Devitrification and vitrification of tellurite glasses

The crystallization of pure tellurite glass during various heating rates was studied. The activation energy for crystallization was 115 × 10²² eV mol^–1. The glass transformation, T _g, starting crystallization, T _x, crystallization, T _c and melting temperatures, T _m, have been reported for binary tellurite glasses of the form (1 – x) TeO₂–xA_nO_m [A_nO_m = MnO₂, Co₃O₄ and MoO₃]. Among many different parameters of the glass forming potential the two-thirds rule, T _g/T _m, the glass stabilization range, T= T _x – T _g, and the glass forming tendency, K _g= (T _c– T _g)/(T _m – T _c), are reported for the first time for tellurite glasses.     

Deformation, stress relaxation, and crystallization of lithium silicate glass fibers below the glass transition temperature

The deformation and crystallization of Li₂O·2SiO₂ and Li₂O·1.6SiO₂ glass fibers subjected to a bending stress were measured as a function of time over the temperature range 50 to 150°C below the glass transition temperature (T _g). The glass fibers can be permanently deformed at temperatures about 100°C below T _g, and they crystallize significantly at temperatures close to, but below T _g, about 150°C lower than the onset temperature for crystallization for these glasses in the no-stress condition. The crystallization was found to occur only on the surface of the glass fibers with no detectable difference in the extent of crystallization in tensile and compressive stress regions. The relaxation mechanism for fiber deformation can be best described by a stretched exponential (Kohlrausch-Williams- Watt (KWW) approximation), rather than a single exponential model.The activation energy for stress relaxation, E _s, for the glass fibers ranges between 175 and 195 kJ/mol, which is considerably smaller than the activation energy for viscous flow, E (400 kJ/mol) near T _g for these glasses at normal, stress-free condition. It is suspected that a viscosity relaxation mechanism could be responsible for permanent deformation and crystallization of the glass fibers below T _g.     

New multicomponent fluoride glasses

New glass compositions have been investigated in the barium-, indium- and gallium-based glass system (BIG). Radioactive thorium has been completely removed and replaced by gadolinium, and these new glasses have been stabilized by substituting barium by alkali ions. Characteristic temperatures T _g, T _x, and T _l, and the Hruby factor, H _r, have been determined for each glass. The most significant critical cooling rates, R _c, are also given: both the highest H _r, and the lowest R _c, values for BIG thorium-free glasses were obtained by coupling two alkali ions. Some attempts have also been made to replace thorium by cerium (IV), leading to cerium(III)-containing glasses. The synthesis of cerium(IV)-containing fluoride glasses seemed to be impossible. Nevertheless, CeF₄ has been synthesized, the knowledge of its lattice parameters being improved.     

Viscous flow behaviour of NixZr100−x metallic glasses from Ni30Zr70 to Ni64Zr36

The compositional dependence of viscous flow in the Ni _x Zr_100–x amorphous system was investigated under non-isothermal conditions at a heating rate of 10 K min^–1in the compositional range fromx=30 tox=64 at % with the aid of a Hereaus TMA 500 dilatometer. The crystallization behaviour of the same glassy alloys under the same non-isothermal conditions was studied with a Perkin-Elmer DSC 7 differential scanning calorimeter. The characteristic crystallization and viscous flow parameters (the onset temperature,T _x, of crystallization; the temperature,T _m, of maximum heat evolution of the first crystallization step; the enthalpy, H _x, of crystallization; the activation energy,Q _x, of crystallization; the glass transition temperature,T _g; the viscosity values (T _g) and _min; and the activation energy for viscous flowQ (T>T _g), were shown to be dependent on composition. This dependence was examined on the basis of the equilibrium phase diagram of the Ni-Zr-system, and it is shown that glassy alloys possessing eutectic compositions manifest the greatest thermal stability because of the long-range atomic diffusion needed for crystallization to occur. Glassy alloys with nearly peritectoid compositions show low thermal stability, as no long-range diffusion is needed for the formation of the stable crystallization end-products NiZr₂ and NiZr. In all cases, the crystallization process is governed by viscosity flow of these glassy alloys.     

Preparation and characterization of rapidly quenched glasses in the systems R2O—WO3 (R=Li,Na, K)

In the systems R₂O-WO₃ (R=Li, Na, K), the glass-forming region was extended by use of the rapid quenching technique combining a thermal-image furnace with a twin roller. The infra-red spectra revealed that the Li₂WO₄ glass contains no condensed macro-anions but only discrete ions of Li⁺ and WO ₄ ^2– . The glass transition temperature (T _g) and the crystallization temperature (T _c) were determined.T _g is affected very slightly by the type of alkali cations, whereas it is decreased with increase in the alkali oxide content. More than oneT _g was observed in Li₂O-WO₃ glasses, which is due to the formation of metastable phases. The conductivity of glasses containing the same amount of alkali oxide (of different types) increased with decrease in the radius of the alkali cation. Electrochromism was observed in the glasses with R=Li, Na; the electric field required for colouration is lower than that in WO₃ films. The electro-colouration is due to H_xWO₃ formed in the glass matrix by the double injection of protons and electrons.     

Crystallization of Te-Si-Pb glasses exhibiting double T g effect

Results of calorimetric, X-ray and transmission electron microscopy investigations of the glasscrystal transition in Te₈₀Si_20–xPb_x glasses are presented. The investigated glasses exhibit a double-stage crystallization and a double glass-transition temperature, T _g, within a composition range of 2.5 to 10 at % Pb. The second glass transition appears after partial crystallization of the glass. The double glass-transition phenomenon is discussed in relation to the crystallization process.     

Er- and Nd-doped yttrium aluminosilicate glasses: Preparation and characterization

The undoped, and Nd-, Er-doped low silica high alumina yttrium aluminosilicate (YAS) glasses were prepared by flame synthesis in the form of transparent glass microbeads with diameters ranging from a few to several tens of micrometers. The silica content ranged between 5 and 20 mol.%. The prepared glass microbeads were characterized by optical microscopy, SEM XRD, FT-IR and UV–VIS–NIR spectroscopy. The glass forming ability of glasses, expressed in terms of the difference between the glass transition temperature, T_g, and the onset of crystallization, T_x, improved with increasing silica content. Doping of YAS15 glass with neodymium or erbium at the level of 1–5 mol.% leads to decrease of both the T_g and T_x. However, the glass forming ability was not affected. The UV–VIS–NIR reflectance spectra in the spectral range from 300 to 1800 nm shows characteristic absorptions, due to the optically active Nd³⁺ and Er³⁺ ions in the host glass.     

Thermal properties of multicomponent tellurite glass

Quaternary tellurite glass systems of the form 80TeO₂–5TiO₂–(15 − x)WO₃ − xA_nO_m where A_nO_m is Nb₂O₅, Nd₂O₃, and Er₂O₃, x = 0.01, 1, 3, and 5 mol% for Nb₂O₅ and x = 0.01, 0.1, 1, 3, 5, and 7 mol% for Nd₂O₃ and Er₂O₃, have been prepared by the melt quenching. Density and molar volumes have been measured and calculated for every glass system. The thermal behavior of the glass series was studied by using the differential thermal analysis DSC. Glass transition temperature T _g, crystallization temperature T _c, and the onset of crystallization temperature T _x were determined. The glass stability against crystallization S (≈100 °C) and glass-forming tendency K _g (≈0.3) have been calculated. Specific heat capacity C _p (≥1.4 J/g °C) was measured from room temperature and above the T _g for every composition in each glass series. Quantitative analysis of the above thermal properties of these new tellurite glass with the structure parameters like average cross-link density (≥2.4), number of bonds per unit volume n _b (≥8 × 10²⁸ cm⁻³), and the average stretching force constant (F) have been studied for every glass composition.     

Crystallization kinetics of selenium-tellerium glasses

Bulk glasses of the compositions Se₇₀Te₃₀ and Se₈₀Te₂₀ were prepared by the melt quenching technique. Differential thermal analyses were performed at different heating rates. The values of the glass transition temperature,T _g, the crystallization temperature,T _c, and the peak temperature of crystallization,T _p, were found to depend on the composition and the heating rate. The activation energy for the glass transition,E _i, as well as the activation energy for crystallization,E _c, were evaluated from the heating rate dependence ofT _g andT _p. The crystallization mechanism was examined through analysis of the data under non-isothermal conditions. The results indicated that surface crystallization is dominant for both compositions.     

Properties of glasses in the ZrF4-AlF3-BaF2-RF (R = Li,Na or K) system

Densities, molar volumes, refractive indices, molar refractivities, glass transition temperatures and elastic constants of glasses with compositions (100–x) (0.6ZrF₄ · 0.1 AlF₃ · 0.3BaF₂) · xRF, where R = Li, Na or K, were studied. All the properties vary continuously with increasing RF concentration for all three alkali atoms. The calculated molar refractivities of LiF, NaF and KF are constants which are very close to the values of the crystalline compounds LiF, NaF and KF, respectively, throughout the composition range studied, suggesting that no change occurs in the coordination state of the alkali ions by fluorine atoms. The comparison with crystalline compounds suggests that Li⁺, Na⁺ and K⁺ ions are all coordinated by six fluorine atoms.     

Thermomechanical properties and stability of mixed strontium-zinc borophosphate glasses

Borophosphate glasses of the SrO-ZnO-B₂O₃-P₂O₅ system were prepared in 3 compositional series of 25ZnO-25SrO-xB₂O₃-(50 – x)P₂O₅, (58.3 – x) ZnO-xSrO-16.6B₂O₃-33.33P₂O₅ and (50 – x)ZnO-20B₂O₃-30P₂O₅ and the changes in their thermomechanical and thermal properties with changes in their composition were investigated. The thermal expansion coefficient decreases with increasing B₂O₃ content, whereas the values of glass transformation temperature T _g and dilatation softening temperature T _d increase. The replacement of ZnO by SrO increases both T _g and T _d. From the DSC results the value of the the Hruby's criterion K _gl was calculated and changes in the glass-forming tendency with changes in glass composition were discussed.     

Preparation and study of Ca1 − xMgx(PO3)2 glassy and crystalline phases

The structure of Ca_{1 – x} Mg _x (PO₃)₂ crystalline and glassy samples was investigated in the whole concentration region of x = 0–1.0. From X-ray diffraction data it was found that, in the crystalline samples, solid solutions are formed for x < 0.3 on the calcium-side of the system, with the structure of -Ca(PO₃)₂, and for x > 0.6 on its magnesium-side, with the structure of Mg₂P₄O₁₂. Similar results were obtained from the study of their infrared and Raman spectra. In the glassy state, homogeneous glasses were formed within the whole concentration region. The values of their transformation temperatures, T _g, and crystallization temperature, T _c, change slightly with the composition and lie within the region of T _g = 529–544 °C and T _c = 631–677 °C.     

Local structure,glass transition,structural relaxation,and crystallization of functional oxide glasses investigated by Mössbauer spectroscopy and DTA

This review paper summarizes early Mössbauer and DTA studies of different oxide glasses containing small amounts of iron (III) or tin (IV) as the probe. A lot of valuable information of the atomic level has been obtained about the role of nonbridging oxygen (NBO), network former (NWF), network modifier (NWM), local network structure, glass transition, structural relaxation, crystallization, etc. Introduction of alkali oxide into iron (III)-containing oxide glass causes a marked decrease in glass transition temperature (T_g) amounting to 100 °C and a concordant decrease in quadrupole splitting (Δ) of Fe^III, which reflects decreased distortion of NWF–oxygen polyhedra and formation of NBO. By contrast, introduction of non-alkali oxide into oxide glass causes an increase in T_g amounting to more than 100 °C and a concordant increase in Δ, reflecting increased distortion of NWF–oxygen polyhedra in highly cross-linked network. These experimental results led to a discovery of “T_g-Δ rule”, which was consistent with the “conformer model” proposed for polymers by Matsuoka and Quan. Debye temperatures (θ_D) obtained by low-temperature Mössbauer measurements proved to be useful to determine short- and long-range structures of glass and glass ceramics. Isothermal annealing of vanadate glasses at temperatures higher than T_g or crystallization temperature (T_c) causes a “tunable” decrease in DC-resistivity from the order of MΩ cm to Ω cm. Introduction of metal oxide with a narrow bandgap (E_g) is highly effective to increase the conductivity after the annealing. It was proved that “structural relaxation” of NWF–oxygen polyhedra and resultant decrease in the activation energy (E_a) for conduction are responsible for the improved conductivity. Heat treatment of IR-transmitting aluminate, gallate, and tellurite glasses at temperatures higher than T_g or T_c revealed that crystallization was triggered by the cleavage of NWF–oxygen bonds. These findings will contribute to the development of functional glass and glass ceramics such as smart glass and eco-friendly glass.

     

Ga as an additive in the As2Te3 glass

Results of measurements of the mean atomic volume (V), the glass transition temperature (T _g), the activation energy for glass transition (E _t) and the d. c. electrical conductivity () are reported and discussed for ten glass compositions of the Ga–As–Te system. The glasses studied can be represented as Ga _x (As_0.4Te_0.6)_100–x glasses, with the additive Ga ranging from 0 to 12 atomic percent (at.%) in the parent As₂Te₃glass. In the Ga _x (As_0.4Te_0.6)_100–x glasses, changes in slope are observed in the V, T _g, E _t, and other electronic properties, at the composition with a Ga content of 2 at.%. The results are compared with those obtained on introduction of Ag and Cu to the As₂Te₃and the [0.5As₂Te₃–0.5As₂Se₃] glasses. Analysis of the data suggest formation of GaAs, Ga₂Te₃and excess Te structural units (s.u.) in lieu of some of the original As₂Te₃s.u., for addition of Ga up to 2 at.% to the parent As₂Te₃glass; for higher Ga contents, formation of GaAs, GaTe and excess Te s.u. are indicated.     

The influence of composition on the formation and stability of Ni-Si-B metallic glasses

The influence of composition on the formation and stability of Ni-based glassy alloys containing Si and B has been investigated systematically. Depending on the SiB ratio certain compositions, with total metalloid contents in the range 17 to 49 at% (a wider range than has hitherto been reported for metal-metalloid glasses), have been vitrified by melt-spinning to an average thickness of 17 m. For metalloid concentrations greater than 36 to 40 at% the amorphous phase is brittle in the as-quenched state. The highest crystallization temperatures, T _x, occur at 32 to 38 at% metalloid, the actual value of T _x again depending on the SiB ratio. It is shown that, over a wide composition range, the glass-forming boundary corresponds closely with a value for the reduced crystallization temperature isometric of 0.52. This value corresponds to a critical cooling rate for glass formation of about 10⁶ K sec^–1, predicted from kinetic theories and assuming that T _x is a good estimate of the glass transition temperature, T _g. This agrees quite closely with the cooling rate for approximately 17 m thick tape predicted from thermal studies of the melt-spinning process. Hence, T _x/T _liq usefully describes the glass-forming ability (GFA) for much of the composition range studied, although preliminary results suggest that near the centre of the glass-forming range the GFA may be over-estimated. Substitution of Si by other metalloid elements from Groups IIIb–Vb in Ni₇₈Si₁₀B₁₂ glassy alloy generally decreases the thermal-stability.     

Studies of the effect of paramagnetic impurity in the structure of sodium disilicate glass

Magic angle spinning nuclear magnetic resonance (MAS NMR) is used to measure the ²⁹Si chemical shift and full width at half maximum in Na₂O·2SiO₂·xMnO for 0.0x0.8 glasses. The large space varying magnetic field of the paramagnetic centers increases the line width and moves the chemical shift to a lower frequency, i.e. more shielded, with the increase of Mn⁺⁺. The T₁ relaxation time falls rapidly and the relaxation rate is shown to be proportional with the concentration of MnO. The differential thermal analysis (DTA) results show that the glass transition temperature, T_g, crystallization temperature, T_x, and liquidus temperature T_L do not change due to addition of MnO. This suggests that Mn⁺⁺ does not form chemical bond with the network. However, the Mn⁺⁺ acts as nucleating agent and enhances the crystallization and the rate of crystallization can be tailored by judicious choice of the amount of MnO.     

Verres aux halogenures de cadmium : I.- Verres fluores

New halide glasses based on cadmium halides as glass progenitors have been discovered. This paper describes fluoride glasses in the CdF₂-BaF₂-ZnF₂ ternary system. Binary glasses Cd_xBa_1?xF₂ (0.45 < x < 0.52) have been obtained. The limits of the glassy area are: 5–55 % CdF₂, 15–55 % BaF₂, 0–60 % ZnF₂. Numerous fluorides such as the alkali fluorides, AlF₃, YbF₃ and ThF₄ may be included in the glass composition. Only thin glass samples may be prepared because of the high crystallization rate. The characteristic temperatures of the Cd_0.3Ba_0.4Zn_0.3F₂ glass are 290° C for T_G, 330° C for T_C and 620° C for T_F. The optical transmission spectrum of a thin sample shows an extrinsic absorption band at around 9000 nm and the multiphonon IR absorption edge beyond 10500 nm. The optimization of these glasses will make them attractive materials for IR transmittance and IR optical fibres. The crystallization of the Cd_0.5Ba_0.5F₂ glass results in two solid solutions Ba_1?xCd_xF₂ and Cd_1?xBa_xF₂ deriving from the fluorite structural type. Thus, cadmium fluoride glasses appear as the result of the disordering of the fluorite type structure in the same way as ZrF₄-based glasses derive from the disordering of the ReO₃-type structure. Both glasses may be described as the random insertion of cations into an aperiodic array of F^? anions.     

Density andT g of glasses of the As-Al-Te system

Results of measurement of the density and the glass transition temperature (T _g) on 25 glass compositions of the As-AI-Te system are reported and discussed. A linear increase of density and a decrease ofT _g are seen with increasing As content across the AsAlTe₄- As₂Te₃ tie-line compositions. In the density-composition and mean atomic volume-composition dependences for the As _x ,Al₅Te_95–x , As _x Al₈Te_92–x and As _x Al₁₂Te_88–x glasses, a change in slope is seen at the respective tie-line compositions, indicating chemical ordering in these glasses. When the dependence of density on the average coordination numberZ are considered, the effects of chemical ordering are obscured; instead, the effects of topological nature emerge, with changes in slope at the topological thresholds ofZ2.40 andZ2.67. The results further indicate that the dependence of mean atomic volumev onZ is controlled by the Te content in these glasses. Due to this, the generally observed features in the dependence ofv onZ for chalcogenide glasses, namely a minimum inv forZ2.40, a peak inv forZ2.67 and a decrease ofv forZ > 2.67, are suppressed.     

Crystallization and properties of CaO-P2O2-B2O3 glasses

Glasses were prepared with compositions (50–0.5 x) CaO.(50–0.5 x) P₂O₅ · x B₂O₃ with B₂O₃ contents (x) from 0 to 45 mol%. The glass transformation temperature (T _g), dilatational softening temperature (T _D) and Vickers hardness (H _V) initially increased with x, but showed maxima at about x=20 for T _g and T _D and at about x=35 for H _V. The thermal expansion coefficient decreased with x, levelling off at about 35 mol% B₂O₃. The maximum tendency to crystallize occurred at around 25 mol% B₂O₃. Volume nucleation (and hence glass-ceramic formation) and surface nucleation were obtained for x between 15 and 25 mol%. The first phase to appear was BPO₄, which was probably homogeneously nucleated. Subsequently the 4CaO · P₂O₅ phase was heterogeneously nucleated on the BPO₄. For 10 x 35 only surface nucleation was observed. The kinetics of nucleation were investigated in the 20 mol% B₂O₃ glass. The changes in properties and crystallisation behaviour with B₂O₃ content were related to short-range structural information. Infrared spectra and literature data indicated a threedimensional network of B-O-B and B-O-P linkages in the glasses.     

Kinematical transformations in amorphous selenium alloys used in xerography

Differential scanning calorimetric (DSC) studies were carried out on xerographic photoreceptor materials of amorphous selenium (a-Se), a-Se_1&#x2203-xTe_x and a-Se:0.5 wt% As alloys with various amounts of chlorine doping. Glass transformation kinetics of bulk and vapour-deposited film samples were essentially identical indicating a similar amorphous structure for both forms. The glass transition temperature,T _g, in the a-Se_1&#x2203-xTe_xsystem was found to increase monotonically with the tellurium content up to 13 wt% which was accompanied by a fall in the activation energy,E _gl, for the structural relaxation. 0.5wt% As increasedT _g, whereas chlorine doping in the amounts used (100 ppm) had no effect onT _g. Crystallization exotherms were analysed using the Kissinger method which was shown to remain a valid analysis by integrating the fundamental Kolmogoroff equation describing crystallization transformations by nucleation and growth. The signifiance of the activation energy,E _A, and the frequency factor,C ₀, in the Kissinger analysis is discussed and used in interpreting the different crystallization behaviour exhibited by the various alloys. The apparent activation energy,E _A, of the crystallization kinetics was found to fall with the temperature range accessed in the DSC experiment. When other authors' previous crystallization studies by other methods were also considered, then 1/E _A was found to fall nearly linearly with the mean reciprocal temperature, 1/T, used in the experiment. It is argued that this behaviour arises if the crystal growth rate scales with the inverse viscosity of the melt and the latter is described by a Vogel-Tammann-Fulcher type of behaviour. The additives tellurium and arsenic retard the crystallization essentially by increasing the viscosity. Chlorine doping was also found to have an inhibitory effect but due probably to a change in the nucleation kinetics, rather than a viscosity rise.