High and Frequency‐Insensitive Converse Piezoelectric Coefficient Obtained in AgSbO3‐Modified (Li,K, Na)(Nb,Ta)O3 Lead‐Free Piezoceramics

AgSbO₃ was doped into KNN‐based lead‐free piezoceramics with an optimized composition of Li_0.02(Na_0.53K_0.48)_0.98Nb_0.8Ta_0.2O₃ (abbreviated as LKNNT) to further enhance its piezoelectric property. The doping of AgSbO₃ was found to be effective in reducing the grain sizes, resulting in more uniform microstructure in AgSbO₃‐doped LKNNT ceramics. AgSbO₃ lowers tetragonal‐orthorhombic phase transition point (T_T‐O), but with a more gentle rate as compared with other dopants. A large converse piezoelectric coefficient d₃₃* up to 598 pm/V under a relatively low electric field of 1 kV/mm was obtained in the LKNNT‐5 mol% AgSbO₃ composition, whose tetragonal‐orthorhombic phase transition point (T_T‐O) was controlled near room temperature, but its Curie temperature was kept at 235°C. The d₃₃* obtained in the present material is a very high value for nontextured KNN‐based ceramics, which is attributed to the polymorphism phase transition effect and “soft” behavior caused by the addition of AgSbO₃.     

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.     

Orthorhombic to tetragonal phase transition due to stress release in (Li,Ta)-doped(K,Na)NbO3 lead-free piezoceramics

XRD and Raman scattering experiments revealed an interesting finding that phase structure changed from orthorhombic to tetragonal symmetry when pulverizing (Li,Ta)-doped (K,Na)NbO₃ lead-free piezoelectric ceramics (sintered body) to powder. Both orthorhombic and tetragonal phases coexist in the Li_0.05(Na_0.51K_0.49)_0.95Nb_0.80Ta_0.20O_3.00 (LKNNT) sintered bulk sample at room temperature, but almost only the tetragonal phase is observed in the ground powder. In addition, annealing experiment enhanced the formation of tetragonal phase and improved the temperature stability of piezoelectricity. It is revealed that the internal stress existing in the LKNNT ceramics favors the formation of orthorhombic phase, which transfers to tetragonal phase when the stress was released.     

New Insight on Sintering Progress of KNN‐Based Lead‐Free Ceramics

0.96(K_0.5Na_0.5)_0.95Li_0.05Nb_0.93Sb_0.07O₃–0.04CaZrO₃ (0.96KNLNS–0.04CZ) lead‐free piezoelectric ceramics have been prepared by a new ceramics sintering progress—three‐step sintering method, via adjusting every step sintering temperature and holding time to improve piezoelectric properties. The result shows that the phase structure of the ceramics was changed from single phase to two phase coexisted by three‐step sintering, meanwhile, orthorhombic–tetragonal phase transition temperature was modified to around zero degree. Remarkably, piezoelectric properties has been obtained in 0.96KNLNS‐0.04CZ ceramics, which piezoelectric parameter is d₃₃ =420 pC/N, K_p =0.485.     

Sintering behavior,structural phase transition,and microwave dielectric properties of La1‐xZnxTiNbO6‐x/2 ceramics

La_1‐xZn_xTiNbO_6‐x/2 (LZTN‐x) ceramics were prepared via a conventional solid‐state reaction route. The phase, microstructure, sintering behavior, and microwave dielectric properties have been systematically studied. The substitution of a small amount of Zn²⁺ for La³⁺ was found to effectively promote the sintering process of LTN ceramics. The corresponding sintering mechanism was believed to result from the formation of the lattice distortion and oxygen vacancies by means of comparative studies on La‐deficient LTN ceramics and 0.5 mol% ZnO added LTN ceramics (LTN+0.005ZnO). The resultant microwave dielectric properties of LTN ceramics were closely correlated with the sample density, compositions, and especially with the phase structure at room temperature which depended on the orthorhombic‐monoclinic phase transition temperature and the sintering temperature. A single orthorhombic LZTN‐0.03 ceramic sintered at 1200°C was achieved with good microwave dielectric properties of ε_r~63, Q×f~9600 GHz (@4.77 GHz) and τ_f ~105 ppm/°C. By comparison, a relatively high Q × f~80995 GHz (@7.40 GHz) together with ε_r~23, and τ_f ~?56 ppm/°C was obtained in monoclinic LTN+0.005ZnO ceramics sintered at 1350°C.     

Improved thermal stability of the piezoelectric properties of (Li,Ag)‐co‐modified (K,Na) NbO3‐based ceramics prepared by spark plasma sintering

High‐performance lead‐free piezoelectric ceramics 0.94(K_0.45Na_0.55)_1?xLi_x(Nb_0.85Ta_0.15)O₃–0.06AgNbO₃ (KNNLTAg‐x) were successfully prepared by spark plasma sintering technique. The doping effect of Li on the structural and electrical properties of KNNLTAg‐x (x=0, 0.02, 0.04, 0.06, 0.08 and 0.10) ceramics was studied. The lattice structure, ferroelectric and piezoelectric properties of the KNLNTAg‐x ceramics are highly dependent on the Li doping level. In particular, the Li dopant has a great impact on both Curie temperature T_c and orthorhombic‐tetragonal transition temperature T_O‐T. The 4% Li‐doped sample exhibited relatively high T_O‐T of 95°C, leading to a stable dynamic piezoelectric coefficient (d₃₃*) of 220‐240 pm/V in a broad temperature range from 25°C to 105°C. Additionally, the 2% Li‐doped sample shows a high d₃₃* of 320 pm/V at 135°C, suggesting its great potential for high‐temperature applications.     

Low‐Temperature Synthesis of CoNb2O6 Microwave Dielectric Ceramics

Low‐fired cobalt niobate (CoNb₂O₆) microwave dielectric ceramics were prepared through a developed sol–gel process using Nb₂O₅·nH₂O as starting source. A metal‐dioxo‐bridged complex precursor was described on the basis of FT‐IR spectrum. The crystalline phases of calcined powders were characterized by X‐ray diffraction. Nanosized CoNb₂O₆ particles with orthorhombic α‐PbO₂‐type structure were obtained above 750°C. There was no subsequent phase change upon sintering, and all compounds sintered to at least 94% of theoretical density. At 1000°C/4 h, CoNb₂O₆ ceramics exhibited ε_r ~ 21.9, Q × f ~ 66 140 GHz (at 8.9 GHz) and τ_f ~ ?39.7 ppm/°C, having a good potential for low‐temperature cofired ceramic applications.     

Structural evolution of the R‐T phase boundary in KNN‐based ceramics

Although a rhombohedral‐tetragonal (R‐T) phase boundary is known to substantially enhance the piezoelectric properties of potassium‐sodium niobate ceramics, the structural evolution of the R‐T phase boundary itself is still unclear. In this work, the structural evolution of R‐T phase boundary from ?150°C to 200°C is investigated in (0.99?x)K_0.5Na_0.5Nb_1?ySb_yO₃–0.01CaSnO₃–xBi_0.5K_0.5HfO₃ (where x = 0‐0.05 with y = 0.035, and y = 0‐0.07 with x = 0.03) ceramics. Through temperature‐dependent powder X‐ray diffraction (XRD) patterns and Raman spectra, the structural evolution was determined to be Rhombohedral (R, <?125°C)→Rhombohedral + Orthorhombic (R + O, ?125°C to 0°C)→Rhombohedral + Tetragonal (R + T, 0 °C to 150°C)→dominating Tetragonal (T, 200°C to Curie temperature (T_C)) → Cubic (C, >T_C). In addition, the enhanced electrical properties (e.g., a direct piezoelectric coefficient (d₃₃) of ~450 ± 5 pC/N, a conversion piezoelectric coefficient () of ~580 ± 5 pm/V, an electromechanical coupling factor (k_p) of ~0.50 ± 0.02, and T_C~250°C), fatigue‐free behavior, and good thermal stability were exhibited by the ceramics possessing the R‐T phase boundary. This work improves understanding of the physical mechanism behind the R‐T phase boundary in KNN‐based ceramics and is an important step toward their adoption in practical applications.     

Temperature dependence of field‐responsive mechanisms in lead zirconate titanate

An electric field loading stage was designed for use in a laboratory diffractometer that enables in situ investigations of the temperature dependence in the field response mechanisms of ferroelectric materials. The stage was demonstrated by measuring PbZr_1?xTi_xO₃ (PZT) based materials—a commercially available PZT and a 1% Nb‐doped PbZr_0.56Ti_0.44O₃ (PZT 56/44)—over a temperature range of 25°C to 250°C. The degree of non‐180° domain alignment (η₀₀₂) of the PZT as a function of temperature was quantified. η₀₀₂ of the commercially available PZT increases exponentially with temperature, and was analyzed as a thermally activated process as described by the Arrhenius law. The activation energy for thermally activated domain wall depinning process in PZT was found to be 0.47 eV. Additionally, a field‐induced rhombohedral to tetragonal phase transition was observed 5°C below the rhombohedral‐tetragonal transition in PZT 56/44 ceramic. The field‐induced tetragonal phase fraction was increased 41.8% after electrical cycling. A large amount of domain switching (η₀₀₂=0.45 at 1.75 kV/mm) was observed in the induced tetragonal phase.     

Low Temperature Sintering and Enhanced Piezoelectricity of Lead‐Free (Na0.52K0.4425Li0.0375)(Nb0.86Ta0.06Sb0.08)O3 Ceramics Prepared from Nano‐Powders

(Na_0.52K_0.4425Li_0.0375)(Nb_0.86Ta_0.06Sb_0.08)O₃ powders were synthesized via sol–gel and solid‐state reaction methods as a raw material for the preparation of the ceramics. Dependence of piezoelectric properties and microstructure on sintering temperatures was investigated in this study. Sol–gel‐derived nano‐powders could be densified at a lower temperature of 940°C and exhibited excellent electrical properties after sintering at 1020°C (d₃₃ = 424 pC/N, d₃₃^* = 780 pm/V, k_p = 52.1%, and T_c = 265°C). The enhanced electric properties were most likely due to the coexistence of orthorhombic and tetragonal phase in the samples at room temperature, homogenous microstructure with fine grain and high density.     

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.     

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.     

Seed‐Free Solid‐State Growth of Large Lead‐Free Piezoelectric Single Crystals: (Na1/2K1/2)NbO3

Large Na_0.5K_0.5NbO₃ (NKN) piezoelectric single crystals were obtained by seed‐free solid‐state crystal growth method, which is a traditional sintering grain growth process, with LiBiO₃ used as a sintering aid. The largest dimension of the single crystals obtained was 11 mm × 9 mm × 3 mm. In addition to the LiBiO₃ doping content, temperature, and time effect of the crystal growth process was systematically investigated and considered from the kinetics point of view. With the assistance of Avrami analysis, parameters relevant to the crystal growth process were determined. Laue diffraction and transmission electron microscopy suggested an orthorhombic symmetry for the single crystalline structure. Dielectric‐frequency‐temperature measurements revealed an orthorhombic‐tetragonal and tetragonal‐cubic phase transition at 155°C and 405°C, respectively, both of which are typical of first‐order transitions, and have a well‐defined thermal hysteresis. Rayleigh analysis was performed regarding to the extrinsic reversible and nonreversible piezoelectric properties, and the result suggested a dominant intrinsic reversible piezoelectric contribution of 91.5% under E₀ = 1 kV/cm AC amplitude. Such a single crystal growth process route is low cost and a relative simple preparation process.     

Dielectric and impedance spectroscopic study of lithium doped potassium tantalum niobium oxide

The (K_0.90Li_0.10) (Nb_0.80Ta_0.20)_0.99Mn_0.01O₃ (KLTN) ceramic has been synthesized by the conventional solid-state reaction route. The Rietveld refinement X-Ray diffraction (XRD) patterns confirmed the single-phase orthorhombic crystal structure with space group Amm2. The frequency dependent electrical properties were examined by the complex dielectric and impedance spectroscopy in the temperature range 30 °C–500 °C where multiple structural phase transition was observed with high dielectric constant, low tangent loss and well saturated electric polarization. The shifting of the ferroelectric phase transition temperature by 30 °C in heating and cooling mode suggests the irreversible motion of the domains and domain walls and significant effects on grain boundaries on structural phase transition temperature. A series of phase transitions from orthorhombic to tetragonal (∼190 °C) and tetragonal to Cubic (∼390 °C in cooling and 420 °C in heating) have been obtained in the wide range of temperature. The different type of analogy such as Modulus formalism, complex impedance spectra, frequency dependent conductivity, the activation energy of charge carriers has been used to understand the microstructure-electrical properties relation.     

Sealing Glass‐Ceramics with Near Linear Thermal Strain,Part I: Process Development and Phase Identification

A widely adopted approach to form matched seals in metals having high coefficient of thermal expansion (CTE), e.g. stainless steel, is the use of high CTE glass‐ceramics. With the nucleation and growth of Cristobalite as the main high‐expansion crystalline phase, the CTE of recrystallizable lithium silicate Li₂O–SiO₂–Al₂O₃–K₂O–B₂O₃–P₂O₅–ZnO glass‐ceramic can approach 18 ppm/°C, matching closely to the 18 ppm/°C–20 ppm/°C CTE of 304L stainless steel. However, a large volume change induced by the α‐β inversion between the low‐ and high‐ Cristobalite, a 1^st order displacive phase transition, results in a nonlinear step‐like change in the thermal strain of glass‐ceramics. The sudden change in the thermal strain causes a substantial transient mismatch between the glass‐ceramic and stainless steel. In this study, we developed new thermal profiles based on the SiO₂ phase diagram to crystallize both Quartz and Cristobalite as high expansion crystalline phases in the glass‐ceramics. A key step in the thermal profile is the rapid cooling of glass‐ceramic from the peak sealing temperature to suppress crystallization of Cristobalite. The rapid cooling of the glass‐ceramic to an initial lower hold temperature is conducive to Quartz crystallization. After Quartz formation, a subsequent crystallization of Cristobalite is performed at a higher hold temperature. Quantitative X‐ray diffraction analysis of a series of quenched glass‐ceramic samples clearly revealed the sequence of crystallization in the new thermal profile. The coexistence of two significantly reduced volume changes, one at ~220°C from Cristobalite inversion and the other at ~470°C from Quartz inversion, greatly improves the linearity of the thermal strains of the glass‐ceramics, and is expected to improve the thermal strain match between glass‐ceramics and stainless steel over the sealing cycle.     

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.     

Phase Transition and Large Electrostrain in Lead‐Free Li‐Doped (Ba,Ca)(Ti,Zr)O3 Ceramics

Large piezoelectric effect is achieved in Li‐doped Ba_0.85Ca_0.15Ti_0.90Zr_0.10O₃(BCTZ) ceramics by use of tuning the phase boundaries. Rhombohedral–orthorhombic (R–O) and orthorhombic–tetragonal (O–T) multiphase coexistence is constructed in the ceramics by changing Li contents. The high piezoelectric constant d₃₃ (493 pC/N) and large electrostrain (dS_max/dE_max = 931 pm/V) have been observed in the Li‐doped (Ba, Ca)(Ti, Zr)O₃ ceramics at low sintering temperature (1350°C/2 h). The significant enhancement in materials properties is ascribed to the multiphase region around room temperature induced by Li‐doped effect.     

Raman scattering and X-ray diffraction investigations of sol–gel derived SBN powders

Sol–gel derived Sr_xBa_1-xNb₂O₆ (SBN) powders were prepared at an annealing temperature of 1200°C. Their structural changes were characterized by Raman spectroscopy and X-ray diffractometry. Changes in the peak position and the relative intensity of the Raman spectra and X-ray patterns were examined for different values of x. Our results suggest that the SBN powders consist of a mixture of orthorhombic phase BaNb₂O₆ (BN) and tetragonal tungsten-bronze phase (TTB) SBN powders for x<0·5, and orthorhombic phase SrNb₂O₆ (SN) and TTB phase SBN powders for x>0·5. Pure TTB type SBN powders with x≠0·5, however, were obtained at higher annealing temperatures. A possible formation route of the sol–gel derived SBN powder is discussed.     

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.