Showing papers on "Devitrification published in 2009"

Structural investigation of bismuth borate glasses and crystalline phases

Abstract: Glasses of the system: xBi2O3–(100−x)B2O3 (x = 20 to 66 mol%) were prepared and characterized by density, DSC, UV–visible absorption and 11B MAS-NMR spectroscopy. Glass molar volume increases while the glass transition temperature decreases with Bi2O3 concentration. Densities of some bismuth borate glasses are found to be greater or very close to those of single crystal phases with equal composition. B11 MAS-NMR studies determined that the fraction of tetrahedrally coordinated borons (N4) is maximum at 42 mol% of Bi2O3 and that there is a local maxima in N4 at Bi2O3 concentration of 50 mol%. Glasses containing Bi2O3 concentration of 33 mol% and higher show an unusual, intense absorption band just below the optical band gap. Two crystalline phases: Bi3B5O12 and Bi4B2O9 were prepared by devitrification of glasses and characterized by X-ray diffraction, FTIR and 11B MAS-NMR studies. Both crystalline phases contained significantly lower N4 than glasses with equal composition.

All-solid bandgap guiding in tellurite-filled silica photonic crystal fibers

Abstract: We report all-solid bandgap-guiding fibers formed by pumping molten tellurite glass into silica-air photonic crystal fiber at high pressure. The spectral positions of the guidance bands agree well with multipole simulations and bandgap calculations. The micrometer-diameter tellurite strands are found to contain microheterogeneities (most probably originating from devitrification), which increase the fiber attenuation, although no evidence of crystallization is seen in the bulk tellurite glass. The technique offers a potential route to employing difficult-to-handle glasses, or glasses unsuitable for fiber drawing, in fiber-based amplifiers, modulators, filters, and nonlinear devices.

Conversion of a glassy antiferromagnetic-insulating phase to an equilibrium ferromagnetic-metallic phase by devitrification and recrystallization in Al substituted Pr(0.5)Ca(0.5)MnO(3).

Abstract: We show that Pr0.5Ca0.5MnO3 with 2.5% Al substitution and La0.5Ca0.5MnO3 (LCMO) exhibit qualitatively similar and visibly anomalous M–H curves at low temperature. Magnetic field causes a broad first order but irreversible antiferromagnetic (AF)–insulating (I) to ferromagnetic (FM)–metallic (M) transition in both and gives rise to a soft FM state. However, the low temperature equilibrium state of Pr0.5Ca0.5Mn0.975Al0.025O3 (PCMAO) is FM-M whereas that of LCMO is AF-I. In both systems the respective equilibrium phase coexists with the other phase with contrasting order, which is not in equilibrium, and the cooling field can tune the fractions of the coexisting phases. It is shown earlier that the coexisting FM-M phase behaves like 'magnetic glass' in LCMO. Here we show from specially designed measurement protocols that the AF-I phase of PCMAO has all the characteristics of magnetic glassy states. It devitrifies on heating and also recrystallizes to an equilibrium FM-M phase after annealing. This glass-like AF-I phase also shows a similar intriguing feature observed in FM-M magnetic glassy state of LCMO, that when the starting coexisting fraction of glass is larger, successive annealing results in a larger fraction of the equilibrium phase. This similarity between two manganite systems with contrasting magnetic orders of respective glassy and equilibrium phases points to a possible universality.

Crystallization of monoclinic WO3 in tungstate fluorophosphate glasses

Abstract: Tungstate fluorophosphate glass compositions with high WO3 concentration were prepared in the ternary system (80−0.8x)NaPO3–(20−0.2x)BaF2–xWO3 with x = 40,50 and 60 mol%. Transparency decreases as WO3 concentration increases but can be improved by addition of oxidizing systems such as CeO2 or Sb2O3/NaNO3. Characterizations by thermal analysis (DSC) point out that an increase in the amount of WO3 results in a higher glass transition temperature. In addition, such compositions are very stable against devitrification since samples containing 40% and 50% of WO3 donot even exhibit the expected crystallization event. In these samples, the stability against crystallization decreases with the WO3 content and vitreous sample containing 60% of WO3 exhibits an endothermic event around 620 °C due to crystallization of monoclinic WO3 phase. In these glasses, it was shown that the nucleation stage can be induced by thermal-treatment when external nucleating agents such as Ti or Sb are used. Finally, gold-doped samples exhibit a higher crystallization tendency and monoclinic WO3 phase can be grown in such glasses.

Nanoscale Solute Partitioning in Bulk Metallic Glasses

Abstract: challenge to determine the fine structure of these precipitates, and no one technique can provide all the answers. In this paper, we report the experimental study of a multicomponent BMG alloy, Zr52.5Cu17.9Ni14.6Al10Ti5, utilizing several state-of-the-art characterization techniques. Nanoscale solute partitioning due to strong chemical order is revealed at unprecedented detail by a new wide-field atom probe. This level of details is crucial for understanding the interference peaks observed in small-angle X-ray and neutron scattering experiments, an unsolved mystery for over a decade. A core/shell structure is formed as a result of nanoscale solute partitioning, which poisons the growth and helps stabilize the nanocrystalline particles. Zr52.5Cu17.9Ni14.6Al10Ti5 is a widely studied BMG with excellent glass forming ability. [5‐11] Upon devitrification, crystalline precipitates of 10‐20nm diameter emerge, as evidenced by high-resolution transmission electron microscopy. [9] Moreover,

Devitrification of Fe-Based Amorphous Metal SAM 1651 and the Effect of Heat-Treatment on Corrosion Behavior

Abstract: Devitrification of amorphous alloys is a process in which thermodynamically preferable crystalline phases form from the metastable amorphous phase. The corrosion behavior of the crystalline phases and the remaining amorphous material is likely to be different from that of the fully amorphous material and could have detrimental effects on the corrosion resistance of the devitrified material. The effect of heat-treatment on the corrosion behavior of an Fe-based bulk metallic glass Fe 48 Cr 15 Mo 14 C 15 B 6 Y 2 [structural amorphous metal (SAM) 1651] was examined, which revealed a degradation in the corrosion resistance of SAM 1651 upon annealing at 700°C. However, the partially devitrified material still exhibited good corrosion resistance even in the highly aggressive environments of 6 M HCl. The preferential corrosion sites of nanoscale in both fully amorphous and partially devitrified materials were identified with transmission electron microscope analysis. The corrosion resistance of both fully amorphous and partially devitrified materials was explained in terms of chemical and structural characteristics of the alloys. © 2009 The Electrochemical Society. [DOI: 10.1149/1.3148325] All rights reserved.

Microstructural Evolution of Silica on Single-Crystal Silicon Carbide. Part I: Devitrification and Oxidation Rates

Abstract: Dry thermal oxidation of single-crystal silicon carbide (6H-SiC) at 1400°C in low-water oxygen using an alumina tube furnace initially yields a predominantly vitreous oxide scale. After 28 h, approximately one quarter of the oxide scale's surface becomes crystalline, with disk-like cristobalite aggregates (radialites) statistically distributed within the vitreous matrix. Crystalline areas were found to be thinner than vitreous regions using optical microscopy and atomic force microscopy/hydrofluoric acid (AFM/HF)-etching analysis, providing evidence for different oxidation rates of SiC covered by vitreous silica and cristobalite. As the bulk oxide scale continues to grow during devitrification, the radialites assume their characteristic morphology with a deepened center. Line profiles of the oxide scale's thickness across radialites obtained from AFM/HF etching were used to determine the oxidation rate of SiC covered by crystalline silica and the crystallisation rate applying a two-stage parabolic equation. As a result, it was found that the parabolic rate-constant Bvitreous is ≈4.2 times larger than the corresponding rate-constant in crystalline areas (Bcristobalite), suggesting similar differences in effective oxygen diffusion coefficients. For the crystallization rate ν, we determined a value of 1.5±0.1 μm/h.

Devitrification of Fe-Based Amorphous Metal SAM 1651: A Structural and Compositional Study

Abstract: The effect of heat treatment on the chemistry and structure of an Fe-based bulk metallic glass (BMG) Fe48Cr15Mo14C15B6Y2 (SAM 1651) was examined experimentally. Chemical segregation was found in the as-received material, with islands 10 to 200 nm in diameter enriched in Y, Mo, and C and depleted in Fe and Cr, with respect to the surrounding matrix. Heat treatment in the range of 600 °C to 800 °C caused partial devitrification of the BMG with the formation of nanocrystalline (Fe, Cr)23C6 and (Fe, Cr)7C3 carbides in a matrix of the remaining amorphous phase. The devitrification process followed a primary crystallization route. Amorphous particle-free zones (PFZs) in the devitrified material were found, corresponding to the Y-Mo–rich islands in the fully amorphous SAM 1651. The formation of Cr-rich carbide during devitrification caused the formation of nanometer-sized Cr-depleted zones surrounding the carbide particles, which is detrimental to the corrosion performance of the alloy after thermal exposure.

Fluorogermanate glass with reduced content of OH-groups for infrared fiber optics

Abstract: The hydroxyl (OH − ) impurities in germanium oxide (GeO 2 )-based glasses cause a strong absorption band between 2.4 and 3.6 μm, which makes this region of the electromagnetic spectrum inaccessible for signal transmission. To eliminate the OH − absorption, PbF 2 was employed by partially replacing the PbO content in the following compositions: 56GeO 2 −(31- x ) PbO–9Na 2 O–4Ga 2 O 3 – x PbF 2 ( x = 0, 3, 6, 9, 15, 25, 31). The removal of OH - ions in relation to the concentration of PbF 2 incorporated in the glass-forming liquid was systematically studied. Differential thermal analysis (DTA) and Fourier transform infrared (FTIR) spectroscopy were respectively used for the analysis of glass devitrification and optical properties. The results of FTIR suggest that over 99% OH − impurity was removed, as compared to glasses without a purification process. An optimized core/cladding glass pair was then selected and the rod-in-tube technique was used. Fiber drawing conditions were established using the measured viscosity data and the devitrification range. The low OH − absorption fiber has demonstrated the transmission window up to 4 μm with a loss of 2.34 dB/m at 1.49 μm.

Effects of Doping Trivalent Ions in Bismuth Borate Glasses

Abstract: Bismuth borate glasses from the system: 40Bi2O3–59B2O3–1Tv2O3 (where Tv=Al, Y, Nd, Sm, and Eu) and three glasses of composition: 40Bi2O3–60B2O3, 37.5Bi2O3–62.5B2O3 and 38Bi2O3–60B2O3–2Al2O3 were prepared by melt quenching and characterized by density, UV-visible absorption spectroscopy and differential thermal analysis (DTA) studies. Bismuth borate glasses exhibit a very strong optical absorption band just below their absorption edge. Glasses were devitrified by heat treatment at temperatures above their glass transition temperatures and the crystalline phases produced in them were characterized by Fourier transform infrared (FTIR) absorption spectroscopy and X-ray diffraction (XRD). Bi3B5O12 was found to be the most abundant phase in all devitrified samples. DTA studies on glasses and FTIR and XRD analysis on crystallized samples revealed that very small amounts of trivalent ion doping causes significant changes in the devitrification properties of bismuth borate glasses; rare-earth ions promote the formation of metastable BiBO3–I and BiBO3–II phases during glass crystallization.

High-strength Al 87 Ni 8 La 5 bulk alloy produced by spark plasma sintering of gas atomized powders

Abstract: In situ devitrification and consolidation of gas atomized Al87Ni8La5 glassy powders into highly dense bulk specimens was carried out by spark plasma sintering. Room temperature compression tests of the consolidated bulk material reveal remarkable mechanical properties, namely, high compression strength of 930 MPa combined with plastic strain exceeding 25%. These findings demonstrate that the combined devitrification and consolidation of glassy precursors by spark plasma sintering is a suitable method for the production of Al-based materials characterized by high strength and considerable plastic deformation.

Design of LTCC with High Thermal Expansion

Abstract: New applications of low-temperature co-fired ceramics (LTCC), such as pressure sensors or integrated functional layers, require materials that possess higher coefficients of thermal expansion (CTE). To fabricate LTCC with elevated CTE, two methods of material design are examined: firstly, glass ceramic composites (GCC), which consist of > 50 vol% glass in the starting powder, and, secondly, glass-bonded ceramics (GBC), where glass is added as a sintering aid only. The CTE of GBC is mainly determined by the crystalline component. For GCC, the CTE can be well predicted, if CTE and elastic data of each phase in the microstructure are known. A nonlinear characteristic of the CTE versus phase composition was found with increasing £ crystals/- £ glass ratio and absolute CTE difference between the components. The glass composition and glass amount can be used to compensate the fixed properties of a crystalline material in a desired way. However, because the CTE and permittivity of a glass cannot be chosen independently, an optimum glass composition has to be found. For a given LTCC, it is possible to control the devitrification by shifting the glass composition. In this way, the resulting CTE values can be predicted more exactly and tailoring becomes possible. Different LTCC materials, based on the crystalline compounds Ba(La,Nd) 2 Ti 4 O 12 , ZrO 2 (Y-TZP), SiO 2 (quartz), and specially developed glasses, possessing an elevated CTE of around 10 x 10 -6 K -1 while showing permittivity e r between 6 and 63, are introduced.

Crystallization kinetics and phase transformation in superionic lithium metaphosphate (Li2O–P2O5) glass system

Abstract: The ionic conductivity of mol% 50Li2O–50P2O5 melt quenched glass shows an anomalous increase after its glass transition temperature (Tg) around 590 K. On further increasing the temperature gradually, the conductivity decreases owing to the devitrification of Li2O–P2O5 glass. The evolution of devitrified crystallites was evidenced by XRD patterns. To understand the devitrification process, isothermal and non-isothermal DSC studies have been carried out on mol% 50Li2O–50P2O5 glass. Tg as well as Tc values are found to increase monotonically with increasing heating rates. Variation of Tg as a function of heating rates has been investigated to evaluate the lower limiting temperature of Tg and the activation energy for structural relaxation. Results of the DSC studies indicate (i) single-stage bulk crystallization of the glass, with DSC traces exhibiting a single transition, (ii) an order parameter (Avrami constant) of 2.8 ± 0.1, suggesting internal (bulk) crystallization of the glass, (iii) an activation energy for crystallization equal to 121.7 kJ mol−1 and (iv) the activation energy for structural relaxation, Eg, to be 558.8 kJ mol−1. The crystallization mechanism is closely associated with the JMA model and the experimental dataset have been fitted to a non-isothermal Avrami expression and the obtained parameters confirm the experimental results.

Structure of Na2O–CaO–P2O5–SiO2 Glass–Ceramics with Multimodal Porosity

Abstract: We have investigated the evolution of the structure of nano–macro porous CaO–Na2O–P2O5–SiO2 bioactive glass–ceramics by Fourier transform infrared (FTIR) and Raman spectroscopies, and X-ray diffraction (XRD) A controlled devitrification, followed by a chemical leaching treatment is used to produce a multimodal distribution of nano/macro pores that are expected to improve cell attachment Data show that the leaching process removes the sodium- and calcium-containing crystalline phases that are formed during the ceramming heat treatment The primary Si–O peaks in the infrared spectra blue shift with leaching, indicating that the sample becomes SiO2 rich In parallel, the fraction of nonbridging oxygen decreases These results suggest a restructuring of the glass network far below the glass transition temperature The stresses from leaching, capillary forces, and subsequent restructuring develop and grow, eventually producing cracks in the sample

Structural Similarity on Multiple Length Scales and Its Relation to Devitrification Mechanism: A Solid-State NMR Study of Alkali Diborate Glasses and Crystals

Abstract: The nature of the devitrification process is a fundamental problem in glass science and governs the ultimate stability of glass. It is hypothesized that the devitrification mechanism of a given glass composition is strongly correlated with the extent of structural similarity at the intermediate-range level between the glass and crystal phase to which it transforms. However, relatively little information is available to test this hypothesis because of the lack of long-range periodicity in glass and the absence of effective analysis methods. In this work Li2O−2B2O3 (L2B) and Na2O−2B2O3 (N2B) glasses, which respectively exhibit homogeneous (internal) and heterogeneous (surface) devitrification mechanisms, and corresponding crystals were prepared and comprehensively studied by using multiple solid state NMR techniques. Of those, 11B MAS and MQMAS experiments were applied to reveal short-range order information; 7Li and 23Na spin−echo decay and 11B{6Li} and 11B{23Na} rotational echo double resonance (REDOR) ex...

A low silica, barium borate glass-ceramic for use as seals in planar SOFCs

Abstract: A low silica, barium borate glass–ceramic for use as seals in planar SOFCs containing 64 mol%BaO, 3 mol%Al 2 O 3 and 3 mol%SiO 2 was studied. Coefficient of thermal expansion (CTE) between 275–550 °C, glass transition temperature ( T g ), and dilatometric softening point ( T s ) of the parent glass were 11.9 × 10 −6 °C −1 , 552 °C, and 558 °C, respectively. Glass–ceramic was produced by devitrification heat treatment at 800 °C for 100 h. It was found that nucleation heat treatment, seeding by 3 wt%ZrO 2 as glass-composite and pulverization affected the amount, size and distribution of crystalline phases. SEM-EDS and XRD results revealed that crystalline phases presented in the devitrified glass–ceramic were barium aluminate (BaAl 2 O 4 ), barium aluminosilicate (BaAl 2 Si 2 O 8 ) possibly with boron associated in its crystal structure, and barium zirconate (BaZrO 3 ). CTE of the devitrified glass–ceramic was in the range of (10.1–13.0) × 10 −6 °C −1 . Good adhesion was obtained both in the cases of glass and devitrified glass–ceramic with YSZ and AISI430 stainless steel. Interfacial phenomena between these components were discussed.

Crystallization of bismuth borate glasses

Abstract: Bismuth borate glasses with Bi2O3 concentration of 20?66?mol% were prepared by melt quenching and devitrified by heat treatment above their glass transition temperatures. All glasses show a strong tendency towards crystallization on annealing that increases with Bi2O3 concentration. The crystalline phases formed on devitrification were characterized by FTIR absorption spectroscopy and DSC measurements. Our studies reveal that phases produced in glasses are strongly determined by initial glass composition and the two most stable crystalline phases are: Bi3B5O12 and Bi4B2O9. The metastable BiBO3 phase can also be formed by devitrification of glass with 50?mol% of Bi2O3. This phase is, however, unstable and decomposes into Bi3B5O12 and Bi4B2O9 on prolonged heat treatment.

Interfacial study of Crofer 22 APU interconnect-SABS-0 seal glass for solid oxide fuel/electrolyzer cells

Abstract: In planar solid oxide fuel and electrolyzer cells, compatibility and thermochemical stability of interconnect-seal glass interface is essential in order to avoid mixing and leakage of different gases and degradation of cell performances. In the present work, interfacial compatibility and thermochemical stability are studied for an alkaline earth silicate based glass (SABS-0) and Crofer 22 APU interconnect system with respect to thermal treatment temperature (700–850 °C) and time (0–100 h). The study has been carried out in argon to avoid complications from oxidation. Even though pore and crack-free interface is obtained and maintained for all the thermal treatment conditions, there are simultaneous diffusion of the Crofer 22 APU and the SABS-0 glass elements, chemical reaction at the Crofer 22 APU/SABS-0 interface, and devitrification of the SABS-0 glass itself.

Investigation of the devitrification and microwave penetrating properties of fused silica

Abstract: Four kinds of fused silica (SiO2) bulk materials were prepared by the hot-pressing method. Bulk SiO2 made from sol–gel powders was optically transparent and exhibited the best microwave penetrating properties. Commercial SiO2 obtained from melted quartz increased crystallization. The addition of polyvinyl alcohol (PVA) (aqueous solution) blocked the crystallization of commercial SiO2 to a limited extent. In contrast, PVA powders accelerated the heterogeneous nucleation and growth of crystalline silica. In this case, the bulk material of SiO2–PVA was visibly darkened and exhibited the worst microwave penetrating properties. X-ray diffraction patterns and electron probe X-ray microanalysis confirmed that crystallization was correlated with the optical transmission loss. Crystallized areas randomly dispersed in the continuous amorphous body of fused SiO2, served as scattering centers which affected the optical transparency and also served as microwave absorbing or scattering centers which limited the microwave penetration. The connection between devitrification and microwave penetrating properties of fused SiO2 could therefore be used as a reference to produce other microwave penetrating materials with better performance.

Microstructural Evolution of Silica on Single Crystal Silicon Carbide. Part II: Influence of Impurities and Defects

Abstract: Passive oxidation of single crystal silicon carbide (6H–SiC) resulted in the formation of a vitreous silica layer which crystallized gradually at temperatures near 1300°–1400°C. During this process, statistically distributed devitrification centers appeared and disk-like aligned crystal plates (radialites) formed. The crystallization process did not necessarily start at structural defects although these often act as areas of preferred nucleation. A second structural transition from disk-like radialites to small crystalline spheres (globulites) was not connected to structural defects but a consequence of the presence of impurities. Alkaline and earth-alkaline elements are common contaminations within the atmospheres of typical furnaces fitted with alumina tubes. Globulite formation was a process of recrystallization catalyzed by an impurity-related melt formed on top of the devitrified areas. Crystallization caused exsolution of reaction gas and local impurity enrichments as the solubility for these phases is much higher in the devitrifying vitreous silica matrix. Both clean and impurity-loaded oxidation produced specific morphologies of scales and interfaces.

Glass composition and glass substrate for flat panel display using the same

Abstract: PROBLEM TO BE SOLVED: To provide a glass composition which is suitable for a glass substrate to be used in a flat panel display such as a liquid crystal display device, keeps high thermal stability, does not substantially contain BaO but has low devitrification temperature, is suitable for production of the glass substrate by a downdraw process. SOLUTION: The glass composition contains 54-62 mass% SiO 2 , 4-11 mass% B 2 O 3 , 15-20 mass% Al 2 O 3 , 2-5 mass% MgO, 0-7 mass% CaO, 0-13.5 mass% SrO, 0-1 mass% K 2 O, 0-1 mass% SnO 2 and 0-0.2 mass% Fe 2 O 3 but does not contain BaO substantially. The total content of alkaline earth metal oxides (MgO+CaO+SrO) is 10-18.5 mass%. The devitrification temperature of the glass composition is ≤1,200°C. COPYRIGHT: (C)2011,JPO&INPIT