Liquid-assisted laser ablation has the advantage of relieving thermal effects of common laser ablation processes, whereas the light scattering and shielding effects by laser-induced cavitation bubbles, suspended debris, and turbulent liquid flow generally deteriorate laser beam transmission stability, leading to low energy efficiency and poor surface quality. Here, we report that a continuous and directional high-speed microjet will form in the laser ablation zone if laser-induced primary cavitation bubbles asymmetrically collapse sequentially near the air-liquid interface under a critical thin liquid layer. The laser-induced microjet can instantaneously and directionally remove secondary bubbles and ablation debris around the laser ablation region, and thus a very stable material removal process can be obtained. The shadowgraphs of high-speed camera reveal that the average speed of laser-induced continuous microjet can be as high as 1.1 m/s in its initial 500 μm displacement. The coupling effect of laser ablation, mechanical impact along with the collapse of cavitation bubbles and flushing of high-speed microjet helps achieve a high material removal rate and significantly improved surface quality. We name this uncovered liquid-assisted laser ablation process as laser-induced microjet-assisted ablation (LIMJAA) based on its unique characteristics. High-quality microgrooves with a large depth-to-width ratio of 5.2 are obtained by LIMJAA with a single-pass laser scanning process in our experiments. LIMJAA is capable of machining various types of difficult-to-process materials with high-quality arrays of micro-channels, square and circle microscale through-holes. The results and disclosed mechanisms in our work provide a deep understanding of the role of laser-induced microjet in improving the processing quality of liquid-assisted laser micromachining.

No abstract

No abstract

No abstract

Nano‐ZnO powder was synthesized by the plasma vapor‐phase reaction (PVPR) process. Most of the ZnO particles obtained are rod‐like, with diameters of between 20 and 100 nm. ZnO‐based varistors were fabricated by using the synthesized nano‐ZnO powder and commercial French process submicrometer ZnO powder, respectively. The sintering activity of the samples prepared with the nano‐ZnO powder is higher, but there is only a little difference between the mean grain sizes of the samples sintered at the same temperature. The current–voltage (I–V) characteristics in the low‐current region are similar. However, when subjected to surge currents, the residual voltage ratio of the samples prepared using the PVPR ZnO powder is much lower. Comparative analysis of the dielectric property and the grain‐boundary barrier height reveals that the donor concentration of the ZnO crystal grains in the sample prepared using the PVPR ZnO powder is higher than that prepared using French process ZnO powder. A higher donor concentration gives rise to a higher grain conductivity, a higher nonlinear coefficient in the high‐current region and a better protection level in applications.