Heat capacities and electrical conductivities of ( U1 − yEuy) O2 ( y = 0.044 and 0.090) from 300 to 1550 K

Abstract

Heat capacities and electrical conductivities of ( U 1 − y Eu y ) O 2 ( y = 0.044 and 0.090) were measured by means of direct heating pulse calorimetry over the temperature range from 300 to 1550 K. An anomalous increase in the heat capacity curve of each sample of (U 1 − yEu y)O 2 was observed similarly to the cases of ( U 1 − y M y ) O 2 ( M = Gd, La and Sc) found previously by the authors. As the increase of the europium content in ( U 1 − y Eu y ) O 2 from y = 0.044 to 0.090, the onset temperature of an anomalous increase in the heat capacity decreased from about 1150 to 950 K. The values of the enthalpy ( ΔH f ) and the entropy for defect formation ( ΔS f ) in (( U 1 − y Eu y ) O 2 were calculated from the excess heat capacity assuming the presence of Frenkel pairs of oxygen, and were found to be similar to those of UO 2 doped with La, Gd and Sc previously obtained by the authors. On the other hand, no anomaly was seen in the electrical conductivity curve around the onset temperature of the anomalous increase in the heat capacity. It is conducted that the occurrence of the excess heat capacity of ( U 1 − y Eu y ) O 2 originates from the predominant contribution of the formation of Frenkel pair-like defects of oxygen similarly to the cases of UO 2 doped with other trivalent cations. The difference in the onset temperatures of ( U 1 − y Eu y ) O 2 from those of UO 2 doped with other trivalent cations was thought to be originated from that of the elastic strain induced by the lattice parameter change.