Redox ratio and optical absorption of polyvalent ions in industrial glasses

Abstract

The changes in glass structure and redox ratio, R (reduced ion to oxidized ion) of Mn2+-Mn3+, Cu+-Cu2+, Cr3+-Cr6+, Ni2+-Ni3+ and Co2+-Co3+ couples and optical absorption due to Mn3+, Cu2+, Cr3+, Ni2+ and Co2+ ions in industrial soda-lime-silica glass were investigated as a function of Na2O concentration in the range 11–19 mol%. With increasing Na2O concentration in the experimental glasses, the basicity, expressed as calculated basicity, Λcal, increased. 29Si NMR and X-ray diffraction were used to investigate the structural change in glasses. The NMR spectra showed high non-bridging oxygens (NBOs) when the basicity of glass was increased. The results were interpreted to be due to the tetrahedral networks; Q 4 species were depolymerized by replacing the bridging oxygens (BOs) with NBOs to Q 3 species. These results confirmed the shift of broadening peaks of XRD patterns. The redox reactions of the Mn2+-Mn3+, Cu+-Cu2+ and Cr3+-Cr6+ couples shifted more toward their oxidized ions due to the oxygen partial pressure, p(O2), during melting and the oxide ion activity, a O2−, increased with increasing glass basicity. These changes caused the redox ratio of these ion couples to decrease. The Ni2+-Ni3+ and Co2+-Co3+ couples were assumed to be present only in the Ni2+ and Co2+ ions in these glasses, respectively. The optical absorption bands due to Mn3+, Cu2+, Cr3+, Ni2+ and Co2+ ions were also investigated. Their spectra occurred at constant wavelengths with different optical densities or intensities as a function of glass basicity. The increase in the intensities of the absorption bands of these absorbing ions, except for Cr3+ ion, at the maximum wavelength, depends not only on the ion concentration but also on the increase of polarizability of oxide (−II) species, α oxide(−II), surrounding the ions. This value affected directly the extinction coefficients of the ions, ɛ ion. The increase of ɛ ion caused the colour of glasses appearing in high intensity. In the case of Cr3+ ion, the results were reversed such that the lower the concentration, the higher the intensities of colour.