Reference

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Reference

• C. J. Brinker and G. W. Scherer, Sol-Gel Science
  - The Physics and Chemistry of Sol-Gel
  Processing, New York, Academic Press, 1990.
• Sol-gel chemistry http//www.psrc.usm.edu/mauritz/
  solgel.html
• The Sol-gel gateway
• http//www.solgel.com

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• The Sol-Gel process allows to synthesize ceramic
  materials of high purity and homogeneity by a
  process occurs in liquid solution of
  organometallic precursors (TMOS, TEOS,
  Zr(IV)-Propoxide, Ti(IV)-Butoxide, etc. ), which,
  by means of hydrolysis and condensation
  reactions, lead to the formation of a new phase
  (SOL). M-O-R H2O ?M-OH R-OH (hydrolysis)
  M-OH HO-M?M-O-M H2O (water condensation)
  M-O-R HO-M ? M-O-M R-OH (alcohol
  condensation) (M Si, Zr, Ti)

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• The SOL is made of solid particles of a diameter
  of few hundred of nm suspended in a liquid phase.
  
• Then the particles condense in a new phase (GEL)
  in which a solid macromolecule is immersed in a
  liquid phase (solvent). Drying the GEL by means
  of low temperature treatments (25-100 C), it is
  possible to obtain porous solid matrices
  (XEROGELs).
• The fundamental property of the solgel process is
  that it is possible to generate ceramic material
  at a temperature close to room temperature.
• Therefore such a procedure opened the possibility
  of incorporating in these glasses soft dopants,
  such as fluorescent dye molecules and organic
  chromophores.

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Precusorgt Oligomergt Polymergt Colloidsgt Sol
gt gel

• Xerogel, Aerogel Organic/Inorganic

Ceramics, Glass Porous Materials
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Traditional Sol-Gel Methodology
Hydrolysis
Condensation
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Random Network of SiO2
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Hydrolysis

• (8) Olation and (9) Oxolation

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Factors affecting Reactivity

• pH, 
• Water content, 
• Concentration, 
• Temperature,
• Drying conditions 

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Condensation rate of silica
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How to control hydrolysis rates?

• By contrast to silicon alkoxides whose hydrolysis
  requires catalysts for efficient gelation rates,
  hydrolysis of most metal alkoxides is rapid and
  can lead to uncontrolled precipitation. The
  electronegative alkoxide groups make the metal
  highly prone to nucleophilic attack by water. The
  more electrophilic metal centres as compared to
  silicon- as well as a larger and thus more
  stereolabile coordination sphere result in a
  higher hydrolytic susceptibility. The following
  sequence of reactivity is usually found
• Si(OR)4 ltlt Sn(OR)4 Ti(OR)4 lt
• Zr(OR)4 Ce(OR)4.7

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Strategy for slowing down hydrolysis

• Changing the nature of the organic group R
  alkoxides with primary organic groups such as
  n-butoxides are less sensitive to hydrolysis than
  secondary ones such as isopropoxides 
• Increase of the metal coordination number thus
  hindering attack of water and formation of the
  metal hydroxyl bond, M-OH, necessary for the
  development of the network 
• Decreasing the functionality of the precursor by
  partial substitution of the OR ligands by anionic
  ligands such as carboxylates or b-diketonates
  leading to M-Z bonds less susceptible to
  hydrolysis

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Typical Starting Materials
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Sol, Gel and Flocculate
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The sol-gel process (a) sol (b) gel.
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Gel Process
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The problem of Drying

• Freshly prepared silica gels contain an
  appreciable amount of solvent (usually 70 to 90
  wt ) which must be eliminated. The solid silica
  network formed by hydrolysis and polycondensation
  of Si alkoxides is made up by Silica species of a
  few tens of nanometer size.Capillary stress
  appears when the liquid move inside the pores
  during drying and form a liquid-gas curve
  interface

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The problem of Drying
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Supercritical Drying
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Monolithic Aerogels
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Hydrophobic Surface
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Contact Angle of Hydrophobic surface
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Applications of Aerogel

• Thermal Insulation
• Acoustic Insulation
• Catalyst Support
• Optical applications
• Nuclear Waste Storage
• Filler for paints or others
• low dielectric constant materials
• Batteries

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• Hydrolysis
• The reaction of a metal alkoxide (M-OR) with
  water, forming a metal hydroxide (M-OH).
• Condensation
• A condensation reaction occurs when two metal
  hydroxides (M-OH HO-M) combine to give a metal
  oxide species (M-O-M). The reaction forms one
  water molecule.
• Sol
• A solution of various reactants that are
  undergoing hydrolysis and condensation reactions.
  The molecular weight of the oxide species
  produced continuously increases. As these species
  grow, they may begin to link together in a
  three-dimensional network.
• Gel Point
• The point in time at which the network of linked
  oxide particles spans the container holding the
  Sol. At the gel point the Sol becomes an Alcogel.
  
• Alcogel (wet gel)
• At the gel point, the mixture forms a rigid
  substance called an alcogel. The alcogel can be
  removed from its original container and can stand
  on its own. An alcogel consists of two parts, a
  solid part and a liquid part. The solid part is
  formed by the three-dimensional network of linked
  oxide particles. The liquid part (the original
  solvent of the Sol) fills the free space
  surrounding the solid part. The liquid and solid
  parts of an alcogel occupy the same apparent
  volume.
• Supercritical fluid
• A substance that is above its critical pressure
  and critical temperature. A supercritical fluid
  possesses some properties in common with a
  liquids (density, thermal conductivity) and some
  in common with gases. (fills its container, does
  not have surface tension).
• Aerogel
• What remains when the liquid part of an alcogel
  is removed without damaging the solid part (most
  often achieved by supercritical extraction). If
  made correctly, the aerogel retains the original
  shape of the alcogel and at least 50 (typically
  gt85) of the alcogel's volume.
• Xerogel
• What remains when the liquid part of an alcogel
  is removed by evaporation, or similar methods.
  Xerogels may retain their original shape, but
  often crack. The shrinkage during drying is often
  extreme (90) for xerogels.

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Hybride Synthesis Strategy

• Although they initially have been worked out by
  chemists from the sol-gel scientific community
  today hybrids are elaborated by researchers
  coming from a variety of disciplines, polymers
  chemists, solid state chemists, catalysis,
  materials researchers etc. Each of these
  communities elaborate hybrids using their own
  tools, specific disciplinary methods and more
  important their own raw materials. It is not
  seldom to see that a polymerist will work out an
  hybrid system having an emphasis on the polymer
  side of the hybrid, using even pre-formed
  polymers, capped oligomers etc. Sol-gel and
  inorganic chemists will preferably use as
  precursors, silicon or metal alkoxides or even
  inorganic building units such as clusters or
  nanoparticles. They can also use lamellar
  inorganic compounds as host for organic
  components. Many names have been given to these
  materials Ceramers, Polycerams, Ormosils or
  Ormocers.
• However it is now commonly accepted that a
  molecular approach for the synthesis of hybrids
  reflects better the wide opportunities offered by
  this compounded chemistry.

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Schematic illustration of the organic template
approach
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Shape of different products available through
processing by sol-gel
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• Organo(alkoxy)silanes and metal alkoxides serving
  as precursors for sol-gel-derived materials.

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Cross-linking of inorganic clusters or
nanoparticles via photochemically induced
polymerization of (meth)acrylic functional
groups.
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Selected Organic Dye for Sol-gel
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One-pot synthesis of chromophoric sol_gel hybrid
materials.
Angew. Chem. Int. Ed. 2002, 41, No. 10
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xerogels prepared from fluoroaromatics 1 to
5(from left to right) and APS or MAPS in TEOS
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Industrial ApplicationsFunctional Coating for
Glass
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Commercial magnifying lens(PMMA) euipped with an
abrasion resistant, UV cured hybrid coating