Experimental design for CO2 laser cutting of sub-millimeter features in very large-area carbon nanotube sheets

In this work, laser cutting methodologies have been demonstrated for defining submillimeter features in commercially available carbon nanotube (CNT) sheet materials (24 and 74-mu m thick) using a commercial CO2 laser source with a maximum power output of 30 W. Full factorial and central composite designs of experiments were used to investigate the effect of various factors, including the laser power (10-90% of max), speed (10-90% of max), and pulses per inch (100-1000 PPI), on the resulting kerf width and the kerf width variation for 5 mm long single straight-line features. Maximizing the pulses per inch yielded laser cuts with reduced variability, while the power and speed were determined to be the significant factors affecting the kerf width. Fabrication of more complex grid structures, consisting of 100 square openings (0.838 mm wide), revealed that the conditions used for cutting straight lines were insufficient for cutting grids. Thus, for a fixed number of pulses-per-inch, the ratio of the values of power and speed, P/S ratio, was determined to be most significant for affecting the feature quality of these grid structures, as it determines the heat input energy and the delivered energy density. It was determined that, for a fixed PPI of 1000, a percent power value 1.6 x higher than the percent speed value was needed as the input parameter to cut the 24 mu m thick CNT sheet, and a percent power value 3.5 x higher than the percent speed value was required to successfully cut the 74 mu m thick CNT sheet. Ultimately, refinement in the laser cutting conditions by increasing the energy delivered per pulse and using two passes to account for sheet non-uniformities, enabled patterning of large area (up to 280 mm x 280 mm) CNT sheets with close-packed circles. A structure of this size has over 48,000 openings, and analysis via optical microscopy determined the features have a measured diameter of 0.78 mm +/- 0.01 mm, exhibiting reproducibility over the entire CNT sample. Finally, multiple intricate geometries with variable feature spacing were fabricated all on a single CNT sample, demonstrating the utility of laser machining as an alternative technology solution to traditional CNT patterning techniques. These results provide a scalable methodology to determine the laser cutting parameters for use in fabricating very large-area CNT structures with intricate features.