Date of Award

1992

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Tryfon Charalampopoulos

Abstract

The complex refractive index of carbonaceous particulates is an important quantity in many areas of combustion research and practical applications. This property characterizes the radiative transport in luminous flames and plays a key role in the interpretation of conventional light scattering measurements. The purpose of the present study is to develop a technique which will allow the experimental determination of the temperature dependence of the refractive indices of carbonaceous materials and provide a predictive model for the variation of the indices. A high temperature ellipsometer system is developed that allows measurements of the intensity of polarized light reflected from the surface of a bulk sample enclosed in a temperature controlled environment. Measurements are possible in the range of angles of incidence 40$\sp\circ$ to 50$\sp\circ$ and in the temperature range 25$\sp\circ C$ to 2300$\sp\circ C$ under rough vacuum and inert conditions. The reflected light intensities are measured with respect to the variation of the angular polarization state and the data are reduced using Fourier analysis. The optical components of the experimental system are arranged to form a PSA ellipsometer. The effects of cell window birefringence, sample surface roughness and polarizer leakage on ellipsometry measurements are discussed. In addition, the role of the angle of incidence and the analyzer azimuth on numerical precision are assessed. The refractive indices of three carbonaceous samples (amorphous carbon, pyrolytic graphite and flame soot) are determined over the temperature range 25-600$\sp\circ{C}$ and the spectral range 400-700 nm. It was seen that for all three of these materials, the measured refractive index shows insignificant temperature dependence. These results differ by 30 percent or more from the predictions of the Drude-Lorentz dispersion model, which has been used extensively to predict the variation of the optical properties of carbonaceous particulates. A new set of dispersion constants is developed from the inferred refractive indices that accurately predict the indices in the temperature range 25-600$\sp\circ C$ and in the wavelength range 400-700 nm.

Pages

256

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