Enhanced electrocaloric effect in BaSn/TiO3 ceramics by addition of CuO

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Highlights

  • CuO was added into BaSn/TiO3 as an additive for optimizing ferroelectric and electrocaloric effect (EC) properties.

  • EC temperature change was increased from −0.15°C to −0.33°C at the electric field of 10kV/cm after adding 2 mol%CuO.

  • A relatively large EC temperature change of −0.48°C at the electric field of 20kV/cm was achieved.

  • The relationship between the EC effect and intrinsic properties was explained based on Landau-Devonshire theory.

Abstract

For several decades, ferroelectrics with enhanced electrocaloric (EC) effect have served as competitive alternatives for solid-state cooling devices. In order to enhance the EC effect, there has been increasing research interest on exploring novel systems of EC materials and efficient cooling cycles. In contrast, optimizing intrinsic properties through doping effect has been relatively ignored. In this work, we introduce the binary compound CuO as an additive into the conventional lead-free BaSn0·11Ti0·89O3 (BST) ferroelectric ceramic. This leads to a remarkably large adiabatic temperature change of ΔT = −0.33 °C under a relatively low electric field (10 kV/cm). This is higher than the ΔT of pure BST (ΔT = −0.15 °C, under 10 kV/cm). The largest of ΔT = −0.48 °C was achieved under 20 kV/cm. We assume that the large temperature change was achieved due to the addition of CuO additive, which improves the ferroelectric properties (e.g., a higher polarization and a lower coercive field). In addition, by introducing CuO additives, the breakdown electric field of the ceramics was also enhanced at high temperatures (above the Curie temperature, TC) and the working temperature range was greatly broadened (30 °C–60 °C) under a slightly high electric field. Our findings present a promising approach to enhance the EC effect by tuning the intrinsic properties of EC materials. We expect that our work emphasizes the importance of additives in enhancing the EC properties.

Introduction

The electrocaloric (EC) effect is a reversible adiabatic temperature change or entropy change that occurs in a polar material with the application or removal of an external electric field (ΔE) [1]. Generally, the EC effect is characterized by the adiabatic temperature change (ΔT) and the EC strength (ΔTE) [2]. After intensive research of many years on the EC effect, we are now able to synthesize ferroelectric materials with excellent EC properties. Until now, a number of different types of ferroelectrics as potential EC materials have been studied, such as single crystals [[2], [3], [4]], thin or thick films [[5], [6], [7], [8]], polymers or polymer composites [9], and conventional ferroelectric ceramics [[10], [11], [12], [13], [14], [15], [16], [17], [18], [19]]. To further improve the EC effect, considerable effort has been devoted to exploring new EC materials and novel EC device concepts [20]. This includes synthesizing materials containing several co-existing polar states near the critical point in the temperature-composition phase diagram [[17], [18], [19]], fabricating composite materials containing both nano-sized ferroelectric ceramics and ferroelectric polymers [21], modifying strain, employing mechanical stress and other physical parameters in the samples [22,23], and so forth [24,25]. On the other hand, theoretical studies and modeling approaches are helpful for clarifying physical mechanisms underlying the EC effect [26,27]. In spite of the important progress made, the search for novel material systems still attracts significant interest in this field. However, it remains as a big challenge to increase EC effect further. Therefore, what we need is to develop an alternative method to resolve this problem.

Defect engineering represents a powerful technique to tune the ferroelectric and piezoelectric properties of ferroelectric materials [[27], [28], [29], [30]]. Very recently, Anna Grünebohm et al. [31] studied the influence of different kinds of defects, such as nonpolar and polar defects, on ECE by using molecular dynamics simulations. They found that those defects enhanced the EC temperature range and still capable a large EC response. Yang-Bin Ma et al. [32] showed that both positive and negative ECE could be achieved in the BaTiO3 samples with anti-parallel defect dipoles by tuning the density of defects and the external electric field.

To understand the relationship between the doping effect and the EC effect in experiment, here we introduce a binary compound to a specific ferroelectric to adjust its properties. Here, we focus on the BaSn0·11Ti0·89O3 (BST11) ceramic system. Recently, Sanlialp et al. systematically studied the EC effect of the BaSnxTi1-xO3 ceramics, and showed that the coexistence of multiple phases would enhanced the EC performance [23]. More importantly, their work provides solid evidence for the stability and reliability of direct and indirect measurements for the EC effect in BaSnxTi1-xO3 ceramics. Ye et al. studied the effects caused by different sintering aids added into the BaZr0·2Ti0·8O3 ceramics and thick films, and concluded that binary compounds, such as PbO and B2O3, will reduce the sintering temperature of the ceramic effectively without sacrificing its dielectric and EC properties [25]. On the other hand, the binary compound CuO has been used as an additive to reduce the final sintering temperature during fabrication and enhance the density of the ceramics [[33], [34], [35]]. Zhou et al. showed that dense (Ba0·85Ca0.15) (Ti0·9Sn0.1)O3-CuOx ceramics could be fabricated at a low temperature of 1250 °C without sacrificing the piezoelectric response with d33 = 683 pC/N [27]. However, there is still no systematic discussion or studies on how the binary compound dopants influence the intrinsic properties of these ceramics and consequently their EC effect.

In this work, a series of BaSn0·11Ti0·89O3-xCuO (BST-xCuO) ceramics with x = 0, 1 mol%, 2 mol% and 3 mol% were carefully synthesized. We discuss the influence of CuO addition on the polarization and the coercive electric field (EC) and optimize the composition of CuO to achieve a relatively large EC effect. This optimization step led to an adiabatic temperature change of ΔT = −0.33 °C under a relatively low electric field (ΔE = 10 kV/cm), as well as a high electrocaloric strength (ΔTE = −0.33 K mm/kV). Table S1 gives the EC effect comparison between this work and other different types of materials. All the direct measurements were performed using a modified Differential Scanning Calorimeter (DSC).

Section snippets

Preparation of samples

The CuO-modified BaSn0·11Ti0·89O3-xCuO (x = 0, 1 mol%, 2 mol% and 3 mol%) ceramics were fabricated by a conventional solid-state reaction method using BaCO3, SnO2, TiO2 and CuO powders (Alfa Aesar). The stoichiometric weights of BaCO3, SnO2, TiO2 powders were first well-mixed by ball milling for 6 h in ethanol and then calcined at 1300 °C for 3 h in air. The mixture was ball milled again with different amounts of CuO additive and pressed into disks under 200 MPa by isostatic pressing. Final

Results and discussions

Fig. 1a shows the room temperature XRD patterns of the BST-xCuO ceramics with x = 0, 1 mol%, 2 mol% and 3 mol% at room temperature. All results exhibit pure perovskite structures without any secondary phase, which indicates that the Cu atoms diffuse into the lattice and form a solid solution. Fig. 1b–e presents the SEM surface micrographs of BST-xCuO ceramics (x = 0, 1 mol%, 2 mol% and 3 mol%). As shown in Fig. 1b, the grain size of the pure BST ceramic is clearly not uniform whereas Fig. 1c–e

Conclusions

In summary, we have introduced CuO as an additive to fabricate lead-free CuO-modified BST ceramics through a conventional solid-state reaction method. The influence of CuO additive on the intrinsic properties of the ceramic, such as ferroelectric polarization and specific heat capacity, and subsequently the EC response were systematically investigated. By direct EC measurements using a modified DSC, we found that the CuO-modified BST ceramics show relatively good EC performance. In particular,

CRediT authorship contribution statement

Yuting Wang: Materials fabricating; Materials Characterization, Data curation, Formal analysis, Writing - original draft. Junning Li: Materials Characterization, Formal analysis, Writing - review & editing. Ruihao Yuan: Materials Characterization, Data curation. Hongcheng Gao: Materials Characterization, Data curation. Jing Lv: Materials Characterization, Data curation. Dezhen Xue: Writing - review & editing. Xihong Hao: Writing - review & editing. Yaodong Yang: Formal analysis, Writing -

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by National Science Foundation of China (NSFC No. 51372195, and 51772238). X. Lou would like to thank the “One Thousand Youth Talents” program for support. Support from National Key R&D Program of China (2017YFA0208000) is appreciated.

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