Occurrence of Ti3+ and Fe2+ in Mullite

Electron paramagnetic studies showed that Ti³⁺ and Fe²⁺ occur in mullites taken from a refractory material which was fused-cast under a reducing atmosphere. Exposure of the mullite samples to temperatures >1600°C caused oxidation of Ti³⁺ and Fe²⁺ to Ti⁴⁺ and Fe³⁺, respectively.     

The Iron-Oxygen Equilibrium in Glass: Effect of Platinum on the Fe2+/Fe3+ Equilibrium

A number of experiments were performed on iron-containing sodium disilicate melts in air. It was found that it was not possible to obtain an equilibrium between Fe²⁺ and Fe³⁺ in platinum crucibles owing to the reaction between platinum and iron, whereas in alumina crucibles the equilibrium was rapidly established. Thermodynamic calculations of the reaction 2FeO (in Na²O-2SiO²) +½O² (g) = Fe²O³ (in Na²O-2SiO²) showed that the equilibrium went more and more to the right with increasing temperature. The standard free energy, enthalpy, and entropy for the reaction were calculated.     

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.     

Influence of Fe3+/Fe2+ Ratio on the Crystallization of Iron-Rich Glasses Made with Industrial Wastes

The influence of the Fe³⁺/Fe²⁺ ratio on the crystallization of iron-rich glasses was investigated in this study. The glass batches were made from two hazardous industrial wastes: mud (goethite and jarosite) originating from the zinc hydrometallurgical process and electric arc furnace dust (EAFD). Glass compositions were prepared by adding different percentages of carbon powder. The crystallization process was investigated by a combined thermogravimetry/differential thermal analysis technique, in air or nitrogen atmospheres, using powder and bulk glass samples. The crystalline phases formed, i.e., pyroxene and spinels, and their relative ratio were determined by X-ray diffractometry. The experimental results indicated that melting temperature and crystallization behavior were influenced by the initial Fe³⁺/Fe²⁺ ratio and by the amount of carbon added to the glass batch. For goethite and jarosite glass compositions, decreasing the Fe³⁺/Fe²⁺ ratio increased the crystallization rate by favoring magnetite formation. For EAFD glass compositions, the addition of carbon to the batch inhibited chromite–magnetite spinel formation and favored the attainment of an amorphous glassy phase.     

Glass Formation in the System SiO2–PbO Containing Transition-Metal Oxides

Glass-forming regions, valence states, and viscosities in SiO₂–PbO systems containing various transition-metal oxides as a third component were investigated. The glasses were prepared by melting in an open atmosphere. The glass-forming regions ranged as follows: MnO≡ZnO > FeO_1.5>NiO. The ratios Fe²⁺/(Fe²⁺+ Fe³⁺) and Mn³⁺/ (Mn³⁺+ Mn²⁺) in the glasses were determined by chemical analysis. The Fe²⁺/ (Fe²⁺+ Fe³⁺) ratio in SiO₂–PbO–FeO_1.5 glasses ranged from 0.016 to 0.050. The Mn³⁺/ (Mn³⁺+ Mn²⁺) ratio in SiO₂–PbO–MnO glasses ranged from 0.056 to 0.30. The fraction of manganese (III) ions in the glasses varies considerably with the glass composition. The effects of transitionmetal oxides on the viscosity are discussed.     

Factors Affecting the Preparation of Sr2Fe2−xMoxO6

Samples of Sr₂Fe_{2− x} Mo _x O₆were prepared by solid-state reaction in air and 5%H₂–95%N₂. X-ray diffractometry was used to identify the phases and evaluate the lattice parameters. It is found that molybdenum ions can dissolve in the SrFeO₃ even if the sample is heated in air but the solubility is limited. The solubility can be enhanced by heating the sample in low oxygen partial pressure, which is attributed to the larger ionic radii of Fe³⁺ and Mo⁵⁺ than that of Fe⁴⁺. The degradation of Sr₂Fe_{2− x} Mo _x O₆in water and air is also reported.     

Electrical Resistivity Surface for FeO-Fe2O3-P2O5 Glasses

The dc electrical properties and microstructure of x (FeO-Fe₂O₃)-(100 – x )P₂O₅ glasses were investigated up to a maximum of x = 75 mol%. Results indicate that, in general, the minimum resistivity of the glass does not occur at equal Fe^2+] and Fe^3+] concentrations, although for the special case where x = 55 mol% the minimum does occur at Fe^2+]/Fe total = 0.5, as reported by other investigators. Evidence presented shows that the position of the minimum resistivity is a function of total iron content. The minimum shifts to glasses richer in Fe^2+] at higher total iron concentrations.     

Redox State of Iron and Its Related Effects in the CaO–P2O5–Fe2O3 Glasses

Iron exists in Fe²⁺ and Fe³⁺ states in CaO–P₂O₅–Fe₂O₃ glasses and they impart characteristic optical absorption bands that allow analysis of relative proportion of the two species. This communication reports the redox states of glasses melted under air, argon, and oxygen atmospheres and relates them to the dissolution rates. The dissolution rate was found to be related to the redox state and it is lowered if the glass is melted under oxidizing atmospheres.     

Effect of Fe4+ in the System SrFeO3-SrTiO3

Perovskites of the system SrFeO₃-SrTiO₃ were prepared, and measurements were made of their magnetic and electrical behavior. Chemical analysis showed that the percentage of Fe⁴⁺ varied from 72.5% for SrFeO_2.86 to about zero for Sr(Fe_0.1Ti_0.9)O_2.95; the remainder of the iron was in the Fe³⁺ state and electrical balance was achieved by oxygen loss. Sr(Fe_{1- x} Ti _x )O₃ was antiferromagnetic between x = 0 and x = 0.9, with a Néel temperature below 60°K. A parasitic ferromagnetic component developed when these compounds were cooled in a magnetic field, the magnitude of this component being dependent on the cooling field. The conductivity of these perovskites ranged from 10⁻⁸ ohm⁻¹ cm⁻¹ for x = 1.0 to 10⁻² for x = 0.0 and showed a marked change at x = 0.8. The break corresponded to a change in slope of the lattice parameter and the disappearance of Fe⁴⁺. The Fe⁴⁺ content depended on the heat treatment and atmosphere during formation.     

Rutherford Backscattering Spectroscopy Study of the Kinetics of Oxidation of (Mg, Fe)2SiO4

Single crystals of oilivine, (Mg_0.9Fe_0.1)₂SiO₄, have been oxidized in air at temperatures between 700° and 1100°C for times from 0.5 to 100 h. Both an internal and an external oxidation layer developed. Transmission and analytical electron microscopy observations reveal that the internal oxidation layer is composed of precipitates of magnetite plus amorphous silica, which nucleated heterogeneously on dislocations and grew in an Fedepleted matrix of olivine. Rutherford backscattering spec-trometry (RBS) demonstrates that the thin external oxidation layer is free of Si; that is, it is made up of Mg-Fe oxide phases. Thus, the oxidation process is primarily controlled by diffusion of Fe²⁺ and Mg²⁺ ions toward the surface with Si⁴⁺ and O^2- remaining largely immobile. The kinetics of oxidation, as determined from RBS analyses of the external oxidation layer, are parabolic with an activation energy of 140 kJ/mol. Although this activation energy is lower than that reported for self-diffusion of Mg in Mg₂SiO₄, the diffusivity calculated from the reaction rate constant is in good agreement with published values for lattice diffusion of Mg in the limited temperature range in which data overlap. However, the rate of accumulation of Fe in the external layer is more rapid than expected for lattice diffusion, indicating that the transport of Fe is dominated by short-circuit diffusion along the precipitate complexes which decorate dislocations.     

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.     

Influence of Cr and Fe on Formation of α-Al2O3 from γ-Al2O3

The effect of Cr and Fe in solid solution in γ-Al₂O₃ on its rate of conversion to α-Al₂O₃ at 1100°C was studied by X-ray diffraction. The δ form of Al₂O₃ was the principal intermediate phase produced from both pure γ-Al₂O₃ and that containing Fe³⁺ in solid solution, although addition of Fe greatly reduced crystallinity. Reflectance spectra and magnetic susceptibilities showed that Cr exists as Cr⁶⁺ in γ-Al₂O₃ and as Cr³⁺ in α-Al₂O₃, with θ-Al₂O₃ as the intermediate phase. The intermediates formed rapidly, and the rates of their conversion to α-Al₂O₃ were increased by 2 and 5 wt% additions of Fe and decreased by 2 and 4 wt% additions of Cr. An approximately linear relation observed between α-Al₂O₃ formation and decrease in specific surface area was only slightly affected by the added ions. This relation can be explained by a mechanism in which the sintering of δ- or θ-Al₂O₃, within the aggregates of their crystallites, is closely coupled with conversion of cubic to hexagonal close packing of O^2- ions by synchro-shear.     

Phase Diagram of the System BaO-Fe2O3

The phase diagram of the system BaO-Fe₂0₃ was determined by X-ray diffraction, melting-point measurement, and microscopic methods. Since the reduction of Fe³⁺ to Fe²⁺ was observed by chemical analysis in the samples heated at high temperature, especially in molten samples, the samples were heated at 1 atmosphere pressure of oxygen in the temperature region in which the liquid was in equilibrium; 1 atmosphere pressure of oxygen was su5cient to restrain the reduction of Fe³⁺. In the temperature region of solid-solid equilibrium, the dissociation was not observed even when the samples were heated in air. BaO - 6Fe₂O₃ formed a solid solution with BaO.Fe₂0₃. The BaO:Fe₂0₃ ratio of the solid solution was BaO.4.5Fe₂0₃ at 1350°C. and BaO. 5.0Fe₂0_a at 800°C. The precipitation micro-structures of each primary solid solution were observed.     

A Study of Conductivity in the Sr(Fe1−xTix)O3−δ System

The conductivity and conductivity-temperature characteristics of the Sr(Fe_1−xTi_xO_3−δ system fired in air are studied by means of Mössbauer spectroscopy at room temperature and at 78 K. On the basis of quantitative analysis of the Fe⁴⁺ content in the solid solution, the concentrations of oxygen vacancies are calculated and the relationship between Fe⁴⁺ content and conductivity is found. It is also found that the turning points of conductivity and material constant B, as well as Fe⁴⁺ and the percentage of oxygen vacancies, occur at x=0.7 on the curve. When x=0.7, Fe³⁺ (II), which has never been reported before, disappears and the lattice parameters no longer vary with x. Besides Fe⁴⁺, Fe³⁺(II) is also a factor affecting the conductivity of this system.     

Thermal Grooving of MgO and Al2O3

High-purity polycrystalline MgO and Al₂O₃ were thermally grooved at 1500° and 1600°C. Accurate techniques were developed for following the growth of a single groove. For high-purity samples growth kinetics were essentially similar to those reported in the literature but were determined to be controlled by volume diffusion. Specimens for thermal grooving were prepared from Al₂O₃ to which transition metal oxides (Fe₂O₃₉, MnO, and TiO₂), which are known to accelerate shrinkage and sintering of Al₂O₃ powder compacts, had been added; the rate of groove growth was increased remarkably by minor amounts of these additives. Control of partial pressure indicated that Fe²⁺ and Ti⁴⁺ are the species active in promoting groove growth. Substantial evidence was found for volume diffusion as the mechanism controlling groove formation.     

Determination of the Fe2+/Fe3+ Ratio in Nuclear Waste Glasses

The Fe²⁺/Fe³⁺ ratios of 47 simulated nuclear waste glass samples with ratios varying from 0.01 (oxidized) to 1.6 (reduced) were determined by wet-chemical and Mössbauer spectral analyses. The wet-chemical method involved the spectrophotometric determination of Fe²⁺ and total iron using remote spectroscopy with fiber optic chemical sensing. Interferences from other species present in these glasses were examined and alternative analytical techniques were investigated. Results of wet-chemical and Mössbauer spectral analysis were comparable; however, the wet-chemical method is probably preferable for the analysis of highly radioactive glasses until such glasses have been shown to have satisfactory Mössbauer spectra.     

Oxidation-Reduction Equilibria in Molten Na2O.2SiO2 Glass

The oxidation-reduction equilibria in molten glasses having a soda/silica ratio of 1/2 and containing small amounts of variable-valence ions of the transition elements titanium, vanadium, iron, cobalt, and nickel, the post-transition elements tin and antimony, and the rare-earth element cerium were obtained by equilibrating the melts with various atmospheres. The simple mass expression
was always applicable. In the expression, n is the number of electrons involved in the valence change of the metal M. The value of n is 1 for all systems except for the ions of antimony and tin where n is 2. The ions found are those generally accepted as existing in glass melts which are Ti³⁺, Ti⁴⁺; Fez+, Fe³⁺; Ce³⁺, Ce⁴⁺; Mn²⁺, Mn³; Co²⁺, Co³⁺; Ni²⁺, Ni³; Sb³⁺, Sb⁵⁺; and Sn²⁺, Sn⁴⁺. Two mass action expressions were needed for vanadium-containing glasses to describe the equilibria between 5+, 4+, and 3 f species.     

Sintering, Crystallization, and Properties of B2O3/P2O5-Doped Li2O·Al2O3·4SiO2 Glass-Ceramics

Sintering, crystallization, microstructure, and thermal expansion of Li₂O·Al₂O₃·4SiO₂ glass-ceramics doped with B₂O₃, P₂O₅, or (B₂O₃+ P₂O₅) have been investigated. On heating the glass powder compacts, the glassy phase first crystallized into high-quartz s.s., which transformed into β-spodumene after the crystallization process was essentially complete. The effects of dopants on the crystallization of glass to high-quartz s.s. and the subsequent transformation of high-quartz s.s. to β-spodumene were discussed. The major densification occurred only in the early stage of sintering time due to the rapid crystallization. All dopants were found to promote the densification of the glass powders. The effect of doping on the densification can fairly well be explained by the crystallization tendency. All samples heated to 950°C exhibited a negative coefficient of thermal expansion ranging from about −4.7 × 10^-6 to −0.1 × 10^-6 K^-1. Codoping of B₂O₃ and P₂O₅ resulted in the highest densification and an extremely low coefficient of thermal expansion.     

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.     

Thermal Expansion Behavior of NaZr2(PO4)3Type Compounds

The thermal expansion of the skeletal framework was essentially zero for NaZr₂(PO₄)₃-type compounds; the interstitialion, e.g., Na⁺, was primarily responsible for the total thermal expansion. The composition dependence of the thermal expansion is discussed in terms of the amounts, crystallographic sites, and ionic radii of the interstitial ions. The mechanism which results in low thermal expansion was clarified, particularly for KZr₂(PO₄)₃, in which a larger ion is substituted for Na⁺, and NbZr(PO₄)₃, which does not contain Na⁺. Polycrystalline ceramics formed from these crystals might be useful as thermal-shock-resistant materials.