Refractive index and density of Li-, Na- and K-germanosilicate glasses

Refractive index and density measurements in the ternary systems Li₂O-SiO₂-GeO₂, Na₂O-SiO₂-GeO₂ and K₂O-SiO₂-GeO₂ are given. Molar refraction data are correlated to the structure of these glasses. In the Li₂O-SiO₂-GeO₂ system, phase separation occurs which can be suppressed by rapid cooling. In the systems Na₂O-SiO₂-GeO₂ and K₂O-SiO₂-GeO₂ no such phase separation is observed; in these systems the molar refraction data indicate that, contrary to the Li₂O-SiO₂-GeO₂ system, the four-six co-ordination change of Ge⁴⁺ is not affected by the presence of SiO₂ as second network former.     

Effect of the size of nuclei on crystallization behavior of Li2O · 2SiO2 glass

Crystallization peak temperature was measured for Li₂O · 2SiO₂ glass with various number densities of nuclei induced by long heat-treatment (405 h) at 460°C. Its crystallization process of Li₂O · 2SiO₂ glass was simulated numerically in order to examine the effect of the initial size of nuclei on crystallization behavior, and the relationship between crystallization peak temperature and the number density of nuclei in the glass was considered. It was found that the effect of nuclei size is not significant when the heat-treatment temperature is lower than 460°C. By considering the effect of nuclei size, number density of nuclus was evaluated by DTA method accurately.     

Mössbauer studies of the effect of nucleating agents on the crystallization of YIG in the Na2O-SiO2-Y2O3-Fe3O3 glass system

The effect of 2 mol % of P₂O₅ or TiO₂ on the crystallization of yttrium-iron-garnet (YIG) in Na₂O-SiO₂-Y₂O₃-Fe₂O₃ glass ceramics has been investigated by X-ray and Mössbauer spectroscopic studies. These studies indicate that the glass ceramics containing P₂O₅ are more favourable for the crystallization of YIG. Analysis of the results suggest that an optimum growth temperature for crystallization of the YIG phase is 800° C in the sample with P₂O₅ as the nucleating agent. The sample heat-treated at the growth temperature for 40 h is not as favourable for crystallization of the YIG phase as the sample which is given a two-stage heat-treatment.     

Studies on Si-O bonding in silicate and aluminosilicate glasses based on SiKβ emission X-rays

The SiK X-ray emission spectra of silicate and aluminosilicate glasses and crystals have been measured with an X-ray fluorescence spectrometer and the chemical shifts of the SiK peak relative to that of SiO₂ glass have been determined. Trends in the chemical shift of the SiK peak for Na₂O-SiO₂, Li₂O-SiO₂ and Li₂O-Al₂O₃-SiO₂ glasses were similar to those of their crystalline counterparts, indicating that the strength of the Si-O bond varies in the same way with composition in both glass and crystal. The chemical shift in alkali aluminosilicate glasses containing equal molar proportions of aluminium oxide and alkali oxide increased with decreasing SiO₂ content, indicating that the Si-O bond also weakened in aluminosilicate glasses. The same effect was also observed when zirconia was added. The SiK bandwidths for Na₂O-Al₂O₃-SiO₂ and Li₂O-Al₂O₃-SiO₂ glasses with the Al₂O₃: M₂O ratio equal to one, remained almost the same as for SiO₂ glass, indicating that the addition of alkali does not result in the formation of non-bridging oxygens in the framework if accompanied by Al₂O₃.     

Nucleation kinetics of a Li2O·2SiO2 glass based on a liquid model

A liquid model was proposed wherein crystal embryos should exist in equilibrated melts at temperatures even above the melting point or liquidus temperature and the number density of embryos should decrease as the melting temperature increases. Crystallization behavior of a Li₂O·2SiO₂ glass was studied. Secondary heat-treatment (600 °C for 20–60 min) of the Li₂O·2SiO₂ glass induced different number of crystals depending upon the melting temperature in the range 1040°–1300 °C and duration of heat-treatment t. The number density of crystals N decreased with increasing temperature of melting. The number density of pre-existing embryos was estimated from (N – t) diagram and was found consistent with what the proposed model claims.     

Transmission electron microscopy of partially crystallised glasses

The early stages of crystallisation in a Li₂O-SiO₂-P₂O₅ glass were studied by transmission electron microscopy using thin sections prepared from the bulk material by chemical thinning. Small crystalline regions of lithium disilicate were formed, often as several small single crystals joined together around a central core. Possible explanations of the core are discussed in terms of a region of different chemical composition or a central region of disorder. The observations are also compared with the growth of crystal spherulites in polymers.     

Estimated immiscibility isotherms of the Li2O-Al2O3-SiO2 system

Using the immiscibility temperature estimation method, recently developed by the present author and Tomozawa, immiscibility isotherms of the Li₂O-Al₂O₃-SiO₂ system were estimated. High reliability of the estimated immiscibility isotherms was confirmed by observing the morphologies of phase-separated glasses, and also by comparing the estimated and observed immiscibility temperatures at several compositions. The determined immiscibility isotherms revealed that, in the Li₂O-Al₂O₃-SiO₂ system, only composition regions near the Li₂O-SiO₂ and Al₂O₃-SiO₂ binary edges are phase-separable. In composition regions where base glasses for commercial glass-ceramics are located, the immiscibility temperatures were much lower than the glass transition temperatures, implying that no phase separation actually occurs. Accordingly the phase separation in practical glasses for producing glass-ceramics may be attributed to increased immiscibility resulting from various additives.     

Structure of rapidly quenched glasses in the system Li2O-SiO2

Raman spectra of glasses in the Li₂O-SiO₂ system (41.3 Li₂O 61.3 mol%), prepared by rapid quenching, were measured. The proportions of SiO₄ units with 1–4 non-bridging oxygens per silicon (NBO/Si) and the fractions of bridging oxygen, non-bridging oxygen and free or full-active oxygen were determined for these glasses from quantitative analysis of the Raman spectra obtained. X-ray structural analysis of Li₂O-SiO₂ showed an increasing elongation of the average atomic distance of the Si-O pair with increase in Li₂O content due to weakening of the Si-O bond.     

Crystalline phase formation, microstructure and mechanical properties of a lithium disilicate glass–ceramic

In this work, crystalline phase formation, microstructure and mechanical properties of a lithium disilicate (LS2, Li₂Si₂O₅) glass–ceramic in the SiO₂–Li₂O–Al₂O₃–P₂O₅ system were investigated. A four-stage heat treatment process was used for crystallization of the glass. The effects of holding time and temperature at the final stage on the crystalline morphology of the glass–ceramic were studied. The experimental results revealed the two crystallization peaks at 672 and 839 °C. At a temperature lower than 770 °C and holding time of 20 min, the lithium metasilicate (Li₂SiO₃) phase dominates. On the other hand, when the glass was heated to a higher temperature or held for a longer time, the LS2 phase dominates and some other minor phases such as cristobalite and lithium phosphate emerge. Scanning electron microscopy and energy dispersive spectrometry revealed a large number of nanosized ZrO₂ particles when the crystallization temperature was above 790 °C. Vickers hardness of the LS2 glass–ceramic was about 8.1–8.4 GPa and flexural strength was in the range of 282–307 MPa. Crack deflection was observed along the LS2 cluster boundaries. The crystallization sequence was proposed to explain the observed microstructure and phases.     

The problem of incorporating high levels of sodium nitrate into silica gels without segregation

Na₂O-SiO₂ sols were prepared using silicon alkoxide and colloidal silica precursors with sodium nitrate as the salt. The experimental procedure carried out was changed depending on the precursors used. Several tests with dialysis membranes were performed in order to study the possibility of incorporating Na⁺-ions into the sols. These ions were introduced into silica sols contained in membranes from an exterior solution. In addition, washing tests were carried out with Na₂O-SiO₂ sols contained inside the membranes and with several exterior washing solutions. Resistivity of the washing solutions was measured against time. The washing solutions were changed periodically. After drying and heat-treatment, Na₂O content in the gels was determined using flame photometry. The crystalline phases formed were detected by X-ray diffraction. The results show a small incorporation of alkaline ions in Na₂O-SiO₂ gels, even in those samples prepared through the dialysis membranes tests.     

A transmission electron microscope study of glass-ceramics

A technique for preparing thin foils of glass-ceramics for transmission electron microscopy has been developed, using a combination of chemical, abrasion and ion-beam machining. This has permitted a study of the microstructural development in a number of glasses of different chemical compositions subjected to various heat-treatments. Results are presented and discussed for materials based on the Li₂O-SiO₂ system and on a more complex Li₂O-K₂O-ZnO-SiO₂ material employing phosphorus pentoxide as a nucleation catalyst.     

Thermal Diffusivity Measurements of Liquid Silicate Melts

The effect of structure on the thermal diffusivities/conductivities for liquid silicates have been summarized based on recent experimental work carried out by the Royal Institute of Technology, Stockholm and the Tokyo Institute of Technology using the laser-flash and the hot-wire methods, respectively. In the former case, the effective thermal diffusivity was measured by a three-layer method. The relationship proposed by Mills that the thermal conductivity of silicates increases with a decrease in the ratio of NBO/T (number of non-bridging oxygens per tetrahedrally coordinated atom) has been well supported by the effective thermal diffusivity data for the liquid CaO-Al₂O₃-SiO₂ slags. However, it has been shown that for the slags having a higher CaO/Al₂O₃ ratio, the effective thermal diffusivity is roughly constant independent of the ratios of NBO/T. It has been concluded that when the silicate network is largely broken down, the phonon mean free path is not affected by the structure. It has been found by the hot-wire method that the magnitudes of thermal resistivity are in the hierarchy Li₂O-SiO₂2O-SiO₂2O-SiO₂ despite their similar values of NBO/T. It has been concluded that the ionicity of non-bridging oxygen ions is also a factor controlling the thermal conductivity of silicates as well as the number of broken bridges in the silicate network. The effective thermal diffusivity was measured for the CaO-Al₂O₃-SiO₂-FeO system to elucidate the radiation contribution to the effective thermal diffusivity. It has been found that the effective thermal diffusivity increases with an increase in FeO content. It can be considered that the strong absorption and emission within the liquid slag films caused by the Fe²⁺ ions enhances the photon heat transfer.     

Bioactive pyrophosphate glass/beta-tricalcium phosphate composite with high mechanical properties

A composition of phosphate glass was incorporated into the beta-tricalcium phosphate (β-TCP) ceramic by a weight ratio of 10% aiming to study the effects of the phosphate glass on the mechanical and biological properties of β-TCP matrix. X-ray diffraction analysis showed that the phase composition of the phosphate glass was in the pyrophosphate region which consisted of Na₄P₂O₇ and β-Ca₂P₂O₇ (β-DCP) phases. Density and bending strength assessment had been conducted and the results showed that the pyrophosphate glass/β-TCP (PG/β-TCP) system presents a high mechanical properties. The biological characteristics of the composite had been evaluated by means of in vivo testing and in vitro testing. Leaf-like particles with Ca/P molar ratio of 1.46 was observed under electron microscopy (SEM) after soaking for 5 days on the surface of composite but no similar structure was detected on the surface of pure β-TCP. Osteoblast-like cells 3T3-E1 were seeded onto the PG/β-TCP composite and pure β-TCP matrix. MTT results show that the composite is non-toxic and biocompatible and furthermore, the living cell number adhered on the surface of matrixes reveals that the addition of pyrophosphate glass can significantly facilitate cells adhesion.     

The effect of ZrO2 in 30K2O-70SiO2 glass: a comparison with 30Li2O-70SiO2

The influence of the introduction of ZrO₂ in concentrations of up to 5 mol% into a 30K₂O-70SiO₂ base glass composition has been investigated with the aid of differential thermal analysis (DTA), FT-IR spectroscopy, as well as chemical durability tests (pH, conductivity and AES-Inductively Coupled Plasma measurements) and refraction index determination. Several differences have been found by comparison with similar compositions belonging to the Li₂O—ZrO₂—SiO₂ system. The higher ionic radius and the lower field strength of K⁺ with respect to Li⁺ is responsible for higher stability towards crystallisation and lower chemical durability. Moreover, even though Zr⁴⁺ increases glass polymerisation (higher glass transition temperature, chemical durability and refraction index), the presence of high water content as highlighted by spectroscopy measurements, seems to weaken the glass structure.     

Phase separation in Li2O-SiO2 glasses with additions of V2O5, MnO2 and Cr2O3

A number of lithium silicate glasses, inside and just outside the miscibility gap, containing low percentages of V₂O₅, MnO₂ and Cr₂O₃ were examined by electron microscopy techniques. Upon heat treatment in the range 450 to 550° C these glasses exhibited phase separation preceding extensive crystallization. The Li₂O contents (mol%) of the base glass compositions were: 26 (C-I), 30 (C-II), 34 (C-III) and 37 (C-IV). The V₂O₅ and MnO₂ additions were 1 mol% and the Cr₂O₃ addition was 0.1 mol%. Globular or sectorized domains were obtained for compositions C-I to C-III containing V₂O₅, for formulations C-II to C-IV containing Cr₂O₃ and for glass C-I containing MnO₂. In nearly every case a much finer liquid-phase separation of droplet type was also detected. The results indicate that V₂O₅ and MnO₂ widen the Li₂O-SiO₂ miscibility gap and that the Cr₂O₃ may cause fine-scale crystallization.     

Preparation and characterization of Li<Subscript>2</Subscript>O–CaO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript>–SiO<Subscript>2</Subscript> glasses as bioactive material

The aim of the present investigation was to study the role of Al₂O₃ in the Li₂O–CaO–P₂O₅–SiO₂ bioactive glass for improving the bioactivity and other physico-mechanical properties of glass. A comparative study on structural and physico-mechanical properties and bioactivity of glasses were reported. The structural properties of glasses were investigated by X-ray diffraction, Fourier transform infrared spectrometry, scanning electron microscopy and the bioactivity of the glasses was evaluated by in vitro test in simulated body fluid (SBF). Density, compressive strength, Vickers hardness and ultrasonic wave velocity of glass samples were measured to investigate physical and mechanical properties. Results indicated that partial molar replacement of Li₂O by Al₂O₃ resulted in a significant increase in mechanical properties of glasses. In vitro studies of samples in SBF had shown that the pH of the solution increased after immersion of samples during the initial stage and then after reaching maxima it decreased with the increase in the immersion time. In vitro test in SBF indicated that the addition of Al₂O₃ up to 1.5 mol% resulted in an increase in bioactivity where as further addition of Al₂O₃ caused a decrease in bioactivity of the samples. The biocompatibility of these bioactive glass samples was studied using human osteoblast (MG-63) cell lines. The results obtained suggested that Li₂O–CaO–Al₂O₃–P₂O₅–SiO₂-based bioactive glasses containing alumina would be potential materials for biomedical applications.     

Glass-ceramics with random and oriented microstructures

The technique of hot extruding a glass-ceramic to produce a material with an aligned crystal microstructure is described. The results of a statistical investigation to analyse the degree of morphological orientation of an extruded glass-ceramic based on the Li₂O-SiO₂ system are reported. Details are also presented of the determination of the crystallographic orientation in the aligned glass-ceramic.     

Porous shell/dense core structures prepared in tungsten phosphate glass through template-free route

The preparation of the 11.7Li₂O–39.7WO₃–10.9CaO–37.7P₂O₅ glass (pgLWCP) was based on a one-step heat treatment of the 6Li₂O–18WO₃–43CaO–33P₂O₅ (gLWCP) glass followed by leaching of the β-Ca₂P₂O₇ phase formed during the crystallization process. The porous structure was formed in the region formerly occupied by the β-Ca₂P₂O₇ phase. The gLWCP undergoes devitrification through surface crystallization. This process occurs after a thermal treatment in lower temperature and in a shorter period of time than that required for the complete crystallization. After acid leaching treatment, we obtained a core-/shell-like structure with a very well-defined dense glass (gLWCP)/porous glass (pgLWCP) interface. The pgLWCP exhibits reversible coloration–decoloration reactions.     

Phase separation behaviour of a metal-organic derived sodium silicate glass

The phase separation behaviour of an 18.56 mol % Na₂O-SiO₂ glass prepared via the standard glass preparation technique and the metallic-organic derived (MOD) method have been studied and contrasted. Our results suggest that at this composition the miscibility temperature is significantly elevated when a MOD preparation procedure is used. The origin of this effect appears to be related to the enhanced water content in the glass prepared by the MOD procedure.     

控制聚合与沉淀协同作用改善高铁轨道板涂料用水玻璃性能

CaO可能对水玻璃酸碱度、Zeta电位产生影响,引起调控水玻璃中硅氧聚合体的聚合度和形成沉淀的协同作用,有望改善水玻璃的力学强度和耐水性。欲提高环保型高铁轨道板涂料用水玻璃的力学强度和耐水性,本实验拟采用CaO对水玻璃进行改性,并探索改性的相关机理。实验结果表明,CaO改性水玻璃力学强度提高77%,耐水性也明显提高。Zeta电位仪、pH计、X射线衍射仪、傅里叶红外光谱仪及固体核磁共振仪检测结果表明:添加CaO使水玻璃Zeta电位值、pH值均增大;在形成Ca²⁺的协同作用下,导致硅氧聚合体解聚、重组并限制聚合,主要以链状聚合体(Q²)存在。同时Ca²⁺与Q²结合形成耐水性较好、力学强度较高的CaO·xSiO₂·yH₂O(CSH)凝胶,而不是含大量Na⁺、耐水性差的硅氧聚合体。可以推断,CaO可产生控制聚合度和沉淀协同效应改善水玻璃性能,有望应用于高铁轨道板涂料用水玻璃。