UC-UN Solid Solutions Research Articles

Heat of miving in UC-UN solid solution

The vapour pressures of nitrogen and uranium over UC-UN solid solutions were measured in a temperature range 1900 to 2300 K using a mass spectrometer with a Knudsen cell. The evaluated chemical activities of uranium, nitrogen and UN were appreciably smaller than those assumed in the ideal solid solution. This shows that the chemical stability of UN at high temperature is considerably improved by an addition of UC. The heat of miving between UC and UN was estimated using these chemical activities. The heat of miving was calculated to be (−23.4 ± 9.0) kcal mol −1 at 2073–2273 K in a wide compositional range of the solid solution. This large heat of miving indicates that UC-UN may be looked upon as a regular solution.

Investigations on uranium carbonitrides: III. Nitrogen vapour pressures and thermodynamic properties

Nitrogen pressures of UN and UC-UN solid solutions have been measured between 1900 and 2300 K by mass spectrometry with the Knudsen effusion technique. For the reaction UN(s) → U(1) + 0.5 N 2 (g) it has been found by third-law calculations that ΔH°(298 K) = 280.7 ±1.3 kJ/mol (67.1 ± 0.3 kcal/mol) . Reliable uranium vapour pressure data could not be obtained. The thermodynamic properties of UN and the UC-UN solid solution will be discussed in relationship with previous investigations in the literature. These properties cannot be described by the regular solution model.

Investigations on uranium carbonitrides (I): The lattice parameters of the solid solutions

The lattice parameters of the UC-UN solid solutions have been measured at room temperature as a function of composition. Equilibrium has been approached from two directions, by starting from UC + UN or from UN + graphite mixtures. After equilibrium, which was readily obtained at 1800°C, a careful chemical characterization was carried out, including chemical analyses of all major elements.

Influence of energetically active carbon on thermodynamics and phase relationship of U-C-N system

The reactions of UN with graphite and of UN with active carbon have been investigated at 1300°C and 1400°C under a variety of pressures of nitrogen gas. Results show that, at fixed pressure, the nitrogen content of the equilibrium UC-UN solid solution formed in the presence of graphite is higher than in the equilibrium UC-UN solid solution formed in the presence of active carbon. A brief thermodynamic argument explains the result in terms of the differing chemical potentials between graphite and active carbon. Les réactions de UN avec le graphite et de UN avec le carbone actif ont été étudiées à 1300 et 1400°C dans différentes conditions de pression de l'azote gazeux. Les résultats montrent qu'à une pression donnée, la teneur en azote de la solution solide d'équilibre UC-UN formée en présence de graphite est plus élevée que dans la solution solide d'équilibre UC-UN formée en présence de carbone actif. Un bref argument thermodynamique explique ce résultat en termes des potentiels chimiques différents du carbone et du carbone actif. Die Reaktionen zwischen UN und Graphit sowie UN und aktivem Kohlenstoff wurden bei 1300 und 1400°C unter verschiedenen Stickstoff-Drucken untersucht. Bei gegebenem Druck ist die Stickstoff-Konzentration des im Gleichgewicht mit Graphit gebildeten UC-UN-Mischkristalls höher als jene des im Gleichgewicht mit aktivem Kohlenstoff gebildeten UC-UN-Mischkristalls. Durch eine kurze thermodynamische Begründung wird das Ergebnis in Hinblick auf das unterschiedliche chemische Potential zwischen Graphit und aktivem Kohlenstoff erklärt.

Compressive creep of sintered UC-UN solid solutions

Compressive creep of slightly hypo-stoichiometric uranium carbonitrides sintered from UC and UN powder mixture was studied at temperatures of 1310 and 1500° C. The creep rate as a function of C/N mole fraction showed a maximum near the composition N/(C+N) = 1 4 (in moles) and a minimum near N/(C+N) = 3 4 . This behaviour was correlated with the recent experimental evidence of the anomalies in the elastic constant and melting point, and thermodynamic discussions on the deviation of uranium carbonitrides from the ideal solution of UC and UN.

On the formation of UC-UN solid solutions

In order to elucidate the mechanisms of the formation of the solid solution between UC and UN, some experiments were carried out using several kinds of combination as the starting materials; (i) UC, (ii) UC 2, (iii) UC + UN and (iv) UN+graphite. The conclusions drawn from these experiments are: 1. (1) Free carbon precipitating through the reaction of UC with nitrogen gas is not graphite but amorphous carbon with more or less incomplete graphitization. The degree of graphitization is a function of temperature. These amorphous carbons may have been a frequent cause of the scattering results obtained by many workers up to the present time. 2. (2) The mixture of UN and graphite is the best combination as the starting materials to investigate the equilibrium of the region consisting of UC-UN solid solution and free carbon, from both equilibrium and kinetic points of view. 3. (3) During the course of the reactions, UC + N 2 and UN + graphite, an intermediate state may appear, which includes two phases of UC-UN solid solutions, one having the composition corresponding nearly to the equilibrium one and the other being a solid solution with the composition different from the equilibrium one. The diffusion of non-metal atoms through the UC-UN solid solution-layer may control the overall reaction.

Graphitization of free carbon precipitating through the reaction of UC 2 with N 2

The degree of graphitization of the carbon formed through the reaction of UC 2 with nitrogen gas was studied as a function of temperature and nitrogen pressure. The degree of graphitization increases with increasing temperature and with decreasing nitrogen pressure. From these results, it might be concluded that the scattering results obtained by the reaction of UC with nitrogen gas is due to the different degree of graphitization of the precipitating carbons. In order to study the equilibrium of the two-phase region of UC-UN solid solution and free carbon, that is, the relation between temperature, nitrogen pressure and the composition of the UC-UN solid solution, it should be necessary to employ the mixture of UN and graphite as the starting material.

Consideration on the ideality of UC-UN solid solutions

The monocarbides and mononitrides of uranium and plutonium as well as their solid solutions could obtain certain importance as fuel for high-power reactors. From the thermochemical point of view the question whether the mixed carbonitride crystal shows ideal, regular or abnormal behaviour is of decisive importance. In the present paper the published data on measurements of equilibrium between UC-UN solid solutions and nitrogen, carbon and uranium are studied critically with the aid of a new method of calculation. The advantage of this method is that it enables the enthalpy of mixing to be determined as a function of the temperature and the solid solution composition directly from the relevant UC 1- x - N x measurement data, instead of those relating to the pure UC and UN components. The paper also discusses the ideal behaviour of the solid solution series on the basis of molecular volumes, which are calculated from the published data. Both theoretical considerations lead to the conclusion that the UC 1- x - N x solid solutions do not show ideal behaviour.

518. UC-UN固溶体のVegardの法則の成立性

The validity of Vegard's law for UC-UN solid solutions is still in dispute. A discussion is given on the possible causes of this disagreement among published papers on this subject. The factors taken up include oxygen contamination, errors of chemical analysis in determining the composition of solid solution, slower than expected reaction rate, non-stoichiometry of solid solution, coexisting second phase, and the state of distribution of carbon and nitrogen atoms in the UC-UN lattice.Experiments were performed in vacuo and in nitrogen gas in order to obtain the relation between the lattice parameter and the composition of solid solution, and to determine the effects brought upon this rebtion by the factors cited above.The lattice parameters of UC-UN solid solution obtained by vacuum sintering at 1, 700° and 1, 800°C showed a positive deviation from Vegard's law. The same parameters when the solid solution coexisted with UC2 deviated similarly from Vegard's law. The presence of oxygen in the UC-UN solid solution caused little change of lattice parameters in the vicinity of a composition of UC0.5N0.5.

Melting point maxima of ThC-ThN and of UC-UN solid solutions

Melting points of ThC-ThN and UC-UN mixtures in nitrogen atmospheres were determined. Maxima occur at 2910 ± 35° C with the compositions ThC 0.35 N 0.65 ± 0.1 and UC 0.3 N 0.7 ± 0.1. From a determination of the nitrogen activity the excess free energies of mixing the terminal mononitrides at 2820°C in the respective pseudo-binary solid solutions are inferred to be −3.2 ± 2.0 kcal/mole at the ThN mole fraction of 0.45 and −0.7 ± 1.5 kcal/mole at the UN mole fraction of 0.50.

Change of Composition of UC1-xNx in Nitrogen Gas

The UC-UN solid solution, UC1-xNx, exists stably in monophase within a certain range of nitrogen pressure at constant temperature. At the upper limit of this range, UC1-xNx, coexists with UC2, or with carbon, and it precipitates metallic uranium at the lower limit which corresponds to the decomposition pressure of UC1-xNx. In order to measure the decomposition temperature of UC1-xNx with given x under constant nitrogen pressure, it is necessary to heat UC1-xNx without changing its composition, at a constant pressure. In the present work, preliminary considerations have been given to change in composition. Experiments were also performed in which the solid solution was heated at a rate of 200°C/min under different nitrogen pressures. From the results, it is concluded that there exists a temperature range within which the value of x in UC1-xNx, is maintained constant when the nitrogen pressure is fixed.