Reactive Nanocomposite Materials: Challenges and Perspectives

Edward L. Dreizin

New Jersey Institute of TechnologyNewark, NJ 07102

Primary research sponsors: DTRA, TACOM-ARDEC Picatinny

Presented at Workshop on Nano-energetics

Rutgers University, Piscataway, NJ, February 28, 2008

Reactive Nanocomposite Materials: A simple concept

• Metal-based fuels have extremely high combustion enthalpies

• Relatively low rate of energy release in heterogeneous reactions presents the major bottleneck for most applications

• Increase of reactive surface or reduction of reactive elements to a nano-size results in a quantitative increase in the reaction rate

• Reactive Nano-materials: – high specific reactive interface area + high energy density

• Two main classes– Based on mixing reactive nanopowders– Based on fully-dense nano-composite structures

• Two synthesis approaches– Bottom-up (from atoms and molecules to nanoparticles)– Top-down (from bulk materials to nanostructures)

• NJIT work: fully dense nanocomposite materials produced by top-down techniques

Reactive nanomaterials: main challenges

• How far down “nano” will we go?– Haw far can we go?– When does it make sense to stop?

• How to describe properties of nanocomposite reactive materials?– Experimental reactivity assessment– Models

• Fundamental descriptions (reactions, mechanics)– Molecular dynamics

• Performance modeling– Chemical +mechanical processes combined– Simplified integrated models

• Do reaction mechanisms depend on the initiation?– Thermal versus shock– Spark versus laser

• Is nanoscale a deciding factor?– Will the same compositions mixed on the same scale behave identically?

• Is manufacturing approach important?• How to correlate lab evaluations/tests with performance metrics?

How far down “nano” can we go? Fundamental limits

• What is the nature of the interface between the reactive components?– Why aren’t they reacting?

• Phase boundaries: significant part of the bulk material• Metastable phase boundaries – specific phases or class of materials by themselves

• What is the thickness (volume) of the interface “phase”?– How does it depend on specific chemical components or component types?– Is it being affected by manufacturing process?

• Processing temperature• Processing time• Pressure/shear/strain rate

– Is it aging?

• Other fundamental limitations– How do material properties change for the nano-domains?– Mechanical strength– How to study nano-scaled materials experimentally

• Electron microscopy• X-ray diffraction; absorption (synchrotron)• Thermal analysis

How far “nano” do we want to go? Practical restrains

• Technology limitations– Sizes of nanoparticles that can be handled, processed/mixed– Control of the layer thickness– Degree of homogeneity achievable by

• Mixing; sol-gel synthesis • Sputtering; vapor deposition• Mechanical refinement

– Cost• Added cost to produce nanostructure• Must be offset by performance benefits

• Storage– Aging studies needed– Correlated with reaction mechanisms– Other issues: unmixing, large surface area, low density, cost

• Handling– Safety, cost– Convenience– New standards, guidelines, and tools needed

• Health hazards– Not studied– Modeling can help

Describing properties: Reactivity Assessment I

• Select the appropriate lab technique to study reaction– Open tray burn

• Results depend on poorly controlled– Packing density, aspect ratio of the channel– Extremely difficult to model

• What do we learn?– Constant volume explosion

• Energy/rate of reaction• Packing density can be controlled• Still difficult to model

– Heated filament ignition• Purely thermal initiation• Ignition kinetics quantified• Only suitable for ignition (not for combustion

studies)

Describing properties: Reactivity Assessment II

• Select the appropriate lab technique to study reaction (continued)– Laser ignition

• Packing density must be controlled/reported• Uncouple laser/material interaction from

combustion • Can be used with individual

particles/granules/small pellets• Ignition delays can be quantified for different laser

energies• Combustion can be observed

– Shock initiation• Correlation with thermal ignition• Difficult to work with

– Spark ignition• Correlation with thermal ignition• Mechanism of spark interaction with material

remains unclear• Has not been used

Describing properties: Reactivity Assessment III

• Select the appropriate lab technique to study reaction (continued)– Thermal Analysis (DSC/TGA)

• Understanding of heterogeneous reactions• Analysis of intermediate products (on intermediately recovered samples)• Good control of environment • Quantification of reaction kinetics• Smaller peaks become important• Difficult to uncouple multiple reactions

onset

majoridentifiable peaks

40

20

10

5

2

10.05

0.10

0.25

400 500 600 700 800 900 1000

T [K]

Hea

t Flo

w, e

xoth

erm

ic =

up

0.50

1.0

2.0

[K/min]

[mW/mg]

90 %

10 %

40 %

1 %

Reaction progress α

0.0010 0.0015 0.0020 0.0025

1/T [1/K]

-5

-4

-3

-2

-1

0

1

2

ln(dα

/dt)

Reaction modeling: example: Al-MoO3

• Reaction rates need to be described as a function of :– Processes at the interface – Morphology– Domain dimensions

Al

MoO3

Al

Mo9O26

Al

Mo4O11

Al

MoO2

Reaction progress, α

Temperature

Al2O3 (amorphous)

Al2O3 (amorphous) γ-Al2O3

γ-Al2O3

…

• Each Al2O3 polymorph presents a different diffusion resistance

• Each Mo oxide phase is a different source for oxygen ions– Nano-sized dimensions can limit the oxygen availability at a specific temperature

Performance Modeling

• Macroscopic models– Mechanical

• Validate existing concepts (strength tests; ballistic tests)• Experimentally determine properties of “interface phases”• Most bulk material properties likely irrelevant

– Chemical• Reactions insignificant for macro-materials may become rate-limiting steps• New reactions likely to be discovered

– Both: expected to depend on manufacturing approach– Simplified integrated models to be developed based on validated

mechanical and chemical submodels• Development of relevant submodels is needed now

• Molecular dynamics– Eventually necessary– Appropriate potentials are largely unknown– Computers still need to be faster– Validations?

• Close collaborations needed between modelers and experimentalists

Initiation role

• Different applications produce different ignition stimuli– Thermal– Shock– Shear– Combined

• Hypothesis: ultimately, every initiation process can be reduced to thermal initiation

– Hot spots– Shear-induced heating– New interfaces produced are ignited thermally

• Main challenges– Fundamental understanding of individual processes involved (submodels needed)– Laboratory scale validations

• Experimental Options– Laser ignition (pellets, particles)– Shock initiation (pellets)– Spark ignition (packed powder)– Primer initiation (packed powders, pellets)– “Blue pig” (consolidated materials)

• How to interpret results• Appropriate time scales

Is nanoscale a deciding factor?

• Manufacturing may affect performance– Type/morphology of interface

• Points of contact• Extended area subject to high T during synthesis• Extended area subject to strong deformation during synthesis

– Mechanical properties of components• Work-hardened• Annealed

– Porosity– Protective layers (nature, thickness)– Purity of components– Types of components that can be processed– Scale, morphology, uniformity of mixing

• Manufacturing determines the scale of production and ultimate cost

Laboratory tests and performance metrics

• Any well documented experiment with reproducible and quantifiableresults is valuable– Data on different materials tested using the same technique are of

specific interest– Data that can be modeled quantitatively are of specific interest

• Simplified laboratory configurations, well-quantified conditions/sample parameters• Correlate the existing data• Define sample characteristics

– Sample size, density, initiation approach• Obtain data in a systematic way to detect correlations (coordinated

effort needed) – Standard laboratory measurements

• Constant volume explosion• Laser ignition• …

– Standard performance tests• Blue pig• Filled tube ignition with a primer• …

Future research (experimental)

• Real time measurements of combustion reactions involving heterogeneous processes may not be currently feasible/useful

• Alternative approach:– Detailed measurements in slow reactions (as in DSC) to establish

mechanisms– Outputs available from rapid combustion reactions

• Optical (pyrometry, spectroscopy)• Pressure• Other... (mass spec?)

– Correlate predicted time scales, reaction sequences• Specific measurements of interest

– High resolution structural studies– High sensitivity measurements defining the atomic environments/states– Detailed thermal analysis studies– Any well-documented and reproducible ignition and combustion

experiments • Specific emphasis on experiments performed consistently with different types of

reactive nanomaterials

Future research (modeling)

• Development of individual submodels– Reaction steps– Kinetics

• Aging, initiation– Effect of size distributions, morphology

• Quantitative description of properties of nano-scaled domains– Thermodynamic– Mechanical– Need experimental validation

• Conceptual description of the interface phases• Molecular dynamics

– Validations needed

Future NJIT efforts

• Use well-characterized materials

• Perform systematic thermal analysis studies (low heating rates)– Vary material characteristics, e.g., particle size distributions, oxidizing

environment, etc., in a systematic way – Analyze phases produced at intermediate temperatures– Interpret results in terms of a detailed reaction model

• Perform independent ignition experiments (high heating rates), e.g., laser ignition, heated filament ignition, etc.– Enable experimental variation of heating rate– Identify ignition kinetics from specific experiment

• Compare results and validate the detailed reaction model• Complementary combustion tests (Constant volume explosion, others)

• Simplify the validated model to enable its usefulness for large scale hydrocodes and other practical applications