Synthesis and characterization of sol–gel derived highly condensed fluorinated methacryl silica and silica–zirconia hybrid materials

Sol–gel reaction of 3-trimethoxysilylpropyl methacrylate (MPTS), heptadecafluorodecyltrimethoxysilane (PFAS), and zirconium n-propoxide (ZPO) chelated with methacrylic acid (MAA) was investigated depending on the catalyst concentration, the reaction time and the zirconium alkoxide addition for the fabrication of highly condensed fluorinated methacryl silica and silica–zirconia hybrid material (MF and MFZ hybrimer). Controlling the catalyst concentration and the reaction time has been found to be important parameters for the strengthening of the silica networks in MF hybrimer. The addition of zirconium alkoxide enhanced the condensation degree in short reaction time, which could efficiently fabricate the highly condensed MFZ hybrimer. In particular, their molecular sizes have been found to be highly dependent on the extent of sol–gel reaction and were experimentally calculated.     

Fabrication and characterization of photocurable inorganic-organic hybrid materials using organically modified colloidal-silica nanoparticles and acryl resin

Photocurable inorganic-organic hybrid materials were prepared from colloidal-silica nanoparticles synthesized through the sol-gel process and using acryl resin. The synthesized colloidal-silica nanoparticles had uniform diameters of around 20 nm and were organically modified, using methyl and methacryl functional silanes, for efficient hybridization with acryl resin. The organically modified and stabilized colloidal-silica nanoparticles could be homogeneously hybridized with acryl resin without phase separation. The successfully fabricated hybrid materials exhibit efficient photocurability and simple film formation due to the photopolymerization of the organically modified colloidal-silica nanoparticles and acryl resin upon UV exposure as well as an excellent optical transmission of above 90% in the visible region and an enhanced surface smoothness of around 1 nm RMS roughness. They likewise exhibit improved thermal and mechanical characteristics, much better than those of acryl resin. Lastly and most importantly, these photocurable hybrid materials fabricated through the synergistic combination of colloidal-silica nanoparticles with acryl resin are candidates for optical and electrical applications.     

Fabrication of Yb-doped silica glass through the modification of MCVD process

A method to deliver Yb and Al compounds in vapor phase to the reaction/deposition zone has been devised for modified chemical vapor deposition (MCVD) process. By using this MCVD setup, we succeeded to prepare silica glass preforms presenting uniform dopants concentrations in the radial and longitudinal directions with good reproducibility. Preforms with a core diameter larger than 5 mm were easily prepared by depositing around 40 layers. By changing some parameters in the deposition step, such as carrier gases flow and temperatures of Yb(DPM)₃ and AlCl₃ furnaces, different concentrations of Yb and Al were incorporated into the core region of the silica glass preforms. By adjusting the Yb and Al concentrations, we succeeded to prepare preforms for large mode area (LMA) fiber with a change less than 10% of nominal refractive index.     

Fabrication of mesoporous polymer/silica hybrid using surfactant-mediated sol–gel method

A mesoporous polymer/silica hybrid was fabricated by a surfactant-mediated sol–gel method. Under our experimental conditions, acrylonitrile (AN) monomer was located at the exterior of micelles and the sol–gel reaction of tetraethoxyorthosilicate (TEOS) proceeded concurrently with the polymerization reaction of the AN monomer. In other words, the micelle/polyacrylonitrile/silica precursor was synthesized through the radical polymerization accompanied with a hydrolysis/condensation single reaction in a reaction system. This is a unique characteristic of our methodology, which embraces the concept of ‘micelle templating’. The pore diameter of the mesoporous polymer/silica hybrid could be tuned by varying the spacer length and concentration of surfactants. Furthermore, compared with conventional mesoporous carbons, the carbonized mesoporous polymer/silica hybrids displayed an enhanced electrical performance favorable for use as a supercapacitor.     

Synthesis and characterization of SiO2-PEG hybrid materials

Sol-gel is a promising technique for the synthesis of organic-inorganic hybrid materials both of class I and II. In materials of class I organic molecules are physically entrapped in an inorganic matrix, while in those of class II organic and inorganic parts are connected by covalent bonds. In this paper a sol-gel procedure to obtain SiO₂-PEG hybrids of class I, in which PEG is simply mixed at the sol stage, is compared to a sol-gel procedure to obtain SiO₂-PEG hybrid materials of class II, where a particular sol-gel Si-C precursor is synthesized. XPS analyses showed the different distribution of the organic phase in the SiO₂ matrix and the bond between PEG and SiO₂ for hybrids of class II. The PEG molecule in hybrid of class II showed an enhanced thermal stability up to 350 °C. Doping with a lithium salt was performed on hybrids of class II, and the ionic conductivity was measured.     

Design,growth and characterization of PbTe-based thermoelectric materials

Thermoelectric devices convert thermal energy, i.e. heat, into electric energy. With no moving parts, the thermoelectric generator has demonstrated its advantage of long-duration operational reliability. The IV–VI compound semiconductor PbTe-based materials have been widely adopted for the thermoelectric applications in the medium temperature range of 350–650°C. In most of the reports, thermoelectric materials were manufactured by a hot pressing or quench and annealing method. The recent advancements in the converting efficiency of thermoelectrics, including PbTe-based materials, have been attributed to the modification on material inhomogeneity of microstructures by hot pressing or simply cooling the melt to reduce the thermal conductivity. On the other hand, due to its time-consuming preparation/processing and unnecessary good crystalline quality (for thermoelectric applications), the processing of thermoelectric materials by crystal growth resulted in very few investigations. In this report, the design and growth of the PbTe-based materials solidified from the melt for thermoelectric applications as well as the results of their thermoelectric characterizations will be reviewed. It shows that, besides its Figure of Merit comparable to other processing methods, the melt grown PbTe material has several additional capabilities, including the reproducibility, thermal stability and the functional gradient characteristics from the variation of properties along the growth length.     

Fabrication of fluorine-doped silica glasses by the sol-gel method

Fluorine-doped silica glasses are produced by the sol-gel method for optical fiber preforms. In order to dope fluorine into silica glass, fluorinated silicon alkoxide, Si(OC₂H₅)₃F, is titrated into SiO₂ sol solutions. The fluorine content in silica glass depends on: the fluorine concentration in the gel, the specific surface area of SiO₂ particles and the heating rate in the sintering process. Fluorine-doped silica glass with a maximum relative refractive index difference of −0.93% is obtained. Using this technique, optical fibers with a triangular refractive index profile are fabricated with a minimum optical loss of 1.6 dB/km at 1.69 μm wavelength.     

Formation of silica/epoxy hybrid network polymers

Epoxy-based inorganic–organic hybrid polymers, for use as a matrix in coatings, have been prepared from 3-glycidoxypropyltrimethoxysilane by a sol–gel process. The precursor molecule possesses both epoxy and silicon alkoxide functionality and so interlinked inorganic–organic networks can be formed. Diethylenetriamine was used to open the epoxy rings and form the organic network to an extent determined by the initial ratio of amine to epoxy groups. The materials were cured either at room temperature or with an additional heat treatment at 150 °C. Structural characterisation of the cured hybrid materials was performed using a combination of Raman, and ²⁹Si and ¹³C MAS NMR spectroscopies. These show that the formation of the two networks does not occur independently and the rate or extent of organic cross-linking has a direct effect on the extent of the inorganic network formation, and vice-versa.     

Fabrication of PZT materials from nanostructured powders

The nanostructured PbZr_0.52Ti_0.48O₃ powders (PZT) were prepared by urea hydrolysis and subsequent thermal treatment. Phase pure PZT is obtained after calcination of the gel at 873 K for 4h in air atmosphere. The XRD pattern showed the tetragonal symmetry with lattice parameters: a_t = 3.97 Å and c_t = 4.15 Å. The XRD crystallite size of the PZT material heat-treated at 873 K for 4h was 6.0 nm. TEM analysis showed that the powder consisted of small clusters of crystallites of 30 to 40 nm in size. PZT materials fired at 1373 K showed nearly 99% density with equiaxed grains of 2–3 μm. The good sinterability relatively at lower temperatures is thought to result from enhanced diffusivities in nanosized particles.     

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.     

Preparation and characterization of ordered mesoporous silica membrane

Hexagonal mesoporous silica (MCM-41) membranes were prepared at air-water interface by means of an interfacial silica-surfactant self-assembly process. The free-standing and oriented mesoporous silica membranes with pore size ≈2.9-3.8 nm were synthesized at room temperature in acidic media and were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) observations. Small-angle X-ray diffraction (SAXRD) patterns of membranes clearly indicated that as-synthesized membranes were typical of MCM-41 materials with a periodic hexagonal structure with the channels parallel to the surface. SEM images showed that the as-synthesized membrane was continuous and crack-free. In this paper, some novel findings are reported.     

Characterization of surface and porous properties of synthetic hybrid lamellar silica

Synthetic lamellar silica and hybrid lamellar silicas have been prepared by liquid crystal templating, template extraction and silanization. The samples have been characterized by thermogravimetric analysis (TGA), carbon analysis, spectroscopy, X-ray diffraction (XRD) and nitrogen adsorption. The XRD analyses have shown that the lamellar periodic stacking is preserved for all samples. The quantity and type of organic molecules at the silica surface have been evaluated by carbon analysis, TGA and spectroscopy. The covalent grafting of the solvent used for extraction of the initial surfactant has been highlighted by these analyses. The nitrogen adsorption analyses have revealed three categories of pores and two types of samples. The initial lamellar silica exhibits a very low specific surface area and plate-like type of pores. The second type of samples is made up of the hybrid samples and the initial substrate from whom the surfactant has been extracted. These samples show a significantly higher specific surface area with interlamellar spaces corresponding to narrow-slit like mesopores around 4 nm. The nitrogen adsorption data analysis has highlighted the presence of micropores within the silica sheets. The difference of the specific surface is due to pore blocking by the surfactant impeding the access to nitrogen into interlamellar spaces and by the silanes covering the pores once the surface modified. The presence of micro and mesopores combined to a high BET specific surface of 612 m²/g makes these lamellar silicas interesting materials for catalysis applications.     

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.     

Multigram scale synthesis and characterization of low-density silica xerogels

The synthesis of silica with preserved porosity and tailored morphology by sol–gel process can be achieved by hybrid organic–inorganic synthesis: a modified alkoxide, viz. 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS), is introduced during the base catalysed synthesis with TEOS as main silica precursor. Additives with methoxy groups induce a nucleation mechanism because of their higher reactivity compared to main reagents with ethoxy groups. The nucleation model presented in previous papers was refined by taking into account the porosity of the particles and calculating the number of additive molecules by nucleus for each value of the ratio of additive/main reagent. The extrapolation of the synthesis process to semi-industrial scale goes through the replacement of laboratory grade reagents by industrial grade reagents and the scaling up to the production of higher quantities. At each of these two steps, the morphology and porosity of the samples has been compared to those of laboratory grade samples. It was shown that the texture and particle size has quasi totally been preserved.     

Textural characterization of high temperature silica aerogels

Silica alcogels were synthetized by the sol–gel polymerization of tetraethylorthosilicate in acid media. Conventional and supercritical drying was performed in order to obtain xerogels and aerogels. Different process parameters of the supercritical drying were altered in order to control the texture of the resulting gel. The texture and the structural evolution of xero- and aerogels were studied by thermogravimetric-differential thermal analysis, Fourier-transform infrared spectroscopy, transmission electron microscopy and N₂ physisorption at 77 K. ²⁹Si magic angle spinning nuclear magnetic resonance experiments on silica samples were used to resolve various silicon local environments. Hydrophilic microporous xerogels and hydrophobic micro- or mesoporous silica aerogels were obtained, whose microscopic structure is very similar. However, the samples obtained by different drying procedures exhibit a different structural evolution with temperature.     

Laser damage of glycidoxypropyltrimethoxysilane based hybrid materials

The preparation of solid samples, having specific optical properties, is one of the main goals for the realization of devices in the photonic field. To this end, it is important to attain the best control of the optical properties in the final materials. The sol-gel technique is a powerful synthesis method allowing the preparation of matrices with high stability, mechanical resistance and high optical quality. The use of hybrid organic-inorganic precursors permits samples with different thicknesses, ranging from films of hundreds of nanometers to bulk samples of millimeters, to be obtained. The control of the synthesis protocol and the choice of precursors and catalyst allows the control of the final matrix microstructure, which is related to optical properties, like the laser damage threshold. In this work four different matrices, based on glycidoxypropyltrimethoxysilane and Zr alcoxide, have been prepared through sol-gel synthesis. An interpenetrating organic and inorganic network, controlled by the synthesis protocol, characterizes these matrices. The obtained materials show high resistance to the optical damage and long term stability.     

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.     

Fabrication of inorganic–organic hybrid films for optical waveguide

Inorganic–organic hybrid guiding and buffer layers for optical waveguides were synthesized by the sol–gel process. An acid-catalyzed solution of 3-(trimethoxysilyl)propylmethacrylate and tetraethylorthosilicate was used as a precursor to produce thick films by spin-coating. Incorporation of a UV-sensitive functional group leads to the fabrication of the photo-patternable guiding layer. High-resolution patterned films with 8 μm linewidth were obtained using a conventional photo-lithography. Furthermore, the amount of phenyltrimethoxysilane added as a refractive index modifier varied in order to control the refractive index of the underlying buffer layer. The refractive index variations and structural changes of these inorganic–organic hybrid films as a function of the processing conditions were analyzed using the prism coupling technique and a Fourier transform infrared spectrometer. We have produced optically transparent films at above 500 nm with a propagation loss of 0.84 and 1.49 dB/cm at 1310 and 1550 nm, respectively.     

Characteristics of colored inorganic–organic hybrid materials

Inorganic–organic hybrid glasses are relatively new nanometric materials of Ormosil’s group (organic modified silicates). There co-existence, on a molecular scale, exists between inorganic structures in the form of silica-oxide network and organic structures based on carbon links. Properties of these materials are intermediate between those of inorganic glasses (hardness, chemical and thermal resistance) and organic polymers (low temperature of obtaining, elasticity of structure). The hybrid materials are compatible matrices for organic compounds such as organic dyes, laser dyes, photo-chromic compounds, etc. Inorganic–organic hybrid glasses are usually produced in the form of thin coatings on various bases using a low-temperature sol-gel process. These coatings, depending on the kind and amount of units, building their structure, show various properties: refractive index changing in a wide range, anti-static properties, anti-reflection, corrosion protection, intensive color, luminescence and others. That is why these materials found application as protective and colored covering of glass articles as well as in new technical areas. The aim of this paper is obtaining and characterizing colored inorganic–organic coatings on glass, considering both protective and colored properties. These materials have been produced from phenyltriethoxysilane (PhTES), 3-glycidoxypropyltrimethoxysilane (GPTMS), aluminium tri-sec-butylate (TBA); (PGT matrix). The structure of PGT matrix was determined using the FTIR, ²⁹Si MAS NMR and ²⁷Al MAS NMR examinations. It has been found that chemical bonds occur between structural units. The two groups of organic dyes were used for coloring the coatings. The first group consisted of ORASOL dyes, chiefly based on various metal complexes. These dyes have a wide range of commercial utilization. The second group included the organic, intensive dyes obtained in the laboratory and are inaccessible for sale. The coloring coatings were coated on flat glass using the dip-coating method. The samples were submitted for thermal treatment at temperatures of 100 and 200 °C. Investigation of chemical resistance (boiling in water for 1 h) was made for coated materials after thermal treatment at 100 °C. UV–VIS transmission of colored coatings was examined after each stage of thermal treatment and also after hydrolytic resistance examination. The quality of the coatings and their thickness were estimated by SEM observations. The obtained, inorganic–organic coatings were characterized by good chemical resistance and stability of color.     

Raman characterization of semiconducting materials and related structures

Raman spectroscopy is discussed as a versatile tool for the characterization of semiconductors and related materials. Raman scattering by intrinsic phonon modes is shown to provide information on the crystallinity and composition of bulk materials as well as on the periodicity of artificial superlattices. Inelastic light scattering gives further insight into the properties of electron or hole gases present in doped materials. Raman scattering by electronic or vibronic excitations of impurities is discussed as a quantitative technique for the assessment of, e.g., residual impurities in substrate materials as well as for the analysis of the dopant incorporation in heavily doped semiconductors.