Effect of morphology of boron-potassium nitrate composite powders on their ignition and combustion

Composite powders of boron and potassium nitrate (PN) with spherical and irregular particle shapes are prepared by milling, respectively, with and without an emulsion as process control agent. Powders are characterized using electron microscopy, x-ray diffraction and custom ignition and combustion tests. The mixing between boron and PN is more homogeneous for the spherical powders. When coated on an electrically heated filament, spherical powders ignite at lower temperatures and have lower apparent activation energies for ignition. Spherical composite powders also exhibit shorter ignition delays and greater rates of pressure rise when laser-initiated in a closed vessel, while the total pressure rise is similar for both spherical and irregularly-shaped powders. Ignition delays of laser-heated powders increase with ambient air pressure suggesting that, in the present experiments, air does not participate in the reactions leading to ignition, but serves instead as a heat sink.