The influence of lattice defects, recombination, and clustering on thermal transport in single crystal thorium dioxide

Cody A Dennett,Marat Khafizov,J Matthew Mann,Timothy A Prusnick,Tiankai Yao,Aaron French,Amey Khanolkar,Krzysztof Gofryk,Lin Shao,Narayan Poudel,Zilong Hua,Phyllis K Morgan,Lingfeng He,David H Hurley,David B Turner

doi:10.1063/5.0025384

Abstract

Thermal transport is a key performance metric for thorium dioxide in many applications where defect-generating radiation fields are present. An understanding of the effect of nanoscale lattice defects on thermal transport in this material is currently unavailable due to the lack of a single crystal material from which unit processes may be investigated. In this work, a series of high-quality thorium dioxide single crystals are exposed to 2 MeV proton irradiation at room temperature and 600 °C to create microscale regions with varying densities and types of point and extended defects. Defected regions are investigated using spatial domain thermoreflectance to quantify the change in thermal conductivity as a function of ion fluence as well as transmission electron microscopy and Raman spectroscopy to interrogate the structure of the generated defects. Together, this combination of methods provides important initial insight into defect formation, recombination, and clustering in thorium dioxide and the effect of those defects on thermal transport. These methods also provide a promising pathway for the quantification of the smallest-scale defects that cannot be captured using traditional microscopy techniques and play an outsized role in degrading thermal performance.

Highlights

Actinide and lanthanide fluorite oxides, ThO2, UO2, and CeO2, form an important family of high temperature ceramics for a variety of energy applications
Defected regions are investigated using spatial domain thermoreflectance to quantify the change in thermal conductivity as a function of ion fluence as well as transmission electron microscopy and Raman spectroscopy to interrogate the structure of the generated defects
For all spatial domain thermoreflectance (SDTR) measurements, this global optimization results in slopes that quite accurately capture the far-field phase lag for multiple frequencies simultaneously

Summary

Introduction

Actinide and lanthanide fluorite oxides, ThO2, UO2, and CeO2, form an important family of high temperature ceramics for a variety of energy applications. Exhibiting scitation.org/journal/apm similar behavior in many respects, ThO2 has several key distinctions from other heavy metal fluorite oxides that make it attractive for the above mentioned applications including a fixed tetravalent cation oxidation state, extremely high melting temperature, and large electronic bandgap.. Exhibiting scitation.org/journal/apm similar behavior in many respects, ThO2 has several key distinctions from other heavy metal fluorite oxides that make it attractive for the above mentioned applications including a fixed tetravalent cation oxidation state, extremely high melting temperature, and large electronic bandgap.13,14 For many of these applications, thermal transport is a key property that determines the suitability of a particular oxide for a particular use, controlling, for example, the peak center line temperature in nuclear fuels and heat dissipation ability in large bandgap electronics UO2 forms the basis for the large majority of commercial nuclear fuels worldwide. CeO2 is utilized in electrochemical applications as a catalysis material due to its ability to store and transport oxygen and as a solid oxide fuel cell material. Given these technological implications, the thermophysical properties and performance characteristics of UO2 and CeO2 have been the subject of detailed study for decades. In contrast, ThO2 has been less widely investigated to date despite potential applications as a fertile fuel in advanced, proliferation-resistant nuclear reactors and as a high reflectivity material for extreme ultraviolet optics. Exhibiting scitation.org/journal/apm similar behavior in many respects, ThO2 has several key distinctions from other heavy metal fluorite oxides that make it attractive for the above mentioned applications including a fixed tetravalent cation oxidation state, extremely high melting temperature, and large electronic bandgap. For many of these applications, thermal transport is a key property that determines the suitability of a particular oxide for a particular use, controlling, for example, the peak center line temperature in nuclear fuels and heat dissipation ability in large bandgap electronics

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