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.     

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.     

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.     

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.     

Thermodynamic properties of Si(OH)4(g) based on combined experimental and quantum chemistry data

The volatility of silicon‐based ceramics in combustion environments is primarily controlled by the formation of gaseous Si(OH)₄. The heat capacities and entropies of this species at 298.15‐2100 K and P = 0.1 MPa have been studied with the B3LYP density functional theory for the 6‐311+G(d,p) basis set in different approximations: a harmonic oscillator, an anharmonic oscillator, and with corrections for hindered rotors. Experimentally based Gibbs energies of Si(OH)₄(g) at 424‐1661 K have been employed to evaluate the Gibbs energy of formation, , and the entropy, , of Si(OH)₄(g) at = 298.15 K and P = 0.1 MPa. We found that the QC and “experimental” values are very close for the harmonic and anharmonic oscillator approximations, but not for the “hindered rotor” approximation. This conclusion is also supported by calculations of the OH rotational energy for Si(OH)₄ molecule, where the potential barrier was found to exceed 12 kJ/mol. Finally, we recommend the thermodynamic properties of Si(OH)₄ in the ideal gas state at P = 0.1 MPa over the temperature range of 298‐2100 K.     

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.     

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.     

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.     

Simulation Studies of the Phase Stability of the Ruddlesden–Popper Phases

Atomistic simulation techniques are used to examine the stability of Ruddlesden–Popper (R–P) phases Sr(n = 1, 2, 3, 4 and ∞). Various sets of empirical pair potentials are employed to determine the formation energies of the R–P phases. Formation energies are also calculated with Density Functional Theory (DFT). The tendency of a given R–P phase to dissociate into a lower order R–P phase plus SrTiO₃ perovskite is found to increase with increasing n. The results obtained are compared with experiment and previous computational studies. The stability of intergrowth phases with respect to the pure R–P compounds is examined. In all cases the intergrowths are calculated to be thermodynamically less stable than the pure R–P phase, but the differences are in some cases negligible. Finally, the energy for SrO partial Schottky disorder in strontium titanate is computed taking the formation of R–P phases into account.     

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.     

Enhanced O2 Flux of CaTi0.85Fe0.15O3−δ Based Membranes by Mn Doping

Dense symmetric membranes of CaTi_0.85?xFe_0.15Mn_xO_3?δ (x = 0.1, 0.15, 0.25, 0.4) are investigated in order to determine the optimal Mn dopant content with respect to highest O₂ flux. O₂ permeation measurements are performed as function of temperature between 700°C–1000°C and as function of the feed side ranging between 0.01 and 1 bar. X‐ray photoelectron spectroscopy is utilized to elucidate the charge state of Mn, and synchrotron radiation X‐ray powder diffraction (SR‐XPD) is employed to investigate the structure symmetry and cell volume of the perovskite phase at temperatures up to 800°C. The highest O₂ permeability is found for x = 0.25 over the whole temperature and ranges, followed by x = 0.4 above 850°C. The O₂ permeability for x = 0.25 reaches 0.01 mL(STP) min^?1 cm^?1 at 925°C with 0.21 bar feed side and Ar sweep gas. X‐ray photoelectron spectroscopy indicates that the charge state of Mn changes from approx. +3 to +4 when x > 0.1, which implies that Mn mainly improves electronic conductivity for x > 0.1. The cell volume is found to decrease linearly with Mn content, which coincides with an increase in the activation energy of O₂ permeability. These results are consistent with the interpretation of the temperature and dependency of O₂ permeation. The sintering behavior and thermal expansion properties are investigated by dilatometry, which show improved sinterability with increasing Mn content and that the thermal expansion coefficient decreases from 12.4 to 11.9 × 10^?6 K^?1 for x = 0 and x = 0.25, respectively.     

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.     

Microwave Dielectric Properties and Thermally Stimulated Depolarization Currents of (1 − x)MgTiO3–xCa0.8Sr0.2TiO3 Ceramics

(1 ? x)MgTiO₃–xCa_0.8Sr_0.2TiO₃ (0.04 ≤ x ≤ 0.2, MT‐CST) composite ceramics were prepared by the conventional solid‐state reaction process. The phase composition, microwave dielectric properties, and microwave dielectric loss mechanisms were studied. Ca_0.8Sr_0.2TiO₃ was employed as a τ_f compensator for MgTiO₃, and they coexisted well without forming any secondary phases. Interestingly, significant dielectric relaxations associated with oxygen vacancy defects were observed in all the MT‐CST ceramics through the dielectric‐temperature spectra. Thermally simulated depolarization current was therefore conducted to obtain the defects associated with extrinsic dielectric loss mechanisms. The concentrations of both defect dipole and in‐grain oxygen vacancies increased with the increasing x, which could induce microwave dielectric loss consequently. It demonstrated that the behaviors of Q × f were basically influenced by phase composition and defects here. Temperature‐stable ceramics can be achieved at x = 0.06, where the microwave dielectric properties were ε_r = 21.19, Q × f = 110 900 GHz (f = 9.295 GHz), and τ_f = ?0.9 ppm/°C, respectively.     

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.     

Domain engineering enhanced microwave tunability in nonstoichiometric Ba0.8Sr0.2TiO3

Domain engineering via oxygen vacancy, , loading achieved by A/B modification as well as quenching treatment, was utilized for Ba_0.8Sr_0.2TiO₃ (0.8 BSTs) in an attempt to enhance the microwave tunable characteristics. For similar grain sizes, the domain sizes were notably reduced for all nonstoichiometric BSTs, indicating that the loaded (as a consequence of Ti defects, ) played a role in the nuclei for new domain walls. The tunability T at 100 MHz under a direct current field of 30 V/20 μm increased steadily as the domain size (d.s.) declined for all BSTs, regardless of the A/B ratio, due to the d.s. effect. The tunable characteristics in nonstoichiometric BSTs having a similar d.s. of ?190 nm were then compared. The tunability and tan δ decreased for A/B = 1.002 (0.2 mol% Ti defects). The introduced formed pinning centers that restricted domain wall motion, leading directly to lower tunability and smaller dielectric loss. However, ‐overloaded samples (i.e., A/B ≥ 1.005) exhibited increased values for tan δ due to conduction in the domains. The quench treatment of 0.8 BST (with A/B = 1.002) samples resulted in a d.s. reduction from 191 to 170 nm. These quenched specimens showed greater tunability, T_total, originating from the strengthened dipole contribution, T_dipole, as a consequence of the d.s. effect. The tan δ of the quenched specimens was essentially unchanged, indicating a homogenous distribution via the quench, effectively reducing the mobile (which contributes to electrical conduction) in the domains. Consequently, the achieved figure of merit via domain engineering was 2.25 at 100 MHz for the quenched BST with A/B = 1.002, which was 1.54 times larger than that of unmodified BST.     

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.     

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.     

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.     

Enhanced Light Storage of SrAl2O4 Glass‐Ceramics Controlled by Selective Europium Reduction

A luminescent Eu, Dy: SrAl₂O₄ glass‐ceramics with high transparency in the visible region was successfully synthesized using the frozen sorbet technique with the control of O₂ partial pressure () for the oxidation of Eu²⁺ ions. The glass‐ceramics include Eu²⁺, Eu³⁺, and Dy³⁺ ions, and thus exhibits three characteristic types of emission bands, 4f–5d at around 520 nm (Eu²⁺ ions), 4f–4f at 610 nm (Eu³⁺ ions), and 480 nm (Dy³⁺ ions). The Eu, Dy: SrAl₂O₄ glass‐ceramics provide remarkable long‐persistent luminescence under dark condition. The glass‐ceramics also exhibits color‐changing luminescence in the visible region based on their remarkable light storage properties. The luminescent Eu, Dy: SrAl₂O₄ glass‐ceramics using the frozen sorbet technique with control of are promising materials for application in novel photonic and light storage materials.