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Research to Measure the Evaporation and Condensation Coefficients on Certain Refractory Compounds. Part I. The Evaporation Mechanism of Si3N4. Part II. A Knudsen Cell Heating System for Mass Spectrometric Study of the Vaporization of Graphite at Elevated Temperatures.

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Technical rept. 1 Feb 69-31 Jan 69,

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The vaporization of silicon nitride was investigated by heating powder compacts in vacuum using graphite Knudsen cells. After heating in vacuum, the compacts developed a thin white surface layer surrounding a gray bulk material. The thickness of the white surface layer was studied as a function of heating time, temperature, and the orifice size of the Knudsen cell. Using x-ray diffraction powder methods, the composition of the white surface layer and of the interior of the compacts were examined and found to be a two phase mixture of silicon and silicon nitride. The design and fabrication of an induction heated flux concentrator capable of obtaining temperatures in the range 2200-3000K to study the vaporization of graphite by a Bendix time-of-flight mass spectrometer is outlined. The performance of the flux concentrator in heating Knudsen cells made of tungsten, tantalum, and graphite to 2800K is described. The triple Knudsen cell technique will be used to study the vaporization of different grades of graphite. The condensation coefficients of the high temperature carbon species such as C2 and C3 will be determined by the isotopic exchange method. A discussion of the problems encountered in heating graphite to high temperatures is given. Author

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  • Ceramics, Refractories and Glass
  • Atomic and Molecular Physics and Spectroscopy

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