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 (∼mean diameter of 1 μm) ignite much faster than micron-sized ones (30–150 μm 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.