Surface Chemistry Promoting Energetic Material Combustion
Abstract
Harnessing more of the abundant chemical energy stored in an aluminum (Al) particle (i.e., theoretically estimated specific energy is 31 MJ/kg) will have tremendous benefits for the use of Al as an energy generating material in many applications. Micron scale Al particles are notoriously plagued with incomplete combustion and sluggish ignition. The goal of this project is to improve the reactivity of micron scale Al fuels that will have wide sweeping impact on every ordnance system that includes Al. To accomplish this goal, this research will focus on examining the surface chemistry of aluminum particles: specifically, reactions at the interface of the alumina shell and the oxidizing agent. The exothermic contribution from alumina surface reactions with halogens will be explored toward functionalizing Al particles with greater reactivity. Through deeper understanding of surface hydroxyl moieties and the knowledge of aluminum and oxygen atom coordination in the crystal lattice of alumina, models will be developed that describe local electronic charge and the acid-base characteristics of these sites. Surface-sensitive, non-linear vibrational spectroscopy will reveal properties of aluminum oxide surface hydroxyl groups that include the structure within the water hydration layers. Surface hydroxyl coverage and acid-base properties play a very important role in promoting molecular adsorption and heterogeneous catalytic chemical reactions. Even with background understanding of alumina and its application in the field of heterogeneous catalysis, there is little understood about the surface chemistry of aluminum oxide surrounding an aluminum core particle from the energetic material science perspective. In particular, this knowledge when applied to functionalizing the inherent passivation shell may transform harvesting the potential chemical energy stored in a micron scale Al particle. The objectives of this research are to (1) design and perform experiments that identify hydroxyl species that promote alumina surface reactivity; (2) synthesize aluminum particles with tailored surface features that promote pre-ignition reaction kinetics; and, (3) characterize macroscopic combustion behavior of composites to resolve the influence of surface chemistry on global reaction mechanisms. A combination of experimental methods using spectroscopy (FTIR), equilibrium diagnostics (DSC-TG), and non-equilibrium combustion studies (pressure cells, Parr Bomb, flame speed) will be used to accomplish these objectives. Numerical simulations are also planned to complement the experimental work. This project will answer scientific questions that include: which surface hydroxyl sites are more reactive and how can a particle surface structure be tailored to promote exothermic surface reactions and enhance overall particle reactivity. The scientific contribution from this study will extend beyond aluminum fuel particles toward other metal fuels with catalytic oxide shells that may be enhanced in similar ways.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Oct 06, 2018
- Source ID
- W911NF1710387
Entities
People
- Michelle L. Pantoya
Organizations
- Army Contracting Command
- Texas Tech University
- United States Army