Optimizing the piezoelectric properties of Ba0·85Ca0·15Zr0·10Ti0·90O3·lead-free ceramics via two-step sintering

The two-step sintering of lead-free Ba₀·₈₅Ca₀·₁₅Zr₀·₁Ti_0·9O₃·(BCZT) ceramics was investigated as a way to enhance its piezoelectric properties. The variations in grain size as a function of the calcination and sintering conditions and its effect on performance is discussed. Results indicate that as the calcination and first-step sintering temperatures increased, grain size became large and was independent of the second sintering step. Large grains were responsible for the enhanced piezoelectric properties by causing lattice distortion, larger domains, and easy motion of domain walls. The BCZT ceramic calcined at 1200 °C and sintered at 1540 °C without holding and then cooled to 1400 °C and held at 1400 °C for 4 h exhibited optimal performance with the highest remnant polarization P_r ∼13.5 μC/cm², the largest piezoelectric constant d₃₃ ∼ 529 pC/N at room temperature, and the highest Curie temperature T_c ∼125 °C. Two-step sintering has been turned out to be an effective method to realize high-performance BCZT ceramics by microstructure optimization.     

Electrochemical characterization of BaCe0·7Zr0·1Y0·16Zn0·04O3-δ electrolyte synthesized by combustion spray pyrolysis

BaCe_0·7Zr_0·1Y_0·16Zn_0·04O_3-δ perovskite has been investigated due to its potential as an electrolyte in industrial steam electrolysis applications. The lowest area specific resistance (ASR) is achieved as 4.0 Ω cm² at 711 °C under 3% wet Ar atmosphere. The conductivity is calculated as 2.93 × 10^?2 S cm^?1 and kept stable for a ~70 h testing period. ASR increased at lower temperature (511 °C) under the same atmosphere and a new impedance arc (with 4.5 Ω cm² ASR and 2 × 10^?8 F equivalent capacitance) is formed, indicating second phase formation. No second phase formation is observed at the same temperature under dry 5% H₂ in Ar. The second phase formation/degradation of the electrolyte is attributed to Ba(OH)₂ and CeO₂ formations around 500 °C under wet atmospheres. At elevated temperatures, ~700 °C, BaCe_0·7Zr_0·1Y_0·16Zn_0·04O_3-δ exhibits both excellent protonic conductivity and stability which makes it a great candidate for both industrial fuel cells and steam electrolysers.     

Synthesis and characterisation of La0·95Sr0·05GaO3−δ, La0·95Sr0·05AlO3−δ and Y0·95Sr0·05AlO3−δ

The oxide-ion conducting phases La_0·95Sr_0·05GaO_3−δ (LSG), La_0·95Sr_0·05AlO_3−δ (LSA) and Y_0·95Sr_0·05AlO_3−δ (YSA) have been synthesised, either by solid state reaction or through a chemical route. Structural and microstructural characterisation was carried out using XRD and SEM/EDS techniques and a.c. impedance spectroscopy (250–1000°C) was employed to determine the electrical properties. Although low-temperature impedance results were strongly dependent on phase purity and microstructure, high temperature electrical conductivity data could be used to compare the electrical conductivities of LSG, LSA and YSA. Chemical modification from the well-known Sr-doped lanthanum gallate via replacement of different cations on the A and/or B site decreased the conductivity, increased the activation energies and decreased the ionic conductivities. From the results, there is no simple correlation between oxygen ion conductivity and geometric aspects related to unit cell parameters and cation radii. Instead, cation polarisability seems to play an extremely important role in designing improved oxygen ion conductors based on the perovskite structure.     

Dielectric properties of porous Ba0·997La0·003Ti1·0045O3 ceramics

An oxalate method was used in this work to produce La–BaTiO₃ materials. The obtained powders had small particle size and were disaggregated. However, the sintering process is difficult due to the presence of large pores uniformly distributed in the matrix. Grain size is not uniform, and the type of microstructure obtained suggests that clusters of particles readily sinter and densify, while leaving between them large pores. Impedance spectroscopy was done up 230°C. The variation of the permittivity with temperature showed that lanthanum changes the transition temperature of BaTiO₃, to T_o ∼115°C. One of the resistances of the proposed equivalent circuit was related to grain boundary conductance, since its associated activation energy, 0·72 eV, is too high to be assigned to a donor species. It is proposed that La substituting Ba is compensated by Ba vacancies, which turns the material highly resistive, not showing the PTC effect.     

Effects of rapid thermal annealing treatment on 0·95(Na0·5Bi0·5)TiO3-0·05BaTiO3 thin films

Abstract

Abstract

A 0·95(Na_0·5Bi_0·5)TiO₃-0·05BaTiO₃+1?wt-%Bi₂O₃ (NBT-BT3) ceramic was used as target to deposit the NBT-BT3 thin films. The excess 1?wt-%Bi₂O₃ was used to compensate the vapourisation of Bi₂O₃ during the depositing processes. The optimal deposition parameters were substrate temperature of 500°C, radio frequency power of 100?W, chamber pressure of 10?mtorr and oxygen concentration of 40%. Rapid thermal annealing treated processes were carried out on NBT-BT3 thin films from 600 to 800°C in ambient or in oxygen atmosphere for 1?min. The surface morphologies and thicknesses of NBT-BT3 thin films were characterised by field emission scanning electron microscopy, and the thickness was ～216?nm. It would be shown that the NBT-BT3 thin films annealed in oxygen atmosphere had the smaller grain size, larger memory window, smaller leakage current density, larger remanent and saturation polarisation and smaller coercive field than the NBT-BT3 thin films annealed in ambient atmosphere.     

Effects of sintering temperature on properties of Na0·5K0·5NbO3-LiSbO3-BiFeO3 lead free ceramics

Abstract

Abstract

Lead free 0·95K_0·5Na_0·5NbO₃-0·05LiSbO₃ (KNN-LS) ceramics doped with 0·4?mol.-% BiFeO₃ (BF) have been prepared by the conventional mixed oxide method with sintering temperature at 1065-1135°C in this paper. The samples are characterised by X-ray diffraction analysis and scanning electron microscopy. The dielectric and piezoelectric properties are also investigated. The results present that initially the increase in the sintering temperature is very effective in improving the density and electric properties. However, the properties of the samples would be deteriorated as they are sintered over the optimum temperature. The KNN-LS (doped with 0·4%BF) ceramics shows excellent properties with sintering temperature at 1100°C.     

Preparation and study as positive electrode of Li0·33La0·56TiO3–PANI nanocomposite

Abstract

Abstract

A new electrode material based on the Li_0·33La_0·56TiO₃–polyaniline nanocomposite is reported. This material is prepared by in situ polymerisation of aniline in the presence of Li_0·33La_0·56TiO₃ nanoparticles. The nanocomposite and its precursors are characterised by X-ray diffraction, thermogravimetry, Fourier transform infrared, TEM, SEM, electrical conductivities and electrochemical measurements. The structural and electrochemistry study reveals the existence of relatively strong interactions between the conducting polymer and the oxide particles to assure good synergy in the transport process. The dc and ac electrical conductivities and diffusion coefficient measurements at room temperature indicate that the conductivity values are several orders of magnitude higher in the composite than in the oxide alone. This behaviour determines better reversibility for Li insertion in charge–discharge cycles compared to the pristine oxide and polymer when it is used as electrode of lithium batteries.     

Energy storage properties of MgO-doped 0.5Bi0·5Na0·5TiO3-0.5SrTiO3 ceramics

0.5Bi_0·5Na_0·5TiO₃-0.5SrTiO₃-x wt% MgO (x = 0, 0.5, 1.0, 1.5, 2.0, 3.0) ceramics were fabricated via solid-state method. The effect of MgO doped on energy storage properties, dielectric performance, phase structure and microstructures of 0.5Bi_0·5Na_0·5TiO₃-0.5SrTiO₃ (BNST) ceramics were studied systemically. The Mg²⁺ substituted Ti⁴⁺ site in BNST, which was confirmed by X-ray diffraction (XRD) result. The scanning electron microscope (SEM) images show that all the ceramic samples exhibit uniform and compact morphologies. The temperature dependent permittivity exhibits frequency dispersion, indicating that the ceramic samples are typical relaxor ferroelectrics. It was found that MgO doped BNST can significant improve the breakdown strength (E_b) of samples from 109 kV/cm to 227 kV/cm, which results in a great enhancement on energy storage density. The sample of x = 3.0 has the largest energy storage density (2.17 J/cm³), which is twice as much as the BNST. Consequently, we consider that MgO-doped BNST ceramics are able to be a promising candidate in the field of pulsed-power devices.     

Preparation and properties of (Ba0·6Sr0·4)Bi2Ta2O9 ceramic

(Ba_0·6Sr_0·4)Bi₂Ta₂O₉ (BSBT) ceramic materials with plate-like grains were prepared by the conventional mixed oxide method. The microstructures of BSBT ceramic sintered at different temperatures were studied. The variations of the saturation polarization (P_s), remanent polarization (P_r) and coercive field (E_c) with applied electric field and temperature were studied by hysteresis (D–E loop) measurements. The permittivities, piezoelectric and pyroelectric coefficients of BSBT ceramic poled at various electric fields and temperatures were measured.     

Phase structure and electrical properties of lead free Na0·5Bi0·5TiO3–CaTiO3 ceramics

Abstract

Abstract

(1?x)Na_0·5Bi_0·5TiO_3?xCaTiO₃ ceramics with x?=?0–0·2 were prepared by solid state sintering method. Structural and morphology studies carried out by X-ray diffraction and scanning electron microscopy indicate the change in crystal structure from rhombohedral to orthorhombic symmetry (R3C to Pnma). The morphotropic phase boundary of this system was found to lie around x?=?0·08–0·14, where the orthorhombic and rhombohedral symmetries coexist. The orthorhombic phase is stabilised for x>0·14, indicating that the rhombohedral phase of Na_0·5Bi_0·5TiO₃ is susceptible to orthorhombic distortion brought about by Ca substitution. Calcium substitution in Na_0·5Bi_0·5TiO₃ caused an obvious decrease in peak temperature and a decrease in relative permittivity. The compositional variation of the fundamental dielectric behaviour is discussed in relation to the crystal chemistry of the system. The highest piezoelectric constant d₃₃ of 85 pC N^?1 is achieved for x?=?0·1, with the coercive field of 18 kV cm^?1 and the dielectric maximum temperature of 148°C.     

Humidity sensitive electrical properties of a novel ceramic heterocontact structure ZnO/BaPb0·8Bi0·2O3

The humidity dependence of the voltage-current characteristics of nonsymmetrical and symmetrical heterocontacts ZnO/BaPb_0·8Bi_0·2O₃, ZnO/BaPb_0·8Bi_0·2O₃/ZnO and ZnO/ZnO have been studied at fixed temperature in air with different relative humidity values. Nonlinear characteristics have been observed for all the contacts at any humidity values. The increase in the nonlinearity coefficient for the ZnO/BaPb_0·8Bi_0·2O₃/ZnO contact with respect to the ZnO/ZnO contact, and the voltage-current relationship at forward bias for ZnO/BaPb_0·8Bi_0·2O₃ heterocontacts are attributed to surface states. The increase in relative humidity causes the decrease in the Schottky potential barrier height, thereby resulting in a redistribution of the voltage drop at the heterocontact interface and in the current growth at lower voltages.     

Dielectric behaviour and magnetoelectric effect in Ni0·25Co0·75Fe2O4-Ba0·8Pb0·2TiO3 ceramic composites

Abstract

Dielectric and magnetoelectric properties are reported in magnetoelectric (ME) composites containing lead doped barium titanate as the electrical component and a mixed Ni-Co ferrite as the magnetic component. Changes were observed in dielectric properties as well as in the ME effect as the molar ratio of the components was varied. A maximum value of ME conversion factor of 0·42 mV cm ^{- 1} Oe ^{- 1} (52·78 VA ^{- 1}) was observed in the composite containing 15 mol-% ferrite. Larger ME effects were observed in composites with higher contents of ferroelectric phase. This conclusion differs from earlier theoretical studies which predict a large ME interaction in composites with a 1:1 molar ratio of participating phases. Apart from the technological importance of the ME effect, some anomalies in magnetic phase transition induced by electric fields are also of scientific interest. The variation in dielectric properties with temperature resulted in two maxima, corresponding to the ferroelectric and ferrimagnetic Curie temperatures. This behaviour is unlike anything previously observed in other ME composites.     

Nonstoichiometry and relaxation kinetics of nanocrystalline mixed praseodymium–Cerium oxide Pr0·7Ce0·3O2−x

The oxygen deficiency and kinetics of oxygen uptake and release of nanocrystalline mixed praseodymium–cerium oxide with composition Pr_0·7Ce_0·3O_2−x were investigated by combining coulometric titration and potentiometric measurements using stabilised zirconia oxygen concentration cells. The P(O₂) versus composition isotherms indicate a two-phase region at high P(O₂) [P(O₂)>0·1 bar at 560°C] and a single-phase region at lower P(O₂). The oxygen pressure dependence in the homogeneous region can be described by a power law with an exponent (−1/6), in accordance with doubly charged oxygen vacancies as majority defects. The enthalpy of reduction amounts to (2·9±0·3) eV. The chemical diffusion coefficients are of the order of 10⁻⁶ cm² s⁻¹ at 640°C with an activation energy of ≈0·3 eV. The low activation energy for diffusion may be related to the high density of interface sites in the nanocrystalline material.     

Electrical transport properties of Pr1−xSrxMnO3 (x = 0 to 0·45)

Abstract

Abstract

The electrical transport properties of polycrystalline Pr_1?xSr_xMnO₃ (x = 0 to 0·45) perovskite manganites prepared by the solid state reaction method have been studied. The insulator-metal transition temperature T _im shifted to higher temperatures when Sr content was increased. Generally, the average grain size decreased with increase in Sr content. The resistivity versus temperature curves in the metallic (ferromagnetic) and insulating (paramagnetic) region were fitted with several models. The activation energy of electrons hopping and the density of states at the Fermi level were estimated from the curve fitting. The ρ-T² curve was found to be nearly linear in the metallic region, but the ρ-T^2·5 curve deviated from linearity for x = 0·2 and 0·33. The variable range hopping model and small polaron hopping models fit well in the high temperature region. Sr substitution led to the decrease in the activation energy and the T _o values.     

Impact of crystallisation processes on depth profile formation in sol–gel PbZr0·52Ti0·48O3 thin films

Abstract

Abstract

This study revealed the influence of crystallisation processes on the homogeneity of the sol–gel PbZr_0·52Ti_0·48O₃ thin films, allowing identification and further optimisation of thin film performance. Crystallisation processes determine the optical gradient appearance, irrespective of the chemical solvents used in this work. X-ray diffraction analysis showed that a refractive index gradient was apparent in the samples which had dominant (001)/(100) orientation and significant change of lattice parameters with thickness.     

AC behaviour and conductive mechanisms of 2·5 mol% La2O3 doped PbZr0·53Ti0·47O3 ferroelectric ceramics

Impedance spectroscopy (IS) is an effective characterization method for the investigation of many ceramic systems including the ferroelectric systems. It is employed to analyze the properties of the intragranular and interfacial regions, their interrelations, their temperature and frequency dependences and the DC and AC phenomena in order to separate the individual contributions from the total cell impedance. Using this technique and thermally stimulated current measurements we have studied the ac behavior and the conductive mechanisms in lanthanum-doped lead zirconate titanate, a donor-doped ferroelectric ceramic system. AC and DC conductivity components were found; a Jonscher power-law behavior was obtained and a strong low frequency dispersion was observed in a wide frequency range. A mixture of ionic–electronic conductive behavior was analyzed from the experimental results and the energy activation values obtained. ©     

Preparation and properties of (Ba0·7Sr0·3)TiO3 powders and thin films using precursor solutions formed from alkoxide-hydroxide

(Ba_0·7Sr_0·3)TiO₃ powders and thin films were prepared using alkoxide-hydroxide route. Solutions of Ba and Sr hydroxides dissolved in methanol were reacted with Ti-isopropoxide under conditions of stirring at room temperature for 15 h, and then dried under reduced pressure at ≦40°C to prepare precursors of (Ba_0·7Sr_0·3)TiO₃ powder. The amorphous precursors were hydrolyzed at 100°C by introducing nitrogen gas containing water vapor. The hydrolyzed products were crystalline nanosize powders of (Ba_0·7Sr_0·3)TiO₃. As-hydrolyzed (Ba_0·7Sr_0·3)TiO₃ powders showed a good crystallinity with cubic phase. (Ba_0·7Sr_0·3)TiO₃ thin films were also successfully prepared at 650°C on Pt/Ti/SiO₂/Si substrates from precursor solutions obtained by the reaction of alkoxide with hydroxides. The (Ba_0·7Sr_0·3)TiO₃ thin films exhibited the microstructure with fine grains as small as 20–60 nm. The dielectric constants of the thin films ranged from 600 to 800 at room temperature.     

Compatibility and conductivity of La2Ni1−xFexO4+δ and LaNi0·6Fe0·4O3−δ with GDC electrolyte

The compatibilities and conductivities of K₂NiF₄ typed La₂Ni_0·9Fe_0·1O_4+δ (L₂NF91) and LaNi_0·6Fe_0·4O_3?δ (LNF64) perovskites, promising cathode materials for solid oxide fuel cell, with Gd_0·1Ce_0·9O_1·95 (GDC) electrolyte were investigated. L₂NF91 and LNF64 were synthesised using citrate and modified citrate methods with the calcination temperature of 1000°C for 5 h. The single phased oxides with the average particle sizes of L₂NF91 and LNF64 ～0·2 μm were obtained. The thermal expansion coefficients of L₂NF91 and LNF64 were 12·7×10^?6 and 13·2×10^?6 K^?1 respectively. The mixtures of cathode materials and the electrolytes were heated between 800 and 1200°C to observe the formation of secondary phases at the operation temperatures of solid oxide fuel cell. The X-ray diffraction and scanning electron microscopy–energy dispersive X-ray results indicated that L₂NF91 and LNF64 had good chemical compatibility with GDC from room temperature up to 900°C. Both L₂NF91 and LNF64 showed higher conductivities when in contact with GDC electrolyte than with Zr_0·92Y_0·08O_1·96 electrolyte.     

H400—6.5／0／97离心式压缩机喘振的原因分析

分析离心式压缩机产生喘振的原因，并根据压缩机的运行状况，提出了防喘振的措施，以确保压缩机安全运行。