K. E. Sickafus

University of Tennessee, Knoxville, TN, USA © 2012 Elsevier Ltd. All rights reserved.

1.05.1. Introduction 123

1.05.2. Radiation Effects in Ceramics: A Case Study — a-Alumina Versus Spinel 124

1.05.2.1 Introduction to Radiation Damage in Alumina and Spinel 124

1.05.2.2 Point Defect Evolution and Vacancy Supersaturation 125

1.05.2.3 Dislocation Loop Formation in Spinel and Alumina 127

1.05.2.3.1 Introduction to atomic layer stacking 127

1.05.2.3.2 Charge on interstitial dislocations 127

1.05.2.3.3 Lattice registry and stacking faults I: (0001) Al2O3 129

1.05.2.3.4 Lattice registry and stacking faults II: {111} MgAl2O4 129

1.05.2.3.5 Lattice registry and stacking faults III: {1010} Al2O3 130

1.05.2.3.6 Lattice registry and stacking faults IV: {110} MgAl2O4 130

1.05.2.3.7 Unfaulting of faulted Frank loops I: (0001) Al2O3 131

1.05.2.3.8 Unfaulting of faulted Frank loops II: {111} MgAl2O4 132

1.05.2.3.9 Unfaulting of faulted Frank loops III: {1010} Al2O3 132

1.05.2.3.10 Unfaulting of faulted Frank loops IV: {110} MgAl2O4 133

1.05.2.3.11 Unfaulting of faulted Frank loops V: experimental observations 133

1.05.2.4 Amorphization in Spinel and Alumina 134

1.05.3. Radiation Effects in Other Ceramics for Nuclear Applications 136

1.05.3.1 Radiation Effects in Uranium Dioxide 136

1.05.3.2 Radiation Effects in Silicon Carbide 136

1.05.3.3 Radiation Effects in Graphite 137

1.05.3.4 Radiation Effects in Other Ceramics 138

1.05.4. Summary 138

References 139

Abbreviations

dpa Displacements per atom BF Bright-field

TEM Transmission electron microscopy

i Interstitial

v Vacancy

ccp Cubic close-packed

hcp Hexagonal close-packed

SHI Swift heavy ion

PKA Primary knock-on atom

CVD Chemical vapor deposition

1.05.1. Introduction

Ceramic materials are generally characterized by high melting temperatures and high hardness values. Ceramics are typically much less malleable than metals and not as electrically or thermally conduc­tive. Nevertheless, ceramics are important materials in fission reactors, namely, as constituents in nuclear fuels, and are widely regarded as candidate materials for fusion reactor applications, particularly as electri­cal insulators in plasma diagnostic systems. These applications call for highly robust ceramics, materials that can withstand high radiation doses, often under very high-temperature conditions. Not many cera­mics satisfy these requirements. One of the purposes of this chapter is to examine the fundamental mechanisms that lead to the relative radiation toler­ance of a select few ceramic compounds, versus the susceptibility to radiation damage exhibited by most other ceramics.

Ceramics are, by definition, crystalline solids. The atomic structures of ceramics are often highly complex compared with those of metals. As a conse­quence, we lack a detailed understanding of atomic processes in ceramics exposed to radiation. Never­theless, progress has been made in recent decades in understanding some of the differences between radi­ation damage evolution in certain ceramic com­pounds. In this chapter, we examine the radiation damage response of a select few ceramic compounds that have potential for engineering applications in nuclear reactors. We begin by comparing and contrasting the radiation damage response of two particular (model) ceramics: a-alumina (Al2O3, also known as corundum in polycrystalline form, or ruby or sapphire in single crystal form) and magnesio- aluminate spinel (MgAl2O4). Under neutron irradia­tion, alumina is highly susceptible to deleterious microstructural evolution, which ultimately leads to catastrophic swelling of the material. On the other hand, spinel is very resistant to the microscopic phenomena (particularly nucleation and growth of voids) that lead to swelling under neutron irradiation. We consider the atomic and microstructural mechan­isms identified that help to explain the marked dif­ference in the radiation damage response of these two important ceramic materials. The fundamental prop­erties of point defects and radiation-induced defects are discussed in Chapter 1.02, Fundamental Point Defect Properties in Ceramics, and the effects of radiation on the electrical properties of ceramics are presented in Chapter 4.22, Radiation Effects on the Physical Properties of Dielectric Insulators for Fusion Reactors.

It is important to be cognizant of the irradia­tion conditions used to produce a particular radiation damage response. Microstructural evolution can vary dramatically in a single compound, depending on the following irradiation parameters: (1) irradiation source-irradiation species and energies — these give rise to the so-called ‘spectrum effects,’ (2) irradia­tion temperature, (3) irradiation particle flux, and (4) irradiation elapsed time and particle fluence. Throughout this chapter, we pay particular attention to the variations in radiation damage effects due to differences in irradiation parameters. A single ceramic material can exhibit radiation tolerance under one set of irradiation conditions, while alter­natively exhibiting damage susceptibility under another set of conditions. A good example of this is MgAl2O4 spinel. Spinel is highly radiation tolerant in a neutron irradiation environment but very suscepti­ble to radiation-induced swelling when exposed to swift heavy ion (SHI) irradiation.

Finally, it is important to note that radiation tolerance refers to two distinctly different criteria: (1) resistance to a crystal-to-amorphous phase trans­formation; and (2) resistance to dislocation and void nucleation and growth. Both of these phenomena lead (usually) to macroscopic swelling of the material, but the causes of the swelling are completely different. The irradiation damage conditions that produce these two materials’ responses are also typically very differ­ent. We examine these two radiation tolerance criteria through the course of this chapter.