Application of laser-based diagnostics for nanomaterials synthesis.

摘要

Spectroscopic laser-based diagnostics are applied in the gas-phase synthesis of nanostructrued materials to make non-intrusive, in-situ, spatially-precise measurements of gas-phase temperatures and relevant chemical species. For the nanomaterials themselves, a novel application of Raman spectroscopy is developed to characterize nanoparticles in-situ, during flame and plasma synthesis. As result, the local conditions for gas-phase synthesis can be determined for a given nanomaterial property, so that fundamental mechanisms can be revealed and process conditions can be optimized.;The synthesis configurations investigated in this work are (i) the inverse co-flow diffusion flame (IDF), (ii) the counter-flow diffusion flame (CDF), (iii) the low-pressure burner-stabilized premixed stagnation-point flame, and (iv) the inductively-coupled plasma (ICP) impinging on a cold substrate.;Spontaneous Raman spectroscopy (SRS) is used to measure local gas-phase conditions in the 2-D axi-symmetric IDF and the quasi 1-D CDF, where nanomaterials are grown on inserted substrates of various compositions. Nitrogen-diluted methane-air flames are examined. Carbon nanotubes (CNTs) are grown catalytically on metal-alloy substrates, and their morphologies are correlated with the local gas-phase temperature and the concentrations of carbon-based precursor species (e.g. C2H2, CO). Zinc oxide (ZnO) nanostructures are grown directly on zinc-plated steel substrates, and their morphologies are correlated with the local gas-phase temperature and the concentrations of oxidative (e.g. O2, H2O, and CO2) and reducing (e.g. H2) species. Computational simulations in 1-D, involving detailed chemical kinetics and transport properties, and in 2-D, using simplified kinetics and transport, are used to validate and improve the measurements.;Laser-induced fluorescence (LIF) is employed to measure the gas-phase temperature profile and OH radical species concentration distribution in a low-pressure, premixed, nitrogen-diluted hydrogen-oxygen, burner-stabilized, stagnation-point flame. Titania nanoparticles are synthesized using a metalorganic precursor. The LIF measurements are compared with computational simulations with detailed chemical kinetics and transport, to affirm the quasi 1-D flow field, as well as to investigate the effects of precursor addition and uniform electric-field application.;SRS is utilized to characterize in-situ the composition and crystallinity of nanoparticles, in aerosol form, produced in the aforementioned low-pressure premixed flame and in the ICP synthesis setup. The Stokes spectra are identified for crystalline phases of TiO2 (and Al2O 3 in a different flame setup) and c-BN based on ex-situ-taken spectra from the literature. The in-situ technique is able to delineate the phase conversion of nanoparticles (including amorphous to crystalline) as they evolve in the flow field.