Laser Ignition of Pyrotechnics – Effects of Wavelength,Composition and Confinement

Ignition tests were carried out using three different laser systems and three different pyrotechnic compositions. Pyrotechnic materials investigated are: sulfur/charcoal/potassium nitrate based composition (gunpowder, GP), Shellac binder‐based boron/potassium nitrate composition (SR 44) and acaroid resin binder based magnesium/potassium nitrate composition (SR 371C). The laser sources were the multimode output from an Ar‐ion laser (λ=500 nm average), a high‐power commercial diode laser (λ=784 nm) and a small laser diode operating at around the same wavelength but controlled by a customized electronic circuitry. Lasers operating in the visible wavelength range provided more reproducible and quicker ignition than the infrared output from the diode lasers. It was found that unconfined gunpowder exhibits more reproducible ignition for both the visible and the infrared wavelengths compared to the other two compositions. The composition based on magnesium, SR 371C appeared to be very sensitive to laser intensity variations and gave erratic and therefore, irreproducible ignition delay times. The threshold laser energies to initiate reproducible ignition for the different wavelengths were measured and ignition maps were constructed. From these maps, the required laser power density for any value of the ignition delay time, i.e. laser energy density was determined. Tests were also conducted on gunpowder samples, partially confined in a modified pyrogen igniter capsule and a small laser diode. The diode was operated in single pulse mode using a current surge, which was much higher than the recommended value for CW operation. This provided ~1 W pulses at the end of a 1 mm diameter fiber optic cable and caused reproducible ignition in the semi‐confined pyrotechnic bed within the capsule. The threshold ignition energy under semi‐confined conditions was found to be substantially less than that required in the unconfined environment under similar experimental conditions.     

Studies into Laser Ignition of Confined Pyrotechnics

Ignition tests were carried out on three different pyrotechnics using laser energy from the multimode output from an Ar‐Ion laser (av) at 500 nm and a near‐IR diode laser pigtailed to a fibre optic cable and operating at 808 nm. The pyrotechnics investigated were: G20 black powder, SR44 and SR371C. The confined ignition tests were conducted in a specially designed ignition chamber. Pyrotechnics were ignited by a free space beam entering the chamber through an industrial sapphire window in the case of the Ar‐ion laser. For the NIR diode laser, fibre was ducted through a block into direct contact with the pyrotechnic. The Ar‐Ion laser was chosen as this was found to ignite all three pyrotechnics in the unconfined condition. It also allowed for a direct comparison of confined/unconfined results to be made. The threshold laser flux densities to initiate reproducible ignitions at this wavelength were found to be between ∼12.7 and ∼0.16 kW cm⁻². Plotted on the ignition maps are the laser flux densities versus the start of ignition times for the three confined pyrotechnics. It was found from these maps that the times for confined ignition were substantially lower than those obtained for unconfined ignition under similar experimental conditions. For the NIR diode laser flux densities varied between ∼6.8 and ∼0.2 kW cm⁻². The minimum ignition times for the NIR diode laser for SR371C (∼11.2 ms) and G20 (∼17.1 ms) were faster than those achieved by the use of the Ar‐ion laser. However, the minimum ignition time was shorter (∼11.7 ms) with the Ar‐ion laser for SR44.     

丁羟复合推进剂的辐射点火

采用CO2激光点火装置,对丁羟复合推进剂的点火过程进行了实验研究,利用描述固体推进剂物化现象的一维传热模型对复合推进剂的辐射点火特性进行了理论分析。通过最小二乘法拟合实验数据得到了丁羟复合推进剂的点火准则。结果表明,丁羟复合推进剂的点火过程主要包括惰性加热及气相点火过程,惰性加热时间和点火延迟时间随热流密度的增大而减小,且随着热流密度的增大,热流密度的影响逐渐降低。固相传热数学模型能够比较准确地描述复合推进剂的辐射点火特性。     

Radiative Ignition of Fine‐Ammonium Perchlorate Composite Propellants

Radiative ignition of quasi‐homogeneous mixtures of ammonium perchlorate (AP) and hydroxyterminated polybutadiene (HTPB) binder has been investigated experimentally. Solid propellants consisting of fine AP (2 μm) and HTPB binder (~ 76/24% by mass) were ignited by CO₂ laser radiation. The lower boundary of a go/no‐go ignition map (minimum ignition time vs. heat flux) was obtained. Opacity was varied by adding carbon black up to 1% by mass. Ignition times ranged from 0.78 s to 0.076 s for incident fluxes ranging from 60 W/cm² to 400 W/cm². It was found that AP and HTPB are sufficiently strongly absorbing of 10.6 μm CO₂ laser radiation (absorption coefficient ≈250 cm⁻¹) so that the addition of carbon black in amounts typical of catalysts or opacitymodifying agents (up to 1%) would have only a small influence on radiative ignition times at 10.6 μm. A simple theoretical analysis indicated that the ignition time‐flux data are consistent with in‐depth absorption effects. Furthermore, this analysis showed that the assumption of surface absorption is not appropriate, even for this relatively opaque system. For broadband visible/near‐infrared radiation, such as from burning metal/oxide particle systems, the effects of in‐depth absorption would probably be even stronger.     

Effect of the Phase Transition on the Ignition Process

A direct relation in the form of a firstorder differential equation is obtained between the phasetransformation depth (phaseboundary position) and the FrankKamenetskii parameter characterizing the intensity of chemical heat release. As a particular case, this equation yields a solution of the problem obtained previously for cylindrical and planeparallel reactors. The presence of a stable intermediate position of the interphase boundary is generalized to symmetric regions of an arbitrary (even fractional) dimension, which appears to have some physical meaning.     

Potential Biocides: Iodine‐Producing Pyrotechnics

Currently there is a need for specialized pyrotechnic materials to combat the threat of biological weapons. Materials have been characterized based on their potential to produce heat and molecular iodine gas (I₂) to kill spore‐forming bacteria (e. g. anthrax). One formulation, already proven to kill anthrax simulants, is diiodine pentoxide with aluminum; however, it suffers from poor stability and storage problems. The heat and iodine gas output from this mixture and candidate replacement mixtures were measured with bomb calorimetry and extraction and analysis of I₂ by UV‐Vis. Of the mixtures analyzed, calcium iodate and aluminum was found to be the highest producer of I₂. The heat output of this mixture and others can be tuned by adding more fuel, with the cost of some iodine. Products of combustion were analyzed by thermal analysis (SDT), XPS, XRD, and LC/MS. Evidence for various metal iodides and metal oxides was collected with these methods.     

硝基胍对硝胺发射药点火性能影响的实验研究

采用差示扫描量热仪（ Ｄ Ｓ Ｃ）和点火燃烧模拟装置,研究了硝基胍（ Ｎ Ｇｕ）对 Ｎ Ｃ Ｎ Ｇ Ｒ Ｄ Ｘ 系列发射药点火性能的影响。实验证明： Ｎ Ｇｕ 可以改善 Ｎ Ｃ Ｎ Ｇ Ｒ Ｄ Ｘ 类硝胺火药的点火性能,且当 Ｎ Ｇｕ 与 Ｒ Ｄ Ｘ 之比大于１ 时效果才更显著。     

Combustion Process of Mg/TF Pyrotechnics

The combustion process of pyrotechnics was studied in order to obtain informations of the rate control parameters of burning rate. The pyrotechnics tested was made of Mg (magnesium) and TF (polyfluoroethylene). The burning rate measurements revealed that the burning rate of the Mg/TF propellants (pellet in shape) increases with increasing the weight fraction (ξ) of Mg in the range of ξ > 0.33. Though the adiabatic flame temperature is the maximum at ξ = 0.33, the burning rate increases with decreasing the flame temperature. The total burning surface area of the Mg particles mixed with in the unit mass of propellant plays an important role on the oxidation process in the gas phase just above the propellant burning surface. The heat flux feedback from the gas phase to the propellant burning surface increases with increasing ξ. Therefore, the burning rate increases as ξ increases.     

铝热剂对双基推进剂激光点火特性的影响

用纳米及微米铝粉、纳米氧化铅、纳米氧化铜和纳米三氧化二铋为原料,采用超声分散复合的方法,制备了铝热剂AI/PbO、Al/CuO和Al/Bi2O3.采用XRD、SEM-EDS和FTIR对原料和产物的物相、组成、形貌和结构进行表征;采用CO2激光点火实验方法,研究了含不同铝热剂双基(MIC-DB)推进剂在不同热流密度下的点...