Effect of current step-down on the growth and hardness of PEO coatings on Al6061 alloy

AbstractIn this study, plasma electrolytic oxidation (PEO) treatments were conducted on Al6061 plates using silicate containing alkaline electrolyte. The hardness variation with the growth of the PEO coating was investigated under two different regimes of applied AC current. The first was a continuous supply with a constant current density of 500mA/cm2 for 60 or 90min. The other was a current supply of 500mA/cm2 for 60min followed by a current step-down to 200mA/cm2 and continuous treatment for 30min. In the case of a constant current for 60min, the coating thickness and Vickers hardness increased to 150μm and 1300 Hv, respectively. With further processing in the same manner, the coating grew thicker while its hardness gradually decreased to 900 Hv. In the case of a current step-down, no significant thickness increase was observed, but there was a hardness increase up to 1800 Hv. Analysis of the PEO coatings by SEM/EDS and XRD revealed that all their function layers were comprised mainly of α-Al2O3 with hardness dependent on density. The current step-down method contributes to the densification of the function layer in a PEO coating, and thus increases the hardness

Black political leaders and black political participation: The role of perceptions

The purpose of this dissertation is to investigate the relationship between black political perceptions toward black political leaders and black political participation. The dissertation confirms that blacks have developed positive perceptions toward the role of black political leaders in the American political system in improving black conditions. The dissertation also finds that some factors of black political leadership have influenced black political participation when they decide to participate in election related activities (e.g., help campaign, talk to someone to persuade why they should vote for a certain candidate). This dissertation adds to our understanding of theories of black political participation by suggesting another determinant (positive perceptions among blacks toward the role of black political leaders in the American political system in improving black conditions) of black political participation. The dissertation also contributes to the study of black political leadership by pointing out factors of black political leadership (e.g., black political candidates\u27 natural dispositions, policy orientations) that have influenced black political participation

Effects of a Carbon Nanotube Additive on the Corrosion-Resistance and Heat-Dissipation Properties of Plasma Electrolytic Oxidation on AZ31 Magnesium Alloy

Plasma electrolytic oxidation (PEO) coating was obtained on AZ31 Mg alloy using a direct current in a sodium silicate-based electrolyte with and without a carbon nanotube (CNT) additive. The surface morphology and phase composition of the PEO coatings were investigated through field emission scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The corrosion-resistance properties of the PEO coatings were evaluated using potentiodynamic polarization measurements and electrochemical impedance spectroscopy (EIS) in a 3.5 wt.% NaCl solution. Furthermore, the heat-dissipation property was evaluated by a heat-flux measurement setup using a modified steady-state method and Fourier transform infrared spectroscopy (FT-IR). The results demonstrate that, by increasing the concentration of CNT additive in the electrolyte, the micropores and cracks of the PEO coatings are greatly decreased. In addition, the anticorrosion performance of the PEO coatings that incorporated CNT for the protection of the Mg substrate was improved. Finally, the coating’s heat-dissipation property was improved by the incorporation of CNT with high thermal conductivity and high thermal emissivity

Thermal Radiative Copper Oxide Layer for Enhancing Heat Dissipation of Metal Surface

The heat dissipation of a metal heat sink for passive cooling can be enhanced by surface modifications to increase its thermal emissivity, which is reflected by a darker surface appearance. In this study, copper electrodeposition followed by heat treatment was applied to a copper substrate. The heat treatment formed a nanoporous oxide layer containing CuO and Cu2O, which has a dark blackish color and therefore increased the thermal emissivity of the surface. The heat dissipation performance was evaluated using the sample as a heat sink for an LED module. The surface-treated copper heat sink with a high thermal emissivity oxide layer enhanced the heat dissipation of the LED module and allowed it to be operated at a lower temperature. With an increase in the heat treatment, the thermal emissivity increases to 0.865, but the thermal diffusivity is lower than the copper substrate by ~12%. These results indicate that the oxide layer is a thermal barrier for heat transfer, thus optimization between the oxide thickness and thermal emissivity is required by evaluating heat dissipation performance in operating conditions. In this study, an oxide layer with an emissivity of 0.857 and ~5% lower thermal diffusivity than the copper substrate showed the lowest LED operating temperature

Effects of Anodizing Conditions on Thermal Properties of Al 20XX Alloys for Aircraft

Anodizing was applied to improve the heat dissipation performance of aluminum (Al) alloys, by forming an oxide layer, such that they could be employed in aerospace applications. The methods employed were hard sulfuric acid (high hardness), soft sulfuric acid (low hardness), boric-sulfuric mixed acid, tin-sulfuric mixed acid, and chromic acid solutions. Each process was completed under optimized conditions. The surface morphology was observed using field emission scanning electron microscopy (FE-SEM) and a digital camera. For the determination of thermal performance, Fourier transform infrared spectroscopy (FT-IR) was used to measure the emissivity at 50 °C, and laser flash analysis (LFA) was utilized to analyze the thermal diffusivity at room temperature to 300 °C. The radiative property of metals is often ignored because of their low emissivity, however, in this research, the emissivity of the metal oxides was found to be higher than that of bare metal series. This study improved the heat dissipation properties by oxidization of Al via the anodizing process

Effects of Anodizing Conditions on Thermal Properties of Al 20XX Alloys for Aircraft

Anodizing was applied to improve the heat dissipation performance of aluminum (Al) alloys, by forming an oxide layer, such that they could be employed in aerospace applications. The methods employed were hard sulfuric acid (high hardness), soft sulfuric acid (low hardness), boric-sulfuric mixed acid, tin-sulfuric mixed acid, and chromic acid solutions. Each process was completed under optimized conditions. The surface morphology was observed using field emission scanning electron microscopy (FE-SEM) and a digital camera. For the determination of thermal performance, Fourier transform infrared spectroscopy (FT-IR) was used to measure the emissivity at 50 °C, and laser flash analysis (LFA) was utilized to analyze the thermal diffusivity at room temperature to 300 °C. The radiative property of metals is often ignored because of their low emissivity, however, in this research, the emissivity of the metal oxides was found to be higher than that of bare metal series. This study improved the heat dissipation properties by oxidization of Al via the anodizing process