Simulation of phase explosion in the nanosecond laser ablation of aluminum

Wetting State Transition of Laser Direct Writing Aluminum Surface Based on Coupling Effect of Micro/Nanoscale Characteristics

Micro/nanostructured metal surfaces fabricated by laser direct writing (LDW) have been widely used in wettability-related fields. Previous studies focused on the effects of surface structural patterns or chemical composition on wettability, while the coupling mechanism and respective contributions of the two are not distinct. This paper reveals the coupling effect of micro/nanoscale characteristics on the wettability of LDW aluminum surfaces and elucidates the transition mechanism between wetting states on the surfaces with linear laser energy density. Through the contact angle experiments, a wetting state transition of the LDW surface is found from a more hydrophilic than pristine rose petal effect to lotus effect. Based on the bionic analysis method of the superhydrophobicity factors of lotus leaves, the contributions to the wettability of LDW surfaces are divided into the micro/nanoscale characteristics. The theoretical model for identifying the wetting state of a rough surface is proposed. Based on this model, the average Young's contact angle, θ̅Y, is calculated, which indicates the contribution of the nanoscale characteristics. During the transition process from rose petal effect to lotus effect, θ̅Y > 90° is a necessary condition for detachment from the rose petal effect, which is contributed by the high specific surface organic adsorption at the nanoscale. What is more, the wetting state determined by the microscale characteristics further enhances its hydrophobicity, leading to the lotus effect. Based on the wetting state identification model and the Cassie-Baxter equation, the change of micro/nanoscale characteristics on aluminum surfaces after LDW treatment is presented, and the influence of micro/nanoscale characteristics on the wetting state is decoupled and quantified. This research helps to coordinate the effects of surface structure and chemical composition on wettability in the design of specific wettability functional surfaces and can also be applied to other high heat density surface processing fields.

Data-Driven Predetermination of Cu Oxidation State in Copper Nanoparticles: Application to the Synthesis by Laser Ablation in Liquid

Copper-based nanocrystals are reference nanomaterials for integration into emerging green technologies, with laser ablation in liquid (LAL) being a remarkable technique for their synthesis. However, the achievement of a specific type of nanocrystal, among the whole library of nanomaterials available using LAL, has been until now an empirical endeavor based on changing synthesis parameters and characterizing the products. Here, we started from the bibliographic analysis of LAL synthesis of Cu-based nanocrystals to identify the relevant physical and chemical features for the predetermination of copper oxidation state. First, single features and their combinations were screened by linear regression analysis, also using a genetic algorithm, to find the best correlation with experimental output and identify the equation giving the best prediction of the LAL results. Then, machine learning (ML) models were exploited to unravel cross-correlations between features that are hidden in the linear regression analysis. Although the LAL-generated Cu nanocrystals may be present in a range of oxidation states, from metallic copper to cuprous oxide (Cu2O) and cupric oxide (CuO), in addition to the formation of other materials such as Cu2S and CuCN, ML was able to guide the experiments toward the maximization of the compounds in the greatest demand for integration in sustainable processes. This approach is of general applicability to other nanomaterials and can help understand the origin of the chemical pathways of nanocrystals generated by LAL, providing a rational guideline for the conscious predetermination of laser-synthesis parameters toward the desired compounds.

Damage Characteristics of Aluminum-Coated Grating Irradiated by Nanosecond Pulsed Laser

An aluminum-coated grating (ACG) is a core component of laser systems and spectrometers. Understanding damage to the ACG induced by nanosecond lasers is critical for future high-power laser applications. In this study, we applied finite element simulation and practical experimentation to investigate the characteristics of ACG damage. Based on a coupling model using fluid heat transfer with the level-set method, we simulated the damage caused to an ACG by a 1064 nm nanosecond single pulse laser. The theoretical modeling showed that the ridge and bottom corners of the grid will be preferentially damaged, and the simulated damage threshold will range from 0.63 J/cm2 to 0.95 J/cm2. We performed a one-on-one damage test according to the ISO21254 standard to investigate the failure condition of 1800 l/mm ACGs; the laser-induced damage threshold (LIDT) was 0.63 J/cm2 (1064 nm, 6.5 ns). Microscopy images showed that the damaged area decreased with decreasing laser fluence, and scanning electron microscopy measurements showed that the main damage mechanism was thermodynamic damage, and that damage to the grid occurred first. The results of the experiments and simulations were in good agreement.

Suppression of spallation induced nanoparticles by high repetition rate femtosecond laser pulses: realization of precise laser material processing with high throughput

This paper reports a mechanism to suppress nanoparticle (NP) generation during femtosecond laser processing of 64FeNi alloy (Invar) to realize high precision fine metal masks. Nanoparticle redeposition during processing can reduce precision and ablation efficiency. Since Gaussian laser beams have spatially distributed fluence, NP types can vary even within a laser spot. Surface areas irradiated by the beam center with high peak fluence can be decomposed into vapor and liquid droplets by phase explosion; whereas positions irradiated by the beam edge, where fluence is close to ablation threshold, can be decomposed by stress confinement under the surface, known as spallation. Spallation characteristics were verified from target surfaces covered with exfoliation and fragments. It occurred above a certain number of pulses, indicating a significant incubation effect. Spallation induced NPs, i.e., agglomerated fragments, distort micro-hole size and shape, but were effectively suppressed by increasing repetition rate, due to increased surface temperature, i.e., heat accumulation. Suppression also occurred from direct sample heating using a hot plate. Thus, thermal energy can relax stress confinement and inhibit spallation induced NPs. Numerical simulation for heat accumulation also confirmed that suppression arises from thermal effects. Increasing repetition rate also helped to increase productivity.

Solar Concentration for Wastewaters Remediation: A Review of Materials and Technologies

As the effectiveness of conventional wastewater treatment processes is increasingly challenged by the growth of industrial activities, a demand for low-cost and low-impact treatments is emerging. A possible solution is represented by systems coupling solar concentration technology with advanced oxidation processes (AOP). In this paper, a review of solar concentration technologies for wastewater remediation is presented, with a focus on photocatalyst materials used in this specific research context. Recent results, though mostly on model systems, open promising perspectives for the use of concentrated sunlight as the energy source powering AOPs. We identify (i) the development of photocatalyst materials capable of efficiently working with sunlight, and (ii) the transition to real wastewater investigation as the most critical issues to be addressed by research in the field.