Oxidative defects characteristics

The ionic defects characteristic of the fluorite lattice are interstitial anions and anion vacancies, and the actinide dioxides provide examples. Thermodynamic data for the uranium oxides show wide ranges of nonstoichiometry at high temperatures and the formation of ordered compounds at low temperatures. Analogous ordered structures are found in the Pa-O system, but not in the Np-O or Pu-O systems. Nonstoichiometric compounds exist between Pu02 and Pu016 at high temperatures, but no intermediate compounds exist at room temperature. The interaction of defects with each other and with metallic ions in the lattice is discussed. 

Olpin, S.E., Manning, N.J., Pollitt, R.J. Clark, S. (1997). J. Inherited Metab. Dis. 20, 415-419. Improved detection of long-chain fatty acid oxidation defects in intact cells using [9,10- H]oleate. Vianey-Saban, C., Divry, C., Brivet, M., Nada, M., Zabot, M.T., Mathieu, M. Roe, C. (1998) Clin. Chim. Acta, 269, 43-62. Mitochondrial very-long-chain acyl-coenzyme A dehydrogenase deficiency clinical characteristics and diagnostic considerations in 30 patients. 

In this section, we discuss atomistic modeling studies of the interaction of model passivated surfaces with halide ions. First, the study using conventional DFT of chloride adsorption and subsurface penetration on defect-free hydroxylated nickel oxide surfaces characteristic of passivated nickel surfaces is presented. Then the implications of using DFT-l-U are discussed as well as the interaction with other halides. This is followed hy one example on the effect of implementing surface defects characteristic of those observed experimentally on a passivated nickel surface. Finally, the application of reactive MD modeling to more complex systems including a substrate metal (copper) covered by a passive film (copper oxide) in interaction with a chloride-containing aqueous solution is presented. 

Figure 9.17. Mars-Van Krevelen mechanism for the oxidation of CO on a metal oxide surface. A characteristic feature is that lattice oxygen is used to oxidize the CO, leaving a defect that is replenished in a separate step by oxygen from the gas phase.

An unfortunate characteristic of early zinc polycarboxylate cements was the early development of elastomeric characteristics- cobwebbing -in the cement pastes as they aged, thus shortening working time (McLean, 1972). Improvements in cement formulation, the addition of stannous fluoride to the oxide powder (Foster Dovey, 1974, 1976) and modifications in the polyacid have eliminated this defect. However, the cements have to be mixed at quite a low powder/liquid ratio, 1 -5 1 0 by mass, when used for luting. 

A detailed description is given to the role of point defects available in the volume and on the surface of oxide adsorbents on adsorption-induced change of electrophysical characteristics. We try to deduce the impact of the proper nature of adsorbent as well as the nature of adsorption centers. 

In above sections the main attention has been paid to adsorption-caused change in electrophysical characteristics of semiconductor adsorbent caused by surface charging effects. However, as it was mentioned in section 1.6, the change in electrophysical characteristics of such adsorbents can be caused by other mechanisms, e.g. by direct interaction of absorbate with the surface defects provided (as in the case of oxide adsorbents) by superstoichiometric atoms of metals and oxygen... 

As it has been mention in preceding section, the vast effect of the mechanism of adsorption-caused change in electrophysical characteristics of adsorbent is provided by availability of defects [32]. However, various admixtures play similarly important role on effects of properties of oxides including the sensitivity of their electrophysical properties to adsorption [4, 5]. Small amounts of admixtures (of the order of 0.5 -1 mol.-%) can both increase the sensitivity of oxide for instance to oxygen (addition of Y2O3 to calcium oxide over pressure interval lO -10 Torr [189]) and decrease it (addition of Ga203 to ZnO [190]), or can result in insensitivity of electric conductivity on the pressure of the gas in question (as it is the case with respect to O2 while adding 0.5 -1 mol.-% of lithium to NiO [190]). 

The experimental studies of the surface properties of monocrystals of oxides of various metals recently conducted at well-controlled conditions [32, 210] enable one to proceed with detailed analysis of separate effects of various factors on characteristics of semiconductor gas sensors. In this direction numerous interesting results have been obtained regarding the fact of various electrophysical characteristics of monocrystalline adsorbents on the value of adsorption-related response. Among these characteristics there are crystallographic orientation of facets [211], availability of structural defects, the disorder in stoichiometry [32], application of metal additives, etc. These results are very useful while manufacturing sensors for specific gases with required characteristics. 

The microstructure of commercial varistors is extremely complex, and commercial preparations also contain other dopants, mainly oxides of cobalt, manganese, chromium, and antimony, that are used to fine tune the varistor characteristics. The transition-metal dopants are chemically similar to Zn2+ and mainly form substitutional defects within the ZnO grains, such as CoZn, that modify the n-type behavior of the grain interior. (See also Chapter 8 for further discussion of the electronic... 

There is considerable evidence that defective homeostasis of redox-active metals, i.e. iron and copper, together with oxidative stress, contributes to the neuropathology of AD. The characteristic histology of AD is the deposition of both Ap, as neurotic plaques (Figure 18.12a), and of the protein tau, as neurofibrillary tangles NFT (Figure 18.12b), predominantly in the cerebral cortex and hippocampus. 

---