The stability and durability of micro-nano structures is the key to influence the structural materials surface towards practical engineering applications. In this paper, the stability of micro-nano structures fabricated by femtosecond laser is systematically studied. It is found that the hierarchical micro-nano hybrid structures seriously affect absorbing properties and stability of the material surface due to the poor crystallinity. In order to enhance efficient optical absorption and stability, the annealing method is applied to further crystallization of the micro-nano structures. As a result, hierarchical micro-nano hybrid structures with large depth-to-width ratios are fabricated. The experimental results demonstrate that an average reflectance of 3.43% is realized in the waveband of 300-2400nm, and the reduction rate of the reflectance reaches 39.6%. Meanwhile, the water jet impact experiment verifies that the stability of the structure is further improved.
Machining micro-holes with high-power pulsed laser is very common, but due to the mechanism of laser processing, micro-cracks, recast layer and heat-affected zones in hole walls are difficult to avoid. Hybrid processing of laser beam machining with Electrochemical Machining (ECM) can effectively eliminate these defects and improve processing efficiency. This paper proposes to use a synchronized laser and electrochemical machining method. A metal tube electrode is used to guide and transmit the laser to the machining area to achieve high coaxial coupling of laser and electrochemical energy. An experimental system of laser beam machining with ECM is developed, and the surface quality of hole walls are studied on aluminum alloy and stainless-steel workpiece. The effects of ECM voltage and current on the rate of laser beam machining with ECM and the surface quality of hole walls (recast layer, micro-cracks, surface roughness, etc.) are analyzed. The experimental results show that the machining current increases with the increase of pulse voltage, the material removal rate increases, the micropore clearance increases, and the machining precision decreases. By adopting appropriate ECM parameters, the fast processing of micro-holes is realized, with smooth side wall, small taper, good entrance roundness, and the recast layer of the hole wall is fully removed.
Femtosecond laser processing has been extensively used in micromachining, especially for the precision processing of hard and brittle materials. However, the precision of the materials ablated by femtosecond laser is not easy to control. This paper reports an experimental and theoretical study on the ablation characteristics of fused silica using high repetition rate femtosecond laser. An experimental study of microchannels milling on the fused silica was carried out. The influence of pulse energy, repetition rate, scanning velocity, scanning times on the size and morphology of the microchannels was obtained. Simultaneously, the experimental data on the depth and width of microchannels under different parameter combinations were acquired through the orthogonal experiment. The prediction model of aspect ratio was obtained by BP neural network algorithm. Finally, the verification test was established and showed that the experimental results were consistent with the theoretical results. It would provide a theoretical basis for further study on the microchannels fabrication of femtosecond laser.
Reducing the reflection of silicon surface is an effective way to enhance its optical absorption performance in optical and optoelectronic devices. In this paper, the influence mechanism of heat accumulation effect existing in the material substrate on the multi-scale porosity properties of surface structure during femtosecond laser irradiation is investigated. Micro-nano structures will lose their multi-scale porous properties at high-repetition-rate laser irradiation due to excessive agglomeration, nucleation or melting. By rapidly cooling the material substrate, the porosity of surface micro-nano structure are optimized, and the antireflection performance of the material surface is improved obviously. Our study opens a novel and convenient route for preparation of broadband antireflective black silicon surfaces for various applications.
As direct band gap semiconductor, CIGS thin film solar cells has developed rapidly in recent years. Nano-ripples on transparent layer of CIGS solar cells can improve the light absorption. The photoelectric conversion efficiency improved and the thickness of solar cell decreased with the requirement of optical absorptivity. This study proposed a key technological point concerning the obtainment of perfect LIPSSs using picosecond laser with different beam pattern. Linear sweep in parallel to the laser polarization direction was performed using a Nd:VAN laser system with 10-ps Qswitched pulsed at a central wavelength of 532 nm with a repetition rate of 1 kHz. The nanostructure with different characteristics were obtained at different laser fluence and scanning speed which was focused with circular spot. The linear spot pattern can improve the processing efficiency greatly. In this way, three kind of morphologies were obtained successfully at the same parameter, which was useful for processing different kind of nanostructures. To our knowledge, it is the first time that we report the LIPSSs formed on CIGS absorber layers. Thus, the proposed technique can be considered to be a promising method for the laser machining of special nonmetal films.
Micro-structures offer superior functions such as superhydrophobicity, selfcleaning, anti-wear and drag reduction. In this paper, various microstructures were fabricated by rear-side picosecond laser irradiation of two-layer materials. The material of underlying layer was commonly commercial available ink; the material of surface layer was PMMA. The high light absorptivity of underlying material result in significantly reduced absorption depth. The laser source could therefore be regarded as plane heat source, leading to better surface morphologies after the mater-laser interaction. The results showed that convex structures were obtained at a lower laser fluence; with increase of laser fluence, a doughnut-like structures were obtained; with further rise of laser fluence, bowl-like structures would be obtained. Moreover, the size of microstructures could be tuned by adjusting laser processing parameters such as laser power, frequency and laser-mater interaction time. This method provides an insight for fabrication of functional surface.
This study was performed using picosecond pulses to obtain the three-dimensional micro-nano-hierarchical porous structures on the surface of titanium via the combination of the ablation and the deposition of ablated particles. For the repetition rate of 100 kHz and the scanning speed of 10 mm/s, there were secondary nano-tree-like micro-nano structures via the deposition of the ablated material formed on the primary microstructures. However, when the scan speed decreased, the primary microstructures were invisible owing to too much material deposition. When the repetition rate increased to 500 kHz, the ablated particles were irradiated by the posterior pulse before deposition onto the surface of material, and agglomerated into spidernet-like nanostructures. Then, the upper layer of the secondary micro-nano structure was molten and came into micro-nano porous fractal structures. Both the secondary micro-nano porous structures showed the hierarchy in the vertical and horizontal direction of the surface of titanium. Contact angle measurement after 3 months indicated that the nano-tree-like micro-nano structures showed super-hydrophilcity, and the spidernet-like nano and micro-nano porous fractal structures showed super-hydrophobicity.
This study presents the comparison of laser drilling of nickel based superalloy in air and vacuum environment. A 300W Nd:YAG laser was used to carry out the experimental study, pulse width and pulse peak power were studied of their influence on the hole quality and appearance, the experiments were respectively conducted in air and vacuum. It was found that the holes drilled in vacuum were much different to those drilled in air. Laser of shorter pulse width may drill the better holes in the air, but in the vacuum it was the pulse width around 1ms that produced holes of the best quality.
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