Experimental Graphitization and Oxidation Kinetic of Nanodiamond: Implication for Nebular Thermal Processing

Introduction: Nanodiamonds are present in several primitives chondrites [1], from petrologic type 1 to 3.8, together with presolar SiC. They originate from several sources and mechanisms, as shocks in supernovae or condensation in different environments. It is generally believed that they are homogeneously distributed in the nebular gas. Nevertheless, in ordinary chondrites, [2] found a correlation between increasing asteroidal metamorphism and progressive disparition of these presolar grains. Later, [3] also suggested that nanodiamonds from carbonaceous chondrites could have been submitted to a nebular thermal event, as their initial distribution in various unmetamorphosed classes was correlated with bulk meteorite fractionation. In both cases, (asteroidal and nebular), the reaction mechanism of nanodiamond disparition with metamorphism remains unknown, as well as the length and temperature of the episod. In this work, we have studied experimentally the ‘graphitization’ (formation of polyaromatic structures) and oxidation kinetics of synthetic nanodiamonds to determine which of those two suspected reactions was relevant to asteroïdal and nebular conditions. Those data are then compared to SiC volatilization rate determined by [4] and a time-temperature couple can be proposed. Experimental: Experimental ‘graphitization’ was achieved by pyrolysis of synthetic detonation nanodiamonds (ND) (~5 nm) and treated in HOCLO4 at 190°C to clean potential graphitic impurities. Runs temperatures between 600°C and 1500°C, and duration between 90 s and 15 hours were performed. For each run, the transformed mass fraction of nanodiamonds (sp carbons) into onion-like carbon (sp carbons) was then quantitavely determined by X ray diffraction analysis. Experimental oxidation was achieved through Thermo Gravimetric Analysis. The temperature ranged from 300°C to 800°C with a rate of 10°C/min in an atmosphere of dry air (fO2 = 0.2). The reaction rate of ND was determined through mass evolution with temperature and activation energy could be extracted. Results: Graphitization of nanodiamonds, i.e. apparition of the first aromatic units, starts at ~ 1000°C. Above this temperature, a first aromatic layer forms at the diamond surface and concentric layers (onion-like carbon) then progress forward to the heart of the initial crystal [5]. Fig.1 illustrates the transformed volume fraction versus time at 1400°C.