Showing papers in "Journal of Non-crystalline Solids in 1987"

Hydrolysis of titanium alkoxides: modification of the molecular precursor by acetic acid

Abstract: Monolithic TiO 2 gels can be reproducibly obtained when the hydrolysis of titanium alkoxides is performed in the presence of acetic acid. This carboxilic acid does not act only as an acid catalyst, but also as a ligand and changes the alkoxide precursor at a molecular level therefore modifying the whole hydrolysis condensation process. Infra-red experiments show that bidentate acetates replace OR groups and are directly bounded to the titanium. Both, chelating and bridging acetates, are observed, leading to Ti(OR) x (Ac) y . oligomers. Hydrolysis of this new molecular precursor removes first (OR) groups and bridging acetates. Chelating acetates are still observed in the gel. They can only be removed upon heating above 200 °C.

NMR study of the sol/gel polymerization

Abstract: Kinetics and mechanisms of the hydrolysis-condensation reactions, starting with tetraethyl orthosilicate (TEOS), have been studied by 29 Si NMR. A standard experimental procedure for the sol/gel process was used: acid-catalyzed reactions and different concentration ratios of alkoxide and water. The inorganic polymerization was influenced by the following experimenta) parameters: — The starting alkoxides which may exhibit weakly reactive “Si-OR groups”. — The use of gel precursors with different chain lengths which lead to parasite ester exchange reactions. — The variations of pH which modify hydrolysis and condensation rates. — The H 2 O content where an excess of water was required to fully separate hydrolysis and condensation domains, and to eliminate “OR groups” in the gel.

Guiding principle for preparing highly photosensitive Si-based amorphous alloys

Abstract: A guiding principle was proposed for the preparation of highly-photosensitive hydrogenated amorphous SiGe (a-SiGe:H) as well as SiC (a-SiC:H) alloy materials via the plasma chemical-vapor-deposition (CVD), which was derived from the experimental analyses of the behavior of adsorbed precursors on the growing surface by means of a radical separation technique. According to this principle, Triode method and H2-dilution method of starting gas materials were used for the deposition of a-SiGe:H and a-SiC:H, and highly-photoconductive alloy materials were prepared in the optical-gap range between 1.3 and 2.3eV.

Isothermal and adiabatic nucleation in glass

Abstract: A summary of both classical (isothermal) and adiabatic theories of nucleation is presented. Using experimentally determined parameters, such as viscosity, specific heat, glass transition temperature (Tg), melting temperature (Tf) and heat of fusion, the temperatures of maximum crystal nucleation rates, Tmax, were calculated for several glass forming systems and compared with experimental data. It is shown that both theories give a good estimate for Tmax. For systems which do not show volume (homogeneous) nucleation, Tmax is lower than Tg. For systems which show volume nucleation, Tmax >Tg. It is concluded that both theories can be used to predict the occurrence of internal crystal nucleation in glasses and that, in general, a high value of Tg (Tg/Tf > 0.58) indicates the absence of internal nucleation.

Short range order in the network of the borophosphate glasses: Raman results

Abstract: This paper presents a Raman investigation of glasses of the system MX:M 2 O:B 2 O 3 :P 2 O 5 (M = Li, Ag; X = Cl, 1). For the phosphate glasses, we discuss in detail the relationship among glass composition, density of the different phosphate (PO 4 ) units, and Raman spectra. For the borophosphate glasses, we analyze how the oxygens introduced with the modifier oxide M 2 O are shared among borate and phosphate units. When P 2 O 5 is partially substituted by boron oxide, and the M 2 O content remains constant, the negative charge of the phosphate units increases, on average. Comparison with the results of a companion 31 P NMR-MAS study allows identification of bands associated with structures containing the P-O-B motives. A discussion is also given of the changes induced into the borophosphate glass network by addition of a “doping” MX salt.

Spectroscopic investigations of borosiloxane bond formation in the sol-gel process

Abstract: The formation of borosilicate glass by the sol-gel process has been monitored during all stages, from the initial alkoxide solution to the final heat-treated product, using nuclear magnetic resonance and infrared spectroscopy as well as chemical analysis. The reactions of boric acid and trialkyl borates with silicon alkoxides and their hydrolysis mixtures have been studied for various compositions and catalysts. For all samples it was found that there exists only a small number of borosiloxane (-B-O-Si-) bonds at the time of gelation, the majority of boron being in the form of boric acid. This boric acid is condensed to borosiloxane bonds only upon heat treatment of the dried gel. For compositions having higher boron contents a small number of -B-O-B- bonds are also formed, which do not convert to -B-O-Si- upon heating to 500 °C.

Infrared reflectivity spectroscopy of silicate glasses

Abstract: Infrared reflection spectra of more than fifty silicate glasses have been obtained with a Fourier-transform infrared spectrometer and are reported in the range from 20 to 4000 cm −1 . Results have been systematically analysed with both Kramers-Kronig inversion and fit of the factorized form of the dielectric function to reflectivity spectra. Good agreement between both methods has been found. The detailed analysis of this set of data shows localized modes characteristic of the response of cations like Na, K or Ca, as opposed to the coupled-mode behavior of Al-O-Si vibrations. The degree of coherence of vibrations is also discussed. The specific potentialities of this spectroscopy and the kind of conclusions that can be drawn about the structure of silica glasses — when complete spectra are available — is illustrated by several examples in silica glasses containing Na, K, Ca, Al, Zr, F, B or Mg ions.

Rheological properties of silica sol-gel materials

Abstract: Rheological properties of silica sol-gel materials were determined using dynamic and steady shear flow measurements. During aging, transitions from Newtonian to shear thinning to thixotropic flow behavior were observed. Yield stresses were also observed during the later stages of aging. Measurements of storage and loss moduli indicated that dissipative (viscous) processes dominated during the Newtonian flow period. Elastic interactions became increasingly important during the shear thinning and thixotropic flow periods. These changes in rheological properties during aging reflected the structural evolution from a dilute sol (containing isolated, non-interacting polysilicate species) to the development of polysilicate structures with a three-dimensional network character. As observed by previous workers, silica sols prepared with acid catalyst and low water content exhibited “spinnability” after sufficient aging. Optimum spinnability was observed when the flow behavior was highly shear thinning, but not thixotropic. Freestanding fibers could be formed with sols having high polysilicate species concentration.

Kinetic simulations and mechanisms of the sol−gel polymerization

Abstract: High resolution 29 Si NMR experiments concerning acid-catalyzed sol-gel polymerization from tetraethyl-orthosilicate have been interpreted by means of simulated kinetic curves. In the first hours, the growth process is controlled by the chemical reactivity. The rate constants of the preponderant reactions (hydrolysis-condensation-reesterification) are influenced by the H + concentration and the nature of oligomers (degree of hydrolysis and degree of condensation of silicon atoms). When the size of the polymers becomes sufficiently large, the kinetics of the growth process is mainly controlled by the diffusion which depends on the mobility of the particles and on the molecular separation.

Short time reactions of a na2o-cao-sio2 glass with water and salt solutions

Abstract: The pH of Corning 015 soda-lime silicate glass was measured in water and salt solutions at different temperatures as a function of treatment time. The pH increased to a maximum and then decreased. This pH change was modeled as resulting from a sequence of chemical reactions, and the resulting rate constants discussed. The infrared absorption of the hydrated layers showed contributions from SiOH groups, molecularly adsorbed water and hydrogen bonding. Reaction rates were higher in salt solutions than in water.

Analysis of ac conductivity of glasses by a power law relationship

Abstract: The frequency dependence of electrical conductivity of many germanate and borate glasses has been analyzed in terms of an empirical power law at different temperatures. The power law has often been used in determining the concentration of mobile carriers in various materials, but our analysis does not agree with some of the assumptions and, hence, limits the physical interpretation of the law.

Speciation and local structure in alkali and alkaline earth silicate glasses: Constraints from 29Si NMR spectroscopy

Abstract: 29 Si NMR spectroscopy provides important contributions to the understanding of silicate glass structure, as the resonance frequency is sensitive to several aspects of the local chemical environment (including intertetrahedral bond angles, bond distances, and number of bridging oxygens per silicon atom). New data on a series of binary alkali and alkaline earth glasses are presented which highlight the effects of composition, as well as the complications that can arise in the interpretation of spectra. In binary alkali silicate glasses, two partially resolved peaks may be observed, which are attributable to Si in sites with different numbers of bridging oxygens. These are present even in glasses which are almost certainly single, homogeneous phases. In alkaline earth and in aluminosilicate glasses, single, broad, often asymmetric resonances are found, whose linewidths span the frequency ranges of three or more structural species. Using a combination of slow sample spinning and non-spinning techniques, NMR spectra also provide data on local electronic symmetry, and indicate that silicon sites in glasses can be more symmetrical than those in crystalline materials.

Scattering function of vitreous silica

Abstract: The scattering function S(Q, E) of vitreous silica a-SiO 2 was measured with inelastic neutron scattering over a wide range of variables (1 Q −1 , 15 E 170 meV), using a chopper spectrometer at a pulsed source. The data are analyzed in terms of the Q dependence of S(Q, E) at different energies, and in terms of the E dependence of the effective one-phonon density of states G(E) . Implications for the structure and dynamics of a-SiO 2 are discussed.

Optical properties of Tm3+ in fluorozirconate glass

Abstract: The optical properties of Tm 3+ in fluorozirconate glass are reported. Optical absorption, emission and lifetimes for different levels have been obtained. Measured oscillator strengths and radiative rates for several transitions are compared with calculated values using the Judd-Ofelt theory with a reasonable agreement between theory and experiments. Multiphonon emission rates for the 1 I 6 , 1 D 2 , 1 G 4 and 3 F 4 levels are discussed and compared with radiative rates. The stimulated emission cross section for the 1 D 2 → 3 H 4 transition has also been calculated. It is smaller than in fluorophosphate glasses.

Absolute raman intensities in glasses: II. Germania-based heavy metal oxides and global criteria

Abstract: The theoretical expressions for absolute Raman intensities derived in part I have been generalized in two ways; one for application to multicomponcnt glasses and the other to enable qualitative global predictions to be made in terms of such fundamentals as valency, bond length and mass. From the latter it is anticipated that the most intense Raman spectra among good glass formers will most likely be found in multicomponent heavy-metal oxide glasses. Using the full multicomponent theory, a detailed comparison is then made with experimental Raman spectra for ten different binary through quaternary germanate glasses containing Tl 2 O, PbO, Bi 2 O 3 and/or Sb 2 O 3 . Adopting a simple model which is based on the assumption of a dominantly four-fold coordinated germanium site leads to estimates for the number of bridged-anion and non-bridged-anion modes which, when coupled with the theoretical intensity expressions for such components, explains in a surprisingly quantitative manner the measured intensities of the corresponding Raman spectral features. An equally detailed discussion is presented of the lowest frequency acoustic-Raman or ‘boson’ peak and absolute intensity predictions again compare well with experiment for all ten germanate compositions.

Short range order in the network of the borophosphate glasses: A 31P NMR-MAS (Magic Angle Spinning) study

Abstract: We analyze the 31P NMR-MAS (Magic Angle Spinning) spectra in a number of borophosphate glasses. These spectra consist of several, relatively broad, Gaussian lines which are due to different types of PO4 tetrahedra. In addition to the phosphate units occurring in the M2O: P2O5 systems (i.e., branching, middle, end, and monomeric units) structural units typical of borophosphates have been identified in the MAS spectra: the BPO4 unit, two types of boron-bonded middle units (MB1 and MB2), and a boron-bonded end unit (EB). The oxygens introduced with the metal oxide M2O are preferentially taken by the phosphate units. It is also shown that, in contrast with the behavior of borates and phosphates, substantial changes are induced into the borophosphate glass network by addition of a doping salt (AgI or LiCl). The doping salt appears to favor the transfer of negative charge from the borate to the phosphate units. In the conclusions, we discuss the connections of the 31P NMR-MAS spectra with optical basicity data and 11B NMR findings.

On the structures of hydrogen-associated defect centers in irradiated high-purity a-SiO2:OH

Abstract: Electron spin resonance (ESR) studies of γ-irradiated high OH and high OH + OD silica have shown that a weak pair of lines with a 238-Gauss separation is the 29 Si hyperfine structure (hfs) associated with the wellknown 74-Gauss proton doublet. This result supports the models of Vitko and Radtsig for this defect. In addition, the ESR spectrum of the well-known 10.4-Gauss doublet was computer simulated using the same distribution of g values as characterizes the silicon E′ center. It is concluded that both the 74- and 10.4-Gauss doublets are manifestations of E′-type defects associated with protons.

Alcohol is an unnecessary additive in the silicon alkoxide sol-gel process

Abstract: It is unnecessary to add alcohol to a polymerizing silicon tetramethoxide-H2O sol-gel system as a co-solvent. The alcohol released during the reaction is sufficient to convert the initially biphasic system into a homogeneous one, resulting in the usual highly transparent, porous xerogel. We suggest that the alcohol-free sol-gel system may replace in many cases the traditional H2O-alcohol-alkoxide system

The formation and thermal stability of amorphous Ni-Nb alloy powder synthesized by mechanical alloying

Abstract: NixNb1−x alloys were synthesized from pure crystalline Ni and Nb powder by mechanical alloying under a controlled atmosphere. The mechanically alloyed powders were amorphous in the range 0.20

Recent progress in superionic conducting glasses

Abstract: Critical review is given on the recent progress in superionic conducting glasses. The ion species of Ag + , Cu + , Li + , Na + , and F − are separately treated since the level of progress is different in glasses of each ion species. The development of glasses as active cathode materials in the rechargeable secondary cells is also mentioned.

The density of sodium borosilicate glasses related to atomic arrangements

Abstract: Densities of sodium-borate glasses are reported over an extremely wide range of compositions. A semi-empirical model for density based on the ideas of Krogh-Moe and recently applied to the lithium borates is here applied to the sodium borates. The analysis was able to yield the volumes of the structural units present in the glasses and these were found to be larger than in the corresponding lithium borates. Carbon dioxide retention occurs for high sodium content glasses prepared from sodium carbonate and is shown not to have an effect on the densities. The volume of the carbonate unit in the glasses was also found.

Skeletal density of silica aerogels

Abstract: The skeletal density of silica aerogels, produced by hypercritical drying of gels, is studied by helium pycnometry. The bulk and the skeletal densities vary as a function of parameters such as TMOS concentration. pH and densifying heat treatment. Skeletal densities were found to be slightly lower than that of vitreous silica. The results are compared to values obtained on xerogels.

Electrical properties of n-type semiconducting chalcogenide glasses in the system Pb-Ge-Se

Abstract: The electrical properties such as thermoelectric power, electrical conductivity and drift mobility have been studied on the chalcogenide glasses in the system Pb-Ge-Se. It has been found that the glasses in a certain composition range exhibit n-type conduction, as the second case in chalcogenide glasses, following the Bi-containing glasses. There were little differences in the electron and hole drift mobilities in a given glass, regardless of its conduction type. Above finding implies that the change in conduction type is not related to the relative drift mobilities of electrons and holes, but to the shift of the Fermi level toward the conduction band. The appearance of n-type conduction has been discussed in terms of chemical bonds.

Shape of the Delaunay simplices in dense random packings of hard and soft spheres

Abstract: Two novel shape parameters for Delaunay simplices are proposed which allow one to recognize slightly distorted tetrahedral and octahedral simplices. These simplex types are found to be the basic building units in dense packings of hard and soft spheres.

Fluorinated amorphous silicon-germanium alloys deposited from disilane-germane mixture

Abstract: We have grown amorphous silicon-germanium alloys by rf glow discharge decomposition of disilane, germane, silicon tetrafluoride and hydrogen. Films with an optical band gap in the range of 1.72 to 1.2e V have been made. The optimized films show the following properties: 1) incorporation of germanium is uniform across the length of the sample; 2) hydrogen is bonded to silicon and germanium uniformly without any preference; 3) the slope of the valence band tail remains constant as the optical gap is lowered down to 1.25eV and 4) use of silicon tetrafluoride in the gas mixture improves the quality of the material. Photothermal deflection spectroscopy and space charge limited conduction were used to obtain information about the deep states. Even for the lowest band gap material, the density of states at the Fermi level is about 10 17 cm −3 eV −1 which demonstrates the high quality of the material.

Site selection spectroscopy in Eu3+-doped fluorozirconate glass

Abstract: Optical absorption and emission spectra are presented for Eu3+ ions in fluorozirconate (ZBL) glass at room temperature. The energy levels, responsible for the various transitions, are assigned. The point symmetry of the Eu3+ probes is investigated using site selection spectroscopy techniques. Emission and excitation spectra at 4.4 K are presented and two major types of sites are observed. Comparison of the 5D0 → 7F1 and 5D0 → 7F2 intensities suggests that the Eu3+ ion sites have a high symmetry. This is confirmed by decay-time measurements which have been carried out for both glassy and crystalline hosts. Agreement with previous structural results is discussed and a model is proposed.

X-ray diffraction investigations of silica gel structures

Abstract: We used small and wide-angle X-ray diffraction to examine the structure of silica gels prepared by hydrolysis and polycondensation of tetraethoxysilane in dependence on the preparation conditions and following heat-treatment. It is shown that all investigated silica gels have nearly the same dimension of primary particles (≈ 1 nm in diam.), which associate in the course of the gelation to secondary chain-like clusters varying in dependence on the preparation conditions in shape and size. By temperature-dependent in situ measurements, a displacive phase transition like the α-β transition of cristobalite occurring in the amorphous, discontinuous gel network is proved. We propose a cristobalite topology with low-temperature character for the structure of silica gel. That means that one polyhedron type similar to that in a-cristobalite is predominant. Two of such polyhedrons with four six-membered Si-O rings chiefly constitute one primary particle. The gel-to-glass conversion is determined by a continuous sintering process of primary particles and results at 800 ° C in a glass-like structure with a higher order (smaller Tg) than in the structure of conventional melted silica (Tg ≈ 1400 ° C). However, the pores, nearly 6 nm in diam., remain unchanged in dimension up to a temperature of 800 ° C.

Drying gels. II: Film and flat plate

Abstract: An analysis is presented of stresses and strains that develop during the drying of a gel. The theory presented in Part I of this series is used to simulate the drying behavior of a film of gel on a rigid substrate and of a flat plate. The stress in the film is always tensile. The plate contains regions of compressive, as well as tensile, stress. If the permeability is low enough to inhibit contraction of the gel, tension appears at the exterior suface; otherwise, tension appears in the wet (solvent-saturated) region. The dried gel is predicted to contain a density gradient. The exterior is less dense than the interior if permeability does not restrict contraction during drying, and conversely.

Local atomic structure of amorphous and crystalline alloys: Computer simulation

Abstract: The microscopic stresses defined at each atom, or the atomic level stresses, are effective parameters to describe the local variations in the atomic arrangement in liquids and glasses. We have calculated them for model binary alloy systems both in the amorphous and in the crystalline (fcc and bcc) states. It was found that alloying two elements does not change the average atomic level stresses in the amorphous state when normalized by the elastic modulus, while in the crystalline state alloying sharply increases the stresses, in agreement with the theoretical prediction. Some of the basic issues regarding the nature of the amorphous alloys, namely the glass transition and the criteria for glass formation, are discussed based upon these results.