PDF Journal of the European Ceramic Society - NJU

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ceramics synthesized by rapid hot press sintering

Xianlu Gao

^a

, Ye Li

^a

, Jianwei Chen

^a

, Chen Yuan

^a

, Min Zeng

^a,⁎

, Aihua Zhang

^a

, Xingsen Gao

^a

, Xubing Lu

^a

, Qiliang Li

^a,b

, Jun-Ming Liu

^a,c,⁎

aInstitute for Advanced Materials and Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China

bDepartment of Electrical and Computer Engineering, George Mason University, Fairfax, VA 22033 USA

cLaboratory of Solid State Microstructures and Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 21009, China

A R T I C L E I N F O

Keywords:

Lead-free ceramics BiFeO3

Energy-storage density Hot press sintering

A B S T R A C T

Lead-free Bi_1−xSm_xFe_0.95Sc_0.05O₃(x= 0.15–0.19) ceramics were fabricated by rapid hot press sintering, and their structure, ferroelectric and energy storage properties were comprehensively investigated. All the samples are in the mixed phases withR3crhombohedral andPbnmorthorhombic structures. With increasingx, the ferroelectric polarization decreases gradually, while the polarization loop becomes gradually slimed too. An high recoverable energy density (˜2.21 J/cm³) and a large efficiency (˜76%) with good thermal stability (20 °C–120 °C) are obtained under electricfield (230 kV/cm) for the optimized samplex= 0.17. Moreover, transmission electron microscopy and piezo-response force microscopy measurements reveal that the presence of two-phase coexistence favors the formation of polar nano-regions, leading to the linear-towards polarization behaviors and the enhanced dielectric breakdownfield, which is responsible for the superior energy storage performance of Bi1−xSmxFe0.95Sc0.05O3ceramics. These results indicate a significant step to tailor lead-free BiFeO3-based ceramics towards high dielectric energy storage applications.

1. Introduction

Energy storage devices, such as dielectric capacitors, electrochemical capacitors, lithium ion batteries, and fuel cells, play an important role in modern electronic and electrical systems [1–4]. Among these candidates, dielectric capacitors have the highest power density and fastest charging/discharging speed, suitable for pulsed high-power equipment [3]. Besides, the dielectric capacitor is an all-solid-state structure without any chemical reaction, and also exhibits excellent mechanical properties, high temperature stability, and good fatigue properties, which are more reliable and safer than batteries and chemical capacitors. Thus, the dielectric capacitors have attracted ex- tensive research in energy storage applications.

Dielectric energy storage materials have been extensively explored in the antiferroelectric (AFE) and relaxor ferroelectric materials due to their characterizations of small remnant polarization, large saturated polarization and slim hysteresis loop [5–10]. Therefore, these materials could exhibit excellent energy storage density. For examples, Zhao et al.

reported a high recoverable energy density (Wre) of 38 J/cm³in AFE

(Pb1-3/2xLax) (Zr1-yTiy)O3(PLZT)ﬁlms [5]. Hu et al. achieved a highWre

of 61 J/cm³in Pb0.96La0.04Zr0.98Ti0.02O3thinﬁlms with relaxor char- acters [7]. In addition, their thermal stability was studied for practical application in real environments [8–10].

Although remarkable progress has been obtained in PLZT-based materials, these lead-based materials are harmful to health and environment. It is important to develop environmental-friendly, lead-free dielectric materials. Currently, reported works on the lead-free dielectric materials for energy storage applications were mainly focused on traditional relaxor ferroelectrics and solid solutions [11–26]. For examples, Pan et al. fabricated new lead-free relaxor ferroelectric Mn- doped 0.4BiFeO3-0.6SrTiO3thinﬁlm capacitors with an ultrahighWre˜ 51 J/cm³andη˜75% [13]. Yang et al. reported a highWreof 54.9 J/

cm³and a highηof 74.4% in lead-free BiFeO3/Bi3.25La0.75Ti3O12thin ﬁlms [14]. Actually, the reported highWrewas only achieved in the thinﬁlms, while the density in bulk ceramic state would be one order of magnitude smaller due to the thickness limitation. Thus, the study on energy storage density in lead-free bulk materials is practical. However, so far reportedWreis still low (˜1 J/cm³) due to various defects, such as

https://doi.org/10.1016/j.jeurceramsoc.2019.02.009

Received 23 November 2018; Received in revised form 27 January 2019; Accepted 3 February 2019

⁎Corresponding authors at: Institute for Advanced Materials and Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou 510006, China.

E-mail addresses:[email protected](M. Zeng),[email protected](J.-M. Liu).

Available online 05 February 2019

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fracture, porosity and impurity [15–17,20–23].

The lead-free BiFeO3(BFO) is a well-known multiferroic material with a very large polarization (˜100μC/cm²in single crystal or epi- taxialfilms) [27,28], intentionally attractive for energy storage application. However, oxygen vacancies can be induced during the sintering process owe to the volatility of Bi element, resulting in a large leakage current in ferroelectric test. Consequently, the realization of large polarization in pure BFO ceramics is difficult. Alternately, a relatively large polarization could still be achieved by various doping modifica- tions and novel fabrication methods, such as rapid liquid-phase sintering, hot press sintering, chemical leaching, and microwave-hydro- thermal route [29–33]. Wang et al. reported a high saturation polarization (42μC/cm²) in the Sm and Sc co-doped BFO ceramics by the rapid liquid phase sintering [29].

It should be mentioned that for doped BFO ceramics, the mixed phase has been extensively revealed for these compositions [29,32,34,35]. Especially, Xu et al. analyzed the AFE physical mechanism of La- and Nd-doped BFO throughfirst-principle calculations, and predicted that the (Bi0.5Nd0.5)FeO3materials would have a ultra- highWreof up to˜150 J/cm³[33]. These studies expected that BFO- based ceramics have a potential for energy storage applications. Under this motivation, Sm/Sc co-doped Bi1−xSmxFe0.95Sc0.05O3ceramics with xvarying from 0.15 to 0.19 were adopted since these compositions are in the two-phase coexistence region with possible AFE characterizations [31–33]. In this study, the Bi1−xSmxFe0.95Sc0.05O3ceramics were synthesized by a rapid hot press sintering. Their crystal structure, microstructure, ferroelectric, and energy storage properties were system- atically studied. It was revealed that the mixed phases of R3c rhombohedral andPbnmorthorhombic structures could be observed for all the samples (x= 0.15 - 0.19). These ceramics have dense microstructure. Most importantly, a relatively largeWreof˜2.21 J/cm³along with a high efficiencyηof˜76% were obtained atx= 0.17. In addition, the thermal stability from 20 °C to 120 °C was verified forx= 0.17.

These results indicate that the Sm/Sc co-doped BFO-based ceramics are a promising candidate for energy storage applications

2. Experimental details

The Sm/Sc co-doped Bi1−xSmxFe0.95Sc0.05O3(x= 0.15, 0.16, 0.17, 0.18, 0.19) samples were prepared by the rapid hot press sintering method using high-purity powders of Bi2O3 (≥99.99%), Sm2O3

(≥99.99%), Fe2O3(≥99.9%), Sc2O3(≥99.9%) as starting materials.

All the stoichiometric starting materials were ball milled using plane- tary milling with zirconia balls in ethanol for 24 h. After the ball milling and drying, the mixture was pre-sintered at 850 °C for 5 min, followed by a second ball milling. The resultant powder was putted into a gra- phite module with a diameter of 12 cm and sintered in a fast DC hot press oven (DCHP-2000A-03) at 830 °C for 3 min with a stress of 120 MPa and a heating rate of 150 °C/min. Finally, the disk samples were cut and polished into slices with˜0.2 mm in thickness. As a reference, pure BiFeO3(BFO) was also fabricated using the same process as that for the Sm/Sc co-doped BFO ceramics.

The crystal structure of non-doped and co-doped BFO ceramics was characterized by the standard X-ray diffraction (XRD, PANalyticalX’ Pert PRO diffractometer) with CuKαradiation. The microstructure was determined by field emission scanning electron microscopy (SEM, ZEISS Ultra 55). For the SEM observation, the samples were polished and chemically etched with 4% hydrofluoric acid for 3 min.

The dielectric characterizations were measured by a high-performance frequency analyzer (Alpha-A, Novocontrol Technology) withac drive amplitude of 100 mV from 100 Hz to 1 MHz. For the ferroelectric measurement, the Au electrodes were sputtered on top surface of the samples with the diameter of 2 mm, and Au bottom electrodes were deposited on whole surface. The polarization-electricfield (P-E) hysteresis loops were investigated by a ferroelectric tester (Radiant Technology) under different temperatures and frequencies. Domain observations were carried out by combining the brightfield transmission electron microscopy (TEM, JEOL 2011) and the piezo response force microscopy (PFM, Cypher, Asylum Research) measurements.

3. Results and discussion

Fig. 1(a) shows the room temperature XRD patterns of non-doped BFO ceramic and Bi1−xSmxFe0.95Sc0.05O3ceramics (x = 0.15, 0.16, 0.17, 0.18, and 0.19). Clearly, the non-doped BFO ceramic shows very good crystallinity without detectable impure phase (e.g. Bi2Fe4O9). This indicates that the rapid hot press sintering is a practical method to prepare pure phase BFO-based ceramics. The diffraction peaks of the non-doped BFO ceramic can be indexed to the rhombohedral phase (JCPDS No.: 00-014-0181). At doping contentx≥0.15, some diffraction peaks, e.g., (006), (116) and (018), disappear, accompanying with the occurrence of new diffraction peaks. In addition, the broadened peak in the 2θrange of 31°-33° begins doubly splitting, seeFig. 1(b). It should be noted that the intensity of these new peaks is gradually Fig. 1.Room temperature XRD diffraction patterns for non-doped BFO ceramic, and Sm/Sc co-doped Bi_1−xSmxFe0.95Sc0.05O3(x= 0.15, 0.16, 0.17, 0.18, 0.19) ceramics. (b) The enlarged diffraction peaks in the 2θrange of 31°-33° for all the samples.

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enhanced with increasingx. Some peaks in sample atx= 0.19 can be indexed clearly to the SmFeO3 phase with an orthorhombic crystal structure (JCPDS No.: 01-074-1474). This means that the Bi1−xSmxFe0.95Sc0.05O3(x = 0.15 - 0.19) ceramics are almost in the mixed phases with a rhombohedral structure (space groupR3c) and an orthorhombic structure (space groupPbnm). Similar results have been extensively reported in previous papers [29,36–38].

Fig. 2(a)-(e) display the typical morphologies recorded by SEM on the polished and chemically etched fractured surfaces of non-doped BFO and Bi1−xSmxFe0.95Sc0.05O3ceramics (x= 0.15, 0.16, 0.17, 0.18, and 0.19) ceramic samples. It can be seen that all the samples exhibit ﬂaky-like morphology, and the particles are irregular with grain size ranging from 200 nm˜2μm. Noting that the polished surfaces present some pores due to the chemically etched process, implying that some amount of glass phase is presented in the samples. The reason is attributed to the nature of BFO-based ceramics with rapid sintering procedure [29,30].

The relative densityρrof the as-prepared samples was evaluated and displayed inFig. 3(a) as a function of the Sm doping levelx. Theρrfor

the non-doped BFO sample is˜90.6%, much larger than those prepared using traditional solid-state sintering method or sole-gel method (˜85%) [39,40]. Obviously, the dense microstructures in our samples can be attributed to the large mechanical pressure which plays an important role in eliminating pores during the sintering process. Withxincreasing from 0.15 to 0.18, theρrgradually increases and reaches the maximum value of 94.7%. This value is close to other ceramics prepared by hot pressing method [41,42]. A further increase inxto 0.19, theρrdoes not follow the increase behavior, which can probably be due to the non- uniformity of ceramic surfaces. Surely, it should be mentioned that the measured properties such as dielectric and leakage current data (see Fig. S1, support information) are somehow composition-dependent.

The electrical breakdown strength (Eb) is an important physical quantity for energy-storage capability.Ebcan be achieved by analyzing the Weibull distribution function, described by [11,12]:

=

Xi ln( )Ei (1)

= − − +

Y_i ln( ln (1 i n/( 1))) (2)

whereXi andYi are the two parameters in the Weibull distribution Fig. 2.SEM images for the polished and chemically etched fractured surfaces of (a) non-doped BFO ceramic, and Sm/Sc co-doped Bi_1−xSmxFe0.95Sc0.05O3ceramics for (b)x= 0.15, (c)x= 0.16, (d)x= 0.17, (e)x= 0.18, and (f)x= 0.19.

Fig. 3.(a) Relative density (ρr) as a function of Sm doping content in Bi_1−xSm_xFe_0.95Sc_0.05O₃(x = 0.15-0.19) ceramics, the reference line is for the non-doped BFO ceramic. (b) Weibull distribution for the dielectric breakdown strength (Eb) in all the ceramics.

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function,Eiis the specific breakdown voltage,iis the serial number of specimens, andnis the sum of specimens of each sample. According to this function, the Weibull distribution plots are shown inFig. 3(b) for the samples with differentx. The value ofEbcould be extracted from the intercept on the x-axis of the linearfitting lines in the Weibull distribution plot. As seen inFig. 3(b),Ebfor the non-doped BFO ceramic is

˜ 190 kV/cm, consistent with previous report. Asxincreases, Ebﬁrst increases and then decreases. The maximumEbis˜233 kV/cm atx= 0.17. The enhancedEbmay possible be induced by the two-phase coexistence with densiﬁed structure, and further discussion will be presented below.

Fig. 4(a) presents the room temperature polarization - electricﬁeld (P - E) hysteresis loops of non-doped BFO and Bi_1−xSmxFe0.95Sc0.05O3

(x = 0.15 to 0.19) ceramics. The measurement was performed at 200 kV/cm with a frequency of 100 Hz. The non-doped BFO ceramic exhibits a saturatedP - Eloop with a large remnant polarization (Pr)

value of˜34.6μC/cm², and a coerciveﬁeld (Ec) of˜96 kV/cm, consistent with previous reports [29,30]. In contrast,Pratx= 0.15 is almost the same with the non-doped BFO ceramic, implying that a small amount of Sm doping (x≤0.15) cannot narrow the loop hysteresis.

With increasingx, both maximum polarization (Pm) andPrare gradually decreased due to the contribution of the non-polar orthorhombic phase. Oncex =0.17, thePrandEcdecreased sharply from˜20.5μC/

cm²and˜104 kV/cm atx= 0.16 to ˜4.6μC/cm²and˜33 kV/cm, respectively, implying a phase transition from the ferroelectric to relaxor- like, and the shape ofP - Eloops is signiﬁcantly slimed, which is expected to have a potential in energy storage applications.

In term of theP - Eloops inFig. 4(a), the energy storage density (W) can be calculated using the formula [5,8,24,25]:

∫

=

W EdP (3)

Fig. 4.(a) Room temperature polarization-electricﬁeld (P-E) hysteresis loops of non-doped BFO ceramic, and Sm/Sc co-doped Bi_1−xSmxFe0.95Sc0.05O3(x= 0.15- 0.19) ceramics, and (b) the corresponding recoverable energy storage density (W_re) and the energy storage eﬃciency (η) versus Sm content.

Fig. 5.(a) Room temperature polarization-electricﬁeld (P-E) hysteresis loops under various electricﬁelds for the ceramic atx= 0.17 and (b) the correspondingWre

andηversus the electricﬁelds. (c)P-Eloops of the ceramics atx= 0.17 at diﬀerent temperatures at 200 kV/cm and (d) the correspondingW_reandηversus temperatures.

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ﬁ

Theηis almost saturation atx≥0.17 with a value of˜74%.

Note thatWregreatly depends on the measuredE. In order to illustrate their relationship, the optimized samplex= 0.17 was selected and itsP-Eloop was measured under increasingEtill the sample was breakdown. As shown in Fig. 5(a), all of the Pm,Pr, andEc increase linearly with increasingE. Consequently, the evaluatedWre, as shown in Fig. 5(b), increases linearly, while the evaluated ηalmost keeps a constant value. The maximumWreandηvalues are˜2.21 J/cm³and˜ 76%, respectively, obtained atEb˜230 kV/cm.

The thermal stability is one of important factors for the energy storage applications. Here, the samplex= 0.17 with the optimizedWre

was selected to illustrate its thermal stability. TheP - Eloops at frequency (f= 100 Hz) and electricﬁeld strength (E= 200 kV/cm) under various temperatures (T ∈ 20 °C–170 °C) were measured. The re- presentativeP - Eloops at selectedTare shown inFig. 5(c). It is revealed thatThas a slight impact on the shape ofP-Eloops. BothPrand Pmincrease gradually with increasing temperature. The corresponding Wreandη, as shown inFig. 5(d), remains constant until T≥120 °C, indicating a good thermal stability. Furthering increasingT, the Wre

presents a slight decrease due to the thermal induced enhanced con- ductivity [8–10]. Also, the frequency independentP - Eloops (see Fig.

S2, support information) further prove that theWreis reliable, which is related to polarization switching, but not leakage current injection.

Note that these high energy storage properties in our BFO-based ceramics may be associated with the rapid hot press sintering procedure, in which the pressure load does help the sintering process of the samples increasing the relative density, so that the ceramics can stand higher voltages and show higher energy storage properties [43]. In addition, our hot press sintering process can be seen as a relative sealed environment with short sintering time (˜3 min). Thus, the Bi volatili- zation is weaker compared with traditional rapid sintering, which can also eﬀectively improve the polarization and energy storage performances [15,29,30,43]. Using comparative analysis, several lead-free ferroelectric ceramics about their ferroelectric and energy storage

storage properties of Bi1−xSmxFe0.85Sc0.05O3ceramics, we performed the domain structure measurements for three representative samples, i.e., the non-doped BFO, and the Sm doped ceramics atx= 0.15, and 0.17 by transmission electron microscope (TEM) and piezo response force microscope (PFM).Fig. 6presents the brightﬁeld TEM images for the three samples. It can be found that the non-doped BFO ceramic, see Fig. 6(a), exhibits an obvious stripe domain structure with the width ranging from 80 nm to 200 nm, implying the decent ferroelectricity.

The corresponding selected area electron diﬀraction (SEAD), see Fig. 6(b), taken along [100] plane, reveals a pseudo-cubic symmetry with the equald-spacing of (001) and (010) planes and nearly 90° axial angle, which is consistent with Rhombohedral phase identiﬁed by XRD.

As for x = 0.15, the microstructure has only a slight diﬀerence, as shown inFig. 6(c), in spite of a moderate Sm doping into BFO matrix.

The stripe domain structure is still observable, but the domain width is decreased. It is noted that some splitting spots can be found, see the red dot inFig. 6(d) for [100] SAED, implying a secondary phase may be formed, consistent with the XRD results. In contrast, a random nanodomains (or polar nanoregions), seeFig. 6(e) is presented in thex= 0.17 sample, implying the coexistence of ferroelectricity and relaxor- like [11]. In particular, the two extra sets of 1/2{000} superlattice re- ﬂections in the [100]/[010] SAEDs are observed, see Fig. 6(f), indicating that the symmetry structure is lowered on the local scale with oxygen octahedral tilting, in comparison with the pseudo cubic matrix of non-doped BFO. It is impossible that chemical inhomogeneity is responsible for the formation of mixed structure in Sm/Sc modiﬁed BFO ceramics, as evidenced by the homogeneous distribution of all elements (see Fig. S3 and Fig. S4 for the element composition and mapping of samplex= 0.17, respectively, support information).

Actually, the domain structure of Bi1−xSmxFe0.85Sc0.05O3ceramics can be directly observed by PFM measurement, and the results are shown inFig. 7. Similar to the TEM results, the non-doped BFO, see Fig. 7(a), exhibits a clear stripe contrast ferroelectric domain with the width of ˜200 nm and lengthiness of ˜1μm. As for x = 0.15, see

Table 1

Comparison of ferroelectric and energy storage properties between Bi1−xSmxFe0.95Sc0.05O3(x= 0.17) ceramic and other lead-free ceramics.

Compounds Eb(kV/mm) Pm(μC/cm²) Pr(μC/cm²) Wre(J/cm³) η(%) Ref.

Ba0.3Sr0.7TiO3 21 12 2.1 1.13 86.8 [15]

(Bi0.5Na0.5)TiO3-SrTiO3 6.5 30 3.5 0.65 73.6 [20]

(Bi0.5Na0.5)TiO3-BaTiO3-KNbO3 10 28 2.6 0.89 73 [17]

(Bi0.5Na0.5)TiO3-BaTiO3-NaNbO3-ZnO 14 20 3.2 1.03 72 [12]

(Bi0.5Na0.5)TiO3-Ba0.85Ca0.15Ti0.9Zr0.1O3 9.35 26 3.5 0.87 81 [16]

Bi0.5-xLaxNa0.40K0.10Ti0.98Zr0.02O3 8 38 2.1 1.00 70 [19]

BaTiO3-Bi(Mg0.5Zr0.5)O3 27 23 2.3 2.9 86.8 [11]

BaTiO3-Bi(Zn2/3Nb1/3)O3 13.1 8 0.1 0.79 93.5 [21]

BaTiO3-Ba(Mg2/3Nb1/3)O3 14 14 0.4 1.13 93 [22]

Srx(Bi1-xNa0.97-xLi0.03)0.5TiO3 13 34 1 1.7 87.2 [45]

Bi0.5K0.5TiO3-0.06La(Mg0.5Ti0.5)O3 17 28 6 2.08 82 [43]

BiFeO3-BaTiO3-Nb2O5 9 25 5.3 0.71 58 [23]

BiFeO3-BaTiO3-Ba(Mg1/3Nb2/3)O3 12.5 38 5.7 1.56 75 [24]

BiFeO3-BaTiO3-La(Mg1/2Ti1/2)O3 13 37.5 4.2 1.66 82 [25]

BiFeO3-BaTiO3-Ba(Zn1/3Ta2/3)O3 18 36.2 2.6 2.56 80 [27]

Bi0.83Sm0.17Fe0.95SC0.05O3 23 25.2 4.4 2.21 76 This work

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Fig. 7(b), the striped phase contrast is still observed, but the lengthiness of domains become shorter. While for x = 0.17, see Fig. 7(c), it is diﬃcult to observe homogeneous and continuous stripe domains, and

the domain structure becomes to be a maze-type nanodomains with random configuration, implying a transition from the ferroelectric to like relaxor states. Generally, the shape of domains can affect the Fig. 6.Brightfield TEM micrographs of the Bi_1−xSm_xFe_0.95Sc_0.05O₃ceramics for (a)x= 0, (b)x= 0.15, and (c)x= 0.17.

Fig. 7.Domain structures of Bi_1−xSmxFe0.95Sc0.05O3ceramics observed by PFM: (a)x= 0, (b)x= 0.15, (c)x= 0.17.

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phases withR3crhombohedral andPbnmorthorhombic structures for all the samples. Scanning electron microscopy revealed that all the samples possess pore-free and dense microstructure. The slim P-E hysteresis loops were exhibited forx≥0.17. Interestingly, superior energy storage performances with aWreof˜2.21 J/cm³and anηof˜76% were observed forx= 0.17. Furthermore, the thermal stability from 20℃to 120℃was explored atx= 0.17. TEM and PFM tests revealed that the nano-domains are presented atx≥0.17, which increase the threshold ﬁeld, and result in the slim loops compare to the typical ferroelectric domains, which is responsible for the high energy storage performances in Bi1-xSmxFe0.95Sc0.05O3lead-free ceramic. These results indicate that lead-free BFO-based ceramics are tailorable for promising candidates in high energy storage dielectric capacitors.

Acknowledgements

This work was supported by the National 973 Projects of China (2015CB654602), the National Key Research Program of China (No.

2016YFA0201004), the National Natural Science Foundation of China (Grant Nos.: 51332007, and 11574091), the Natural Science Foundation of Guangdong Province of China (No. 2015A030313375), and X. B. Lu thanks for the support from the Project for Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2016).

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jeurceramsoc.2019.02.

009.

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