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OLD GUARD TECHNICAL WEB PAGE COMPETITION

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,

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OLD GUARD TECHNICAL WEB PAGE COMPETITION

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  1. 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

  2. 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+MoO3→ Al2O3+Mo • Use nanocomposite powders to achieve high reaction rate • Determine the minimum concentration of MoO3 necessary for rapid and complete combustion of aluminum with external oxidizer • Target applications: metallized propellants, thermobaric explosives

  3. 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: MoO3

  4. Technical approach • Prepare a set nano-composite materials xAl+MoO3using 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

  5. Time of Reaction For Reactive Milling Temperature of Milling Vial Milling Time Arrested Reactive Milling • Starting components: powders of Al and MoO3 • 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

  6. Al2O3 Mo Al MoO3 Epoxy Nanocomposite morphology: 4Al+MoO3 (A)

  7. Nanocomposite morphology: 4Al+MoO3 (B) Nano-scale network of reactive boundaries

  8. Nanocomposite morphology: 8Al+MoO3

  9. Phase analysis of nanocomposite powders XRD Analysis: undesirable partial reaction reducing MoO3 begins

  10. Particle size distributions Shows average size to be too large.

  11. Reducing particle sizes for 8Al+MoO3 Actual mixture used. • Wet milling found ineffective • Sifting used • Sifting performed under hexane in a glovebox to ensure safety Average size is too large. • Sifting reduces the volume fraction of larger particles to approximate reference size • The position of the size distribution mode changes only slightly

  12. Test Setup : Constant Volume Explosion • Constant volume explosion (9.2 liter explosion vessel) • Pressure tracesand ignition pulse recorded with digital oscilloscope

  13. 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-MoO3 powders selected to ensure the constant fuel volume (equal to that of pure Al) to produce legitimate comparisons of the reaction rates and energies

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

  15. Fuel Load Matrix • Gas Composition: 22.5% O2 , 77.5% N2 • 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

  16. Pressure Traces Al 4Al+MoO3 8Al+MoO3 Estimated flame temperatures, K: 1780 1730 1350

  17. Pressure differentials (dP/dt)

  18. 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+MoO3 and followed by that for 4Al+MoO3 • 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+MoO3 performed better than other materials • Equivalent heat of reaction is proportional to (Tflame-Troom)/mAl • the heat of reaction can be estimated as (mair+mfuel)Cp∙ DT/mAl

  19. Conclusions • Powders of aluminum-rich thermites with MoO3 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 MoO3 • 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

  20. 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

  21. 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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