Characterization of irradiation-induced novel voids in α-quartz

Naoki Okada,Ippei Maruyama,Takahiro Ohkubo,Kiyoteru Suzuki,Kenta Murakami

doi:10.1063/5.0029299

Abstract

The macroscopic volume expansion of minerals subjected to high-energy irradiation typically occurs because of structural disordering. However, the mechanisms involved in this swelling associated with structural disordering have not been fully clarified. In particular, the role of the voids resulting from the aggregation of defects (which, in contrast to irradiated materials, are not observed in disordered melt-quenched amorphous glass) is still poorly understood. Here, we employ molecular dynamics simulations of α-quartz to examine a model that involves three stages of amorphization and volume expansion. The collapse of the crystalline structure is directly evaluated in terms of structural ordering based on symmetry operations, which enables the estimation of isolated defects. In the first stage, with increasing deposited energy, crystallinity decreases sharply compared with the decrease in density, which is linked to the formation of under- and over-coordinated atomic structures. Large voids (≥7.0 Å), which are not present in melt-quenched glass, are created at a deposition energy of 4 eV/atom, and in the second stage, the volume fractions of the large voids increase during subsequent irradiation from this energy up to 25 eV/atom. In the final stage, at higher deposited energies, the volume fraction of the large voids and the density fluctuate and become saturated owing to the balance between generation and annihilation of the large voids.

Highlights

Irradiation of SiO2 in both its crystalline (α-quartz) and glassy forms has been extensively investigated with the aim of understanding the physical properties of radiation-induced defects and the mechanisms of their formation.1–9 In particular, the transmission efficiency of optical fibers made from SiO2 glass is strongly reduced in the presence of radiation-induced oxygen vacancies.10–13 Optical fibers are widely used in industrial environments, and these vacancies have been thoroughly investigated
The crystallinity of the irradiated α-quartz as a function of the deposited energy is shown in Fig. 1, with density taken to represent the macroscopic change in volume [the large voids (LVs) whose volume fraction is plotted in this figure will be discussed in Sec
The amorphous structure, which is characterized by loss of translational symmetry, is induced by a process called the collision cascade in which atoms accelerated as a result of the irradiation [primary knock-on atoms (PKAs)] collide with and accelerate other atoms in the cell, which collide with further atoms, and so on

Summary

Introduction

Irradiation of SiO2 in both its crystalline (α-quartz) and glassy forms has been extensively investigated with the aim of understanding the physical properties of radiation-induced defects and the mechanisms of their formation. In particular, the transmission efficiency of optical fibers made from SiO2 glass is strongly reduced in the presence of radiation-induced oxygen vacancies. Optical fibers are widely used in industrial environments, and these vacancies have been thoroughly investigated. Irradiation of SiO2 in both its crystalline (α-quartz) and glassy forms has been extensively investigated with the aim of understanding the physical properties of radiation-induced defects and the mechanisms of their formation.. There have been a number of reports on radiation-induced volume changes (15%–20%) in α-quartz that were large enough to significantly reduce the mechanical strength of concrete during long-term operation of nuclear reactors.. There have been a number of reports on radiation-induced volume changes (15%–20%) in α-quartz that were large enough to significantly reduce the mechanical strength of concrete during long-term operation of nuclear reactors.3,6,14 Evaluation of this agerelated degradation is a critical aspect of the safety assessment of recent nuclear power plants, and an understanding of the mechanism of the volume expansion of α-quartz due to irradiation is highly desirable There have been a number of reports on radiation-induced volume changes (15%–20%) in α-quartz that were large enough to significantly reduce the mechanical strength of concrete during long-term operation of nuclear reactors. Evaluation of this agerelated degradation is a critical aspect of the safety assessment of recent nuclear power plants, and an understanding of the mechanism of the volume expansion of α-quartz due to irradiation is highly desirable

Methods

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Discussion

Conclusion