The emissivity of micro- and nano- particles in non-reacting environments

Pyrometry measurements of clouds of high temperature particles require an estimate of the the spectral dependence of the particle emissivity. Typically this dependence is assumed anywhere from ε ∼ λ^-2 to ε ∼ constant. Depending upon the assumption used, there is uncertainty in the temperature of 100s to 1000 K, which may be undetectable based solely on goodness of fit of spectral data. A lack of data in the visible to near-IR for alumina at temperatures above 2500 K provides the motivation for this experimental study of the emissivity of micro- and nano-alumina in a controlled, high temperature environment. A heterogeneous shock tube was used to measure the emissivity of aluminum oxide in an inert environment as a function of temperature (2000-3500 K), wavelength (550-950nm), and particle diameter (50nm-10μm). In micron sized alumina particles, the spectral dependence upon temperature transitions from decreasing with wavelength to increasing with wavelength, with the dependence being roughly grey around 3000 K. Because of local minima in the ε vs. λ curve, a power law (λ ⁿ) dependence is insuficient to describe the emissivity. However, if such a dependence is assumed, n transitions from -1.4 to 0.5 as temperature increases from 2500 K to 3500 K. Nano-sized alumina particles exhibit an even stronger spectral dependence, n is approximately -1.4 at 2678 K, but reaches as high as 2.1 at 3052 K.