Effect of multiple scattering on the critical density of the energy used to initiate a PETN–aluminum compound by a neodymium laser pulse

The radiative transfer equation and the Mie theory are used in this paper to determine the optical properties of PETN–aluminum nanoparticles compounds. In the case of laser initiation with a wavelength of 1064 nm, the illumination gain at a depth of 100 μm from the surface of the sample varies in the range of 1.070–3.308 for nanoparticles with radii of 20–200 nm. The minimum of the relationship between the density of the energy required to initiate an explosive decomposition and the mass fraction of nanoparticles can be determined by the maximum illumination gain in the sample. The relationships between the critical density of the energy required to initiate PETN–aluminum nanoparticles compounds by the pulses of the first and second harmonics of a neodymium laser and the nanoparticle radius with account for multiple light scattering are determined. It is shown that account for multiple light scattering improves the agreement between theory and experiment.     

Effect of laser radiation absorption efficiency on the heating temperature of inclusions in transparent media

The efficiency of light absorption by inclusions of various metals in transparent media is calculated using as an example silver azide, lead, and PETN. It is shown that the absorption efficiency, along with the laser pulse energy density, has a decisive influence on the maximum temperature of heating of the inclusion. Dependences of the maximum heating temperature on the radius of the inclusions are plotted for a pulse duration of 30 ns. Asymptotic expressions are obtained for the dependence of the maximum heating temperature on the pulse duration for an ensemble of inclusions.     

Simulation of initiation of PETN by a nanosecond laser pulse in the weak absorption region

A numerical simulation of the initiation of PETN by a laser pulse was performed. The heat-conduction equation was solved in a cylindrical coordinate system taking into account multiple reflection of the light beam, zero-order exothermic reaction, and melting. A criterion for the ignition of explosives by a laser pulse with multiple reflection of the light flow was obtained. The calculation results are in satisfactory agreement with experiment and the ignition criterion. Dependence of the critical energy density on the light beam radius is due to radial heat transfer. The ignition threshold can be controlled by changing the reflection coefficient of the back surface of the sample.     

Initiation of PETN explosion by the second harmonic pulse of a neodymium laser

The explosion of PETN of density 1.73 g/cm³ was first initiated by the second-harmonic pulse of a Q-switched neodymium laser. It is shown that the primary process of energy absorption in this case is PETN molecule ionization involving two-photon absorption. The critical initiation energy density corresponding to a 50-% probability of explosion is 12.3 J/cm².     

Paradox of small particles in the pulsed laser initiation of explosive decomposition of energetic materials

The dependences of the critical energy density required to initiate the explosive decomposition of lead azide and the radius of the most heated nanoparticle on the pulse duration of the first harmonic of neodymium laser (1064 nm) are calculated within the framework of the micro-hotspot model of thermal explosion. The calculations are carried out with account for the dependence of the absorption efficiency factor of the laser pulse on the lead nanoparticle radius. With the maximum value of the absorption efficiency factor (1.18), the lead nanoparticle radius (in lead azide) becomes 74 nm. If the pulse duration is short (smaller than 40 ns), the radius of the most heated lead nanoparticle in the lead azide matrix varies slightly (less than 15%) and equals 63.5 nm within the range of short pulse durations. Accounting for the dependence of the absorption efficiency factor of the laser pulse on the nanoparticle radius makes it possible to resolve the paradox of small particles.     

Effect of the temperature dependence of the absorption coefficient on the critical energy of ignition of condensed substances by a laser pulse

This paper presents an analytical criterion of ignition of condensed explosives by a short laser pulse with a Gaussian energy distribution over the beam cross section taking into account the temperature dependence of the absorption coefficient. The dependence of the critical ignition energy density on the beam radius is due to the radial component of heat release from the reaction volume. Estimates of the critical energy density of the laser pulse by this criterion are consistent with the results of numerical solution of the heat-conduction equation.     

Laser initiation of low-density mixtures of PETN with metal additives

This paper presents data on laser initiation of low-density mixtures of PETN with metal additives with varying dispersity of PETN and particle size of the additive. A laser with a wavelength of 1.06 μm and a pulse length of 40 and 30 ns was used. Curves of the threshold initiation parameters on the additive content are shown to have minima. For coarse additives, no significant dependence of the initiation threshold of the mixtures on the nature of the metal at its optimal content (except for aluminum) was observed. For PETN mixtures with an optimal amount of fine aluminum, a significantly greater (a factor of 6.2) decrease in the threshold initiation parameters compared to direct initiation of PETN was found. It is shown that the initiation thresholds of the mixtures do not depend on the dispersity of PETN with the optimal additive content. Increasing the dispersity of PETN extends the dependences of the threshold parameters on the additive content while the optimal additive content is shifted to higher values. The initiation thresholds are found to strongly depend on the density of the mixture charge. The key points of the mechanism of laser initiation of PETN mixtures with additives are formulated.     

Influence of laser wavelength on the critical energy density for initiation of energetic materials

Critical densities of the energy of laser initiation of PETN containing nanoscale aluminum inclusions at radiation wavelengths of 1064 and 532 nm were measured experimentally. The critical initiation-energy density that corresponds to a 50%th probability of explosion was 1.15 J/cm² for the first harmonic of a neodymium laser and 0.7 J/cm² for the second. The dependence of the efficiency of radiation absorption by aluminum on the size of metal nanoparticles for the first and second harmonics of a neodymium laser is calculated. It is shown that the particle diameter corresponding to the absorption efficiency maximum and the amplitude of the maximum depend on the radiation wavelength. The absorption efficiency maximum for the first harmonic is observed in an inclusion 204 nm in diameter, and for the second, in an inclusion 96 nm in diameter. The amplitude of the maximum increases from 0.351 at a wavelength of 1064 nm to 0.490 at a wavelength of 532 nm. Dependences of the critical initiation energy density for energetic materials on the radius of metallic nanoparticles are calculated. Qualitative agreement between theoretical and experimental results is shown.     

Simulation of the ignition of organic explosives by a laser pulse in the weak absorption region

Numerical simulation of the ignition of RDX, HMX, and TATB by a nanosecond laser pulse was performed. The heat-conduction equation was solved in cylindrical coordinates with allowance for the multiple reflection of the light beam, a zero-order exothermic reaction, and melting. Despite the small temperature gradient due to the smallness of the radiation absorption coefficient, violation of thermal equilibrium due to Arrhenius nonlinearity leads to ignition of energetic materials from the surface. The critical energy density for ignition of PETN, RDX, HMX, and TATB by a nanosecond laser pulse was determined. Calculations have shown that with identical absorption and reflection coefficients, PETN is the most sensitive and TATB is the most heat-resistant.     

Initiation of a reactive material by a radiation beam absorbed by optical heterogeneities of the material

The heating dynamics and thermal ignition of a condensed transparent material by a radiation pulse absorbed by optical micro-heterogeneities present in the material were studied numerically and approximately analytically over a wide range of pulse durations. For quasistationary heating conditions of micro-heterogeneities, an analytical dependence of the radiation power density critical for ignition on the formal kinetic parameters of the chemical process and the radius of the particle absorbing the radiation was obtained. The critical energy characteristics of ignition are compared with experimental data on the initiation threshold of lead azide by laser pulses of different duration.     

Influence of the thickness and absorption coefficient of a copper oxide film on the ignition delay of PENT by a laser pulse

Numerical modeling of PETN ignition by a copper oxide film absorbing laser radiation has been performed. The calculation results showed the presence of a minimum in the curve of the dynamic delay of PETN ignition by a rectangular laser pulse versus thickness of the absorbing film. This effect is due to the fact that when the film thickness is commensurate with the reciprocal of the absorption coefficient, the amount of heat generated in the thin film due to the multiple reflection of the light flux is proportional to its thickness. Therefore, the smaller the film thickness, the more time is required to heat it to the ignition temperature of PETN. In the case of a thick film, additional energy and time are required to heat its cold part to the ignition temperature of PETN.     

Measuring the temperature of PETN explosion products with iron inclusions

The spectral-kinetic characteristics of luminescence of PETN with iron nanoparticle inclusions are measured in real time in the case of a laser initiated explosion (the wavelength is 1064 nm, and the pulse duration is 14 ns). During the action of the laser radiation pulse, the luminescence of the samples is observed, and the explosive decomposition occurs in a microsecond time interval. The spectral pyrometry method is used to establish the thermal nature of explosive luminescence. The explosion temperature is estimated to be 3400 ±100 K.     

Laser initiation of PETN-based composites with additives of ultrafine aluminium particles

This paper describes the study on the dependence of the critical energy density of the explosive decomposition of PETN on the mass concentration of the additives of ultrafine Al particles (100–120 nm) in the range of 0.025–1% under the influence of the first and second harmonics of a neodymium laser (12 ns, 1064 and 532 nm). It is shown that the critical energy of the explosion initiation by the first and second harmonics of the laser radiation reaches a minimum value at the same absorption rates, but different concentrations of additives. The photoacoustic method is used to show that, as the laser energy radiation is absorbed, the pressure amplitude in the heated layer reaches a maximum value when the concentration of additives corresponds to the minimum value of the critical energy density.     

Influence of the mass fraction of oxide in aluminum nanoparticles on the explosive decomposition threshold and light absorption efficiency in PETN-based compounds

Pulsed laser initiation is used to experimentally investigate the thresholds of explosive decomposition of PETN-based compounds with aluminum nanoparticle inclusions depending on the core-shell (Al/Al₂O₃) mass ratio in the particle. It is found that, with decreasing mass fraction of Al from 74 to 13%, the explosive decomposition threshold increases by a factor of 12.5. Light absorption efficiency calculations for the core-shell system performed by the Aden-Kerker method for Al/Al₂O₃ inclusions in PETN show that this efficiency decreases with decreasing mass of metal in the particle. The experimental and theoretical results can be regarded as an additional confirmation of the hot-spot concept of laser initiation of PETN containing nanoscale inclusions of metals.     

Effect of melting on the critical ignition energy of condensed explosives by a short laser pulse

A criterion for the initiation of explosives with a melting point below the ignition temperature by a short laser pulse is obtained. The criterion obtained is in good agreement with numerical solutions of the heat conduction equation in the cylindrical coordinate system. This criterion made it possible to explain experiments on the initiation of PETN from an open surface by a laser pulse in the range of transparency with variation in the diameter of the light beam. The calculation results coincided with the experiment at α = 0.065 cm^?1 and the Fresnel reflection coefficient.     

Laser initiation of mixtures of PETN and aluminum by a deposit

The formation of a deposit from mixtures of PETN with fine aluminum under the action of neodymium laser radiation in a free generation mode is studied. This paper also describes the efficiency of a deposit as a means to reduce the energy of laser initiation of mixtures as a function of fineness of PETN, aluminum content in initiated mixtures and deposit, mixture density, and diameter of the laser action region on the explosive. The compositions of mixtures for preparing deposits optimal in composition, which reduce the initiation energy of mixtures of PETN and aluminum down to 3.75 times, are determined. The functioning mechanism of a deposit during laser initiation of mixtures of PETN and aluminum is under discussion.     

Analysis of the Critical Conditions for Ignition of Gas–Particle Mixtures by a Heated Body with Pulsed Energy Supply

The critical conditions for the ignition of solid particles suspended in a gas by a heated body with pulsed energy supply are determined using numerical and approximate methods. It is shownthat in the kinetic (onetemperature) ignition regime, the critical duration of the thermal pulse is equal to the time of establishment of a zero gradient on the interface between the heaterand the gas–particle mixture. In the diffusive (twotemperature) ignition regime (small coefficients of heat transfer between the particles and the gas), the critical duration of the thermal pulse is much shorter than the time of establishment of a zero gradient. It is established that the critical duration of the thermal pulse is determined from the condition that the time of complete particle burnout on the interface between the heater and the gas–particle mixture is equal to the time of repeated heating of the gas to the temperature of the transition of the particle oxidation reaction tothe diffusive reaction regime. An approximate method for calculating the critical duration of the thermal pulse for the diffusive ignition regime is proposed. Numerical calculations show that the minimum time of establishment of the hightemperature combustion regime is reached when the thermal pulse duration is equal to the time of attainment of a zero pressure gradient on the interface between the heater and the gas–particle mixture.     

Numerical modeling of TATB shock-wave heating

A model of shock-wave heating of condensed high-explosives (HE) in which a refined dependence of the heat capacity of an HE on temperature is used and the effect of the initial density of the HE is taken into account is given. The dependences of HE (TNT, PETN, and TATB) heating on pressure in the shock-wave front are calculated. Modeling of TATB heating is of interest for understanding the shock-wave detonation initiation, including the dependence of the shock-wave sensitivity on the initial density and temperature of an HE. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 2, pp. 94–99, March–April, 2000.     

熔盐储热材料比热容强化的研究进展

强化熔盐材料比热容可以有效增强熔盐材料的蓄热能力,减小蓄热系统面积及热损失,进而降低蓄热成本,是近年来中高温储能领域的研究热点。本文主要从熔盐储热材料比热容强化的必要性、强化方法和强化机理等方面综述了近年来熔盐传热蓄热材料比热容强化的研究进展。具体阐述了添加可溶性添加剂和掺杂异质纳米颗粒形成纳米流体两种强化熔盐比热容的方法及目前存在的问题,重点探讨了熔盐纳米流体的制备方法、异质纳米颗粒体系、强化效果及比热容强化机理等问题。此外,指出了当前利用纳米流体强化熔盐储热材料比热容方面存在的不足：研究体系单一、悬浮稳定性差和比热容强化机理不完善等,并对熔盐纳米流体的未来发展方向,即多体系熔盐纳米流体的开发,多手段比热容强化机理的揭示和多方法熔盐纳米流体物性的测量进行了展望。     

Effect of interfacial heat transfer on the critical conditions of shock-wave initiation of chemical reaction in porous energetic materials

This paper presents the results of numerical analysis of a viscoplastic model for hotspot formation based on the solid-state mechanism of hotspot ignition of an energetic porous material under shock-wave loading. The highly viscous pore collapse regime is considered, which is of great interest for theoretical studies of the shock-wave initiation of heterogeneous energetic materials. Interfacial heat transfer was described under the assumption that the gas is ideal and hence homobaric (uniform in pressure). Parametric analysis was conducted, and characteristic features of the effect of heat transfer and interfacial heat transfer on the critical conditions of shock-wave initiation of chemical reaction in the energetic porous material were determined.