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Temperature-Stable Energy Storage Properties of Tungsten Bronze Type Compounds

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14 June 2023

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14 June 2023

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Abstract
The temperature-dependent energy storage properties of four tungsten bronze phase compounds are studied together with an investigation of their structure and temperature-dependent permittivity response, i.e., Ba6Ti2Nb8O30 (BTN), Ba6Zr2Nb8O30 (BZN), Sr3TiNb4O15 (STN) and Sr3ZrNb4O15 (SZN) ceramics. It was found that BZN has smaller grains and a more porous structure than BTN. SZN shows no clear grain boundaries with the most porous structure among all samples, exhibiting a much lower permittivity response than other samples with no signs of phase transitions from room temperature to 400 °C. Though the energy storage response of those samples is generally quite low, it exhibits rather good temperature stability. It was suggested that by obtaining a denser structure through chemical modification or other methods, those tungsten bronze ceramics with good temperature stability could be promising as energy storage devices when improved energy storage properties are achieved.
Keywords: 
Energy storage properties
;  
Tungsten bronze phase
Subject: 
Chemistry and Materials Science  -   Materials Science and Technology

Introduction

Dielectric ceramic capacitors show high power density (~ 108 w/kg), fast charge-discharging rate (<1 µs), and good thermal stability, therefore are crucial as energy storage devices in applications such as pulse power industries, medical equipment, and electronic devices [1], etc. Among those dielectric ceramic capacitors, relaxor ferroelectrics show better energy storage properties (such as recoverable energy storage density, Wrec, and energy efficiency, ƞ) than ferroelectrics and paraelectric materials due to the higher maximum polarization, lower energy loss, and higher breakdown field attributed from their unique microstructure with polar nano regions [1]. The typical material systems include PbZrO3-based and lead-free perovskite-structured systems including BaTiO3 (BT)-based, K0.5Na0.5NbO3 (KNN)-based, and Na1/2Bi1/2TiO3 (NBT)-based ceramics. Since the usage of lead is restricted due to its toxicity to the environment, research is intensively focused on developing promising lead-free systems, and great research advances have been achieved. For example, a high Wrec of 1.76 J/cm3 was reported in 0.94Bi0.55Na0.45TiO3-0.06BaTiO3 ceramics under 75 kV/cm [2], BT based 0.7BT-0.3BiScO3 ceramics obtained a Wrec of 2.3 J/cm3 under 225 kV/cm [3], 0.85KNN-0.15ST showed a Wrec of 4.03 J/cm3 under a field of 400 kV/cm [4].
Tetragonal tungsten bronze (TTB) structure with polar distortion is one of the major categories of lead-free ferroelectrics [5,6,7,8]. Over the recent years, TTB ferroelectrics have been researched due to their excellent non-linear optic, pyroelectric, and optoelectronic properties [9,10]. In the meantime, tetragonal tungsten bronze (TTB) structured materials could also be potential energy storage materials, such as Sr1-xBaxNb2O6 (SBN), Ba6-3xNd8+2xTi18O54 (BNT), Ba6Ti2Nb8O30 (BTN), Sr3TiNb4O15 (STN). They have a general formula (A1)2(A2)4(C)4(B1)2(B2)8O30. A1 represents a 12-coordinate site, A2 is a 15-coordinate site, and B1 and B2 are 6-coordinate octahedral sites [11]. Smaller triangular (C) sites could be (partially) filled by small low-charged cations which are generally empty, therefore the formula becomes A6B10O30. TTB materials are normally porous due to the abnormal grain growth during sintering [12] and the properties would greatly vary when different elements are located at A, B, and C sites, allowing the manipulation of their structure and properties [13]. TTB ceramics with a relaxor ferroelectric nature are especially attractive as energy storage devices. For example, Fe-doped tungsten bronze Sr1−xBaxNb2O6 (SBN) ceramics could achieve Wrec of 0.68 J/cm3 and ƞ of 83.6 % [14]. MgO-Sr0.7Ba0.3Nb2O6 composites show Wrec of 0.93 J/cm3 together with ƞ of 89.4 %. The (Sr0.7Ba0.3)5LaNb7Ti3O30 tungsten bronze ceramic was reported to have a Wrec of 1.36 J/cm3 and ƞ of 91.9 % [6].
Ba6Ti2Nb8O30 (BTN) ceramic was first reported in 1965 [15]. It has a tetragonal structure (space group P4bm), with lattice parameters a = b = 12.53 Å and c = 4.01 Å. The structure consists of NbO6 octahedra sharing corners occupied by Nb and Ti forming two main interstices (A1 and A2) [16]. Both pentagonal A1-sites and tetragonal A2-sites are predominantly occupied by Ba. The co-existence of pentagonal and tetragonal sites results in trigonal vacant C-sites [16]. BTN ceramic was initially studied as a ferroelectric due to its low dielectric loss and low-temperature coefficient of relative permittivity [17,18]. It was reported that its electrical properties are preferable when sintering in a reducing atmosphere leading to more electrons generated together with oxygen vacancies [19]. Sr3TiNb4O15 (STN) tungsten bronze phase has been recently determined to have an orthorhombic structure with unit cell a = 12.363 Å, b = 12.4 Å, and c = 7.76 Å [11,20], different from BTN. It is suggested the smaller ionic radius of Sr2+ compared to Ba2+ allows for more flexibility in the structure leading to this orthorhombic structure [21]. Pure STN is known to show low conductivity and owns a relaxor nature due to the composition disorder at A and B sites with Curie temperature at around 600 °C [22]. Unfortunately, there is a lack of fundamental understanding of those materials as well as their performance as potential energy storage devices.
In this work, we have looked into the structural features and small signal dielectric properties of four representative tungsten bronze compounds, i.e., Ba6Ti2Nb8O30 (BTN), Ba6Zr2Nb8O30 (BZN), Sr3TiNb4O15 (STN), Sr3ZrNb4O15 (SZN) ceramics as well as systematically studied their energy storage properties at different temperatures, which could provide insights for implementing them as temperature stable energy storage devices, provided with improved energy storage response.

Methodology

Ba6Ti2Nb8O30 (BTN), Ba6Zr2Nb8O30 (BZN), Sr3TiNb4O15 (STN), Sr3ZrNb4O15 (SZN) tungsten bronze ceramics were fabricated using the conventional solid-state reaction method using high-purity reagent powders in stoichiometric ratios: BaCO3 (Sigma Aldrich, 99.9 %), TiO2 (Sigma Aldrich, 99.9 %), Nb2O5 (Sigma Aldrich, 99.99 %), SrCO3 (Sigma Aldrich, 99.9 %) and ZrO2 (Sigma Aldrich, 99 %). The mixed powders for each composition were milled for 24 h in ethanol and calcined at 950 °C in air for 42 hours. After the second milling, the powders were pressed into green bodies and were further densified using a cold isostatic press. Sintering of pellet-shaped samples was done at 1300 °C for a total of 7 days in air.
X-ray diffraction (XRD) patterns were collected from the ground sample surfaces using a Bruker D8 Advance diffractometer (Bruker AXS GmbH, Germany). The microstructure of all sintered samples was examined under a scanning electron microscope (SEM) (Quanta 200, FEI Co., USA). The relative permittivity was measured with an LCR meter (E4980AL, Keysight, USA) from room temperature to 400 °C with 2 K/min heating/cooling rate on samples with Pt electrodes (Balzers SCD040) on both sides. The energy storage property test at different temperatures was based on measuring the polarization-electric field (PE) loops of ground samples (around 0.5 mm thick). Before measurement, the ground samples are annealed in air at 500 °C for 1 hour to remove the stress effects from mechanical processing and then sputtered with Pt electrodes. The tests are performed using a unipolar waveform under a frequency of 100 Hz with a maximum field of 140 kV/cm on a TF Analyser 2000 (aixACCT (aixPES), Aachen, Germany). Samples were immersed in insulating silicon oil (Wacker®-AP 100 Silicone Fluid) during the tests.

Results and Discussion

The X-ray diffraction (XRD) patterns shown in Error! Reference source not found. indicate that both BTN (Ba6Ti2Nb8O30) [16] and BZN (Ba6Zr2Nb8O30) [13] have a tetragonal structure with the space group P4bm. For the XRD pattern belonging to BTN (Ba6Ti2Nb8O30), the strongest peak at 2 θ = 31.77° (see the star label in Figure 1b) and peaks at 2 θ = 29°, 43° correspond to a secondary phase Ba5Nb4O15, based on a previous study on BTN using Rietveld structural refinement method [19]. The peaks at 2 θ = 25.5° correspond to another secondary phase Ba3Ti4Nb4O21. Ba6Zr2Nb8O30 (BZN) has the same structure as BTN and is also a relaxor ferroelectric with the equal-valence B-site substitution of Zr4+ for Ti4+ [13]. For BZN, the strongest peak located at 2 θ = 31.6° also corresponds to Ba5Nb4O15, same as BTN (see the labeled peaks in Figure 1b), of which the location of the peak is a bit right-shifted compared to an earlier work [13]. As the Sr3Ti1−xZrxNb4O15 (STZN) compound, the Ti-rich end member Sr3TiNb4O15 (STN) (x = 0.0) has the orthorhombic structure with Pna21 symmetry [11]. For STN, the peaks at 2 θ = 23°, 46° and the highest peak located at 2 θ = 32.5° (see the labelled peaks in Figure 1b) correspond to a secondary phase Sr(Ti0.5Nb0.5)O3 with cubic perovskite structure (Pm 3 ¯ m), the peaks at 2 θ = 26°, 35° corresponds to another secondary phase Ti0.67Nb0.33O2 [23]. The Sr3ZrNb4O15 (SError! Reference source not found.ZN) end member (x = 1.0) has the same crystal symmetry as STN despite being substituted with larger cations of Zr4+ (radius 0.72 Å) instead of Ti4+ (radius 0.605 Å), consistent with the earlier study [11].
Error! Reference source not found. shows the microstructure features of the surfaces of the as-sintered samples under SEM. It is seen that they all have a porous structure with different extents of porosity. In Figure 2a, BTN shows uneven equiaxed grains with an average grain size of 10 µm, consistent with earlier studies [16]. Among the four samples, BZN has the smallest grains (size of around 1-2 µm) and an intergranular array of pores and grains (Figure 2b), in agreement with a previous study [13]. Compared with other samples, STN has more round-shaped grains (around 5 µm in size) (Figure 2c). As seen in Figure 2d, SZN is quite porous with connected irregular grains and unclear grain boundaries [13], which could influence its permittivity response and energy storage properties. It is suggested the intensively distributed pores and unclear grain boundaries in those samples (especially in BZN and SZN) are due to the release of CO2 gas during the sintering process, resulting in low sample density [13].
Error! Reference source not found. shows the temperature-dependent permittivity of the four TTB samples. Except for SZN which has a rather low relative permittivity (< 150) which is probably associated with its porous structure (see Figure 2d), the maximum permittivity of other samples is all above 1000.
The transition at around 200 °C in BTN and 150 °C for STN (See Figure 3a and c) indicates ferroelectric to paraelectric transition across Curie temperature, in agreement with previous works [22,24]. Also, it is noted that this transition is broadened, possibly arising from the order-disorder variations of TiO6 and NbO6 for BTN and STN [25]. It has been reported that the diffuse exponent, γ , of STN and BTN is 1.41, and 1.95 respectively (when γ = 1 fits the normal ferroelectric when γ = 2 fits the typical relaxor) [25]. This type of diffused ferroelectric nature was also seen in other TTB ceramics [26,27]. It is also seen that the dielectric loss of BTN and STN show a sudden increase from 200-250 °C together with the corresponding permittivity. Based on similar studies it can be concluded the corresponding peaks are not signals of transitions [11,25]. Instead, it could be due to the motion of charge carriers including oxygen vacancies which are thermally activated at higher temperatures, with higher conductivity for samples [28]. Also, the increased dielectric loss could arise from the reduced contribution of ferroelectric domain walls at high temperatures [29]. Nevertheless, all samples have a rather low dielectric loss (less than 0.4 at frequencies above 1 kHz) till 400 °C, showing great potential in electronic applications, etc.
In Figure 3b, BZN seems to show the strongest relaxor behavior among all samples (see Figure 3b), showing strong frequency dependence in permittivity and loss tan   δ , where the permittivity peaks move to higher temperatures with increasing frequency. In a similar study for porous BZN, the maximum permittivity also occurs around room temperature with strong frequency dispersion and γ of reaches 1.87 [13]. Similar to BTN and SZN, the permittivity and dielectric loss of BZN significantly increase from 200 °C and then decline at 300 °C. The corresponding peak at 300 °C could be associated with the dynamics of polar nano regions in this relaxor composition [13] or space charge effects.
Interestingly, SZN shows stable permittivity and dielectric loss values with increasing temperature, indicating no phase or structural transitions in this range (Figure 3d). SZN could be ferroelectric with small domains or only piezoelectric [11]. Another study [11] suggested that there is a phase transition for SZN at 500 °C. Compared with STN, the shift of phase transitions to higher temperatures in SZN leads to more stable piezoelectric properties in this wide temperature range [11].
Error! Reference source not found. a and b show the temperature-dependent P(E) loops of BZN and STN samples under the maximum electric field of 140 kV/cm and 100 kV/cm, respectively. The maximum polarization (Pmax) for BZN is below 6 µC/cm2 under 140 kV/cm and below 8 µC/cm2 for STN under 100 kV/cm. With temperature, the maximum polarization shows a slight decline for both samples. It is also found that the remanent polarization (Pr) gets larger and the P (E) loop becomes lossy at higher temperatures. The highest endurable electric field for BZN and STN samples is 140 and 120 kV/cm. BZN has a more porous structure than STN, as shown in Error! Reference source not found.. Pores would reduce the effective field applied on the sample, so the porous BZN can sustain a higher field [13] than STN with a denser microstructure. As seen in Figure 4c, the Wrec for both samples is less than 1 µC/cm3, which is rather low to serve as energy storage materials. A similar energy storage response is also observed for the other two compositions, i.e., BTN and SZN (the data for BTN and SZN are not shown due to lack of complete data at all temperatures). At room temperature, the Wrec of BZN at 140 KV/cm is 0.29 J/cm3 and shows a minor decline with temperature till 0.17 J/cm3 at 150 °C due to increased conductivity. In contrast, STN shows a similar Wrec at room temperature at 100 kV/cm with a slightly increased energy storage response at 150 °C of 0.29 J/cm3 due to reduced energy loss. The ƞ (%) shows a similar trend as Wrec for both BZN and STN respectively. As seen in Figure 4d, it declines significantly from around 80 % to 39 %, due to the increased conductivity with the sample with temperature for BZN, while for STN it overall increases from 70 % to 79 % from room temperature to 150 °C.
The porous structure of those TTB samples could lead to a low breakdown field and therefore limit their energy storage response, as shown in Error! Reference source not found.. Those energy storage properties could be potentially improved by chemical doping. For example, B site Ta-doped Sr2NaNb3.5Ta1.5O15 shows an increased breakdown strength and improved energy density of 3.99 J/cm3 and ƞ of 91.7 % [30]. Sb-doped Sr2Na0.8Ag0.2Nb4.7Sb0.3O15 tungsten bronze ceramics show enhanced relaxor nature and could achieve Wrec of 2.27 J/cm3 and ƞ of 93.3 % [31]. Those doped samples show dense microstructure and no abnormal growth crystal grains. Similar enhanced relaxor nature and energy storage properties are also seen in other ceramics such as Sb-modified (Sr0.515Ba0.47Gd0.01) (Nb1.9-xTa0.1Sbx)O6 (SBGNT-based) [32], CuO-modified Sr2NaNb5O15-based [33] lead-free tungsten bronze relaxor ceramic, etc. Alternatively, sintering or post-annealing at a reducing atmosphere could also reduce the porosity and improved the electrical and energy storage properties [19].

Conclusions

The four compositions of tungsten bronze phase ceramics, i.e., Ba6Ti2Nb8O30 (BTN), Ba6Zr2Nb8O30 (BZN), Sr3TiNb4O15 (STN), Sr3ZrNb4O15 (SZN) ceramics are systematically studied with a focus on their microstructure, dielectric response and its correlation with their temperature dependence of energy storage properties. Due to the different A and B site cations, those tungsten bronze ceramics show significantly different microstructure, permittivity response, and different extents of relaxor nature. Nevertheless, it was found that they generally have a low recoverable energy storage density (lower than 1 J/cm3) due to their porous structure, especially SZN with an expected poorest energy storage response. Doping would be an ideal method to achieve a dense structure with smaller grains, enhanced relaxor nature, and promising energy storage properties.

Declaration of Competing Interest

The authors declare no competing interests that could influence the work in this paper.

Acknowledgments

X.S. and N.H.K. gratefully acknowledge the financial support for this work from the Deutsche Forschungsgemeinschaft under KH 471/2 and the ETI-funding of Friedrich-Alexander-University Erlangen-Nürnberg.

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Figure 1. Normalized XRD patterns of BTN, BZN, STN and SZN at room temperature between 10° and 90° (a); enlarged 2θ range between 20° and 40° with the strongest peak marked with star labels (b).
Figure 1. Normalized XRD patterns of BTN, BZN, STN and SZN at room temperature between 10° and 90° (a); enlarged 2θ range between 20° and 40° with the strongest peak marked with star labels (b).
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Figure 2. Microstructure image of (a) BTN; (b) BZN; (c) STN; and (d) SZN ceramic samples.
Figure 2. Microstructure image of (a) BTN; (b) BZN; (c) STN; and (d) SZN ceramic samples.
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Figure 3. Temperature-dependent permittivity of BTN (a); BZN (b); STN (c) and SZN (d) (heating round) at different frequencies between room temperature and 400 °C.
Figure 3. Temperature-dependent permittivity of BTN (a); BZN (b); STN (c) and SZN (d) (heating round) at different frequencies between room temperature and 400 °C.
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Figure 4. Temperature-dependent energy storage properties of BZN (a) and STN (b) from room temperature to 175 °C under field of 140 kV/cm and 100 kV/cm respectively; calculated recoverable energy density (Wrec) and energy storage efficiency (ƞ) from room temperature to 150 °C for BZN (c) and STN (d).
Figure 4. Temperature-dependent energy storage properties of BZN (a) and STN (b) from room temperature to 175 °C under field of 140 kV/cm and 100 kV/cm respectively; calculated recoverable energy density (Wrec) and energy storage efficiency (ƞ) from room temperature to 150 °C for BZN (c) and STN (d).
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