Experimentelle Untersuchungen zur Diamantbildung in Astrophysikalischen Umgebungen
Zusammenfassung der Projektergebnisse
The research project was devoted to the study of diamond formation at conditions which are prevailing in the interstellar medium, in stellar envelops of evolved stars, and in protoplanetary and planetary disks. The project consisted of two subproject. The first one was dedicated to the study of the erosion of graphitic and diamond samples by exposure to hydrogen atoms at temperatures between 300 and 1000 K. The results provide important information on the stability of carbon in astrophysical environments under atomic hydrogen exposure. In our experimental studies, the erosion of graphite was found to be temperature independent. Methane, CH3, CH2, but also higher alkanes were detected as major degradation products. Diamond erosion rates were found to be 10 times lower than those determined for microcrystalline graphite. In the second part of the project, we have studied the diamond formation in heterogeneous condensation processes that mimic the formation conditions of refractory carbon material in astrophysical environments. We have designed an ultra-high vacuum setup that included an atomic hydrogen source and a newly developed atomic carbon source, which provides an intense and pure flux of low-energy atomic carbon. The combination of these two atomic sources allowed us to use the most abundant cosmic species for the condensation of diamonds on varying substrates at different temperatures. The carbonaceous condensates were characterized by high-resolution transmission electron microscopy (HRTEM) in combination with electron energy loss spectroscopy. The study of the energy-loss near edge structure of the carbon K edge allowed us to clearly distinguish between different carbonaceous structures including diamonds. In addition, UV-Raman spectroscopy was applied for studying the condensation products. Small nanometer-sized diamond crystals were detected in samples produced at substrate temperatures between 15 and 770 K. Higher temperatures seem to foster the evolution of graphitic substructures in the condensates. The diamond crystals were embedded in amorphous carbon structures. The nanodiamonds generated by the deposition of C atoms or C atoms along with hydrogen in our experiments show typical twin structures, a characteristic property also observed in presolar meteoritic diamonds extracted from the Murchison or Allende meteorite. Such typical twin structures are formed in CVD-type processes. Our experiments have demonstrated that nanodiamonds can be formed in the interstellar medium as well as in protoplanetary and planetary disks. They might be embedded in an amorphous carbon matrix.
Projektbezogene Publikationen (Auswahl)
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Low-temperature condensation of carbon, Astrophys. J. 847, 89/1-89/7 (2017)
S. A. Krasnokutski, M. Goulart, E. B. Gordon, A. Ritsch, C. Jäger, M. Rastogi, W. Salvenmoser, Th. Henning, and P. Scheier
