Investigation of the mechanism of sol-gel formation in the Sr(NO3)(2)/citric acid/ethylene glycol system by solution state Sr-87 nuclear magnetic resonance spectroscopy
Xh. Li et al., Investigation of the mechanism of sol-gel formation in the Sr(NO3)(2)/citric acid/ethylene glycol system by solution state Sr-87 nuclear magnetic resonance spectroscopy, J MATER RES, 15(11), 2000, pp. 2393-2399
The polymerization mechanism of a modified Pechini process was investigated
by Sr-87 nuclear magnetic resonance spectroscopy for the mixed solution of
strontium nitrate, citric acid, and ethylene glycol. The C-ratio (the rati
o of citric acid to metal ions in the polymer complex) is suggested to have
a strong influence on the quality of the derived film. An analysis of the
chemical shift variation, as a function of C-ratio, indicates the presence
in the solution of two species: solvated strontium ions and strontium ions
bound to the polymer complex, with a stoichiometry of 1:7. A polymeric prec
ursor model based on this stoichiometry is proposed. Through a relaxation r
ate study of the strontium sites, it was found that the polymerization mech
anism is predominantly bimolecular within the concentration region bring st
udied. The equilibrium rate constant for the polymerization was calculated
to be 10(4) s(-1). A kinetic study of the fast cation exchange between the
two identified strontium sites indicates that the inhomogeneity of the poly
meric network leads to film cracking during pyrolysis.