OLD GUARD TECHNICAL WEB PAGE COMPETITION Prepared By: Ben Tsui, College of Staten Island Assisted by: Kushal Jain, College of Staten Island Combustion of Off-Stoichiometric Al-MoO3 Nano-composite Powders in Dry Air Original research by: Soumitri S. Seshadri, Swati Umbrajkar, Vern Hoffmann, and Edward L. Dreizin

ABSTRACT Aluminum as a fuel –Advantages High oxidation enthalpy High combustion temperature Environmentally benign products –Problems Ignition delay leading to agglomeration of molten particles –Low bulk burn rates, incomplete combustion Approach –Enable rapid ignition assisted by thermite reaction: Al+MoO 3 → Al 2 O 3 +Mo –Use nanocomposite powders to achieve high reaction rate –Determine the minimum concentration of MoO 3 necessary for rapid and complete combustion of aluminum with external oxidizer Target applications: metallized propellants, thermobaric explosives

Proposed Concept ? Nanocomposite thermite: A very rapid reaction No free Al left to react with external oxidizer –AP in propellants –Air in thermobaric explosives Metal-rich thermites: Free Al remains Find the minimum concentration of thermite oxidizer required to achieve the high reaction rates? This work: Fuel: Al; Oxidizer: MoO 3

Technical approach Prepare a set nano-composite materials xAl+MoO 3 using Arrested Reactive Milling (ARM), with x > 4 (stoichiometric x=2) Characterize prepared powders –Particle size distributions; low angle laser light scattering, Coulter LS 230 –Particle composition, morphology: XRD, SEM Carry out equilibrium thermodynamic calculations for combustion of the prepared materials in air –Determine optimum metallic fuel loads for the constant volume explosion experiments –Use NASA CEA code Perform constant volume explosion experiments to assess the reaction rate and completeness for different materials –Use results for spherical Al powders as a baseline

Arrested Reactive Milling Starting components: powders of Al and MoO 3 Process –Mill with steel balls to prepare nanocomposite powder –Use hexane as a process control agent –Stop milling before the thermite reaction is mechanically triggered Product: micron-sized nanocomposite powders Time of Reaction For Reactive Milling Milling Time Temperature of Milling Vial

Nanocomposite morphology: 4Al+MoO 3 (A) Al MoO 3 Epoxy Mo Al 2 O 3

Nanocomposite morphology: 4Al+MoO 3 (B) Nano-scale network of reactive boundaries

Nanocomposite morphology: 8Al+MoO 3

Phase analysis of nanocomposite powders XRD Analysis: undesirable partial reaction reducing MoO 3 begins

Particle size distributions Shows average size to be too large.

Reducing particle sizes for 8Al+MoO 3 Wet milling found ineffective Sifting used –Sifting performed under hexane in a glovebox to ensure safety Sifting reduces the volume fraction of larger particles to approximate reference size The position of the size distribution mode changes only slightly Actual mixture used. Average size is too large.

Test Setup : Constant Volume Explosion Constant volume explosion (9.2 liter explosion vessel) Pressure traces and ignition pulse recorded with digital oscilloscope

Selecting experimental conditions Use comparable particle sizes Fuel load selected to enable the maximum flame temperature at given initial pressure (1 atm) selected based on the vessel pressure rating Mass of Al determined based on thermodynamic equilibrium calculations Mass of Al-MoO 3 powders selected to ensure the constant fuel volume (equal to that of pure Al) to produce legitimate comparisons of the reaction rates and energies

Thermodynamic Calculations CEA Code by Gordon and McBride (NASA), constant volume condition For pure Al burning in air, T max occurs at an equivalence ratio of  = 1.02 For 9.2 liter vessel filled with air at 1 atm, this corresponds to 2.89 g load of Al

Fuel Load Matrix FuelMass Load, gAluminum Mass, g Al2.89 4Al+MoO Al+MoO Gas Composition: 22.5% O 2, 77.5% N 2 Mass loads of thermite mixtures are determined using their theoretical maximum densities and selecting the mass, for which the volume is equal to that of 2.89 g of pure Al i.e. constant volume. The overall mass of available Al is smaller for thermite mixtures

Pressure Traces Al4Al+MoO 3 8Al+MoO 3 Estimated flame temperatures, K:

Pressure differentials (dP/dt)

Discussion of Results Flames produced by nanocomposite thermites propagate much faster than those produced by pure Al powder Reaction pressure is highest for Al, closely followed by that for 8Al+MoO 3 and followed by that for 4Al+MoO 3 The reaction pressures indicate that the flame temperatures are much lower than calculated adiabatic flame temperature Despite larger size particles and smaller mass of Al available, the nanocomposite powder 8Al+MoO 3 performed better than other materials Equivalent heat of reaction is proportional to (T flame -T room )/m Al –the heat of reaction can be estimated as (m air +m fuel )C p ∙  T/m Al

Conclusions Powders of aluminum-rich thermites with MoO 3 as an oxidizer are produced by Arrested Reactive Milling Produced powders comprise micron-sized, pore-free particles Each particle is a nanocomposite of Al and MoO 3 Powder size distributions measured using low-angle laser light scattering –Any coarser powder is sieved to approach the size distribution of pure Al powder used as the reference Constant volume explosion experiments used to compare reaction rates and energies of different materials –Air is used as an oxidizer –Fuel load for Al is selected based on thermodynamic calculations showing the maximum adiabatic flame temperature –Fuel loads for nanocomposite powders selected to match the volume of solid fuel for Al

Applicability Due to these higher reaction efficiencies, nanocomposite thermites prove to be effective components for propellants, thermobaric explosives and pyrotechnics. This will result in safer, more reliable and higher efficiency products. The reaction energies for Al and 8Al+MoO3 are nearly the same The reaction rates for nanocomposite fuel-rich thermite powders are much higher than that for Al

Additional Research Effects of even higher concentrations of Molybdenum Oxide will be investigated Reaction completeness will be determined by analysis of collected combustion products Ignition kinetics for metal-rich thermites will be quantified by thermal analysis and additional experiments Additional thermite compositions (e.g., using CuO as an oxidizer) will be studied

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