Anodization electropolishing

The solutions cease to electropolish well once they contain substantial amounts of metal or when the potential is incorrectly set the departure from good finishing is recognised by the development of etch or pit features, often at edges or recesses, and this led Hoar etal. to express the critical parameters in diagrammatic form (Fig. 11.8) where the line x-x represents the ideal condition of anodic electropolishing. 

Under diffusion-controlled dissolution conditions (in the anodic direction) the crystal orientation has no influence on the reaction rate as only the mass transport conditions in the solution detennine the process. In other words, the material is removed unifonnly and electropolishing of the surface takes place. 

In uniform corrosion the superficial or geometrical area of the metal is used to evaluate both the anodic and cathodic current density, although it might appear to be more logical to take half of that area. However, surfaces are seldom smooth and the true surface area may be twice to three times that of the geometrical area (a cleaved crystal face or an electropolished single crystal would have a true surface area that approximates to its superficial area). It follows, therefore, that the true current density is smaller than the superficial current density, but whether the area used for calculating /, and... 

Electropolishing which exploits a generally similar type of solution, but introduces anodic currents as an additional means of dissolution thereby providing better control of rapid processing. Electrosmoothing and electrobrightening are terms used to describe inferior finishes which may have lustre but have lower specular reflectivity. 

Electropolishing techniques utilise anodic potentials and currents to aid dissolution and passivation and thus to promote the polishing process in solutions akin to those used in chemical polishing. The solutions have the same basic constitution with three mechanistic requirements—oxidant (A), contaminater (B) and diffusion layer promoter (C) —but, by using anodic currents, less concentrated acid solutions can be used and an additional variable for process flexibility and control is available. 

Fig. 11.7 Annotated anodic polarisation curve for the electropolishing of copper in...

Electropolishing surface finishing of a metal by making it the anode in an appropriate solution, whereby a bright and level surface showing specular reflectivity is obtained. 

Different views exist as to the reasons for selective dissolution of the asperities. According to older concepts, convection of the liquid is hindered in the solution layers filling recesses hence, reaction products will accumulate there and raise the concentration and viscosity in these layers. Both factors tend to lower a metal s anodic dissolution rate relative to that at raised points. According to other concepts, a surface condition close to passive arises during electropolishing. In this case, the conditions for passivation of the metal at raised points differ from those in recesses. 

The need for defect-free, flat silicon surfaces led to the first investigations in this field, which were performed as early as 1958 [Tul]. It was found that electropolishing of silicon is possible in HF if the applied anodic potential is sufficient to produce current densities in excess of the critical value JPS. 

Electropolishing under galvanostatic conditions can be used to remove bulk silicon in a well-defined manner. This can for example be used to profile doping density or diffusion length versus the thickness of the sample, as discussed in Sections 10.2 and 10.3. The thickness D of the removed silicon layer can be calculated from the applied current density J, the anodization time t, the dissolution valence nv, the atomic density of silicon Nsi and the elementary charge e. 

Electropolishing is well established as a simple, in situ method to separate porous silicon layers from the silicon electrode. By switching the anodic current density from values below JPS to a value above JPS, the PS film is separated at its interface to the bulk electrode. The flatness of a PS surface separated by electropolishing is sufficient for optical applications, as shown in Fig. 10.10. 

Anodic oxide formation suggests itself as a passivating mechanism in aqueous electrolytes, as shown in Fig. 6.1a. However, pore formation in silicon electrodes is only observed in electrolytes that contain HF, which is known to readily dissolve Si02. For current densities in excess of JPS a thin anodic oxide layer covers the Si electrode in aqueous HF, however this oxide is not passivating, but an intermediate of the rapid dissolution reaction that leads to electropolishing, as described in Section 5.6. In addition, pore formation is only observed for current densities below JPS. Anodic oxides can therefore be excluded as a possible cause of pore wall passivation in PS layers. Early models of pore formation proposed a... 

Another way to use silicon wafers as DLs was presented by Meyers and Maynard [77]. They developed a micro-PEMFC based on a bilayer design in which both the anode and the cathode current collectors were made out of conductive silicon wafers. Each of fhese componenfs had a series of microchannels formed on one of their surfaces, allowing fhe hydrogen and oxygen to flow through them. Before the charmels were machined, a layer of porous silicon was formed on top of the Si wafers and fhen fhe silicon material beneath the porous layer was electropolished away to form fhe channels. After the wafers were machined, the CEs were added to the surfaces. In this cell, the actual diffusion layers were the porous silicon layers located on top of the channels because they let the gases diffuse fhrough fhem toward the active sites near the membrane. 

In electropolishing, the metal workpiece is made the anode rather than the cathode. Instead of deposition onto the surface of the workpiece, some of the metal dissolves, leaving a bright, polished surface. High points dissolve at a faster rate than recessed areas. Electropolishing is performed to improve adhesion of subsequent electroplates, to deburr and Finish parts, and for decorative purposes (Schaer 1971). 

The term electropolishing is used when an electric potential is applied through the chemical solution using the specimen as the anode. A simple... 

Lausmaa, J., Kasemo, B., Matsson, H., and Odelius, H., Multi-technique surface characterizations of oxide films on electropolished and anodically oxidized titanium App. Surf. Sci. 45, 189-200 (1990). 

Various pre-treatment protocols have been developed including pickling and anodic/cathodic pulses to remove the oxide films. It was apparent that different types of steel require different pre-treatments, i.e. cast pieces behave differently to rolled pieces. Significant success was achieved in electropolishing cast pieces and the finish obtained with the ionic liquid was superior to that with phosphoric add, however, the converse was true for rolled pieces because the oxide film is thicker in the latter samples and hence slower to dissolve in the ionic liquid. 

This technology was scaled-up to a 1.3 tonne plant by Anopol Ltd (Birmingham, UK). Results have shown that the technology can be applied in a similar manner to the existing technology. The ionic liquid has been found to be compatible with most of the materials used in current electropolishing equipment, i.e. polypropylene, nylon tank and fittings, stainless steel cathode sheets and a titanium anode jig. 

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