Microstructure and grain growth in Li2O-Al2O3-SiO2 glass ceramics

Microstructure and grain growth were studied in two glass ceramics of the Li₂O-Al₂O₃-SiO₂ system, one an experimental material of basic composition Li₂O-Al₂O₃-4SiO₂ and the second a commercial material of approximately 0.7Li₂O-Al₂O₃-6SiO₂ composition with small amounts of other oxides. There was evidence from transmission electron microscopy that the commercial material contained residual glass at grain-boundary triple points and glass layers at some but not all grain boundaries. No definite evidence was found for residual glass in the experimental material. The composition of secondphase regions in the commercial material was studied by STEM microanalysis. Al-rich regions of irregular morphology were found but there was no evidence that residual glass was SiO₂-rich, as has been previously suggested for this type of glass-ceramic. Grain growth showed a fairly similar time dependence in the two materials with growth more rapid in the commercial material at a given temperature. It is suggested that grain growth is controlled by precipitate particles rather than by residual glass phase.     

Incorporation of transition metal in porous glass-ceramics of TiO2-SiO2 system

Incorporation of transition metals in porous glass-ceramics of TiO₂-SiO₂ system was made by the phase separation and crystallization of the glasses of TiO₂-SiO₂-Al₂O₃-P₂O₅-CaO-MgO system containing various kinds of transition metals. The amount of transition metals incorporated in the skeleton of the porous glass-ceramics was dependent on both chemical composition of mother glass and conditions of heat treatment. In general the amount decreased with the increasing amount of rutile in the skeleton. In the glass of high TiO₂/SiO₂ ratio, the incorporation of relatively large amounts of transition metals was possible even if the precipitation of a fairly large amount of rutile occurred. The crystallization of rutile and in porous glass-ceramics was essential to fabricate rigid platelet porous glass-ceramics.     

Preparation and crystallization of ultrafine Li2O-Al2O3-SiO2 powders

Ultrafine powders of Li₂O-Al₂O₃-SiO₂ (LAS) glass-ceramic were prepared by the sol-gel process using tetraethoxysilane, titanium butoxide, lithium, magnesium, aluminium (and zinc) inorganic salts as starting materials. The effect of pH on the sol-gel transition and particle sizes of the Li₂O-Al₂O₃-SiO₂ system was studied. The nucleation and crystallization process of LAS powders were also investigated by differential thermal analysis and X-ray diffraction. The results show that a surface nucleation process occurs for ultrafine LAS powders. The LAS glass-ceramics fabricated from ultrafine LAS powders have a low thermal expansion coefficient, <10×10^–7 °C.     

Characterization of crystal phases,morphology and crystallization processes in lithium aluminosilicate glass-ceramic

The nucleation and crystallization processes of Li₂O-Al₂O₃-SiO₂ glass-ceramics were investigated by differential thermal analysis. The crystalline phases produced during thermal treatment at different temperatures and the residual glassy phase were characterized by X-ray diffraction, SEM and image analysis techniques. The activation energy of the crystallization process was calculated as E=380±20 kJ mol^–1. The influence of nucleating agents (TiO₂, ZrO₂) was evaluated to obtain glass-ceramics transparent to visible light. The stability of the glassy phase in cooling was determined by means of temperature-time-transformation curves.     

The stability of mother glass for porous glass-ceramics of the TiO2-SiO2 system

The stability of the glasses of TiO₂-SiO₂-Al₂O₃-B₂O₃-CaO-MgO and TiO₂-SiO₂-Al₂O₃-P₂O₅-CaO-MgO systems during formation has been investigated as a basic study on the preparation of porous glass-ceramics. The main factor affecting the stability of these glasses which are used as mother glasses of porous glass-ceramics was CaO content. The stability was remarkably improved by increasing the CaO content.The preparation of porous glass-ceramics with an appropriate amount of pore volume was possible from a glass containing CaO up to 30 mol%. The DTA trace of the glasses showed two distinct exothermic peaks in the ranges 30–130° C and 140–300° C above glass transition temperatures. The porous glass-ceramics of TiO₂-SiO₂ system containing more than 60 mol% TiO₂ and having a surface area larger than 400m²g^–1, a pore volume of 0.3–0.5 ml g^–1, and average pore radius between 1 and 20 nm were fabricated.     

Effects of MO (M = Mg, Ca, Ba) on crystallization and flexural strength of semi-transparent lithium disilicate glass-ceramics

In this study, semitransparent lithium disilicate glass-ceramics in the Li₂O-K₂O-Al₂O₃-Y₂O₃-La₂O₃-SiO₂ system was investigated by incorporation of P₂O₅ as nucleation agent and alkaline earth oxides as additive. The influence of alkaline earth oxides on the structure of glasses network, crystalline phases, microstructure and mechanical properties were investigated by means of Raman spectra, differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The mechanical strength was measured corresponding to norm ISO 6872. The Raman spectra predominantly showed that small additions of alkaline earth oxides not only form asymmetrical vibrations of the M-Si-O bonds, but also enhance the intensities of symmetrical vibrations of the P-O bonds, making the glass network more stable. And the small additions of CaO or BaO has more influence on the crystallization behaviour, crystalline phase, microstructure and mechanical properties of the glass-ceramics than the addition of MgO. The additions of alkaline earth oxides enhanced the first exothermic peak temperature but decreased the flexural strength of lithium disilicate glass-ceramics.     

Metastable liquid immiscibility and Vycor-type glass in phosphate-silicate systems

Glass formation and metastable liquid immiscibility in Na₂O-P₂O₅-SiO₂ glasses were studied. Phase separation in some of these glasses resulted in formation of an interconnected two-phase amorphous structure, similar to that in glasses of the Na₂O-B₂O₃-SiO₂ system used for production of Vycor glass. Leaching of the high-phosphate phase yielded a high-silica (over 90% SiO₂) skeleton, which could be sintered to a bulk glass.     

Structural and microstructural study of glasses in the Li2O-TiO2-SiO2 system

Glasses from the Li₂O-TiO₂-SiO₂ system with TiO₂ content ranging between 5 and 20 mol % were prepared. The glasses studied have an opal appearance and when their microstructure is studied through transmission electron microscopy, a strong phase separation in droplet form is observed, with the size of the droplets depending on the TiO₂ and the Li₂O content. X-ray diffraction showed that these glasses are non-crystalline and identified the crystalline phases which take place after the corresponding thermal treatment to enhance liquid immiscibility. A structural study of the samples prepared was carried out through infrared spectroscopy. Titanium oxide is incorporated into glasses as a network modifier, so that titanium atoms place themselves in octahedral positions [TiO₆]. This is attributed to the presence of Li₂O which breaks up the glass network, favouring the hexacoordination of titanium atoms.     

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.     

The role of TiO2 and composition in the devitrification of near-stoichiometric cordierite

Devitrification behavior and thermal expansion of glasses and glass-ceramics, doped with TiO₂, near the stoichiometric cordierite composition were investigated. The activation energy for growth of surface nucleated crystals was shown to be approximately 433 kJ/mol. and was independent of TiO₂ content in the glass. Volume nucleation was achieved by the addition of approximately 8 wt% TiO₂, but the mechanism of volume nucleation was different depending upon cordierite composition. Regions in the MgO-Al₂O₃-SiO₂ phase diagram near the stochiometric cordierite compound were investigated using thermal analysis, X-ray powder diffraction and dilatometry. Glass in glass phase separation was postulated for MgO-lean glasses, whereas precipitation of mullite and Al₂TiO₅ preceded devitrification in other compositions. In each case, the formation of a silica-rich glass is believed to initiate the devitrification. Coefficients of thermal expansion for the glass-ceramics increased with increasing TiO₂ content resulting from increasing levels of uncrystallized glass and the formation of mullite and rutile during crystallization.     

锂铝硅系光敏微晶玻璃的研究进展和应用

自Li₂O-Al₂O₃-SiO₂系光敏微晶玻璃诞生以来,人们便期望将其优秀的异向刻蚀能力应用于微型结构件的制造中。目前,对锂铝硅系光敏微晶玻璃材料的研究主要集中在成核机理、析晶过程和组分、性质间的关系等方面。光敏微晶玻璃的光敏性和结晶性能密不可分,光敏性决定了晶核的形成,而生成的偏硅酸锂晶体则决定了其异向刻蚀能力。因而对成核机理和析晶过程的研究至关重要,亦有利于了解玻璃的本质。而随着科学技术的发展进步,光敏微晶玻璃的某些性能已不能很好地满足应用要求,如介电损耗和化学稳定性,因此提高光敏微晶玻璃的性能变得必要。随着研究的日益深入,光敏微晶玻璃在实际应用中也暴露出一些问题,如刻蚀精度、壁角倾斜度、内壁光滑度、介电损耗高等,这些问题制约着该材料的发展,亟待解决。 本文阐述了锂铝硅系光敏微晶玻璃的光敏化诱导析晶原理,详细介绍了Li₂O-Al₂O₃-SiO₂系光敏微晶玻璃的研究进展以及在三维集成电路、微流控芯片和微通道板等方面的应用,分析了现阶段存在的问题,并指出了今后光敏微晶玻璃的研究方向。     

Preparation of porous glass-ceramics of the TiO2-SiO2 system

Porous glass-ceramics of the TiO₂-SiO₂ system of high titania content have been prepared by heat treatment and subsequent acid leaching of phase-separated glasses of the TiO₂-SiO₂-Al₂O₃-B₂O₃-CaO-MgO (or -Na₂O) system. The porous glass-ceramics obtained in the study had a surface area of 100 to 300 m²g^–1, with an average pore radius of 3 to 9 nm. The ceramics which contained a large amount of anatase and rutile were expected to be applied in the field of photocatalysts.     

Transparency of LiTaO3-SiO2-Al2O3 glass-ceramics in relation to their microstructure

The glasses with various compositions in the LiTaO₃-SiO₂-Al₂O₃ system were heated from room temperature to temperatures ranging from 750° to 1050° C at a rate of 5° C min^–1. From the glasses in the LiTaO₃-SiO₂ system no transparent glass-ceramic was obtained even when their LiTaO₃/SiO₂ mole ratios were as high as 2.33. The diameter and number of the LiTaO₃ crystal grains precipitated in the glasses were 5–15 m and 10⁸–10¹⁰ grains cm^–3, respectively. On the contrary, transparent glass-ceramics were obtained from the glasses containing Al₂O₃; their compositions covered a fairly large area in the LiTaO₃-SiO₂ -Al₂O₃ system, which encompasses the compositions with the LiTaO₃/SiO₂+AlO_1.5 mole ratio as low as 0.25. The diameter and number of the LiTaO₃ crystal grains precipitated in the transparent glass-ceramics were as small as 10–20 nm and as many as 10¹⁶–10¹⁸ grains cm^–3, respectively. High nucleation rates of the LiTaO₃ crystals in the Al₂O₃-containing glasses were interpreted in terms of structural inflexibility induced in the glass-network by the addition of Al₂O₃ to the LiTaO₃-SiO₂ system.     

Study of Al2O3 effect on structural change and phase separation in Na2O-B2O3-SiO2 glass by NMR

The effect of Al₂O₃ on the structure change and the phase separation in Na₂O-B₂O₃-SiO₂ glass was investigated using ¹¹B nuclear magnetic resonance (NMR), ²⁹Si MAS NMR, and ²⁷Al MAS NMR together with infrared absorption spectroscopy and field emission scanning electron microscopy (FE-SEM). The results show that the structure change from the introduction of Al₂O₃ contributes greatly to the inhibition of phase separation. First, the introduction of Al₂O₃ imparts an ionic character to the boron-oxygen network, resulting in the formation of B-O-Al-O-Si bonds and thus increases the compatibility of the silicon network with the boron-oxygen network. Second, the addition of Al₂O₃ causes the sodium ion to transfer from the boron-oxygen network to AlO₄ tetradedra, changing a number of four-coordinated borons into three-coordinated borons. As the bond energy of the four-coordinated boron is weaker than that of the three-coordinated boron, the -B-O-Si- bond with the four-coordinated boron in Na₂O-B₂O₃-SiO₂ glasses is easily broken and results in severe phase separation during heat treatment. However, the -B-O-Al- bond with the three-coordinated boron formed in Na₂O-B₂O₃-SiO₂-Al₂O₃ glasses is difficult to be broken due to the high bond energy. In addition, the silicon network in Na₂O-B₂O₃-SiO₂-Al₂O₃ glasses is also strengthened by the addition of Al₂O₃, which prevents [BO] groups from further aggregation. As a result, the tendency of the glass towards phase separation is greatly suppressed in the Na₂O-B₂O₃-SiO₂-Al₂O₃ system.     

Phase separation in tellurite glass-forming systems containing B2O3,GeO2,Fe2O3,MnO,CoO,NiO and CdO

The boundaries between the regions of single-phase and two-phase glasses were established in tellurite glass-forming systems containing B₂O₃ and one of the following oxides: GeO₂, Fe₂O₃,CoO, NiO, MnO and CdO. The character of the microstructures inside and outside the regions of stable phase separation were determined by electron microscopy. It was shown that the existing microheterogeneities may either result from incomplete liquid immiscibility during melting and supercooling or be due to typical metastable separation.     

Some phase relationships within the system Na2O/B2O3/Nb2O5

Phase behaviour over regions of the ternary system Na₂O/B₂O₃/Nb₂O₅ has been explored.Liquidus temperatures and the stability regions of primary phases have been determined over selected composition ranges by high temperature microscopy. Crystallisation processes in melts and corresponding glasses have been followed using both conventional methods of thermal analysis and newly developed micro techniques combined with hot stage microscopy.An electron microscope has been employed to follow changes in the microstructure of quenched glasses after controlled heat treatments.It has been shown that the system contains a liquid immiscibility gap, and some attention is given in the discussions to the influence that can be assigned to cations in determining the extent of such gaps and general structural relationships in borate/oxide systems.     

Transparent Ni-doped ZnO-Al2O3-SiO2 system glass-ceramics with broadband infrared luminescence

We report transparent Ni²⁺-doped ZnO-Al₂O₃-SiO₂ system glass-ceramics with broadband infrared luminescence. After heat-treatment, ZnAl₂O₄ crystallite was precipitated in the glasses, and its average size increased with increasing heat-treatment temperature. No infrared emission was detected in the as-prepared glass samples, while broadband infrared luminescence centered at 1310 nm with full width at half maximum (FWHM) of about 300 nm was observed from the glass-ceramics. The peak position of the infrared luminescence showed a blue-shift with increasing heat-treatment temperature, but a red-shift with an increase in NiO concentration. The mechanisms of the observed phenomena were discussed. These glass-ceramics are promising as materials for super broadband optical amplifier and tunable laser.     

Compositional study and properties characterisation of alkaline earth feldspar glasses and glass-ceramics

Fields in which metastable crystallisation of feldspars may occur as a result of heterogeneous nucleation were defined at 1000° C for the systems BaO-Al₂O₃-SiO₂, SrO-Al₂O₃-SiO₂, and CaO-Al₂O₃-SiO₂. The metastable fields of the above barium and calcium systems differ considerably from the respective primary liquidus fields of celsian and anorthite with the tendency for the stoichiometric compositions to be situated at the centre of the metastable region.Various physical and dielectric properties of the glasses and glass-ceramics of the three systems are presented and compared. High linear thermal expansion, high electrical resistivity, and low loss tangent and dielectric constant characterise these glass-ceramics. One of them (No. 2, table I) has, in addition, high heat resistance and translucency at high temperatures.     

The mechanism of nucleated crystallisation of glasses in lithia-alumina-silica and cordierite systems

Two principal lines of investigation are discussed. In the first of these, in which glasses were crystallised under high pressure, it was shown that the low temperature -forms of spodumene and eucryptite could be obtained. In further experiments the effects of substituting GeO₂ for SiO₂, Ga₂O₃ for Al₂O₃, MgO for Li₂O and TiO₂ for Al₂O₃ in Li₂O-Al₂O₃-SiO₂ glasses were studied. It was found possible to crystallise both and -forms of germanium and gallium containing spodumene and eucryptite. Infra-red absorption spectra, X-ray diffraction data and thermal expansion data are presented for the various crystal types. Based on the results of the investigation, it is suggested that nucleated crystallisation in the glasses may be due to the initial precipitation of a crystal phase which is isomorphous with the predominant crystal phase that is to be developed in the glass-ceramic, and that during the heat-treatment the crystals of the latter phase grow on the crystals of the primary phase.     

Fabrication of Li2O-Al2O3-SiO2 glass-ceramic ferrules by precision drawing of crystallized preforms

In this paper, we describe the fabrication of Li₂O-Al₂O₃-SiO₂ (LAS) crystallized preforms with suitable drawing formability, and the characteristics of drawn LAS glass-ceramic capillaries. It was found that addition of K₂O to the LAS glasses reduces crystallinity of the crystallized preforms and forms low-viscosity glass matrices. It was confirmed that, by using a parallel plate viscometer, such crystallized preforms show suitable viscosity for drawing below the melting temperatures of the crystalline phase. We also proved that the addition of K₂O is effective in stabilizing the preforms by preferential precipitation of stable -spodumene solid solution (s.s.), which is essential in preventing the deposition of coarse surface crystals during the drawing. The LAS glass-ceramic capillaries drawn from the crystallized preform containing 50 mass% -spodumene s.s. showed a dimensional accuracy within sub-micrometers. It was confirmed that the glass-ceramic capillaries have a low thermal expansion coefficient (25 × 10^–7/K) and excellent mechanical and chemical durability, making them promising candidate components of fiber-optic devices.