Protective coating, electrode

Parylene s use in the medical field is linked to electronics. Certain pacemaker manufacturers use it as a protective conformal coating on pacemaker circuitry (69). The coated circuitry is sealed in a metal can, so that the parylene coating serves only as a backup should the primary barrier leak. There is also interest in its use as an electrode insulation in the fabrication of miniature electrodes for long-term implantation to record or to stimulate neurons in the central or peripheral nervous system, as the "front end" of experimental neural prostheses (70). One report describes the 3-yr survival of functioning parylene-coated electrodes in the brain of a monkey (71). 

Welding (qv) of titanium requires a protected atmosphere of iaert gas. Furthermore, parts and filler wire are cleaned with acetone (trichloroethylene is not recommended). The pieces to be welded are clamped, not tacked, unless tacks are shielded with iaert gas. A test sample should be welded. Coated electrodes are excluded and higher purity metal (lower oxygen content) is preferred as filler. Titanium caimot be fusion-welded to other metals because of formation of brittle intermetallic phases ia the weld 2oae. 

Often it is necessary in designing a cathodic-protection system to know the conductivity of a protective coating (e.g. bitumen enamel) on a structure. This measurement is usually carried out by finding the resistance between an electrode of known area placed in contact with the coating and the structure itself. The electrode placed on the structure can be either of thin metal foil or, preferably, of material such as flannel soaked in weak acidic solution. The resistance between the pad and the metal is measured by means of either a resistivity meter, as previously described, or a battery with a voltmeter and an ammeter or microammeter. Generally speaking, in field work where such measurements have to be made, a resistivity meter is preferable. 

Dr. Hui has worked on various projects, including chemical sensors, solid oxide fuel cells, magnetic materials, gas separation membranes, nanostruc-tured materials, thin film fabrication, and protective coatings for metals. He has more than 80 research publications, one worldwide patent, and one U.S. patent (pending). He is currently leading and involved in several projects for the development of metal-supported solid oxide fuel cells (SOFCs), ceramic nanomaterials as catalyst supports for high-temperature PEM fuel cells, protective ceramic coatings on metallic substrates, ceramic electrode materials for batteries, and ceramic proton conductors. Dr. Hui is also an active member of the Electrochemical Society and the American Ceramic Society. 

The electrodes and electrode protective coating of the oxygen sensor play a crucial role in determining the performance characteristics and durability (2). The electrodes used are the inner or air-reference electrode and the outer or exhaust gas electrode. The protective coating goes over the outer or exhaust electrode. While a complete discussion of the requirements and properties of the electrodes and electrode protective coating is beyond the scope of this paper, a brief description will be given. 

Finally, the protective coating over the outer electrode must resist abrasion by particulates in the exhaust gas, must be porous to allow access of the exhaust to the outer electrode, must adhere to the outer electrode, must have expansion characteristics compatible with the electrolyte, and must be stable under oxidizing and reducing conditions at temperatures up to 1000 C. A flame sprayed spinel (MgAl20 ) coating as shown in Figure 9 meets these requirements. 

An elegant variant of the method has been used for SrTi03 where all these problems have been overcome by working in the kinetic regime in which the electrode and surface reactions are completely hindered.247 Special electrodes as well as special protecting coatings are not necessary (see Figure 45). For more details the reader is referred to the Ref.248... 

Early experiments were concerned with the protection against water-vapor adsorption afforded a pair of soft steel electrodes by coating each with FEP Teflon resin. The two coated electrodes were cleaned as before, mounted in the gas cell, and then discharged with gaseous ions while the metal pins were grounded. The resulting contact potential difference was zero. The same result was obtained using a pair of resin-coated aluminum electrodes. 

Chromium is a white, hard, lustrous, and brittle metal (mp 1903 10°C). It is extremely resistant to ordinary corrosive agents, which accounts for its extensive use as an electroplated protective coating. The metal dissolves fairly readily in nonoxidizing mineral acids, for example, hydrochloric and sulfuric acids, but not in cold aqua regia or nitric acid, either concentrated or dilute. The last two reagents passivate the metal in a manner that is not well understood. The electrode potentials of the metal are... 

Use Alloys (low-alloy steels, stainless steel, copper and brass, permanent magnets, electrical resistance alloys), electroplated protective coatings, electro-formed coatings, alkaline storage battery, fuel cell electrodes, catalyst for methanation of fuel gases and hydrogenation of vegetable oils. 

Similarly, anodic oxidation of p-methoxyphenol (140a) was carried out in the presence of the substituted propenylbenzene 143 using teflon-coated electrode to afford the corresponding dihydrobenzofuran 144 in 80% yield (Scheme 27). The hydrophobic coating on the electrode protected the highly reactive intermediate from the solvent and enhanced the reaction with 143. 

Titanium diboride is an advanced ceramic material with properties similar to those of metals such as high thermal and electrical conductivity. The other distinguishing features of this material are its excellent oxidation resistance and chemical corrosion resistance at elevated temperatures. It can be used as an abrasive and oxidation protection coating as well as the electrode material in aluminium refining. In addition, like other borides it can also be used as a neutron absorber. 

Harrison et al. (1988) have shown that GOD-modified platinum electrodes can be effectively protected from electrode fouling by dip-coating with a perfluorosulfonic acid polymer. The dip-coating formed a polymer layer of 2 pm thickness on the electrode. After 6 days of continuous in vitro measurement in whole blood at 37°C the sensitivity was decreased by only 6%. Interferences from electroactive anions were reduced due to the Donnan exclusion of these species. Nevertheless, differential measurements were necessary for complete elimination of interferents. 

Discovery of polymer coatings that maintain sensitivity, promote selectivity, protect the electrode from the biological fluid, and provide a biocompatible surface to the measured system... 

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