The Spontaneous Escape Behavior of Silver from Graphite-like Carbon Coatings and Its Effect on Corrosion Resistance.

Silver-doped graphite-like carbon (Ag-GLC) coatings were prepared on the surface of aluminum alloy and single-crystal silicon by magnetron sputtering under different deposition parameters. The effects of silver target current and deposition temperature, as well as of the addition of CH4 gas flow, on the spontaneous escape behavior of silver from the GLC coatings were investigated. Furthermore, the corrosion resistance of the Ag-GLC coatings were evaluated. The results showed that the spontaneous escape phenomenon of silver could take place at the GLC coating, regardless of preparation condition. These three preparation factors all had an influence on the size, number and distribution of the escaped silver particles. However, in contrast with the silver target current and the addition of CH4 gas flow, only the change in deposition temperature had a significant positive effect on the corrosion resistance of the Ag-GLC coatings. The Ag-GLC coating showed the best corrosion resistance when the deposition temperature was 500 °C, which was due to the fact that increasing the deposition temperature effectively reduced the number of silver particles escaping from the Ag-GLC coating.

Use of Green Fs Lasers to Generate a Superhydrophobic Behavior in the Surface of Wind Turbine Blades.

Ice generation on the surface of wind generator blades can affect the performance of the generator in several aspects. It can deteriorate sensor performance, reduce efficiency, and cause mechanical failures. One of the alternatives to minimize these effects is to include passive solutions based on the modification of the blade surfaces, and in particular to generate superhydrophobic behavior. Ultra-short laser systems enable improved micromachining of polymer surfaces by reducing the heat affected zone (HAZ) and improving the quality of the final surface topography. In this study, a green fs laser is used to micromachine different patterns on the surface of materials with the same structure that can be found in turbine blades. Convenient optimization of surface topography via fs laser micromachining enables the transformation of an initially hydrophilic surface into a superhydrophobic one. Thus, an initial surface finish with a contact angle ca. 69° is transformed via laser treatment into one with contact angle values above 170°. In addition, it is observed that the performance of the surface is maintained or even improved with time. These results open the possibility of using lasers to control turbine blade surface microstructure while avoiding the use of additional chemical coatings. This can be used as a complementary passive treatment to avoid ice formation in these large structures.

Highly Regular Hexagonally-Arranged Nanostructures on Ni-W Alloy Tapes upon Irradiation with Ultrashort UV Laser Pulses.

Nickel tungsten alloy tapes (Ni-5 at% W, 10 mm wide, 80 µm thick, biaxially textured) used in second-generation high temperature superconductor (2G-HTS) technology were laser-processed in air with ultraviolet ps-laser pulses (355 nm wavelength, 300 ps pulse duration, 250-800 kHz pulse repetition frequency). By employing optimized surface scan-processing strategies, various laser-generated periodic surface structures were generated on the tapes. Particularly, distinct surface microstructures and nanostructures were formed. These included sub-wavelength-sized highly-regular hexagonally-arranged nano-protrusions, wavelength-sized line-grating-like laser-induced periodic surface structures (LIPSS, ripples), and larger irregular pyramidal microstructures. The induced surface morphology was characterized in depth by electron-based techniques, including scanning electron microscopy (SEM), electron back scatter diffraction (EBSD), cross-sectional transmission electron microscopy (STEM/TEM) and energy dispersive X-ray spectrometry (EDS). The in-depth EBSD crystallographic analyses indicated a significant impact of the material initial grain orientation on the type of surface nanostructure and microstructure formed upon laser irradiation. Special emphasis was laid on high-resolution material analysis of the hexagonally-arranged nano-protrusions. Their formation mechanism is discussed on the basis of the interplay between electromagnetic scattering effects followed by hydrodynamic matter re-organization after the laser exposure. The temperature stability of the hexagonally-arranged nano-protrusion was explored in post-irradiation thermal annealing experiments, in order to qualify their suitability in 2G-HTS fabrication technology with initial steps deposition temperatures in the range of 773-873 K.

Improved Copper-Epoxy Adhesion by Laser Micro- and Nano-Structuring of Copper Surface for Thermal Applications.

The objective of this work is the enhancement of metal-to-metal bonding to provide high thermal conductivity together with electrical insulation, to be used as heat sinks at room and cryogenic temperatures. High thermal conductive metal (copper) and epoxy resin (Stycast 2850FT) were used in this study, with the latter also providing the required electrical insulation. The copper surface was irradiated with laser to induce micro- and nano-patterned structures that result in an improvement of the adhesion between the epoxy and the copper. Thus, copper-to-copper bonding strength was characterized by means of mechanical tensile shear tests. The effect of the laser processing on the thermal conductivity properties of the Cu/epoxy/Cu joint at different temperatures, from 10 to 300 K, is also reported. Using adequate laser parameters, it is possible to obtain high bonding strength values limited by cohesive epoxy fracture, together with good thermal conductivity at ambient and cryogenic temperatures.

Surface Superconductivity Changes of Niobium Sheets by Femtosecond Laser-Induced Periodic Nanostructures.

Irradiation with ultra-short (femtosecond) laser beams enables the generation of sub-wavelength laser-induced periodic surface structures (LIPSS) over large areas with controlled spatial periodicity, orientation, and depths affecting only a material layer on the sub-micrometer scale. This study reports on how fs-laser irradiation of commercially available Nb foil samples affects their superconducting behavior. DC magnetization and AC susceptibility measurements at cryogenic temperatures and with magnetic fields of different amplitude and orientation are thus analyzed and reported. This study pays special attention to the surface superconducting layer that persists above the upper critical magnetic field strength Hc2, and disappears at a higher nucleation field strength Hc3. Characteristic changes were distinguished between the surface properties of the laser-irradiated samples, as compared to the corresponding reference samples (non-irradiated). Clear correlations have been observed between the surface nanostructures and the nucleation field Hc3, which depends on the relative orientation of the magnetic field and the surface patterns developed by the laser irradiation.

Laser-induced transient skin disruption to enhance cutaneous drug delivery.

The use of pressure waves (PW) to disrupt the stratum corneum (SC) temporarily is an effective strategy to increase the deposition of drug molecules into the skin. However, given the rather modest outcomes when compared with ablation-assisted drug delivery, its potential has been underestimated. Accordingly, the aim of this study was to examine the impact of Resonant Amplitude Waves (RAWs) on increasing cutaneous delivery. RAW phenomena are triggered by focusing a high-peak-power pulsed laser onto an appropriate transducer structure, under space- and time-controlled resolution. In order to determine the optimal conditions for the generation and use of RAWs, a screening of laser parameters setting and an analysis of different geometries of the impact pattern over diverse materials used as transducers was performed, analyzing the footprint of the RAW waves in an agarose gel. The results obtained were then checked and fine-tuned using human skin samples instead of agarose. Furthermore, ex vivo experiments were carried out to characterize the effect of the RAWs in the cutaneous delivery of diclofenac (DIC) and lidocaine (LID) administered in the form of gels. The application of RAWs resulted in an increased delivery of DIC and LID to the skin, whose intensity was dependent on the composition of the formulation. In fact, the maximum observed for DIC and LID in short-time experiments (39.1 ± 11.1 and 153 ± 16 µg/cm2, respectively) was comparable to those observed using ablation-assisted drug delivery under the same conditions. In conclusion, the combination of RAWs with specific formulation strategies is a feasible alternative for the cutaneous delivery of drug candidates when short onset of action is required.

Hydrophobicity, Freezing Delay, and Morphology of Laser-Treated Aluminum Surfaces.

Until recently, superhydrophobicity was considered as a hint to predict surface icephobicity, an association of concepts that is by no means universal and that has been proven to depend on different experimental factors and material properties, including the actual morphology and chemical state of surfaces. This work presents a systematic study of the wetting and freezing properties of aluminum Al6061, a common material widely used in aviation, after being subjected to nanosecond pulsed IR laser treatments to modify its surface roughness and morphology. All treated samples, independent of their surface finishing state, presented initially an unstable hydrophilic wetting behavior that naturally evolved with time to reach hydrophobicity or even superhydrophobicity. To stabilize the surface state and to bestow the samples with a permanent and stable hydrophobic character, laser-treated surfaces were covered with a thin layer of CF x prepared by plasma-enhanced chemical vapor deposition. A systematic comparison between freezing delay (FD) and wetting properties of water droplets onto these plasma-/polymer-modified laser-treated surfaces that, under conditions where a heterogeneous nucleation mechanism prevails, surface morphology rather than the actual value of the surface roughness parameter the key feature for long FD times. In particular, it is found that surface morphologies rendering a Cassie-Baxter wetting regime longer FDs than those characterized by a Wenzel-like wetting state. It is that laser treatment, with or without additional coverage with thin CF x coatings, affects wetting and ice formation behaviors and might be an efficient procedure to mitigate icing problems on metal surfaces.

'Laser chemistry' synthesis, physicochemical properties, and chemical processing of nanostructured carbon foams.

Laser ablation of selected coordination complexes can lead to the production of metal-carbon hybrid materials, whose composition and structure can be tailored by suitably choosing the chemical composition of the irradiated targets. This 'laser chemistry' approach, initially applied by our group to the synthesis of P-containing nanostructured carbon foams (NCFs) from triphenylphosphine-based Au and Cu compounds, is broadened in this study to the production of other metal-NCFs and P-free NCFs. Thus, our results show that P-free coordination compounds and commercial organic precursors can act as efficient carbon source for the growth of NCFs. Physicochemical characterization reveals that NCFs are low-density mesoporous materials with relatively low specific surface areas and thermally stable in air up to around 600°C. Moreover, NCFs disperse well in a variety of solvents and can be successfully chemically processed to enable their handling and provide NCF-containing biocomposite fibers by a wet-chemical spinning process. These promising results may open new and interesting avenues toward the use of NCFs for technological applications.

Laser control of zeolite nucleation.

Precursor solutions for the synthesis of zeolites are irradiated by means of a Nd-YAG laser. These solutions are subsequently submitted to a hydrothermal treatment and the results analyzed by X-ray diffraction and electron microscopy. Laser irradiation promotes the formation of silica nanoparticles that nucleate into zeolite (silicalite-1), following a hydrothermal treatment. The average crystal size (in the 0.6-3.6 µm range) of the zeolite exponentially decreases as a function of laser irradiation time. In addition, a longer irradiation time results in a narrower crystal size distribution.