Novel Sol−Gel Synthesis of Sodium Aluminophosphate Glass Based on Aluminum Lactate

A novel sol−gel route based on aluminum lactate and sodium polyphosphate aqueous solutions has been developed to produce sodium aluminophosphate gels and glasses with the composition 43.8:12.5:43.8 Na2O/Al2O3/P2O5. Preparations were optimized on the basis of detailed 27Al solution- state NMR data, revealing the nature of the dominant precursor species as a function of pH and gelation temperature. A pH range of 1.8 to 3.6 was found to provide optimum conditions for the formation of homogeneous and transparent glasses. This pH range corresponds to the maximum concentration of mixed Al(lact)1(H2O)4 2+ and the Al(lact)2(H2O)2 + precursor complexes, which appear to have the highest reactivity toward phosphate. The further replacement of lactate by phosphate ligands is favored by increasing the gelation temperatures above ambient. Upon heating to 400 °C the gels are converted to bulk glasses with thermal and structural characteristics identical to those of glasses prepared via traditional melt-cooling from 1350 °C. Solid-state 27Al and 31P NMR data and 27Al{31P} rotational echo double resonance (REDOR) spectra in particular confirm that the degree of Al/P connectivity is virtually identical in the glassy materials prepared by both routes. Thin films of this material have been prepared successfully on silica substrates and subsequently characterized by atomic force microscopy (AFM) and secondary neutral mass spectrometry (SNMS). These data indicate that the amount of residual carbon can be removed oxidatively upon heating by using HClO4 rather than HCl as the proton source.