Showing papers on "Fluence published in 2021"

Removal mechanism of surface cleaning on TA15 titanium alloy using nanosecond pulsed laser

Abstract: Nanosecond laser cleaning provides an alternative cleaning method that displays a significantly improved cleaning efficiency without environmental pollution. This paper reports the results of an investigation into the nanosecond pulsed laser cleaning of TA15 titanium alloy oxide film using the laser fluence and the laser head moving speed as variables. The TA15 titanium alloy oxide film changed from original silver gray to silvery white with the increase of laser fluence. However, the material surface transformed from silvery-white into yellowish-brown when the laser energy fluence exceeded 5.97 J/cm2. This phenomenon was attributed to the formation of titanium alloy oxides (Ti6O, TiO2-anatase, γ-TiO2, and TiO) after removing the oxide film using a high laser fluence at atmospheric conditions. The best surface properties were obtained when the laser fluence and the laser head moving speed were set to 3.98 J/cm2 and 5 mm/s, respectively. The oxidation content and roughness were 2.08 wt% and 37 μm, respectively. The removal effect of the oxide film was best when the surface temperature was slightly higher than the boiling point of the oxide film, as shown by numerical calculations and analysis. The main mechanism of laser cleaning was laser ablation.

Wavelength-dependent time-dose reciprocity and stress mechanism for UV-LED disinfection of Escherichia coli.

Abstract: Ultraviolet (UV) disinfection efficiency by low-pressure (LP) mercury lamp depends on the UV fluence (dose): the product of incident irradiance (fluence rate) and exposure time, with correction factors. Time–dose reciprocity may not always apply, as higher UV–LP inactivation of E. coli was obtained at a higher irradiance over shorter exposure time, for the same UV fluence. Disinfection by UV LEDs is limited by low radiant flux compared to mercury LP lamps. Our goal was to determine the UV-LED time–dose reciprocity of E. coli for four different central LED wavelengths (265, 275, 285 and 295 nm) under different fluence rates. Inactivation kinetics determined at UV-LED265 was not affected by the fluence rate or exposure time for a given UV fluence. In contrast, UV-LED275, UV-LED285, and UV-LED295 led to higher inactivation at low fluence rate coupled to high exposure time, for the same UV fluence. The intracellular damage mechanisms for each LED central wavelength were determined by using the bioreporters RecA as an indicator of bacterial DNA damage and SoxS as an indicator of oxidative stress. For 265 nm, higher DNA damage was observed, whereas for 285 and 295 nm, higher oxidative stress (possibly due to reactive oxygen species [ROS] damage) was observed. ROS inactivation of E. coli was predicted to be more effective when keeping the ROS concentration low but allowing longer exposure, for a given UV fluence.

Ultra-short pulsed laser powder bed fusion of Al-Si alloys: Impact of pulse duration and energy in comparison to continuous wave excitation

Abstract: Laser assisted powder bed fusion of Al-40 wt%Si using ultra-short laser pulses is presented. The effects of the pulse duration as well as the pulse energy on the melt pool size, solidification morphology and material properties have been investigated. The experiments have been carried out with a mode-locked fiber laser system delivering pulses from 500 fs up to 800 ps at a wavelength of 1030 nm. Comparative investigations have been performed using a continuous wave Yb-fiber laser operating at 1070 nm. The results show that the melt pool width is reduced at shorter pulse durations and higher repetition rates while maintaining the same average power. Additionally, keyhole melting is achieved in pulsed operation after exceeding the threshold fluence for single pulse ablation. In comparison to continuous wave radiation, powder bed fusion using ultra-short laser pulses leads to a more uniform melt pool shape with refined primary Si and eutectic structure that is accompanied with an improvement of the mechanical properties.

The impact of fluence dependent 120 MeV Ag swift heavy ion irradiation on the changes in structural, electronic, and optical properties of AgInSe2 nano-crystalline thin films for optoelectronic applications

Abstract: Swift heavy ion (SHI) irradiation in thin films significantly modifies the structure and related properties in a controlled manner. In the present study, the 120 MeV Ag ion irradiation on AgInSe2 nanoparticle thin films prepared by the thermal evaporation method and the induced modifications in the structure and other properties are being discussed. The ion irradiation led to the suppression of GIXRD and Raman peaks with increasing ion fluence, which indicated amorphization of the AgInSe2 structure along the path of 120 MeV Ag ions. The Poisson's fitting of the ion fluence dependence of the normalized area under the GIXRD peak of AgInSe2 gave the radius of the ion track as 5.8 nm. Microstructural analysis using FESEM revealed a broad bi-modal distribution of particles with mean particle sizes of 67.5 nm and 159 nm in the pristine film. The ion irradiation led to the development of uniform particles on the film surface with a mean size of 36 nm at high ion fluences. The composition of the film was checked by the energy dispersive X-ray fluorescence (EDXRF) spectrometer. The UV-visible spectroscopy revealed the increase of the electronic bandgap of AgInSe2 films with an increase in ion fluence due to quantum confinement. The Hall measurement and EDXRF studies showed that the unirradiated and irradiated AgInSe2 films have n-type conductivity and vary with the ion fluence. The changes in the films were tuned with different ion fluence and are favorable for both optical and electronic applications.

Mechanisms of femtosecond laser ablation of Ni3Al: Molecular dynamics study

Abstract: Detailed knowledge of the physical essence of femtosecond laser and Ni3Al interactions is of great significance for the effective processing of film-cooling holes in Ni3Al-based single crystal superalloy turbine blades. Although attempts have been made to understand the femtosecond laser processing of Ni3Al-based single crystal superalloys via experiments or mesoscopic simulation, several phenomena and material removal mechanisms at the atomic level remain obscure. The information obtained in previous studies cannot be directly related to the femtosecond laser processing of Ni3Al-based single crystal superalloys due to the differing intrinsic properties of materials. In this paper, using TTM-MD, three regimes of material response to femtosecond laser irradiation are identified in simulations with a single pulsed 500 fs laser: melting, photomechanical spallation and phase explosion. Several key physical parameters related to the thermodynamic and dynamic behaviour of Ni3Al are obtained at the atomic level, including the fluence threshold, crystal lattice thermal stability and thermodynamic critical temperature. At low fluence, the material behaviour is characterized by heterogeneous surface melting and homogeneous subsurface melting. Photomechanical spallation occurs at higher fluence, which can be interpreted as void nucleation and growth, microcrack formation and eventual large liquid layer ejection. Under extremely high laser fluence, the material behaviour follows the phase explosion mechanism, forming an ablation plume with a layered structure. The dominant driving force change under different fluences causes the mechanism change. Additionally, an integral visual picture is successfully created based on the key parameters obtained, through which the mechanisms throughout the laser spot are revealed.

A computational study of heat transfer and material removal in picosecond laser micro-grooving of copper

Abstract: A 2D two-temperature model (TTM) is developed for picosecond laser micro-grooving of copper (Cu) and discretized using the finite difference scheme, to explore the temperature evolution and groove formation process. For lattice and electron subsystems, as well as the coupling strength between them, comprehensive temperature-dependent properties are employed and updated in real time to make the calculation more accurate. The model verification is then conducted and a reasonable good agreement has been found between predicted and measured groove depths. Using the verified model, computational studies of temperature field evolution are carried out in different fluence levels. For low fluence of around 0.283 J/cm2, phase change can be neglected, and the temperature difference between two subsystems decreases with an increase in pulse width and can be neglected when the pulse width is over 200 ps. In contrast, for high fluence of about 10.61 J/cm2, the maximum lattice temperature increases rapidly to boiling point within 3 ps, and the peak electron temperature is around 10 times higher, leading to material removal via evaporation. Under this high fluence level, groove depth around 0.25 μm is obtained within a 12 ps single pulse irradiation. In addition, thermal diffusion occurs from the residual hot zone adjacent to groove edge towards surrounding area during the off-pulse period, as the heat affected zone depth is up to 1 μm after 500 ps and over 4 μm after 6 ns, and the peak residual lattice temperature drops to about 396 K from boiling point within 53 ns. Moreover, the groove depth and groove profile show a periodically evolving manner with respect to scanning times, and for each scanning, a slow-fast-slow pattern has been observed in groove depth increase and profile development.

Influence of fractal and multifractal morphology on the wettability and reflectivity of crystalline-Si thin film surfaces as photon absorber layers for solar cell

Abstract: Crystalline Si films incorporated with Al are important for applications in microelectronics and solar cells. In this paper, we report on the morphology of crystalline Si surfaces in Al/amorphous-Si bilayer thin films under ion beam irradiation at 100 °C. Micro-Raman and transmission electron microscopy studies show that best crystallization is achieved at a fluence of 1 × 1012 ions cm−2. The contact angle of Si surfaces (after chemically etched unreacted Al), referred to as absorber surfaces, decreases with increasing ion fluence. These surfaces are hydrophobic in nature and the hydrophobicity decreases with increasing ion fluence. Fractal and multifractal analysis of atomic force microscopy images, along with system energy/unit cell and Laplace pressure calculations, supports our observations. Moreover, the calculated multiple scattering cross sections of light, along with reflectivity measurements, indicate that absorber surfaces of best crystalline films have the lowest reflectivity. The present results suggest that such surfaces having low optical reflectance and a hydrophobic nature can be used as photon absorber layers for advanced solar cell devices.

Assessment of the Irradiation Exposure of PET Film with Swift Heavy Ions Using the Interference-Free Transmission UV-Vis Transmission Spectra.

Abstract: This paper presents the results of a study of polyethylene terephthalate (PET) films irradiated with Ar and Kr ions at both normal orientation and an angle of 40° to the normal. Normal irradiation was performed using Ar8+ and Kr15+ ions with an energy of 1.75 MeV/au and fluences in the range (2–500) × 1010 cm−2 for Ar8+ ions and (1.6 − 6.5) × 1010 cm−2 for Kr15+ ions. Kr ions with an energy of 1.2 MeV/au and charges of 13+, 14+, and 15+ were used for angled irradiation. For each Kr ion charge value, three fluence values were used: 5 × 1010, 1 × 1011, and 2.5 × 1011 cm−2. It is well known that irradiation of PET films by swift heavy ions results in a red shift of the UV-vis transmission spectra absorption edge. The experimental transmission spectra exhibit well-defined interference fringes, which obscure the underlying transmission response. Using an existing technique to obtain interference-free transmission curves Tα(λ) for both pristine and irradiated PET film samples, we found that S, the total radiation-induced absorption of light by the PET film, is proportional to the logarithm of the fluence F. In addition to this dependence on the irradiating fluence, we also found that the charge of the irradiating ion has a significant influence on the position of the absorption edge in the UV-vis spectra. This provides experimentally independent evidence to confirm our previous results showing that ion charge has an effect on the post-irradiation state of PET films. We present a physical interpretation of the observed absorption edge red shift in irradiated PET films as being due to the growth of extended conjugated systems via the formation of intermolecular helical structures. Our investigations into the stability of irradiation-induced effects in PET films show that comparison of UV-vis transmission spectra before and after annealing can provide information about the structure of deep traps in PET.

Synthesis of MOF-5 nanostructures by laser ablation method in liquid and evaluation of its properties

Abstract: Capability of pulsed laser ablation (PLA) in liquid environment as a physical bottom-up method to synthesis metal–organic framework MOF-5 has been investigated experimentally for the first time. In this experiment, a high-purity zinc (Zn) target was irradiated by the fundamental wavelength of a pulsed Nd:YAG laser in a solution of dimethylformamide (DMF) containing terephthalic acid. A variety of diagnostics were employed to investigate the properties of MOF-5 nanostructures and to study the effects of laser fluence and concentration of acid on their characteristics. The porous and rod structure of synthesized MOF-5s observed in SEM images and their cubic shape observed in TEM microimages in good agreement with XRD and FTIR spectra confirm the production of MOF-5 nanostructures by PLA method. Results show that size of synthesized MOF-5 nanostructures was decreased with increasing the laser fluence. Furthermore, an amount of produced nanostructures was increased with increasing the ligand concentration in the liquid environment of ablation and laser fluence.

Thermal damage free material processing using femtosecond laser pulses for fabricating fine metal masks: Influences of laser fluence and pulse repetition rate on processing quality

Abstract: We demonstrate thermal damage free processing by using femtosecond (fs) laser pulses on thin 64FeNi alloy (Invar) sheets for making fine metal masks (FMMs). To achieve high throughput as an essential prerequisite for the industrial application, high-fluence pulses can be exploited to increase ablation rates. However, the processing quality can be significantly degraded due to thermal damage induced by the high-effective-penetration-depth. To avoid the detrimental effect on the sample, a low fluence is an advantageous condition to realize cold processing. Meanwhile, the low fluence causes low productivity, and which can be compensated by applying high-repetitive pulses. But the accumulative heat on the surface induced by megahertz-repetition-rates should be suppressed. To investigate the influence of the repetition rates on the heat accumulation, the surfaces irradiated by the pulse trains were inspected by the scanning electron microscopy (SEM). A numerical estimation for the heat accumulation was also conducted by a simple toy model, and the calculation well coincided with the experimental observations.

In-flow optical characterization of flame-generated carbon nanoparticles sampled from a premixed flame.

Abstract: In this work, the optical absorption properties of carbon nanoparticles are investigated by applying in-flow extinction and laser-induced incandescence measurements. Carbon nanoparticles are produced in an ethylene/air premixed flame and sampled at different heights above the burner. From extinction measurements, the absorption coefficient is obtained in a wide spectral range, considering the negligible scattering under our experimental conditions. With the application of Tauc plot the optical band gap is evaluated at the sampling heights under analysis. The increase of this value with the decrease in the height is consistent with the quantum confinement effect detected in the inception region of the flame. Two-color laser induced incandescence measurements are performed at relatively high laser fluence. The fluence curves, given by the particle temperature under laser irradiation versus laser fluence, are also obtained. A significant difference in the optical properties of these particles is observed by changing the sampling height. Moreover, considering the fluence curve in the low laser fluence regime, the refractive index absorption function E(m) is evaluated at an excitation wavelength of 1064 nm. Finally, the knowledge of the behavior of the absorption coefficient in a wide spectral range allows retrieving the values and the behavior of E(m) as a function of wavelength.