Cars Diagnostic on a Photochemical Reactor for IR Laser Induced Production of Si and Si₃N₄ Powders

Abstract

In a flow reactor a low power (up to 50 W) CW CO₂ laser tuned at 944,19 cm⁻¹ has been focussed in order to produce Si and Si₃N₄ ultrafine powders from SiH₄ and SiH₄/NH₃ mixtures.Among possible on-line optical diagnostics, two different CARS techniques have been used to monitor the excitation process and to measure average reaction temperatures in collinear geometry. In broad-band CARS at low resolution (≈6.0 cm⁻¹) the reactant temperature is measured from the attenuation of the corresponding integrated peak intensity below and at the dissociation threshold. In narrow-band experiments the temperature reached by the dissociating reactants below and above the threshold is inferred from the spectral shape (measured with 0.2 cm⁻¹ resolution) of the envelope of rovibrational CARS transitions involved.Results obtained at the threshold for SiH₄ dissociation are in agreement with previous data on gas-phase pyrolysis in a thermal process. For the SiH₄/NH₃ reaction the difficulty in obtaining stoichiometric Si₃N₄ has been related to the cooling effect of large NH₃ addition to the SiH₄ warmed up in the laser absorption.