Thermal conductivity of neptunium dioxide

Revisiting the melting temperature of NpO2 and the challenges associated with high temperature actinide compound measurements

“…22,23 Upon deeper examination, experiments on the thermal conductivity of NpO 2 were preformed only up to 1473 K (Ref. 21) and show a decreasing trend with temperature, in qualitative agreement with molecular dynamics calculations up to 2200 K. 19 However, the calculations only consider lattice terms, and the measurements are below the threshold of significant polaron contribution in UO 2 . Therefore, we add the UO 2 polaron term 20 to the lattice terms for the thermal conductivity of NpO 2 .…”

The high temperature heat capacity of NpO2

“…As mentioned in Section 1 the high temperature heat capacity of pure NpO 2 has been experimentally determined in two previous studies, by Arkhipov et al [3] and Nishi et al [4]. Since the results of these two studies are in significant disagreement, we present new experimental data in this work.…”

“…The mass of samples ranged from (140 to 190) mg. The prepared pellets were sintered in a furnace at 1800 K for 7.5 h under the constant flow of air (300 ml Á min À1 ) which guaranteed that no hypostoichiometric NpO 2Àx phase would be formed as used earlier by Nishi et al [4]. Since NpO 2 is known not to become hyperstoichiometric (NpO 2+x ) and hence cannot be further oxidized, the oxygen rich atmosphere (air, pure oxygen) is required to condition the pure NpO 2 phase.…”

“…The enthalpy increments of NpO 2 have been measured by Arkhipov et al [3] and recently by Nishi et al [4], both using a drop calorimetry technique and covering a temperature range from (300 to 1100) K. Furthermore several estimations of the high temperature heat capacity have been made: Sobolev [5] made an estimation based on the theoretical approach presented in his earlier study [6], Serizawa et al [7] determined the heat capacity as a sum of phonon vibrations, dilatation contributions and Schottky specific heat whereas Kurosaki et al [8] derived the heat capacity of NpO 2 using a molecular dynamic study. Since all these high temperature results are not in adequate agreement, particularly the measurements deviate significantly, we present new experimental data of the enthalpy increments of NpO 2 measured using a drop calorimeter.…”

(Solid+gas) equilibrium studies for neptunium dioxide

“…The uncertainties of the two investigation methods are the total combined uncertainties with a coverage factor of 2. The sintering was performed as described in a recent publication [6] under flowing air at about 300 cm 3 Á min À1 at T = 1723 K for 7.5 h. The X-ray diffraction analysis with Cu Ka radiation performed on the disks after sintering indicated a single fluorite-type phase. The lattice parameter of the NpO 2 sample was found (0.5434 ± 0.0001) nm, consistent with the results from the earlier studies [7][8][9][10][11][12].…”