Crystallization behavior and microstructure of powdered and bulk ZnO-Al2O3-SiO2 glass-ceramics

The crystallization behavior of glass with the composition: 55.6 mol% SiO₂, 22.8 mol% Al₂O₃, 17.7 mol% ZnO and 3.84 mol% of TiO₂ as nucleating agent and with different particle sizes has been studied by differential scanning calorimetry (DSC), X-ray diffraction (XRD) and tranmission electron microscopy (TEM). In glass powders two crystalline phases: zinc-aluminosilicate s.s. with high-quartz structure, Zn_x/2Al_xSi_3−xO₆, (x varies dependent on heat-treatment temperature) and gahnite are formed. The ratio of these phases depends on particle sizes. In bulk glass, however, gahnite is the sole crystalline phase. The composition of initially formed zinc-aluminosilicate s.s. was determined by Rietveld refinement of XRD patterns to be Zn_0.69Al_1.38Si_1.62O₆. With temperature increase, the amount of zinc-aluminosilicate s.s decreased with simultaneous reduce of zinc and aluminum incorporated in the structure. Eventually at 1423 K almost pure high-quartz structure was formed. The activation energies of zinc-aluminosilicate s.s. and gahnite crystallization were determined by non-isothermal method to be 510 ± 18 and 344 ± 17 kJ mol⁻¹, respectively. The latter value matches well with those cited in literature for crystal growth of gahnite in similar glasses. That is attributed to the fact that the high-quartz structure acts as a precursor for gahnite crystallization.     

Structure and crystallization of potassium titanium phosphate glasses containing B2O3 and SiO2

Transparent glasses composition of which can be expressed by the formula: (100−x) · (K₂O · 2TiO₂ · P₂O₅) · x(K₂O · 2B₂O₃ · 7SiO₂), where x=5, 10, 15 and 20 mol% (KTP-xKBS), were obtained by melt quenching technique. The structure and crystallization behavior of these glasses have been examined by Fourier transform infrared spectroscopy, differential thermal analysis and X-ray diffraction. In spite of their nominal composition, the studied glasses exhibit a similar oxygen polyhedra distribution. However, significant differences were found in the trigonal BO₃ units amount. During DTA runs all the examined glasses devitrify in two steps. In the former, very small crystals of an unknown crystalline phase are produced. In KTP-5KBS and KTP-10KBS glasses anatase phase was also detected. Attempts were made in order to identify the unknown phase (UTP) for which a AB₃(XO₄)₂(OH)₆ Crandallite-type structure was proposed where the A, B and X sites were occupied by K, Ti and/or Al, and P, respectively. In the second devitrification step the crystallization of the KTiOPO₄ phase occurs while the UTP phase previously formed disappears. Isothermal heat treatments performed at temperature just above T_g have allowed one to obtain transparent crystal-glass nanocomposites, formed by crystalline nanostructure of the UTP phase uniformly dispersed in the amorphous matrix.     

Glass-forming ability and crystallization of bulk metallic glass (HfxZr1−x)52.5Cu17.9Ni14.6Al10Ti5

We have produced a series of bulk metallic glasses of composition (Hf_xZr_1−x)_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ (with x=0-1) by an arc melting/suction casting method. The density of these alloys increases by nearly 67% with increasing Hf content from 6.65 g/cm³ (x=0) to 11.09 g/cm³ (x=1). Over the same composition range the glass-forming ability decreases, as demonstrated by the size of the largest amorphous ingots that can be cast without crystallization. Although both the glass transition temperature and the melting temperature increase linearly with increasing Hf content, the reduced glass transition temperature (T_g/T_m) decreases, from 0.64 (x=0) to 0.62 (x=1), which suggests that the `confusion principle' correlating increased glass-forming ability with increased number of components, does not apply in this case due to the chemical similarity between Zr and Hf. A different crystallization behavior is observed for Zr-based and Hf-based glasses. The final crystalline phases are CuZr₂ and Zr₂Ni for Zr-based alloys, and Al₁₆Hf₆Ni₇ and CuHf₂ for Hf-based alloys.     

Crystallization processes in amorphous Zr54Cu46 alloy

The crystallization of amorphous Zr₅₄Cu₄₆ alloy was investigated by using X-ray diffraction (XRD), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM) techniques. The experimental results show that an endothermic peak in DSC traces for amorphous Zr₅₄Cu₄₆ alloy exists at about 1006 K, indicating following eutectoid reaction occurs, namely, Cu₁₀Zr₇+CuZr₂↔CuZr in amorphous Zr₅₄Cu₄₆ alloy during heating. With increasing the heating rate, the glass transition temperature T_g and onset crystallization temperature T_x of amorphous Zr₅₄Cu₄₆ alloy increase in parallel, and the supercooled liquid region ΔT_x (=T_x−T_g) holds almost constant with an average value of 44 K. Both XRD and TEM results prove that Cu₁₀Zr₇ and CuZr₂ are main crystallization products for amorphous Zr₅₄Cu₄₆ alloy under continuous heating conditions. No CuZr phase is identified because of its small precipitation amount. Finally, the crystallization processes of amorphous Zr₅₄Cu₄₆ alloy were summarized.     

Studying of optical and morphological properties of SiO2-MOx (M: Co/Cu) glasses prepared by the sol-gel method

The cobalt oxide-silica and copper oxide-silica glasses are prepared by the sol-gel method. Tetra-Ethyl-Ortho-Silicate (TEOS) is used as source of the silica and transition metal nitrate is used as the dopant. The morphological, structural and optical properties of the samples are investigated by Scanning Electron Microscope (SEM), Energy Dispersion Analysis by X-ray (EDAX), X-ray diffraction (XRD) and Ultra-Violet/Visible (UV/Vis) spectrophotometer. The influences of heat treatment temperature and withdrawal rate on absorption/transmission spectra are investigated in the range 400-800 nm. In the cobalt oxide-silica, the intensity and position of absorption bands (λ_max) are changed by heat treatment. These behaviors are explained through the Bathochromic and the Hyperochromic effects. In addition, the effect of the withdrawal rate on transmission curves is explained in terms of the Levich equation and Beer-Lambert law.     

Glass formation and crystallization behavior in Mg65Cu25Y10−xGdx (x=0, 5 and 10) alloys

The glass forming ability and crystallization behavior of Mg₆₅Cu₂₅Y_10−xGd_x (x=0, 5 and 10) alloys have been investigated. The glass forming ability (GFA) is significantly improved when Y in Mg₆₅Cu₂₅Y₁₀ is substituted with Gd. Ternary Mg₆₅Cu₂₅Gd₁₀ bulk metallic glass (BMG) with diameter of at least 8 mm was successfully fabricated by conventional Cu-mold casting method in air atmosphere. Mg₂(Y, Gd) is the first competing crystalline phase against the glass formation in the Mg₆₅Cu₂₅Y_10−xGd_x (x=0, 5) alloys, while Mg₂Cu and Cu₂Gd are the competing crystalline phases in the Mg₆₅Cu₂₅Gd₁₀ alloy. Therefore, the suppression of the formation of Mg₂(Y, Gd) during cooling from the liquid improves the GFA significantly.     

Crystallization of SiO2Al2O3MgO gel glasses

Basic glasses are prepared by chemical polymerization in a sol-gel process. Nucleation and crystallization of these glasses are analyzed in dependence of the composition of the basic glasses and “additions” of TiO₂ and LiO₂. A comparison of gel glasses with conventionally molten glasses is made. Gel glass-ceramics are prepared as bulk materials and thin coatings.     

Surface crystallization of β-BaB2O4 phase using a CO2 laser source

This letter describes decrystallization process of the β-BaB₂O₄ phase on the surface of 63BaO-26B₂O₃-11TiO₂ glassy sample when irradiated by a CO₂ laser beam for 30-40 s, using a laser power of ≈1.7 W. An analysis by optical microscopy of the irradiated region showed that the glass surface contained crystallites with the same morphology as those observed after traditional crystallization in an electric furnace. However, the crystal growth rate and the crystallite sizes appeared to be higher than those obtained by heating in an electric furnace. X-ray diffraction of the irradiated region showed a preferential orientation of the β-BaB₂O₄ crystallites on the glass surface.     

Properties and structure of RO---Na2O---Al2O3---P2O5 (R = Mg, Ca, Sr, Ba) glasses

The properties and structure of (45 - x)RO · xNa₂O · 2.5Al₂O₃ · 52.5P₂O₅ (R = Mg, Ca, Sr, Ba, 0 x 31 mol%) glasses were investigated. The variation in the molar volumes of glasses in the MgO series is closely related to the formation of the end groups in the glasses with the substitution of Na⁺ ions for Mg²⁺ ions, resulting in a variation of the density and refractive index of the glasses. The properties of glasses containing CaO in terms of Na₂O substitution depend mainly on the low field strength of Na⁺ ions substituting for CaO even though the end groups occurring in the glasses increased. The variation in properties of the glasses containing SrO and BaO, some of which were substituted by Na₂O, could be explained by differences in masses, field strength and polarizability between the Na⁺ ions and the alkaline-earth ions due to a small variation in the structure of the glasses despite Na₂O substitution.     

X-ray diffraction studies of glasses in the systems Na2OMeOSiO2 (MeMg,Zn, Ca,Ba)

X-ray diffraction studies of glasses in the following ternary systems have been made: Na₂OMgOSiO₂, Na₂OZnOSiO₂, Na₂OCaOSiO₂ and Na₂OBaOSiO₂. The following heavy atom substitutions have been used: Ag for Na and Ge for Si. The changes in the electron radial distribution curves resulting from AgNa replacement can be explained as amplifications of relatively well-defined NaSi distances, which are nearly the same in all the glasses investigated. The GeSi substitution causes changes which can be explained on the basis of isostructural GeSi substitutions.     

Crystallization kinetics and optical band gap studies of Se96In4 glass before and after slow neutron irradiation

Results of differential scanning calorimetry (DSC) under non-isothermal condition on Se₉₆In₄ semiconducting chalcogenide glass before and after slow neutron irradiation, for different exposure times, have been reported and discussed. Some of Sn atoms have been injected into the glass by nuclear transmutation processes and the binary glass is converted into a ternary. This is accompanied by an increase in the activation energy of crystallization, E_c, and in the glass transition temperature, T_g and a decrease in the glass transition activation energy, E_t, in the onset crystallization temperature, T_c and in the peak temperature of crystallization T_p. Optical band gap measurements have also been carried out, before and after irradiation, on identical thin pellets of Se₉₆In₄ glass. The energy band gap, E_g, is found to decrease upon irradiation. These effects have been attributed to a structural change upon doping and to irradiation induced defects.     

A molecular dynamics study on vibration spectra of a-SiO2 surface

The charge equilibration (QEq) method was used in the molecular dynamics study of the vibration spectra on the a-SiO₂ surface. The vibrational density of states (VDOS) of a silica glass surface and partial contribution of Si and O atoms were compared with those obtained from the fixed-charge (FQ) model. The VDOS in each of the samples has two groups of vibrational bands, i.e., wide-lower-frequency band (WB) between 0-25 THz and narrow-higher-frequency band (NB) between 25 and 40 THz, which is similar to the bulk VDOS obtained experimentally. The QEq surface exhibits a peak and two shoulders while two peaks are found for the FQ surface. At the surface, an excess peak (at 1-2 THz) was observed by substracting the Debye VDOS from the calculated VDOS, which is regarded as the experimentally observed the boson peak (BP). The position of the BP is shorter than that in the case of the bulk. In the FQ surface, the VDOS consists of narrower bands, while in the QEq sample, wider bands exist due to charge variation.     

Semiconducting TiO2SiO2 glasses (II): Optical properties

The refractive index, optical absorption coefficient α and the thermomodulated absorption dα/dT have been measured on 70% TiO₂?30% SiO₂ glasses with up to 8% Ti³⁺. The direct absorption data show intense ligand field absorption at photon energies hν = 1.9 and 3.0 eV, arising from Ti³⁺ in a distorted octahedral environment. In the bandgap region at 3.5 eV α obey αhν ～ (hν ? E_g)²; it is qualitatively different from the bandgap absorption in crystalline TiO₂. The da/dT spectra show peaks in the bandgap region and at 1.1 eV in the near IR. This last peak is attributed to absorption by small polarons, and its line shape is compared with theoretical predictions.     

Refractive index-structure correlations in Na2O-Al2O3-SiO2 glasses

The current structural models have been used to analyse the refractive index data of Na₂O-Al₂O₃-SiO₂ glasses (Al₂O₃/Na₂O?1). The SiO₂ content is the sole factor that controls the refractive index. Values could be obtained for the factors with which each structural unit contributes in the refractive index. The content of Al₂O₃ or Na₂O has no effect on the refractive index. The factors (differential refraction) are constant and do not change with composition. They have the same values for Na₂O-SiO₂ glasses. The differential refraction of a structural unit increases linearly with increasing the number of non-bridging oxygen ions. The difference of the contribution to the refractive index from a silicate unit to the next equals to a half of that for AlO₄ tetrahedron. The effect could be attributed to the change in both the concentration and differential refraction of structural units. The obtained factors for the structural units are useful in calculating the refractive index with a high degree of accuracy.     

Sintering process of glasses in the system Na2O-B2O3-SiO2

In this work, the sintering process of different glasses in the system Na₂O-B₂O₃-SiO₂ has been studied. The studied compositions are suitable for sealing the gas manifolds of molten carbonate fuel cells. Sealing glasses are usually applied on the surfaces to be sealed using powder glass mixed with an organic medium. The agglomerant elimination and the sintering of the glass powder take place during the thermal treatment. Three different particle sizes of glass powder and different sintering temperatures and times have been used to reveal the influence of the specific surface area and viscosity on sintering. The control of these parameters allows optimization of the sealing conditions of the glasses. Dense materials have been characterized as well as the sintering mechanism. Two processes take place during the thermal treatment: the sintering process and the quartz crystallisation. Both processes act in opposite directions on the glass densification. Crystallisation is the dominant process at long times and high temperatures. The viscous flow Scherer model has been adequately applied to the experimental data.     

Thermally modulated differential scanning calorimetry of a glass of composition 45Na2O-40B2O3-10Al2O3-5In2O3

The kinetics of structural relaxation and of glass transition of the 45Na₂O-40B₂O₃-10Al₂O₃-5In₂O₃ glassforming melt is studied by means of standard DSC and of temperature modulated DSC. In this way the dependence of the fictive temperature on cooling rate is determined simultaneously with the determination of the dependence of the dynamic glass transition temperature on modulation frequency. Both sets of data are fitted together in terms of the equation of Ritland-Bartenev. It was found that the activation energy of the structural relaxation exhibits a moderate dependence on temperature with the dimensionless fragility parameter α=3.3 (for strong systems, α is about 1 and increases to about 8 for some very fragile polymeric systems).     

Characterization of chemosynthetic Al2O3-2SiO2 geopolymers

Pure chemosynthetic Al₂O₃-2SiO₂ geoploymers displaying positive alkali-activated polymerization properties and high compressive strength at room temperature were effectively fabricated utilizing a sol-gel method. The molecular structure of the precursor powder and resulting geopolymers were investigated by X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) analysis. In addition, the mechanical and alkali-activated polymerization properties of these materials were also studied. NMR data revealed that the chemosynthetic powders began to contain 5-coordinated Al atoms when the calcination temperatures exceeded 200 °C. These calcined powders were capable of reacting with sodium silicate solutions at calcination temperatures exceeding 300 °C, which is, however, much lower than the temperature required to convert kaolin to Metakaolin.     

Characterization of (Ba, Sr)RuO3 films deposited by metal organic chemical vapor deposition

(Ba, Sr)RuO₃ oxide electrodes have been studied for high dielectric (Ba, Sr)TiO₃ film in DRAM capacitors. Metal organic chemical vapor deposition (MOCVD) is used for large-scale deposition and provides better step coverage properties. In this work, methoxyethoxytetramethylheptanedionate (METHD) precursor and solvent [n-butylacetate(C₆H₁₂O₂)] were mixed together into a single solution source. Post deposition annealing is carried out in oxygen atmosphere using rapid thermal annealing (RTA) to investigate the effect of organic impurities such as carbon during deposition. After annealing, resistivity of the BSR film decreased drastically compared to the as-deposited film. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analysis were used to describe this phenomenon accurately. The decrease in carbonate with increasing annealing time was confirmed by XRD analysis.