combustion of environmentally altered molybdenum trioxide nanocomposites

Full Papers

Combustion of Environmentally Altered Molybdenum TrioxideNanocomposites

Kevin Moore and Michelle L. Pantoya*

Mechanical Engineering Department, Texas Tech University, 2500 Broadway, Lubbock, TX 79409 (USA)

DOI: 10.1002/prep.200600025

Abstract

Nanocomposite thermite mixtures are currently under develop-ment for many primer applications due to their high energydensities, high ignition sensitivity, and low release of toxins intothe environment. However, variability and inconsistencies incombustion performance have not been sufficiently investigated.Environmental interactions with the reactants are thought to be acontributing factor to these variabilities. Combustion velocityexperiments were conducted on aluminum (Al) and molybdenumtrioxide (MoO3) mixtures to investigate the role of environmentalinteractions such as light exposure and humidity. While the Alparticles were maintained in an ambient, constant environment,the MoO3 particles were exposed to UV or fluorescent light, andhighly humid environments. Results show that UVand fluorescentlighting over a period of days does not significantly contribute toperformance deterioration. However, a humid environmentseverely decreases combustion performance if the oxidizerparticles are not heat-treated. Heat treatment of the MoO3

greatly increases the material;s ability to resist water absorption,yielding more repeatable combustion performance. This workfurther quantifies the role of the environment in the decrease ofcombustion performance of nanocomposites over time.

Keywords: Thermites, Oxidizers, Combustion Velocity, Humidity,Aging

1 Introduction

The photochromic properties of metal oxides have beenstudied extensively for over forty years [1 – 8]. Ultraviolet(UV) light has been shown to cause changes in severalchemical, mechanical, and electrical properties of metaloxides. Indium oxide thin films have been shown to becomemuchmore electrically conductive through exposure to UVradiation [6]. When in an oxygen-present environment,amorphous zinc oxide thin films will crystallize due to UVirradiation [7]. Photodarkening in these same zinc oxidefilms has been attributed to photoreduction effects. Li hasshown similar UV-induced color formation and photo-reduction observations in tungsten oxide nanoparticles,which he claims to describe a change of the oxide fromWO3

to W2O5 [9].

Molybdenum trioxide also exhibits photochromic proper-ties. S. K. Deb reports the formation of “color centers”,which result in a significant increase in optical density due toUV irradiation [8]. He describes this color change as beingdue to an increase in oxygen vacancies in the latticestructure. He also describes a thermal bleaching process,which entails heating the sample to 300 8C in the presence ofoxygen.Thebleachedmolybdenumoxideparticles are rid ofthese color centers and do not form new color centers in thepresence of UV irradiation. In the past ten years, newtechnologies have allowed the formation of nanoparticlemolybdenum oxide (MoO3), which has stronger photo-chromic properties. Absorbance measurements by Li haveshown nanoparticleMoO3 to be 15 times more absorbent ofUV radiation than the bulk MoO3 counterparts.Raman spectroscopy has also been used to characterize

MoO3 [10 – 11]. According to S. H. Lee, heat treatment ofamorphous thin films, in a manner similar to the thermalbleaching described previously, causes a-MoO3 crystalformation to occur [10]. Raman spectroscopy experimentsfrom Lee show sharp peak formation previously not seenwith the amorphous samples.Molybdenum trioxide particles are often used as oxidizing

agents in nanocomposite energetic materials. Research innanocomposite formulations as primers is growing due tothe formulation;s high energy density and environmentalconsiderations of current primers [12]. More information inthis area of research is needed to describe variability andinconsistency seen in performance parameters, such ascombustion rate and ignition sensitivity. Plantier has shownstandard deviations of Al/Fe2O3 formulations to be as highas 15% of the mean combustion velocity [13]. Granier hasshown ignition time and combustion speed standard devia-tions in Al/MoO3 formulations of greater than 20% of themean values [14].Although the ability ofMoO3 optical properties to change

through exposure to UV radiation is known, little is knownabout how the combustion properties are affected byenvironmental factors. A change in combustion character-istics could lead to a greater understanding of causes ofvariations in the performance of nanocomposites. This* Corresponding author: email: [email protected];

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knowledge could lead to the development of more consis-tent nanocomposites and expedite the use of environ-mentally responsible primer formulations. Therefore, thescope of this work is to evaluate how the combustionbehavior of nanocompositeAl/MoO3 formulations is affect-ed by MoO3 particles that have been altered throughexposure to fluorescent light, UV radiation and high rela-tive humidity levels, all common environmental factorswhich may change the combustion performance of thesematerials.

2 Experimental Setup

2.1 Material Preparation

All experimentation was completed using Al fuel par-ticles and MoO3 as the oxidizer particles. The Al had anaverage diameter of 120 nm and was formed through a gascondensation process by Nanotechnologies, Inc. (Austin,TX). Aluminum particles are naturally pyrophoric, there-fore, a passivation layer of Al2O3 was formed during theproduction process to decrease the risk of unintentionalignition. Themanufacturer used abase hydrolysismethod todetermine that the activeAl content of this powderwas 82%by weight, which leaves 18% of the powder Al2O3. Themanufacturer also reported the specific surface area of thepowder to be 18.4 m2/g, based on the BET method.The MoO3 particles were acquired from Climax Technol-

ogies (Phoenix, AZ) with a sheet-likemorphology andweredetermined by BET method to have a specific surface areaof 42 m2/g. MoO3 particles have been shown to form colorcenters at temperatures below300 8C[8].After being heatedto 300 8C, these samples did not form any color centers andretained their original color [8]. In this study, a similar heattreatment process was conducted to compare the burningbehavior of heat-treated and untreated MoO3 particles.Along these lines, two separate batches of MoO3 weretested. One batch will be defined as heat-treated, and 1 gsamples of MoO3 particles, housed in an open ceramiccrucible, were heat-treated to 400 8C in a furnace for twohours. After heat treatment, the MoO3 particles were thenmixed and exposed under the same conditions as theuntreated MoO3 samples. The second batch will be definedas untreated and were not exposed to any heating con-ditions.Composites were prepared by suspending Al and MoO3

particles in hexanes and sonicating themixture using a sonicwand (Misonix, Farmingdale, NY) for two minutes. Thesonication breaks up agglomerates and allows for theoptimum intermixing of fuel and oxidizer particles. Aftersonication, the solution of hexanes and particles werepoured into a metal tray to allow the hexanes to evaporate.The mixture was then brushed out of the pan into a finemesh, which aids in breaking up any agglomerates formedduring the evaporation process.

2.2 Material Testing

The burn velocity describes the rate of flame propagationthrough the loose powder, unconfined sample. The velocitywas determined using a fabricated burn tray, consisting of analuminum block with a square channel cut in the top side.The square channel is 42 mmwide and high with a length of6 cm.At a distance of 35 mmand 55 mm from the beginningof the channel, two 1 mm diameter holes were drilledthrough the block to allow light to be emitted from thereaction to two fiberoptic cables connected to the other endof these holes. The energetic mixture was ignited using aspark igniter at one end of the channel, and the reactionwould self-propagate until all reactants were consumed. Asthe reaction passed over each hole, light traveled to thefiberoptic and was transmitted to a photodiode. The photo-diode converted the light intensity to a voltage which wasread by an oscilloscope. The two voltage traces were thenread off of the oscilloscope display. The time delay betweenthe two sharp pulses was used to determine the time neededfor the reaction to pass from the first hole to the second.Since the distance between the holes is known, the velocityof the flame propagation was determined. The 35 mmchannel distance to the first hole allows the acceleration ofthe reaction to cease, and the velocity determination is thesteady state velocity of the mixture.

2.3 Molybdenum Trioxide Environmental Modifications

The MoO3 powder was exposed to three differentenvironmental conditions: (1) fluorescent light; (2) a UVlamp; and, (3) an environmentof 99%relative humidity. TheMoO3 powderwas first added to hexanes, sonicated to breakup agglomerates, then poured into a glass pan to evaporatethe hexanes. The glass pan with a thin layer of MoO3 wasused to house the MoO3 during exposure to light andhumidity. Having theMoO3 in a thin layer allows for a largeand consistent amount of surface area for each sampleduring exposure periods.For fluorescent light exposure, the glass pans were set in a

room lit by fluorescent lighting. For the UV irradiation, thepans were housed in a box opaque to all outside light. Insidethis box, a 365 nmUVlamp (Fisher Scientific, Pittsburg,PA)was used for irradiation. To create a humid environment, anair-tight Lexan container was used to house the pans. Awarm water container inside the Lexan container main-tained an environment of 99% relative humidity. Duringhumid environment exposure, the MoO3 was also exposedto fluorescent lights. The Al used in the thermite mixturescontaining these environmentally altered MoO3 particleswere not exposed to any environment besides normalambient environments (25 8C, 22% relative humidity).

Combustion of Environmentally Altered Molybdenum Trioxide Nanocomposites 183

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3 Results

3.1 Scanning Electron Microscope Imagery

Figure 1 shows SEM images of MoO3. Initially, the MoO3

samples are colored in a bright yellow,which is the conditionof the MoO3 sample in this figure, but in a matter of hoursthe MoO3 samples begin to retain a bluish tint. The SEMimages of Figure 2 show MoO3 that had been exposed tofluorescent light for four days. After this amount ofexposure, the MoO3 had changed from yellow to a greenishblue. Figure 3 shows SEM images of MoO3 that had beenheat-treated to 400 8C. This powder had a very bright whitecolor.

3.2 Burn Velocity Results

Al/MoO3 formulations were produced at varying equiv-alence ratios to obtain the optimum stoichiometry based onthe highest velocity. The average burn velocities as afunction of equivalence ratios for heat-treated and un-treated MoO3 are shown in Figure 4. The Al and untreatedMoO3 mixtures have an optimum equivalence ratio of 1.6

and the head-treated MoO3 mixtures have an optimumequivalence ratio of 2. These optimum equivalence ratioswere used to prepare mixtures for the following environ-mental impact tests (i.e. Figures 5 – 7).The burn velocity results for the mixtures with MoO3

subjected to fluorescent light exposure are presented inFigure 5 and those subjected to UV light exposure inFigure 6. Figure 7 displays the burn velocity results from thehumidity tests. The uncertainty bars shown in Figures 4 – 7are based on one standard deviation above and below theaverage burn velocity,which is based on six burn tests. Basedon these multiple tests for the repeatability of a measure-ment, the average uncertainty in a series of experiments isroughly 15 %.

4 Discussion

4.1 SEM Imaging

Untreated MoO3 crystals shown in Figure 1 are mostlynanometer size crystal particles. Much larger sheet-likecrystals, several micrometer in length and width but onlyabout a hundred nanometers thick, are scattered sporadi-cally amongst the smaller crystal particles. These unexposedMoO3 particles appear in a bright yellow color. After only afew hours of exposure to fluorescent light, the yellowparticles take on a bluish tint. After four days of fluorescentexposure, the MoO3 particles turned completely blue, andSEM images of these particles are shown in Figure 2. Theseblue particles look very similar to the unexposed yellowparticles of Figure 1. Based on these images, it appears the

Figure 1. SEM micrographs of untreated MoO3 exposed to nolight (yellow).

Figure 2. SEM micrographs of MoO3 exposed to 4 days offluorescent light (blue).

Figure 3. SEM micrographs of heat-treated MoO3.

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transition from yellow to blue color has no effect on thecrystal structure of the MoO3 crystals. Previous work [8]attributed the color change of MoO3 particles to desorptionof oxygen but no other physical changes are observed viaSEM analysis. Depending on the amount of oxygenremoved from the matrix, the average oxygen content permolecule may decrease to 2.5 or 2 atoms instead of 3. This isa significant factor that potentially impacts the fuel tooxidizer ratio, making the actual mixture fuel rich.Heat-treating the MoO3 particles yields a significant

visual change in crystal structure. SEM images of these heat-treated particles are shown in Figure 3. The SEM imagesshow that these particles are several micrometer in lengthand width, while between 100 and 300 nm in thickness.These particles are very similar in size to the large crystalsseen in Figure 1 and Figure 2. These heat-treated particlesappear white in color, and do not change color afterexposure to fluorescent or UV light for several days. Thissuggests that the heat-treated crystals do not undergo aphotoreduction and do not lose oxygen atoms to theatmosphere.

The crystal change observed here has been described as“thermal bleaching” byDeb, and only occurswhen the heat-treating occurs in the presence of oxygen, since oxygen fromthe atmosphere will fill the vacancies created by thephotoreduction [8]. Lee [10] refers to this heat-treatedMoO3 as the a-phase crystalline structure. Whether therelatively large sheet particles of Figure 3 are of the samecrystalline structure as these heat-treated particles is notknown at this time.

4.2 Burn Velocity

4.2.1 Equivalence Ratio Testing

The equivalence ratio was calculated based on the globalreaction shown in Equation (1).

2AlþMoO3!Al2O3þMo (1)

When mixed with Al, the yellow, untreated MoO3

particles have an average maximum burn velocity of about

Figure 4. Burn velocity as a function of equivalence ratio.

Figure 5. Burn velocity as a function of fluorescent lightexposure.

Figure 6. Burn velocity as a function of UV exposure.

Figure 7. Burn velocity as a function of increased relativehumidity.




420 m/s at an equivalence ratio of 1.6. The white, heat-treated MoO3 particles have an average maximum burnvelocity of about 360 m/s at an equivalence ratio of 2. Theseexperiments were conducted in an ambient air environmentand the loose powder is only at a density of about 5%TMD,which leaves a significant volume of air among the particles.As the aluminum from the reaction is first oxidized uponmelting of the Al core or mechanical failures of the Al2O3

shell, gaseous oxygen in the air could more easily diffuse tothe molten Al than the solid MoO3. In this scenario, Al isreacting with gaseous oxygen which is not accounted for inEq. (1). Although the balance appears fuel rich with anequivalence ratio greater than one, reactions with air maymake the balance more stoichiometric.The untreated MoO3 mixtures display higher burn

velocities than the heat-treated MoO3 mixtures at everyequivalence ratio tested. The untreated MoO3 particles areon averagemuch smaller in size than the heat-treatedMoO3

particles (see Figs. 1 – 3). Their smaller size allows for betterintermixing between these MoO3 particles and the alumi-num particles, creating decreased diffusion distances. Withthese smaller diffusion distances, the reaction can take placemore rapidly, which results in higher burn velocities.

4.2.2 Environment Exposure Testing

Fluorescent and UV light exposure to the untreatedMoO3 has little effect on burn velocity (Figs. 5 and 6). After4 days exposure to fluorescent light, the burn velocitydecreased by about 35 m/s, which is less than 10% of thetotal velocity. After 4 days of exposure to UV radiation, theburn velocity decreased even less, only about 15 m/s. Thissmall decrease could be due to oxygen desorption from theMoO3 matrix, creating an oxygen poor environment andincreasing the actual equivalence ratio of the mixture. Asseen in Figure 4, a slight change in the equivalence ratio canreduce the burn velocity by 25 m/s or 30 m/s. The smalldecrease shows that light affects can slightly contribute tothe variability of performance parameters, but does notimpede or hinder the reaction.The humid environment testing resulted in a significant

difference between the twoMoO3 powders (Figure 7).Afterexposure to 99% relative humidity conditions for only oneday, the untreated MoO3 mixture had a reduction in burnvelocity of over 99%. It is obvious that the untreated MoO3

absorbs water from the air, which is detrimental to the Al/MoO3 reaction. This water absorption could be due tooxygen vacancies in the matrix. These vacancies create ionson the MoO3 particle surface and could more easily attractcharged water molecules from the air. The water moleculeswould continue to be attached until exposed to an environ-ment of low relative humidity or temperatures around100 8C. During the mixing process, water attached tothe MoO3 molecules could come in contact with Al,significantly lowering the Al reactivity. The decreased Alreactivity would account for the loss of performance of themixture.

In the humid environment, the heat-treatedMoO3 mixtureshowed no change after two days exposure but decreased inperformance during the final two days of testing (Fig. 7). Theheat-treated MoO3 is more resistant to absorbing water fromthe environment. The characteristics of the heat-treatedMoO3 crystal structure provide the reasoning for the differ-ence in water absorption abilities. Since oxygen does notdesorb from the surface of theMoO3, the surface has less freeions. A more electronically stable surface would attract fewwatermolecules, keeping the surfaceof theparticles relativelydry. More analysis of the crystal structures of the treated anduntreated powders is vital to understanding this property.Based on this drastic performance reduction, it seems the

water content in the environment is an important factor tobe considered. These results show that an increase inatmospheric water content could significantly alter theperformance of some or all of the material in a nano-composite mixture.Although the heat-treated MoO3 did not perform as well

as the untreated MoO3 before being exposed to light orhumidity, the potential for its practical use can be great. If amaterial of this nature is needed in an environment thattypically has increased humidity levels or is exposed tohumidity for a very long time, the heat-treated MoO3 is thebetter choice. This material would resist the ill effects ofwater absorption for a much longer period of time than theuntreatedMoO3.On theother hand, if a primermaterialwasneeded in a dry environment or in a formulation thatincludes awater-proof binder, the untreatedMoO3would bethe best choice because it performs as consistently as theheat-treated MoO3 and burns at a faster velocity, whichtends to correlate with higher pressure output.

5 Conclusions

Nano-scale MoO3 particles were exposed to three differ-ent environmental conditions to determine factors thatcontribute to the increased variability and reduced com-bustion performance when mixed with 120 nm Al particles.Aluminum particles were held in a constant environmentsuch that they were not exposed to anything different fromnormal preparation conditions. There were no significantdecreases in burn velocity when theMoO3 particles exposedto fluorescent and UV light were combined with Al. Burnvelocities of MoO3 mixtures exposed to a humid environ-ment for one day decreased by 99%.Heat-treating MoO3 nanoparticles to 400 8C yields sig-

nificantly larger MoO3 particles due to a crystallographicphase change. Heat-treated MoO3 particles, based on burnvelocities, were not affected by fluorescent light but showedminor decreased burn velocities when exposed to UVradiation. After heat-treated MoO3 particles were exposedto a humid environment for two days, burn velocities of themixture were unchanged, but decreased when exposed formore than two days. The heat-treated particles show morepotential for increased lifetime in applications involvinghumid environments.

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6 References

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Acknowledgements

Dr. Pantoya and Mr. Moore gratefully acknowledge the supportof the Army Research Office (Contract Number W911NF-04-1-0217) and our program manager, Dr. David Mann.

(Received April 20, 2005; Ms 2005/112)




combustion of environmentally altered molybdenum trioxide nanocomposites

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