Understanding Limits of the Thermal Mechanism of Laser Initiation of

Oct 26, 2012 - Laser initiation of energetic materials has a potential to shift a paradigm in the development of novel explosive technologies with a s...
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Understanding Limits of the Thermal Mechanism of Laser Initiation of Energetic Materials Edward D. Aluker,† Alexander G. Krechetov,† Anatoliy Y. Mitrofanov,† Anton S. Zverev,† and Maija M. Kuklja*,‡ †

Department of Physical Chemistry, Kemerovo State University, Kemerovo 650043, Russia Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, United States

‡

ABSTRACT: Laser initiation of energetic materials has a potential to shift a paradigm in the development of novel explosive technologies with a stunningly broad range of applications. Once interactions of laser irradiation with energetic materials are better understood, dramatically improved safe explosive devices, intricate high-precision tools in micromedicine, miniaturized cutting and drilling tools, synthesis of new promising materials with tailored properties, and fundamentally new concepts of converting energy become possible. We consider the interplay between optical and thermal energies, analyze the applicability of the thermal mechanism of initiation, and estimate the limits of its efficiency in the process of laser initiation of energetic materials. We propose a simple demonstration of a feasibility of nonthermal selective photoinitiation while challenging the widely popular perception of the thermal nature of laser initiation.

I. INTRODUCTION Laser initiation of energetic materials opens up intriguing prospects in the development of novel explosive technologies ranging from the emerging nanotechnology, which promises multifunctionality of materials,1,2 to the vastly improved safety of explosive devices,3−5 to intricate high-precision tools in micromedicine,6 and to the synthesis of new promising materials with tailored properties.7 Forecasts of these exciting applications coupled with the underutilized potential of energetic materials drive the ever-increasing interest in laser initiation studies.1−6,8−13 However, fundamental understanding of mechanisms of laser initiation is far from complete, and ways to control chemical reactions in energetic materials with laser excitation are yet to be established. Illustratively, one appealing way to solve safety issues of explosive devices is related to the development of economical miniature laser detonators, which is currently hampered by the lack of suitable energetic materials and systems. The ideal energetic material has to satisfy contradictory requirements. On one hand, the material should exhibit a relatively high sensitivity (i.e., a low initiation threshold) to laser irradiation to ensure that the used source of initiating pulses, say a low-power laser diode, is fairly small and affordable. On the other hand, the same energetic material should have a low sensitivity (i.e., a high initiation threshold) to unintended external perturbations such as impact, shock, spark, or friction to provide sufficient and reliable safety of the material’s use and handling. Existing primary explosives that satisfy the photosensitivity requirements are also too sensitive to other perturbations and therefore do not satisfy the safety requirements. Most known secondary explosives that comply with the safety demands do not exhibit necessary photosensitivity and hence cannot be initiated by the laser excitation. © 2012 American Chemical Society

Over the years, there has been a great deal of effort to decrease the sensitivity of energetic materials. An alternatively compelling approach is to develop novel methods and techniques to selectively increase the photosensitivity of secondary explosives without changing the materials’ response to other possible accidental perturbations. This way, it would be possible to control the earliest stages of initiation of explosive decomposition chemical reactions in materials to a high degree, which would facilitate the emerging new technologies based on laser initiation of energetic materials. For the time being, there are two main concurrent research directions aimed at resolving this problem: a traditional thermochemical approach4 and a relatively new and much less popular photochemical approach.8,9,13−15 The thermochemical approach is essentially based on classical ideas of “hot spots” introduced by Bowden and Yoffe.16 It is believed that the laser excitation heats the light-absorbing inclusions, which, in turn, heat the surrounding explosive material and initiate a thermal chemical decomposition reaction.4,17 In accordance with the photochemical approach, the optical absorption of laser excitation by explosive molecules or lightsusceptible additives leads to the formation of free radicals that trigger a chain decomposition reaction.15,18 A series of recent investigations raise a compelling case to demonstrate the possibility of an athermal mechanism of laser initiation. A study of Nd:YAG laser excitation of cyclotrimethylenetriamine (RDX) samples under pressures up to 5.0 GPa and laser fluences between 1.0 and 10 J/cm2 buttressed the idea that electronic excitations play a critical role in initiation and Received: August 30, 2012 Revised: October 23, 2012 Published: October 26, 2012 24482

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Hcæ = cρTc

suggested that the observed optical absorption was associated with crystallographic defects, vacancies and dislocations.19 An observed selective photoinitiation of pentaerythritol tetranitrate (PETN) was attributed to generation of active radicals by direct photoexcitation, which provided a propagation of the explosive decomposition, probably by way of a chain reaction.13,18 [The term “selective” is not accidental here and reflects the fact that the wavelength of the initiating pulse is not important in the case of regular (nonselective) initiation and only the intensity of the pulse plays a role. In the case of selective initiation, the wavelength plays a crucial role as it has to fall into the narrow band in which the material can absorb (see more details in ref 18).] Note that the light-scattering additives, such as MgO, BaSO4, Al, and Ag, used in the experiments allowed for a considerable reduction of the laser initiation threshold.18 One can argue that both approaches present certain challenges in describing the initiation of detonation phenomena in energetics. Assuming that the laser initiation is activated by a thermal mechanism,3−5,10−12 it is challenging to consistently explain why numerous attempts to reduce the laser initiation threshold by introducing various forms of carbon, the classical light-absorbing additives, have fallen short in yielding desirable results thus far.5,11,20 A proposition that the laser initiation triggers chemistry by athermal optical excitation faces challenges in explaining how a fairly low energy of laser irradiation (1.17 eV)13,18 is able to excite secondary explosives, which are typically wide-gap dielectrics.18,19 In this work, we analyze the applicability of the thermal mechanism of initiation and estimate the limits of its efficiency in the process of laser initiation of energetic materials. We propose a simple demonstration of the possibility of nonthermal selective photoinitiation and dispute the widely popular opinion of the thermal nature of laser initiation.

(1)

where Hc is the initiation threshold (the deposited surface energy density of the laser irradiation necessary to initiate the sample), æ is the absorption coefficient, c and ρ are the heat capacity and density of the sample, respectively, and Tc = Ti − T0 (Ti is the flash temperature, and T0 is the initial sample temperature). The left-hand side of eq 1 represents the energy density absorbed from the laser pulse, while the right-hand side denotes the energy density needed for heating the sample to the flash temperature. By using typical values for the majority of secondary explosives, c ≈ 10 J/(g K), ρ ≈ 2 g/cm3, and Ti ≈ 550 K, we obtain the threshold Hcæ ≈ 5 × 103 J/cm3, which corresponds to Hc ≈ 5 × 103 J/cm2 at æ ≈ 1 cm−1. The initiation thresholds registered in the experiments happen to be at least 1 or 2 orders of magnitude lower than that even at æ ≈ 0.1−0.01 cm−1, which rules out thermally activated decomposition and suggests that the resultant explosive initiation is associated with nonthermal processes.18 This analysis suggests that attempts to observe thermal initiation of homogeneous energetic materials are likely to be successful only with an increase of æ, for example, by introducing dyes or by using wavelengths of initiating light at the edge of the material's optical absorption band. The lack of laser diodes that can provide intensive irradiation with wavelengths of 300 nm or lower makes the transition to the initiation at the edge of the band gap problematic. The introduction of suitable dyes is quite possible but is generally less effective than the use of heterogeneous energetic systems.

III. HETEROGENEOUS ENERGETIC MATERIALS The situation with the heterogeneous samples, especially containing light-absorbing particles, is considerably more complex. Numerous experiments aimed at lowering (or tuning) the laser initiation threshold by introducing those additives have failed so far.5,11,20 Certain hopes based on rather simple speculations are placed in the use of nanoparticles.22,23 The absorbed energy of light is proportional to the particle cross section, d2, while its mass is proportional to d3, where d is the diameter of a spherical particle. Therefore, the heat of the particle should be ∼d−1 and at the nanoscale can reach very large values (∼104 K for particles of ∼100 nm). Nevertheless, it is worthwhile to analyze possible reasons for the failure of the attempts at threshold reduction by using nanoparticle additives.11 In the line of reasoning presented above, the efficiency of nanoparticle additives is speculated in terms of heating these particles only. From the explosive initiation point of view, one should be interested in heating the surroundings of the nanoparticle and not the nanoparticle itself, which is obviously affected by the size, shape, and configurations of the lightabsorbing particles. Let us consider a model experiment. Take a fragment of a homogeneous sample of mass me that is transparent to the initiating light pulse and contains a light-absorbing additive, for example, a small piece of soot. The magnitude of me, hereafter referred to as the critical mass, is the minimum mass of an energetic material, the heating of which to the flash temperature triggers exothermic reactions leading to an explosion of the entire sample. Let us then consider the adiabatic heating of this sample due to the light pulse absorbed by the soot, assuming that the sample is surrounded by a vacuum. The most interesting and important parameter in the process is the

II. EFFICIENCY OF LASER-INDUCED THERMAL INITIATION IN HOMOGENEOUS ENERGETIC MATERIALS Let us consider the efficiency of the thermal mechanism versus a nonthermal photochemical mechanism of initiation. In accordance with the thermochemical mechanism,3−5,12 it is assumed that the high efficiency of laser initiation is provided by a highly focused laser beam that locally heats the material. We argue that the thermal efficiency of the laser initiation mechanism is significantly lower than necessary, which makes the thermally activated process unlikely. Indeed, most laser experiments use a wavelength of 1060 nm generated by a phosphate glass Nd3+ laser,8,18 which falls into the optical band gap of the studied materials,13,18,19 meaning that the samples can hardly absorb the light waves. Typically, secondary explosives (such as RDX, PETN, cyclotetramethylene-tetranitramine (HMX), triamino-trinitrobenzene (TATB), diaminodinitroethene (DADNE), trinitrotoluene (TNT), etc.) are wide-gap dielectrics with the band gap exceeding 4 eV.21 To make thermal initiation work, the sample needs to be heated, while the only heat source is the laser-generated light, the absorption of which is extremely small in the forbidden band gap. This delimits a very low efficiency of the thermal initiation of homogeneous transparent samples. Let us make simple estimates to illustrate this initial proposition. Thermal initiation of a sample occurs at or above the flash temperature, Ti. An adiabatic heating process during the typical initiating pulse of ∼10−8 s can be described as 24483

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Figure 1. Configurations of the model system ignited by the laser pulse for (a) a hot sphere, (b) a hot plate, and (c) a hot shell. The shaded part represents the light-absorbing material with the characteristic size a, and the white part shows the energetic material of size h.

a. Hot Sphere. Consider a light-absorbing sphere with a diameter h, surrounded by a layer of an energetic material with a thickness a (Figure 1a). By using simple geometric relationships, we obtain from eq 3

threshold exposition of the ignition, Hc, i.e., the minimal exposition that heats the sample’s fragment to the flash temperature. (Typically, in explosive initiation studies, instead of an accepted in physical optics exposition, authors often use the term surface energy density.) The Hc is not to be confused with the threshold exposition of initiation, Ht, which is usually measured in laser initiation experiments and determined as a probability of the entire sample to explode and not the ignition of the isolated material's fragment as in our case.24 The adiabatic heating of the sample’s fragment can be described as Hcσ = (ceme + chmh )Tc

Hc =

(4)

Usually, the densities and heat capacities of energetic ingredients and light-sensitive additives are fairly close to each other; therefore, we will assume chρh = ceρe = cρ (5)

(2)

and simplify eq 4:

where σ is the effective cross section of the light-absorbing additive, ce, ch, me, and mh are specific heat capacities and masses of the energetic material and light-absorbing additive, respectively, and Tc = Ti − T0 (Ti and T0 are the flash temperature and the fragment’s initial temperature, respectively). In this model aimed at estimating the maximal possible heating of the material, the thermal diffusion and other processes of heat transfer from the considered fragment to the adjacent material’s layers are neglected. It is clear that an account for the heat transfer can only reduce the value of this heating. Similarly to eq 1, the left-hand side of eq 2 describes the energy absorbed by the light-absorbing additive, while the right-hand side defines the energy necessary for heating the whole fragment (including the light-absorbing additive and the critical mass of the energetic material) to the flash temperature. It is convenient to express me and mh through the corresponding volumes Ve and Vh and densities ρe and ρh: Hcσ = (ceρe Ve + chρh Vh)Tc

⎡⎛ ⎤⎫ 2 ⎧ 2a ⎟⎞ Tch⎨chρh + ceρe ⎢⎜1 + − 1⎥⎬ ⎣⎝ ⎦⎭ 3 ⎩ h⎠

Hc =

2 cρTc (h + 2a)3 3 h2

(6)

Figure 2 (curves 1) illustrates the obtained dependence of the threshold of ignition of the fragment, Hc, with the size of a light-absorbing particle, h, together with the effect of the thickness of an energetic material, a. It is shown that, for h ≫ a, the heating of the particle to T = Ti is the limiting factor. An increase of Hc with an increase of h is related to the obvious fact that the absorbed energy of the laser irradiation is proportional to the particle cross section, ∼h2, and the energy needed to heat the particle is inversely proportional to its volume, ∼1/h3, which results in the dependence Hc ≈ h at h ≫ a. At h ≪ a, the limiting factor is determined by the ability of the heated particle to transmit the energy needed to heat the energetic layer to T = Ti. The amount of thermal energy accumulated in small particles (heated to T > Ti) may or may not be sufficient to heat the adjoined energetic layer to T = Ti, which defines the dependence Hc ≈ a/h 2 at h ≪ a. We draw attention to the fact that the minimum of Hc falls in the region where h ≈ a (Figure 2, curves 1). We therefore suggest that it is impossible to tune the sensitivity of energetic materials to laser excitation by introducing nanoparticles as nanosizes correspond to the region of high Hc, shown at the left branch of curves 1 (Figure 2), and not to the Hc minimum. We conclude that an analysis of the size range is absolutely necessary while choosing appropriate light-sensitive nano- or microparticles that would yield the minimum of Hc (Figure 2, curves 1). b. Hot Plate. We now describe a flat light-absorbing layer with thickness h that covers the sample of an energetic material

(3)

Furthermore, we simulate the three most interesting, in our opinion, configurations of the fragment exposed to initiation (Figure 1): a hot sphere (light-absorbing spherical particle surrounded by a spherical layer of an energetic material), a hot plate (light-absorbing plane adjoined to a layer of an energetic material, perpendicular to the light beam), and a hot shell (spherical particle of an energetic material embedded into a layer of the light-absorbing shell). In all cases, the mass of the energetic material is equal to the critical mass me. All three configurations can be easily realized in experiments; for example, a soot powder can serve as hot spheres, and a coat of black paint can serve as the hot surface or shell. 24484

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IV. SUMMARY AND CONCLUSION Assuming the thermal nature of laser initiation of energetic materials, the efficiency of the process of converting laser irradiation energy into thermal energy to trigger chemical decomposition has been estimated in homogeneous and heterogeneous energetic materials, primarily secondary explosives. The performed analysis demonstrated that the thermal energy required to initiate chemistry in the energetic systems is significantly higher (an order of magnitude or more) than that the used lasers can generate due to the low optical absorption of energetic materials in the range of irradiation wavelengths. This inconsistency suggests that the chemical decomposition may be initiated by nonthermal selective photoinitiation rather than thermally as usually supposed. From a fundamental chemistry point of view, the choice of a photochemical or thermochemical approach is crucial because it largely defines the strategy of both experimental and theoretical research to achieve the desirable control of the sensitivity of explosive systems to initiation of detonation. The thermochemical mechanism dictates adding light-absorbing particles to the energetic material, primarily soot,17 while the photochemical mechanism suggests adding light-sensitive or light-scattering particles.18 The thermochemistry requires modeling and simulations of ground-state chemical reactions of particular decomposition pathways coupled with heat transfer, while photochemistry expects to study decomposition through excited-state potential surfaces and changes in optical properties induced by defects and/or deformations in crystals. A comparative analysis of heterogeneous energetic systems with the three particle configurations (Figure 2) revealed that the hot sphere configuration, which is most often used in laser initiation experiments, is the least efficient at small particle sizes. We suggest that the hot plate and hot shell configurations will be more efficient in utilizing heat absorbed from the initiating pulse to tune the sensitivity of energetic materials to laser ignition. Hence, it is probably worth trying experimentally as the estimates obtained here imply that coating technologies could provide useful and simple means of improving the efficiency of the thermal mechanism of initiation by choosing the correct geometric configuration and sizes of additives to energetic ingredients. In addition, we stress here that such lightabsorbing coatings preclude (or even exclude) the penetration of laser irradiation into the energetic sample and therefore fully eliminate initiation through nonthermal mechanisms, thus allowing for the elucidation of the peculiarities of the thermal processes in great detail. We emphasize here that the obtained estimates and recent experiments give evidence in favor of photochemistry in energetic materials, which opens up new prospects in both fundamental and applied studies. We propose a rather simple explanation. In the case of the thermochemical initiation, the absorbed energy of the igniting pulse is spent for heating an entire crystalline lattice of a homogeneous energetic material or for heating an entire light-absorbing particle in heterogeneous systems. In the case of photochemical initiation, the absorbed energy is directly channeled to form active excited radicals, which then provide a propagation of the explosive decomposition reaction.25

Figure 2. Threshold exposition Hc as a function of h. The characteristic size of the light-absorbing material is shown for the energetic fragment of size (a) a = 10 μm and (b) a = 20 μm and (c) for the hot sphere configuration. Curves 1 illustrate the results obtained from eq 5 for the hot sphere (see Figure 1 a), curves 2 are obtained from eq 8 for the hot plate (see Figure 1b), and curves 3 are obtained from eq 10 for the hot shell (see Figure 1c). The parameters used in the calculations are cρ = 20 J/(cm2 K) and Tc = 250 K.

of thickness a (Figure 1b). Equation 3 for this configuration will take the form Hc = Tc(ceρe a + chρh h)

(7)

and by substituting eq 4, it becomes

Hc = cρTc(h + a)

(8)

Figure 2 (curves 2) shows that the thermal effect is more pronounced in this geometry. A light-sensitive paint layer of the minimal thickness h would provide a sufficiently full absorption of the initiating pulse. c. Hot Shell. Finally, we consider a light-absorbing layer that covers an individual spherical grain of an energetic material (Figure 1c). Equation 3 transforms into 3 3 3 2 chρh [(a + 2h) − a ] + ceρe a Hc = Tc 3 (a + 2h)2

and taking into account eq 4, it becomes 2 Hc = cρTc(a + 2h) 3

(9)

(10)

showing that eq 10 is very similar to eq 8. We conclude here that the use of heterogeneous systems can appreciably increase the efficiency of the thermal mechanism of the laser initiation with the preference given to the configurations of the hot plate and hot shell over the hot sphere, which is frequently employed in experimental studies.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest. 24485

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Chem. Phys. 2007, 126, 114701. (d) Wu, C. J.; Yang, L. H.; Fried, L. E.; Quenneville, J.; Martinez, T. J. Phys. Rev. B 2003, 67, 235101. (e) Dreger, Z. A.; Gruzdkov, Y. A.; Gupta, Y. M.; Dick, J. J. J. Phys. Chem. B 2002, 106, 247−256. (f) Marinkas, P. L. J. Lumin. 1977, 15, 57−67. (22) Assovskiy, I. G. Fizika Goreniya i Vnutrennyaya Ballistika; Nauka: Moscow, 2005; p 357 (in Russian). (23) Dreizin, E. L. Prog. Energy Combust. Sci. 2009, 35 (2), 141−167. (24) There are at least two reasons for Hc and Ht to differ from each other. First, due to energy dissipation and heating of the surroundings, heating of the fragment inside the sample is lower than that of an isolated fragment. Second, some interference effects, occurring if the amount of light-absorbing inclusions is large, would lead to a reduction of the initiation threshold. Nevertheless, configurations and sizes of light-absorbing particles should similarly affect Hc and Ht. Indeed, both the shape and size of the inclusions can affect only the earliest stages of the initiation process, light absorption and heat transfer from a lightabsorbing inclusion to the energetic layer. Clearly, an isolated fragment and a fragment inside the considered sample participating in these processes would undergo identical conditions. (25) The photochemical decomposition is extraordinarily complex (see, for example, refs 8, 9, 14, 15, 18, and 19). Detailed estimates of the efficiency of the photoinitiation mechanism in specific explosives and some ways for its realization, in particular, the introduction of light-scattering particles, will be communicated elsewhere.

ACKNOWLEDGMENTS This research was supported in part by the Federal Program “Scientific and Scientific Pedagogical Cadre of Innovative Russia” (State Contract 02.740.11.0554), by the National Science Foundation (NSF), and by the Office of Naval Research (Grant N00014-12-1-0529). M.M.K. is grateful to the Office of the Director of the NSF for support under the IRD program. Any appearance of findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF.

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REFERENCES

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