Article pubs.acs.org/JPCC
Comparing Boron and Aluminum Nanoparticle Combustion in Teflon Using Ultrafast Emission Spectroscopy Rusty W. Conner and Dana D. Dlott* School of Chemical Sciences, University of Illinois, 600 South Mathews Avenue, Box 76-6 MC-712, Urbana, Illinois 61801-3364, United States ABSTRACT: Boron and aluminum nanoparticle combustion in TeflonAF was studied using time-resolved emission spectroscopy after laser flash-heating. When a threshold energy density was reached by the nanoparticles, a picosecond emission burst was observed. This was attributed to a dense metal plasma with an emission profile that fit well to a graybody model. Thermodynamics showed that this plasma only represented a small fraction of the nanoparticle mass. Depassivation of the boron particles occurred when the oxide shell boiled off and the metal core was melted. Depassivation of aluminum occurred when the oxide shell melted and the metal core was almost vaporized. The lifetimes for exothermic chemistries could be determined from their effects on the plasma lifetime. The rate of energy release from vapor Al + Teflon reactions has a ∼100 ps lifetime, while energy release from liquid B + Teflon was observed to occur with a ∼200 ps lifetime.
1. INTRODUCTION In this study, we investigate the combustion of boron nanoparticles with a Teflon oxidizer. Results are compared to previous measurements on aluminum nanoparticles in Teflon.1−5 The materials, consisting of fuel nanoparticles embedded in a polymer oxidizer, are flash-heated using ∼100 ps laser pulses, and chemical reaction dynamics are probed by time-resolved optical emission spectroscopy.4,5 Greatly simplified, the reactions of these materials can be expressed as
Al(s) + (3/2)CF2(s) → AlF3(s) + (3/2)C(s)
In practice though, the full energy potential of boron has yet to be realized. This is largely due to the formation of HBO2, a stable intermediate that bypasses the reaction pathway leading to the formation of B2O3 and maximum energy release. When fluorine is added to the combustion environment, the intermediate BFO becomes the dominant species at combustion temperatures, even in hydrogenated atmospheres.17 BFO will then undergo further reaction to make the desired B2O3. Beyond their energetics, the aluminum and boron systems have notable differences that may affect their reaction dynamics. Table 1 lists the phase transition temperatures for
(1)
and
B(s) + (3/2)CF2(s) → BF3(g) + (3/2)C(s)
Table 1. Phase Transition Temperatures of Metal Fuels, Oxides, and Reaction Productsa
(2) −3
with volumetric heats of combustion of about 20 kJ cm and 13 kJ cm−3, respectively. For comparison, the heat of detonation of TNT is about 7.6 kJ cm−3.6,7 Interest in Al nanopowders has grown recently with their increased availability.8,9 With the greatly enhanced surface area associated with nanoparticle fuels, these materials can react much faster than conventional formulations containing micrometer-sized metal particles.10−14 Al or B nanopowders can be added to explosives such as RDX to induce further reactions with detonation products such as CO2 and H2O or even with ambient O2 to create an explosive where additional energy is released over multiple time scales. Nanoparticles can be added to an oxidizing polymer, such as Teflon or epoxy, to create a reactive material, a material that combines structural integrity with explosive power.15,16 Boron is isoelectronic with Al and has a high heat of combustion. The smaller atomic weight and lower density of boron makes it attractive for applications such as rocket propellants, where being lightweight is paramount.17−21 © 2011 American Chemical Society
Al Al2O3 AlF3 B B2O3 BF3 a
mp (K)
bp (K)
933 2327
2792 3270 1276sp
2348 723 146
4273 2133 173
mp = melting point, bp = boiling point, sp = sublimation point.
the metals, their oxide layers, and expected final combustion products.22 Keep in mind these values are for pure materials not in nanometric form but are still useful as a framework for discussion. Boron has a much higher melting and boiling point than aluminum, and the aluminum oxide melts at a much higher temperature than boron oxide. According to Table 1, if we heat aluminum particles, the metal cores melt first. Then the Received: October 14, 2011 Revised: December 7, 2011 Published: December 28, 2011 2751
dx.doi.org/10.1021/jp209912t | J. Phys. Chem. C 2012, 116, 2751−2760
The Journal of Physical Chemistry C
Article
oxide layer will melt, which is followed by the metal vaporization. For boron, the oxide melts first and then boils off, leaving the boron core to melt and finally vaporize. Aluminum fluoride (AlF3) will condense at 1276 K, but boron fluorination leads to products that are gaseous under ambient conditions, creating the potential for more explosive work. In our experiments, we use TeflonAF, a copolymer of Teflon (polytetrafluorethylene) with fluorinated dioxole units added to improve processability. Unlike Teflon, TeflonAF has some oxygen available in addition to fluorine, and the fluorine/ oxygen ratio is 7:1. Previous experiments using femtosecond mid-IR transient absorption techniques showed that the first chemical bonds to break in Al/Teflon after flash-heating were in the CFO regions of the polymer,3 presumably caused by Al atoms attacking the C−F and C−O bonds. The onset of widespread chemical reactivity in energetic materials is referred to as initiation, and initiation in flash-heated Al/Teflon occurred in about 50 ps. There was also a subsequent 700 ps decrease in intensity for all vibrational bands, which has since been attributed to laser ablation, resulting in structural breakdown of the polymer and motion of material out of the probed volume.5 Since then, we have focused on the optical emission of flashheated Al/TeflonAF.4,5 Two distinct dynamic behaviors were observed, depending on whether the samples were flash-heated above or below the ablation threshold. Figure 1 is a schematic
matrix, the emission burst was more intense and persisted longer. The additional intensity and extended duration of the emission burst was attributed to energy-releasing chemistries. The onset of energy-releasing chemistries is termed ignition. The additional duration of the emission burst can be used to determine the ignition time constant. For Al/Teflon, the energy release from condensed-phase reactions occurs in ∼100 ps.4,5 Above the ablation threshold, the polymer structure breaks down, and the expanding material can fill any gaps left by imperfect confinement by the windows (see Figure 1c). In this case we observe a second emission component with a delayed rise due to confined ablation.4,5 For Al/Teflon, the rise of the second emission burst occurs over ∼0.5−1.0 ns before decaying in a few nanoseconds. The second emission burst has both a BB component and narrowband (NB) features attributed to atomic Al and molecular AlF transitions. The atomic Al features were observed as both emission bands and absorption bands, and AlF was observed as an absorption band.4,5 The absorption bands were observed against the backlighting provided by the BB emission. The AlF reaction product appeared after it escaped the solid material ∼200 ps after flash-heating. To ensure an even comparison between the Al and B materials, we have matched the following conditions. Every sample had approximately the same quantity of polymer oxidizer, to fabricate films of comparable thicknesses. The thickness, ∼300 nm, was chosen to be much greater than the mean nanoparticle diameter yet be optically thin at the flashheating wavelength to produce uniform heating throughout the films. Even at stoichiometric equivalence, the thin film samples absorbed no more than 20% of the flash-heating pulses. The Al and B nanoparticles have similar sizes (Table 2). We compare Table 2. Nanoparticle Properties mean diameter (nm) oxide layer thickness (nm) absorption cross-section at 1053 nm (cm2)
aluminum
boron
50 2 8.8 (× 10−12)
62 3 8.1 (× 10−12)
Al and B samples not with similar fuel mass loads, but with similar stoichiometric equivalences. A fractional equivalence (eq) of 1.0 means the quantity of added metal fuel is theoretically just enough to consume all of the oxidizing species. This means a 1.0 eq mixture of TeflonAF and metal fuel will have an oxidizer-to-fuel mass ratio of 2.3:1 for Al and 5.75:1 for B. Since the core−shell nanoparticles have somewhat different absorption cross sections23,24 (Table 2) and different crosssectional areas, we compare measurements made not at equal values of incident laser fluence J but instead at equivalent values of energy absorbed per mole of metal atoms. The average laser fluence Javg = Ep/πr02, where Ep is the energy of the laser pulses and r0 is the (1/e2) radius of the Gaussian laser beam. The fluence at the beam center Jc is 2Javg, and the detection system was arranged to observe sample material only at the beam center. In this case, the absorbed energy density per nanoparticle Enp is given by23
Figure 1. Schematic of flash-heating process and sample confinement. (a) The flash-heating laser pulses are incident on the side where emission is collected. The pulses are absorbed by Al or B fuel nanoparticles but not by the unreactive polybutadiene (PB) or strongly oxidizing Teflon polymer matrix. (b) At lower fluences, reactions between the flash-heated nanoparticles and the polymer occur at roughly condensed-phase densities. (c) At higher fluences, a confined ablation process occurs where the ablation plume expands into a larger volume due to imperfect confinement.
representing those processes. When flash-heated to near the Al vaporization temperature yet below the ablation threshold of the Al/Teflon composite, the aluminum nanoparticles react with their immediate surroundings but leave the polymer mostly intact (Figure 1b). A broadband (BB) emission burst is generated that decays in picoseconds. When the polymer was nominally unreactive polybutadiene (PB), the emission burst lifetime was ∼40 ps. We say “nominally unreactive PB”, but it is certainly possible that hot Al or B atoms could react with a hydrocarbon matrix. However, in the strongly oxidizing Teflon
Enp = JC σ
(3)
where σ is the absorption cross-section, which is slightly different for Teflon and PB.23 Using the average nanoparticle size and the oxide thickness and assuming the core has the density of bulk metal, we can estimate the average mass per 2752
dx.doi.org/10.1021/jp209912t | J. Phys. Chem. C 2012, 116, 2751−2760
The Journal of Physical Chemistry C
Article
equipped with a video imaging system. The thresholds for the bleach and ablation processes were determined using a previously described method for measuring threshold fluences when Gaussian radial profile pulses are used. With Gaussian pulses, the sample is exposed to a range of fluences, and the size of a spot for a threshold process increases with increasing laser fluence as the edges of the laser beam rise above the threshold. In our analysis method, the exposed area was plotted as a function of the log of the pulse energy.27−29 The intercept with the x-axis, which in principle yields the energy needed to expose a spot of zero area, gives the threshold energy density or fluence.
nanoparticle, which allows us to determine the absorbed energy density per mole. This energy density ranged from about 100 kJ mol−1 to a few thousand kJ mol−1.
2. EXPERIMENTAL METHODS The combustion dynamics of reactive materials were studied using laser flash-heating and an ultrafast UV/visible detector. Samples were composite thin films of metal nanoparticle fuels embedded in nominally unreactive PB or strongly oxidizing TeflonAF. The nanoparticles were 50 nm aluminum (Novacentrix, Inc.) and 62 nm boron (SB Boron). The procedure to make uniform films was described in detail previously.4 Briefly, the nanoparticles were suspended, using a siloxane surfactant25 to reduce aggregation, in a solvent that also dissolves the polymer. This suspension was then spincoated onto 1/4 in. thick quartz substrates to create a uniform thin film of ∼300 nm thickness. The samples were confined to maximize the time period for reactions to occur in a condensed-phase environment. We sandwiched each thin film between its quartz substrate and a second glass or quartz substrate and compressed the sandwich with a vacuum (Figure 1a). A schematic of the vacuum cell has been published previously.4,5 However, the windows imperfectly confine the samples on the nanoscale. This is due to the surface roughness of the quartz and the lumpiness of the samples themselves due to the fuel nanoparticles (Figure 1b). Even if the polymer does a good job of filling the scratches when spun onto the first quartz substrate, this will not be the case for the second substrate, which was applied after the polymer had dried. The sample films were rastered across the flash-heating laser beam to expose a fresh region of material on each shot. For a Nd:YLF laser with mode locking, Q-switching, and cavity dumping26 producing Gaussian profile 1.053 μm, 120 ps pulses could pump up to ∼350 μJ into a 180 μm spot (1/e2 diameter) at the sample. Upon flash-heating, a visible burst of emission was collected from the side being pumped and imaged onto a spectrograph (Oriel MS-257) and streak-camera detector (Hamamatsu C4334-01). The vertical slit of the spectrograph and the horizontal slit of the streak camera defined a 100 μm × 70 μm region, so that only sample material in the center ∼100 μm region of the 180 μm diameter pump beam was observed. In this study we detected the emission from the same side the flash-heating pulses entered, a major improvement over our previous work. When the emission had to pass through the sample before reaching the detector, the UV part was strongly scattered, distorting the spectrum. In that arrangement, we were also limited to lower nanoparticle fuel loads due to the nanoparticle scattering and absorption. This fact limited us to equivalence ratios 4000 K), which also generates more light in Teflon than in PB. Al vaporizes (
