Metastable carbon

A new form of carbon coating is now available which is neither diamond nor graphite and is known as diamond-like carbon (DLC).bl DLC can be considered as a metastable carbon produced as a thin coating with a broad range of structure and composition.l b[42]... 

The work on carbon nitride solids is strongly related to research on diamondlike carbon (DLC) materials [5, 6]. DLC materials are thin film amorphous metastable carbon-based solids, pure or alloyed with hydrogen, which have properties similar to that of crystalline diamond (high hardness, low friction coefficient, high resistance to wear and chemical attack). This resemblance to diamond is due to the DLC structure, which is characterized by a high fraction of highly cross-linked sp -hybridized carbon atoms. To obtain this diamond-like structure... 

It is well known that the stable crystalline form of carbon at ambient conditions is graphite, which is fully sp hybidized. The synthesis of the fully sp -hybridized crystalline diamond is performed at high temperatures and pressures. So, the production of metastable carbon solids with a high fraction of sp -hybridized... 

E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. 

Towe, K. M., and Malone, P. G. Precipitation of metastable carbonate phases from seawater. Nature 226, 348-349 (1970). 

Rietmeijer, F. J. M. MacKinnon, I. D. R. 1987 Metastable carbon in two chondritic porous interplanetary dust particles. Nature, Land. 326, 162-165. 

The source materials for both the modern calcareous sands and young calcarenites of Bermuda are characterized by a complex metastable carbonate assemblage of skeletal aragonite, magnesian calcite, and calcite. The limestone... 

Further processes leading to the chemical stabilization of these periplatform metastable carbonates and chalk formation have been documented by Malone et al. (1989) for carbonate sediments found at ODP site 716, Maldives Archipelago, Indian Ocean. This site, one of only a few such periplatform sites drilled by the ODP, represents a reasonably continuous recovery of sediments deposited at a nearly constant sedimentation rate. 

In all the experiments and thermodynamic calculations, it was assumed that carbon monoxide was stable at the time of decomposition of iron carbonate and was removed from the rocks being metamorphosed. Nor were possible secondary reactions in the case of oxidation of FeC03 by water taken into account—the result of the reactions would be the formation of some CO, CH4 and more complex hydrocarbons. Such assumptions are valid in examining the results of short-term experiments in which metastable carbon monoxide arises and does not undergo subsequent decomposition. However, in long-term natural processes the establishment of complete equilibrium between all the components of the gas phase, with the decom-... 

The main issue is that carbynes are metastable carbon crystals [13] that ... 

So far, no carbynes were reported in the completely hydrated CI carbonaceous meteorites. It is likely that pervasive hydration had destroyed metastable carbons originally present. If so, they should still be present in the anhydrous proto-CI materials, which are not yet recognized among the collected meteorites. 

Fig. 6. Percentages of carbonate cement (fringe and late calcite cement) and carbonate grains in lenses and host rocks. Host rocks contain only late calcite cement. Percentages were obtained by point counting thin sections. In host rocks, a statistically significant linear relationship exists between the two variables (expressed by the linear function Y= 1.80 A- 0.87 / = 0.88 n = 18). This suggests that late calcite cementation occurred relatively late after most metastable carbonate grains had been dissolved. Cementation only occurred when carbonate grains were available as nuclei for cement precipitation.

The amounts of calcite cement in the host rocks show a positive correlation with the amount of carbonate grains (Fig. 6). This indicates calcite cement precipitation after dissolution of metastable carbonate grains. Stable carbonate grains (which survived dissolution) served as nuclei for late calcite cement in host rocks. Within lenses, late calcite cement merely filled all available pores, which can be ascribed to either an abundance of nuclei or locally low permeability. The lack of correlation between calcite cement and secondary porosity suggests relatively long-range transport of dissolved CaCOj. Late calcite cement precipitated preferentially within lenses that contained more calcite nuclei than surrounding host rocks. 

The industrially made butadiene—styrene mbber of mark SKS-30, which contains 7.0-12.3 per cent cis and 71.8-72.0 per cent trans-bonds, with density of 920-930 kg/m was used as matrix polymer. This mbber is fully amorphous one. Fullerene-containing mineral shungite of Zazhoginsk s deposit consists of 30 per cent globular amorphous metastable carbon and 70 per cent high-disperse silicate particles. Besides, industrially made technical carbon of mark no 220 was used as nanofiller. 

In the light of these findings, in a recent paper [15] the doping problem in carbon has been attacked from a different point of view. Instead of searching for other atom types or complexes that effectively would act as shallow donors metastable carbon modifications which accept both boron and nitrogen as substitutional impurities... 

The recombination process is governed by the same selection rules as spectroscopic transitions. Let us consider the recombination of a carbon ion in its metastable state, Is 2s 2p This means that we have to study the spectroscopic transition from the highly excited atomic state Is 2s 2p ( P)nx, where n is a large number and x is s, p, d, f,..., to one of the low states of the carbon atom. The ground state is Is 2s 2p P and the other low states are Is 2s 2p and and Is 2s 2p As the highly excited atom is in a triplet or quintet state, only transitions to P or are allowed. Therefore the recombination of the metastable carbon ion must leave the carbon atom in the P or state. The selection rules have been discussed in more detail earlier. ... 

The reactivity of crystalline silicon with tetrafluoromethane, CF4, has been investigated by Holcombe et al. [36]. They found a mild exothermic reaction with onset at about 500 °C (peak at 530 °C) with no significant mass change and the onset of a strongly exothermic reaction at 990 °C (peak at 1020°C) with a significant mass loss indicative of volatile SiFx formation. Samples held under CF4 atmosphere for 75 min at 1030 °C exhibited both S-SiC and metastable carbon formation on the sttrface of Si. The exotherm at 500°C is believed to occur from highly reactive amorphous silicon present on the surface of crystalline Si. The formation of metastable carbon, particularly diamond phase on both Si and SiC substrates by thermal treatment in fluorocarbon atmosphere has been the subject of a disclosure by Holcombe et al. [37]. 

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