Interdiffusion and Association Phenomena in the System NiO-Al2O3

The rate of formation of NiAl₂O₄ by reaction between single crystals of NiO and Al₂O₃ can be described by k = 1.1 × 10⁴ exp (−108,000 ± 5,000/ RT ) cm²/s. In NiO the behavior of D as a function of concentration supports the Lidiard theory of diffusion by impurity-vacancy pairs. A good fit of the theory to the experimental results was obtained by assuming that Al³⁺ ions diffuse as [Al_Ni· V_Ni]'pairs. The diffusion coefficient of pairs, D_p , obeys the equation 6.6 × 10⁻² exp (−54,000 ± 3,000/ RT ) cm²/s. The free energy of association for pairs was calculated to range from 6.5 kcal/mol at 1789°C to 9.0 kcal/mol at 1540°C. The interdiffusion coefficients in the spinel showed a constant small increase with increasing concentration of Al³⁺ dissolved in the spinel.     

Kinetics of NiCr2O4 Formation and Diffusion of Cr3+ Ions in NiO

The reaction kinetics for NiCr₂O₄ formation and the diffusion of Cr³⁺ ions into single-crystal NiO were studied between 1300° and 1600°C in air. The experimental activation energy for NiCr₂O₄ formation was about 83 kcal/mol. After incubation, NiCr₂O₄ formed by a diffusion-controlled process. The origin of pores at the NiO/NiCr₂O₄ interface is discussed. The concentration profiles of Cr³⁺ in NiO were linear because the interdiffusion coefficient was directly proportional to the mol fraction Cr³⁺. Theoretical considerations indicate that the interdiffusion coefficient equals 3/2 the self-diffusion coefficient of Cr³⁺, which is rate-determining. The interdiffusion coefficient at 1 mol% Cr₂O₃ can be expressed as =4×10⁻³ exp (−55,000/RT) cm² s⁻¹.     

Fast Li+ Ion Conduction in Li2O-Al2O3-TiO2-SiO2-P2O5 Glass-Ceramics

Fast lithium ion conducting glass-ceramics have been successfully prepared from the pseudobinary system 2[Li_{1+ x} Ti₂Si _x P_{3− x} O₁₂]-AlPO₄. The major phase present in the glass-ceramics was LiTi₂P₃O₁₂ in which Ti⁴⁺ ions and P⁵⁺ ions were partially replaced by Al³⁺ ions and Si⁴⁺ ions, respectively. Increasing x resulted in a considerable enhancement in conductivity, and in a wide composition range extremely high conductivity over 10⁻³ S/cm was obtained at room temperature.     

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.     

Effects of M3+ Ions on the Conductivity of Glasses and Glass-Ceramics in the System Li2O—M2O3—GeO2—P2O5 (M = Al, Ga, Y, Dy, Gd, and La)

Glasses with compositions Li_1.2M_0.2Ge_1.8(PO₄)₃ (M = Al, Ga, Y, Gd, Dy, and La) were prepared and converted to glass-ceramics by heat treatment. The effects of the M³⁺ ions on the conductivity of the glasses and glass-ceramics were studied. The main phase present in the glass-ceramics was the conductive phase LiGe₂(PO₄)₃. Al³⁺ and Ga³⁺ ions entered the LiGe₂(PO₄)₃ structure by replacing Ge⁴⁺ ions, but lanthanide ions did not. The glass-ceramics exhibited much higher conductivity than the glasses. With increased ionic radius of the M³⁺ ions, the conductivity remained almost unchanged at ∼3 × 10⁻¹² S/cm for the glasses, but it decreased from 1.5 × 10⁻⁵ to 8 × 10⁻⁹ S/cm for the glass-ceramics at room temperature. The higher conductivity for Al³⁺- and Ga³⁺-containing glass-ceramics was suggested to result from the substitutions of Al³⁺ and Ga³⁺ ions for Ge⁴⁺ ions in the LiGe₂(PO₄)₃ structure.     

Phase Equilibria in the Spinel Region of the System FeO-Fe2O3-Al2O3

Phase relations in the spinel region of the system FeO-Fe₂O₃-Al₂O₃ were determined in CO₂ at 1300°, 1400°, and 15000°C and for partial oxygen pressures of 4 × 10⁻⁷ and 7 × 10⁻¹⁰ atmospheres at 15OO°C. The spinel field extends continuously from Fe₃O_4-x to FeAl₂O_4+z.     

Formation of ZnAl2O4 in the Presence of Cl2

The formation of ZnAl₂O₄ spinel in diffusion couples of Al₂O₃ and ZnO was investigated between 1000° and 1390°C in air and in air containing 4.8 vol% Cl₂ by X-ray diffraction, electron probe microanalysis, and scanning electron microscopy. The rate of formation of a spinel layer obeyed a parabolic rate law and was accelerated remarkably by the presence of Cl₂. The interdiffusion coefficient, , and the activation energy, E, were calculated to be 10⁻⁸ to 10⁻⁹ cm²/s and 123 kcal/mol (514 kJ/mol) in air and 10⁻⁷ cm²/s and 31 kcal/mol (130 kJ/mol) in air containing 4.8 vol% Cl₂, respectively.     

Thermal Expansion of the Low-Temperature Form of BaB2O4

Thermal expansion of the low-temperature form of BaB₂O₄ (β-BaB₂O₄) crystal has been measured along the principal crystallographic directions over a temperature range of 9° to 874°C by means of high-temperature X-ray powder diffraction. This crystal belongs to the trigonal system and exhibits strongly anisotropic thermal expansions. The expansion along the c axis is from 12.720 to 13.214 Å (1.2720 to 1.3214 nm), whereas it is from 12.531 to 12.578 Å (1.2531 to 1.2578 nm) along the a axis. The expansions are nonlinear. The coefficients A, B , and C in the expansion formula L _t = L ₀(1 + At + Bt ²+ Ct ³) are given as follows: a axis, A = 1.535 × 10⁻⁷, B = 6.047 × 10⁻⁹, C = -1.261 × 10⁻¹²; c axis, A = 3.256 × 10⁻⁵, B = 1.341 × 10⁻⁸, C = -1.954 × 10⁻¹²; and cell volume V, A = 3.107 × 10⁻⁵, B = 3.406 × 10⁻⁸, C = -1.197 × 10⁻¹¹. Based on α _t = (d L _t /d t )/ L ₀, the thermal expansion coefficients are also given as a function of temperature for the crystallographic axes a , c , and cell volume V.     

Measurement of Oxygen Diffusion in Dy2O3 and Gd2O3 by Studying High-Temperature Oxidation of the Metals

Studies of the oxidation of Gd and Dy at P _O2's from 10^−0.3 to 10^−14.5 atm and temperatures from 727° to 1327°C indicate both semiconducting and ionic-conducting domains in the sesquioxides formed. At higher temperatures, where dense coarsegrained oxide layers developed, the rate of oxidation in the high- P ₀₂ semiconducting domain yielded oxygen diffusion coefficients in Dy₂O₃ in excellent agreement with literature values derived from oxidation of partially reduced oxide single crystals. Under the same conditions, the oxidation of Gd yielded oxygen diffusion coefficients in cubic Gd₂O₃ which are considerably below literature values for monoclinic single-crystal Gd₂O₃. At lower temperatures, porous scales were formed, and apparent diffusion coefficients derived from oxidation rates show a smaller temperature dependence than the high-temperature data. At low P _O2, the oxides behave as ionic conductors, and metal oxidation rates result in estimates of the electronic contribution to the electrical conductivity of the order of 10⁻⁶ to 10⁻⁷Ω⁻¹ cm⁻¹.     

The System MgO–MgAl2O4

In the determination of the liquidus, solidus, and subsolidus of the system MgO-MgAl₂O₄ the limits of the solid solution of A1 ions in periclase and Mg ions in spinel were measured. By using both X-ray diffraction and optical techniques, the maximum periclase solid solution was found at 82 wt% MgO, 18 wt% A12O3 (9.5% A1³⁺) and maximum spinel solid solution at 39% MgO, 61 % A1203 (6% Mg⁺⁺). Periclase and spinel solid solutions existed stably in easily detectable amounts at temperatures above approximately 1500°C.     

Vacuum Vaporization in the System MgO-Cr2O3

The vaporization of the system MgO-Cr₂O₃ was studied in a vacuum of 10⁻⁵ torr (10⁻³N/m²) at 1500° to 1700°C using the Langmuir and Knudsen methods. It was found that the phases in the system vaporize nearly congruently and the logarithm of the vaporization coefficient, α, of MgCr₂O₄ increases linearly with increasing reciprocal temperature. Alpha tends to unity at a temperature near the melting point (2525±23°C). The additivity rule can be applied to the Langmuir vaporization rates on the basis of the surface area ratios of the phases in the 2-phase system MgO-Cr₂O₃. The enthalpies of vaporization of MgCr₂O₄ were 695.0 and 549.2 kcaVmol for activated and equilibrium processes, respectively.     

Diffusion of 51Cr in MgO Crystals

An arc fusion technique was used to grow single crystals of MgO with an aliovalent impurity level of ∼ 250 ppm. Diffusion coefficients for Cr³⁺ in single-crystal MgO were measured from 1353° to 1553°C using a high-specific-activity isotope,⁵¹ Cr. The diffusion coefficient of ⁵¹Cr for both arc-grown and vapor-deposited MgO crystals can be characterized by an activation energy of 70.0±2.1 kcal/mol. These results are comparable with the interdiffusion coefficients in Cr₂O₃-MgO when the interdif-fusion D is extrapolated to low Cr concentrations.     

Energetics of Substituted Pollucites Along the CsAlSi2O6–CsTiSi2O6.5 Join: A High-Temperature Calorimetric Study

Enthalpies of drop solution for a suite of substituted pollucites with the compositions CsTi _x Al_{1− x} Si₂O_{6+0.5 x} , 0 ≤ x ≤ 1, which are synthesized using the sol–gel method, have been measured in molten lead borate (2PbO·B₂O₃) at 701°C. As Ti⁴⁺ substitutes for Al³⁺, the enthalpies of drop solution become less endothermic and show exothermic heats of mixing within the composition range from x = 0.3 to 1. This nonideal mixing behavior is consistent with the trend seen in variation of lattice parameters, and we interpret it to be a result of the short-range order associated with the framework cations Al³⁺, Si⁴⁺, and Ti⁴⁺ in the structure. Using enthalpies of drop solution of SiO₂, Al₂O₃, TiO₂, and Cs₂O, standard molar enthalpies of formation of these phases from their constituent oxides and from the elements are derived for the first time.     

Structure of Liquid Al6Si2O13 (3:2 Mullite)

We report the first measurements of the structure factor, S ( Q ), and the pair distribution function, G ( r ), of Al₆Si₂O₁₃ (3:2 mullite) in the normal and supercooled liquid states in the temperature range 1776–2203 K. Measurements are obtained by synchrotron X-ray scattering on levitated, laser-heated liquid specimens. The S ( Q ) shows a prepeak at 2.0 Å⁻¹ followed by a main peak at 4.5 Å⁻¹ and a weak feature at 8 Å⁻¹. The G ( r ) shows a strong (Si,Al)–O correlation at 1.80 Å at high temperature that moves to 1.72 Å as the liquid is supercooled. The second and third nearest neighbor peaks at 3.0 and 4.25 Å sharpen with supercooling. The short-range structure of the high-temperature liquid is similar to the corresponding glasses produced by rapid quenching. Supercooling causes an increase in the concentration of tetrahedral Si⁴⁺ ions, which is manifested by the large shift in the first peak to lower ionic distance, r , values in G ( r ). The increase in tetrahedrally coordinated Si⁴⁺ ions is offset by an increase in octahedral Al³⁺ ions. The clustering of the SiO₄⁴⁻ tetrahedral units results in increased viscosity of the liquid at temperatures below the melting point, which is consistent with Al₆Si₂O₁₃ being a fragile liquid.     

High-Strain-Rate Superplasticity in Y2O3-Stabilized Tetragonal ZrO2 Dispersed with 30 vol% MgAl2O4 Spinel

High-strain-rate superplasticity is attained in a 3-mol%-Y₂O₃-stabilized tetragonal ZrO₂ polycrystal (3Y-TZP) dispersed with 30 vol% MgAl₂O₄ spinel: tensile elongation at 1823 K reached >300% at strain rates of 1.7 × 10⁻²– 3.3 × 10⁻¹ s⁻¹. The flow behavior and the microstructure of this material indicate that the MgAl₂O₄ dispersion should enhance accommodation processes necessary for grain boundary sliding. Such an effect is assumed to arise from an enhancement of the cation diffusion by the dissolution of Al and Mg ions into the ZrO₂ matrix and from stress relaxation due to the dispersed MgAl₂O₄ grains.     

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

The effect on the γ-Al₂O₃-to-α-Al₂O₃ phase transition of adding divalent cations was investigated by differential thermal analysis, X-ray diffractometry, and surface-area measurements. The cations, Cu²⁺, Mn²⁺, Co²⁺, Ni²⁺, Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺, were added by impregnation, using the appropriate nitrate solution. These additives were classified into three groups, according to their effect: (1) those with an accelerating effect (Cu²⁺ and Mn²⁺), (2) those with little or no effect (Co²⁺, Ni²⁺, and Mg²⁺), and (3) those with a retarding effect (Ca²⁺, Sr²⁺, and Ba²⁺). The crystalline phase formed by reaction of the additive with γ-Al₂O₃ at high temperature was a spinel-type structure in groups (1) and (2) and a magnetoplumbite-type structure in group (3). In groups (2) and (3), a clear relationship was found between the transition temperature and the difference in ionic radius of Al³⁺ and the additive (Δ r ): The transition temperature increased as Δ r increased. This result indicates that additives with larger ionic radii are more effective in suppressing the diffusion of Al³⁺ and O²⁻ in γ-Al₂O₃, suppressing the grain growth of γ-Al₂O₃, and retarding the transformation into α-Al₂O₃.     

Compressive Creep and Oxidation Resistance of an Si3N4 Material Fabricated in the System Si3N4-Si2N2O-Y2Si2O7

The compressive creep behavior and oxidation resistance of an Si₃N₄/Y₂Si₂O₇ material (0.85Si₃N₄+0.10SiO₂+0.05Y₂O₃) were determined at 1400°C. Creep re sistance was superior to that of other Si₃N₄ materials and was significantly in creased by a preoxidation treatment (1600°C /120 h). An apparent parabolic rate constant of 4.2 × 10⁻¹¹ kg²·m-⁴·s⁻¹ indicates excellent oxidation resistance.     

Ion-Probe Measurement of Oxygen Self-Diffusion in Single-Crystal Al2O3

Anion self-diffusion coefficients normal to (1102) were obtained for single-crystal Al₂O₃ in a 1.3 × 10 ³ N/m² (10⁻⁵ torr) vacuum at 1585° to 1840°C. Tracer was supplied from an initial 650 to 1300 A Al₂¹⁸O₃ layer produced by the oxidation of vapor-deposited Al metal films in an ¹⁸O₂ atmosphere at 520°C. Concentration gradients extended over depths of 3000 to 5000 A and were measured by mass spectrometry of material sputtered from the samples with a beam of Ar⁺ ions. Crystals which had not been preannealed to remove surface damage displayed enhanced diffusion. Diffusion coefficients from preannealed crystals may be described by D₀ =6.4×10⁵cm²/s, with an activation energy of 188 ± 7 kcal/mol. The diffusion is interpreted as an extrinsic vacancy mechanism.     

Formation of Sm2+ lons in Sol-Gel-Derived Glasses of the System Na2 O-Al2O3-SiO2

Samarium ions (Sm²⁺) incorporated into aluminosilicate glasses by a sol-gel process showed persistent spectral hole burning at room temperature. Gels of the system Na₂O-Al₂O₃SiO₂ synthesized by the hydrolysis of Si(OC₂H₅)₄, Al(OC₄H₉)₃, CH₃ COONa, and SmCl₃·6H₂O were heated in air at 500°C, then reacted with H₂ gas to form Sm²⁺ ions. Whereas Al³⁺ ions effectively dispersed the Sm³⁺ ions in the glass structure, Na⁺ ions were not effective. The Al₂O₃-SiO₂ glasses proved appropriate for reacting the Sm³⁺ ions with H₂ gas and exhibited the intense photoluminescence of Sm²⁺ ions. The reaction of Sm³⁺ ions with H₂ in the Al₂O₂-SiO₂ glasses was determined by first-order kinetics, and the activation energy equaled 95 kJ/mol. At 800°C, the maximum photoluminescence of the Sm²⁺ ions was achieved within 20 min.     

Preliminary Observations on Phase Relations in the "V2O3–FeO" and V2O3–TiO2 Systems from 1400°C to 1600°C in Reducing Atmospheres

Phase relations within the "V₂O₃–FeO" and V₂O₃–TiO₂ oxide systems were determined using the quench technique. Experimental conditions were as follows: partial oxygen pressures of 3.02 × 10⁻¹⁰, 2.99 × 10⁻⁹, and 2.31 × 10⁻⁸ atm at 1400°, 1500°, and 1600°C, respectively. Analysis techniques that were used to determine the phase relations within the reacted samples included X-ray diffractometry, electron probe microanalysis (energy-dispersive spectroscopy and wavelength-dispersive spectroscopy), and optical microscopy. The solid-solution phases M₂O₃, M₃O₅, and higher Magneli phases (M _n O_{2 n −1}, where M = V, Ti) were identified in the V₂O₃–TiO₂ system. In the "V₂O₃–FeO" system, the solid-solution phases M₂O₃ and M₃O₄ (where M = V, Ti), as well as liquid, were identified.