The Ceramic Method

The reaction rate increases by using powdered reactants with large specific surface area (SSA) and high contact area between their particles the diffusion lengths are also reduced. The SSA of a nonporous solid increases as the size of the particle decreases assiuning homogeneous, nonporous, spherical particles, the relationship SSA-r-5 = 3 holds, where r stands for the radius of the particle and 5 for its density. 

The reactants should be powdered and intimately and stoichiometrically mixed and heated. Then they should be thoroughly pulverized and heated a second or even a third time the advance of the reaction can be checked from the powder X-ray diffraction diagram, although the method is of low accuracy for 

The specific nature of the reactants can permit lower reaction temperatures. 

Alikhanzadeh-Arani and Salavati-Niasari [5] have reported the preparation of YBaCuO at 870 °C, a temperature somewhat lower than that required in the conventional ceramic method, when starting from [tris(2-hydroxyacetophenato) triaqua yttrium(III)] instead of 2(003)3. 

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

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... 

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. 

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... 

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... 

Chevrel compounds of general formula ArM06S8 with A= Cu, Pb, La etc. (Fig. 5) are generally prepared by the ceramic method. A novel precursor compound has been employed [22] to obtain these compounds by a one-step reduction as given by the reaction ... 

The sol-gel method has been conveniently employed for the synthesis of 123 compounds such as YBa2Cu307 and the bismuth cuprates. Materials prepared by such low-temperature methods have to be annealed or heated under suitable conditions to obtain the desired oxygen stoichiometry as well as the characteristic high Tc value. 124 cuprates, lead cuprates and even thallium cuprates have been made by the sol-gel method the first two are particularly difficult to make by the ceramic method. Coprecipitation of all the cations in the form of a sparingly soluble salt such as carbonate in a proper medium (e.g. using tetraethyl-ammonium oxalate), followed by thermal decomposition of the dried precipitate has been employed by many workers to prepare cuprates. 

The M-NM transition has been studied in powder samples (prepared by the ceramic method) of the series of perovskite oxides Lai- TiOs with 0 temperature dependencies to the magnetic susceptibility (Fig. 7.6a) and electrical resistivity (Fig. 7.6b) for different compositions, as well as the cell parameters for the phases ... 

Such electrodes have mostly been used as anodes, but cathodic redox catalysis is also possible. In Ti/Ti02 composite cathodes, prepared by the ceramic method, Ti(OH)4 is... 

The majority of useful complex materials are oxides, but the ceramic method can also be used to prepare metal halides and nitrides by using controlled atmospheres and appropriate reaction vessels. For example, the formation of calcium gold nitride from calcium nitride and gold in a sealed gold capsule under nitrogen ... 

The long reaction time is only one of the problems associated with the ceramic method. Finding the right annealing conditions and temperature is primarily a matter of trial and error, as the method of preparation of new materials cannot be easily predicted. Often, the product is not totally homogeneous in composition, even when the reaction proceeds almost to completion. If there is any impurity left at the end of the reaction, it is normally impossible to remove. 

Figure 4.4 Reaction scheme for the formation of lanthanum iron oxide using the ceramic method.

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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