Defect chemistry boundaries

Materials Research Strategy Boundary Defect Chemistry... 

Volknann M., Hagenbeck R., and Waser R., Grain-boundary defect chemistry of acceptor-doped titanates Inversion layer and low-field conduction, J. Am. Ceram. Soc., 80, 2301-2314, 1997. 

M. Vollman, R. Waser Grain Boundary Defect Chemistry of Acceptor-Doped Titanates Space Charge Layer Width J Am.Ceram.Soc. 77[1] 235-43 (1994)... 

One consequence of the modified defect chemistry being restricted to the boundary is readily seen in Fig. 27. Unlike in the case of homogeneous doping, in heterogeneous doping the transition from interstitial to vacancy type does not show up as a knee in the conductivity curves, since, as soon as the boundary zone becomes less conductive, the bulk which is in parallel, dominates.113... 

Figure 36. Defect concentration and conductance effects for three different thicknesses Li L2 Lj. The mesoscale effect on defect concentration (l.h.s.) discussed in the text, when L < 4J, is also mirrored in the dependence of the conductance on thickness (r.h.s.). If the boundary layers overlap , the interfacial effect previously hidden in the intercept is now resolved. It is presupposed that surface concentration and Debye length do not depend on L. (Both can be violated, c , at sufficiently small L because of interaction effects and exhaustibility of bulk concentrations.)36 94 (Reprinted from J. Maier, Defect chemistry and ion transport in nanostructured materials. Part II. Aspects of nanoionics. Solid State Ionics, 157, 327-334. Copyright 2003 with permission from Elsevier.)...

The limits of integration are the oxygen partial pressures maintained at the gas phase boundaries. Equation (10.10) has general validity for mixed conductors. To carry the derivation further, one needs to consider the defect chemistry of a specific material system. When electronic conductivity prevails, Eqs. (10.9) and (10.10) can be recast through the use of the Nemst-Einstein equation in a form that includes the oxygen self-diffusion coefficient Dg, which is accessible from ionic conductivity measurements. This is further exemplified for perovskite-type oxides in Section 10.6.4, assuming a vacancy diffusion mechcinism to hold in these materials. 

Ikeda, J.A.S. and Chiang, Y.-M. (1993) Space charge segregation at grain boundaries in titanium dioxide 1, Relationship between lattice defect chemistry and space charge potential. f Am. Ceram. Soc., 76, 2437—2446. 

In order to establish the model of intergranular impedance for doped barium titanate, it is important to notice that miorostructure properties of BaTiOj based materials, expressed in their grain boundary contacts, are of basic importance for electric properties of these materials. The barrier character of the grain boundaries is especially pronounced for doped BaTiOs materials which are used as PTC resistors. Basically two types of dopants can be introduced into BaTiOs large ions of valence 3+ and higher, can be incorporated into Ba positions, while the small ions of valence 5+ and higher, can be incorporated into the Ti sublattice [9-11], Usually, the extent of the solid solution of a dopant ion in a host structure depends on the site where the dopant ion is incorporated into the host structure, the compensation mechanism and the solid solubility limit [12], For the rare-earth-ion incorporation into the BaTiOs lattice, the BaTiOs defect chemistry mainly depends on the lattice site where the ion is incorporated [13], It has been shown that the three-valent ions incorporated at the Ba -sites act as donors, which extra donor charge is compensated by ionized Ti vacancies (V -), the three-valent ions... 

Using the model compoimd, of which the bulk defect chemistry is treated in Section II. A. 1 of this chapter, Maier constracted a Kr6ger- fink diagram of the model compoimd MX for bulk and boundary layer. His results are schematically included in Figure 5.2, and presented in Figure 5.6. 

While it is the bulk defect chemistry that determines the efficiency of homogeneous doping, it is the defect chemistry of boundary regions that determines charge carrier variations in the vicinity of the interface. The defect model to be described below is able to solve the first problem, the elucidation of the boundary problems deserves much more further work. If we ignore effects at very small sizes at which interfacial effects dominate the whole carrier chemistry within the particle, the material s properties are expected to be dominated by bulk phenomena. 

We term it the degree of influence [249]. As can be seen from Fig. 5.75 t = 0, if the boundary layer defect chemistry does not differ fi om the bulk defect chemistry, i.e. 

Bulk and boundary conductivity sensors have already been discussed in Chapter 5 in relation to equilibrium defect chemistry, potentiometric sensors in the previous section. Nevertheless, we wish — in view of the importance of this application — to sketch out some of the fundamentals of electrochemical (composition) sensors . The fact that a variation in the chemical composition (ck) ehcits a physical signal is the rule rather than the exception. This is merely a necessary sensor criterion. In addition, it is important that a sensor signal exhibits adequate sensitivity , is sufficiently selective, stable and as free from drift as possible , and displays an ad-... 

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