Microstructure Investigation and Cleaning of Chirped-FLSP Copper

Authors

Mark Anderson, University of Nebraska - LincolnFollow

First Advisor

Dr. Jeffery Shield

Date of this Version

12-2019

Comments

A THESIS Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Master of Science, Major: Mechanical Engineering and Applied Mechanics, Under the Supervision of Professor Jeffrey E. Shield. Lincoln Nebraska: October, 2019

Copyright 2019 Mark Anderson

Abstract

With the advancement of technology, comes an increased demand to find materials that can provide high heat transfer and large critical heat flux surfaces in applications like electronics. Research has shown that by using techniques that add micro/nanostructures to the surface the transfer of heat and the critical heat flux can be increased in two-phase heat transfer. A common technique to do this is Femtosecond Laser Surface Processing (FLSP). The surfaces after FLSP increase the active surface area, surface roughness, and result in superhydrophilic wicking surfaces benefiting the heat transfer performance; however, when FLSP of copper is performed heat transfer characteristics are decreased. Not only do the heat transfer characteristics decrease, but the ability to consistently produce FLSP copper samples has proven difficult. To control the FLSP of copper, the investigation of microstructure is necessary to provide insight on how the copper surfaces are changed due to the laser.

In the present study, copper surfaces were processed using the chirping capabilities of FLSP. An investigation of the surface structure and subsurface that formed at various pulse counts for chirped-laser processing of copper substrates was conducted. After a specific pulse count was reached, a layered microstructure formed with alternating layers of Cu and Cu₂O accompanied by an outer layer of CuO. The formation of these oxides reduce heat transfer properties. This study then investigated the removal of copper oxides using citric acid. The chirping of the FLSP setup generated consistent copper samples that increased the heat transfer coefficients at large superheats as compared to polished samples. The removal of surface and subsurface oxides caused in chirped FLSP samples produce pool boiling results that increased heat transfer coefficients compared to their counterparts over a large range of superheats.

Adviser: Jeffrey E. Shield