Formation of Hexagonal Metastable Phases from an Undercooled LuFeO3 Melt in an Atmosphere with Low Oxygen Partial Pressure

Controlling the oxygen partial pressure ( P _O2) in the ambient atmosphere is an important parameter for material processing because the transition metal ion changes its valence depending on P _O2. In the present study, containerless solidification of the LuFeO₃ melt, where the undercooling level can be treated as another experimental parameter, was carried out in order to explore the unidentified metastable phases under the controlled P _O2 using an aerodynamic levitator. Decreasing P _O2 down to 1 × 10³ Pa, the unidentified phase was solidified from the undercooled melt. The X-ray diffractometry results after annealing at 1 × 10³ Pa showed the peak profile of the stable perovskite LuFeO₃ phase, suggesting that this unidentified phase was thermodynamically metastable. Thermogravimetric analysis showed that the mass of the sample solidified at P _O2= 1 × 10³ Pa significantly increased, suggesting that the formation of the metastable phases might be related to the presence of Fe²⁺ ions.     

Electronic Conductivity of La-Doped Ceria Ceramics

Lanthanum-doped ceria powder with a composition Ce_0.8La_0.2O_1.9 was prepared by heating the oxalate solid solution (Ce_0.8La_0.2)₂(C₂O₄)₃ at 873 K in air. As-prepared powder was densified to 96%–97% relative density by sintering in air at 1773 K for 4 h. The electronic current of the disk sample was measured in a temperature range from 773 to 1113 K by the direct current polarization method using a Hebb–Wagner ion-blocking cell. A linear relationship, which was theoretically predicted, was measured between log σ_e (electronic conductivity) and E (applied voltage) in the applied voltage range of 0.2–1.0 V. The σ_e was proportional to P _O2^−1/4.3≈ P _O2^−1/4.6 in the oxygen partial pressure range of 10⁻²–10⁻⁸ Pa, and to P _O2^−1/6.7≈ P _O2^−1/7.1 in the oxygen partial pressure range of 10^−7.5–10⁻²² Pa. The apparent activation energy of the electronic conduction was 1.87–1.94 eV. The hole conduction was also measured in the P o₂ range of 10²–10⁵ Pa. The transport number of oxide ion was 0.96–1.00 at 773–1113 K under an oxygen partial pressure of 10⁻⁵ Pa.     

Use of Ozone to Prepare Silver Oxides

The possible use of ozone to produce higher oxides was analyzed thermodynamically. Because the activity of oxygen in ozone is ∼10¹⁸ at room temperature and ∼10⁸ at 773 K, ozone may react with metals to yield higher-oxidation-state products than does an ambient pressure of pure oxygen. In agreement with this thermodynamic prediction, silver oxides (AgO and Ag₂O) were synthesized experimentally by blowing a gas mixture of 6 vol% ozone and oxygen through a water-cooled lance. Neither Ag₃O₄ nor Ag₂O₃ was detected. Slow oxygen diffusion into a metal and/or reaction products and decomposition of unstable oxides that are formed successively are obstacles to practical applications of syntheses via ozone oxidation.     

Oxidation of Low-Oxygen Silicon Carbide Fibers (Hi-Nicalon) in Carbon Dioxide

Polycarbosilane-derived low-oxygen SiC fibers, Hi-Nicalon, were heat-treated for 36 ks at temperatures from 1273 to 1773 K in CO₂ gas. The oxidation of the fibers was investigated through the examination of mass change, crystal phase, resistivity, morphology, and tensile strength. The mass gain, growth of β-SiC crystallites, reduction of resistivity of the fiber core, and formation of protective SiO₂ film were observed for the fibers after heat treatment in CO₂ gas. SiO₂ film crystallized into cristobalite above 1573 K. Despite the low oxygen potential of CO₂ gas ( p _O2= 1.22 Pa at 1273 K − 1.78 × 10² Pa at 1773 K), Hi-Nicalon fibers were passively oxidized at a high rate. There was a large loss of tensile strength in the as-oxidized state at higher temperatures because of imperfections in the SiO₂ film. On the other hand, the fiber cores showed better strength retention even after oxidation at 1773 K.     

Oxygen-Ion Diffusion in a 110 K Phase BiPbSrCaCuO Superconductor

The oxygen-ion conductivity of a hot-pressed 110 K phase (2223) superconductor was determined by using an O₂,Pt/ZrO₂/110 K phase/ZrO₂/Pt,O₂ cell and impedance spectroscopy at temperature of 600°-800°C and oxygen partial pressures ( P _O2) of 0.21-0.001 atm (∼21-0.1 kPa). The oxygen-ion diffusivities were calculated via the Nerst-Einstein equation and were greater in air than at P _O2= 0.001 atm. Decomposition of the 110 K phase at 800°C and P _O2= 0.001 atm increased the bulk diffusivity but had a lesser effect on the grain-boundary diffusivities. The activation energies for oxygen diffusion, at different P _O2 values, have been determined.     

Defect Structure and Thermodynamic Properties of the Wustite Phase (Fe1-yO)

The defect structure model in the wustite phase is considered involving the 4:l cluster as a basic defect structure unit. Interactions between defects are considered in terms of the Debye-Hückel theory for strong electrolytes. It has been shown that the available data on wustite nonstoichiometry fit well the 4:l cluster model. Above 1173 K the degree of ionization of the cluster is −5. The equilibrium constant of the cluster formation determined in this work is K _c = [(( V _Fe)₄Fe _i )⁵⁻][h]⁵ [Fe_Fe]⁻⁶[V _i ]⁻¹P_O2^−3/2 f ⁶_±= 1.07 × 10⁻¹⁴ exp[396.7 (kJ)/ RT ]. The equilibrium constant is quite consistent with such thermodynamic parameters as partial molar enthalpy and entropy of oxygen in Fe_{1- y} O within the entire stability range of the wustite phase.     

Solubility of Platinum and Rhodium in Lime–Alumina–Silica Melts at 1700 K

Three oxide melts of composition 10-20-70, 30-13-57, and 40-20-40 (wt%) in the ternary system CaO–Al₂O₃–SiO₂ were equilibrated at 1700 K in platinum or platinum–rhodium boats, under oxygen partial pressures ( P _O2) ranging from 10⁻¹³ to 1 atm, and fixed by various gaseous mixtures (H₂/H₂O, CO/CO₂, and Ar/O₂). The melts were analyzed for platinum and rhodium content after quenching. The complex dependence of solubility of these elements with P _O2 was explained by their speciation. Chronopotentiometric measurements confirmed the passivation of the metallic boats. For increased P _O2, this speciation followed the series Pt, PtO, PtO₂ or RhO for the solid phases covering the metallic boats and Pt⁰, Pt²⁺, Pt⁴⁺, Pt⁶⁺ or Rh⁰, Rh²⁺, Rh³⁺, Rh⁴⁺ in solution in the oxide melt. The predominance diagrams were determined for the three compositions in the ternary system.     

Defect Structure of Cobalt Monoxide: III, The Cluster Model

The 4:1 cluster model has been considered and its application to the available experimental data for the deviation from stoichiometry, electrical conductivity, and the Seebeck coefticient has been analyzed. It has been shown that the degree of ionization of the cluster is −5. The equilibrium constant of formation is K ₅= [( V _Co)Co _i ]⁵⁻[h^˙]⁵ p _O2^3/2= 4.6 exp[−167.5 (kJ)/ RT ]. It has been shown that the applicability of this model in describing CoO properties is limited to higher values of the deviation from the stoichiometric composition (y). At low y , defects in CoO can be considered as isolated. Then doubly ionized cobalt vacancies are assumed to be the predominant lattice defects. Their equilibrium constant of formation, determined from experimental data in the vicinity of the Co/CoO phase boundary by using an ideal approximation, is K _V = [ V ⁿ _Co][h^˙]² p _o2^−1/2= 3.2 × 10⁻⁴ exp[−121.1 (kJ)/ RT ]. Excellent agreement between the equilibrium constant K _V determined for low p _{O 2} and that resulting from the Debye Hückel model within the entire range of y has been found.     

Structure and Properties of Silver Borate Glasses

Clear glasses form in the system Ag₂O-B₂O₃ up to about 35 mol% (65 wt%) Ag₂O. Infrared absorption, thermal expansion, and density data indicated an analogy to the Na₂O-B₂O₃ system. Pentaborate-triborate group pairs appear to be formed on addition of Ag₂O to B₂O₃ up to 20 mol% Ag₂O and diborate groups from 20 to 33 mol% Ag₂O. This interpretation is supported by the comparison of the infrared absorption spectra of quenched and crystallized glasses. One crystallization product, Ag₂O-4B₂O₃, was identified previously. A new compound starts to appear at 28 mol% Ag₂O. The theory that silver is generally present as a network modifier like sodium was substantiated by the comparison of the molar volume of sodium and silver borate glasses. Above 27 mol% Ag₂O some atomic silver is assumed to be present; below 15 mol%, exploratory studies indicate a two-phase structure within an immiscibility gap. A low-temperature internal friction peak in the glasses up to 28 mol% Ag₂O corresponds to the alkali peak in other glasses; a high temperature peak appearing in the 34 mol% Ag₂O glass is associated with the appearance of nonbridging oxygen in the system.     

Determination of CaO–SiO2–MgO–Al2O3–CrOx Liquidus

In this work, the liquidus of synthetic CaO–SiO₂–MgO–Al₂O₃–CrO _x slags is evaluated in the industrially relevant compositional domain. Equilibrium experiments are carried out at 1500°C and partial oxygen pressure ( p _O2) 10^−11.04 atm, and at 1600°C and p _O2=10^−10.16 and 10^−9.36 atm. The studied basicities (CaO/SiO₂) are 1.2 and 0.5. Al₂O₃ levels range from 0 to 30 wt%. Oversaturated liquid is sampled and phase relations are measured with quantitative electron probe microanalysis–wavelength dispersive spectroscopy (EPMA–WDS). The results are compared with the commercially available FactSage thermodynamic databases. Qualitative agreement is always obtained. Also a good quantitative agreement is found at the higher basicity, especially for the spinel liquidus. A minor but systematic deviation can be observed for the eskolaite liquidus. At the lower basicity, the calculated phase diagram deviates strongly from the experimental results, probably due to missing ternary interactions in the database.     

Electrical Conductivity and Thermoelectric Power of Uranium Dioxide

The electrical conductivity and thermoelectric power of an uranium dioxide single crystal were measured between 908 and 1697 K and for P _O2 included between 10⁻²⁴ atm and the boundary UO_{2+ x} /U₄O₉. For T <1273 K and near stoichiometry, the electrical conductivity versus P _O2 shows a plateau characteristic of an extrinsic regime. For T ≥1273 K, the conductivity exhibits a minimum. The Seebeck coefficient ̄ _UO2 also exhibits a behavior change when the temperature is increased. For T <1200 K, ̄ _UO2 remains positive whatever the values of P _O2. For T>1200 K, ̄ _UO2 changes from negative to positive values when P _O2 increases. This set of results shows that the stoichiometric oxide presents a p extrinsic to intrinsic transition near 1273 K. At T ≤1273 K, the extrinsic conductivity values versus temperature confirm that the mobility of the holes occurs by a small polaron process, with an activation energy of 0.17 eV. At T >1273 K, the electrical conductivity minima are characteristic of a p → n transition. A band gap energy of 2.0 eV has been calculated from the temperature dependence of these conductivity minima values. Furthermore, this set of results has allowed us to determine the relative oxygen partial molar free energy of stoichiometric UO₂.     

Synthesis and Characterization of Aurivillius Phases in the Bi–Ag–Ti–O System

New Aurivillius phases in the Bi–Ag–Ti–O system were investigated by means of a solid-state reaction and X-ray diffraction. We found that the oxygen partial pressure has a significant influence on the synthesis of the Aurivillius phases. The mixed-layer Aurivillius phase Ag_0.5Bi_8.5Ti₇O₂₇ was observed after firing in an O₂ flow, but a single-phase material is difficult to obtain. A single-phase compound of the four-layer Aurivillius phase Ag_0.5Bi_4.5Ti₄O₁₅ was obtained on firing in an oxygen partial pressure of 10 bar (1 × 10⁶ Pa). The dielectric properties (at 1 MHz) of the Ag_0.5Bi_4.5Ti₄O₁₅ compound were as follows: T _max=687°C, ɛ _r =166 (∼20°C), and tan δ=0.004 (∼20°C).     

Limits of the Thermodynamic Stability of Cobalt—Iron—Manganese Mixed Oxides at 1200°C

The thermodynamic stability of spinel and rock-salt structure phases has been investigated for the quasi-ternary mixed oxide system Co-Fe-Mn-O at 1200°C and at total pressures of the order of 1 atm (∼1.01 × 10⁵ Pa) using thermogravimetric and electrical conductivity measurements. The results reveal the stability limits of the spinel and rock-salt structure phases with varying cationic composition and oxygen partial pressure at 1200°C. The oxygen partial pressure was varied over 12 orders of magnitude, from log₁₀ a _o2= 0 to log₁₀ a _o2= -12, by using CO/CO₂ and N₂/O₂ gas mixtures and monitored by an electrochemical, stabilized zirconia-based EMF cell. The maximum cobalt mole fraction used in the investigation was about 0.33.     

Direct Measurement of Relative Partial Molar Enthalpy of SiO2 in SiO2–M2O (M=Li, Na, K, Cs) Binary and SiO2–CaO–Al2O3 Ternary Melts

The relative partial molar enthalpies, Δ _SiO2, of SiO₂ in SiO₂–M₂O (M = Li, Na, K and Cs) binary and SiO₂–CaO–Al₂O₃ternary melts were directly measured by drop-solution calorimetry at 1465 K and 1663 K. Δ _SiO2 changes from exothermic to endothermic as silica content increases, confirming the tendency toward immisciblity seen from activity measurements. It is concluded that Δ _SiO2 is negative due to acid-base reactions and charge-coupled substitutions when the melt is composed of fewer Q ⁴ and more Q ³ and Q ² species, but positive due to structural strain when the melt is composed of mostly Q ⁴ species. The Δ _SiO2 obtained by calorimetry is a useful measure of basicity, when comparing different alkali and alkaline earth oxides.     

Defect Equilibria and Transport in YBa2Cu3O7-x at Elevated Temperatures: I, Thermopower, Electrical Conductivity, and Galvanic Cell Studies

Studies of the Y-Ba-Cu-O system at elevated temperatures (300 to 1173 K) and under controlled oxygen activity (10² to 10⁵ Pa) were performed by using electrical methods such as thermopower, electrical methods such as thermopower, electrical conductivity, and emf of solid-state galvanic cell measurements. The discontinuities of measured electrical parameters demarcate regions of different properties and phenomena: (a) the transition between the range when the oxide sample is quenched on one side and the equilibrium range on the other side and (b) the transition between the quasi-metallic and semiconducting (m/s) regimes. The temperature of the m/s transition increases from 950 K at p o₂= 10² Pa to 1150 K at 10⁴ Pa. These data have served to derive a T-p o₂ diagram for YBa₂Cu₃O_7-x. NO discontinuity of the measured electrical parameters has been observed under conditions accompanying the phase transition between the tetragonal and orthorhombic structures. A p-n type transition has been observed within the semiconducting regime. Electrochemical oxygen titration has led to the determination of the relationship between the lattice oxygen content and the oxygen activity in the ambient gas atmosphere.     

Nonstoichiometry and Mixed Conduction in α-Ta2O5

Electrical conductivity of single-crystal and polycrystalline Ta₂O₅ was measured as a function of composition, temperature, and oxygen partial pressure ( P _O2) by both dc and ac complex impedance methods. A defect model based on the assumption of predominant disorder on the oxygen sublattice is proposed to explain the observed nonstoichiometry of α-Ta₂O₅. The defect model is consistent with the observed P _O2-independent ionic conductivity and the P ^−1/4_O2-dependent electronic conductivity. Blocking of the ionic component was observed at low P _O2. Both the electronic and ionic components of conductivity were found to decrease with time. This aging phenomena is related to destabilization of the a phase.     

Nonlinear Optical Properties of B2O3-Based Glasses: M2O-B2O3(M = Li, Na, K, Rb, Cs, and Ag) Binary Borate Glasses

The third-order nonlinear optical susceptibilities χ⁽³⁾ of M₂O-B₂O₃ (M = Li, Na, K, Rb, Cs, and Ag) binary borate glasses have been measured by the third harmonic generation (THG) method. It is found that the enhancement of χ⁽³⁾by the structural change of BO₃ units to BO₄ units is small, while the enhancement of χ⁽³⁾ due to the formation of non- bridging oxygen is rather significant. The effects of alkali cations on the χ⁽³⁾ of alkali borate glasses are discussed in terms of the M-O bond character, focusing on the covalency of Li₂O-B₂O₃ glasses. Comparison of the χ⁽³⁾ values for Cs₂O-B₂O₃ and Ag₂O-B₂O₃ glasses which contain cations of comparable polarizability reveals that the χ⁽³⁾ value is much greater for Ag₂O-B₂O₃ glasses than for Cs₂O-B₂O₃glasses, which is possibly due to the great contribution of Ag(4 d_z2 + 5 s + 5 p_z ) hybrid orbitals to the nonlinear optical response.     

Chemical Compatibility between Silver Electrodes and Low-Firing Binary-Oxide Compounds: Conceptual Study

The chemical compatibility of silver electrodes and low-firing ceramics has been considered, in terms of the existence of a tie line between silver (and/or Ag₂O) and the binary oxide compound in the corresponding ternary phase diagram. The probability of the existence of the tie line is related to the conditions in the subordinated silver-based isothermal binary systems. Greater probabilities have been calculated for the systems with fewer silver-based binary compounds. Based on the concepts that have been developed, several silver-based isothermal binary systems have been investigated to identify the oxides suitable for the development of low-temperature cofired ceramics. The developed concept has been tested by investigating the phase relations in the Bi₂O₃–Nb₂O₅ and Bi₂O₃–V₂O₅ ternary systems with silver. X-ray and microstructural investigations of bismuth niobates and bismuth vanadates reveal that, as a result of the inertness of Bi₂O₃ and the reactivity of Nb₂O₅ and V₂O₅ toward silver, compounds that are rich in niobium or vanadium react with silver to form ternary Nb/V-Bi-Ag oxide compounds, whereas for compounds that are rich in bismuth, tie lines to silver and Ag₂O do exist.     

Thermodynamic Study of the Cu-Sr-O System

The Gibbs energies of formation of the inter-oxide compounds in the Cu-Sr-O system have been obtained from solid-state electrochemical measurements in the temperature range 900 to 1300 K. Cells employing yttria-stabilized zirconia or single-crystal calcium fluoride as electrolyte were used in studies of SrCu₂O₂, Sr₂CuO₃, SrCuO₂, and "Sr₃Cu₅O_{8+ z} ." The oxygen potential vs stoichiometry dependence was studied, particularly for this last compound, by thermogravimetry at a few selected temperatures in the oxygen partial pressure interval 1 × 10¹ to 1 × 10⁵ Pa. Based on the results of emf measurement, thermogravimetric study, and X-ray powder diffraction, phase relations in this system were calculated.