Alkaline earth titanate

Most glass-ceramics have low dielectric constants, typically 6—7 at 1 MHz and 20°C. Glass-ceramics comprised primarily of network formers can have dielectric constants as low as 4, with even lower values (K < 3) possible in microporous glass-ceramics (13). On the other hand, very high dielectric constants (over 1000) can be obtained from relatively depolymerized glasses with crystals of high dielectric constant, such as lead or alkaline earth titanate (11,14). 

The main electroceramic apphcations of titanium dioxide derive from its high dielectric constant (see Table 6). Rutile itself can be used as a dielectric iu multilayer capacitors, but it is much more common to use Ti02 for the manufacture of alkaline-earth titanates, eg, by the cocalciuation of barium carbonate and anatase. The electrical properties of these dielectrics are extremely sensitive to the presence of small (<20 ppm) quantities of impurities, and high performance titanates require consistently pure (eg, >99.9%) Ti02- Typical products are made by the hydrolysis of high purity titanium tetrachloride. 

Table 14. Alkali Metal and Alkaline-Earth Titanates...

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. 

Pechini, M., Method of preparing lead and alkaline earth titanates and niobates and coating method using the same form a capacitor, U.S.Pat. 3330697, July 11, 1967. 

To obtain effective materials for catalytic combustion, Arai and co-workers developed a preparation method based on the hydrolysis of the alkoxides of the components.12 This sol-gel method was originally developed to prepare high-purity submicron powders of alkaline or alkaline-earths titanates or zircon-ates.13,14 The method is particularly suitable for low-temperature production of mixed oxides due to the effective mixing of the components at the atomic level that can be obtained in the gel precursor. 

The simplest conceptual way in which an insulating perovskite such as the archetypal alkaline earth titanates CaTiOj, SrTiOj and BaTiOj can be turned into a conductor is to introduce electrons into the empty 3d tj band formed from the orbitals on the TF" cations. This will occur if the compound can be appropriately doped with aliovalent ions or alternatively reduced chemically. 

Degradation of Doped Alkaline Earth Titanates, J. Am. Ceramic Soc. 72, 2234. 

Lencka, M.M. and Riman, R.E. (1995) Thermodynamics of the hydrothermal synthesis of calcium titanate with reference to other alkaline-earth titanates. Chem. Mater., 7 (1), 18-25. 

Garcia, A., Galindo, R., Gargori, C., Cerro, S., Llusar, M., and Monrds, G. (2013) Ceramic pigments based on chromium doped alkaline earth titanates. Ceram. Int, 39, 4125-4132. 

The Pechini method was originally developed for preparing lead and alkaline-earth titanates and niobates and combinations thereof by way of resin intermediates [34]. On ignition of the resin the organic portion is removed, leaving the selected composition of mixed oxides chemically combined. The oxides are then sintered into dense ceramic bodies. This method is also applicable to solid electrolytes, allowing a very intimate mixture of the starting materials. 

When orbital overlap is not too marked and the bands are not very broad (cf. band width. Chapter 2) there is significant interaction between the lattice and the electrons. The electrons (or holes) then polarize their environment (see Section 5.3). The electron -I- distortion field state is known as a polaron. The semiconductor InSb is a typical example of a solid containing large polarons . Here the effective mass is increased very slightly, the mobility is not greatly reduced, and the band model for transport is a good approximation, in short the polarization effect is not too strong. Typical mobilities are of the order of 10 (alkaline earth titanates) and... 

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