High-current substrates

As a result, ceramics with thick high-conductivity metallizations are the norm for these types of applications. Ceramics make the most sense for these applications because of tiieir high tiiermal conductivity and stability at high 

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By virtue of the high breakdown potential of the oxide film (approximately 155 V in sea water and 280 V in low conductivity water of pH = 7) tantalum has found use as a substrate for platinum in impressed-current cathodic-protection anodes, which can be used at high impressed voltages (50 V) and high current densities. However, because of its lower cost, niobium is preferred for systems that have to operate at high voltages... 

The properties of platinum as an inert electrode in a variety of electrolytic processes are well known, and in cathodic protection it is utilised as a thin coating on a suitable substrate. In this way a small mass of Pt can provide a very large surface area and thus anodes of this type can be operated at high current densities in certain electrolyte solutions, such as seawater, and can be economical to use. 

Attack on the substrate by contact with Mg(OH>2 and Ca(OH)2 (calcareous scale) can also cause deplatinisation to occur. Anodes located close to the cathode or operating at high current densities can lead to a rapid build up of calcareous deposit, the major constituents of which are Mg(OH)2 and Ca(OH>2. The alkaline conditions so generated can lead to rapid dissolution of the platinum. The calcareous deposit can be removed by washing with dilute nitric acid. 

Electroplating passive alloys Another application of strike baths reverses the case illustrated in the previous example. The strike is used to promote a small amount of cathode corrosion. When the passivation potential of a substrate lies below the cathode potential of a plating bath, deposition occurs onto the passive oxide film, and the coating is non-adherent. Stainless steel plated with nickel in normal baths retains its passive film and the coating is easily peeled off. A special strike bath is used with a low concentration of nickel and a high current density, so that diffusion polarisation (transport overpotential) depresses the potential into the active region. The bath has a much lower pH than normal. The low pH raises the substrate passivation potential E pa, which theoretically follows a relation... 

The formation of two-layer PS on p-Si involves two different physical layers in which the potential-current relations are sensitive to the radius of curvature. The space charge layer of p-Si under an anodic potential is thin, which is responsible for the formation of the micro PS. The non-linear resistive effect of the highly resistive substrate is responsible for that of macro PS. The effect of high substrate resistivity should also occur for lowly doped n-Si. However, under normal conditions, the thickness of the space charge layer under an anodic potential, at which macro PS is formed, is on the same order of magnitude as the dimension... 

Let us now consider the charge state of the electrode. The emitter is positively biased. A p-type silicon electrode is therefore under forward conditions. If the logarithm of the current for a forward biased Schottky diode is plotted against the applied potential (Tafel plot) a linear dependency with 59 meV per current decade is observed for moderately doped Si. The same dependency of 1EB on VEB is observed at a silicon electrode in HF for current densities between OCP and the first current peak at JPS, as shown in Fig. 3.3 [Gal, Otl]. Note that the slope in Fig. 3.3 becomes less steep for highly doped substrates, which is also observed for highly doped Schottky diodes. This, and the fact that no electrons are detected at the collector, indicates that the emitter-base interface is under depletion. This interpretation is sup-... 

Success in indirect electrooxidation with a metal redox carrier depends on the choice of a metal ion (1) that is best suited for the desired functionalization (2) that is soluble in the electrolysis media in both the high and the low oxidation states (3) that is expected to undergo electrooxidative regeneration with high current efficiency as well as to react with the substrate in a high yield (4) that can be readily separated from the products ... 

Along with electronic transport improvements must come attention to substrate transport in such porous structures. As discussed above, introduction of gas-phase diffusion or liquid-phase convection of reactants is a feasible approach to enabling high-current-density operation in electrodes of thicknesses exceeding 100 jxm. Such a solution is application specific, in the sense that neither gas-phase reactants nor convection can be introduced in a subclass of applications, such as devices implanted in human, animal, or plant tissue. In the context of physiologically implanted devices, the choice becomes either milliwatt to watt scale devices implanted in a blood vessel, where velocities of up to 10 cm/s can be present, or microwatt-scale devices implanted in tissue. Ex vivo applications are more flexible, partially because gas-phase oxygen from ambient air will almost always be utilized on the cathode side, but also because pumps can be used to provide convective flow of any substrate. However, power requirements for pump operation must be minimized to prevent substantial lowering of net power output. 

Evaporation is a process in which the target metal is heated in a high-vacuum environment and deposited onto the substrate. There are two main ways to heat the target metal resistive heating, where a high current is passed through a boat or... 

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