Materials degradation in hot environments
Heat exposure is a challenge to many technologically applicable materials – especially when it is part of their operating conditions. The LINX teams at Aarhus University and Technical University of Denmark studied degradation and aging of materials operating at high temperatures.
Evaluating the long-term stability and performance of materials working at high temperatures is crucial in order to assess product lifetimes. In this LINX project, two different types of material: steel from thermal gasification reactors and advanced semiconductors, were under investigation, where both must withstand hot working environments. Reactors used for thermal gasification of biomass operate at very high temperatures, and the steel components in such reactors are exposed to intense heat for extended periods of time. Advanced semiconductor materials, which can convert heat directly to electricity, can be exploited in the so-called “thermoelectric” technology to harvest energy from waste heat. These materials must be able to function ceaselessly for decades under a heat gradient without intervention, and structural changes can be detrimental to the electrical properties.
In collaboration with Frichs Ecotech A/S and TEGnology ApS, the LINX teams at Aarhus University and at the Technical University of Denmark (DTU) used X-ray diffraction and 3D-imaging, respectively, to investigate structural changes at the atomic level and all the way up through the microstructure in the reactor steel and semiconductor materials operating at high temperature.
Read the interesting one pager from this project that involved TEGnology and Aarhus University: Degratation of thermoelectric materials in hot environments.
Techniques and Methods
For both Frichs Ecotech A/S (Frichs) and TEGnology ApS (TEGnology), it was important to understand the degradation and aging of their materials under high temperature operating conditions. TEGnology were interested in the migration of ions in their thermoelectric module components, since ion migration decreases the performance. Frichs were interested in knowing whether possible phase changes or microcracks affect the stability of their thermal gasification reactors, and if the reactor material is transferred into the processed feedstock.
In this LINX project, the LINX teams at Aarhus University (AU) and at the Technical University of Denmark (DTU) used X-ray diffraction and 3D-imaging, respectively, in order to study these materials after exposure to typical operating conditions. These methods allowed us to investigate structural changes at the atomic level and all the way up through the microstructure, making them ideal for the study of subtle changes in materials that are exposed to high temperatures.
The LINX team at AU performed position resolved X-ray diffraction experiments on different thermoelectric module components after operation. This type of measurement allowed us to identify phase composition through the components, and thus whether ion migration takes place. These experiments confirmed that a method developed by TEGnology to prevent ion migration was successful.
DTU used X-ray 3D-imaging techniques to study the possible occurrence of micro-cracks in the reactor steel samples. AU performed X-ray diffraction experiments on reactor steel material as well as the reactor feedstock before and after exposure in the reactor. This was done to characterize the crystalline phases present in the materials. Supplementary elemental analysis was carried out to investigate whether elements from the reactor were present in the processed feedstock material. The investigations of the Frichs reactor showed that the exposed surface of the reactor material is partially transformed from steel to rust under operation. Although, a significant amount of reactor material was not found to be transferred into the processed feedstock.
Participants: Frichs Ecotech, TEGnology, Aarhus University, Technical University of Denmark.
Start date, end date: April 2016 – April 2018.
Title: Aging of materials (at high temperatures) (FP08.002, Materials at operating and processing conditions).