BORON PARTICLE SIZE DEPENDENCE OF IGNITION CHARACTERISTICS OF B/KNO_3 MIXTURES IGNITED BY A DIODE LASER

摘要

The effect of boron particle size on the ignition and combustion characteristics of B/KNO_3 pyrotechnic mixtures is examined experimentally. Following ignition by a high power diode laser, 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 (～1 μm) ignite much faster than micron-sized ones (30-150 μm). The sub-micron samples are also characterized by high intensity light emission during combustion, due primarily to BO_2 emission whereas in the larger boron samples black-body radiation dominates the spectra. The data were 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; adding convective heat transfer from particle combustion yields a satisfactory fit to the data. This additional heat source can contribute as much heat flux as the laser source itself, and unlike the laser source is affected by external condition such as pressure and surrounding air flow.