The thermal conductivity of oxides of uranium, neptunium and americium at elevated temperatures

Two different methods were used to measure the thermal conductivity of oxides of uranium, americium and neptunium: diffusivities were determined in the temperature range from 1000 to 1800 K by a modulated electron beam technique and converted to conductivities whereas a direct electrical heating method was used to deduce mean conductivities between about 1600 K and the melting temperature of the oxides. The principal result of this investigation is the observation that the thermal conductivity of U-Am dioxide exhibits an upswing above 1800 K as in the case of uranium dioxide and uranium-plutonium dioxide, whereas the conductivity of uranium-neptunium dioxide continues to decrease with temperature, like that of thoria, as one would expect for a pure phonon conductor. The high-temperature rise has been attributed to an electronic contribution which, apparently, is related to the ability of the oxide to accommodate the metals in question in another than the four-valent state. (orig.)