Sol-Gel Science

The Physics and Chemistry of Sol-Gel Processing

C. Jeffrey Brinker, George W. Scherer

• Publisher: Academic Press
• Year: 2013
• eBook ISBN: 9780080571034
• Topic: Technology & Engineering
• Subtopic: Physical & Theoretical Chemistry
• Index: Technology & Engineering

Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing presents the physical and chemical principles of the sol-gel process. The book emphasizes the science behind sol-gel processing with a chapter devoted to applications. The first chapter introduces basic terminology, provides a brief historical sketch, and identifies some excellent texts for background reading. Chapters 2 and 3 discuss the mechanisms of hydrolysis and condensation for nonsilicate and silicate systems. Chapter 4 deals with stabilization and gelation of sols. Chapter 5 reviews theories of gelation and examines the predicted and observed changes in the properties of a sol in the vicinity of the gel point. Chapter 6 describes the changes in structure and properties that occur during aging of a gel in its pore liquor (or some other liquid). The discussion of drying is divided into two parts, with the theory concentrated in Chapter 7 and the phenomenology in Chapter 8. The structure of dried gels is explored in Chapter 9. Chapter 10 shows the possibility of using the gel as a substrate for chemical reactions or of modifying the bulk composition of the resulting ceramic by performing a surface reaction (such as nitridation) on the gel. Chapter 11 reviews the theory and practice of sintering, describing the mechanisms that govern densification of amorphous and crystalline materials, and showing the advantages of avoiding crystallization before sintering is complete. The properties of gel-derived and conventional ceramics are discussed in Chapter 12. The preparation of films is such an important aspect of sol-gel technology that the fundamentals of film formation are treated at length in Chapter 13. Films and other applications are briefly reviewed in Chapter 14. Materials scientists and researchers in the field of sol-gel processing will find the book invaluable.

CHAPTER 1

Introduction

Our goal is to present the fundamental principles of sol-gel processing. This requires that we explore areas of physics (e.g., fractal geometry and percolation theory), chemistry (mechanisms of hydrolysis and condensation), and ceramics (sintering and structural relaxation) that may be unfamiliar to many readers, including those involved in sol-gel research. Therefore we introduce each topic on an elementary level and develop it only so far as necessary to make our point; numerous references are provided for those who want more detail. In this chapter we begin by stepping through the stages of the process, which parallel the structure of the book, and by introducing the terminology. Then we provide a brief historical sketch of the evolution of this field of study plus a list of “essential reading.”

Figure 1 presents a schematic of the routes that one could follow within the scope of sol-gel processing, with each stage labelled by the number of the relevant chapter in this book. This illustration recurs as a “locator” at the beginning of each chapter to provide a perspective on the place of that step in the overall process.

Fig. 1 Overview of the sol-gel process with steps labelled by numbers of corresponding chapters of this book.

1 SOL-GEL PROCESSING

A colloid is a suspension in which the dispersed phase is so small (∼ 1–1000 nm) that gravitational forces are negligible and interactions are dominated by short-range forces, such as van der Waals attraction and surface charges. The inertia of the dispersed phase is small enough that it exhibits Brownian motion (or Brownian diffusion), a random walk driven by momentum imparted by collisions with molecules of the suspending medium. A sol is a colloidal suspension of solid particles in a liquid. An aerosol is a colloidal suspension of particles in a gas (the suspension may be called a fog if the particles are liquid and a smoke if they are solid) and an emulsion is a suspension of liquid droplets in another liquid. All of these types of colloids can be used to generate polymers or particles from which ceramic materials can be made. A ceramic is usually defined by saying what it is not: it is nonmetallic and inorganic; some would also say it is not a chalcogenide. We thus include all metal oxides, nitrides, and carbides, both crystalline and noncrystalline. In the sol-gel process, the precursors (starting compounds) for preparation of a colloid consist of a metal or metalloid element surrounded by various ligands (appendages not including another metal or metalloid atom). For example, common precursors for aluminum oxide include inorganic (containing no carbon) salts such as Al(NO₃)₃ and organic compounds such as Al(OC₄H₉)₃. The latter is an example of an alkoxide, the class of precursors most widely used in sol-gel research. An alkane is a molecule containing only carbon and hydrogen linked exclusively by single bonds, as in methane (CH₄) and ethane (C₂H₆); the general formula is C_nH_2n+2. An alkyl is a ligand formed by removing one hydrogen (proton) from an alkane molecule producing, for example, methyl (•CH₃) or ethyl (•C₂H₅) (where the dot • indicates an electron that is available to form a bond). An alcohol is a molecule formed by adding a hydroxyl (OH) group to an alkyl (or other) molecule, as in methanol (CH₃OH) or ethanol (C₂H₅OH). An alkoxy is a ligand formed by removing a proton from the hydroxyl on an alcohol, as in methoxy (•OCH₃) or ethoxy (•OC₂H₅). A list of the most commonly used alkoxy ligands is presented in Table 1.

Table 1

Commonly Used Ligands.

Alkyl		Alkoxy
methyl	•CH3	methoxy	•OCH3
ethyl	•CH2CH3	ethoxy	•OCH2CH3
n-propyl	•CH2CH2CH3	n-propoxy	•O(CH2)2CH3
iso-propyl	H3C(•C)HCH3	iso-propoxy	H3C(•O)CHCH3
n-butyl	•CH2(CH2)2CH3	n-butoxy	•O(CH2)3CH3
sec-butyl	H3C(•C)HCH2CH3	sec-butoxy	H3C(•O)CHCH2CH3
iso-butyl	•CH2CH(CH3)2	iso-butoxy	•OCH2CH(CH3)2
tert-butyl	•C(CH3)3	tert-butoxy	•OC(CH3)3
Other

Dot (•) indicates bonding site. Parentheses indicate atom with available bond. n = normal (meaning a linear chain), sec = secondary, tert = tertiary.

Metal alkoxides are members of the family of metalorganic compounds, which have an organic ligand attached to a metal or metalloid atom. The most thoroughly studied example is silicon tetraethoxide (or tetraethoxysilane, or tetraethyl orthosilicate, TEOS), Si(OC₂H₅)₄. Organometallic compounds are defined as having direct metal-carbon bonds, not metal-oxygen-carbon linkages as in metal alkoxides; thus, alkoxides are not organometallic compounds, although that usage turns up frequently in the literature. Metal alkoxides are popular precursors because they react...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Dedication
• Preface
• Acknowledgments
• Chapter 1: Introduction
• Chapter 2: Hydrolysis and Condensation I: Nonsilicates
• Chapter 3: Hydrolysis and Condensation II: Silicates
• Chapter 4: Particulate Sols and Gels
• Chapter 5: Gelation
• Chapter 6: Aging of Gels
• Chapter 7: Theory of Deformation and Flow in Gels
• Chapter 8: Drying
• Chapter 9: Structural Evolution during Consolidation
• Chapter 10: Surface Chemistry and Chemical Modification
• Chapter 11: Sintering
• Chapter 12: Comparison of Gel-derived and Conventional Ceramics
• Chapter 13: Film Formation
• Chapter 14: Applications
• INDEX