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Lunar and Planetary Science XXX 1753.Pdf Lunar and Planetary Science XXX 1753.pdf INTERSTELLAR DIAMOND. II. GROWTH AND IDENTIFICATION. Andrew W. Phelps, University of Dayton Research Institute, Materials Engineering Division (300 College Park, Dayton OH, 45469-0130, [email protected]). likely form clumps of carbon that look much like the Introduction: The first abstract in this series starting material – hydrogen rich sp3 bound clusters. suggested that many types of diamond are found in many types of meteoritic material and that the The compounds that fit the requirements above variation in polytype abundance appears to reflect the include the cycloalkanes such as adamantane [21-30] nature of the host. How the diamonds got there and (C10H16) and bicyclooctane (C6H8) (BCO) . what they mean will now be examined. Opinions These compounds ‘look’ like small cubic diamonds regarding the formation mechanism(s) of meteoritic and lonsdaleite respectively. If these particular and interstellar diamond grains have changed over compounds are suggested as models for diamond the years as new methods of making synthetic nuclei then a number of structural and diamond were developed[1,2]. Meteoritic diamonds thermodynamic predictions can be made about the were once believed to be the result of high pressure - types of diamond that can form and its relative temperature processes in the interior of planetary abundance with regard to the physics and chemistry bodies[3,4]. Impact formation of diamond became of its growth environment. A few examples of this popular after the development of shock wave are: Adamantane is a lower energy form and would diamond synthesis[5] because shock synthesis was tend to form in an environment of sufficient thermal able to form lonsdaleite and meteorites were full of energy to allow the molecule to organize into this lonsdaleite. This formation method fit the observed stable molecule. BCO is a higher energy form with data better than HPHT synthesis particularly when identical bonding but the bonds are slightly strained diamonds were found in meteorite impact ejecta and due to the eclipsed carbons in the walls of the in heavily sheared ureilites[6-8]. Diamond formed by molecule. BCO formation would be favored in a terrestrial or pre-terrestrial impact was to remain the hydrogen-rich environment over adamantane and so accepted explanation until the discovery of a pre- on. Again, the presence of diamond in so many solar character for some diamonds suggested that the different hosts immediately questions the need for diamond had to have formed before their multiple diamond grain formation routes. The pure incorporation in meteorites[9]. However, a shock number-density of diamond grains in space reduces history was still being assigned to hexagonal the number of likely formation mechanisms to those diamond component in meteorites even when that could produce diamond on a constant flow basis intimately intermixed with ‘interstellar’ cubic rather than in an individual batch-type process. [10-12] diamond. Frenklach and co-workers [31] demonstrated that hexagonal and rhombohedral Recently, Clayton suggested that diamond polytypes of diamond are co-formed with cubic may form under oxygen rich conditions. This has diamond from the gas phase at low pressures. Some actually been observed for some time in the of the diamond found in meteorites may result from laboratory synthesis of diamond where oxygen rich shock synthesis but hexagonal and rhombohedral atmospheres are used to enhance diamond growth [13] rates. This is illustrated in the ‘phase diagram’ of polytypes can form by other, non-shock, means . [32] This is an important consideration when polytypes diamond CVD by Bachmann . The diagram other than cubic diamond make up a significant clearly shows that the field of diamond growth amount of the diamond recovered[8]. extends well into the oxygen rich side of the ternary diagram. The nucleation of diamond may be difficult Grain Growth: Recent calculations have in an oxygen rich environment but grain growth does suggested that diamond may be as stable as graphite not appear to suffer significantly from an excess of at very small diameters[14-16]. The calculated sizes oxygen. (angstroms to nanometers) were similar to the Diamond Grain Stability: A few basics appear measured diameters of interstellar diamond recovered to hold constant with respect to diamond stability. from meteorites[17]. The important question is: how These include the relative stability of cubic diamond do those small diameter diamonds form in the first to the other polytypes, the etch rate of diamond in place? The previous abstract suggested that the hydrogen, the stability of sp3 versus sp2 bonding, carbon available in space to form diamond would be and thermal sensitivity. These factors are similar to mainly a mixture of hydrocarbons[18,19] and carbon all forms of diamond in that their variations can be monoxide. Ostwald’s Rule suggests that the first understood in light of the particular synthesis system. species to form will be those least stable forms that Bundy et al.[33] point out an area in the carbon phase are the closest in energy to the starting materials[20]. diagram where only cubic diamond has been found to A chemical system full of light hydrocarbons will exist. This area is within a larger phase field where Lunar and Planetary Science XXX 1753.pdf INTERSTELLAR DIAMOND. I. CONDENSATION AND NUCLEATION: A. W. Phelps cubic and hexagonal diamond can coexist and Carli P. S., in International Conference on High graphite may exist as a metastable phase. The field Pressure, 6th, University of Colorado, 1977., edited where only cubic diamond exists is bounded on one by K. D. Timmerhaus and M. S. Barber (Plenum side by a thermodynamic curve and on the other by a Press, New York, 1979), V. 1. Physical Properties kinetic curve. Lonsdaleite is stable to any and Materials Synthesis, pp. 940-3. [6]Lipschutz M. experimental pressure below a certain temperature E. and Anders E., Geochim. Cosmochim. Acta 24, 83- where there is not enough thermal energy to 105 (1961). [7]Lipschutz M. E., Sci. 143, 1431-4 rearrange the bonds even though at high pressures, (1964). [8]Hanneman R. E. et al., Sci. 155, 995-7 the denser cubic phase would be more stable. The (1967). [9]Lewis R. S. et al., Nature 326, 160-2 idea of there being a critical amount of thermal (1987). [10]Howard W. N. et al., 2nd International energy to rearrange the diamond structure is Conference, New Diamond Science and Technology, important to the study of crystal nucleation and Pittsburgh, 1991. [11]Howard W. N. et al., J. Appl. growth. A diamond will crystallize as the cubic Phys. 68, 1247-51 (1990). [12]Frenklach M. et al., J. phase if sufficient time and thermal energy is Appl. Phys. 66, 395-9 (1989). [13]Rietmeijer F. J. M. available, otherwise non-cubic diamond phases may and Mackinnon I. D. R., Nature 326, 162-5 (1987). form. Industrial shock formation is capable of being [14]Badziag P. et al., Nature 343, 244-5 (1990). tuned to form cubic or hexagonal diamond based on [15]Nuth J. A., Nature 329, 589 (1987). [16]Tauson the thermodynamics of the shock wave[34]. V. L. and Abramovich M. G., Doklady Akademii Hexagonal diamond is usually formed under Nauk SSSR 287, 201-5 (1986). [17]Lewis R. S. et al., conditions that require rapid growth. This explains Nature 339, 117-21 (1989). [18]Kvenvolden K. et the amount of hexagonal polytypes of diamond found al., Nature 228, 923-6 (1970). [19]Anders E., Annals during gas phase studies of diamond growth. A of the New York Academy of Sciences 93, 649-64 flowing reactor system nucleated and grew the (1962). [20]Verma A. R. and Krishna P., hexagonal polytypes far more rapidly than the cubic Polymorphism and Polytypism in Crystals (John phase could form. Wiley & Sons, New York, 1966). [21]Schulman J. [10,11] M. and Disch R. L., 106, 1202-4 (1984). [22]Amzel Howard et al. report the formation of cubic L. M. et al., 75 (8), 1073-9 (1971). [23]Boyd R. H. and hexagonal diamond in an acetylene-oxygen et al., 75 (9), 1264-71 (1971). [24]Bruesch P., plasma. Diamond particles formed over a wider Spectrochimica Acta 22, 861-5 (1966). [25]Clark T. range of compositions in a diffusion arrangement et al., 101 (9), 2404-10 (1979). [26]MacFarlane J. J. rather than the narrow range observed in premixed [10] and Ross I. G., , 4169-76 (1960). [27]Mansson M. et gas systems. Howard et al. state that the al., 92 (25), 7296-9 (1970). [28]Schleyer P. v. R. et 'crystallite size and morphology varied with reaction al., 85 (9), 1358-9 (1963). [29]Wong W.-K. and pressure. The particle size was generally smaller at Westrum E. F., 74 (6), 1303 (1970). [30]Srivastava higher pressures and at lower C to O ratios'. They S. P. et al., 38, 1216-7 (1977). [31]Clayton D. D., also found that the growth was reaction time limited. LPSC XXIX, 1998. [32]Bachmann P. K. et al., Particular importance to the formation of interstellar Diamond and Rel. Mat. 1, 1-12 (1991). [33]Bundy F. diamond is the observation that with lower P. et al., Carbon 34 (2), 141-53 (1996). hydrocarbon to oxygen ratios the effects of nutrient [34]Heimann R. B. and Kleiman J., Crystals 11, 1-73 depletion become marked in the growth region. '... (1988). that residual hydrocarbon concentrations downstream from the primary flame front are smaller at lower C to O ratios. A decrease in the particle size at lower C to O ratios was observed...' The results of thermodynamic calculations and laboratory experiments will be presented to add to the previous arguments.
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