Ultrasonically Assisted Process for Sol-Gel Production

By AZoNano

Table of Contents

Introduction
Sol-Gel Process
High Power, Low Frequency Ultrasound
Sono-Gels
Sono-Ormosil
Nanocoating through Ultrasonic Sol-Gel Reaction
Mesoporous TiO2 through Ultrasonic Sol-Gel Synthesis Mesoporous
Conclusion
About Hielscher

Introduction

A number of potential additives such as thin film coatings, ultrafine nano-sized particles and spherical-shaped particles, fibers, porous aerogels and xerogels, porous and dense material are used for producing high-performance materials. Using the sol-gel technique, materials like highly porous, ultralight aerogels, ceramics and organic-inorganic hybrids can be produced from colloidal suspensions.

Since the sol particles range in the nanometer size, the material exhibits special properties. Hence, the sol-gel process is especially important in nanochemistry. In this technical note, the production of nano-sized material through ultrasonically assisted sol-gel methods is studied in detail.

Sol-Gel Process

Sol-gel and associated processing comprise a number of steps: precipitating powder or making sol, gelling the sol on a substrate or in a mold, or making a second sol from the precipitated powder and its gelation, or molding the powder into a body by non-gel methods. This is followed by drying, firing and sintering processes.

Figure 1. Steps of sol-gel synthesis

Sol-gel process is a wet-chemical technique of synthesis used for producing a network of metal oxides or hybrid polymers. Generally, metal alkoxides and metal chlorides are used as precursors.

The sol which is present in a suspension of the precursors converts into a gel-like diphasic system, which in turn occurs in liquid as well as solid phase. During a sol-gel process, the chemical reactions that take place are hydrolysis, poly-condensation, and gelation.

During poly-condensation and hydrolysis, a colloid (sol), which are nanoparticles dissolved in a solvent, is produced. The existing sol phase then changes into a gel. The ensuing gel-phase is produced by particles, whose size and formation can differ considerably from distinct colloidal particles to continuous chain-like polymers. The morphology depends on the chemical conditions.

Based on the observations on SiO2 alcogels, it can be concluded that a catalyzed sol results base- in a distinct species produced by the combination of monomer-clusters that are dense and highly branched.

The development of a continuous chain of low-density polymers offers various benefits with respect to physical characteristics in the formation of ceramic and glass components in two and three dimensions. With additional processing steps, like dip-coating or spin-coating, it is possible to coat substrates with thin films to make a wet gel.

After drying and heating processes, a dense material can be obtained. This material can be further processed. Ultrafine and uniform powders can be made through spray pyrolysis, precipitation, or emulsion methods; or aerogels can be formed by extracting the liquid phase of the wet gel.

High Power, Low Frequency Ultrasound

For chemical processes, high power, low frequency ultrasound proves quite useful. During low-pressure cycle, tiny vacuum bubbles are created in the liquid by high power ultrasound. These bubbles grow over a number of cycles.

Depending on the intensity of ultrasound, liquid compresses and stretches to varying degrees, and at low ultrasonic intensities, the cavitation bubbles oscillate to some equilibrium size for numerous acoustic cycles. This occurrence is called as stable cavitation.

At high intensity of ultrasound, the cavitational bubbles form within a few acoustic cycles to a radius of double their original size and disintegrate at a point of compression when the bubble is unable to absorb more energy. This phenomenon is called as inertial cavitation.

Sono-Gels

Ultrasound is applied to the precursors during sol-gel reactions. The ensuing materials that exhibit novel properties are dubbed as sonogels. Owing to the lack of additional solvent along with ultrasonic cavitation, a special environment is created for sol-gel reactions, and this environment enables the development of specific features in the resulting gels such as fine texture, high density, and homogeneous structure.

These properties ascertain the development of sonogels on additional processing and the final material structure.

For preparing silica gels from Si(OC2H5)4, ethanol is commonly utilized because of the non-solubility of Si(OC2H5)4 in water. However, ethanol can promote cracking during the drying process. Here, ultrasonication provides a suitable solution. This results in a high-density silica sono-gel when compared to traditionally produced gels.

Figure 2. Ultrasonic glass flow cell for continuous sonication

Sono-Ormosil

Sonication is ideal for synthesizing polymers. In contrast to standard gels, sono-ormosils are characterized by a higher density and better thermal stability.

Nanocoating through Ultrasonic Sol-Gel Reaction

In nanocoating, a material is covered with a nano-scaled layer, thus encapsulated or core-shell structures are carried. Such nano composites have high physical and chemical properties because of the combined properties and structuring effects of the components.

Mesoporous TiO2 through Ultrasonic Sol-Gel Synthesis Mesoporous

Mesoporous TiO2 is generally used as photocatalyst in sensor technology, electronics and environmental remediation. For enhanced material properties, it is used to create TiO2 with large surface area and high crystallinity. The ultrasonic assisted sol-gel process ensures that the extrinsic and intrinsic characteristics of TiO2, like the surface area, particle size, crystallinity, pore-diameter, pore-volume, rutile, anatase and brookite phase ratios are influenced by controlling the parameters. The comparison of both TiO2 samples, in presence and absence of ultrasonic irradiation is illustrated in the image below.

Figure 3. SEM images of TiO2 powder in the presence and absence of ultrasound, respectively.

Figure 4. Ultrasonic sol-gel synthesis of mesoporous TiO2

The above images of the surface of the powder samples indicate that the application of ultrasonic waves results in smaller particles and also promotes greater homogeneity in the typical size of the particles.

Conclusion

The use of high-power ultrasound in sol-gel processes allows for better mixing and deagglomeration of the particles, thus promoting spherical, low- dimensional particle shape, smaller particles size, and improved form and size.

The application of intense ultrasound enables in customizing special materials from sol-gel processes. It is usually applied to produce fine dispersions and emulsions, thus making it suitable for chemistry and materials' research and development.

Figure 5. High power ultrasound setup for sonochemical applications.

About Hielscher

Hielscher Ultrasonics is a family business, located in Teltow near Berlin (Germany). The main emphasis of its activities is the conception, development and production of ultrasonic devices for the use in laboratory and industrial applications. Technological innovations together with the realization of new ultrasound based processes substantiated the company growth and its market acceptance.

Today, ultrasonic devices made by Hielscher Ultrasonics are being used in laboratories and production plants on all continents across the world. More than 70% of the total sales is based on export. Almost every second device is supplied to customers outside Europe. Hielscher Ultrasonics integrates the ultrasonic devices into complex ultrasonic systems, such as wire cleaning systems, too. The systems are produced to meet the customers requirements in terms of power, extended range of accessories and steady state proof equipment.

Hielscher USA, Inc. is the representative for Hielscher ultrasonic equipment in the North American market. It is located in Ringwood, NJ.

This information has been sourced, reviewed and adapted from materials provided by Hielscher.

For more information on this source, please visit Hielscher.

Date Added: Mar 4, 2013 | Updated: Jun 11, 2013
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