Christian Rüssel

Thermodynamics of iron in alkali–magnesia–silica glasses

Abstract Alkali–magnesia silica glasses with lithium (74 SiO 2 , 16 Li 2 O, 10 MgO), sodium ((90xa0−xa0 x ) SiO 2 , x Na 2 O, 10 MgO ( x =10, 15, 20, 25)), potassium ((90xa0−xa0 x ) SiO 2 , x K 2 O, 10 MgO ( x =10, 15, 20, 25)) and caesium (74 SiO 2 , 16 Cs 2 O, 10 MgO) (all compositions given in mol%) doped with 0.2 mol% Fe 2 O 3 were studied by means of square-wave voltammetry. Decreasing Na 2 O and K 2 O contents lead to an increase in the standard potentials of the Fe 3+ /Fe 2+ redox pair. A comparison of the different alkali ions shows that the standard potentials increase from lithium to caesium. The standard potentials, standard enthalpies and standard entropies of iron in the alkali–magnesia–silica glasses are compared with literature data of alkali–lime–silica glasses. Empirical correlations enable a quantitative calculation of the standard potentials from the chemical composition.

XPS investigations on coordination and valency of Ti in fresnoite glasses and glass ceramics

Abstract Crystalline fresnoite (Ba 2 TiSi 2 O 8 ), attributed glasses and glass ceramics of the systems Ba 2 TiSi 2 O 8 xa0+xa0 x SiO 2 ( x =0, 0.75) and Sr 2 TiSi 2 O 8 have been studied by X-ray photoelectron spectroscopy (XPS). To understand the process of crystallization, changes in coordination of the constituting elements have been studied in glassy and crystalline samples. It was observed that changes in coordination took place at the Ti-sites. It is shown that in fresnoite glass (Ba 2 TiSi 2 O 8 ), fivefold coordinated Ti predominates (≈60%) while fourfold (≈25%) and sixfold (≈15%) coordinated Ti is present as well. In another series of experiments, the effect of reducing Ti(IV) to Ti(III) by adding carbon to the glass batches was studied. It was shown that XPS could resolve the two valency states and gave Ti(IV)/Ti(III) ratios equal to that obtained from thermogravimetry. By comparison of all samples measured, XPS-line positions for different coordinations and valencies of Ti in these samples are given.

Diffusivities of polyvalent elements in glass melts

Abstract Self-diffusion coefficients of polyvalent elements have been measured as a function of temperature in silicate melts of different compositions using square-wave voltammetry, a fast potentiostatic pulse method. In the temperature range of 700 to 1500°C, a linear correlation between log( D ) and 1/ T is observed for all polyvalent elements and glass melt compositions. The self-diffusion coefficients are highest for Ag + and up to six orders of magnitude lower for Cr 6+ . Self-diffusion coefficients of polyvalent ions with valencies two to four lie between these limits. The self-diffusion coefficients also strongly depend on the melt composition. In melts possessing higher alkali content and hence lower viscosity, higher diffusion coefficients are observed.

Self-diffusivity of iron in alkali–lime–alumosilicate glass melts

Abstract Self-diffusion coefficients of iron were measured in glass melts with the basic mol% composition of 16 R2O, 10 CaO, x Al2O3 and (74xa0−xa0x) SiO2 with x=0, 5, 10, 15 and R=Li, Na, K and Cs in the temperature range of 900–1300°C using square-wave voltammetry. All diffusion coefficients had an Arrhenian dependence which depended on the type of alkali present and the Al2O3-concentration. Larger alkali cations, e.g. Cs+, as well as an increase in the Al2O3-content led to a decrease in the diffusion coefficients and also to an increase in viscosity. Within one glass composition, the Stokes–Einstein equation is fulfilled with respect to the dependence of the diffusivity upon viscosity. At constant viscosity, however, increasing size of the alkali cation and increasing Al2O3-content led to larger iron diffusion coefficients.

Self-diffusion of iron in alkali–magnesia–silica glass melts

Abstract Alkali–magnesia–silica glass melts with lithium (74 SiO2, 16 Li2O, 10 MgO), sodium ((90−x) SiO2, x Na2O, 10 MgO (x=10, 15, 20, 25)), potassium ((90−x) SiO2, x K2O, 10 MgO (x=10, 15, 20, 25)) and caesium (74 SiO2, 16 Cs2O, 10 MgO) (all compositions given in mol%) doped with 0.2 mol% Fe2O3 were studied by means of square-wave voltammetry. Decreasing Na2O- and K2O-contents lead to a decrease in the self-diffusion coefficients of iron and an increase in the viscosities of the liquids. A comparison of diffusivities of different alkali ions shows that at constant temperature, the self-diffusion coefficients decrease from lithium to caesium. Within the error limits, in most of the glasses studied, the Stokes–Einstein equation is fulfilled with respect to the dependence of the diffusivity on the viscosity. At constant viscosity, i.e., mobility of the glass network, the self-diffusion coefficients decrease with increasing alkali concentrations and decreasing size of the alkali cation. These effects are discussed using a structural model for the incorporation of iron into alkali containing glasses.

Zirconia-doped Mg-Ca-Al-Si-O-N glasses: preparation and properties

Abstract Premelted oxide glasses were mixed with a polymeric, electrochemically derived precursor solution, dried, calcined in gaseous ammonia and subsequently melted in nitrogen. A rapid melting route was applied using soaking times in the range of 4 to 8 min. TEM micrographs as well as the systematic variation of the glass properties did not reveal evidence of incomplete dissolution of the nitride component or inhomogeneous nitrogen distribution. The most notable physical property of the oxynitride glasses observed was their hardness to 9.7 GPa. No phase separation was observed in zirconia-free oxynitride glasses but incorporation of ZrO 2 causes strong phase separation.

Zirconia-doped Mg-Ca-Al-Si-O-N glasses: crystallization

Abstract Crystallization of Mg-Ca-Si-Al-O-N glasses containing up to 8 wt% ZrO2 has been studied. Crystallization takes place at temperatures above 900–1000°C. The main oxide phases are anorthite and spinel. Further phases formed are diopside, gehlenite, ZrO2 and ZrN, depending on the zirconia and nitrogen content as well as on the crystallization temperature. The only oxynitride phase observed is Mg-petalite. Nitrogen seems to stabilize the tetragonal modification of ZrO2. The microstructure obtained is extremely fine-grained. This is caused by the phase separation of the oxynitride glasses used. Mechanical properties such as hardness and fracture toughness increase with increasing nitrogen content, simultaneously they are influenced by the microstructure of the glass ceramics and, therefore, by the ZrO2 content.