Effects of K/Na ratio on the phase structure and electrical properties of 0.98(KxNa1−x)NbO3–0.02BiScO3 lead-free ceramics

Lead-free piezoelectric ceramics 0.98(K_xNa_1?x)NbO₃–0.02BiScO₃ (0.98 K_xN_1?xN–0.02BS) (x = 0.30–0.60) doped with 0.8 mol% Mn were prepared by conventional solid-state sintering. The effects of K/Na ratio on the phase structure and electrical properties of the Mn doped 0.98 K_xN_1?xN–0.02BS (0.98 K_xN_1?xN–0.02BS–Mn) were mainly studied. It is experimentally demonstrated that the electrical properties strongly depend on K/Na ratio in the 0.98 K_xN_1?xN–0.02BS–Mn ceramics and when x = 0.45 the ceramics exhibit optimum electrical properties: d₃₃ ～ 308 pC/N, k_p ～ 0.495, ?_r ～ 1577, tan δ ～ 0.028. These results show that the 0.98 K_xN_1?xN–0.02BS–Mn ceramic with x = 0.45 is a promising lead-free piezoelectric material.     

Dielectric properties and diffuse phase transition behavior of CuO-doped lead-free Ba(ZrxTi1−x)O3 ceramics

CuO-doped lead-free Ba(Zr_0.2Ti_0.8)O₃ (BZT20) ceramics were prepared through a solid state processing technique, and the effects of CuO on microstructure, dielectric properties and diffuse phase transition behavior were investigated. The average grain sizes were increased by CuO doping. The temperature and frequency dependences of the dielectric constant revealed that CuO-doped BZT20 ceramics exhibited broad diffuse phase transition behavior. The dielectric constant increased with increasing CuO concentration. The value of T_m and degree of diffusion (γ) changed regularly in the studied compositional ranges. The BZT20 samples with 1.0 mol% CuO doping, sintered at 1310 °C, showed excellent dielectric property and lower diffusivity with ε_m=21,371 and γ=1.87. These results can be explained by the disordered distribution of Cu ions in the B sites and the weakened bonding force with oxygen ions in Cu substituted BZT20 structure.     

Temperature dependent phase transition of (Bi0.5Na0.5)1−xBaxTiO3 lead-free piezoelectric

Phase transformation and electric properties of lead-free piezoceramics (Bi_0.5Na_0.5)_1?xBa_xTiO₃ with x=0.05, 0.06, and 0.07(BNB5T, BNB6T and BNB7T) were investigated using dielectric, piezoelectric and ferroelectric measurements. Electric field induced strain measurement shows “W” shape bipolar strain characteristics for BNB5T with typical ferroelectric P–E curve, while BNB6T and BNB7T, possessing pinch-off P–E, exhibit “V” shape field-induced strain. All the BNBxT specimens exhibit relaxor characteristic, identified by the Debye Law. Dielectric properties measured at elevated temperatures with the frequency variation (10–500 kHz) reveal frequency dispersion below the T_d point, but no dispersion between T_d and T_m, which may be ascribed to an intermediate phase transition. By adding more Ba²⁺ ions, the region of intermediate phase, distinguished by frequency dependence dielectric constant, expands to lower temperature. Moreover, the ferroelectric properties measured at elevated temperature were carried out below and at the depolarization temperature to well investigate the existence of this phase. Much less ε–T profile dispersion were observed during the investigation of BNB6T and BNB7T, leading to possible existence of an intermediate phase in the investigated compositions. The results suggest that the linear field-induced-strain of (Bi_0.5Na_0.5)_1?xBa_xTiO₃ are expected to be attributed to the intermediate phase.     

Diffuse phase transition and high electric field properties of BaCeyTi1−yO3 relaxor ferroelectric ceramics

Pure perovskite BaCe_yTi_1−yO₃ ceramics with compositions y=0.10, 0.20 and 0.30 have been prepared by solid state reaction. The temperature and frequency dependence of dielectric properties (permittivity, dielectric loss and dielectric modulus) of the ceramics has been investigated. A diffuse phase transition is typical for all the compositions, with a reduction of the Curie temperature with increasing Ce addition. If in the case of the sample with 0.10 Ce content, no frequency shift of the phase transition temperature (T_m) is noticed, a relaxor-like ferroelectric character become predominant for concentrations 0.20 and 0.30. The ceramic with y=0.20 presents higher tunability, a reduced hysteretic behavior and reasonable low dielectric losses at room temperature, which makes this composition a very good candidate for tunable capacitors applications.     

Microstructure and microwave dielectric properties of Li2Ti1−x(Zn1/3Nb2/3)xO3 ceramics

Li₂Ti_1?x(Zn_1/3Nb_2/3)_xO₃ (0≤x≤0.5) ceramics were prepared by a solid state ceramic route, and the phase purity, microstructure, and microwave dielectric properties were investigated. The XRD results suggest the formation of solid solutions for all studied compositions (0≤x≤5). The dielectric properties are strongly dependent on the compositions, the densifications and the microstructures of the samples. The Q×f value increases with x up to x=0.2 and then decreases with the further increase of x. The best microwave dielectric properties of ε_r=20.5, Q×f =75,257 GHz, and τ_f =15.4 ppm/°C could be obtained when x=0.2.     

Enhanced piezoelectric strain of BiFeO3–Ba(Zr0.02Ti0.98)O3 lead-free ceramics near the phase boundary

The xBiFeO₃-(1-x)Ba(Zr_0.02Ti_0.98)O₃ + 1.0 mol% MnO₂ (xBF-BZT) lead-free piezoelectric ceramics were prepared by conventional solid-state reaction method. The structure, dielectric, and piezoelectric properties were studied. X-ray diffraction (XRD) analysis showed that xBF-BZT ceramics exhibited pure perovskite structure with the coexistence of tetragonal and rhombohedral phases (0.66 ≤ x ≤ 0.74). The Curie temperature T_c, the dielectric constant ε_r (1 kHz), dielectric loss tanδ (1 kHz), piezoelectric constant d₃₃, coercive field E_c (80 kV/cm), and remnant polarization P_r (80 kV/cm) of 0.7BF-0.3BZT-Mn ceramics were 491°C, 633, 0.044, 165 pC/N, 35.6 kV/cm, and 22.6 μC/cm², respectively. The unipolar strain of 0.7BF-0.3BZT reached up to 0.20% under the electric field of 60 kV/cm, which is larger than that (0.15%) of BiFeO₃–BaTiO₃ ceramics. These results indicated that the xBF-BZT ceramics were promising candidates for high-temperature piezoelectric materials.     

Synthesis and thermoelectric properties of (NaxCa1−x)3Co4O9 ceramics

(Na_xCa_1−x)₃Co₄O₉ (x=0.05−0.2) ceramics with a layered crystal structure were prepared by a sol–gel method followed by a low-temperature sintering procedure. The electrical conductivity and Seebeck coefficient of the complex oxide ceramics were measured from 400 to 900 °C. Their electrical conductivity and power factor increase with increasing temperature, while the thermal conductivity is very weakly dependant on the temperature. Na dopant amount has a remarkable effect on electrical and thermal transport properties. The figure of merit in the ceramic samples is smaller than that of traditional thermoelectric alloys.     

Enhanced temperature stability in the O–T phase boundary of (K,Na)NbO3-based ceramics

In order to obtain excellent electrical properties and its temperature stability of KNN-based ceramics to meet the practical applications, a new lead-free material system of (1−x)K_0.5Na_0.5Nb_0.96Sb_0.04O₃-xBi_0.5Na_0.5Zr_0.8Sn_0.2O₃ (KNNS-xBNZS, 0 ≤ x ≤ 0.060) was designed, and the enhanced electrical properties (eg, d₃₃ ~ 465 pC/N, ε_r ~ 3318, S_uni ~ 0.133%) is obtained in the ceramics with x = 0.04. The physical origin of enhanced electric properties should be ascribed to the phase instability of R-T, resulting in a low-energy barrier, which can greatly facilitate the polarization switching. Moreover, the temperature stable of piezoelectric constant (d₃₃ or d₃₃*) is measured by three distinctive methods in different phase boundaries (O, O-T, R-T) at the temperature range 20°C-180°C, revealing that the O-T phase boundary has a relatively good temperature stability. A mode is used to show how to effectively modified the piezoelectric constant (d₃₃ or d₃₃*) and its temperature stability, we believe that such a strategy may further improve the temperature stability of d₃₃ or d₃₃* value in KNN-based ceramics.     

Fabrication and phase transition of La2−xLuxZr2O7 transparent ceramics

A series of transparent ceramics with the composition of La_2−xLu_xZr₂O₇ (x = 0−2.0) were prepared by solid-state reactive sintering in vacuum. With the increase of Lu content (x), phase transition from pyrochlore to defective fluorite occurred and a two-phase region existed in the range of x = 0.6−1.2. Grain sizes of the pyrochlore phase dominated samples (x < 0.5) were 11−14 μm, and that of the defective fluorite phase dominated samples were larger than 60 μm. However, grain sizes of the samples in the two-phase region were smaller than 3 μm. The La_0.8Lu_1.2Zr₂O₇ ceramic with the smallest grain size (∼2.5 μm) reached a highest in-line transmittance of 72.4% at 1100 nm among all the samples.     

X-ray diffraction,dielectric and Raman studies of the Ba1−xNaxTi1−x(Nb1−ySby)xO3 ceramics

Ba_1−xNa_xTi_1−x(Nb_1−ySb_y)_xO₃ (BNTNSO) ceramics with compositions (x=0.05; y=0; 0.10; 0.20 and 0.30) have been prepared by conventional solid-state method and sintered in the temperature range 1350–1400 °C. Phase purity and structure are investigated using X-ray diffraction (XRD) data. The structural study showed that our synthesized compounds are single phase and crystallize in the tetragonal system with P4mm space group at room temperature. Based on a phenomenological model, dielectric and Raman properties of BNTNSO compounds have been explored. Referring to this model, dielectric properties of ceramics have been investigated in broad ranges of temperature (100–500 K) and frequency 1–10³ kHz). The dielectric permittivity evolution as a function of temperature and frequency has exhibited a classical ferroelectric character for 0≤y≤0.20 and a relaxor type behavior for y=0.30. The investigation of Raman spectra as a function of temperatures and compositions, confirmed the dielectric behavior. These results indicate that the y=0.20 composition is of extreme significance as far as its technological and industrial applications are concerned, which refers basically to its interesting physical properties and environmentally friendly characters, especially as its transition temperature is equal to the room temperature. The used samples show that the substitution of Nb by Sb favors and maintains the relaxor characters without changing the transition temperature.     

Investigation on the phase transition behavior and electrical properties of textured Lix(K0.48Na0.52)1−xNbO3 piezoelectric ceramics

Lead-free Li_x(K_0.48Na_0.52)_1−xNbO₃ (KNN–xLi) ceramics were successfully prepared by the tape casting method and solid-state reaction. The effects of Li content on phase transition, microstructure and electrical properties were investigated. XRD results show strong preferred orientation, indicating the presence of textured structure in the samples. With increasing Li content, the Currie Temperature shifts to higher temperature while the phase transition temperature between orthorhombic and tetragonal phases shifts to lower temperature. The sample with x=0.05 is determined to contain two perovskite phases with orthorhombic and tetragonal structures at room temperature, respectively, and exhibits the maximum values of piezoelectric coefficient of 286 pC/N and planar electromechanical coupling factor of 0.45.     

Temperature stable and high-Q microwave dielectric ceramics in the Li2Mg3−xCaxTiO6 system (x=0.00–0.18)

Microwave dielectric properties of Li₂Mg_3−xCa_xTiO₆ (x=0–0.18) ceramics were studied using a conventional solid-state route to find temperature stable and high Q microwave ceramics. As the calcination temperature was 500 °C, the Li₂TiO₃ phase with monoclinic rock salt structure in C2/c space group started to form. When the samples were calcined from 600 °C to 900 °C, the XRD patterns exhibited a remarkable chemical reaction between the MgO and Li₂TiO₃ phases, which eventually formed the Li₂Mg₃TiO₆ phase. The results indicated the Li₂Mg₃TiO₆and CaTiO₃ co-existed with each other and formed a stable composite system when the calcium content was added. The SEM photographs indicated that the pores caused by the Li evaporation could be effectively reduced due to the appearance of CaTiO₃. As x was increased from 0 to 0.18, the relative density was significantly improved due to the elimination of pores. As the Ca content increased, the dielectric constant (εr) increased from 14.8 to 20.6; the quality factor (Q×f) decreased from 148,713 GHz to 79,845 GHz, and the temperature coefficient of resonant frequency (τf) significantly increased from −42.4 to +10.8 ppm/°C due to the increased amount of CaTiO₃. Therefore, at x=0.12, the LMCxT ceramics sintered at 1280 °C for 6 h displayed excellent comprehensive properties of εr=17.8,Q×f=102,246 GHz and τf=−0.7 ppm/°C.     

p-Type/n-type behaviour and functional properties of KxNa(1-x)NbO3 (0.49 ≤ x ≤ 0.51) sintered in air and N2

Potassium sodium niobate (KNN) is a potential candidate to replace lead zirconate titanate in sensor and actuator applications but there are many fundamental science and materials processing issues to be understood before it can be used commercially, including the influence of composition and processing atmosphere on the conduction mechanisms and functional properties. Consequently, KNN pellets with different K/Na ratios were sintered to 95% relative density in air and N₂ using a conventional mixed oxide route. Oxygen vacancies (V_O^??) played a major role in the semi-conduction mechanism in low p(O₂) for all compositions. Impedance spectroscopy and thermo-power data confirmed KNN to be n-type in low p(O₂) in contradiction to previous reports of p-type behaviour. The best piezoelectric properties were observed for air- rather than N₂-sintered samples with d₃₃?=?125?pC/N and k_p?=?0.38 obtained for K_0.51Na_0.49NbO₃.     

Synthesis and characterization of quarternary Ti3Si(1−x)AlxC2 MAX phase materials

Ti₃Si_(1−x)Al_xC₂ (x=0–1) quarternary MAX phase materials were prepared by spark plasma sintering of TiC, Ti, Si and Al powder mixtures at 1200 °C. Effect of Al addition on lattice parameters, density and hardness were investigated. Impurities are limited to binary phases of TiC and Ti₅Si₃. No multinary compound other than Ti₃Si_(1−x)Al_xC₂ can be detected. TiC exists as impurity in all samples and trace amount of Ti₅Si₃ can be detected in Samples x=0.1–0.6. Oxidation of Al cannot be avoided although all sintering were performed under vacuum and trace amount of Al₂O₃ can be found in all samples with Al addition. Experimental results show that the lattice parameters a and c increase linearly with increasing Al content for x=0–1. The lattice variations are strongly anisotropic and follow Vegard׳s law. Both density and hardness decrease as Al content increases. The linear variation of lattice parameters, d spacings of crystalline faces and density against Al concentration suggest that continuous solid solutions of Ti₃Si_(1−x)Al_xC₂ (x=0–1) may have been formed between Ti₃SiC₂ and Ti₃AlC₂.     

New scheelite-type Cd1−3x⌷xGd2x(MoO4)1−3x(WO4)3x ceramics – their structure,thermal and magnetic properties

Polycrystalline samples of scheelite-type Cd_1−3x⌷_xGd_2x(MoO₄)_1−3x(WO₄)_3x solid solution with limited homogeneity (0<x≤0.25) and cationic vacancies (denoted as ⌷) have successfully prepared by a high-temperature annealing of CdMoO₄/Gd₂(WO₄)₃ mixtures composed of 50.00 mol% and less of gadolinium tungstate. Initial reactants and obtained ceramic materials were characterized by XRD, simultaneous DTA–TG, and SEM techniques. A phase diagram of the pseudobinary CdMoO₄–Gd₂(WO₄)₃ system was constructed. The eutectic point corresponds to 1404±5 K and ~70.00 mol% of gadolinium tungstate in an initial CdMoO₄/Gd₂(WO₄)₃ mixture. With decreasing of Gd³⁺ content in a CdMoO₄ framework, the melting point of Cd_1−3x⌷_xGd_2x(MoO₄)_1−3x(WO₄)_3x increases from 1406 (x=0.25) to 1419 K (x=0.0833), and next decreases to 1408 K (x=0). EPR method was used to identify paramagnetic Gd³⁺ centers in Cd_1−3x⌷_xGd_2x(MoO₄)_1−3x(WO₄)_3x for different values of x parameter as well as to select biphasic samples containing both Cd_0.25⌷_0.25Gd_0.50(MoO₄)_0.25(WO₄)_0.75 and Gd₂(WO₄)₃.     

Relationships between crystal structure and electrical properties of Li0.055[Agx(K0.5Na0.5)1−x]0.945(Nb1−yTay)O3 ceramics

Electrical properties of perovskite Li_0.055[Ag_x(K_0.5Na_0.5)_1?x]_0.945(Nb_1?yTa_y)O₃ (0.00 ≤ x ≤ 0.04, 0.01 ≤ y ≤ 0.09) ceramics were investigated based on the structural characteristics. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal phase was detected through the entire range of compositions. With increasing of Ta⁵⁺ content, the dielectric constant (?_r), piezoelectric coefficient (d₃₃) and electromechanical coupling factor (k_p) of Li_0.055(K_0.5Na_0.5)_0.945(Nb_1?yTa_y)O₃ ceramics were increased up to y = 0.07 and then decreased, while mechanical quality factor (Q_m) was increased. However, the ?_r, d₃₃, k_p and Q_m of Li_0.055[Ag_x(K_0.5Na_0.5)_1?x]_0.945(Nb_0.07Ta_0.03)O₃ ceramics were not changed remarkably with Ag⁺ content. The dependence of temperature coefficient of k_p (TCk_p) on the oxygen octahedral distortion was also discussed by Raman-active vibrations modes.     

Preparation and electrical properties of (1−x)SrBi2Nb2O9−xBiFeO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics, (1−x)SrBi₂Nb₂O₉−xBiFeO₃ [(1−x)SBN−xBFO] (x=0.0, 0.03, 0.05, 0.07, 0.10) were prepared by a conventional solid-state reaction method. The crystal structure, microstructure and electrical properties were systematically investigated. All compositions formed layered perovskite structure without any detectable secondary phases. Plate-like morphology of the grains which is characteristic for layer-structure Aurivillius compounds was clearly observed. The excellent electrical properties (e.g., d₃₃~19 pC/N, 2P_r~18.8 μC/cm²) and a high Curie temperature (e.g., T_c~449 °C) were simultaneously obtained in the ceramics with x=0.05. Additionally, thermal annealing studies indicated that the BFO modified SBN ceramics system possessed stable piezoelectric properties, demonstrating that the modified SBN-based ceramics are the promising candidates for high-temperature applications.     

Preparation and microwave dielectric properties of Ca0.6La0.8/3(SnxTi1−x)O3 ceramics

Ca_0.6La_0.8/3(Sn_xTi_1−x)O₃ ceramics were prepared via a conventional solid state reaction method, and the effect of Sn doping on their crystal phase structure and microwave dielectric properties was investigated. Results showed that Sn doping could hinder the formation of the rutile TiO₂ detrimental phase of Ca_0.6La_0.8/3TiO₃ ceramic. Also, the Q×f₀ value was enhanced and the τ_f value was lowered by Sn doping. The best microwave dielectric properties, i. e. ε_r=113 and Q×f₀=8487 GHz were obtained for a Sn doping content of 0.02. The mechanism of the improved properties deriving by Sn doping is discussed.     

Effect of isovalent substitution on phase transition and negative thermal expansion of In2−xScxW3O12 ceramics

Solid solutions of In_2−xSc_xW₃O₁₂ (0≤x≤2) were successfully synthesized using the solid state reaction method. Effects of substituted scandium content on the phase composition, microstructure, phase transition temperatures and thermal expansion behaviors of the resulting In_2−xSc_xW₃O₁₂ (0≤x≤2) samples were investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and thermal mechanical analyzer (TMA). Results indicate that the obtained In₂W₃O₁₂ ceramic undergoes a structure phase transition from monoclinic to orthorhombic at 248 °C. This phase transition temperature of In₂W₃O₁₂ can be easily shifted to a lower temperature by partly substituting the In³⁺ with Sc³⁺. When the x value increased from 0 to 1, the phase transition temperatures of In_2−xSc_xW₃O₁₂ (0≤x≤2) samples decreased from 248 to 47 °C. All the In_2−xSc_xW₃O₁₂ (0≤x≤2) ceramics show fine negative thermal expansion below their corresponding phase transition temperatures. The negative thermal expansion coefficients of the In_2−xSc_xW₃O₁₂ (0≤x≤2) ceramics change in the range from −1.08×10⁻⁶ °C⁻¹ to −7.13×10⁻⁶ °C⁻¹.     

Synthesis and lithium ionic conductivity of Li3−2x(In1−xZrx)2(PO4)3 (0≦x≦0·20)

Lithium ion conductors, Li_3−2x(In_1−xZr_x)₂(PO₄)₃ (0≦x≦0·20), were synthesized by a solid state reaction. A superionic conductive phase of Li₃In₂(PO₄)₃ was stabilized down to room temperature, assisted by the substitution of Zr⁴⁺ for In³⁺ sites of Li₃In₂(PO₄)₃. TG-DTA analysis indicated no phase transition in the samples with x superior to 0·05. The substituted samples showed much higher ionic conductivity by a couple of magnitude than that of Li₃In₂(PO₄)₃. In particular, the highest conductivity at room temperature was 1·42×10⁻⁵ Scm⁻¹ for Li_2·8(In_0·9Zr_0·1)₂(PO₄)₃. Thin films with the composition of Li_2·8(In_0·9Zr_0·1)₂(PO₄)₃ was prepared by a sol–gel method. A coating solution was made from lithium isopropoxide, indium isopropoxide, zirconium isopropoxide and diphosphorus pentaoxide. Well crystallized films were obtained on silicon dioxide and quartz glass substrates by dropping the coating solution, followed by firing over 873 K. In the temperatures above 473 K the lithium ionic conductivity of the film was slightly higher than that of sintered samples prepared by the solid state reaction at 1373 K.