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.     

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.     

Composition‐induced structural transitions and enhanced strain response in nonstoichiometric NBT‐based ceramics

In this work, the nonstoichiometric 0.99Bi_0.505(Na_0.8K_0.2)_0.5‐xTiO₃‐0.01SrTiO₃ (BNKST(0.5‐x)) ceramics with x=0‐0.03 were synthesized by conventional solid‐state reaction method. The composition‐induced structural transitions were investigated by Raman spectra, dielectric analyses, and electrical measurements. It is found that the relaxor phase can be induced through the modulation of the (Na, K) content. The (Na, K) deficiency in BNKST(0.5‐x) ceramics favors a more disordered local structure and can result in the loss of long‐range ferroelectricity. The x=0.015 critical composition possesses relatively high positive strain S_pos of 0.42% and large signal piezoelectric constant d₃₃* of 479 pm V^?1 at 6 kV mm^?1, along with the good temperature (25‐120°C) and frequency (1‐20 Hz) stability. The recoverable large strain responses in nonstoichiometric ceramics can be attributed to the reversible relaxor‐ferroelectric phase transition, which is closely related to the complex defects (, , and ) and the local random fields. This work may be helpful for the exploration of high‐performance NBT‐based lead‐free materials by means of A‐site compositional modification.     

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.     

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.     

Oxygen Nonstoichiometry and Thermodynamic Explanation of Large Oxygen‐Deficient Ruddlesden–Popper Oxides LaxSr3−xFe2O7−δ

The oxygen nonstoichiometry of large oxygen‐deficient Ruddlesden–Popper oxides La_xSr_3?xFe₂O_7?δ (LSFO7‐x) (x = 0, 0.25, 0.5) was measured by the high‐temperature gravimetry and the coulometric titration. In the composition series, the P(O₂) dependencies exhibited typical plateaus at δ = (2?[])/2. Meanwhile, La_0.5Sr_2.5Fe₂O_7?δ showed the smallest oxygen nonstoichiometry and was the most thermochemically stable compound against P(O₂), temperature, and the La content. Based on the defect equilibrium model and the statistical thermodynamic calculation derived oxygen nonstoichiometric data, the substitution of La for Sr‐site can promote the forward reaction of oxygen incorporation, the backward reaction of the disproportionation of the charge carriers, and oxygen redistribution between the O1 and O3 sites, resulting in the reduction of oxygen‐deficient and the lower decomposition P(O₂). The obtained thermodynamic quantities of the partial molar enthalpy of oxygen, , and the partial molar entropy of oxygen, , calculated from the statistical thermodynamic calculation are in good agreement with those using the Gibbs–Helmholtz equation.     

Effect of the (Ba + Sr)/Ti ratio on the microwave‐tunable properties of Ba0.6Sr0.4TiO3 ceramics

The impact of the (Ba + Sr)/Ti (A/B) ratio on the microwave‐tunable characteristics of diffuse phase transition (DPT) ferroelectric Ba_0.6Sr_0.4TiO₃ (0.6‐BST) ceramics was investigated. The reduction in the lattice constant with increasing nonstoichiometry was attributed to introduced partial Schottky defects, i.e., and . The magnitude of the dielectric constant, ε′, at room temperature in the absence of an applied electric field was governed by the shift in the dielectric maximum temperature, T_m, because T_m was close to room temperature for the 0.6‐BST. The dielectric loss, tanδ, diminished as the ε′ decreased for 0.98≤A/B≤1.05, while the tanδ was much higher for A/B=0.95 having the greatest A‐site vacancy loading. The negatively charged and were mainly compensated by oxygen vacancies and likely partly compensated by holes, h^?, which contributed to the electrical conduction. The tunability, T, at 100 MHz was almost constant at 20%–25% for A/B≥1.00 despite the reduction of the ε′, whereas T decreased for A/B<1.00 to ca. 10% for A/B=0.95 having the greatest A‐site vacancy loading. The results implied that the for larger A/B values was more efficient in generating nucleation sites in the polar nanoregions (PNRs) than the for smaller A/B values, thereby providing greater dipole polarization. Consequently, the figure of merit, FOM, reached its maximum of 250 at A/B=0.9875, which was ca. 155% higher than that of the stoichiometric BST.     

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.     

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.     

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.     

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.     

Temperature stability and electrical properties of MnO‐doped KNN‐based ceramics sintered in reducing atmosphere

Lead‐free MnO‐doped 0.955K_0.5Na_0.5NbO₃‐0.045Bi_0.5Na_0.5ZrO₃ (abbreviate as KNN‐0.045BNZ) ceramics have been prepared by a conventional solid‐state sintering method in reducing atmosphere. The MnO addition can suppress the emergence of the liquid phase and improve the homogenization of grain size. All ceramics sintered in reducing atmosphere show a two‐phase coexistence zone composed of rhombohedral (R) and tetragonal (T) phase. MnO dopant results in the content increase in R phase and slight increase in Curie temperature T_C. For KNN‐0.045BNZ ceramics, Mn²⁺ ions preferentially occupy the cation vacancies in A‐site to decrease oxygen vacancy concentration for 0.2%‐0.4% MnO content, whereas Mn²⁺ ions substitute for Zr⁴⁺ ions in B‐site to form oxygen vacancies at x ≥ 0.5. The defect dipole is formed at the moderate concentration from 0.5 to 0.6, which can provide a preserve force to improve the temperature stability of piezoelectric properties for k_p and . The Mn0.4 ceramics show excellent electrical properties with quasistatic piezoelectric constant d₃₃ = 300 pC/N, electromechanical coupling coefficient k_p = 51.2%, high field piezoelectric constant  = 430 pm/V (at E_max = 25 kV/cm) and T_C = ~345°C, insulation resistivity ρ  =  6.13 × 10¹¹ Ωcm.     

A piezomicrobalance system for high‐temperature mass relaxation characterization of metal oxides: A case study of Pr‐doped ceria

A system for mass relaxation studies based on a gallium phosphate piezocrystal microbalance has been developed, built, and successfully used to characterize a representative mixed ionic and electronic conducting material. The apparatus is constructed to achieve reactor gas exchange times as short as 2 seconds and temporal resolution in mass measurement of 0.1 seconds. These characteristics enabled evaluation of mass relaxations that occurred on the 6 seconds time scale. Proof of concept for materials characterization capabilities of the system was carried out using 10% praseodymium‐doped cerium oxide (PCO), a material that undergoes, at selected temperatures and oxygen partial pressures, changes in mass but not in conductivity. Thin films were deposited on the piezocrystals via pulsed laser deposition (PLD). Mass relaxation curves were collected at 700°C upon application of a small step change in oxygen partial pressure, . Using two different films, the surface reaction constant, k_S, was obtained over the range from 10^?4 to 0.1 atm. Its value is found to vary between 9.7 × 10^?6 and 1.7 × 10^?4 cm/s, displaying a power law dependence on , with a law exponent of 0.67 ± 0.02, as averaged over the two sets of results. This steep dependence of k_S on is surprisingly independent of a change in dominant defect type within the range of measurement.     

Cationic effect of charge compensation on the sulfide capacity of aluminosilicate slags

The effect of CaO on the sulfide capacity of CaO‐Al₂O₃‐SiO₂ slags was studied from the viewpoint of the ionic structure of alumina in slag. The aluminum coordination number was analyzed using ²⁷Al 500‐MHz solid nuclear magnetic resonance spectroscopy and the results were compared with those of the sulfide capacity analysis. The sulfide capacity of slag, in the peralkaline region (), exhibited a linear relationship with respect to basicity () as excess free Ca²⁺ formed a 4‐coordinated aluminum unit structure (^[IV]Al; ) and stabilized the sulfide ions (). However, sulfide capacity in the peraluminous region () exhibited a nonlinear relationship with respect to basicity () owing to the structure of higher‐coordinated aluminum units (^[V]Al, ^[VI]Al; Al³⁺) and the relative lack of Ca²⁺. Therefore, the sulfide capacity of high Al₂O₃‐bearing slags strongly depended on the basicity () and stability of sulfide ions (), which depended on the competitive behavior of Ca²⁺ owing to the structural changes in Al₂O₃. The effect of the aluminum coordination number on the sulfide capacity was discussed in detail using an analysis of the slag structure and thermodynamics model.     

Low‐temperature synthesis of uranium monocarbide by a Pechini‐type in situ polymerizable complex method

Uranium monocarbide (UC) was successfully synthesized by the Pechini‐type in situ polymerizable complex technique (IPC) with the organic matter as the only carbon source. In the aqueous process, a mixture of citric acid (CA) and mannitol with was polymerized to form a spongy‐like organic polymeric precursor without any precipitations. The structural evolution and formation mechanism of the precursor were investigated using XRD, DSC‐TG, SEM (EDX), TEM, and FT‐IR. XRD results demonstrated that UC was obtained with the /mannitol/CA molar ratios of 1.0/0.3/1.0 at a low temperature of 1400°C. SEM and TEM analyses revealed that the UO₂ nanoparticles were uniformly distributed in the carbon matrix to form UO₂/C nanocomposites, and submicrometer‐sized ellipsoidal UC particles cemented together. FT‐IR showed that a ‐CA chelated structure was firstly obtained, achieving the molecular scale mixing of uranium and C. Then the in situ charring guaranteed the intimate contact of UO₂ and C, leading to a low reaction temperature in carbothermal reduction owing to a short diffusion distance.     

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.     

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.     

Structure and Microwave Dielectric Behavior of A‐Site‐Doped Sr(1−1.5x)CexTiO3 Ceramics System

The ion valence state, phase composition, microstructure, and microwave dielectric properties of Sr_(1?1.5x)Ce_xTiO₃ (x = 0.1–0.67, SCT) ceramics were systematically investigated. Sr_(1?1.5x)Ce_xTiO₃ ceramics were produced with gradual structural evolution from a cubic to a tetragonal and turned to an orthorhombic structure in the range of 0.1 ≤ x ≤ 0.67. Above a critical Ce proportion (x = 0.4), microstructural changes and normal grain growth initially occurred. On the basis of chemical analysis results, the reduction of Ti⁴⁺ ions was hastened by tetravalent ions (Ce⁴⁺). By contrast, this reduction was inhibited by trivalent ions (Ce³⁺). The observed dielectric behavior was strongly influenced by phase composition, oxygen vacancies (), and defect dipoles, namely, () and (). Temperature stable ceramics sintered at 1350°C for 3 h in air yielded an intermediate value of dielectric constant (ε_r = 40), with the smallest reported value of temperature coefficient of resonant frequency (τ_f = +0.9 ppm/°C), and quality factor (Q × f = 5699 GHz) at x = 0.6.     

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.