Metallic Clusters, Metallic Clusters, MesoscopicMesoscopicA t d th i R ti A t d th i R ti Aggregates and their Reactive Aggregates and their Reactive

CharacterizationCharacterization

Bryan Eichhorn Rich Yetter and Bryan Eichhorn, Rich Yetter and Michael R. Zachariah

MURI: SMART FUNCTIONAL NANOENERGETIC MATERIALS

It Is Well Known That Going Smaller Results In Faster ChemistryIn Faster Chemistry

a. How can we make smaller length scale materials ?

b. In what form can they be assembled to be utilized effectively?

AFOSR/MURI

Sintering of NanoAluminum

Rapid Loss of Nanostructure at High Temperatures and High Heating Rates. 

f

Dynamic TEM shows rapid loss of Surface Area. 

A) Before B) After

12ns Laser Pulse

Image of aggregate taken with continuous wave (CW)  electron beam before and after heating

0.68( 1)Dfpd

CoalescenceTime Nη

= −

E) Aft

heating 

Image of the same aggregate taken with DTEM  electron pulse before ,during, and, after heating 

( )σ

Reaction Products are large

100 nm

• Loss of nanostructure (surface area) in: < 20 usE i i 10

• Loss of nanostructure (surface area) in: < 20 usE i i 10

C) Before E) AfterD) t=0‐12ns

With T. LaGrange and K. Sullivan @ LLNL

• Expt pressure rise times ~ 10 us . 

• Thus loss of nanostructure may occur before combustion 

• Expt pressure rise times ~ 10 us . 

• Thus loss of nanostructure may occur before combustion 

Understanding the extent of reaction in nanoenergetics:

20 μmΦ=1 Φ=0 52 5 μmΦ=1 5 20 μmΦ=1 Φ=0.5

Void

2.5 μmΦ=1.5

Reaction is not

10 μmAl rich phase

Cu richReaction is not going to completion 

Eq. Ratio Al Cu O Al2OX

Al/CuO (Φ=0.5) 20 57 23 2.3

Al/CuO (Φ=1) 36 25 39 2 2Al/CuO (Φ=1) 36 25 39 2.2

Al/CuO (Φ=1.5) 49 14 37 1.5

AFOSR/MURIJ. Conny ‐ NIST

• Reactions appear not to be going to completion.

This is Bad News:This is Bad News:These results imply that simply  going smaller has diminishing returns because sintering ( i e loss of surface area) competesreturns because sintering ( i.e. loss of surface area) competes with reaction.  

i.e. Sintering times and Reaction times are sufficiently close that the nanostructure is lost before it can be effectively utilized. 

We need an approach that enables us too:1 Go to smaller length scales1. Go to smaller length scales.2. Disables sintering

AFOSR/MURI

Strategy for this Project:Develop a mesoparticle comprised of  ultra‐small nanostructures that can be rapidly disassembled releasing highly reactive nanostructures.

1. Develop very small energetic clusters  < 2 nm and nanoparticles that are passivated.

2.   Assemble these clusters and NPs into a meso‐scale particle with gas generators.

3.   Study and optimize mesoparticle disassembly and cluster combustion.

Gas generatorAl Cluster (e.g. Al77)

Controlled 

Mesoscale compositeof Al cluster and gas generator

Aerosol  Assembly

evaporation 

Heating leads to gas generation and  Individual cluster

AFOSR/MURI

cluster ejection Combustion 

ElectroElectro--hydrodynamic spray to create polymerhydrodynamic spray to create polymer--particle compositesparticle composites

Micro‐scale particles of nanomaterials 

Direct Spray formation of microparticles with a gas generatorDirect Spray formation of microparticles with a gas generator

Reaction products smaller i.e. less sintering

• Faster burn times• Diminished sintering

p g

Nanoaluminum Microparticles

Wide range of burn times due to agglomeration/sintering

More compact burning with a much narrower burn time distribution

Mesoparticle burn times have a narrow distribution and burn as fast as the fastest nanoparticle.

Avg Burn time :~4000 us

Avg Burn time :~800us

i.e. formulation of a micronscale material with a nanoscale burn time.

Application of Aluminum Application of Aluminum MesoparticlesMesoparticles in Composite Solid Rocket Propellantsin Composite Solid Rocket PropellantsResearch in collaboration with G. Young NSWC‐IH

• In estigate al min m mesoparticles as an ingredient for solid composite rocket propellants

Enhanced Propellant Burn

• Investigate aluminum mesoparticles as an ingredient for solid composite rocket propellants.  • Potential Benefits: easier processing, and potential benefits resulting from reduced sintering prior 

to combustion. 

Mesoparticles

HTPB/AP/Al (20/70/10 by wt%)

H2 Aluminumn

1 .3

1 .4

1 .5

urni

ng R

ate

Enhanced Propellant Burn Mesoparticles H2 Aluminumn

1

1 .1

1 .2

0 5 1 0 1 5 2 0

Nor

mal

ized

Bu

Burning rate normalized by H2 aluminum shows up to 1.36 X combustion rate.

% N C in M e so p a rtic le A d d itiv e

1

m/s

)

rb (cm/s) = 0.417 * P(MPa)0.497

Baseline PropellantMesoparticle Propellant

Bur

ning

Rat

e (c

m

rb (cm/s) = 0.304 * P(MPa)0.487

AFOSR/MURIZachariah and Eichhorn, UMD

Greater degree of luminosity particularly with respect to the propellant surface. Indication of aluminum particle ignition in close proximity to the propellant surface.

0.10.1 1 10

Mesoparticle Propellant

Pressure (MPa)

3 El fil d 3 DPolyvinalidine fluoride

Direct Printing to incorporate material

3. Electrospray to create films and 3‐D structures.  ‐ Higher polymer content and fast deposition.

Al +

U t 50 t % N AlUp to 50 wt % Nano Al

AFOSR/MURI

ElectroElectro--hydrodynamic spray to create polymerhydrodynamic spray to create polymer--particle compositesparticle compositesNano‐Composite Laminates

Tensile: Laminate > Single layerStrain: Laminate >> Single layer Faster Burn

Toughness: Laminate > Single layer        

Thi L i h bl• High metal loadings• Better mechanical properties• Faster burn rates

This Laminate approach enables

This general approach offers the potential to make graded material to tune the propellant burnand to enable access to higher density materials.

Al( ) + HBr( ) 4AlBr NEt31000°C 10 5 t

Tol/NEt3 ‐78°C             RT Al4Br4(NEt3)4

Low Oxidation State Al clustersLow Oxidation State Al clustersAl(s) + HBr(g)                            4AlBr NEt31000°C, 10‐5 torr Al4Br4(NEt3)4

Al+0 23Al+0.23

[AlBrNEt3]4 [AlCp*]4

Al77[N(SiMe3)2]202-

AFOSR/MURI

C.Dohmeier, C.Robl, M.Tacke, H.Schnockel; Angew.Chem.,Int.Ed. (1991), 30, 564.

Mocker, M.; Robl, C.; Schnöckel, H. Angew. Chem. Int. Ed. 1994, 33, 1754–1755.

Drop Diameters versus Frame Number

Tetramer Solution Burning in Liquid FuelsTetramer Solution Burning in Liquid FuelsSingle Droplet Burn Studies

0.4

0.5

0.6

(mm)

Drop Diameters versus Frame Number

Gas generation 

0

0.1

0.2

0.3

Diameter (

Gas Generation, Droplet Inflation, and Eruptions throughout lifetime.

0450 455 460 465 470 475 480 485 490

Frame Number

Final Droplet Diameter

, p , p g

Decreased burn time by:• ~20%  with only 0.13 wt % added  to Toluene/Ether

Mechanism of gas generation

• ~15%   with Kerosene

We observe significant burn enhancement in liquid f l i h l ll f ddi ifuel with  only a very small amount of additive.

Behavior is different than that observed by adding nanoaluminum to fuel

SUMMARYSUMMARY

Better Utilization of Traditional NanomaterialsBetter Utilization of Traditional Nanomaterials• Developed  spray approach to generate variety of microstructures that enable  new 

methods for incorporation/utilization of nanomaterials. • Enhanced performance, although the exact mechanism is not fully understood. 

Better Utilization of Traditional NanomaterialsBetter Utilization of Traditional Nanomaterials

p , g y• These assembly approaches are in there infancy but seem to offer a way forward to 

implement nanomaterials at practical scales. 

Cluster Based Materials in Liquid FuelsCluster Based Materials in Liquid Fuels

•  We have shown that homogenous hydrocarbon soluble clusters and molecules have a  different mechanism of combustion relative to  heterogeneous solutions of NPs and supported NP composites in the same hydrocarbons

• Homogenous solutions show continuous Al combustion at droplet surface in addition to disruptive gas generation (HBr ) from reactions with water.   This is in sharp contrast to the disruptive Al combustion after hydrocarbon 

b ti f d ith Al ll id

AFOSR/MURIZachariah and Eichhorn, UMD

combustion found with Al colloids.

•

Simultaneous  0.006 2 105

Tz4 bound to FGS (JL58)Our TOur T--Jump TOFMS coupled to Jump TOFMS coupled to NanocalorimeterNanocalorimeter

measurement of temporally resolved thermal and speciation data at high heating rates

Vin

Iin

Iout

Vout

Pt heater 0.004

0.0051.5 105

m/z = 152Heating Rate

sity

(a.u

.)

He

high heating rates up to ~105 K/s

0.001

0.002

0.003

5 104

1 105

Sig

nal I

nten

s eating Rate (

0

0.001

0

S K/s)

Tz4:Time

1000

1200

T tg. C

)

(FGS)Time Resolved MS of Tz4 bound to FGS (JL58):

400

600

800Temperature

erat

ure

(deg

(FGS)

0

200

400

0 0.002 0.004 0.006 0.008Te

mpe

time (s)

Calorimeter developed by  David Levan @ NIST

Seen for Al and B Combustion Sintering of Fractal Aggregates

0 68( 1)Dfpd

t Nη

ng Tim

e

tburn ~d2

Example:

0.68( 1)pt Nσ

= −

Burnin

tburn ~d1

Df = 1.8dp =  50 nmN = 100 primary particle in agg.

tburn ~d0.3‐0.5

Fusion time + heating time < 15 μs

Characteristic Reaction Time = 10 μs, an experimentally measured pressure

Particle Size

an experimentally‐measured pressure rise time

“50 nm ALEX”i.e. aggregated 

An aggregate of 100, 50 nm primaries when sintered yields a 230 nm sphere.

particles with anaverage primary 

particle size of 50nm

AFOSR/MURICharacteristic  pressurization time ~ Sintering time. 

Droplet Combustion Experiment PSU drop tower at UMD

Objective: Probe Reactivity of Precursors and Nanofluids

Operating PrincipleOperating Principle: 

1. Generate Drop in Inert Environment 

2. Ignite in Reactive Environment

3 Image Burning to Measure

Suitable for air iti l ith

3. Image Burning to Measure Rate of Reaction

sensitive samples with solid precipitation e.g. tetramer solutions and 

fl idnanofluids.