4)The Pechini method

This method was originally applied to fabricate perovskite powders. In this process a chelate is formed between mixed captions (dissolved as salt in a water solution) with a hydroxyl carboxylic acid (citric acid). The nitrate and citric acid solution are mixed with a polyhydroxyl alkohol and stirred while  heating at around 80°C. Heating to moderated temperature (150°C, 250°C) causes a condensation with the formation of a water molecule. During heating, polyesterification occurs and most of the excess water is removed, resulting a polymeric resin. Polyacrylics can substitute citric acid or can be used as ion exchange resin.

5)Modified Pechini process

The modification consists of using C3S hydrate instead of calcium salts during the phase of chelation. This improves the combination during the heating, (calcium silicate hydrate gel acting as a seed).The reaction schematically as follow  (figure 3) give an organo mineral compound organo hydrate  C3S,  (HO)x -C3S-OR ( R organic radical). In a  first step an amorphous mass of hydrated C3S -O R is obtained ( Figure 3) .

Figure 3
Amorphous mass of organo hydrate tricalcium silicate
(microstructure shown through SEM )
Figure 4 shows the mechanism of chelation of  Ca++ and cross linking polymer. Figure 4 a, shows  polyvinyl alcohol ( obtained from polyvinyl acetate) . Figure 4b shows the mechanism of chelation obtained from citric acid . Figure 4 c shows the total mechanism of chelation and cross linking polymerisation  ( M2+ being calcium  and X 4+ being silicon or aluminum )

Figure 4
Mechanism of cross linking polymerisation
A cross linking agent (triethanolamine titanate ) can be used as  chelating agent. Finally the product is heated at about 1450°C during a short time ( less than 60 minutes dependent on the quantity).