Ignition and combustion of boron particles coated by modified materials with various action mechanisms

Abstract

Coating boron particles with materials having multiple mechanisms of action is a promising way to promote the ignition and combustion properties of boron. The differences in thermal oxidation properties and ignition combustion characteristics of micron-sized boron composite particles (namely, AP@B, FR@B, and GAP@B) coated with 20 wt% ammonium perchlorate (AP), fluoropolymer (FR), or glycidyl azide polymer (GAP) were comparatively investigated using TG–DSC and CO2 laser ignition test systems. The results showed that the starting reaction temperature and activation energy of the composite particles were significantly reduced by the lower pyrolysis temperatures of the three modified materials. However, their oxidation phase lagged behind that of the original boron particles, most likely because the coating agglomerated 27% of the fine boron particles (<0.1 μm) into micron-sized particles. In addition, the slow heating approach did not allow the particles to reach the ignition temperature immediately and may indirectly aggravate the oxide layer thickness. Nevertheless, the coating modifier effectively promoted energy release (the heat release was increased by 93.76%, 28.65%, and 24.20% for FR@B, GAP@B, and AP@B, respectively). Generally, the ignition delay time (ti) decreases, whereas the combustion time (tc), self-sustaining combustion time (tf), maximum combustion temperature (Tmax) and burnout rate (α) increase after the coating of modified materials. First, AP decomposition produced O2 and other gases to provide an oxidant for boron combustion and disperse the particles into space, which promotes extremely rapid energy release. Thus, the Tmax and α of AP@B were significantly enhanced. Second, FR pyrolysis gas production can vaporize B2O3 and then promote the exposure of internal boron to react with oxygen. Thus, the tf and α of FR@B were significantly enhanced. However, the Tmax value was not the highest since the highest calorific value (up to 15,094 J•g−1) of FR@B is released at a slow rate. Finally, GAP can significantly shorten ti due to its lower activation energy, and this modification effect can be significantly improved by increasing the O2 content. Since boron is characterized by high oxygen consumption, increasing the oxygen partial pressure can significantly promote ignition and combustion. The Tmax values of four samples in a mixed O2–N2 environment under 1.0 MPa even exceeded the boiling point of B2O3, which is favorable for the reaction of internal boron with oxygen due to the gasification of the oxide layer at this time.