Chao-Lei Ban

Modelling specific capacitance of D.C. etched aluminium foil for aluminium electrolytic capacitor

Abstract The morphology of etched aluminum foil was observed using scanning electron microscopy, which led to the establishment of a cubic tunnel etch model and a trench tunnel etch model. With these two modes, the theoretical maximum specific capacitance values for the anode foil used in aluminum electrolytic capacitors were calculated with Matlab at various formation voltages. The corresponding optimum values for tunnel size, density, distribution and geometrical shape were also given as a function of oxide thickness. The experimental and theoretical capacitance values are compared. It is concluded that the theoretical maximum specific capacitance for trench etch tunnel model is higher than that for cubic etch tunnel one at fixed formation voltage. Promoting tunnel-merging in rows rather than in clusters is preferred during electro-etching process, leading to formation of trench tunnels on the Al foil. For high formation voltages, measured capacitances approach the optimum values and enlargement of surface area by electrochemical etching was faced with the limit. But for low formation voltages, the experimental capacitance value obtained is far behind the optimized one and the tunnels size, distribution, shape and density must be optimized to achieve high capacitance.

Effect of mechanical attrition on anodizing of DC-etched aluminum foil for electrolytic capacitor

Abstract On tunnel-etched aluminum foil for electrolytic capacitor, the barrier dielectric film, γ-Al2O3, was formed by a mechanical attrition enhanced anodizing process. The anodizing was conducted in a traditional boric acid solution and the mechanical attrition (MA) action was applied by impact of nano-sized α-Al2O3 particles on the sample surface with a special vibrating frequency. The surface morphology of anodized foil was observed with SEM. The morphology and microstructure of the anodized oxide were examined by SEM, TEM and XRD. The capacitance and withstanding voltage of the oxide film were also determined with LCR meter and small-current charging. The results show that MA has significant effects on the microstructure and performance of Al anodic film. The MA-assisted barrier film becomes thinner, more crystallized but with less structure defects than the traditional one without MA, leading to increase in the film capacitance and withstanding voltage.

Mechanical attrition assisted DC etching aluminum foils with enhanced capacitance

The aluminum foils for high voltage aluminum electrolytic capacitors were DC etched in HCl–H2SO4 hybrid acids and the effects of in-situ mechanical attrition on the etching were studied in this work. Compared with the conventional etching, the distribution of etch tunnels was, under in-situ mechanical attrition, more uniform with high density of etch pits and long tunnels, which expands the surface area of etched Al electrode and greatly increases its capacitance. This effect is probably due to the mechanical attrition effects which are beneficial for the breakdown of the protective film and reducing concentration polarization in the bulk etchant solutions, inducing higher pit and tunnel density and increasing the survival rate of tunnels.