Zinc oxide reaction sintering

The measures of solid state reactivity to be described include experiments on solid-gas, solid-liquid, and solid-solid chemical reaction, solid-solid structural transitions, and hot pressing-sintering in the solid state. These conditions are achieved in catalytic activity measurements of rutile and zinc oxide, in studies of the dissolution of silicon nitride and rutile, the reaction of lead oxide and zirconia to form lead zirconate, the monoclinic to tetragonal transformation in zirconia, the theta-to-alpha transformation in alumina, and the hot pressing of aluminum nitride and aluminum oxide. 

Lee, V. J. Parravano, G. (1959). Sintering reactions on zinc oxide. Journal of Applied Physics, 30, 1735-40. 

The extraction of zinc by carbothermic reduction of zinc oxide sinter at 1,100°C can be represented by the reaction... 

DoUimore and Tonge [15] ascribed the deceleratory decomposition of zinc formate in air (0 < nr < 0.3) to an initial instantaneous and extensive nucleation of reactant crystalhte surfaces with product zinc oxide and the operation of a contracting sphere mechanism. For 0.3 < nr < 0.8 the reaction rate is almost constant, probably as a result of reactant cracking. for both processes is 67 kJ mol". During the course of reaction the yields of hydrogen and carbon monoxide increased, while that of carbon dioxide decreased. This was attributed to a decrease in the catalytic activity of the product oxide, possibly as a result of sintering. The formation of higher molecular mass products was mentioned. 

Systems in which two simple oxides react to form a complex oxide are usefiil models for investigating the production of single-phase ceramics by reaction sintering because of the available data on the reaction mechanisms (91) and the relative simplicity of the reactants and product. Two examples are the formation of zinc ferrite by Eq. (11.67) and the formation of zinc aluminate spinel by the reaction between ZnO and AI2O3 powders. 

In this zone, immediately above the tuyere level, reduction reactions predominate, particularly the reaction of COj with coke to form CO at temperatures above 1000°C. Sinter commences to melt in this zone and lead oxide in the form of glassy lead silicates is reduced by CO to metallic lead. Zinc oxide is also reduced to form zinc metal vapour, which rises with the gas stream. Any lead sulfide present can be volatihsed and may react with zinc vapour in colder adjacent regions such as the furnace walls or centre of the furnace, to form zinc sulfide and lead metal. This can give rise to the formation of acaetions. 

On the other hand, while chlorides accumulate near the top of the catalyst, they are more mobile and can be detected in significant concentrations, up to 0.05%, at all levels in a deactivated bed. Although reasonable hves of at least two years can often be achieved in the presence of chloride there is more rapid movement of the peak in temperature profile, and the concentration of carbon monoxide in the outlet gas increases more rapidly. Surface chlorides, which are formed by reaction with zinc oxide, are mobile and sinter the catalyst surface. Chlorides are also soluble in condensed steam and can be washed down onto lower, more active catalyst layers. 

For strontium-doped lanthanum chromites, the precipitation of SrO is not possible, because strontium ions are more stable in the perovskite stmctuie than the lanthanum ions. Lanthanum oxide, La203, is often precipitated instead of SrO. This causes the disintegration of sintered specimens at ambient temperatures by the volume change associated with the formation of lanthanum hydroxide, La(OH)3, due to the reaction of La203 with the water vapor in the air (Meadowcroft, 1969 Sakai et al., 1990a). The substitution of transition metals such as zinc (Hayashi et al., 1988), cobalt (Zhou et al., 1996), or vanadium (Larsen et al., 1997) have been used to improve the sintering of strontium substituted lanthanum chromite. The substitution of strontium and transition metals is often utilized for intercoimects of the planar type... 

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