Particulate sol

Either particulate sol or polymeric sol has been used for thin film coatings. The polymeric sol was fabricated by partial hydrolysis of corresponding metal alkoxide. If the rate of hydrolysis or condensation is very fast, then some kinds of organic acids, beta-dicarbonyls, and alkanolamines have been used as chelating agent in sol-gel processes to control the extent and direction of the hydrolysis-condensation reaction by forming a strong complex with alkoxide. [2]. 

Parameter that provides a mathematical description of the fractal structure of a polymer network, an aggregated particulate sol, or of the particles that comprise them. 

Concentration of an added electrolyte above which a particulate sol undergoes coagulation instead of gelation. 

In the particulate-sol method a metal alkoxide dissolved in alcohol is hydrolyzed by addition of excess water or acid. The precipitate that results is maintained as a hot solution for an extended period during which the precipitate forms a stable colloidal solution. This process is called peptization from the Greek pep—to cook (not a misnomer many descriptions of the sol-gel process have a strong culinary flavor). The colloidal solution is then cooled and coated onto the microporous support membrane. The layer formed must be dried carefully to avoid cracking the coating. In the final step the film is sintered at 500-800 °C. The overall process can be represented as ... 

In the first route where typically a metal salt or a hydrated oxide is added to a non-stoichiometrically excess amount of water, a "particulate sol is precipitated which consists of gelatinous hydroxide particles. The hydrolysis rate is fast. The primary colloidal particles so obtained are usually in the range of 5 to 15 nm. 

The hydrolysis can be acid- or base-catalyzed. In the case of silica, acid-catalyzed hydrolysis results in sols with weakly-branched polymers that lead to final products having smaller pores while base-catalyzed hydrolysis produces particulate sols and final porous bodies showing larger pores [Brinker ct al.. 1982 Shafer ct al.. 1987]. 

Alumina is much more adapted to the particulate sol route while silica and titania are more flexible in that they can lead to both particulate and polymeric gels as the membrane precursors. This is particularly true with the silica system. Hydrolysis and condensation reactions in silica systems are known to be slower than most other alkoxides. Their reactions, therefore, are easier to control. The fact that it is much easier to prepare alumina sol-gel membranes than their titania equivalents may suggest that the plate-shaped particles like alumina can be more readily processed to form defect-free membranes than spherical particles such as titania [van Praag et al., 1989]. 

FIGURE 26.2 (a) The capacity versus cycle number showing the first 30 cycles for LiNiOj synthesized using the particulate sol-gel process, and (b) the first cycle voltage vs. lithium content for the low defect concentration of LiNi02-... 

Chang, C.-C., Particulate sol-gel (PSG) synthesis, structural and electrochemical characterization of LiM02 (M = Ni, Nio75Coo.25), Ph.D. dissertation, Carnegie Mellon University, Pittsburgh, PA, 1999. 

There is no principal difference in particle packing from suspensions and particulate sols. In both cases the densification of the dispersion may be accompanied by agglomeration of the particles cind the formation of a continuous... 

In sol-gel processing, particulate sols of hydrous metal oxides can be formed using a peptization reaction to prevent aggregation of the primary particles. This is the case for y-alumina obtained from boehmite. Usually mesoporous membranes with a pore diameter down to 5 nm are easily elaborated from commercial boehmite using nitric acid as the peptization agent. Larbot et al. [53]... 

Colomer, M.T. 2006. Proton conductivity of nanoporous anatase xerogels prepared by a particulate sol-gel method. Journal of Solid State Electrochemistry 10, 54—59. 

FIGURE 8.17 The polymerization routes of silica in acidic and alkaline solutions. In acidic solutions the polymeric species aggregate and form three-dimensional networks. In alkaline solutions the sufficient solubility produces particulate sols. (From Her, R.K., The Chemistry of Silica, lohn Wiley Sons, New York, 1979, 174. With permission.)... 

Compared to particulate sols, spinnable sols also show a greater molecular-weight dependence of the intrinsic viscosity ... 

Fig. 5.52 Schematic representation of changes during the gelation of particulate sols. Dots represent hydrogen bonds curved lines are micelle surfaces.

M.R. Mohammadi, D.J. Fray, M.C. Cordero-Cabrera, Sensor performance of nanostructured Ti02 thin films derived from particulate sol-gel route and polymeric fugitive agents , Sensors and Acmators B Chemical, 124, 74-83, (2007). 

Mohammadi, M. R. and Fray, D. J. (2007) Semiconductor TiOj-GajOj thin film gas sensors derived from particulate sol-gel route , Acta Materialia, 55(13), 4455 466. 

Particulate sols are often prepared via the following steps ... 

Table 6 summarizes the pore structure of sol-gel derived alumina coated with various active species by the solution-sol mixing method. Synthesis of the boehmite sol was described above. Nitric acid was used to peptize the particulate sol. The sol was mixed with the aqueous solution of corresponding salt solution in a predetermined ratio to give the loading listed on the second column in Table 6. The coated samples were dried and calcined in air at 450°C for 3 hours.

---