Dielectric Loss in CaF2 Doped with Oxygen Ions

Dielectric loss was measured on CaF₂ single crystals heated in air at 750° C for 30 hr to introduce O^2- ions. The loss measurements were made in parallel and in perpendicular directions to the CaF₂ surface region which contained most of the O^2-. The layer containing O^2- was more conducting and therefore produced a space charge relaxation near 10 kcps at 210°C. A similar loss peak was previously reported as due to oxygen ion-fluorine vacancy dipole relaxation.     

Oxidation Kinetics of Single-Crystal SrTiO3

The oxidation kinetics were determined for single-crystal SrTiO₃ by measuring the time and temperature dependence of the weight gain of reduced crystals. The oxidation can be described as a diffusion-controlled process. The calculated diffusion coefficients between 850° and 1460°C are represented by D = 0.33 exp (-22.5 ± 5.0 kcal/ RT ) cm²/sec. Directly measured oxygen ion diffusion coefficients in the same temperature interval reported earlier are interpreted as being extrinsic and can be represented by D = 5.2 × 10⁻⁶ exp (-26.1 ± 5.0 kcal/ RT ) cm²/sec, where the activation energy is for mobility only. Assuming that the calculated diffusion coefficients are for vacancy diffusion and the two activation energies are equivalent within experimental error, a vacancy concentration (fraction of vacant lattice sites), [O□], fixed by impurities in the fully oxidized crystal is calculated to be 1.6 × 10⁻⁵ by virtue of the relation between the oxygen self-diffusion coefficient, D_02-, and the oxygen vacancy diffusion coefficient, D_o□ ; D _o2-= [O□] D _o□ where the oxygen ion concentration [O^2-] is taken as unity.     

Phase Equilibria and Structural Species in MgF2-MgO, MgF2-CaO, and MgF2-Al2O3 Systems

Partial equilibrium phase diagrams for the systems MgF₂-MgO, MgF₂-CaO, and MgF₂-Al₂O₃ were determined by differential thermal analysis. Simple eutectics were observed at 8.5 mol% MgO and 1228°± 3°C in the MgF₂-MgO system, at 7.5 mol% CaO and 1208°± 3°C in the MgF₂-CaO system, and at 2.5 mol% Al₂O₃ and 1250°± 3°C in the MgF₂-Al₂O₃ system. On the basis of agreements between the activities calculated by the Clausius-Clapeyron equation and Temkin's model using the present data, the eutectic melt consists of Mg²⁺, F^-, and O^2- ions in the MgF₂-MgO system; Mg²⁺, Ca²⁺, F^-, and O^2- ions in the MgF₂-CaO system; and Mg²⁺, Al³⁺, F^-, and AlO_2¯ ions in the MgF₂-Al₂O₃ system. Well-defined long needles of MgO in the MgF₂-MgO system, less defined needles of CaO in the MgF₂-CaO system, and Al₂O₃ grains in the MgF₂-Al₂O₃ system were observed by optical microscopy.     

CO2-Catalyzed Surface Area and Porosity Changes in High-Surface-Area CaO Aggregates

The changes in surface area and mesoporosity in aggregates of ∼0.01 μm cross-section CaO particles when heated in CO₂at 686°C were determined from N₂ adsorption isotherms. Initially, the surface area decreases rapidly with little change in porosity. When the surface area has decreased below ∼90 m²/g, surface area and porosity variations become consistent with expectations for coarsening by grain-boundary or bulk diffusion. The initial rapid decrease in surface area must result from CO₂-catalyzed surface diffusion, but the data suggest that surface diffusion is not rate-limiting. The rate-limiting step may be reaction of CO₂ to form surface CO₃^2- ions or decomposition of these ions to O^2- ions and CO₂ gas.     

Sulfate Decomposition and Sodium Oxide Activity in Soda–Lime–Silica Glass Melts

Reaction equilibrium constants for the sulfate decomposition process, which releases oxygen and sulfur oxide gas in soda–lime–silica glass melts, have been determined. The chemical solubility of SO₂, probably in the form of sulfite ions in soda–lime–silica melts, has also been determined. The chemical solubility value of SO₂, dissolving as sulfite, ranges between 0.02 and 0.06 wt% SO ₃ ²⁻ at 1 bar SO₂ pressure in the temperature range of 1600–1800 K. Results of square-wave-voltammetry studies and measurements of the temperature-dependent sulfur retention after the fining process of commercial float glass melts and a model soda–lime–silica melt, with 74 wt% SiO₂, 16 wt% Na₂O, and 10 wt% CaO, are presented. The measured sulfur retention data and the results of the square-wave-voltammetry studies are used to determine the equilibrium constant of the sulfate decomposition reaction in the temperature range of 1600–1800 K. The thermodynamic relations and properties found for sulfate decomposition are used to derive activities of sodium oxide in soda–lime–silica melts. Literature values for sodium oxide activities in these glass melts are rare. In this study, these activities have been determined by a method, based on the measurement of sulfate decomposition equilibrium constants and the residual sulfate concentrations in glass melts, equilibrated with almost pure sodium sulfate galls.     

Mass Spectrometric Identification of Si–O–H(g) Species from the Reaction of Silica with Water Vapor at Atmospheric Pressure

A high-pressure sampling mass spectrometer was used to detect the volatile species formed from SiO₂ at temperatures between 1200° and 1400°C in a flowing water vapor/oxygen gas mixture at 1 bar total pressure. The primary vapor species identified was Si(OH)₄. The fragment ion Si(OH)₃⁺was observed in quantities 3 to 5 times larger than the parent ion Si(OH)₄⁺. The Si(OH)₃⁺ intensity was found to have a small temperature dependence and to increase with the water vapor partial pressure as expected. In addition, SiO(OH)⁺, believed to be a fragment of SiO(OH)₂, was observed. These mass spectral results were compared to the behavior of silicon halides.     

Phase Equilibria and Structural Species in NdCl3–NaCl, NdCl3–CaCl2, PrCl3–NaCl, and PrCl3–CaCl2 Systems

Equilibrium phase diagrams for the systems NdCl₃–CaCl₂ and NdCl₃–NaCl were determined by differential thermal analysis. A simple eutectic was observed at 59 ± 1 mol% CaCl₂ and 600°± 2°C in the NdCl₃–CaCl₂ system. A compound NaCl.3NdCl₃ which melts incongruently at 545°± 5°C to NdCl₃ and a liquid containing approximately 47 mol% NaCl, and a eutectic at 68 mol% NaCl and 439°± 2°C were found in the NdCl₃–NaCl system. On the basis of agreements between the activities calculated by the Clausius–Clapeyron equation and Temkin's model using the present data for the NdCl₃–CaCl₂ system and the literature data for the PrCl₃–CaCl₂ system, the melts in the former system consist of Nd³⁺, Ca²⁺, and Cl⁻ ions and in the latter system of Pr³⁺, Ca²⁺, and Cl⁻ ions. The above approach indicates the presence of Na⁺, Cl⁻, and NdCl^2-₅ ions in the NaCl-rich melts and Nd³⁺, Cl⁻, and NdCl⁻₄ in the NdCl₃-rich melts in the NdCl₃–NaCl system. Analogous ions were indicated in the melts of the PrCl₃–NaCl system.     

High-Temperature Creep and Kinetic Demixing in (Co,Mg)O

The creep behavior of fine-grained (Co_0.5Mg_0.5)O and (Co_0.25Mg_0.75)O has been characterized as part of an investigation of kinetic demixing in solid-solution oxides due to a nonhydrostatic stress. (i) For low stresses and small grain sizes, the dominant deformation mechanism for both compositions is diffusional creep limited by the transport of oxygen along grain boundaries. The oxygen grain-boundary diffusivity, D _o ^b is independent of oxygen partial pressure. The values of ω D _o ^b , where ω is the grain-boundary width, that have been determined from the steady-state diffusional creep rates are given by ω D _o ^b =4.7×10⁻⁸ exp[-230 (kJ/mol)/ RT ] (cm³/s) for (Co_0.5Mg_0.5)O in the range 950° to 1200°C and ω D _o ^b =7.4 × 10⁻⁸ exp[-263 (kJ/mol)/ RT ] (cm³/s) for (Co_0.25Mg_0.75)O in the range 1100° to 1250°C. Since oxygen diffusion controls the rate of diffusional creep, kinetic demixing is not observed in deformed samples of either composition. (ii) For high stresses and large grain sizes, the dominant deformation mechanism in both cases is dislocation-climb-controlled creep, where the rate of dislocation climb is controlled by oxygen lattice diffusion. Based on the positive dependence of creep rate on oxygen partial pressure, it is concluded that oxygen diffuses through the lattice by an interstitial mechanism.     

Oxidation Equilibrium of Iron in Borosilicate Glass

Ferrous/ferric equilibria were determined in alkali-alkaline-earth borosilicate glass as a function of temperature, oxygen partial pressure, and glass composition. Expected linear relations are found between log(Fe²⁺/Fe³⁺) and log( p O₂) or 1/ T . The slopes of the correlation with 10g( p O₂) are near the expected value of -0.25, but are found to decrease with increasing temperature. Reaction enthalpies determined from the correlation with 1/ T of –100 to –116 kJ/mol are similar to those reported for other silicate glasses. The ferroudferric equilibrium is not dependent on total iron content in the range 0.5 to 0.09% Fe₂O₃. More reducing conditions are required at lower temperatures to stabilize the amber chromophore. The ferroudferric equilibria are correlated to the number of bridging and nonbridging oxygen ions in the glass. The results suggest that the oxidation-reduction reaction can be written as: Fe²⁺+ (¹/₄)O₂+ (³/₂)O^2-= FeO₂⁻     

Relationship between Doping and Ordering (1/3<111> and 1/2<111> Types) in A(B2+B5+)O3 and A(B3+B6+)O3 Systems

The Madelung energies of A²⁺ (B_1/3²⁺ B_2/3⁵⁺) O₃^2- and A²⁺ (B_2/3³⁺ B_1/3⁶⁺) O₃^2- systems were calculated for different dopants on the A-site. The theoretical results show that the higher-valence dopant enhances the 1/2<111>-type (1:1) ordered arrangement of B-site cations in A²⁺ (B_1/3²⁺ B_2/3²⁺)O₃^2-, whereas the lower-valence dopant promotes the 1:1 ordering in A²⁺ (B_2/3³⁺ B_1/3⁶⁺)O₃^2-, and no doping promotes the 1/3<111>-type (1:2) ordering in the both systems. These calculation results are in agreement with the experimental observations of donor- and acceptor-doped PMN systems.     

Electrical Properties and Cation Valencies in Mn3O4

The electrical conductivity and thermopower of Mn₃O₄ were measured in the temperature range 920° to 1530°C. Electrical conduction in cubic Mn₃O₄ is explained by the small polaron hopping of electron holes between Mn⁴⁺ and Mn³⁺ on octahedral sites. The concentrations of Mn⁴⁺ and Mn³⁺ are governed by the disproportionation equilibrium 2Mn _oct ³⁺⇄Mn _oct ⁴⁺+ Mn _oct ²⁺. This model also explains the electrical behavior of NiMn₂O₄ and CuMn₂O₄.     

Thermodynamic Assessment of the Lithium-Borate System

The Li₂O–B₂O₃ quasi-binary system is assessed. A two-sublattice ionic solution model, (Li¹⁺) _P (O²⁻, BO₃³⁻, B₄O₇²⁻, B₃O_4.5) _Q , is adopted to describe the liquid phase. All solid phases are treated as stoichiometric compounds. A set of parameters consistent with most of the available experimental data on phase diagram and thermodynamic properties is obtained by using the CALPHAD technique.     

Phase Diagram of the Pseudobinary System Ga2O3-Bi2-x3+Bix5+O3+x2-

The pseudobinary system Ga₂O₃-Bi_{2- x} ³⁺Bi _x ⁵⁺O_{3+ x} ^2- was studied in view of its importance for growth of SrGa₁₂O₁₉ crystals from a bismuth oxide flux. A subsolidus transition of γ*-Bi₂O₃ to a β'-phase and a strictly stoichiometric 1:2 phase with 33.3 mol% bismuth oxide were found. The single-crystal data for the compound indicated space group P2₁2₁2, with lattice constants a=0.79180±0.0003 nm, b =0.8288±0.0003 nm, and c =0.5889±0.0003 nm; the measured density was 7.1±0.3 g/cm³ and the cell content (Z) 2.     

Phase Equilibria and Physical Properties of Oxygen-Deficient Zirconia and Thoria

The compositions of anion-deficient zirconia and thoria in equilibrium with O₂ were measured from 1 to 10⁻⁶ atm and 1400° to 1900°C; for ZrO_{2- x} (po₂ in atm, and T in °K), log x∼−0.890-[(0.400×10⁴)/ T ]-[(log p )/6]; for ThO_{2- x} , log x∼−1.870-[(0.340×10⁴)/ T ]-[(log p )/6]. The ZrO_{2- x} -Zr boundary was located at x=0.014 at 1800°C; thoria was single-phase over the entire range. Consistent results were obtained when O₂/inert gas mixtures were used, but use of H₂/H₂O and CO/CO₂ at 1000° to 1200°C gave abnormal and, in the latter case, erratic data; side reactions in these atmospheres are inferred. The monoclinic-tetragonal phase change of ZrO₂ and the lattice thermal expansion, room-temperature Young's modulus, and strength properties of ZrO₂ and ThO₂ bodies were not appreciably altered by oxygen deficiency. The lattice dimensions decreased slightly with departure from stoichiometry.     

Hydroxyapatite Coating on Thermally Oxidized Titanium Substrates

Titanium substrates were oxidized in oxygen or air at temperatures of 600°–800°C, then immersed in solutions of 2.0m M – 20.7m M CaCl₂ and 1.2m M –12.4m M KH₂PO₄ for aging periods of 0.5–10 d. The titanium surface was successfully coated with hydroxyapatite (HAP) when the substrates were oxidized in oxygen gas at 610°C for 1 h and then aged in a solution of 2.00m M Ca²⁺ and 1.20m M PO₄³⁻. The Ca/P ratio of the surface coating increased toward its stoichiometric HAP value (return 10/6) as the aging time increased; the Ca/P ratio attained a value of 1.66 after 10 d.     

Relative Activity of Oxygen Ions in Molten Sodium Phosphate as Determined from the Solubility of Sulfur Trioxide

The solubility of sulfur trioxide in sodium phosphate melts with Na₂O/P₂O₅ ratios from 1.0 to 1.4 was investigated at 653° to 1021°C. The oxygen-ion activity estimated from the solubility of SO₃ increased exponentially as the Na₂O/P₂O₅ ratio increased. This compositional dependence of the activity is discussed on the basis of a polymer model.     

Preparation and Unit-Cell Parameters of Single Crystals of Ba2Ti9020

Ba₂Ti₉O₂₀ crystallizes in the monoclinic system with α= l.4818(5) nm, b = 1.4283(6), and c = 0.7109(2) with β = 98.37°±0.07°. The most likely space group is P 2₁/ m , Z = 4 with a calculated density 4.58 g/cm³. The powder pattern was indexed. The Ba₂Ti₉O₂₀ crystals form as stellated groups when melts of BaCl₂+ 20 to 50% TiO₂ cool from 1275°C.     

Oxygen-Pressure Dependence of Cation Interdiffusivity in Magnesiowustite Solid Solutions

Cation interdiffusivity in magnesiowustite solid solutions was measured as a function of temperature and oxygen pressure. During diffusion, the local composition of Fe²⁺ and Fe³⁺ was always equal to the equilibrium composition with the oxygen pressure in the. gas phase. The interdiffusivity is empirically expressed as D=DP₀ P ₀₂^1/n expβX_Fe . The interdiffusivity increases exponentially with iron mole fraction between 30 and 80 mol% Fe at a given oxygen pressure and temperature. The value of β was between 8 and 10 with the 1/n value 1/2 to 1/5 at 1250° to 1350°C. This oxygen-pressure dependence of the interdiffusivity is equal to that of the product of defect concentration (Roth-type cluster) and correlation factor. From the result, iso-interdiffusivity lines are drawn in the broad range of the magnesiowustite solid solution in the MgO-FeO-Fe₂/₃O quasi-ternary system.     

Solid-State SO2 Sensor Using a Sodium-Ionic Conductor and a Metal–Sulfide Electrode

All solid-state sulfur oxides (SO _x ) sensor devices combined with a sodium ionic conductor (Na₅DySi₄O₁₂) disk and metal sulfide-sensing electrodes synthesized via solution routes have been systematically investigated for the detection of SO₂ in the range of 20–200 ppm at 150–400°C. Among the various sulfide-sensing electrodes tested, the metal monosulfide-based electrodes gave good SO₂ sensitivity at 400°C. The Pb_{1− x} Cd _x S ( x =0.1, 0.2)-based solid electrolyte sensor element showed the best sensing characteristics, i.e., the EMF response was almost linear to the logarithm of SO₂ concentration in the range between 40 and 400 ppm, with a 90% response time to 100 ppm SO₂ of about 3–15 min, and also showed high selectivity to SO₂ at 400°C.     

Ion Exchange in Alkali Layers of Potassium β -Ferrite ((1 + x)K2O · 11Fe2O3) Single Crystals

The potassium ions in potassium β-ferrite ((1 + x)K₂O ·11Fe₂O₃) crystals were exchanged with Na⁺, Rb⁺, Cs⁺, Ag⁺, NH₄⁺, and H₃O⁺ in molten nitrates or in concentrated H₂SO₄. On the other hand, spinel and hexagonal ferrites were formed by soaking the crystals in the melt of divalent salts. The crystals of K⁺, Rb⁺, and Cs⁺β-ferrites decomposed to form α-Fe₂O₃ at high temperatures of 800° to 1100°C. In addition, H₃O⁺, NH₄⁺, and Ag⁺β-ferrites decomposed to form α-Fe₂O₃ at relatively low temperatures of 350° to 650°C, in accordance with the stabilities of the inserted ions. The electrical properties of some β-ferrites were measured.