Raman Spectroscopic Investigations of the Effects of Ag+ and Ce3 + Doping on the Densification of Nanoporous Silica Xerogels

Doped and undoped 50 Å porous silica xerogels were heat-treated at various annealing temperatures ranging from 800°C to 1150°C. The heating was performed in air for 1 hour at each temperature. Raman spectroscopy was used to follow the structural changes occurring at various stages of the gel-to-glass transformation, as well as to investigate the effects of metal ions on the densification of these nanoporous silica xerogels. Raman data and density measurements showed that while densification is completely achieved at 1050°C for undoped xerogels, it occurs at 950°C for Ag⁺-doped samples. On the other hand, Ce^{3 +} doping was found to slow down the densification process, with complete densification occurring at 1100°C.     

High Temperature Behavior of a Gel-Derived SiOC Glass: Elasticity and Viscosity

The high temperature behavior of a sol-gel derived silicon oxycarbide glass containing 12 at.% carbon has been characterized by means of creep and in-situ ultrasonic echography measurements. Temperature induced changes include structural relaxation and densification from 1000 to 1200°C, and crystallization to form a fine and homogeneous -SiC/glass-matrix nanocomposite with 2.5 nm large crystals above 1200°C. Young's modulus measurements clearly reveal a consolidation of the material upon annealing below 1200°C. Crystallization is almost complete after few hours at 1300°C and results in a significant increase in Young's modulus. The viscosity of the oxycarbide glass is much higher than that of fused silica, with two orders of magnitude difference at 1200°C, and the glass transition temperature ranges from 1320 to 1370°C.     

Study of the crystallization kinetics in amorphous Fe83P17 alloy

The crystallization kinetics of Fe₈₃P₁₇ amorphous alloy has been studied by Mössbauer spectroscopy and X-ray diffractometry. The samples were annealed isothermally at two different temperatures (315 °C and 325 °C). During isothermal annealing of the samples three phases were observed: crystalline Fe₃P phase, crystalline -Fe phase and the amorphous phase. The value of the Avrami exponent was found to be about 2.0 at each annealing temperature. This suggests that the growth rate of the crystals is controlled by volume diffusion and the nucleation rate decreases during crystallization. The activation energy obtained for the overall crystallization process was 193±43 kJ mol^–1.     

Structural and Photoluminescence Behaviors of Nano-Structure Thin Film and Bulk Silica Gel Derived Glasses

Nano-structure bulk and thin film silica gel derived glasses were prepared by sol-gel technique. Both samples were derived from the same precursor and subjected to the same heat-treatment regime. Structural information about prepared samples are obtained by analyzing the XRD patterns and TEM micrographs. The bulk samples phase changes from amorphous to -crystoballite at higher temperature (1300°C) than that in the thin film (500°C). The crystallite size depends to a large extent on the heat-treatment temperature. Bulk sample heat treated at 1400°C was as small as 10.4 nm. Thin film samples show higher response to heat-treatment temperature than the bulk samples, where the film is denser, has smaller pores and seems more homogeneous at lower temperature than bulk sample as revealed by SEM. The observed Raman spectra for bulk and thin film samples are in accordance with that of the -crystoballite. The Raman peak intensity is higher for thin film than bulk samples. The photoluminescence PL measurements for bulk samples show a broad intense peak at 532 nm combined with three weak peaks at longer wavelength 587, 635 and 666 nm. The PL peak intensity shows a reasonable decrease with increasing the heat-treatment temperature while the peak position shifted slightly to a lower wavelength. While the thin film samples show a unique peak at wavelength = 523 nm. The appearance of PL bands are interpreted on the light of non-bridged oxygen hole center as well as the structure defects.     

Nucleation of Titania Nanocrystals in Silica Titania Waveguides

SiO_{2(1 – x)}-TiO_2(x) monomode waveguides at 632.8 nm, with x in the range 0.07–0.2 and thickness of about 0.4 m, were deposited on silica substrates by a dip-coating technique. Nucleation of TiO₂ nanocrystals and the growth of their size by thermal annealing up to 1300°C were studied by waveguided Raman scattering in the SiO_2(0.8)-TiO_2(0.2) composition. In the low frequency region (5–50 cm^–1) of the VV and HV polarized Raman spectra the symmetric and quadrupolar acoustic vibrations are observed. The mean size of the titania particles are obtained from the frequencies of the Raman peaks. The results are compared with those obtained from the measure of the linewidths in the X-ray diffraction spectra. Nanocrystals with a mean size in the range 4–20 nm are obtained by thermal annealing in a corresponding range of 700–1300°C.     

Silicon Carbide Modified Carbon Materials. Formation of Nanocrystalline SiC from Thermochemical Processes in the System Coal Tar Pitch/Poly(carbosilane)

Poly(carbosilane) or PCS, {–CH₂–SiH(CH₃)–} _n , is used as a Si-bearing precursor in combination with a coal tar pitch to study thermally induced transformations toward SiC-modified carbon composites. Following mixing of the components in the molten pitch at 160°C, the mixture is heated under argon atmosphere at 500°C yielding a solid carbonizate that is further subjected to separate pyrolysis experiments at 1300°C or 1650°C. At temperatures up to 500°C, the PCS reacts with suitable pitch components as well as undergoing decomposition reactions. At higher temperatures, clusters of prevailingly nanocrystalline -SiC are confirmed after the 1650°C pyrolysis step with indications that the formation of the compound starts at 1300°C. ²⁹Si MAS NMR, XRD, FT-IR, XPS, and elemental analysis are used to characterize each pyrolysis step, especially, from the viewpoint of transformation of silicon species to silicon carbide in the carbon matrix evolved from the pitch.     

Preparation of Porous Cordierite with Thermally Stable Pore Structure

A cordierite ceramic with a thermally stable pore structure was prepared by a simple modification of a sol-gel reaction of alkoxide precursors, synthesized from Mg metal or Mg acetate, Al(i-OPr)₃, and partially prehydrolyzed Si(OEt)₄. For aging and drying, wet cordierite gel was treated with water vapor at 150°C to strengthen the gel network through enhanced hydrolysis and condensation reactions. The cordierite xerogels showed BET surface areas between 220–410 m²/g, depending on the catalyst and treatment conditions used. In particular, water vapor treated xerogel displayed comparatively thermally stable pore characteristics, which exhibited only a 4–17% decrease in BET surface area up to 700°C, while samples prepared using the conventional sol-gel method showed a 55–91% reduction. Both the DTA and XRD patterns showed that crystallization began at 900°C leading to the -Cordierite phase and the subsequent phase transition to -cordierite at temperatures between 1050 to 1250°C.     

Generation of WO3-ZrO2 catalysts from solid solutions of tungsten in zirconia

WO₃-ZrO₂ samples were obtained by precipitating zirconium oxynitrate in presence of WO₄ species in solution from ammonium metatungstate at pH=10.0. Samples were characterized by atomic absorption spectroscopy, thermal analysis, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy and energy filtered-TEM. The ammonia retained in the dried sample produced a reductive atmosphere to generate W⁵⁺ ions coexisting with W⁶⁺ ions to produce a solid solution of tungsten in the zirconia lattice to stabilize the zirconia tetragonal phase when the sample was annealed at 560 °C. When the sample was annealed at 800 °C, the W atoms near crystallite surface were oxidized to W⁶⁺, producing patches of WO₃ on the zirconia crystallite. The HR-TEM analysis confirmed the existence of the solid solution when the sample was annealed at 560 °C, and two types of crystalline regions were identified: One with nearly spherical morphology, an average diameter of 8 nm and the atomic distribution of tetragonal zirconia. The second one had a non-spherical morphology with well-faceted faces and dimensions larger than 30 nm, and the atom distribution of tetragonal zirconia. When samples were annealed at 800 °C two different zirconia crystallites were formed: Those where only part of the dissolved tungsten atoms segregated to crystallite surface producing patches of nanocrystalline WO₃ on the crystallite surface of tetragonal zirconia stabilized with tungsten. The second type corresponded to monoclinic zirconia crystallites with patches of nanocrystalline WO₃ on their surface. The tungsten segregation gave rise to the WO₃-ZrO₂ catalysts.     

Modification of Non-Hydrolytic Sol-Gel Derived Alumina by Solvent Treatments

The effect of wetting non-hydrolytic derived alumina xerogels with water and organic solvents in the 20–70°C range on the alumina's properties was investigated. Wetting with organic solvents does not affect the alumina. However, contact with water was found to change the sharp crystallization at 800°C to a continuous crystallization starting at 450°C. Water treatment for a day at room temperature (RT) followed by second calcination decreased the surface area by 10%. This decrease in surface area is less pronounced with increasing wetting periods. On the other hand water treatment at 50–70°C followed by a second calcination resulted in a surface area increase of up to 15%. Upon water treatment the total pore volume has decreased from 0.65 (cm³/gr) to 0.48 (cm³/gr) and the average pore size decreased from 6.8 nm to 4.1 nm. The Cl content was found to be uneffected by the water treatment, remaining at 2.5% wt. Wetting with water at elevated temperature (70°C) accelerated the morphological changes, eliminating the crystallization peak at 800°C in one hour. A dissolution-reprecipitation mechanism is suggested to explain the results. In addition, Mass-Spectroscopy of the effluent gas during heat treatment revealed the emission of CO₂ and water upon phase transition into -Al₂O₃, at 1150–1300°C.     

Structural investigations of sol-gel-derived LiYF4 and LiGdF4 powders

A soft synthesis route based on the sol-gel process was used for preparing rare-earth tetrafluoride powders from alkoxide precursors. In-situ fluorination was performed by decomposition of a fluorine containing organic compound named 1,1,1-trifluoro-5-methyl-2,4-hexanedione when sintering the as-prepared xerogel to produce crystallized samples. Both to insure complete departure of organic residues as well as to avoid any oxidation into oxyfluoride, annealing treatment was carried out under fluorine atmosphere. Free-oxygen content of resulting samples was evidenced by infrared and Raman spectroscopies. X-ray absorption spectroscopies (XAS) and ¹⁹F nuclear magnetic resonance (NMR) studies showed that samples heat treated at 300 °C are already crystallized but for a full crystallization in LiGdF₄ and LiYF₄ a thermal treatment at 550 °C is needed. Temperature dependence of powder morphology was analyzed by scanning electron microscopy (SEM).     

Preparation and Characterization of TiO2 Thin Films by Sol-Gel Method

Titanium dioxide (TiO₂) thin films have been deposited on silicon and glass substrates by the sol-gel process using titanium iso-propoxide [Ti(O-i-C₃H₇)₄]. The bond configuration of the TiO₂ thin films was analyzed by using FTIR in the wavenumber range from 400 to 4000 cm^–1. The spectral transmittance of as-deposited TiO₂ films deposited on fused silica glass was measured in the wavelength range from 200 to 900 nm. X-ray diffraction measurements were performed to determine the crystallinity of the TiO₂ films. As-deposited films were amorphous. As the film was annealed at higher temperature, the structure was transformed from amorphous to the anatase crystalline state. The chemical composition of the deposited film was investigated using X-ray photoelectron spectroscopy (XPS). The films are essentially stoichiometric with carbon as the dominant impurity on the surface. Raman spectra show the characteristic of TiO₂ anatase phase. The electrical properties of the TiO₂ films were measured using capacitance-voltage (C-V) and current-voltage techniques. From C-V measurements, the dielectric constants were calculated to be approximately 26 for the as-deposited films and 75–82 for films annealed at 700°C in different atmosphere. For the as-deposited samples, the breakdown voltage was 2.7 MV/cm, and for an electric field of 1 MV/cm, the leakage current was 5 × 10^–5 A/cm² and the resistivity was 2.2 × 10¹⁰ -cm.     

Crystallization Behavior of Alkoxy-Derived Cordierite Gels

Alkoxy-derived cordierite gels were synthesized from tetraethylorthosilicate (TEOS), aluminum isopropoxide (Al(OPr ⁱ )₃), and magnesium ethoxide (Mg(OEt)₂). TEOS was partially hydrolyzed at molar ratios H₂O/TEOS = 1.2, in the presence of hydrochloric acid as a catalyst, HCl/TEOS = 0.1. Aluminum and magnesium alkoxides were added successively or as a double alkoxide. Phase transformations occurring in the gel were studied by differential thermal analysis, x-ray diffractometry, and Fourier-transform infrared spectroscopy. In all cases, -cordierite crystallized at similar temperatures (950–1000°C) with small amounts of spinel, which confirms dominant influence of the optimal conditions for partial hydrolysis of TEOS on the gels homogeneity. The transformation of - into -cordierite began at about 1100°C. Broadening of diffraction peaks and appearance of new bands in the FT IR spectra confirmed the transformation of - into modulated -cordierite at temperatures above 1300°C. Differential thermal analysis under nonisothermal conditions also proved homogeneous nucleation with constant rate and three-dimensional crystallite growth during -cordierite crystallization. The overall activation energy of the crystallization of -cordierite is 580 ± 81 kJ/mol.     

Synthesis and Characterization of the β-BaB2O4 Phase Obtained by the Polymeric Precursor Method

A modified polymeric precursor method based on the Pechini process was used to synthesize -BaB₂O₄ (-BBO) crystalline phase. D-sorbitol (C₆H₁₄O₆) was used as a polymerizing agent to avoid the loss of boron during the samples' calcination and crystallization. The -BBO stoichiometric crystalline phase was only obtained when sorbitol was added to the solution. The results of Raman spectroscopy show that the amorphous phase is only completely eliminated when the samples are heat-treated at 750°C for 20 h. Thin films of -BBO phase displaying a preferred orientation were obtained when crystallized at 750°C for 2 h and deposited on sapphire substrate.     

Site selection spectroscopy of SiO2:Eu3+ gels

Silica gels doped with Eu³⁺ ions were studied at temperatures between 10 K and 300 K by site selection spectroscopy in samples heated up to 200°C. The ⁵D₀ ⁷F₀ transition shows internal structures due to the different environments of the europium ions. Lifetimes, energy levels and homogeneous linewidths are site dependent. In the wet gel the Eu³⁺ ions prefer a liquid-like environment and only when the liquid is removed by heat treatment, the ion is linked more strongly to the silica network.     

Surface Energy Changes of Heat Treated TEOS Derived Silica Xerogels

The surface energy of monolithic silica xerogels was examined by measuring the interaction of organic probes with xerogels heated at temperatures close to the gel-to-glass transition temperature. Values of the dispersive component of the surface energy, , between 60 and 80 mJ m^-2 have been observed using n-alkanes for silica xerogels heated at 700, 800 and 900°C. At 1000°C, decreases to 8.37 mJ · m^-2. Also the differential heat of adsorption, variation of standard free energy and entropy of adsorption decrease when the silica xerogel is heated at 1000°C, showing a lower interaction potential of the organic probes with the silica surface. For the silica xerogels heated between 700 and 900°C, the acid character varies in accordance with the variation of the chemical nature of the silica. Upon heating at 1000°C, both acid and base characters are very close in accordance with a neutral surface. Within the experimental conditions used in this work, the surface of the obtained monolithic silica xerogels behaves as a glass surface when the treating temperature is 1000°C.     

Crystalline Inclusions Formed in C+N+BF2 Coimplanted on Silicon (111)

In the present work, high-doses (10¹⁷–10¹⁸cm^–2) of carbon, nitrogen, and boron (BF₂⁺) ions were coimplanted on silicon (111) substrates at 21, 25 and 77keV, respectively. Two series of samples have been implanted (series A and B) and subsequently annealed. Series A samples have been implanted at room temperature and treated one minute by rapid thermal annealing (RTA) and 3 hours at 1200°C. Series B samples have been implanted at 600°C and subsequently annealed at 1200°C during 3 hours. The annealing in both series has been carried out in N₂ at atmospheric pressures.The structure of the buried layers has been determined by conventional and high resolution transmission electron microscopy (CTEM and HRTEM). Polycrystalline silicon and new crystalline phases are observed by electron diffraction patterns. The polycrystalline silicon inclusions have been confirmed from analysis of HRTEM images.     

Preparation of octadecyl modified column gels using heat-treated silicas and their retention behaviour in high-performance liquid chromatography

Summary The effect of silanol groups on three types of octadecylmodified column gels using heat-treated silicas by calcination has been studied by high-performance liquid chromatography. After heat-treating at 180°C, 500°C and 950°C, the silicas treated with octadecyldimethylchlorosilane were used for the measurement of physical and chemical analysis. From elemental carbon analysis data, the reactive silanol group concentrations, _OH(s), were determined to be 2.0 in the 180°C treated silica, 2.1 in the 500°C treated silica and 1.6 in the 950°C treated silica, respectively (original silica: mean pore diameter 116 Å, specific surface area 298 m²/g, pore volume 1.22 ml/g, particle size 5.0 m). The separation factors, , of pyridine versus phenol were measured to be 0.79 on 180°C treated silica, 0.91 on 500°C treated silica and 1.98 on 950°C treated silica, using acetonitrile-water mixtures as the eluent. And then, on the basis of the physico-chemical and chromatographic data, the three types of octadecyl modified column gels using heat-treated silicas by calcination have been compared.     

Effect of the Measurement Temperature on the Dispersive Component of the Surface Free Energy of a Heat Treated SiO2 Xerogel

The surface free energy of a monolithic silica xerogel treated at 1000°C has been measured by inverse gas chromatography in the temperature range 25–150°C using n-alkanes. Values of the dispersive component, _S ^D, vary from 49.07 mJ·m^–2 at 25°C to 17.20 mJ·m^–2 at 150°C. The _S ^D value obtained at 25°C is lower than that found for amorphous and crystalline silicas but higher than that found for glass fibres meaning that the heat treatment at 1000°C changes drastically the structure of the silica xerogel showing a surface similar to a glass. However, the higher value of _S ^D in comparison to glass fibres can be attributed to the mesoporous structure present in the silica xerogel. In the temperature range of 60–90°C there exists an abrupt change of the _S ^D values as well as in the dispersive component of the surface enthalpy, h _S ^D. Such abrupt change can be attributed to an entropic contribution of the surface free energy.     

UV-Visible Absorption Characteristics of TEOS-Derived and Cr-Doped Silica Sonogels Aged in Different pH Solutions and Heat Treated up to 600°C

Cr-doped xerogels were obtained by sol-gel process from the acid-catalyzed and ultrasound-stimulated hydrolysis of tetraethoxysilane (TEOS) with addition of CrCl₃6H₂O in water solution during the liquid step of the process. The gels were aged immersed in different pH solutions for about 30 days, after that they were allowed to dry. The samples were annealed at temperatures ranging from 40 to 600°C and analyzed by UV-visible absorption spectroscopy. Cr³⁺ is the preferable oxidation state of the chromium ion in the gels annealed up to 250–300°C, in the case of aging in solutions of pH = 5 and 11. A high UV absorption below 320 nm, due to the host gel, and different absorption bands, depending on the temperature, due to the chromium ion were observed in the xerogels at temperatures below 250°C, in the case of aging in solutions of pH = 1 and 2. These absorption bands have not been assigned. Above 300°C up to 600°C, Cr⁵⁺, and possibly Cr⁶⁺, are the preferable oxidation states of the chromium ion independent of the pH of the aging solution, so the xerogels turn to a yellowish appearance in all cases.     

Raman-microanalysis of strain and crystal orientation in laser-crystallized silicon

Summary In this work we report on the use of Micro-Raman-Spectroscopy (MRS) to determine residual strain in cw-laser crystallized silicon on insulator (SOI)-thin layers and to measure slight deviations in crystal orientation which occur along the crystallization path during the crystallization process. The spatial resolution achieved is 0.8 m. Relative peak positions of the silicon phonon line have been measured with a reproducibility better than ±0.02 cm^–1. Frequency shifts in the crystalline layer were in the range of –1.5 cm^–1. This corresponds to a tensile biaxial stress of 3.8 kbar. Polarization dependent intensity monitoring has been carried out to measure the local tilt of crystal orientation in crystallized thin layers. For this we use polished cross-cut specimens which enable a change in observation direction, thus achieving an angular resolution of ±1°. At a crystallization path length of 90 m the measured backward tilt of the [001]-axis was 3°.