Effect of alkali metal perchlorate and iodate type on boron ignition: The role of oxidizer phase change

Abstract

Boron often demonstrates low reactivity despite having high gravimetric and volumetric energy density. Here we explore a family of oxidizers with very similar chemistries to understand controlling mechanisms in boron ignition. Alkali metal perchlorates as well as iodates (LiClO4, NaClO4, KClO4, LiIO3, NaIO3 and KIO3) were investigated under rapid heating conditions (∼105 K/s) by temperature-jump (T-Jump) ignition and time-of-flight mass spectrometry (TOFMS). T-Jump ignition tests in atmospheric pressure argon show that B/LiClO4, B/NaClO4, B/LiIO3, and B/NaIO3 ignite while B/KClO4 and B/KIO3 do not, despite the near identical chemistries. T-Jump TOFMS results demonstrate a dramatic increase of Cl and I containing species for Li and Na systems relative to K when B is added. Thermogravimetry/differential scanning calorimetry (TGA/DSC) analysis reveals that there is a temperature gap between melting and decomposition onset of LiClO4, NaClO4, LiIO3, and NaIO3, respectively, whereas KClO4 and KIO3 melt and decompose concurrently. We find that the larger interval between melting and decomposition renders more time for the oxidizer to melt and surround/wet the nanoscale B before decomposing, providing instant access to oxygen for B nanoparticles that results in vigorous ignition. This study of a set of equivalent oxidizers illustrates the important role of transport phenomena and physical properties have on ignition.