Preparation and properties of ceramics based on (Na0.5Bi0.5)1 ? xAxTiO3 (A = Sr, Cd) solid solutions

This paper describes the preparation and properties of (Na_0.5Bi_0.5)_{1 − x} A _x TiO₃ (A = Sr, Cd) lead-free ferroelectric ceramics. The (0.63–0.66)Na_0.5Bi_0.5TiO₃ · (0.37–0.34)Sr_0.7Bi_0.2TiO₃ ceramics are shown to have the best dielectric and piezoelectric properties: tanδ = 0.013–0.009, ɛ_RT = 1200–1500, d ₃₁ = (370–400) × 10⁻¹² C/N, and k _ρ = 0.4–0.58. The cadmium-containing ceramics offer low dielectric losses and high dielectric permittivity at room temperature.     

Production and dielectric properties of lead-free ceramics with the formula [(Na0.5K0.5)1 ? xLix](Nb1 ? y ? zTaySbz)O3

The influence of past history on the possibility of obtaining high-density ferroelectric ceramics of solid solutions [(Na_0.5K_0.5)_{1 − x} Li _x ](Nb_{1 − y − z} Ta _y Sb _z )O at x = 0–0.14, y = 0, 0.1, 0.2, and z = 0–0.1 is identified. The dynamics of behavior of materials under a field in the process of polarization is evaluated. The relative dielectric permeability of samples in a wide range of temperatures (20–700°C) and frequencies (25 Hz-1 MHz) of a variable electric field is studied. On the basis of study of dielectric, piezoelectric, and elastic properties of solid solutions, prospective objects for further technological elaboration and modification of chemical composition with the goal of improving the electrophysical parameters are selected. Original Russian Text ? I.A. Verbenko, O.N. Razumovskaya, L.A. Shilkina, L.A. Reznichenko, K.P. Andryushin, 2009, published in Neorganicheskie Materialy, 2009, Vol. 45, No. 6, pp. 762–768.     

Structural, spectroscopic and microwave characterizations of (Sm0.5Y0.5)Ti(Nb1?xTax)O6 ceramics

Microwave dielectric ceramics of the type (Sm_0.5Y_0.5)Ti(Nb_1−x Ta _x )O₆ were prepared for x = 0, 0.2, 0.4, 0.6, 0.8 and 1 through the conventional solid state ceramic route. The ball-milled compositions were calcined at 1,250 °C. The cylindrical pellets were sintered at temperatures between 1,385 and 1,450 °C. The densities were determined by Archimedes method. The structure was analyzed using X-ray diffraction method. The microwave dielectric properties of the polished pellets were measured using cavity resonator method. The morphological studies were done using Scanning Electron Microscopy and Transmission Electron Microscopy. The Raman spectra were recorded and analyzed to confirm the structural change. The photoluminescence spectra were also taken and the emission lines were identified. A correlation study was done among the measured properties and parameters. Most of the samples have high dielectric constant, high quality factor and low temperature coefficient of resonant frequency and hence suitable for microwave applications.     

Construction of new morphotropic phase boundary in 0.94(K0.4?xNa0.6BaxNb1?xZrx)O3–0.06LiSbO3 lead-free piezoelectric ceramics

0.94(K_0.4−x Na_0.6Ba _x Nb_1−x Zr _x )O₃–0.06LiSbO₃ ceramics were prepared by conventional technique, and the effect of BaZrO₃ on the phase transitions, dielectric, ferroelectric, and piezoelectric properties of the ceramics were investigated. The phase transitions for the ceramics were determined by the temperature dependence of dielectric properties and X-ray diffraction patterns. BaZrO₃ changes the symmetry of the ceramics from tetragonal dominant phase with x = 0–0.06 to rhombohedral phase with x = 0.07–0.09. The phase transition near room temperature for the composition with x ~ 0.06 is different from previously reported phase transition between orthorhombic and tetragonal phases. It is suggested that a new morphotropic phase boundary (MPB) is constructed with both rhombohedral–orthorhombic and orthorhombic–tetragonal phase transitions near room temperature, and the enhanced piezoelectric properties (d ₃₃ = 344 pC/N and k _P = 32.4% with x = 0.06) are obtained. The results indicate that the construction of new MPB is of significance for further development of KNN-based ceramics.     

Preparation and thermophysical properties of (NdxGd1?x)2Zr2O7 ceramics

Ceramic powders of (Nd _x Gd_1−x )₂Zr₂O₇ (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0) were synthesized by chemical-coprecipitation followed by calcination method, and were then pressureless-sintered at 1,600 °C for 10 h in air. Phase constituents and morphologies of the synthesized powders and sintered ceramics were identified by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). A high-temperature dilatometer and a laser-flash method were used to analyze the thermal expansion coefficient and thermal diffusion coefficient of different ceramics from room temperature up to 1,400 °C. Thermal conductivity was calculated from thermal diffusivity, density, and specific heat. (Nd _x Gd_1−x )₂Zr₂O₇ (0.1 ≤ x ≤ 1.0) ceramics are with a pyrochlore-type structure; however, pure Gd₂Zr₂O₇ exhibits a defective fluorite-type structure. The average linear thermal expansion coefficients of different (Nd _x Gd_1−x )₂Zr₂O₇ ceramics decrease with increasing the value of x from 0 to 1.0 in the temperature range of 25–1,400 °C. The thermal conductivities of (Nd _x Gd_1−x )₂Zr₂O₇ ceramics are located within the range of 1.33 to 2.04 W m⁻¹ K⁻¹ from room temperature to 1,400 °C.     

Microstructure and Young’s modulus of high-pressure LixNa1 ? xTayNb1 ? yO3 ceramics

The microstructure and elastic properties of ceramic samples of high-pressure Li _x Na_{1 − x} Ta _y Nb_{1 − y} O₃ perovskite solid solutions have been studied for the first time in relation to their composition and the synthesis temperature. The results demonstrate that, with increasing sintering temperature, the Young’s modulus of the Li_0.17Na_0.83Ta _y Nb_{1 − y} O₃ solid solutions decreases, which can be understood in terms of the recrystallization behavior of the disordered solid solutions under high-pressure synthesis conditions.     

Frequency effect on the electrical and dielectric properties of (TlGaS2) 1 ? x(TlInSe2)x (x = 0.005, 0.02) single crystals

The electrical properties (loss tangent (tanδ), real (ɛ) and imaginary (ɛ″) parts of complex dielectric permittivity, and ac conductivity across the layers (σ_ac)) of (TlGaS₂)_{1 − x} (TlInSe₂) _x (x = 0.005, 0.02) layered single crystals have been studied in the frequency range f = 5 × 10⁴ to 3.5 × 10⁷ Hz. The results demonstrate that the dielectric dispersion in the crystals has a relaxation nature. Almost throughout the frequency range studied, their ac conductivity follows the relation σ_ac ∼ f ^0.8, characteristic of hopping conduction through localized states near the Fermi level. The Fermi-level density of states (N _F ), the spread of their energies, the mean hop time τ and distance R, and the concentration of deep traps determining the ac conductivity of the crystals (N _t ) have been estimated. With increasing x in (TlGaS₂)_{1 − x} (TlInSe₂) _x , N _F and N _t increase, while τ and R decrease.     

Characterization of ternary (Na0.5K0.5)1?xLixNbO3 lead-free piezoelectric ceramics prepared by molten salt synthesis method

(Na_0.5K_0.5)_1−x Li _x NbO₃ powders and ceramics were prepared by molten salt synthesis method. Pure perovskite-phase powder was obtained at a low temperature of 740 °C with a grain size of below 800 nm. The effects of the LiNbO₃ on phase transition, microstructure, electrical properties, and temperature stability were investigated. A morphotropic phase boundary was identified. The scanning electron microscopy indicated that the (Na_0.5K_0.5)_1−x Li _x NbO₃ powders and ceramics obtained by the molten salt synthesis method have a relatively uniform particle size and microstructure. The results indicate that these materials are promising candidates for lead-free piezoelectric ceramics for practical applications.     

Self-propagating combustion synthesis of Pb1?xSrxZrO3 (0 ≤ x ≤ 0·20) ceramics and their dielectric properties

Lead strontium zirconate, Pb_1−x Sr _x ZrO₃ (0 ≤ x ≤ 0·20) ceramics, were prepared by novel glycinenitrate self-propagating combustion technique. The crystal structure of the ceramics was investigated as a function of composition via X-ray diffraction (XRD). The XRD patterns obtained on these powders showed the formation of pure orthorhombic phase of lead strontium zirconate without impurities and all the compounds retained the orthorhombic structure. The samples were sintered at 900–1100°C for 2 h. It has been observed that the dielectric constant decreases with increase in strontium content (100 kHz). In all compositions, dielectric constant showed a peak at transition temperature and the magnitude of the peak was found to decrease with strontium doping level.     

Lattice parameters and dielectric properties of (1 ? x)Bi(Mg1/2Ti1/2)O3 · xBiCoO3 perovskite solid solutions

(1 − x)Bi(Mg_1/2Ti_1/2)O₃ · xBiCoO ceramics have been prepared at high pressures (6 GPa) and temperatures (1370–1570 K). Perovskite solid solutions have been obtained in the composition ranges 0 < x < 0.6 and 0.8 < x < 1. The Bi(Mg_1/2Ti_1/2)O₃-based perovskite phase (x < 0.6) has an orthorhombic structure (sp. gr. Pnnm), which persists up to the decomposition temperature of the material. The BiCoO₃-based phase (x > 0.8) has a tetragonal structure (sp. gr. P4mn). In the range 0.6 < x < 0.8, the ceramics consisted of the two perovskite phases. The lattice parameters of the solid solutions with x < 0.6 are linear functions of composition. The dielectric properties of the orthorhombic solid solutions have been studied using impedance spectroscopy. The temperature dependences of dielectric permittivity ɛ′ and dielectric loss tanδ exhibit different behaviors at low and high temperatures. With increasing x, the boundary between these regions shifts from about 450 to 300 K. At high temperatures, both ɛ′ and tanδ rise steeply. The dc conductivity of the solid solutions with x < 0.6 exhibits Arrhenius behavior. The activation energies for charge transport in the ceramics studies are presented.     

Oxidation state of chromium in (Na0.5Bi0.5TiO3)(1?x)(BiCrO3)x solid solution; investigated by XAS and impedance spectroscopy

X-ray absorption spectroscopy (XAS) and impedance spectroscopy have been used to study the oxidation state of chromium in (Na_0.5Bi_0.5TiO₃)_(1−x)(BiCrO₃) _x solid solution. XAS measurements reveal that chromium ion occupies the octahedral site in Na_0.5Bi_0.5TiO₃ (NBT). The increase in chromium content increases the distortion in chromium-oxygen octahedral. No shift in the Cr–K edge was observed with increase in chromium content. The XAS measurements suggest that chromium exists either as +3 or +5 state in (Na_0.5Bi_0.5TiO₃)_(1−x)(BiCrO₃) _x . The impedance measurements show a considerable increase in the electrical conductivity with increase in chromium content. The activation energy for conduction mechanism was found to lie between 0.50 and 0.7 eV for all the samples. These measurements indicate that main contribution to the conductivity is because of oxygen defects generated by the incorporation of chromium at B-site of NBT.     

Sol–gel synthesis, densification, and electrical properties of CuO–B2O3 doped Ba6?3xR8+2xTi18O54 (R?=?Nd) microwave dielectric ceramics

The low-temperature confirmable Ba_6−3x R_8+2x Ti₁₈O₅₄ (BNT, R = Nd, x = 2/3) ceramics were prepared by means of a citrate sol–gel soft-chemical method and the addition of sintering aid. Nano-sized BNT crystallite powders (~80 nm) were successfully synthesized as indicated by transmission electron microscopy and X-ray diffractometry. The powder compacts exhibit enhanced sintering activity and can be well densified at 925 °C with the aid of a small amount of CuO and B₂O₃. Compared to pure BNT ceramics prepared by a solid-state reaction method, not only was the sintering temperature significantly decreased, but also the good microwave dielectric properties of dielectric constant ε_r = 63 and quality factor Q × f = 5200 GHz were maintained. Moreover, the relationship between the microstructure, densification, and electrical properties was discussed.     

The morphotropic phase boundary and electrical properties of (1?? x )Pb(Zn1/2W1/2)O3– x Pb(Zr0.5Ti0.5)O3 ceramics

Ceramics in the solid solution of (1 − x)Pb(Zn_1/2W_1/2)O₃–xPb(Zr_0.5Ti_0.5)O₃ system, with x = 0.80, 0.85, 0.90, and 0.95, were synthesized with the solid-state reaction technique. The perovskite phase formation in the sintered ceramics was analyzed with X-ray diffraction. It shows that the rhombohedral and the tetragonal phases coexist in the ceramic with x = 0.90, indicating the morphotropic phase boundary (MPB) within this pseudo-binary system. Dielectric and ferroelectric properties measurements indicate that the transition temperature decreases while the remanent polarization increases with the addition of Pb(Zn_1/2W_1/2)O₃. In the composition of x = 0.85 which is close to the MPB in the rhombohedral side, a high piezoelectric property with d ₃₃ = 222 pC/N was observed.     

Dielectric properties and electrical conductivity of LixNa1?xTa0.1Nb0.9O3 ferroelectric solid solutions

We have studied the dielectric properties and electrical conductivity of Li _x Na_{1 − x} Ta_0.1Nb_0.9O₃ (x = 0.03−0.135) ferroelectric solid solutions at temperatures from 290 to 700 K and frequencies from 25 to 10⁶ Hz. The results demonstrate that charge transport in these materials is due to the Li⁺ ion and that their conductivity is dominated by volume ion transport. In the temperature range studied, the Li _x Na_{1 − x} Ta_0.1Nb_0.9O₃ solid solutions undergo a first-order ferroelectric phase transition close to second order. Increasing the lithium content enhances features characteristic of second-order transitions.     

Stabilization of the anatase phase of Ti1?xSnxO2 (x < 0.5) nanofibers

We experimentally investigate the stabilization of the anatase phase of Ti_1−x Sn _x O₂ (x < 0.5) nanofibers when synthesized by an electrospinning method. The as-spun nanofibers became nano-grained, polycrystalline nanofibers after calcination and the diameters of the nanofibers depend on Sn content. Stabilization of the anatase phase in Ti-rich compositions and incorporation of Sn ions were confirmed by X-ray diffraction, Raman, X-ray absorption near-edge structure, and photoluminescence (PL) spectroscopies. Results from the PL study also demonstrated the tunable nature of the optical properties, with the emission maximum shifting towards higher wavelength with increasing Sn concentration.      

Electrical properties of (K0.5Na0.5)1? x Ag x NbO3 lead-free piezoelectric ceramics

(K_0.5Na_0.5)_1−x Ag _x NbO₃ lead-free piezoelectric ceramics have been fabricated by an ordinary ceramic technique. The results of XRD reveal that Ag⁺ diffuses into the K_0.5Na_0.5NbO₃ lattices to form a new solid solution with an orthorhombic perovskite structure and the solubility of Ag⁺ into A-sites of K_0.5Na_0.5NbO₃ is about 0.20. The ceramics can be well-sintered at 1,100–1,110 °C. The partial substitution of Ag⁺ for A-site ion (K_0.5Na_0.5)⁺ decreases slightly both paraelectric cubic-ferroelectric tetragonal (T _C) and ferroelectric tetragonal-ferroelectric orthorhombic phase transition temperatures (T _O−T). The ferroelectricity of the ceramics becomes weak at high Ag⁺ concentration. The ceramic with x = 0.10 possesses optimum electrical properties: d ₃₃ = 135 pC/N, k _P = 0.43, k _t = 0.46, ε _r = 470, tanδ = 3.39%, and T _C = 394 °C.     

Microwave properties of the systems Ba1?xSrx(Zn1/3Ta2/3)0.94T{i}0.06 O3 and Ba(Zn1/3Nb2/3)1?xZrxO3

By means of a solid-phase synthesis two types of microwave dielectric materials are obtained as follows: Ba_1–xSr_x(Zn_1/3Ta_2/3)_0.94Ti_0.06O₃, where x = 0.20, 0.25, 0.30, 0.35 and 0.40 at sintering temperature T_S = 1350, 1400 and 1450 C; Ba(Zn_1/3Nb_2/3)_1–xZr_xO₃, where x = 0.04, 0.06, 0.08 and 0.10 at T_S = 1300, 1350, and 1400 C. The microwave characteristics of the materials are investigated at f = 10 GHz. The composition Ba(Zn_1/3Nb_2/3)_1–xZr_xO₃ demonstrates _r = 38, Q = 6100 and _f = +15 ppm C^–1 and the composition Ba_0.80Sr_0.20- (Zn_1/3Ta_2/3)_0.94Ti_0.06O₃ has _r = 42, Q = 8200 and _f = –13 ppm C^–1. The composition Ba_0.75Sr_0.25(Zn_1/3Ta_2/3)_0.94Ti_0.06O₃ has _r = 40, Q = 6500 and low _f = –13 C^–1 ppm. This composition could be used successfully for realisation of dielectric microwave resonators for the satellite television.     

Temperature dependence of dielectric and piezoelectric properties of (1?x)(BiScO3–0.64PbTiO3)–xLiNbO3 high-temperature relaxor ferroelectric ceramics

(1−x)(0.36BiScO₃–0.64PbTiO₃)–xLiNbO₃ (BSPT64–xLN) high-temperature relaxor ferroelectric ceramics near the morphotropic phase boundary (MPB) composition were investigated. X-ray diffraction showed a change in symmetry from MPB phase to rhombohedral phase with LiNbO₃ content increasing. A change from normal ferroelectric features to relaxor ferroelectric features was observed with LiNbO₃ substitution up to x = 0.06, while high-temperature dielectric relaxation was exhibited at T _max ~230 to 383°C for 0.02 ≤ x ≤ 0.06. The BSPT64–xLN ceramics with x = 0.02 LiNbO₃ exhibited good piezoelectric properties compared with BS–0.64PT ceramics: piezoelectric coefficients d ₃₃ = 505pC/N, planar electromechanical coupling factors k _p = 0.47, and remnant polarization P _r = 40 μC/cm², respectively. The annealing temperature dependence of piezoelectric response for BSPT64–xLN ceramics was measured and it was found that the piezoelectric properties decreased slightly before 210°C for x = 0.02 and 0.04, showing excellent thermal stability, which indicated that BSPT64–xLN relaxor ferroelectric ceramics can be used in the range of high temperature compared to Pb-based relaxor ferroelectrics.     

Structural stability and sintering properties of Fe/Nb/Ti co-substituted Sr(Co0.8Fe0.1Nb0.1)1?xTixO3?δ (x?=?0.00, 0.20, 0.40) oxides

The novel Fe/Nb/Ti co-substituted Sr(Co_0.8Fe_0.1Nb_0.1)_1−x Ti _x O_3−δ (x = 0.00, 0.20, 0.40) oxides have been synthesized by the solid-state reaction method. These co-substituted strontium cobaltates possess a cubic perovskite-type structure at room temperature. Structural stability and sintering properties of the samples x = 0.00, 0.20, 0.40 were investigated by X-ray diffraction (XRD), thermogravimetry (TG), and scanning electron microscopy. The combined TG and XRD results demonstrate that the structural and chemical stability of the Fe/Nb/Ti co-substituted Sr(Co_0.8Fe_0.1Nb_0.1)_1−x Ti _x O_3−δ (x = 0.20, 0.40) oxides is improved greatly compared with the sample x = 0.00 and the Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ oxide.     

Structure of Sr1 ? xPbxFeO3 ? δ (0 < x < 0.5) perovskite-like materials

We have synthesized Sr_{1 − x} Pb _x FeO_{3 − δ} (x = 0, 0.05, 0.1, 0.15, 0.2, 0.3, 0.5) perovskite-like materials and studied their structure by X-ray diffraction, M?ssbauer spectroscopy, and electron microscopy. According to the X-ray diffraction data, the Pb solubility limit in the perovskite structure is x ≈ 0.15. The materials with x = 0.05 and 0.1 contained Pb_1.33Sr_0.67Fe₂O₅ inclusions 10–30 nm in size. Using chronopotentiometry and temperature-programmed desorption, we have estimated oxygen mobility in the materials with x = 0.05 and 0.1. The results demonstrate that Pb doping increases oxygen mobility in the strontium-ferrite-based materials.