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Sol-Gel process

Dr. Dmitri Kopeliovich

So-Gel process is a synthesis of an oxide ceramic in a liquid solution of an alkoxide based precursor in which submicron particles of a solid phase suspended in the liquid (Sol) are obtained and then condensed forming Gel - a 3-D network of polymerized macromolecules surrounded with the solution.

Applications of Sol-Gel process:

  • Thermal insulation.
  • Acoustic insulation.
  • Protective optical coatings.
  • Lightweight materials.
  • Tough ceramics.
  • Membranes and microfilters.
  • Nuclear waste storage.
  • Ultra fine powder abrasives.
  • Encapsulation of biomolecules for controlled drug release.

The stages of the Sol-Gel process

Hydrolysis of precursor (sol formation)

The precursor is an aqueous solution of the metal M alkoxide: M-OR, where R is the alkyl group (e.g., C2H5).

The metal alkoxide reacts with the surrounding water and forms the colloidal suspension (sol) of the metal hydroxide M-OH according to the hydrolysis reaction:

M-OR + H-OH = M-OH + R-OH

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Polycondensation (gelataion)

Polycondensation of the hydrolyzed precursor occurs according to the reactions:

Reaction with the non-hydrolyzed metal alkoxide M-OR resulting in alcohol formation R-OH:
M-OH + M-OR = M-O-M + R-OH

Reaction with the hydrolyzed metal alkoxide M-OH resulting in water formation H2O:
M-OH + M-OH = M-O-M + H2O

Polycondensation stage results in a formation of the Gel - a rigid 3-D network built of polymeric molecules and surrounded with the solvent.

The gel structure obtained from the acidic solutions with PH<6 (acid catalyzed) is uniform and weakly cross-linked.
The gel structure obtained from the basic or neutral solutions with PH>6 (base catalyzed) consists of separate clusters .

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During the aging stage the ploycondensation reactions continue completing the formation of the gel.
The gel structure is reinforced with additional cross-links, which cause contraction of the gel matrix and expulsion of the solution from the shrinking pores.

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The water and other liquids entrapped within the pores of the gel structure are removed during this stage.
Drying is performed at a temperature of about 400ºF (~200ºC).
After drying the gel converts into a monolith micro-porous structure called Xerogel.
Drying at super-critical conditions preventing collapsing of the gel network results in a formation of a macro-porous low density structure called Aerogel.

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Calcination is performed at increased temperatures varying within the range 750-1470ºF (400-800ºC).
During the calcination stage the dry gel structure is dehydrated according to the reaction:

2M(OH)n = 2MOn/2 + nH2O

2Al(OH)3 = 2Al2O3 + 3H2O

Volatile organics are also removed in this stage.

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Properties of some aerogels

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sol-gel_process.txt · Last modified: 2012/06/01 by dmitri_kopeliovich
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