Multistep thermal decomposition of energetic NMe₄B₃H₈: insights from combined kinetic and microscopic analyses

Tetramethylammonium octahydrotriborate (NMe₄B₃H₈) is a promising high-energy fuel component for solid ramjets due to its high hydrogen-boron content and excellent chemical stability. This study elucidates its physicochemical properties and complex thermal decomposition mechanism. Characterization via SEM, EDS, and FTIR revealed that the synthesized samples comprised high-purity micron-sized spherical particles with an onset decomposition temperature of approximately 227 °C. Non-isothermal decomposition behaviors were investigated using simultaneous TG-DSC. To resolve the severe overlap of exothermic peaks in DSC curves, the Fraser-Suzuki function was employed for deconvolution, isolating three distinct reaction stages. A global fit using Combined Kinetic Analysis based on the Sestak-Berggren (SB) model accurately described the entire process (R2 > 0.99), yielding activation energies of 101.20, 101.05, and 152.07 kJ mol^-1 for the consecutive stages. Mechanistic analysis, corroborated by microscopic morphological evolution, indicates that the first two stages follow a three-dimensional random nucleation and growth model (A3). The third stage marks a transition from random chain scission (L3) to a phase boundary-controlled model (R3), corresponding to the degradation of polymer intermediates into a porous skeletal residue. These findings provide a theoretical basis for formulating and predicting the combustion performance of boron-based propellants.