(189w) Methods for Analysis of Laser Cutting to Assess Patterned Feature Quality in CNT Sheets

Carbon nanotube (CNT) sheets are well known for their excellent electrical properties which are leveraged in in applications such as electrodes and electromagnetic shielding. Furthermore, their mechanical and structural properties extends their utility even further to areas such as such as strain sensors, water purification, and filtration. Many of these applications require CNT sheets to be precisely shaped or patterned to function effectively and thus it is of significant importance to develop methods to efficiently, accurately, and reliably shape these materials. Laser patterning is a particularly promising direct-cutting method for patterning CNT sheets that has speed and design creation and adjustability advantages over photolithographic and reactive chemical etching methods and precision, reproducibility, and design scale advantages over other direct cutting methods. For this reason, laser patterning has become a widely explored technique for fabricating CNT sheets with complex features. These studies have used a variety of different laser types and systems including CO₂, diode-pumped solid state, excimer, femtosecond, and fiber lasers, across many different formulations of CNT materials. The next step to building upon this body of work is to establish standardized methods of laser comparison that can translate across different laser systems and CNT materials in order to enhance the practical value of CNT sheet laser patterning research across different systems and materials.

To support this goal, a detailed laser cutting quality comparison between two distinct laser systems, a CO₂ laser system (CO₂-LS) and a fiber laser system (Fiber-LS), was demonstrated on commercially available carbon nanotube (CNT) sheet material. This analysis was geared towards a thorough analysis of the edge of the sheet where the laser cut through, hereafter referred to as the cut edge, as well as the area surrounding it and is particularly relevant to the laser cutting of line, circle, and corner features and larger scale mesh structures. Laser cutting parameters for the comparison were first determined by generating a laser cutting parameter optimization matrix with each laser system. Power and speed combinations were varied across a matrix pattern laser cut into CNT sheets and an optimized combination was identified. Optical microscope imaging of laser patterned features was performed along with image analysis methods including axial and radial dimensional accuracy and feature pitch limitation analysis to assess each laser’s cutting accuracy and precision, cut edge variability, and spacing of microscale features. The Fiber-LS was shown to produce visually smoother features, while the CO₂-LS produced features with an undesirable wave-like pattern in the cut edge. The fiber laser also exhibited an eccentricity value max 1.5 times lower and an average 2.4 times lower (for circular feature diameters below 700 µm), an average difference from design diameter 3 times lower and a range of variability 2.8 times lower, and the ability to cut both circular and line features at a slightly reduced pitch, 50 µm shorter than the CO₂-LS. For both laser systems, SEM was used to obtain a detailed small-scale characterization of laser patterned CNT material behavior such as cut edge width, curvature, steepness, and corner feature sharpness. The features produced by the Fiber-LS were observed to have a cut edge with shorter width and a higher steepness and higher corner sharpness compared to that of the CO₂-LS. Additionally, two features unique to CNT sheets and previously unreported were imaged which included a full cut edge cross sectional profile of the laser cut CNT sheet material and a metal particle band phenomenon that appeared near the cut edge after laser cutting. Finally, Raman D/G mapping and analysis were used to assess the damage the lasers caused to the CNT materials. The Fiber-LS average D/G returned to control CNT material levels about 100 µm from the cut edge and exhibited 80% less distance of CNT damage from the cut edge when compare to the CO₂-LS. Definitively, the fiber laser surpassed the CO₂-LS in laser cutting quality when working with CNT materials.

This work supports the CNT sheet laser patterning field though several contributions of note. Examples of systematic characterization methods for assessing cut quality, especially methods for measuring the accuracy and precision of laser cut features at the microscale which have not been widely reported in the literature. All methods are sufficiently detailed and compatible with different CNT sheet materials and laser systems such that they are readily transferable to other studies. Furthermore, the laser system comparison between the CO₂ and Fiber laser systems presents practical ways for different laser types and systems to be compared in order to determine the optimal laser choice for an application while simultaneously establishing that the Fiber-LS produced features with superior cut quality to those produced by the CO₂-LS. Ultimately, this work aims to enrich the field of CNT sheet laser patterning by presenting a systematic approach for evaluating cut quality across different laser systems.