Reactive Nanocomposite Materials: Challenges sdytse/NanoE-Workshop2008/ Nanocomposite

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  • 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 arent 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


  • 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


    majoridentifiable peaks









    400 500 600 700 800 900 1000

    T [K]


    t Flo

    w, e



    ic =







    90 %

    10 %

    40 %

    1 %

    Reaction progress

    0.0010 0.0015 0.0020 0.0025

    1/T [1/K]











  • Reaction modeling: example: Al-MoO3 Reaction rates need to be described as a function of :

    Processes at the interface Morphology Domain dimensions









    Reaction progress,


    Al2O3 (amorphous)

    Al2O3 (amorphous) -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 exp