Radiation isn’t just a danger to living things; over time, constant irradiation can cause everything from electronics to structural materials used in nuclear or space applications to degrade and fail.
Izabela Szlufarska
That’s one reason materials scientists are always searching for new materials that can withstand harsh radiation environments. One strategy that has shown promise for making radiation-resistant metallic systems is layering of different materials, which produces a high density of interfaces. Interfaces can absorb materials defects, leading to recovery and healing of radiation damage. In addition, those interfaces can also be designed to improve strength, toughness and oxidation resistance, thus playing an important role in materials performance.
Now, engineers at the University of Wisconsin-Madison have applied an analogous layering strategy to a class of radiation-resistant ceramics called MAX phases, carefully examining what happens at the interfaces of such multi-layers. This work opens the door to creating novel, layered ceramics with applications as structural and coating materials in nuclear reactors and electronic components in semiconductors. The research appears in the June 25, 2021, issue of the journal Science Advances.
“Ceramics often have good corrosion resistance and high-temperature stability, so they have a special role to play in nuclear applications,” says Izabela Szlufarska, a professor of materials science and engineering at the University of Wisconsin-Madison. “Layering is successful in metallic systems. But ceramics have very different behaviors than metals. One of the things in question is whether interfaces are beneficial in ceramics because of the more complex behavior of defects in these materials. Moreover, ceramics are often comprised of elements that are quite distinct from each other and each of these elements may interact differently with interfaces, leading to a complex response to radiation.”
Hongliang Zhang
To investigate, the team, which included postdoctoral scholars Hongliang Zhang and Jianqi Xi, created a layered system using titanium silicon carbide (Ti3SiC2), the MAX phase ceramic material with the highest known stability of the crystalline structure under radiation. However, although the crystal structure of titanium silicon carbide remains stable, this material tends to accrue defects over time in such a way that leads to a phase transformation at high irradiation levels.
Zhang, an expert in ceramic deposition, used a technique called radiofrequency magnetron sputtering to coat the titanium silicon carbide with nanoscale layers of silicon carbide and titanium carbide, two other ceramics known to have good radiation resistance.
The team then irradiated this “sandwich” of layered materials with carbon ions in the UW-Madison Ion Beam Laboratory before using transmission electron microscopy to determine its resistance to radiation.
Jianqi Xi
They found that when it comes to radiation resistance, an interface can be either boon or bane, depending on the atomic-level details of the interface. At the border between the MAX phase and titanium carbide, the radiation resistance was improved. There, the radiation-induced phase transformation was suppressed because the interface with the titanium carbide acted as a defect sink, allowing the defects that formed within the MAX phase to migrate into the interface and into the titanium carbide.
But the opposite was true of the silicon carbide, which turned out to act as a source of defects; defects that developed in that material were transferred into the MAX phase —accelerating its degradation.
Szlufarska says the study shows that layering and creating interfaces in MAX phase ceramics provides a highly promising pathway to engineering new materials with improved radiation resistance. However, the interfaces need to be carefully selected and designed because not all interfaces are beneficial to annealing radiation damage.
Design of interfaces requires an understanding of the evolution of the atomic-level structure and chemistry of the near-interfacial regions in materials. And, Szlufarska says, we need to understand how defects in materials on both side of the interfaces are coupled to each other. “This complexity makes a priori predictions of radiation response difficult.”
However, the team plans to use atomistic simulations to identify promising ceramic interfaces in the future before such materials are synthesized and tested experimentally in the laboratory. “Because of the complexity and richness of defect behavior in ceramics, there’s a great potential for design of new types of multilayer materials. The space to explore is large and we have barely scratched the surface,” says Szlufarska.
Izabela Szlufarska is the Harvey D. Spangler Professor of Engineering in materials science and engineering and nuclear engineering and engineering physics at UW-Madison, and is a core faculty member at the UW-Madison Institute for Nuclear Energy Systems (INES).
Other UW-Madison authors include Ranran Su, Xuanxin Hu, Jun Young Kim, Shuguang Wei and Chenyu Zhang. Liqun Shi of Fudan University in Shanghai also contributed.
The authors acknowledge support from U.S. Department of Energy award # DEFG02-08ER46493.