Crystallization Kinetics and Mechanism of Low‐Expansion Lithium Aluminosilicate Glass‐Ceramics by Dilatometry (original) (raw)
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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.
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 .
European Journal of Glass Science and Technology Part a Glass Technology
In this work we propose an ion exchange (IE) treatment to minimize surface crystallization and thus improve the sintering of compacts made of glass particles. This concept was tested by subjecting glass powders of the LiO₂–Al2O₃–SiO₂ (LAS) system to molten KNO₃ IE treatment aimed at developing a low thermal expansion glass-ceramic via sinter–crystallisation. The relative densities of treated and untreated glass powder compacts sintered at different temperatures were compared and suitable IE and sintering temperatures were selected. Optical microscopy, x-ray diffraction, dilatometry and scanning electron microscopy were used to characterise the sintered samples. The proposed IE decreased the optimum sintering temperature from 1100 to 1000°C and yet allowed the formation of virgilite, a desirable β-quartz solid solution having a negative thermal expansion coefficient. The compositional changes in surface of the glass particles caused by IE diminished the crystallisation tendency, whic...
Journal of The American Ceramic Society, 2008
We report on the role of Al2O3 and K2O on crystallization in glasses featuring a SiO2/Li2O ratio (3.13 to 4.88) far beyond that of lithium disilicate (LD, Li2Si2O5) stoichiometry. Glasses in both bulk and frit form were produced by the conventional melt-quenching technique. Scanning electron microscopy analysis revealed surface nucleation as the dominant crystallization mechanism in glass–ceramics (GCs) derived from bulk glasses richer in Al2O3 and K2O in the temperature range 800°–900°C and dendritic skeletal surface growth of lithium metasilicate crystalline phase (LS,Li2SiO3). The glasses with lower amounts of Al2O3 and K2O showed an intermediate type of crystallization mechanism (simultaneous surface and volume nucleation) resulting in the preferential formation of Li2Si2O5. The formation of LD GCs by sintering and the crystallization of glass-powder frits seems to occur via the precursor phase of LS, resulting in high-strength materials.
Crystallization pathways and some properties of lithium disilicate oxynitride glasses
Ceramics International
Lithium silicates have been used as model glasses for scientific and technological studies of glass-ceramics because they easily crystallize in the interiors, even without the addition of any nucleating agent. On the other hand, partial replacement of oxygens with nitrogens affects most properties of oxide glasses, but its effect on the crystallization kinetics of the glasses has been poorly documented. In this work, we report, for the first time, on the crystallization kinetics of nitrited lithium silicate glasses. The oxynitride glasses were prepared by partial substitution of oxygen by nitrogen, up to 6 at.% N/(N+O), by melt-quenching the liquid under N 2 atmosphere inside a glove box. As expected, the density, microhardness, and Young's modulus of the glasses improved with increasing nitrogen content. Higher values of glass transition and crystallization peak temperatures were also obtained with an increase in the nitrogen content. Rietveld refinement analysis after adequate thermal treatment revealed that addition of nitrogen led to increasingly higher contents of lithium metasilicate at the expense of the (expected) lithium disilicate crystal phase. Crystallization kinetic parameters such as the activation energy and 2 Avrami index were calculated using Ozawa's equations. These two parameters also increased with increasing nitrogen content, whereas the crystal growth rates decreased with increasing nitrogen content. The above-described changes in the properties of the oxynitride glasses are straightforwardly explained by the increase in the connectivity of the glass network, which results in enhancement of the atomic packing density owing to partial substitution of twocoordinated oxygens by three-coordinated nitrogens.
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.
Mechanism and kinetics of glass-ceramics formation in the LiO2-SiO2-CaO-P2O5-CaF2 system
Central European Journal of Chemistry, 2009
Two glasses based on lithium disilicate (LS2), with and without fluorapatite (FA), were synthesised in the Li2O-SiO2-CaO-P2O5-CaF2 system with P2O5: CaO: CaF2 ratios corresponding to fluorapatite. Glass-ceramics have then been prepared by thermal treatment. The mechanism and kinetics of crystallization as functions of grain size and rate of heating were investigated using thermal analysis methods. The smaller particles crystallize preferentially by surface crystallization, which is replaced by volume crystallization at larger particle sizes. Inclusion of FA in the LS2 favours crystallization through the surface mechanism. The onset limit for volume crystallization replacing the surface mechanism is at about 0.3 mm for pure LS2 glass and 0.9 mm for glass containing FA. The calculated activation energies of the glasses (299 ± 1 kJ mol-1 for pure LS2 glass and 288 ± 7 kJ mol−1 for glass containing FA according to Kissinger, or 313 ± 1 kJ mol-1 for pure LS2 glass and 303 ± 8 kJ mol-1 fo...
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.
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.