Relaxor ferroelectrics relative permittivity

Relaxor Ferroelectrics. The general characteristics distinguishing relaxor ferroelectrics, eg, the PbMg 2N b2 302 family, from normal ferroelectrics such as BaTiO, are summari2ed in Table 2 (97). The dielectric response in the paraelectric-ferroelectric transition region is significantly more diffuse for the former. Maximum relative dielectric permittivities, referred to as are greater than 20,000. The temperature dependence of the dielectric... 

Relaxor ferroelectrics differ from the ferroelectric materials described previously in many ways. Of fundamental importance is the fact that the relative permittivity shows a wide diffuse peak which is approximately the same magnitude as in a normal ferroelectric. The broad peak is relatively temperature insensitive, but the transition is frequency dependent, with a maximum value at a temperature T, rather than a sharp transition at a temperature (Figure 6.18a and b). The region under the curve represents a state called the ergodic relaxor (ER) state (see following text). 

Figure 6.18 Relative permittivity versus temperature behaviour (a) a canonical relaxor (b) non-canonical relaxor (c) normal ferroelectric...

The relative permittivity and other dielectric behaviour of a relaxor ferroelectric as a function of temperature is often different depending upon whether the sample is cooled from higher temperatures in an electric field (FC) or in no electric field (ZFC). For example, the non-ferroelectric low-tenperature state of a canonical relaxor can be transformed irreversibly into a ferroelectric state by the application of a sufficiently high electric field or if the material is cooled in the presence of an electric field. As with non-canonical relaxors, this state transforms to the ergodic state above... 

In the second group, the diffuse peak in relative permittivity is steeper on the low-temperature side compared to the canonical relaxors, and at a temperature generally similar to the value of for a canonical relaxor, the PNRs amalgamate into domains and form a ferroelectric solid (Figure 6.20e). In these materials, no non-ergodic state forms. 

Figures 1 and 2 respectively show the temperature dependence of the relative permittivity and loss tangent of relaxor ferroelectric PLZT (9.5/65/35). As the temperature increases from -60°C to 100°C, the relative permittivity generally increased due to the unfreezing of domains. Between 0°C and 10°C, a broad peak can be seen in the lower frequency curves. This peak corresponds to the diffuse phase transition in this relaxor ceramic from the ferroelectric to the paraelectric state (also called the relaxor phase). Further heating continued to increase the relative dielectric permittivity until a maximum was achieved, at which point, the crystal s structure became cubic. This maximum in the permittivity, which is frequency dependent, occurs at the Curie temperature. Evidence of these phase transitions can also be seen in the loss tangent graph in figure 2.

In addition to the high k of many relaxor compositions they also have a broad peak in the permittivity versus temperature range, even in the absence of additives and even in the form of single crystals. This behavior is attributed to nanoscale (-lOnm)-ordered regions, which are too small to yield the sharp phase transition of normal ferroelectrics. As a result, spontaneous polarization and associated ferroelectric properties are retained over a very broad temperature range. Another attractive feature of relaxors is that dense polycrystalline ceramics are achievable at relatively low sintering temperatures (<900°C), which allows a significant reduction in the amount of Pd used in Ag-Pd metallizations for electrodes in multilayer capacitors (see Section 31.7). 

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