Grain‐Boundary and Thermally Stimulated Current Characteristics of Y2O3‐Doped ZnO Varistor

The doping of rare‐earth oxides can greatly improve the electrical characteristics of ZnO varistors. Thermally stimulated current (TSC) characteristic test, capacitance voltage (C–V) characteristic test, scanning electron microscope (SEM) test, and voltage current (V–I) test were carried out to study the influence of Y₂O₃ content on the electrical properties of ZnO varistors in this study. The results show that the grain size decreases while the voltage gradient increases as the Y₂O₃ content is increased. The reaction of Y₂O₃ with other additives leads to the decrease in grain‐boundary defects, which accounts for the decrement of barrier height, donor density, and surface state density. The trap level and trapped charge of ZnO varistors decrease as the Y₂O₃ content is increased from 0.3 to 0.9 mol%, which means the shallow traps inside ZnO varistors reduce, and the Y₂O₃ additive can greatly improve the TSC characteristic of ZnO varistors.     

Influence of WO3‐Doping on the Microstructure and Electrical Properties of ZnO–Bi2O3 Varistor Ceramics Sintered at 950°C

The phase evolution, microstructure, and electrical properties of WO₃‐doped ZnO–Bi₂O₃‐based varistors were investigated for different amounts x (0 ≤ x ≤ 1.60 mol%) of the dopant. When x was less than 0.40, the dissolved W⁶⁺ in the β‐Bi₂O₃ acted as a donor in the grain boundaries and reduced the electrical properties of the ZnO varistors. However, when x was 0.40 mol%, which meant an amount of WO₃ equal to that of Bi₂O₃, the electrical properties dramatically increased, which means the W⁶⁺ donor effect is removed at the grain boundaries because a new Bi₂WO₆ phase was formed in the grain‐boundary regions. The Bi₂WO₆ phase has high oxygen conductivity at high temperatures; it transfers more oxygen to the grain boundaries in order to further enhance the electrical properties. For x values higher than 0.40 (i.e., an addition of WO₃ that is greater than the content of Bi₂O₃), the electrical properties were steadily reduced in comparison to the composition with x = 0.40. This could be explained by the reduced amount of Co, Mn, and Al at the grain boundaries and in the ZnO grains as a result of their incorporation into the ZnWO₄ phase. The electrical properties of the ZnO grains and the grain boundaries were in agreement with the results of the impedance spectroscopy analysis.     

Er2O3 Doping Effect on Varistor Properties and Multiimpulse Stress Behavior of ZnO–PrO1.83–CoO–Cr2O3–Dy2O3 Ceramics

The Er₂O₃ doping effects on varistor properties and impulse aging behavior of the ZnO–PrO_1.83–CoO–Cr₂O₃–Dy₂O₃ ceramics were investigated in the range of 0–2.0 mol%. The nonlinear coefficient increased from 42 to 56 with an increase in the amount of Er₂O₃. The clamp voltage ratio (K) decreased with an increase in the amount of Er₂O₃. The varistors doped with 2.0 mol% in the amount of Er₂O₃ exhibited the best clamping characteristics, with K = 1.43–1.83 at an impulse current of 1–50 A. The varistors doped with 0.25 mol% in the amount of Er₂O₃ exhibited the strongest electrical stability, with the variation rate of the breakdown field of ?0.5%, the variation rate of the nonlinear coefficient of ?5.5%, and the variation rate of the leakage current of ?1.5% after applying 400 times at an impulse current of 400 A     

Synthesis of Single‐Phase β‐Yb2Si2O7 and Properties of Its Sintered Bulk

Single‐phase β‐Yb₂Si₂O₇ was synthesized by solid‐state reaction using Yb₂O₃ and SiO₂ gel. SiO₂ gel significantly decreased the synthesis temperature and shortened the holding time. Bulk Yb₂Si₂O₇ was obtained by pressureless sintering. Grain size, relative density (92.9%), and flexural strength [(182.3 ± 2.0) MPa] were enhanced as the sintering temperature increased and equiaxed grains were obtained with an average grain size of approximately 3 μm. Bulk Yb₂Si₂O₇ possessed a suitable thermal expansion coefficient [(4.64 ± 0.01) × 10^?6/K] between 473 and 1573 K, and the thermal conductivities at 300 and 1400 K were 4.31 and 2.27 W/m·K, respectively.     

Novel BiFeO3–BaTiO3–Ba(Mg1/3Nb2/3)O3 Lead‐Free Relaxor Ferroelectric Ceramics for Energy‐Storage Capacitors

A novel lead‐free relaxor ferroelectric ceramic of (0.67?x)BiFeO₃–0.33BaTiO₃–xBa(Mg_1/3Nb_2/3)O₃ [(0.67?x)BF–0.33BT–xBMN, x = 0–0.1] was prepared by a solid‐state reaction method. A relatively high maximum polarization P_max of 38 μC/cm² and a low remanent polarization P_r of 5.7 μC/cm² were attained under 12.5 kV/mm in the x = 0.06 sample, leading to an excellent energy‐storage density of W ~1.56 J/cm³ and a moderate energy‐storage efficiency of η ~75%. Moreover, a good temperature stability of the energy storage was obtained in the x = 0.06 sample from 25°C to 190°C. The achievement of these characteristics was basically attributed to an electric field induced reversible ergodic to ferroelectric phase transition owing to similar free energies near a critical freezing temperature. The results indicate that the (0.67?x)BF–0.33BT–xBMN lead‐free realxor ferroelectric ceramic could be a promising dielectric material for energy‐storage capacitors.     

Perovskite‐Structured BiFeO3–Bi(Zn1/2Ti1/2)O3–PbTiO3 Solid Solution Piezoelectric Ceramics with Curie Temperature About 700°C

In this article, perovskite‐structured BiFeO₃–Bi(Zn_1/2Ti_1/2)O₃–PbTiO₃ (BF–BZT–PT) ternary solid solutions were prepared with traditional solid‐state reaction method and demonstrated to exhibit a coexistent phase boundary (CPB) with Curie temperature of T_C~700°C in the form of ceramics with microstructure grain size of several micron. It was found that those CPB ceramics fabricated with conventional electroceramic processing are mechanically and electrically robust and can be poled to set a high piezoelectricity for the ceramics prepared with multiple calcinations and sintering temperature around 750°C. A high piezoelectric property of T_C = 560°C, d₃₃ = 30 pC/N, ε₃₃^T/ε₀ = 302, and tanδ = 0.02 was obtained here for the CPB 0.53BF–0.15BZT–0.32PT ceramics with average grain size of about 0.3 μm. Primary experimental investigations found that the enhanced piezoelectric response and reduced ferroelectric Curie temperature are closely associated with the small grain size of microstructure feature, which induces lattice structural changes of increased amount ratio of rhombohedral‐to‐tetragonal phase accompanying with decreased tetragonality in the CPB ceramics. Taking advantage of structural phase boundary feature like the Pb(Zr,Ti)O₃ systems, through adjusting composition and microstructure grain size, the CPB BF–BZT–PT ceramics is a potential candidate to exhibit better piezoelectric properties than the commercial K‐15 Aurivillius‐type bismuth titanate ceramics. Our essay is anticipated to excite new designs of high–temperature, high–performance, perovskite‐structured, ferroelectric piezoceramics and extend their application fields of piezoelectric transducers.     

Y3Al5O12–SiO2 Glasses: Structure and Polyamorphism

Aerodynamic levitation and CO₂ laser melting have been used to synthesize the yttrium aluminosilicate glasses zY₂O₃–yAl₂O₃–xSiO₂ with z/y = 3/5 corresponding to the YAG (Y₃Al₅O₁₂) composition and x between ~5 and ~45 mol%. The low‐ and high‐density (LDA inclusion and HDA matrix) polyamorphic phases in glasses with less than ~14 mol% SiO₂ were identified with backscattering electron imaging. Polarized and depolarized Raman spectra show the formation of various Qⁿ SiO₄ species whose relative populations change smoothly as the SiO₂ content is altered. The AlOs (s = 4–6) and YOz (z = 6–9) polyhedra formed in the YAG glass are preserved upon silica additions while the terminal oxygens of the Q²AlO₄ tetrahedra are gradually bridged to the Qⁿ‐SiO₄ species. The low‐frequency Boson Peak overlaps with the vibrational spectrum and its maximum is redshifted with increasing silica content. Micro‐Raman spectra measured for the LDA and HDA amorphous phases are found to be similar to the spectra of the bulk glass indicating common structural characteristics. The stability of the LDA phase against crystallization appears to be lower than that of the HDA phase. The crystallinity on certain inclusions consisted of YAG microcrystals and a new unidentified microcrystalline phase within Y₄Al_2(1?x)Si_2xO_(9+x) solid solution.     

Sintering and Dielectric Properties of Li2O–B2O3–Al2O3–SiO2 Glass‐Added (Ca0.7Sr0.3O)1.03(Ti0.1Zr0.9)O2 for Copper Electrode

When 1.5 wt% of Li₂O–B₂O₃–SiO₂ and 1.5 wt% of Li₂O–B₂O₃–Al₂O₃ glass‐added (Ca_0.7Sr_0.3O)_1.03(Ti_0.1Zr_0.9)O₂ batch was ball milled for 10~30 h followed by sintering at 950°C in flowing N₂‐10%H₂ atmosphere, an apparent density of approximately 4.5 g/cm³, a dielectric constant of approximately 26, and a quality factor of roughly about 3300 GHz were demonstrated. A prolonged ball mill time thereafter significantly decreased both of the dielectric properties because of the enhanced reduction of the specimens during sintering. The apparent evidence of a material reaction between the dielectric material and the copper electrode was not observed.     

Phase Evolution of SiO2–Al2O3–ZnO–CaO–ZrO2–TiO2‐Based Glass with Added Y‐PSZ Particles

Yttria partially stabilized zirconia Y‐PSZ/glass‐ceramic composites were prepared by reaction sintering using powder mixtures of a SiO₂–Al₂O₃–ZnO–CaO–ZrO₂–TiO₂‐based glass and yttria partially stabilized zirconia (Y‐PSZ). The glass crystallized during sintering at temperatures of 1173, 1273, and 1373 K to give a glass‐ceramic matrix for high‐temperature protecting coatings. With the increasing firing time, the added zirconia reacted with the base glass and a glass‐ceramic material with dispersed zircon particles was prepared in situ. Furthermore, the added zirconia changed the crystallization behavior of the base glass, affecting the shape, amount, and distribution of zircon in the microstructure. The bipyramid‐like zircon grains with imbedded residual zirconia particles turned out to have two growth mechanisms: the inward growth and the outward growth, and its rapid growth was mainly dominated by the later one. For comparison, the referenced glass‐ceramic was prepared by sintering using exclusive glass granules and its crystallization behavior at 1173–1373 K was examined as well. Scanning electron microscopy (SEM), energy dispersive X‐ray spectroscopy (EDS), transmission electron microscopy (TEM), and X‐ray diffraction (XRD) were used to characterize the crystallization behavior of the base glass and the phase evolution of the Y‐PSZ/glass‐ceramic composites.     

High–Energy‐Storage Density Capacitors of Bi(Ni1/2Ti1/2)O3–PbTiO3 Thin Films with Good Temperature Stability

High–energy‐storage density capacitors with thin films of 0.5Bi(Ni_1/2Ti_1/2)O₃–0.5PbTiO₃ (BNT–PT) were fabricated by chemical solution deposition technique on Pt/Ti/SiO₂/Si substrates. The dense thin films with pure‐phase perovskite structure could be obtained by annealing at 750°C. High capacitance density (~1925 nF/cm² at 1 kHz) and extremely high‐energy density (~45.1 J/cm³) under an electric field of 2250 kV/cm were achieved at room temperature. The energy‐storage density and efficiency varied little in a wide temperature range from ?190°C to 250°C. The high–energy‐storage density and good temperature stability make BNT–PT films promising candidates for high power electric applications.     

Stability and electrical properties of MoSi2‐ and WSi2‐oxide electroconductive composites

MoSi₂‐ and WSi₂‐based electroconductive ceramic composites were fabricated using 40‐80 vol% fine‐ and coarse‐Al₂O₃, and ZrO₂ particles (refractory oxides) after sintering in argon. Their chemical and thermal stability was tested between 1400°C‐1600°C for up to 48 hours. X‐ray diffraction analysis showed the formation of secondary 5‐3 metal silicide (Mo₅Si₃, W₅Si₃) and silica phases on the grain boundaries and surface. The fraction of the W₅Si₃ (11.4‐38.8 vol%) was significantly higher than that of the Mo₅Si₃ (3.3‐7.3 vol%) in the composites after annealing at 1400°C for 48 hours. The rates of grain growth in the composites (0.013‐0.023 μm/h) were highly decreased by a grain‐boundary pinning effect. This effect was relatively better with the addition of the coarse‐grained oxides due to their more homogeneous distribution throughout the microstructure. The 20–80 vol% MoSi₂‐Al₂O₃ (fine‐grained) composite exhibited an electrical conductivity of 8.8 S/cm at 900°C. At the 60 vol% silicide content, MoSi₂–Al₂O₃ (coarse‐grained) and WSi₂–Al₂O₃ (fine‐grained) showed higher electrical conductivity (126‐128 S/cm) at 900°C. The density, porosity level, particle distribution, intrinsic conductivity of silicide phase, particle size, and fraction of the secondary 5‐3 silicide phase highly influenced their electrical properties.     

New calcium‐free Na2O–Al2O3–P2O5 bioactive glasses with potential applications in bone tissue engineering

Sodium aluminophosphate glasses were evaluated for their bone repair ability. The glasses belonging to the system 45Na₂O–xAl₂O₃‐(55‐x)P₂O₅, with x = (3, 5, 7, 10 mol%) were prepared by a melt‐quenching method. We assessed the effect of Al₂O₃ content on the properties of Na₂O–Al₂O₃–P₂O₅ (NAP) glasses, which were characterized by density measurements, DSC analyses, solubility, bioactivity in simulated body fluid and cytocompatibility with MG‐63 cells. To the best of our knowledge, this is the first investigation of calcium‐free Na₂O–Al₂O₃–P₂O₅ system glasses as bioactive materials for bone tissue engineering.     

Infrared‐Transparent Y2O3–MgO Nanocomposites Fabricated by the Glucose Sol–Gel Combustion and Hot‐Pressing Technique

A glucose sol–gel combustion method has been developed to synthesize composite nanopowders with equal volume fractions of Y₂O₃ and MgO. The synthesis involves the generation of precursor foam containing Y³⁺ and Mg²⁺ cations via the chemical and thermal degradation of glucose molecules in aqueous solutions. Subsequent calcination of the foam gave the composite nanopowders uniform composition and surface areas of 44–62 m²/g depending on the relative amount of glucose. Then the nanopowder with an average particle size of 19 nm was consolidated by the hot‐pressing technique with different sintering temperatures. The fabricated nanocomposite is mid‐infrared transparent as the result of fine grains, narrow grain size distribution, and uniform phase domains. The transmittance increases with increase in the sintering temperature and reaches 83.5% at 3–5 μm mid‐infrared wave range once the temperature reaches 1350°C, which is close to the theoretical value of 85%. And it is noteworthy that the cutoff wavelength reaches 9.6 μm, which is superior to those of spinel, AlON, and sapphire. And the Vickers hardness of the sample reaches 10.0 ± 0.1 GPa, which is significantly higher than those of the coarse grained single‐phase MgO and Y₂O₃. The results indicate that the glucose sol–gel combustion and hot‐pressing technique is an effective method to fabricate infrared transparent Y₂O₃–MgO nanocomposites.     

A Complete Solid Solution with Rutile‐Type Structure in SiO2–GeO2 System at 12 GPa and 1600°C

High pressure and temperature synthesis of compositions made of (Si_1?x,Ge_x)O₂ where x is equal to 0, 0.1, 0.2, 0.5, 0.7, and 1 was performed at 7–12 GPa and 1200–1600°C using a Kawai‐type high‐pressure apparatus. At 12 GPa and 1600°C, all the run products were composed of a single phase with a rutile structure. The lattice constants increase linearly with the germanium content (x), which indicates that the rutile‐type phases in the SiO₂–GeO₂ system form a complete series of solid solutions at these pressure and temperature conditions. Our experimental results show that thermodynamic equilibrium state was achieved in this system at 12 GPa and 1600°C, but not at 1200°C. At lower pressures (7 and 9 GPa) and 1600°C, we observed the decomposition of (Si_0.5,Ge_0.5)O₂ into SiO₂‐rich coesite and GeO₂‐rich rutile phases. The silicon content in the rutile structure increases sharply with pressure in the vicinity of the coesite–stishovite phase transition pressure in SiO₂.     

Preparation and electrical properties of ZnO‐Bi2O3‐based multilayer varistors with base metal nickel inner electrodes

In this study, ZnO‐Bi₂O₃‐based multilayer varistors (MLVs) cofired with nickel (Ni) inner electrodes were prepared by tape‐casting method. Samples were sintered in pure nitrogen (N₂) to keep Ni from being oxidized, and then reoxidized in air to obtain the nonlinear properties (reduction‐reoxidation method). The EDAX results showed that Ni inner electrodes are stable and have no evident migration into ZnO‐Bi₂O₃‐based ceramics when sintered in N₂. The influence of reoxidation temperature on microstructures and nonlinear properties of samples were studied. Samples reoxidized at the temperatures lower than 650°C showed poor nonlinear properties. After reoxidized in air at 700°C for 2 hours, samples exhibited nonlinear properties of V_1 mA=16.3 V, α=26.5, I_L=0.68 μA. At the reoxidation temperature higher than 750°C, the oxidation of Ni inner electrodes deteriorated the nonlinear properties of samples. It demonstrated that ZnO‐Bi₂O₃‐based MLVs with base metal Ni inner electrodes proposed in this work are suitable for reduction‐reoxidation method. The replacement of noble metals Pt or Ag/Pd alloys by base metal Ni is expected to lower the cost of ZnO‐Bi₂O₃‐based MLVs.     

Thermodynamic Stability of Spinel Phase at the Interface Between Alumina Refractory and CaO–CaF2–SiO2–Al2O3–MgO–MnO Slags

The interfacial reaction between alumina refractory and CaO–CaF₂–SiO₂–Al₂O₃–MgO–MnO slag was observed at 1873 K to estimate the stability of the spinel phase using computational thermodynamics under refining conditions of Mn‐containing steels. The concentration of MnO formed by the slag–steel reaction in the CaO–CaF₂–SiO₂–Al₂O₃–MgO melts generally increased by decreasing the CaO/SiO₂ ratio of the initial melts. No intermediate compounds were formed at the refractory–slag interface when the initial CaO/SiO₂ ratio was 0.5, whereas CaAl₁₂O₁₉ (CA6) and Mg(Mn)Al₂O₄ (spinel), identified from TEM analysis using EDS mapping and SAED patterns, were observed at the refractory–slag interface when the CaO/SiO₂ ratio was 1.0 or greater. The (at.%Mg)/(at.%Mn) ratio in the spinel solution increased by increasing the CaO/SiO₂ ratio, which originated from the fact that MgO activity continuously increased as the CaO/SiO₂ ratio increased. From thermodynamic analysis considering the equilibrium constant (K_SP) and activity quotient (Q_SP) of the spinel formation reaction at the slag–refractory interface and the bulk slag phase, the precipitation–dissolution behavior of the spinel phase was predicted, which exhibited good consistency with the experimental results. Hence, the dissolutive corrosion mechanism of alumina refractory into the CaO–CaF₂–SiO₂–Al₂O₃–MgO–MnO slag was proposed.     

Grain size effect and microstructure influence on the energy storage properties of fine‐grained BaTiO3‐based ceramics

The dependence of energy storage properties on grain size was investigated in BaTiO₃‐based ferroelectric ceramics. Modified BaTiO₃ ceramics with different grain size were fabricated by two‐step sintering method from BaTiO₃ powders doped with Al₂O₃ and SiO₂ by aqueous chemical coating. For samples doped with ZnO sintering aid in addition to Al₂O₃‐SiO₂, the density and breakdown strength increased significantly. In general, samples with smaller grains have lower polarization but higher energy storage efficiency. Al₂O₃‐SiO₂‐ZnO‐doped samples with average grain size of 118±2 nm have an energy density of 0.83±0.04 J/cm³. Obvious segregation of doping elements in second phase and grain boundary was observed by TEM‐EDS. Impedance spectroscopy further explains the relationship between microstructure and properties. Compared to common energy storage ceramics, the grain size of this low‐cost ceramics sintered at relatively low temperature is small, and the pilot scale production has been well completed. All these features make the utilization in multilayer devices and industrial mass production possible. In addition, the obtained rules are helpful in further development of energy storage ceramics.     

Relaxor Ferroelectric BaTiO3–Bi(Mg2/3Nb1/3)O3 Ceramics for Energy Storage Application

Perovskite solid solution ceramics of (1 ? x)BaTiO₃–xBi(Mg_2/3Nb_1/3)O₃ (BT–BMN) (x = 0.05–0.2) were synthesized by solid‐state reaction technique. The results show that the BMN addition could lower the sintering temperature of BT‐based ceramics. X‐ray diffraction results reveal a pure perovskite structure for all studied samples. Dielectric measurements exhibit a relaxor‐like characteristic for the BT–BMN ceramics, where broadened phase transition peaks change to a temperature‐stable permittivity plateau (from ?50°C to 300°C) with increasing the BMN content (x = 0.2), and slim polarization–electric field hysteresis loops were observed in samples with x ≥ 0.1. The dielectric breakdown strength and electrical resistivity of BT–BMN ceramics show their maxima of 287.7 kV/cm and 1.53 × 10¹³ Ω cm at x = 0.15, and an energy density of about 1.13 J/cm³ is achieved in the sample of x = 0.1.     

Dielectric Properties of Ultra‐Low Sintering Temperature Al2O3–BBSZ Glass Composite

Ceramic composites of B₂O₃–Bi₂O₃–SiO₂–ZnO (BBSZ) glass mixed with Al₂O₃ (10–50 vol%) were sintered at 450°C, and their microstructural and dielectric properties investigated. Dense structures were obtained when the Al₂O₃ content was lower than 30 vol%. Raman, XRD, and FESEM showed the existence of a secondary phase, Bi₂₄Si₂O₄₀, in all samples. The dielectric properties of the composite with 30 vol% addition of Al₂O₃ showed good dielectric properties with ε_r of 14.8 and 20.8 and 32.5 at 100 kHz and 100 MHz and 1 GHz, respectively. The tanδ values at the same frequencies were 0.004 and 0.006 and 0.016. The results show that BBSZ glass with different amounts of Al₂O₃ exhibit widely applicable relative permittivity values and affordable loss and are thus promising candidates for ultra‐low sintering temperature applications.     

Order–Disorder Phase Transition and Magneto‐Dielectric Properties of (1−x)LiFe5O8–xLi2ZnTi3O8 Spinel‐Structured Solid Solution Ceramics

In this study, the spinel solid solution ceramics (1?x)LiFe₅O₈–xLi₂ZnTi₃O₈ (0 ≤ x ≤ 1) were prepared via the solid‐state reaction method. The phase evolution, sintering behaviors, microstructures, magneto‐dielectric properties, and microwave dielectric properties were systematically investigated. The XRD and SEM analysis indicated that the LiFe₅O₈ phase and the Li₂ZnTi₃O₈ phase were almost fully soluble in each other at any proportion. Meanwhile, the evidence of ionic substitution has been directly observed at the atomic scale by means of scanning transmission electron microscopy, which is further confirmed by the Raman spectroscopy. Evidence shows that the magnetic and dielectric properties are quite sensitive to the compositions. The optimal results with remarkable magneto‐dielectric properties of μ′ = 38.2, tanδ_μ = 0.25, ε′ = 19.6, tanδ_ε = 8 × 10^?3 at 1 MHz, and ε′ = 19.1, Q × f = 10 400 GHz at about 7 GHz have been obtained in 0.25LiFe₅O₈–0.75Li₂ZnTi₃O₈ sample. The design of complex spinel solid solution can generate novel magneto‐dielectric single‐phase ceramics combining both high permeability and good dielectric properties, which provides a way in developing multifunctional materials for applications in electronic devices.