Structural instability of metallic glasses under radio-frequency-ultrasonic perturbation and its correlation with glass-to-crystal transition of less-stable metallic glasses

It has been reported that the structural stability is significantly deteriorated under radio-frequency-ultrasonic perturbation at relatively low temperatures, e.g., near/below the glass transition temperature T(g), even for thermally stable metallic glasses. Here, we consider an underlying mechanism of the ultrasound-induced instability, i.e., crystallization, of a glass structure to grasp the nature of the glass-to-liquid transition of metallic glasses. Mechanical spectroscopy analysis indicates that the instability is caused by atomic motions resonant with the dynamic ultrasonic-strain field, i.e., atomic jumps associated with the beta relaxation that is usually observed for low frequencies of the order of 1 Hz at temperatures far below T(g). Such atomic motions at temperatures lower than the so-called kinetic freezing temperature T(g) originate from relatively weakly bonded (and/or low-density) regions in a nanoscale inhomogeneous microstructure of glass, which can be straightforwardly inferred from a partially crystallized microstructure obtained by annealing of a Pd-based metallic glass just below T(g) under ultrasonic perturbation. According to this nanoscale inhomogeneity concept, we can reasonably understand an intriguing characteristic feature of less-stable metallic glasses (fabricated only by rapid melt quenching) that the crystallization precedes the glass transition upon standard heating but the glass transition is observable at extremely high rates. Namely, in such less-stable metallic glasses, atomic motions are considerably active at some local regions even below the kinetic freezing temperature. Thus, the glass-to-crystal transition of less-stable metallic glasses is, in part, explained with the present nanoscale inhomogeneity concept.     

Thermal and structural studies of nanocrystallization of oxyfluoride glasses

The influence of fluorine content on the structure and crystallization of oxyfluoride glasses from the Na₂O–Al₂O₃–SiO₂–LaF₃ system was studied by DTA/DSC, XRD, FTIR and SEM methods. It has been found that the increase in the fluorine content in the structure of oxyfluoride glasses causes the increase of the flexibility of their structure, which inhibits the process of crystallization of the silicate- aluminium matrix. Simultaneously the ability of the glass for LaF₃ crystallization, which shows a multistage character, is increasing. Analysis of the local atomic interactions in the structure of glasses has been used to explain the course of the crystallization.     

Kinetics of phase transition and thermal stability in Se80?x Te20Zn x (x?=?2, 4, 6, 8, and 10) glasses

Se_80?x Te₂₀Zn _x (x?=?2, 4, 6, 8, and 10) glasses have been prepared using conventional melt quenching technique. The kinetics of phase transformations (glass transition and crystallization) have been studied using differential scanning calorimetry (DSC) under non-isothermal condition at five different heating rates in these glasses. The activation energy of glass transition (E _t), activation energy of crystallization (E _c), Avrami exponent (n), dimensionality of growth (m), and frequency factor (K _o) have been investigated for the better understanding of growth mechanism using different theoretical models. The activation energy is found to be highly dependent on Zn concentration. The rate of crystallization is found to be lowest for Se₇₀Te₂₀Zn₁₀ glassy alloy. The thermal stability of these glasses has been investigated using various stability parameters. The values of these parameters were obtained using characteristic temperatures, such as glass transition temperature T _g, onset crystallization temperature T _c, and peak crystallization temperature T _p. In addition to this, enthalpy-released during crystallization has also been determined. The values of stability parameters show that the thermal stability increases with the increase in Zn concentration in the investigated glassy samples.     

Effect of silver on phase separation and crystallization of niobium oxide containing glasses

Effect of silver introduction in sodium phosphate and sodium borophosphate glasses containing large amount of niobium oxide have been investigated using differential scanning calorimetry and XRD. Same sodium niobate phase in the Nb₂O₅-NaNbO₃ based solid solution have been observed following two heat treatments designed for nucleation and growth of the crystalline phase. Silver introduction in the glass composition is clearly responsible for increasing the crystallization rate. Its effect after nucleation and crystallization treatments has been shown. Phase metastable separation is occurring during heat treatment with formation of a phosphate rich and niobium rich phase. Crystallization effect on optical transparency of glasses and on Raman scattering response have been investigated.     

Thermal analysis and thermochemistry of vitreous into crystalline state transition

Thermochemistry and structural mechanism of crystallization of MgO-Al₂O₃-SiO₂ glasses with TiO₂ as crystallization activator were studied. Thermal and HREM investigation proved that near the T _g temperature crystallization is going by rearrangement of glass structure elements and part of its components redistribution like at disorder — order phase transition in solid bodies. Nanocrystals of Mg-titanate and high quartz structure solid solution are formed then. Next enstatite and cordierite are crystallizing. Thermochemical and chemical bonds strength analysis indicate that during multistage crystallization of glasses, kind and order of crystal phase formation, is determined by the glass structure decomposition progress and its particular components release accompanying increase of temperature. It has been proved that molar heat capacity change (ΔC _p) accompanying the glass transition is the significant measure of degree of changes in the structure of glass preceding crystallization. This revised version was published online in July 2006 with corrections to the Cover Date.     

Atomic‐Level Mechanisms of Nucleation of Pure Liquid Metals during Rapid Cooling

To obtain a material with the desired performance, the atomic‐level mechanisms of nucleation from the liquid to solid phase must be understood. Although this transition has been investigated experimentally and theoretically, its atomic‐level mechanisms remain debatable. In this work, the nucleation mechanisms of pure Fe under rapid cooling conditions are investigated. The local atomic packing stability and liquid‐to‐solid transition‐energy pathways of Fe are studied using molecular dynamics simulations and first‐principle calculations. The results are expressed as functions of cluster size in units of amorphous clusters (ACs) and body‐centered cubic crystalline clusters (BCC‐CCs). We found the prototypes of ACs in supercooled liquids and successfully divided these ACs to three categories according to their transition‐energy pathways. The information obtained in this study could contribute to our current understanding of the crystallization of metallic melts during rapid cooling.     

The influence of minor components on the structural reorganization in glassforming materials

Correlation of structure parameters of glasses and related crystals formed in homogeneous or heterogeneous nucleation processes by thermal treatment is discussed on the basis of DTA, TG and EGA measurements in relation to the textural patterns of the materials. For cordierite glass, crystallization of metastable disordered cordierite polymorphs is related to an exothermic heat evolution and simultaneous with a small weight loss (appr. 0.025%). By MS-EGA, evolution of water was determined during the transformation of the metastable melt to a metastable intermediate crystalline phase. Interpretation of the crystallization by comparing the available structure parameters of cordierite glasses and crystals alone is insufficient to explain the role of water in the kinetics of crystallization. Optical and electron microscopy of the primary crystallization phenomena show the metastable solid solution with low quartz-type structure. Interpretation of the crystallization behaviour in terms of conventional theory of nucleation and crystal growth is impossible.     

Dynamics of lithium ions in calcium bismuthate glasses

Ion transport in Li(2)O-CaO-Bi(2)O(3) glasses has been studied in the frequency range from 10 Hz to 2 MHz and in the temperature range from 293 to 543 K. The variation of the dc conductivity and the activation energy of these glasses with composition have been compared with those of binary lithium bismuthate glasses. It has been observed that the introduction of CaO in small amount does not effect the dc conductivity and the corresponding activation energy, but the larger amount of CaO changes them to some extent. The frequency-dependent conductivity has been studied using both conductivity and modulus formalisms. The values of the nonexponential parameter and frequency exponent are found to differ for the Li(2)O-CaO-Bi(2)O(3) glasses from those for the binary Li(2)O-Bi(2)O(3) glasses. The concentration of mobile Li(+) ions does not change appreciably with the change in the Li(2)O as well as CaO content in the compositions. The increasing amount of CaO in the glass compositions for fixed Li(2)O content points out that CaO dilates the glass network, enhancing the migration of Li(+) ions.     

Thermal properties of oxide glasses

A criterion based on the length of induction period of crystallization was used to evaluate the thermal stability of M₂O·SiO₂ (M?=?Li, Na) glasses against crystallization. It was founded out that the stability of studied glasses against crystallization is Li₂O·SiO₂?<?Na₂O·SiO₂. The results coincide with the order determined by stability criteria based on temperatures and the values of activation energy. A criterion based on the length of induction period enables to discriminate among the thermal stabilities of the silicate glass systems.     

Study on the Stability of GaF3-based Glasses by Differential Scanning Calorimetry

The glass formation and devitrification of GaF₃-based glasses were studied by differential scanning calorimetry. A comparison of various simple quantitative methods to assess the level of stability of multicomponent fluoride glass systems is presented. Most of these methods are based on critical temperatures. In this paper a new parameter k _b(T) is added to the stability criteria. The stability of several GaF₃-based glasses were experimentally evaluated and correlated with the activation energies of crystallization via this new kinetic criterion and compared with those evaluated by other criteria. This revised version was published online in August 2006 with corrections to the Cover Date.     

Influence of thermal stability on dielectric properties of SiO<Subscript>2</Subscript>–K<Subscript>2</Subscript>O–CaO–MgO glasses

Compositions of 55SiO₂–10K₂O–(35–x)CaO–xMgO are prepared by melt and quench technique. Thermal parameters of the as-prepared glasses are studied using the differential thermal analyzer under non-isothermal conditions. Kissinger, Augis–Bennett and Lasocka models are employed to investigate the kinetics of crystallization and thermal stability of these glasses. Based on this, it is concluded that CM-15 glass exhibits highest thermal stability. Raman spectroscopy is used to reveal the structural units of the glasses. Dielectric properties are observed through impedance spectroscopy. All the glasses are phase separated. The ratio of CaO/MgO influences the thermal stability, which leads to affect the dielectric properties. The highest dielectric permittivity is observed ~22 at room temperature and 100 Hz for CM-15 glass, where CaO/MgO ratio is ~1.33.     

The elementary softening event in glassy systems in terms of the model of the excited state

The pressure dependence of the glass-transition temperature (glass-transition lines) is described through a relationship similar to the Clausius-Clapeyron equation. The criterion for the glass-liquid transition for polymer and other glasses is calculated. According to the proposed speculations, an elementary softening event in glasses is reduced to the critical deformation of interatomic (intermolecular) linkage, which corresponds to the maximum force of attraction between atoms. A glass (an amorphous polymer) softens when the mean energy of the thermal motion of the kinetic units responsible for the viscous flow is ∼3 times higher than the work of the ultimate deformation of the interatomic bond. The nature of structural changes occurring in the course of critical displacement (excitation) of kinetic units in liquids and glasses is discussed.     

Na-Gd phosphate glasses

Summary The present paper deals with preparation, thermal properties and radioluminescence of Ce-doped Na-Gd phosphate glasses. Thanks to their high radioluminescence intensity, three times greater than that of BGO, these glasses are promising materials for the detection of neutrons, - and X-rays. The glasses with a Gd concentration up to 89 mol% were prepared by a rapid quenching technique in air. Their thermal properties were determined using DSC and temperatures of glass transition were measured in addition using TMA. Temperatures of glass transition, crystallization and melting depend on Gd concentration and they follow the liquidus line in a phase diagram of binary system in which two eutectics and a congruently melting compound exist. High glass-forming ability of this glass system was found evidenced. The glasses containing at least 30 mol% Gd were moisture resistant.     

Inertial suppression of protein dynamics in a binary glycerol-trehalose glass

The traditional approach used to predict the ability of a glassy matrix to maximally preserve the activity of a protein solute is the glass transition temperature (T(g)) of the glass. Recently it has been shown that the addition of a low T(g) diluent (glycerol) can rigidify the structure of a high T(g) glassy matrix in binary glycerol-trehalose glasses. The optimal density of glycerol in trehalose minimizes the average mean square displacements of non-exchangeable protons in the glass samples. The amount of glycerol added to a trehalose glass coincides with the maximal recovery of biological activity in a separate study using similar binary glass samples. In this study, we use molecular dynamics (MD) simulations to investigate the dynamics of a hydrated protein encased in glycerol, unary trehalose and binary glycerol-trehalose glasses. We have found that we are able to reproduce the rigidification of the glycerol-trehalose glassy matrix and that there is a direct correlation between bulk glass dynamics and the extent of atomic fluctuation of protein atoms. The detailed microscopic picture that emerges is that protein dynamics are suppressed mainly by inertia of the bulk glass and to a lesser extent specific interactions at the protein-solvent interface. Thus, the inertia of the glassy matrix may be an influential factor in the determination of pharmaceutically relevant formulations.     

Thermal analysis of mechanochemically activated glass

Thermal analysis (TG, DTG, DTA, DSC) has been used to study the influence of mechanochemical activation of glass making batch on its melting and obtained glass crystallization. It revealed, that long time (12 h) mechanical treatment accelerates the glass batch melting and also improving network homogeneity influence the internal structure of the obtained glass. Refinement of the glass, at the beginning reduces temperature and increases degree of crystallization as a result of nucleating action of the increasing surface area of grains. However long time activation lowers the crystallization ability of glass and increases crystallization temperature. This is attributed to the deformation of the structure surface layer and surface energy increase, hampering the crystals nuclei formation.     

原子尺寸差异与非晶形成能力

采用分子动力学模拟技术，研究了纯Au及AuCu合金的熔化、非晶化和晶化过程.模拟结果表明,在冷却速率为5×1011 K•s－1至4×1012 K•s－1的范围内，液态Au总是形成晶体，且冷速越快，结晶温度越低；而AuCu合金则形成非晶，且冷速越快，非晶转变温度越高.验证了原子尺寸的不匹配有利于非晶形成这一规律.     

Size-dependent growth of polymorphs in nanopores and Ostwald's step rule of stages

More than 100 years after Ostwald postulated his step rule of stages, predictive understanding as to early crystallization stages of polymorphic materials is still premature. We studied crystallization of the polymorphic pharmaceutical acetaminophen in nanoporous glasses as a model for early stages of bulk crystallization since the surface energy significantly contributes to the total Gibbs free energy of nanosized crystals in both cases. Systematic studies of transitions between different polymorphs inside nanoporous glasses show that the thermodynamic stability of the polymorphs depends on the crystal size. Accordingly, the transient occurrence of different polymorphs during crystal growth in bulk systems can be related to surface energy contributions to the total Gibbs free energy of the developing crystals. In nanosized early-stage crystals with high surface-to-volume ratios other polymorphs may be stable than in large crystals with low surface-to-volume ratios. Improved control of the crystallization of polymorphic materials by imposing well-defined confinement is a promising strategy to tailor release of polymorphic drugs and to optimize optical, electronic, magnetic and ferroelectric properties of polymorphic materials.     

Crystallization at the glass transition in supercooled thin films of methanol

The stability of an amorphous material depends on how fast and by what mechanism crystallization occurs. Based on crystallization rate measurements through optical reflectivity changes in supercooled methanol thin films, it is observed for the first time that there is a definitive and detectable change of the crystallization mechanism at the glass transition temperature T(g). For methanol glasses below T(g)=103.4 K, crystallization occurs as an interface controlled, one-dimension process at frozen-in embryo sites, while in the deep supercooled liquid phase above T(g) crystallization is diffusion controlled in two dimensions with a constant nucleation rate and an activation energy of 107.8(+/-4.7) kJ/mol.     

Influence of fluorine on thermal properties of lead oxyfluoride glass

Oxyfluoride glasses are the basic materials for obtaining transparent glass–ceramic (TGC) which can be used in a wide range of optoelectronics devices such as: amplifiers, up-conversion, telescopes, laser sources. Oxyfluoride TGC is obtained by the control heat treatment of the parent glass due to low phonon nanocrystalline phases. The oxyfluoride glasses from the sodium–lead–silica system were the object of investigation. The influence of fluoride content on the thermal properties of glasses was analyzed. Thermal characteristics of glasses like the transition temperature T _g, the temperature for the crystallization onset T _x, and the maximum crystallization temperature T _c, thermal stability parameter were determined by DTA/DSC method. The linear expansion coefficients of oxyfluoride glasses as a function of temperature were measured using a thermo-mechanical analyzer (TMA 7 Perkin-Elmer). The effect of crystallization on the thermal expansion coefficient and softening temperature T _s was found.     

Influence of particle size on the crystallization process and the bioactive behavior of a bioactive glass system

Bioactive glasses have attracted considerable interest in recent years, due to their technological application, especially in biomaterials research. Differential scanning calorimetry (DSC) has been used in the study of the crystallization mechanism in the SiO₂–Na₂O–CaO–P₂O₅ glass system, as a function of particle size. The curve of the bulk glass presents a slightly asymmetric crystallization peak that could be deconvoluted into two separate peaks, their separation being followed in the form of powder glasses. Also, a shift of the crystallization peaks to lower temperatures was observed with the decrease of the particle size. FTIR studies – that are confirmed by XRD measurements – proved that the different peaks could be attributed to different crystallization mechanisms. Moreover, it is presented the bioactive behavior of the specific glass as a function of particle size. The study of bioactivity is performed through the process of its immersion in simulated human blood plasma (simulated body fluid, SBF) and the subsequent examination of the development of carbonate-containing hydroxyapatite layer on the surface of the particles. The bioactive response is improved with the increase of the particle size of powders up to 80 μm and remains almost unchanged for further increase, following the specific surface to volume ratio decrease.