Electrocaloric effects in spark plasma sintered Ba_0.7Sr_0.3TiO₃-based ceramics: Effects of domain sizes and phase constitution

Abstract

The crystal structures, domain structures, dielectric strengths and electrocaloric effects of spark plasma sintered Ba_0.7Sr_0.3TiO₃ and Ba_0.7Sr_0.3Ti_0.997Mn_0.003O₃ ceramics have been investigated. The densification can be enhanced by the spark plasma sintering (SPS) process without obvious grain growth, where the fine ferroelectric domain structures are obtained. The content of tetragonal phase in Ba_0.7Sr_0.3TiO₃ ceramics prepared by the SPS process is lower than that in the counterpart prepared by the conventionally sintering process, but it will increase again by partially substituting titanium with 0.3 mol% manganese ions. The dielectric strength can be significantly enhanced by the spark plasma sintering process, which can be improved further by Mn-substitution, and consequently the electrocaloric temperature change, ΔT, increases. Meanwhile, the electrocaloric strength has been enhanced by the spark plasma sintering process, which should be attributed to combined effects of domain wall density the content of ferroelectric phase in the present ceramics.

Introduction

Electrocaloric effect (ECE) is a change in the temperature and entropy of a material when an external electrical field is applied or removed under adiabatic conditions, and it may provide an efficient way to realize the solid-state cooling device [1], [2], [3], [4], [5], [6]. A lot of interest had been concentrated on the ECE of bulk ferroelectric materials in the 1960s and 1970s, but no commercial application has been exploited because of the small ECE [3], [7]. The research interest has been reactivated recently after discovering giant ECEs in thin-film PbZr_0.95Ti_0.05O₃ and [P(VDF-TrFE)] copolymer [1], [2], and then numerous studies of ECEs on all kinds of materials have been conducted [8], [9], [10], [11]. One of most important parameter to characterize the ECE is EC temperature change, ΔT (in Kelvin). So far, the highest EC ΔT of 45.3 K has been obtained by a field-induced antiferroelectric to ferroelectric phase transition around room temperature in Pb_0.8Ba_0.2ZrO₃ thin film [12], while only several Kelvin has been observed in the bulk materials. A new parameter, electrocaloric strength ΔT/ΔE, has been introduced in order to compare the ECEs of different materials in various electrical fields, and the ferroelectric oxides have superior EC performance in this manner [4], [13]. Although thin films have the advantage on the application in small solid-state cooling devices, the bulk materials are essential to the EC application in the middle and/or large scale devices, such as solid state refrigeration [4], [5]. However, the highest EC temperature change in bulk materials is low, and they cannot fulfill the requirement of the practical application in the solid state cooling device [14], [15]. Furthermore, all bulk materials with high EC temperature change contain lead ions, which are not desired in the practical application since they harm our environment. The EC temperature changes of lead-free bulk materials are very small, so it is a big challenge to achieve high EC temperature changes in the lead-free bulk materials.

According to the previous work [5], the ECEs of bulk materials are restricted by dielectric strengths, that is, the low dielectric strengths of bulk materials lead to the small EC temperature changes. Thus, the ECE can be improved by enhancing the dielectric strength in the bulk materials. Recently, our group has attained a high EC temperature change, 3.08 K, in the Mn-substituted Ba_0.65Sr_0.35TiO₃ ceramics prepared by the spark plasma sintering (SPS) process at room temperature [16]. The dielectric strength of Ba_0.65Sr_0.35TiO₃ ceramics can be improved from 40 kV cm⁻¹ to 90 kV cm⁻¹ by the spark plasma sintering process, and it can be enhanced further to 130 kV cm⁻¹ by substituting part of titanium with manganese ions. As a result, the EC temperature changes of corresponding ceramics increase from 0.83 K to 2.10 K, and then 3.08 K. Although the high ECE has been achieved in the above ceramics, the correlation between structures, especially microstructures and electrocaloric effects is not clear.

In the present work, the detail phase constitutions, microstructures and electrocaloric properties of Ba_0.7Sr_0.3TiO₃-based ceramics have been investigated, and the correlation between these parameters is tried to be revealed in the paper.