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Chem. Rev. 1990, 90. 3-72 93

The Sol-Gel Process

LARRY L. HENCH' and JON K. WEST Dq"nt of Materials Sc*mce and Enghwrhg, Advanced Materials Research Center, UnimrMy of Floryle. oakresville. Fknm 326 1

Received May 16. 1989 (Revised Manuscript Received October 27, 1989)


1. Introduction 1. Sol-Gel Process Steps: An Overview 1. Hydrolysis and Polycondensation IV. Gelation

VI. Aging VII. Drying

V. Theoretical Studies

VIII. Stabilization IX. Densification

XI. Conclusions I. Infroducf/on

X. Physical Properties

Interest in the sol-gel processing of inorganic ceramic and glass materials began as early as the mid-1900s with Ebelmanl,* and Graham's3 studies on silica gels. These early investigators observed that the hydrolysis of tet- raethyl orthosilicate (TEOS), Si(OC2H5),, under acidic conditions yielded Si02 in the form of a "glass-like material".l Fibers could be drawn from the viscous gel,

and even monolithic optical lenses? or composites formed? However, extremely long drying times of 1 year or more were necessary to avoid the silica gels fracturing into a fine powder, and consequently there was little technological interest.

For a period from the late 18008 through the 19209 gels became of considerable interest to chemists stim- ulated by the phenomenon of Liesegang Rings4& formed from gels. Many noted chemists, including OstwaldE and Lord Rayleigh,? investigated the problem of the periodic precipitation phenomena that lead to the formation of Liesegang rings and the growth of crystals from gels. A huge volume of descriptive literature re- sulted from these studiesg'O but a relatively sparse understanding of the physical-chemical principles?

Roy and co-workersll-" recognized the potential for achieving very high levels of chemical homogeneity in colloidal gels and used the sol-gel method in the 19509 and 1960s to synthesize a large number of novel ceramic oxide compositions, involving AI, Si, Ti, Zr, etc., that could not be made using traditional ceramic powder methods. During the same period Iler's pioneering work in silica chemistry15 led to the commercial development of colloidal silica powders, Du Pont's colloidal Ludox spheres. Stober et a1.I6 extended Iler's findings to show that using ammonia as a catalyst for the TEOS hy- drolysis reaction could control both the morphology and size of the powders, yielding the so-called Stober spherical silica powder.

The final size of the spherical silica powder is a function of the initial concentration of water and am- monia, the type of silicon alkoxide (methyl, ethyl, c ,

Lany L. Hend, is a eadaduate Research Professa in tb Cep" 01 Materials Science and Engineering at the University of Florida. where he has taught since 1964 after receiving B.S. (1961) and

Ph.D. (1964) degrees in Ceramic Engineering at The Ohm State Universny. He is the Director of lha Boglass Research Center and

CoDirectof of the Advanced Materials Research Center at the University of Florida. He has published more than 250 research articles and is the coauthor or coeditor of 12 books in the fields of biomateriak. ceramic processing. ceramic characterization. glass surfaces. electronic ceramics. nuclear waste disposal, and sol-gel processing.

Jon K. West received his Ph.D. at the Univerrity of FkMa in 1979 while wwking full lime as an engineering manager wilh tb Battery Business Department of General Electric Co. His cment position is Associate-in-Engineering with the Department of Materials Science and Engineering at the University of Florida. His work in sol-gel silica includes mechanical testing, process control and instrumentation. and theoretical studies based on molecular wbtal calculations. He is the author of eight publications Including the recently published textbook Principles of Electronic Ceramics. by Hench and West. from John Wilev 8 Sons.

pentyl, esters, et4 and alcohol (methyl, ethyl, butyl, pentyl) mixture used,16 and reactant tempe~ature.~' An example of a typical colloidal silica powder is shown in

Figure la, made by the Stober process, and its uniform distribution of particle sizes is shown in Figure lb from Khadikar and Sacks work.ls

0009-2665/90/0790-0031$09.50/0 0 1990 American Chemical Society

.J .nE .5 .s .I

34 Chemical Reviews. 1990. Vol. 90. No. 1

Figure I. Top SEM of Stober spherical silica powders. Bottom: Histogram (number of particles in a given diameter class versus particle diameter) of a typical batch of Stoher spherical silica powders. Reprinted from ref 18: copyright 1988 University of Florida.


O~erbeek'~3 and SugimotoZ1 showed that nucleation of particles in a very short time followed by growth without supersaturation will yield monodispersed col- loidal oxide particles. Matijevic and co-~orkers~~-~~ have employed these concepts to produce an enormous range of colloidal powders with controlled size and morphologies, including oxides (TiO,, a-FepO3, Fe304, BaTiO,, Ce02), hydroxides (AIOOH, FeOOH, Cr(OH),), carbonates (Cd(OH)CO,), Ce,O(CO3),, Ce(I)/Y HC03), sulfides (CdS, ZnS), metals (Fe(I), Ni, Co), and various mixed phases or composites (Ni, Co, Sr ferrites), sulfides (Zn, CdS), (Pb, CdS), and coated particles (Fe304 with AKOH), or Cr(OH),).

The controlled hydrolysis of alkoxides has also been used to produce submicrometer Ti0z,26 doped Ti02.27

Zr0z,28 and doped Zr0z,28 doped SiOz,TJ SrTiO,." and even cordierite30 powders.

Emulsions have been employed to produce spherical powders of mixed cation oxides, such as yttrium alu- minum garnets (YAG), and many other systems such as reviewed in Hardy et al."

Sol-gel powder processes have also been applied to fissile elements31 where spray-formed sols of U02 and U02-PuO, were formed as rigid gel spheres during passage through a column of heated liquid. Both glass and polycrystalline ceramic fibers have been prepared by using the sol-gel method. Compo- sitions include TiOz-Si02 and Zr02-Si02 glass high-purity SiO, waveguide fiber~?~.~~ A120b ZrO,,

Hen& and West

Tho,, MgO, TiO,, ZrSiO,, and 3AI2O3.2SiOz fibers.- Abrasive grains based upon sol-gel-derived alumina are important commercial products."

A variety of coatings and films have also been de- veloped by using sol-gel methods. Of particular im- portance are the antireflection coatings of indium tin oxide (ITO) and related compositions applied to glass window panes to improve insulation chara~teristics.~~

Other work on sol-gel coatings is reviewed by Schroe- der,'8 Macken~ie,'~," and Wenzel.S1 Mackenzie's re- vie~s~~50 include many other applications of the sol-gel process, proven, possible, and potential.

The motivation for sol-gel processing is primarily the potentially higher purity and homogeneity and the lower processing temperatures associated with sol-gels compared with traditional glass melting or ceramic powder methods. Ma~kenzie'~fl summarizes a number of potential advantages and disadvantages and the relative economics of sol-gel methods in general. Hench and colleague" compare quantitatively the merits of sol-gel-derived silica optics over the alternative high-temperature processing methods.

During the past decade there has been an enormous growth in the interest in the sol-gel process. This growth has been stimulated by several factors. On the basis of Kistler's early work,% several teams have pro- duced very low density silica monoliths, called aerogels, by hypercritical point drying.5B Zarzycki, Prassas, and Phalippo~~~.~ demonstrated that hypercritical point drying of silica gels could yield large fully dense silica glass monoliths. Yoldassg showed that large monolithic pieces of alumina could be made by sol-gel methods. These demonstrations of potentially practical routes for production of new materials with unique properties coincided with the growing recognition that powder processing of materials had inherent limitations in ho- mogeneity due to difficulty in controlling agglomera- tion.60 ,

The first of a series of International Conferences on

Ultrastructure Processing was held in 1983 to establish a scientific basis for the chemical-based processing of a new generation of advanced materials for structural, electrical, optical, and optoelectronic applications. Support by the Directorate of Chemical and Atmos- pheric Sciences of the Air Force Office of Scientific Research (AFOSR) for the Ultrastructure Conferences in 1983,6l 1985,6, 1987," and 198ga and the Materials

Research Society Better Ceramics Through Chemistry annual meetings in alternate years in 1984,65 19S6,6 and 198Sa has provided constant stimulation for the field.

In addition, AFOSR has provided a stable financial base of support for a number of university programs in sol-gel science throughout the 1980s under the technical monitoring of D. R. Ulrich.

The primary goal in these conferences and the AFOSR research and development program was to es- tablish a scientific foundation for a new era in the manufacture of advanced, high-technology ceramics, glasses, and composites. For millennia, ceramics have been made with basically the same technology. Pow- ders, either natural or man-made, have been shaped into objects and subsequently densified at temperatures close to their liquidus. The technology of making glass has also remained fundamentally the same since pre- history. Particles are melted, homogenized, and shaped

The Sol-Gel Process Chemical Reviews, 1990, Vol. 90, No. 1 35 f

Figure 2. Change in the roles of physics and chemistry as ce- ramics move toward ultrastructure processing. Reprinted from ref 61; copyright 1984 Wiley.

into objects from the liquid.

The goal of sol-gel processing and ultrastructure processing in general is to control the surfaces and in- terfaces of materials during the earliest stages of pro- duction.61 Long-term reliability of a material is usually limited by localized variations in the physical chemistry of the surface and interfaces within the material. The emphasis on ultrastructure processing is on limiting and controlling physical chemical variability by the pro- duction of uniquely homogeneous structures or pro- ducing extremely fine-scale (10-100 nm) second phases. Creating controlled surface compositional gradients and

achieving unique physical properties by combining in- organic and organic materials are also goals of ultra- structure processing.

The concept ofmolecular manipulation of the pro- cessing of ceramics, glasses, and composites requires an application of chemical principles unprecedented in the history of ceramics. Modern ceramics are primarily the products of applied physics, as indicated in Figure 2. During the past decade there has been enormous progress made in the shifting of the emphasis of ceramic science to include a larger overlap with chemistry, as also illustrated in Figure 2.61 The extensive literature represented by the conference proceedings cited above6147 contains excellent examples of this shift to- ward chemical-based processing in materials science.

Another essential factor for the increased scientific understanding of the sol-gel process is the availability of new analytical and calculational techniques capable of investigating on a nanometer scale the chemical processes of hydrolysis, polycondensation, syneresis, dehydration, and densification of materials. Many of the concepts of molecular control of sol-gel processes are a result of the use of nuclear magnetic resonance (NMR), X-ray small-angle scattering (XSAS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC), dielectric re- laxation spectroscopy (DRS), etc., that have been de- veloped during the past three decades.

The difference between the modern development of sol-gel-derived materials, such as gel-silica opti~s,~~-~~ and the classical work of Ebelman1*2 is that now drying of the monolithic silica optics can be achieved in days rather than years. The primary problem that had to be overcome was cracking during drying due to the large shrinkage that occurs when pore liquids are removed from the gels. For small cross sections, such as in powders, coatings, or fibers, drying stresses are small and can be accommodated by the material. For mon- olithic objects greater than about 1 cm in diameter, drying stresses developed in ambient atmospheres can introduce catastrophic fracture. To prevent fracture during drying, it is essential to control the chemistry of each step of the sol-gel process carefully. Likewise, to densify a dried gel monolith, it is essential to control the chemistry of the pore network prior to and during pore closure. The objective of this review is to describe the chemistry of the seven steps of the sol-gel process that can yield monoliths under ambient pressures. This review also describes how sol-gel-derived monoliths can be processed to result in fully dense components or with precisely controlled and chemically stable porosities. Most detail exists for Si02, and therefore the emphasis in this review is on silica sol-gel processing. The pro- cessing of silica monoliths by alkoxide methods will be compared with more traditional colloidal sol-gel methods.

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