Laser drilling of high-aspect-ratio holes in biochar

Nanosecond pulses at 532 nm from a Nd:YAG laser were used to examine the feasibility of machining grids of high-aspect-ratio holes in the biochar layers formed on partially burned organic samples. To examine the effects of pulse energy and frequency on machining quality, pulse energies ranging from 0.1 to 3.5 mJ were used to drill holes approximately 50 µm in diameter at pulse frequencies of 1, 5, and 10 Hz. Next, three distinct scanning schemes were used to create 10x10 grids of holes of varying hole spacings to optimize the packing density of the holes without removing the surface char. Post-processing examinations of all samples were conducted using optical and scanning electron microscopes to observe the quality of the machined holes. Our initial results show that laser incising can be used effectively on the biochar layer, however, the depth and feature size achievable are limited by the difference in ablation thresholds of the charred layer and the underlying organic material. We were able to consistently machine grids of 50 μm diameter holes 150 μm apart up to the depth of the biochar layer but we had to roughly double the hole spacing to extend the holes past the biochar layer due to the increased heat affected zone that occurs at the pulse energies needed to ablate the underlying material.