Showing papers in "Journal of Non-crystalline Solids in 1984"

Atomic size effect on the formability of metallic glasses

Abstract: The minimum solute concentration in a binary alloy system necessary to obtain a stable amorphous phase by rapid quenching, C B min , collected from the published reports on glass formation of 66 systems, was found to be inversely correlated with the atomic volume mismatch, | (v b −v A v A | , where v A is the atomic volume of the matrix and v B is the atomic volume of the solute. The atomic scale elasticity theory was developed to calculate the atomic level stresses in solid solution, which led to the stress criteria for the topological instability of solid solution. It was found that C B min is closely correlated with the critical solute concentration for the topological instability of solid solution. This result indicates that the atomic size ratio between the constituent elements is the most important factor in the determination of value of C B min , and the amorphous alloys are stabilized partly because the solid solutions of the corresponding compositions are topologically unstable.

New materials for contact lenses prepared from Si- and Ti-alkoxides by the sol-gel process

Abstract: Methods of synthesizing materials for hard contact lenses were developed by hydrolysis and condensation of an epoxide substituted alkoxysilane and Ti-alkoxides. To enforce sufficient tensile strength, polymethacrylates were incorporated as linear crosslinking elements using a methacryloxy substituted alkoxysilane as a hook between the siliceous network and the polymer chain. The incorporation of titania led to dense monolithic products which could be cured only with some minor shrinkages. Good wettabilities [contact angles with water: (25 ± 5)°] are due to glycol groupings formed by the epoxide radicals. O 2 -permeabilities of P = (13 ± 1) × 10 −11 ml O 2 cm 2 ml −1 s −1 mm Hg −1 result from the silicone-like structure elements, where R denotes a silicon-carbon bound organofunctional radical.

Phosphate glass dissolution in aqueous solutions

Abstract: Rates of aqueous dissolution are reported for a series of phosphate glasses having the composition (50 − X)M2O·(X)CaO·50P2O5. Dissolution reactions involving the consumption of H+ and OH− were monitored using pH stat titration techniques, solution analyses using inductively coupled plasma emission, and depth profiling of corroded glass surfaces for H and other elements using elastic recoil detection analysis and Rutherford backscattering. These analytical results indicate that the phosphate glasses dissolve uniformly due to acid- or base-catalyzed hydration of the polymeric phosphate network. The rate and nature of this hydration appears to be controlled by the surface pH and/or charge which develops at the glass/ solution interface. The implication of these results on dissolution models for both the phosphate and silicate glasses is discussed.

Principles of hydrolysis and condensation reaction of alkoxysilanes

Abstract: Alkoxysilanes are used as starting materials for network formers in the sol-gel process. Substitution of an SiO-bond by an SiC-bond allows the introduction of organic radicals into a siliceous network. Since the hydrolysis and condensation reactions strongly effect the properties of the developed materials, it is necessary, in order to prepare specific materials, to understand the basic mechanisms of these reactions. Hydrolysis and condensation of alkoxysilanes have a long history and basic investigations with regard to mechanisms are carried out from very different points of view. Papers from different fields are reviewed and basic results of the co-condensation of ≡ SiC-containing with non ≡ SiC-containing silanes are reported.

Formation of sheets and coating films from alkoxide solutions

Abstract: Fundamental problems in preparing thin sheets and coating films of oxides have been discussed on the basis of experiments made in the author's laboratory. It has been found that thin sheet formation is only possible with the starting solutions exhibiting spinnability in the course of hydrolysis and polymerization of the metal alkoxide. This requirement is satisfied by the formation of linear polymers in the solution. Coating films have been prepared by the dip-withdrawal technique. It has been found that coating films of 0.05–0.5 μm in thickness can be provided by one dip-coating run and repetition of the application leads to thicker coating films. The preparation of SiO2, BaTiO3 and transition metal oxide-SiO2 coating films has been discussed.

Modification of polymer-gel structures

Abstract: In metal-organic driven systems, gelling occurs as a result of the formation of a polymeric network by simultaneous and competing chemical reactions. The resultant polymeric units consist of an inorganic network, similar to that of oxide glasses, framed by hydroxyl and organic groups. The extent of polymerization as well as the structure of the polymeric network are determined by the hydrolysis and polymerization reactions. By controlling the kinetics of these reactions, it is possible to alter the polymeric structure of the gels. The effect of these molecular-structural variations in the gel is maintained in the glass and ceramic materials derived from the gels, thus allowing property modification of ceramic materials without compositional variations.

Glasses formed by hypervelocity impact

Abstract: This paper presents description, classification, and geological setting of impact glasses, which are formed as a result of meteorite impacts with the planetary surface, and discusses the impact-glass formation process in the context of cratering mechanics. Impact glasses can be classified as belonging to two major groups: (1) mineral glasses, which are identical in composition to a mineral, and (2) rock glasses, which have the composition of a rock or a mixture of various rocks. Rock glasses may be (1) melt ejecta, (2) parts of a coherent melt layer inside the crater cavity, or (3) dikes or veins. The composition of rock glasses at a particular crater can be matched by that of the target. In nonporous rocks, the formation of rock glasses requires peak pressures in excess of 60-80 GPa, while mineral glasses are formed in the pressure range of about 25 to 55 GPa; in porous rocks, interstitial glass forms at pressures as low as 5 GPa.

The elastic behaviour of TeO2 glass under uniaxial and hydrostatic pressure

Abstract: TeO2 glass of purity exceeding 98.5 mol.% has been made, despite earlier suggestions that some 7.5–10 mol.% of modifier is required to form vitreous telluride networks. It is argued that in view of the high purity of the glass obtained, TeO2 may well be able to form glass by itself, given an appropriate thermal history in preparation. The hydrostatic and uniaxial pressure dependences of ultrasonic waves propagated in this glass at room temperature have been measured. The results provide the second and third order elastic constants of the glass. The bulk modulus is consistent with a ring diameter averaging about 8 atoms (Te4O4 rings) suggesting that the glass is a disordered version of paratellurite; however, if the TeO bending force constant were to be unusually strong, then a larger ring diameter (as in tellurite) would be indicated. Although the anion coordination number is only 4, the pressure derivatives of the second order elastic constants are positive and the third order elastic constants are negative, in marked contrast to the anomalous behaviour of silica-based glasses. These findings suggest that bond bending motions of bridging atoms between the trigonal bypyramidal groups (which are the structural units) do not play an important role in the elastic properties of TeO2 glass. In consequence the shear and oongitudinal acoustic mode Gruneisen parameters are both positive (γ1 = +2.14, γs = +1.11): the long wavelength acoustic modes stiffen under hydrostatic pressure.

The structure of soda-silica glasses: A mas NMR study

Abstract: “Magic-angle spinning” NMR of sodium silicate glasses has shown that the introduction of Na2O into the SiO2 structure causes the progressive replacement of [SiO4] tetrahedra containing four bridging oxygens by [SiO4] tetrahedra with three bridging and one non-bridging oxygens. At 33.3 mol.% Na2O all [SiO4] are of the latter type and on further increase of Na2O content they are progressively replaced by [SiO4] with two non-bridging oxygens until complete replacement is achieved at 50 mol/% Na2O. The chemical shifts of both [29SiO4] types and also 23Na vary continuously with composition, i.e. there is no evidence for clustering of either species. The spectra of glasses of the disilicate and metasilicate compositions have been compared with those of the corresponding crystal phases and differences are observed between the sodium sites in both compositions and also for the silicon sites in the disilicate. This indicates that there are no “crystal-like” areas in the glass structure. In fact the increase in the 29Si resonance halfwidth on approaching a “crystal” composition suggests an increased range of SiOSi bond angles at these compositions.

Thermodynamic model of natural, medieval and nuclear waste glass durability

Abstract: A thermodynamic model of glass durability based on the hydration of “structural units” has been applied to natural glass, and glasses containing nuclear waste. The relative durability, predicted from thermodynamic calculations, correlates directly with the experimentally observed release of silicon to the leaching solution in short-term laboratory tests. By choosing natural glasses and ancient glasses, whose long-term performance is known and which bracket the durability of waste glasses, the long-term stability of nuclear waste glasses can be interpolated. The current Savannah River defense waste glass formulation is a durable as natural basalt from the Hanford Reservation (106 y old). The thermodynamic hydration energy is shown to be related to the bond energetics of the glass.

Thermal bleaching of x-ray-induced defect centers in high purity fused silica by diffusion of radiolytic molecular hydrogen

Abstract: Isochronal and isothermal anneal curves are presented for paramagnetic defect centers in high purity Type III (1200 ppm OH) fused silicas which had been y irradiated. Successful mathematical fits to the data were achieved by means of a modified second order kinetic theory formulation under the operating hypothesis that E’ centers are annealed in the temperature range 500–700K by reaction with radiolytically-formed interstitial water molecules. Thus, H2O becomes the third small molecule (after O2 and H2) recently demonstrated to account for the thermal bleaching of radiation-induced defect centers in amorphous silica by diffusion-limited processes. Significant dependences of the annealing behavior on γ-ray dose and time after irradiation are reported and discussed in the context of the H2O diffusional model. Finally, it is emphasized that removal of the paramagnetic centers by this mechanism would leave behind an ensemble of charged, diamagnetic defects which could be reactivated by subsequent irradiations unless care is taken to remove them by higher temperature anneals.

The densification of monolithic gels

Abstract: The densification of gels to glasses with composition SiO 2 and 8Al 2 O 3 92SiO 2 has been studied as a function of heat treatment between 120 and 1200°C using Raman spectroscopy together with measurements of density and specific surface area. Monolithic dry gels are prepared at 120°C. In the case of SiO 2 these have a density of 1.39 g/cm 3 , a specific surface area of 790 m 2 /g and a mean pore radius of 10 A. In the case of 8Al 2 O 3 92SiO 2 gels these values are 0.76 g/cm 3 , 535 m 2 /g and 50 A, respectively. The Raman spectra of monolithic dry SiO 2 gels, measured as a function of temperature, show that both surface and bulk SiOH groups are present. The relative concentration of these groups is temperature dependent. The total OH content decreases with increasing temperature. The combined results of Raman, density and specific surface area measurements show that during densification two, partly related, processes occur, viz. pore collapse and the condensation of surface SiOH groups to SiOSi bonds.

Brillouin and raman scattering study of borate glasses

Abstract: We report light scattering results obtained from Brillouin and Raman scattering experiments in a series of B 2 O 3 − x M 2 O glasses (M  Li, Na, K, Rb, Cs and Tl). Raman lines related to the internal vibrations of structural units give information on the composition and concentration dependence of the short range arrangement in the glasses. The “Boson peak” is analysed on the basis of the Martin-Brenig model. The short correlation range is lower than 10 A and increases with the cation radius. From these experiments the very low frequency light scattering (Light Scattering Excess) is extracted. The temperature dependence of the LSE intensity in pure B 2 O 3 glass as well as its variation with composition at room temperature are studied. The Theodorakopoulos-Jackle theory relates this intensity to the acoustic attenuation deduced here from Brillouin linewidth measurements. While a theoretical description of the relation between the temperature variation of both quantities agrees well with experimental results, disagreement is apparent in their composition dependence. Finally, the sound velocity plotted versus concentration gives evidence of the contradictory influence of the hardening due to the change of the boron atom coordination number and the softening related to the distortion of the network by large cations.

Sol-gel transition in the hydrolysis of silicon methoxide

Abstract: The sol-gel transition in the hydrolysis of silicon methoxide in acidic, basic and neutral pH media has been investigated by means of gas chromatography and small angle X-ray scattering measurements to elucidate the effect of a catalyst on gelling time and the properties of the obtained gel. In an acidic medium, the evolution of methanol immediately after the addition of water was remarkable, and only a trace of silicon methoxide could be detected after 30 min, although the gelling time was as long as 150 h. In an ammonia catalyzed basic medium, the rates of the polycondensation reaction and the coagulation of particles were very large and the gelling occurred after several hours. But about half of the silicon methoxide remained unreacted in the sol sampled after two thirds of the gelling time. In a neutral pH medium, both the hydrolysis and polycondensation reactions proceeded at the intermediate rates of those in the acidic and the basic media.

Thermodynamic properties of undercooled liquid metals

Abstract: In the form of a droplet emulsion, a number of liquid metals and alloys have been undercooled by substantial amounts (0.3–0.4 T m ) before crystallization. For example, pure Hg, In, Sn, and Bi can be undercooled by 90°C, 110°C, 187°C and 227°C respectively. Differential scanning calorimetry has been applied to determine the heat capacity difference, ∪Cp, between the undercooled liquid and crystalline solid. With decreasing temperature an increasing value of ∪Cp is observed which implies a reduction in configurational entropy of the liquid. In addition the heat capacity measurements allow for the determination of the free energy and the onset of the hypercooled regime which are useful in the evaluation of the crystallization kinetics.

Amorphous zirconium-nickel films formed by solid state reactions

Abstract: We have formed an amorphous zirconium-nickel phase by annealing vapor deposited crystalline layers of elemental zirconium and nickel. Auger depth profiling and x-ray diffraction have been used to monitor the reaction. The thermodynamics of the process is explained in terms of a metastable free energy diagram which reflects the large negative heat of mixing between zirconium and nickel. The formation of an amorphous layer occurs via a diffusion limited reaction occurring at the zirconium-nickel interface.

Raman study of the structure of glasses along the join SiO2GeO2

Abstract: In order to better understand the distribution of tetrahedra in multicomponent tetrahedral network structures of melts and glasses, we have investigated the Raman spectra of binary SiO 2 GeO 2 glasses. We compare the Raman spectral features of the end-member glasses and discuss their vibrational origins. The mixing of GeO 2 and SiO 2 melts results in a continuous random network structure of TO 4 tetrahedra (T  Si, Ge) in the glass. Raman bands corresponding to the asymmetric stretch ( v as ) of oxygen in GeOGe, SiOSi and SiOGe bonds are observed in the glasses having intermediate compositions along the SiO 2 GeO 2 join. The presence of three distinct v as (TOT) bands in the spectrum of a glass having Si/Ge one reveals that a considerable degree of SiGe disorder exists in the glass. The presence of a single symmetric oxygen stretching band in the spectra of binary SiO 2 GeO 2 glasses indicates that the symmetric stretch modes ( v s ) of oxygen in SiOSi, SiOGe and GeOGe bonds are strongly coupled. An observed decrease in the halfwidth of the v s (TOT) band in the spectra of SiO 2 GeO 2 glasses with increasing concentration of GeO 2 may be attributed to a decrease in the average TOT bond angle and a predominance of six-membered ring structures. Results of the present study support the assignment of the bands in the 900–1200 cm −1 region of the alumino-silicate glasses, spectra to the v as (AlOSi) and v as (SiOSi) modes. In contrast to the alumino-silicate glasses, however, the SiO 2 GeO 2 glasses have a much higher degree of disorder of the network-forming cations.

Crystallization in fluoride glasses: I. Devitrification on reheating

Abstract: The devitrification of glass from the BaF 2 LaF 3 ZrF 3 AlF 3 quaternary system is studied as a function of sample size and heating rate by differential scanning calorimetry. Results are analyzed in terms of a theory of non-isothermal crystallization kinetics similar to that of Matusita and Sakka but modified to allow for diffusion controlled growth of the crystallites.

Atomic transport and structural relaxation in metallic glasses

Abstract: Structural relaxation effects in metal-metalloid glasses are reviewed. Data on viscous flow and atomic diffusion are presented, and possible mechanisms of structural relaxation are discussed.

Raman studies of Al coordination in silica-rich sodium aluminosilicate glasses and some related minerals

Abstract: Polarized Raman spectra have been obtained for a series of sodium aluminosilicate glasses and selected minerals, in order to investigate changes in the glass structure with composition. Particular attention is paid to characterization of the Al coordination, and to test Lacy's hypothesis concerning the role of “triclusters” in Al-rich glasses (Al/Na = R > 1.0). A comparison of five glasses ranging from nearly sodium trisilicate composition glass (R = 0.03) to albite composition glass (R = 1), and beyond to a glass of R = 1.61, leads to the following observations with increasing R: the polarized bands previously assigned to T-O (non-bridging) vibrations (950, 1110 cm−1) decrease in intensity, while those assigned to T-O (bridging) vibrations (485, 980 cm−1) either remain the same or increase in intensity. The narrow band at 495 cm−1 and a somewhat wider one at 595 cm−1, may be related to Raman active “defects” that have been observed in vitreous SiO2 at essentially the same frequencies. Our observations indicate a general increase in tetrahedral polymerization and random incorporation of Al into tetrahedral sites. Comparison of the Raman spectra of mullite (which contains (Si, Al)3O10 triclusters) with sillimanite (which contains no triclusters) shows that the bands at 418 and 1200 cm−1 in mullite are absent for sillimanite. Assuming that these “extra” bands are due to the triclusters in mullite, we have to conclude that similar triclusters do not exist in any of the glasses we examined. If these “extra” bands in mullite are not due to triclusters, we can draw no such conclusion. Our study indicates that observed changes in physical properties of sodium aluminosilicate glasses as R increases are not due to an Al coordination change.

Further investigations on Raman spectra of silica gel evolving toward glass

Abstract: In a previous investigation a continuous evolution of silica gel structure toward that of v-SiO 2 was verified by Raman spectroscopy. Modifications on the intensity of some Raman bands usually associated in the literature to “defects” of the structure were observed. In the present work the study has been extended to the transformation with temperature of silica gels obtained from tetraethoxysilane (TEOS) with different H 2 O/TEOS molar ratios. Raman spectra as well as thermogravimetric data are discussed in terms of the effect of this ratio on the local structure and the sintering process of silica gel.

Preparation and characterisation of V2O5B2O3 glasses

Abstract: The binary V2O5B2O3 glasses were prepared. With low V2O5 content the samples were amorphous but very unstable due to their hygroscopic character; these developed crystallinity due to B2O3 content. There was some crystallinity in compositions ranging from 30 mol.% to 50 mol.% V2O5. In higher compositions good amorphous structure was noticed. The structure was also highly dependent on the quenching rate. The physical parameters such as density, glass transition temperature and the X-ray diffraction, infrared absorption spectra are discussed with respect to all compositions. The dc electrical conductivity was measured for only those samples (70 to 90 mol.% of V2O5) which were found to be amorphous in nature. It is observed that in this glass system too, the high temperature conduction phenomenon is due to adiabatic hopping like that in V2O5TeO2 glasses. The electrical properties are highly sensitive to the thermal history of preparation of the samples.

Glasses from colloids

Abstract: A process is described for preparation of glass from colloidal dispersions of particles produced by flame oxidation. Particles of SiO2 made from SiCl4 (BET diameter ∼ 60 nm) are dispersed in a hydrophobic solvent by an adsorbed layer of alcohol molecules. The adsorption sites are shown by photoacoustic spectroscopy to be isolated silanol (SiOH) groups on the surface of the particles. The alcohol presents a steric barrier to coagulation of the colloid, so that a suspension of 13 vol.% of SiO2 retains a viscosity below 50 cP for weeks. The silanol is acidic, so more basic alcohols adsorb more strongly; e.g. 1-propanol disperses silica, but methanol does not, nor does more acidic fluorinated propanol (C3F7OH). The colloid is readily gelled by exposure to a base, such as an alkyl amine or NH3, which deprotonates the silanol. The resulting charges cause gelling, in qualitative agreement with the theory of Feat and Levine. Evaporation of the solvent is readily achieved to obtain a hydrophobic gel containing ∼ 65–75% porosity. A cylindrical monolith ∼ 1.3 × 19 cm can be dried in 3 days, then sintered to clear glass at 1300–1400°C.

Properties of amorphous semiconducting multilayer films

Abstract: The structural, optical, and electrical properties of amorphous multilayer films containing up to 180 double layers of a-Si:H/a-SiNx and a-Si:H/a-SiOx were studied. For a-Si:H layer thickness d >100 A the transport properties are dominated by space-charge doping with a-SiNx positive and a-SiOx negatively charged. For d A quantum-well effects increase the optical and electrical gaps. A d =12 A multilayer film shows no evidence for the predicted loss of extended states in two-dimensional disordered systems.

Determination of energy barrier for structural relaxation in a-Si and a-Ge by Raman scattering

Abstract: We have used the expressions for the shifts due to changes in bond angle with stress previously determined for c-Si and c-Ge to calculate the mean bond angle deviation, Δθ, from the measured linewidth of the T0-phonon under various annealing temperatures. T a . The resulting strain energy corresponding to Δθ(T a ) can be fitted by the relation U 0 -U 1 exp(−E a /k b T), where E a is the energy barrier for structural relaxation in the amorphous phase under annealing. For both a-Si and a-Ge, we have determined the energy barrier E a to be approximately 0.2 eV.

Monolithic aerogels in the systems SiO2B2O3, SiO2P2O5, SiO2B2O3P2O5

Abstract: The gels in the binary and ternary systems: SiO 2 B 2 O 3 , SiO 2 P 2 O 5 , SiO 2 B 2 O 3 P 2 O 5 ; were prepared by hydrolysis and polydensation of metalorganic compounds. The gelling times vary with the molar percentage of SiO 2 . The solvent was evacuated under hypercritical conditions in an autoclave in order to obtain aerogels free of cracks. The monolithicity of the aerogels is influenced by the method of preparation of the alcogels. The crystallization of BPO 4 was observed in the ternary system only. These materials can be converted into glasses by heat treatment. The structural evolution was followed by means of infrared spectroscopy and textural evolution by dilatometric measurements.

Raman spectra and structure of natural glasses

Abstract: The prominent features in most synthetic silicate glass Raman spectra are intense broad bands in the 850–1150 cm −1 region due to symmetric stretching motions of silica tetrahedra with varying numbers of non-bridging oxygens, and an intense band in the 400–600 cm −1 region due to a stretch-bend mixed vibration across SiOSi bridging bonds. The wavenumber of the 400–600 cm −1 band varies with degree of polymerization of the network and with SiOSi (and SiOAl) bridging bond angle. Raman spectra were measured on several suites of natural glasses including desert glass, tektites, obsidians of varying composition, and some lunar glasses. Some natural glasses, particularly the lunar glasses produce very broad and poorly defined spectra, a result that is interpreted in terms of additional structural disorder beyond the conventional random network. Desert glass, as expected, is essentially silica glass and its structure differs from laboratory-produced silica glass. Obsidians, in spite of their chemical complexity, produce Raman spectra with the major features found in compositionally simpler alkali silicate and alkali-alumina-silicate glasses. Spectra vary systematically from rhyolitic to basaltic compositions reflecting the depolymerization of the network. Tektite spectra are similar to spectra of obsidians but differ in detail. All spectra correlate with the number of non-bridging oxygens calculated from bulk chemical composition.

Electronic structure calculations for amorphous alloys

Abstract: The electronic structure of amorphous metals is calculated selfconsistently by the LMTO-ASA method using various structural models. The s- and p-states have been included and the hybridization with the d-states produces structure in the density of states. The possibility of ferromagnetism is discussed.

The mixed alkali effect in lithium-sodium borate glasses

Abstract: The electrical conductivity of a series of 0.35 (Li, Na)2O·B2O3 glasses shows a minimum at the composition Na/(Na+Li)∼0.6, which becomes stronger as the temperature is decreased; the activation enthalpy for electrical conductivity shows a maximum at this composition. In general, replacing 1% of the total oxygen concentration by chlorine or bromine (keeping the total alkali content fixed) in these glasses increases the conductivity; fluorine doping has an opposite effect. The mixed alkali effect, expressed in terms of the compositional dependence of the activation enthalpy for conductivity, is enhanced when borate glass is doped with fluorine, but is slightly diminished when doped with chlorine or bromine. The results are explained in terms of the structure of halogenated alkali-borate glasses, and discussed in relation to the origin of the mixed alkali effect.

Sol-gel derived transparent IR-reflecting ITO semiconductor coatings and future applications

Abstract: Heat insulating properties of windows can be improved by thin indium-tin oxide films which reflect the long wave IR-radiation. In this way, the heat transport by radiation exchange is suppressed. These ITO-films are highly transparent for solar radiation. Therefore, windows composed of ITO-coated glasses are good passive solar collectors. We have now developed a dip coating process for sol-gel-derived ITO-layers. The characteristics of such ITO-coatings are described. It is shown that such coatings have excellent properties and therefore good chances for technical applications.