials: …”