Barium titanate, 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. 

Figure 1.7 Various properties of barium titanate as a function of temperature. Anisotropic properties are shown with respect to the lattice direction, (a) Lattice constants, (b) spontaneous polarization Ps and (c) relative permittivity er.

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). 

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 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... 

P. Saini, M. Arora, G. Gupta, B. K. Gupta, V. N. Singh, V. Choudhary, High Permittivity Polyaniline-Barium Titanate Nanocomposites with Excellent Electromagnetic Interference Shielding Response. Nanoscale 2013, 5, 4330. 

Barium titanate has been studied extensively since the end of World War II when it was identified independently in Russia, Japan and USA as a promising material with high permittivity for ceramic capacitors. Its ferroelectric activity is known since 1946 when it was discovered on ceramic samples. Since 1950 s also the single-crystals of desired size and quality are available. BaTiOs undergoes several phase transitions... 

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... 

Barium strontilun titanate - (Ba, Sr)Ti03 Ferroelectrics spontaneous polarization that can be switched with an electric field. They have a structural phase transition at a temperatiu-e (Curie temperature (TJ) at which permittivity has a strong anomaly. For example, lead zirconate titanate - Pb(Zr, Ti)Os, barium titanate - BaTiOs... 

A real capacitance behavior is observed only for frequencies less than 1/(2jiRC). A wide frequency range is obtained by increasing the grain conductivity as much as possible. This is the reason why strontium titanate was preferred to barium titanate, with the mobihty of electrons being greater (see Table 11.4). Typically, conductivity is 10 Q l cm i and the cut-off frequency 1 GHz. The apparent relative permittivity is of the order of 20,000. 

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. 

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