Aluminum - Fluoropolymer Combustion Prof. Michelle Pantoya J. W. Wright Regents Endowed Chair Professor Mechanical Engineering Department Acknowledgements: Army Research Office Dr. Ralph Anthenien Office of Naval Research Dr. Cliff Bedford October 2014
Overview Vision - Promote cleaner, safer, and more effective energetic composites through an understanding of basic combustion behaviors. Objective - Examine surface chemistry promoting Al reactivity with fluoropolymers Reaction Kinetics Synthesis Strategies 2
Al Powder Production < 25 microns Amorphous Shell Crystalline Core High purity Al introduced to a heated ceramic (2000 C) with an inert (Ar) gas flow. Vapor phase Al travels, nucleates, and coagulates Cools and crystalizes as a solid Oxygen introduced after solidification (<660 C). Typically in amorphous phase (~440 C - ambient) 3 Pesiri et al J. Pyro 2004 Johnson & Higa MRS Proccedings 1996 Kerns et al. Mat Sci. & Eng 2004 Amorphous g d/q a 0 200 400 600 800 1000 1200 1400 g-phase starts at 440 C
Exothermic Surface Chemistry Al-F Kinetics of Al + F Reactions Equilibrium Reactions DSC-TGA Analysis PTFE 50nm Al 10 mg samples 4 Osborne et al. Comb Sci Tech 2007 Dean et al. Thermochimica Acta 2009 Mulamba et al. Applied Surface Sci. 2014
50nm Al / PTFE (70/30) Fluorination of Al core Decomposition of PTFE PIR: Fluorination of Al 2 O 3 shell PTFE melt: 322 C Al melt 5 Osborne et al. Comb Sci Tech 2007
15-30% PTFE: 50 nm Al 30% PTFE PIR same for all stoichiometries 25% PTFE 20% PTFE 15% PTFE 6
Nano vs Micron Al - PTFE Kinetics Mass change Heat flow 50nm Al / Teflon 70/30 Nano-Al/Teflon more energetic than micron- Al/Teflon Nano-Al/Teflon shows 2- stage reaction Nano-Al/Teflon lost only 6% of mass Micron-Al/Teflon lost 25% of its mass Micron-Al/Teflon left largely unreacted Osborne et al. Comb Sci Tech 2007 1-3 um Al / Teflon 70/30 7
Hydroxyl Bonds to Alumina Surface III FT-IR of g-al 2 O 3 - Hydroxyl groups bound to surface in many ways II o Tetrahedrially coordinated aluminum (I) I g-al 2 O 3 o Two alumina ions with one in the tetrahedral coordination and the other in octahedral coordination (II) o Three octahedrally coordinated aluminum ions (III) F-Treated g-al 2 O 3 Sarbak Cryst. Res. Tech. 1997 Peri J. Phys. Chem. 1965 8
Role of surface hydroxyl groups To test the PIR dependence on OH surface groups, Al 2 O 3 was calcined at 550 C to decrease OH surface sites and then mixed with Teflon and analyzed. Smaller exotherm signifies less Al 2 O 3 fluorination, supporting reliance of Al 2 O 3 fluorination on surface OH groups. o Hydrated Al 2 O 3 used in water treatment fluoride extraction Extent of substitution depends on hydration of alumina Saniger et al., J. of Fluorine Chem., 88 (1998) 117-125 Al 2 O 3 /Teflon Calcined Al 2 O 3 /Teflon Fluorine OH substitution Sarbak, Cryst. Res. Technol., 32 (4) (1997) 491-497. 9
Exothermic Surface Reactions: Pre-Ignition Reaction (PIR) AlF 3 reaction is exothermic starting at 400 C (b-alf 3 ) AlF 3 transition from b to a phase causes exothermic peak at 552 C. o Also corresponds to Aloxidation contributing to overall heat of combustion 15nm g-al 2 O 3 +Teflon b-alf 3 formed b a 10
11 PIR effects on Teflon degradation PIR causes Teflon to degrade at lower temperatures In case of g-al 2 O 3 - Teflon o 60 C lower onset temperature. Stripping fluoride ions from polymer during PIR causes chain to become unstable, requiring less energy to degrade. Al 2 O 3 /Teflon Mass change Teflon 60 C
Al/Teflon Thermal Degradation ~ 400 C 400-500 C ~ 550 C 1. Fluorination of amorphous Al 2 O 3 via surface OH groups 2. Teflon becomes unstable and degrades 3. Al 2 O 3 g crystallization enables diffusion of fluoride ions to Al core for fluorination 12
Loose powder confined in a tube: Energy Propagation Acrylic tubing 10.0 cm length Instrumented with detectors spaced 1 cm apart 6 photo-detectors 6 piezo-crystal pressure sensors The Bockmon Tube Bockmon et al, J of Applied Physics 2005
Flame Speeds Flame speed - optic signals & high speed camera Pressure history mode of propagation & t rxn Bockmon et al, Journal of Applied Physics 2005
Affect of Fluoropolymer Chemistry Al/MP 1600 Al/MP 1000 Al/MP 1400 Al/MP 1400 Al/MP 1000 Al/MP 1600 Al/MP 1100 4 DuPont Zonyl MP PTFE vary in chain length MP1100 < MP1600 < MP1000< MP1400 Also in melt temperature 324 < 328 < 330 < 332 C Particle size constant ~10m PIR only observed for longer chain lengths Flame speed higher for PIR kinetics Mulamba et al. Ap. Surf. Sci. 2014 15
Flame speed (m/s) PIR and Flame Speed: Al + MoO 3 + PTFE 9 Al+MoO 3 8 7 6 5 4 3 2 1 0 400 450 500 550 600 6 5 4 3 2 Al + MoO 3 + 5%PTFE 280 260 240 220 200 6 5 4 3 2 1 272 252 229 0 5 10 %PTFE Al + MoO 3 + 10% PTFE 1 0 300 350 400 450 500 550 600 650 700 0 300 350 400 450 500 550 600 650 16
Flame Propagation Velocity (m/s) Aluminum + DuPont Zonyl MP 1100 500 460 406 400 300 165 200 100 0 Hexane 17 Hexane 165 m/s Nonpolar Isopropanol Isopropanol 406 m/s Protic Acetone Acetone 460 m/s Aprotic
-OH bonding via FTIR Wavenumbers for OH IR absorption are well defined Type Wavenumber Range Net charge on -OH Basicity is as follows: A > D > B > E > C No. of oxide nearest neighbor A 3785-3800 -0.5 4 B 3740-3745 0 2 C 3700-3710 +0.5 0 D 3760-3780 -0.25 3 E 3730-3735 +0.25 1 Knozinger and Ratnasamy Catal. Rev. Sci. Eng 1978 18 Peri J. Phys. Chem. 1965
Analysis: FTIR A b s o r b s n c e U n i t s 0.006 0.005 0.004 0.003 0.002 0.001 0.002 Hexane Type D OH bonds 0.0022 Isopropa nol 0.0051 Acetone 0.006 0 3771.34 3784.18 3785.18 Wavenumber (1/cm) A b s o r b s n c e U n i t s 0.005 0.004 0.003 0.002 0.001 Type C OH bonds 0.002 Hexane 0.0018 Isoprop anol 0.0049 Acetone Acetone and Isopropanol are strong Lewis acids & promote OH growth on the alumina surface Type D OH bond Strong Lewis Base 0 19 3703.89 3700.49 3700.89 Wavenumber (1/cm)
Analysis: Review Effect of Solvent Energy propagation rates are significantly affected by the solvent used to mix Al + PTFE Flame speeds Acetone > Isoporponal > Hexane No difference in physical mixing (SEM) Acetone and Isopropanol are strong Lewis acids & promote OH growth on the alumina surface Type D OH bond Strong Lewis Base From Sarbak s study: OH bonds on the surface of Al2O3 are dislodged by F produces exothermic chemistry Observed to be a rate determining step in energy propagation Greater contribution of OH from Acetone and Isopropanol may promote Al + PTFE reactivity By introducing more surface sites that catalyze the PTFE + Al reaction 20
Flexible Free-Standing Al-M x O y -F blends Develop deposition method for thin film thermites. Started with Mg-MnO based thermites Al-MoO 3 also successful Binder-Solvent systems: PVDF-NMP Viton-Acetone Paraffin Paraffin-Xylene Viton PVDF H H H H F H F F F H F H C C C C C C C C C C C C n H H H H n F H F CF3 n F H F H Films are cast using a double blade film applicator 21 Meeks et al. Combustion and Flame 2014
SEM Images Cross Section (1% PVDF, 40% Solids, ER:1, 4.39 mm) 22 Meeks et al. Combustion and Flame 2014
Characterizations 23 Meeks et al. Combustion and Flame 2014
Flame Speeds: Mg-MnO 2 -PVDF Mg-MnO 2 -PVDF most effective for preparation as a thin film coating Greater homogeneity of the mixture Increasing PVDF concentration resulted in higher heat of combustion Decreased flame speeds 24 Meeks et al. Combustion and Flame 2014
Al-I 2 O 5 PIR I 2 O 5 Kinetics I2O5 Heat Flow Reaction Stage Dissociation Δ r H Onset Temperature Stage A Endotherm I 2 O 5 IO 2 + IO 3 37.8 405 o C Stage B Exotherm I 2 O 3 + O 2-12.0 432 o C Stage C Endotherm IO 2 + IO 23.2 447 o C Stage D Exotherm I 2 + O 2-25.8 450 o C 25 Mulamba et al. J. Nanoparticle Research 2014 Chaudhuri et al. J. Chem Phys 2013.
Al/I2O5 Heat Flow and Mass loss PIR PIR occurs at lower temp for I 2 O 5 than C 2 F 2 26 Mulamba et al J. Nano Research 2014
Al2O3/I2O5 :Exothermic Surface Chemistry 27 Mulamba et al J. Nano. Research 2014
Leaner mixtures delay onset of PIR 28 Mulamba et al J. Nano Research 2014
Varying Heating Rates Consistent Stoichiometry A 10 o C min -1 B 20 o C min -1 C 25 o C min -1 D 30 o C min -1 E 40 o C min -1 29 Consistent PIR activation post 25 o C min -1 Activation energy (PIR) = 1.42 kj/g Mulamba et al J. Nano. Research 2014
Conclusions Fundamental Al-F surface chemistry analyzed o PIR caused by fluorination of the oxide shell via surface OH Fluoropolymer stability / Al surface area / hydroxyl concentration Newly synthesized Al-F based formulations show potential for a variety of tailorable composites o Fibers o Moldable flexible materials o Films Al-I exhibits similar surface exotherms - PIR o Heating rate governs manifestation o Stoichiometry governs onset o Activation energy related to propagation speed 30