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.
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