Showing papers in "Journal of Non-crystalline Solids in 1974"

Photostructural transformations in amorphous As2Se3 and As2S3 films

Abstract: The influence of illumination and annealing on thin films of amorphous As2Se3 and As2S3 has been studied through their effects on the structure and optical properties. It is shown that the dominant photostructural change in evaporated films is the polymerization of the as-deposited molecular (As4S6 or As4Se6) glass through either illumination or heating. This irreversible polymerization gives rise to a large absorption edge shift to smaller energy and to an appreciable index of refraction change. In addition there is a small reversible absorption edge shift, due presumably to trapping of photo-generated electrons and holes which is larger in As2S3 than in As2Se3. This reversibility persists in both well-annealed evaporated films and in sputtered films which appear to be structurally indistinguishable both from each other and, aside from defects or voids, from the bulk glass. The reversible effect, called photo-darkening, is accompanied by a negligible refractive index change when compared with the irreversible polymerization change. The application of the present results to photochemical effects is discussed.

Thermodynamic investigation on glassy states of pure simple compounds

Abstract: The present review deals primarily with glass transition phenomena in pure simple compounds and pays special attention to the thermodynamic aspects of the vitrification process. The concept of glassy state is extended to liquid crystalline and even to crystalline materials which have any type of disorder. Thus the familiar supercooled liquid-glass transition is shown to be just one example of a class of ‘glass transitions’ due to loss of equilibrium which must occur quite frequently in condensed matter. Evidence of several glass transition phenomena in one and the same compound is given. The fact that glass should be considered as one of the states of aggregation of matter, irrespective of either the method of formation or the existence of three-dimensional periodicity in molecular arrangements, is stressed.

Composition—property relationships in inorganic oxide glasses

Abstract: By regarding glasses as oxygen polymers with a cross-linked network structure that can have different packing densities and varying degrees of cross-linking according to which network-forming elements and cations are present, the way in which many of their properties change with composition can be explained, at least qualitatively, in terms of the corresponding changes in cross-link density and network packing. The packing of an oxide network is conveniently measured by the oxygen density of the glass, and this and the cross-link density are readily calculated from the composition and actual density.

Ultraviolet spectra of silicate glasses: A review of some experimental evidence☆

Abstract: Ultraviolet spectroscopy is a useful experimental tool for the determination of the electronic structure of glasses. This paper discusses the ultraviolet spectra of SiO2 and high-purity silicate glasses. It considers in turn, absorption intrinsic to the SiO network, the effects of network modifiers, impurity-induced absorption, luminescence and radiation damage, both permanent and transient, in the wavelength region extending from 90 to 350 nm.

The temperature dependence f photoconductivity in a-Si

Abstract: The photoconductivity and its dependence on light intensity have been investigated in a-Si as a function of temperature between 100 and 500 K. In most experiments a photon energy of 2 eV was used. Specimens were deposited on to a substrate held at a temperature between 300 and 600 K by the r.f. decomposition of silane. Graphs of the photocurrent versus 1/T show a photoconductive maximum and the general features of the curves are similar to those found for the chalcogenide glasses. The main emphasis of the paper lies in the interpretation of the results in the light of the information on transport properties and the density of state distribution obtained from drift mobility and field effect experiments. It is shown that recombination takes place predominantly between two groups of localized states, which have been identified in the previous work. The initial state at ϵA is situated about 0.18 eV below geC in the electron tail states, the final state lies in a density of state maximum, 0.4 eV above ϵV. Above about 250 K, the photocurrent is carried by electrons in extended states, but below this temperature transport is by phonon assisted hopping through states near ϵA. A recombination process involving two states of a structural defect centre is discussed on the basis of the results and appears to be a feasible interpretation.

Vicker's Hardness of glass

Abstract: An equation has been derived for the direct calculation of Vicker's hardness number of glasses from their chemical compositions. The method is based on a consideration of deformation during indentation, which was related to Young's modulus and the average single bond strength of a glass. Excellent agreement is obtained between calculated and measured values of Vicker's hardness number for a variety of glasses.

Relaxed continuous random network models: (I). Structural characteristics☆

Abstract: Two elastic-energy-relaxed continuous random network (Polk) models for tetrahedrally bonded amorphous semiconductors have been obtained: a 201-atom model built entirely at Yale and a 519-atom model relaxed from a structure built by Polk and Boudreaux which originated at Harvard. In relaxing the coordinates to minimize the total energy the Keating potential was used for the interatomic interactions. The models are analyzed in terms of density, elastic distortion energy, elastic constants, numbers of five-, six- and seven-fold rings, distribution of dihedral angles, and radial distribution functions. We find that, despite their different origins, the models have essentially identical characteristics. Our principal conclusions are as follows: (a) The density of the CRN model is, to within 1%, that of diamond cubic. (b) The bulk modulus is about 3% lower than that for the diamond cubic structure and the shear modulus lies between the two diamond cubic shear moduli. (c) There are, to within ± 10% (and with corrections for surface effects), 0.38 five-fold, 0.91 six-fold and 1.04 seven-fold rings per atom. (d) For a reasonable value of the bond bending force constant, rms bond length distortions are about 1.0% and bond angle distortions are about 7.0°. (e) The radial distribution function agrees very well with experiment for all four principal peaks.

E.S.R. studies of radiation damage and structure in oxide glasses not containing transition group ions: A contemporary overview with illustrations from the alkali borate system☆

Abstract: Electron spin resonance (e.s.r.) is a particularly powerful method for identifying and characterizing radiation-induced defects in vitreous materials. This paper reviews the fundamental principles and the present state of the art in applying e.s.r. techniques to achieve these ends in oxide glasses which do not contain transition group ions. A wide variety of generic defect types are discussed in some detail by means of examples from the alkali borate system. These include oxygen-associated trapped hole centers and oxygen-vacancy-associated trapped electron centers, as well as several electron and hole-type centers which are associated with interstitial cations and anions, respectively. Probable analogs in other oxide glasses are pointed out. Special emphasis is placed on the kinds of structural information which can be obtained from these studies.

Absorption currents and steady currents in polymer dielectrics

Abstract: It is shown that published experimental data on absorption currents in polymers (mainly polyethylene terephthalate) are consistent with the Walden model of injection, followed by build up of trapped space charge. The published results do not agree with the tunnelling process, dipolar relaxation or electrode polarisation. Detailed analysis indicates that the injection efficiency rises sharply with field, and that the observed current-time plots arise from the change in field at the injecting electrode, rather than from a modification of the barrier shape. It follows that the knee in the current-time plot does not reflect a transit time, but is a measure of t0, a complex function of field and injection mechanism. It also follows that the steady-state currents are controlled by the transport mechanism in the bulk.

Diffraction studies of glass structure: (V). The structure of some arsenic chalcogenide glasses

Abstract: X-ray and neutron diffraction experiments have been performed on vitreous arsenic sulphide close to the composition As 2 S 3 and on a series of vitreous arsenic selenides including compositions both arsenic- and selenium-rich with respect to As 2 Se 3 The data have been analysed by direct examination of the diffraction patterns, by Fourier analysis and by comparison both with quasi-crystalline calculations based on the orpiment structure and with models of the first neighbour correlations using the experimental peak functions Sheet structures occur in all the glasses, the sheet separations being larger than in the corresponding orpiment structures, and the intersheet correlation perpendicular to the sheets extends for 15 A in arsenic sulphide and 20 A in arsenic selenide For the selenide glasses the sheet structure becomes more pronounced with increasing arsenic concentration For all the glasses the data are consistent with full satisfaction of the respective two- and threefold covalency requirements of chalcogenide and arsenic atoms, excess atoms being incorporated into the structure by the formation of covalent bonds between like atoms The distribution of arsenic-chalcogen bond lengths is shown to be symmetrical about the average values of 224 A and 244 A for sulphide and selenide respectively In the selenide glasses close to the As 2 Se 3 composition, the average first SeSe distance is 365 A The advantages and limitations of the experimental and analytical techniques employed are discussed and a simple model based on steric considerations is advanced to account for the influence of arsenic concentration on the inter-sheet correlation

Thermal expansion and density of glassy PdNiP and PtNiP alloys

Abstract: Thermal expansion and density of (Pd1−xNix)0.80P0.20 and (Pt1−yNiy)0.75P0.25 alloys in their various states have been measured from room temperature to the glass transition temperature Tg. The thermal expansion of the glassy alloys at room temperature varies linearly with x and y and is 10 to 20% higher than that of corresponding pure metals. The thermal expansion of the undercooled alloy liquids near Tg as well as the molar volume v deduced from the density of glasses in contrast exhibits a negative deviation with composition x and y. This behavior is in line with the previously reported negative deviation of the glass transition temperature of these glassy alloys with metal content and may be explained in terms of excess volume associated with a mixture of hard spheres.

Systematic generation of random networks

Abstract: This paper reports the computer generation of atomic models for amorphous tetrahedrally coordinated materials. Several approaches are used and their results are intercompared in terms of careful statistical studies on each structure. The best approach is capable of automatically building large models with no included voids or dangling bonds. The strain energies are minimized for each structure implying that they represent metastable equilibrium arrangements; the radial distribution functions are in excellent agreement with experimental data. The programs developed are capable of building models which vary systematically as measured by several parameters including the ratio of five to six membered rings and the average distortion of the tetrahedra formed by near neighbors.

Radiative and non-radiative transition probabilities and quantum yields for excited states of Er3+ in germanate and tellurite glasses

Abstract: Absorption and fluorescence spectra of Er3+ in germanate and tellurite glasses were obtained. Spontaneous transition probabilities of the 4 S 3 2 and 4 F 9 2 to all terminal levels of Er3+ were calculated using the Judd-Ofelt theory and intensity parameters obtained from measured intergrated absorption coefficients. Quantum efficiencies of the 4 S 3 2 and 4 F 9 2 fluorescences were measured by the comparative method and by the use of measured decay times. Multiphonon relaxation rates for 4 S 3 2 → 4 F 9 2 and 4 F 9 2 → 4 I 9 2 were calculated using the experimental data. The average rate for 4 S 3 2 → 4 F 9 2 in germanate is 1.16 × 105 sec−1 and in tellurite is 1.60 × 104 sec−1, and the rate for 4 F 9 2 → 4 I 9 2 is 2.85 × 10 5 sec −1 in germanate and 2.33 × 105 sec−1 in tellurite. The higher rates in germanate glasses are explained by the stronger interaction of the glass phonons with the electronic states of Er3+ in germanate than in tellurite glasses. This also explains the higher quantum yield of the visible fluorescence of Er3+ in tellurite glasses compared to other glasses.

Low frequency storage and loss moduli of soda-silica glasses in the transformation range

Abstract: The storage and loss moduli of sodium silicate glasses in the transformation range are presented. The data cover the frequency range from 10 −3 to 1 rad/sec. Using linear viscoelastic theory, the frequency-temperature dependence is separated into master curves for the storage and loss moduli at the glass transition temperature and shift factor data describing the shift of the curves with temperature along the frequency scale. These binary glasses are concluded to be thermorheologically simple. The relationship between the shape of the master curves and the configurational entropy is shown, and various available theories for the shift factor data are discussed.

Investigation of surface layers, formed on glass electrode membranes in aqueous solutions, by means of an ion sputtering method

Abstract: A method of measuring concentration profiles in glass surface layers by photon emission during ion sputtering, that offers a depth resolution of 30 to 50 A, is applied to leached surface layers of pH-electrode membranes. Two steady state layer regions different with respect to ion diffusion are found at low pH. Within the outer layer the interdiffusion coefficient is constant. Within the transition layer interdiffusion coefficients are observed with a minimum that is two orders of magnitude lower than the lithium diffusion coefficient within the glass and is explained by a low diffusion coefficient of interdiffusing protons within a highly glass-like structure. The explanation is supported by the similar magnitude of the diffusion coefficient obtained here and that of protons which have replaced lithium ions within this glass in an electric field. The corresponding maximum resistivity agrees with measurements by Wikby. Surface layer properties and additional experimental results suggest that the equilibrium determining the pH-membrane potential is the dissociation of SiOH-groups at the immediate layer surface rather than an ion exchange of glass alkali and solution hydrogen ions.

Monte Carlo models of atomic arrangements in arsenic-selenium glasses

Abstract: Models of the atomic arrangements for a series of bulk AsSe glasses with compositions from Se to 50 at % As50 at % Se have been developed using a computer-generated Monte Carlo procedure in conjunction with atomic radial distribution functions. These models demonstrate the gradual growth of interconnected chains from a mixture of chains and rings in pure selenium up to 40 at % As followed by a gradual breakdown of such structures. This behavior correlates well with a number of properties dependent on molecular morphology such as the glass transition temperature, viscosity, electric conductivity and photoconductivity. The models indicate that the mean Se vertex bond angle is about 104° with an rms deviation of 13.5° while the mean As vertex bond angle is 102° with an rms deviation of 15°. An addition model based on the crystalline As2Se3 structure has been developed for 40 at % As60 at % Se. It indicates that distortion of the crystalline layer structure must involve breaking about 1 3 of the AsSe rings but that no joining of layers is required. Models were also developed for sputtered films of these materials. Qualitatively, the models are very similar to those for bulk materials, showing rings in pure selenium, gradual interconnection of rings and chains to 40 at % As and then a breakdown of the network above 40 at % As. However there is an increase in the distribution of near neighbor distances and bond angles, some increase in the mean bond angle and at 40 at % As and above, a decrease in the mean near-neighbor distance. Below 40 at % As mean near-neighbor distances for film and bulk materials are the same.

Voids in amorphous semiconductors

Abstract: Small angle X-ray scattering (SAXS) has been used to investigate density fluctuations occurring in amorphous semiconductors prepared by sputtering, evaporation and electrodeposition. Correlations of the total number of dangling bonds determined by SAXS, optical absorption, and ESR signals have been made. The density deficits from the density of the FC-2 crystal are in some cases accounted for by the voids. It is argued that models based on domains 10–15 A in size are not supported by the SAXS data.

Mechanical properties of NiFe based alloy glasses

Abstract: The ductility and tensile strength of NiFe based alloy glasses prepared by the ‘roller quenching’ technique were investigated. These alloy glasses are intrinsically ductile and possess a high tensile strength. However, the mechanical properties of glasses which are predominantly Fe are very susceptible to the quenching conditions, and such glasses tend to be brittle. It is suggested that the hot rolling of the alloy during quenching is responsible for the brittle behavior. Tensile strengths as high as 230 000 psi were obtained for the NiFe based alloy glasses.

Role of structure in amorphous magnetic materials: Dense random packing of hard spheres of two sizes

Abstract: We investigate the structure of amorphous rare-earth transition-metal alloys and its influence on their magnetic properties in the context of a simple model. The packing fraction of the alloys is discussed using both ball-bearing assemblies and also computer-generated clusters of randomly close-packed hard spheres, and the latter clusters are used to discuss the radial distribution functions (RDF's) and the magnetic anisotropy. The computer-generated RDF's are compared with published neutron diffraction results on amorphous TbFe 2 .

Dielectric properties of fine-grained PbTiO3 crystals precipitated in a glass

Abstract: The spontaneous deformation of PbTiO3 crystals precipitated in a PbOTiO2Al2O3SiO2 glass was found to decrease with decreasing grain size. This trend remains even after the glass matrix surrounding PbTiO3 crystals is removed completely by HNO3 acid immersion. Consequently, a linear electro-optic effect cannot be expected from transparent glass-ceramics which necessarily comprise very-fine-grained crystals. The dielectric constant of PbTiO3 crystals surrounded by a glassy matrix shows two maxima at grain sizes 0.15 μm and 250 A, but is still fairly high even at a grain size as small as 160 A. This suggests that a material with a large quadratic electro-optic effect may be produced from a transparent glass-ceramic. The maxima of the dielectric constant at grain sizes 0.15 μm and 250 A were interpreted in terms of the internal stress and internal electric field, respectively.

Multichannel conduction in alkali silicate glasses

Abstract: The qualitative change of the electrical conduction of alkali silicate glasses with alkali mixing is discussed in connection with dielectric and mechanical relaxation. The observed conduction is understood as the additive effect of two kinds of independent diffusion processes — I and II. In single alkali glass, diffusion process-I is always observed as the dominant conduction process. However, when one alkali is progressively substituted by another, the dominant conduction process is altered from diffusion process-I to II. In the mixed alkali glass containing equal amounts of two alkali ions, diffusion process-I disappears, so that only diffusion process-II is observed. Consequently, it becomes evident that the mixed alkali effect is not only the successive change of conductivity, but the alternation of the dominant conduction process.

Nonlinear ultrasonic attenuation in glasses

Abstract: Measurements of the attenuation of longitudinal sound waves of frequencies between 730 MHz and 1400 MHz were made on a borosilicate glass at temperatures from 0.4 K to 2 K. For high acoustic intensity J the absorption decreases with decreasing temperature. At low temperatures the attenuation is strongly dependent on the acoustic input power and was found to be proportional to J−12 indicating a saturation of the absorption. For low acoustic intensities (J ⪅ 2 × 10−7 W/cm2) an increase of the attenuation with decreasing temperature below T < 0.6 K was observed. The experimental results are compared with recent data in vitreous silica and can qualitatively be described by current theories based on a tunneling model.

Optical properties of bismuth in germanate, borax and phosphate glasses

Abstract: Absorption, emission, excitation spectra and quantum efficiencies of luminescence of bismuth were obtained in germanate, borax and phosphate glasses. Decay times were measured for fluorescence of bismuth in germanate glass at room temperature and liquid air temperature. A non-exponential decay of fluorescence at room temperature is observed. A thermal equilibrium between the 3P1 and 3P0 states is discussed. The dependence of fluorescence intensity and decay times on temperature is presented. The amount of covalency between bismuth and glass matrix was calculated from the nephelauxetic effect.

Electrical and optical properties of amorphous boron and amorphous concept for ß-rhombohedral boron

Abstract: Measurements of the conductivity (σ), thermoelectric power (S) and thermal conductivity (κ) of amorphous boron are made over wide temperature ranges (T = 77–850 K for σ, T = 300–850 K for S and T = 80–1100 K for κ). The room temperature spectral dependencies of the reflection (R) and absorption (α) coefficients are determined for the wavelength intervals 2–25 μm and 1.3–25 μm respectively. The I–V characteristics are also studied and shown to be consistent with the Poole-Frenkel law. The value obtained for the thermal energy gap of amorphous boron (1.3 eV) is slightly smaller than that of crystal s-rhombohedral boron (1.4 eV). The temperature dependence of the electrical conductivity can be satisfactorily described by the Mott law ln σ ≈ −(T 0 /T) 1 4 , where T 0 ⋍ 10 8 K . This gives an estimate, N ≈ 1018 cm−3, for the concentration of trapping levels responsible for the hopping conduction. The value ϵ 0 ⋍ 9 is found from the spectral dependence of R while α has Urbach-like character − α ≈ exp ( h ω/Δ) , where Δ ⋍ 0.19 eV . A comparison is made between amorphous boron and crystalline s-rhombohedral boron. Because of the very complex crystal structure and the large dimensions of the unit cell of s-boron, some of its physical properties could be qualitatively described on the basis of the so-called ‘amorphous concept’.

Photoconduction of glasses in the TeSeSb system

Abstract: The photoconductivity of bulk glasses of the TeSeSb system is measured as a function of light intensity and photon energy. The relative sensitivity ( ΔI / I d ) has linear and square-root dependences on light intensity in low and high illumination intensities, respectively, and is nearly proportional to the square-root of the resistivity at room temperature. The spectral response of photoconductivity, which is calculated by taking into account the effect of surface recombination of carriers, agrees qualitatively with the experimental results. The experimentally determined broad spectral response suggests the presence of band tails below the conduction band and above the valence band. The large residual dark conductivity in the decay response is associated with the presence of many deep trapping centers.

Elongation and spheroidization of phase-separated particles in glass

Abstract: When certain phase-separated glasses are elongated by redrawing near their softening point, the spherical second-phase particles are found to elongate as well. The elongation process is discussed, and the conditions under which high particle elongations are possible are estimated. Annealing the glass tends to spheroidize cylindrical-shaped particles by an ‘ovulation’ process. The spacing of the resulting spheroidal particles is compared with theoretical predictions.

Internal friction of mixed alkali metaphosphate glasses: (I). Results

Abstract: The internal friction of LiNa, LiK, LiCs, LiAg, NaCs and NaAg metaphosphate glasses was measured at 0.5 Hz and 2 kHz. The dielectric losses were also measured from 40 to 160°C, at frequencies of 300, 3 000 and 30 000 Hz. The densities of the glasses were determined and the molar volume of oxygen was calculated. In general, the mixed alkali behaviour of metaphosphate glasses is very similar to the mixed alkali behaviour of silicate glasses. Silver behaves in this respect like an alkali ion with approximately the same size as a sodium ion.

Electron microscopic study of the crystallization kinetics of the amorphous alloy Pd80Si20

Abstract: Amorphous Pd 80 Si 20 foils, prepared by the rapidly rotating mill device method, were quenched from the liquid state, temperatures of the melt being 1100, 1500 and 1600°C. The foils were linearly heated at a rate of 1.5 °C/min over a temperature range of 20–600°C. The structural changes were observed by methods of optical microscopy, transmission electron microscopy and electron diffraction. The beginning of crystallization of the amorphous phase is affected by the temperature of the melt. The crystallization of the amorphous phase was terminated at a temperature of 390–400°C. Silicides Pd 9 Si 2 and Pd 3 Si, the morphology of which varied with temperature, were formed during crystallization.

Monte-Carlo models of amorphous materials

Abstract: A Monte-Carlo technique is described for the determination of atomic and molecular arrangements in simple amorphous solids. This method appears to be capable of describing configurations of atoms in glasses with sufficient detail to reproduce an experimental radial distribution function. The radial distribution function is combined with data on chemical bonding and with data on other physical properties to generate a model of the atomic arrangements which provides a view of the local order in an amorphous material. The entire procedure involves two steps. In the first, a quasi-random arrangement of atoms is constructed by a stochastic Monte-Carlo procedure. In the second stage the atomic arrangement is subjected to an extended Monte-Carlo perturbation, whereby the final configuration is described by the experimental radial distribution function.

Nucleation kinetics and critical cooling rate of glass-forming liquids

Abstract: The critical cooling rate of a substance is the minimum linear rate at which its melt must be cooled to prevent crystallization entirely. In a number of systems this critical cooling rate has proved to be a useful quantitative experimental measure of the tendency to glass formation. The present paper describes a method of deriving theoretical values of the critical cooling rate from classical nucleation kinetics. In general, the calculated values show the correct tendency to glass formation.