Effect of crystal content on the properties of willemite glass-ceramics

Helium permeability, thermal expansion properties, and density have been measured for a series of glass-ceramics based on willemite as a lone crystalline phase. These glass-ceramics were prepared such that the compositions of both the glassy and crystalline phases were fixed, and only the relative concentrations of glass and crystal varied. Results of this study demonstrate that the helium permeability and glass transformation temperature of glass-ceramics are controlled by the glassy phase composition, whereas the density, thermal expansion coefficient, and dilatometric softening temperature are a function of both phases. All of these conclusions can be explained readily by simple continuity and mixing arguments.     

Experimental study of stress relaxation during annealing

In order to calculate the residual stresses in a flat sheet of glass during annealing on the basis of its thermal history, the viscosity and coefficient of thermal expansion must be determined throughout the transformation range. The structural modifications of the material are taken into account by using fictive temperature concept. Three constitutive assumptions for the rate of the fictive temperature are discussed. A very good agreement is obtained between measured and calculated residual stresses.     

Surface and bulk residual stresses in Li2O · 2SiO2 glass–ceramics

In the present work surface and bulk residual stresses generated in partially crystallized Li₂O · 2SiO₂ glass–ceramics are analyzed after different heat treatments. The phase specific residual stresses in the crystalline Li₂Si₂O₅-phase are evaluated for the first time in the near-surface zone and the bulk of the samples using both medium and high energy synchrotron radiation. The results reveal that in the crystals within the bulk of the samples micro residual stresses generated by the thermal anisotropy of the isolated individual crystallites depend on the crystallographic direction. In contrast, the residual stress state in the near-surface zone is isotropic due to the superposition of thermal residual stresses in and around the crystals of the near-surface area. Residual stress calculations using a modified Selsing’s model yield a good estimate of the anisotropic residual stresses in the bulk crystallites, whereas the isotropic residual stress state in the crystallized surface layer can be described by an elastic stress model for thin films.     

Elasto-plastic simulation of stress evolution during grain growth using a phase field model

Coupling effects among phase transformation, temperature and stress/strain are formulated based on thermodynamics using a phase field model. The elasto-plastic constitutive relationship is applied for stress analysis considering thermal expansion, transformation dilatation, and the effect of stress on phase transformation is included in the formulation. Subsequently, equations are numerically solved using a finite element method and stress evolution due to phase transformation and residual stress distribution are simulated. The results obtained show that large stresses are generated around phase interfaces. The high-stress regions move as grains grow, and finally residual stress distribution is observed along grain boundaries. The distribution and values at the grain boundaries are revealed to depend on the timing of collision and positioning of nuclei.     

Modeling and measurement of residual stresses in a bulk metallic glass plate

The recent advent of multi-component alloys with exceptional glass forming ability has allowed the processing of large metallic specimens with amorphous structure. The possibility of formation of thermal tempering stresses during the processing of these bulk metallic glass (BMG) specimens was investigated using two models: (i) instant freezing model, and (ii) viscoelastic model. The first one assumed a sudden transition between liquid and elastic solid at the glass transition temperature. The second model considered the equilibrium viscosity of BMG. Both models yielded similar results although from vastly different approaches. It was shown that convective cooling of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 plates with high heat transfer coefficients could potentially generate significant compressive stresses on the surfaces balanced with mid-plane tension. The crack compliance (slitting) method was then employed to measure the stress profiles in a BMG plate that was cast in a copper mold. These profiles were roughly parabolic suggesting that thermal tempering was indeed the dominant residual stress generation mechanism. However, the magnitude of the measured stresses (with peak values of only about 1.5% of the yield strength) was significantly lower than the modeling predictions. Possible reasons for this discrepancy are described in relation to the actual casting process and material properties. The extremely low residual stresses measured in these BMG specimens, combined with their high strength and toughness, serve to further increase the advantages of BMGs over their crystalline metal counterparts.     

Sol–gel synthesis and characterization of barium (magnesium) aluminosilicate glass sealants for solid oxide fuel cells

Solid oxide fuel cells (SOFC) correspond to efficient energy conversion systems coupled with low emissions of pollutants. In the aim to fabricate high temperature planar SOFC, glass and glass-ceramic sealants are developed to associate several criteria and properties : high thermal expansion (11.0 to 12.0 ? 10^? 6 K^? 1), high electrical resistance > 2 kΩ/cm², good thermochemical compatibility with the other active materials of the fuel cell, and stability under H₂ and H₂O atmospheres at an operation temperature of 800 °C for a long time. According to these requirements, new BAS (BaO–Al₂O₃–SiO₂) and BMAS (BaO–MgO–Al₂O₃–SiO₂) glass-ceramic sealants have been developed by sol–gel route which is a non-conventional process for such applications. By this soft chemistry process, we anticipate a decrease in the glasses processing temperature due to a better homogeneity between cationic precursors in the mixture and a more important reactivity of materials. Experimental results in terms of thermomechanical properties, thermal expansion coefficient, crystalline phase content, and microstructure were discussed. In particular, the influence of the %BaO on the thermomechanical properties of glass-ceramics was described. Changes in properties of glass-ceramics were closely related to the microstructure. The influence of MgO on glass processing temperatures, on the structure and on the microstructure is evaluated in order to confirm that these glass-ceramics are promising candidates to SOFC applications. So, after performing a systematic investigation to the various systems, the properties of suitable glass were proposed.     

Glasses and glass-ceramics in the oxyfluoride ternary system Pb(PO3)2-WO3-PbF2

Glasses were prepared by the melt-quenching method in the ternary system Pb(PO₃)₂-WO₃-PbF₂ and doped with Er³⁺ in order to prepare luminescent transparent glass-ceramics. This work focused on thermal and structural characterization of tungsten lead-phosphate glasses and crystallization study for preparing transparent glass-ceramics. Thermal properties such as thermal stability and crystallization behavior upon heating were investigated by DSC in function of PbF₂ content. For low PbF₂ concentrations, only one crystallization peak due to Pb₃(PO₄)₂ is observed whereas samples containing more than 15% of PbF₂ present another exothermic event at lower temperatures related with precipitation of PbF₂, Pb₂P₂O₇ and Pb₂OF₂. Structural investigations by Raman spectroscopy suggest that PbF₂ modifies the tungsten-phosphate network through the formation of P―F and P―O―Pb bonds but the average network connectivity remains almost constant. A crystallization study has been performed by DSC to investigate the dominant crystallization mechanisms in these glasses and it has been established that Pb₃(PO₄)₂ is nucleated on the surface whereas PbF₂, Pb₂P₂O₇ and Pb₂OF₂ crystallize dominantly from the glassy bulk. Transparent glass-ceramics containing nanosized PbF₂ crystallites were also prepared by suitable heat-treatment on the glass sample containing 20% of PbF₂ and Raman microscopy of these glass-ceramics supports the crystallization mechanisms determined by DSC.     

Structure,chemical durability and crystallization behavior of incinerator-based glassy systems

Vitrification by melting is being proposed as a convenient method to solidify different kinds of silicate and other oxide-based inorganic wastes. Incinerator bottom and fly ashes have been mixed with glass cullet, feldspar and clay by-products as melting fluxing agents. Washing, drying, and grinding pre-treatments followed by melting at 1450 °C lead to the formation of glasses and glass-ceramics, depending on the starting materials composition and thermal treatment. The obtained glasses have been studied by SEM, chemical durability tests in aqueous and alkaline environment, leaching test (UNI 10802), and by differential thermal analysis. The glass-ceramics morphology was investigated by XRD and SEM. The results were explained by the structure of the glasses caused by the presence of different amount of modifiers in the glassy lattice. The obtained glasses show good chemical resistance, in particular in alkaline environment and thermal characterization highlighted that the materials are also suitable to obtain glass-ceramics.     

Stress measurements in aluminium alloy films by the sin2 ψ method

The lattice parameters and the homogeneous elastic stresses in evaporated Al, AlSi and AlCu films 1 μm in thickness with, in part, a high degree of fibre texture have been determined. The stresses measured at room temperature as a function of the substrate temperature can be explained by the difference in the coefficients of thermal expansion between film and substrate in connection with creep processes. The different lattice parameters of the films as compared with bulk aluminium were found to be due to homogeneous elastic stresses and alloying elements.     

晶核剂对锂锌硅系微晶玻璃析晶及显微结构的影响

采用差热分析方法研究了LZS系玻璃陶瓷的析晶动力学,讨论了晶核剂对微晶玻璃的晶相、显微结构及析晶动力学参数的影响.结果表明:与TiO2相比,P2O5能够更加有效地促进玻璃析晶,但两者效果相差不大.使用不同的晶核剂不会影响玻璃析出的晶相,但使用P2O5作为晶核剂能得到均匀细小的晶体.     

Epoxy based coatings on glass: strengthening mechanisms

Glass may be strengthened by epoxy coatings although the strengthening mechanisms remain unclear. Possible strengthening mechanisms are reviewed and are used to analyse strength data for both a solvent based and a water based coating system. The coatings either fill (solvent based coatings), or partially fill (water based coatings) surface cracks and it is shown that closure stresses arising from the thermal expansion mismatch of the coating within these cracks can account for the observed degrees of strengthening. It is also demonstrated that other suggested mechanisms such as flaw healing cannot fully account for the observed degree of strengthening.     

Crack nucleation in ultrathin blown silicate glasses

Ultrathin blown glasses, owing to the very high cooling rate, have an open structure, characterized by lower hardness and higher deformability, and no tempering residual stresses. In these glasses, due to their high densificability, the threshold load for crack nucleation is higher than in air-cooled plates, and it is also above the values estimated by present theoretical formulations.     

Peculiarities of the melt growth of metal single crystals with specified crystal-geometric and structural parameters

This paper describes the authors' modifications of Kapitza and Bridgman apparatus and growth methods of metal single crystals with specified crystal geometric and structural parameters. The effects associated with defect formation are discussed. The features of the effects in their relation to the crystal lattice types and crystallographic orientation are indicated. It is concluded that one of the main factors responsible for the defect formation during single crystal growth in the mould is strain. This is due to thermomechanical stresses arising during the crystal growth and cooling to room temperature, from the difference in thermal expansion coefficients of the crystal and mould materials.     

The use of flow behavior and thermal expansion to monitor structural change of amorphous Fe78Si9B13 ribbon

Samples of the amorphous Fe₇₈Si₉B₁₃ ribbon were annealed isothermally and isochronally under different tensile stresses. These samples were also investigated using thermal expansion tests, DSC and XRD methods. The results show that a structural transition similar to the glass transition was found using thermal expansion method. Comparing the XRD patterns of the tensile stress annealed samples with those of stress free annealed samples, it is suggested that tensile stress promotes the crystallization.     

Float glass fracture toughness determination by Hertzian contact: experiments and analysis

In this work, we use a Hertzian indentation test to estimate the fracture toughness of float glass. Reference is made to two models for fracture toughness determination, that are based on two different physical approaches, the first one on the crack appearing at the surface of the sample and the second one on the cone crack developed under the surface. Our experimental fracture toughness values were higher than the one expected from the literature. By analysing the reasons for errors and limitations of the models, it has been possible, by adapting the latter to our special case of indentation, to derive a toughness value close to the one generally assumed for glass and obtained by other techniques. When using indenters with mechanical properties different from those of glass, we show that the friction between indenter and sample has an important influence on the fracture toughness determination. But other aspects, such as residual stresses or mechanical properties changes under loading, can also to some extent yield faulty estimations of the fracture toughness of glass in the Hertzian indentation test.     

Mössbauer and EPR spectra for glasses and glass-ceramics prepared from simulated compositions of Lunar and Martian soils

In situ resource processing and utilization on planetary bodies is an important and integral part of NASA’s space exploration program. Within this scope and context, our general effort is primarily aimed at developing glass and glass-ceramic type materials using Lunar and Martian soils, and exploring various application potentials of these materials for planetary surface operations. This paper reports the successful preparation of glasses from the melts of simulated composition of Lunar and Martian soils, and thermal and structural characterization using differential thermal analysis (DTA), Mössbauer and electron paramagnetic resonance (EPR) spectroscopy. Glass-ceramic materials were developed from these glasses using selective heat treatment, and were also analyzed by Mössbauer and EPR spectroscopy. The crystalline phases formed at different stages of heat treatment were analyzed and identified by X-ray diffraction (XRD). Potential applications envisaged up to this time of these glasses/glass-ceramics on the surface of Moon or Mars are discussed.     

Effect of heat-treatment condition on crystallization behavior and thermal expansion coefficient of Li2O-ZnO-Al2O3-SiO2-P2O5 glass-ceramics

Utilizing P₂O₅ as nucleation agent, a Li₂O-ZnO-Al₂O₃-SiO₂ glass was prepared by conventional melt quenching technique and subsequently converted to glass-ceramics with different crystal phases. During the processing, two-step heat-treatments including nucleation and crystallization were adopted. The effects of heat-treatment on the crystal type, the microstructure and the thermal expansion behavior of the glass-ceramics were studied by means of differential scanning calorimetry, X-ray powder diffraction analysis, scanning electron microscopy and thermal expansion coefficient tests. It was shown that the crystallization of occurred after the glass was treated at 580 °C. As the temperature increased from 580 °C to 630 °C, cristobalite and were identified as main and second crystal phases, respectively, in the glass-ceramic. An increase in the temperature to 700 °C, the β-quartz solid solution in the glass-ceramic accompanied by a decrease in cristobalite content. The transformation from to γ₀-Li₂ZnSiO₄ took place from 700 °C to 750 °C. The resulting crystallization phases in the glass-ceramics obtained at the temperature higher than 750 °C were β-quartz solid solution and γ₀-Li₂ZnSiO₄. The glass-ceramics containing or β-quartz solid solution crystal phase possessed a microstructure formed by the development of dendritic crystals. The thermal expansion coefficient of the glass-ceramics varied from 36.7 to 123.8 × 10⁻⁷ °C⁻¹ in the temperature range of 20-400 °C, this precise value is dependent on the type and the proportion of the crystalline phases presented.     

烧结法制备高炉渣CMAS系统微晶玻璃的研究

利用高炉渣及其它辅助原料制备基础玻璃,采用一步烧结法制备主晶相为辉石的CaO-MgO-Al2O3-SiO2(CMAS)微晶玻璃.综合运用DSC,XRD以及场发射扫描电子显微镜等测试手段,分析热处理制度对高炉渣CMAS微晶玻璃的析晶行为及性能的影响.结果表明:随着热处理温度上升,微晶玻璃的主晶相均为辉石,次晶相均为长石,晶相析出量增加,微晶玻璃的体积密度及抗折强度均呈现先增后减趋势.随着热处理时间增加,微晶玻璃的体积密度及抗折强度均呈现下降趋势.当热处理温度为1020 ℃,晶化时间30 min时,样品的机械性能最好,体积密度为2.690 g·cm-3,抗折强度为67.00 MPa.     

Volume relaxation of a-Se studied by mercury dilatometry

Mercury dilatometry was used to study volume relaxation of amorphous selenium (a-Se). Influence of experimental setting and reactivity of a-Se with mercury was studied using two differently constructed dilatometers (dissimilar sensitivity of sample volume change and dissimilar values of characteristic time constants of the dilatometers). Three types of volume–time–temperature experiments were made. Temperature of glass transition and thermal expansion coefficients in glassy and liquid state were determined from non-isothermal experiments. Volume relaxation was studied mainly on the basis of isothermal experiments. It was found that classical isothermal relaxation experiments are strongly influenced by the loss of information at the experiment beginning due to the proceeding relaxation. On the other hand, more complex thermal history experiments seem to be less dependent on these procedural parameters and were considered as most suitable for studying structural relaxation. The reaction of a-Se and mercury was found not to be influencing the results of volume relaxation measurements.     

Leaching of some lithium silicate glasses and glass-ceramics by HCl

The leaching of some binary and ternary lithium silicate glasses and their respective glass-ceramics by HCl is reported.The leaching rate of lithium silicate glasses gradually decreases with the decrease of the percentage of Li₂O or by the introduction of small amounts of a third component, e.g. Al₂O₃, MgO, ZnO or B₂O₃. With a further increase in the proportions of B₂O₃ or ZnO the rate of leaching increases. The rate of leaching is also substantially modified by the conversion of glasses into glasses-ceramics.The results obtained are discussed in terms of the effects of the different ions on the rate of the interdifussion of the lithium and hydrogen ions in the glass and the leached layer, the phase separation developed in the glass, the type and concentration of crystalline phases developed in glass-ceramics and the composition of the residual glass phase.