(493d) Combustion of Multi-Stage Ball Milled Ternary B/Al/PTFE Nano-Sale Composites

Boron has high gravimetric and volumetric combustion enthalpies, almost two times higher than aluminum, and has high energy release potential in a number of applications involving air combustion such as solid fueled ramjet, blast enhancement and pyrotechnic applications. However, it is difficult to extract from boron its full combustion enthalpy due to the surface oxide layer (B₂O₃), especially in micron size boron particles. Thus, the efficiency of boron combustion is low. In contrast to boron, aluminum particles burn more favorably due to the presence in many environments of homogenous flame zones and the differing thermodynamics/physics of combustion-generated oxides. Fluorine has shown the ability to lower the ignition temperature of boron by removal of the oxide layer chemically and to accelerate boron combustion by converting the high boiling temperature oxide product to gaseous BF₃.Specifically, Al/PTFE energetics have shown favorable combustion performance such as high heat/light release and low ignition temperature. While PTFE addition may further reduce fuel content, it may be possible to develop ternary B/Al/PTFE nanoscale composite energetics that exploit the drastically higher combustion enthalpy of boron and ignition/combustion enhancements of aluminum and PTFE in order to produce fuel particles with favorable ignition and combustion energy release properties that have heating values in excess of neat aluminum.

In this work, Al/PTFE was used to improve boron combustion through use of multi-stage ball milling processes in order to explore the effects of staged ball milling on the combustion of the B/Al/PTFE system. The favorable combustion energy properties of the ternary composites are explored through equilibrium calculations and oxygen calorimetry. The ignition performance of ternary composites are measured using T-jump hot wire experiments. The combustion rate and combustion species are explored using optically accessible small volume combustion bomb experiments. In particular, we investigate differences in performance which can occur specifically from variation of ball milling staging rather than overall stoichiometry and further explore reasons for these differences through characterization of the composites and their combustion products. This work provides a fundamental understanding of reactions between B/Al/PTFE ternary compositions in systems with combustion performance that exceeds aluminum and provides insight into how the nanostructure can affect changes in combustion.