Optical Absorbance and Fluorescence in Pure and Doped ThO2

Fluorescence spectra resulting from uv excitation of pure, Ca²⁺-doped, and Y³⁺-doped ThO₂ were resolved into discrete bands at 395.4, 434.6, and 464.0 nm. These bands are related to absorption bands in the pure material at 238.9, 227.3, and 252.8 nm, respectively. The Ca²⁺ - and Y³⁺-doped ThO₂ specimens showed an apparent absorption edge at 240 nm (213 nm is the true edge in pure ThO₂). However, the fluorescence studies showed that this edge is, in fact, spurious, resulting from enhancement of the high-energy absorbance bands by doping.     

Defect Centers in Gamma-Irradiated Single-Crystal Al2O3

The γ-irradiation-induced optical absorption spectra of annealed Al₂O₃ single crystals were analyzed. The samples were beat-treated in O₂ or vacuum from 1600° to 1800°C. Four absorption bands are reported with average peak positions at 3.4,4.8,5.5, and 6.6 eV for samples annealed in O₂ at ≥ 1750°C. Vacuum anneals up to 1800°C and O₂ anneals at < 1700°C failed to generate observable bands in the energy range studied. There is also evidence that one or more bands exist at >6.6 eV. The radiation-induced change in the absorption spectra is reported and the defect centers responsible for the measured optical absorption bands are discussed with respect to the aliovalent impurities in the host lattice. Special attention is given to the role of the Fe³⁺ impurity ion.     

Thermal Simulation of In-Reactor Microstructures in ThO2-UO2

A device was built to provide the capability for out-reactor simulation of the microstructures seen in irradiated fuel. Out-reactor simulation of nuclear heating makes possible the precise measurement of temperature gradients in the fuel and provides data for correlating microstructural alterations with temperature. The simulation apparatus was designed to produce controlled thermal gradients sufficient to produce both equiaxed and columnar grain growth in the specimens. Provision was made for calorimetric measurements from which thermal conductance could be determined. Micro-structural alterations in UO₂ and ThO₂-UO₂ materials were achieved, and thermal conductivity data were recorded. Microstructural alterations in the ThO₂-10 wt% UO₂ specimens were similar to those that other investigators have observed in UO₂ at lower temperatures.     

Formation of Small-Sized CdSxSe1-x Crystals in Sol-Gel-Derived SiO2 Glasses

Silica glasses doped with small-sized CdS_x-Se_1-x crystals were prepared by the sol-gel method. Gels synthesized by the hydrolysis of Si(OC₂H₅)₄, in the presence of CdSeO₄ with NH₄SCN dissolved in HNO₃ or NH₄OH, were heated in H₂-N₂ atmosphere. The pH value of solutions for CdSeO⁴ and NH⁴SCN primarily determined the fraction of anions in CdS^xSe^1-x crystals. The anion content in crystals was dependent on the pH value of the solvent and/or heat-treatment temperature, and the sulfur fraction changed from 0.1 to 0.6. The optical absorption spectra were red-shifted as the selenium content and the crystal size increased, and the emission spectra showed a sharp band near the absorption edge position and a broad tail extending into the long wavelength. The optical band gap energies increased reciprocally proportional to the square of the crystal size.     

Localized Cooling in Flux Crystal Growth

Large crystals of ZnO, Al₂O₃, Ga₂O₃, ThO₂, Y₃Ga₅O₁₂, ZnGa₂O₄, and ZrO₂ were grown from PbO-PbF₂ flux systems using a temperature gradient and localized cooling induced by a jet of cold air directed on the crucible bottom. The temperature gradients in the crucible were much less than that in the furnace. The technique is described, and crystals grown with and without a temperature gradient are compared.     

Activated Sintering of ThO2 and ThO2-Y2O3 with NiO

The effect of additives on the sintering of ThO₂ and ThO₂-Y₂O₃ compacts and loose powders was studied by isothermal shrinkage measurements and by scanning electron micrography. Small amounts of the oxides of Ni, Zn, Co, and Cu reduced the sintering temperature. The behavior of NiO at a concentration of 0.8 wt% (2.5 mol%) was studied in detail and found to yield high-density bodies at temperatures below 1500°C. The presence of Y₂O₃ as a separate phase increases the rate of sintering of ThO₂, but smaller amounts of NiO are much more potent. The major portion of the densification occurs very rapidly and is followed by a much slower sintering process typical of volume diffusion. The fast early shrinkage may be caused by the capillary forces of a liquid, but since no evidence of melting was found, a solid-state mechanism may be responsible.     

Raman Spectrum of PbZrO3

Laser-induced Raman spectra were obtained for powders and multidomain single crystals of PbZrO₃ at room temperature. Factor-group analysis was used to interpret the spectra on the basis of the space group C_2v⁸-Pba 2. Raman spectra were also observed for single crystals of PbZrO₃ at several other temperatures between 76° and 520°K and related to crystal structural changes. No first-order Raman bands were noted above the Curie temperature, indicating a phase transformation to a cubic zirconate structure. The interpretation of Raman spectra for thermally decomposed PbZrO₃ is discussed.     

Phase Stability, Phase Transformation Kinetics, and Conductivity of Y2O3—Bi2O3 Solid Electrolytes Containing Aliovalent Dopants

Single-phase, cubic solid solutions of baseline composition 25% Y₂O₃—75% Bi₂O₃ with and without aliovalent dopants were fabricated by pressureless sintering of powder compacts. CaO, SrO, ZrO₂, or ThO₂ was added as an aliovalent dopant. Sintered samples were annealed between 600° and 650°C for up to 4000 h. Samples doped with ZrO₂ or ThO₂ remained cubic, depending upon the dopant concentration, even after long-term annealing. By contrast, undoped, CaO-doped, and SrO-doped samples transformed to the low-temperature, rhombohedral phase within ∼ 200 h. Conductivity measurements showed no degradation of conductivity in samples that did not undergo the transformation. In samples that underwent the transformation, a substantial decrease in conductivity occurred. The enhanced stability of the ZrO₂- and ThO₂-doped samples is rationalized on the basis of suppressed interdiffusion on the cation sublattice.     

Influence of Pulling Rates on the Thermoluminescence and Optically Stimulated Luminescence of α-Al2O3:C Crystals Grown by the Edge-Defined, Film-Fed Growth Technique

In this work, α-Al₂O₃:C crystals were grown by the edge-defined, film-fed growth method with different pulling rates. The influence of pulling rate on the optical, thermoluminescence (TL), and optically stimulated luminescence (OSL) properties of the as-grown α-Al₂O₃:C crystals was investigated. All the α-Al₂O₃:C crystals grown with different pulling rates show a single TL glow peak and an invariable blue emission band at 415 nm. With an increasing pulling rate in the 0.1–0.7 mm/min range, the absorption coefficient of optical absorption bands at 206 and 256 nm increased, the TL and OSL dose response improved, the TL peak shifted to higher temperatures, and the OSL decay rate increased. When the pulling rate is increased to 1 mm/min, the absorption coefficients of the bands at 206 and 256 nm, the TL temperature and intensity, the OSL intensity, and the decay rate declined. The TL and OSL dose response of the as-grown α-Al₂O₃:C crystals shows a linear–sublinear saturation characteristic. The pulling rates almost show no influence on the crystals' saturation dose. The α-Al₂O₃:C crystal grown with the pulling rate of 0.7 mm/min shows the best TL and OSL properties.     

Grain-Boundary Segregation and Final-Stage Sintering of Y2O3

The addition of ThO₂ to Y₂O₃ inhibits grain growth during sintering and allows the sintering process to proceed to theoretical density by maintaining a high diffusion flux of vacancies from the pores to the grain boundaries. The inhibition of grain growth is accomplished by the segregation of ThO₂ solute at the grain boundaries, causing a decrease in the grain-boundary mobility. The segregation of ThO₂ at the grain boundaries can be inferred from the results of the microhardness and grain-growth studies presented. Further evidence for segregation is provided by quenching experiments and surface activity experiments.     

Revised ThO2-Nb2O5 Phase Diagram

The phase equilibrium diagram of the system ThO₂-Nb₂O was redetermined near the composition Th₂Nb₂O₉. This phase was found to melt incongruenlly at 1362°C, with a eutectic temperature at ∼1350°C. The peritectic and eutectic compositions must occur between 60 and ∼64 mol % ThO₂. From single crystal and powder X-ray diffraction data, Th₂ Nb₂O₉ was found to have a primitive monoclinic unit cell with a = 6.711(1), b = 25.254(5), c=7.757(1)×10⁻¹nm, β=90.46 (1)°.     

The System ThO2-P2O5

The existence of compounds with 1:1, 3:2, and 3:1 ThO₂:P₂O₅ ratios in the system ThO₂-P₂O₅ was confirmed. A 1:2 compound found by previous workers was not investigated, and their 2:1 compound was not detected; however, extensive solid solution on either side of the 3:2 compound was established. The linear thermal expansion behavior of the compounds and solid solutions was determined.     

Transition from Ionic to Electronic Conduction in Pure ThO2 Under Reducing Conditions

Open-circuit emf and ac conductivity studies were conducted on two batches of dense polycrystalline ThO₂. The open-circuit emf data were used to delineate the low- p o₂ ionic domain boundary for "pure" ThO₂, which is presented as a log P_θ line on a log Po₂-1/ T diagram. In addition the ionic conductivity, σ_ion, and the high-Po₂ log P_θ boundary were also determined, mainly from ac conductivity measurements, which also confirmed the Po₂^I/4 dependence of σ_p, the p-type electronic conductivity, shown by other investigators. The main results are, for the first batch, log Pθ= 12.7−220.2 × 10³/4.575T, log σ_ion= 1.9−44.3×10³/4.575T, and log P_θ=−1.0−31.4 × 10³/4.575T; for the second batch, log Pθ=11.2−219.7 × 10³/4.575T, log σ_ion= 1.7−41.6 × 10³/4.575T, and log Pθ=0.6−40.4 × 10³/4.575T. The oxygen permeability of ThO₂ tubes and the oxidation rate constant of Th were predicted from the conductivity and emf data and compared with direct measurements previously reported. The calculated and previously measured permeabilities agreed very well; however, the correlation between the predicted and previously measured oxidation kinetics was somewhat less satisfactory.     

Optical Absorption Spectra of Chlorine-Associated Defect Centers in an Irradiated ZrF4-Based Glass

The optical absorption spectra of an irradiated zirconium-barium-lanthanum aluminum- lithium fluoride glass prepared with 1 wt% BaC12 were resolved into Gaussian bands identified as the V_k, and V_H centers, Zr³⁺, and two bands associated with Cl, i.e., FCl⁻ at 4.78 eV and Cl⁻₂ at 3.90 eV. These latter bands are more thermally stable than the VH and V, trapped hole centers, persisting to 450 and 550 K, respectively.     

Optical Absorption Spectra of Cr3+ in MgO·Al2O3-MgO·3.5Al2O3 Spinels

Spinel crystals in the system MgO· n Al₂O³:Cr³⁺ (1.04≤ n ≤3.51) were synthesized by flame fusion; their absorption spectra were measured in transmission from 400 to 50,000 cm⁻¹. The O-H band spectra suggest the presence of protons in octahedral site vacancies which appear for n >1. The spectra for Cr³⁺ indicate clearly that these ions occupy only octahedral sites. However, the effective trigonal field, which varies with n , causes the major absorption bands to split in the form of a lower-energy shoulder on a higher-energy component. The red-green transition in these spinels was rationalized and observed to arise from a somewhat different mechanism than that in other oxides in which the site radii and Cr³⁺ concentration vary considerably.     

Pore Growth in Sintered Thoria

Sintered ThO₂ will sometimes undergo a decrease in density, i.e. become more porous, if held at high temperature for a sufficient time. The bulk density of sol-gel ThO₂ sintered to 99% of theoretical density at 1200°C will decrease steadily at 1800°C in air to <90% of theoretical after ∼ 15 h. Gas-release measurements from such porous ThO₂ specimens annealed in air or in vacuum indicate that some gases move through the material freely at 1800°C. The only gases that could contribute to the decrease in density are the oxides of sulfur and carbon, which, if they exist, could be produced or eliminated rapidly by controlling the annealing environment. However, the density decrease appears to depend on factors that affect grain growth, rather than on the generation or elimination of postulated gases.     

Local Structures of Bismuth Ion in Bismuth-Doped Silica Glasses Analyzed Using Bi LIII X-Ray Absorption Fine Structure

Local structures around a bismuth ion in bismuth-doped silica glasses (BiSG) are analyzed using a Bi L_III X-ray absorption fine structure (XAFS). The XAFS spectrum of crystalline α-Bi₂O₃ was also measured for comparison with BiSG whose first peaks are about 0.1 Å less than α-Bi₂O₃ in radial structure functions. According to curve-fitting results using FEFF 8.2, the Bi–O distances of the first and second neighboring coordination spheres are estimated to be 2.1 and 2.3 Å, respectively. The former corresponds to Bi⁵⁺–O bonding and the latter to Bi³⁺–O bonding compared with the Bi–O distances in typical crystals including Bi³⁺ and Bi⁵⁺ ions. The above results show that Bi ions in BiSG exist as both Bi³⁺ and Bi⁵⁺ states, which is also supported by X-ray absorption near the edge structure spectra of BiSG.     

Preparation of CdS-doped Glasses from Gels Containing Diethyldithiocarbamatocadmium

CdS-doped SiO₂ glasses were prepared via silica gels containing diethyldithiocarbamatocadmium Cd(S₂CN(C₂H₅)₂)₂. Heat treatment of the gels gave transparent yellow SiO₂glasses doped with hexagonal CdS crystals. In optical absorption and fluorescence spectra, the optical absorption edge and the emission peak clearly exhibited a blue shift, which was attributed to the quantum size effect of the carrier confinement, as the CdS content was decreased. In the fluorescence spectra of the CdS-doped silica glasses, the emission peak was observed only near 500 nm and not observed at longer wavelengths which were known to be present if there were sulfur vacancies.     

Defect Structure and Electrical Conductivity of ThO2-Y2O3 Solid Solutions

Compositions in the system ThO₂-YO_1.5 were coprecipitated as oxalates and converted to oxides. Disks were pressed and sintered in oxygen at 1400° to 2200°C. Densities of the sintered disks were 96 to 98% of theoretical. Solid solutions with the fluorite-type structure were formed up to 20 to 25 mole % YO_1.5 at 1400°C and up to 45 to 50 mole % YO_1.5 at 2200°C. Density data showed that these solid solutions correspond to Th_{1— x} Y _x O_{2—0.5 x} , having a complete cation sub-lattice filled by Th⁴⁺ and Y³⁺ ions, and vacancies in the anion sublattice. The observed increase in electrical conductivity with increase in YO_1.5content is consistent with charge transport by oxygen ions through a vacancy mechanism. Approximately 7 mole % ThO₂ is soluble in YO_1.5 at 2200°C. Density results indicate an anion interstitial structure for the Y₂O₃ phase. Transference number measurements indicate that the electrical conductivities are only partly due to ions.     

Preparation and Physical Properties of Radiofrequency-Sputtered Amorphous Films in the System AlPO4–TiO2

Amorphous films in the system AlPO₄–TiO₂ were prepared by an rf-sputtering method, and their physical properties, such as density, refractive index, and thermal expansion coefficient, and the infrared absorption spectra were measured. The thermal expansion coefficient increased linearly with increasing TiO₂ content. The results of the molar refractivity and the infrared absorption spectra indicated that the coordination number of titanium ions in these films is higher than that in SiO₂–TiO₂ glasses with a negative thermal expansion, in which Ti⁴⁺ ions are tetrahedrally coordinated. In order to confirm the coordination state of the titanium ions in these amorphous films, titanium K -band emission spectra were obtained by X-ray emission spectroscopy, revealing sixfold coordination. The higher coordination state of Ti⁴⁺ was considered to account for these amorphous films not exhibiting negative thermal expansion, as in the SiO₂–TiO₂ system.