Ceramic, Heal Thyself!
Study shows how defect-engineered zirconia repairs radiation-induced damage
In yttria-stabilized zirconia (top), the remaining defects are few and far between, having less impact on the properties of the material. In zircon (bottom), the defects are clustered. Enlarged View
Results: The restless movement of oxygen atoms enabled by designer defects in a ceramic material heals radiation-induced damage, according to a computer simulation study by Dr. Ram Devanathan and Dr. Bill Weber at Pacific Northwest National Laboratory. Using supercomputers, the scientists modeled the interactions among millions of atoms in the ceramic known as yttria-stabilized zirconia and other materials to see how each responded at the atomic level to radiation.
Why It Matters: Materials that can resist radiation damage are needed to expand the use of nuclear energy. Inside nuclear power plants, materials capable of handling high-radiation doses could improve the durability of key equipment and reduce the costs of replacements. Outside the plant, new materials could aid in waste storage.
"If you want a material to withstand radiation over millennia, you can't expect it to just sit there and take it, it must have a mechanism for self healing," said Devanathan.
Methods: The scientists performed simulations of the interactions of millions of atoms on two massively parallel supercomputers. The computers were located at the Department of Energy's Environmental Molecular Sciences Laboratory at PNNL and National Energy Research Scientific Computer Center at Lawrence Berkeley National Laboratory.
The researchers analyzed yttria-stabilized zirconia, created by adding yttria to zirconia to engineer specific defects in the structure. Yttrium has a smaller charge than the zirconium, so when a zirconium atom is replaced with yttrium, it creates a local charge imbalance. To redress this imbalance, the material gives up oxygen atoms. The loss of these oxygen atoms leaves empty oxygen sites called "vacancies" in the atomic structure. The remaining oxygen atoms constantly jump in and out of these vacant sites.
"It is like a classroom full of fidgety kids," said Devanathan. "When the teacher turns her back, the kids constantly jump into empty chairs, leaving their own chairs vacant until another kid leaps into the seat."
Next, the scientists simulated an atom releasing an alpha particle, a nuclear reaction of concern. An alpha particle shoots out of an atom with such force that the remainder of the atom recoils in the opposite direction. The recoiling atom can cause significant damage to surrounding atomic structures.
Then, using data analysis algorithms developed at PNNL, the researchers analyzed gigabytes of data, looking for atoms knocked out of place in fractions of a nanosecond after the recoiling atom hits the yttria-stabilized zirconia. Returning to the classroom analogy, when empty chairs are available, a child who is knocked out of his seat can hop onto a nearby vacant chair. A displaced oxygen atom fills the vacancy built into the structure, realigning the atomic structure, and when the oxygen moves on, the damage is repaired.
This self-healing process does not completely repair the material, but defects are isolated, unlike those found in zircon. In zircon simulations, the defects clustered together, changing the material's properties. Clustered defects are much more difficult to repair than isolated defects.
"This research raises the possibility of engineering mobile defects in ceramics to enhance radiation tolerance for applications such as nuclear waste disposal, where the material is expected to resist radiation damage over millennial time scales," said Weber.
What's Next: The researchers plan on conducting additional simulations that focus on a finer scale at the electronic excitations and their effect on the self-healing process.
Acknowledgments: The DOE Office of Basic Energy Sciences funded this research.
Citation: Devanathan, R and WJ Weber. 2008. "Dynamic annealing of defects in irradiated zirconia-based ceramics." Journal of Materials Research 23(3):593-597.