Metal zoning

Orebody zoning (Park and Macdiarmid, 1963) is observed in Cu-Pb-Zn deposits. For example, in Osarizawa deposit, which is one of the largest Cu-Pb-Zn deposits in Japan, ore metal zoning from deeper to shallower parts is Cu —> ZnPb —> AuAg. 

The deposits are characterized by conspicuous metal zoning and polymetallic mineralization. From the centre to margin of the mine district, the following zonings are recognized Sn-W-Bi-Cu zone, Cu-As-Zn zone, and Zn-Pb-Cu-As zone (Nakamura, 1970). 

Ewers, G.R. and Keayse, R.R. (1977) Volatile and precious metal zoning in the Broadlands geothermal field. New Zealand. Econ. Geol., 72, 1337-1354. 

Govett, G.J.S. Whitehead, R.E.S. 1974. Origin of metal zoning in stratiform sulfides a hypothesis. Economic Geology, 69, 551-556. 

The effect of a centrifugal force on SHS reactions has been investigated with a primary focus on the influence of this force on the dynamics of the combustion process [9-11]. In this paper we present the results of an investigation on the formation of a functionally-graded ceramic/metal zone in a composite formed by combustion synthesis under the influence of a centrifugal force. 

In the case of (11 l)-metal//(001)-support contacts, a parallel and a rotated orientation relationships have also been observed. Figure 4.20 shows examples recorded on a Pt/Ceo,8Tbo,202.x catalyst. Figure 4.20(a) illustrates the rotated relationship, for which the [110]-metal zone axis is perfectly aligned with the [010] zone axis of die support. The parallel orientation relationship involves an alignment of the [110] zones axis of metal and support. Figure 4.20(c). Note diat for this type of interface the only planes which lie strictly parallel are, in both cases, the (002)-support planes and the (1-1 l)-metal planes, see DDPs in Figures 4.20(b) and 4.20(d). 

On the semiconducting side, the interfacial layer has two zones. The first zone lies within the evanescent tail of the metal. The second zone is the remaining region where, due to interdiffusion, a composition or doping different from the original bulk semiconductor exists. A similar description can be characterized on the metal side and the new alloyed metal zone may be of sufficient width to become the metal forming the barrier. This new interfacial metal can have different characteristics from the originally deposited metal. 

The deposition technique of the coating, for example, in the case of immersion in molten metals, zones of possible liquid retention or tightly closed zones can lead to explosions. 

Figure 8.10. Evolution of the conductance of sensors disposing of two metallic zones submitted to different atmospheres at 450°C...

To confirm or invalidate such a hypothesis, a new series of experiments must be carried out on the second kind of sample, which has a metallic zone deposited at the surface of the internal zone. This metallic zone, without any connection to the system of electric measurement, is a mere element of the oxide-metal-gas system. 

Firstly, as indicated by fields A and B in Figure 8.11, we note that with the same gaseous concentrations, the conductance of the sample with three metallic zones is slightly lower than the conductance of the sample with two metallic zones. This indicates that adding a metal in the internal zone entails another effect of electric blocking. In any case, the effects are perfectly reversible, as indicated in field C, in which the oxygen concentration is 20%. 

The situation described in field E is associated with a concentration of 1% at the level of the electrodes and a concentration of 20% in the internal zone. The results are significant we observe an electric blocking effect much larger for the sample disposing of three metallic zones than with the sample disposing of two metallic zones. 

Finally, in the reverse case, where the external compartment has an oxygen concentration of 20% and the internal compartment a concentration of 1%, the sample with three metallic zones shows an electric blocking slightly larger than the one observed in the case of the device possessing two metallic zones. 

Thus, in all the cases, the presence of a third metallic zone amplifies the effect of electric locking by oxygen on tin dioxide. 

In a third series of experiments, we have tried to assess the effect of a reducing gas on the external and internal zones of a sensor possessing of two metallic zones, which are located in the external parts of the sensor. 

The morphologies shown in Figirre 9.20 are somewhat idealized. In fact, the oxide scales resulting from simrrltaneous oxidation of alloys often have a more complicated structure several stacked layers, presence of precipitates in the adjacent metal zone due to internal oxidation, presence of metal islands in the oxide films, formation of pores and of cavities, etc. In addition, the oxidation of an alloy often exhibits non steady state behavior involving an initial phase during which the structure and the composition of the scales change transient oxidation). 

Fig. 1.31 Metal zoning in Osarizawa ore deposit (Saisei-hi, cross section, (b)) and Ashio deposit, cross section, (a) (Horikoshi 1977)...

Fig. 6 is a diagrammatic sketch of the five most common metals that occur in a roll front. The metal zones usually overlap so the redox boundary between selenium and vanadium may be the only visible line. In addition, one or more of the zones is likely to be absent in any given area for example, if vanadium is absent, uranium is found at the redox boundary with selenium. 

Repetitive passage of a molten zone along a bar of the impure actinide metal (zone melting), particularly when followed by the induced migration of impurities in the solid by an impressed electric field (electrotransport), can be used to reduce the impurity level in Pu from 200 ppm to 20 ppm. 

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