Grain boundary, electroceramics

Fig. 3. An overview of atomistic mechanisms involved in electroceramic components and the corresponding uses (a) ferroelectric domains capacitors and piezoelectrics, PTC thermistors (b) electronic conduction NTC thermistor (c) insulators and substrates (d) surface conduction humidity sensors (e) ferrimagnetic domains ferrite hard and soft magnets, magnetic tape (f) metal—semiconductor transition critical temperature NTC thermistor (g) ionic conduction gas sensors and batteries and (h) grain boundary phenomena varistors, boundary layer capacitors, PTC thermistors.

A book edited by Levinson (1981) treated grain-boundary phenomena in electroceramics in depth, including the band theory required to explain the effects. It includes a splendid overview of such phenomena in general by W.D. Kingery, whom we have already met in Chapter I, as well as an overview of varistor developments by the originator, Matsuoka. The book marks a major shift in concern by the community of ceramic researchers, away from topics like porcelain (which is discussed in Chapter 9) Kingery played a major role in bringing this about. 

The bulk and grain boundary behaviour of a polycrystalline electroceramic material can be described by a Voigt structure consisting of two RC circuits. This simple Voigt structure is shown in Figure 4.24a. The parameters of this model all have direct physical meanings Rl =Rh, ci = R2 = Rgb, and C2 = Cgb, where b refers to bulk and gb refers to grain boundary. 

As is the case in many fields, for example, microelectronics, catalysis, and medidne, much interest has developed in understanding the role of nanodimensions in influencing and ideally optimizing properties. In this chapter, attention is focused on the impact of nanoscale dimensions on the properties of electrically conductive electroceramics and the implications that this may have on applications. This is particularly relevant since electroceramics, by their nature, are dominated by boundaries (grain boundaries, electrode interfaces, surfaces, etc.), with the notable exception of epitaxial thin films, which are optimized for dielectric and ferroelectric applications but are beyond the scope of this chapter [5]. Some boundaries are detrimental to... 

Litzelman, S.J., De Souza, R.A., Butz, B Tuller, H.L., Martin, M and Gerthsen, D. (2008) Heterogeneously doped nanocrystaUine ceria films by grain boundary diffusion Impact on transport properties. J. Electroceram.,... 

Eleig, /., and Maier, J. (1999). Einite-element calculations on the impedance of electroceramics with highly resistive grain boundaries 1. Laterally inhomogeneous grain boundaries. /. Am. Ceram Soc. 82 3485-3493. 

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