Staff

Abstract:
Carbon nanodots (CNDs) produced in pure water by the ablation of graphite with a nanosecond laser pulse exhibit weak photoluminescence. A small addition of polyethyleneimine (PEI) to the aqueous suspension of CNDs causes a significant increase in emissions. This paper presents experimental and theoretical studies of the emission properties of CND/PEI systems. The obtained CNDs responded to even trace amounts of PEI in solution (~0.014% v/v), resulting in a significant increase in the initial weak blue emission of CNDs and PEI taken separately. Morphology and size measurements showed that particle aggregation occurred in the presence of the polymer. A decrease in the calculated Stokes shift values was observed with increasing PEI content in the solution. This indicates a reduction in the number of non-radiative transitions, which explains the increase in the emission intensity of the CND/PEI systems. These results therefore confirmed that the increase in the emission of CND/PEI systems is caused by particle aggregation. Kinetic studies proved that the process is controlled mainly by diffusion, the initial stage of which has a dominant influence on determining the optical properties of the system.

Keywords:
aggregation-induced emission, carbon nanodots, polyethyleneimine, intrinsic fluorescence, adsorption kinetic study

Abstract:
This work presents a growth of Fe3O4 nanoparticle films on silicon substrate. The iron oxide is deposited applying a pulsed laser deposition technique. The process is performed in open air in the absence and presence of external magnetic field. In fact, the morphologies of the obtained Fe3O4–Si samples are similar. The Fe3O4 nanoparticles are spherical with average diameters of 30 nm and are densely agglomerated on the Si substrate. The Fe3O4–Si material prepared in the absence of magnetic field has revealed more intense signals during X-ray diffraction and Raman measurements. The magnetic investigations indicate that the Fe3O4 nanoparticles are significantly coupled with the Si substrate and do not exhibit superparamagnetic behavior. Moreover, the Verwey transition is 98 K for both investigated Fe3O4–Si samples.

Keywords:
Fe3O4 nanoparticles,magnetic materials,pulsed laser deposition

Abstract:
A new method of surface modification based on the method of electrospark alloying (ESA) using carburizer containing nitrogen—carbon components for producing coatings is considered. New processes have been proposed that include the step of applying saturating media in the form of paste-like nitrogenous and nitrogenous-carbon components, respectively, onto the surface without waiting for those media to dry, conducting the ESA process with the use of a steel electrode-tool, as well as with a graphite electrode-tool. Before applying the saturating media, an aluminium layer is applied onto the surface with the use of the ESA method at a discharge energy of Wp = 0.13–6.80 J. A saturating medium in the form of a paste was applied to the surfaces of specimens of steel C22 and steel C40. During nitriding, nitrocarburizing and carburization by ESA (CESA) processes, with an increase in the discharge energy (Wp), the thickness, micro hardness and continuity of the “white layer” coatings, as well as the magnitude of the surface roughness, increase due to saturation of the steel surface with nitrogen and/or carbon, high cooling rates, formation of non-equilibrium structures, formation of special phases, etc. In the course of nitriding, nitrocarburizing and CESA processing of steels C22 and C40, preliminary processing with the use of the ESA method by aluminum increases the thickness, microhardness and continuity of the “white layer”, while the roughness changes insignificantly. Analysis of the phase composition indicates that the presence of the aluminum sublayer leads to the formation of the aluminum-containing phases, resulting in a significant increase in the hardness and, in addition, in an increase in the thickness and quality of the surface layers. The proposed methods can be used to strengthen the surface layers of the critical parts and their elements for compressor and pumping equipment

Keywords:
electrospark alloying,coatings,roughness,structure,microhardness,continuity,X-ray diffraction analysis,nitriding,nitrocarburizing,carburization

Abstract:
Despite the existence of many more efficient methods of producing carbon nanoparticles, ablation of a carbon target by a laser pulse remains important. It enables studying the bare properties of nanoparticles, not contaminated with reagents or reaction products. The present work analyses the mechanisms of nucleation and growth of nanoparticles in carbon vapours generated during ablation of graphite with a nanosecond laser pulse. The role of both the homogeneous and the heterogeneous (ions) nucleation was investigated, defining the areas of their occurrence. It has been shown that the most favourable conditions are high pressure of the order 1 GPa and relatively low temperature of about 15 000 K. Such conditions are obtainable when ablation occurs in a liquid and the fluence of the laser pulse is low, exceeding the ablation threshold about 2.7 times only. The resulting nanoparticles are relatively homogeneous and have a diameter of approximately 2.5–5 nm.

Keywords:
laser ablation in liquids, nucleation, plasma expansion, laser produced plasma

Abstract:
The article presents theoretical and experimental study of shock waves induced by a nanosecond laser pulse. Generation of surface plasma pressure by ablation of the graphite absorption layer in water medium and shock wave formation were analyzed theoretically and experimentally. The amplitude and temporal variation of the shock wave pressure was determined basing on a proposed hydrodynamic model of nanosecond laser ablation and experimentally verified with use of a polyvinylidene fluoride (PVDF) piezoelectric-film sensor. The determined pressure wave was used for examination of adhesive strength of tungsten–zirconium–boride coatings on steel substrate. The magnetron sputtered (MS) W–Zr–B coatings show good adhesion to the steel substrate. The obtained experimental results prove the correctness of the proposed model as well as the suitability of the procedure for assessment of adhesive strength.

Keywords:
laser pulse, shock wave, MS coatings, ternary borides, laser adhesion test

Abstract:
An improved method for the production of luminescent carbon nanoparticles is proposed in this work. The new method overcomes the disadvantages of commonly used approaches. It involves two-stage laser ablation in water and in aqueous solutions, where the first stage is the laser ablation of a graphite target and the second is the shredding of particles produced in the first step. The two-stage method offers the optimization of the laser pulse fluence for the performance of each process. It was found that the two-stage process of laser ablation allows producing photoluminescent carbon structures in pure water. The additional reagent may be added either in the first or second stage. The first stage performed in pure water allows avoiding the contamination of the target. Moreover, it simplifies the identification of the origin of photoluminescence. Two synthesis routes for the preparation of carbon nanoparticles by the proposed method using pure water as well as urea aqueous solution are investigated. It was found that the use of urea as a reagent results in luminescence properties similar to those obtained with other more hazardous amine-based reagents. The influence of the synthesis approach and process parameters on the structural and luminescent properties of nanoparticles is also explored in this work.

Keywords:
pulsed laser ablation in liquid, carbon nanoparticles, photoluminescent particles

Abstract:
Fluorescent carbon dots (CDs) synthesized by pulsed laser ablation in liquid (PLAL) are still interesting materials due to their possible applications. However, unlike CDs produced by the hydrothermal method, CDs produced the synthesis products by the PLAL method were never separated by dialysis, which differentiates the synthesis products and allows the identification of the main source of fluorescence. In this work, the synthesis of fluorescent carbon dots (CDs) was performed by nanosecond laser ablation of a graphite target immersed in polyethyleneimine (PEI) and ethylenediamine (EDA), and the synthesis products were separated by dialysis. The results of optical measurements showed that the main source of luminescence of the obtained nanostructures are fluorescent particles or quasi-molecular fluorophores created in the ablation process. In the case of ablation in PEI, most of the produced molecular fluorophores are associated with carbogenic nanostructures, while in the case of EDA, free fluorescent molecules dominate.

Keywords:
carbon dots, photoluminescence, laser ablation

Abstract:
With increasing demand for high-performance and long-lasting cutting and forming tools, the members of this expanding class of superhard metals hold promise to address the shortcomings of traditional tool materials. Those shortcoming include their high cost (silicon nitride, cubic boron nitride, and diamond), their inability to cut ferrous metals due to chemical reactions (diamond), instability in the presence of humidity (cubic boron nitride) and relatively low hardness (tungsten carbide). Also the increasing industrial demand for protective coatings with high hardness, good elastic properties and thermal stability calls for the investigation of new material systems. Although transition metal (TM) nitrides are successfully applied for different tasks in automotive or aero-space industries, the search for improved materials is an ongoing topic, being far from its end. In this work the study on deposition of thin films made of new super-hard materials (SHM) such as tungsten boride are presented. Additionally, the influence of doping by titanium of those materials is investigated. Investigated films were deposited by the pulsed laser deposition method. The used targets were synthetized by SPS method. The powders of boron and tungsten in 4.5 to 1 molar faction were used. The films deposited by PLD method have stoichiometric composition such as used targets. The WB2 and WB3 phase are dominant. Research carried out using SEM, XRD and nanoindentation test showed that the phase composition of the targets is more important in the case of laser deposition than magnetron. All obtained layers are very hard and thermally stable. In the case of magnetron sputtering, smooth layers were obtained while the layers deposited by the laser have a very high roughness. Titanium doping increases the amount of WB3 phase in the sintered discs, while it has no significant effect on the properties of the deposited layers.
Z rosnącym zapotrzebowaniem na niezawodne, a jednocześnie zapewniające dużą wydajność, narzędzia do skrawania i obróbki plastycznej, coraz większego znaczenia nabiera rozszerzająca się grupa supertwardych ceramik przewodzących prąd. Materiały te dobrze rokują w związku z rozwiązaniem problemów tradycyjnych materiałów narzędziowych, których niedoskonałości obejmują wysoką cenę (azotek krzemu, azotek boru), niezdolność do cięcia stopów żelaza w wyniku reakcji chemicznych (diament), niestabilność w obecności wilgoci (azotek boru) i względnie małą twardość (węglik wolframu). Również rosnący popyt na powłoki ochronne o wysokiej twardości, o dobrych właściwościach sprężystych i stabilności termicznej powoduje, że badania nad nowymi systemami materiałowymi prowadzone są coraz intensywniej. Pomimo że azotki metali przejściowych są już z powodzeniem stosowane do różnych zadań w przemyśle samochodowym i lotniczym, poszukiwanie ulepszonych materiałów jest tematem wciąż aktualnym. W pracy przedstawiono badania nad osadzaniem cienkich powłok z nowych supertwardych materiałów (SHM), którymi są borki wolframu. Dodatkowo zbadano wpływ domieszkowania tych materiałów tytanem. Warstwy osadzane były metodą ablacji laserowej PLD. Tarcze do osadzania zsyntetyzowane zostały metodą spiekania plazmowego SPS proszków boru i wolframu o stosunku atomów 4,5 do 1. Osadzane z użyciem lasera warstwy mają skład stechiometryczny podobny do użytych tarcz. W warstwach tych dominuje faza WB3. Badania przeprowadzone z użyciem SEM, XRD i nanoindentacji wykazały, że skład fazowy tarcz jest odwzorowany w warstwach osadzonych laserem. Wszystkie uzyskane warstwy są bardzo twarde i stabilne termicznie. Warstwy osadzane laserem odznaczają się dużą chropowatością. Domieszkowanie tytanem zwiększa ilość fazy WB3 w spiekanych tarczach i osadzanych warstwach oraz zmniejsza ilość i wielkość naniesionych na powierzchnię cząstek.

Keywords:
laser ablation, tungsten borides doped titanium, plasma sintering / ablacja laserowa, borki wolframu domieszkowane tytanem, spiekanie plazmowe

Abstract:
The laser ablation of graphite in ambient argon is studied both experimentally and theoretically in conditions corresponding to the initial conditions of carbon nanotube synthesis by the laser vaporization method. The results of the experiment show that the maximum plasma temperature of 24 000 K is reached 25 ns after the beginning of the laser pulse and decreases to about 4000–4500 K after 10 μs. The maximum electron density of 8 × 1025 m−3 is reached 15 ns from the beginning of the laser pulse. The hydrodynamic model applied shows comparable plasma temperatures and electron densities. The model also replicates well a shock wave and plume confinement—intrinsic features of supersonic flow of the ablated plume in an ambient gas. The results show that the theoretical model can be used to simulate nanosecond laser ablation in an ambient gas from the beginning of the process up to several microseconds.

Abstract:
Tungsten borides belong to the group of potentially superhard materials which hardness could be compared to cubic boron nitride and diamond. However, difficulty in fabrication of single phase material using conventional methods is the main drawback of this group of ceramics. In order to overcome this problem material can be deposited as a thin layer e.g. in the pulsed laser deposition process. In this paper, the effect of laser wavelength and energy density of nanosecond Nd:YAG laser on the WBx-type layers were analyzed using wavelengths 355 and 1064 nm with the energy density of laser beam from 1.7 to 5 J/cm2 and from 1.7 to 9.3 J/cm2, respectively. The WB2.5 and WB4.5 targets synthesized in Spark Plasma Sintering process were used and the layers were deposited onto Si (100) substrate heated to a temperature of 570 °C. Layers' microstructure were analyzed using X-ray Diffraction and scanning electron microscope equipped with energy dispersive X-ray spectrometer. Change of laser wavelength and energy density resulted in variations of the chemical composition and morphology of deposited layers. Finally, W2B-βWB, αWB-WB-WB3 and WB3, and boron layers were deposited wherein WB3 structure is formed in a wide range of laser fluences and at both investigated wavelength. Next, WB3 layers were investigated in the indentation test at a load of 5–30 mN and its hardness was up to 50 ± 10 GPa

Keywords:
Pulsed laser deposition, Super-hard materials, Tungsten borides, Tungsten triboride

Abstract:
The laser spallation technique was applied for measurement of residual stress and adhesion of thin films. Two films of different properties, ductile and soft Ti, and hard and brittle TiN, were studied. The films were produced on 304 steel substrate by PVD method. The residual stress value obtained by laser spallation technique LST were compared with stress value from X-ray diffraction method. Good agreement of stress values measured by both methods was attained. Additionally, the interface strength of the films was tested by laser adhesion spallation technique LASAT with use of VISAR system. It was shown that shock wave induced by a nanosecond laser pulse adequately determines properties of PVD thin films on metal substrate

Keywords:
Laser spallation technique, Residual stress, Adhesion, Thin layer, PVD, VISAR system

Abstract:
The plasma induced during underwater pulsed laser ablation of graphite is studied both experimentally and theoretically. The results of the experiment show that the maximum plasma temperature of 25000 K is reached 20 ns from the beginning of the laser pulse and decreases to 6500 K after 1000 ns. The observed OH absorption band shows that the plasma plume is surrounded by the thin layer of dissociated water vapour at a temperature around 5500 K. The hydrodynamic model applied shows similar maximum plasma temperature at delay times between 14 ns and 30 ns. The calculations show also that already at 14th ns, the plasma electron density reaches 0.97·1027 m−3, which is the critical density for 1064 nm radiation. At the same time the plasma pressure is 2 GPa, which is consisted with earlier measurements of the peak pressure exerted on a target in similar conditions.

Keywords:
underwater laser processing, pulsed laser ablation in liquid, laser induced plasma, numerical modelling

Abstract:
The numerical model of laser welding is presented. The time dependent set of equations describing heating, melting and solidification is solved using ANSYS-Fluent package and adopted to the problem using the external user-defined functions. The developed model is used for investigation of laser welding of Ir pad with spark plug electrode made of Ni. These spark plugs are key parts of industrial internal combustion engines using biogas as fuel. Problems in welding arise from significant difference of melting points of both metals; moreover, the boiling point of Ni is close to melting point of Ir. Theoretical model of the welding process is a useful and cost lowering tool providing guidance for selection of parameters and reducing significantly number of expensive and time consuming experiments

Keywords:
Nd:YAG laser, iridium, Ir, nickel, Ni, spark plug, laser welding, numerical model, dissimilar metals welding, temperature dependent material properties

Abstract:
The most promising areas of research of new super-hard materials are transition metal borides. These materials are one of the candidates for future superhard layers that will be competitive to DLC and c-BN layers. In this paper MoB-type tungsten boride (WB) layers were examined. WB layers have been deposited by radio frequency magnetron sputtering on Silicon (100), 304 stainless steel (SS 304) and Inconel 601 substrates. Measured thickness of herein prepared layers was about 1 µm, and all studied samples were dense, uniform and smooth. Surface investigation was performed by using an optical profilometer, atomic force microscopy, and scanning electron microscopy. The structure analysis was examined by using X-ray diffractometer (XRD) and transmission electron microscopy (TEM) techniques. Results from the XRD and TEM analysis showed that WB layers were dominated by (101) reflection and indicated a fine grain structure with a grain size of 20–40 nm. The effect of target sputtering power and ambient gas pressure was investigated. The hardness of WB layers deposited on silicon substrate was compared under the load from 1 mN to 5 mN. The hardness of WB layers deposited on SS 304 and Inconel was measured up to 50 mN. All layers of WB revealed excellent hardness exceeding 40 GPa.

Keywords:
Borides, Nanohardness, RF magnetron sputtering, Superhard layers, Tungsten boride

Abstract:
The electric characteristics of a segmented plasmatron and the results of optical emission spectroscopy of Ar-air, N2, and N2–CO2 are presented. The main working gas forming the plasma stream was fed near the cathode into the arc region and another additional gas was injected into the plasma stream beyond the arc. It is shown that the gas injected into the plasma stream is drawn to the arc area due to arc spot movement and cyclic arc shrinking and expanding due to the power supply pulsation. It was found that when the anode spot moves upstream, the additional gas is retracted into the arc region, changing the operating conditions of the plasmatron. The retraction mechanism depends on the gas type and is different in argon and molecular plasmas. The results of the plasma emission spectroscopy show differences in the electron excitation and rotational temperatures for the plasmas studied and are used to explain the mechanism of functioning of a segmented plasmatron.

Keywords:
segmented plasmatron, electric properties, optical emission

Abstract:
Reactive spark plasma sintering (SPS) of WB2/WB3 ceramics from elements is studied; the sintering pressure dependence of the ratio of WB3 to WB2 in samples produced by SPS is discussed. Regardless of the sintering pressure, the obtained samples are very hard ~20 GPa. WB3 superhard films prepared by pulsed laser deposition (PLD) from selected SPS targets are presented.

WB3 coatings were prepared on Si (100) substrates using a nanosecond, Nd:YAG laser operating at a 355 nm wavelength. The phase analysis, crystallography, and orientations have been studied using X-ray diffraction (XRD). A WB2 to WB3 phase transformation from 8.2% WB3 in a sintered target to 93.3% WB3 in a deposited film was observed. Additionally, the surface of a SPS sintered WBx target after the ablation process was examined. XRD studies show that already during the laser ablation there is a significant WB2 to WB3 phase transformation. Vickers hardness of sintered samples was measured in macro- and micro-scale, and PLD films in the nanoscale.

Keywords:
Films, Tungsten borides, SPS – sintering, Pulsed laser ablation/deposition

Abstract:
Rhenium diboride (ReB2) thin films have been deposited by a nanosecond pulsed laser deposition method on Si (100) substrate heated to 570°C. The coatings were formed in the ablation process of SPS sintered ReB2 target. The effect of laser wavelength, energy density and postannealing on the films' properties was studied. Investigated wavelengths were 355 and 1064 nm of Nd:YAG nanosecond laser. Laser beam energy density varied from 2.1 to 6.1 J cm− 2 and from 4.1 to 9.4 J cm− 2 at 355 and 1064 nm, respectively. Layer thickness was of the order of several hundred nanometres. Deposition efficiency increases with an energy density in quasi linear way and strongly grows for shorter wavelength. The layers consist of two characteristic microstructures: a smooth basis and stick debris (typical diameters of several hundred nanometres). Lower energy density of laser beam and longer wavelength favour dominance of smooth basis and minimization of debris. The XRD analysis of all samples indicate the crystalline ReB2 with preferred (002) orientation and fine grain size of about 20 nm. Shorter wavelength and higher energy density foster stronger (002) orientation. Moreover, an annealing right after the deposition (25 min in 350°C) causes minimization of degree of orientation and decrease of hardness. The Vickers hardness of ReB2 films is at about 60 GPa and is reduced to about 40 GPa after the annealing process. Deposition efficiency and physical and chemical structures of layers produced under variety of conditions were studied and compared.

Keywords:
Ceramics ablation, Pulsed laser deposition, Rhenium diboride, ReB2 optical properties, Super-hard films, Wavelength influence

Abstract:
Our experiments with the ablation of graphite by a nanosecond laser pulse showed the formation of craters with a depth of upto 60 µm. The creation of such deep craters is hard to explain solely by evaporation. Existing models should be supplemented by an additional mass removal process that ensures penetration of the material. The recoil pressure at the surface of the target generates a compression wave propagating deep into the material. Possible mechanisms of fracture by the longitudinal compression wave are discussed. A phenomenological model of material fragmentation is proposed. Modelling results are in good agreement with the experiment. The model may be used for polycrystalline graphite as well as other brittle materials treated by the nanosecond laser pulse.

Keywords:
laser ablation, recoil pressure, rigid unloading, graphite

Abstract:
In this paper, the effect of absorption of the laser beam and subsequent radiation on the dynamics of a tungsten plasma plume during pulsed laser ablation is analyzed. Different laser wavelengths are taken into consideration. The absorption and emission coefficients of tungsten plasma in a pressure range of 0.1–100 MPa and temperature up to 70 000 K are presented. The shielding effects due to the absorption and radiation of plasma may have an impact on the course of ablation. The numerical model that describes the tungsten target heating and the formation of the plasma and its expansion were made for 355 nm and 1064 nm wavelengths of a Nd:YAG laser. The laser beam with a Gaussian profile was focused to a spot size of 0.055 mm2 with a power density of 1 × 109 W/cm2 (10 ns full width half maximum pulse duration). The plasma expands into air at ambient pressure of 1 mPa. The use of the shorter wavelength causes faster heating of the target, thus the higher ablation rate. The consequences of a higher ablation rate are slower expansion and smaller dimensions of the plasma plume. The higher plasma temperature in the case of 1064 nm is due to the lower density and lower plasma radiation. In the initial phase of propagation of the plasma plume, when both the temperature and pressure are very high, the dominant radiation is emission due to photo-recombination. However, for a 1064 nm laser wavelength after 100 ns of plasma expansion, the radiation of the spectral lines is up to 46.5% of the total plasma radiation and should not be neglected.

Keywords:
Plasma temperature, Tungsten, Laser ablation, Laser beams, Plasma pressure

Abstract:
The effect of laser wavelength on single-wall carbon nanotubes synthesis yield and their properties was studied. A double-pulse Nd:YAG laser, working at a wavelength of 355 or 1064 nm, was used for carbon nanotubes production. The synthesized carbon nanotubes (CNTs) were investigated using the SEM/STEM microscopy and Raman spectroscopy. The results show that the useful range of UV laser radiation fluence is narrower and the properties of synthesized CNTs depend much more on the laser fluence than in the case of infrared laser radiation.

Keywords:
carbon nanotubes, laser radiation fluence, pulsed laser vaporization, Raman spectra

Abstract:
The ablation of graphite is studied as a function of laser fluence for 355, 532 and 1,064 nm wavelength generated by a nanosecond Nd:YAG laser. It has been found that in the case of lower wavelengths, the transition from the thermal ablation to the phase explosion takes place at lower laser fluences. The change of crater shape due to the effect of deep drilling in the proximity of the phase explosion threshold was observed. The calculations of plasma radiation flux to the target surface were made, and the considerable increase of absorbed energy density was found in the case of 355 nm wavelength.

Keywords:
laser ablation, ablation rate, carbon

Abstract:
A Nd:YAG laser operating at a wavelength of 266 or 355 nm is used to deposit a thin layer of copper on the (0 0 0 1)α-Al2O3 surface. The formation process is precisely controlled by identification of time distribution of two characteristics: energy and flux density of particles incident on the substrate. For this purpose, the Cu-plasma expansion is described by means of an analytical hydrodynamic model whose self-similar solutions are fitted to the experimental plasma images and time-of-flight spectra. The obtained nanocomposite is examined by the aberration-corrected high-resolution transmission electron microscopy (Cs-HRTEM) method. The results reveal that copper crystals assume one main orientation relative to the substrate (1 1 1)[2 −1 −1]Cu∥ (0 0 0 1)[−1 −1 2 0]α–Al2O3 and the formed interface has a specific microstructure. To reconstruct the phase boundary region, molecular dynamic (MD) and static (MS) simulations are carried out. The results show that strong bonding between copper and sapphire induces structural changes in the (1 1 1) Cu layer nearest the substrate and leads to formation of the system of partially dissociated dislocations in the next layer. In consequence, the Cu/α–Al2O3 interface becomes the semicoherent system. The lattice matching regions of the individual Cu layers are significantly lowered, which results in strong deformations along the closed packed planes. The reconstructed interface is used for Cs-HRTEM image simulation. A good accordance with the experimental results indicates that the MD model correctly maps the microstructure at the phase boundary of the synthesized nanocomposite.

Abstract:
Nitrogen and argon plasmas with a small admixture of air produced in a segmented plasmatron are studied both experimentally and theoretically. A two-temperature hydrodynamic model is used to simulate the plasma flow inside the plasmatron. The calculated plasma temperature and electron density are in reasonable agreement with the experimental values obtained from emission spectroscopy. The electron temperatures are several thousand kelvins higher than the atom temperatures, showing that the plasmas produced in the segmented plasmatron are in non-equilibrium.

Keywords:
segmented plasmatron, plasma expansion, emission spectroscopy

Abstract:
This contribution proposes a description of selected experimental activities conducted in aerospace sciences and dedicated to generate experimental data to assess atmospheric entry plasma models. In order to provide comprehensive set of experimental data, high enthalpy shock tube facilities have been developed to generate plasma representative of entry plasma for broad range of trajectory entry conditions. The shock-heated plasma is obtained through adiabatic compression and the resulting post-shock plasma flow exhibits thermodynamic state analogous to actual entry plasma. However, significant insight can be obtained through experiments conducted also with non-equilibrium plasma flows obtained with other methods. The typical methodologies adopted to provide experimental data of interest to enhance entry plasma modeling are sketched for four distinct non-equilibrium plasma kinds produced respectively by four specific ground facilities. The contribution firstly will consider experimental campaigns conducted with a high enthalpy shock tube in order to document in absolute radiance the radiative signature in the UV spectral range of an Earth entry plasma. Then, the investigations of the interaction between a shock wave and an electrical discharge will be described. These investigations were performed to identify the role of the internal degrees of freedom of molecular gases on the propagation of the shock. Also, the contribution covers investigations devoted to the thermodynamic state characterizations by means of spectroscopic diagnostics in the cases of the non-equilibrium plasmas flows generated by plasma wind tunnels. The examination of the Saha-Boltzmann equilibrium is proposed in the case of a subsonic plasma flow. And at last, the characterization methods of air supersonic plasma jet are presented and the 2D distributions of the subsequently measured plasma properties are documented for a straight comparisons with non-equilibrium plasma jet computations.

Keywords:
Abel transform, multi-temperature model optical emission spectroscopy, non-equilibrium plasma, particle in cell - Monte Carlo computation, plasma wind tunnel, Saha balance, shock tube, supersonic plasma

Abstract:
The effect of laser wavelength on parameters of laser-ablated carbon plume is studied. A theoretical model is applied, which describes the target heating and formation of the plasma and its expansion, and calculations are made for the fundamental and third harmonic of a Nd:YAG laser. The calculated distributions of plasma temperature and electron density in the early phase of expansion show that plasma temperatures are higher in the case of 1064 nm but the electron densities are higher in the case of 355 nm, which is in agreement with experimental findings. It has been shown that while a higher plasma temperature in the case of 1064 nm is the result of stronger plasma absorption, the greater ablation rate in the case of 355 nm results in larger mass density of the ablated plume and hence, in higher electron densities. An additional consequence of a higher ablation rate is slower expansion and smaller dimensions of the plume.

Keywords:
Plasma temperature, Laser ablation, Laser beams, Absorption coefficient, Carbon

Abstract:
The characteristics of segmented plasmatron are presented. The arc is sustained in argon plasma and in plasma stream, argon, nitrogen and carbon-dioxide are introduced. The numerical simulation of argon electric arc and plasma stream is carried out and the temperatures and densities of electron and heavy particles are presented. Experimental and theoretical investigations determine and explain the influence of natural gas and flow rate injected into plasma stream on working plasmatron conditions. The anode voltage drop analysis shows that in plasmatron working at a current between 40 and 340 A, gas flow rate between 0.4 and 2 g/s and pressure from 5 to 100 kPa, this drop changes from negative to positive, influencing arc voltage value. The gas injection into plasma stream results in anode spot dynamic. Electric breakdowns accompanied by high voltage fluctuations are observed when argon or nitrogen is introduced while carbon dioxide eliminates this type of breakdown reducing the voltage fluctuations and ablation of electrodes.

Keywords:
segmented plasmatron, re-entry in planetary atmosphere, plasma jet

Abstract:
The plasma plume induced during laser ablation of graphite is studied both experimentally and theoretically. The hydrodynamic model applied shows that the high velocities of the plume observed in the experiment result from the large pressure gradient built up during laser–plume interaction. This means that at a low laser intensity of ∼109 W cm−2 the acceleration mechanism is rather gasdynamical than electrostatic. The calculated temporal profiles of plasma temperature and electron density in the early phase of expansion are in reasonable agreement with experimental profiles obtained from emission spectroscopy. The effect of evolution of the plasma temperature on time-of-flight signals is presented. It has been shown that the appearance of ions with higher charge prior to ions with lower charge often ascribed to the effect of the induced electric field can be explained by the evolution of the plume temperature.

Keywords:
carbon, laser ablation, plasma plume acceleration, hydrodynamic model

Abstract:
Laser ablation of graphite is studied both theoretically and experimentally. Plasma temperature and electron density in the early phase of expansion into vacuum are measured as a function of distance from the target. The experimental results agree well with the theoretical simulations. The simulation of ablation shows that plasma plume considerably affects the ablation rate

Keywords:
laser ablation, plasma plume

Abstract:
The effect of laser wavelength on heating of the ablated carbon plume is studied. The plasma absorption coefficients are calculated in order to analyze the results and an experiment is conducted using the first, second, and third harmonic of a Nd:YAG laser. Plasma temperature and electron density in the early phase of expansion in vacuum are studied as a function of distance from the target. The calculations show that the ratio between absorption coefficients for 1064, 532, and 355 nm is approximately 9:2:1. The experimental results do not agree well with the theoretical predictions. Indeed, the plasma temperatures are highest in the case of 1064 nm but no clear differences between 532 nm and 355 nm have been found.

Keywords:
plasma plume absorption, PLD, carbon, laser wavelength dependence

Abstract:
The interaction of laser beam with a target and next with the evaporated material is studied theoretically. In the case of a nanosecond laser pulse with 1064 nm wavelength, the ablation is thermal and therefore the interaction of the laser beam with a target is studied with the use of thermal model. The model which describes both the target heating, formation of the plasma and its expansion consists of equations of conservation of mass, momentum and energy and is solved with the use of Fluent software package. The calculations show a sharp increase of the plume temperature and pressure after plasma formation and following it, a considerable increase of the velocity of plasma plume. Maximum plasma pressure of 2 × 108 Pa, temperature of 61 500 K and front velocity of 3.8 × 104 m · s−1 have been found. The results show that the Mie absorption cannot be neglected in the phase of plasma formation. The shape of the plume and plasma front velocity obtained from the model are close to that observed in the experiment carried out in similar conditions.

Keywords:
laser ablation, plasma formation, plasma expansion

Abstract:
The characteristics of hydroxyapatite (Ca10(PO4)6(OH)2) thin films deposited by the pulsed laser deposition technique have been describrd. The laser used was a Nd:YAG, operating at the wavelength of 355 nm. All films were deposited at room temperature, either in ambient water vapour or in vacuum, and were annealed, after deposition in air, at 600 °C. Next, they were examined with the use of an X-ray diffractometer, Fourier transform infrared spectrometer, atomic force microscope, micro scratch tester and scanning electron microscope. The analyses showed that crystalline films exhibiting very strong
adhesion to the substrate have been obtained.

Keywords:
pulsed laser deposition, hydroxyapatite, biomaterials

Abstract:
The paper presents studies of the application of ion diagnostics and optical spectroscopy for on-line measurement of the amount and characteristics of co-deposits from the laser-ablated surface of the plasmafacing components (e.g. graphite tiles). For removal of the co-deposit layer a repetitive Nd:YAG laser was used. Determination of the characteristics of ions emitted from the laser-illuminated targets was performed using ion collectors (on the basis of a time-of-flight method) and an optical spectrometer. The main ion stream parameters and spectral lines of deuterium and carbon or tungsten ions were measured depending on laser pulse parameters. The research proved that optical spectroscopy could be a convenient method for on-line observation of the co-deposited layer removal by means of laser ablation. In combination with the investigation of collected co-deposit dust, the performed study made it possible to state that laser-induced breakdown spectroscopy can be useful as a diagnostic method for the ablative co-deposited layer removal and the wall conditioning. The properties of modified surfaces of samples and collected dust (evaporated co-deposit) were determined using different measuring methods.

Keywords:
laser ablation, laser-induced co-deposit removal, ion diagnostics and optical spectroscopy

Abstract:
Plasma plume induced by ArF excimer laser ablation of a hydroxyapatite (Ca10(PO4)6(OH)2) target was studied during expansion into a vacuum or water vapour. The ArF laser operated at a wavelength of 193 nm with a pulse energy of 300–350 mJ and a 20 ns pulse duration. The emission spectra of the plasma plume were registered with the use of a spectrograph and an ICCD camera. The expansion of the plasma plume was studied using the time of flight method. The time-dependent radiation of the Ca I and Ca II lines was registered with the use of a monochromator and photomultiplier at various distances from the target. The dynamics of the plasma plume was also imaged by means of fast photography. It was found that during expansion into a vacuum, the plasma front moved with a constant velocity of 1.75x10^4 m/s, while in thecase of ambient water vapour at a pressure of 20 Pa, velocities of 1.75x10^4–1.5x10^3 m/s were found depending on the distance from the target. Electron densities of 1.2x10^24–4.5x10^21 1/m3 were determined from the Stark broadening of the Ca II and Ca I lines at distances of 1–25 mm from the target.
Temperatures of 11,500–4500 K were determined from the relative intensities of carbon lines and continuum radiation at distances of 4–29 mm from the target. The results allowed the estimation of thermal and kinetic energies of ablated particles. During expansion into a vacuum, the kinetic energies of Ca, P and O atoms were 64, 49 and 25 eV, respectively. During expansion into water vapour, kinetic energies dropped to 0.47, 0.36 and 0.19 eV, respectively at a distance of 25 mm from the target and were comparable to the energies of thermal motion.

Keywords:
Laser ablation, Hydroxyapatite, Plasma plume

Abstract:
The plasma plume induced during ArF laser ablation of a graphite target is studied. Velocities of the plasma expansion front are determined by the optical time of flight method. Mass center velocities of the emitting atoms and ions are constant and amount to 1.7×104 and 3.8×104 m s−1, respectively. Higher velocities of ions result probably from their acceleration in electrostatic field created by electron emission prior to ion emission. The emission spectroscopy of the plasma plume is used to determine the electron densities and temperatures at various distances from the target. The electron density is determined from the Stark broadening of the Ca II and Ca I lines. It reaches a maximum of ∼9.5×1023 m−3 30 ns from the beginning of the laser pulse at the distance of 1.2 mm from the target and next decreases to ∼1.2×1022 m−3 at the distance of 7.6 mm from the target. The electron temperature is determined from the ratio of intensities of ionic and atomic lines. Close to the target the electron temperature of ∼30 kK is found but it decreases quickly to 11.5 kK 4 mm from the target.

Abstract:
Net emission coefficients of low temperature thermal iron-helium plasma mixture at atmospheric pressure are presented. The calculations are made assuming the plasma is in the local thermodynamic equilibrium at a pressure of 0.1 MPa. The results are presented for several values of helium mass fraction in the mixture (between 0 and 1), for a temperature range 3000–25000 K and three characteristic plasma dimensions; 0 – corresponding to the optically thin case, and 1 mm and 10 mm. The values of net emission coefficients allow the estimations of total radiation losses in iron-helium plasmas.

Keywords:
net emission coefficients, iron-helium plasma

Abstract:
A theoretical modelling of the plasma plume induced during welding of iron sheets with CO2 laser is presented. The set of equations consists of the equations of conservation of mass, energy, momentum and the diffusion equation and is solved with the use of the commercially available program Fluent 6.1. The computations are made for a laser power of 1700 W and for two shielding gases—argon and helium. The results show a significant difference between these two cases. When helium is used as the shielding gas, the plasma is much smaller and burns only where the metal vapour is slightly diluted by helium. In the case when argon is the shielding gas, there are actually two plasmas: argon plasma and metal plasma. The flowfield shows that the velocity increases in the hot region but only part of the mass flux enters the plasma core. In the case when argon is used as the shielding gas, the total absorption of the laser radiation amounts to 18–33% of the laser power depending on argon and iron vapour velocities. In the case of helium the total absorption is much lower and amounts to ~5% of the laser power.

Keywords:
laser welding, CFD, numerical model

Abstract:
Theoretical modelling of the plasma plume induced during welding of iron sheets with CO2 laser are presented. The set of equations consists of equation of conservation of mass,energy, momentum and the diffusion equation and is solved with the use of commercially available program Fluent 6.1. The computations are made for the laser power of 1700 W and shielding gas argon. Two solutions are taken into account stationary and non–stationary. The results show significant difference between these two cases.

Keywords:
laser welding, simulations, plasma

Abstract:
An experimental set–up and spectroscopy diagnostic method for laser–induced fuel removal and decomposition of co–deposited layers on plasma–facing components from tokamaks are described. For irradiation of a graphite limiter tile from the TEXTOR tokamak Nd:YAG 3.5-ns pulse laser with a repetition rate of 10 Hz and single pulse energy of up to 0,8 J at 1,06 µm has been used. The spectroscopy system allowed recording of spectra in the visible wavelength range including CII and Dα spectral lines. The evolution of CII and Dα spectral lines was observed pulse–by–pulse during the co–deposit removal. The efficient ablation of the 45 µm thick co–deposit occured after approximately 50 laser pulses.

Keywords:
tokamaks, laser ablation, spectroscopy, co–deposit removal

Abstract:
Momentary emission spectra of iron and argon lines were measured in a plasma plume induced during welding with a continuous wave CO2 laser. Time-dependent spectra were registered using a fast gate, lens coupled microchannel plate image intensifier placed between a spectrograph and a 1254 silicon intensified target detector connected to an optical multichannel analyser. The results, together with the analysis of the colour images from a fast camera, show that in the case when argon is the shielding gas, two plasmas exist: the argon plasma and the iron plasma. It has been found that during strong bursts the plasma plume over the keyhole consists mainly of metal vapour, not being diluted by the shielding gas. No apparent mixing of the metal vapour and the shielding gas has been observed. The space-averaged electron densities determined from the Stark broadening of the 7503.87, 7514.65 Å Ar I lines amounts to (0.75–1.05) × 1023 m−3 depending on the distance from the surface. Assuming that argon is not mixed with the metal vapour and is in local thermodynamic equilibrium these electron densities correspond to the temperatures of 12–13 kK. At the peaks of strong vapour bursts the space-averaged electron densities determined from the Stark broadening of the 5383.37 Å Fe I line are (0.6–1) × 1023 m−3. Numerical simulations showed that the maximum densities in the plasma centre are considerably higher and amount to ~1.8 × 1023 m−3 and ~2.45 × 1023 m−3 in the case of the argon and metal plasma, respectively. Consequently the absorption of the laser beam in the plasma plume amounts to ~5% of the beam power in the case of argon and 10% in the case of metal plasma.

Abstract:
Iron plasma appears during material processing with laser, electric arc etc., and has considerable influence on the processing conditions. In this paper emission coefficients of low temperature thermal iron plasma at atmospheric pressure are presented. Net emission coefficients εN have been calculated for pure iron plasma as well as for Fe–Ar and Fe–He plasma mixtures. To calculate the recombination radiation the knowledge of the Biberman factors ξz fb(Te, λ) is necessary and they have been calculated from the iron photo–ionization cross sections. The calculations allow estimation of energy losses, energy radiated by plasma plume and its comparison with the energy absorbed from laser beam.

Keywords:
absorption coefficient, plasma emission, Biberman factor

Abstract:
The dynamics of the plasma plume produced during laserwelding is quite complex. The keyhole wall oscillates and this results in oscillations of the plasma plume over the keyhole mouth. The metal vapour, which appears in irregular bursts, interacts with the shielding gas flowing from the opposite direction. In the present work, temporary electron densities and temperatures are determined in the peaks of plasma bursts during welding with a continuous wave CO2 laser. It has been found that during strong bursts the plasma plume over the keyhole consists of metal vapour only, without being diluted by the shielding gas. The results, together with the analysis of the colour pictures from sreak camera, allow interpretation of the dynamics of the plasma plume. No apparent mixing of metal vapour and the shielding gas has been observed. In typical bursts the electron density determined from the Stark broadening of Ar I lines varies from 0.9x1023 m-3 near the metal surface to 0.5x1023 m-3 at a distance of 1.5 mm from the surface. Assuming that argon is not mixed with the metal vapour and is in local thermal equilibrium these electron densities correspond to temperatures 12.7 kK and 11.5 kK, respectively. In strong bursts the electron density varies, along the same distance, from 1.6x1023 m-3 to 0.6x1023 m-3, which corresponds to the temperatures of 14.2 kK, respectively.

Keywords:
laser welding, plasma diagnostics

Abstract:
An analysis is presented of the oscillations of keyhole pressure and plasma radiation emitted during welding with a continuous wave (CW) CO2 laser. Welding was done with a CW CO2 laser, Photon Sources VFA 2500, operating at the power of 1.75 kW. The welded materials were mild and stainless steel sheets, 0.8–2 mm thick. The shielding gas was argon or helium. Oscillations of plasma radiation were registered in monochromatic or broad band radiation with the use of a photomultiplier or photodiode and pressure variations with a microphone in the frequency range of 20–2 × 104 Hz. It has been found that the optical signal from the plasma plume is closely connected with the acoustic signal and that the source of the acoustic signal is the pulsating movement of the plasma plume. Spectral analysis of the measured oscillations shows differences in power spectra depending on the welding conditions. Generally, two intrinsic frequency peaks in the range of 0.5–4 kHz are always present but the amplitude, frequency and width of the peaks depend on the material and welding conditions. The results show that the optical and acoustic signals emitted during the welding process can be useful for process monitoring. The behaviour of the observed oscillations is characteristic for deterministic chaos. Considerable regularization of the process was observed as an effect of modulation of the laser beam. The modulation factor (Pmax−Pmin)/Pmax was equal to 0.2 and the modulation frequency was 2 kHz. In this case, the intense peak corresponding to the modulation frequency was observed in the power spectrum together with smaller peaks corresponding to the harmonic frequencies.

Abstract:
A spectroscopic study of a low-pressure supersonic jet of laser-heated argon plasma is presented. The experimental set-up consisted of a high-pressure convergent nozzle and a supersonic nozzle. The supersonic nozzle was placed just behind the convergent nozzle and was connected to a low-pressure chamber. A continuous wave laser with output power of 2 kW was used to maintain the plasma in the stream of argon gas flowing from the convergent nozzle. The plasma then expanded through the supersonic nozzle. Emission spectra from the laser-sustained plasma and supersonic jet were measured with a 1.3 m focal length spectrograph and 1254 silicon intensified target (SIT) detector connected to an EG&G PARC optical multichannel analyser (OMA) III. We found that the supersonic stream of argon plasma had an electron density of - and a temperature of 6 - 7 kK.

Abstract:
Deux plasmatrons a arc non transferes sont utilises pour simuler les proprietes du gaz ionise entourant une sonde d'exploration planetaire. Le plasmatron de I'IEPE (Poznan) d'une puissance maximale de 25 kW fonctionne a pression atmospherique et a pression reduite (p>1 kPa) avec de l'argon, de l'azote et des melanges CO2-N2. La source a arc non transfere du LAEPT (Clermont-Ferrand) d'une puissance applicable maximale de 100 kW fonctionne a pression atmospherique avec de nombreux melanges gazeux dont les melanges N2-O2 et CO2-N2. Une modelisation fluide du plasmatron de I'IEPE est presentee pour de l'argon en tenant compte d'un desequilibre thermique Te-T. Des mesures de temperature par spectroscopie d'emission ont ete realisees dans les jets de plasma obtenus avec les deux plasmatrons. La temperature d'excitation de l'azote atomique et la temperature de vibration de la molecule CN ont ete determinees (IEPE). Les temperatures d'un plasma d'air et d'un plasma de CO2-N2 ont ete determinees a partir de l'emission du cuivre (LAEPT)

Keywords:
nanoparticles, carbon dots, laser ablation, nucleation, ion-induced nucleation

Keywords:
pulsed laser ablation in liquids, carbon nanoparticles, optical properties

Keywords:
carbon nanoparticles, C-dots, laser ablation in liquid, photoluminescence

Keywords:
laser ablation, plasma plume

Keywords:
magnetron sputtering, pulsed laser deposition, superhard materials, tungsten borides

Keywords:
RF magnetron sputtering, hard materials, PLD, tungsten boride

Abstract:
Tungsten boride compounds are very promising new hard and super - hard materials. This kind of materials could be used in production of high-speed tools, durable bearings and nonabrasive surfaces. The hardness of tungsten borides strongly depends on boron content as well as material microstructure. For example hardness of W2B is about 12.4 GPa, WB hardness is 18 – 36 GPa, WB2 hardness is 28.5–39.7 GPa and WB3 hardness is 28.6 – 36.9 GPa. Moreover production of tungsten borides in the form of layers or nano -grained structured increase its hardness even above 40 GPa. Although the properties of tungsten boride structures are known from theoretical calculations, the data concerning structures produced experimentally are scare. Therefore, the properties of tungsten boride layers deposited by pulsed laser deposition (PLD) process are investigated in this paper. Layers were deposited using two types of targets: with boron to tungsten ratio of 2.5:1 and 4.5:1 and with the use of two laser wavelengths: 355 and 1064 nm.

Keywords:
pulsed laser deposition, tungsten borides

Keywords:
pulsed laser deposition, rhenium borides, tungsten borides

Keywords:
pulsed laser deposition, rhenium borides

Keywords:
pulsed laser deposition

Keywords:
pulsed laser deposition, RF magnsteron sputtering, tungsten borides

Abstract:
Pulsed laser deposition is a method frequently used for creating thin films of various materials on
solid substrates. High energy laser pulse causes evaporation of the target material, forming a
plume which subsequently
expands and moves with high speed from the target. Thin film of the
evaporated material is deposited on the substrate placed at some distance in front of the target.
The behavior of the plume influences both the stoichiometry and homogeneity of the deposit
ed
layer
–
the final product of the process. Better understanding of the process of expansion of the
plume, variation of its structure as well as deposition of the material itself is therefore very
important and should give us opportunity for better contro
l of formation of the deposited layer.

Keywords:
laser ablation, plume expansion, DSMC

Keywords:
laser ablation, graphite, carbon nanotubes

Abstract:
Inexpensive Tungsten Tetraboride super hard coatings prepared by pulsed laser deposition (PLD) method are investigated. WB4 coatings were prepared on Si (100), by PLD technique using nanosecond, Nd:YAG laser operating at 355 nm wavelenght. Additionally the surface of SPS sintered WBx target before and after ablation process was examined. Phase change of WB2 to WB4 at the surface of target is observed. The phase analysis, crystallography and orientations have been studied using X-ray diffractometry. The surface layer of target after ablation and deposited films are found to be mostly WB4 phase with hardness HV>28 GPa.

Keywords:
laser mikro/nano engineering, thin films, pulsed laser ablation/deposition, tungsten tetraboride WB4

Abstract:
The interaction of laser beam with dissimilar metals during welding process was studied theoretically. A finite element based three-dimensional transient heat transfer and fluid flow model was applied for prediction temperature distribution and material mixing field as well as weld dimensions. The model was used for study and optimization process parameters of welding of Ni – Pt spark plug for bio-fuel engine. The laser pulse duration and energy, and the angle of incidence of the laser beam to thesurface were analyzed. The shape of the melting pool obtained from the theoretical model was close to experimental results.

Abstract:
In answer to research interest in production of super hard coatings prepared by pulsed laser deposition (PLD) method, Rhenium Diboride was taken under consideration. PLD is characteristic by necessity to define deposition parameters best for particular substance. ReB2 coatings were prepared with the use of 355 nm and 1064 nm wavelengths of Nd:YAG laser and the influence of laser wavelength on those films preparation was investigated. Deposition efficiency increased with shorter wavelength, however layer’s smoothness was better for longer wavelength. The XRD analysis shows crystalline ReB2 character of achieved samples.

Keywords:
pulsed laser deposition, rhenium diboride

Abstract:
The experimental studies are carried out to adapt the plasmatron functioning to simulate re-entry conditions in a planetary atmosphere. The plasma flow is produced using argon or nitrogen arc and nitrogen or carbon dioxide is introduced into plasma jet. Although the gas introduced into plasma jet is injected behind the arc it influences the arc characteristics and dynamics of flow. This effect is studied in detail. Emission spectra of atomic nitrogen and ionized molecular nitrogen (N2+) are also recorded and analyzed. The atomic emission is studied in the infra-red region and has allowed the determination of an excitation temperature. The electron density is determined from the continuum radiation, and the rotational and vibrational temperatures are determined from the 1stnegative system of N2+