采用非均匀防热材料进行高速飞行器防热设计的防热效果研究

本文提出采用非均匀防热材料进行高超声速飞行器防热设计的设想,设计出外层部分为抗烧蚀、抗剪切的紧密结构材料,内层为密度轻、隔热效率高的稀松材料,中间为逐渐过渡层的非均匀防热材料,数值模拟了非均匀防热材料的烧蚀防热情况,探讨了非均匀防热材料在飞行器防热设计中应用的可行性和防热效果,通过本文研究发现采用非均匀防热材料可以降低原始材料质量消耗,大大减轻防热结构重量,减少向内部结构传导的热量,降低内部结构的温升,这些优点显示了非均匀防热材料具有很高的防热效率和广阔的应用前景.     

Sub-picosecond UV laser ablation of metals

Laser ablation of Nickel, Copper, Molybdenum, Indium, Tungsten and Gold by short ultraviolet laser pulses (0.5 ps, 248 nm) in vacuum is reported for the first time. For Nickel and Indium, ablation is also studied in air to demonstrate the influence of the ambient atmosphere. Metal ablation in air is significantly less efficient than in vacuum due to redeposition of ablated material. The ablation rates in vacuum are discussed using a thermal model, which also allows to estimate ablation rates for other metals from basic optical and thermal properties. A comparison of the morphology of ablation sites after nanosecond and sub-picosecond ablation shows unequivocally the advantages of short-pulse laser ablation for high-precision patterning of thermally good conducting materials in micron-scale dimensions.     

飞船返回舱气动热及烧蚀防热的不确定性初步研究

由于飞船返回舱的气动加热和热化学烧蚀是非常复杂的变化过程,许多参数很难测定,并且与材料制造工艺水平密切相关,目前使用的数据存在较大散布,需要研究这些不确定因素对防热效果的具体影响。本文数值模拟了炭化材料的烧蚀热响应过程,研究了气动加热及材料参数的不确定性对计算结果的影响,在对计算结果进行分析的基础上,定量地研究了这些不确定量对烧蚀防热影响大小,说明了防热材料的各物理量对烧蚀防热影响重要程度完全不同,应该对几个关键参数进行深入细致的研究。     

Theoretical model of CO2 laser ablation of soft-tissue phantoms

Summary A model of thermal laser ablation of soft tissues is developed taking into consideration two mechanisms: evaporation and liquid moving, due to vapour pressure gradient. Usually a soft tissue is modelled as a single-component material with thermal and optical properties very similar to those of water. We examined the non-stable kinetics of the evaporation process, for short-pulse infrared laser ablation of soft tissues, and we also calculated the average liquid velocity and the ablation rates under vapour pressure gradient. The theoretical results are in good agreement with previous reported experimental data on gelatin and polyacrylamide tissue phantoms The authors of this paper have agreed to not receive the proofs for correction     

Efficient and controllable thermal ablation induced by short-pulsed HIFU sequence assisted with perfluorohexane nanodroplets

A HIFU sequence with extremely short pulse duration and high pulse repetition frequency can achieve thermal ablation at a low acoustic power using inertial cavitation. Because of its cavitation-dependent property, the therapeutic outcome is unreliable when the treatment zone lacks cavitation nuclei. To overcome this intrinsic limitation, we introduced perfluorocarbon nanodroplets as extra cavitation nuclei into short-pulsed HIFU-mediated thermal ablation. Two types of nanodroplets were used with perfluorohexane (PFH) as the core material coated with bovine serum albumin (BSA) or an anionic fluorosurfactant (FS) to demonstrate the feasibility of this study. The thermal ablation process was recorded by high-speed photography. The inertial cavitation activity during the ablation was revealed by sonoluminescence (SL). The high-speed photography results show that the thermal ablation volume increased by ∼643% and 596% with BSA-PFH and FS-PFH, respectively, than the short-pulsed HIFU alone at an acoustic power of 19.5 W. Using nanodroplets, much larger ablation volumes were created even at a much lower acoustic power. Meanwhile, the treatment time for ablating a desired volume significantly reduced in the presence of nanodroplets. Moreover, by adjusting the treatment time, lesion migration towards the HIFU transducer could also be avoided. The SL results show that the thermal lesion shape was significantly dependent on the inertial cavitation in this short-pulsed HIFU-mediated thermal ablation. The inertial cavitation activity became more predictable by using nanodroplets. Therefore, the introduction of PFH nanodroplets as extra cavitation nuclei made the short-pulsed HIFU thermal ablation more efficient by increasing the ablation volume and speed, and more controllable by reducing the acoustic power and preventing lesion migration.     

Thermal melting and ablation of silicon by femtosecond laser radiation

The space-time dynamics of thermal melting, subsurface cavitation, spallative ablation, and fragmentation ablation of the silicon surface excited by single IR femtosecond laser pulses is studied by timeresolved optical reflection microscopy. This dynamics is revealed by monitoring picosecond and (sub)nanosecond oscillations of probe pulse reflection, which is modulated by picosecond acoustic reverberations in the dynamically growing surface melt subjected to ablation and having another acoustic impedance, and by optical interference between the probe pulse replicas reflected by the spalled layer surface and the layer retained on the target surface. The acoustic reverberation periods change during the growth and ablation of the surface melt film, which makes it possible to quantitatively estimate the contributions of these processes to the thermal dynamics of the material surface. The results on the thermal dynamics of laser excitation are supported by dynamic measurements of the ablation parameters using noncontact ultrasonic diagnostics, scanning electron microscopy, atomic force microscopy, and optical interference microscopy of the modified regions appearing on the silicon surface after ablation.     

Phase explosion and recoil-induced ejection in resonant-infrared laser ablation of polystyrene

We investigate the phenomenon of resonant-infrared laser ablation of polymers using polystyrene as a model material. Ablation is carried out using various mid-IR laser wavelengths that are resonant with vibrational modes of a polystyrene target. Time-resolved plume imaging coupled with etch-depth measurements and thermal calculations indicate that ablation begins after a superheated surface layer reaches a temperature of ∼1000°C and undergoes spinodal decomposition. The majority of the ablated material is then expelled by way of recoil-induced ejection as the pressure of the expanding vapor plume compresses a laser-melted area.     

Pulsed laser ablation of metals in vacuum: DSMC study versus experiment

We propose a hybrid model describing the processes of heating and ablation of a metal target with expansion of vaporized material into a vacuum under the action of nanosecond laser pulses of moderate intensity. The model is based on the integration of the thermal problem of laser-induced material heating and the direct Monte Carlo simulation method to describe the hydrodynamics of the ablation products. Modeling has been performed for niobium as an example and the results obtained are in good agreement with experimental data on mass removal and time-of-flight (TOF) measurements. Application of the commonly used fitting procedures for characterization of the laser plume parameters by the TOF signal is discussed. PACS 52.38.Mf; 79.20.Ds     

Material ejection in nanosecond Er:YAG laser ablation of water, liver, and skin

We investigated the mechanisms of material ejection in Q-switched Er:YAG laser tissue ablation (70-ns pulse duration) where moderate and large radiant exposures are associated with large volumetric energy densities in the target material. For water, an initial phase of non-equilibrium surface vaporization is followed by an explosive vaporization of the superficial liquid volume from a supercritical state. The ablation of deeper layers with lower peak temperatures proceeds as phase explosion. For mechanically strong tissues, non-equilibrium surface vaporization is followed by a vapour explosion coupled with thermal dissociation of the biomolecules into volatile products. In deeper layers, ablation proceeds as confined boiling with mechanical tearing of the tissue matrix by the vapour pressure. The recoil stress induced at a radiant exposure of 5.4 J/cm² is in the order of 500–900 MPa. For water and soft tissues such as liver, the recoil causes a powerful secondary material expulsion. For stronger tissues such as skin, no secondary expulsion was observed even though the recoil stress largely exceeds the static tensile strength of the tissue. Recoil-induced material expulsion results in an increase of both ablation efficiency and mechanical side effects of ablation. Theoretical modelling of the succession of phase transitions in nanosecond-laser tissue ablation and of recoil-induced material expulsion remain a major challenge for future work. PACS  42.62.Be; 79.20.Ds     

分形理论在碳化材料三维烧蚀热防护计算中的应用

烧蚀过程中的传热传质对烧蚀防护工程具有重要意义,准确的温度评估可以为高超声速飞行器的热防护结构和烧蚀材料的设计提供有效的支持.由于材料烧蚀形成的碳化物是一种典型的多孔介质,其结构具有自相似性,可以用分形理论来描述.在烧蚀计算中引入了分形渗透率张量模型进行计算.针对烧蚀过程中热解气体在碳化层中的扩散方程、温度、材料等参数...     

Optodynamic study of multiple pulses micro drilling

This paper describes an analysis of pulsed lasers micro-drilling of different metals. Study focuses to an optodynamic phenomenon which appears as thermal effects induced by laser light pulses and leads to dynamic process manifested as ultrasonic shock waves propagating into the sample material. The shock waves are detected by a non-contact optical method by using arm compensated Michelson. Monitoring of the main parameters of the micro drilling such as material ablation rate and efficiency was realized by analysis of the optodynamic signals. The process is characterized by decreasing ablation rate that leads to the finite hole depth. The experimental part of study comprehends a comparison between various metals. In order to describe decreasing ablation rate a theoretical model based on the energy balance is proposed. It considers the energy/heat transfer from the laser beam to the material and predicts a decreasing drilling rate with an increasing number of successive laser pulses. According to the proposed model, the finite depth of the hole appears as a consequence of the increasing surface area through which the energy of the laser beam is conducted away to the material around the processed area. Decreasing ablation rate and the finite hole depth predicted by model were in good agreement with the experimental results.     

Robust plasmonic substrates

We report on resonant infrared laser ablation of polystyrene using single 8 ps pulses at a wavelength of 3.31 μm generated by a MgO:PPLN optical parametric amplifier pumped by a Nd:YLF laser. We determined the single-pulse ablation threshold to be 0.46 J/cm², about a factor of five smaller than in previous free-electron-laser studies. Time-resolved imaging of the laser–target interaction reveals that the detailed dynamics of the ablation process begin with thermal expansion of a large volume of hot material from which a less dense plume of polymeric material evaporates. This plume disappears on a time scale of 0.75 μs and the hot polymer material recedes back into the crater from which it was expelled. Subsequently, and on a much longer time scale, structural alterations in the ablation crater continue to evolve for at least another millisecond. Our results suggest that single picosecond pulses are effective for the ablation of polymers and exhibit dynamics similar to those observed in studies using a free-electron laser.     

Laser ablation of silicon and copper targets. Experimental and finite elements studies

The ablation process induced by excimer lasers is a collective phenomenon that basically involves two phenomena: the laser radiation–matter interaction and the dynamic of the ablation plume. The laser parameters, the thermal and optical properties of the material, and the surface morphology are critical factors in the ablation mechanisms affecting the direction of the ablation plume expansion. In this study, the role of the surface roughness and the evolution of its morphology under the laser irradiation were investigated. Assuming a thermal ablation model, a theoretical study of the initial steps of the laser ablation process by a finite element method using ANSYS (6.1) was performed. Different ablation experiments were carried out on silicon and copper targets using a XeCl laser. The target surface morphology changes were observed by SEM and the plume deflection was recorded by a digital camera. An acceptable agreement between the experimental and simulated results was found. This study contributes to a better understanding of the physical processes involved in the laser ablation and the relations between the plume deflection angle and the surface roughness. PACS 79.20.Ds; 81.40.Gh; 44.05.+e     

Photoablation by UV and visible laser radiation of native and doped biological tissue

Photoablation studies of biological material (human cornea) with UV and visible laser light show that effective, apparently non-conventional thermal photoablation can be achieved by introducing energy absorbing dopants in the tissue. Previously unknown high ablation rates of 80 Gmm/pulse have been observed. The results allow one to clearly postulate different ablation mechanisms for increasing laser fluence. The results are compared with the photoablation rates observed with 193 nm UV laser light on undoped human cornea. Explosive desorption has been found the dominant process involved.     

The influence of substrate temperature on femtosecond laser micro-processing of silicon,stainless steel and glass

We report the influence of substrate temperature on femtosecond laser ablation of silicon, stainless steel, and glass. Remarkable decrease in surface roughness was observed under high substrate temperature for silicon and stainless steel. While the ablation efficiency of glass as a typical wide band-gap material is scarcely altered at 900 K, the efficiency for stainless steel as a conductor apparently increased about 20% accompanied to the elevation of substrate temperature from 300 to 900 K. Silicon wafer results in slight increase of the ablation efficiency with decreasing the ablation threshold. Considering that the melting temperature of glass is much lower than those of silicon and steel, the observations from this work suggests that the material ablation caused by the ultrafast laser irradiation could not be explained in term of only laser-induced thermal excitation.     

Early stage of the material removal during ArF laser ablation of graphite

Material removal during ArF excimer laser ablation of graphite at atmospheric pressure was investigated by two independent methods; 1) by observation of the propagating properties of the shock wave generated by the carbonaceous ejecta and 2) by in situ measurement of the size distribution of carbon nanoparticles condensing in the ablation plume. This latter was carried out by a scanning mobility particle sizer system based on a differential mobility analyser. The performed measurements indicate that the material removal during ArF laser ablation consists of two steps at fluences above the threshold fluence. First, a thin layer of carbon (of the order of 1 nm) is removed by a quick desorption process, leading to shockwave formation. This process takes place in a ns time scale, and desorption rate estimations reveal that this can not be explained by thermal surface evaporation. Since to our knowledge there is no thermal process that could account for the estimated desorption rate, it is argued that this is a fast photochemical (i.e. non-thermal) process. The size distribution of the condensed nanoparticles related to this step shows a rising edge at diameters below 10 nm. At fluences above the ablation threshold, the majority of the material is ejected in the second phase, resulting in condensation of carbon nanoparticles, peaking at 50 nm diameters in the size spectrum. Both shockwave formation and material removal are also detected well below the ablation threshold fluence, which is attributed to the photochemical process. PACS 61.46.+w; 81.16.Mk     

Influence of scanning velocity on bovine shank bone ablation with pulsed CO2 laser

The influence of scanning speed on hard bone tissue ablation is studied with a 10.6-μm laser. The groove morphology and the thermal damage created in bovine shank bone by pulsed CO2 laser are examined as a function of incident fluence by optical microscope following standard histological processing. The results show that ablation groove width, depth and ablation volume, as well as the zone of thermal injury, increase gradually with incident fluence. As compared to the result for high scanning speed, the lower scanning speed always produces larger ablation volume but thicker zone of thermal injury. It is evident that scanning speed plays an important role in the ablation process. In clinical applications, it is important to select appropriate scanning speed to obtain both high ablation rates and minimal thermal injury.     

Short-pulse UV laser ablation of solid and liquid metals: indium

2 to 2.5 mJ/cm² when a 0.5 ps pulse is used instead of a 15 ns laser pulse. Measurements on liquid indium show a different behavior. With 15 ns laser pulses the threshold fluence is lowered by a factor of ∼3 from 100 mJ/cm² for solid indium to 30 mJ/cm² for liquid indium. In contrast, measurements with 0.5 ps laser pulses do not show any change in the ablation threshold and are independent of the phase of the metal at 2.5 mJ/cm². This behavior could be explained by thermal diffusion and heat conduction during the laser pulse and demonstrates in an independent way the energy lost into the material when long laser pulses are applied. Time-of-flight measurements to investigate the underlying ablation mechanism show thermal behavior of the ablated indium atoms for both ps and ns ablation and can be fitted to Maxwell-Boltzmann distributions. Received: 2 December 1996/Accepted: 11 December 1996     

Femtosecond laser ablation characteristics of nickel-based superalloy C263

Femtosecond laser (180 fs, 775 nm, 1 kHz) ablation characteristics of the nickel-based superalloy C263 are investigated. The single pulse ablation threshold is measured to be 0.26±0.03 J/cm² and the incubation parameter ξ=0.72±0.03 by also measuring the dependence of ablation threshold on the number of laser pulses. The ablation rate exhibits two logarithmic dependencies on fluence corresponding to ablation determined by the optical penetration depth at fluences below ∼5 J/cm² (for single pulse) and by the electron thermal diffusion length above that fluence. The central surface morphology of ablated craters (dimples) with laser fluence and number of laser pulses shows the development of several kinds of periodic structures (ripples) with different periodicities as well as the formation of resolidified material and holes at the centre of the ablated crater at high fluences. The debris produced during ablation consists of crystalline C263 oxidized nanoparticles with diameters of ∼2–20 nm (for F=9.6 J/cm²). The mechanisms involved in femtosecond laser microprocessing of the superalloy C263 as well as in the synthesis of C263 nanoparticles are elucidated and discussed in terms of the properties of the material.