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Abstract: Using high-resolution atomic force microscope we observed in ambient atmosphere the slow morphological transitions of the incipient adlayer of gold grown on (0001) sapphire substrate by pulsed laser deposition. The equivalent average uniform thickness of the gold deposition was about 0.55Å, which is about one-fourth of its monolayer. A dynamic simulation revealed that about 10% of the gold was implanted into the substrate up to the depth of about 3.3nm and the top monolayer of the sapphire surface was almost completely depleted of oxygen atoms due to the preferential sputtering by the plume particles. The gold adlayer transformed into a labile phase which enhanced the surface roughness and had a preferred orientation of a wavy structure during 24h of the deposition. The auto-correlation function of this wavy structure in labile metastable phase revealed two-fold symmetry and provided a preferential size of about 4nm (peak to peak) with a mean separation of 8nm. At the end of about 6 days this phase was found to completely transform into an apparently de-wetted phase of beads with average in-plane diameter of ∼20nm and height of ∼7nm having large size distribution. Each bead was seen to have coating of a concentric corona layer, which might be that of the condensed moisture or other gaseous species from atmosphere because subjecting these samples to vacuum removed this layer. These observations shed light on the dynamics of the pulsed laser deposited metastable gold adlayer in the incipient stage of its growth on sapphire and their wetting or de-wetting mechanisms in ambient atmosphere. [Copyright &y& Elsevier]

Abstract: Pulsed Laser Deposition of high crystalline quality thin ZnO films on sapphire substrates necessitates a growth temperature of about 750 °C or higher while sharp interfaces in a multilayer structure suitable for optoelectronic devices are obtained at comparatively low growth temperatures. To meet these contradictory requirements a growth scheme has been evolved in which a ZnO buffer layer deposited at about 750 °C was found to facilitate high crystalline quality growth of a ZnO over layer even at 400 °C. The buffer layer grown at 750 °C was found to have its in plane lattice 30° rotated compared to that of the sapphire substrate and so was the case with the ZnO over layer grown at 400 °C on the buffer. The full width at half maximum of the high resolution X-ray diffraction rocking curves of (00.2) and (10.4) peaks of ZnO films grown at low temperature on the buffer were found to be ∼0.18° and ∼0.28°, respectively, which are comparable to 0.11° and 0.23° as observed in the case of films deposited directly at 750 °C and much better than the values of 0.46° and 0.48° seen in case of the films grown at 400 °C without any buffer layer. The buffer assisted low temperature ZnO growth also decreased the r.m.s. value of the surface roughness of the film to about 8 Å from about 40 Å of the one grown at high temperature without any buffer. [Copyright &y& Elsevier]

ZnO band-gap was engineered for and by growing its quantum wells and nanoparticle matrices using pulsed laser deposition. Alloy thin films of MgxZn1-xO grown with high crystalline quality by variable oxygen ambient pressure methodology developed by us could continuously increase the band gap up to about 3.8 eV. Increasing Cd concentration in the alloy thin films of CdyZn1-yO grown with good crystalline quality monotonically decreased the band gap up to about 2.9 eV. Multilayer matrices of alumina capped ZnO nanoparticles with mean radii in the range of 1.8–3.6 nm were grown to obtain size-dependent band gap up to about 4.5 eV. These nanoparticles are understood to grow without the formation of a wetting layer on the substrate. [Copyright &y& Elsevier]

The composition of nitrogen laser (wavelength 337 nm, pulse width 3 ns) induced ablation plumes from ZnSe and ZnO targets was studied at different laser fluences using a time-of-flight mass spectrometer. In the case of ZnSe, abundant ion signals corresponding to Se and ZnSe clusters, some of which were Se-rich, were detected with fluence-dependent distributions. At a laser fluence of 1250 J m[sup -2], clusters with elevated intensity were observed at sizes of 6, 13, 19, 23 and 33 ZnSe molecules (‘magic numbers’), which match quite well with the earlier observation by others of magic numbers of chemically similar ZnS produced by a conventional vaporization and quenching scheme. In the case of ZnO, we detected the presence of atomic Zn and molecular species of ZnO, as well as a series of (ZnO)[sub n]-type clusters with fluence-dependent distributions. Unlike the case of ZnSe, no magic numbers were observed for ZnO. [ABSTRACT FROM AUTHOR]

Oriented growth of hexagonal CdS nanoparticulate film with the C-axis perpendicular to the plane of the surface has been achieved up to a film thickness of ∼1 μm, using the combination of Langmuir–Blodgett (LB) and chemical bath deposition techniques. This combination is shown to eliminate the inherent limitation of the LB technique on the film thickness (up to∼300 A˚). The average size of the particles has been found to be constant with the film thickness. [Copyright &y& Elsevier]

We report the doping of C[sub 60] in porous glass by diffusion in solution phase at room temperature. The presence of C[sub 60] in the doped porous glass was confirmed spectroscopically. We also report the changes in optical absorption spectrum and intensity-dependent transmission of 30 ns laser pulses at 527 nm in these materials. [ABSTRACT FROM AUTHOR]

Comments on the nanosecond photoacoustic studies on ultraviolet laser ablation of organic polymers. Importance of understanding time evolution of the ablation process; Assessment of the presented results; Response of the researchers to the comments.

Abstract: The use of porous surface structures is gaining popularity in biomedical implant manufacture due to its ability to promote increased osseointegration and cell proliferation. Laser direct metal deposition (LDMD) is a rapid manufacturing technique capable of producing such a structure. In this work LDMD with a diode laser in continuous mode and with a CO2 laser in pulsed modes are used to produce multi-layer porous structures. Gas-atomized Ti–6Al–4V and 316L stainless steel powders are used as the deposition material. The porous structures are compared with respect to their internal geometry, pore size, and part density using a range of techniques including micro-tomography. Results show that the two methods produce radically different internal structures, but in both cases a range of part densities can be produced by varying process parameters such as laser power and powder mass flow rate. Prudent selection of these parameters allows the interconnected pores that are considered most suitable for promoting osseointegration to be obtained. Analytical models of the processes are also developed by using Wolfram Mathematica software to solve interacting, transient heat, temperature and mass flow models. Measured and modelled results are compared and show good agreement. [Copyright &y& Elsevier]

Abstract: Laser piercing is one of the inevitable requirements of laser profile cutting process and it has a direct bearing on the quality of the laser cut profiles. We have developed a novel power ramped pulsed mode (PRPM) laser piercing technique to produce much finer pierced holes and to achieve a better control on the process parameters compared to the existing methodology based on normal pulsed mode (NPM). Experiments were carried out with both PRPM and NPM laser piercing on 1.5-mm-thick mild steel using an in-house developed high-power transverse flow continuous wave (CW)-CO2 laser. Significant improvements in the spatter, circularity of the pierced hole and reproducibility were achieved through the PRPM technique. We studied, in detail, the dynamics of processes involved in PRPM laser piercing and compared that with those of the NPM piercing. [Copyright &y& Elsevier]

Abstract: Temperature dependence of the photoluminescence (PL) transitions in the range of 10–300K was studied for ZnO thin films grown on sapphire by pulsed laser deposition. The low temperature PL spectra were dominated by recombination of donor bound excitons (B X) and their phonon replicas. With increasing temperature, free exciton (F X) PL and the associated LO phonon replicas increased in intensity at the expense of their bound counterparts. The B X peak with line width of ∼6meV at 10K exhibited thermal activation energy of ∼17meV, consistent with the exciton-defect binding energy. The separation between the F X and B X peak positions was found to reduce with increasing temperature, which was attributed to the transformation of B X into the shallower donor bound exciton complexes at consecutive lower energy states with increasing temperature, which are possible in ZnO. The energy separation between F X peak and its corresponding 1-LO phonon replica showed stronger dependence on temperature than that of 2-LO phonon replica. However, their bound counterparts did not exhibit this behavior. The observed temperature dependence of the energy separation between the free exciton and it is LO phonon replicas are explained by considering the kinetic energy of free exciton. The observed PL transitions and their temperature dependence are consistent with observations made with bulk ZnO crystals implying high crystalline and optical quality of the grown films. [Copyright &y& Elsevier]

Abstract: We have studied temperature dependent photoluminescence (PL) from ZnO Multiple Quantum Wells (MQWs) of different well layer thicknesses in the range ∼1–4 nm grown on (0001) sapphire by a novel in-house developed buffer assisted pulsed laser deposition. At 10 K the PL peak shifted toward blue with decreasing well layer thickness and at constant well layer thickness the PL peak shifted towards red with increasing temperature. To the best of our knowledge we have observed for the first time an efficient room temperature (RT) PL emanating from such MQWs. The red shift of the PL peak with increasing temperature has been found to be due to the band gap shrinkage in accordance with the Varshni’s empirical relation. The spectral linewidth was found to increase with increasing temperature due to the scattering of excitons with acoustic and optical phonons in different temperature regimes. Both at RT and at 10 K the PL peak shifted with respect to the well layer thickness in the range of ∼3.35–∼3.68 eV with decreasing thickness in agreement with the calculated values. [Copyright &y& Elsevier]

Abstract. Cadmium sulphide nanocrystals were grown at room temperature (20 Celsius) under arachidic acid monolayers floating over an aqueous solution of CDCl[sub 2] inside an enclosed Langmuir-Blodgett set-up, through slow infusion of H[sub 2]S gas. X-ray diffraction spectra suggest an oriented growth of the crystallites. The particle sizes were found to increase with duration of exposure to the H[sub 2]S gas. Atomic force microscopy indicated that the particles were nearly circular pellets with uniform morphology throughout. In Raman spectra, the FWHM of the LO phonon was found to be large (Is approxiamtely equal to 20 cm[sup -1]) for all the films grown with different exposure times in H[sub 2]S gas, and was found to reduce to 8 cm[sup -1] after annealing a typical sample at 500 Celsius for 45 min. [ABSTRACT FROM AUTHOR]

Abstract. Reflectivity studies have been done on nanocrystalline hexagonal CdS particulate films grown by the Langmuir-Blodgett (LB) technique under a floating monolayer of arachidic acid. The band-edge peak (E[sub 0]) in the reflectivity spectra has been found to be blue-shifted with respect to that for bulk CdS. The appearance of the E[sub 1] (A) peak at about 5 eV is evidence for the hexagonal nature of the particles with the c axis normal or nearly normal to the plane of the film. This suggests the epitaxial nature of the grown particulate films. Another reflectivity peak has been observed at about 320 nm. However, its origin is not yet clear. [ABSTRACT FROM AUTHOR]

Correlation between size, bonding of Si and O at the surface/interface of Si nanocrystals and frequency of Si phonons in Si-SiO 2 nanocomposites is established using Raman mapping and X-ray photoelectron spectroscopy. Corroboration of these results with absorption spectroscopy shows that lowering of direct transition of light electron conduction band to heavy hole valence band ∼2.8 and 2.95 eV is a result of the interplay between effect of electron confinement (increases band gap) and oxidation bonding environment at the surface/interface (decreases band gap) of a nanocrystal in Si-SiO 2 nanocomposites. These studies have generated deeper insight into the fundamental understanding of the nanocomposite, which can allow easy optical microscopy monitoring of Si-SiO 2 based device fabrication. [ABSTRACT FROM AUTHOR]

Raman and atomic force microscopy (AFM) mapping on the same selected area are used to get unique information about the morphology of Si nanocrystals (NCs) embedded in SiO 2 , which is difficult to obtain by any other conventional technique. The sensitivity of Raman spectroscopy to surface/interface and confinement effects in NCs is effectively used to correlate the Raman intensity profile in Raman mapping with the topography obtained from AFM to understand that Si NCs are clustered in i) smaller clusters (∼100 nm) organized closely in two dimensions (2D) and ii) big (∼2 μm) three dimensional (3D) isolated clusters, although the growth is carried out to be multilayer (Si/SiO 2 ). Raman mapping performed by varying the focal spot along the depth shows stacking of larger (>∼60 Å) to smaller sizes (<∼40 Å) Si NCs from bottom to top for some clusters. To understand the observed morphologies, further study of specially grown Si–SiO 2 nanocomposites is performed, which suggest formation of smaller Si NCs at the top due to annealing at 800 °C in Si rich SiO 2 and possible existence of thermal gradient in an insulating matrix of SiO 2. Larger Si NCs are formed in the laser induced plume (plasma) itself. [ABSTRACT FROM AUTHOR]

The results of a numerical simulation method that estimate various discharge parameters in the positive column of a DC glow discharge controlled by ambipolar diffusion are presented. The parameters like reduced electric field ( E/N ), electron temperature, ionization rates, ambipolar diffusion losses and the average gas temperature were numerically evaluated for several mixtures of CO 2 , N 2 and He in low pressure regime. The estimated E/N value which is a primary governing parameter of positive column was verified experimentally using a double probe in diffusion controlled CW CO 2 laser for a variety of CO 2 , N 2 and He mixtures. The role of auxiliary ionization source like pulser used for pre-ionization and its effect on the steady state E/N value was also studied. A reasonably good agreement was found between the theoretical and the experimental results. Based on the results of this simulation a zigzag folded, diffusion-cooled, 500 W CW CO 2 laser has been designed and developed for research in gas phase nanoparticle synthesis. [ABSTRACT FROM AUTHOR]

We studied the band offset and alignment of pulsed laser deposited TiO 2 /CuGaO 2 hetero-structure using x-ray photoelectron spectroscopy. Valance band offset (VBO) of TiO 2 /CuGaO 2 interface was calculated using Kraut equation as 2.15 eV, which was in corroboration with VBO obtained directly from valance band onsets. A schematic band alignment diagram for the TiO 2 /CuGaO 2 interface was constructed which showed a type II band alignment with a significant band bending of 0.48 eV. Interface studies of TiO 2 /CuGaO 2 hetero-structure, showing type II band alignment, is important for gaining insight to the design of various photovoltaic devices based on such hetero-structures. [ABSTRACT FROM AUTHOR]

We have grown thin films of Dy doped ZnO (Dy:ZnO) on sapphire substrate using pulsed laser deposition with Dy concentration varying from 0.05 at.% to 1.8 at.%. These as grown Dy:ZnO thin films were characterized using low temperature four probe electrical measurements (in the range ~ 300 K to 4 K). All the Dy:ZnO thin films showed metal like behavior with a transition from positive to negative temperature coefficient of resistivity (TCR) and again to positive TCR with an increase of Dy concentration. This anomaly in conduction mechanism was investigated by considering quantum corrections to classical Boltzmann conductivity. All the Dy:ZnO thin films also exhibited negative magneto-resistance that supported for a significant contribution of weak localization in quantum corrections to conductivity. [ABSTRACT FROM AUTHOR]

Localized surface plasmon resonance (LSPR) characteristics of gold nanoparticles films grown at different substrate temperatures and mass thicknesses with and without alumina capping were studied. At different film mass thicknesses, the LSPR response was observed mainly in the films grown at high substrate temperatures. About 300 °C substrate temperature was found to be optimum for producing narrow and strong LSPR band in both uncapped and alumina capped gold nanoparticles films. The LSPR wavelength could be tuned in the range of 600–750 nm by changing either number of ablation pulses or decreasing target to substrate distance (TSD) and alumina layer capping. Though the alumina capping re-sputtered the gold films still these films exhibited stronger LSPR response compared to the uncapped films. Atomic force microscopic analysis revealed formation of densely packed nanoparticles films exhibiting strong LSPR response which is consistent with the package density of the nanoparticles predicted by the theoretical calculations. The average size of nanoparticles increased with substrate temperature, number of ablation pulses and decreasing the TSD. For the same mass thickness of gold films grown at different substrate temperatures the surface enhanced Raman scattering (SERS) intensity of Rhodamine6G dye was found to be significantly different which had direct correlation with the LSPR strength of the films at the excitation wavelength. [ABSTRACT FROM AUTHOR]

We have grown Ga doped ZnO (GZO) thin films at moderate temperatures with Ga concentrations in the range varying from 0.25 to 3 at.% on sapphire substrates using buffer assisted pulsed laser deposition. Room temperature resistivity measured was ∼5.1 × 10 −5 Ω cm with a electron mobility of ∼41.9 cm 2 /V s for an optimum Ga concentration of ∼0.75 at.% in the GZO films. Buffer assisted growth methodology maintains relatively good crystalline quality of the GZO thin films, thereby improving the electron mobility even at high dopant concentrations. This plays a key role in decreasing the resistivity of GZO films to the aforementioned value, which to the best of our knowledge is the lowest so far. These highly conducting GZO thin films with good mobility are potential candidates for transparent conducting oxide (TCO) applications in various optoelectronic devices. [ABSTRACT FROM AUTHOR]

Among the various shaped memory alloys (SMA), nitinol (Ni–Ti alloy) finds applications in automotive, aerospace, biomedical and robotics. The conventional route of fabrication of SMA has several limitations, like formation of stable secondary phases, fabrication of simple geometries, etc. This paper reports a novel method of fabricating SMA using a laser based additive manufacturing technique. Three different compositions of Ni and Ti powders (Ni-45% Ti-55%; Ni-50% Ti-50%; Ni-55% Ti45%) were pre-mixed using ball-milling and laser based additive manufacturing system was employed to fabricate circular rings. The material properties of fabricated rings were evaluated using Scanning Electron Microscopy (SEM), Differential scanning calorimeter (DSC), X-ray diffraction (XRD) system and micro-hardness test. All the characterized results showed that SMA could be manufactured using the laser based additive manufacturing process. The properties of laser additive manufactured SMA (Ni-50% Ti-50%) were found to be close to that of conventionally processed SMA. [ABSTRACT FROM AUTHOR]

Two order higher ionic conductivity was reported by us (J. Khare et al., Ceram. Int. 40 (2014) 14677) in pellet made from yttria stabilized zirconia nanoparticles grown under pulsed mode in comparison with pellets made from yttria stabilized zirconia nanoparticles grown under CW mode of CO 2 laser based vaporization method. To understand possible cause of difference in conductivity, the local structure around Zr +4 ion and Y +3 ion has been investigated using X-ray absorption fine structure spectroscopy. The local structure around Zr has been found to depend on the mode of laser vaporization for nanoparticles generation. In case of pulsed mode of vaporization method the charge compensating oxygen vacancies created by the substitution of Y are preferentially forming dipole type defect configuration leading to high ionic conductivity. While formation of immobile tripole type defects are responsible for low ionic conductivity in pellets made from nanoparticles generated under CW mode of laser vaporization. [ABSTRACT FROM AUTHOR]

Epitaxy of ZnO layers on cubic GaP(1 1 1) substrates has been demonstrated using pulsed laser deposition. Out of plane and in-plane epitaxial relationship of ZnO layer with respect to GaP substrate determined using phi ( ϕ ) scans in high resolution X-ray diffraction measurements are (0 0 0 1) ZnO ‖ (1 1 1) GaP and 〈 - 1 2 - 1 0 〉 ZnO ‖ - 1 1 0 GaP respectively. Our results of epitaxy of ZnO and its intense excitonic photoluminescence with very weak defect luminescence suggest that (1 1 1) oriented GaP can be a potential buffer layer choice for the integration of ZnO based optoelectronic devices on Si(1 1 1) substrates. [ABSTRACT FROM AUTHOR]

We have studied the band offset and alignment of pulsed laser deposited Al 2 O 3 /CuGaO 2 and ZnO/CuGaO 2 hetero-structures using photoelectron spectroscopy. Al 2 O 3 /CuGaO 2 interface exhibited a type I band alignment with valance band offset (VBO) of 4.05 eV whereas type II band alignment was observed in ZnO/CuGaO 2 hetero-structure with a VBO of 2.32 eV. Schematic band alignment diagram for the interface of these hetero-structures has been constructed. Band offset and alignment studies of these heterojunctions are important for gaining insight to the design of various optoelectronic devices based on such hetero-structures. [ABSTRACT FROM AUTHOR]

TiO 2 thin films of widely different structural and morphological characteristics were grown on Si (1 0 0) substrates using Atomic Layer Deposition (ALD) by varying the substrate temperature ( T s ) in a wide range (50 °C ≤ T s ≤ 400 °C). Spectroscopic ellipsometry (SE) measurements were carried out to investigate the effect of growth temperature on the optical properties of the films. Measured SE data were analyzed by considering double layer optical model for the sample together with the single oscillator Tauc-Lorentz dispersion relation. Surface roughness was taken into consideration due to the columnar growths of grains in crystalline films. The refractive index was found to be increased from amorphous ( T s ≤ 150 °C) to the nanocrystalline films (250 0 < T s ≤ 400 °C). The pronounced surface roughening for the large-grained anatase film obtained at the amorphous to crystalline phase transformation temperature of 200 °C, impeded SE measurement. The dispersions of refractive indices below the interband absorption edge were found to be strongly correlated with the single oscillator Wemple–DiDomenico (WD) model. The increase in dispersion energy parameter in WD model from disordered amorphous to the more ordered nanocrystalline films was found to be associated with the increase in the film density and coordination number. [ABSTRACT FROM AUTHOR]

An improved mass addition approach based on enthalpy balance is used for the numerical simulation of the temperature distributions and geometries during laser rapid manufacturing (LRM) of a multi-layered thin-wall. The approach involves the estimation of the local track height at every node along the track width on the substrate/previously deposited layer by simultaneous balancing of the excessive enthalpies above solidus temperature about the laser axis and the material interface at the substrate/previously deposited layer for material addition during the laser rapid manufacturing of a multi-layer thin wall. It takes laser power, laser beam size, scan-speed, powder feed rate, powder stream diameter, and time-delay between the deposition of two subsequent tracks as user-defined input, and computes the temperature distributions and the geometries of the deposited layers across the process domain in a dynamic fashion. In the present study, the laser rapid manufacturing of five layered thin walls of SS 304L on a work piece of the same material was simulated for various combinations of processing parameters and compared with experimental results. The percentage errors in simulated and corresponding experimental cumulative track heights along with track width were calculated and compared with those of other existing models and the results of present approach were found to be the least. The result indicates that the height and width of the layer under deposition depends on the geometry and temperature distribution of previously deposited layer and, consequently, governed the final geometry of thin wall. [ABSTRACT FROM PUBLISHER]

We deployed a 2kW continuous wave fiber laser integrated with the 5-axis workstation to understand the effect of various processing parameters (laser power, welding speed, beam spot size and chamfer at welded edges) on depth of penetration, angular distortion and welded bead surface profile during autogenous laser welding of 6mm thick austenitic stainless steel type 304 plates. Full penetration with reduced weld bead surface undulation (<100µm) and least angular distortion (<0.8°) was achieved for butt joints having chamfered edges of 0.8mm. The microscopic studies revealed grossly defect-free fusion zone with a few porosities at isolated locations. The microstructure at the fusion zone was largely austenitic with few ferrites and the direction of growth was epitaxial towards the fusion line. The measured values of microhardness at base material and fusion zone were 208±4 HV0.1 and 235±10 HV0.1 respectively. The tensile testing of laser welded samples indicated the ultimate strength >605MPa and these samples could be bent for an angle >170° without noticeable crack during bend test. The study opened the avenues for the deployment of fiber laser welding technology for applications demanding critical values of surface weld bead profile and distortion. [ABSTRACT FROM AUTHOR]

Highlights: [•] Laser peening significantly extended fatigue life of pre-fatigued spring steel. [•] Increase in fatigue life of laser peened specimens was more than 15times. [•] Black PVC tape is an effective coating for laser peening of ground surfaces. [•] Repeat peening repaired local surface melted regions on laser peened surface. [•] Technique is effective for life extension of in-service automobile parts. [Copyright &y& Elsevier]

Highlights: [•] AISI 316LN is a suitable material for vacuum chamber of Injector Microtron. [•] Formed component need not be annealed to obtain low magnetic permeability. [•] GTAW with high Mn filler produced a crack-free non-magnetic weld. [•] The weld exhibited acceptable mechanical properties. [•] Results are also important for future indigenous accelerators. [ABSTRACT FROM AUTHOR]

Abstract: Fatigue is a major life limiting factor affecting service life of low pressure steam turbine blades which, depending on turbine rating, are generally made of martensitic stainless steel or Ti6Al4V alloy. The results of the present study has demonstrated that with respect to conventional shot peening, laser peening brought more than two times longer mean fatigue life of Ti6Al4V alloy. Enhanced fatigue performance of laser peened specimens was attributed to the deeper peened layer and smoother finish. In the case of DIN X20Cr13 stainless steel, laser peening did not register significant improvement in fatigue resistance over shot peening. At maximum test stress of 550MPa, DIN X20Cr13 stainless steel specimens, shot peened and laser peened specimens exhibited comparable fatigue lives. However, at lower magnitude of maximum test stress of 400MPa, laser peened specimens displayed about 30% longer fatigue lives than their shot peened counterparts. Similar fatigue lives of shot peened and laser peened specimens of DIN X20Cr13 stainless steel specimens is attributed to comparable magnitude of surface residual stress and case depth produced by the two peening treatments. [Copyright &y& Elsevier]

Abstract: Gas dissociation in a high energy, high repetition rate Transversely Excited Atmospheric (TEA) CO2 laser in both sealed-off and gas replenishment modes were studied for nitrogen lean gas mixture. A comprehensive theoretical model based on the Boltzmann transport equation and the discharge excitation circuit equations was adopted to calculate the amount of CO2 dissociated during a single discharge pulse. Theoretically it is shown that inclusion of superelastic collisions in the Boltzmann transport equation is necessary for precise estimation of dissociation per pulse, particularly at high discharge energy loadings and for nitrogen rich gas mixtures. Gas lifetime for repetitively pulsed operations was found experimentally by measuring the amount of CO formed when frequent arcing sets in under sealed off operation. Using this model, the optimum replenishment rate of CO2 either by gas purging and/or by catalytic regeneration needed for arc free long term operation of the laser was estimated. The measured saturation values of CO concentration in the laser chamber agreed well with the calculated values for various operating conditions. Arc free, long term repetitively pulsed operation of the laser was achieved in the gas replenishment mode with gas purging and/or catalytic regeneration. [Copyright &y& Elsevier]

Abstract: An experimental study is carried out on a novel surface thermo-mechanical treatment involving conventional shot blasting and laser surface treatment, for enhancing inter-granular corrosion resistance of type 304 stainless steel. The modified surface exhibited fine re-crystallized austenite grains. A non-destructive approach of monitoring width of γ(311) diffraction peak through a portable X-ray diffraction was adopted, to determine micro-structural state on the thermo-mechanically treated surface. The results of the study have demonstrated that laser-assisted surface thermo-mechanical treatment of type 304 stainless steel is effective not only for enhancing material's inherent resistance against sensitization and inter-granular corrosion due to its subsequent exposure to susceptible temperature regime but also for reducing inter-granular corrosion susceptibility of pre-sensitized specimens. [Copyright &y& Elsevier]

Abstract: We investigated the solid particle erosion behaviour of WC-reinforced Ni-matrix based laser clad layers to improve the performance of engineering components for potential power plant applications. WC-reinforced Ni-matrix based laser clad layers having various compositions of WC (5, 10 and 15wt%) were deposited on austenitic stainless steel substrates. The laser clad layers were characterised using optical and scanning electron microscopy, microhardness testing and air-jet erosion testing. In solid particle erosion studies using the air-jet erosion tester, the set of testing parameters, including air-erodent compositions, erodent particle velocities and impact angles, was selected by using the Taguchi technique. The morphologies of the worn surfaces were used to predict the wear mechanisms. The results of a microstructural examination of the cross-sections of laser clad revealed a good metallurgical bond between the WC-reinforced Ni matrix and the austenitic stainless steel substrate. Dissociation/partial melting/full melting of WC particles was not observed in the laser clad layers. The microhardness value in the laser cladding zone was between 900–2400 VHN, while it was 230–270 VHN on the substrate. The results of erosion wear studies of the WC-Ni laser clad surface revealed that the erosion behaviour of the WC-Ni laser clad is primarily governed by erodent jet velocity followed by impact angle. The erosion does not much depend on the Ni-concentration in the MMC or the erodent feed rate. The wear signature at the erosion wear surface indicated that the erosion was primarily governed by a ductile erosion mechanism followed by the removal of WC particles from the matrix. The erosion resistance of the Ni-clad layer with WC was found to be at least four times higher than that without WC particles. The quantified contribution of various erosion parameters is useful for function-based design of components with extended service life. [Copyright &y& Elsevier]

Abstract: This research article describes the large scale fabrication of ZnO nanorods of various shapes on Si(100) substrate, by using metalorganic precursor of Zn in solutions with microwave as the source of energy. This is a low temperature, environmental friendly and rapid thin film deposition process, where ZnO nanorods (1–3 μm length) were grown only in 1–5 min of microwave irradiation. All as-synthesized nanorods are of single crystalline grown along the <0001> crystallographic direction. The coated nanorods were found to be highly dense having a thickness of ∼1–3 μm over the entire area 20 mm × 20 mm of the substrate. The ZnO thin film comprising of nanorods exhibits good adhesion with the substrate. A possible mechanism for the initial nucleation and growth of ZnO is discussed. A cross over from a strong visible light emission to an enhanced UV emission is observed, when the nature of the surfactants are varied from polymeric to ionic and nonionic. The position of the chromaticity coordinates in yellow region of the color space gives an impression of white light generation from these coatings by exciting with a blue laser. [Copyright &y& Elsevier]

Abstract: Analytical and experimental studies on geometrical aspects of the deposited tracks were carried out at different processing parameters for laser rapid manufacturing (LRM) in vertical surface configuration using AISI type 304 stainless steel powder on the substrate of the same material. The vertical downward shift of the deposited track and its peak due to the gravity flow of the melt were found to follow quadratic dependence on the track height. The downward rounded bulging was found to be quite significant for the scan speeds lesser than 200mm/min, while this was insignificant for the scan speeds more than 400mm/min. A set of consolidated processing parameters for continuous material deposition was identified. The threshold value of laser energy and powder fed, both per unit traverse length for the continuous deposition were found to be ~96J/mm and ~0.006g/mm respectively. The maximum powder catchment efficiency was ~42% for stand-off distances in the range of 15–18mm. The surface waviness factor was found to decrease from ~0.95 to ~0.05 when the overlap index was increased from 30% to 80%. The study provides a deeper insight into the ensuing geometrical aspects of the tracks using LRM in vertical configuration. [Copyright &y& Elsevier]

Abstract: It is shown that the threshold fluence for laser paint stripping can be accurately estimated from the heat of gasification and the absorption coefficient of the epoxy-paint. The threshold fluence determined experimentally by stripping of the epoxy-paint on a substrate using a TEA CO2 laser matches closely with the calculated value. The calculated threshold fluence and the measured absorption coefficient of the paint allowed us to determine the epoxy paint thickness that would be removed per pulse at a given laser fluence even without experimental trials. This was used to predict the optimum scan speed required to strip the epoxy-paint of a given thickness using a high average power TEA CO2 laser. Energy Dispersive X-Ray Fluorescence (EDXRF) studies were also carried out on laser paint-stripped concrete substrate to show high efficacy of this modality. [Copyright &y& Elsevier]

Abstract: Growth of hexagonal shaped single crystal yttria stabilized zirconia nanoparticles has been observed in the vapors emanating from solid targets vaporized using CO2 laser in CW and pulsed modes of operation. The mean size and yttrium concentration in these nanoparticles are found to be in the range of 5–40nm and ∼10.4–11.6wt% respectively, which depend on the laser power density and the gaseous environment used during the growth process. Nanoparticles synthesized with pulsed mode of laser operation were found to be smaller in size compared to those generated in the CW mode. It was also found that formation of these nanoparticles was more pronounced in helium gas environment compared to those in nitrogen and argon. X-ray and electron diffraction studies showed that in all experimental conditions the generated nanoparticles were in cubic phase. From high resolution TEM it was observed that the nanoparticles with identical orientations coalesced with each other in line with the oriented attachment theory. [Copyright &y& Elsevier]

Abstract: Impedance matching in an indigenously developed 2kW RF-excited fast axial flow (RF-FAF) CO2 laser was seen to be significantly influenced by the capacitance formed between the laser head and the electrodes. We find in the published literature that this capacitance is generally assumed to be negligible while designing the matching network in the RF-FAF CO2 lasers. It is shown that this capacitance could have a significant value and consequently would modify the design of the matching network remarkably for a specific laser design studied in the present case. The widely used discharge load model for capacitively coupled RF (CCRF) discharge excited CO2 laser plasma is extended to include this capacitance. The method of measurement of this capacitance using a bridge and the ANSYS software based simulation of its formation due to the laser head structure are discussed. [Copyright &y& Elsevier]

Abstract: The pitting corrosion and sensitization in laser rapid manufactured specimens of type 316L stainless steel are studied and compared with its wrought and weld deposit (by plasma transferred arc welding-PTAW) counterparts. With respect to wrought specimens, the laser rapid manufactured specimens exhibited lower pitting corrosion resistance as well as higher degree of sensitization. However, pitting corrosion resistance of laser rapid manufactured specimens is largely similar to that of PTAW-deposited specimens. Relatively lower pitting corrosion resistance of laser rapid manufactured and PTAW-deposited specimens are attributed to their cast microstructure and associated micro-segregation effects as expected. Repeated exposure to sensitization temperature range (773–1073K) during multiple layer deposition coupled with slower rate of cooling during later part of laser rapid manufacturing resulted in sensitization, giving rise to continuous corrosion attack along the grain boundaries. Solution annealing is found to be helpful in healing the associated chromium depletion and decreasing the degree of sensitization with modest improvement in pitting resistance. Primary mode of solidification in laser rapid manufactured specimens of type 316L stainless steel influenced potential sites for initiation of pitting corrosion. In primary ferrite solidified regions, initiation of pitting corrosion occurred preferentially at δ-ferrite/austenite boundaries whereas core of austenite cells/dendrites and grain boundaries were the preferred sites for initiation of pitting corrosion in primary austenite solidified regions. These studies provide insight into the basic corrosion mechanisms of laser rapid manufactured specimens of type 316L stainless steel. [Copyright &y& Elsevier]

Abstract: Present experimental laser shock peening study on SAE 9260 spring steel, performed with an in-house developed 2.5J/7ns pulsed Nd:YAG laser, aimed to evaluate laser shock peening process as a possible alternative to existing shot peening practice for enhancing fatigue life of leaf springs. In the investigated range of process parameters, laser shock peening yielded largely comparable magnitude of surface compressive stress and shallower compressed surface layer than those achieved with existing shot peening practice. In contrast to considerably rougher shot peened surface with numerous defects, laser shock peening produced largely unaltered surface finish without peening-induced defects. With respect to shot peening, laser shock peening brought about significant increase in fatigue life. Improved fatigue performance of laser shock peened specimens is attributed to their better surface finish without peening-induced surface defects, which were potential fatigue crack nucleation sites in shot peened specimens. [Copyright &y& Elsevier]

Abstract: We simulated the temperature distribution and single track geometry in Laser Rapid Manufacturing (LRM) of metal structures using two dimensional FEM with a finer modeling approach. This approach involved the calculation of excessive enthalpies above melting point for all nodal points in the process domain and using those for the computation of local track height at every node along the track width on the substrate. Laser power, laser beam size with its spatial profile, scan speed, powder feed rate and powder stream diameter with its flow distribution were taken as user-defined input parameters to simulate the single track geometry during LRM. The simulated results were experimentally verified. The percentage errors in simulated and corresponding experimental track heights along the normalized track width were calculated and compared with those of other existing models. With our modeling approach, the localized errors in predicting track geometries were found to be the least. This model is capable of dynamically predicting the temperatures and track geometry at various user-defined input parameters. [Copyright &y& Elsevier]

Abstract: The optical responses of gold nanoparticle films and thin continuous films grown by pulsed laser deposition (PLD) in a wide range of process parameters were studied. The growth conditions for obtaining strong surface plasmon resonances (SPR) were optimized. Laser fluence and substrate temperature were found to play key roles in the SPR characteristics of the grown films. The SPR wavelength was tuned in the range of 580–720nm by varying number of pulses and target-to-substrate distance during the PLD. Atomic force microscopic analysis revealed that the size of metal nanoparticles systematically increased with increasing number of pulses and decreasing target-to-substrate distance. Morphology of the films, which exhibited strong SPR showed densely packed spheroid nanoparticles of gold. Theoretical calculations of SPR wavelengths for the presently studied densely packed gold nanoparticle films were carried out using modified Yamaguchi model which incorporate particle–substrate and particle–particle interactions. From these calculations it was found that the observed SPR responses and their tuning arise from the combination of high volume filling fraction and aspect ratios of the gold nanoparticles. [Copyright &y& Elsevier]

Abstract: Highly transparent and conducting Chromium doped ZnO (Cr:ZnO) thin films with preferential c-axis orientation were grown on (0001) sapphire substrates using buffer assisted pulsed laser deposition. The resistivity of Cr:ZnO thin films was found to decrease to a minimum value of ∼1.13×10−3Ωcm with the increasing Cr concentration up to ∼1.9at.% and then increase with further increase of Cr concentration. On the contrary, the band gap and carrier concentration of Cr:ZnO thin films increased up to ∼3.37eV and ∼2×1020 cm−3, respectively, with the increase of Cr concentration up to ∼1.9at.%, then decreased with further increase of Cr concentration. The increase of carrier concentration and conductivity with Cr doping at low Cr concentrations (<1.9at.%) could be attributed to the presence of Cr in +3 valence state in ZnO thus acting as donor while decrease of carrier concentration beyond ∼1.9at.% of Cr concentration could be attributed to the charge compensating effect due to the presence of acceptor like point defects such as oxygen interstitials. This was experimentally confirmed using x-ray photoelectron spectroscopy. The observed variation in the band gap of Cr:ZnO thin films with increasing Cr doping was attributed to the competing effects of the high free carrier concentration induced Burstein-Moss blue shift and band gap narrowing. [Copyright &y& Elsevier]

Abstract: A simple and sensitive technique, which utilizes the sensing of both the discharge current and the visible light emission from the discharge, has been developed to detect the arc formation during the operation of a high repetition rate TEA CO2 laser. It is shown that the optical detection in the visible spectral region is more sensitive than that it is in the UV region and hence can be used more effectively to monitor the onset of arc formation, while the current signal is useful in the strong arc formation regime. We demonstrate that the trigger signals generated by both the methods can be employed for reliable protection of the laser system from catastrophic damage due to deleterious arc formation. [ABSTRACT FROM AUTHOR]

Abstract: Photoluminescence (PL) of high quality GaN epitaxial layer grown on β-Si3N4/Si (111) substrate using nitridation–annealing–nitridation method by plasma-assisted molecular beam epitaxy (PA-MBE) was investigated in the range of 5–300K. Crystallinity of GaN epilayers was evaluated by high resolution X-ray diffraction (HRXRD) and surface morphology by Atomic Force Microscopy (AFM) and high resolution scanning electron microscopy (HRSEM). The temperature-dependent photoluminescence spectra showed an anomalous behaviour with an ‘S-like’ shape of free exciton (FX) emission peaks. Distant shallow donor–acceptor pair (DAP) line peak at approximately 3.285eV was also observed at 5K, followed by LO replica sidebands separated by 91meV. The activation energy of the free exciton for GaN epilayers was also evaluated to be ∼27.8±0.7meV from the temperature-dependent PL studies. Low carrier concentrations were observed ∼4.5±2×1017 cm−3 by measurements and it indicates the silicon nitride layer, which not only acts as a growth buffer layer, but also effectively prevents Si diffusion from the substrate to GaN epilayers. The absence of yellow band emission at around 2.2eV signifies the high quality of film. The tensile stress in GaN film calculated by the thermal stress model agrees very well with that derived from Raman spectroscopy. [Copyright &y& Elsevier]

Abstract: Non-polar a-plane GaN films were grown on an r-plane sapphire substrate by plasma assisted molecular beam epitaxy (PAMBE). The effect of growth temperature on structural, morphological and optical properties has been studied. The growth of non-polar a-plane (11−20) orientation of the GaN epilayers were confirmed by high resolution X-ray diffraction (HRXRD) study. The X-ray rocking curve (XRC) full width at half maximum of the (11−20) reflection shows in-plane anisotropic behavior and found to decrease with increase in growth temperature. The atomic force micrograph (AFM) shows island-like growth for the film grown at a lower temperature. Surface roughness has been decreased with increase in growth temperature. Room temperature photoluminescence shows near band edge emission at 3.434–3.442eV. The film grown at 800°C shows emission at 2.2eV, which is attributed to yellow luminescence along with near band edge emission. [ABSTRACT FROM AUTHOR]

Abstract: Fatigue crack growth and fracture toughness characteristics of laser rapid manufactured (LRMed) Inconel 625 compact tension specimens of thickness 12 and 25mm were investigated. Fatigue crack propagation in all the specimens investigated in the stress intensity range (ΔK) of 14–38MPa√m, exhibited stage II crack growth in Paris’ regime with nearly same slopes of crack growth per cycle versus ΔK plot. Fatigue crack growth rates in the LRMed specimens of present study were found to be lower than the reported values for wrought Inconel 625 in the ΔK range of 14–24MPa√m and above this range they tended to coincide. X-ray diffraction patterns of the fractured surfaces revealed that the crack propagated along the growth direction of the specimens which was predominantly along the (111) plane. The fracture toughness values (J 0.2) for LRMed Inconel 625 specimens were found to be in the range of about 200–255kJ/m2. The LRMed specimens exhibited stable crack growth during the J-integral test. [ABSTRACT FROM AUTHOR]

Abstract: High-quality GaN epilayers were grown on Si (111) substrates by molecular beam epitaxy using a new growth process sequence which involved a substrate nitridation at low temperatures, annealing at high temperatures, followed by nitridation at high temperatures, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. The material quality of the GaN films was also investigated as a function of nitridation time and temperature. Crystallinity and surface roughness of GaN was found to improve when the Si substrate was treated under the new growth process sequence. Micro-Raman and photoluminescence (PL) measurement results indicate that the GaN film grown by the new process sequence has less tensile stress and optically good. The surface and interface structures of an ultra thin silicon nitride film grown on the Si surface are investigated by core-level photoelectron spectroscopy and it clearly indicates that the quality of silicon nitride notably affects the properties of GaN growth. [ABSTRACT FROM AUTHOR]

Abstract: Present paper describes microstructural and mechanical characterization of laser-clad composite joint made of Stellite 21 and type 316L stainless steel (SS). The study has been broadly performed on two kinds of laser-clad specimens viz. (i) involving direct deposition of Stellite 21 on SS and (ii) involving gradient in chemical composition across substrate/clad interface. Both kinds of specimens exhibited nearly similar tensile strength, which was higher than that of SS in the interface region. Inter-dendritic carbides provided low energy fracture path in laser-clad deposits of Stellite 21. Both direct and graded interfaces of laser-clad specimens exhibited superior fatigue strength than the SS substrate. Instrumented impact testing brought out distinct difference in the mode of crack propagation across direct and the graded clad specimens. In contrast to initiation-controlled brittle crack propagation across the interface region in “direct clad” specimens, crack propagation across “graded interface” was marked with significant plastic deformation. [Copyright &y& Elsevier]

Abstract: Transparent conductive dysprosium doped ZnO (Dy:ZnO) thin films with preferential orientation in the (0002) direction were deposited on (0001) sapphire substrate by buffer assisted pulsed laser deposition. The experimental results show that the resistivity of Dy:ZnO thin films decreased to a minimum value of ∼7.6×10−4 Ωcm with increasing Dy concentration up to ∼0.45at%, then increased with the further increase of Dy concentration. On the contrary, the band gap and carrier concentration of Dy:ZnO thin films initially increased, then decreased with increase of Dy concentration. The blue shift of band gap of Dy:ZnO thin films with increasing carrier concentration was attributed to the competing effects of Burstein-Moss shift and band gap narrowing. A bright room temperature photoluminescence observed at ∼575nm in all the Dy:ZnO thin films, with maximum intensity at ∼0.45at% of Dy doping, was attributed to be due to intra-band transitions of Dy3+ in ZnO. Near band edge photoluminescence of ZnO was observed at ∼380nm with photoluminescence intensity decreasing with increase of Dy concentration. Such Dy:ZnO thin films are found to be suitable candidate for luminescent device applications. [Copyright &y& Elsevier]