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The addition of polyvalent transition metal ions to the usually insulating traditional soda-lime–silica glasses can lead to semiconducting properties. We report on synthesis of glasses and glass-ceramics in a soda-lime-silicate based system containing Fe2O3 in the concentration range from 5 to 30 mol%. Two sub-systems were considered, in one of them the ratio [Na2O]/[Fe2O3] was varied while in the other one, the ratio [SiO2]/[Fe2O3] was changed. The phase composition of the synthesized products was characterized by X-ray diffraction and energy dispersive X-ray analysis, while the electrical properties were studied by impedance spectroscopy. Partially crystallized non-reduced samples are semiconducting even at room temperature while the glassy samples (both reduced and non-reduced) exhibit semiconducting properties at temperatures equal or larger than 100 °C. An attempt is done to predict the physical approximation explaining the conduction process in the glasses.

Redox processes in a tin doped melt with the basic composition 20Na 2 O · 80SiO 2 were studied by square-wave voltammetry and impedance spectroscopy. At temperatures of 1350 and 1450 °C, the recorded square-wave voltammograms exhibited a distinct peak attributed to the reduction of Sn 4+ to Sn 2+ . Impedance spectra recorded at the same temperatures could be simulated with a simple equivalent circuit assuming a diffusion controlled electrode reaction. At temperatures T ⩽ 1250 °C, and small step times, the voltammograms exhibited a second peak which was much stronger affected by the frequency (step time) applied than expected from a diffusion controlled electron transfer reaction. In order to fit the impedance spectra at these temperatures, a more complex equivalent circuit was necessary, taking into account adsorption phenomena. In all impedance spectra, the Warburg parameter obtained from simulation, which stands for the diffusion from and to the electrode, exhibited a minimum which was equal to the voltammetric peak at more negative potentials. The voltammetric peak at less negative potentials is caused by the reduction of Sn 4+ to an adsorbed state. Within this reduction step, a monolayer of Sn 2+ species is formed.

The mechanism of electrochemical nucleation is studied using electrochemical methods and differential thermal analysis. A platinum crucible containing an undercooled melt with the composition 2BaO · TiO 2 · 2.75SiO 2 was contacted with a platinum wire and another platinum wire was inserted into the melt. Then a dc-potential leading to a current flow was attached. At the cathode nucleation and subsequent crystal growth occurred. This is due to the electrochemical reduction of Ti 4+ to Ti 3+ . Assuming Ti 4+ and Ti 3+ to be incorporated as network formers and modifiers, respectively, this leads to a decrease of the mean number of bridging oxygens per network former. This can be explained by percolation theory which can be used to calculate the undercooling temperature necessary to achieve nucleation directly from the mean number of bridging oxygens per network former. Since for nucleation, a certain Ti 3+ -concentration is necessary, a certain threshold with respect to the current density is required to achieve nucleation. This threshold increases with increasing temperature.

Glasses with the compositions Li 2 Si 2 O 5 and 9Li 2 Si 2 O 5 · BaSiO 3 were crystallized using electrochemically induced nucleation. A platinum wire was inserted into the melt. A dc-current was supplied between the platinum wire (cathode) and the crucible (anode) at various temperatures below the liquidus temperature. This led to nucleation at the cathode and subsequently to radial growth of lithium disilicate crystals. The crystals exhibit dendritic growth. The main crystal growth direction is the crystallographic c -axis of lithium disilicate. This axis is oriented perpendicular to the electrode surface. A crystal growth selection near the cathode results in a highly textured glass–ceramics. The electrochemically induced nucleation is based on the reduction of the silicate glass to elementary silicon and an alloy formation between Si and Pt with a depletion of Si in the melt near the cathode. The crystalline products were characterized and the effects of experimental parameters were studied.

The electrical impedances of glasses in the system BaO ∗ Al 2 O 3 ∗ B 2 O 3 were studied in the temperature range from 550 to 1150 °C using different heating and cooling rates. The specific conductivities were illustrated by fitting Arrhenius functions to the data and the derivative (−d ln σ /d(1/ kT )). In this system, the activation energy of the electrical conductivity depends on temperature. Crystallization of the melt affects the derivative. Depending on the specific conductivities and the respective activation energies of the liquid and the crystalline phase, maxima or minima (or both) are observed in the derivative. The melting of the crystalline phase during heating results in a maximum of the derivative. The shape of the derivative can be explained taking into account the specific conductivities and the attributed activation energies of the liquid and crystalline phase.

Glasses with the basic composition x Li 2 O · 15Al 2 O 3 · (85− x )SiO 2 ( x =8.5, 11, 13.5, 16 and 18.5) were doped with 0.5 mol% SnO 2 and studied by square-wave voltammetry and impedance spectroscopy in the temperature range from 1400 to 1600 °C. All square-wave voltammograms recorded exhibited a well pronounced peak attributed to the Sn 2+ /Sn 4+ -redox pair. Impedance spectra were recorded in a wide frequency range from 10 −2 to 10 6 s −1 as a function of the dc-potential superimposed. The spectra were simulated using an equivalent circuit taking into account an electron transfer reaction from an adsorbed state. The Warburg parameter obtained from spectra simulation exhibited a minimum which within the limits of error occurred at the same potentials as the voltammetric peak. This is evidence for a electrode reaction solely controlled by diffusion. The peak potentials in melts with Li 2 O-concentrations of up to 13.5 mol% are equal within the limits of error, further increasing Li 2 O-concentration resulted in more negative peak potentials. Then, the Sn 2+ /Sn 4+ -equilibrium is shifted to the oxidized state.

A model is introduced to describe the effect of nucleation time and heating rate on the induction period observed during crystallization of glass. Nuclei formed during thermal treatment of a glass will grow with time. During further heating up, besides an accelerated crystal growth, also the critical nucleus of the crystalline phase increases. If during subsequent rapid heating, the crystals get smaller than the critical radius, they will be dissolved again. If the glass is kept only for a short time at nucleation temperature, all nuclei will be dissolved; however, if they are kept for a long time, an increasing quantity of nuclei will be large enough to survive the subsequent heating process. Numerical calculations using experimental data for the nucleation of lithium disilicate show that the induction period of the nucleation process can quantitatively be explained assuming steady state nucleation.

A glass with the composition 48.5SiO 2 · 48.5B 2 O 3 · 3Na 2 O was studied at temperatures in the range from 340 to 860 °C using impedance spectroscopy. The spectra obtained were simulated using a simple equivalent circuit. During cooling of a sample, the specific conductivity of the sample decreased according to Arrhenius equation until at temperatures 2 O/B 2 O 3 -rich) which runs parallel to deviations from linearity in the Arrhenius plot. In the temperature range from 670 to 630 °C, specific conductivities at constant temperature depend on time due to decreasing Na 2 O concentrations in the matrix during cooling and increasing Na 2 O-concentrations during heating. The specific conductivities during heating and cooling exhibit a hysteresis which is explained by changes in the chemical compositions of matrix and droplet phase during thermal treatment which, however, always deviate from the equilibrium concentrations at temperatures

The dependence of viscosity on composition of silicate melts is described by means of a structure based model. It distinguishes between network forming oxides, such as SiO 2 , and network modifying ones, such as Na 2 O. The structural assumption is that there are two different types of bonds in the system: rigid ones like Si–O–Si and floppy bonds like Si–O − Na + . The activation energy for shear viscosity is reduced in the presence of floppy bonds. Therefore, even low concentrations of modifying oxides reduce considerably the viscosity of the melt, a lubricant effect.

The formation of fluorapatite crystals with needle-like morphology was studied in glass-ceramics (GCs) of the system SiO2–Al2O3–CaO–P2O5–K2O–F−. X-ray diffraction (XRD), 31 P and 27 Al nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), temperature-dependent electric conductance measurements, and transmission electron microscopy (TEM) with energy dispersive X-ray analysis were used to study the influence of the Ca/P ratio of the starting glass composition on the apatite crystal morphology. The composition of the ambient glass hosting apatite crystals and particularly the calcium supersaturation herein is shown to be the key to an understanding of the apatite crystal morphology observed.

Glasses in the system Na 2 O–MgO–CaO–SiO 2 –F − were crystallized using electrochemically induced nucleation. For that purpose, a platinum crucible containing the melt was used as anode while a platinum wire inserted into the melt acted as cathode. A dc-potential of 1 V supplied between the electrodes led to a current flow of 10 mA. This resulted in nucleation at the cathode and subsequently to radial growth of fluororichterite and diopsid crystals. Both crystalline phases were oriented with their crystallographic c -axis radially, i.e., perpendicular to the electrode surface. The structures formed are described and the mechanism leading to nucleation and subsequent crystal growth is discussed.

The local order in the vicinity of Ti and Ba atoms in Ba 2 TiSi 2 O 8 + x SiO 2 ( x =0.0, 0.1, 0.5 and 0.75) glasses was studied by X-Ray absorption Spectroscopy (XAS). The coordination number of Ti 4+ atoms was identified by using the fingerprint X-Ray absorption near-edge structure (XANES) spectra of well-defined crystalline references. Coordination numbers 4, 5 and 6 could be observed in the glass. With increasing the amount of SiO 2 , the Ti 4+ coordination number shifts to higher values. The coordination of Ba 2+ atoms is not influenced by the composition of the glass. This may result from the large coordination number of this ion. The behavior of Ti as a network former or a network modifier depends on the coordination number. Additionally, the coordination number of Ti depends on the relation between network former and network modifier in the glass.

Glasses with the composition of 2BaO·TiO2·2SiO2·xB2O3 were melted. A platinum wire was inserted into the melt and at 1220°C a dc-current of 10 mA was supplied between the wire (cathode) and the crucible (anode). This led to a crystallization of fresnoite (Ba2TiSi2O8) at the cathode. The crystallization front proceeded to the crucible wall. The result was a highly aligned structure. The crystallographic c-axis of the fresnoite was oriented perpendicularly to the electrode surface. These glass-ceramics are mechanically stable and can easily be cut, ground and polished, which is in contrast to those which do not contain boron oxide.

It is a usual assumption that oxide glasses build up continuous random networks. A further and very important development of this approach is the rigidity networks idea. According to it, the network switches from floppy to rigid if the number of bridges between the network formers exceeds a certain threshold value. In oxide glasses, only properties which require a motion over a sufficiently large distance will be subject to angular constraints at oxygen bonds thus there can be a switch from floppy to rigid modes. The kinetics of phase transition (and most nucleation processes) are very sensitive to the rigidity of the network. Slightly above the threshold concentration of rigid bonds some tiny floppy regions still exist inside the rigid network. They can serve as active centers for the nucleation process. Much later, nucleation can appear in the rigid part of the network. Therefore, we predict a certain interval of concentration of rigid bonds in which a bimodal size distribution function of precipitated particle will be observed. This prediction is confirmed by experimental data.

Glass samples of the 0.30Li2O·0.70(xTeO2·(1−x)SiO2) system were synthesized. The difference between the melting points of silica and tellurium oxide, which melts at 733°C with further volatilization, makes this synthesis a complex one. However, transparent and homogeneous glasses were obtained in the SiO2-rich domain up to x=0.30 by mixing appropriate amounts of the two binary systems, 0.30Li2O·0.70SiO2 and 0.30Li2O·0.70TeO2, in an induction furnace. With the increase of the TeO2 content in the samples investigated a decrease of Tg and an increase in the samples’ densities were measured. The results of electrical measurements, carried out by impedance spectroscopy, show an increase in electrical conductivity when TeO2 is added to the Li2O·SiO2 glass, which we attribute to a mixed former effect between TeO2 and SiO2. Both the activation energy and the σ0 parameter of the Arrhenius equation increase with increased TeO2 content in the samples. A carbon replica analysis of the samples shows a phase separation in the ternary glasses.

We interpret the crystallization of several glass-forming systems in which fresnoite crystals are formed in terms of the rigidity percolation theory. The structure of glass-forming melts can be described as a network with some rigid and some floppy regions. A jump of some properties of the system is expected when the average number of constraints per molecule exceeds a threshold value. In particular, we observed a drastic change in the critical undercooling at which crystallization begins. Above the rigidity threshold, nucleation takes place predominantly in some small floppy regions. This is the reason why the condition for the start of crystallization can be reformulated: the threshold undercooling is so large that the size of the critical nucleus does not exceed the size of the floppy regions.

A dc-potential of 1.5 V between a platinum wire and a platinum crucible containing a magnesia–titania alumosilicate melt is sufficient to cause nucleation and subsequent crystal growth of mullite at the cathode at a temperature high enough to prevent nucleation under normal conditions. This crystal growth is fast in the crystallographic (0 0 1)-direction and leads to a glass–ceramic with a highly oriented microstructure. The mechanism of this process is discussed as well as the crystallization behavior under normal conditions.

Crystalline fresnoite (Ba 2 TiSi 2 O 8 ), attributed glasses and glass ceramics of the systems Ba 2 TiSi 2 O 8 + 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.

During cooling of a glass melt with the composition of 2BaO · TiO 2 · 2.75SiO 2 crystallization takes place at 1030°C during DTA measurements and at 1220°C, if the glass melt is placed in a rotation viscosimeter. If higher temperatures are used, e.g., 1300°C, a cathodic current flow of 45 mA/cm 2 is achieved by attaching a dc-potential of 1.2 V and crystallization of fresnoite takes place at the cathode, leading to a highly oriented structure. The crystallites show fast dendritic growth and the crystallographic c -axis is oriented perpendicular to the electrode surface. Electrochemically induced nucleation runs parallel with the reduction of Ti 4+ of Ti 3+ which is supposed to decrease the viscosity, increase the diffusion coefficient and possibly alter the surface tension of the nuclei formed.