Laser Heating Conditions Research Articles

Homologous distortion of the aluminum crystal structure under laser radiation

It was shown by X-ray diffraction that the aluminum crystal structure is distorted under conditions of nonequilibrium laser heating, which appears in lowering the lattice symmetry. A method for describing the observed distortions, based on the transition to a new unit cell, was proposed. It was shown that the distorted aluminum crystal structure can be described using the transition from the face-centered cubic cell to the monoclinic body-centered cell. The parameters of the aluminum unit cell after laser irradiation were determined as a = 0.2870 nm, b = 0.2860 nm, c = 0.4060 nm, and β = 90.013° (for the axes of the monoclinic body-centered lattice).

The dynamics of expansion of gas and condensed particles under conditions of local laser heating of metal surface

A model of formation of the products of interaction between focused electromagnetic radiation and metal surface is considered. A description of the dynamics of expansion of gas cloud is given; estimates are obtained of the amount, size and velocity of scattering of condensed particles being formed. The obtained results enable one to gain a better notion of the possibilities of analysis of emission spectra for qualitative and quantitative investigation of the atomic composition of solid sample under the effect of laser radiation.

FEM-Based Process Design for Laser Forming of Doubly Curved Shapes

This study presents a finite element method (FEM) based three-dimensional laser forming process design methodology for thin plates to determine the laser scanning paths and heating conditions. The strain fields were first calculated via FEM. The laser scanning paths were chosen perpendicular to the averaged principal minimum strain direction. The ratios of in-plane and bending strain were calculated to help determine the heating condition. Two typical doubly curved shapes—a pillow shape and a saddle shape—were studied, and the overall methodology was validated by experiments.

Prospects for laser-ultrasonic treatment for surface modification, welding, and pattern cutting

Heat and mass transport, structurization, and residual-stress formation in the course of laser-ultrasonic hardening, near-surface alloying, rapid crystallization of melts, and pattern cutting are experimentally studied with allowance made for the theoretical concept that ultrasound influences the processes of phase transformations and diffusion under conditions of high-rate laser heating and subsequent cooling. It is shown that harder and deeper hardening zones and melt baths are formed under the laser-ultrasonic treatment. Plastic deformation of the surface by ultrasound significantly affects the completeness of austenitic transformation and thus makes it possible to efficiently control the strained state of the surface in the zone exposed to laser radiation. Acoustic flows emerging in the laser-induced melt smooth the temperature nonuniformities and the topography of the melt surface efficiently even when the duration of the laser exposure is about 1 ms. The influence of the geometry of injecting the ultrasound on the structurization and formation of residual stresses during rapid crystallization of the laser-induced melt and the subsequent tempering was observed. It was established that the injection of ultrasound into the laser-induced melt brings about a more efficient alloying and better micromechanical characteristics of the surface. The laser-ultrasonic pattern cutting of metal without the formation of flashes was realized.

Nonlinear optical study of the Si(111)7 x 7 to 1 x 1 phase transition: Superheating and the nature of the 1 x 1 phase.

The reversible 7\ifmmode\times\else\texttimes\fi{}7\ensuremath{\leftrightarrows}1\ifmmode\times\else\texttimes\fi{}1 phase transition of Si(111) is investigated by means of optical second-harmonic (SH) generation. The SH response increases sharply at ${\mathit{T}}_{\mathit{c}}$\ensuremath{\simeq}1130 K. The symmetry and magnitude of the SH signal are compatible with an adatom gas model for the 1\ifmmode\times\else\texttimes\fi{}1 phase in which the number of dangling bonds is increased by roughly 25% with respect to the 7\ifmmode\times\else\texttimes\fi{}7 structure. The behavior observed under conditions of laser heating provides clear evidence of superheating of the Si(111)7\ifmmode\times\else\texttimes\fi{}7 phase by more than 50 K above ${\mathit{T}}_{\mathit{c}}$ on the nanosecond time scale.

Variable strain energy in amorphous silicon

Different Raman experiments on structural relaxation of a-Si and a-Ge are reviewed and discussed in relation to calorimetric measurements on a-Ge. On the basis of the correlation found between results from Raman spectroscopy and results from calorimetry in the case of a-Ge and of the strong similarity between a-Si and a-Ge in terms of their Raman spectra, it is suggested that the strain energy in a-Si may vary considerably with preparation conditions and subsequent treatments. Under this assumption the a-Si Gibbs free energy versus temperature has been constructed for material in different initial states of relaxation. It is shown that the melting temperature of amorphous silicon should increase when relaxation occurs during the heating phase prior to melting. Thus differences in apparent melting temperature, as observed under different laser heating conditions, may be explained.

Kinetics of subthreshold luminescence and microwave conductivity of glasses under laser heating conditions

An experimental investigation was made of the luminescence and microwave conductivity of K8 and ZhS 11 optical glasses under conditions of subthreshold irradiation by a quasi-cw neodymium laser. For ZhS 11 glass it was established that the luminescence depended on temperature and the influence of photostimulation was observed at the 1.06 μ wavelength. The activation energies of the microwave conduction and subthreshold luminescence processes were determined for ZhS 11 glass.

Experimental determination of vapor species from laser-ablated carbon phenolic composites

Experimental determination of vapor species from laser-ablated carbon phenolic composites

The mechanism and kinetics of evaporation from laser irradiated UO2 surfaces

A theoretical analysis of laser evaporation experiments for the determination of the high temperture saturated vapor pressure of uranium dioxide is presented. The interaction of laser radation with the condensed phase is discussed and the timescales involved in the energy equilibration processes are given. The mechanism of evaporation is described within the framework of the Terrace–Ledge–Kink model, where it is shown that by using symmetry properties of ionic crystal lattices, the surface Madelung potentials and binding energies of ions and neutral ionic units may be determined. Results are presented for NaCl, CsCl, and UO2 surfaces. Based on these calculations, general conclusions are drawn on the nature of the evaporating species. In addition, it is shown that the multispecies evaporation from uranium dioxide is fundamentally related to the amount of atomic and electronic disorder in the surface layer. The effects of the kinetics of surface reactions on the rate of evaporation are investigated. Parallel reaction paths such as direct evaporation from kink sites are treated in addition to surface diffusion followed by desorption series reactions. Finally, the possibility of producing superheated metastable surface layers under pulsed laser heating conditions is investigated using models for the kinetics of atomic and electronic disorder in the condensed phase.

Use of argon laser radiation in restoration of single-crystal state of ion-implantation-amorphized silicon surface

High-power (up to 500 W) radiation of a cw argon laser was used to restore the single-crystal state of a silicon surface made amorphous by ion implantation. The experiments were carried out in air and the power density of the laser radiation was 100 W/cm2. The duration of illumination did not exceed several seconds. The experiments were carried out on single-crystal silicon plates doped by implantation of arsenic ions at room temperature (ion energy 100 keV, dose 1800 μC/cm2). Surface melting was observed after several seconds under soft laser heating conditions. Reflection electron micrographs indicated that laser radiation restored the single-crystal structure of an ion-implanted amorphous layer of silicon even when the illumination was brief (3 sec). Complete recovery was achieved after melting the surface.