Grain Boundary Curvatures Measurements in Annealed Yttria‐Stabilized Zirconia (3Y‐TZP) and Their Relation to Mean Grain Size

It was determined that the mean grain boundary radius of curvature in 3 mol% yttria‐stabilized zirconia isothermally annealed without and with a DC electric field  = 18 V/cm was uniquely proportional to the mean linear intercept grain size , the proportionality constant α = 3/2 being in accord with the Rios‐Fonseca stereological model.     

Superior Reliability Via Two‐Step Sintering: Barium Titanate Ceramics

Two‐step sintering (TSS) in a reducing atmosphere has been employed to obtain fine‐grain BaTiO₃ ceramics with a core‐shell microstructure, a more uniform grain‐size distribution, and superior reliability for multilayer ceramic capacitor applications. Compared to ceramics of the same composition conventionally sintered for about the same time, TSS ceramics feature a thinner shell thickness thus a stronger dopant localization, which leads to a lower concentration, higher internal resistance and more dopant‐ association. Improved reliability is manifest in a 50% higher breakdown strength at ambient temperature and a 400% longer endurance time to withstand DC stress at 185°C, in addition to a less field‐and‐temperature‐dependent capacitance. A scaling analysis of the redistribution and endurance dynamics identifies transmission across the shell‐grain‐boundary region as the critical element beneficially impacted by core‐shell structure and two‐step sintering.     

Effect of Surface Concentration of Lu on Oxygen Permeation of Polycrystalline Alumina at High Temperatures

The effect of Lu surface concentration on oxygen permeation in polycrystalline α‐alumina wafers was determined at 1773 K under limited oxygen potential gradients (Δ), where the two surfaces of the wafer were deliberately subjected to different oxygen partial pressures [ (I) ?  (II)]. When oxygen permeation occurred mainly by oxygen GB diffusion under a Δ generated by a combination of low values, the Lu‐coating on the (I) [ (II)] surface decreased [increased] the oxygen permeability constants. When Δ was the result of a combination of high values, where oxygen permeation proceeded mainly by aluminum GB diffusion, the oxygen permeability constants were decreased only by the Lu‐coating on the (I) surface. The analysis of mass transfer parameters, such as the chemical potentials, GB diffusion coefficients, and fluxes of aluminum and oxygen in the wafers, suggested that ambient oxygen molecules were effectively attracted toward Lu‐coated surfaces exposed to low‐ environments, leading to a change in oxygen permeability.     

Estimation of Stress Exponent and Activation Energy for Rapid Densification of 8 mol% Yttria‐Stabilized Zirconia Powder

The rapid densification behavior of 8 mol% Y₂O₃‐stabilized ZrO₂ polycrystalline (8Y‐SZP) powder compacts at the initial stage of pressure sintering (relative density () below 0.92) has been investigated using an electric current‐activated/assisted sintering (ECAS) system. Data points corresponding to a fixed heating rate were extracted from the densification rate () versus ρ and versus temperature (T) curves. These curves were obtained experimentally by consolidation at a fixed current. Under fixed current ECAS, the heating rate () decreases continuously over sintering time. Using a quasi‐ constant heating rate (CHR) method, data points were extracted to plot vs. ρ, vs. T, and ρ vs. T curves at a fixed . The stress exponent (n), estimated from a log‐log plot of grain size (d)‐corrected /ρ and effective stress (σ_eff) at 1300–1400 K, shows an almost constant value of 1. In addition, the activation energy (Q) for rapid densification, estimated from an Arrhenius plot of d‐corrected /ρ also shows an almost constant value of 350 kJ/mol, which is considerably lower than the previously reported value of the activation energy for Zr⁴⁺ lattice diffusion of about 440 kJ/mol. These results suggest that rapid densification of 8Y‐SZP by ECAS seems to proceed by diffusional creep controlled by grain‐boundary diffusion of Zr⁴⁺ ions.     

Hydroxyl Defect Effect on Reoxidation of Sc‐Doped (Ba,Ca)(Ti,Zr)O3 Fired in Reducing Atmospheres

The behavior of grain and grain‐boundary conductivity of acceptor (Sc)‐doped (Ba,Ca)(Ti,Zr)O₃ ceramics sintered in moist reducing atmosphere and subsequently reoxidized in dry and moist atmosphere was investigated by means of impedance spectroscopy (IS). In moist firing atmosphere, water vapor was found to react with oxygen vacancies, forming positively charged hydroxyl defects on regular oxygen sites in the crystal lattice. Proton hopping is considered to raise the ionic conductivity significantly. Therefore, hydroxyl defects in turn influence the grain conduction. Hydroxyl defects are also considered to be responsible for alternations of the dielectric maximum at the Curie point.     

Layered Structure Induced Anisotropic Low‐Energy Recoils in Ti3SiC2

Low‐energy recoil events in Ti₃SiC₂ are studied using ab initio molecular dynamics simulations. We find that the threshold displacement energies are orientation dependent because of anisotropic structural and/or bonding characteristic. For Ti and Si in the Ti–Si layer with weak bonds that have mixed covalent, ionic, and metallic characteristic, the threshold displacement energies for recoils perpendicular to the basal planes are larger than those parallel to the basal planes, which is an obvious layered‐structure‐related behavior. The calculated minimum threshold displacement energies are 7 eV for the C recoil along the direction, 26 eV for the Si recoil along the direction, 24 eV for the Ti in the Ti–C layer along the direction and 23 eV for the Ti in the Ti–Si layer along the direction. These results will advance the understanding of the cascade processes of Ti₃SiC₂ under irradiation and are expected to yield new perspective on the MAX phase family that includes more than 100 compounds.     

Defects and Transport Properties in TiNb2O7

The electrical conductivity of TiNb₂O₇ was characterized as a function of temperature, and . The total conductivity was independent of in the low oxygen partial pressure regime, while a dependency of was observed at higher oxygen partial pressures. The conductivity increased with increasing under oxidizing conditions below 700°C. Mixed electronic and protonic conduction was indicated by H/D isotope exchange and transport number measurements. A defect model based on interstitial type of hydration was established and fitted to the conductivity data allowing for determination of physicochemical parameters of hydration and electron migration.     

Control of Oxygen Permeability in Alumina under Oxygen Potential Gradients at High Temperature by Dopant Configurations

The oxygen permeability of polycrystalline α‐alumina wafers, which served as models for alumina scales on alumina‐forming alloys, under steep oxygen potential gradients () was evaluated at 1873 K. Oxygen permeation occurred by the grain‐boundary (GB) diffusion of oxygen from the higher‐oxygen‐partial‐pressure () surface to the lower‐ surface, along with the simultaneous GB diffusion of aluminum in the opposite direction. The fluxes of oxygen and aluminum at the outflow side of the wafer were significantly larger than at the inflow side. Furthermore, Lu and Hf segregation at the GBs selectively reduced the mobility of oxygen and aluminum, respectively. A wafer with a bilayer structure, in which a Lu‐doped layer was exposed to a lower and an Hf‐doped layer was exposed to a higher , decreased the oxygen permeability. When the sign of was reversed, however, the oxygen permeability of the wafer was comparable to that of a nondoped wafer. Co‐doping with both Lu and Hf markedly increased the oxygen permeation, presumably because the Lu‐stabilized HfO₂ particles that were segregated at the GBs acted as extremely fast diffusion paths for oxygen through the large number of oxygen vacancies introduced by the solid solution of Lu in the particles.     

Confirmation of the Dominant Defect Mechanism in Amorphous In–Zn–O Through the Application of In Situ Brouwer Analysis

The dominant point defect mechanism of amorphous (a‐) indium zinc oxide (IZO) was probed through in situ electrical characterization of sputtered a‐IZO thin films in response to changes in oxygen partial pressure (pO) at 300C. The results yielded a power law dependence of conductivity (σ) versus pO of ～?1/6. This experimental method, known as Brouwer analysis, confirms doubly‐charged oxygen vacancies as the dominant defect species in a‐IZO. The success of this study suggests that Brouwer analysis is a viable method for studying the defect mechanisms of amorphous oxides.     

Microstructure and Magnetic Properties of Metastable RFeO3 (R: Rare‐earth element) Formed from Undercooled Melt

Containerless levitation technique, where the undercooling can be treated as one of the major thermodynamic parameters, was used to study the influence of oxygen partial pressure () on the microstructure and physical properties of rare‐earth orthoferrites RFeO₃ (where R = Rare‐earth element) in the ranges from 10⁵ to 10^?1 Pa. The microstructure of the as‐solidified samples changed into orthorhombic RFeO₃ (o‐RFeO₃), metastable hexagonal RFeO₃ (h‐RFeO₃), and Fe²⁺‐containing RFe₂O₄ and a new metastable R₃Fe₂O₇ phases with decreasing . The effect of on the magnetic properties was indicated as that the saturation magnetization gradually increased for R = La to Yb and decreased for R = Lu with decreasing due to the formation of metastable and magnetic phases such as Fe₃O₄ and Fe.     

First‐Principles Study of Sc1−xTixF3 (x ≤ 0.375): Negative Thermal Expansion,Phase Transition,and Compressibility

We present the first‐principles investigation of (x ≤ 0.375). Controllable thermal expansion of is achieved by different Ti contents. The negative thermal expansion (NTE) behavior is weakened gradually with increasing Ti content, which is consistent with experimental measurements. The Jahn–Teller effect plays an important role in the cubic‐to‐rhombohedral phase transition, which stems from the enhanced energy stability when the 3d orbitals of cation split into triply degenerate and sets. The unusual thermal stiffening of is found, which is similar to that of and but contrary to other NTE materials.     

Stability and Decomposition of Ca‐Substituted Lanthanum Ferrite in Reducing Atmospheres

Calcium‐substituted lanthanum ferrites (La_1?xCa_xFeO_3?δ x = 0, 0.1, 0.2, 0.3, 0.4) were synthesized in air and subsequently decomposed in reducing atmospheres. The partial pressure of oxygen () was controlled by varying the H₂/H₂O ratio by bubbling hydrogen/argon mixtures through water baths at controlled temperatures. Three regions of mass loss were identified as the was reduced, two of which were determined to be associated with decomposition reactions. Calcium was shown to decrease the thermal stability of the perovskite compound, but rather than incrementally increasing the required for decomposition proportional to calcium concentration, all samples partially decomposed at a single . The extent of the partial decomposition was dependent on the amount of calcium substitution and temperature. The perovskite phase remaining after the partial decomposition was found to fully decompose at the same oxygen partial pressure as pure lanthanum ferrite.     

Effect of Oxygen Vacancy on Electrical Property of Acceptor Doped BaTiO3–Na0.5Bi0.5TiO3–Nb2O5 X8R Systems

In this study, we reported a new BaTiO₃–Na_0.5Bi_0.5TiO₃–Nb₂O₅–Mn₂O₃/Fe₂O₃/Co₃O₄/In₂O₃ X8R system with high dielectric constant (>2100) at room temperature. The impacts of oxygen vacancy ( ) on dielectric, electrical conductivity, and ferroelectric properties were systematically studied. The Curie point is largely depended on the concentration, which can be confirmed by the dielectric behavior and A_1g octahedral breathing modes in Raman spectrum. In addition, the activation energy of diffusion is greatly reduced with the increase in concentration. It was found that the remnant polarization and coercive field were both decreased with increasing concentration, due to the facilitated defect dipoles reorientation and domain switching.     

Effects of K2O Evaporation on the Structural Properties of KSbO3 Compounds

A specimen having a stoichiometric composition of KSbO₃·(KSb) calcined at 800°C has an R rhombohedral structure (RS), and changes to a Pn cubic structure (CS) when calcined at 1100°C. Finally, a <111>‐oriented rhombohedral phase is formed in the specimen calcined at 1230°C. K/Sb ratio decreases from 1.0 in RS, 0.93 in CS, and finally to 0.85 in <111>‐oriented rhombohedral phases. On the other hand, a specimen having a K‐excess composition of K_1.1SbO₃ calcined at 800°C shows a RS that is maintained in the K‐excess specimen calcined at 1230°C. The composition of these specimens is very close to KSb. Therefore, the RS with a space group of R is a stable form of KSbO₃. The formation of Pn cubic and <111>‐oriented R phases can be explained by the evaporation of K₂O during the calcination process at temperatures above 1100°C.     

Electric field‐assisted precipitation of lead selenide quantum dots in borosilicate glass

The influence of electric field on the precipitation and optical properties of PbSe quantum dots (QDs) in borosilicate glasses was investigated. Diameters of the PbSe QDs increased as was increased from 0.0 to 0.6 kV/mm even at the same subsequent heat treatment at 510°C for 2 hours. Increase in to 0.9 kV/mm caused red‐shift of the absorption and photoluminescence (PL) bands of QDs. Application of also led to decreases in viscosities that accelerated diffusion of ions; this phenomenon led to the formation of large QDs and resulted in the red‐shifts of the absorption and PL bands. Increase in the temperature due to Joule heating was ~38°C that led to an observed decrease in viscosities.     

Growth of diopside crystals in CMAS glass‐ceramics using Cr2O3 as a nucleating agent

CaO–Al₂O₃–MgO–SiO₂ (CAMS)‐based glass‐ceramics were prepared using body crystallization method. Adding Cr₂O₃ into the ceramics not only effectively lowered the crystallization temperature, but also led to significant grain refinement of diopside that crystallized in the CAMS glass‐ceramic after crystallization treatment at 900°C for 2 hours. Experimental work verified that the epitaxial growth of the diopside on the spinel particles, which formed during nucleation treatment when fabricating the glass‐ceramics, facilitated the heterogeneous nucleation of diopside on the spinel and refined the diopside. In addition, two energetically favored crystallographic orientation relationships between the epitaxial growth diopside and spinel were experimentally observed. They are //[001]_diopside,////(200)_diopside and //[101]_diopside, (311)_spinel//. These two novel results can be potentially used to develop new glass‐ceramic materials with improved performance.     

Oxidation Behavior of ZrB2–SiC Composites at Low Pressures

ZrB₂‐60 mol%SiC composite with a eutectic microstructure was oxidized at 1573 to 1873 K with reduced total pressures (P_tot) and low oxygen partial pressures (). The mass change was continuously measured by a thermobalance, and then fit with a multiple paralinear model. Oxidation scale of SiO₂/ZrO₂+SiO₂/ZrO₂/ZrB₂ was formed at  > 0.13 kPa, whereas only porous ZrO₂ remained at  < 0.13 kPa, P_tot < 1.33 kPa and higher than 1773 K. With increasing , the parabolic oxidation constant decreased, whereas the linear oxidation constant increased.     

Single‐Crystal Elastic Properties of Aluminum Oxynitride (AlON) from Brillouin Scattering

The Brillouin light scattering technique was used to determine experimentally the three independent elastic constants of cubic aluminum oxynitride at the ambient condition. They are , and . Its bulk modulus is 221.2 GPa. The magnitude of Zener anisotropic ratio is 2.1 similar to other spinels. The anisotropic nature of the material is shown by a large variation in the Young's modulus and Poisson's ratio with crystallographic directions. The material was found to be auxetic in certain orientations.     

First‐principles investigation of the thermodynamic stability of MB2 materials surfaces (M = Ti/Zr/Hf)

Some of the renewed interest in transition metal diborides (MB₂, M = Ti/Zr/Hf) arises from their potential use as matrices in ultrahigh‐temperature ceramic matrix composites (UHTCMCs). Crucial to the understanding of such composites is the study of the fiber/matrix interfaces, which in turn requires a deep knowledge of the surface structures and the thermodynamics of the matrix material. Here we investigate the surface stability of MB₂ compounds by first‐principles calculations. Five surfaces are stabilized when going from a M‐rich to a B‐rich environment, respectively (0001)_M, (100)_M, (101)_B(M), (113)_M and (0001)_B, with the highly stable (100)_M, (101)_B(M) and (113)_M surfaces being discussed here for the first time. The mechanism behind the surface stability is analyzed in terms of cleavage energy, surface strain and surface bonding states. Our results provide important information for a better understanding of the most likely surfaces exposed to the fibers in UHTCMCs, thereby for the construction of reliable interfaces and ultimately UHTCMCs models.     

Defect engineering on phase structure and temperature stability of KNN‐based ceramics sintered in different atmospheres

Lead‐free MnO‐doped 0.955K_0.5Na_0.5NbO₃‐0.045Bi_0.5Na_0.5ZrO₃ (Abbreviated as KNN‐0.045BNZ) ceramics have been prepared by the conventional solid‐state sintering method in reducing atmosphere ( = 1 × 10^?10 atm) and air. For ceramics sintered in reducing atmosphere, only Mn²⁺ ions exist in ceramics who preferentially occupy the cation vacancies in A‐site at x = 0.2‐0.4, whereas Mn²⁺ ions substitute for Zr⁴⁺ ions in B‐site to form defects () at x > 0.4. For ceramics sintered in air, mixed Mn²⁺, Mn³⁺, and Mn⁴⁺ ions coexist here. The Mn²⁺ ions preferentially occupy the cation vacancies in A‐site at x = 0.2‐0.4 and then Mn²⁺ ions substitute for Zr⁴⁺ ions in B‐site at x > 0.4. Meanwhile, the Mn³⁺ ions and Mn⁴⁺ ions substitute for Nb⁵⁺ ions in B‐site to form defects () at x = 0.2‐0.8. The (, , and ) dipolar defects show a positive dipolar defect contribution (DDC) to the , whereas the dipolar defects () show a negative DDC to the . The dipolar defects ( ‐ and ) can help improve the temperature stability of . The 0.4% MnO‐doped KNN‐0.045BNZ ceramics sintered in reducing atmosphere show excellent piezoelectric constant d₃₃ = 300 pC/N and 0.2% MnO‐doped KNN‐0.045BNZ ceramics sintered in air possess optimal piezoelectric constant d₃₃ = 290 pC/N.