Intergranular Stress Induced Phase Transition in CaZrO3 Modified KNN‐Based Lead‐Free Piezoelectrics

The phase transition temperatures of CaZrO₃‐doped (Na_0.49K_0.49Li_0.02)(Nb_0.8Ta_0.2)O₃ lead‐free piezoelectrics (LKNNT‐CZ5) have been studied with X‐ray diffraction and Raman scattering. It is found that LKNNT‐CZ5 ceramics and powders with the same crystallite size (~2–5 μm) exhibit distinctly different phase transition behaviors versus temperature. The ceramic bulk changes its orthorhombic phase to tetragonal structure abruptly after being pulverized to powders at room temperature(RT), and follows a phase transition sequence with increasing temperature: orthorhombic (RT–65°C), orthorhombic and tetragonal (65°C–105°C), and tetragonal phase (105°C to ~220°C). Contrastively, the powders hold a tetragonal structure without any phase transition within the whole temperature region (RT–220°C). These differences might be induced by the existing internal stress or residual strain, which could be released during the grinding process.     

Investigation of the Structure and Electrical Properties of (KxNa0.96−xLi0.04)(Nb0.96−yTaySb0.04)O3 Piezoelectric Ceramics Modified with Manganese

The effect of high doping levels of manganese (Mn) on the structure and electrical properties of (K_xNa_1?x)NbO₃ (KNN) ceramics containing Li, Ta, and Sb has been investigated. The samples were measured using synchrotron X‐ray diffraction whereas Rietveld refinement with Fullprof was used to determine the structural information as a function of temperature. Temperature‐dependent dielectric measurement was used to compare the phase transition temperatures. The results show that Mn decreases the temperature range of phase coexistence between the orthorhombic and tetragonal phase from ~180°C to ~120°C. The Curie temperature remained unchanged with Mn addition while the dielectric constant and dielectric loss increased with Mn addition. High amounts of Mn led to a reduction in both piezoelectric and ferroelectric properties. The remnant polarization, remnant strain, and piezoelectric coefficient values decreased from 24 μC/cm², 0.000824, 338 ± 37 pm/V to 13 μC/cm², 0,00014 and 208 ± 27 pm/V, respectively for the undoped and 5 mol% Mn‐doped sample.     

Anisotropic field induced phase transitions and negative electrocaloric effect in rhombohedral Mn doped Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals

The electrocaloric effect (ECE) of Mn doped Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ (PIN-PMN-PT:Mn) single crystals with particular emphasis on the impact of crystallographic orientations and phase transitions were investigated systematically. Orientation-dependent phase transitions have been demonstrated by the dielectric and strain behaviors. Intriguingly, the negative ECE of –0.02?°C and –0.002?°C were obtained firstly in [001]-oriented PIN-PMN-PT:Mn crystals near the rhombohedral→tetragonal phase transformation and in [011]-oriented crystals near the rhombohedral→orthorhombic phase transformation, respectively. However, only the positive ECE was found in [111]-oriented crystals near the tetragonal→rhombohedral phase transition. Additionally, the maximum ECE temperature changes calculated in [001]-, [011]- and [111]-oriented crystals were 0.33?°C, 0.46?°C and 0.38?°C, respectively. Our results suggest that the negative ECE is attributed to electric field-induced phase transitions, whose critical field decreases with the increase of temperature. The phase transition-mediated coexistence of positive and negative effects in the relaxor-ferroelectric single crystals is beneficial to enhance the efficiency of the solid-state cooling devices.     

Influence of barium borosilicate glass on microstructure and dielectric properties of (Ba,Ca)(Zr,Ti)O3 ceramics

Barium borosilicate (BBS) glass was added as a sintering aid to (Ba_0.7Ca_0.3)TiO₃-Ba(Ti_0.8Zr_0.2)O₃ (BCZT) ceramics at levels from 2 to 15?wt%, yielding enhanced densification. The addition of BBS also induced changes in phase composition, from predominantly tetragonal to orthorhombic at room temperature. It is shown that the changes in phase content are caused by a shift of the orthorhombic to tetragonal phase transformation from below room temperature to ≈50?°C. An additional high temperature transition around 120?°C was also identified. These observations are interpreted in terms of the development of chemical heterogeneity associated with the redistribution of dopant elements (particularly Zr and Ca) through the liquid phase during sintering. The relative permittivity and electric field-induced polarisation values were generally degraded by the presence of the glass phase, but a reduction in ferroelectric hysteresis and improved densification behaviour have potential benefits in dielectric energy storage applications.     

Temperature-insensitive dielectric and piezoelectric properties in (1-x)K0.5Na0.5Nb0.997Cu0.0075O3-xSrZrO3 ceramics

Systematic investigation on phase transition, dielectric and piezoelectric properties of (1-x)K_0.5Na_0.5Nb_0.997Cu_0.0075O₃-xSrZrO₃ (x = 0, 0.03, 0.06, 0.09, 0.12, 0.15, abbreviated as KNNC-100xSZ) ceramics was carried out. Due to the coexistence of orthorhombic and tetragonal phase in a wide temperature range, a diffused polymorphic phase transition (PPT) region was achieved in KNNC with x ≥ 0.06. KNNC-12SZ ceramics exhibited high dielectric permittivity (∼1679), low dielectric loss (∼0.02) and small variation (Δe'/ε'_25 °C ≤ 15%) in dielectric permittivity from −78 °C to 237.3 °C. KNNC-6SZ ceramic possessed a high level of unipolar strain (∼0.15%) and maintained a smaller variation of ±12% under the corresponding electric field of 60 kV cm⁻¹ at 10 Hz from 25 °C to 175 °C. d₃₃^*, which was calculated according to the unipolar strain at 60 kV cm⁻¹, was 230 pm V⁻¹ and remained stable below 100 °C. Therefore, our work provided a new promising candidate for temperature-insensitive capacitors and piezoelectric actuators.     

Temperature dependent dielectric properties of (na,K) NbO3, near equimolar composition

Pellet samples of Na_1-xK_xNbO₃ (0.490 ≤ x ≤ 0.510) were prepared using solid state reaction method with double sintering. Dielectric measurements were carried out on the prepared samples, from 30 to 500 °C, at 1 MHz. Dielectric constant and electrical conductivity were found minimum, for x = 0.500 among all the prepared compositions, at room temperature and at all the measured frequencies. Three distinguished phase transitions, at 200, 380 and 460 °C, were observed in dielectric measurements of the prepared compositions. The high temperature X-ray diffraction (HT- XRD) of the samples with x = 0.500, shows monoclinic phase between room temperature and 200 °C, tetragonal between 200 and 380 °C, tetragonal with increased lattice parameters, between 380 and 460 °C, and cubic phase beyond 460 °C. The tetragonal to tetragonal phase transition at 380 °C, and tetragonal to cubic at 460 °C was distinctly observed from dielectric and HT- XRD measurements in the present study.     

Structural,thermal, dielectric and ferroelectric properties of K0.5Bi0.5TiO3 ceramics

Dense K_0.5Bi_0.5TiO₃ (KBT) lead-free ceramics were prepared by conventional solid reaction route. Their temperature behavior (up to 600 °C) was investigated by X-ray diffraction, DSC, dielectric spectroscopy and electric field-polarization technique. The first temperature dependent Raman scattering studies were also performed. X-ray and Raman scattering results show that samples exhibit a single perovskite structure with cubic symmetry at temperatures higher than approximately 400 °C and with coexistence of the cubic and tetragonal phases below this temperature. Two structural phase transitions between tetragonal phases in temperature range 200–225 °C and between tetragonal and cubic ones near 400 °C are observed. The content of the tetragonal phase increases with decreasing temperature and at room temperature it reaches more than 70%. Temperature- dependent P-E loops and pyroelectric data revealed a polar behavior in KBT up to about 400 °C, which means that the intermediate phase (～270–380 °C) is rather ferroelectric than antiferroelectric.     

On the Yttrium Tantalate – Zirconia phase diagram

The phase diagram for the YTaO₄-ZrO₂ quasi-binary has been determined up to 1600?°C. There are three distinct compositional regimes: an extensive YTaO₄ solid solution, an extensive ZrO₂ solid solution and a two-phase intermediate region. The addition of ZrO₂ to YTaO₄ decreases the M–T transition temperature almost linearly from 1426?°C to approximately 450?°C at the solubility limit (～28?mol% ZrO₂), and then remains constant until the ZrO₂ solid solution phase boundary is reached. Within the intermediate region, there exists an extensive two-phase tetragonal (T?+?t) phase field above the M–T transformation temperature. The transformation exhibits no hysteresis on heating and cooling but nonetheless there is a distribution with temperature in the mass fraction of the monoclinic and tetragonal phases so no unique transformation temperature can be identified. No other high temperature phases were observed but it is suggested that a higher temperature solid solution phase is likely above 1700?°C, based on the similarity in crystallographic relationship between the two tetragonal solid solution structures.     

Dielectric and Raman Spectroscopic Studies of Na0.5Bi0.5TiO3–BaSnO3 Ferroelectric System

A series of lead‐free perovskite solid solutions of (1 ? x) Na_0.5Bi_0.5TiO₃(NBT)—x BaSnO₃(BSN), for 0.0 ≤ x ≤ 0.15 have been synthesized using a high‐temperature solid‐state reaction route. The phase transition behaviors are studied using dielectric and Raman spectroscopic techniques. The ferroelectric to relaxor phase transition temperature (T_FR) and the temperature corresponding to maximum dielectric permittivity (T_m) are estimated from the temperature‐dependent dielectric data. Dielectric studies show diffuse phase transition around ~335°C in pure NBT and this transition temperature decreases with increase in x. The disappearance of x‐dependence of A₁ mode frequency at ~134 cm^?1 for x ≥ 0.1 is consistent with rhombohedral‐orthorhombic transition. In situ temperature dependence Raman spectroscopic studies show disappearance and discontinuous changes in the phonon mode frequencies across rhombohedral (x < 0.1)/orthorhombic (x ≥ 0.1) to tetragonal transition.     

Phase formation and electrical properties of Ba(ZrxTi1−x)O3 ceramics synthesized through a novel combustion technique

High quality Ba(Zr_xTi_1?x)O₃ (BZT, 0.025≤x≤0.150, step=0.025) ceramic has been prepared by the combustion technique. The raw materials were mixed with CH₄N₂O and calcined at 1000 °C and sintered at 1375 °C. For Ba(Zr_xTi_1?x)O₃ with x=0.025, the ceramics exhibited orthorhombic structure at room temperature. The crystal structure was transformed to rhombohedral, tetragonal and cubic phase, respectively with increasing zirconium content. The effect of x on dielectric properties has been studied intensively. It is found that the phase formation, which was affected from zirconium substitution, strongly influences the dielectric behavior. The rhombohedral phase decreases the maximum dielectric constant while the tetragonal phase enhances it. An extrapolation studied revealed the phase transition peaks merged into one peak at x ～0.094. The highest diffuseness constant of 1.95 was observed in BZT with 0.075 mol% zirconium. This was caused by the broadest dielectric peak of ferroelectric phase transition and the imminent diffusion between ferroelectric phase transition peak and Curie phase transition peak. The ferroelectric properties were sensitive to the phase exhibited in BZT system.     

In situ hot-stage TEM of the phase and domain evolution in quenched Na1/2Bi1/2TiO3–BaTiO3

Quenching relaxor ferroelectric 0.94(Na_1/2Bi_1/2)TiO₃–0.06BaTiO₃ (NBT-6BT) enhances the depolarization temperature (T_d), linked to the stabilization of ferroelectric order. The thermal evolution of the domain structure and phase assemblage provides insights about the onset of ferroelectric order in quenched materials. Unpoled furnace cooled and quenched NBT-6BT ceramics were studied using in situ temperature-dependent transmission electron microscopy. The rhombohedral to tetragonal structural transition in furnace cooled and quenched samples occurs in a comparable temperature range of 120°C–220°C. While the tetragonal phase is characterized by polar nanoregions (PNRs) and no domain contrast in the furnace cooled state, the quenched composition exhibits an increased fraction of lamellar domains, which are partially stable up to 300°C, thus benefiting the delayed depolarization. This is further corroborated by the dielectric data indicating earlier freezing of PNR dynamics in the quenched state. The reversibility of the phase transition is demonstrated by successive cooling, where quenched NBT-6BT features an increased grainy PNR contrast after the experiment, followed by a kinetically delayed coalescence of PNRs back into lamellar domains. This demonstrates that the stabilized ferroelectric state upon quenching is associated with the conversion of polar units on the nanometer scale into long-range domain structures.     

Structure Evolution and Electrical Properties of Y3+‐Doped Ba1−xCaxZr0.07Ti0.93O3 Ceramics

Lead free piezoelectric ceramics of Y³⁺‐doped Ba_1?xCa_xZr_0.07Ti_0.93O₃ with x = 0.05, 0.10, and 0.15 were prepared. Composition and temperature‐dependent structural phase evolution and electrical properties of as‐prepared ceramics were studied systematically by X‐ray diffraction, Raman spectroscopy, impedance analyzer, ferroelectric test system, and unipolar strain measurement. Composition with x = 0.10 performs a good piezoelectric constant d₃₃ of 363 pC/N, coercive field E_c of 257 V/mm, remanent polarization P_r of 14.5 μC/cm², and a Curie temperature T_m of 109°C. High‐resolution X‐ray diffraction was introduced to indicate presence of orthorhombic phase. Converse piezoelectric constant d₃₃* of x = 0.10 composition performed better temperature stability in the range from 50°C to 110°C. That means decreasing orthorhombic–tetragonal phase transition temperature could be an effective way to enlarge its operating temperature range.     

Structural phase transition,electrical and photoluminescent properties of Pr3+-doped (1-x)Na0.5Bi0.5TiO3-xSrTiO3 lead-free ferroelectric thin films

Lead-free ferroelectric Pr³⁺-doped (1-x)Na_0.5Bi_0.5TiO₃-xSrTiO₃ (x?=?0–0.5) (hereafter abbreviated as Pr-NBT-xSTO) thin films were prepared on Pt/Ti/SiO₂/Si and fused silica substrates by a chemical solution deposition method combined with a rapid thermal annealing process at 700?°C, and their structural phase transition, dielectric, ferroelectric, and photoluminescent properties were investigated as a function of STO content. Raman analysis shows that with increasing STO content, the phase structures evolve from rhombohedral phase to coexistence of rhombohedral and tetragonal phases (i.e. morphotropic phase boundary), and then to tetragonal phase. The structural phase transition behavior has been well confirmed by temperature- and frequency- dependent dielectric measurements. Meanwhile, the variation in photoluminescence intensity of Pr³⁺ ions with different STO content in the NBT-xSTO thin films also indicates that there exists a clear structural phase transition when the film composition is close to the morphotropic phase boundary. Superior dielectric and ferroelectric properties are obtained in the Pr-NBT-0.24STO thin films due to the formation of morphotropic phase boundary. Our study suggests that Pr-NBT-xSTO thin films be promising multifunctional materials for optoelectronic device applications.     

(K,Na)NbO3-based ceramics with excess alkali oxide for piezoelectric energy harvester

(K_0.44Na_0.52Li_0.04)(Nb_0.86Ta_0.1Sb_0.04)O₃ (KNLNTS) ceramics were prepared by a solid-state reaction. The effect of excess (K,Na)₂O alkali oxide on the densification, phase evolution, microstructure development, and piezoelectric properties was investigated. The figure of merit (FOM) (d₃₃·g₃₃) for piezoelectric energy harvesting applications was also compared between the samples with and without excess alkali oxide. The addition of the excess alkali oxide changed the tetragonal crystal structure to orthorhombic and decreased the sintering temperature by about 100 °C. The dielectric constant of the orthorhombic phase is much lower than that of the tetragonal phase. The orthorhombic sample with excess alkali oxide sintered at 1020 °C demonstrated higher FOM in spite of having a smaller piezoelectric constant (d₃₃) than the stoichiometric sample sintered at 1100 °C. This result indicates that a KNN ceramic with an orthorhombic composition near the MPB with a moderate piezoelectric constant and smaller permittivity is more advantageous for an energy harvesting application than that with a morphotropic phase boundary (MPB) or a tetragonal composition.     

Structure and dielectric behavior of (1-x)Pb(Sc1/2Ta1/2)O3-xPb(In1/2Nb1/2)O3 solid solution

A solid solution system of (1-x)Pb(Sc_1/2Ta_1/2)O₃-xPb(In_1/2Nb_1/2)O₃ (x = 0.2, 0.4, 0.6, and 0.8) was synthesized by conventional solid-state reaction technique. The optimum sintering temperatures of ceramics with x = 0.2, 0.4, 0.6, and 0.8 were 1400?°C, 1400?°C, 1300?°C, and 1200?°C, respectively. At these temperatures, the densest samples and the maximum dielectric constant were obtained. With increasing x, the percentage of pyrochlore phase increased, indicating a decrease in the solubility of solid solution. For x = 0.2, with the sintering temperature increasing, the ordering degree decreased while the dielectric constant increased. For x = 0.6 and 0.8, at the highest sintering temperature, the most pyrochlore phase appeared and the minimum dielectric constant was obtained. In addition, the relaxor characteristics of solid solution ceramics were systematically investigated. It was found that the dielectric maximum decreased and the temperature at dielectric maximum shifted to higher temperature with x increasing. All compositions exhibited the second-order phase transition due to the analysis of dielectric behaviors on heating and cooling. Interestingly, the difference in dielectric maximum between heating and cooling became larger with PIN content increasing. The diffuseness exponents of all compositions were calculated to be in the range of 1.53–1.66, suggesting the typical relaxor. The polarization-electric field (P-E) hysteresis loops of all solid solutions showed the shapes of slim loop. Meanwhile, the coercive field and remnant polarization of all compositions were analyzed in detail.     

The effect of zirconia substitution on the high-temperature transformation of the monoclinic-prime phase in yttrium tantalate

Amorphous yttrium tantalate, as well as solid solutions containing zirconia, transform on heating to a monoclinic-prime phase and then, with further heating, to a crystalline tetragonal (T) solid solution phase at ～1450?°C. On subsequent cooling the tetragonal phase converts by a second-order displacive transformation to a different monoclinic phase not to the monoclinic-prime phase. On subsequent reheating and cooling, the phase transformation occurs between the monoclinic (M) and tetragonal phases, and the monoclinic-prime phase cannot be recovered. The limit of zirconia solubility in both the monoclinic-prime and monoclinic phases lies between 25 and 28?m/o ZrO₂, consistent with previous first-principles calculations. The monoclinic-prime phase is stable up to at least 1400?°C for 100?h for zirconia concentrations from 0 to ～60?m/o ZrO₂. This temperature exceeds the temperature of the equilibrium M-T phase transformation suggesting that the monoclinic-prime phase transforms directly to the tetragonal phase by a reconstructive transformation and is unaffected by the zirconia in solid solution.     

Physical properties of the (K0·44Na0·52Li0.04)0.97La0·01Nb0·9Ta0·1O3 ceramic with coexisting tetragonal and orthorhombic monocrystalline grains at room temperature

A structural and physical properties study on ferroelectric (K_0·44Na_0·52Li_0.04)_0.97La_0·01Nb_0·9Ta_0·1O₃ ceramics with monocrystalline grains of orthorhombic (O) and tetragonal (T) perovskite phases coexisting at room temperature (RT) is presented. Different sintering temperatures were evaluated. XRD analysis demonstrates high crystalline quality of both phases with volume fractions depending of the sintering temperature. SEM shows grain facets and morphologies of both phases in all samples. Raman analysis confirms the dopant incorporation and the coexistence of both phases. The contributions of each phase in the dielectric response are deconvoluted using the frequency-temperature dielectric analysis. The orthorhombic-to-tetragonal (T_O-T) and tetragonal-to-cubic (T_T-C) phase transition temperatures shift ~100 °C below those reported for pure-KNN in all samples, as consequence of the Li, La, and Ta doping combination. Li doping promotes T-phase grain growth and decreases T_O-T below RT. La incorporation promotes the O-phase grain growth and decreases T_O-T and T_T-C. The optimal sintering temperature is 1180 °C with 50-50 volume fractions of T-O phases. Piezoresponse studies show a complex 180°-domain structure.     

Composition-dependent xBa(Zr0.2Ti0.8)O3-(1-x)(Ba0.7Ca0.3)TiO3 bulk ceramics for high energy storage applications

This work reports the composition dependent microstructure, dielectric, ferroelectric and energy storage properties, and the phase transitions sequence of lead free xBa(Zr_0.2Ti_0.8)O₃-(1-x)(Ba_0.7Ca_0.3)TiO₃ [xBZT-(1-x)BCT] ceramics, with x?=?0.4, 0.5 and 0.6, prepared by solid state reaction method. The XRD and Raman scattering results confirm the coexistence of rhombohedral and tetragonal phases at room temperature (RT). The temperature dependence of Raman scattering spectra, dielectric permittivity and polarization points a first phase transition from ferroelectric rhombohedral phase to ferroelectric tetragonal phase at a temperature (T_R-T) of 40?°C and a second phase transition from ferroelectric tetragonal phase - paraelectric pseudocubic phase at a temperature (T_T-C) of 110?°C. The dielectric analysis suggests that the phase transition at T_T-C is of diffusive type and the BZT-BCT ceramics are a relaxor type ferroelectric materials. The composition induced variation in the temperature dependence of dielectric losses was correlated with full width half maxima (FWHM) of A₁, E(LO) Raman mode. The saturation polarization (P_s) ≈8.3?μC/cm² and coercive fields ≈2.9?kV/cm were found to be optimum at composition x?=?0.6 and is attributed to grain size effect. It is also shown that BZT-BCT ceramics exhibit a fatigue free response up to 10⁵ cycles. The effect of a.c. electric field amplitude and temperature on energy storage density and storage efficiency is also discussed. The presence of high T_T-C (110?°C), a high dielectric constant (ε_r ≈?12,285) with low dielectric loss (0.03), good polarization (P_s ≈?8.3?μC/cm²) and large recoverable energy density (W?=?121?mJ/cm³) with an energy storage efficiency (η) of 70% at an electric field of 25?kV/cm in 0.6BZT-0.4BCT ceramics make them suitable candidates for energy storage capacitor applications.     

Domain Reengineering of the [011]‐Poled PMN–0.35PT Single Crystals for Dielectric Bolometer Arrays

The domain configuration was reengineered with a modified poling procedure for the [011]‐poled single‐domain PMN–0.35PT crystals located at the morphotropic phase boundary. As a consequence, the dielectric constant ε_r at room temperature was significantly enhanced by more than 10 times to about 18 000, extremely higher than the reported (1 ? x)Pb(Mg_1/3Nb_2/3)O₃‐xPbTiO₃ ferroelectric crystals. Besides, the decreasing rate of the dielectric constant (dε_r/dT) was about 300/K with a temperature coefficient (α) of 1.7%/K, comparable to the BST materials for dielectric bolometer applications. The ferroelectric phase transition behavior was investigated to establish the poling procedure and a thermal hysteresis of about 25°C was indicated across the room temperature for the orthorhombic–tetragonal phase transition, which contributed to the revolution of the domain pattern.     

Orthorhombic‐pseudocubic phase transition and piezoelectric properties of (Na0.5K0.5)(Nb1−xSbx)‐SrZrO3 ceramics

0.96(Na_0.5K_0.5)(Nb_1?xSb_x)‐0.04SrZrO₃ ceramics with 0.0≤x≤0.06 were well sintered at 1060°C for 6 hours without a secondary phase. Orthorhombic‐tetragonal transition temperature (T_O‐T) and Curie temperature (T_C) decreased with the addition of Sb₂O₅. The decrease in T_C was considerable compared to that in T_O‐T, and thus the tetragonal phase zone disappeared when x exceeded 0.03. Therefore, a broad peak for orthorhombic‐pseudocubic transition as opposed to that for orthorhombic‐tetragonal transition appeared at 115°C‐78.2°C for specimens with 0.04≤x≤0.06. An orthorhombic structure was observed for specimens with x≤0.03. However, the polymorphic phase boundary structure containing orthorhombic and pseudocubic structures was formed for the specimens 0.04≤x≤0.06. Furthermore, a specimen with x=0.055 exhibited a large piezoelectric strain constant of 325 pC/N, indicating that the coexistence of orthorhombic and pseudocubic structures could improve the piezoelectric properties of (Na_0.5K_0.5)NbO₃‐based lead‐free piezoelectric ceramics.