Titanates, dielectric permittivity

Barium titanate and BaTi03-based materials are most commonly used for ceramic capacitors with high dielectric permittivity. BaTi03 powder of extremely high quality (in respect of its purity, stoichiometry, particles morphology) is required for most of the modem applications. This characteristic may be considerably improved by the application of alkoxide precursors. Thus, it is of no surprise that synthesis of BaTi03 and BaTi03-based materials from metal alkoxides attracted considerable attention for several decades. The first works on... 

A ferroelectric model material is barium titanate BaTi03. On cooling from high temperatures, the permittivity increases up to values well above 10,000 at the phase transition temperature Tc. The inverse susceptibility as well as the dielectric permittivity follows a Curie-Weiss law x1 f 1 oc (T — O). The appearance of the spontaneous polarization is accompanied with a spontaneous (tetragonal) lattice distortion. 

The compositions of most dielectric materials used for ceramic capacitors are based on ferroelectric barium titanate. As discussed in detail in Pragraph 1.3 the permittivity of ferroelectric perovskites shows marked changes with temperature, particularly close to the phase transition. From the device point of view a high dielectric permittivity with stable properties over a wide temperature range is required. There are various specifications which have to be fulfilled (e.g. X7R AC/C(T = 25°C) < 0.15 in a range between -55°C and 125°C). 

For practical purposes it is important that the properties of titanate dielectrics can be deliberately adjusted. This concerns in particular the temperature dependence of permittivity which can be controlled either by shifting the maximum in the direction of the temperature coordinate while maintaining the maximum value, or by flattening the maximum. 

A significant increase in permittivity of barium titanate dielectrics, required very small size capacitors, can be attained by utilizing the barrier phenomena whi are based on the formation of thin insulating layers in semiconducting BaTi< ceramics. These systems exhibit permittivity values of the order 10. ... 

The introduction of neodymium into barium titanate leads to a product with a high dielectric permittivity of 80, which is attributed to a single-phase BaNd2TisOi4 (NBT), analogous to the phase described above in samarium-doped barium... 

Interest turned to compositions of high permittivity titanates and zirconates in which one member exhibited a negative and the other a positive TC , the objective being to achieve xf = 0. The effect of composition on TC, for two such systems is shown in Fig. 5.35. Another system which attracted interest is MgTi03-CaTi03 in which the end members have the dielectric properties shown in Table 5.7. The composition Mg0 95Ca0 05TiO3 yields xf 0, er = 21 and Q = 8000 at 7 GHz. 

Other microwave dielectrics have been identified and developed into commerical products and those currently exploited are listed in Table 5.8. The modified neodymium titanates are particularly important because of their high permittivity values and the reduction in resonator size this offers. 

The relative permittivity values normally encountered in crystals are rather small (Table 11.1). Some crystals, however, exhibit relative permittivity values many orders of magnitude higher than those in normal dielectrics. For example, one crystallographic polymorph of barium titanate, BaTiOs. has a relative permittivity, e, of the order of 20 000 (more values are given in Table 11.2.). By analogy... 

PBNT. Lead Barium Neodymium Titanate has very low dielectric loss and its high frequency permittivity is stable over a wide temperature range, and so it is used in microwave resonators and filters. 

Lower- and medium-K values (ranging from 10 to 160) are achieved with nonferroelectric dielectric materials. Magnesium titanate or titaniumoxide are typical exponents. These dielectrics are not subject to a permittivity reduction over time and can be tailored to negative positive zero (NPO) characteristics. 

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