Enhanced energy density in Mn-doped (1-x)AgNbO3-xCaTiO3 lead-free antiferroelectric ceramics

Journal of Alloys and Compounds xxx (xxxx) xxx

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Enhanced energy density in Mn-doped (1-x)AgNbO3-xCaTiO3 lead-free antiferroelectric ceramics Yonghao Xu a, *, Yan Guo a, Qian Liu a, Yuehong Yin a, Jiale Bai a, Long Lin b, Jingjing Tian a, Ye Tian c, ** a b c

School of Physics and Electronic Information Engineering, Henan Polytechnic University, Jiaozuo, 454003, China School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo, 454003, China Electronic Materials Research Laboratory, Xi’an Jiaotong University, Xi’an, 710049, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 August 2019 Received in revised form 16 November 2019 Accepted 2 December 2019 Available online xxx

Ceramics of 0.2 wt% Mn-doped (1-x)AgNbO3-xCaTiO3 (x ¼ 0.00e0.04) were prepared in flowing oxygen with the solid state method. The microstructure, antiferroelectricity and energy storage performance were investigated to explore the potential for use in energy storage capacitors. Incorporation of CaTiO3 in AgNbO3 effectively inhibits the grain growth and gives rise to high dielectric breakdown strength. The temperature-dependent dielectric property and Raman spectroscopy measurements demonstrate the enhanced stability of antiferroelectricity by CaTiO3 addition through adjusting the M1eM2 phase transition. As a result of both improved antiferroelectricity and dielectric breakdown strength, a high recoverable energy density of 3.7 J/cm3 was achieved in the 2 mol% CaTiO3-modified AgNbO3 ceramic, which represents one of the highest recoverable energy density in recently studied lead-free ceramics. Furthermore, the energy storage performance displays an excellent thermal stability with a low variation (3%) over a wide temperature range (20e120  C). These results indicate that modifying AgNbO3 simultaneously on the A- and B-sites in the ABO3 perovskite structure may lead to the discovery of new antiferroelectric materials with a high energy density. © 2019 Elsevier B.V. All rights reserved.

Keywords: AgNbO3 Antiferroelectric materials Phase transition Energy storage density

1. Introduction Dielectric materials play a crucial role in modern electronics and associated devices, especially in energy storage capacitors. Currently, the world research communities have been actively seeking advanced dielectric materials that exhibit higher energy storage performance, in order to meet the increasing requirements of miniaturization of electronic devices. The recoverable energy storage density (Wrec) of a dielectric capacitor can be determined by measuring the electric polarization hysteresis loop of the dielectric RP material and calculated according to Wrec ¼ Prm EdP, where Pm is the maximum polarization at the peak electric field and Pr is the remanent polarization when the applied field is unloaded. In comparison to ferroelectric (FE) and linear dielectric materials, antiferroelectric (AFE) materials have been considered as the

leading contender for energy storage capacitors due to their high Wrec and fast charge-discharge rates [1e3]. The charge-discharge process of an AFE capacitor is realized through the reversible AFE-FE phase transition. The most extensively studied AFE materials are based on the prototype compound lead zirconate (PbZrO3) [4,5]. However, the toxicity of lead-based derivatives raises great environmental and human health concerns [6e8]. Thus, numerous efforts have been made to explore suitable lead-free alternatives. Most recently, silver niobate (AgNbO3) is considered to be a most promising lead-free AFE material due to its double-like polarization hysteresis loops, large maximum polarization (~52 mC/cm2) [9], and high Wrec of 2.1 J/m3 [10]. AgNbO3 possesses a complex series of polymorphic phase transitions accompanying with a series of dielectric anomalies upon heating [9e13]: 70 C

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Xu), [email protected] (Y. Tian).

270 C

350 C 380 C 580 C

M1 / M2 / M3 / O / T / C; where M1, M2 and M3 denote the phases with orthorhombic symmetry in rhombic orientation, while O denotes the phase with

https://doi.org/10.1016/j.jallcom.2019.153260 0925-8388/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Y. Xu et al., Enhanced energy density in Mn-doped (1-x)AgNbO3-xCaTiO3 lead-free antiferroelectric ceramics, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153260

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orthorhombic symmetry in parallel orientation. T and C are the phases with tetragonal and cubic symmetries, respectively [14e16]. The M2 and M3 phases exhibit a centrosymmetric Pbcm space group, where antiparallel displacements of Agþ and Nb5þ cations are found in the unit-cell (i.e., AFE) [17,18]. In contrast, the detailed structure of the M1 phase is still under debate. It was believed in early studies that all the M phases share the same AFE structure [17]. However, the convergent-beam electron diffraction (CBED) study reported by Yashima et al. [19] indicated that M1 phase is ferroelectric, where the unit-cell exhibits a non-centrosymmetric polar Pmc21 structure. Most recently, Tian et al. demonstrated that a phase transition involving the polar nature of the structural occurs at a critical temperature Tf (~170  C) [10]. Tf is higher than the M1-M2 transition temperature and was thought to be associated with the partial freezing of antipolar dipoles in the past. Moreover, theory calculations by Niranjan et al. [20] and Moriwake et al. [21] indicate that the centrosymmetric Pbcm and non-centrosymmetric Pmc21 phases may coexist due to their similar free energies. The polymorphic phase transitions in AgNbO3 are summarized in Fig. 1 [10,11,14,17,19,22]. The occurrence of anti-/ferroelectricity in AgNbO3 at room temperature brings a series of polarization development events upon increasing applied electric-fields, resulting in an apparent non-zero Pr in double polarization hysteresis loops, which leads to a much reduced energy storage performance. In order to enhance energy storage performance, tailoring phase transitions in AgNbO3 with chemical modification is believed to be an effective strategy. In ABO3 perovskite oxides, the A- and B-site cations can be partially replaced by iso- or hetero-valent cations. For example, chemical modifications using Bi3þ or La3þ on A-site, or Ta5þ on B-site in previous works can progressively stabilize the AFE phase, leading to a significantly enhanced energy storage property (Wrec can reach up to ~4 J/cm3) [23e25]. However, reports are rare in the literature on simultaneous A- and B-site doping in AgNbO3. Such a modification strategy may lead to the discovery of new AFE materials with a high energy density, as has proven with La3þ and Ti4þ co-doping in AFE PbZrO3 [26]. It is well known that Wrec of a linear dielectric capacitor is limited by its maximum polarization and dielectric breakdown strength (BDS), while Wrec of an AFE capacitor is related to phase transition fields EF and EA. AFEs with high phase transition fields are capable of giving rise to a high Wrec. However, the AFE capacitor has to work under electric fields below the BDS of its dielectric material. In our previous work, it was found that introducing modifying perovskite oxides with a low tolerance factor t, such as CaZrO3 (CZ),

Fig. 1. Permittivity change and phase evolution as a function of temperature in AgNbO3.

to form a solid solution with AgNbO3 can promote the stability of antiferroelectricity and give rise to an enhanced Wrec [27]. However, the low BDS in the AgNbO3-CZ solid-solution does not allow application of an ultrahigh electric field to trigger the phase transition, which significantly limits the energy storage performance. Therefore, both BDS and antiferroelectricity have to be improved to realize a higher Wrec in AgNbO3-based AFE materials. It is reported that CaTiO3 is a linear dielectric with a high BDS (435 kV/cm with Wrec of 1.5 J/cm3) [28]. Recent publications also showed that the addition of CaTiO3 can inhibit the grain growth and contribute to a high BDS in ferroelectric materials [29,30]. Moreover, the tolerance factor t of CaTiO3 is smaller than that of AgNbO3, which could be beneficial to stabilizing antiferroelectricity in AgNbO3. It is known that synthesizing AgNbO3 ceramics with high electric insulation is quite challenging; Mn-doping at a very minor amount in AgNbO3 seems to be effective in achieving high insulation and low leakage current under ultrahigh field [31]. Based on these observations, 0.2 wt% Mn-doped (1-x)AgNbO3-xCaTiO3 (abbreviated as Mn-(1-x)AN-xCT, x ¼ 0.00e0.04) ceramics are designed and prepared to study how the phase transition behavior is impacted and whether the energy storage performance of AgNbO3-based materials can be improved. We successfully demonstrate an ultrahigh Wrec of 3.7 J/cm3 in our AgNbO3-based ceramic system. Additionally, we find the AgNbO3-based ceramic possesses a good thermal stability (the fluctuation in Wrec is less than 3%) over the temperature range of 20e120  C at 225 kV/cm. Our work shows that enhancing BDS in AFE materials should be paid more attention for practical applications.

2. Experimental Polycrystalline ceramics of 0.2 wt.% Mn-doped (1-x)AN-xCT (x ¼ 0.00e0.04) were prepared using the conventional solid-state reaction method. Ag2O (99.7%), CaCO3 (99%), TiO2 (99%), Nb2O5 (99.99%) and MnO2 (99%) were the raw materials. The raw powders were weighed at stoichiometric proportion and then mixed using planetary ball milling in polyethylene containers with ZrO2 media for 24 h, and the dried mixture was calcined at 900  C for 6 h in flowing O2. MnO2 was added to the calcined powders and milled for another 24 h. 5 wt% solution polyvinyl alcohol binder (PVA) was mixed with the dried powders and then pressed into pellets under 400 MPa. Sintering was carried out at 1100  Ce1130  C for 6 h in flowing O2. The surface of each as-sintered pellet was examined with a scanning electron microscope (SEM, Zeiss Supa 50VP, Oberkochen, Germany). After the surface layer of each pellet was removed by grinding, the pellets were crushed into fine powders and annealed at 600  C for 10mins to remove residual stress. Then the phase purity and crystal structure were analyzed with X-ray diffraction (XRD, Model D/Max-IIIC, Rigaku, Tokyo, Japan) using Cu Ka radiation in the 2q range from 20 to 70 . The relative dielectric permittivity (εr) and dielectric loss (tand) were measured using an LCR meter (I M 3536, Hioki, Nagano Prefecture, Japan) at a fre quency of 100 kHz during heating at a rate of 3 C/min from 100  C to 500  C. The DC BDS was measured at room temperature through a high-potential test instrument (Meiruike, RK2671AM, China). The hysteresis loops were measured at 10 Hz with a ferroelectric measurement system (Premier II-30V, Radiant, Novato, CA, USA). The X-ray photoelectron spectrometer (XPS, PHI 5000 VersaProbe III, Ulvac-Phi, Japan) was employed to explore the valence states of manganese. The Raman spectra were recorded from the aspolished pellets using a micro-Raman spectroscope (Renishaw, inVia, UK) with Ar-ion laser as an excitation source at wavelength of 532 nm.

Please cite this article as: Y. Xu et al., Enhanced energy density in Mn-doped (1-x)AgNbO3-xCaTiO3 lead-free antiferroelectric ceramics, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153260

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3. Results and discussion Fig. 2 depicts the XRD patterns and lattice parameters of the Mn-(1-x)AN-xCT ceramics. As shown in Fig. 2aee, all compositions display a single perovskite structure without any trace of secondary phases, indicating that Mn, Ca and Ti cationic dopants thoroughly diffuse into the lattice of AgNbO3 to form the perovskite solid solution. All of labeled diffraction peaks are well consistent with the standard data of centrosymmetric Pbcm space group of AgNbO3 (PDF#70-4738). The Rietveld refinement using GSAS software was carried out to further quantitatively analyze the lattice parameter change with various CaTiO3 concentrations. The Rietveld refinement fitting of Mn-(1-x)AN-xCT ceramics show satisfactory “goodness of fit” as labeled in Fig. 2aee. As shown in Fig. 2f, the lattice parameters a and b decrease with increasing substitution concentrations, while the c-axis parameter exhibits an increase with increasing CaTiO3 modification. The unit cell volume decreases monotonically, indicating the lattice dimension contraction due to the substitution of smaller Ca2þ and Ti4þ for Agþ and Nb5þ on A- and B-sites, respectively (Agþ: 1.28 Å, CN ¼ 8; Ca2þ: 1.12 Å,

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CN ¼ 8; Nb5þ: 0.64 Å, CN ¼ 6; Ti4þ: 0.605 Å, CN ¼ 6) [32]. However, the phase transition induced by addition of CaTiO3 cannot be clearly revealed by the XRD results due to the insufficient resolution power. The purpose of adding 0.2 wt% Mn to AgNbO3 is to reduce the leakage as defects associated with Ag in AgNbO3 were reported to be responsible to enhanced conductivity upon applying electric field [33,34]. For a Mn-doped ceramic sintered at high temperature, the Mn cation is likely to exist in multiple oxidation states. By determining the exact oxidation state in Mn-doped AgNbO3 could help to analyze the site occupancy of Mn in the ABO3 perovskite lattice. Thus, XPS measurements of 0.2 wt% Mn-doped AgNbO3 were conducted for this purpose and the result are displayed in Fig. 3. The survey scanned XPS spectra demonstrated the existence of elements of Mn, Ag, Nb and O. From the spectrum of Mn 2p (Fig. 3b), two main asymmetric peaks (641.2 eV and 652.2 eV) and two weak peaks (646.4 eV and 656.9 eV) are observed. Usually, for each valence state of Mn 2p, it should have two orbital spin-split peaks as Mn 2p3/2 and Mn 2p1/2 with peak area ratio is 2:1 and the peak energy difference should be around 11.05eV. Based on the

Fig. 2. X-ray diffraction patterns and Rietveld profile fitting of Mn-(1-x)AN-xCT ceramics: (a) x ¼ 0.00, (b) x ¼ 0.01, (c) x ¼ 0.02, (d) x ¼ 0.03 and (e) x ¼ 0.04. The whole pattern is indexed according to PDF#70-4738. (f) Composition variation of the lattice parameters and unit cell volume.

Please cite this article as: Y. Xu et al., Enhanced energy density in Mn-doped (1-x)AgNbO3-xCaTiO3 lead-free antiferroelectric ceramics, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153260

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Fig. 3. X-ray photoelectron spectroscopy (XPS) spectrum of the 0.2 wt% Mn-doped AgNbO3 ceramic: (a) survey scan, (b) narrow spectrum of Mn 2p and (c) narrow spectrum of Mn 3s.

peak position and peak shape, the peak identification and curve fitting indicates the main peaks appeared at 641.2eV and 652.2eV should be assigned to Mn 2p3/2 and Mn 2p1/2, respectively. While another two weak peaks at 646.5eV (near Mn 2p3/2) and 656.9eV (near Mn 2p1/2) should be the satellite peaks of Mn2þ, indicating the existence of Mn2þ [35]. Meanwhile, the Mn 3s peak as shown in Fig. 3c also demonstrates the existence of Mn2þ. In addition, Mn ions possessed different valence states at different temperatures: MnO2 (<535  C), Mn2O3 (<1080  C), Mn3O4 (<1650  C) and MnO (>1650  C) [36]. In this work, the 0.2 wt% Mn-doped AgNbO3 ceramics were sintered at 1100  C in O2 atmosphere, the existence of Mn3þ and Mn4þ cannot be excluded from the XPS spectra. In view of the radii of Mn2þ (0.83 Å for CN ¼ 6; 0.96 Å for CN ¼ 8), Mn3þ (0.65 Å for CN ¼ 6), Mn4þ (0.53 Å for CN ¼ 6), Agþ (1.28 Å for CN ¼ 8), and Nb5þ (0.64 Å for CN ¼ 6), substitution of Mn2þ for Agþ at A sites and Mn3þ/Mn4þ occupancy at B sites could both be possible. Such distribution compensates the local charge nonneutrality and results in the reduced leakage current. Fig. 4 presents SEM images of the as-sintered surface of the Mn(1-x)AN-xCT ceramics with various CaTiO3 contents. All of the investigated compositions show dense microstructures and almost no pores are observed between grains. The grain size distributions and the average grain size of various CaTiO3 contents were statistically measured using the Nano-Measure software, which are

shown in the insets of Fig. 4. The average grain size of the ceramics with x ¼ 0.00, 0.01, 0.02, 0.03, and 0.04 is 3.69 mm, 2.68 mm, 2.35 mm, 2.26 mm, and 2.14 mm, respectively. It is observed that smaller and uniform grains are obtained after CaTiO3 modification, which is expected to be beneficial to enhancing the BDS and improving the high energy storage performance of bulk ceramics. Meanwhile, a high relative density (>95%) is obtained for all the compositions. The BDS of Mn-(1-x)AN-xCT ceramics are also evaluated according to the Weibull distribution [37,38]. The value of BDS can be described by:

Xi ¼ lnEi   Yi ¼ ln  ln 1 

(1) i nþ1

 (2)

where Ei is the breakdown electric field for the ith specimen in ascending order, and n is the sum of the specimens tested. The intercept of the fitted lines between Xi and Yi determines the value of dielectric breakdown strength. The result indicates that the BDS augments from 206.4 kV/cm (x ¼ 0.00) to 254.5 kV/cm (x ¼ 0.04) (Fig. 4f). The reduction of the average grain sizes with CaTiO3 incorporation disperses the electric-field intensity through

Please cite this article as: Y. Xu et al., Enhanced energy density in Mn-doped (1-x)AgNbO3-xCaTiO3 lead-free antiferroelectric ceramics, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153260

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Fig. 4. SEM images and grain size distributions of Mn-(1-x)AN-xCT ceramics. (a) x ¼ 0.00, (b) x ¼ 0.01, (c) x ¼ 0.02, (d) x ¼ 0.03, (e) x ¼ 0.04. (f) Grain size and BDS with various CaTiO3 content.

enlarging the surface area, and enhances BDS to higher magnitudes [39,40]. According to the empirical rules, the decreased tolerance factor t should favor a more stable AFE state in AgNbO3-based ceramics, which will shift the temperature-driven M-type phase transition toward low temperatures (see Fig. 1), since a smaller t factor generally suppresses the ferroelectricity in AgNbO3. To verify whether the smaller t factor favors a more stable AFE state, the doping effect on the temperature of polymorphic phase transition is examined by analyzing the temperature-dependent dielectric spectra of Mn-(1-x)AN-xCT ceramics. As shown in Fig. 5, a series of dielectric anomalies were observed in the plots, being similar to those observed in undoped AgNbO3. The dielectric properties were measured in different chamber as the presented break in Fig. 5. Here, the temperatures of M1eM2, M2eM3, and M3eO phase transitions are labeled as T1, T2 and T3, respectively. The shift of the polymorphic phase transition with the increase of doping level is shown in Fig. 5f. It can be clearly seen that with increasing doping concentration x, the dielectric peaks corresponding to T1 and T2 continuously shift to lower temperatures while T3 remains nearly unchanged. The result indicates that CaTiO3 modification broadens the temperature ranges of high temperature M-type AFE phases (M2 and M3), verifying that the smaller t factor favors the more stable AFE state. Additionally, the chemical modification significantly suppresses the dielectric loss to 3  103 (at x ¼ 0.04) compared with 1.6  102 in un-doped AgNbO3. Such suppression is essential for power electronics applications. To further clarify the effect of CT modification on the antiferroelectricity of Mn-(1-x)AN-xCT solid solutions, roomtemperature hysteresis loops under various electric fields at 10 Hz were recorded and presented in Fig. 6a. At low doping concentration (x  0.02), typical double polarization hysteresis loops with weak remanent polarization were observed, which indicates the occurrence of electric-field-induced AFE-to-FE transition upon loading the ceramic with a high electric-field. The maximum Pm remains almost unchanged (Pm ¼ 41.5e43.7 mC/cm2). At higher doping levels (x  0.03), slant hysteresis loops are seen even under

the maximum applied field of 245 kV/cm, where Pm significantly decreases to ~20 mC/cm2. These samples experienced dielectric breakdown prior to the electric-field-induced AFE-to-FE phase transition. The critical fields (EF and EA) are shown in the inset; that both EF and EA shift to higher electric fields upon increasing x suggests a tendency of enhanced antiferroelectricity. The increase in EF indicates a higher energy required to stabilize the FE state, which, therefore, confirms the enhanced antiferroelectricity in its virgin state. The results are consistent with the results of dielectric measurement. The increased EF and EA and the retentive Pm will be conducive to Wrec, while the increase in DE goes against the energy efficiency (h). Fig. 6b presents the temperature-dependent hysteresis loops of x ¼ 0.02 sample over the temperature range of 20e120  C under a maximum applied field of 225 kV/cm. Upon heating, the Pm remains constant. On account of the lower free energy barrier between AFE and FE states at higher temperatures, EF gradually decline while EA gradually increase, giving rise to a reduced DE at higher temperatures. This characteristic, therefore, demonstrates an excellent temperature stability of recoverable energy storage performance. The results from physical properties measurements (including dielectric properties and hysteresis loops) consistently show that CT doping effectively enhances the stability of the AFE state. Theoretical calculations previously suggested that antiferroelectricity originated from a linear coupling between AFE polarization and octahedral distortion (i.e., antiferrodistortive, AFD) [41]. Therefore, to further clarify the enhanced antiferroelectricity originated from strengthened octahedral distortion, the Raman spectroscopy measurements were conducted and the results are displayed in Fig. 7a. According to previously reported Raman spectra [42], three types of vibrational modes for NbO6 octahedron were observed, namely non-degenerate OeNb symmetric stretching vibration v1, doubly degenerated symmetric OeNb asymmetric stretching vibration v2, and triply degenerated symmetric OeNb symmetric bending vibration v5. Consistent with the spectra observed for Ta doped AgNbO3 [25], the coalescence of v1 and v2 was evident with the increase of CaTiO3 content, which indicates the M1eM2 phase structure evolution and enhanced structure

Please cite this article as: Y. Xu et al., Enhanced energy density in Mn-doped (1-x)AgNbO3-xCaTiO3 lead-free antiferroelectric ceramics, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153260

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Fig. 5. Temperature dependence of dielectric constant and dielectric loss of Mn-(1-x)AN-xCT ceramics for (a) x ¼ 0.00, (b) x ¼ 0.01, (c) x ¼ 0.02, (d) x ¼ 0.03, and (e) x ¼ 0.04 at 100 kHz. (f) Phase diagram determined from the dielectric measurement.

symmetry. The mode broadening and reduction in the intensity of v5 peak, associated with Nb5þ cation displacement and NbO6 octahedral distortion, also appears as CaTiO3 concentration increases. The chemical pressure from smaller Ca2þ cations on A-sites indirectly impacts on the NbO6 octahedral compression, and induces a reduction in the correlation length of cation displacements, which is usually related to the mode broadening within the Pbcm symmetry [43]. The decrease in intensity is attributed to the lower polarizability of Ti4þ (2.93 Å3) [44] compared to that of Nb5þ (3.10 Å3) [45]. In addition, as shown in Fig. 7b, the shift of v5 in CTdoped AgNbO3 is evident due to the strengthened NbO6 octahedral distortion. Fig. 8a shows the energy storage performance of x ¼ 0.00e0.04 at room temperature. Wrec increases initially with increasing CT content, reaching the maximum at x ¼ 0.02, and then decreases gradually. The optimal Wrec with a value of 3.7 J/cm3 was achieved at x ¼ 0.02 under an applied field of 235 kV/cm, a 176% enhancement over the unsubstituted AgNbO3 ceramic (2.1 J/cm3). The enhanced Wrec is attributed primarily to the improved BDS and enhanced antiferroelectricity by CT addition in the range of x ¼ 0.00e0.02. Different from Wrec, h increases monotonically with

increasing CT content from 44% to 73%. The higher h for x  0.03 compared with that of x ¼ 0.02 (53%) comes from the incomplete AFE-to-FE phase transition at the applied fields. Fig. 8b shows the energy storage properties of the x ¼ 0.02 ceramic over the temperature range of 20e120  C under the applied field of 225 kV/cm. It is evident that h increases with increasing temperature due to the reduced electric hysteresis DE. Moreover, the ceramic shows a good thermal stability of Wrec (the fluctuations in Wrec are less than 3% in the measurement temperature range) as a result of the stabilized AFE phase at high temperatures; it maintains a high value above 3.6 J/cm3. To make comparison with the ceramic of x ¼ 0.02, the recoverable energy storage density Wrec of recently studied leadfree ceramics are summarized in Fig. 9. It can be seen that the Wrec of 3.7 J/cm3 achieved in the x ¼ 0.02 ceramic is amongst the highest values [10,22e25,31,43,46e63], indicating x ¼ 0.02 is a promising candidate composition for application in high power energy storage capacitors. 4. Conclusions In this work, a high recoverable energy storage density (Wrec) of

Please cite this article as: Y. Xu et al., Enhanced energy density in Mn-doped (1-x)AgNbO3-xCaTiO3 lead-free antiferroelectric ceramics, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153260

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Fig. 8. Energy storage performance of (a) Mn-(1-x)AN-xCT ceramics at room temperature under 230 kV/cm and (b) x ¼ 0.02 at various temperature during 20e120  C under 225 kV/cm.

Fig. 6. (a) Hysteresis loops of at ambient temperature for Mn-(1-x)AN-xCT ceramics. (b) Hysteresis loops as a function of temperature for x ¼ 0.02 under 225 kV/cm.

3.7 J/cm3 with an excellent thermal stability (the fluctuations in Wrec are less than 3%) over the temperature range of 20e120  C is achieved in the 0.2 wt% Mn doped 0.98AgNbO3e0.02CaTiO3 ceramic. The reduction of the average grain sizes with CaTiO3 incorporation dispersed the electric-field intensity through enlarging the surface area, and pushed the BDS to higher magnitudes. Through modulating the M2 phase to room temperature by

adding CaTiO3, the more stable AFE state was confirmed by the dielectric characterization, Raman spectroscopy, and hysteresis measurements. The combined high BDS and more stable AFE order lead to the enhanced energy storage density. The result will stimulate further research on A- and B- site co-doped lead-free AgNbO3 AFE compounds to realize highly efficient and environmentally friendly energy storage capacitors over a broad temperature range. Author contribution statement Yonghao

Xu:

Conceptualization,

Methodology,

Funding

Fig. 7. (a) Raman spectra recorded at ambient condition in the range of 50e1000 cm1 with changing CaTiO3 content for Mn-(1-x)AN-xCT ceramics. (b) The close examination of the v5 mode at normalized intensity for x ¼ 0.00 and 0.04.

Please cite this article as: Y. Xu et al., Enhanced energy density in Mn-doped (1-x)AgNbO3-xCaTiO3 lead-free antiferroelectric ceramics, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153260

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Fig. 9. Comparison of the recoverable energy density Wrec of x ¼ 0.02 with recently published lead-free ceramic systems: AgNbO3 (AN)-based10,2225,31,43,46; BaTiO3 (BT)based4750; Bi0.5Na0.5TiO3 (BNT)-based5155; NaNbO3 (NN)-based56,57; K0.5Na0.5NbO3 (KNN)-based5860; SrTiO3 (ST)-based61,62; CaTiO3 (CT)-based63.

acquisition, Writing - Original Draft, Writing - Review & Editing, Supervision. Yan Guo: Investigation, Resources, Data Curation, Writing Original Draft. Qian Liu: Resources, Data Curation. Yuehong Yin: Formal analysis. Jiale Bai: Resources, Data Curation. Long Lin: Funding acquisition. Jingjing Tian: Resources. Ye Tian: Writing - Review & Editing, Validation. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 51702089), the Key Scientific Research Project of Colleges and Universities in Henan Province (Grant No. 18A430016), the Young Core Instructor Foundation of Henan Polytechnic University (Grant No. 2017XQG-11), and the Doctoral Fund Project of Henan Polytechnic University (Grant No. B2017-59 and B2019-20). The authors are also grateful to Professor Xiaoli Tan at the Department of Materials Science and Engineering, Iowa State University, USA, for valuable discussions and critical reading of the manuscript. References [1] Z. Liu, T. Lu, J. Ye, G. Wang, X. Dong, R. Withers, Y. Liu, Antiferroelectrics for energy storage applications: a review, Adv. Mater. Technol. 3 (2018), 1800111. [2] B. Chu, X. Zhou, K. Ren, B. Neese, M. Lin, Q. Wang, F. Bauer, Q.M. Zhang, A dielectric polymer with high electric energy density and fast discharge speed, Science 313 (2006) 334e336. [3] Q. Li, L. Chen, M.R. Gadinski, S. Zhang, G. Zhang, U. Li, E. Iagodkine, A. Haque, L.Q. Chen, N. Jackson, Q. Wang, Flexible high-temperature dielectric materials from polymer nanocomposites, Nature 523 (2015) 576e579. [4] X. Tan, C. Ma, J. Frederick, S. Beckman, K.G. Webber, The antiferroelectric 4 ferroelectric phase transition in lead-containing and lead-free perovskite ceramics, J. Am. Ceram. Soc. 94 (2011) 4091e4107. [5] X. Hao, J. Zhai, L.B. Kong, Z. Xu, A comprehensive review on the progress of lead zirconate-based antiferroelectric materials, Prog. Mater. Sci. 63 (2014) 1e57. [6] D. Damjanovic, N. Klein, L. Jin, V. Porokhonskyy, What can be expected from

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Please cite this article as: Y. Xu et al., Enhanced energy density in Mn-doped (1-x)AgNbO3-xCaTiO3 lead-free antiferroelectric ceramics, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.153260