The study of Ca-doped BCTZ ceramics sintered in reducing atmosphere

Effects of Ca²⁺ doped into (Ba_1???xCa_x)(Ti_1???yZr_y)O₃ (BCTZ) ceramics on microstructure and insulation resistance have been investigated. It is found that the shift of Ca²⁺ from A-sites to B-sites greatly influences the unit cell volume and the Curie temperature of BCTZ ceramics. During the firing process in reducing atmosphere, the large amounts of additive Ca²⁺ dispense among the grain boundaries and lead to the restrain of the grain growth. Within the same volume, the specimen with smaller grain size provides more grain boundaries, which helps in providing an efficient diffusion pathway for oxygen during annealing step and results in the improvement of the insulation resistance of the samples.     

Synthesis and sintering properties of (La0.8Ca0.2−x Sr x )CrO3 perovskite materials for SOFC interconnect

Synthesis and sintering properties of the (La_0.8Ca_0.2−x Sr _x )CrO₃ samples doped by two alkaline earth metals in comparison to the doped only by one alkaline earth metal were evaluated by phase analysis, sintering properties, thermal expansion behaviors, and electrical conductivity. The sintered (La_0.8Ca_0.2−x Sr _x )CrO₃ (x = 0, 0.05, and 0.1) and (La_0.8Ca_0.2−x Sr _x )CrO₃ (x = 0.2) were found to have orthorhombic and rhombohedral symmetries, respectively. Relative density of the (La_0.8Sr_0.2)CrO₃ sample sintered at 1500^∘C for 5 h was lower than that of the (La_0.8Ca_0.2−x Sr _x )CrO₃ (x = 0, 0.05, and 0.1) sample. TECs of the (La_0.8Ca_0.2−x Sr _x )CrO₃ (x = 0, 0.05, 0.1, and 0.2) in air were 11 × 10⁻⁶/^∘C, 11.2 × 10⁻⁶/^∘C, 11.2 × 10⁻⁶/^∘C, and 11.3 × 10⁻⁶/^∘C, respectively. The electric conductivity of the (La_0.8Ca_0.2−x Sr _x )CrO₃ sample was determined.     

Magnetization and Electric Properties of Pr-doped ZnO

Magnetic susceptibility of ZnO-varistors doped with Pr-ion was measured to elucidate the valence state of the Pr-ion and the effect of thermal treatment. Magnetic susceptibility _χ(T) of the Pr-ion obeyed Curie-Weiss law, from which its valence state was estimated. The valence state of the Pr-ion varied with annealing condition of specimens, and it was indicated that the Pr-ion in ZnO ceramics was more reducible in comparison with pure \hboxPrO_x ceramics. Nonlinear current-voltage characteristics was diminished with the reduction of the valence state of Pr ions. According to Auger electron spectra, the Pr-ions segregated at grain boundaries in ZnO ceramics.     

Ionic conduction and crystal structure of aluminum doped NASICON-type LiGe<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript> glass-ceramic crystallized at different times and temperatures

In this communication, NASICON-type glass-ceramic (lithium germanium phosphate, LiGe₂(PO₄)₃) was prepared as lithium super ionic conductor using aluminum as dopant for ionic conduction improvement. The solid solution was Li_1?+?xAl_xGe_2-x(PO₄)₃ (x?=?0.5) that Ge⁴⁺ ions were partially substituted by Al³⁺ ions in crystal structure. Initial glasses were converted to glass-ceramics at different times and temperatures for maximum ionic conduction achievement. The crystals were characterized by X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Energy-Dispersive X-ray spectroscopy (EDX), Differential Scanning Calorimetry (DSC) and Complex Impedance Spectroscopy (CIS) methods. The maximum lithium ion conductivity for glass-ceramic, 5.32?×?10^?3 S/cm at 26 °C was obtained for specimen crystallized at 850 °C for 8 h with minimum activation energy of 0.286 eV. Increasing the crystallization temperature results in secondary phase formation in grain boundary and increasing in crystallization time results in microcracks formation in specimen. Both phenomena decreased the ionic conductivity.     

Effect of La3+ substitution on microwave dielectric properties of (Pb0.5Ca0.5)(Fe0.5Nb0.5)O3 ceramics

Microwave dielectric properties of the [(Pb_0.5Ca_0.5)_1?x La_2x/3](Fe_0.5Nb_0.5)O₃ and [(Pb_0.5Ca_0.5)_1?x La _x ](Fe_0.5Nb_0.5)O₃ ceramics were investigated as a function of La³⁺ content $ {\left( {0.0 \leqslant \times \leqslant 0.2} \right)} $ . A single perovskite phase was detected in [(Pb_0.5Ca_0.5)_1?x La_2x/3](Fe_0.5Nb_0.5)O₃, while Pb₃Nb₄O₁₃ were detected as a secondary phase in [(Pb_0.5Ca_0.5)_1?x La _x ](Fe_0.5Nb_0.5)O₃ beyond x?=?0.05 due to the excess of unbalanced charge. The amount of Pb₃Nb₄O₁₃ was proportional to the unbalanced charge. Qf value of [(Pb_0.5Ca_0.5)_1?x La_2x/3](Fe_0.5Nb_0.5)O₃ decreased remarkably with La³⁺ substitution due to the increase of oxygen vacancy. For [(Pb_0.5Ca_0.5)_1?x La _x ](Fe_0.5Nb_0.5)O₃ ceramics, dielectric constant and Qf value increased with La³⁺ content up to x?=?0.03 due to an increase of density and grain size. Temperature coefficient of resonant frequency (TCF) was depended on B-site bond valence in single perovskite phase.     

Fractal Approach to ac Impedance Spectroscopy Studies of Ceramic Materials

A novel fractal model for grain boundary regions of ceramic materials was developed. The model considers laterally inhomogeneous distribution of charge carriers in the vicinity of grain boundaries as the main cause of the non-Debye behaviour and distribution of relaxation times in ceramic materials. Considering the equivalent circuit the impedance of the grain boundary region was expressed. It was shown that the impedance of the grain boundary region has the form of the Davidson–Cole equation. The fractal dimension of the inhomogeneous distribution of charge carriers in the region close to the grain boundaries could be calculated based on the relation d _s = 1 + , where is the constant from the Davidson–Cole equation.     

Ni–Mn–Fe–Cr–O negative temperature coefficient thermistor compositions: Correlation between processing conditions and electrical characteristics

A study has been carried out to correlate the effect of sintering temperature on the microstructural, electrical and reliability aspects of Ni_0.75Mn_{(2.25−x−y)}Cr _x Fe _y O₄ (x = 0 to 0.3 and y = 0 to 0.3) negative temperature coefficient thermistor compositions prepared by solid-state route. The calcined and sintered compositions were characterized by X-ray diffraction and Scanning Electron Microscopy. The existence of cubic spinel single-phase region was determined by sintering Ni_0.75Mn_{(2.25−x−y)}Cr _x Fe _y O₄ samples in air at temperatures 1150 to 1250 °C. X-ray diffraction patterns of samples sintered above 1200 °C shows additional Bragg reflections of a rock salt structured NiO phase besides normal cubic spinel. A maximum B-value of 4044 K was obtained for Ni_0.75Mn_1.95Cr_0.25Fe_0.05O₄ composition at a sintering temperature 1250 °C/3 h. The reliability of the thermistor compositions were evaluated by performing accelerated ageing based on thermal cycling test. We found that chromium enhances the reliability of Ni_0.75Mn_{(2.25−x−y)}Cr _x Fe _y O₄ (x = 0 to 0.3 and y = 0 to 0.3) based NTC thermistor compositions. A maximum reliability of +0.25% resistance drift was observed at sintering temperature 1200 °C for 0.25 mol% chromium content. Excellent reliability of Ni_0.75Mn_{(2.25−x−y)}Cr _x Fe _y O₄ NTC thermistor compositions makes it ideal candidates for high-performance thermal sensor applications.     

Impedance spectroscopy of Ba4Ti13O30 film prepared by laser chemical vapor deposition

The impedance and electrical conductivity of Ba₄Ti₁₃O₃₀ film prepared by laser chemical vapor deposition were investigated as functions of temperature (300–1073?K) and frequency (10²–10⁷?Hz). The impedance response was resolved into two contributions, which were associated with the grains (R _g C _g) and grain boundaries (R _gb C _gb). With increasing temperature, both R and C decreased. R _gb was much higher than R _g and C _g were close to C _gb. Below 876?K, a step-like behavior was observed in the ac conductivity plots as functions of frequency due to two dispersion regions. The low-frequency dispersion was associated with grain boundaries and the high-frequency one corresponded to grains. The activation energies of grain conduction (E _g) and grain boundary conduction (E _gb) were 0.20 and 1.49?eV, respectively, which indicated that the relaxation process in grains arose from hopping electron and that in grain boundaries from the diffusion of oxygen vacancies.     

Effect of Ca co-dopant on the electrical conductivity of Gd-doped ceria

Co-doped ceria of Ce_0.8Gd_0.2?x Ca _x O_2-δ (x?=?0?0.2), were prepared by oxalate co-precipitation method. Their structures and conductivities were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and AC impedance spectroscopy (IS). All the electrolytes were found to be single phase with cubic fluorite structure. SEM cross-section image showed relatively uniform grains with distinct and clean grain boundaries. The chemical states of the surface of the prepared samples were analyzed by XPS. Though Gd and Ca were present in their characteristic chemical state, Ce was found in single Ce⁴⁺ state or in mixed Ce⁴⁺ and Ce³⁺ states. IS measurements indicated that the conductivities for Ce_0.8Gd_0.2–x Ca _x O_2-δ pellets increased with increasing the sintering temperature. Moreover, co-doping with appropriate ratio of gadolinium and calcium was found to effectively enhance the conductivity in comparison to the singly doped ceria. The isothermal conductivity plots showed that sample Ce_0.8Gd_0.1Ca_0.1O_2-δ had the maximum conductivity with minimum activation energy (σ _700°C?=?0.0742?S/cm, Ea?=?0.58?eV), which is much higher compared to the conductivity exhibited by most of the reported codoped ceria compositions.     

Unique Growth and Optical Behaviors of Chromium Doped ZnSe Nanoparticles Induced by ns Laser Pulse

We have synthesized chromium doped ZnSe nanocrystalline particles (Cr²⁺:ZnSe NCPs) by using nanosecond pulsed laser ablation of polycrystalline Cr²⁺:ZnSe micron-sized powder in liquid environment. The scanning electron micrscope and X-ray diffraction results reveal that the products are ZnSe cubic sphalerite structure with an average size around 60 nm. Based on the Cr²⁺:ZnSe nanoparticles, typical random laser emissions centering at 2175 nm with a threshold of 0.4 mJ/pulse were observed. Compared to the Cr²⁺:ZnSe bulk laser, the central wavelength shows a ～175 nm blue-shift. The photoluminescence lifetime of Cr²⁺:ZnSe NCPs is shorter than that of Cr²⁺:ZnSe bulk.     

Investigation on the design and synthesis of new systems of BNT-based lead-free piezoelectric ceramics

Three primary differences between BNT- and PZT-based ceramics were analyzed from the composition and the active component of the materials. Based on the analysis the authors’ group developed the new idea of the design of the multiple complex in the A-site ions of BNT compounds. (Bi_0.5Na_0.5)²⁺, Bi³⁺ and Na⁺ in the ABO₃ structure are defined as A-site, A₁-site and A₂-site ions, respectively, and A, A₁ and A₂-site ions can be simultaneously or singly substituted partially by alkaline-earth metal ions, metal ions with +3 valence and metal ions with +1 valence, respectively. Under this consideration, Several new systems of Bi_0.5Na_0.5TiO₃ (abbreviated as BNT)-based lead-free piezoelectric ceramics were proposed. These ceramics can be prepared by conventional ceramic techniques and have excellent piezoelectric performance. Among these materials, Bi_0.5(Na_1−x−y K _x Li _y )_0.5TiO₃ possesses higher piezoelectric constant (d ₃₃ = 230 pC/N), higher electromechanical couple factor (k _p = 0.40), larger remanent polarization (P _r = 38.9 μC/cm²) and a better P-E hysteresis loop until about 200^∘C. This work was supported by the projects of NSFC (50410179), (50572066), and (59972020), and NAMMC (2001-AA325060).     

Statistical inference for ratiometric imaging of excitable cells: a self‐organizing state‐space model

Concentrations of free intracellular calcium ions ([Ca²⁺]_i) in excitable cells are often measured using indicator dyes, such as fura‐2. Of note, however, these indicator dyes are divalent metal ion chelators that affect intrinsic changes in [Ca²⁺]_i. To examine whether indicator dyes alter calcium signaling, we estimated [Ca²⁺]_i using a novel statistical inference method that examines fluorescence signals and the calcium current through the cell membrane. We first constructed a model of transient [Ca²⁺]_i, which was then translated into a state‐space model with such state variables as [Ca²⁺]_i, endogenous calcium buffer, and calcium indicators. Then, a self‐organizing state‐space model was defined by augmenting a state vector with unknown parameters from the original state‐space model. In the model, some unknown parameters were estimated with the original state vector. Next, we used a recursive Bayesian estimation to obtain a set of state vectors and the unknown parameters associated with a set of observation vectors. To calculate the recursive Bayesian estimation, we used a sequential Monte Carlo method, which is referred to as a particle filter method. To verify the effectiveness of the proposed method, we carried out experiments with a set of test data from a model with known parameters. The results show that our proposed method properly estimated the temporal profiles of [Ca²⁺]_i, the indicator dye concentration, and certain model parameters in a noisy environment. © 2012 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Electrical characterization of TiO2-based ceramics for VOCs

Cr doped TiO₂ and TiO₂/Cr₂O₃ powders are prepared by sol–gel technique and solid state reaction respectively. The structural and electronic properties of these powders are investigated. The responses of the materials made from the powders to benzene and formaldehyde vapors are studied. The research results show that Cr³⁺ can enter into the lattice as substitutional metal dopant by sol gel route at lower temperature. The segregation of Cr₂O₃ happens when Cr content exceed 8 at.%. The resistance of TiO₂ can be remarkable decreased by doping Cr³⁺ as substitutional metal dopant. Besides Cr³⁺, the segregation of Cr₂O₃ also contributes to the alteration of conduction from n- to p-type. With some other catalyzer, both of Cr doped TiO₂-based and Cr₂O₃/TiO₂-based ceramics can detect benzene and formaldehyde vapors at 21 ppm.     

Electrical conductivity of Nd1−x Ca x Sc1−y Mg y O3 perovskite-type oxides

The electrical conductivities of Nd_1−x Ca _x Sc_1−y Mg _y O₃ were measured in the temperature range from 673 to 1273 K under both wet and dry conditions. The optimum Ca²⁺ doping concentration to the Nd³⁺ site in NdScO₃ was found to be the range from 5 to 10%. Hole conduction was predominant under dry and highly oxidized conditions of P(O₂) > 10⁻² kPa in the temperature range from 973 to 1273 K, and proton conduction was predominant under wet and reduced conditions in the temperature range from 673 to 973 K. Oxide ion conduction was predominant under wet and reduced conditions in the temperature range from 1073 to 1273 K. The Mg²⁺ doping concentration limit for the Sc³⁺ site to hold a single crystal phase was 2%, and this doping enhanced hole conduction.     

Normal-to-relaxor ferroelectric phase transition and electrical properties in Nb-modified 0.72BiFeO<Subscript>3</Subscript>-0.28BaTiO<Subscript>3</Subscript> ceramics

Preparation and electrical properties of sintered bodies consisting of monophase cubic spinel oxides, Cr_XMn_1.5Co_(1.0-X)Ni_0.5O₄ (0 ≦ X ≦ 0.42), were investigated. Specimens with compositions within X = 0.42 were prepared as starting materials. The element of Cr was used to exchange Co³⁺ in octahedral sites (B sites) with Cr³⁺ so that the hopping mechanism can be discussed. The sintered bodies with mono cubic spinel structure were confirmed to be prepared by heat-treatment for 48 h in air at 1000 °C to convert them into a cubic spinel structure after sintering at 1400 °C. The semiconductive characteristics of the sintered bodies were determined as p-type because the Seebeck coefficients were all positive. The electrical conduction of the sintered bodies was concluded to be controlled by the small polaron hopping mechanism. In the region 0.1 ≦ X ≦ 0.42, the lattice constant increases and electrical conduction (σ) decreases linearly with increasing Cr concentration. The decrease in σ and the increase in the lattice constant corresponded to the increase in Cr concentration by which the jumping distance of electrons between Mn³⁺ and Mn⁴⁺ is lengthened.     

Interface mediated transport properties in n-type SrTiO3 : Induced dipole alignment at oxide grain boundaries

Low temperature potentiometry and capacitance measurements based on noncontact atomic force microscopy were used to quantify local properties due to grain boundaries at a 0.05 wt.% Nb-doped SrTiO₃ [001] surface. Local I-V curves were constructed by combining potential steps and transport currents measured at individual grain boundaries (GBs) under different lateral biases. The GBs exhibit a positive temperature coefficient of resistivity (PTCR) effect. A comparison of transport properties and calculations suggest that SrTiO₃ grain boundaries undergo a non-polar to polar state phase transition induced by the large electric field associated with the boundary charge. This is supported by the temperature dependence of the barrier height and the boundary charge obtained by numerical simulation of I-V curves using a double Schottky barrier model. The built-in potential associated with the boundary was directly imaged with frequency-modulated Kelvin probe force microscopy at different temperatures and the results support the previous conclusion.     

Impedance spectroscopy of acceptor-doped CaZrO3 with cation nonstoichiometry

The electrical conductivities of acceptor-doped Ca_1−x Zr_0.99M_0.01O_3−δ (M = Mg²⁺, In³⁺) systems have been investigated as a function of cation nonstoichiometry (0 ≤ x ≤ 0.05). The electrical characterization was carried out using impedance spectroscopy at different temperatures in dry air. The contributions of grain (R _g) and grain boundary resistance (R _gb ) to the total resistance (R _t) were examined with the impedance and/or modulus plane representation. For the stoichiometric composition, the R _t of the specimen decreased with acceptor doping. When Ca deficiency was small, both the R _g and the R _gb slightly increased with nonstoichiometry (x). However, when Ca deficiency was large and excess zirconia was found as a second phase, the R _gb and the contribution of the R _gb to the R _t significantly increased with x.     

Dielectric,ferroelectric, and piezoelectric properties of lead-free Ba0.95Ca0.05Ti1-xZrxO3 ceramics

Abstract

Ba_0.95Ca_0.05Ti_1-xZr_xO₃ (BCTZO) ceramics were prepared by a solid state reaction method. The samples were characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM) and X-ray absorption near edge structure (XANES). The ceramics exhibit a pure perovskite structure. The average grain size gradually decreases with increasing Zr concentration. XANES results indicate that the intensities of pre-edge peaks dropped with increasing Zr concentration. The BCTZO ceramic of x?=?0.05 has the optimum electrical properties with the maximum dielectric constant (ε'_m), remanent polarization (2P_r), coercive electric field (2E_c) and piezoelectric charge constant (d₃₃) of 7,244, 12.54 (μC/cm²), 5.29 (kV/cm) and 288 (pC/N), respectively.     

Dielectric Properties and Sintering Characteristics of CaTiO<Subscript>3</Subscript>-(Li<Subscript>1/2</Subscript>Nd<Subscript>1/2</Subscript>)TiO<Subscript>3</Subscript> Ceramics

The dielectric properties and the sintering effect upon microstructure of (1–x) CaTiO₃-x(Li_1/2Nd_1/2)-TiO₃ Ceramics are investigated in this paper. Nd³⁺ and Mg^{2 +} ions co-substitution for Ca^{2 +} on A site improves the sintering characteristic of CaTiO₃ ceramics with forming orthorhombic perovskite structure. The structure of (1 – x) CaTiO₃-x(Li_1/2Nd_1/2)TiO₃ changes from orthorhombic to tetragonal as (Li_1/2Nd_1/2)TiO₃ addition increasing. Limited solubility of (Li_1/2Nd_1/2)TiO₃ in CaTiO₃ forming a part solid solution compound achieves the adjustment of for CaTiO₃ at low sintering temperature. The proper dielectric properties with = 78, tan = 0.0006, = +7 ppm/C are obtained for 0.8Ca_0.67(Nd,Mg)_0.22TiO₃-0.2(Li_1/2Nd_1/2)TiO₃ ceramics.