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.     

Replica field theory for anharmonic sound attenuation in glasses

A saddle-point treatment of interacting phonons in a disordered environment is developed. In contrast to crystalline solids, anharmonic attenuation of density fluctuations becomes important in the hydrodynamic regime, due to a broken momentum conservation. The variance of the shear modulus Δ² turns out to be the strength of the disorder enhanced phonon-phonon interaction. In the low-frequency regime (below the boson peak frequency) we obtain an Akhiezer-like sound attenuation law Γ ∝ Τω². Together with the usual Rayleigh scattering mechanism this yields a crossover of the Brillouin linewidth from a ω² to a ω⁴ regime. The crossover frequency ω_c is fully determined by the boson peak frequency and the temperature. For network glasses like SiO₂ at room temperature this crossover is predicted to be situated one order of magnitude below the boson peak frequency.     

Vanadium contribution in different glasses in view of the ligand field theory

Spectrophotometric and magnetic studies were carried out on different glasses containing known amounts of V₂O₅ in order to throw some light on the valence states and coordination of vanadium in such glasses (in view of the ligand field theory).The results obtained were found to be in agreement with those of aqueous solutions, e.g. [V(H₂O)₆]³⁺, [VO(H₂O)₅]²⁺, and VO²⁺ which showed a slight difference in band positions due to the distortion as well as the difference between the ligand field strength of glass and water. Generally it may be concluded that vanadium is present in borate and silicate glasses in three possible valencies, V⁵⁺, V⁴⁺ and V³⁺, while in phosphate glasses it is generally present in two valencies V⁴⁺ and V³⁺.     

Advanced approaches to functional glasses

Two key technologies were reviewed to propose advanced approaches to functional glasses. The first is combinatorial glass research system. The combinatorial glass research system consists of an automatic batch preparation apparatus, two types of automatic apparatuses for melting of glass batches in crucibles and two types of apparatuses for evaluation of thermal stability, namely a DTA apparatus equipped with sample changer and a temperature gradient furnace. The combinatorial system was effective in searching for new glass forming systems and to speed up preparation of glass samples. The second technology is a new technique based on anodization of thin film aluminum. The anodic oxidation of sputtered aluminum film was applied to preparation of alumina nanostructures possessing nanopore arrays on glass surfaces. Oxides or metallic compounds could be introduced into nanopores using sol–gel technique or electro deposition technique. The introduction of TiO₂ by sol–gel process yielded high performance photo catalysts and the impregnation of Ni metal or Fe–Pt alloy into nanopores by electro deposition gave high density magnetic rod array of about 1 Tbit/in.². The new processes based on anodic oxidation of sputtered aluminum and titanium would be very effective to develop functional nanostructure layers on glass surfaces.     

Silver to erbium energy transfer in phosphate glasses

Silver and erbium co-doped sodium metaphosphate glasses were prepared by standard melt-quench technique. The samples underwent heat or 355 nm laser radiation exposure treatments in order to promote silver clustering. Absorption and photoluminescence spectra allowed distinguishing several active species related to silver, whose concentration in the samples depends on the treatment. A broad emission of silver related species has been detected in the visible region of the spectrum. The excitation spectra of the band and of the 1.5 μm luminescence of Er demonstrated energy transfer from Ag aggregates to Er. Differently from silicate glasses, in phosphates the silver to erbium energy transfer is very weak.     

Sintering inhomogeneous glasses: Application to optical waveguides

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

The barrier–cluster model applied to chalcogenide glasses

The aim of this article is to familiarize the readers with barrier–cluster model of the non-crystalline semiconductors, which is based on the assumption that in non-crystalline semiconductors, there exist micro-regions separated from each other by potential barriers. We suppose that these micro-regions in chalcogenide glasses are created by closed clusters. This model allows explanation not only of a number of important optical and electrical features of chalcogenide glasses, but also the results of X-ray structure measurements and ESR experiments. This concept gives a new look at the density of states within the forbidden band and at the exponential tails of the optical absorption.     

Lattice vibrations of glasses

Measurements of the thermal conductivity of glasses have led to a model of their lattice vibrations. They are not described as waves, as they are in crystals, but as uncorrelated harmonic motions of individual atoms. The lattice vibrations of fully cerammed, crystalline glass ceramics have been found to be either waves, as in crystals, or vibrations of individual atoms, as in glasses. The understanding of the cause for this different behavior has been aided by studies of systematically doped crystals.     

Crystallization of gel-derived glasses

The crystallization behaviors of gels, gel-derived glasses and melted glasses are reviewed, and comparisons are effected among the observed behaviors. These comparisons are related to the behavior expected on the basis of models for the nucleation frequency and crystal growth rate, as well as the anticipated differences in material characteristics among gels and the two types of glasses. Different temperature regimes are identified where different comparisons between melted and gel-derived materials are expected. It is concluded that a need exists for more definitive, detailed data on the crystallization behavior of gel-derived materials.     

Optical spectra of glasses

This presentation focuses on several ways in which structural and dynamical information can be obtained from optical spectroscopy by examining results of infrared and Raman studies of selected oxide glasses. Though these studies represent only a small fraction of the work on most of the glasses mentioned, and many other important forms of optical spectroscopy are not treated here, they illustrate useful approaches to the study of amorphous materials by optical methods.     

Onset of rigidity in glasses: From random to self-organized networks

In this paper, we review the signatures of a new elastic phase that is found in glasses with selected compositions. It is shown that in contrast with random networks, where rigidity percolates at a single threshold, networks that are able to self-organize to avoid stress will remain in an almost stress-free state during a compositional interval, a Boolchand intermediate phase, that is bounded by a flexible phase and a stressed rigid phase. We report the experimental signatures and describe the theoretical efforts that have been accomplished to characterize the Boolchand intermediate phase. We illustrate one of the methods used in more detail with the example of Group III chalcogenides and finally suggest further possible experimental signatures of self-organization.     

Formation of quasicrystals and metallic glasses in relation to icosahedral clusters

It has been widely accepted that quasicrystals and at least some metallic glasses are built up with icosahedral clusters. Information about the cluster structures can be obtained from crystalline counterparts. In this paper, we will describe the formation rules of bulk metallic glasses, originally developed for quasicrystals, by combining cluster structures with phase diagram features. We will introduce the e/a-constant and e/a-variant criteria for ternary systems, and e/a-constant and atomic size constant criteria for quaternary systems. We will show how the glass forming composition optimization is realized by applying these rules in the Zr–Al–Ni and Zr–Al–Ni–Cu systems. In both systems the optimized glass-forming composition is related to a common binary icosahedral cluster Zr₉Ni₄ derived from the fcc Zr₂Ni phase. A novel route to reach amorphous forming composition is also attempted by mixing Al-based quasicrystal-forming compositions with Zr.     

Radial segregation due to weak convection in a floating zone

The influence of weak convection, caused by surface tension forces, on radial dopant segregation occurring in crystals grown under microgravity conditions is studied numerically. The geometry considered corresponds to a floating-zone configuration with partially coated melt surfaces consisting of small evenly distributed spots of free surfaces. In order to distinguish dopant distribution due to weak convection clearly from distribution due to diffusion the spots only cover one quarter of the periphery. Thus, surface tension-driven convection is allowed only over one quarter of the floating-zone configuration resulting in an asymmetric dopant distribution. The percentage of free surfaces present is varied in order to alter the Marangoni flow rates. The maximum dopant concentration due to radial segregation is plotted as a function of a certain convection level. The results of the present numerical study are supposed to be used to design corresponding space experiments launched at the end of the year 2000.     

Electrical properties of phosphate glasses

The electrical properties of glasses in the Na₂OP₂O₅, Ag₂OP₂O₅ and (1?x)Na₂OxAg₂OP₂O₅ systems have been measured over a range of temperature and composition.The properties of the Na₂OP₂O₅ and Ag₂OP₂O₅ glasses have been compared within the phosphate system as well as with silicate glasses. The silver-containing glasses show higher conductivity and lower temperature coefficients when compared with the sodium-containing glasses. A maximum in the room temperature resistivity of the (1?x)Na₂O?xAg₂O?P₂O₅ system was found around the mole ratio of 0.16:0.84 Ag₂O:Na₂O, indicating a mixed-alkali effect. A similar effect was seen in the tan δ, but not in the T_g-against-composition plots. A linear relationship was noted for the tan δ-versus-log₁₀ (resistivity) plot, as has been seen in other glass-forming systems.