The Condensation of Coefficient of Si3N4
Samples of silicon nitride containing high concentrations of the N14 and N15 isotopes of nitrogen were prepared by reacting compressed pellets of powdered silicon with nitrogen gas in an induction heated vacuum furnace. The condensation coefficient, beta, of these materials was evaluated by a nitrogen isotope exchange technique in a Knudsen type effusion apparatus attached to a time-of-flight mass spectrometer. The condensation coefficient was determined to decrease linearly with increasing temperature, and was evaluated in the temperature range 1625K to 1710K as beta x 1000 = 20.26 - 9.4 x 0.001T where T is the temperature in degrees K. The vaporization of silicon nitride proceeds by a mechanism which depletes the interior of the solid in nitrogen, leaves a thin coating of 'stoichiometric' silicon nitride on the surface, and decreases the apparent density of the solid. (Author).