Feature Article pubs.acs.org/JPCC
Time-Resolved Spectroscopy of Initiation and Ignition of FlashHeated Nanoparticle Energetic Materials Rusty W. Conner† and Dana D. Dlott* School of Chemical Sciences, University of Illinois at Urbana−Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801, United States ABSTRACT: Nanotechnology has brought a great deal of excitement to research in energetic materials (EMs). Nanoparticle EMs have high densities of stored energy and the potential for multifunctionality. Here we discuss research on fundamental mechanisms of initation and ignition of EM with Al or B fuel nanoparticles and TeflonAF or nitrocellulose (NC) oxidizer. Polybutadiene (PB) was also used as an inert. The thin-film samples were confined between two windows and were activated by flash-heating the metal nanoparticles with picosecond laser pulses. Reactions of isolated nanoparticles with their surroundings were studied by measuring ablation thresholds. A shock-induced polymer dissociation model was needed to explain the growth of the reaction volume surrounding a flash-heated fuel particle. Thicker oxide passivation layers confined the nanoparticle allowing the pressure to build up to higher values during flash-heating. Initiation, as the onset of chemical reactivity, was probed using time-resolved Raman or infrared to monitor disappearance of nitrato (ONO2) of NC or CF2, CF3,or CFO of TeflonAF. Ignition, the onset of energy-releasing processes, was studied by analyzing time-dependent spectra of emission bursts. In flash-heated Al/Teflon, initiation occurs in ∼50 ps and ignition occurs in ∼100 ps. In B/Teflon, ignition occurs in ∼200 ps. Transient opacity measurements of Al/Teflon suggest that chemical reactivity beyond the initial exothermic formation of AlF occurs within ∼5 ns. become comparable to chemical reaction rates,15−17 so neither chemical reaction dynamics nor heat and mass transfer may be neglected. Furthermore, many chemical reactions are interfacial, occurring where fuel and oxidizer particles contact. Another important feature of reactive nanomaterials is the ease in formulating new materials, which provides the potential of engineering EM for specific applications from the bottom up.16 Re-engineering molecular explosives using organic synthesis is difficult. In general, the first characteristic one screens for in target compounds is volumetric heat of explosion, ΔHex. In the more than 150 years since the invention of TNT (2,4,6-trinitrotoluene) with ΔHex = 7.6 kJ cm−3, thousands of candidate EMs have been synthesized and found to be not useful, usually due to too-low material density or too-high sensitivity to accidental detonation. The highest performance molecular EM in common use is HMX,18 first widely produced in the 1940s with ΔHex = 11.7 kJ cm−3. A few better molecular EMs have been developed, such as CL-2019 and octanitrocubane (ΔHex ≈ 12 kJ cm−3),20,21 but they are not yet economical. More than a century’s effort in organic synthesis has increased the energies of useful molecular EM by just 60%. There are at least three important types of reactive nanomaterials. For all three, the fuel is a late transition metal, typically Al, and the oxidizer is either a polymer or a nanoparticle of an early transition-metal oxide or an early transition metal. Examples of the three types are Al + fluorocarbon polymer22−24 or Al + epoxy; Al + MoO33,25−27 or Al + WO3;27 and Al + Ni,28 and every one of these reactions
1. INTRODUCTION This Article describes studies of the fundamental mechanisms of initation and ignition of nanotechnology energetic materials1−4 (EMs) using picosecond laser flash-heating. The introduction of nanotechnology is revolutionizing the field of EMs,5 allowing the creation of new EM with higher energy densities and multifunctionality.6 In EMs, the chemistries, while complicated, can usefully be described as fuel + oxidizer. With homogeneous molecular materials, for instance, RDX (1,3,5-trinitroperhydro-1,3,5triazine), the fuels (e.g., C and H atoms) and oxidizers (O atoms and to a lesser extent N atoms) generate energy by producing very stable, lower energy species such as CO, CO2, OH, H2O, and HCN, and fuel and oxidizer are located on the same or on adjacent molecules.7−9 Explosives with micrometersized metal particle additives are sometimes described as nonideal, and the fuels such as Al and oxidizers such as NH4NO3 or NH4ClO4, which produce aluminum oxides such as Al2O3, are located on different particles. Therefore the chemistries in molecular EM are usually dominated by processes understood in the context of chemical reaction dynamics,10−13 and diffusive heat and mass transfer are not considered. In the nonideal EM, the exothermic chemistries are usually rate-limited by heat and mass transfer,14 and the details of the chemical kinetics are ordinarily not needed to describe performance. With reactive nanomaterials, the particle size is reduced to a level where reactions in nonideal EM that might ordinarily be described as combustion become so fast that they are better described as deflagration (ultra-high-speed combustion) or thermal explosion. Understanding the reactivity of nanomaterials requires new thinking. The rates of mass and heat transfer © 2012 American Chemical Society
Received: March 30, 2012 Revised: May 12, 2012 Published: May 30, 2012 14737
dx.doi.org/10.1021/jp303077f | J. Phys. Chem. C 2012, 116, 14737−14747
The Journal of Physical Chemistry C
Feature Article
can generate energies in the 15−20 kJ cm−3 range, substantially greater than RDX. Some other intriguing possibilities for nanoparticle EM involve replacing Al fuel with B,29−31 Si,32 and C.33 The simplest way to use reactive nanomaterials is to incorporate them into conventional explosives.34 For instance, nano-Al may be added to RDX so the Al can undergo a delayed reaction with H2O produced by RDX detonation,35 or the bomb can disperse the Al as an aerosol that reacts with ambient oxygen. Nanoparticle EM in the future may involve organizing components over large length scales, for instance, with local fuel-oxidizer as repeating subunits, or structures with composition gradients to modify the dynamic performance.16,36 Many nanoparticle EMs such as epoxy composites or sintered all-metal composites can be multifunctional.6 A conventional gravity bomb, for instance, consists of an explosive charge packed into an iron or steel casing that is mostly dead weight. Energetic nanomaterials may serve as both energetic and structural elements, allowing the elimination of the steel casing. For instance Al/Teflon composites22,24,32 can be machined into projectiles with armor-piercing capabilities. Sintered all-metal composites such as Ni + Al might be used as reactive armor or reactive bomb casings. As illustrated in Figure 1, a reactive nanomaterial is composed of at least three components:37 a fuel, an oxidizer,
C13F27, whose carboxylate can bond to Al or Al2O3 and whose fluorine atoms are useful oxidizers. In our experiments, nanoenergetic materials were activated by flash-heating with picosecond laser pulses24,43 that are absorbed by the metallic cores of the fuel particles but hardly absorbed by the passivation layers or the oxidizers. EM activation may occur via several processes depending on the heating rate and the core and shell materials, for instance, liquid metal diffusing through the oxide,15,44,45 oxygen diffusing into the core,46 liquid metal leaking out through cracks in the oxide created by metal thermal expansion,38,47−49 a phase explosion of the liquid core,37 or melting or vaporizing the shell material.50−52 Any of these activations would diminish the passivation and allow the fuel to react with the oxidizer. The onset of substantial chemical reactivity is frequently called “initiation”. Most initiation processes are endothermic, for instance, the abstraction of O or F atoms from the oxidizer by fuel atoms. Exothermic reactions are subsequent, for instance, when Al joins with O or F to form aluminum oxides such as AlO or fluorides such as AlF. These nascent molecules, due to their strong chemical bonding, would form in high-lying vibrational and/or electronic excited states whose subsequent nonradiative relaxation would release energy into the surroundings. The onset of energy-releasing processes is frequently called “ignition”. Most previous spectroscopic studies of EM dynamics looked at gas-phase products or the fireball.53,54 In this Article we will describe novel spectroscopic methods to study the dynamics of flash-heated nanoenergetic materials with a focus on looking inside the EM to probe chemistries in the condensed phase and on measuring and differentiating the time scales for initiation and ignition.
2. MATERIALS AND METHODS To make an exploding EM sample where we could probe the interior and mimic the inertial confinement of the interior of a large explosive charge, we made optically thin EM films sandwiched between windows. 50 The thin films were suspensions of nanoparticles in oxidizing or inert polymers that were spin-coated from solution onto 100 cm2 optical substrates (see Figure 1).50 The homogeneity of the composite was enhanced using surfactants to disperse the nanoparticles.50,55 The films were 0.3−4 μm thick and were optically thin at the flash-heating wavelength of 1.053 μm, so the laser pulses would uniformly heat the sample interior. The nanoparticles in initial studies37,43 were oxide-passivated micrometer-sized aggregates of 50−250 nm spherical Al particles termed ALEX,56,57 created by exploding aluminum wires in an inert atmosphere.58 We subsequently used unaggregated, spherical, oxide-passivated Al or B nanoparticles with relatively narrow size distributions.38,59 The oxidizers were either highly nitrated nitrocellulose (NC) (12% N), TeflonAF, a processable fluorocarbon consisting of tetrafluoroethylene (TFE) and fluorinated dioxole units24 (hereafter simply Teflon unless the distinction is necessary), or polybutadiene (PB) which is nominally unreactive.50−52 The 1.053 μm, 100 ps flash-heating pulses were generated with a Nd:YLF laser.60 The pulse energies ranged from 10 to 350 μJ.37 The Gaussian-profile pulses were focused to 100 μm diameter (1/e2) spots. With 100 ps pulses, heating of the metallic cores of the fuel nanoparticles was approximately, but not entirely, adiabatic and isochoric.37 The samples were mounted on a motorized positioner, as depicted in Figure 1c,d,
Figure 1. (a) Nanoparticle energetic materials studied here consisted of nanoparticle fuel (Al or B) with native oxide passivation and average separation davg, embedded in the polymer oxidizers TeflonAF or nitrocellulose (NC). The stoichiometric shell with diameter dsh is an imaginary sphere containing just enough oxidizer to fully consume the fuel. (b) After flash heating, a volume with diameter drxn has reacted. Ablation threshold is where drxn ≈ davg. (c) Open-faced geometry. The sample is mounted on a motorized translation stage. (d) Sandwich geometry for confined material and confined ablation studies.
and a barrier layer to prevent fuel-oxidizer reactions. With Al or B nanoparticles, the barrier is created by oxides that passivates the fuel particles. On Al particles, the native oxide is typically a few nanometers in thickness,38−40 so Al nanoparticles are core−shell particles with a metallic core and an oxide shell. It is often noted that the native oxide is dead weight. For instance, with 30 nm particles with a 3 nm oxide, the oxide will be ∼30% of the total mass. Self-assembled monolayers have been proposed as passivation materials,41,42 for instance, CO2H− 14738
dx.doi.org/10.1021/jp303077f | J. Phys. Chem. C 2012, 116, 14737−14747
The Journal of Physical Chemistry C
Feature Article
volumes do not overlap, and a flash-heated region may have many localized reaction volumes, but it otherwise would remain intact. When davg ≥ drxn, the reaction volumes overlap and the flash-heated region is destroyed.63 In our experiments, the nanoparticle concentration is specified by the fractional stoichiometric equivalence (eq). When the sample load is 1.0 eq, the fuel-oxidizer load is balanced. Less than 1.0 eq denotes a fuel-poor sample, and greater than 1.0 eq (not used here) denotes a fuel-rich sample. For Teflon oxidizers, the chemistry can be expressed in a greatly simplified form as50,52
which exposed a fresh volume for each laser shot. About 10 ng of EM was ignited on each shot. In initial experiments and in ablation-specific studies,37,43,55,61 an unconfined sample (Figure 1c) was used, but later we developed a confinement method where a thin film of EM was sandwiched between windows (Figure 1d) compressed with a vacuum chuck.50 The confinement was not perfect, however, because gas-phase species could expand into the scratches and other surface imperfections of the windows as well as spaces created by window hydrodynamic expansion. Experiments were conducted as a function of laser fluence. Because Gaussian beams were used, only the center of the beam was imaged into the spectrograph. The fluence Jc at the center of the beam when the pulse energy was Ep and the (1/ e2) beam radius was r0 was twice the average fluence Jc = 2Javg = 2Ep/πr02. All fluence values quoted in this study are the center fluence Jc. The absorption cross sections σ of the nanoparticles used here were measured using an integrating-sphere to distinguish scattering from absorption,62 so we were able to determine the energy Enp deposited into the metallic core of each nanoparticle using the relation Enp = 2Javg σ
Al(s) + (3/2)CF2(s) → AlF3(s) + (3/2)C(s)
(2)
B(s) + (3/2)CF2(s) → BF3(g) + (3/2)C(s)
(3)
or
with volumetric heats of combustion of about 20 and 13 kJ cm−3, respectively. Accounting for the oxide passivation layer and for the oxygen content of the dioxole moieties of TeflonAF, we were able to determine the equivalence for the nanoparticle/oxidizer combinations used here.50,52 For instance, a 1.0 eq mixture of TeflonAF with 50 nm Al particles having 2 nm oxide shells results in an oxidizer-to-fuel mass ratio of 2.3:1,50 and with 62 nm Boron particles, 1.0 eq denotes a boron-toTeflon mass ratio of 5.8:1.52 Using this knowledge, we can show, for instance, that a 60 nm Al nanoparticle in TeflonAF would have an oxidizer shell diameter dsh = 90 nm.63 The average distance davg between welldispersed nanoparticles at a given concentration is given by63 davg =
(1)
3
1 + VAl ρox w/mAl
(4)
where ρox is the density of the NC (1.23 g/cm3) or Teflon (2.1 g/cm3) oxidizers, VAl is the volume fraction of Al nanoparticles in the sample, w is the weight fraction (wt %) of Al in the sample, and mAl is the mass of a single nanoparticle. In the example above with the 60 nm Al particles in Teflon, davg = 90 nm when w = 12%, corresponding to 30% eq. Several time-resolved spectroscopy methods were developed to study flash-heated EMs. Initial studies probed initiation via the disappearance of ONO2 (nitrato) stretching vibrations from ALEX/NC samples using broadband multiplex coherent antistokes Raman spectroscopy (CARS) with 30 ps time resolution.37 Time-resolved emission detected by a 2 ns photomultiplier tube established that the emission bursts that characterized ignition were a few nanoseconds or less.37 Subsequently, more sophisticated techniques were developed. Initiation was probed using a transient infrared (IR) with a broadband IR laser monitored by an IR spectrograph with a 2 × 32 channel LN2-cooled HgCdTe (MCT) array.64 Emission bursts were probed using a 15 ps streak camera and spectrograph system with visible and UV sensitivity.50−52 The growing opacity of the samples was monitored using a continuous 532 nm laser beam transmitted through the sample onto the streak camera.
Using the average nanoparticle diameter and oxide-layer thickness from the manufacturer, assuming the metal core had the density of bulk metal, we could estimate the average mass of metal per nanoparticle and then determine the absorbed energy density E v . Then, the temperature T f immediately after the flash-heating pulse could be determined if the heat capacity Cv(T) was known, Ev = ∫ TTfi Cv(T) dT. There are many issues associated with knowing this heat capacity, including finite size effects, nonadiabaticity, and ultrarapid heating effects. For a simple estimate, we assumed the flashheated metal had the same heat capacity as bulk metal heated gradually,50 ignoring radiative heat losses. Despite the simplicity of this method, the estimated errors were small enough that we could make educated guesses of the state of the nanoparticles after flash-heating, for instance, whether the metal cores had been melted or vaporized.52 Experiments were also conducted as a function of nanoparticle concentration. The reactions of flash-heated spherical nanoparticles extend outward into nominally spherical reaction volumes. To describe these reactions, we introduce three characteristic diameters, dsh, drxn, and davg. We define dsh as the diameter of the hypothetical oxidizer shell surrounding a nanoparticle, which contains just enough oxidizer to consume the fuel (Figure 1a). A fuel + oxidizer reaction would therefore consume a sphere whose diameter would, at minimum, be equal to dsh. However the reaction might spread even beyond dsh, driven by the heat or shock waves generated by the combination of laser energy and chemical reaction energy. If the average distance between nanoparticles is davg, then a low nanoparticle concentration means davg ≪ drxn, so the reaction
3. PHYSICAL EFFECTS OF FLASH-HEATING Three distinct effects of irradiation by single pulses could be identified by post mortem microscope observations.50,52 Figure 2 compares microscope images from Al/Teflon and B/Teflon at indicated values of Ev. The transients to the right of each image illustrate the time dependence of the emission burst. At the lowest fluence threshold, we saw a “bleaching” effect, 14739
dx.doi.org/10.1021/jp303077f | J. Phys. Chem. C 2012, 116, 14737−14747
The Journal of Physical Chemistry C
Feature Article
the metal core temperatures63,68 at threshold.50−52 The conclusions we drew were as follows. The bleaching threshold resulted from processes involving individual nanoparticles.69 The threshold was a bit higher in nominally unreactive PB than in Teflon, indicating that fueloxidizer reactions might play a minor role in bleaching. The threshold in Al/Teflon occurred with metallic cores in the 2300−2800K range, near what is needed to melt the Al2O3 shell with the core melted but not vaporized. In B/Teflon, the threshold occurred with metallic cores near 2350K, about what is needed to melt boron or to vaporize a B2O3 shell. The bleaching is believed to result from nanoparticle depassification and melting, followed by coagulation into larger lumps because larger metal lumps have less absorption than the original nanoparticles.62,70 In Al/Teflon and B/Teflon, ablation thresholds were concentration-dependent.55,63,68 In Al/Teflon, the ablation threshold occurred at about twice what was needed to vaporize the metal cores. Above the B/Teflon ablation threshold, when boron is heated to ∼4300 K, about its boiling point, the usual dual-peak emission burst is observed (Figure 2l), but right near threshold, B/Teflon can undergo ablation with B in the liquid state. When B/Teflon ablates from the molten boron state, its emission burst has only a single peak, as seen in Figure 2k, and this is the most significant difference between B/Teflon and Al/ Teflon ablation.52 The molten B must vaporize surrounding Teflon, and the heat from exothermic chemistries may help vaporize some of the boron.
Figure 2. Photos of samples exposed to single flash-heating pulses delivering the indicated energy densities to the metallic cores and the corresponding emission bursts, as characterized by the time dependence at 500 nm. In (a) and (g), the samples were bleached. In (b), the sample was darkened. In (c) and (i), the samples were ablated.
characterized by a loss of absorption in the irradiated spot (Figure 2a,g) accompanied by emission bursts lasting for ∼100 ps in Al/Teflon and ∼200 ps in B/Teflon (Figure 2d,j). Next, we saw a “darkening” effect that was more prominent with Al (Figure 2b) than with B (Figure 2h). Finally, laser ablation occurred, removing material from the ablated spot (Figure 2c,h,i). The ablation threshold was lower in B/Teflon than in Al/Teflon, and in fact we did not see darkening in B/Teflon without at least some ablation (Figure 2h).52 With confined samples, the confined ablation process showed dual-burst emission with the second burst lasting for a few nanoseconds (Figure 2f,l).50 One exception was close to threshold in B/ Teflon, where a bit of ablation was observed without the second emission burst (Figure 2h,k).52 High-speed photos of the flashheating-induced ablation of an unconfined Al/Teflon sample are shown in Figure 3.65 A hemispherical blast wave can be seen in the air.65−67 We studied the nanoparticle concentration dependence of the thresholds for bleaching and ablation and made estimates of
4. ABLATION: THE FIRST MICROMETER We developed a novel method to measure drxn in Al/Teflon and Al/NC based on measuring the concentration dependence of the ablation threshold.63,68 The idea is illustrated in Figure 1a,b. When drxn < davg, the reaction volumes do not coalesce significantly, so chemical reactivity is spotty and substantial mass removal does not occur.63 When drxn > davg, the reaction volumes from individual nanoparticles overlap, so the entire flash-heated volume can be reacted and ablated away. At ablation threshold, we have argued that drxn ≈ davg,63 so threshold measurements at known values of davg can be used to determine drxn. Plots of drxn versus input 100 ps duration laser energy density Ev for different size Al nanoparticles are shown in Figure 4. drxn increases linearly with Ev. In Figure 4, both the slopes and y-axis intercepts of the linear plots increase with nanoparticle diameter. When a larger
Figure 4. Reaction volume diameter drxn versus absorbed energy unit volume Al for the indicated particle sizes. The linear dependence on energy density was explained by a spherical shock-induced polymer dissociation model. The intercept is approximately dsh, as shown in Table 1. Adapted from ref 63. Copyright 2004 American Institute of Physics.
Figure 3. Time-resolved microscope images of laser ablation of Al/ Teflon (30 nm Al, 30% eq) after flash-heating. The thin hemispherical line is the blast wave in air. Reproduced with permission from ref 65. Copyright 2005 Materials Research Society. 14740
dx.doi.org/10.1021/jp303077f | J. Phys. Chem. C 2012, 116, 14737−14747
The Journal of Physical Chemistry C
Feature Article
particle is flash-heated, the reaction propagates farther than with a smaller particle because at constant Ev with a larger particle the laser deposits more energy and the reaction produces more heat.63,68 Figure 4 also shows that reactions propagate farther in Teflon than in NC. That might seem counterintuitive because NC has highly exothermic reactions whereas Teflon does not, but the NC energy release must be preceded by endothermic initiation steps, and there is no a priori reason why initiation in Teflon cannot be more facile than in NC, and if the shock duration is short, then ignition in NC might be suppressed. This is apparently the case based on studies of short-duration shock-induced chemistry in these materials. Nakamura et al.71 used nanosecond 4 GPa shock waves and found efficient depolymerization of Teflon. Moore and coworkers72,73 used 100 ps shock waves and found that NC does not initiate or ignite even at pressures of tens of gigapascals. The intercepts in Figure 4 represent a minimum distance for reaction propagation at the lowest values of Ev.63 We might expect these intercepts to be equal to dsh because at minimal Ev an Al core would react only with the material in its stoichiometric shell, but the heat from fuel-oxidizer chemistries may increase the reaction volume beyond dsh, and this effect would be larger for larger nanoparticles. Table 1 compares the
Figure 5. Reaction volume diameter drxn versus absorbed energy unit volume Al for 60 nm Al particles having thicker (6 nm) and thinner (2.5 nm) oxide layers. With picosecond pulses, drxn ∝ Ev, in agreement with a shock-induced dissociation model. Particles with the thicker oxide have greater hydrodynamic confinement during the pulses, which increases the initial shock pressure. With longer duration 10 or 25 ns pulses, the thicker oxide has little effect on drxn, and the data were fit with a thermal explosion model, where drxn ∝ Ev1/3. Adapted with permission from ref 68. Copyright 2005 Wiley-VCH.
used, each having 60 nm cores but with either a thicker 6 nm or a thinner 2.5 nm oxide shell. Laser pulses were used, having durations of 100 ps, 10 ns, or 25 ns. The 10 and 25 ns pulses were much longer than nanoparticle hydrodynamic expansion time constants, so the shock waves generated by flash heating were weak. In such cases, the expectations of thermal-explosion theory were met, drxn ∝ (Ev)1/3, and drxn did not depend on oxide layer thickness, but when 100 ps pulses were used, the pulse durations were comparable to nanoparticle hydrodynamic expansion time constants, which allowed high pressures to build up during flash-heating, and the shock dissociation linear dependence drxn ∝ (Ev) was observed. In this case drxn was significantly larger with the thicker 6 nm oxide shells. The oxide shell thickness dependence points to a physical rather than chemical explanation because the oxide is nominally chemically inert. This unusual effect of the oxide shell was explained, at least qualitatively, by the shock-induced chemistry model. With a 100 ps pulse and a 60 nm particle, flash-heating is not entirely isochoric.37 The particles in the absence of confinement by an oxide shell would have some time to expand during the laser pulse, but a thicker oxide layer will confine the hot metallic cores for a longer time, allowing greater pressures to build up before the core−shell particles explode and shock waves are generated. A closely related concept has been invoked to explain the unusually high combustion velocities observed in loosely packed mixtures of fuel and oxidizer nanoparticles by Levitas, Pantoya, and coworkers,48 where the propagation of a combustion wave occurs as rapidly heated nanoparticles explode out of their oxide shells. These experiments provided useful insight into the flashheating of individual nanoparticles in oxidizing media with the 100 ps pulses used in most of our studies. At the lower laser fluences, the Al nanoparticles were activated when they were vaporized, and the vapor could react with adjacent oxidizer. The reaction volume has a diameter drxn that is slightly larger than dsh because of the extra energy provided by the laser and the exothermic reaction, but at higher laser fluences the laser energy becomes much greater than the chemical energy, and it causes the nanoparticles to explode, launching powerful spherical shock waves. The shocks were more intense when a thicker oxide confinement layer was used. In this nanoparticle
Table 1. Al Nanoparticle and Polymer Shell Parameters (Reproduced with permission from ref 63. Copyright 2004 AIP.) particle diameter, dAl (nm)
oxide layer (nm)
diameter of NC shell, dsh (nm)
intercept from Figure 4a (nm)
diameter of Teflon shell, dsh (nm)
intercept from Figure 4b (nm)
30 62 111
2.8 6.0 5.6
46 96 185
57 ± 6 122 ± 11 240 ± 25
43 88 170
40 ± 6 110 ± 11 260 ± 25
measured intercepts to computed values of dsh.63,68 For the smallest particles (30 nm), there was excellent agreement between measured intercepts and computed values of dsh. With larger particles, the intercepts are consistently somewhat larger than dsh. The linear dependence of drxn on energy, drxn ∝ Ev, seen in Figure 4 was unexpected. Fast reactions of EM are usually described by thermal explosion models,74−77 where chemical reactions occur when a critical temperature Tc is reached. One would then expect the volume of reacted material to increase linearly with Ev, which would imply that drxn ∝ (Ev)1/3.63 To explain the linear dependence on Ev, we postulated a shockwave dissociation reaction as an alternative to a thermal explosion.63 In this process, the nanoparticles with their adjacent oxidizer explode, launching spherical shock waves whose pressure P declines as the shock propagates outward. In shock-induced dissociation reactions, the likelihood of a reaction is usually proportional to Pnτ, where P is the pressure, τ is the duration of the pressure pulse, and the exponent n is typically in the 1 to 2 range. Combining these models showed that a linear dependence of dsh on Ev would result when the value of n was near unity so that Pτ = constant.63 Further support for this picture of a shock-induced polymer dissociation reaction was found in experiments on Al/NC, where ablation thresholds were measured as a function of oxide-layer thickness and laser pulse duration.68 The results are summarized in Figure 5. Two kinds of Al nanoparticles were 14741
dx.doi.org/10.1021/jp303077f | J. Phys. Chem. C 2012, 116, 14737−14747
The Journal of Physical Chemistry C
Feature Article
would be fit by one value of T for each spectrum and a single emissivity for all spectra in the series. It took us awhile to discover the graybody nature of the emission burst spectra in Figure 6a,b. With our original emission system, we missed the rollover in the near UV. With our improved system that features achromatic collection optics and better UV sensitivity, the graybody nature emerged immediately.52 In Figure 6a,b, the temperatures used to fit the data were in the 8000K to 25 000K region, and therefore the emitting species is a dense plasma. These graybody spectra posed a conundrum, inasmuch as our thermochemical calculations showed that the laser pulses did not have enough energy to heat the fuel nanoparticles to such high temperatures. Figure 6c shows the relationship between the plasma emission temperature and Ev. At the emission burst threshold, the absorbed laser energy Ev, if uniformly distributed among the nanoparticles, would be 400 kJ mol−1 for Al and 70 kJ mol−1 for B. Our thermochemical estimates give Ev needed to vaporize Al (with a liquid oxide shell) as 416 kJ mol−1, and Ev to vaporize B2O3 (with solid B) as 71 kJ mol−1. Therefore, these high-temperature plasmas were created with barely enough Ev to boil the nanoparticles if the particles were uniformly heated. These considerations led us to realize that the plasma formation must involve mechanisms that localize heating to a small portion of the nanoparticle cores. The plasma generation mechanisms must involve a localized thermal runaway. Plasmas generated by short laser pulses have been studied extensively in the context of laser ablation, and the most likely mechanism for energy concentration is avalanche photoionization. During a laser pulse, a fluctuation causes a small number of atoms, possible just one, to ionize. The free electrons are accelerated by the intense fields and repeatedly collide with and ionize nearby atoms, producing a localized avalanche of hot electrons. The plasma resulting from this avalanche can be much hotter than the bulk metal. Therefore, avalanche ionization provides a reasonable explanation for the high temperatures associated with the plasma emission bursts.52
explosion regime, drxn can be much greater than dsh, and the reactions are not fuel + oxidizer because there is not much fuel. Instead, the reactions are shock-induced depolymerization occurring behind the shock front.
5. EMISSION BURSTS It was difficult to extract information about ignition from the emission bursts because a portion of the nanoparticles was flash-heated to unexpectedly high temperatures. We now describe the characteristics of the emission bursts, and in Section 7 we will discuss how the bursts provide information about chemical reactivity. The emission burst spectra from Al and B samples were ∼100 ps duration50−52 broad bands peaked in the near UV (Figure 6a,b), except during confined ablation where a second
Figure 6. (a) Emission burst spectra from B/Teflon and (b) from Al/ Teflon. The dashed curves are fits to a graybody model. (c) Energy density versus plasma temperature. Parts (a) and (b) are adapted from ref 52. Copyright 2012 American Chemical Society.
6. INITIATION PROBED BY TIME-RESOLVED VIBRATIONAL SPECTROSCOPY Initiation was studied using time-resolved vibrational spectroscopy to probe the disappearance of oxidizer transitions: ONO2 (nitrato) in NC37 and CF2, CF3, and CFO in TeflonAF.64 At that time, we were not using confined samples. This had no effect on the shorter delay time data, say a few hundred picoseconds, because samples were inertially confined, but on the nanosecond time scale the disappearance of oxidizer transitions may be caused by physical motion of the ablation plume as well as initiation chemistries. Our first initiation experiments used CARS to monitor nitrato groups of 2 μm thick NC with 3% eq Al nanoparticle aggregates (ALEX). Such a low fuel concentration was needed to minimize scattering of the visible beams used in CARS measurements. Figure 7 shows CARS data from samples 2 μm thick, where the ALEX concentration was 3% eq. A representative CARS spectrum shown in Figure 7 has a sharp ONO2 stretch transition near 1300 cm−1 atop a nonresonant background. The nitrato survival fraction data in Figure 7 were computed from such spectra by fitting the resonant and nonresonant parts, keeping in mind that CARS signals are proportional to [concentration].2 The nitrato disappearance occurred in two phases. The first phase lasted ∼300 ps, and the
nanosecond burst appeared (Figure 2f,l).50−52 Some superimposed narrow-band features were observed in both absorption and emission, originating from Al or B atoms and AlF.51 We did not detect BF because its transitions were too deep in the UV for our apparatus. All other things being equal, the emission bursts with the oxidizing polymers Teflon and NC were always more intense than with PB.50−52 We should note that the metal particles have a high emissivity and the (transparent) polymer matrices have quite low emissivities, so most of the emission we observed originated from metal or dense plasmas created by flashheating. The broadband emission bursts could be described by a graybody model.52 With a blackbody, the emissivity is unity, and the emission spectra are characterized by a single parameter, temperature T, which determines both the spectral distribution and the absolute intensity. In the graybody model used here, we used one additional parameter, an emissivity
