Showing papers on "Devitrification published in 2021"

Structure determines where crystallization occurs in a soft colloidal glass

Abstract: Glass is inherently unstable to crystallization. However, how this transformation occurs while the dynamics in the glass stay frozen at the particle scale is poorly understood. Here, through single-particle-resolved imaging experiments, we show that due to frozen-in density inhomogeneities, a soft colloidal glass crystallizes via two distinct pathways. In the poorly packed regions of the glass, crystallinity grew smoothly due to local particle shuffles, whereas in the well-packed regions, we observed abrupt jumps in crystallinity that were triggered by avalanches—cooperative rearrangements involving many tens of particles. Importantly, we show that softness—a structural-order parameter determined through machine-learning methods—not only predicts where crystallization initiates in a glass but is also sensitive to the crystallization pathway. Such a causal connection between the structure and stability of a glass has so far remained elusive. Devising strategies to manipulate softness may thus prove invaluable in realizing long-lived glassy states. Softness, a machine-learned structural quantity, has been recently identified as a parameter that characterizes glassy dynamics. Here, the authors observe devitrification in 3D soft colloidal glasses and find that softness may indicate regions predisposed to crystallization.

Devitrification and sliding wear behaviors of AlFeSi metallic glass coatings

Abstract: Thermally-induced structure evolutions and wear behaviors of the AlFeSi metallic glass coatings were investigated. Annealing below the onset crystalline temperature, the microstructure of the coating consists of the amorphous phase coexistence with α-Al crystals. The partially devitrified annealing (350 °C) promotes new Al9Fe2Si2 and Al9FeSi3 phases precipitations. The fully crystallized annealing (≥400 °C) produces three crystalline phases of α-Al, Al9Fe2Si2, and Al9FeSi3. The hardening responses of the coatings depend on the annealing temperature. Under dry friction conditions, wear rates of the full crystallization coatings are lower than that of the partial crystalline and as-sprayed coatings. The wear resistance of the coating annealed at 450°C is about 4 times that of the as-sprayed coating. In 3.5wt.% NaCl solution, current density induced by sliding wear increases as a function of the annealing temperature. The as-sprayed coating displays excellent resistance to tribocorrosion. The microstructural evolutions responsible for annealing-induced changes in wear behaviors are discussed.

The influence of potassium substitution for barium on the structure and property of silver-doped germano-gallate glasses

Abstract: The structure and crystallization of silver-doped germano-gallate glasses for substitution of potassium for barium ions have been investigated. By means of Raman spectroscopy, a structural study has been carried out revealing a glass network depolymerization with the formation of non-bridging oxygens likely localized on germanium tetrahedra when the barium concentration increases. These structural modifications do not appear to significatively change the silver ion environments or ionic states, as reported on luminescence and optical transmission spectroscopies. A combined differential scanning calorimetry, X-ray diffraction characterization, scanning electron microscopy and micro-luminescence spectroscopy demonstrate that the barium ions introduction tends to stabilize the silver ions and to influence drastically the devitrification and the crystal growth kinetics. Correlations have been established between ionic conductivity, silver distribution and crystal growth mechanisms.

Effect of MoO3 on structural, thermal and transport properties of lithium phosphate glasses

Abstract: A melt quenching method has been adopted to synthesize the glasses 50Li2O–хMoO3–(50−х)P2O5 (х = 0, 5, 10, 15, 20, 25 and 27.5 mol%) which show the tendency of devitrification with increasing content of MoO3. The glass transition temperatures are found in the range between 332.3 °C and 361.9 °C when the glass forming domain at the fixed concentration of Li2O is limited to 0 ≤ x ≤ 27.5. Variation of local structure has been studied by Raman spectroscopy. The presence of different phosphate and molybdate structural units is observed. The electrical conductivity of the glasses at room temperature increases with x. The maximum conductivity is 5.0•10−7 S/cm in 50Li2O–27.5MoO3–22.5P2O5 composition at room temperature.

Effects of composition and phase relations on mechanical properties and crystallization of silicate glasses

Abstract: Crystallization, mechanical properties and workability are all important for commercialization and optimization of silicate glass compositions. However, the inter-relations of these properties as a function of glass composition have received little investigation. Soda-lime-silica glasses with Na2O-MgO-CaO-Al2O3-SiO2 compositions relevant to commercial glass manufacture were experimentally studied and multiple liquidus temperature and viscosity models were used to complement the experimental results. Liquidus temperatures of the fabricated glasses were measured by the temperature gradient technique, and Rietveld refinements were applied to X-Ray powder diffraction (XRD) data for devitrified glasses, enabling quantitative determination of the crystalline and amorphous fractions and the nature of the crystals. Structural properties were investigated by Raman spectroscopy. Acoustic echography, micro-Vicker’s indentation and single-edge notched bend testing methods were used to measure Young’s moduli, hardness and fracture toughness, respectively. It is shown that it is possible to design lower-melting soda-lime-silica glass compositions without compromising their mechanical and crystallization properties. Unlike Young’s modulus, brittleness is highly responsive to the composition in soda-lime-silica glasses, and notably low brittleness values can be obtained in glasses with compositions in the wollastonite primary phase field: an effect that is more pronounced in the silica primary phase field. The measured bulk crystal fractions of the glasses subjected to devitrification at the lowest possible industrial conditioning temperatures, indicate that soda-lime-silica glass melts can be conditioned close to their liquidus temperatures within the compositional ranges of the primary phase fields of cristobalite, wollastonite or their combinations.

DSC Analysis of Thermophysical Properties for Biomaterials and Formulations.

Abstract: The development of freezing and freeze-drying processes for biological samples requires knowledge of the thermophysical properties of the biomaterial and protectant solutions involved. This chapter provides an introduction on the use of differential scanning calorimetry (DSC) to study thermophysical properties of biomaterials in protective solutions. It covers specific methods to study thermal events related to freezing and drying processes including crystallization, eutectic formation, glass transition, devitrification, recrystallization, melting, molecular relaxation, and phase separation.

The crystallization phenomenon on a modified phosphate glass interpreted through the correlation between the a.c. electrical behaviour and the non isothermal nucleation

Abstract: The electrical behavior of a bismuth-phosphate glass lithium ion conductor is studied in a very wide temperature range, from 39 K to 843 K, encompassing the glassy state, passing through the glass transition temperature, and reaching devitrification The relationship between the structural changes caused by the thermal treatment and the resulting electrical response is interpreted through the dc conductivity and ac permittivity An isothermal and a non-isothermal treatment allows to propose a simple analysis of the energy distribution and atoms rearrangement times involved in devitrification, using a Gaussian approximation method considering the fundamental elements in the M Avrami’s generalized theory

Analysis of elemental distributions and phase separation in rare-earth-doped silica-based fiber preforms and optical fibers

Abstract: The elemental distribution, phase separation and nanocrystallization process in rare-earth-doped silica-based fiber preforms and optical fibers were studied by using analytical electron microscopy. The particular properties of devitrification processes depending on the fiber core compositions and concentration of rare earth elements was systematically studied in aluminophosphosilicate fibers. It was investigated that the record high (for Modified Chemical Vapour Deposition manufactured samples) concentrations of rare-earth ions (up to 3 at% Yb3+) can be achieved alongside with a reasonably low background optical losses using aluminosilicate glass matrix. Two types of phase separation were revealed in the investigated samples of different composition: the droplet separation and the formation of two interpenetrated glass phases. It was concluded that the second type phase separation can be accompanied by the formation of nanocrystalline inclusions. Possible applications of the obtained glasses in practice for creation of highly-absorbing layers in optical fibers were discussed in the paper.

A Ni–CoWB composite developed by devitrification of Ni–Co–W–B bulk metallic glass

Abstract: The effects of annealing on microstructure and mechanical properties of Ni36.3Co25W23.7B15 bulk metallic glass are investigated. Composite samples were produced by annealing amorphous samples of the alloy above the crystallization temperature. Microstructural examinations revealed that nickel solid solution and CoWB phase precipitate upon annealing. Fully crystallized composite samples contain about 60 vol% nickel solid solution and 40 vol% CoWB phase. Microhardness of the amorphous alloy is determined to be 1168 HV. Composites having microhardness values higher than 1400 HV are obtained. Improvement in microhardness is caused by the formation of CoWB phase, whose microhardness is determined to be between 4500 and 5000 HV. This is the first composite reinforced by CoWB phase. Indentation fracture toughness of the composite having peak hardness ranges between 2.44 and 4.67 M P a m . It is found that further annealing results in slightly lower microhardness but higher indentation fracture toughness, which is between 3.55 and 6.45 M P a m . The effects of microstructure on microhardness and indentation fracture toughness of the composites are discussed.

Evolution of the structure and chemical composition of the interface between multi-component silicate glasses and yttria-stabilized zirconia after 40,000-hour exposure in air at 800°C

Abstract: The chemical and structural stability of two commercial multicomponent silicate glasses (SCN and G6) in contact with yttria-stabilized zirconia (YSZ) was investigated after exposure times of up to 40,000 hours in air at 800 °C. With exposure time, interfacial layers develop at the SCN-YSZ and G6-YSZ interfaces, which were characterized in detail using both quantitative chemical analysis and atomic-resolution imaging. At the SCN-YSZ interface, a Ca-Ba-Si-O reaction phase was found to grow by diffusion control. In G6-YSZ, Raman spectroscopy and electron microscopy revealed a disorganized interfacial reaction later between G6 and YSZ, and the occurrence of cubic to tetragonal to monoclinic phase transformations in YSZ. This microstructural evolution is discussed in terms of devitrification resistance of glass and diffusion processes at interfaces.

Thermal Properties of Glass and Melts of the CaO–B2O3–Al2O3–CuO System

Abstract: The temperatures of devitrification of cold crystallization, melting, and glass transition of samples of the CaO–B2O3–Al2O3–CuO system are determined by the methods of differential thermal analysis. The viscosity of alumina–calcium borate melts containing up to 8.1% of CuO is measured in the temperature range of 880 to 1300°C. It is shown that the addition of copper oxide reduces the viscosity and activation energy of the viscous flow.

Study on the structure change and devitrification of high silica glass particles with B2O3 modification by Raman, IR and MAS NMR spectroscopies

Abstract: The devitrification behavior of glass depends on its network structure. High silica glass particles were prepared using a sol-gel method. The network structure modification was achieved by heat treatment at 1050 °C for 1 h after coating with 3 wt%∼ 5 wt% B2O3. The relationship between structure change and devitrification behavior of high silica glass particles with B2O3 modification was investigated by Raman, IR, 11B MAS NMR,29Si MAS NMR, and X-ray powder diffraction. The results demonstrated that the network structure units of high silica glass were comprised of BO (bridging oxygen), NBO (non-bridging oxygen, Q2 and Q3), [BO4], and [BO3]. It was found that Q2, Q3 and [BO3] could be converted into Q4 and [BO4] under the effect of B2O3modification. As the network structure of high silica glass was repaired by B2O3 modification, the devitrification behavior of high silica glass was prevented.

Internal examination of mid-infrared chalcogenide glass optical fiber preforms and fiber using near-infrared imaging [Invited]

Abstract: We report on the internal examination of mid-infrared chalcogenide glasses using near-infrared light to reveal light-scattering defects The technique is demonstrated on imperfectly made chalcogenide glass rods and fiber This simple, non-destructive technique enables assessment of the interior of glasses and convenient detection of regions containing defects hidden due to the glass opacity to visible light This method will reveal the presence of unwanted light-scattering defects including nucleated crystals, dust, striae, and bubbles Hence, this method will help to optimize both chalcogenide glass chemical formulations, against devitrification, and process design to manufacture glass rods and fiber with minimized light scattering defects

Spontaneously formed quasicrystal grains in a pure metal

Abstract: Pure metals so far have been obtained in quasicrystal (QC) forms only by templation – epitaxial growth on a QC substrate. Here, we report spontaneous formation of dodecagonal QC (DDQC) grains in pure tantalum (Ta), an early transition metal normally in a body-centered-cubic crystal structure. The DDQC grains comprise icosahedral clusters assembled in accordance with the Stampfli triangle–square tiling scheme and are formed directly from the supercooled liquid and the β-Ta phases during thermal devitrification of a Ta metallic glass in molecular dynamics simulations using a realistic quantum mechanically based interatomic potential. They co-exist with β-Ta and are retainable to and stable at room temperature, with a slightly lower configurational energy than β-Ta.

Structure-property relations characterizing the devitrification of Ni-Zr glassy alloy thin films

Abstract: The investigation of devitrification in thermally annealed nanodimensional glassy alloy thin films provides a comprehensive understanding of their thermal stability, which can be used to explore potential applications. The amorphous to crystalline polymorphous transformation of cosputtered NiZr alloy (Ni78Zr22 at%) films, with a thickness lower than the reported critical limit of devitrification, was studied through detailed structural characterization and molecular dynamics (MD) simulations. Devitrification to a nanocrystalline state (Ni7Zr2 structure) was observed at 800 degC, with an increase in density (approx 3.6%) much higher than that achieved in bulk alloys. Variation in the magnetic property of the films and the overall physical structure including morphology and composition were examined before and after annealing. MD simulations were employed to effectively elucidate not only the high densification but also the increased magnetic moment after annealing, which was correlated with the simulated change in the coordination number around Ni atoms. The structural relaxation process accompanying devitrification was described as a disorder-to-order transformation while highlighting the crucial role played by chemical short range order prevalent in glassy materials.

Specific trends in phosphate glass crystallization

Abstract: This paper focusses on investigating and comparing the congruent crystallization of phosphate glasses with different degrees of polymerization. The study was performed both on powders, with different size fractions, and coarse particles which can be assimilated to bulk. From DSC experiments, corroborated by SEM analysis, it was demonstrated that LiPO3 crystallizes from surface whereas LiGe2(PO4)3 crystallizes in the whole volume. Sn2P2O7 presented both phenomena, the nucleation time lag being short enough to observe internal crystallization at the laboratory time scale. Using the non-isothermal Ozawa method, the kinetic parameters of the overall devitrification process were determined in terms of the Avrami exponent and of the activation energy for crystallization. The temperature of the maximum nucleation rate was calculated by using the nucleation adiabatic theory. For the achievement of this calculation, the heat capacity temperature dependence up to melting was determined from DSC experiments. The results were found in a good agreement with the SEM observation and the results of the non-isothermal crystallization study.

The Anomalous Nucleation in Al-Tb Metallic Glasses

Abstract: The effects of amorphous structure on the devitrification of Al90Tb10 marginal glass former system were investigated in detail by a combined study of high-energy X-ray diffraction (HEXRD), X-ray absorption fine structure (EXAFS), reverse Monte Carlo simulations (RMC), transmission electron microscopy (TEM) and thermal analyses. The atomic structures of melt-spun ribbons and magnetron sputtered amorphous samples with the same composition were simulated using RMC constrained by XRD, EXAFS, and ab-initio results. The fcc-Al nanocrystals nucleated and grown in thin-film specimens have a limited size with almost perfect spherical morphology. The population of these nanocrystals is three orders of magnitude higher as compared to ribbon specimens. The differences in the devitrified ribbon and thin-film metallic glasses were traced back to structural differences in the amorphous state. The amorphous melt-spun ribbons and magnetron sputtered thin- films were found to have different degrees of short-range order and clustering.

A new iron-phosphate compound (Fe7P11O38) obtained by pyrophosphate stoichiometric glass devitrification.

Abstract: Iron phosphates are a wide group of compounds that possess versatile applications. Their properties are strongly dependent on the role and position of iron in their structure. Iron, because of its chemical character, is able to easily change its redox state and accommodate different chemical surroundings. Thus, iron-phosphate crystallography is relatively complex. In addition, the compounds possess intriguing magnetic and electric properties. In this paper, we present crystal structure properties of a newly developed iron-phosphate compound that was obtained by devitrification from iron-phosphate glass of pyrophosphate stoichiometry. Based on X-ray diffraction (XRD) studies, the new compound (Fe7P11O38) was shown to adopt the hexagonal space group P63 (No. 173) in which iron is present as Fe3+ in two inequivalent octahedral and one tetrahedral positions. The results were confirmed by Raman and Mossbauer spectroscopies, and appropriate band positions, as well as hyperfine interaction parameters, are assigned and discussed. The magnetic and electric properties of the compound were predicted by ab initio simulations. It was observed that iron magnetic moments are coupled antiferromagnetically and that the total magnetic moment of the unit cell has an integer value of 2 µB. Electronic band structure calculations showed that the material has half-metallic properties.

Immobilization of Pb in vitrified and devitrified industrial wastes: Evaluation of structural stability using XAFS spectroscopies

Abstract: X-ray Absorption Fine Structure (XAFS) spectroscopies are implemented in a series of thermally treated vitrified industrial wastes in an effort to explain the structural role of Pb in the formation of stable, solidified products. In the vitrified wastes, Pb forms trigonal and tetragonal pyramids and acts as a network modifier. Consequently, Pb is successfully immobilized into a vitreous matrix. Devitrification promotes the formation of magnetoplumbite microcrystallites; nevertheless, the majority of Pb continues to modify the glassy network. The chemical stability of the vitroceramics correlates to the type of microcrystallites formed. Lead is safely trapped in both the crystalline and vitreous regions when microcrystalline growth proceeds via the depletion of both Pb and Fe from the silica matrix. However, if the formation of FexOy microcrystallites prevails, structural integrity is lost due to Fe removal, leading to a weak glassy network that can potentially permit the escape of Pb.

Promoting vitreous silica devitrification by placement on a NaCl grain at 800 °C–1150 °C

Abstract: This study investigated devitrification of a silica glass substrate put on a NaCl grain heated at temperatures of 800 °C–1150 °C. The devitrified areas are concentric double circles with respective diameters of ca. 1.5 and 1 cm. Devitrification was especially eminent in the area within the inner circle. No change in diameter of outer and inner circles was observed, at least for heating times of 10–480 min. Only the depth of the region within the inner circle increased concomitantly with increasing heating time. No marked difference was found among silica glass species with different amounts of OH and oxygen-deficient silica glasses. The heating temperature depth-dependence characteristics of the center and inner circular region at a heating time of 8 h were compared. The depth of the center of the circular region increased linearly at temperatures greater than about 1000 °C. No temperature dependence was observed at temperatures less than about 900 °C. Devitrification did not proceed under vacuum. The circular region showed alkalinity. Based on these findings, we proposed a model to elucidate the devitrification mechanism.

Ellipsometric analysis of isothermally devitrified metallic glasses

Abstract: Effects of devitrification in metallic glasses are of particular interest for their utilization in various applications as the phase transformation from amorphous to crystalline state is known to significantly change their properties. In this work, we study the effect of devitrification on the optical properties of metallic glasses using the spectroscopic ellipsometry technique. Ellipsometry measurements on isothermally crystallized samples at controlled temperatures between the glass transition temperature and the crystallization temperature revealed characteristic dependence of the complex index of refraction on the controlled devitrification temperature, suggesting an effective optical approach which can be prospectively used for characterization of structural changes during devitrification. The crystallinity of devitrified samples was verified using a combination of differential scanning calorimetry and X-ray diffraction measurements.

Glass devitrification and electrical properties of LiNaGe4O9

Abstract: The glass of lithium-sodium tetragermanate LiNaGe4O9 was prepared by fast quenching the melt. The glass was devitrified on heating under the control of differential scanning calorimetry. It was sho...