Silica Glass Matrix Research Articles

Synthesis and characterization of ZnAl 2O 4/SiO 2 nanocomposites by sol–gel method

Transparent bulk ZnAl 2O 4/SiO 2 nanocomposites, containing ZnAl 2O 4 nanocrystals dispersed in silica glass matrix, were obtained by the sol–gel method. The gels of composition 89SiO 2–6Al 2O 3–5ZnO (ZAS) were prepared at room temperature, and heat-treated at different temperatures. The microstructural evolution of gels was characterized by differential scaning calorimetry (DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM) and infrared spectroscopy. ZnAl 2O 4 nanocrystals were precipitated from ZAS system and dispersed in the SiO 2-based glass during heat-treatment in the temperature range of 900–1100°C. The nanocrystalline particles appear spherical and are estimated to be 10–18 nm in diameter.

Reactions at the liquid silicon/silica glass interface

Experiments have been carried out to determine the nature and origin of the spots growing on silica glass surfaces in contact with liquid silicon during CZ–Si crystal growth. Silica glass ampoules were filled with silicon and tempered between 5 min and 40 h at a temperature (1693 K) slightly above the melting point of silicon. Cross sections of the ampoules with solidified silicon have been examined by scanning electron microscopy and optical polarization microscopy. In addition cross sections from commercial silica glass crucibles used in the Czochralski process or dipped into the silicon melt were investigated with the same methods. At the silicon/silica glass interface different reaction zone morphologies were detected. A solution-precipitation mechanism is suggested for the fast lateral growth of the reaction zone, which is proposed to consist of small cristobalite crystals embedded in a silica glass matrix.

Silicate-silicate liquid immiscibility and graphite ribbons in Libyan desert glass

Transmission electron microscopic (TEM) investigation of the dark (brown or bluish) streaks occurring in Libyan Desert Glass reveals the common presence of small glass spherules. The spherules, mostly 100 nm in size, are homogeneously dispersed within the silica-glass matrix. The complete absence of electron diffraction effects confirms their amorphous nature. The spherules are Al-, Fe- and Mg-enriched with respect to the surrounding silica matrix and their (Mg + Al + Fe) : Si ratio is close to 1.The silica-glass matrix and amorphous spherules form an emulsion texture (i.e., globules of one glass in a matrix of another glass), which originates from silicate-silicate liquid immiscibility. This texture has also been observed in other impact-derived glasses.The silica glass also contains carbonaceous inclusions consisting of 5–50 nm thick, polygonalized graphite ribbons that form closed structures up to 200 nm in diameter.These observations are in agreement with an impact origin for Libyan Desert Glass.

The Influence of Silica Glass Matrix on the Spectra of Interstitial Molecules S2

The luminescence of interstitial molecules S2 in silica glass upon excitation with synchrotron radiation is investigated at room and liquid-helium temperatures. The additional photoluminescence band with a maximum in the range of 440 nm at 10 K is assigned to the transition B""3Π u → X3Σ g – in the S2 molecule. The spectra obtained are compared with the spectra of S2 molecules in the gas phase and in the solid inert-gas matrix. It is demonstrated that the influence of the glass matrix on the spectra of S2 molecules manifests itself in two effects. First, the spectral lines are broadened as a result of collisions between the molecules and pore walls in the glass. Second, the luminescence from the molecular terms with energies higher than the energy corresponding to the band gap or the work function of the glass is quenched through the transfer of electronic excitations to the glass network. It is indirectly established that S2 molecules in the glass are located in small pores and interstices whose sizes are only 0.15 A larger than the molecular size. This radically limits the possible chemical reactions accompanying the photodestruction of molecules.

Crowding and hydration effects on protein conformation: a study with sol-gel encapsulated proteins

We are developing an experimental system for testing the effects of macromolecular crowding and molecular confinement on protein structure. In the present study, solvent effects on the secondary structure of two proteins were examined by circular dichroism following encapsulation in the hydrated pores of a silica glass matrix by the sol-gel method. Changes in the unfolded conformations of encapsulated apomyoglobin and reduced serum albumin were analyzed after equilibration with aqueous solutions of natural osmolytes, short-chain alcohols, polyethylene glycol, and a complete series of Hofmeister cations. In many instances, the α-helical content of the encapsulated protein was increased by addition of solutes at concentrations that have no effect on the protein in the absence of the glass. The results are discussed from the perspective of water structure. We argue that perturbed water at the silica interface causes an increase in the average free energy of the bulk water phase which, consequently, diminishes the strength of the hydrophobic effect inside the glass matrix and destabilizes the conformation of encapsulated proteins. We propose that solutes can increase the strength of the hydrophobic effect and influence folding equilibria without directly interacting with the protein. A hypothesis is provided for the apparent paradox that kosmotropic (strongly water binding) anions favor native protein structure, whereas chaotropic (weakly water binding) cations enhance native protein structure. The encapsulation results suggest that macromolecular crowding and molecular confinement are accompanied by hydration effects that may oppose or potentiate the stabilizing effects of excluded volume on protein structure, depending on the surface chemistry of the crowding agent and its influence on bulk water structure. In the crowded environment of a living cell, excluded volume effects, surface-induced water structure, and compatible solutes are expected to complement the dominant forces in protein folding.

Cluster glass structure in nanohybrids of nonstoichiometric zinc ferrite in silica matrix

A cluster glass structure has been observed to occur in nanohybrids consisting of zinc ferrite in an amorphous silica matrix. Such a unique cluster glass structure has been supported by studies of x-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, and magnetic characterization. By increasing the Fe/Zn ratio in silica gel to 10, crystallites of zinc ferrite of partially inverted structure were nucleated and grown in amorphous iron oxide pockets dispersed in the silica network. The zinc ferrite crystallites, together with the confinement of silica matrix, suppress nucleation and crystallization of α-Fe2O3 to a temperature above 900 °C, preserving the cluster glass structure. The resulting cluster glass structure consists of amorphous iron oxide pockets of ∼10 nm in dimensions together with zinc ferrite crystallites of ∼3 nm in sizes, which are uniformly dispersed in the amorphous silica glass matrix.

Second harmonic generation in transparent KTiOPO 4/SiO 2 nanocomposite glasses prepared by the sol–gel method

Transparent bulk xKTP–(1− x)SiO 2 nanocomposites, containing nanocrystalline KTiOPO 4 (KTP) dispersed in silica glass matrix, were fabricated by the sol–gel method. The transmission properties of KTP/SiO 2 have been measured between 2500 and 300 nm and show transmission of 30% at 0.532 μm and 60% at 1.064 μm. The nanocomposite glasses exhibit an UV cut-off at 350 nm. The second harmonic generation (SHG) was observed from xKTP–(1− x)SiO 2 nanocomposites or glass-ceramics. The intensity of the SHG signal in the nanocomposites increases as a function of KTP concentration.

Electron-beam induced growth of Cu nanoparticles in silica glass matrix

The growth of nanometer-scale Cu metal particles in an amorphous SiO2 film was observed under a finely focused, intense electron beam in a dedicated scanning transmission electron microscope. Annular dark-field images show that the particle illuminated by the beam grows at the expense of surrounding Cu particles. Electron energy-loss spectroscopy was used for identifying the oxidation states of particles containing Cu atoms. An electric-field induced diffusion model explains the growth of metal particles in a dielectric medium.

Synthesis and linear and non-linear optical properties of (0.8PPV+0.2DMPPV)/silica glass composites by sol–gel process

The (0.8 Poly(1,4-phenylene vinylene) + 0.2 Poly(2,5 dimethoxy-1,4-phenylene vinylene))/silica glass composites for non-linear optical applications were synthesized by the sol–gel process and the linear and non-linear optical properties were investigated. Microstructural evolution of the composites was measured by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), infrared (IR) and ultra-violet/visible (UV/Vis) spectroscopy, and transmission electron microscopy (TEM). The conjugated (0.8PPV + 0.2DMPPV) copolymer was found to be incorporated into the silica glass and we confirmed it by the trans-CC bands in the UV/Vis spectra and the TEM results. The UV/Vis absorption bands of the composites were found to shift toward the short wavelength side and the amount of the blue shift increased with increasing heat treatment temperature and time due to the reduction of the effective conjugation length of the copolymers. Spherical particles 20 nm in diameter of the (0.8PPV + 0.2DMPPV) copolymer were found to be embedded and uniformly distributed in the silica glass matrix by TEM. Third-order non-linear optical susceptibility, χ (3), was measured by the Z-scan method and found to be 5.9 × 10 −10 esu.

Optimization of the Selectivity of Pyrene Immunoaffinity Columns Prepared by the Sol−Gel Method

The sol−gel method opened a new path for synthesizing immunoaffinity column packing materials by encapsulating antibodies in the pores of a silica glass matrix. This paper describes different strategies for the optimization of the selectivity of pyrene immunoaffinity columns synthesized by encapsulating the immunoglobulin (IgG) fraction of an anti-pyrene antiserum in a porous silica glass. The selectivity of the columns which are designed for the determination of pyrene in aqueous samples is limited by the retention of cross-reacting and nonspecifically adsorbed polycyclic aromatic hydrocarbons (PAHs). An optimum in selectivity is achieved by a new method to wash the columns with a mixture of a high molecular weight polymer which blocks nonspecific adsorption sites and a surfactant which removes unwanted substances by inclusion into micelles. Optimal results were achieved by washing with 5% (v/v) Roti-Block in 10:90 acetonitrile−water (v/v). Roti-Block is the trade name for a mixture of a high molecular weight poly(vinylpyrrolidone) with a nonionic surfactant dissolved in PBS buffer.

Electrical conduction in composites of nanosized iron particles and oxide glasses

Nanocomposites involving iron particles in silica glass matrix have been synthesized by the hot pressing of suitably reduced precursor gel powders. The metal particles have diameters in the range 3.8 to 10.2 nm. An almost four orders of magnitude resistivity range at room temperature has been obtained by such changes in particle diameters. The resistivity in the temperature range 200-340 K shows a fractional temperature dependence with an average value of n ∼ 0.69. The resistivity changes in this temperature region can be explained on the basis of an electron tunneling mechanism.

Nonlinear optical properties of Sn+ ion-implanted silica glass

The absolute value of the third-order nonlinear optical susceptibility, ¦χ(3)¦, of Sn+ ion-implanted silica glass was found to be ∼ 10−6 esu. This value is as large as those reported for semiconductor-doped glasses. Silica glass substrates were implanted with Sn+ ions at an acceleration energy of 400 keV to a dose of 2 × 1017 ions/cm2 at room temperature. Metallic Sn microcrystallites of 4–20 nm in diameter were found to be embedded in the silica glass matrix. The average volume fraction of the Sn microcrystallites was evaluated to be 28%. ¦χ(3)¦and the imaginary part of the dielectric function, Im ϵ, had peaks at the same wavelength of 500 nm owing to surface plasmon resonance. The peak width of ¦χ(3)¦was nearly half of that of Im ϵ, which can be explained by an effective medium theory.

Corrosion behavior of a powdered simulated nuclear waste glass: A corrosion model including diffusion process

Static corrosion tests were performed with a powdered simulated waste glass in deionized water at 90°C for periods of up to 130 days. It was observed that normalized elemental mass loss (NL) values for soluble elements (Li, B, Na and Mo) were larger than those for Si by a factor of three and continued to increase after saturation of Si. A corrosion model (diffusion-combined model), where a diffusion model is combined with a dissolution/precipitation model (reaction path model), was developed and applied to the analysis of experimental results. In the diffusion-combined model, it is assumed that less-soluble elements dissolve into the solution congruently with the silica glass matrix (glass matrix dissolution). On the other hand, it is assumed that soluble elements diffuse through the glass to the surface and dissolve into the solution, in addition to the glass matrix dissolution. The diffusion-combined model can explain the experimental results well, and it is found that the diffusion coefficient is the most effective parameter determining the corrosion behavior.

γ-Radiation sol–gel synthesis of glass–metal nanocomposites

Glass–metal nanocomposites of nanocrystalline silver in a silica glass matrix have been successfully synthesized by γ- radiation combined with the sol–gel method.

Synthesis of nanocrystalline nickel-zinc ferrite by the sol-gel method

Ni 0.5Zn 0.5Fe 2O 4 crystals having dimensions varying from 13 to 35 nm have been synthesized within a silica glass matrix by a sol-gel method. Particle sizes in the above range can be controlled by a suitable heat treatment schedule at temperatures varying from 973 to 1323 K. The coercivity values for different heat-treated samples are found to be in the range 20–95 Oe, which are significantly larger than that for samples having grain sizes of the order of a few microns. The Mössbauer spectra of the samples show the presence of ultrafine particles exhibiting superparamagnetic relaxation.

Resistivity of Ultrafine Iron Particles in a Glass Matrix

Bulk samples containing nanometer-sized iron particles in a silica glass matrix have been prepared by hot pressing sol-gel derived precursor powders subjected to reduction treatment. All samples exhibit metallic behaviour. The resistivity variation as a function of temperature in the range of 80 to 340 K shows a phonon softening effect in these ultrafine particles.

Synthesis of nanometre-sized quasi-crystals in the Al-Cu-Fe system by the sol-gel technique

The discovery of an icosahedral phase with a five-fold symmetry in A I M n alloy [1] has sparked off intense research in this fascinating field of materials science. A rapid-solidification technique was originally used for the synthesis of these quasi-crystals. A number of other methods have recently been used for preparing these materials, e,g. sputtering [2], ion-beam heating for deposition [3], controlled heat treatment of amorphous phase [4], interdiffusion treatment [5] and mechanical alloying [6, 7]. Most of the quasi-crystalline alloys prepared by these techniques are in metastable states. Tsai et al. [8, 9] reported on the preparation of thermodynamically stable quasi-crystalline alloys in the A1-Cu-M system (M = Cr or Fe) by rapid solidification as well as by conventional solidification. In all of these methods high-temperature processing is necessary at some stage. The sol-gel method has emerged as a versatile technique for the synthesis of a wide variety of materials [10-12] because of its advantage of low-temperature processing. We showed earlier that F e C r alloy of nanometre dimensions could be precipitated within a silica glass matrix by the reduction of a suitable precursor gel [13]. We have now used the sol-gel route for the preparation of quasi-crystalline alloy of composition A165Cu20Fe15 dispersed in a silica glass matrix. The details are reported in this letter. The metal precursors and the amounts used were Al(NO3)3 • 9H20 1.245 g, Cu(NO3)2 • 3H20 0.246 g and Fe(NO3)3 '9H20 0.309 g. These were first dissolved in 5cm 3 distilled water and stirred thoroughly. A few drops of concentrated nitric acid were added to this solution, followed by the addition of 2 cm 3 Si(OC2H5)4. The mix was stirred for 30 min to obtain a clear sol. This gelled within a period of 3-4 h. This alcogel was heated in hydrogen atmosphere at 923 K for about 45 min. For characterization the heat-treated gel pieces were ground and mounted on a carbon-coated grid for transmission electron microscopy, which was carried out in a JEM 200 CX electron microscope. The M6ssbauer spectrum of the sample was recorded in a conventional set-up operating in constant-acceleration mode. The details of the M6ssbauer measurements were published in [13]. Fig. la is an electron micrograph of the sample and Fig. lb is the selected-area electron diffraction pattern. The statistical average of the particle diameters as calculated by the procedure described in [12] was 15 + 1.8 nm. The interlattice spacings d, calculated from the electron diffraction rings, are given in Table I. Also shown in this table are the

Electrical conduction in sol-gel derived glass-metal nanocomposites

Glass-metal nanocomposites involving iron, nickel and copper in a silica glass matrix have been prepared as films on glass substrates by the sol-gel route. The metal phase is introduced as chloride and silicon tetraethoxide is used as the glass precursor. DC electrical resistivity of these nanocomposites has been measured over the temperature range 100 to 350 K. Resistivities varying from 10 Omega m to 106 Omega m have been obtained by controlling the volume fraction of the different metal phases within the silica glass. Low-temperature resistivities of the samples are controlled by a simple activation with energies in the range 0.01-0.1 eV. At temperatures above 150 K some of the nanocomposites exhibit another activated mechanism. The latter is believed to arise due to hopping of electrons between the localised states formed by the distributed metal atoms within the silica glass matrix.

Glass-metal nanocomposite synthesis by metal organic route

Glass-metal nanocomposites incorporating ultrafine particles of iron, nickel, cobalt and manganese, respectively, in a silica glass matrix have been prepared by heat treatment of a gel derived from a sol containing silicon tetraethoxide and a suitable metal organic compound. Metal particles in all the nanocomposites are isolated and spherical-shaped with diameters ranging from 3 to 10 nm. Films of these nanocomposites with thickness of the order of a few micrometres have been prepared on glass slides by a simple dip-and-pull technique. Optical absorption spectra of the nanocomposite films have been measured over the wavelength range 200 to 2000 nm. Effective medium theories of Maxwell-Garnett and Bruggeman, respectively, have been used to calculate theoretically the optical absorption of these materials. The Maxwell-Garnett theory gives results which are in better agreement with experimental data than those obtained from Bruggeman formalism. The filling factor f as estimated from the least-squares fit of the experimental results with the Maxwell-Garnett theory has a value in the range 1 to 4%.

Natural shock behavior of almandite in metamorphic rocks from the ries crater, Germany

An extensive study of a big number of gneiss specimens with various shock features from the suevite allowed unravelling of the shock behavior of almandite garnets. Almandites in shocked metamorphic rocks show with increasing dynamic pressures strong irregular fracturing. differently oriented sets of planar fractures or elements, brown turbidity and nucleation of minute crystals of an unknown phase in solid garnets. At higher peak pressures garnet was found to break down to (1) orthopyroxene + spinel + glass, and to (2) spinel + glass due to fast shock-melting. Extensive quantitative electron microprobe studies of almandite garnets and their breakdown products were carried out. The breakdown products within the original grain boundaries of the garnets consist of an alumina-rich orthopyroxene (with up to 10 wt. % Al 2O 3), hercynite to pleonaste spinels and a silica and calcium-rich glass matrix. The chemical zonation of magnesium and manganese of the former garnets is inherited in the composition of the newly formed orthopyroxenes. Petrographic evidence and chemical composition suggest a fast breakdown of the almandite garnets after passing of shock waves at rapidly falling pressures and very high post-shock temperatures within the ejected gneissic rock material.