Boron particle size effect on B/KNO3 ignition by a diode laser

The effect of boron particle size on the combustion characteristics of B/KNO3 pyrotechnic mixtures is examined experimentally. Following ignition by a high power diode laser, the pressure and light emission were recorded as a function of time. Ignition delay times and combustion spectra were found to vary significantly upon changing the boron particle size. Samples containing sub-micron boron particles (similar to mean diameter of 1 mu m) ignite much faster than micron-sized ones (30-150 mu m mean diameter). The sub-micron samples are also characterized by high intensity light emission during combustion, due primarily to BO2 emission whereas in the larger boron samples black-body radiation dominates the spectra. The data were analyzed and compared with predictions of standard thermal ignition theory. It was found that a semi-inert solid model in which the laser is the sole heat source does not reproduce the experimental results. Instead a model of a single spherical boron particle embedded in a mixture of boron and potassium nitrate and heated by a laser source is introduced. In this model boron particles act as hot spots absorbing the laser energy and dissipating it in the matrix. Ignition is assumed to occur when the B/ KNO3 matrix surrounding the hot particle reaches the decomposition temperature of potassium nitrate. A reasonable correlation between experimental and predicted ignition delay times is obtained for all intensities and boron particle sizes studied. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.