Showing papers on "Fluence published in 2016"

Effect of laser ablation surface treatment on performance of adhesive-bonded aluminum alloys

Abstract: This study was conducted to investigate the effect of laser ablation surface treatment on the joint performance of AA6022-T4 2.0 mm adhesively bonded to itself. It was found that laser ablation treatment at higher energy fluence (i.e., 19.01 J/cm 2 ) improved the joint strength by 25% versus untreated material as measured after water soak exposure. Furthermore, a greater proportion of fracture surface exhibited pure cohesive failure following laser ablation treatment. Results revealed that while laser ablation treatment at lower energy fluence had little influence on the surface topography (though it did act to remove any sheet metal lubricant or other contaminants), treatment at higher energy fluence increased both surface roughness and surface area. This is considered a contributing factor to the improved adhesive bond performance. However, the higher energy fluence were also found to modify the surface chemistry and created a more uniform and thicker aluminum oxide layer, which was likely to be another contributing factor to the improved bond performance. Additionally, the effect of aging (up to 6 weeks) in ambient conditions of laser ablated aluminum on adhesive bonding performance appeared to have no statistical influence.

Effect of magnetic field on laser-induced breakdown spectroscopy of graphite plasma

Abstract: The effect of transverse magnetic field on laser-induced breakdown spectroscopy of graphite plasma as a function of fluence has been investigated. Graphite targets were exposed to Nd:YAG (1064 nm, 10 ns) laser pulses at various laser fluences ranging from 0.4 to 2.9 J cm−2 under two different environment of air and Ar at a pressure of 150 and 760 torr. A transverse magnetic field of strength 0.5 tesla was employed by using permanent magnets. It is revealed that due to the presence of the magnetic field the emission intensity, electron temperature and number density of graphite plasma have been increased at all fluences and for all environmental conditions. The enhancement in plasma parameters is attributed to magnetic confinement effect and Joule heating effect. Initially by increasing the fluence from 0.4 to 1.5 J cm−2 (in air) and 0.4 to 1.8 J cm−2 (in Ar), the emission intensity, electron temperature and number density have been increased and have attained their maximum values. Further increase in fluence was responsible for the decreasing trend in all plasma parameters. More increase in fluence (beyond 1.8 J cm−2 in case of air and 2.2 J cm−2 in case of Ar) up to a maximum value of 2.9 J cm−2, the saturation or self-sustained regime was achieved, which is responsible for insignificant changes in plasma parameters. The value of plasma parameter “β” was also evaluated analytically, and it was less than one for all conditions (fluences as well as environments), which confirmed the existence of confinement effect.

Ultrashort laser pulse ablation of copper, silicon and gelatin: effect of the pulse duration on the ablation thresholds and the incubation coefficients

Abstract: In this paper, the influence of the pulse duration on the ablation threshold and the incubation coefficient was investigated for three different types of materials: metal (copper), semiconductor (silicon) and biopolymer (gelatin). Ablation threshold values and the incubation coefficients have been measured for multiple Ti:sapphire laser pulses (3 to 1000 pulses) and for four different pulse durations (10, 30, 250 and 550 fs). The ablation threshold fluence was determined by extrapolation of curves from squared crater diameter versus fluence plots. For copper and silicon, the experiments were conducted in vacuum and for gelatin in air. For all materials, the ablation threshold fluence increases with the pulse duration. For copper, the threshold increases as τ 0.05, for silicon as τ 0.12 and for gelatin as τ 0.22. By extrapolating the curves of the threshold fluence versus number of pulses, the single-shot threshold fluence was determined for each sample. For 30 fs pulses, the single-shot threshold fluences were found to be 0.79, 0.35, and 0.99 J/cm2 and the incubation coefficients were found to be 0.75, 0.83 and 0.68 for copper, silicon and gelatin, respectively.

Formation of copper tin sulfide films by pulsed laser deposition at 248 and 355 nm

Abstract: The influence of the laser wavelength on the deposition of copper tin sulfide (CTS) and SnS-rich CTS with a 248-nm KrF excimer laser (pulse length τ = 20 ns) and a 355-nm frequency-tripled Nd:YAG laser (τ = 6 ns) was investigated. A comparative study of the two UV wavelengths shows that the CTS film growth rate per pulse was three to four times lower with the 248-nm laser than the 355-nm laser. SnS-rich CTS is more efficiently ablated than pure CTS. Films deposited at high fluence have submicron and micrometer size droplets, and the size and area density of the droplets do not vary significantly from 248 to 355 nm deposition. Irradiation at low fluence resulted in a non-stoichiometric material transfer with significant Cu deficiency in the as-deposited films. We discuss the transition from a non-stoichiometric material transfer at low fluence to a nearly stoichiometric ablation at high fluence based on a transition from a dominant evaporation regime to an ablation regime.

Laser fluence dependence on emission dynamics of ultrafast laser induced copper plasma

Abstract: The characteristic emission features of a laser-produced plasma depend strongly on the laser fluence. We investigated the spatial and temporal dynamics of neutrals and ions in a femtosecond laser (800 nm, ∼40 fs, Ti:Sapphire) induced copper plasma in vacuum using both optical emission spectroscopy (OES) and spectrally resolved two-dimensional (2D) imaging over a wide fluence range of 0.5–77.5 J/cm2. 2D fast gated monochromatic images showed a distinct plume splitting between the neutrals and ions, especially at moderate to higher fluence. OES studies at low to moderate laser fluence confirm intense neutral line emission over ion emission, whereas this trend changes at higher laser fluence with dominance of the latter. This evidences a clear change in the physical processes involved in the femtosecond laser-matter interaction at high input laser intensity. The obtained ion dynamics resulting from OES and spectrally resolved 2D imaging are compared with charged particle measurement employing Faraday cup and Langmuir probe; results showed good correlation.

Charge collection studies in irradiated HV-CMOS particle detectors

Abstract: Charge collection properties of particle detectors made in HV-CMOS technology were investigated before and after irradiation with reactor neutrons. Two different sensor types were designed and processed in 180 and 350 nm technology by AMS. Edge-TCT and charge collection measurements with electrons from 90Sr source were employed. Diffusion of generated carriers from undepleted substrate contributes significantly to the charge collection before irradiation, while after irradiation the drift contribution prevails as shown by charge measurements at different shaping times. The depleted region at a given bias voltage was found to grow with irradiation in the fluence range of interest for strip detectors at the HL-LHC. This leads to large gains in the measured charge with respect to the one before irradiation. The increase of the depleted region was attributed to removal of effective acceptors. The evolution of depleted region with fluence was investigated and modeled. Initial studies show a small effect of short term annealing on charge collection.

Nano- and Submicrometer-Sized Spherical Particle Fabrication Using a Submicroscopic Droplet Formed Using Selective Laser Heating

Abstract: Detailed characterization of products by pulsed laser melting in liquid was performed for TiO2 particles obtained at different laser fluences. The size, crystal structure, and inner structure of obtained spherical particles depended on the irradiating laser fluence. Single crystalline submicrometer spherical TiO2 particles of 200 nm were obtained using nanosecond pulsed laser irradiation at 100 mJ cm–2 pulse–1 onto raw particles dispersed in ethanol using cross-sectional high-resolution transmission electron microscopy observation. At higher laser fluence (e.g., 225 mJ cm–2 pulse–1), large submicrometer spheres with strain and twin structures 500 nm in diameter and nanometer spheres of 70 nm were simultaneously observed. The formation of such bimodal size distribution of obtained particles was explained based on the phase transition fluence curves deduced by Mie theory and the optical absorption increase induced by the formation of nonstoichiometric TiO2 particles. Thus, with the appropriate laser fluence...

A complete residual stress model for laser surface hardening of complex medium carbon steel components

Abstract: A numerical model is presented for evaluation of residual stresses following laser surface treatment of mechanical components with arbitrary geometry. Following on from previous temperature and microstructural models, stress evaluation is performed by considering the resulting deformation from thermal expansion, elastic and plastic deformation, and microstructural changes. A 3.3 kW diode laser with wavelength of 930 nm and 34 mm × 2 mm rectangular spot is utilized to perform heat treatment experiments on an AISI 9810 steel cam, with x-ray diffraction measurements performed before and after laser exposure to determine circumferential and axial surface stresses. Verification of model accuracy is performed by comparing calculated stresses with the measured values. The influence of incident laser fluence and scanning velocity on the hardened depth and residual stress state is then investigated numerically for the same component. It is found that higher laser fluence, or an increase in exposure velocity at constant fluence, leads to an increase in the hardened depth and a reduction in compressive residual stresses.

The fluence threshold of femtosecond laser blackening of metals: The effect of laser-induced ripples

Abstract: With the primary controlling factor of the laser fluence, we have investigated femtosecond laser blackening of stainless steel, brass, and aluminum in visible light range. In general, low reflectance about 5% can be achieved in appropriate ranges of laser fluences for all the treated metal surfaces. Significantly, towards stainless steel and brass a fluence threshold of blackening emerges unusually: a dramatic reflectance decline occurs in a specific, narrow fluence range. In contrast, towards aluminum the reflectance declines steadily over a wide fluence range instead of the threshold-like behavior from steel and brass. The morphological characteristics and corresponding reflectance spectra of the treated surfaces indicates that the blackening threshold of stainless steel and brass corresponds to the fluence threshold of laser-induced subwavelength ripples. Such periodic ripples growing rapidly near ablation threshold absorb visible light efficiently through grating coupling and cavity trapping promoted by surface plasmon polaritons. Whereas, for aluminum, with fluence increasing the looming ripples are greatly suppressed by re-deposited nanoparticle aggregates that present intrinsic colors other than black, and until the formation of large scale “ravines” provided with strong light-trapping, sufficient blackening is achieved. In short, there are different fluence dependencies for femtosecond laser blackening of metals, and the specific blackening fluence threshold for certain metals in the visible range originates in the definite fluence threshold of femtosecond laser-induced ripples.

Evolution of extended defects in polycrystalline UO2 under heavy ion irradiation: Combined TEM, XRD and Raman study

Abstract: A well-suited way to understand the behavior under irradiation of the UO2 nuclear fuel is to use ion implantations and further analyze the microstructure. The present work is focused on the characteristics of extended defects created by Kr implantations in UO2 polycrystals. The influence of many parameters on these extended defects has been studied separately: irradiation temperature (room temperature, 500 and 600 °C), ion fluence (5 × 1015 and 1.26 × 1017 i/cm2), ion energy (4 and 27 MeV) and conditions of subsequent thermal annealing. The evolution of extended defects characteristics and density was determined using transmission electron microscopy and additional damage characterizations were performed by X-ray diffraction and Raman spectroscopy. Results obtained by the different analytical techniques are shown to be in excellent agreement and to complement literature data. In addition, this work presents the extended defect transformation induced by fluence (dislocation loops and lines → tangled dislocation network) and shows that temperature influences directly their evolution kinetics. Moreover we demonstrate that the temperature to allow extended defects mobility is different during irradiation and under thermal annealing conditions. Finally dislocation line density and swelling obtained from this study are found in the same order of magnitude as the ones obtained from in pile irradiations.

Transient enhancement of magnetization damping in CoFeB film via pulsed laser excitation

Abstract: Laser-induced spin dynamics of in-plane magnetized CoFeB films has been studied by using time-resolved magneto-optical Kerr effect measurements. While the effective demagnetization field shows little dependence on the pump laser fluence, the intrinsic damping constant has been found to be increased from 0.008 to 0.076 with the increase in the pump fluence from 2 mJ/cm2 to 20 mJ/cm2. This sharp enhancement has been shown to be transient and ascribed to the heating effect induced by the pump laser excitation, as the damping constant is almost unchanged when the pump-probe measurements are performed at a fixed pump fluence of 5 mJ/cm2 after irradiation by high power pump pulses.

How photon pump fluence changes the charge carrier relaxation mechanism in an organic-inorganic hybrid lead triiodide perovskite.

Abstract: This study explores the excitation wavelength and fluence dependence of processes occurring in formamidinium lead triiodide (FAPbI3) film using time-resolved transient absorption and terahertz spectroscopies. The results indicate that second-order processes are responsible for charge carrier recombination at low fluences of the absorbed photons (below 8.4 × 1012 ph per cm2). An increase in fluence leads to the appearance and successive reduction of the time component assigned to the Auger recombination of free charge carriers (240–120 ps). Simultaneously, the bimolecular recombination time decreases from ∼1400 to ∼700 ps. Further increasing the pump fluence produces an exciton population that recombines in 6 ps. The comparison of two characteristic bleaching bands located at 480 and 775 nm provides evidence for the validity of the two valence bands model. Excitation with higher fluences results in a marked difference in the probed dynamics at these bands, reflecting the action of two excited states at the conduction band. Our results demonstrate that a single model cannot be applied in characterizing the perovskite absorber transitions at all pump fluences. These findings are relevant in understanding their operating mechanism under specific experimental conditions, which should differ for perovskite based solar cells, lasing media or photon detectors.

Retention in tungsten resulting from extremely high fluence plasma exposure

Abstract: PISCES-B was used for a series of high-fluence plasma exposures to investigate the deuterium fuel retention properties of tungsten, when exposed to continuous plasma irradiation. The goal was to determine whether the fuel retention in the tungsten saturates at sufficiently high fluence, or continues to increase as a function of the plasma fluence. During pure deuterium plasma exposure, up to a maximum deuterium fluence of 2 × 1028 m−2, retention results indicate that saturation is not reached and that retention scales as the square root of time, indicative of diffusion dominating the fuel uptake of the tungsten. However, measurements performed while sculpting the PISCES plasma to replicate a burning plasma, by adding a small amount (5%) of helium ions to the incident deuterium plasma, indicate the deuterium uptake in the target is severely inhibited.

Effect of fluence on carbon nanostructures produced by laser ablation in liquid nitrogen

Abstract: Effects of laser fluence on the properties of carbon nanostructures produced by laser ablation method in liquid nitrogen have been studied experimentally. The beam of a Q-switched Nd:YAG laser of 1064-nm wavelength at 7 ns pulse width and different fluences is employed to irradiate the graphite target in liquid nitrogen. Properties of carbon nanostructures were studied using their UV–Vis–NIR spectrum, TEM images, and Raman scattering spectrum. Two categories of graphene nanosheets and carbon nanoparticles were observed due to variation of laser fluence. Results show that in our experimental condition there is a threshold fluence for producing carbon nanoparticles. With increasing the laser fluence from the threshold, the amount of carbon nanoparticles in suspensions was increased, while the amount of graphene nanosheets was decreased.

Measurements of fluence profiles in femtosecond laser filaments in air.

Abstract: We introduce a technique to measure fluence distributions in femtosecond laser beams with peak intensity of up to several hundred terawatts per square centimeter. Our approach is based on the dependence of single-shot laser ablation threshold for gold on the angle of incidence of the laser beam on the gold sample. We apply this technique to the profiling of fluence distributions in femtosecond laser filaments at a wavelength of 800 nm in air. The peak intensity is found to be clamped at a level that depends on the external beam focusing. The limiting value of the peak intensity attainable in long-range 800 nm air filaments, under very loose focusing conditions (f-number above ∼500), is about 55 TW/cm2.

Modified structural, surface morphological and optical studies of Li3+ swift heavy ion irradiation on zinc oxide nanoparticles

Abstract: Nanoparticles of ZnO were synthesized by a co-precipitation method. The ZnO samples were irradiated by Li3+ swift heavy ions (SHI) with fluences of 5 × 1011 and 1 × 1013 ions per cm2, constant current of 1 particle nano ampere (PNA) and Li3+ ion energy of 50 MeV. Structural parameters of pristine and SHI-irradiated ZnO nanoparticles were investigated by XRD. It was explicitly confirmed by XRD data that pristine and SHI-irradiated ZnO samples show wurtzite structures. The particle sizes of pristine and SHI-irradiated ZnO samples were estimated using XRD data and found to be in the range of 20 nm to 22 nm. SEM images show that the pristine sample has a spherical shape, whereas the ZnO sample SHI-irradiated with 5 × 1011 ions per cm2 fluence exhibits enlargement in grain size, and the sample irradiated with 1 × 1013 ions per cm2 fluence has a rod-like structure. EDAX was employed to investigate the elemental composition of the materials. Raman spectra show phonon interaction and vibration bands in the materials at 438.69 cm−1 and 435.95 cm−1 due to ZnO bonding. Atomic force microscopic (AFM) studies evince enhancement in roughness and track formations after SHI irradiation of ZnO samples as compared to the pristine counterpart. The UV-vis spectroscopy results show a decreased energy band gap (red shift) with enhanced Li3+ swift heavy ion irradiation on the ZnO nanoparticles.