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.     

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.     

Polymorphism of BaTiO3 Acceptor Doped with Mn3+, Fe3+, and Ti3+

The effect of Mn doping on the cubic to hexagonal phase transition temperature in BaTiO₃ has been determined by quenching samples with different Mn contents from a range of temperatures. Under conditions of equilibrating samples in air over the range 1000°–1400°C, cubic solid solutions BaTi_{1− x} Mn _x O_3−δ form over the range 0≤ x ≤0.015(5), whereas hexagonal solid solutions form for x ≥0.02, depending on the temperature. The results are compared with those on doping BaTiO₃ with Fe³⁺ and observations made concerning acceptor doping with Ti³⁺.     

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.     

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.     

Kinetic Reactions in Quenched MgO:Fe3+ Crystals

Quenched single crystals of trivalent iron-doped MgO were studied using etching and EPR spectroscopy. Quench-rate measurements indicated that a liquid nitrogen quench from 1100°C is not rapid enough to prevent localized association and clustering. The distribution, size, and concentrations of precipitates, clusters, and associates in quenched crystals are critically dependent on both solute concentration and quench rate. At T<600°C, the mobility of iron is reduced as a result of cluster formation. Above 600°C, clusters dissociate, liberating mobile associates and vacancies.     

The Effect of Atmosphere on the Sintering of Fe3O4 and Fe2O3 Powders at Low Temperatures

The sintering kinetics of submicrometer Fe₃O₄ and Fe₂O₃ powders were investigated at 300° to 500°C. Using measurements of the rate of reduction of surface area, the coefficients of surface diffusion on the oxides are estimated for a range of oxygen partial pressures. The surface-diffusion coefficients appear to be independent of P _O2 for magnetite and only slightly dependent on P _O2 for hematite.     

Nucleation and Growth of Magnesioferrite in MgO Containing 0.9% Fe3+

The initial stage of precipitation of magnesioferrite from a supersaturated solid solution of 0.9 cation % Fe³⁺ in MgO at 500 °C was studied by fitting the magnetization curves to 7000 Oe at low temperatures with the Brillouin function. The averase umber of Fe³⁺ ions in a precipitate particle, increased monotonically with aging time from 9 in the as-quenched condition to 88 after 16 h. The average spin quantum number per Fe³⁺ ion decreased to near its final value at an aging time corresponding to = 50. Thus the volume fraction of precipitate is near the final value when the average particle size is only about three unit cells of magnesioferrite.     

Structural and Optical Characterization of Flower-Like Rutile Nanostructures Doped with Fe3+

Flower-like agglomerates with sizes of 200–400 nm of pure and Fe³⁺-doped TiO₂ with rutile crystalline structure were synthesized by the coprecipitation method. The morphology of the agglomerates was determined by electron microscopy (TEM and HRTEM). TiO₂ agglomerates consist of nanorods with clearly visible crystalline faces, parallel to the axis of elongation whose direction was along the [101] direction of pure TiO₂ and the [111] direction of doped TiO₂. Furthermore, nanorods consist of "chains" of spherical particles, most likely interconnected through the so-called oriented attachment or grain-rotation-induced grain coalescence (GRIGC) process. UV/Vis reflection measurements revealed that the absorption of pure TiO₂ was significantly shifted from UV toward the visible spectral region upon the incorporation of Fe³⁺ into the TiO₂ host.     

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.     

Upconversion Luminescence in Er3+ Doped and Yb3+/Er3+ Codoped Yttria Nanocrystalline Powders

Y_1.9Er_0.1O₃ and Y_1.7Yb_0.2Er_0.1O₃ nanocrystalline powders were prepared via a reverse-strike coprecipitation method using nitrates and ammonia as raw materials. The obtained powders were of cubic-phase structure of Y₂O₃ and the particle size was in the range of ∼60–80 nm. Strong red (⁴F_9/2–⁴I_15/2) and green (²H_11/2/⁴S_3/2–⁴I_15/2) upconversion luminescence were observed in all the samples when excited with a 980-nm continuous wave diode laser. The possible upconversion mechanisms in Y_1.9Er_0.1O₃ and Y_1.7Yb_0.2Er_0.1O₃ were discussed. Power studies indicated that two-photon processes are responsible for the green and red upconversion luminescence in these systems. The codoping of Yb³⁺ greatly enhanced the red (⁴F_9/2–⁴I_15/2) upconversion emission.     

2.0 μm Emission Properties and Energy Transfer between Ho3+ and Tm3+ in PbO—Bi2O3—Ga2O3 Glasses

Emission properties of 2.0 μm fluorescence and the energy transfer between Ho³⁺ and Tm³⁺ in 57PbO·25Bi₂O₃·18Ga₂O₃ (mol%) glass codoped with Ho³⁺ and Tm³⁺ were investigated. Cross-relaxation rates in Tm³⁺ increased approximately 5 times when the Tm₂O₃ concentration was increased from 1.0 to 1.5 wt%. Coefficients of the forward Tm³⁺→ Ho³⁺ energy transfer were about 15 times larger than those of the Tm³⁺← Ho³⁺ backward transfer. Analysis of the energy transfer and gain spectra indicated that the highest gain at the 2.0 μm wavelength region could be achieved from the glass with 1.5 wt% of Tm₂O₃ and 0.3 wt% of Ho₂O₃.     

Mössbauer Study of the Influence of Fe-Si-N Liquid in the Synthesis of β-Si3N4 from Silica

Mössbauer spectra of the products obtained by carbothermal reduction and distribution of silica in the presence of iron in the temperatures range 1200^o to 1540^o were studied. The preponderance of β- Si₃N₄ over the α form at a higher reaction temperature were assumed to be related to the formation of an Fe-Si-N liquid. The liquid did not alter its composition with the variation of reduction-temprature, Iron had no effect on the reaction mechanism below 1300^oC.     

On the Formation of Fe2+ in the System MgO-Fe2O3-MgFe2O4 at High Temperatures

The formation of ferrous iron in the system MgO-Fe₂O₃MgFe₂O₄ is of interest in connection with its deleterious effect on the microwave performance of magnesium ferrite, MgFe₂O₄. The partial reduction of ferric iron in the system at relatively low temperatures is discussed in terms of the preferential diffusion of iron, and its partial stabilization as ferrous iron from lattice energy considerations.