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.     

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.     

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.     

0.6Mg4Nb2O9-0.4SrTiO3复相陶瓷中温烧结和微波介电性能

采用传统固相反应法制备了0.6Mg4Nb2O9-0.4SrTiO3复合陶瓷.研究了LiF掺杂对其烧结特性、显微组织和微波介电性能的影响.实验结果表明:通过添加一定量的LiF,可将Mg4Nb2O9/SrTiO3陶瓷的致密化烧结温度降至1100 ℃;其中掺杂1.5wt% LiF、 1100 ℃下烧结5 h的0.6Mg4Nb2O9-0.4SrTiO3陶瓷微波介电性能为:ε=20.6,Q·f=4057 GHz; 样品的微波介电性能与杂相Sr(Ti1-xNbx)O3+δ和残留液相有关.     

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。     

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.     

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.     

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.     

Voltage‐induced wrinkling behavior of dielectric elastomer

Wrinkles, with regular periodic patterns in soft dielectric membrane, are interesting, since they are induced electrically by applying a voltage. An experimental investigation is presented to study the wrinkling behavior of dielectric elastomer. Steady wrinkles, without the accompany of electrical breakdown were attained. According to the relationship between wrinkling and breakdown, the electromechanical behaviors of DE membrane can be divided into the following types: Type A: breakdown directly without wrinkles; Type B: wrinkle and immediate breakdown; Type C: form steady wrinkles within a voltage span. Three different electromechanical behaviors of DE membrane are classified in a phase chart. A theoretical analysis is presented and discussed, involving the effect of prestretch and configurations to predict the relationship between mechanical wrinkling and electrical breakdown. Wrinkles at on‐demand location can be triggered. The results agree with the experiments. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43258.     

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.     

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.     

掺MgO和SrTiO3对PMN-PT-BT系铁电陶瓷材料介电性能的影响

采用二次预合成法制备了PMN-PT-BT弛豫铁电陶瓷材料,研究了添加MgO和SrTiO3对于PMN-PT-BT铁电陶瓷介电性能的影响.结果表明,上述两种掺杂物质均使PMN-PT-BT瓷体的居里峰宽化,向低温方向移动,介电温度特性比较好.添加10wt%的MgO和3wt%的SrTiO3,在1100℃的温度下烧结,可以获得性能为εγ=5650(20℃),△C/C(-30～+80℃)≤±15%,tgδ=9×10-4,Eb=6.2 kV/mm的陶瓷电容器材料.     

SrTiO3复合功能陶瓷的AC阻抗谱研究

本文采用一次烧成法制备出SrTiO3电容-压敏复合功能陶瓷,测试了样品的交流阻抗谱,确定了样品的等效电路,分析了不同氧化热处理条件下的晶粒和晶界电学特性.结果表明,SrTiO3复合功能陶瓷晶界存在多种受主态形态,氧化热处理过程中晶粒保持其半导体特性,晶界绝缘程度显著提高.     

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.     

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.