Add time:09/24/2019         Source:sciencedirect.com

An aqueous solution route has been developed to prepare calcium sodium borate compositions of the formula xB2O3·(100–x)(0.5CaO·0.45Na2O·0.05P2O5) (36.2 ≤ x ≤ 61.3 mol%). The effect of heat-treatment and melt-quenching on the structure and physical properties was investigated using differential thermal analysis, X-ray diffraction, FTIR spectroscopy, 11B MAS NMR and density. Dried powders were heat treated at temperatures 400–800 °C for 3h. XRD revealed formation of crystalline Na2B4O7·5H2O, Ca5(PO4)3OH, Na3B5O9H2O and CaB3O5(OH) in the samples treated up to 500 °C. Samples treated at higher temperature show the presence of CaNaB5O9, CaNa3B5O10 and CaNaPO4. Quenched solids obtained by melting powders are amorphous. It is deduced from FTIR spectra that CaO and Na2O modify the borate network forming BO4 and BO3 with non-bridging units, which depend on the molar ratio R =(Na2O+CaO)/B2O3 and temperature. The fraction N4 of four coordinated boron atoms changes with R and can be predicted by treating the compositions as if they are mixtures of Na2O–B2O3 and CaO–B2O3 matrices. N4 values have been used to calculate the fraction of all units in the glass structure. The results are confirmed by 11B MAS NMR spectra that show the presence of symmetric and asymmetric trigonal BO3 groups and splitting line shape of [4]B. The latter is assumed being related to different four-coordinated boron environments. Density and molar volume obtained by using mixed matrices concept agree well with the experimental data.

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