Showing papers in "Journal of Non-crystalline Solids in 1986"

Sol-gel processing of silica. II: The role of the catalyst

Abstract: In the sol-gel synthesis of silica, a silicon-containing raw material, a solvent, water, and a catalyst are utilized. By varying the catalyst, dramatic effects on gelation time, porosity, bulk and apparent density, and volume shrinkage on drying were observed. For example, porosities ranging from two to sixty-eight per cent can be obtained for dried and fired gels. Mechanisms for catalysis have been proposed for the catalysts considered.

Calorimetric measurements on as-sb-se glasses

Abstract: Results of thermal analysis performed at different heating rates on eight glasses of the As-Sb-Se system, with compositions represented by (As, Sb) 40 Se 60 and As x Sb 15 Se 85 − x are reported and discussed. The values of the glass transition temperature ( T g ), the crystallisation temperature ( T c ) and the peak temperature of crystallisation ( T p ) are found to be independent of particle size. From the heating rate dependence of T g and T p values of the activation energy for glass transition ( E t ) and the activation energy for crystallisation ( E c ) are evaluated and their composition dependence discussed. The crystallisation data are examined in terms of recent analyses developed for non-isothermal crystallisation studies to arrive at E c and details of the crystallisation mechanism. The results indicate bulk nucleation and crystallisation with two and three dimensional growth respectively for the As x Sb 15 Se 85 − x and (As, Sb) 40 Se 60 glasses.

A unified model for reversible photostructural effects in chalcogenide glasses

Abstract: A general mechanism is proposed for the production of reversible photo-induced changes in chalcogenide glasses in terms of the scission of bonds, both inter-molecular and intra-molecular. Which mechanism is dominant is determined by several factors, in particular the type of chalcogen atom present in the material. The consequences of the various bond-breaking models are examined in detail for the cases of photo-induced changes in the structural, optical and mechanical properties of such materials.

Dc and ac conductivity in wide composition range Li2OP2O5 glasses

Abstract: The electrical conductivity of lithium phosphate glasses has been studied over wide frequency and temperature ranges, using a fully automated system capable of yielding data of high precision. Due to a “total drybox” sample preparation facility, samples containing P2O5 contents up to 65 mol% have been included without sacrifice of data quality. Comparisons with the behavior of the corresponding silicate, borate and boroaluminate glasses have been made and analyzed in terms of microscopic models. At the other composition extreme, measurements have been extended to 62.5 mol% Li2O without finding a conductivity maximum. The ac conductivity data have been analyzed using the modulus formalism, and systematic trends in the Kohlrausch parameter β with composition have been recorded. β decreases from 0.60 to 0.49 over the composition range 37–62.5 mol% li2O and seems to correlate best with the isothermal conductivity. Attention is drawn to anomalous structuring effects in P2O5-rich glasses.

Structural considerations about SiO2 glasses prepared by sol-gel

Abstract: Silica gels were prepared from alcoholic solutions of tetraethylorthosilicate (TEOS) with different H2O/TEOS molar ratios In the present work the transformation with temperature of these gels has been studied using IR spectroscopy together with measurements of density and specific surface area, thermal analysis and SEM The IR spectra of these gels, measured as a function of temperature and H2O/TEOS ratio, confirm that the relative concentration of OH groups is temperature dependent The total OH content, also depends on the H2O/TEOS ratio in the gel The combined results of IR, density and specific surface area measurements allow some structural considerations to be advanced about the final SiO2 glasses obtained

A new family of organically modified silicates prepared from gels

Abstract: A new family of organically modified silicates (ORMOSILS) prepared from gels, are obtained by mixing the appropriate amounts of tetraethoxy (or-methoxy) silanes and polyethylene glycol. The solvolysis has been studied by gas chromatography and NMR spectroscopy. The gels are heat treated at 150°C under a flow of dried air. Chemical analysis and weight measurement suggest a formula involving one poly (ethylene oxide) oligomer for two silicons. Large pieces of transparent dried gels are obtained, with densities ranging around 1.5 and a low specific pore surface area. By dissolution of various lithium salts, doped gels are easily obtainable with high Li+ conductivities.

Sol-gel chemistry studied by 1H and 29Si nuclear magnetic resonance

Abstract: The sol-gel process for forming inorganic glasses and ceramics can be studied uniquely at the molecular level by using 1 H and 29 Si nuclear magnetic resonance (NMR). A variety of reaction conditions have been investigated, and resonances for hydrolysis products, oligomers, and polymers have been identified. Differences resulting from variations in pH, water/alkoxide ratio, starting material, solvent, and addition of a second network-forming oxide are discussed. The effects of ester interchange were observed in 29 Si NMR spectra. The structural makeup of the polymer formed was studied as a function of pH. The hydrolysis was followed with 1 H NMR spectroscopy. All reactions were of tetramethyl or tetraethyl orthosilicate in methanol or ethanol.

Structural changes of silica xerogels during low temperature dehydration

Abstract: In this work we investigated the temperature evolution of the structure of silica thin films by Fourier Transform Infrared (FTIR) Spectroscopy. The gels, prepared by hydrolysis and polycondensation in the presence of an excess of water to tetramethoxysilane, were thin enough to be studied in transmission. It was found that the removal of water from the pores and gel surface is easier when the gels are prepared in basic conditions. During the drying heat treatment, residual methanol oxidizes and produces formaldehyde, formic acid and formates. These species are still present in the gel at 300 °C.

On the activation energy of crystallization in metallic glasses

Abstract: A new equation is derived from Johnson-Mehl-Avrami equation to calculate the activation energy of crystallization for metallic glass E, d [ ln ( d x d t ) p ]/ d (1/T p ) = − E R , where x is the volume fraction of transformation, t the effective time, Tp the peak temperature of crystallization at d2x/dt2=0, R the gas constant. It demonstrates that (dx/dt)p can be used instead of the cooling rate B in a Kissinger plot. This new equation has successfully predicted the activation energy of crystallization for Pd76Y9Si15 during non-isothermal annealing; and indicated, as expected, that the maximum rate of crystallization in DSC increases by the same factor as the heating rate does.

Transient nucleation effects in glass formation

Abstract: We extend to the non-isothermal case a numerical technique that was developed to treat transient homogeneous nucleation in a one-component system by modeling directly the reaction by which clusters are produced. Calculations are presented for the nucleation frequency during the quench and for the number of nuclei produced and the volume fraction transformed at the end of quench for different rates of cooling from the melt. Three model systems are considered: an alkali silicate which is a relatively good glass former, and two metallic glasses. These show a wide range of critical cooling rates for glass formation. In some systems transient effects are predicted to be critical for glass formation. A simple technique is presented for determining when transient effects are important based on a calculation using steady state nucleation frequencies and macroscopic growth velocities.

Structure of poorly crystalline MoS2 - A modeling study

Abstract: Current interest in amorphous and poorly crystallized phases of the layered transition metal dichalcogenides arises primarily from their unique catalytic and electrochemical properties not present in the corresponding crystalline phases. Computer calcultions of the scattered X-ray intensity from model structures of poorly crystalline (px-) MoS2 are compared here with experimental patterns from materials prepared at different temperatures. The results are used to identify the salient features of the observed diffraction patterns of px-MoS2 in which the measured diffuse scattered intensity may be clearly attributed to the stacking and rotational disorder of the layers as well as the limited number of these layers. A procedure for the interpretation of the X-ray diffraction pattern of px-MoS2 has thereby been developed in which the entire scattering pattern may be calculated without separation into Bragg peaks (of varying breadth and shape) and diffuse background.

About the structure and bonding of laser generated carbon films by raman and electron energy loss spectroscopy

Abstract: Laser generated amorphous carbon films have been investigated by Raman and electron energy loss spectroscopy to obtain information on bonding type and structure of carbon. Frequency shifts of the dominant first-order Raman lines and a strongly diminished π electron signal in the plasmon region as well as in the K absorption edge clearly indicate a fraction of sp3 bonds in the carbon films. Our model of disordered carbon containing sp2 and sp3 bonding fractions consists of small graphitic islands linked by a tetrahedrally bound network. Calculations within a force constant description confirm a fraction of 4… 20% sp3 bonds depending on the preparation history of the samples. A size of L ≤ 50 A of the graphitic clusters has been estimated by analysis of the intensity ratio of the shifted G and D Raman lines. Different graphitic target materials cause laser generated carbon films with a varying amount of sp3 bonds.

Sol-gel processing of silica: I. The role of the starting compounds

Abstract: The preparation of silica by the sol-gel process usually involves the use of a silicon alkoxide. When the solvent, catalyst and ratios of different components are kept constant, the particular alkoxide used can be varied. This paper represents a systematic study of how the alkoxide can affect the gelation rate and the properties of the gel formed. In addition, silicon acetate has also been used as the metal organic compound.

Transparent zirconia gel-monolith from zirconium alkoxide

Abstract: Transparent zirconia gel-monoliths were prepared by controlled hydrolysis of stabilized zirconium alkoxide. The gel after drying at 60°C was amorphous to electron diffraction. Infrared spectroscopic study indicated that the molecular structure of the dried gel was different from that of the precipitated zirconia as reported in the literature. The pore-structure of the dried gel when examined from its N2-desorption isotherm and the pore size distribution curve obtained by the BJH method indicated that the gel was microporous with most of the pore sizes less than ∼ 15 A in radius.

The factors affecting the thickness of sol-gel derived silica coatings prepared by dipping

Abstract: A partially hydrolysed solution of tetraethoxysilane was used to prepare thin glassy films on glass microscope slides by dipping. The thickness of these films after heat treatment to 500 °C was determined as a function of the withdrawal speed and the properties of the coating solution, such as viscosity, density and surface tension. The results were fitted to available theories of dip coating and acceptable agreement was obtained for coating solution viscosities below around 8 mPas and withdrawal speeds in the range 5 to 15 cm/min. In addition, the thickness of selected coatings were obtained at a range of heat treatment temperatures. Differences in the thickness of coatings prepared after different maturing times of the solution are also discussed.

The reactivity of TiO2 and ZrO2 in organically modified silicates

Abstract: The objective of this investigation was to compare the influence of titania, zirconia and silica on synthesis and properties of epoxide group containing organically modified silicates. These materials were prepared from the epoxysilane ( MeO ) 3 SiC 3 OCH 2 CHCH 2 O and titanium, zirconium and silicon alkoxides by the sol-gel process. The results show that titanium and zirconium, beside their role as network formers, act as efficient catalysts for epoxide polymerization. Zirconium promotes thermically the polymerization of epoxides to polyethylene oxide crosslinking chains, whereas titanium promotes this reaction photochemically. The incorporation of crosslinking polyethylene oxide decreases the scratch resistance of the silicate condensates. The activity of silicon in both reactions is very poor.

The effect of molecular structure on borosilicate glass leaching

Abstract: The leaching behavior of several sodium borosilicate glasses has been characterized using a combination of pH stat titrations. elemental solution analyses, elemental depth profiling via elastic recoil detection analysis and Rutherford backscattering. sodium diffusion measurements, and 11B NMR measurements on both leached and unleached glass. Leaching results indicate that the molecular structure of the glass controls glass dissolution by establishing the distribution of ion exchange sites, hydrolysis sites, and the access of water to those sitas. There is no correlation between sodium leaching and sodium diffusion in the unaltered glass. For most borosilicates in most environments, network hydrolysis controls the kinetics of glass dissolution.

Sol-gel transition in simple silicates: III. Structural studies during densification☆

Abstract: Raman spectroscopy and DSC were used to determine the structure and average heat of formation of siloxane species formed during the gel-to-glass conversion. Based on structural studies of cyclic model compounds and MO calculations of ring strain energies, we conclude that 3 and 4-fold siloxane rings are responsible for the strong Raman bands at 608 and 490 cm−1, respectively.

Effect of formamide additive on the chemistry of silica sol-gels: Part I: NMR of silica hydrolysis

Abstract: Monolithic gels can be more easily obtained when prepared with drying control chemical additives (DCCAs). The influence of formamide DCCA on the hydrolysis and early condensation of tetramethoxysilane has been studied by 29 Si NMR. It was found that formamide decreases the hydrolysis rate by 6 × but increases the polymerization rate slightly. The rate constants for both hydrolysis and condensation reactions depend upon the concentration of formamide and solution pH.

DC conductivity of V2O5Bi2O3 glasses

Abstract: Semiconducting vanadium-bismuth oxide glasses have been prepared in the composition range 80–95 mol% V2O5 The DC conductivity of the annealed glasses has been measured in the temperature range 80–480 K and compared with those of phosphate and tellurite glasses The conductivity is sensitive to microstructure The data have been analysed in the light of models of polaronic hopping conduction Hopping of the polaron is shown to be adiabatic in nature Mott's T 1 4 analysis is made at low temperature

Alpha alumina transformation in seeded boehmite gels

Abstract: The kinetics and surface area changes during the γ to α-Al 2 O 3 transformation sequence of an α-Al 2 O 3 seeded boehmite sol-gel are compared to an unseeded boehmite sol gel. The seeded gels exhibited increased kinetics, however the activation energy for the γ to α-Al 3 O 3 transformation was lowered only 17% indicating the α-Al 2 O 3 transformation is growth controlled. Transmission electron microscopy showed that each α-Al 2 O 3 seed acts as a multiple nucleation site for the θ to α-Al 2 O 3 transformation. The higher surface area of the seeded boehmite results from the prevention of vermicular microstructure development. Thus, by seeding boehmite sol gels with α-Al 2 O 3 the growth process during transformation to α-Al 2 O 3 is controlled resulting in a fine, uniform microstructure which sinters at significantly lower temperatures.

Physical and chemical characteristics of lead-iron phosphate nuclear waste glasses

Abstract: Experimental determinations of the properties of lead-iron phosphate glasses pertinent to their application to the problem of the permanent disposal of high-level nuclear wastes have been carried out. These investigations included studies of the composition and physical properties of nuclear waste glasses (NWG), as well as the effect of preparation conditions. Lead-iron phosphate nuclear waste glasses were prepared by dissolving simulated US defense wastes or simulated commercial power reactor wastes in molten lead-iron phosphate melts at temperatures between 900 and 1050°C. The measured physical and chemical properties of the nuclear waste glasses formed by cooling these melts and annealing included the following: (1) aqueous corrosion resistance as a function of the solution pH, solution temperature, and glass composition, (2) glass density, (3) thermal expansion coefficient, (4) glass transition temperature and softening point, (5) heat capacity, (6) critical rate (7) temperature for the maximum crystallization rate, (8) relative solubility of waste oxides in the glass melt, (9) reactions between the molten glass and the melting crucible (Pt, ZrO2, Al2O3), and (10) studies of possible metal cannister materials. Experimental results for the lead-iron phosphate NWG are compared to available data for borosilicate NWG. Relative to borosilicate NWG, the lead-iron phosphate glasses have several distinct advantages which include a much lower aqueous corrosion rate, a lower preparation temperature, and the ability to immobilize many types of commercial and defense-related high-level radioactive wastes.

Drying gels: I. General theory

Abstract: An assessment is given of the driving forces causing shrinkage during drying of organometallic gels. Both the elastic and viscous responses of the gel to those forces are analyzed, so that the stresses and strains resulting from drying can be calculated. The principle causes of contraction are assumed to be the large interfacial energies between the solid, liquid, and vapor phases. The extent of chemical reactions that occur during drying is assumed to be proportional to the change in interfacial area, so the corresponding change in chemical potential can be included as a contribution to interfacial energy. Three contributions to the capillary stress are identified, the largest being the redistribution pressure that drives the liquid from the wet region of the gel into the dry exterior region. This causes the contraction rate of the wet region to exceed that of the dry region. The rate of liquid flow through the pores of the gel is analyzed. The shrinkage rate is shown to be controlled by the viscosity of the solid phase of the gel. as well as the rate of transport of pore liquid.

Study of polymerization processes in acid and base catalyzed silica sol-gels

Abstract: Raman spectroscopy has been employed in conjunction with 29 Si NMR and the molybdenum chemical technique to study the polymerization process of silica sol-gels under pH conditions ranging from 1 to 9. A qualitative correlation between the rate of hydrolysis and the size of the resulting polymeric particles has been tentatively proposed. The extent to which the relative Raman intensity simultaneously proves information regarding particle dimensions, shape and degree of condensation has been discussed. A quantitative comparison between the monomer concentration as determined by the molybdenum chemical technique and by 29 Si NMR has been provided.

Neutron diffraction and exafs evidence for TiO5 units in vitreous K2O · TiO2 · 2SiO2

Abstract: A combination of neutron diffraction and X-ray absorption spectroscopic techniques has been used to investigate the Ti coordination in vitreous K2O· TiO2 · 2SiO2. In the former case, the Tizz.sbnd;O bond lengths have been studied using the technique of isotopic substitution. XANES (X-ray absorption near-edge structure) data indicate that there is little or no evidence for a centre of inversion symmetry at the Ti site in the glass. TiO bond distances derived from neutron diffrraction and EXAFS (extended X-ray absorption fine structure) agree to within 0.03 A and it is concluded that the Ti is present as TiO5 tetragonal pyramids containing one non-bridging oxygen atom associated witth a titanyl bond (Ti=0 = 1.65 A). The bridging TiO bond length is 1.96 A and TiTi correlations occur at 3.4 A and 6.1 A.

Characterization of various stages of the sol-gel process

Abstract: Monolithic samples of silica and cobalt-doped silica glasses were prepared from the hydrolysis-gelling process of TEOS in ethanol solution. Cobalt was used as a “colour indicator”. The sol-gel transition was studied by measuring the specific and intrinsic viscosities of the solution. The gel-glass transition was studied by infrared spectroscopy. TGA, DTA, density and BET measurements. Infrared spectra were recorded at different temperatures in the near-infrared region (4000 to 10 0000 cm−1) as well as in the region of fundamental absorption (200 to 4000 cm−1). Molecular water was distinguished from silanol groups and from organic constituyents. Organic components were totally removed at temperatures between 325°C and 350°C and an amorphous silica was produced. Isolated and bound silanol groups were observed which disappeared at temperatures above 1100°C. Reaction of active surface sites with ethanol and water vapors at room temperature was studied and a method was applied to determine at various temperatures the chemical absorbed water and the number of silanol groups per surface area. Two different silanol groups decreasing rates with temperatures were observed. The experiments indicate that the hydrolysis-gelling process is somewhat similar to that of polycondensation of silicic acid and some degree of ordering must be present in the silica particles formed.

Solvent effects on the condensation stage of the sol-gel process

Abstract: Effects produced by changing the solvent medium on the sol-gel polymerization process have been investigated by Raman spectroscopy, the molybdenum chemical reaction and electron microscopy methods. The mechanism of particle aggregation as well as the extent of condensation of the polymeric network have been shown to be dramatically affected by the presence of organic additives, such as formamide, dimethyl formamide, acetonitrile and dioxane. These observations have been explained in terms of hydrogen bonding and electrostatic interactions which modulate the nucleophilic substitution mechanism associated with the sol-gel condensation process.

The structure of binary alkali silicate glasses

Abstract: The role of alkali metal cations in determining the structure of binary silicate glasses has been studied by magic angle spinning NMR of 29Si and 133Cs in Rb2OSiO2 and Cs2OSiO2. Glasses with up to ≈ to mol% alkali metal oxide have been investigated and the results compared with those for Li2OSiO2 and Na2OSiO2. At high concentrations of Rb2O or Cs2O there is a deviation from the formation of solely Qn and Qn−1 species which was found for Na2OSiO2. We suggest that the structure of binary silicates is determined by a balance between the repulsion of non-bridging oxygens on different Q species and the mutual attraction or repulsion of the alkali metal cations.

Non-isothermal transformation kinetics: Remarks on the Kissinger method

Abstract: It has been demonstrated that the activation energy of a solid state reaction can be determined by Kissinger's method whatever the reaction mechanism. On the other hand the influence of both the kinetic law obeyed by the reaction and the values of E/RT on the error in the activation energy determined from Kissinger's method has been analyzed. It has been shown that whatever the reaction mechanism this error is lower than 5%, provided that E/RT > 10.

Redox processes in glass-forming melts

Abstract: Oxidation-reduction (redox) equilibria of multivalent elements in glass-forming melts can be expressed by several different, yet similar, chemical equations. Each equation attempts to describe the effects that melt temperature, base composition, imposed oxygen fugacity, multivalent element concentration, and the presence of other redox components induce on the established redox equilibrium. Redox kinetics, or the rate at which equilibrium is attained in the melt, demonstrates the interrelationship between the dissolved multivalent species and the gaseous redox components. The diffusion of redox gases, such as oxygen into or out of a static glass-forming melt controls how fast the redox state of the melt can change. Both equilibrium and kinetic redox processes are thus integral to the development of desired glass/melt properties.