This document discusses nanomaterials produced via the sol-gel method, specifically focusing on aerogels. It defines sol-gel processing and aerogels, describing aerogels as highly porous gels with over 50-99.5% air content, making them very light. The document outlines the sol-gel synthesis process for aerogels and discusses their properties including low thermal conductivity, hydrophobicity or hydrophilicity depending on surface chemistry, and mechanical brittleness. It also covers types of aerogels and their applications as composites, absorbents, sensors, and thermal insulators.
2. Content
• What is Sol-Gel
• Sol-Gel Method
• Aerogel
• Synthesis of aerogels
• Properties of Aerogels
• Types of Aerogels
• Application
• Advantages
• Disadvantages
3. What is Sol-Gel ?
The sol-gel process is a more chemical method (wet chemical method)
for the synthesis of various nanostructures, especially metal oxide
nanoparticles.
Sol-gel processing designates a type of solid material synthesis,
performed in a liquid and at low temperature.
It’s a typical nanotechnology because all gel products contain
nanoparticle or are nanocomposites.
Synthesis schemes used depends on initial precursors.
The precursors for synthesizing these colloids consist Metal alkoxides e.g.
Tetramethoxysilane (TMOS) and Tetraethoxysilane (TEOS) and metal
chlorides ( they readily react with water) .
4. Sol-Gel Method :
• Reference - Z. Li, Y. Shi, A. Zhu et al., “Light‐Responsive, reversible emulsification and demulsification of oil‐in‐water pickering
emulsions for catalysis,” Angewandte Chemie InternationalEdition, vol. 60, no. 8, pp. 3928–3933, 2021.
5. Aerogel :-
When most of the solvent is removed from the gel, the liquid content in the gel is
much less than the solid content, or the medium in the spatial network structure of
the gel is gas, and the appearance is solid, that is, dry gel, also called it aerogel.
Aerogels also have the properties of gels. The aerogel is the lightest solid known. Its
density is only three times that of air, so it is sometimes called “solidified smoke”.
Aerogels are gels with nanometer pores, low density, porosity, and high internal
area. Due to these properties, aerogels are widely used in applications that require
sound insulation as well as high light transmittance.
Aerogels are known to be the lightest and least dense solids, with about 50–99.5%
of their volume being air. Aerogels usually have amorphous structures and lack
crystalline order in their microstructure. For this reason, these nanomaterials are very
brittle and have very high transparency.
7. Synthesis of Aerogels :-
The synthesis of aerogels using the sol-gel process consists of two steps,
which are as follows
(i) The first stage involves the formation of separate colloidal solid
particles with nanometer dimensions.
(ii) The second stage involves colloidal particles in the solvent joining
together to form a gel.
The most obvious feature of this reaction is the transition from a
colloidal solution (liquid) into a di- or multiphase gel (solid) that led to
the expression “sol-gel process”.
8. Properties of Aerogels :-
A. Thermal conductivity of Aerogels :
The main reason for the low thermal conductivity of these nanomaterials is their
porous structure, in such a way that the porosity in their structure prevents
continuous and proper flow of heat.
B. Hydrophobicity of Aerogels :
After the aerogel is synthesized, large amounts of hydroxyl groups are formed on its
surface. These groups cause the aerogel to react with water and cause it to dissolve
severely in it. One way to make hydrophilic aerogels hydrophobic is to replace their
hydroxyl (-OH) group with nonpolar (-OR) groups. Therefore, it can be concluded
that aerogels are inherently hydrophilic, but they can be dehydrated by chemical
operations.
A. Mechanical Properties of Aerogels :
The mechanical properties of aerogels are highly dependent on their structural
conditions such as the quality of cavity cohesion and density.
9. Types ofAerogels :-
1. Inorganic Aerogels: It can be prepare by sol-gel polymerization of
Tetramethoxysilane (TMOS), or the less toxicTetraethoxysilane (TEOS).
2. Organic Aerogels: Condensation of resorcinol-formaldehyde and
melamine-formaldehyde in alkaline aqueous solution is used for the
synthesis of organic aerogel.
3. Carbon Aerogels: Carbon aerogels are synthesized by the pyrolysis
of organic aerogels at temperature above 500°C.
4. Hybrid Aerogels: These can be synthesized by using organic
mineral compound.
5. Metal Oxide Aerogels: Metal oxide aerogels are the inorganic
cousins of the more common silica aerogels- each type with its own
unique properties.
Reference - D. W. Schaefer and K. D. Keefer, “Structure of random porous materials: silica aerogel,” Physical Review Letters, vol. 56, no. 20, pp. 2199–2202, 1986
A. V. Rao and D. Haranath, “Effect of methyltrimethoxysilane as a synthesis component on the hydrophobicity and some physical properties of silica
aerogels,” Microporous and Mesoporous Materials, vol. 30, no. 2-3, pp. 267–273, 1999.
E. Cuce, M. C. Pinar, P. M. Cuce, C. J. Wood, and S. B. Riffat, “Toward aerogel based thermal superinsulation in buildings: a comprehensive review,” Renewable and
Sustainable Energy Reviews, vol. 34, pp. 273–299, 2014.
10. Applications :-
Aerogel as Composite
Aerogel as an Absorbent
Aerogel as a Sensor
Aerogel as material with Low-Dielectric Constant
Aerogel as Catalyst
Aerogel as Storage Media
Aerogel as Template
Aerogel as a Thermal Insulator
11. Advantages of Sol-Gel Method :-
a) Mono sized nano particles are produced by this method.
b) It is useful for the synthesis of glass, glass ceramics or ceramic materials
at lower at lower temperature.
c) Can produce thin bond-coating to provide excellent adhesion between
the metallic substrate and the top.
d) Can have low temperature sintering, usually 200-600°C.
e) Can produce a simple, economic and effective method to produce high
quality coatings
Disadvantages :-
a) Controlling the growth of the particle.
b) There is often a large volume shrinkage and cracking during drying.
c) The cost of the raw materials may be high.
