High emissivity ZnO coatings prepared from chloride electrolyte by electrodeposition on a “Dynamic Hydrogen Bubble Template”

Abstract

In this paper the potential thermal of zinc oxide (ZnO) coatings for space applications (e.g., satellite components), obtained by a cathodic electrodeposition on a “Dynamic Hydrogen Bubble Template“ (DHBT) using chloride electrolyte (without and with potassium chloride (KCl)) under different conditions, was studied. The effect of increasing the active area, deposition time and current density of the electrodeposition procedure was also studied. A detailed physico-chemical characterization was performed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). The results show that morphology structure, pore size and film thickness vary in a systematic fashion as the electrodeposition procedure is altered. It was also verified that the electrodeposition of DHBT using chloride electrolyte can be used for the production of ZnO coatings with highly 3-dimensional micrometric pore structures. An interesting aspect of the results was the observation that the deposition quality of ZnO coating was higher when using chloride electrolyte solution with KCl. The optimum compromise was found for a ZnO coating with “dendrites” structures using it = 24.07 Acm-2s, where it is the applied current density by deposition time. Under these conditions the film thickness was 398 μm, the macropore mean diameter was 155 μm and the current efficiency was 97%. Preliminary studies of infrared emissivity were carried out and a good correlation between infrared emissivity and textural properties (film thickness and pore diameter), was found. The results indicated that the highly 3-dimensional micrometric pore structures of ZnO coating obtained by chloride electrolyte with addition of KCl can further improve the infrared emissivity with selective temperature dependence. Based on the experimental results, a mathematical relationship between infrared emissivity of smooth surface and infrared emissivity of rough surface was established. As a result, estimative of roughness factor of ZnO coatings was determined. The results obtained allowed us to conclude that ZnO coatings (ZnO_E2_17724 and ZnO_E2_17727) with highly 3-dimensional micrometric pore structures provide an infrared emissivity dependent on temperature (from 321 K to 531 K) which has a great potential to be used as an additional control of cooling heat transfer specially for satellite components.