Date of Award

December 2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Pradeep K. Rohatgi

Committee Members

Michael Nosonovsky, Hugo Lopez, Ben Church, Chan S. Kim

Keywords

Copper Alloys, Self Cleaning, Superhydrophobicity, Superoleophobicity

Abstract

Thermal oxidation and chemical oxidation of C36000 and C84400 copper alloys in different oxidizing N2-O2 mixtures and in different concentrations of stearic acid (STA) and arachidic acid (ARA) generated surface morphologies that affected the wetting of the copper alloys. XRD and EDX analyses of the thermally oxidized surface layers of the C36000 copper alloys confirmed the formation of ZnO and PbO with the nanowires formed consisting of ZnO in terms of the chemical composition. Equally, the XRD analyses of the oxidized surface layer of the C84400 copper alloy at the different oxidizing temperatures (550 °C and 650 °C) showed that a mixture of ZnO, PbO and CuO were formed. In the thermally oxidized C36000 copper alloy, the growth of the ZnO nanaowires on the convoluted oxide layers sufficiently altered the contact angle behaviour of water as the surfaces on which the ZnO nanowires grew were anti-wetting to water. The calculated values of the fsl, for the surfaces of the thermally oxidized C36000 substrates with ZnO nanowires, ranged from 0.002 to 0.004. A stable Cassie-Baxter wetting state, with contact angle up 142 ° for water, was generated due to a combination of water trapped in the convoluted oxide layer and the low fsl. However, surfaces that showed high water contact angle, > 130 °, were oleophilic under water to oil drops. Superoleophobicity was achieved through a porosity driven mechanism that was dependent on the thickness of the oxide layer in the thermally oxidized samples of the C84400 copper alloys. The measured under water oil contact angles increased with the oxide layer thickness, as the porosity of the oxide layer increased due to the irregular packing of oxide grains. At a peak oxide layer thickness of 25.69 ± 9.14µm, oil contact angle of 154.4 ± 1.2 confirming underwater superoleophobicity was achieved. The oxide granule morphologies changed with the amount of oxygen in the oxidizing N2-O2 gas mixture used. In N2-0.75 wt.-% O2 gas mixture, the granules were larger and showed lateral growth. However, the kinetics of the thermal oxidation processes showed that the growth of the oxide layer followed the parabolic law. A combination of the thermal oxidation and reaction of the oxide formed with STA and ARA created superhydrophobic surfaces made of varying forms of nanostructures. Superhydrophobicity was achieved with the maximum water contact angles of 150.2 ° ± 1.5 ° and 153.8 ° ± 3.5 ° for the C36000 as well as 150.5 ° ± 1.5 ° and 152.9 ° ± 2.1 ° for the C84400 copper alloys that were processed through this method. However the hierarchical rough surface degraded and the superhydrophobicity of the surfaces cease after 18 hours of the process. FT-IR and XRD of the surfaces confirmed the oxides reacted to form the stearates and arachidates of Zn, Cu and Pb. Similar anti-wetting, superhydrophobic behaviour was achieved through the direct chemical oxidation process in different concentrations of STA and in 0.005M ARA at 30 °C and 40 °C process temperature. The nano-features formed at 40 °C were thicker than those at 30 °C. The XRD studies on the sample surfaces after 5 days of chemical oxidation in 0.005M STA confirmed the formation of zinc stearate and zinc arachidate superhydrophobic coatings.

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