Relaxor Dielectrics

These lead-based materials (PZT, PLZT, PMN) form a class of ceramics with either important dielectric, relaxor, pie2oelectric, or electrooptic properties, and are thus used for appHcations ia actuator and sensor devices. Resistive properties of these materials ia film form mirror the conduction processes ia the bulk material. Common problems associated with their use are low dielectric breakdown, iacreased aging, and electrode iajection, decreasiag the resistivity and degrading the properties. 

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... 

Because of very high dielectric constants k > 20, 000), lead-based relaxor ferroelectrics, Pb(B, B2)02, where B is typically a low valence cation and B2 is a high valence cation, have been iavestigated for multilayer capacitor appHcations. Relaxor ferroelectrics are dielectric materials that display frequency dependent dielectric constant versus temperature behavior near the Curie transition. Dielectric properties result from the compositional disorder ia the B and B2 cation distribution and the associated dipolar and ferroelectric polarization mechanisms. Close control of the processiag conditions is requited for property optimization. Capacitor compositions are often based on lead magnesium niobate (PMN), Pb(Mg2 3Nb2 3)02, and lead ziac niobate (PZN), Pb(Zn 3Nb2 3)03. 

Ferroelectric Thin-Film Devices. Since 1989, the study of ferroelectric thin films has been an area of increasing growth. The compositions studied most extensively are in the PZT/PLZT family, although BaTiO, KNbO, and relaxor ferroelectric materials, such as PMN and PZN, have also been investigated. Solution deposition is the most frequentiy utilized fabrication process, because of the lower initial capital investment cost, ease of film fabrication, and the excellent dielectric and ferroelectric properties that result. 

Fig. 10 Relaxation time distribution function/(r) describing the dielectric dispersion in relaxor PMN. The short timescale maximnm describes the glassy-type dynamics, whereas the long timescale part refers to the polar clnster dynamics. The same featnres are obtained in PMN, PLZT, and SEN relaxors...

Relaxor materials as Pb(Mgi/3Nb2/3)03 (pmn), Pb(Zni/3Nb2/3)03 (PZN), and (Pbo.92 La0.o8)(Zro.7Ti0.3)03 (plzt) are a subgroup of ferroelectrics with diffuse phase transistions. Characteristic behavior of this class are the strong dielectric dispersion related with high dielectric losses, see Figure 1.18. 

Figure 1.18 (a) Normalized polarization for first-order, second-order and diffuse phase transition in ferroelectric and relaxor materials and (b) dielectric behavior of relaxor-type... 

However, the temperature, at which the maximum of the initial scattered light occurs, seems to be related to the scattering angle 9S and thus to the period Ag , respectively. Figure 9.14(b) shows the correspondence between the temperature Tm of maximum intensity Ig and the spatial period Agn. A spatial disorder of the smallest polar structures occurs at Tm = 45 °C, while the spatial orientation of the largest structures remains stable up to Tm = 60 °C. Such big dispersion of the thermal decay of polar structures over Agn unambiguously illustrates the relaxor behavior of sbn. At the same time it is a key point to understand the bandwidth in the determination of the phase transition temperature Tm in sbn from different methods. For example, in sbn doped with 0.66 mol% Cerium, Tm detected from the maximum of the dielectric permittivity e at 100 Hz (e-method) equals Tm = 67 °C [20], Determination of Tm from the inflection point of the spontaneous electric polarization P3... 

Figure 15.2 Dielectric responses of RLs both without (a) and with (c) electric field bias as discussed in the text. Response (b) defines all the various characteristic transition temperatures of a relaxor (from [14]).

Figure 15.6 The relaxor dielectric response of PMN at 1 and 8x 103 bar. The frequencies from left-to-right are 102, 103, 104 and 106 Hz (from [14]).

Close parallels can be drawn between the dielectric response of a relaxor and that of a random assembly of electric dipoles - a dipolar glass . The matter is clearly one of very considerable complexity depending as it must upon the sizes, charges and polarizabilities of the ions, and thermal history. 

Kanai, H. et al. (1998) Effects of microstructure on insulation resistance degradation of relaxors, in Advances in Dielectric Ceramic Materials, Vol. 88, The Am. Ceram. Soc., 295-9. 

Relaxor ferroelectrics47-49 (RFEs) have attracted considerable attention in recent years due to their unusual physical behaviour. Relaxors are technologically important as transducer/actuator materials. Relaxors are intermediate between dipolar glasses and classical FEs and exhibit both substitutional and charge disorder. They exhibit very large dielectric, piezoelectric, and electromechanical... 

Fig. 7.9 shows the temperature dependence of the dielectric constant and dielectric loss at 1 kHz for the PMN-PT ceramics obtained by sintering the calcined powders from a soft-mechanochemical route at 1200°C for 2 h. A diffuse phase transition, being typical for a relaxor, is observed for each ceramics. As x increases from 0 to 0.2, the maximum dielectric constant, K, , increases from 13000 to 27000. The temperature correspondent to K ,... 

Even though some ferroelectric materials, especially the relaxor ferroelectrics, have an extremely large dielectric constant, which is a very desirable property for the dielectric layer of capacitors in ULSI DRAMS, the usually large dielectric loss prevents the materials from being used in the DRAMS. Furthermore, the quite large number of component cations of the relaxor ferroelectric materials makes it almost impossible to deposit thin films using chemical vapor deposition (CVD) which is believed to be the method of choice for mass production of the devices. 

The use of excess PbO to achieve densifrcation is quite common in the sintering of PZT. Accurate control of the atmosphere in binder burnout and sintering is important to assure reproducible electrical, dielectric, and piezoelectric properties. The same understanding of processing and control is needed in the manufacture of relaxor dielectric materials and relaxor MLC using the excess PbO route. Successful implementation can result in MLC reliability equal to that of BaTi03 MLC [41]. 

Another field of intensive research is the insulating perovskite alloys with exceptional dielectric and piezoelectric properties [74], like the so-called relaxor ferroelectric alloys PZT (PbZrxTii-xOs), PZN-PT (Pb(Zni/3Nb2/3)03-... 

Relaxor ferroelectric polymers are intimately related to the ferroelectric polymers described above. All known relaxor ferroelectric polymers are based on the P(VDF-TrFE) copolymer. As the name suggests, these polymers behave as relaxor ferro-electrics, which is distinguished by a broad peak in dielectric constant and a strong frequency dispersion [99,100]. There are two major limitations of the P(VDF)-based ferroelectric actuators. First, the electrically induced paraelectric-ferroelectric transition that allows for actuation only occurs at temperatures above the Curie... 

Recent work by Bao et al. has shown that P(VDF-TrFE) synthesized via reductive dechlorination from P(VDF-CTFE) exhibits ferroelectric relaxor behavior at high temperature ( 100°C) with a melting point near 200°C [111]. This result is important as it provides another avenue to study the relaxor phenomena which are still not completely understood. The high melting point coupled with the high dielectric response of these materials at high temperature makes them attractive for use in high-temperature capacitors. 

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