Spinels chemical zoning

It provides a sensitive check for pyroxene entry as a crystallizing phase. V-Cr and V-Al substitutional trends provide a good control for the positions of both pyroxene and plagioclase in the crystallization sequence. The positive V-Al sympathetic trend and the negative V-Cr relationship of early spinels (Fig. 5a b) demonstrate the unique chemistry of those early chromites and indicate crystallization before pyroxene and plagioclase. The trends for chromites with the second chemical zoning trend and late ulvbspinel are antipathetic both for V-Cr and V-Al (Fig. 

Residual methane is present at the exit of the combustion zone. In the catalytic bed, the methane steam-reforming and the water shift reactions take place. The gas leaving the ATR reactor is in chemical equilibrium. Normally, the exit temperature is above 900-1100°C. The catalyst must withstand very severe conditions when exposed to very high temperatures and steam partial pressures. One example of an ATR catalyst is nickel supported by magnesium aluminum spinel. For compact design, the catalyst size and shape is optimized for a low pressure drop and high activity. 

Chemical vapour deposition (CVD), or more specifically, organo-metallic CVD can also be used to prepare ferrite thin films (Itoh, Takeda Naka, 1986). A mixture of acetylacetonate complexes of the desired metal is evaporated in a quartz boat (number 1 in Fig. 3.27) and is reacted with O2, which is introduced directly in the deposition zone. By adding a second furnace (B), an additional component evaporating at a different temperature can be transported to the substrate. Glass or MgO single crystals can be used as the substrates an annealing up to 1(X)0°C is performed on the film to improve its crystallisation. Spinel thin films (Ni, Ni-Zn) have been prepared by this method. 

In the case of the sulfidation of this alloy, alternating repetition of compact and microcrystallized layers is observed. The chemical (X ray energy dispersive spectrometry) and structural (X ray diffraction) analyses show that compact layers are rich in iron and contain mostly pyrrothite (FeS sulfide) whereas the microcrystallized layers are poor in iron and contain daubreelite (Fe(Cr, A1)2 S4 spinel). Very near the metal, a thin zone of variable composition and thickness (about several micrometers) is noticed. It can be verified that the layers are formed successively from this internal zone. An external zone of sulfide FeS develops during the reaction (see Figure 2a). 

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