Passivity of iron and nickel in a CH3OHOH2OH2SO4 system

The passivity of iron and nickel has been investigated in methanolic solutions of sulphuric acid with various water content. The stable oxide-hydroxide film can be formed on the metal surface only in the solutions with the molar ratio of acid to water lower than 1:4. In the solutions with higher ratio water molecules are bound in the hydratation shell of hydrogen ions. In such solutions the anodic behaviour of iron and nickel is similar to the behaviour of these metals in aqueous concentrated solutions of sulphuric acid. A thick unprotective anodic layer covers the nickel surface during anodic polarization whereas iron undergoes anodic dissolution without forming an anodic film.     

Behaviour of the FeOSH system under coal liquefaction conditions

The behaviour of the iron-sulphur system in the presence of water and hydrogen under coal liquefaction conditions has been investigated by using a high-pressure differential thermal analysis technique. The iron compounds used were Fe₂O₃, Fe₃O₄ and FeS₂. DTA was performed under 10.1 MPa (initial pressure) N₂ or H₂, heating to 450 °C (2.5 °C min⁻¹). Sulphate ions were detected by BaCl₂ addition to the product aqueous solution; sulphides were formed as gaseous and solid products. The results suggest that the catalytic behaviour of the sulphate and sulphides formed under the conditions employed must be considered for the iron-sulphur system in coal liquefaction.     

Mechanical properties of highly toughened ZrO2Y2O3

Y-PSZ (partially stabilized zirconia) and Y-TZP (tetragonal zirconia polycrystals) containing from 1·5 to 5·0 mol% Y₂O₃ were prepared by hot isostatic pressing. The K_IC value nonlinearly increases with the decrease from 2·5 to 2·0 mol % Y₂O₃, and has a maximum K_IC value of 20 MPa √m at 2·0 mol% Y₂O₃. In order to clarify the toughness creating mechanism of the above materials, the experimental results of K_IC, σ_f and transformation zone size were examined, with emphasis on Y₂O₃ composition, Al₂O₃ additives and grain size.     

Stabilite thermodynamique des phases dans le systeme CaOAl2O3CO2H2O

The paper presents a theoretical investigation on the nature of thermodynamical stable precipitating phases in the CaOAl₂O₃CO₂H₂O system when monocalcium aluminate (CA) with or without an excess of lime or alumina is mixed with water in equilibrium with gaseous carbon dioxide. Basing on the nature of ions in solution (A10₂⁻, Ca⁺⁺, CO_{3_}⁻⁻, OH⁻ and H⁺). and supposing that only $\text{C}\text{C}$, AH₃, CH, CAH₁₀, C₂AH₈ and C₄ACH₁₁ are the possible stable phases, the system was mathematically described taking into account the values of solubility products, the equilibrium between atmospheric carbon dioxide and the solution, ionic dissociation constant of water, matter balance of calcium and aluminate ions and condition of the solution electroneutrality. For a given system of initial parameters, computer calculation has allowed to determine the only possible solid-liquid system among the 22 theoretically possible cases (zero, one or two solids in equilibrium with the solution). So, varying the initial parameters has led to draw isobaric and isosteric phase diagramms, what shows that, in the conditions of the study, CAH₁₀ is the only stable phase at 21°C. The result suggests that the precipitation of C₂AH₈ and $\text{C}{}_4\text{A}\text{C}\text{H}{}_{11}$ can only be a consequence of saturations of the solution with respect to CAH₁₀.     

Phase relations between Al2O3Cr2O3 and FeAl2O4FeCr2O4 solid solutions at 1823 K

Tie-lines between the corundum and spinel solid solutions have been determined experimentally at 1823 K. Next, activities of FeCr₂O₄ and FeAl₂O₄ in the spinel solid solution were determined by combining the tie-line data with literature values for the activities of Cr₂O₃ and Al₂O₃ in the corundum phase. Activities and the Gibbs energy of mixing for the spinel solid solution were also obtained from a model based on cation distribution between nonequivalent crystallographic sites in the oxide lattice. The difference between the Gibbs energy of mixing obtained experimentally and from the model has been attributed to a strain enthalpy term which is relatively unchanged in magnitude from the reported at 1373 K. The integral enthalpy of mixing obtained from experimental data at 1373 and 1823 K using the second law is compared with the model result.     

Flame-spraying synthesis of aluminate glasses in the Al2O3–La2O3 system

The paper deals with the synthesis and characterisation of binary aluminate glasses in the La₂O₃–Al₂O₃ system with Al₂O₃ contents changing between 74.6 and 86.9 mol% (48–65 wt.%), and of ternary glasses with 75.7 mol% Al₂O₃ doped with 1 mol% of Nd₂O₃ or Er₂O₃. Six binary and two ternary compositions were prepared. Flame synthesis facilitated the preparation of X-ray amorphous microspheres in the systems with 58 wt.% Al₂O₃, and with eutectic composition in the pseudobinary LaAlO₃–LaAl₁₁O₁₈ system doped with Er. Other systems contained low fractions of crystalline LaAlO₃ perovskite, regardless of the composition. The diameter of prepared microspheres ranged between 2 and 10 μm. They were transparent for visible light, as well as in the IR wavenumber range from 1300 to 4000 cm^?1.     

Onset of perovskite formation in the catalytic system La2O3/γ-Al2O3

Molecular dynamics simulations of model systems for the surface interaction of lanthanum oxide supported on -alumina have been carried out at 1500 K. The onset of formation of perovskite-like phases has been analysed in samples containing four different concentrations of lanthanum oxide. A mechanism of the formation of perovskite-like polyhedra is proposed. This mechanism essentially involves a displacement of an oxide ion associated to an octahedral aluminum by a lanthanum ion and appears to be independent of La₂O₃ loadings.     

Properties of the Cr2O3/MgF2 system — catalytical aspect

The influence of Cr₂O₃ on properties of the Cr₂O₃/MgF₂ catalysts obtained by impregnation and precipitation-deposition is described. The IR technique enabled monitoring of changes in the structure of the MgF₂ surface after introducing chromium ions. Tetrahedral chromium complexes as well as structures of Cr₃O₃ type were observed. The deposited species generated the Lewis and BrØnsted acidity which in turn was responsible for the catalytic activity in the reactions of 2-propanol decomposition, butène double bond isomerization, cumene cracking, dehydrogenation of cyclohexane and decomposition of methyl alcohol.     

Phase equilibria in the La2O3-Y2O3-Nd2O3 system at 1500 °C

The phase equilibria in the ternary La₂O₃-Y₂O₃-Nd₂O₃ system at 1500 °C were studied by X-ray diffraction, petrography, and electron microscopy in the overall concentration range. The samples of different compositions have been prepared from nitrate acid solutions by evaporation, drying, and calcination at 1100 and 1500 °C. The solid solutions based on various polymorphous forms of source components and ordered phase of LaYO₃ were revealed in the system. The isothermal section of the phase diagram for the La₂O₃-Y₂O₃-Nd₂O₃ system has been developed. It was established that in the ternary La₂O₃-Y₂O₃-Nd₂O₃ system there exist fields of solid solutions based on hexagonal (A) and monoclinic (B) modifications of La₂O₃ and Nd₂O₃, cubic (C) modification of Y₂O₃, as well as perovskite-type structure of LaYO₃ (R) with rhombic distortions. The systematic study that covered the whole composition range excluded the formation of new phases. The refined lattice parameter of the unit cell and the boundaries of the homogeneity fields for solid solutions were determined.     

Intrinsic stress evolution in diamond films prepared in a CH4H2NH3 hot filament chemical vapor deposition system

The intrinsic stress in diamond films prepared in a CH₄H₂NH₃ hot filament chemical vapor deposition system has been investigated by the substrate curvature technique as a function of film thickness (2.2–50 μm) and ammonia concentration (0–1.4%). Our results indicated that the film stress changed from compressive to tensile with the increase of film thickness and diamond quality at a constant ammonia concentration of 0.5%. The existence of a non-diamond phase was found to be beneficial to the relaxation of intrinsic tensile stress in the films. The intrinsic stress in diamond films was tensile at an ammonia concentration from 0 to 1.4%, while the maximum tensile stress existed at 0.75% NH₃. The possible origin of intrinsic tensile stress was discussed.     

Thermodynamic calculation of Ms in ZrO2–CeO2–Y2O3 system

The difference of Gibbs free energy between tetragonal and monoclinic phases in ZrO₂–CeO₂–Y₂O₃ as a function of composition and temperature is thermodynamically calculated from the three related binary systems. In 8 mol% CeO₂–0.5 mol% Y₂O₃–ZrO₂, the equilibrium temperature between tetragonal and monoclinic phases, T₀, is obtained as 832.5 K and the M_s temperature of this alloy with a mean grain size of 0.90 μm is calculated as 249.9 K using the approach derived by Hsu et al. [J. Mater. Sci., 18(1983)3206; 20(1985)23; Acta Metall., 37(1989)3091; Acta Metall. Mater., 39(1991)1045; Mater. Trans. JIM, 37(1996)1284], which is in good agreement with the experimental one of 253 K.     

Effects of a low amount of C on the phase transformations in the AlN–Al2O3 pseudo-binary system

The effects of a low amount of C on the phase transformations in the AlN–Al₂O₃ pseudo-binary system are reported in samples having an AlN content in mol% ranging between 44 and about 0. Various complementary experimental techniques were used to determine the nature of the phase equilibria. Carbon is embedded in the components of three eutectics as a function of the average chemical composition of the sample in AlN. In two of them, a component belonging to the quaternary system Al–O–N–C and having a wide composition range was found. Its X-ray and neutron diffraction spectra are well refined with a hexagonal crystalline structure.     

Internal cation mobilities in the molten binary system Li2CO3K2CO3

Internal mobility ratios of two cations in the molten binary system (Li, K)(CO₃)_0.5 have been measured with the Klemm method. From these and available data on the densities and conductivities, the internal mobilities of Li⁺ and K⁺ have been calculated. Except at a region of relatively high concentration of Li₂CO₃, the Chemla effect occurs, that is, the mobility of the large cation K⁺ is greater that of the small one Li⁺. The internal mobilities of Li⁺ ions are well expressed by b_Li = [A/(V − V₀)] exp (− E/RT), where A, V₀ and E are constants characteristic of the cation and independent of temperature T, V the molar volume of mixtures, and R the gas constant. Negative deviations from this equation at a small V region may be attributed to the free space effect. For b_K such a simple equation does not hold over an extended region of concentration, which suggests that the agitation effect and the free space effect would also play a role for the mobility.     

Crystallisation and characterisation of dielectric glass ceramics in Li2O–Al2O3–SiO2 system

Abstract

Glass ceramics in the Li₂O–Al₂O₃–SiO₂ system have been synthesised to produce bulk materials grown in a glass phase via quenching followed by controlled crystallisation. The crystallisation and microstructure of Li₂O–Al₂O₃–SiO₂ (LAS) glass–ceramic with nucleating agents (B₂O₃ and/or P₂O₅) are investigated by differential thermal analysis, X-ray diffraction and scanning electron microscopy and the effects of B₂O₃ and P₂O₅ on the crystallisation of LAS glass are also analysed. The introduction of both B₂O₃ and P₂O₅ promotes the crystallisation of LAS glass by decreasing the crystallisation temperature and adjusting the crystallisation kinetic parameters, allows a direct formation of β spodumene phase and as a result, increases the crystallinity of the LAS glass ceramic. Microstructural observations show that the randomly oriented, nanometre sized crystalline is found with residual glass concentrated at crystallite boundaries. Furthermore, it is interesting that codoping of B₂O₃ and P₂O₅ creates not much effect on the crystallisation temperature. The dielectric properties of the glass–ceramics formed through controlled crystallisation have a strong dependence on the phases that are developed during heat treatment. The dielectric constant is continuously increased and the dielectric loss is decreased with addition of additives where mobile alkali metal ions (e.g. Li⁺) are incorporated in a crystal phase and minimise the residual glass phase.     

Phase diagram study and thermodynamic modeling of the Na2O–Al2O3–ZrO2 system

The phase diagram of the Na₂O–Al₂O₃–ZrO₂ system was experimentally studied at 1500°C–1650°C by a classical equilibration/quenching method and differential thermal analysis followed by X-ray diffraction phase analysis and electron probe micro-analysis. A sealed Pt crucible was utilized to prevent the volatile loss of Na₂O during high-temperature phase equilibrium experiments and the hydration upon quenching. The phase diagram of the Na₂O–Al₂O₃–ZrO₂ system was revealed for the first time. Based on the present experimental data and available binary modeling results in literature, the thermodynamic modeling of the ternary system was performed using the Calculation of Phase Diagram method and the phase diagram of the entire the Na₂O–Al₂O₃–ZrO₂ system was constructed and the optimized thermodynamic properties for all solids and liquid phase within the ternary system were obtained.     

Stability in the system CaO–Al2O3–H2O

The solubility of AH₃, CAH₁₀, C₂AH_7.5, and C₃AH₆ was determined experimentally at 7 to 40 °C and up to 570 days. During the reaction of CA, at 20 °C and above initially C₂AH_7.5 formed which was unstable in the long-term. The solubility products calculated indicate that the solubilities of CAH₁₀, C₂AH_7.5 and C₄AH₁₉ increase with temperature while the solubility of C₃AH₆ decreases. Thus at temperatures above 20 °C, C₃AH₆ is stable, while at lower temperature also CAH₁₀ and C₄AH₁₉ are stable, depending on the C/A ratio.At early hydration times, CAH₁₀ can be stable initially at 30 °C and above, as the formation of amorphous AH₃ stabilises CAH₁₀ with respect to C₃AH₆ + 2AH₃. With time, as the solubility AH₃ decreases due to the formation of microcrystalline AH₃, CAH₁₀ becomes unstable at 20 °C and above.