john joseph granier- ignition and combustion behaviours of nano-composite energetic materials

8/3/2019 John Joseph Granier- Ignition and Combustion Behaviours of Nano-Composite Energetic Materials

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IGNITION AND COMBUSTION BEHAVIORS OFNANO-COMPOSITE ENERGETIC MATERIALS

byJOHN JOSEPH GRANIER, B.S.M.E.

A THESISIN

MECHANICAL ENGINEERINGSubmitted to the Graduate Faculty

of Texas Tech University inPartial Fulfillment ofthe Requirements forthe Degree ofMASTER OF SCIENCE

INMECHANICAL ENGINEERING

Approved

May, 2003


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ACKNOWLEDGEMENTS

My deepest appreciation goes to Dr. Michelle Pantoya for her encouragement,vision, and selfless efforts to promo te my success. I am grateful to my long-time bestfriend, S ara who I am lucky to call my wife. Thank you for nurturing my thirst for lifeand hunger for know ledge . Finally, I thank God for entreating me with such wonderfulgifts and opportunities.

I would like to gratefully acknowledge the Army Research Office (Contract NumberDA AD 19-02-1-0214 ) and our ARO program m anager. Dr. David Mann. I would like toacknowledge Mr. Steven Nicolich and Ms. Neha Mehta of the US Army TACOM-ARD EC for the support provided under contract number DA AE30-02-C-1132.I would also like to acknowledge that the electron micrographs of Aluminum andM olybdenum -trioxide powders were taken by Los Alamos National Laboratory. Thanksto Dr. Steven F. Son for permission to use these images.

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TABLE OF CONTENTS

ACKNOWLEDGEMENTS i iABSTRACT vLIST OF TABLE S viLIST OF FIGURES viiNOMENCLATURE viiiChapter

L INTRODUCTION 11.1 Therm ites 11.2 Com posite versus Monom olecular 21.3 Nanocom posite Therm ites 2

n . SAMPLE PREPARATION 52.1 Comp osite Powders 52.2 Pressed Com posite Pellets 7

2.2.1 Com position Study 92.2.2 Particle Diameter Study 12

m . EXPERIMENTAL SETUP 143.1 Lase r 153.2 Measuring Ignition Time 163.3 Measuring Flame Propagation 17

IV. THEO RY 184.1 Ignition Tim e Calculations 184.2 Diffusion vs. Chem ical Kinetics 21

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V. RESULTS 245.1 Com position Study Results 24

5.1.1 Ignition Time M easurem ents 245.1.2 Bum Rate M easurements 26

5.2 Al Particle Diam eter Study 285.2.1 Ignition Time M easurements 285.2.2 Bum Rate M easurements 30

VL DISCUSSION 326.1 Comp osition Study 326.2 Particle Diam eter Study 336.3 Effect of Alum inum Oxide Layer 366.4 Ignition Tem perature Estimates 386.5 Nan o versus. Micron Therm ite Kinetics 39

VIL CONCLUSIONS 43REFERENCES 45

IV


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ABSTRACT

Advances in material processing have created nano-sized fuel particles withenhanced properties ideal for exothermic reactions compared with those of largerparticles of the same material. As a result the combustion associated with nano-scaleparticles has recently shown the potential for enhanced energetic behavior of thermites.In this study, the ignition sensitivity of an aluminum molybdenum-trioxide (AI/M0O3)composite mixture was examined.

Laser ignition experiments were performed to determine the ignition time and bumrates of newly developed nano-Al mixed with M0O3. The scope of this work can best besummarized in two stages: first, the ignition time and bum rates for three nano-regime Almixtures were studied over a varied composition range, the results were then used tofurther study ignition time and bum rate as a function of Al particle diameter, whichranged from 17.4 nm to 20 [xm at the optimum stoichiometric composition. A 50-W CO2laser provided the ignition source and high-speed digital images were used to determineignition time and bum ra tes. Results indicate that a slightiy fuel rich (equivalence ratio of1.2) mixture presents the earliest ignition times and fastest bum rates for the nano Alparticle diameters. Experimen ts using a slightly fuel rich mixture for ten Al particlediameters show an increased sensitivity to ignition with decreasing particle size, adecreased bum rate with decreasing particle size and less variability of ignition times andbum rates for nano-regime A l.


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LIST OF FIGURES

1.1 Surface area to volume ratio as a function of Al particle diam eter 32.1 SEM of Nano-Al spheres 62.2 TE M of Nano-Al particle with AI2O3 oxide layer 62.3 SEM of Al (spheres) and M0O3 (crystals) mixture 72.4 Pressed pellet sample (1:1.5) 82.5 Acrylic die assembly 92.6 Outer cylinder flame propagation (39.2 nm AL 1:1.5, (|)=1.02) 112.7 Pressed pellet sample (1:1) 133.1 Equ ipmen t setup 143.2 Exp erimen tal apparatus 153.3 Gaussian beam intensity Profile (Spiricon, Inc.) 163.4 Ignition timing sequence 174.1 Flame structure diagram 184.2 Constant surface heat flux diagram 195.1 Ignition sequence (39.2 nm AL 1:1.5, (|)= 1.02) 245.2 Ignition time as a function of comp osition 255.3 Planar flame propagation (39.2 nm AL 1:1.5, (1)=1.02) 275.4 Bu m rate as a function of com position 275.5 Ignition time as a function of Al particle diameter ((t)=1.2) 295.6 Bu m rate as a function of Al partic le diam eter ((|)=1.2) 306.1 Non-dim ensiona l ignition time and deviation ratios 346.2 Non-dim ensiona l bum rates and deviation ratios 346.3 Ignition time as a function of bum rate 356.4 Ignition time effects on pre-reaction zone temperatures 376.5 Calculated temperature history 386.6 Con tact points illustration 406.7 Bu m rate as a function of nanometer regime Al particle diameter ((t)=L2) 42

vn


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NOMENCLATURE

Variable

A AreaA Arrhenius pre-exponential factorB Mass transfer num berC Species concentrationd Particle diameterD Pellet diameterD Mass diffusivityEa Activation energy/ Fuel/ox idizer contact frequencyk Thermal conductivityk Surface reaction rate constantL Pellet length

Sf M ass flux (Solid fuel gaseous oxid izer)^ Adjusted mass flux (Solid fuel - solid oxidizer)q" Constant Heat fluxr Rad ial dimensionR Gas constantR Spherical fuel particle outer radiusS Particle surface areaT Temperatureu Bum vleocity

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VXY

Greeka8.-e

P0 )

Subscripts20|imCalcdelay

FG

i, in iignmpoxPR

Particle volumeAxial dimensionMass fraction

Thermal diffusivityReaction zone thicknessEquivalence ratioPure fuel densityReaction rate

Correspond to data point of 20fxm Al particlesCalculated valueElectro-mechanical shutter delayFuelCorresponding to "Go/no g o" criteriaInitialIgnitionMelting pointOxidizerLaser pulseOuter particle radius

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CHAPTER IINTRODUCTION

1.1 ThermitesThermite reactions can be described as exothermic reactions involving the reduction

of metallic oxides with aluminum to form aluminum oxide and metals or alloys.Thermite reactions are characterized by large heat release, which is often sufficient toheat the product phases above their melting points. For exam ple, a common thermitereaction between aluminum (Al) and molybdenum trioxide (M0O3) has an adiabaticflame temperatures of 3253C, which well exceeds the melting points of molybdenumand aluminum oxide (AI2O3) [1].

2A I -f-M0O3 -^ Mo -h AI2O3 AH^b = 4279cal/cm'Because of the large heat generation, thermite reactions can usually be initiated locallyand the reaction becomes self-sustaining.

Thermites can be used to create a new material through combustion synthesis orcan be used strictly for their energy release rates in military applications. A variety ofmetallurgical applications take advantage of the thermite reaction products in a moltenmetallic phase and oxide phase. Because of the control of product formation, thermitesare used in the synthesis of ceramic and composite materials and in the preparation ofceramic lining s in metallic pipes. The high temperatures achieved in this reaction allowthe formation of a new material under conditions (i.e., high heating rates, high flametempe ratures) that are otherwise impossible to achieve. Therefore, the synthesizedproduct can be tailored to have specific physical properties based on the selection ofreactants [2]. Som e thermites are used specifically for ordinance applications. Thesetypes of reactions (i.e., Al and M0O3) give off so much energy that the products arespewed in all direction s. In these reactions, the goal is to combine reactants such that thesensitivity to ignition and energy release rate are optimized.


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potential to produce high energy release rates and approach those of a monomolecularenergetic material.

The ignitability of a thermite is an extremely important performance parameter.Knowledge of the ignition energy and delay time will allow materials to be tailored forspecific applications. For example, parameters such as ignition energy and ignition delaycould dictate which composition is most appropriate as a primer in a weapon system.Determining if ignition sensitivity increases with decreasing particle size may allowlighter and more mobile energy sources to become incorporated in the weapon system.Discovering thermites that require less ignition energy could potentially reduce theoverall payload of the ordnance system. Also, understanding the ignition sensitivity ofthermites is critical for their safe use and handling.Reduction of aluminum particle size from the micron- to nano-scale dimensionsconsiderably increases the surface area to volume ratio and potentially enhances thematerial's reactivity (Figure 1.1). A larger surface area to volume ratio will allow greaterintermixing and will reduce the diffusion distance between fuel and oxidizer particles.Because these reactions are diffusion controlled, decreasing the diffusion distance couldpotentially permit increased sensitivity to ignition. Essentially the increased SA^ ratioallows more ftiel to be in contact with oxidizer and increases the amount of material thatwill react at any instant in time.

0) 0.4I 0.35o 0.3^ 0.25^ 0.28 0.1"5 0.05m n

- - '._

. > ^ ^ - , ^ -50 100 150 200 250Aluminum Particle Size (nm)

Figure 1.1 Surface area to volume ratio as a function of Al particle diameter


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Generally, nano-particles are approaching molecular dimensions and intermixing atthis level enhances the characteristics of the mixture over the traditional micro-sizedtherm ite mixes [5]. Therm ites are traditionally considered a difficult class ofpyro techn ics to ignite due to the large thermal stimulus required [6]. This is becauseignition is usually described by the melting of one of the two components followed by adiffusion-controlled reaction. The melting point for aluminum is 660 C and M0O3beg ins to sublime at 750 C. Nano-scale particles may be so highly reactive thatsolid/solid physical contact may be sufficient for ignition. Ignition temperatures atmelting or near melting of the M0O3 and Al may not be a prerequisite for the reaction.

Ignition sensitivity is determined by the time to ignition. A thermally ignitionsensitive com posite w ill ignite faster than a reference standard for the same material. Inthe following experiments a standard ignition time for traditional micron-scale Aldiam eter compo sites of AI/M0O3 are 0.09 to 6 sec. Therefore improving ignitionsensitivity would require a reduction in ignition time by at least 50%.

The objective of this study is to compare the ignition time and bum rate ofcom pressed pellets of nano and micron-scale AI/M0O3 com posites. Laser ignitionexperiments were performed on various composition samples and high speed imagingallowed analysis of ignition time and reaction propagation. Three Al particle sizes wereexamined: 108, 39.2, and 29.9 nm diameter to determine an optimum fuel/oxidizercom position of AI/M0O3 based on earliest ignition time and highest bum rate. A slightlyfuel rich mixture was then further used to analyze the effect of varying Al particlediame ters from 17.4 nm to 20 |im. Ignition time and bum rates were measured andanalyzed for 10 nano-regime and 3 micron-regime Al particle diameters and the M0O3remained constant


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Figures 2.1 and 2.2 are electron micrographs representative of the Al shown inTable 2.1. Figure 2.1 clearly shows the solid spherical geometry of the nano-Al powders.Figure 2.2 illustrates the existence of AI2O3 oxide layer formed around the surface of theAl particles. Figure 2.3 displays the heterogeneity of the AI/M0O3 mixtures and themeshing the solid Al spheres with the sheet-like M0O3. All of these images were taken atLos Alamos National Lab under the direction of Dr. Steven F . Son.

]0.1 \im\Figure 2.1 SEM of Nano-Al spheres

Figure 2.2 TEM of Nano-Al particle with AI2O3 oxide layer


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MAG= 5 2 KX5 0 0 kV

Figu re 2.3 SEM of Al (spheres) and M0O3 (crystals) mixture

The Al and M0O3 powders were first measured to achieve a specific mass ratio orequivalence ra tio. The stoichiometric mass ratio for the reaction is 0.375 based onassuming the following global reaction.

2A1 -I- M0O3 -^ AI2O3 + M oThe equivalence ratio ((])) is determined from Eq. (2.1) and considers the actual Alcontent for each mixture (Table 2.1) [8]

< t ) = CT (2 .1)ST

In this equation, F represents fuel (Al), A is oxidizer (M0O3) and the subscripts ACT andST indicate the actual and stoichiometric ratios.

The powder mixture was suspend in a solution and mechanically mixed using sonicwaves. The sonication process helps break up macro-scale agglomerates and ensure bettermixing of fuel and oxidizer. The mixture was heated to a low temperature, which allowsthe solution to evaporate leaving a well-mixed pow der composite. The powder was siftedthrough a 200-|j,m pore wire mesh to eliminate macro-scale agglomerates formed duringthe drying process . The dry sifted p ow der was then used for further testing.

2.2 Pressed C omp osite Pellets


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The final mixture was divided into either 180 or 125 mg proportions and pressed intosohd cylindrical pellets at a 1:1.5 diameter to length ratio (D/L) as seen in Figure 2.4 or a1:1 D /L ratio as seen in Figu re 2.7. The pellet lengths were decreased with anappropriate mass decrease (to achieve the same bulk density) for the particle diameterstudy to create stronger pellets with a more uniform density. Both the 1:1.5 and 1:1pellets were pressed to 38% theoretical m aximum density (TM D) (See Section 2 .2.1).

Figure 2.4 Pressed pellet sample (1:1.5)

The pellets were pressed in acrylic dies (Figure 2.5). The plunger is purposely madelonger than the acrylic die to extrude the solid pellet after compression. The bottomplunger is inserted into the die until sealing with the shelf of the inner diameter (barrel).The barrel of the acrylic die is then filled with loose powder and pressed to a specificpellet leng th. The leng th is fixed to achieve a specific bulk pellet density and guaranteedby shim s inserted between the top plunger head and the top of the die. Once com pressed,the shims and bottom plunger are removed. The solid cylindrical pellet is then extrudedinto the larger diameter barrel of the bottom plunger. This technique has proven to be asafe and effective method for pressing composite pellets and has not resulted inspontaneous ignition.


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Figure 2.5 Acrylic die assembly

Initial efforts to compress the powder between equivalent plunger diameters fromeach end of the barrel resulted in spontaneous pre-ignition of the mixture. Duringcompression, the pellet locally stresses and expands the inner diameter of the die (evenwith a tool grade steel barrel). The barrel was reamed to 4.366 mm diameter andcom pressed pellets wou ld locally expand to 4.521 mm in diameter. Upon extmsion, thelarger diameter pellet would be forced into a smaller diameter barrel creatingcom pression ignition w ithin the die. Problems also arose from po wder creeping up theedge s of the barrel and wedging between the barrel and the plunger shaft. This created anunsafe situation by requiring tremendous force on the trapped energetic material in theprocess of freeing the wedged plung er. The acrylic dowels were chosen because they areeasily disposable (i.e., less expensive than steel) and replaceable if cracked or destroyedfrom a wedged plunger.

2.2.1 Com position StudyThree Al particle diameters were tested over a range of five fuel to oxidizer mass

ratios (30% to 70% A l). The equivalence ratios for each mixture are tabulated in Table2.2. It is noted that (|)


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Tab le 2.2 Equ ivalence ratios

ParticleDiameter

108 nm39.2 nm29.9 nm

Fuel / Oxidizer Ratio30/70 40/60 50/50 60/40 70/30Equivalence Ratio (t)0.830.660.51

1.291.020.791.941.531.18

2.912.301.774.453.582.76

Approximately 700 mg of AI/M0O3 were created for each composition and diameterpresented in Table 2.1 . The theoretical maxim um density was calculated for eachmixture as a weighted average of the pure solid densities of the three constituentmolecules (A l, M0O3, and AI2O3). From the 15 stock powders, four cylindrical pelletsamples of approx imately 150 mg were created at 38% theoretical maximum density(TM D). The pellets were compressed to an axial length of 6 mm (to provide a longerbum propagation distance) with a diameter of 4.52 mm, approximately a 1:1.5 diameterto length ratio . Figu re 2.4 is a representative pellet at the 1:1.5 diameter to length ratio.

One concern that should be addressed is the possibility of density gradients withinthe pellet. Each pellet likely contains radial density gradients due to adaptations in theacrylic die design. As mentioned earlier, the bottom plunger was made with a largerdiam eter to preven t ignition upon extm sion of the solid pellet. This adaptation creates anuneven p ressure distribution between the top and bottom plungers. It is expected that thestress distribution within the composite material was non-uniform due to the unequalplunger diameters making the solid cylindrical pellet denser in the radial center withdensity decreasing towards the outer radius. The density gradient may not be acontinuous slope from center to outside edge, but more likely a like less dense layerexists on the outer curved wall of the cylinder.

The density gradient was proven in a few recorded videos by the pellet igniting onthe front plane center, propagating radially to the outer edge then racing down the outeredge wall in the axial direction. This may be a result of less dense material along the

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outer radial portion of the pellet. Less dense powders have higher bum rates thancompressed pellets [10].

Figure 2.6 supports the argument for lower outer surface density in the pressedcylindrical pellet. Ignition occurs in the center of the front p lane at frame 1839 or time33.25 ms. The reaction zone quickly expands to near the outer surface (1880) thenprop agates ax ially under a small subsurface shell. Frame 1920 shows that the reactionhas reached the end of the pellet and then the outer shell is thrown off in frames 1940-2020. The pellet begins to lift off the stand at the front end in frame 1940. Frames 2040-2080 show the cylindrical pellet core spinning above the sample-mounting stand. Inframe 2080, the pellet has rotated and the front plane is actually facing the camerashow ing that the pellet is now burning from the outer radius inward. For each pictureshown in Figure 2.6, 20 frames were taken in between, but not shown for conc iseness.

Illuminated sample

188012.8125 ms190019.0625 ms192025.3125 ms194031.5625 ms196037.8125 ms

Figure 2.6 Outer cylinder flame propagation (39.2 nm Al, 1:1.5, (t)=1.02)

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198044.0625 ms200050.3125 ms202056.5625 ms204062.8125 ms206069.0625 ms208075.3125 ms

The outer shell may result from sample contamination due to die cleaners andacrylic wear. In the process of machining the acrylic dies, special care was taken toprevent frictional m elting and to provide a clean inner barrel surface finish. A water-soluble coolant was used along with small interval bites with the tool steel drill bits.Normal cleaning chemicals such as acetone, isopropyl alcohol and methyl alcohol reactedwith the acrylic die weakening the barrel surface and the entire die's strength whenstressed in com pression.

Also by using a metal plunger with an acrylic die, micro acrylic shavings could bescraped off the barrel walls and pressed into the sides of the solid pellet. This wouldleave a non-reactive zone near or at the surface of the pellet, which acts as a conduction-inhibiting wall and promotes a non-symmetric density gradient forming stress cracks inthe sohd cylinder.

2.2.2 Particle Diameter StudyIt will be shown in the following chapter, that the slightiy fuel rich mixtures were

more sensitive to ignition and burned faster for the three diameters study. Based on the12


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CHAPTER 111EXPERIMENTAL SETUP

A 50 W CO2 laser was used to ignite the flat end of a horizontal cylindrical pellet.An electro-mechan ical shu tter controlled the samples exposure to the irradiation. APhantom IV high-speed camera was triggered to record from the instant of sampleexposure through ignition at 32,000 frames per second (fps). A schematic of theexperimental set-up is shown in Figure 3.1

5 0 W C Oj La se i1 0 .6 nm wave le ng t i i

Shut te i

Powei Mete i w i thDigi ta l LCD Display

P r essed S amp le

PC withImag ing Sof t .va ie

T i i gge i i ng Dev i ce

H igh - S pee t t - Came fa^

Figure 3.1 Equipment setup

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Power Meter Shutter COs Laser

Figure 3.2 Experimental apparatus

3.1 LaserThe experiments were carried out using a 50 W CO2 laser (Universal Laser UL-50-

OEM ) with a wavelength of 10.6^m. The beam diameter is 4 mm with beam divergenceof 5 1 mR ad, producing a maxim um power density of approximately 100 W/cm". Thelaser was operated in continuous w ave (CW) mode and held constant at 50 W. The beampower was measure at regular intervals with a thermal power meter seen in Figure 3.2.The laser was operated in its original state of a Gaussian beam intensity distributionsimilar to the intensity profile seen in Figure 3.3. A Gaussian beam can be described asmajority of the power intensity is at the center of the beam and trails off to near zero atthe outer edge of beam.

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'^


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the same triggered shutter and recording the red dot's first appearance with the highspeed camera at the same time resolution of 32 kfps. The electro-mechanical delay wascalculated as the time difference between time zero (actually triggering the shutter) andfirst view of the red diode laser. Figure 3.4 is a simplified diagram illustrating theove riap of reco rded and laser irradiation time intervals. In this diagram tdeiay correspo ndsto the shutter delay time, tign is the ignition time and tp is the laser pulse tim e.

Signals

10 1

. . ./ Igr ition

~ [delay tig

c' '

,t [ms]

Figure 3.4 Ignition timing sequence

3.3 Measuring Flame PropagationThe burn rate was measured with the high-speed camera and imaging software.

The Vision Research software provided with the Phantom IV camera also provided ameans of post processing video data. By establishing a reference length (such as the mlerin Figure 2.4), the software determines speed based on a distance between sequential timeframes. Using a "find ed ge" image filter, the planar reaction front location could easilybe identified and marked for speed measurements. The "find edg e" feature, distinguishespatterns in light intensity between adjacent pixels creating an emphasized contrasting lineat the actual flame front.

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CHAPTER IVTHEORY

4.1 Ignition Time CalculationsIt is traditionally thought that thermite reactions such as Al and M0O3 begin by the

me lting or sublim ation of one of the reactants. Since the bimolecular reaction is diffusioncontrolled, micron scale and largerreactants are more likely to diffuse after melting of Alat 660C or sublimation of M0O3 at 750C. The flame stmcture can be simplisticallyillustrated in Figure 4.1 (corresponding to progressively later times).

Laser ->^

flux > />

Phase Transfoniiation^ ZoneU-

^ T Sohdi f 1 melt^"s.,. ^^ Reactants T-"-ini \1 >

^llw -^^

Reaction Pre-heatZone Zone

MoltenTPiodiicts i. V Solid:^i t ReactantsFlame Propagat ion

Bum Rate , / /

Figure 4.1 Flame stmcture diagram

X

b)

c)

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The laser heat flux irradiates the front surface of the AI/M0O3 solid pellet raising thebulk solid temp erature (Figure 4.1 ). Laser heating continues until one or both of the solidreactants reach its melting or sublimation tem perature and creating a phase transition. Asthe temperature increases, the phase transformation zone is formed where reactants maybe in a solid, liquid or gaseous state. The multi-phase nature of this zone enhancesmo lecular diffusion (Figure 4.1 b). Further increase in temperature leads to initiation ofthe chemical reaction and formation of a flame and reaction zone. Exothermic reactionsallow a self-sustained flame propagation. For thermites the reaction zone temperature ismuch greater than the melting temperature of both reactants leaving the products in amolten sta te. Th is wou ld best be represented as a step function in the temperature profilebetween the phase transition zone and the reaction zone but is shown as a gradual curvefor graphical purposes (Figure 4.1 c).

Based on inert heating up until ignition, the ignition time can be estimated based ona semi-infinite solid analysis. An analytical solution can be derived for a semi-infinitesolid exposed to a constant surface heat flux (CO2 laser irradiation) as seen in thediagram below.

^Ic >50 \V Lcisei

KI .Mmcl i ica l Pel le t

(k . a iFigure 4.2 Constant surface heat flux diagram

Beginning with the 1-D heat diffusion equation,ar d-T = oc rdt dx' (4.1)

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Boundary conditions are applied for a constant unifonn heat flux and assuming aninitial temperature (7,)

T{x,0) = T,

- k dx(4.2)

= ^ox= 0

The solution becomes [11]

T{x,t)-T,= Iq'Scalny" -exp^-x'^Aoa Ro^ erfc ^lyfat j (4.3)Since the energy input is the largest at the front surface, ignition is most like to occur atx=0 and Eq. (4.3) is reduced to

^x=0.r=0 ~ 'i *""2?;'- TT (4.4)

Eq. (4.4) is solved directiy for ignition time base on an initial temperature of 37C and anignition temperature of 660C corresponding to the melting of aluminum.

'ign.CalcKa

nT,-T,)2^: ) (4.5)

Note that the thermal properties k and a must be estimated as a weighted average of thethermal prop erties of A l, AI2O3, and M0O 3. The thermal properties are also assumed toremain con stant w ith increasing tem perature. The individual thermal properties for threeconstituent molecules in the composite and four representative composite mixtures (basedon ranges of pure Al concentration) are presented in Table 4.1.

Table 4.1 Com posite thermal properties [11]

k [W/m-K]Cp [J/kg-K]a [m^/s]

Al237903

9.71 E-05

AI2O346

7651.51 E-05

M0O350

8823.78E-05

30-45% A lmixture83.85

875.306.39E-05

45-60% Almixture92.54879.38

7.02E-05

60-82% Almixture101.09883.40

7.63E-05

97-99% A lmixture121.84887.58

9.15E-05

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The pure Al concentrations used for grouping the composite mixture properties areshown in Table 2.1 and averaged for the four ranges shown in Table 4.1. The ignitiontimes calculated from Eq. (4.5) based on the thermal properties in Table 4.1 range from 2to 3 sec.

Table 4.2 Ignition time calculations

tjgn.Calc [S]

30-45% Almixture2.12




4.2 Diffusion vs. Chemical KineticsThe combustion behavior of these reactions can be explained by describing the rate

of conversion from reactants (Al and M0O3) to products (AI2O3 and Mo). The reactioncan be reduced to considering a single small sphere of fuel (Al) and surrounding oxidizer.First assum e that the reaction o ccurs at the surface of the solid Al sphere and the reactionis either limited by the chem ical reaction rate or the oxidizer diffusion rate. The reactionrate (CO) can be exp ressed as

(0 ^C^jCj^oOj (4.6)assuming the following global reaction.

2A1 + M 0 O 3 -^ A I2O 3 + ]VIoIn Eq. (4.6) C^i and C^^Q^ are the concentrations of the reacting chemical species Aland M0O3 respectively. Each concentration is raised to the pow er equal to thecon-esponding stoichiom etric coefficient. The specific reaction-rate constant, k can beexpressed as an Arrhenius law according to Eq. (4.7).

k = A exp RT ; (4.7)

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Assuming that the spherical nano-scale particles do not vaporize prior to combustion, i.e.,they burn on the surface as oxidizer diffuses to the surface. The mass-bum ing rate (r&.)of the sphere is given by

fe= AnR^r^ = 47ci?pZ) ln(l + B) (4.8)where / i ^ i s the mass flux at the surface, D is the mass diffusivity and B is the masstransfer num ber. The m ass flux from the particle surface would equal the ftiel reactionrate. The reaction rate is first order in oxygen concentration and second order in fuelconcentration as in Eq. (4.6).

n ^ = c o = / :Y ,^ Y^ % (4.9)Assuming that the reaction rate occurs at the particle surface, the fuel mass fraction {YFM)in the gas phase is small compared to oxygen such that:

Y^,co - 0Y ,, . = 1Y o . , . l .The mass transfer number can be expressed as^ = ox,o.-\x,R (4.10)

andY o . . < Y , , , ^ < l

The small number approximation can replace the natural log function in the mass fluxterm.

4 = ^ l n ( l + B ) = = ^ ( Y , , , . - Y , , , , ) (4.11)Comb ining Eq. (4.9) and E q. (4.12), the mass flux at the particle surface is given in

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"^ = Y, . Rk + pD

R J(4.12)

In diffusion controlled reactions, the reaction rates are fast com pared to the oxygendiffusion rates. Here k p D / R and the mass flux is function of the particle radiusaccording to Eq. (4.13).

< = \ pDR (4.13)If the reaction rates are comp arable to or slower than the oxygen diffusion rate, then thechem istry contro ls the reaction. In this case k f>D IR and the mass flux isindependent of particle radius (Eq. (4.14)).

A=k o., (4.14)For diffusion controlled rea ctions, the mass flux (Eq. (4.13)) and subsequent

reaction rate are inversely proportional to particle radius and directly proportional tooxygen conce ntration . Eq . (4.13) suggests that smaller particles (nano-regime) willproduce higher mass flux reaction rates than larger particles (micron regime).

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CHAPTER VRESULTS

5.1 Composition Study Results5.1.1 Ignition Time M easurements

The sequence of ignition and flame propagation for a 40/60 and 39.2 nm Al sampleis shown in Figure 5.1 By comparing images 1109 through 1117 to the first image,ignition starts in the center of the cylinder face, and propagates to the outer radius, thenpropagates in the axial direction down the outside wall of the cylinder.

Image 00.0 s11090.03465625 s

11100.03468750 S

11110.03471875 s

11120.03475000 S

11130.03478125 s

11140.03481250 s

11150.03484375 s

Figure 5.1 Ignition sequence (39.2 nm Al, 1:1.5, (t)=1.02)

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11160.03487500 s11170.03490625 s

Ignition times were recorded for three to four pellets for each composition and particlediameter and the results are shown in Figure 5.2.

CO

EE

300250200150

g1100_D)

50

1 1

A1

1

108 nm 39.2 nm 29.9 nm

i0 1 2 3 4Equivalence Ratio

Figure 5.2 Ignition time as a function of composition

Figure 5.2 indicates that ignition time is a strong function of composition. For eachAl particle diameter studied, the ignition time corresponding to slightiy fuel richcondition was the fastest at approximately 20 ms. The largest variability in ignition timewas observed for the largest Al particle diameter composites, which ranged from 20-260ms. As the 108 nm Al particle composite becomes increasingly fuel rich, considerablemore energy is required to ignite the pellet. This can be determined from the laser power(50 W) and the ignition time (Figure 5.2). For example, 13 J is required to ignite the 108nm Al particle composite with a 70/30 AI/M0O3 ratio {^=4.45). For the same diameter(108 nm) at a composition of 40/60 ((j)=1.29) only 1 J is required.

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The 39.2 nm Al particle composites exhibited a reduced range of ignition times,from 7-50 ms. Further decreasing the Al particle size to 29.9 nm leads to an increasinglymore narrow range of ignition time, 21 to 37 ms, for various compositions.

Overall, the smallest particle size composite exhibits more consistent ignition timesregardless of composition. This behavior may be attributed to the increased homogeneityassociated w ith the smallest particle compo sites as seen in Figure 2.3. Exam ining thesurface area to volume ratio of Al particles allows a quahtative comparison of sohd-solidcontact between Al and M0O3. The surface area to volume ratio (S/V) can be expressedin terms of particle diameter (d) as 6/d assuming spherical particles. As the particle sizedecreases from 108 to 29.9 nm, the S/V increases from 0.06 to 0.20 (Figure 1.1).Assuming the same level of mixing for each sample, the smallest Al particle diametersallow increased solid/solid contact between Al and M0O3, thereby increasing thehomogeneity of the mixture. This increased homogeneity may promote a more narrowrange of ignition time as a function of particle diameter.

Based on preliminary results from the composition study, nano-composite samplesignite on the order of ms contrary to the ignition times predicted in Table 4.2. Thissuggests that nano AI/M0O3 mixtures do not require heating to phase transition (meltingor sub liming). Further suggesting that nano-com posite thermite reactions may not bediffusion dependent.

5.1.2 Bum Rate MeasurementsThe corresponding bum rates were determined by following the luminescent flame

front in the axial direction along the cylindrical sam ple. Figure 5.3 shows a sequence ofimages that illustrate the fairly planar flame propagation through the pellet. The bumrates for all compositions and particle diameters are shown in Figure 5.4.

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11170.0349375 s11230.03509375 s11280.03525000 s11330.03540625 s11380.03556250 s11430.03571875 s11480.03587500 s

Figu re 5.3 Planar flame propagation (39.2 nm Al, 1:1.5, (t)=1.02)

20I 15^ 10DCc00

0

-

y^

A 1

A i 1

108 nm 39.2 nmA29.9nm

0 1 2 3 4 5

Equivalence RatioFigure 5.4 Bum rate as a function of compo sition

Figure 5.4 indicates that the bum rate is also a strong function of samplecom position and directly related to the ignition time. Again the largest variability is

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displayed by the 108 nm Al mix tures, ranging from 1-20 m/s. Similar to ignition timetrends, the variability between compositions decreases with decreasing particle diameter.

For all three Al diameters shown, it is obvious that the fastest bum rates are atslightiy fuel rich com positions. It is noted that for approxim ately equal ignition times forall three d iameters (?,. = 20m s for ( ^ - 1 .2 ) , the 108 nm Al bum rates are twice themag nitude of the 39.2 and 29.9 nm Al mixtures. This may be attributed to the fact thatthe 108 nm Al mixtures have 72.7% pure Al (Table 2.1) resulting in 0.3 aluminum oxideto aluminum ratio.

Table 5.1 Alu min um o xide to pure Al ratiod

[nm]108

39.229.9

Al (AI+AI2O3powders)r% i72.757.544.3

AI2O3Al ' ^^^

0.380.741.26

This ratio is approximately a half to a fourth the value for the 39.2 and 29.9 nm Almix tures. Th is oxide layer concentration is further discussed in Chapter V.

5.2 Al Particle Diameter StudyTo study the effect of particle size for the optimum slightly fuel rich mixtures, seven

pellets were created for the 10 nano-Al diameters listed in Table 2.1 and 30 pellets werecreated for the micron A l diameters (3-4, 10-14, and 20 ^im). As mentioned previouslythe pellets were compressed to 38% TMD with a 1:1 diameter to length ratio.5.2.1 Ignition Time M easurements

Ignition times were recorded for six to seven pellets for each nano-Al particlediam eter mixtu re and 10 to 20 pellets for each micron-Al particle diameter mixture. Theignition tim e as a function of Al particle diameter is illustrated in Figure 5.5. The samplenumber decreased for the micron-size Al diameter mixtures because the ignition timesoften exceeded the allotted camera memory or samples simply did not ignite. The

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deviation bars in Figure 5.5 are generated based on the standard deviation of the datacollecte d. It is noted that the error bars for the micron-A l particle diameter samples aredeceiving due the log scaling of ignition time. The actual standard deviation values canbe seen in Table 5.2.

10000

0) 1 0 0 0E

"EO )O )o

1 10 100 1000 10000 100000log Al Particle Dia [nm]

Figu re 5.5 Ignition time as a function of Al particle diameter (0=1 .2)

Figure 5.5 clearly shows that the nano-regime Al particle diameters are far moresensitive to thermal ignition because they ignite faster than the micron Al particlemix tures. Tab le 5.2 show s that the nano-regime Al particles consistently ignite in 10-20ms for all nano-Al diam eters investigated (17.4 to 202 nm ). But unlike initialconclusions made by the composition analysis, ignition times within the nano-regime Almixtures do not necessarily increase with diameter or even show a consistent trend inreference to particle diame ter. The nano-diam eter Al samples also exhibited the mostcons istent and repeatab le results (in terms of measurem ent standard deviation). Thenano-scale Al diameter mixtures display a maximum ignition time standard deviation of12.6 m s.

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The micron-scale Al diameter mixtures result in more variability than the nano-scaleAl particles. For exam ple the mixtures comp osed of 3-4 |im Al diameter particles igniteat about 90 ms, which is four times slower than the nano-regime samples. Also, the 3-4(Xm samp les hav e a larger standard deviation than observed with the nano-regimecom posites. Th e 3-4 |im samples display a four times increase in standard deviation fromthe nano regime standard d eviation.5.2.2 Bum Rate Measurements

Bum rates were recorded for four to six pellets for each nano-Al particle diameterand 8 to 20 pellets for each micron -Al partical diameter. The results are shown in Figure5.6. The sample num ber decreased for the nano and micron samples because bum rateswere only measured for samples with approximate planar propagation.

50.0040.00

10 100 1000 10000 100000log Al Particle Dia [nm ]Figu re 5.6 Bu m rate as a function of Al particle diameter (0=1.2)

For each Al particle diameter, the fastest bum rate corresponds to the shortestignition time and the slightly fuel rich case. Generally, bum rate varied from 1 to 30 m/s.The largest va riability in bum rate , 14 - 17 m/s, was associated with the largest Al

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particle composites {X0-20\im). As Al particle diameter is decreased, a correspondingreduction in the variability of the bum rate is observed.

Table 5.2 shows the actual data points plotted in Figures 3.6 and 3.8 and thecorresponding standard deviations.

Table 5.2 Al particle d iameter num erical resultsAl Particle[nm]

17.424.929.939.252.775.9100.9108153.820 23-4 \im10-14 ^m

20 nm

Ignition Timeavg [ms] J24.2121.7318.3921.9315.5520.7614.5617.3125.4912.4089.431384.13

6039.43

St dev [ms]8.7612.6010.3812.006.576.904.694.3711.882.6852.82736.05

847.18

Burn Rateavg [m/s]2.163.231.643.1711.236.815.556.406.048.261.2029.9222.91

St dev [m/s]1.391.270.141.754.120.731.650.961.335.480.8517.1414.89

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CHAPTER VIDISCUSSION

6.1 Composition StudyPrice et al. showed that the oxide coating formed on the surface of aluminum

particles forms a reaction barrier or inhibition for the initiation of the aluminum oxidationreaction [9]. In the three diameters studied in the composition study, the oxide shellvaried from 3.9 nm to 2.5 nm (Table 2.1). However, the increased thickness of the shellwas not found to inhibit the ignition time for slightly fuel rich compositions. In fact, theoxide shell thickness and Al particle diameter appear independent of ignition time for theslightly fuel rich case (Figure 5.2). But the oxide layer concentration may effect bu mveloc ities and reaction zone propa gation. In Figure 5.2, all three Al particle diametersignite in 10-25 ms for an equivalence ratio of 1 to 1.4. Figure 5.4 illustrates that bumrates are highest for all three Al particle diameters for an equivalence ratio of 1.2 to 1.6.Due to the trends shown in Figures 5.2 and 5.4, a slightly fuel rich mixture (0=1.2) waschosen as the optimal fuel/oxidizer composition for all Al particle diameters in theparticle diameter study.

Two explanations can be suggested for the performance of the slightiy fuel richmixtures:

1. For surface ignition, the front surface of the cylindrical pellet is exposed togaseou s oxygen in the adjacent air. The gaseous oxygen m olecules are also fasterdiffusers and do not require a phase transformation to initiate the reaction. Thisrapid diffusion of gaseou s oxid izer to the Al fuel at the pellet surface is sure toeffect surface reaction ch arac teristic s. At the surface, the fuel not only reacts withthe solid ox idizer, but also the air in the environmen t. Also , since the solid pelletsare only compressed to 38% TMD, this suggests that a finite quantity of air istrapped in the interstitial spaces of the Al and M0O3 mix tures. The trappedgaseou s oxygen is a faster diffuser and may enhance the reaction rates for slightiyfuel rich solid composition producing a solid/air effective composition closer to

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stoichiom etry. Hav ing slightiy fuel rich conditions allows plenty of fuel for bothreactions.

2. A second explanation is the higher thermal conductivity associated with the fuelme tal, enhances heat transfer. The increased Al concentration in a slightiy fuelrich mixture im proves cond uctive heat transfer. It will be shown in Section 5.3,that improved heat transfer mechanisms may increase reactant temperatures andpromote faster reaction zone propagation.

The first theory suggests that the largest oxidizer concentration (solid and gaseous)occurs at all of the outer surfaces of the cylind rical pellet. If the solid/air interface doescreate a closer to stoichiometric mixture then the results in Figure 2.6 may becon tradicting. To summ arize. Figure 2.6 shows the reaction zone propagating the fastestjust below the outer diameter of the cylindrical pellet and not at the solid/air interface.Figu re 2.6 was initially included as evidence of radial density gradients. The lowerdensity of the subsurface zo ne would again support suggestion 2 from above. The lowerdensity zone (subsurface shell) would allow more interstitial air, thus affecting the localfuel/oxidizer ratios.

6.2 Particle Diameter StudyIgnition times and bum rates were compared for all Al diameter mixtures in the

nano and micron regim es (Table 5.2). It is interesting to observe how the ignition timeand standard deviation vary with Al particle diameter. Figure 6.1 shows the ignition timeas a percentage of the 20|J.m diameter Al data point reference (tign20nm); this is an arbitrarypoint for comparison. The data points and exponential curve fit show that the change inignition time increases proportionately to the change in standard deviation forcorresponding Al diameter mixtures. As the ignition time increases, the standarddeviation (i.e., variability in ignition time between experiments) also increases.

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10.00

E ICM

c

O )^ 0 ,

,00 H,10,01

0.001

t I gnSt dev

-Expon. (t Ign)^ Expon. (St dev)

' ^ ' ^

Figure10 100 1000 10000 10000log Particle Diameter [nm ] 0

6.1 Non-dimensional ignition tim e and deviation ratiosA similar trend is represented with bum rates. All of the nano-scale Al mixtures

exhibit bum rates on the order of 5 m/s while the micron-scale samples bum faster.Figure 6.2 illustrates that the change in bum rate increases proportionately to the changein standard deviation for corresponding Al diameter mixtures.

Fs-aac3IDo>

1.00

0.10

0.01

0 . 0 0 -

^Vt

BR St dev

^ ^ ^ E x p o n . ( B R )" ^E x p on . (S t dev )

10 100 1000 10000 100000log Particle Diameter [nm]

Figure 6.2 Non-dimensional bum rates and deviation ratios

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Results from the particle diameter study (Figures 5.5 and 5.6) have shown thatincreased homogeneity in nano-scale composites actually decreases the variability inIgnition sensitivity and burn ra tes. Figures 6.1 and 6.2 show that the data points (ignitiontimes and burn rates) and corresponding standard deviations decrease proportionally toone ano ther. For exam ple, the 20 |im A l shows the slowest ignition time and the largestvariability in ignition tim e. For a representative nano-pow der Al, the 17.4 nm Al showsan equal percent decrease in ignition time and standard deviation of ignition time fromthe 20 |im Al.

Figure 6.3 shows the relationship between burn rate and ignition time. Trends inthis figure are further explained on page 33.

10000

SUI

tme

io

EO)o

1000

100

10

3-4Lim

s!1

10 10 0log Burn Rate [m/s]

Figure 6.3 Ignition time as a function of burn rate

As the particle size increases the bum rate increases. This phenomenon may be contraryto what might be expected because as particle size decreases, burn rates increase in loosepowder media [13]. Smaller particles have a higher surface to volume ratio and maypromote improved homogeneity between fuel and oxidizer particles and increase thereactivity of the mixture. These advantages of nano-scale materials were hypothesized tolead to increases in ignition sensitivity. However, as compressed pellets, the burn rate

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actually decreases with decreasing particle size. Therefore, although the media may bemore sensitive to ignition, the bum rate is inhibited in nano-scale particle composites.This behavior may be a direct result of the decrease in active Al content in the nano-regime powders.

6.3 Effect of Aluminum Oxide LayerFor both the composition and the particle diameter study, only the active Al

concentration was used to determine stoichiometry and similarly the equivalence ratio.The pure Al concentrations in Table 2.1 can be justified by the increasing volumepercen tage of the AI2O3 for decreasing particle diameters. The concentration of the un-reactive oxidation layer becomes much larger in percentage for the smaller Al particles(Table 2.1). As shown in Table 4.1, AI2O3 has a significantly lower thermal conductivitythan pure Al. W hile not acting as a reactant, the aluminum ox ide concentration doesinhibit conduction.

Considering the air concentration of a 38% TMD pellet all forms of heat transfermust be considered . The Peclet Num ber can be estimated to determine the significanceof conduction and convection [12].

uSPe = ' - (6.1)aAssum ing a therm al diffusivity of air to be 2.25x10"^ m^/s, an effective a of 3.8x10' m^/scan be estimated for the 38 % TMD pellet. For solid/solid reactions the flame thickness isdifficult to measure experim entally, but is estimated to be on the order of 1 mm . Thereaction zone thickness (5r) can be estimated as 10% of the flame zone thickness. Usingthe range of bum velocities for the nano Al mixtures in Table 5.2, the Peclet number canbe estimated as

M = 1 . 2 - > 1 2 m / 5 (6.2)

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This shows that convection and conduction are significant for the lower burn rates with aPeclet number on the order of one, and at higher burn rates with a Peclet number on theorder of ten, convection becom es the more dom inant heat transfer m echanism. As shownin Figure 4.1, the solid reactants in front of the reaction zone are preheated to a thresholdtemp erature (usually T^p) before the reaction will propaga te. Since conduction issignificant and the average thermal conductivity is steadily decreases (due to AI2O3concentration) with decreasing particle diameter, there is an expected lag in raising thepreheat zone to the threshold tem perature. This time lag then contributes to the slowerburn rates of the nano-regime Al mixtures.

Similarly, the micron Al mixtures undergo a large bulk preheat due laser heating.As seen in Figure 6.3, the ten and twenty micron mixtures heat for seconds beforeignition. Th is inert heating raises the temp erature of the entire pellet prior to theformation of any reaction zone. Thus once a reaction zone is formed, the preheat zonecan reach the threshold temperature faster which promotes a faster flame propagation andburn rate. Schematic illustrations of temperature profiles resulting from ms and secondlaser heating are shown in Figure 6.4.

< / f l , -

T >

VTemperature, Profile Solid Rencnnts.AlMo(:)3 y X

flux

T1- -- i \

ReactionZone\ ^1 Molten H SIProduct.s H

^

yi

Pre-heatZoneSolidReactants -^

1 *

Reaction Pie-heatT . Zone ZoneMol te i ]I Piochicts

Flame PropagationBum Rate. //Flame PropagationBum Rate, n

X

a) Laser Heating for nm Al mixturesmilliseconds of laser exposure

b) Laser heating for ^im Al mixturesseconds of laser exposure

Figure 6.4 Ignition time effects on pre-reaction zone temperatures

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6.4 Ignition Temperature EstimatesEstimates of ignition temperatures were made based on experimental ignition times.

Using the same thermal property grouping scheme as Table 4.2, temperature curves couldbe created based on Eq. (4.4). Figure 6.5 shows four temperature history curves with thecalculated ignition points presented in Table 4.2 and the experimental ignition pointspresented in Table 5.2. As to be expected, the nano-Al samples present much lowerignition temperatures corresponding to the much lower ignition times compared to themicron-Al samples.

1200

Ignition (Expe rimental) tign (Calculated)

0 1000 2000 3000 4000 5000 6000 7000t [ m s ]Figure 6.5 Calculated temperature history

Figure 6.5 suggests that the nano-Al powders ignite below lOO^C and the 10-20 mmpellets ign ite closer to the me lting point of Al (660C). This may also verify theimpo rtance of the small diffusion distances acquired by the nano-composites. The nano.Al may never melt as suggested in the flame structure diagram (Figure 4.1) and thereaction initiates in a solid reactant state. The refore, the increased thermal sensitivity ofthe nano-Al composite may be attributed to the chemical kinetics that occur in the solid-

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solid state for nano-scale Al and diffusion kinetics that occur in the solid-liquid state forthe micron-scale composites.

6.5 N ano v ersus. Micron Thermite KineticsAs stated previously, thermite reactions have traditionally been classified as

diffusion controlled reactions, inferring that the reaction rate is a function of particlediam eter as shown in Eq. (4.13). The data presented in Figure 5.6, indicates that fornano-scale Al mixtures the bum rate and reaction rate are not strong functions of particlesize. In Figure 5.6, there is very little difference in bum rates for the ten nano-scalediam eters studied. This suggests that the diffusion-controlled analysis may be altered fornano-scale thermites.

The solid sphere analysis presented in Section 4.2 is common for a solid fuelparticle surrounded in a gaseous oxygen environm ent (i.e., carbon particles in air). Themass flux (Eq. (4.12)), is in terms of oxidizer concentration at the solid particle surface{YOX,R)- This oxidizer concentration (also in Eq. (4.13) and Eq. (4.14)) assumes that thesame ox idizer environ men t surroun ds the entire particle surface. This is not the case forthe solid/solid reaction between Al and M0O3. As shown in the SEM images (Figure2.3), the Al and M0O3 interactions only occur at specific contact points. A contactfrequency parameter is estimated to approximate the difference between gaseous oxidizerenvironment and solid oxidizer contact points.

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Figure 6.6 Con tact points illustration

Due to the geometry of this specific reaction, the contact frequency is approximated asthe ratio of surface area of the M0O3 and projected area of a single Al sphere.

/ = ' Mo O, (6.3)'A l

Since the M0O3 are crystalline sheets, the surface area can be approximated as a dualsided rectang ular sheet. The projected area of the Al spheres is essentially the area of acircle.

' Mo O, = 2wh (6.4)A A , = ^ ' (6.5)

Since th e M0O3 dim ension s are constant for all Al particle diameters studied, / willremain in terms of AJ^^Q .

/ = ' Mo O,70^' (6.6)The mass flux presented in Eq. (4.12), is now converted to a mass flux per contact pointbetween solid fuel and solid oxidizer.

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f4=Y R

\ f

k + pDR Jv/J= Y TTR'

'^MoO, k +pDRpDR J

(6.7)

For diffusion-controlled reactions, a new expression for mass flux is developed in Eq.(6.8).

=Y_- (P ) (6.8)Similarly, for chemistry-controlled reactions, Eq. (6.9) describes the mass flux bum rate.

' ^ = \ KR' (k) (6.9)For micron size particles, the surface area of the M0O3 and projected area of Al are onthe same order of magnitude and the contact frequency will reduced to 1 (f=X). Thecontact frequency param eter becom es negligible for micron Al and the diffusion limitedmass flux is inversely proportional to particle radius as shown by the micron data inFigure 5.6.

- 1 = 4 =Y. .^ / - I (6.10)If the contact frequency parameter is not equal to one, as for nano-scale Al particles themass flux is directiy proportional to the particle radius (Eq. (6.11)).

4 =^=\TlR ipD)^ / ! (6.11)

The change in mass flux for micron and nano-scale particles is correlated to the change inparticle size for the two limiting cases off.

dR = -Y .pDR' , / - I (6.12)//m

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d 4dR = Y TT0X,o (pD)^ / ! (6.13)oxLooking at the nanometer region of Figure 5.6, the bum rate appears to be linear withparticle diameter as shown below confirming the constant slope predicted by Eq. (6.13).

I U . \ J \ J14.00

12.00J O 10.00 -Rae

0 b o

E 6.003C D 4.002.00o nrt

o

T% iP-i- ^ f ^ >

10 2100 110 160log Al Particle Dia [nm ]

Figure 6.7 B um rate as a function of nanometer regime Al particle diameter {^=X.2)

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CHAPTER VIICONCLUSIONS

Ignition times were shown to be shortest (approximately 20 ms) for slightly fuel richmix tures of Al and M0O 3. As the mixture becomes increasingly fuel rich, the ignitiontime increases accordingly.

For all Al particle mixtures, the maximum bum rate corresponds to the slightiy fuelrich mixture and the shortest ignition time. Although the smallest Al particle mixtureexhibited a more narrow range of bum rate for varying compositions, the largest Alparticle diameter mixture showed the fastest bum rate, more than a factor of 2 increaseover the smallest Al particle mixture.

Th is work also explains the difficulty in pressing pellets such that pre-ignition canbe avoided. The acrylic die was designed to avoid pre-ignition based on the sensitivity ofthis mixture to frictional/compressional ignition. As a result, potentially slight variationsin density gradient lead to interesting buming behavior. For example, some pellets wereobserved to bum in the radial direction first, followed by flame propagation along theouter sides of the pellet. The pellet was then observed to bum from the outside inward asthe pellet was propelled in a circular motion, spinning in the horizontal plane.

It has been shown that decreasing Al particle diameter from micron- to the nano-scale in an AI-M0O3 composite consistently decreases the ignition time and improves thereliability of the com po site's response to ignition. These results suggest that theincreased sensitivity to ignition may be a direct result of reaction initiation in the solidrather than liquid state. Ignition temperature estimates suggest that ignition for nano-scale particles occurs at temperatures well below the melting point of Al and the reactionis initiate in the solid state.

Bum rates in these pellets were shown to increase as particle size increases. Thisbehavior was attributed to the increased thermal properties associated with micron-Alcomposites. Each Al particle is coated with a passivation layer of AI2O3 which acts as adiluent in the AI/M0O3 reaction. As the surface area to volume ratio increases withdecreasing particle size, the AI2O3 content increases. Although nano-scale Al particles

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may be more sensitive to ignition, these particles do not promote increased flamepropagation rates in packed media because the increased amount of AI2O3 in the mixtureresults in an overall lower thermal conduc tivity. Because the diffusive mode ofcombustion is significant in these reactions, lowering the thermal conductivity of themixture inhibits energy transport and result in slower bum rates. The micron-scale Alparticle composites actually have a higher active Al content and promote faster bum ratesin packed media. However, the nano-scale Al composites are significantiy more sensitiveto ignition.

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REFERENCES

[1] Fischer, S.H., Gm belich, M .C., "Theoretical Energy Release of Thermites,Intermetallics, and Combustible Metals," Proceedings of the 24"' InternationalPyrotechnics Seminar (1998).[2] W ang , L. L., Munir, Z. A., M axim ov, Y. M., "Therm ite reactions: their utilizationin the synthesis and processing of materials," J. of M aterials Science 28: 3693-3708(1993) .[3] Sim pson , R.L., Tillotson, T., Hrubesh, L., Gash, A., "Nanostmctured EnergeticM aterials Derived from Sol-gel Chemistry," Submitted to 3 1" International AnnualConference of ICT, Karisruhe, Germany, June 27-30 (2000).[4] S. F. Son, H. L., B. W . Asay, J. R. Busse, B. S. Jorgensen, B. Bo ckm on, and M.Pantoya, "R eaction Propagation Physics of AI/M0O3 Nanocomposite Therm ites,"The Intem ational Pyrotechnics Society, The Twenty-Eighth IntemationalPyrotechn ics Sem inar, Adelaide, Austraha, November 4-9 (2001).[5] Au ma nn, C. E., Skofronick, G. L., and Martin, J. A., "Oxidation Behaviors ofAluminum Nanopo wders," y. of Vac. Sc. & Tech. B 13(2): 1178-1183 (1995).[6] de Yong, L. Park, B and Valen ta, F., "A Study of the Radiant Ignition of a Rangeof Pyrotechnic M aterials using a C 0 2 Laser," MR L Technical Report, MRL -TR-90-20 (1990).[7] Free ma n, E. S. and Gordon, S., "The Application of the Absolute Rate Theory tothe Ignition of Propagatively Reacting Systems. The Thermal Ignition of theSystems Lithium Nitrate-Magnesium, Sodium Nitrate-Magnesium" J. ofPhys.

C/iem 60: 867-871 (1956).[8] Turns, S.,An Introduction To Combustion: Concepts and Applications, McGraw-Hill Companies, Inc. New York, 2000.[9] Price, E. W ., "Com bustion of M etallized Particles," Chapter 9 pages 479-5 13,Progress in Astm autics and A eronautics, Martin-Summerfield Series, Vol. 90"Fun dam entals of Solid Propellant Co mbu stion," Edited by Kuo, K., Summerfield,M. published by the American Institute of Aeronautics and Astronautics Inc.(1984).[10] Con kling, J. A., Chemistry of Pyrotechnics: Basic Principles and Theory, MarcelDekker, Inc., New York (1985).

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[11] Incropera, F. P., DeWitt, D. P., Fundamentals of Heat and Mass Transfer, 4"' Ed.,John Wiley and Sons Inc., New York, (1996).[12] Glassman, I., Combustion, 3"* Ed., Academic Press, San Diego, CA, (1996)[13] Bockmon, B. S., Pantoya, M. L., Son, S. F. and Asay, B. W., "Bum RateMeasurements in Nanocomposite Thermites," Proceedings of the AmericanInstitute of Aeronautics and A stronautics Aerospace Sciences Meeting, Paper No.AIAA-2003-0241 (2003).

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