Impurity Effects on Heterogeneous Nucleation from the Vapor III—Mercury on Pyrex Glass

Abstract

The thermal accommodation coefficient (TAC) and critical supersaturation for nucleation (αcr) of mercury vapor on Pyrex glass have been studied over a wide range of vapor pressure and temperature conditions for both rigorously outgassed blown Pyrex surfaces, and for the same surfaces contaminated by the adsorption of water vapor or CO. Measurements were made by observing mass spectrometrically the mercury vapor flux leaving a substrate exposed to a flux of mercury atoms from a static vapor at fixed temperature and pressure. By appropriate manipulation of the substrate temperature both the TAC and αcr could be measured at essentially the same time and under identical conditions of vapor pressure, vapor temperature, and substrate contamination. Observed values of the TAC ranged from 0.6 to 1.0 depending on substrate condition. The αcr ranged from sightly more than unity to about seven over the temperature range from 263 °K to 225 °K. Impurity adsorption generally increased αcr, but not greatly. Results are interpreted in terms of the effect of impurity adsorption on the TAC and the substrate-nucleus contact angle.