Dopants chromium

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... 

Elemental forms of gallium and arsenic, plus small quantities of dopant material — silicon, tellurium or chromium — are reacted at elevated temperatures to form ingots of doped single-crystal GaAs. 

The control of electrical resistivity in these ceramics is, as noted above, very desirable. There has been considerable success in the use of uranium as a dopant for this function in a variety of compositions [36-38], Recently, chromium has also been shown to exhibit some promise for this [39],... 

Extension of the lifetime measurements to 77 K can be done easily by immersing the ruby sample in liquid nitrogen in a clear glass Dewar. In addition, spectral studies of the absorption and emission of ruby can be used to determine the Cr dopant levels and, at higher concentrations, the spectra of chromium-chromium pairs." ... 

In order to ensure the transfer of knowledge from the alumina system to other materials, a mathematical model was developed. This model is based on the balance of the dopant, e.g. chromium, in the molten layer An increase of chromium concentration in the supplied powder leads to an enrichment of the melt with chromium and by this way to an increasing amount of chromium in the crystaUizing material. This case is written in the following equation ... 

An example of a complex material used in syngas applications is Lag jSro gFeo g. Coo.iCro.203 [26]. This composition contains many of the features previously discussed. It is predominately an iron-based perovskite, providing both electronic and ionic conductivity. In order to improve the chemical stability and resistance to chemical expansion, the B-site is doped with chromium. A small amount of cobalt is added in order to improve the electronic conductivity, and cobalt wiU also increase the ionic conductivity slightly. Strontium is used as the A-site dopant in order to avoid the problems associated with calcium, and particularly with barium, in regard to reaction with CO2. In addition, the material has been made substantially A-site deficient to improve its stability. 

Replacement of Cr with Al has been investigated extensively in recent times. Several groups concluded that Cr can be successfully replaced with Al. In 2000 Araujo and Rangel [33] first time ever reported Fe-Al-Cu catalysts for WGS reaction. They prepared Fe-Al, Fe-Cu, Fe-Al-Cu catalysts by coprecipitation. They maintained the iron to dopant molar ratio of 10. Addition of Al to the iron oxide increases the catalytic activity of iron oxide slightly. However, addition of both Al and Cu to the iron oxide increases the WGS activity tremendously (34 x lO" mol g h ). The catalyst with both dopants showed higher activity than a chromium- and copper-doped commercial catalyst (25 X lO" mol g h ). This sample produces the active phase more easily than the other catalysts and shows resistance to a further magnetite reduction. 

Although coordination complexes are molecules, units can be identified in atomic solids that behave optically and electronically like fixed molecular complexes although there are no molecules in atomic solids. There are strong interactions among all the atoms in the lattice. The optical spectrum of a chromium dopant in oxide lattices, e.g., in a ruby laser, is like that of chromium coordination complexes in oxidic solvents. 

The nonstoichiometry of an oxide strongly depends on the presence of alio-valent impurities and dopants affect the number of thermal defects in non-stoichiometric oxides and their electrical conductivity because the solutes have a valence other than the atoms they replace. This is illustrated in the case of lithium and chromium doping of nickel and zinc oxides in equilibrium with gaseous oxygen. 

By definition, iron containing between 0.15 and 1.4 wt.% C is typically referred to as steel.Hence, although we typically think of steel as containing chromium and other metal dopants, some y- and 5-phases of pure iron could also be considered forms of steel. Steels with a carbon concentration of 0.83 wt.% undergo a transformation from austenite into two intimately mixed solid phases at a temperature of 723°C. Although this phase transformation looks like a eutectic, the material above... 

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