Showing papers on "Devitrification published in 2018"

Magnetic and spectroscopic studies of an iron lithium calcium silicate glass and ceramic

Abstract: A glass forming matrix consisting of SiO2 – Li2O – CaO was doped with 10% and 15% molar of Fe2O3 and investigated for changes in structural and bulk physical properties of the host when doped with the iron oxide. The vitreous nature of the samples decreases with increase of Fe2O3 from 10 mol% to 15 mol% which is confirmed by the increase in intensity of Bragg's peaks of the XRD patterns and a decrease in the temperature difference between glass transition (Tg) and glass crystallisation (Tc) as found by DTA. Devitrification in this system was achieved by over-doping with Fe rather than heat treatment near Tg. FTIR analysis also reveals that the introduction of Fe ions into the glass matrix causes the anti-symmetric Si-O-Si to shift to lower frequencies on account of the presence of the Fe ions in their modifying structural locations. The optical band gap for all the samples falls in the insulator region. Magnetisation studies of the glass samples show them to be paramagnetic throughout the temperature range of 300 K to 5 K, whereas the glass ceramic sample shows room temperature ferrimagnetism with paramagnetic behaviour at low temperatures attributed to anti-ferromagnetic interactions. Moreover, samples containing iron show a decrease in band gap to almost half of that without iron ions (from 4.05 eV to 2.60 eV) with a further decrease in band gap with increase of iron content. Thus, in principle an increase in Fe content or relative ratios of Fe2 + and Fe3 + as would result from oxygen deficiencies around these ions can be used to achieve desired magnetic and optical properties.

Kinetic arrest of front transformation to gain access to the bulk glass transition in ultrathin films of vapour-deposited glasses

Abstract: Physical vapour deposition has emerged as the technique to obtain glasses of unbeatable stability. However, vapour deposited glasses exhibit a different transformation mechanism to ordinary glasses produced from liquid. Vapour deposited glasses of different thermodynamic stability, from ultrastable to those similar to ordinary glasses, transform into the liquid state via front propagation starting at the most mobile surfaces/interfaces, at least for the first stages of the transformation, eventually dynamiting the high thermal stability achieved for some of these glasses. A previous study showed that it was possible to avoid this transformation front by capping the films with a higher Tg material. We show here fast calorimetry measurements on TPD and IMC vapour deposited glasses capped respectively with TCTA and TPD. This capped configuration is very effective in suppressing the heterogeneous transformation of the stable glasses into the supercooled liquid and shifts the devitrification temperature to much higher values, where the bulk homogeneous mechanism becomes active. This approach may be useful to further study the bulk glass transition in thin films.

Electrical switching in Si 20 Te 80 - x Bi x (0 ≤ x ≤ 3) chalcogenide glassy alloys

Abstract: Chalcogenide glasses have attained enormous research interest due to their importance in finding electronic memories. Here we report electrical switching and thermal crystallization behavior of Si20Te80 − xBix (0 ≤ x ≤ 3) glasses. We observe a significant decrease in the threshold voltage (VT) and the thermal stability (ΔT), indicating that in Si20Te80 − xBix glasses, the resistivity of the additive element Bi plays a dominant role over network connectivity/rigidity. The variation of VT with respect to thickness and temperature of the sample indicates that the memory switching observed in Si20Te80 − xBix glasses is influenced by the thermally induced transitions (thermal mechanism). Scanning electron microscopy (SEM) studies on pre-switched and post switched samples reveal the morphological changes on the surface of the sample, and serve as an experimental evidence for the formation of the crystalline filament between two electrodes during switching. Furthermore, the decrease in ΔT values indicates that the Si-Te glasses become de-vitrifiable more easily with the addition of Bi, influencing the decrease of VT. Structural evaluation like thermal devitrification studies and morphological changes elucidate the restricted glass formability of the studied glass system.

Thermal Study of Polyols for the Technological Application as Plasticizers in Food Industry.

Abstract: In this work is presented the complete thermal analysis of polyols by direct methods such as simultaneous thermogravimetric and differential thermal analyzer (TGA-DTA), differential scanning calorimetry (DSC), modulated DSC (MDSC), and supercooling MDSC. The different thermal events in the temperature range of 113–553 K were identified for glycerol (GL), ethylene glycol (EG), and propylene glycol (PG). Boiling temperature (TB) decreased as GL > EG > PG, but increased with the heating rate. GL showed a complex thermal event at 191–199 K, identified as the glass transition temperature (Tg) and devitrification temperature (Tdv), and a liquid–liquid transition (TL-L) at 215–221 K was identified as the supercooling temperature. EG showed several thermal events such as Tg and Tdv at 154 K, crystallization temperature (Tc) at 175 K, and melting temperature (Tm) at 255 K. PG also showed a complex thermal event (Tg and Tdv) at 167 K, a second devitrification at 193 K, and TL-L at 245 K. For PG, crystallization was not observed, indicating that, during the cooling, the liquid remained as an amorphous solid.

Microstructure and soft magnetic properties of (Fe0.9Co0.1)73.5Si13.5B9−xNb3Cu1Gex nanocrystalline alloys

Abstract: The nanocrystalline (Fe0.9Co0.1)73.5Si13.5B9−xNb3Cu1Gex alloys were obtained by partial devitrification of their amorphous precursors. The influence of partial substitution of B by Ge on the microstrucural evolution of these alloys was studied by means of X-ray diffraction (XRD) and differential scanning calorimeter (DSC). The temperature dependence of initial permeability (μi-T curves) for (Fe0.9Co0.1)73.5Si13.5B6Nb3Cu1Ge3 alloys heating–cooling cycled at 450–650 °C was mainly measured. It was found that the Ge doping into (Fe0.9Co0.1)73.5Si13.5B9Nb3Cu1 alloy can reduce the onset primary crystallization temperature (Tx1), enlarge the interval temperature (ΔTx) from 129 °C to 180 °C and improve the high temperature soft magnetic properties, especially for (Fe0.9Co0.1)73.5Si13.5B6Nb3Cu1Ge3 alloy. After annealed at 450–650 °C for the alloy with x = 3, the optimum high temperature magnetic softness was observed in 600 °C-annealed sample, the μi above 1000 at 10 kHz can be kept up to 550 °C, which is due to the higher crystallization phase volume fraction and the thinner amorphous layer thickness and an enhancement of the exchange stiffness in the intergranular region.

Glass-ceramics: their production from wastes

Abstract: GLASS-CERAMICS: THEIR PRODUCTION FROM WASTES . Glass-ceramics are fine, polycrystalline materials that are produced by the controlled crystallization (devitrification) of a glass. The fundamental principles of the crystallization are presented briefly and the various processing methods for glass-ceramics described in light of these fundamental principles. Two examples of the production of glass- ceramics from wastes are discussed in detail; the wastes are slag from steel production and fly ash from incineration.

Current assisted extrusion of metallic alloys: Insight into microstructure formation and mechanical properties

Abstract: We present the devitrification behavior of an Fe-based amorphous metal (SAM2×5) during a current-assisted extrusion process. The alloy is of composition Fe49.7Cr17.7Mn1.9Mo7.4W1.6B15.2C3.8Si2.4 and contains Y2O3 nanoparticles as a reinforcing phase. Detailed analysis of phase formation and microstructural analysis verifies that the SAM2×5 devitrified (SAM2×5-Dev) materials can be successfully extruded up to several centimeters in length with extrusion temperatures in the range between 1000 °C and 1100 °C. Subsequent analysis of mechanical properties via microhardness testing, together with microstructural characterization by scanning electron microscopy, demonstrates that the properties of the extruded SAM2×5-Dev composites significantly surpass those of common steels. These results open new horizons in the area of structural amorphous materials and expand their potential industrial processing.

Optical and structural properties of BCBS glass system with and without alumina

Abstract: BaO CaO Al2O3 SiO2 (BCAS) glass and their derivatives have gained extreme importance for their high endurance to elevated temperatures and being suitable for various electrochemical applications. Two glass systems, one being 50mol% [SiO2 B2O3]-xBaO-(45-x)CaO 5Al2O3 called as BCBSA and another without Al2O3 termed as BCBS were synthesized using melt quenching technique in the present work. Addition of ZnO and MgO as flux helped in melting them at 1300 °C which is much lower than the usual melting temperature of these glasses [1–4]. Density of the quenched glasses was measured by Archimedes method and structural bond vibrations were confirmed through FTIR. UV Visible spectroscopy was used to determine band gap energy and confirm the insulating nature of the synthesized glasses. The samples were isothermally heated at 700 °C, 800 °C for 50 h and at 900 °C for 50 and 100 h duration in air to allow the devitrification process to take place. The heat treated samples were analyzed by X-ray diffraction to identify the developed phases. Five Al2O3 free samples synthesized at 1300 °C by regular melt quenching technique were found to be devoid of the monocelsian phase. This is a detrimental phase for high temperature sealant applications as it has a very low coefficient of thermal expansion (CTE). Al2O3 free BCBS glasses, properties of which are being reported for the first time and glasses with low BaO concentrations are found to meet the requirements for high temperature applications as sealants in Solid Oxide Fuel Cell (SOFC).

Devitrification Behavior of Sol-Gel Derived ZrO2-SiO2 Rare-Earth Doped Glasses: Correlation between Structural and Optical Properties

Abstract: Optical and structural properties of glasses and glass-ceramics (GC) obtained by different heat-treatment of Tb and Tb-Yb doped sol-gel derived 30ZrO2-70SiO2 materials were investigated. A glass was formed after treatment at 700 °C whereas devitrification of the media after the treatment at 1000 and 1100 °C, led to the formation of GC containing up to three different crystalline phases, namely, tetragonal ZrO2, Yb-disilicate and cristobalite. The modification of the optical properties through the heat treatment was caused by redistribution of the rare earth elements (REE) among the different phases: both Tb and Yb entered the t-ZrO2 lattice, Yb can also be present in the form of a Yb2Si2O7 crystal. Devitrification led to an increase in Tb→Yb energy transfer efficiency as compared to the glass, though it was higher in the samples heat-treated at 1000 °C than in those treated at 1100 °C. The most intensive Yb3+ luminescence, induced by the energy transfer from the Tb3+ ion, was observed at the interface between t-ZrO2 and the glassy phases, due to the high concentration of REE in this area caused by the inability of ZrO2 to accept larger amounts of the REE. The mechanisms of the Tb→Yb energy transfer vary between different phases of the GC. The results obtained in this study are important for the development of spectral down-converters for potential solar energy applications based on Tb-Yb co-doped glass-ceramics.

Suppression effects of cooling rate on crystallization in ZBLAN glass

Abstract: ZBLAN glass is a heavy metal fluoride glass that shows great potential in the application of long-haul telecommunication cables. However, during processing in the fiber-drawing temperature region, the material tends to undergo heavy devitrification, resulting in a crystalline fiber that is not usable as an optical waveguide. In this study, ZBLAN glass was processed with different cooling rates to discern how the rate of cooling affects the process of crystallization. Rapidly cooled ZBLAN glass test samples were then analysed with a wide range of spectroscopy and imaging techniques including EDS, SEM, TEM and XRD. These techniques revealed there are two predominant crystal phases, one rich in zirconium and barium, the other in zirconium and sodium. Ultimately, a critical cooling rate was established to be somewhere between 900 °C/min and 4000 °C/min, with a theoretically predicted value of 1081 °C/min for a test sample volume size of 9.4 × 10−8 m3. Cooling above this critical cooling rate yielded fully amorphous ZBLAN test samples that were completely free of nano-crystalline inclusions. Using Kramers-Kronig relations, an attenuation loss for the fully amorphous ZBLAN test samples was estimated to be 0.09 dB/Km at a wavelength of 1800 nm.

Glass Transition Characteristics and Thermodynamic Functions of (1–x)(0.75TeO2–0.25WO3) + xLa2O3 Glasses

Abstract: The temperature-dependent heat capacity of three high-purity (1–x)(0.75TeO2–0.25WO3) + xLa2O3 tellurite glass samples (x = 0.02, 0.04, and 0.06) has been determined by dynamic calorimetry in the range 320–1000 K. The experimental devitrification and heat capacity data have been used to evaluate standard thermodynamic functions: C o (T), enthalpy, entropy, and Gibbs energy in glassy and supercooled liquid states. We have demonstrated model-parameter similarity of properties in a statistical approach and found correlation relationships for parameters as functions of composition, which make it possible to predict the thermodynamic functions of unexplored glasses of this series. We have identified crystallization-resistant compositions of (1–x)(0.75TeO2–0.25WO3) + xLa2O3 glasses as a basis for the development of optically active elements.

Crystallization Kinetics and Consolidation of Al82La10Fe4Ni4 Glassy Alloy Powder by Spark Plasma Sintering

Abstract: The mechanically alloyed Al82La10Ni4Fe4 glassy powder displays a two-step devitrification characterized by the precipitation of fcc-Al together with small amounts of the intermetallic Al11La3 phase in the first crystallization. The interface-controlled growth mechanism governed the first crystallization event. Calculations of the activation energy, using the methods of Kissinger, Ozawa, and Augis-Bennett gave values of 432.33, 443.2, and 437.76 kJ/mol, respectively. The calculated Avrami exponent (n) for the first crystallization peak was about 1.41, suggesting an almost zero nucleation rate. On the other hand, the value of n for the second peak related to the residual amorphous phase completely transformed into the intermetallic phase Al11La3 was about 3.61, characterizing diffusion controlled three-dimensional crystal growth with an increasing nucleation rate. Samples sintered at 573 K kept an amorphous structure and exhibited a high compressive strength of 650 MPa with a maximum elongation of 2.34% without any plastic deformation. The failure morphology of the sintered sample surface presented a transparticle fracture mechanism, indicating the efficiency of the sintering processing.

Prediction and Measurement of the Kinetics of Isothermal Crystallization and Devitrification of Mold Powder Slags

Abstract: The rate and extent of formation of crystalline phases in the slag film that develops between the mold and the strand in steel continuous casting determine heat transfer and lubrication between them and therefore play crucial roles in shell integrity. In this work, a commercial mold powder for casting medium carbon (MC) thin steel slabs is selected for carrying out detailed comparisons between predicted and experimental results of the rates of isothermal crystallization and devitrification of liquid and glassy slag, respectively. Predictions are obtained with a kinetic model that combines the induction period (of Simon and Kolman) and the Kissinger methods to evaluate time–temperature–transformation (TTT) diagrams for the devitrification and crystallization reactions associated with the peaks appearing in non-isothermal thermo-analysis traces. To obtain experimental validation of the diagram, a high-temperature confocal laser scanning microscope (HT-CLSM) is used to rapidly heat or cool a sample, ~ 140 mg, of glassy powder or liquid slag, respectively, to selected isothermal conditions, where it is held for a prescribed length of time before quenching. Identification and quantification of the phases present in the quenched samples are done by a quantitative X-ray powder diffraction analysis (QXRPDA) method that uses a PONKCS (partial or not known crystal structure) procedure based on Le Bail and Rietveld strategies for refinement of diffraction patterns. The many experimental results indicate that the predicted TTT diagrams for devitrification and crystallization of the mold slag give a good estimation of the rate of the transformations that take place at different temperatures and reveal the differences in the reactions that occur along both paths.

Commercial alkaline earth boroaluminosilicate glasses for sealing solid oxide cell stacks. Part II: Characterization of devitrification and glass-ceramic phase assemblages

Abstract: The devitrification process and formation of crystalline phases from commercial alkaline earth boroaluminosilicate glasses containing 48-61 mol% SiO2, 18-28 mol% CaO, 1-7 mol% MgO, 7-10 mol% Al2O3, 1-11 mol% B2O3 plus minor amounts of Na2O, K2O, FeO and TiO2 were quantified through analysis of phase assemblages as function of heat treatments above the glass transition temperatures using the electron microprobe and powder X-ray diffraction. Treatments at 800 °C and 850 °C lasted up to 6 weeks. Results indicate that devitrification was strongly activated through presence of heterogeneous nucleation, and that the growth mechanism gradually changed from three-dimensional growth at the onset of devitrification towards one-dimensional growth in later stages, when heterogeneous nucleation was absent or less dominating. A cc ep te d A rt ic le This article is protected by copyright. All rights reserved. Most glasses developed entangled and fibrous microstructures with little or no residual glass phase, which are adequate for rigid sealants, and only one of the laboratory analogue glasses, MCAS, developed microstructures with both more equiaxed grains and a considerable amount of residual glass phase, which may be adequate for more compliant and self-healing sealants as often required in SOC-applications. Even though the glasses lie within a relatively narrow compositional range, resulting phase assemblages differed significantly. Anorthite (plagioclase) developed as the main crystalline phase in all samples together with pyroxene (or pyroxenoide) and cristobalite. Calciummagnesium-silicate pyroxene (diopside) was in a large part replaced by the calcium-silicate pyroxenoid (wollastonite) in the samples where the mol-proportion MgO:CaO was 1:5 or lower. In samples with a very low MgO proportion and consequently a high CaO proportion, calcium metaborate and calcium aluminum borosilicate (okayamalite) crystallized among the main phases and these glasses crystallized completely within the period of heat treatment. Although cristobalite is metastable at the annealing temperatures, both α and β forms were rapidly formed in most of the samples, likely due to kinetic reasons. The presence of the latter is explained by the stabilization effect of Al and B substitution for Si compensated by Ca stuffing in the structure. The stuffed cristobalite transformed with time to quartz (at 800 °C) or quartz plus tridymite (at 850 °C). Boron was incorporated in the first crystallizing phases, especially diopside, substituting for Al and Si, but the so established substitution partly disappeared with time during the heat

A comparative study of Sm networks in Al-10 at.%Sm glass and associated crystalline phases

Abstract: The Al–Sm system is selected as a model system to study the transition process from liquid and amorphous to crystalline states. In recent work, we have shown that, in addition to long-range translational periodicity, crystal structures display well-defined short-range local atomic packing motifs that transcends liquid, amorphous and crystalline states. In this paper, we investigate the longer range spatial packing of these short-range motifs by studying the interconnections of Sm–Sm networks in different amorphous and crystalline samples obtained from molecular dynamics simulations. In our analysis, we concentrate on Sm–Sm distances in the range ~5.0–7.2 A, corresponding to Sm atoms in the second and third shells of Sm-centred clusters. We discover a number of empirical rules characterising the evolution of Sm networks from the liquid and amorphous states to associated metastable crystalline phases experimentally observed in the initial stages of devitrification of different amorphous samples. As ...

Devitrification of the Kob-Andersen glass former: Competition with the locally favored structure

Abstract: Supercooled liquids are kinetically trapped materials in which the transition to a thermodynamically more stable state with long-range order is strongly suppressed. To assess the glass-forming abilities of a liquid empirical rules exist, but a comprehensive microscopic picture of devitrification is still missing. Here we study the crystallization of a popular model glass former, the binary Kob-Andersen mixture, in small systems. We perform trajectory sampling employing the population of the locally favored structure as order parameter. While for large population a dynamical phase transition has been reported, here we show that biasing towards a small population of locally favored structures induces crystallization, and we estimate the free energy difference. This result sheds new light on the competition between local and global structure in glass-forming liquids and its implications for crystallization.

Optical glass, optical glass preformed part, optical component and optical instrument

Abstract: The invention provides optical glass. The optical glass comprises, by weight, 0-10% of SiO2, 5-25% of B2O3, 0-15% of ZrO2, 10-25% of ZnO, 2-30% of TiO2+Nb2O5+WO3 and 30-55% of Ln2O3 in total of La2O3,Gd2O3, Y2O3 and Yb2O3, wherein B2O3/Ln2O3 is 0.3-0.8, and Y2O3/B2O3 is 0.15-2. The optical glass has a refractive index (nd) of 1.84-1.87 and an Abbe number (vd) of 38-41 and excellent in bubble degree and devitrification resistance and high in stability.

Electrical Conductivity and 7Li NMR Spin-Lattice Relaxation in Amorphous, Nano- and Microcrystalline Li2O-7GeO2

Abstract: Differential scanning calorimetry showed that on heating Li2O-7GeO2 glass crystallized in stages. X-ray phase analysis and atomic force microscopy were used to study the structure and morphology of the phase states obtained at glass devitrification. It was shown that glass devitrified through an intermediate state in which the sample volume was occupied by nanometer-sized ordered phase nuclei with Li2Ge4O9 and Li2Ge7O15 structures surrounded by an amorphous medium. Further heating resulted in complete sample crystallization and transformation of nanometer-sized nuclei into micrometer-sized Li2Ge7O15 crystallites. It was shown that in comparison with amorphous and completely crystallized polycrystalline states, the intermediate nanocrystalline state has an increased electrical conductivity σ. Complete crystallization on heating was accompanied by sharp and irreversible decrease of σ. Charge transfer in amorphous, nano- and microcrystalline states of Li2O-7GeO2 composition was associated with motion of lithium ions which were weakly bound to the germanium-oxygen structural framework. Complex impedance spectra were studied in the glass, intermediate and polycrystalline states of Li2O-7GeO2. It was shown that the hodographs for the intermediate nanocrystalline state reflected charge transfer within the ordered nuclei and the embedding amorphous medium. The results of conductivity and impedance spectra measurements were supplemented by 7Li NMR spin-lattice relaxation studies. Comparative analysis of the data of electrical properties measurements and NMR relaxation studies gave evidence that increased conductivity of the intermediate nanocrystalline state resulted from high mobility of the Li+ ions.

Glass powder for three-dimensional molding

Abstract: PROBLEM TO BE SOLVED: To provide a glass powder for three-dimensional molding that resists devitrification even when irradiated with laser, in three-dimensional molding.SOLUTION: A glass powder for three-dimensional molding has a difference between a crystallization initiation temperature and a softening point of 50°C or more, and contains, as a glass composition in mass%, SiO: 35-80%, AlO: 0-30%, BO: 0-30%, RO: 1-40% (R is at least one selected from Mg, Ca, Sr, Ba and Zn), LiO: 0-5%, NaO+KO: 0-17%, and further contains at least one transition metal oxide selected from CuO, FeO, CoO, CrO, NiO, VOand MnOof 0.01% or more. Preferably the glass powder for three-dimensional molding has a thermal expansion coefficient at 30-380°C of 120×10/°C or less and a softening point of 500-1000°C.SELECTED DRAWING: None

Microcrystalline glass for dentistry and preparation method thereof

Abstract: The invention discloses microcrystalline glass for dentistry and a preparation method thereof. The glass is prepared from the following raw materials in parts by mass: 50 percent to 65 percent of SiO2, 20 percent to 30 percent of Li2CO3, 0 to 4 percent of K2CO3, 2 percent to 5 percent of K2SO4, 2 percent to 7 percent of P2O5 and 2 percent to 5 percent of ZrO2; the sum of the mass percent of the raw materials is 100 percent. A glass block body is obtained through a melting and casting method, and the coordination effect of P and S is expressed; more crystal nucleuses are obtained under a low-temperature state; a glass intermediate which is easily machined is obtained through devitrification treatment; then heat treatment is carried out to obtain a microcrystalline glass finished product which takes Li2Si2O5 as a main crystal phase. The microcrystalline glass disclosed by the invention has good mechanical properties including bending strength, rigidity and the like and also has good light transmission performance and biocompatibility; the microcrystalline glass can be used for an all-ceramic repairing material for the dentistry.