Formation of sub-micron size carbon structures by plasma jets emitted from a pulsed capillary discharge

We have performed an experimental investigation of the potential use of intense plasma jets produced in a repetitive pulsed capillary discharge (PCD) operating in methane gas, to irradiate Si (1 0 0) substrates. The surface modifications induced by the plasma jet using two different material inserts at the capillary end, graphite and titanium, are characterized using standard surface science diagnostic tools, such as scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis and Raman spectroscopy (RS). It has been found that the application of methane plasma jet results in the formation of sub-micron size carbon structures. It is observed that the resulting plasma irradiated surface morphologies are different, depending on the different material inserts used at the capillary end, at otherwise identical operational conditions. To investigate the species responsible for the observed surface changes in different material inserts to the capillary, optical-emission spectroscopy (OES) was recorded using a 300-1000 nm spectrometer. The OES results show the presence of H, CH and C₂ Swan band in the discharge plasma, which play a significant role in the formation of the carbon structures.     

Microstructure and corrosion behavior of austenitic stainless steel treated with laser

Surface modification of AISI316 stainless steel by laser melting was investigated experimentally using 2 and 4 kW laser power emitted from a continuous wave CO₂ laser at different specimen scanning speeds ranged from 300 to 1500 mm/min. Also, an investigation is reported of the introduction of carbon into the same material by means of laser surface alloying, which involves pre-coating the specimen surfaces with graphite powder followed by laser melting. The aim of these treatments is to enhance corrosion resistance by the rapid solidification associated with laser melting and also to increase surface hardness without affecting the bulk properties by increasing the carbon concentration near the surface. Different metallurgical techniques such as optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to characterize the microstructure of the treated zone. The microstructures of the laser melted zones exhibited a dendritic morphology with a very fine scale with a slight increase in hardness from 200 to 230 Hv. However, the laser alloyed samples with carbon showed microstructure consisting of γ dendrite surrounded by a network of eutectic structures (γ+carbide). A significant increase in hardness from 200 to 500 Hv is obtained. Corrosion resistance was improved after laser melting, especially in the samples processed at high laser power (4 kW). There was shift in I_corr and E_corr toward more noble values and a lower passive current density than that of the untreated materials. These improvements in corrosion resistance were attributed to the fine and homogeneous dendritic structure, which was found throughout the melted zones. The corrosion resistance of the carburized sample was lower than the laser melted sample.     

Accelerated life ac conductivity measurements of CRT oxide cathodes

The ac conductivity measurements have been carried out for the activated Ba/SrO cathode with additional 5% Ni powder for every 100 h acceleration life time at the temperature around 1125 K. The ac conductivity was studied as a function of temperature in the range 300-1200 K after conversion and activation of the cathode at 1200 K for 1 h in two cathodes face to face closed configuration. The experimental results prove that the hopping conductivity dominate in the temperature range 625-770 K through the traps of the WO₃ associate with activation energy E_a = 0.87 eV, whereas from 500-625 K it is most likely to be through the traps of the Al₂O₃ with activation energy of E_a = 1.05 eV. The hopping conductivity at the low temperature range 300-500 K is based on Ni powder link with some Ba contaminants in the oxide layer stricture which indicates very low activation energy E_a = 0.06 eV.     

Characteristics of polymer velvet as field emitters under high-current pulsed discharge

In this paper we investigated the surface morphology and emission property of polymer velvet in a cathode test system powered by a ∼400 ns, ∼400 kV pulsed generator. After a series of pulse shots, the velvet surface exhibited an obvious decrease in the amount of emitters, namely, the smoothing of microprotrusions, indicating a lower field enhancement factor or a higher turn-on electric field than that for no shots. As the velvet cathode lifetime proceeded, the beam degradation was observed in terms of the voltage pulse length, maximum emission current, and rise time of diode current. Further, the average current density significantly decreased during a 100 pulse shot test, from 280 to 160 A/cm². The surface discharge caused many plasma spots on the velvet surface. The cathode plasma expands towards the anode, directly leading to the diode gap closure. The degradation in the velvet performance after high-current emission may be related to this behavior of cathode plasma. Finally, the electron emission mechanisms, how to affect the surface morphology of velvet, are presented.     

Microstructure evolution occurring in the modified surface of 316L stainless steel under high current pulsed electron beam treatment

High current pulsed electron beam (HCPEB) surface treatment of 316L stainless steel (SS) was carried out with a wide spectrum of treating parameters. Microstructure changes occurring in the modified surface were characterized with microscopy, X-ray diffractometry and electron backscatter diffractometry (EBSD) techniques. The evolution regularities of surface craters and microstructure refinement, as well as preferred orientation of (1 1 1) crystal planes were discussed on considering the coupled temperature-stress fields formed in surface layers after an absorption of HCPEB energy.     

Effect of pulse repetition rate and number of pulses in the analysis of polypropylene and high density polyethylene by nanosecond infrared laser induced breakdown spectroscopy

Pulse repetition rates and the number of laser pulses are among the most important parameters that do affect the analysis of solid materials by laser induced breakdown spectroscopy, and the knowledge of their effects is of fundamental importance for suggesting analytical strategies when dealing with laser ablation processes of polymers. In this contribution, the influence of these parameters in the ablated mass and in the features of craters was evaluated in polypropylene and high density polyethylene plates containing pigment-based PbCrO₄. Surface characterization and craters profile were carried out by perfilometry and scanning electron microscopy. Area, volume and profile of craters were obtained using Taylor Map software. A laser induced breakdown spectroscopy system consisted of a Q-Switched Nd:YAG laser (1064 nm, 5 ns) and an Echelle spectrometer equipped with ICCD detector were used. The evaluated operating conditions consisted of 10, 25 and 50 laser pulses at 1, 5 and 10 Hz, 250 mJ/pulse (85 J cm⁻²), 2 μs delay time and 6 μs integration time gate. Differences in the topographical features among craters of both polymers were observed. The decrease in the repetition rate resulted in irregular craters and formation of edges, especially in polypropylene sample. The differences in the topographical features and ablated masses were attributed to the influence of the degree of crystallinity, crystalline melting temperature and glass transition temperature in the ablation process of the high density polyethylene and polypropylene. It was also observed that the intensities of chromium and lead emission signals obtained at 10 Hz were two times higher than at 5 Hz by keeping the number of laser pulses constant.     

Enhanced field emission from LaB6 thin films with nanoprotrusions grown by pulsed laser deposition on Zr foil

Lanthanum hexaboride (LaB₆) films have been deposited on a zirconium foil by pulsed laser deposition method. The field emission studies of the LaB₆ deposited film have been performed in the planar diode configuration under ultra high vacuum conditions. The Fowler-Nordheim plots were found to be linear in accordance with the quantum mechanical tunneling phenomenon. A typical field emission current of 7.02 μA was drawn at an applied electric field of 2 V/μm. The field enhancement factor is calculated to be 8913 cm⁻¹, indicating that the field emission is from nanoscale protrusions present on the emitter surface. The atomic force microscope (AFM) investigation of the surface clearly shows the conical shaped nanoprotrusions of few hundred nanometers with asperities of 20-40 nm on its top. The emission current-time plot recorded at the pre-set value of emission current of 5 μA over a period of more than 3 h exhibits an initial increase and subsequent stabilization of the current. The results reveal that the LaB₆/Zr field emitter obtained by the pulsed laser deposition (PLD) is a promising cathode material for practical applications in field emission-based devices.     

Iron disilicide formation by Au-Si eutectic reaction on Si substrate

We have investigated the growth of iron disilicide on Au-coated Si(0 0 1) substrates and its photoluminescence behaviour. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy observations revealed that the Si surface above 380 °C was melted as a result of the Au-Si eutectic reaction and that coarse island disilicide grains with sizes of several micrometres were formed on the Si surface. The full width at half maximum of 0.056° on the rocking curve of α-FeSi₂004 was observed on the sample deposited at 800 °C, and indicated the high crystal quality in perfection of orientation. The photoluminescence spectrum of β-FeSi₂ grains, which were deposited at 750 °C, was observed. The melted Si surface contributed to the improved crystallinity of α-FeSi₂ and β-FeSi₂.     

Decay kinetics of the defect-based visible luminescence in ZnO

Photoluminescence spectra and decays under pulsed N₂ (337 nm) laser excitation were measured for hydrothermally grown bulk and liquid-phase epitaxy (LPE)-grown film ZnO samples within 9-300 K. Temperature dependence of integrated spectra over the exciton and visible spectral regions was evaluated using a model involving standard energy barrier processes. Decay curves measured within a broad time window (10 ns-1 ms) and with extreme signal/background ratio (five orders of magnitude) point to complex decay mechanism in which the exponential and inverse power-law processes can coexist. There is no straightforward interconnection between the observed temperature dependence of integrated visible photoluminescence intensity and its decay shape over the 9-300 K temperature interval.     

A detailed study through the focal region of near-threshold single-shot femtosecond laser ablation nano-holes in borosilicate glass

A detailed study of the morphology of nano-craters drilled in borosilicate glass by single shot femtosecond laser ablation near the ablation threshold has been performed by scanning electron microscopy, atomic force microscopy and scanning electron microscopy imaging after focused ion beam sectioning. The influence of the numerical aperture (NA = 0.4 and 0.8), the pulse energy (16 nJ < E_p < 600 nJ) and the position of the specimen surface into the focal region were systematically investigated, leading to nanometric or micrometric scales in every spatial dimension. The nanocrater’s size is not restricted by the diffraction limit but determined by the laser pulse stability and the material properties. If the beam is focused inside the glass, two craters are drilled, shaping very distinct morphologies. Their dimensions have been studied in details and different relationships have been proposed for the evolutions of the depths and of the various diameters of these craters as functions of the pulse energy, the numerical aperture and the position of specimen surface in the beam-material interaction region. It is suggested that the long, thin conical profile with very high aspect ratio of the secondary craters is due to a spontaneous reshaping of the beam which transforms the incoming Gaussian pulse into a Gaussian-Bessel pulse. As proposed in the developed model the geometry of the second craters seems to be connected with the one of the main craters.     

Characterization of scandia doped pressed cathode fabricated by spray drying method

Scandia doped pressed cathode was prepared by a new method of spray drying combined with two-step hydrogen reduction process. The Sc₂O₃ and barium-calcium aluminate co-doped powders have sub-micrometer size in the range of 0.1-1 μm and scandium oxide and barium-calcium aluminate are distributed evenly in the powders. The cathodes sintered by powder metallurgy at 1600 °C_b have a smooth surface and sub-micrometer grain structure with homogeneous distribution of scandium, barium, calcium and aluminum which are dispersed over and among the tungsten grains. This cathode has good emission, e.g., the current density of this cathode reaches 31.50 A/cm² at 850 °C_b. After proper activation, the cathode surface is covered by a Ba-Sc-O active substances layer with a preferable atomic ratio, leading to its good emission property. The evaporation activation energy of SDP cathode with 4.58 eV is the highest among the Ba-W, M-type and SDP cathodes, and the average evaporation velocity v_t of SDP cathode with 1.28 × 10⁻⁸ g cm⁻² s⁻¹ at 1150 °C_b is the lowest one.     

Surface modification of Al-20Si alloy by high current pulsed electron beam

Hypereutectic Al-20Si (Si 20 wt.%, Al balance)alloy surface was treated with high current pulsed electron beam (HCPEB) under different pulse numbers. The results indicate that HCPEB irradiation induces the formation of metastable structures on the treated surface. The coarse primary Si particle melts, producing a “halo” microstructure with primary Si as the center on the melted surface. A supersaturated solid solution of Al is formed in the melted layer caused by Si atoms dissolving into the Al matrix. Cross-section structure analysis shows that a 4 μm remelted layer is formed underneath the top surface of the HCEPB-treated sample. Compared with the matrix, the Al and Si elements in the remelted layer are distributed uniformly. In addition, the grains of the Al-20Si alloy surface are refined after HCPEB treatment, as shown by TEM observation. Nano-silicon particles are dispersed on the surface of remelted layer. Polygonal subgrains, approximately 50-100 nm in size, are formed in the Al matrix. The hardness test results show that the microhardness of the α(Al) and eutectic structure is increased with increasing pulse number. The hardness of the “halo” microstructure presents a gradient change after 15 pulse treatment due to the diffusion of Si atoms. Furthermore, hardness tests of the cross-section at different depths show that the microhardness of the remelted layer is higher than that of the matrix. Therefore, HCPEB technology is a good surface modification method for enhancing the surface hardness of hypereutectic Al-20Si alloy.     

Cathode Plasma Formation in Pulsed High Current Vacuuum Diodes

Cathode plasmas in pulsed high current vacuum diodes have been studied using optical interferometry and spectroscopy. Both aluminum and graphite cathodes were used and the diode current density was varied over a factor of ten. The cathode plasma inventory was seen to increase during the length of the pulse and the plasma density was seen to increase with increasing current density. Spectral line emission from H, CI, CII, and CIII was observed when either cathode was used. It is concluded that cathode plasma expansion is dominated by protons from cathode surface contaminants.     

Dynamics of pulsed magnetron discharge in high-current electron guns

High-speed video filming is applied to investigate low-pressure (argon 0.053–0.133 Pa) pulsed gas discharge dynamics in a planar magnetron built-in the explosive emission cathode of a high-current electron gun. It has been found that, in the beginning, the discharge starts at the side surface of the cathode and then spreads to its face. The stability of the discharge ignition instant at the cathode face as a function of the working pressure of the gas and the cathode design has been studied. It has been found that the pulsed longitudinal magnetic field results in the discharge switching over in the axial direction onto the electron gun collector. A qualitative explanation of the observed discharge behavior has been given.     

Study of microstructure and nanomechanical properties of Zr films prepared by pulsed magnetron sputtering

The present work studies the effect of substrate temperature on the growth characteristics of zirconium films prepared by pulsed magnetron sputtering. Formation of α-phase of zirconium was observed in the temperature range 300-873 K. X-ray diffraction of Zr films revealed predominantly [0 0 1] texture. It is noticed that crystallite size increases with increasing substrate temperature. Hexagonal shaped crystallites seem to grow along the surface normal of the substrate for the films deposited at 773 K. Nanoindentation measurements showed that the hardness of the films is in the range 6-10 GPa. The scratch test indicated that the films deposited at higher substrate temperatures had excellent bonding with the substrate and no significant critical failure was noticed up to an applied load of 20 N.     

Laser energy density, structure and properties of pulsed-laser deposited zinc oxide films

Zinc oxide thin films were deposited on soda lime glass substrates by pulsed laser deposition in an oxygen-reactive atmosphere at 20 Pa and a constant substrate temperature at 300 °C. A pulsed KrF excimer laser, operated at 248 nm with pulse duration 10 ns, was used to ablate the ceramic zinc oxide target. The structure, the optical and electrical properties of the as-deposited films were studied in dependence of the laser energy density in the 1.2-2.8 J/cm² range, with the aid of X-ray Diffraction, Atomic Force Microscope, Transmission Spectroscopy techniques, and the Van der Pauw method, respectively. The results indicated that the structural and optical properties of the zinc oxide films were improved by increasing the laser energy density of the ablating laser. The surface roughness of the zinc oxide film increased with the decrease of laser energy density and both the optical bang gap and the electrical resistivity of the film were significantly affected by the laser energy density.     

Detection and characterization of absorption heterogeneities in KH2PO4 crystals

The SOCRATE facility is used for the detection of heterogeneous absorption in large and rapidly grown KH₂PO₄ (KDP) crystals. The different regions are subsequently characterized by spectrophotometry and laser-damage measurement. In accordance with previous results from the literature we found that, by using the SOCRATE damage measurement set-up with a large beam (300 ∗ 600 μm²), the absorption level does not play a major role in the damage mechanism at 355 nm. However, by testing the damage resistance of the same KDP component with a focused beam (9 μm diameter) at 355 nm, we highlight for the first time significantly different damage resistances between highly and weakly absorbing regions from a KDP crystal. Indeed, with a focused beam, we demonstrate that the laser-damage threshold may be lower in some sectors cut in the pyramidal part than in sectors from the prismatic part. This means that the absorbing defects at high concentration are not the predominant damage precursors at 355 nm.     

Structure and properties of the bronze laser alloyed with titanium

The paper describes the microstructure and properties (microhardness and wear resistance) of the bronze laser alloyed with titanium. The laser alloying was done using a pulsed Nd:YAG laser with a generated beam energy of 25-35 J. A very fine microstructure was formed under such rapid solidification conditions like laser treatment. The high chemical homogeneity and fine structure of the melted zone were attributed to high cooling rates due to the short interaction time with Nd:YAG pulsed laser radiation and relatively small volume of the melted material. The structure obtained in the surface layer after laser alloying permits to get a high level of hardness and an improved wear resistance.     

Phase transformation of graphite irradiated by high-intensity pulsed ion beams

The microstructure and morphology of graphite irradiated by high-intensity pulsed ion beams (HIPIB) has been studied by varying the ion current density as 200, 350 and 1500 A/cm² with one to five shots. Phase transformation from graphite to diamond-like carbon (DLC) on the HIPIB-irradiated graphite was confirmed by Raman spectroscopy where a typical broadened asymmetric peak appeared in the wavenumber range of 1100-1700 cm⁻¹. Formation of DLC on the irradiated graphite strongly depended on the HIPIB parameters and preferably took place at the medium ion current density of 350 A/cm² up to five shots. Numerical simulation of ablation process was performed to explore the transformation mechanism of DLC from graphite irradiated by HIPIB. The calculation showed that the temperature profile in irradiated graphite at 350 A/cm² is almost identical to that at 200 A/cm², showing a deeper heat-affected zone in comparison with that of 1500 A/cm². Moreover, the ablation depth per shot is around 0.8 μm at 350 A/cm², higher than that of 0.4 μm at 200 A/cm² and much lower than that of 8.4 μm at 1500 A/cm², respectively. The experimental and numerical results indicate that a proper temperature and pressure repetitively created in the top layer of ablated graphite during HIPIB irradiation facilitates the phase transformation.     

Comparison of continuous-wave (CW) and tone-burst (TB) excitation modes in vibro-acoustography: Application for the non-destructive imaging of flaws

This paper describes the application of continuous-wave (CW) and tone-burst (TB) vibro-acoustography (VA) experiments for imaging a flawed composite plate. For both modes, the ultrasound frequency is set at f₁ = 3 MHz and f₂ = 3 MHz + ∣Δf∣. The plate was placed at the focus of the transducer and scanned point-by-point over an area of 60 mm by 50 mm on its frontal face with an increment step equal to 0.25 mm/pixel. The resulting acoustic emission amplitude at ∣Δ f∣ is recorded. For the CW mode the difference frequency was set at ∣Δf∣ = 12.9 kHz. For the TB mode, the burst-emitted signal was 100 μs long at a pulse repetition frequency (PRF) of 100 Hz corresponding to bursts of 300 cycles at 3 MHz, and the difference frequency was set at ∣Δf∣ = 44 kHz. The resulting VA images readily show the shape of the flaws. The images also reveal considerable detail of internal substructures such as the fibers used to reinforce the plate. However, the CW VA image shows an artifact caused by the effect of ultrasound standing waves established between the plate and the concave surface of the transducer, resulting in masking some of the flaws. On the other hand, the TB-VA image is free from such artifact. Despite some advantages of using TB-VA, there are some limitations related to this mode. Advantages and limitations of using the two modes are discussed.