Alumina conductive pathways

The activation energy represents the ease of ion hopping, as already indicated above and shown in Fig. 2.5. It is related directly to the crystal structure and in particular, to the openness of the conduction pathways. Most ionic solids have densely packed crystal structures with narrow bottlenecks and without obvious well-defined conduction pathways. Consequently, the activation energies for ion hopping are large, usually 1 eV ( 96 kJ mole ) or greater and conductivity values are low. In solid electrolytes, by contrast, open conduction pathways exist and activation energies may be much lower, as low as 0.03 eV in Agl, 0.15 eV in /S-alumina and 0.90 eV in yttria-stabilised zirconia. 

The conductivity, due to Na ions, passes through a maximum at intermediate x. It is optimised at x 2, where the values approach those of Na / "-alumina, especially at high temperature, >300°C, Fig. 2.11. At the solid solution limits, x = 0 and 3, the conductivity is very low, for the same reasons given in the discussion of Fig. 2.3. The crystal structure of NASICON is a framework, built of (Si, P)04 tetrahedra and ZrOg octahedra which link up in such a way as to provide a relatively open, three-dimensional network of sites and conduction pathways for the Na ions, Fig. 2.12(a). Two Na sites are available, Nal and Na2. The former is a six-coordinate site while the latter is an irregular eight-coordinate site. These sites are partially occupied at intermediate x. 

In molten salt sublattice materials, practically all the ions in the sublattice are available for motion with an excess of available sites per cation as in e.g. Na -alumina, (a25 c = 1 4 x 10 Q cm S E = 0.16 eV). This leads to a high degree of disorder of these cations. The site occupancies and the conductivity characteristics for some of these salts are given in reference 12, pp. 49 and 53. Conduction is favoured by a levelling of the energy profile along the conduction pathway... 

The ionic conductivity for various ions in the piP"-alumina structure along the conduction planes shows a maximum for an optimum size of the ions. It should be neither too small nor too big to fit the available pathways in the lattice [8]. 

Dechlorination of 1,1,2,2-TCA can follow one of the three pathways in Figure 3 elimination and formation of a partially chlorinated ethylene sequential dechlorination or direct transformation to ethane. The first pathway is unlikely, given the lack of chlorinated ethylene intermediates and the fact that the transformation rate of 1,1,2,2-TCA is an order of magnitude lower than that of PCE, which has a similar rate to TCE, the DCEs and VC. (Schreier 1996) The recalcitrance of the DCA isomers to transformation (Lowry and Reinhard 1999 McNab and Ruiz 1998) implies that sequential dechlorination through DCA does not occur, but this pathway cannot be ruled out because the DCA tests were conducted using Pd/alumina catalysts, rather than the Pd/C used for the 1,1,2,2-TCA and 1,1,2-TCA tests. However, the available data (lack of chlorinated intermediates and the low reactivity of the DCA isomers) suggest that direct transformation is the most probable pathway. 

Fig. 4.30 Structure of /f -alumina (a) alternating spinel blocks and conduction planes (b) migration pathway of Na+ ions indicating paths of concerted motion.

The addition of fillers such as carbon and ceramics (silica, alumina, aluminum nitride, etc.) is commonly used to induce thermal conductivity into conventional polymers. The higher thermal conductivity can be achieved by the addition of high volume fractions of suitable use of a filler. Fillers have to form a random close packed structure to maximize a pathway for... 

Figure 29.2 Structure of -alumina showing (a) the altemation of spinei biocks and conduction planes (b)the honeycemb pathway along which Na ions migrate and the kind of concerted mechanism that is envisaged (Courtesy ef Chleride Silent Power)...

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