Article pubs.acs.org/JPCC
Examining Hydroxyl−Alumina Bonding toward Aluminum Nanoparticle Reactivity Richa Padhye,† Jena McCollum,† Carol Korzeniewski,‡ and Michelle L. Pantoya*,† †
Department of Mechanical Engineering and ‡Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409, United States S Supporting Information *
ABSTRACT: The stabilizing, amorphous alumina (Al2O3) passivation layer surrounding aluminum (Al) particles participates in reactions that lower barriers to bulk Al oxidation. The behavior has been observed in thermites comprised of nanoscale Al particles (nano-Al) dispersed within a fluoropolymer matrix. Studies reported herein show the oxide passivation shell on nano-Al particles is affected by the polarity and hydrogen bonding properties of the solvent employed for thermite dispersal, resulting in enhanced thermal energy propagation during Al combustion in nano-Al + poly(tetrafluoroethylene) (PTFE) mixtures. Relative to conventional treatments that employ hexane for thermite dispersal, the speed of flame front movement measured in a Bockmon Tube apparatus under steadystate conditions increased more than 2-fold following treatments in acetone or 2proponal. Differential scanning calorimetry and infrared spectroscopy measurements indicate contact with the polar solvents increases the amount and accessibility of hydroxyl species on the nano-Al oxide shell, which in turn participates in a preignition reaction (PIR) that activates PTFE and likely weakens the Al passivation layer. A molecular-scale mechanism is proposed for the PIR that derives from catalytic reactions of alumina and halocarbon fluorinating reagents. Additionally, infrared spectra show evidence for a greater fraction of disordered, liquid-like hydrogen-bonded water molecules within the alumina layer of nano-Al particles after treatment in the polar solvents studied. The O−H vibrational features suggest solvent treatment may affect the structure of nanoAl surface oxides and PIR kinetics. This study reveals potential strategies for optimizing fuel particle reactivity that include modification of the Al particle passivation shell using polar solvents to promote early preignition exothermic reaction.
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INTRODUCTION Polytetrafluoroethylene (PTFE) is commonly used as an oxidant and binder in energetic composites.1 Fluorocarbonbased polymers have a high energy density (i.e., close to 60 kJ/ g compared with TNT of 2.8 kJ/g).1,2 When combined with a metal fuel, such as aluminum (Al) or magnesium (Mg), fluorocarbon polymers are reduced through a reaction that is highly exothermic (i.e., heat of reaction >9 kJ/g)1−4 and can be optically bright. In fact Al + PTFE and Mg + PTFE compositions were originally used as infrared tracking flares in the 1950s.1 Osborne et al.5 examined the reactivity of nanoscale Al (nano-Al) combined with PTFE in argon and found that a preignition reaction (PIR) occurs around 400 °C, well below the oxidation temperature of nano-Al particles reported to be between 500 and 660 °C. This preignition reaction was attributed to surface exothermic chemistry between the alumina passivation shell surrounding the Al core particle and the decomposing PTFE. Interestingly, the PIR exists for micronscale Al particles combined with PTFE but is orders of magnitude reduced such that its contribution to the overall reactivity of micron-scale Al had not previously been considered. The increased surface area to volume ratio of nanoparticles makes the PIR significantly more pronounced © 2015 American Chemical Society
and contributes to the overall enhanced reactivity of nano-Al particles. Also, the PIR weakens the PTFE polymer and hence activates PTFE toward further energetic reactions. Two further investigations on the PIR were pursued to resolve the effect of particle surface area to volume ratio. The first, by Pantoya et al.,6 studied alumina particles combined with PTFE and varied the alumina particle size. They showed that as the alumina particle size decreased the PIR reaction shifted toward lower onset temperature and increased exothermicity. Their results indicated that the contact surface between alumina and PTFE is critical to promoting the PIR. The second study by Mulamba et al.7 employed Al particles and kept particle size constant while varying the chain length of the PTFE molecular structure, as synthesized by DuPont, Inc. They showed that longer PTFE chains produced a PIR, whereas shorter chain length PTFE molecules were more stable and less reactive with the alumina passivation shell. All of these studies were performed using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), but Mulamba et al.7 coupled their kinetic analysis to the measurement of flame Received: August 28, 2015 Revised: November 10, 2015 Published: November 17, 2015 26547
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mechanism is proposed to explain reactivity within the oxide phase and suggest further studies aimed at scrutinizing specific structural sites that have potential to enhance reaction kinetics through the preignition temperature region.
speeds as the reaction energy propagated through a powder mixture. Interestingly, they showed a direct correlation between flame speed and onset of the PIR: higher flame speeds correlated to reduced onset temperatures for the PIR.7 One important observation made by Osborne et al.5 was that when the nano-Al particles were calcined to 550 °C the magnitude of the PIR was significantly reduced. This result suggests that the PIR is linked to hydration,8 or more specifically to the density of reactive −OH groups within the alumina.8−10 In other words, the reactive −OH groups within the oxide passivation shell surrounding the core of nano-Al particles help to activate the fluorination reactions in the PIR. This realization focused our subsequent research efforts on the possible interfacial processes that can take place within the alumina passivation shell. Crystalline alumina surfaces have been extensively studied.8−14 Basic surface hydroxyl types have been identified, and by invoking understanding of aluminum and oxygen atom coordination within an aluminum oxide lattice, models for the local electronic charge and acid−base characteristics at these sites have been developed.9−11 In recent years, further insights into the properties of aluminum oxide surface hydroxyl groups, including structure within water hydration layers, have been gleaned from surface-sensitive, nonlinear vibrational spectroscopy measurements at water/aluminum oxide interfaces.15−17 Surface hydroxyl coverage and acid−base properties can play an important role in affecting the performance of aluminum oxide materials in promoting molecular adsorption or heterogeneous catalytic chemical reactions.8−10,18−20 Despite this knowledge and its application in the area of heterogeneous catalysis, a molecular-level understanding of aluminum oxides has not yet been applied in the field of energetic materials and, in particular, to the passivation shell surrounding Al fuel particles. The manufacturing of Al fuel particles involves quick evaporation of a bulk quantity when introduced to high temperatures (i.e., temperatures above 2000 °C).21−23 When these particles condense and coagulate, their spherical size is controlled in the presence of an inert gas, such as argon. Once particles solidify and are at temperatures below 440 °C, a controlled concentration of oxygen is introduced that oxidizes the surface to form a passivation shell around the pyrophoric core. This shell is amorphous. However, Levitas et al.24 found that the overall reactivity of Al particles surrounded by an amorphous shell compared with Al particles with crystalline γAl2O3 phase shells was nearly the same. For this reason, the literature on heterogeneous catalysis relevant to γ-Al2O3 is assumed to be applicable for surface hydration of the amorphous shell. The objective of this study is to assess effects of treatment by solvents containing various concentrations of water on the energetic properties of Al fuel particles. As-received Al nanoparticles, together with the PTFE oxidant, were dispersed in hexane, acetone, or 2-propanol carrier fluid solvents to aid intermixing. Following removal of the solvents, a fuel/oxidant mixture was recovered as a dry powder. The sensitivity of flame speed and differential scanning calorimetry (DSC) measurements to the water content and hydrogen bonding ability of the dispersal solvent is shown. Additionally, the O−H stretching vibrations of the hydration layer surrounding oxide-passivated Al particles could be detected by applying infrared spectroscopy. The energy released in both the preignition and the Al oxidation reactions correlated with the intensity of vibrational bands associated with the hydration layer. An atomic-scale
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EXPERIMENTAL SECTION Reagents. Aluminum particles with an average spherical diameter of 80 nm and oxide shell (ca. 2.7 nm thickness) were supplied by NovaCentrix (Austin, TX). The aluminum oxide stabilizing layer on the particles enabled the samples to be handled openly in the laboratory atmosphere. A transmission electron microscope image of the particles is included in the Supporting Information (Figure S1). Spherical poly(tetrafluoroethylene) (PTFE) particles (Zonyl MP 1100) that are 3 μm diameter were procured from Dupont (Wilmington, DE). The solvents, hexane (Fisher), isopropanol (Macron Fine Chemicals), and acetone (Mallinckrodt), were reagent grade, or better, and used without further purification. The water content of each solvent was determined by Coulombic Karl Fischer volumetric titration (Mettler-Toledo model DL36, Columbus, OH). While hexane showed negligible water, acetone and isopropanol contained 0.9 vol % and 0.1 vol %, respectively. Sample Preparation. For reactivity studies, Al and PTFE powders were combined to give a mixture of 40 wt % Al with PTFE. This composition, which corresponds to an Al:PTFE equivalence ratio of 1.5, has been shown to be optimal for producing high flame speeds.21 To ensure good dispersion, the Al and PTFE powders were suspended in solvent (1.0 g of solid in 60 mL of solvent) and sonicated at 600 W for 60 s, using a programmed cycle of 10 s on and 10 s off (Sonicator 3000, Misonix, Farmingdale, NY). Afterward, an aliquot of solution was poured into a shallow Pyrex dish that was placed in a fume hood, and the dried mixture (Al + PTFE) was reclaimed following solvent evaporation. Scanning electron microscopy (SEM) techniques were used to assess the uniformity of mixing between Al and PTFE phases in Al + PTFE samples. Figure S2 shows the dispersion of phases is typical for organic solvent dispersed Al + PTFE thermite mixtures25 and indicates differences in the physical mixing among samples should not significantly influence the variations in reactivity reported. Instrumentation. A flame tube apparatus, popularly known as a Bockmon Tube,26 was used to measure flame speeds and examine energy propagation in Al + PTFE samples. The apparatus is diagrammed in Figure 1. Combustion takes place inside the powder-filled quartz tube indicated in the upper inset photograph of Figure 1. Progress of the reaction is followed by a high-speed camera system aligned to collect images perpendicular to the direction of flame propagation. The quartz tube was 10.5 cm in length with inner and outer diameters of 3 and 8 mm, respectively. Powder (ca. 0.35 g) is loaded with care into the tube to a constant bulk density for all samples and with negligible density gradients. The tube was positioned inside a combustion chamber with the long axis in the camera focal plane. The camera is a Phantom v 7.1 (Vision Research; Wayne, NJ USA) equipped with a Nikon AF Nikkor 52 mm 1:2:8 lens. Sample ignition, triggered by a voltage pulse (15−18 V) into a nichrome wire fuse in contact with the powder, was synchronized with image collection (Vision Builder software, Vision Research). Flame front progression was measured from still-frame images captured at 5.1 × 104 fps. Thermal equilibrium analysis was performed using a Netzsch Jupiter simultaneous thermal analyzer (STA) 449 differential scanning calorimeter (DSC). Approximately 10 mg samples 26548
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Figure 1. Arrangement of instrumentation and components used in flame speed measurements. The sample is loaded into the quartz tube positioned in the combustion chamber. An enlargement of the tube showing the nichrome ignition fuse is included as an inset. Progression of the combustion reaction is captured by the lens system of a highspeed camera positioned perpendicular to the direction of flame propagation. Image collection is synchronized with the ignition voltage pulse.
were loaded into alumina crucibles and then into the STA sample holder. Each sample was heated at 10 °C/min from 45 to 600 °C in an 80 vol % Ar/20 vol % O2 environment. To ensure repeatability three experiments were performed for each sample. Infrared spectra were recorded using a Bruker Tensor (Bruker Optics; Billerica, MA, USA) Fourier transform infrared spectrometer equipped with a narrow band liquid nitrogen cooled mercury−cadmium−telluride (MCT) detector. A purge gas generator (Balston 75−52, Parker-Hannifin Corp., Haverhill, MA, USA) continuously supplied dry, CO2-free air to the instrument. Spectra were collected using an infrared transmission cell fitted with a single CaF2 window and 6 mm diameter aperture. The samples were adhered to the window.19,20 A few milligrams was placed in the center of the window, and a thin layer was spread evenly across the aperture by gentle agitation. Spectra were computed from the average of 128 interferograms recorded at 4 cm−1 resolution and apodized using a Blackman−Harris function. Back-reflection of light by sample in the optical beam path as measured by a positive baseline offset from zero absorbance through the featureless region at 2800 cm−1 was used to estimate the sample mass in the beam path.
Figure 2. (A) Representative still frame images showing flame propagation in a nano-Al/PTFE mixture following treatment in acetone. The numbers indicate time in microseconds. (B) Plot of flame front position versus time for the images shown in (A). The inset shows the average flame speed for Al + PTFE mixtures following treatment in the indicated solvents. The error bars give the 95% confidence limits based on three repetitive measurements.
samples investigated, the characteristics of the flame front evolution were similar to that depicted for the acetone-treated sample in Figure 2A, and the reported flame speeds were measured through the linear response region, as depicted in Figure 2B. The bar chart in the inset to Figure 2B shows the sensitivity of flame speed to sample treatment conditions. Typically, Al + PTFE mixtures are prepared by dispersal in a highly nonpolar solvent, such as hexane.5,7,25,26 The bar chart in Figure 2B shows flame speeds increase more than 2-fold relative to hexane treatment following processing of Al + PTFE in acetone or 2propanol. The faster flame speeds that accompany Al + PTFE dispersal in polar versus nonpolar solvents are an indication of different reaction mechanisms affecting energy propagation in the samples. To investigate possible energy pathways, experiments that interrogated the structure and possible role of the hydroxyl binding and oxide passivation shell surrounding the Al particle core were conducted through application of DSC and FTIR techniques. Figure 3 shows DSC responses over the 200−450 °C range for Al + PTFE mixtures prepared by either dry mixing or dispersal in different solvents. An endotherm arising from PTFE decomposition is present in all traces near 320 °C.5 The preignition exotherm associated with surface chemistry on nano-Al particles appears near 380 °C and is clearly evident only for the solvent-treated samples. For these samples, the specific energy (energy per gram of reactant) released during
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RESULTS Effects of solvent processing on flame speeds for Al + PTFE mixtures are demonstrated in Figure 2. The sequence of still frame images (Figure 2A) tracks the flame front following the ignition voltage pulse for a representative sample of acetonetreated Al + PTFE. Due to the low density of the Al + PTFE mixtures, the high rate of gas generation during reaction, and the limited space for gas escape in the burning environment, convection is the dominant mode of heat transfer during flame propagation.25 As a result, at early times (within the first 1−2 cm of front development) velocities vary nonlinearly with time as the combustion reaction evolves toward a steady state.25−30 The images in Figure 2A capture the flame front movement as it enters the second phase of evolution associated with steadystate propagation and characterized by a constant velocity.25−30 A plot of flame front position versus time is shown in Figure 2B. The linearity confirms the steady-state nature of the reaction beyond the first 2 cm of front movement. For all 26549
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nano-Al particles when exposed to solvents containing a few percent water since the oxide passivation shell is only a few nanometers in thickness. The PIR onset temperatures in Figure 3 and Table 1 are well above the boiling points of water and the dispersal solvents. When physisorbed to aluminum oxide, water, 2-propanol, and acetone are readily driven off at temperatures near their boiling points.33−35 However, 2-propanol34 and acetone33 are capable of forming strongly bound states on aluminum oxides. Desorption steps for strongly bound 2-propanol and acetone, as well as some dehydration steps that remove lattice −OH groups from alumina,10 take place at temperatures in the 300− 500 °C range.10,33,34 The possibility these transformations may contribute to the specific energy through the preignition region in Figure 3a was investigated using nano-Al particles conditioned in the same manner as the Al + PTFE mixtures. Figure 3b shows that the DSC traces for these aluminum samples are nearly featureless, particularly relative to the responses in Figure 3a, indicating the preignition exotherms are not likely affected by the release of strongly adsorbed solvent or solvent decomposition products, incorporated into the passivation shell of the nano-Al particles. Figure 4 shows DSC traces over the temperature range in which reactions occur between PTFE and the Al-rich core of
Figure 3. (a) DSC traces through the temperature region of the preignition reaction are shown for nano-Al/PTFE mixtures without solvent processing (untreated) and following processing in the indicated solvents (as labeled). The specific energies determined from integration of the wave near 380 °C are included on the traces for each solvent processed material. (b) DSC curves showing the relatively featureless responses for samples of aluminum particles conditioned in the same manner as the nano-Al/PTFE mixtures.
the PIR was determined from the area under the exotherm. The values are given in the figure and also in Table 1 and indicate Table 1. Values for the Onset Temperature and Enthalpy from DSC Analysis of Figures 3 and 4 pre-ignition reaction
Figure 4. DSC traces through the temperature region of the aluminum combustion reaction for nano-Al/PTFE mixtures without solvent processing (i.e., untreated) and following processing in the indicated solvents. The specific energies determined from integration of the reaction peak are included on the traces.
main reaction
treatment
onset temp. (°C)
enthalpy (J/g)
onset temp. (°C)
enthalpy (J/g)
untreated hexane isopropanol acetone
--387.3 377.4 396.9
--6.7 28.7 14.2
535.4 513.7 512.0 515.2
812 861 1292 1119
the Al nanoparticles. The onset temperature for particles treated in hexane, 2-propanol, and acetone is about the same, roughly 512−515 °C (Table 1), but for the untreated sample, the onset temperature is shifted considerably higher to 535 °C. The trends are consistent with earlier studies that showed the PIR activates the nano-Al+PTFE mixture toward reaction at lower temperatures.5,7 Although the onset temperatures are similar, the extent of reaction as indicated by the specific energy shown on the traces in Figure 4 and Table 1 increases in relation to the specific energy through the PIR range. While Mulamba et al.7 showed a direct relation between flame speed and onset temperature of the PIR (i.e., higher flame speeds correlated to reduced onset temperature for the PIR), the enthalpy for their PIR and main reaction also correlated with onset temperatures (lower onset temperature produced higher enthalpies). The results shown in Table 1 provide new information for this correlation. These results indicate that enhancing Al reactivity is achieved by polar solvents that (1) produce a PIR (regardless of onset temperature) and (2) have appreciably high enthalpies for both the PIR and main reaction. These results imply that polar solvents may facilitate more complete combustion (and enhanced reactivity) via added
the thermal energy released during the PIR is sensitive to the processing conditions and increases in relation to the polarity and hydrogen bonding ability of the dispersal solvent. The heat released is smallest for samples processed in hexane, the driest and least polar of the solvents employed, and the greatest for 2propanol, which is both polar and capable of forming hydrogen bonds to exposed sites within the alumina passivation shell covering the Al core. Additionally, 2-propanol has a tendency to absorb water when exposed to ambient atmosphere, up to about 10 vol % (0.33 mole fraction).31 Trace water present in the solvent can promote oxide layer hydration and potentially increase the density of −OH groups within the passivation shell.9,10 Lefevre and co-workers demonstrated the common γalumina structure can readily transform to phases rich in Al− OH groups when particles are dispersed in aqueous solutions.10 Phambu showed evidence for a related phase at the metal− oxide interface on nano-Al particles.32 We anticipate this type of oxide layer hydration may be both complete and fast for the 26550
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from the protective aluminum oxide shell, leaving the hydration layer within the shell dominated by more strongly bound structured water. In contrast, a greater fraction of liquid-like water is retained following dispersal of nano-Al particles in the polar solvents employed, which are also capable of hydrogen bonding with sites on the oxide lattice. It is noted however that the vibrational spectra observed for crystalline Al2O3 are assumed similar to amorphous Al2O3 forming the shell surrounding Al particles.
chemical energy liberated upon alumina shell surface reactions with fluorine. Figure 5 displays infrared spectra obtained from samples of nano-Al particles that had dried in air following solvent
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DISCUSSION The effectiveness of the PIR in accelerating combustion of Al + PTFE mixtures appears linked to molecular structure within the oxide layer surrounding the aluminum core. Disruption of the protective oxide layer is required to expose the core and initiate the energetic, flame-producing oxidation of zerovalent aluminum (Al0) atoms. The discussion below considers possible routes whereby solvent treatment can facilitate the PIR and enhance access to Al0 beneath the oxide layer. At temperatures near 350 °C, just before the onset of the PIR, PTFE becomes unstable leading to the formation of small, reactive fluorocarbon fragments.1,2,36 Since small fluorocarbon species are known to undergo reaction with aluminum oxides within the PIR temperature range (360−420 °C),18−20 it is likely fragments formed during the initial stages of PTFE thermal decomposition begin to consume the passivation layer on oxide-stabilized aluminum particles. Scheme 1 shows an
Figure 5. Transmission infrared spectra obtained from an ultrathin layer of nano-Al particles adsorbed to a CaF2 window. The nano-Al samples were dispersed in the indicated solvents and dried prior to measurements. The spectral intensities reflect a constant mass of the nano-Al particles in the beam path as estimated from the positive shift of the baseline from zero absorbance measured at 2800 cm−1 due to back-reflection of light by the particles. The inset shows the relationship between the integrated intensities of the infrared spectral bands between 2800 and 3800 cm−1 and the energy released through the PIR temperature range (Figure 3a).
Scheme 1. Transformation of a Surface Alkoxy Species Leading to Fluorination at an Octahedral (Oh) Site within an Al2O3 Lattice
treatment. The features arise from stretching of −OH groups within the oxide layer surrounding the aluminum core of the particles. The peaks are broad and characteristic of water molecules within the oxide layer interacting through hydrogen bonding forces.9,13,15,17 A main peak near 3240 cm−1 is present in all spectra. Near 3450 cm−1, a shoulder is evident particularly in spectra of nano-Al particles that had been dispersed in polar solvents. The 3450 cm−1 feature is considerably weaker for the sample that had been dispersed in hexane. The plot in the inset to Figure 5 shows that the integrated intensity of the O−H stretch feature across the 2800−3800 cm−1 range follows the trend of the specific energies through the PIR temperature range for the nano-Al samples (Figure 3). The dashed line in the plot (Figure 5, inset) extrapolates to zero PIR energy. The x-intercept value is close to the integrated intensity for the hexane-treated sample and is consistent with a base amount of oxide required to stabilize nano-Al particles in the ambient atmosphere. Insights into the chemical environment of the hydration layer surrounding solvent-treated nano-Al particles can be derived from vibrational spectra of aluminum oxide/water interfaces.15−17 Peaks near 3200 and 3450 cm−1 in sum frequency vibrational spectra have been attributed to hydrogen-bonded H2O species in environments near or within the aluminum oxide lattice, depending upon the structure of the exposed aluminum oxide surface plane.15−17 At the smooth crystalline α-Al2O3(0001) plane, a broad 3200 cm−1 peak has been ascribed to water molecules in structured, ice-like environments adjacent to the oxide surface, whereas the 3450 cm−1 feature is thought to arise from more disordered, liquid-like water environments at the interface.15 By comparison, the spectra in Figure 5 suggest that treatment of nano-Al particles in the highly nonpolar solvent hexane displaces any liquid-like water
intermediate surface alkoxy species and its reaction leading to fluorination at an octahedral site within an Al2O3 lattice with release of a carbonyl compound. Steps of this nature have been proposed to explain the action of halocarbon fluorinating agents on high surface area aluminum oxides.18 Gray and coworkers showed evidence for a nucleophilic reaction leading to octahedral aluminum fluorinated methoxy, (AlOh)−O−CF2H, type intermediates upon exposure of γ-phase-rich alumina materials to CHClF2 at 400 °C.18 Decomposition of the surface alkoxy species transfers fluorine to aluminum sites, eventually converting oxide to AlF3.18 We anticipate that processes analogous to those in Scheme 1, and discussed in refs 18−20, play a role in the early stages of nano-Al+PTFE combustion and contribute to the energy release through the PIR region in DSC measurements (Figure 3). The reactions not only can increase accessibility of Al0 sites essential for the highly energetic phase of nano-Al+PTFE combustion (Figures 2 and 4) but also activate the fluorocarbon by producing fragments and oxygenated byproducts that are expected to have lower barriers to reaction relative to PTFE. Scheme 1 provides a model for possible chemical steps associated with the PIR. The PIR in turn has been shown to accelerate energy release during nano-Al+PTFE combustion as measured in flame-speed experiments.7 Studies that examine 26551
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The Journal of Physical Chemistry C the link between the PIR and Al0 combustion are in progress. However, we anticipate conversion of the oxide passivation shell and initiation of fluorocarbon fragmentation at PIR temperatures likely play a key role in enhancing the accessibility of reactive fluorocarbon fragments to Al0 sites. The results in Figures 2−5 show the PIR and flame speeds are sensitive to the nature of the solvents employed for thermite dispersal. For the liquid solvents studied, the native oxide stabilizing the aluminum particles can retain the contacting solvent to varying extents through the interplay of dispersion, dipole−dipole, and hydrogen bonding forces. Since aluminum oxides are hydrophilic and easily wet by polar solvents, 2-propanol and acetone are expected to be more strongly retained than hexane by the aluminum oxide protective layer. This more thorough wetting of the oxide layer by polar solvents can have a few different effects leading to enhancement of the PIR. First, the polar solvents may purge contaminants, such as airborne species retained by the native oxide during storage, from the framework and thereby increase the availability of sites in the lattice for reaction (i.e., via Scheme 1). Second, residual water in the polar solvents may penetrate the framework and gain access to Al0 sites at the Al0/oxide interface, leading to further growth and increased coverage of the oxide phase. Third, water and polar solvents can be retained by aluminum oxides, even after air-drying, due to the strength of dipolar and hydrogen bonding forces.33−35 Although the physisorbed forms of these species are driven off at temperatures near their boiling points,33−35 movement of the vapor molecules through disordered regions of the framework has the potential to disrupt structure and increase the density of the most highly reactive oxide sites (vide inf ra). Finally, 2-propanol and acetone have been shown to dissociate over aluminas, leaving fragments that have desorption temperatures in the PIR range.33,34 For nano-Al samples dispersed in 2-propanol, or acetone, desorbing fragments formed from solvent decomposition over the protective aluminum oxide layer can contribute to the energy release at PIR temperatures and, similar to physisorbed solvents, may alter oxide structure as the vapors transport within highly roughened or disordered regions. In addition to wetting the surface of aluminum oxides, solvents can change the catalytic properties by modifying the density and acid/base characteristics of surface hydroxyl groups.9 Insights into these properties can be gained from infrared spectroscopy measurements that probe the surface hydroxyl O−H stretching vibrations. The transitions are observed as narrow peaks in the 3700−3800 cm−1 range. The bands have been associated with particular Al3+ (i.e., octahedral versus tetrahedral) and oxygen atom (i.e., terminal versus bridging) coordination geometries and correlated with local acidity and charge.9,11−14,32 In Figure 5, the bands detected arise mainly from O−H stretching vibrations of water molecules retained by the aluminum oxide layer. The water molecules interact through hydrogen bonding forces with exposed lattice O and OH groups. As a result of the hydrogen bonding interactions, the O−H stretching vibrations of the aluminum oxide hydroxyl groups shift to lower energies and overlap the region of the water O−H stretches,8,13,14 obscuring details related to surface hydroxyl group coordination. However, water hydrating aluminum oxides can be removed by chemical treatments33 or thermal treatments above about 400 °C.10,33,34 With the removal of water, the spectral features of surface hydroxyl groups appear in the 3700−3800 cm−1
region. An objective of ongoing work is to probe the protective oxide layer on nano-Al particles at temperatures above ambient, and near the PIR, with the aim of desorbing hydrating water molecules from the oxide and probing the properties of hydroxyl groups bonded to the lattice. The insights into the acidity and charge of hydroxyl groups within the protective oxide layer on nano-Al particles will be applied to identify thermite conditioning strategies that achieve optimal flame speeds. The possibility will be explored of tuning the polarity and hydrogen bonding characteristics of the Al + PTFE dispersal solvent to create aluminum oxide functionality that enhances the rate of the fluorination reactions (i.e., Scheme 1)18−20 and coupling of the PIR to the main energy-releasing pathways. Further, it is anticipated that conditioning of Al particles in polar or hydrogen bonding solvents will affect reactions of the particles with other halogenated reagents. For example, in mixtures of nano-Al and iodine pentoxide (I2O5) (Al + I2O5), we have observed a PIR that likely arises from reaction of the Al surface oxide layer with traces of −IO or I2 fragments in the matrix.37 Solvent conditioning of the surface hydroxyl coverage and coordination geometries on the Al particles may lower barriers for Al + I2O5 reactivity by facilitating the PIR. For mixtures of Al particles and electron acceptor reagents that do not exhibit a PIR with the Al particle passivation shell (i.e., metal oxides, such as copper oxide and molybdenum trioxide), solvent conditioning of the Al particles is not anticipated to affect Al combustion.
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CONCLUSIONS The PIR that occurs near 350 °C in advance of bulk Al oxidation in Al + PTFE mixtures likely results from attack of PTFE thermal decomposition fragments on the oxide passivation layer surrounding the nano-Al particles. Fluorination of the oxide exothermically contributes to the overall reaction and further activates PTFE toward reaction with Al0. These chemical processes occurring at PIR temperatures lower the onset temperature for the bulk Al oxidation reaction (to temperatures of 512−515 °C), produce greater PIR and main reaction enthalpies, and increase the flame speed. The observation of greater than 2-fold enhancement in flame speed following dispersal of Al + PTFE mixtures in acetone or 2-proponal compared to conventional treatments in hexane likely can be attributed to conditioning of the oxide layer through polar and hydrogen bonding interactions with the solvents. The ability of the solvents to affect structure within the oxide passivation shell, and coordination and acid/base characteristics at surface hydroxyl groups, is under investigation with the aim of developing solvent treatments that enhance the reactivity of nano-Al fuel particles.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b08408. A transmission electron microscope (TEM) image of nanoaluminum (nano-Al) particles employed in the current studies is shown in Figure S1. The particles had an average spherical diameter of 80 nm and were stabilized by an aluminum oxide shell (ca. 2.7 nm thickness). Figure S2 shows SEM images of Al + PTFE samples following solvent processing. The images show 26552
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(18) Chupas, P. J.; Grey, C. P. Surface modification of fluorinated aluminas: Application of solid state NMR spectroscopy to the study of acidity and surface structure. J. Catal. 2004, 224, 69−79. (19) Vaynberg, J.; Ng, L. M. Surface chemistry of fluoroethanols - I. A FTIR study of the reaction of 2,2-difluoroethanol on Al2O3 surface. Surf. Sci. 2005, 577, 175−187. (20) Vaynberg, J.; Ng, L. M. Surface chemistry of fluoroethanols - II. A FTIR study of the reaction of 2,2,2-trifluoroethanol on Al2O3 surface. Surf. Sci. 2005, 577, 188−199. (21) Zheng, B.; Lin, Y.; Zhou, Y.; Lavernia, E. J. Gas atomization of amorphous aluminum: Part I. Thermal behavior calculations. Metall. Mater. Trans. B 2009, 40, 768−778. (22) Ö zbilen, S.; Ü nal, A.; Sheppard, T. Influence of atomizing gases on the oxide-film morphology and thickness of aluminum powders. Oxid. Met. 2000, 53, 1−23. (23) Panda, S.; Pratsinis, S. Modeling the synthesis of aluminum particles by evaporation-condensation in an aerosol flow reactor. Nanostruct. Mater. 1995, 5, 755−767. (24) Levitas, V. I.; Pantoya, M. L.; Chauhan, G.; Rivero, I. Effect of the alumina shell on the melting temperature depression for aluminum nanoparticles. J. Phys. Chem. C 2009, 113, 14088−14096. (25) Watson, K. W.; Pantoya, M. L.; Levitas, V. I. Fast reactions with nano- and micrometer aluminum: A study on oxidation versus fluorination Combust. Combust. Flame 2008, 155, 619−634. (26) Bockmon, B.; Pantoya, M.; Son, S.; Asay, B.; Mang, J. Combustion velocities and propagation mechanisms of metastable interstitial composites. J. Appl. Phys. 2005, 98, 064903−064907. (27) Kappagantula, K. S.; Farley, C.; Pantoya, M. L.; Horn, J. Tuning energetic material reactivity using surface functionalization of aluminum fuels. J. Phys. Chem. C 2012, 116, 24469−24475. (28) Dikici, B.; Dean, S. W.; Pantoya, M. L.; Levitas, V. I.; Jouet, R. J. Influence of aluminum passivation on the reaction mechanism: Flame propagation studies. Energy Fuels 2009, 23, 4231−4235. (29) Yarrington, C. D.; Son, S. F.; Foley, T. J.; Obrey, S. J.; Pacheco, A. N. Nano aluminum energetics: The effect of synthesis method on morphology and combustion performance. Propellants, Explos., Pyrotech. 2011, 36, 551−557. (30) Dutro, G. M.; Yetter, R. A.; Risha, G. A.; Son, S. F. The effect of stoichiometry on the combustion behavior of a nanoscale Al/MoO(3) thermite. Proc. Combust. Inst. 2009, 32, 1921−1928. (31) CRC Handbook of Chemistry and Physics (Internet Version 2016), 96th ed.; Haynes, W. M., Ed.; CRC Press: Taylor and Francis Group: Boca Raton, FL. (32) Phambu, N. Characterization of aluminum hydroxide thin film on metallic aluminum powder. Mater. Lett. 2003, 57, 2907−2913. (33) Zaki, M. I.; Hasan, M. A.; Al-Sagheer, F. A.; Pasupulety, L. Surface chemistry of acetone on metal oxides: IR observation of acetone adsorption and consequent surface reactions on silica-alumina versus silica and alumina. Langmuir 2000, 16, 430−436. (34) Hamad, M. Conversion of isopropanol over treated CuO supported on gamma-alumina. J. Appl. Sci. Res. 2010, 6, 1247−1264. (35) Men, Y.; Gnaser, H.; Ziegler, C. Adsorption/desorption studies on nanocrystalline alumina surfaces. Anal. Bioanal. Chem. 2003, 375, 912−916. (36) Drobny, J. G. Technology of Fluoropolymers, 2nd ed.; CRC Press: Boca Raton, FL, 2009. (37) Farley, C.; Pantoya, M. L. Reaction kinetics of nanometric aluminum and iodine pentoxide. J. Therm. Anal. Calorim. 2010, 102, 609−613.
uniformity of mixing between Al and PTFE phases in Al + PTFE samples (PDF)
AUTHOR INFORMATION
Corresponding Author
*Phone: 806-834-3733. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are thankful for support from the Army Research Office grant no. W911NF-14-1-0250 and equipment grant W911NF-14-10417 and encouragement from our program manager, Dr. Ralph Anthenien.
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REFERENCES
(1) Crouse, C. Fluorinated polymers as oxidizers for energetic composites In Advances in Fluorine-Containing Polymers; Smith, D. W., Jr., Iacono, S. T., Boday, D. J., Kettwich, S. C., Ed.; American Chemical Society: Washington, DC, 2012; pp 127−140. (2) Koch, E. C. Metal-Fluorocarbon Based Energetic Materials; WileyVCH: Weinheim, 2012. (3) Kuehnel, M. F.; Lentz, D.; Braun, T. Synthesis of fluorinated building blocks by transition-metal-mediated hydrodefluorination reactions. Angew. Chem., Int. Ed. 2013, 52, 3328−3348. (4) Kiplinger, J. L.; Richmond, T. G.; Osterberg, C. E. Activation of carbon-fluorine bonds by metal complexes. Chem. Rev. 1994, 94, 373− 431. (5) Osborne, D. T.; Pantoya, M. L. Effect of al particle size on the thermal degradation of Al/Teflon mixtures Combust. Combust. Sci. Technol. 2007, 179, 1467−1480. (6) Pantoya, M. L.; Dean, S. W. The influence of alumina passivation on nano-Al/Teflon reactions. Thermochim. Acta 2009, 493, 109−110. (7) Mulamba, O.; Pantoya, M. L. Exothermic surface chemistry on aluminum particles promoting reactivity. Appl. Surf. Sci. 2014, 315, 90−94. (8) Knözinger, H.; Ratnasamy, P. Catalytic aluminas: surface models and characterization of surface sites. Catal. Rev.: Sci. Eng. 1978, 17, 31−70. (9) Morterra, C.; Magnacca, G. A case study: Surface chemistry and surface structure of catalytic aluminas, as studied by vibrational spectroscopy of adsorbed species. Catal. Today 1996, 27, 497−532. (10) Lefevre, G.; Duc, M.; Lepeut, P.; Caplain, R.; Fedoroff, M. Hydration of gamma-alumina in water and its effects on surface reactivity. Langmuir 2002, 18, 7530−7537. (11) Shirai, T.; Watanabe, H.; Fuji, M.; Takahashi, M. Structural properties and surface characteristics on aluminum oxide powders. Ceramics Res. Lab 2009, 9, 23−31. (12) Contescu, C.; Jagiello, J.; Schwarz, J. A. Hetrogeneity of proton binding-sites at the oxide solution interface. Langmuir 1993, 9, 1754− 1765. (13) Peri, J. B.; Hannan, R. B. Surface hydroxyl groups on gammaalumina. J. Phys. Chem. 1960, 64, 1526−1530. (14) Peri, J. A model for the surface of gamma-alumina. J. Phys. Chem. 1965, 69, 220−230. (15) Zhang, L.; Tian, C.; Waychunas, G. A.; Shen, Y. R. Structures and charging of alpha-alumina (0001)/water interfaces studied by sum-frequency vibrational spectroscopy. J. Am. Chem. Soc. 2008, 130, 7686−7694. (16) Sung, J.; Zhang, L.; Tian, C.; Waychunas, G. A.; Shen, Y. R. Surface structure of protonated R-sapphire (11̅02) studied by sumfrequency vibrational spectroscopy. J. Am. Chem. Soc. 2011, 133, 3846−3853. (17) Sung, J.; Zhang, L.; Tian, C.; Shen, Y. R.; Waychunas, G. A. Effect of pH on the water/alpha-Al2O3 (11̅02) interface structure studied by sum-frequency vibrational spectroscopy. J. Phys. Chem. C 2011, 115, 13887−13893. 26553
DOI: 10.1021/acs.jpcc.5b08408 J. Phys. Chem. C 2015, 119, 26547−26553
