Explosive decomposition of PETN with nanoaluminum additives under the influence of pulsed laser radiation at different wavelengths

The results of measurements of the threshold of explosive decomposition of PETN with 0.1 wt % additives of aluminum nanoparticles under the action of the first and second harmonics of a YAG:Nd³⁺ laser with a pulse duration of 12 ns are presented. A comparison of experimental and theoretical results is performed. It is concluded that the absorption of radiation heats the nanoparticles to form chemical decomposition kernels in the vicinity of the hotspot, so that the initiation of explosive decomposition is not associated with optical breakdown of the sample.     

Characteristics of the initiation of chain and thermal explosions of energetic materials by pulsed laser radiation

A comparative experimental and theoretical study of the initiation of silver azide (SA) single crystals and pressed pentaerythritol tetranitrate (PETN)–metal nanoparticles by a neodymium laser pulse is performed. The main differences in the explosive decomposition of the samples are associated with the absence of the induction period and the presence of subthreshold effects in the initiation of PETN-based composites. By contrast, the initiation of SA single crystals always features an induction period, but no subthreshold effects. It is shown that the observed differences in the explosive decomposition are due to the fact that SA single crystals decompose by the chain explosion mechanism, whereas pressed PETN–metal nanoparticles samples, by the thermal explosion mechanism in the micro-hotspot mode. The kinetic parameters of the initiation of the decomposition reaction calculated within the framework of the existing model are consistent with the available experimental data. An experimental criterion for distinguishing between the chain and thermal (in micro-hotspot) mechanisms of the initiation of an explosion under the action of a laser radiation pulse is formulated, according to which the absence of the induction period and a pronounced manifestation of subthreshold effects are indicative of a thermal explosion, whereas the presence of the induction period and the absence of subthreshold effects are characteristic of a chain explosion.     

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.     

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 λ _m = 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     

Photochemical and photothermal dissociation of PETN during laser initiation

The temperature dependence of the efficiency of laser initiation (λ = 1060 nm) of pure PETN with an open and closed surface and PETN with a light-scattering additive (0.5% MgO) and an open surface was studied. It was found that the closing of the surface and introduction of light-scattering additives leads to a change in the temperature dependence of the initiation threshold. The observed effect is associated with the specificity of the photoinduced fragmentation of PETN molecules in defective areas of the crystal lattice.     

Pressure of the products of the explosive decomposition of a mixture of pentaerythritol tetranitrate and nickel monocarbide nanoparticles initiated by laser radiation

Relative characteristics of the pressure created by the products of explosive decomposition of a mechanical mixture of pentaerythritol tetranitrate and nickel monocarbide (NiC) nanoparticles upon laser initiation are determined. It is demonstrated that the explosion of the mechanical mixture is caused by the absorption of laser radiation by NiC nanoparticles, a process accompanied by the heating and exothermic decomposition of NiC to the nickel and carbon phases, which, in turn, give rise to the formation of hotspots. The optimal concentration of NiC at which the maximum pressure of the explosion products is achieved is determined.     

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.     

Dispersion of zirconium dioxide by pulsed laser radiation

A technique for producing ZrO₂ dioxide nanoparticles under the action of pulsed laser radiation is developed. By the methods of transmission electron microscopy and X-ray phase analysis, it is shown that the high-temperature cubic phase of ZrO₂ is formed during laser ablation. The dependence of the size of ZrO₂ dispersed particles on the laser radiation intensity is determined. A thermodynamic one-dimensional model of laser ablation of zirconium dioxide is analyzed. The results of analytical computations of ablation of ZrO₂ particles are confirmed by experimental data.     

Interaction of pulsed laser radiation with a polycarbonate-based composite

Abstract—The interaction of pulsed laser radiation with foil-clad polycarbonate containing diffraction gratings is studied. It is found that the laser beam reflected from diffraction gratings is selffocused in polycarbonate bulk and becomes a seed for changing the polycarbonate structure, which leads to the glass transition and growth of ordered structures with symmetry features, which gives rise to the light dispersion effect in composites. A probable formation mechanism of ordered structures in polycarbonate during irradiation is proposed.     

Critical parameters of a micro-hotspot model of the laser-pulse initiation of the explosive decomposition of energetic materials

The work continues a series of studies of the micro-hotspot model a thermal explosion. The dependences of the critical energy fluence and the temperature of the reaction kernel in pentaerythritol tetranitrate (PETN) on the radius of aluminum nanoparticles at half-maximum pulse durations of 10 to 150 ns are calculated. For each pulse duration, there is an optimal nanoparticle radius at which the critical energy fluence is minimal. The dependences of these parameters on the pulse duration are derived. It is shown that there is a universal relationship, independent of the pulse duration, between the normalized critical energy fluence and the nanoparticle radius.     

Investigation of early stages of explosive pentaerythritol tetranitrate crystal decomposition initiated by pulsed electron beams

The method of high-resolution pulsed optical spectroscopy is used to investigate the spectral-kinetic characteristics of explosive emission of pentaerythritol tetranitrate crystal excited by a pulsed electron beam (0.25 MeV, 35 ns, and 15 J/cm²). It is established that emission with the spectrum comprising three components with maxima at 3.1, 2.4, and 1.5 eV is initiated in the edge of the excitation pulse. Moreover, transformation of the emission spectrum is observed directly within the time over which the excitation pulse acts followed by the emission decay; however, emission develops again on the subnanosecond scale; moreover, its spectrum contains only the 1.5-eV band, which appears to be an indicator of the self-sustaining chemical reaction of explosive decomposition. The 3.1-and 2.4-eV bands are identified as emissions of an exciton and a primary NO ₂* radiative defect, and the 1.5-eV band is probably connected with the NO ₃ radical. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 3–9, February, 2007.     

Second-harmonic conversion of partially coherent radiation of neodymium glass laser

The experimental results on the conversion of multimode radiation of a neodymium glass laser with controlled spatial and temporal coherence to the second optical harmonic in nonlinear KDP crystal during the oee interaction implementation are presented. The dependence of the efficiency of the conversion to the second harmonic on the fundamental radiation power density on the crystal in the range I = 0.2 ? 5 GW/cm² is studied at the number of transverse modes in the cavity N = 1000; the radiation divergence φ = 3.5, 4.7, and 5 mrad; the emission spectral width δλ = 5and 42 Å; and the pulse duration of 2.5 ns. The breakdown power density of the KDP crystal is determined, the interference properties of the converted radiation of the second harmonic are studied.     

Visualization of the spatio-temporal structure of a pulsed photoelectron beam formed by femtosecond laser radiation

A method based on an original electron microscope created for investigating photoelectron beams is presented. It ensures a nanometer spatial resolution and picosecond time resolution. Electrons appearing when a metal needle is irradiated by femtosecond laser pulses are transmitted through a dielectric microcapillary and are subjected to a ponderomotive potential created by femtosecond laser radiation focused near the capillary tip. The position-sensitive detection scheme allows for the detection of the spatial profile of a photo-electron beam with a magnification of K ≅ 4 × 10⁴. The time structure of the photoelectron beam is visualized by scanning the delay time between laser pulses irradiating the needle and a laser pulse focused near the capillary tip.     

Characteristics of a built-in wavemeter for a tunable pulsed laser

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 53, No. 6, pp. 895–899, December, 1990.