Electrical properties of Sb and Cr-doped PbZrO3–PbTiO3–PbMg1/3Nb2/3O3 ceramics

The pyroelectric, dielectric and DC resistive properties of Sb and Cr-doped ceramics with a base composition of Pb(Mg_1/3Nb_2/3)_0.025(Zr_0.825Ti_0.175)_0.975O₃ have been studied. Sb doping has been shown to produce a linear reduction in Curie temperature (T_C=−22z+294 °C) with concentration (z) and to give an increase in pyroelectric coefficient from 250 to 310 μCm⁻² K⁻¹ for z increasing from 0 to 3 at.%. It also produces first a reduction and then an increase in both dielectric constant and loss, so that the 33 Hz pyroelectric figures of merit (FOM's) are as follows: F_V peaks at 3.8×10⁻² m² C⁻¹ and F_D peaks at 1.2×10⁻⁵ Pa^−1/2. The resistivity is increased substantially from 1.1×10¹¹ to ca 6×10¹¹ Ωm with 1 at.% Sb, thereafter changing little. The behaviour has been explained in terms of Sb acting as a donor ion, reducing oxygen vacancy concentrations up to 1 at.%, with conductivity dominated by hole hopping between traps (E_a=0.59±0.05 eV) that are not changed by the Sb doping. It is concluded that additions of higher levels of Sb do not produce electron-mediated hopping conduction. The Cr additions have no effect upon T_C, but reduce dielectric constant and loss, pyroelectric coefficient and resistivity at doping levels up to 3 at.%. The FOM F_V peaks at 3.6×10⁻² m² C⁻¹ and F_D at 1.9×10⁻⁵ Pa^−1/2. The behaviour of the electrical resistivity as a function of dopant level is shown to produce a linear ln(σ_o) vs z^−1/3 dependence (σ_o=DC conductivity), as would be expected for hole hopping conduction between Cr³⁺ sites, with an E_a=0.38±0.03 eV.     

Dielectric behaviors of Nb2O5–Co2O3 doped BaTiO3–Bi(Mg1/2Ti1/2)O3 ceramics

Nb₂O₅ and Nb–Co doped 0.85BaTiO₃–0.15Bi(Mg_1/2Ti_1/2)O₃ (0.85BT–0.15BMT) ceramics were investigated. From XRD patterns, undesired phase was observed when the (Nb₂O₅/Nb-Co) doping levels exceed 3 wt.%/2 wt.%, giving rise to the deteriorate dielectric constant. The 0.85BT–0.15BMT ceramics doped with 2 wt.%Nb₂O₅ was found to possess a moderate dielectric constant (? ～ 1000) and low dielectric loss (tan δ = 0.9%) at room temperature and 1 kHz, showing flat dielectric behavior over the temperature range from ?55 to 155 °C. It was found that the formation of core–shell structure in the BT based ceramics is controlled by the doping sequence of Nb- and Bi-oxides.     

Effects of microstructure on the microwave dielectric properties of Ba(Co1/3Nb2/3)O3 and (1−x)Ba(Co1/3Nb2/3)O3–xBa(Zn1/3Nb2/3)O3 ceramics

Ba(Co_1/3Nb_2/3)O₃(BCN) has a 1:2 ordered hexagonal structure. A large amount of the liquid phase, which contains high concentrations of Ba and Nb ions was found in the BCN ceramics. Q-values of BCN increased with increasing sintering temperature; however, it significantly decreased when the sintering temperature exceeded 1400 °C. The presence of a large amount of liquid phase could be responsible for the decrease of the Q-value. For (1−x)Ba(Co_1/3Nb_2/3)O₃–xBa(Zn_1/3Nb_2/3)O₃ [(1−x)BCN–xBZN] ceramics, the 1:2 ordered hexagonal structure was observed in the specimens with x⩽0.3 and the BaNb₆O₁₆ second phase was found in the specimens with x⩾0.6. Grain growth, which is related to the BaNb₆O₁₆ second phase occurred in the specimens with x⩾0.5. In this work, the excellent microwave dielectric properties of τ_f=0.0 ppm/°C, ε_r=34.5 and Q×f=97,000 GHz were obtained for the 0.7BCN–0.3BZN ceramics sintered at 1400 °C for 20 h.     

Microwave dielectric properties of Bi2O3-doped Ca[(Li1/3Nb2/3)1−xTix]O3−δ ceramics

The effect of the additive on the densification, low temperature sintering, and microwave dielectric properties of the Ca[(Li_1/3Nb_2/3)_1−xTi_x]O_3−δ(CLNT) was investigated. Bi₂O₃ addition improved the densification and reduced the sintering temperature from 1150 to 900 °C of CLNT microwave dielectric ceramics. As the Bi₂O₃ content increased, the dielectric constant (ε_r) and bulk density increased. The quality factor (Q·f₀), however, was decreased slightly. The temperature coefficient of resonant frequency (τ_f) shifted to a positive value with increasing Bi₂O₃ content. The dielectric properties (ε_r, Q·f_0, τ_f) of Ca[(Li_1/3Nb_2/3)_0.95Ti_0.05]O_3−δ and Ca[(Li_1/3Nb_2/3)_0.8 Ti_0.2]O_3−δ with 5 wt.% Bi₂O₃ sintered at 900 °C for 3 were 20, 6500 GHz, −4 ppm/°C, and 35, 11,000 GHz, 13 ppm/°, respectively. The relationship between the microstructure and dielectric properties was studied by X-ray diffraction (XRD), and SEM.     

La(Mg1/2Ti1/2)O3–La2/3TiO3 microwave dielectric ceramics

Structure and microwave dielectric properties were studied in the (1−x)La(Mg_1/2Ti_1/2)O₃–xLa_2/3TiO₃ system. Ceramics with this composition in the 0⩽x⩽0.5 range were processed from powders obtained by a citrate-based chemical route. Structure of these perovskite solid solutions changed from orthorhombic for x=0.1 and 0.3 to pseudocubic for x=0.5. Microwave and radio frequency measurements revealed increase in permittivity and temperature coefficient of the resonant frequency τ_f with increasing of La_2/3TiO₃ content. Close to zero τ_f value was found near to x=0.5 composition of (1−x)La(Mg_1/2Ti_1/2)O₃–x La_2/3TiO₃.     

Magnetic and dielectric properties of BaFe1/2Sn1/2O3-δ ceramics

Ceramic samples of barium ferrostannate BaFe_1/2Sn_1/2O_3-δ have been prepared by the sol-gel and hydrothermal synthesis methods. To reduce the number of oxygen vacancies, annealing in oxygen has been performed. The heat capacity, magnetization, Mössbauer and dielectric spectroscopy data point to the appearance of antiferromagnetic ordering below ～55 K followed by further spin glass magnetic ordering below 15 K. The giant dielectric response seems to be due to the relaxation of electrons trapped by oxygen vacancies. Comparison of the Fe K-edge X-ray absorption spectra of BaFe_1/2Sn_1/2O_3-δ confirmed substantial difference from those of the similar stoichiometric BaFe_1/2Nb_1/2O₃ perovskite.     

Densification behavior and electrical properties of CuO-doped Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 ternary ceramics

CuO modified Pb(In_1/2Nb_1/2)O₃–Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PIN–PMN–PT) ternary relaxor based ferroelectrics with the composition near the morphotropic phase boundary were synthesized by two-step columbite precursor method. The introduction of CuO significantly improved the sinterability of PIN–PMN–PT ceramics, resulting in the full densification of samples at lower sintering temperatures. It also profoundly modified the crystal structure and fracture mode of the ceramics. Properly increasing CuO content led to the disappearance of rhombohedral-tetragonal phase transition, remarkably improved the Curie temperature (T_c), and made the ceramics more relaxorlike. The ternary ceramics doped with 0.25 wt% CuO possessed optimum piezoelectric properties (d₃₃=584 pC/N, d₃₃^*=948 pC/N, and k_p=0.68), high ferroelectric properties (E_c=9.9 kV/cm, and P_r=33.1 μC/cm²), low dielectric loss (tan δ=0.9%), and wider temperature usage range (T_c=225 °C). The obtained properties are much higher than those of previously reported PIN–PMN–PT based ceramics, indicating that CuO doped PIN–PMN–PT is a promising candidate for electromechanical applications with high performance and wide temperature/electric field usage ranges.     

Large dielectric constants and good thermal stability of Nb2O5-doped BaTiO3–Bi(Mg1/2Ti1/2)O3 ceramics with core–shell structure

0.96BaTiO₃-0.04Bi(Mg_1/2Ti_1/2)O₃ (0.96BT-0.04BMT) + y wt.% Nb₂O₅ ceramics (0.0 ≤ y ≤ 2.0) were sintered at 1275 °C to fabricate a ceramic with a large ε_r for an X8R multilayer ceramic capacitor (MLCC). Addition of Nb₂O₅ afforded a core–shell structure, and the compositions of the core and shell regions were similar to those of BT and BT-BMT, respectively. The sample (y = 1.25) exhibited a large ε_r of 2285 with a good temperature stability satisfying the X8R specification because of a broad shell-region phase-transition peak at −17 °C and a decreased ε_r of the core-region phase-transition peak. The 0.1 wt% BaO–CaO–SiO₂ (BCS) was used to reduce the sintering temperature, and the 9-layered MLCC produced using a BCS-doped 0.96BT-0.04BMT + 1.25 wt% Nb₂O₅ ceramic at 1200 °C showed a large capacitance of 67 nF with a good temperature stability thus complying with EIA-X8R regulations.     

Gelcasting of Al2O3–W composites: Broadband dielectric spectroscopy and rheological studies of tungsten influence on polymerisation kinetics

Gelcasting is a method which allows for the production of high-quality ceramic-metal composites. In this work, the problem of local polymerisation in gelcasting of alumina-tungsten composite and metal influence on activation energy of glycerol monoacrylate was investigated. The broadband dielectric spectroscopy method was used for the first time to study the kinetics of gelation in ceramic processing. Polymerisation reaction rates constant of pure monomer, and monomer with ceramic and the metal filler, within the temperature range from 313 K to 333 K were determined. The obtained results were compared with rheological measurements. The calculated activation energy of polymerisation for monomer with 0.5 vol% of tungsten is 66 kJ/mol in comparison to a pure monomer which is 290 kJ/mol. This indicates the catalytic influence of metal on the polymerisation. Therefore, in the alumina suspension with tungsten polymerisation begins near the surface of the metal particles resulting in polymer structure inhomogeneity and lower mechanical properties of green bodies.     

0.74NaNbO3–0.26Sr(Mg1/3Nb2/3)O3 lead-free dielectric ceramics with high energy storage properties

Sodium niobate (NaNbO₃) ceramics are commonly investigated for use as energy storage ceramics because of their excellent properties. NaNbO₃ ceramics are modified mainly by doping with a Bi-based composite perovskite, that is, by the nonequivalent doping of Bi³⁺ at the A site of the NaNbO₃ ceramic. In addition, the high volatility of Bi at high temperatures increases the defects in the ceramics. This paper provides a new idea of doping modification of sodium NaNbO₃-based energy storage ceramics. Here, (1?x)NaNbO₃–xSr(Mg_1/3Nb_2/3)O₃ (x = 0.17, 0.20, 0.23, 0.26) ceramics were prepared by doping NaNbO₃ with an Sr-based composite perovskite. Compared with Bi-based composite perovskite, Sr-based composite perovskite doping of NaNbO₃ ceramics can also obtained good energy storage properties: a total energy storage density of 4.28 J/cm³ and an energy storage efficiency of 89.3%. In addition, the ceramics exhibited good frequency stability (2–200 Hz) and a high charge/discharge rate (1.06 μs).     

Crystallisation and characterisation of dielectric glass ceramics in Li2O–Al2O3–SiO2 system

Abstract

Glass ceramics in the Li₂O–Al₂O₃–SiO₂ system have been synthesised to produce bulk materials grown in a glass phase via quenching followed by controlled crystallisation. The crystallisation and microstructure of Li₂O–Al₂O₃–SiO₂ (LAS) glass–ceramic with nucleating agents (B₂O₃ and/or P₂O₅) are investigated by differential thermal analysis, X-ray diffraction and scanning electron microscopy and the effects of B₂O₃ and P₂O₅ on the crystallisation of LAS glass are also analysed. The introduction of both B₂O₃ and P₂O₅ promotes the crystallisation of LAS glass by decreasing the crystallisation temperature and adjusting the crystallisation kinetic parameters, allows a direct formation of β spodumene phase and as a result, increases the crystallinity of the LAS glass ceramic. Microstructural observations show that the randomly oriented, nanometre sized crystalline is found with residual glass concentrated at crystallite boundaries. Furthermore, it is interesting that codoping of B₂O₃ and P₂O₅ creates not much effect on the crystallisation temperature. The dielectric properties of the glass–ceramics formed through controlled crystallisation have a strong dependence on the phases that are developed during heat treatment. The dielectric constant is continuously increased and the dielectric loss is decreased with addition of additives where mobile alkali metal ions (e.g. Li⁺) are incorporated in a crystal phase and minimise the residual glass phase.     

BaMg1/3Nb2/3O3–Mg4Nb2O9 composite microwave ceramics with high Q-factor and low sintering temperature

Revised thermodynamic equilibrium in the BaO–MgO–Nb₂O₅ pseudo-ternary system has lead to development of a novel composite dielectric material with dielectric constant, ?′ = 25.5, efficacy factor, Q × f = 160 THz, and temperature coefficient of the resonant frequency, τ_f = +0.5 ppm/K. The material shows one of the highest Q-factors among the Ta-free microwave dielectric resonators. It also does not contain volatile Zn and Co elements. Other important property of the title compound is low sintering temperature of 1320 °C which significantly reduces the processing cost.     

Characterisation of SnO2–CoO–MnO–Nb2O5 ceramics

Varistors based on SnO₂ have attracted increasing interest in recent years. However, the combined effect of CoO–MnO on SnO₂ ceramics is still unclear. In this study, the non-Ohmic behaviour of the 98.95 mol%SnO₂–0.5 mol%CoO–0.5 mol%MnO–0.05 mol%Nb₂O₅ system, the microstructures and the influence of sintering temperature were investigated. The samples were prepared by the mixed oxide route, and were sintered at temperatures in the range 1250–1450 °C. SEM observation and EDS analysis revealed that the ceramics have a two-phase microstructure comprising SnO₂ primary grains and a Mn, Co rich secondary phase of small particles. The sintered density of the samples increased with the increase in sintering temperature. The maximum non-linear coefficient (α = 10) was obtained at a sintering temperature of 1350 °C.     

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.     

Characterization of 0.93Pb(Zn1/3Nb2/3)O3–0.07BaTiO3 ceramics derived from a novel Zn3Nb2O8 B-site precursor

In this work, perovskite relaxor ferroelectric lead zinc niobate–barium titanate (0.93PZN–0.07BT) ceramics were fabricated by using a combination of Zn₃Nb₂O₈ B-site precursor and reactive sintering process. The effects of sintering condition on phase formation, densification, microstructure and dielectric properties of the final products have been investigated using a combination of X-ray diffraction, Archimedes density measurement, scanning electron microscopy and dielectric measurement techniques. It is seen that pure perovskite phase of PZN-BT solid solutions can be achieved in all samples. Density and average grain size values of sintered samples increased with sintering temperatures and dwell time. With appropriate sintering at 1150 °C for 5 h, 0.93PZN–0.07BT ceramics exhibited a peak dielectric constant of 11,497 and dielectric loss of 0.05 at the Curie temperature of 99 °C measured at 1 kHz.     

Microstructure evolution and reaction mechanism of Pb(Zr1/2Ti1/2)O3-Pb(Zn1/3Nb2/3)O3–Pb(Ni1/3Nb2/3)O3 piezoelectric ceramics with plate-like PbTiO3 template

0.83 Pb(Zr_1/2Ti_1/2)O₃-0.11Pb(Zn_1/3Nb_2/3)O₃-0.06Pb(Ni_1/3Nb_2/3)O₃ (PZNNT) samples with plate-like PbTiO₃ (PT) template were prepared using tape casting technology. The microstructure evolution and reaction mechanism between the matrix and PT template was investigated systematically. The quench heat treatment experiment was designed and the microstructure was evaluated. The results showed that the plate-like PT template has relatively low thermal stability which would decompose to form Pb-rich liquid phase and Ti-rich region at the sintering temperature of 900 °C–1050 °C. Plate-like PT template reacted with the PZNNT matrix materials during the sintering process, which did not contribute to the grain growth orientation for PZNNT matrix. Finally, the mechanism of grain growth for the PZNNT ceramics with plate-like PT template is clarified. This work demonstrated that the thermal stability of plate-like template is one of the key factors for fabricating textured piezoelectric ceramics.     

Sintering behavior of Cr2O3–Al2O3 ceramics

We investigated the sintering behavior of Cr₂O₃–Al₂O₃ ceramic materials. In our observation of the isothermal shrinkage behavior of Cr₂O₃–Al₂O₃ ceramic, the activation energy of sintering reaction was measured to be 102 kJ/mol, that is, the near value of the activation energy of diffusion of Al ions in Al₂O₃ single crystal. Therefore the diffusion of cations is believed to control the sintering behavior of this material. With the addition of TiO₂, (the compound chosen to accelerate the diffusion of cations) to Cr₂O₃–Al₂O₃, the sintering behavior was accelerated.     

Investigation of dielectric and piezoelectric properties in Pb(Ni1/3Nb2/3)O3–PbHfO3–PbTiO3 ternary system

The dielectric and piezoelectric properties were investigated in the (1 ? x)Pb(Hf_1?yTi_y)O₃–xPb(Ni_1/3Nb_2/3)O₃ (PNN–PHT, x = 0.05–0.50, y = 0.55–0.70) ternary system. The morphotropic phase boundary (MPB) was determined by X-ray powder diffraction analysis. Isothermal map of Curie temperature (T_C) related to the compositions in the phase diagram was obtained. The optimum dielectric and piezoelectric properties were achieved in ceramics with the MPB compositions, with the maxima values being on the order of 6000 and 970pC/N, respectively. Rayleigh analysis was used to study the extrinsic contribution (domain wall motion) in PNN–PHT system, where the extrinsic contribution was found to be ～30% for composition 0.49PNN–0.51PHT(30/70), showing a high nonlinearity.     

Crystal structure of corundum type Mg4(Nb2–xTax)O9 microwave dielectric ceramics with low dielectric loss

New microwave dielectric ceramics, i.e. Mg₄(Nb_2−xTa_x)O₉ (MNT) solid solutions, were synthesized and their microwave dielectric properties and crystal structure were investigated in this study. From the discrete variational Xα (DV-Xα) method, it was found that the Ta–O bonds in the TaO₆ octahedron become more covalent than Nb–O bonds in the NbO₆ octahedron; this result leads to the decrease of the ionicity in the Ta⁵⁺ ion. The dielectric constants of MNT were slightly decreased from 12.4 to 11.5; this result might be due to the covalent interaction of Ta–O bonding. The quality factors of the samples were found to exhibit high value (Q·f≒350 000 GHz for x=2) which is comparable to those of Al₂O₃.     

Microwave dielectric characteristics of 0.75(Al1/2Ta1/2)O2–0.25(Ti1−xSnx)O2 ceramics

The microwave dielectric characteristics of 0.75(Al_1/2Ta_1/2)O₂–0.25(Ti_1−xSn_x)O₂ ceramics were investigated. The microwave dielectric properties of 0.75(Al_1/2Ta_1/2)O₂–0.25TiO₂ sintered at 1450 °C exhibited a dielectric constant (ϵ_r) of 31.2, a Q·f₀ of 54,590 GHz, and the temperature coefficient of resonant frequency (τ_f) of +12.8 ppm/°C. To control of the τ_f and enhance the Q·f₀ for 0.75(Al_1/2Ta_1/2)O₂–0.25TiO₂, Sn⁴⁺ was substituted for Ti⁴⁺. With an increase of Sn content from 5 to 50 mol%, the ε_r slightly decreased, the Q·f₀ increased and the τ_f shifted from positive to negative value. The τ_f within ±10 ppm/°C of zero was realized for the Sn content below 30 mol% and the microwave dielectric properties had the ε_r value of 31.2–26.3, the Q·f₀ of 54,600–70,700 GHz, and τ_f of +12.8–−9.3 ppm/°C for this compositions. The relationship between microstructure and microwave dielectric characteristics was investigated.