Wet-Chemical Routes

There are two main routes to produce ceramic powders fiom a solution (i) evaporation of the liquid and (ii) precipitation by adding a chemical reagent that reacts with components in the solution. Other wet-chemical routes include sol-gel, gel combustion, emulsion, and so on. 

24 In-line optical transmittance spectra of the Zr02-doped Ho Y203 ceramics (2.14 mm thick) from the powders with different slurry concentrations. Reproduced with permission from [101]. Copyright 2015, Elsevier 

The kinetics and mechanism of precipitation have been well studied and widely available in the open literature [8, 22, 23, 102-107]. Precipitation from solution involves two basic steps (i) nucleation of fine particles and (ii) growth of the nuclei. The control of the powder characteristics is realized by controlling the reaction conditions for both nucleation and growth, as well as the extent of coupling between the two processes. 

The first term on the right-hand side is the intrinsically positive contribution of the siuface free energy, while the second term is due to the contribution by the bulk free energy change. With unit volume of the liquid, the free energy change due to the transfer from vapor to hquid is given by [108]  

Putting Eq. (3.23) into Eq. (3.22), the height of the free energy activation barrier can be derived, which is  

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J. Cerdk, A. Cirera, A. Vilk, A. Cornet, and J.R. Morante. Deposition on micromachined silicon substrates of gas sensitive layers obtained by a wet chemical route a CO/CH4 high performance sensor , Thin Solid Films 391 (2001), 265-269. 

Up to 48 ternary catalyst mixtures were prepared simultaneously in less than 1 h. Hence the sputtering procedure is much faster than the wet chemical route and in fact one of the fastest syntheses available. This advantage is gained at the expense of low layer porosity. Thus, sputtered catalysts are new artificial catalysts and not directly comparable to catalysts prepared by wet-chemical procedures. These catalysts offer the advantage of quick preparation and characterization compared with alumina-based catalysts. They can also be used for obtaining so-called intrinsic kinetics because there is no influence of diffusion. 

As discussed in Chapter 3, the fabrication of bulk oxide ceramics usually starts with a powder and this can be prepared via solid state reaction (also loosely referred to as the mixed oxide route), or by wet chemical routes. 

These recent works show that there are many opportunities for using CBD in large area photovoltaics. In the future it is possible that not only window layers but also absorber layers will be made by CBD (first attempts have been made to deposit CuInS2 [104], CuInSc2 [110] and CdTe [94] by CBD), leading to a completely wet chemical route which is highly relevant to industrial applications as shown for CdTe... 

E. R. Camargo, M. Popa, J. Frantti, and M. Kakihana. Wet-chemical route for the preparation of lead zirconate An amorphous carbon- and halide-free precursor synthesized by the hydrogen peroxide based route. Chem. Mater, 13 3943-3948, 2001 E. R. Camargo, J. Erantti, and M. Kakihana. Low-temperature chemical synthesis of lead zirconate titanate (PZT) powders free from halides and organics. J. Mater. Chem., 11 1875-1879,2001... 

In recent years, several methods have been described in the literature concerning the so-called wet-chemical routes, including the co-precipitation method [1]. This method has been used for oxide nanopowders preparation... 

A wide range of methods have been used to synthesize ceramic powders, with different compositions. Although most of these methods are applicable to transparent ceramic power synthesis, some of them have not been employed in the fabrication of transparent ceramics. Solid-state reaction is still the key method, while wet-chemical routes are increasingly used, whereas gas-phase reactions are only limited to nonoxide ceramics. For large scale applications, cost-effectiveness, environmental friendliness and less energy consumption, are all very important considerations, when selecting synthetic methods. 

Precipitation (or coprecipitation) method is also a simple and efficient wet-chemical route for preparing ceramic nanoparticles. A carefuUy developed precipitation method that optimizes processing parameters such as reactants and their concentrations, pH, temperature, and calcination conditions can produce a massive and reproducible quantity of ceramic nanoparticles with high purity and crystallinity [10]. For example, one of the most important ceramics in orthopedic applications, nanocrystaUine hydroxyapatite (Ca,(,(PO )/OH)2, HA), can be produced in large quantity through the aqueous reaction ... 

Ciampi S, Harper JB, Gooding JJ (2010) Wet chemical routes to the assembly of organic monolayers on silicon surfaces via the formation of Si-C bonds surface preparation, passivation and functionalization. Chem Soc Rev 39 2158-2183... 

Fig. 23.11 Influence of Pd bulk doping on (a) XPS valence band spectra and (b) high-resolution transmission electron microscopy (HRTEM) images of a SnO nanoparticles synthesized by wet chemical route (a) (SnOj Pd), 7 =450 °C (Dieguez et al. 2000 Cabot et al. 2000) (b) (SnO iPd) (8 wt%) (Adapted with permission from Nayral et al. 2000, Copyright 2000 Elsevier)...

Nanoscaled powders are the main materials used to prepare ceramics and thick fihns for a variety of applications during gas sensor design and fabrication. It was fonnd that the varions synthesis techniqnes nsed for producing nanoscaled powders can generally be divided into three major types assembly of clnsters/nano-particles produced by (1) wet-chemical routes, (2) gas-phase synthesis, and (3) electrolytic deposition. 

Table 1. Wet-chemical routes for synthesizing ZrTi04 materials...

Among the oxygen reduction catalysts, Pt, Pt-based catalyst, or Ag dispersed in carbon is the most powerful catalyst. However, there are some problems such as the cost for the material, cohesion of the metals, and deactivation from dropping out of the catalyst from the base electrode. Nano-sized perovskite-type oxide electrocatalyst, which could be synthesized by a wet chemical route, is one of the most... 

Pale yellow cerium(IV) hydroxide can be prepared by addition of bases, such as aqueous auunonia, to solutions of cerium(IV) salts, e.g., cerium(IV) nitrate or CAN. Nanocrystalline cerium(IV) hydroxide (NCs-Ce(OH)4) is an intermediate in the production of cerium dioxide, which has been synthesized successfiilly using a novel and simple wet chemical route at ambient temperature for the preparation of nanocrystalline CeOi powder and films on mass scale for various purposes. The average crystalhte size of NCs-Ce(OH)4 has been estimated by the Scherrer equation to be 3 4nm. Absorption and luminescence spectroscopic studies have been examined for future application in the development of optical devices. ... 

The nanotechnology wave will likely change the way materials and devices are produced in the future. The abiUty to synthesize crystallites at the nanometer scale with precisely controlled size and composition and to assemble them into large structures with unusual properties and functions, will revolutionize all segments of material manufacturing for industrial applications. Among the main techniques for generating nanoparticles via the wet chemical route, combustion synthesis is the one that stands out. 

Wu XD, Zheng LJ, Wu D. (2005) Fabrication of superhydrophobic surfaces from microstmcmred ZnO-based surfaces via a wet-chemical route. Langmuir 21 ... 

The conventional solid-phase reaction method of synthesizing SOFC materials requires high calcinations and sintering temperatures. The main drawbacks include poor compositional homogeneity, uncontrollable particle size distribution, low surface area, and poor sintering of the products, which worsen their electrochemical properties. Various wet chemical routes have recently been developed to synthesize submicrometer to nanosized oxide powders. The sol-gel method is one of them, and their implementation would yield nanostruc-tured materials with well-defined stoichiometries at temperatures substantially lower than those used in solid-state reaction. 

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