Good Glass Formers Research Articles

Molecular dynamics in amorphous pharmaceutically important protic ionic liquid–benzalkonium chloride

In this paper, we report the dielectric measurements at ambient pressure to portray the molecular dynamics of a glass forming pharmaceutical as well as a protic ionic conductor, benzalkonium chloride [BaCl] by means of broadband dielectric spectroscopy. The measurement in macroscopic nature of a stochastic transport theory and of the electric modulus formalism stanchly reproduces the shape of the dispersion of the microscopic ionic movement in terms of translational and ionic diffusive motion of molecular ions. The situation is analogous to most ionic conductors, BaCl showed only single resolved peak in the electric modulus formalism and doesn't show any crystallisation tendency during cooling as well as heating. The temperature dependence of the conductivity relaxation has been fitted by single Vogel–Fulcher–Tamman (VFT) equation. It is found that the value of Tg observed from DSC is significantly higher than one obtained from the α-conductivity relaxation corresponding to 100s indicating that the structural τα(Τ) is longer than conductivity τσα(Τ) for about 10K. Finally, the results were compared with data for other protic ionic conductors like Procaine HCl, Procainamide HCl, Verapamile HCl, Lidocaine HCl, Tramadole HCl and Chlorpromazine HCl and found that BaCl is a good strong glass former.

On the high glass-forming ability of Pt-Cu-Ni/Co-P-based liquids

The continuous and isothermal crystallization diagrams of the Pt42.5Cu27Ni9.5P21 and the Pt60Cu16Co2P22 bulk glass forming compositions are determined using calorimetric experiments. In the case of the Pt42.5Cu27Ni9.5P21 bulk metallic glass, the formation of the primary crystalline phase can be prevented by rapid cooling in a conventional DSC. In contrast, for similar cooling conditions, the formation of the primary precipitating compound in Pt60Cu16Co2P22 cannot be prevented in a conventional DSC as also observed in in-situ synchrotron X-ray scattering experiments. This is attributed to a critical overheating, above which remaining structures dissolve, resulting in a drastic increase of the degree of undercooling, similar to what is observed in Zr-based BMGs. Using the classical nucleation theory, the combined thermodynamic and kinetic data are used to model the isothermal crystallization data for Pt42.5Cu27Ni9.5P21, yielding an interfacial energy value of 0.11 J/m2 between the primary nucleating crystal and the liquid. This value is three times higher than the value for good Zr-based glass-formers, suggesting that the interfacial energy plays a pivotal role in the exceptionally high glass-forming ability of Pt-P-based systems and compensates for the fragile liquid behavior and the large driving force for crystallization.

Composition formulas of Fe-based transition metals-metalloid bulk metallic glasses derived from dual-cluster model of binary eutectics

It is known that bulk metallic glasses follow simple composition formulas [cluster](glue atom)1 or 3 with 24 valence electrons within the framework of the cluster-plus-glue-atom model. Though the relevant nearest-neighbor cluster can be readily identified from a devitrification phase, the glue atoms remains poorly defined. The present work is devoted to understanding the composition rule of Fe-(B,P,C) based multi-component bulk metallic glasses, by introducing a cluster-based eutectic liquid model. This model regards a eutectic liquid to be composed of two stable liquids formulated respectively by cluster formulas for ideal metallic glasses from the two eutectic phases. The dual cluster formulas are first established for binary Fe-(B,C,P) eutectics: [Fe-Fe14]B2Fe + [B-B2Fe8]Fe ≈ Fe83.3B16.7 for eutectic Fe83B17, [P-Fe14]P + [P-Fe9]P2Fe≈Fe82.8P17.2 for Fe83P17, and [C-Fe6]Fe3 + [C-Fe9]C2Fe ≈ Fe82.6C17.4 for Fe82.7C17.3. The second formulas in these dual-cluster formulas, being respectively relevant to devitrification phases Fe2B, Fe3P, and Fe3C, well explain the compositions of existing Fe-based transition metals-metalloid bulk metallic glasses. These formulas also satisfy the 24-electron rule. The proposition of the composition formulas for good glass formers, directly from known eutectic points, constitutes a new route towards understanding and eventual designing metallic glasses of high glass forming abilities.

Machine Learning Approach for Prediction and Understanding of Glass-Forming Ability.

The prediction of the glass-forming ability (GFA) by varying the composition of alloys is a challenging problem in glass physics, as well as a problem for industry, with enormous financial ramifications. Although different empirical guides for the prediction of GFA were established over decades, a comprehensive model or approach that is able to deal with as many variables as possible simultaneously for efficiently predicting good glass formers is still highly desirable. Here, by applying the support vector classification method, we develop models for predicting the GFA of binary metallic alloys from random compositions. The effect of different input descriptors on GFA were evaluated, and the best prediction model was selected, which shows that the information related to liquidus temperatures plays a key role in the GFA of alloys. On the basis of this model, good glass formers can be predicted with high efficiency. The prediction efficiency can be further enhanced by improving larger database and refined input descriptor selection. Our findings suggest that machine learning is very powerful and efficient and has great potential for discovering new metallic glasses with good GFA.

Gaining Thermodynamic Insight From Distinct Glass Formation Kinetics of Structurally Similar Organic Compounds

Thermodynamic and kinetic aspects of crystallization of 12 structurally similar organic compounds were investigated from the supercooled liquid state by calorimetric and rheologic measurements. Based on their crystallization behaviors, these compounds were divided into 3 categories: stable glass formers, poor glass formers, and good glass formers with poor stability on reheating. Correlation was sought between thermodynamic quantities and glass formation based on nucleation and crystal growth theories. Larger values of enthalpy of fusion and melting point were found to correlate with poor glass-forming ability. Conversely, lower entropy of fusion was found to correlate with glass formation. Examination of kinetic aspects of glass formation revealed 2 important facets of good glass formers, that is, rapid increase in viscosity on supercooling and high melting point viscosity compared with non-glass formers. A broader relationship was sought between entropy of fusion and glass formation by including several glass formers from literature. Our analysis indicated that good glass formers tend to have an entropy of fusion closer to 0.3 J cm−3 K−1. The structural similarity of the compounds in this study provides insights regarding the nature of intermolecular interactions responsible for the observed effect on entropy of fusion, viscosity, and crystallization kinetics.

Crystallization kinetics of tellurium-rich Se–Te–Sn glassy alloys

Differential scanning calorimetry (DSC) has been used to study the non-isothermal crystallization kinetics of Se30Te70−xSnx (for x = 0, 1.5 2.5 and 4.5) glassy alloys. Characteristic kinetic parameters say glass transition temperature (T g), peak crystallization temperature (T p) and melting temperature (T m) are determined from the DSC curves, recorded at four different heating rates, i.e., 9, 12, 15, 18 K min−1. The characteristic temperatures T g, T c and T m are found to increase with an increase in Sn concentration as well as with the heating rate (α). The increase in T g, T p and T m may be ascribed to the increase in cross-linking of amorphous network due to the dispersion of Sn into the Se chains. The activation energy of glass transition (E g), crystallization activation energy (E c) and Avrami exponent (n) have been evaluated from the heating rate dependence of T g, T c and T p using Kissinger, Mahadevan, Augis-Bennett and Moynihan methods. Thermal stability and glass-forming ability of amorphous Se30Te70−xSnx alloy have also been examined. Our results show that Se30Te70−xSnx composition, with thermal stability parameter H R > 0.5, is thermally stable and has characteristics of good glass former. Further, the composition corresponding to x = 1.5 is found to be thermally more stable as compared to the other compositions.

Enhancement of tensile properties by the solid solution strengthening of nitrogen in Zr-based metallic glass composites

The effects of nitrogen on both the glass forming ability (GFA) and mechanical properties of Zr-based bulk metallic glasses were studied experimentally. The results show that the doping of nitrogen could induce the precipitation of N-rich Zr3Ni-type crystals, and the precipitated crystals don’t trigger heterogeneous nucleation. Therefore, the remainder molten metal contains very lower nitrogen content and keeps good glass formation ability. Based on this, using the precipitated β-Zr(Ti, Nb) acted as a solid solvent for absorbed nitrogen, the optimum mechanical properties of bulk metallic glass composites can be obtained by doping an appreciate amount of nitrogen. This finding not only gives us a clue to avoid the detriment of light interstitial impurity to glass formation, but also opens a window to adjust the mechanical properties of the composites.

Influence of preparation pathway on the glass forming ability

The glass forming ability (GFA), i.e. the ease of amorphization of drugs, is mostly investigated using the critical cooling rate upon melt quenching to generate an amorphous product via the thermodynamic pathway. However, amorphous materials can also be prepared via the kinetic pathway by milling. In this case, the time required to generate an amorphous product is called the minimal milling time. This study investigates the correlation of the GFA between these two pathways. Eighteen compounds were chosen and their GFA was investigated by determining the critical cooling rate and the minimal milling time. It was observed that drugs, which turned amorphous upon cooling from the melt at slow cooling rates also had a low minimal milling time and vice versa. It was found that the GFA of the studied set of drugs was inherent and independent of the preparation method. It can be concluded that a drug with low critical cooling rate will also have a low minimal milling time and is thus a good glass former.

Covalent to Ionic Behaviour of Cadmium Doped PbO-B2O3 Glasses

Background: Spectroscopic studies in the past have exposed that PbO-B2O3 glasses are mostly classified as non-crystalline materials. These glasses prove to good host for incorporating RE (rare earth) and transition metal oxides due to its low phonon energy, large ion Polarizability and higher covalency. TM’s are widely used in the glass due to their two or more valence states. These states affect optical as well as structural properties. Due to commercial importance, B2O3 is good glass former. It is frequently used as a dielectric material. The transition metals incorporated borate glasses have great importance in the field of luminescent solar energy concentrators, gamma ray shielding materials, opto-acoustical electronics, solid state lasers, optical fibres, UV filters and ultrasonic devices in nonlinear devices for frequency conversion in the ultraviolet region and piezoelectric actuator. In transition metals cadmium oxide incorporated glasses has great importance because it can act both as network former and network modifier. Another important factor of cadmium glass is low electrical resistivity, wide band gap and high transmission in the visible region. It has different applications such as photo transistors, diodes, transparent electrodes, solar cells and gas sensors. Method: Glass samples xCdO-(35-x) PbO–65B2O3 with x varying from 0 to 15 mol % have been formed by the conventional melt quench technique. The raw materials of cadmium oxide (CdO), lead oxide (PbO) and boric oxide (B2O3) of suitable amounts are mixed properly and melted crucible at temperature 1000-1100°C in silica till the formation of bubble free liquid. This melt is then transferred in to a heated steel mould. These samples are annealed at temperature of 380°C for 1 hour so that samples do not break due to remaining internal strains. These samples are grinded with different grades of SiC and polished with CeO2 so as to obtain maximum flatness. The composition of the glass samples is shown in Table 1. The structural and physical properties are examined by using XRD (X-ray diffraction), Fourier transform infrared spectroscopy (FTIR), density and molar volume techniques. Results: Cadmium oxide doped glasses have been synthesized by using melt quench technique. XRD, density, molar volume and FTIR spectroscopy have been used to characterize the glasses. XRD confirms the glassy behaviour of prepared samples. There is a rise in density of glasses (4.11 to 4.94 g/cm3) and consistent decline in molar volume (30.02 to 22.10) of glasses with incorporation of cadmium. FTIR spectroscopy shows that addition of cadmium oxide helps to translate [BO3] units into [BO4] units. Also the corporation of cadmium effect on the ionic/covalent behaviour of glasses; covalency decreases and ionicity increases. Conclusion: (1) Xrd shows the amorphous nature of prepared samples. (2) Results of boron-boron separation show that addition of cadmium oxide makes compaction in glass network which is due to the presence of BO4 groups. These factors help to increase the density and reduce the molar volume. (3) Ionicity and covalency results confirmed that addition of cadmium oxide increased the ionicity of glass sample which may be used to make some ionic applications. Keywords: Covalency, FTIR, glasses, Cadmium doping, non-crystalline materials.

Composition dependence of the glass forming ability in binary mixtures: The role of demixing entropy.

We present a comparative study of the glass forming ability of binary systems with varying composition, where the systems have similar global crystalline structure (CsCl+fcc). Biased Monte Carlo simulations using umbrella sampling technique show that the free energy cost to create a CsCl nucleus increases as the composition of the smaller particles is decreased. We find that systems with comparatively lower free energy cost to form CsCl nucleus exhibit more pronounced pre-crystalline demixing near the liquid/crystal interface. The structural frustration between the CsCl and fcc crystal demands this demixing. We show that closer to the equimolar mixture, the entropic penalty for demixing is lower and a glass forming system may crystallize when seeded with a nucleus. This entropic penalty as a function of composition shows a non-monotonic behaviour with a maximum at a composition similar to the well known Kob-Anderson (KA) model. Although the KA model shows the maximum entropic penalty and thus maximum frustration against CsCl formation, it also shows a strong tendency towards crystallization into fcc lattice of the larger "A" particles which can be explained from the study of the energetics. Thus for systems closer to the equimolar mixture although it is the requirement of demixing which provides their stability against crystallization, for KA model it is not demixing but slow dynamics and the presence of the "B" particles make it a good glass former. The locally favoured structure around "B" particles is quite similar to the CsCl structure and the incompatibility of CsCl and fcc hinders the fcc structure growth in the KA model. Although the glass forming binary systems studied here are quite similar, differing only in composition, we find that their glass forming ability cannot be attributed to a single phenomenon.

Dramatically growing shear rigidity length scale in the supercooled glass formerNiZr2

Finding a suitably growing length scale that increases in tandem with the immense viscous slowdown of supercooled liquids is an open problem associated with the glass transition. Here, we define and demonstrate the existence of one such length scale which may be experimentally verifiable. This is the length scale over which external shear perturbations appreciably penetrate into a liquid as the glass transition is approached. We provide simulation based evidence of its existence, and its growth by at least an order of magnitude, by using molecular dynamics simulations of NiZr2, a good fragile glass former. On the probed timescale, upon approaching the glass transition temperature from above, this length scale, {\xi} is also shown to be consistent with Ising-like scaling. Furthermore, we demonstrate the possible scaling of {\xi} about the temperature at which super-Arrhenius growth of viscosity, and a marked growth of the penetration depth sets in. Our simulation results suggest that upon supercooling, marked initial increase of the shear penetration depth in fluids may occur in tandem with the breakdown of the Stokes-Einstein relation.

Physical, thermal and structural properties of Calcium Borotellurite glass system

In this work the glass forming ability in Calcium Borotellurite (CBTx) glass system was studied. Six glass samples were prepared by melt-quenching technique and the obtained samples are transparent, lightly yellowish, with no visible crystallites. The structural studies were carried out by using XRD, FTIR, Raman Spectra, density measurements, and the thermal analysis by using DTA and specific heat. The results are discussed in terms of tellurium oxide content and their changes in structural and thermal properties of glass samples. The addition of TeO2 increased the density and thermal stability values and decreased glass transition temperature (Tg). Raman and FTIR spectroscopies indicated that the network structure of CBTx glasses is formed by BO3, BO4, TeO3, TeO3+1 and TeO4 units. CBTx system showed good glass formation ability and good thermal stability, which make CBTx glasses suitable for manufacturing process and a candidate for rare-earth doping for several optical applications.

Glass transition of two-dimensional 80–20 Kob–Andersen model at constant pressure

We reconsider numerically the two-dimensional version of the Kob–Andersen model (KA2d) with a fraction of 80% of large spheres. A constant moderate pressure is imposed while the temperature T is systematically quenched from the liquid limit through the glass transition at down to very low temperatures. Monodisperse Lennard-Jones (mdLJ) bead systems, forming a crystal phase at low temperatures, are used to highlight several features of the KA2d model. As can be seen, e.g. from the elastic shear modulus G(T), determined using the stress-fluctuation formalism, our KA2d model is a good glass-former. A continuous cusp-singularity, with , is observed in qualitative agreement with other recent numerical and theoretical work, however in striking conflict with the additive jump discontinuity predicted by mode-coupling theory.

Influence of particle size on crystallization and relaxation behavior of Ge20Se4Te76 glass for infrared optics

Influence of particle size on relaxation and crystallization behavior of the Ge20Se4Te76 chalcogenide glass was studied by differential scanning calorimetry (DSC). The structural relaxation kinetics was described by means of the Tool–Narayaswamy–Moynihan (TNM) model; it was found that the structural relaxation processes are not influenced by powder particle size or by the mechanically induced defects and heterogeneities originating from the grinding procedures — the evaluated TNM parameters were constant for all tested powders. On the other hand, the crystallization kinetics of the Ge20Se4Te76 glass was found to be significantly influenced by the powder size. While the coarser powders exhibited a single-peak behavior corresponding to the nucleation-growth Johnson–Mehl–Avrami (JMA) kinetics, the crystallization of finest powders proceeded in two distinct steps. XRD analysis identified the two DSC signals as crystallization of hexagonal Te and rhombohedral GeTe, indicating that large amounts of mechanically induced defects accelerate the Te precipitation. Regarding the overall thermal characterization, the evaluated glass-stability criteria have shown that although the studied chalcogenide glass is a very good glass-former, its stability largely depends on the sample form.

Preparation and effects of glass-coatings on BaTiO3-based PTC thermistors

Glasses in the Na2O–K2O–K2SiF6–ZnO–Al2O3–B2O3–SiO2 system were investigated as potential intermediate temperature sealing materials to protect BaTiO3-based positive temperature coefficient (PTC) thermistors. The influence of compositional changes on the crystallization tendency and sintering behavior of the glass was reported. Additionally, the electrical properties of PTC thermistors with glass-coatings were compared with those without glass-coatings. The results showed that the formulation (in wt%), 3.1Na2O–9.4K2O–10.6K2SiF6–4.3ZnO–4.3Al2O3–20.3B2O3–48SiO2 (G2), performed better on thermal properties, had a good glass formability within a relatively wide range of sintering temperature and was compatible with the PTC thermistors (no cracks or pinholes). Furthermore, the glass layer did not affect the characteristics of the PTC thermistors too much.

Effect of alloy substituents on soft magnetic properties and economics of Fe-based and Co-based alloys

Abstract

Development of Fe-based bulk metallic glasses as potential biomaterials.

A new series of Fe80-x-yCrxMoyP13C7 (x = 10, y = 10; x = 20, y = 5; x = 2 0, y = 10, all in at.%) bulk metallic glasses (BMGs) with the maximum diameter of 6mm have been developed for biomedical implant application by the combination method of fluxing treatment and J-quenching technique. The corrosion performance of the present Fe-based BMGs is investigated in both Hank's solution (pH = 7.4) and artificial saliva solution (pH = 6.3) at 37 °C by electrochemical measurements. The result indicates that the corrosion resistance of the present Fe-based BMGs in the above two simulated body solutions is much better than that of biomedical 316 L stainless steel (316 L SS), and approaching that of Ti6Al4V biomedical alloy (TC4). The concentrations of Fe, Ni and Cr ions released into the Hank's solution and artificial saliva solution from the present Fe-based BMGs after potentiodynamic polarization are significant lower than that released from 316 L SS. The biocompatibility of the present Fe-based BMGs is evaluated through the in vitro test of NIH3T3 cells culture in the present Fe-based BMG extraction media for 1, 3 and 5 days. The result indicates that the present Fe-based BMGs exhibit no cytotoxicity to NIH3T3 cells. And the test result of the cell adhesion and growth on the surface of the samples indicates that the present Fe-based BMGs exhibit the better cell viability compared with 316 L SS and TC4 biomedical alloys. The present Fe-based BMGs, especially Fe55Cr20Mo5P13C7 BMG, exhibit good glass formation ability, the high corrosion resistance and excellent biocompatibility, suggesting their promising potential as biomaterials.

A new method locating good glass-forming compositions

A new method was proposed to pinpoint the compositions with good glass forming ability (GFA) by combining atomic clusters and mixing entropy. The clusters were confirmed by analyzing competing crystalline phases. The method was applied to the Zr–Al–Ni–Cu–Ag alloy system. A series of glass formers with diameter up to 20 mm were quickly detected in this system. The good glass formers were located only after trying 5 compositions around the calculated composition. The method was also effective in other multi-component systems. This method might provide a new way to understand glass formation and to quickly pinpoint compositions with high GFA.

Crystallization behavior of Fe- and Co-based bulk metallic glasses and their glass-forming ability

In the present work we study and compare the crystallization behavior of Fe- and Co-based good bulk glass formers with an exceptionally high glass-forming ability leading to the critical thickness of cast samples reaching 1 cm. For Fe-based alloys we also investigate the effect of opposite C/B content ratio on the glass-forming ability and the crystallization behavior. The structure and phase composition of the glassy samples were examined by conventional X-ray diffractometry and transmission electron microscopy while thermal stability and phase transformations were studied by differential scanning calorimetry. The reasons for high glass-forming ability are discussed. The glass-forming ability of the studied alloys depends on both factors: the type of crystallization reaction and characteristic temperatures.

Role of different factors in the glass-forming ability of binary alloys

In the present work, we discuss the glass-forming ability of various binary alloys in which the glassy phase was not formed even by melt spinning technique with high cooling rate of the melt up to 1 MK/s (some consisted of partly glassy phase), though by commonly accepted guidelines, these alloys could be as good glass-formers as many other binary glasses. The alloys studied belong to binary systems with multiple eutectics; the constituent elements have a negative enthalpy of mixing, and a significant variability of atomic size differences is observed from system to system. The results indicate the necessity of taking into account simultaneously various factors influencing the glass-forming ability including melt fragility.