Carbon nanotubes (CNTs), with their unique graphitic structure, and superior mechanical, electrical, and optical properties, have attracted intense interest for use in composite applications. Specifically, space systems and components stand to benefit from their light weight, specific strength, fatigue resistance, fracture toughness, and potential sufficient thermal expansion coefficient properties. However, CNT composites may suffer property changes when exposed to actual or simulated space conditions like thermal cycles, electromagnetic radiation, and high vacuum. The low earth orbit (LEO) is of particular interest because it is the region that harbors the International Space Station (ISS) and the growing commercial space economic activities like satellites and Planet Labs constellation. LEO exposes materials to temperature cycles, atomic oxygen and a that can lead to fatigue cracking surface erosion, among other effects.
It is worth noting that most reports about CNT composites focus on characterizing their mechanical performance under LEO conditions. There are limited studies on the effects of space conditions have on the electrical properties of CNT composites, especially on applications focusing on electromagnetic interference and charge dissipation. In-depth knowledge and understanding of the instantaneous changes in electrical properties that the CNTs composites have when exposed to space conditions, and the magnitude and variation of such changes, are of great importance to the design and performance of systems and components containing them.
To this note, researchers at the Naval Postgraduate School: Dr. Brian Earp, Mr. Joel Hubbard, Mr. Alexander Tracy, Mr. Dan Sakoda and led by Professor Claudia Luhrs investigated the electrical behaviors of CNT epoxy composites with low (less than 1%) CNT loadings subjected to diverse in-situ simulated space conditions. Specimens containing 0.014%, 0.2%, and 0.75% CNT loadings were exposed to simulated sunlight, temperature, and pressure conditions that would be expected in an actual space environment. Their research work is currently published in the journal, Composites Part B: Engineering.
In their approach, the research team employed various means to achieve the space conditions in a simulated environment. A vacuum chamber produced the low pressures and temperatures associated with LEO. The actual solar irradiance was replicated in this experiment using a solar simulator. The effects associated with the temperature variations were determined via a conventional oven. For all the individual scenarios, the changes in resistivity for the composite samples and possible underlying mechanisms explaining the observed electrical behaviors were reported. The authors also characterized the microstructural and thermogravimetric properties of the loadings.
Under high vacuum, low pressures, and high temperatures exceeding 60 °C, an instant decrease in resistivity by a minimum of 20% and a maximum of 40% was reported. Exposure to simulated sunlight also significantly influenced the electrical properties of the composites, leading to a resistivity reduction of up to 60%. Surface porosity played a vital role in reducing the composite resistivity under all the studied conditions. It is worth noting that the observed changes in the electrical properties were partially reversed when the samples were exposed to atmospheric conditions.
In summary, the authors measured the instantaneous changes in electrical properties of CNT epoxy composites with low CNT loadings. They studied the electrical behaviors of the specimens under a simulated LEO environment. Overall, the space conditions exhibited a substantial instantaneous impact on the electrical properties of CNT composites. The results were consistent with mass loss due to volatile species outgassing and the reduction in resistivity was attributed to the extra charge carriers generated by light exposure.
Earp, B., Hubbard, J., Tracy, A., Sakoda, D., & Luhrs, C. (2021). Electrical behavior of CNT epoxy composites under in-situ simulated space environments. Composites Part B: Engineering, 219, 108874.