Flow characteristics and micro-scale metallic particle formation in the laser supersonic heating technique

The characteristics of the supersonic flow of the laser heating technique for producing micro-scale metallic particles were investigated in this study. A numerical model was established to predict the flow fields and particle trajectories leaving a spray nozzle with shock wave effects. The compressible flow of the shock waves and the trajectories of particles in diameters of 1–20 μm were simulated and compared with the flow visualization. In the experiment, a pulsed Nd-YAG laser was used as heat source on a carbon steel target within the nozzle, and the carbon steel particles were ejected by high-pressure air. The result shows that the shock wave structures were generated at various entrance pressures, and there is a significant increase in the amount of carbon steel particles and the spraying angles by increasing the entrance air pressure.     

影响激光加热运动圆柱体的加热线型的分析

 采用几何分析的方法研究激光加热运动圆柱体的过程，在说明加热线型的概念和意义后，讨论了在地平面上各点处所具有的加热线型的时间宽度，从而指出，为了激光有效加热运动圆柱体，激光器应当怎样在地平面上放置的原则。即根据激光器有效工作时间与圆柱体抛射时间的比例关系选择一定的相对时间宽度等值线，以此等值线为界在地平面上划出一有效区域，并从中选取距离轨道最近的点作为激光器的放置点。当考虑大气对激光的衰减作用时，激光以小于一定入射角阈值进行动态加热的总能量将出现极大值，在此处放置激光器最有利于动态加热。     

Searching for minicharged particles via birefringence,dichroism and Raman spectroscopy of the vacuum polarized by a high-intensity laser wave

Absorption and dispersion of probe photons in the field of a high-intensity circularly polarized laser wave are investigated. The optical theorem is applied for determining the absorption coefficients in terms of the imaginary part of the vacuum polarization tensor. Compact expressions for the vacuum refraction indices and the photon absorption coefficients are obtained in various asymptotic regimes of interest. The outcomes of this analysis reveal that, far from the region relatively close to the threshold of the two-photon reaction, the birefringence and dichroism of the vacuum are small and, in some cases, strongly suppressed. On the contrary, in a vicinity of the region in which the photo-production of a pair occurs, these optical properties are manifest with lasers of moderate intensities. We take advantage of such a property in the search of minicharged particles by considering high-precision polarimetric experiments. In addition, Raman-like electromagnetic waves resulting from the inelastic part of the vacuum polarization tensor are suggested as an alternative form for finding exclusion limits on these hypothetical charge carriers. The envisaged parameters of upcoming high-intensity laser facilities are used for establishing upper bounds on the minicharged particles.     

Thermoelectric temperature measurements in laser pulsed heating of metals

Measurements of temperature pulses following focused laser irradiation on metals were performed with a specially designed thermocouple. This detector had a hot junction of an average of 5 μ in size and a rise time of less than 1 μsec. Experimental results are compared to thermal-model predictions calculated numerically. Thermal responses were also measured for laser intensities above the damage threshold of the material, in close vicinity of the damaged area. This demonstrates possible applications of the experimental technique to the investigation of damage mechanisms.     

激光器与轨道的相对位置对动态加热的影响

 通过分析光束入射的角度因子将激光器在地平面上的安放位置进行分类,采用了几何分析和数值模拟的方法,探讨了聚焦激光束在加热运动圆柱体过程中,为了达到一定的加热功率水平和能量水平以取得最好的加热效果而应采取的措施,即应当以激光垂直入射的轨道点为中心,以激光的有效工作时间为取值范围,并参考大气对激光能量的衰减,相对于轨道选取适当的距离进行加热。该研究从理论上说明了激光器与圆柱体运动轨道的相对位置对激光动态加热的影响可以看作一个优化问题,存在最优解,进而对于合理使用激光器进行动态加热具有参考价值。     

激光对运动圆柱体加热的几何学研究

 当激光无论从地面或是从空中辐照运动的圆柱体时，都涉及到入射激光相对于圆柱体的辐照方式。在一定假设下，导出了激光对抛射圆柱体表面加热时圆柱表面热加载功率密度分布公式，并结合一理想情况，针对光束入射角度因子、光斑中心的偏移、光斑形状的变化进行了解析计算分析。分析表明，即使圆柱体本身不做自转，圆柱体表面的光强分布中心也会发生偏移，分布也会发生变化，尤其沿圆柱体轴向的分布变化较大，必须结合实际加以细化研究。     

Elastic displacement of surface due to laser picosecond pulse heating of gold

Laser picosecond pulse heating initiates nonequilibrium energy transport in the surface vicinity of the metallic substrates. In this case, electron temperature rises rapidly while lattice site temperature is slow during the heating period. Although the rise of lattice site temperature is low, the temperature gradient is high. This results in elastic displacement of the surface. To model the heating process thermomechanical coupling needs to be introduced in the analysis. In the present study, picosecond pulse heating of gold surface is considered. Three-dimensional analysis of electron kinetic theory is introduced when formulating the electron and the lattice site temperatures. The analysis is extended to include the thermomechanical coupling due to mechanical response of the substrate material. The lattice site temperatures obtained from the electron kinetic theory are compared with the predictions of the two-equation model. It is found that both models predict almost identical temperature profiles in the surface vicinity of the substrate material. In addition, the surface displacement on the order of 10⁻¹¹ m is predicted.     

Analytical investigation into laser pulse heating and thermal stresses

Laser pulse heating of metallic surfaces results in rapid rise of temperature in the region irradiated by the laser beam. This in turn results in high temperature gradient in this region. The irradiated substrate material expands as a response to the temperature gradient. Consequently, high thermal stress levels are developed in the region of the high temperature gradient. In the present study, closed form solutions for temperature and stress fields due to a laser pulse decaying exponentially in time are presented. A Laplace transformation method is employed in the analysis. The resulting equations are non-dimensionalized with the appropriate parameters. It is found that temperature rises rapidly during the early heating period in the surface region. In this case, internal energy gain dominates the conduction losses from the surface vicinity. The thermal stress levels attain high values in the surface region. The stress wave developed is compressive and it propagates with a wave speed c₁ inside the substrate.     

短脉冲激光加热模型的遴选原则

 利用推导出的1维半空间光致热弹性应力响应的精确解,对激光与材料相互作用的加热模型的选取进行了研究。理论分析和计算结果表明：常用的表面加热模型只有在热穿透深度远大于光吸收长度时有效,若将其应用于超短脉冲激光（ps以下量级）加热的情况,可能会导致较大的误差;皮秒和飞秒激光辐照的热效应须用体加热模型计算。     

Analytical solution for multilayer assembly including heating and cooling cycles with laser pulse parameter variation

Laser heating of surfaces is involved with heating and cooling cycles. Material response to a laser pulse in the heating cycle is rapid while in the cooling cycle it is gradual. In this case, temperature rises rapidly in the heating cycle while temperature decay is gradual in the cooling cycle. Depending on the laser pulse properties (pulse length and intensity), the rise and fall of temperature profiles change in the surface region of the substrate material. In the present study, an analytical solution for laser heating pulse is presented and a closed-form solution for temperature distribution inside the multilayer assembly is obtained. Steel is considered as top layer while copper is situated below steel in the multilayer assembly. It is found that the analytical solution agrees well with the numerical predictions. Temperature rise in steel is higher than copper. This is due to the thickness of steel, which is larger than the absorption depth. In this case, internal energy gain dominates over the heat conduction in the energy transport process.     

Ultraviolet laser removal of small metallic particles from silicon wafers

Laser removal of small 1 μm sized copper, gold and tungsten particles from silicon wafer surfaces was carried out using ultraviolet radiation at 266 nm generated by Nd:YAG harmonic generation. Successful removal of both copper and gold particles from the surface was achieved whereas tungsten particles proved to be difficult to remove. The cleaning efficiency was increased with an increase of laser fluence. The optimum processing window for safe cleaning of the surface without any substrate damage was determined by measuring the damage threshold laser fluence on the silicon substrate and the required fluence for complete removal of the particles. The different cleaning efficiencies with particle type are discussed by considering the adhesion force of the particle on the surface and the laser-induced cleaning force for the three different particles.     

铜微颗粒碰撞阻尼特性

碰撞阻尼在机床、机器人、透平机械、飞机以及运载火箭等领域具有重要的应用价值. 在碰撞阻尼器中加入微颗粒材料, 可以利用颗粒的细化和塑性变形而有效地吸收振动能量, 为碰撞阻尼的研究和发展开辟了一条新途径. 本文讨论了带有中值粒度为50 μm的铜颗粒碰撞阻尼器在96 h内对正弦激励悬臂梁的阻尼减振特性. 研究表明, 在所考察的时间段内, 主系统的响应经历了先上升、再下降和再上升的过程. 这三个阶段的响应对应着铜颗粒微观结构变化的三个阶段. 在初始阶段, 铜颗粒主要表现为弹性变形, 能耗较低, 而钢球的次谐波共振可能将部分能量返回给主系统, 使主系统响应随时间呈现近似线性的上升; 在第二阶段, 当主系统响应增加到一定程度时, 钢球对铜粉的冲击力超出铜颗粒的屈服应力, 铜颗粒发生屈服, 不可逆能耗使主系统的响应震荡下降; 到了第三阶段, 铜颗粒在钢球冲击下发生硬化, 其应变和层错概率上升, 应变能和层错能下降, 主系统的响应再次持续震荡上升. 本文的结果对振动的被动控制以及材料塑性变形机理研究具有参考 意义. 关键词： 振动控制 碰撞阻尼 颗粒减振剂 微结构分析     

强激光与超音速自由剪切层的相互作用

 以波长为10.6μm,半径0.1m,焦距106m的激光束穿过二维超音速自由射流剪切层流场为物理模型，研究了由于辐射加热强激光束对流场的干扰以及激光束穿过流场后光束远场强度的分布。结果表明：由于流场的存在，使光束的远场强度分布产生了较为明显的变化，对于射流出口处不同的马赫数以及出口压力，流场对光场的远场强度分布影响不同；对于没有引起气体介质电离的强激光束，由辐射加热引起的对超音速自由射流剪切层流场的影响可以忽略。     

Production of microscale particles from fish bone by gas flow assisted laser ablation

Recycled wastes from fish and seafood can constitute a source of precursor material for different applications in the biomedical field such as bone fillers or precursor material for bioceramic coatings to improve the osteointegration of metallic implants.In this work, fish bones have been used directly as target in a laser ablation system. A pulsed Nd:YAG laser was used to ablate the fish bone material and a transverse air flow was used to extract the ablated material out of the interaction zone. The particles collected at a filter were in the micro and nanoscale range. The morphology as well as the composition of the obtained particles were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and transmission electron microscopy (TEM). The results reveal that the composition of the analyzed particles is similar to that of the inorganic part of the fish bone.     

Repetitive laser pulse heating analysis: Pulse parameter variation effects on closed form solution

Laser conduction limited heating finds wide application in surface processing industry. Modelling of the heating process gives insight into the physical processes involved in conduction limited heating. Moreover, analytical solutions provide functional relation among the parameters that influence the heating process. In the present study, repetitive laser pulse heating of a solid substrate is considered. A closed form solution for the temperature rise including the cooling cycle is obtained using a Laplace transformation method. It is found that the results obtained from the closed form solution agree well with the numerical predictions. The maximum surface temperature rises rapidly once the cooling period between the consecutive pulses reduces.     

Photo-production of scalar particles in the field of a circularly polarized laser beam

The photo-production of a pair of scalar particles in the presence of an intense, circularly polarized laser beam is investigated. Using the optical theorem within the framework of scalar quantum electrodynamics, explicit expressions are given for the pair production probability in terms of the imaginary part of the vacuum polarization tensor. Its leading asymptotic behavior is determined for various limits of interest. The influence of the absence of internal spin degrees of freedom is analyzed via a comparison with the corresponding probabilities for production of spin-1/2 particles; the lack of spin is shown to suppress the pair creation rate, as compared to the predictions from Dirac theory. Potential applications of our results for the search of minicharged particles are indicated.     

Mold-free fs laser shock micro forming and its plastic deformation mechanism

Mold-free micro forming using a fs laser was investigated by producing micro pits on pure aluminum foil. The characteristics of the pit profiles, their forming mechanisms, and the influences of some important parameters on the pit profiles were investigated by measuring the profiles and the surface morphologies of the pits. The microstructures of the shocked aluminum foil were observed through transmission electron microscopy (TEM). Pits obtained through fs laser shock forming are composed of two regions: the directly impacted region and the plastically bending region. Diameters of the former strongly depend on laser beam sizes. The plastically bending region has a negative effect on forming precision. Shorter laser pulse width is beneficial for narrowing the range of the plastically bending region and enhancing the forming precision. Using a single-side clamping mode can also narrow the plastically bending region through buffering the local bending. Fs laser-induced microstructures are characteristic of fragmentary short dislocation lines and parallel slip lines, which are the results of the ultrafast and ultrahigh pressure loading. The localization of the fs laser shock forming induced by ultrafast loading can enhance the precision of mold-free forming.     

Roughness measurement of metallic surfaces based on the laser speckle contrast method

A method of measuring surface roughness of flat lapped, ground and polished metallic surfaces, by the far-field speckle contrast method is presented in this paper. The laser speckle contrast technique depends on the existence of an approximately linear relationship between the speckle contrast and the roughness of the illuminated surface. Initially it was shown that the linear relationship existed up to 0.1 μm Ra (centre-line average) roughness using Helium–Neon light, after which a saturation effect was observed. The effect of varying the incident angle of illumination was investigated with a view to extending the measurement range. The use of high incident angles of illumination has been found to increase the surface roughness range up to 0.4 μm measurement Ra.     

自悬浮定向流法制备纳米铜微粒及其结构表征

 采用自悬浮定向流法制备金属铜纳米微粒，并用TEM，XRD和AES等分析手段研究了铜纳米微粒的形貌、粒度、结构及其表面氧化层特性。结果表明，在一定的参数条件下采用自悬浮定向流法可制备出单晶纳米铜微粒，并且通过工艺参数的调控可达到对微粒粒度的控制。