A study of Raman spectroscopy and low-temperature specific heat in gel-synthesized amorphous silica

A comparative study of low-temperature specific heat (1.5–25 K), C_p, and low-frequency Raman scattering (<150 cm⁻¹) has been performed in amorphous silica samples synthesized by sol–gel method (xerogels) and thermally densified in a range of densities, from ρ=1250 kgm⁻³ to ρ=2100 kgm⁻³, close to the density of the melt quenched vitreous silica (v-SiO₂). The present analysis concerns the application of the low-energy vibrational dynamics as an appropriate tool for monitoring the progressive thermal densification of silica gels. By comparison with v-SiO₂, the Raman and thermal properties of xerogels with increasing thermal treatment temperature revealed the following important results: (i) the existence of a critical treatment temperature at about 870°C, where a homogeneous viscous sintering produces full densification of the samples. This effect is detected by the observations of the Boson peak in Raman spectra at about 45 cm⁻¹ and of a peak in C_p(T)/T³, very close to those observed in v-SiO₂; (ii) in silica xerogels treated at temperatures less than about 800°C, the low-frequency Raman scattering is greater, with a continuous decreasing unstructured shape, and the Boson peak is not detected in the spectra.     

Preparation of zirconia aerogel by heating of alcohol–aqueous salt solution

Using zirconyl nitrate dihydrate as starting material, zirconia aerogel with high specific surface area was prepared by heating zirconyl nitrate dihydrate solution with an alcohol–aqueous mixture and supercritical drying method (SCFD). It was characterized by XRD, TEM and BET methods. The specific area of primary ZrO₂ aerogel could reach 675.6 m²/g which was much higher than that of documents so far. While both the specific surface area and pore volume decreased markedly on increasing the calciniation temperature. Contrast with the powder synthesized by ordinary trend, the content of t-ZrO₂ in the aerogel increased when the calcination temperature improved from 500 to 700 °C, the percentage of tetragonal phase was 86% at 700 °C. And it was very interesting that the particle size of the aerogel was bigger than 30 nm after being roasted in 1000 °C, the tetragonal phase still existed.     

Erbium-activated silica xerogels: spectroscopic and optical properties

Silica-based sol–gel glasses activated by Er³⁺ ions are attractive materials for integrated optics (IO) devices such as frequency upconverters and optical amplifiers. Monolithic erbium-activated silica xerogels with erbium content ranging from 0 up to 40 000 ppm were prepared by the sol–gel technique. Samples were densified by thermal treatment in air at 950°C for 120 h. The densification degree and the relative content of hydroxyl groups were studied by Raman spectroscopy. Refractive indices were measured at 632.8 and 543.5 nm by a prism coupling technique. Green to blue and violet upconversion luminescence upon continuous-wave excitation at 514.5 nm was observed for all samples. Emission at 1.5 μm, characteristic of the ⁴I_13/2→⁴I_15/2 transition of Er³⁺ ions, was observed at room temperature for all samples upon continuous-wave excitation at 980 nm. For the 5000 Er/Si ppm-doped xerogel, a photoluminescence was observed and a lifetime of 8 ms for the metastable ⁴I_13/2 level was measured.     

Arylene- and alkylene-bridged polysilsesquioxanes

Bis(triethoxysilyl)arylene monomers 1–5 and the 1,6-bis(trimethoxysilyl)hexylene monomer 6 were hydrolytically condensed under sol-gel conditions using both acid and base catalysts to produce their respective arylene- and alkylene-bridged polysilsesquioxane materials (X-1 through X-6). Polymerization reactions yielded gels within 24 h. One notable exception was the acid-catalyzed polymerization of 2,5-bis(triethoxysilyl)thiophene (X-5-S1) which required approximately 1 month to gel. The gels were processed by extracting with low dielectric solvents or by aqueous extraction. Solid state ¹³C and ²⁹Si CP MAS-NMR, infrared spectroscopy, and gas sorption porosimetry were measured on the xerogels. The materials were transparent, glass-like xerogels with surface areas as high as 1100 m²/g and porosity primarily confined to a micropore region (< 20 Å diameters). Xerogels prepared using the aqueous extraction had surface areas between 500 and 956 m²/g.     

Colloidal sol/gel processing of ultra-low expansion TiO2/SiO2 glasses

A series of titania-silica glasses with 0–9% TiO₂ were fabricated using a sol/gel process. The sol was prepared by dispersing colloidal silica fume in an aqueous solution of titania which was synthesized through the acid-catalyzed hydrolysis of titanium isopropoxide. The sols gelled in 2–4 days, and then were dried for 6–8 days. The dry gels were sintered at 1450–1500°C to produce clear, dense, microstructure-free glasses. The gels underwent a total shrinkage of 50% to yield glass rods about 50 mm long and 5 mm in diameter, or glass discs about 4 cm in diameter and 5 mm thick. The drying step was most critical in the production of crack-free specimens.

In the gel, the transmission electron microscope (TEM) revealed the presence of 1–5 nm rutile microcrystallites uniformly distributed within a network of colloidal silica particles. After sintering to 1450–1500°C, though, a dense, transparent, microstructure-free glass was created. Fourier transform infrared spectroscopy (FTIR) verified the formation of an amorphous solid-solution of titania and silica after sintering.

The thermal expansion of the glasses was measured using a differential dilatometer. The average linear coefficients of thermal expansion (CTE @ 25–675°C) varied between +5 × 10⁻⁷ and −0.2 × 10⁻⁷°C⁻¹ in the range 0 to 9% TiO₂. The glass with 7.2% TiO₂ exhibited a zero thermal expansion coefficient at 150–210°C. The hysteresis in CTE on heating and cooling was of the order of 0.01–0.02 ppm.     

Synthesis of fluorine doped silica gel and its properties at high temperature

Fluorine doped silica gels were synthesized by using the sol-gel processes of (A) SiF₄(g) and H₂O(1) and (B) the mixed solution of Si(OC₂H₅)₄, C₂H₅OH, H₂O and H₂SiF₆. By the former process we obtained a gel of relatively high fluorine content (8–12 at.%F), while we could synthesize the gel of 0–12 at.% F by adjusting the F/Si ratio of the starting solution mixtures by the latter process.

The defluorination behavior and the structural change of these gels at high temperature were studied by heating-mass spectrometry, IR and ESR measurements. The results revealed the following: (1) defluorination by liberation of SiF₄(g) was admitted from temperatures at about 400°C and was controlled by the diffusion of fluorine in the gel bulk. (2) The peak separation analysis for the IR band of 1300-900 cm⁻¹, where the stretching vibrations of Si---O and Si---F appear, showed that the change of the band shape resulted from the increase or the decrease of the Si---F bonds and the change of the bond angle of Si---O---Si as well as the change of the force constant accompanied by fluorination or defluorination. (3) The defects of the Si E′ center were induced by X-ray irradiation depending on the degree of the defluorination, and were reduced by the heat treatment. However, with the heat treatment at temperatures higher than 1000°C, the E′ center increased again. The IR spectra suggest that this behavior might correspond to the gel-glass trasition.     

Cation site occupation by Ag/Na ion-exchange in R2O–Al2O3–SiO2 glasses

Ag⁺/Na⁺ ion-exchanged R₂O–Al₂O₃–SiO₂ glasses with uniform concentration profile of Ag⁺ and Na⁺ were prepared by heat treatment in molten silver salt followed by holding at the same temperature in an ambient atmosphere. Their glass transition temperature (T_g) and thermal expansion coefficient (TEC) were measured and structures were investigated using ²⁹Si-MAS NMR, ²⁷Al-MAS NMR, IR and Raman spectroscopies. Both T_g and TEC decreased with increase of the exchange ratio, but T_g was still above the ion-exchange temperature of 400°C even for the fully exchanged sample. The ²⁹Si- and ²⁷Al-MAS NMR spectra were mostly unchanged and no sign of the structural alteration of the glass network was observed. On the other hand, the vibrational spectra showed remarkable peak shifts depending on the exchange ratio. From these structural results, it was found that when the exchange ratio was low, the introduced Ag⁺ ions were stabilized at the non-bridging oxygen (NBO) site, and then Na⁺ ions in AlØ₄ site were exchanged by Ag⁺ ions after full replacement of NBO sites, where Ø represents the bridging oxygen.     

ESR and Mössbauer studies of dispersed states of Fe ions in silica gels

0.01Fe₂O₃---0.99SiO₂ gels were prepared by three different sol-gel methods and the effects of the heat-treatment temperature of the gels on the ESR and Mössbauer parameters were investigated to obtain information about the change in the state of iron ions in the gel. All the gels were found to be amorphous under 600 °C, and among these three methods, the best dispersed state of Fe³⁺ ions was obtained by the method where TEOS was partially hydrolyzed with the same mole content of water before the addition of Fe(OEt)₃. It was indicated that the reaction at the sol-stage plays an important role in the degree of dispersal of Fe³⁺ ions in this system.     

Different morphologies of the dry physical gels as a function of the sample preparation method

Xerogels obtained by drying the gels formed by glucofuranose derivatives with organic solvents were studied. The xerogels were characterized using the SEM, XRD, DSC and OPM techniques. The morphology of a xerogel observed by SEM may change from ‘amorphous, fibrillar’ to ‘crystal-like’, which may be caused by time or temperature. The results suggest that a similar transition may take place when the xerogel is being prepared. Thus SEM pictures of xerogels should be treated with great caution as they may not reflect the gelator network morphology in the bulk gel.     

Hydrothermal synthesis of MoS2 nanowires

The MoS₂ nanowires with diameters of 4 nm and lengths of 50 nm were synthesized by a hydrothermal method using 0.36 g MoO₃ and 1.8 g Na₂S as precursors in 0.4 mol/l HCl solution at 260°C. The products are characterized by XRD, XPS, TEM, HTEM and BET. Results show that the as-prepared MoS₂ nanowires consist of 1–10 sulfide layers with BET surface areas of 107 m²/g. The possible reaction route and the formation mechanism of the MoS₂ nanowires are discussed. The effects of exterior conditions such as pH value, temperature, concentration of precursors and additives on the particle size and morphology of MoS₂ crystallites were investigated.     

High porosity silica xerogels prepared by a particulate sol–gel route: pore structure and proton conductivity

The lower cost and higher hydrophilicity of silica xerogels could make them potential substitutes for perfluorosulfonic polymeric membranes in proton exchange membrane fuel cells (PEMFCs). For that purpose, we need to obtain micro or micro and mesoporous silica xerogels with a high porosity. The preparation of micro (<2 nm) and micro and mesoporous silica xerogels (2<d_poresize10 nm) from particulate as oppossed to polymeric suspensions of silica using tetraethyl orthosilicate (TEOS) as precursor is used. Two techniques of varying packing density have been performed in this work: (1) Control of the aggregation degree in the sol by adjusting its pH before gelation (pH 5, 6 and 8) and (2) Mixture of sols with a different average particle size (particles formed under acid and base catalyzed reactions). Proton conductivity of the obtained xerogels was studied as a function of temperature and relative humidity (RH). High pore volume, high porosity and small pore size SiO₂ xerogels have been achieved in the calcination temperature range from 250 to 550 °C. The calcined xerogels showed microporosity or micro and mesoporosity in the whole range of calcination temperatures. By mixing sols (molar ratio: acid/base=4.8) porosities up to 54.7±0.1% are achieved, at 250 °C of firing temperature. According to EMF measurements, electrical transport is due to protons in this kind of materials. The proton conductivity of the studied xerogels increased linear with measured temperature. A S-shaped dependence of the conductivity with the RH was observed with the greatest increase noted between 58% and 81% RH. Xerogels with a low porosity (40.8±0.1%) and an average pore size less than 2.0 nm showed lower values of proton conductivity than that of xerogels with a higher porosity and a higher average pore size in the whole range of temperature and RH. When silica xerogels, with the highest conductivity, are treated at pH 1.5, that property increased from 2.84×10⁻³±5.11×10⁻⁵ S/cm to 4.0×10⁻³±7.2×10⁻⁵ S/cm, at 81% RH and 80 °C. It indicates that the surface site density of these materials has a strong effect on conductivity. Proton conductivity values achieved are less than one order of magnitude lower than that of Nafion, under the same conditions of temperature and RH.     

Photoassisted growth and nitrogen doping of ZnSe

ZnSe was grown on GaAs(100) substrates by photoassisted MOVPE using the precursors dimethylzinc-triethylamine (DMZn-TEN) and ditertiarybutylselenide (DtBSe). The optimal growth temperature was T_g = 300°C. Above 300°C no enhancement of growth rate was observed under illumination. Furthermore, nitrogen doping experiments were performed using phenylhydrazine, allylamine and tert-butylamine as nitrogen precursors. Nitrogen concentrations up to 10¹⁹ cm⁻³ (SIMS) were achieved with input flow ratios as low as [PhHz]/[Se] = 4 × 10⁻⁴. All as-grown samples were highly compensated. Successful nitrogen incorporation was also observed with allylamine. However, the use of tert-butylamine together with the adduct compound DMZn-TEN showed no incorporation of nitrogen.     

Structural transformation of the TiO2---SiO2 system gel during heat-treatment

The gel formation of the (100-x)TiO₂·xSiO₂ (x = 0–10 mol%) system has been studied. The progressive elimination of residues was followed by DTA and TGA curves. DTA curves showed that the formation of anatase during heat treatment could be sensibly slowed down with the increase of SiO₂. The relationship between the gel composition and crystallization temperature of anatase has been systematically investigated. The X-ray diffraction spectra demonstrated that the crystallization temperature of anatase is 400°C for TiO₂ gel and 430°C for 90TiO₂ - 10SiO₂ gel. The infrared absorption spectra were used to follow the structural transformation of gels heat-treated at different temperatures. With the help of EPR it is evident that titanium ions exist only in tetravalence.     

Mixed-metal oxide aerogels for oxidation of volatile organic compounds

Aerogels of 100% silica, 8 wt% Zr in silica, 5 wt% V in silica and 100% zirconia were synthesized and tested as oxidation catalysts in the temperature range of 300–700°C for destruction of volatile organic compounds. The silica-based aerogels were all amorphous and had surface areas above 600 m²/g after oxidation. The zirconia aerogel was crystalline with a relatively low surface area of 250 m²/g. As catalysts for oxidation (using O₂ in He) of CH₃OH to CO₂, the zirconia aerogel exhibited the highest activity and best selectivity while the silica aerogel exhibited the poorest. Inclusion of Zr at levels as low as 8 wt% into the silica aerogel framework produced activities and selectivities which were very much like the zirconia aerogel. These properties have the impact of producing a Zr based catalyst with high activity, but with thermal stability afforded by Zr–silica mixtures.     

Water and the glass transition temperature of silicate melts

Literature data on the effect of water on the glass transition in silicate melts are gathered for a broad range of total water content c_w from 3 × 10⁻⁴ to 27 wt%. In terms of a reduced glass transition temperature T_g^*=T_g/T_g^GN, where T_g^GN is T_g of the melt containing c_w≈0.02 wt% total water, a uniform dependence of T_g^* on total water content (c_w) is evident for silicate melts. T_g^* decreases steadily with increasing water content, most strongly at the lowest water content where H₂O is dominantly dissolved as OH. For water-rich melts, the variation of T_g^* is less pronounced, but it does not vanish even at the largest water contents reported (≈27 wt%). T_g^* vs. c_w is fitted by a three-component model. This approach accounts for different transition temperatures of the dry glass, hydroxyl and molecular water predicting T_g^* as a weighted linear combination of these temperatures. The required but mostly unknown water speciation in the glasses was estimated using IR-spectroscopy data for hydrous sodium trisilicate and rhyolite.     

Behaviour of vicinal InP surfaces grown by MOVPE: exploitation of AFM images

InP substrates and epilayers grown by MOVPE have been studied by AFM. For different misorientation angles, we observed the surface of the substrate after annealing under phosphine (PH₃) and of the epilayers under different growth conditions such as growth temperature T_g and trimethylindium (TMI) partial pressure. After annealing the terrace width corresponds to the nominal value of misorientation angle measured by X-ray diffraction. We observed different topographies and roughnesses for the grown layers corresponding to different growth modes. We propose, taking into account the roughness of the surface, a calculation of the step height and terrace width. For 2D nucleation (θ ≤ 0.2° and T_g = 500°C) and step flow mode, the roughness is low while it is increased by step bunching (θ ≥ 0.5° and T_g ≥ 580°C). Moreover we have examined the surface morphology for different misorientation angles and determined the influence of growth conditions (growth temperature, indium partial pressure) on the growth mechanism. At T_g = 580°C the increase of the TMI partial pressure in the reactor enhances the step bunching and leads to larger terraces.     

SIMS study of the concentration of dopants in LPCVD silicon thin films

Measurements of solid phase dopant concentration (S) of LPCVD Si thin films as a function of substrate temperature (T_s = 500−640 ° C) and gas phase doping ratio (R = 1 × 10⁻⁵ −4 × 10⁻²) by SIMS indicate different behaviors of P and B in the films. A linear relation S = b(T)R is observed for B-doped film with b(T) varying from 4 to 50 depending on T_s. Boron-doped microcrystalline film has a higher doping efficiency than that of P-doped ones.     

Hydrolytic resistance of Na2O---B2O3---SiO2 gels prepared by sol-gel process

Alkoxide derived gels were prepared in the system Na₂O---B₂O₃---SiO₂. The gel compositions were situated in the liquid-liquid immiscibility area of the phase equilibrium diagram.

Hydrolytic resistance tests were performed on the gels heat-treated at temperatures ranging between 120 to 850 °C. The Na₂O, B₂O₃ and SiO₂ extracted from the attack gels were analyzed. The experimental results indicate that the amount of B₂O₃ has a significant influence on the chemical durability of the heat-treated gels. At temperatures of 850 °C the greater the B₂O₃ mol% the greater are the amounts of Na₂O and B₂O₃ extracted. Different behaviour was observed for gels heat-treated at 600 °C where the amounts of B₂O₃ and Na₂O extracted slightly increases as the B₂O₃ mol% increases. Small amounts of extracted SiO₂ were always observed.

These results are complemented with other measurements so that an explanation of the controlling mechanism is given.     

Densification kinetics of porous colloidal P2O5-doped silica gels using constant heating rate measurements

Commercially available fumed colloidal silica was gelled in an aqueous solution containing PO₄H₃. Instantaneous gelation was obtained by adding several drops of HF (48 wt%). The objective of this paper was to study the gel-to-glass conversion of these colloidal gels using constant heating rate (CHR) experiments.

Gel densification was measured at temperatures ranging from 50 to 1550°C at different heating rates (1 to 10°C/min) using a dilatometer.

Shrinkage and shrinkage rate as a function of temperature were measured and CHR equations were used to derive information on the densification mechanisms.

The experimental results show that small additions of PO₄H₃ into pure silica dispersions give rise to gels which densify to high silica glass at much lower temperatures than pure colloidal silica gels. The bloating effects produced by pure colloidal silica gels at temperatures above 1280°C were also eliminated. For these P₂O₅-doped silica gels maximum shrinkage rates were found at temperatures between 1050 and 1150°C according to the heating rate used.

The CHR analysis showed that several different mechanisms seem to operate in a complex interdependence in the whole range of temperature studied. This CHR analysis was compared with those results obtained from isothermal shrinkage experiments in the range where viscous sintering is the predominant shrinkage-controlling mechanism (between 1000 and 1100°C).     

Characteristics of ZnO–B2O3–CaO–Na2O–P2O5 glass powders prepared by spray pyrolysis

Fine-sized ZnO–B₂O₃–CaO–Na₂O–P₂O₅ glass powders with spherical shape were directly prepared by high temperature spray pyrolysis. The ZnO–B₂O₃–CaO–Na₂O–P₂O₅ powders prepared by spray pyrolysis at temperatures above 1200 °C had broad peaks at around 30° in the XRD patterns. The glass transition temperatures (T_g) of the glass powders obtained by spray pyrolysis at preparation temperatures between 900 °C and 1400 °C were near 480 °C regardless of the preparation temperatures. The dielectric layers formed from the glass powders prepared by spray pyrolysis at preparation temperatures above 1300 °C had clean surface and dense inner structure at the firing temperature of 580 °C. The transmittance of the dielectric layer formed from the glass powders obtained by spray pyrolysis at preparation temperature of 1400 °C was 90% at the firing temperature of 580 °C, in which the thickness of the dielectric layer was 13 μm. The UV cutoff edges gradually shift towards longer wavelength with increasing the preparation temperature of glass powders and the firing temperature of dielectric layers.