Michael R. Zachariah Professor

Transcription

1 Michael R. Zachariah Professor Probing the Reaction Dynamics of Nanoenergetic Materials Department of Chemistry and Biochemistry

2 NanoEnergetic Materials 2Al + 3MO Al O + 3M H Fuel Oxidizer METAL/ OXIDIZER Al/MoO 3 Al/Ni 2 O 3 Al/Teflon Al/KClO 4 H volume (kcal/cc) HIGH EXPLOSIVE TNT HMX PETN CL-20 H volume H mass H mass (kcal/g) Fischer and Grubelich, 32nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Lake Buena Vista, FL Thomas N. Hall and James R. Holden, Navy Explosives Handbook, NSWCMP , 1988.

3 Thermodynamics Motivation Energy release from Thermite > CHNO chemistry. Kinetics Thermites are much slower than CHNO Systems; - Smaller thermites are faster => Nano But, we know very little about mechanisms of ignition and propagation. We are still groping to establish a conceptual mechanism, let alone advance to a mathematical model.

4 Metastable Interstitial Composites How can we study the reactivity of nanocomposites? MoO 3, CuO, etc. Al Al 2 O 3 CuO Al Al203??????

5 How to characterize the reactivity of powder of ultrafast nanocomposites? A New Approach: T-Jump Mass Spectrometry/Optical Emission Basic Approach: I or R e- ions Mass-Spec Fine wire coated and rapidly heated Photons Optical Emission Coat wire with: Organics, Organics+ binder Thermites, Thermites + organics Sputtered thin films Etc.

6 T-Jump Wire Ignition Example of heating rate of 1.7 X 10 5 C/s Wire temperature determined by resistance. Temp 6 11:50:52 AM 10/24/ Temp (K) Ignition temperature ~ 1250 K Normalized Intensity (a.u.) Time (ms)

7 T-Jump Mass-Spectrometry Possibility to study the ignition kinetics of Nanocomposites using mass spectrometry. Detector Oscilloscope Computer Positively charged ions accelerated by Electric field move up time-of-flight tube to the detector V Vp V Electron Gun CAN BE PULSED UP TO 20kHz, to obtain multi-mass spectra from each combustion event.

8 Ignition of NanoThermite in T-Jump Mass-Spec: Al + CuO => Al2O3 + Cu 70 µm High Speed X-Ray Movie, 135,000 fps (total time = 1.8 msec) High speed video 33,000 fps Obtained at Argonne National Lab Collaboration with T. Hufnagel, JHU.

9 Time Resolved Thermite Mass-Spectrum Stoichometric Al/CuO- one spectrum every 100 µs. Al +CuO => Al2O3 + Cu Al/CuO nanocomposite under heating rate K/s

10 More Details in Thermite Spectra Al +CuO => Al2O3 + Cu 100 H O + (a). background 2 + N 2 + O H O O 2 (b). Al/CuO thermite CO 2 + Al + Cu + Al O H O + 2 (c). Al/Fe 2 O 3 thermite + Al + CO Fe + 2 Al O M/z Difference between two systems: Oxygen species Al2O is reaction product Background: H2O mass 18 N2 mass 28 O2 mass 32 Al/CuO thermite Oxygen species mass 16, 32 Al mass 27 Cu mass 63,65 Al2O mass 70 Al/Fe2O3 thermite Al mass 27 Fe mass 56 Al2O mass 70 All have: CO mass 28 CO2 mass 44

11 Comparison of O2 Release from CuO, Fe2O3, ZnO 200 Oxygen release from CuO, Fe2O3, ZnO 1500 Cuo Temp Zno Temp Fe2O3 Temp 1400 Intensity (a.u.) Temp of Fe2O3 O2 release Temp of CuO O2 release O2 from CuO O2 from Fe2O Temperature (C) Gas Release from CuO X-Ray 135,000 fps Total time = 3 msec O2 from ZnO Time (ms) Reactivity From Pressure Cell: CuO >> Fe2O3 >> ZnO (doesn t even burn) Oxidizer Decomposition (Melting) Point: CuO < Fe2O3 < ZnO Results suggest that oxidizer decomposition to release O2 is important for ignition and that the amount of O2 release correlates with the reactivity.

12 O2 Release Temperature vs. Ignition Temperature Comparing thermite ignition with O2 release from the neat oxide powders: 1600 Al/SnO 2 Al/WO 3 O2 Release Temp (C) Al/CuO Al/Fe 2 O Ignition Temp (C) There is a strong correlation between O2 release and the ignition temperature.

13 Fuel Rich VS Stoichiometric O Al O 2 Al/CuO (Fuel RichEq ratio=3) AlO CO 2 Cu Al 2 O -Shift from excess oxidizer (high O2 peak) to excess fuel (high Al peak) O Al O 2 CO 2 Al/CuO (Stoichiometric) Cu Al 2 O Enhanced Peaks Al AlO (new peak) Al 2 O Diminished Peaks O, O 2 CO 2 Cu m/z

14 How does the Al get out, since it has an oxide shell? Diffusion mechanism for nano-aluminum Molten aluminum diffuses outward. Aluminum reacts with oxygen. We have observed hollow particles observed in Oxidation of Nanoaluminum Melt Dispersion Hollow Product MDM was proposed to explain the high reactivity in MICs (Levitas et al. 2006, 2007) Aluminum particle with core shell structure Molten Al Spallation of alumina shell Molten Al clusters

15 What is Role of Oxide Thickness on Reaction Time Al nanoparticles ( ~50 nm dia) usually have an oxide shell of ~ 2nm. 50 nm Al core Oxide shell 50 nm Prepare Al with different Oxide Shell thickness Al particles are 400 C for different lengths of time : a) 6 mins - 3 nm shell (based on wt. gain) b) 12 mins - 4 nm shell

16 Center for Energetic Concepts Development Ignition Delay vs Oxide Shell Thickness Pulse turned off Norm. Intensity (a.u.) 2 nm 3 nm 4 nm 1200 Temperature (K) Ignition delay increases with shell thickness - Suggests a transport limit Time (ms)

17 Effective Diffusion Coefficient in Oxide Shell from Ignition Delay 50 nm 2 nm Delay time (us) Ignition Ignition Delay Delay Time Time No Steady State No Steady State Tsteady = 730 K MS_Cu Signal Tsteady = 730 K Effective diffusion coeff., D eff = L 2 / t Steady State Temperature = 730 K L (nm) t (us) D eff (cm 2 /s) e e e-10 Shell thickness (nm) Vashista and co-workers 10E-6 cm2/sec Mass spectrometric data shows same trend in ignition delay Effective Diff. Coeff. are consistent. Park et al., J. Phys. Chem. B, 2005 D= 10E-8 10E-9 cm2/sec

18 NanoCrystals of Al Core-Shell Reactive Oxidizers Fe2O3 KMnO4 Hi-Res Image Coating Al with Ni Elemental Map Fe O Mn CuO Al coated with Perfluorohexadecanoic Acid KClO4

19 Conclusions Development of new mass spectrometry tool enables us to probe reaction dynamics of ultra-fast condensed state systems at high heating rates. Metal oxides appear to release O2 and the ignition temperature would appear to correlate with O2 release. However, some other systems not discussed today show indications of heterogeneous reactions. Al from oxide coated particles seem to leak out by a diffusion mechanism. A very short transient ion generation pulse ( 20 us) occurs just at the ignition point ( not discussed today) New classes of nanocomposites offer opportunities to tune reaction profiles. Publications may be downloaded from