Vacuum outgassing and gas phase thermal conduction of a microgravity physical vapor transport experiment

Abstract

An experiment to study the effects of microgravity on the deposition of organic thin films using physical vapor transport (PVT) was successfully flown and operated on the Space Shuttle mission 51I. Nine PVT ampoules contained three types of organic solids, buffer gases from He to Xe, and special substrates. The sealed ampoules, heated to 400 °C by concentric heater assemblies, were contained in nine evacuated and sealed UHV quality stainless-steel cells. The outgassing of a cell’s contents and resultant heat loss by gas phase conduction are important parameters for the cell power requirements and ampoule boundary conditions. We report the measurement of the absolute partial pressures of nine residual gases generated within the cells and the radial gas phase heat conductivity in the absence of gravity. H2, CH4, and CO2 were the dominant gases, with total pressures ranging from 0.3 to 2.5 Torr. The heat loss was observed to be pressure dependent, ∼15 W/Torr, and thus an important loss mode.