Structure and physicochemical characteristics of the glasses in the system Na2O/BaO/ZrO2/TiO2/SiO2/B2O3/Al2O3 – Influence of the ZrO2 addition on the physico-chemical and mechanical properties

Glasses with the compositions 20.1Na2O∙(23.1-y)BaO∙yZrO2∙23TiO2∙9.8B2O3∙21SiO2∙3Al2O3 with y = 0, 0.5, 1, 2, 3 and 6 mol% were prepared using a traditional melt-quenching technique. The thermophysical properties, i.e. glass transition temperature, Tg and crystallization peak maximum temperature, Tc were determined for the obtained glasses by differential scanning calorimetry and showed that increasing zirconium oxide concentrations resulted in an increasing Tc and an initial decrease followed by an increase of Tg values. The difference Tc-Tg, used as glass stability criterium, increases for ZrO2 concentrations up to 2 mol% and then starts to decrease for the compositions with higher zirconium oxide concentration. The densities were measured by using a helium pycnometer and served to determine main physico-chemical characteristics of the prepared glasses such as molar volume, oxygen packing density and refractive index. The obtained results reveal that the values of the density and of all estimated physico-chemical characteristics increase with increasing ZrO2 concentrations. X-ray photoelectron spectroscopy showed that all elements from the glass composition are present in oxide states with Zr4+ ions being only in octahedral coordination while Ti4+ ions are of 6-, 5- and 4-fold coordination to oxygen. The structure of the glasses was studied by Fourier Transformed Infrared Spectroscopy on powdered samples and the existence of Si–O–Si/O–B–O bending, Si–O–Si rocking, B–O–B bending and B–O–B/B–O–Si stretching vibrations was observed. The presence of BO3, BO4 and SiO4 polyhedra as interconnected as well as isolated structural units and the occurrence of some TiO62− and a larger number of TiO4 and mainly of TiO5− units is suggested. At 570-580 cm−1, a peak typical for the compound BaTiO3 is also registered for all samples. The conducted depth-sensing indentation on the obtained glasses allowed to determine mechanical properties such as universal and indentation hardness, elastic modulus and elastic part of indentation work. Increasing the ZrO2 concentration resulted in increasing elastic moduli which together with the DSC data implies that the addition of ZrO2 leads to structural changes in the glass network and enhanced mechanical properties which results in increased hardness.