K. Park, A. Prakash, A. Rai, D. Muckergee, A. McCormick and M. R. Zachariah

University of Maryland and University of Minnesota

Army-DURINT Program Review November 17, 2005

• Kinetic considerations will fall into two categories:

  • Intrinsic nanoparticle reactivity

  • Energetic nanocomposite mixtures

Single Particle Mass Spectrometer (SPMS)

:Efficient particle transport with aerodynamic lens

Relative Intensity

.25

.2

.15

.1

.05

0

0 .000002 .000004 .000006 .000008 .00001

Time, s

Aero. Sci. Tech. submitted

Single Particle Mass Spectrometer (SPMS)

Rate-limiting step ?

Furnace temperature = 1100 oC

t (sec)

15

10

5

0

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4

α (extent of conversion)

Streitz-Mintmire potential

Hawa and Zachaeriah Phys Rev. B., 2005

Diffusion in spherical geometry

∇

J=− cD− cv,

where vpD

Tk

B

2

∂ c⎧∂ c+∂ c⎡ 2 −γ⎥⎤ − 2cγ⎫

= D⎨⎬

∂ t⎩∂ r2 ∂ r⎣⎢ r⎦ r⎭

FLUX = DIFFUSION + CONVECTION

Diffusion by conc. gradient, Fick’s Law = state steady 0 & Convection induced by press gradient

γ− = ∇ pvO2

Tk

B

Diffusion Coefficient of Al

Source Expression of D, m2/s Value at 1000 K, m2/s
Gall & Lesage, 1994 )/849282(10 3.1 10 RTExpD−×= 4.1*10-35
Garcia-Mendez et al, 1980 1.5*10-19 at 773 K
Campbell et, 1999 (MD) 1.2*10-8 (for 10 nm rad.
particle) (not at 1000 K)

Mechanism τ

Aerosol Particle Mass analyzer (APM)

Aeroso entrance

High voltage

Outer electrode

Mass classified aerosol exit Z ve2

mrω2 =πdρtruerω= neEAPM

(Ehara et al., 1997) 6

500 Aluminum linescan
EDX Al
400

300

200 100 0

0 10203040506070 Position, nm

300

HRTEM of Oxidized O

Elemental line-scan

100

particle at 1000 oC

across a hollow

particle showing Al

confirming hollow

nature of particles

0

0 102030405060

Position, nm

and O in particle

• Hollow aluminum oxide particles have potential applications in drug delivery, catalysis and as low thermal conductivity material.

Onset of 1st exothermic peak is

600

560

540

[oC] -DSC

Tonset-DSC

logarithmic increasing with particle

size – TGA and DSC

[oC] - TGA

500

234

101010

Particle Size d [nm]

580

560

234

101010

520

Particle Size d [nm]

Professor Michelle Pantoya

  • TGA Results suggest that oxidation of Aluminum starts around 550 C. (Mench et al., 1998). This occurs over 10’s minuites. i.e. slow rxn not see in our experiment.

  • Above melting point, pressure gradients present in the particle may cause the oxide coating to rupture or thin the shell, and thus increasing the rate of diffusion/reaction. (Rai et al., 2004; Storaska and Howe, 2004)

  • MORE LIKELY: Above melting point molten aluminum may also diffuse out. Aluminum is a smaller ion should diffuse faster. => enhancing oxidation rate above melting point. Fast reaction regime probed by our experiment.

100 400 700 1000 1300 Oven Temperature, TOven oC

  • How to change the onset of reaction.

  • How to make Nanocomposites react faster.

  • How to make Nanocomposites react slower.

Increasing reactivity

Nickel Aluminum Magnesium

Composite particles with desired reactivity

Less Reactive Highly Reactive

Mg

1.2 1

0.8

0.6

0.4

0.2 0

Mg/(Mg+Al)

  • Studying the reactivity of Al/Mg composites using single-particle mass-spectrometry (SPMS).

  • Formation of Al/Ni composites in aerosol phase with desired (core-shell) architecture to tune the reactivity.

Ni Shell Desired Architecture

Al Core

• Advantages with the core shell structure

-Coating tunes the reactivity of the nanoparticle.

-Prevents formation of aluminum oxide coating and thus results in increased energy release per unit mass as compared to oxide coated particle.

Al + MO => Al2O3 + M +ENERGY

Rxn rate

Reaction of Al with Synthesis /source Mean particle size (nm) Wt % of Al at (dP/dt)max (dP/dt)max (psi/µs)
Fe2O3 NanoCat 80 -~0.017
Fe2O3 Aero-sol-gel 200 -~0.000096
CuO Spray-pyrolysis 250 30 % 4.45
MoO3 -~40 45% 8

1

• Strong correlation between equilibrium composition of radical oxygen and reaction rate.

• Higher the mole fraction of free O, higher the observed pressurization rate.

  • Potassium permanganate is a very strong oxidizer and makes the MIC very unstable.

  • Can we passivate the surface of this oxidizer with a mild oxidizer?

Weak Oxidizer

Strong Oxidizer

Metal oxide Fuel metal aero-sol-gel aerosol nanoparticles nanoparticles

Advantages

  • Fast aerosol processing of nanoenergetic composite particles

  • Enhance the interconnected mixing between fuel and oxidizer nanoparticles

Creation of High Surface Area Metal Oxides

Aero-Sol-Gel Synthesis

Objective: Marry Aerosol Processes with Sol-Gel chemistries to build new types of particle level architecture.

  • Produce porous matrix on nanoparticles ( on-the-fly)

  • Explore range of porosities and gel architectures

Approach: Carry out bulk reaction in a micro-droplet to obtain oxidizer

particles in the powder form.

Process window

Surface Area Measurements

Control on surface area by varying concentration

1000 nm droplet from atomizer Drying