Yuji Arita

Defect structures in doped CeO2 studied by using XAFS spectrometry

Abstract The local structures around Ce and Ln atoms in Ce 1− x Ln x O 2− x /2 (Ln=Sc, Y, Nd, Sm, Gd, Yb, x =0–0.30) were studied by using EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure) spectroscopy. The oxidation state of Ce was found to be present predominantly as the tetravalent Ce 4+ cations in all Ce 1− x Ln x O 2− x /2 samples. Except for the Ce–O distances in Sc-doped CeO 2 showing the presence of two-phase of Sc 2 O 3 and Ce 1− x Sc x O 2− x /2 , the Ce–O distances in Ce 1− x Ln x O 2− x /2 were seen to decrease with increasing dopant concentration, suggesting that oxygen vacancies introduced by doping trivalent cations Ln to CeO 2 were thought to be located around Ce ions. With dopant content, the Gd–O distances decreased but Y–O distances were seen almost constant. Several complex defect structures composed of Ce, Gd or Y and oxygen atoms were proposed.

EXAFS study of Ce1-xGdxO2-x/2

Abstract The local structures around Ce and Gd atoms in Ce1−xGdxO2−x/2 (x=0–0.30) were studied by means of extended X-ray absorption fine structure (EXAFS) spectrometry. Oxygen vacancies introduced by doping trivalent Gd ion to CeO2 were seen to be located around both Gd and Ce ions. The association of two Gd ions and one oxygen vacancy is proposed in heavily doped solid solutions, supporting the decrease in the ionic conductivity in the heavily doped region previously reported.

High temperature phase transitions of CaTiO3 and (Ca0.85Nd0.15)TiO3 by X-ray diffractometry and differential thermal analysis

Abstract The two phase transitions of CaTiO3 and (Ca0.85Nd0.15)TiO3 were studied by high temperature X-ray diffraction and differential thermal analysis in the temperature range 300–1723 K. The presence of the two kinds of phase transitions in CaTiO3 and (Ca0.85Nd0.15)TiO3 at about 1390 K and about 1530 K recently found by the heat capacity measurement in our laboratory was confirmed. From the X-ray diffraction patterns of CaTiO3 and (Ca 0.85Nd0.15)TiO3, the first phase transition at lower temperature was due to the change from the orthorhombic Pbnm to the orthorhombic Cmcm structure, and the second one at higher temperature was due to change from Cmcm to the cubic Pm3m structure, originating from the sequential rotations of the TiO6 octahedra about one of the axes. Since no clear peak was seen at the phase transition around 1390 K in the DTA curve in contrast with the presence of the endothermic peak at the transition around 1530 K, it is thermodynamically considered that the former and the latter transition were second- and first-order phase transition, respectively.

Thermoelectric properties of Rh-doped Ru2Si3 prepared by floating zone melting method

Precipitation-free samples of Rh-doped Ru 2 Si 3 were prepared by the floating zone (FZ) method, The temperature dependences of the electrical resistivities and the Seebeck coefficients of Rh-doped Ru 2 Si 3 (Rh content = 0, 4, 6 mol%) were measured. The electrical resistivities of both 4% and 6% Rh-doped Ru 2 Si 3 were smaller than those of undoped Ru 2 Si 3 prepared by the FZ method and 4% Rh-doped one prepared by other methods. The maximum value of the Seebeck coefficients for all samples was -175 μV/K at 673 K for 4% Rh-doped Ru 2 Si 3 . The dimensionless thermoelectric figure of merit reached 0.8 for 4% Rh-doped Ru 2 Si 3 at 1073 K, which was about 50% larger than that of optimized n-type SiGe.

Removal of platinum group metals contained in molten glass using copper

Removal of platinum group metals (PGMs) such as Pd, Ru, and RuO2 from molten glass by using various amounts of liquid Cu was done as a basic study on a new vitrification process for a high-level radioactive waste. We prepared two types of borosilicate glasses containing PGMs and Cu, respectively. These glasses were mixed together and heated at 1,473 K for 4 h in Ar atmosphere. More than 95% of Pd were removed as a spherical metal button composed of Pd-Cu alloy when Cu was added in an amount 0.5 times the weight of Pd. Nearly 95% of Ru was also removed as a spherical button with 2.5–5 times as much Cu addition as Ru in weight. Ruthenium oxide was reduced to metallic Ru by a reaction with Cu in the molten glass. The removal fraction was increased by increasing the amount of Cu and reached 63% when Cu addition was 7.5 times as much as RuO2 in weight. By addition of Si as a reducing agent, nearly 90% of Pd and Ru were removed with Cu and Si metal composites even under O2:Ar = 20:80 (v/v) condition.

EXAFS analyses of CaTiO3 doped with Ce, Nd and U

Abstract The EXAFS (extended X-ray absorption fine structure) analyses were carried out for Ti–K, Ce–L3, Nd–L3 and U–L3 edges to make clear the local structures in (Ca 1− x Ce x )TiO 3 ( x =0.10, 0.20), (Ca 1− x Nd x )TiO 3 ( x =0.05, 0.10, 0.15, 0.20) and (Ca 1− x U x )(Ti 1−2 x Al 2 x )O 3 ( x =0, 0.05). With increasing neodymium content in (Ca 1− x Nd x )TiO 3 the first nearest titanium–oxygen distances decreased. On the other hand, with increasing cerium content in (Ca 1− x Ce x )TiO 3 , the first nearest titanium–oxygen distances increased. The first nearest titanium–oxygen distance in (Ca 1− x U x )(Ti 1−2 x Al 2 x )O 3 was larger than that in undoped CaTiO 3 . With increasing dopant content, the first nearest dopant (cerium or neodymium)–oxygen distances increased. From the result of the compositional dependencies of bonding length (cation–oxygen distance), which is inversely proportional to the bonding energy, it is estimated that magnitude of the leaching rate of Ce-doped CaTiO 3 is larger than those of Nd-doped and undoped CaTiO 3 . The experimental results on the leaching rates of doped and undoped CaTiO 3 were consistent with this estimation. The structural change of (Ca 0.80 Ce 0.20 )TiO 3 after a leaching test in HCl at 363 K for 140 days was also measured by X-ray diffraction and XANES analysis.

High temperature heat capacities of (U0.91 M0.09)O2 (where M is Pr, Ce, Zr) from 290 to 1410 K☆

Abstract Heat capacities of (U 0.91 M 0.09 )O 2 , where M is Pr, Ce and Zr, were measured by direct heating pulse calorimetry over the temperature range 290–1410 K. No anomalous increase in the heat capacity curve of each sample was observed at temperatures up to 1410 K, dissimilarly to the cases of (U 1− y M y )O 2 (M is a trivalent cation (Gd, Eu, La) or simulated fission product composed of several tri- and tetravalent cations; y = 0.044–0.142) found previously by the present authors. It is proposed that the occurrence of the heat capacity anomaly of (U,M)O 2 , where M is a trivalent cation or simulated fission product, originates from the predominant contribution of Frenkel pair-like defects of oxygen formed by the introduction of aliovalent cations (M 3+ ) in UO 2 from the electroneutrality condition, and that the introduction of the tetravalent cations which are the same valency as uranium ions in UO 2 results in insignificant effect on the total number of the oxygen defect, producing no heat capacity anomaly. The difference in the onset temperatures of the heat capacity anomaly of UO 2 doped with various trivalent cations is discussed from the viewpoint of the difference between the averaged cationoxygen interatomic distance calculated from the ionic radii and that obtained from the experimental lattice constants on the assumption of the perfect fluorite structure, reflecting the variety of the distribution of oxygen around cations in the lattice, i.e. that of the local structural environments for cationoxygen bonding. A linear relation between the onset temperature of the heat capacity anomaly and the difference in the cationoxygen interatomic distance was found, supporting the importance of the variety of local structures in UO 2 as the origin of the heat capacity anomaly.

Isotope Effects on Thermal Conductivity of Boron Carbide

The isotopically enriched disk samples of 10B4 12C, (10B0.75 11B0.25)4 12C, (10B0.20 11B0.80)4 12C, 10B4 13C, 10B4 (12C0.50 13C0.50), 11B4 (12C0.50 13C0.50), 11B4 12C, 11B4 13C and natural nB4 nC were prepared by the spark plasma sintering method. The thermal diffusivity for each sample was measured by the laser flash method over the temperature range from 290 to 1,450 K. The isotope effects on the thermal conductivities, which were calculated from the measured thermal diffusivities, the heat capacity for natural B4C and densities, were discussed in terms of the phonon-velocity and the scattering probability of phonon by isotopic impurity. The thermal conductivity decreased only with increasing the mean mass of boron. In the single-atom model, the phonon-velocity decreases with increasing the mass of atoms. The thermal conductivity obtained in this study was seen to be independent on the scattering probability of phonon by isotopic impurity, but to depend on the phonon-velocity. Isotope effects are thought to be difficult to detect in polycrystalline boron carbide.

High temperature heat capacities and electrical conductivities of UO2 doped with yttrium and simulated fission products

Abstract Heat capacities and electrical conductivities of (U 0.190 M 0.090 )O 2 (M = Y and simulated fission products (FP) for 10% burnup) were measured simultaneously by means of direct heating pulse calorimetry in the temperature range from 300 to 1500 K. Anomalous increases in the heat capacity curves of UO 2 doped with yttrium and simulated fission products were observed above about 1100 and 600 K, respectively. These anomalous increases were similar to those of ( U 1− y M y ) O 2 (M = Gd, La, Eu and Sc) found previously by the authors. The values for the enthalpy and entropy of defect formation in (U 0.910 M 0.090 )O 2 (M = Y and FP) were calculated from the excess heat capacity assuming the presence of Frenkel pairs of oxygen, and were found to be similar to that for UO 2 doped with Gd, La, Eu and Sc. On the other hand, no anomaly was seen in the electrical conductivity curve around the onset temperature of the anomalous increase in the heat capacity of doped UO 2 . It was, therefore, concluded that the excess heat capacity originates from the predominant contribution of the formation of Frenkel pair-like defects of oxygen and from the small contribution of the formation of electron-hole pairs. The difference in the onset temperatures of UO 2 doped with various cations was thought to be originated from that of the elastic strain induced by the change of the lattice parameters of doped UO 2 .

Heat capacity measurement of U1−yLayO2 (y=0.044, 0.090, 0.142) from 300 to 1500 K

Heat capacities of U1−yLayO2 were measured by means of direct heating pulse calorimetry in the temperature range from 300 to 1500 K. An anomalous increase in the heat capacity curve of each sample was observed similarly to the case of U1−yGdyO2, found recently in our laboratory. As the lanthanum content of U1−yLayO2 increased, the onset temperature of an anomalous increase in the heat capacity decreased and the excess heat capacity increased. The enthalpy of activation (ΔHf) and the entropy of activation (ΔSf) of the thermally excited process, which cause the excess heat capacity were obtained to be 2.14, 1.63 and 1.50 eV and 39.4, 34.2 and 31.8 J·K−1·mol−1 for U0.956La0.044O2, U0.910La0.090O2 and U0.858La0.142O2, respectively. The values at zero La content extrapolated by using the data of ΔHf and ΔSf for U1−yLayO2 were in good agreement with the experimental values of undoped UO2 so far reported, similarly to the case of Gddoped UO2. The electrical conductivities of U1−yLayO2 (y=0.044 and 0.142) were also measured as a function temperature. No anomaly was seen in the electrical conductivity curve. It may be concluded that the excess heat capacity originates from the predominant contribution of the formation of oxygen clusters and from the small contribution of the formation of electron-hole pairs.