Conduction Mechanisms in Ceramics

The importance of Eq. 30.2 is that it applies to all materials and it shows that the two factors affecting o are 

When we consider the effect of variables such as composition, structure, and temperature on o we are concerned with their effect on n and p. 

If more than one type of charge carrier is contributing to o then we can define a partial conductivity for each. Eor example, if o were due to the movement of electrons and cations with a charge Z, then for electrons 

TABLE 30.2 Transference Numbers of Cations, t+, Anions, t, and Electrons or Holes, 4.hSeveral Materials  

The transference or transport number is the fraction of Otot contributed by each charge carrier. 

---

We will begin by describing the conduction mechanisms in ceramics and looking at some specific applications. We will finish by describing one of the most fascinating developments in ceramics—high-temperature superconductors. 

The conduction mechanisms in ceramics can be quite complex and may involve the movement of electrons. 

Ion movement can make a major contribution to o, particularly if the material has a large Eg. Conductivity resulting from ion migration is important in several ceramics. It is also the major conduction mechanism in ionic salts such as the halides. 

Several kinds of conduction mechanisms are operative in ceramic thermistors, resistors, varistors, and chemical sensors. Negative temperature coefficient (NTC) thermistors make use of the semiconducting properties of heavily doped transition metal oxides such as n-ty e Ti O andp-ty e... 

A polymer electrolyte with acceptable conductivity, mechanical properties and electrochemical stability has yet to be developed and commercialized on a large scale. The main issues which are still to be resolved for a completely successful operation of these materials are the reactivity of their interface with the lithium metal electrode and the decay of their conductivity at temperatures below 70 °C. Croce et al. found an effective approach for reaching both of these goals by dispersing low particle size ceramic powders in the polymer electrolyte bulk. They claimed that this new nanocomposite polymer electrolytes had a very stable lithium electrode interface and an enhanced ionic conductivity at low temperature. combined with good mechanical properties. Fan et al. has also developed a new type of composite electrolyte by dispersing fumed silica into low to moderate molecular weight PEO. 

Solid-solid synthesis reactions operate by different mechanisms, which include solid state diffusion and chemicsd reaction. Diffusion in ceramic solids is always ionic in nature and depends on defect or hole diffusivity, as well as, electron conductivity. Once the ionic reactants are in close association, chemical reactions can take place. 

Although the emphasis here will, by necessity, be placed on more recent data, several key reviews of transport in nanocrystalline ionic materials have been presented, the details of which will be outlined first. An international workshop on interfacially controlled functional materials was conducted in 2000, the proceedings of which were published in the journal Solid State Ionics (Volume 131), focusing on the topic of atomic transport. In this issue, Maier [29] considered point defect thermodynamics and particle size, and Tuller [239] critically reviewed the available transport data for three oxides, namely cubic zirconia, ceria, and titania. Subsequently, in 2003, Heitjans and Indris [210] reviewed the diffusion and ionic conductivity data in nanoionics, and included some useful tabulations of data. A review of nanocrystalline ceria and zirconia electrolytes was recently published [240], as have extensive reviews of the mechanical behavior (hardness and plasticity) of both metals and ceramics [13, 234]. 

The thermal-mechanical coupling model for suddenly-heated ceramic-metal FGMs has been developed by the present authors in Ref[l]. To consider the plastic deformation effect on the heat conduction in the materials, the coupled heated conduction equation in Ref.[l] is now modified as ... 

---