Showing papers on "Devitrification published in 2013"

Model vapor-deposited glasses: Growth front and composition effects

Abstract: A growing body of experimental work indicates that physical vapor deposition provides an effective route for preparation of stable glasses, whose properties correspond in some cases to those expected for glasses that have been aged for thousands of years. In this work, model binary glasses are prepared in a process inspired by physical vapor deposition, in which particles are sequentially added to the free surface of a growing film in molecular dynamics simulations. The resulting glasses are shown to be more stable than those prepared by gradual cooling from the liquid phase. However, it is also shown that the composition of the resulting glass, which is difficult to control in physical vapor deposition simulations of thin films, plays a significant role on the physical characteristics of the material. That composition dependence leads to a re-evaluation of previous results from simulations of thinner films than those considered here, where the equivalent age of the corresponding glasses was overestimated. The simulations presented in this work, which correspond to films that are approximately 38 molecular diameters thick, also enable analysis of the devitrification mechanism by which vapor-deposited glasses transform into the supercooled liquid. Consistent with experiments, it is found that this mechanism consists of a mobility front that propagates from the free interface into the interior of the films. Eliminating surface mobility eliminates this route of transformation into the supercooled liquid.

Model Vapor-Deposited Glasses: Growth Front and Composition Effects

Abstract: A growing body of experimental work indicates that physical vapor deposition provides an effective route for preparation of stable glasses, whose properties correspond in some cases to those expected for glasses that have been aged for thousands of years. In this work, model binary glasses are prepared in a process inspired by physical vapor deposition, in which particles are sequentially added to the free surface of a growing film. The resulting glasses are shown to be more stable than those prepared by gradual cooling from the liquid phase. However, it is also shown that the composition of the resulting glass, which is difficult to control in physical vapor deposition simulations of thin films, plays a significant role on the physical characteristics of the material. That composition dependence leads to a re- evaluation of previous results from simulations of thinner films than those considered here, where the equivalent age of the corresponding glasses was overestimated. The simulations presented in this work, which correspond to films that are approximately 38 molecular diameters thick, also enable analysis of the devitrification mechanism of vapor-deposited glasses. Consistent with experiments, it is found that this mechanism consists of a mobility front that propagates from the free interface into the interior of the films. Eliminating surface mobility eliminates this route of transformation into the supercooled liquid.

Devitrification of arc-sprayed FeBSiNb amorphous coatings: Effects on wear resistance and mechanical behavior

Abstract: FeBSiNb amorphous coatings were synthesized by arc spray processing The effects of annealing treatments on the microstructure, elastic and mechanical properties of the coatings were investigated Annealing below 650 °C induces the formation of α-Fe phase nanocrystals Annealing at 650 °C promotes the formation of Fe 3 B, Fe 23 B 6 phases and α-Fe solid solution The as-sprayed coating displays ultra-high hardness ( H > 16 GPa), high reduced Young's modulus ( E r > 210 GPa) and good wear resistance These properties are further improved after annealing treatments ( H > 20 GPa and E r > 260 GPa are achieved in the fully crystallized coating) The mechanical hardening is accompanied with an increase of the ratio of hardness to elastic modulus ( H / E r ), the yield pressure ( H 3 / E r 2 ) and the elastic recovery ( η value) The different microstructural mechanisms responsible for these annealing-induced changes in mechanical and elastic properties are discussed

Glass Crystallization Research — A 36-Year Retrospective. Part I, Fundamental Studies †

Abstract: Over the past 36 years, our research has mainly focused on the crystallization kinetics and properties of glasses and glass-ceramics. A sampling of the most conclusive papers are reviewed here and include the following topics: the effects of liquid phase separation on crystal nucleation; tests and development of models of crystal nucleation, growth, and overall crystallization; metastable phase formation; surface crystallization; glass stability against devitrification; glass-forming ability; possible correlations between the molecular structure and the nucleation mechanism; sintering with concurrent surface crystallization; and diffusional processes that control crystallization. We conclude that despite significant advancement in the knowledge about several aspects of phase transformations in glasses that resulted from our own research and of other groups, which is not described in this review, glass crystallization remains an open, rich field to be explored.

Effect of magnesium ion incorporation on the thermal stability, dissolution behavior and bioactivity in Bioglass-derived glasses

Abstract: There is a strong discrepancy in the literature regarding the effect of magnesium on bioactive glasses. Hence the present study is focused on the physical and chemical behavior of the “golden standard” 45S5 glass and magnesium-containing bioactive glasses developed here to evaluate their reactivity and in vitro bioactivity. The aim of this study was to analyze the influence of CaO replacement by MgO, especially its effect on the rate of formation of the apatite-like layer at the glass surface, the reaction kinetics between the glasses and simulated body fluid (SBF-K9) and on the glass stability against devitrification during heating. Five melt-derived bioactive glasses of the system 24.3Na 2 O–26.9(xCaO − (1 − x)MgO)–46.3SiO 2 –2.5P 2 O 5 (x = 1; 0.875; 0.75; 0.625 and 0.5) were synthesized with CaO progressively replaced by MgO. Their thermal stability on heating was characterized by DSC analysis. Their degradation and ability to form an apatite-like layer were evaluated through in vitro tests by immersion in SBF-K9; FTIR, ion selective electrode analysis and by solid state nuclear magnetic resonance (NMR) spectroscopy. Our results indicate that magnesium plays an important role in the stability of this glass family, defined as the difference between the glass transition temperature T g and crystallization temperature T x . The lower T g observed in the MgO-rich glasses and insignificantly changed solubilities, as well as the 29 Si NMR results suggest that in this glass system MgO does not act as a network intermediate or former oxide, but as network modifier, as we expected. Dissolution kinetics, FTIR, and solid state 31 P and 1 H MAS-NMR consistently indicate that partial replacement of CaO by MgO in the bioglass does not influence the rate at which the initial amorphous calcium phosphate (ACP) layer is precipitated when the glass is exposed to SBF. In contrast it greatly reduces the rate of conversion of this precursor phase to the crystalline hydroxycarbonate apatite (HCA)-layer.

Infrared to Visible Up-Conversion Emission in Er3+/Yb3+ Codoped Fluoro–Phosphate Glass–Ceramics

Abstract: Erbium Er3+ and ytterbium Yb3+ codoped fluoro-phosphate glasses belonging to the system NaPO3–YF3–BaF2–CaF2 have been prepared by the classical melt-quenching technique. Glasses containing up to 10 wt% of erbium and ytterbium fluorides have been obtained and characterized using differential scanning calorimetry (DSC) and UV–visible and near-infrared spectroscopy. Transparent and homogeneous glass–ceramics have been then reproducibly synthetized by appropriate heat treatment above glass transition temperature of a selected parent glass. Structural investigations of the crystallization performed through X-ray diffractometry (XRD) and scanning electron microscopy (SEM) have evidenced the formation of fluorite-type cubic crystals based during the devitrification process. Finally, infrared to visible up-conversion emission upon excitation at 975 nm has been studied on the Er3+ and Yb3+ codoped glass–ceramics as a function of thermal treatment time. A large enhancement of intensity of the up-conversion emissions–about 150 times- has been observed in the glass–ceramics if compared to the parent glass one, suggesting an incorporation of the rare-earth ions (REI) into the crystalline phase.

Crystallization-aided extraordinary plastic deformation in nanolayered crystalline Cu/amorphous Cu-Zr micropillars

Abstract: Metallic glasses are lucrative engineering materials owing to their superior mechanical properties such as high strength and great elastic strain. However, the Achilles' heel of metallic amorphous materials — low plasticity caused by instantaneous catastrophic shear banding, significantly undercut their structural applications. Here, the nanolayered crystalline Cu/amorphous Cu-Zr micropillars with equal layer thickness spanning from 20–100 nm are uniaxially compressed and it is found that the Cu/Cu-Zr micropillars exhibit superhigh homogeneous deformation (≥ 30% strain) rather than localized shear banding at room temperature. This extraordinary plasticity is aided by the deformation-induced devitrification via absorption/annihilation of abundant dislocations, triggering the cooperative shearing of shear transformation zones in glassy layers, which simultaneously renders the work-softening. The synthesis of such heterogeneous nanolayered structure not only hampers shear band generation but also provides a viable route to enhance the controllability of plastic deformation in metallic glassy composites via deformation-induced devitrification mechanism.

The role of Bi2O3 on the thermal, structural, and optical properties of tungsten-phosphate glasses.

Abstract: Glasses in the ternary system (70 – x)NaPO3-30WO3-xBi2O3, with x = 0–30 mol %, were prepared by the conventional melt-quenching technique. X-ray diffraction (XRD) measurements were performed to confirm the noncrystalline nature of the samples. The influence of the Bi2O3 on the thermal, structural, and optical properties was investigated. Differential scanning calorimetry analysis showed that the glass transition temperature, Tg, increases from 405 to 440 °C for 0 ≤ x ≤ 15 mol % and decreases to 417 °C for x = 30 mol %. The thermal stability against devitrification decreases from 156 to 67 °C with the increase of the Bi2O3 content. The structural modifications were studied by Raman scattering, showing a bismuth insertion into the phosphate chains by Bi–O–P linkage. Furthermore, up to 15 mol % of Bi2O3 formation of BiO6 clusters is observed, associated with Bi–O–Bi linkage, resulting in a progressive break of the linear phosphate chains that leads to orthophosphate Q0 units. The linear refractive index, n0,...

Glass substrate for cu-in-ga-se solar cell, and solar cell using same

Abstract: The present invention provides a glass substrate for Cu-In-Ga-Se solar cells that contains, as given in standard mole percentages for the oxides, 55 - 70% SiO2, 6.5 - 12.6% Al2O3, 0 - 1% B2O3, 3 - 10% MgO, 0 - 4.8% CaO, 0 - 2% SrO, 0 - 2% BaO, 0 - 2.5% ZrO2, 0 - 2.5% TiO2, 5.3 - 10.9% Na2O, and 0 - 10% K2O, such that the MgO + CaO + SrO + BaO is 7.7 - 17%, the Na2O + K2O is 10.4 - 16%, the MgO/Al2O3 is 0.9 or less, (2Na2O + K2O + SrO + BaO)/(Al2O3 + ZrO2) is 2.2 or less, and (Na2O + K2O)/Al2O3 × (Na2O/K2O) is 0.9 or greater. The glass transition temperature of the glass substrate is 650 - 750°C; the average coefficient of thermal expansion is 75 × 10-7 - 95 × 10-7/°C at 50 - 350°C; the relationship between the temperature (T4) at which the viscosity is 104 dPa·s and the devitrification temperature (TL) is T4 - TL ≥ -30°C; the density is 2.6 g/cm3 or less; and the brittleness index value is less than 7000 m-1/2. Thus, a glass substrate for CIGS solar cells that satisfies the characteristics for high power generation efficiency, high glass transition temperature, prescribed average coefficient of thermal expansion, high glass strength, low glass density, and devitrification prevention during the formation of plate glass with a good balance can be provided.

Sintering and devitrification of glass-powder compacts in the akermanite–gehlenite system

Abstract: The sintering and devitrification behavior of glass-powder compacts with four compositions, Ca2Mg0.5Al1.0Si1.5O7, Ca2Mg0.6Al0.8Si1.6O7, Ca2Mg0.7Al0.6Si1.7O7, and Ca2Mg0.8Al0.4Si1.8O7, corresponding to akermanite–gehlenite ratios (mol%) of 50/50, 60/40, 70/30, and 80/20 were investigated. Glass frits were prepared by the classical melt quenching technique in water. The structure of the glasses was investigated using FTIR and NMR, whereas the sintering behavior was studied by DTA and HSM. Sintering precedes crystallization only in Ca2Mg0.5Al1.0Si1.5O7 glass while in the remaining glass compositions maximum densification was achieved slight after the onset of crystallization. However, the ratios of final area/initial area (A/A 0) of the glass-powder compact ranging from 0.63 to 0.66 imply towards good densification levels (95–98 %) achieved in the investigated glasses. Qualitative and quantitative XRD analyzes were performed in glass-powder compacts heat treated at 900 and 1000 °C. Merwinite was found to crystallize first followed by decomposition at higher temperatures to form akermanite-like phase.

High-strength glass-ceramics in the system MgO/Al2O3/SiO2/ZrO2/Y2O3 – microstructure and properties

Abstract: Glasses with the composition 51.9SiO2·21.2MgO·21.2Al2O3·5.7ZrO2 (in mol%) were crystallized in different annealing steps at 950 °C and 1060 °C. Using X-ray diffractometry and high resolution (scanning) transmission electron microscopy with energy-dispersive X-ray spectroscopy, it is found that the annealing-induced devitrification results in glass-ceramics with α-quartz solid solution as the main crystal phase, with a few additional ZrO2 and MgAl2O4 crystals as secondary crystal phases. If 2.5 mol% Y2O3 are added to the base glass, however, an identical heat treatment leads to glass-ceramics with dendritic ZrO2 crystals as the main crystal phase, and subsequently grown MgAl2O4 as the secondary crystal phase. The quartz solid solution was found to be absent in those Y2O3-bearing samples, as Y2O3 is incorporated in the residual glass matrix, thus stabilizing it. It was observed that the bending strength was comparable for both sample sets, thus indicating that the existence of α-quartz is not necessarily an important prerequisite for excellent mechanical properties of glass-ceramic materials based on the MgO/Al2O3/SiO2 system. Strengthening mechanisms for both Y2O3-bearing and Y2O3-free glass-ceramics are discussed.

Liquidus surface of MgO–CaO–Al2O3–SiO2 glass-forming systems

Abstract: Alkaline earth aluminosilicate systems are of widespread technological importance, and it is therefore essential to have accurate phase equilibrium diagrams available. While the phase equilibria for the single alkaline earth CaO–Al 2 O 3 –SiO 2 and MgO–Al 2 O 3 –SiO 2 systems are fairly well established, the mixed MgO–CaO–Al 2 O 3 –SiO 2 systems have received little attention despite their important role in the high-tech glass industry. Here, the liquidus surface (liquidus temperature T liq and primary devitrification phase) is determined by x-ray diffraction phase analyses of isothermally reacted samples from powder mixtures. The phase equilibrium diagrams are presented based on measurements on a total of 120 compositions. In the composition range of interest for industrial glasses, T liq tends to increase with increasing silica content and decreasing alkaline earth-to-alumina ratio. Cristobalite, mullite, anorthite, and cordierite are found to be the dominant devitrification phases for the compositions with high SiO 2 , Al 2 O 3 , CaO, and MgO contents, respectively. Anorthite extends further into the cristobalite and mullite fields for the [CaO]/[MgO] = 3 system compared to the cordierite behavior in systems with [MgO]/[CaO] = 3. For systems with [MgO] = [CaO], the liquidus temperatures are shifted to lower values since the formation of anorthite and cordierite requires some critical concentration of CaO and MgO, respectively.

Microstructural evolution in two alkali multicomponent silicate glasses as a result of long-term exposure to solid oxide fuel cell environments

Abstract: The microstructural evolution in two potential solid oxide fuel cell (SOFC) sealing glass materials exposed to air and a gas mixture of steam + H2 + N2 at 800 °C up to 10000 h was determined. The glass exposures were performed on common SOFC substrates like alumina and zirconia. Characterization of the crystalline phases and pore size distribution was performed for the specimens with various exposure conditions. Comparison of the microstructural and chemical stability of the two glasses was performed based on known trends related to glass chemistry. It was observed that multicomponent glasses followed few rules for chemical and microstructural stability reported in the literature for glasses with fewer components. The two glasses examined in this study displayed adequate resistance to devitrification but marginal resistance to porosity changes in the SOFC environment exposure. The implications of the results for the design and long-term performance of SOFC seals are discussed.

Preparation method and thermal properties of samarium and europium-doped alumino-phosphate glasses

Abstract: The present work investigates alumino-phosphate glasses from Li2O–BaO–Al2O3–La2O3–P2O5 system containing Sm3+ and Eu3+ ions, prepared by two different ways: a wet raw materials mixing route followed by evaporation and melt-quenching, and by remelting of shards. The linear thermal expansion coefficient measured by dilatometry is identical for both rare-earth-doped phosphate glasses. Comparatively to undoped phosphate glass the linear thermal expansion coefficient increases with 2 × 10−7 K−1 when dopants are added. The characteristic temperatures very slowly decrease but can be considered constant with atomic weight, atomic number and f electrons number of the doping ions in the case of Tg (vitreous transition temperature) and Tsr (high annealing temperature) but slowly increase in the case of Tir (low annealing temperature–strain point) and very slowly increase, being practically constant in the case of TD (dilatometric softening temperature). Comparatively to undoped phosphate glass the characteristic temperatures of Sm and Eu-doped glasses present lower values. The higher values of electrical conductance for both doped glasses, comparatively to usual soda-lime-silicate glass, indicate a slightly reduced stability against water. The viscosity measurements, showed a quasi-linear variation with temperature the mean square deviation (R2) being ranged between 0.872% and 0.996%. The viscosity of doped glasses comparatively to the undoped one is lower at the same temperature. Thermogravimetric analysis did not show notable mass change for any of doped samples. DSC curves for both rare-earth-doped phosphate glasses, as bulk and powdered samples, showed Tg values in the range 435–450 °C. Bulk samples exhibited a very weak exothermic peak at about 685 °C, while powdered samples showed two weak exothermic peaks at about 555 °C and 685 °C due to devitrification of the glasses. Using designed melting and annealing programs, the doped glasses were improved regarding bubbles and cords content and strain elimination.

The effect of composition on pressure-induced devitrification in metallic glasses

Abstract: Long-range topological order (LRTO) was recently revealed in a Ce75Al25 metallic glass (MG) by a pressure-induced devitrification (PID) at 300 K. However, what compositions may have PID and an understanding of the physical and chemical controls behind PID are still not clear. We performed in situ high pressure x-ray diffraction measurements on CexAl1−x (x = 65, 70, and 80 at. %) MGs. Combining our experimental results and simulations, we found PID is very sensitive to compositions and can only exist over narrow compositional ranges. These results provide valuable guidance for searching for PID in MGs.

Structure–property correlations in lead silicate glasses and crystalline phases

Abstract: Lead silicate glasses containing 40–65 mol% of PbO were prepared at two melt-quenching rates and characterized by X-ray diffraction, UV-Visible absorption spectroscopy, density, microhardness, thermo-mechanical analysis, differential scanning calorimetry and Raman scattering studies. On increasing the PbO concentration, density increases, glass transition temperature decreases and the optical absorption edge shifts towards longer wavelength. An intense optical absorption band was observed just below the absorption edge in glasses with 55 mol% and higher concentration of PbO. Dilatometric measurements show an unusual property that glasses do not show any abrupt increase in volume near the glass transition temperature but transform directly into the liquid state. Raman spectroscopy confirmed that the concentration of SiO4 tetrahedra containing one or more NBOs increase with PbO mol%. Devitrification studies on lead silicate glasses found that samples with 40–45 mol% of PbO do not crystallize, whereas sample...

Melilite glass–ceramic sealants for solid oxide fuel cells: effects of ZrO2 additions assessed by microscopy, diffraction and solid-state NMR

Abstract: The influence of adding 0–5 mol% zirconia (ZrO2) to a series of melt-quenched alkaline-earth aluminosilicate glasses designed in the gehlenite (Ca2Al2SiO7)–akermanite (Ca2MgSi2O7) system has been investigated for their potential application as sealants for solid oxide fuel cells (SOFCs). The work was implemented with a dual aim of improving the sintering ability of the glass system under consideration and gaining insight into the structural changes induced by ZrO2 additions in the glasses consequentially leading to their enhanced long-term thermal stability. That the degree of condensation of SiO4 tetrahedra increased with increasing amounts of zirconia was confirmed by 29Si magic-angle (MAS) NMR. 1D 27Al, 11B MAS as well as two-dimensional (2D) 11B MQMAS/STMAS NMR experiments gave structural insight into the number and nature of aluminum and boron sites found in the glass and glass–ceramic (GC) samples. Irrespective of the heat treatment time, increasing the zirconia content in glasses suppressed their tendency towards devitrification, while the glasses exhibited good sintering behavior resulting in mechanically strong GCs with higher amounts of residual glassy phase making them suitable for self-healing during SOFC operation. All the GCs exhibited low total electrical conductivity; appropriate coefficients of thermal expansion (CTE), good joining and minimal reactivity with SOFC metallic components at the fuel cell operating temperature, thus, qualifying them for further appraisal in SOFC stacks.

Glass and Glass-Ceramics

Abstract: The classic definition of glass is based on the historical method of formation: This is a very unusual way of defining any material. The result is that glass is now defined in several different ways.

Effect of ZnO on Crystallization of Zircon from Zirconium-Based Glaze

Abstract: The promotion of zircon (ZrSiO4) crystallization by ZnO from a zirconium-based frit glaze was studied and the possible mechanism was discussed. X-ray diffraction was used to analyze the relative quantities of zircon and other transitional crystals in the samples. The results show that ZnO can significantly decrease the crystallization temperature of zirconium-based glaze, depress the formation of Ca2ZrSi4O12, and promote the devitrification of transitional crystals t-ZrO2 and Ca2ZnSi2O7, as well as lead to the formation of more zircon than the ZnO-free glaze. It was also found that zircon not only can form from the interaction between t-ZrO2 and SiO2 but also can devitrify directly from the glass phase of zirconium-based glaze.

Novel microcrystal glass ceramic composite board and preparation method thereof

Abstract: The invention discloses ingredients, manufacturing techniques and technological parameters of a novel microcrystal glass ceramic composite board. The novel microcrystal glass ceramic composite board is characterized in that the ingredients contain no zinc oxide. The ingredients comprise 50-65% of SiO2, 5-20% of Al2O3, 5-20% of CaO, 5-20% of MgO, 2-8% of K2O+Na2O, 0-1% Li2O, 2-8% of NaO, 3-8% B2O3 and balanced 5-20% of clarifying agent, nucleation agent and coloring agent. A pyroxene microcrystal phase is separated out from glass ceramics, the devitrification breaks through particle boundaries, and crystallization textures in various patterns are grown. The novel microcrystal glass ceramic composite board achieves purposes of low cost and high decorative art levels.

Effect of deviations of composition from the quasi-binary section TiNi-TiCu on structural and phase transformations in rapidly quenched alloys

Abstract: Methods of Xray diffraction, transmission an d scanning electron microscopy, and selectedarea electron diffraction (SAED) have been used to study the phase and elemental composition and structure of alloys close to the stoichiometric Ti50Ni25Cu25 alloy. Based on the method of rapid quenching of the melt (freejet melt spinning), alloys of the quasibinary T iNi-TiCu section have been prepared, which in the ini� tial ascast state exhibited the ther moelastic martensitic transformations B2 ↔ B19 and related shapemem� ory effects. The chemical composition of the Ti50 + xNi25Cu25 - x alloys was varied by changing titanium and copper concentrations within x ≤ ±1 at % (from Ti49Ni25Cu26 to Ti51Ni25Cu24). It has been established that quenching at a cooling rate equal to 10 6 K/s leads to the amorphization of all the alloys under consideration. Heating to 723 K and higher leads to the devitrification of the alloy with the formation of a nanocrystalline or submicrocrystalline structure of the B2 austenite. The mechanical properties of these alloys have been measured in the initial amorphous state and in the polycrystalline martensitic state. It has been shown that, depending on the extent of the deviations of the alloy composition from the stoichiometry, which cause the decomposition of the alloys in the process of nanocrystallization, regular changes are observed in their mechanical properties and in the shapememory effects. The kinetics of the processes of the devitrification of the alloys, as well of the forward and reverse martensitic transformations, have been studied, their charac� teristic temperatures have been determined, and a diagram of the dependence of the characteristic tempera� tures on the chemical composition of the alloys has been constructed.

Effect of antimony addition on thermal stability and crystallization kinetics of germanium-selenium alloys

Abstract: Chalcogenide glasses are promising materials for threshold and memory switching. Alloys of Ge19Se81 − xSbx (x = 0, 4, 8, 12, 16, 17.2, 20) have been prepared by melt quench technique. Differential thermal analysis has been used to characterize samples at different heating rates (α = 5, 10, 15, 20 K/min). Glass transition temperatures, crystallization temperature, melting temperature, activation energy for glass transition and crystallization have been determined. Thermal stability of alloys has been analyzed in terms of ease by which atomic rearrangements take place and the resistance to devitrification. The correlation between thermal stability and glass forming ability has also been studied.