Mixed oxides, electrical properties

Alkaline-Earth Titanates. Some physical properties of representative alkaline-earth titanates ate Hsted in Table 15. The most important apphcations of these titanates are in the manufacture of electronic components (109). The most important member of the class is barium titanate, BaTi03, which owes its significance to its exceptionally high dielectric constant and its piezoelectric and ferroelectric properties. Further, because barium titanate easily forms solid solutions with strontium titanate, lead titanate, zirconium oxide, and tin oxide, the electrical properties can be modified within wide limits. Barium titanate may be made by, eg, cocalcination of barium carbonate and titanium dioxide at ca 1200°C. With the exception of Ba2Ti04, barium orthotitanate, titanates do not contain discrete TiO ions but ate mixed oxides. Ba2Ti04 has the P-K SO stmcture in which distorted tetrahedral TiO ions occur. 

The titanates are an important group of mixed oxides with unusual optical, electrical, and mechanical properties. Many of these materials can be deposited by CVD, and CVD may soon become an economical production process.t" i] The most important titanates are as follows. 

One of the most exciting developments in materials science in recent years involves mixed oxides containing rare earth metals. Some of these compounds are superconductors, as described in our Chemistry and Technology Box. Below a certain temperature, a superconductor can carry an immense electrical current without losses from resistance. Before 1986, it was thought that this property was limited to a few metals at temperatures below 25 K. Then it was found that a mixed oxide of lanthanum, barium, and copper showed superconductivity at around 30 K, and since then the temperature threshold for superconductivity has been advanced to 135 K. 

It has often been pointed out that the electrical conductivity of sintered samples of ZnO and of other n-conducting oxides is frequently caused by the conductivity of thin layers near the surface, and not by the conductivity of the bulk (25-28). According to our present knowledge, these thin layers near the surface of oxides are caused by electron transfer from the layers to the chemisorbate during the chemisorption, and the amount of chemisorption may be related to the electronic properties of the gas molecules and of the solids. The dependence of the electrical conductivity of some semiconductors on the pressure of CO, COj, and on the vapor pressure of ethanol, methanol, acetone, and water, as observed by Ljaschenko and Stepko (29), can be explained by the same mechanism. The dependence of conductivity of some mixed oxides at high temperatures can be explained in a similar way (30). 

Liou and W.L. Worrell, Electrical properties of novel mixed-conducting oxides. Appl. Phys. A., 49 (1989) 25-31. 

W.L. Worrell, Electrical properties of mixed-conducting oxides having high oxygen-ion conductivity. Solid State Ionics, 52 (1992) 147-51. 

The structure of MgV Og consists of layers of CO 5 and MgOs pyramids whose bases are coplanar but whose apices alternate in the z direction. A series of oxides and mixed oxides containing have been obtained by hydrogen reduction of alkali-metal vanadates, and the electrical and spectroscopic properties of some vanadium-... 

The electrical and/or magnetic properties of a number of mixed oxides of A1 and other metals including members of the spinel family (see Box 12.6) and sodium P-alumina (see Section 27.3) have extremely important industrial applications. In this section, we single out Ca3Al205 because of its role in cement manufacture, and because it contains a... 

Electrical conductivity and catalytic properties of R-mixed oxides" 1 ... 

These reactions require ionic intermediates and are catalyzed by acidic or basic solids hke AI2O3 or CaO and especially mixed oxides such as Al203/Si02 and MgO/ Si02. Electronic effects can also successfully explain the phenomena of catalyst promotion and catalyst poisoning. Solid-state catalysts can be classified according to their electrical conductivity and electron-transfer properties as shown in Table 5-14. 

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