Aluminum oxide electrical conductivity

Further improvements in anode performance have been achieved through the inclusion of certain metal salts in the electrolyte, and more recently by dkect incorporation into the anode (92,96,97). Good anode performance has been shown to depend on the formation of carbon—fluorine intercalation compounds at the electrode surface (98). These intercalation compounds resist further oxidation by fluorine to form (CF ), have good electrical conductivity, and are wet by the electrolyte. The presence of certain metals enhance the formation of the intercalation compounds. Lithium, aluminum, or nickel fluoride appear to be the best salts for this purpose (92,98). 

For many electronic and electrical appHcations, electrically conductive resias are required. Most polymeric resias exhibit high levels of electrical resistivity. Conductivity can be improved, however, by the judicious use of fillers eg, in epoxy, silver (in either flake or powdered form) is used as a filler. Sometimes other fillers such as copper are also used, but result in reduced efficiency. The popularity of silver is due to the absence of the oxide layer formation, which imparts electrical insulating characteristics. Consequently, metallic fibers such as aluminum are rarely considered for this appHcation. 

Aluminum metal is produced from aluminum oxide by electrolysis using the Hall-Heroult process, whose story is detailed in our Chemical Milestones Box. The melting point of AI2 O3 is too high (2015 °C) and its electrical conductivity too low to make direct electrolysis commercially viable. Instead, AI2 O3 is mixed with cryolite (Na3 AlFfi) containing about 10% CaF2. This mixture has a melting point of 1000 °C, still a high temperature but not prohibitively so. Aluminum forms several complex ions with fluoride and oxide, so the molten mixture... 

Figure 39. Comparison of theoretical j- U curves with experimental data on electrical conduction of aluminum (curve I) and tantalum (curve II) oxides.62...

Aluminum Oxide Moisture Sensor. This type of sensor is a capacitor, formed by depositing a layer of porous aluminum oxide onto a conductive substrate, and then coaling the oxide with a thin film of gold The conductive base and the gold layer become the capacitor s electrodes. Water vapor penetrates the gold layer and is absorbed by the porous oxidation layer The number of water molecules absorbed determines the electrical impedance of the capacity, which is. m turn, a measure of water vapor pressure. 

Some applications, however, must conduct heat but not electricity. In these applications the adhesive must permit high transfer of heat plus a degree of electrical insulation. Fillers used for achieving thermal conductivity alone include aluminum oxide, beryllium oxide, boron nitride, and silica. Table 9.9 lists thermal conductivity values for several metals as well as for beryllium oxide, aluminum oxide, and several filled and unfilled resins. 

The Aluminum Company of America, currently referred to as Alcoa, tested a new and interesting concept of using a cermet as the base material [245], It was composed of a mixture of Ni—Fc—O with excess Ni or Cu. The recommended composition was NiFe204 + 18% NiO + 17% Cu. The excess metal provided good electrical conductivity and a simple connection to the current lead. The surface layer of the anode was completely oxidized, but this outer oxide layer could be kept fairly thin. 

The two metallizations most commonly used to fabricate transducers on AW devices are gold-on-chromium and aluminum. Au is often chosen for chemical detection applications because of its inertness and resistance to corrosion a layer 100-200 nm diick is necessary to provide adequate electrical conductivity. Unfortunately, the inertness of Au also prevents its adhesion to quartz and other oxides utilized for AW device substrates. Therefore, an underlayer of Cr (2-10 nm thick) is utilized to promote the adhesion of Au to the substrate the electropositive (reactive) nature of Cr allows it to form strcxig bonds with oxide surfaces, while alloying between the Cr and Au chemically binds the two metal layers... 

Electrolysis is used in a wide variety of ways. Three examples follow (1) Electrolysis cells are used to produce very active elements in their elemental form. The aluminum industry is based on the electrolytic reduction of aluminum oxide, for example. (2) Electrolysis may be used to electroplate objects. A thin layer of metal, such as silver, can be deposited on other metals, such as steel, by electrodeposition (Eig. 14-2). (3) Electrolysis is also used to purify metals, such as copper. Copper is thus made suitable to conduct electricity. The anode is made out of the impure material the cathode is made from a thin piece of pure copper. Under carefully controlled conditions, copper goes into solution at the anode, but less active metals, notably silver and gold, fall to the bottom of the container. The copper ion deposits on the cathode, but more active metals stay in solution. Thus very pure copper is produced. The pure copper turns out to be less expensive than the impure copper, which is not too surprising when you think about it. (Which would you expect to be more expensive, pure copper or a copper-silver-gold mixture )... 

The important properties of aluminum oxide ceramics are their high temperature stability (melting point of AI2O3 2050°C), their good thermal conductivity, their high electrical resistivity and their high chemical resistance. Their mediocre thermal shock resistance is a disadvantage. All these properties are dependent upon the chemical purity and particle size distribution of the oxide powder and the density, structure and pore size di.stribution of the ceramic. 

In another article, Wang et al. (1993) used the CVCC conductivity cell for measurement of a variety of molten cryolite melts with additives of aluminum fluoride, aluminum oxide, calcium fluoride, magnesium fluoride, and lithium fluoride. On the basis of the measured results, a multiple regression equation for the electrical conductivity of cryolite melts was derived. Influence of the bath composition on the electrical conductivity at different bath temperatures was discussed. A comparison of the measured results with the published electrical conductivity values for cryolite melts was made. The new regression equation can be used to calculate electrical conductivity of cryolite melts in modern industrial bath chemistry. 

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