Crystallization of Polymer-Derived Silicon Carbonitride at 1873 K under Nitrogen Overpressure |
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Authors: | Martin Friess Joachim Bill Jerzy Golczewski re Zimmermann Fritz Aldinger Ralf Riedel Rishi Raj |
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Affiliation: | Pulvermetallurgisches Laboratorium, Max Planck Institut für Metallforschung and Institut für Nichtmetallische Anorganische Materialien, Universität Stuttgart, D-70569 Stuttgart, Germany;Fachbereich Materialwissenschaft, Technische Universität Darmstadt, D-64287 Darmstadt, Germany;Department of Mechanical Engineering, University of Colorado at Boulder, Boulder, Colorado 80309-0427 |
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Abstract: | The chemical stability of an amorphous silicon carbonitride ceramic, having the composition 0.57SiC·0.43Si3N4·0.49C is studied as a function of nitrogen overpressure at 1873 K. The ceramic suffers a weight loss at p N2 < 3.5 bar (1 bar = 100 kPa), does not show a weight change from 3.5 to 11 bar, and gains weight above 11 bar. The structure of the ceramic changes with pressure: it is crystalline from 1 to 6 bar, amorphous at ~10 bar, and is crystalline above ~10 bar. The weight-loss transition, at 3.5 bar, is in excellent agreement with the prediction from thermodynamic analysis when the activities of carbon, SiC, and Si3N4 are set equal to those of the crystalline forms; this implies that the material crystallizes before decomposition. The amorphous to crystalline transition that occurs at ~10 bar, and which is accompanied by weight gain, is likely to have taken place by a different mechanism. A nucleation and growth reaction with the atmospheric nitrogen is proposed as the likely mechanism. The supersaturation required to nucleate α-Si3N4 crystals is calculated to be 30 kJ/mol. |
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Keywords: | silicon carbonitride polymers/polymerization crystals/crystallization nitrogen |
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