Crystallization Heat Treatment Research Articles

The effect of photo-thermo-induced crystallization on the properties and microstructure of transmission volume Bragg gratings

Volume Bragg gratings (VBGs) recored on photo-thermo-refractive (PTR) glass are extensively utilized in high power lasers, optical communication, holographic display fields, etc. In the majority of application fields, high refractive index modulation (RIM) value and high transmittance are the required parameters for transmission Volume Bragg gratings (TVBGs) devices. This study indicates that the RIM values and transmittance of TVBGs strongly depend on the size and concentration of sodium fluoride (NaF) crystals within the grating periodic structure. Increasing the UV exposure dose causes a reduction in the size of NaF crystals, leading to a finitely rise in the RIM value of TVBGs. Prolonging the crystallization heat treatment time and elevating the temperature effectively enhance the RIM value of TVBGs, which also significantly enlarge the size of NaF crystals in the grating periodic structure. By fine-tuning the process parameters of photothermal induced crystallization, a RIM value of 20–320 ppm is freely controlled. For 1.5 mm thick TVBGs, the above RIM value range can meet the application requirements of achieving high diffraction efficiency in the visible to near-infrared range. Following UV exposure at a dose of 5 J/cm2 and nucleation at 480 °C for 120 min, crystallization treatments at 550 °C for 30–60 min and at 570 °C for 60–120 min are determined as optimal for fabricating TVBGs with high diffraction efficiency and transmittance in the visible (532 nm) and near-infrared (1053 nm) bands, respectively.

Effect of TiO2 and CaO Addition on the Crystallization and Flexural Strength of Novel Leucite Glass-Ceramics.

The aim of this study was to investigate the effects of TiO2/CaO addition on the crystallization and flexural strength of leucite glass-ceramics (GC). Synthesis of translucent and high strength GCs is important for the development of aesthetic and durable dental restorations. To achieve this, experimental aluminosilicate glasses (1-3 mol% TiO2 and CaO (B1, B2, B3)) were melted in a furnace to produce glasses. Glasses were ball milled, screened and heat treated via crystallization heat treatments, and characterized using XRD, differential scanning calorimetry, dilatometry, SEM and biaxial flexural strength (BFS). Increasing nucleation hold time (1-3 h) led to a reduction in crystallite number for B2 and B3 GC, and significant differences in leucite crystal size at differing nucleation holds within and across test groups (p < 0.05). A high area fraction of leucite crystals (55.1-60.8%) was found in the GC, with no matrix microcracking. Changes in the crystal morphology were found with higher TiO2/CaO addition. Mean BFS of the GC were 211.2-234.8 MPa, with significantly higher Weibull modulus (m = 18.9) for B3 GC. Novel glass compositions enriched with TiO2/CaO led to crystallization of leucite GC of high aspect ratio, with high BFS and reliability. The study's findings suggest a potential high performance translucent leucite GC for use in the construction of dental restorations.

Effect of striae on the high temperature dielectric and electrical properties of lithium aluminosilicate glass-ceramics

Lithium Aluminosilicate glass-ceramics with striae (LAS-WS) and without striae (LAS-WoS) were processed using melt casting technique. LAS-WS and LAS-WoS samples were characterized and found stress birefringence >10 nm/cm and <10 nm/cm, respectively. LAS-WS and LAS-WoS samples were subjected to structural, microstructural, high temperature dielectric and electrical characterization. The presence of striation did not significantly affected the formation of β-spondumene structure during crystallization heat treatment. TEM image confirms the formation of nanocrystals in the glass matrix of lithium aluminosilicate glass-ceramics. The homogeneous distribution of LAS nanocrystals of average size of 3–6 nm is evident from TEM results. Stress birefringence distribution in lithium aluminosilicate glass-ceramic clearly shows the striae formation. Shadowgraphy images confirmed the striae free and striae containing regions in the glass-ceramic. The effect of striae on dielectric properties and AC conductivity was analysed using impedance spectroscopy and it was found that the value of the relative permittivity decreased from 415 (1.2 Hz) to 9.76 (1.2 MHz) for LAS-WoS and from 401 (1.2 Hz) to 10.3 (1.2 MHz) for LAS-WS, respectively. It was found that striae in LAS glass-ceramics influence the electrical impedance and AC conductivity relaxation process rather than relative permittivity. The effect of striae on crystallization and morphology are presented. Both the samples have shown crystallinity of more than 75 %. The electrical conductivity of σ = 2.0 × 10−3 S/cm for LAS-WoS and 2.7 × 10−3 S/cm for LAS-WS at 300 °C as well as σ = 4.6 × 10−3 S/cm for LAS-WoS and 6.9 × 10−3 S/cm for LAS-WS at 700 °C are found to be in good agreement due to increase in the electrical conductivity with temperature. The ionic conductivity of the material due to Li-ion mobility with respect to temperature and electric field and possible mechanism of AC conductivity are discussed. The results presented indicates the potential application of this material that demands high temperature operation of lithium aluminosilicate glass-ceramics.

Epitaxial growth of gehlenite in CaO–MgO–Al2O3–SiO2 based glass ceramic induced by Nb2O5 addition

In this study, the crystallization behavior of a CaO–MgO–Al2O3–SiO2 based glass ceramics with Nb2O5 addition was investigated by using DSC, XRD and TEM techniques. It was found that three crystalline phases, namely, CaNb2O6, diopside and Gehlenite (Ca2Al2SiO7), were formed from the glass matrix during the crystallization heat treatment. The CaNb2O6 phase was the first to precipitate from the glass matrix at around 850 °C, acting as a nucleating agent that facilitated the formation of Gehlenite (Ca2Al2SiO7) at 900–950 °C. The TEM observation revealed a consistent orientation relationship between the CaNb2O6 and the Gehlenite (Ca2Al2SiO7) phases, indicating that CaNb2O6 could induce the epitaxial growth of Gehlenite (Ca2Al2SiO7). This result provides a new insight into the preparation of Gehlenite epitaxial films and glass ceramics.

Preparation of Li2O–Al2O3–SiO2–B2O3（LASB）glass-ceramic with low thermal expansion coefficient from spent hydrogenation catalysts and crystallization kinetics

The recovery of spent hydrogenation catalysts occupies advantages in both alleviating environmental burden and resource shortage. Whereas, the development of conventional hydrometallurgical recovery of molybdenum (Mo) and nickel (Ni) from spent MoNi/Al2O3 catalysts has been limited by the numerous processes and the accompanying generation of waste liquids and residues. Herein, A new recycling route for the preparation of spent hydrogenation catalysts with low thermal expansion coefficients (CTE) was proposed, which has the advantages of short process, green environment and high value utilization. The Li2O–Al2O3–SiO2–B2O3 (LASB) parent glass containing Mo and Ni were obtained by melting with different contents of the spent catalysts. The thermal expansion coefficient of the glass was minimized to 6.08 × 10−6/°C at an addition of 25 wt% of the spent catalysts. Moreover, according to the crystallization peak in DTA curve, glass-ceramics were prepared by one-step crystallization heat treatment at different crystallization temperatures. The main crystalline phase of the glass-ceramic precipitated by heating at 685 °C is β-quartz (Li2Al2Si3O10), which gives the glass-ceramics the lowest coefficient of thermal expansion (0.96 × 10−6/°C) of glass-ceramic as well as the maximum density (2.45 g/cm3) and Vickers hardness (705.8 Hv). Furthermore, the activation energy (354.65 ± 17.42) kJ/mol) and Avrami index (5.39) associated with crystallization were extracted from the fitting of different theoretical model, indicating the three-dimensional crystallization of the LASB glass-ceramics. This technology provides a waste-to-material strategy for hazardous waste and a clean production route for the preparation of the functional glass-ceramics.

Crystallization heat treatments for the fabrication of volume Bragg gratings based on photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a functional optical material with broad applications in holographic optics. In this work, we examine the effect of crystallization heat treatment on the growth and distribution of sodium fluoride (NaF) nano-crystals in PTR glass using DSC, XRD, SEM, and TEM. The optical properties of the crystallized PTR glass are analyzed using a spectrophotometer. The results indicate that the NaF nano-crystals precipitated in PTR glass matrix are in the form of polycrystalline, and the growth of which is highly dependent on temperature. The growth of NaF nano-crystals saturates during the crystalline phase transition, resulting in decreased distribution uniformity. Higher crystallization temperature and longer crystallization time lead to an increase in absorption and scattering losses in the visible band. However, the crystallized PTR glass exhibits favorable absorption and scattering loss characteristics in the near-infrared band. By adjusting the crystallization treatments within the temperature range of 530 °C–570 °C for 10–120 min, refractive index modulation (RIM) values of the transmitting volume Bragg gratings (TVBG) ranging from 50 to 350 ppm can be achieved, which allows for maximum relative diffraction efficiencies exceeding 90% at 532 nm and 1053 nm using different crystallization processes. This research enhances the understanding of NaF nano-crystal growth mechanisms and provides insights into the application of holographic optical elements based on PTR glass in both the near-infrared and visible bands.

Microstructure and mechanical properties of SiC joint using anorthite based glass-ceramic and first principles calculation of joint interface

A specialized CaO–Al2O3–SiO2 (CAS) glass-ceramic was innovated to join SiC ceramics. The crystallization kinetics of the CAS glass was studied, revealing distinct anorthite precipitates in the CAS glass-ceramic. The impact of process parameter variations on microstructure and mechanical properties was studied. It revealed that post-joining crystallization heat-treatment could change the size and the content of crystals and significantly improve the shear strength of the joint. It was duly observed that a remarkable shear strength of 86 MPa was achieved when the joint was formed at the temperature of 1450 °C and treated at 1050 °C for crystallization. Notably, this exceptional mechanical endurance remained impressively high, maintaining a value of 85 MPa at 800 °C. Based on density functional theory calculations, it was confirmed that glass/C–SiC (0001) has a stronger interface bond than glass/Si–SiC (0001), resulting in a reliable joining at the glass/C–SiC (0001) interface for CAS glass and SiC.

Investigation of Spark Plasma Sintering on Microstructure-Properties of Zirconia Reinforced Fluormica Glass for Dental Restorations.

Conventional sintering methods of dental ceramics have limitations of high temperature and slow cooling rates with requirements of additional heat treatment for crystallization. Spark plasma sintering (SPS) is an emerging technique that has the potential to process dental restorations with dense microstructures and tailor-made clinically relevant properties with optimized processing parameters. This study explored the potential of the SPS of zirconia-reinforced fluormica glass (FM) for dental restorative materials. FM glass frit was obtained through the melt-quench technique (44.5 SiO2-16.7 Al2O3-9.5 K2O-14.5 MgO-8.5 B2O3-6.3 F (wt.%)). The glass frit was ball-milled with 20 wt.% of 3 mol% yttria-stabilized zirconia (FMZ) for enhanced fracture toughness. The mixtures were SPS sintered at a pressure of 50 MPa and a heating rate of 100 °C/min for 5 min with an increase in temperature from 650-750 °C-850 °C-950 °C. Phase analysis was carried out using XRD and microstructural characterization with SEM. Micro-hardness, nano-indentation, porosity, density, indentation fracture toughness, and genotoxicity were assessed. The increase in the SPS temperature of FMZ influenced its microstructure and resulted in reduced porosity, improved density, and optimal mechanical properties with the absence of genotoxicity on human gingival fibroblast cells.

Highly efficient and thermally robust phosphor-in-glass ceramic (PiGc) for high-brightness laser-driven lighting

How to improve the luminous efficiency (LE) and thermal stability of phosphor conversion materials have always been the focus of high-brightness laser-driven lighting. In this study, we successfully prepared a series of phosphor-in-glass ceramics (PiGc) by conducting a crystallization heat treatment on phosphor-in-glass (PiG) made from LuAG:Ce3+ phosphor and Li2O–Al2O3–SiO2 (LAS) glass via a spark plasma sintering (SPS). Comparing to PiG, the optical and thermal performances of PiGc have been enhanced significantly, including a higher internal quantum effect of 81.2% and lower thermal quenching (it remains 86% at 473k). By precipitating β-spodumene crystals, the coefficient of thermal expansion (CTE) of the PiGc decreased by 15% at 700 °C. Meanwhile, the use of precipitated nanocrystals as 'light-scattering centers' improved luminous efficiency remarkably. Under excitation of 450 nm blue laser with 7.04W/mm2, the LE of the most optimal sample was improved from 209.5 lm/W to 232.2 lm/W. These results indicate that crystallization heat treatment is an effective method for improving the LE and thermal stability of PiG. The PiGc is a highly efficient and thermally robust phosphor conversion materials for high-brightness laser-driven lighting.

Effect of thermocompression on properties of transparent glass-ceramics containing quantum dots

A novel strategy for preparing transparent glass-ceramics with a uniform quantum-dot size and high transparency via thermocompression is reported. Borophosphate glass containing the Cs–Pb–Br component is prepared using the conventional melting method. The glass is then pressed with a piece of stainless steel to generate thermocompression during the crystallization heat treatment, by which small and uniform-sized nanocrystals of CsPbBr3 quantum dots are produced in glass. Thermocompression reduces the specific surface energy of the nanocrystals and inhibits the abnormal growth of microcrystals, thereby reducing the average particle size from 7 to 4 nm and completing the growth of microcrystals. This significantly increases the transmittance of the glass-ceramics and enhances the luminescence intensity by approximately threefold. The experimental results show that low-intensity pressure can control the structure and properties of glass-ceramics during the growth of microcrystals in glass, which is a new process for preparing transparent microcrystalline glass.

Direct synthesis of TiO2 nanoparticles without heat treatment: Effect of order of addition and precursor/reducing ratio

Titanium dioxide (TiO2) is the most used semiconductor for heterogeneous photocatalysis. However, there is a limitation on its application since TiO2 works as catalyst only under ultraviolet radiation. Therefore, the synthesis of high-performance TiO2 NPs by a direct method without further calcination could improve the use of this catalyst. The current work aimed to obtain anatase/rutile TiO2 nanoparticles for photocatalysis under UV light by a direct method without further heat treatment. A 2k factorial design was used to evaluate the effect of the order of addition and ratio of the precursor/reducing agents on the synthesis of the NPs. TiO2 nanoparticles with large surface area were synthesized via the controlled precipitation method. The NPs were characterized by the XRD, DSC/TG, FTIR, BET, DLS, ELS, UV–Vis, FESEM-EDS techniques, and by the photocatalytic activity test. The commercial TiO2 NP P25 Evonik was used as standard photocatalyst for comparison with the obtained samples. The specific surface area (BET) of the TiO2 NP samples was three to five times higher in comparison to P25. The photoactivity of the TiO2 NP samples and P25 was determined as the discoloration of methylene blue and rhodamine B dyes under UVA radiation (330 nm, 12 W/m2) for 180 min. The results showed that the discoloration efficiency of the synthesized TiO2 NP samples is equivalent to that of the P25 sample. The synthesis of TiO2 nanoparticles via the controlled precipitation method using only two reagents proved to be a viable and promising route. The method does not require heat treatment for titanium dioxide crystallization and is still capable of producing a mixture of ideal phases to delay the recombination of the photogenerated electron-hole pair, contributing to the increase of the photocatalytic efficiency. The TiO2 NP samples will be used to the degradation of recalcitrant pollutants in water as textile dyes and pharmaceutical byproducts.

Tunable Raman Gain in Transparent Nanostructured Glass-Ceramic Based on Ba2NaNb5O15 †.

Stimulated Raman scattering in transparent glass-ceramics (TGCs) based on bulk nucleating phase Ba2NaNb5O15 were investigated with the aim to explore the influence of micro- and nanoscale structural transformations on Raman gain. Nanostructured TGCs were synthesized, starting with 8BaO·15Na2O·27Nb2O5·50SiO2 (BaNaNS) glass, by proper nucleation and crystallization heat treatments. TGCs are composed of nanocrystals that are 10-15 nm in size, uniformly distributed in the residual glass matrix, with a crystallinity degree ranging from 30 up to 50% for samples subjected to different heat treatments. A significant Raman gain improvement for both BaNaNS glass and TGCs with respect to SiO2 glass is demonstrated, which can be clearly related to the nanostructuring process. These findings show that the nonlinear optical functionalities of TGC materials can be modulated by controlling the structural transformations at the nanoscale rather than microscale.

One-Step Microcrystalline Glass Preparation Using Smelting Slag from Waste Automobile Three-Way Catalysts through Iron Collection

The smelting slag obtained through iron collection from waste automobile three-way catalysts was used as a raw material to prepare microcrystalline glass through a one-step crystallization heat treatment. The phase composition and microstructure of the prepared glass were analyzed through X-ray diffraction and scanning electron microscopy–energy dispersive X-ray spectroscopy, respectively. Single-factor experiments were conducted to investigate the effects of crystallization temperature from 900 to 950 °C and crystallization time from 0.5 to 4 h on the physical and chemical properties of the microcrystalline glass. The results indicated that the optimum crystallization temperature and time for preparing microcrystalline glass with glass smelting slag through the proposed one-step crystallization heat treatment process were 950 °C and 3 h, respectively. Under these experimental conditions, the number of crystalline phases of the microcrystalline glass was high, the grains were mainly spherical and columnar particles, the sample structure was dense, and the best results were obtained: the density was 2.72 g/cm3, the water absorption was 1.55%, the porosity was 4.2%, the Vickers hardness was 618 HV, the acid resistance was 2.6%, and the alkali resistance was 0.04%. In addition, the results of the toxicity characteristic leaching procedure indicated that the leaching concentrations of heavy metals such as Zn, Cr, and Pb in the microcrystalline glass were lower than those in the base glass and were considerably lower than the acceptable limits. The microcrystalline glass obtained from final smelting slag through heat treatment can enhance the stabilization of harmful elements. The findings of this study can be applied to the treatment of bulk solid waste.

Study on Crystallization Process of Li2O-Al2O3-SiO2 Glass-Ceramics Based on In Situ Analysis.

In this paper, we used differential scanning calorimetry (DSC), high-temperature X-ray diffraction (HT-XRD), and confocal scanning laser microscopy (CSLM) to investigate the Li2O-Al2O3-SiO2 glass crystallization process. At 943 K, lithium disilicate (Li2Si2O5) phase crystals began to precipitate in the Li2O-Al2O3-SiO2 glass with a crystal size of 50-70 nm. At the temperature of 1009 K, petalite (LiAlSi4O10) crystals began to precipitate in the vitreous phase, forming composite spherical crystals of LiAlSi4O10 and Li2Si2O5 with size in the range of 90-130 nm. Furthermore, the Kissinger method and KAS method of the JMAK model were used to calculate the crystallization activation energy and the Avrami index "n". It was found that the precipitation mechanism of the two kinds of crystals is whole crystallization; accordingly, the selection of crystallization heat treatment system was guided to determine the nucleation and crystallization temperature.

A new rejuvenation heat treatment of crept Ni-based superalloy single crystals

Ni-based superalloys are widely used in harsh environments due to their exceptional high-temperature mechanical properties, and their microstructures are inevitably subjected to degradation, including accumulation of strain/stress, generation of high density of dislocations, and rafting of γ′-particles, under creep damage. To prolong their service life, rejuvenation heat treatment is proposed to restore their microstructure. In this study, a Ni-based superalloy single crystal specimen with 0.15% creep strain is taken as a surrogate of partially served components. The microstructure, including the dislocation density and distribution as well as the size and morphology of γ′-particles, is investigated. Then two comparative heat treatment protocols are applied to the crept superalloy single crystal. Because super-solvus homogenization treatment triggers recrystallization, a recovery pre-annealing step at sub-solvus temperature is designed to relieve the stored deformation energy. After homogenization and aging heat treatment, the single crystalline structure is preserved, and in the meanwhile the γ′-particles are restored to uniform cuboidal morphology and the dislocations are fully eliminated. Our findings provide a route to rejuvenate crept superalloy single crystals.

Improved ferroelectric characteristics of ALD lanthanum-doped hafnium oxide thin film by controlling post-cooling time

A study was conducted on how the subsequent cooling time affects the ferroelectric (FE) characteristics of atomic layer deposited (ALD) lanthanum (La) doped HfO2 thin films after the crystallization heat treatment in the form of TiN/La-doped HfO2/TiN metal-ferroelectric-metal (MFM) device structure. The ALD La-doped HfO2 thin film was cooled to room temperature in a chamber (“chamber cooling” - slower cooling) or in an air atmosphere (“air cooling” - faster cooling). Based on the polarization vs electric field (P-E) hysteresis curve, the double remanent polarization (2Pr) values of the chamber cooling and air-cooling are 9.5 μC/cm2 and 40 μC/cm2, respectively. This drastic improvement stems from more orthorhombic phase formation within La-doped HfO2 thin film, which is confirmed by smaller grain size, higher amount of oxygen vacancies, and more tensile stress within HfO2 thin film. Our results suggest that FE properties of La-doped HfO2-based thin film can be significantly modulated by post cooling processes.

Effect of heat treatment on the roughness and mechanical properties of dental lithium disilicate glass-ceramics

In dental clinics, it is common to perform small fitting adjustments in dentures using a micro-grinding tool after testing them in the patient's mouth. This procedure increases local roughness and can lead to formation of microcracks on the prosthesis surface. This study aimed to investigate the benefits of a post-finishing heat treatment to surface roughness and crack healing and its effect on the flexural strength of lithium disilicate (LD) dental glass-ceramics. Commercially available lithium metasilicate, Li2SiO3, samples were heat treated at 840 °C for 7 min to induce the phase transformation into LD, Li2Si2O5. The LD samples were characterized by X-ray Diffraction, Scanning Electron Microscopy, Vickers hardness, Young’s modulus, and fracture toughness. One of the surfaces of the LD samples was sanded aiming to simulate the denture fitting adjustments performed in the dentist’s laboratory, generating a rough surface, Group 1. Half of the LD samples had their biaxial flexural strength evaluated by the piston-on-three-ball test (P–3B) and the other half were submitted to a second short-term heat treatment (840 °C - 5 min), Group 2, and later assessed by the P–3B. Roughness parameters in both groups were measured by 3D optical profilometry. After the crystallization heat treatment, formation of elongated LD crystals, Li2Si2O5, 35% amorphous phase, and residual Li3PO4 was observed. In addition, the following mechanical property values were obtained: Vickers hardness = 5.8 ± 0.1 GPa, fracture toughness = 2.2 ± 0.1 MPa m1/2, and Young’s modulus = 100.3 ± 0.3 GPa. The samples in Group 1 showed bending strength of 206 ± 30 MPa and the following roughness parameters: Ra = 0.45 ± 0.16 μm, Rz = 22.7 ± 6.7 μm, and PV = 27.7 ± 7.1 μm. In the samples in Group 2, the Ra, Rz and PV roughness parameters were 0.31 ± 0.12 μm, 5.2 ± 2.5 μm, and 9.2 ± 4.7 μm, respectively. With this decrease in roughness, the bending strength increased by 62%, with a mean value of 331 ± 59 MPa. In the need for machine finishing of LD-based glass-ceramic dental prostheses, the use of a second short-term heat treatment at 840 °C for 5 min generates considerable gains in bending strength, increasing the lifecycle of the prosthesis as a result of reduced surface roughness caused by softening of the remaining amorphous phase in the glass-ceramic. These conditions can be adapted to each chemical and crystallographic composition of the glass-ceramic under study.

Microstructure and phase composition evolution of dual-phase ytterbium silicate coatings plasma sprayed from stoichiometric Yb2Si2O7 feedstock powder

The main challenges of atmospheric plasma spraying (APS) when dealing with rare earth silicate are the amorphous phase formation and silica volatilization in as-sprayed coatings. In this study, the microstructure and phase composition evolution of dual-phase ytterbium silicate environmental barrier coatings (EBCs) plasma sprayed from stoichiometric Yb2Si2O7 feedstock powder are investigated. The unavoidable silica volatilization leads to the coating composition deviate from stoichiometry of disilicate composition and result in the presence of Yb2SiO5 in coatings. By controlling the phase fraction of Yb2SiO5 through modulation of plasma character allows minimizing the effects of thermal residual stress on the coating durability in thermal cycling. The high crystallinity and crack-free coatings were achieved through optimized crystallization heat treatment. The presence of dislocation and deformation twins endow the dual-phase coating with intrinsic quasi plasticity and improvement in reliability. The present study provides useful inspirations on the optimization of spraying parameters and design of advanced EBC systems.

5 - Effect of Roughness on Flexural Strength of Dental Lithium-Disilicate

A common process in dental clinics is to perform small fitting adjustments of dentures after testing in the patient's mouth using the micro-grinding tool. This procedure promotes a local roughness increase and can lead to the formation of microcracks on the prosthesis surface. The aim of this work was to investigate the benefits of a post-finishing heat treatment on the surface roughness smoothing and crack healing of lithium disilicate dental glass ceramics, and its effect on the flexural strength. Commercially available lithium metasilicate, Li 2 SiO 3 (IPS e-maxCAD), samples were heat treated at 840C-7min. The material was characterized by X-ray diffraction, Scanning Electron Microscopy, Vickers hardness, Youngs modulus and fracture toughness. One of the surfaces of the samples was machined, aiming to simulate the fitting procedure of dentures in the dentist's laboratory, generating a rough surface. Half of the samples were tested by biaxial flexural testing (3B-P) and the other half were submitted to a rapid heat treatment (840C-3min) and later tested by biaxial strength, with statistical analysis by Weibull. The Roughness parameters (Ra, PV and Rz) were measured using Zygo New View 7100 Optical Profiler. After the crystallization heat-treatment protocol suggested by the manufacturer, the formation of elongated lithium disilicate crystals, Li 2 Si 2 O 5 , with 36% residual amorphous phase was observed. In addition, the Vickers hardness of 6.1±0.3 GPa, fracture toughness of 1.90± 0.2 MPa.m1/2 and modulus of elasticity values of 91 ±2 GPa were obtained. The samples from the group without rapid heat-treatment showed bending strength of 203.5 ±33 MPa, with roughness values of Ra=0.6 ±0.2 μm, Rz=22 ±6 μm and PV=28 ±7 μm. After the rapid heat treatment, the roughness parameters Ra, Rz and PV were 0.3 ±0.2 μm, 9 ±6 μm and 5 ±4 μm, respectively. With this reduction in roughness, the flexural strength increased by 65%, with mean values of 336.6 ±32 MPa. Weibull modulus values were in the order of 5 for both groups. In the need for finishing machining of lithium disilicate-based glass-ceramic dental prostheses, the use of a rapid heat treatment at 840 C allows considerable gains in flexural strength. Increasing the service life of the prosthesis, due to a reduction on the surface roughness, promoted by the softening of the remaining amorphous phase in the glass-ceramic.

Effect of high pressure and high temperature on the Na2O · 2CaO · 3SiO2 glass-ceramic’s structural properties

In this study, the Soda-lime-silicate system, with the Na2O·2CaO·3SiO2 (N1C2S3) composition, was used to study the effect of high pressure on structural properties of this glass-ceramic. Samples were produced and submitted to a single stage crystallization heat treatment at 720 °C for 30 min simultaneously in atmospheric pressure, 2.5 GPa, 4.0 GPa or 7.7 GPa. All characterizations were performed ex-situ. The major crystalline phase obtained is the Combeite, regardless of the pressure used, with indications of a β−CaSiO3 or Cristobalite-II phase transition happening due to the high pressure. Raman and Infrared spectroscopy showed that the phase transition could be a CaO precipitation on the material, due to the breaking of Si–O bonds and presence of Ca–O vibrational modes present on the high pressure samples. All results point to the Combeite crystalline phase being highly stable under pressures up to 7.7 GPa.