Electrical Conductivity and Thermoelectric Power of Uranium Dioxide

Abstract

The electrical conductivity and thermoelectric power of an uranium dioxide single crystal were measured between 908 and 1697 K and for PO2 included between 10−24 atm and the boundary UO2+x/U4O9. For T<1273 K and near stoichiometry, the electrical conductivity versus PO2 shows a plateau characteristic of an extrinsic regime. For T≥1273 K, the conductivity exhibits a minimum. The Seebeck coefficient Q̄UO2 also exhibits a behavior change when the temperature is increased. For T<1200 K, Q̄UO2 remains positive whatever the values of PO2. For T>1200 K, Q̄UO2 changes from negative to positive values when PO2 increases. This set of results shows that the stoichiometric oxide presents a p extrinsic to intrinsic transition near 1273 K. At T≤1273 K, the extrinsic conductivity values versus temperature confirm that the mobility of the holes occurs by a small polaron process, with an activation energy of 0.17 eV. At T>1273 K, the electrical conductivity minima are characteristic of a p→n transition. A band gap energy of 2.0 eV has been calculated from the temperature dependence of these conductivity minima values. Furthermore, this set of results has allowed us to determine the relative oxygen partial molar free energy of stoichiometric UO2.

First-principles calculations of uranium diffusion in uranium dioxide

“…36 In addition to this, first principles calculations 22 have shown that two U 5+ ions are more stable than one U 6+ ion upon introduction of an oxygen interstitial. Hence, we follow here the assumption of many other authors 35 that electronic disorder is controlled by the disproportionation of two U 4+ (5f 2 ) ions to one U 5+ (5f 1 ) ion and one U 3+ (5f 3 ) ion, which may be written as follows using the Kröger-Vink notation:…”

Anisotropic thermal conductivity in uranium dioxide

“…The temperature gradient alone does not give rise to this behaviour, but instead we propose that the small electric field generated by the temperature gradient is the source. This behaviour is facilitated by UO 2 having a large Seebeck coefficient 27 . We suggest that this field interacts with the electric moments on uranium ions in UO 2 to change the [x00] magnon properties.…”

Dense and nanometric electronic excitations induced by swift heavy ions in an ionic CaF2crystal: Evidence for two thresholds of damage creation

“…Also sputtering yields 117 were measured by the collector technique. Sputtering yields and track data were calculated within the framework of the inelastic thermal spike model 65 using λ = 6 nm deduced from the localized 5f electrons in the band gap, 118,119 which needs only 2 eV to reach the conduction band. The results are reported in Fig.…”