Excellent energy storage performance in NaNbO 3 ‑based relaxor antiferroeic ceramics under a low electric field

NaNbO 3 -based antiferroelectric (AFE) ceramics have the prominent advantages of stable performance and low cost. However, its energy storage property is often remarkably limited by the hysteresis of the antiferroelectric to ferroelectric phase transfor-mation. In this work, 0.88Na(Nb 1− x Ta x )O 3 –0.12Bi 0.2 Sr 0.7 TiO 3 ( x = 0–0.075) antiferroelectric ceramics were synthesized using a conventional mixed oxide route. Ta 5+ were completely dissolved into the lattice of 0.88NaNbO 3 –0.12Bi 0.2 Sr 0.7 TiO 3 to form a pure perovskite structure. With increased Ta content, the AFE orthogonal P phase was replaced by AFE orthogonal R phase progressively. Meanwhile, the dielectric constant curve showed relaxor-like properties. As a result, slender P–E curves with reduced hysteresis loss and decreased residual polarization were achieved. Interestingly, a large recoverable energy storage density ( W rec ~ 2.16 J cm −3 ) and high energy storage efficiency ( η ~ 80.7%) were obtained simultaneously under a low driving electric field of 15 kV mm −1 at doping ratio ( x ) of 0.075. In addition, the 0.88Na(Nb 0.925 Ta 0.075 )O 3 –0.12Bi 0.2 Sr 0.7 TiO 3 sample exhibited excellent temperature stability, indicating an ideal candidate in future pulsed power capacitor.

The lead-free AFE material in high-power capacitors is attracting a growing body of research because of its advantages of high energy and high power density. It has become one of the most popular and promising research directions. Compared with (Pb,La)(Zr,Sn,Ti)O3 and AgNbO 3 -based AFE ceramics, NaNbO 3 (NN)-based AFE ceramics have many advantages, such as nontoxicity, stable performance, and low cost [39][40][41]. At room temperature (RT), NN exhibits AFE orthorhombic P phase, and it turns into AFE orthorhombic R phase at about 360 °C [42]. However, the AFE P phase in undoped modified NaNbO 3 -based ceramics is unstable and easily transformed into the orthorhombic FE phase, resulting in an irreversible AFE-FE phase transition and displaying a square P-E loop similar to that of FE materials [40,41], thus decreasing its η. The two aspects in the current research on the modification of NaNbO 3 -based AFE ceramics are as follows. 1) A stable AFE P phase can be obtained by doping BaZrO 3 , CaZrO 3 , CaSnO 3 , and other perovskite structural materials [36,[39][40][41][42], which can stabilize the AFE P phase under RT, realize reversible AFE-FE phase transition, and obtain typical AFE double-hysteresis loop (beam waist type). This phenomenon can improve the W rec and η of materials, which are only 3 J cm −3 and 74%, respectively. 2) The stable antiferroelectric R phase under RT can be obtained by introducing 12 mol% Bi 0.5 Na 0.5 TiO 3 into NaNbO 3 -based AFE phase, It has a thinner hysteresis loop than the antiferroelectric P phase can be obtained under RT. Thus a high W rec of 12.2 J cm −3 and an acceptable η of 69% were achieved [36,43].
Ta has been displayed to be a high-quality dopant in AgNbO 3 for improving the AFE phase stability [31][32][33]. For example, 15 mol.% Ta doping in AgNbO 3 can enhance E b by nearly 23 kV mm −1 , thus obtaining a large recoverable energy storage density of 4.2 J cm −3 and increased η of 69%. Results show that the antiferroelectricity can be enhanced by replacing Nb with Ta due to decreased polarizability of B-site cations in distorted AFE configuration. In addition, the addition of Ta can increase the bulk density and reduce the grain size of AgNbO 3 ceramics, thus remarkably improving the E b of materials [31]. However, research on Ta doping in NN-based system with stable AFE P phase is lacking. This finding is a good example to show that NN is different from other AFE perovskite systems in nature.
In this paper, 12 mol% Sr 0.7 Bi 0.2 TiO 3 was firstly chosen to form solid solutions with NaNbO 3 to enhance the antiferroelectricity by reducing the tolerance factor [25,41,46]. Although the characteristics of double hysteresis loop are obtained, the hysteresis loss and residual polarization P r are still very large, resulting in low W rec and η. Therefore, Ta was chosen to replace the B-site in 0.88NaNbO 3 -0.12Sr 0. of structure with ferroelectric, dielectric and energy storage performance are systematically investigated. The phase transition from the AFE P phase to AFE R phase caused by Ta doping contributes to enhanced energy storage performance. As a result, a large W rec of 2.16 J cm −3 and a high η of 80.7% under 15 kV mm −1 are obtained in the 0.88Na(Nb 0.925 Ta 0.075 ) O 3 -0.12Bi 0.2 Sr 0.7 TiO 3 (NN-BST-0.075Ta) sample. Notably, few reports on lead-free dielectrics can obtain a high η (> 80%) and large W rec (> 2 J cm −3 ) simultaneously under 20 kV mm −1 . The NN-BST-0.075Ta sample has a remarkable promise in energy storage dielectric capacitors.

Experimental procedure
.050, and 0.075) ceramics were fabricated by a conventional solid phase method by using Nb 2 O 5 (> 99.5%), Na 2 CO 3 (> 99.5%), Ta 2 O 5 (> 99.9%), SrCO 3 (> 99.5%), Bi 2 O 3 (> 99.5%), and TiO 2 (> 99.5%) powders. Dry raw powders were weighed using the stoichiometric ratio of NN-BST-xTa and subjected to wet-milling for 36 h in a teflon jar with isopropanol. Dry mixed powders were calcined at 950 °C for 2 h and remixed for 10 h to obtain NN-BST-xTa as-calcined powders. The refined powder was uniaxially pressed into disk-shaped samples with a thickness of about 1 mm and a diameter of 10 mm. The disk-shaped samples were sintered at 1160 °C for 3 h in an air atmosphere to obtain high density. Discs were bedded in the same composition of as-calcined powders to resist the volatilization loss of sodium and bismuth. Sintered disk-shaped samples were ground and polished to obtain a thickness of 0.1 mm and measure the electrical performance. The silver electrode with diameter of 0.5 mm was coated in the middle of the polished disc surface by using the screen-printing method and heated at 550 °C.
Field-emission scanning electron microscope (SEM) was used to examine the surface morphology of samples. The crystal structures of NN-BST-xTa ceramic samples were performed through X-ray diffraction (XRD). The dielectric constant ε r -temperature (T) and dielectric loss (tan δ)-temperature (T) curves were tested by an LCR meter at a heating speed of 120 °C h −1 . Polarization (P)-electric field (E) curves were characterized using an ferroelectric tester under 10 Hz at various temperatures. All specimens were submerged in silicone oil to prevent surface flash over.

Results and discussion
The XRD analysis of unpoled NN-BST-xTa ceramics were presented Fig. 1(a). All the ceramics reveal perfect perovskite structure. No other impuritiesare contained.
It was suggesting that Ta 5+ are dissolved in the NN-BST ceramic lattice and form a uniform perovskite solid solution. The enlarged XRD diffraction patterns in the range of 31.5°-46.8° ( Fig. 1(b)) further present variations in (110) and (200) reflection peaks. The (110) and (200) diffraction peaks are not obvious split, which indicates that NN-BST-xTa ceramics basically belong to the pseudo-cubic phase. As x increases from 0 to 0.075, the positions of (110) and (200) diffraction peaks remain unchanged, indicating that the lattice volume has not changed. This phenomenon occurs because the radii of Ta 5+ (0.64 Å, CN = 6) and Nb 5+ (0.64 Å, CN = 6) [31,44].
The SEM images of NN-BST-xTa ceramics were exhibited in Fig. 2(a)-(d). As can be seen from the figures, NN-BST-xTa ceramics are sintered compactly. We used a linear interception method by Nano Measurer software to calculate the average grain size of NN-BST-xTa. Results are displayed in Fig. 2(e)-(h). The average grain sizes of NN-BST-xTa are 1.95, 1.72, 1.63, and 1.34 μm at x = 0, 0.025, 0.050, and 0.075, respectively. The average grain size decreases with the increase of Ta content, which is caused by the low ion mobility of Ta [31,44]. NN-BST-0.075Ta ceramics have a dense microstructure and small grains that may be responsible for increasing W rec and E b . Figure 3 shows the ε r -T and tan δ-T curves of unpoled NN-BST-xTa ceramics at 1, 10, 100, and 1000 kHz frequencies. The dielectric peaks of x = 0 and 0.025 components are evident at 160 °C, corresponding to the antiferroelectric P to R phase transition temperature (T P-R ). In addition, at x = 0-0.05, ε r at RT increases significantly and decreases with further increase in x. It can be noted that the P-R phase transition peak for x = 0.05 sample. However, the ε r at RT is higher than that for samples with x = 0 and 0.025, which shows that the ε r of x = 0.05 sample is affected by the antiferroelectric R phase. In addition, due to the thermal hysteresis of antiferroelectric P-R phase transition during heating, antiferroelectric P phases are few. When x = 0.075, the the large ε r at RT and disappearing dielectric peak T P-R at RT indicate that the sample should consist of the antiferroelectric R phase. Thw similar result has been found in other NN-based relaxor AFEs [30,34,[45][46][47]. Figure 4(a) shows the dielectric performance of undoped NN-BST-xTa ceramics at the temperature range of − 170 °C to 100 °C. NN-BST-0.075Ta has evident dielectric dispersion and small frequency shift at high temperature. Thus, NN-BST-0.075Ta ceramics has class relaxor characteristics. This relaxor behavior is associated with the different polarizability of perovskite B-site ions. The polarizability of Nb 5+ (3.10 Å 3 ) is higher than that of Ta 5+ (2.82 Å 3 ). This phenomenon leads to disordered B-site and random electric field. Therefore, the long-distance driving dipoles are weakened [30,34]. The r and tan δ as functions of frequency are shown in Fig. 4(b). The r and tan δ almost remain at the same level at testing frequencies. The dielectric performance shows outstanding frequency stability and is beneficial to the practical application of pulse power transistor. These results are related to the large P and high E b [34,[48][49][50][51].
The unipolar curves of P-E tested under different electric fields under 10 Hz for NN-BST-xTa ceramics are displayed in Fig. 5. Figure 5(a) and (b) present that x = 0 and 0.025 samples have stable antiferroelectricity. P m increases when the E is more than 12 kV mm −1 , indicating that evident AFE-FE phase transition occurs. For 0.025 < x ≤ 0.075 samples, increasing Ta content gradually results in slender P curve. P properties show stable relaxor characteristics, resulting in increased E b (Fig. 5(c)-(d)). This result is because the polarizability of Ta 5+ is lower than that of Nb 5+ , which leads to decreased sensitivity of B-site cations to the E [27]. Generally, the substitution of Ta 5+ for Nb 5+ will not lead to reduced tolerance factor due to the same radii. In addition to the tolerance factor, the different polarizability between Nb 5+ and Ta 5+ reduces the average off-centering of the B-site ion in the octahedra and modifies the cell parameter [31]. Therefore, the addition of Ta in NN-12SBT ceramics leads to the phase transition from antiferroelectric to relaxor antiferroelectric, and the E AF of the AFE-FE phase transition is getting blurry. Above results are consistent with the dielectric performance. Figure 6(a)-(c) compare the curves of P-E, P r , P m , and energy storage properties of NN-BST-xTa ceramic samples at a low E (15 kV mm −1 ). NN-BST-0.025Ta and NN-BST have relatively stable antiferroelectric properties and large P r (25.2 and 19.9 μC cm −2 , respectively) and P m (53.4 and 44.1 μC cm −2 , respectively). With increasing Ta content, P m decreases gradually, and P r decreases rapidly. Zhang  [42] studied the NN-xBi 1/2 Na 1/2 TiO 3 (NN-xBNT) system and observed similar results. BNT (24 mol.%) is dissolved in NN, and a kind of antiferroelectric material with relaxor characteristics is obtained. In addition, larger nanodomains (30-50 nm) than polar nanoregions (PNAs) are observed in the material. As mentioned previously, the dopant of the perovskite B-site by Ta results in a local random field, weakening the long-range driving dipole and enhancing the dielectric relaxation. Despite the decreases in P m , W rec increases when 0.025 ≤ x ≤ 0.075 due to the P-E curve becomes slender and long, thereby broadening the integral area and increasing W rec . Such changes in P-E curves caused by the transition from the antiferroelectric P phase to antiferroelectric R phase are helpful to improve the energy storage performance [52][53][54][55]. Subsequently, the W rec and η of NN-BST-xTa ceramics are compared with several representative lead-free ceramics for energy storage reported recently at low E (Fig. 6(d)) [15,16,31,36,[45][46][47]. Considering its practical application, temperature dependence is one of the important indices in the design of dielectric capacitor. Figure 7 displays the (a) unipolar P-E curves, (b) W rec , and η of NN-BST-0.075Ta ceramics as a function of temperature under 15 kV mm −1 . The unipolar P-E curve displays an excellent temperature stability, which may be due to the stable R phase coexisting in NN-BST-0.075Ta over a wide range of temperature, as displayed in Fig. 7(a),. The W rec and η vary from 1.98 J cm −3 and 80.7%, respectively, to 2.2 J cm −3 Fig. 4 (a) The dielectric constant (ε r )-temperature (T) and loss (tan δ)-temperature (T) curves of NN-BST-0.075Ta ceramics at the temperature range of − 170 °C to 100 °C. (b) ε r and tan δ of NN-BST-0.075Ta ceramics as a function of frequency  (Fig. 7(b)). The variations in W rec and η are less than 11.1% and 6.3%, respectively. The good temperature stability of NN-BST-0.075Ta indicates its potential as lead-free relaxor AFE material for use in temperature-stable pulse capacitor.

Conclusion
A novel Ta-doped NN-BST dielectric ceramics with a pure perovskite phase is successfully synthesized by a traditional solid state reaction route. The effect of Ta 5+ in the perovskite B-site increases the relaxation of the material, thus improving E b and decreasing P r . Dielectric and ferroelectric performances reveal that the NN-BST-0.075Ta ceramics belong to relaxor AFEs. Therefore, a large recoverable W rec of 2.16 J cm −3 and η of 80.7% at a low electric field of 15 kV mm −1 are achieved in NN-BST-0.075Ta ceramics due to improved relaxation proper ty. NN-BST-0.075Ta ceramics also exhibit excellent energy storage temperature stability. These excellent properties in NN-BST-0.075Ta relaxor AFE ceramics indicate its potential as a candidate in future dielectric capacitors.