Sintering of metal oxides

In Chapter 1 we emphasized that the properties of a heterogeneous catalyst surface are determined by its composition and structure on the atomic scale. Hence, from a fundamental point of view, the ultimate goal of catalyst characterization should be to examine the surface atom by atom under the reaction conditions under which the catalyst operates, i.e. in situ. However, a catalyst often consists of small particles of metal, oxide, or sulfide on a support material. Chemical promoters may have been added to the catalyst to optimize its activity and/or selectivity, and structural promoters may have been incorporated to improve the mechanical properties and stabilize the particles against sintering. As a result, a heterogeneous catalyst can be quite complex. Moreover, the state of the catalytic surface generally depends on the conditions under which it is used. 

An important attribute of these materials comes from the two distinct cationic sites (Oh and Td), and the feasibility of cations migration among them due to the redox nature of metalions, while keeping the spinel structure intact. This particular aspect helps to avoid possible segregation/sintering of metal-ions and remain stable for longer period, compared to a mixed metal oxide catalyst. This unique property of spinels makes them an attractive candidate for number of catalytic reactions. 

As for the complete oxidation of propene, propane and methane, Nieuwenhuys and coworkers studied the influence of metal oxides additives on the catalytic activity of Au/Al203 [109-115], The addition of 3d transition metal oxides (MnOx, CoOx or FeOx), which were active by themselves, or ceria that was poorly active by itself promoted the catalytic activity of Au/Al203 in the total oxidation of propene [112]. The most active catalyst was Au/Ce0x/Al203, with a T95 at 497 K and with a high stability. In these cases, ceria and the transition metal oxides may act as co-catalysts and the role is twofold it stabilizes the Au NPs against sintering (ceria)... 

In the polymeric sol-gel process, partial hydrolysis of a metal alkoxide dissolved in alcohol is accomplished by adding the minimum of water to the solution. The active hydroxyl groups on the alkoxides then react to form an inorganic polymer molecule that can then be coated onto the ceramic support. On drying and sintering, the metal oxide film forms. Chemically the polymeric sol-gel process can be represented as ... 

The particle size of Pu aerosols is very variable, depending on the mode of formation. In Fig. 5.2, curves A, B and C show size spectra obtained by Carter Stewart (1971) in laboratory experiments on the oxidation of Pu metal in air. In controlled oxidation at temperatures below the ignition point (about 500°C), scaly, friable, oxide particles were produced, with median diameter increasing with temperature. Few particles less than 1 jum in diameter were found. When the delta alloy of Pu was used, the oxide was more adherent, and the particle size larger. Increase of particle size with increase of temperature was also found in laboratory oxidation of uranium metal (Megaw et al., 1961), and was ascribed to sintering of the oxide layer. 

An important property of a passivating solid coating is that it should be stable in spite of subsequent thermal treatments, such as the sintering of metal connections. The problem of thermal stability is especially worrisome in the case of a-Si H that is known to evolve hydrogen above about 350°C (Pankove and Carlson, 1977). Heating the sample for 30 min at 500°C caused the leakage current to increase by a factor of about three, as shown by the dashed line in Fig. 3. Note that the current for the heated sample is still about one order of magnitude lower than that of the oxide-passivated p-n junction, which is well known for its thermal stability. 

Consider the sintering process of compacted metal powders. Would the resulting sintered material be more or less desirable (from a mechanical standpoint), if an excessive amount of metal oxides were present in the presintered matrix How would you design the sintering conditions (co-reactant gases, temperature, etc.) for these matrices ... 

The dynamic membranes originally developed by Union Carbide are protected by three core patents U.S, 3977967, 4078112, and 4412921 (Trulson and Litz, 1976 Bibeau, 1978 and Leung and Cacciola, 1983) and their foreign equivalents. Those patents cover a broad range of metal oxides such as zirconia, gamma alumina, magnesia>alumina spinel, tantalum oxide and silica as the membrane materials and carbon, alumina, aluminosilicates, sintered metals, fiberglass or paper as the potential porous support materials. However, their marketed product, trade named Ucarscp membranes, focused on dynamic membranes of hydrous zirconium oxide on porous carbon support. 

Pure Si3N4 powders are difficult to densify, even under high-pressure conditions. Therefore, sintering aids consisting of metal oxides (MeO) are... 

The instrumentation for fabrication of the ET normally employs a thermistor as a temperature transducer. Thermistors are resistors with a very high negative temperature coefficient of resistance. These resistors are ceramic semiconductors, made by sintering mixtures of metal oxides from manganese, nickel, cobalt, copper, iron and uranium. They can be obtained from the manufacturers in many different configurations, sizes (down to 0.1-0.3 mm beads) and with varying resistance values The best empirical expression to date describing the resistance-temperature relationship is the Steinhart-Hart equation ... 

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