Powder synthesis

In this equation the concentrations are not the equilibrium values but the actual values in the reactor. This definition corresponds to that of the supersaturation used with physical condensation. Physical supersaturation is a special case of chemical supersaturation as defined above if the condensation reaction is A g) - C(5), the definition given above reduces to S = [A]reactor/[ ]equii. which defines physical supersaturation. There is saturation when S = 1, and p(A) can be used instead of [A]. 

Homogeneous chemical nucleation is similar both to stepwise polymerization reactions in fluid media and to spinodal decomposition in mixtures. The characteristic differences with a binodal-type nucleation reaction are a strong sensitivity to the presence of impurities, the absence of an activation barrier and a concentration threshold, and no incubation time. 

Very small nuclei that might form in the gas phase can deposit on the substrate and contribute to heterogeneous deposition and accelerate growth. This accounts for blanket deposition (irrespective of substrate) at high supersaturation and also explains the morphological features of high-temperature deposition discussed below. 

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Powder Preparation. The goal in powder preparation is to achieve a ceramic powder which yields a product satisfying specified performance standards. Examples of the most important powder preparation methods for electronic ceramics include mixing/calcination, coprecipitation from solvents, hydrothermal processing, and metal organic decomposition. The trend in powder synthesis is toward powders having particle sizes less than 1 p.m and Httie or no hard agglomerates for enhanced reactivity and uniformity. Examples of the four basic methods are presented in Table 2 for the preparation of BaTiO powder. Reviews of these synthesis techniques can be found in the Hterature (2,5). 

J. M. Halstead, V. Venkateswaran, and B. Mehosky, "SiC Powder Synthesis," presented at the 24thMutomotive Technology development Contractors Coordination Meeting Dearborn, Mich., 1986. 

Vapor-Phase Techniques. Vapor-phase powder synthesis teclmiques, including vapor condensation, vapor decomposition, and vapor—vapor, vapor—Hquid, and vapor—soHd reactions, employ reactive vapors or gases to produce high purity, ultrafine, reactive ceramic powders. Many nonoxide powders, eg, nitrides and carbides, for advanced ceramics are prepared by vapor-phase synthesis. 

SOFC Anode, cathode, electrolyte Powder synthesis... 

The physical and mechanical properties of all the final products made of nitride UFPs depend intimately on the smallness and uniformity of its particle size and also on the purity of nitride particles used. For three decades the major efforts have been related to the development of powder synthesis technology. 

In general, carbides, nitrides, and borides are manufactured in the vapor phase in order to form high-purity powders. This procedure is fundamentally different than a strict CVD process, since in powder synthesis reactors, deposition on seed particles may be desirable, but deposition on the reactor walls represents a loss of product material. As we will see, in CVD, heterogeneous deposition on a surface will be sought. Aside from this issue of deposition, many of the thermodynamic and kinetic considerations regarding gas phase reactions are similar. 

Chapter 5 will be devoted to solid phase synthesis of ceramic powders Chapter 6, to liquid phase synfiiesis and Chapter 7, to gas phase synthesis. Other miscellaneous methods of ceramic powder synthesis are discussed in Chapter 8. All of these ceramic powder synthesis methods have one thing in common, the generation of particles with a particular particle sized distribution. To predict the particle size distribution a population balance is used. The concept of population balances on both the micro and... 

Ceramic Powder Synthesis with Solid Phase Reactant... 

This chapter discusses the fluid-solid and solid-solid reactions used to produce ceramic powders. The first aspect of this discussion is the spontaneity of a particular reaction for a given temperature and atmosphere. Thermodynamics is used to determine whether a reaction is spontaneous. The thermod3mamics of the thermal decomposition of minerals and metal salts, oxidation reactions, reduction reactions, and nitridation reactions is discussed because these are often used for ceramic powder synthesis. After a discussion of thermodynamics, the kinetics of reaction is given to determine the time necessary to complete the reaction. Reaction kinetics are discussed in terms of the various rate determining steps of mass and heat transfer, as well as surface reaction. After this discussion of reaction kinetics, a brief discussion of the types of equipment used for the synthesis of ceramic powders is presented. Finally, the kinetics of solid—solid interdiffusion is discussed. 

A classic example of a solid—fluid ceramic powder synthesis reaction is that of calcination and dehydration of natural or synthetic raw materials. Calcination reactions are common for the production of many oxides from carbonates, hydrates, sulfates, nitrates, acetates, oxalates, citrates, and so forth. In general, the reactions produce an oxide and a volatile gaseous reaction product, such as CO2, SOg, or HgO. The most extensively studied reactions of this type are the decompositions of magnesium hydroxide, magnesium carbonate, and calcium carbonate. Depending on the particular conditions of time, temperature, ambient pressure of CO2, relative humidity, particle size, and so on, the process may be controlled by a surface reaction, gas diffusion to the reacting... 

The reduction of oxides in reducing atmospheres is also an important industrial fluid—solid reaction that produces a powder. Because these types of reactions can affect ceramic powder synthesis, they are included in this chapter. However, these reduction reactions are frequently used to produce metal powders and are not often used to produce ceramic powders. These reduction reaction can, however, be the first step in a sequence of steps to produce carbide and nitride powders. Several examples of fluid—solid reduction reactions are... 

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