Rotation, tool-electrode

The rotating hemispherical electrode (RHSE) was originally proposed by the author in 1971 as an analytical tool for studying high-rate corrosion and dissolution reactions [13]. Since then, much work has been published in the literature. The RHSE has a uniform primary current distribution, and its surface geometry is not easily deformed by metal deposition and dissolution reactions. These features have made the RHSE a complementary tool to the rotating disk electrode (RDE). 

With its axisymmetric transport and current distribution, the rotating hemispherical electrode complements the rotating disk as a tool for studying electrode processes. Der-Tau Chin provides a valuable overview and summary of the fundamental theory and applications of this interesting device. 

A precondition for an appropriate decision in the planning of a preparative electroorganic synthesis is sufficient information about the electrochemical reaction. As far as possible, knowledge about the influence of parameters such as temperature, solvent, pH value, and stirring rate should be included. Electroanalytical standard methods to acquire such data have been discussed in Chapter 1 cyclovoltammetry as an especially valuable tool and its combination with the rotating disk electrode method for additional knowledge. At... 

Albery, W. J. and Hitchman M. L., Ring-Disc Electrodes, Oxford University Press, Oxford, 1971. This now-classic book describes one of the most formidable tools in the arsenal of the electroanalyst, i.e. the rotated ring-disc electrode (RRDE). Its first two chapters are a clear and lucid introduction to the basic rotated disc electrode (RDE) and the multi-faceted problems of mass transport. Well worth a read, if only for the occasional dip into this field. 

Looking at Eq. 25D we note that is a linear function of concentration. Hence the rotating disc electrode can be used as a tool in electroanalytical measurements. It has also been used extensively to determine the diffusion coefficients of different electroactive species in solution. 

Figure 4-1. Protein film voltammetry as a technique for studying redox enzyme mechanisms. The catalytic current-potential profile provides information on the rate-defining catalytic processes occurring within the enzyme. It is important that interfacial electron transfer is facile and information is not masked by limitations due to tlie transport of substrate and product for this reason the rotating disc electrode is an important tool in these studies.

With a few exceptions, the fluid flow must be simulated before the mass-transfer simulations can be rigorously performed. Nevertheless, here are several important situations, such as that at a rotating disk electrode, where the fluid flow is known analytically or from an exact, numerical solution. Thus there exists a body of work that was done before CFD was a readily available tool (for example, see Refs. 34-37). In many of these studies, a boundary-layer analysis, based on a Lighthill transformation (Ref. 1, Chapter 17), is employed. 

Figure 7.4 Comparison of microholes drilled by gravity feed at 40 V using (a) a 0.2 mm cylindrical tool-electrode (b) a flat sidewall (0.1 mm thickness) shaped tool-electrode. In both cases a tool-electrode rotation of 500 rpm is used. Reprinted from [136] with the permission of the Journal of Micromechanics and Microengineering.

As in the case of tool-electrode vibration, the electrolyte flow can be promoted by tool-electrode rotation. An example combining gravity-feed drilling with tool-electrode rotation is shown in Fig. 7.6. A tungsten carbide flat sidewall tool-electrode (Fig. 7.3b) with pulsed voltage supply was used [136]. The drilling time for the fixed depth of 450 p,m increases with the tool-electrode rotation rate due to the reduced heat power. The entrance diameter shows an inverse volcano dependence on the tool-electrode rotation rate. This effect was attributed by the authors to the competition between the promotion of the electrolyte flow and the increased drilling time [136]. 

Figure 7.6 Effect of tool-electrode rotation in gravity-feed drilling. Reprinted from [136] with the permission of the Journal of Micromechanics and Microengineering.

Adding abrasive material to the electrolyte does not itself promote the local chemical etching. This effect can, however, be achieved in combination with the appropriate tool-electrode motion (e.g., rotation or vibration). In this way, machining quality is improved by reducing the surface roughness [133]. 

The heat transfer through the electrolyte can be influenced by local hydrodynamic flows (by convection) or by changing the heat conductivity of the electrolyte. The first strategy can be implemented by adding the appropriate tool-electrode motion such as vibration or rotation. Both do not only promote the heat transfer but also the local high-temperature chemical etching of the workpiece as discussed in Section 7.2. 

The combination of tool-electrode rotation, tool shape, and optimal pulse duty ratio results in this excellent result. 

A further increase in the tool-electrode rotation improves the channel quality (Fig. 7.14). The sidewall taper angle of the microchannel profile decreases to almost 0° and the machining over-cut is also reduced. This effect is due to the improved electrolyte flow, promoting chemical etching of the glass. 

One way to overcome this difficulty was proposed by Jain et al. [62]. They used abrasive cutting tools. The feed of the tool-electrode was chosen in such a manner so as to always keep contact between the tool and the workpiece. Thus, the machining gap is of the same order of magnitude as the size of the abrasive particles on the tool-electrode (a few micrometres). The researchers showed that the material removal rate achieved with the abrasive tools is higher than that obtained with the conventional tools. This effect may be attributed to the increased gap, but, at the same time, as tool rotation (20 rpm)... 

By mounting the tool-electrode holder on a motor-controlled stage, drilling can be done at a constant speed. Depending on the motor controller used, more or less complex velocity profiles can be used. By synchronising the vertical motion with tool rotation, it is possible to drill threads in glass as shown by Lee et al. [79]. 

The tool-electrode is mounted on a flexible structure that controls the vertical guidance of the tool. An optical sensor measures the tool displacement. An optional voice-coil motor can be added in order to control the force at which gravity-feed drilling is done. This motor can also be used to add vertical vibrations to the tool to promote the flow of the electrolyte inside the microhole. Rotation can also be included. 

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