Circular disc electrode

For preliminary work, a circular disc electrode with a diameter in the 1-5 mm range is usually used. The substrate concentrations are typically between 1 and 10 mM and the voltage sweep rates are usually between 50 mV s 1 and 500 V s 1. Sometimes the voltage sweep is continued to include several E — t half-cycles. 

The rotated disc electrode (RDE) is one of the most commonly employed hydro-dynamic electrodes. Figure 7.1 shows a schematic representation of a typical RDE. The electrode itself is a flat, circular disc of metal, graphite or an other conductor, and has a radius of r its area A, therefore, is straightforwardly nr. The disc is embedded centrally into one flat end of a cylinder of an insulatory material such as Teflon or epoxy resin. Behind the face of the electrode is an... 

The ceramic circular discs (1cm diameter, 0.1cm thick) were prepared by pressing the powder, using a pressure of 8 MPa, with adding in 8 % PVA solution as binder, and sintering at 1 300 °C for 2 hours. Then the ceramic samples were prepared by electroding the sintered disc with silver paint. 

The dielectric material in the form of a flat circular disc of thickness d, is placed between the electrodes to form a parallel-plate capacitor of plate area A. The upper or high electrode (H) is directly connected to the high-voltage terminal of the measuring unit, whereas the lower electrode (L) is similarly connected to the low-voltage terminal. These two terminals are Isolated from the metal screen of the BNC-type connector and cables. The third or guard electrode (C) is held at the same potential as the lower electrode, but is isolated from it by a Teflon guard. The... 

In the case of pipe flow (tubular flow, also tubular or tube electrode), the electrolyte solution is pumped through a circular tube at a rate (flow rate V"f) low enough to secure laminar flow (for the distinction from turbulent flow, see below). The working electrode is embedded as a ring (annulus) in the wall of the pipe a double ring can be mounted also to enable mechanistic studies like with a ring-disc electrode (see above). 

Fig. 8.2 Circular iridium electrodes used for impedance spectroscopy and electrode model extraction in this study. Upper picture shows the surface of a whole MEA, lower picture illustrates several disc electrodes zoomed on...

The electrochemical cell used consisted of a circular disc of Pt (1.5 cm2) to which a Pt wire was spot welded. This electrode was placed inside the flat surface of a plastic vial cap (2.2 cm diameter) with the Pt wire pointing upwards, perpendicular to the Pt disc. A Pyrex glass tube (3 cm long, 1.0 cm internal diameter 0.8 cm ) was placed on the Pt disc. The tube was then clamped and molten paraffin wax was poured into the outside hollow area between the tube and the wall of the vial cap until the wax level reached the top of the cap. The clamp was removed after 15 minutes at which time the Pt disc was attached firmly to the bottom of the glass tube by the paraffin wax with the Pt wire protruding vertically through the paraffin wax seal. 

Lens components can also be constructed out of disc electrodes with circular apertures. Here the key fringing fields fan out from the apertures and it is straightforward to extend the arguments above to explain why these aperture lenses exhibit a focusing effect. Figure 3.5 shows a simulation of the focusing response of a three-element aperture lens. 

The most common form of solid macrosized bulk working electrodes for electrochemical measurements is a circular disc shape embedded in a usually round holder. Contact is made on the back of the electrode either simply mechanically or by soldering. Older approaches with mercury contacts are getting out of fashion due to a worldwide mercuriphobia. In fact any shape other than a circular disc is possible, but care has to be taken that there is a good electric cmitact with the electrode material and that the insulation is sufficient in order to avoid resistive and parasitic effects. 

In K. Birkeland and S. Eyde s furnace, the arc between the electrodes is maintained by an alternating current of about 5000 volts and 50 periods per second, and is spread by a strong magnetic field into two semicircular discs of flame at right angles to the axis of the electrodes. The sheets of flame alternately rise and break with great rapidity in the upper and lower half of the reaction chamber. The impression on the eye is that of a steady circular sheet of flame about 1-8 metres in diameter. The electric flame is produced in a flat box of refractory... 

A disc capacitor of thickness 1 mm carries circular electrodes of diameter 1 cm. The real and imaginary parts of the relative permittivity of the dielectric are 3000 and 45 respectively. Calculate the capacitance and the power dissipated in the dielectric when a sinusoidal voltage of amplitude 50 V and frequency 1 MHz is applied to the capacitor. [Answer 245 mW]... 

A disc of reduced semiconducting rutile crystal 2 cm in diameter and 2 mm thick is heated in air for 10 s at 300 °C. After cooling, circular electrodes, lcm in diameter, are applied symmetrically to the two major surfaces. The chemical diffusion coefficient D for the oxidation reaction in reduced single-crystal TiC is given by... 

This electrode is uniformly accessible [4], in that during a reaction the flux (and hence the current density) is the same across the entire disc surface. Also, the system possesses circular symmetry about the z-axis. This greatly simplifies the mathematical description of the hydrodynamics, and allows an analytical solution of the convective-diffusion equation [5]. 

The disc system consisted of a truncated cone with a small base angle, the cross section of which generated the flow under investigation. It was made of an insulating material (Plexiglass) and equipped, in the disc plane, with a small circular electrode ( = 80j m) located at r = 2.7 cm from the rotation axis. The external disc radius was equal to 3 cm. 

Fig. 73. Flow patterns at a rotating disc. (A) Perpendicular view of the electrode surface of a circular velocity Uc = cOrRD he radial distance Rq (B) side view of the stream lines concentric around the z-axis u velocity along the rotation axis, u centrifugal velocity, perpendicular to the rotation axis.

---