Experimental studies of the vapor phase nucleation of refractory compounds. I. The condensation of SiO

Abstract

An experimental apparatus has been built in which a quantitative study of the vapor phase nucleation of refractory particles can be performed. With this equipment, measurements have been made of the critical partial pressure (Pc) of SiO necessary to initiate avalanche nucleation in the SiO–H2 system as a function of ambient temperature in the range 750 K⩽T⩽1000 K. The condensate produced at low temperatures is Si2O3 rather than the more stable oxides SiO or SiO2. At high temperatures a mixture of Si2O3 and amorphous SiO2 is found. The SiO2 may be the result of annealing or oxidation on the collector after the particles had been formed as less stable clusters in the vapor. Analysis of Pc vs T using classical nucleation theory yields a surface energy of ∼500 ergs/cm2 for the particles. Despite the fact that this is in reasonable agreement with values for Si2O3 obtained from the literature, several inconsistencies in the analysis are noted. It is argued that classical nucleation theory is not applicable to this system.