Alumina process zone

A is a constant with a value close to unity, Vzirc is the volume fraction of the metastable particles, Et is the volume strain accompanying the transformation, Em is Young s modulus of the matrix (often alumina), and w is the width of the process zone around a crack containing transformed particles (shown in Figure 18.22). 

Figure 14.20 Microcrack process zone around a crack in an alumina-zirconia duplex ceramic, where the stress-induced phase transformation of zirconia particles induces microcracking. From Ref. [21],...

In the laser heated float zone (LHFZ) or pedestal (LHPG) growth process, a circumferential laser is placed around a preform rod (e.g. polycrystalline alumina) to zone refine a segment of the material while at the same time updrawing a single crystal fiber (e.g., sapphire). 

The continuous sintering is mainly a zone sintering process in which the electrolyte tube is passed rapidly through the hot zone at about 1700 °C. This hot zone is small (about 60 mm) in zone sintering, no encapsulation devices are employed. The sodium oxide vapor pressure in the furnace is apparently controlled by the tubes themselves. Due to the short residence time in the hot zone, the problem of soda loss on evaporation can be circumvented. A detailed description of / "-alumina sintering is given by Duncan et al. [22]. 

Steinfeld et al. [133] demonstrated the technical feasibility of solar decomposition of methane using a reactor with a fluidized bed of catalyst particulates. Experimentation was conducted at the Paul Scherrer Institute (PSI, Switzerland) solar furnace delivering up to 15 kW with a peak concentration ratio of 3500 sun. A quartz reactor (diameter 2 cm) with a fluidized bed of Ni (90%)/Al2O3 catalyst and alumina grains was positioned in the focus of the solar furnace. The direct irradiation of the catalyst provided effective heat transfer to the reaction zone. The temperature was maintained below 577°C to prevent rapid deactivation of the catalyst. The outlet gas composition corresponded to 40% conversion of methane to H2 in a single pass. Concentrated solar radiation was used as a source of high-temperature process heat for the production of hydrogen and filamentous... 

Two different approaches have been used to graft molybdenum on alumina, namely, either a two-step process involving gas-phase impregnation and further decomposition at high temperature (GPID) or the direct contact of [Mo(CO),5] vapor with the alumina support placed in a hot zone so as to achieve its decomposition. All of the relevant studies point to the existence of a close relationship between the OH group density on the support and the amount of deposited molybdenum as well as the chemical nature of such deposits. Hence, we successively deal with three types of alumina highly, partially and fully dehydroxylated surfaces. 

In support of the conclusion based on silver, series of 0.2, 0.5, 1.0, 2.0, and 5.0 % w/w of platinum, iridium, and Pt-Ir bimetallic catalysts were prepared on alumina by the HTAD process. XRD analysis of these materials showed no reflections for the metals or their oxides. These data suggest that compositions of this type may be generally useful for the preparation of metal supported oxidation catalysts where dispersion and dispersion maintenance is important. That the metal component is accessible for catalysis was demonstrated by the observation that they were all facile dehydrogenation catalysts for methylcyclohexane, without hydrogenolysis. It is speculated that the aerosol technique may permit the direct, general synthesis of bimetallic, alloy catalysts not otherwise possible to synthesize. This is due to the fact that the precursors are ideal solutions and the synthesis time is around 3 seconds in the heated zone. 

This paper is devoted to the sorption of uranyl, which exhibits a complex aqueous and surface chemistry. We review briefly the sorption behaviour of An in the environment, and illustrate the variety of environmental processes using published data of uranyl sorption in the Ban-gombe natural reactor zone. After summarizing the general findings of the mechanisms of An sorption, we then focus particularly on the current knowledge of the mechanisms of uranyl sorption. A major area of research is the influence of the aqueous uranyl speciation on the uranyl surface species. Spectroscopic data of U(VI) sorbed onto silica and alumina minerals are examined and used to discuss the role of aqueous uranyl polynuclear species, U02(0H)2 colloids and uranyl-carbonate complexes. The influence of the mineral surface properties on the mechanisms of sorption is also discussed. 

This process of separation, also known as column chromatography, was first developed in 1900by Day, an American petroleum chemist. However, a more extensive study was made in 1906 by Tswett, a Polish botanist. He observed that when a solution of plant pigments in petroleum ether is passed slowly through a column packed with alumina, a number of horizontal bands of different colours are produced in the column. This is, evidently, due to the fact that different constituents of the mixture are adsorbed to different extents. The most readily adsorbed constituent is held at the top. The others with decreasing order of adsorbabilities are held up in different zones down the column in the same order. This, of course, gives only a partial separation of the various constituents as some of the less readily adsorbed... 

A novel application of a symmetric porous membrane as a catalyst carrier but not as a permselective barrier is to use the membrane itself as the reaction zone for precise control of the stoichiometric ratio [Sloot et al., 1990]. In this case, the reactants are fed to the different sides of the membrane which is impregnated with a catalyst for a heterogeneous reaction. The products diffuse out of the membrane to its both sides. If the reaction rate is faster than the diffusion rate of the reactant in the membrane, a small reaction zone or theoretically a reaction plane will exist in the membrane. An interesting and important consequence of this type of membrane reactor is that within the reaction zone the molar fluxes of the reactants arc always in stoichiometric ratio and the presence of one reactant in the opposing side of the membrane is avoided. The reaction zone can be maintained inside the membrane as long as the membrane is symmeuic and not ultrathin. Therefore, membrane reactors of this fashion are particularly suited for those processes which require strict stoichiometric feed rates of premixed reactants. A symmetric porous a-alumina membrane of 4.5 mm thick was successfully tested to demonstrate the concept [Sloot et al., 1990]. 

In 1999, Brasseur-Tilmant [56] presented a work dealing with modification of macroporous alumina media by TiOi particles deposition using supercritical isopropanol. The aim was to prepare inorganic membranes for cross-fiow filtration. Anatase particles were deposited on plane alumina support after thermal decomposition of titanium alkoxide precursors. A slight infiltrated zone was observed and a pore size reduction was achieved from 110 to 5 nm, leading to obtain fine ultrafiltration membranes. The main problem was to control the reaction at the membrane interface and not in the porosity, and moreover, this process was suitable for mbular membrane preparation. 

The concept that the removal of an undesired reaction product by selective adsorption from the reaction zone of an equilibrium-controlled reaction increases the conversion and the rate of formation of the desired component (based on Le Chatelier s principle) was used to develop a novel PSA process concept called SERP for direct production of fuel cell-grade hydrogen by steam reforming of methane (CH4 + 2H20 44 C02 + 4H2).57 61 The concept uses a physical admixture of a reforming (noble metal on alumina) catalyst and a chemisorbent (K2C03 promoted hydrotalcite), which selectively and reversibly chemisorbs C02 from a gas at a temperature of -450 °C in the presence of steam. The cyclic SERP steps consisted of the following ... 

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