Relaxor materials

Fig. 11. Fundamental characteristics of relaxor materials compared to BaTiO. Temperature dependence for the relaxor ferroelectric 0.93...

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... 

In this section, we will be concentrating mainly on the use of NMR technique based literature on dipolar glasses and the information obtained from them. The NMR nuclei studied in dipolar glasses are 1H, 2H and 87Rb and 31P. 207Pb, 45Sc and 93Nb are other nuclei which are studied in relaxor materials. 

The NH- - -N-bonded crystals revealed yet another exeiting property of the hydrogen-bonded materials. In a typical ferroelectric crystal, domains of different spontaneous polarization are formed, as illustrated in Fig. 3a. Most recently also relaxor behaviour has been observed in the dabco monosalts [36]. In the relaxor materials, small nano-sized domains are formed, which have properties considerably different from the bulk of the crystal (see Fig. 3b). The structure of these domains may be different, although it should be stabilized by their surroundings. The properties of these nanoregions can considerably influence the macroscopic properties of the crystal, which despite the... 

We thank W. W. Rhodes of AT T Bell Laboratories at Murray Hill, New Jersey for supporting our work in the relaxor materials. Subsequent work on the processing and reliability of multilayer ceramic capacitors was achieved through many useful collaboration and discussions with our colleagues at AT T Bell Laboratories Engineering Research Center at Princeton, New Jersey. 

It is clear that a sharp Curie tenperature cannot be defined for relaxor materials in the same way as for simple materials such as BaTiOj. To take account of this, a depolarisation temperature, is taken as the temperature at which a partial or complete loss of polarisation, and a consequent degradation in piezoelectric performance, occurs. In materials with relative permittivity behaviour similar to that in Figure 6.18b,... 

In addition to the multilayered ceramic capacitors just described, many of the barium titanate-based compounds that exhibit high dielectric constants are used in single-layer tape-cast capacitor devices. Relaxor materials such as lead magnesium niobate (PMN), which are characterized by high dielectric constants, broad dielectric maxima, and low sintering temperatures, have been manufactured in thin sheets by tape casting. 

Relaxor ferroelectrics can be prepared either in polycrystalline form or as single crystals. They differ from the previously mentioned normal ferroelectrics in that they exhibit a broad phase transition from the paraelectric to ferroelectric state, a strong frequency dependence of the dielectric constant (i.e. dielectric relaxation) and a weak remanent polarization. Lead-based relaxor materials have complex disordered perovskite structures. 

Lead magnesium niobate, Pb(Mg 3Nb2/3)03, is a ferroelectric relaxor material which is paraelectric at room temperature. The perovskite phase is typically modifield with addition of lead titanate or barium titanate, which raises the Curie temperature to near room temperature. Very high dielectric constants can be achieved, >20,000, but at the expense of temperature stability. 

Other materials that possess high dielectric constants are relaxor materials such as lead magnesium niobates. The structure of relaxor materials is extremely sensitive to stoichiometry and processing. Small changes in processing conditions or stoichiometry lead to formation of undesirable paraelectric phases and a resultant low dielectric constant. The extreme sensitivity of these materials has limited their use in capacitor dielectrics. 

Under the effect of a stress (X ) or an electric field (E. ), a relaxor material gets deformed. The relation x(Xjj,Pj.) is quadratic in P ... 

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. 

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. 

In such a measurement, the sample is clamped as lightly as possible, and the displacement of the surface in monitored. The amount of sample clamping is important, because the mechanical constraints can impact the ferroelastic response of the sample. That is, in samples where the mechanical coercive stress is low, it is possible to change the domain state of the material by improperly clamping it in the sample fixture. This is especially important in elastically soft piezoelectrics, such as many of the relaxor ferroelectric PbTiC>3 single crystals. 

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. 

Strontium barium niobate is a single-crystal material with the tungsten bronze type of structure which is made by the Czochralski method but has yet to find a major use. It has relaxor characteristics of the type shown in Fig. 7.1 which give it a high pyroelectric coefficient and detectivity, but its high permittivity lowers the figure of merit l. ... 

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... 

It should be stressed that the diffraction methods do not provide complete characterization of lattice distortions and ionic shifts in relaxors due to the compositional disorder of these materials and nanometric scale of polar order. Thus, local methods such as magnetic resonance and, in particular, NMR can be extremely useful in this case. In NMR experiments, the nuclei are sensitive to their local environment at a distance less than 1-2 nm. In addition, NMR operates at a much longer time scale (105-108 s) in comparison with the neutron or X-ray... 

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. 

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