Modification of the LSMPS method for the conservation of the thermal energy in laser irradiation processes

In this work, new least-square moving particle semi-implicit (LSMPS) formulations for the modeling of the heat conduction in laser irradiation processes for both thick blocks and thin plates are developed. These new LSMPS formulations guarantee the conservation of the total thermal energy during the heat exchange between particles. The conservation of the thermal energy in the LSMPS method was implemented together with multiresolution techniques for the discretization of the domain with particles of different sizes so that a better characterization of the thermal gradients in the vicinity of the laser beam can be obtained. The simulation of laser irradiation processes for thin plates is still very challenging for particle methods with spherical particles and this is essentially because it is difficult to accommodate a minimum number of particles along the thickness direction without increasing considerably the resolution or the number of particles in the entire plate. In order to overcome this difficulty, a new multiresolution method based on particles with ellipsoidal shapes was also developed for a more efficient modeling of the laser irradiation in thin plates. By conducting the heat conduction simulations, in which the standard LSMPS method can provide accurate temperature distribution and by comparing the results with an analytical solution, it was confirmed that the proposed method is as accurate as the standard LSMPS method. Moreover, the heat conduction with an external heat source, in which the total thermal energy is not conserved by using the standard LSMPS method, was successfully simulated by using the proposed method. The simulations of laser irradiations were also conducted, and the validity of the proposed method has been confirmed by comparing numerical results with experimental data.     

Numerical modelling of laser beam-target interaction near the plasma-formation threshold

Experimental investigations of laser-produced plasmas often determine plasma parameters as averaged values only and these data cannot be compared with actual conditions at a target surface. In this paper, numerical modelling of target heating with non-constant target material properties was used to estimate the temperature at the surface of the target as a function of time. Three target materials (magnesium, aluminum and silicon) and an experimentally determined laser pulse profile were used for modelling. Gray-body radiation of the target surface and thermal diffusion losses are taken into account during numerical modelling. It was assumed that radiation from the laser impacts perpendicularly onto the target surface. The results obtained by modelling are in good agreement with the experimentally observed plasma parameters and they are consistent with thermal target heating in which the laser beam is treated as a source of very intense input heat energy.     

Pulsed laser treatment of plasma-sprayed thermal barrier coatings: effect of pulse duration and energy input

Pulsed laser treatments of plasma-sprayed thermal barrier coatings can provide good corrosion resistance of protected components without impairing thermal fatigue resistance of the ceramic layers. Laser treatments are performed over a wide range of pulse durations and energy inputs, and their effects on microstructure, crystalline grain size and chemical composition of the remelted thin upper layer are investigated. Particular attention is given to macro and microcracking originating on the surface, gas bubble motion inside the melted layer and consequent surface crater formation. Density, shape, dimension and distribution of craters in the laser-irradiated zone are correlated with pulse duration and energy input of the laser beam. A numerical simulation of temperature distributions and heat phenomena originating in the ceramic coating during laser irradiation is presented, in order to explain the influence of laser characteristics on the quality of the coating surface.     

The interaction between a pulsed laser beam and a steel surface

The interaction between a pulsed laser beam and the surface of a carbon steel has been investigated. The variables of the irradiation were the power intensity of the laser beam and the number of pulses. At the area of impact of the incoming laser beam concentric ripples were observed. The number of ripples and their displacement was found to be dependent on both the above parameters. No transformation products, such as martensite or bainite, or any evidence of a heat affected zone have been observed at the immediate vicinity of the impact area.     

Numerical and experimental study of the thermal stress of silicon induced by a millisecond laser

A spatial axisymmetric finite element model of single-crystal silicon irradiated by a 1064 nm millisecond laser is used to investigate the thermal stress damage induced by a millisecond laser. The transient temperature field and the thermal stress field for 2 ms laser irradiation with a laser fluence of 254 J/cm(2) are obtained. The numerical simulation results indicate that the hoop stresses along the r axis on the front surface are compressive stress within the laser spot and convert to tensile stress outside the laser spot, while the radial stresses along the r axis on the front surface and on the z axis are compressive stress. The temperature of the irradiated center is the highest temperature obtained, yet the stress is not always highest during laser irradiation. At the end of the laser irradiation, the maximal hoop stress is located at r=0.5 mm and the maximal radial stress is located at r=0.76 mm. The temperature measurement experiments are performed by IR pyrometer. The numerical result of the temperature field is consistent with the experimental result. The damage morphologies of silicon under the action of a 254 J/cm(2) laser are inspected by optical microscope. The cracks are observed initiating at r=0.5 mm and extending along the radial direction.     

Influence of thermal deformations of the output windows of high-power laser systems on beam characteristics

By using the well-known Green's function methods, we study the three-dimensional temperature distributions and thermal deformations of the output windows of unstable optical resonators induced by an incident annular laser beam. Some expressions and theoretical profiles of the temperature distributions and thermal deformations as functions of the radius and of the thickness of optical windows are obtained. Moreover, the influence of the thermal deformations of sapphire, silica, and silicon windows within unstable optical resonators on the Strehl ratio and on the far-field laser intensity distribution is also discussed. Under conditions of 50-kW intense laser irradiation during 5 s, the maximum thermal deformation in sapphire, silica, and silicon substrates is 1.993, 0.393, and 6.251 microm, respectively. Under the same conditions the Strehl ratio of sapphire is higher than that of silica.     

Evaluation of the Effective Sample Temperature in Thermal Diffusivity Measurements Using the Laser Flash Method

In the measurement of thermal diffusivity by the laser flash method, a temperature rise occurs in the sample as a pulsed laser hits on the sample surface. Due to the temperature dependence of thermal diffusivity of the sample, the thermal diffusivity corresponds to a temperature that is larger by T _eff than the temperature before laser irradiation is applied. This effective temperature rise, T _eff, has been investigated by using a numerical simulation. The results indicate that the effective temperature rise is almost equal to a maximum temperature rise, T _M, of the back surface of the sample in cases where both linear and nonlinear temperature variations of thermal diffusivity are considered.     

Bimorph变形镜10.6μm薄膜研究

基于压电驱动器的Bimorph变形镜是10.6 μm系统的一个重要元件.为了镀制薄膜,本文首先利用有限元软件对两种镀膜夹持方式与沉积温度进行了计算,对热应力产生的热变形进行了分析,选择了合适的镀膜夹持方式.为了预测bimorph变形镜受激光辐照后的温升,对单晶硅与石英玻璃制作的bimorph变形镜有限元模型进行了计算与分析.最后,利用光度计对镀制的薄膜进行了反射率测量.试验结果显示反射率测量值大于99.5％,满足实际系统的需要.     

Probing the photoresponse of individual Nb2O5 nanowires with global and localized laser beam irradiation

Photoresponse of isolated Nb(2)O(5) nanowires (NW) padded with platinum (Pt) at both ends were studied with global irradiation by a laser beam and localized irradiation using a focused laser beam. Global laser irradiation on individual NW in ambient and vacuum conditions revealed photocurrent contributions with different time characteristics (rapid and slowly varying components) arising from defect level excitations, thermal heating effect, surface states and NW-Pt contacts. With a spot size of < 1 μm, localized irradiation highlighted the fact that the measured photocurrent in this single NW device (with and without applied bias) depended sensitively on the photoresponse at the NW-Pt contacts. At applied bias, unidirectional photocurrent was observed and higher photocurrent was achieved with localized laser irradiation at reverse-biased NW-Pt contacts. At zero bias, the opposite polarity of photocurrents was detected when the two NW-Pt contacts were subjected to focused laser beam irradiation. A reduced Schottky barrier/width resulting from an increase in charge carriers and thermoelectric effects arising from the localized thermal heating due to focused laser beam irradiation were proposed as the mechanisms dictating the photocurrent at the NW-Pt interface. Comparison of photocurrents generated upon global and localized laser irradiation showed that the main contribution to the photocurrent was largely due to the photoresponse of the NW-Pt contacts.     

Thin-Film Thermophysical Property Characterization by Scanning Laser Thermoelectric Microscope

This work presents a scanning laser-based thermal diffusivity measurement technique for thin films as well as for bulk materials. In this technique, a modulated laser beam is focused through a transparent substrate onto the film–substrate interface. The generated thermal wave is detected using a fast-responding thermocouple formed between the sample surface and the tip of a sharp probe. By scanning the laser beam around the thermocouple, the amplitude and phase distributions of the thermal wave are obtained with micrometer resolution. The thermal diffusivity of the film is determined by fitting the obtained phase signal with a three-dimensional heat conduction model. Experimental results are presented for a 150-nm gold film evaporated on a glass substrate.     

A Numerical Simulation to Propose a Flash Method for In Situ Detection of the Thermal Diffusivity of Anisotropic Thin Film Materials

Strong anisotropy of thermal diffusivity is frequently observed in thin film materials. We propose an in situ experimental method to remotely measure radial and axial components of the thermal diffusivity. The method is based on the traditional laser flash technique but is specialized to also highly challenging experimental situations such as sample manufacture and use phase when thin films may be exposed to very high pressures or temperatures and to high temperature gradients. The method requires laser pulses of very short duration and fast measurement of transient temperature excursions in only radial directions on the surface of the thin film samples. The accuracy of the method is checked by comparison with results from a finite element calculation for a graphite sheet with high anisotropic conductivity that simulates a thermo-physical experiment.     

The mechanism of increase of the thickness of the zone of thermal effect during pulse-periodic treatment of a target by an electron beam

Based on the results of numerical simulation of temperature fields, the reasons for the growth of the layer thickness of the zone of thermal effect in a target of 45 steel during pulse-periodic stimulation by a low-energy high-current electron beam in the mode of initial melting are analyzed. It is concluded that the growth of thickness of these layers is primarily caused by the spread of the beam energy density from pulse to pulse and by the presence of individual pulses with a higher-than-average value of energy density. It is shown that the variation of the thermal properties of steel, which occurs during irradiation due to the carbon saturation of the surface region of the target, results in the increase of the thickness of only those layers of the zone of thermal effect which are formed in the region of former melt. The dependence of the target temperature on the number of irradiation pulses is measured and calculated numerically. It is demonstrated that the increase of the target temperature has no noticeable effect on the growth of the thickness of the zone of thermal effect     

Computational simulations of pulsed laser induced melting and solidification of monocrystalline GaSb

Numerical analysis of pulsed laser induced phase change processes in the near-surface region of monocrystalline bulk GaSb is done using a thermal nonequilibrium model. The calculations of basic parameters of the processes such as surface melt duration, maximum reflectivity of the probe laser beam, etc., are verified using results of experimental work on GaSb samples irradiated by the ruby (694 nm, 35 ns FWHM) and ArF (193 nm, 13 ns FWHM) lasers. The comparison of experimental data with numerical predictions shows that while for the ruby laser a reasonable agreement in surface melt duration is achieved, the results for the ArF laser differ quite a lot. The amorphization of the topmost surface layer is identified as a main reason for these differences. The subsequent experimental analysis of the surface structure by low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) confirms the theoretical predictions and clearly indicates the appearance of an amorphous GaSb structure after ArF laser irradiation.     

Beam quality after propagation of Nd:YAG laser light through large-core optical fibers

Laser beam characteristics are altered during propagation through large-core optical fibers. The distribution of modes excited by the input laser beam is modified by means of mode coupling on transmission through the fiber, leading to spatial dispersion of the profile and, ultimately and unavoidably, to degradation in the quality of the delivered beam unless the beam is spatially filtered with consequent power loss. Furthermore, a mismatch between the intensity profile of a typical focused high-power laser beam and the profile of the step-index fiber gives rise to additional beam-quality degradation. Modern materials processing applications demand ever higher delivered beam qualities (as measured by a parameter such as M(2)) to achieve greater machining precision and efficiency, a demand that is currently in conflict with the desire to utilize the convenience and flexibility of large-core fiber-optic beam delivery. We present a detailed experimental investigation of the principal beam-quality degradation effects associated with fiber-optic beam delivery and use numerical modeling to aid an initial discussion of the causes of such degradation.     

Adaptivity for finite volume on unstructured triangular meshes: a study of thermal injury in teeth

This paper presents a numerical study of thermal injury in teeth, caused both by convective heating, due to drinking of hot beverage and mastication of foods, and by laser irradiation in dental treatment. The numerical study employs an adaptive finite volume method on unstructured triangular meshes to solve the governing equations. An adaptive time stepping methodology was also used in order to control the solution error. Adaptive methodologies are adequate to solve such problems since steep gradients will develop at specific locations in the domain of study. The convective heating results were compared to experimental data available in the literature. Laser treatment results are in agreement to the temperature increasing observed in literature. The simulation results demonstrate that both the error estimate and adaptive methodology herein proposed are suitable and reliable for the controlled solution of parabolic problems. Copyright © 2004 John Wiley & Sons, Ltd.     

Thermal Diffusivity Measurements of Candidate Reference Materials by the Laser Flash Method

The National Metrology Institute of Japan (NMIJ) in AIST has investigated the laser flash method in order to establish a thermal diffusivity standard for solid materials above room temperature. A uniform pulse-heating technique, fast infrared thermometry, and a new data analysis method were developed in order to reduce the uncertainty in thermal diffusivity measurements. The homogeneity and stability of candidate reference materials such as isotropic graphite were tested to confirm their qualification as thermal diffusivity reference materials. Since graphite is not transparent to both the heating laser beam and infrared light for thermometry, the laser flash method can be applied to graphite without black coatings. Thermal diffusivity values of these specimens with different thicknesses, were measured with changing heating laser pulse energies. A unique thermal diffusivity value can be determined for homogeneous materials independent of the specimen thickness, by extrapolating to zero heating laser pulse energy on the plot of apparent thermal diffusivity values measured with the laser flash method as a function of heating laser pulse energy.Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22--27, 2003, Boulder, Colorado, U.S.A.     

Effects of pulsed laser radiation on epitaxial self-assembled Ge quantum dots grown on Si substrates

Laser irradiation of Ge quantum dots (QDs) grown on Si(100) substrates by solid-source molecular beam epitaxy has been performed using a Nd:YAG laser (532 nm wavelength, 5 ns pulse duration) in a vacuum. The evolution of the Ge QD morphology, strain and composition with the number of laser pulses incident on the same part of the surface, have been studied using atomic force microscopy, scanning electron microscopy and Raman spectroscopy. The observed changes in the topographical and structural properties of the QDs are discussed in terms of Ge-Si diffusion processes. Numerical simulations have been developed for the investigation of the temperature evolution of the QDs during laser irradiation. The obtained results indicate that the thermal behaviour and structural variation of the nanostructures differ from conventional thermal annealing treatments and can be controlled by the laser parameters. Moreover, an unusual island motion has been observed under the action of subsequent laser pulses.     

Luminescence quenching and recovering in photo-thermo-refractive silver-ion doped glasses

A spectroscopic investigation of luminescent centers transformation in photo-thermo-refractive glass by using ultraviolet (UV) nanosecond laser pulses with radiation wavelength 355 nm was performed. Initially the glass was irradiated by UV lamp and thermal-treated that causes a neutral silver molecular clusters luminescence in a visible spectral region. After the laser irradiation a luminescence quenching was observed in the irradiated region. The thermal treatment below glass transition temperature restores the luminescence of silver molecular clusters with complex spatial distribution of luminescence intensity inside the irradiated region. UV lamp irradiation achieves the same result without any inhomogeneity. The observed effects are caused by photoionization and reduction of subnanosized silver molecular clusters with the participation of cerium and antimony ions.     

Laser pulsing of field evaporation in atom probe tomography

The processes by which field evaporation in an atom probe is momentarily stimulated by impingement of a laser beam on a specimen are considered. For metals, the dominant and perhaps only sensible mechanism is energy absorption leading to thermal pulsing, which has been well established. The energy of a laser beam is absorbed in a thin optical skin depth on the surface of the specimen. For materials with a band gap such as semiconductors and dielectrics, it is found that energy absorption in a thin surface layer dominates the process as well and leads to similar thermal pulsing. The relative amount of surface absorption versus volume absorption can strongly influence the heat flow and therefore the mass spectrum of the specimen. Thus it appears for very different reasons that all materials behave similarly in response to laser pulsing in atom probe tomography.     

The processing of SiC/SiC ceramic matrix composites using a pulsed Nd–YAG laser: Part I: Optimisation of pulse parameters

The machining of a composite material comprising silicon carbide (SiC) fibres in a chemical vapour infiltrated SiC matrix has been investigated using a 400 W pulsed Nd–YAG laser. The principal aim of this work has been to determine, by comparison with the results obtained from other ceramic matrix composites optimum processing conditions for these materials with regard to both material removal rate and cut surface quality. Previous trials involving a borosilicate glass matrix composite and a magnesium–alumino–silicate glass-ceramic matrix composite, both incorporating the same SiC fibres, have highlighted the importance of the coupling of the matrix phase with the emitted laser radiation. The various phase's resistance to oxidative degradation at high temperatures is another influential factor. The material considered in this work has been shown to be particularly suitable in these respects, with the result that both cut rate and quality are significantly enhanced. Report is made of the effect of varying the laser pulse parameters such as pulse energy, duration and intensity and concentrates on the material removal rate. Part II of this work addresses the influence of process variables, such as choice and pressure of assist gas and the point of focus of the laser beam on the quality of the cut surface. © 1998 Chapman & Hall