Current electropolishing

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. 

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. 

Electropolishing is performed in concentrated mixtures of acids (sulfuric, phosphoric, chromic, etc.). Often, organic acids and glycerol are added. It is somewhat inconvenient that almost all metals and alloys require their own solution composition. For electropolishing, intermediate and high current densities are used, between about 0.1 and 500 mA/cm. Depending on current density, the process requires between 30 s and 20 to 30 min. Usually, a metal layer 2 to 5 pm thick is removed under these conditions. 

Electropolishing region does not occur in anhydrous organic solutions due to the lack of water which is required for the formation of oxide film. Figure 5, as an example, shows that in anhydrous HF-MeCN solution the current can increase with potential to a value of about 0.5 A/cm2 without showing a peak current. The relationship between current and potential is linear due to the rate limiting effect of resistance in solution and silicon substrate. 

While n.. shows no dependence on doping density, current density or electrolyte concentration in the electropolishing regime, it does in the PS regime [Le23, Fr6]. enerally n., increases with current density. This is shown for the mesoporous rein Fig. 6.9 a, and microporous regime in Fig. 4.6. From the data of the latter gure the dependence of n.. on formation current density J (in mA cm4) in etha-HF can be fitted to ... 

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. 

As shown in Fig. 4.5, the dissolution valence n., shows a relatively constant value of 4 for electropolishing current densities well above JPS and a bias below 10 V. 

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... 

The geometry of the pore tips is pyramidal, with facets formed by (111) planes, for Jap < JPS. This is the case when the current density is limited by the applied bias, as is the case for the samples shown in Fig. 9.13 b and c. If the bias is increased, dissolution at the pore tip occurs partially in the isotropic electropolishing regime (/tip=/ps)- This reduces the tendency to form facets and the tip geometry becomes almost hemispherical, as shown in Fig. 9.13d. 

The IR filter is realized by a PS layer with a modulation of porosity, which constitutes an interference filter as described in detail in the next section. The 30 pm thick porous layer is then released from the substrate by electropolishing, which is easily done in situ by increasing the etching current density above JPS. This process is commonly applied to form free-standing PS membranes and PS tubes [Tj 1], The internal strain between the Si3N4 layer used for masking and the porous layer lifts the filter up to its rest position, as shown in Fig. 10.10. The filter is suspended at two microactuator arms, which work as thermal bimorph actua-... 

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. 

The lack of the surface effect in the Ni-Bi couples was in all probability due to the presence onto the specimen surface of a very thin protective layer occurred at the end of electropolishing when the current was already switched off, while the specimen surface still continued to contact with the electrolyte. Though transparent and undetectable by EPMA, this layer consisting presumably of bismuth oxide was nonetheless sufficient to... 

While the subject of this chapter may seem counter to the title of the book, metal dissolution is vital in numerous aspects of metal deposition, counter electrode processes, pre-treatment protocols and electropolishing. This chapter outlines the current state of understanding of metal dissolution processes and discusses in some detail an electropolishing process that has now been commercialised using a Type III ionic liquid. 

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