Solids ceramic method

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. 

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... 

Most modern materials are formed empirically by solid-state methods. These methods generally involve more processing activity than chemical synthesis (for example, sintering of ceramic powders, modifying concrete by polymers, thermomechanical processing of alloys, layering polymeric membranes for... 

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. 

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... 

Ford Motor Co [312] obtained a patent for a method of producing sodium which uses a solid ceramic (P-alumina) that conducts sodium ions as a divider for a two-compartment cell. This method allows low melting sodium salts or salt mixtures to be used to produce sodium [312-315], The cell operates at 200°C and 6 V instead of the conventional 7 V for Downs cells, making sodium at 600°C. The average current efficiency is 100% compared to 85-92% for Downs cells. The power consumption of the process at the same productivity is 20-30% lower than for the Downs cell. This process has, however, not been operated on a large scale because the P-A1203 does not have sufficient lifetime. 

As mentioned earlier, diffusion distances for the reacting cations are rather large in the ceramic method. Diffusion distances are markedly reduced by incorporating the cations in the same solid precursor. Synthesis... 

Two types of transformations can be very broadly distinguished. The first is the formation of a solid solution, in which solute atoms are inserted into vacancies (lattice sites or interstitial sites) or substitute for a solvent atom on a particular sublattice. Many types of synthetic processes can result in this type of transformation, including ion-exchange reactions, intercalation reactions, alloy solidification processes, and the high-temperature ceramic method. Of these, ion exchange, intercalation, and other so-called soft chemical (chimie douce) reactions produce no stmctural changes except, perhaps, an expansion or contraction of the lattice to accommodate the new species. They are said to be under topotactic, or topochemical, control. 

In solid phase reaction s3mthesis, there are three types of chemical reactions oxidation or reduction of a solid, thermal decomposition of a solid, and solid state reaction between two t3 s of solid. With liquid phase ssmthesis of ceramic powders, there are five different methods drying of a liquid, precipitation, sol-gel sjmthesis, hydrothermal S5m-thesis, and reactions of a liquid metal melt with a gas to give a solid ceramic. There are basically three operational principles for precipitation temperature change, evaporation, and chemical reaction. Sol—gel... 

This method involves dissolving metal salts (often nitrates, hydroxides or oxalates) in a suitable solvent and then evaporating the mixture to dryness. The dried residue is then reacted as in the ceramic method. This procedure only works well if the components have similar solubilities, otherwise the solid with the lowest solubility starts precipitating out first, giving no better mixing than the normal ceramic method. 

Solid-solid reactions are the basis of the most frequently used procedures for preparing mixed oxides, especially when the surface areas of the resulting solids are not an important parameter. Indeed, these high-temperature methods are essential for preparing perovskites with special morphologies, such as monocrystals or thin layers. Because this kind of method is most frequently used for the preparation of ceramic materials, it is usually referred to as the ceramic method. ... 

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