I am employed at the ISIS Facility of the Rutherford Appleton Laboratory, where I am responsible for the neutron diffractometer GEM (GEneral Materials). I am responsible for experiments on GEM to study the structure of glasses, liquids and disordered crystals.
My personal research is into the structure of oxide glasses, such as germanate glasses, and chalcogenide glasses, such as arsenic sulphide glasses. As well as developing general techniques for studying glasses by neutron diffraction, I have particular interests in studies of bonding in glasses, and the variation of coordination numbers with composition.
I am an editor of the Society’s journal Physics and Chemistry of Glasses, and I am the chairman of the international organising committee of the International Conference on Borate Glasses, Crystals, and Melts.
Wide range structural study of sodium silicate glasses
Alex C. Hannon*1, Paul A. Bingham2, Steve A. Feller3, Shuchi Vaishnav2, Martin C. Wilding2, Greg Guokas3, Michael Packard3, Hayley Austin4, Collin Flynn3, Hector Rea3, Bruno Vallim3, Wataru Takeda3
ISIS Faculity, Ruthergord Appleton Laboratory, Chilton,Didcot,Oxon, OX11 0QX UK
2 Materials and Engineering Research Institute, Sheffield Hallam University, Howard Street,
Sheffield S1 1WB, UK
3 Department of Physics, Coe College, Cedar Rapids, Iowa 52402, United States
4 Johns Hopkins University, Baltimore, MD 21218, USA
A preliminary report is given of a wide ranging structural study of sodium silicate glasses.
Vitreous silica (v-SiO2) is sometimes referred to as ‘the archetypal glass’, since it is the example usually cited to explain basic ideas about the structure of network glasses; according to the random network model, the structure of silica glass is formed by corner sharing between SiO4 tetrahedra, with a distribution of bond and torsion angles at the Si-O-Si bridges, leading to a topology that is ‘random’ in the sense that it is non-repeating, and no site is exactly the same as any other site.
Sodium silicate is the example that is usually cited to explain the effect of network modifier (in this case Na2O) on a random network structure. Each extra oxygen atom introduced from Na2O leads to the replacement of one bridging oxygen by two (negatively charged) non-bridging oxygens, which are charge-balanced by the Na+ ions. Given its status as the archetypal modified network glass, it is surprising to find that the literature contains relatively little structural study of sodium silicate glasses. Thus we have embarked on a wide-ranging, comprehensive study of the structure of silicate glasses.
Glass samples with nominal Na2O-SiO2 compositions of 20, 25, 30, 35, 40, 45, 50 were made by conventional melt quenching, and crystalline Na2SiO3 was made by crystallising glassy material with a composition 50 mol% Na2O. (It was held at 620 ºC for 14 hours.) Glass samples were also made by using twin rollers to rapidly quench the melt, producing samples with nominal compositions of 45, 50, 55, 60, 65 and 70 mol% Na2O.
A comprehensive set of neutron diffraction measurements has been made on the samples, and other spectroscopic and physical property measurements are underway. The figure shows preliminary (uncorrected) measurements of the differential neutron correlation functions of these samples. There are strong peaks at about 1.6 Å and 2.6 Å due respectively to Si-O bonds and the O-O distance in SiO4 tetrahedra. For high Na2O contents, two additional features become very apparent at about 1.3 Å and 2.3 Å. The second of these arises from Na-O bonds; we have recently developed the ability to calculate a close simulation of the O-O peak in the correlation function, and this will enable us to determine more reliable information on the Na-O coordination number and distribution of distances than has hitherto been available. The peak at about 1.3 Å has a quite different origin; it is due to C-O bonds as a result of a significant amount of CO2 retention in the glass structure. Sodium silicate glass is usually made by melting a mixture of silica and sodium carbonate, Na2CO3. For low and intermediate Na2O contents, all of the carbon is lost from the melt in the form of CO2 gas. However, for very high Na2O contents, there is significant and increasing CO2 retention, so that ternary Na2O-CO2-SiO2 glasses are formed. The C-O bond length ~1.3 Å observed in the neutron correlation functions is consistent with the presence of CO32- carbonate groups in the glass. Carbonate groups present a challenge for the random network description of glass structure, since an oxygen atom in a C-O bond cannot bridge to a silicon atom. It is likely that instead the CO3 groups are balanced by forming bridges to Na+ ions.
This work supported in part by the US National Science Foundation under grant number DMR-1407404.