A study of tin oxides in silicate based glasses
The roles of tin in two silicate based glass systems have been investigated by NMR and Môssbauer spectroscopies and by physical property measurements of the glasses. The first glass system investigated was the stannous silicate (binary SnO-Si02) glass. Glasses with SnO contents ranging from 17 to 72 mol.% have been made by melting pelleted powder in an alumina crucible. It was found that alumina crucibles are unsuitable for making glass with <20 mol.% SnO because of attack on the crucible at the high melting temperature (_>_1600°C). Silica crucibles will not withstand such high temperature and tin will attack a platinum crucible. The ability of this system to form glass past the orthosilicate composition has been discussed in terms of the polarizing power of Sn2+ and the structure of SnO. The 119Sn NMR results did not give much structural information due to the high chemical shift anisotropy of Sn 2+ site but they showed that the glass also contains trace amounts of Sn4+species. The 29Si MAS NMR results showed that SnO does not depolymerise the silicate network to the same extent as Na20 or even Pb0. Computer simulations of the 29 Si MAS NMR spectra showed that, for SnO <-30 mol.%, the disposition of Qn species is consistent with the binary model, which means that SnO is acting the role of modifier. For compositions > 30 mol.% SnO, the Qn distribution follows the statistical model and this has been interpreted as SnO now acting as an intermediate. The 119Sn MOssbauer results confirmed this interpretation. The Sn2+ isomer shift decreases with increase of SnO which is indicative of increasing covalent character of the Sn—O bonds while the larger quadrupole splitting suggests distortion of the SnO polyhedral structure in the glass. The relation of the Sn 2+ isomer shift to the quadrupole splitting and the temperature dependence of the isomer shift of Sn2+ indicate the formation of Si—O—Sn linkages at high SnO contents. The decrease of the viscosity of the glass with increasing SnO is small when compared to the decrease of the viscosity in alkali metal and alkaline-earth oxides silicates when the respective modifier oxide is increased in those glasses. The variation of the density, thermal expansion and refractive index with SnO content showed discontinuities in the region of 30-45 mol.% SnO. This has been interpreted as being the point where SnO changes its role from that of modifier to intermediate. The results of differential thermal analysis and devitrification of SnO-Si02 glasses showed that glass with 40 mol.% SnO can be heat treated in the temperature range of 570° to 680 °C to produce metastable SnSiO3 crystals. SnSiO 3 decomposed to SnO + Si02 at temperatures above —700°C and, at temperatures greater than 720°C, oxidation of SnO to Sn02 and Si02(glass) to Si02 (cristobalite) took place. The second glass system is tin-doped float glass. This is glass of the float composition remelted with tin(II) oxalate in silica crucibles under normal atmosphere conditions. In this way it has been demonstrated that we can mimic the tin oxide distribution found within the tin diffusion region in float glass. Synthesis of the glass has shown that both Sn2+ and SO+ can be assimilated simultaneously in the glass but there is a solubility limit for SO+. The 1195n Mbssbauer results showed that Sn2+ and SO+ played different structural roles in the glass. The environment of Sn2+ in glass is similar to that in amorphous SnO while the SO + structure in glass does not change significantly compared to crystalline Sn02. The Debye temperatures and recoil free fractions showed that Sn2+ is less rigidly bound to the network modifier site while SO + is rigidly bound at network former sites in the glass. The different structural roles of 5n 2+ and SO+ in the glass were reflected in the some of the physical properties of the glasses.