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Title:Thermal conductivity of UO2 and U3O8 epitaxial layers damaged by ion irradiation
Author(s):Weisensee, Patricia
Advisor(s):Cahill, David G.
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Thermal Conductivity
Uranium Dioxide
Ion (Ar) Irradiation Damage
Thin Film
Abstract:This thesis presents experimental results on thermal conductivity measurements on both irradiated and unirradiated uranium oxides between 90 K and 658 K. The irradiation doses range from 5x10^13 argon ions/cm² to 2x10^16 Ar+/cm² at 2 MeV and room temperature. The main focus lies on 360 – 500 nm single crystal UO2 and U3O8 thin films grown on YSZ substrates. The U3O8 samples are formed by heating the UO2 in air for 2.5 hours at 600°C and slow cooling. From a comparison between data measured by an optical pump-probe technique – time-domain thermoreflectance (TDTR) – and a heat transport model the thermal conductivity of the sample under study can be determined. For UO2, the thermal conductivity is found to be 10.2 W/m.K at 323 K and with increasing temperatures its behavior is inversely proportional to the temperature. At 648 K the thermal conductivity is 4.9 W/m.K. The thermal conductivity peaks between 200 and 250 K and decreases for decreasing temperatures. Upon irradiation the thermal conductivity at 323 K decreases to 8.6 W/m.K for a dose of 5x10^13 Ar+/cm² and to 4 W/m.K for 7x10^14 Ar+/cm² and 1x10^16 Ar+/cm². At 648 K the corresponding values are 5.2 W/m.K, 4.2 W/m.K and 2.5 W/m.K for the mentioned doses. A modified model by Klemens is used to predict the irradiation effect on the thermal conductivity for low and medium doses. For high doses non-linearities during the irradiation limit the further decrease in thermal conductivity and cannot be captured by the model. Due to self-annealing during the experiments, irradiated samples do not show a 1/T behavior. U3O8 is studied with irradiation doses of 0 Ar+/cm², 7x10^14 Ar+/cm², 2x10^15 Ar+/cm² and 2x10^16 Ar+/cm². Their thermal conductivities at 333 K are 1.67 W/m.K, 0.96 W/m.K, 1.2 W/m.K and 1.97 W/m.K, respectively. The self-annealing is found to be stronger than in UO2 so that the thermal conductivities at 658 K are 1.3 W/m.K, 1.18 W/m.K, 1.36 W/m.K and 1.86 W/m.K, respectively, for the above mentioned doses. For lower doses the thermal conductivity decreases with increasing dose but then starts increasing again for higher doses. This is probably caused by re-crystallization and the formation of a second phase of UO2+x in U3O8. In general, it is found that oxidation of UO2 has a stronger influence on the thermal conductivity than irradiation with argon ions.
Issue Date:2012-02-01
Rights Information:© 2011 Patricia B. Weisensee
Date Available in IDEALS:2014-02-01
Date Deposited:2011-12

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