High Energy Storage Performance in Pb_1−xLa_x(Hf_0.45Sn_0.55)_0.995O₃ Antiferroelectric Ceramics

Abstract

Energy storage efficiency (η) and large recoverable energy density (W_re) are necessary for antiferroelectric materials in order to develop antiferroelectric-based dielectric capacitors with exceptional energy storage capacity. In the present paper, the effect of doping La³⁺ on the energy storage capacity of Pb_1−xLa_x(Hf_0.45Sn_0.55)_0.995O₃ antiferroelectric ceramics was studied. Adjusting the content of La and changing the phase structure of PLHS from antiferroelectric to relaxor ferroelectric gradually, which narrowed its hysteresis loop, yielded a high energy storage efficiency of 81.9% and the maximum breakdown field strength of 200 kV/cm when x = 2 mol%. In addition, the recoverable energy density and energy storage efficiency both showed excellent temperature stability and frequency stability in the temperature range of 10–110 °C and the frequency range of 10–100 Hz, suggesting that Pb_0.98La_0.02(Hf_0.45Sn_0.55)_0.995O₃ are favorable materials candidates for the preparation of pulsed-power capacitors that can be used in a wide range of conditions.

1. Introduction

As the preferred material for dielectric capacitors, antiferroelectric materials are widely used in automotive electronics, communications, aerospace, and other fields because of their reversible antiferro–ferro phase transition, exhibiting the typical traits of double P-E hysteresis loops, shown in Figure 1d, and achieving high energy density (W_st) easily [1,2,3].

The antiferroelectric materials that many researchers have focused on include PZT [4,5,6]: PbHfO₃ [7], AgNbO₃ [8,9], and NaNbO₃ [10] systems.

As lead-free antiferroelectric materials, AgNbO₃ and NaNbO₃ have attracted much attention due to being environmentally friendly. However, they all exhibit complex temperature-induced phase transition behavior, generally low energy storage efficiency, low breakdown field strength, and harsh sintering process requirements, all of these factors limit their use in the energy storage industry [11,12,13,14,15,16,17]. Therefore, researchers have turned their attention back to lead-containing materials.

Because of the narrow antiferroelectric phase region of pure PZT, slight composition fluctuation can easily lead to the deviation of the FE-AFE phase boundary; thus, researchers have improved the energy storage performance through A/B-site-doping-formed PLZT, PLZS, and other systems. However, when E_A-F exceeds the breakdown strength (E_b) it is challenging to acquire a high recoverable energy density (W_re).

PbHfO₃(PHO)-based ceramics are similar to PZT-based ceramics in phase structure and electrical properties. Studies have shown that their valence band and conduction band electron distribution are similar, while the band gap is quite different (PZO is about 3.6 eV, whereas PHO is about 2.6 eV) [18]. The multi-phase transitions of PHO are complex, and the mesophase is also controversial. The doping modification of PHO (such as A-site doping, B-site doping, or A/B-site co-doping) can efficiently and effectively adjust the electric field strength for the phase transition of AFE-FE or optimize the breakdown strength, thereby improving the energy storage performance [19,20,21,22,23].

The phase stability of the ABO₃ perovskite structure can be calculated using Formula (1) [22,24], in which, R_A, R_B, and R_O represent the ionic radius of A-site, B-site, and oxygen cations, respectively.

$$\mathrm{t}=\left(R_A+R_O\right)/\sqrt{2}\left(R_B+R_O\right)$$

(1)

High t is conducive to the formation of the ferroelectric phase, while low t is crucial for the antiferroelectric phase. If the antiferroelectric phase is too stable to be transformed into the ferroelectric phase before breakdown, the P-E loop will appear as a linear dielectric, resulting in a low energy density [25]. As a result, it is necessary to select the appropriate radius of ions for doping at different sites for optimum electrical performance. At the same time, acquiring excellent recoverable energy density (W_re) and energy efficiency (η) and improving the stability of its antiferroelectric phase are the main research hotspots regarding PHO ceramics.

In multiple studies, La³⁺ with a smaller ionic radius was introduced into the A-site of Pb(Hf,Sn)O₃ (PHS) to reduce its tolerance factor and strengthen the stabilization of the antiferroelectric stage, thus enhancing the energetic storing density [26,27,28,29,30,31,32,33,34,35]. In addition, the introduction of higher-valence cations into A site of Pb(Hf,Sn)O₃ can produce Pb vacancies, which helps to establish chemical pressure in the lattice, resulting in the tilt and compression of the oxygen octahedron of Pb(Hf,Sn)O₃ and structural heterogeneity, thereby enhancing the stabilization of the antiferroelectric stage [29,30,34].

At the same time, by adjusting the doping of La³⁺, the crystal plane spacing of the sample decreases as the La doping content increases, so the lattice size and lattice constant also decrease accordingly. The basic relationship between ceramic grain size and breakdown field strength (E_b) is expressed as follows:

$$E_{\mathrm{b}}\propto {\displaystyle \frac{1}{\sqrt{G}}}$$

(2)

where G is the mean grain dimension. From Equation (2), it can be inferred that the lower the ceramic grain size, the larger the breakdown field strength. Meanwhile, the energy storage efficiency (η), recoverable energy density (W_re), and energy density (W_st) of the permittivity of the dielectric materials are directly related to the breakdown electric field.

$$W_{\mathrm{st}}={\displaystyle {\int }_0^{P\max }E\mathrm{dP}\left(\mathrm{upon}\mathrm{charging}\right)}$$

(3)

$$W_{\mathrm{re}}=-{\displaystyle {\int }_{P\max }^{P\mathrm{r}}E\mathrm{dP}\left(\mathrm{upon}\mathrm{discharging}\right)}$$

(4)

$$\eta ={\displaystyle \frac{W_{\mathrm{re}}}{W_{\mathrm{st}}}}\times 100\%$$

(5)

Therefore, to obtain ceramic materials with high W_st and η, many researchers are also committed to reducing the porosity of ceramics, making grains uniform, or performing surface modification or the doping of ceramic particles.

In the current work, we examined how La³⁺ doping affects the energy storage characteristics of ceramics Pb_1−xLa_x(Hf_0.45Sn_0.55)_0.995O₃. By utilizing the solid-phase sintering process, the optimal grain size of the PLHS ceramic material was determined, leading to an increase in its energy storage efficiency (η) and energy density (W_st). The recoverable energy density and the energy storage efficiency both showed excellent temperature stability and frequency stability in a wide temperature range and frequency scale.

2. Experimental Procedures

The formula Pb_1−xLa_x(Hf_0.45Sn_0.55)_0.995O₃ (x = 0, 0.01, 0.02, 0.03) was followed in the preparation of the PLHS ceramics with a conventional solid-state reaction process. The raw materials were La₂O₃, PbO, SnO₂, and HfO₂ powders with a chemical purity of not less than 99.0%. The mixed oxides were milled with ethanol for 12 h and then dried at 120 °C. The dried material was pre-sintered with 100~150 °C/h, heated to a temperature of 800~1200 °C, and kept for 2~4 h, and then naturally reduced to room temperature. After that, the second milling and drying procedure was carried out with the same conditions as those of the first, and then the samples were mixed with 95% alcohol and 6% PVA and sieved (60~80 mesh), ensuring their good fluidity. The obtained material was pressed into cylindrical pellets of 10 mm in diameter and 2 mm in height at 6 MPa, degreased at 600 °C for 4 h, and subsequently cooled to ambient temperature naturally. The material was then subjected to sintering for 2.5 h at 1150–1250 °C, after which it was polished, and the entire piece was silver-plated on both sides and then sintered for 30 min at 550 °C. Finally, the samples were polarized in silicone oil for dielectric testing.

X-ray diffraction (D8-Advanced, Bruker AXS Inc., Madison, Germany) was applied to measure the crystal structure at ambient temperature. SEM (JSM 6510LV, Jeol, Tokyo, Japan) was utilized for observing surface morphology. The variation in dielectric performance with temperature was measured with a TH2827C LCR instrument (Changzhou Tonghui, Changzhou, China) at 1 kHz frequency, and p-e hysteresis loops were determined with a high-voltage amplifier (Trek 610E; Trek, Lockport, NY, USA) and a ferroelectric test system (PolyK Technologies, North Philipsburg, PA, USA) in silicone oil at 10 Hz. Energy storage efficiency (η) along with energy density (W_st) were detected through the p-e hysteresis loops.

3. Results and Discussion

Figure 2a presents the XRD images of PLHS ceramic samples, which were subjected to sintering for 2.5 h at 1150–1250 °C and polished with various La amounts at ambient conditions. Figure 2a displays that there are no impurities or secondary phases in any of the ceramics; they are all pure perovskite phase structures. To better show the phase structure of the ceramics, we performed a narrow-spectrum XRD scan of the sample in the range of 43.5°–45.0° in 2θ, as shown in Figure 2b. The ceramics have a tetragonal phase structure, and there are two obvious splittings at the (200) and (002) lattices in Figure 2b. At the same time, it can be seen that as the La amount increases from x = 0 to x = 3 mol%, the (200) and (002) diffraction peaks gradually move to a high angle (right direction). According to Bragg’s formula, the crystal plane spacing of samples reduces with an increase in the La doping amount, and therefore the lattice size and lattice constant decrease accordingly. The analysis shows that the ionic radius of La³⁺ (1.36 Å) is lower than that of Pb²⁺ (1.49 Å) (coordination number is 6), and lattice distortion occurs when La³⁺ replaces Pb²⁺, so the ceramic grain size reduces in microns. According to Formula (2), the smaller the ceramic grain, the greater the strength of the breakdown field, which is conducive to obtaining excellent capacity storing performance.

Figure 3 shows the SEM pictures of PLHS ceramics, which were subjected to sintering for 2.5 h at 1150–1250 °C and polished with various La contents. From the graph, we can observe that the growth of the La³⁺ level has a strong influence on grain size. Grain size clearly increases as the La content increases to x = 1 mol%. This may be because the ionic size of La³⁺ is smaller than Pb²⁺, and once La³⁺ replaces Pb²⁺, the internal diffusion rate is accelerated, so the growth of grains is accelerated. When the content of La increases to x = 2 mol%, the ceramics’ grain size starts to reduce significantly. Because the introduction of extra La³⁺ accelerates the formation of negatively charged Pb²⁺ vacancies, it is easy to separate at grain boundaries. These generated cavities induce the attraction of directly electrically loaded La³⁺ ions, which in turn generate vacancy ion couples in the vicinity of the grain boundaries via Coulombic function. These vacancy ion pairs hinder the movement of grain boundaries and suppress grain growth, causing a reduction in grain size. Moreover, once the content of La is less than 2 mol%, the surface morphology of all ceramics is relatively dense, and when the content is further increased to 3 mol%, pores begin to appear in the ceramic. It is possible that the lattice distortion of the ceramics will increase when the doping amount of La increases, resulting in more defects.

The amount of La not only influences the microstructures of PLHS but also has a significant impact on their energetic behavior. Consequently, to achieve the best energy storage performance, the appropriate amount of La doping should be determined. Figure 4 shows the P-E hysteresis loops and P_max, P_r, and P_max-P_r, as well as the energy density (W_st) and the recoverable energy density (W_re), of all ceramic samples that had been tested under ambient conditions and 10 Hz with 200 kV/cm of maximum electric field strength. As illustrated in Figure 4a, when x = 0, the hysteresis loop corresponds to the antiferroelectric form. However, as the content of La increases, the hysteresis loops change from antiferroelectric to relaxed ferroelectric gradually. This may be due to the addition of La³⁺ ions, which enhances the local random electric field and random stress field, breaks the long-range order of antiferroelectrics, and transforms antiferroelectrics into relaxor ferroelectrics [36,37], which shortens the hysteresis loop of the antiferroelectric phase and enhances its stability. As the electric field strength involved in the antiferroelectric-to-relaxed-ferroelectric phase transition (E_A-RF) increases, and the electric field strength (E) reduces, all P-E hysteresis loops become thinner, showing that PLHS ceramics present excellent energetic storing properties. When x = 2 mol%, the p-e loop is the narrowest, which shows that the residual polarization is the smallest, which can minimize the heat generated inside the material and enhance the reliability of the device. As can be seen from Figure 4b, P_max and P_r decreases from 16.9 μC/cm² to 8.3 μC/cm² and from 1.1 μC/cm² to 0.9 μC/cm², respectively, as the content of La increases from x = 0 to x = 3 mol%. Since La³⁺ ions occupy the crystallographic position of Pb²⁺, the composition is disordered, and the disordered ions diffuse and exchange positions, a process that tends to be ordered. This process will form a strong random field to break the long-range ordered antiferroelectric state and form a relaxed ferroelectric state, thereby reducing the residual polarization (P_r), enhancing the breakdown field strength (E_b), and improving the energy storage performance of the material. As illustrated in Figure 4c, the maximum η of 81.9% and W_re of 0.77 J/cm³ are observed in Pb_0.98La_0.02(Hf_0.45Sn_0.55)_0.995O₃.

Based on previous reports, it is almost impossible to obtain both a large W_re and high η in AFE ceramics. For example, Wang et al. [35] obtained the highest restorable storing energy intensity of 16.4 J/cm³ in (Pb_0.99Nb_0.02) (Zr, Sn, Ti)_0.98O₃ (PNZST) antiferroelectric thin films, but the efficiency was only 57%. The efficiency of other studies was mostly between 60 and 75% and rarely more than 80% [38,39,40,41,42]. This study also revealed a remarkably high energy storage efficiency (η) of 81.9% and recoverable energy density (W_re) of 0.77 J/cm³, making it generally better than alternative Pb-based AFE ceramics with respect to energy storage characteristics. Ultra-high efficiencies were obtained at moderate energy storage densities. High energy storage efficiency means limited internal dissipated energy, thus avoiding the influence of internal heat accumulation on material stability and enhancing the service life of ceramic capacitors. Figure 5 shows the recently reported performance parameters of antiferroelectric ceramics.

In practical applications, performance stabilization is also a key criterion for evaluating the usage of energy storage materials. Therefore, at an electrical amount of 150 kV/cm, the energy storage characteristic of the x = 2 mol% element varies with frequency and temperature, as presented in Figure 6 and Figure 7. In Figure 6a, the ceramics show a consistent narrow hysteresis loop at different frequencies, and the residual polarization is generally small. As shown in Figure 6b, P_max and P_r decrease slowly from 9.74 μC/cm² and 1.26 μC/cm² to 9.37 μC/cm² and 0.52 μC/cm² in the range of 0.5–10 Hz, respectively, and then stabilize in the range of 10–100 Hz. Meanwhile, W_st decreases in the range of 0.5–10 Hz and then stabilizes at about 0.71 J/cm³ in the range of 10–100 Hz. W_re increases from 0.56 J/cm³ to 0.62 J/cm³ in the range of 0.5–10 Hz, so η also increases from 71.3% to 84.5% and then stabilizes to about 88.1% in the range of 10–100 Hz. At the same time, it can be observed from Figure 7 that in the temperature scale of 10–110 °C, the shape of the hysteresis loop almost does not change, with P_max and P_r increasing from 8.91 μC/cm² and 0.56 μC/cm² to 9.42 μC/cm² and 0.95 μC/cm², respectively, so P_max-P_r tends to be stable throughout the temperature range. Therefore, in the 10–100 Hz range and the 10–110 °C temperature interval, the ceramics have good frequency and temperature stability and have broad application prospects. The above findings show that when x = 2 mol%, the ratio of Pb and La at point A contributes to PLHS obtaining suitable tolerance in terms of the t factor according to Formula (1), which improves the ceramic’s AFE phase stability.

4. Conclusions

In the present study, Pb_1−xLa_x(Hf_0.45Sn_0.55)_0.995O₃ ceramics with different La contents were successfully gained through a conventional solid-phase sintering procedure, and the relationships among microstructures, crystal structures, and energy storage performance of ceramics and the La amount were investigated systematically. With a higher La concentration, the antiferroelectric state gradually transforms into the relaxor ferroelectric state with narrower hysteresis loops, which leads to an increase in E_A and E_b and a decrease in ΔE. The efficiency (η) increased from 71.4% at x = 0 to 81.9% at x = 2 mol% with a W_re of 0.77 J/cm³. However, with an increase in the La content to x = 3 mol%, pores began to appear in the ceramics; thus, the efficiency (η) decreased to 75.6%. The energy storage characteristics exceed most of antiferroelectric ceramics. When x = 2 mol%, Pb_1−xLa_x(Hf_0.45Sn_0.55)_0.995O₃ ceramic exhibits excellent temperature and frequency stability at 10–100 Hz and 10–110 °C and has broad application prospects. The Pb_0.98La_0.02(Hf_0.45Sn_0.55)_0.995O₃ ceramic is a prospective material for use in improved pulsed power capacitors, according to all of these data.