Ceramics grain boundaries

E., (2000), Adsorption and wetting mechanisms at ceramic grain boundaries , Ceramic Transactions, 118, 427 -44. 

Eqs (2.88) and (2.89) cannot be applied to all solids but they are valid for the many high symmetry ionic structures that are non-polar and non-conductive. Also it has to be borne in mind that in the case of ceramics, grain boundaries can give rise to anomalies in the applied field distribution, and occluded layers of water can contribute to increased permittivities. R.D. Shannon [9] and others have calculated polarizabilities using Eq. (2.89) with the established values of molecular volume (Fm= 1 /N) and permittivity. They find that each constituent ion can be assigned a unique polarizability which is the same whatever other ions they are associated with. Table 2.5 gives the polarizabilities of a wide selection of ions and using these it is possible to calculate the permittivity of any combination... 

When an ordered array of monosized particles is sintered, the vacancies and grain boundaries create pores, cracks, and other flaws which are difficult to remove dming sintering. Pores are flaws about the size of the original ceramic particles in the final sintered ceramic. Grain boundaries are flaws that lead to cracks of a size equivalent to the size of the ordered domain. Both these flaws lead to the failure of the final ceramic piece according to Griffiths s analysis ... 

Kingery, W. D. (1974). Plausible concepts necessary and sufficient for interpretation of ceramic grain-boundary phenomena I, Grain boundary characteristics, structure and electrostatic potential II, Solute segregation, grain-boundary diffusion and general discussion. Journal of the American Ceramic Society, 57, 1-8 and 74-83. 

The variety of mixed conductors is also very high and is associated with their applications in solid oxide fuel cells as electrode materials. The addition of the transition metal oxides to these conductors limits the decrease of the total conductivity and maintenance of its ionic nature. Further doping leads to a separation of a transition metal oxide phase inducing a correlative decrease of the ionic conductivity and an increase of the electronic conductivity, especially at low temperatures. Mixed conductivity was observed only in transition metal oxide phases and electronic conductivity in the zirconia-rich parts of the binary and ternary systems. The phase separation of a transition metal oxide starts at the ceramic grain boundaries. Then, the second phase forms grains and cross-linked structures, and the conductivity approaches to that of the pure transition metal oxide. The enhancement of the solid solubility of the transition metal cations with the increase of the temperature is characteristic of most studied ternary systems. When the concentration of the stabilizing dopant is changed, the... 

At lower temperatures and for fine-grained ceramics grain boundary diffusion may be fhe dominanf pafh. In fhese sifuafions the process is termed Coble creep (Coble, 1963) and the creep rate is... 

Sintering. A ceramic densiftes duriag sintering as the porosity or void space between particles is reduced. Additionally, the cohesiveness of the body iacreases as iaterparticle contact or grain boundary area iacreases. Both processes depend on and are controlled by material transport. 

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