Rheological and thermodynamic behaviors of different calcium aluminosilicate melts with the same non-bridging oxygen content

The relationships between the chemical composition and the derivative rheological and thermodynamic values have been determined for two melt series in the anorthite-wollastonite-gehlenite (An-Wo-Geh) compatibility triangle. The melt series have 0.5 and 1 non-bridging oxygens per tetrahedrally coordinated cation (NBO/T), respectively. The influences of the ratio Si/(Si + AlCa_1/2) and NBO/T on the fragility and the configurational entropy at T_g are evaluated. Linear dependencies of the viscosity, the glass transition temperature and the fragility on the ratio Si/(Si + AlCa_1/2) are found for the two melt series. A crossover in the viscosity-temperature relationship is observed for both series, i.e. an inverse compositional dependence of viscosity in the high and low viscous range. The crossover presumably reflects different responses of the adjustment of melt structure to the substitution of Al³⁺ + 1/2Ca²⁺ for Si⁴⁺ in the low versus the high viscous ranges. The crossover shifts to higher temperature with increasing NBO/T.     

Creep characterization of alumina particulate reinforced alumino-silicate glass

Compressive creep studies on alumino-silicate glass reinforced with alumina powders of 0.2 μm particle size as a function of volume fraction showed that there was a significant increase in the creep resistance by about an order of magnitude in the 20 vol.% composite. In order to study the effect of particle size on creep resistance, composites reinforced with 20 vol.% of 1.2 and 6.5 μm alumina particle size were also characterized. It was found that the composite with 1.2 μm particle sized reinforcement gave the highest creep resistance. All the composites exhibited Newtonian viscous creep and showed no strain hardening even at strains up to 10%.     

Preparation, characterization and catalytic properties of singly and doubly titanium-modified mesoporous silica gel

A series of titanium-modified mesoporous silica gel have been synthesized using tetrabutyl titanate. The samples were characterized by nitrogen adsorption-desorption, FT-IR and Raman spectroscopy, as well as by solid state diffuse reflectance UV-VIS spectroscopy. Physicochemical characterization of the materials showed that Ti atoms were part of the framework of silica gel, and it was probably in a tetrahedral coordination for low Ti contents. The resulting titania modified the inner walls of the mesoporous silica gel after hydrolysis and calcination. Actually, the titanium precursor reacted and condensed with the active silanol groups on silica gel via Si-O-Ti bonds. In addition, the titanium-modified mesoporous silica gel showed distinct activity behavior in the catalytic oxidation of cyclohexene and styrene with hydrogen peroxide as oxidant.     

Causal Voigt profile for modeling reflectivity spectra of glasses

A new dielectric function model appropriate for a quantitative analysis of infrared spectra of amorphous solids and glasses is introduced and validated by the study of the infrared reflectivity spectra of two different glasses, a calcium aluminosilicate and a borosilicate. Modeling results show its superiority to the classical sum model and confirm its efficiency in the reproduction of the absorption bands of glasses located in the far infrared range.     

Photoluminescence characterization of aged and regenerated mesoporous silica

The investigation of the photoluminescence features of aged and regenerated mesoporous silica is reported. The emission spectrum of aged samples displays a blue band peaked at about 450 nm at room temperature with excitation channels at 250 and 200 nm. No UV emission band is detected. The regenerated samples recover the optical transparency of the samples, the vibrational properties in the 3000–3800 cm^? 1 but for the isolated silanols band and the UV-blue emission features. Indeed beside the blue band peaked around 450 nm a UV band centred at 340 nm is reported whose relative contribution depends on the excitation wavelength. The aging and regeneration processes are discussed in view of the attribution of the observed emission features to specific surface defects.     

Partitioning of gadolinium and its induced phase separation in sodium-aluminoborosilicate glasses

Phase separation in sodium-aluminoborosilicate glasses was systematically studied as a function of Gd₂O₃ concentration with transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), and electron energy loss spectroscopy (EELS) methods. Gadolinium-induced phase separation in the glasses can be consistently explained by proposing that Gd cations partition to the borate-rich environments and subsequent agglomeration of the Gd-borate moieties, or short-range ordered structural groups, in the glass. Agglomeration of the Gd-borate rich environments is further discussed within the context of excess metal oxides, [Na₂O]_ex or [Al₂O₃]_ex=|Na₂O-Al₂O₃|, and excess B₂O₃, [B₂O₃]_ex, available for incorporating Gd cations. Results showed that agglomeration of the Gd-borate rich environments occurred at a much lower Gd₂O₃ concentration in the glass without [Na₂O]_ex or [Al₂O₃]_ex and at a significantly higher Gd₂O₃ concentration in the glass with either [Na₂O]_ex or [Al₂O₃]_ex. Assuming 1BO₄:1Gd:2BO₃ (based on literature-reported Gd-metaborate structure) as a local Gd-borate environment in glass, we introduced the saturation index of boron, SI_[B]=Gd₂O₃/(1/3[B₂O₃]_ex), to examine the glass susceptibility to Gd-induced phase separation for all three alkali-aluminoborosilicate systems. While our results have provided some insight to the glass structure, they also provide insight to the mechanism by which the metal oxide is dissolved into the melt. This appears to occur predominately through boron complexation of the metal oxide.     

Liquid properties of Pd-Ni alloys

Liquid properties of Pd-Ni metal alloys are computed by molecular dynamics (MD) simulation with the use of quantum Sutton-Chen potential (Q-SC) model. The thermodynamical, structural, and transport properties of the alloy are investigated. The melting temperatures for Pd-Ni system are predicted. The temperature and concentration dependence of diffusion coefficient and viscosity are reported. The transferability of the potential is tested by simulating the liquid state. The values of melting point are in excellent agreement with the experiment. Comparison of calculated structural and dynamical properties with the available experiments and other calculations shows satisfactory consistency.     

An investigation of the local iron environment in iron phosphate glasses having different Fe(II) concentrations

The local environment around iron ions in iron phosphate glasses of starting batch composition 40Fe₂O₃-60P₂O₅ (mol%) melted at varying temperatures or under different melting atmospheres has been investigated using Fe-57 Mössbauer and X-ray absorption fine structure (XAFS) spectroscopies. Mössbauer spectra indicate that all of the glasses contain both Fe(II) and Fe(III) ions. The quadrupole splitting distribution fits of Mössbauer spectra show that Fe(II) ions occupy a single site whereas Fe(III) ions occupy two distinct sites in these glasses. When melted at higher temperatures or in reducing atmospheres, the Fe(II) fraction in the glass increases at the expense of Fe(III) ions at only one of the two sites they occupy. The pre-edge feature in the XAFS data suggests that the overall disorder in the near-neighbor environment of iron ions decreases with increasing Fe(II) fraction. The XAFS results also show that the average iron-oxygen coordination is in the 4-5 range indicating that iron ions have mixed tetrahedral-octahedral coordination.     

Acidity-dependent mesostructure transformation of highly ordered mesoporous silica materials during a two-step synthesis

Highly ordered mesoporous silica materials have been synthesized under mildly acidic conditions by templating with a nonionic triblock copolymer (Pluronic P104) in a two-step process. It was found that a transformation from the SBA-15 type 2-dimensional (2D) hexagonal channel mesostructure (p6mm symmetry) to the MSU-X type 3-dimensional (3D) worm-like mesostructure could be induced by varying the pH whilst keeping all other conditions constant. The transformation between two types of mesoporous silica materials can be attributed to the effect of varying proton concentration on the interaction between organic micelles and inorganic species. Both types of mesoporous materials have high surface areas, large pore volumes, thick pore walls, large mean pore sizes, and narrow pore size distribution.     

Nanostructural damage associated with isostatic compression of silica aerogels

Isostatic compression of silica aerogels is known to allow densification of these highly porous materials. However, at the onset of compression, hydrophobic and consequently slightly reacting aerogels, exhibit a decrease in bulk modulus. This unusual behavior is associated with damage occurring at low pressures which recovers with further density increase. Damage development and healing are analyzed measuring elastic modulus and, for the first time, internal friction as a function of compression. It is proposed that the origin of damage and healing could be associated with the rupture of tenuous links between clusters of dense silica particles at low density levels, and with the creation of new links between the resulting arms and reacting species that are revealed at cluster interface under higher pressure.     

The photochemical metal organic deposition of manganese oxide films from films of manganese(II) 2-ethylhexanoate: a mechanistic study

The photochemistry of thin films of manganese(II) 2-ethylhexanoate to produce manganese oxide films is presented. Thin films of manganese(II) 2-ethylhexanoate can be cast onto silicon wafers by spin coating. The FTIR spectroscopy of these films suggests they exist as chains or clusters with the manganese(II) centers linked by bridging 2-ethylhexanoate ligands. The initial photochemical reaction results in the loss of the bridging 2-ethylhexanoate ligands and the formation of monomeric manganese(II) 2-ethylhexanoate. In this process approximately 60% of the bridging 2-ethylhexanoate ligand is converted to chelating 2-ethylhexanoate while the other 40% fragments to form CO₂, heptane and heptene. Further photolysis resulted in the conversion of the remaining 2-ethylhexanoate ligand to form CO₂, heptane and heptene. The film consisted of manganese(II) oxide although the surface of the film was more oxidized and had carbon impurity on it. Annealing the film resulted in the conversion to polycrystalline α-Mn₂O₃.     

Devitrification of Ni-based glassy alloys containing noble metals in relation with the supercooled liquid region

The formation of the supercooled liquid region and devitrification behavior of Ni-based glassy alloys were studied by using X-ray diffraction, transmission electron microscopy, differential scanning calorimetry and isothermal calorimetry. oC68 Ni₁₀Zr₇-type phase is primarily formed in the studied alloys in the initial stage of the devitrification process by nucleation and three-dimensional diffusion controlled growth. The replacement of Cu by Ni in Cu₅₅Zr₃₀Ti₁₀Pd₅ glassy alloy induces precipitation of oC68 Ni₁₀Zr₇ phase directly from the glassy phase. The reasons for such a behavior are discussed taking into account mixing enthalpy in a liquid state and the interval of the supercooled liquid region.     

Synthesis and characterization of high nickel-containing mesoporous silica via a modified direct synthesis method

A method by modifying tetraethyl orthosilicate (TEOS) with nickel species has been developed for the synthesis of mesoporous silica with high nickel content (up to 14.7 wt% of Ni). Using the method, nickel-containing mesoporous materials were obtained with high BET surface area and pore volume. The materials were characterized by means of X-ray powder diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy, N₂ adsorption, Fourier transform infrared. Nickel species were incorporated into the silica frameworks. Formation of nickel phyllosilicates was also confirmed. After activation, mesostructures are still intact. Small nickel clusters embedded in the silica walls were found.     

Interpretation of the critical length scale determining the conductivity in ionically conducting silicate glasses

Implications of basic percolation theories based on a random-barrier model for hopping conductivity have been explored for the interpretation of length scales determining dc conduction in ionically conducting alumina-silicate glasses. Independent measurements of activation energy E_c, conductivity spectra and Mott-Schottky capacitance-voltage characteristics indicate a common typical size for the average extent of displacement at the frequency of onset of ac conduction, L_c∼10 nm, which is interpreted as the effective correlation length of the infinite cluster determined by the percolation threshold ξ_c=E_c/k_BT. The localization of carriers in clusters of sites between critical barriers causes a reduction of the effective mobile ion concentration (≈4 × 10¹⁶ cm⁻³) by many orders of magnitude below the nominal alkali concentration.     

Preparation and characterization of amorphous silica nanowires from natural chrysotile

By chemical dispersing and acid leaching, silica nanowires have been prepared from the natural mineral, chrysotile. X-ray fluorescence analysis (XRF), thermogravimetric analysis and differential thermogravimetric analysis (TGA–DTA), powder X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) were used to characterize the silica nanowires. The results indicate that the chemical composition of the silica nanowires is SiO_1.8 · 0.6H₂O, and although the silica is amorphous, its structure is regular to some extent. The structural unit of the silica nanowires is the [SiO₄] tetrahedron and six-member silicon–oxygen ring with the tetrahedral positioned alternately up and down in the six-member ring. The silica nanowires are well-dispersed and have cylindrical morphology and smooth surface, with lengths over 10 μm and diameters of 30–60 nm.     

Structural properties of liquid Fe-Si alloys

The atomic structure of liquid Fe-Si alloys within the whole concentration range (including pure components) was investigated at the temperature of 1550 °C by means of X-ray diffraction. Analysis of the reported data on physical properties of Fe-Si melts has been carried out. Interrelation between liquid-state structure and solid-state properties was considered. Fe-Si alloys in the liquid state are shown to be micro-inhomogeneous and they contain atomic micro-formations (clusters) that differ by atomic composition and packing. There are five types of clusters: two of them consist of atoms of one kind (Fe or Al); the composition of the other clusters depends on the stoichiometry of solid Fe₃Si, FeSi and Fe₂Si₅ phases. The entire concentration range of the Fe-Si system consists of four concentration intervals. Within each interval melts are constituted of the clusters of two kinds. The variation of the component concentrations in Fe-Si alloys results in changing of volume fraction of each type clusters, whereas the atomic composition and arrangement inside the clusters remains constant.     

Study of layered and defective amorphous solids by means of thermal wave method

In this work the thermal properties and the thickness calibration in layered thick acrylic films using the thermal wave method are reported. The results from acrylic films on two different substrates showed that thermal diffusivity and conductivity can be measured for both supports, metallic and glassy. The experimental applicability of a 1-D model and accuracy of the thickness measurement are also discussed.     

An X-ray absorption spectroscopy study of FeCo alloy nanoparticles embedded in ordered cubic mesoporous silica (SBA-16)

Nanocomposites containing FeCo alloy nanoparticles dispersed in a highly ordered cubic mesoporous silica (SBA-16) matrix were prepared using two different synthetic methods, co-precipitation and impregnation. Extended X-ray Absorption Spectroscopy (EXAFS) technique at both Fe and Co K-edges was used to investigate the structure of FeCo nanoparticles and the presence of additional disordered oxide phases. EXAFS technique gives evidence of differences in the oxidation degree of the FeCo nanoparticles depending on the synthetic method used.     

Facile synthesis of ordered large-pore mesoporous silica thin film with Im3m symmetry using n-butanol as the cosurfactant

A facile synthesis route to ordered large-pore (10.7 nm) mesoporous silica film with the cubic Im3m mesostructure is reported in a TEOS–F127–BuOH–HCl–H₂O system through dip-coating method. Characterization by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and nitrogen sorption reveals that the obtain mesoporous silica material possessed high surface area and large pore diameter. A relative comparison between the mesoporous silica films synthesized with and without BuOH is also presented. A reasonable formation mechanism of the large-pore mesoporous silica film is depicted in this work.