DTIC ADA545032: Effect of Nano-Aluminum and Fumed Silica Particles on Deflagration and Detonation of Nitromethane

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4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Effect of Nano-Aluminum and Fumed Silica Particles on Deflagration and W911NF-04-1-0178
Detonation of Nitromethane
5b. GRANT NUMBER
5c. PROGRAM ELEMENT NUMBER
6. AUTHOR(S) 5d. PROJECT NUMBER
Justin L. Sabourin a, Richard A. Yetter a, Blaine W. Asay b, Joseph M. Lloyd
b, Victor E. Sanders b, Grant A. Risha c, Steven F. Son d
5e. TASK NUMBER
5f. WORK UNIT NUMBER
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION
REPORT NUMBER
a The Pennsylvania State University, University Park, PA, USA
b Los Alamos National Laboratory, Los Alamos, NM, USA
c The Pennsylvania State University—Altoona, Altoona, PA, USA
d Purdue University, West Lafayette, IN, USA
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S)
U. S. Army Research Office
P.O. Box 12211
Research Triangle Park, NC 27709-2211 11. SPONSOR/MONITOR'S REPORT
NUMBER(S)
12. DISTRIBUTION/AVAILABILITY STATEMENT
Approved for public release; federal purpose rights.
13. SUPPLEMENTARY NOTES
The views, opinions and/or findings contained in this report are those of the author(s) and should not be construed as an official

1T4h.e A vBiSewTRsA, oCpTi nions and/or findings contained in this report are those of the author(s) and should not be construed as an official
DTheep ahretmtereongt eonfe tohues A inrmterya pctoisointi obnet,w peoelinc yn iotrro dmeectihsiaonne, (uNnlMes)s, psoa rdtiecsliegsn oaft enda nboys octahleer a dluomcuimnuemnt a(3ti8o na.nd 80 nm diameter), and fumed
silica is examined in terms of the deflagration and detonation characteristics. Burning rates are quantified as functions of pressure
using an optical pressure vessel up to 14.2 MPa, while detonation structure is characterized in terms of failure diameter.
Nitromethane is gelled using fumed silica (CAB-OSIL), as well as by the nanoaluminum particles themselves. Use of
nanoaluminum particles with fumed silica slightly increases burning rates compared to the use of larger diameter Al particles;
however distinct increases in burning rates are found when CABO-SIL is removed and replaced with more energetic aluminum
nanoparticles, whose high surface area allows them to also act as the gellant. Mixtures including fumed silica yield a reduced
burning rate pressure exponent compared to neat NM, while mixtures of aluminum particles alone show a significant increase.
Failure diameters of mixture detonations are found to vary significantly as a function of 38 nm aluminum particle loading, reducing
more than 50% from that of neat nitromethane with 12.5% (by mass) aluminum loading. Failure diameter results indicate a relative
15. SUBJECT TERMS
minimum with respect to particle separation (% loading) which is not observed in other heterogeneous mixtures.
Burning Rate, Deflagration, Detonation, Nanoaluminum, Nitromethane
16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF RESPONSIBLE PERSON
a. REPORT b. ABSTRACT c. THIS PAGE ABSTRACT OF Richard A. Yetter
PAGES
UU UU UU UU 10 19b. TELEPHONE NUMBER (Include area code)
814-863-6375
Standard Form 298 (Rev. 8/98)
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Prescribed by ANSI Std. Z39.18
PropellantsExplos.Pyrotech.2009,34,385–393 385
Full Paper
Effect of Nano-Aluminum and Fumed Silica Particles on
Deflagration and Detonation of Nitromethane
Justin L. Sabourin*, Richard A. Yetter
The Pennsylvania State University, University Park, PA, 16802 (USA)
Blaine W. Asay, Joseph M. Lloyd, Victor E. Sanders
Los Alamos National Laboratory, Los Alamos, NM, 87545 (USA)
Grant A. Risha
The Pennsylvania State University, Altoona, PA, 16601 (USA)
Steven F. Son
Purdue University, West Lafayette, IN, 47907 (USA)
Received: December 03, 2008; revised version: April 27, 2009
DOI:10.1002/prep.200800106
Abstract 1 Introduction
The heterogeneous interaction between nitromethane (NM), Nitromethane (NM) is a highly energetic, detonatable,
particles of nanoscale aluminum (38 and 80nm diameter), and liquid organic nitro compound (CH NO) which may be
fumed silica is examined in terms of the deflagration and 3 2
usedasamonopropellant,abipropellantcomponent,ora
detonation characteristics. Burning rates are quantified as func-
tionsofpressureusinganopticalpressurevesselupto14.2MPa, highexplosive.Thenon-toxicnatureofNM,energycontent,
while detonation structure is characterized in terms of failure lowviscosity,easeofmanufacture,andhandlingmakeitan
diameter. Nitromethane is gelled using fumed silica (CAB-O- interestingprospectforpresentandfuturepropulsionand
SIL(cid:2)),aswellasbythenanoaluminumparticlesthemselves.Use
explosive technologies.Although NMhaslong been iden-
of nanoaluminum particles with fumed silica slightly increases
tified as a monopropellant [1, 2], interest in NM has been
burningratescomparedtotheuseoflargerdiameterAlparticles;
howeverdistinctincreasesinburningratesarefoundwhenCAB- stagnantuntilthelastdecade[3–8]astoxicpropellantssuch
O-SIL is removed and replaced with more energetic aluminum as hydrazine are replaced with non-toxic alternatives. As
nanoparticles,whosehighsurfaceareaallowsthemtoalsoactas withmanyliquidmono-andbi-propellantscurrentlyinuse,
the gellant. Mixtures including fumed silica yield a reduced
or under development, there is interest in improving
burning rate pressure exponent compared to neat NM, while
mixturesofaluminumparticlesaloneshowasignificantincrease. handling characteristics and energy content by adding a
Failure diameters of mixture detonations are found to vary gellantand/ormetallicparticles.Gelledmetallizedpropel-
significantly as a function of 38nm aluminum particle loading, lantshavelongattractedattentionfromthespaceexplora-
reducing more than 50% from that of neat nitromethane with
tion and defense communities in search of improved
12.5% (by mass) aluminum loading. Failure diameter results
propulsionsystemswithincreasedsafetyandenergydensity
indicatearelativeminimumwithrespecttoparticleseparation(%
loading)whichisnotobservedinotherheterogeneousmixtures. [9]. Gelled propellants share some of the advantageous
propertiesofbothliquidandsolidpropellants.Thegellingof
Keywords: Burning Rate, Deflagration, Detonation, liquid mono- and bi-propellants reduces risk of leakages
Nanoaluminum,Nitromethane
whilemaintainingtheirabilitytobepumpedandthrottled,
unlike solid propellants. Gelled propellants are generally
lesssensitivetoimpact,friction,andelectrostaticdischarge
thansolidpropellants.Additionally,propellantcracksarea
majorconcerninsolidpropellants,butarenotanissuewith
* Correspondingauthor;e-mail:[email protected] gels. From a performance standpoint, gelled propellants
(cid:3)2009Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim
386 J.L.Sabourin,R.A.Yetter,B.W.Asay,J.M.Lloyd,V.E.Sanders,G.A.Risha,S.F.Son
offerhighspecificanddensityimpulses,whicharecompa- Louazeetal.[18]lookedatboththeburningrateandflame
rable to liquid systems; however, performance may be temperature effects of Aerosil 200 (5, 6, 8% loading) by
increased with more energetic materials such as metal itself. Their results showed reductions in mixture burning
particles. rates and an increase in pressure dependence with the
Energy densities of propellants are increased with the additionofgellingagent;howeverthelatterresultwasnot
addition of metals such as aluminum (Al), which also conclusivegiventheuncertaintyinthedata.
increase burning rate, due to the exothermic Al oxidation Detonation characteristics of highly energetic heteroge-
reaction. With the development of nanoparticle manufac- neousmixturesareimportantfromasafetyaspectandtothe
turing technologies, researchers have found that burning design of future weapon systems. The failure diameter is
ratesofpropellantsmaybeincreasedfurtherwiththeuseof used as a measure of the ideality of an explosive and is
nanoscale particles [10]. Micron-sized particles, which are relatedtothereactionzonelength[19]andisafunctionof
used in the majority of metallized propellants, demand many variables. It is known that the addition of an inert
longer combustion times for complete combustion and material,e.g.,glassmicro-balloons,hasapronouncedeffect
requirehigherignitiontemperaturesthansmallernanosized onthefailurediameterofliquidexplosives[20].Theseinert
particles. Nanoparticles are desirable since they offer particles introduce inhomogeneities which produce local-
shortened ignition delay, decreased burning times, more izedheating(“hotspots”)asopposedtoahomogeneousone
completecombustion,higherspecificsurfacearea,andthe [21]. This difference results in a very different behavior
ability to act as gelling agents for liquids, replacing tradi- when measuring the detonation velocity as a function of
tionallyusedlowenergygellingagentssuchasfumedsilica failure diameter. Heterogeneous materials have large
(SiO). Due to the decreased ignition delay and burning reductions in velocity before failure, while homogeneous
2
times, nanoparticle reactions contribute more efficiently explosiveshaveverysmalldeficits.Ithasbeenshownthat
within the primary reaction zone of liquid propellants, theeffectoftheaddedmaterialisaresultofaphysicaleffect
feeding more energy back to the propellant surface, (local shock heating) rather than a chemical effect (e.g.,a
increasing reaction and burning rates of the propellant surfacecatalyzedchemicalreaction)[22].
comparedtolargerscalemetalparticles. Theeffectoftheaddedinclusionsonfailurediameterisa
The history of experimental and theoretical research of function of both the particle density and mixture fraction
metallizedgelledpropellantsdatesbackdecades,withmost (andthustheintra-particlespacing)andthebeaddiameter.
oftheworkdevotedtoaerospaceapplications[9,11].Much Ifareactivematerialisadded,ratherthananinertmaterial,
oftheworkbeganinthe1950sand1960swiththestudyof thenonemightexpectbothphysicalandchemicaleffects.It
slurryfuelswithvariousmetals.Inthe1970s,hydrocarbons, is known that the addition of aluminum to explosives
hydrazine derivatives,andinhibited redfumingnitricacid increases bubble energies in underwater weapons, raises
(IRFNA) began to be studied. The majority of the work reaction temperatures, and creates incendiary effects.
involving metallized gelled propellants in the last decade However, when the particles are large with respect to the
andahalfhasbeendevotedtobi-propellantsystems[12– reaction zone length, much of the added metal is not
14], and very few of these studies have involved using consumedinthereactionzone,whichyieldslongerburning
nanoscale particles (d <100nm) [15]. Recent work has times,higherignitiontemperatures,anddelays.Mostofthe
p
been completed involving the study of metallized gelled reaction occurs late in the process, and thus does not
nitromethane (NM), using ultra fine aluminum particles contribute directly to the detonation energy. It has been
(ALEX) from Argonide Corp. (Sanford, FL) and 5mm postulated that by incorporating nanoscale metals, the Al
diameterparticles[16].Althoughparticlesize distribution reactionwilloccurinthereactionzone,thusdirectlyfeeding
of the ALEX powders was not given, generally they are energy into the propagating shock wave, and dramatically
found to have a relatively large size distribution and a enhancing detonation performance. The results to date
nominalparticlediametergreaterthan100nm[17].Weiser howeverhavebeenmixed.Brousseauetal.[23]foundthat
etal. [16] completed theoretical calculations of specific nanoaluminum (nAl), 100–200nm in diameter, did not
impulse and equilibrium flame temperature, and experi- increase the velocity of detonation for a range of plastic-
mental measurements of flame structure, temperature bonded explosives, and in some cases actually decreased
distribution,andburningratesupto13MPa.Allmixtures performance.However,theyfoundthateveniftheperfor-
were gelled using Aerosil(cid:2) 200, which is a commercially mance was decreased, in at least one case, the heat of
available fumed silica. Using 5% Aerosil and ALEX detonationwasincreased.Theyalsofoundthatwhenusing
particle loadings of 5 and 10%, burning rates of (cid:2)4 and TNT, nAl increased both the heat of detonation and the
5mms(cid:3)1werefoundatthehighestpressurestudied((cid:2)12– detonationvelocity,particularlyatchargediameterscloseto
13MPa).AlthoughnotstateddirectlybyWeiseretal.,itis thecriticaldiameter,andimportantlythefailurediameter
assumedthattheALEXparticleloadingisinclusiveofoxide decreased [23]. LeFrancois etal. [24] found similar results
coating.Fromthetheoreticalcalculations,itwasdetermined for plastic bonded explosives (100nm Al) and ascribe the
thatflametemperatures(confirmedfromexperiments)and lackofimprovementtothelargefractionoftheAlparticle
specificimpulsemaybeincreasedwithaluminumaddition thatispresentasaluminumoxide.However,theydidfind
to gelled NM. However, above (cid:2)5% Al, specific impulse that energy release is much better for the explosive
beginstodecreasewithfurtherloading.Inaseparatereport, containingnAl,whichwasalsofoundbyotherworkers.
PropellantsExplos.Pyrotech.2009,34,385–393 www.pep.wiley-vch.de (cid:3)2009Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim
EffectofNano-AluminumandFumedSilicaParticlesonDeflagrationandDetonationofNitromethane 387
Due to the large specific surface area of the aluminum After loading the quartz tubes, air bubbles were removed
nanoparticlesused,theycansimultaneouslyactasagelling usingvibrationandthesamplewasweighedandmeasured
agent.Inthisstudy,NMwasgelledwithtraditionallyused sothatpackingdensitycouldbedetermined.
fumedsilica,aswellassolelybythepassivatedAlparticles. The burning rates of the NM/nAl/CAB-O-SIL mixtures
Thepurposeofthecurrentstudywastoexploretheeffectof were obtained using an optical pressure vessel (i.e., win-
nanoscale metallic particle addition on the combustion dowed chamber) under well-controlled operating condi-
characteristicsofaliquidmonopropellantorexplosive,from tions.Thechamber,constructedfrom316stainlesssteel,is
both a detonation and deflagration perspective. These equippedwithfouropticalviewingportsandhasatotalfree
deflagrationanddetonationexperimentsweredesignedto volumeof23ltominimizethepressurevariationcausedby
be an initial survey that when coupled together would the generation of gaseous combustion products during an
providethebasisformorein-depthworktoexamineeffects experiment.Theopticalchamberwasbroughttothedesired
such as particle size and reaction zonelength as well. The initial pressure using argon as the pressurant gas by
reactionzonelengthofNMdetonationhasbeenreportedto regulating the inlet and exhaust valves. The continuous
range between 35 [25] and 200mm [26], depending on the purgeofargonkepttheproductgasesfreefromtheviewing
confinement and other factors, while NM deflagration area.Oneoftheopticalviewingportswasbacklitusingan
reaction zone thicknesses have been determined to vary optical diffuser. The opposite viewing port of the diffuser
from900to200mm,at3and15MPa,respectively,usinga wasusedforreal-timerecordingoftheburningprocesswith
detailedmodelandreactionmechanism[7]. adigitalvideocamera.
TheinstantaneouspressurewasmonitoredusingaSetra
206pressuretransducer.Ignitionwasobtainedbyresistance
heating either a coiled nichrome wire or a double base
2 Experimental Approach boosterpropellant(NOSOL363)threadedontoanichrome
wire. Ignition sans propellant was preferred since it was
The effects of the addition of nanoscale aluminum found that the combustion process reaches steady-state
particles to NM were studied in both deflagration and quicker;howeveratlowpressureandaluminumloadingit
detonationcombustionmodesinordertodetermineif,by was found that the booster was required for ignition. A
reducingthesizeoftheAltothesub-100nmscale,apositive NicoletGenesisdataacquisitionsystemwasusedtorecord
effectwouldbeobtained.Inthedeflagrationstudy,burning the pressure transducer output at a sampling rate of
rates were determined using a large, nearly constant 1000Hz.Thepositionandtimeoftheregressingluminous
pressurewindowedvessel,whiledetonationcharacteristics front were tracked using the video recorder. From these
were determined using a conically shaped test fixture to data, the linear burning rate (r ) was determined using a
b
evaluatecriticalfailurediameters. curvefittopositionvs.timeplot,andmassburningrateper
unit area was determined by multiplying by the packing
density (m ¼r·r ). Further details of the strand burner
b b
2.1 Deflagration Study setupmaybefoundinRishaetal.[27].
Over the course of this study two different aluminum
nanoparticles were used. The first were 38nm average 2.2 Detonation Study
diameterparticlesproducedbyTechnanogy,L.L.C.((cid:2)54%
active Al, 54m2 g(cid:3)1), the second were 80nm average Inordertominimizethenumberofexperiments,aconical
diameter particles produced by NovaCentrix, Corp. (for- confinement was used to determine the critical diameter.
merly Nanotechnogies, Inc., (cid:2)81% active Al, 26m2 g(cid:3)1). Thismethodhasbeenusedinthepastwithgoodresults[28,
The fumed silicon oxide (CAB-O-SIL(cid:2)TS-720) used was 29]. However, with non-ideal explosives in particular,
produced by Cabot Corporation and was treated with a overdrivendetonationprocessesresultinalowermeasured
dimethylsiliconefluidtoincreasehydrophobicity,andhada failurediametercomparedtoexperimentsperformedwitha
specificsurfaceareaof115m2g(cid:3)1(BET). constant diameter. Overdriven detonations occur when
Constituentsweremeasuredintosmallpolyethylenebags maximum properties predicted by the Chapman–Jouguet
beginning with all dry components. Mixture weight meas- theoryareexceededduetoupstreampressures.
urements were made using a Mettler-Toledo analytical All constituents used in failure diameter studies are
balance (AB265-S) with a resolution as low as 0.01mg. equivalent to those in the burning rate tests. We experi-
After thoroughly mixing by hand, or in some cases an mented with gelling the NM with the addition of 3% by
ultrasonicbath(e.g.,lowparticleconcentrations),mixtures weightoftheTS-720fumedsilicatokeepthealuminumin
wereimmediatelyloadedin10mmOD(8mmID)quartz suspension.However,wefoundthatthesilicahadaneffect
tubes(cid:2)70mmlongwhicharecappedatoneend.Depend- on the failure diameter on its own, and thus we used neat
ingonthemixtureconsistency,a100cm3syringe,equipped NM and mixed the 38nm aluminum and initiated the
with varying gauges of long hypodermic needles, was detonation within 5min to minimize the settling. The
employed to load the tube. The syringe allowed for the mixture waspoured into a thin-walled Mylar(cid:2) conewhich
gel-like mixtures to be loaded with minimal air pockets. wasplacedagainstasteelwitnessplatetodeterminecritical
(cid:3)2009Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim www.pep.wiley-vch.de PropellantsExplos.Pyrotech.2009,34,385–393
388 J.L.Sabourin,R.A.Yetter,B.W.Asay,J.M.Lloyd,V.E.Sanders,G.A.Risha,S.F.Son
diameter.Themixturewasstirredwithalong,thinrodasit 38nmAlwasusedbyitselfaswell,actingasagellantand
waspouredintotheconetoagitateanybubblestothetop. significantlyincreasingenergycontentofmixture.Although
The cones were made of 0.127mm thick optically clear the rheological behavior is not known for NM–nAl
Mylar glued together with a 100%-methyl cyanoacrylate mixtures, previous studies of NM and nanoparticles of
(Permabond 910) adhesive. The Mylar film was chosen to SiO haveshownashearthinningbehaviorwithyieldstress
2
minimize the amount of confinement. The cone diameter [30].
ranged from 30mm at the top to 2mm at the base over a Inordertodevelopanunderstandingoftheeffectsofthe
distanceof48.3cm.Gluedtothetopofthecone,alsowith CAB-O-SIL/NM mixtures alone, initial testing was com-
Permabond 910, was a 10.2cm, 30mm diameter cylinder pleted to explorethe effect of particle loading on burning
(Figure1) to allow for the development of a steady-state rate(Figure2).Capturedimagesoftheburningprocessfor
detonationwave.Afterthemixtureswerepouredintothe twoseparatemixturesareillustratedinFigure3.Asinthe
cones,a1.6mmpieceofaluminumsheetwasplacedoverthe detonation study, Figure2 demonstrates that small addi-
openingsothattheboosterwasnotindirectcontactwiththe tionsoftheoxidegellant(i.e.,upto4–5wt.-%)affectthe
NM.A37g,30mmdiameterPBX-9501boosterwasglued combustionprocessonitsown,increasingtheburningrates
to aluminum sheet, which was in turn initiated with a in this case. This would indicate that the heterogeneous
standard#8non-electricdetonator.Thetemperatureofthe interactionbetweenthehighsurfaceareaparticlesandNM
liquid was controlled during the experiments to between increasestheheattransfercouplingbetweenthecondensed
19.0and28.48C.Thetemperaturewasmonitoredbyplacing
athermocoupleatthetopofthecylindricalsection,directly
into the liquid. As the detonation propagated, it left a
roughenedsectiononthewitnessplate.Thepointatwhich
the detonation ended was inferred to be the failure
diameter.
3 Results and Discussion
3.1 Deflagration Study
Linearandmassburningratetestingincludedtheuseof
CAB-O-SIL as a gellant due to its high surface area and
chain-like structure, making it an excellent thixotrope.
However,sincetheCAB-O-SILusedisprimarilymadeof Figure2. Constantpressurelinearburningrateasafunctionof
silica(SiO),whichlowerstheenergydensityofthemixture, fumedsilicaaddition.
2
Figure1. Schematicofconeusedtodeterminefailurediameter
(notdrawntoscale). Figure3. Capturedimagesofdeflagrationprocesses.
PropellantsExplos.Pyrotech.2009,34,385–393 www.pep.wiley-vch.de (cid:3)2009Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim
EffectofNano-AluminumandFumedSilicaParticlesonDeflagrationandDetonationofNitromethane 389
andgasphases,oraffectsreactionoftheNMitself,suchasa
possible catalytic effect, enhanced by the extremely high
surface area. It can also be expected to affect the thermal
diffusivitywhichwouldaffectburningrate.AthigherCAB-
O-SILparticleconcentrationsburningratesaredecreased
duetolowerenergydensities(orflametemperature).The
lattereffectwasalsoobservedbyLouazeetal.[18],aswere
long cinders created by coalesced molten silica during the
combustionprocess(Figure4).
The energetics of the mixtures studied are shown in
Table1.Theheatofreaction(DH )ofeachindividual
Reaction
systemiscalculatedassumingthereactiongiveninEq.(1).
Equilibrium compositions and mixture enthalpies were
calculatedusingNASAChemicalEquilibriumwithAppli-
cations (CEA) software [31], assuming a temperature and
pressureof298Kand1atm.Thedilutingeffectsofthesilica
areconfirmed,asaretheenhancingeffectsofthealuminum.
aCH NO þbAlþcAl O þdSiO
!3(cid:2)cþ2b(cid:3)Al O þdS2iO3þaN þ2wCO þxCO
2 2 3 2 2 2 2
þyH OþzH ð1Þ
2 2
Figure5. Linear burning rates of various mixtures of NM with
Figure5illustratestheresultsoftheburningratesofthree
nAlparticlesandfumedsilicaasafunctionofpressure(thepure
different mixtures studied here as a function of pressure.
NMcaseisshownforcomparison).
Only the 12.5% 38nm Al mixture and 4% 38nm Al/3%
CAB-O-SIL mixture may be considered gel-like. The
mixture containing the 80nm particles was much less are presented in Figure5, at a pressure of 5MPa, the
viscous due to the lower specific area of the particles burningrateofpureNMisincreasedbyfactorsof2.29and
compared to the 38nm particles. For comparison, linear 3.89with4%38nmAl(3%CAB-O-SIL)and12.5%38nm
burningratesofNMwithoutanyadditivesareshown.Using Aladditions,respectively.Inthelowerhalfofthepressure
theburningrateequationsdevelopedfromthedata,which rangeconsideredthereislittletonodifferencebetweenthe
useof38and80nmdiameterparticles.Giventheburning
rates of these mixtures, differences in individual particle
burningtimesbetweenthe38and80nmparticles,described
bythed2-lawofdiffusionflametheory(t, /d 2)andthe
bdiff p
d1-lawforkineticallylimitedcombustion(t, /d )[32],as
bkin p
wellasdifferencesinignitiondelaysandreactionenergetics,
arenotenoughtosignificantlyaffectburningrates.Itshould
be noted that all mixture percentages are on a mass basis,
andallaluminumloadingsareexclusiveoftheoxidecoating
(i.e., a 12.5% 38nm Al mixture contains 77% NM and
10.5%AlO).Forthisreasonthe38nmparticle mixtures
2 3
containmorealuminathanthe80nmAlparticlemixtures.
Theeffectsofthisonthereactionenergeticsareshownin
Figure4. Combustion products of NM and CAB-O-SIL mix-
tures. Table1.However,basedonthedatainFigures2and6,this
Table1. Combustionenergeticsofmixtures.
%Al %SiO AlParticleSize,nm(%AlO) DH ,(kJ/gm) %DeviationfromPureNM
2 2 3 Reaction
0 0 – (cid:3)4.59 –
0 3 – (cid:3)4.45 (cid:3)3.1
1 3 38(0.84) (cid:3)4.53 (cid:3)1.5
1 3 80(0.23) (cid:3)4.55 (cid:3)0.9
4 3 38(3.37) (cid:3)4.73 þ2.9
4 3 80(0.94) (cid:3)4.84 þ5.2
10 3 38(8.42) (cid:3)5.19 þ11.6
10 3 80(2.35) (cid:3)5.42 þ15.3
12.5 0 38(10.5) (cid:3)5.45 þ15.8
(cid:3)2009Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim www.pep.wiley-vch.de PropellantsExplos.Pyrotech.2009,34,385–393
390 J.L.Sabourin,R.A.Yetter,B.W.Asay,J.M.Lloyd,V.E.Sanders,G.A.Risha,S.F.Son
differenceisnotbelievedtoaffectburningratesappreciably, nearthesurfaceaswell,increasingthetemperaturegradient
anddifferencesinburningratesareattributedmoretothe andthusburningrates.Equilibriumpredictions,aswellas
particle ignition and burning times. The slight increase in spectroscopic studies indicate that the disparity between
burning rate at higher pressure with decreased particle neat NM and nAl/NM flame temperatures increases with
diameter is ascribed to the reduced flame zone thickness, pressure, which leads to the hypothesis that pressure
which reduces the energy contribution of the larger Al exponents should increase with Al addition, as seen with
particle reactions because of the longer burning time and the 12.5% Al mixture. Wheneffects of the aluminum and
reducedresidencetime.Overtherangeofpressuresstudied, silica particles are combined, the reduction in pressure
thepredictedcombustionzonethicknessofNMisreduced dependence with these mixtures indicates that the silica
byafactorof(cid:2)4.5[7]. alonedrivesthepressuredependencelower.Thismayimply
Forcomparison,resultsfromWeiseretal.[16]areshown that the burning rate pressure exponent of other liquid
in Figure5 as well. The Weiser etal. data were obtained propellantsmaybereducedwithinertparticleadditionas
usingmixturesof5%ALEXparticlesand5%Aerosil200. well. The mechanism for this may be attributable to a
This mixture is similar to the 38 and 80nm CAB-O-SIL catalytic effect or a physical “flame holding” effect where
mixturesofthisstudyintermsofconstituentconcentrations. the gas phase flame is stabilized closer to the surface by
Again,anincreaseinburningrateisobservedwiththeuseof solidscollectingintheproducts.
the smaller particles, particularly at high pressures. These In each case considered above, the effect of aluminum
results may be explained using the same interpretation as loading was considered as well. At a nominal chamber
discussed above. Interestingly, if the pure NM data are pressure of (cid:2)4.3MPa (625psi), burning rates were deter-
extrapolatedtohigherpressuresthereislittletonodiffer- minedasafunctionofAlconcentration(Figures6and7).In
encebetweenthedataofWeiseretal.[16],andthepureNM Figure6, a constant CAB-O-SIL loading of 3% (wt.-% of
data.Thistrendmayresultfromtheinertgellantaddition, totalmixture)isusedwhilevaryingthealuminumloading
which acts as a heat sink and changes phase (i.e., melts) up to (cid:2)10%. For comparison, and to determine effects of
duringreaction.Inallcaseswherefumedsilicaisusedthe aluminum oxide, which is present in large quantities on
burningratepressureexponentsarelessthanthepureNM nanoscale aluminum particles, burning rates of mixtures
case.Thelowestpressureexponent,assumingapowerlaw with pure aluminum oxide are presented as well. Larger
fit to the Weiser etal. data, occurs with the greatest silica aluminum concentrations werenotused sincethemixture
loading. The greatest pressure exponent occurred with becameverythickandclay-like,makingloadingthequartz
12.5%aluminumloadingwithnoCAB-O-SILpresent. sample tube without cracks and air pockets extremely
When treated separately, the 4% Al and 3% silica difficult. The plot shows that the smaller Al particles
mixtures have two effects. Although the silica acts as a produce greater burning rates, particularly at larger con-
diluent,asFigure2shows,burningratesareincreasedatlow centrations. As previously indicated, the smaller particles
concentrations.Thealuminumincreasestheenergycontent are reacting more efficiently within the flame zone, and
ofthesystem,yieldinghigherflametemperatures.Thesmall contributing more energy to the combustion process.
diameterAlnanoparticlescontributetoreactionsoccurring Though there is scatter in the data, it appears as if mass
burningrateperunitareaexhibitsalinearrelationshipwith
Al loading. Figure7 illustrates the effect of aluminum
concentration using the 38nm particles while using no
gelling agent. The trends in Figure6 display noticeable
Figure6. Mass burning rates per unit area of mixtures of NM
with 38 and 80nm Al particles with a constant 3% CAB-O-SIL Figure7. Linearandmassburningratesofmixturesof38nmAl
loadingasafunctionofAlconcentration. particlesandNMasafunctionofAlloading.
PropellantsExplos.Pyrotech.2009,34,385–393 www.pep.wiley-vch.de (cid:3)2009Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim
EffectofNano-AluminumandFumedSilicaParticlesonDeflagrationandDetonationofNitromethane 391
differenceswiththatofFigure7,althoughalargerrangeof
Al concentrations was studied without the CAB-O-SIL
addition.InFigure7,alinearrelationshipexistsupto5%Al
loading.Above this concentration, theburning rate is less
dependent on Al loading until (cid:2)13%. For Al loadings
greater than 13%, the burning rate increases significantly.
Therapidincreaseabove13%correspondswithachangein
mixture consistency from a gel-like to clay-like substance.
Thisresultsinburningthatmaynotbeplanar.
As a caveat, long-term particle dispersion is a practical
concern at low concentrations of aluminum particles and
CAB-O-SIL.Insomecases,evenwhenusinganultrasonic
bath, significant settling of the mixture may occur. Strat-
ificationofaluminumand3%CAB-O-SILmixtureswasnot
seen in the time between sample preparation and testing;
however, without CAB-O-SIL use, stratification was evi-
Figure9. Comparison of NM critical diameter as a function of
dentwithmixturesbelow10%aluminumloading.
particle size and concentration. This study used Al, while the
othertwousedglassmicro-balloons.
3.2 Detonation Study
Figure8showstheresultsofthecriticalfailurediameter dependstronglyonthediameterofthecharge,theconfine-
study of 38nm Al and NM mixtures. A common temper- ment, the homogeneity of the explosive, reaction zone
aturebasewasobtainedbycorrectingthefailurediameter thickness,particleshape,andsizedistributionamongothers
by0.25mm/8Ctoaccountfortheeffectoftemperature[33]. thatmayhavenotyetbeenidentified.Clearlytheaddition
There is a precipitous drop at approximately 9% Al, with of Al has similar effects to those found with glass micro
verylittleeffectfrom0to8%,andagradualrisefrom9%.It balloons. The initial slopes found in Figure9 are similar
isusefultoexaminetheresultsinadifferentwayusingthe amongallthedifferentparticlesizes.However,ourresults
intra-particle spacing (Eq.(2)) instead of concentration. with aluminum show a minimum while the others do not.
TheresultsareplottedinFigure9alongwithdatafromtwo ThedataofPreslesetal.[20]showanindicationofone,but
otherstudiesusinginertglassmicro-balloons[20,25].Ris they did not go high enough in density to have it fully
theparticleradius,r istheparticledensity,W istheweight described.Certainly,asthenumberofparticlesisincreased,
g g
fractionofparticles,andristhedensityofthemixture. thefailurediametermustincrease,justbydilutioneffects.
We have demonstrated that the addition of 38nm
"4pR3r ðW rÞ(cid:3)1#1=3 aluminum does indeed have a significant effect on the
L¼ g g ð2Þ failure diameter; however it appears that the chemical
3
oxidation process is not contributing appreciably to the
mechanismdrivingthedetonation;mostlikelythereaction
Several studies have examined the effect of nAl on zoneand residencetimes aretooshort. Theoretical calcu-
detonation performance, but unfortunately as indicated lations of the detonation energies using CHEETAH 4.0
earlier, the results are somewhat ambiguous. Results software [34] predict a linear increase with aluminum
particleaddition(Table2).At4%38nmaluminumparticle
loading, the thermal detonation energy increases nearly
20%, while the mechanical energy increases by (cid:2)7%
compared to pure NM. The fact that chemically inactive
additives behave similar to reactive particles supports the
conclusionthatreactionzonesaretoothinforthealuminum
to make appreciable chemical contribution to the detona-
tionprocess.Additionally,althoughdetonationenergiesare
predicted to increase linearlywith Aladdition, thefailure
diameter is highly nonlinear with concentration, again
implying that physical rather than chemical interactions
are significantly affecting the detonation processes. Using
an addition of diethylene triamine or diluants we could
adjustthereactionzonethicknessindependentlyandrepeat
theexperimentsinordertobegintoquantifyitseffectand
Figure8. Failure diameter of NM as a function of 38nm Al thisissuggestedforfuturework.
concentration.
(cid:3)2009Wiley-VCHVerlagGmbH&Co.KGaA,Weinheim www.pep.wiley-vch.de PropellantsExplos.Pyrotech.2009,34,385–393
392 J.L.Sabourin,R.A.Yetter,B.W.Asay,J.M.Lloyd,V.E.Sanders,G.A.Risha,S.F.Son
Table2. Theoreticaldetonationenergies.
%Al %AlO Density Thermaldetonation Mechanicaldetonation Totaldetonation
2 3
(g/cm3) energy(kJ/cm3) energy(kJ/cm3) energy(kJ/cm3)
0 0 1.14 (cid:3)1.68 (cid:3)6.07 (cid:3)7.75
4 3.37 1.20 (cid:3)2.00 (cid:3)6.50 (cid:3)8.50
8 6.73 1.26 (cid:3)2.41 (cid:3)6.91 (cid:3)9.32
16 13.5 1.40 (cid:3)3.30 (cid:3)7.73 (cid:3)11.0
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