Graduate Studies
First Advisor
Craig Zuhlke
Date of this Version
5-2024
Document Type
Article
Citation
A thesis presented to the faculty of the Graduate College at the University of Nebraska in partial fulfillment requirements for the degree of Master of Science
Major: Electrical Engineering
Under the supervision of Professor Craig A. Zuhlke
Lincoln Nebraska, May 2024
Abstract
This thesis investigates the challenges and advancements in femtosecond laser surface processing (FLSP) to facilitate the creation of antimicrobial surfaces for condensing heat exchangers. Through an exploration of dual-pulse FLSP on silver, the research showcases successful microstructure creation at lower pulse energies, reaching as low as 2 mJ. However, limitations arise due to the interplay between required fluence and the minimum spot size necessary for self-organization, establishing a threshold for pulse energy. To address scalability concerns, a higher average power laser is tested, employing a Light Conversion Pharos operating at 20 W with a 10 kHz repetition rate and "BiBurst" mode. This mode allows for pulse trains with 200 ps interpulse spacing, streamlining the process of applying dual-pulse FLSP to silver by excluding the need for an interferometer setup. Experimental trials demonstrate a significant 5.93x increase in processing rate compared to previous methods utilizing a Coherent Astrella femtosecond laser. To further enhance throughput for large-area applications, the use of a galvo scanner for dual-pulse FLSP is explored. The study examines the difference between rastering using a galvo to translate the laser beam versus mechanical stages for translating the sample, observing a statistically significant decrease in surface-area to area ratio with beam translation while maintaining nearly equal surface roughness. This discrepancy stems from variations in structure development, particularly in the non-uniform formation of mounds, requiring further investigation to elucidate the differences between beam and sample translation. Additionally, the application of FLSP on electroplated materials is studied for the first time, specifically silver-plated steel, in collaboration with NASA's efforts to develop antimicrobial solutions for next generation condensing heat exchangers (CHXs). The successful processing of a full-scale electroplated CHX packet to create micro/nano-textured surfaces that have been shown in previous studies to have antimicrobial properties marks a significant milestone, showcasing the potential of FLSP to be a solution for antimicrobial surfaces for full-scale CHXs. This comprehensive study not only contributes to the understanding of FLSP methodologies but also brings the LPCHX one step closer to mission readiness.
Advisor: Craig A. Zuhlke
Comments
Copyright 2024, Garrett Beard. Used by permission