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 mis- sion is carbon ber, 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 ber 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 ow 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 ndings are the rst to directly observe the structural dependence of carbon oxidation rates at these length-scales while also giving impor- tant insight into the onset of pitting at various active surface sites, important pieces in fundamentally understanding of carbon oxidation. 1 Keywords Oxidation, in situ STEM, MEMS-based closed-cell gas, thermal protection systems TPS, ablation, carbon bers Introduction Atmospheric entry spacecrafts are equipped with thermal protection systems (TPS) to shield the payload from extreme heating. The material used for TPS is usually a composite com- posed of a solid porous matrix and a polymer.1One such example is PICA2(phenolic- impregnated carbon ablator) which was used on the NASA Stardust Sample Return Capsule3 as well as on the Mars Science Lab4and Mars 2020 entry vehicles.5PICA is a low-density, high-porosity ablative material made of a carbon ber preform, FiberForm,rimpregnated with phenolic resin. During the heating phase of the atmospheric entry, the phenolic rapidly pyrolyzes, with only the charred preform remaining.6{10The exposed carbon bers interact with the oxygen in the atmosphere { or species containing oxygen, such as CO 2{ and start to oxidize. The ability of the charred carbon bers to survive the harsh entry conditions is directly linked to the carbon oxidation rates at high temperature and under an oxygen-rich environment. From a fundamental standpoint, the chemistry of carbon oxidation remains poorly un- derstood. Decades of research, starting in the 1950's,11have yielded global models of carbon oxidation where all the elementary processes are lumped into a few global reactions. Other global models have been developed using chemical equilibrium assumptions, and were veri ed using high enthalpy experiments.12Detailed kinetic mechanisms, including molecular-scale processes, remain elusive despite recent e orts to bridge the gap between microscopic and empirical approaches.13{18 More recently, molecular beam experiments have been performed to provide a better understanding of the interaction between the hot carbon char and oxygen.19,20These ex- 2 periments have led to the construction of a nite-rate kinetic model based on reaction probabilities.21,22Contrary to previous models which are largely based on empirical mea- surements,23,24all the individual rates are directly calculated from molecular beam data. One important aspect of oxidation relates to the evolution of the carbon surface topology, the phenomenon generically called \pitting".25{28Past experimental studies have found that in certain conditions, oxidation does not occur uniformly but takes the form of pits or crater-like cavities, randomly scattered across the otherwise smooth carbon surface.29{33 These pits initiate at atomic vacancies, defects, or basal plane carbon atoms,33,34eventually reaching the scale of the diameter of the bers.35Hahn34observed that the frequency, size and initiation sites of the pits on highly oriented pyrolytic graphic (HOPG) varies with temperature. At temperatures of less than 875 °C, pits were only initiated by defects, while at higher temperatures, they were also initiated from basal plane layers. Over the years, two main techniques have been used to perform in situ environmental transmission electron microscope (TEM) studies. The rst method involves directly intro- ducing gas in the high-vacuum column of a dedicated environmental TEM (ETEM).36{38 However, this method severely limits the partial pressure of the reactive gas, as the column must remains at very low pressure. Moreover, the method is not without risks as the TEM column can be contaminated with the reactive gases. A more recent approach makes use of a specially designed closed cell gas reaction gas-cell holder.39{44With this method, the sample is contained within electron transparent silicon nitride membranes and the temper- ature, pressure and gas environment is controlled. The environment within the gas cell can be controlled by setting: a static gas quantity or gas ow, the temperature from ambient up to 1000 °C and the pressure up to 1 atm. Additionally, the layer of gas in the cell is thin, thereby maintaining high spatial resolution.45 The surface roughness due to pitting a ects the reactive area of the material, and there- fore, the net oxidation rate. The current state-of-the-art resorts to ad hoc models of surface evolution.46Since the onset and propagation of this phenomenon is not fully understood, it 3 is dicult to accurately predict the available reactive area or the oxidation rate. A better understanding of the evolution of pitting on a much smaller length scale is thus needed. The goal of this work is to study the oxidization of carbon at the microstructural-level. Previous work35,47has determined the bulk properties of FiberForm during oxidation in air, O 2and CO 2. The current work builds on that to illuminate the microstructural-dependent prop- erties of FiberForm and carbon such as the mechanism of pitting as opposed to the bulk properties such as mass loss and recession at given temperatures and reactant ow rates in previous work.35To this end, a thin section of FiberFormrwas heated in a closed gas-cell under a steady ow of air while being examined in real-time using a scanning transmission electron microscope (STEM). Results Characterization of Carbon Fiber A carbon specimen was sectioned from a carbon ber within a sample of FiberForm and then thinned, using focused ion beam (FIB) techniques. The sample, taken from the interior of the ber, was parallel to the longitudinal axis of the ber, as shown in Fig. 1. The interior section specimen was then transferred to the top window of an Atmosphere E-chip as shown in the SEM image in the rightmost panel of Fig. 1 and hereafter called \FF-IS". The E-chip was sealed and held at a pressure of 76 torr with a small ow of air supplied throughout the experiment. The chip was resistively heated at 10 °C/s in 50 °C steps followed by isothermal holds for10 min. Annular dark- eld STEM images were acquired over the course of the experiment as shown in Fig. 2. Region 1, highlighted by the red box in Fig. 2(a), corresponds to the top portion of the specimen shown in the SEM image in Fig. 1, and is characterized by a uniform appearance. This region is shown to be uniform in thickness, based upon imaging with STEM and SEM. The dark region below Region 1, labeled \Background" in Fig. 2(a), is the electron transparent Si xNywindow on the E-chip shown in Fig. 1. Region 2, shown by 4 the green box in Fig. 2(a), corresponds to the 1m strip of material running across the specimen and parallel to the ber axis. This structure is also apparent in the SEM image in Fig. 1 and is characterized by a more heterogeneous micro-structure with what appears to be inclusions and voids. Multiple studies have shown the presence of microvoids or pores in both polyacrylonitrile- based (PAN) and mesophase pitch-based carbon bers.48{53Guigon et al.52attributes the irregular pores to the void space between the crumbled sheets of carbon similar to what would happen when crumpling a sheet of paper. Qin et al.48attributed microvoids on the surface and interior of PAN-based carbon bers to the release of nitrogen and the rearrangement of crystallite during high-temperature heat treatments. Other studies51,53point to needle- shaped voids that can been seen using TEM but were rst proposed by Perret and Ruland54. Perret and Ruland54attributes these needle-shaped voids to the non-ideal stacking of the micro brils of carbon such that the space between these nearly parallel stacked ribbons is not completely lled. Due to the relatively large size of the voids shown in Figs. 1 and 2, and the many crystallites seen in Region 2, it seems most likely that these voids are due to the rearrangement of crystallites during a heat treatment similar to what is described by Qin et al.48. Since the specimen was thinned to a relatively uniform thickness using FIB, the contrast di erences between Regions 1 and 2 are assumed to be due to di raction e ects and density di erences. The major nding from the reported experiment is the di erent oxidation behaviors observed by the two signi cantly di erent microstructural regions: Region 1 and 2. Based on the stark di erences in appearance of Regions 1 and 2, High-resolution TEM (HRTEM) was performed on the specimen and similarly structured carbon ber sections to study the di erences. HRTEM prior to oxidation con rmed that the structure of the two regions is in fact very di erent. Fig. 3 shows HRTEM images of a similar structured FiberForm specimen where the center section contains inclusions { like Region 2 in FF-IS { and is surrounded by a uniform region { like Region 1 in FF-IS. The continuous and wavy lattice fringes of the 5 (002) crystallographic planes are running vertically in the top inset of Fig. 3, indicative of the inclusions being large single crystallites. In this particular crystallite, shown in Fig. 3, the basal planes stack laterally along the larger image. Needle-shaped voids oriented vertically and parallel with the lattice fringes can also be seen in Fig. 3. More detailed images are included in the Supporting Information (Fig. 5) and show these voids to be between ribbons in the structure close to where the carbon folds over on itself, which is strikingly similar to the mechanism proposed by Perret and Ruland54. Based on the structure in Fig. 3 and the appearance of the inclusion marked by the green cross in Fig. 2(a), it is concluded that the basal planes stack approximately normal to the image. The wavy pattern has been seen in the lattice fringes of many studies on PAN-based carbon bers as pointed out in the review by Edie55. The appearance is proposed to be from the interlinked turbostratic layers of carbon present in the precursor PAN bers that have a bril structure.55A measurement of multiple lattice fringes shows that the d-spacing is 0.38 nm, which is signi cantly larger than the (002) plane spacing of graphitic carbon, 0.34 nm. Edie55reported that multiple studies on carbon bers have observed interlayer spacings larger than those of graphite with a d-spacing of 0.38 nm being within the range of values reported by Bennett and Johnson53. It should be noted that this microstructure is generally attributed to the \skin" of carbon bers, which does not seem to be the case here since Region 2 occurs in the ber interior and is thicker than the skin regions commonly reported. However, Region 2 does have similar characteristics such as the highly aligned large crystallites. It should be noted that there is a subtle line running along the bottom of FF-IS parallel to the ber axis shown in Fig. 1. This line is located 400 nm from the exterior surface of the FiberForm, and may represent the transition between skin and core regions discussed in the literature. For example, Frank et al.51showed a dark- eld TEM image of a 6-7m diameter PAN-based carbon ber with a skin region that is 150-250 nm thick. The structure of Region 1 in Fig. 2, on the other hand, is the same as the lower inset in Fig. 3, which appears to be much more disordered with only small regions of lattice fringes visible 6 and at random orientations to one another. This structure resembles the core region of a PAN-based carbon ber proposed by Johnson56in which the structural organization in cross section is random due to the extensive folding of the core layer planes. More HRTEM images can be found in the Supporting Information documents (Fig. 5), showing more detailed views of the needle-shaped voids and sheet folding present in these bers. Based on the random structure exposed during the FIB process (Region 1), it was ex- pected that numerous active surface sites would be available for O 2during oxidation. This would result in uniform and rapid oxidation normal to the surface of the carbon. On the other hand, the highly ordered structure of the inclusions in Region 2 would mean far fewer active surface sites, and results in slower, more directional oxidation from the edges of the basal planes and defects. Since sites other than basal plane edge sites and defects do not favor oxidation,57oxidation normal to the basal plane would be much lower than lateral oxidation, and would strongly dependent on surface defects. Both of these predictions are veri ed by the oxidation experiment. Oxidation of Carbon Fiber The grey-scale values in the regions discussed above were analyzed over the course of the experiment to quantify the oxidation of the carbon microstructures. Fig. 4(a) shows the percentage of the original grey-scale value averaged over the entire Region 1, and the center of the inclusion particle in Region 2, marked by a green cross. To account for image variabilities, those values were normalized by the background value of the window in the FF-IS chip. This method provides an approximation of the relative thickness of the FiberForm specimen as long as the grey-scale of the background remains constant. Jonge et al.58used a similar approach to calculate the thickness of gold nanoparticles in a STEM liquid ow-cell. They used a Rutherford scattering approach to model the elastic scattering of electrons in ADF imaging mode, which they use to compare the thickness of gold nanoparticles against known values. Sacci et al.59used this approach to identify lithium-containing deposits formed 7 during potential sweeping of a model Li-ion battery system in a liquid-cell STEM experiment. The background grey-scale value of FF-IS remains constant until 70 min (or 950 °C) into the experiment at which point (Figs. 2(i) to 2(l)) the Si xNywindow becomes heterogeneous in brightness, with lighter spots forming. A lighter spot in ADF mode indicates an increase in thickness or an increase in density. SEM images from another FiberForm specimen and E-chip, taken before and after a similar oxidation experiment, show that only the Si xNy window with a FiberForm specimen on top undergoes noticeable di erences in appearance (Fig. 6 of the Supporting Information), suggesting that the change is the result of deposi- tion of oxidation products or tungsten impurities from the FIB transfer procedure and not an oxidation of the Si xNymaterial. Post-mortem images of FF-IS were not taken due to extensive oxidation to the carbon specimen. Fig. 4(a) shows that the greyscale of all regions did not change noticeably until a temper- ature of 750-800 °C was reached, indicating that the specimen thickness remains essentially the same and little oxidation occured. By the time the temperature rose to 900 °C, the grey- scale value of Region 1 was still 80% of the original value, having decreased by 2 percentage points per minute (pp/min) or less. The greyscale value decreased at a rate of 3 pp/min (R2= 0.88) during the isothermal hold at 900 °C, and then the rate increased to 7 pp/min (R2= 0.97) at 950 °C until the background level changed. At this point the greyscale value of Region 1 leveled o , even increasing slightly. This is likely due to the changes of the SixNywindow that occurs at 70.0 min, but may also be a result of the appearance of small white spots in Region 1 (Fig. 2(i) to 2(l)). A previous study utilizing energy dispersive X-ray spectroscopy (EDS) by Panerai et al.47found these white spots to be rich in calcium and oxygen, with trace amounts of silicon, sodium, sulfur and potassium. X-ray di raction (XRD) of the white residue left from oxidizing FiberForm in a conventional furnace showed these white deposits to be a mixture of calcium oxide and calcium silicate (Fig. 7 of the Supporting Information). These calcium- and silicon-rich particles could be responsible for the change to the Si xNywindow. 8 Examining Fig. 2 shows that Region 1 started to darken in ADF mode at 750°C and continued to darken with increasing time/temperature, indicative of uniform thinning of the region. Brightness in ADF mode of STEM imaging is proportional to atomic number and thickness. Therefore if the image is darkening with no composition changes, then the thickness is decreasing. However, the behavior observed in Region 2 is di erent than that in Region 1. For one, thinning of Region 2 is not uniform. As can be seen in the square inclusion in Fig. 2 in which the grey-scale decreases from the outside of the particle inward. This behavior is consistent with lateral oxidation from the edges of basal plane layers and stacking of basal planes normal to the image. The onset of oxidation at the edges of the inclusion shown in Region 2 of Fig. 2 is similar to Region 1 as can be seen by comparing Fig. 4(b) with Fig. 4(a) and consistent with the temperature Hahn34reports for pitting initiated from basal plane layers. From Fig. 4(a), the grey-scale value of the center of the inclusion remains very stable until the temperature reaches 1050 °C, indicating that substantial oxidation is not occurring at the inclusion center until that temperature is reached. Additionally, if only considering the square inclusion in Fig. 2, the anisotropic oxidation extends to the vertical and horizontal dimensions of the particle as shown in Fig. 4(b). As de ned in Fig. 4(b), the vertical and horizontal directions of the square inclusion begin noticeable reductions at 850°C. Above 850 °C, the rate of vertical reduction is greater than the horizontal reduction indicating that the oxidation rate is more rapid in the vertical direction. Since the vertical and horizontal directions in Fig. 4(b) are orthogonal and the a-axes in the basal plane of carbon are at 60 °to each other, this would explain the reported di erences in rate along directions 90 °from each other. It is not known with certainty however what crystallographic directions are parallel to the vertical and horizontal directions. This preferential oxidation within the basal plane of carbon has been observed in HOPG using scanning tunneling microscopy (STM) by the appearance of hexagonal pits once the pits reached a certain sizes.33,60 The results shown in Fig. 4(a) correlates to the meso-scale study of Panerai et al.35. In 9 that experiment, a 20 mm thick sample of FiberForm was oxidized for 10 min in a quartz tube of owing oxygen, at temperatures ranging from 200 to 1100 °C. The results show that the sample remains non-reactive until a temperature of 500°C is reached, at which point a sudden mass loss is observed. The mass loss steadily increases with increasing temperature up to1000°C where another steep increase is noted. Analysis of the product gas composition showed that the change in slope corresponds to an increase in CO and decrease in CO 2 following a Boudouard reaction at temperatures above 800°C. The current results did not show a signi cant decrease in thickness until 750-800 °C. This could be due to di erences in ow rates or ow conditions. In the ow tube experiment, gas is forced to ow through the porous FiberForm that is pressured t within the tube, while the same is not true for the ow within the E-chip. The onset of thickness decrease shown in Fig. 4(a) does correspond to the temperature in Panerai et al.35at which point signi cant amounts of CO are observed. This suggests that the decrease in thickness is attributed to oxidation of carbon to produce mostly CO and that in this study minimal oxidation was observed in the temperature range in which a majority of CO 2is produced. The signi cant mass loss and recession rates at temperatures as low as 400 °C in Panerai et al.35would also suggest that bulk FiberForm is more closely related to Region 1 in the current study, whereas more ordered inclusions may only be present in small amounts. One last observation to point out pertains to the potential \skin" region described above. While the subtle line can be seen in Fig. 1, it cannot be resolved prior to the oxidation experiment in ADF STEM (Fig. 2(a)). However, at 700 °C a line parallel to the ber axis can be seen developing at the bottom of FF-IS in the same location as the subtle line was seen in SEM. The development of this line could be the result of delamination during heating or lateral oxidation at basal plane edges. Interestingly, this edge region remains brighter than the surrounding Region 1 throughout the rest of the oxidation experiment (Fig. 2(e-l)) suggesting more similarities to the oxidation mechanism of Region 2 than Region 1. This would support the notion that this edge region is in fact the skin region. 10 In summary, di erent microstructures of a thin carbon ber specimen were character- ized and exhibited many similarities to PAN-based carbon bers reported in the literature. These carbon microstructures showed unique oxidation behavior at temperatures ranging from 25-1050 °C as observed using in situ TEM in a closed-environmental cell. These direct observations of carbon oxidation suggest the microstructural-dependent oxidation mecha- nisms proposed above to be true for Regions 1 and 2. These results provide further insight into the oxidation mechanisms of carbon bers at the microstructure-level using in situ gas- cell STEM. The microstructural-dependent oxidation results provided by this state-of-the-art technique add to what our group has already reported about bulk oxidation of these carbon ber materials in ow-tube experiments.35,47These insights allow for the design of advance TPS architectures and development of novel materials for future space missions. Materials and Methods As-received FiberFormr(Fiber Materials, Inc., Biddeford, ME, USA) was sectioned and thinned for imaging using a Hitachi NB5000 dual-beam focused-ion beam (FIB). A Pro- tochips Atmosphere 200 ™(Morrisville, NC, USA) was used to study the oxidative behavior of FiberForm. FIB prepared FiberForm samples were transferred onto the Si xNywindow of an Atmosphere E-chip while also being in physical contact with the SiC surrounding the window to enable resistive heating during the experiment. The E-chip was then installed into the Atmosphere holder according to the manufacturer's speci cations creating a gas-tight seal that was veri ed in a vacuum chamber up to 1 106mbar. Ar and O 2(Ultra-high purity, Airgas) gases were supplied to the E-chip through the included gas manifold. The system was set for gas ow and air was constantly supplied throughout testing at a constant pressure of 76 torr. More experimental details on the use of the Protochips Atmosphere setup for studying closed-cell high-temperature gas reactions can be found in Unocic et al.44.In situimaging was simultaneously captured in bright- eld (BF) and annular dark- eld (ADF) 11 imaging modes using an FEI Titan S aberration-corrected STEM operating at 300 kV, in the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory. Videos of the experiment were collected using Camtasia, a screen capture software, while continuously scanning the specimen. High-resolution TEM (HRTEM) imaging was conducting before and after the experiment using the FEI Titan along with additional post-mortem imaging taken by the dual-beam. Acknowledgments The authors thank D. Co ey (Oak Ridge National Laboratory) for her expert FIB specimen preparation and useful discussions, N. Briot (Electron Microscopy Center, University of Kentucky) for his help in preparing the samples as well as T. Dolan (College of Medicine, University of Kentucky) for his illustrations. This research was supported in part by a grant from the Kentucky Science and Engineering Foundation as per Grant/Award Agreement KSEF-3939-RDE-020 with the Kentucky Science and Technology Corporation, as well as from NASA Space Technology Research Grants Program, Early Stage Innovations (ESI) award NNX15AD73G. In situ STEM experiments and HRTEM imaging were conducted at the Center for Nanophase Materials Sciences, which is a DOE Oce of Science User Facility.