Aluminum electrolytic etching

Figure 103. Boron fibers in an aluminum alloy, electrolytically etched, BF. The boron fibers have a bright tungsten core.

Figure 128. SiC fibers in an aluminum matrix. Electrolytically etched, surface layer removed, BF. The layer of fibers consisting of substoichiometric SiC and carbon protects the matrix.

Hydrogen fluoride Catalyst in some petroleum refining, etching glass, silicate extraction by-product in electrolytic production of aluminum Petroleum, primary metals, aluminum Strong irritant and corrosive action on all body tissue damage to citrus plants, effect on teeth and bones of cattle from eating plants... 

Aluminum foil capacitors occupy an important position in circuit applications due to their unsurpassed volumetric efficiency of capacitance and low cost per unit of capacitance.328 Together with tantalum electrolytic capacitors, they are leaders in the electronic discrete parts market. Large capacitance is provided by the presence of extremely thin oxide layers on anodes and cathodes, and high surface areas of electrodes could be achieved by chemical or electrochemical tunnel etching of aluminum foils. The capacitance of etched eluminum can exceed that of unetched metal by as much as a factor of 50.328... 

The same investigation showed that copper-precoated ceramic plates coated with aluminum can be etched like copper circuit boards. A comparative study of galvanoaluminum layers and other electrolytically precipitated deposits was recently published [140]. 

Then the anodic alumina layer formed was removed chemically in the selective etchant composed of phosphoric (6 wt.%) and chromic (1.8 wt.%) acids at 60 C. Hemispheric etching pits - replica of the alumina cell bottoms - remain on the surface of the aluminum foil. The second porous anodization of aluminum was made. At this stage, the pores on the aluminum foil surface arise not in random way but at the sites of primary alumina cell Imprints to repeat the cell size. The pore diameter and spacing are dictated by the parameters of the anodization process, specifically by the electrolyte composition and the anodization voltage. The alumina film thickness is defined by the anodization time and the anodization current density. The second stage provides a continuous development of the alumina film. Total etching process takes 10-20h to get pores of approximately 100 pm lengths. 

The growth of porous oxide on aluminum in various electrolytes under anodic bias has been studied for some time [10]. When aluminum is anodized in certain electrolytes, like phosphoric, sulfuric or oxalic acids, using a two-step process followed by etching, an array of close packed cells (pores) arranged in a near hexagonal pattern develops. Each cell contains a cylindrical pore and the axis of the cell is perpendicular to the surface. 

Another way to template thin films of nano-sized cylinders perpendicular to the surface is to start with a preformed membrane of track-etched polycarbonate or nanoporous alumina. A fiuid dispersion of a filler material can be drawn into the pores. Anodized aluminum oxide was the template for construction of lithium ion nanobatteries having many parallel cells filled with the solid state electrolyte PEO-LiOTf (poly(ethylene oxide)-lithium trifluoromethanesulfonate) and the electrodes coated on the top and bottom surfaces of the film (41). 

Thermal functionalization methods make up the majority of methods reported in the literature for Si-C bond formation on porSi (Tables 1,2,3,4,5, and 6). The procedures are quite straightforward samples can be placed in small flask or vial (Fig. 2a), immersed in or coated with the reactant, and heated (if required). If a vial is used, the cap should be lined with material that is inert to the vapors from the liquid. For electrochemical functionalization methods (Table 7), use of the same etching cell setup used originally to prepare the porSi works well (Fig. 2b). The electrolyte/reactant and electrode (usually Pt) are placed within the well above the porSi wafer. The wafer sits on a rectangular aluminum electrode, which in Fig. 2b was cut from a weighing dish. 

Figure C.7 illustrates the steps required to electrochemically etch Si. A Teflon cell fitted with a Viton o-ring is shown in (a). A silicon wafer is placed polished side down onto the top of the o-ring, (b). A piece of aluminum foil is placed on top of the wafer backside and the plastic backplate is screwed into place, (c). The polished side of the wafer is shown from the top of the Teflon cell in photograph (d). The wafer surface is treated with 10% HF(aq) for 10 min. to remove any native oxide layer, followed by rinsing with water and ethanol. The cell is then filled with an electrolyte consisting of 12.5% HF (HF H20 Et0H of 1 4 3), and a platinum electrode is immersed into the solution, (e). Electrical connections to the platinum and aluminum electrode surfaces are made (note current flows from the bottom to top) and appropriate current started. Photograph (g) shows the presence of tiny bubbles that indicate the electrochemical anodization of the silicon substrate. The final etched wafer is shown in (h), which is rinsed with water and ethanol and dried under a flow of nitrogen. Using a current of 54 mA.cm for 20 min. for a p-type Si(lOO) substrate with a resistivity of 20-50 fl.cm results in a macroporosity (70%) with pores 2-3 pm in diameter and 40-50 pm depth (e.g.. Figure C.8).

At an electrolyte pH of 8, the passivation region extended for over 0.5 V. Electrochemical tests were performed on acid etched, bare A1 2024-T3 panels with artificial pits as a control experimental to determine if the increased passivation with increasing electrolyte pH was a result of self-passivation of the bare aluminum alloy surface (Fig. 6.12(b)). The polarization curves for the bare A12024-T3 showed no appreciable passivation and no significant difference with pH, indicating that the passivation observed in the primers was not a result of self-passivation of the substrates, but activity from the primer. This behavior provides important clues not only to the inherent electrochemical properties of the initial coatings, but also the mechanisms responsible for corrosion protection. 

The production of pSi microparticles generally starts with the porosification of a silicon wafer in an apposite etch tank (Figure 11.4). The tank is composed of HF-resistant materials such as polytetrafluoroethylene or aluminum oxide. The wafer is placed in the tank with the frontside facing the etch solution and the rearmost side in contact with the anode of a power supply. If no backside illumination of the wafer is required, a metallic layer is deposited over the entire wafer backside to provide a better current uniformity otherwise, an annular contact along the wafer edge is employed. As an alternative, a backside electrolytic solution can be used to obtain a better contact, but at the expense of flexibility [12, 19]. 

Figure 11.20 Schematic diagram of a polypyrrole-based electrolytic capacitor. Surface etching of the aluminum substrates increases the anode area and the capacitance.

---