The atomic structure of an irradiated material is closer to a liquid than a glass, according to a team of researchers in the US. Glasses have been used by researchers to study and predict possible effects of radiation damage, but the engineers behind the new research say that studying liquid states may be more appropriate. They add that the findings from their molecular-dynamic stimulations could help to identify novel radiation-resistant materials.
Exposure to neutron radiation can cause significant structural damage to materials. Understanding the effects of this damage is important for applications such as the construction of nuclear facilities, and the storage of nuclear waste.
“When exposed to radiation, materials undergo some disordering of their atomic structure,” explains Mathieu Bauchy, a civil engineer at the University of California, Los Angeles (UCLA). “In turn, this disordering can affect properties such as density, stiffness, strength and toughness. Therefore, it is essential to understand the effect of irradiation on the atomic structure of materials in order to ensure their integrity.”
The disordered atomic network resulting from irradiation resembles the disordered non-crystalline state of glassy materials. Glasses are formed when a liquid material is rapidly cooled, or quenched, through a process known as vitrification. Instead of forming an ordered crystalline solid, the rapid cooling causes the atoms to become stuck in a non-crystalline state.
Irradiation versus vitrification
Because of their similarities, glasses have been used to predict the properties of irradiated materials. But some differences have been noticed between the materials, leading to questions about whether irradiation and vitrification have equivalent affects. To address this, Bauchy and colleagues at UCLA and Oak Ridge National Laboratory used reactive molecular-dynamic simulations to compare the atomic structures of irradiated quartz and glassy silica, which are both forms of silicon dioxide (see video).
It is essential to understand the effect of irradiation on the atomic structure of materials in order to ensure their integrity
Mathieu Bauchy, UCLA
The effect of radiation on quartz – one of the most abundant minerals on Earth and a major component of many sands used for building – is important as it has many civil-engineering applications, including in the building of nuclear facilities and waste repositories.
After running simulations of both irradiation damage and heating followed by rapid cooling – vitrification – on quartz, the researchers compared the atomic structures of the resulting materials. They found significant differences in the disorder created by irradiation and vitrification. The irradiated material was more disordered than the glass and had an atomic structure closer to that of a liquid.
Counter-intuitive result
“Since irradiation results in the disordering of the atomic structure, it is intuitive to assume that, upon exposure to radiations, crystals should evolve towards a glassy state,” explains Bauchy. “However, by comparing the structure of irradiated quartz with that of glassy silica, we found that this assumption does not hold true.”
Team member N M Anoop Krishnan adds: “We observed that irradiated quartz exhibits more disorder than glassy silica, both in the short- and the medium-range environment of the atoms. Interestingly, we found the structure and thermodynamic properties of irradiated quartz to be equivalent to those of a silica-liquid melt.”
Indeed, the atomic structure of irradiated quartz features co-ordination defects, edge-sharing units, and large silicate rings. These are all absent from glassy silica that is produced through vitrification.
Damage slowdown
The researchers say that their finding that irradiated materials have a liquid-like structure has important implications. Bauchy says that from a fundamental perspective, it explains why structural damage slows after prolonged radiation exposure, rather than continuously increasing. “Once the material reaches a liquid-like structure it becomes easier for the atoms to move and reorganize, which prevents the accumulation of any further damage.”
The result also “suggests that the structure and properties of irradiated materials can be predicted from those of their corresponding liquid,” explains Krishnan. According to the researchers, this understanding could help to identify novel radiation-resistant materials.
The research is described in The Journal of Chemical Physics.
