Non-isothermal crystallization and nanostructuring in potassium niobium silicate glasses

Potassium niobium silicate (KNS) glasses the composition of which is characterized by the K₂O/Nb₂O₅ molar ratio ranging from 0.85 to 1.2 and SiO₂ 50-54 mol% were examined in order to clarify the influence of chemical composition on formation of transparent nanostructured state of glasses. Differential thermal analysis, X-ray diffraction and scanning electron microscopy were used to study the non-isothermal crystallization of the KNS glasses as well as their morphological features. It was found that all glasses devitrify in three steps forming unidentified phases at the first two ones while at higher temperature (1000-1100 °C) the crystallization of K₃Nb₃O₆Si₂O₇ takes place. For prolonged heat treatment time (more than 5 h) at high temperature (1050-1100 °C) the transformation of this phase into the KNbSi₂O₇ ferroelectric one occurs in some extent. Nanostructuring occurs at the first stage of the devitrification process. It results from two partially overlapped processes: amorphous phase separation and subsequent crystallization. It was shown that only for the glass with the K₂O/Nb₂O₅ molar ratio equal to 0.85 and SiO₂ 50 mol% it is possible to separate the above processes by isothermal heat treatments at 680 °C obtaining fully transparent nanostructured samples. These samples contain nanocrystals 10 times smaller than the amorphous inhomogeneities of the phase separated matrix in which are dispersed.     

Experimental and theoretical studies of flame hydrolysis deposition process for making glasses for optical planar devices

We have here presented experimental and theoretical studies of a flame hydrolysis deposition (FHD) process for making glasses for optical planar devices. FHD involves deposition of soot particles generated in the flame on a planar surface to form a porous layer. In order to function as a waveguide core, the porous soot deposit must be sintered at high temperatures to form a dense glass. However, these temperatures are high enough to cause the dopants to volatilize at the surfaces of the deposit. As a result, dopant concentration gradients and compositional inhomogeneities are created in the densified glass layer which result in inferior optical properties. If the layers could be deposited pre-sintered during laydown, these problems could be minimized or avoided. To understand the FHD process and its effect on the morphology of the resulting deposit, a number of models have been developed. We present models to predict the structure of the soot-laden flame along with the methods to estimate soot number density and mean size at different locations in the flame. A simple method is presented to predict morphology of the suspended soot and soot deposit formed during deposition. Motivated by the predictions of the model, process changes were made which resulted in the morphology of the core layers to change from sooty to pre-sintered during deposition. These process changes resulted in core glasses with significantly improved core roughness, index uniformity and thickness uniformity which helped reduce the straight waveguide losses from 0.2-0.3 to 0.02-0.05 dB/cm. Details of underclad and overclad glasses which resulted in low warpage and polarization sensitivity of 0.1 nm for phasar or phasar-arrayed waveguide devices are also presented.     

The influence of strontium substitution in fluorapatite glasses and glass-ceramics

Strontium is often substituted for calcium in order to confer radio-opacity in glasses used for dental cements, biocomposites and bioglass-ceramics. The present paper investigates the influence of substituting strontium for calcium in a glass of the following composition: 4.5SiO₂3Al₂O₃1.5P₂O₅3CaO2CaF₂, having a Ca:P ratio of 1.67 corresponding to calcium fluorapatite (Ca₅(PO₄)₃F). The glasses were characterized by magic angle spinning nuclear magnetic resonance (MAS-NMR), by differential scanning calorimetry (DSC) and X-ray powder diffraction (XRD). The ²⁹Si, ²⁷Al and ³¹P NMR spectra for the glasses with different strontium contents were identical. The ¹⁹F spectra indicated the presence of F-Ca(n) and Al-F-Ca(n) species in the calcium glasses and in the strontium glasses F-Sr(n) and Al-F-Sr(n). It can be concluded that strontium substitutes for calcium with little change in the glass structure as a result of their similar charge to size ratio. The low strontium glasses bulk nucleated to a calcium apatite phase. Intermediate strontium content glasses surface nucleated to a mixed calcium-strontium apatite and the fully strontium substituted glass to strontium fluorapatite.     

Superplasticity and superplastic forming ability of a Zr-Ti-Ni-Cu-Be bulk metallic glass in the supercooled liquid region

The superplastic deformation behavior and superplastic forming ability of the Zr_41.25Ti_13.75Ni₁₀Cu_12.5Be_22.5 (at.%) bulk metallic glass (BMG) in the supercooled liquid region were investigated. The isothermal tensile results indicate that the BMG exhibits a Newtonian behavior at low strain rates but a non-Newtonian behavior at high-strain rates in the initial deformation stage. The maximum elongation reaches as high as 1624% at 656 K, and nanocrystallization was found to occur during the deformation process. Based on the analysis on tensile deformation, a gear-like micropart is successfully die-forged via a superplastic forging process, demonstrating that the BMG has excellent workability in the supercooled liquid region.     

The influence of Mg substitution for Al on the properties of SiMeRE oxynitride glasses

The thermal-mechanical properties of 60Si20Mg20RE (RE: rare earth) oxynitride glasses can be tailored by the substitution of rare earths of decreasing ionic size. The Young’s modulus, hardness, glass transition temperature and viscosity all increase with the substitution La by Gd and Gd by Lu while the thermal expansion coefficients decrease. Compared to the 55Si25Al20RE oxynitride glasses, replacement of Al by Mg lowers the glass transition temperatures and viscosities and raises the thermal expansion coefficients substantially. On the other hand, the Young’s moduli are higher in the Mg-bearing glasses. These behaviors are seen to be a result in changes in the nature of the bonding in the glass structure.     

Mechanical properties of a TAS fiber: a preliminary study

The mechanical properties, including elastic moduli, hardness, fracture toughness and tensile strength of a glass fiber in the Te-As-Se system (TAS) were studied. The values for the hardness (1.4 GPa) and the fracture toughness (0.18 MPa √m) show that this glass is both soft and brittle in comparison to glasses from other systems. However, indentation measurements should be interpreted with caution due to an indentation creep phenomenon and to a delayed fracture process. In addition, the effect of treatments in air (relative humidity about 60%) at different temperatures below T_g were investigated. The main result of this study is that the studied TAS glass is sensitive to the presence of humidity, and aging treatments have a pronounced detrimental effect on the strength of the uncoated fibers.     

Structure and properties of lithium thio-boro-germanate glasses

Structural studies of the ternary xLi₂S + (1 − x)[0.5B₂S₃ + 0.5GeS₂] glasses using IR, Raman, and ¹¹B NMR show that the Li₂S is not shared proportionately between the GeS₂ and B₂S₃ sub-networks of the glass. The IR spectra indicate that the B₂S₃ glass network is under-doped in comparison to the corresponding composition in the xLi₂S + (1 − x)B₂S₃ binary system. Additionally, the Raman spectra show that the GeS₂ glass network is over-modified. Surprisingly, however, the ¹¹Boron static NMR gives evidence that ∼80% of the boron atoms are in tetrahedral coordinated. A super macro tetrahedron, B₁₀S₁₈⁻⁶ is proposed as one of the structures in these glasses in which can account for the apparent low fraction of Li₂S present in the B₂S₃ sub-network while at the same time enabling the high fraction of tetrahedral borons in the glass.     

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.     

Effect of fluorspar and alumina on the viscous flow of calcium silicate melts containing MgO

For the investigation of the effect of fluorspar and alumina on the viscous flow and the crystallization behavior of the silicate melts, the viscosity of the CaO-SiO₂-10 wt%MgO-CaF₂ (or -Al₂O₃) system was measured. The addition of CaF₂ decreased the crystallization temperature (T_CR) of the CaO-SiO₂-MgO-CaF₂ system, while the T_CR of the CaO-SiO₂-MgO-Al₂O₃ (wt%CaO/wt%SiO₂=1.0) system exhibited a minimum value at 10 wt% Al₂O₃. The activation energy was decreased in the CSMF system and vice versa in the CSMA system by addition of CaF₂ or Al₂O₃ in each system. From the relationship between the activation energy and the composition of the glasses, the following equation could be obtained. , where the B^total is the ratio of (wt%CaO + wt%CaF₂ + wt%MgO)/(wt%SiO₂ + wt%Al₂O₃). This means that CaO, CaF₂, and MgO would behave as a network modifier, while SiO₂ and Al₂O₃ behave as a network former in the glasses investigated.     

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 of 2(Ca,Sr,Ba)O-TiO2-2SiO2 composition glasses

Crystallization was examined for glasses having chemical composition of 2(Ca,Sr,Ba)O-TiO₂-2SiO₂ in which the CaO/SrO/BaO molar ratio varied. Powdered glass samples were pelleted into disks and sintered at 950 °C for 2 h. The major phase precipitated in the sintered samples was (Ca,Sr,Ba)₂TiSi₂O₈ and minor phase of perovskite such as CaTiO₃ or SrTiO₃ increased with CaO content in the samples containing more than 40 mol% of CaO in total CaO+SrO+BaO. Three regions having different slopes were found in linear relationships between SrO mol% and exothermal peak temperature on DSC curves or d[0 0 2] values determined by powder XRD method. These facts suggested that the major phase precipitated in each region was a solid solution containing a different amount of CaO, SrO, BaO and that these compositions varied depending on SrO content in the sample. The micro-crystalline structure, which could be useful in fabricating a dielectric dense body, was observed for samples containing 30-70 mol% of SrO.     

Effects of Sn addition on the glass forming ability and crystallization behavior in Ni-Zr-Ti-Si alloys

The effect of Sn substitution for Si on the glass forming ability (GFA) and crystallization behavior has been studied in Ni₅₉Zr₂₀Ti₁₆Si_5 − xSn_x (x=0, 3, 5) alloys. A bulk amorphous Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ alloy with diameter up to 3 mm can be fabricated by injection casting. Partial substitution of Si by Sn in Ni₅₉Zr₂₀Ti₆Si_5 − xSn_x alloys improves the glass forming ability. The improved GFA of the Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ alloy is can be explained based on the lowering of liquidus temperature. The crystallization sequence becomes completely different with addition of Sn. The amorphous Ni₅₉Zr₂₀Ti₁₆Si₅ alloy crystallizes via precipitation of only a cubic NiTi phase in the first crystallization step, whereas the amorphous Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ alloy crystallizes via simultaneous precipitation of orthorhombic Ni₁₀(Zr,Ti)₇ and cubic NiTi phases. Addition of Sn in the Ni₅₉Zr₂₀Ti₁₆Si₅ alloy suppresses the formation of the primary cubic NiTi phase. The bulk amorphous Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ alloy exhibits high compressive fracture strength of about 2.7 GPa with a plastic strain of about 2%.     

Making monosaccharide and disaccharide sugar glasses by using microwave oven

We studied how to make sugar glasses without caramelization. A saccharide easily becomes a caramel during melting because it is very sensitive to heat. We have successfully made glasses of monosaccharide and disaccharide sugars by using a microwave oven heating method. It was necessary to heat sugars quickly and uniformly to avoid a caramelization and the microwave oven was particularly effective for this purpose. The glass phase of the sample was tested by X-ray powder diffraction experiment.     

Glass transition and fragility of Na2O-TeO2 glasses

The heat capacity (C_p) change in the glass transition region for the xNa₂O ·(100−x)TeO₂, mol%, glass forming melts with x=7.5, 11.1, 15.0, 20.0 and 25.0 was measured as a function of heating rate (2, 4, 6, 10 and 15 °C/min) using differential scanning calorimetry. The glass transition properties that have been measured and reported in this paper include the glass transition temperature (T_g), glass transition width (ΔT_g), heat capacities in the glassy and liquid state (C_pg and C_pl) and the activation enthalpy for glass transition (). T_g for these sodium tellurite melts decreased and increased with increasing Na₂O. Values of the ratio C_pl/C_pg ranged between 1.28 and 2.47, and the fragility parameter ranged between 100 and 130, suggesting that these glass forming melts may be classified as intermediate between typical strong and fragile liquids. The viscosity, η, calculated at a few selected temperatures near the glass transition region decreased with increasing Na₂O at any given temperature, which is also expected.     

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.     

Electrical current and laser radiation effects on amorphous solids

In the present paper a phenomenological theory of external factor effects upon amorphous solid properties is presented and applied to some examples. Electric current and laser light effects are considered, but the theory can be generalized to include other effects, e.g., ionizing radiation, chemical reactants etc. Changes of amorphous solid properties are considered as initiated locally and propagated along certain lines, narrow stripes or tubes, which will be called current- or laser traces. The proposed theory consists of the kinetics of the traces’ appearance, development and disappearance, and of the kinetics of processes (physical, chemical a. o.) which occur inside traces. This phenomenological theory is applied to determine the molecular mechanisms of the considered processes by comparing the kinetics obtained theoretically and experimentally for relevant cases. The comparison also allows determination of constants and functions that cannot be calculated theoretically. A possibility was considered that properties change by crystallization, e.g., initiated by resonance electron collisions with atoms, molecules and ions within the amorphous solid. It is shown how the obtained kinetic dependencies can be compared with experimental data to check the hypothesis itself on the resonance mechanism and to determine what are namely resonances contributing to these processes.     

Studies of structures and properties of polymeric systems containing bis-(hydroxy-arylidene)alkanones as NLO-active chromophores

NLO-properties of polymer systems containing bis-(hydroxy-arylidene)alkanone chromophores were studied experimentally and analyzed using ab initio quantum chemical calculations. A monoclinic crystal structure (space group P2₁11) of the polyester containing fragments of such chromophores in the backbone was simulated and a reasonable agreement between the experimental and simulated X-ray powder diffraction patterns was achieved. Ab initio quantum-mechanical estimations of the SHG-observable macroscopic second-order non-linearity tensor coefficients, obtained for the polymer crystal structure at the HF SCF level, led to the major d_XZZ-coefficient of 1.9 pm/V. Films of ionic complexes of strongly basic poly-1,10-decamethylene-acetamidine with various bis-(hydroxy-arylidene)alkanone chromophores were prepared and shown to display considerable THG-efficiencies. The molecular structure of these complexes is discussed on the basis of the experimental spectroscopic data and calculated relative stabilities of different forms of the complexes. The formation of the complexes results in a considerable increase of the glass transition temperature by ≈70°C.     

Compositional dependence of ultraviolet fluorescence intensity of Ce in silicate, borate, and phosphate glasses

Fluorescence spectra of Ce³⁺ ions in silicate, borate, and phosphate glasses melted in Ar were measured. The relative fluorescence intensity of Ce³⁺ in the ultraviolet region increased in the order of R = Ba, Ca, Sr, and Mg in the 20Li₂O-20RO-60SiO₂ glass samples and with decreasing BaO content in the BaO-B₂O₃ glass samples, respectively. In contrast, the relative fluorescence intensity of Ce³⁺ did not change with varying the glass composition in phosphate glass samples. The compositional dependence of the relative fluorescence intensity of Ce³⁺ is discussed in terms of redox reaction of Ce³⁺-Ce⁴⁺ in oxide glasses.     

Sol-gel preparation and optical properties of SiO2-Ta2O5 glass

Tantalum-doped silica glass was fabricated by the sol-gel process in order to obtain a glass with a high refractive index for optical use. A crack-free, clear glass rod was successfully prepared from a low-density gel and used as the core material for fabricating optical fibers. Transmission loss in the fabricated fibers was high, in the range of 10³-10⁴ dB/km, which may be caused by coloration due to the multivalency of tantalum; however, the loss was reduced by nearly one order of magnitude by heat treatment at 800 °C, that is, to 75 dB/km at a wavelength of 0.8 μm.     

Glass foams: formation, transport properties, and heat, mass, and radiation transfer

Energy efficiency, environmental impact, and quality of the final product in glass manufacturing depend, to a large extent, on foams formed on the surface of the molten glass and of the batch due to entrapment of gas bubbles generated by the batch fusion and refining chemical reactions during the melting process. Hence, understanding the mechanisms of foam formation as well as development of theoretical models for thermophysical and transport properties and heat, mass, and radiation transfer in glass foams are not only a problem of significant fundamental interest but also of tremendous practical impact. In this paper, the review of the current state-of-the-art in our understanding of glass foams is provided, including some of our recent results in modeling the dynamics of the foam growth and its steady-state thickness, prediction of gas diffusion through glass foams, and thermal radiative properties of glass foams. In addition, the new results on simulation of combined conduction and radiation heat transfer in glass foams and radiative transfer in primary (batch) foams are presented and discussed in some detail. The paper also presents practical means available for reducing foaming in glass melting and concludes with the discussion of unresolved problems and summary of the directions for the future work in the area.