Thermal stability of lithium metasilicate produced under high pressure from lithium disilicate glass

Abstract

Crystalline lithium disilicate, Li 2 Si 2 O 5 (LS 2c ) is usually produced at atmospheric pressure by suitable heat treatment of the glass with the same stoichiometric composition (LS 2g ). At this pressure, the orthorhombic phase of lithium disilicate is thermodynamically stable. The heat treatment of LS 2g under high pressure (7.7 GPa) induces the crystallization of lithium metasilicate Li 2 SiO 3 (LS m ) phase, which remains metastable after pressure release. In this work, distinct heat treatments under high pressure were chosen to produce lithium metasilicate crystals with different grain sizes embedded in the glass matrix: smaller than 10 µm, about 100 µm and larger than 100 µm. Thermal stability of this set of samples was investigated during subsequent heat treatments at atmospheric pressure. All samples were analyzed by X-ray diffraction, Raman spectroscopy and optical microscopy. Small LS m crystals produced under high pressure remained detectable only up to 1 hour of heat treatment at 610°C at atmospheric pressure while for longer heating times, the formation of quartz and crystalline LS 2c was observed. On the other hand, for the samples containing larger LS m crystals produced under high pressure, the metasilicate phase remained detectable up to 9.5 hours of subsequent heat treatment, forming spherulitic configurations. K E Y W O R D Sglass-ceramic, high pressure, lithium disilicate, lithium metasilicate, thermal stability

Lithium disilicate glass produced at high pressure: Characterization of structural, thermal and mechanical properties

“…Irreversible changes were also observed by thermal analysis, since the crystallization temperature peak (T c ) decreased as the processing pressure increased from 2.5 to 7.7 GPa 23 . In addition, by controlling the crystallization process of LS 2 glass under HPHT in the range from 2.5 GPa to 7.7 GPa, it was observed the formation of glass‐ceramics containing grains of crystalline lithium disilicate (Li 2 Si 2 O 5 ), lithium metasilicate (Li 2 SiO 3 ), quartz (SiO 2 ), and coesite (SiO 2 ), depending on pressure and temperature conditions 12,22,24,25 . Values of H IT and E IT and the refractive index increased significantly in the glass‐ceramics produced in situ under high pressure 3,22 …”

“…Table 1 presents the experimental conditions used to produce the LS 2 glasses under high pressure. Melting times of 5 and 10 min were chosen based on previous works on densification of glasses under pressure 3,12,22‐25 as well as on the mechanical stability of the apparatus for processing at HPHT.…”

“…Table 1 presents the experimental conditions used to produce the LS 2 glasses under high pressure. Melting times of 5 and 10 min were chosen based on previous works on densification of glasses under pressure 3,12,[22][23][24][25] as well as on the mechanical stability of the apparatus for processing at HPHT. LS 2 monolithic samples (labeled B and C) and precompacted powder sample (labeled D) of cylindrical shape (2.6 mm high × 3 mm diameter) were placed inside a hexagonal boron nitride (hBN) cylinder used as a soft pressure transmitting medium as well as an electric insulator.…”

“…23 In addition, by controlling the crystallization process of LS 2 glass under HPHT in the range from 2.5 GPa to 7.7 GPa, it was observed the formation of glass-ceramics containing grains of crystalline lithium disilicate (Li 2 Si 2 O 5 ), lithium metasilicate (Li 2 SiO 3 ), quartz (SiO 2 ), and coesite (SiO 2 ), depending on pressure and temperature conditions. 12,22,24,25 Values of H IT and E IT and the refractive index increased significantly in the glass-ceramics produced in situ under high pressure. 3,22 In this work, the goal was to produce lithium disilicate glass by quenching the melt at 7.7 GPa to investigate the thermal and mechanical properties of the glass produced under high pressure.…”

“…In previous papers, Buchner et al 3,12,[22][23][24][25] have investigated the effect of high-pressure processing in the structure and properties of LS 2 produced by the conventional melt and quenching process at ambient pressure. They observed permanent changes in the short-range structural order compatible with polyamorphism after densification of LS 2 glass at 7.7 GPa during 5 min at room temperature.…”