Bromine-associated defect centers in irradiated ZrF4-based glasses

ZrF₄---BaF₃---LaF₃---AlF₃---NaF (ZBLAN) glasses batched with small additions of NaBr were examined by electron spin resonance (ESR) following their exposure to 100-keV X-rays at 77 K or ⁶⁰Co γ-rays at room temperature. Two sets of radiation-induced ESR spectral features observed for certain of the doped glasses, but not for the undoped control samples, are identified, with the assistance of computer line shape simulation methods, as arising from B₂⁻ and FBr⁻ molecular ions. It is suggested that ESR detection of these species may serve as a diagnostic for bromide impurities in fluoride glasses. The method is used to investigate the retention of bromides by ZBLAN glasses melted under various conditions.     

A model for the Ge–O coordination in germanate glasses

A successful model for the composition-dependence of the coordination number in borate glasses has been adapted for germanate glasses. The predicted coordination number has then been compared with neutron diffraction results for caesium and sodium germanate glasses, which show strong evidence of a significant growth and decline in the coordination number as modifier is added to the glass. The comparison with the predictions of the model support an interpretation in which the predominant higher coordination of germanium is five-fold, not six-fold.     

A Comparison with empirical results of the interstitialcy theory of condensed matter

A quantitative and comprehensive comparison of the predictions of the interstitialcy theory of condensed matter with empirical results for characteristic and universal properties of simple dense liquids and glasses is given. The most distinctive thermodynamic properties are the unusually large shear susceptibilities and entropies. These are essentially the same for interstitials in crystals, liquids and glasses.     

Fast ion transport in oxide glasses

The discovery of fast ion transport in solids has in recent years stimulated much interest in the scientific community both with regard to obtaining improved understanding of the phenomenon and applying such materials in advanced battery systems. More recently the phenomenon of fast ion transport has also been observed to occur in increasing numbers of amorphous systems. In this paper we review recent transport data obtained in over 100 glasses which appear to exhibit exceptionally high Ag, Li, Na and F ion conductivities at temperatures far from the melting points. We also include results recently obtained in our laboratory for glasses in the lithium borate system. Common characteristics of these glasses are summarized and are compared with predictions of classical diffusion theory. Relatively low and composition-independent values of σ₀ indicate poor agreement with a simple isolated-ion diffusion model, while composition-dependent activation energies are related to structural changes. Some glasses share similar properties with their crystalline counterparts while others do not. This is discussed in terms of the relative disorder already existing within the crystals. The need for improved characterization of glasses is also discussed.     

Elongation and spheroidization of phase-separated particles in glass

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.     

The application of small polaron theory to transition metal oxide glasses

A review of the experimental data on a range of transition metal oxide glasses shows a strong relationship between activation energy and site spacing for glasses based on V₂O₅. The behaviour confirms that transport in amorphous V₂O₅ and vanadate glasses is by adiabatic hopping with an appreciable polaron bandwidth, while non-adiabatic hopping between isolated centres is apropriate for glasses based on most other transition metal oxides. The behaviour of vanadium is linked to the retention of a well-defined VO^5? structural unit in the glass network and to optimisation of the packing of this unit within the glass.     

Mechanical properties of NiFe based alloy glasses

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.     

Evaluating Mohr–Coulomb yield criterion for plastic flow in model metallic glasses

The yielding of metallic glasses (MGs) is generally considered to be dependent on pressure or normal stress, which is usually described by the Mohr–Coulomb yield criterion. Experimentally, the orientation of the shear band angle or fracture angles in MG is roughly 40° to 42° in uniaxial compression and 50° to 59° in uniaxial tension, both with respect to the loading direction. However, the amounts of deviation from the maximum shear stress direction (45°) under uniaxial tension and compression are quite different, which seems to be inconsistent with the Mohr–Coulomb yield criterion. Here we carried out molecular dynamics simulations to study the incipient plasticity of model Dzugutov glasses subjected to various uniaxial and biaxial stress states. The yield points are defined as the sharp rise of the population of atoms participating in non-affine deformations, in addition to the traditional offset method. The yield surfaces thus created agree well with the Mohr–Coulomb yield criterion. Furthermore, the orientations of the embryonic shear bands (instead of the fully-grown shear bands, or fracture planes) are measured, which also follow predictions based on the Mohr–Coulomb criterion. Our simulation results call for orientational measurement of incipient shear bands in experiments to critically examine the Mohr–Coulomb yield criterion in MG systems.     

二维层状金属碘化物的制备及光电磁器件的应用进展

二维金属碘化物,呈现范德瓦耳斯层状堆垛结构,平均原子序数大,具有合适的能带间隙、强的磁电耦合效应,是一种新型的光电探测材料,能被用于制备高能射线探测器,且一些磁性二维金属碘化物能被用于磁电器件。由于其在光电磁器件方面的潜在应用,近期成为了低维材料研究的热点。并且在材料制备方面,层状金属碘化物一般熔点较低,制备条件温和简单,可用于二维层状材料生长机理的研究。本文首先介绍了层状金属碘化物的结构、性质,然后着重阐述了二维层状金属碘化物的制备方法,最后讨论了层状金属碘化物在光电磁器件方面的应用。期望读者对二维层状金属碘化物有更深入的了解,更好地推动二维层状金属碘化物的应用。     

Crystallization statistics. A new tool to evaluate glass homogeneity

We propose and test a new method to evaluate the chemical homogeneity of glasses based on statistical analyses of the volume distribution of crystals developed through thermal treatments. The method is based on the fact that each volume element of a glass piece subjected to a proper thermal treatment should exhibit a certain number of crystals, which is dictated by its chemical composition. We performed numerical simulations to interpret the experimental results obtained for some glasses, and demonstrate that this new method is adequate to determine the degree and scale of heterogeneity of glasses that display volume crystallization.     

An X-ray diffraction study of the structure of barium fluorozirconate and fluorohafnate glasses

Two binary glasses have been prepared in the ZrF₄BaF₂ and HfF₄BaF₂ systems, containing 64 mol.% ZrF₄ and 66 mol.% HfF₄, respectively. The two plate-like glass samples were studied by X-ray diffraction, and the resulting data were subjected to a Fourier analysis. The pair correlation functions thus obtained yield a ZrF (or HfF) nearest neighbor coordination number of ～ 7.6 for both glasses. The second peak in the correlation functions has been assigned to a combination of FF and BaF interactions, while the third peak is probably due in part to ZrZr (Hf-Hf) interactions. The diffraction data have been compared with the predictions of structural models for barium fluorozirconate and fluorohafnate glasses.     

Prediction of maximum homogeneous nucleation temperature for crystallization of metallic glasses

A graphical method is used to find out the viscosity at any temperature between the melting temperature and glass transition temperature, which in turn is used to predict the temperature corresponding to the maximum in homogeneous nucleation rate. The present approach does not need any critical experimental data and the model predictions are very close to those experimentally measured in known metallic glasses. This model can be applied for both pure metals and glass forming alloys.     

Entropy model for flow behavior in metallic glasses

A model based on configurational entropy is proposed to interpret phenomenologically the flow behavior in metallic glasses. From available thermal and viscosity measurements, the effect of temperature, strain rate, pressure and composition on flow stress of metallic glasses have been evaluated. The values calculated agree fairly well with experimental data. The failure of atomic rearrangements to relieve the imposed strain results in stress concentrations and local structural disorder at deformed regions. This leads to inhomogeneous flow and softening of the metallic glasses.     

Direct calculation of Young's moidulus of glass

An equation has been derived for the direct calculation of the Young's modulus of oxide glasses from their chemical compositions. The method is based on a consideration of dissociation energy of the oxide constituents per unit volume and the packing density. For borate glasses, the ratio of four-coordinated to three-coordinated borons must be taken into consideration. Excellent agreement is obtained between calculated and measured values of Young's modulus for over thirty different glasses.     

Lutetium and thulium based rare earth bulk metallic glasses

We report the formation of lutetium and thulium based bulk metallic glasses based on the correlations between the thermodynamic, kinetic, elastic and other properties of metallic glasses. The two novel rare earth based bulk metallic glasses (REBMGs) exhibit excellent glass formation ability, high elastic moduli, considerable smaller Poisson’s ratio, high thermal stability, and even higher mechanical strength than that of typical high strength Zr-based BMGs. The reasons for the properties of the Lu- and Tm-based and other REBMGs are discussed.     

Glass microstructure evaluations using high temperature mechanical spectroscopy measurements

This paper compares the microstructures of several glasses by measuring the Young’s modulus and internal friction as a function of temperature, using the impulse excitation technique (IET). IET is based on the analysis of the resonant vibration of a solid material sample, induced by an impulse excitation. IET determines the mechanical resonant frequencies (f_r) from which the elastic moduli can be calculated, and for each f_r the corresponding internal friction (Q⁻¹). It was found that the stiffness of quartz and borosilicate glasses increases with temperature. The stiffness of soda-lime and alumino-silicate glasses decreases with the increase of temperature. The change of stiffness of quartz and alumino-silicate glasses during heat-treatment is reversible, but that of borosilicate and soda-lime glasses is not. Explanations for the irreversibility are suggested based on the Q⁻¹-features of the glasses. Diffusion of network modifier ions in the glass network holes is proposed to cause a non-reversible stiffness change, whereas localised anelastic relaxation of network modifier ions leads to a reversible stiffness change.     

Chemical annealing of oxygen hole centers in bulk glasses

Chemical annealing of the oxygen hole centers in bulk glasses by using hydrogen (or deuterium) has been extensively reported in the literature. Hydrogen chemically anneals these defect centers by reacting with them to form hydroxyl species. We have here presented a reaction-diffusion model to predict the chemical annealing rates of these defect centers in bulk glasses. The model considers diffusion of hydrogen (or its isotopes, e.g., deuterium) through the glass and its simultaneous reaction with the oxygen hole defect centers, resulting in the formation of the hydroxyl species in the glass. The predictions from the model are in excellent agreement with the experimental data presented in [J. Appl. Phys. 60 (12) (1986) 4325]. The model also establishes the precise connection between the dependence of effective hydrogen diffusivity on its intrinsic diffusivity, temperature, hydrogen and defect concentration. We also discuss methods for understanding chemical annealing behavior at high temperatures, where hydrogen not only reacts with the oxygen hole centers, but also with the silica matrix. For the case where hydrogen interaction with the silica matrix is the dominant reaction, a reaction-diffusion model is presented and the equilibrium constant (between 500 and 900 °C) for the reaction is estimated by comparing the predictions from the model with the hydroxyl profiles reported in [Appl. Phys. Lett. 47 (3) (1985) 328; J. Appl. Phys. 61 (12) (1987) 5447].     

X-ray scattering studies of the short and medium range ordering in AsxTe1−x glasses

The structures of As_xTe_1−x(x = 0.2, 0.4, and 0.5) glasses are studied using the differential X-ray anomalous scattering technique. The partial distribution functions have also been obtained for the stoichiometric As₂Te₃ glass, giving information on the medium range ordering. All the glasses show chemical disorder to differing extents, the As₂Te₃ glass being the most disordered. The Te coordination number undergoes a change at x=0.4 where it is 2.5 compared with 2 for AsTe. This change indicates the existence of about 50% of threefold Te sites in the stoichiometric glass, as well as in the Te-rich glasses. Some of the physical properties of the glasses may be explained based on these results.     

Oxynitride glasses, their properties and crystallisation - a review

Oxynitride glasses were first discovered as intergranular phases in silicon nitride based ceramics in which the composition and volume fraction of such oxynitride glass phases determine the properties of the material. In particular they have been shown to control high temperature mechanical properties. The desire to understand the nature of these grain boundary phases has resulted in a number of investigations on oxynitride glass formation and properties which have shown oxynitride glasses to possess refractory behavior and higher, elastic modulus, viscosity and hardness compared to the corresponding oxide glasses. This paper provides a review of the preparation and characterisation of oxynitride glasses and outlines the effect of composition, especially nitrogen content, on properties such as glass transition temperature, hardness, Young’s modulus and viscosity.     

Infrared absorption and structure of chlorophosphate glasses

The infrared absorption spectra of glasses in the NaPO₃---ZnCl₂ system were studied in the range of 4000−350 cm⁻¹. The results were interpreted using a local mode approach based on existing frequency charts. A continuous structural breakdown of the glass structure occurs upon addition of ZnCl₂ to the NaPO₃ polymer, with a consequent decrease in its chain length. The actual structure of the chlorophosphate glasses is discussed on the basis of possible anionic groups present. There is evidence for the occurrence of mixed oxychloride coordination shells. The density and glass transition temperature of the glasses were also determined.