Yoldas process

Flow chart of the Yoldas process for preparing alumina. From Assih el at. (172),... 

Actual and simulated A1 NMR spectra of an alumina sol prepared by the Yoldas process. From Olson and Bauer 1I78. ... 

Monolithic alumina xerogels produced by the Yoldas process and heated to SOO C. From Yoldas (164),... 

Relative gelling volumes versus the acid-io-aluminum ratio for alumina sols prepared by the Yoldas process. Shaded areas denote the range of acid-io-aluminum ratios yielding transparent, monolithic gels or films Curve A. inorganic acids Curve B, organic acids. From Yoldas (165). 

Structural changes that occur during the Yoldas process (and variations of this general procedure) have been characterized by several groups employing NMR [177,178] or Al NMR in combination with Raman [172] and IR [174] spectroscopy, gel permeation chromatography [175], or viscosity measurements [173]. Olson and Bauer [178] closely followed the Yoldas procedure (Fig. 22) and observed A1 NMR spectra (Fig. 26) consistent with... 

Compared to amorphous systems, structural evolution in crystalline systems may be dominated by the effects of phase transformations [190] which often occur in conjunction with dehydration or when metastable transitional phases are involved. The most thoroughly studied crystalline gel system is alumina prepared by the Yoldas process [47,191,192]. (See Chapter 2.) Xerogels, which are often completely transparent, are typically composed of... 

The investigations on the preparation and properties ofalkoxy-derived sols and gels of A1203 performed by Yoldas [1793], along with the numerous works on hydrolysis of Si(OR)4, laid the foundation of the sol-gel method as the technique employing the process of continuous transformation from colloid (sol) to gel. The structures of sols and gels prepared in accordance with the procedures described by Yoldas were in detail characterized by means of NMR,... 

Gamma-alumina membranes were the first and most investigated mesopor-ous membranes to follow the colloidal preparation method. Based on a sol-gel process developed by Yoldas [13], a boehmite sol can be prepared by hydrolysis... 

Stable particulate alumina sols are precursors for many materials such as controlled porosity catalysts, ceramics, and coatings. They are commonly formed by peptization of pseudoboehmites (AIOOHKH2O) by dilute monoprotic acids. They can be prepared either by the process developed hy Yoldas [4] from hydrolyzed aluminum alkoxides or by acidic peptization of the precipitates derived from hydrolyzed aluminum salts [5,48]. As will be seen below, these alumina sols are usually multi-component fluids that also contain soluble aluminum species. Furthermore, colloidal aluminas can also be produced by the polymerization of hydrolyzed aluminum cations. Therefore, a brief review on the characterization of aluminum species in solution by At NMR is presented in Sec. III.B.l. The NMR studies on colloidal aluminas are then discussed in Sec. III.B.2. 

Earlier in this chapter we discussed the sol-gel processing of aqueous silicates and mentioned the preparation of Si02 gels from fine particles made by flame oxidation (2)-(4). The preparation of alumina sols from alkoxides has been characterized by Yoldas (45). Aluminum alkoxides such as aluminum sec-butoxide and aluminum isopropoxide are readily hydrolyzed by water to form hydroxides. Which hydroxide is formed depends on the conditions used in the hydrolysis. The initial hydrolysis reaction of aluminum alkoxides can be written... 

Yi G., Sayer M. An acetic acid/water based sol-gel PZT process I modification ofZr and Ti alkoxides with acetic acid. J. Sol-Gel Sci. Technol. 1996a 6 65-74 Yi G., Sayer M. An acetic acid/water based sol-gel PZT process II formation of a water based solution. 1. Sol-Gel Sci. Technol. 1996b 6 75-82 Yoldas B.E. Hydrolysis of aluminium alkoxides and bayerite conversion. J. Appl. Chem. Biotechnol. 1973 23 803-809... 

Yoldas B.E. Alumina sol preparation from alkoxides. Am. Ceram. Soc. Bull. 1975 54 289-290 Yoshida M., Lai M., Kumar N.D., Prasad P.N. Ti02 nano-particle-dispersed polyimide composite optical waveguide materials through reverse micelles. J. Mater. Sd. 1997 32 4047-4051 Yoshizawa K., Sugoh Y., Ochi Y. Controlled growth of monodispersed spherical silica by a new synthetic process. In Science ofCeramics Vol. 14, D. Taylor, ed. Stoke-on-Trent, UK The Institute of Ceramics, 1988... 

Osmosis is a process of diffusion driven by a chemical potential gradient. For example, if pure liquid is separated from a salt solution by a membrane that is permeable to liquid, but not to salt, pure liquid will diffuse into the salt-rich side. This type of transport occurs in general when liquid A and B are separated by a membrane permeable only to B. A dramatic example is shown in Fig. 1 Yoldas [2] prepared a sol of alumina in water, placed it into a bag made from a dialysis membrane, and immersed the bag in alcohol as the water diffused out of the sol into the alcohol bag, the sol contracted and finally gelled. Of course, it is not necessary to have a membrane to have... 

The Yoldas [164,165] process outlined schematically in Fig. 22 is the most common method of forming transparent, monolithic alumina gels. (See Fig. 23.) It consists of hydrolyzing an aluminum alkoxide, Al(OR)j, normally AI(OBu )3, in a large excess of water (r = 100-200) at 80-100°C, resulting in precipitation of fibrillar boehmite, followed by peptization with a mineral acid (HNO3) to stable particulate sol. Cold-water hydrolysis... 

The second approach, sequential additions of different alkoxides to partially hydrolyzed precursors, was invented by Thomas [166] and popularized by Yoldas [167,168]. In this process alkoxides are added in the reverse order of their respective reactivities (least reactive precursors first) and a partial hydrolysis step is performed after each addition. The idea is that the newly added, unhydrolyzed alkoxides will condense with partially hydrolyzed sites on the polymeric species formed by the preceding hydrolysis steps (heterocondensation) rather than with themselves (homocondensation). Unlike the previous method where molecular-level homogeneity is predicted, the homogeneity of the product will depend on the size of the polymeric species to which the last component is added. Two sequential hydrolysis schemes to prepare a multicomponent borosilicate glass composition are shown in Fig. 66 where alkali and alkaline earth metals are introduced as salts or alkoxides. 

Xerogel monoliths are semiporous monoliths (usually 30-70% porosity). Yoldas [40] in the 1970s was the first to demonstrate fabrication of monoliths via a sol-gel process. Since then numerous routes for fabrication of xerogel silica monoliths and nonsilica monoliths were suggested and demonstrated. In this section a few examples of xerogel monolith febrication will be presented. 

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