Precursor Powder Synthesis

When creating supersaturation levels sufficient to induce particle formation, precipitation of sparingly soluble salts and sol-gel processes are viewed differently. Precipitation normally involves mixing a cation solution with a precipitant solution. For example, consider preparation of an oxalate precursor to a CoO- and MnO-doped ZnO powder. In this process, the Zn, Mn, and Co are coprecipitated with controlled stoichiometry and the precipitate is calcined to the oxide. To form the oxalate, a state of supersaturation is created by mixing an aqueous solution of the metal nitrates or chlorides with an oxalate precipitant solution. The system is supersaturated with respect to the different metal oxalate phases and a crystalline coprecipitate forms. Depending on precipitation conditions (pH, concentrations, temperature, etc.), different metal complexes are present in solution. The form and concentration of these complexes determine the phase, morphology, and particle size distribution of the resulting precipitate. 

The sol-gel process is normally defined as the formation of a sol, gelation of the sol, and removal of the solvent to produce ceramic powders or monoliths. The sol is formed by hydrolysis and condensation reactions of soluble metal salts (e.g., aluminum chloride or nitrate) or metalorganics such as alkoxides (e.g., Al(0/ )3 where R = -CH3, -C(CH3)3, etc.). Examples of hydrolysis and condensation reactions for metal (A/) alkoxides are 

The kinetics of the two competing reactions are controlled by the precursors used M and R) and the reaction conditions. For example, rapid addition of excess water to an alcoholic alkoxide solution generally leads to complete hydrolysis with little condensation. Because a hydroxide or hydrous oxide precipitate usually forms, this process resembles the sparingly soluble salt case. Polymeric species form when conditions are adjusted such that condensation occurs. It is the type and distribution of species in solution and how they react with water (hydrolysis) or among themselves (condensation) that determine the form and structure of the particulate or gel product. The ability to form large polymeric species is one of the primary differences between the sol-gel and salt precipitation processes. 

in the previous example, the amount of added base is increased (m 2.5), the aluminum solubility limit is exceeded and a precipitate spontaneously forms. The driving force for this process is a function of the supersaturation, 5, which is the 

1 Diffusion of lattice ion-containing species to the crystal surface 

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E. Bermejo, T. Becue, C. Lacour, M. Quarton, Synthesis of nanoscaled iron particles from freeze-dried precursors, Powder Technol. 94 (Issues 1) (1997) 29-34. 

Mazdiyasni, K.S., Powder synthesis from metal-organic precursors, Ceram. Int., 8, 42, 1982. 

FIGURE 6.2 TG/DTA curves for the precipitate obtained from [Nb(02)4] and Na solutions used as a precursor for synthesis of NaNbOj powder. 

The foregoing powder synthesis techniques, along with others such as polymerization and decomposition of complex double alkoxides and the melt and spray decomposition of mixed nitrates, produce precursor powders with a high degree of chemical homogeneity. However upon subsequent calcination of the products of these reactions, the crystallization path invariably passes through the following sequence of reactions ... 

Historically, stabilized (and partially stabilized) zirconia ceramics were prepared from powders in which the component oxides are mechanically blended prior to forming and sintering. Because solid state diffusion is sluggish, firing temperatures in excess of 1800°C are normally required. Furthermore, the dopant was nonuniformly distributed, leading to inferior electrical properties. Trace impurities in the raw materials can also lead to enhancement of electronic conductivity in certain temperature ranges, which is also undesirable. To overcome these problems, several procedures have been developed to prepare reactive (small particle size) and chemically pure and homogeneous precursor powders for both fully stabilized and partially stabilized material. Two of these are alkoxide synthesis and hydroxide coprecipitation. 

Zr + 8 mole%Y203 Defect structure and mechanism of ionic conduction 18, 17.3.7.3 Oxygen ion resistivity of polycrystalline material 18,17.3.7 Preparation of precursor powders for polycrystalline ceramics 18, 17.3.7.3.1 Alkoxide synthesis 18, 17.3.7.3.1 Hydroxide co-precipitation 18, 17.3.7.3.1 Sintering (densification) characteristics 18, 17.3.7.3.3... 

Two commercial stoichiometric Sp powders (Mg Alsl.0) were used. The first (labeled here Nl) is derived from A1 and Mg hydrated sulfate salts, using solution chemistry it is supplied by Baikowski (La Balme de Silligny, France). The calcination temperature is of-1100°C. The second, produced by Nanocerox (Ann Arbor, Ml, USA) was synthesized by flame-spray pyrolysis from a double Al-Mg alkoxide precursor. The synthesis product is calcined at 650°C. 

Powder synthesis using the alkoxy precursor technique exhibits processing flexibility not available in traditional high-temperature solid-state reaction of the mechanically mixed and milled oxides and/or the inorganic salt constituents. With proper process control, impurities can be reduced to very low levels, yielding the desired electrical, optical, thermal, and mechanical properties. In addition, selected elements may be purposely added in controlled amounts to influence these properties. 

Mechanochemical processing has been recently applied to the synthesis of a wide range of nanocrystalline rare earth oxides. Milling of precursor powders leads to the formation of a composite of the starting materials which react during milling or subsequent heat treatment to form a mixture of separated fine particles of the desired phase within a soluble salt matrix. 

Ceramic processing typically consists of three main steps (i) synthesis or preparation of precursor powders, (ii) consolidation or packing of the powders into green bodies, and (iii) sintering [44]. Every step has a significant effect on the microstructure and optical performance of the final transparent ceramics. There are a number of parameters relevant to the quality of the powders, facility, and way of consolidation and techniques of sintering, which can be used to optimize the fabrication process as a whole. 

Precursor powders of ceramics can be prepared using solids, such as oxides, hydroxides, and carbonates, as starting materials [45-50]. In this case, it is called the solid-state reaction process. The precursor powders can also be synthesized by wet-chemical methods, such as chemical precipitation or co-precipitation [51-59], sol-gel [60, 61], gel combustion [62-65] and hydrothermal synthesis [66-69]. The... 

Chapter 3 covers synthesis of precursor powders of transparent ceramics. These synthetic methods are also widely used for other ceramics or other materials. For each method, a brief description, together with examples of transparent ceramics derived from the powder synthesized by using the method, will be presented. There is no attempt to comment on whether one method is superior to another. In fact, every method has its own advantages and disadvantages. Different methods could be suitable for different transparent ceramic materials. 

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