Metal alkoxide precursors

Sol-gel chemistry offers a unique advantage in the creation of novel organic-inorganic hybrids. The sol-gel process begins with a solution of metal alkoxide precursors [M(0/f) ] and water, where M is a network-forming element, and R is typically an alkyl group. Hydrolysis and... 

The sol-gel process involves hydrolysis of alkoxide precursors under acidic or basic conditions, followed by condensation and polycondensation of the hydroxylated units, which lead to the formation of porous gel. Typically a low molecular weight metal alkoxide precursor molecule such as tetramethoxy silane (TMOS) or tetra ethoxysilane (TEOS) is hydrolyzed first in the presence of water, acid catalyst, and mutual solvent... 

In some instances, this approach has proven successful, with comparatively low crystallization temperatures being observed. For example, Eichorst and Payne in the synthesis of LiNb03 noted crystallization temperatures of 400-500 °C for a mixed-metal alkoxide precursor.111 In other instances, these attempts have proven less successful. Numerous attempts have been made to synthesize Pb-Zr and Pb-Ti precursors, each with the 1 1 cation stoichiometry of the desired PbZr03 and PbTi03 compounds.83,84 Unfortunately, 1 1 stoichiometric ratio compounds have not always been obtained, with crystalline compounds of other stoichiometries precipitating from the solution, as illustrated in Fig. 2.11.83 This figure shows the crystal structure of PbTi2[p(4)—... 

Another challenge faced by sol-gel technologies involves controUing the dispersion of different metals within a mixed metal (e.g., sihcon and titanium) matrix. The solvolysis and condensation steps for metal alkoxide precursors involved in sol-gel reactions can be quite different from that of orthosilicates, which often leads to the loss of dispersion and formation of separate silica and other metal oxide domains [54]. 

Controlled hydrolysis and condensation of metal alkoxide precursors, followed by drying at ambient pressure and temperalure. 

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,... 

IR spectra measurements as well as variation of the film thickness, shrinkage, and refractive index demonstrated substantial differences in the mechanisms of thermal decomposition of films prepared from the exclusively metal alkoxide precursor and from the metal alkoxides modified by 2-ethylhexanoic acid. These differences affect the evolution of film microstructure and thus determine the different dielectric properties of the obtained films. The dielectric permittivity of the films prepared from metal alkoxide solutions was relatively low (about 100) and showed weak dependence ofthe bias field. This fact may be explained by the early formation of metal-oxide network (mostly in the... 

The same metal alkoxide precursor solutions in EtOH and methoxyetha-nol, which were used for preparation ofpowders were also suggested for application of LiNb03 and NaNb03 films on Pt and quartz substrates [524, 1778]. The choice of solution concentration and thickness ofthe layer, which undergoes calcination, is very important for the preparation of films without cracks or bubbles. Oriented LiNbxTa,.x03 (0 < x < 1) films were obtained on the... 

Chemical modification of the metal alkoxide precursors can dramatically affect the nature of the final product. Most additives are nucleophilic XOH molecules that react with the alkoxide to give... 

Both homometallic and heterometallic oxo-alkoxides can be produced by partial hydrolysis of metal alkoxide precursors via dehydroxylation and dealkoxolation of metal hydroxo alkoxides. The crystal structures of a number of polynuclear titanium oxo alkoxides have been elucidated, these structures having evolved during the slow addition of small amounts of water to Ti(OEt)4 47,48,76-79... 

C. Sanchez and J. Livage, Sol-gel chemistry from metal alkoxide precursors. New. J. Chem., 14 (1990) 513. 

Tetramethoxysilane, Si(OCH3)4, abbreviated TMOS, and tetraethoxysi-lane, Si(OCH2CH3)4, abbreviated TEOS, are the most commonly used metal alkoxide precursors in sol-gel processing of silicates (24). Silicate gels are most often synthesized by hydrolyzing the alkoxides dissolved in their parent alcohols with a mineral acid or base catalyst. At the functional group level, three bimolecular nucleophilic reactions are generally used to describe the sol-gel process (24) ... 

As polymer spheres are easily accessible with low polydispersity, they are attractive templates. Coating in non-aqueous solvents is usual as most metal alkoxide precursors rapidly undergo hydrolysis and condensation in the presence of water. 

Traditionally, sol-gel technique refers to inorganic polymerization reactions based on the hydrolysis and polycondensation of metal-organic compounds, in particular metal alkoxides, carried out under controlled pH, amount of water, and precursor concentration. The process essentially consists of four steps (i) hydrolysis, (ii) condensation, (iii) drying, and (iv) heat treatment. Typically, homogeneous solution of metal alkoxide precursors (M(OR) or MM (OR)j4.j,) is activated by controlled hydrolysis (stoichiometric amount of protic reagents) to initiate polymerization reactions that... 

Figure 12 Nucleophilic attack by water molecule on metal alkoxide precursor.

This chapter focuses only on the syntheses of oxide ceramic powders. The precursors of these powders are metal-organic compounds, mainly metal alkox-ides. The different varieties of ceramic powders synthesized by mixed metal alkoxide precursors are the focal points of this chapter. Numerous references on the fundamental principles of sol-gel processing are available in the literature and within other chapters of this book [10-12]. 

This chapter is divided into three major sections (1) metal alkoxide precursor synthesis methods used by the present authors, (2) silicate powder syntheses, and (3) nonsilicate powder syntheses. 

Most gels, however, form structures between these two extremes. The hydrolysis and polycondensation of metal alkoxide precursors yield an initial formation of fractal clusters, which upon reaching a critical size begin to form a continuous network or skeleton via cluster-cluster aggregation. Both the initial cluster formation and the subsequent aggregation of clusters are fractal in nature. There is a very distinct difference between these two processes, however, as is more evident in Sec. X. 

Recent advances in sol-gel science allowed us to go further in controlling the individual particle size at the sol stage, resulting in individual nanoscale grains in the ceramic. This new development is based on the chemical modification of both metal salt and metal alkoxide precursors to modify their reactivity and to achieve a nanopore structure for the membranes [34]. Some ex-... 

Closely related to the salt infiltration method discussed above is the utilization of metal alkoxide precursors in a sol-gel process to fill the interstitial sites in an opal template. The template is first infiltrated with an alkoxide (neat or as solution), then the hydrolysis and condensation reaction following the infiltration transforms the alkoxides into an extended network of the corresponding metal oxide. By this method Ti02 networks besides other oxides were initially prepared with a strong focus on their optical properties as photonic crystals [30,50]. For this purpose a high refractive index material is desired, which would be the rutile modification of titania with the highest refractive index. Nevertheless, the sol-gel procedure provides an amorphous or anatase modification with lower refractive index. By calcination of the anatase... 

If mixed-metal alkoxide precursors can be prepared and their reactivity can be controlled, ordered macroporous mixed metal oxides can be produced by a sol-gel method using a colloidal crystal template. Alkoxides of Ti and Zr with other metals are easily prepared by mixing titanium alkoxide and zirconium alkoxide with alkoxides of other metals or metal salts, and the formed mixed metal alkoxides can infiltrate the voids of templates. Therefore, production of mixed metal oxides with Ti and Zr is straightforward. 

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