Barium Disilicate Research Articles

Unusual crystallization pathways revealed in six barium disilicate (BaSi2O5) glasses

Due to the unusual crystallization of (nominally) stoichiometric BaSi2O5 (BS2) glass, which shows unexpected and diverse crystal phases, a series of six glasses with different chemicals and melting procedures were prepared in three laboratories and characterized before and after crystallization by differential scanning calorimetry, density measurements, X-ray diffraction, FTIR, and Raman spectroscopy. The aim of this study was to assess whether there is systematic behavior in the crystallization pathways in relation to precursor chemicals, impurities, and hydroxyl content of this glass. Small glass monoliths were treated at the first DSC crystallization peak and quenched to determine which phases formed in the early-stages of crystallization. The glass transition temperatures (Tg) divide these six glasses between those with a Tg near 690 °C versus those near 700 °C. The DSC peak crystallization temperatures varied even more; from 855 to 917 °C. In these six glasses, our results are best explained by a combination of metastable high-BaSi2O5 and Ba6Si10O26. Monotonic trends in crystallization show that the DSC signal from the Ba-rich phases increases as the Tg and the crystallization temperatures increase. The BS2 glasses with both the lowest Tg and lowest DSC crystallization temperatures show the most barium disilicate crystal. This leads to the conclusion that the metastable monoclinic high-BaSi2O5 is favored in these conditions. The small differences in glass synthesis conditions and chemicals used strongly influence the relative proportions of phases which crystallize and their kinetics. In-situ and ex-situ diffraction measurements confirm the conclusions above. The structural distinctions between the barium silicate crystals and the BS2 supercooled liquid, and the implications for the role of structural polymerization are discussed. We conclude that high-BaSi2O5 or Ba6Si10O26 are the predominant phases in the earliest stages of crystallization. This study highlights the extreme sensitivity of BS2 glass crystallization kinetics and pathways to minor differences in composition and synthesis conditions and explains the different conclusions reached by distinct authors that worked on the crystallization of BS2 glasses.

Effect of high pressure in barium disilicate glass investigated by DTA and Raman spectroscopy

AbstractThis work aimed to explore the effect of high pressures at room temperature on the vibrational modes and the thermal properties and stability of barium disilicate glass (BaO‐2SiO2) (BS2). The BS2 was prepared through the conventional melting route at atmospheric pressure. After being submitted to different pressure values (2.5 GPa, 4.0 GPa, and 7.7 GPa) and room temperature, the samples were characterized ex situ by Raman spectroscopy and differential thermal analysis. With this, it is possible to see that the pressure modifies the Raman vibrational modes of the glass, in the region associated with the Si‐O‐Si symmetrical bending modes. The Si‐O symmetrical stretching vibrational modes (Qn) remain unchanged, however there are changes on values of glass transition and decrease crystallization temperature, as well as the parameters of thermal stability of the glass.

Microstructural and residual stress effects on toughening of stoichiometric BaO.2SiO2 glass-ceramics

Barium disilicate (BaO.2SiO2=BS2) glass is one of the few stoichiometric glasses that nucleates internally, homogeneously via thermal treatment. This system has been scarcely assessed in microstructure-property studies. Here we address fracture strength and toughness (KIC) variation as a function of crystallized volume fraction and crystal size, as well as the possible effect of residual stresses (RS) in BS2 glass-ceramics (GCs) by independently varying these two microstructural parameters. KIC increased with spherulite size and crystallized volume fraction. KIC variation with crystallized volume fraction was similar for GCs with different crystal sizes. Combination of the current findings on BaO.2SiO2 (crystals under tensile RS) with previous studies of Li2O.2SiO2 GCs (compressive RS) indicates that crystallization of a tougher phase – not type of residual stress in the crystals – is the crucial parameter controlling fracture toughness and strength. These findings are quite useful to design novel strong and tough GCs.

Crystallization heat treatment curves of barium disilicate glass / Curvas de tratamento térmico de cristalização de vidro de dissilicato de bário

The transformation of glasses into glass-ceramics by heat treatment has been widely studied to increase the toughness of glasses. Barium disilicate is a material that can be toughened this way, greatly increasing its range of applications. To predict the fraction of crystals as a function of heat treatment time and temperature, it is necessary to draw the curves relating these three quantities. For this, data on the nucleation rate and growth rate of barium disilicate crystals were obtained from the literature and processed in order to obtain equations as a function of temperature. These equations were applied to the Johnson-Mehl-Avrami (JMA) equation and integrated in the intervals corresponding to the isothermal treatments, constant cooling from the melting temperature and constant heating from the glass transition temperature, thus obtaining the curves for these three types of heat treatment.

Energy landscape modeling of crystal nucleation

Nucleation is a crucial phase transformation process across all materials science. In the context of crystal nucleation, accurate knowledge of the nucleation rate is essential for the design of next-generation glass-ceramic and other composite materials; however, the quantitative prediction of crystal nucleation rate versus temperature remains elusive. This work proposes an energy landscape method to account for the thermodynamic and kinetics aspects of nucleation. This energy landscape approach shows fair agreement with experimental nucleation data for the barium disilicate system used as a model. It is also used to gain insight into the fundamental physics of classical nucleation theory (CNT) and a new method to compute nucleation rates.

Decoupling Microstructural and Residual Stress Effects on Glass-Ceramic Toughening

Barium disilicate (BaO.2SiO 2 = BS2) is one of the very few stoichiometric glasses that allows for accurate control of the desired microstructure via thermal treatment, making it a (scarcely studied) model system for microstructure-property studies. Here, we performed a systematic research work on the variation in hardness, elastic modulus, fracture strength, and toughness as a function of the crystalline volumetric fraction, crystal size, and residual stresses in BS2 glass-ceramics (GC). These microstructural features were independently modified; the average crystal diameter varied from 5 to 100 μm and the crystallized volumetric fraction from 0 to 68%. The internal residual stresses in the crystals, are tensile in this system . Samples with an average spherulite size above 30 µm showed spontaneously fractured crystals due to the residual stresses. The samples with 5 and 10 µm spherulites have not cracked because the spherulites have a high volume fraction of residual glass (74%) and moderate internal stresses (40-70 GPa). The fracture toughness, K IC , increased with the spherulite size and volume fraction. However, the residual glass inside the spherulites rendered crack bowing, bridging, and trapping ineffective. The variation of K IC with the crystallized volume fraction is similar for GCs with different crystal sizes. Also, a comparison with a lithium silicate glass-ceramic showing compressive residual stresses yielded similar results. These combined findings indicate that the crystallization of a tougher phase is the crucial parameter controlling fracture toughness in these materials. The results with this model material can be extended to design novel strong and tough glass-ceramics.

Effect of high pressure on the structure of barium disilicate glass-ceramics

A systematic study of crystallization of barium disilicate at high pressures and temperatures was undertaken. Two groups of samples were submitted to nucleation at 710°C, and then submitted to a crystal growth at either 860°C or 960°C respectively. Both groups were heat treated at 1 atm, 2.5 GPa, 4 GPa, and 7.7 GPa. The diffraction results show that at atmospheric pressure, the phases formed are, monoclinic Ba6Si10O26, orthorhombic low-Ba2Si4O10, and monoclinic high-Ba2Si4O10 depending on the temperature treatment. In contrast, at 7.7 GPa orthorhombic BaSiO3, monoclinic phase Ba6Si10O26 and coesite formed when treated at 860°C, whereas monoclinic Ba6Si10O26 is predominantly formed when treated at 960°C. At 2.5 and 4 GPa orthorhombic low-Ba2Si4O10 formed when treated at 960°C. The high frequency Raman modes are strongly affected by the different pressure and temperature treatments. The shifts and formations of distinct vibrations modes are discussed as function of pressure.

Nucleation and early stage crystallization in barium disilicate glass

Barium disilicate is one of the glass-ceramic systems where internal nucleation and crystallization can occur from quenched glass upon heat treatment without requiring nucleating agents. The structural origin of the nano-clusters formed during low temperature heat treatment is of great interest in gaining a fundamental understanding of nucleation kinetics in silicate glasses. Here, we present experimental investigations on the low temperature heat treatment of barium disilicate (BaO•2SiO2) glass. Several experimental techniques were used to characterize the structural nature of barium disilicate glasses that were heat treated between the glass transition temperature, Tg, and the peak temperature of crystal growth, Tcr. The data show that small amounts of crystallites including BaSi2O5 as well as other higher Ba/Si ratio phases are formed. Moreover, unlike that reported for lower BaO content (BaO<33mol%) barium silicate glass or the analogous Li2O−SiO2 glasses, no clear evidence is observed for liquid/liquid phase separation in barium disilicate glass.

Effect of non-stoichiometry on the crystal nucleation and growth in oxide glasses

Non-stoichiometric glasses (NSG) are much more common than stoichiometric compositions. However, due to inherent difficulties, fundamental studies of crystallization kinetics of NSG are much less frequent. To shed light on the crystal nucleation and growth kinetics of NSG, we adopted a nucleation kinetics model, leaving the interfacial energy and diffusion coefficient as free parameters, to explain experimental nucleation data of glasses of three compositions in the pseudo-binary Li2O·2SiO2–BaO·2SiO2 model system. We show that, as the glass composition approaches the eutectic, the nucleation rates drop drastically, mainly due to an increase in the interfacial energy. This result corroborates the common empirical observation that eutectic compositions tend to show good glass-forming ability. We also found that the structural relaxation times are significantly shorter than the characteristic nucleation times, validating the widely used and scarcely tested assumption that relaxation does not play a noticeable role in crystal nucleation. For a stoichiometric glass the fitted diffusion coefficient determining nucleation, D, is lower than that determining the growth of macro crystals, DU, and this difference significantly increases with decreasing temperature. On the other hand, the diffusion coefficient calculated from viscosity, Dη, is close to DU at high temperatures and approaches D in the glass transition range. Finally, after crystallization of the primary phase (lithium disilicate), barium disilicate and more lithium disilicate crystals precipitate in the diffusion zone existing in the residual glass.

The origin of the unusual DSC peaks of supercooled barium disilicate liquid

Crystallization proceeds through high-temperature and non-stoichiometric phases which recrystallize to become sanbornite (BaSi2O5) during heating or through time.

A Raman investigation of the structural evolution of supercooled liquid barium disilicate during crystallization

AbstractThe homogeneous and stoichiometric crystallization of the barium disilicate (BaSi2O5) glass makes it possible to follow the structural evolution of Raman bands from the supercooled liquid to the crystalline phase. We monitored the crystallization of supercooled liquid BaSi2O5 at 790°C over 440 minutes revealing a three‐stage crystallization process: stage 1 involves changes in the barium sites toward a bonding environment that is similar to that in orthorhombic low barium disilicate. The end of stage 1 is marked by the loss of the Q4 species vibration at 1170 cm−1 and the creation of a crystalline band at 490 cm−1. Stage 2 is marked by the transition of the Q3 peak (1060 cm−1) from a Gaussian‐like to a Lorentzian curve, indicating the formation of well‐developed sheet structures. These observations lead us to conclude that the Raman spectra are resolving both the crystalline L‐BS2 phase and the BS2 melt. The final stage involves a rapid decrease in the remaining Qn species to form Q3. This crystallization process involves structural modifications that occur on the tens of microns scale. The barium cations drive the initial stages of crystallization, providing direct evidence that the short‐range order, around network modifiers, is a critical factor involved in homogeneous crystallization.

Crystal growth and viscous flow in barium disilicate glass

Decoupling between the diffusion coefficients that control crystal growth (DU) and viscous flow (Dη) in deeply supercooled glass-forming liquids is one of the most celebrated, still unsolved, phenomena in glass science because it strongly impacts the analysis and prediction of crystallization kinetics. To further understand the decoupling phenomenon, we measured viscosity and crystal growth rates (of the crystalline surface layer growth and internally nucleated crystals) of a supercooled liquid with barium disilicate composition. We performed all measurements using glass samples from the same batch, a rare case that we call a “clean” experiment. This type of analysis is uncommon, and its aim is to minimize the uncertainty due to the composition variation that is inherent in laboratory glass production. The decoupling temperature (Td), the glass transition temperature (Tg), the melting point (Tm), and the liquid fragility index were obtained and successfully used to test a recently derived relationship which allows one to estimate Td by knowing only Tm and the viscosity curve. We also show, for the first time, that the reduced decoupling parameter, which gauges the degree of deviation between DU and Dη, does not depend on the crystal growth mechanism for the cases of Normal or Screw Dislocation growth.

Structural and dynamic properties of vitreous and crystalline barium disilicate: molecular dynamics simulation and Raman scattering experiments

In this paper, we use a molecular dynamics simulation and Raman scattering measurements to study the vibrational and structural characteristics of barium disilicate, BaSi2O5, in vitreous and crystalline states. Our proposed atomic interaction potential describes the structural and dynamic behaviour of this material very well and can also be used for further extended studies. In addition, Raman spectroscopy enabled validation of the predictions of the potential by comparing the simulated vibrational density of states with the spectrum of the material in its vitreous state. Furthermore, we characterized the kinetics of the crystallization process through in situ Raman measurements as a function of temperature.

Residual glass and crystalline phases in a barium disilicate glass–ceramic

Investigations about the presence of residual glass are scarce, despite its fundamental role in the crystallization kinetics and luminescent properties of barium disilicate glass–ceramics (BaO·2SiO2–BS2) with a quasi-stoichiometric composition. Non-isothermal (DTA/DSC) experiments have demonstrated that BS2 presents a polymorphic transformation, where the h-BS2 (monoclinic structure) phase is completely transformed in l-BS2 (orthorhombic structure) at temperatures higher than 1020°C (10°C/min). In this study, BS2 monolithic samples were heat-treated at 1000°C (BS2-10) and 1100°C (BS2-11) in a DSC furnace at a heating rate of 10°C/min. In addition, the crystalline and amorphous phases were characterized and quantified by Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD) experiments, respectively. Although the complete polymorphic transformation from h-BS2 to l-BS2 was achieved at 1100°C, our results demonstrated that BS2-11 contains a minor, albeit not negligible, amount of residual glass.

A Novel Alkali and Alkaline Earth Silicate Glass for Marine Applications

Aims: To test the corrosion properties of a novel alkali and alkaline earth silicate glass. Place and Duration of Study: Department of Metallurgical and Materials Engineering, between 1999-2001. Methodology: We included a glass which contains 75% mole lithium disilicate (Li2O.2SiO2-LS2) and 25% mole barium disilicate (BaO.2SiO2-BS2) phases. 1 mole% of P2O5 was used as nucleating agent. The scanning electron microscope (JEOL JSM-330) was used to observe the glass microstructures. X-ray diffraction (XRD) was carried out on the samples using a diffractometer (Philips PW1820) employing Cu Kα. In the chemical durability tests, the corrosive environment was boiling solutions of 0.625N HCl and 5% wt. NaOH along with boiling water. Another reference composition was also used, which included equal moles of LS2 and BS2, for the corrosion tests. Results: When A and B glass were compared, the basic and boiling water corrosion value of B glass in terms of weight loss, (3.12; 0.38) is higher than that of A glass (4.2; 0.70). Slight differences were observed in the corrosion properties in basic and boiling water environment when A and B glass were compared. However, the weight loss of A glass in acidic environment, was 3.55% wt whereas that of B glass was determined to be 0.4% wt. Conclusion: The chemical durability of A and B glasses against acidic environment was comparable to that of soda-lime glass. It was concluded that this novel composition is a candidate for the substitution of soda-lime glass in marine applications. Original Research Article Ertug; BJAST, 7(3): 333-341, 2015; Article no.BJAST.2015.150 334

Determining the Crystal Volume Fraction of BS2 Glass by Differential Scanning Calorimetry and Optical Microscopy

In this work, the crystal volume fraction, α(t), of a barium disilicate (BS2) glass‐ceramic was carefully investigated by optical microscopy (OM) and differential scanning calorimetry (DSC). X‐ray diffraction experiment revealed that the reflected peaks of the glass‐ceramic, which was prepared by heat treatment at 1000°C for 12 h, were indexed as the low‐temperature orthorhombic sanbornite mineral phase. This result was confirmed by the refinement of the crystal structure parameters. Bulk samples were then heat‐treated in the range of 760°C to 830°C. In each case, the α(t) values obtained by DSC were higher than those determined by OM due to surface crystallization and the formation of new nuclei during the heating/cooling steps in the DSC experiments. OM and DSC techniques were also used to estimate the number of preexisting nuclei, Nq, in a set of samples heat‐treated at 790°C directly in the DSC furnace. At this temperature, it was found that the Nq obtained directly by OM measurements were in reasonable agreement with those calculated from the combination of overall crystallization with crystal growth kinetics.

Controlled Crystallization of and Spherulitic Morphology in Li2O.2SiO2-BaO.2SiO2 Glasses

The crystallization process of Li2O.2SiO2-BaO.2SiO2 glasses was investigated using DTA, XRD and SEM methods. The incorporation of P2O5 did not result in volume crystallization; a surface crystallization was dominant for all the compositions studied according to DTA data. Lithium disilicate, barium disilicate and different forms of SiO2 were the identified phases after two-stage heat treatment. The spherulitic morphology was observed by SEM after primary crystallization temperature. After the crystallization of barium disilicate phase, the spherulitic morphology disappeared and massive polycrystalline microstructures were formed.

Composition and texture of barium silicate crystals in fresnoite glass-ceramics by various scanning electron microscopic techniques

Highly oriented fresnoite glass-ceramics were prepared from a non-stoichiometric melt (Ba2TiSi2O8 + 0.75 SiO2) by electrochemically induced nucleation. By using electron microscopy, cathodoluminescence (CL), electron backscatter diffraction (EBSD) and energy dispersive X-ray (EDX) spectroscopy as well as wavelength dispersive X-ray (WDX) spectroscopy, two additional barium silicate phases free of titanium were identified besides the fresnoite phase predominantly occurring throughout the sample. A barium disilicate phase (BaSi2O5) was found within a radius of approximately 300 µm from the Pt-wire where the nucleation was induced. At distances >300 µm a phase with the composition BaSiO3 was detected which occurred in between the large dendritic fresnoite crystals next to an additional glassy phase which mainly consists of SiO2. Both barium silicate phases show more intense cathodoluminescence than the fresnoite phase and the maxima in the attributed spectra were shifted to smaller wavelengths in comparison to fresnoite.

Glass-Ceramic Composite As a New Sealing Material for SOFCs

A composite material consisting of BaO.2SiO 2 (BS 2 ) as crystalline component in a CaO-MgO-Al 2 O 3 -SiO 2 glass matrix was evaluated as sealing material for solid oxide fuel cell stacks. Microstructural measurements using XRD and EDX indicated a reaction of the barium disilicate to barium aluminosilicate BaO.Al 2 O 3 .2SiO 2 (celsian). Dilatometric studies were performed on sintered composites. Although the CTE is lower than required, a good joining behavior and thermal cycling behavior with high chromium femtic steels were obtained. Investigations of the chemical stability of the composite sealant under simulated SOFC conditions in a dual atmosphere revealed a good compatibility with the steel.

Proton conduction in glass – an impedance and infrared spectroscopic study on hydrous BaSi 2O 5 glass

Hydrous barium disilicate glasses (BaSi 2O 5) containing 2.75 and 3.54 wt% dissolved water (corresponding to a molar concentration of hydrogen atoms of 11.1 and 14.2 mol/l, respectively) were synthesized by high temperature fusion in an internally heated gas pressure vessel. Near-infrared spectroscopy gives evidence that both OH groups and H 2O molecules are present in the glasses. The maximum intensity in the range of OH stretching vibrations is at 2800 cm −1 indicating strong hydrogen bonding in the glasses. Electric conductivity measurements were carried out at temperatures up to 523 K without significant alteration of the sample. At higher temperatures, OH groups are converted to molecular H 2O and water diffuses out of the sample resulting in a continuous decrease of the conductivity. An activation energy of 87 kJ/mol was derived for the dc conductivity in the unaltered glasses similar to the activation energy for bulk water diffusion in other silicate glasses. Because the dry barium disilicate glass is an electrical insulator at experimental conditions, we infer that the dc conductivity of the hydrous glasses is due to proton conduction.