Showing papers on "Devitrification published in 2002"

High-Speed Calorimetry for the Study of the Kinetics of (De)vitrification, Crystallization, and Melting of Macromolecules †

Abstract: This paper reports on the characteristics and use of a new mode of measurement: High Performance DSC (HPer DSC), which represents a major step forward in high-speed calorimetry, as compared to standard DSC. It facilitates the study of the kinetics and metastability of macromolecular systems, especially the analysis of rate-dependent phenomena in real time. Controlled and constant scan rates at hundreds of degrees per minute and combinations thereof both in cooling and in heating are possible. Heats of transition, heat capacities, temperature-dependent crystallinities, etc. can be established at the extreme rates applied. Examples of the utilization of HPer DSC are given for polymers with respect to the effective hindrance of crystallization and cold crystallization, avoidance of recrystallization and the rate dependency of vitrification and devitrification. Low, milligram-scale sample masses, even down to the microgram level, are utilized. The short measuring times also provide the high throughput needed...

Synthesis of Cu47Ti34Zr11Ni8 Bulk Metallic Glass By Warm Extrusion of Gas Atomized Powders

Abstract: Cu47Ti34Zr11Ni8 amorphous gas atomized powders were consolidated by warm extrusion. After consolidation near 723 K using an extrusion ratio of 5, the material retains between 88% and 98% of the amorphous structure found in the gas atomized powder. The onsets of the glass transition and crystallization temperatures of this extruded material are observed respectively at slightly higher and lower temperatures than those of the starting powders. These temperature shifts are attributed to a composition change in the remaining amorphous phase during partial devitrification throughout the extrusion process. Powders extruded at the same temperature, but using higher extrusion ratios of 9 and 13, exhibit substantial devitrification during the consolidation process yet still deform homogeneously.

Crystallization in SiO2–metal Oxide Alloys

Abstract: HfO2–SiO2 and La2O3–SiO2 amorphous alloys were prepared, and their crystallization behavior was studied. The results suggest that higher permittivities can be achieved in the La-containing system without devitrification. The crystallization mechanisms between systems are distinctly different, yet observations are consistent with bulk material. Hf-containing materials tend toward phase separation, while La-containing materials tend toward silicate formation. For Hf-containing films, negligible thickness or time dependence was observed. In La-containing films, rapid thermal anneals could improve crystallization resistance, and thickness effects related to interface reactions were observed. These behaviors are discussed in the context of phase diagrams and metastable immiscibility.

Lead-free thick film resistors: an explorative investigation

Abstract: Pastes prepared with seven lead-free glass particles were screen-printed and fired on 96%-Al2O3 substrates. The fired films were studied in terms of sintering, devitrification, crack propagation and bleeding. The most promising glass compositions were selected for the preparation of RuO2-based thick film resistors (TFRs), on pre-fired PtAu- or PdAg-based terminations; electrical properties of the resistors (sheet resistance vs. RuO2 volume fraction, temperature coefficient of resistance and size effects) were investigated. The study evidenced a myriad of complex phenomena, including devitrification, relevant bleeding of the glass on alumina substrates, anomalous distribution of conductive grains in the glassy matrix, phase separations. Contrarily to what happens in traditional systems, it is difficult to get a wide resistance range simply by changing the fraction of the conductive phase. These results emphasize the criticality in the choice of the glass composition for the preparation of lead-free thick film resistors.

Apparatus and method for making sheet glass by the overflow downdraw fusion process

Abstract: A molten glass delivery system for use in producing sheet glass by the overflow downdraw fusion process is provided. The delivery system includes a first conduit (9) which surrounds a portion of a second conduit (19) with a free surface (21) of molten glass (31) being formed between the two conduits. The first (9) and second (19) conduits are positioned with respect to one another so that the spatial relationship between the exit end (20) of the second conduit (19) and the free surface (21) results in neither substantial numbers of devitrification defects (27, 29) nor substantial numbers of blister defects (35) in finished sheets of devitrification sensitive glass, e.g., high silica LCD glass.

Between glass and crystal: Glass–ceramics, a new way for optical materials

Abstract: We report results of controlled devitrification of rare-earth-doped glasses made in order to improve the initial spectroscopic properties of the as-melted glass to obtain crystal-like properties. This study is made to allow the design of reduced-size photonic devices owing to the use of more efficient active materials. The main possibilities of devitrification of glasses are evaluated in two glassy systems and compared. The key point is the choice of the glass composition allowing partial crystallization during thermal treatments adapted to the thermal stability of the material. A strong improvement in the spectroscopic properties of the rare-earth ions has been obtained in a heterogeneously nucleated glass (GeO2–PbO–PbF2). The final glass-ceramic is ultratransparent with weak additional losses mainly in the short-wavelength region. Another possibility has been evaluated through the spinodal decomposition of a fluorozirconate glass (ZrF4–LaF3–AlF3–GaF3–ErF3). Its spectroscopic properties are also...

Glass-forming ability and crystallization behavior of Co62−xFexNb6Zr2B30 (x=0,16) amorphous alloys with large supercooled liquid region

Abstract: The kinetics of the glass transition and the crystallization process of Co62−xFexNb6Zr2B30 (x=0,16) amorphous alloys with large supercooled liquid region was investigated by differential scanning calorimetry. The dependence of the glass transition on the heating rate was analyzed in terms of the Vogel–Fulcher–Tamman equation. The glass-forming ability of these alloys is discussed with respect to the width of the supercooled liquid region, the reduced glass transition temperature, and the fragility parameter. The studied alloys are found to be rather strong glass formers. The crystalline phases formed after the devitrification are identified by means of x-ray diffraction and thermomagnetic measurements. The complicated crystallization process requiring long-range atomic rearrangements seems to contribute to the high stability of the supercooled liquid against crystallization.

Phase Transformations in Al87Ni7Ce6 and Al87Ni7Nd6 Amorphous Alloys

Abstract: This paper reports on studies of the effect of different rare earth (RE) elements on the devitrification behaviour of alloys of general composition Al 87 Ni 7 RE 6 (here RE = Ce, Nd). We have evidenced two crystallisation mechanisms as a function of the type of rare earth element. When RE = Ce the transformation proceeds in two steps as shown by Differential Scanning Calorimetry (DSC) traces. When RE = Nd the transformation sequence has an additional step. There is also evidence of a calorimetric continuous background for all alloys which is attributed to diffusional homogenisation of the matrix. The glass transition becomes manifest for both alloys when fast enough rates are used in DSC (higher than 10 K/min and 40K/min, respectively for Al 87 Ni 7 Ce 6 and Al 87 Ni 7 Nd 6 ). The first crystallisation step implies the precipitation of nanocrystalline Al. The kinetics of this process is influenced by the simultaneous occurrence of the glass transition. In Al 87 Ni 7 Ce 6 when it takes place above T g (i.e. at high heating rates) it also implies the formation of an intermetallic compound. Kissinger plots for the first transformation display a kink at temperatures corresponding to the glass transition range marking the change in mechanism for the transformation. In Al 87 Ni 7 Nd 6 the glass transition remain visible after partial crystallisation showing that the matrix is readily homogenised. Therefore the molten state of these alloys appears rather fragile.

Devitrification and microstructural coarsening of a fluoride-containing barium aluminosilicate glass

Abstract: Barium aluminosilicate (BAS) glass-ceramics have the potential to be used in the production of cast prostheses for biomedical applications because of their radiopacity and increased strength compared with traditional feldspathic porcelains. It is essential to understand the crystallization kinetics of these materials in order to fabricate products with increased fracture resistance rapidly. It was hypothesized that the addition of fluoride (F) to the composition of BAS glass would reduce the necessary processing time and temperatures by obviating the need for a separate crystal nucleation treatment. BASF glass was subjected to both linear non-isothermal and one-stage isothermal crystallization treatments, and the resulting glass-ceramics were characterized using x-ray diffraction, differential thermal analysis, and stereology. BASF glass had a low energy barrier to crystallization (397 kJ/mol) and transformed to 76 ± 2% crystallinity within 30 min at 975°C. A fine-grained microstructure was produced by bulk crystallization without the need for a separate crystal nucleation stage. After the initial crystal precipitation, the mean crystal size and mean free path between crystals increased over time at elevated temperature by a diffusion rate-limited coarsening mechanism.

Defect control in the making of sheet glass by the fusion process

Abstract: A molten glass delivery system for use in producing sheet glass by a fusion process is provided. The delivery system includes a first conduit ( 9 ) which surrounds a portion of a second conduit ( 19 ) with a free surface ( 21 ) of molten glass ( 31 ) being formed between the two conduits. The first ( 9 ) and second ( 19 ) conduits are positioned with respect to one another so that the spatial relationship between the exit end ( 20 ) of the second conduit ( 19 ) and the free surface ( 21 ) results in neither substantial numbers of devitrification defects ( 27, 29 ) nor substantial numbers of blister defects ( 35 ) in finished sheets of devitrification sensitive glass, e.g., high silica LCD glass.

Synthesis route-dependent formation of quasicrystals in Zr70Pd30 and Zr70Pd20Cu10 amorphous alloys

Abstract: The devitrification of mechanically milled amorphous Zr70Pd30 and Zr70Pd20Cu10 powders occurs via a single-step, first-order transformation to a stable Zr2Pd tetragonal structure. This is in sharp contrast to the devitrification of the same amorphous alloys prepared by melt spinning, in which a primary metastable quasicrystalline phase forms. Since the mechanical milling process does not involve direct liquid phase formation of an amorphous structure, it is inferred that the short-range order in the solid state derived amorphous powder is different from that in the melt spun ribbon.

Devitrification kinetics of PbGeO3

Abstract: The devitrification of glassy PbGeO3 was studied and interpreted by means of isothermal and non-isothermal Johnson-Mehl-Avrami equations. In the case of the non-isothermal approach, several approximated equations proposed by various authors were considered in order to obtain both the activation energy Ea and the Avrami morphological coefficient n of the crystallisation process. A critical discussion of the Avrami coefficient on the basis of experimental morphological evidence is also presented.

A Combined Nuclear Magnetic Resonance and X-ray Absorption Fine Structure Study on the Local Structures of Ge and Pb in PbO−GeO2 Glasses and Their Relationships with Thermal Properties and Devitrification Products

Abstract: Thermal and structural properties of xPbO/(1 - x)GeO 2 glasses with 0.0 ≤ x ≤ 0.50 were studied by means of differential scanning calorimetry (DSC), X-ray absorption fine structure (XAFS) on Pb L I I I and Ge K edges, and 2 0 7 Pb solid-state NMR. Extended X-ray absorption fine structure (EXAFS) evidences on Ge indicated that most of the Ge is in tetrahedral coordination, which exhibits an increasing distortion toward higher distances for x ≥ 0.10. Concerning the Pb environment, both EXAFS and NMR have shown that in all glasses most of the Pb ions are at the apex of a trigonal (PbO 3 ) or square (PbO 4 ) pyramid or both as in crystalline PbGeO 3 . NMR has also shown that Pb enters as a modifier and, by increasing x, the PbO 3 /PbO 4 ratio increases. The thermal behavior of the system can be interpreted by dividing the investigated compositional range into three regions: (i) 0 < x ≤ 0.05; (ii) 0.05 < x < 0.25; (iii) 0.25 ≤ x ≤ 0.50. In both region i and region ii, a nanoscale separation can be inferred because of the presence of nanocrystalline GeO 2 dispersed in the glassy matrix. In region iii, a complete miscibility was observed. The Pb environment and the nanoscale separation can clarify the complex devitrification processes found in this system.

Kinetics of vitrification, glass relaxation and devitrification: a unified treatment

Abstract: A generalized survey is made of a series of recent investigations in which a new, generic theory of vitrification is developed. First a derivation is given of the geometry of the thermodynamics of vitrification and of glass stabilization. Out of the momentary values of the thus constructed thermodynamic functions follows, in a consequent way, the phenomenology of the kinetics of glass transition: of vitrification and of glass stabilization (relaxation). Both processes are treated in the framework of an enlarged, non-linear (or quasi-linear) formulation of irreversible thermodynamics as processes of freezing-in and of stabilization of structural disorder, using a generalized internal structural parameter concept. Thus the basic kinetic dependences governing vitrification, glass stabilization and devitrification (including the Kohlrausch stretched-exponent formula, the Stokes–Einstein relation, the Avrami dependence) follow as particular cases of a formalism, directly based on the main principle of thermodynamics of irreversible processes – the phenomenological law – employed here in a non-linear way. The kinetic equations derived from this new approach are compared with existing experimental evidence. In a second step the summarised phenomenological analysis crystallization kinetics in glass-forming systems (and especially the kinetics of overall devitrification) is derived, considering glasses and glass-forming melts as viscoelastic bodies. Significant results, also confirmed by experiment, follow from the interlocking of crystallization and relaxation kinetics.

Optical glass, glass material for press molding, optical element, and methods of manufacturing them

Abstract: PROBLEM TO BE SOLVED: To provide an optical glass having excellent devitrification resistance, high refractive index, and high dispersibility; a glass material for press molding comprising the optical glass; and an optical element. SOLUTION: The optical glass contains SiO 2 of ≥18 and <30, BaO of ≥12 and <23, TiO 2 of 22-37, Nb 2 O 5 of ≥7 and <16, Na 2 O of 5-20, K 2 O of 0-6, CaO of 0-5, SrO of 0-5, ZrO 2 of 0-4, Ta 2 O 5 of 0-3, Sb 2 O 5 of 0-1, and P 2 O 5 of ≥0 and <0.5, in terms of wt.%, and does not substantially contain PbO, As 2 O 3 and F. Alternatively, the optical glass contains SiO 2 , BaO, and TiO 2 as indispensable ingredients, and has a refractive index (nd) of≥1.80, an Abbe's number (νd) of≤30, and such a devitrification resistance that the numerical density of crystallized particles is ≤12pieces/mm 3 deposited after heated at a temperature by 20°C higher than the glass transition temperature for 5 hours and then kept at 900°C for 5 minutes. The glass material for press molding and the optical device are composed of the optical glass. COPYRIGHT: (C)2004,JPO

Molecular dynamics simulations on devitrification: The PbF2 case

Abstract: In this work molecular dynamics simulations were performed to reproduce the kinetic and thermodynamic transformations occurring during melt crystallization, vitrification, and glass crystallization (devitrification) of PbF2 Two potential parameters were analyzed in order to access the possibility of modeling these properties These interionic potentials are models developed to describe specific characteristic of PbF2, and thermodynamic properties were well reproduced by one of them, while the other proved well adapted to simulate the crystalline structure of this fluoride By a modeled nonisothermal heat treatment of the glass, it was shown that the devitrification of a cubic structure in which the Pb–Pb distances are in good agreement with theory and experiment

Method of manufacturing thin sheet glass and apparatus for manufacturing the same

Abstract: PROBLEM TO BE SOLVED: To provide an apparatus for manufacturing thin sheet glass which is capable of easily forming even the glass, such as non alkaline glass applied to sheet glass for a display substrate of a liquid crystal display, having a high devitrification temperature. SOLUTION: A cooling member 8 is disposed at a heating source 6 of side end members 2 and the downflow glass temperature of an inner layer verging on the heating source 6 is kept at the devitrification temperature of this glass or above and the outside surface temperature of the downflow glass is kept below the devitrification temperature of the glass.

Devitrification-resistant chalcogenide glasses

Abstract: The invention resides in a molecular, inorganic glass and a method of making the glass, the glass being vitreous and resistant to devitrification, that is composed, in substantial part at least, of thermally-stable, zero-dimensional clusters or molecules, composed of four atoms of arsenic and three atoms of sulfur, the glass further containing up to 12 atomic percent of germanium, adjoining clusters being bonded to each other primarily by van der Waals forces, and at least 95% of the glass composition consisting of 42-60% arsenic, 37-48% sulfur plus selenium, the selenium being 0-14%.

Formation and crystallization of Ti–Hf–Ni metallic glasses

Abstract: The formation of a Ti25Hf50Ni25 metallic glass and the phase evolution upon crystallization (devitrification) are reported. The glass is strongly metastable, with a 65 °C separation between the glass transition temperature, Tg=335 °C, and the onset temperature for primary crystallization to a nanoscale microstructure consisting of an icosahedrally symmetric ordered phase, that can not be reported as the icosahedral phase (i-phase). There is no distinction between an i-phase and a high-order crystal rational approximant for such small grains, since the unit cell size of a reasonable high-order approximant is of the order of the grain size. Upon annealing at higher temperatures, the metastable icosahedrally symmetric phase transforms to a stable Ti2Ni-type crystal phase.