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Special Issue "Sol-Gel Technique"

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A special issue of Materials (ISSN 1996-1944). This special issue belongs to the section "Manufacturing Processes and Systems".

Deadline for manuscript submissions: closed (15 December 2010)

Special Issue Editor

Guest Editor
Prof. Dr. Jay B. Benziger (Website)

Department of Chemical Engineering, Princeton University, Engineering Quad., Room A-407, Princeton, NJ 08544-5263, USA
Phone: 01-609-258-5416
Fax: +1 609 2580211
Interests: reaction engineering of organic vapor phase deposition; dynamics of polymer electrolyte membrane fuel cells; recovery of thiols for hydrocarbon streams; developing a large-scale liquid scintillation detector

Special Issue Information

Dear Colleagues,

Sol-gels are versatile materials made by condensing a solution (sol) of metal oxide precursors into three dimensional networks. The gels are bi-phasic systems in which a continuous fluid phase fills the space inside a polymerized network. The gels can be dried in controlled fashion to produce porous solids with unique thermal, mechanical, optical and chemical properties. Sol-gel materials have grown in importance over the past 30 years as chemists and engineers have learned how to vary the reactants and processing conditions to tailor material properties for specific applications.

The early work with sol-gels focused on those made of silica, derived by condensation of silanols groups (Si-OH), as illustrated in by reaction (1).

Reaction 1

Reaction 1. Condensation of silanols into a gel. The silanols condense by forming water leading to a network of Si-O-Si bonds. The quaternary functionality of the Si results in a three dimensional network.

The silanols groups may be on the surface of nanometer sized silica particles or could be formed by hydrolysis of silicone alkoxides as illustrated in reaction (2).

Reaction 2

Reaction 2. Hydrolysis of silicon alkoxides to produce silanols. The silanols subsequently undergo condensation reactions to produce silica gels.

Silicon has four functional groups that can undergo condensation. By altering the reaction conditions (temperature, concentration, pH, solvent and reactant), the degree and nature of the condensation reactions can be controlled. At low pH conditions the rate of condensation slows down with degree of branching resulting in low cross-link density and very porous gels. In contrast high pH will accompany rapid condensation that can produce dense particles that precipitate from the solution. The gels are filled with liquid, generally a water alcohol solution. Controlled drying of the gel is employed to tailor the porosity and composition for specific applications. Highly porous materials can be produced that are exceptional thermal insulators. Alternatively dense gels may be employed as thin film protective coatings for lenses.

Advances in chemistry and the chemical precursors available for sol-gel processing have made it an extremely flexible process for materials synthesis. We are no longer limited to silica gels or even to metals oxides. It is now possible to make sol gels materials from almost any transition metal, as well as make composite materials. The applications of sol-gel materials have grown as the synthesis and processing methods have opened new vistas of material properties. A list of some of major applications of sol-gels is given in Table 1.

Table 1. Applications of Sol-gel materials.

 

Application
Sol-Gel Material
Optical fibers
High purity doped silica gel films for optical fiber precursors
Protective optical coatings
Abrasion resistance silica gel coatings on plastic substrates
Anti-reflective optical coatings
Laser windows, smart windows
Thermal insulation for windows
Aerogel window spacers, solar collector coatings
High Temperature Refractory Insulation
Ceramic foams
Chemical Sensors
Thin film NOx sensors, sol-gel coated crystal oscillators
Catalysts and Adsorbents
Silica alumina solid acid catalysts, high surface area catalyst supports, Silica gel desiccant
Ceramic membranes
Sol-gel molecular sieves, antibacterial filters
Abrasives
Alumina abrasives
Dental sealants and fillers
Hydroxyapatite

 

Sol-gel technology offers many important advantages in materials processing. The nanometer structure of the gels permits low temperature processing of ceramic materials so that ceramics and plastics can be combined in hybrid materials. The introduction of metal alkoxides precursors for sol-gels made possible the production of high purity materials that dramatically improved the quality of optical fibers. The pore structure and large surface areas associated with sol-gel materials has been essential to the development of catalysts and adsorbents making possible improved production of gasoline and removing impurities for automobile exhausts and new photocatalysts for splitting water. We now have a tool box of chemical and processing methods to tailor sol-gels to tackle new materials technologies. In this special issue we present reviews of the synthesis and processing techniques to produce sol-gel materials.

Prof. Dr. Jay B. Benziger
Guest Editor

Keywords

  • silica gels
  • aerogels
  • xerogels
  • optical fiber cladding
  • sol gel derived waveguides
  • abrasive resistant coatings
  • hybrid glass coatings
  • antireflective coatings
  • titania sol gel photocatalysts
  • aerogel windows
  • sol gel catalysts
  • adsorbents
  • ceramic foams
  • sol gel membranes
  • sol gel NOx sensors
  • colloidal gels
  • sol gel transition
  • self cleaning coatings
  • ceramic spin coating
  • ceramic dip coating
  • sol gel composites

Related Special Issue

Published Papers (5 papers)

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Research

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Open AccessArticle Sol-Gel Entrapped Levonorgestrel Antibodies: Activity and Structural Changes as a Function of Different Polymer Formats
Materials 2011, 4(3), 469-486; doi:10.3390/ma4030469
Received: 17 January 2011 / Revised: 14 February 2011 / Accepted: 21 February 2011 / Published: 25 February 2011
Cited by 7 | PDF Full-text (1809 KB) | HTML Full-text | XML Full-text
Abstract
The paper describes development of a sol-gel based immunoaffinity method for the steroid hormone levonorgestrel (LNG) and the effects of changes in the sol-gel matrix format on the activity of the entrapped antibodies (Abs) and on matrix structure. The best sol-gel format [...] Read more.
The paper describes development of a sol-gel based immunoaffinity method for the steroid hormone levonorgestrel (LNG) and the effects of changes in the sol-gel matrix format on the activity of the entrapped antibodies (Abs) and on matrix structure. The best sol-gel format for Ab entrapment was found to be a tetramethoxysilane (TMOS) based matrix at a TMOS:water ratio of 1:8, containing 10% polyethylene glycol (PEG) of MW 0.4 kDa. Addition of higher percentages of PEG or a higher MW PEG did not improve activity. No activity was obtained with a TMOS:water ratio of 1:12, most likely because of the very dense polymer that resulted from these polymerization conditions. Only minor differences in the non-specific binding were obtained with the various formats. TMOS was found to be more effective than tetrakis (2-hydroxyethyl)orthosilicate (THEOS) for entrapment of anti-levonorgestrel (LNG) Abs. However, aging the THEOS-based sol-gel for a few weeks at 4 °C stabilized the entrapped Abs and increased its binding capacity. Confocal fluorescent microscopy with fluorescein isothiocyanate (FITC) labeled immunoglobulines (IgGs) entrapped in the sol-gel matrix showed that the entrapped Abs were distributed homogenously within the gel. Scanning electron microscopy (SEM) images have shown the diverse structures of the various sol-gel formats and precursors. Full article
(This article belongs to the Special Issue Sol-Gel Technique)
Open AccessArticle Iron Oxide Silica Derived from Sol-Gel Synthesis
Materials 2011, 4(2), 448-456; doi:10.3390/ma4020448
Received: 28 December 2010 / Revised: 31 January 2011 / Accepted: 16 February 2011 / Published: 17 February 2011
Cited by 14 | PDF Full-text (501 KB) | HTML Full-text | XML Full-text
Abstract
In this work we investigate the effect of iron oxide embedded in silica matrices as a function of Fe/Si molar ratio and sol pH. To achieve homogeneous dispersion of iron oxide particles, iron nitrate nonahydrate was dissolved in hydrogen peroxide and was [...] Read more.
In this work we investigate the effect of iron oxide embedded in silica matrices as a function of Fe/Si molar ratio and sol pH. To achieve homogeneous dispersion of iron oxide particles, iron nitrate nonahydrate was dissolved in hydrogen peroxide and was mixed with tetraethyl orthosilicate and ethanol in a sol-gel synthesis method. Increasing the calcination temperature led to a reduction in surface area, although the average pore radius remained almost constant at about 10 Å, independent of the Fe/Si molar ratio or sol pH. Hence, the densification of the matrix was accompanied by similar reduction in pore volume. However, calcination at 700 °C resulted in samples with similar surface area though the iron oxide content increased from 5% to 50% Fe/Si molar ratio. As metal oxide particles have lower surface area than polymeric silica structures, these results strongly suggest that the iron oxides opposed the silica structure collapse. The effect of sol pH was found to be less significant than the Fe/Si molar ratio in the formation of molecular sieve structures derived from iron oxide silica. Full article
(This article belongs to the Special Issue Sol-Gel Technique)

Review

Jump to: Research

Open AccessReview Sol-Gel Synthesis of Non-Silica Monolithic Materials
Materials 2010, 3(4), 2815-2833; doi:10.3390/ma3042815
Received: 15 February 2010 / Revised: 13 April 2010 / Accepted: 16 April 2010 / Published: 21 April 2010
Cited by 36 | PDF Full-text (592 KB) | HTML Full-text | XML Full-text
Abstract
Monolithic materials have become very popular because of various applications, especially within chromatography and catalysis. Large surface areas and multimodal porosities are great advantages for these applications. New sol-gel preparation methods utilizing phase separation or nanocasting have opened the possibility for preparing [...] Read more.
Monolithic materials have become very popular because of various applications, especially within chromatography and catalysis. Large surface areas and multimodal porosities are great advantages for these applications. New sol-gel preparation methods utilizing phase separation or nanocasting have opened the possibility for preparing materials of other oxides than silica. In this review, we present different synthesis methods for inorganic, non-silica monolithic materials. Some examples of application of the materials are also included. Full article
(This article belongs to the Special Issue Sol-Gel Technique)
Open AccessReview Novel Materials through Non-Hydrolytic Sol-Gel Processing: Negative Thermal Expansion Oxides and Beyond
Materials 2010, 3(4), 2567-2587; doi:10.3390/ma3042567
Received: 11 January 2010 / Revised: 9 March 2010 / Accepted: 2 April 2010 / Published: 6 April 2010
Cited by 20 | PDF Full-text (497 KB) | HTML Full-text | XML Full-text
Abstract
Low temperature methods have been applied to the synthesis of many advanced materials. Non-hydrolytic sol-gel (NHSG) processes offer an elegant route to stable and metastable phases at low temperatures. Excellent atomic level homogeneity gives access to polymorphs that are difficult or impossible [...] Read more.
Low temperature methods have been applied to the synthesis of many advanced materials. Non-hydrolytic sol-gel (NHSG) processes offer an elegant route to stable and metastable phases at low temperatures. Excellent atomic level homogeneity gives access to polymorphs that are difficult or impossible to obtain by other methods. The NHSG approach is most commonly applied to the preparation of metal oxides, but can be easily extended to metal sulfides. Exploration of experimental variables allows control over product stoichiometry and crystal structure. This paper reviews the application of NHSG chemistry to the synthesis of negative thermal expansion oxides and selected metal sulfides. Full article
(This article belongs to the Special Issue Sol-Gel Technique)
Figures

Open AccessReview Chemical Sensors Based on Molecularly Imprinted Sol-Gel Materials
Materials 2010, 3(4), 2196-2217; doi:10.3390/ma3042196
Received: 26 November 2009 / Revised: 27 January 2010 / Accepted: 23 March 2010 / Published: 24 March 2010
Cited by 31 | PDF Full-text (547 KB) | HTML Full-text | XML Full-text
Abstract
The sol-gel technique is earning the worldwide attention of researchers in the field of material science, due to its versatility in synthesizing inorganic ceramic materials at mild conditions. High purity, homogeneity, controlled porosity, stable temperature and nanoscale structuring are the most remarkable [...] Read more.
The sol-gel technique is earning the worldwide attention of researchers in the field of material science, due to its versatility in synthesizing inorganic ceramic materials at mild conditions. High purity, homogeneity, controlled porosity, stable temperature and nanoscale structuring are the most remarkable features offered by this method for generating highly sensitive and selective matrices to incorporate analyte molecules. The crafting of sol-gel sensors through molecular imprinting has put great influence on the development of innovative chemical sensors, which can be seen from the growing number of publications in this field. The review provides a brief overview of sol-gel sensor applications, and discusses the contribution of molecular imprinting in exploring the new world of sensors. Full article
(This article belongs to the Special Issue Sol-Gel Technique)

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