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.     

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.     

Phase Equilibria in the ZrO2–MgO–MnOx System

Phase equilibria were experimentally investigated in the MgO–MnO_x and the ZrO₂–MgO–MnO_x systems for different oxygen partial pressures by powder X‐ray diffractometry, scanning electron microscopy, and differential thermal analysis. The formation of two compositionally and structurally different β‐spinel solid solutions was observed in the MgO–MnO_x system in air in the temperature interval 1473–1713 K. Isothermal sections of the ZrO₂–MgO–MnO_x phase diagram were constructed for air conditions ( = 0.21 bar) at 1913, 1813, 1713, 1613, and 1523 K. In addition, isothermal sections at 1913 and 1523 K were constructed for = 10^?4 bar. The β‐spinel and halite phases of the MgO–MnO_x system were found to dissolve up to 2 and 5 mol% ZrO₂. A continuous c‐ZrO₂ solid solution forms between the boundary ZrO₂–MnO_x and ZrO₂–MgO systems. It stabilizes in the ZrO₂–MgO–MnO_x system down to at least 1613 K in air and down to 1506 K at = 10^?4 bar.     

Electrochemical Determination of Gibbs Energy of Formation of LaCrO3 Using a Composition‐Graded Bielectrolyte

Presented are new measurements of the standard Gibbs free energy of formation of rhombohedral LaCrO₃ from component oxides La₂O₃ and Cr₂O₃ in the temperature range from 875 to 1175 K, using a bielectrolyte solid‐state cell incorporating single crystal CaF₂ and composition‐graded solid electrolyte (LaF₃)_y·(CaF₂)_1?y (y = 0–0.32). The results can be represented analytically as (±2270)/J·mol^?1 = ?72329 + 4.932 (T/K). The measurements were undertaken to resolve serious discrepancies in the data reported in the literature. A critical analysis of previous electrochemical measurements indicates several deficiencies that have been rectified in this study. The enthalpy of formation obtained in this study is consistent with calorimetric data. The standard enthalpy of formation of orthorhombic LaCrO₃ from elements at 298.15 K computed from the results of this study is /kJ·mol^?1 = ?1536.2 (±7). The standard entropy of orthorhombic LaCrO₃ at 298.15 K is estimated as 99.0 (±4.5) J·(mol·K)^?1.     

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.     

Diffusion Kinetics of Indium in TiO2 (Rutile)

This work determines the self‐diffusion coefficients of indium in TiO₂ single crystal (rutile). Diffusion concentration profiles were imposed by deposition of a thin surface layer of InCl₃ on the TiO₂ single crystal and subsequent annealing in the temperature range 1073–1573 K. The diffusion‐induced concentration profiles of indium as a function of depth were determined using secondary ion mass spectrometry (SIMS). These diffusion profiles were used to calculate the self‐diffusion coefficients of indium in the polycrystalline In₂TiO₅ surface layer and the TiO₂ single crystal. The temperature dependence of the respective diffusion coefficients, in the range 1073–1573 K, can be expressed by the following formulas: and The obtained activation energy for bulk diffusion of indium in rutile (316 kJ/mol) is similar to that of zirconium in rutile (325 kJ/mol). The determined diffusion data can be used in selection of optimal processing conditions for TiO₂–In₂O₃ solid solutions.     

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.     

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.     

(K,Na, Li)(Nb,Ta)O3:Mn Lead‐Free Single Crystal with High Piezoelectric Properties

Lead‐free single crystal, (K, Na, Li)(Nb, Ta)O₃:Mn, was successfully grown using top‐seeded solution growth method. Complete matrix of dielectric, piezoelectric, and elastic constants for [001]_C poled single crystal was determined. The piezoelectric coefficient d₃₃ measured by the resonance method was 545 pC/N, which is almost three times that of its ceramic counterpart. The values measured by the Berlincourt meter ( = 630 pC/N) and strain–field curve ( = 870 pm/V) were even higher. The differences were assumed to relate with the different extrinsic contributions of domain wall vibration and domain wall translation during the measurements by different approaches, where the intrinsic contribution (on the order of 539 pm/V) was supposed to be the same. The crystal has ultrahigh electromechanical coupling factor (k₃₃ ~95%) and high ultrasound velocity, which make it promising for high‐frequency medical transducer applications.     

Optimized Growth of Heteroepitaxial (111) NiFe2O4 Thin Films on (0001) Sapphire with Two In‐Plane Variants via Chemical Solution Deposition

(111)‐oriented epitaxial thin films of nickel ferrite (NFO) are grown on c‐plane sapphire [α‐Al₂O₃(0001)] substrates using a chemical solution deposition technique. The processing conditions, including pyrolysis and annealing temperatures, are varied to achieve a film that shows maximum texture and epitaxy. It is shown that increasing the pyrolysis temperature to 400°C and decreasing the annealing temperature to 750°C for 10 min result in the highest degree of texture in the films. Lower film thickness also leads to a higher degree of texture. Microstructural studies confirm an in‐plane epitaxial relationship between the (111) NFO film and the (0001) Al₂O₃ substrate in two variants, [110]_NFO || or .     

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 Quantities of La2Ni0.95Al0.05O4.025 + δ

The oxygen nonstoichiometry of La₂Ni_0.95Al_0.05O_4.025 + δ (LNAO) is measured as a function of oxygen partial pressure (pO₂) and temperature by coulometric titration method and thermogravimetric analysis (TGA) in 800°C–1000°C temperature range and 10^?16‐1 atm pO₂ range. The partial molar quantities for mixing of oxygen were calculated and results were compared with the literature results on La₂NiO_4 + δ (LNO). The variation in activity coefficient of holes versus oxygen nonstoichiometry illustrated an early positive deviation of the activity coefficient of holes from unity, leading to  ≈  7 at δ  ≈  0.08, which was lower than the literature value of  ≈  14 for La₂NiO_{4  +  δ} at δ  ≈  0.08, indicating lesser deviation of LNAO from ideal solution behavior. The effective mass of holes () was 1.02–1.21 times the rest mass (m_o), which indicated the band‐like conduction and allowed the effect of the small degree of polaron hopping to be ignored. The comparison of oxygen nonstoichiometry and partial molar quantities showed that incorporation of interstitial oxygen is less favorable in LNAO in comparison to LNO.     

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.     

Lattice and Grain‐Boundary Diffusion of Al in Tetragonal Yttria‐Stabilized Zirconia Polycrystalline Ceramics (3Y‐TZP) Analyzed Using SIMS

Aluminum oxide was deposited on the surface of 3 mol% yttria‐stabilized tetragonal zirconia polycrystals (3Y‐TZP). The samples were annealed at temperatures from 1523 to 1773 K. Diffusion profiles of Al in the form of mean concentration vs. depth in B‐type kinetic region were investigated by secondary ion mass spectroscopy. The experimental results for the lattice diffusion (D_B) and grain boundary diffusion (D_GB) are as follows: and where δ is the grain‐boundary width and s is the segregation factor.     

Solute Diffusion of Platinum in Rutile Titanium Dioxide

¹⁹⁵Pt solute diffusion coefficients in undoped single‐crystal rutile TiO₂ have been determined parallel to the c‐axis over the temperature range of 1073–1523 K and oxygen partial pressure of 101 kPa. This has yielded the following temperature dependence: On the basis of this result, it has been concluded that ¹⁹⁵Pt diffuses via an interstitial mechanism involving rapid transport parallel to the c‐axis through open interstitial diffusion channels. While other divalent cations have also displayed diffusion via the same mechanism, and report diffusion rates that greatly exceed that for cation self‐diffusion, the overall rate of ¹⁹⁵Pt diffusion is slow. This has been attributed to the size of Pt²⁺ (0.80 Å) which is comparative to the dimension of interstitial channels (0.77 Å) through which transport is believed to be taking place.     

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.     

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.     

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 interfacial energy on microstructure of a directionally solidified Al2O3/YAG eutectic ceramic

An Al₂O₃/Y₃Al₅O₁₂ eutectic ceramic was prepared with a c‐axis sapphire seed by an optical floating zone furnace. The crystallographic relationships in the initial and the steady growth sections of the as‐grown eutectic ceramic were investigated by electron backscattering diffraction and transmission electron microscopy. The corresponding results were: <111 > {101}Y₃Al₅O₁₂ || <0001 > {}Al₂O₃ and <110 > {}Y₃Al₅O₁₂ || < > {0001}Al₂O₃, respectively. The steady orientation of the latter one shows smaller planar disregistry. Interfacial strain played the decisive role in affecting the solidification behavior of the Al₂O₃/Y₃Al₅O₁₂ eutectic ceramic. The stability of interfaces with minimum interfacial strain and better ionic charge balance in irregular microstructure prevail upon the constraint of the seed. These results might cast light for the interfacial design of the Al₂O₃/Y₃Al₅O₁₂ binary eutectic ceramic.     

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.