Polyamorphism in a metallic glass

A metal, or an alloy, can often exist in more than one crystal structure. The face-centred-cubic and body-centred-cubic forms of iron (or steel) are a familiar example of such polymorphism. When metallic materials are made in the amorphous form, is a parallel 'polyamorphism' possible? So far, polyamorphic phase transitions in the glassy state have been observed only in glasses involving directional and open (such as tetrahedral) coordination environments. Here, we report an in situ X-ray diffraction observation of a pressure-induced transition between two distinct amorphous polymorphs in a Ce(55)Al(45) metallic glass. The large density difference observed between the two polyamorphs is attributed to their different electronic and atomic structures, in particular the bond shortening revealed by ab initio modelling of the effects of f-electron delocalization. This discovery offers a new perspective of the amorphous state of metals, and has implications for understanding the structure, evolution and properties of metallic glasses and related liquids. Our work also opens a new avenue towards technologically useful amorphous alloys that are compositionally identical but with different thermodynamic, functional and rheological properties due to different bonding and structural characteristics.     

Amorphous silicon exhibits a glass transition

Amorphous silicon is a semiconductor with a lower density than the metallic silicon liquid. It is widely believed that the amorphous-liquid transition is a first-order melting transition. In contrast to this, recent computer simulations and the experimental observation of pressure-induced amorphization of nanoporous silicon have revived the idea of an underlying liquid-liquid phase transition implying the existence of a low-density liquid and its glass transition to the amorphous solid. Here we demonstrate that during irradiation with high-energy heavy ions amorphous silicon deforms plastically in the same way as conventional glasses. This behaviour provides experimental evidence for the existence of the low-density liquid. The glass transition temperature for a timescale of 10 picoseconds is estimated to be about 1,000 K. Our results support the idea of liquid polymorphism as a general phenomenon in tetrahedral networks.     

Metamaterial-plasmonic absorber structure for high efficiency amorphous silicon solar cells

We show that a planar structure, consisting of an ultrathin semiconducting layer topped with a solid nanoscopically perforated metallic film and then a dielectric interference film, can highly absorb (superabsorb) electromagnetic radiation in the entire visible range, and thus can become a platform for high-efficiency solar cells. The perforated metallic film and the ultrathin absorber in this broadband superabsorber form a metamaterial effective film, which negatively refracts light in this broad frequency range. Our quantitative simulations confirm that the superabsorption bandwidth is maximized at the checkerboard pattern of the perforations. These simulations show also that the energy conversion efficiency of a single-junction amorphous silicon solar cell based on our optimized structure can exceed 12%.     

Correlation between Structures of Bulk Amorphous Zr-Ti-Ni-Cu-Be Alloy in Different States

The structures of the bulk amorphous Zr41Ti14Cu12.5Nil0.0Be22.5 alloy have been analyzed in solid, supercooled liquid and liquid with X-ray diffraction. The first coordination sphere radii and the first coordination numbers are 0.312 um, 11.2 in solid state, 10.932 nm, 10.932 in supercooled liquid region and 0.305 urn, 11.296 in liquid state. The structures are the same in different states. But it shows some tendency to crystallizing that the first coordination sphere radius and the first coordination number drop in supercooled liquid region.     

Amorphous polymorphism

Recent studies of amorphous solid materials have revealed the possibility that more than one distinct amorphous phase may be formed from the same substance. In analogy with the phenomenon of crystalline polymorphism, this behavior has been termed “amorphous polymorphism”. We review the experimental manifestation of amorphous polymorphism, especially in tetrahedrally coordinated materials such as H₂O and SiO₂. Guided by computer simulation results on these substances we show how a thermodynamic explanation of these phenomena is possible, specifically that amorphous polymorphism occurs in substances where the thermodynamic behavior of the liquid state exhibits liquid-liquid phase separation, or a tendency toward it. We identify a number of systems which may also display amorphous polymorphism, and emphasize the central role to be played by computer simulation in the elucidation of this phenomenon.     

Glow-discharge-produced amorphous semiconductors and their application to solar coatings

Recent developments in the physics and technology of tetrahedrally bonded amorphous semiconductors are reviewed. First, some unique advantages of these materials are discussed from the point of view of their basic physical properties and of their technological applications. The current state of the art in the development and technology of amorphous silicon solar cells is surveyed, and some new approaches and key technologies which enable efficiencies of the order of 10% to be attained are introduced. The principle advantages of photothermal conversion in p-type hydrogenated amorphous siliicon films are described. Some fundamental properties of photothermal conversion using this material are presented. Finally, a newly developed amorphous silicon photovoltaic-photothermal hybrid solar coating device with a conversion efficiency in excess of 55% is described.     

Liquid-liquid phase transition in supercooled silicon

Silicon in its liquid and amorphous forms occupies a unique position among amorphous materials. Obviously important in its own right, the amorphous form is structurally close to the group of 4-4, 3-5 and 2-6 amorphous semiconductors that have been found to have interesting pressure-induced semiconductor-to-metal phase transitions. On the other hand, its liquid form has much in common, thermodynamically, with water and other 'tetrahedral network' liquids that show density maxima. Proper study of the 'liquid-amorphous transition', documented for non-crystalline silicon by both experimental and computer simulation studies, may therefore also shed light on phase behaviour in these related materials. Here, we provide detailed and unambiguous simulation evidence that the transition in supercooled liquid silicon, in the Stillinger-Weber potential, is thermodynamically of first order and indeed occurs between two liquid states, as originally predicted by Aptekar. In addition we present evidence to support the relevance of spinodal divergences near such a transition, and the prediction that the transition marks a change in the liquid dynamic character from that of a fragile liquid to that of a strong liquid.     

Fabrication of a carbon-nanotube-based field-effect transistor by microcontact printing

The fabrication of a field-effect transistor with both channel material and source and drain electrodes made from carbon nanotubes (CNTs) through patterned deposition of CNT films by microcontact printing is described. Surfactant-dispersed single-walled CNTs are first separated into semiconducting and metallic fractions by gel filtration. The semiconducting and metallic CNTs are then sequentially transferred by dendrimer-coated polydimethylsiloxane stamps onto dendrimer-coated silicon wafers following a printing protocol optimized for this purpose. The resulting CNT micropatterns are visualized by atomic force microscopy. Semiconducting as well as metallic CNTs preserve their characteristic electronic properties within the transferred films. A device composed of a rather thick (ca. 5 nm) and densely patterned film of metallic CNTs cross-printed on top of a thinner (ca. 1.5 nm) and less dense film of semiconducting CNTs shows the typical properties of a field-effect transistor with the metallic CNT stripes as electrodes, the semiconductive CNT stripes as channel material, and the silicon substrate as gate electrode.     

Spreading of liquid lead droplets on metallic iron covered by silicon oxide particles or films

As well known, the spreading of a liquid metal droplet on a solid metal is very sensitive to the presence of chemical heterogeneities on the solid metal. In this study, wetting experiments with liquid lead on heterogeneous surfaces composed of iron and silicon oxide particles or films were performed using the dispensed drop technique. High purity iron and binary iron–silicon substrates with different silicon contents were studied. Before the wetting experiments, the substrates are annealed at 850 °C in a N₂–H₂ atmosphere in order to reduce iron oxides and to form silicon oxide particles or films on the surface. The liquid lead droplet is then released onto the metallic substrate partly or wholly covered by the oxides. The spreading of the liquid metal droplet strongly depends on the surface area fraction covered by the oxides.     

Electrical resistivity and thermoelectric power of polycrystalline and amorphous Se-Te-Cu system

The dark electrical resistivity and thermoelectric power have been measured for the bulk ternary alloy Se-Te-Cu. The samples were both polycrystalline and amorphous in structure. The measurements were carried out below room temperature. Depending on Cu addition, crystallographic structure, and amorphous or polycrystalline state, the samples manifested semiconducting or metallic behaviour. The maximum difference in electrical resistivity magnitude was of 14 orders. The activation energy ΔE of charge carriers determined for all semiconducting samples ranged from 0.07 to 0.25 eV. An increase in thermoelectric power resulting from the electron–phonon mass enhancement was estimated.     

液态和非晶态Cu-Zr合金的研究进展

从性质、结构以及相图等方面论述了近年来液态和非晶态Cr Zr合金的研究现状。指出目前研究的主要内容集中在固态尤其是非晶态的结构和晶化以及相分离过程等方面 ,而对与凝固过程密切相关的液态Cu Zr合金的结构和性质方面的研究则涉及非常少。因而从液态Cu Zr合金的结构和性质入手 ,寻求液态、非晶态与晶态之间的相互关系 ,是一个值得研究的新课题     

Rapid solidification of zirconium-based alloys

Zr based metal-metal binary and ternary alloys can be obtained in the amorphous state in very wide composition ranges. Several eutectic reactions and intermetallic compounds are present in these alloy systems which provide opportunities for examining the validity of different theories on glass formation. The amorphous phases in these alloys decompose by a variety of crystallization mechanisms. Instances of polymorphic, primary and eutectic crystallization have been encountered in these glasses. Zr-based metallic glasses possess excellent corrosion resistance and mechanical properties. In several studies their properties have been compared with that of their crystalline counterparts and interesting differences have emerged. In the solute lean Zr-based alloys very large freezing ranges are available for studying the liquid to solid transformation. It has been possible to study the formation of some of the low temperature phases directly from the liquid. This paper describes some of the aforementationed studies carried out on Zr-based amorphous and crystalline alloys.     

Preparation of amorphous FexSi(1 − x) film using unbalanced magnetron sputtering

The preparation of iron-silicon films was performed onto Si (100) substrates by microwave electron cyclotron resonance (ECR) plasma source enhanced unbalance magnetron sputtering. The compositions, microstructures and properties of films under different sputtering powers and annealing conditions were characterized by AES, GAXRD, TEM and absorption spectrum techniques. The results described that the amorphous iron silicon films can be easily prepared by unbalance magnetron sputtering. Even the Fe/Si ratio deviated far from 1:2, such as Fe/Si = 1:14.8 or 1:10, the amorphous iron silicon film with semiconductor properties can also be obtained, which suggests that the Fe/Si ratio is not the only factor to determine whether the samples have semiconducting properties in iron silicon amorphous. After annealing at 850 °C for 4 h, the microstructure of nanometer β-FeSi₂ embedded into amorphous Si still possesses semiconducting characteristics.     

Pressure-induced amorphization and polyamorphism: Inorganic and biochemical systems

Pressure-induced amorphization (PIA) is a phenomenon that involves an abrupt transition between a crystalline material and an amorphous solid through application of pressure at temperatures well below the melting point or glass transition range. Amorphous states can be produced by PIA for substances that do not normally form glasses by thermal quenching. It was first reported for ice I_h in 1984 following prediction of a metastable melting event associated with the negative initial melting slope observed for that material. The unusual phenomenon attracted intense interest and by the early 1990s PIA had been reported to occur among a wide range of elements and compounds. However, with the advent of powerful experimental techniques including high resolution synchrotron X-ray and neutron scattering combined with more precise control over the pressurization environment, closer examination showed that some of the effects previously reported as PIA were likely due to formation of nanocrystals, or even that PIA was completely bypassed during examination of single crystals or materials treated under more hydrostatic compression conditions. Now it is important to understand these results together with related discussions of polyamorphic behavior to gain better understanding and control over these metastable transformations occurring in the low temperature range where structural relaxation and equilibration processes are severely constrained. The results will lead to useful new high-density amorphous materials or nanocrystalline composites containing metastable crystalline varieties and the experiments have driven new theoretical approaches to modeling the phenomena. Here we review the incidence and current understanding of PIA along with related phenomena of density- and entropy-driven liquid–liquid phase transitions (LLPT) and polyamorphism. We extend the discussion to include polymeric macromolecules and biologically-related materials, where the phenomena can be correlated with reversible vs irreversible unfolding and other metastable structural transformations.     

Thermodynamic properties of amorphous silicon via tight binding simulations

An atomic-scale structure of amorphous silicon, generated by reverse Monte Carlo, has been used as a starting configuration for finite temperature molecular dynamics simulations performed by an orthogonal tight binding Hamiltonian. Structural, dynamic, elastic and electronic structure properties have been investigated in the range of temperatures up to and above the melting transition. The amorphous silicon structure undergoes a melting transition at a temperature sensibly smaller than that of the crystalline structure. Above this temperature, the structure has the same properties of an under-cooled liquid and it has a metallic behavior.     

Contact reactions between amorphous silicon and single-crystal metallic films

Single-crystal palladium, chromium and gold films with (100) orientation were made by electron beam evaporation of the metal onto a rock-salt substrate maintained at about 400°C. Subsequently, an amorphous silicon film was deposited at room temperature onto the metal film in the same pump-down to form a reaction couple. Reactions between the amorphous silicon and the single-crystal metallic films were studied by transmission electron diffraction and microscopy. We found that metals such as palladium and chromium which form silicides react uniformly over the entire interface with the silicon, but gold which forms no stable silicides shows a non-uniform interfacial reaction. The nucleation and growth rate of silicon crystals in the Au-Si reaction was measured.     

Preparation and properties of radiofrequency sputtered X-ray amorphous films in the system SiO2–ZrO2

Amorphous films in the system SiO₂–ZrO₂ were prepared by radiofrequency sputtering method and their density, refractive index, elastic constants, and thermal expansion coefficient were measured. All of the physical properties had a similar compositional dependence; that is, they increased, but not proportionally, with increasing ZrO₂ content. The coordination states of cations in these amorphous films were estimated by the compositional dependence of volume and molar refractivity. The coordination state of silicon ions in the amorphous films did not change, but the coordination number of zirconium ions changed from 8 to 6, depending on ZrO₂ content. These results indicate that, in amorphous films in the system SiO₂–ZrO₂, the change of the coordination state of zirconium ions in the amorphous films has an important effect on the properties.     

Preparation of a new class of semiconductors: bulk amorphous tetrahedral solid solutions Ge1−x (GaSb)x

A new method of preparation of bulk amorphous solids is successfully applied to the semiconductor solid solutions Ge_1–x (GaSb) _x .The method consists in the solid-state disordering of a high-pressure phase on decompression. Large mutual solubility can be achieved for high-pressure phases of Ge and GaSb. Amorphization of metallic solid solutions occurs on decompression even at room temperature. Some data concerning the structure and stability of the amorphous semiconducting solid solutions a-Ge_1–x (GaSb) _x are presented.     

Macro-scale complexity of nano- to micro-scale architecture of olivine crystals through an iodine vapour transport mechanism

The production of nano- to micro-scale olivine (magnesium and iron silicate) crystals has been achieved at relatively low temperatures through an iodine vapour transport of the metal onto amorphous silicon dioxide. The process occurs down a temperature gradient from 800 to 600 °C yielding high quality crystals with long range crystallinity, highly complex interconnectivity and intricate macroscale architecture. Scanning electron microscopy (SEM) imaging of the substrate before and after the reaction reveals that the amorphous silicon oxide species is mobile, due to the lack of correlation between the silicon oxide layer and the final olivine particles, leading to a vapour–liquid–solid or vapour–solid growth mechanism. This technique demonstrates a facile, low temperature synthetic route towards olivine crystals with nano- to micro-scale dimensions.