Papers by Keyword: Laser Irradiation

Abstract: In recent years, nickel tungstate has attracted considerable research interest as an electrode material for supercapacitors. In this work, NiWO₄ was synthesized by co-precipitation and exposed to laser irradiation. The structure of nickel tungstate was investigated by X-ray diffraction and its electrochemical properties by potentiodynamics, galvanostatic and impedance spectroscopy methods. The results show that NiWO₄ subjected to laser irradiation for 180 s showed higher specific characteristics than the initial material. Namely, at a discharge current of 1 mA, NiWO₄ achieves a specific capacitance of 153 F/g, and this value is 48% higher than that of the initial material. The higher specific characteristics of laser-modified NiWO₄ result from the ability of the material to interact better with electrolyte ions due to the passage of fast redox reactions and the capacitance of the electrical double layer, which is confirmed by impedance studies.

Abstract: Possible causes of mass transfer acceleration of carbon atoms and alloying elements in the surface layers of steels and alloys under extreme heating, under pulsing laser irradiation in particular, are considered. The research shows that the anomaly accelerated mass transfer, including diffusion in particular, in steels and alloys under fast laser heating has a cooperative character and is a result of a simultaneous action of several processes of different physics. It is proved that the carbon atoms mass transfer parameters and alloying elements depend on the scale and the level of emerging tension, relaxation of which goes along with a local plastic deformation, and occurrence of increased number of linear defects in crystal structure.

Abstract: The results of experimental studies of the structure and properties of steels and alloys subjected to laser chemical-heat treatment of carbide and nitride coatings created by electro-spark alloying and ion-plasma sputtering are achieved. It is shown that laser processing of coatings allows to change purposefully the structural state and set properties of the surface layers of the product, and, therefore, modify the basic characteristics of process operation and control the most important output parameters – wear products, and the quality of the surface layers of the irradiated parts. It is established that a rational choice of chemical composition and method of applying coatings on the surface of metal products of various functionalities helps to improve the quality of the surface layers, to increase the hardness of the irradiated working areas by 30-50%, wear resistance-2-3 times, as compared to volumetric-hardened steels.

Abstract: Thermal barrier coating (TBC) is deposited onto the gas turbine blade surface in order to protect the substrate from high-temperature combustion gas. Cracks and delamination of the ceramic coating which come from high heat flux loading are serious problem in TBC. In this study, the rapid thermal cycling device utilizing laser irradiation was developed. It was then investigated how the damage progresses in the ceramic coating exposed to cyclic rapid thermal loading. As a result, a sintering layer was formed in the surface of the ceramic coating, although such phenomenon was not recognized in TBC sample tested by the conventional thermal cycling test using an electric furnace. It was also revealed from the cross-sectional observation that the vertical crack was initiated at the surface of TBC and propagated into sintering layer. Finally, mechanical factors of those damages from finite element analysis using the TBC model including sintering progress was discussed

Abstract: To grind fused silica in ductile mode, it was proposed to repair surface and subsurface micro cracks of fused silica by CO₂ laser irradiation. However, excessive residual stress remains on the surface because the melt fused silica on the surface quenches in air. It causes the critical depth of cut for ductile grinding fused silica to be smaller than 0.2μm. To investigate the distribution of the residual stress and look for an optimal manner of irradiation to control residual tensile stress, a numerical model of was built for simulating the dynamic behavior of fused silica when irradiated by CO₂ laser. Laser energy absorption, heat transmission, viscoelastic behavior of fused silica and thermally induced stress were considered in the numerical simulation. The results show how the residual stress is formed and distributed. We found that an appropriate control of the temperature field as a function of time and position in the laser process is the key to reduce the residual stress. Therefore, three kinds of processes were proposed to reduce residual tensile stress on the surface of fused silica introduced by laser irradiation. The residual stress distributions of these three processes were compared by numerical analysis to decide a better method of laser irradiation.

Abstract: Low temperature photoluminescence (PL) of high-resistivity detector-grade Cl-compensated CdTe semiconductor crystals subjected to irradiation with nanosecond (τ = 7 ns) laser pulses of the second harmonic (λ = 532 nm) of a YAG:Nd laser is studied. Irradiation of CdTe crystals within the certain range of laser pulse energy densities results in a relative decrease in the emission intensity in both the deep energy level and edge regions and an increase in the exciton band intensity in the PL spectra. The evolution of the PL spectra depending on laser energy density, excitation level and temperature under excitation are analyzed. Laser-stimulated transformation of the point defect structure of the CdTe surface region and mechanisms of laser-induced defect formation are discussed. The optimal regimes of laser processing have been obtained which result in the minimum ratio of the defect and exciton bands that is an evidence of an increase in the structural perfection of the irradiated crystals.

Abstract: The photoconductivity (PC) spectra of CdZnTe (CZT) semiconductor solid solutions obtained from different manufacturers were investigated before and after irradiation with nanosecond laser pulses. Depending of energy density, irradiation of CZT crystals resulted in an increase in the photosensitivity, high-energy shift of the PC spectrum maximum and transformation of its profile. Laser processing with the certain energy densities improved homogeneity in the crystal surface area and provided equalization of structural characteristics in the surface region and bulk of the samples. Irradiation of CZT crystals pre-coated with an In electrode film caused rectification in the I-V characteristics because of the formation of a barrier structure. The application of the obtained In/CZT/Au diodes with low leakage current as X/γ-ray detectors is considered.

Abstract: Laser reduction is a facile method for the synthesis of laser converted graphene from graphite oxide (GO) The laser power density of semiconductor laser can effect on the reduced graphene quality. The research on relationship between laser power density and reduction of graphene oxide can help to determine the optimum technology parameter in the synthesis of laser covered graphene.

Abstract: An experimental study was conducted to get the ignitibility of NEPE propellant under different CO₂ laser heat fluxes. The combustion flame temperature distribution of NEPE propellant was measured using an infrared thermometer. Results show that the ignition delay time tends to decrease with the increase of laser heat flux, and there exists a significant value. The ignition delay time decreases fast when the heat flux is less than this value, but varies little when the heat flux is greater than this value. Laser irradiation had a significant effect on the combustion of NEPE propellant and after the laser unloading, the flame temperature of the propellant dose not decline immediately, but fall rapidly after a short delay.

Abstract: Simulation of transient bioheat transfer in a two dimensional (2D) human eye model is conducted using a newly developed hybrid fundamental solution-finite element method (HFS-FEM) coupling with the radial basis function (RBF) approximation. Firstly, a time stepping scheme based on the finite difference method (FDM) is used to handle time variable in the transient Pennes bioheat equation. Secondly, the particular solution of the governing equation is approximated by a RBF approach. Then, the homogeneous solution is calculated by means of HFS-FEM. The obtained results are compared with those from ABAQUS and a good agreement between them is observed.