A-Site Doping Effect on PLZT Relaxor Ferroelectric Glass-Free Medium-Temperature Sintering Ceramics

Abstract

The high-temperature sintering characteristics of PLZT not only lead to lead volatilization and component mismatch but also limit its compatibility with low-cost electrode materials (such as Cu), making it a key bottleneck in its industrialization. In this study, PLZT dielectric ceramics were prepared using a glass-free densification process. Additionally, rare earth element Nd³⁺ was used for A-site doping to regulate the phase composition and domain structure of the material, and the relaxation characteristics and energy storage performance of PLZT were investigated. The results show that Nd³⁺ doping shifts the Raman 144 cm⁻¹ peak redward by 2.7 cm⁻¹. The P-E loop exhibits a narrow double-loop characteristic, with residual polarization reduced to 0.7 μC/cm² and maximum polarization reaching 17.7 μC/cm². When x = 0.07, a high energy storage density (W_rec = 3.98 J/cm³ and efficiency (η = 85%, x = 0.05) were achieved at 500 kV/cm. Through charge–discharge testing, the power density was determined to be 172.23 MW/cm³, with a discharge time τ_0.9 = 9.17 ns. This work could facilitate its application in multilayer ceramic capacitors (MLCCs) and embedded energy storage devices.

1. Introduction

Electrostatic capacitors, as core energy storage components in pulsed power systems, exhibit performance that critically depends on the energy density (W_rec) and charge/discharge efficiency (η) of their dielectric materials [1,2]. Antiferroelectric lead lanthanum zirconate titanate (PLZT) ceramics demonstrate significant advantages in high-power energy storage due to their unique double hysteresis loop and near-zero residual polarization characteristics. However, conventional PLZT ceramics require sintering at temperatures exceeding 1250 °C to achieve densification [3]. This not only leads to lead volatilization and composition mismatch but also limits compatibility with low-cost electrode materials (e.g., Cu), forming a critical bottleneck hindering industrialization.

During the process of reducing sintering temperatures, conventional methods typically rely on glass phase formers (such as Bi₂O₃, B₂O₃, etc.) to promote sintering. However, these amorphous phases often remain at grain boundaries, degrading the material’s dielectric properties and breakdown strength. For instance, the glass-modified PLZST system reported by Chen et al. reduced the sintering temperature to 1130 °C, yet suffered over 30% E_B loss due to grain boundary heterogeneities [4]. Xie et al. prepared a novel Pb_0.91La_0.06(Zr_0.552Sn_0.368Ti_0.08)O₃ @PbO-B₂O₃-SiO₂-Al₂O₃-ZnOMnO₂ antiferromagnetic particle core–shell structure via sol–gel synthesis [5]. The sintering temperature of PLZST@PBSAZM AFE ceramics was reduced from 1250 °C to 1100 °C, achieving an energy storage density of 7.4 J/cm³. Huang et al. incorporated 0.4 wt% BaO-B₂O₃-Al₂O₃-SiO₂ into (Pb_0.91Ba_0.015La_0.05) (Zr_0.6Sn_0.4)O₃ AFE ceramics from 1300 °C to 1100 °C by incorporating 0.4 wt% BaO-B₂O₃-Al₂O₃-SiO₂ (BBAS) glass, achieving a high energy storage density of 6.3 J/cm³ [6]. Sun et al. significantly reduced the sintering temperature of Pb_0.95La_0.02Sr_0.02(Zr_0.50Sn_0.40Ti_0.10)O₃ AFE ceramics to 1040 °C by adding Al₂O₃ [7], achieving an effective energy storage density of 3.23 J/cm³. Most reported strategies for lowering sintering temperatures typically involve adding glass-phase sintering aids. We are committed to exploring glass-free cooling strategies, such as nanopowder activation, liquid-phase-assisted sintering, or defect engineering regulation, which can achieve densification without introducing harmful second phases [8]. Additionally, A-site doping (e.g., Sr²⁺, Ba²⁺, or rare-earth ions) effectively modulates the lattice distortion and domain structure of PLZT, thereby optimizing its relaxation properties and antiferromagnetic-ferromagnetic phase transition behavior [9].

This study focuses on medium-temperature sintered PLZT relaxor ferroelectric ceramics, investigating the effects of a glass-free densification process on their phase structure, microstructure, and dielectric properties. Additionally, rare earth element Nd³⁺ is used for A-site doping to regulate the material’s phase composition and domain structure, elucidating its mechanism of action on relaxor properties and energy storage performance. This study aims to provide new control strategies for the preparation of medium-temperature sintered PLZT relaxor ferroelectric ceramics, promoting their application in multilayer ceramic capacitors (MLCCs) and embedded energy storage devices [10,11].

2. Experimental Procedures

Traditional solid-state methods were used to prepare Pb_0.97-xLa_0.03Nd_x(Mg_0.8Mo_0.2Zr_0.95Ti_0.05O₃) relaxor antiferromagnetic ceramics with x = 0.01, 0.03, 0.05, and 0.07. The following materials were weighed according to their nominal stoichiometric compositions: Pb₃O₄ (99%), La₂O₃ (99.9%), ZrO₂ (99%), TiO₂ (99.8%), Nd₂O₃ (99.9%), MoO₃ (99.9%), MgO (98%). Additionally, 2 mol% Pb was added in excess to compensate for Pb volatilization during high-temperature sintering. The mixture was ground in a planetary ball mill using anhydrous ethanol and zirconia balls as the medium for 12 h. The resulting slurry was dried at 80 °C for 2 h, then sieved using an 80-mesh screen. The dried powder was calcined at 700 °C for 3 h. To ensure uniformity of the powder, the calcined powder was ground again in a ball mill for 12 h. Subsequently, the ceramic powder was granulated with a binder (10% PVA), sieved, and then pressed to obtain ceramic green bodies. Finally, the pressed ceramic green bodies were placed in a sealed crucible filled with pre-sintered powder and sintered at 1030 °C for 2 h. After sintering, the samples were polished to 100 µm and then sputtered with a silver electrode.

The crystal structure of the sintered ceramics was determined using an X-ray diffractometer (XRD; Bruker D8, Bruker, Berlin, Germany). The feasibility of low-temperature sintering was demonstrated by studying the ceramic powders using a thermogravimetric analyzer (DSC; STA449 F5, NETZSCH, Zerbst, Germany). The microstructure of the ceramics was analyzed using a scanning electron microscope (SEM; SU5000, HITACHI, Tokyo, Japan). Raman spectra of finely ground powders were acquired using an in situ confocal Raman spectrometer (Raman; LabRAM HR Evolution, Horiba Scientific, Kyoto, Japan). Oxygen vacancy information was obtained via X-ray photoelectron spectroscopy (XPS; Thermo Escalab 250 Xi, Thermo Fisher, Waltham, MA, USA). Polarization-electric field loops (P-E loops) were measured at 10 Hz using a ferroelectric analyzer (TF Ferroanalyzer; TF2000, aixACCT, Aachen, Germany). Dielectric spectra of the samples were analyzed over the frequency range of 100 Hz to 1 MHz using a precision impedance analyzer (4294A, Agilent, Santa Clara, CA, USA). The energy release characteristics of ceramic disk capacitors were investigated using a commercial charge–discharge platform (CFD-001, Gogo Instruments Technology, Shanghai, China).

3. Results and Discussion

Figure 1a–d displays the cross-sectional scanning electron microscopy (SEM) morphology of PLZTN ceramics. A dense microstructure with low porosity and well-defined grain boundaries is observed in all samples. Furthermore, all specimens exhibit intergranular fracture, indicating high grain-forming strength [12,13]. The inset displays the mean grain size normal distribution for PLZTN1, PLZTN2, PLZTN3, and PLZTN4 samples. The mean grain size decreases with an increase in Mo_0.8Mg_0.2O_1.4 additive content, indicating synergistic realization of effective low-temperature densification. Figure 1e presents the TG-DSC analysis of PLZTN ceramics. The curve exhibits distinct endothermic and exothermic phenomena. Between room temperature and 100 °C, a pronounced endothermic response occurs, attributed to the physical adsorption of water and the volatilization of residual anhydrous ethanol solvent [14]. A pronounced endothermic reaction occurs between 260 °C and 586 °C, attributed to the oxidative decomposition of the organic binder (PVA) backbone and the initiation of minor Pb volatilization. From 586 °C to 742 °C, Pb volatilization dominates, potentially accompanied by minor oxygen loss due to oxygen vacancy formation [15]. Between 742 °C and 840 °C, Pb volatilization remains predominant but at a slower rate [16]. This indicates Pb volatilization ceases as the primary crystalline phase forms. As temperature continues to rise, significant weight loss reappears near 1030 °C. Further temperature increases may cause additional Pb volatilization, potentially degrading ceramic sample performance. This also demonstrates the reliability of PLZTN ceramics sintered at 1030 °C. Figure 1f shows that the breakdown voltage (E_B) progressively increases with rising Nd³⁺ content, with values of 445, 457, 469, and 491 kV/cm for 100 μm thick ceramics. These results demonstrate that a dense microstructure and fine grain size effectively enhance the E_B of PLZTN ceramics [17,18,19].

Figure 2a–d shows the phase composition and crystal structure of PLZTN ceramics with different compositions analyzed by powder X-ray diffraction (XRD). All polymers consist of a single perovskite phase (PDF#87-0569), with no secondary phase formed in the main perovskite phase, demonstrating that Mo/Mg and Nd³⁺ have been completely solid-solved into the main crystal phase. The XRD pattern (b) exhibits a double peak at approximately 2θ = 30.5°, corresponding to the (110) and (101) planes, with the (110) peak intensity being about half that of the (101) peak. Figure 2c shows an unsplit and nearly perfectly symmetrical (111) peak near 2θ = 38°, while Figure 2d displays (200) and (002) double diffraction peaks near 44° with a peak intensity ratio of 2:1, exhibiting typical tetragonal phase structural characteristics [20,21]. The angles of these main peaks exhibit non-monotonic changes with increasing Nd³⁺ doping concentration: a shift toward lower angles corresponds to lattice expansion, while a shift toward higher angles indicates lattice contraction [22,23,24]. When Nd replaces Pb at the A site, Pb vacancies form to maintain charge balance. These vacancies repel surrounding cations, causing distortion and expansion of the oxygen octahedra. Additionally, a single vacancy induces relaxation of neighboring atoms toward the vacancy, locally increasing the lattice constant and resulting in lattice expansion [25]. Lattice contraction, however, is primarily driven by changes in the valence state of B-site ions and the “average ionic radius” effect [26]. When Nd ions are added beyond a critical threshold, the contraction effect caused by ionic size differences becomes dominant, ultimately overpowering the expansion effect induced by lead vacancies. High concentrations of point defects also induce substantial internal stresses. To relax these stresses, the lattice may reduce its energy through slight overall contraction [27]. This is consistent with the previously studied PLZT phase structure, indicating that Mo/Mg and Nd³⁺ doping did not alter the crystal structure.

Figure 3a shows the Raman scattering spectrum test results for PLZT ceramics. As shown in Figure 3a, the addition of Nd³⁺ causes disorder in the local composition of the ceramics, and all vibration modes exhibit diffuse reflection peaks [28,29]. For perovskite structures, Raman spectroscopy can be divided into three vibrational regions: the peak intensity in the region where the wavenumber is less than 150 cm⁻¹ represents the vibration of the a-site ions; the peak intensity in the region where the wavenumber ranges from 150 cm⁻¹ to 450 cm⁻¹ indicates the vibration of the B-O bond; When the wavenumber is between 450 cm⁻¹ and 800 cm⁻¹, the peak intensity is related to the stretching of the oxygen octahedron [30]. As shown in Figure 3b, with the addition of Nd³⁺, a strong vibrational peak (marked with a plum blossom symbol) appears near 144 cm⁻¹ and gradually broadens. The peak shifts from 145.2 cm⁻¹ (x = 0.01) to 142.5 cm⁻¹ (x = 0.05), with a shift of 2.7 cm⁻¹, indicating that the introduction of Nd³⁺ affects the A site. The lattice expansion weakens the vibrational recoil constant of the A site ions, and the cation disorder increases [31,32].

The P-E hysteresis loops and I-E curves (10 Hz) of PLZTN ceramics at room temperature (x = 0.01, 0.03, 0.05, and 0.07) are shown in Figure 4a–d. As the Nd³⁺ doping concentration increases, the P-E hysteresis loops gradually exhibit linear polarization response, with the residual polarization intensity Pr decreasing from 1.4 to 0.7 µC/cm², and the maximum polarization intensity Pm increasing from 10.5 to 17.7 µC/cm². This is attributed to the increased Nd³⁺ doping causing the destruction of the antiferromagnetic order, with macroscopic ferroelectric domains being refined into polar nanoregions (PNRs), resulting in elongated loops [33,34]. Additionally, the electric field-induced EAFE-FE phase transition can also be observed in PLZTN ceramics [35]. At room temperature, the AFE phase belongs to the Pbam space group, while the electric field-induced FE(I) phase belongs to the C2mm space group. As the electric field further increases, the dipoles align along the [111] direction, and the polarization increases rapidly [36]. For PLZTN (x = 0.07) ceramics, in the first stage, when the applied electric field is less than 280 kV/cm, the polarization increases linearly and slowly, with a polarization intensity of only 7.4 µc/cm². AFE matrix contains randomly distributed polar nanoregions (PNRs). When the applied electric field increases to 350 kV/cm, due to the AFE-FE phase transition, the PNRs gradually fuse into FE long programs, and the polarization enters the next stage, with the polarization intensity rapidly increasing to 11 µc/cm², approaching 1.5 times that of the first stage; In the final stage, when the applied electric field exceeds 350 kV/cm, the FE long domain dissociates into PNRs, ultimately reconstructing the AFE order. Polarization increases sharply, reaching a maximum of 17.7 µc/cm² at 500 kV/cm. Figure 4e,f shows the frequency dependence of the dielectric constant and dielectric loss of PLZTN ceramics, demonstrating excellent frequency stability across the broad frequency range of 100–1 MHz. With increasing Nd³⁺ doping concentration, the dielectric constant exhibits an overall decreasing trend throughout the entire frequency range [37]. Concurrently, dielectric loss significantly increases in the low-frequency region (<10⁴ Hz). This decrease in dielectric constant coupled with the rise in dielectric loss precisely correlates with the progressively increasing breakdown electric field [38]. This correlation indicates that microstructural evolution systematically influences the material’s dielectric properties. As doping content increases, the average grain size decreases. A finer microstructure may lead to the formation of high grain boundary density [39], thereby reducing the overall dielectric constant. Grain refinement also implies a sharp increase in the number of grain boundaries. These grain boundaries act as electromigration barriers, accumulating substantial space charges under alternating electric fields. This leads to significant interfacial polarization relaxation, manifesting as heightened low-frequency losses [40]. The dense grains and numerous grain boundaries effectively impede the propagation of breakdown trees. More importantly, these grain boundaries efficiently trap charge carriers, preventing their acceleration and collision ionization, thereby substantially enhancing the intrinsic breakdown field strength [41].

Generally speaking, the polarization-electric field (P-E) hysteresis loop can reflect the energy storage performance of antiferromagnetic materials. The evaluation includes total energy storage density (W_tot), recoverable energy storage density (W_rec), loss energy storage density (W_loss), and energy storage efficiency (η), which can be calculated using the following formulas [42,43]:

$$W_{tot}={{\displaystyle \int }}_0^{P_m}EdP$$

(1)

$$W_{rec}={{\displaystyle \int }}_{P_r}^{P_m}EdP$$

(2)

$$W_{loss}=W_{tot}-W_{rec}$$

(3)

$$\eta ={\displaystyle \frac{W_{rec}}{W_{tot}}}\times 100\%$$

(4)

P_m is the maximum polarization intensity, P_r is the residual polarization intensity, E is the applied electric field, W_rec is the recoverable energy storage density, W_loss is the energy loss, W_tot is the total energy storage density, and η is the energy storage efficiency [44,45].

As shown in Figure 5a–d, the total energy storage density (W_tot), recoverable energy storage density (W_rec), and efficiency (η) of PLZTN ceramics under different electric fields are demonstrated. When the E_AFE-FE phase transition occurs, W_tot and W_rec increase rapidly [46], while η decreases slowly with increasing electric field. As the Nd³⁺ content increases, η reaches its maximum (85%) in PLZTN-3, demonstrating that Nd³⁺ can effectively enhance antiferroelectricity, increase the breakdown field strength, and thereby improve the effective energy storage density.

Figure 5e shows the W_rec and η of PLZTN ceramics with different Nd³⁺ contents, with the recoverable energy storage density of PLZTN ceramics increasing from 1.95 J/cm³ to 3.98 J/cm³. The energy storage efficiency improved from 69% to 83%, indicating that Nd³⁺ doping disrupts the long-range ordered structure of FE domains and improves the P-E hysteresis loop. It also shows that the addition of the Mo_0.8Mg_0.2O_1.4 additive does not affect the energy storage density and efficiency of PLZTN ceramics [53]. Although Nd³⁺ doping stabilizes the AFE phase, excessive Nd³⁺ ions entering the crystal structure increase the loss energy storage density (W_loss), thereby reducing energy storage efficiency and affecting energy storage performance. Therefore, at x = 0.05, the PLZTN ceramic achieves a maximum η of 85%, while at x = 0.07, the PLZTN ceramic achieves a maximum W_rec of 3.98 J/cm³, but η drops to 83%. Additionally, the W_rec and η values of various types of ceramics reported in recent years are shown in Figure 5f. These ceramics exhibit different polarization characteristics, and due to the electric field-induced phase transition, AFE typically demonstrates higher energy storage density. In this work, the PLZTN (x = 0.07) ceramic achieves a maximum W_rec of 3.98 J/cm³ at 500 kV/cm. This indicates that PLZTN ceramics hold promising potential for applications in the field of energy storage.

The energy storage characteristics derived from the hysteresis loop reflect the performance under quasi-static conditions and are insufficient to explain the actual behavior in practical pulse power applications. Therefore, the discharge current curves under underdamped and overdamped conditions were measured, and the current density (C_D), power density (P_D), and discharge energy density (W_dis) were calculated using the following formulas:

$$C_D={\displaystyle \frac{I_{max}}{S}}$$

(5)

$$P_D={\displaystyle \frac{E{I}_{max}}{2S}}$$

(6)

$$W_{dis}={\displaystyle \frac{R\int i{\left(t\right)}^{2}dt}{V}}$$

(7)

In the equation, I_max is the peak current, S is the electrode area, E is the electric field, R is the load resistance (205 Ω), and V is the sample volume.

The underdamped and overdamped discharge current curves for PLZTN-3 and PLZTN-4 samples are shown in Figure 6a,b,d,e. As the electric field increases, C_D and P_D exhibit a consistent upward trend. Under an electric field of 380 kV/cm, PLZTN-3 achieved CD and PD values of 844.24 A/cm² and 134.85 MW/cm³, respectively (Figure 6g). Under an electric field of 420 kV/cm, PLZTN-4 achieved C_D and P_D values of 958.514 A/cm² and 172.23 MW/cm³, respectively (Figure 5f). The maximum W_dis values for PLZTN-3 and PLZTN-4 samples were 2.25 J/cm³ and 2.95 J/cm³, respectively, with ultra-fast discharge rates of τ_0.9= 11.38 ns and 9.17 ns. The excellent discharge performance of PLZTN relaxor antiferromagnetic ceramics doped with Nd³⁺ ions indicates that this ceramic has great application potential in pulse power capacitors [54].

The doping of rare earth elements into PLZT may occur at the A site or the B site. When doping occurs at the A site, it plays a significant role in enhancing the ceramic’s oxidation resistance. Figure 7a–d show the valence states and oxidation states of the main elements in ceramics with different Nd³⁺ doping ratios, as analyzed using XPS. These spectra were fitted using Gaussian-Lorentzian functions. As shown in Figure 7a, the high-resolution XPS spectra of Pb 4f exhibit highly symmetric and consistent binding energies, confirming that an increase in Nd³⁺ doping does not lead to a significant shift of Pb elements to lower oxidation states. The O 1s spectra described in Figure 7b typically decompose into three peaks with low, medium, and high binding energies, corresponding to lattice oxygen, adsorbed oxygen, and oxygen vacancies. This indicates that increased Nd³⁺ doping reduces oxygen vacancies and increases lattice-bound oxygen. This phenomenon can be explained by the donor or acceptor effect of the dopant. When Nd³⁺ is doped into the A site, replacing Pb²⁺ ions, the donor Nd³⁺ acts as a source of additional electrons, thereby reducing the number of oxygen vacancies while maintaining electrical neutrality [55]. Figure 7c,d shows the Zr 3d and Nd 3d spectra, with all Zr 3d peaks exhibiting high symmetry. The binding energies of Nd 3d5/2 and Nd 3d3/2 increase with increasing Nd³⁺ concentration. This phenomenon is attributed to lattice contraction caused by high Nd doping concentrations, Simultaneously, Nd³⁺ replaces Pb²⁺ at site A, where a significant difference in electronegativity exists between them. Following substitution, the bonding characteristics between Nd and O undergo alteration. The more electronegative O atom exerts a stronger attraction on the bonding electrons, resulting in reduced electron density around Nd. This collectively leads to a weakened screening effect, requiring higher energy to excite Nd 3d electrons and thereby increasing the binding energy [56].

4. Conclusions

This work employs a glass-free densification process to prepare PLZT at reduced temperatures, achieving glass-free densification at 1030 °C (grain size ~1.2 μm). Concurrently, A-site doping with the rare earth element Nd³⁺ is utilized to regulate the phase composition and domain structure of the material, with the aim of investigating its mechanism of action on relaxation properties and energy storage performance. Nd³⁺ doping induces cation disorder at the A site (red shift of the Raman 144 cm⁻¹ peak by 2.7 cm⁻¹), promoting the stabilization of the antiferromagnetic phase and the formation of polar nanodomains, significantly optimizing the polarization characteristics (P_r ≈ 0.7 μC/cm², P_m increased to 17.7 μC/cm²). Under the synergistic effect, the ceramic achieves a releasable energy storage density of 3.98 J/cm³ and an energy efficiency of 85% under a field strength of 500 kV/cm, while demonstrating a high power density (172.23 MW/cm³) and nanosecond-level discharge response (τ_0.9= 9.17 ns). XPS analysis confirmed that the proportion of oxygen vacancies decreases with increasing Nd³⁺ doping, indicating that the acceptor element successfully reduces oxygen defects. These results could facilitate the application of PLZT in multilayer ceramic capacitors (MLCCs) and embedded energy storage devices.