Liquid sintered aluminas

In order to extract small fluid samples from the reaction cell at high temperatures, a capillary with a tip of sintered alumina, can be introduced sidewise into the center of the reaction cell. The samples pass into a section of stainless steel capillary between two high pressure valves mounted on a heated metal block. The capillary section serves as a pipette from which liquid and gaseous components can be extracted for analysis. Another method is, to use a small spindle press. By turning it back very slowly, samples can be sucked out steadily from the reaction cell. 

However, there have been suggestions in the literature for reference electrodes based on the aluminalaluminium couple. An electrode comprising a molten salt and aluminium was first described by Drossbach [81]. Common types of reference electrodes involve a molten aluminium pool covered by molten cryolite contained within a thin walled tube of sintered alumina or boron nitride. This housing must be electrically insulating but able to transfer ions (i.e. porous and ionically conductive). Also it must be resistive to corrosion. Electrolytes must be in contact and a tortuous path is required such that the different melts do not mix but establish a stable liquid junction. 

The multitude of refractory aggregates (for example, calcined clay, flint clay, andalusite, mulhte, bauxite, fused white or brown alumina, sintered alumina, etc.) and the different grain sizes of these materials—in addition to the big variety of bonding agents, additives, and mixing liquids—enable endless combination possibihties in development work on unshaped refractory products. Therefore, a qualitative standardization is not really possible as it is the case for refractory bricks. As a result of the ENV 1402-1 standard, which now provides a more precise definition of the products, there will probably be adjustments on the market in the long run. If also considering economic aspects, suitable products will obviously be developed on the basis of all the available constituents. 

The capillary disc filter looks like any other standard rotary vacuum disc filter, as can be seen in Figure 3.25, but the filter medium is a finely porous ceramic disc, which draws filtrate through the disc material by capillary action, under the applied vacuum. The filter discs are made of sintered alumina with uniform micropores less than 1 pm in size, which allows only liquid to flow through it. Despite an almost absolute vacuum, no air penetrates the filter material. The disc material is inert,... 

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. 

As shown in Fig. 8.14 the cell is formed in principle by two liquid electrodes, the sodium negative and the sulphur positive, separated by a tube of sintered polycrystalline /3-alumina. Since sulphur is an insulator, the compartment containing the sulphur electrode is fitted with a carbon felt current collector. The cell, which may be written as... 

The industrially important acetoxylation consists of the aerobic oxidation of ethylene into vinyl acetate in the presence of acetic acid and acetate. The catalytic cycle can be closed in the same way as with the homogeneous Wacker acetaldehyde catalyst, at least in the older liquid-phase processes (320). Current gas-phase processes invariably use promoted supported palladium particles. Related fundamental work describes the use of palladium with additional activators on a wide variety of supports, such as silica, alumina, aluminosilicates, or activated carbon (321-324). In the presence of promotors, the catalysts are stable for several years (320), but they deactivate when the palladium particles sinter and gradually lose their metal surface area. To compensate for the loss of acetate, it is continuously added to the feed. The commercially used catalysts are Pd/Cd on acid-treated bentonite (montmorillonite) and Pd/Au on silica (320). 

A similar comparison of rate constants for 0.6 and 5% Pt/alumina and 1,8% Ag/alumina catalysts at 673 K in oxygen atmosphere reveals that sintering rates for the Ag catalyst are roughly 40-50 times higher than for either Pt catalyst. Thus, Pt is clearly much more thermally stable than Ag under oxidizing conditions. These results are consistent with those from a model catalyst study [44] of sintering of Pt and Ag on alumina in vacuum in which it was observed that Pt/alumina was thermally stable in vacuum to about 873 K, above which temperature liquid-like particle migration was observed, while Ag/alumina was stable to only 723 K, above which temperature evaporation of the metal was observed. This latter result is... 

Prior to the sintering and annealing operations in the fabrication of electrolyte tubing, the green-formed shapes are bisque-fired (<1200°C) to remove binders and lubricants During reactive liquid phase sintering of / "-alumina at 1575-1585° C, four processes normally occur in parallel " ... 

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