Multifunctional All-Inorganic Flexible Capacitor for Energy Storage and

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Functional Inorganic Materials and Devices

Multifunctional All-Inorganic Flexible Capacitor for Energy Storage and Electrocaloric Refrigeration over a Broad Temperature Range Based on PLZT 9/65/35 Thick Films Bingzhong Shen, Yong Li, and Xihong Hao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b12353 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 26, 2019

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Multifunctional All-Inorganic Flexible Capacitor for Energy Storage and Electrocaloric Refrigeration over a Broad Temperature Range Based on PLZT 9/65/35 Thick Films Bing-zhong Shen,†,‡ Yong Li,† and Xihong Hao*,†,‡ †Inner

Mongolia Key Laboratory of FE-Related New Energy Materials and Devices, Inner

Mongolia University of Science and Technology, Baotou 014010, China ‡Key

Laboratory of Integrated Exploitation of Bayan Obo Multi-Metal Resources, Inner

Mongolia University of Science and Technology, Baotou 014010, China

KEYWORDS: multifunctional capacitors, flexible, relaxor FE, PLZT 9/65/35 thick film, energy-storage performance, electrocaloric refrigeration

ABSTRACT: Multifunctional capacitors can efficiently integrate multiple functionalities into a single material to further down-scale state-of-the-art integrated circuits, which are urgently needed in new electronic devices. Here, an all-inorganic flexible capacitor based on Pb0.91La0.09

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(Zr0.65Ti0.35)0.9775O3 (PLZT 9/65/35) relaxor ferroelectric thick film (1 μm) was successfully fabricated on LaNiO3/F-Mica substrate for application in electrostatic energy storage and electrocaloric refrigeration simultaneously. The flexible PLZT 9/65/35 thick film presents a desirable breakdown field of 1998 kV/cm, accompanied by a superior recoverable energy density (Wrec) of 40.2 J/cm3. Meanwhile, the thick film exhibits excellent stability of energy-storage performance,

including

a

broad

operating

temperature

(30-180

oC),

reduplicative

charge-discharge cycles (1 × 107 cycles) and mechanical bending cycles (2000 times). Moreover, a large reversible adiabatic temperature change (ΔT) of 18.0 oC, companied by an excellent electrocaloric strength of 22.4 K cm/V and refrigerant capacity (RC) of 11.2 J/cm3, is obtained at 80 oC in the flexible PLZT 9/65/35 thick film under the moderate applied electric field of 850 kV/cm. All of these results shed light on that flexible PLZT 9/65/35 thick film capacitor open up a route to practical applications in micro-energy-storage system and on-chip thermal refrigeration of advanced electronics.

1. INTRODUCTION An urgent requirement for highly efficient, lightweight and miniaturized smart circuit systems has risen to the forefront of materials science as the arrival of Internet of Things (IoT).1-3 In such a case, multifunctional flexible electronic materials have attracted tremendous attention. It is because they can further down-scale electronic devices and integrated circuits via the simultaneous integration of multiple separate functional components into a single material system.4-6 Accordingly, there are in pressing need of designing or discovering an extraordinary material/structure to achieve multitask performance simultaneously. Recently, ferroelectric (FE) materials are extensively investigated for fabricating multifunctional electronic devices, because they could exhibit the great superiority simultaneously in dielectric, piezoelectric and

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pyroelectric properties together with a photovoltaic response. Therefore, they are widely explored for applications in energy-storage capacitors, microactuators, pyroelectric security sensors, electrocaloric refrigeration devices and hybrid energy harvesters.7-9 At present, several types of FEs with flexible structure, such as FE polymer-based nanocomposite, all-inorganic FE functional oxides grown on ultra-thin metal substrates and some epitaxial FE thin films on flexible substrates via a conventional peeling-transfer multi-step process, have been reported for applications in next-generation flexible electronic devices.10-13 Although the flexible polymer-based composites possess high dielectric breakdown strength, their thermal stability, lifetime, and radioresistance fail to satisfy the rigorous requirements for flexible smart electronic devices operating in harsh environment.14 In comparison, inorganic FE oxides overcome above disadvantages, therefore they are greatly potential for applications in flexible microelectronic devices.15-18 More recently, the flexible inorganic FE materials grown on the metallic foil have been reported for application in harsh conditions. However, high growth/annealing temperature for obtaining good crystalline quality and electrical performance of the films inevitably causes the interdiffusion between bottom electrode and film. It leads to a high leakage current density and an electrical shorting in films, which severely deteriorate the final electrical performances of the inorganic FE materials. Therefore, it is a big challenge in obtaining the flexible inorganic FE films with excellent crystalline quality and performance.19 Although the current reported technique of “grow-transfer”, i.e., the first growing films on rigid substrates and then transferring to flexible substrates, permits the flexible films with outstanding electrical performances, it suffers from tedious multistep process, high cost and size limitation.20 Thus employing a cost-effective one-step or transfer-free fabrication method on suitable flexible

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substrates to achieve a high-performance inorganic dielectric films for flexible device applications is highly urgent.21 Currently, it is demonstrated that fluorophlogopite mica (F-Mica) as the substrate exhibits a multitude of advantages, including excellent chemical stability, atomically smooth surface, mechanical flexibility, high transparency and insulation together with desired thermal stability (> 950 oC).22 All these features of F-Mica provide new opportunities for the design of advanced flexible inorganic FE films. Multifunctional FE thin/thick film capacitors that combine the energy-storage and refrigeration performance will boost the development of smart microelectronic systems.23,24 As of our best knowledge, the investigation on energy-storage performance and electrocaloric refrigeration simultaneously of inorganic FE films grown on flexible F-Mica substrate has not been reported so far. In this work, Pb0.91La0.09(Zr0.65Ti0.35)0.9775O3 (PLZT 9/65/35) relaxor FE was selected as the ideal multifunctional thick film material and grown on flexible F-Mica substrate by a radio-frequency (RF) magnetron sputtering method. The composition of PLZT 9/65/35 thin/thick films usually exhibit relatively high dielectric constant (εr), large maximum polarization (Pmax), small remanent polarization (Pr) and moderate electric breakdown field (BDS), which are favorable for high energy-storage applications. Moreover, this relaxor composition is close to morphotropic phase boundary (MPB) of rhombohedral and tetragonal phase, where a large number of disordered fluctuating polarizations can lead to extra entropy contributions.25,26 This is conducive to achieve excellent electrocaloric effect (ECE) over a wide operating temperature range.27 As expected, the 1-μm-thick flexible PLZT 9/65/35 relaxor FE thick film possesses both a large energy-storage properties and a high ECE over a wide operating temperature range. Furthermore, the measurements of the electrical characteristics under various bending radii and

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after intensive bending cycles demonstrate that the PLZT 9/65/35 thick film show a good flexible property. This work open up new highlights for flexible electrostatic energy storage and electrocaloric refrigeration applications. 2. EXPERIMENTAL SECTION 2.1 Preparation of PLZT 9/65/35 Ceramic Target. PLZT 9/65/35 RFE thick films were prepared on LaNiO3(LNO)/F-Mica substrate by the RF magnetron sputtering technique from the sintered

ceramic

target.

PLZT

9/65/35

ceramic

target

with

the

composition

of

Pb0.91La0.09(Zr0.65Ti0.35)0.9775O3 was prepared using a conventional solid state reaction route. Stoichiometric amounts of Pb3O4 (95.0%), La2O3 (99.99%), TiO2 (98.0%) and ZrO2 (99.0%) powders were thoroughly mixed with alcohol and then ball mill for 24h. An excess amount of 15mol% Pb3O4 was added to the raw materials to compensate the volatile part of Pb3O4 during high-temperature sintering and to enhance the perovskite phase formation. The mixed precursors were calcined at an optimized temperature of 850 oC for 3h. The calcined powder was reground and then mixed with a binder (polyvinyl alcohol) in order to press into disks of 60 mm in diameter and 3.5 mm in thickness. The disks were subsequently sintered at an optimized temperature of 1180 oC for 3h in air. 2.2 Thick Films Deposition. Both the LNO and PLZT 9/65/35 films were deposited on 20-µm-thick F-Mica substrates via RF magnetron sputtering method. Firstly, 315-nm-thick LNO thin films as the bottom electrode was deposited on F-Mica in Ar and O2 mixed atmosphere by a commercialized LNO ceramic target. Then the PLZT 9/65/35 RFE thick film was deposited by RF magnetron sputtering at room temperature from the above sintered ceramic target. The deposition parameters such as the sputtering time, sputtering power, sputtering pressure and

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sputtering atmosphere of the PLZT 9/65/35 thick films were optimized as a means to achieve the desired film thickness and pronounced energy-storage properties and ECE. The Ar pressure of 2.2 Pa and RF power of 130 W were selected as the optimal deposition parameters. Finally, the PLZT 9/65/35 thick film undergone heat treatment with rapid thermal annealing (RTA) at 650 oC for 30 min to form the perovskite phase. The design schematic diagram and optical images of PLZT 9/65/35/LNO/F-Mica structure were presented in Figure 1a. 2.3 Characterization and Measurements. Crystallization phase structures of the PLZT 9/65/35 RFE thick film were investigated by an X-ray diffraction system (XRD, MiniFlex 600, Rigaku, Japan). The surface microstructure and the thickness were studied by using field-emission scanning electron microscopy (FE-SEM, ZESIS Supra 55, German). The transmitted spectra were obtained by using an ultraviolet and visible spectrophotometer (UV-VIS, K.Satou U-3900, Japan). For the measurements of electrical properties, gold pads of 0.2 mm in diameter were coated on the film surfaces as top electrodes by DC sputtering. The frequency, DC electric field and temperature-dependent dielectric properties were carried out by using a computer-controlled TH2828 LCR analyzer, and the testing temperatures were controlled by the Linkam stage (HFS600E-PB2, England). The electric field-polarization loops (P-E), electric field-leakage current density curves (J-E) and fatigue cycles tests of the PLZT 9/65/35 thick film were obtained by using a FE tester (Radiant Technologies, Inc., Albuquerque, America). A series of predesigned semicircular-shaped copper bars moulds with various curvature radii were used to introduce different tensile and compressive bending strains in the sample, and the mechanical bending cycles were performed by a homemade bending stage.28 3. RESULTS AND DISCUSSION

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The surface and cross-sectional morphologies of the PLZT 9/65/35 thick film are investigated by FE-SEM as shown in Figure 1b and c, respectively. The thick film deposited on LNO/F-Mica is extremely dense without notable pinholes, which is a result of the effective arrangement of the target particles during the thick film deposition. It can be found from the cross-sectional image that the PLZT 9/65/35 thick film and LNO bottom electrode are well integrated on F-Mica substrate. The clear and sharp interfaces in PLZT/LNO and LNO/F-Mica layers are also observed, suggesting the high quality of the multilayer structure without substantial interdiffusion across interfaces. In addition, the thickness of LNO and PLZT 9/65/35 layers are about 315 nm and 1 μm, respectively. Figure 1d shows the X-ray diffraction (XRD) patterns of the LNO thin film and PLZT 9/65/35 thick film grown on F-Mica substrates after annealing at 650 oC. It is noted that the film presents a pure perovskite structure without secondary phase. Moreover, the PLZT 9/65/35 thick film exhibits a preferred (100)-orientation because the (100) and (200) reflection intensities are distinctly higher than of the other diffraction peaks, which may be induced by the same grown orientation of LNO conductive oxide electrodes. Thus, LNO is the ideal bottom electrode to grow high quality PLZT 9/65/35 thick film at the room temperature due to both of them sharing the same structure of the perovskite.29,30 Figure 1e shows the UV-visible transmission spectra as a function of incident light wavelength of the pure F-Mica substrate, LNO/F-Mica and PLZT/LNO/F-Mica structure. The transmittance of the pure F-Mica substrate is 92% in a wavelength range of 350-750 nm, while only 50% and 40% transmittance are obtained in LNO/F-Mica and PLZT/LNO/F-Mica within the same wavelength ranges, respectively. The results show that the F-Mica substrate has an outstanding transparent feature, while PLZT/LNO/F-Mica is semitransparent. Although the PLZT 9/65/35

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possesses a wide band gap of 3.2 eV and it is highly transparent, the LNO bottom electrode has a narrow band gap of 2.2 eV,31 which can effectively absorb incident visible light and thus decrease the transmittance of PLZT/LNO/F-Mica (see Figure S1 and S2, Supporting Information).

Figure 1. (a) Photograph of the PLZT 9/65/35 thick film grown on a flexible LNO/F-Mica substrate and corresponding schematic diagram. (b) The FE-SEM morphology and (c) cross-sectional images of the PLZT/LNO/F-Mica thick films, respectively. (d) XRD patterns of

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PLZT 9/65/35 and LNO films grown on F-Mica substrates. (e) UV-visible optical transmission spectra of pure F-Mica substrate, LNO/F-Mica and PLZT/LNO/F-Mica structures. Figure 2a illustrates the dielectric constant (εr) and dielectric loss tangent (Tanδ) of the PLZT 9/65/35 thick film as functions of frequency measured from 2 kHz to 1 MHz at room temperature. The εr of the PLZT 9/65/35 thick film monotonically decreases with the frequency increasing, demonstrating a typical characteristic of FEs. The dielectric constant of the PLZT 9/65/35 thick film slightly decreases with the frequency increasing before the frequency reaches 105 Hz owing to long-time polarization process of some frameworks. Then it sharply decreases over the frequency range of 105-106 Hz, which can be ascribed to the declined dipole orientation polarization at high frequency.32 On the contrary, a rapid increase of Tanδ is observed at the same frequency range of 105-106 Hz, which may be caused by the high electrical resistance of the bottom electrode LaNiO3.33 The minimum value of the dielectric constant can still reach up to 1500 at 1 MHz, which is much higher than most of the dielectric capacitors, and it is crucial for achieving the high energy-storage density (P = ε0εrE).33,34 Figure 2b shows the electric field-dependent dielectric constants (εr-E) of the PLZT 9/65/35 RFE thick film, which was measured at room temperature and at 100 kHz. Strong nonlinear behavior can be observed and the dielectric constant gradually decrease from 2000 to 400 as the DC electric-field increase from 0 to 400 kV/cm. Hereby, the relative dielectric tunability (nr) can be calculated by:

nr =

ε (0) - ε ( E 0 ) ε (0)

(1)

where ε(0) and ε(E0) represent the dielectric constants at zero and under a certain electric-field, respectively. The dielectric tunability of the PLZT 9/65/35 thick film reaches up to 80%, which is much higher than that of a majority of dielectric tunable materials like PbZrO3, Ba0.5Sr0.5TiO3

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and 0.96NaNbO3-0.04CaZrO3/0.88(K0.5Na0.5)NbO3-0.12SrZrO3 films.35,36 Overall, the flexible PLZT 9/65/35 thick film has a huge dielectric tunability, which endows it with the potential to apply in tunable devices. The temperature-dependent dielectric properties of the PLZT 9/65/35 thick film as a function of frequency (10 kHz-100 kHz) are shown in Figure 2c. A typical diffused phase transition is observed, which indicates a remarkable thermal stability of both εr and Tanδ in a wide temperature range. Moreover, a frequency-dependent εr and Tanδ of the PLZT 9/65/35 thick film are also observed in the T-εr and T-Tanδ curves. The temperature corresponding to the maximum dielectric constant (Tm) is shifted to higher temperature with the frequency increasing. The maximum dielectric constant (εm) decreases monotonically with the frequency increasing. The phenomena mentioned above are a typical characteristic of relaxor behavior, which can be quantitatively analyzed by a modified Curie-Weiss law:37 (T - Tm ) γ 1 1 = ε εm C

(2)

where εm is the maximum dielectric constant and Tm represents the corresponding temperature. γ is the degree of phase transition diffuseness representing the degree of relaxation. In general, γ = 1 represents the Curie-Weiss behavior of normal FEs, while γ = 2 describes a typical relaxor FEs that undergo a complete diffuseness phase transition. The plot of ln(1/ε – 1/εm) as a function of ln(T – Tm) is shown in the inset of Figure 2d, in which the linear relationships can be observed. The slop of fitting line (γ) is 1.99, indicating that the PLZT 9/65/35 has a high degree of relaxor dispersion, which in turn favors a wide operating temperature range in ECE.

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Figure 2. (a) Frequency-dependent and (b) DC electric field-dependent dielectric properties of PLZT 9/65/35 thick film measured at room temperature. (c) Temperature-dependent dielectric properties of PLZT 9/65/35 thick film at various frequencies. (d) The inverse dielectric constant (10 k/εr) as a function of temperature for the PLZT 9/65/35 thick film at 100 kHz. The inset shows the plots of ln(1/εr – 1/εm) as a function of ln(T – Tm) of the PLZT 9/65/35 thick film. For the energy storage and electrocaloric refrigeration applications, it is essential to explore the intrinsic breakdown strength and leakage current density of capacitors. As shown in Figure 3a, the average breakdown strength of the PLZT 9/65/35 thick film is analyzed by a two-parameter Weibull distribution, which can be described as:38,39,40 X i = ln( Ei )

(3)

Yi = ln(- ln(1 - i /(n + 1)))

(4)

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where Xi and Yi are two parameters in Weibull distribution, Ei is the measured breakdown field of each specimen, i is the serial number of specimens and n is the total number of specimens. The Ei of samples are arranged in ascending order of E1 ≤ E2 ≤ ... Ei ... ≤ En. According to the Weibull distribution, Yi and Xi possess a corresponding linear relationship. The average Eb could be extracted from a point where the fitting lines intersect with the horizontal line through Yi = 0, indicating the BDS value of the PLZT 9/65/35 thick film is 1998 kV/cm. Yoshimura et al.41 and Bowen et al.42 concluded that the relationship between Eb and thickness d could be expressed as follows:

E b = cd - α

(5)

where α and c are constants. Obviously, the Eb of films usually decreases with the thickness increasing from formula (5). The dense microstructure is one reason for the high Eb of PLZT 9/65/35 thick film. Another possible reason is the low leakage current density of PLZT 9/65/35 thick film which has been proved from the logarithmic plots of current density as a function of electric field of PLZT 9/65/35 thick film, as shown in Figure 3b. The leakage current density of PLZT 9/65/35 is merely 5×10-7 A/cm2 at the fix applied electric field of 200 kV/cm at room temperature, which is a rather low value. Figure. 3c presents the P-E loops in the first quadrant of PLZT 9/65/35 thick film under various electric fields at room temperature. As the electric field increases from electric field of 200 kV/cm to the average electric breakdown field of 1998 kV/cm, the maximum polarization (Pmax) and remnant polarization (Pr) of PLZT 9/65/35 thick film increase continuously and reach to the values of 95 μC/cm2 and 30 μC/cm2, respectively. In addition, the P-E loops of PLZT 9/65/35 thick film is still keeping slim under high electric field applications due to the large

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difference between Pmax and Pr, which is in favour of obtaining high energy-storage performance. The energy-storage performance can be calculated from the P-E loops by the formula:43,44,45

Wtot =



Pmax 0

EdP

Wrec =



Pmax Pr

EdP

η=

(6)

(7)

Wrec * 100% Wtot

(8)

where Wtot is stored total energy-storage density, Wrec is recoverable energy-storage density, η is the energy-storage efficiency and E is the applied electric field. As expected from the definitions given above, both Wtot and Wrec are increase with the electric fields. The maximum Wrec value of 40.2 J/cm3 is obtained in the PLZT 9/65/35 thick film at its average breakdown field of 1998 kV/cm and room temperature. Meanwhile, it is delightful that the values of energy-storage efficiency of the PLZT 9/65/35 thick film are relatively stable under the measurement electric field ranges, which are only slightly fluctuation between 62% and 58%, as shown in Figure 3d. Figure 3e shows the energy-storage performance comparisons among the representative flexible and inflexible thin/thick film capacitors. It is evident that the thin film capacitors always possess excellent dielectric breakdown strength and large energy-storage density. For example, the Mn-doped 0.97(0.93Na0.5Bi0.5TiO3-0.07BaTiO3)-0.03BiFeO3 and Ba(Zr0.35Ti0.65)O3 thin films possess ultrahigh recoverable energy-storage density of 81.9 and 65.1 J/cm3, respectively.21,17 However, their thicknesses are only 0.35 and 0.13 μm, respectively, which extremely restrict the overall discharged energy of thin films. By contrast, thick film capacitors

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possess relatively low breakdown strength and recoverable energy-storage density less than 40 J/cm3 due to the declined dielectric breakdown field with film thickness increasing.48-51 Although the larger recoverable energy-storage density can be achieved in thin films, the overall discharged energy is greater for thick films due to the large volume. Encouragingly, the flexible PLZT/LNO/F-Mica capacitor in this work exhibits a higher breakdown field of about 1998 kV/cm along with a larger recoverable energy-storage density of about 40.2 J/cm3, as compared with other lead-base thick film capacitors grown on rigid substrates.

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Figure 3. (a) The Weibull plot of the electric breakdown strength of the PLZT 9/65/35 thick film. (b) Electric field versus leakage current density of the PLZT 9/65/35 thick film at 200kV/cm. (c) The P-E loops in the first quadrant of PLZT 9/65/35 thick film under different electric fields. (d) Electric field dependence of energy-storage properties of PLZT 9/65/35 thick film. (e) A comparison of energy-storage properties of newly developed flexible thin film and thick film capacitors, inflexible lead-based thin film and thick film capacitors on rigid substrates.

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Thermal stability is a key factor for the practical application of capacitors in the electronic devices. Thus, the Pmax, Pr, ΔP (ΔP = Pmax – Pr), energy-storage density and efficiency for the PLZT 9/65/35 thick film are measured as a function of temperature from room temperature to 180 oC, as shown in Figure 4a and b. The measured electric field is deliberately carried out at 850kV/cm and is about half lower than Eb on the basis of the theoretical calculations of the devices with a lifetime greater than 10 years.54,55 As the temperature increases from room temperature to 180 oC, the Pmax and Pr are reduced slightly, while the ΔP is maintained. With the temperature increasing, Wtot initially decreases and then increases once the temperature is above Curie point. While, η exhibits the reverse tendency. It is attractive that the variation of Wrec is less than 2% over the whole temperature ranges, indicating the excellent thermal stable energy-storage performance in the PLZT 9/65/35 thick film. Strong fatigue resistance is another essential prerequisite for capacitors to ensure long-term charge-discharge cycle stability. In this study, the fatigue behavior of the PLZT 9/65/35 thick film was measured up to 107 electric cycles at 1 kHz and 850 kV/cm. Figure 4c shows Pmax, Pr and ΔP of the PLZT 9/65/35 thick film before and after different electric cycles, and the inset shows the corresponding first quadrant of the P-E loops. Both the Pmax and the Pr slightly decrease, but the change of ΔP is negligible with the fatigue cycles increasing. The calculated Wrec slightly fluctuates between 16.5 and 15.1 J/cm3 and η varies between 62.1% and 59.2%. Hence, these results demonstrate that the prepared PLZT 9/65/35 thick film capacitor possesses an excellent fatigue endurance.

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Figure 4. (a) The Pmax, Pr and Pmax – Pr of the PLZT 9/65/35 thick film as a function of temperatures. The inset shows the P-E loops in the first quadrant of the PLZT 9/65/35 thick film measured from room temperature to 180 oC at 1 KHz under 850 kV/cm. (b) The Wtot, Wrec and η of the PLZT 9/65/35 thick film as a function of temperatures. (c) The Pmax, Pr and Pmax – Pr of the PLZT 9/65/35 thick film as a function of electric field cyclings. The inset shows the P-E loops in the first quadrant of the PLZT 9/65/35 thick film during the 1 × 107 charging-discharging switching cycles at room temperature under 850 kV/cm. (d) The Wtot, Wrec and η of the PLZT 9/65/35 thick film as a function of electric field cyclings. To evaluate the flexible electronic applications, we investigated the influence of the tensile bending, compressive bending and mechanical bending cycles on the energy-storage performances of the Au/PLZT/LNO/F-Mica capacitor. The measurement schematic for the capacitor at different tensile and compressive bending radii are shown in Figure 5a. The bending states of the capacitor are maintained by rolling the PLZT 9/65/35 thick film on homemade

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semicircular shaped copper bars mould with a certain convex and concave surface bending radii of 10, 7.5 and 5 mm. The tensile and compressive surface strain are 0.21%, 0.28% and 0.42%, correspondingly. The strain is obtained according to the Equation S2, as shown in the Supporting Information. Figure 5b presents the in-situ J-E curves of the flexible PLZT 9/65/35 thick film under the flat state and various curvature radii. Obviously, the measured leakage current density has a similar value of about 3 × 10-8 A/cm2 at flat state and various curvature radii, and the fluctuations of the leakage current density are within the measurement error range. It is demonstrated that the insulating property of the flexible PLZT 9/65/35 thick film is not deteriorated by the mechanical bendings in our experiment. Figure 5c shows the in-situ P-E loops of the flexible PLZT 9/65/35 thick film, which measured at 850 kV/cm and 1 kHz under the flat state and various tensile and compressive bending radii. The corresponding changes in Pmax, Pr and ΔP as a function of tensile and compressive bending radii are shown in Figure 5d. The P-E loops of the PLZT 9/65/35 thick film are almost regardless of the bending strain, and corresponding Pmax, Pr and ΔP also show the slight fluctuation around the constant of Pmax ≈ 79.5 µC/cm2, Pr ≈ 25.9 µC/cm2 and ΔP ≈ 53.6 µC/cm2. All of these results are extremely beneficial to achieving stable energy-storage performances. Specifically, compared with the flat state, the Wtot, Wrec and η of the PLZT 9/65/35 thick film with various tensile and compressive radii show the negligible variations within 2.3%, 4.5%, and 4.1%, respectively, as shown in Figure 5e.

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Figure 5. (a) The schematic diagram of electrical measurements of the flexible Au/PLZT 9/65/35/LNO/F-Mica capacitor on homemade semicircular shaped copper bars mould. (b) Room-temperature J-E plots of the flexible Au/PLZT 9/65/35/LNO/F-Mica capacitor measured in the flat, 5 mm, 7.5 mm and 10 mm tensile/compressive bending radii, respectively. (c) P-E loops in the first quadrant of the flexible Au/PLZT 9/65/35/LNO/F-Mica capacitor at various tensile/compressive bending radii. (d) The Pmax, Pr, Pmax – Pr and (e) Wtot, Wrec and η as a function of bending radii. (f) The photographs of the flexible Au/PLZT 9/65/35/LNO/F-Mica capacitor were bent to 4.5 mm by the mechanical bending stage. (g) The Wrec and η as a function of

bending

cycles.

The

inset

shows

the

P-E

hysteresis

loops

of

the

flexible

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Au/PNZSTBL/LNO/F-Mica capacitor under tensile bending of 4.5 mm before and after 2000 bending cycles. Figure 5f shows the photograph of mechanical bend test with the tensile bending radius of about 4.5 mm for the PLZT 9/65/35 thick film. The dynamics bending processes were performed by a homemade bending stage at 2 Hz and repeated physical bending 2000 times.56 Note that 4.5 mm are generally regarded as bending radii for wearable devices on a human finger. Figure 5g shows the P-E loops and calculated Wrec and η of the PLZT 9/65/35 thick film at pristine and after bending cycles of 2000 times. It is found that the measured P-E loops of the PLZT 9/65/35 thick film almost remain stable before and after bending cycles. The maximum changed values of the Wrec and η are just 3.6% and 1.2%, respectively. These demonstrate that the flexible PLZT 9/65/35 thick film has a great foreground for applications in wearable and flexible devices. It is well known that the continuous phase transition in relaxor FEs is favorable to a large ECE over a wide temperature range. The adiabatic temperature changes (ΔT) and isothermal entropy changes (ΔS) of the FEs at an applied electric field can be calculated by an indirect method based on Maxwell equation:

ΔT = -

1 ρ∫

E2 E1

ΔS = -

1 ρ∫

E2 E1

T ∂P ( ) dE C ∂T E

(9)

∂P ) dE ∂T E

(10)

(

where molar heat capacity C and mass density ρ can be taken as constant in the temperature range of 293-423 K. T is the absolute temperature, and P is the maximum polarization at applied field E. E1 and E2 are the initial and final applied fields, respectively. There have been reported that the values of heat capacity C is 300 J/K kg for PLZT 8/65/35 and theoretical density ρ is

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7.84 g/cm3 for PLZT 9/65/35,57,58 which are selected for PLZT 9/65/35 thick film for ECE calculations. To evaluate the ECE, the P-E loops of PLZT 9/65/35 thick film were measured at 15 oC intervals in the temperature ranges between room temperature and 150 oC at 850 kV/cm, as shown in Figure 6a. The slim P-E loops together with the persistence of ferroelectricity above Tm further confirm the relaxor nature of PLZT 9/65/35 thick film. Figure 6b exhibits the polarization versus temperature data P(T) at different electric fields, and the values of (∂P/∂T)E are obtained from a four-order polynomial fits of raw P(T) data, as show in Figure S3. Accordingly, the ΔS and ΔT in operating temperature range at selected electric fields (i.e., at 50 kV/cm intervals in the electric field ranges between 150 and 850 kV/cm) are calculated based on equation (10) and (11) as shown in Figure 6c and d, respectively. As expected, the large EC response with a wide operating temperature is achieved, and the values of ΔS and ΔT are enhanced with the applied electric field increasing. The largest values of ΔS and ΔT are 15 J/K kg and 18 oC at electric field of 850 kV/cm and 80 oC, respectively, which mainly benefit from the increase of ∂P/∂T (Figure S3, Supporting Information) owing to a drastic variation of electric displacement with temperature.59,60 In addition, it is found that the temperature (80 oC) corresponding to the peak of ΔS and ΔT are slightly inferior its Curie temperature, which is consistent with the previously reported by Zhao et al.61 and Gao et al.62 The largest ΔS and ΔT of PLZT 9/65/35 thick film are 15 J/K kg and 18 oC, respectively, tested at 80 oC and 850 kV/cm. Moreover, the temperature span (Tspan) over which the ΔT maintains 0.8 × ΔTmax of the PLZT 9/65/35 thick film is larger than 60 oC, which indicates that the proposed material in this work possesses an enormous ΔT over a broad temperature range.

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Figure 6. (a) The P-E loops of PLZT 9/65/35 thick film measured from the room temperature to 150 oC and at fixed field 850 kV/cm. (b) Fourthorder polynomial fits of maximum polarization with respect to temperature (T) at different electric fields. (c) Isothermal entropy change (ΔS) and

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(d) adiabatic temperature change (ΔT) of PLZT 9/65/35 thick film as functions of temperatures at different electric fields. (e) ΔT/ΔE and (f) ΔS/ΔE of PLZT 9/65/35 thick films as functions of temperatures. (g) The changes in the refrigerant capacity (RC) as a function of ΔTHL under different electric fields. In order to quantitatively measure the effectiveness of applied electric field change (ΔE) for an ECE in dielectrics, the EC strength ΔT/ΔE and ΔS/ΔE are given in the Figure. 6e and f, respectively. Even though a human body safety voltage (35 V, i.e., 350 kV/cm) is applied on the PLZT 9/65/35 thick film, it is enough to yield large EC strength of ΔT/ΔE ~ 15.2 K cm/V and ΔS/ΔE ~ 12.9 J cm/K kg V at 80 oC. The remarkable EC strength of the PLZT 9/65/35 thick film appears near the ferroelectric-paraelectric phase transition, i.e, at Curie temperature (TC), where there exists a sharp variation of polarization (P) with temperature (T), i.e., ∂P/∂T. Notability, the composition of PLZT 9/65/35 is near the morphotropic phase boundary (MPB) between rhombohedral and tetragonal phase, which means more polarization variants in this composition. Hence, compared with the single composition the increased number of polarization variants lead to a significant increase in the entropy when the phase transition occurs at TC.26,63 These results indicate that flexible PLZT 9/65/35 EC thick film can generate remarkable ΔT and ΔS under relatively low electric fields, so it can meet the requirements of EC materials in flexible wearable personal refrigeration devices. In addition, the refrigerant capacity (RC), which shows the magnitudes of heat that can be transferred in one thermodynamic cycle, is also an inevitable parameter to the comprehensive evaluation the refrigeration performance of the EC materials for assuring equipment operation in harsh conditions.5,64 RC can be calculated by the integral as follows:

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RC = ∫

TH TL

Δ SdT

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(11)

where TL (fixed at 30 oC) and TH are the cold and hot temperatures decision by temperature span of the refrigeration cycle. The changes in the RC as a function of ΔTHL under different electric fields are shown in Figure 6g. RC of the PLZT 9/65/35 thick film is increased with ΔTHL and E increasing and reaches up to a relatively high value of 11.2 J/cm3 at 120 oC and 850 kV/cm. It is superior to the most film materials.5,65,66 The combination of large RC and superior EC strength over the wide temperature range allow the PLZT 9/65/35 thick film to work reliably in practical applications. 4. CONCLUSIONS In summary, the high energy-storage performance and large ECE have been achieved in the all-inorganic flexible PLZT 9/65/35 relaxor ferroelectric thick film simultaneously. The high recoverable energy-storage density of 40.2 J/cm3 together with the energy efficiency of 61% is obtained at the BDS of 1998 kV/cm. The PLZT 9/65/35 thick film retains the superior energy-storage performances at elevated temperature, multiple electric cycles and repetitive mechanical bending cycles. Moreover, the PLZT 9/65/35 thick film possesses a large ΔT, ΔS, ΔT/ΔE, and ΔS/ΔE of 18.0 oC, 15 J/K kg, 15.2 K cm/V, and 12.9 J cm/K kg V, respectively, at TC and 850 kV/cm. The temperature span (Tspan) over which the ΔT maintains 0.8 × ΔTmax of the PLZT 9/65/35 thick film is larger than 60 oC. The combination of these fascinating performances endows flexible PLZT 9/65/35 thick film with huge potential for simultaneous applications in energy storage and electrocaloric refrigeration over a broad temperature range.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (X.H) Author Contributions B.-Z.S. conceived the idea, prepared the samples, performed major tests, analyzed the data, prepared figures and wrote the main manuscript text. Y.L. conducted FE-SEM measurement. X.H. and Y.L. provided advice for the research and revised the manuscript. X.H. supervised this study, and provided intellectual and technical guidance. All authors discussed the results and commented on the manuscript. ORCID Xihong Hao: 0000-0001-8128-6466 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors would like to acknowledge the financial support from the Program for Innovative Research Team in Universities of Inner Mongolia Autonomous Region (NMGIRT-A1605), the Grassland Talent Plan of Inner Mongolia Autonomous Region, the Innovation Guide Fund of Baotou (CX2017-58) and the Innovation Guide Fund for Science and Technology of Inner Mongolia Autonomous Region (KCBJ2018034).

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(48) Xu, X.; Liu, W.; Li, Y.; Wang, Y.; Yuan, Q.; Chen, J.; Ma, R.; Xiang, F.; Wang, H., Flexible Mica Films for High-Temperature Energy Storage. J. Mater. 2018, 4, 173-178. (49) Liu, Y.; Wang, Y.; Hao, X.; Xu, J., Preparation and Energy-Storage Performance of PLZT Antiferroelectric Thick Films via Sol-Gel Method. Ceram. Int. 2013, 39, S513-S516. (50) Zhao, Y.; Hao, X.; Zhang, Q., Energy-Storage Properties and Electrocaloric Effect of Pb(1-3x/2)LaxZr0.85Ti0.15O3 Antiferroelectric Thick Films. ACS Appl. Mater. Interfaces 2014, 6, 11633-11639. (51) Park, C. K.; Lee, S.; Lim, J. H.; Ryu, J.; Choi, D.; Jeong, D. Y., Nano-Size Grains and High Density of 65PMN-35PT Thick Film for High Energy Storage Capacitor. Ceram. Int. 2018, 44, 20111-20114. (52) Hao, X.; Wang, Y.; Yang, J.; An, S.; Xu, J., High Energy-Storage Performance in Pb0.91La0.09(Ti0.65Zr0.35)O3 Relaxor Ferroelectric Thin Films. J. Appl. Phys. 2012, 112, 114111. (53) Zhang, T. D.; Li, W. L.; Zhao, Y.; Yu, Y.; Fei, W. D., High Energy Storage Performance of Opposite Double-Heterojunction Ferroelectric-Insulators. Adv. Funct. Mater. 2018, 28, 1706211. (54) Zhang, G.; Li, Q.; Gu, H.; Jiang, S.; Han, K.; Gadinski, M. R.; Haque, M. A.; Zhang, Q.; Wang, Q., Ferroelectric Polymer Nanocomposites for Room-Temperature Electrocaloric Refrigeration. Adv. Mater. 2015, 27, 1450-1454. (55) Zhang, G.; Zhang, X.; Yang, T.; Li, Q.; Chen, L.; Jiang, S.; Wang, Q., Colossal Room-Temperature Electrocaloric Effect in Ferroelectric Polymer Nanocomposites Using Nanostructured Barium Strontium Titanates. ACS Nano 2015, 9, 7164-7174.

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JH.;

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H.;

Wang

Q.,

Bio-Inspired

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(62) Zhao, Y.; Hao, X.; Zhang, Q., Enhanced Energy-Storage Performance and Electrocaloric Effect in Compositionally Graded Pb(1−3x/2)LaxZr0.85Ti0.15O3 Antiferroelectric Thick Films. Ceram. Int. 2016, 42, 1679-1687. (63) Lu, S. G.; Rozic, B.; Zhang, Q. M.; Kutnjak, Z.; Li, X. Y.; Furman, E.; Gorny, L. J.; Lin, M. R.; Malic, B.; Kosec, M.; Blinc, R.; Pirc, R., Organic and Inorganic Relaxor Ferroelectrics with Giant Electrocaloric Effect. Appl. Phys. Lett. 2010, 97, 162904. (64) Lu, S. G.; Zhang, Q. M., Electrocaloric Materials for Solid-State Refrigeration. Adv. Mater. 2009, 21, 1983-1987. (65) Hamad, M. A., Detecting Giant Electrocaloric Properties of Ferroelectric SbSI at Room Temperature. J. Adv. Dielect. 2013, 03, 1350008. (66) Correia, T. M.; Kar-Narayan, S.; Young, J. S.; Scott, J. F.; Mathur, N. D.; Whatmore, R. W., Pst Thin Films for Electrocaloric Coolers. Journal of Physics D: Applied Physics 2011, 44, 165407. BRIEFS This work is the first time demonstration of a multifunctional all-inorganic flexible Pb0.91La0.09(Zr0.65Ti0.35)0.9775O3 (PLZT 9/65/35) relaxor Ferroelectric (RFE) thick film capacitor, which possesses an excellent energy-storage properties and an outstanding ECE simultaneously over a wide operating temperature range and opens up a new route to practical applications in micro-energy-storage system and on-chip thermal refrigeration of advanced electronics. SYNOPSIS

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Multifunctional all-inorganic flexible thick film capacitor for today’s micro-energy-storage system and on-chip thermal refrigeration of advanced electronic applications.

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