Structural characteristics of silica sonogels prepared with different proportions of TEOS and TMOS

Sonohydrolysis of mixtures of tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS) with different TMOS/(TMOS + TEOS) molar ratio R was carried out to obtain ∼2.0 × 10⁻³ mol SiO₂/cm³ and ∼86%-volume liquid phase wet gels. Aerogels were obtained by supercritical CO₂ extraction in autoclave. The samples were analyzed by small-angle X-ray scattering (SAXS) and nitrogen adsorption. The structure of the wet gels can be described as a mass fractal structure with fractal dimension D ∼ 2.2 and characteristic length ξ increasing from ∼4.6 nm for pure TEOS to ∼6.4 nm for pure TMOS. A fraction of the porosity is eliminated with the supercritical process. The fundamental role of the TMOS/(TMOS + TEOS) molar ratio on the structure of the aerogels is to increase the porosity and the pore mean size as R changes from pure TEOS to pure TMOS. The supercritical process increases the mass fractal dimension and shortens the fractality domain in the mesopore region. A secondary structure appearing in the micropore region of the aerogels can be described as a mass/surface fractal structure with correlated mass fractal dimension D_m ∼ 2.6 and surface fractal dimension D_s ∼ 2.3.     

Mass fractal characteristics of sonogels prepared from sonohydrolysis of tetraethoxysilane with additions of dimethylformamide

Wet silica gels with ∼1.4 × 10⁻³ mol SiO₂/cm³ and ∼90 vol.% liquid phase were prepared from the sonohydrolysis of tetraethoxysilane (TEOS) with different additions of dimethylformamide (DMF). Aerogels were obtained by CO₂ supercritical extraction. The samples were studied mainly by small-angle X-ray scattering (SAXS) and nitrogen adsorption. Wet gels exhibit a mass fractal structure with fractal dimension D increasing from 2.23 to 2.35 and characteristic length ξ decreasing from ∼9.4 nm to ∼5.1 nm, as the DMF/TEOS molar ratio is increased from 0 to 4. The supercritical process apparently eliminates some porosity, shortening the fractality domain in the mesopore region and developing an apparent surface/mass fractal (with correlated mass fractal dimension D_m ∼ 2.6 and surface fractal dimension D_s ∼ 2.3) in the micropore region. The fundamental role of the DMF addition on the structure of the aerogels is to diminish the porosity and the pore mean size, without, however, modify substantially the specific surface area and the average size of the silica particle of the solid network.     

Structural evolutions of hydrothermally prepared mesostructured MCM-48 silica using differently manufactured amorphous silica powders

The hydrothermal synthesis of Si-MCM-48 mesoporous molecular sieves was carried out using a ternary SiO₂:CTAOH:H₂O system wherein differently manufactured amorphous silica powders such as fumed silica (FMDS), spray dried precipitated silica (SDPS) and flash dried precipitated silica (FDPS) were used as silica source materials. The changes in structural/textural properties were evaluated using powder XRD, N₂ adsorption-desorption and scanning electron microscopy techniques. Studies on the progressive development of MCM-48 mesophases revealed that, the reactivity of the silica source follow the trend: FMDS > SDPS > FDPS. MCM-48 synthesized using low cost FDPS has exhibited thicker pore walls but poorer orderness, while MCM-48 prepared from relatively expensive FMDS has thinner pore walls and more ordered structure. Moreover, the extent of contraction caused by calcinations, agglomerate size and structural stability were found to depend on the reactivity of the silica source used.     

Growth of (1 0 1¯ 3¯ ) semipolar GaN on m-plane sapphire by hydride vapor phase epitaxy

The crystalline, surface, and optical properties of the (1 0 1¯ 3¯) semipolar GaN directly grown on m-plane sapphire substrates by hydride vapor phase epitaxy (HVPE) were investigated. It was found that the increase of V/III ratio led to high quality (1 0 1¯ 3¯) oriented GaN epilayers with a morphology that may have been produced by step-flow growth and with minor evidence of anisotropic crystalline structure. After etching in the mixed acids, the inclined pyramids dominated the GaN surface with a density of 2×10⁵ cm⁻², revealing the N-polarity characteristic. In the low-temperature PL spectra, weak BSF-related emission at 3.44 eV could be observed as a shoulder of donor-bound exciton lines for the epilayer at high V/III ratio, which was indicative of obvious reduction of BSFs density. In comparison with other defect related emissions, a different quenching behavior was found for the 3.29 eV emission, characterized by the temperature-dependent PL measurement.     

Three-dimensional structure of fast ion conducting 0.5Li2S + 0.5[(1 − x)GeS2 + xGeO2] glasses from high-energy X-ray diffraction and reverse Monte Carlo simulations

A high-energy X-ray diffraction study has been carried out on a series of 0.5Li₂S + 0.5[(1 − x)GeS₂ + xGeO₂] glasses with x = 0.0, 0.1, 0.2, 0.4, 0.6 and 0.8. Structure factors were measured to wave vectors as high as 30 Å⁻¹ resulting in atomic pair distribution functions with high real space resolution. The three dimensional atomic-scale structure of the glasses was modeled by reverse Monte Carlo simulations based on the diffraction data. Results from the simulations show that at the atomic-scale 0.5Li₂S + 0.5[(1 − x)GeS₂ + xGeO₂] glasses may be viewed as an assembly of independent chains of (Li^+-S)₂GeS_2/2 and (Li^+-O)₂GeO_2/2 tetrahedra as repeat units, where the Li ions occupy the open space between the chains. The new structure data may help understand the reasons for the sharp maximum in the Li⁺ ion conductivity at x ∼ 0.2.     

RDF analysis, positron annihilation and Raman spectroscopy of xTiO2-(60 − x)SiO2-40Na2O non-linear optical glasses: III. Non-bridging oxygen bonds tracing and structure analysis

xTiO₂-(60 − x)SiO₂-40Na₂O glasses have proven an interesting linear and non-linear optical properties [M. Abdel-Baki, F. Abdel Wahab, F. El-Diasty, Mater. Chem. Phys. 96 (2006) 201]. The investigated glasses show one order of magnitude enhancement for the second-order index of refraction and third-order optical susceptibility over some TiO₂ silicate glasses. In this work, we continue studying these glasses using three different techniques to analyze the glass structures seeking to provide a deep insight for the relation between structure, compositions and the optical characteristics of these glasses. Radial Distribution Function analysis (RDF) combined Raman spectroscopy are used to study these glasses. Positron annihilation lifetime spectroscopy and Doppler broadening measurements are carried out to investigate the change in the glass structure as the incorporation of TiO₂ concentration into glass. The origin of the non-bridging oxygen (NBO) bonds has been traced to correlate their existence with the measured non-linear optical properties of the investigated glasses.     

Influence of La/Ce-doping on phase transformation and crystal growth in TiO2-15 wt% ZnO gels

Titania based ceramic is a good candidate for environmental sensing materials. Ion doping is an effective method to improve the properties by modifying their microstructure and phase composition. By using differential thermal analysis and X-ray diffraction methods, the transformation behaviors of La³⁺/Ce³⁺-doped TiO₂-15  wt% ZnO gels were studied so as to modify the phase transformation and decrease the granularity of crystals. Experimental results show that, anatase, rutile and ZnTiO₃ nanocrystals can be tailored by varying La³⁺/Ce³⁺ contents and sintering temperatures. La³⁺/Ce³⁺ doping decreases the transformation temperature of gel to anatase and the forming temperature of ZnTiO₃ phase, enhances the transformation temperature of anatase to rutile and results in appearance of Zn₂Ti₃O₈ interphase. With the increases of La³⁺/Ce³⁺ contents, the transformation rates of ZnTiO₃, gel to anatase and anatase to rutile, as well as the granularity of the crystals are reduced. Ultimately, the action mechanism of La³⁺/Ce³⁺ doping was discussed.     

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.     

Preparation of a crack-free AlN template layer on sapphire substrate by hydride vapor-phase epitaxy at 1450 °C

A crack-free aluminum nitride (AlN) template layer was grown on a (0 0 0 1) sapphire substrate at 1450 °C using a thin (100 nm) protective AlN layer grown at 1065 °C by hydride vapor-phase epitaxy (HVPE). Full-width at half-maximum (FWHM) values of X-ray rocking curves (XRCs) for (0 0 0 2) and (1 0 1¯ 0) planes of the AlN layer were 378 and 580 arcsec, respectively. The formation of voids was observed at the interface between the thin protective AlN layer and the sapphire substrate due to decomposition reaction of sapphire during heating up to 1450 °C. The voids relaxed the tensile stress in the AlN layer, which resulted in the suppression of cracks.     

Synthesis and Judd-Ofelt analysis of a sol-gel material doped with a Ln2(Benzoate)6(Phen)2 (Ln = Er/Yb) complex

In this work, we have prepared a sol-gel derived hybrid material directly doped with Er_1.4Yb_0.6(Benzoate)₆(Phen)₂ (Phen = 1,10-phenanthroline) complex, which was reported with intramolecular Yb-Er energy-transfer process in our previous work. The infrared (IR) spectra of the pure complex and hybrid gel material were investigated. The NIR photoluminescence (PL) spectrum of hybrid gel material shows strong characteristic emission of Er³⁺ with broad full width at half-maximum (FWHM) of 70 nm. Judd-Ofelt theory was used in order to analyze the optical properties of Er³⁺ ions in the hybrid gel material.     

Epitaxial films of metals from the platinum group (Ir,Rh, Pt and Ru) on YSZ-buffered Si(1 1 1)

In the present work we have grown twin-free single crystal metal films of iridium (Ir), rhodium (Rh), platinum (Pt) and ruthenium (Ru) on silicon (1 1 1) substrates via an yttria-stabilized zirconia (YSZ) buffer layer. A prerequisite for the realisation of heteroepitaxial metal films without additional texture components was the twin-free deposition of the YSZ films by pulsed laser deposition (PLD). For the metal films on top, a novel two-step growth process was applied with an extremely low deposition rate for the first 20 nm. For all metals, a drastic texture improvement by up to a factor of 9 could be observed compared to the oxide buffer layer. Minimum values were 0.18° (Ir) and 0.12° (Rh) for tilt and twist, respectively. For all four metals investigated, twin-free epitaxial films could be grown on YSZ/Si(1 1 1) whereas the twinning problem for platinum films was solved by decoupling the Pt-YSZ interface via an additional iridium interlayer. The grown metal/YSZ/Si(1 1 1) multilayer samples offer the possibility to integrate a variety of interesting nanostructures and functional materials on silicon. They are now available in 4 in wafer size.     

Local atomic and electronic structures of equiatomic liquid alloy KSb from 923 to 1773 K

The local atomic and electronic structures of equiatomic liquid alloy KSb have been investigated using ab initio molecular dynamics simulations. There is a small peak located at about 62° in the covalent SbSb bond-angle distribution function. The height of 62° peak increases with temperature. The further analysis show that this peak attributes to the triangular structures with each Sb atom covalently bonded to the other two in the liquid KSb and their number increases with temperature. Therefore, there exist isolated Sb atoms, covalently bonded Sb dimers, triangles with three covalent SbSb bonds and short covalently bonded Sb chains in the liquid KSb. Upon temperature increasing the covalent Sb chains tend to break into short pieces so that the isolated Sb atoms, covalently bonded Sb dimers and triangles with three covalent SbSb bonds increase in numbers. The obtained total electronic densities of states at different temperatures give a reasonable explanation of the electronic conductivity increasing with temperature.     

Structural,electrical and optical properties of SnO2 films deposited on Y-stabilized ZrO2 (1 0 0) substrates by MOCVD

SnO₂ films have been deposited on Y-stabilized ZrO₂ (YSZ) (1 0 0) substrates at different substrate temperatures (500–800 °C) by metalorganic chemical vapor deposition (MOCVD). Structural, electrical and optical properties of the films have been investigated. The films deposited at 500 and 600 °C are epitaxial SnO₂ films with orthorhombic columbite structure, and the HRTEM analysis shows a clear epitaxial relationship of columbite SnO₂(1 0 0)||YSZ(1 0 0). The films deposited at 700 and 800 °C have mixed-phase structures of rutile and columbite SnO₂. The carrier concentration of the films is in the range from 1.15×10¹⁹ to 2.68×10¹⁹ cm⁻³, and the resistivity is from 2.48×10⁻² to 1.16×10⁻² Ω cm. The absolute average transmittance of the films in the visible range exceeds 90%. The band gap of the obtained SnO₂ films is about 3.75–3.87 eV.