Ceramic Conductors

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. ... 

High-Temperature Proton Conductors Ceramic Oxides... 

Figure 6.35. DTA calorimeter cell described by Barral) et al. (94). A. B. copper sample cups. 4-mra O. D. by 6 mm C,copper reference cup D. two-conductor ceramic supports. 3-mm diameter by 50 mm E. copper radiator shield, 35-mm diameter by 53 mm F, program-sensing thermocouple G. liquid CO cooling gas jet H, electric furnace. 45-mm id by 100 cm 1. copper base plate. 38-mm diameter.

In this chapter we described tape casting. This process is used to make fiat sheets. Although this is a means of shaping and could have been described in the shaping chapter we described it here because it shares a common feature with the other thick film coating methods it uses a slurry. It also is the process most often used to make ceramic substrates for thick-film circuits. Ceramics are a major component of thick-film circuits. Even in thick-film conductors, ceramics are important in ensuring adhesion between the metal layer and the substrate (which is invariably also a ceramic). 

As it is comparatively easy to combine Low Temperature Cofired Ceramics (LTCC) with materials that have different characteristics, it is possible to integrate and build the different types of components into the ceramic. Furthermore, while it is possible to incorporate low loss metal into LTCCs as a conductor, ceramic has low dielectric loss at high frequencies making it effective for achieving low loss performance, compared with other materials such as resin and the like. In addition, its thermal expansion coefficient compared with resin materials and other ceramic materials is low, and it has the merit of excellent connection reliability for high density packaging of LSI components. For these reasons, LTCCs are regarded as a... 

Electronic properties of surfaces and interfaces in semi-conductor ceramic materials... 

Potential barriers can be commonly seen in semi-conductor ceramics. The applications include the use of SiC, thus insulated, as substrate (see section 11.6.1), grain boundary layer capacitors (section 11.6.2), PTC thermistors (section 11.6.6), varistors (section 11.6.7), ferrites (section 11.6.8) and gas detectors [WOL 91]. 

Ionic conductivity is used in oxygen sensors and in batteries (qv). Stabilized zirconia, Zr Ca 02 has a very large number of oxygen vacancies and very high conductivity. P-Alurnina/72(9(9j5 -4< -(y, NaAl O y, is an excellent cation conductor because of the high mobiUty of Na" ions. Ceramics of P-alurnina are used as the electrolyte in sodium-sulfur batteries. 

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. 

Properties and Mature of Bonding. The metaUic carbides are interesting materials that combine the physical properties of ceramics (qv) with the electronic nature of metals. Thus they are hard and strong, but at the same time good conductors of heat and electricity. 

Polymer thick films also perform conductor, resistor, and dielectric functions, but here the polymeric resias remain an iategral part after cuting. Owiag to the relatively low (120—165°C) processiag temperatures, both plastic and ceramic substrates can be used, lea ding to overall low costs ia materials and fabrication. A common conductive composition for flexible membrane switches ia touch keyboards uses fine silver particles ia a thermoplastic or thermoset polymeric biader. 

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... 

The transport of charged ions in alkali halides and, later on, in (insulating) ceramics is a distinct parepisteme, because electric fields play a key role. This large field is discussed in Schmalzried s 1995 book, already mentioned, and also in a review by one of the pioneers (Nowick 1984). This kind of study in turn led on to the developments of superionic conductors, in which ions and not electrons carry substantial currents (touched on again in Chapter 11, Section 11.3.1.1). 

This kind of microstructure also influences other kinds of conductors, especially those with positive (PTC) or negative (NTC) temperature coefficients of resistivity. For instance, PTC materials (Kulwicki 1981) have to be impurity-doped polycrystalline ferroelectrics, usually barium titanate (single crystals do not work) and depend on a ferroelectric-to-paraelectric transition in the dopant-rich grain boundaries, which lead to enormous increases in resistivity. Such a ceramic can be used to prevent temperature excursions (surges) in electronic devices. 

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