Growth of ThO₂, UO₂, U_xTh_1-xO₂ and CeO₂ Single Crystals for Thermal Property Measurements

High quality single crystals greatly simplify measurements of thermal properties and microstructure defects, especially for complex assessments such as nuclear fuel utility. High melting points, typical of nuclear fuel materials and their surrogates, present a difficult challenge in the synthesis of single crystals for such studies. Materials like UO₂ and ThO₂ that melt at 2865°C and 3390°C, respectively, are beyond traditional melt growth techniques like Czochralski. Therefore, exotic melt techniques including solar furnace, cold crucible, and arc melting have been used to produce crystalline samples. These samples tend to suffer from thermally induced defects, multiple grains, and/or small sample sizes that limit the types of characterization that can be performed. Solution growth techniques, including hydrothermal and flux growth, enable the dissolution of refractory materials and recrystallization at significantly lower temperatures.<br/><br/>This talk will focus primarily on the crystal growth of ThO₂, UO₂, U_xTh_1-xO₂, and CeO₂ as nuclear fuel materials or surrogates. The uranium and thorium bearing oxides were synthesized in supercritical hydrothermal solutions between 650 and 750°C, which is over 2000°C lower than the melting points and thereby avoids thermal defect formation. In these solutions the samples are grown near equilibrium, which further reduces the defects. Under these thermodynamic growth conditions, highly faceted single crystals displaying identifiable crystallographic planes were grown, enabling plane-specific thermal measurements. The CeO₂ single crystals could not be grown in hydrothermal solutions and therefore a Na₂O-B₂O₃ flux was used to produce (111) oriented samples. The crystals spontaneously nucleated on cooler sections of their container’s platinum walls as the growth solution was slowly cooled from 1200°C.<br/><br/>The latter half of the talk will discuss the electrical and thermal property measurements of the fuel and fuel candidates before and after irradiation. The crystal samples were irradiated with 2MeV protons at both room temperature and elevated temperature (600°C) to generate desired defect structures such as point detects and dislocation loops. Thermal conductivities of the pristine and irradiated samples were measured in a temperature range of 77-300K using spatial-domain thermoreflectance, and the results were compared to the modeling predictions from density functional theory and Boltzmann transport equations to investigate the scattering mechanisms between phonon-phonon, phonon-electron, and phonon-defect. Important modeling inputs, such as the lattice structure of pristine and irradiated samples and quantitative information of defects, were investigated by using characterization techniques including single crystal X-ray diffraction, Raman spectroscopy, photoluminescence spectroscopy, X-ray fluorescence spectroscopy, Hall, and advanced electron microscopy. This multi-institute effort has provided unique insights into energy transport and defect diffusion of nuclear fuels and fuel candidates in extreme environments.