Photon Doppler Velocimetry and Simulation of Ejection of Particles from the Surface of Shock-Loaded Samples

The results of photon Doppler velocimetry of ejecta from shock-loaded metal samples are reported. The experiments have been performed with tin and lead samples of a given thickness and a given surface roughness. The direct numerical simulation of the process of mass ejection from the surface of shock-loaded samples is performed for conditions close to experimental by the smoothed particle hydrodynamics method. The areal density and initial velocity distribution of the volume density of ejecta are determined. Using these results, we calculate the time dependence of the profile of the volume density at the expansion of the formed dust cloud to air. Applying an approach based on the transport equation for the correlation function of the scattered field, the main parameters of the velocity distribution of ejecta, areal density of ejecta, etc. are reconstructed from spectral photon Doppler velocimetry data. The experimentally observed temporal dynamics of spectra, which is caused by the drag of dust in air, is described at an appropriately chosen size dispersion of dust particles. The masses of ejecta reconstructed from experimental data are in agreement with the smoothed particle hydrodynamics results.     

Visualization of microparticle explosion and flow field in nanoparticle synthesis by pulsed laser ablation

Laser ablation of microparticles is one of the promising methods for efficient nanoparticle synthesis but the physical mechanisms of nanoparticle formation are not yet clearly understood, particularly in the case of metallic or ceramic particles. In this work, we report, for the first time, the results of in situ visualization of the metal microparticle explosion. Ablation of a Cu microparticle in CuO nanoparticle synthesis by a Q-switched Nd:YAG laser is visualized using laser-flash shadowgraphy. The dynamics of the ablation plume and shock wave is also analyzed by monitoring the photoacoustic probe-beam deflection. Strong shock-wave emission from a single particle has been observed in the velocity range 1000–4000 m/s even at near-threshold fluences. The threshold laser fluence for particle ablation has been found to be substantially lower than that required for bulk metal ablation. Consequently, the results confirm that the photomechanical mechanism associated with the shock-wave generation plays a significant role in the ablation process. PACS 81.07.-b; 81.07.Wx; 42.62.Cf     

Analysis of the sizes and number of microparticles in the ablation plume,which attenuates laser radiation during ablation of a constructive material

The effect of laser radiation with a wavelength of 1.3 μm, power of 25 kW, pulse width of 1 s, and irradiated spot area of 9 cm² on carbon silicon carbide composite material (CSCCM) is analyzed. The formation of an ablation plume (which consists of vapor of irradiated material, burning products, drops and microparticles of various chemical composition and size) above the irradiated surface leads to a significant loss of laser energy. The fractions of the scattered and absorbed laser radiation in the plume are determined, the size and mass distributions of microparticles in the plume are estimated, their concentration is calculated, and the microparticle escape velocities from the irradiated CSCCM surface are evaluated.     

Influence of shock-wave pressure up to 65 GPa on the crystal structure and superconducting properties of MgB2

The influence of shock-wave pressure treatment up to 65 GPa on the crystal structure and the superconducting transition temperature of a polycrystalline MgB₂ sample has been investigated. X-ray diffraction measurements have revealed that the shock-wave pressure does not result in any irreversible structural phase transitions in the MgB₂, except for microdistortions formed in the crystal structure of the shock-wave pressure-treated MgB₂ sample. This conclusion is in agreement with the results of superconducting transition temperature measurements of a MgB₂ sample performed before and after its shock-wave pressure treatment.     

Mechanisms of fracture of the free surface of shock-compressed metals

The mechanisms of the ejection of aluminum and copper microparticles from the free surfaces of these metals have been studied under conditions of the escape of a moderate-intensity shock wave from a sample. The free surfaces of samples contained 0.7–0.9 mm deep artificial wells and protrusions simulating (on a greater scale of 10: 1) the natural surface roughness retained upon mechanical processing. The pressure in a shock-wave pulse at the base of a protrusion was controlled within P = 5–20 GPa (i.e., below the melting region), and the variable duration of pressure pulses was 0.02, 0.2, and 1 μs. Analysis of the free surfaces of postloaded samples showed that, for certain loading and roughness parameters, the ejection of metal from vertices of protruding ridges or pyramids (as a result of the longitudinal fracture) was about ten times greater than the amount of metal ejected in the form of cumulative jets from wells. The amount of ejected metal and the size distribution of metal microparticles were quantitatively characterized using “soft collecting targets” and by measuring mass losses of samples upon fracture.     

Role of shock waves in initiating the detonation of pentaerythritol tetranitrate by a pulsed electron beam

The results of measurements of the velocity of shock waves generated in pressed pentaerythritol tetranitrate samples by a pulsed electron beam (0.25 MeV, 15 J/cm², and 20 ns) and of the velocity of expansion of the explosion products into vacuum are presented. It was demonstrated that, during the interaction of the electron beam with pentaerythritol tetranitrate, it experiences decomposition accompanied by a pressure rise high enough to produce a shock-wave initiation of the sample.     

Response of high-purity titanium to high-pressure impulsive loading

Abstract

Measurements of free surface velocity profiles of high-purity titanium samples under shock-wave loading were performed to study the dynamic strength and phase transition parameters. The peak pressure of the initial compression waves was within the range of 4 to 40 GPa, and the load duration was vaned between 10^?8 and 10^?6 s. An anomalous structure of shock waves was observed at pressures of ～ 2.0 to 5.0 GPa due to the α-ω phase transition. The dynamic strength of pure titanium is lower than that of titanium alloys but exceeds the spall strength of commercial grade titanium.     

On the interaction of microparticles with ion flux in gas discharge plasma

The charging and interaction of microparticles in the gas discharge near-electrode region, where the role of the external electric field and ion drift is significant, are considered. It is shown that the ion focusing, responsible for, as is generally accepted, the formation of vertical linear dust structures, is strongly suppressed under conditions typical of experiments with dusty plasma (i.e., the gas discharge at pressures of 10–100 Pa). The contribution of ions trapped by an ion microparticle to the dusty particle charge is estimated. It is shown that trapped ions can appreciably reduce the microparticle charge.     

Mathematical simulation of the shock compression of porous molybdenum using a heterogeneous model

The shock compression of a heterogeneous material is numerically simulated. The physical model used for the simulation is based on a layered model of a porous material and consists of a set of thin matrix plates with a known equation of state that are separated by filler layers also with a known equation of state. The model is intended to calculate the parameters (pressure, temperature, mass velocity) of shock compression of the matrix and the filler of heterogeneous materials during their one-dimensional shock compression in terms of a developed hydrodynamic code. The adequacy of the proposed model is tested on porous molybdenum during shock-wave loading to a pressure of 15–70 GPa and a temperature of 4000 K.     

Superconductivity at 50 K in a mixed-phase Ca–P–O compound system treated by shock-wave pressure up to 65 GPa

Shock-wave pressure treatment up to 65 GPa has been applied to samples representing powdered mixtures of calcium and calcium phosphate in 1:1 volume ratio. Magnetometric measurements have revealed superconductivity of the samples at 50 K. By means of X-ray powder diffraction and magnetometric measurements, it has been found that superconductivity observed in the samples corresponds to instability at ambient pressure and room temperature of the phase/s formed in the samples during their shock-wave pressure treatment.     

Densification of Crystalline Boron Carbide during Shock-Wave Loading

Crystalline powder boron carbide samples are subjected to explosive loading by 2-μs shock waves with an amplitude of 38 GPa and shock heating to 700 K and to subsequent conservation. The samples recovered after shock-wave loading are studied by X-ray diffraction, and new effects of shock-wave loading on boron carbide are revealed. The explosive treatment is shown to shift the X-ray diffraction reflections of initial boron carbide toward high angles, which is attributed to an increase in the boron carbide density at the level of atomic volume in the unit cell of boron carbide. The X-ray diffraction reflections are found to broaden, which is interpreted as an increase in the coherent scattering region in the crystalline boron carbide subjected to the explosive treatment.     

动载荷下金属板表面的微物质喷射

 用石英晶体传感器技术，测量了冲击波作用下铝合金（Ly-12）和纯铅样品自由表面的微物质喷射量。在冲击压力为32 GPa时，测得光洁度为3.2、0.4、0.1 μm的铝合金的微物质喷射量分别为1.53～3.28 g/m²，0.2～0.3 g/m²和0.053～0.096 g/m²，对光洁度为3.2 μm的纯铅样品，在压力为13 GPa和47 GPa时，微物质喷射量分别微26.4～42.4 g/m²和183～328 g/m²。在最高冲击压力约为20 GPa时，做了多次冲击下的微物质喷射量测量，发现比单次冲击加载下的喷射量有很大的减少。结果表明，微物质喷射量与自由表面的加工条件、局部熔化和加载方式等因素有关。     

碳水混合物相互作用势及混合法则的实验研究和理论探索

 利用二级轻气炮加载技术研究了碳水混合物的冲击压缩特性。研究发现：冲击压力p低于19.0 GPa时,石墨与水混合物冲击压缩特性明显不同于金刚石与水混合物的冲击压缩特性;p大于23 GPa后,它们的特性十分接近;当p为52.9 GPa时,石墨与水混合物表现出反常的冲击压缩特性,压力增加而体积出现膨胀,这与高压下碳与水发生化学反应产生气体相关。还对碳水混合物的相互作用势及混合法则的选取进行了讨论。     

斜波压缩下锡的相变动力学特性

利用磁驱动加载装置(CQ-4)和高精度速度测试装置(DPV),开展了斜波加载下锡的动态压缩实验。实验结果表明:锡在加载阶段经历了弹塑性转变和相变等物理过程,相变压力约为7.5 GPa。β–γ相变对应的特征速度随着锡厚度的增加,从676.3 m/s减小到636.8 m/s,对应的压力从7.62 GPa降低到7.11 GPa。结合Hayes多相状态方程和非平衡相变动力学模型,对锡的斜波压缩实验过程进行了模拟,数值计算结果可以较好地描述锡在加载阶段的弹塑性转变和相变等物理过程。讨论了体模量在不同热力学过程中的物理形式,计算结果显示,斜波压缩过程需考虑压力对体模量的修正。分析了相变弛豫时间、体模量等典型物理参数对速度波形的影响,结果表明,相变弛豫时间和各相初始自由能主要影响混合区部分速度波形,γ相的体模量参数只影响相变后的速度波形,β相的体模量参数会影响整体速度波形。     

具有速度谱分辨能力的角色散FP干涉测量技术

 发展了可用于速度谱诊断的角色散Fabry-Perot干涉测量技术,使不同频移探测激光形成的干涉条纹在空间分离,从而可对单一或混合运动源引起的多普勒频移成分进行检测,并给出物体速度谱分布。利用Fabry-Perot干涉测量技术,开展了激光驱动铝箔飞片实验。随着驱动激光能量的变化,观察到了铝箔飞片的不同速度谱分布,包括没有速度空间分散的速度谱、分裂成几片并以不同速度飞行的离散速度谱、碎片云或射流状态下的连续速度谱分布,此外还观测到了冲击下铝箔/玻璃界面分离引起的条纹分裂现象。     

Structural Transformations of Amorphous Carbon (Glassy Carbon) at High Shock Pressures

Amorphous carbon (glassy carbon) samples were shock compressed up to 80 GPa and temperatures up to 1700 K for several microseconds. Glassy carbon samples before and after an explosive action are analyzed by X-ray diffraction, electron microscopy, and electron-probe microanalysis. It is shown that as a result of microsecond shock pressure exposure, glassy carbon is compacted to ρ_CG ≈ 2.3(5) g/cm³ and is partly transformed into a graphite-like nanomaterial with a cellular structure. At the level of crystallites, the density of glassy carbon increases via a decrease in the interplanar spacings and an increase in the crystallite thickness and width. Spheres from 20 nm to 80 μm in diameter are found to be formed during shock-wave compression of glassy carbon in a copper container and high-temperature shock heating posteffects. Spheres 20 μm in diameter consist of a copper-rich core and a carbon shell.     

Impacts of fast meteoroids and a plasma–dust cloud over the lunar surface

The possibility of the formation of a plasma–dust cloud in the exosphere of the Moon owing to impacts of meteoroids on the lunar surface is discussed. Attention is focused on dust particles at large altitudes of ~10–100 km at which measurements were performed within the NASA LADEE mission. It has been shown that a melted material ejected from the lunar surface owing to the impacts of meteoroids plays an important role in the formation of the plasma–dust cloud. Drops of the melted material acquire velocities in the range between the first and second cosmic velocities for the Moon and can undergo finite motion around it. Rising over the lunar surface, liquid drops are solidified and acquire electric charges, in particular, owing to their interaction with electrons and ions of the solar wind, as well as with solar radiation. It has been shown that the number density of dust particles in the plasma–dust cloud present in the exosphere of the Moon is ?10^?8 cm^?3, which is in agreement with the LADEE measurements.     

z切LiTaO3单晶的冲击相变研究

利用二级轻气炮加载下的冲击Hugoniot线(冲击波速度D-粒子速度u关系)和粒子速度剖面测量，结合基于密度泛函理论的平面波赝势计算研究了z切LiTaO₃单晶的高压相变.实验发现，D-u关系在u=0.95km/s附近出现明显拐折；实测波剖面中25.9 GPa和32.6 GPa时观测到弹-塑性双波结构，而终态压力为42.7 GPa和53.0 GPa时则为三波结构.上述结果都清楚地表明z切LiTaO₃单晶冲击相变的发生，相变起始压力约为37.9 GPa.同时，理论计算的菱形相(R3c对称群)压缩线与低压实验数据符合较好，而正交相(Pbnm对称群)压缩线则与扣除热压贡献的高压实验数据相符，由此推断z-切LiTaO₃的高压相为正交结构.从实验和理论上澄清了z切LiTaO₃的相变起始压力和高压相晶体结构的认识，研究工作亦对类似单晶材料的冲击相变研究有参考价值.     

室温高压下天然锡石状态方程研究

 利用金刚石压腔同步辐射X射线就位衍射技术,在室温、最高压力达13 GPa条件下,对采自云南个旧锡矿的天然锡石做了X衍射测量。结果表明,在实验压力范围内锡石未发生相变,其晶胞参数随压力增大而逐渐减小。与前人合成锡石实验结果相比,本次实验测定的晶胞参数偏大。由Birch状态方程对实验结果p-V进行最小二乘法拟合,得出锡石的K_T(0)＝228 GPa。分析认为,在天然矿物晶体中,Zr对Sn的类质同象置换是晶胞参数偏大的主要原因。     

Levitation of charged macroparticles in the anode region of a glow discharge

Levitation of dust particles in the anode region of a dc glow discharge was observed for the first time. A dust cloud of several tens of particles formed at a distance of several millimeters above the central part of the anode. When the discharge parameters were varied, the shape of the cloud and its position above the anode varied. An analysis of the experimental conditions revealed that these particles are positively charged in contrast to other experiments on the levitation of dust particles in a gas-discharge plasma. An estimate of the particle charge taking into account processes of electron emission from their surface is consistent with results of measurements of the electric field strength.