Conductor ceramic oxides

High-Temperature Proton Conductors Ceramic Oxides... 

Solid Oxide Fuel Cell In SOF(7s the electrolyte is a ceramic oxide ion conductor, such as vttriurn-doped zirconium oxide. The conduetKity of this material is 0.1 S/ern at 1273 K (1832°F) it decreases to 0.01 S/ern at 1073 K (1472°F), and by another order of magnitude at 773 K (932°F). Because the resistive losses need to be kept below about 50 rn, the operating temperature of the... 

These, therefore, constitute the guidelines for finding superconductors or how to raise the superconducting temperature. Since Covalon conduction is a nucleus to superconductivity and covalent bond is a poor conductor at room temperature, a good conductor at room temperature implies a poor covalent bond and therefore will not be a superconductor or will be a poor superconductor at best at low temperature. Inasmuch as a good covalent bond can come from compound formation, good superconductors, particularly Type-II, shall be expected to come from intermetallic compounds or special type of ceramic oxides and nitrides. 

Aluminum is the most abundant metal and the third most abundant element in the Earth s cmst, behind only oxygen and silicon. Its low weight and useful properties make aluminum and its alloys valuable materials for manufacturing and electrical applications. Inorganic compounds of aluminum are plentiful and used as absorbents, catalysts, ionic conductors, ceramics, and electrical materials. Organometalhc compounds of aluminum are also of great industrial importance and fundamental discoveries continue to be made regarding the variety of coordination numbers, structures, oxidation states, and reactivity exhibited by aluminum. ... 

In the models discussed above, the constituent phases have different values of conductivity, but the conductivities are of the same type, i.e. they are both ionic or both electronic therefore, no special conditions apply at the boundaries between the phases. Examples of systans where this situation holds are polyphase zirconia ceramics (oxide ion condnctors), discussed in Section 4.1.3, and PTCR materials (electronic conductors). 

The major components of an individual SOFC cell include the electrolyte, the cathode, and the anode. Fuel cell stacks contain an electrical interconnect, which links individual cells together in series or parallel. The electrolyte is made from a ceramic such as yttria-stabilized zirconia (YSZ) and functions as a conductor of oxide ions. Oxygen atoms are reduced into oxide ions on the porous cathode surface by electrons, and then flow through the ceramic electrolyte to the fuel-rich porous anode where the oxide ions react with fuel (hydrogen), giving up electrons. The interconnect serves to conduct the electrons through an external circuit. 

Figure 1.1 shows an SOFC scheme. It contains a solid oxide electrolyte made from a ceramic such as yttria-stabilised zirconia (YSZ) which acts as a conductor of oxide ions at temperatures from 600 to 1000°C. This ceramic material allows oxygen atoms to be reduced on its porous cathode surface by electrons, thus being converted into oxide ions, which are then transported through the ceramic body to a fuel-rich porous anode zone where the oxide ions can react, say with hydrogen, giving up electrons to an external circuit as shown in Figure 1.1. Only five components are needed to put such a cell together electrolyte, anode, cathode and two interconnect wires.

Electronic Applications. The PGMs have a number of important and diverse appHcations in the electronics industry (30). The most widely used are palladium and mthenium. Palladium or palladium—silver thick-film pastes are used in multilayer ceramic capacitors and conductor inks for hybrid integrated circuits (qv). In multilayer ceramic capacitors, the termination electrodes are silver or a silver-rich Pd—Ag alloy. The internal electrodes use a palladium-rich Pd—Ag alloy. Palladium salts are increasingly used to plate edge connectors and lead frames of semiconductors (qv), as a cost-effective alternative to gold. In 1994, 45% of total mthenium demand was for use in mthenium oxide resistor pastes (see Electrical connectors). 

Silicon carbide has very high thermal conductivity and can withstand thermal shock cycling without damage. It also is an electrical conductor and is used for electrical heating elements. Other carbides have relatively poor oxidation resistance. Under neutral or reducing conditions, several carbides have potential usehilness as technical ceramics in aerospace appHcation, eg, the carbides (qv) of B, Nb, Hf, Ta, Zr, Ti, V, Mo, and Cr. Ba, Be, Ca, and Sr carbides are hydrolyzed by water vapor. 

It is well known that dense ceramic membranes made of the mixture of ionic and electron conductors are permeable to oxygen at elevated temperatures. For example, perovskite-type oxides (e.g., La-Sr-Fe-Co, Sr-Fe-Co, and Ba-Sr-Co-Fe-based mixed oxide systems) are good oxygen-permeable ceramics. Figure 2.11 depicts a conceptual design of an oxygen membrane reactor equipped with an OPM. A detail of the ceramic membrane wall... 

Ionic conductors, used in electrochemical cells and batteries (Chapter 6), have high point defect populations. Slabs of solid ceramic electrolytes in fuel cells, for instance, often operate under conditions in which one side of the electrolyte is held in oxidizing conditions and the other side in reducing conditions. A signihcant change in the point defect population over the ceramic can be anticipated in these conditions, which may cause the electrolyte to bow or fracture. 

Fig. 4. Configuration of a ceramic membrane reactor for partial oxidation of methane. The membrane tube, with an outside diameter of about 6.5 mm and a length of up to about 30 cm and a wall thickness of 0.25-1.20 mm, was prepared from an electronic/ionic conductor powder (Sr-Fe-Co-O) by a plastic extrusion technique. The quartz reactor supports the ceramic membrane tube through hot Pyrex seals. A Rh-containing reforming catalyst was located adjacent to the tube (57).

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. 

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