High-Pressure Chemistry of Carbon Nitride Materials
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Mat. Res. Soc. Symp. Proc. Vol. 499 © 1998 Materials Research Society
EXPERIMENT Paracyanogen (pCN) is an amorphous carbon nitride polymer with a relatively high thermal stability[11]. Its structure is composed sp2-bonded chains, -N=C-N=C-, with crosslinking at every carbon. A weak infrared absorption peak at 2200 cm1 shows that there are nitrile (-CN ) residues in the polymer. Paracyanogen can be readily made in high purity and well defined stoichiometry of C INI [11]. Its properties and stability were monitored as a function of pressure and temperatures in the ranges of 3 to 19 GPa and 475 to 850 'C. Experiments were conducted in a multi-anvil high-pressure system[12] using comer-truncated tungsten carbide cubes as the final anvil stage and a magnesium oxide pressure medium. A coiled rhenium tube acted as both a heater and reaction vessel and imposed a thermal gradient from its center (hot) to either end of the sample volume (Fig. 1). The sample volume was 64 mm 3 for the pressure range of 3 to 8 GPa and and 27 mm 3 for the range of 10 to 19 GPa. This relatively large volume permitted extensive analysis of the product. Pressure annealing to 17 GPa yields a solid with a density of 2.1 g/cm3 from a starting density of roughly 1.6 g/cm 3. Infrared absorption spectroscopy remains virtually identical, suggesting that local bonding order is not affected and that nitrile residues do not continue polymerization. All analysis reported here was made ex-situ. Composition analysis of quenched and degassed samples (e.g., the release of molecular nitrogen product) was made by electron microprobe. Structural changes were monitored by x-ray and electron diffraction. Hardness and density were measured by simple indentation and flotation procedures, respectively. The kinetics of decomposition were determined by quenching the reaction after varying time intervals ranging from 1 minute to 22 hours. Temperatures were measured using a standard tungsten-rhenium thermocouple with the geometry shown in Fig. 1. In these experiments the uncertainty in temperature is less than 10 "C and the uncertainty in the pressure is 5%.
Fig. 1. A picture of a multi-anvil high pressure sample sectioned through the center of the heater cylinder. The thermocouple measures the maximum temperature of the rhenium heater cylinder.
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RESULTS
The most distinctive feature in the high-pressure/temperature processing of pCN is a dramatic boundary between a region of dense undecomposed material and one with significant decomposition and high porosity from a post-decompression evaporation of molecular nitrogen. (Fig. 1). The temperature above which this rapid decomposition occurs is well defined and increases with pressure (Fig. 2). Above this critical temperature paracyanogen decomposes rapidly in a wave with a propagation velocity of about 1 mm per minute from the center towards the ends. This self-propagating decomposition is due to the large excess heat release in the reaction which exceeds the thermal conductivity of pCN. Another important observa
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