A glass with high crack initiation load: Role of fictive temperature-independent mechanical properties

The crack initiation load of a series of calcium aluminosilicate glasses and selected commercial glasses were evaluated using Vickers indentation. The results showed that a calcium aluminosilicate glass containing 80 mol% SiO₂, 10 mol% Al₂O₃ and 10 mol% CaO exhibited a high crack initiation load comparable to that of the less-brittle glass (LB glass) developed by Asahi Glass Co., Ltd. It has previously been determined that glasses experience a fictive temperature increase by indentation. The indented region of a glass, therefore, acquires, in general, different mechanical properties, such as hardness and elastic moduli, from the original, unindented glass. The extent of these mechanical property changes depends upon the glass composition and a certain glass composition with fictive temperature-independent mechanical properties can have the deformed region with matching mechanical properties to those of the undeformed region of the glass. It was found that the calcium aluminosilicate glass having no fictive temperature dependence on elastic moduli gave the highest crack initiation load. However, this composition did not coincide with fictive temperature-independence of hardness or density.     

Tantalum based bulk metallic glasses

Ta-based bulk metallic glasses with high strength (2.7 GPa) and hardness (9.7 GPa), high elastic modulus (170 GPa) and high density (12.98 g/mm³) were developed. The best glass forming ability so far for a Ta-Ni-Co system reaches a critical diameter of 2 mm by the copper mold casting method. It shows an exceptionally high glass transition temperature of 983 K and a high crystallization temperature up to 1023 K. The unique mechanical and physical properties make them a promising high strength material.     

Tin valence and local environments in silicate glasses as determined from X-ray absorption spectroscopy

X-ray absorption spectroscopy (XAS) was used to characterize the tin (Sn) environments in four borosilicate glass nuclear waste formulations, two silicate float glasses, and three potassium aluminosilicate glasses. Sn K-edge XAS data of most glasses investigated indicate Sn⁴⁺O₆ units with average Sn-O distances near 2.03 Å. XAS data for a float glass fabricated under reducing conditions show a mixture of Sn⁴⁺O₆ and Sn²⁺O₄ sites. XAS data for three glasses indicate Sn-Sn distances ranging from 3.43 to 3.53 Å, that suggest Sn⁴⁺O₆ units linking with each other, while the 4.96 Å Sn-Sn distance for one waste glass suggests clustering of unlinked Sn⁴⁺O₆ units.     

Studies of some mechanical and optical properties of: (70 − x)TeO2 + 15B2O3 + 15P2O5 + xLi2O glasses

Specimens of the glassy system: (70 − x)TeO₂ + 15B₂O₃ + 15P₂O₅ + xLi₂O, where x = 5, 10, 15, 20, 25 and 30 mol% were prepared by the melt-quenching. An ultrasonic pulse-echo technique was employed, at 5 MHz, for measuring: the ultrasonic attenuation, longitudinal and shear wave velocities, elastic moduli, Poisson ratio, Debye temperature and hardness of the present glasses. It is found that the gradual replacement of TeO₂ by Li₂O in the glass matrix up to 30 mol% leads to decrease the average crosslink density and rigidity of prepared samples which affects the properties, i.e., the hardness, ultrasonic wave velocities and elastic moduli are decreased, while the Poisson ratio and the ultrasonic attenuation are increased. Also, optical absorption spectra were recorded in the range, 200-800 nm for these glasses. The obtained results showed that a gradual shift in the fundamental absorption edge toward longer wavelengths occurred. Values of both of the optical energy gap, E_opt, and width tails, ΔE, are determined. It is observed that E_opt is decreased and ΔE increased with the increase of Li₂O in the glass matrix up to 30 mol%. The compositional dependences of the above properties are discussed and correlated to the structure of tested glasses.     

Nano- and micro-indentation studies on lithium borate-barium bismuth niobate glasses

The structure and mechanical properties of multifunctional lithium tetra-borate based glasses and glass-ceramics of the system (100 − x) Li₂B₄O₇₋x(BaO-Bi₂O₃-Nb₂O₅) with x = 10, 20 and 30 in molar ratio, have been characterized. Nano- and micro-indentation techniques were employed to evaluate the elastic modulus, hardness and toughness of the as-cast and annealed glasses. These were complemented with detailed structural investigations using X-ray diffraction, microscopy (optical, scanning electron and high resolution transmission electron microscopies) and nuclear magnetic resonance spectroscopy. These investigations reveal a smooth variation of the mechanical properties with composition except for the composition corresponding to x = 20. This deviation has been attributed to subtle changes in the glass-structure due to amorphous phase-separation and heat-treatment-assisted nano-crystallization.     

Nanoindentation of glass wool fibers

The nanoindentation technique is used to analyze the depth dependence of the hardness and the reduced elastic modulus of bulk glasses and glass wool fibers (4-12 μm in diameter) of calcium aluminosilicate composition. In spite of the fiber geometry and the delicate sample mounting-technique, nanoindentation proves to be a relatively accurate method that provides reproducible data for both hardness (H) and reduced elastic modulus (E_r) of thin glass fibers. It is found that H and E_r are generally lower for the fiber than for the bulk sample. Within a given fiber, both H and E_r are approximately constant with increasing indentation depth. However, both of these parameters decrease with diminishing fiber diameter. This trend is attributed to an increase of the free volume of the fibers with decreasing fiber diameter, i.e. to an increase of the fictive temperature.     

Development and properties of a glass made from MSWI bottom ash

A uniform shiny black-coloured glass was obtained using bottom ash produced by a Portuguese municipal solid waste incinerator (MSWI). The bottom ash was the single batch material used in the formation of the glass, which was obtained by vitrification of the solid waste at 1400 °C for 2 h. Under these conditions, a homogeneous melt with an appropriate viscosity to be shaped was obtained, indicating the suitability of this waste material to be employed in the development of vitreous products. The characterization of the resulting glass was performed in order to assess its structural, physical, mechanical, thermal and chemical features. The glass had a density of 2.69 g cm⁻³, a hardness of 5.5 GPa, a fracture strength of 75 MPa, a thermal expansion coefficient of 9.5 × 10⁻⁶ °C⁻¹ and it exhibited a very good chemical stability. In summary, the MSWI bottom ash glass has good mechanical and chemical properties and may, therefore, be used in several applications, particularly as a construction material.     

Elasticity, thermal expansion and compressive behavior of Mg65Cu25Tb10 bulk metallic glass

The existence of special covalently bonded short-range ordering structures in a Mg₆₅Cu₂₅Tb₁₀ bulk metallic glass (BMG) is confirmed by thermal expansion and compression behavior. Under ambient conditions the linear thermal expansion coefficient obtained is almost constant in the glassy state with a value of 4.0 × 10⁻⁵ K⁻¹. By fitting the static equation of state at room temperature under ambient conditions we find the value for bulk modulus B of 48.7 GPa, which is in excellent agreement with the experimental study by pulse-echo techniques of 44.7 GPa. Unlike many bulk metallic glasses, such as Zr- and Pd-based, which bulk modulus is much larger than 100 GPa, the value B of Mg₆₅Cu₂₅Tb₁₀ BMG falls into the range of SiO₂ and fluorozirconate glass ZBLAN. Moreover, the elastic constant of the Mg₆₅Cu₂₅Tb₁₀ BMG is almost the same as those of ZBLAN. No evidence for the high-pressure phase transitions of the Mg₆₅Cu₂₅Tb₁₀ BMG has been found up to 31.19 GPa at room temperature.     

Mechanical properties of a TAS fiber: a preliminary study

The mechanical properties, including elastic moduli, hardness, fracture toughness and tensile strength of a glass fiber in the Te-As-Se system (TAS) were studied. The values for the hardness (1.4 GPa) and the fracture toughness (0.18 MPa √m) show that this glass is both soft and brittle in comparison to glasses from other systems. However, indentation measurements should be interpreted with caution due to an indentation creep phenomenon and to a delayed fracture process. In addition, the effect of treatments in air (relative humidity about 60%) at different temperatures below T_g were investigated. The main result of this study is that the studied TAS glass is sensitive to the presence of humidity, and aging treatments have a pronounced detrimental effect on the strength of the uncoated fibers.     

Synthesis and mechanical properties of Zr-based bulk amorphous alloys

Design and characterization of bulk amorphous alloys has been an active area of research due to their promising thermal and mechanical properties. An alloy composition Z₆₅Cu₁₇Ni₁₀Al₈ was designed and synthesized by Cu mold casting. In the base alloy 2 at.% Gd was added to study its effect on thermal and mechanical properties. Characterization of the alloys was done by techniques of X-ray diffraction (XRD), differential scanning calorimetry (DSC) and field emission scanning electron microscopy (FESEM). Many thermal parameters were evaluated to investigate the thermal stability and glass-forming ability of the alloys. In addition, the mechanical properties like nanohardness, elastic modulus and elastic recovery were measured. Thermal properties and activation energy reduced while mechanical properties like nanohardness, elastic modulus and percentage elastic recovery (% R) increased with Gd addition.     

Oxygen and silver diffusion into float glass

In the float glass process, molten glass is floated on a molten metallic tin bath, such that tin penetrates the glass surface. Consequently, the glass has distinctly two different faces; the tin-penetrated face (bottom face) and the opposite face (top face). In this paper, the effects of tin on oxygen and silver diffusion into the top and bottom faces of a soda–lime–silica float glass are reported. It was revealed that oxygen diffusion from the atmosphere into the bottom face at temperatures above glass transition temperature was extremely suppressed near the surface region of the glass. This was not observed for the top face. This effect was ascribed to chemical reactions between the diffused oxygen and Sn²⁺ near the surface of the glass. Silver diffusion was also influenced by the tin due to chemical reactions of Ag⁺ ions with Sn²⁺, leading to the precipitation of nanometer-sized Ag crystals. As a result, the coloration due to the surface plasmon resonance of the Ag crystals was significantly different between the top and bottom faces because of differences in the nucleation and growth processes associated with the Ag crystals.     

Effects of Nb2O5 Replacement by Er2O3 on elastic and structural properties of 75TeO2-(10 − x)Nb2O5-15ZnO-(x)Er2O3 glass

Tellurite 75TeO₂-(10 − x)Nb₂O₅-15ZnO-(x)Er₂O₃; (x = 0.0-2.5 mol%) glass system with concurrent reduction of Nb₂O₅ and Er₂O₃ addition have been prepared by melt-quenching method. Elastic properties together with structural properties of the glasses were investigated by measuring both longitudinal and shear velocities using the pulse-echo-overlap technique at 5 MHz and Fourier Transform Infrared (FTIR) spectroscopy, respectively. Shear velocity, shear modulus, Young's modulus and Debye temperature were observed to initially decrease at x = 0.5 mol% but remained constant between x = 1.0 mol% to x = 2.0 mol%, before increasing back with Er₂O₃ addition at x = 2.5 mol%. The initial drop in shear velocity and related elastic moduli observed at x = 0.5 mol% were suggested to be due to weakening of glass network rigidity as a result of increase in non-bridging oxygen (NBO) ions as a consequence of Nb₂O₅ reduction. The near constant values of shear velocity, elastic moduli, Debye temperature, hardness and Poisson's ratio between x = 0.5 mol% to x = 2.0 mol% were suggested to be due to competition between bridging oxygen (BO) and NBO ions in the glass network as Er₂O₃ gradually compensated for Nb₂O₅. Further addition of Er₂O₃ (x > 2.0 mol%) seems to further reduce NBO leading to improved rigidity of the glass network causing a large increase of ultrasonic velocity (v_L and v_S) and related elastic moduli at x = 2.5 mol%. FTIR analysis on NbO₆ octahedral, TeO₄ trigonal bipyramid (tbp) and TeO₃ trigonal pyramid (tp) absorption peaks confirmed the initial formation of NBO ions at x = 0.5 mol% followed by NBO/BO competition at x = 0.5-2.0 mol%. Appearance of ZnO₄ tetrahedra and increase in intensity of TeO₄ tbp absorption peaks at x = 2.0 mol% and x = 2.5 mol% indicate increase in formation of BO.     

Anionic structure and elasticity of Na2O-MgO-SiO2 glasses

The structure and elastic properties of a series of xNa₂O · MgO · 4SiO₂ glasses have been studied using both Raman and Brillouin spectroscopy. Relative to Na₂O-SiO₂ glasses, the maximum abundance for phyllosilicate structural units in the present glasses shows a lag of 0.5 units in the number of non-bridging oxygen per silicon atom (NBO/Si). This phenomenon has been attributed to the decrease in the average coordination number of modifying cations due to the presence of Mg²⁺. It has also been found that the decomposition of both metasilicate and disilicate (dimerized SiO₄) anionic structural units in Na₂O-SiO₂ glasses are enhanced by the addition of MgO. However, the presence of Mg²⁺ does not cause a considerable effect on the decomposition of phyllosilicate structural unit. The acoustic data have revealed that both shear and Young’s moduli of the present glasses decrease with increasing NBO/Si (the variation in bulk modulus is reversed, however). The resistance to shear deformation for the anionic structural units in silicate glasses has been found to decrease in the following order: tectosilicate > phyllosilicate > metasilicate > disilicate > orthosilicate. The relative contribution of the various anionic structural units to the bulk modulus of a glass remains to be determined. The ideal mixing model using Makishima-Mackenzie’s relationship for predicting Young’s modulus is not applicable to the present glasses.     

Surface oxidation of basalt glass/liquid

To evaluate surface oxidation of Fe²⁺-rich multi-component silicate glass, powder and pieces of natural basalt glass are heat-treated in Ar and subsequently investigated by Mössbauer spectroscopy to monitor the increase of Fe³⁺. Glass pieces show no increase in oxidation with time or temperature, suggesting that the oxygen potential between glass and Ar is insufficient to cause volume oxidation. In contrast, glass powder oxidizes readily to a degree comparable with that of powder oxidation in air, suggesting that surface oxidation does not depend on the oxygen potential. No cation diffusion to the glass surface is detected in Ar, though it is observed upon heat treatment in air; cation diffusion is therefore unlikely to be involved in oxidation. We suggest the following mechanism for surface oxidation: (1) adsorption of water on the glass surface as OH⁻, by exposure to air and (2), a concomitant reaction, i.e., oxidation with the glass, upon heating (chemisorption). Hereby, either oxygen of air or residual oxygen in Ar would react with the hydrogen of the -OH, liberating the oxygen for oxidation of iron. Heat treatment in vacuum of 10⁻⁸ mbar does not result in any oxidation, and we assume that the adsorbed OH is exhausted from the glass surface.     

Mechanical behavior under nanoindentation of a new Ni-based glassy alloy produced by melt-spinning and copper mold casting

An investigation was made into the thermal stability and mechanical behavior under nanoindentation of a new glassy alloy with composition Ni₅₀Nb₂₈Zr₂₂, produced in the form of melt-spun ribbons and copper mold-cast wedges. The alloy composition was designed based on the lambda criterion combined with the electronegativity difference among the elements. X-ray diffraction and scanning electron microscopy confirmed that the ribbons and wedges (up to 200 μm in thickness) are amorphous. The thermal properties of these samples were evaluated by differential scanning calorimetry (DSC). Nanoindentation revealed that the hardness of this alloy, around 10 GPa, is among the highest reported for metallic glasses. Remarkably, the cast wedge exhibits greater hardness and higher elastic modulus than the ribbon. This correlates with the larger amount of frozen-in free volume in the ribbons than in the cast wedges, as evidenced by DSC. In addition, finite element simulations of nanoindentation curves were performed. The Mohr-Coulomb yield criterion allows for better adjustment of the experimental data than the pressure-independent Tresca yield criterion. The simulations also reveal that the cohesive stress in the ribbons is lower than in the wedges, which explains the difference in hardness and Young's modulus between the two samples.     

Synthesis and optical properties of purified translucent,orthorhombic boron nitride films

Large-area (>1 cm²) freestanding translucent orthorhombic boron nitride (oBN) films have been synthesized by magnetron sputtering at a low radio-frequency power of 120 W. The structural characterizations were performed by means of X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. It is demonstrated that oBN is a direct band gap semiconductor (E_g∼3.43 eV). Excited by ultraviolet laser (wavelength at 325 nm), the oBN films emit strong white light, which can be seen by the naked eyes in the dark. In the photoluminescence spectrum, besides the ultraviolet near-band-edge radiative recombination emission, there are three visible emission bands (centered at 400, 538, and 700 nm) arising from the defect-related deep-level centers of oBN, which are mixed to form the white light emission. The hardness and elastic modulus of oBN films are 11.5 and 94 GPa, respectively, examined by nanoindentation measurements.     

Thermal properties and crystallization of iron phosphate glasses containing up to 25 wt% additions of Si-, Al-, Na- and U-oxides

The thermal properties (expansion, T_g and T_SOFT.) of glasses, having 56-66% P₂O₅, 14.8-34.2% Fe₂O₃ and 2-25 wt% additions of SiO₂, Al₂O₃, Na₂O and UO₂, were comparatively estimated from dilatometric measurements in similar conditions. The T_g reversibility was clearly verified by varying the heating rates between 1 and 5 °C min⁻¹. From linear equations fits of the various glass properties as functions of the six components it is suggested the iron, sodium and uranium oxides decrease the thermal expansion (for 50 < T ? 300 °C), T_g and T_SOFT. From DTA/XRD analysis of three glasses it was confirmed the crystallization tendency decreased with increasing the UO₂ level in the glasses. Leaching test data for two compositions containing Na₂O suggest addition of UO₂ increases the chemical durability of the related glass. The roles of UO₂, Na₂O and Fe-oxide species as structural components of the glass network are discussed.     

Silicate nanoparticles by bioleaching of glass and modification of the glass surface

Bioleaching is examined as a low temperature (50 °C) soft chemical approach to nanosynthesis and surface processing. We demonstrate that fungus based bioleaching of borosilicate glass enables synthesis of nearly monodispersed ultrafine (∼5 ± 0.5 nm) silicate nanoparticles. Using various techniques such as X-ray diffraction, X-ray photoelectron spectroscopy and FTIR we compare the constitution and composition of the nanoparticles with that of the parent glass, and establish the basic similarities between the two. The bioleaching process is shown to enhance the non-bridging oxygen component and correspondingly influence the Si-O-Si network. The root mean square roughness of glass surface is seen to increase from 1.27 nm for bare glass to 2.52 nm for 15 h fungal processed case, this increase being equivalent to that for glass annealed at 500 °C.     

Preparation and characterization of monolithic alumina aerogels

Alumina aerogels were prepared by a sol-gel method combined with the ethanol supercritical drying technique using aluminum tri-sec butoxide and nitric acid as the precursor and catalyzer respectively. This method affords high-surface-area alumina aerogel monoliths without the use of complexing agents. The structure and morphology of the aerogels were investigated by TEM, XRD, FTIR and BET techniques. The results confirmed that the as-prepared alumina aerogel possessed a network microstructure made up of pseudoboehmite fibers and a surface area of 690 m²/g. It was transformed to γ-Al₂O₃ after heat treatment at 800 °C without a significant loss in surface area. DMA analysis and hotdisk thermal analysis were performed to characterize the mechanical and thermal properties of the samples. The results indicated that the alumina aerogel was robust and exhibited excellent thermal insulating properties. The elastic modulus was up to 11.4 MPa after drying, which is the one of the highest modulus of alumina aerogels ever reported. The thermal conductivities at 30 °C and 400 °C were 0.028 W/mK and 0.065 W/mK respectively.     

Precipitation of nanometer-sized SnO2 crystals and Sn depth profile in heat-treated float glass

The effect of oxygen diffusion from the atmosphere on tin depth profile in the bottom face of a soda-lime-silica float glass at temperatures above T_g was investigated. The heat treatment was performed in ¹⁸O₂/N₂ and argon (Ar) atmospheres. The significant diffusion of tin to the surface was observed for the glass heat-treated in ¹⁸O₂/N₂ atmosphere, resulting in the formation of a tin-enriched layer near the surface region. It was found that the tin was supplied from the region shallower than the ‘hump’ which is commonly observed in the tin profile of a commercial soda-lime-silica float glass. No significant change in the tin depth profile was observed for the glass heat-treated in Ar atmosphere. These results indicate that ¹⁸O diffusion into the glass, which causes the change in chemical state of tin from Sn²⁺ to Sn⁴⁺, induces the significant diffusion of tin. Furthermore, the precipitation of crystalline SnO₂ particles with a diameter of ∼1 nm was clearly recognized in the tin-enriched layer. This fact indicates that a phase separation was induced by the oxygen diffusion into the glass. Consequently, Sn²⁺ may be supplied to the surface in order to compensate for the marked decrease in Sn²⁺ concentration in the glass system. The significant diffusion of tin to the surface was suppressed by increasing the iron content in the glass. This suppression was ascribed to the increase in Sn⁴⁺ concentration as a result of the redox reaction between tin and iron because the diffusion coefficient of Sn⁴⁺ is much smaller than that of Sn²⁺.