DEFECTS IN CRYSTAL LATTICE SERIES

These nanoscale to microscale features have been revealed to drastically reduce thermal conductivity. Potential operating environments for these materials include high radiation fields and extreme temperatures that promote or directly generate lattice defects in otherwise perfect fluorite structures. 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. 12 Exhibiting similar behavior in many respects, ThO 2 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. 5–8 In contrast, ThO 2 has been less widely investigated to date despite potential applications as a fertile fuel in advanced, proliferation-resistant nuclear reactors 9–11 and as a high reflectivity material for extreme ultraviolet optics. 2–4 Given these technological implications, the thermophysical properties and performance characteristics of UO 2 and CeO 2 have been the subject of detailed study for decades. 1 CeO 2 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. UO 2 forms the basis for the large majority of commercial nuclear fuels worldwide. 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.Īctinide and lanthanide fluorite oxides, ThO 2, UO 2, and CeO 2, form an important family of high temperature ceramics for a variety of energy applications.

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. 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.

DEFECTS IN CRYSTAL LATTICE SERIES

In this work, a series of high-quality thorium dioxide single crystals are exposed to 2 MeV proton irradiation at room temperature and 600 ☌ to create microscale regions with varying densities and types of point and extended defects. 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. Due to the entropy term in the thermodynamic potentials, intrinsic lattice defects are inevitable, they must always be present in thermodynamic equilibrium.Thermal transport is a key performance metric for thorium dioxide in many applications where defect-generating radiation fields are present. An important fact to realize is that there is no such thing like a perfect crystal. Famous examples of three-dimensional defects are the microtubes in SiC or the As precipitates in semiinsultating GaAs. Stacking faults can give rise to the growth of twin crystals. Screw dislocations play a role in crystal growth, resulting in growth spirals. Dislocations may give rise to an inhomogeneous distribution of donors or acceptors, and they may act as recombination centers. Point defects play an important role in diffusion processes. In that way, one distinguishes between point defects like vacancies and interstitials, line defects like edge dislocations and screw dislocations, two-dimensional defects like stacking faults, and three-dimensional defects like precipitates and microtubes. They are often categorized according to their dimensionality. This chapter is dealing with intrinsic defects. Defects often determine the essential electronic properties of a semiconductor. Or they may just represent a disarrangement of the regular atoms of the solid, those are “intrinsic defects”. In this case, we speak of extrinsic defects. Lattice defects are deviations from the strictly periodic arrangement of atoms in a solid.

Defects in crystal lattice