Investigation of the mechanism of sol-gel formation in the Sr(NO₃)₂/citric acid/ethylene glycol system by solution state ⁸⁷ Sr nuclear magnetic resonance spectroscopy

The polymerization mechanism of a modified Pechini process was investigated by ⁸⁷Sr nuclear magnetic resonance spectroscopy for the mixed solution of strontium nitrate, citric acid, and ethylene glycol. The C-ratio (the ratio 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 precursor model based on this stoichiometry is proposed. Through a relaxation rate study of the strontium sites, it was found that the polymerization mechanism is predominantly bimolecular within the concentration region being studied. The equilibrium rate constant for the polymerization was calculated to be 10⁴ s⁻¹. A kinetic study of the fast cation exchange between the two identified strontium sites indicates that the inhomogeneity of the polymeric network leads to film cracking during pyrolysis.