Lead-Free Bilayer Thick Films with Giant Electrocaloric Effect near

A giant adiabatic temperature change ΔT~5.2 K and a large isothermal entropy change ΔS~6.9 J kg-1 K-1 ..... direct measurement setup (PDF). AUTHOR ...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 23346−23352

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Lead-Free Bilayer Thick Films with Giant Electrocaloric Effect near Room Temperature Jinglei Li,†,# Yunfei Chang,*,# Shuai Yang,† Ye Tian,† Qingyuan Hu,† Yongyong Zhuang,† Zhuo Xu,† and Fei Li*,† †

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Key Laboratory of the Ministry of Education and International Center for Dielectric Research, School of Electronic Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China # Condensed Matter Science and Technology Institute, School of Instrumentation Science and Engineering, Harbin Institute of Technology, Harbin 150080, China S Supporting Information *

ABSTRACT: Electrocaloric refrigeration utilizing ferroelectrics has recently gained tremendous attention because of the urgent demand for solid-state cooling devices. However, the low room-temperature electrocaloric effect and narrow operation temperature window hinder the implementation of lead-free ferroelectrics in high-efficiency cooling applications. In this work, chemical engineering and thick-film architecture design strategies were integrated into a BaTiO3-based system to resolve this challenge. Novel environmental-friendly Ba(Zr0.20Ti0.80)O3−Ba(Sn0.11Ti0.89)O3 (BZT-BST) bilayer films of ∼13 μm in single-layer thickness were prepared by the tape casting process. A giant adiabatic temperature change, ΔT ∼ 5.2 K, and a large isothermal entropy change, ΔS ∼ 6.9 J kg−1 K−1, were simultaneously achieved at room temperature based on the direct measurements, which are much higher than those reported previously in many lead-free ferroelectrics. Moreover, the BZT-BST thick films exhibited a remarkably widened operation temperature range from about 10 to 60 °C. These outstanding properties were mainly attributed to the multiphase coexistence near room temperature, relaxor ferroelectric characteristics, and improved electric-field endurance of the bilayer thick films. This work provides a guideline for the development of environment-friendly electronic materials with both ultrahigh and stable electrocaloric performance and will broaden the application areas of lead-free ferroelectrics. KEYWORDS: electrocaloric effect, bilayer thick film, relaxor ferroelectric, lead-free ceramics, phase transition, direct measurement

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Curie temperature (Tc) and low coercive field (Ec). Pure BaTiO3 bulk ceramics have sharp first-order ferroelectricparaelectric (FE-PE) phase transition at Tc ∼ 120 °C, which endows it with maximized ΔT value (∼0.4−1.4 K) near Tc.9,10 However, the low room-temperature ECE and narrow operation temperature range seriously hindered its commercialization into EC devices which are expected to preferably work at room temperature. Chemical modification strategies through substitution and doping have been proved to be an effective way to shift Tc and tailor ECE in BaTiO3-based ceramics.11−22 For example, Tc values of Ba(Zr0.25Ti0.75)O3,11 (Ba0.65Sr0.35)TiO3,12 and (Ba0.94La0.04)TiO313 ceramics were all shifted toward room temperature, with maximized ΔT values about 0.2−1.5 K achieved near there. Until now, despite remarkable progress made through chemical modification strategies, there are still two challenges in BaTiO3 based ceramics for electrocaloric applications. On one hand, as a

INTRODUCTION Electrocaloric (EC) refrigeration has attracted extensive attention recently because of the high energy conversion efficiency and easy miniaturization, driven by the urgent demand of realizing next-generation solid-state cooling devices for various applications, such as temperature regulation for electronics and on-chip cooling.1−3 Intriguing electrocaloric refrigeration is based on electrocaloric effect (ECE) of a material, which is parametrized by temperature (ΔT) and entropy (ΔS) changes of the material induced by polarization state variations with applying/removing an electric field.4 To date, a lot of investigations on electrocaloric effect have focused on lead-based materials, such as (Pb, La)(Zr, Sn, Ti)O3, Pb(Zr, Ti)O3 and Pb(Mg1/3Nb2/3)O3−PbTiO3, among which encouraging results have been obtained.5−8 However, because of growing environmental and human health concerns on the toxicity of lead, developing sustainable lead-free materials with high EC performance becomes a very important topic. Barium titanate (BaTiO3) based materials are among the most attractive lead-free EC candidates because of the tunable © 2019 American Chemical Society

Received: April 10, 2019 Accepted: June 11, 2019 Published: June 11, 2019 23346

DOI: 10.1021/acsami.9b06279 ACS Appl. Mater. Interfaces 2019, 11, 23346−23352

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic illustration of the approach followed in this study.

free ferroelectrics9−22,25−31 and even higher than several widely used lead-based ones,32−36 including the well-known PMN-PT relaxor ferroelectrics. The underlying mechanisms responsible for the dramatically improved EC performance of the BZTBST bilayer thick films were discussed.

result of the fundamental limitation, broader EC operation temperature range was always gained at the cost of ECE values, and vice versa.11−21 For example, ΔT peak values decreased by more than 50% in Ba(Zr, Ti)O311 and (Ba, Ca)(Hf, Ti)O315,16 ceramics with widening EC temperature usage range. On the other hand, low breakdown electric fields of bulk ceramics make it very challenging to increase ΔT values over 2.0 K.9−17,19−22 These disadvantages seriously hindered the implementation of BaTiO3-based ferroelectrics in highefficiency solid-state cooling applications. Therefore, the vital question is how to explore novel lead-free materials with both high room-temperature ECE and superior temperature stability. Because the breakdown strength of ferroelectrics exponentially increases with the reduction in the thickness of single ceramic layer,23,24 the large ΔT values have been achieved in thin films, where very high electric fields can be applied because of the geometry.1,2,6−8 Because of their small volumes, however, the lower heating/cooling capacity of thin films hinders their application in medium and large-scale solid-state cooling devices. Thick-film architecture enables access to highbreakdown electric fields and larger overall volumes, potentially offering a feasible solution to avoid the disadvantages of bulk ceramics and thin films. In this work, we integrated chemical engineering and thick-film architecture design strategies to develop novel 0.90Ba(Zr0.20Ti0.80)O3− 0.10Ba(Sn0.11Ti0.89)O3 (BZT-BST) bilayer thick films with high room-temperature ECE and wide operation temperature range. As illustrated in Figure 1, to significantly enhance ECE values at room temperature, both Zr4+ and Sn4+ were judiciously substituted into BaTiO3 to tune FE-PE phase transition toward room temperature; meanwhile, bilayer thick film structure was employed to take advantage of the EC amplifying effect as a result of improved breakdown electric field. Furthermore, to broaden EC operation temperature range, the characteristic of FE-PE phase transition was modified from a normal sharp one to a diffused one (i.e., a relaxor ferroelectric). Encouragingly, our direct measurement results indicate the BZT-BST bilayer thick films possessed a very high adiabatic ΔT ∼ 5.2 K and a large isothermal ΔS ∼ 6.9 J kg−1 K−1 at room temperature. Besides, the newly designed BZT-BST exhibited much wider operation temperature range from about 10 to 60 °C. The ECE values obtained here are much superior to those of previously reported lead-

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EXPERIMENTAL SECTION

0.90Ba(Zr0.20Ti0.80)O3-0.10Ba(Sn0.11Ti0.89)O3 (BZT-BST) bilayer thick films were prepared by the tape casting process, and the detailed procedures were illustrated in Figure S1. BZT-BST equiaxed powder with ∼180 nm in average size was synthesized through solidstate reaction using BaCO3 (99.8%), TiO2 (99.8%), SnO2 (99.99%), and ZrO2 (99.9%) raw materials. Calcined powder and 1 wt % LiBiO2 sintering aid were mixed and then tape cast using a nonaqueous formulation.24 To achieve high density and good crystallinity, the sintering temperature of BaTiO3-based systems is generally above 1325 °C,11−14,16,18,20−22 which is too high for the internal electrodes cofired with the ceramic layers. To broaden choice ranges for internal electrodes, LiBiO2 was employed in this work to reduce the sintering temperature of BZT-BST bilayer thick films without sacrificing EC performance. Dried green tapes printed with Pt electrodes were precisely aligned and stacked, followed by isostatic lamination. After the organics were removed at 500 °C, specimens were sintered at 1170−1230 °C for 2 h to form bilayer thick films, and then, their opposite ends were terminated using silver paste as outer electrodes for property characterizations. Phase structure was evaluated using X-ray diffraction (XRD, D/max 2400, Rigaku, Tokyo, Japan). Microstructure and composition distribution were observed using scanning electron microscopy (SEM, NanoSEM 630, Nanolab Technologies, Milpitas, CA) combined with energy dispersive spectroscopy (EDS). Dielectric permittivity (εr) and loss (tan δ) vs temperature were tested using an Agilent E4980A LCR meter (Agilent Technologies, Santa Clara, CA). A modified Sawyer−Tower circuit was used to characterize polarization−electric field (P-E) loops. A uniquely designed calorimeter with a heat flux sensor was used to directly measure the heat Q generated/absorbed owing to ECE, with the detailed setup illustrated in Figure S2a. When applying a constant voltage V with a pulse time duration t to a reference resistor (resistance R), a joule heat Qr = (V2/R)t generates and can be detected by the heat flux sensor as a signal (Figure S2b shows an example). When applying/removing an electric field to an EC sample, the same heat flux sensor can detect the generated/absorbed heat QECE as a signal (Figure S2c presents an example). On the basis of the ratio α between such two signals, QECE was determined using QECE = αQr. The adiabatic temperature change ΔT and isothermal entropy change ΔS were, respectively, obtained from the relationships of QECE = CΔT and QECE = TΔS, where C is the specific heat capacity. 23347

DOI: 10.1021/acsami.9b06279 ACS Appl. Mater. Interfaces 2019, 11, 23346−23352

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ACS Applied Materials & Interfaces

Figure 2. (a) XRD pattern of BZT-BST ceramic material, with the inset showing its crystal structure. (b) Cross-sectional microstructure image of BZT-BST bilayer thick film. (c) Respective element distribution maps of BZT-BST bilayer thick film.

Figure 3. (a) Dielectric permittivity and loss as functions of temperature for BZT-BST bilayer thick films. The inset shows characterization of its diffuseness parameter γ using the modified Curie−Weiss equation. (b) Temperature dependence of polarization for BZT-BST bilayer thick films measured under different electric fields. A bipolar P−E loop measured at room temperature is displayed in the inset.

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RESULTS AND DISCUSSION Figure 2a shows room-temperature XRD pattern of BZT-BST ceramic material. It is of pure perovskite phase without any secondary phases, suggesting the formation of 0.90Ba(Zr0.20Ti0.80) O3-0.10Ba(Sn0.11Ti0.89)O3 solid solution. The single diffraction peak around 45° indicates pseudocubic structure of the samples. Figure 2b presents detailed morphologies of BZT-BST bilayer thick film. Both ceramic and electrode layers show high continuity without observable cracks, which enables the heat to flow more easily in the samples. Benefited from the tape casting process and lowtemperature sintering, each ceramic layer with ∼13 μm in thickness exhibits dense and fine-grained microstructure. Here the addition of LiBiO2 sintering aid helps to form liquid phases at ≤800 °C,37 which can facilitate dissolution/migration of the substances during the sintering process, thus promoting densification and grain growth of the BZT-BST ceramic materials. Both the reduced thickness and improved densification behavior of the bilayer thick films contribute to the substantially enhanced breakdown electric field. Elemental distribution maps (Figure 2c) show that Ba2+, Ti4+, Zr4+, and Sn4+ elements are homogeneously distributed in the ceramic layers, and there is no detectable ion diffusion behavior observed between the ceramic and electrode layers.

Figure 3a presents temperature dependences of dielectric permittivity εr and loss tan δ for the BZT-BST bilayer thick films measured at different frequencies. Different from the three (rhombohedral-orthorhombic, orthorhombic-tetragonal, and tetragonal-cubic) phase transitions observed in the εr-T curves for pure BaTiO3 (Figure 1),9,10 only one dielectric anomaly was detected from our BZT-BST material, possibly suggesting the existence of invariant critical point (ICP)18,38−40 in the Zr4+ and Sn4+ cosubstituted BaTiO3 ceramics, where cubic, tetragonal, orthorhombic, and rhombohedral phases coexist. Besides, compared to the narrow FE-PE phase transitional peak observed at ∼120 °C (Tc) for pure BaTiO3 ceramics,9,10 the BZT-BST bilayer thick films possess a very broad FE-PE phase transitional peak near room temperature. Temperature Tm corresponding to maximum dielectric permittivity (εm) is around 0 °C at 1 kHz, and shifts to be ∼5 °C with increasing frequency to 100 kHz. The broadened FE-PE phase transition peak, variation of Tm with frequency, and frequency dispersion of εr suggest typical relaxor ferroelectric behavior of the BZT-BST material. To further analyze phase transition characteristic of BZT-BST, the degree of diffuseness γ was determined using modified Curie− Weiss law 1/εr − 1/εm = (T−Tm)γ/C,41 where γ is 1 for normal ferroelectrics and 2 for ideal relaxor ferroelectrics. 23348

DOI: 10.1021/acsami.9b06279 ACS Appl. Mater. Interfaces 2019, 11, 23346−23352

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Figure 4. (a) Directly recorded room-temperature ECE signal for BZT-BST bilayer thick films as a 33 MV/m electric field was turned on and off, respectively. (b) Electric field dependences of EC-induced ΔT and ΔS measured at room temperature.

Figure 5. (a) Temperature dependences of ΔT and ΔS for the BZT-BST bilayer thick films under different electric fields. (b) Comparisons of ΔT and its temperature stability among the BZT-BST bilayer thick films and other typical lead-based/lead-free ferroelectrics.11,16,18,19,27−29,32,34

direct measurement. Figure 4b displays electric field dependences of ECE values (ΔT and ΔS) for BZT-BST bilayer thick films measured at room temperature. The thick films possessed room-temperature ΔT ∼ 1.6 K and ΔS ∼ 2.2 J kg−1 K−1 at a relatively low electric field of 4.6 MV/m, which are much higher than many previously reported BaTiO3-based bulk ceramics and single crystals upon the same or similar electric fields,11−14,16,20−22,26 possibly owing to the operation near invariant critical point. Multiphase coexistence near ICP supplies more available polarization states and more randomness in the system.38 Moreover, vanishing energy barriers near ICP makes the switching between different states much easier.42 All these can help to improve EC performance of the BZT-BST bilayer thick films. ECE values increased dramatically with the increase of applied fields, with giant values of ΔT = 5.2 K and ΔS = 6.9 J kg−1 K−1 achieved at 33 MV/m, which significantly outperformed previously reported BaTiO3 based ferroelectrics.9−22,25,26 In this work, we fabricated two active thick layers on the film substrate with the same composition, which overcomes the limitation imposed by low dielectric strength of bulk ceramics, substantially improving breakdown electric field to be above 33 MV/m. Benefited from the amplifying effect in this unique bilayer thick-film architecture, room-temperature EC responses were significantly enhanced. Besides, internal stresses which come from the mechanical constraint of the substrate, should exist in this bilayer thick-film architecture during application/removal of electric fields. Such internal stresses could tailor the coexisting phases in the BZTBST bilayer thick films, further contributing to the enhanced EC responses near room temperature.

BZT-BST possesses a γ value of 1.88 according to the inset of Figure 3a, further confirming the relaxor state (i.e., diffusive phase transition characteristic) of those samples. Such diffuseness could be associated with the structural disorder and compositional fluctuation induced by competitions among Zr4+, Sn4+, and Ti4+ cations in B-sites of the ABO3 perovskite structure. Figure 3b demonstrates the polarization vs temperature, extracted from the P-E loops taken from the BZT-BST bilayer films under different electric fields at 1 Hz. A slim P−E loop measured at room temperature is shown in the inset. At Tm (∼0−5 °C), a polarization value of about 0.20 C/m2 was achieved at 15 MV/m, and remarkably increased to around 0.27 C/m2 at 30 MV/m. When applying the same electric field, the induced polarization keeps almost the same as temperature increases from −10 to 15 °C, after which it drops slightly with further increasing temperature to 50 °C. Presence of polarization above Tm and no obvious peak near Tm are also fingerprints for relaxor behavior in the BZT-BST material. Figure 4a shows the directly measured ECE signal from BZT-BST bilayer thick films at room temperature. Upon applying a 33 MV/m electric field, ΔT increased dramatically by ∼5.2 K owing to electrocaloric heating. With this field still on, the thermal energy gradually leaked to the environment, resulting in the drop of the temperature to the ambient temperature. When removing this electric field, ΔT decreased suddenly by ∼5.2 K because of electrocaloric cooling, leading to the presence of the endothermic peak shown in the figure. Because the exothermal and endothermal peak values were almost the same, and the initial temperature was recovered after sufficient thermal equilibrium time in both heating and cooling processes, Joule heating was negligible during such 23349

DOI: 10.1021/acsami.9b06279 ACS Appl. Mater. Interfaces 2019, 11, 23346−23352

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ACS Applied Materials & Interfaces

a large ΔS ∼ 6.9 J kg−1 K−1 were simultaneously achieved at room temperature. Moreover, they maintained superior temperature stability within ±3.9% variations in ΔT over a wide temperature range from about 10 to 60 °C. The ECE values of BZT-BST bilayer films significantly outperformed previously reported lead-free bulk ceramics, being attributed to the multiphase coexistence near room temperature, the substantially improved electric-field endurance, and the presence of relaxor ferroelectric feature. This study may shed light on designing EC materials with significantly enhanced performance for high-efficiency solid-state cooling applications.

In addition to high room-temperature EC performance, superior temperature stability is expected for electrocaloric refrigeration applications. Figure 5a demonstrates temperature dependences of ΔT and ΔS for the BZT-BST bilayer thick films measured under various electric fields. With increasing applied field from 5 to 33 MV/m, the temperature for maximum ECE shifted from about 10 to 20 °C, higher than the Tm (∼0−5 °C) observed in Figure 3a. A similar phenomenon can be found in other ferroelectric materials and was attributed to the shifts in the phase-transition temperatures induced by electric fields.28,43 Such a shift suggests the reinforcement of the electric field-induced order state when competing with thermal agitation. Larger electric fields favor ferroelectric order state and place higher energy barriers for the transition to paraelectric phase, thus shifting the ΔT peak to higher temperatures. Such phenomenon needs to be taken into consideration when designing new ferroelectrics for electrocaloric refrigeration in the future. Besides, different from the strong temperature dependence of maximum ECE for pure BaTiO3,9,10 maximized ECE values kept very stable over a broad temperature range for the BZT-BST bilayer thick films. For example, when measured under 33 MV/m, they maintained very high ΔT values (>5.0 K) within only ±3.9% variations detected from 10 to 60 °C, and this temperature range is wide enough to conduct a commercially viable solidstate cooling cycle. Because relaxors possess a complex polar structure consisting of polar nanoregions (PNRs), applying electric field will induce strong entropic contribution from formation/alignment of PNRs in the relaxor ferroelectrics, leading to ECE peak temperature above Tm. This feature improves the potential of relaxor ferroelectrics to be utilized in electrocaloric cooling applications, for those PNRs remain well several hundred degrees above the phase transition temperature,44 thus contributing to the superior ECE temperature stability of the BZT-BST bilayer thick films. Figure 5b compares ΔT and its temperature stability among BZT-BST bilayer thick films and previously reported representative lead-based/lead-free ferroelectrics. Encouragingly, with the integration of chemical engineering and thickfilm architecture design strategies, our newly designed BZTBST bilayer thick films exhibited both ultrahigh roomtemperature ΔT of ∼5.2 K and superior temperature stability of at least 50 K (from about 10 to 60 °C). These properties significantly outperform previously reported KNN-, NN-, BT-, and BNT-based lead-free bulk ceramics,9−22,25−31 and are even superior to many lead-based counterparts well-known for their strong ECE, including the attractive PMN−PT relaxor bulk ceramics and multilayers.32−36 Besides, BZT-BST bilayer thick films exhibit higher EC properties than many previously reported BaTiO3-based thin films under the same level of electric fields.45−48 Multiple phase coexistence near room temperature and improved electric field endurance should be responsible for the giant ECE of the BZT-BST bilayer thick films; meanwhile, ferroelectric relaxor behavior contributes to the stably maintained ECE over a broad temperature range. All of these suggest the tremendous potential of unique BZT-BST bilayer thick films for high-efficiency solid-state cooling applications.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06279. Supported illustrations of the fabrication process for the BZT-BST bilayer thick films and ECE direct measurement setup (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jinglei Li: 0000-0002-4013-0059 Yunfei Chang: 0000-0003-2830-5730 Fei Li: 0000-0002-4572-0322 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 51802182, 51831010, 51761145024, and 11572103), the Fundamental Research Funds for the Central Universities (Grant sxjh012019011), the Natural Science Foundation of Shaanxi Province (Grant 2018KJXX-081 and 2019ZDLGY04-09), and the Natural Science Foundation of Heilongjiang Province (Grant YQ2019E026).

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REFERENCES

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CONCLUSIONS Novel BZT-BST bilayer thick films were developed with integrating both composition engineering and thick-film architecture design strategies, where a giant ΔT ∼ 5.2 K and 23350

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Research Article

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DOI: 10.1021/acsami.9b06279 ACS Appl. Mater. Interfaces 2019, 11, 23346−23352

Research Article

ACS Applied Materials & Interfaces (46) Engelhardt, S.; Molin, C.; Gebhardt, S.; Fahler, S.; Nielsch, K.; Huhne, R. BaZrxTi1‑xO3 Epitaxial Thin Films for Electrocaloric Investigations. Energy Technol. 2018, 6, 1526−1534. (47) Akcay, G.; Alpay, S. P.; Rossetti, G. A., Jr.; Scott, J. F. Influence of Mechanical Boundary Conditions on the Electrocaloric Properties of Ferroelectric Thin Films. J. Appl. Phys. 2008, 103, No. 024104. (48) Yamada, T.; Matsuo, S.; Kamo, T.; Funakubo, H.; Yoshino, M.; Nagasaki, T. Experimental Study of Effect of Strain on Electrocaloric Effect in (001)-epitaxial (Ba, Sr)TiO3 Thin Films. Jpn. J. Appl. Phys. 2017, 56, 10PF15.

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DOI: 10.1021/acsami.9b06279 ACS Appl. Mater. Interfaces 2019, 11, 23346−23352