Uranium dioxide is oxidised by two distinct processes in the temperature ranges -130" to 50" and 130" to 180°, though in each case oxygen is absorbed into interstitial positions in the lattice. Between -130" and 50", the oxygen absorbed increases as the logarithm of the time ; the same law is followed a t the same temperatures by solid solutions of uranium dioxide and thoria. The oxidation virtually ceases at temperatures below 50" at a composition which depends upon the surface area of the uranium dioxide. Between 130" and 1 80" the kinetic law followed is characteristic of a difiusion-controlled process but both the rate of reaction and, to some degree, the composition of the oxide finally formed depend upon the pressure. The reaction with nitrogen dioxide is much faster than the reaction with oxygen. * Part VI, preceding paper.
The magnetic susceptibilities of UO,, UO,.,, U,O,, U307, and U,O, a t temperatures between 1.5" and 4 4 " ~ have been determined using an a.c. inductance method. The susceptibility of U,O, rises to a sharp maximum at 4 . 2 " ~, with smaller effects a t 8" and a t 25.3'~. The susceptibility of UO, reaches a maximum a t 2 9 ' ~ and remains constant between 25" and 2 " ~. A maximum in the susceptibility-temperature plot for U,O, a t 6 . 4 " ~ occurs also in the plots for UO,., and U307 and appears to be primarily due to the inclusion of interstitial oxygen ions in the UO, lattice, independent of the precise nature of subsequent ordering effects.
By L. E. J. ROBERTS.[Reprint Order No. 5302.1 Oxygen is rapidly chemisorbed by freshly reduced surfaces of uranium dioxide at -183", and also by surfaces of mixed crystals of uranium dioxide and thorium dioxide if these surfaces are prepared by crushing macrocrystalline samples. One oxygen molecule reacts with a single U4+ site in the primary act of chemisorption. At least half the U4+ ions in the surface layers react with oxygen and only about 30% of the occupied sites become vacant on high-temperature evacuation. There is evidence that Th4+ ions concentrate preferentially in the surface layers of some mixed-crystal preparations.URANIUM dioxide is commonly prepared by reduction with hydrogen or carbon monoxide at high temperatures, and the oxide so prepared can be shown to be UO,.,, as closely as analytical accuracy permits, The oxide is oxidised readily in air at temperatures between 100" and 180", taking up additional oxygen to a composition at least U0,.,5 without any change in the original fluorite crystal structure except a slight contraction of the unit cell (Anderson and Alberman, J., 1949, S 303). The oxygen enters interstitial positions in the uranium dioxide lattice without the nucleation of a new phase at low temperatures, and the surface area remains unchanged as oxidation proceeds (Anderson and Roberts, to be published). It has been found that the solution of oxygen in uranium dioxide commences at temperatures even lower than the 27 kcal./mole activation energy of the bulk oxidation hitherto studied would lead one to expect ; uranium dioxide preparations that have been exposed to air at room temperature always contain oxygen in excess of the stoicheiometric formula, in amounts proportional to their specific surface area. The present work is concerned with a chemisorption of oxygen on uranium dioxide, which is practically instantaneous even at -183", and which must therefore occur with zero or very low activation energy. The study was extended to include uranium dioxide-thorium dioxide mixed crystals in order to discover the effect on the surface reaction of diluting the sites capable of reacting with oxygen, presumably the U4+ ions in the crystal surface, with Th4+ ions which would not be expected to react, because of their inability to donate electrons. EXPERIMENTALUranium Dioxide Prepavations .-The preparation and properties of the uranium dioxide specimens used are listed below. Starting materials were " AnalaR " uranyl nitrate or the very pure grade of " ammonium diuranate " ayailable in the atomic energy project. The preparations were characterised by : (i) X-Ray diffraction patterns, all of which showed the lines expected for the dioxide, with a cube cell edge of 5.457 f 0.002 k.X.U. and a theoretical density of 10.96 g./c.c. The effective crystallite diameter, 6, was estimated from the halfwidth of the X-ray lines by comparison with a well-crystallised sample of particle size -2-5 p, the calcuIations being made by Jones's methods (Proc. Roy. Soc., 1938, A , 166, 16). (ii) The real density as...
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