Showing papers in "Journal of Non-crystalline Solids in 1971"

Structural model for amorphous silicon and germanium

Abstract: A model for amorphous germanium and silicon has been constructed using the random network concept. Each atom has a first coordination number of four, and only a small variation in the nearest neighbor distance is allowed. Non-crystallinity is due to variations in the tetrahedral bond angle and rotations about bonds. No difficulty was encountered in continuing to make the model larger and no difference in structure between the central and outer regions could be observed. The radial distribution function and the density of the model agree well with recent measurements.

Optical and electrical energy gaps in amorphous semiconductors

Abstract: The experiments are analyzed which yield information about the density of localized gap states g l (E) in amorphous semiconductors. A conflict is found between optical absorption data which reveal only a very small g l (E) and electrical measurements, which predict orders of magnitude larger g l (E) . A model proposed resolves this conflict by assuming that because of spatial potential fluctuations electrons and holes are trapped in different regions and that near the mobility edge the extended electron wavefunctions are somewhat excluded from regions in which electrons are localized. The consequences of this model regarding the interpretation of the optical and the electrical gap are discussed.

Relation between apparent glass transition temperature and liquids temperature for inorganic glasses

Abstract: The ratio of the glass transition temperature T g to the liquidus temperature T l (or T m ) was calculated for a wide variety of inorganic glass forming systems including the elements, oxides and sulfides and technologically important systems such as borates and silicates. It was found that a simple empirical rule T g /T l = 2 3 which was suggested for organic polymers holds suprisingly well for inorganic systems.

Hall conductivity of amorphous semiconductors in the random phase model

Abstract: Kubo's formula for the electrical conductivity is used to evaluate the Hall conductivity in the Random Phase Model of Amorphous Solids appropriate to the diffusive, short mean free path regime. The Hall mobility is found to be small in magnitude, temperature insensitive, and to exhibit the same anomalies in sign as found in the localized regime: namely, negative for a non-degenerate distribution of holes on electrons due to diffusive motion about a closed path of three sites.

Properties and structure of glasses in the system AsS

Abstract: In obtaining the glass, at first, the raw materials were mixed up and melted by using porcelain crucible, but in this case it was found by infra-red absorption method that the glass contains some oxygen impurities. Then a range of glasses was prepared by melting elementary pure As and S in definite proportions in a sealed vacuum tube. The infra-red absorption, molecular volume, molecular refraction, hardness, thermal expansion and viscosity of sample glasses with various compositions were investigated and, additionally, solubility of these glasses into CS2 was measured. The structure of glasses in the system AsS was studied by X-ray diffraction and a structural model was set up. From the measurements the following conclusions were made. The structure of As2S3 glass is a distorted form of the crystalline orpiment structure. With increasing S content above As2S3, S is likely to exist in the chain-like form, but when the S content is greater than that in As2S8–10, both chain-like and ring type forms co-exist. With decreasing S content below As2S3, S between As and As is eliminated and AsAs bonds are formed. Consequently a deformation of layer occurs and an expansion of the layer distance was observed.

The mixed alkali effect in the Na2OCs2OSiO2 glasses

Abstract: As an approach to the mixed alkali effect in glass, the self-diffusion coefficients of sodium and cesium ions in Na 2 OCs 2 OSiO 2 glasses were measured at temperatures 350–550°C. Electrical conductivity of the glasses and the transport number for sodium ions were also measured. The substitution of the alkali ions in the glass by different alkali ions caused the mobility of each alkali ion to decrease pronouncedly and the activation energy for migration to increase rapidly. The increase of activation energy was attributed to an increase in alkali-oxygen bond strength resulting from the presence of two kinds of alkali ions. This is related to the expectation that the activity of the alkali ions decreases when two alkali ions are mixed.

The elastic and anelastic behavior of a metallic glass

Abstract: The temperature dependence of the internal friction and Young's modulus of the glassy and crystalline phases of a Pd 0.76 Au 0.75 Si 0.165 alloy have been determined by a resonance technique. The internal friction of the glassy alloy increases exponentially above 150°C at a frequency of 140 Hz. The apparent activation energy of the relaxation process increases upon annealing from 50 to 80 kcal/mole. It is suggested that relaxation processes possessing a broad spectrum of activation energies are active in the glassy alloy. Young's modulus of the glassy alloy decreases gradually with increasing temperature and increases approximately 28% upon crystallization.

Dielectric and optical properties of transparent ferroelectric glass-ceramic systems

Abstract: The properties and behavior of optically transparent materials prepared by controlled crystallization of a ferroelectric phase from a glass are discussed. The dielectric and optical properties are different from those of the bulk materials and are shown to be a function of the crystallite size. The control of the crystallite size to values below 0.2 μm allows one to prepare transparent specimens which exhibit a large electrooptic effect. The dielectric and electrooptic properties are shown for a representative number of glass-ceramic systems.

Effect of phase separation on the properties of simple glasses II. Elastic properties

Abstract: Available theories for the variation of elastic moduli in two-phase materials are reviewed. It appears that only three rigorous independent solutions currently exist: the upper Voigt limit, the lower Reuss limit, and the interdependent boundaries obtained by Hashin and Shtrikman. The lower Hashin-Shrikman boundary appears to best describe the behavior of elastic properties over the complete range between the end-members. This boundary was previously derived by Kerner for the case of spherical particles. Plots of moduli against volume fraction always have positive (concave-upward) curvature, with the exception of the linear behavior of the Voigt model. In terms of weight percent, the curvatures may be positive, negative, or zero, according to the relative values of the densities and moduli of the end-members. Sonic velocities and adiabatic moduli are reported for PbOB2O3, Li2OSiO2, and PbOSiO2 glasses. The Hashin-Shtrikman/Kerner relations describe the moduli fairly well over known or suspected two-phase composition regions. Maxima and minima previously reported for certain properties in the LiOSiO2 and the PbOSiO2 systems were not verified in this investigation. The Hashin-Shtrikman/Kerner relations fit the experimental data fairly well over the two-phase regions in the sodium silicate system and in the alkali borate glass systems, with the fit being poorest for the potassium borates. No immiscibility is expected near 12 wt % (24 mole %) Na2O in the Na2OP2O2O5 system, or over the range 0–10 wt% Na2O(O-16 mole % Na2O) in the Na2OGeO2 system.

Extrusion of glass

Abstract: Extrusion is especially suitable for glasses to which the usual technologies of casting, blowing or drawing are difficult or impossible to apply. This holds for glasses with a narrow working range (short glasses), for glasses which strongly tend to crystallize and for glasses with a very high softening point. Due to the extrusion pressure, the working range is enlarged to higher viscosities, the manufacturing process is shifted into a range of lower cyrstallization rates and moreover the lower working temperature can be controlled in a more convenient way. Furthermore, extrusion could be applied to conventional glasses, if products with other than circular cross sections are to be manufactured. For the process frit can be used. Quantitative measurements showed that the flow behaviour of the glass is in satisfactory agreement with the results obtained from the Hagen-Poiseuille law.

The effect of copper and silver on the conductivity and photoelectric properties of glassy arsenic selenide

Abstract: A study was made of the effect of copper and silver on the electric properties, photoconductivity and its kinetics in glassy arsenic selenide. Introduction of copper and silver from 0 to 2.4 at% was shown to increase the conductivity from 5.6 × 10 −13 ohm −1 cm −1 to 5.8 × 10 −11 ohm −1 cm −1 and 2.3 × 10 −12 ohm −1 cm −1 , respectively At the same time, the activation energy decreases from 1.71 (for glassy As 2 Se 3 ) to 1.21 eV and 1.46 eV, respectively. The photoconductivity spectra shift to the infrared with an increase of the copper and silver content. The values of the drift mobility and their temperature dependences were calculated for all the compositions studied. A study of the photoconductivity kinetics showed the existence of trapping centres for the majority and minority current carriers in the band gap of the materials studied. We found depths of these centres for all compositions, and also found for the minority current carriers trapping centres the cross-sections for the trapping of electrons and holes.

ESR and optical studies of alkali-associated trapped-electron centers in alkali borate glasses irradiated at 77°K☆

Abstract: Lithium, sodium, and potassium borate glasses have been X-irradiated at 77°K and studied by ESR and optical techniques. In addition to the familiar boron-oxygen hole center and a recently-reported electron-type center associated with boron, another class of defect has been recognized. This center gives rise to broad ESR signals with overall widths which are independent of boron isotope but which vary with the type of alkali. It is shown that this type of defect comprises an electron trapped on a complex of one or more alkali ions and that it is probably responsible for the familiar radiation-induced optical absorption in the range 1.5–2.4 eV.

Metastable equilibrium density of hydroxyl-free synthetic vitreous silica

Abstract: It is shown that the volume behaviour of hydroxyl-free vitreous silica (type IV silica glass) is similar to that of the so-called natural vitreous silica (fused quartz, type I and II silica glasses) with low hydroxyl content. Thus, in view of that, a distinction may be made between two groups, types I, II and IV on the one hand and type III, the hydroxyl-rich silica glasses, on the other hand, although a distinction may also be made between the group of “natural” (type I and II) and “synthetic” silica glasses (type III and IV) in view of production method and starting material (quartz or SiCl 4 ). It is further shown that the type III silica glasses give an example for a change from the anomalous volume behaviour of “pure” silica glasses (thermal expansion, high-temperature minimum of volume) to the common behaviour of silicate glasses.

Experimental evidence of filament “forming” in non-crystalline chalcogenide alloy threshold switches

Abstract: The results of quantitative electrical measurements on threshold switching devices fabricated by photoengraving techniques from non-crystalline chalcogenide alloy thin films are presented. An electrical forming process is observed prior to the establishment of a stable switching characteristic, and the results are used to demonstrate that the actual threshold switching occurs in a filament produced during this forming process, with a much higher conductivity than the as-deposited thin film. The implications of these observations for theories of switching are discussed.

Direct observation of the double glass transition in a phase-separated glass

Abstract: Heat capacity measurements have been performed for the phase-separated pseudobinary chalcogenide glass system X PbSe-(1− X )Ge 1.5 As 0.5 Se 3 in the composition range X = 0.0 to 0.6. The heat capacity versus temperature plots for the phase-separated glasses exhibit two glass transition breaks. The composition dependences of the heat capacity changes, ifΔC p , at the two transitions are shown to be in accord with the behavior predictable from the phase diagram of a glass-forming system exhibiting liquid-liquid immiscibility and with the structure of these glasses as determined by electron microscopy. The two composition points on the immiscibility dome at which the temperature is equal to the higher glass transition temperature have been determined from the composition dependence of the ΔC p 's and indicate that the dome spans almost the entire composition range for this system.

The effect of gallium, indium and thallium on the conductivity and photoelectric properties of glassy arsenic selenide

Abstract: A study has been made of the effect of Ga, In and Tl on the conductivity, the photoconductivity, and the kinetics of photoconductivity of glassy arsenic selenide. It has been shown that as the content of these elements in As2Se3 increases, the conductivity increases, the activation energy decreases, and the spectral response of photoconductivity shifts to the infrared. The magnitude of the drift mobility and its temperature dependence have been determined for all compositions. As shown by a study of the kinetics of photoconductivity, for all glasses, the existence of trapping centres for the majority and minority current carriers in the band gap is typical. Experimental data were used to determine the depth of these centres. Cross-sections for the capture of the electrons and holes by the minority carriers trapping centres have been calculated.

Heat capacities and thermal behavior of alkali borate glasses

Abstract: The heat capacities of selected glasses in the five alkali borate systems have been measured over a range of high temperatures which includes the respective glass transition regions. The heat capacities per gram atom at 350°K show little variation with composition while those at the low temperature ends of the glass transition, Tg-, show some systematic variations. When compared with the respective 3R values, the heat capacities at Tg- range from about 0.75 for B2O3 to values in excess of unity for various alkali borate compositions. The present data on heat capacity have been combined with previous data on the elastic moduli, densities and thermal expansion coefficients to evaluate the Gruneisen constant, γ, for each composition for temperatures around ambient. For B2O3, γ has a low value of about 0.25. It decreases with additions of Li2O and increases with additions of the other alkali oxides.

Preparation and electrical properties of amorphous InSb

Abstract: Layers of amorphous InSb are prepared by flash evaporation on cold substrates. The main properties of these layers are: The electrical band gap E g el increases from 0.17 eV in crystalline up to 0.65 eV in amorphous InSb. E g el , however, has no characteristic value, but can be varied by annealing below the transition temperature between 0.4 and 0.65 eV. The absorption edge as well as the photoconduction edge are shifted to higher photon energy in the amorphous state.

Optical properties of amorphous compounds As2S3 and As2Se3 in the infrared region

Abstract: Optical properties of amorphous As2S3 and As2Se3 at room temperature in the region between 30 and 8000 cm−1 are described. Absorption peaks at 38 cm−1 for both As2S3 and As2Se3 are assigned as vibration modes v AS concerning with only arsenic atoms. Kramers-Kronig analysis of reflectivity indicates only one reststrahlen band at 301 cm−1 for As2S3 or at 226 cm−1 for As2Se3 which is assigned as AsS or AsSe stretching vibration vR. Bands at 698 and 991 cm−1 for As2S3 or 487 and 732 cm−1 for As2Se3 are interpreted as overtones of vR. The shape of the reststrahlen bands fits to a Gaussian form, and broadening of the band can be attributed to a Gaussian distribution of oscillator frequencies. It is found from the value of the force constants (1.18 and 1.19 md/A for As2Se3 for the stretching vibration and the difference between low and high frequency dielectric constants (1.63 and 1.92 for As2S3 and As2Se3) that the compounds have a large degree of covalency.

Nd3+ in germanate glasses: spectral and laser properties

Abstract: The absorption and fluorescent properties of Nd3+ in potassium-barium germanate glasses have been determined. The quantum efficiency of the 1.06 μ emission in the germanates is higher than that in silicates at low Nd3+ concentration; however, concentration quenching sets in at lower activator concentrations in the germanates. 3 × 1 4 inch laser rods had slope efficiencies approaching one percent, and a threshold of 12.4 joules. The gain in a germanate rod, as determined by measurements of threshold vs. resonator reflectivity, was higher than that in barium crown silicate.

A study of the distribution of boron sites in glassy B2O3 using 11B NMR

Abstract: A study has been made of the 11 B NMR spectrum of glassy B 2 O 3 with the aid of computer-simulated NMR spectra. It is shown that the glass contains a distribution of boron sites centered on an average or most probable site characterized by a quadrupole coupling constant of 2.66 MHz and an electric field gradient asymmetry parameter of 0.11. It is concluded that 11 B NMR alone cannot determine the exact nature of the distribution. The implications of the NMR data with respect to the suggested existence in the glass of distorted BO 4 “tetrahedra” is discussed.

Interpretation of the switching effect in amorphous semiconductors as a recombination instability

Abstract: For an explanation of the switching effect appearing on thin layers of amorphous semiconductors, a double-injection model is developed where the presence of diffusion currents in front of the electrodes is assumed in the high-resistance state. In the diffusion regions recombination takes place. The cause of the switching effect is seen in a recombination instability. Because of a saturation of the recombination current the life time of the free carriers becomes long enough for the diffusion length to extend over the full length and the high-resistance state to collapse.

The contamination of glass by platinum

Abstract: The contamination of glass by platinum due to melting in platinum crucibles has been investigated. It was determined that unstirred melts prepared by induction melting in nonoxidizing atmospheres suffered only surface contamination due to a process involving atmospheric oxidation of the crucible. In this process oxygen is contributed to the atmosphere by dissociation of volatile components of the melt and is a function of both the glass composition and the melting temperature. A new laser glass matrix composition has been developed which minimizes platuinum contamination.

Phase separation in SiO2B2O3Na2O system

Abstract: Isothermal studies of the phase separation occurring in the SiO 2 B 2 O 3 Na 2 O system near the center of the miscibility gap are made at three temperatures. Early stage observations of the phase morphology indicate that phase interconnectivity is probably due to nucleation and subsequent intersecting growth of silica-rich particles. It is found also that the extent of interconnectivity is strongly dependent on the volume ration of the coexisting phases. Evidence is also presented that the glass of present composition is in a two-phase region rather than the three phase region previously reported. It is also shown that there is very little rotation of the tie-lines between 748°C and 650°C.

Analysis of the specific heat and thermal conductivity of vitreous selenium above 1 K

Abstract: The excess specific heat and the dip in the thermal conductivity of vitreous selenium near 5 K are analysed in terms of a band of additionnal modes which scatter the acoustic phonons strongly. The value of the lowest frequency of this band is h ω R /k B = 11 K , a value very similar to the case of vitreous silica and germania.

Photoinduced injection of electronic charge carriers from phthalocyanine into selenium

Abstract: Efficient photoinduced charge injection from metal-free phthalocyanine into amorphous selenium has been observed. The sensitization is strongly field-dependent, and its spectral dependence corresponds to the absorption spectrum of the phthalocyanine. Under high field conditions the maximum quantum efficiency of sensitization by red ( > 6000 A ) radiation, absorbed selectively by the sensitizer, has been observed to be 0.6 electron per absorbed photon. The results suggest that the absorption of red radiation causes the photogeneration of mobile charge carriers in the phthalocyanine layer. In the presence of an external field these may be injected across the interface and move through the adjacent selenium layer. The dependence of injection current on electric field is analyzed in terms of Schottky-type emission across a barrier between sensitizer and substrate. The observations are interpreted in terms of published data on the relative energy levels of sensitizer and substrate.

Supercritical fluctuations in the B2O3PbOAl2O3 system

Abstract: A small-angle X-ray diffraction study of the supercritical region of the B 2 O 3 PbOAl 2 O 3 immiscibility surface is presented. This is the first time that diffraction methods have been applied to the study of critical opalescence in a system of oxides at high temperatures. Debye's theory is shown to be applicable and, from the spectra correlation lengths and the range of molecular interactions l is calculated. The topography of the supercritical region is determined and the relationship between l and the main interatomic distances in the melt discussed. The spectra of supercritical fluctuations in the liquid are compared to the spectra of vitreous specimens obtained by quench. This permits the quality of the quench to be assessed and an accurate determination made of the initial condition for the study of spinodal decomposition in the miscibility gap.