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.     

Electrochemical Determination of the Gibbs Energy of Formation of MgAl2O4

The standard Gibbs energy of formation of the spinel MgAl₂O₄ from component oxides, MgO and α-Al₂O₃, has been determined in the temperature range 900 to 1250 K using a solid-state cell incorporating single-crystal CaF₂ as the solid electrolyte. The cell can be represented as—Pt,O₂,MgO+MgF₂|CaF₂|MgF₂+MgAl₂O₄+α-Al₂O₃,O₂,Pt—The standard Gibbs energy of formation from binary oxides, computed from the reversible emf, can be represented by the expression—capdelta G °_f,ox=−23600 − 5.91 T (±150) J/mol—The 'second-law' enthalpy of formation of MgAl₂O₄ obtained in this study is in good agreement with high-temperature solution calorimetric studies reported in the literature.     

Synthesis of Titanate Derivatives Using Ion-Exchange Reaction

Two types of titanate derivatives, layered hydrous titanium dioxide (H₂Ti₄O₉· n H₂O) and potassium octatitanate (K₂Ti₈O₁₇) with a tunnellike structure, were synthesized using an ion-exchange reaction. Fibrous potassium tetratitanate (K₂Ti₄O₉· n H₂O) was prepared by calcination of a mixture of K₂CO₃ and TiO₂ with a molar ratio of 2.8 at 1050°C for 3 h, followed by boiling-water treatment of the calcined products for 10 h. The material then was transformed to layered H₂Ti₄O₉· n H₂O through an exchange of K⁺ ions with H⁺ ions using HCl. K₂Ti₈O₁₇ was formed by a thermal treatment of KHTi₄O₉· n H₂O. Pure KHTi₄O₉· n H₂O phase was effectively produced by a treatment of K₂Ti₄O₉ with 0.005 M HCl solution for 30 min. Thermal treatment at 250°–500°C for 3 h resulted in formation of only K₂Ti₈O₁₇.     

Vaporization Studies of the La2O3–Ga2O3 System

The vaporization of the samples of the compositions Ga₂O₃+ LaGaO₃, LaGaO₃+ La₄Ga₂O₉, and La₄Ga₂O₉+ La₂O₃ was investigated using Knudsen effusion mass spectrometry in the temperature range 1494–1937 K. The partial pressures of the gaseous species O₂, Ga, GaO, Ga₂O, and LaO were determined over the samples investigated. The equilibrium partial pressures were used for the calculation of the thermodynamic activities of the components at 1700 K. Gibbs energies of formation of LaGaO₃( s ) and La₄Ga₂O₉( s ) at 1700 K from the component oxides were derived from the thermodynamic activities as −46.4 ± 4.7 and −99.2 ± 7.9 kJ·mol⁻¹, respectively. The results were compared with the literature data obtained using other methods.     

Liquid Immiscibility in the System K2O-B2O3-SiO2

The subliquidus miscibility gap in the system K₂O-B₂O₃-SiO₂ has been determined for compositions with molar ratios SiO₂/B₂O₃<2 and T≥550°C. The shape of the miscibility gap is an elongated dome similar in form to, but less extensive than those in the lithium and sodium borosilicate systems. The consolute composition (molar) and temperature are estimated to be 4 ± 1 K₂O -30±8 B₂O₃-66±8 SiO₂ and 629±5°C, respectively .     

Modification of αAI2O3 Platelet/ZrO2 Matrix Interface by Epitaxial Growth of β-AI2O3 on the α-AI2O3

An epitaxial β-alumina crystal growth method was used to modify α-AI₂O₃ platelet surfaces before inclusion as a reinforcing phase in partially stabilized zirconia (3Y-TZP). The as-grown surface phase was Na-β"-AI₂O₃. This was converted to Ca-β"-AI₂O₃ by ion exchange, as the latter is more temperature-stable at composite sintering temperatures. The conditions of formation, thermal stability, and chemical compatibility of these interfacial phases were examined. α-AI₂O₃ platelets with Ca-β"-AI₂O₃ film were incorporated into 3Y-TZP. The β"-AI₂O₃/ZrO₂ interface was found to promote platelet debonding and pullout, thus enhancing the α-AI₂O₃ platelet/crack interactions during the fracture process.     

Thermodynamics of CuAlO2 and CuAl2O4 and Phase Equilibria in the System Cu2O-CuO-Al2O3

The standard Gibbs free energies of formation of CuAlO₂ and CuAl₂O₄ were determined in the range 700° to 1100°C, using emf measurements on the galvanic cells (1) Pt,CuO +] Cu₂O/CaO-ZrO₂/O₂,Pt; (2) Pt,Cu +] CuAlO₂+] Al₂O₃/CaO-ZrO₂/ Cu +] Cu₂O,Pt; and (3) Pt,CuAl₂O₄+] CuAlO₂+]Al₂O₃/CaO-ZrO₂/O₂,Pt. The results are compared with published information on the stability of these compounds. The entropy of transformation of CuO from tenorite to the rock-salt structure is evaluated from the present results and from earlier studies on the entropy of formation of spinels from oxides of the rock-salt and corundum structures. The temperatures corresponding to 3-phase equilibria in the system Cu₂O-CuO-Al₂O₃ at specified O₂ pressures calculated from the present results are discussed in reference to available phase diagrams.     

Mixed Protonic–Electronic Conduction in "α-Al2O3": An Alternative Hypothesis

As an alternative, the voltage data of Kurita et al . recently published on galvanic cells with commercial α-Al₂O₃ as a solid electrolyte and with O₂, H₂O/α-Al₂O₃ as well as H₂, H₂O/α-Al₂O₃ as electrodes can be quantitatively described by assuming that α-Al₂O₃ represents a mixed sodium ionic–electronic conductor rather than a protonic–electronic conductor. From the evaluation of the experimental data, numerical values for the p -type electronic conduction parameter are obtained that agree sufficiently well with the data known to date for the sodium ion conductor Na-beta-Al₂O₃.     

High-Pressure Sintering of Nanocrystalline γAl2O3

High-pressure sintering of nanocrystalline γ-A1₂O₃ has been studied over a temperature range of 923-1323 K and at a pressure of 1 GPa. The γ-Al₂O₃ to α-Al₂O₃ transformation temperature changed from 1473 K without pressure to ∼1023 K at 1 GPa. Full density was obtained at 1273 and 1323 K in 10 min. The microhardness value of fully dense α-alumina with a grain size of 142 nm was found to be 25.3 ± 0.8 GPa. The Hall-Petch slope for the very fine grain size range is different from that of the coarse-grained alumina.     

Cassiterite Solubility in High-Silica K2O-Al2O3-SiO2 Liquids

The saturation surface of cassiterite, SnO₂, was determined for liquids in the system K₂O–Al₂O₃–SiO₂ as a function of bulk composition and temperature. At fixed K₂O/Al₂O₃ cassiterite solubility varies weakly with SiO₂ concentration (76 to 84 mol%), temperature (1350° to 1550°C), and log ( f _O2) (−0.7 to −5.3). Cassiterite solubility is also approximately independent of composition in liquids with molar ratios of K₂O/Al₂O₃ lessthan equal to 1 (peraluminous liquids). As K₂O/Al₂O₃ increases from 1 (peralkaline liquids), however, cassiterite solubility increases steeply and approximately linearly with K₂O in excess of Al₂O₃. It is proposed that potassium in excess of aluminum combines with Sn⁴⁺ to form quasi-molecular complexes with an effective stoichiometry of K₄SnO₄.     

Ultraviolet Absorption of Pentavalent Vanadium in Binary Alkali Borate Classes

The ultraviolet absorption of vanadium(V) in R₂O-B₂O₃ glasses (R represents Li, Na, or K) and in aqueous buffer solutions was investigated. In glasses of variable R₂O:B₂O₃ ratio, a change in the absorption spectra similar to the [VO₄]³- → [VO₃(OH)]^2- change in aqueous solution was observed. It is suggested that vanadium(V) in these glasses may be present as either [VO₄]^3- or [VO₃O_1/2]^2- (O_1/2 indicates a bridging oxygen ion), depending on the basicity of the melt. The critical R₂O concentration, above which this change in vanadium(V) environment occurs with increasing alkalinity, is 24, 26, and ≅30 mol%, for K₂O, Na₂O, and Li₂O, respectively.     

Phase Equilibria in the Systems NiO-Cr2,O3,-O2, MgO-Cr2O3−O2, and CdO-Cr2,O3,-O2, at High Oxygen Pressures

Phase equilibria were determined for the systems NiO-Cr₂O₃−O₂, MgO-Cr₂O₃,-O₂, and CdO-Cr₂O₃−O₂ from 450° to above 850° C and at oxygen pressures of from 2 to 3500 atm. Only two intermediate phases were found in the nickel system: NiCrO., (CrVO₄ structure) and the spinel NiCr₂O₄. The magnesium and cadmium systems are similar in that they have three analogous phases: the low-temperature α-MgCrO₄ and α-CdCrO₄ (both with the CrVO₄ structure), the high-temperature β-MgCrO₄ and β-CdCrO₄ (both with the α-MnMoO₄ structure), and the spinels MgCr₂O₄ and CdCr₂O₄. The cadmium system contains an additional phase, Cd₂CrO₅, which is primitive monoclinic.     

Purification of Mullite by Reduction and Volatilization of Impurities

Mullite materials usually contain a residual glassy phase rich in SiO₂, which concentrates impurities as Na₂O, K₂O, Fe₂O₃, and other minority compounds. A suitable way to minimize this glassy phase is the reduction and volatilization of its components by calcination at high temperatures (1300–1450°C) in atmospheres with a very low partial pressure of O₂. Over 95% of the Na₂O, K₂O, and Fe₂O₃ in mullite can be removed in this way, leaving concentrations lower than 0.02% by weight. To avoid the degradation of mullite that occurs when the partial pressure of O₂ is too low, the material to be purified is covered with TiO₂ plates.     

Growth of α-AI2O3 Platelets in the HF-γy-AI2O3 System

In the presence of a fluorine mineralizer, highly aggregated, <5 μm α-Al₂O₃ platelet particles form by vapor transport during the thermal transformation of γ-alumina. Platelet aggregation was determined to occur by platelet inter-growth and by edge nucleation on primary α-Al₂0₃ platelets. The addition of 10¹⁰α-alumina seed particles/cm³γ–Al₂O₃ resulted in the development of discrete particles during the initial stage of transformation. Impingement of the growing platelets during the latter stage of transformation, however, resulted in intergrowth, a process which was not changed by seeding. Particle size distribution broadening was observed to increase with increasing HF and H₂O concentrations because vapor reactant supersaturation increases the degree of edge nucleation. When initially low HF and H₂O concentrations were used in seeded systems, however, essentially aggregate-free α-Al₂O₃ platelets of 10–15 μm were obtained.     

Phase Relations in the Si3N4-Rich Portion of the Si3N4–AlN–Al2O3–Y2O3

The phase relations in the Si₃N₄-rich portion of the Si₃N₄–AlN–Y₂O₃ rystem were investigated using hot-pressed bodies. The one-phase fields of β3 and α, the twophase fields of β+α, β+M (M=melilite), and α+M, and the three-phase fields of β+α+M were observed in the Si₃N₄-rich portion. The α- and β-sialons are not two different compounds but an allotropic transformation phase of the Si–Al–O–N system, and an α solid solution expands and stabilizes with increasing Y₂O₃ content. Therefore, the formulas of the two sialons should be the same.     

Hydration of Sodium β- and β'-Aluminas

The enthalpy and entropy for the hydration of sodium β - and β '-aluminas have been determined by measuring the infrared absorbance of single-crystal samples following equilibration at elevated temperatures in water vapor pressures ranging from 3 to 70 kPa. The equilibrium water concentrations can be varied continuously and reversibly up to saturation concentrations of about 0.9H₂O-Na_1.2AI₁₁O_17.1 for the β-phase and 0.4H₂O.Na_1.67Mg_.67Al_10.33O₁₇ for the β'-phase. The hydration reactions are exothermic; Δ H =−56 ± 2 for sodium β '-aIumina and ΔH =−67 ± 2 kJ/mol for sodium β-alumina. The entropies of hydration are about −143 ± 6 J/(mol-K) for both sodium β - and β '-aIuminas.     

Growth of β-Alumina (Na2O · 11Al2O3) Single Crystals by the Flux Method

The crystal-growth process and growth conditions of β-alumina (Na₂O · Al₂O₃) were investigated using the Na₂B₄O₇-Na₃AlF₆ flux method. β-Alumina (electric fusion brick) was used as both nutrient and seed. Weight loss of the flux varied widely for various runs: ≅ 10 wt% of flux evaporated at 100 h, ≅ 17 wt% at 150 h, and 43 wt% at 600 h. When β-alumina crystal was grown, only 20 wt% Na₂B₄O₇ was added to the Na₃AlF₆ flux. The linear growth rates of the β-alumina single crystal grown by an Na₃AlF₆-20 wt% Na₂B₄O₇ flux method at 1040°C and Δ t = 18°C were ≅ 1.0 × 10⁻³ mm/h ( a face) and ≅0.3 × 10⁻³ mm/h ( c face). The β-alumina single crystals grown were bounded by only c [001] and a [100] and were colorless and transparent.     

Effect of Monovalent Cation Additives on the γ-Al2O3-to-α-Al2O3 Phase Transition

The effect of monovalent cation addition on the γ-Al₂O₃-to-α-Al₂O₃ phase transition was investigated by differential thermal analysis, powder X-ray diffractometry, and specific-surface-area measurements. The cations Li⁺, Na⁺, Ag⁺, K⁺, Rb⁺, and Cs⁺ were added by an impregnation method, using the appropriate nitrate solution. β-Al₂O₃ was the crystalline aluminate phase that formed by reaction between these additives and Al₂O₃ in the vicinity of the γ-to-α-Al₂O₃ transition temperature, with the exception of Li⁺. The transition temperature increased as the ionic radii of the additive increased. The change in specific surface area of these samples after heat treatment showed a trend similar to that of the phase-transition temperature. Thus, Cs⁺ was concluded to be the most effective of the present monovalent additives for enhancing the thermal stability of γ-Al₂O₃. Because the order of the phase-transition temperature coincided with that of the formation temperature of β-Al₂O₃ in these samples, suppression of ionic diffusion in γ-Al₂O₃ by the amorphous phase containing the added cations must have played an important role in retarding the transition to α-Al₂O₃. Larger cations suppressed the diffusion reaction more effectively.     

Phase Equilibrium Investigations on Na–K–(β+β")-Alumina

The activities of Na₂O and K₂O dissolved in mixed-alkali Na–K–(β+β")-Al₂O₃ (NKBA) have been determined by using yttria stabilized zirconia (YSZ) as a solid electrolyte in the following galvanic cells: The approach enables to verify in situ the establishment and maintenance of the β/β"-equilibrium, and to characterize it as a function of the phase composition of NKBA. The results can be expressed as follows:      

Preparation of Larnthanum β-Alumina with High Surface Area by Coprecipitation

Mixtures of La₂O₃ and Al₂O₃ with various La contents were prepared by co-precipitation from La(NO₃)₃ and Al(NO₃)₃ solutions and calcined at 800° to 1400°C. The addition of small amounts of La₂O₃ (2 to 10 mol%) to Al₂O₃ gives rise to the formation of lanthanum β-alumina (La 2 O₃·11–14Al₂O₃) upon heating to above 1000°C and retards the transformation of γ-Al₂O₃ to α-Al₂O₃ and associated sintering.