Heterogeneous gas bubble formation during GaP solution growth

The gas bubble formation on seed surface, during growth of GaP crystals from the solution, is considered on the basis of the model for heterogeneous nucleation. It is shown that the bubble formation in this case could be a consequence of the interaction between gallium oxide, Ga₂O₃, and gallium generating gaseous gallium protoxide, Ga₂O₃, during heating. The size of the bubbles incorporated in the crystal depends on the kinetics of this reaction and the dissolution rate of Ga₂O₃ in liquid gallium. The values of forces that act on gaseous bubbles in the liquid zone are estimated. It is shown that the migration behaviour of these bubbles is mainly determined by Archimed and thermocapillary forces and depends on growth situations. The methods for avoiding bubble formation in GaP solution growth are suggested.     

Strength of multi-component glass fibers

The initial strength and degradation of strength in multi-component glass fibers are reported. Plastic-coated glass fibers with an initial strength of 300–350 km/mm² are obtained by excluding the causes of extrinsic origins of fracture; bubbles, small crystals and platinum particles. Degradation of strength is strongly dependent on the glass composition, especially on the network modifier content. The addition of Al₂O₃ and B₂O₃ is useful for suppressing degradation of strength. Multi-component glass fibers which show no degradation of strength for 5000 h under stress-free and atmospheric conditions have been prepared with borosilicate glasses containing 1–5 mol.% Al₂O₃.     

Bubble transport in three-dimensional laminar gravity-driven flow - mathematical formulation

This paper presents a complete set of coupled equations that govern the bubble transport in three-dimensional gravity-driven flow. The model accounts for bubble growth or shrinkage due to pressure and temperature changes as well as for multiple gas diffusion in and out of the bubbles but neglects bubble coalescence, break-up, and nucleation. The model applies to glass melting furnaces but it could be extended to other two-phase flow applications such as metal and polymer processing, passive cooling systems, and two-phase flow around naval surface ships. Governing equations are given for the key variables which are, in the present case, (1) the refining agent concentration, (2) the gas species dissolved in the liquid phase, and (3) the bubble radius, gas molar fraction, and density function. The method of solution based on the backward method of characteristics is briefly discussed.     

Gas bubble associated mass transport in Bridgman-Stockbarger melt growth system

Experimental evidence has been obtained showing that gas bubbles may be responsible for melt stirring in Bridgman-Stockbarger growth system which results in Pfann type impurity distribution profiles along the crystal length. The hypothesis of the gas bubble associated mass transfer mechanism is supported by the production of Y₃Al₅O₁₂–Nd³⁺ single crystals under conditions which eliminate or limit gas bubble nucleation in the melts with Nd³⁺ distribution profiles similar to those generally observed in melt growth systems where the mass transport in the melts is limited to diffusion.     

Mass spectrometric analysis of gaseous crystallization medium,growing, and some properties of α-Al2 18O3 crystals

Some gas phase processes during the vacuum vertical directed crystallization of α-Al₂ ¹⁸O₃ are studied in this paper. An effect of these processes on the H, C, N concentrations in the crystals is discussed. The detected pulsations of partial H₂ and C¹⁸O pressures are a result of bubble destruction at the melt-gas interface. The C¹⁸O bubbles are formed because of chemical interaction between the melt and carbon-containing phases of container material. Besides the container wall, the H₂ bubbles can be generated also at the crystallization front supersaturated by H₂. The perfect α-Al₂O₃ and α-Al₂ ¹⁸O₃ crystals grown under the same conditions have noticeable differences of their density, microhardness and lattice parameters. This is explained by an isotopic substitution effect.     

Experimental study of bubble generation during β‐BaB2O4 single crystal growth

The generation of bubble‐inclusions during BaB₂O₄ (BBO) crystal growth from high temperature solution has been optically observed by an in situ observation technique. It was found that bubbles are formed from the peripheries of some hexagonal defects in the (0001) plane of the growing crystal, which may be caused by the evaporation of the air‐opened interface at the high temperature. In addition, atomic force microscope (AFM) was used to investigate the distribution of bubbles. Results revealed that the bubble generation and distribution depend strongly on the microscopic structure of the interface: on a rough interface, bubbles are easily formed and grow rapidly; however, they are greatly suppressed by step trains on a vicinal interface. In the latter case, the height value of a bubble is close to that of the step, which is in the order of several tens of nanometers. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Self‐template hydrothermal synthesis ZnS microspheres

Zinc sulfide (ZnS) microspheres were synthesized by a self‐template hydrothermal route using thiourea as sulphur source. The formation of these hollow spheres was mainly attributed to the oriented aggregation of ZnS nanocystals around the gas‐liquid interface between gas (H₂S, NH₃, or CO₂) and water followed by an Ostwald ripening process. The gas bubbles of H₂S, NH₃, or CO₂ produced during the reaction might play a soft‐template to form ZnS hollow microspheres. The products were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), electron diffraction (ED), and photoluminescence (PL). The crystal structure of prepared ZnS microspheres is hexagonal phase polycrystalline. The average microspheres diameter is 1.5 ‐ 6 µm. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effects of reactor type and mass transfer on the morphology of CuS and ZnS crystals

For the precipitation of CuS and ZnS, the effects of the reactor/precipitator type, mass transfer and process conditions on crystal morphology were studied. Either H₂S gas or a S^2– solution were applied. Three different types of reactors have been tested, namely a laminar jet, a bubble column and an MSMPR reactor. The choice of reactor type as well as mass transfer and metal concentration all have a considerable influence on the morphology of the produced crystals. A well mixed bubble column with H₂S containing gas as feed yields the coarsest crystals. Use is then made of the surface active properties of CuS‐particles, which induce agglomeration at the gas‐liquid interface, where as the low metal concentrations inside the reactor also contribute to the formation of coarser particles (especially for ZnS). (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synthesis and characterization of MnTe nanocrystals in glass

The first evidence of the growth of MnTe nanocrystals successfully synthesized in a glass matrix (SiO₂–Na₂CO₃–Al₂O₃–B₂O₃) using the fusion method is reported. Measurements of optical absorption, photoluminescence, electronic paramagnetic resonance and atomic force microscopy were carried out in order to characterize the produced nanocrystals.     

Characteristics and biocompatibility of Na2O–K2O–CaO–MgO–SrO–B2O3–P2O5 borophosphate glass fibers

A novel Na₂O–K₂O–CaO–MgO–SrO–B₂O₃–P₂O₅ borophosphate glass fiber is prepared. The thermal properties including differential thermal analysis (DTA) and viscosity measurement of the glass were presented. The tensile strength of the glass fiber is measured. The reaction of the glass fibers in the SBF solution is characterized by XRD, FTIR and SEM. XRD and FTIR indicate that the carbonate hydroxyapatite has formed rapidly on the glass. Cell attachment, spreading and proliferation on the glass are determined by MTT [3-(4,5-Dimethylthiazol-2-yl)-2,5-Diphenyltetrazolium Bromide] assay method using Human osteosarcoma MG-63 cells. The bioactivity and biocompatibility of the glass fiber make it a good potential prospect in the field of tissue engineering.     

Conversion of batch to molten glass, I: Volume expansion

Batches designed to simulate nuclear high-level waste glass were compressed into pellets that were heated at 5 K/min and photographed. Three types of batches were prepared, each with different amounts of nitrates and carbonates. The all-nitrate batches were prepared with varying amounts of sucrose. The mixed nitrate-carbonate batches were prepared with silica particles ranging in size from 5 to 195 μm. One batch containing only carbonates was also tested. Sucrose addition had little effect on expansion, while the size of silica was very influential. Sucrose addition reduced primary foam for batches containing 5-μm silica, but had no effect on batches containing larger particles. Excessive amounts of sucrose increased secondary foam. The 5-μm grains had the strongest effect, causing both primary and secondary foam to be generated, whereas only secondary foam was produced in batches with grains of 45 μm and larger. We suggest that the viscosity of the melt and the amount of gas evolved are the main influences on foam production. As more gas is produced in the melt and as the glass becomes less viscous, gas bubbles coalesce into larger cavities until the glass can no longer contain the bubbles and they burst, causing the foam to collapse.     

Growth of bubble-free Ti-doped Al2O3 single crystal by the czochralski method

The behaviour of bubble entrapment in Ti-doped Al₂O₃ single crystals grown by the Czochralski method was studied from the view point of crystal growth conditions such as the crystal rotation rate and kinds of growth atmosphere. The entrapment of bubbles was not correlated with the shape of solid-liquid interface dependent on crystal rotation rate. We found that the use fo He atmosphere, instead of the conventional atmosphere such as Ar, suppresses the formation of bubbles.     

Effect of boron addition on the structure and properties of iron phosphate glasses

Effects of boron addition on the glass forming characteristics, structure and properties of iron phosphate glasses with nominal compositions of xB₂O₃-(40−x)Fe₂O₃-60P₂O₅ (x = 2-20, mol%) and xB₂O₃-(100−x)[Fe₂O₃-60P₂O₅] (x = 2-20, mol%) have been investigated by DTA, XRD, IR and Mössbauer spectroscopy. Although there were some weak local surface crystallizations on especially most of the glasses in group B, all of the compositions formed glass. DTA spectra showed two exothermic peaks corresponding to crystallizations along with an endothermic glass transition peak. T_g increased with increasing B₂O₃ content for the glasses in the first series which indicates that the addition of B₂O₃ increases the thermal stability of glasses in this series while the opposite is observed in the second series. The dissolution rates of boron containing bulk glasses were found to be around 10⁻⁹ gr/cm² min which are comparable to that of the base iron phosphate glass. When the B₂O₃ content was above 14%, new bands related to BO₄ tetrahedral groups have been observed in the IR spectra. The Mössbauer isomer shift values of boron doped glasses were found to be a little lower than that of base glass but both iron ions had distorted octahedral coordination in all glasses. The fraction of Fe²⁺ ions in glasses (Fe²⁺/∑(Fe²⁺ + Fe³⁺)) was found to be 23% for the base glass while it was 10-22% for the boron doped glasses.     

The origin of electrooptical sensitivity of glassy materials: crystal motifs in glasses

The results of studying electrooptical Kerr sensitivity in heavy metal silicate and phosphate glasses and glass-ceramics are presented. A niobium-lithium-silicate glass demonstrating a record Kerr coefficient (266×10⁻¹⁶ m/V²) has been formed. Formation of the transparent glass-ceramics containing electrooptical sodium niobate microcrystals has been studied, and glass-ceramics demonstrating Kerr coefficients higher than 6000×10⁻¹⁶ m/V² have been elaborated. On the base of the effective medium approximation, it is shown that the Kerr coefficient of these glass-ceramics depends on the volume fraction of sodium niobate microcrystals, v_c as a linear function of v_c(1−v_c)⁻² A conception of the origin of electrooptical sensitivity of glasses is proposed. This conception is based on the hypothesis that in glasses there exist regions with exactly crystalline ordering within 2-3 coordination spheres, with these regions having no phase boundaries. These regions are named the crystal motifs (CM). Due to the highly effective mechanism of nuclear polarizability of the electrooptical crystals, the motifs with the symmetry of such crystals are responsible for high permittivity and Kerr sensitivity of the glasses, and they play a role of pre-nuclei while electrooptical glass-ceramics are forming under glass heat treatment. It has been found that synthesized barium-titanate-silicate and niobium-lithium-phosphate glasses demonstrate extremely low Kerr coefficients, and they do not form transparent glass-ceramics with any electrooptical precipitates. This contradicts literature data and is explained by the difference in the conditions of glass synthesis, which are supposed to be responsible for the formation of proper CMs.     

Spectroscopic and electrical studies of sodium-digermanate glasses containing iron oxide

The infrared, optical absorption spectra and electrical conductivity of sodium-digerminate glasses containing iron oxide have been studied as a function of iron content. Addition of iron does not introduce any new absorption band in the infrared spectrum of pure sodium-digermanate glass. A small shift of the existing absorptions toward lower wavenumbers is observed. These are in agreement with the spectra of Fe₂O₃Na₂O2SiO reported earlier. The optical absorption spectra also indicated that the non-bridging oxygen present in pure sodium-digermanate glass was unaffected by the addition of Fe₂O₃ in a small quantity. The DC conductivity measurements revealed “mixed conduction” phenomenon in which ionic as well as electronic conduction occur in the glass.     

Growths of bubble/pore sizes in solid during solidification—an in situ measurement and analysis

The sizes of a bubble trapped in solid after nucleation on the solidification front during an upward freezing of water containing a dissolved oxygen or carbon dioxide gas are experimentally measured and quantitatively predicted in this work. The sizes of the bubble include the height, radius and contact angle of the cap on the solidification front and the length of the bubble in the solid. From in situ measurements of bubble shapes in solid at cold temperatures of −25° and −15°C, it quantitatively shows that pore formation can be divided into five regimes: (1) nucleation on the solidification front, (2) spherical growth, (3) solidification rate-controlled elongation, (4) disappearance of the bubbles, and (5) formation of the pores in solid. To interpret experimental results, equations incorporated with the growth rate of a spherical bubble and solidification rate to predict bubble shapes in the solid during the spherical growth and solidification rate-controlled elongation are successfully proposed. It is found that the time to reach the regime of solidification rate-controlled elongation corresponding to the maximum radius of the bubble is increased by decreasing solidification rate and increasing spherical growth rate of the bubble. Experimental data show the effects of initial gas concentration and solidification rate on geometries of the bubble in solid. Valuable database for more systematical studies of pore formation in solids are provided.     

Structural role of MgO and PbO in MgO–PbO–B2O3 glasses as revealed by FTIR; a new approach

FTIR spectra of three MgO–PbO–B₂O₃ glass series have been analyzed. There is a decrease in the fraction N₄ of four coordinated boron with increasing the MgO content, at the expense of PbO. A new technique has been presented to make use of the N₄ data and follow the change in the modifier and former fractions of PbO and MgO. These fractions change markedly, at different rates, with the glass composition. The fraction of modifier MgO is always less than the MgO content, which suggests a former role of this oxide in the studied glasses. The ability of the glass to include MgO increases with increasing PbO content.     

Dehydration of Er-doped phosphate glasses using reactive agent bubble flow method

This paper investigated on the dehydration of Er³⁺-doped phosphate glasses using O₂ + CCl₄ bubble flow method. The influence of parameters of dehydration effect was studied systemically. It was found that the dehydration rate was rather rapidly at the initial stage of bubbling. Increasing gas flow rate and temperature could drive the dehydration process toward lower OH concentration in the melts. The dehydration process was carried out in open system and analyzed by thermodynamic theory of open system, which clarified the process of eliminating of OH groups from the glass melts with bubbling time and gas flow rate. A time-dependent empirical equation about reaction rate constant was derived, and the relationships between rate constant and bubbling flow rate, and temperature were also discussed. This analytical method could be applicable to the dehydration of glass melt using other dehydration agents.     

Dynamical scaling analysis using the Lillie Number for vitrification of deeply supercooled glycerol

Thermal response was measured for a deeply supercooled glycerol specimen by applying calorimetric temperature scanning rate spectroscopy, cooling the specimen from liquid at a slow constant cooling rate until glass transition was observed. The effective fraction of glass as a function of temperature was determined and a new definition of glass transition temperature, T_gC, as the temperature at which the effective glass fraction to be 0.5 was presented. The relation between this and the cooling rate showed the Arrhenius behavior. The effective glass fraction curves shifted linearly as a function of ln(cooling rate). When T was scaled to the Lillie Number, the glass fraction lay on a master curve, which was successfully fitted with a Kohlrausch–Williams–Watts function. The Kohlrausch exponent, the relaxation time as a function of temperature and the kinetic fragility index were determined. The results were compared with literature values.     

The structural role of titanium in Ca-Sr-Zn-Si/Ti glasses for medical applications

Glasses for medical applications are used in particulate form or as a cement component. This work was undertaken to determine structural changes in 0.48SiO₂-0.36ZnO-0.12CaO-0.04SrO glass when the SiO₂ is substituted with 5 mol% increments of TiO₂. X-ray Diffraction (XRD) was used to determine the presence of crystallinity. This occurred after additions of 20 mol% TiO₂. Differential Thermal Analysis (DTA) and Network connectivity (NC) calculations determined that by increasing the TiO₂ content, the T_g and NC reduced (T_g 670 °C to 632 °C, NC 1.83 to −1.14) suggesting that TiO₂ acts as a modifying oxide. X-ray Photoelectron Spectroscopy (XPS) was used to determine the glass composition and the relative fraction of Bridging Oxygens (BO) to Non-Bridging Oxygens (NBO). XPS revealed that by increasing the concentration of TiO₂, the NBO concentration increases, further suggesting the modifying role of Ti. The NBO/BO ratio was found to increase from 1.2 to 9.0 as the TiO₂ content increased from 0 to 20 mol% additions. Raman spectroscopy was used to determine the Q-Structure of the glass series and found that the addition of TiO₂ reduced the Raman shift from containing predominantly Q¹/Q² units when no Ti was present to Q⁰/Q¹ with TiO₂ additions.