Crystallization Process and Some Properties of Li 2 O-SiO 2 Glass-Ceramics Doped with Al 2 O 3 and K 2 O (original) (raw)
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Synthesis and properties of lithium disilicate glass-ceramics in the system SiO2–Al2O3–K2O–Li2O
Ceramics International, 2009
The purpose of this study was the synthesis of lithium disilicate glass-ceramics in the system SiO 2-Al 2 O 3-K 2 O-Li 2 O. A total of 8 compositions from three series were prepared. The starting glass compositions 1 and 2 were selected in the leucite-lithium disilicate system with leucite/lithium disilicate weight ratio of 50/50 and 25/75, respectively. Then, production of lithium disilicate glass-ceramics was attempted via solid-state reaction between Li 2 SiO 3 (which was the main crystalline phase in compositions 1 and 2) and SiO 2. In the second series of compositions, silica was added to fine glass powders of the compositions 1 and 2 (in weight ratio of 20/100 and 30/100) resulting in the modified compositions 1-20, 1-30, 2-20, and 2-30. In the third series of compositions, excess of silica, in the amount of 30 wt.% and 20 wt.% with respect to the parent compositions 1 and 2, was introduced directly into the glass batch. Specimens, sintered at 800 8C, 850 8C and 900 8C, were tested for density (Archimedes' method), Vickers hardness (H V), flexural strength (3-point bending tests), and chemical durability. Field emission scanning electron microscopy and X-ray diffraction were employed for crystalline phase analysis of the glass-ceramics. Lithium disilicate precipitated as dominant crystalline phase in the crystallized modified compositions containing colloidal silica as well as in the glass-ceramics 3 and 4 after sintering at 850 8C and 900 8C. Selfglazed effect was observed in the glass-ceramics with compositions 3 and 4, whose 3-point bending strength and microhardness values were 165.3 (25.6) MPa and 201.4 (14.0) MPa, 5.27 (0.48) GPa and 5.34 (0.40) GPa, respectively.
Journal of the European Ceramic Society, 2012
The effects of K 2 O content on sintering and crystallization of glass powder compacts in the Li 2 O-K 2 O-Al 2 O 3 -SiO 2 system were investigated. Glasses featuring SiO 2 /Li 2 O molar ratios of 2.69-3.13, far beyond the lithium disilicate (LD-Li 2 Si 2 O 5 ) stoichiometry, were produced by conventional melt-quenching technique. The sintering and crystallization behaviour of glass powders was explored using hot stage microscopy (HSM), scanning electron microscopy (SEM), differential thermal (DTA) and X-ray diffraction (XRD) analyses. Increasing K 2 O content at the expense of SiO 2 was shown to lower the temperature of maximum shrinkage, eventually resulting in early densification of the glass-powder compacts. Lithium metasilicate was the main crystalline phase formed upon heat treating the glass powders with higher amounts of K 2 O. In contrast, lithium disilicate predominantly crystallized from the compositions with lower K 2 O contents resulting in strong glass-ceramics with high chemical and electrical resistance. The total content of K 2 O should be kept below 4.63 mol% for obtaining LD-based glass-ceramics.
Materials Chemistry and Physics, 2008
The crystallization characteristics of glasses based on the Li 2 O-CaO-SiO 2 eutectic (954 ± 4 • C) system containing Al 2 O 3 , MgO and ZnO has been investigated by differential thermal analysis (DTA), X-ray diffraction analysis (XRD), and scanning electron microscopy (SEM). The partial replacement of Li 2 O by Al 2 O 3 and CaO by MgO or ZnO in the studied glass-ceramics led to the development of different crystalline phase assemblages, including lithium meta-and di-silicates, lithium calcium silicates, ␣-quartz, diopside, clinoenstatite, wollastonite, -eucryptite ss, -spodumene, ␣-tridymite, lithium zinc orthosilicate, hardystonite and willemite using various heat-treatment processes. The dilatometric thermal expansion of the glasses and their corresponding glass-ceramics were determined. A wide range of thermal expansion coefficient values were obtained for the investigated glasses and their corresponding crystalline products. The thermal expansion coefficients of the investigated glasses were decreased by Al 2 O 3 , MgO or ZnO additions. The˛-values of the investigated glasses were ranged from (+18) to (+108) × 10 −7 K −1 (25-300 • C), while those of the glass-ceramics were (+3) to (+135) × 10 −7 K −1 (25-700 • C). The chemical durability of the glass-ceramics, towards the attack of 0.1N HCl solution, was markedly improved by Al 2 O 3 with MgO replacements. The composition containing 11.5 mol% Al 2 O 3 and 6.00 mol% MgO exhibited low thermal expansion values and good chemical durability. (S.M. Salman). because of their very low thermal expansion, transparency, high chemical durability, and strength. Some of many widespread applications of such glass-ceramics are telescope mirror blanks, ring-laser gyroscopes and optically stable platforms .
Strength and Microstructure in Lithium Disilicate Glass-Ceramics
Journal of The American Ceramic Society, 1975
The effect of heat treatment on the microstructure of Li2O-Al2O3·SiO2 glass-ceramics which contain crystals of either Li2SiO3, Li2Si2O5, or both was investigated quantitatively. Strength determinations for abraded rods were correlated with heat treatment on the basis of both size and distribution of crystals and the type and amount of crystal phases present. The presence of Li2Si2O5 crystals enhanced the strength, whereas the presence of Li2SiO3 crystals did not change the strength of the abraded parent glass. The interrelation between strength and microstructure is discussed.
Journal of Thermal Analysis and Calorimetry, 2011
This article aims to shed some light on the structure and thermo-physical properties of lithium disilicate glasses in the system Li 2 O-SiO 2 -Al 2 O 3 -K 2 O. A glass with nominal composition 23Li 2 O-77SiO 2 (mol%) (labelled as L 23 S 77 ) and glasses containing Al 2 O 3 and K 2 O with SiO 2 /Li 2 O molar ratios (3.13-4.88) were produced by conventional melt-quenching technique in bulk and frit forms. The glass-ceramics (GCs) were obtained from nucleation and crystallisation of monolithic bulk glasses as well as via sintering and crystallisation of glass powder compacts. The structure of glasses as investigated by magic angle spinning-nuclear magnetic resonance (MAS-NMR) depict the role of Al 2 O 3 as glass network former with fourfold coordination, i.e., Al(IV) species while silicon exists predominantly as a mixture of Q 3 and Q 4 (Si) structural units. The qualitative as well as quantitative crystalline phase evolution in glasses was followed by differential thermal analysis (DTA), X-ray diffraction (XRD) adjoined with Rietveld-reference intensity ratio (R.I.R.) method, Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The possible correlation amongst structural features of glasses, phase composition and thermo-physical properties of GCs has been discussed.
Ceramics International, 2014
The role of each oxide component on the structure of molten glass and on their densification and crystallization behaviours is expected to be more easily assessed for compositions comprising a small number of components. In the present work, binary (Li 2 O-SiO 2 ), ternary (Li 2 O-Al 2 O 3 -SiO 2 ), and quaternary (Li 2 O-K 2 O-Al 2 O 3 -SiO 2 ) glass compositions were selected. The aim was to investigate the relationships between the compositions and structure of the glasses, namely, the effects of each component on the extent of liquid-in-liquid phase separation, their thermal behaviour and the phase assemblage in the disilicate glass-ceramics. The distribution of structural units in the experimental glasses was assessed using 29 Si MAS-NMR spectroscopy. Adding Al 2 O 3 played a dual role as network former and modifier when added to the binary system by enhancing Q 2 and diminishing Q 3 structural units. The processing window for sintering, the difference between the onset of crystallisation and glass-transition temperature (T c −T g ), was too narrow for the less polymerised network structures (binary and ternary systems), hindering the densification of the corresponding glass powder compacts. Oppositly, compositions in the quaternary system featured excellent densification behaviour coupled with high mechanical strength. These features derive from a more rigid glass network comprising four coordinated (AlO 4/2 ) − units and K + cations in its vicinity.
Al2O3/K2O-containing non-stoichiometric lithium disilicate-based glasses
Journal of Thermal Analysis and Calorimetry, 2013
The crystallisation kinetics of experimental glasses in 3 different systems: (were studied under non-isothermal conditions. The DTA results revealed a stronger tendency to crystallisation of binary compositions in comparison to the ternary and quaternary compositions comprising Al 2 O 3 and K 2 O which present the lower crystallisation, i.e. the crystallisation propensity follows the trend A [ B [ C. The devitrification process in the Li 2 O-SiO 2 and Li 2 O-Al 2 O 3 -SiO 2 systems began earlier and the rate was higher in comparison to that of glasses in the quaternary Li 2 O-K 2 O-Al 2 O 3 -SiO 2 system. Thus, addition of Al 2 O 3 and K 2 O to glasses of Li 2 O-SiO 2 system was demonstrated to promote glass stability against crystallisation. However, the activation energy for crystallisation was shown to depend also on the SiO 2 /Li 2 O ratio with the binary system showing a decreasing trend with increasing SiO 2 / Li 2 O ratio, while the opposite tendency was being observed for compositions with added Al 2 O 3 and K 2 O.
Journal of Materials Science, 1992
The nucleation and crystallization processes of Li20-AI203-SiO 2 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 4-20 kJ mo1-1. The influence of nucleating agents (Ti02, Zr02) 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.
Journal of Materials Science, 2013
The aim of this study was to investigate the effects of different nucleating agents (P 2 O 5 , TiO 2 and ZrO 2 ) on the crystallization behaviour and the properties of a parent glass with composition 23.7 Li 2 O-2.63 K 2 O-2.63 Al 2 O 3 -71.78 SiO 2 (mol%) and SiO 2 /Li 2 O molar ratio far beyond that of stoichiometric lithium disilicate (LD, Li 2 Si 2 O 5 ). The scanning electron microscopy examination of the as-cast non-annealed glasses revealed the occurrence of liquidliquid phase separation for all the compositions. P 2 O 5 revealed to be effective in promoting bulk crystallization of LD, while TiO 2 and ZrO 2 led to surface crystallization. Moreover, ZrO 2 enhances the glass polymerization and shifts T p to higher temperatures, hindering crystallization. At 900°C, TiO 2 -containing glasses feature LD and lithium metasilicate (LMS, Li 2 SiO 3 ), while P 2 O 5 -and ZrO 2 -containing ones present monophasic LD and LMS glassceramics, respectively.
Effect of K2O on structure–property relationships and phase transformations in Li2O–SiO2 glasses
Journal of the European Ceramic Society, 2012
Glass compositions with formula (71.78 − x)SiO 2 -2.63Al 2 O 3 -(2.63 + x)K 2 O-23.7Li 2 O (mol.%, x = 0-10) and SiO 2 /Li 2 O molar ratios far beyond that of stoichiometric lithium disilicate (Li 2 Si 2 O 5 ) were prepared by conventional melt-quenching technique to investigate the influence of K 2 O content on structural transformations and devitrification behaviour of glasses in the Li 2 O-SiO 2 system. The scanning electron microscopy (SEM) examination of as cast non-annealed glasses revealed the presence of nanosized droplets in glassy matrices suggesting occurrence of liquid-liquid phase separation. An overall trend towards depolymerization of the silicate glass network with increasing K 2 O content was demonstrated by employing magic angle spinning-nuclear magnetic resonance (MAS-NMR) spectroscopy. The distribution of structural units in the experimental glasses was estimated using 29 Si MAS-NMR spectroscopy suggesting the appearance of Q 2 , enhancement of Q 3 and diminishing of Q 4 groups with increasing K 2 O contents. X-ray diffraction (XRD) and differential thermal analysis (DTA) were used to assess the influence of K 2 O on devitrification process and formation of lithium disilicate (Li 2 Si 2 O 5 ) and/or lithium metasilicate (Li 2 SiO 3 ) crystalline phases. (H.R. Fernandes). GC manufacture, 4 the GCs derived from this parent binary system exhibit some unfavourable characteristics in terms of their mechanical and chemical properties which hinder their potential applications in several technological areas.