Precursor methods

The solid-state method requires high temperature heating or high mechanical energy for diffusion of reactants. Synthesis in liquid phase is useful to prepare solid compounds that contain the different cations in an ideally atomic dispersion. There are two methods concerning this concept one is a precursor method and the other is a co-precipitation method. 

Ceria-based oxides can be obtained by the decomposition of some compound precursor, such as hydroxide, nitrate, halides, sulfates, carbonates, formates, oxalates, acetates, and citrates.For example, nanosize or porous cerium oxide particles have been prepared at low temperatures by pyrolysis of amorphous citrate, which is prepared by the evaporation of the solvent from the aqueous solution containing cerium nitrate (or oxalate) and citric acid. In the case of mixed oxides, the precursor containing some cations in the same solid salts is prepared. In the same manner of ceria particles, the precursors complexing some cations with citrates are useful to synthsize ceria-zirconia mixed oxides and their derivatives. Also, Ce02-Ln203 solid solutions, where Ln = La, Pr, Sm, Gd, and Tb, have been synthesized from the precursors obtained by the evaporation of nitrate solutions at 353 K in air from an intimate mixture of their respective metal nitrates. The precursors are dried and then heated at 673 K to remove nitrates, followed by calcination at 1073 K for 12h. 

A variation of the precursor method is that the mixed precursors are polymer complexes. A powder of composition Ceo,i2Zro,8802 has been synthesized at mild temperatures (873 - 1073 K) by a polymer complex solution method using polyvinylalcohol (PVA) and by a polymerizable complex method using ethylene glycol. In these methods, a solution of polyvinylalcohol (PVA) (or ethylene glycol), citric acid, and metal ions are polymerized to form a gel precursor with randomly distributed cations. Heating of these precursors at a mild temperature produces mixed oxides that are compositionally homogenous at an atomic level. 

In addition, the combustion process that modifies the precursor method is also attractive. It requires only a short duration of few minutes to produce metal oxides having fine size and large surface area. The required quantities of metal nitride are mixed with urea, and the mixture is dissolved in a minimum amount of water. The content is transferred to a Pyrex dish and then is introduced into a preheated muffle furnace maintained at 773 K. The water evaporated in a few minutes to produce a fluffy powder. Ceria and Cei xPrx02.y fine powders have been synthesized by this method. -  

---

Production of net-shape siUca (qv) components serves as an example of sol—gel processing methods. A siUca gel may be formed by network growth from an array of discrete coUoidal particles (method 1) or by formation of an intercoimected three-dimensional network by the simultaneous hydrolysis and polycondensation of a chemical precursor (methods 2 and 3). When the pore Hquid is removed as a gas phase from the intercoimected soHd gel network under supercritical conditions (critical-point drying, method 2), the soHd network does not coUapse and a low density aerogel is produced. Aerogels can have pore volumes as large as 98% and densities as low as 80 kg/m (12,19). 

The continued development of new single-source molecular precursors should lead to increasingly complex mixed-element oxides with novel properties. Continued work with grafting methods will provide access to novel surface structures that may prove useful for catalytic apphcations. Use of molecular precursors for the generation of metal nanoparticles supported on various oxide supports is another area that shows promise. We expect that the thermolytic molecular precursor methods outlined here will contribute significantly to the development of new generations of advanced materials with tailored properties, and that it will continue to provide access to catalytic materials with improved performance. 

In the via precursor method, however, it is difficult to prepare the ji-conjugated polymers with ideally developed -conjugation system the -conjugated polymer chains contain many conformational defects because the jc-conjugated chains are caused to develop from disordered precursor polymer, which form random coil conformation, in solid state. For the preparation of polymers with well-developed jc-conjugation system by the via precursor method, accordingly, it is necessary to introduce orientational and conformational orderliness of the precursor polymers in the films. 

Third-order nonlinear optical effect in polyarylenevinylene cast films prepared by via-precursor method... 

The slurry process has been enhanced with vacuum to fabricate planar SOFCs [78], This method is of low cost and thus has been widely used to develop low-cost SOFCs. However, together with other liquid precursor methods such as sol-gel and spray pyrolysis, it is time, labor, and energy intensive because the coating-drying-sintering has to be repeated in order to avoid cracking formation. 

As noted above, the synthesis of dimer involved a complicated synthetic procedure and produced very low yields. Alternative routes for deposting poly-(tetrafluoro-p-xylylene) thin film were studied the precursor method and a new... 

Accidently, using hexafluoro-p-xylene with the contaminated copper wire obtained from the precursor method experiments, a polymer film was deposited on the silicon substrates. Obviously, some dibromotetrafluoro-p-xylene from the precursor method that adhered to, or reacted with, the metal could somehow initiate this VDP process. However, a complete explanation of these results is not yet available. As an extension of this discovery, commercially available 1,4-bis(trifluoromethyl)benzene in conjunction with a catalyst/initiator has proved to be a potential alternative by which to deposit poly(tetrafluoro-p-xylylene) film successfully.23... 

It is possible to extract the nanocolloids from aqueous solution into an organic phase or to support them onto inorganic supports by what is called the precursor method (described in Section 3.5) to generate heterogeneous catalysts. Such catalysts find application in chemical catalysis, e.g., in selective hydrogenation of fatty acids. 

The precursor method achieves mixing at the atomic level by forming a solid compound, the precursor, in which the metals of the desired compound are present in the correct stoichiometry. For example if an oxide MM 204 is required, a mixed salt of an oxyacid such as an acetate containing M and M in the ratio of 1 2 is formed. The precursor is then heated to decompose it to the required product. Homogeneous products... 

However, for modern electronic circuits, it is important to have a product of controlled grain size and the precursor method is one way to achieve this. (Another way is the sol-gel method which has also been applied to this material.) The precursor used is an oxalate. The first step in the preparation is to prepare an oxo-oxalate of titanium. Excess oxalic acid solution is added to titanium butoxide which initially hydrolyses to give a precipitate which then redissolves in the excess oxalic acid. 

The products of the precursor method are usually crystalline solids, often containing small particles of large surface area. For some applications, such as catalysis and barium titanate capacitors, this is an advantage. 

In what form do you want the product to be You might choose, for example, vapour phase epitaxy because an application requires a single crystal. Alternatively, you might choose a particular method, such as the precursor method or hydrothermal synthesis, because you need a homogeneous product. 

You should consider the availability of reactants required for a particular method. If you are considering a precursor method, is there a suitable precursor with the right stoichiometry The CVD method needs reactants of similar volatility do your proposed reactants meet this requirement In microwave synthesis, does at least one of your starting materials absorb microwaves strongly ... 

Definitions. Colloids are solid particles with diameters of 1 100 nanometers, A sol is a dispersion of colloidal particles in a liquid. A gel is an interconnected rigid network of sub-micrometer dimensions. A gel can be formed from an array of discrete colloidal particles (Method I) or the 3-D network can be formed from the hydrolysis and condensation of liquid meial alkoxide precursors (Methods 2 and 3). shown in Fig. 11. The metal alkoxide precursors used in Methods 2 and 3 are usually Si(OR)4 where R is CHj. C-Hj. or C3H7. The metal ions can be Si, Ti. Sn. Al, and so on,... 

---