10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Measurement of the Size Distribution of Stable Clusters during Silane Pyrolysis in a Helium Atmosphere
MIGUEL VAZQUEZ PUFLEAU, Yang Wang, Elijah Thimsen, Pratim Biswas, Washington University in St. Louis
Abstract Number: 416 Working Group: Aerosol Physics
Abstract Nucleation is the first stage required for gas phase species to become solid or liquid. It is highly relevant for both industrial and environmental applications, yet the phenomenon is not well understood. This has constrained optimal design of efficient processes in numerous industries including semiconductors, photovoltaics and ceramics, and limited the accuracy of weather modeling and forecasting. Current approaches to describe the initial stages of aerosol formation are based on the classical nucleation theory (CNT), which predicts the existence of a critical size above which particles are stable and grow larger and below which particles are unstable and can easily break apart. CNT was conceived for physical systems. But, by analogy of principles, the concept of chemical equilibrium as given by the free Gibbs energy has been used to predict critical nucleus sizes. For silane, at given process conditions, stable silicon hydride molecules, with a given amount of silicon atoms, have been determined based on ab initio calculations. However, experimental evidence of the mobility of such silicon clusters produced during silane pyrolysis was not available.
In the present work, we measured for the first time the mobility of sub nm silicon hydride stable clusters using a half mini differential mobility analyzer. The clusters were produced during silane pyrolysis at different conditions in a flow reactor. The clusters measured at different temperatures displayed a peak at around 0.75 nm mobility. The effect of residence times was also assessed. Transmission Electron Microscopy (TEM) characterizations and particle size analysis were undertaken to provide insights into the larger particle mode at conditions with partial silane conversion in which other standard online characterization instruments could be biased due to the unreacted precursor interfering with the measurement. In addition, a dependence of stable cluster abundance as a function of process parameters was observed, analyzed and compared with silicon hydrides predicted to be stable from ab initio calculations reported in the literature, showing good agreement.