COMBUSTION BEHAVIOR OF SOL-GEL SYNTHESIZED

ALUMINUM AND TUNGSTEN TRIOXIDE

by

DANIEL PRENTICE, B.S.M.E.

A THESIS

IN

MECHANICAL ENGINEERING

Submitted to the Graduate Faculty of Texas Tech University in

Partial Fulfillment of the Requirements for

the Degree of

MASTER OF SCIENCE

IN

MECHANICAL ENGINEERING

Approved

Michelle Pantoya Chairperson of the Committee

Darryl James

Jordan M. Berg

Accepted

John Borrelli Dean of the Graduate School

May, 200

ii

ACKNOWLEDGEMENTS

I would like to thank my advisor Dr. Michelle Pantoya for introducing me to this

exciting field of nanotechnology research. Thank you for being so generous in your

advice and in allowing me to be a part of the growing literature on nano-composite

energetic materials. I would like to acknowledge Dr. Alex Gash from Lawrence

Livermore National Laboratory for providing the aerogel materials studied as well as the

SEM micrographs and XRD analysis. Thank you to Dr. John Granier and Mr. Keith

Plantier for help in setting up and conducting experiments. Without your help, I would

not have known where to start. I would also like to acknowledge Mr. Dustin Osborne for

conducting the TGA work and Dr. Mark Grimson and Mr. Peter Jung for their help and

advice with the SEM work.

Thank you to my wife Sara for your support during this exciting but stressful time

in our lives. And finally, thank you to my parents for encouraging me to always do my

best and for supporting me in whatever I chose to do.

iii

TABLE OF CONTENTS

ACKNOWLEDGEMENTS.............................................................................................ii

ABSTRACT...................................................................................................................vi

LIST OF TABLES........................................................................................................vii

LIST OF FIGURES......................................................................................................viii

CHAPTER

I. INTRODUCTION .........................................................................................1

1.1 Traditional Thermites .........................................................................1

1.2 Nano-composite Thermites.................................................................2

1.3 Advantages of Tungsten Trioxide.......................................................3

II. EXPERIMENTAL.........................................................................................5

2.1 Sample Preparation ............................................................................5

2.1.1 Aerogel Tungsten Trioxide Properties........................................5

2.1.2 Commercial Tungsten Trioxide Properties .................................9

2.1.3 Thermite Synthesis .................................................................. 10

2.1.4 Pellet Synthesis........................................................................ 11

2.3 Open Channel Burn Testing Apparatus............................................. 11

2.4 Pellet Combustion Rate Testing Apparatus....................................... 12

III. THEORY..................................................................................................... 15

3.1 Motivation to Research Aerogel Composites .................................... 15

iv

3.2 Aerogel Synthesis............................................................................. 16

3.3 Advantages of Aerogels.................................................................... 17

IV. RESULTS.................................................................................................... 19

4.1 Open-Channel Combustion Wave Speed .......................................... 19

4.2 Pellet Combustion Wave Speed........................................................ 20

4.3 Pellet Ignition Sensitivity ................................................................. 21

V. DISCUSSION.............................................................................................. 23

5.1 Open-Channel Combustion Wave Speed .......................................... 23

5.1.1 Optimum Equivalence Ratio .................................................... 23

5.1.2 Optimum Oxidizer Composition .............................................. 24

5.2 Pellet Combustion Wave Speed and Sensitivity ................................ 26

5.2.1 Combustion Wave Speed as a Function of Bulk Density .......... 26

5.2.3 Combustion Wave Speed as a Function of Composition .......... 27

5.3 Aerogel Combustion Dominant Mechanism Transition..................... 36

VI. CONCLUSIONS ......................................................................................... 39

VII. ALUMINUM AND TUNGSTEN TRIOXIDE AGING EFFECTS............... 41

7.1 Aluminum and Tungsten Trioxide Aging Study................................ 41

7.1.1 Introduction............................................................................. 41

7.1.2 Experimental Setup.................................................................. 44

7.1.3 Results..................................................................................... 50

7.1.4 Discussion ............................................................................... 51

7.1.4 Conclusions ............................................................................. 54

v

7.2 Long Term Aerogel Aging ............................................................... 55

REFERENCES ............................................................................................................. 57

vi

ABSTRACT

Calcined (to remove impurities) and non-calcined tungsten trioxide (WO3)

aerogels as well as micron-scale and nano-scale commercial WO3 powders were mixed

with nano-scale aluminum (Al) and their combustion performance in the form of

combustion wave speeds was compared in loose powder and pressed pellet

configurations. Results show that both the calcined and non-calcined, aerogel based

mixtures outperformed the commercial based mixtures in both configurations.

Combustion wave speed was also found as a function of mixture bulk density. Results

show that conduction is the dominant energy transfer mechanism in pressed pellets while

convection is the dominant mechanism in loose powder form. This causes material purity

to be the most important factor for pressed pellets and oxidizer particle size to be the

most important factor for loose powders. A preliminary aging study was conducted which

showed a 7% performance reduction after 4 days of laboratory air exposure and 91-98%

performance reduction after 22 months of exposure.

vii

LIST OF TABLES

1. Oxidizer Properties..................................................................................................5

2. Calculated Andreev numbers and physical parameters for Al+WO3 composites at 31.7-34% TMD ............................................................................... 29

3. Normalized Andreev number for Aerogel 120 and Aerogel 400 in pellet and powder configuration............................................................................. 30

4. Combustion Wave Speeds ..................................................................................... 50

viii

LIST OF FIGURES

1. X-ray diffraction analysis of Aerogel 120 and Aerogel 400 ......................................7

2. Thermal gravimetric plots of Aerogel 120 and Aerogel 400 .....................................8

3. SEM micrographs of Aerogel 120 and Aerogel 400 .................................................9

4. Photograph of the open channel burn testing apparatus .......................................... 12

5. Schematic of the laser ignition and flame propagation diagnostic system ................................................................................................................... 14

6. Plot of open-channel combustion wave speed as a function of equivalence ratio for Aerogel 120 and Aerogel 400................................................ 19

7. Plot of combustion wave speed as a function of pellet density ............................... 21

8. Plot of ignition delay time as a function of pellet density ....................................... 22

9. Photograph of an Aerogel 400 pellet at 49.6% TMD.............................................. 27

10. Andreev number as a function of pellet oxidizer composition at a TMD range of 31.7-34%........................................................................................ 29

11. Calculated adiabatic flame temperature as a function of mass percent water in the oxidizer .............................................................................................. 32

12. Estimated drop in combustion wave speed as a function of mass percent water in the oxidizer.................................................................................. 33

13. Combustion wave speed as a function of oxidizer surface area and pellet density................................................................................................... 34

14. Semi-Log plot of combustion wave speed as a function of TMD for Aerogel 120 and Aerogel 400 ........................................................................... 37

15. Photograph of sample storage containers ............................................................... 46

16. Schematic of flame propagation apparatus............................................................. 48

17. Photograph of burn channel ................................................................................... 49

ix

18. Plot of combustion wave speed as a function of days aged..................................... 51

19. Thermal gravimetric plot of original and aged Aerogel 120 and Aerogel 400 ........................................................................................................... 56

1

CHAPTER I

INTRODUCTION

1.1 Traditional Thermites

Thermite reactions are characterized by an oxidation-reduction reaction between a

metal and a metallic oxide. The reactions are highly exothermic and typically self-

sustaining. Wang et al. [1] conducted a thorough literature review on typical thermite

applications ranging from welding [2, 3] to the synthesis of metals and alloys [4] or

ceramic and composite materials [5-7]. Other applications include a cost-effective

method for storing radioactive wastes [8], using gas-generating additives to create

thermites that could be used for the demolition of concrete [9] or in self-destruct

electrical components [10].

Because thermites are a mixture, an ideal balance of fuel and oxidizer components

can be attained resulting in high energy densities. The energy density is typically higher

than that of explosives because monomolecular energetic materials often have an

imperfect fuel-oxidizer balance. The reaction rate for a thermite is limited by the mass

transport between the reactants. In monomolecular energetic materials such as explosives,

the reaction rate is governed by chemical kinetics and thus explosives have a faster

energy release rate or greater power than thermites.

2

1.2 Nano-composite Thermites

Because thermite reactions are mass diffusion limited, decreasing the reactant

particle size decreases the diffusion distances and increases the reaction rate [11]. By

reducing the size of the reactants from micron-scale to nano-scale, flame propagation

rates (i.e. speeds) have been measured up to 1000 times faster than traditional thermites

[12]. Other studies have similarly confirmed that nanocomposite thermites exhibit greater

speeds than their micron scale counterparts [13-16]. In light of the increased speeds found

in nanocomposite thermites, researchers are beginning to develop materials that have

very high energy densities like traditional thermites but also have high energy release

rates (power) like explosives or other monomolecular energetic materials.

Researchers at Lawrence Livermore National Laboratory (LLNL) created a new

class of energetic materials using sol-gel chemistry [17-23]. Their method allows for not

only nanocomposite but actually nanostructured thermite production. They are able to

create a nano-scale structure with a skeletal sol-gel matrix composed of the oxidizer with

the fuel imbedded in the voids between the matrix [22]. The nanostructure allows for a

level of homogeneity and intimate mixing that is otherwise not easily achieved by simply

mixing nano particles. The sol-gel method offers the possibility to control the

composition, morphology and reactivity of the thermite at the nanometer scale [17].

LLNL researchers first synthesized Fe2O3 aerogel and xerogels [17]; then

synthesized composites such as Fe2O3+Al aerogel and xerogel thermites [18]; and further

expanded the technique to synthesize other oxides (Cr2O3, Al2O3, In2O3, Ga2O3, SnO2,

ZrO2, HfO2, Nb2O5, and WO3) [20, 21] and included silicon gas generators into the

3

thermites [24-26]. One result was that water or hydroxyl groups are an inherent

component in the sol-gel synthesis of these thermites [17, 20]. Plantier et al. conducted

combustion experiments using LLNL sol-gel derived Fe2O3 aerogel and xerogel and

compared it to commercially available nanoscale Fe2O3 [27]. They measured the

combustion wave speeds in open and confined burning experiments and showed

relatively poor burning performance of the aerogel and xerogel Fe2O3 compared to the

commercially available nanopowder. This was attributed to the high amount of impurities

(hydroxyl groups and organic contamination) found in the solids. New aerogel and

xerogel powders were then calcined to 410° C to remove impurities and results showed

the largest improvement in combustion wave speed from ~10 m/s to over 900 m/s in

confined burning conditions.

1.3 Advantages of Tungsten Trioxide

For ordnance applications, desirable properties for the mixture include a high

adiabatic flame temperature and large enthalpy of reaction [28]. Tungsten trioxide (WO3)

combined with aluminum has a high adiabatic flame temperature (5544 K [41]) and large

enthalpy of reaction (3801 cal/cc [41]). Another important property when high energy

density is desired is high material density. Tungsten trioxide has high material density

(7.20 g/cc compared with 5.25 g/cc for Fe2O3). Furthermore, oxidizers such as WO3 and

MoO3 sublime at relatively low temperatures unlike Fe2O3 which remains in the solid

phase. The gas generated during sublimation could facilitate a diffusion limited reaction.

Also, Balakir et al. found that oxidizers with high vapor pressure such as WO3 gave the

4

fastest rates of combustion [30]. For these reasons, this study focused on characterizing

the combustion behavior of Al and WO3.

There were three objectives in this study of nano-Al and WO3 combustion. The

first objective was to determine the optimum stoichiometry. Because it has been shown

that varying the bulk density of the thermite has a significant affect on the combustion

behavior of Al and MoO3 [29], the second objective was to determine how the bulk

density affects the combustion wave speed of nano-Al and WO3. The final objective was

to identify the dominant mechanisms controlling the combustion behaviors corresponding

to the different thermite mixes in loose powder and pressed form.

Optimum stoichiometry was determined by varying the fuel/oxidizer ratio of the

mixtures where the ratio that exhibits the fastest combustion wave speed is deemed

optimum. The thermites were composed of nano-scale aluminum powder mixed with four

types of WO3 oxidizers. The oxidizers were calcined and non-calcined WO3 aerogels

prepared at LLNL, and commercially available nano and micron-scale WO3 powders.

These experiments were performed by burning loose powders of various fuel/oxidizer

ratios in an open channel and measuring the combustion wave speed and flame

propagation behavior via high-speed photography and imaging diagnostics. Bulk density

combustion behaviors were determined by preparing samples of various densities and

examining flame propagation behavior also using high-speed imaging diagnostics.

Identifying the dominant mechanisms controlling the combustion behaviors was

accomplished by using an Andreev numerical analysis and by analyzing the visual data

recorded for the combustion wave speed experiments.

5

CHAPTER II

EXPERIMENTAL

2.1 Sample Preparation

The WO3 oxidizers vary according to manufacturing methods and physical

properties described in Table 1. The Al fuel particles were 80 nm average diameter

supplied by Nanotechnologies Inc. (Austin, Texas) and consistently used for all mixtures

in this study. An alumina shell that is roughly 4 nm thick encapsulates the core Al

particle such that this powder has an active Al content of 84 %. Active Al is defined as

the portion of powder not in the form of Al2O3.

Table 1. Oxidizer Properties Oxidizer Name Oxidizer Description (WO3) Particle Surface Area % WO3

Aerogel 120Nano-Scale,Cubic Tungsten Trioxide

(Calcined to 120° C)77.1 m

2/g (BET) 91

Aerogel 400Nano-Scale, Orthohombic Tungsten Trioxide

(Calcined to 400° C)28.2 m

2/g (BET) 96

nm WO3 Nano-Scale Tungsten Trioxide 16.9 m2/g (Manufacturer

Provided)

100

(Assumed)

Micron-Scale Tungsten Trioxide 0.026 m2/g (Calculated by

Visual Method)

100

(Assumed)

2.1.1 Aerogel Tungsten Trioxide Properties

Both of the WO3 aerogels were synthesized at Lawrence Livermore National

Laboratories (LLNL). The Aerogel 120 was calcined to 120°C and the Aerogel 400 was

calcined to 400°C to remove hydroxyl groups (OH) and impurities that have been shown

to be present during the sol-gel synthesis process [17, 20, 27]. Chemical analysis of the

6

aerogel derived oxidizers was performed at LLNL and the purity is recorded in Table 1.

Aerogel 120 has 1.2% carbon and 1.0% hydrogen impurities by weight. Aerogel 400 has

trace carbon and hydrogen impurities (<0.5 wt.%). Surface area measurements were

made using a gas adsorption analyzer and using BET (Brauner, Emmett, and Teller)

theory. Crystalline structure was determined by x-ray diffraction (XRD) performed at

LLNL as seen in Fig. 1. The Aerogel 120 is highly disordered and is classified as cubic

tungsten oxide as determined from Fig. 1a. The Aerogel 400 is highly crystalline and is

classified as orthorhombic tungsten oxide as determined from Fig. 1b.

To confirm the presence of bonded water, thermal analysis of the Aerogel 120 and

Aerogel 400 oxidizers was conducted using a thermal gravimetric (TG) analyzer

(Netzsch, STA 409 PC). The TG curves for the Aerogel 120 and Aerogel 400 are shown

in Fig. 2. A decrease in the TG curve as temperature increases represents a weight loss of

the sample as it is heated. The TG curve shows that the Aerogel 120 sample had a weight

loss at about 150° C. This confirms the presence of bonded water in the Aerogel 120. The

TG curve for Aerogel 400 was nearly constant which shows there was little to no bonded

water in the sample. The presence of water in the Aerogel 120 but not in the Aerogel 400

was expected because Plantier et al. observed a similar phenomena in sol-gel synthesized

Fe2O3 [27].

7

a.

b.

Figure 1. X-ray diffraction analysis of a. Aerogel 120 and b. Aerogel 400 (Courtesy LLNL)

8

Figure 2. Thermal gravimetric plots of Aerogel 120 and Aerogel 400

9

Figure 3 shows scanning electron microscope (SEM) micrographs of the Aerogel

120 and Aerogel 400 oxidizers. As seen in Fig. 3b, the Aerogel 400 particles sintered

together upon heating to form large agglomerates.

a. b.

Figure 3. SEM micrographs of a. Aerogel 120 and b. Aerogel 400 (Courtesy LLNL)

2.1.2 Commercial Tungsten Trioxide Properties

The commercial nano-scale WO3 was donated by Nanomat (North Huntingdon,

PA) and will be referred to as nm WO3. The commercial micron-scale WO3 was

purchased from Acros Organics (Morris Plains, NJ) and will be referred to as µm WO3.

The surface area measurement for the nm WO3 was determined using a gas adsorption

analyzer and BET theory. The mass percent WO3 content was assumed to be 100 % for

both of these materials. The surface area measurement for the µm WO3 was estimated by

calculating the surface area of representative single particles viewed from SEM

micrographs. The surface areas and purities of the WO3 oxidizers can be found in Table

1.

10

2.1.3 Thermite Synthesis

The mixtures were prepared by combining the powders in pre-specified

proportions in a suspension solution of isopropanol. The stoichiometric fuel/oxidizer

mass ratio for the Al+WO3 reaction is 0.2327 (Eq. (2.1)).

3232 OAlWWOAl +!+ (2.1)

The equivalence ratio (Φ) was calculated according to Eq. (2.2).

!

" =

MF

MOx

# $ %

& ' ( act

MF

MOx

# $ %

& ' ( st

(2.22)

The subscripts F and Ox represent fuel and oxidizer respectively. The M represents mass

and act is actual while st is stoichiometric. For the fuel, only the active Al content was

considered. For the oxidizer, only the pure WO3 was considered. An equivalence ratio of

1.4 was shown to be optimum (slightly fuel rich).

The powders and solution were mechanically mixed using an ultrasonification

process. This process allowed for a more homogeneous mixture by breaking up large

agglomerates. The mixture and solution were heated at a low temperature, approximately

70° C, in a glass dish to evaporate the isopropanol. The composite dried powder was

recovered from the dish and stored in a vial until tested in open channel burn experiments

or immediately cold-pressed using a hydraulic press and uni-axial die if the thermite was

to be tested in pellet form. Care was taken to perform the combustion experiment for the

particular composition within 24 hours of their creation to minimize any aging effects of

the Al from oxidation in air.

11

2.1.4 Pellet Synthesis

Using a standard axial press, the dried powder mixtures of Φ=1.4 were pressed

into a solid cylindrical pellet with a constant diameter of 6.5 mm and varying length,

dependent on the pellet density desired, on the order of 3 mm. The theoretical maximum

density (TMD) was calculated as a weighted average of the pure solid densities of the

three reactants (Al, WO3, and Al2O3). The pellets were pressed to various percentages of

the TMD ranging from 20 to 50 % for each mixture. All of the pellets had a mass of

approximately 250 mg.

2.3 Open Channel Burn Testing Apparatus

Figure 4 shows the open burn channel used to determine the loose powder

combustion wave speeds. A 10 x 0.3175 x 0.3175 cm square channel cut into an acrylic

block was filled with 250 mg of thermite to be tested. The top of the channel was open to

ambient air. The composites were ignited using a spark ignition system of which the

terminals for the spark gap can be seen in Fig. 4. A Phantom IV high-speed camera from

Vision Research (Wayne, NJ) captured images of the reacting thermite perpendicular to

the open burn channel and at frames rates of 32,000 frames per second (fps) with a

resolution of 128 x 32 pixels. The camera interfaced with a computer that used Vision

Research software to post process the photographic data. By establishing a reference

length, the software determines speed based on a distance between sequential time

frames.

12

Figure 4. Photograph of the open channel burn testing apparatus

2.4 Pellet Combustion Rate Testing Apparatus

Figure 5 shows a schematic of the apparatus used to ignite the pellets and measure

the combustion wave speed. The energetic pellet is housed in a reaction chamber with

various ports for the laser beam and viewing not shown in the schematic. The flat end of

the cylindrical pellet was carefully aligned with the beam path of a 50-W CO2 laser (10.6

µm wavelength). The beam delivery was controlled by a pushbutton operated, electro-

mechanical shutter. The laser output was measured with a thermal power meter that

adjusts in and out of the beam path. The laser output was kept at 50 W continuous wave.

A triggering device was used to simultaneously open the shutter and start recording from

the high-speed camera. The same high-speed camera and image analysis software used in

the open channel burn experiments was used to measure the pellet burn speeds. Because

13

the trigger is a human controlled pushbutton without any electronic timing device and

because the pellets burn very quickly, it is probable that some laser radiation is imparted

onto the pellet after ignition. By using the enthalpy of reaction, the laser power and an

average pellet burn time, it was calculated that no more than 0.35% of the total pellet

energy released could come from laser exposure. This value assumes a worst-case

scenario where the shutter remained open for the entire pellet burn. Further information

regarding the testing apparatus can be found elsewhere [13].

14

Figure 5. Schematic of the laser ignition and flame propagation diagnostic system

15

CHAPTER III

THEORY

3.1 Motivation to Research Aerogel Composites

Monomolecular energetic materials are only able to achieve approximately half of

the energy density of energetic composites due to chemical stability issues and because

modern synthesis procedures limit the oxidizer-fuel balance and the physical density

obtainable [19]. Micron-scale energetic composites are only able to achieve reaction rates

on the order of meters per second compared to kilometers per second for monomolecular

energetic materials. It has been shown that for a given bulk density, decreasing the

reactant particle sizes increases the combustion rate [11, 30, 35]. Synthesizing the

oxidizer using sol-gel chemistry, to form an aerogel, is one way to decrease reactant

particle sizes from the micron-scale to the nano-scale.

Tillotson et al. showed that aerogel composites are very sensitive to thermal

ignition [18]. They theorized that because of the very low thermal conductivity of an

aerogel based composite, there is the rapid formation of “hot spots” or localized high

temperature regions that ignite the composite. The high sensitivity of aerogel composites

would be an advantage in terms of performance but possibly a disadvantage in terms of

safety.

16

3.2 Aerogel Synthesis

Gash et al. describe an aerogel as a highly porous lightweight solid [17]. They

explain that aerogels are created using sol-gel chemistry. Furthermore, they describe the

sol-gel chemistry process where hydrolysis and condensation of molecular chemical

precursors, in solution, is used to produce nanometer-sized primary particles called sols

[36] which are linked through further condensation to form a three dimensional solid

network, or a gel, with the solvent liquid in its pores. An aerogel is formed when the

solvent is removed by supercritical extraction [17]. This leaves a three dimensional

porous structure made up of nano-scale particles. Tungsten trioxide aerogels are formed

by the dissolution of the metal salt, WCl6 in this case [21], in a solvent followed by the

addition of an epoxide which induces gel formation in a timely manner [17]. A more in-

depth review of sol-gel chemistry and aerogels is found elsewhere [17-21].

One problem with this method of aerogel synthesis was that hydroxyl groups or

bonded water was found in the aerogel. Water in the form of either hydration of the

precursor salt or as the solvent was found to be a necessary component in the synthesis of

the aerogels [17]. Because the water is so detrimental to combustion performance,

calcination of the aerogel was used to remove the hydroxyl groups. Fourier Transform

Infrared Spectroscopy (FTIR) confirmed that a large percentage of the hydroxyl groups

were removed when the aerogel was calcined to 200° C under a dynamic vacuum [17].

The downside to calcining the aerogels is that the surface areas and pore volumes were

found to decrease while the pore diameters increase with increasing drying temperatures

[17]. Because the hydroxyl groups are surface bound, heating induces condensation of the

17

groups which pulls together the small particles that make up the microstructure of the gel

[17].

3.3 Advantages of Aerogels

Aerogels composites have some distinct advantages over micron-scale composites

or even nano-scale composites not based on aerogels. One of which is the possibility to

precisely control the composition, morphology and reactivity of the composite at the

nanometer scale [17]. Sol-gel chemistry can produce material with variable and uniform

densities [18].

A major advantage of aerogel composites over both micron-scale and other types

of nano-scale composites is its level of homogenous mixing. The aerogels feature

intimate mixing of components at the nano-meter scale [18]. Tillotson et al. [18]

explained that in conventional mixing, domains of either fuel or oxidizer can exist which

limit the mass transport which decreases the efficiency of the reaction. Sol-gel derived

nanocomposites such as aerogels should be more uniformly mixed thereby increasing the

reaction efficiency.

A long-term goal is to create aerogel based nanocomposites where the aluminum

is integrated into the oxidizer during the aerogel creation. This would produce a lattice

structure in which the fuel (Al) resides within the pores of the solid oxidizer (WO3)

matrix [18]. The nanocomposites featured in this study were not constructed this way, but

fundamental knowledge gained here will allow the creation of an integrated

nanocomposite aerogel as described above.

18

Finally, the production of aerogels can allow for the materials to be synthesized in

the final desired shape and density. Gash et al. [17] created monolithic materials in a

variety of shapes and sizes. They recognized that this might eventually eliminate the need

for the time-consuming, expensive and potentially dangerous pressing and machining of

energetic solids.

19

CHAPTER IV

RESULTS

Because the same nano-aluminum was mixed with all of the oxidizers studied, the

thermite mixes will be referred to by their representative oxidizer (e.g. an Al+Aerogel

120 sample will simply be called Aerogel 120).

4.1 Open-Channel Combustion Wave Speed

Figure 6 shows the open-channel combustion wave speed plotted as a function of

thermite equivalence ratio for the Aerogel 120 and Aerogel 400 samples. Maximum

speeds of 490 and 310 m/s were seen at an equivalence ratio of 1.4 for the Aerogel 120

and Aerogel 400 samples respectively.

Figure 6. Plot of open-channel combustion wave speed as a function of equivalence ratio for Aerogel 120 and Aerogel 400

20

4.2 Pellet Combustion Wave Speed

Figure 7 shows the combustion wave speed as a function of pellet density for all

four thermite mixtures investigated. The error bars do not represent experiment

repeatability but rather measurement errors that occur while using the Phantom imaging

software. Due to the relatively fast burn speeds of the pellets and the limited data

acquisition rate of the high-speed camera, 32,000 frames per second allowed roughly 15

frames of data for each experiment. Furthermore, the resolution of each frame was 128 x

32 pixels which made it difficult to visually observe the combustion wave. Therefore,

five speed measurements were made for each mixture using the Vision Research imaging

software. The speed was measured as an average speed between “first light” and the

completion of the self-propagating wave. The standard deviation was plotted as error bars

for each experiment. As the speed became faster, as in the Aerogel 400 experiments, the

measurement error increased. This was because fewer frames were captured for

successively higher speeds.

21

Figure 7. Plot of combustion wave speed as a function of pellet density

As seen in Fig. 7, the Aerogel 400 exhibited the fastest combustion wave speeds

for all densities. The combustion wave speed slightly increased with an increase in pellet

density from 20-30% TMD for the Aerogel 400 samples while staying constant or

slightly decreasing for the other samples. The increased speeds of the Aerogel 400 over

the Aerogel 120 is the opposite result of that seen in the open burn experiments. As a

loose powder, the Aerogel 120 produced consistently higher speeds by 58% over the

Aerogel 400 at an optimum stoichiometric ratio of 1.4. For pellets, the Aerogel 400

produced consistently higher speeds by at least 376% over the Aerogel 120.

4.3 Pellet Ignition Sensitivity

One important factor to consider before analyzing trends in the pellet combustion

wave speed data is the initial condition of the pellet before combustion. Figure 8 shows

22

the ignition time as a function of pellet density for all of the same data points seen in Fig.

7. The ignition time dramatically increased for all samples above a theoretical maximum

density of 35%. Above this critical density, the pellets are highly reflective. No attempt

was made to reduce reflectivity (i.e. by coating the front face to enhance absorption).

Therefore, only combustion behaviors below 35% TMD were further analyzed.

Figure 8. Plot of ignition delay time as a function of pellet density

23

CHAPER V

DISCUSSION

5.1 Open-Channel Combustion Wave Speed

5.1.1 Optimum Equivalence Ratio

The slightly fuel rich optimum equivalence ratio shown in Fig. 6 is similar to

results for nano-Al+MoO3 [13, 29] and nano-Al+Fe2O3 combustion [27]. Pantoya et al.

[29] attributed higher combustion wave speeds for nano-particles at slightly fuel rich

equivalence ratios to possible Al oxidation with the ambient air that is not taken into

account in the stoichiometric calculations. A slightly fuel rich composition may enhance

the thermal transport properties of the mixture because the Al has a higher thermal

diffusivity and conductivity than WO3 or air. Plantier et al. [27] attributed the slightly

fuel rich optimum equivalence ratios to enhanced thermal properties; namely, increased

thermal conductivity. They concluded that for highly compacted composites, a higher

thermal conductivity will enhance heat transfer in the diffusive dominant reaction thereby

increasing combustion wave speeds.

The Aerogel 120 and Aerogel 400 samples in this study likely experienced a

similar phenomena as in the nano-scale MoO3 and Fe2O3 studies [13, 29 and 27]. Namely

the thermal transport properties are enhanced for a slightly fuel rich thermite mixture.

Beyond this critical mixture ratio, too little solid oxidizer is available to further increase

speed. Because nano-scale thermites experience diffusion controlled reactions, the

24

enhanced heat transfer contributes towards pre-heating the reactants which increases the

combustion wave speed.

5.1.2 Optimum Oxidizer Composition

The Aerogel 120 and Aerogel 400 both exhibit higher combustion wave speeds

than traditional micron-scale Al+WO3 composites. Maximum combustion wave speeds

of 490 and 310 were seen for the Aerogel 120 and Aerogel 400 samples respectively

while similar open burn experiments using µm WO3 produced a maximum combustion

wave speed on the order of 40 m/s.

The aerogel derived oxidizers react faster because the smaller particle diameters

of the aerogels decrease the diffusion distances thereby increasing the combustion wave

speed. Also, the smaller particle sizes lead to increased surface area and increased contact

between the Al fuel and the WO3 oxidizer which also promotes faster combustion wave

speeds.

Bockmon et al. found that strong convective mechanisms likely control the

combustion wave speed for loose powders of nano-Al+MoO3 thermites burning in

confined geometries [14]. When convective processes are significant, a pressure gradient

propels the combustion wave forward. This was found in nano-Al+MoO3 thermites

because the molybdenum trioxide sublimes at a lower temperature than the adiabatic

flame temperature for the reaction. Therefore, the MoO3 expels gas during combustion.

Tungsten trioxide, like MoO3, sublimes below the adiabatic flame temperature of the

reaction (sublimation temperature of 3680 K [1], reaction adiabatic flame temperature. of

25

4280 K [1]). This causes WO3 to also expel gas during the combustion reaction. Balakir

et al. concluded that the Al+WO3 reaction proceeds primarily in the oxide gas phase [30].

The gaseous intermediate products cause the reacting particles to be violently propelled

in all directions. Some of these particles are ejected in the direction of the unburned

thermite in the channel. This ignites the unburned thermite much sooner than if the

reaction was not violent and only conductively driven. Therefore, the Al+WO3 reaction is

diffusion controlled and convection dominant in a loose powder form.

The Aerogel 120 has the highest combustion wave speed because it has the

smallest particle diameter therefore the smallest diffusion distance. Even though the

Aerogel 400 has fewer impurities than the Aerogel 120, it is limited by a larger particle

diameter due to sintering during calcination. Because convection is dominant in the loose

powder thermite, the small particle diameter of Aerogel 120 outweighs the benefit of

higher purity with larger particle diameters found in the Aerogel 400. The faster the

reactants diffuse, the faster gas is produced. In summary, the Aerogel 120 has faster

combustion wave speeds than the Aerogel 400 due to a faster reaction rate which causes a

greater rate of ejected particles (convection) that advance the combustion wave.

It should be noted that the products of the Al+WO3 reaction are solids. The

tungsten is only in a gaseous form during combustion but the products (i.e. W and Al2O3)

condense back into solids as they cool. The reaction is classified as gasless combustion.

The highly energetic nature of the reaction carried nearly all of the residue away such that

products were not available for further analysis.

26

5.2 Pellet Combustion Wave Speed and Sensitivity

5.2.1 Combustion Wave Speed as a Function of Bulk Density

The combustion wave speed increases as pellet density increases for Aerogel 400

as seen in Fig. 7. It also appears that the Aerogel 120, nm WO3, and µm WO3 samples

speeds either stay constant or slightly decrease as pellet density increases. Combustion

wave speed data is valid up to pellet densities of 35% TMD. After which, the initial

conditions are not constant due to differing heating rates from laser ignition as seen in

Fig. 8.

As the pellets were pressed to higher densities, the surface finish became much

more reflective. Figure 9 shows the surface finish for an Aerogel 400 pellet at 49.6 %

TMD. The increasing sheen found at higher pellet densities could reflect some of the

laser radiation. Although some radiation would be reflected, the remaining radiation

would continue to heat the pellet at a slower rate. The goal is to only heat a thin planar

surface of the pellet but the long laser exposure times cause the entire pellet to pre-heat.

This slower heating rate effect can increase the combustion wave speed and at the very

least causes the initial conditions to be different for pellets with greatly varying ignition

delay times.

27

Figure 9. Photograph of an Aerogel 400 pellet at 49.6% TMD

The remaining combustion wave speed data below 35% TMD shows an apparent

trend of increasing wave speed as density increases for Aerogel 400 and no definitive

trends for Aerogel 120, µm WO3 and nm WO3 as seen in Fig. 7. The increasing trend in

Aerogel 400 cannot be confirmed because it only involves two data points without

experimental repeatability. If the trend is valid, it can be explained by the fact that pellets

of higher densities have higher thermal conductivity. As explained earlier, higher thermal

conductivity leads to higher combustion wave speeds.

Based on previous unpublished research, it appears that the pellet pressing process

does not affect parameters such as particle size. More densely pressed pellets have fewer

voids as expected but the particles are not sintered together.

5.2.3 Combustion Wave Speed as a Function of Composition

Because the ignition delay times were similar for the four different sample

compositions at a given pellet density, the combustion wave speed as a function of

28

thermite composition can be studied. The combustion wave speeds for the Aerogel 400

were dramatically higher than the Aerogel 120, nm WO3, and µm WO3 at all pellet

densities. Strictly focusing on the aerogels, this is the opposite trend observed for the

open channel burns. To explain the reverse trend, the dominant mechanism for the

reaction must first be determined. To quantify whether each reaction was conduction or

convection dominant, a stability criterion known as the Andreev number was used.

The Andreev number is a non-dimensional parameter similar to the Peclet number

but tailored to reacting flow through porous media at constant pressure as seen in these

thermite pellets [32]. Equation 5.1 shows the calculation used to find the Andreev

number (An).

!

An ="bUdhcp

kg= const. (5.1)

In this equation, ρb is the bulk density of the composite, U is the combustion wave speed,

dh is the hydraulic pore diameter, cp is the specific heat of the composite and kg is the

thermal conductivity of the gas. The hydraulic pore diameter was estimated using

Eq. (5.2) [33].

!

dh

=4"

Ao(1#")

(5.2)

In this equation, ε is the void volume, Ao is the specific surface area, calculated as the

solid surface area divided by the solid volume (As/Vs), and (1-ε) is the solid volume

fraction. The Andreev number is relative so it is useful for comparing convection and

conduction dominance between two samples. Larger Andreev numbers indicate

convection dominance while smaller numbers indicate conduction dominance.

29

Because the pellets corresponding to different composites were not all produced

at the exact same density, the Andreev number was found for the narrowest range of

densities from 31.7 to 34% TMD. The Andreev number corresponding to the previous

range is plotted as a function of oxidizer composition in Fig. 10 with the corresponding

values used in calculation shown in Table 2.

Figure 10. Andreev number as a function of pellet oxidizer composition at a TMD range of 31.7-34%

Table 2. Calculated Andreev numbers and physical parameters for Al+WO3 composites at 31.7-34% TMD

Avg. Bulk

Density

(kg/m^3)

Combustion

Velocity

(m/s)

Surface Area

of WO3

(m^2/g)

Total Surface

area in Pellet

(m^2)

Total Volume

in Pellet

(m^3)

Total Heat

Capacity

Cp (J/kg K)

Potential

Volume

(m^3)

Void

Volume

(m^3)

Hydraulic

Diameter, dh

(m)

Andreev

Number

2012.31 0.1018 0.026 1.242709043 1.2648E-07 472.6271 4.244E-08 8.404E-08 3.4216E-14 2.418E-08

nm WO3 1923.2 2.1988 15.7 3.863403864 1.2418E-07 471.49765 3.992E-08 8.425E-08 1.0833E-14 1.577E-07

Aerogel 400 2015.28 7.1642 28.2 5.923260903 1.1627E-07 468.05365 3.953E-08 7.674E-08 6.0255E-15 2.972E-07

Aerogel 120 1850 2.309 77.1 14.02803719 1.2319E-07 462.18209 3.917E-08 8.402E-08 2.9514E-15 4.253E-08

The Andreev analysis showed that the combustion reactions were strongly

conduction dominant for all of the oxidizers at all densities. Moreover, any hot particles

30

ejected by gaseous intermediate products were propelled away from the pellet due to

planar burning as confirmed by high-speed photography. Similar Andreev analysis

performed on the Aerogel 120 and Aerogel 400 powders found is shown in Table 3. The

Aerogel 120 is nearly 2600 times as convective in powder configuration as it is in pellet

configuration. Similarly, the Aerogel 400 is nearly 600 times as convective in powder

configuration as it is in pellet configuration.

Table 3 Normalized Andreev number for Aerogel 120 and Aerogel 400 in pellet and powder configuration

Thermite

Normalized

Andreev Number

Aerogel 120 (Pellet) 1

Aerogel 400 (Pellet) 7

Aerogel 120 (Powder) 2583

Aerogel 400 (Powder) 3965

It should be noted that the Andreev analysis only focuses on physical parameters

such as particle size, surface area, density, etc., and not on chemical composition. This

causes the Andreev analysis performed above to not fully account for the phenomena

taking place in Al+WO3 reactions; namely the gaseous intermediate products created

from WO3 sublimation. Nonetheless, it is still a useful tool to provide insight into the

thermal transport mechanisms occurring.

Focusing on the aerogel based pellets specifically, it now becomes clear why the

Aerogel 400 outperformed the Aerogel 120. Because conduction is the dominant

mechanism for the pellet combustion, the bonded water impurities found in the Aerogel

120 hinder the reaction. The bonded water acts as a heat sink. No large-scale effects such

as convection are present in the pellet combustion to outweigh the impurity effects as was

31

seen in the open channel burn experiments. Removing impurities to improve combustion

performance was the initial reasoning for calcining the aerogel.

Equation 5.3 was used to model the combustion occurring in the Al+WO3

reactions.

!

1.4Al + 0.0706Al2O3

+1/2WO3

+ x H2O"1/2W + 0.4Al +

0.5706Al2O3

+ x H2O

(5.3)

The subscript x represents the appropriate moles of water for the oxidizer considered.

This equation accounts for the alumina inherent in nano-Al as well as the bonded water

impurities found in the aerogels. It was assumed that all of the impurities in the aerogels

are bonded water.

Flame temperatures of 2530 K and 2847 K were calculated for the Aerogel 120

and Aerogel 400 samples respectively using Eq. 5.3. A flame temperature of 3078 K was

calculated for an ideal, water-free reaction. The effect of water content on adiabatic flame

temperatures is plotted in Fig. 11 for a range or water content of 0-25% oxidizer mass.

This shows that the adiabatic flame temperature decreases rapidly as the water content is

increased.

32

Figure 11 Calculated adiabatic flame temperature as a function of mass percent water in the oxidizer

To estimate the effect adiabatic flame temperature and therefore bonded water

content would have on the pellet combustion wave speed, a model derived by Armstrong

[42] was used and is shown in Eq. 5.4.

!

u2

=12A* exp("EA /RTf )(RTf /EA )#

2

d2((Tf "T0) /Tf )

(5.4)

In this equation, u is the combustion wave speed, A* is a constant based on the inverse

Lewis number, EA is the activation energy, R is the gas constant, Tf is the adiabatic flame

temperature, α is the thermal diffusivity, d is the particle diameter and T0 is the initial

temperature. Because the adiabatic flame temperature was the only parameter to be

varied, Eq. 5.4 can be reduced to the proportion seen in Eq. 5.5.

33

!

u2~Tf exp("EA /RTf )

(Tf "T0Tf

#

$ % %

&

' ( (

(5.5)

Using this proportion, the adiabatic flame temperatures calculated previously and an

estimated activation energy of 250 kJ/mol [43], the effect of bonded water content on

combustion wave speed was approximated as seen in Fig. 12. This shows that as the

water content in the oxidizer is increased, there is a dramatic drop in combustion wave

speed. This analysis predicted that the just a 5% mass water content in the oxidizer would

result in a 41% decrease in the combustion wave speed compared with no water content.

This verifies why the Aerogel 400 with low water content, greatly outperformed the

Aerogel 120 with higher water content, in the pellet configuration.

Figure 12 Estimated drop in combustion wave speed as a function of mass percent water in the oxidizer

34

The reason the Aerogel 400 had a higher combustion wave speed than the µm

WO3 and nm WO3 can be clearly seen in Fig. 13. By definition, diffusion distance again

plays the dominant role for oxidizer performance. The Aerogel 400 oxidizer had the

largest surface area (i.e. smallest particle diameter) which causes it to have the highest

combustion wave speeds.

Figure 13. Combustion wave speed as a function of oxidizer surface area and pellet density

One aspect that has not been analyzed is radiative transport mechanisms. Plantier

et al. observed that the flame is highly luminescent in nano-scale Al+Fe2O3 thermites

[27]. Because of the high luminescence, they claim that radiation transport may be a

significant factor. These nano-scale Al+WO3 thermites also exhibit extremely

luminescent combustion properties. To achieve high-speed photographs that are not

35

overexposed, a NDx2 filter, NDx4 filter or a series of both are commonly needed at a f-

stop value of 32 even with 1/32000 second exposure times.

Yang et al. measured absorption coefficients for nano and micron-scale aluminum

particles in the UV and VIS spectrum [31]. They found that nano-scale Al particles have

significantly higher absorption coefficients than micron-scale particles. To the author’s

knowledge, similar experiments have not been performed on nano and micron-scale WO3

particles.

Pantoya and Granier also recognized that absorption properties of nano-scale

particles might be an important contributing factor to the increased ignition sensitivity of

Al+MoO3 nanocomposites [29]. They stated that the high flame temperatures coupled

with the solid products would provide an intense source for radiation transport. They

further explained that because the nano-scale Al particles are smaller than the wavelength

of light in the thermal radiation wavelength regime, dependent scattering may increase

the effective penetration depth of radiative transport. They claim that this increased

penetration depth may cause a larger pre-heat zone to be formed preceding the reaction

zone during pellet combustion. They concluded that this enhanced pre-heating of

unreacted material would affect the combustion wave speed but not necessarily the

ignition sensitivity.

Alternatively, an initial study of Al+MoO3 nanothermites conducted by Son et al.

concluded that radiation is of little importance and that convective heat transfer is the

dominant mechanism during combustion[34]. Therefore, radiative transport mechanisms

may or may not play an important role in Al+WO3 thermites.

36

At the very least, radiation transport mechanisms may be important in

understanding ignition sensitivity of these pellets because laser radiation was used to

ignite the thermites. Different oxidizer and thermite absorption coefficients could affect

ignition characteristics such as pre-heating and hot spot formation. Further research is

needed before a conclusion can be reached about radiation transports role in

nanocomposite thermites.

5.3 Aerogel Combustion Dominant Mechanism Transition

The most interesting and important finding from this study is the observed

transition from convection based heat transfer to conduction based heat transfer as

density increases for the aerogel samples. Figure 14 is a semi-logarithmic plot of the

combustion wave speed of the Aerogel 120 and Aerogel 400 as a function of theoretical

maximum density. It was formed by combining the open burn data found in Fig. 6 and

the pellet burn data for the aerogels found in Fig. 7.

37

Figure 14 Semi-Log plot of combustion wave speed as a function of TMD for Aerogel 120 and Aerogel 400

The Aerogel 120 produced a higher combustion wave speed than the Aerogel 400

in the low-density, open channel configuration. Conversely, in high-density, pressed

pellet configuration, the Aerogel 400 consistently produced higher combustion wave

speeds than the Aerogel 120. In the open channel configuration, convection heat transfer

is the dominant mechanism as confirmed by Andreev analysis as seen in Table 3. Low

normalized Andreev numbers correspond to conduction while high numbers correspond

to convection. When convection is the dominant mechanism, the most important

parameter affecting the combustion wave speed is the particle diameter. Aerogel 120 is

composed of smaller particles than Aerogel 400, therefore the Aerogel 120 produces

faster combustion wave speeds.

As density is increased, the air pockets found in the material are removed which

lowers the convective effects. Andreev analysis confirms that conduction is the dominant

heat transfer mechanism for the thermites in pellet form (Table 3). The bonded water

38

found in Aerogel 120 impedes the energy transport through conductive mechanisms. For

this reason, the Aerogel 400, which does not contain bonded water impurities,

outperforms the Aerogel 120 in pellet form.

The transition from convection to conduction as density is increased causes the

important material parameters to be different depending on the material density. In

essence, at the low densities found in powders, convection is dominant and the particle

diameter is the most important parameter. At high densities found in pellets, the purity of

the oxidizer outweighs the benefits of smaller particle diameter because conduction is the

dominant mechanism.

Unfortunately, Fig. 14 merely suggests the previous trends. The original set of

experiments did not test the aerogels at densities other than the natural density produced

through mixing. It would be desirable to find the combustion wave speed for several

densities slightly above the natural powder density of each aerogel Also, repeatability

was not conducted due to the limited amount of Aerogel 120 and Aerogel 400 available

and the uncertainty in the total number of experiments that would eventually be

conducted. At the end of this study, when it was apparent that more data would be

beneficial for low-density combustion data, new experiments were conducted. Even

though new nano-Al was used, the aerogels had aged over a period of 22 months which

greatly decreased the performance of the Aerogel 120 and Aerogel 400 (see Chapter VII).

Therefore, new data was unable to be obtained for low-density combustion of Aerogel

120 and Aerogel 400. This causes the transition phenomenon to be suggested but not

proven.

39

CHAPTER VI

CONCLUSIONS

An optimum equivalence ratio of 1.4 was found for thermites composed of nano-

Al and Aerogel 120 or Aerogel 400 oxidizer. The slightly fuel rich composition is

explained by the increased thermal transport properties found when more nano-Al is

used. This pre-heats the reactants and increases the combustion wave speed. Because

thermite reactions are diffusion limited, both the Aerogel 120 and Aerogel 400 exhibit

higher combustion wave speeds than µm WO3 in an open-channel burning configuration.

Convection was determined to be the dominant mechanism for the advance of the

combustion wave in the loose powder open-channel burn experiments. This was

explained by the fact that WO3 sublimes below the adiabatic flame temperature of the

combustion reaction. This causes gas to be expelled during the reaction creating

convection in the form of the violent ejection of burning particulate in all directions.

Andreev analysis further suggests convection dominance. This convection causes the

Aerogel 120 to burn faster than the Aerogel 400 even though the Aerogel 400 has fewer

impurities. The large-scale effect of convection combined with the smaller particle

diameter of Aerogel 120 outweighs the purity benefits of Aerogel 400 in loose powder

form.

The increase in Aerogel 400 combustion wave speed observed as density

increases was attributed to the associated increase in thermal conductivity. Aerogel 400

composites exhibited faster combustion wave speeds at all pellet densities than the

40

Aerogel 120, nm WO3 or µm WO3. Andreev analysis showed that conduction is the

dominant mechanism for combustion in pellet form. This explains why the Aerogel 400

outperformed the Aerogel 120 which is the opposite trend seen in loose powder form.

Bonded water impurities act as a heat sink and impede thermal transport through

conduction. Therefore, even though the Aerogel 400 had larger a particle diameter than

Aerogel 120, its lack of bonded water causes it to perform better. The Aerogel 400

outperformed the nm WO3 and µm WO3 because of its smaller particle diameter and

therefore reduced diffusion distance over the other oxidizers. It was noted that radiation

effects might play a role in the ignition sensitivity and combustion wave speed of these

materials. To date, little research has focused on the radiation effects.

41

CHAPTER VII

ALUMINUM AND TUNGSTEN TRIOXIDE AGING EFFECTS

A preliminary study was conducted to quantify aging effects (i.e. decreased

combustion wave speeds) that had been observed qualitatively. It was know that

aluminum oxidizes in air over time but the length of time and associated environmental

factors were unknown or not quantified. The goal was to obtain a length of time for

which a mixed thermite could be stored before its performance was significantly reduced.

7.1 Aluminum and Tungsten Trioxide Aging Study

7.1.1 Introduction

One of the unique properties of aluminum is that its surface will naturally oxidize

with air to form a layer of alumina (Al2O3). There are a few studies that investigate the

oxidation of nanosized aluminum powders [12 and 37]. Aumann et al. [12] showed that

nano-Al powder consists of an aluminum core with a shell of amorphous alumina. They

said that the shell of alumina is what keeps the mixed thermite from reacting until an

external energy source stimulates the reaction. They exposed the nano-Al to a controlled

amount of oxygen gas at various temperatures for a short time. This showed that the

oxide growth was temperature dependent for time periods on the scale of hours. They

observed no growth in the alumina shell for temperatures less than 350°C. Pesiri et al.

[37] also showed that the oxide shell thickness can be controlled by carefully introducing

42

oxygen gas to the aluminum nanopowder during production. They showed that the nano-

Al powder would still react with the oxygen in the atmosphere even with the protective

alumina layer. This reaction was claimed to increase the thickness of the alumina shell

for 3 months of continuous exposure at which point it stabilizes. Finally, they showed

that the nano-Al powder is very sensitive to any water vapor in the air and will absorb 5%

wt. water from the atmosphere within minutes. Therefore, they recommend handling and

storing the nano-Al powder under dry Nitrogen gas.

Because of the nano-Al’s sensitivity to air and water exposure, aging studies have

been conducted to quantify the effects of exposure to the atmosphere [38-40]. Walter et

al. [38] not only studied the effect of aging on nano-Al, but also on the oxidizers nano-

MoO3 and nano-CuO. They showed that the surface area of nano-MoO3 decreased

twofold from 10-12 days exposure to light and air. The nano-CuO showed only a slight

decrease in particle size with exposure to light and air. The nano-Al metal content

decreased by 50% wt. from two years of exposure to air. This showed that for a thermite

of Al and MoO3, the aging effects of the MoO3 would have a greater impact sooner than

the aging effects of the aluminum. Bulian et al. [39] and Puszynski [40] investigated the

aging of nano-Al powder exposed to air with high moisture content. Bulian not only

looked at the physical properties of the aluminum, but also the effect on the open burn

rate of the exposed aluminum mixed with CuO. Bulian showed very large speed

decreases when the aluminum was exposed to 97% relative humidity air for less than 3

days. After 3 days, the thermite wouldn’t react at all. The major decrease in speed was

even observed when the decreasing reactive aluminum content of the exposed nano-Al

43

was accounted for by increasing the nano-Al mixed with CuO. Bulian attributed the

decrease in speed to two factors: the increase in the Al2O3 thickness and the increase in

the water content in the aluminum. Puszynski further showed that a large reduction in the

percent reactive aluminum content was seen in nano-Al exposed to air with as low as

43% R.H. The time for aluminum content reduction of 50% or greater was on the order of

5-40 days. Finally, both Bulian and Puszynski showed that the effect of water vapor on

nano-Al powder could be stopped by coating the aluminum with oleic acid or Dow

Corning Z-6124 silane. Additionally, the coatings increased the open burn speeds when

the coating weight percent was five or less due to the coating increasing the mixing of the

aluminum and the oxidizer.

Walter et al. [38] only studied the effect of aging on the nano-Al, nano-MoO3 and

nano-CuO separately. The effect of aging on an already mixed thermite was not

investigated. Furthermore, there are other relevant oxidizers, such as WO3, which need to

be investigated. Bulian et al. [39] and Puszynski [40] also chose to study the aging of the

nano-Al and not the thermite itself. Bulian only investigated the effect of very high

relative humidity air exposure and Puszynski didn’t conduct burn speed measurements on

the thermites containing aluminum exposed to lower relative humidity air.

Often times, in a lab environment, it is impractical to test a thermite immediately

after it has been mixed. The thermites are stored in non-airtight plastic or glass

containers. Because of the unknown effect of the exposure of the thermite to the

atmosphere, there is no fixed rule for how long the thermite is good after it has been

mixed. This study will focus specifically on the effect of atmospheric air exposure on

44

thermites consisting of nano-Al mixed with nano-WO3. Separate mixes will be exposed

to the air in an indoor lab environment and sealed in a container for a period of four days.

Combustion wave speed measurements of the loose powders will be made from samples

of the mixes at various times of exposure by using a high-speed camera. This will extend

previous studies by being the first study, to the author’s knowledge, to investigate the

effect of aging on the mixed thermite itself. Furthermore, it will investigate mixes that

include the commonly used oxidizer WO3. Finally, it will quantify the change in

performance of thermites exposed to atmospheric conditions common to a lab

environment and explain the relevant phenomena using previously published studies.

7.1.2 Experimental Setup

The aluminum powder was obtained from Nanotechnologies (Austin, TX) and has

a 120 nm average particle diameter. The average particle diameter is calculated by the

manufacturer using a gas adsorption analyzer using Brauner, Emmett, and Teller (BET)

theory to measure the surface area. The diameter is calculated by assuming the particles

are spherical. These Al particles are passivated with an alumina (Al2O3) shell that is

roughly 4 nm thick and encapsulates the core Al particle. Based on thermal analysis

(Base Hydrolysis Method), the manufacturer determined that this powder has an active

Al content of 81%. Active Al is defined as the portion of powder not in the form of

Al2O3. The previous material specifications were measured on June 25, 2004 for the

particular batch of Al used in this study. Also, the Al used was originally opened on July

14, 2004 after being received from the manufacturer packed in argon gas. After being

45

opened, it was sealed in an airtight container until mixed with the oxidizer on October 18,

2004.

The tungsten trioxide (WO3) powder was purchased from Aldrich (St. Louis, MO)

and has an average particle diameter of 49.6 nm. The manufacturer supplied the particle

diameter data using an unspecified method. Also, because no purity data was provided, it

was assumed that the WO3 was 100% pure. This assumption was valid because the WO3,

unlike the Al, doesn’t have a passivation shell or any other characteristic which would

cause a loss of purity. The WO3 used in this study was originally opened from the

manufacturer on September 9, 2004 and again stored in an airtight container until mixed

with the fuel on October 18, 2004.

It was calculated that approximately 6 grams of Al+WO3 mixture was needed.

Due to safety considerations, three separate but chemically equivalent batches of 2 grams

each were prepared as described below. Each batch was prepared by combining the

powders in controlled proportions in a suspension solution of isopropanol. The powders

and solution were mechanically mixed using an ultra-sonification process. This process

allowed for a more homogeneous mixture by breaking up large agglomerates. The

mixture and solution was heated at a low temperature in a glass dish to evaporate the

isopropanol solution. After all of the isopropanol had evaporated, the composite powder

was scraped from the dish and placed into a vial to be immediately combined with the

other batches. The three batches were mechanically mixed and then separated into two

equal amounts. One half was placed into a 4 oz. polypropylene vial with a lid from Tyco

46

Healthcare Group LP. (Mansfield, MA) and the other half was placed into a glass dish

open to the ambient laboratory environment.

Both of the thermite samples, one open to the air and one enclosed in the vial,

were stored next to each other in the ambient lab environment as seen in Fig. 15. The

temperature and relative humidity was measured and recorded every 24 hours using a

digital hygro-thermometer calibrated by Control Company (Friendswood, TX) on June 4,

2001. The temperature and relative humidity measurements were averaged for the five

days of aging to get an overall average temperature of 23.6°C, and an overall average

relative humidity of 38%.

Figure 15 Photograph of sample storage containers

47

The performance of the thermite was quantified by measuring the combustion

wave speed of loose powder burning in a channel exposed to an ambient air environment.

The schematic of the full apparatus used to measure the combustion wave speed is found

in Fig. 16. A square channel 0.3175 cm wide and 10 cm long was used to burn the loose

powders in as seen in Fig. 17. The channel was made of acrylic so that the light from the

thermite combustion could pass through. The powder was ignited by a spark gap that

passed through the channel. A trigger fired the spark gap ignition and triggered the

camera at the same time. A Phantom IV high-speed camera from Vision Research

(Wayne, NJ) was used to record the flame propagation. The camera interfaced with a

computer that used Vision Research software to post process the videos. By establishing a

reference length, the software determines speed based on a distance between sequential

time frames.

48

Spark Gap

Ignition

Open Burn Channel

Trigger

High Speed Camera

PC with Imaging

Software

Figure 16 Schematic of flame propagation apparatus

49

Figure 17 Photograph of burn channel

Every day for five days, two samples from the vial and two samples from the tray

were burned and the combustion wave speeds were recorded. Each sample was 250 mg.

Two samples were burned daily to obtain repeatability data. It was observed that the

combustion wave speeds were not constant throughout the burn. Because the speed was

not constant, either the measurement time or the measurement length was required to be

kept constant to achieve comparable speed measurements. It was impractical to keep the

length viewed in the videos constant because the camera was moved daily for other

experiments so the time was held constant. The average speed was measured between the

frame where first light was visible and 40 frames later. Because the camera ran at 32000

50

frames/sec, 40 frames is equal to 1.25 ms. The standard deviation was calculated for all

of the repeated tests.

7.1.3 Results

Table 4 presents the combustion wave speeds recorded over the four-day period.

It shows the individual and average speeds starting on day 0 until day 4. It can be seen

that one repeatability test was conducted for each sample.

Table 4 Combustion Wave Speeds Velocities (m/s)

Days Aged 0 1 2 3 4

Vial Burn 1 36.176 33.591 30.843 31.168 31.446

Vial Burn 2 31.734 30.305 33.260 31.751 31.445

Avg. 33.955 31.948 32.052 31.460 31.446

Std. Dev. 3.141 2.324 1.709 0.412 0.001

Tray Burn 1 37.446 33.042 30.842 29.951 27.936

Tray Burn 2 33.878 36.073 33.829 33.242 32.893

Avg. 35.662 34.558 32.336 31.597 30.415

Std. Dev. 2.523 2.143 2.112 2.327 3.505

In Fig. 18, the average combustion wave speed for the tray and vial mixes is

plotted as a function of the number of days the mix has aged. The error bars represent a

standard deviation between the repeated tests for each sample. The points on the plot

were slightly separated for each day to show the error bars more clearly. All tests were

actually conducted within 30 minutes of each other on each day so the daily points would

truly be on top of one another.

51

26

28

30

32

34

36

38

40

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Days Aged

Co

mb

usti

on

Wave S

peed

(m

/s)

Vial Contained

Tray Contained

Figure 18 Plot of combustion wave speed as a function of days aged

7.1.4 Discussion

Figure 18 shows that there is a decrease in the combustion wave speed for

thermites that have aged for a period of four days. This result was expected because it

was already known that Al particles form thicker alumina shells the longer they are

exposed to air [12 and 37] which reduces the amount of active aluminum or fuel. Using

the data from Table 4, it was calculated that over a period of 4 days, the sample stored in

the vial had a reduction in combustion wave speed of 7.4% and the sample stored in the

tray open to the air had a reduction in combustion wave speed of 14.7%. This shows that

it is certainly better to place a mixed thermite in some type of container than to allow it to

52

be exposed to air freely. Even though the vial was not advertised as air tight, it did have

enough benefit to reduce the speed degradation by half.

Bulian et al. [39] showed a reduction in the combustion wave speed of over 50%

within two days exposure to air at 40°C and 97% relative humidity. This study shows that

samples exposed to air at a lower temperature and a lower relative humidity experience a

much smaller reduction in the combustion wave speed performance. Bulian et al. [39]

also theorized that the decrease in combustion wave speed was due to the increase in the

alumina barrier on the aluminum particles and increase in the physisorbed and

chemisorbed water. This study shows that the increase in the water content in the

aluminum plays a greater role than the increase in the alumina shell. Far less reduction in

wave speed was seen at double the exposure time at similar dry air exposures but reduced

water vapor exposures.

Of course, Bulian et al. [39] also used copper oxide as the oxidizer and mixed the

thermite after the aluminum exposure. This suggests that a thermite that is already mixed

is less susceptible to environmental exposure. This could be due to the fact that the

oxidizer blocks some of the aluminum particles contact with the environment. Tungsten

trioxide doesn’t experience an effect like the alumina shell formation when exposed to air

so the air exposure by itself would not affect the WO3 as severely as the aluminum.

Further study is needed to prove this theory. Chemical analysis to determine the chemical

composition of the thermite at different stages of exposure would show whether the

alumina formation is hindered in a premixed thermite.

53

There were several problems encountered with this study. The greatest problem

encountered was in measuring an accurate combustion wave speed. Because the wave

speeds did not vary by a large degree, very accurate measurements were needed to find a

trend in the data. Because the combustion wave was identified by visual methods,

problems arose from the brightness of the thermite when burnt. The brightness can be

reduced mechanically and electronically by filters and image processing but reflections

off of the burning channel and the smoke from the combustion made finding the

combustion wave difficult. Other difficulties came from the combustion waves being so

fast in a Al+WO3 thermite. There was only a limited amount of data that could be

recorded with the equipment on hand. The camera recorded at 32,000 frames/sec which

was not ideally fast enough for these very high-speed thermites. Finally, the Al+WO3

thermite exhibited non-constant combustion wave speeds. Other thermites such as

Al+Fe2O3 exhibit much more constant combustion wave speeds when they are burnt in

loose powder form. All of these factors, which would be minor if examining greater

speed variations, become large sources of error when such accuracy is needed. This is

represented by the relatively large error bars seen in Fig. 18.

This study showed that there is a reduction in the degradation of a thermites

performance when it is mixed before being exposed to the environment and when the

ambient conditions have low relative humidity. This produces a need for further

investigation into these phenomena. Besides combustion wave speeds, data about the

thermites surface area is also needed. This would more directly compare to the work that

Bulian et al. [39] and Puszynski [40] have completed. Also, conducting chemical analysis

54

on the thermites at different levels of environmental exposure would show whether or not

the oxidizer impedes the alumina growth. Scanning electron microscope (SEM) pictures

might also provide some insight to the oxidizer protecting factor. Finally, studies which

are conducted for longer time periods with more accurate measurement techniques such

as photo diodes could be conducted.

7.1.4 Conclusions

It has been shown that Al+WO3 thermites exposed to ambient laboratory air

found in an arid climate will reduce the combustion wave speed of the loose powder by

15% within four days. If the thermite is stored in a standard laboratory vial, the

combustion wave speed is reduced by 7% within four days. This shows that even an

imperfect, or not airtight, vial should be used to store mixed thermites. Also, if the study

being conducted with the thermite requires high accuracy, it is necessary to burn the

thermite on the same day as it is mixed.

It has also been shown that a mixed thermite exposed to relatively dry air

experiences a far smaller amount of performance degradation compared to an unmixed

thermite exposed to high humidity air. Therefore, the amount of water vapor in the air

plays a very important role in the rate of decreasing performance. Further study is needed

to show if the mixed thermite may resist environmental exposure to a greater degree than

an unmixed thermite.

55

7.2 Long Term Aerogel Aging

In an effort to obtain new data to reinforce the trends seen in Fig. 14, a new batch

of Aerogel 120 and Aerogel 400 was mixed 22 months after the initial batch. New nano-

Al was used in the mixes but it was required to use the original supply of both aerogel

oxidizers. Open-channel combustion burns for the new Aerogel 120 and Aerogel 400

were conducted using the same procedures described in Section 2.1.3 and 2.3. Both

thermites were mixed at Φ=1.4 as before. The original Aerogel 120 had a combustion

wave speed of 490 m/s. The new Aerogel 120 sample had a combustion wave speed of 8

m/s, a decrease of 98%. The original Aerogel 400 sample had a combustion wave speed

of 310 m/s. The new Aerogel 400 sample had a combustion wave speed of 29 m/s, a

decrease of 91%. Because the aluminum was new, this shows that the aerogels had

almost totally degraded over a period of less than two years.

Considering the preliminary aging study mentioned previously, the most likely

degrading factor was the absorption of water through water vapor in the atmosphere. An

aging study of Al and MoO3 showed that the aging effects of MoO3 were greater than the

aging effects of Al [38]. Thermal gravimetric analysis was conducted on the aged aerogel

samples as shown in Fig. 19. The original TG analysis is plotted along with the new

analysis. It can be clearly seen that the aged oxidizers experience a greater reduction in

mass as they are heated. This confirms the presence of additional water in the aged

samples.

56

Figure 19 Thermal gravimetric plot of original and aged Aerogel 120 and Aerogel 400

57

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