Nanoscale oxidation behavior of carbon fibers revealed with in situ gas cell STEM

Abstract

Thermal protection systems (TPS) are used to protect spacecraft payloads during the extreme conditions of atmospheric entry. The backbone of the composite TPS material used in the NASA Stardust Sample Return Capsule and the Mars 2020 mission is carbon fiber, which oxidizes at these temperatures and atmospheric conditions. This study presents the direct observation of carbon oxidation using in situ Scanning Transmission Electron Microscopy (STEM). A thin section of a commercially-available carbon fiber material containing multiple carbon structures was examined by STEM in a closed-environmental cell in which temperature was raised from 25 to 1050°C under a steady flow of air. Results show that the random polycrystalline carbon structure oxidized more uniformly and rapidly than the single crystallite region, which oxidized more anisotropically. These findings are the first to directly observe the structural dependence of carbon oxidation rates at these length-scales while also giving important insight into the onset of pitting at various active surface sites, important pieces in fundamentally understanding of carbon oxidation.

Oxidation of Multi-layer Graphite Using an Atomistic Multi-lattice Kinetic Monte Carlo Model

“…At 1223 K, we approximate the etch rate of a disordered carbon fiber structure to be 1.75 nm/s from their results (by assuming the thickness of their carbon fiber strip to be 1.5 μm). We expect the value of this rate to be higher than the in-depth etch rate of pure graphite due to the higher number of active sites available for oxygen adsorption in carbon fiber than in graphite . In extrapolating our curve to slightly lower temperatures, around 1223 K, our prediction for the in-depth etch rate at this temperature would be approximately 1 nm/s.…”

“…There may also be other reactions occurring at higher temperatures that are not included in our KMC model. The result of their experiments at higher temperatures (2478 K) at 1 atm (101,325 Pa) is on the same order of magnitude, 10 4 nm/s, as the in-depth etch rate calculated in our work at high temperatures (2500 K) at 0.099 atm (10,000 Pa) when the curve in Figure is extrapolated to 2500 K. Cochell et al conducted experiments on the oxidation of a thin strip of carbon fiber at a fixed pressure of 10,132 Pa and varying temperature . At 1223 K, we approximate the etch rate of a disordered carbon fiber structure to be 1.75 nm/s from their results (by assuming the thickness of their carbon fiber strip to be 1.5 μm).…”

“…We expect the value of this rate to be higher than the in-depth etch rate of pure graphite due to the higher number of active sites available for oxygen adsorption in carbon fiber than in graphite. 63 In extrapolating our curve to slightly lower temperatures, around 1223 K, our prediction for the in-depth etch rate at this temperature would be approximately 1 nm/s. This value is in the range of the experimental observation, but to the extent that our result is higher than expected at this temperature, we note that experimental conditions, such as the presence of other gases, could influence the rate of oxidation.…”

“…The result of their experiments at higher temperatures (2478 K) at 1 atm (101,325 Pa) is on the same order of magnitude, 10 4 nm/s, as the in-depth etch rate calculated in our work at high temperatures (2500 K) at 0.099 atm (10,000 Pa) when the curve in Figure 6 is extrapolated to 2500 K. Cochell et al conducted experiments on the oxidation of a thin strip of carbon fiber at a fixed pressure of 10,132 Pa and varying temperature. 63 At 1223 K, we approximate the etch rate of a disordered carbon fiber structure to be 1.75 nm/s from their results (by assuming the thickness of their carbon fiber strip to be 1.5 μm). We expect the value of this rate to be higher than the in-depth etch rate of pure graphite due to the higher number of active sites available for oxygen adsorption in carbon fiber than in graphite.…”

Direct Observation of Sample Dynamics in the Gaseous Environment: A Perspective on Current Trends and Future Directions of In Situ Gas Cell Transmission Electron Microscopy

“…This study revealed the nonuniform oxidative mechanism of the material due to its dependence on microstructure. 58 Reductive mechanisms can be similarly investigated using reducing gases such as H 2 in the gas cell. The effect of silica on iron oxide reduction was reported under 700−950 mbar H 2 atmosphere at 750 °C, where comparative studies were performed on silica-promoted and unpromoted iron oxide samples.…”