Temperature‐Dependent Deformation and Dislocation Density in SrTiO3 (001) Single Crystals

This study evaluates the change of flow stress as related to dislocation density in SrTiO₃ single crystals in order to provide guidance for later electrical studies. The key parameters varied are temperature and loading rate during the deformation. It is found that in <100>‐oriented SrTiO₃ single crystals, the dislocation density is enhanced by plastic deformation, more so at higher temperature as compared to room temperature. The experimental approach of quantifying the dislocation density through a determination of ex situ X‐ray diffraction rocking curves was successfully applied over the upper temperatures region of the lower temperature ductility zone for strontium titanate, i.e., in the so‐called “A‐regime”. For 1.0% deformed samples deformed at 300°C, a fourfold increase in dislocation density to 1.4 × 10¹³ m^?1 was found as compared to the nondeformed state (3.7 × 10¹² m^?1). Cross‐section techniques confirmed that the observed dislocation densities measured at the surfaces were identical to those seen in the core of the crystals. The use of rapid changes in loading rate provided an estimate for activation volume of the dislocation core for both 25°C and 300°C.     

Frequency‐Dependent Qf Value of Microwave Dielectric Ceramics

The microwave dielectric properties of some typical low‐loss dielectric ceramics (CaNdAlO₄, Ba(Mg_1/3Nb_2/3)O₃, Ba(Zn_1/3Nb_2/3)O₃, Ba₄Sm_9.33Ti₁₈O₅₄, Ca_0.71Nd_0.26TiO₃ and SrTiO₃) were evaluated at different frequencies by using high‐order TE_0np resonant modes of resonant cavity method. The dielectric constant shows little dependence on frequency for these microwave dielectric ceramics, while the Qf value increases with frequency in the measurement frequency range of 3–14 GHz. The results indicate that the Qf value as a frequency‐independent constant at microwave frequencies is not valid for these microwave dielectric ceramics, while the constant Qf value has been accepted as a common recognition for a long period. The frequency dependence of Qf value is attributed to the defects‐induced extrinsic dielectric loss, which cannot be avoided in actual microwave dielectric ceramics.     

Preparation of Micro‐/Mesoporous SiOC Bulk Ceramics

Micro‐/mesoporous SiOC bulk ceramics with high surface area and bimodal pore size distribution were prepared by pyrolysis of polysiloxane in argon atmosphere at 1100°C–1400°C followed by etching in hydrofluoric acid solution. Their thermal behaviors, phase compositions, and microstructures at different nano‐SiO₂ filler contents and pyrolysis temperatures were investigated by XRD, SEM, DSC, and BET. The SiO₂ fillers and SiO₂‐rich clusters in the SiOC matrix act as pore‐forming sites and can be etched away by HF. At the same time, the SiO₂ filler promotes SiOC phase separation during the pyrolysis. The filler content and pyrolysis temperature have important effects on phase compositions and microstructures of porous SiOC ceramics. The resulting porous SiOC bulk ceramic has a maximum specific surface area of 822.7 m²/g and an average pore size of 2.61 nm, and consists of free carbon, silicon carbide, and silicon oxycarbide phases.     

Structure‐Dependent Microwave Dielectric Properties and Middle‐Temperature Sintering of Forsterite (Mg1–xNix)2SiO4 Ceramics

The crystal structure, microstructure, and microwave dielectric properties of forsterite‐based (Mg_1–xNi_x)₂SiO₄ (x = 0.02–0.20) ceramics were systematically investigated. All samples present a single forsterite phase of an orthorhombic structure with a space group Pbnm except for a little MgSiO₃ secondary phase as x > 0.08. Lattice parameters in all axes decrease linearly with increasing Ni content due to the smaller ionic radius of Ni²⁺ compared to Mg²⁺. The substitution of an appropriate amount of Ni²⁺ could greatly improve the sintering behavior and produce a uniform and closely packed microstructure of the Mg₂SiO₄ ceramics such that a superior Q × f value (152 300 GHz) can be achieved as x = 0.05. The τ_f value was found to increase with increasing A‐site ionic bond valences. In addition, various additives were used as sintering aids to lower the sintering temperature from 1500°C to the middle sintering temperature range. Excellent microwave dielectric properties of ε_r~6.9, Q × f~99800 GHz and τ_f~?50 ppm/°C can be obtained for 12 wt% Li₂CO₃‐V₂O₅‐doped (Mg_0.95Ni_0.05)₂SiO₄ ceramics sintered at 1150°C for 4 h.     

Composition‐Dependent Microstructures and Properties of La‐, Zn‐, and Cr‐Modified 0.675BiFeO3–0.325BaTiO3 Ceramics

Pure and 1.0 mol% La₂O₃, ZnO, and Cr₂O₃‐modified 0.675BiFeO₃–0.325BaTiO₃ (BF–BT) multiferroic ceramics were prepared and comparatively investigated. For pure and La‐, Zn‐, and Cr‐modified BF–BT, the average grain size is 415, 325, 580, and 395 nm, and the maximum dielectric constant temperature is 460°C, 430°C, 465°C, and 445°C, respectively. All additives weaken the ferroelectricity slightly. Zn‐ and Cr‐modifications dramatically enhance the room‐temperature magnetic properties, whereas La‐modification has almost no effect on magnetic property. Especially, the Cr‐modified BF–BT ceramics show switchable polarization and magnetization of 4.9 μC/cm² and 0.27 emu/g at room temperature, the magnetoelectric coupling is confirmed by the magnetization‐magnetic field curves measured on ceramics before and after electric poling. The mechanism responsible for the different effects of additive on microstructures and properties are discussed based on additive‐induced point defect and second phase as well as diffusion‐induced substitution. These results not only provide a promising room‐temperature multiferroic material candidate, but also are helpful to design new multiferroic materials with enhanced properties.     

Low Loss Magneto‐Dielectric Composite Ceramics Ba3Co2Fe24O41/SrTiO3 for High‐Frequency Applications

Magneto‐dielectric composite ceramics Ba₃Co₂Fe₂₄O₄₁/SrTiO₃ (Co₂Z/STO) loading with high volume fraction of hexaferrite Co₂Z ( = 60%–95%) are successfully prepared by a hybrid process. The microstructures with homogeneously dispersed constituent grains are observed in these composites. The composites loading with 60%–80% hexaferrite possess stable magneto‐dielectric properties in the frequency range from 10 MHz to 1 GHz with both low dielectric loss and magnetic loss. For the composite loading with 60% Co₂Z, the relative permittivity is 28.08 and permeability is 4.46 (at 10 MHz), the dielectric loss tangent keeps below 0.009 within the frequency range from 10 MHz to 1 GHz. Also, it possesses a magnetic loss tangent of 0.006 and 0.144 at 50 MHz and 1 GHz, respectively, which are much lower than 0.056 and 1.242 of the single Co₂Z phase at the same frequency. These excellent properties indicate that the low‐loss Co₂Z/STO composite ceramic is a new kind of multifunctional magneto‐dielectric material with potential for high‐frequency electromagnetic device applications.     

Photoluminescence and Temperature Dependent Electrical Properties of Er‐Doped 0.94Bi0.5Na0.5TiO3‐0.06BaTiO3 Ceramics

Er‐doped 0.94Bi_0.5Na_0.5TiO₃‐0.06BaTiO₃ (BNT‐6BT: xEr, x is the molar ratio of Er³⁺ doping) lead‐free piezoceramics with x = 0–0.02 were prepared and their multifunctional properties have been comprehensively investigated. Our results show that Er‐doping has significant effects on morphology of grain, photoluminescence, dielectric, and ferroelectric properties of the ceramics. At room temperature, the green (550 nm) and red (670 nm) emissions are enhanced by Er‐doping, reaching the strongest emission intensity when x = 0.0075. The complex and composition‐dependent effects of electric poling on photoluminescence also have been measured. As for electrical properties, on the one hand, Er‐doping tends to flatten the dielectric constant‐temperature (ε_r‐T) curves, leading to temperature‐insensitive dielectric constant in a wide temperature range (50°C–300°C). On the other hand, Er‐doping significantly decreases the ferroelectric‐relaxor transition temperature (T_F–R) and depolarization temperature (T_d), with the T_F–R decreasing from 76°C to 42°C for x = 0–0.02. As a result, significant composition‐dependent electrical features were found in ferroelectric and piezoelectric properties at room temperature. In general, piezoelectric and ferroelectric properties tend to become weaker, as confirmed by the composition‐dependent piezoelectric coefficient (d₃₃), planar coupling factor (k_p), and the shape of polarization‐electric field (P–E), current‐electric field (J–E), bipolar/unipolar strain‐electric field (S–E) curves. Furthermore, to understand the relationship between the T_F–R/T_d and the electrical properties, the composition of x = 0.0075 has been intensively studied. Our results indicate that the BNT‐6BT: xEr with appropriate Er‐doping may be a promising multifunctional material with integrated photoluminescence and electrical properties for practical applications.     

Temperature‐ and Frequency‐Dependent Properties of the 0.75Bi1/2Na1/2TiO3–0.25SrTiO3 Lead‐Free Incipient Piezoceramic

The dielectric and electromechanical properties of 0.75Bi_1/2Na_1/2TiO₃–0.25 SrTiO₃ (25ST) as a function of temperature and frequency were studied. It is shown that the 25ST is a relaxor ferroelectric as evidenced by the temperature‐dependent dielectric relaxations with an incipient piezoelectricity featured by the presence of a reversible electric‐field‐induced phase transformation at room temperature. The transition occurs on a broad electric field strength range depending on field amplitude and frequency. It is also accompanied by a huge strain that is attributed to repetitive poling and depoling originating due to the reversibility of the phase transition. The 25ST makes an attractive lead‐free candidate for stack actuators as it presents a high normalized d₃₃* of ~600 pm/V at a low electric field of 4 kV/mm for frequencies ranging from 0.1 up to 100 Hz.     

烧成对钛酸锶压敏陶瓷电学性能的影响

本文研究了烧结助剂、烧结温度和烧结气氛对SrTiO3压敏陶瓷材料性能的影响。结果表明：在烧结过程中使用Al2O3和H2SiO3作烧结助剂比用Li2CO3和H2SiO3效果更好；在1420℃烧结的样品与1400℃及1440℃相比，具有较好的综合电性能；烧结过程中，除要保证还原性气氛外，还要有一定的氧分压。     

Temperature‐Dependent Scaling Behavior of Dynamic Hysteresis in Pb(Zr,Ti)O3 Ceramics

The dynamic hysteresis scaling behaviors of Nb‐doped Pb(Zr_0.52Ti_0.48)O₃ ceramics have been investigated as a function of electric field amplitude (E₀) and frequency (f) at different temperatures (T). The loop area <A> of saturated loops is found to follow various power laws as <A> ∝ E₀^0.3065 at fixed f and <A> ∝ f ^0.0120 at fixed E₀. Furthermore, the linear scaling relation <A> = k₃ f^αE₀^β + b₃ is estimated under various temperatures. The exponents α (=0.01) and β (=0.10) are T‐independent, whereas the slopes k₃ and y‐intercepts b₃ are T‐dependent because the increasing temperature in the same phase range only decreases the threshold field of the reversal rather than change the dynamic reversal process.     

High Electric‐Induced Strain and Temperature‐Dependent Piezoelectric Properties of 0.75BF–0.25BZT Lead‐Free Ceramics

0.75BiFeO₃–0.25Ba(Zr_xTi_1?x) + 0.6 wt% MnO₂ (0.75BF–0.25BZT) ceramics with Mn addition were prepared by the solid‐state reaction method. The high‐field strain and high‐temperature piezoelectric properties of 0.75BF–0.25BZT ceramics were studied. Introduction of Zr in the solid solutions decreased the Curie temperature slightly, and improved the dielectric and piezoelectric properties obviously. The piezoelectric properties of 0.75BZT–0.25BT ceramics reached the maximum at Zr content of 10 mol%. The Curie temperature T_c, dielectric constant ε and loss tanδ (1 kHz), piezoelectric constant d₃₃, and planner electromechanical coupling factor k_p of 0.75BF–0.25BZT ceramics with 10 mol% Zr were 456°C, 650, 5%, 138 pC/N, and 0.30, respectively. The high‐field bipolar and unipolar strain under an electric field of 100 kV/cm reached up to 0.55% and 0.265%, respectively, which were comparable to those of BiScO₃–PbTiO₃ and “soft” PZT‐based ceramics. The typical “butterfly”‐shaped bipolar strain and frequency‐dependent peak‐to‐peak strain indicated that the large high‐field‐induced strain may be due to non‐180° domain switching. Rayleigh analysis reflected that the improved piezoelectric properties resulted from the enhanced extrinsic contribution by Zr doping. The unipolar strain of 0.75BF‐0.25BZT ceramics with 10 mol% Zr was almost linear from RT to 200°C. These results indicated that 0.75BF–0.25BZT ceramics were promising candidates for high‐temperature and lead‐free piezoelectric actuators.     

Dielectric Relaxation in Zr‐Doped SrTiO3 Ceramics Sintered in N2 with Giant Permittivity and Low Dielectric Loss

SrTiZr_xO₃ (x = 0, 0.002, 0.006, 0.01, and 0.014) ceramics with a weak temperature‐dependent giant permittivity (>10⁴) and a very low dielectric loss (<0.01) were fabricated using the conventional solid‐state reaction method by sintering them in N₂ at 1500°C. With increasing Zr content, the permittivity decreased from approximately 48 000 to 18 000 and the dielectric loss decreased from approximately 0.005 to 0.003. According to the XRD, XPS, and ac conductivity analysis, the dielectric properties of pure SrTiO₃ ceramics sintered in N₂ were due to the existence of the giant defect dipoles generated by the fully ionized oxygen vacancies and Ti³⁺ ions, while the dielectric properties of SrTiZr_xO₃ (x > 0) ceramics were also influenced by the defect dipoles (). The giant permittivity and low dielectric loss phenomenon could be explained by giant defect dipoles related to oxygen vacancies.     

Temperature‐Insensitive High Strain in Lead‐Free Bi0.5(Na0.84K0.16)0.5TiO3–0.04SrTiO3 Ceramics for Actuator Applications

Lead‐free piezoelectric ceramics, 0.96[{Bi_0.5 (Na_0.84K_0.16)_0.5}_1?xLi_x(Ti_1?yNb_y)O₃]–0.04SrTiO₃ (BNKLiTN–ST) with x, y = 0–0.030, were synthesized by solid‐state reaction method. X‐ray diffraction patterns indicated that Li and Nb successfully diffused into the BNKT–ST lattice and formed a pure perovskite structure with x, y ≤ 0.025. Increasing the Li and Nb contents (x, y = 0.020) induced a phase transformation from the coexistent rhombohedral–tetragonal phases for pure BNKT–ST ceramics to a pseudocubic phase, resulting in degradation of the remnant polarization and coercive field. However, the field‐induced strain was markedly enhanced at x, y = 0.020, giving rise to a giant dynamic piezoelectric constant (d₃₃* = S_max/E_max = 800 pm/V). Furthermore, the temperature dependence of the field‐induced strain response showed temperature‐insensitivity up to 120°C. To explore its potential for device applications, a 10‐layered stack‐type multilayer actuator was fabricated from the optimal composition (x, y = 0.020). This actuator showed a large S_max/E_max of 600 pm/V at a relatively low driving field of 4.5 kV/mm suggesting highly promising results in lead‐free BNT‐based ceramics.     

Low‐Temperature Ionic Conductivity of an Acceptor‐Doped Perovskite: I. Impedance of Single‐Crystal SrTiO3

Low‐temperature conductivity mechanisms were identified in acceptor‐doped SrTiO₃ single crystals quenched from high temperatures under reducing conditions. Impedance spectroscopy measurements made on samples of the prototypical perovskite structure doped with iron provided a framework for creating a complete conductivity model for a well‐defined point defect system. The dominant conductivity mechanism in the room‐temperature range was identified as being controlled by oxygen vacancy hopping. The activation energy for oxygen vacancy migration, an often debated value in the perovskite community, is determined to lie within the range of 0.59–0.78 eV for the iron‐doped system with the bottom of this range approaching the intrinsic value for oxygen vacancy hopping in an undoped single crystal. At low temperatures, oxygen vacancies form defect complexes with iron impurities, and the observed range of activation energies is explained and modeled in terms of an oxygen vacancy trapping mechanism.     

Optical and Tunable Dielectric Properties of K0.5Na0.5NbO3–SrTiO3 Ceramics

Pure perovskite K_0.5Na_0.5NbO₃–xSrTiO₃ (x = 0.16, 0.17, 0.18, and 0.19) ceramics were prepared by using a solid‐state reaction process. The ceramics were optically transparent for visible and near‐infrared wavelengths. Then, high tunability (24.1%) and low dielectric loss (0.016) for the x = 0.18 sample indicated the transparent ceramics could be used in tunable devices. The Lorentz‐type relation fitting for the temperature dependence of dielectric permittivity showed that these ceramics had a typical relaxor behavior, and the polar nanoregions were related to the tunable dielectric properties. The nonlinear dielectric behavior was further explored by the Johnson model combined with Langevin terms, which revealed that the polar nanoregions contributed to the nonlinear ε(E) dependencies with contributions of 12.3%, 11.6%, 5.9%, and 3.6% for x = 0.16, 0.17, 0.18, and 0.19, respectively.     

BaTiO3–Bi(Mg2/3Nb1/3)O3 Ceramics for High‐Temperature Capacitor Applications

Solid solutions of (1?x)BaTiO₃–xBi(Mg_2/3Nb_1/3)O₃ (0 ≤ x ≤ 0.6) were prepared via a standard mixed‐oxide solid‐state sintering route and investigated for potential use in high‐temperature capacitor applications. Samples with 0.4 ≤ x ≤ 0.6 showed a temperature independent plateau in permittivity (ε_r). Optimum properties were obtained for x = 0.5 which exhibited a broad and stable relative ε_r ~940 ± 15% from ~25°C to 550°C with a loss tangent <0.025 from 74°C to 455°C. The resistivity of samples increased with increasing Bi(Mg_2/3Nb_1/3)O₃ concentration. The activation energies of the bulk were observed to increase from 1.18 to 2.25 eV with an increase in x from 0 to 0.6. These ceramics exhibited excellent temperature stable dielectric properties and are promising candidates for high‐temperature multilayer ceramic capacitors for automotive applications.     

Growth and Microstructure‐Dependent Hardness of Directionally Solidified WC–W2C Eutectoid Ceramics

Directionally solidified WC–W₂C ceramics containing 40 at% carbon, corresponding to the WC–W₂C eutectoid composition, were produced by laser surface melt processing. The resulting microstructures showed a lamellar‐type eutectic/eutectoid microstructure with the WC minor phase embedded in the W₂C matrix phase. The interlamellar spacing (λ) in the eutectoid regions followed the relationship Vλ^3.8 = constant, with the smallest spacing of 331 ± 36 nm achieved in the 3.24 mm/s processed sample. The indentation hardness increased with decreasing interlamellar spacing, and a Vickers indentation hardness of 28.5 GPa was achieved in the sample with the smallest interlamellar spacing. The directionally solidified WC–W₂C materials show enhanced indentation mechanical properties in comparison to previously reported WC–Co composites and WC‐based materials.     

Scale Up and Anode Development for La‐Doped SrTiO3 Anode‐Supported SOFCs

The possibility of developing large solid oxide fuel cell (SOFC) stacks based upon 25 cm² ceramic oxide anode‐supported cells is investigated. Planar fuel cells comprising strontium titanate‐based anode support impregnated with active catalysts were prepared using a combination of deposition techniques. The fuel cell tests performed in a semisealed rig have shown power densities of 185 mW cm^?2 at 850°C using humidified hydrogen as fuel and air as oxidant. The structure and evolution of the catalytically active impregnated materials‐10 mol% Gd‐doped CeO₂ and nickel‐ are analysed using electron microscopy at the end of the fuel cell test, revealing that a ceria and nickel layer surrounds the titanate backbone grains while ~50–150 nm spherical‐like nickel particles uniformly decorate this top layer.     

Thermoelectric Efficiency of Reduced SrTiO3 Ceramics Modified with La and Nb

Ceramics of La_xSr_1?xNb_yTi_1?yO₃ (LSNT) were synthesized under various reducing atmospheres. Covering the specimens with graphite carbon felt under an Ar‐gas flow during sintering drastically enhanced the electrical conductivity, σ. Ti K‐edge absorption spectra indicated the presence of Ti³⁺ for heavily reduced specimens. The increase in conductivity was attributed to the 3d band of Ti³⁺. The maximum value for the figure of merit, ZT, was obtained for strontium titanate ceramics modified with both 5 mol% La and 5 mol% Nb, namely 5/5‐LSNT, exhibiting a ZT value of ~0.221 at 473 K. This high ZT value was almost 1.5 × larger than that of the conventional 10 mol% La‐doped sample, 10/0‐LSNT (ZT~0.144), and was mainly attributed to the larger Seebeck coefficient of the material.