NOx generation in laser-produced plasma in air as a function of dissipated energy

An experimental study on the production of NO_x as a function of dissipated energy in laser-produced plasma in air is presented. A plasma was produced by focusing a (60–180) mJ, 5 ns, 532 nm pulse from a Q-switched Nd:YAG laser. The results show that for laser energy in the range of 13–99 mJ the laser plasma generates 6.7×10¹⁶ NO_x molecules per joule and 4.6×10¹⁶ NO molecules per joule. An order of magnitude estimate of the NO and NO_x production per unit volume of heated gas based on a simple model show that the NO_x and NO production efficiency in air are about 3×10²² and 2×10²² molecules J⁻¹ m⁻³.     

Real time NDE of laser shock processing with time-of-flight of laser induced plasma shock wave in air by acoustic emission sensor

Acoustic emission sensor is used to research the time-of-flight of the shock wave induced by laser-plasma in air for real time nondestructive evaluation (NDE) of laser shock processing. The time-of-flight of the shock wave propagating from the source to the sensor declines nonlinearly and similarly at the different distances for different laser energies. The velocity of the shock wave at the distance of 30 mm increases faster than that of the distance of 35 mm. The relationship between the laser energy and the distance is almost linearly when the signal with distortion is measured by acoustic emission sensor. Finally, Taylor solution is used to analyze the experimental results, and the empirical formula between the energy of the shock wave and the laser energy is established, which will provide a theoretical basis for real time NDE of laser shock processing.     

激光能量对激光诱导Cu等离子体特征辐射强度、电子温度的影响

利用Nd:YAG激光(波长1 064 nm,脉宽10 ns)烧蚀金属Cu靶获得等离子体 .改变激光脉冲能量,观测到Cu的原子谱线和离子谱线随激光脉冲能量有不同的变化关系, 但都在330 mJ/pulse时,谱线强度达到最大,随后在330 mJ～370 mJ/pulse间出现一小平台 ,能量继续增加,各谱线强度减小.同时,使用烧蚀Cu靶产生的五条原子谱线(465.11 nm,5 10.55 nm,515.32 nm,521.82 nm,529.25 nm)的相对强度,在局部热力学平衡近似下,利用B oltzmann图的最小二乘法拟合,测定了不同激光能量下Cu等离子体的电子温度.随激光能量的增加,电子温度近似单调地从1.02×104 K上升到1.46×104 K后,反而有所下降.     

激光波长对水中金属元素激光诱导击穿光谱探测的影响

针对激光诱导击穿光谱(LIBS)在海洋应用中的问题,对1 064和532nm两个激发波长下水中LIBS光谱特性进行探测分析,以比较其烧蚀效果。通过激光诱导等离子体的时间分辨光谱,分析水下等离子体电子密度随时间的演化规律,1 064nm激光诱导等离子体寿命约为1 200ns,而532nm激光激发情况下等离子体寿命仅约为600ns。基于光在水中的传输特性和LIBS的实验结果,建立了获得最佳LIBS探测效果所需的入水前激光脉冲能量Eiopt(r)与探测距离r的关系,并应用到水下原位探测的模拟分析。结果表明,当探测距离不大于5cm时,所需的入水前1 064nm激光单脉冲能量小于100mJ,该激发波长可用于LIBS的水下探测;当探测距离增至10cm时,所需的入水前532nm激光单脉冲能量只需30mJ左右。因此,当原位探测距离增加时,则需考虑选择532nm激光作为烧蚀光源。     

Spatial and time resolved analysis of CN bands in the laser induced plasma from graphite

Analysis of the emission bands of the CN molecules in the plasma generated from a graphite target irradiated with 1.06 μm radiation pulses from a Q-switched Nd:YAG laser has been done. Depending on the position of the sampled volume of the plasma plume, the intensity distribution in the emission spectra is found to change drastically. The vibrational temperature and population distribution in the different vibrational levels have been studied as function of distance from the target for different time delays with respect to the incidence of the laser pulse. The translational temperature calculated from time of flight is found to be higher than the observed vibrational temperature for CN molecules and the reason for this is explained.     

气体压强对纳秒激光诱导空气等离子体特性的影响

环境气体的压强对激光诱导等离子体特性有重要影响.基于发射光谱法开展了气体压强对纳秒激光诱导空气等离子体特性影响的研究,探讨了气体压强对空气等离子体发射光谱强度、电子温度和电子密度的影响.实验结果表明,在10-100 kPa空气压强条件下,空气等离子体发射光谱中的线状光谱和连续光谱依赖于气体压强变化,且原子谱线和离子谱线强度随气体压强的变化有明显差别.随着空气压强增大,激光击穿作用区域的空气密度增加,造成激光诱导击穿空气几率升高,从而等离子体辐射光谱强度增大.空气等离子体膨胀区域空气的约束作用,增加了等离子体内粒子间的碰撞几率以及能量交换几率,并且使离子-电子-原子的三体复合几率增加,因此造成原子谱线OⅠ777.2 nm与NⅠ821.6 nm谱线强度随着气体压强增大而增大,在80 kPa时谱线强度最高,随后谱线强度缓慢降低.而离子谱线N Ⅱ 500.5 nm谱线强度在40 kPa时达到最大值,气体压强大于40 kPa后,谱线强度随压强增加而逐渐降低.空气等离子体电子密度均随压强升高而增大,在80 kPa后增长速度变缓.等离子体电子温度在30 kPa时达到最大值,气体压强大于30 kPa后,等离子体电子温度逐渐降低.研究结果可为不同海拔高度的激光诱导空气等离子体特性的研究提供重要实验基础,为今后激光大气传输、大气组成分析提供重要的技术支持.     

激光烧蚀硬铝产生等离子体温度和力学效应测量

 为了研究激光推进技术中激光与材料相互作用的机制,获取等离子体状态参数及力学参数,采用Nd:YAG被动调Q固体激光器烧蚀硬铝,通过激光诱导等离子体光谱技术测得等离子体光谱和温度,由冲量摆测得力学参数。实验结果显示：在激光功率密度0.534×10⁸ W/cm²时,靶材表面的等离子体温度在等离子体辐射过程中呈二次曲线衰减;改变靶材等离子体点燃阈值附近的激光功率密度时,随着功率密度的增加,等离子体温度、冲量耦合系数也随着增大,当功率密度达到靶材的等离子体点燃阈值时,各参数达到最大,此后随着功率密度增加,由于等离子体对能量的屏蔽作用,导致靶材表面的等离子体温度降低,等离子体获得的动能减少,靶材耦合的冲量降低。     

Effect of energy above laser-induced damage thresholds in the micromachining of silicon by femtosecond pulse laser

A 400 nm second harmonic Ti : sapphire femtosecond laser was applied to structure silicon base on a direct-write process in air. A series of lines were ablated with pulses of 300-fs duration at varying power densities ranging from 50 to 100 nJ of energy on 2″ silicon (1 1 1) wafers. In this event, we investigate and report extensive laser induced thermal damage and redeposition encompassing the ablated lines at high energy levels above the damage threshold of the silicon. In addition, the effect of polarisation on the direction of micromachining is also observed and discussed. The resolution and quality of these lines were also found to hold a linear relationship to the laser energy up to its thermal threshold limit.     

Theoretical and experimental study of laser induced damage on GaAs by nanosecond pulsed irradiation

The surface damage experiments of gallium arsenide (GaAs) single crystal irradiated by 1.06 and 0.53 μm nanosecond irradiations are carried out with fundamental and frequency-doubled Nd:YAG laser, respectively. The surface damage thresholds for both wavelengths are experimentally determined and the damaged morphologies and elementary component are analyzed with electron probe microanalyzer (EPM). It is found that the components of Ga and As almost keep constant in our experiments when the irradiated fluence is just around the surface damage threshold and no oxygen is found at all. The theoretical calculations on temperature rise for both wavelengths are carried out using the purely thermal model. It is shown that for irradiation with photon energy above the corresponding band gap the theoretical calculation is in good agreement with the experimental results; however, for that with photon energy just below the band gap, the experimental results cannot be effectively explained by the purely thermal heating mechanism. Combining with the experiment of multi-shot damage from references we finally conclude that the damage by laser irradiation with photon energy below the band gap should be explained by the micro-defect accumulation and consequently enhanced absorption heating mechanism.     

飞秒激光等离子体通道传导能量特性的研究进展

基于强飞秒激光诱导等离子体通道的基本性质,分析了等离子体通道在传导电能和射频电磁能时的一些基本问题。综述了飞秒激光等离子体通道传输电能和电磁能方面的新概念和新技术,重点描述了该领域近几年所取得的一些理论及应用的最新成果。最后,展望了飞秒激光等离子体通道在传导能量技术领域中的发展趋势。     

Effect of leakage transformer on laser power and mode pattern of CO2 laser excited by 60 Hz AC discharges

The 60 Hz AC discharges generated by a leakage transformer instead of an isolation one, which is usually used in the power supply of DC CO₂ laser, are employed as a pumping source in this experiment. The laser performance characteristics as functions of pressure and discharge current have been investigated. The maximum laser power is about 40 W at conditions of 18 Torr and 35 mA. The output power is not less than that of DC discharge at the same condition. In addition, the laser outputs of 60 Hz AC discharges obtained by different transformers such as a leakage type and an isolation one have been measured. It was found that the laser power obtained by the leakage transformer is saturated at the higher current compared with the isolation transformer.     

Melting of carbon heated by focused laser radiation in air at atmospheric pressure and temperature below 4000 K

The melting of carbon at a pressure of about 1 atm is observed. The escape of liquid carbon from the heating region and its spread over the sample surface are observed in the experiments. The structural changes in graphite occurring in the melting region are determined from scanning tunneling microscope (STM) images obtained by means of an atomic force microscope and from Raman spectra. The STM images exhibit significant rearrangement of the structure of the graphite surface in the region of the escape of liquid carbon. The Raman spectra show that the graphite ordering degree increases significantly.     

Study of the electric field and wall voltage in a high pressure ac-PDP cell by laser induced fluorescence spectroscopy

The electric field in a surface discharge type ac-PDP cell with He or He/Xe(0.1%) mixture has been measured over a wide range of pressure (5－50 kPa) using laser induced fluorescence detection. The wall voltage was estimated from the measured electric field. The Stark manifolds of triplet atomic helium Rydberg state (2s^3S) with principal quantum numbers (n=8 and 9) have been used to measure the electric field, as the lifetime of 2s^3S is longer than the single atomic helium Rydberg state (2s^1S) in high pressure discharge. Comparison of the Stark manifolds between the n=9 and n=8 shows that the measurement accuracy of electric field can be increased by 10%. The maximum electric field strength during discharge and the wall voltage at the end of pulse decreases with the increase of pressure. The comparison of He and He/Xe(0.1%) discharge at 13 kPa showed that He/Xe gas mixture discharge can accumulate more wall charge on MgO surface and the electric field was somewhat higher than those of pure helium discharge during pulse off period under the same discharge conditions.     

Effect of laser energy, substrate film thickness and bioink viscosity on viability of endothelial cells printed by Laser-Assisted Bioprinting

Biofabrication of three dimensional tissues by Laser-Assisted Bioprinting (LAB) implies to develop specific strategies for assembling the extracellular matrix (ECM) and cells. Possible strategies consist in (i) printing cells onto or in the depth of ECM layer and/or (ii) printing bioinks containing both cells and ECM-like printable biomaterial. The aim of this article was to evaluate combinatorial effects of laser pulse energy, ECM thickness and viscosity of the bioink on cell viability. A LAB workstation was used to print Ea.hy926 endothelial cells onto a quartz substrate covered with a film of ECM mimicking Matrigel™. Hence, effect of laser energy, Matrigel™ film thickness and bioink viscosity was addressed for different experimental conditions (8-24 μJ, 20-100 μm and 40-110 mPa s, respectively). Cell viability was assessed by live/dead assay performed 24 h post-printing. Results show that increasing the laser energy tends to augment the cell mortality while increasing the thickness of the Matrigel™ film and the viscosity of the bioink support cell viability. Hence, critical printing parameters influencing high cell viability have been related to the cell landing conditions and more specifically to the intensity of the cell impacts occurring at the air-ECM interface and at the ECM-glass interface.     

Adsorption of N2 and N2O on Ni(7 5 5) surface: ab initio periodic density functional study

Adsorption of N₂ and N₂O at various sites on Ni(7 5 5) has been investigated by density functional theory (DFT) method (periodic DMol³). Several possible adsorption structures (attaching the nitrogen atom to the surface, or lying parallel) are found for both molecules. There is a clear binding energy preference of N₂ and N₂O for step sites in contrast to the case of CO. It is revealed that the decomposition of N₂O occurs exclusively near the step, but not on the terrace. Two decomposition channels can be considered; dissociative adsorption and spontaneous decomposition during TPD ramp. Three possible candidates for the precursor of the spontaneous decomposition of N₂O during TPD ramp are discussed.     

A study on microstructures and properties of P/M valve seats of hot forging by laser irradiation

A CO₂ laser was used as heating source for hot forging. Powder/metallurgy (P/M) valve seats for car engine was studied by this novel process. The effects of compression ratio and tempering temperature followed by laser irradiation on the microstructures and mechanical properties were investigated. It was indicated that the amount of the recrystallized grains increased with increasing compression ratio, especially when the compression ratio was above a certain value (18.70%), resulting in more refined grains. Good properties including density, radial crushing strength and hardness can be obtained by optimizing processing parameters.     

A study of TiB2/TiB gradient coating by laser cladding on titanium alloy

TiB₂/TiB gradient coating has been fabricated by a laser cladding technique on the surface of a Ti–6Al–4V substrate using TiB₂ powder as the cladding material. The microstructure and mechanical properties of the gradient coating were analyzed by SEM, EPMA, XRD, TEM and an instrument to measure hardness. With the increasing distance from the coating surface, the content of TiB₂ particles gradually decreased, but the content of TiB short fibers gradually increased. Meanwhile, the micro-hardness and the elastic modulus of the TiB₂/TiB coating showed a gradient decreasing trend, but the fracture toughness showed a gradient increasing trend. The fracture toughness of the TiB₂/TiB coating between the center and the bottom was improved, primarily due to the debonding of TiB₂ particles and the high fracture of TiB short fibers, and the fracture position of TiB short fiber can be moved to an adjacent position. However, the debonding of TiB₂ particles was difficult to achieve at the surface of the TiB₂/TiB coating.     

Effect of pulse repetition rate and number of pulses in the analysis of polypropylene and high density polyethylene by nanosecond infrared laser induced breakdown spectroscopy

Pulse repetition rates and the number of laser pulses are among the most important parameters that do affect the analysis of solid materials by laser induced breakdown spectroscopy, and the knowledge of their effects is of fundamental importance for suggesting analytical strategies when dealing with laser ablation processes of polymers. In this contribution, the influence of these parameters in the ablated mass and in the features of craters was evaluated in polypropylene and high density polyethylene plates containing pigment-based PbCrO₄. Surface characterization and craters profile were carried out by perfilometry and scanning electron microscopy. Area, volume and profile of craters were obtained using Taylor Map software. A laser induced breakdown spectroscopy system consisted of a Q-Switched Nd:YAG laser (1064 nm, 5 ns) and an Echelle spectrometer equipped with ICCD detector were used. The evaluated operating conditions consisted of 10, 25 and 50 laser pulses at 1, 5 and 10 Hz, 250 mJ/pulse (85 J cm⁻²), 2 μs delay time and 6 μs integration time gate. Differences in the topographical features among craters of both polymers were observed. The decrease in the repetition rate resulted in irregular craters and formation of edges, especially in polypropylene sample. The differences in the topographical features and ablated masses were attributed to the influence of the degree of crystallinity, crystalline melting temperature and glass transition temperature in the ablation process of the high density polyethylene and polypropylene. It was also observed that the intensities of chromium and lead emission signals obtained at 10 Hz were two times higher than at 5 Hz by keeping the number of laser pulses constant.     

Self-focusing and defocusing of TEM0p Hermite-Gaussian laser beams in collisionless plasma

The authors have investigated the self-focusing and defocusing of first six TEM_0p Hermite-Gaussian laser beams in collisionless plasma. In case of collisionless plasma the nonlinearity in the dielectric constant is mainly due to the ponderomotive force. It is found that modes with odd p-values defocuses and that with even p-values exhibit oscillatory as well as defocusing character of beam-width parameters variation during their propagation in collisionless plasma. The entire theoretical formulation is established under parabolic wave equation approach. The numerical computation is completed by using fourth order Runge-Kutta method. Finally the behavior of beam-width parameters with the dimensionless distance of propagation is presented graphically.     

Theoretical investigations on KClxBr1−x, KClxI1−x and KBrxI1−x: A first-principles study

Using first-principles total energy calculations within the full-potential linearized augmented plane wave (FP-LAPW) method, we have investigated the structural, electronic and thermodynamic properties of potassium halides (KCl_xBr_1−x, KCl_xI_1−x and KBr_xI_1−x), with x concentrations varying from 0% up to 100%. The effect of composition on lattice constants, bulk modulus, band gap and dielectric function was investigated. Deviations of the lattice constants from Vegard's law and the bulk modulus from linear concentration dependence (LCD) were observed for the three alloys. The microscopic origins of the gap bowing were explained by using the approach of Zunger and coworkers. On the other hand, the thermodynamic stability of these alloys was investigated by calculating the excess enthalpy of mixing ΔH_m as well as the phase diagram.