large enhancement of the electrocaloric effect in plzt ceramics prepared by hot pressing

The resulting superior breakdown strength of hot-pressed PLZT enables the application of high electric field to induce a giant electrocaloric effect, in which the adiabatic change of tem

Large enhancement of the electrocaloric effect in PLZT ceramics prepared by hot-pressing

Guangzu Zhang, Zhibiao Chen, Baoyan Fan, Jianguo Liu, Mo Chen, Meng Shen, Pin Liu, Yike Zeng, Shenglin Jiang, and Qing Wang,

Citation: APL Mater 4, 064103 (2016); doi: 10.1063/1.4950844

View online: http://dx.doi.org/10.1063/1.4950844

View Table of Contents: http://aip.scitation.org/toc/apm/4/6

Published by the American Institute of Physics

Large enhancement of the electrocaloric effect

in PLZT ceramics prepared by hot-pressing

Guangzu Zhang,1,2Zhibiao Chen,2Baoyan Fan,1,2Jianguo Liu,3Mo Chen,2

Meng Shen,2Pin Liu,2Yike Zeng,2Shenglin Jiang,2and Qing Wang1, a

1Department of Materials Science and Engineering, The Pennsylvania State University,

University Park, Pennsylvania 16802, USA

2School of Optical and Electronic Information, Huazhong University of Science

and Technology, Wuhan, Hubei 430074, China

3Wuhan National Laboratory for Optoelectronics, Huazhong University of Science

and Technology, Wuhan, Hubei 430074, China

(Received 27 February 2016; accepted 29 April 2016; published online 16 May 2016)

In this contribution, we demonstrate the optimization of the microstructures of the Pb0.85La0.1(Zr0.65Ti0.35)O3(PLZT) relaxor ferroelectric ceramics and subsequent enhancements in their polarization and electrical resistivity by using a hot-pressing process The resulting superior breakdown strength of hot-pressed PLZT enables the application of high electric field to induce a giant electrocaloric effect, in which the adiabatic change of temperature (∆T ) and the isothermal change of entropy (∆S) are around 2 times greater than those of the samples prepared by the conventional sintering approach using muffle furnace Moreover, the addition of extra PbO to make

up the loss of Pb in the high-temperature sintering leads to the further improvements

in the phase composition and electrical properties of PLZT, due to inhibition of the pyrochlore phase formation The relationship among the sintering conditions, the content of excess PbO, and the microstructure as well as the electrical character-istics of PLZT have been investigated in a systematic manner This work provides

a facile approach to enhanced electrocaloric effect in bulk ceramics.C 2016 Au-thor(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4950844]

The electrocaloric effect (ECE) is the reversible thermal changes of a ferroelectric material upon the application and removal of an electric field.1 6 ECE is of great potential in realizing advanced solid-state cooling devices for integrated electronics and sensors Also, EC cooling would

be an alternative to the conventional compression-expansion cooling technology, due to its supe-rior cooling efficiency and more importantly, elimination of the greenhouse-gas coolants such as Freon gases, chlorofluorocarbons (CFCs), and hydrochlorofluorocarbons (HCFCs) that can react with the atmospheric ozone and eventually form ozone holes when entering the atmosphere.2 Be-fore 2006, only weak ECE, for example, a ∆T less than 1◦C, was obtained in ferroelectrics.5

Recently, remarkable ECE has been demonstrated in lead zirconate titanate (PZT) thin films and poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene)s, both of which exhibit a ∆T of

12◦C.7 , 8Since then, ECE has been extensively investigated in ferroelectric ceramics,1 3polymers, and polymer nanocomposites.9 12While ferroelectric polymers have high breakdown strength and can be easily processed, the electrocaloric strength of ferroelectric polymers is low, indicating that

a large electric field (>1 MV cm−1) is needed for sizable ECE.9 12 Although ceramic thin films possess excellent electrocaloric strength, they are limited by the heat absorption capacity.13On the other hand, ferroelectric bulk ceramics enjoy high electrocaloric strength and large heat absorption capacity However, the breakdown strength of the bulky ceramics needs to be further improved so that a giant ECE can be obtained in bulk ceramics at high fields.15

a Author to whom correspondence should be addressed Electronic mail: wang@matse.psu.edu

064103-2 Zhang et al. APL Mater 4, 064103 (2016)

The poor breakdown strength of bulk ceramics can be attributed to the non-ideal density of the samples.14Although glasses can serve as sintering aids to decrease the porosity and enhance the breakdown strength of ferroelectric ceramics,15 – 17 the introduction of glasses usually deterio-rates the ferroelectric properties.15 , 16While the sintering atmosphere can be controlled to increase the density of ferroelectric ceramics, the densification capacity is strongly dependent on various complicated factors such as the compositions of the materials.18Different from the above-mentioned approaches, hot-press sintering has been utilized to fabricate high-density ceramics, in which the ce-ramics are directly pressed by a high mechanical pressure during the high-temperature densification process, effectively removing pores from samples.14 , 19 – 22

In this contribution, a typical relaxor ferroelectric material, PLZT, in which the giant ∆T and

∆S are attributable to the specific composition that locates at the morphotropic phase boundary (MPB),23 , 24has been prepared by the hot-press and solid-state sintering processes, respectively The hot-pressed samples present superior dielectric, ferroelectric, and electrocaloric properties to those

of specimens sintered via the common method, owing to the high density and favorable micro-structure benefited from the introduction of mechanical pressure during the hot-press sintering and densification processes We also demonstrated that the phase composition and electrical properties

of PLZT can be further optimized by adding an appropriate content of excess PbO

Analytical-grade PbO (99.9%), La2O3(99.9%), TiO2(99.8%), and ZrO2(99.6%) were mixed according to the formula Pb0.85La0.1(Zr0.65Ti0.35)O3to prepare the PLZT ceramics To investigate the influence of lead volatilization during the sintering process, excess PbO with a variable amount was introduced into the samples The mixed powders were milled for 4 h, calcined at 900◦C for 3 h, and milled again for 4 h Subsequently, the green compacts were pressed at a pressure of 10 MPa The samples were then sintered by the muffle and hot-pressing furnaces, respectively The samples with 0, 2, 4, 6, 8, and 10 mol% excess PbO sintered at 1250◦C in the muffle furnace for 3 h were marked as S0, S1, S2, S3, S4, and S5, respectively In the hot-pressing process, the green compacts were buried in zirconium oxide powders and pressed with a silicon carbide die (50 mm in diameter)

at 1120◦C for 2 h under a pressure of 100 MPa The PLZTs with 0, 2, 4, 6, 8, and 10 mol% excess PbO were labeled as H0 to H5, respectively After hot-pressing, the samples were annealed in air

at 980◦C for 2 h Next, all the sintered PLZTs were cut into pieces (8 mm × 8 mm) and polished

to a thickness of 80 µm Lastly, gold electrodes with a diameter of 5 mm were sputtered (Denton Vacuum, Desk IV) on both sides of the samples

The X-ray diffraction (XRD) analysis was carried out using PANalytical X’pert Pro MPD theta-theta diffractometer The microstructures of the samples were analyzed by using Hitachi S-4800 field emission electron microscope (SEM) Densities of the specimens were measured by the Archimedes method Dielectric spectra of the unpoled samples were acquired over a broad temperature range using a Hewlett Packard 4284A LCR meter in conjunction with a Delta De-sign oven model 2300 High-field ferroelectric hysteresis loops were collected using a modi-fied Sawyer-Tower circuit The resistivity of the samples was tested under an applied field of

30 kV cm−1 Dielectric breakdown strength was measured by the TREK P0621P instrument The breakdown results were statistically calculated by the Weibull distribution function.10The details of ECE measurement are presented in our previous work.10 , 11

It is well known that the pyrochlore phase is likely to form in the PZT systems when volatilized lead cannot be adequately compensated during the high-temperature sintering and densification pro-cesses, which have a negative impact on the dielectric and ferroelectric properties of the ferroelec-tric ceramics.21 , 25Since the quantity of Pb loss is strongly associated with the sintering methods and processes, we analyzed the XRD diffraction patterns of the PLZT ceramics with various quantities

of excess PbO prepared by using the two different processes, to determine the optimized content of excess PbO in different sintering methods As indicated in Fig.1(a), in addition to the reflections

of the perovskite phase, the diffraction peaks arising from the pyrochlore phase are evident in the samples with less amounts of excess PbO, whose intensity decreases with the increase of excess PbO content It is noted that the characteristic peaks corresponding to the pyrochlore phase in the hot-pressed samples are much weaker than those of the conventional sintered ceramics with the same concentration of PbO The characteristic peak of the pyrochlore phase in the hot-pressed samples can be fully depressed with 4 mol% excess PbO, whereas 8 mol% excess PbO is required

FIG 1 (a) XRD di ffraction patterns and (b) relative densities of the PLZT ceramics with various contents of excess PbO sintered by the conventional and the hot-press sintering methods.

for the PLZTs sintered by the conventional method It is believed that, compared to the conven-tional sintering method, the sintering temperature and holding time required for the solid-state mass transfer and densification in the hot-press sintering approach are lower and shorter, limiting the va-porization of lead Figs S1 and S226present the cross-sectional SEM images of the PLZTs sintered with the two different processes Obviously, there exist a large number of pores distributed in the conventional sintered samples whose porosity is significantly higher than those of the hot-pressed samples; this is in accordance with the density data obtained via the Archimedes method (Fig.1(b)) These results explicitly demonstrate that the hot-press sintering is an efficient method to enhance the density of the PLZT ceramics by the application of a high mechanical pressure on the sam-ples during the high-temperature densification process Furthermore, as revealed in Fig 1(b)and Figs S1 and S2,26the densities are maximized at 4 mol% and 8 mol% loadings of excess PbO in the hot-pressed and the conventionally sintered samples, respectively, suggesting that a proper amount

of excess PbO can further benefit the densification of the PLZT ceramics However, excessive PbO results in the decrease of density This is because the excessive PbO leads to the formation of a liquid phase during the inchoate sintering stage, creating closed pores which are difficult to be eliminated from the ceramics by the subsequent solid-state mass transfer process.25 It has been well established that the electrical properties, especially the electrical resistivity and the breakdown strength which determine the highest electric field that can be applied on ferroelectric ceramics, rely largely on the density of the ceramic samples.14Consequently, a high ECE is expected in the hot-pressed PLZT with an optimized content of excess PbO

The temperature dependence of dielectric constants and losses at different frequencies is shown

in Figs S3-S6.26All the samples exhibit broad dielectric constant peaks whose positions shift progres-sively to high temperatures with the increase of frequency (Figs S3 and S4),26indicative of the relaxor ferroelectric nature of the prepared PLZT samples Since ECE is associated with the ∆S of phase tran-sition in ferroelectric materials induced by the applied electric field, a strong ECE can be achieved in

a ferroelectric material containing more polar-states with a similar energy level, for example, ferro-electrics with a composition located at the MPB.27As Pb0.85La0.1(Zr0.65Ti0.35)O3is a typical relaxor ferroelectric material with a composition close to the MPB,28it generally has a larger ECE than other ferroelectric ceramics.23 , 24It can be seen from Figs S5 and S626that the hot-pressed samples have higher dielectric constants and lower dielectric losses than those of the conventional sintered specimens containing the same content of PbO, owing to their higher density and lower content of pyrochlore phase A decrease of the dielectric constant, accompanied by the increase of the dielectric loss, is observed for both the conventionally sintered and the hot-pressed PLZTs when excessive PbO is intro-duced In addition, the Tm(i.e., the temperature corresponding to the dielectric constant peak) of the hot-pressed PLZTs is lower when compared to those of the samples sintered with the conventional process It is accepted that the internal stress among the grains of samples with higher density is larger than that of low-density ceramics The paraelectric phase stability increases as the increase of internal stress,21 , 29giving rise to a lower Tcof the hot-pressed PLZTs The polarizations of the hot-press sintered samples are greater than those of the conventionally prepared specimens, and the hot-pressed PLZT with 4 mol% of excess PbO exhibits the highest polarization (Fig.2), because of its highest density

064103-4 Zhang et al. APL Mater 4, 064103 (2016)

FIG 2 Hysteresis loops of the PLZT ceramics with various contents of excess PbO sintered by (a) the conventional and (b) the hot-press sintering methods (c) The polarizations of the PLZTs with various contents of excess PbO sintered by the two

di fferent approaches.

and the absence of pyrochlore phase and excessive PbO Since ECE stems from the change of dipole orientation, high polarization of the hot-pressed samples indicates their strong ECE.6

For ferroelectrics, a high electrical resistivity is essential for a large dielectric breakdown strength.30 Otherwise, a large electric current and a breakdown in the sample are likely to occur

FIG 3 (a) Electrical resistivity, (b) Weibull plots of breakdown strength, and (c) Weibull breakdown strength of the PLZT ceramics sintered by the conventional and the hot-press sintering methods.

FIG 4 (a) ∆T and (b) ∆S of the PLZT ceramics with various contents of excess PbO sintered by the conventional and the hot-press sintering processes.

when subjected to a high electric field because of the formation of the conduction channels.30,31

A high resistivity of >1013 Ωm, 2 orders of magnitude higher than those of the PLZTs sintered

in the muffle furnace, has been obtained in the hot-pressed samples, attributable to the elimination

of pores which act as defects to negatively affect the high-field resistivity of the ceramics As a result, the breakdown strength increases from ∼100 kV cm−1in the conventionally sintered PLZTs

to >200 kV cm−1 in the hot-pressed samples and reaches the highest value of 230 kV cm−1 in H2 (Figs 3(b)and 3(c)) These findings confirm that hot-press sintering and introduction of an adequate quantity of excess PbO significantly contribute to the high breakdown strength in the PLZT ceramics

Fig.4 plots the ECE of the PLZTs measured at room temperature As expected, the ∆T and

∆Sof the hot-pressed samples are higher than those of the PLZTs prepared from typical solid-state process at an electric field of 100 kV cm−1, owing to the optimized ferroelectric properties in the hot-pressed PLZTs derived from their higher density It is important to note that the best ECE, for either the conventionally sintered or the hot-pressed PLZTs, is achieved at an appropriate quantity

of excess PbO content, indicating that both the pyrochlore phase and excessive PbO should be avoided in the ECE materials Because of the limited breakdown strength, only an electric field

of 100 kV cm−1 is allowed to be applied on the conventionally sintered PLZTs Notably, the hot-pressed samples display much larger breakdown strengths which are more than two times those

of the conventionally sintered samples At 200 kV cm−1, a giant ECE, including a ∆T of 3.1◦C and a

∆Sof 3.3 J kg−1K−1, has been achieved in the hot-pressed PLZT with 4 mol% of excess PbO, which are 100% higher than the best ECE obtained in the PLZT fabricated by the conventional sintering approach, with a ∆T of 1.5◦C and a ∆S of 1.6 J kg−1K−1measured at 100 kV mm−1

In summary, the microstructures, phase compositions, and electrical performances of the PLZTs fabricated via these two different sintering methods have been systematically compared The results demonstrate that the hot-press sintering can be implemented to improve the phase compo-sition and density of the ferroelectric ceramics and enhance its polarization, electrical resistivity, and breakdown strength In addition, the electrical and dielectric properties of PLZT can be further tailored by introducing an appropriate amount of excess PbO Benefiting from the optimization in the microstructures, phase compositions, and especially, the breakdown strength which allows the application of a relatively high electric field, a giant ECE has been achieved in the hot-pressed PLZT, which is more than 2 times that of the conventionally sintered specimens We envisage that this work would contribute to the development of high-performance electrocaloric ceramics and their fabrication technology for practical cooling applications

We acknowledge the support from the National Science Foundation (No CMMI 1361713) and the National Science Foundation of China (Grant Nos 61378076 and U1532146)

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