Properties Of Amorphous Materials Research Articles

Symmetry and order of the kinetic heterogeneity in Pd-Si amorphous alloys

In amorphous alloys, the atomic arrangement exhibits short-range order while lacking long-range order. Despite this absence of long-range order, the local atomic arrangements and interactions can still significantly influence the movement of atoms.The microstructural features and structural evolution mechanisms of amorphous materials are key areas of research, and the dynamics of amorphous alloys can provide insights into their formation processes and structural evolution. The cage effect refers to the phenomenon where atoms are trapped by their surrounding atoms, making it difficult for them to migrate or diffuse freely. This leads to slower diffusion rates and higher viscosities in these materials. Atomic concentration is one of the crucial factors that influence the structure and properties of amorphous materials. Changes in concentration can significantly alter the material’s structure. Adjusting the atomic concentration can lead to differences in the diffusion rates between elements in the amorphous alloys, resulting in a heterogeneous distribution of elements in different regions, which in turn affects the deformation characteristics of amorphous materials.This study aims to investigate the effect of Pd atomic concentration on the diffusion hindrance of Si atoms, as well as its impact on the local symmetry and order of the system. To achieve this objective, molecular dynamics simulations are employed to explore the relaxation process of atoms in Pd-Si amorphous alloys at different Pd atomic concentrations, and parameters related to atomic diffusion, displacement distribution, system symmetry, and order are analyzed. The results show that increasing the concentration of Pd atoms leads to a more pronounced hindrance in the diffusion of Si atoms, manifested by an increase in the abnormal peak values of the non-Gaussian parameters and a decrease in the standard deviation of the displacements. This indicates that a higher Pd atom concentration enhances the cage effect of Si atoms, thus restricting their diffusion. Additionally, the increase in Pd concentration promotes the transition from unsaturated to saturated bond types in the Pd-Si amorphous alloys, and also leads to a decrease in the system's configurational entropy. This consequently enhances the local symmetry and order of the Pd-Si amorphous alloys, resulting in Si atoms locate at the center of more closed, higher-symmetry, and more compact cluster structures, which strengthens their cage effect and local symmetry. This study investigates the impact of Pd atom concentration on the diffusion behavior and local environment of Si atoms, providing a new insight into the structural evolution of amorphous alloys.

Engineering medium-range order and polyamorphism in a nanostructured amorphous alloy

Like crystalline materials, the properties of amorphous materials can be tailored by tuning the local atomic-to-nanoscale structural configurations. Polyamorphism is evident by the coexistence of kinetically stabilized amorphous structures with tailorable short-to-medium-range orders, providing a viable means to engineer the degree of local order and heterogeneity. Here, we report experimental evidence of the coexistence of liquid-like and solid-like amorphous phases in a Ni82P18 amorphous alloy with enhanced thermal stability and plasticity prepared by pulsed electrodeposition. The two amorphous phases, of comparable volume fraction of ~50% each, have similar short-range order but are distinguished by packing at the medium-range length scale (>6 Å). Upon heating, a structure crossover at ~450 K was observed, where the liquid-like structure transforms to the solid-like structure, as evidenced by the enthalpy release and an anomalous contraction of atomic structure over the medium-range length scale, due to the metastable nature of the liquid-like structure.

Analysis of medium-range order based on simulated segmented ring detector STEM-images: amorphous Si

Properties of amorphous materials are connected to the local structure at the nanoscale, which is typically described in terms of short- and medium-range order (SRO, MRO). Variable resolution fluctuation electron microscopy (VR-FEM) is a sensitive method to characterize the underlying characteristic length scale of MRO of amorphous samples (Voyles, Gibson and Treacy, J. Electron Microsc. 49 (2000) 259). VR-FEM data was acquired using scanning transmission electron microscopy (STEM), collecting a large number of nano-beam diffraction patterns (NBDPs) with various probe sizes. Here we present an advanced method to accelerate the calculation of simulated FEM normalized variance profiles using a newly developed simulation and analysis approach with segmented ring detectors using the program STEMcl (Radek et al., Ultramicroscopy 188 (2018) 24). VR-FEM simulations are based on structures obtained from molecular dynamics (MD) simulations. A comparison between simulated and experimental VR-FEM profiles with respect to peak position, ratio and shape (and intensity) show good agreement. Moreover, a crystalline cluster of 1 nm in size was embedded into the MD box to test the validity of the paracrystalline approximation with the pair-persistence analysis suggested by Gibson et al. (Gibson, Treacy and Voyles, Ultramicroscopy 83 (2000) 169). The corresponding VR-FEM simulation and calculation of MROs yield close results to the size of the initially embedded crystalline cluster, which supports both the paracrystalline approach and the validity of the segmented detector simulation. Additionally, we conclude that continuous random network (CRN) amorphous silicon models contain a higher degree of MRO than experimentally expected.

Mechanical Properties of Nanocrystalline Coatings Prepared by Thermal Spraying Process

The uniaxial tensile tests are generally used to measure the yield stress and/or tensile stresses. The majority of the results, resulting from this method and deferred in the literature on the mechanical properties of amorphous materials, were obtained in the ribbons form generally produced by melt spinning. This limitation in the shape of the specimens for tensile tests was inherent in the manufacturing techniques of the amorphous materials which required cooling rates of about 106°C/s. The fabrication of amorphous coatings by plasma spraying, in tubular form, should improve rigidity of the specimens, increase their size and especially get a more reliable means to the amorphous coatings under the tensile test. Amorphous coatings of NiCrBSi alloys are produced by thermal plasma spraying. The thickness of the resulting amorphous deposit may be up to 500 μm. The uniaxial tensile tests were carried on Instron machine at room temperature. Microstructural studies are investigated by means of X-ray diffraction (XRD), scanning (SEM) and transmission (TEM) electron microscopy. The test methodology used in this study shows that it is possible to obtain a comprehensive assessment of the mechanical properties of the amorphous coatings deposited by thermal plasma in tubular form. Plasma sprayed coatings in tubular form, under the uniaxial tensile test in the amorphous state, crystallized and nanocrystallized, fracture without bursting compared to samples in the form of amorphous ribbon.

Measuring acoustic properties of amorphous materials

Measuring acoustic properties of amorphous materials

Doping effects in amorphous oxides

Impurity doping of crystalline Si is one of the most striking techniques in semiconductor technology. A rigid and perfect crystalline lattice is prerequisite for effective doping. However, it has been reported to date that introducing a small amount of impurities drastically improves also the properties of amorphous materials. This paper reviews three pronounced doping effects on optical and electrical properties of amorphous oxides; i.e., (i) F-doping of silica glass to improve the vacuum-ultraviolet optical transmission and radiation toughness, (ii) codoping effects on solubility enhancement of rare earth ions in silica glass melt, and (iii) electron-carrier generation in transparent amorphous oxide semiconductors. It is emphasized that effectiveness of electron doping is determined by the magnitude of electron affinity and stabilization energy of a dopant. Importance of the local structure formed around a dopant ion and the location of conduction band minimum measured from the vacuum level is addressed to understand the doping effects in amorphous oxides.

Influences of Extra Magnets on Giant Magnetoimpedance Effect in Amorphous Wires

Adjacent extra magnets can induce an additional impedance to an amorphous wire. We have studied the additional impedance passing through a magnetic ring and found that it is related to the electromagnetic characteristics, shapes, and locations of the extra magnet and the frequency of the ac excitation, regardless of the amorphous material properties and the extra magnetic field conditions. According to our theoretical analysis, the additional impedance severely reduces the magnetoimpedance change of the amorphous wire and destroys the giant magnetoimpedance characteristic. We therefore propose a return structure to diminish the additional impedance. Our experiments measured the impedances of an amorphous wire, a Cu wire, and the return structure with a variety of extra magnetic fields. The results confirmed the characteristics of the additional impedance and verified the effectiveness of the return structure.

An Investigation into the Influence of Counterion on the Properties of Some Amorphous Organic Salts

Amorphous solids and crystalline salts are both of interest as a means of improving the dissolution characteristics and apparent solubility of poorly water soluble active pharmaceutical ingredients which have low bioavailability in humans. The theory and selection of both crystalline drug substance salt forms and amorphous products have been extensively studied. However, less is known about the impact of different counterions on the properties of amorphous drug substance salts. In this study, several salts of either nicardipine or propranolol were prepared and characterized with respect to glass transition temperature, crystallization tendency and moisture sorption behavior. Although the moisture sorption behavior and crystallization tendency varied depending on the counterion used, no trends were readily apparent. The glass transition temperature was found to be dependent on the counterion used to form the salt, and was higher in all instances for the salts than for the neutral compound. Several molecular descriptors were calculated for the various counterions, and multivariate analysis was used to build a model that successfully correlated Tg with a number of these parameters. Important parameters which influenced Tg included counterion pKa and electrophilicity index. In conclusion, it is apparent that, as for crystalline salts, the counterion has an effect on the properties of amorphous materials.

Bond constraint theory and the quest for the glass computer

Electronic switching in amorphous chalcogenide semiconductors has been observed and studied for nearly forty years. Technological exploitation of this phenomenon has most recently emerged in DVD’s where GST, a compound of germanium, antimony, and tellurium, is used to store information. We explain how GST behaves as a switch and how X-ray absorption fine structure can be used to unlock the specifics of the switching process. The tool that leads to this deeper understanding is the bond constraint theory. We explain how this theory leads to an explanation of switching and of the behavior and properties of amorphous materials in general. Finally, the prospects for developing GST-related materials into non-volatile memory media that could be the basis for glass computers are discussed.

Fictive temperature measurement of amorphous SiO2 films by IR method

The structure and properties of amorphous materials, in general, change with their thermal history. This is usually explained using the concept of fictive temperature, i.e., the temperature at which the super-cooled liquid state turned into a glassy state. In earlier studies, a simple IR method was used to determine the fictive temperature of silica glasses, both bulk and fiber. In the present study the applicability of the same technique for thin amorphous silica films on silicon was examined. It was found that the IR absorption as well as reflection peak wavenumber of the silica structural band can be used to determine the fictive temperature of amorphous silica films on silicon with an unknown thermal history. Specifically, IR absorbance spectra of an amorphous silica film of thickness greater than 0.5μm grown on silicon can be taken before and after etching a thin surface layer of 20–30nm and the peak wavenumber of the difference signal can be compared with the pre-determined calibration curve to convert the peak wavenumber to the fictive temperature. For a film thicker than ∼2μm, IR reflection peak wavenumber can be converted directly to the fictive temperature of the film by using the calibration curve.

Effect of Nuclear Quadrupole Interaction on the Relaxation in Amorphous Solids

Recently it has been experimentally demonstrated that certain glasses display an unexpected magnetic field dependence of the dielectric constant. In particular, the echo technique experiments have shown that the echo amplitude depends on the magnetic field. The analysis of these experiments results in the conclusion that the effect seems to be related to the nuclear degrees of freedom of tunneling systems. The interactions of a nuclear quadrupole electrical moment with the crystal field and of a nuclear magnetic moment with magnetic field transform the two-level tunneling systems inherent in amorphous dielectrics into many-level tunneling systems. The fact that these features show up at temperatures $T<100mK$, where the properties of amorphous materials are governed by the long-range $R^{-3}$ interaction between tunneling systems, suggests that this interaction is responsible for the magnetic field dependent relaxation. We have developed a theory of many-body relaxation in an ensemble of interacting many-level tunneling systems and show that the relaxation rate is controlled by the magnetic field. The results obtained correlate with the available experimental data. Our approach strongly supports the idea that the nuclear quadrupole interaction is just the key for understanding the unusual behavior of glasses in a magnetic field.

Molecular Dynamics Investigation of Relaxations in Zeolite ZSM-5 Based Amorphous Material

We present results from molecular dynamics computer simulations of the dynamic properties of amorphous material derived from zeolite ZSM-5, in particular secondary relaxations for temperatures below the critical temperature. We observe local jumps where groups of atoms move collectively. At low temperatures the relaxations comprise mainly of one-dimensional chains of atoms. The dimensionality of the relaxing centers increases with the temperature due to side branching. Both the displacement vector and the number of atoms participating in the jumps increase with temperature. The possibility of having reversible jumps decreases with increasing temperature due to a strong drop in the potential energy during aging. There exist very prominent peaks in the van Hove correlation functions as a manifestation of the hopping processes. The dynamics of the oxygen atoms is found to be more heterogeneous than those of the silicon atoms.

Amorphous Pharmaceutical Solids.

Amorphous forms are, by definition, non-crystalline materials which possess no long-range order. Their structure can be thought of as being similar to that of a frozen liquid with the thermal fluctuations present in a liquid frozen out, leaving only "static" structural disorder. The amorphous solids have always been an essential part of pharmaceutical research, but the current interest has been raised by two developments: a growing attention to pharmaceutical solids in general, especially polymorphs and solvates and a revived interest in the science of glasses and the glass transition. Amorphous substances may be formed both intentionally and unintentionally during normal pharmaceutical manufacturing operations. The properties of amorphous materials can be exploited to improve the performance of pharmaceutical dosage forms, but these properties can also give rise to unwanted effects that need to be understood and managed in order for the systems to perform as required.

Characterization of nanometer-scale defects in metallic glasses by quantitative high-resolution transmission electron microscopy

Although defects can have a significant effect on the properties of amorphous materials, in many cases these defects are poorly characterized and understood. This is at least partly due to the difficulty of imaging defects in amorphous materials in the electron microscope. In this work, we demonstrate the utility of quantitative analysis of high-resolution transmission electron microscopy for the identification and characterization of nanometer-scale defects in metallic glasses. For a proper identification of such defects, it is important to carefully consider the effects of the imaging conditions and thickness variations in the sample, both of which we describe in detail. As an example, we show that regions of localized plastic deformation (shear bands) in bulk metallic glasses contain a high concentration of nanometer-scale voids. These voids apparently result from the coalescence of excess free volume once the applied stress is removed.

Influence of the treatment of melt on the properties of amorphous materials: ribbons, bulks and glass coated microwires

Modern concepts of the structure of liquid metals and alloys are analyzed. Several types of microinhomogeneity and microheterogeneity of liquid metallic solutions are shown to exist. Their structure depends on their composition and pre-history. Using temperature and pressure variations and/or other physical effects, one can modify the structure. These changes retain down to liquidus and affect the structure and properties of the solidified alloy if the melt is cooled at appropriate cooling rate. This investigation focus on the effects of melt overheating above the liquids. Optimal thermal melting modes are specified using the results on liquid alloys structure and properties. Such optimized thermal treatment of melts (OTTM) is shown to be most efficient for rapidly quenched (RQ) metallic materials. The OTTM effects upon the structure and properties of amorphous ribbons, bulk amorphous alloys and glass coated microwires are analyzed.

Vibrational spectrum and temperature behavior of thermal conductivity and specific heat in amorphous dielectrics

A microscopic model is proposed to describe the temperature behavior of the thermal conductivity and specific heat of amorphous dielectrics. The consideration is carried out in a lattice model of the amorphous state. The main model points are connected with the one-site matrix of phonon scattering on the defects which are impurity oscillators vibrating in a symmetric two-well potential. Depending on the value of the only random parameter of the model, a tunnel state can arise or not on the defect. The mass operator of the system is written in a general form which involves some parameters whose values are unknown. The analytical expressions obtained make it possible to find the model parameter values before numerical calculations. The results demonstrated good agreement with experiments and confirm the validity of the model for describing the properties of amorphous materials over a wide temperature range.

Fractal fracture of polyarylatesulfone film specimens

Actual cracks on the mesoscale have practically nothing to do with ideal cracks with smooth lips traditionally considered in the theory of fracture. Actual fracture surfaces are, as a rule, quite irregular and are characterized by the presence of hills and pits of various sizes. The observation enables one to use fractal models for the analysis of the process of fracture [I -3]. Recent years are marked by the appearance of numerous experimental works based on the use of the fractal concept of fracture for the description of the behavior of cracks. Unfortunately, most of these works were carried out for crystal solid bodies and, in particular, for metals [3, 4]. We made an attempt to study the relationship between the fractality of fracture and the structure and properties of amorphous materials. As an example, we took polyarylatesulfone (PAS)---a linear amorphous copolymer extensively used as a heat resistant structural material [5].

Modelling non-Arrhenius temperature dependence of transport properties in amorphous materials

Based on the series hopping scheme proposed earlier, numerical simulation has been conducted to reproduce the non-Arrhenius temperature dependence for the transport properties of amorphous materials modelled by hopping in a two-dimensional square lattice. Although the Arrhenius temperature dependence was assumed for individual hopping steps, τhop∫ exp(E/kT), where E varies with the barriers heights, the resulting logarithmic diffusivity plotted against reciprocal temperature produces a convex curvature observed by many experiments.

Magnetoelastic properties of amorphous materials and of transducers based on them

The range of application of amorphous magnetically-soft materials is fairly extensive. After it had been established that these materials are very sensitive to external effects, it became possible to use them for making various sensors and transducers. The literature contains insufficient data on the magnetoelastic characteristics of amorphous magnetically-soft materials. The article presents the results of investigations of these characteristics of amorphous alloys with various compositions.

Thermal induced transformations in glassy chalcogenides SixTe60?xAs30Ge10

Temperature-induced transformations are considered to be interesting characteristic properties of amorphous materials including the Si x Te60−x As30Ge10 system, withx=5, 10, 12 and 20. Density (ϱ), X-ray diffraction and differential thermal analysis (DTA) were used to characterize the compositions. DTA traces of each glass composition at different heating rates from 5 to 30° C min−1 were obtained and interpreted. Fast and slow cooling cycles were used to determine the rate of structure formation. Cycling studies of materials show no memory effect but only ovonic switching action. The compositional dependence of the crystallization activation energy (E) and the coefficient of glass-forming tendency (K gl) have been calculated. The thermal transition temperatures and associated changes in specific heat have been examined as a function of the Te/Si ratio by differential scanning calorimetry. It was found thatϱ andE increase linearly with increasing tellurium content, while the heat capacity (c p) andK gl, decrease with increasing tellurium content.E=1.54 eV andc p=0.246 J g−1 K−1 forx=20 whileE=2.74eV andc p=0.22 J g−1 K−1 forx=5.