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.     

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⁺.     

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 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.     

Formation of Core-Shell Structure in the BaTiO3-SrTiO3 System

When a 1-mol%-Fe₂O₃-added 0.67BaTiO₃-0.33SrTiO₃ (mole ratio) powder compact was sintered at 1380°C, a core-shell structure was developed. The core phase formed via solid-state interdiffusion of barium and strontium ions between BaTiO₃ and SrTiO₃ particles. In contrast, the shell phase formed via a solution-precipitation process in the presence of an Fe₂O₃-containing liquid phase. Energy-dispersive X-ray analysis showed that the core was a strontium-rich paraelectric phase and the shell was a barium-rich ferroelectric phase at room temperature. The core-shell structure developed in the BaTiO₃-SrTiO₃ system suggests the possibility of obtaining a variety of phase distributions with different Curie temperatures.     

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.     

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.     

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.     

Compressive Creep Performance of SrFeO3

Compressive creep performance of strontium-deficient and strontium-excess SrFeO_3-δ materials has been investigated in the temperature range 800°–1000°C and in the stress range 2.5–25 MPa. The absolute densities of the strontium-deficient and strontium-excess materials are 4.99 and 5.25 g/cm³, respectively, which corresponds to porosities of ∼2% and 5%, respectively. Both materials contain secondary phases because of the cation nonstoichiometry. The creep rate is faster for the strontium-deficient material than the strontium-excess material. The stress exponent is approximately unity, and the activation energy is 260± 30 kJ/mol for both materials. The results can be explained by a cation diffusion mechanism. The present findings are discussed in relation to previous sintering data and the possible application of these materials as oxygen-permeable membranes.     

Electrical and Magnetic Properties of the System SrFeO3–BiFeO3

Perovskite phases containing iron in both the tetravalent and trivalent states were prepared by equilibration at high oxygen pressures. Small bismuth additions to SrFeO₃ resulted in high conductivity (10¹ to 10³ (ohm-cm)^–1 at 26°C) and antiferromagnetic order below 130°K. Distortion to tetragonal symmetry occurs approximately at the composition Bi_0.3Sr_0.7FeO_2.87, but higher bismuth concentrations (up to 80 mole %) give rise to a second cubic phase region which shows decreased conductivity (∼10^–5 (ohm-cm)^–1) and weak ferromagnetism with ordering temperatures higher than 26°C. Rhombohedral specimens Bi_0.9Sr_0.1FeO_2.96 and BiFeO_3.01 are anti-ferromagnetic and have comparatively low conductivity.     

Influence of Interdiffusion on Solid Solution Formation and Sintering in the System BaTiO3-SrTiO3

Formation of BaTiO₃-SrTiO₃ solid solution during sintering of powder mixtures is characterized by preferential diffusion of Ba²⁺ ions. As a consequence, several nonequilibrium phases are temporarily formed; they were identified by X-ray and microprobe analysis. Eutectic liquid appears below 1300°C, which may explain exaggerated grain growth during sintering of BaTiO₃-SrTiO₃ mixtures. Disturbance in neck growth and Kirkendall-type porosity hamper densification in the heterogeneous system as compared with the pure titanates.     

Control of Grain-Boundary Pinning in Al2O3/ZrO2 Composites with Ce3+/Ce4+ Doping

The control of the microstructure of Ce-doped Al₂O₃/ZrO₂ componsites by the valence change of cerium ion has been demonstrated. Two distinctively different types of microstructure, large Al₂O₃ grains with intragranular ZrO₂ particles and small Al₂O₃ grains with intergranular ZrO₂ particles, can be obtained under identical presintering processing conditions. At doping levels greater than ∼ 3 mol% with respect to ZrO₂, Ce³⁺ raises the alumina grain-boundary to zirconia particle mobility ratio. This causes the breakaway of grain boundary from particles and the first type of microstructure. On the other hand, Ce⁴⁺ causes no breakaway and produces a normal intergranular ZrO₂ distribution. The dramatic effect of Ce³⁺ on the relative mobility ratio is found to be associated with fluxing of the glassy boundary phase and is likewise observed for other large trivalent cation dopants. The ZrO₂ second phase acts as a scavenger for these trivalent cations, provided their solubility limit in ZrO₂ is not exceeded.     

Incorporation of Er3+ into BaTiO3

The incorporation of Er³⁺ into BaTiO₃ ceramics was investigated on samples containing 0.25, 0.5, 1, 2, 8, and 10 at.% of dopant, after sintering at 1350–1550°C in air. For Er³⁺ concentrations ≤1 at.%, dense and large-grained ceramics with low room-temperature resistivity (10²–10³Ω·cm) were obtained. The observed properties are largely independent of stoichiometry. Simultaneous substitution of Er³⁺ at both cation sites, with higher preference for the Ba site, is proposed. The behavior of heavily doped ceramics depends on stoichiometry. When Ba/Ti < 1, the electrical properties change from slightly semiconducting to insulating as Er concentration increases from 2 to 8 at.%. The ceramics have tetragonal perovskite structure and contain a large amount of Er₂Ti₂O₇ pyrochlore phase. On the other hand, when Ba/Ti > 1, the ceramics are insulating, fine-grained, and single phase. In this case, incorporation of Er³⁺ predominantly occurs at the Ti site, with oxygen vacancy compensation. Incorporation is accompanied by a significant reduction of tetragonality and by expansion of the unit cell. The different results indicate that Er³⁺ solubility at the Ba site does not exceed 1 at.%, whereas solubility at the Ti site is at least 10 at.%. However, the incorporation of Er³⁺ and the resulting properties are also strongly affected by sintering conditions.     

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.     

Lattice Parameters, Ionic Conductivities, and Solubility Limits in Fluorite-Structure MO2 Oxide [M = Hf4+, Zr4+, Ce4+, Th4+, U4+] Solid Solutions

Changes in the lattice parameters of fluorite type MO₂ oxides (M = Hf⁴⁺, Zr⁴⁺, Ce⁴⁺, Th⁴⁺, U⁴⁺) due to the formation of solid solutions can be predicted by proposed empirical equations. The equations show the generalized relationship between dopant size and ionic conductivity in the binary systems of these oxides, illustrating that the smaller the difference between the dopant ionic radius and the critical dopant radius, the higher the conductivity. The solubility limit of the same periodic group elements in fluorite-structure MO₂ oxides decreaes linearly with the square of Vegard's slope for each solute as determined from the proposed equations.     

High-Brightness LaPO4:Ce3+, Tb3+ Nanophosphors: Reductive Hydrothermal Synthesis and Photoluminescent Properties

This paper definitely reveals that LaPO₄:Ce³⁺, Tb³⁺ (LAP) nanophosphors prepared by normal hydrothermal method suffer significant loss of luminescence due to the oxidation of Ce³⁺–Ce⁴⁺ at hydrothermal stage. To effectively protect Ce³⁺ from oxidation, a reductive hydrothermal process using hydrazine hydrate as a protecting agent is proposed to synthesize LAP nanophosphors with different Ce³⁺ and Tb³⁺ concentrations, which exhibited much stronger green photoluminescence (PL) and longer lifetime than the products prepared by normal hydrothermal method. Furthermore, the high-brightness LAP nanophosphors exhibited high-quenching concentration of Tb³⁺; the La_0.4Ce_0.4Tb_0.2PO₄ nanophosphor showed almost the same PL intensity as that of the commercially used La_0.7Ce_0.2Tb_0.1PO₄ bulk powder.