Vapor Pressure of Silicon Monoxide

Abstract

Silicon monoxide is a material that is used extensively in the glass and metallurgical industries. In addition, silicon monoxide is also particularly important to the field of astrophysics where it is theorized to play a vital role in the production of metal silicate dust grains in the condensing outflows of dying stars. In this work, the evaporation coefficients and vapor pressure of commercially available amorphous silicon monoxide were measured over the temperature range (1301 to 1519) K using a Knudsen effusion cell and a commercial, thermogravimetric balance. A second- and third-law analysis of the vapor pressure data yielded (351 ± 11) and (359.1 ± 2.0) kJ·mol−1 for the enthalpy of the vaporization reaction at 298.15 K, respectively. It is also shown that a thermodynamic assessment of silicon monoxide vapor pressure that has been used in the modeling of astrophysical condensation at 1000 K and below greatly overpredicts silicon monoxide vapor pressure.

Laboratory measurement of optical constants of solid SiO and application to circumstellar dust

“…in the temperature range 1175 K to 1410 K. Similar results for α are found by Ferguson & Nuth (2008 and are essentially confirmed by our own measurements (Wetzel et al 2012b, Gail et al, in prep. ) which can be approximated by…”

“…Calculations with T c = 800 K result in model spectra that are not significantly different. The old results of Gail & Sedlmayr (1986) seem to indicate a temperature for onset of SiO condensation as low as 600 K, but this conclusion can no longer be maintained since that calculation is based on older vapour-pressure measurements of solid SiO for which it is now known that they have to be revised downwards substantially (Ferguson & Nuth 2008;Wetzel et al 2012b;Ferguson & Nuth 2012), which increases the condensation temperature.…”

“…New determinations of the vapour pressure of solid SiO (Ferguson & Nuth 2008Wetzel et al 2012b) have now shown that the older measurements were seriously in error and had strongly overestimated the vapour pressure. Our own results (Wetzel et al 2012b, Gail et al, in prep.…”

A Model of Silicate Grain Nucleation and Growth in Circumstellar Outflows

“…S is calculated using the vapor pressure measurements presented in Ferguson & Nuth (2008), and a dilution factor, W, is introduced to account for departure from vibrational equilibrium, following Nuth & Ferguson (2006), who suggest that such a departure can promote nucleation. The dilution factor is used in the calculation of the vibrational temperature, which is given by…”

“…More recent measurements of the SiO vapor pressure (Nuth & Ferguson 2006;Ferguson & Nuth 2008) indicate that the vapor pressure curve commonly used (Schick 1960) is significantly in error when applied to circumstellar outflows. The lower vapor pressures measured by Nuth & Ferguson are not, by themselves, enough to promote SiO nucleation, but a consideration of vibrational disequilibrium (Nuth & Donn 1981;Nuth et al 1985) together with the new vapor pressure measurements suggests that silicate grains can nucleate in circumstellar outflows.…”