Putting Stars in the Gap: in situ Irradiation and Heating of Synthetic SiC and Implications for the Origins of C-rich Circumstellar Materials TJ Zega 1,2* , J Bernal 3 , JY Howe 4 , P Haenecour 1, , S Amari 5 , and LM Ziurys 3,6 1. Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA. 2. Dept. of Materials Science and Engineering, University of Arizona, Tucson, AZ, USA. 3. Dept. of Chemistry and Biochemistry, University of Arizona, Tucson, AZ, USA. 4. Dept. of Materials Science and Engineering, and Dept. of Chemical Engineering and Applied Chemistry, University of Toronto, Ontario, Canada. 5. Physics Dept. and McDonnell Center for the Space Sciences, Washington University, St. Louis, MO, USA. 6. Dept. of Astronomy, University of Arizona, Tucson, AZ, USA. * Corresponding author: tzega@lpl.arizona.edu Primitive meteorites contain within them the preserved ashes of ancient stars. These ‘presolar’ grains condensed in gaseous circumstellar environments and made their way through the interstellar medium to our local part of the galaxy where the solar system was forming some 4.56 billion years ago. Graphite and SiC are among the largest (≤ 30 m) types of presolar materials and thus, also the most well studied [1]. SiC, in particular, was shown to occur mostly in the 3C polytype, reflecting the thermodynamic conditions in which it formed [2]. Moreover, grains of SiC and other metal-carbides were observed inside of graphite [3], raising questions regarding the sequence in which these materials condensed in their host circumstellar envelopes. To better understand such materials, and moreover, how reduced C can form in what is known from astronomical observations [4] to be an otherwise H-rich environment (along with other abundant elements such as O, N, and S), we report here results on in situ heating and irradiation experiments inside the TEM with synthetic SiC. A 3C polytype of SiC (99% purity) was dropcasted onto SiN support films as part of the Norcada microelectromechanical systems (MEMS) chip used for in situ heating, which we have previously demonstrated [5,6]. The MEMS chips were loaded into a Hitachi ‘Blaze’ heating holder and together loaded into the Hitachi H9000 transmission electron microscope (TEM) located at the intermediate voltage electron microscope tandem facility at Argonne National Laboratory. We heated the SiC grains at 5 °C/min to 1000 °C, and then held it isothermally and irradiated it with Xe at 150 keV to 15 displacements per atom (dpa) over approximately two hours. After heating and irradiation, the sample was analyzed at the University of Arizona using a Hitachi HF5000 aberration-corrected scanning TEM (S/TEM). The HF5000 is equipped with energy-dispersive X-ray and electron energy-loss spectrometers (EDS/EELS, respectively) for chemical analysis. The imaging analysis was carried out at 200 keV, whereas EELS analyses were performed at the C, K edge at 60 keV. Figure 1 shows one of the grid locations that was monitored in the experiment. Particles are mostly monodispersed, but localized areas contain clumps of several particles. High-resolution (phase-contrast) TEM (HRTEM) imaging shows that the bulk of the interior of the large (≥100 nm) particles contains the original 3C SiC structure. In comparison, smaller (<100 nm) particles show breakdown of the 3C structure on their edges. HRTEM reveals lattice fringes containing interplanar d-spacings of 0.34 nm, which correspond to the (002) spacing for graphite, occur on the edges of most grains. In addition, localized parts of the edges contain complete breakdown of long-range order, whereas others contain small (1 nm) 2488 doi:10.1017/S1431927619013175 Microsc. Microanal. 25 (Suppl 2), 2019 © Microscopy Society of America 2019 https://doi.org/10.1017/S1431927619013175 Downloaded from https://www.cambridge.org/core. IP address: 196.19.150.128, on 07 Aug 2019 at 10:10:05, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms.