Reactive Nanocomposite Materials: Challenges and Perspectives New Jersey Institute of Technology Newark, NJ 07102 Edward L. Dreizin 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 topdown 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) ln(dα/dt) 2 1 0-1 -2-3 Understanding of heterogeneous reactions Analysis of intermediate products (on intermediately recovered samples) Good control of environment Quantification of reaction kinetics major Smaller peaks become important Difficult to uncouple multiple reactions onset identifiable peaks [mw/mg] 40 90 % 40 % 10 % 1 % Reaction progress α Heat Flow, exothermic = up 2.0 1.0 0.50 0.25 0.10 [K/min] 20 10 5 2-4 0.05 1-5 0.0010 0.0015 0.0020 0.0025 1/T [1/K] 400 500 600 700 800 900 1000 T [K]
Reaction modeling: example: Al-MoO 3 Reaction rates need to be described as a function of : Processes at the interface Morphology Domain dimensions Al Al Al Al Al 2 O 3 (amorphous) Al 2 O 3 (amorphous) γ-al 2 O 3 γ-al 2 O 3 MoO 3 Mo 9 O 26 Temperature Mo 4 O 11 MoO 2 Reaction progress, α Each Al 2 O 3 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 quantifiable results 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