Yoichi Takahashi

Non-stoichiometry and its effect on thermal properties of Li2TiO3

Abstract Li 2 TiO 3 is among the most promising candidates for solid breeder materials. However, its structural property, especially the possibility of non-stoichiometric composition, has not been well established yet. Therefore, the composition of Li 2 TiO 3 has been further investigated by means of thermogravimetry, XRD etc. Also, thermal diffusivity measurement was carried out. Mass of Li 2 TiO 3 was found to change after the change of the atmosphere from hydrogen to oxygen as observed by means of thermogravimetry. At the same time, the color was observed to change from dark blue to initial color, white. These behaviors suggest not only oxygen deficient but also lithium oxide deficient defects formation. Thus, the doubly non-stoichiometric composition, Li 2− x TiO 3− y , has been confirmed. Further, it was shown by thermal diffusivity measurement that 95Li 2 TiO 3 (i.e. Li 2 O/TiO 2 =0.95) has a higher thermal conductivity than 100Li 2 TiO 3 .

Miscibility gap in CeO2–ZrO2–YO1.5 system as an electrode of solid oxide fuel cell

Abstract To detect miscibility gap in [(CeO2)x(ZrO2)1−x]0.8(YO1.5)0.2 (x=0–1), powders prepared by liquid-mixing method were investigated by conventional X-ray diffraction (NXRD) and Synchrotron X-ray diffraction (SXRD). The NXRD pattern of the samples, calcined at 873 K, exhibited a broadened cubic structure without any secondary phases. The lattice constants or samples annealed at different temperatures were determined, and they show linear dependence on cerium content (x) except for 1273 K, which indicates that a complete solid solution can be obtained above 1473 K. The NXRD results on samples annealed at 1273 K exhibit deviation from Vegards law. The SXRD patterns confirmed the existence of miscibility gap in the samples of x=0.5–0.8 annealed at T=1273 K for 778 h.

High temperature liquid metal corrosion and high temperature electrical conductivity of Y2O3

Abstract Yttrium sesquioxide has been proposed as a promising candidate material for collector electrodes used in the laser enrichment system of uranium-235. For this purpose, yttria is expected to be compatible with molten uranium and electrically conductive. A corrosion test of yttria with molten lanthanum as a simulating metal for uranium and a measurement of its electrical conductivity under extremely low oxygen pressure were performed. It was shown from the corrosion test that a yttria sample was considerably corroded by the molten lanthanum at 1513 K and the maximum corrosion depth for 5 Ms was 0.162 mm. The electrical conductivity of hypo-stoichiometric yttria reduced by titanium was higher than that of pure germanium at room temperature (2.1 S/m). The oxygen pressures equilibrated with the yttria specimens were estimated to discuss the relation to measured conductivities.

Characterization of unsaturated organic molecule - alkali metal - ternary graphite intercalation compounds

Abstract Raman scattering and X-ray diffraction measurements were made to identify stage structure of CsC24(C2H4)n and CsC24(C2H3CN)n, which were exposed to air for a long time. Their electric conductivities were also measured during the long exposure to air. Raman spectra suggest that the gradual degradation of their electric conductivities in air is caused by the slow decomposition of the ternary GICs from the periphery region, although results of X-ray diffraction for all samples indicate the retention of the initial stage structure. Among the samples, CsC24(C2H4)1.4 which absorbed large amount of ethylene, was found to be exceptionally stable both in its stage structure and electric conductivity.

Heat capacity and phase transition of Zircaloy-4

Abstract Heat capacity from 300 K to 1100 K and enthalpy change of the phase transition of Zircaloy-4 between 1080 K and 1270 K were measured with a triple-cell-type differential scanning calorimeter and a twin-type drop calorimeter. Heat capacities measured with these two apparatuses were in good agreement with each other, and they were several % less than the other literature values. They were also in good agreement with the values calculated by assuming the additivity of the contributions of the elements and the values of pure zirconium. On the phase transition, two peaks were observed in the temperature range from 1080 K to 1270 K. The first peak was assigned to a eutectic temperature of the ZrFe system or the ZrCr system, and the second peak was considered to come from the α−β phase transition of Zr affected by Sn and O. The enthalpy change of the α-β phase transition (Δ H tr ) was determined to be 4.12 ± 0.35 kJ mo −1 which was in good agreement with literature values for pure Zr.

Behavior of unsaturated organic molecules in the nanospace of stage-2 cesium-graphite intercalation compounds

Intercalation reaction of benzene (C6H6) into CsC24 and the characteristics of resulting ternary graphite intercalation compound (GIC) have been investigated, in comparison with the similar CsC24–ternary GICs with ethylene (C2H4) and acrylonitrile (C2H3CN), and the behavior of these organic molecules within the interlayer nanospace has been discussed. The stability of these ternary GICs under exposure in air, in both stage structure and electrical conductivity was found to be in the order of C2H4–, C6H6– and C2H3CN–GICs. This result was reasonably explained as follows: the C2H4 molecules in the interlayer space oligomerize considerably and form network preventing diffusion of the Cs atoms in the interlayer, while the C6H6 molecules form only dimers or trimers and unable to form network, and the C2H3CN molecules, in spite of their strong tendency to polymerize, apparently do not form oligomers in the interlayer space, and its ternary GIC tends to degrade easily. By the electrical and galvanomagnetic measurements, it is revealed that the dominant carrier of these ternary GICs is electrons, even when the electrical conductivity decreases considerably after prolonged exposure in air, while the host graphite, Grafoil, shows two-carrier conduction. This fact indicates that the degradation of the ternary GICs occurs mainly at the periphery or surface layer of the crystallite, and at the inside of the crystallite, the ternary stage structure is retained unchanged.

Host Effect on the Properties of AM-GICs

Abstract Carbon materials (A-1, A-2 and A-3 derived from pitch cokes) with different graphitization degree, were allowed to react with potassium. The hydrogen-sorption behavior at 77 K, electrical resistivity. ESR and Raman spectra of the resulting compounds with the composition of KC60 were determined. The sorbed amount at saturation, (n H2/n K)sat, was 0.55 and 1.43 for KC60s prepared from A2 (d 002 = 0.3377 nm) and A3 (d 002 = 0.3361 nm), respectively. No H2 sorption was observed for KC60 from A-1 (d 002=0.342 nm). Temperature dependence of the resistivity of KC60s from A-2 and A-3 showed metallic behavior, contrary to semiconductive one for the host materials. However, KC60 (A-1) showed semiconductive temperature dependence, similarily to the host material. Raman spectra of KC60s from A-2 and A-3 showed doublet structure, similarly to that of K-GICs from HOPG, characteristic for graphite intercalation compounds with stage n > 2. On the contrary, KC60 (A-1) gave single peak at around 1597 cm−1. Those facts suggest that potassium exists in the interlayer spaces for KC60s (A-2, A-3), but not for KC60 (A-1). It was also shown that g-factor of ESR spectra of KC60 can be useful to predicting the H2-sorption behavior.

The heat capacity of tantalum from 80 to 1000 K

Abstract The heat capacity of metallic tantalum has been measured in the temperature range 80–1000 K by laser-flash calorimetry. The results are compared with available low- and high-temperature heat capacities, and the thermodynamic values are given.

Phase equilibria of NdC13NaClKCl

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. Molten chloride is considered to be applied to a fast-breeder-reactor fuel and a solvent in the pyrochemical reprocessing of spent nuclear fuel. In this work, phase diagrams for molten chloride systems were constructed, using NdCl 3 as an imitative substance in place of UCl 3 or PuCl 3 . A compound of 3NdCl 3 .NaCl, which melts incongruently at 540°C to NdCl 3 and liquid and a eutectic at 437°C were found in the NdCl 3 -NaCl system. In the NdCl 3 -KCl system, many invariant reactions were observed: the decomposition of NdCl 3 .2KCl and 2NdCl 3 .KCl at 444 and 474°C, respectively; a eutectic at 489°C; a peritectic at 506°C and a monotectic at 624°C. It is thought that there should be a peritectic compound of 6NdCl 3 .KCl above 474°C. A compound of 2NdCl 3 .NaCl.KCl was considered to exist in the ternary system of NdCl 3 -NaCl-KCl, to which attention should be paid in determining the composition of the fuel of the molten-salt fast breeder reactor.