High-energy storage and temperature stable dielectrics properties of lead-free BiScO

Lead-free (1–x)(0.4BiScO3–0.6BaTiO3)-x(Bi0.5Na0.5)TiO3 (BSBT–xBNT) ceramics were prepared by traditional ceramics preparation method. The energy storage characteristics, as well as dielectric spectrum curves of BSBT–xBNT ceramics, were systematically used in research. The ternary system formed perovskite solid solution with dense and homogenous microstructure. BSBT–xBNT ceramics exhibited slimmer and slanted hysteresis loops followed by higher saturation polarisation (Pmax) and lower remnant polarisation (Pr), which induced high-energy storage density of 1.39 J/cm3 (E = 180 kV/cm). Ranging from 200 to 400°C, BSBT–xBNT ceramics had high permittivities, and a matching dielectric loss, which exhibited flat temperature coefficients of permittivity (TCɛ). The impedance spectroscopy analysis indicated that the non-Debye relaxation mechanism existed in BSBT–xBNT ceramics. BSBT–xBNT ceramics may have potential application value for the high-temperature capacitor.


Introduction
Owing to its excellent ferroelectric, piezoelectric, dielectric, energy storage and strain properties, lead-based ceramics with perovskite structure are extensively applied in sensors, actuators and pulse power [1].The urgent research work of lead-free ceramics replacing lead-based ceramics cannot be postponed for the serious environment issues.Recently, lead-free materials such as BaTiO 3 (BT), (Bi 0.5 Na 0.5 )TiO 3 (BNT) and (KNa)NbO 3 -based ceramics have been investigated widely, because they can gather a large number of energy in a short time and discharge instantaneously [2][3][4].BNT-based ceramics, accompanied with relatively large P r value and high Curie temperature, were among the most prospective competitors for replacing lead-based ceramics, but the shortcomings such as high coercive field (E c ) and high electrical conductivity limited its applications [5].For further improving its energy storage properties, various modifications such as doping or adding a new component to pure BNT matrix have been taken to decrease its coercive field and conductivity.BT-based ceramics have been extensively applied in electrical devices, its phase transition temperature, and electrical properties can be regulated by forming solid solutions [6].The BNT-BT solid solutions obtain excellent electric properties around the morphotropic phase boundary.On this foundation, further studies centring on elevating energy storage capabilities of BNT-BT binary system have been implemented through incorporating new composition (K 0.5 Na 0.5 NbO 3 , calcium zirconate, strontium titanate) [7][8][9], single ion (La 3+ , Sn 4+ ) [10,11], double ions (La 3+ /Zr 4+ ) [12], and complex ions [(Al 1/2 Nb 1/2 ) 4+ , (Li 1/2 Nd 1/2 ) 2+ ] [13,14].
Recently, Wu et al. [5] found that the modulated diffusions of bismuth and scandium in core-shell BT@BiScO 3 ceramics suppressed polarisation non-linearity and reduced the degree of hysteresis, which could be taken into consideration in enhancing energy storage capability.Subsequently, Ogihara et al. [15] found that 0.7BT-0.3BSceramics could achieve high permittivities stability over a wide-temperature range.The energy density of 0.7BT-0.3BScapacitor with a thickness of 15 µm reached up to 6.1 J/cm 3 (E = 730 kV/cm) [16].Lim et al. [6] also found that (1-x) (0.4BiScO 3 -0.6BT)(BSBT)-x(K 0.5 Bi 0.5 )TiO 3 ceramics possessed a considerable recoverable energy storage density (∼4.0 J/cm 3 ) and a stable permittivity with low dielectric loss range from 100 to 300°C.In addition, BT-xBS thin film with high polarisabilities and permittivities was acquired [17].Although the above-mentioned systems achieved excellent energy storage and dielectric properties, the demand for the externally applied field was beyond the practical application level, energy storage efficiency was also unrecognised, and the high-temperature dielectric stability was confined to relatively low-temperature regions.
Combining the excellent properties of the BNT-BT and BT-BS systems, here, we designed BSBT-BNT solid solution and its excellent electrical behaviours were reported.The large energy storage density (1.39 J/cm 3 ) and energy storage efficiency (88%) were obtained synchronously under the more modest field (E = 180 kV/cm).The high-temperature dielectric stability displayed a higher-temperature range from 200 to 400°C.The findings reveal that BSBT-xBNT ceramics are more available in the dielectric energy storage field.

Experiment
Bulk BSBT-xBNT ceramics (x = 0, 0.05, 0.10, 0.15, 0.20, 0.30) were fabricated via traditional ceramic preparation method.The initial reagent grade powers of barium carbonate (99.0%), sodium carbonate (99.8%), titanium dioxide (99.0%), bismuth (III) oxide (99.95%), and scandium oxide (99.9%) were utilised as source materials.The materials required by stoichiometric properties were demanded to be milled evenly and calcined at 880°C to form the desired perovskite phase.To increase the strength of the embryonic sample, 5 wt% solution of polyvinyl alcohol (PVA) was added to the resulting powders.These powders were compressed into a circular plate under 40 MPa and sintered at 1150°C for 2 h afterwards.For the sake of facilitating the test of electrical performance, the sintered samples were grounded to about 0.25 mm thickness and printed by silver electrodes on both sides.
The X-ray diffraction technique (XRD, AXS D8-ADVANCE, Bruker) was utilised to confirm the crystal structure of the samples.The microstructure of ceramics was examined by field-emission scanning electron microscopy (FESEM, quanta 450 FEG, FEI).A computer-controlled impedance analyser (4294A, Agilent) was applied to determine the temperature-dependent dielectric properties from ambient temperature to 600°C.Agilent 4294 A impedance analyser was also collected to measure the temperaturedependent impedance spectrum from 40 Hz to 1 MHz.The hysteresis loops and strain curves were tested at 1 Hz and ambient temperature via the ferroelectric materials measurement system (P-PMF, Radiant).

Results and discussion
Fig. 1a shows the XRD patterns of BSBT-xBNT ceramics.We can observe all the samples have a single perovskite structure without other impurity phases, which indicate that the BNT composition has absolutely dissolved into the BSBT binary system and creates a new solid solution.Magnified profiles of the ( 111) and ( 200) peaks are displayed in Fig. 1b and c.The ( 111) and ( 200) diffraction peaks BSBT-xBNT ceramics shift toward higher angle with increasing BNT content.Moving toward higher angle indicates that BNT substitution to the BSBT solid solution leads to the contraction of lattice constants ascribed to the smaller ionic radius of (Bi 0.5 Na 0.5 ) 2+ (1.015 Å) than that of Ba 2+ (1.35 Å) and Na + (1.02 Å) on A-site.The analogous situation also occurs in B-site for Ti 4+ (0.605 Å) replacing Sc 3+ (0.745 Å).
Fig. 2 shows SEM surface morphologies of BSBT-xBNT ceramics.For BSBT-xBNT ceramics, a dense and homogeneous structure is obtained with clear grain boundaries, and there are no distinct pores on the structure surface.Fig. 3 displays the average grain size for BSBT-xBNT ceramics.As we can see, the average grain size only increases from 1.9 µm for pure BSBT ceramic to 2.15 µm for BSBT-0.3BNTceramic with the doping content increasing, indicating that the BNT content is not significant for the grain size of BSBT-xBNT ceramics.In addition, BSBT-xBNT ceramics display the spherical-like grains and many small grains.The crystal morphology is often sensitive to the amounts of solid or liquid in the samples [6].Hence, we can speculate that BNT possibly existing as the liquid phase influences the grain shape during the sintering process.Fig. 4a displays the P-E loops of BSBT-xBNT ceramics tested under the dielectric breakdown strength (DBS) and room temperature.As a whole, BSBT-xBNT ceramics show the slimmer P-E loops accompanied by the low P r and E c , which are conductive to the enhancement of energy storage capabilities.We can see from Fig. 4a, the variations of E c and P r are not obvious with the BNT content increasing.Conversely, the P max reaches a maximum and then decreases.Accompanied by the introduction of BNT, BSBT-xBNT ceramics exhibit high P max value that is obviously more than that of the pure BSBT ceramics.The BSBT-0.15BNT ceramic possesses a larger value P max (18.01 μC/cm 2 ) and a smaller P r (0.81 μC/cm 2 ).In this work, BSBT-xBNT ceramics, followed by slanted and pinched P-E loops, mean excellent energy storage characteristics, which are propitious to the use in capacitor energy storage devices.
In accordance with the P-E results, the calculated energy storage capability for BSBT-xBNT ceramics is plotted in Figs.4b  and c.On the electric field increasing, we can see that the energy storage density shows a sharp upward trend, while energy storage efficiency exhibits the opposite trend due to a sudden growth in the density of energy loss.For BSBT-0.15BNT ceramic, its maximum energy storage density is up to 1.39 J/cm 3 (E = 180 kV/cm), and the relevant energy storage efficiency reaches 88%.In addition, the dielectric breakdown field of BSBT-xBNT ceramics excesses 140 kV/cm.When the critical breakdown strength increases, the energy storage characteristics will also be affected accordingly.Gerson and Marshall [18] demonstrated that DBS is negatively correlated with the thickness of the samples [19].As a result, reducing the thickness of the samples would realise the better energy storage density further for BSBT-xBNT ceramics.Fig. 5 displays the P-I-E characteristics of BSBT-xBNT ceramics tested at 80 kV/cm, 1 Hz, and room temperature.When 0 ≤ x ≤ 0.15, the P-E loops express as a linear type, along with rectangle-like I-E loops with no apparent current peaks.As the amount of solid solution expands further, the P-E loops gradually evolve from non-linearity to saturation, that is, the characteristic of the ferroelectrics.According to the characteristics of the hysteresis loop, when the doping content increases, a phase transition process to a ferroelectric state occurs in BSBT-xBNT ceramics.Moreover, when the doping content excesses 0.15, four weak current peaks can be observed in the current curves around E = ±35 kV/cm.The peaks 1 and 3 represent the polarisation switching with ergodic relaxor-ferroelectric type, whereas the appearing peaks 2 and 4 indicate a reverse transition process to ergodic relaxor phase with electric field revoked [20].We also become conscious of the similar situation in other BNT-based ceramics [10,11,21,22].dielectric constant curves display apparent frequency dispersion phenomenon below the temperature of maximum dielectric permittivity (T m ), and T m shows a little shift toward higher temperature with the increase in frequency.These features have a resemblance to another relaxor-ferroelectric ceramics, as reported in the previous literature such as BiScO 3 -Pb(Mg 1/3 Nb 2/3 )TiO 3 -PbTiO 3 and BiScO 3 -BaTiO 3 -(K 0.5 Bi 0.5 )TiO 3 ceramics [23,24].Meanwhile, the maximum dielectric constant is put up from about 806 to 1520 by degrees, and the T m shifts from 148 to 298°C at 1 kHz, which are similar to those shown in BaTiO 3 -xBiScO 3 ceramics and thin films [15,17].It means that T m is sensitive to the BNT introduction.
As shown in Fig. 6, BSBT-xBNT ceramics display hightemperature dielectric stability that demonstrates flat coefficients of temperature (TCɛ) within a wide-temperature range from 200 to 400°C.On the basis of the following equation [25], the dielectric permittivity value of BSBT-xBNT ceramics tested at 1 kHz was applied to calculate TCɛ: where ɛ 200 , ɛ 300 , and ɛ 400 correspond to the dielectric constant at 200, 300, and 400°C, respectively.To achieve the optimum performance of BSBT-xBNT ceramics in the high-temperature capacitor, we look forward to the TCɛ value approaching zero.Except for the BSBT-0.3BNTceramic, solid solution content dependent on the TCɛ value for BSBT-xBNT ceramics is reacted in Fig. 7a.Notably, the magnitude of TCɛ is ∼−388 ppm/°C for BSBT-0.1BNTceramic.Furthermore, the dielectric loss of BSBT-0.1BNTceramic gains constant near 0.01 in this temperature range, indicating a good resistivity.After 400°C, dielectric loss increases dramatically with increasing temperature further, which may result from the increase of thermally excited ionic conductivity [26].
These characters indicate that BSBT-xBNT ceramics are a greatly competitive contender for the high-temperature energy storage capacitor applications.
Frequency dispersion and diffuse phase transition of the dielectric spectrum are usually regarded as the criteria for the relaxor characteristics of ferroelectrics [27,28].In this paper, the variations of dielectric properties in the high region are described by the modified Curie-Weiss law [29-31]  promotion of the relaxor nature.This probably derived from the growing disorder of cation owing to the replacement on the A/Bsite by Na + , Bi 3+ , and Ti 4+ [32].
The electrical properties were also investigated by the impedance spectrum analysis technique.Fig. 8a presents the Cole-Cole plots of the impedance of the BSBT-0.15BNTceramic as a function of temperature at 40-10 MHz.Below 450°C, high resistivity causes no apparent semi-circle, while the semi-circles behave normally and the radii of semi-circles become smaller with the temperature increasing, which indicates low resistivity and negative temperature coefficient of resistance (NTCR) phenomena at higher temperatures.Owing to a distribution of relaxation time, the semi-circles exhibit a certain degree of contraction, and the centre of semi-circle deviates from the horizontal axis of Z′.These indicate the non-Debye-type relaxation mechanism occurs in BSBT-0.15BNTceramic, and BSBT-0.15BNTceramic contains essentially one electrical component around this temperature range [33,34].Fig. 8b shows the frequency-dependent imaginary parts of impedance in 450-540°C temperature range.The shift and broadening of the peaks are associated with temperature variation.On an increase in temperature from 450 to 540°C, the peaks shift to higher-frequency region and it merges together, finally indicating a reduction in the mobility of the space charge carriers and relaxation time of the BSBT-xBNT ceramics [35].Meanwhile, the broadening peaks mean the emergence of a temperature-dependent relaxation phenomenon [35].The existence of defects in elevated temperature stage and immobile species at low-temperature region may lead to this relaxation [35,36].

Conclusion
High-energy storage capacity and dielectric stability of BSBT-xBNT ceramics were fabricated by traditional ceramics synthesis method.All tested samples are pure perovskite phases and show the dense structure and homogenous grains.The slimmer P-E loops with large DBS demonstrate high-energy storage density, the maximum energy storage density of 1.39 J/cm 3 (E = 180 kV/cm) is obtained at x = 0.15 with 0.25 mm thickness.Moreover, BSBT-xBNT ceramics exhibit high dielectric with a maximum flat temperature coefficient permittivity of −388 ppm/°C.The impedance measurements indicate the occurrence of non-Debyetype relaxation mechanism and NTCR in BSBT-xBNT ceramics.This novel ternary system with high dielectric constant, excellent energy storage properties have potentially significant values for high-temperature energy storage capacitor devices.

Fig. 6
Fig. 6 draws the curves of dielectric spectrum for BSBT-xBNT ceramics from ambient temperature to 600°C.It is clear that no sharp dielectric peaks emerge for BSBT-xBNT ceramics, and the dielectric permittivity puts up a broad feature.Meanwhile, the