Other dopants

Irreversible reaction can occur with other dopants such as iodine (at elevated temperatures) [69] and FeCl3 [70], and a loss of conductivity is seen over a period of months for p-doped material even at room temperature, although n-doped material appears to have a higher degree of thermal stability [71]. 

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... 

The diagram then clearly shows the situation for a particular temperature and so can be used to determine the most important conductivity mechanism for a particular operating condition at the temperature of the diagram. Similar diagrams must be constructed for other temperatures, which, when stacked up, will give a three-dimensional representation of the way in which the predominant conduction mechanism changes with temperature. Another set of diagrams will also need to be constructed for other dopant concentrations. 

In 1949, the development of a catalyst based on a combination of platinum and an acidic component (e.g. A1203, A1C13) allowed the use of lower reaction temperatures than with the early catalysts.6 However, problems were still encountered with chlorine corrosion. In the 1960s, Universal Oil discovered that the addition of rhenium to a bifunctional Pt/Al203 catalyst resulted in slower deactivation by carbon deposition, and other dopants have since been found to modify the catalyst acidity and resistance to poisons, e.g. Cl, Sn, Ir. More recently, catalysts based on zeolites and noble metals have been shown to be more resistant to nitrogen and sulphur compounds, while giving a high activity and selectivity to branched alkanes. 

The analytical solutions to Fick s continuity equation represent special cases for which the diflusion coefficient, D, is constant. In practice, this condition is met only when the concentration of diffusing dopants is below a certain level ( 1 x 1019 atoms/cm3). Above this doping density, D may depend on local dopant concentration levels through electric field effects, Fermi-level effects, strain, or the presence of other dopants. For these cases, equation 1 must be integrated with a computer. The form of equation 1 is essentially the same for a wide range of nonlinear diffusion effects. Thus, the research emphasis has been on understanding the complex behavior of the diffusion coefficient, D, which can be accomplished by studying diffusion at the atomic level. 

GC-MIP systems have been investigated in considerable detail. Because of the low power of the plasma, it is easily quenched if the normal, atomic spectrometric sample introduction techniques, such as nebulisation, are used. Capillary columns overcome this problem as they require only low flow rates and small sample sizes more compatible with stable plasma operation. The capillary columns can be passed out of the oven, down a heated line, and the end of the column placed in the plasma torch just before the plasma, thus preventing any sample loss. A makeup gas is usually introduced via a side arm in the torch to sustain the plasma (Fig. 4.1, Greenway and Barnett, 1989). Other dopant gases can also be added in this way to prolong the lifetime of the torch and improve the plasma characteristics. 

A short review of the OVPO and the MCVD method is given. A more detailed discussion of the PCVD method is presented. Emphasis has been laid on the description of experiments in which pure and Ge02 -oxide doped silica have been deposited. It turns out that the PCVD method has some unique properties such as deposition without soot formation in the gas phase, moderate substrate temperature, high deposition efficiency and the possibility of rapid reactor movement. Besides germanium, other dopant elements such as boron and fluorine have also been successfully deposited simultaneously with silica. 

In brief, the core-shell technique allows one to prepare doped nanoparticles that contain no other dopant sites than those known from the corresponding bulk material, despite their small size, below 15 nm. The quantum efficiency of lanthanide ions can also be improved by surface modification of the nanoparticles. 

In addition to the optical activity of the samples, CSA also has beneficial effects on the conductivity of the polymer when compared with other dopants [58,59], and can be exploited for systems used for chiral recogni-... 

Wang et al. [59] obtained a Tc value of 90 K with a nominal starting composition of Tl2 2Ba2CeCu309 3. Since the product is a multiphase with mainly Tl-2201 phase, the research is still open to find whether or not Ce is really going inside the Tl-2201 lattice. Following this work, Thomas et al. [60] have attempted to incorporate Th, Pr, Tb, Pb, and Te. However, they also obtained multiphase samples with mainly Tl-2201 phase. Also, the Tc value obtained was 75 K for Th, and other dopants showed still lower Tc. Since all these samples are multiphasic we cannot comment about the variation of hole concentration with Tc. 

Though we will not imdertake any systematic analysis of the electronic implications of the presence of boron (or other dopants) in Si, it is evident that questions surrounding the energetics of impurities in Si are an important part of the whole picture of defects in semiconductors. One question of obvious significance is that... 

Recently Cabrera et al. [47] developed a new synthesis of ordered mesoporous solids involving the use of atrane complexes as precursors. This technique was tested to obtain silica, Al and Ti mesoporous oxides and simply and doubly doped oxides with a large list of metals [see Table 3 in reference 47]. Other dopants or chemical groups were added to MCM type materials. For... 

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