Lithium Metasilicate Crystals Research Articles

Exploring the effect of facile i-line photolithography on the structure and physicochemical properties of photosensitive glass ceramics

Photosensitive glass-ceramics exhibit significant potential to replace silicon materials in the microfabrication of micro-electro-mechanical-system (MEMS) devices. However, they are constrained by the limitations of a conventional 320-nm photolithography process. Therefore, developing an ideal industrial i-line (365 nm) photolithography for Li2O–Al2O3–SiO2 photosensitive glass-ceramics is urgently needed. This study initially explores the impact of a facile i-line photolithography on the structure and physicochemical properties of photosensitive glass ceramics. We experimentally showcase a direct, scalable, and straightforward i-line photolithography technique for Li2O–Al2O3–SiO2 photosensitive glass ceramics. Our research concentrates on how the exposure time affects the properties of Li2O–Al2O3–SiO2 photosensitive glass ceramics, particularly regarding nucleation and crystallization. This is achieved by adjusting the exposure time parameter and utilizing XRD, etching experiments, SEM, TEM, and other tests under a i-line light source. By optimizing the exposure process parameters, we also modify the annealing process parameters affecting the crystallization of lithium metasilicate in the exposed areas. Under a treatment process involving an exposure time of 20 min, a nucleation temperature of 500 °C, a crystallization temperature of 630 °C, and a nucleation/crystallization duration of 2 h, the sample achieves the highest crystal quantity and the optimal thickness-etching rate ratio.

Edge chipping damage in lithium silicate glass-ceramics induced by conventional and ultrasonic vibration-assisted diamond machining.

Diamond machining of lithium silicate glass-ceramics (LS) induces extensive edge chipping damage, detrimentally affecting LS restoration functionality and long-term performance. This study approached novel ultrasonic vibration-assisted machining of pre-crystallized and crystallized LS materials to investigate induced edge chipping damage in comparison with conventional machining. The vibration-assisted diamond machining was conducted using a five-axis ultrasonic high-speed grinding/machining machine at different vibration amplitudes while conventional machining was performed using the same machine without vibration assistance. LS microstructural characterization and phase development were performed using scanning electron microscopy (SEM) and x-ray diffraction (XRD) techniques. Machining-induced edge chipping depths, areas and morphology were also characterized using the SEM and Java-based imaging software. All machining-induced edge chipping damages resulted from brittle fractures. The damage scales, however, depended on the material microstructures; mechanical properties associated with the fracture toughness, critical strain energy release rates, brittleness indices, and machinability indices; and ultrasonic vibration amplitudes. Pre-crystallized LS with more glass matrix and lithium metasilicate crystals yielded respective 1.8 and 1.6 times greater damage depths and specific damage areas than crystallized LS with less glass matrix and tri-crystal phases in conventional machining. Ultrasonic machining at optimized amplitudes diminished such damages by over 50 % in pre-crystallized LS and up to 13 % in crystallized LS. This research highlights that ultrasonic vibration assistance at optimized conditions may advance current dental CAD/CAM machining techniques by significant suppression of edge chipping damage in pre-crystallized LS.

Failure mechanisms in in-situ SEM micropillar compressions of pre-crystallized and crystallized zirconia-containing lithium silicate glass-ceramics

Microscale microstructure-property relations for zirconia-containing lithium silicate glass-ceramics (ZLS) are critical for their applications as load-bearing restorations. This study investigated in-situ scanning electron microscopy (SEM) micropillar compression responses of pre-crystallized and crystallized ZLS materials with a flat diamond punch. Micromechanical properties (moduli, strength, ductility, resilience and toughness) of the two materials were determined. Pre-crystallized and crystallized ZLS materials showed distinct ductile-brittle failure mechanisms. Pre-crystallized ZLS containing weaker lithium metasilicate crystals and higher concentrated glass matrix revealed its ductile mode featured with bending and delamination, and side-splitting fractures originated from fragmentation and microcracking. Crystallized ZLS with stronger lithium disilicate crystals and lower concentrated glass matrix demonstrated bending and shear-band initiation, shear fracture from shear-band expansion and fragmentation. Further, crystallized ZLS yielded much higher moduli, strength, ductility, resilience and toughness than pre-crystallized ZLS. This study provides vital insights into the basic understanding of micromechanical behavior of ZLS materials for load-bearing applications.

Overview of Lithium Disilicate as a restorative material in dentistry

Lithium disilicate was first introduced to the dental field as an indirect restorative material in 1998. It was marketed under the name IPS Empress 2, and was intended for use with press technology. It was later replaced by modified versions including IPS e.max® Press and IPS e.max® CAD. Newer versions have since emerged,namely Amber Mill GC Initial and CEREC Tesseratwo.The latter has part crystal composition of lithium disilicate,embedded in a glassy zirconia matrix. The CAD versionis provided in a meta-silicate state, characterised by40% platelet-shaped lithium meta-silicate crystals and aglassy matrix that is bluish in colour. To obtain the desired lithium disilicate structure and tooth shades, a process ofcrystallization is required. This involves firing at 840 °C,for 25 minutes. The resulting glass-ceramic material has the benefit of providing maximum aesthetic translucency along with good fracture resistance of about 2MPa, and mechanical strength of 360MPa. Developments in the all-ceramic dental materials have led to improvements in their physical properties and aesthetic appeal, leading to a substantial increase in their clinical use. This paper present a review of lithium disilicate with particular reference to its chemical composition, aesthetic versatility, and durability for use in crowns, veneers, and implant retained restorations. It also covers the recommended techniques prescribed to ensure predictable bonding and cementation. An electronic literature search on the use of lithium disilicate in dentistry was carried out using EBSCOhost search engine. This included all papers relating to its use for conventional veneers, crowns and bridge work, for CAD/CAM restorations, dentine bonding procedures and luting agents. It covered all papers published in peer reviewed journals from 1988 to 2021. The review indicates that lithium disilicate can be a useful and versatile material in dentistry providing it is handled correctly and the recommended tooth and restoration surface preparations and bonding procedures are carried out. The latter involves tooth etching and silane treatment of the fitting surfaces of restorations prior to cementation to improve adhesion and fracture resistance.

Effect of preheating and isothermal holding time on the crystallization, densification and properties of a sintered lithium silicate glass-ceramic

The objective of this work was to study the effect of different heating-up procedures and isothermal treatment times on crystallization, densification and properties of a sintered non-stoichiometric lithium silicate glass-ceramic. Glass samples containing SiO 2 , Li 2 O and K 2 O as main constituents, besides minor amounts of other oxides such as Al 2 O 3 , P 2 O 5 and ZrO 2 , were melted at 1500 °C - 60 min and subsequently fragmented (<25 μm) into a fine powder. The obtained glass particles were characterized by scanning electron microscopy SEM, X-ray diffraction, XRD and differential thermal analysis, DSC. The thermal analysis indicated the presence of two exothermal peaks, the first at approximately 640 °C, characteristic of the crystallization of lithium metasilicate, Li 2 SiO 3 , and the second close to 820 °C, corresponding to the crystallization of lithium dissilicate, Li 2 Si 2 O 5 . In order to study the crystallization behavior the glass powder was uniaxially compacted and investigated by dilatometry. The samples were sintered under different conditions: Group 1: sintering at 840 °C with isothermal holding times of 5, 15, 60 and 180 min; Group 2: preheating to 660 °C with an isothermal plateau for 720 min and further heating up to 840 °C with identical holding times of 5, 15, 60 and 180 min. The sintered glass-ceramics were characterized by XRD, Rietveld refinement, SEM, relative density, hardness, fracture toughness and flexural strength. The results showed that the samples of group 1, directly heated up to 840 °C, achieved a relative density ranging between 60 and 70% and a hardness and fracture toughness lower than the second group that was subjected to a differentiated preheating process. The sintered samples for less than 60 min, presented Li 2 SiO 3 and Li 2 Si 2 O 5 as crystal phases and a microstructure formed by low aspect ratio crystals, besides apparent porosity. The samples sintered at 840 °C for 180 min, independently of the heating up procedure, presented only Li 2 Si 2 O 5 as crystal phase with elongated grains with an aspect ratio higher than 2 and relative densities in the order of 84 to 91%. The hardness ranged in the order of 5.6 to 6 GPa and the average fracture toughness was 1.5 MPa m 1/2 . The materials subjected to the preheating process and treated at 840 °C for 180 min exhibited a flexural strength of 160 MPa, while those directly heated to 840 °C and treated for 180 min showed a flexural strength of only 75 MPa.

Effect of K2O:Al2O3 Ratio on Crystallization Phenomena and Microstructure of Lithium Silicate Glass-Ceramics

K2O plays an important role as glass modifier in glass systems. In the present study,the effect of K2O:Al2O3 ratio on crystallization and microstructural evolution in Li2O-SiO2- K2O-MgO-Al2O3-ZnO-ZrO2-P2O5 glass system has been reported. K2O mol% has been varied from 1 to 3 mol% while keeping Al2O3 at 2 mol%. The glass structure was analyzed by MAS-NMR spectroscopy. Heat treated glass systems were studied by using XRD, SEM and DTA to analyse the sintering and crystallization behavior. Increased amount of K2O modified the role of Al2O3, which acted as glass network former and formed [AlO4/2] tetrahedron unit, which eventually polymerized the glass structure. Consequently, lithium disilicate (LS2) crystallization was suppressed and crystallization of lithium metasilicate (LS) was promoted. As a result, while in the sample with 1 mol% K2O LS2 appeared as the main crystalline phase and LS as minor phase, 3 mol% K2O samples did not show any lithium disilicate throughout the sintering temperature range. The K2O:Al2O3 ratio did not have a significant impact on densification of these glass-ceramics.

Crystallization of the glasses within the SiO2-Li2O-TiO2 system

Glasses from the ternary system SiO2-Li2O-TiO2 with increasing titania content replacing silica were prepared by melting-annealing technique. Structural FTIR, thermal and differential thermal analysis (DTA) properties of the glasses were measured and evaluated. X-ray diffraction and scanning electron microscope investigations were done to identify the crystalline phases formed together with revealing their morphological features. The coefficient of thermal expansion (CTE) data of glass-ceramic samples are identified to be between 80.83 × 10−7 °C−1 and 120.04 × 10−7 °C−1 (23–500 °C). The increase of the CTE is assumed to be concomitant with the increase of the crystallization of lithium metasilicate (Li2SiO3) phase or quartz (SiO2). Titania is assumed to behave primarily partly as a modifier oxide and at high titania content; it is assumed to act as former (TiO4) groups. X-ray diffraction patterns indicate that the pure lithium silicate glass produces lithium disilicate (LS2) and quartz (SiO2) as crystalline phases and with increasing TiO2, first it acts as a nucleating agent initiating the formation of different crystalline silica phases beside (LS2) then titania is separated at 15 %TiO2 and 30%TiO2 as lithium titanate. The microhardness and densities values are calculated. The studied samples show promising thermal and mechanical properties especially for both CTE and microhardness values, beside their chemical stability which could be applied for commercial uses.

New, tough and strong lithium metasilicate dental glass-ceramic

Glass-ceramics have found widespread use in dentistry due to their favorable properties: biocompatibility, chemical inertness, high fracture strength and toughness, superior esthetics, color stability, and translucence. The objective of this work was to develop a new tough, strong and machinable glass-ceramic based on the lithium metasilicate (LS) crystal phase. We designed a glass composition aimed to yield LS crystals after proper treatment. We melted, casted, and crystallized the glass for a favorable microstructure. We then characterized its microstructure and relevant mechanical, optical and chemical properties with several experimental tools. We also measured the residual stresses. This newly developed glass-ceramic shows a house-of-cards microstructure composed of 50% vol. plate-like LS crystals of 5–25 μm, randomly dispersed in a glassy matrix. Lithium disilicate (12% vol.), two minor crystal phases, and 34% vol. residual glass are also present. The average fracture toughness measured by the double torsion technique is 3.5 ± 0.5 MPa m1/2. The average fracture strength, evaluated by the ball-on-three balls (B3B) technique, is 450 ± 40 MPa. The elastic modulus, determined by nanoindentation, is 124 ± 2 GPa, the linear thermal expansion coefficient is 13.6 × 10−6 °C-1, and the solubility in 4%vol. acetic acid is 215 ± 30 μg/cm2, which is below the limit established by the ISO 6872 standard for some applications, but improper for uncoated use. These properties could still be optimized. The improved toughness, strength, and reasonable machinability indicate that, after optimization, this glass-ceramic could be a very promising candidate for dental restorative applications.

Effects of heat treatment on the crystallization behavior, microstructure and thermal properties of a complex Li2O-SiO2 glass system

In this paper, glass-ceramics in the SiO2-Li2O-MgO-Al2O3-Rb2O-P2O5 system were fabricated by the two- and three-stage heat treatment at different temperatures and holding times. The effects of heat treatment on the crystallization behavior, microstructure and thermal properties of the glass-ceramics were investigated in detail by DSC, XRD, Raman spectroscopy, SEM and CTE. The results suggest that the lithium metasilicate (LM) phase with a lower crystallization activation energy precipitated from the parent glass before the lithium disilicate (LD) phase. Additionally, only LM crystals with the spherical nano-grains were precipitated after two-stage heat treatment. After three-stage heat treatment, LD was the primary crystalline phase with the minor phases of lithium phosphate, coesite and LM. Raman spectroscopy shows that Q2 and Q3 tetrahedra were the primary microstructural units of LM and LD glass-ceramics, respectively. With increasing temperature and prolonging holding time, LD crystals evolved from the uniform submicron equiaxed grains to the coarse rod-like grains. LD glass-ceramics have a lower CTE (9.51~9.70 × 10-6K-1) and higher softening point (730 ~ 760 °C) than LM glass-ceramics and the parent glass.

Phase and microstructural evolution in lithium silicate glass‐ceramics with externally added nucleating agent

AbstractDevelopment of lithium disilicate‐based glass‐ceramics critically depends on use of nucleating agent in the glass matrix. The present study reports the effect of externally added nucleating agent Li3PO4 in Li2O–K2O–MgO–ZnO–ZrO2–Al2O3–SiO2 system which is compared with a reference composition (GC1) (SiO2:Li2O = 2.16:1) prepared with in situ formed Li3PO4. For externally added Li3PO4, two compositions were studied. In one case (GC2) before addition of Li3PO4, SiO2:Li2O ratio in glass was maintained as 2.87:1 and in another case (GC3) SiO2:Li2O ratio in glass was maintained same as reference GC1 that is, 2.16:1. The glasses were characterized by using MAS‐NMR spectroscopy. Sintering and crystallization behavior of the glass‐ceramics was characterized by using XRD, SEM, DTA. Due to in situ formation of Li3PO4, GC1 resulted in a dense sample with finer crystals of lithium disilicate. In GC2 and GC3, externally added lithium phosphate, which was in the form of ultrafine aggregated particles, formed flower‐like colonies of radially outward crystals. Higher SiO2:Li2O ratio in GC2 resulted in lithium disilicate crystals and high viscous glass causing large air entrapment and so less densification. GC3 with higher lithia in glass showed higher densification than GC2 but only lithium metasilicate crystals were formed.

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

AbstractCrystalline lithium disilicate, Li2Si2O5 (LS2c) is usually produced at atmospheric pressure by suitable heat treatment of the glass with the same stoichiometric composition (LS2g). At this pressure, the orthorhombic phase of lithium disilicate is thermodynamically stable. The heat treatment of LS2g under high pressure (7.7 GPa) induces the crystallization of lithium metasilicate Li2SiO3 (LSm) 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 LSm 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 LS2c was observed. On the other hand, for the samples containing larger LSm crystals produced under high pressure, the metasilicate phase remained detectable up to 9.5 hours of subsequent heat treatment, forming spherulitic configurations.

Crystallization of lithium disilicate-based multicomponent glasses – effect of silica/lithia ratio

Two glass compositions were prepared from the system SiO2-Li2O-K2O-ZrO2-P2O5 with different SiO2/Li2O ratio (2.39 and 3.39) and the crystallization behavior was investigated by differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The crystallization kinetic parameters (activation energy of crystallization and Avrami exponent) were evaluated by different methods from the data obtained by DTA performed at different heating rates. For both glasses, two exothermic peaks were observed in the DTA curves, and the crystallization peak temperatures increased with SiO2/Li2O ratio. XRD analysis revealed that the first peak corresponds to the crystallization of lithium metasilicate (Li2SiO3) and the second to the formation of lithium disilicate (Li2Si2O5). After heating the glasses at a temperature above the second crystallization peak (900°C), both Li2Si2O5 and Li2SiO3 were found in samples having the lowest SiO2/Li2O ratio, whereas no Li2SiO3 was detected in samples with the highest SiO2/Li2O ratio. For both glasses, the value obtained by different methods for the activation energy of crystallization was in the range of 225–275kJmol−1 for the first exothermic peak and in the range of 425–500kJmol−1 for the second peak. The estimated Avrami exponent was close to 1 for the first exothermic peak, indicating surface crystallization, and close to 3 for the second exothermic peak, suggesting volume crystallization. This was confirmed by the morphological study made by SEM that showed needle-like crystals in the microstructure of samples with lithium metasilicate and granular crystals in the microstructure of samples having lithium disilicate.

Non-stoichiometric crystallization of lithium metasilicate–calcium metasilicate glasses. Part 2 — Effect of the residual liquid

Crystallization of non-stoichiometric glasses of the Li2O·SiO2–CaO·SiO2 joint (which has a simple eutectic) at deep undercooling proceeds in two stages. In the first stage, lithium metasilicate (LS) crystals are formed. In the second stage, after a considerable delay, it is supplemented by the evolution of a calcium metasilicate (CS) phase. The analysis of the evolution of the residual liquid composition and its effect on the crystallization process revealed a novel phenomenon. The formation of LS crystals in the first stage of the process leads to the establishment of a temporary equilibrium between the residual liquid and the LS-crystal phase. As a result, LS crystal growth is temporarily arrested. This termination of LS crystal growth occurs when the composition of the residual liquid reaches that of the metastable liquidus for LS at given temperature and pressure. The growth of LS crystals is resumed only in the second stage of the process when the CS crystals form. The evolution of the CS-crystal phase changes the composition of the residual liquid and shifts it away from the composition corresponding to the metastable liquidus. As the result, the LS crystals again become capable of further growth. The above crystallization pathway is expected to be a general phenomenon in phase formation in multi-component systems with different mobilities of the components determining the kinetics of formation of the different phases.

Nucleation Kinetics of Lithium Metasilicate in ZrO2‐Bearing Lithium Disilicate Glasses for Dental Application

The effect of ZrO2 on the nucleation of lithium metasilicate crystals in lithium disilicate‐based multicomponent glasses of composition in wt% x ZrO2 + (1 − (x/100)) (73.82 SiO2 + 3.99 K2O + 15.33 Li2O + 3.51 Al2O3 + 3.35 P2O5) with x = 0, 3, 4, 6, 8, and 12 was studied by combining the results from differential scanning calorimetry (DSC), micropenetration viscometry (MPV), scanning electron microscopy (SEM), and X‐ray diffraction (XRD). Peak shift of the first exotherm of DSC upscans indicated an increase in the temperature of maximum nucleation from 793 K (x = 0) to 873 K (x = 12), while number densities of relict structures of SEM images after nucleation at these temperatures and etching with HF led to an increase in the maximum nucleation rate from 2 × 1017/m3s (x = 0) to 6 × 1018/m3s (x = 12). MPV showed that viscosity increased with the ZrO2 content but nucleation was fastest close to 109.7 Pa s for all glasses studied. XRD revealed lithium metasilicate crystals within the etched volumes of approximately one order of magnitude smaller sizes compared to SEM relict structures, which were assumed to result from early‐stage phase separation and heterogeneous nucleation processes.

Nonstoichiometric crystallization of lithium metasilicate–calcium metasilicate glasses. Part 1 — Crystal nucleation and growth rates

The initial stages of the crystallization kinetics of lithium metasilicate (LS) in glasses of the Li2O·SiO2–CaO·SiO2 join – which has a simple eutectic – was investigated at high undercoolings, somewhat above the glass transition temperatures. Calcium metasilicate crystals precipitate and grow only in the advanced stages of crystallization. The variation of glass composition from the eutectic (26.5mol% Li2O) towards lithium metasilicate (50mol% Li2O) results in a sharp increase of the internal nucleation rate of LS crystals, whereas the growth rates increase only weakly. This strong increase of the nucleation rate is primarily caused by a decrease of the thermodynamic barrier for nucleation – due to an increase of the thermodynamic driving force for crystallization and a decrease of the nucleus/liquid interfacial energy – as the glass composition approaches the crystal composition.

Synthesis of monodispersed lithium silicate particles using the sol–gel method

Lithium silicate particles were prepared by the sol–gel process based on the Stober method using tetraethoxysilane and lithium ethoxide as starting materials; lithium dodecyl sulfate (LDS) was used as a surfactant. Lithium ion concentration of the obtained particles increased with an increase of Li/Si ratios from 1 to 4. Scanning electron microscope images showed that the obtained particles were rather monodispersed with diameter of 100–300 nm, and the particle size was not influenced by the amount of added LDS but the Li/Si ratios. Fourier-transform infrared spectra of the particles showed that the intensity of the peaks due to CO32− increased with an increase of the Li/Si ratios. X-ray diffraction patterns and 29Si magic-angle spinning-nuclear magnetic resonance spectra of the particles indicated that Q3 and Q2 units were present as amorphous state in the particles prepared with Li/Si ratios of 1 and 2, respectively. In the case of Li/Si ratios of more than 3, lithium metasilicate crystals formed, and Q1 and Q2 units were dominant.

Real time neutron diffraction and NMR of the Empress II glass-ceramic system

Objective. This study reports real time neutron diffraction on the Empress II glass-ceramic system. Methods . The commercial glass-ceramics was characterized by real time neutron diffraction, 31P and 29Si solid-state MAS-NMR, DSC and XRD. Results. On heating, the as-received glass ceramic contained lithium disilicate (Li 2Si 2O 5), which melted with increasing temperature. This was revealed by neutron diffraction which showed the Bragg peaks for this phase had disappeared by 958 °C in agreement with thermal analysis. On cooling lithium metasilicate (Li 2SiO 3) started to form at around 916 °C and a minor phase of cristobalite at around 852 °C. The unit cell volume of both Li-silicate phases increased linearly with temperature at a rate of +17 × 10 −3 Å 3.°C −1. Room temperature powder X-ray diffraction (XRD) of the material after cooling confirms presence of the lithium metasilicate and cristobalite as the main phases and shows, in addition, small amount of lithium disilicate and orthophosphate. 31P MAS-NMR reveals presence of the lithiorthophosphate (Li 3PO 4) before and after heat treatment. The melting of lithium disilicate on heating and crystallisation of lithium metasilicate on cooling agree with endothermic and exotermic features respectively observed by DSC. 29Si MAS-NMR shows presence of lithium disilicate phase in the as-received glass-ceramic, though not in the major proportion, and lithium metasilicate in the material after heat treatment. Both phases have significantly long T 1 relaxation time, especially the lithium metasilicate, therefore, a quantitative analysis of the 29Si MAS-NMR spectra was not attempted. Significance. The findings of the present work demonstrate importance of the commercially designed processing parameters in order to preserve desired characteristics of the material. Processing the Empress II at a rate slower than recommended 60 °C min −1 or long isothermal hold at the maximal processing temperature 920 °C can cause crystallization of lithium metasilicate and cristobalite instead of lithium disilicate as major phase.

Effect of Al 2O 3 and K 2O content on structure, properties and devitrification of glasses in the Li 2O–SiO 2 system

The effect of Al 2O 3 and K 2O content on structure, sintering and devitrification behaviour of glasses in the Li 2O–SiO 2 system along with the properties of the resultant glass–ceramics (GCs) was investigated. Glasses containing Al 2O 3 and K 2O and featuring SiO 2/Li 2O molar ratios (3.13–4.88) far beyond that of lithium disilicate (Li 2Si 2O 5) stoichiometry were produced by conventional melt-quenching technique along with a bicomponent glass with a composition 23Li 2O–77SiO 2 (mol.%) (L 23S 77). The GCs were produced through two different methods: (a) nucleation and crystallization of monolithic bulk glass, (b) sintering and crystallization of glass powder compacts. Scanning electron microscopy (SEM) examination of as cast non-annealed monolithic glasses revealed precipitation of nanosize droplet phase in glassy matrices suggesting the occurrence of phase separation in all investigated compositions. The extent of segregation, as judged from the mean droplet diameter and the packing density of droplet phase, decreased with increasing Al 2O 3 and K 2O content in the glasses. The crystallization of glasses richer in Al 2O 3 and K 2O was dominated by surface nucleation leading to crystallization of lithium metasilicate (Li 2SiO 3) within the temperature range of 550–900 °C. On the other hand, the glass with lowest amount of Al 2O 3 and K 2O and glass L 23S 77 were prone to volume nucleation and crystallization, resulting in formation of Li 2Si 2O 5 within the temperature interval of 650–800 °C. Sintering and crystallization behaviour of glass powders was followed by hot stage microscopy (HSM) and differential thermal analysis (DTA), respectively. GCs from composition L 23S 77 demonstrated high fragility along with low flexural strength and density. The addition of Al 2O 3 and K 2O to Li 2O–SiO 2 system resulted in improved densification and mechanical strength.

Electrical conductivity of gamma-irradiated V 2O 5 doped lithium disilicate glasses doped and their glass–ceramics derivatives

Some physical properties of the lithium disilicate (Li 2Si 2O 5) glasses doped with different ratios of V 2O 5 were investigated before and after gamma-rays irradiation. Increasing V 2O 5 ratio causes remarkable changes in the properties studied. The observed variations in the properties may be correlated with the changes in internal glass network with changes in the chemical composition. Vanadium ions are believed to be present in three possible valence states V 3+, V 4+ and V 5+, and the ratios of these states depend on glass composition. Observed increase in electrical conductivity is assumed to be related to several parameters including the creation of localized state, which increase the charges carrier to conduction band, increase of the number of nonbridging oxygen’s and/ or electron hopping between vanadate ions in different valence states. However, the results indicate that the heat-treatment of amorphous samples accelerate the sequence of formation of lithium metasilicate crystals, causing a decrease in the electrical conductivity. The changes obtained due to gamma-irradiation are correlated to several factors, such as polarization and field strength of the respective cations and to the amount of induced defect centers created upon gamma-irradiation.

Examination of the laser-induced variations in the chemical etch rate of a photosensitive glass ceramic

Previous studies in our laboratory have reported that the chemical etch rate of a commercial photosensitive glass ceramic (FoturanTM, Schott Corp., Germany) in dilute hydrofluoric acid is strongly dependent on the incident laser irradiance during patterning at λ=266 nm and λ=355 nm. To help elucidate the underlying chemical and physical processes associated with the laser-induced variations in the chemical etch rate, several complimentary techniques were employed at various stages of the UV laser exposure and thermal treatment. X-ray diffraction (XRD) was used to identify the crystalline phases that are formed in Foturan following laser irradiation and annealing, and monitor the crystalline content as a function of laser irradiance at λ=266 nm and λ=355 nm. The XRD results indicate the nucleation of lithium metasilicate (Li2SiO3) crystals as the exclusive phase following laser irradiation and thermal treatment at temperatures not exceeding 605 °C. The XRD studies also show that the Li2SiO3 density increases with increasing laser irradiance and saturates at high laser irradiance. For our thermal treatment protocol, the average Li2SiO3 crystal diameters are 117.0±10.0 nm and 91.2±5.8 nm for λ=266 nm and λ=355 nm, respectively. Transmission electron microscopy (TEM) was utilized to examine the microscopic structural features of the lithium metasilicate crystals. The TEM results reveal that the growth of lithium metasilicate crystals proceeds dendritically, and produces Li2SiO3 crystals that are ∼700–1000 nm in length for saturation exposures. Optical transmission spectroscopy (OTS) was used to study the growth of metallic silver clusters that act as nucleation sites for the Li2SiO3 crystalline phase. The OTS results show that the (Ag0)x cluster concentration has a dependence on incident laser irradiance that is similar to the etch rate ratios and Li2SiO3 concentration. A comparison between the XRD and optical transmission results and our prior etch rate results show that the etch rate contrast and absolute etch rates are dictated by the Li2SiO3 concentration, which is in turn governed by the (Ag0)x cluster concentration. These results characterize the relationship between the laser exposure and chemical etch rate for Foturan, and permit a more detailed understanding of the photophysical processes that occur in the general class of photostructurable glass ceramic materials. Consequently, these results may also influence the laser processing of other photoactive materials.