Bismuth rich phase

Both, spinel and pyrochlore accommodate excess dopants, the concentrations of which exceed their solubility limits in ZnO, and therefore they concentrate at grain boundaries [113]. The microstructure of a ZnO varistor will then comprise ZnO grains, a bismuth-rich phase, and spinel grains, which can be located either inter- or intragranularly (Figure 1.5). 

Figure 1.5 A backscattered-electron microscopy image of a polished ZnO varistor ceramics sintered at 1170 °C. The dark gray phase is ZnO, the light-gray is spinel, and the white is the bismuth-rich phase [131].

The maximum in catalytic activity observed for the multiphase region of the phase diagram necessarily arises from interactions between the separate phases. The bismuth rich and cerium rich solid solutions can readily form coherent interfaces at the phase boundaries due to the structural similarities between the two phases which can permit epitaxial nucleation and growth. A good lattice match exists between the [010] faces of the compounds, this match is displayed in Figure 6. We have also shown that regions of an [010] face of a Ce doped bismuth molybdate crystal resembles cerium molybdate compos tionally. This means that the interface between the two compounds need not have sharp composition gradients. It is structurally possible for the Bi-rich phase to possess a metal stiochiometry at the surface that matches that of the Ce-rich phase. 

Physically, the relationship between catalytic activity and Z f can be understood from a study of single phase bismuth cerium molybdate solid solutions. The results show that maximum activity is achieved when there exists a maximum number and optimal distribution of all the key catalytic components bismuth, molybdenum and cerium in the solid. Therefore, it reasonably follows that the low catalytic activity observed for the two phase compositions where Af Af(min) results from the presence of interfacial regions in the catalysts where the compositional uniformity deviates significantly from the equilibrium distribution of bismuth and cerium cations present in the solid solutions. These compositions may contain areas in the interfacial region which are more bismuth-rich or cerium-rich than the saturated solid solutions. Conversely, at Af(min), the catalyst is similar to an ideal mixture of the two optimal solid solutions. The compositional homogeneity of the interfacial region approaches that of the saturated solid solutions. Therefore, the catalytic behavior of compositions at Af(min) is similar to that of the saturated solid solutions. 

It is quite difficult to explain this difference in the behavior of similar compounds. The factors which can account for these differences in the evaporation mechanisms can be reduced to two. First, we have the presence of intermediate phases which are less rich in Se or Te, and which are not observed in the bismuth—sulfur system [20-22]. The presence of intermediate phases, even though they decompose peritectically below the meltii points of Bi2X3, should stabilize bismuth selenide and telluride against thermal dissociation. Secondly, bismuth selenide and telluride have one type of crystal structure and bismuth sulfide has a different structure. The telluride and selenide have layered lattices and the sulfide has a chain lattice but with some sulfur atoms outside the chains [23, 24]. It is natural to assume that such sulfur atoms are bound less strongly to the lattice and this accounts for the ease of thermal dissociation in bismuth sulfide. 

Pig. 5.33c TViple-grain junction in ZnO with vanishing dihedral angle (cf Fig. 5.34). The grain boundary phase is rich in bismuth oxide, wets extremely well and is essential for the varistor properties (see Section 5.8). FVom Ref. [164]. 

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