High energy storage properties of lead-free Mn-doped (1-x)AgNbO3-xBi0.5Na0.5TiO3 antiferroelectric ceramics

Journal of the European Ceramic Society 40 (2020) 56–62

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Original Article

High energy storage properties of lead-free Mn-doped (1-x)AgNbO3xBi0.5Na0.5TiO3 antiferroelectric ceramics

T ⁎

Yonghao Xua, Yan Guoa, Qian Liua, Guodong Wanga, Jiale Baia, Jingjing Tiana, Long Linb, , Ye Tianc a

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 c Electronic Materials Research Laboratory, Xi’an Jiaotong University, Xi'an, 710049, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: AgNbO3 Antiferroelectricity Phase transition Energy storage density Efficiency

In this work, 0.2 wt.% Mn-doped (1-x)AgNbO3-xBi0.5Na0.5TiO3 (x = 0.00–0.04) ceramics were synthesized via solid state reaction method in flowing oxygen. The evolution of microstructure, phase transition and energy storage properties were investigated to evaluate the potential as high energy storage capacitors. Relaxor ferroelectric Bi0.5Na0.5TiO3 was introduced to stabilize the antiferroelectric state through modulating the M1-M2 phase transition. Enhanced energy storage performance was achieved for the 3 mol% Bi0.5Na0.5TiO3 doped AgNbO3 ceramic with high recoverable energy density of 3.4 J/cm3 and energy efficiency of 62% under an applied field of 220 kV/cm. The improved energy storage performance can be attributed to the stabilized antiferroelectricity and decreased electrical hysteresis ΔE. In addition, the ceramics also displayed excellent thermal stability with low energy density variation (< 6%) over a wide temperature range of 20−80 °C. These results indicate that Mn-doped (1-x)AgNbO3-xBi0.5Na0.5TiO3 ceramics are highly efficient lead-free antiferroelectric materials for potential application in high energy storage capacitors.

1. Introduction Dielectric materials play a key role in modern day electronic devices, especially in energy storage and power electric applications. Among the popular dielectric materials, antiferroelectric (AFE) materials have been proposed as leading contenders due to their high phase transformation fields, low dielectric loss and remanent polarization (Pr), high recoverable energy storage density (Wrec) and fast chargedischarge rates compared with ferroelectrics (FE) and linear dielectrics [1–8]. Therefore, antiferroelectric materials have good scope for development in the field of commercial capacitor applications. The Wrec and the energy efficiency (η) of AFE materials can be estimated according to the hysteresis loop using the following formula: Pm

Wrec = ∫ EdP , Wloss = ∫ PdE , and η = Pr

Wrec ; Wrec + Wloss

where E is the ap-

plied electric field, Pm is the maximum polarization, Pr is the remanent polarization, and Wloss is the area of the hysteresis loop. The most extensively studied AFE materials until now are based on the prototype compound lead zirconate (PbZrO3) [9,10]. However, the toxicity of lead-based derivatives raises great environmental and human health concerns [11–13]. Thus, numerous efforts have been made to explore ⁎

suitable lead-free AFE alternatives with properties comparable to leadcontaining AFE materials to circumvent this issue [14]. Recently, silver niobate (AgNbO3), a lead-free AFE-like material with ABO3 type perovskite structure, is considered to be a promising candidate for AFE energy storage capacitors due to its high Wrec of 2.1 J/cm3 [15]. AgNbO3 can not only meet the requirements of energy storage capacitors with high Wrec, but also overcome the environmental issues caused by commonly used lead-containing materials. AgNbO3 undergoes a complex series of phase transitions (M1, M2, M3, O, T and C) as a function of temperature upon heating [16–18]. The high temperature phases O, T and C corresponding to orthorhombic, tetragonal, and cubic symmetry, respectively, have been widely studied [19–21]. The M2 and M3 phases are recognized as centrosymmetric Pbcm symmetry with partial AFE ordering [20,22,23]. On the other hand, the nature of the room-temperature M1 phase remains controversial. The M1 phase presents both FE and AFE behavior, as characterized by high-resolution X-ray powder diffraction, neutron powder diffraction, and electron diffraction [19,22,24]. Based on the first principle calculation reported by M. K. Niranjan et al. [25], who suggested that a very small energy difference between the nonpolar Pbcm and polar Pmc21 phases (∼0.1 meV/f.u.), indicating that both phases

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

https://doi.org/10.1016/j.jeurceramsoc.2019.09.022 Received 19 June 2019; Received in revised form 8 September 2019; Accepted 11 September 2019 Available online 12 September 2019 0955-2219/ © 2019 Elsevier Ltd. All rights reserved.

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2. Experimental procedure

(Pbcm and Pmc21) coexist in AgNbO3 samples at room temperature. Recently, based on X-ray diffraction, selected-area electron diffraction (SAED) and second harmonic generation (SHG) experimental results, Tian et al. [15] suggested that the average structure of the M1 phase possessed Pbcm space group. The phase transition behavior of AgNbO3 ceramic can be effectively tuned by chemical modification to obtain enhanced energy density properties. Most recently, the investigation of energy density properties of A- or B-site modified AgNbO3-based AFE ceramics has drawn extensive attention due to the improved Wrec and η [26–32]. High energy densities of 4.4 J/cm3 and 4.2 J/cm3 were achieved for La-doped [32] and Ta-doped [29] AgNbO3 ceramics, respectively. These results have strengthened the potential of AgNbO3-based ceramics as energy storage capacitors. However, the previous studies were mainly focused on only the A- or B-site modified AgNbO3-based ceramics. Further modifications with A- and B- sites simultaneously may lead to the discovery of new AFE materials with high energy density, similar to the case of PbZrO3. Generally, for perovskite structure, the decrease in tolerance factor t would result in enhanced stability of antiferroelectricity and increased EF. The tolerance factor is defined as t = (RA + RO )/[ 2 (RB + RO )], where RA, RB, and RO are the ionic radius of A-sites cation, B-sites cation and oxygen anion, respectively. Modifications with oxides such as Bi2O3 [28] and La2O3 [32] in AgNbO3based ceramics were reported to be significantly beneficial for the stability of antiferroelectricity through reducing the tolerance factor. In our previous study [33], CaZrO3 dopant effectively stabilized the antiferroelectricity of AgNbO3 by dramatically decreasing the tolerance factor t. However, when the doping concentration exceeded 2 mol%, the double hysteresis loop with saturation polarization could not be obtained prior to the dielectric breakdown strength (BDS). This was due to the significantly enhanced free energy barrier between the virgin AFE state and the field-induced metastable FE state. It should be noted that, although a high Wrec was obtained for pure AgNbO3 ceramics, the η was merely 40% at ∼175 kV/cm [15]. Therefore, enhancement of η for AFE AgNbO3-based materials should be given more attention for practical capacitor applications. Sodium bismuth titanate (Bi0.5Na0.5TiO3, BNT) is a lead-free relaxor ferroelectric ceramic with a rhombohedral phase structure at room temperature [34,35]. As one of the most promising lead-free FE materials, the energy storage density of BNT-based FE relaxors has been extensively investigated. Shi et al. [36] obtained Wrec of 1.17 J/cm3 with high η of 91% for (0.94-x)Bi0.5Na0.5TiO3 - 0.06BaTiO3xSrTi0.875Nb0.1O3 system. Zhang et al. [37] studied the influence of BaZrO3 additive on the energy storage properties of 0.775Bi0.5Na0.5TiO3-0.225BaSnO3 relaxor FEs and achieved Wrec of 2.08 J/cm3 with high η of 88.8%. Furthermore, the tolerance factor t of Bi0.5Na0.5TiO3 (t = 0.975) is smaller than that of AgNbO3 (t = 0.988), which would be beneficial to stabilize the antiferroelectricity of AgNbO3. Considering the relatively high η of BNT-based relaxor FEs and high Wrec of AgNbO3 ceramic, the incorporation of BNT into AgNbO3 would enhance the energy efficiency while maintaining a high energy storage density. In this work, 0.2 wt.% Mn-doped (1-x)AgNbO3-xBi0.5Na0.5TiO3 (abbreviated as Mn-(1-x)AN-xBNT, x = 0.00–0.04) ceramics were prepared by the traditional solid state sintering method and the effects of BNT addition on the microstructure, phase transition behavior and energy storage properties were investigated. A small amount of Mn (0.2 wt.%) was added to reduce the electrical conduction and electrical hysteresis ΔE (ΔE = EF-EA) [27]. As expected, enhanced energy storage performance was achieved with relatively high Wrec of 3.4 J/cm3 and η of 61.7% for the 3 mol% BNT doped AgNbO3 ceramic. In addition, the as-prepared ceramic also exhibited good thermal stability with low fluctuation less than 6% from room temperature to 80 °C at 210 kV/cm.

Polycrystalline ceramic samples of Mn-(1-x)AN-xBNT (x = 0.00–0.04) were prepared using the conventional solid-state reaction method. Ag2O (99.7%), Bi2O3 (99%), Na2CO3 (99%), TiO2 (99%), Nb2O5 (99.99%) and MnO2 (99%) were used as the raw materials. The raw materials were weighed according to the stoichiometric proportion and then mixed homogenized by planetary ball milling in a polyethylene container with ZrO2 media for 24 h. The dried mixture was calcined at 900 °C for 6 h in flowing oxygen. The calcined powders and MnO2 were then milled for another 24 h. Polyvinyl alcohol (PVA, 5 wt.%) binder solution was mixed with the dried powders and then pressed into pellets under 150 MPa. The pellets were heated to 600 °C and held for 2 h to burn out the PVA binder. Then, the pellets were subjected to cold isostatic pressing under 300 MPa for 1.5 min. Sintering was carried out at 1110 °C to 1140 °C for 6 h in flowing oxygen. The grain morphology was examined with a scanning electron microscope (SEM, Zeiss Supa 50 V P, Oberkochen, Germany) on the fractured surfaces of the sintered samples. Before the SEM observation, the fractured surface was polished and thermally etched at temperature 100 °C lower than respective sintering temperature for 30 min. After the surface layer of each pellet was removed by grinding, the pellets were crushed into refined powders, and were annealed at 600 °C for 10 min to remove residual stress. Then, the phase purity and crystal structure were analyzed by X-ray diffraction (XRD, Model D/Max-IIIC, Rigaku, Tokyo, Japan) using Cu Kα radiation in the 2θ range from 20° to 90°. The relative dielectric permittivity (εr) and dielectric loss (tanδ) were measured using an LCR meter (IM 3536, Hioki, Nagano Prefecture, Japan) at frequency of 10 kHz, 100 kHz and 1 MHz during heating at a rate of 3 °C/min from -100 °C to 500 °C. The sintered pellets were polished to a thickness of 0.15 mm and point electrodes (1.5 mm in diameter) were obtained at room temperature by magnetron sputtering (VTC-16-SM, HF-Kejing, Hefei, China) for hysteresis loop measurement. The hysteresis loops were measured with a ferroelectric measurement system (Premier II-30 V, Radiant, Novato, CA, USA) at 10 Hz, and the current density curves were calculated from the corresponding hysteresis loops. 3. Results and discussion Fig. 1 shows the SEM micrographs of polished and thermally etched fracture surfaces of Mn-(1-x)AN-xBNT ceramics. All the Mn-(1-x)ANxBNT samples exhibited dense and homogeneous morphology. The statistical summary of the average grain size of ceramics with various BNT contents was also obtained using the Nano-Measure software, as shown in the insets of Fig. 1. The average grain size of Mn-(1-x)ANxBNT ceramics decreased from 1.56 μm to 1.23 μm with increase in the BNT content. Compared with pure AgNbO3 ceramic in our previous work (average grain size of ∼3 μm) [33], BNT addition significantly decreased the average grain size, which was beneficial for obtaining high BDS. Moreover, high relative density (> 95%) was obtained for all the compositions. Fig. 2 depicts the XRD patterns and cell parameters of Mn-(1-x)ANxBNT ceramics. As shown in Fig. 2a, pure perovskite structure was observed for all samples with no impurity or secondary phase. In the present work, the structure refinement of Mn-(1-x)AN-xBNT ceramics was carried out using the Pbcm space group. The lattice parameters and unit cell volume are shown in Fig. 2b. The lattice parameters a, b and the unit cell volume generally decreased with increase in BNT content due to the smaller radius of Bi3+ (1.17 Å, CN = 8), Na+ (1.18 Å, CN = 8) and Ti4+ (0.605 Å, CN = 6) compared with Ag+ (1.28 Å, CN = 8) and Nb5+ (0.64 Å, CN = 6) [38], respectively. On the other hand, the c-axis parameter showed a general increase with increasing level of BNT substitution. Moreover, phase transitions arising from the BNT dopant could not be definitely distinguished from the XRD 57

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Fig. 1. SEM micrographs of the fractured surface and grain size distribution of Mn-(1-x)AN-xBNT ceramics. (a) x = 0.00; (b) x = 0.01; (c) x = 0.02; (d) x = 0.03; (e) x = 0.04; (f) grain size of Mn-(1-x)AN-xBNT ceramics.

This result indicated that the M1–M2 phase transition occurred when the BNT doping amount increased, or a certain composition possessed M2 phase. To verify the effect of BNT addition on the temperature involving phase transitions, the relative dielectric permittivity (εr) and loss tangent (tanδ) of Mn-(1-x)AN-xBNT ceramics were measured on heating at 10 kHz, 100 kHz and 1 MHz as shown in Fig. 3a–e. The dielectric properties were measured in different chamber as the presented break. A series of dielectric anomalies were observed in the relative permittivity plots for Mn-(1-x)AN-xBNT ceramics, similar to those of pure AgNbO3. Here, the temperatures of M1–M2, M2–M3, and M3–O phase transitions were denoted as T1, T2 and T3, respectively. Based on the temperature-dependence of the dielectric properties, the phase diagram

patterns, due to the similar lattice peaks compared with pure AgNbO3. According to the design plan of the ceramics, the decreased tolerance factor t should induce the enhanced AFE state in AgNbO3-based ceramics, which can shift the M1–M2 phase transition to ambient temperature. On the basis of temperature evolution of the crystal structure of pure AgNbO3, the lattice parameter ratios vary regularly in the phase sequence of M1, M2, and M3 [19]. The b/a and c/a lattice parameter ratios of the investigated compositions at room temperature were compared with those of pure AgNbO3 at rising temperature, as shown in Fig. 2c and d, respectively, to study the possible M1–M2 phase transition with BNT modification. It was found that the trends of b/a and c/a with increasing BNT doping-level were in accordance with the results observed in pure AgNbO3 with rising temperature calculated from Ref. 19.

Fig. 2. (a) X-ray diffraction patterns of Mn-(1-x)AN-xBNT (x = 0.00–0.04) ceramics. The whole pattern was indexed according to PDF#70-4738. (b) Composition variation of the lattice parameters and unit cell volume. (c) Lattice parameter ratios for Mn-(1-x)AN-xBNT at room temperature. (d) Calculated lattice parameter ratios for pure AgNbO3 under various temperatures from Ref. [19]. 58

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Fig. 3. Dielectric properties of Mn-(1-x)AN-xBNT (x = 0.00–0.04) upon heating. (a) x = 0.00; (b) x = 0.01; (c) x = 0.02; (d) x = 0.03; (e) x = 0.04; (f) Phase diagram of Mn-(1-x)AN-xBNT determined from dielectric constant..

tilting. It is also worth noting that, for x≥0.01 compositions, the M1–M2 phase transition occurred below room temperature, indicating more stable AFE state at the room-temperature range. The measured dielectric properties show that BNT modification effectively enhanced the stability of the AFE order. To further confirm the impact of BNT doping on the AFE state of Mn-(1-x)AN-xBNT solid solutions, the room-temperature hysteresis loops under different electric fields at 10 Hz were recorded, as shown in Fig. 4a. For all the investigated compositions, slant double hysteresis loops with weak Pr were observed, indicating that the electric-field-induced AFE-to-FE phase transformation occurred prior to BDS. Moreover, the Pm remained between 33 μC/cm2 and 43 μC/cm2. The corresponding current density response (J vs. E) was also calculated to provide more detailed picture of the AFE ↔ FE phase switching process (Fig. 4b), and the detailed phase transition properties including EF, EA and ΔE are summarized in Fig. 4c. With increase in BNT concentration, EF increased from 122 kV/cm to 186 kV/cm, while EA clearly increased from 48 kV/ cm to 114 kV/cm. The electric hysteresis ΔE increases first, and reaches the maximum at x = 0.01, then decreases with further increase in BNT

of Mn-(1-x)AN-xBNT is obtained and shown in Fig. 3f. With increasing dopant concentration, the dielectric peaks corresponding to T1, T2 and T3 monotonically shifted towards lower temperatures. Generally, the low temperature M-type phase transitions are associated with Ag+ and Nb5+ cations displacement, and Nb5+ cations contribute significantly to the stability of ferroelectricity [39]. On the other hand, the phase transition between M3–O is related to different oxygen octahedral tilting systems [19]. As discussed above, the tolerance factor t of Mn-(1x)AN-xBNT solid solution decreased with increasing BNT-doping level, and resulted in enhanced octahedral tilting. The reduction of the oxygen octahedron size caused by enhanced octahedral tilting diminished the displacement freedom of Nb5+ cations, and suppressed the ferroelectricity stability, which led to the gradual decrease in temperature of M-type phase transitions (i.e., T1, T2 and T3). It has been reported that the phase transformation temperature involving oxygen octahedral tilting increases with decreasing tolerance factor t. However, for the M3-O phase transition in Mn-(1-x)AN-xBNT ceramics, the phase transition temperature gradually decreased, which was mainly due to the competition between Nb5+ off-centering and NbO6 octahedral 59

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Fig. 4. (a) Hysteresis loops, (b) Current density curves, (c) Phase transition fields including EF, EA and ΔE, and (d) Wrec and η of Mn-(1-x)AN-xBNT (x = 0.00-0.04) ceramics at ambient temperature.

concentration. The increased phase transition fields (EF and EA) and the retentive Pm are conducive for high energy storage density. The energy storage performances including Wrec and η at room temperature were investigated, as exhibited in Fig. 4d. As expected, both Wrec and η mainly increased with increase in BNT-doping level, reached the maximum at x = 0.03, and then slightly decreased. The optimal Wrec of 3.4 J/cm3 with a high η of 62% was achieved at x = 0.03. Both enhanced Wrec and η, compared to the parent AgNbO3, indicate the potential of application in high energy storage capacitors. Fig. 5a depicts the hysteresis loops of x = 0.03 over the temperature range of 20–80 °C under a maximum applied field of 210 kV/cm. Due to the lower free energy barrier between the AFE state and FE state at high temperature, the EF gradually decreased with rising temperature (as shown in the inset of Fig. 5a). To the contrary, EA showed a general increase with increasing temperature, due to enhanced AFE stability at elevated temperature. The decreased ΔE, which resulted from the contrary trend of EF and EA, and a constant Pm (∼35 μC/cm2) in the measured temperature range guarantee the temperature stability of energy storage properties. Fig. 5b shows the temperature-dependent energy storage properties of x = 0.03 ceramic. As shown, the ceramic showed good thermal stability of Wre with the fluctuations less than 6%, and maintained a high value for Wre (> 3.1 J/cm3). The energy efficiency η remained above 64% over the temperature range of 20–80 °C. The recoverable energy storage density Wrec and the energy efficiency η of Mn-(1-x)AN-xBNT ceramic (x = 0.03) were compared with those of other recently studied lead-free ceramics, as shown in Fig. 6 [15,27,29,31,32,36,40–53]. Although the BaTiO3 (BT)-based, Bi0.5Na0.5TiO3 (BNT)-based, NaNbO3 (NN)-based, K0.5Na0.5NbO3 (KNN)-based, SrTiO3 (ST)-based and CaTiO3 (CT)-based ceramics have higher η, the Wrec values of these materials are lower than that of 3 mol % BNT doped AgNbO3 ceramic. It can be seen that the Wrec of 3.4 J/cm3

Fig. 5. (a)Hysteresis loops and (b) Energy-storage properties of Mn-(1-x)ANxBNT ceramic (x = 0.03) at 20 − 80 °C. The inset shows the corresponding phase transition fields EF, EA and ΔE.

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Fig. 6. Comparison of the recoverable energy density Wrec and the energy efficiency η of Mn-(1-x)AN-xBNT ceramic (x = 0.03) with recently published lead-free ceramic systems. AgNbO3 (AN)-based [15,27,29,31,32,40]; BaTiO3(BT)-based [41–45]; Bi0.5Na0.5TiO3(BNT)-based [36,46–48]; NaNbO3 (NN)-based [49]; K0.5Na0.5NbO3 (KNN)-based [50,51]; SrTiO3(ST)-based [52]; CaTiO3 (CT)-based [53].

achieved in x = 0.03 is one of the highest values of recently studied lead-free ceramics, and the η is improved significantly compared with pure AgNbO3. The result demonstrates that the strategy of co-doping Aand B- sites is effective for enhancing the energy storage performance of lead-free AgNbO3-based AFE compounds. 4. Conclusions In this work, Mn-(1-x)AN-xBNT (x = 0.00-0.04) ceramics were prepared by conventional solid-state reaction method in oxygen atmosphere, and their crystal structure and energy storage properties were investigated. BNT effectively enhanced the AFE stability by shifting the M1−M2 phase transition toward lower temperature. The electrical hysteresis ΔE was reduced due to the modification of BNT with relaxor property. As expected, high recoverable energy storage density Wrec of 3.4 J/cm3 and high η of 62% were achieved for the 3 mol% BNT doped AgNbO3 ceramic. Moreover, this composition possessed good thermal stability with Wrec variation less than 6% over the temperature range 20 − 80 °C. The result demonstrates that Mn-(1-x)AN-xBNT AFE compounds are promising lead-free candidates for high energy storage capacitors over a broad temperature range. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 51702089), the Key Scientific Research Projects of Henan Colleges and Universities (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). 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) 334–336. [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) 576–579. [4] B. Rangarajan, B. Jones, T. Shrout, M. Lanagan, Barium/lead-rich high permittivity glass-ceramics for capacitor applications, J. Am. Ceram. Soc. 90 (2007) 784–788. [5] H. Ogihara, C.A. Randall, S. Trolier-McKinstry, High-energy density capacitors utilizing 0.7BaTiO3-0.3BiScO3 ceramics, J. Am. Ceram. Soc. 92 (2009) 1719–1724.

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