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71.
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A λ-type specific heat anomaly of U4O9?y, which occurs slightly above room temperature, was measured for the samples with different O/U ratios. The transition temperatures obtained from the peak of λ-type specific heat anomaly are in fairly good agreement with the data by X-ray diffraction and electrical conductivity measurements. The entropy increment for the transition was obtained to be 0.46 e.u. for UO2.250, 0.56 e.u. for UO2.240 and 0.70 e.u. for UO2.228, depending on O/U ratio. On the basis of the entropy change with O/U ratio, the mechanism of the phase transition is discussed. The entropy change due to the phase transition is mainly divided into two terms: one is due to the order-disorder rearrangement of U(IV) and U(V) ions,the other is associated with the displacement of oxygens ions. The former term is estimated to be rather smaller than the entropy change calculated by the Bragg-Williams approximation, and the latter is calculated using the X-ray diffraction data with the assumption of the Willis model. 相似文献
74.
In order to gain an insight into creep crack propagation problems a mechanics approach is presented in this paper on the basis of the plastic singular stress field near a crack tip under the Mode III and I steady-state creep conditions, in combination with a generalized creep damage hypothesis. Closed form equations which predict rates of creep crack propagation are obtained analytically. According to this analysis, it is found that is the index which characterizes the effect of damage accumulation in front of a crack, where n is the creep exponent and b1 is the exponent involved in the creep damage hypothesis used in this analysis. The tendency of the predicted crack propagation behaviors is consistent with experimental data available in the literature. 相似文献
75.
76.
Phase relations in the composition range from to were studied by electrical-conductivity measurements and X-ray diffraction in the ranges and . The plot of log σ versus log showed straight lines with distinct slopes, which corresponded to four regions (, , and ). The existence of the hyperstoichiometric phase was suggested in the temperature range from 1025 to 1126°C. The peritectoid temperature () was estimated to be present between 1126 and 1131°C. The partial free enthalpies and entropies for the two-phase equilibrium (, and ) were calculated and compared with previous results. From the dependence of the electrical conductivity on the oxygen partial pressure the nonstoichiometric defect structures of and were interpreted as consisting of doubly charged oxygen interstitials (Oi'') and doubly charged oxygen vacancies (VO''). At room temperature, the homogeneity range of the U4O9 phase was investigated with a Debye-Scherrer camera. 相似文献
77.
The nonstoichiometric composition of was measured by means of thermogravimetry in the range of and . The compositional deviation from stoichiometry, , in the hyperstoichiometric Cr2O3+x phase was observed to be smaller than 2 × 10?4, irrespective of temperatures, provided that the hyperstoichiometric Cr2O3+x exists. The existence of the hypostoichiometric Cr2O3?x phase was first established in this study in the region of low oxygen partial pressure below 10?5 Pa. From the oxygen partial pressure dependence of in Cr2O3?x, the defect structure was discussed with the neutral chromium interstitials in the composition near stoichiometry and with the triply charged ones far from stoichiometry. The partial molar enthalpy and entropy of oxygen of Cr2O3?x showed the complex compositional dependences, suggesting the change of the type of the predominant defect. 相似文献
78.
The heat capacity of U3O8−z with various O/U ratios was measured in the range from 250 to 750 K, and λ-type heat capacity anomalies were found in each sample. The transition temperatures were 487 and 573 K for UO2.663, 490 and 576 K for UO2.656 and 508, 562 and 618 K for UO2.640. The entropy changes of the transitions were 0.44 and 0.39 J K−1mol−1 for UO2.663, 0.58 and 0.47 J K−1mol−1 for UO2.656 and 0.62, 0.51 and 0.25 J K−1mol−1 for UO2.640, increasing as O/U decreases. The enthalpy change due to the transition varied linearly with the transition temperature except for UO2.640, showing the presence of the same mechanism of phase transition among the samples with various O/U ratios. The mechanism of the phase transition was discussed on the assumption that the transition is originated from the order-disorder rearrangement of U5+ and U6+ with a consequent displacement of atoms, similarly to the case of U4O9−y. 相似文献
79.
The vapor pressures over nonstoichiometric NbO2 ± x(s) (1.972?2.037) were measured by mass-spectrometric method in the temperature range 1958–2326 K. The congruently vaporizing composition in the NbO2 ± x phase was determined to be stoichiometric NbO2.000 from the composition dependence of the total vapor pressures. The partial pressures of oxygen were calculated as a function of temperature and O/Nb composition from the partial pressures of the gaseous species NbO2(g) and NbO(g) over nonstoichiometric NbO2 ± x, from which the partial molar enthalpies and entropies of oxygen were calculated as a function of O/Nb composition. The composition dependence of the partial molar enthalpy and entropy obtained suggested the existence of some kind of short-range ordering in the nonstoichiometric NbO2 ± x(s) phase. The enthalpies of formation of nonstoichiometric NbO2 ± x(s) were also determined as a function of composition by combining the partial molar enthalpies of oxygen with the enthalpy of formation of stoichiometric NbO2.00(s). The phase diagram around NbO2 ± x at high temperatures was determined from the vaporization study. 相似文献
80.
Male Fischer 344 rats were subcutaneously injected with azoxymethane (AOM) twice weekly at a dose of 15 mg/kg and were fed with freeze-dried (FD) samples of beer brewed without hops (non-hops beer), beer with hops at 4 times the amount of regular lager beer (x 4-hops beer), and isomerized hop extract (IHE) for the whole experimental period (I/PI) or for the post-initiation period (PI) only. Feeding FD beer samples at a dose of 1% significantly decreased the number of aberrant cryp foci (ACF) in the PI protocol over five weeks.x4-hops beer showed stronger inhibitory effects on the development of the numbers of aberrant crypts per focus and large ACF with four or more crypts than non-hops beer. Feeding IHE to rats at a dose of 0.01% or 0.05% in either the I/PI or PI experiment significantly reduced the numbers of ACF. Prostaglandin E2 (PGE2) levels in colonic mucosa of AOM-treated rats were significantly reduced by feeding of IHE. PGE2 production induced by lipopolysaccharide/interferon-gamma (LPS/IFN-gamma) in RAW264.7 cells was also reduced by treatment with IHE and isohumulone in a dose-dependent manner. These observations suggest that isohumulones show chemopreventive effects on ACF formation in rat colon by inhibiting the production of PGE2. 相似文献