NEEM MURI

Multi-Scale Modeling ofMulti-Scale Modeling ofMulti-Scale Modeling of Nano Aluminum Particle Ignition and Combustion

Multi-Scale Modeling of Nano Aluminum Particle Ignition and Combustion

Vigor Yang, Dilip Sundaram, and Puneesh Puri Georgia Institute of Technology

Atlanta, GA 30332-0150and

The Pennsylvania State UniversityUniversity Park PA16802University Park, PA16802

Presented at 2010 MURI NEEM Program Reviewg

Aberdeen, Maryland

March 15, 2010

NEEM MURI

Multi-Scale Modeling of N Al i P ti l I iti d C b ti

Multi-Scale Modeling of N Al i P ti l I iti d C b tiNano Aluminum Particle Ignition and CombustionNano Aluminum Particle Ignition and Combustion

• Development of a unified model for ignition and combustion of aluminum i l li bl ll lparticles applicable at all scales

• Investigation of the essential difference in physiochemical mechanisms atmicro and nano scales St d f th ll ti b h i f ti l d t b ti i fl i t• Study of the collective behavior of particle dust combustion in flow environments

• Coupling the studies of the USC group at meso/micro scalesGT

Quantum Micro MesoNano Macro

USC

Q

Length (m)

10-12 10-9 10-6 10-3 100

NEEM MURI Various Stages of Particle Behavior

RoutO Anions Rout Rout

Rin

δAl Cations

Rint

δO2 Molecules

Rint

δO2Molecules

Phase TransformationsStage I

(particle heating/phase transformations)

Rin

Stage II(core melting and ignition due to melting/cracking)

Stage III(heterogeneous reactions/healing of cracks)

O2 Molecules

O id

Al (g)

oxidizer Al2O3(s)Al(s)Al(l)

Stage IV

melting of oxide layer to formcap (micro)

particle consumed due to heterogeneous reactions (nano)

Detached flame front

Oxide cap

Stage Vdetached flame front (micro)

Al2O3(s)Al(s)Al(l) detached flame front (micro)Al(l)

NEEM MURI Ignition Temperature of Single Aluminum Particle in Air as Function of Particle Size

Micron and larger size particles:

• For particles ( > 100 microns)

Micron and larger size particles:

• For particles ( > 100 microns)3500

Parr et al. [11]B li t l [36]

3500Derevyaga et al. [30]M h t l [29]For particles ( > 100 microns),

ignition occurs at temperature near the melting point of aluminum oxide (2350 K) F i l (1 100 )

For particles ( > 100 microns), ignition occurs at temperature near the melting point of aluminum oxide (2350 K) F i l (1 100 ) e,

K

2500

3000

Bulian et al. [36]Assovskiy [35]Yusasa e tal. [33,34]Brossard et al. [32]Ermakove tal. [31]

e,K

2500

3000

Merzhanov et al. [29]Friedman et al. [27,28]Trunov et al. [15]CurveFit

• For particles (1~100 microns), ignition over a wide range of temperature from 1300 to 2300 K

• For particles (1~100 microns), ignition over a wide range of temperature from 1300 to 2300 K

Tem

pera

ture

2000

2500

Tem

pera

ture

2000

2500

Nano size particles:

• Ignition reported to occur at temperature as low as 900 K

Nano size particles:

• Ignition reported to occur at temperature as low as 900 K

Igni

tion

T

1000

1500

Igni

tion

T

1000

1500

• Trunov et al. (2005) suggested that aluminum oxidation and polymorphic phase transformation of the alumina shell are responsible

• Trunov et al. (2005) suggested that aluminum oxidation and polymorphic phase transformation of the alumina shell are responsible 10-2 10-1 100 101 102 103 104500

1000

10-2 10-1 100 101 102 103 104500

1000

of the alumina shell are responsible for these diverse ignition temperatures

of the alumina shell are responsible for these diverse ignition temperatures

Particle Diameter, μmParticle Diameter, μm

NEEM MURI Burning Time of Single Aluminum Particle in Air as Function of Particle Diameter

Micron and larger size particles:

B i d diff i

Micron and larger size particles:

B i d diff i105

Wilson and Willams [27]• Burning under diffusion-controlled conditions

• Beckstead’s particle burning time correlation based on various

• Burning under diffusion-controlled conditions

• Beckstead’s particle burning time correlation based on various s 103

104

Wilson and Willams [27]Wong and Turns [29]Prentice [28]Olsen and Beckstead [30]Hartman [26]Friedman and Macek [21]

experimental measurements:experimental measurements:1.8

0.2 0.11 0

beff

dC T p X

τ =

ngtim

e,m

s102

103 Friedman and Macek [21]Davis [25]Parr et al. [9] (T0=1500 K)Parr et al. [9] (T0=2000 K)Models

d1.8

Nano size particles:

• Burning under kinetically-controlled conditions

Nano size particles:

• Burning under kinetically-controlled conditions

ffB

urni

n

100

101

1500 K

controlled conditions• d1-model from theoretical

prediction; however, d0.3 law based on experimental data of P l 2003

controlled conditions• d1-model from theoretical

prediction; however, d0.3 law based on experimental data of P l 2003 10-2 10-1 100 101 102 103

10-1

10d0.32000 K

3500 K

Parr et al., 2003 Parr et al., 2003 Particle diameter, μm10 10 10 10 10 10

NEEM MURIResearch Topics

• Phenomenological development of aluminum particle ignition and combustion over a wide range of length scales based on g gcharacteristic time and dimensional analysis.

• Molecular dynamics simulations of small aluminum particles– the effect of pressure and voids

the effect of the oxide shell– the effect of the oxide shell – the effect of a nickel coating

• Macroscale modeling of flame propagation– the ignition and combustion processes of nAl with liquid

oxidizers

NEEM MURI Ignition Criteria Heating, Cracking, and Healing of Oxide Layer

• Ignition at nano scale occurs at much lower temperature (~ 940K) than at micron scale

• Ignition at nano scale occurs at much lower temperature (~ 940K) than at micron scale

• Two schools of thoughts on ignition criteria at nano scale:– cracking due to thermal stress– polymorphic phase transformation of oxide layer

• Two schools of thoughts on ignition criteria at nano scale:– cracking due to thermal stress– polymorphic phase transformation of oxide layerp y p p y

• Characteristic time scales for heating, melting, and healing must be considered

• Fourier number ~ A/V

p y p p y• Characteristic time scales for heating, melting, and healing must be

considered• Fourier number ~ A/V

20* fg

meltp f

D ht

c Tα=

Δ

• If the characteristic time for shell growth through direct oxidation is small as compared to that of melting and cracking then the oxidation for the

Rout

δO2 Molecules2

* 0heat

f

Dtα

=

p f

2dN P Dπ

melting and cracking, then the oxidation for the rest of time can be modeled as that of diffusion through the layer.

• If the characteristic time for shell growth is larger the oxidation process must be modeled as

Rint

O2 Molecules

R2 2

2O O pdN P D

dt mKT

π

π=

larger, the oxidation process must be modeled as the direct attack of oxygen on aluminum surface with cracking.

Rin

NEEM MURI Mode of CombustionDiffusion vs. Kinetically Controlled

dρ

Diffusion ControlledDiffusion Controlled Kinetically ControlledKinetically Controlled2dρ

• To determine the dominant combustion mechanism a Damkohler number• To determine the dominant combustion mechanism a Damkohler number

0,

,2p

b kinp o

dt

MW kPXρ

∞

=0,

,8 ln(1 )p

b diffO

dt

D iYρ

ρ ∞

=+

• To determine the dominant combustion mechanism, a Damkohler number, Da, for surface reaction is defined as

• To determine the dominant combustion mechanism, a Damkohler number, Da, for surface reaction is defined as

, 0 ,b diff p ot MW kPd XDa ∞= =

• Small particles at low pressures burn under kinetically controlled conditions • Small particles at low pressures burn under kinetically controlled conditions

, ,4 ln(1 )b kin o

Dat D iYρ ∞

= =+

• Large particles and high pressures favor diffusion controlled mechanism.• The characteristic burning time follows d1 law and is inversely proportional

to pressure under kinetically controlled mechanism. The burning time

• Large particles and high pressures favor diffusion controlled mechanism.• The characteristic burning time follows d1 law and is inversely proportional

to pressure under kinetically controlled mechanism. The burning time p y gfollows the d2 law and is independent of pressure under diffusion controlled mechanism.

p y gfollows the d2 law and is independent of pressure under diffusion controlled mechanism.

NEEM MURI Mode of CombustionNano vs. Micron Scale

• Heterogeneous oxidation more favorable for nano particles as compared to a detached fl f i ti l

Rout

δ

O Anions

Al Cations

Al2O3(s)Al(s)Al(l)

flame for micro particles.• In case of direct oxidation, the process is

kinetically controlled due to small diffusion time scales.

Heterogeneous(diffusion controlled)

Rin

Phase Transformations R• In case of heterogeneous oxidation through oxide layer, the process is diffusion controlled due to slow diffusion of Al cationsor O anions through the oxide layer.

Rout

Rint

δO2 Molecules

• The melting and boiling points of aluminum are 933 and 2791 K, respectively; and for alumina, the melting and boiling points are2327 and ~3700 K.

Heterogeneous(kinetically controlled)

Rin

idi• For micron-scale particles, as oxide melts, Al can vaporize forming a detached flame. But if the rate of heterogeneous oxidation is very fast, particle can self heat to melting point

d t d i h tOxide cap

Al (g)

oxidizer

and get consumed in a pure heterogeneous fashion. Homogeneous (microscale)Detached

Flame front

NEEM MURI Fourier Analysis)

Al2O3(s)Al(s)Al(l)

O Anions

Al Cations

O Anions

Al Cations

me

scal

e(p

s

6

8

Core Melting/Fragmentation of Shell

Al Cations

Phase Transformations

Al Cations

Phase Transformations

acte

ristic

tim

4

Phase Transformations

O2 Molecules

Phase Transformations

O2 Molecules

O id hi k ( )

Cha

ra

1 2 3 4 5 6 7 8 9 10

2Melting of Shell/Heterogeneous Oxidation of core

Oxide thickness (nm)O2 Molecules

Al (g)

oxidizer

Detached flame front

Oxide cap

NEEM MURIVarious Regimes of Particle Ignition

NEEM MURI Characteristic Time Scale Study (I)

tmelt c> treac V1000 K, 1 atm

O Anions

Al Cations

Al2O3(s)Al(s)Al(l)

O Anions

Al Cations

O Anions

Al Cations

s(nm

)

101

tmelt,s=tmelt,ctmelt,c=treactmelt,s=treac

melt,c reac

tmelt c< tmelt sIIIV

V Al Cations

Phase Transformations

Al Cations

Phase Transformations

Al Cations

Phase Transformations

eth

ickn

es

100

melt,c melt,s

IIV V

Phase TransformationsPhase Transformations

O2 Molecules

Phase Transformations

Oxi

de

10-1

tmelt,s< treac

IIIVIVI

Core size (nm)20 40 60 80 100 O2 Molecules

Al (g)

oxidizer

Al (g)

oxidizer

Detached Flame front

Oxide cap

Detached Flame front

Oxide cap

NEEM MURI Characteristic Time Scale Study(Effect of Pressure)

m)

1

tmelt,s=tmelt,ctmelt,c=treactmelt s=treac

tmelt,c> treacV1000 K, 5 atm

m)

tmelt,s=tmelt,ctmelt,c=treac

tmelt,c> treacII V1000 K, 10 atm

deth

ickn

ess(

nm

100

101 melt,s reac

tmelt,c< tmelt,s

I

IIIV

IVV

deth

ickn

ess(

nm

100

101 tmelt,s=treac

tmelt,c< tmelt,s

III

IV

IV V

Core size (nm)

Oxi

20 40 60 80 100

10-1tmelt,s< treac

I

III VIVI

Core size (nm)

Oxi

d

20 40 60 80 100

10-1 tmelt,s< treac

I

VIVI

ess(

nm)

101

tmelt,s=tmelt,ctmelt,c=treactmelt,s=treac

tmelt,c< tmelt,s

V

1000 K, 50 atm

ess(

nm)

101

tmelt,s=tmelt,ctmelt,c=treactmelt,s=treac

tmelt,c< tmelt,s

V1000 K, 100 atm

Oxi

deth

ickn

e

10-1

100

tmelt,s< treac

IIIV

VI

Oxi

deth

ickn

e

10-1

100

t < t

III

V

Core size (nm)20 40 60 80 100

10 VI

Core size (nm)20 40 60 80 100

tmelt,s< treac

NEEM MURI

Molecular Dynamics Study of Melting of Nano Aluminum Particlesg

NEEM MURI Properties of Aluminum Particles at Nano Scales

• Aluminum particle of 1 nm consists of about 32 aluminum molecules. • A large fraction of constituent particles are on the surface • Aluminum particle of 1 nm consists of about 32 aluminum molecules. • A large fraction of constituent particles are on the surface • Melting and boiling temperatures of particles decrease with decreasing particle size.

Melting temperature could be as low as 400 K for 1-nm particle • Liquid and solid phases may coexist in dynamic equilibrium over a range of temperatures

• Melting and boiling temperatures of particles decrease with decreasing particle size. Melting temperature could be as low as 400 K for 1-nm particle

• Liquid and solid phases may coexist in dynamic equilibrium over a range of temperatures

K

1000

Aluminum melting temperature as function of particle size obtained by molecular dynamics simulations (Thompson et al.

Aluminum melting temperature as function of particle size obtained by molecular dynamics simulations (Thompson et al.

Tem

pera

ture

,K

600

800

2005), experiment measurements (Eckert et al., 1993), and theoretical predictions 2005), experiment measurements (Eckert et al., 1993), and theoretical predictions

Mel

ting

T

400

600 Bulk AluminumTheory [23]MD Simulations [22]Experiment [21]

Particle Diameter, nm0 100 200 300

NEEM MURI Melting Point of Nano-Aluminum Particle as Function of Size

• The melting point of aluminum nano-particles increases monotonically as a

• The melting point of aluminum nano-particles increases monotonically as a p yfunction of particle size and approaches bulk melting point for particle size of 8 nm and larger.

• Two body Lennard-Jones fails to

p yfunction of particle size and approaches bulk melting point for particle size of 8 nm and larger.

• Two body Lennard-Jones fails to ypredict the melting point. Sutton-Chen validated against structural properties also fails to capture the melting phenomenon.

ypredict the melting point. Sutton-Chen validated against structural properties also fails to capture the melting phenomenon.

• Glue and Embedded-atom potentials predict comparable melting points with results from Glue potential slightly higher in magnitude

• Glue and Embedded-atom potentials predict comparable melting points with results from Glue potential slightly higher in magnitude

ergy

,eV

-2.90

-2.85

-2.80Glue PotentialStreitz Mintmire PotentialSutton Chen PotentialEmbedded Atom Potential

• Equilibrium potential energy vs. temperature study done for clusters of aluminum atoms less than 800 atoms e.g. 256 atoms (2 nm). Dynamic coexistence of solid and liquid phase

• Equilibrium potential energy vs. temperature study done for clusters of aluminum atoms less than 800 atoms e.g. 256 atoms (2 nm). Dynamic coexistence of solid and liquid phase

Pote

ntia

lEne

-3.05

-3.00

-2.95

coexistence of solid and liquid phase was observedcoexistence of solid and liquid phase was observed

Temperature, K300 400 500 600 700 800

-3.10

NEEM MURI

Effects of Pressure and Void Size on Melting of AluminumMelting of Aluminum

NEEM MURI Different void geometries considered for defect-nucleated melting of 5.5 nm aluminum nano particle

0.00 nm3 0.32 nm3 0.98 nm3

2.94 nm3 4.91 nm3 6.88 nm3

8.84 nm38.19 nm3

NEEM MURI Effect of Void Size on Melting of 8.5 nm Aluminum Particle

1000ur

e(T

m)

950

Tem

pera

tu

850

900

Mel

ting

T

800

850

M

0 20000 40000 60000 80000750

Void Volume (A3)

NEEM MURI Evolution of density contours with time showing mechanism of melting for 8.5 nm nanoparticle

0.0 ps 90.9 ps 106.8 ps 119.1 ps

No Void

0.0 ps 91.8 ps 98.7 ps 109.8 ps

Void size of 5.0 nm3

0.0 ps 57.4 ps 93.9 ps 97.8 ps

Structural Collapse (20.0 nm3)

NEEM MURI Melting of Al Particle with Oxide Coating(8 nm Particle)

ex,δ 0.08

0.10

0.12

dex,δ

0.010

0.015

940 K

Lind

eman

nIn

de

0.02

0.04

0.06

Lind

eman

nIn

d

0.005

0.0 0~940 K

~1100 Kcore oxide

Iterations0 5000 10000 15000-0.02

0.00

Iterations0 5000 10000 15000

0.000

Aluminum oxide (Al2O3) may melt at a temperature (less than 1200K)substantially lower than its bulk value

NEEM MURI Thermo-Mechanical Behavior of Ni-Coated Nano-Al Particles

• Tight-binding force scheme (Cleri & Rosato); vacuum conditionE ilib i ti f b lk Al & Ni i NVE bl• Equilibrium properties of bulk Al & Ni using NVE ensemble

• Melting point & latent heat of melting of bulk Al, bulk Ni, nAl, & nNi particles with NPT ensemble– Homogenous melting at 1030 K (Al), 1880 K (Ni) -- no free surface for

liquid nucleation– Heterogeneous melting for nano-particles --inward propagating liquid phase

front

Cohesive Energy, eV Lattice Constant, Å Latent Heat of Melting, kJ/mol

Prediction Kittel et al. Prediction Kittel et al. Prediction Brandes & Brook

Al -3.3315 -3.339 4.043 4.05 9.65 9.82Al 3.3315 3.339 4.043 4.05 9.65 9.82

Ni -4.415 -4.435 3.474 3.519 18 17.16

NEEM MURINi-Coated Nano-Al Particles –Results (1/2)

• NPT calculations for Al core (3-10 nm), Ni shell (0.5-2 nm) to study thermo-mechanical behaviorThi k f h ll l ti t di t f

6 nm Al, 2 nm Ni shell

Lindemann index

• Thickness of shell relative to diameter of core → integrity of shell upon core melting

• Melting of core delayed due to cage-like effect -- Al t i t lli b d ith Ni th i t fatoms in metallic bond with Ni near the interface

• Inter-metallic reactions decrease potential energy --heat release

• 6 nm Al, 2 nm Ni – solid Ni withstands tensile stressTotal potential energyRadius of Al coreThermal evolution of 6 nm particle

NEEM MURINi-Coated Nano-Al Particles –Results (2/2)

• Adiabatic simulations to predict heat release due to inter-metallic reactions

Thermal evolution of 11 nm particle

• 10 nm Al core, 1 nm Ni shell (30 wt % Al) : solid Ni shell unable to withstand tensile stress

• Inter-metallic heat release → self-heating of particle;Inter metallic heat release self heating of particle; adiabatic reaction temperature of ~ 2150 K

• Low wt % of Al (3 nm Al core, 2 nm Ni shell) --insufficient Al atoms to exothermically react with Niinsufficient Al atoms to exothermically react with Ni → insignificant heat release

PhenomenonTemperature, K

Adiabatic simulation for 11 nm particle

Phenomenon 3 nm 6 nm 10 nm1 nm 2 nm 1 nm 2 nm 1 nm

Al core melting 800 800 990 990 1050

Ni-Al reactions 1370 – 1100 1580 1120

Reaction temperature 1900 – 2000 – 2150

NEEM MURI

Burning Characteristics of Nano Aluminum ParticlesBurning Characteristics of Nano Aluminum Particles in Flow Environments

i t t-- air -- steam -- water

NEEM MURI Flame Speed as Function of Particle Diameter in Mono-Dispersed Aluminum/Air Mixture

Kinetic-Controlled RegionKinetic-Controlled Region Diffusion-Controlled RegionDiffusion-Controlled Region

• For non-preoxidized particles, ti l t b l

• For non-preoxidized particles, ti l t b l

10-7 10-6 10-5 10-4

101

φ = 0.85

non-preoxidatedparticles

Kn > 1 Kn < 1particles at sub-nano scales are assumed to behave as large molecules. The maximum flame speed is achieved with particle size

particles at sub-nano scales are assumed to behave as large molecules. The maximum flame speed is achieved with particle size

s

100

d−0.59preoxidatedparticles

approaching to its molecular limit.

• For pre-oxidized particles, as the percentage of active aluminum and

approaching to its molecular limit.

• For pre-oxidized particles, as the percentage of active aluminum and

S L,m

/s10-1

Risha et al. [8]Boichuk et al. [6]Goroshin et al. [4]Goroshin et al. [3]B ll l [2] d−0 98

the energy content of the particle drop below a critical point, the flame speed of the particle-laden flow begins to decrease with

the energy content of the particle drop below a critical point, the flame speed of the particle-laden flow begins to decrease with

7 6 5 4

10-2Ballal [2]Cassel [1]Molecular limit (present)Theory (present)

d 0.98gdecreasing particle size. At the extreme situation, the energy release from particle oxidation may not even be able to sustain a flame

gdecreasing particle size. At the extreme situation, the energy release from particle oxidation may not even be able to sustain a flame

particle diameter, m10-7 10-6 10-5 10-4

0not even be able to sustain a flame. not even be able to sustain a flame.

NEEM MURICombustion of Nano-Aluminum and Liquid Water

Al-water zone

Mass conservation of mixture u L ign LS u Vρ = ρ = ρ

2

2 p,2 2 2

dT d TC u

dx dxρ = λ

Energy conservation

TvapTu

Al+ liquid water

Al +steam

Reaction zone

Post-flame zone

X= - ∞ X= - t

u

vap

x ,T T

x t, T T

→ −∞ →

= − =

Boundary conditionvap

• Analytical solution for preheat zoneX ∞ X t

Al-steam zone

2dT d TEnergy conservation

• Analytical solution for preheat zone • Thickness (t), flame speed unknown• Solution iterated until T-t=Tvap,H2O

1 p,1 1 2

dT d TC u

dx dxρ = λ

x 0, T T= =Boundary condition

Heat fluxbalance

Tign

• Volume and mass fractions based on experimental packing density . Large particles, lean mixture → thick paste

2

ign

1 2 fg t H O ( l )

t t

x 0, T T

dT dTx t, h V

dx dx+ −

= − λ λ + ρ=x= - t x=0

NEEM MURI Combustion of Nano-Aluminum and Liquid Water (Modeling Details and Results 2/3)

Reaction zone

dT dT

Particle consumption Boundary condition

d

dp=38 nm, Φ=1

0

1

03

dxdTdx

+

−

=

λλ

ad

p

dT 0dxT Td 0

=

=

→p0

b0

MdMudx

= −τ

p

p0

p

dx 0,

d

dx L

1

0

=

→

=

=

Mixture energy equation

dT d dT⎛ ⎞

Boundary conditionp0

x L,d

0→ =

ignx 0,T T= =

x= 0 x=L

• Ignition temperature and burning time from Huang

3 p3 3 FdT d dTuC QWdx dx dx

⎛ ⎞ρ = λ +⎜ ⎟⎝ ⎠

g

x L,dT 0dx

→ = P = 3.65 MPa, Φ=1

g p g get al., 2009

• Smaller particle → higher wt % of oxide → Lower flame temperaturep

• Reaction zone thinner at higher pressures & for smaller particles → smaller combustion time scales

NEEM MURI Combustion of Nano-Aluminum and Liquid Water (Modeling Details and Results 3/3)

P = 3.65 MPa, Φ=1 P = 3.65 MPa, dp=38 nm Φ=1, dp=38 nm

• Burning rates decrease significantly with particle size in spite of high flame temperatures

• Increase in flame speed with equivalence ratio

Φ=1, dp=38 nm

• Increase in flame speed with equivalence ratio attributed to increase in energy release rate

• Reduction of flame thickness with pressure due to quicker burnout of particles (kineticallyto quicker burnout of particles (kinetically controlled combustion)

NEEM MURIPublications

1. P. Puri, V. Yang, Effect of Particle Size on Melting of Aluminum at Nano Scales, Journal of Physical Chemistry C, Vol.111,2007, pp.11776-11783

2. P. Puri, V. Yang, Effect of Voids and Pressure on Melting of Nano-Particulate and Bulk Aluminum, Journal of NanoparticleResearch, Vol. 11 (5), 2009, pp.1117-1127( ) pp

3. Y. Huang, G.A. Risha, V. Yang, R.A. Yetter, Effect of Particle Size on Combustion of Aluminum Particle Dust in Air,Combustion and Flame, Vol.156, 2009, pp.5-13

4. Y. Huang, G.A. Risha, V. Yang, R.A.Yetter, Combustion of Bimodal Nano/Micro-Sized Aluminum Particle Dust in Air,Proceedings of the Combustion Institute, Vol. 31, 2007, pp. 2001-2009.

5. P. Puri, V. Yang, Pyrophoricity of Aluminum at Nano Scales, Combustion and Flame, In review, g, y p y , ,6. P. Puri, V. Yang, Thermo-mechanical Behavior of Nano Aluminum Particles with Oxide Layers During Melting, Journal of

Physical Chemistry, In review7. D.S. Sundaram, Y. Huang, P. Puri, G.A. Risha, R.A. Yetter, V. Yang, Flame Propagation of Nano-Aluminum-Water Mixture,

Combustion and Flame, In Preparation8. D.S. Sundaram, P. Puri, V. Yang, Thermo-Mechanical Behavior of Nickel-Coated Nano-Aluminum Particles, Journal of, , g, , f

Physical Chemistry, In Preparation9. P. Puri, D.S. Sundaram, V. Yang, Ignition and Combustion of Aluminum Particles at Micro and Nano Scales, Progress in

Energy and Combustion Science, In Preparation10. P. Puri, D.S. Sundaram, V. Yang, A Multi-Scale Theory on Ignition and Combustion of Aluminum Particles, Combustion and

Flame, In PreparationFlame, In Preparation11. P. Puri, V. Yang, Thermo-Mechanical Behavior of Nano Aluminum Particles with Oxide Layers, AIAA Paper 2008-93812. P. Puri, V. Yang, Molecular-Dynamics Simulations of Effect of Pressure and Void Size on Melting of Aluminum, AIAA Paper

2007-564413. P. Puri, V. Yang, Molecular Dynamics Study of Melting of Nano Aluminum Particles, AIAA Paper 2007-142914. Y. Huang, G. A. Risha, V. Yang, and R.A. Yetter, Flame Propagation in Bimodal Nano/Micro-Sized Aluminum Particle/Air14. Y. Huang, G. A. Risha, V. Yang, and R.A. Yetter, Flame Propagation in Bimodal Nano/Micro Sized Aluminum Particle/Air

Mixtures, AIAA Paper 2006-115515. G. A. Risha, Y. Huang, R. Yetter, and V. Yang, Combustion of Aluminum Particles with Steam and Liquid Water, AIAA Paper

2006-1154

NEEM MURI Acknowledgements

This work was sponsored by the U.S. Army Research Office under the M l i U i i R h I i i i d C N W911NF 04 1Multi-University Research Initiative under Contract No. W911NF-04-1-0178. The support and encouragement provided by Drs. Ralph Anthenien is gratefully acknowledged.

NEEM MURI

THANKS !

NEEM MURI Characteristic Time Scale Study(Effect of Temperature)

nm)

101

tmelt,s=tmelt,ctmelt,c=treactmelt,s=treac

tmelt,c> treacV1500 K, 1 atm

m)

101

tmelt,s=tmelt,ctmelt,c=treactmelt,s=treac

tmelt,c> treac

tmelt,c< tmelt,sII

V2000 K, 1 atm

xide

thic

knes

s(n

100

10tmelt,c< tmelt,s

tmelt,s< treac

I

IIIV

IV V

ide

thic

knes

s(nm

100

10 melt,c melt,s

tmelt,s< treac

I

IIIV

IV V

Core size (nm)

Ox

20 40 60 80 100

10-1 IIIVIVI

Core size (nm)

Oxi

20 40 60 80 100

10-1III

VIVI

ess(

nm)

101

tmelt,s=tmelt,ctmelt,c=treactmelt,s=treac

tmelt,c> treac

tmelt,c< tmelt,sII

IV V2500 K, 1 atm

ess(

nm)

101

tmelt,s=tmelt,ctmelt,c=treactmelt,s=treac

tmelt,c> treac

tmelt,c< tmelt,s

IIIV

V3000 K, 1 atm

Oxi

deth

ickn

e

10-1

100

tmelt,s< treac

I

III

IV V

VIVI O

xide

thic

kne

10-1

100

tmelt,s< treac

I

III

IV V

VIVI

Core size (nm)20 40 60 80 100

10 VI

Core size (nm)20 40 60 80 100

10 VIVI

Nano Engineered Energetic Materials (NEEM)Vigor Yang, Georgia Institute of Technology

Objective: A comprehensive theory and predictive methodology for ignition & combustion of aluminum particles at different length scales

Scientific issues: Stage I (particle heating/phase transformations) Stage II (core melting and ignition due to melting/cracking)

• Thermo-mechanical behavior of nano aluminum particles with and without coatings at different length scales

Effect of particle size on ignition and combustion

Stage I (particle heating/phase transformations) Stage II (core melting and ignition due to melting/cracking)

Stage III (heterogeneous reactions/healing of cracks)

melting of oxide layer to form cap (Micro)

particle consumed due to heterogeneous reactions

(Nano)

• Effect of particle size on ignition and combustion characteristics of nano particulate aluminum

• Collective behavior of particles in energetic materials and flow environments Stage V (detached flame front (micro))

Major Accomplishments:

• Studied thermophysical behaviors of bulk and particulate aluminum with different coatings over a broad range of scales

Army Relevance: A comprehensive and quantitative knowledge of combustion and ignition of nano aluminum particles. The theoretical framework and computational methodology g

• Established a unified theory accommodating the various processes and mechanisms involved in the ignition & combustion of aluminum particles at micro and nano scales

developed can be applied to a variety of nano metal particulates.

Funding Profile: $100K per year

Grant # W911NF-04-1-0178• Examined the burning characteristics of aluminum

particles in flow environments

Graduate Students: Puneesh Puri, Dilip Sundaram

Grant # W911NF 04 1 0178

PI Contact information: Vigor Yang

Email: vigor@gatech.edu Ph: 404-894-3002

NEEM MURI Ignition Criteria Two Different Thoughts

• Melting of aluminum core and ensuing volume expansion lead to pressure buildupvolume expansion lead to pressure buildup

• Due to low surface curvature of small particles, oxide layer is subject to higher tension as compared to large particles.p g p

• This causes rupture of the oxide shell and hence ignition.

20 30 h ti i t t 873 & 1173 K20 30 h ti i t t 873 & 1173 K

• Ignition criteria is determined by self heating, involving phase transformations and cracking in oxide shell.

• Ignition criteria is determined by self heating, involving phase transformations and cracking in oxide shell.

20-30 nm; heating air temperature: 873 & 1173 KRef: Rai et al., JPC, 200420-30 nm; heating air temperature: 873 & 1173 KRef: Rai et al., JPC, 2004

gg

• At micron scales, particles ignite at melting point of alumina with formation

• At micron scales, particles ignite at melting point of alumina with formation

Ignition temperature as function of particle diameter. Ref: Dreizin et al., C & F, 2005Ignition temperature as function of particle diameter. Ref: Dreizin et al., C & F, 2005

of an oxide cap.of an oxide cap.

NEEM MURI Shape Assumed by Nano-Particles due to Surface Tension

2 nm 3 nm 4 nm

6 nm5 nm 7 nm

Bulk8 nm 9 nm

NEEM MURICharge Development on Nano Particle

• Melting of nano-particle and bulk aluminum different on the basis of

• Melting of nano-particle and bulk aluminum different on the basis of charges

• Simulations performed using complete S-M potential with charge

l i (E b dd d

charges

• Simulations performed using complete S-M potential with charge

l i (E b dd devolution (Embedded atom + Electrostatic part of S-M potential) and Embedded atom potential (Embedded atom part of S-M

evolution (Embedded atom + Electrostatic part of S-M potential) and Embedded atom potential (Embedded atom part of S-M without charge evolutionpotential)

• Surface charge development on aluminum is too small in case of the

potential)

• Surface charge development on aluminum is too small in case of the S-M potential to make electrostatic forces substantial.

• Similar melting temperature from b th E b dd d t d S M

S-M potential to make electrostatic forces substantial.

• Similar melting temperature from b th E b dd d t d S Mboth Embedded-atom and S-M potential for a 3 nm particleboth Embedded-atom and S-M potential for a 3 nm particlewith charge evolution

NEEM MURIParticulate vs. Bulk Aluminum Melting

Melting simulation for bulk aluminum with periodic boundary conditions

Melting simulation for bulk aluminum with periodic boundary conditions

Melting simulation for aluminum in particulate phase

Melting simulation for aluminum in particulate phase

NEEM MURI Effect of Oxide Thickness and Core Size

5 nm (diameter) Al core + 2 nm thick Al2O3 (9 nm particle)

9 nm (diameter) Al core + 3 nm thick Al2O3 (15 nm particle)

NEEM MURI Summary and Conclusions

The bulk melting is characterized by a sharp increase in structural and thermodynamic properties, whereas the particulate phase involves surface pre-melting. p p , p p p gA perfect crystal with periodic boundary conditions is associated with structural melting which predicts the melting point greater than the thermodynamic melting point. Structural melting occurs at 1244 K. From the study of bulk crystals with 864 and 2048 atoms, it was concluded that the ratio y y ,between the structural and thermodynamic melting points for aluminum is 1.32. The range of critical void size increases as the number atoms considered to represent the bulk phase increases (1.7 and 5.0 nm3 for 864 and 2048 atoms, respectively). Irrespective of particle size, the effect of defect nucleated melting is negligible in case ofIrrespective of particle size, the effect of defect nucleated melting is negligible in case of nanoparticles because of the presence of surface which acts as nucleation site. It can be concluded that the primary mechanism of melting is nucleation at a surface or void. Phenomena like generation of dislocation were observed in the current study, but theirPhenomena like generation of dislocation were observed in the current study, but their impact is negligible as compared to nucleation which is the main mechanism of melting. Melting temperature is independent of shape and type of void which is fully consistent with previous studies. The effect of pressure on the defect nucleated melting of aluminum has also beenThe effect of pressure on the defect nucleated melting of aluminum has also been investigated and is negligible for pressures up to 300 atm.

NEEM MURI Modeling of Bimodal Aluminum Dust Flame at Fuel-Lean Conditions

preheat flame post flameflame flame

a) overlapping flame

preheat flame post flameflame

b) separated flame

preheatzone zone III zone

Tparticles heatedby local gas

gas heated byburning of

l i l

zone I zone II

,2ignT

preheatzone I

flamezone II

post flamezone

Tparticles heatedby local gas

burning of

flame zone I

,2ignT

preheatzone II

x

,1ignTgas heated byconduction fromflame zone

large particles

burning ofsmall particles

overlappingburning

gas heated byconduction fromflame zone

x

,1ignTbu g o

large particles

burning ofll ti l

,1bx v= τ

x0x =

small particles0x Z= 0 ,2bx Z v= + τ

,1bx v= τ

x

0x =small particles

0x Z= 0 ,2bx Z v= + τ

• Flame configuration depends on the mass concentration, particle size, ignition • Flame configuration depends on the mass concentration, particle size, ignition g p , p , gtemperature, and burning time of each group of aluminum particles.

• Ignition temperature and burning time of aluminum particle are needed as input parameters

g p , p , gtemperature, and burning time of each group of aluminum particles.

• Ignition temperature and burning time of aluminum particle are needed as input parametersparameters. parameters.

NEEM MURI Laminar Flames of Mono- and Bimodal-Dispersed Aluminum Particles/Air Mixtures

100% micro particles (5–8 μm)100% micro particles (5–8 μm) 20% nano particles (100 nm) addition20% nano particles (100 nm) addition

Bimodal particle flame features increased flame speed and thicker flame zone. Bimodal particle flame features increased flame speed and thicker flame zone.

NEEM MURI Characteristic Time Scale Study (II)

t l =t l

tmelt,c> treac V1000 K, 1 atm

O Anions

Al Cations

O Anions

Al Cations

O Anions

Al Cations

ss(n

m)

101

tmelt,s tmelt,ctmelt,c=treactmelt,s=treac

tmelt,c< tmelt,sIIIV

Phase TransformationsPhase TransformationsPhase Transformations

eth

ickn

es

100

, ,

IIV VO2 Molecules

Oxi

de

10-1

tmelt,s< treac

IIIVIVI

Core size (nm)20 40 60 80 100

Oxide cap

Al (g)

oxidizer

Oxide cap

Al (g)

oxidizer

Detached Flame front

Oxide cap

Detached Flame front

Oxide cap