Synthesis and structural evolution of Al2O3-B2O3-P2O5 gels and glasses

Aluminium borophosphate gels and glasses with different compositions are prepared, for the first time, via the sol-gel process using aluminium lactate, boric acid and phosphoric acid as precursors. This glass-forming range is significantly wider than that accessible via the melt cooling route. The route from the solution to the gel and the final glass is monitored ex-situ by multinuclear high-resolution liquid and solid state NMR techniques, characterizing the influence of composition and temperature on the hydrolysis, polymerization, and vitrification processes. Addition of Al(lact)(3) to H3BO3 aqueous solution results in the formation of [B(lact)(2)](-) species. In the dried xerogels, the boron structure is dominated by the three tetrahedrally coordinated environments involving variable lactate, phosphate and water ligands-[B(lact)(2)](-), [B(lact)(OH)(2-x)(PO3)(x)](-), and [B(lact)(OH)(4-x)(PO3)(x)](-)-the fractional concentrations of which are significantly dependent on the gel-forming conditions (e.g. gel-processing temperature and Al/B/P molar ratio). Al-27 -B-11 double resonance experiments suggest that B-O-Al connectivities are not present. The dominant aluminium species present in the xerogel are [Al(lact)(3)], [Al(lact)(2)(PO3)(2)](-), [Al(lact)(1)(PO3)(4)](2-), and [Al(PO3)(6)](3-), involved in various degrees of polymerization. Following the heat treatment of the gel towards glass, the lactate ligands (linked to both B and Al units) are successively replaced by phosphorus ligands, resulting in further structural polymerization. Upon heating to 500 degrees C, the gels are finally converted to bulk glasses with thermal and structural characteristics close to those of glasses accessible via traditional melt cooling from 1550 degrees C. Single resonance and B-11{P-31} and Al-27{P-31} double resonance NMR results indicate that the extent of B/P and Al/P connectivity in the glasses is generally maximized.