Showing papers in "Journal of Non-crystalline Solids in 1979"

Topology of covalent non-crystalline solids I: Short-range order in chalcogenide alloys

Abstract: The pronounced glass-forming tendencies of alloys of S and Se with Ge and/or As are discussed topologically. An atomic model is introduced which for predominantly covalent forces constitutes the first microscopic realization of Kauzmann's description of the glass transition as an entropy (not enthalpy or volume) crisis. The model contains no adjustable parameters and predicts the glass-forming tendency as a function of composition in excellent agreement with experiment. Several related properties, including phase diagrams, radial distribution functions and crystal structures are discussed in the context of chemical bonding and short-range order in the non-crystalline covalent networks of these materials.

Thermal analysis of non-isothermal crystallization kinetics in glass forming liquids

Abstract: A brief review is given of previously suggested methods for analysis of experimental data of non-isothermal crystallization kinetics based on the Johnson-Mehl-Avrami transformation rate equation. Conditions for applicability are outlined and an estimate of error is made. A method of non-isothermal analysis is presented which facilitates rapid data evaluation over large temperature ranges and which justifies the previous applications in the literature of Kissinger's method to the analysis of crystallizations kinetics. An extension of these methods is made to the case of growth rates which obey a Vogel-Further temperature relation.

Electrical properties of semiconducting oxide glasses

Abstract: Studies of the electrical conductivity of semiconducting oxide glasses are reviewed in the framework of Mott's theory for such materials. An examination of the conduction processes in semiconducting oxide glasses confirms the applicability of the polaronic hopping model of electrical transport. Some deviation from Mott's theory are observed in phase-separated glasses. The thermal activation energy for conduction appears to be the dominating factor which controls conductivity, although in many cases the pre-exponential factor also has a great influence on conductivity. The diffusion-like conduction mechanism in a system of randomly distributed ions is probably not applicable to glasses that exhibit some kind of order in the structure, such as clustering.

Oxygen-associated trapped-hole centers in high-purity fused silicas

Abstract: Two distinct oxygen-associated trapped-hole centers (OHCs) are identified in samples of room-temperature γ-irradiated, high-purity fused silica. One, which we label the “wet” OHC, predominates in the high-OH-content (wet) silicas while the other, the “dry” OHC, is more prevalent in low-OH (dry) silicas. Excellent computer simulations of the low-temperature electron-spin-resonance spectra are obtained for both wet and dry silicas using only the relative abundance of the “wet” and “dry” OHCs as an adjustable parameter. Analysis of the 17 O-hyperfine structure which occrs in samples of wet silica enriched in 17 O provides direct confirmation that the “wet” OHC is a hole trapped in a single nonbonding 2p-orbital of an oxygen (presumed nonbridging). Correlation of optical absorption and electron spin resonance via isochronal pulse anneals indicates that the “dry” OHC has an optical transition ay 7.6 eV. In addition, it is reported that the “dry” OHC can be induced in the dry silicas by the fiber drawing process. From the present results, an O 2 − molecular ion model appears most attractive for the “dry” OHC.

Indentation deformation/fracture of normal and anomalous glasses

Abstract: Vickers deformation/fracture indentations have been investigated in six silicate glasses. The characteristic damage patterns fall into two distinct groups, according to whether the glass shows “normal” or “anomalous” mechanical behaviour. Observations of the damage morphology during and after contact, of the scales of the deformation and fracture zones, and of the residual stress intensity about the impressions, all point to a basic difference in the local stress/strain micromechanics. This difference is discussed in relation to the factors which control the brittleness of glass.

A new structural model for amorphous transition metal silicides, borides, phosphides and carbides

Abstract: A new model for the so-called transition metal—metalloid glasses has been constructed in which groups of atoms - coordination polyhedra with defined local geometry - are packed randomly to form a dense, three-dimensional array. Specifically, the coordination polyhedra are trigonal prisms containing six metallic atoms in a nearest neighbour sub-shell around a central non-metallic element with three further metal atoms at somewhat larger distances. They are essentially identical to the structural units observed in crystalline transition metal borides, carbides, silicides and phosphides. Computed density, neutron structure factor and radial distribution functions are in agreement with experimental data for a-Pd 4 Si. The local coordination can be distorted and changes in the properties of this structure provide some insight into the atomic rearrangements which take plave when metallic glasses are annealed to low temperature. Such “stabilized” glasses appear to contain structural units which are more regular than those found in rapidly-quenched glasses.

Hydration of soda-lime glass

Abstract: The hydration of soda-lime glass is studied using resonant nuclear reactions to measure the hydrogen and sodium profiles of hydrated glasses. The rate of growth of the surface layer of hydrated glass is initially proportional to the square root of time as is characteristic of diffusion controlled processes. After longer exposure a steady-state hydration profile is observed, which indicates that in addition to the diffusion controlled reaction there is a slow etching of the glass surface. The measured hydration profiles are discussed in relationship to the Doremus model of interdiffusing ions, which is found to be in good agreement with the data. This model is also discussed in relationship to measured hydration profiles of vacuum heated samples of hydrated glass.

DC and AC conductivity in hopping electronic systems

Abstract: The various theories of dc and ac conductivity in materials with hopping electronic charge carriers are critically examined in the light of the available experimental evidence. The temperature dependence of the dc conductivity suggests that in most materials the transition from nearest-neighbour hopping to variable-range hopping has been observed. However the theory in this region is insufficiently developed to enable meaningful determinations of the relevant parameters to be made from experimental data. The situation with ac theory is even less satisfactory, none of the existing theories being capable of accounting adequately for the observed behaviour. Neglect of many-body interactions and of the effect of the dielectric lattice, in addition to that of the charge carriers, are seen as the principal shortcomings. It follows, then, that the question of any relationship between the dc and ac conductivities cannot be answered at present, because of the lack of reliable data and because of insufficient theoretical understanding of the carrier processes in question.

Defects in plasma-deposited a-Si: H

Abstract: The relationships between hydrogen vibrational spectra, electron spin densities and refractive index are investigated for a range of plasma-deposited amorphous silicon-hydrogen alloys. Results are also presented on the morphology of thick films as shown by scanning electron microscopy. A model is proposed for the structural origin of defects in these alloys based on voids that grow perpendicular to the film surface and are associated with coupled SiH2 units.

X-ray photoelectron spectroscopy of sodium phosphate glasses

Abstract: X-ray photoelectron spectroscopy (XPS, ESCA) has been applied to sodium phosphate glasses with compositions between 15 and 50 mol% Na2O and for the purpose of comparison also to crystalline compounds containing oxygen and phosphorus. From the measurements a discrimination between bridging and non-bridging oxygen atoms was possible. The method provides a quantitative technique for structural analysis of phosphate glasses.

The structure of lithium, sodium and potassium germanate glasses, studied by Raman scattering

Abstract: Polarized and depolarized Raman spectra of alkali germanate glasses are given, together with Raman powder spectra of the crystalline compounds Li2O · 2 GeO2; 3 Li2O · 8 GeO2; 2 Li2O· 9 GeO2; Li2O · 7 GeO2; 2 Na2O · 9 GeO2; K2O · 2 GeO2; K2O · 4 GeO2 and K2O · 8 GeO2. The alkali germanate glasses: xA2O (1−x) GeO2 are studied in the composition range 0 < x < 0.333. The vibrational modes observed in the high energy range of the Raman spectra of the crystalline compounds are interpreted in terms of symmetrical and antisymmetrical O-Ge-O and Ge-O− stretch vibrations. The molecular structure of the germanate glasses is deduced from a comparison of the Raman spectra of the glasses with those of the crystalline compounds, together with a study of the polarization properties of the glass spectra. It is observed that 6-coordinated Ge atoms occur in a network structure which resembles the structures occurring in the crystalline compounds 2 Li2O · 9 GeO2 and 2 Na2O · 9 GeO2. In the region of 0.18 < x < 0.33 it is found that tetrahedra with one non-bridging oxygen atom are formed. These tetrahedra are probably present in a network as occurs in the crystalline digermanates Li2O · 2 GeO2 and K2O · 2 GeO2.

Raman scattering in pure and hydrogenated amorphous germanium and silicon

Abstract: The Stokes Raman spectra of pure and hydrogenated amorphous germanium and silicon are discussed. First- and second-order structures due to the vibration of GeGe and SiSi bonds and also first-order structures due to GeH and SiH bonds are observed. The resonance behaviour and the polarization ratio of the observed structures are presented and interpreted in terms of a dielectric constant-bond polarizability model.

Crystallization of Al23Te77 glasses

Abstract: The crystallization of Al23Te77 glasses has been studied by DSC techniques. Two peaks occur, showing an earlier excess Te crystallization and a later one of the remaining amorphous matrix. Isothermal determination of the kinetics is only possible for the second process, so a model for non-isothermic crystallization is developed on the basis of the Avrami theory, which is in agreement with isothermic results. The shift of the crystallization peaks with the scan rate allows knowledge of the activation energy for both processes. Those are found to be 1.9 and 2.8 eV respectively. Fitting of the experimental data with this model also indicates a diffusion mechanism for the Te crystallization and a homogeneous nucleation and growth process for the second stage. Re-scanning of the sample after the completion of the first peak shows a second Tg commonly associated with phase separation. Results are discussed in terms of the studies of structure recently reported.

Sintering inhomogeneous glasses: Application to optical waveguides

Abstract: Variations in viscosity or pore size within a glass body cause uneven contraction during sintering. Consequently, stresses develop which alter the local sintering rate and, in some cases, produce bulk flow. This paper illustrates how these stresses can be analyzed by analogy to thermal stress. As a particular example, sintering of optical waveguide preforms made by the OVPO process is examined in detail. The magnitude of the self-stresses, and the conditions required for bulk flow are determined.

Electron spin resonance and hopping conductivity of a-SiOx

Abstract: Amorphous SiOx-layers with O < x < 2 have been prepared by evaporation of Si at oxygen pressures of 10−6 … 10−3 mbar. The composition of the samples was determined by proton backscattering. The band gap, derived from optical measurements, increases with rising oxygen content relatively little at first and the main enhancement occurs above x = 1.5. The ESR spectrum consists of two structures: (1) a relatively broad line A with g-value and linewidth depending considerably on composition, particularly after irradiation with He+-ions of 250 keV energy, and (2) a narrow line B with g ≈ 2.0001 which corresponds to the E′-center in SiO2 and increases in intensity with rising oxygen content. The properties of the broad line A can be described by a superposition of 3 lines with different g-values and linewidths. The hopping conductivity after He+-irradiation decreases strongly with increasing oxygen content indicating that the E′-center does not contribute to the hopping transport.

Properties of heavily doped GDSi with low resistivity

Abstract: Significant decrease in resistivity has been observed in glow-discharge-produced silicon (GDSi) containing 1019−1021 cm−3 phosphorous atoms. At the highest doping level a resistivity of 0.01 ω cm at room temperature was obtained. The temperature dependence of the resistivity follows the form, ϱ = ϱ0exp(T0/T)14, over a temperature range from 80–400 K. Optical absorption, which increased with wavelength and was roughly proportional to the conductivity, was observed in the longer wavelength side of the intrinsic absorption edge and it was ascribed to mobile charge carriers. Hall effect measurements have shown that μH of phosphorus doped GDSi is about 1 cm2 V−1s−1 and has a normal (negative) sign.

Intensity parameters and laser analysis of Pr3+ and Dy3+ in oxide glasses☆

Abstract: Intensity parameters ( Ω Λ ) of Pr3+ and Dy3+ have been obtained in tellurite, borate and phosphate glasses. It has been found that Ω Λ calculated by exlusion of hypersensitive transitions, gives a better fit between measured and calculated lifetimes and branching ratios than those including hypersensitive transitions. Using these parameters and calculated matrix elements U(Λ), radiative transition probabilities, branching ratios and integrated cross sections for stimulated emission were calculated for 3P0, 3P1 and 1D2 excited states of Pr3+ and 4 F 9 2 excited state of Dy3+. Potential transitions are indicated.

Correlation effects in hopping transport

Abstract: The effects of intersite Coulomb interactions on transport in a system of localized states is examined. Correlation effects are seen to inhibit the DC conduction but to enhance the AC conduction. It is shown that the interactions lead to a Coulomb gap in the one-electron excitation spectrum for DC conduction of width E g ∝ E c (E c /W) 1 2 , where E c is the average intersite Coulomb energy and W is the random potential. The need to excite across the Coulomb gap can be alleviated by multi-electron hopping, which can lead to a non-activated T − 1 4 -like behavior as transport becomes collective in nature. Examples of systems where correlations effects have been observed are given. The possibility of a many-electron delocalization is also discussed.

Dielectric relaxation study of chalcogenide glasses

Abstract: The paper reports dielectric measurements carried out for a variety of threshold and memory alloys of glassy AsGeTe and SeGeTe at different temperatures (83 to 373 K) and various frequencies (0.2, 0.5, 1.0, 2.6 and 5.0 MHz). It is found that the glassy system of chalcogenides exists in the form of molecular dipoles which remain frozen at low temperatures and, as the temperature is increased, the molecules attain freedom of rotation at temperatures which are sometimes as low as 253 K. All the materials displayed dielectric dispersion in the radio frequency range. Gevers' formula has been used to calculate the dielectric loss (ϵ′') and loss-angle (tan δ) from the measured values of the real part of dielectric constant (ϵ′). The curves: log ϵ′ versus temperature, ϵ′ versus log ω, ϵ″ versus log ω, tan δ versus log ω and tan δ versus temperature, gave a direct evidence of the existence of a Debye-type relaxation having a wide distribution of relaxation times. Cole-Cole diagrams have been used to determine the distribution parameter (α) and the molecular relaxation time (τ). The temperature dependence of α and τ for all the alloys is consistent with the “molecular relaxation mechanism”. The paper also reports accurate values of the static and optical dielectric constants for all the alloys. Eyring's relaxation rate equations have been used to determine the free energy of activation (ΔF), and enthalpy of activation (ΔH) for all the alloys. These results indicate the existence of a stronger intermolecular interaction for SeGeTe alloys. mott's concept of “dangling bonds” has also been used to explain the existence of a stronger intermolecular interaction, and hence a greater density of defect states in case of SeGeTe as compared with AsGeTe alloys. It has been finally concluded that the dielectric behaviour of chalcogenide glasses, in general, can be successfully explained by using the theory of molecular relaxation.

Relationship between density, viscosity and structure of GeO2 melts at low and high pressures

Abstract: A correlative study of the viscosity, density and structure of GeO 2 melts has been carried out at low and high pressures. It is observed that under isothermal (1425°C) conditions the viscosity decreases from 6.0 × 10 3 poise at 1 bar to 1.2 × 10 3 poise at 9.5 kbar. The maximum variation in the density of quenched GeO 2 glasses is from 3.62 ± 0.02 g cm −3 for glass formed from a liquid quenched at 1 atm and 1425°C to 3.95 ± 0.04 g cm −3 for glass formed from a liquid quenched at 18 kbar and 1700°C. The similarity of the Raman spectra of GeO 2 melt quenched at 1 atm (1425°C) and at 18 kbar (1700°C) implies that the scattering units in GeO 2 glasses quenched at low and high pressures are the same. The intensity of the Rayleigh tail, however, decreases in glasses quenched at successively higher pressures, the structure apparently becoming more ordered with increasing pressure. The Raman spectra of the GeO 2 polymorphs with quartz and rutile structures, crystallized at 1150°C and at 4 and 6 kbar, respectively, are distinct because of the difference in Ge coordination, resulting in a large difference in the frequency and intensity of the GeO symmetric stretching mode in the two polymorphs. A comparison of the Raman spectra of GeO 2 glasses with those of crystalline GeO 2 polymorphs shows that the local ordering in GeO 2 glass is similar to that of hexagonal GeO 2 in which Ge is tetrahedrally coordinated. The decrease in the viscosity of GeO 2 melt with increasing pressure cannot be attributed to a pressure-induced coordination change [1]. More likely, there is a systematic change in the nature of the GeO bond with increasing pressure. The increase in the degree of local ordering in GeO 2 melts quenched at high pressures does explain the progressive increase in melt density.

Thermal expansion of amorphous metals

Abstract: The thermal expansion of three commercial amorphous metals has been measured from room temperature to above the crystallization temperature. The results clearly demonstrated the glassy nature of these materials and show the reduction in volume which results from crystallization. The glass transformation temperature is more evident here than on the usual differential scanning calorimeter curves. The activation energies associated with the glass transformation and crystallization are derived from data taken as a function of heating rate. The measured temperatures for the glass transformation and crystallization from the thermal expansion measurements are the same as those obtained from DSC measurements at the same heating rate.

Lanthanum fluorozirconate glasses

Abstract: Investigations in the ZrF 4 -BaF 2 -LaF 3 ternary system show the existence of a vitreous area in the range: ZrF 4 : 56–67%, BaF 2 :25–36%, LaF 3 :3–13%. For a standard glass of molar composition 0.62 ZrF 4 , 0.30 BaF 2 , 0.08 LaF 3 , the refractive index is 1.5248 with the glassy transition occuring at 310°C and fusion occuring at 545°C. Quaternary glasses have been obtained by including a M F fluoride ( M = Na or Li) with the previous ternary glass according to the formula: (0.62 – 0.4 x )ZrF 4 (0.30 –0.5 x )BaF 2 (0.088 – 0.1 x )LaF 3 xM F. Evolution of glassy transition, refractive index, density and molar refractivity is discussed in relation to the structural role of the Na + and Li + ions. In these zirconium fluoride glasses, lithium could act as a network former at low concentration and as a network modifier at higher concentration.

A test of classical nucleation theory: crystal nucleation of lithium disilicate glass

Abstract: For several inorganic glasses predictions have been made of the homogeneous crystal nucleation rate using classical nucleation theory. However, in none of these cases were comparisons made with experiment, due primarily to the inability of being able to guarantee homogeneous conditions. Evidence has been provided that crystalline formation in Li2O · 2 SiO2 glass may initiate by a homogeneous mechanism. Thus, we have computed the nucleation rate curve of lithium disilicate crystals in this glass. It is found that not only do all forms of the classical theory predict nucleation rates many orders of magnitude smaller than those observed, but also that the temperature dependence of the theoretical rate is quite different from that observed.

Structural analysis of anodic alumina films

Abstract: Structures of amorphous anodic alumina films have been investigated by the X-ray radial distrubution function and the correlation method. The amorphous anodic alumina films are considered to contain AlO5 coordination polyhedra as well as AlO4 tetrahedra and AlO6 octahedra. Two types of conformation are taken into account by the correlation method; one (model A) is the conformation in which the above coordination polyhedra are arranged in a similar way to that found in the crystalline modifications of γ-Al2O3 within a short range of r A and the other (model B) is the conformation in which coordination polyhedra are randomly arranged. The final model formed by mixing these two conformations (model A and B) in 1 : 1 ratio describes quantitatively the observed scattering intensity curves of the amorphous anodic alumina films.

Structure characterization of CVD amorphous Si3N4 by pulsed neutron total scattering

Abstract: The structure factor S ( Q ) of chemically vapor-deposited (CVD) amorphous Si 3 N 4 has been measured by pulsed neutron diffraction over the range of the scattering vector Q from 1–330 nm −1 . The oscillatory behavior in the S ( Q ) persists up to Q = 300 nm −1 and there is appreciable small angle scattering intensity. The SiN bond length is l SiN = 0.1729 nm, and its coordination numbers n SiN and n NSi are 3.70 and 2.78 respectively. The bond angles around a Si and a N atom are found to be 109.8 and 121°. Analysis of the small angle scattering intensity shows the existence of voids with an average diameter of about 1 nm and a volume fraction of about 4%, which may stabilize the amorphous structure of Si 3 N 4 having rigid covalent bonds due to relaxing the strain energy accumulated in the matrix.

ESR and optical absorption of Cu2+ in Na2OSiO2 glasses

Abstract: ESR and optical absorption spectra of Cu2+ in xNa2O(100−x)SiO2 glasses were measured, where x ranges from 12 to 70 mol% Na2O. This glass system was divided into three composition regions, 12 ≲ x ≲ 37, 37 ≲ x ≲ 55 and 55 ≲ x, from the composition dependence of the ligand field transition energy and spin hamiltonian parameters of Cu2+. Two boundary compositions (37 and 55 mol%) between the two adjacent regions agreed with the eutectics in the equilibrium phase diagram. Two types of Cu2+-complexes, with less basic ligands (HFS-1) and much more basic ones (HFS-2), were detected in ESR for ultra-high soda glasses (x ≳ 55). The distribution of the ESR parameters due to the fluctuation of ligand fields was negligible for HFS-2 compared with that for other glasses. The Cu2+ ion responsible for HFS-2 was considered to distribute in the microphase of orthosilicate. Imagawa's basicity, the covalency of the bondings between Cu2+ and ligands, was calculated by using Maki and McGarvey's analysis. The basicity of σ-type symmetry remained constant, irrespective of the glass composition, and the value was identical with those for other oxyanionic glasses. The π-type basicity was also constant for the glasses of x ≲ 55. Two different basicities, each corresponding to HFS-1 or 2, were obtained for the glasses of x ≳ 55. The value derived from HFS-1 was identical with those for x

Glass transition and specific heats in the systems PS, PSe, AsS and AsSe

Abstract: The glass transition temperatures were measured in the systems AsS, As0.5P0.5S, PSe, AsSe and PAsSe. Heat capacities of the glasses in the selenium systems were obtained by differential scanning calorimetry. As shown by the residual entropies departures from ideality are high in the chalcogen glasses. The results are discussed in terms of the structure of glasses in these systems. The thermodynamic data of glasses and liquids in these systems indicate a balance of intra- and intermolecular saturation of bonds. The amount of polymerization increases with increasing average molecular weight in the glass and with increasing temperature in some of the investigated liquids.

An X-ray diffraction study of the amorphous structure of chemically vapor-deposited silicon nitride

Abstract: X-ray diffraction measurements have been carried out for two kinds of samples of CVD-amorphous silicon nitride with different values of the density. From radial distribution analysis, it became clear that the short range structure in these samples resembles that of the crystalline β-phase. Coloring in CVD-amorphous Si 3 N 4 was found to be induced by X-ray irradiation. Based on the deficient numbers of the first nearest neighbours in SiN bondings, it was proposed that CVD-amorphous Si 3 N 4 consists of small clusters including vacancies in their matrix.