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Abstract  

Uranium oxides are known as nonstoichiometric compounds whose composition changes according to external conditions such as temperature and oxygen partial pressure. The change of composition caused by the formation of defect structure results in a change of their properties. In this paper, the compositional changes of UO2 and doped UO2 [(U, M)O2; M=La, Ti, Pu, Th, Nb, Cr, etc.] and also those of other uranium oxides (U4O9, U3O8) are shown against oxygen partial pressure. From the results of doped UO2, it is concluded that the valence control rule holds to a first approximation. The defect structures are estimated both from log x vs. log Po2 (x: deviation from the stoichiometric composition and Po2: oxygen partial pressure) and log vs. log Po2 (: electrical conductivity) relations. The defect structures of UO2 and doped UO2 are derived based on the Willis model for UO2+x. The detect structure of U4O9 phase is similar to that of UO2+x, but the defect structures of U3O8 phase are complicated due to the existence of many higher-order phase transitions. The thermodynamic data such as the partial molar enthalpy and entropy and the heat capacity are important to characterize the defect structure. The high temperature heat capacities of UO2 doped with Gd show pronounced increases at high temperatures the onset temperature decreases as the dopant content increases. The increase of heat capacity is interpreted to be due to the formation of lattice defects. The heat capacity measurements on U4O9 and U3O8 clucidate the presence of the phase transition. The mechanisms of these phase transitions are discussed.

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Abstract  

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 (Hf) and the entropy of activation (Sf) 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 Hf and Sf 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.

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Summary Proton concentration in SrZr0.95M0.05O3-α (M=Ga, Sc, Y and Nd) was measured with a thermobalance at different temperatures (T=673-973 K) and water vapor pressures (P H2O=0.9-12.8 kPa). At all the pressures and temperatures examined, the amount of proton concentration in these samples is in the order of Sc>Y>Ga>Nd. By an equilibrium evaluation, we estimated the maximum possible proton concentration about 2.0 mol% in these samples. Infrared (IR) absorption spectra are measured in these samples. The absorption bands can be fitted by four (M=Ga, Y, Nd) or three (M=Sc) Gaussian bands.

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Abstract  

The heat capacities of binary aqueous solutions of 1,2-ethanediol, 1,2-propanediol and 1,2-butanediol were measured at temperatures ranging from 283.15 to 338.15 K by differential scanning calorimetry. The partial molar heat capacities at the infinite dilution were then calculated for the respective alkanediols. For 1,2-ethanediol or 1,2-propanediol, the partial molar heat capacities at the infinite dilution of increased with increasing temperature. In contrast, the partial molar heat capacities of 1,2-butanediol at the infinite dilution decreased with increasing temperature. Heat capacity changes by dissolution of the alkanediols were also determined. Heat capacity changes caused by the dissolution of 1,2-ethanediol or 1,2-propanediol were increase with increasing temperature. On the other hand, heat capacity changes caused by the dissolution of 1,2-butanediol are decrease with increasing temperature. Thus our results indicated that the structural changes of water caused by the dissolution of 1,2-butanediol differed from that of the two other alkanediols.

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Abstract  

Heat capacities, electrical conductivities and phase transition temperature of hafnium hydrides, HfHx (0.99≤x≤1.83), were studied using a direct heating pulse calorimeter and a differential scanning calorimeter from room temperature to above 500 K. The heat capacity of HfH1.83 was larger than that of pure hafnium and showed no anomaly of heat capacity. In contrast, there were λ-type peaks for the heat capacity and DSC curves for HfHx (1.1≤x≤1.6) near 385 and 356 K. The anomalies of heat capacity and electrical conductivity of HfHx (1.1≤x≤1.6) were considered the result of phase transition and order-disorder phase transition for hydrogen in the hafnium hydride lattice for HfHx (1.1≤x≤1.3).

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Abstract  

The most probable value theory has been applied to analytical radioassay for mixture samples of six pure-beta emitters by using a liquid scintillation spectrometer. Activities of each nuclide in the mixture sample can be determined by solving normal equations which are derived from twelve observation equations, the number of which is twice that of nuclides to be radioassayed. Requirement for the technique is to construct quenching correction curves of the respective nuclides. This technique has been tested to3H–63Ni–14C–45Ca–36Cl–32P mixture samples, and found to be very useful with the aid of computerized data processing.

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Abstract  

Effects of fission (fragment) damage on the magnetic properties were investigated for some uranium compounds with NaCl-type crystal structure, such as uranium monocarbide (UC, paramagnetic) monophosphide (UP, antiferro) and monosulfide (US, ferro). The induced changes in the magnetic properties due to the fission damage were much pronounced in the magnetically ordered state. A shift of the magnetic transition point (either the Neel (TN) and Curie (TC) temperature) was observed, together with the changes of the magnetic parameters. In some cases, a new magnetically ordered phase was revealed by the fission damage even at room temperature irradiation.

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Journal of Thermal Analysis and Calorimetry
Authors: H. Shigematsu, Y. Akishige, T. Matsui, T. Tojo, H. Kawaji, and T. Atake

Summary The phase diagram of the mixed crystal (K1-xRbx)2SeO4 was determined by means of thermal analysis and neutron scattering experiments. The hexagonal to orthorhombic phase transition line exists for any x. The normal-incommensurate phase transition temperature decreases continuously with increasing Rb content. However, the incommensurate-commensurate phase transition was not observed except for K2SeO4. According to the clear softening of the Σ2- Σ 3 phonon branches and the finite frequency at 0 K for x>0.34, an existence of the hypothetical phase transition was confirmed.

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