Structure aspects of the crystallization of silicate glasses

Translated from Steklo i Keramika, No. 6, pp. 34–35, June, 1989.     

Investigation into doping of silica glasses with fluorine by modified chemical vapor deposition

The equilibrium observed during modified chemical vapor deposition (MCVD) of silica glasses doped with fluorine is investigated by chemical thermodynamics. The influence of the flow rate of Freon-12 and sulfur hexafluoride on the efficiency of the incorporation of fluorine into the condensed phase is analyzed. It is revealed that, at very low flow rates of fluorinating reactants, more than 50% of fluorine can be incorporated into the glass. The refractive index and the efficiency of the conversion of SiCl₄ into the fluorine-doped silica glass are studied as functions of the vapor-gas phase composition.     

Structural dependence of crystallization in phosphorus-containing sodium aluminoborosilicate glasses

The article reports on the structural dependence of crystallization in Na₂O–Al₂O₃–B₂O₃–P₂O₅–SiO₂-based glasses over a broad compositional space. The structure of melt-quenched glasses has been investigated using ¹¹B, ²⁷Al, ²⁹Si, and ³¹P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, while the crystallization behavior has been followed using X-ray diffraction and scanning electron microscopy combined with energy dispersive spectroscopy. In general, the integration of phosphate into the sodium aluminoborosilicate network is mainly accomplished via the formation of Al–O–P and B–O–P linkages with the possibility of formation of Si–O–P linkages playing only a minor role. In terms of crystallization, at low concentrations (≤5 mol.%), P₂O₅ promotes the crystallization of nepheline (NaAlSiO₄), while at higher concentrations (≥10 mol.%), it tends to suppress (completely or incompletely depending on the glass chemistry) the crystallization in glasses. When correlating the structure of glasses with their crystallization behavior, the MAS NMR results highlight the importance of the substitution/replacement of Si–O–Al linkages by Al–O–P, Si–O–B, and B–O–P linkages in the suppression of nepheline crystallization in glasses. The results have been discussed in the context of (1) the problem of nepheline crystallization in Hanford high-level waste glasses and (2) designing vitreous waste forms for the immobilization of phosphate-rich dehalogenated Echem salt waste.     

Unveiling crystallization mechanism for controlling nanocrystalline structure in glasses

Controlling nanocrystalline structure in glasses renders the exploration of new composite multiphase (glass-ceramic) materials with novel functionalities that determined by the precipitated nanocrystals and residual glassy matrix. Previous microstructural investigation of glass-ceramics focused only on one aspect of nanocrystalline structures, e.g., nano-polycrystalline or single nanocrystalline. The recognition of the microscopic mechanism of nanostructure formation in glasses is absent. Here, we use advanced microscopic techniques to show the formation of different nanocrystalline structures composed of nano-polycrystals and single nanocrystals in 80GeS₂·20In₂S₃ and 72.5GeS₂·14.5Sb₂S₃·13RbCl glasses, respectively. Crystallization mechanism for controlling the nanocrystalline structure in glasses was revealed to depend on whether the glass network former participates in crystallization process. The results may shed light not only on glass crystallization mechanism, but also on the fundamental nature of the network structure of chalcogenide glasses.     

Preparation and crystallization of glasses in the system tetrasilicic mica-fluorapatite-diopside

The production of glasses whose composition ranged between tetrasilicic mica and fluorapatite-diopsite 50/50 (in wt.%) was investigated. Glass-ceramics were obtained by both bulk crystallization and sintering of glass powder compacts. The experimental results showed that increasing amount of apatite and diopsite components in the ternary system until 50% mica content generally caused decrease of melting temperature and increasing stability of glass against spontaneous crystallization during cooling after casting. Liquid immiscibility, whose features depend on the particular glass composition, characterized all the investigated glasses but it was more pronounced in the glasses with higher amount of apatite and diopsite components. The investigated glasses are preferably crystallized in bulk form between 700 and 900 °C, resulting in formation of different combinations between mica, fluorapatite and diopsite, depending on the particular composition. The obtained glass-ceramics exhibited attractive aesthetics, structural integrity and dense structure.     

PbWO4 formation during controlled crystallization of lead borate glasses

Lead borate glasses were prepared and then heat treated in order to obtain transparent glass-ceramics. Controlled crystallization of precursor lead borate glass at appropriate annealing temperature and time led to formation of the PbWO₄ crystallites. The observed broad blue emission band is related to the PbWO₄ crystallites. The influence of PbX₂ content (X=F, Cl, Br), PbF₂ concentration and lanthanide doping (Eu, Dy) on the excitation and emission spectra of lead borate glass-ceramics containing PbWO₄ phase was examined. The relationship between Pb–X bond and spectral line width of the blue emission can be successfully observed, when halogen X ions (X=F, Cl or Br) are also present in the distorted PbWO₄ crystallites.     

Precipitation of metallic nanoparticles inside silicate glasses by femtosecond laser pulses

Precipitation behaviors of metallic nanoparticles inside silicate glasses by femtosecond laser irradiation are studied in this work. Laser irradiation results easily in precipitation of metallic nanoparticles where colors depend in part on their sizes and quantities. Laser irradiation induced precipitation of metallic nanoparticles displays the different behaviors from that in the unirradiated area inside glasses and their mechanisms are investigated.     

Behavior of copper oxide in silicoborate glazed glasses

Copper-containing glazed glasses in the B₂O₅-SiO₂-Al₂O₃-CaO-CuO system with a copper oxide molar content of 5 to 25%, added during synthesis of the glasses, was investigated. The glasses with a minimum content of 15% B₂O and 15–25% CuO crystallized during processing with separation of fine-needle CuO crystals. However, all glasses formed a good glaze coating when applied on ceramics at temperatures of 900–1000°C. The CuO crystals totally dissolved in the melt. __________ Translated from Steklo i Keramika, No. 8, pp. 9–11, August, 2007.     

Local surface modification of aluminum by laser irradiation

A novel method involving a combination of pulsed YAG laser irradiation and electrochemistry for chemical and physical modification of aluminum surface at a selected area is reviewed. Local metal deposition is carried out by successive steps of anodizing, laser irradiation, and electroplating (or electroless plating), and the fabrication of a prototype for printed circuit boards is attempted. Local organic film deposition with anodizing, laser irradiation, and electrophoretic deposition is introduced, and the formation of microtrenches and pores on aluminum surfaces with anodizing, laser irradiation, and electrochemical etching (or chemical etching) is also described. It is emphasized that the key technology in these procedures is to use aluminum as a target for the laser irradiation in solution, and that anodic oxide films on aluminum play an important role in the processes.     

Photo-thermal chemical vapor deposition of graphene on copper

We demonstrate the synthesis of single-layer graphene films on copper by photo-thermal chemical vapor deposition (PTCVD) realized using a rapid thermal processing system typically used in CMOS processing. Influence of the temperature on the low-pressure (10 mbar) graphene synthesis using methane precursor was characterized by analyzing the crystalline quality, thickness and electronic properties of the films. Using a growth time of only 60 s, for graphene fabricated at 950 °C the sheet resistance and mobility show equivalent quality compared to thermal CVD graphene. Moreover, μ-Raman mapping reveals very low defect density and high 2D to G band ratio similar to the fingerprint of exfoliated single-layer graphene. The synthesis process was found to exhibit a threshold at around 900 °C at which (and below) the single-layer graphene film does not contain adlayer flakes typically observed in high temperature CVD graphene on copper. Our study shows that PTCVD can be used for the high throughput fabrication of high-quality single-layer graphene on copper and is therefore a promising method while pursuing cost-effective graphene fabrication.     

Growing graphene on polycrystalline copper foils by ultra-high vacuum chemical vapor deposition

We show that monolayer graphene can be grown isothermally on polycrystalline copper foils via ultra-high vacuum chemical vapor deposition (UHV-CVD), using acetylene as a carbon precursor. The growth is self-limiting, yielding monolayer graphene with a quality comparable to that of graphene grown by atmospheric- or low-pressure chemical vapor deposition. Copper sublimation, a typical concern for UHV-CVD, is shown to be suppressed by growing graphene domains. Further, the roughness of the copper surface after growth is similar to that of copper foils after growth processes at higher pressures. A dependency of the growth kinetics on the surface orientation of the copper grains is observed and a growth model including all stages of growth is presented and discussed. Similar to observations at higher growth pressures, the graphene domains possess sigmoidal growth, however the overall growth behavior is more complicated with two subsequent growth modes. The role of hydrogen is investigated and shows that, contrary to reports for higher growth pressures, dissolved hydrogen in the copper foil plays an essential role for graphene growth whereas ambient hydrogen does not have a noticeable influence.