Ceramic methods

Finally, the obtained wafers are thermally treated in a furnace at around 500°C-2000°C for several hours depending on the raw material and the application of the membrane. This methodology is principally appropriate for materials with high chemical, thermal, and mechanical stabilities [183], 

Because diffusion has an activation energy barrier, higher temperatures result in higher diffusion coefficients. Furthermore, smaller particle sizes mean smaller Ax required for a complete reaction. Therefore, the traditional approach to solid-solid reactions is to use fine powders and high reaction temperatures. This direct combination of two or more powdered solids at high temperature, referred to as the ceramic method of synthesis, is discussed in Section 5.2. 

As discussed in Chapter 3, the deposition in molecular beam epitaxy is kept to small thicknesses, on the order of a monolayer, so that a very small diffusion coefficient, D, does not prevent a reaction from occurring on a reasonable time scale. In the ceramic method, we are again presented with the interdependence of the diffusion coefficient, reaction temperature, and displacement distance, Eqs. 5.1 and 5.2. 

When ceramic compounds are used as reactants in solid-state synthesis, the reaction conditions are often extreme because of the refractory nature of ceramics. 

The traditional ceramic method of solid-state synthesis is designed to overcome the problems of slow ionic migration over long distances. We discuss the classic 

The starting materials are mixed in the molar ratio needed in the final product. In the case of ZnCr2C 4, 1 mol of ZnO and 1 mol of CdC would be needed. Fine powders of each would be used. Powders are normally sold with purity and mesh designations. The purity may read 3 N7, which means three nines followed by a seven 99.97%. Sometimes all the digits are nines 5 N means 99.999%. The mesh designation follows one of the standard mesh sizes. Sample U.S. standard sieve sizes include  

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As well documented in the literature, spinels are basically known for its magnetic properties. Generally, high temperature preparation methods (ceramic methods) have... 

The word ceramic is a misnomer since these oxide compounds are all thermally unstable and tend to release oxygen or decompose on melting. Most ceramics are stable at high temperatures and normally do not melt. However, the so-called "ceramic method" is used in the preparation of these cuprate compounds. 

FIGURE 3.1 The basic apparatus for the ceramic method (a) pestles and mortars for fine grinding (b) a... 

The sol is either then treated or simply left to form a gel over time by dehydrating and/or polymerizing. To obtain the final product, the gel is heated. This heating serves several purposes—it removes the solvent, it decomposes anions such as alkoxides or carbonates to give oxides, it allows rearrangement of the structure of the solid, and it allows crystallisation to occur. Both the time and the temperature needed for reaction in sol-gel processes can reduced from those in the direct ceramic method in favourable cases, the time from days to hours, and the temperature by several hundred degrees. 

Lithium niobate is a ferroelectric material used as an optical switch. Preparation by the simple ceramic method leads to problems in obtaining the correct stoichiometry, and a mixture of phases often results. Several sol-gel preparations have been described, their advantage being the lower temperature required for the preparation and the greater homogeneity of the product. One such preparation starts with lithium ethoxide (LiOC2H5 (or LiOEt)) and niobium ethoxide Nb2(OEt)io. Each ethoxide was dissolved in absolute ethanol and the two solutions mixed. The addition of water leads to partial hydrolysis giving hydroxy-alkoxides, for example ... 

The Chevrel phase CuMoeSs was prepared by a ceramic method. What would be suitable starting materials and what precaution would you have to take ... 

A compound (NH4)2Cu(Cr04)2.2NH3 is known. How could this be used to prepare CuCr204 What would be the advantage of this method over a ceramic method Suggest which solvent was used to prepare the ammonium compound. 

The ideal condition for carrying out a solid state reaction in order to obtain a homogeneous product in the shortest time at the lowest possible temperature is to ensure homogeneous mixing of the reactants on an atomic scale. This, however, cannot be achieved in the ceramic method or its modifications. The only way to achieve this is to prepare a single phase (a chemical compound) in which the reactants are present in... 

Ceramic Method. Mixtures of barium carbonate and iron oxide are reacted at 1200-1350 °C to produce crystalline agglomerates which are ground to a particle size of ca. 1 pm. This method is only suitable for the high-coercivity pigments required for magnetic strips [5.35],... 

Single crystals of K NbeOn can be obtained by slow cooling of the melt near 1200°C.4,34,355 Powder samples can be prepared by conventional ceramic methods at about 1100°C. X-ray powder patterns have been reported in the literature.4 The BET surface area of the ceramic preparation85 is typically of the order of 1 m2g-1. Hydrothermal synthesis can be used to prepare the single-nhase oxide at temperatures as low as 280°C with four times the surface area, as reported in the literature.365 Catalytic properties of such preparations have not oeen examined. 

For the high temperatures used in the ceramic method, special attention must be paid to the reaction vessel. Materials that are normally considered inert, such as platinum, will react with certain elements (e.g., phosphoms) at high-temperatures. Much of this interesting high-temperature chemistry is learned the hard way after an unexpected reaction of the cmcible. For open cmcible reactions, the common cmcible compositions are alumina, zirconia, quartz, and platinum, all available commercially in sizes ranging from under 1 inch in diameter up to... 

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