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Laser-based diagnostics for the in situ characterization of non-soot nanoparticles

Subject Area Chemical and Thermal Process Engineering
Energy Process Engineering
Term from 2013 to 2021
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 222540104
 
Final Report Year 2021

Final Report Abstract

In this project, non-perturbative optical techniques for the in situ particle diagnostics of gasborne nanoparticle synthesis of non-soot materials has been performed. The intention is on the one hand to develop suitable spectroscopic methods and on the other hand to use these methods to further characterize the optical properties of these materials. The main accomplishments in this proposal period was the in situ characterization of Si and Ge nanoparticles, mainly in terms of their optical properties. The particles are synthesized in an optically accessible microwave plasma reactor using silane and/or germane as precursor gases. Extinction/emission measurements were performed with a broadband laser-driven light source (LDLS), a spectrometer, and an EMCCD camera. The optical properties of liquid Si and Ge were modeled based on line-of-sight attenuation (LOSA) spectra using Drude and Mie theory. The uncertainties in the properties for Ge also were determined by Bayesian analysis. From the significantly different extinction spectral signatures, it was possible to determine the particle phase (liquid/solid) in the probe volume – which is a novel and previously not expected possibility. Furthermore, the gas-phase temperature and the particle volume fraction of liquid silicon and germanium nanoparticles were retrieved using tomographic reconstruction of lineof-sight emission and extinction spectroscopy. The experiments were performed by either using a focused beam through different locations of the particle stream or with a diffuse light beam of the LDLS source. Furthermore, the line-of-sight averaged gas-phase temperature was determined from the wavelength-dependent incandescence of hot particles. For gas-phase temperature measurements, some oxygen was introduced into the reaction zone to produce SiO, and the measured absorption spectra allowed us to determine the gas-phase temperature by spectral fitting. The measurements were also performed for silane/germane precursor mixtures for the synthesis of nanoparticle composites to later determine their optical properties. Along with the in situ characterization, ex situ TEM analysis of the particle was performed by collecting the samples using a pneumatically-driven thermophoretic sampler at the same measurement location as for LOSA measurements. An EDX analysis shows the composition of mixture of Si and Ge nanoparticles. Moreover, the purity of the Si nanoparticles was validated by recording the Raman spectrum of Si powder in a different experiment.

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