NSUF 16-713: Improving understanding of defect evolution in neutron-irradiated MAX phases
The objective of this project is to investigate the defect type responsible for the thermal and electrical recovery of MAX phases that have been irradiated at ~400°C. Previous studies have indicated this process occurs somewhere between 400 and 700 °C, since irradiations at these temperatures respectively yield ~20% and 95% nominal thermal conductivity. These approximate to values of 10-20 W/mK and 60-80 W/mK respectively. The fundamental question what kind of defect configuration exists at ~400°C, and what process prevents the A-layer atoms from returning to their correct Wykoff position after irradiation 400°C. Electrical properties remain within the same order of magnitude. Since the temperatures at which recovery occurs correspond to 2-4 eV/atom, and interstitial migration of the A-layer atom is available even at room temperature, it suggests another process limits the A-layer recovery (e.g. an activation for a vacancy migration). The defect configuration and activation energy data can be acquired by TEM and electrical resistivity measurements respectively. The defect cluster configuration, density and even chemical composition can be quantified and visualized by high resolution TEM/STEM, coupled with either EDS or a HAADF detector. This is the de facto method for defect characterization. This will provide insight into the concentration of surviving defects, correlated to other data such as lattice parameter swelling to determine the defect free volume caused by the strain field associated with interstitial point and line defects. The electrical resistivity measurements can be used in a similar way to annealing experiments on metals because of the A-layer atom is responsible for electrical conductivity. Thermal diffusivity experiments have established an Arrhenius relationship to temperature, but the MAX phases are ceramics and thus have a small 1/T contribution from phonon-phonon processes. Logically, the electrical conductivity can be used to quantify the exact activation energy for defect migration related to the A-layer atom recovery, using incremental annealing experiments, with temperatures bracketed by earlier thermal experiments. Overall, information gained will have broad reaching impacts on the question of whether MAX phases can or cannot be used in advanced reactors. The first empirical data of its kind, it can likely be used to model other MAX phases, as this migration threshold (e.g. vacancy) will represent an approximate operational temperature threshold. The project is expected to take no more than 3 months to complete, as fabrication of two MAX phase FIB foils can be conducted as the other samples are annealed and tested. Data analysis should take approximately 1 month from the time of the initial data collection.
Additional Info
| Field | Value |
|---|---|
| Abstract | The objective of this project is to investigate the defect type responsible for the thermal and electrical recovery of MAX phases that have been irradiated at ~400°C. Previous studies have indicated this process occurs somewhere between 400 and 700 °C, since irradiations at these temperatures respectively yield ~20% and 95% nominal thermal conductivity. These approximate to values of 10-20 W/mK and 60-80 W/mK respectively. The fundamental question what kind of defect configuration exists at ~400°C, and what process prevents the A-layer atoms from returning to their correct Wykoff position after irradiation 400°C. Electrical properties remain within the same order of magnitude. Since the temperatures at which recovery occurs correspond to 2-4 eV/atom, and interstitial migration of the A-layer atom is available even at room temperature, it suggests another process limits the A-layer recovery (e.g. an activation for a vacancy migration). The defect configuration and activation energy data can be acquired by TEM and electrical resistivity measurements respectively. The defect cluster configuration, density and even chemical composition can be quantified and visualized by high resolution TEM/STEM, coupled with either EDS or a HAADF detector. This is the de facto method for defect characterization. This will provide insight into the concentration of surviving defects, correlated to other data such as lattice parameter swelling to determine the defect free volume caused by the strain field associated with interstitial point and line defects. The electrical resistivity measurements can be used in a similar way to annealing experiments on metals because of the A-layer atom is responsible for electrical conductivity. Thermal diffusivity experiments have established an Arrhenius relationship to temperature, but the MAX phases are ceramics and thus have a small 1/T contribution from phonon-phonon processes. Logically, the electrical conductivity can be used to quantify the exact activation energy for defect migration related to the A-layer atom recovery, using incremental annealing experiments, with temperatures bracketed by earlier thermal experiments. Overall, information gained will have broad reaching impacts on the question of whether MAX phases can or cannot be used in advanced reactors. The first empirical data of its kind, it can likely be used to model other MAX phases, as this migration threshold (e.g. vacancy) will represent an approximate operational temperature threshold. The project is expected to take no more than 3 months to complete, as fabrication of two MAX phase FIB foils can be conducted as the other samples are annealed and tested. Data analysis should take approximately 1 month from the time of the initial data collection. |
| Award Announced Date | 2016-08-16T00:00:00 |
| Awarded Institution | Illinois Institute of Technology |
| Facility | Materials Research Collaborative Access Team (MRCAT) |
| Facility Tech Lead | Alina Zackrone, Jeff Terry, Kory Linton |
| Irradiation Facility | None |
| PI | Caen Ang |
| PI Email | [email protected] |
| Project Type | RTE |
| RTE Number | 713 |