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1 Metallic Clusters, Mesoscopic Aggregates and their Reactive Characterization Bryan Eichhorn, Rich Yetter and Michael R. Zachariah MURI: SMART FUNCTIONAL NANOENERGETIC MATERIALS

2 It Is Well Known That Going Smaller Results In Faster Chemistry a. How can we make smaller length scale materials? b. In what form can they be assembled to be utilized effectively?

3 Rapid Loss of Nanostructure at High Temperatures and High Heating Rates. Sintering of NanoAluminum Dynamic TEM shows rapid loss of Surface Area. Image of aggregate taken with continuous wave (CW) electron beam before and after heating Image of the same aggregate taken with DTEM electron pulse before,during, and, after heating 12ns Laser Pulse A) Before B) After C) Before D) t=0 12ns E) After ηd D p 0.68 f CoalescenceTime = ( N 1) σ Reaction Products are large With T. LaGrange and K. LLNL 100 nm Loss of nanostructure (surface area) in: < 20 us Expt pressure rise times ~ 10 us. Thus loss of nanostructure may occur before combustion

4 Understanding the extent of reaction in nanoenergetics: Φ= μm 2.5 Φ=1 20 μm Φ=0.55 Cu rich Void Al rich phase 10 μm Reaction isnot going to completion Eq. Ratio Al Cu O Al 2 O X Al/CuO (Φ=0.5) Al/CuO (Φ=1) Al/CuO (Φ=1.5) Reactions appear not to be going to completion. J. Conny NIST

5 This is Bad News: These results imply that simply going smaller has diminishing returns 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 scales. 2. Disables sintering

6 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 generator Al Cluster (e.g. Al 77 ) Controlled evaporation Aerosol Assembly Mesoscale composite of Al cluster and gas generator Heating leads to gas generation and cluster ejection Individual cluster Combustion

7 Electro-hydrodynamic spray to create polymer-particle particle composites Micro scale particles of nanomaterials Direct Spray formation ofmicroparticles with a gas generator Reaction products smaller i.e. less sintering Faster burn times Diminished sintering 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.

8 Application of Aluminum Mesoparticles in Composite Solid Rocket Propellants Research in collaboration with G. Young NSWC IH Investigate aluminum m mesoparticles as an ingredient for solid composite rocket propellants. Potential Benefits: easier processing, and potential benefits resulting from reduced sintering prior to combustion. Mesoparticles H2 Aluminumn HTPB/AP/Al (20/70/10 by wt%) Enhanced Propellant Burn Normalized Bu urning Rate Burning rate normalized by H2 aluminum shows up to 1.36 X combustion rate %NC in Mesoparticle Additive 1 r b (cm/s) = * P(MPa) Burning Rate (cm m/s) r b (cm/s) = * P(MPa) Greater degree of luminosity particularly with respect to the propellant surface. Indication of aluminum particle ignition in close proximity to the propellant surface. Baseline Propellant Mesoparticle Propellant Pressure (MPa) Zachariah and Eichhorn, UMD

9 Direct Printing to incorporate material 3. Electrospray to create films and 3 D structures. Higher polymer content and fast deposition. Polyvinalidine fluoride Al + Up to 50 wt % Nano Al

10 Electro-hydrodynamic spray to create polymer-particle particle composites Nano Composite Laminates Tensile: Laminate > Single layer Strain: Laminate >> Single layer Toughness: Laminate > Single layer Faster Burn This Laminate approach enables High metal loadings Better mechanical properties Faster burn rates This general approach offers the potential to make graded material to tune the propellant burn and to enable access to higher density materials.

11 Low Oxidation State Al clusters Tol/NEt 3 78 C RT 1000 C, 10 5 torr Al 4 Br 4 (NEt 3 ) 4 Al () (s) + HBr () (g) 1000 C 10 4AlBr NEt 5 t 3 Al Al77[N(SiMe3)2]20 [AlBrNEt 3 ] 4 [AlCp*] 4 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,

12 Tetramer Solution Burning in Liquid Fuels Diameter (mm) Drop Diameters versus Frame Number Single Droplet Burn Studies Gas generation Frame Number Final Droplet Diameter Gas Generation, Droplet Inflation, and Eruptions throughout lifetime. Decreased burn time by: ~20% with only 0.13 wt % added to Toluene/Ether ~15% with Kerosene Mechanism of gas generation We observe significant burn enhancement in liquid fuel with ih only a very small amount of additive. i Behavior is different than that observed by adding nanoaluminum to fuel

13 SUMMARY Better 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. 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 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 combustion found with Al colloids. Zachariah and Eichhorn, UMD

14 Our T-Jump TOFMS coupled to Nanocalorimeter V in I in I out V out Tz4 bound to FGS (JL58) Simultaneous measurement of temporally resolved Heating Rate m/z = 152 thermal and speciation data at high heating rates up to ~10 5 K/s Signal Intens sity (a.u.) Pt heater Time Resolved MS of Tz4 bound to FGS (JL58): Tz4: (FGS) Tempe erature (deg g. C) Temperature time (s) He eating Rate ( K/s) Calorimeter developed by David NIST

15 Burnin ng Time Seen for Al and B Combustion t burn ~d 2 t burn ~d 1 t burn ~d Sintering of Fractal Aggregates Example: ηd p t= ( N 1) σ Df = 1.8 dp = 50 nm N = 100 primary particle in agg. Fusion time + heating time < 15 μs Characteristic Reaction Time = 10 μs, an experimentally measured measured pressure rise time D f Particle Size An aggregate of 100, 50 nm primaries when sintered yields a 230 nm sphere. 50 nm ALEX i.e. aggregated particles with an average primary particle size of 50nm Characteristic pressurization time ~ Sintering time.

16 Droplet Combustion Experiment PSU drop tower at UMD Objective: Probe Reactivity of Precursors and Nanofluids Operating Principle: 1. Generate Drop in Inert Environment 2. Ignite in Reactive Environment 3. Image Burning to Measure Rate of Reaction Suitable for air sensitive samples with solid precipitation e.g. tetramer solutions and nanofluids.