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Nano-Scale Studies for Environmentally Benign Explosives and Propellants

Alba Lalitha Ramaswamy ECE Dept.

University of Maryland College Park, MD 20742

U.S.A.

Pamela Kaste, Sam Trevino US Army Research Laboratory

Aberdeen Proving Grounds APG, MD 21005-5066

U.S.A.

ABSTRACT

Novel nano-scale technologies are being applied for the realization of next generation energetic propellants and explosives with increased energy outputs, maintained or improved IM characteristics, non-toxic emissions and efficient, rapid degradation both for end of life disposal and post-functional biodegradation. For example nano-scale ingredient formulations potentially promote a rapid burning or hyper-reactivity upon initiation leaving little or no residue after burning and thus minimizing the environmental impact. In particular carbon nanotube incorporation in propellant formulations offer the potential for burning rate and combustion tailoring. Moreover, because of the unique structure of carbon nano-tubes, they may be used for confining the “nano-energetic” crystals in a nano-matrix with a positive effect on vulnerability properties. Thus, the possibility exists for simultaneously improving reactivity and stability of energetic materials, usually opposing characteristics. To this end, research is being pursued to derivitize these compounds, ultimately with energetic functional groups that would improve performance and facilitate dispersion into a polymer matrix. Novel techniques for incorporating “nano-energetic” material into nanotubes are under investigation. Finally future environmentally benign formulations can be obtained through the incorporation of bio-degradable capsules, which open up on stimuli from the sun, soil, humidity and ph releasing their content. Propellant residues on soils will be caused to biodegrade due to the action of the micro-capsule contents, consisting of bio-flora and/ or bio-stimulants or various environmentally friendly chemical agents.

BACKGROUND

Nano-Energetics The number and type of nano-energetic materials produced world-wide is increasing due to the potential impacts on future energetic formulations. Novel technologies for the production of new nano-scale materials are developing at a rapid pace. The incorporation of nano-energetic components in formulations such as oxidizers or fuels has shown the potential to create or promote an extremely efficient and rapid degradation of propellants, leaving little or no un-reacted residue. This will lead to environmentally clean or benign formulations. Furthermore the introduction of nano-energetic components may potentially improve the performance properties of propellant formulations by increasing on the control and rate of the localized “energetic” chemical reactions. A distribution homogenization of the “reaction centers” throughout the formulation, by the incorporation of nano-energetics, gives greater control of the overall burning by producing a smoother burning spread or reduced localized “hick-ups”. Different architectures in which nano-scale energetic materials are embedded in energetic matrixes can thus be studied and designed with the added support of theoretical modeling at the molecular level. Typical nano-energetic ingredients, which are being investigated for incorporation in formulation mixes can be sub-divided into inorganic nano-fuels, nano-oxidizers and/or nano-explosives.

RTO-M

Paper presented at the RTO AVT Specialists’ Meeting on “Advances in Rocket Performance Life and Disposal”,held in Aalborg, Denmark, 23-26 September 2002, and published in RTO-MP-091.

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Carbon Nanotubes Carbon nano-tubes (CNTs) have a unique coiled graphitic structure. CNTs were first discovered in 1991. [1] They can grow with a single cylindrical wall (SWNT) or multi-walls (MWNT). The larger the diameter of the tubes, the more likely they are to be multi-wall; e.g. a 50 nm internal diameter tube typically has up to 6 multi-walls and a 1.2-1.4 nm diameter tube is single-walled. The lengths and internal diameters can vary significantly depending on the production conditions. Thus the typical dimensions of CNTs can vary from 1-100 nm internal diameter and 2-10 µm in length (though tubes up to 2 mm in length have been reported). [2] With recent improvements in production conditions, the dimensions and quality of tubes can be better controlled. The crystal structure of carbon nanotubes has been measured by various techniques and determined to consist of the coiled graphitic structure. The Young’s modulus of elasticity for single-wall CNTs has been found to be close to 1 TPa with a maximum tensile strength close to 30 GPa. [3,4] The longitudinal tensile strength is reported to be up to one hundred times that of steel. [2] There are variations in the reported values, measurement techniques utilized and calculated values. |The research is ongoing.

The morphology of the CNTs is very important in characterizing their strength or mechanical properties. Thus CNTs can come in three typical morphologies: the “bamboo” or platelet-type, the “stacked cups” type or the “perfect tube”. Also, the CNTs when first grown, typically have a hemispherical “cap” at each end. The caps contain 5-membered carbon rings, where the C-C bonds are strained and can hence be etched or removed by the action of dilute nitric acid. The “bamboo” or “stacked cup” morphologies consist of chemical termination of dangling carbon bonds at the end of each platelet or cup with carboxyl (COOH) groups. Thus under the action of mechanical stress the line of weak hydrogen-bonding between the “platelets” or “cups” can break open. The perfect carbon nanotubes instead are very strong. Different functional groups can be attached through the COOH groups. This can be done for all CNT types. The advantage of the “bamboo” type is that functional groups can be attached all along the length of the tubes rather than just at the ends.

The electrical conductivity along their lengths is very high, reaching stable current densities of up to 1013 Ω/cm2, which corresponds to 120 billion to 3 trillion electrons per second [5,6]; in comparison, a typical 18-gauge copper wire carries a current density of up to 102 Ω/cm2. [7] The CNTs have thus been considered as the “nano-wires” of future molecular computers where the individual diodes and transistors are considered to be built out of single organic molecules combined and doped appropriately. The conductance has been found to be and to vary according to the chirality of the tube. [5, 8, 9] Thus the CNT has the ability to be metallic or semi-conducting depending on the twist of the tube. [10]

Finally the thermal conductivity is reported to be about 2000 W/m/K, which approaches that of diamond. [11, 12] However there is still a lot of controversy on the actual thermal conductivity and its temperature dependence. The purity of carbon nanotubes is critical in determining their thermal stability. When heated in a thermogravimetric analyzer (TGA), CNTs produced by NanoLab (Brighton, MA), which are about 96-98% pure, decompose between 450-500 °C. The main contaminants are silicon and nickel catalyst, which come from the production process. Some amorphous carbon is also present. CNTs from other sources burn at around 250˚C but have a 15% residual catalyst impurity. Since the catalyst remains as a tip on the nanotube, most can be removed through an acid-etching process.

The standard CNT fabrication process consists in sputtering a nickel or cobalt catalyst onto the surface of a substrate, which can consist of molybdenum, titanium, graphite, quartz, silicon or alumina. The substrate is heated to 600-800 °C in the presence of a DC plasma, where acetylene and ammonia gases are introduced. The acetylene decomposes and diffuses through the catalyst and crystallizes as a nanotube on the substrate. For nano-arrays, application of an electric field during growth aligns the tubes.

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The coiled graphitic structure of carbon nano-tubes (Fig.6) confers unique properties to the nanotubes. Because the diameters of CNTs are nano-scale while their lengths can be micro-scale, they also have exceedingly high aspect ratios. This offers the potential for achieving a maximum effect with a minimum level of parasitic material. Such properties make nanotubes attractive as a new candidate ingredient for propellant formulations with the potential for improving the ignition and combustion properties. Moreover, since the tubes consist of pure carbon, they pose no adverse effect to the environment. Through the use of carbon nanotubes it may also be feasible to enhance the specificity of a propellant formulation to initiation by plasma, due to the high electron density and conductance localized on the nanotube walls. Stability, which is a property that is generally diametrically opposed to reactivity, may be augmented by encapsulation of nanoenergetic materials within the carbon nanotube. The carbon nanotube encapsulation technology has the potential for providing safer and more efficient handling and compaction of energetic materials by facilitating insertion and packing into the formulations. Methods for preparation of energetic crystals of nano-scale dimensions and methods for insertion into the tubes are technology efforts that are being developed and investigated.

Studies are being performed to assist in the research effort to incorporate/bind the carbon nano-tubes within propellant matrixes, and to understand their effects on the initiation, performance, safety and mechanical properties of formulations containing them. A key to the effectiveness of nanotubes will be in the ability to thoroughly disperse them in the propellant matrix. One approach to increase the affinity of the tubes for energetic binders is to functionalize the tubes. Ultimately derivitization with energetic groups is desired, but an initial approach is to prepare a nanotube precursor which can undergo further reaction with monomers so that a polymer-substituted carbon nanotube is obtained.

Biodegradation The disposal of energetic material organic wastes has been performed since World War II by the technique of bio-oxidation. Nearly all U.S, explosive and ammunition plants utilized bio-oxidation also known as biological oxidation or biochemical oxidation. This process does not include TNT and some other explosives for which no effective bacteria were found at the time. It is a method for the disposal of organic process water-borne wastes by the action of living organisms. A vast knowledge-basis has been accumulated since those times on the biological treatment of energetic material organic waste. Such a knowledge-basis has been extended/ applied herewith for the development of environmentally benign explosives and propellants. This means that the wastes or residues remaining on the grounds after firing of the charges should be capable of natural self-degradation in a rapid fashion to produce benign gaseous components. The principle being used here is that the combination of soil, bacteria, pH, temperature and radiation acting on the residues effectively functions as a biological bed. In addition, to activate or concentrate the biological growths on the propellant residues, micro-capsules containing bacterial nutrients are being considered which is analogous to passing an activated sludge through the wastes. Finally, depending on the particular chemical make-up of the formulation, micro-capsules containing catalysts, oxidizers, bio-enzymes and free-radical generators can be utilized to further enhance or facilitate the self-degradation reaction. Thus by coupling the present-day bio-treatment knowledge-basis of energetic materials with innovative applications, it may be possible to achieve full biodegradability of propellant formulations, in the near future.

EXPERIMENTAL

High Resolution Microscopy Carbon nano-tubes manufactured by Nanolab were examined with a high resolution electron microscope JEOL (Japanese Electron Optics Limited) Model 1200 EX-2 with the microscope set for the high-resolution transmission mode. Reactions used in preparation of polyethyleneimine- and

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poly(methylmethacrylate)-substituted carbon nanotubes, exploiting naturally-occuring carboxylic acid impurities of the nanotubes, are found in Bratcher, 2001. [13] The carbon nano-tubes were placed in 100µl of water inside a small test-tube and dispersed by agitation of the tube in a vortex mixer for 10-20 seconds. This allowed the individual tubes to separate for examination under the microscope. Various magnifications were used to image both the individual tubes and the cluster agglomerates.

Desorption-Gas Chromatography-Mass Spectroscopy Desorption or pyrolysis was achieved via a CDS Model 2000 Pyroprobe® (coil type) connected through a heated interface chamber to the splitless injector of an Agilent GC-MS system (Model 6890N GC and Model 5973N MSD). The GC Column used was an Agilent Technologies Inc. capillary column (30 m x 25 mm; 0.25 um HP-5MS film). The injector temperature was 200 °C; the Pyroprobe® interface temperature was 250 °C. The GC oven temperature program was as follows: 50 °C isothermal for 1 min; 50-250 °C at 40°C/min; 250 °C isothermal for 1 min. The Pyroprobe® was programmed to give a 20-s desorption pulse at either 400 or 700 °C (heating rate: 1000 °C/sec) as indicated for each particular experiment. Samples were held within the coil of the Pyroprobe® by first placing them in a quartz tube containing a small plug of glass wool, and then inserting the entire tube into the coil.

FTIR Spectroscopy FTIR spectra were collected with a Nicolet Model 970 Spectrometer operating with OMNIC software and using 4 cm-1 resolution. A Spectra-Tech infrared microscope in the specular mode with narrow-band MCT detector was used to collect the spectrum of the methylmethacrylate piece isolated from the nanotube material; a gold coated microscope slide was used to obtain the background spectrum. A Nicolet OMNI Sampler attachment was used in the main compartment with a triglycine sulfate (TGS) detector for the unmodified and modified carbon nanotube samples. Appropriate scanning velocities are preset in the instrument for each of the detectors.

X-Ray Diffraction

X-ray diffraction data were collected on a Model D5005 X-ray diffractometer with DIFFRAC Plus software and analyzed with EVA software. Sample surfaces were pressed as flat as possible. The instrument was set at 40 volts. The x-rays have a wavelength of 1.54 Å. In most trials the samples were scanned from 2-Theta values of 2.5-40 degrees (theta), thus enabling detection limits to d-spacings to about 30 Å (from n(lambda) = 2d sin). The instrument was run at 40kV and 40 A, with a step size of 0.5 degrees and 4 seconds per time step.

RESULTS

Fig. 1-3 show the high-resolution electron micrographs of the unmodified, PEI- and PMMA-modified carbon nanotubes for magnifications of 40,000x. As can be seen from the micrographs, the tubes are distinct with a clear center core and a ‘thick’ outer wall. The outer wall actually consists of a series of graphitic layers, forming a multi-wall structure. There are typically of the order of six to nine layers, as determined from transmission electron microscopy of microtoned slices of the carbon nano-tubes. The lengths of the tubes are on the order of 20 µm, while the average diameter is approximately 20 nm. There is a large variation in tube sizes as indicated by the values summarized in Table 1. The data given in Table 1 was generated by measuring the distribution of the structural dimensions from a large sample of carbon nanotubes as viewed/ examined from high resolution electron micrographs. The “repeat defect distance” referred to in the table is the size of the segments in a single CNT as depicted in Fig. 4 and the distribution “modes” represent the observed maximae in the distribution curves.

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Table 1: Carbon Nanotube Structural/Physical Dimensions

Structural dimension Range (nm) Distribution Modes (nm)

No. of CNTs sampled

Mean (nm)

Inner core diameter(I.D.) 1.25 - 50 2.5, 6.5, 10.5 153 6.25 ± 1.25

Wall thickness 0.625 – 37.5 2.5, 6.9, 10.0 139 5.0 ± 0.5 Outer diameter (O.D.) 2.50 - 85.7 12.50,18.75,25 153 19.0 ± 2.0 Repeat defect distance 20 - 65 n/a 12 43.0 ± 1.0

Figure 1: High-resolution Transmission Electron Micrograph of Unmodified

Carbon Nanotube (40,000x).

Figure 2: High-resolution Transmission Electron Micrograph of Polyethyleneimine-

Modified Carbon Nanotube (40,000 x).

Figure 3: High-resolution Transmission Electron Micrograph of Amine-modified Carbon Nanotube (40,000x).

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CNT tube with “bamboo” morphology

Defect sites with COOH groups around perimeter of tube

Segment

C

O

O H

Figure 4: Carboxylic Defects in CNT “Bamboo” Structure.

The morphology of the tubes is the “bamboo” structure with clear surface features or defects occurring at regular intervals of about 20-60 nm. These defect areas consist of terminated carbon bonds with COOH groups (see Fig. 4). The presence of the carboxylic defect sites has been determined by titration. [13] The carbon nanotubes were functionalized at these defect sites, with the ultimate goal of incorporating them into propellant formulations. The line of weak hydrogen bonding between the platelets of the bamboo structure can be opened by the action of mechanical stress. However, functionalization has conferred to them a unique structure: the carboxylic defect sites have become very evident and some of the tubes show “surface undulations”. The nature of the amine and PMMA attachment to defect sites in relation to the rest of the tube is very important. Some of the PMMA is attached to the surface of the tube through reactions with the carboxylated defect sites, however, there is also evidence of PMMA around and detached from the tubes. In order to characterize the structure of modified CNTs, a gold atom or gold micro-sphere will be attached to each functional group through a sulphur atom. In this way the tubes can be viewed with the high resolution electron microscope and the location of the amine groups can be detected; prompt gamma activation analysis (PGAA) analysis, sensitive to the gold atoms, can then be performed to accurately determine the number of attached functional groups and thus if all defect sites have reacted.

Experiments performed by Nanolab have shown that under some conditions of mixing, the carbon nano-tubes tend to break at the “bamboo” junctions. [14] However, conditions of mixing for which the stresses do not break the tubes are being investigated. Prompt gamma activation analysis (PGAA) of NanoLab CNTs was used to determine a C/H ratio of 600:1. From known carbon-to-carbon bond distances (See Fig. 5) and the fact that defect sites occur at regular intervals, the defect density can also be computed from PGAA data. The computation can be done accurately through a computer code. A rough estimate was obtained as follows: It is assumed that each carboxylic group is attached to a 6-membered carbon ring at the tube termination. (This is only an approximation since there can be 5-membered rings.) Since each carbon atom in “bulk” is shared by three other carbon atoms, except at the ends of the tubes,

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each hexagon in the “bulk” contains an average of two carbon atoms. If n is the number of hexagons, a row contains (2n+2) carbon atoms, and each “row” is bonded to two hydrogen atoms. The ratio of carbon to hydrogen atoms can therefore be approximated by:

C/H ~ (2n+2)/2 = n+1

Given that the ratio of carbon to hydrogen atoms is 600:1, 2.45 Å x 600 (i.e. 147 nm) would represent the average repeat defect distance. The actual repeat defect distance measured from the micrographs is only 20-60 nm. While this may be due in part to some chirality or twist in the tubes as depicted in Fig. 6, or the presence of more 5-membered rings than expected, or the inner tube containing a lower density of defect sites, it is most likely due to the fact that the measured values of repeat defect distance were made on select tubes with regular defects, while PGAA was run on a bulk sample (no discrimination between types of tubes). Tubes with a different chirality or twist have been reported where the lattice parameter, density and interlayer spacing varies. [15]

2.83 A

2.456 A

2.45 A

1.42 A

Figure 5: Distances of Carbon Atoms in CNT.

Figure 6: CNT Structure for C/H Ratio showing line through a row of Six-membered Rings and the Twist or Chirality that may exist in CNTs.

Desorption Gas Chromatography-Mass Spectroscopy The D-GC-MS results (Table 2) for the virgin CNT samples showed no indication of organic solvent present, consistent with the fact that these materials are produced under conditions of high temperature and pressure with metal catalysts (although water could be absorbed from the environment afterward). The only compound detected at 400 °C was benzene, while the 700 °C treatment yielded benzene, toluene and styrene – all probable decomposition products of the CNT structure. Because the modified CNT

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samples appeared to contain significant solvent levels (see below), all the CNT samples were run as provided, and after being exposed to house vacuum for about 30 minutes. The result for the unmodified CNT samples was surprising. There appeared to be a greater number and level of the decomposition products obtained with the vacuumed unmodified samples (V-CNTs) compared to those not exposed to vacuum (CNTs). Also, only with the vacuumed samples were higher molecular weight hydrocarbon species, both aliphatic and aromatic/fused ring systems detected. The reason that the vacuumed samples yielded a greater quantity of decomposition products, and a larger molecular-weight distribution, is unknown. It may be possible that the CNT samples retain water that could modify the thermal profile to which the sample is exposed, and mitigate the degradation affect. It is possible to use selective ion analysis (M/Z=18) with weighed samples to determine characterize the water content with D-GC-MS, but this was not done in this analysis.

Table 2: Components Identified in the D-GC-MS Analyses of the CNT Samples.

Species T(ret) V-4 V-4/7 4 4/7 V-4 V-4/7 4 4/7 ref V-4 V-4/7 4 4/7Methane, chloro- 1.70 VS HAcetone 1.76 H HAcetic acid, me-ester 1.93 VS VSAcetic acid 1.92 M MButanone 2.04 M H HAcetaldehyde 2.07 M-S VSSTHF 2.15 M1,3-Cyclohexadiene 2.27 M MCyclobutane, ethenyl- 2.23 M MButen-2-one, 3-methyl 2.29 M MBenzene 2.38 M V-W M M M2-Propenoic acid, methyl ester 3.58 H VS VS VSBenzaldehyde 3.74 V-SPropanone, Hydroxy 2.35 M2 Pentanone, 3-Me (others) 2.65 M S M SToluene 2.82 M W M MFuran (dime-ethyl)-dihydro 3.42 S SStyrene 3.43 M W MPhenol 3.80 W S MDecane 3.87 MBenzene, triMe 3.89 MLimonene 4.05 MCyclohexene (me-(me-ethnyl) 4.05 M MUndecane 4.31 M3-5 Dimethyl Phenol 4.50 S (ketone) S (ketone)

Dodecane 4.71 MNapthalene 4.77 M M SHigher MWT products 6.00 M-S M-S Many Many

undefined bet 6-7 min 7.50 M-S M-S between between

Cyclotetradecane 6.89 S 4-5 min. 4-5 min.

Eicosene 7.50 HCyclohexadecane 8.18 M

Unmodified CNT's Polyethyleneimine-CNT Poly(methylmethacrylate)-CNT

The D-GC-MS analysis of the polyethylenimine-modified (PEI) and poly(methylmethacrylate) (PMMA) polymeric samples were also performed on the nanotubes as received and after vacuum exposure. The PEI samples desorbed with a 400 °C pulse yielded acetic acid; apparently the acetic acid is held sufficiently

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that it is not removed under the mild vacuum conditions. In addition, the non-vacuumed sample yielded species such as the THF solvent known to be present and 1,3-cyclohexadiene, and ethynlcyclobutane, which were not observed in the samples held under vacuum. Neither sample produced significant single aromatic decomposition fragments, but only the sample held at vacuum had larger molecular weight species that elute after 4 minutes (roughly 10-carbon fragments or greater), albeit at a low level. Compared to the non-modified tubes, the PEI-modified tubes seem to be slightly resistant to thermal decomposition with the 400 °C pulse, particularly in the case of the vacuumed tubes. With the dual 400/700 °C pulse treatment, the PEI-modified samples yielded larger fragments (multiple-ring compounds) than the unmodified nanotubes did.

The PMMA-treated samples had solvent levels that greatly exceeded either the PEI- or unmodified CNT samples. The levels diminished with vacuum treatment, but because the original level was so high, the solvent levels in the vacuumed PMMA samples remained far higher than those discussed above. With a 400 °C pulse, copious amounts of chloromethane and acetic acid methyl ester (methyl acetate) were observed. It is particularly interesting that a solvent as volatile as chloromethane that such a large quantity would remain for weeks, or after vacuum treatment. If this is indeed the case it indicates the ability for carbon nanotubes for retaining this solvent. The very large quantities of acetone (after 700 °C pulse), acetic acid methyl ester (400 °C pulse) and butanone (obtained after pulses at 400 and 700 °C on the same sample) are likely due to decomposition products. The fact that none of these species is present in the PMMA reference material (400 °C) may suggest that these polymers could have quite different polydispersity and could react differently to thermal treatment. The PMMA that polymerizes on the carbon nanotubes at multiple sites could result in many oligomers of various sizes, but most likely much smaller chain lengths than in the reference PMMA material. If poly-dispersity and a larger difference in molecular weight distribution could affect the D-GC-MS product profile to the extent observed would need to be evaluated. It is noted that the major decomposition product of the reference PMMA material, 2-propenoic acid, methyl ester is also observed in large quantities under all treatment conditions in the PMMA-modified tubes.

It is interesting that the only CNT samples that showed significant decomposition products related to the tubes themselves (at 400 °C) were the vacuum-treated unmodified CNT samples. It might be possible that having other functional groups and/or solvent present may actually protect the tube structure from thermal decomposition for some treatments. At the 700 °C pulse, the unmodified, vacuumed samples, and the PEI-modified samples behaved similarly in that products of relatively high molecular weight related to the nanotubes were obtained (e.g. 10 carbon chains and greater). In the case of samples not exposed to vacuum (and possibly with adsorbed water), there are few products at all, even at 700 °C. With samples with substantial levels of PMMA present the products related to the carbon nanotube decomposition have a smaller molecular weight distribution than for the unmodified or PEI-modified samples. It is possible that with the additional energy available from a 700 °C pulse, the PMMA decomposition products might react with the fused ringed compounds to cause further breakdown to 1-or 2-ring systems or smaller aliphatic species.

Although the mechanisms are all very speculative, it is apparent from the D-GC-MS experiments that the carbon nanotubes are quite thermally stable, and that decomposition can be very dependant on factors such as the occluded solvent level and the nature and extent of the modification. The affinity of carbon nanotubes for retaining solvents or species of interest will be further investigated. Such information may be important for assessing the chemical purity of the tubes and to assist in the design of the modified or derivatized and “nano-energetic” filled carbon nanotubes.

FTIR Characterization Obtaining spectra of carbon nanotube was attempted using both specular and diamond microreflectance techniques, but the best results were obtained using the OMNI Sampler macro-ATR accessory that is

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designed for the main compartment of the spectrometer (as opposed to being an attachment for the FTIR microscope). As expected, the spectrum of the unmodified tubes contains no significant absorption bands, as seen in Fig. 7. The spectrum of the PMMA-modified tubes is also presented, however major bands expected for poly(methylmethacrylate), i.e. nominally 1740 cm-1 and 1470 cm-1, are just barely detected, while dominant bands at 1560 cm-1 and 1415 cm-1 are observed. The source of the dominant bands is unknown. They would not be inconsistent with the acid chloride compound used as a precursor for polymer derivitization, however an acid chloride sample (albeit from a different lot) did not show the bands in question. In fact, the acid chloride spectra were very similar to unmodified carbon nanotubes.

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Figure 7: FTIR Omni-Sampler Spectra of PMMA-modified and Unmodified Carbon Nanotubes.

As discussed in the results for the D-GC-MS analyses, significant solvent was evident, particularly for the PMMA-modified nanotubes, and the samples were held under moderate vacuum. Afterwards, the PMMA nanotube particles did not adhere to each other as much, and fine white particles, about the size of a grain of salt, were evident. One of the larger of these was isolated for FTIR analysis by specular microreflectance. The spectrum obtained is shown in Fig. 8, along with a reference spectrum of poly(methylmethacrylate). It is clear that PMMA bands are present in the spectrum of the white grain, but a large, broad band in the 1600 cm-1 region is also present. The particle spectrum is overlaid with that of the bulk PMMA-modified tubes in Fig. 9. The band at 1600 cm-1 appears to be at a higher frequency than that of the nominally 1560 cm-1 band that appears in the bulk; it is noted that the two spectra were run by different methods so that some shift in frequency is possible, although a 40 cm-1 shift seems unlikely. Such a strong band at or just above 1600 cm-1 is typical of an amide, but not many other functional groups. The band is not consistent with any of the components used in the synthesis, with the possible exception of dimethylformamide (DMF). However, DMF was used in the first synthesis step, and it would be unlikely that so much would remain after numerous steps and drying cycles. Moreover, DMF was not found in the D-GC-MS analyses, although other solvents present were detected. The possibility exists that the polymer formed in the derivitization is not solely PMMA. More synthetic work is planned for the future. Material from intermediate steps will be sampled to assist in future characterization work.

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crys1aGrain from PMMA-Modified CNT

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Figure 8: FTIR Micro-Reflectance Spectra of a Polymer Grain isolated from PMMA-CNT and a PMMA Reference Polymer.

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Figure 9: FTIR Spectra of PMMA-modified CNT and a Grain isolated from this Sample. The methacrylate bands at ~ 1740 cm-1 and 1470 cm-1 are not major bands in the bulk PMMA-CNT spectrum.

The spectrum of the PEI-modified sample is shown in Fig. 10, with the spectrum of the unmodified nanotubes provided as reference. The band at 1545 cm-1 is consistent with secondary amines that might be expected in the PEI polymer. Although the bands in the PEI-modified tubes are not as strong as those from the PMMA-modified sample, it is noted that no polymer grains were observed in the PEI sample. The two polymerization reactions reflect different approaches. The PEI-modification is an attempt at attaching a pre-fabricated polymer onto the nanotube, whereas the PMMA-modification relies on polymerization to begin at the defect site of the nanotube. Further work in this area is planned.

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1545

3500 3000 2500 2000 1500 1000

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Arb

itrar

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banc

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Figure 10: FTIR OMNI-Sampler Spectra of PEI-modified CNT and Unmodified CNT Samples.

X-Ray Diffraction

The X-ray scattering results are shown in Fig. 11. The unmodified carbon nanotubes show a peak at 3.4 Å. The PMMA-modified tubes also showed a peak corresponding to nearly the same spacing, but another very weak peak at about 4.4 Å is also evident in the PMMA-modified tubes. It is not yet known to what these spacings correspond. Further x-ray diffraction analyses, to include instrumentation that enables greater spacings to be detected, is in progress.

10 20 30 40

Unmodified dd d

CNT

PMMA Modified

4.4 Å

3.4 Å

Arbitrary Units

2 Theta

Figure 11: X-Ray Scattering Results for Carbon Nanotubes.

Nano-Energetic Encapsulation Finally the technique for nano-energetic encapsulation in the carbon nanotubes has been investigated, including methodology for obtaining nano-scale energetic crystals and their incorporation into nanotubes. The concept is shown schematically in Fig. 12. Details cannot be presented at this time, but will be provided in the near future. Nanotubes are commercially available in an array form, which offers benefits

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for many applications. Details of their use for energetic materials applications will also be forthcoming. The arrays have been thus far evaluated with nano-aluminum particles, which show through the tube walls in high-resolution electron microscopy analysis. The individual carbon nanotubes can be removed from the array substrate by etching.

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Propellant binderNano-energetic particles

Functionalized polymer chains

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Propellant binderNano-energetic particles

Functionalized polymer chains

Multi-walls

50-100 nm

Figure 12: Concept for Nanotube Encapsulation of Energetic Particles.

Biodegradability An in depth analysis of the fungal treatment of soils contaminated with RDX, HMX and TNT is described in the literature [16]. The white rot fungus phanerochaete chrysosporium was found to mineralize both TNT and RDX. These fungi produce extracellular peroxidases that are capable of oxidizing a wide variety of recalcitrant chemicals including lignin and xenobiotics such as polycyclic aromatic hydrocarbons and polychlorinated phenols. Herewith it has been considered to insert dry fungal spores in special biodegradable micro-capsules. These capsules open up on contact with the soil, temperature, sun, humidity, ph, to release the spores. The dry fungal spores are inactive until released. Once in contact with the environment the spores absorb moisture, swell and grow into fungi. The grown fungi would then feed on or mineralize the RDX and TNT components of the formulation.

With regards to the binder components of the formulation, it may be possible to select specific binder molecules, which mimic natural polymers and are thus recognized by soil bacterial enzymes. The biological flora is rich with enzymes from bacteria and fungi, which feed and decompose on the organic wastes in nature. The most common bacteria are known as nitrogen-fixing bacteria and feed on the nitrogen in the atmosphere as well as nitrogen containing compounds such as protein and amino-acids to provide the necessary nitrogen constituent molecules for plants. Thus various bio-stimulants and nutrients can also be included within the micro-capsules to stimulate the growth of the biological flora responsible for the degradation or mineralization of the formulation components. The concepts described above are depicted in Figure 13.

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Action of sun, soil, humidity opens micro-capsules

Micro-capsules release bio-flora, bio-stimuluants, bio-nutrients and chemical agents for the self-degradation of propellant residues

Figure 13: Concept for Micro-capsule Encapsulation of Try Fungal

Spores and Various Bio-ingredients for Formulation Biodegradation.

In addition to the treatment of explosives and propellants by biological living organisms, various chemical materials or additives and methods can be applied to achieve the clean break-up of the energetic materials. These can include catalysts, oxidizers and radical generators. Such chemical additives could also be utilized in combination with biological treatments.

CONCLUSIONS AND FUTURE RESEARCH

Novel nano-scale technologies are being applied for the realization of next generation energetic propellants and explosives with increased energy outputs, maintained or improved IM characteristics, non-toxic emissions and efficient, rapid degradation both for end of life disposal and post-functional biodegradation. In particular the characteristics of carbon nanotube materials offer potentials for improved performance. These include very high thermal and electrical conductivity, and a very high aspect ratio to enable maximum properties modification with minimal material. Key to their practicality will be the ability to disperse them uniformly into the energetic material matrix. To this end, functionalized carbon nanotubes have been prepared and characterized. The results show that polymers have been attached at the defect sites of “bamboo” type nanotubes for both PEI (for which a pre-formed polymer was attached) and PMMA (in which the polymerization actively occurred at the defect site) substitutions. Both high-resolution microscopy and FTIR analysis suggest that not all of the polymerized PMMA that formed was covalently bonded to the nanotubes. D-GC-MS analyses indicate that carbon nanotubes retain significant amounts of solvent; this fact must be taken into account, and may be exploited, for energetic material applications. Initial experiments of producing nanocrystalline oxidizers for incorporation into nanotubes have been performed. In particular, this application offers potential for improved vulnerability properties of energetic materials.

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Plans are being performed for attaching single atoms of gold, through a sulphur atom to each polymer chain. In this way the location of the chains can better be resolved by high resolution electron microscopy, so that a cold neutron Prompt Gamma Activation Analysis on the functionalized tubes with gold atoms, to determine the ratio of carbon to hydrogen to gold atoms and hence infer or compute the average polymer chain length. Also, the carbon nanotubes have been heated to 400 °C on a micro-hot stage at Univ. MD and filmed at the highest magnifications with an ESEM. The tubes have been found to be very stable both in the pure and functionalized form; analysis of the decomposition phenomena is in progress.

In addition the formulation output can further be boosted with the inclusion of novel high-energy density ingredients. For example, high-nitrogen explosives where the chemical energy is stored in the heterocyclic strained ring structure. High-nitrogen explosives decompose with the evolution of nitrogen gas and would thus render the propellant formulations environmentally benign. The incorporation of such energetic ingredients will be studied together with the incorporation of various nano-energetic ingredients. Furthermore environmentally benign formulations can be obtained through the inclusion of bio-degradable capsules, which open up on stimuli from the sun, soil, humidity and ph. Propellant residues on soils will thus biodegrade due to the action of the micro-capsule contents, which have been considered to consist of bio-flora, bio-stimulants or various environmentally friendly chemical agents.

ACKNOWLEDGEMENTS

The authors would like to thank Dave Carnahan of NanoLab for contributing the image of the carbon nanotube array. Dr. Matthew Bratcher is thanked for supplying the modified nanotube samples. Dr. Michael Schroeder, ARL-retired (Voluntary Emeritus Corp), is thanked for providing numerous literature articles on nanotube research that helped to improve our understanding of these materials. Dr. Rose Pesce-Rodriguez is especially thanked for assistance in experimental and data processing techniques for the D-GC-MS characterization.

REFERENCES

[1] Ijima, S., Nature, 354 56, 1991.

[2] Pan, Z.W., Sie, S.S., Chang, B.H., Wang, C.Y., Lu, L., Liu, W., Zhou, W.Y., Li, W.Z., Qian, L.X., Nature, 394 6694, p.631-632, 1998.

[3] Yu, M.F. et al. Phys. Rev. Lett. 84, 5552, 2000.

[4] Schewe, P.F., Stein. B., “Physics News Update, The American Institute of Physics Bulletin of Physics News, Number 279 (Story#2), 15 July. 1996. http://www.aip.org/enews/physnews/1996/ split/pnu279-2.htm.

[5] Frank, S., et al., Science 280 1744, 1998. http://electra.physics.gatech.edu/group/labs/tubelab.html.

[6] Yakobson, B.I., Smalley, R.E., “Fullerene Nanotubes: C(1,000,000) and Beyond,” American Scientist, 85 p324-337, 1997 (see also references therein).

[7] Ellenbogen, J.C., Love, J.C., “Architectures for Molecular Electronic Computers: 1. Logic Structures and an Adder Built from Molecular Electronic Diodes”, Mitre Report, July 1999.

[8] Sanvito, S., Kwon, Y,. Tomanek, D., Lambert, C., “Fractional Quantum Conductance in Carbon Nanotubes”, Phys. Rev. Lett. 84, 1974.

[9] Dekker, C. “Carbon Nanotubes as Molecular Quantum Wires”, Physics Today, p.22, May, 1999.

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[10] Wilder, J.W.G., Venema, L. C., Rinzler, A.G., Smalley, R.E., Dekker, C., Nature 391, 6662, 59-62, 1998.

[11] Hone, J., Whitney, M., Zettle, A., Synthetic Metals 103 2498, 1999.

[12] Berber, S., Kwon, Y., Tomanek, D., “Unusually High Thermal Conductivity of Carbon Nanotubes”, Phys. Rev. Lett. 84, 2000.

[13] Bratcher, M.S., Gersten, B. Kosik, W., Ji, H., Mays, J., “A Study in the Dispersion of Carbon Nanotubes”, Proceedings of the Materials Research Society Fall 2001 Meeting, in press.

[14] Carnahan, D., Nanolab, Inc., Brighton, MA, www.Nano-Lab.com, Personal Communication, 2001.

[15] Gao, G., Cagin, T., Goddard, W., “Energetics, Structure, Mechanical and Vibrational Properties of Single Walled Carbon Nanotubes (SWNT)”, 1997. http://www.wag.caltech.edu/foresight/ foresight_2.html.

[16] Spiker, J.K., Crawford, D.K, and Crawford, D.L.O., “Influence of 2,4,6-trinitrotoluene (TNT) on the Degradation of TNT in Explosive-Contaminated Soils by the White Rot Fungus Phanerochaete chrysosporium”, Appl. Environ. Microbiol., 58, 3199-3202, 1992.

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SYMPOSIA DISCUSSION – PAPER NO: 12

Discusser’s Name: Gary Briggs

Question: Would solid propellants formulated with carbon nano-tubes be expected to exhibit visco-elasticity – a common mechanical feature of many of today’s solid propellants?

Author’s Name: Alba Ramaswamy

Author’s Response: The carbon nano-tubes have a reported tensile mechanical strength of one hundred times that of steel. With regards to the visco-elasticity, we need to actually test and measure what their effect is on the visco-elasticity of propellants.

Discusser’s Name: Ron Derr

Question: In the manufacture of carbon nano-tubes, will the desired nano-tube structure be harvested from a mix of structures or can the nano-tube structure be grown within desired specifications?

Author’s Name: Alba Ramaswamy

Author’s Response: The structure can be grown with the required specificity especially for carbon nano-tube (CNT) arrays. The arrays are formed by the growth of CNTs around microspheres set on a substrate where the catalysts are located. The internal and external diameters of the CNTs can thus be accurately controlled.

Discusser’s Name: Klaus Menke

Question: Carbon nano-tubes appear to have a great potential for burn rate modification of solid rocket propellants. Have you any examples of work done in this field?

Author’s Name: Alba Ramaswamy

Author’s Response: We have not yet formulated the CNTs. It is in the next step/stage of analysis/research. When we can obtain data on the actual effects of CNTs on the burning rate, we can better determine all the beneficial impacts. Only testing and analysis will give the answer. These studies are still in the research and development phase.

Discusser’s Name: Hans Besser

Question: What’s the current production level of CNTs?

Author’s Name: Alba Ramaswamy

Author’s Response: Currently there are several start-up companies in the U.S. that are working on the scale-up of CNT production. The scale-up is still in the development stages so a figure for current production would be difficult to be evaluated.

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