Modified barium titanate dielectrics

The dielectric characteristics of barium titanate ceramics with respect to temperature, electric field strength, frequency and time (ageing) are very dependent on the substitution of minor amounts of other ions for Ba or Ti. 

The principal effects determining properties are now discussed in turn. 

The effect of the AO/B02 ratio varies with different substituents and additives as discussed in the next section. 

Uniformly distributed isovalent substituents do not greatly affect the shape of the st-T curve and other characteristics. Their main effect is to alter the Curie point and the lower transitions of BaTi03. 

Barium can be replaced by isovalent ions with ru radii between 130 and 160 pm. As can be seen in Fig. 5.42 the effect on Tc varies considerably among lead, strontium and calcium which enables the transitions to be shifted to suit 

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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. 

Barium titanate has widespread use ia the electronics iadustry. Its high dielectric constant and the ease with which its electrical properties can be modified by combination with other materials make it exceptionally suitable for a variety of items, ie, miniature capacitors (see Ceramics as electrical materials). 

Bismuth Stannate. Bi2(Sn03)3 sometimes added to barium titanate bodies to modify their dielectric properties particularly to produce bodies having an intermediate dielectric constant (1000-1250) with a negligible temperature coefficient. 

The dielectric constant of barium titanate ranges from 1200-1600 at 1 kHz and 25°C, increasing to -10,000 as the Curie temperature is approached. Under these same conditions, the power factor is a maximum of 1.5%. Because of the large variation of its dielectric properties with temperature and voltage, barium titanate is not, except in rare instances, used as a dielectric without modification. Dielectric properties can be easily modified, however, by combination... 

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