The effect of annealing on the structure of Ge20Te80 glass: atomic-scale imaging using a scanning tunnelling microscope

The effect of annealing on the structure of Ge₂₀Te₈₀ glass has been examined in atomic-scale images obtained using a scanning tunnelling microscope (STM). The STM image has been able in part to answer the question as to how the atomic structure is changed by low-temperature (<T_g) annealing, whereas the effect of annealing on the structure was never observed in previous neutron diffraction study. Scanning tunneling microscope images as large as ≈ 100 nm² have provided atomic-resolution ridges ≈ 3 nm in length. The nearest neighbour distance between peaks in each alignment is equal to ≈ 0.5 nm; the alignments in parallel with each other are a distance of ≈ 0.7 nm apart. These ridges can generate a surface associated with pseudo flatness, the size of which is less than ≈ 10 nm². Thus, the effect of annealing on the structure of as-quenched Ge₂₀Te₈₀ glass has given rise to intermediate-range order in a region less than ≈ 10 nm². The atomic configuration of annealed Ge₂₀Te₈₀ glass is inhomogeneous in the range < 10 nm².     

Numerical simulation of thermoconvective flows and more uniform depositions in a cold wall rectangular APCVD reactor

At atmospheric pressure, the usual flow conditions in the cold wall horizontal rectangular thermal CVD reactors correspond to steady longitudinal thermoconvective rolls that make non-uniform vapour depositions, in shape of longitudinal parallel ridges. In order to get more uniform depositions, the pressure is generally lowered under the atmospheric pressure to promote forced convection flows, instead of mixed convection ones. In the present paper, using three-dimensional direct numerical simulations, we propose and analyse a method to get uniform deposition without lowering the pressure in the reactor. It consists in adequately exciting the parallel thermoconvective rolls at channel inlet to make them unsteady, periodic and sinuous in order to get a uniform time average of the deposition. This method is shown to be adapted for the horizontal and rectangular APCVD reactors with large longitudinal and transversal aspect ratios, when the Reynolds number of the gas flow is O(100), whatever the value of the surface Damköhler number. This situation is encountered in the online or scrolling APCVD reactors used to deposit coatings on float glass in the flat glass industry for instance. The simulations are based on simplified models for the transport equations (Boussinesq model) and the kinetics of the heterogeneous reactions (deposition model of silicon from hydrogen and silane: SiH₄→Si+2H₂).     

Amorphous phase separation and crystallization in a lithium silicate glass prepared by the sol-gel method

A 10Li₂O---90SiO₂ (mol%) gel-glass has been prepared by using tetramethyl orthosilicate and lithium iso-propoxide as starting materials. The phase separation and crystallization behaviour was compared with the corresponding conventionally melted glass using DTA, X-ray diffraction and TEM. The same crystallization phase was found in both the gel glass and melted glass upon heating above 650°C. However, the rate of crystallization in the gel-glass was higher than in the melted glass. TEM revealed amorphous phase separation in the gel glass and melted glass. However, the morphologies were different, an interconnected microstructure being observed in the gel glass and a droplet structure in the melted glass. These differences can be partly attributed to differences in OH content. Other potential influencing factors are also considered. After 650°C for 2 h lithium disilicate crystals were observed in the volume of the gel glass by TEM. As the crystals grew they absorbed Li₂O from the surrounding lithia-rich amorphous phase so that silica-rich (lithia depleted) diffusion zones formed around them.     

Morphology of cells at the grain boundaries of an anisotropic material

We have studied the formation of ridges on the grain boundaries of an anisotropic crystal known as pivalic acid. These results are compared with very well studied another isotropic material succinonitrile to understand the formation of defects in anisotropic crystals. Preliminary results show that along with anisotropy, orientation has a large influence on the morphology of solid-liquid interface formed after planar interface becomes unstable. The formation of ridges and hence the defects at the interfaces are more favoured in highly anisotropic materials.     

The effect of minor additions of P2O5 on some phase transformations of plate glass

The effects of additions of several tenths of a per cent of P₂O₅ on the devitrification behavior of plate glass with the primary phase of tridymite are different from that with the primary phase of wollastonite. But they still follow the phase equilibrium relationship of multicomponents. By examinations with TEM, and measurements of electrical resistivities, it is indicated that phase separation of plate glass may be promoted with a minor addition of P₂O₅. Through XPS measurements it is further suggested that such an addition causes a certain change in the structure of the plate glass.     

Optical and structural properties of calcium silicate glasses

A detailed analysis of the reflectance and transmittance spectra of a set of glasses allowed retrieval, besides the optical functions, of quantitative information on the structural changes occurring in the calcium silicate glass system. The occurrence of a threshold around 46% of CaO content is confirmed and is connected to the appearance in the glass structure of a new configuration of calcium cations. It was also shown that the calcium cations that exceeds the compositional threshold does not act as modifiers of the silicate network since they form Ca–O–Ca bonds in the glass structure. Obvious changes occurring at the threshold in the physical or chemical properties can be understood in the light of the structural changes observed in the system.     

InxGa1−xAs/GaAs quantum wire structures grown on GaAs (100) patterned substrates with [001] ridges

In_xGa_1−xAs/GaAs (x = 0.12-0.23) quantum well (QW) structures were grown by molecular beam epitaxy (MBE) on [001] ridges with various widths (1.1-12 μm) of patterned GaAs (100) substrate. The smallest lateral width of the InGaAs/GaAs quantum wire (QWR) structures was estimated to be about 0.1 μm by high-resolution scanning electron microscope (SEM). The In contents of the grown InGaAs/GaAs QWs on the ridges were studied as a function of ridge top width (ridge width of the MBE grown layer) by cathodoluminescence (CL) measurements at 78 K. Compared to the InGaAs QW grown on a flat substrate, the In content of the InGaAs/GaAs QW on the ridge increases from 0.22 to 0.23 when the ridge top width decreases to about 2.9 μm, but it decreases steeply from 0.23 down to 0.12 with a further decrease of the ridge width from 2.9 to 0.05 μm. A simulation of MBE growth of InGaAs on the [001] ridges shows that this reduced In content for narrow ridges is due to a large migration of Ga atoms to the (100) ridge top region from {110} side facets.     

Glass formation in the ternary Zr-Zr2Cu-Zr2Ni system

Glass formation and glass forming ability in the ternary Zr-Cu-Ni system were studied systematically. For Cu mold casting, the critical size for glass formation was found to be 1 mm in a relatively wide composition range of 56-64  at.% Zr, 16-26 at.% Cu and 16-24 at.% Ni. The phases competing with glass formation were found to be Zr, Zr₂Cu and Zr₂Ni. Despite the fact that Cu and Ni are completely miscible with each other, they cannot be regarded as simple substitution to each other in the case of glass formation in the Zr-Zr₂Cu-Zr₂Ni ternary eutectic system.     

Inward cationic diffusion in glass

We report that inward diffusion of network-modifying divalent cations can occur in iron-containing silicate glasses when they are treated in a reducing atmosphere near the glass transition temperature. As a result of the inward diffusion, a silica-rich nanolayer forms on the glass surface, which increases the hardness and chemical durability. The thickness of the layer can be controlled by varying the heat-treatment conditions. We clarify the mechanism of the inward diffusion and calculate the diffusion coefficient for the network-modifying divalent cations. We demonstrate that the diffusion of these ions is the rate-limiting factor for the reduction process.     

Chemical diffusion and crystalline nucleation during oxidation of ferrous iron-bearing magnesium aluminosilicate glass

Rutherford backscattering spectroscopy (RBS) and transmission electron microscopy (TEM) have been used to evaluate the mechanism and kinetics of oxidation of a Fe²⁺-doped MgO---Al₂O₃---SiO₂ glass (with nominal composition along the enstatite-cordierite-liquid divariant) which was heat treated in air under the time and temperature ranges 10–150 h and 700–800°C, respectively. The results clearly demonstrate that oxidation occurs by a cation diffusion process: specifically, the divalent cations diffuse from the interior of the glass to the free surface where they subsequently react with environmental oxygen to form a two-phase, MgO---(Mg, Fe)₃O₄ crystalline layer which covers the (divalent cation-depleted) glass. Oxidation of some Fe²⁺ within the glass occurs via the inward flux of electron holes (a counterflux to the divalent cation diffusion required to maintain charge neutrality of the glass); this internal oxidation results in the fine-scale ( 1–5 nm), homogeneous nucleation of crystalline (Mg, Fe)₃O₄ within the divalent cation-depleted layer of the glass. Chemical diffusion of an oxygen species is thus demonstrated to be a slower, parallel kinetic process which is not required for oxidation to occur in this material. A first-order analysis of oxidation kinetics in the glass is presented.     

2H NMR studies of supercooled and glassy aspirin

Acetyl salicylic acid, deuterated at the methyl group, was investigated using ²H NMR in its supercooled and glassy states. Just above the glass transition temperature the molecular reorientations, studied using stimulated-echo spectroscopy, demonstrated a large degree of similarity with other glass formers. Deep in the glassy phase the NMR spectra look similar to those reported for the crystal and below 20 K they are indicating that rotational tunneling plays a role. Measurements of the spin–lattice relaxation times for temperatures below 150 K reveal a broad distribution of correlation times in the glass. The dominant energy barrier characterizing the slow-down of the methyl group motion is significantly lower than the well-defined barrier in the crystal.     

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.     

Influence of alumina addition on crystallization and texturing behavior of LaBGeO5 glass

The structure and crystallization behavior of glasses with 25La₂O₃ · 25B₂O₃ · 50GeO₂ composition, melted in platinum (P glass) and corundum (A glass) crucibles, were studied by DTA, X-ray diffraction and FTIR spectroscopy. The Al₂O₃ dissolved from corundum crucible in the A glass was estimated to be in the range 5–7 wt%. This alumina content had almost no influence on glass transition temperature but strongly affected the structure and crystallization behavior of the A glass. In fact, the P glass showed good texture-forming ability: high quality textured glass-ceramic plates based on stillwellite-like LaBGeO₅ crystals were easily obtained. On the contrary, the presence of alumina stabilized the A glass from which binary phases crystallize first, and only afterwards they are transformed in stillwellite by secondary crystallization: so in this glass texturing is hindered. Crystallization and texturing behavior of P and A glasses were well related to FTIR data. P glass contained both threefold and fourfold coordinated boron while in the A glass the presence of aluminum forced boron to assume almost exclusively threefold co-ordination. Hence the easier crystallization of stillwellite phase and the good textures obtained from the P glass contrary to the A glass, can be well understood since all boron atoms have tetrahedral co-ordination in stillwellite LaBGeO₅ crystal.     

A new class of infrared transmitting glass-ceramics based on controlled nucleation and growth of alkali halide in a sulphide based glass matrix

Oxides-based glass-ceramics have been intensively studied and while they exhibit exceptional thermo-mechanical properties, their transparency in the infrared is limited to the 3 μm region. In this paper we describe a new type of glass-ceramics which are transparent up to 11 μm and based on the controlled nucleation and crystallization of cesium chloride sub-micron particles inside a Ge-Sb-S glass matrix. The evolution of the optical transmission versus annealing time and temperature has been investigated. Observations under scanning electronic microscopy as well as X-ray diffraction indicate that the crystalline phase is a primitive cubic cell with a parameter slightly inferior to that of pure CsCl and that the grain sizes are about 100 nm. A preliminary test on fracture propagation shows a much better resistance of glass-ceramics to cracks than the corresponding pure glass matrix.     

Amorphous vanadium pentoxide

Structure and semiconducting properties of amorphous vanadium pentoxide obtained by splat cooling have been studied. Long-range order appears suddenly when the oxide is heated to about 200°C. The short-range order of the glass seems to be almost identical to that encountered in orthorhombic V₂O₅. ESR spectra are typical of localized electrons, they show that the electronic mobility is lower in the amorphous state. The hopping frequency of the unpaired 3d electron from a V⁴⁺ to a V⁵⁺ ion increases when long-range order is established. This phenomenon is related to the electrical resistivity which is about one order of magnitude higher in the glass.     

Finite element analysis of identation on glass(II)

The shapes of indentations made on Na₂O-SiO₂ glasses with differentalkali content were studied by using the finite element method to estimate the basic mechanical properties of the glasses. The yield stress Y, densification factor ¦α¦ and hardenining coefficient H′ of the glasses were successfully estimated, which cannot be determined by the usual methods. The densification factors ¦α¦ obtained for each glass at various temperatures did not change much, and it was considered that they were determined mainly by the structure of the glass. The value of ¦α¦ approached zero when the content of Na₂O exceeded 35 mol%, which corresponds to the stuffing effect of alkali ions in the glass network. The yield stress Y estimated by this method differed considerably from the tensile strength of the glass. The difference was especially large for the glass with low alkali content, i.e. the larger the value of ¦α¦ the larger the difference between Y from the hardness and the tensile strength. The reason for this could be partly attributed to the difference in the mode of deformation.     

Molecular dynamics simulation for the rapid solidification process of MgO–Al2O3–SiO2 glass–ceramics

A molecular dynamics simulation method was used to study the effects of the microstructure on the solidification process of different cooling rates in the MgO–Al₂O₃–SiO₂ glass–ceramics with cordierite as the main crystalline phase. The reasons for changes in the microstructure during the solidification process were analysed by the radial distribution function curve, the bond angular distribution, the coordination number and the volume changes. The results showed that the cooling rate greatly affected the crystallisation process and the glass transition process. When the cooling rate was too fast, the atoms could not undergo a massive displacement before they were “frozen”, and the ability of atoms to achieve an equilibrium position was limited. Some amorphous phases were formed as a result of the disorder of the atomic arrangement, then some crystalline phase precipitated from the vitreous, and a glass–ceramic material was eventually formed.     

On the limitation of quantitative measurements using transmission electron microscopy

This work discussed the limitation of (scanning) transmission electron microscopy (TEM/STEM) techniques in quantitative measurements in electron-beam-sensitive silicate glasses and glass ceramics. Electron beam induced damages in the silicate glasses containing Na and the glass ceramics containing fluorite nanocrystals were demonstrated. The damages were mainly caused by preferentially removing Na and decomposing CaF₂ into Ca. All the damage phenomena were observed under electron beam intensities, which were much weaker than the intensities used in the conventional high-resolution electron microscopy (HREM) and microanalysis in STEM. Therefore, although the advanced TEM/STEM techniques are very promising in the precise measurement of local composition at ultra-high spatial resolution in some materials, they may not be applicable to Na-containing silicate glasses and glass ceramics containing fluorite nanocrystals.     

Fabrication of heavy metal fluoride glass, optical planar waveguides by hot-spin casting

Hot-spin casting is further investigated using a customised rig for making optical planar waveguides from inorganic-compound-glasses. The rig enables a controlled mass of core-glass, held above its liquidus, to be gravity-cast onto the top surface of a spinning cladding-glass substrate that has been pre-heated to around its glass transformation temperature. Spinning encourages the cast liquid to spread as a film over the top surface of the glass substrate. The mass of liquid cast is controlled by the timed opening of an orifice in the base of the core-glass melt-crucible. The resulting step index, slab optical waveguides are annealed, then cooled to room temperature; they comprise a higher refractive index, glass film core, on top of a lower refractive index glass substrate cladding. The glass film core is air-clad. at its upper surface. The process is applied to two heavy metal fluoride core/clad. glass pairs, namely ZBLANPb/ZBLAN and ZBLALiYPb/HBLANY (where ZBLANLiYPb is ZrF₄-BaF₂-LaF₃-AlF₃-LiF-YF₃-PbF₂ and H is HfF₄) to give waveguides of small and large numerical aperture (NA) (e.g. at 643.8 nm wavelength, NA is 0.18 and 0.33, respectively). The Hot-Spin-Cast waveguides exhibit a guiding region whose top surface tends to be parallel to the upper surface of the underlying substrate. However, flatness of the top surface of the guiding region is limited by the flatness of the top surface of the underlying substrate. Multimode slab waveguiding is demonstrated for both NA waveguide types for glass film cores of depths ?10 μm.     

Detection and measurement of strain in glass

Introductory the basic theoretical principles for the assessment of the stress-strain relationship are alluded to, with special respect to the plane state of stress. Then it is shown, in what way permanent residual stresses originate in glass and how they can be detected qualitatively. After that the usual measuring methods for the quantitative determination of stresses are discussed. Beginning with purely manual photo-elastic methods, which use so-called compensators for the determination of the magnitude of stresses, the development of automatical measuring systems, provided that the directions of the principal stresses are known, is described. Finally, a new fully automatic measuring system for stresses is presented. This system is able to determine the maximum stresses, including their sign, without any precondition. In the second part the special properties of residual stresses in the case of drawn and floated glass are discussed. Based on pattern of isoclinics, which have been experimentally determined fundamental statements concerning the state of plane residual stress of flat glass are made. Finally, some typical results of measurement are presented and discussed.