Morphotropic Phase Boundary Region Research Articles

Optimization of pyroelectric figure of merit of PNN-PZ-PT composition at morphotropic phase boundary

Ternary compounds are proven to be a more fascinating, owing to their potential to span a broader composition region of morphotropic phase boundary (MPB) – enhanced piezoelectric, electrostrictive, dielectric and ferroelectric properties. To activate defects dipoles, we perform MnO2 doping in ternary MPB compound 0.55Pb(Ni1/3Nb2/3)O3-0.135PbZrO3-0.315PbTiO3 [PNN-PZ-PT]. Temperature dependent dielectric spectroscopy reveals relaxor-ferroelectric nature of the synthesized ceramics. With the poling treatment, P-E loop of xMn-PNN-PZ-PT is softened, which emphasizes that the poling introduces higher order structural instability in MPB structure. The evolution of structural instability is as evidenced by the emergence of additional anomaly in thermal profile of dielectric constant due to electrical poling of xMn-PNN-PZ-PT and no systematic difference between polarizing behaviour of poled and unpoled specimen (Arrott plots). In association with defects dipoles, pyroelectric response based figure of merits (FOMs) of PNN-PZ-PT are improved. FOMs are characteristics of pyroelectric materials that insights about their suitability for specific application. Fi is suppressed with MnO2 doping and Fv, Fe and increases with MnO2 doping. Our study reveals that tailored and precise acceptor doping is crucial for the simultaneous optimization of all pyroelectric FOMs.

Origin of the ultrahigh field-induced strain in the Gd-doped 0.854Bi0.5Na0.5TiO3-0.12Bi0.5K0.5TiO3-0.026BaTiO3 ternary ceramic system

This study focused on analyzing the ferroelectric, piezoelectric, and dielectric properties of lead-free Bi0.487Na0.427K0.06Ba0.026TiO3 (0.854BNT-0.12BKT-0.026BT) ternary ceramic system by systematically doping 0.001, 0.01, 0.1, 0.5, and 1.0 mol% Gd2O3. The specific composition that was investigated is located at the tetragonal side of the rhombohedral-tetragonal morphotropic phase boundary (MPB) region. Undoped and Gd-doped BNT-BKT-BT ceramics were produced by the conventional solid-state reaction method. Ferroelectric, piezoelectric, and dielectric properties of ceramics were analyzed by carrying out electrical measurements from sintered samples. An ultrahigh field-induced unipolar strain of 0.52% at 65 kV cm−1, with a converse piezoelectric coefficient d33* of up to 795 pm V−1, was achieved with 0.5 mol% Gd doping. This was attributed to the Gd dopant disrupting the normal ferroelectric order and leading to the formation of a nonpolar relaxor phase. The field-induced transition from the nonpolar relaxor phase to the normal ferroelectric phase resulted in relatively large field-induced strain values in the 0.5 mol% Gd-doped ceramics. These results suggest that Gd-doped BNT-BKT-BT ceramics hold promise for digital actuator applications.

Quaternary piezoelectric ceramics with ultra-high mechanical quality factor

The development of piezoelectric ceramics with large piezoelectric coefficient (d33), high Curie temperature (TC) and high mechanical quality factor (Qm) is still a key challenge for practical application for high-power device. Herein, a novel quaternary piezoelectric ceramics of (0.125-x) Pb0.98Sr0.02(Mg1/3Nb2/3)O3–0.445PbTiO3–0.43PbZrO3-xPb(Mn1/3Nb2/3)O3 (abbreviated as xPMnN-PSMN-PZT) were prepared by the solid-state sintering method. The hard dopant PMnN induced the morphotropic phase boundary (MPB) of PSMN-PZT into a tetragonal-rich region. In addition, the XPS and EPR analysis proved the existence of a large amount of oxygen vacancies (Vo••) in PMnN-PSMN-PZT ceramic system. The composition near tetragonal-rich MPB region and the appearance of defect dipoles formed a strong coupling effect on the domain wall motion and leaded to an ultra Qm value. The optimum piezoelectric properties were achieved at x = 9 mol% with d33 = 260 pC/N, Qm = 3880, tanδ = 0.009 and TC = 332 °C. A significant internal bias field (Ebias) of 8.83 kV/cm was also observed in 0.09PMnN-PSMN-PZT ceramics. The origin of this excellent piezoelectric properties was explained by the phase structure, piezoelectric and dielectric analysis. This work demonstrated a rational strategy to obtain ultra Qm value for high-power piezoceramics.

Modified Pb(Mg1/3Nb2/3)O3-PbZrO3-PbTiO3 ceramics with high d33，large Qm and high Tc

The development of piezoelectric ceramics with large piezoelectric coefficient (d33), high Curie temperature (Tc) and high mechanical quality factor (Qm) is still a key challenge for practical application for high-power device. Herein, a series of Sm3+ and Mn2+ ions co-doped 0.15Pb(Mg1/3Nb2/3)O3-(0.85-x)PbZrO3-xPbTiO3 ceramics were prepared by the solid-state sintering method. The optimum piezoelectric properties were achieved at x = 0.435 mol% with a large d33 ∼ 450 pC/N and a high Qm ∼ 1968 near the morphotropic phase boundary (MPB), and with high Tc ∼ 296 °C. The origin of this excellent piezoelectric properties was explained by the phase structure, piezoelectric and dielectric analysis. The composition near MPB region and the appearance of defect dipoles leaded to final excellent piezoelectric properties, and made a harmonization between d33, Tc and Qm. This work demonstrated a rational strategy to obtain large d33, high Tc and high Qm for high-power piezoceramics.

Piezoelectric Properties and Thermal Stability of Pb(Yb1/2Nb1/2)O3-BiScO3-PbTiO3 Ternary Ceramics

Piezoelectric ceramics with excellent piezoelectric properties and a high Curie temperature are important for numerous electromechanical devices in a broad range of temperature environments. In this work, the relaxor ferroelectric Pb(Yb1/2Nb1/2)O3 end member was selected to be introduced into a BiScO3-PbTiO3 high-temperature piezoelectric ceramic to reduce the dielectric loss and improve the piezoelectric properties while slightly reducing the Curie temperature. The phase structure and dielectric, ferroelectric and piezoelectric properties of 0.025Pb(Yb1/2Nb1/2)O3-(0.975−x)BiScO3-xPbTiO3 (0.60 ≤ x ≤ 0.63) ceramics were systematically analyzed, and the best electrical properties were observed in the morphotropic phase boundary region x = 0.61 with d33 = 370 pC/N, kp = 44%, Pr = 33.9 μC/cm2. Importantly, no significant depolarization was observed in the x = 0.61 ceramic from room temperature to 290 °C, demonstrating its good thermal stability and potential applications in a wide range of temperature environments.

Crystal structure of solid solutions 0.65BiFeO3–0.35Ba1-xSrxTiO3 in the region of morphotropic phase boundary

Solid solutions 0.65BiFeO3–0.35Ba1-xSrxTiO3 (0 ≤ x ≤ 1) with the compositions in the vicinity of the morphotropic phase boundary “rhombohedral-cubic” were synthesized by the Solid-state reaction method. The crystal structure and morphology of the ceramics 0.65BiFeO3–0.35Ba1-xSrxTiO3 were studied based on the data obtained by X-ray diffraction, scanning electron microscopy, Raman spectroscopy, as well as energy-dispersive X-ray spectroscopy methods. It was determined that the chemical substitution of barium ions with strontium ions leads to a decrease in the magnitude of rhombohedral distortions, while the unit cell parameters decrease in the whole substitution concentration range. The solid solutions with x ≥ 0.25 are characterized by a single-phase structural state with a cubic unit cell; the average crystallite size decreases with increase of the dopant ions. The results of the structural studies carried out using Raman spectroscopy indicate the presence of rhombohedral distortions in the structure of all studied compounds, which is caused by the presence of nanosized clusters with rhombohedral symmetry. The obtained results made it possible to determine the sequence of the changes occurred in the phase state and the unit cell parameters in the region of the morphotropic phase boundary “rhombohedral -pseudocubic”; the concentration intervals corresponding to the single-phase and two-phase structural states of the compounds were determined. The region of concentration stability of the polar rhombohedral phase was clarified using the structural data obtained by local and microscopic research methods.

Enhanced piezoelectric properties and thermal stability of LiNbO3-modified PNN–PZT ceramics

Piezoelectric PZT ceramics with high piezoelectric properties and good thermal stability are urgently desired concerning the practical application. New compositions of LiNbO3 modified Pb(Ni1/3Nb2/3)O3PbZrO3PbTiO3 ceramics have been prepared in this study. The effects of the introduction of the LiNbO3 on the system were comprehensively investigated in terms of the phase structure, microstructure, electric properties, and thermal stability behavior of the ceramics. All compositions are located in the morphotropic phase boundary (MPB) region, and the ratio of the rhombohedral (R) phase increases obviously with the increase of LiNbO3 concentration. With increasing the LiNbO3 content, the piezoelectric properties were significantly enhanced. The sample added with 2% (in mole) LiNbO3 shows excellent electric properties, including Tm = 185 °C, εr = 5,643, kp = 0.626, Qm = 51, d33 = 902 pC/N. More importantly, no thermal depolarization behavior was observed in the temperature range of 25–100 °C. For PNN-PZT-x%LN ceramics, which is mainly attributed to the pinning effect resulted by the LiPb'−NbZr/Ti∙ defect dipoles.

Piezoelectric, ferroelectric and dielectric characterizations of a new ceramic solid solution (1-x)Ba(Cu1/3Nb2/3)O3-xPbTiO3 with morphotropic phase boundary

The understanding of structure-property co-relations is a key to utilising the materials for their best performances. This paper focuses on the physical property characterization of a new binary solid solution series with formula (1-x)Ba(Cu1/3Nb2/3)O3-xPbTiO3. A unique type of crystal structure evolution as a function of composition has been observed for this solid solution series, where a two-phase coexistence is observed in a very large span of compositions at room temperatures. The microstructure, piezoelectric, ferroelectric and dielectric properties along with their entanglement with crystal structure, have been investigated in detail. The compositional homogeneity of selected compositions has been verified with EDS and XPS. The higher responses in the physical properties have been observed near morphotropic phase boundary compositions. On heating above room temperature, peaks in dielectric permittivity responses have been observed that correspond to the phase transitions of non-centrosymmetric phases. The solid solution's best piezoelectric and ferroelectric responses are observed for the x = 0.70 composition showing higher densification, larger grain size and low dielectric losses, and lies near the morphotropic phase boundary region. The solid solution has enormous potential in piezoelectric industries as it possesses morphotropic phase boundary and high-temperature piezoelectricity.

Effect of bismuth deficiency with K substitution for Na on piezoelectric properties and crystal structure of Bi0.5−x(Na0.8−yK0.2+y)0.5TiO3–3x/2 (0 ≤ x ≤ 0.015, 0 ≤ y ≤ 0.1) ceramics

The piezoelectric properties and crystal structure of Bi0.5−x(Na0.8−yK0.2+y)0.5TiO3–3x/2 (0 ≤ x ≤ 0.015, 0 ≤ y ≤ 0.1) ceramics were characterized in this study. The crystal-structure refinement for Bi0.5−x(Na0.8K0.2)0.5TiO3–3x/2 was carried out using synchrotron X-ray diffraction data and Rietveld analysis and revealed the coexistence of rhombohedral (R3c) and tetragonal (P4mm) phases for the compositions with 0 ≤ x ≤ 0.015, although the fraction of these phases was almost independent of the composition x (49% for the rhombohedral phase and 51% for the tetragonal phase at x = 0.015). The Bi deficiency led to an increase in the lattice distortion of rhombohedral phase and a decrease in the that of tetragonal phase. The piezoelectric constant (d33) value decreased from 151.5 to 120.6 pC/N as x was increased from 0 to 0.015, and a shift of the depolarization temperature to higher temperatures was also confirmed by in situ d33 measurements. These results are attributed to an increase in the lattice distortion of rhombohedral phase enhanced by the Bi deficiency. The K substitution for Na in Bi0.485(Na0.8−yK0.2+y)0.5TiO2.9775 led to the formation of a morphotropic phase boundary (MPB) in the composition range 0 ≤ y ≤ 0.06, resulting in an increase in the fraction of the tetragonal phase. Although a decrease in the mechanical quality factor (Qm) was observed for K-rich compositions, the highest d33 value of 195.4 pC/N with a mechanical coupling factor of 32.9% was obtained for the Bi0.485(Na0.76K0.24)0.5TiO2.9775 (y = 0.04) ceramic because of the enhancement of the tetragonality in the MPB region, resulting in the onset of depolarization at ∼170 °C.

Structural Refinement, Dielectric and Electromechanical Properties of (1-x)NBT-xKBT Piezoceramics in the Morphotropic Phase Boundary Region

Lead-free ferroelectric materials of sodium-potassium bismuth titanate, (1-x)NBT-xKBT systems were synthesized by a hydrothermal process. In this way, the appropriate conditions for the hydrothermal synthesis of NBT and KBT (i.e., concentrations of synthetic precursors, solution pH and temperature) are given graphically. Ceramics of (1-x)NBT-xKBT with (x(mol.%) = 0; 12; 16; 20; 30 and 100) were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The rhombohedral-tetragonal morphotropic phase boundary (MPB) was confirmed to be in the region of 0.12 ≤ x ≤ 0.20 for (1-x)NBT-xKBT at ambient temperature. Scherrer's formula and the Williamson-Hall (W-H) analysis were used to examine the average crystallite size and lattice strain. Raman spectroscopy was effectively applied to study the structural evolution of the (1-x)NBT-xKBT phase. The ceramics exhibited a high temperature of maximum dielectric permittivity at (Tmax = 343 °C at 100 kHz) along with electromechanical coupling factors (kp = 0.34, d33 = 147 pC/N). Based on the composition of all specimens, the results indicate a diffuse phase transition, probably of second order, between ferroelectric and paraelectric phases.

Impedance Spectroscopy and Study of Electrical Properties of Pb(Zr<sub>0.52</sub>Ti<sub>0.48</sub>)<sub>1-x</sub>Fe<sub>x</sub>O<sub>3-x</sub> (0≤x≤0.20) Ceramics

Iron substituted Lead zirconate titanate nanoparticles Pb (Zr0.52Ti0.48) 1-x FexO3-x (PZTFx) (for (x=0.00, 0.025, 0.05, 0.075, 0.10, 0.15 and 0.20) was prepared using the sol-gel route in the morphotropic phase boundary (MPB) region. The X-ray diffraction data revealed the formation of both rhombohedral and tetragonal structures. The microstructural properties of the compounds were examined through the scanning electron microscopy (SEM) technique. The impedance spectroscopy and conductivity spectroscopy were carried out over a wide range of temperatures (RT–400°C) and frequencies (100 Hz–2 MHz) to investigate the grain and grain boundary effect on the electrical properties of PZTFx. The complex impedance analysis data have been presented in the Nyquist plot which is used to identify the corresponding equivalent circuit and fundamental circuit parameters. Cole– Cole plots indicate Debye-type dielectric relaxation and the grain boundaries resistance is dominant at room temperature. The Nyquist plot showed the negative temperature coefficient of resistance (NTCR) character of PZTFx ceramics. The dielectric properties of (PZTFx) ceramics as a function of temperature are studied and displayed a resonance phenomenon for all samples. Temperature-dependent conductivity behavior indicated an Arrhenius type of thermally activated process in the low-temperature region. Activation energy has been calculated from the temperature-dependent DC electrical conductivity measurements for all the samples.

Structural, piezoelectric and ferroelectric analysis of 0.96Bi0.5Na0.5TiO3-0.06BaTiO3: xwt%MnO2 ceramics for high-tech applications

With the rapid growth to design and engineer the smart and efficient electronic devices like piezoelectric sensors, the surge for the materials with multifunctional properties have risen tremendously. Herein, a morphotropic phase boundary (MPB) existent 0.94(Bi0.5Na0.5TiO3)-0.06BaTiO3 (BNBT) ceramics have been fabricated and the influence of additive MnO2 (0–0.20 wt%) has been examined, via structural, morphological, ferroelectric and dielectric analysis. Keeping the MPB region stable, BNBT:0.15Mn ceramic has shown the much improved results with remnant polarization (Pr) of 38.72 μC/cm2, piezoelectric coefficient (d33) of 211 pC/N, and dielectric constant (ϵr) of 4502, sample has maintained its stable d33 value (198 pC/N) till 200 °C. Moreover, the dielectric constant dependence on temperature analysis has been observed to analyze the Curie temperature (TC) of the samples, where BNBT:0.15Mn ceramic has exhibited the TC of ∼335 °C, which is the main reason of achieving the stable piezoelectric effect till 200 °C. Present study not only provides the new direction for the material engineering but also demonstrates the potential of the reported material for high-temperature piezoelectric sensors.

Improving the piezoelectric properties of Pb(Ni, Nb)O3-Pb(Hf, Ti)O3 ceramics through Nd2O3 addition

High-performance piezoelectric materials, whose displacement is sensitive to an external field and vice versa, are constantly required in electromechanical applications. Here, we introduce Nd3+ ions into 0.49Pb(Nb2/3Ni1/3)O3-0.51Pb(Hf0.3Ti0.7)O3 (PNN-PHT) ceramics via the solid-state method. By constructing a ferroelectric system in the morphotropic phase boundary region and exploiting the synergistic effect of polar nanoregions and an enhanced local structural heterogeneity, excellent piezoelectricity and dielectric properties are obtained. The optimized piezoelectric coefficient d33, dielectric constant ε33, electromechanical coupling factor kp, and dielectric loss tanδ are 1125 pC/N, 8667, 0.66 %, and 1.94 %, respectively. The Vogel-Fulcher law and relaxation analysis were adopted to understand the origin of the high piezoelectricity. The Rayleigh analysis method was used to study the intrinsic and extrinsic contributions to the piezoelectricity of the Nd-doped PNN-PHT ceramics, the extrinsic contribution was found to be up to 19 % in the optimal region (PNN-PHT + 0.4 at. % Nd2O3). Moreover, according to in-situ ferroelectric measurements and fatigue measurements, the PNN-PHT + 0.4 at. % Nd2O3 ceramic exhibits excellent fatigue resistance and stable piezoelectric behavior at 60 ℃. This work provides an approach to realize high-performance piezoelectric ceramics with good electromechanical stability under external condition, which is expected to expand the range of application of piezoelectric materials, e.g., for transducers.

Sol-gel synthesis, structure, and dielectric properties of La0.67LixTi1-xAlxO3 solid solutions

La0.67LixTi1-xAlxO3 were synthesized using the sol-gel Pechini route. Ceramic samples were sintered in the temperature range of 1240–1300 °C in the air atmosphere. It was found that in the concentration range of 0.05 ≤ x < 0.15 there is a morphotropic phase boundary region with rhombohedral and tetragonal syngonies. In the concentration range of 0.15 ≤ x ≤ 0.3, a single-phase solid solution with rhombohedral R 3‾ c syngony is formed. As the value of x increases, the average grain size of La0.67LixTi1-xAlxO3 ceramics increases from 5.23 μm (x = 0.05) to 8.76 μm (x = 0.3). All materials of the La0.67LixTi1-xAlxO3 system at 0.05 ≤ x ≤ 0.3 have colossal values of dielectric constant ε′ > 104 at frequencies up to 1 kHz. La0.67LixTi1-xAlxO3 (x = 0.2) solid solution with rhombohedral syngony has the highest value of dielectric constant and the lowest value of the dielectric losses.

Large electrocaloric effect over a wide temperature span in lead-free bismuth sodium titanate-based relaxor ferroelectrics

For efficient solid-state refrigeration technologies based on electrocaloric effect (ECE), it is a great challenge of simultaneously obtaining a large adiabatic temperature change (ΔT) within a wide temperature span (Tspan) in lead-free ferroelectric ceramics. Here, we studied the electrocaloric effect (ECE) in (1-x)(Na0.5Bi0.5)TiO3-xCaTiO3 ((1-x)NBT-xCT) and explored the combining effect of morphotropic phase boundary (MPB) and relaxor feature. The addition of CT not only constructs a MPB region with the coexistence of rhombohedral and orthorhombic phases, but also enhances the relaxor feature. The ECE peak appears around the freezing temperature (Tf), and shifts toward to lower temperature with the increasing CT amount. The directly measured ECE result shows that the ceramic of x = 0.10, which is in the MPB region, has an optimal ECE property of ΔTmax = 1.28 K @ 60 °C under 60 kV/cm with a wide Tspan of 65 °C. The enhanced ECE originates from the electric-field-induced transition between more types of polar nanoregions and long-range ferroelectric macrodomains. For the composition with more relaxor feature in the MPB region, such as x = 0.12, the ECE is relatively weak under low electric fields but it exhibits a sharp increment under a sufficiently high electric field. This work provides a guideline to develop the solid–state cooling devices for electronic components.

Sb2O3‐modified lead zirconate titanate piezoelectric ceramics with enhancing piezoelectricity and low loss

AbstractLead zirconate titanate (PZT)–based piezoelectric ceramics as important functional materials are widely used in many electromechanical devices. The piezoelectric coefficient and mechanical quality factor are vital property parameters for piezoelectric applications. However, the piezoelectric coefficient is inversely proportional to the mechanical quality factor, resulting in a strong limitation among wide applications. Herein, piezoelectric ceramics composed of (xSb2O3, 0.3‐wt% MnCO3)‐doped (Pb0.92Sr0.08)(Zr0.533Ti0.443Nb0.024)O3 ((xSb, Mn)‐PSZTN) were prepared by a conventional solid‐state process. The excellent piezoelectric properties d33 = 554 pC/N, kp = 0.645, and high mechanical quality factor Qm = 540 were simultaneously obtained at x = 0.1 ceramic in the morphotropic phase boundary region. The enhancement of piezoelectric properties was mainly due to the contribution of irreversible domain wall motion. In particular, the regulation of different defect chemical reactions on the properties showed that Sb2O3 could play the role of both donor and acceptor dopant. This work demonstrated that (0.1Sb, Mn)‐PSZTN ceramic was a good candidate material for high‐power piezoelectric devices.

Enhanced piezoelectric properties in BF-BT based lead-free ferroelectric ceramics for high-temperature devices

Due to the high Curie temperature (TC), BiFeO3–BaTiO3 (BF-BT) ceramics have been broadly investigated in high-temperature piezoelectric devices. The piezoelectric constant is one of the most significant factors in determining the sensitivity and reliability of piezoelectric functional components. However, the poor piezoelectric constant (d33) of BF-BT ceramic has prevented the practical application of the material. In this work, we innovatively introduce the 0.93Bi0.5Na0.5TiO3-0.07BaTiO3 (0.93NBT-0.07BT) component to 0.7BF-0.3BT ceramic, to build a morphotropic phase boundary (MPB) for enhancing d33. The XRD analysis shows that the (0.7BF-0.3BT)-x(0.93NBT-0.07BT) ceramics are still in the MPB region with R–PC phases coexistence, and exhibits a homogeneous solid solution. Moreover, the introduction of 0.93NBT-0.07BT ceramic suppresses the generation of defects and facilitates grain growth, thus enhancing piezoelectric property. In consequence, an optimum piezoelectricity d33 = 213 pC/N along with Tc∼450 °C was obtained in (0.7BF-0.3BT)-0.01(0.93NBT-0.07BT). This research provides a new idea for the application of BF-BT ceramics in high-temperature piezoelectric devices.

Bipolar cycling effects in BiFeO3–BaTiO3 piezoelectric ceramics

BiFeO3–BaTiO3 (BF-BT) lead-free piezoelectric system has been paid much attention due to good piezoelectric properties and high Curie temperature. Poling is a process to align ferroelectric domains and increase the piezoelectric coefficients. During the poling process, unipolar direct current (dc) electric fields were applied conventionally, but recently bipolar alternating current (ac) cycling was reported to improve piezoelectric properties in rhombohedral structure piezoelectric materials. We investigated the effects of dc-poling and ac-cycling in BF-BT ceramics. The d33 increased from 210 pC/N with dc-poling to 240 pC/N with ac-cycling in the morphotropic phase boundary region of BF-BT with domain engineering. This improvement of piezoelectric properties with ac-cycling was consistent with the structural evolution related to ferroelectric domains.

Preparation of nanoscale [(Ba0.85Ca0.15)0.995Nd0.005](Ti0.9Hf0.1)O3 ceramics via hydrothermal method and effect of grain size on multifunctional performance

Nanoscale [(Ba0.85Ca0.15)0.995Nd0.005](Ti0.9Hf0.1)O3 (Nd-BCTH) ceramics were prepared via the hydrothermal method, and the effects of sintering temperature and holding time on the phase structure, micromorphology, electrical and optical properties of the Nd-BCTH ceramics were explored. All the Nd-BCTH ceramics present rather pure perovskite structure with composition approaching rhombohedral phase around the morphotropic phase boundary (MPB) region. The highest relative density is obtained for the sample sintered at 1220 °C for 10 h. The existence of Ba2+, Ca2+, Ti4+, Hf4+ and Nd3+ is confirmed and the elements distribute rather uniformly detected by X-ray photoelectron spectrometer (XPS) and energy dispersive X-ray (EDX) analysis. The ceramics present nanoscale grain size, which tends to increase with the increase of sintering temperature and holding time, and significantly affects dielectric constant and Curie temperature. Very thin and narrow ferroelectric hysteresis loops are observed, where a considerable energy storage density (173.88 mJ/cm3) and high energy storage efficiency (80.68%) are obtained at low electric field. The increase of sintering temperature and holding time induces a red shift at 400 nm absorption edge and a blue shift at 300 nm absorption edge in the Nd-BCTH ceramics, and all ceramics have a maximum absorption value at around 260 nm. Under the excitation of 269 nm light, the Nd-BCTH ceramics show the strongest fluorescence peak at 473 nm, corresponding to the 4G3/2→4I9/2 transition, emitting indigo blue fluorescence. When the ambient temperature is above 400 °C, grains conduction dominates the conductive mechanism in the nano-sized Nd-BCTH ceramics. Such conduction can be attributed to oxygen vacancies caused due to the evaporation of alkaline-earth metals during high temperature sintering, and show typically thermally excited relaxation process.

Effect of Different Ca2+ and Zr4+ Contents on Microstructure and Electrical Properties of (Ba,Ca)(Zr,Ti)O3 Lead-Free Piezoelectric Ceramics

In the preparation of (Ba,Ca)(Zr,Ti)O3 lead-free piezoelectric ceramics, different Ca2+ and Zr4+ contents will greatly affect the phase structure, microstructure, and electrical properties of the ceramics. XRD shows that all samples have pure perovskite phase structure, and the (Ba0.85Ca0.15)(ZryTi1−y)O3 ceramics morphotropic phase boundary region from tetragonal phase to rhombohedral phase near 0.08 ≤ y ≤ 0.1. From the dielectric temperature curve, the phase transition temperature (TO-T) was found near room temperature at 0.12 ≤ x ≤ 0.18 for the (Ba1−xCax)(Zr0.1Ti0.9)O3 ceramics. Both Ca2+ and Zr4+ increase have a significant decrease on the Curie temperature Tc. All samples were revealed as relaxers with diffusivities in the range 1.29 ≤ γ ≤ 1.82. Different from the undoped ceramics, ceramics doped with Ca and Zr ions exhibit saturated P–E hysteresis loops, and their ferroelectric properties are significantly optimized. In particular, the (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 ceramic demonstrated optimal properties, namely d33 = 330 pC/N, kp = 0.41, εr = 4069, Pr = 4.8 μC/cm2, and Ec = 3.1 kV/cm, indicating that it is a viable lead-free piezoelectric contender. Variations in Ca and Zr content have a significant effect on the crystal grain sizes and densities of ceramics, which is strongly associated with their piezoelectricity.