Showing papers in "Journal of Non-crystalline Solids in 1985"

EXAFS and the structure of glass

Abstract: The relevance of EXAFS for studying the structure of glass is reviewed. Examples are taken from recent work on SiOx films, soda silica glasses, calcium and iron containing mineral glasses and amorphous biogenic phosphates. The sensitivity of the environment of modifiers to the morphology of the glass network is stressed. The concepts underlying a modified random network (MRN) are introduced and are discussed in relation to the rheological and diffusion properties of glasses.

The synthesis of amorphous NiTi alloy powders by mechanical alloying

Abstract: Crystalline powders of Ni and Ti were mechanically alloyed by high-energy ball milling in an inert atmosphere at T < 240 K. The alloyed NixTi1−x powders were characterized by scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry. For 0.28 < x < 0.72, the powder is amorphous. This composition range is in good agreement with that deduced from a free energy diagram for the crystalline terminal solutions and the amorphous phase. Outside this regime, the powder is a two-phase mixture of an amorphous phase and the crystalline terminal solution of the major element. After ball milling, the solubility limit of Ti in FCC Ni is approximately 28%, which is significantly larger than that in annealed Ni. Atomic pair distribution functions, G(r), were calculated from the X-ray patterns for x = 0.32, 0.4, 0.6, and 0.7. For x = 0.4, the G(r) of the mechanically alloyed powder is almost identical with that of rapidly quenched amorphous Ni40Ti60. For Ni33Ti67, the crystallization temperature, the crystallization enthalpy, and the apparent activation energy for crystallization are 712 K, 0.74 kcal/g at., and 65.8 kcal/g at., respectively. These values are within 10% of those measured by Buschow for rapidly quenched Ni32Ti68. The amorphization by mechanical alloying is attributed to mechanical mixing and to a solid state interdiffusion reaction taking at or near clean boundaries between polycrystalline Ni and Ti.

Sol → gel → glass: I. Gelation and gel structure

Abstract: The mechanisms of gel formation in silicate systems derived from metal alkoxides were reviewed. There is compelling experimental evidence proving, that under many conditions employed in silica gel preparation, the resulting polysilicate species formed prior to gelation is not a dense colloidal particle of anhydrous silica but instead a solvated polymeric chain or cluster. The skeletal gel phase which results during desiccation is, therefore, expected to be less highly crosslinked than the corresponding melted glass, and perhaps to contain additional excess free volume. It is proposed that, during gel densification, the desiccated gel will change to become more highly crosslinked while reducing its surface area and free volume. Thus, it is necessary to consider both the macroscopic physical structure and the local chemical structure of gels in order to explain the gel to glass conversion.

New type of non-crystalline solids between inorganic and organic materials

Abstract: Increasing demands for special materials led to the conception of composites, since valuable properties of different types of materials can be combined. The question arises, whether combinations of different components are reasonable or possible on a molecular scale. General consideration of different principles of combinations are discussed and the sol-gel process as a useful tool in order to obtain glass related combination structures of inorganic and organic components is introduced. Some practical examples are given.

Spectroscopy simulation and scattering, and the medium range order problem in glass

Abstract: Consideration is given to phenomena which we believe to be controlled by the fluctuations in the poorly understood intermediate range order in liquids and glasses. These include structural relaxation, crystal nucleation and incipient liquid-liquid phase separation. Techniques which show promise for investigating the intermediate range order are considered, and predictable or conceivable developments in these techniques which may, by the year 2004 (N.J. Kreidl's 100th), greatly increase or even revolutionize our knowledge of the intermediate range order, are discussed. These include difference spectroscopy, difference scattering, and computer simulation techniques. Finally, we consider possible developments in system preparation or system manipulation techniques which may lead to new insights into relations between physical properties and intermediate range order. An example of special interest is the preparation of noncommunicating microsystems of the same size as the clusters which many believe are the building blocks of the glassy state.

Organic fluorescent dyes trapped in silica and silica-titania thin films by the sol-gel method. Photophysical, film and cage properties

Abstract: The sol-gel process is utilized to solve the notorious problem of incorporating an organic dye in an inorganic oxide thin film. Fluorescent thin films are prepared by this low temperature process with the aid of a surface active agent, with good homogeneity and reproducability. A variety of organic fluorescent molecules are embeded either in silica or in silica-titania films. The dye molecules are not leached out by water. Absorption and emission spectra, enhanced photostability, longer lifetimes and energy transfers between the trapped dye molecules are described and discussed in terms of the effects of molecule matrix-isolation on these properties. Potential uses of the special thin films are numerous, e.g., as laser or solar light guides.

Defect structure of glasses: Some outstanding questions in regard to vitreous silica

Abstract: The defect structure of glasses is discussed primarily in the context of vitreous silicon dioxide (v-SiO2) Our current understanding of the nature of defects in v-SiO2 is reviewed on the basis of the available electron spin resonance data, and a unified picture of radiation-damage processes is formulated for high-purity silicas of both low- and high-OH contents The critical role of hydrogen in the latter case is noted Several presently outstanding questions relating to the optical spectra of defects in fused silica are highlighted with particular emphasis on data obtained in the fiber optic geometry Some potential answers to these questions are suggested, and an attempt is made to chart the directions of the next 20 years' research in glass defect structure

Fast ion conducting glasses

Abstract: General aspects of fast ion conducting glasses are first presented. Conduction processes in such glasses are discussed in terms of the “random site model”, the “weak electrolyte model” and the “conduction path model”; the applicability of the conduction path model to some systems is tested. Four problems are then treated as the “future development” of fast ion conducting glasses; (a) the promising conducting species, (b) the mixed anion effect, (c) the highest conductivity expected to be achieved, and (d) the possible technological applications.

Chemical bond approach to the structures of chalcogenide glasses with reversible switching properties

Abstract: Since the publication of the paper announcing the discovery of reversible switching phenomena in certain types of chalcogenide glasses [1], much effort has been devoted towards the characterization, improvement, and understanding of the properties of these materials [2]. This interest has been stimulated both by the basic scientific questions that have to be answered in order to understand the structures and properties of these alloys, and by their obvious potential as reversible threshold and memory switching elements in a wide variety of electronic devices [2,3].

Kinetics of crystal nucleation in silicate glasses

Abstract: The kinetics of volume nucleation are analysed in a number of “simple” systems in which the crystallising phase has the same composition as the parent glass. The compositions considered are Li2O·2SiO2, Na2O·2CaO·3SiO2, BaO·2SiO2, 2Na2O·CaO·3SiO2, Na2O·SiO2, 3BaO·5SiO2 and CaO·SiO2. In the first three cases classical homogeneous nucleation theory provides a satisfactory description of both the temperature dependence and magnitude of the nucleation rate if the crystal-liquid interfacial energy σ is temperature dependent. All seven compositions showed striking similarities. The values of TMAX/Tm, where TMAX is the temperature of maximum nucleation and Tm the melting point (or effective melting point) were all in the range 0.54 to 0.59. TMAX was always at, or somewhat above, Tg. Values of Td/Tm where Td is the “just detectable” nucleation temperature (I = 106 m−3 s−1) were 0.62, 0.64 and 0.66 in the first three systems. These represent somewhat higher undercoolings for the onset of homogeneous nucleation than observed in “droplet” nucleation studies of non-silicate systems. However, values of the Turnbull α parameter calculated from the σ results were in general accord with droplet studies for non-metals. Values of W ∗ /kT (W ∗ is thermodynamic barrier to nucleation) in the various systems also showed close similarities being ∼ 30 armatTMAX remarkably consistent pattern of the results suggests that the nucleation observed is predominantly homogeneous. Reasons for the failure to observe homogeneous nucleation in other “simple” compositions are discussed.

Experimental tests of the classical nucleation theory for glasses

Abstract: New data for crystal nucleation rates and viscosities were obtained for Li2O·2SiO2 and BaO·2SiO2 glasses. Special efforts were made to minimise impurities in the glasses. Good general agreement with previously published nucleation results was found. The new and previous results were used to test the applicability of classical nucleation theory. For both lithium and barium disilicates the temperature dependence of nucleation rates was satisfactorily described by theory, ln (Iη/T) vs. 1/ΔG2T plots yielding straight lines over a wide range of temperatures (ΔG is the bulk free energy difference per mole between crystal and liquid at temperature T). However, for lithium disilicate, where experimental ΔG data were available, the experimentally determined preexponential factor A in the nucleation equation was much higher than the theoretical value, in agreement with previous studies. This was the case even allowing for reasonable variations in ΔG. A similar result was obtained for barium disilicate using calculated ΔG values. Possible reasons for the discrepancy in A between theory and experiment are discussed including transient nucleation, experimental errors in the nucleation rates and heterogeneous nucleation. None of these effects can account for the observed discrepancy. Other possible explanations are considered.

Glass preparation of the ZrO2SiO2 system by the sol-gel process from metal alkoxides

Abstract: Glasses in the ZrO 2 SiO 2 system containing up to 50 mol.% ZrO 2 were prepared by heating the gels made from the mixed solutions of Zr(OC 3 H 7 ) 4 and partially prehydrolyzed Si(OC 2 H 5 ) 4 . The anionic network structure similar to that of glass was already formed in the gels. And it was observed that the properties of the gels were determined by the porosity in the gel rather than the gel constituting structure. Porous gels converted into the non-porous impervious glasses through three major reactions with increasing temperature: the removal of the adsorbed water, dehydration-condensation and sintering. Density, refractive index and hardness of the non-porous glasses increased with the increase of ZrO 2 content. Infrared spectra and molar refractivity showed that the Zr 4+ ion was incorporated in the interstices of the silica network structure.

Sol → gel → glass: II. Physical and structural evolution during constant heating rate experiments

Abstract: Porous, multicomponent gels were converted to dense glasses at temperatures less than 700°C and at heating rates ranging from 0.5 to 40°C/min. The results of shrinkage, weight loss and differential scanning calorimetry experiments were used to elucidate mechanisms responsible for gel densification. We propose that: (1) capillary contraction, (2) condensation polymerization, (3) structural relaxation, and (4) viscous sintering are the principal gel densification mechanisms. Condensation-polymerization and structural relaxation result in skeletal densification, the magnitude of which closely accounts for all the observed shrinkage between 150 and 525°C. Viscous sintering is the predominant shrinkage mechanism above 525°C. Due to the complex interdependency of the densification mechanisms, the kinetics of gel densification depend strongly on thermal history and, therefore, general constant heating rate analyses are inappropriate for deriving meaningful kinetic information regarding the gel → glass conversion.

Optical transitions of Pr3+ ions in fluorozirconate glass

Abstract: Optical absorption and emission spectra are presented for Pr 3+ ions in fluorozirconate (ZBLA) glass. The measured oscillator strengths and radiative rates for several transitions are compared with calculated values. Radiative transition rates for the excited states are determined by using the Judd-Ofelt theory. Energy transfer effects account for the nonradiative transitions. Although the Judd-Ofelt analysis does not yield the same excellent consistency for oscillator strengths and branching ratios for Pr 3+ as for Er 3+ and Ho 3+ , the experimental observations are consistent with the Judd-Ofelt calculations and the multiphonon emission rates predicted by the phenomenological energy-gap law. The crystallization of ZBLA glass also was studied, using Pr 3+ ions as probes.

Electronic structure and properties of transition-metal glasses: NixZr1−x

Abstract: The linear muffin-tin orbitals (LMTO) method is used to perform self-consistent electronic structure calculations of NixZr1−x glasses for x = 36 and 59. A glass is simulated by a 39-atom cluster with periodic boundary conditions. The atoms are randomly packed and then relaxed with Lennard-Jones potentials. The calculated radial distribution functions are compared with the available experimental data. The computed electronic structure is compared with photoemission data and experimental results for the density of states at the Fermi level. The calculated results are in good overall agreement with experimental data.

Localized states in doped amorphous silicon

Abstract: Doping in amorphous silicon is reviewed, with particular focus on the localized states introduced by doping, and the physical origin of the effects. Many experiments find that doped a-Si:H contains a large density dangling bonds and that the doping efficiency is low and concentration dependent. Recent experiments have given much new information about shallow states near the band tails. Neutral donors are observed directly by their hyperfine structure in ESR. A doping model is described which accounts for most of the data. However, many aspects of the doping process remain to be resolved.

The switching mechanism in amorphous silicon junctions

Abstract: Extensive new results have been obtained on memory switching in a-Si p + ni junctions It is shown that the ON-state has its origins in a highly conducting filament less than lμm in diameter The physical mechanisms that could play a role in the switching operations are discussed

Light-induced metastable defects in hydrogenated amorphous silicon

Abstract: Recent developments in the investigation of light-induced metastable defects in a-Si:H are discussed. A kinetic model that quantitatively describes the creation of metastable dangling bonds in UHV deposited a-Si:H can be extended for a qualitative understanding of defect creation in SiGe alloys, samples containing common impurities (C,N,O), and in compensated a-Si:H. It is also shown that mechanical stress enhances the defect creation efficiency. Detailed information about the energy barrier distribution of the metastable dangling bonds can be obtained by ESR annealing studies. Finally, new results showing a light-induced, reversible quenching of low energy tunneling modes are presented.

Exafs and xanes study of the local coordination environment of sodium in a series of silica-rich glasses and selected minerals within the Na2OAl2O3SiO2 system

Abstract: Photoelectron total-yield Na XANES and EXAFS spectra were gathered to characterize the Na coordination for a collection of Na2OAl2O3SiO2 glasses and minerals. Included in the collection are the crystalline model compounds albite, jadeite and nepheline as well as glasses at these compositions, Na2Si2O5 glass, and a series of glasses containing 75 mol% silica with R (= Al/Na) values ranging from 0.03 to 1.61. High resolution Na K-XANES spectra are reported for the first time and are of good enough quality for the specimens studied to make qualitative comparisons of Na edge features possible. Due to the variable quality of the Na EXAFS data, only Na2Si2O5 glass, three out of five of the glasses in the glass series, nepheline and nepheline glass were worthy of analysis; these samples are the more soda-rich materials used for this study. Comparisons of Na XANES and EXAFS spectra for the glass series, nepheline, nepheline glass and Na2Si2O5 glass indicate little if any change in the Na environment among these materials. Nepheline glass and crystal show reversal of intensity of the 1055 eV and 1058 eV Na edge features. The similarity of the Na coordination environment is also apparent in the EXAFS spectra, which yield NaO distances ranging from 2.56 to 2.62 A. This overall similarity of the Na coordination for samples in this study is not surprising since Na in crystalline compounds within this system is usually coordinated by seven to ten oxygens. The physical property variations for our glass series are not explained by the differences in the Na environments depicted by our Na XANES and EXAFS results. The Na EXAFS for nepheline appears to sense only the closest set of oxygens of those in the entire known Na coordination, and this seems to be true for nepheline glass as well, using high temperature crystal structure studies of nepheline for distance comparisons.

Elastic constants and structure of the vitreous system Co3O4P2O5

Abstract: The elastic moduli of the entire vitreous range of the system CoPO that can be prepared by melting together Co3O4 and P2O5 oxides in open crucibles, have been measured by ultrasonic techniques at 15 MHz. The bulk, shear, longitudinal and Young's moduli and the Poisson ratio are found to be rather sensitive to the glass composition. It is found from this ultrasonic data, that the glass system can be divided into “three compositional regions”. This behaviour is qualitatively interpreted in terms of the cobalt co-ordination, crosslink densities, interatomic force constants and atomic ring sizes. Also presented is a full discussion of effects of annealing on elastic properties.

Magnetic properties of amorphous metals

Abstract: Neutron scattering has provided unique information about the nature of magnetism in amorphous alloys. This paper reviews some of the results obtained principally on two ribbon-form metallic glass systems - (Fe x Ni 1− x ) 75 P 16 B 5 Al 3 and Fe x B 1− x . The former exhibits three states depending on composition: pure ferromagnetism, re-entrant spin glass, and ordinary spin glass. The neutron results provide evidence for the coexistence of spin glass and ferromagnetic correlations in the intermediate regime. The alloys of Fe and B show Invar phenomena which result in anomalously small values of the effective spin-wave stiffness calculated from low-temperature magnetization and Mossbauer data. The long-wavelength spin waves as measured by the neutrons do not reflect the rapid demagnetization found in the bulk results, implying the presence of excitation processes other than long-wave-length spin waves.

Raman and ESR studies of the thermal history of amorphous SiO2

Abstract: The room temperature structure of vitreous SiO 2 is known to change subtly with thermal history. This paper reviews Raman and ESR spectroscopic observations that reveal some of these changes and show that the rate of structural relaxation depends very strongly on trace water (OH) content.

Crystals and quasicrystals in the aluminum-transition metal system

Abstract: Inspection of known Al-TM alloy phase diagrams and structures suggests the ideal composition and structure of the icosahedral quasicrystal phase. The phases α(AlMnSi) and α(AlFeSi) can be decomposed into packings of rhombohedra identical to those in the 3D (icosahedral) Penrose tiling, and Al 13 Fe 4 can similarly be related to the 2D Penrose tiling. Other large-cell phases can be generated by a modification of the projection method of generating Penrose tilings.

EXAFS study of the coordination environment of aluminum in a series of silica-rich glasses and selected minerals within the Na2OAl2O3SiO2 system

Abstract: Photoelectron total yield Al K-XANES and EXAFS spectra were collected for a series of Na2OAl2O3SiO2 minerals and glasses. The purpose of this study is to determine if EXAFS can accurately characterize these known Al environments in terms of AlO distances and coordination numbers, and to characterize the unknown Al environment in the glasses. Al XANES spectra were collected for corundum, sillimanite, albite, jadeite and nepheline, as well as jadeite glass, nepheline glass and a 75 mol% silica glass series having R ( = Al/Na) ranging from 0.28 to 1.61. Al EXAFS spectra were collected for corundum, sillimanite, jadeite and albite, as well as jadeite glass, albite glass and a 75 mol% silica glass with an R of 1.61. Al XANES spectra for our model compounds have two edge maxima at 1568 and 1572 eV for minerals having octahedral Al, while one edge maximum is present at 1566 eV for minerals with tetrahedral Al. The glasses all have a single edge maximum near 1566 eV, indicating tetrahedral-like Al sites in the glasses. The results of our Al EXAFS refinements of the model compounds are in agreement to within ±0.03 A for AlO distances and to within ±23% for coordination numbers of recent crystal structure refinements in the literature. The EXAFS results for the glasses clearly indicate Al in tetrahedral sites, yielding AlO distances near 1.77 A, well within the range of average AlO distances for aluminate tetrahedra of aluminosilicate minerals. Our XANES and EXAFS results indicate tetrahedral Al throughout our 75 mol% silica glass series, which may be consistent with the existence of tricluster units occurring in the Al-rich glasses in the sodium aluminosilicate system.

Characterization studies of molybdophosphate glasses and a model of structural defects

Abstract: Glasses in the system MoO3P2O5K2O have been studied over a wide range of compositions. Molar volumes, microhardnesses, glass-transition temperatures, heat capacities and infrared spectra of selected compositions have been measured and reported. A structural model has been proposed for these glasses in which the basic feature is the presence of an extensive network of corner shared [ MoO 6 2 ] octahedra and [ POO 3 2 ] tetrahedra. The effect of added K2O which degrades such a network and creates unshared corners has been considered in detail. The nature and concentration of various defects which depend upon glass composition and relative energies of the covalent linkages have also been considered in detail. The defects are consolidated into a structural-phase diagram which has been used to interpret the observed properties. The loss of oxygen during the preparation of glasses and its consequence upon the structure have also been discussed.

The optical absorption and luminescence bands near 2 eV in irradiated and drawn synthetic silica

Abstract: The annealing behaviors of the 2 eV optical absorption and luminescence and of the various paramagnetic defect centers (E′, O2− and nonbridging oxygen hole center [HBOHC]) in irradiated synthetic silica rods and unirradiated drawn fibers have been measured. Good agreement was found between the intensities of the 2 eV absorption and luminescence bands and the sum of the intensities of the hole traps in irradiated Suprasil W, suggesting that the optical transitions in this low OH material might be due to a nonparamagnetic electron trap. However, in Suprasil equally good correlations were observed between the absorption and luminescence and the NBOHC intensity for anneal temperatures above 100°C, consistent with the postulate that the 2 eV bands in high OH silica are principally due to the NBOHC. Although the ESR spectrum attributable to NBOHCs was observed for the first time in as-drawn low OH silica fibers, preliminary estimates of oscillator strength support the suggestion that the drawing-induced 2 eV absorption is nonetheless due to the nonparamagnetic electron trap.

Nucleation and crystallization in glass-forming melts: Olds problems and new questions

Abstract: A review is given of basic problems and results in the field of nucleation and crystallization in glass-forming melts. The temperature dependence of the driving force for crystallization and of the free energy of the crystal/melt interface, as well as the connection between the melt viscosity and entropy are considered in some detail. The influence of various factors on the rates of nucleation, crystal growth and overall crystallization is discussed and existing evidence for nonstationary nucleation is surveyed. A generalized kinetic criterion for vitrification is formulated accounting for the effect of nucleation nonstationarity. Finally, an approximate expression is obtained for the minimum cooling rate for glass formation.

Rigidity percolation in glassy structures

Abstract: We review the new concept of rigidity percolation and show that if local flexible units are joined together to form a network, the composite consists of floppy and rigid regions. When the rigid regions percolate, the whole network becomes rigid and resists attempts at elastic deformation. These ideas are applied to network glasses. It is shown that in the floppy region there exist low-frequency modes that should show up in inelastic neutron scattering.

Glasses and glass-ceramics from gels

Abstract: The present state and the future of glasses anf glass-ceramics synthesized by the sol-gel method from metal alkoxides have been critically reviewed and discussed. Glasses and glass-ceramics can be prepared in forms of bulk, fiber, sheet, coating film and particulate. Advantages and disadvantages in applying the sol-gel method for each form are discussed. It is shown that this method is very prospective and many materials will be manufactured by this method in the year 2004.

Structural evolution during the gel to glass conversion

Abstract: We have used differential scanning calorimetry (DSC) to measure quantitatively enthalpic changes which accompany gel densification and have related these changes to the evolving gel structure using Raman spectroscopy, gas adsorption, and thermal analysis. We show that the network structure, which results principally from skeletal dehydration (via condensation) during gel densification, is considerably different from the melt-glass structure. A dramatic reduction in viscosity and the formation of metastable MOM bonds as a product of condensation reactions are examples of these differences. Despite the complex manner in which the gel evolves toward a glass, once the gel has been densified and heated above T g , its structure and properties, e.g. viscosity and distribution of relaxation times, become indistinguishable from those of the conventionally melted glass.