New Formation Route for Carbide-core, Graphitic-carbon Mantle Grains Based on Fullerenes

Introduction: The formation environment of TiCcore, graphitic-mantle spherules was calculated after their discovery in acid residues derived from the Murchison carbonaceous meteorite [1]. The graphitic spherules including metal carbide crystals such as TiC were identified as presolar grains from their isotopic content and assumed to form within the circumstellar envelopes of carbon-rich asymptotic giant branch (AGB) stars [1,2]. The metal carbide crystals were composed of Ti and/or Zr-Mo carbide, were generally located at the center of individual spherules and are surrounded by well-graphitized carbon. Therefore, it has been assumed that TiC condensed prior to carbon. These composite spherules have been called coremantle grains. The radii of the metal carbide core and of the graphitic mantle layer are 5-200 nm and 0.3-9 μm, respectively. Constraints on the formation conditions and environment, such as the C/O abundance ratio and total gas pressure, of the TiC-core, graphiticmantle spherules can be derived from the size of the TiC core and graphitic mantle, and depend on the vapor density and the cooling rate of the grains. Most of the carbon in the outflow of carbon-rich AGB stars is in the form of CO and C2H2 [3]. Carbonaceous materials such as polycyclic aromatic hydrocarbons (PAHs) and fullerenes are believed to form from C2H2 and its derivatives because CO is a very stable molecule [4,5]. Therefore, all of the theoretical calculations were carried out based on C2H2 gas abundances, i.e., no one has ever considered CO gas as a carbon source. If carbonaceous materials are formed not only from C2H2 molecules but also from CO gas, then most predicted formation constraints such as gas outflow velocity, stellar mass loss rate, total gas pressure, temperature and C/O abundance ratio will require reconsideration. Here we demonstrate the production of carbonaceous materials from CO gas in the laboratory and present a possible new formation route for TiC-core, graphitic-mantle spherules around AGB stars. CO gas as the source of carbonaceous grains: We found that single-shell, large-cage-structure, carbon particles are produced by the Boudouard reaction, which is the disproportionation of CO molecules into solid carbon and CO2 gas. In the 1970’s, the Boudouard reaction was used to produce graphite flakes, lamellar graphitic crystallites and filamentous graphite using Mg, Ni, Fe, Co and Mo as catalytic metals [6,7]. Subsequently, the Boudouard reaction has been widely used in the production of carbon nanotubes using Mo, Ni, Co, Ni-MgO and Fe as catalytic metals [8-10]. Recently, we produced large-cage carbon particles by resistive heating of carbon rod at a total pressure of 200 Torr in a gas mixture of He and CO. Since there is no report that fullerenes or nanotubes of this size are produced by the common evaporation of a carbon rod, we believe that our large cage carbon particles were produced by the Boudouard reaction, but in a catalytic metal free system. The high temperature (~3000K) of the evaporation source would provide sufficient energy to induced the Boudouard reaction. As a result of high-resolution transmittion electron microscope (TEM) observations, many large cages, which appear to be short nanotubes or large fullerenes, were visible. We concluded that the large cages are single shell structures, i.e., fullerenes of many sizes, but most are larger than C70, which was determined by the TEM observations including the electron diffraction pattern, by sublimation at temperatures as high as 800°C and by their infrared spectra. Formation of core-mantle grains: Since the large cages are present only on the particles’ surface in the case of production in a gas mixture of He and CO, the large cages are deposited after production of the amorphous carbon particles. Although the evaporation temperature of carbon is quite high (~3000 K), the sublimation temperature of fullerene-like carbon particles is very low. For example, the sublimation temperatures of C60 and C70 are 300 and 350°C, respectively [11]. Therefore, even if amorphous carbon particles formed by the evaporation of the rod and large cages are simultaneously produced around the carbon rod by the Boudouard reaction, the amorphous-core, large-cage-mantle structure would be produced due to the large difference in condensation temperature (i.e, the large cages will remain in the gas-phase until the gas cools down to a considerable degree). In order to make analogs of the TiC-core, graphitic-mantle spherules found in meteorites, TiC grains were produced by coevaporation of carbon and Ti in a CO gas atmosphere. Although we do use Ti wire, the production of large fullerenes was also observed to occur in Ti-free gas mixtures of He and CO and their formation was confirmed using TEM observations. The sizes of the TiC grains are similar to the size of the TiC grains produced in He gas, distributed between 20-40 nm whereas the thickness of the mantle layer is drastically different. Although the production condiLunar and Planetary Science XXXVII (2006) 1073.pdf