A. G. Krechetov

Role of electronic excitations in explosive decomposition of solids

A combined theoretical and experimental study is performed for the initiation of chemistry process in high explosive crystals from a solid-state physics viewpoint. In particular, we were looking for the relationship between the defect-induced deformation of the electronic structure of solids, electronic excitations, and chemical reactions under shock conditions. Band structure calculations by means of the Hartree–Fock method with correlation corrections were done to model an effect of a strong compression induced by a shock/impact wave on the crystals with and without edge dislocations. Based on the obtained results, an excitonic mechanism of the earliest stages for initiation of high explosive solids is suggested with application to cyclotrimethylene trinitramine (also known as RDX) crystal. Experimental tests of this mechanism for AgN3 decomposition controlled by the dislocation density were worked out. The use of pulse radiolysis techniques allows us to observe pre-explosion modifications in properties...

Topography of photochemical initiation in molecular materials.

We propose a fluctuation model of the photochemical initiation of an explosive chain reaction in energetic materials. In accordance with the developed model, density fluctuations of photo-excited molecules serve as reaction nucleation sites due to the stochastic character of interactions between photons and energetic molecules. A further development of the reaction is determined by a competition of two processes. The first process is growth in size of the isolated reaction cell, leading to a micro-explosion and release of the material from the cell towards the sample surface. The second process is the overlap of reaction cells due to an increase in their size, leading to the formation of a continuous reaction zone and culminating in a macro-explosion, i.e., explosion of the entire area, covering a large part of the volume of the sample. Within the proposed analytical model, we derived expressions of the explosion probability and the duration of the induction period as a function of the initiation energy (exposure). An experimental verification of the model was performed by exploring the initiation of pentaerythritol tetranitrate (PETN) with the first harmonic of YAG: Nd laser excitation (1,064 nm, 10 ns), which has confirmed the adequacy of the model. This validation allowed us to make a few quantitative assessments and predictions. For example, there must be a few dozen optically excited molecules produced by the initial fluctuations for the explosive decomposition reaction to occur and the life-time of an isolated cell before the micro-explosion must be of the order of microseconds.

Laser initiation of PETN in the mode of resonance photoinitiation

The effect of the initial temperature of the sample on the efficiency of laser initiation and on the duration of the preexplosion stage (induction period) of PETN is studied. An analysis of the results leads to the conclusion that, in the initiation of PETN with the first harmonic of a neodymium laser (1060 nm), a new selective resonant mode of photoinitiation is realized, through the formation of active radicals providing the development of a chain reaction of explosive decomposition at the preexplosion (solid-phase) stage of the process. For this mode, the threshold volume density of energy absorbed is much lower than that typical of the traditional non-selective mechanisms of initiation. The mode of resonant photoinitiation is, however, difficult to realize because of a low value of the absorption coefficient in the actual band. This difficulty can be easily overcome by introducing light-scattering additives, a modification that opens prospects for practical use of resonant photoinitiation.

Divacancy Model of Heavy Metal Azide Initiation

A hypothetical model of heavy metal azide initiation, in which reaction centers are assumed to be associates of cation and anion divacancies, is described. Empirical foundations for the model and also its physical and mathematical formulations are given. The model qualitatively agrees with available experimental data.

Preexplosion phenomena in heavy metal azides

The paper reports results of investigation of the explosive decomposition of heavy metal azides in real time. The characteristics of the detected predetonation effects — the preexplosion conductance and luminescence of heavy metal azides — are described. The obtained value of the preexplosion conductivity of silver azide indicates that the process is of a chain nature. A model for the development of explosion of heavy metal azides is developed including multiplication of active particles (holes) by a first-order reaction and chain termination by a second-order reaction.

Efficiency of laser initiation and absorption spectra of PETN

A comparative study of the efficiency of the laser initiation of PETN by the first and second harmonics (1060 and 530 nm) of a neodymium-doped phosphate glass laser was performed. A significant difference in the efficiency of PETN initiation by the different harmonics was revealed: as the initial temperature of the sample increased from 373 to 450 K, the threshold initiation fluence decreased from 3.0 to 0.5 J/cm2; at the same time, the second harmonic failed to initiate PETN even at a fluence of 10 J/cm2. The absorption spectrum of PETN was found to have a weak absorption band with a maximum at λm = 1020 nm. It was assumed that the high efficiency of initiation by the first harmonic is associated with light absorption (photo-initiation) by this band

Model of the photostimulated fragmentation of PETN molecules in selective photoinitiation

A model of the photostimulated fragmentation of the PETN molecule irradiated by the first harmonic of a neodymium laser (1060 nm) is proposed. Photoexcitation at 1060 nm leads to the n → π* transition of the 2p nonbinding electron of the oxygen to the π* antibonding orbital, a transition that causes the rupture of the O-N bond and the formation of NO2* radicals, thereby ensuring a further development of explosive decomposition. In the case of a free molecule or a molecule located on the surface of a microcrystal, the process does not require any thermal activation. In the case of a molecule at a regular lattice site, an additional activation energy (0.4 eV) is needed to overcome the potential barrier associated with the passage of through a bottleneck between the nearest-neighbor molecules.

Effect of temperature on the laser initiation of pentaerythritol tetranitrate (PETN)

The temperature dependence of the probability of PETN explosion upon laser pulse initiation (1064 nm, 10 ns, 1–5 J) was examined. As the temperature increases from 393 K to 450 K, the initiation threshold lowers, with the initiation of open-surface samples occurring over the entire temperature range. It was concluded that the initiation mechanism involves thermal-and photoactivation steps. The activation energy for the thermal activation step was found to be 0.35 ± 0.05 eV.

Photo- and thermochemical initiation of PETN under laser excitation

A method for separate control of the efficiencies of the photochemical and thermochemical mechanisms of initiation of energetic materials by laser excitation for a combined action of these mechanisms is proposed. The method is based on the difference in the timescales of the relevant photo- and thermochemical initiation processes, a factor that leads to a different dependence of the efficiencies of these mechanisms on the initiating pulse duration and initiation irradiance. An experimental validation of the method for the initiation of PETN with 1064- and 1070-nm light at irradiances of ∼109 and ∼104 W/cm2, respectively, is performed. The results show that, for the initiation of pure PETN at an irradiance of 109 W/cm2, the photochemical mechanism dominates, ensuring an initiation threshold of ∼4 J/cm2. In the case of samples with light-absorbing inclusions (carbon) and the same irradiance ∼104 W/cm2, only the thermochemical mechanism is operative, with an initiation threshold of ∼15 J/cm2. In the absence of light-absorbing inclusions, the sample does not explode even at a fluence of ∼75 J/cm2; i.e., in this case, the initiation threshold is clearly higher than this value.

Dynamic Topography of Silver Azide Pre‐Explosion Luminescence

For the first time, it is experimentally shown that silver azide pre‐explosion luminescence emerging under weak laser excitation originates and develops locally. As the excitation intensity increases, the mean number of local luminous regions also increases, finally giving rise to homogeneous luminescence.