Dielectric permittivity ceramics

Fig. 107. Temperature dependence of the dielectric permittivity r determined at various frequencies for a ceramic sample ofRbsNb3OF,H.

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... 

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 Ba(ZrxTii x)03 all phase boundaries meet at a Zr content x 0.18, see Figure 1.16. Because of the superposition of the particular phase transitions the resulting transition becomes diffuse with a broad maximum of the dielectric permittivity as shown in Figure 1.16. Therefore, this composition has the potential as suitable temperature-stable dielectric for ceramic capacitors. 

Class I dielectrics usually include low- and medium-permittivity ceramics with dissipation factors less than 0.003. Medium-permittivity covers an sr range of 15-500 with stable temperature coefficients of permittivity that lie between +100 and -2000 MK-1. 

Class II/III dielectrics consist of high-permittivity ceramics based on ferro-electrics. They have er values between 2000 and 20 000 and properties that vary more with temperature, field strength and frequency than Class I dielectrics. Their dissipation factors are generally below 0.03 but may exceed this level in some temperature ranges and in many cases become much higher when high a.c. fields are applied. Their main value lies in their high volumetric efficiency (see Table 5.1). 

Medium-permittivity ceramics are widely used as Class I dielectrics, and in order to be in this category they need to have low dissipation factors. This precludes the use of most ferroelectric compounds in their composition since ferroelectrics have high losses (tan S >0.003), particularly when subjected to high a.c. fields. 

Ceramic capacitors are prepared with their chemical compositions placing them close to a ferroelectric-paraelectric phase boundary, where the dielectric permittivity is anomalously high. These materials are commonly based on BaTiOs which is similar in structure and properties to the piezoelectric ceramics. 

Class II dielectrics are high-permittivity ceramics based on ferroelectrics (see Chap. 15) and have values of k between 2000 and 20,000. 

Inorganic nanoflllers such as clays or ceramics may improve mechanical properties and dielectric properties. An abundant literature has been devoted to layered silicates for applications in the biomedical domain, hydroxyapatite (HAp e.g., nanoparticles of 300 nm in Figure 13.1a) might be of interest. Ferroelectric ceramics are attractive for their high dielectric permittivity and electroactive properties. As an example, BaTiOa particles with d 700 nm are shown in Figure 13.1b. Conductive nanoparticles should induce electrical conductivity in polymeric matrices, but to preserve the mechanical properties, small amount should be used. Consequently, there is great interest in conductive nanotubes [i.e., carbon nanotubes (CNTs)], which exhibit the highest... 

Dielectric Properties Polymers have low dielectric permittivity and it is difficult for them to meet capacitor requirements. Ceramics, on the other hand, have high dielectric permittivity but require high processing temperatures. Dispersion of metal... 

In Fig. 1.14, the phase diagram of mixed relaxors (PSN)i c(PST) c for ordered and disordered ceramic samples, is reported. It is seen that the temperature of dielectric permittivity maximum in the disordered samples is higher than that in ordered PSN samples, while the behavior of PST samples is opposite. This difference has been staying a puzzle for several years. It was shown later (see [38]... 

Fig. 2.12 The temperature dependence of relative dielectric permittivity and losses for bulk PMN ceramics of 0.49 mm thickness for different frequencies, shown in the legends [28]...

The hysteresis loops studies appear to be very informative for nanograin ceramics also. In particular, above we have discussed above the relaxor state induced by the grain sizes. The studies of the relaxor state have been made on the basis of dielectric response analysis. In Ref. [27], the additional firm evidence of relaxor state has been obtained with the help of hysteresis loops measurements. In Fig. 2.16 we report the shape of hysteresis loops in ferroelectric relaxors at different temperatures. It is seen, that at r > (Tm = 363 K is the temperature of dielectric permittivity maximum for PbSci/2Nbi/202 (PSN)) there is no residual polarization, while it is nonzero at T< r. Similar behavior has been observed for 730 nm thick Pbo.76Cao.24Ti03 (PCT) film with average grain sizes 86 nm and Tm = 553 K. 

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