Investigation of the E ﬀ ects of Reduced Sintering Temperature on Dielectric, Ferroelectric and Energy Storage Properties of Microwave-Sintered PLZT 8 / 60 / 40 Ceramics

: In this study, (Pb 0.92 La 0.08 ) (Zr 0.60 Ti 0.40 ) O 3 (PLZT 8 / 60 / 40) ceramics were synthesized using a high-energy ball-milling technique followed by microwave sintering at di ﬀ erent temperatures from 900 ◦ C to 1200 ◦ C. The optimal microwave sintering temperature for the PLZT 8 / 60 / 40 ceramics was found to be 1150 ◦ C, which is relatively low compared with conventional sintering temperature. The sintered ceramics show the pure perovskite phase, uniform grain microstructure (1.2 µ m) and high density (~99.5%). The polarization vs. electric ﬁeld ( P-E ) hysteresis curves were used to investigate the ferroelectric and energy storage properties. The switching characteristic in P-E loops and occurrence of domain switching current in current vs. electric ﬁeld ( I-E ) loops conﬁrms their ferroelectric nature. The PLZT ceramics, which were sintered at 1150 ◦ C, show the highest remnant polarization ( P r ) of ~32.18 µ C / cm 2 and domain switching current ( I max ) of ~0.91 mA with a low coercive ﬁeld ( E c ) of ~10.17 kV / cm. The bipolar and unipolar strain vs. electric ﬁeld ( S-E ) hysteresis loops were also measured and the highest unipolar strain was found to be ~0.26% for the PLZT ceramics sintered at 1150 ◦ C. The unipolar S-E curves were used to derive the piezoelectric coe ﬃ cient ( d 33 ~495 pm / V) and a strain hysteresis loss (~5.8%).


Introduction
Lead lanthanum zirconium titanate (PLZT) is one of the most studied electroceramics due to its technological importance in various applications [1][2][3]. In PLZT ceramics, the ratio of La/Zr/Ti not only decides the morphotropic phase boundary (MPB) but also influence the microstructure and electrical characteristics. Moreover, the nature of the material such as ferroelectric (FE), paraelectric (PE), anti-ferroelectric (AFE) and relaxors ferroelectric (RFE) is also decided by the La/Zr/Ti ratio [4][5][6]. PLZT ceramics of different natures (FE, PE, AFE and RFE) or different compositions (ratio of La/Zr/Ti) possess a higher piezoelectric coefficient, electromechanical coupling coefficients with a wide range of dielectric constants. These properties are sufficiently large to exploit in different types of piezoelectric device such as sensors, actuators and energy harvesting devices. PZT/PLZT materials are used and the dielectric, ferroelectric, energy storage and piezoelectric properties were investigated. The PLZT ceramics sintered at 1150 • C for 20 min possess optimal electrical properties.

Results and Discussion
The detailed phase (X-ray diffraction patterns) and microstructural analysis (scanning electron microscopy images) of the microwave-sintered PLZT 8/60/40 ceramics are already published [35] and a short summary is given here for the completeness of the manuscript. The PLZT ceramics, microwave sintered at 900 • C and 1000 • C show the grain size in the submicron range. When the ceramic microwave sintering temperature was increased from 1050 • C to 1200 • C, the grain size of the ceramics was also increased from the submicron to the micron region. The PLZT ceramic sintered at 1150 • C shows the dense microstructure, uniform, clear and visible grain sizes~1.2 µm and highest density, which affects the electrical properties. The density of the PLZT 8/60/40 ceramics was calculated by the Archimedes method and listed in Table 1 with other microstructural parameters. The effect of frequency on the dielectric properties of PLZT 8/60/40 ceramics microwave sintered at different temperatures was discussed previously [35]. In this study, the effect of temperature on the dielectric properties is discussed in detail. As we know, the dielectric properties of the PLZT materials are greatly affected by the density, grain size, pores, etc. The porosity-corrected dielectric constant is given by the following equation [38][39][40] where, ε is the measured dielectric constant and ε m is the porosity-corrected dielectric constant. The fraction of porosity is given by p = 1 − ρ, where ρ is the experimentally measured density of ceramics. The measured dielectric constant and the porosity corrected dielectric constants for microwave sintered PLZT 8/60/40 ceramics is given in Table 1.
Temperature-dependent dielectric studies were also performed to learn more about FE phase transition. Figure 2a shows the change in dielectric constant (ε r ) and loss (tanδ) with temperature for microwave sintered (900 • C to 1200 • C) PLZT 8/60/40 ceramics measured at 1 kHz. In Figure 2a, the ε r vs. T curves (1 kHz) are broadened curve rather than a sharp peak at a transition temperature (T c ) but in this study considered as dielectric maxima temperature (T m ). This shows the disordered perovskite structure with a diffuse-type phase transition. The ε r vs. T curves show the change in T m and increase in peak dielectric maxima value as a function of microwave sintering temperature except for the PLZT ceramics sintered at 1200 • C. The shift or decrease in the dielectric maxima temperature is due to the change in the grain sizes of the microwave-sintered PLZT ceramics. Not only are the density and the microstructure but also the tetragonality, polarization, dielectric constant, T c and piezoelectric coefficients of the PLZT ceramics are affected by the variation in grain sizes [35]. The value of T m decreases as a function of sintering temperature due to the increase in grain sizes. The dielectric loss for the PLZT ceramics was also measured as a function of temperature and found to be less (Figure 2a). Figure 2b shows the change in T m and dielectric maxima measured at T m as a function of microwave sintering temperature for PLZT ceramics.
The normal type phase transition is a sharp phase transition at which the dielectric constant of the electroceramics show an anomaly (sharp peak in ε r vs. T curves) at T c and the material undergoes FE to PE phase transition. The broadness in the dielectric peak indicates that the phase transition behaviour of the material is shifting from normal type (sharp dielectric peak at T c ) to diffuse or relaxor type (broad dielectric peak at T m ). From Figure 2a it is evident that the dielectric maxima peak is broadened and the phase transition has deviated from the normal type of phase transition. The degree of deviation from the Curie-Weiss law (∆T m ) and diffuseness of the phase transition (∆T diff ) can be evaluated from the following Equations [5,41,42] ∆T m = T cw − T m (2) where T m is the dielectric maxima temperature and the temperature from which permittivity starts to deviate from the Curie-Weiss law is denoted by T cw , which can be from the reciprocal of dielectric constant vs. temperature curves. ε max = maximum value of ε at T m . T 0.9εmax denotes the temperature at high-temperature side of 90% of the ε max . The calculated parameters of PLZT 8/60/40 ceramics are listed in Table 2.

Polarization Versus Electric Field Ferroelectric Hysteresis Loops
The P-E hysteresis loops of a ferroelectric material are unique and used for the confirmation of the ferroelectric nature of the electroceramics. Figure 3a-d show the P-E loops for the PLZT 8/60/40 ceramics, microwave sintered at various temperatures and measured at an electric field of 10, 20, 30 and 40 kV/cm, respectively. The P-E loops for the PLZT ceramics sintered at 1150 • C show a square loop with sharp edges which indicates the homogeneously distributed uniform grain sizes [43,44]. Other P-E loops for the 900-1100 • C sintered ceramics are not saturated due to the low density of the ceramics. Ferroelectric hysteresis loops (P-E and I-E) are used to calculate the values of P r and E c . The actual polarization in the ferroelectric material is always lower than expected values because due to the internal stresses and internal bias electric fields, which affect the domains reorientation and domains back switching. Figure 3e shows the change in P r (increases) and E c (decreases) for PLZT 8/60/40 ceramics as a function of microwave sintering temperature. The PLZT ceramic sintered at 1150 • C shows the highest P r of 32.18 µC/cm 2 and lowest E c of 10.17 kV/cm. Apart from P r and E c , the shape of P-E hysteresis loops is also considered as an important parameter to explore the material nature. The sharp edges of P-E loops show that the material is highly resistive. An ideal ferroelectric material shows the square shape ferroelectric P-E hysteresis loop. The positive and negative value of the coercive field and remnant polarization (E c = −E c and P r = −P r ) should be equal. The difference between E c and −E c as well as P r and −P r values shows that shape of the P-E hysteresis loop is not a square (symmetrical). A high degree of loop squareness is also related to the homogeneous and uniform grains of the ceramics. The squareness (R sq ) of the P-E hysteresis loop can be calculated from Equation (4) [45] where P r = remnant polarization, P sat = saturation or maximum polarization, P 1.1Ec is the polarization, which was measured at 1.1 E c . For an ideal P-E hysteresis loop, R sq is equal to 2.0. The value of R sq is calculated for the microwave sintered PLZT 8/60/40 ceramics and the highest value of~1.06 was found for the ceramics sintered at 1150 • C. The normalized value of volume fraction of back switched domains (V back~Psat − P r ), or considered as "switchable domains" (back switched when E = 0) is also calculated and found to be less for the 1150 • C sintered PLZT 8/60/40 ceramics.

Domain Switching in Current versus Electric Field Loops
Figure 4a-d show the I-E loops of microwave sintered PLZT 8/60/40 ceramics at 10 kV/cm to 40 kV/cm, respectively. When an electric field is applied to FE ceramics, typically the current response came from the linear dielectric response of the ceramics and due to the switching of the domains. At first, when the applied electric field is low, most of the current contribution comes from the dielectric response of the ceramics (Figure 4a). The appearance of a peak signal in the I-E curves, when the applied electric field is~20 kV/cm, shows that the domain switching is happening inside the materials and also confirms the FE nature of PLZT ceramics (Figure 4b). Later, when the applied electric field is sufficiently high to switch the domains, the current response due to domain switching starts dominating the total current of the ceramics (Figure 4c). The magnitude of the current peak increases with the applied electric field and finally attains saturation. At last, at E max , then most of the current response is due to switching of the domains with an almost negligible contribution from the dielectric response ( Figure 4d). A peak in I-E curves before E max indicates the switching of domains and the corresponding electric field is considered as E c . The switching in ferroelectric material takes place by domain nucleation and domain wall movement. The maximum switching current can be described by the following Equation (5) [46][47][48][49][50][51][52] i where α is a constant called as the activation field which is considered as threshold field to initiate nucleation, i ∞ = (i ∞ ) E = ∞ and E is an applied electric field that decides the domain switching current.
With the application of a low electric field, the main contribution in the switching current comes from the nucleation of the domains due to the low domain nucleation rate. On the other hand, the velocity of the domain walls determined the switching of domains at the high applied electric field due to the large rate of nucleation. Both α and i ∞ parameters depend upon the temperature and reduce both switching current and time if the material temperature approaches the Curie temperature. Figures 3d and 4d show the saturated P-E and I-E loops due to the alignment of ferroelectric domains in the direction of the applied electric field. Once the domains start to switch (Figure 4b), the corresponding P-E curve (Figure 3b) shows an improvement in squareness and symmetry in the shape of p-E curves compared to the previous Figure 3a, which is further improved by the application of a higher electric field. Figure 4e shows the switching current (current value at peaks) and leakage current (current value at the maximum electric field) as a function of microwave sintering temperature for PLZT ceramics. It is clear that 1150 • C sintered ceramics show the highest value of domain switching and less leakage current.

Energy Storage Properties of Microwave-Sintered PLZT 8/60/40 Ceramics
P-E hysteresis loops are used not only to determine the ferroelectric properties but also to calculate the energy storage properties of a ferroelectric capacitor. The shape of the P-E loops which depend upon the switching of domains defines the nature of these materials and decides the use of materials in energy storage applications. The recoverable energy storage density (W rec ) is represented by the area between the y-axis and P r to P max branch of the P-E curve. The energy-loss density (W loss ) of the material is represented by a closed area of the P-E loops and can be calculated by numerical integration. Another important parameter energy efficiency (η) of a FE capacitor is a ratio of W rec and W loss . The values of W rec , W loss and η of a FE capacitor are calculated from the Equations (6) and (7) [5,33] where P r = remnant polarization, P max = maximum polarization and E = applied electric field.
Energies 2020, 13, x FOR PEER REVIEW 8 of 18 Figures 3d and 4d show the saturated P-E and I-E loops due to the alignment of ferroelectric domains in the direction of the applied electric field. Once the domains start to switch (Figure 4b), the corresponding P-E curve (Figure 3b) shows an improvement in squareness and symmetry in the shape of p-E curves compared to the previous Figure 3a, which is further improved by the application of a higher electric field. Figure 4e shows the switching current (current value at peaks) and leakage current (current value at the maximum electric field) as a function of microwave sintering temperature for PLZT ceramics. It is clear that 1150 °C sintered ceramics show the highest value of domain switching and less leakage current.

Energy Storage Properties of Microwave-Sintered PLZT 8/60/40 Ceramics
P-E hysteresis loops are used not only to determine the ferroelectric properties but also to calculate the energy storage properties of a ferroelectric capacitor. The shape of the P-E loops which The energy storage density is highly influenced by the polarization (P r , P max ) and maximum applied electric field (E max ) or dielectric breakdown strength (DBS). The DBS of the ceramic capacitor depends upon the doping, sintering temperature, microstructure (density, porosity and grain sizes), conductivity and several extrinsic parameters (thickness and area). As per Equation (6), a ceramic capacitor with high DBS value shows high recoverable energy-storage density [53].
Due to the hysteresis losses in the ferroelectric ceramic capacitor, the microwave-sintered PLZT 8/60/40 ceramics show the moderate energy density values. Typical FE ceramics show the square P-E hysteresis loops with a large hysteresis due to the field-dependent reorientation of the ferroelectric domains, resulting in high energy loss and low recoverable energy efficiency. The energy efficiency of the microwave sintered PLZT ferroelectrics depends upon the W rec and W loss values. The high value of W rec and low W loss is required to achieve high energy-efficiency. Shapes of the P-E loops are seen changing with microwave sintering temperature and their squareness increases, which results in low W rec . Bulk FE ceramic capacitors always show a lower energy-storage density (but higher than other FE ceramics) due to the relatively low DBS and square P-E loop, however, the high dielectric constant of the FE ceramics is always desirable for the energy-storage applications. The very low energy density of~1.24 mJ/cm 3 with energy efficiency~45% is also reported for the lead-free BiFeO 3 based ceramics [54]. The parameters related to the FE and energy storage properties are listed in Table 3. The energy storage properties of the microwave sintered PLZT 8/60/40 ceramics is compared with other PLZT compositions and shown in Table 4.

Strain versus Electric Field (S-E) Hysteresis Loops
In the presence of the applied electric field, piezoelectric materials not only show the polarization and P-E hysteresis loops but also show the strain and S-E hysteresis loops. Generally, two types of S-E loops are reported for piezoelectric ceramics (1) Bipolar S-E hysteresis loops (shape similar as butterfly) used for the domain-switching confirmation (Figure 5a-d) and (2) Unipolar S-E hysteresis loops which are used to calculate piezoelectric charge coefficients (Figure 6a-e). Three types of effects contributes in the butterfly shape of the bipolar S-E loops, converse piezoelectric effect of the lattice, and domain wall switching and movements [46]. Figure 5a-d show the formation of butterfly S-E loops for microwave-sintered PLZT 8/60/40 ceramics measured at the different applied electric fields. At first, the polarization direction and the applied electric field are in the same direction so it shows the electric field-induced strain but when applied electric field direction is reversed then the material shows negative strain. However, a sufficiently large electric field can switch the polarization direction in the same direction which leads to a butterfly shape of the S-E hysteresis loops. Figure 6 shows the unipolar strain curve for microwave-sintered PLZT 8/60/40 ceramics measured at room temperature. From Figure 6a-e, it is observed that the value of strain increases with an applied electric field since an increasing number of domains align with the field. This alignment of domains saturates at high electric fields and, the strain hysteresis loss (area of the loop) decreases and becomes almost negligible. At first, when the electric field is not high enough to switch the domains, the developed strain is almost linear. Later, when the electric field increases, the domain contribution become more dominant and non-linear strain is also observed. The switching of non-180 • domains was present in the FE materials involve a large change in dimensions of the ceramics. In the present case, a maximum unipolar strain of~0.26% is achieved at a field 50 kV/cm for 1150 • C sintered PLZT 860/40 ceramics, which is higher than the other sintering temperatures which can be explained based on the grain sizes [59]. The grain sizes for the microwave-sintered PLZT ceramics was increased from a submicron to the micron range as the sintering temperature increases from 900 • C to 1200 • C. The effect of fine-grain sizes on the electrical properties is explained based on the internal stress of the electroceramics. The material with smaller grains has a greater number of grain boundaries, which results in the pinning of the domain walls and lower piezoelectric response. In the current work, the highest piezo-response was observed for the sample sintered at 1150 • C due to the uniform grain size and dense microstructure. On the other hand, the sample sintered at 1200 • C, in spite of having the biggest grain size, shows the lower electrical properties due to high lead loss at the higher temperatures. The lead loss results in the creation of oxygen vacancies and lower electrical properties. The width of the domains also depends on the grain size and it decreases with a decrease in the grain size. The relation between grain (g) and domain sizes (d) is given as d~g 1/2 [60,61]. Arlt et al. [62] observed that the BaTiO 3 ceramics with grain sizes below 0.7 µm, no longer exhibit 90 • domains. In a study by Tuttle et al. multiple 90 • domain walls were observed only for the grains larger than 1 µm for PZT material [63]. The electric field-induced strain in the ferroelectric material depends upon the 90 • domains. The low strain value for the PLZT ceramics, which were microwave-sintered at 900-1050 • C is mostly due to the switching of the fewer number of 90 • domains. As the sintering temperature increases the grain size also increases which results not only in an increase of domain size but also in the availability of a greater number of 90 • domains and finally in increased strain. The unipolar S-E curves as shown in Figure 6 were measured using a laser interferometry technique. The laser light was focused on the samples during the application of the applied electric field and the displacement in the sample thickness was measured. The strain of the piezoelectric material not only depends upon the 90 • domain wall movements but is also very sensitive to the applied electric field. The ceramics show little change in strain value when the electric field continuously increasing since 90 • domain behaviour is different at each point of the applied electric field. This will lead to wave-type patterns in the electric field induces strain curves. Figure 7a shows the change in the maximum strain of PLZT measured at a field of 50 kV/cm as a function of microwave-sintering temperature. At first, the electric field-induced strain is less (900-1050 • C) due to the low density and small grain size. The temperature more than 1100 • C strain is significantly high and the highest strain was found for the 1150 • C sintered PLZT ceramics. Figure 7b shows the strain hysteresis as a function of microwave sintering temperature at the different applied electric fields for microwave-sintered PLZT ceramics. The strain hysteresis (%) value was calculated from Figure 6 using the following equation: where x max is a maximum strain in the specimen subjected to the maximum applied field, ∆x is the difference between the strain values in both directions at 1 2 E max field. Both directions are related to (1) an increase in strain value with an applied electric field and then (2) after reaching the saturation point if the electric field is reduced, the strain will also reduce with a certain amount of hysteresis due to domain wall movement. The difference ∆x is calculated at 1 2 E max = 25 kV/cm. The strain hysteresis value for the PLZT ceramics (sintered at 1150 • C) was found to be~5% at a maximum applied field. The strain values of ceramics show a good DBS. The piezoelectric charge coefficient (d 33 ) for the poled PLZT ceramics, microwave sintered at 1150 • C, was measured using the d 33 meter (direct method) and found to be~575 pC/N. In a direct method, a variable force is applied to the material and the generated charge was measured. The unipolar S-E curves also can be used as an indirect method to calculate the low and high field piezoelectric charge coefficient denoted as d 33 and d * 33 (S max /E max ), respectively (Figure 7c). In unipolar S-E curves, the slope of the S-E hysteresis loop in a lower electric field region represents an average d 33 . The low and high field d 33 parameters which were calculated from the unipolar S-E loops has their unit in pm/V, since displacement (pm) is divided by the voltage (V). The parameters related with the S-E hysteresis loops are listed in Table 5. The properties of microwave-sintered PLZT ceramics are compared with the conventional sintered ceramics and shown in Table 6. The properties of the microwave-sintered PLZT 8/60/40 ceramics are comparable with the previously studied conventionally sintered ceramics [34].

Conclusions
PLZT 8/60/40 ceramics were fabricated using high-energy mechanical milling and microwave sintering for low lead loss and a cleaner environment. With the help of this study, not only was the processing temperature successfully reduced to 1150 • C (~100 • C less) but also sintering time was decreased to 20 min (~220 min less) without adding excess PbO. The detailed investigation of temperature-dependent dielectric properties, P-E hysteresis loops, and bipolar and unipolar S-E hysteresis loops shows that the PLZT 8/60/40 ceramics which were microwave-sintered at 1150 • C show optimized dielectric, ferroelectric and piezoelectric properties (P r~3 2.18 µC/cm 2 , I max~0 .91 mA, low E c~1 0.17 kV/cm, S max~0 .26%, maximum d 33~4 95 pm/V, maximum S max /E max~5 17 pm/V and minimum strain hysteresis loss~5.8%). The domain switching in P-E, I-E and bipolar S-E loops confirms the ferroelectric nature of the PLZT 8/60/40 ceramics. The investigation of energy-storage properties shows that the microwave-sintered PLZT 8/60/40 ferroelectric ceramic can be used as an energy-storage capacitor.