PHYSICAL PROPERTIES OF UO$sub 2$ SINGLE CRYSTALS. Final Report No. 1, October 1961-February 28, 1963
The sublimation method for the preparation of large single crystals of UO/sub 2/ is described. The first results of the experiments on controlled oxidation of UO/sub 2/ crystals are briefly reported. The experimental methods for the study on diffusion of U and sintering mechanisms in UO/sub 2/ are discussed. Defects in as-grown, annealed, slightly deformed, and irradiated UO/ sub 2/ crystals were studied by transmission electron microscopy. The deformation of the specimens was done in the microscope; irradiations up to 10/ sup 15/ nvt were employed. In order to collect experimental evidence on the formation of fission tracks lattice defects, and fission tracks in other nonmetallic crystals were also investigated. Electrical conductivity measurements between 90 and 800 deg K were performed on UO/sub 2/ single crystals with composition in the range UO/sub 2.000/ to UO/sub 2.007/ using Van der Pauw's method. The activation energies calculated from the log sigma T vs. 1/T curves vary between 0.28 and 0.20 ev depending on the U/O atomic ratio. Hall effect measurements by dc and ac techniques yielded no measurable response ( mu < 0.015 cm/sup 2/ v/sup -1/ sec/sup -1/ at room temperature). An apparatus allowing thermoelectric measurements on UO/sub 2/ single crystals between 80 and 320 deg K was constructed. All samples showed p-type conduction. The Seebeck coefficient was of the order of 0.5 mv/ deg K and remained rather constant in the temperature region investigated. The study of the coupling constant and the jump probability of carriers in UO/sub 2/ shows that the latter are small polarons,'' so that the band model does not apply. The behavior of the carriers as predicted by theory (hopping motion, activated mobility, change of the effective mass with temperature) is described. Tunneling of the carriers is not important. The optical absorption spectrum of UO/sub 2/ single crystals was measured at room temperature. The absorption edge is located at about 2.18 ev. This absorption probably corresponds to band gap transitions. An apparatus for thermal conductivity measurements between liquid helium and room temperature was constructed. The preliminary measurements have already shown the existence of a minimum of the thermal conductivity between 4 and 100 deg K. As to the high- temperature conductivity, the apparatus constructed does not yet give full satisfaction and has to be improved. The results obtained thus far are rather similar, at least up to 500 deg C, to those of J. L. Bates. (auth)
- Research Organization:
- Mol, Belgium. Centre d'Etude de l'Energie Nucleaire
- NSA Number:
- NSA-17-039532
- OSTI ID:
- 4630023
- Report Number(s):
- EURAEC-693
- Country of Publication:
- United States
- Language:
- English
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