Condensation of zinc under conditions of vapour accumulation

Abstract

In vacuum deposition experiments in which only a small proportion of directly incident vapour condenses, the material which fails to condense can accumulate in the vapour phase until a steady state is reached at which supply from the source is balanced by loss due to condensation and to removal by the pumping system. This accumulation effect has been directly demonstrated by deposition of zinc inside a heated ionization gauge envelope. When vapour was allowed to enter the gauge mouth, the pressure increased steadily to a maximum value—the nucleation pressure—at which condensation occurred, and then fell again. With the source off, the vapour pressure of re-evaporating bulk zinc was recorded. The ratio of the nucleation pressure to the vapour pressure gave the supersaturation required for condensation on the tin oxide coating inside the gauge and ranged from 5 at 220 °C to 2 at 300 °C. Analysis of these results using the atomistic model of nucleation theory suggests a critical size of 6 atoms and an adsorption energy of zinc on tin oxide of 0.55 ± 0.10 eV. Classical nucleation theory gives a critical size near 10 atoms and an adsorption energy of about 0.8 eV; it represents the temperature behaviour less accurately than does the atomistic model. The possibility of accumulation of vapour should be recognised in all experiments in which the condensation coefficient is low.