Showing papers in "Journal of Non-crystalline Solids in 1978"

A method for evaluating viscosities of metallic glasses from the rates of thermal transformations

Abstract: The apparent activation energies E(T) for the glass transition and crystallization in Pd77.6Cu6Si16.5 and Pd48Ni32P20 glass are seen to coincide with those for the viscous flow. This implies that both the rates of glass transition and crystallization in metallic glasses scale as the viscosity. Based on this proposition, the viscosity of a Pt45Ni30P25 glass, for example, has been evaluated, by means of thermal measurements, from the glass transition far into the crystallization temperatures. The viscosity η decreases rapidly from 1013 P at 480 K to 107 P at 580 K and is described by a Fulcher's expression as η = 10−3 exp[5950/(T − 320)].

Nuclear magnetic resonance studies of the glasses in the system Na2OB2O3SiO2

Abstract: The 11B NMR spectra have been used to study the structure of glasses in the system Na2OB2O3SiO2. The fraction of BO4 units, and the fraction of BO3 units with one or two nonbridging oxygens, are measured and analyzed according to a structural model. The results indicate that: (1) for a sodium oxide to boron oxide ratio of 0.5 or less, the Na+1 ions are attracted primarily by the borate network; therefore, the ternary glasses can be viewed as binary sodium borate glasses diluted by SiO2; (2) when the sodium oxide to boron oxide ratio exceeds 0.5, the additional Na2O results in the formation of [BSi4O10]−1 units at the expense of diborate and SiO4 units. In this process, Na+1 ions are still taken up only by the borate network. After all the available SiO4 units are consumed to form [BSi4O10]−1 units, additional Na+1 ions are proportionally shared between the borate and silicate networks.

Direct methods for the determination of atomic-scale structure of amorphous solids (X-ray, electron, and neutron scattering)

Abstract: The atomic-scale structure of amorphous solids can be determined from the X-ray, electron, or neutron scattering pattern. The atomic distribution function ϱ ( r ) or the pair correlation function g ( r ) = ϱ ( r )/ ϱ 0 , were ϱ 0 is the average atomiic density, is related to the interference function or structure factor I ( K ) by a Fourier transformation. Unfortunately, I ( K ) is not directly accessible from the scattering experiment, but can be deduced from the scattering pattern after suitable corrections for polarization, absorption, inelastic scattering, multiple scattering, and static approximation, and after normalization to absolute units. In multicomponent systems, the coherent scattering function per atom I a ( K ) is a weighted sum of the partial interference functions I ij ( K ), which represent the Fourier transforms of ϱij ( r ), the number of j -type atoms per unit volume at a distance r from an i -type atom. It is also possible to express I a ( K ) as a weighted sum of number-concentration structure factors I NC ( K ), which are associated with the number-number (density), number-concentration, and concentration-concentration correlations. The long-wavelength limit of the functions I NC ( CK ) can be expressed in terms of the various thermodynamic quantities and their variation with composition. In binary allos, three partial functions are required to describe the atomic arrangements. These functions can be obtained by varying the atomic scattering factors f i through choic of three different radiations (X-rays, electrons, and neutrons), or isotopes in neutron scattering, or anomalous dispersion in X-ray scattering. The latest developments in the experimental techniques for the determination of the interference function I ( K ) are presented, and examples of the scattering patterns of metallic glasses are given, which were obtained by the conventional, variable 2θ technique, and by the variable λ technique. Attempts are discussed to deduce the partial interference functions in binary systems from several scattering experiments, and to determine concentration fluctuations in binary alloys.

Prediction of glass-forming ability for metallic systems

Abstract: The relative glass-forming ability (GFA) of metallic alloys is considered in terms of a parameter ΔT ∗ = (T liq mix − T liq )/T liq mix , which represents the departure of the alloy liquids temperature, Tliq, from that of the simple rule of mixtures liquids temperature, Tliqmix. For values of ΔT ∗ > 0.20 a metallic system is likely to form a glass by melt-quenching in useful thicknesses (i.e. > 20 μm) at a cooling rate of 105−107 K s−1. Hence, a rapid assessment of the GFA of novel compositions may in general be obtained simply from a knowledge of the melting points of the pure components and the liquidust emperatures of the alloys.

Preparation and optical properties of neodymium fluorozirconate glasses

Abstract: A vitreous domain is investigated in the ternary system ZrF 4 BaF 2 NdF 3 . A typical composition for a glass is 60% ZrF 4 , 34% BaF 2 , 6% NdF 3 ; it can be extended to other rare earths. Several spectroscopic properties of this new Nd glass are reported: absorption and fluorescence spectra, fluorescence lifetime and refractive index. The properties of fluorozirconate glasses are compared with those of other known glasses for laser applications.

Mixed alkali glass spectra and structure

Abstract: The far infrared and Raman spectra of several series of mixed alkali metaphosphate glasses have been investigated as a function of the mole fraction x of the network-modifying ionic oxides in xM2O(1−x)M2′O · P2O5 The frequencies of the cation-motion bands in the far infrared spectra, which correspond to cationsite vibrations, do not shift with x, indicating that the vibrationally significant local geometry and forces associated with a particular cation are unaffected by the introduction of the second cation into the glass structure Each Raman-active band due to vibrations of the metaphosphate network occurs at a different frequency for each pure glass (x = 0 or 1), but for mixed alkali glasses only one band occurs for each type of mode and it varies linearly with x This indicates that the cations in these mixed alkali glasses are homogeneously distributed, there is no significant molecular-level domain formation and the phosphate chains are associated with an averaged cation environment whose effect on the chain modes varies with x A simple vibrational model is presented which shows that the cation-dependent shifts are due to small changes in network bond angles and variation of the cationsite forces

Relationship between composition, density and refractive index for germania silica glasses

Abstract: The density and the refractive index for various compositions of binary SiO2GeO2 glass prepared by a flame deposition technique, similar to that used to make low-loss optical waveguides, have been measured by standard Archimedes method and the Becke line method, respectively The density as a function of composition is calculated considering the effective volume of the ions contained in the glass to be invariant The refractive index as a function of composition is also calculated, based on the Lorenz-Lorenz equation, by computing the electronic polarizability of fused silica and germania All calculated results are in good agreement with the observed data

Defects in amorphous insulators

Abstract: Electron spin resonance characterizations are presented for a number of defects in a selection of amorphous insulators, including oxide glasses, oxyhalide glasses, halide glasses, nitrate glasses, oxynitride glasses, and a sulfide glass.

A structural interpretation of B10 and B11 NMR spectra in sodium borate glasses

Abstract: It is shown that the B10 spectra for sodium borate glasses are sensitive to the different structural groups in the glasses. Five boron sites are inferred from the data: two 4-coordinated sites and three 3-coordinated sites. The two 4-coordinated boron sites are identified as BO4 units connected to all BO3 units, and BO4 units connected to one BO4 unit and three BO3 units. The three 3-coordinated boron sites are identified as BO3 units connected to: (1) all BO3 units; (2) a mixture of BO3 and BO4 units; and (3) all BO4 units. These five sites can be interpreted in terms of Krogh-Moe's structural model of alkali borate glasses, wherein the fraction of each structural group can be determined for eight sodium borate glasses spanning the compositional range from 0 to 35 mol% Na2O. The resulting fractions are consistent with Krogh-Moe's structural model.

Effect of metalloidal elements on corrosion resistance of amorphous iron-chromium alloys

Abstract: Corrosion and electrochemical behaviours of amorphous iron-chromium alloys containing different sets of glass-forming metalloidal elements, boron, carbon, silicon and phosphorus, have been investigated. The corrosion resistance in 0.1N H2SO4 increases in the order of alloys containing silicon, boron, carbon and phosphorus, and the corrosion resistance in 3% NaCl increases in the order of alloys containing boron, silicon or carbon, and phosphorus. Difference in the corrosion resistance of amorphous alloys containing various metalloids has been interpreted in terms of the difference in the formation rate of the chromium-enriched protective film on the surface of alloys.

The structure of liquid As2Se3 and GeSe2 by neutron diffraction

Abstract: The liquid structures of As 2 Se 3 and GeSe 2 have been investigated using the neutron diffraction patterns. In both cases the structure factor showed a low first peak maximum which follows a weak but apparent pre-peak at very low momentum transfer. It was also observed that the radial distribution function of both materials are characterized by the well-defined first neighbor shell because of the deep minimum on its right-hand side although in the liquid state. These results indicate that strong covalent bondings between unlike atoms in the solid state still remain when melting. Both the structure factor and the distribution curves of these alloys are, on the whole, similar to those in the amorphous phase which have already been examined. A slight difference in the coordination number, however, is found between amorphous and liquid phases of these materials.

Corrosion behavior of amorphous and crystalline Cu50Ti50 and Cu50Zr50 alloys

Abstract: Corrosion rates and anodic polarization curves of amorphous and crystallinecu 50 Ti 50 and Cu 50 Zr 50 alloys have been examined in various acidic, neutral and alkaline solutions. The amorphous alloys are very stable in acidic and alkaline solutions, but unstable in agressive chloride solutions. The corrosion resistance of these amorphous alloys is higher than that of the crystallized alloys. The high corrosion resistance of amorphous alloys is attributable to the high chemical homogeneity of amorphous alloys without localized crystalline defects such as precipitates, segregates, grain boundaries, etc. Metalloid elements play an important role in the corrosion behavior of amorphous alloys; the addition of phosphorus to amorphous CuTi alloy greatly increases the corrosion resistance, even in 1N HCl.

Nuclear spin lattice relaxation in the amorphous state: Towards an understanding

Abstract: Full reports concerning the nuclear relaxation of 11B, 23Na, 29Si and 77Se in the following glasses: B2O3, Na2B4O7, (Na2O)0.3(SiO2)0.7, Se are presented. Extended measurements confirm the existence of an efficient quadrupolar mechanism typical of glasses and show that the two-phonon process, responsible for the quadropolar relaxation in insulating crystals, is always negligible in our whole range of investigation. The relaxation rate T1−1, field independent in almost all cases, varies like T(1+γ) with 0 < γ < 1 between 1.2 K and 100 K but exhibits different behaviour at higher T depending upon the analysed material. Whereas T1 goes through a minimum at 300 K in B2O3, its T-dependence for 23Na in Na2B4O7 and (Na2O)0.3(SiO2)0.7 becomes faster than in the low-T regime. Several attempts made to interpret the data within the framework of the tunneling model, commonly invoked to explain the anomalous thermal properties of glasses, are reviewed in detail. It is concluded that the T1 results are best accounted for by a process consisting of the modulation of the electric field gradient (EFG) at the nuclear site by its nearest neighbour tunneling defect. This EFG fluctuation is shown to be dominated by interactions between defects and an average correlation time of about 10−9 sec is assigned to it. A complete T1 calculation requires the previous knowledge of the physical nature of the defects, so far not available. Each defect is then tentatively identified with a bridging oxygen atom tunneling between two potential wells, which gives the correct energy dependence for the EEG matrix element. A defect number of 1027 m−3 is shown to be consistent with the results.

The interstitial structure of vitreous silica

Abstract: A careful analysis of gas solubility data provides a unique tool for characterizing glass network geometry. The density of interstitial (solubility) sites in vitreous silica and its crystalline analog, high cristobalite, was reviewed. Close inspection of high cristobalite indicates that there are eight solubility sites per unit cell giving a bulk density of 2.34 × 1022 sites/cm3. A statistical thermodynamic analysis of experimental solubility data for helium and neon in high cristobalite correlates with this calculated value. Correcting this value for the slightly lower bulk density of vitreous silica gives an estimated density of solubility sites in vitreous silica of 2.22 × 1022 sites/cm3. Literature data for solubility site density (obtained from the same statistical thermodynamic analysis) depend strongly on the dissolved gas species. This implies a statistical distribution of effective site diameters. The probability distribution function is adequately represented as log-normal.

Vibrational spectroscopic studies of some lead silicate glasses

Abstract: Results are presented of measurements on the infrared and laser Raman spectra of some lead silicate glasses of general formula (PbO)x(SiO2)y. For the infrared spectra, the frequency dependence of the imaginary component of the complex dielectric constant was determined from a Kramers-Kronig analysis of the reflectance spectra of the glasses at normal incidence. Apparently the addition of small amounts of PbO to vitreous silica serves to modify the continuous three-dimensional silica network; whereas in those glasses with a high lead content, the influence of the cation, Pb2+, appears to lie between that of network former and network modifier.

Structural imperfections in amorphous metals

Abstract: The experimental evidence on the existence and nature of imperfections in amorphous metals is reviewed. It derives from observations of microscopic homogeneity, density measurements, positron annihilation studies, inhomogeneous flow and measurements of the various atomic transport coefficients: shear viscosity, diffusivity, structural relaxation rate and crystal growth rate. In contrast with crystal imperfections, which tend to be discrete and localized due to the constraints imposed by the long range periodicity, imperfections in amorphous metals tend to be more extended and diffuse. This is illustrated by the collapse and annihilation of a vacancy in a two-dimensional amorphous dynamic hard-sphere model, and the observations are generalized for other defects in three dimensions. It is concluded that the most fruitful way of describing defects in amorphous metals is as sites with small local perturbations of the “ideal” short range order, which allows local configurational rearrangements. It is suggested that different types of rearrangement (nearest neighbor changes, shear, volume annihilation) are possible at different sites, and that the concentration and lifetime of each type of site determines the time constant of the corresponding atomic transport process.

Physical and chemical evolutions occurring in glass formation from alkoxides of silicon, aluminum and sodium

Abstract: A multicomponent glass was obtained on using the new method of glass preparation from gel. The reactions occurring in the ethanolic solution of the parent alkoxides give condensation of the monomers so that hydrolysis of the remaining alkoxides groups followed by polycondensation appears to proceed on preformed oligomers. DTA, TGA, density and chemical durability measurements indicate that the transition from gel to the final glass is a continuous process. During the gel to glass conversion, scanning electron microscopy results show considerable compacting of the material.

Local order as determined by electronic and vibrational spectroscopy: Amorphous semiconductors

Abstract: Raman and infrared spectroscopy yield detailed information about the vibrational modes of solids. Photoelectron spectroscopy gives corresponding information on valence and core electronic states. From these spectroscopies the nature of the local atomic order can be inferred. Applications of these methods are illustrated for prototype tetrahedral and chalcogenide amorphous semiconductors. The retention of the crystalline short range order is manifested in the vibrational spectrum of pure amorphous silicon and the III–V compounds. Similar conclusions are obtained from the photoelectron spectra along with additional information on ring configurations. For Se and chalcogenide compounds (e.g. GeTe) vibronic and electronic spectral reveal differences between amorphous and crystalline short range orders. In addition to these intrinsic structures, the local environments and effects of additives, such as hydrogen and conventional dopants in amorphous silicon, are discussed.

Elastic properties of GeSe glass under pressure

Abstract: Elastic moduli and their pressure derivatives of GeSe glass of Ge content up to 30 at% have been determined at 15°C and 1 kbar by ultrasonic interferometry. Sound wave velocities νl and νt, and elastic moduli of this system, increased with Ge content and with pressure. The pressure derivatives K′0 and G′0 are in the range: K′0 = 7.9 ± 0.3 and G′0 = 2.2 ± 0.3. From this measurement it was concluded that GeSe glass is elastically normal. Elastic moduli of GeSe2 glass were estimated from the present data, and the K0 value was compared with those of silica-type glasses, e.g. SiO2, GeO2 and GeS2, based on the 4 3 power law. Elastic normality of GeSe2 glass was also elucidated from the estimated values of K′0 = 8 and G′0 = 2.2 for this glass, despite the structural similarity in Ge coordination between GeO2 and GeSe2 glasses.

Refractive index of chalcogenide glasses over a wide range of compositions

Abstract: The refractive index frequency dependence in the 1–11 μm range for AsSe, GeSe and AsGeSe glassforming systems and for three- and four-component systems obtained by substitution of antimony, bismuth, tin, lead, sulphur and tellurium for arsenic, germanium and selenium, being their periodic system counterparts, is investigated. A comparison of optical constants of chalcogenide glasses with those for other optical materials by means of constructing of an Abbe-type diagram in n2.0 − v2.0 coordinates is considered. The possibility of chalcogenide glass refraction calculation by means of additive formulas is discussed. It is shown that critical points on the property-composition curves coincide with structural region boundaries in all of the investigated systems. These data are considered as further experimental support for the polymer nature of chalcogenide glasses.

The order parameter model for structural relaxation in glass

Abstract: The order parameter model assumes that the state of a glass or liquid depends on T, P and a number of order parameters Zi. Structural relaxation is due to the kinetically impeded evolution of the order parameters following a rapid change in T or P. The linear relaxation function for the evolution of property Q (V or H) in response to a change in X (T or P) is of the form φQX = Σi giQX exp(−tτi). Expressions are derived for the qeighting coefficients giQX in terms of the dependences of V and H on the various order parameters. It is shown that giVT = giHP and that giVTgiHP/giVPgiHT = II, where II is the Prigogine-Defay ratio. The corresponding relations among the relaxation functions are φVT = φHP and φVTφHP/φVPφHT ⩽ II. The predictions of the order parameter model for structural relaxation are compared with and found generally to agree with existing literature data. A number of suggestions for future investigations to test this model are made.

Trap-controlled transient current injection in amorphous materials

Abstract: A theory for trap-controlled transient current injection is presented. The basic equations governing a carrier transport in amorphous materials are solved analytically for an arbitrary energy distribution of traps in the gap. Two special models, opposite with respect to each other, are treated: (1) the model of monoenergetic trapping level; and (2) the model of trapping levels distributed uniformly over a wide energy range. The theoretical results obtained are in agreement with existing transient current injection data on some amorphous materials.

A study of the crystallization of two non-crystalline CuZr alloys

Abstract: Two CuZr alloys, Cu-50 at% Zr and Cu-54 at% Zr, were splat quenched to the non-crystalline state using a piston and anvil device. The glass transition and crystallization temperatures, as well as the enthalpy releases observed during crystallization were measured using differential scanning calorimetry of as-splat specimens and specimens aged below Tg. Transmission electron microscopy and X-ray diffraction experiments were utilized to monitor phase and structural changes in the alloys as they were transformed to the crystalline state. The results of the investigation indicated that the non-crystalline to crystalline transformation of these two alloys in constant heating rate experiments above Tg was a two-step process. The initial step, which is associated with a large exothermic reaction, results in the appearance of crystallites in a matrix of non-crystalline material. The final step, associated with a smaller exothermic reaction, results in the total transformation of glass to the crystalline state and the formation of the equilibium crystalline phases. The effect of aging these splat-quenched non-crystalline alloys at temperatures below Tg was also investigated. It was determined from these experiments that crystallization does occur when the non-crystalline alloys are aged ∼ 15°C below Tg. However, the incubation time for crystalline nucleus formation was found to be substantially greater for the Cu50Zr50 glass. Finally, it was determined that the thermal stability of the aged glass relative to the spontaneous crystallization observed during the constant heating rate experiments above Tg decreases as a function of aging time.

Infrared structural studies of GeySe1−y glasses

Abstract: Bulk and thin-film specimens of GeySe1−y have been made the range 0 < y < 0.50. Transmission and reflectivity measurements at 77 and 300 K are reported in the range 50–400 cm−1. Mode assignment are made and an analysis is presented which supports the chain crossing model for y<0.33. The origin of several small spectral features is discussed in terms of crystal-like stacking within a volume of characterization dimension 10–20 A.

EXAFS studies of disordered solids

Abstract: The availability of intense sources of continuous radiation in the X-ray region has stimulated the development of a technique for structure determination which is particularly suited to studies of local environment in complex systems. The absorption cross section for the photoexcitation of an electron from a deep core state to a continuum state exhibits oscillations as a function of photon energy known as the extended X-ray absorption fine structure, or EXAFS. These oscillations result from the interference between the outgoing photoelectron wavefunction and that portion of it scattered back from neighboring atoms, and therefore contain considerable information about the local environment of the excited atom species. Analysis of the EXAFS can yield not only the distance but also the type and number of the nearest neighbors of the excited atom. The purpose of this paper is to discuss briefly the theoretical background and the practical aspects of EXAFS data analysis. The discussion is then illustrated through application of these techniques to EXAFS studies of arsenic dopants in amorphous silicon and of PdGe metallic glass.

Infrared spectra and structure of superionic conducting glasses in the system AgIAg2OMoO3

Abstract: Thin blown films of glasses with the mole ratio Ag2O/MoO3 = 1 in the system AgIAg2OMoO3 (or the pseudobinary system AgIAg2MoO4) show three absorption bands in the range 4000-200 cm−1; 875 cm−1 (w), 780 cm−1 (s), and 320 cm−1 (m, b), which are characteristic of tetrahedral MoO42− ions The glasses with the ratio Ag2O/MoO3 < 1 have two additional bands at 600 cm−1 (w) and 450 cm−1 (vw), which are characteristic of condensed ions of MoO4 tetrahedra, probably Mo2 O72− ions These glasses are thus composed of Ag+, I−, MoO42−, and probably Mo2O72− ions, and classified as “ionic” glasses containing one type of cations The presence of partial covalency in the Ag+… −OMo link and the influence of ion exchange of Ag+ with K+ on IR spectra are discussed The molar volume of the glasses with the ratio Ag2O/MoO3 = 1 is primarily determined by a fairly dense packing of the constituent anions, I− and MoO42−

Alloying effect on the viscous flow in metallic glasses

Abstract: The temperature dependence of viscosities near the glass transition is measured from the rates of thermal transformation for metallic glasses PtNiP, PdNiP, NiPBA1 and (Fe, Co)PBA1. Alloying among metallic elements which lowers the glass transition temperature Tg lowers the ideal glass transition temperature T0, but raises the residual configurational entropy Sg and the activation energies for “diffusive” rearrangement, Δμ ∗ , of the alloying glasses, while compositional ordering associated with the addition of metalloids raises the Tg and T0 and lowers the Sg and Δμ ∗ . Results are correlated to the atomic ordering and stability of the glasses. The extracted free volume and the critical diffusive volume are much smaller, by a factor of 4, for metallic glasses than for many other glasses.

High corrosion resistance of amorphous FeMo and FeW alloys in HCl

Abstract: The corrosion rates and electrochemical behaviors of amorphous FexMo13P7C and FexW13P7C alloys have been examined in 1 N HCl, in comparison with corrosion behaviors of amorphous FexCr13P7C alloys. The chromium, molybdenum and tungsten increase significantly the corrosion resistance of amorphous FePC alloys. Amorphous FeCr, FeMo and FeW alloys are immune to pitting corrosion in 1 N HCl. The corrosion resistance of amorphous iron alloys containing Cr, Mo or W of 5 at% or more increases in the order of FeW, FeMo and FeCr alloys.

Defect chemistry and states in the gap of lone-pair semiconductors☆

Abstract: The development of models for states in the gap of amorphous semiconductors is reviewed. It is pointed out that early models, the ideas of Anderson, and those of Street and Mott all are essentially ad hoc . On the other hand, ideas developed by Kastner, Ovshinsky, and Kastner, Adler, and Fritzsche, are based on the underlying symmetry of the system: the chemical bonding. Recent successes of the valence-alternation model for defects in lone-pair semiconductors are summarized. Although many results are easily explained, the photoluminescence process appears much more complicated than previously believed.