# Abrupt Transition of Nanothermite Reactivity: The Roles of Loading Density, Microstructure and Ingredients

## TL;DR

This paper studies how the reactivity of nanothermites changes with loading density and material composition, revealing a critical threshold that affects combustion performance.

## Contribution

The study identifies a critical loading density threshold and the role of additives in determining nanothermite reactivity.

## Key findings

- Increasing loading density reduces porosity and convective heat transfer efficiency during combustion.
- Above a critical density, combustion peak pressure drops dramatically, and combustion duration increases significantly.
- Additives like AP, HMX, and CL-20 maintain reactivity better at high loading densities compared to NC.

## Abstract

Nanothermites are widely applied as specific power sources for microscale initiators and pyrotechnics. Increasing the charge density enhances energy storage within a confined combustion chamber, but it also alters the reaction kinetics. To systemically explore this phenomenon, the combustion and pressurization characteristics of electrosprayed nanothermite-based hybrid energetic materials (THEMs) with different metallic oxides (Fe2O3, CuO, and Bi2O3) and various energetic additives (nitrocellulose (NC), octogen (HMX), ammonium perchlorate (AP), and hexanitrohexaazaisowurtzitane (CL-20)) across various loading densities were tested. The results showed that increasing the loading density decreased the porosity of the loaded nanothermites and then rapidly decreased the convective heat transfer efficiency during the combustion propagation process. When the loading density exceeded a critical value, a dramatic decrease in the peak pressure, several orders-of-magnitude decrease in the pressurization rate, and an order-of-magnitude increase in the combustion duration occurred. Due to the dual effects of the porous microstructure on heat and mass transfer, the critical density of both the electrosprayed Al/CuO/NC/CL-20 composites and their physically mixed counterparts is between 37.9 and 43.9% theoretical maximum density (TMD). Because of the different synergistic catalytic effects, the fast reactivity at the high-loading-density maintaining capacity of the applied additives was AP > HMX ≈ CL-20 > NC. Owing to their intrinsic properties of low ignition temperature and high gas yield, the Bi2O3-THEMs could maintain high-speed reactivity even at 59.7% TMD. These results provide valuable insights into the rational design and tailoring of the reactivity of nanothermites for specific applications.