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Characterization of Aluminum Nanopowder Compositions

Queenie S. M. Kwok, Robert C. Fouchard, Anne-Marie Turcotte, Phillip D. Lightfoot, Richard Bowes

and David E. G. Jones*

Canadian Explosives Research Laboratory, 555 Booth St., Ottawa, Ontario K1A 0G1 (Canada)

Summary

The thermal behaviour of two different Al nanopowders and amicron-sized Al powder was studied using DSC, simultaneousTG-DTA, and accelerating rate calorimetry (ARC). The resultsshow that the reactivity of Al powder increases as the particle sizedecreases. The thermal stability of the smaller Al nanopowder(Als) in water and in a humid atmosphere was determined using

ARC and TG-DTA, respectively. Atomic Absorption Spectro-metry (AAS), X-Ray Photoelectron Spectrometry (XPS) andAuger Electron Spectrometry (AES) were used to characterizethe surface chemistry of Alex. The outgassing behaviour formixtures of RDX and the various Al powders was investigatedusing TG-DTA-FTIR-MS. Evolution of NO2 and N2O from achemical interaction between Al nanopowders and RDX wasobserved. The effect of Als and Alex on the thermal stability ofTNT, RDX, Comp B, and AP was determined using ARC.Addition of Als significantly lowered the onset temperature forTNT and RDX decomposition. Electrostatic discharge (ESD)sensitivities of Al nanopowders and their mixtures with TNT,Comp B, RDX and AP were determined. The results show thatthe AP/Als mixture is very sensitive to ESD. Standard dustexplosibility tests demonstrated that Alex is highly explosible.

1 Introduction

Aluminum (Al) powder is commonly added to explosives,propellants and pyrotechnic compositions to improve theirperformance. Al is known to add energy to the burningreaction in propellants and to enhance the blast effect ofexplosives, as well as their underwater performance.Conventional weapons-grade Al powder is typically mi-cron-sized. However, advances in metal processing tech-nology have allowed sub-micron particles to becomecommercially available. One such type of particle is the Alpowder Alex, produced by electro-exploded wire technol-ogy(1). There are also other Al nanopowders produced byplasma technology(2,3).

Because of its large surface area, Al nanopowder canproduce dramatic improvements in the performance ofsome energetic materials. Incorporation of Al nanopowderin propellants results in an increase in the burn rate(48).Some researchers have reported enhancements in detona-tion properties using Al nanopowders(4,914). The replace-ment of conventional Al with nanometric Al has been

shown to increase the detonation velocity and the detona-tion pressure of some explosives(11,14).

Alex displays unusual behaviour in thermal tests(4,1525). Itwas originally claimed that low-temperature exothermicbehaviour was related to some structural energy due todefects in the crystal lattice(4). However, a number ofsubsequent studies have not found evidence for this internal

energy(5,15,17,26)

. Alex exhibitsa strong exothermic peak in airat temperatures that are 100 200 8C below the meltingpoint of Al, due to the oxidation of the particles, aphenomenon only observed with nanoscale powders. Ithasbeen demonstrated that the particle size has an effect onthe reactivity of the Al powders(18,27). Mixtures of nano-metric Al and nitramines showed a gas evolution in theVacuum Stability Test that was dependent on the particlesize(27).

The Canadian Explosives Research Laboratory (CERL)has published results for Al nanopowdersand their mixtureswith various explosives(2125). This paper summarizes ourcontinuing work on the characterization of Alex and a

smaller Al nanopowder (Als) produced by plasma explo-sion. The present paper includes the thermal properties ofvarious Al powders, the stability of Als in water and in ahumid atmosphere, the surface composition of Alex, theoutgassing phenomena for Alex, Als and their mixtures withRDX, and the effects of Alex and Als on the thermalstability of TNT, RDX, Comp B andAP. The explosibility ofdispersed Alex dust, and the electrostatic discharge (ESD)sensitivity of Alex, Als and their mixtures with TNT, RDX,Comp B and AP are also reported. Some comparisons weremade with micron-sized Al powder and paint-fine Al inorder to assess the effect of particle size of the Al powders.

2 Experimental

2.1 Materials

Als, Alex, H-15, TNT, RDX, Comp B, AP, mixtures ofComp B/Alex and Comp B/Als (90 : 10) were obtained fromthe Defence Research Establishment Valcartier (DREV).Als was produced by Tekna Plasma System Inc.(3) and amean particle size of 90 nm was given by DREV. Alex wasmanufactured by Argonide Corporation(1), and has areported mean particle size of 180 nm(28). For H-15, a

2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0721-3113/02/2703/0229 $ 17.50+.50/0

* Corresponding author; e-mail: [email protected]

229Propellants, Explosives, Pyrotechnics 27, 229 240 (2002)


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particle size of about 12 mm was provided by DREV. Anadditional sample of Alex was received directly from themanufacturer (Argonide Corporation). Paint-fine Al man-ufacturedby Carlfors Bruk wasprovided by Orica CanadaInc.

The chunk samples of Comp B, Comp B/Alex and CompB/AlsmixturesweregroundtopowdersusinganIKAmodelA10 analytical mill.RDX wasdried at 658C for a day beforeuse.Other mixtures of AlpowderswithTNT, RDX,CompBor AP were prepared at CERL for the outgassing, thethermal stability and the electrostatic sensitivity studies.

2.2 DSC

A TA 5200 Thermal Analysis System with a 2910 DSCmodule was used for the thermal studies of Al nanopowdersin air. Al pans containing 5.0 mg of samples were heatedfrom 300 or 400 to 600 8C at 0.5 8C min1. Experiments werecarried out in a dry air purge at 50 mL min1. The DSC wascalibrated for heat flow(29) and temperature with SRM

standards indium, lead and zinc(30)

.

2.3 TG-DTA-FTIR-MS

ATA 5200 Thermal Analysis System with a simultaneousTG-DTA 2960 module was used for assessing the thermalbehaviour of Al powders in air and nitrogen. Sample andreference material (Pt foil) of 10 mg were placed in aluminapansand heated at 108C min1 from30 to 12008C. TG mass,DTA baseline and temperature calibrations(30) were per-formed prior to the experiments.

For the outgassing experiments, the TG-DTA was

interfaced to a Bomem MB100 FTIR and a BalzersThermostar GSD300 Quadrupole MS. The TG-DTA-FTIR-MS data were acquired simultaneously to study thethermal behaviour and to identify the gases evolved by thesamples. Samples and reference (Pt foil) of 14 25 mg wereplaced in alumina pans, heated at 5 8C min1 to 120 8C, andthen held isothermally for 40 min in helium.

2.4 ARC

The ARC is a commercial automated adiabatic calori-meter distributed by Arthur D. Little Inc. Samples of about0.5 or 1.0 g were placed in lightweight spherical titaniumvessels. Unless specified otherwise, the ARC experimentswere started at ambient pressure of air and at an initialtemperature of 100 8C. The standard ARC procedure ofheat-wait-search was used(3132).

2.5 ESD Sensitivity

ESD sensitivities of Alex and Als, and their mixtures withTNT,CompB,RDXandAPweredeterminedusinganESDapparatus manufactured by Franklin Applied Physics. The

apparatus is described elsewhere(33). The ESD tests wereperformed with both open and closed sample containers.The limiting energy for the ESD initiation was determinedby subjecting the sample to sparks of varying energies. Apositive reaction was defined when the sample burnedcompletely.

2.6 Dust Explosibility Tests

The measurements of maximum explosion pressure(Pmax), maximum rate of pressure rise ((dP/dt)max) andlimiting oxygen concentration (LOC) of Alex were con-ducted in a stainless steel, spherical vessel (Siwek appara-tus), according to standard procedures(34). The minimumignition energy (MIE) tests were conducted using a deviceknown as the MIKE 3, manufactured by Adolf Kuhner AG(Switzerland).

2.7 Surface Characterization

A UNICAM 929 Atomic Absorption Spectrometer(AAS) was used to analyze the amount of Al in Alex. APHI-5600 X-Ray Photoelectron Spectrometer (XPS) and aPHI-600 Auger Electron Spectrometer (AES) were alsoused to characterize the surface chemistry of Alex.

3 Results and Discussion

3.1 Thermal Behaviour of Al Nanopowders

The TG-DTA results for Als in air are shown in Figure 1.An exotherm was observed at an onset temperature of430 58C. The onset temperature is the temperature atwhich a deflection from the established baseline is observed.This exotherm corresponds to a 23 1% mass gainobservedon the TG curve between 430 and 550 8C. An enthalpychange of 5.5 0.2 kJ g1 was obtained from DSC measure-ments (Figure 2).

As shown in Figure 3, no exotherm and mass gain wereobserved for Als in nitrogen before 550 8C. Therefore, theexotherm and the mass gain observed between 430 and550 8C in air were due to the oxidation of Als:

4 Al(s) 3 O2(g) 2 Al2O3(s) (1)

The expected mass gain for Reaction (1) is 89%. There-fore, the 23 1% mass gain obtained from Als in air impliesthat approximately 26% of Als was oxidized to Al2O3between 400 and 550 8C. The remaining 74% of the samplecorresponds to the unreacted Als, as well as the Al oxide inthe original sample. The melting of Al near 660 8C observedfrom Als in air (DTA curve in Figure 1) indicates that anunreacted Al core remained in Als after the early oxidation.

A second mass gain was observed from Als in air above550 8C (Figure 1). This mass gain may be due to a second

230 Kwok, Fouchard, Turcotte, Lightfoot, Bowes, Jones Propellants, Explosives, Pyrotechnics 27, 229 240 (2002)


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stage of oxidation (Reaction (1)) and/or a nitridation of theunreacted Al core, which is described as follows:

2 Al(s)N2(g) 2 AlN(s) (2)

The second reaction had a mass gain of 27 1% up to1200 8C. However, the baseline had not recovered at1200 8C, which implies the reaction was not complete at1200 8C.

The TG-DTA results for Als in nitrogen show about 33%mass gain, which corresponds witha broad exotherm split bythe melting of Al near 660 8C (Figure 3). The mass gainbetween 550 and12008C is slightly higher in nitrogenthaninair. A possible explanation for the lower mass gain in airis thatan alumina barrier is formed on the outside layer of the Alparticle, hindering further reactionof the remaining Al core.

The TG-DTA and DSC results for Alex received fromDREV are shown along with those for Als in Table 1 andFigures 1 3. The oxidation of Alex (DREV) has a higheronset temperature, a lower mass gain and a lower enthalpychange compared to Als. No oxidation was observed fromH-15 before 790 8C.

The TG-DTA and DSC results suggest that Als is morereactive than Alex in air. The two nanopowders differ intheir particle sizes and the smaller nanopowder (Als)possesses larger surface area. Additionally, the differencesin the reactivity of Al nanopowders may be due to thedifferent thickness of the oxide layer. As compared inTable 1 and Figure 2, a different sample of Alex receiveddirectly from Argonide exhibits an enthalpy change that is70% higher than that obtained for the DREV sample. Theobserved differences between the two samples of Alex may

Figure 1. TG-DTA results for various Al powders heated in air at 10 8C min1

Figure 2. DSC results for various Al powders heated in air at 0.5 8C min1

Characterization of Aluminum Nanopowder Compositions 231Propellants, Explosives, Pyrotechnics 27, 229 240 (2002)


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be due to variations in thickness of theoxide layer that resultfrom differences in the aging of the two samples, i.e.variations in exposure to air and moisture.

The thermal behaviour of Als in ambient and super-ambient pressures of air was studied using ARC. Anexotherm, which is defined as a self-heating rate greaterthan 0.028C min1, was detected between 110 1128C

(Figure 4). No other exotherm was detected up to 400 8C.However, the decrease in pressure (Figure 4), whichcorresponds with the depletion of oxygen (and possiblynitrogen), indicates an oxidation (and possibly nitridation)of Als. The residual pressure, which is the pressure recordedafter the system was allowed to cool to room temperature,was 0.27 MPa (Table 2). The higher value for the residual

Figure 3. TG-DTA results for Als and Alex heated in nitrogen at 10 8C min1

Table 1. Comparison of DSC and TG-DTA results for various Al powders in air

Sample DSC TG-DTA

To/8C DH/kJ g1 Dm% To/8C A/8C min mg

1 Dm1% Dm2%

Als 399 5.5 32 430 4.3 23 27Alex ( DREV) 460 3.9 25 475 4.0 20 38Alex ( Argonide) 478 6.6 35 H-15 790 0.2 11 Paint-fine Al 462 7.8 37

Figure 4. ARC results for Als starting at 0.11 MPa of air



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pressure and the mass loss suggest the production of gases asa result of an outgassing. The outgassing behaviour of Alswill be discussed later in this paper.

In the ARC experiment starting at 0.72 MPa of air, anexotherm was detected at 1058C and this exothermcontinued until the run was stopped manually at 210 8C.This exotherm has a true onset temperature (To) of95 10 8C, determined by extrapolating the self-heatingrateto08C min1. Therefore the oxidation of Als may occur

before 1008C at higher pressures of air. Previous ARC

results for Alex show that no exotherm was detected below150 8C from experiments starting at ambient pressure and0.72 MPa of air (22). Therefore, Alex has a higher To ofoxidation than Als.

3.2 Surface Characterization of Alex

A concentration of 87.14 0.03 mass% of Al wasobtained from an AAS analysis of Alex. The AAS resultsindicate that Alex contains approximately 13 mass% ofoxide layer. The surface characterizationsof Alexusing XPS

and AES indicate the presence of pure metallic Al on thesurface; however, most of theAl on thesurface is mixedwithoxygen, carbon and nitrogen. The XPS results indicate thatAlex has a hydrated oxide layer which also contains sorbed

hydrocarbon vapor, oxy-carbon and oxy-nitrogen species. Athickness of about 10 nm was obtained for the oxide layer.The mean particle size of Alex is approximately 180 nm.Additionally, appreciable AlN was detected at/near thesurface of Alex.

3.3 Stability of Als in Water and in a Humid Atmosphere

The ARC results for a mixture of Als/H2O (10: 1)arealsoshown in Table 2. An exotherm was detected at 32 8C. Theexperiment was stopped automatically at 48 8C, when theself-heating rate exceeded the automatic terminationthreshold (58C min1). This exotherm may result from areaction between Al and water to form Al2O3 or Al(OH)3.

The reactivity of Al nanopowder with water may causeaging of Al nanopowder. As discussed earlier, DSC studieshave demonstrated that thereactivity of Alex in airdependson the aging of the material (Section 3.1). To further studythe aging of Al nanopowder, wetted samples of Als wereprepared by placing Als in a bell jar at a relative humidity>90% at room temperature. TG-DTA experiments were

conducted on the wetted samples of Als in air to determinetheAl2O3 or Al(OH)3 formation froma reaction with water.

As shown in Figure 5, a decrease of the mass gain and thepeak area between 430 and 550 8C was observed after Als

Table 2. Comparison of ARC results for Als, Alex and mixture of Als/H2O

Sample Initial P/MPa Residual P/MPa(a) Mass/g Mass gain% To/8C Rmax/8C min1

Als 0.11 0.27 1.0 0.2 110 0.020.72 0.60 1.0 1.0 105 0.04

Alex(22) 0.11 0.11 1.0 0.15 290 0.020.72 0.68 0.5 0.45 150 0.02

Als/H2O (10 : 1) 0.72 2.0 1.1 1.4 32 5.0(b)

(a) $25 8C(b) Automatic termination of experiment

Figure 5. TG-DTA results for aged Als heated in air at 5 8C min1



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had been exposed to this humid atmosphere for about 47days. This decrease in the mass gain and the peak area, aswell as the presence of white and light grey powders in thewetted Als prior to the TG-DTA experiments, suggest thatAl2O3 or Al(OH)3 was formed from exposure to moisture.Als was completely reacted in 74 days, i.e. no further massgain in the TG study (Figure 5). No significant change inmassgain was obtainedfrom Als exposed todry air for upto62 days. A preliminary aging study conducted at 408Cshowed that Als reacted completely in humid atmosphere inless than three days.

3.4 Outgassing Studies

Mixtures of nanometric Al powder and nitraminesshowed a gas evolution in the Vacuum Stability Test thatwas dependent on the particle size(27). Chemical interactionbetween Alex and RDX above 100 8C has been reported(23).In this study, the outgassing behaviour of mixtures of RDX

with Als and micron-sized Al powders was investigatedusing TG-DTA-FTIR-MS. The outgassing experiments

were also performed on the individual components, todistinguish between the outgassing behaviour due to eachcomponent and the interaction between the Al powders andRDX.

Example plots for the outgassing experiments, using theRDX/Als mixture, are shown in Figure 6. The TG curve andthe sample temperature were plotted against time inFigure 6 (a). The absorbance (A) versus time for theevolved gases detected by FTIR are shown in Figure 6(b).The MS data are presented as ion current versus time for theselected mass fragments (Figure 6 (c)). The results obtainedfor Als, Alex, H-15 and RDX are summarized in Table 3.

No significant mass loss and no evolved gases wereobserved for H-15 and RDX. In contrast, mass losses of0.98 0.07 and 0.46 0.05% were obtained for Als andAlex, respectively. The FTIR results show that the desorbedspecies were CO2 and H2O. Evolution of H2O (m/z 18) wasalso detected by theMS. By comparing themasslosses of thetwo Al nanopowders, Als seems to desorb more than Alex.Therefore, the adsorption of H2O and CO2 is particle size

dependent, with more being adsorbed as the particle sizedecreases.

Figure 6. (a) TG; (b) FTIR; (c) MS results for RDX/Als heated in helium at 5 8C min1 to 1208C and isothermal 40 min



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The outgassing results for the mixtures of RDX andvarious Al powders (50: 50) are compared in Table 3. Masslosses of 1.55 and 0.55% were obtained for RDX/Als andRDX/Alex mixtures, respectively. The estimated masslosses, which are calculated using the TG results for the

neat components, are 0.74 and 0.34% for the RDX/Als andRDX/Alex mixtures, respectively. Since the observed masslosses exceeded the expected values from the individualcomponents, the Al nanopowders appear to interact chemi-cally with RDX. Evolution of N2O, NO2, and H2CO wasdetected by FTIR and the MS starting at about 70 8C. Sincethese gases were not observed from neat RDX or from Alnanopowders, they were generated from the chemicalinteraction between Al nanopowders and RDX. As shownin Table 3, the concentrationof N2O evolved from the RDX/Als mixture is higher thanthat from the RDX/Alex mixture.Apparently, the interaction of Al nanopowder with RDX isparticle size dependent. No such interaction was observed

from the mixture of RDX and H-15.The effect of water on the chemical interaction of Als and

RDX was investigated by conducting outgassing experi-ments on: (i) pre-dried, (ii) pre-wet and (iii) pre-wet/pre-

dried RDX/Als mixtures. The samples were dried at 858Cfor2 hours in theTG-DTAusing a helium purge. The wettedsamples were preparedby placing themixturesin a bell jarata relative humidity>90% for 24 hours. The results for thetreated RDX/Als mixtures are included in Table 3. The

same gases were detected, but they occurred at differentlevels from the treated samples. The concentration of N2Owas highest in the pre-wet mixture, and lowest in the pre-dried mixtures. The presence of water on the surface of thesamples may enhance the interaction between Als andRDX. Although pre-drying reduced the outgassing, it didnot prevent the occurrence of such interaction. Hence, thepresence of water is not necessary for outgassing to occur.

3.5 Effect of Al Nanopowders on Explosives

The results for a typical ARC experiment, using 0.5 g of

TNT/Als (70:30) mixture, are illustrated in Figure 7. Anexotherm was detected with a To 183 9 8C. The run wasstopped automatically at about 2108C, when the self-heating rate exceeded 0.2 8C min1. Figure 8 shows a plot

Table 3. Summary of outgassing studies

Sample Mass/mg TG mass loss%(a) FTIR results 104 N2O/A mg1 MS results(b)

RDX 20 0.08 0.05 nd ndH-15 20 0.05 0.05 nd ndAlex 20 0.46 0.05 H2O, CO2 m/z 18Als 14 0.98 0.07 H2O, CO2 m/z 18RDX/H-15 25 0.09 0.04 nd ndRDX/Alex 25 0.55 0.04 N2O, NO2, HCHO, H2O, CO2 0.88 m/z 18, 29, 30, 44

RDX/Als 20 1.25 0.05 N2O, NO2, HCHO, H2O, CO2 2.1 m/z 18, 29, 30, 44Pre-dried RDX/Als 20 0.84 0.05 N2O, NO2, HCHO, H2O, CO2 1.2 m/z 18, 29, 30, 44Pre-wet RDX/Als 20 1.68 0.05 N2O, NO2, HCHO, H2O, CO2 2.8 m/z 18, 29, 30, 44Pre-wet/pre-dried RDX/Als 20 1.03 0.05 N2O, NO2, HCHO, H2O, CO2 1.7 m/z 18, 29, 30, 44

(a) mass loss at 50 min; the uncertainty of the value represents the TG baseline drift(b) m/z 18H2O; m/z 29H2CO; m/z 30N2O, NO2, HCHO; m/z 44CO2, N2Ondnot detected

Figure 7. ARC results for TNT/Als mixture (T and P vs. time)



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of ln(Rate/8C min1)against103 K/T.Fromtheslopeandtheintercept of the curve, values of E 157 4 kJ mol1 andln(Z/min1) 37 1 were obtained, where E and Z arerespectively the activation energy and the pre-exponentialfactor for the decomposition of the TNT/Als mixture.

As shown in Table 4, the addition of Als and paint-fine Allowered the onset temperature of the TNT decompositionby 40 and 20 8C, respectively. E, Z and the rate constant (k)for the decomposition of the TNT/Als and TNT/paint-fineAl mixtures are significantly different from the othercompositions (Table 4 and Figure 9). The k values werecalculated using the kinetic parameters in Table 4. The

addition of Als or paint-fine Al significantly lowered theonset temperature and increased the rate of the thermaldecomposition of TNT. In contrast, H-15 had no effect onthe decompositionof TNTand theeffect of Alex wasonly toincrease the rate of reaction slightly.

The results for RDX and its mixtures with Als and Alexare compared in Table 4 and Figure 10. The addition of Alsdecreases the onset temperature of the RDXdecompositionby $70 8C (Table 4). The kinetic parameters and the rateconstant for the decomposition of the RDX/Als mixture aresignificantly different fromthose for neat RDX(Table 4 andFigure 10). The lower onset temperature and the higher rateconstant for the decomposition of the RDX/Als mixturesuggest a different decomposition mechanism from that ofRDXalone or forthe RDX/Alex mixture.With theneat RDXand the RDX/Alex mixture, the decomposition occurredwith RDX in the liquid phase, while Als reacted with RDX

below the melting point. Comparing the onset temperaturesof the same types of samples with different mass, no masseffect was observed for either RDX or RDX/Als mixtures.

As shown in Table 4, the addition of Als, Alex, H-15 andpaint-fine Al had little effect on the thermal decomposition

Figure 8. ARC results for TNT/Als mixture (ln(Rate) vs. 1/T)

Table 4. Comparison of ARC results for various formulations

Sample Mass/g To/8C Residual P/MPa(a) E/kJ mol1 ln(Z/min1)

TNT 0.25 220 3 0.2 459 18 107 5TNT/Als (70 : 30) 0.51 183 9 0.2 157 4 37 1TNT/Alex (70 : 30) 0.30 223 4 0.2 415 21 97 5TNT/H-15 (70 : 30) 0.30 224 6 0.3 480 23 112 5TNT/paint-fine Al (70 : 30) 0.30 198 3 0.2 258 27 62 6

RDX 0.2 183 5 0.2 548 111 139 280.4 193 4 0.1 918 10 232 8

RDX/Als (80 : 20) 0.5 120 10 0.4 102 4 25 11.0 119 8 0.4 123 4 32 1

RDX/Alex (80 : 20) 0.2 172 5 0.2 380 138 97 25Comp B 0.20 155 7 0.3 178 1 46 1Comp B/Als (90 : 10) 0.52 152 4 0.2 184 5 48 1Comp B/Alex (90 : 10) 0.20 155 7 0.4 146 3 37 1Comp B/H-15 (90 : 10) 0.31 160 6 0.3 216 18 56 5Comp B/paint-fine Al (90 : 10) 0.30 163 5 0.3 216 3 56 1AP 0.50 173 2 0.4 138 2 32 1AP/Als (80 : 20) 1.0 167 13 0.1 163 13 39 1AP/Alex (80 : 20) 0.50 184 1 0.2 248 4 61 1

(a) $25 8C



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of Comp B, in terms of both onset temperature and kineticparameters.

The ARC results in Table 4 and Figure 11 summarize theeffect of Al nanopowders on the thermal decomposition ofAP. There are insignificant differences between To ofdecomposition for neat AP, and its mixtures with Als andAlex, when the uncertainty of the values are considered(Table 4). However, the addition of Als and Alex seems toslightly increase the rate of AP decomposition (Figure 11).

Overall, Als has a greater effect on the thermal decom-position of TNT and RDX than Alex. Als lowers the onsettemperature by 40 to 708C. It also significantly increases therate of decomposition. The addition of Als has a huge effecton the thermal stability of RDX, therefore a large quantityof RDX/Als mixture may react at a very low temperatureduring production.

3.6 ESD Sensitivity

The ESD sensitivities of Als, Alex, TNT, RDX, Comp B,AP, and their mixtures were determined, and are summa-rized in Table 5. No ignition was obtained when Als wasplaced in a closed sample container and subjected to singlesparks of 25 kV. Hence, the limiting energy is greater than156 mJ (Table 5). However, when Als was placed in opencontainers, it ignited at a charge energy of 6 mJ. Comparedto Alex, which has an ESD limiting energy>156 mJ, Als ismore electrostatically sensitive than Alex. The ESDsensitivity of the Al nanopowder increases as the particlesize decreases, because the smaller Al nanopowder (Als) ismore easily dispersed and has a greater surface area.

For the TNT/Als and Comp B/Als mixtures, no ignitionwas observed at a charge energy of 156 mJ. By contrast, the

Figure 9. Comparison of rate constants for TNT formulations

Figure 10. Comparison of rate constants for RDX formulations



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ESD results show that Als sensitizes RDX and AP toelectrostatic discharge. The RDX/Als mixture would not bedescribed as particularly ESD sensitive. The ESD energy ofthe AP/Als mixture is similar to that of KDNBF (potas-siumdinitrobenzofuroxane), which is a primary explosivesensitive to impact, friction and ESD(33). Alex also appearsto sensitize AP to ESD; however, the material would still notbe described as very ESD sensitive.

3.7 Dust Explosibility

Explosibility tests were conducted to measure themaximum explosion pressure (Pmax), maximum rate of

pressure rise ((dP/dt)max), size-normalized rate of pressurerise (KSt), limiting oxygen concentration (LOC) andminimum ignition energy (MIE) for Alex. Values for eachof these parameters are given in Table 6. Alex can beclassified as being among the most violently reactive (St 3)dusts. It also hasa very low MIE. Alex clearlypresents a dustexplosion hazard and appropriate cautionmust be exercisedin handling this material.

4 Conclusions

TG-DTA and DSC results show an early oxidation of Alnanopowders between 430 and 600 8C. The reactivity of Alpowders in airdepends on the particle size andthe history ofthe sample. Als is very reactive with water, showingexothermic activity at 328C from an ARC experimentconducted on a mixture of Als/H2O. Aging of Als wasobservedfor samplesexposedto humid atmosphere at roomtemperature for more than 47 days, and Als was completelyreacted in 74 days.

A value of approximately 13 mass% of aluminum oxidewas obtained from the AAS analysis for Alex. The XPS andAES results indicate the presence of pure metallic alumi-nium on the surface; however, most of the aluminum on thesurface is the Al3 species, mixed with oxygen, carbon andnitrogen. A thickness of about 10 nm was obtained for theoxide layer. Appreciable AlN was also detected at/near thesurface of Alex.

The outgassing studies using TG-DTA-FTIR-MS, showdesorption of H2O and CO2 from Al nanopowders.

Figure 11. Comparison of rate constants for AP formulations

Table 5. Summary of ESD results

Sample ESD ignition energy/mJ

Closed container Open container

Als >156 6Alex >156 >156TNT >156 >156TNT/Als (70: 30) >156 >156TNT/Alex (70 : 30) 156 >156RDX >156 >156RDX/Als (80 : 20) 100 25RDX/Alex (80: 20) >156 >156

Comp B >156 >156Comp B/Als (90: 10) >156 >156Comp B/Alex (90: 10) >156 >156AP >156 >156AP/Als (80 : 20) 1 6AP/Alex (80 : 20) 100

Table 6. Alex dust explosibility parameters

Pmax/Mpa (dP/dT )max/MPa s1 KSt/MPa m s

1 Dust Class LOC/% MIE/mJ

0.94 118.5 32.2 St 3 5 1 3



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Chemical interactions between these Al nanopowders andRDX, generating NO2 and N2O, were observed starting at70 8C. Both the adsorption of H2O and CO2, and thechemical interaction with RDX increase as the particle sizeof Al nanopowder decreases.

The effects of the addition of Al nanopowder on thethermal stability of TNT, RDX, Comp B, and AP weredetermined using ARC. Als has a greater effect on thethermal decomposition of TNT and RDX than Alex. Alslowers the onset temperature by 40 to 708C. It alsosignificantly increases the rate of decomposition.

Als and Alex appear to sensitize some energetic materialsto ESD. The AP/Als mixture is very sensitive to ESD.Standard dust explosibility tests demonstrated that Alex is ahighly explosible dust.

5 References

(1) Internet WEB Site: ARGONIDE NANOMETAL TECH-NOLOGIES hwww.argonide.comi

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Acknowledgements

We would like to thank Patrick Brousseau of DREV forsupporting and encouraging our research efforts in this work.Additionally, we would like to express our gratitude to Fred Tepperof Argonide Corporation and Joey Viljoen of Orica Canada Inc. forproviding the samples of Alex and paint-fine Al, respectively.Finally, our appreciation is extended to Dr. Jim Brown of NaturalResources Canada and Dr. P. Amyotte of Dalhousie University for

the surface characterization work and for the explosibility tests,respectively.

(Received February 26, 2002; Ms 2002/022)


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