Given the constraints on typical bond energies and the commonality of final products produced from combustion of C H N O based energetic materials, the possibilities for further increases in stored potential energy and thermodynamic performance from these classes of materials are limited. Thus, modulating the energy release to achieve efficiency and effectiveness for desired applications is of great value. Investigation of nanomaterials as energetic materials began more than twenty years ago with much of the interest to increase reaction rates and reduce sensitivity. During this period, research on energetic nanoparticles was devoted to reducing the loss of energy density with metallic materials due to the naturally occurring oxide passivating layer, managing their high surface area preventing high loadings in solids, minimizing particle-particle interactions making dispersion in the gas-phase difficult, and understanding combustion mechanisms. As an outcome, novel synthesis methods of producing nanocomposites, and new fields of applications, such as micro-pyrotechnics, have developed. Yet, the research community is only beginning to understand how to manipulate and build energetic materials at the nanoscale, and what designs are optimal for desired functions. Furthermore, recognizing the difficulties for increased energy density and reduced sensitivity, the development of multifunctional and smart nanoenergetic materials is currently being researched to enable control of energy release rates and material sensitivity on demand. This research is being advanced by assembly of nanoengineered energetic materials to bulk scales by additive manufacturing, the development and application of combustion diagnostics that resolve nanometer and micron scales, and ab initio quantum chemistry and molecular dynamics calculations. The challenges that have been confronted and the directions of continuing research on nanoenergetics are presented and discussed.

Progress towards nanoengineered energetic materials

Understanding the temperature of aluminized, composite-propellant flames is critical to achieving robust rocket motor designs and developing accurate, predictive models for propellant combustion. This work presents measurements of (1) the temperature of CO (within the flame bath gas) and (2) the temperature of AlO (located primarily within regions surrounding the burning aluminum particles) within aluminized, composite-propellant flames as a function of height above the burning propellant surface. Three aluminized, ammonium-perchlorate (AP), hydroxyl-terminated polybutadiene (HTPB) composite propellants with varying aluminum particle size (nominally 31 μ m, 4.5 μ m, or 80 nm) and one non-aluminized AP-HTPB propellant were studied while burning in air at 1 atm. A wavelength-modulation-spectroscopy technique was utilized to measure CO temperature and mole fraction via mid-infrared wavelengths and a conventional AlO emission-spectroscopy technique was utilized to measure the temperature of AlO. The bath-gas temperature varied significantly between propellants, particularly within 2 cm of the burning surface. The propellant with the smallest particles (nano-scale aluminum) had the highest average temperatures and far less variation with measurement location. At all measurement locations, the average bath-gas temperature increased as the initial particle size of aluminum in the propellant decreased, likely due to increased aluminum combustion. The results support arguments that larger aluminum particles can act as a heat sink near the propellant surface and require more time and space to ignite and burn completely. On a time-averaged basis, the temperatures measured from AlO and CO agreed within uncertainty at near 2650 K in the nano-aluminum propellant flame, however, AlO temperatures often exceeded CO temperatures by  ≈ 250 to 800 K in the micron-aluminum propellant flames. This result suggests that in the flames studied here, and on a time-averaged basis, the micron-aluminum particles burn in the diffusion-controlled combustion regime, whereas the nano-aluminum particles burn within or very close to the kinetically controlled combustion regime.

Characterization of the influence of aluminum particle size on the temperature of composite-propellant flames using CO absorption and AlO emission spectroscopy

Burning rate modulation for composite propellants by interfacial control of Al@AP with precise catalysis of CuO

MXene-supported CuO nanocomposites (MCNs) were synthesized through an ice-crystal templating method without using any chemical modifiers. Nano-sized CuO particles were uniformly and stably attached to the surface of MXene nanosheets. The catalysis of the MCNs in ammonium perchlorate (AP) decomposition was studied and the results indicated that the MCNs as catalysts (2 wt%) could significantly decrease the high temperature decomposition (HTD) temperature of AP from 425.0 °C (pure AP) to 324.9 °C (AP with MCNs-3) and increase the exothermic heat from 295.2 J g−1 (pure AP) to 1272.9 J g−1 (AP with MCNs-3). With the increase of the mass ratio, the decomposition temperature decreased at first and then increased. When the ratio of MCNs-3 was 4 wt%, the HTD temperature of AP was reduced by as much as 114.4 °C and the exothermic heat was remarkably increased to 1360.6 J g−1. The high catalytic activity of MCNs-3 for AP thermal decomposition might be owing to the synergistic effect between MXene nanosheets and CuO nanoparticles. In other words, the results proved that the MXene in combination with CuO could be an effective way to enhance the catalytic performance of CuO in AP decomposition, which has potential application value in AP-based composite solid propellants.

Facile synthesis of MXene-supported copper oxide nanocomposites for catalyzing the decomposition of ammonium perchlorate

Kinetic ignition enhancement of CH 4 -air and H 2 -air diffusion flames by a nonequilibrium plasma discharge of air was studied experimentally and numerically through the development of a well-defined counterflow system. Measurements of ignition temperatures and major species, as well as computations of rates of production and sensitivity analyses, were performed to understand the kinetic enhancement pathways for ignition by plasma discharge of air. It was found that plasma discharge of air led to significant kinetic ignition enhancement illustrated by large decreases in the ignition temperatures for a broad range of strain rates. Examination of the radical and NO x production in the plasma showed that the enhancement was caused primarily by the catalytic effect of NO x . The results of numerical simulations of the counterflow burner with preheated air and NO, addition showed the existence of different ignition regimes, which appeared due to the competition between radical production by NO x and other pathways, as well as heat release. There were two ignition regimes for small concentrations of NO, and three ignition regimes for large concentrations of NO,. Numerical simulations agreed well with the experimental measurements and suggested a new strategy for plasma-assisted ignition in supersonic flow, where a combination of thermal and nonthermal plasma would work more efficiently for ignition enhancement.

Kinetic Ignition Enhancement of Diffusion Flames by Nonequilibrium Magnetic Gliding Arc Plasma

Microwave plasma enhancement of multiphase flames: On-demand control of solid propellant burning rate

This effort explores the microwave-supported plasma enhancement of a composite solid propellant flame through a novel alkali metal doping technique. The technique enables formation of a high temperature plasma within the propellant flame in order to perturb the burning rate of a composite solid propellant. The aim of this effort is to investigate the use of a microwave-supported plasma combined with an alkali earth metal doping technique to produce unsteady features within the combustion of aluminized composite solid propellant. The technique employs targeted energy deposition to the propellant flame from a subcritical microwave field through the flame-induced ionization of an alkali earth metal atom (i.e. sodium, in the form of sodium nitrate, NaNO3) and formation of high free electron sites within the flame, which serve as microwave energy deposition sites for plasma kernel formation. Equilibrium propellant combustion calculations using ionic chemistry indicate that propellant formulations containing 10-20 wt.% Al and 15-40 wt.% NaNO3 produce the strongest equilibrium flame electron concentrations. However, using a 1 kW uncharacterized multimodal cavity, this effort shows that at 1 atm pressure, low levels of doping (3.5 wt.%) can result in microwave-supported formation of plasmas within the propellant flame that are capable of enhancing propellant burning rate by as much as 67%. High speed filtered and unfiltered video indicates that plasma enhancement frequently initiates in the near-burning surface portion of the flame, where plasma kernel formation is prevalent due to both the abundance of atomic sodium (inferred regions of high free electron concentration) and high localized flame temperatures. These results also suggest the importance of burning aluminum particles as additional localized energy deposition sites via eddy current heating and as a high temperature source to enhance sodium ionization.

Microwave-Supported Plasma Combustion Enhancement of Composite Solid Propellants Using Alkali Metal Dopants

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1453&context=me_pubs

Microwave-assisted modulation of light emission intensity in alkali-pyrotechnic plumes

This effort demonstrates the development of a novel, graphene oxide nanoscale thermite composite with thermally tunable microwave ignitability. A model thermite system containing nanoscale aluminum and nanoscale iron(II) oxide in a stoichiometric ratio (30/70 wt %) was combined with sheets of graphene oxide (GO) or reduced graphene oxide (rGO) using an immiscible two-fluid sonication and tape casting process. The samples were microwave irradiated within a single-mode resonant microwave cavity to determine the microwave ignition delay. Neat thermites were found to ignite after 4.34 s of microwave illumination, whereas 30 wt % rGO thermite composite ignition delay was an order of magnitude shorter (0.43 s). For most samples (4 of 6 trials), it was found that application of a 30 wt % GO coating inhibits microwave ignition of the thermite. Thermal treatment of the GO thermite composite led to switching of thermites from unignitable to ignitable with microwave field application as short as 0.24 s due to GO reduction. Optimum heat treatment time and GO content are explored with SEM, DSC/TGA-MS, Raman, and XPS deconvolution. This effort demonstrates the use of GO and rGO addition to achieve thermally switchable microwave ignitability to electromagnetically shield or enhance nanoscale energetic ignition by microwave energy.

Stuart J. Barkley

Papers

Microwave plasma enhancement of multiphase flames: On-demand control of solid propellant burning rate

Microwave-Supported Plasma Combustion Enhancement of Composite Solid Propellants Using Alkali Metal Dopants

Microwave-assisted modulation of light emission intensity in alkali-pyrotechnic plumes

Smart Electromagnetic Thermites: GO/rGO Nanoscale Thermite Composites with Thermally Switchable Microwave Ignitability.

Flame temperature measurement of microwave-assisted aluminum particle combustion