Showing papers in "Journal of Non-crystalline Solids in 1992"
TL;DR: In this article, it has been shown for various types of glasses and glass-ceramics that the essential condition for them to bond to living bone is the formation of an apatite layer on their surfaces in the body.
Abstract: It has been shown for various types of glasses and glass-ceramics that the essential condition for them to bond to living bone is the formation of an apatite layer on their surfaces in the body. CaO,SiO 2 -based glasses formed the surface apatite layer in a simulated body fluid, whereas CaO,P 2 O 5 -based glasses did not form it. This means that the rate of the apatite nucleation on the surfaces of the former glasses is much higher than that of the latter glasses. The increase in the degree of the supersaturation of the surrounding fluid with respect to the apatite due to dissolution of the calcium ion from the CaO,SiO 2 -based glasses was almost equal to that due to dissolution of the phosphate ion from the CaO,P 2 O 5 -based glasses. The high rate of the apatite nucleation on the surface of the former glasses is therefore attributed to the lower interface energy between the apatite and the glass surfaces. The CaO,SiO 2 -based glasses form a silica hydrogel on their surfaces prior to formation of the apatite layer. This means that the hydrated silica provides specific favorable sites for the apatite nucleation.
827 citations
TL;DR: In this article, a combination of coating theory and experiment is used to define coating protocols to tailor the deposition process to specific applications, which can be used to tailor coating protocols for specific applications.
Abstract: Sol-gel thin films are formed by gravitational or centrifugal draining accompanied by vigorous drying. Drying largely establishes the shape of the fluid profile, the timescale of the deposition process, and the magnitude of the forces exerted on the solid phase. The combination of coating theory and experiment should define coating protocols to tailor the deposition process to specific applications.
537 citations
TL;DR: In this article, the chemistry-structure-property relationships of organic aerogels were studied and aqueous polycondensation of resorcinol and melamine with formaldehyde was proposed.
Abstract: Traditional inorganic aerogels are mad via the hydrolysis and condensation of metal alkoxides. Recently, we reported the synthesis of organic aerogels based upon the aqueous polycondensation of (1) resorcinol with formaldehyde and (2) melamine with formaldehyde. The former materials can also be pyrolyzed in an inert atmosphere to form vitreous carbon aerogels. In both the inorganic and organic systems, the structure and properties of the dried aerogel are dictated by polymerization conditions. Factors such as pH, reactant ratio, and temperature influence the crosslinking chemistry and growth processes taking place prior to gelation. The ability to tailor the structure and properties of aerogels at the nanometer scale opens up exciting possibilities for these novel materials. This paper addresses the chemistry-structure-property relationships of organic aerogels. 22 refs., 7 figs.
412 citations
TL;DR: The structure of MO 1/2 TeO 2 (M = Li, Na, K, Rb, Cs and Tl) binary glasses has been studied by means of Raman spectroscopy.
Abstract: The structure of MO 1/2 TeO 2 (M = Li, Na, K, Rb, Cs and Tl) binary glasses has been studied by means of Raman spectroscopy. The glasses having low alkali content have a continuous network constructed by sharing corners of TeO 4 trigonal bipyramids (tbp's) and TeO 3 + 1 polyhedra having one non-bridging oxygen atom (NBO). In the glasses containing 20–30 mol% alkali oxide, TeO 3 trigonal pyramids (tp's) having NBOs are formed in the continuous network. When alkali content exceeds 30 mol%, isolated structural units, such as Te 2 O 2− 5 ion, coexist in the continuous network. The fraction of TeO 4 tbp's decreases with increasing alkali content. The glasses, which contain nearly 50 mol% alkali oxide, are composed of a continuous network constituted by TeO 3 + 1 polyhedra and TeO 3 tp's, and of isolated structural units, such as Te 2 O 2− 5 and TeO 2− 3 ions. The structure of thallium tellurite glasses having less than 30 mol% TlO 1/2 is similar to that of alkali tellurite glasses containing equal amounts of MO 1/2 . The fraction of TeO 3 tp's having NBOs in the thallium tellurite glasses, when TlO 1/2 content is equal to or higher than 40 mol%, is larger than that in the corresponding alkali tellurite glasses. In the 66TlO 1/2 ·34TeO 2 glass, most of tellurium atoms are in a form of isolated TeO 2− 3 ion. A new hypothesis is also given for a mechanism for the basic structural changes in the tellurite glasses.
384 citations
TL;DR: In this paper, the spectral and particularly kinetic data show that in all cases luminescence centers with closely similar properties exist which constitute an isoelectronic series and attributed to defects, formed by electrically neutral twofold coordinated Si, Ge, and Sn atoms (T20 centers) in glassy SiO2.
Abstract: Luminescence centers causing optical absorption bands in the 5 eV region in oxygen-deficient pure (‘Si-doped’), as well as in Ge- and Sn-doped glassy SiO2 were studied. The spectral and particularly kinetic data show that in all cases luminescence centers with closely similar properties exist which constitute an isoelectronic series. They are attributed to defects, formed by electrically neutral twofold coordinated Si, Ge, and Sn atoms (T20 centers) in glassy SiO2. Twofold coordinated Si causes the characteristic ‘B2’ optical absorption band at 5.03 eV. Upon reaction with atomic hydrogen, T20 centers convert to the well known ‘H(I)’, ‘H(II)’ and ‘H(III)’ ESR-active defects. The alternative models for these defects are discussed.
357 citations
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TL;DR: The chemical reactivity of metal alkoxides toward hydrolysis, condensation and complexation mainly depends on the electronegativity of the metal atom, its coordination number and the steric hindrance of alkoxide groups.
Abstract: Sol-gel chemistry involves nucleophilic reactions The chemical reactivity of metal alkoxides toward hydrolysis, condensation and complexation mainly depends on the electronegativity of the metal atom, its coordination number and the steric hindrance of alkoxide groups Silicon alkoxides are poorly reactive Hydrolysis and condensation rates have to be enhanced by acid and base catalysis or nucleophilic activation Transition metal alkoxides are usually too reactive They have to be stabilized by complexation in order to avoid fast condensation The molecular design of alkoxide precursors opens the way to tailor-made materials
355 citations
TL;DR: In this article, a two-step solgel process was developed for the production of transparent ultralow-density aerogels, which allowed the formation of ultra-low-density gels in a matter of hours.
Abstract: Conventional silica sol-gel chemistry is limited for the production of transparent ultralow-density aerogels because (1) gelation is either slow or unachievable, and (2) even when gelation is achieved, the large pore sizes result in loss of transparency for aerogels <.020 g/cc. We have developed a two-step sol-gel process that circumvents the limitations of the conventional process and allows the formation of ultralow-density gels in a matter of hours. we have found that the gel time is dependent on the catalyst concentration. After supercritical extraction, the aerogels are transparent, uncracked tiles with densities as low as .003 g/cc. 6 figs., 11 refs.
328 citations
TL;DR: In this article, phase separations of gelling solutions have been investigated for acid-catalyzed, alkoxysilane-based systems containing polyacrylic acid (HPAA).
Abstract: Phase separations of gelling solutions have been investigated for acid-catalyzed, alkoxysilane-based systems containing polyacrylic acid (HPAA). Gels with micrometer-range interconnected porous morphologies were obtained in a composition range broader than that reported for the systems containing sodium polystyrene sulfonate. It is suggested that the data of a light-scattering experiment can be explained by the occurrence of spinodal phase separation which forms an interconnected morphology. The analogy between a chemical crosslinking reaction and a finite-rate cooling of a network-forming liquid is introduced, on which basis the effects of compositional parameters on the resultant gel morphology are explained.
279 citations
TL;DR: In this paper, the short range order of the glasses was deduced with neutron diffraction, infrared and Raman spectroscopy by comparison with the spectra of the synthesized crystalline α-TeO2, Zn2Te3O8 and ZnTeO3.
Abstract: Glass formation occurs in the zinc tellurite system in the region of the eutectic (21 mol% ZnO) on the TeO2-rich side of the phase diagram. Glasses are characterized by a high refractive index which increases with TeO2 content. The glasses are transmitting from about 400 nm to about 6 μm with OH absorption bands at 3.3 and 4.4 μm. Short range order of the glasses was deduced with neutron diffraction, infrared and Raman spectroscopy by comparison with the spectra of the synthesized crystalline α-TeO2, Zn2Te3O8 and ZnTeO3. Glasses consist of disordered TeO4, TeO4, TeO3+ and TeO3 building units. The number of the TeO3+1 units is limited by ZnO addition. There is a relatively strong structural correlation between the glasses and the crystalline compound Zn2Te3O8 in accord with the phase diagram.
233 citations
TL;DR: In this paper, a comparison between colloidal colloidal silica spheres with radii in the range 10-500 nm were prepared by hydrolysis and condensation of tetraethoxysilane (TES) in a mixture of water, ammonia and a lower alcohol at several temperatures.
Abstract: Monodisperse colloidal silica spheres with radii in the range 10–500 nm were prepared by hydrolysis and condensation of tetraethoxysilane (TES) in a mixture of water, ammonia and a lower alcohol at several temperatures. It was attempted to establish a relation between the morphology and the chemical microstructure of the particle. Particle morphologies were examined with transmission electron microscopy and static and dynamic light scattering. The microstructure of the spheres was studied with quantitative direct excitation 29Si nuclear magnetic resonance (NMR) spectroscopy and a combination of qualitative cross-polarization 13C NMR and elemental analysis. A comparison was made between these particles and particles prepared from TES in an ammonia/water in cyclohexane microemulsion and also with Ludox® silica particles. The siloxane microstructure was found to show almost no variation as a function of concentration of reagents, catalyst, co-solvent and temperature. Around 65% of the silicon nuclei was bonded through siloxane bonds with four other silicons, approximately 30% was bonded with three other silicons and a few percent with only two. It was shown that under most experimental conditions several percent of the ethoxy groups never leave the TES molecule and end up inside the silica. The Ludox particles were found to consist of a more condensed silicon structure as compared with particles synthesized in alcohol, ammonia, water mixtures, whereas the spheres prepared in the microemulsion were less condensed and contained more alkoxy groups. The differences in particle morphologies — ranging from irregularly shaped rough particles to perfect, smooth spheres — are not caused by differences in siloxane and ethoxy microstructure. It is proposed instead, that a smooth and spherical particle shape is the result of the growth by monomers or small oligomers, and that a rough, irregular shape is the result of growth by larger silicon structures.
229 citations
TL;DR: In this article, an investigation of the radiative and non-radiative properties of the well known ZBLAN glass composition doped with individual rare earth elements (Pr3+, Nd3+, Dy3, Dy3+, Ho3+, Er3+ and Tm3+) at levels between 0.1 and 4 mol%.
Abstract: An investigation has been made of the radiative and non-radiative properties of the well known ZBLAN glass composition doped with individual rare earth elements (Pr3+, Nd3+, Dy3+, Ho3+, Er3+ and Tm3+) at levels between 0.1 and 4 mol%. The optical absorption and fluorescence spectra of the samples were measured and also the lifetimes for some transitions. Using the Judd-Ofelt model, the radiative decay rates between the different levels, the lifetimes and the branching ratios were calculated. These results and comparison of calculated and measured lifetimes are discussed with respect to radiative and non-radiative rates and quantum efficiencies. The multiphonon rates below 3200 cm−1 are found to be less in ZBLAN than in other host materials (ZBLA, SiO2, YAG).
TL;DR: In this article, the degree of cross-linking of the silica network is inferred from the frequency of the v(Si O Si) skeletal vibration, as monitored by the intensity of a characteristic 945-960 cm−1 vibration, particularly prominent if both steps of the two-stage hydrolysis procedure are performed in an acidic medium.
Abstract: Titania/silica mixed oxides prepared by sol-gel techniques are characterized by diffuse reflectance FTIR, Raman and 29Si solid state CP/MAS-NMR spectroscopy. The degree of cross-linking of the silica network is inferred from the frequency of the v(Si O Si) skeletal vibration. Gels prepared by complete hydrolysis of the alkoxide precursors (tetra-ethoxy-orthosilicate, TEOS, and tetra-isopropoxy-orthotitanate, TIOT, respectively) prior to mixing exhibit TiO2 crystallites embedded in an amorphous silica matrix. Raman spectroscopy is used to study the dependence of TiO2 crystallite size on hydrolysis pH. In an alternative two-stage hydrolysis procedure, 2 mol of water are first added per mole of TEOS, followed by mixing with the appropriate amount of TIOT, and completion of the hydrolysis. No crystalline domains are detected in the resulting oxides. The formation of Si O Ti bridges, as monitored by the intensity of a characteristic 945–960 cm−1 vibration, is particularly prominent if both steps of the two-stage hydrolysis procedure are performed in an acidic medium. A redispersion treatment in base, designed to induce a pore ripening process, if found to be only effective when applied to the wet gel, i.e., prior to drying. 29Si NMR experiments are used to investigate the processes occurring upon drying at 873 K. The extent of condensation reactions, by which Si O Si bridges are formed with concomitant desorption of water, is inferred from the fraction of completely cross-linked Si( O Si)4 sites. 1H-29Si cross-polarization dynamics are found to depend strongly on the degree of cross-linking, as well as on the number of available surface silanol groups.
TL;DR: In this article, a personal review of the experimental Urbach edge phenomena for c-Si and a-Si:H to highlight differences and similarities on the basis of simple physical models and a recent first principles theory of the energy, temperature, and static disorder dependence of the optical absorption edge.
Abstract: The availability of high quality wafers and films, the extensive literature on their optical, electrical and microscopic properties, and the wide spread interest in their applications suggests that a comparison of the absorption edge of c-Si and a-Si:H can be uniquely rewarding. This paper is a personal review of the experimental Urbach edge phenomena for c-Si and a-Si:H to highlight differences and similarities on the basis of simple physical models and a recent first principles theory of the energy, temperature, and static disorder dependence of the optical absorption edge.
TL;DR: In this paper, Fourier transform infrared and Raman spectroscopies were simultaneously used in order to study the time evolution of the acid-catalyzed hydrolysis and condensation of tetraethoxysilane (TEOS) in ethanol, to form silica xerogel.
Abstract: Fourier-transform infrared and Raman spectroscopies were simultaneously used in order to study the time evolution of the acid-catalyzed hydrolysis and condensation of tetraethoxysilane (TEOS) in ethanol, to form silica xerogel. The transformation to vitreous silica by gradual densification of the gel was also monitored after different heat treatments. The extent of hydrolysis was revealed by the changes in intensity of the IR-active bands of TEOS at 793 cm − (Si O asymmetric stretch), 1102 cm −1 (C O asymmetric stretch), 1168 cm −1 (CH 3 rocking) and 1400 cm −1 (CH 2 wagging) and also the TEOS Raman-active bands corresponding to the strongly polarized Si O symmetric stretch (656 cm −1 ) and the depolarized CH 3 rocking (960 cm −1 ). The intensity of all these bands clearly decreased as the reactions proceeded. Good evidence for the formation of ethanol in the initial steps of hydrolysis and for the subsequent predominance of its evaporation was found in the occurrence of maximum intensities for its infrared and Raman-active bands due to deformation of CH 3 or CH 2 groups (881 cm −1 ) and CH 2 bending (1275 cm −1 ), after a period of time. The intensity of these bands later decreased. The gel to glass conversion was followed through the disappearance of the infrared peaks at 554 cm − ) (Si OH rocking) and 940 cm −1 (Si OH stretching), together with the intensity increase of the peaks at 800 and 1078 cm −1 . The complementary nature of the IR and Raman spectroscopies is of great value for a complete understanding of the TEOS to silica glass transformation.
TL;DR: In this paper, the highest quantum yield and longest emission lifetime obtained were 55% and 425 μs, respectively, for an Nd doping level of 0.36 × 1020 cm−3.
Abstract: Nd-doped sol-gel silica glasses possessing optical quality that was adequate for a simple demonstration of laser action have been prepared. The highest quantum yield and longest emission lifetime obtained were 55% and 425 μs, respectively, for an Nd doping level of 0.36 × 1020 cm−3. The optical scattering loss was found to be on the order of 4.5%/cm. The sol-gel samples were prepared using a novel combination of techniques, including, for example, the employment of a particulate silica filler material to ensure crack-free drying, and the addition of propylene oxide to reduce the gelation time.
TL;DR: In this paper, the physical and chemical structures of gels dried from aprotic solvents were studied by a series of techniques (29 Si MAS-NMR, nitrogen adsorption, SAXS, elemental analysis, TGA).
Abstract: A two-step acid/base-catalyzed silica gel has been aged in alcohol and water baths followed by various aprotic solvents with a wide range of surface tensions. Low temperature (CO 2 ) and high temperature (ethanol) aerogels were also prepared. The physical and chemical structures of gels dried from aprotic solvents were studied by a series of techniques ( 29 Si MAS-NMR, nitrogen adsorption, SAXS, elemental analysis, TGA). The aprotic solvents isolated the effects of pore fluid surface tension during drying since they do not participate in condensation and other reactions. For aprotic solvents, a linear decrease in xerogel surface area was observed with increasing surface tension. Pore volume and pore size distribution followed a similar trend. Depending upon whether the gel had been washed in ethanol or water prior to the aprotic solvent, the final pore volume was changed significantly for a given surface tension. This indicates that both surface area and pore volume may be independently controlled.
TL;DR: In this paper, a new crucible drawing method was developed for the drawing of fibers with glass cladding, and the extrinsic losses caused by some oxide impurities were suppressed by the purification of raw elements.
Abstract: Sulfide, selenide and telluride glass fibers were prepared for infrared optical applications. A new crucible drawing method was developed for the drawing of fibers with glass cladding. The extrinsic losses caused by some oxide impurities were suppressed by the purification of raw elements. The transmission loss and mechanical strength, i.e., bending and tensile, of each fiber were investigated before and after the heat treatment under humid conditions. The fibers obtained were used for the power delivery of CO2 laser (10.6 μm) and CO laser (5.4 μm). The antireflection coating onto fiber ends and cooling of fiber with gas or water were examined for the improvement of power transmission efficiency. A fiber cable having ZnSe lens at the output end was prepared for medical and other applications. Temperature monitoring, thermal imaging and gas sensing were also tried using the fibers.
TL;DR: When a polymer such as silanol terminated polydimethylsiloxane (PDMS) is mixed with an alkoxide such as tetraethoxysilane (TEOS) in a suitable solvent and catalyst, gels can be prepared which exhibit rubbery elasticity as discussed by the authors.
Abstract: When a polymer such as silanol terminated polydimethylsiloxane (PDMS) is mixed with an alkoxide such as tetraethoxysilane (TEOS) in a suitable solvent and catalyst, gels can be prepared which exhibit rubbery elasticity The microstructures of such organically modified silicates (ormosils) can be controlled to give solids which are opaque or transparent via experimental variables This paper describes the conditions for the development of various microstructures, the properties of ormosils and how microstructures can control properties Reaction mechanisms and structure developments during heating are followed by liquid and solid state NMR studies and electron microscopy The rubbery ormosils are somewhat unique in that they contain a minor organic component and a major inorganic component, and are therefore potentially useful as a non-toxic rubber at high temperatures When these ormosils are heat-treated in a non-oxidizing atmosphere up to about 1000°C, decomposition and reorganization take place Depending on a number of variables, the products can be a black glass or a black porous nanocomposite The latter can now be heated to 1400°C in air with no loss of carbon and no deformation The porosity, low-expansion coefficient and black color make these nanocomposites useful as high-temperature insulators Transparent ormosils are superior to their pure oxide gel counterparts in terms of stress fracture during processing because of the presence of non-linking organic groups Laser-active organic dyes can be dispersed in ormosil liquid solutions prior to gelation to give transparent solid-state tunable organic dye laser rods or discs Ultrafine crystals of cadmium sulfide have also been dispersed in ormosils to give optical discs for non-linear applications The ormosils contain chemical bonds linking organic polymers to inorganic oxide networks They are likely to be the first group of many new inorganic-organic materials of the future such as multifunctional solids
TL;DR: In this paper, the authors investigated the hydrolysis and initial polycondensation processes of methyltriethoxysilane (MTES) by high-resolution 29Si-nuclear magnetic resonance spectroscopy (29Si-NMR).
Abstract: The hydrolysis and initial polycondensation processes of methyltriethoxysilane (MTES) have been investigated by high-resolution 29Si-nuclear magnetic resonance spectroscopy (29Si-NMR). MTES was hydrolyzed in the system with MTES:ethyl alcohol:water:HCL = 1:4:x:y; x = 1, 2, 3, 6; y = 1 × 10−4, 1 × 10−2. Signals due to end and middle groups of polysiloxanes formed sub-regions on the basis of the number of hydroxyl groups attached to silicon, and the sub-region for silicon possessing more hydroxyl groups was observed at a lower field. Based on signal intensities, the signals due to the dimers (Me(HO)2SiOSi(OH)2Me and Me(HO)2SiOSi(OH)OEt)Me) and the trimer (Me(HO)2SiOSiMe(HO)OSi(OH)2Me) (MeCH3, EtC2H5) were identified.
TL;DR: In this paper, the effects of molecular weight of coexisting polyacrylic acid (HPAA) and reaction temperature on phase separation and gelation behaviors were studied for acid-catalyzed, alkoxy-derived silica systems.
Abstract: Effects of molecular weight of coexisting polyacrylic acid (HPAA) and reaction temperature on phase separation and gelation behaviors were studied for acid-catalyzed, alkoxy-derived silica systems. The range of concentrations of HPAA which resulted in interconnected porous morphologies decreased and shifted toward lower concentrations as the molecular weight increased. The concentration range became larger with increasing reaction temperature. In addition to the relation between rates of phase separation and polymerization, viscosities of gelling silica network and pore-filling liquid during the domain formation process were responsible for the size and connectivity of resultant domains.
TL;DR: The use of 40% or more MeTMOS in the preparation of the aerogels lowers the degree of condensation beyond that of pure ‘SiO2’ as mentioned in this paper.
Abstract: Gels prepared from mixtures of Si(OMe)4 and MeSi(OMe)3 under basic conditions were supercritically dried by the methanol process. In aerogels prepared from only Si(OMe)4, 30% of the silicon atoms still contain an OMe substituent. When Si(OMe)4/MeSi(OMe)3 mixtures are employed, most of the residual methoxy substituents are replaced by methyl substituents. Thus, a permanent hydrophobicity is provided to the aerogels without affecting the average degree of condensation per silicon atom. The use of 40% or more MeTMOS in the preparation of the aerogels lowers the degree of condensation beyond that of pure ‘SiO2’ aerogels.
TL;DR: In this paper, the kinetics of deformation of a rod are analyzed, showing that it is possible to obtain the permeability, D, of the gel from the time-dependence of the load, as well as finding both G p and E p.
Abstract: When a saturated gel is deformed, its mechanical response is profoundly influenced by the flow of liquid in its pores. For example, if a gel bar is suddenly deformed in 3-point bending, the liquid cannot immediately flow out of the pores, so the gel initially responds incompressibly; consequently, its deflection provides a measure of its shear modulus, G p , rather than Young's modulus, E p . In this paper, the kinetics of deformation of such a rod are analyzed, showing that it is possible to obtain the permeability, D , of the gel from the time-dependence of the load, as well as finding both G p and E p . Analyses are presented for the time dependence of the load at constant deflection for a square and cylindrical beam, and for load at constant deflection rate for a square beam. The method is applied to determine G p , E p , and D for a silica gel made from tetraethoxysilane.
TL;DR: In this article, sound propagation in SiO 2 -aerogels and three types of organic aerogels, as well as of the corresponding xerogels was investigated, and the main parameter determining the sound velocity is the density; however, a scaling law for the entire density range investigated cannot be found.
Abstract: Sound propagation in SiO 2 -aerogels and three types of organic aerogels — resorcinol-formaldehyde, melamine-formaldehyde and vitreous carbon — as well as of the corresponding xerogels was investigated. The main parameter determining the sound velocity is the density; however, a scaling law for the entire density range investigated cannot be found. The influence of the constituent material and the production process on the sound velocity is comparably weak. Three types of low density aerogels were observed to become less stiff and the sound velocity was lowered when the specimens are uniaxially compressed. This is explained by a ‘knee model’ for the internal structure. For very low density aerogels, the sound velocity is mainly determined by the elastic properties of the gas within the tenuous structure instead of the skeletal stiffness.
TL;DR: In this article, a transparent xerogel containing various enzymes was obtained by mixing a solution of an enzyme with tetra-methoxy orthosilicate (TMOS) at room temperature followed by gelation and drying.
Abstract: Attachment of enzymes to insoluble matrix is an essential step in the development of biocatalysts. Transparent xerogels containing various enzymes were obtained by mixing a solution of an enzyme with tetra-methoxy orthosilicate (TMOS) at room temperature followed by gelation and drying. Effective immobilization was usually obtained at initial pH values > 7, where there is a change in the gelation mechanism from predominant hydrolysis/condensation to predominant direct polymerization of silicate precursors. The properties of sol-gel matrix, namely, transparency, large hydrophilic surface and good chemical and thermal stability, make it an ideal material for both biocatalysts and optical sensor devices. An example of a simple optical glucose sensor is demonstrated.
TL;DR: In this article, the effects of phosphorus precursors on the phosphosilicate gels were studied by X-ray diffraction, and 29 Si and 31 P magic angle spinning (MAS)-nuclear magnetic resonance (NMR).
Abstract: Gels were prepared using phosphoric acid, triethyl phosphate and trimethyl phosphite as the precursors of phosphorus. The upper limit for P 2 O 5 for gel formation is 90 mol% for gels using phosphoric acid and trimethyl phosphite, and 70 mol% for gels using triethyl phosphate. Gels prepared from phosphoric acid with more than 10 mol% of P 2 O 5 partially crystallize to Si 5 O(PO 4 ) 6 after heat treatment at 200°C for 10 h. Under the same heat treatment condition, all the gels prepared from trimethyl phosphite crystallize. Gels prepared from triethyl phosphate are amorphous even after heat treatment at 800°C for 10 h. However, chemical analysis indicates that most of the phosphorus in the triethyl-phosphate-prepared gels is driven off during the heating process; no more than 8 mol% of the initial amount of P 2 O 5 is retained in the gels. The effects of phosphorus precursors on the phosphosilicate gels were studied by X-ray diffraction, and 29 Si and 31 P magic angle spinning (MAS)-nuclear magnetic resonance (NMR). 29 Si and 31 P spectra show that Si O P and P O P linkages do not form in the xerogels (i.e. gels heat treated to ∼ 60°C). The gels heat treated at 200°C or above (those that did not crystallize) show evidence of Si O P and P O P network formation.
TL;DR: In this paper, several models to evaluate SAXS data are considered in order to decide which model parameters can be reliably related to real geometric sizes of various structural features, such as mean particle and pore sizes, description of network morphology (fractal aspects), derivation of particle and particle size distributions, characterization of particle surfaces and the determination of the skeletal density.
Abstract: Small angle scattering of X-rays or neutrons is known to be a powerful non-destructive technique to explore the structure of aerogels. Several models to evaluate scattering data are considered in order to decide which model parameters can be reliably related to real geometric sizes of various structural features. This includes calculation of mean particle and pore sizes, description of network morphology (fractal aspects), derivation of particle and pore size distributions, characterization of particle surfaces and the determination of the skeletal density. As an example, the appropriate interpretation of SAXS data with respect to structural modifications induced by supercritical drying and isothermal sintering of silica aerogels is quoted. Although some of these considerations refer to the aerogel structure, the results can, in principle, be used for a variety of nanoporous materials.
TL;DR: In this paper, the precursors for the sol-gel synthesis of titanium phosphates have been obtained via the dissolution of P2O5 in alcohols, and NMR experiments show the formation of P-O-Ti bonds during the course of the reactions.
Abstract: Phosphate and silicate materials exhibit similar structures but their solution chemistry is very different Polyphosphates cannot be synthesized under ambient conditions from PO(OR)3 or PO(OH)3 solutions More convenient PO(OH)3−x(OR)x precursors have been obtained via the dissolution of P2O5 in alcohols Their reactivity towards hydrolysis and condensation depends on x and the alkyl group They have been used as precursors for the sol-gel synthesis of titanium phosphates 31P NMR experiments show the formation of P-O-Ti bonds during the course of the reactions
TL;DR: In this paper, different approaches, described in the literature, to prepare catalysts by the sol-gel methods, including both aerogel and xerogel active solids, are reviewed.
Abstract: Sol-gel methods have been recognized as interesting procedures to prepare catalysts. The versatility of the sol-gel techniques allows control of the texture, composition, homogeneity and structural properties of solids, and makes possible production of tailored materials such as dispersed metals, oxidic catalysts and chemically modified supports. In this work, different approaches, described in the literature, to prepare catalysts by the sol-gel methods, including both aerogel and xerogel active solids, are reviewed. In parallel, some of the most relevant catalytic reactions carried out with sol-gel catalysts are evaluated.
TL;DR: The status of the theory and practice of drying is reviewed in this article, with emphasis on work published in the last few years Advances have been made in the theory of drying stresses and the causes of cracking, detailed and precise observations of phenomenology and drying, and preparation of monolithic gels of commercial interest
Abstract: The status of the theory and practice of drying is reviewed, with emphasis on work published in the last few years Advances have been made in (1) the theory of drying stresses and the causes of cracking, (2) detailed and precise observations of phenomenology and drying, and (3) preparation of monolithic gels of commercial interest
TL;DR: In this article, the design, synthesis and characterization of hybrid transition metal oxide gels in which organic polymers are part of the gel network are reported and illustrated with zirconium oxide based materials.
Abstract: Design, synthesis and characterization of hybrid transition metal oxide gels in which organic polymers are part of the gel network are reported and illustrated with zirconium oxide based materials. Zirconium-oxo-poly(AAEM) copolymers have been synthesized by chemical modification at the molecular level of zirconium propoxide with an organically functionalized chelating ligand (acetoacetoxyethylmethacrylate). Both polymerization reactions have been managed simultaneously, leading to hybrid organic-inorganic bushy polymers intimately interpenetrated at the nanosize scale. Zirconium oxo core is made with oxo-alkoxo AAEM modified species where zirconium is likely in sevenfold coordination. The zirconium oxo species are chemically bonded to polymeric methacrylate chains via the β-diketo complexing function. The complexation ratio (AAEM/Zr) has been varied from 0.25 to 0.75. It appears to be the key parameter which controls the structure and the texture of these hybrid materials.