Physical properties of glasses containing several glass-forming oxides

Physical properties, such as dielectric loss, acoustic velocity and electrical conductivity of glasses containing several glass-forming oxides were measured at temperatures in the range from −200°C to 200°C. The relationships between the structure and properties of the glasses, and the change in coordination number of individual glass-forming cations in alkali-containing glasses were analysed. It is shown that the glasses containing several glass-forming oxides are characterized by an inhomogeneous structure.     

A study of crystallization in some Li---Al---Si---O---N system glasses

The crystallization behavior of some Li---Al---Si---O---N system glasses under different conditions was studied. Experimental results show that the addition of nitrogen could decrease the crystallization tendency of glasses. In addition, the atmosphere, chemical composition and different sources of nitride could influence the crystallization of oxynitride glasses.     

Effects of mixed alkaline earth oxides additive on crystallization and structural changes in borosilicate glasses

The effects of mixed alkaline earth oxides on crystallization and structural changes in a multi-component borosilicate glass system are studied by using X-ray diffraction (XRD) and transmission electron microscope (TEM). It is found that the crystallization is decreased with increasing alkaline earth oxide content and there are also a series changes occurred in TEM images. This paper introduces the conception that alkaline earth ions tend to occupy their preferred sites regardless of glass systems. This conception is assisted to explain the TEM images to some extent by suggesting a simple structural model about non-bridging oxygen in glass network. The conception also indicates a ‘blocking effect’ existing in such a multi-component borosilicate glass system, which may be responsible for XRD results in chief. In addition, the structural model suggested by TEM results refers a new unit of Si–O–M²⁺–O–B, which helps in understanding the minimum exhibits in XRD results. Moreover, a dielectric test is taken to study glass properties in detail.     

An X-ray absorption spectroscopy study of the local environment of iron in degradable iron–phosphate glasses

Degradable iron–phosphate glasses with the composition of (CaO)_0.30–(Na₂O)_0.20?x–(Fe₂O₃)_x–(P₂O₅)_0.50, x = 0.01–0.05, were studied by Fe K-edge X-ray absorption spectroscopy (both near-edge, XANES, and extended, EXAFS). The addition of up to 5 mol% iron oxide is known to enhance the durability of the phosphate glass while maintaining biocompatibility. The results from the two techniques used here both show that iron is in the Fe(III) oxidation state and has octahedral coordination. This suggests that Fe is cross-linking the phosphate chains and therefore strengthening the network structure, resulting improved chemical durability of the glasses.     

Spectroscopic and electrical studies of sodium-digermanate glasses containing iron oxide

The infrared, optical absorption spectra and electrical conductivity of sodium-digerminate glasses containing iron oxide have been studied as a function of iron content. Addition of iron does not introduce any new absorption band in the infrared spectrum of pure sodium-digermanate glass. A small shift of the existing absorptions toward lower wavenumbers is observed. These are in agreement with the spectra of Fe₂O₃Na₂O2SiO reported earlier. The optical absorption spectra also indicated that the non-bridging oxygen present in pure sodium-digermanate glass was unaffected by the addition of Fe₂O₃ in a small quantity. The DC conductivity measurements revealed “mixed conduction” phenomenon in which ionic as well as electronic conduction occur in the glass.     

Surface crystallization of leucite in glasses

The aim of this paper was to study surface crystallization of leucite in glasses derived from an SiO₂-Al₂O₃-K₂O base glass in a comparative study with another glass based on SiO₂-Al₂O₃-MgO. Monolithic samples and glass powder of SiO₂-Al₂O₃-K₂O glass were studied and phase formations were determined by using scanning electron microscopy. Heterogeneous nucleation was promoted by seeding the surface of the monolithic glass samples with glass dust to produce a highly disordered crystal as the primary phase. In addition to leucite formation, growth of a highly symmetric, flat, almost two-dimensional crystal phase with controlled diffusion was also observed. In glass granules, leucite demonstrated rapid, almost dendritic growth, from the nucleating centers. Use of leucite ceramic as a restorative dental product is considered.     

Structure and Raman spectra of glasses containing several glass-forming oxides and no glass-modifying oxide

Binary glasses containing no modifying oxides, such as SiO₂---GeO₂, SiO₂---B₂O₃, SiO₂---P₂O₅, GeO₂---B₂O₃, Al₂O₃---P₂O₅ and ternary glasses SiO₂---GeO₂---P₂O₅, Al₂O₃---B₂O₃---P₂O₅, B₂O₃---SiO₂---P₂O₅, Al₂O₃---ZrO₂---P₂O₅ have been prepared by melting and CVD methods. The Raman spectra have also been measured. Structural characteristics of SiO₂, GeO₂, B₂O₃, P₂O₅ in different glass systems are analysed. There exist coordination number changes in B₂O₃- and GeO₂-containing glasses and linkage changes between tetrahedra (SiO₄) and (PO₄) in SiO₂ and P₂O₅ containing glasses. The structure of Al₂O₃ containing glasses is homogeneous and the structure of B₂O₃ containing glasses is inhomogeneous. These experimental results are in coincidence with the X-ray small angle scattering analysis.     

Aligned crystallization in glasses: An analysis of heat transport and interface kinetics

An analysis of heat flow effects which are significant in the development of aligned microstructures in thermally crystallizable glasses is presented. Microstructures developed by heat treatment of a representative glass composition in the CdGeAs system have been interpreted on the basis of a heat flow analysis model developed in this work. The utility and limitations of the current analysis in problems related to interfacial kinetics and microstructure evolution in directionally crystallized glasses are discussed.     

Surface crystallization of rare-earth aluminosilicate glasses

Surface crystallization in a rare-earth aluminosilicate glass (Nd₂O₃–Al₂O₃–SiO₂–TiO₂) was studied using an isothermal method and the crystal growth rate of the glasses was evaluated as a function of the composition. For measuring the surface crystal growth rate, two different methods: measurement of the crystal layer in the longitudinal and lateral growth direction. It was found that crystallization proceeded by surface crystallization only and TiO₂ did not act as a nucleating agent. The growth rate was strongly dependent on the viscosity of glass and agreed with prediction from the Preston model using the known viscosity and melting temperature. As the Si/Nd and Si/Al ratios decreased, the crystal growth rate increased. TiO₂ and Nd₂O₃ played the role of network modifier, which decreased the viscosity of the glass, facilitating crystallization of the rare-earth aluminosilicate glass.     

Thermal properties and crystallization of iron phosphate glasses containing up to 25 wt% additions of Si-, Al-, Na- and U-oxides

The thermal properties (expansion, T_g and T_SOFT.) of glasses, having 56-66% P₂O₅, 14.8-34.2% Fe₂O₃ and 2-25 wt% additions of SiO₂, Al₂O₃, Na₂O and UO₂, were comparatively estimated from dilatometric measurements in similar conditions. The T_g reversibility was clearly verified by varying the heating rates between 1 and 5 °C min⁻¹. From linear equations fits of the various glass properties as functions of the six components it is suggested the iron, sodium and uranium oxides decrease the thermal expansion (for 50 < T ? 300 °C), T_g and T_SOFT. From DTA/XRD analysis of three glasses it was confirmed the crystallization tendency decreased with increasing the UO₂ level in the glasses. Leaching test data for two compositions containing Na₂O suggest addition of UO₂ increases the chemical durability of the related glass. The roles of UO₂, Na₂O and Fe-oxide species as structural components of the glass network are discussed.     

Microheterogeneities in infrared optical selenide glasses

Flocculent inhomogeneities in vitreous Ge₂Se₃ were identified as being SiO₂. The origin of these particles was found in the walls of the vitreous silica tubes in which the glass was prepared. By etching or thermal treatment of the tubes prior to their use the concentration of the microheterogeneities in the selenide glass can be considerably reduced. This is also valid for the preparation of glasses of the system GeAsSe and GeSbSe.     

Dielectric study and ac conductivity of iron sodium silicate glasses

A series of glasses with formula (SiO₂)_0.7−x(Na₂O)_0.3(Fe₂O₃)_x with ( 0.0 ≤ x ≤ 0.20) were prepared and studied by means of AC measurements in the frequency range 20 kHz to 13 MHz at room temperature. The study of frequency dependence of both dielectric constant ε' and dielectric loss ε" showed a decrease of both quantities with increasing frequency. The results have been explained on the basis of frequency assistance of electron hopping besides electron polarization. From the Cole-Cole diagram the values of the static dielectric constant ε_s, infinity dielectric constant ε_∞, macroscopic time constant τ, and molecular time constant τ_m are calculated for the studied amorphous samples. The frequency dependence of the ac conductivity obeys a power relation, that is σ_ac (ω) = Α ω^s. The obtained values of the constant s lie in the range of 0.7 ≤ s ≤ 1 in agreement with the theoretical value which confirms the simple quantum mechanical tunneling (QMT) model. The increase in ac conductivity with iron concentration is likely to arise due to structural changes occurring in the glass network. The structure of a glass with similar composition was published and showed clustering of Fe²⁺ and Fe³⁺ ions which favor electron hopping and provide pathways for charge transport.     

Neutron diffraction study of sodium borosilicate waste glasses containing uranium

The effect of uranium oxide on the structure of sodium borosilicate host glasses has been studied by neutron diffraction. The samples were prepared by quenching the melted mixtures of composition 70 wt% [(65 − x)SiO₂ · xB₂O₃ · 25Na₂O · 5BaO · 5ZrO₂] + 30 wt% UO₃ with x = 5, 10 and 15 mol%. It was found, that the U-loaded glasses posses good glass and hydrolytic stability. An enhanced probability for inter-mediate atomic correlations at around 4.8 Å has been established. The RMC simulation of the neutron diffraction data is consistent with a model where the uranium ions are incorporated into interstitial voids in the essentially unmodified network structure of the starting host glass. The U–O atomic pair correlation functions show a sharp peak at around 1.7 Å, and several farther distinct peaks are at 2.8, 3.6 and 4.1 Å. The uranium ions are coordinated by six oxygen atoms in the 1.6–3.4 Å interval.     

The small anomaly in the temperature dependence of the electrical conductivity of iron containing phosphate glasses

The dc conductivities, σ,of 2 CaO·K₂O·x Fe₂O₃·(5 - x)P₂O₅ glasses (1.4 ?x?2.2) [Fe³⁺/(Fe²⁺ + Fe³⁺)= constant (0.88±0.015)] have been measured in the temperature range from room temperature to 300°C. It has been found that the glass transition temperature increases rapidly with increasing Fe/P ratio and that mixed-valence hopping type electronic conduction takes place in these glasses and every log σ versus 1/T curve obtained shows a change in slope near 100°C, which produces a small increase, 0.14 (for x=2.2)?0.18 eV (for x=1.4), in activation energy at higher temperatures. Both the activation energy and pre-exponential factor for conduction display different dependences on the Fe/P ratio in the different temperature ranges on both sides of the anomaly temperature. These results are discussed in terms of the Mott's equation for small polaron hopping.     

Melt crystallization of zinc alkali phosphate glasses

Melt crystallization of two zinc alkali phosphate glasses was studied with X-ray diffraction (XRD) experiments to accelerate efforts to melt process these glasses with organic polymers. The inorganic glasses differed markedly in chemical durability (water sensitivity) and crystallization rates. They were studied at room temperature prior to and after melt processing with XRD experiments and in situ at melt temperatures without flow in a novel differential scanning calorimeter/XRD apparatus. The glasses were found to be amorphous at room temperature and semi-crystalline above their glass-transition temperatures. Higher temperatures and shear (mixing) rates increased the crystallization rate of the glasses. The non-durable (water-sensitive) glass was observed to contain significant levels of crystalline matter after melt processing at 400°C. This process-induced crystallization of the glasses must be controlled, possibly during processing and/or glass formulation, otherwise it may lead to formation of unwanted phase-separated defects in the glass. If high levels of the crystalline matter are present during melt processing, they may lead to irreversible plugging of the processing equipment.     

Spectroscopic study on iron doped silica-bismuthate glasses and glass ceramics

Fourier transform infrared (FTIR) and electron paramagnetic resonance (EPR) spectroscopic data obtained for iron doped silica-bismuthate glasses were used to investigate the changes induced in the local structure of samples as the ratio between Bi₂O₃ and SiO₂ content changes from 3 to 0.66. The environment of constituent cations was investigated both in vitreous and partially crystallized samples of same composition. Progressive substitution of Bi₂O₃ by SiO₂ contributes to the structural relaxation of vitreous network and enhances glass stability. By crystallization heat treatment the structural units appear to be more uniform as it results from the narrowing of corresponding IR bands and EPR lines. FTIR data show that more affected by composition and heat treatment is the environment of bismuth than that of silicon. EPR data indicate an ordering tendency with SiO₂ content in glasses, while in vitroceramics the iron environment is much uniform in samples with less SiO₂ content, where Bi₁₂SiO₂₀ phase is developed.     

DTA peak shift studies of primary crystallization in glasses

Following the recognition during the last decade that knowledge of the shift in the exothermic crystallization DTA peak as a function of pre-DTA isothermal heat-treatment times and temperatures, can provide quantitative information about the crystallization kinetics, there has been renewed interest in DTA investigations of crystallization of glasses. Most studies to date, however, have focussed on the kinetics of polymorphic crystallization (where the compositions of the crystal and the parent glass are the same). These studies have established that the DTA peak shifts to lower temperatures with increased pre-DTA heat-treatment times and JMAK-based formalisms have been developed to extract the steady state nucleation rate from the DTA peak shift data. In this paper, we report new results on the DTA peak shift in systems undergoing primary crystallization (where the compositions of the crystal and glass are different). The DTA results show that the exothermic peak temperature decreases initially but increases later on, becoming significantly larger than the initial value, with increase in the pre-DTA heat-treatment time at a fixed temperature. This increase at long times has not been reported previously and is qualitatively different than the monotonic decrease reported for polymorphic crystallization. To rationalize these new results, a model of primary crystallization has been developed which includes homogeneous nucleation, diffusion-controlled growth, Gibbs-Thomson effect, and a mean field soft-impingement correction during growth. Based on this model and experimental results, it is concluded that the initial shift to lower temperatures is due to an increase in the number of nuclei (as concluded previously by others for the case of polymorphic crystallization) and the later shift towards high temperatures in our experiments is due to diffusion-controlled growth during the pre-DTA heat treatment.     

An investigation of the local iron environment in iron phosphate glasses having different Fe(II) concentrations

The local environment around iron ions in iron phosphate glasses of starting batch composition 40Fe₂O₃-60P₂O₅ (mol%) melted at varying temperatures or under different melting atmospheres has been investigated using Fe-57 Mössbauer and X-ray absorption fine structure (XAFS) spectroscopies. Mössbauer spectra indicate that all of the glasses contain both Fe(II) and Fe(III) ions. The quadrupole splitting distribution fits of Mössbauer spectra show that Fe(II) ions occupy a single site whereas Fe(III) ions occupy two distinct sites in these glasses. When melted at higher temperatures or in reducing atmospheres, the Fe(II) fraction in the glass increases at the expense of Fe(III) ions at only one of the two sites they occupy. The pre-edge feature in the XAFS data suggests that the overall disorder in the near-neighbor environment of iron ions decreases with increasing Fe(II) fraction. The XAFS results also show that the average iron-oxygen coordination is in the 4-5 range indicating that iron ions have mixed tetrahedral-octahedral coordination.     

Thermophysical properties of Zr-Cu-Al metallic glasses during crystallization

The crystallization behavior of Zr-Cu-Al metallic glasses was studied using thermophysical property measurements. When the Zr content of Zr-Cu-Al metallic glass decreased from 65 at.% to 45 at.%, the thermal conductivity gradually increased and the maximum value obtained was the composition of Zr:Cu:Al = 50:39.3:10.7(at.%). These metallic glasses were not crystallized upon heat treatment below the glass transition temperature T_g, and the thermophysical properties of these metallic glasses were almost constant. In contrast, these metallic glasses started to crystallize upon heat treatment above T_g after a certain derived time, and their thermal conductivity increased with the crystallinity of the metallic glass.