A.G. Gnedovets

Gas dynamics of laser ablation: Influence of ambient atmosphere

A two-stage two-dimensional (2D) gas-dynamic model of laser ablation in an ambient gas atmosphere is proposed. The initial one-dimensional stage of the process is related to the ablation plume formation under the action of a laser pulse (duration of the order of 10 ns; fluence about several J/cm2; laser spot diameter about 1 mm) and describes heating, melting, and evaporation of the target, the target–vapor interaction in the Knudsen layer, and the vapor dynamics. The final 2D stage is responsible for the formation of the energy and angular distributions of the ablated material. Considerable compression of the ambient gas around the expanding plume of the laser-evaporated material and a shock front propagating through the undisturbed ambient gas are found. The pressure of the compressed ambient gas behind the shock may be much higher than the ambient one. However, at the investigated ambient pressures below 100 Pa, it remains still much lower than the vapor pressure during laser evaporation. Therefore, th...

Two-dimensional gas-dynamic model of laser ablation in an ambient gas

Abstract A two-step gas-dynamic model of laser ablation in an ambient gas atmosphere is proposed. The initial 1D stage is related to ablation plume formation and describes heating, melting and evaporation of the target, the target-vapour interaction in the boundary layer, and vapour dynamics. The final 2D stage is responsible for the formation of energy and angular distributions of the ablated material. These distributions are calculated assuming local thermodynamic equilibrium. Interaction between the vapour and ambient gas is taken into account by two-component gas-dynamic equations. Numerical analysis of laser ablation in ambient gas atmosphere revealed that both kinetic energy of ablated atoms and width of their angular distribution decrease with ambient pressure. Dynamics of ablated material expansion and its energy distribution are compared with the experiment.

Simulation of nanoscale particles elaboration in laser-produced erosive flow

Abstract Synthesis of nanoscale particles as a result of vapor condensation under laser vaporisation of materials into high-pressure surrounding atmosphere has been studied. A 2D-hydrodynamic model is developed for a simulation of nanoscale particles formation in a laser-produced erosive flow propagating from the irradiated surface into the ambient gas. The analysis is based on a combined solution of the set of hydrodynamic equations describing the erosive flow and equations of nonequilibrium kinetics of particles nucleation and condensation growth. The detailed structure of the erosive jet is studied: It is shown that nucleation and condensation fronts are separated in space and are formed between the erosive vapor–gas flow and cold ambient gas. The model predicts bimodal particle-size distribution function. The particle size is shown to be independent on the evaporation rate and to increase with the laser beam radius and the ambient gas pressure.

Submicron particles synthesis by laser evaporation at low power density: a numerical analysis

Abstract A model for submicron particles formation by laser evaporation in ambient gas atmosphere is proposed. Heat transport in the target, evaporation, vapour diffusion in the ambient gas, and kinetics of nucleation and particles growth are considered. The diffusion regime of evaporation is studied. Submicron particles formation by millisecond laser pulses of about 104–105 W/cm2 energy density flux is analysed. The nuclei formation and growth rates are estimated and size distributions of the particles are calculated. The mean particle size decreases with the ambient gas pressure. The spatial separation of nuclei formation and the particles growth processes with a deficiency of condensation centres ensures conditions for a nearly uniform growth which results in narrow particle-size distribution.

Study and simulation of the growth of solid lubricant MoSe x coatings during pulsed laser deposition

The chemical composition and tribological properties of the thin-film diselenide molybdenum coatings deposited by pulsed laser deposition in vacuum and a rarefied inert gas (argon) atmosphere are studied. Upon deposition in a gas at a pressure of ∼2 Pa, stoichiometric coatings with improved antifriction properties as compared vacuum-deposited coatings form. However, a too strong increase in the argon pressure (to ∼10 Pa) degrades the tribological properties of the coating. Structure formation in the MoSex coatings grown by pulsed laser deposition on an unheated substrate is investigated. Deposition in vacuum or argon at a pressure of 2 Pa leads to formation of rather smooth coatings with a dense amorphous structure containing molybdenum nanoinclusions. Deposition at a high argon pressure results in a developed surface relief and a loose coating structure. A mathematical model is developed using the kinetic Monte Carlo method in order to describe structure formation in the coatings that grow during physical deposition of an atomic flux. A comparative analysis demonstrates satisfactory agreement between the simulated and experimentally studied structures in the coatings created by pulsed laser deposition at various gas pressures.

Peculiarities of Pulsed Ion Implantation from a Laser Plasma Containing Multiply Charged Ions

A mathematical model describing the dynamics of a pulsed laser plasma with multiply charged ions, as well as the formation of the accelerated ion flow in an external magnetic field, is developed. Experimental studies and mathematical simulation by the particle-in-cell method are used to determine the role of multiply charged ions in the process of ion implantation into a silicon substrate from the pulsed plasma containing singly and doubly charged titanium ions. The plasma spreads between parallel-plate electrodes (Ti target and Si substrate) along the normal to the surface of the target. Ions are accelerated by high-voltage negative pulses applied to the substrate. It is found that doubly charged ions effectively participate in the implantation process when an external electric field is applied very soon after the laser action on the target. The application of a high-voltage pulse with an amplitude of 50 kV 0.5 μs after a laser pulse leads to ion implantation with an energy close to 100 keV. With increasing delay in the application of the high-voltage pulse, the upper boundary of the energy spectrum of implanted ions is displaced towards lower energies. Comparison of the depth profiles of titanium distribution in silicon calculated from the results of simulation are compared with the experimental profiles shows that the model developed here correctly describes the formation of the high-energy component of the ion flow, which is responsible for defect formation and doping of deep layers of the substrate.

Hydrodynamics of laser erosive jet generating nanoscale particles

Abstract A 2D-hydrodynamical model is developed for simulation of ultrafine particle elaboration by means of the laser-evaporation technique. The general picture of the process can be predicted as follows. An erosive flow of the submerged jet type is formed under laser evaporation of a target material into ambient gas atmosphere. Due to the interaction with surrounding gas, the erosive flow is cooled and the vapour condenses generating nanoscale particles. A thin condensation front is formed between the erosive vapour–gas jet and cold ambient inert gas. The gas flow streamlines intersect the condensation surface twice and accordingly the condensation process is divided into two stages. The initial stage corresponds to the inlet part of the front near the laser irradiation spot, where streamlines are directed inward the erosive jet, homogeneous nucleation and condensational growth of nucleation centres take place here. The formed particles of a condensate are transferred by the inert gas into internal flow area, where the condensation is suspended. At the final stage, the particles intersect the condensation front from within the vapour–gas mixture at an outlet section where they continue to grow. The resulting particle size distribution depends both on ambient gas pressure and laser irradiation conditions.

Reactive pulsed laser deposition of WOx layers for SiC-based hydrogen sensor fabrication

Peculiarities of WOx films fabrication by reactive pulsed laser deposition for high temperature Pt-oxide-SiC devices formation were investigated. Deposition of the oxide film was also carried out in such a way as to prevent deposition of droplet fraction (deposition with anti-droplet screen). Direct Simulation Monte Carlo and Kinetic Monte Carlo methods were performed for the deposition processes modeling. The response of the SiC-based devices to hydrogen-containing gases depends on the conditions of deposition of the oxide layer. The best properties were found in the sensor obtained by depositing the scattered flux of W atoms in a shady area on SiC substrate at an oxygen pressure of 10 Pa.

Effect of Debye Screening on Heat Transfer to a Spherical Particle from Rarefied Plasma

The methods of the kinetic theory tire applied for the description of charge and heat transfer front a rarefied plasma to a spherical particle for an arbitrary ratio between Dehye length and particle radius. Different models of the velocity distribution functions of the charged plasma species are considered. The results of the numerical analysis show that the intensity of plasma-particle heat exchange is greatly influenced by gas ionization, participation of electrons and ions in the transfer processes, particle charging, and .screening properties of the plasma. Even at a low degree of ionization, the electron and ion contribution to the heat transfer remains significant.

Condensation in the laminar-free jet of vapor flowing into a cool gas

Laser beam treatment of the surface leads to vaporization. Cooling of the vapor by the surrounded gas results in condensation and the ultrafine particles creation. To make clear conditions of the particles formation, a two-phase hydrodynamic flow of the erosion torch was simulated. A case of the free laminar vapor jet inside the inert gas atmosphere was investigated. The particles are founded to create inside a thin layer -- the condensation front. The particle dimension is proportional to the second power of the ambient gas pressure.