Continuous Wave Laser Heating Research Articles

An Investigation of Oxides of Tantalum Produced by Pulsed Laser Ablation and Continuous Wave Laser Heating.

Recent progress has seen multiple Ta2O5 polymorphs generated by different synthesis techniques. However, discrepancies arise when these polymorphs are produced in widely varying thermodynamic conditions and characterized using different techniques. This work aimed to characterize and compare Ta2O5 particles formed at high and low temperatures using nanosecond pulsed laser ablation (PLA) and continuous wave (CW) laser heating of a local area of tantalum in either air or an 18O2 atmosphere. Scanning electron microscopy (SEM) and Raman spectroscopy of the micrometer-sized particles generated by PLA were consistent with either a localized amorphous Ta2O5 phase or a similar, but not identical, crystalline β-Ta2O5 phase. The Raman spectrum of the material formed at the point of CW laser impingement was in good agreement with the previously established ceramic "H-Ta2O5" phase. TEM and electron diffraction analysis of these particles indicated the phase structure matched an oxygen-vacated superstructure of monoclinic H-Ta2O5. Further from the point of laser impingement, CW heating produced particles with a Raman spectrum that matched β-Ta2O5. We confirmed that the high-temperature ceramic phase characterized in previous work by Raman spectroscopy was the same monoclinic phase characterized in different work by TEM and could be produced by direct laser heating of metal in air.

A comparative study of cerium oxides formed by pulsed laser ablation and continuous wave laser heating

AbstractThe formation of Ce2O3 from cerium metal requires complex reducing conditions. Alternatively, we show that different stoichiometries and structures of cerium oxides can be generated in high‐temperature laser experiments by varying the availability of oxygen and the use of either pulsed laser ablation (PLA) or continuous wave (CW) laser heating of cerium metal. X‐ray diffraction and Raman spectroscopy suggest that a low structural order Ce2O3/CeO2 mixed structure is formed in a 1% O2 atmosphere with PLA. For CW heating in a 1% O2 environment, Raman measurements show the formation of crystalline A‐type Ce2O3 in proximity to the point of laser impingement. These experiments were repeated in 18O2 and allowed us to report for the first time the corresponding Raman spectrum of Ce218O3.

Analysis of Dispersibility Effect of Carbon Additives on Ignitability of Ammonium-Dinitramide-Based Ionic Liquid Propellants Using Continuous Wave Laser Heating

ABSTRACT Ammonium dinitramide-based ionic liquids (ADN-EILPs) are a promising alternative to hydrazine monopropellants. Continuous wave (CW) laser heating using carbon wools is an effective approach to attain the rapid ignition of ADN-EILPs. This study aims to verify the influence of the dispersibility of carbon additives in ADN-EILPs on their ignition. The investigation was performed by performing fluorescence microscopy of samples imitating the mixture of ADN-EILPs with carbon additives and CW laser ignition tests of ADN-EILPs with several yarn-lengths of carbon wools. Based on these results, the dispersity mechanism of carbon additives in ADN-EILPs is proposed, which indicates that the use of high-power laser is not an effective approach to ignite ADN-EILPs consisting of carbon additives with high dispersibility. During sample preparation for the ignition tests, it was verified that the difference in the length of carbon yarns affects the bulk density and morphology of the prepared samples, and dispersibility of carbons. The results of the ignition tests indicate that samples whose morphology altered into a liquid-like morphology cannot be ignited and the ones who retained their original one can be ignited. The physical distribution of the residue of samples with a liquid-like morphology, observed after the ignition tests, agrees with the discussion regarding the dispersibility mechanism of carbon additives, obtained through fluorescence microscopy. Moreover, for the samples exhibiting an ignition capacity, the bulk density of additives would be crucial to be considered to achieve the effective ignition.

Fracture prediction of CFRP laminates subjected to CW laser heating and pre-tensile loads based on ANN

For the application of composite materials in a complex thermal and mechanical environment, we developed a standard artificial neural network (ANN) model for the fracture prediction of carbon fiber-reinforced polymer (CFRP) laminates under continuous wave laser heating and pre-tensile loads. A substantial amount of data was collected through experimentation and from published references, which were converted into 12 800 binary-classification-type input/output data pairs before being used for model training. Different numbers of hidden neurons were evaluated to determine the optimal architecture of the model, while the “early stopping” and “dropout” methods were used to improve its robustness. The trained ANN model functions as a binary classifier that can predict the fracture probability of CFRP laminates after a certain period of laser irradiation. Subsequently, another 14 sets of experimentally collected data were used for ANN model testing. The correct prediction rate of the model reached 86%, which was higher than two other machine learning models (k-nearest neighbors and random forest models) under the same conditions. As the failure behavior of CFRP laminates has a certain degree of randomness, the fracture probabilities predicted by the ANN model have more practical values than the specific fracture times predicted by existing theories. Results indicate that it is feasible to apply the ANN method to predict the failure behavior of composite materials with discrete mechanical properties in complex thermal environments, and this study provides useful guidance for the engineering applications of composite materials in complex thermal and mechanical environments.

Dual Laser Beam Asynchronous Dicing of 4H-SiC Wafer.

SiC wafers, due to their hardness and brittleness, suffer from a low feed rate and a high failure rate during the dicing process. In this study, a novel dual laser beam asynchronous dicing method (DBAD) is proposed to improve the cutting quality of SiC wafers, where a pulsed laser is firstly used to introduce several layers of micro-cracks inside the wafer, along the designed dicing line, then a continuous wave (CW) laser is used to generate thermal stress around cracks, and, finally, the wafer is separated. A finite-element (FE) model was applied to analyze the behavior of CW laser heating and the evolution of the thermal stress field. Through experiments, SiC samples, with a thickness of 200 μm, were cut and analyzed, and the effect of the changing of continuous laser power on the DBAD system was also studied. According to the simulation and experiment results, the effectiveness of the DBAD method is certified. There is no more edge breakage because of the absence of the mechanical breaking process compared with traditional stealth dicing. The novel method can be adapted to the cutting of hard-brittle materials. Specifically for materials thinner than 200 μm, the breaking process in the traditional SiC dicing process can be omitted. It is indicated that the dual laser beam asynchronous dicing method has a great engineering potential for future SiC wafer dicing applications.

High power continuous wave laser heating of graphite in a high temperature range up to 3800 K

As graphite is an important material for several applications, the evaluation of its properties at very high temperatures is of major importance. Conducting studies at temperatures >2800 K is, however, very challenging, and the amount of available data in the literature is, therefore, very limited. This paper presents a methodology that compares experimental data with simulation results following a progressive increase of the temperature range investigated. The study was conducted on a commercial polycrystalline graphite produced by sintering (EDM-3 from POCO) under high power continuous laser heating (kW ytterbium at a wavelength of 1080 nm). Experiments were done inside a vacuum chamber equipped with pyrometers and cameras allowing fine monitoring of the temperature of the samples. A 3D numerical model has been developed based on the finite element method to analyze the experimental results. The evolution of the thermal and optical properties of EDM-3 with temperature is required for correctly modeling laser/material interactions. By running a sequential comparison of calculation with a dedicated experiment, we achieve better knowledge of such properties. From this study, we obtain estimations of the evolution of thermal conductivity, emissivity, and evaporation rate up to 3800 K of amorphous graphite.

Mechanical modeling of bending stiffness degradation for soft-honeycomb sandwich structure under CW laser heating

Based on the theory of Reissner’s sandwich structure bending, a mechanical model of bending stiffness degradation for a soft-honeycomb sandwich structure under continuous wave (CW) laser heating was established by introducing the equivalent debonding coefficient of adhesive and elastic modulus temperature dependence of panels. To verify the rationality of the mechanical model, a real-time bending stiffness measurement system for a soft-honeycomb sandwich structure under CW laser heating was proposed using the overhanging beam three-point bending method. The results show that the mechanical model works good to describe the decreasing trend of the soft-honeycomb sandwich structure under CW laser heating and to predict the residual bending stiffness after CW laser heating, despite the fact that in the early stage of CW laser heating, the bending stiffness of some kind of soft-honeycomb sandwich structure may increase for a short time due to the pyrolysis gas production. However, with the overflowing of pyrolysis gas and the elastic modulus of panel degradation at a high temperature, the bending stiffness of soft-honeycomb sandwich structure declines rapidly and soon fails to withstand any bending load. The research shows that special attention should be paid to the effect of laser heating on the mechanical properties of the soft-honeycomb sandwich structure. It also enlightens the study of mechanical properties and temperature dependence of the honeycomb sandwich structure at a high heating rate.

Comparative study of the influence of pulsed and continuous wave laser heating on the mobilization of carbon and its chemical reaction with iron in a diamond anvil cell

Laser heating in a diamond anvil cell (DAC) is a common method for studying material behavior at high-pressure and high-temperature conditions. It has been previously proven that during continuous wave (CW) laser heating of a sample, carbon of the diamond anvils is mobilized, and its diffusion into the sample can lead to undesirable chemical reactions, which, if not detected, may cause misinterpretations of the results of the experiment. Minimizing the heating time with the use of a pulsed laser (PL) is thought to reduce the risk of possible carbon contamination of the sample; however, this has not been proven experimentally. Here, we report the results of our comparative study of the effect of pulsed and continuous wave (CW) laser heating on the mobilization of carbon and its chemical interaction with iron in a diamond anvil cell. Using X-ray absorption near edge structure spectroscopy, Synchrotron Mössbauer Source spectroscopy, and Synchrotron X-ray diffraction, we examined iron samples that were laser heated in DACs in various pressure transmitting media (neon, argon, and potassium chloride). According to our results, the use of the PL heating does not prevent the sample from carbon contamination. A reaction between carbon and iron happens within a few seconds even at moderate temperatures. We found that one analytical technique was generally insufficient to fully characterize the phase composition of the laser-heated samples.

Melt dynamics and melt-through time in continuous wave laser heating of metal films: Contributions of the recoil vapor pressure and Marangoni effects

The relative contributions of evaporation and melt expulsion due to the recoil vapor pressure and Marangoni effects to the laser damage and melt dynamics in continuous wave (CW) laser interactions with free-standing aluminum films are evaluated in two-phase hydrodynamic simulations. In order the establish the dominant damage mechanisms in different irradiation regimes, the results of hydrodynamic simulations are compared with the predictions of several simplified models that only account for a subset of the involved processes. The hydrodynamic simulations performed in the range of film thickness from 0.2mm to 4mm and laser spot radius from 0.1mm to 1cm reveal only a marginal effect of the Marangoni stresses on the overall picture of melt flow and the melt-through time. The recoil pressure effect, on the contrary, is capable of strongly decreasing the melt-through time in a certain range of laser intensity. At laser intensities below this range the melting process is largely defined by heat transfer in the radial direction, while at laser intensities above this range the thickness of the molten pool and the efficiency of melt expulsion decrease and evaporation becomes the primary mechanism of material removal from the center of the laser spot. The range of laser intensities where the melt-through time is controlled by the recoil pressure effect is not unique and depends on the film thickness. A simple two-phase one-dimensional thermal model of laser melting, where melt expulsion due to the recoil pressure effect is accounted for based on the Bernoulli integral, is developed and found to be capable of accurate prediction of the melt-through time above a certain level of laser intensity. The one-dimensional thermal model captures all qualitative trends revealed in the direct hydrodynamic simulations and can be used as a robust engineering tool for the first-order estimation of the conditions for CW laser damage of metal films.

Heat-Assisted Magnetization Dynamics in GdFeCo Using Field-Induced TR-MOKE

A custom microcoil was used to generate a magnetic field pulse and a continuous wave (CW) laser was used as a heating source to study the heat-assisted effect in magnetization switching on GdFeCo thin films. Four different pulsed field strengths were chosen, and the power of the CW laser heating was varied from 0 to 180 mW. The heat-assisted enhancement was not observed when the pulsed field strength was 2400 Oe. The enhancement was visible when the pulsed field strength was reduced to 2160 Oe, and became obvious when the pulsed field strength was 1750 and 1350 Oe. These interesting results prove that CW laser heating could provide a marked improvement to the magnetization switching when the switching field is relatively small, thus making it a more applicable and economic alternative to pulsed laser heating. Simulations based on the Landau-Lifshitz-Bloch equation have also been performed and agreed well with the experimental data. The CW laser heating scheme provides a more economic and practical way to realize heat-assisted magnetization switching, which could be used for potential future heat-assisted magnetic recording technology.

Molecular Dynamics Simulation of Heat Transfer from a Gold Nanoparticle to a Water Pool

Heat transfer from nanoparticles to a surrounding liquid pool is an essential process for many applications. This work investigates the heat transfer process upon a continuous wave laser heating to a gold nanoparticle (GNP) in a water pool through molecular dynamics (MD) simulations based on realistic potentials. The interactions among gold atoms are described by the embedded atom method (EAM) potentials, both the rigid and flexible TIP3P water models are examined, and a modified Lennard-Jones potential is used for the interaction between the GNP and water. The results show that the interfacial thermal conductance is influenced by the selection of different water models and the interfacial wettability. The interfacial thermal conductance is smaller but increases slightly with the increase of heat flux for the flexible water model, while it keeps almost constant for the rigid model. Increasing the wettability between the particle and the fluid reduces the interfacial resistance. The rise of the interfacial...

Theoretical and experimental study on laser surface hardening by repetitive laser pulses

An analytical solution for the temperature variation in a semi-infinite workpiece heated by repetitive laser pulses has been derived and the effects of various process parameters, viz. laser power, beam diameter, scan speed, pulse duration, repetition frequency and duty cycle on the surface transformation hardening have been investigated. The study shows that the soaking time of the top surface layer at a temperature above the phase transformation temperature, on which the homogeneity of microstructure and the depth of hardening depend, can be increased by repetitive laser pulse heating in comparison to continuous wave laser heating. The depth of hardness increases with the number of incident laser pulses at low frequencies in the range of Hz. This has been demonstrated by comparing the surface hardness characteristics of AISI 1055 steel specimens resulted by the continuous wave and repetitive laser pulse hardening processes.

A virtual experiment control and data acquisition system for in situ laser heated diamond anvil cell Raman spectroscopy

Doubled-sided laser heated diamond anvil cell methods allow simultaneous in situ confocal Raman measurements of materials up to megabar pressures and high temperatures. This paper describes a virtual control and data acquisition system developed to automate setups for simultaneous Raman/laser heating experiments. The system enables reduction of experiment time by ∼90% in comparison to manual operations, allowing measurements of high quality Raman spectra of even highly reactive or diffusive samples, such as hydrogen at extreme conditions using continuous wave laser heating. These types of measurements are very difficult and often impossible to obtain in a manual operation mode. Complete data archiving and accurate control of various experimental parameters (e.g., on-the-fly temperature determination and self-adjusting data collection time to avoid signal saturation) can be done, and open up possibilities of other types of experiments involving extreme conditions.

Effects of layers and vias on continuous-wave laser heating and damage of surface-micromachined structures

The response of microsystem components to laser irradiation is relevant to processes and applications such as laser processing, optical diagnostics, and optical microelectromechanical systems (MEMS) device design and performance. The dimensions of MEMS, which are on the order of several micrometers, are on the same order as infrared laser wavelengths, which results in interference phenomena when the parts are partially transparent. Four polycrystalline silicon structures were designed and irradiated with 532 and 808 nm continuous wave laser light to determine the effects of layers, air gaps, and the presence of a substrate via on the threshold laser power for damage. The presence of a substrate via resulted in lower damage thresholds, and a single-layer structure had the highest damage threshold for structures irradiated with infrared light. Structures irradiated with visible wavelength light exhibited less sensitivity to the underlying via. Optical interference calculations are carried out to evaluate the absorptance of these structures as a feasible explanation for the observed results.

Ultrafast thermoelasticity for short-pulse laser heating

Thermoelastic stress waves in a solid medium caused by subpicosecond-pulse laser heating are investigated with three different approaches: ultrafast thermoelasticity, Lord–Shulman theory, and the classical thermoelasticity. Both the conditions of uniaxial strain and uniaxial stress are considered. A combined finite difference/finite element algorithm is developed for solving the coupled, nonlinear, transient differential equations. It is shown that the thermomechanical results obtained from the ultrafast thermoelasticity are significantly different from those obtained from Lord–Shulman theory and the classical thermoelasticity. The other finding is that in contrast to long pulse and continuous-wave laser heating, thermal stresses in a free-expansion medium induced by an ultrashort, uniform volumetric heat source could be pronounced.