Distinct Amorphous Phases Research Articles

Ultrafast phase transitions in polyamorphic materials triggered by swift heavy ion impacts

We report unexpectedly fast phase transitions inside the track as pressure and temperature develop after a swift heavy ion impact. By means of molecular dynamics simulations we reveal the origin of the core-shell fine structure observed in a-${\mathrm{Si}}_{}{\mathrm{O}}_{2}$ and a-${\mathrm{Si}}_{3}{\mathrm{N}}_{4}$, as well as the dense core in a-Si. The fine structure of the track is composed of two distinct amorphous phases that result from the different pressure levels in the core and in the shell during solidification. We link this behavior to the tetrahedral network present in these materials.

Exploring the interplay between fibrillization and amorphous aggregation channels on the energy landscapes of tau repeat isoforms

Filaments made up of different isoforms of tau protein are associated with a variety of neurodegenerative diseases. Filaments made up of the 4R-tau isoform, which has four repeat regions (R1 to R4), are found in patients suffering from Alzheimer's disease, while filaments made of the 3R-tau isoform, which contains only three repeat units (R1, R3, and R4), are found in patients with Pick's disease (frontotemporal dementia). In this work, a predictive coarse-grained protein force field, the associative memory water-mediated structure and energy model (AWSEM), is used to study the energy landscapes of nucleation of the two different fibrils derived from patients with Pick's and Alzheimer's diseases. The landscapes for nucleating both fibril types contain amorphous oligomers leading to branched structures as well as prefibrillar oligomers. These two classes of oligomers differ in their structural details: The prefibrillar oligomers have more parallel in-register β-strands, which ultimately lead to amyloid fibrils, while the amorphous oligomers are characterized by a near random β-strand stacking, leading to a distinct amorphous phase. The landscape topography suggests that there must be significant structural reordering, or "backtracking," to transit from the amorphous aggregation channel to the fibrillization channel. Statistical mechanical perturbation theory allows us to evaluate the effects of changing concentration on the aggregation free-energy landscapes and to predict the effects of phosphorylation, which is known to facilitate the aggregation of tau repeats.

Solid-state analysis of amorphous solid dispersions: Why DSC and XRPD may not be regarded as stand-alone techniques

Amorphous solid dispersions (ASDs) are single-phase amorphous systems, where drug molecules are molecularly dispersed (dissolved) in a polymer matrix. The molecular dispersion of the drug molecules is responsible for their improved dissolution properties. Unambiguously establishing the phase behavior of the ASDs is of utmost importance. In this paper, we focused on the complementary nature of (modulated) differential scanning calorimetry ((m)DSC) and X-ray powder diffraction (XRPD) to elucidate the phase behavior of ASDs as demonstrated by a critical discussion of practical real-life examples observed in our research group. The ASDs were manufactured by either applying a solvent-based technique (spray drying), a heat-based technique (hot melt extrusion) or mechanochemical activation (cryo-milling). The encountered limiting factors of XRPD were the lack of sensitivity for small traces of crystallinity, the impossibility to differentiate between distinct amorphous phases and its impossibility to detect nanocrystals in a polymer matrix. In addition, the limiting factors of (m)DSC were defined as the well-described heat-induced sample alteration upon heating, the interfering of residual solvent evaporation with other thermal events and the coinciding of enthalpy recovery with melting events. In all of these cases, the application of a single analytical technique would have led to erroneous conclusions, whilst the combination of (m)DSC and XRPD elucidated the true phases of the ASD.

Novel injectable strontium-hardystonite phosphate cement for cancellous bone filling applications

Injectable bone cement (IBC) such as those based on methacrylates and hydraulic calcium phosphate and calcium sulfate-based cements have been used extensively for filling bone defects with acceptable clinical outcomes. There is a need however for novel IBC materials that can address some of the inherent limitations of currently available formulations to widen the clinical application of IBC. In this study, we characterized a novel hydraulic IBC formulation consisting of bioactive strontium-doped hardystonite (Sr-HT) ceramic microparticles and sodium dihydrogen phosphate, herein named Sr-HT phosphate cement (SPC). The resultant cement is comprised of two distinct amorphous phases with embedded partially reacted crystalline reactants. The novel SPC formulation possesses a unique combination of physicochemical properties suitable for use as an IBC, and demonstrates in vitro cytocompatibility when seeded with primary human osteoblasts. In vivo injection of SPC into rabbit sinus defects show minor new bone formation at the SPC periphery, similar to those exhibited in sinus defects filled with a clinically available calcium phosphate cement. The current SPC formulation presented in this paper shows promise as a clinically applicable IBC which can be further enhanced with additives.

Nickel-Based Amorphous Alloys As Electrocatalyst Materials in Alkaline Water Electrolysis

Introduction Alkaline electrolysis with anionic exchange membranes has been shown to be a highly promising technology to produce hydrogen gas. This zero gap system does not require a liquid electrolyte and can utilize non-precious metal electrocatalyst materials. As such, it is possible to produce a low cost and energy efficient water electrolyzer [1]. Production of hydrogen gas from water electrolysis has been largely limited due to a lack of inexpensive and active electrocatalyst materials that do not degrade over time. Amorphous alloys have previously been proposed as electrocatalyst materials due to their unique atomic structure, which offer high corrosion resistance, high number of coordinatively unsaturated sites and electrochemical activity [2]. Their full potential has not been recognized as electrocatalysts largely due to their low surface area when produced and challenges in synthesis [2]. We suggest a novel process to produce high surface area amorphous alloy powders designed specifically as electrocatalysts for hydrogen evolution. This research is focused on Ni-based alloys that show high stability in alkaline solutions and high activity among non-platinum group metals [3]. Specifically, Ni-based alloys containing Nb and Y are studied in this work. Previous work has shown that Ni-Nb-Y amorphous alloys can phase separate into two distinct amorphous phases [4]. Further enhancement of the electrochemically active surface area can be achieved by selectively dealloying one of the two-phases leaving a nano-porous structure [5]. The present work assesses the electrochemical behaviour of these phase-separated Ni-Nb-Y amorphous alloys. Experimental Methods Ni-Nb-Y powder is produced by mechanically alloying high purity elemental powders using a high-energy ball mill. These powders are then chemically etched to selectively dealloy one of the two phases present. These powders can then be pressed into a pellet or mixed into ink and applied (by spraying, printing or brushing) onto a conductive support like carbon paper or glassy carbon rod [1]. Electrochemical characterization is assessed through steady state polarization (Tafel) curves, cyclic voltammetry, and AC impedance spectroscopy. Structural characterization is assessed through x-ray diffraction (XRD) and electron microscopy. Results and Discussion Mechanical alloying of nickel, niobium and yttrium elemental powders was performed, producing Ni58.2-Nb20.25Y21.25with ca 90% amorphous structure. Scanning electron microscopy (SEM) and XRD were used to analyze this alloy (see figure 1). SEM allowed analysis of particle size and morphology (average particle size is approximately 40 micron), and XRD allowed determination of microstructure, alloy formation and degree of crystallinity. The alloy produced a Ni-Nb-Y amorphous phase and a separate Ni-Y amorphous phase, indicating the presence of a phase separated alloy. The synthesized material also contains yttrium oxide, which is believed to reduce electrocatalytic activity [5]. Oxide formation can be limited by reducing exposure of the elemental powders to oxygen during the ball milling processing. Conclusions and Future Work The results suggest that high surface area amorphous Ni58.2-Nb20.25Y21.25 powder can be produced from high-energy ball milling. Future work will be further developing material to achieve fully amorphous structure, characterizing phase separation and evaluating electrochemical properties. [1] D. Pletcher, and X. Li. “Prospects for alkaline zero gap water electrolyzers for hydrogen production.” Int. J. Hydrogen Energy, vol. 36, pp. 15089-15104, (2011) [2] F. Safizadeh, E. Ghali, and G. Houlachi. “Electrocatalysis developments for hydrogen evolution reaction in alkaline solutions – A Review”. Int. J. Hydrogen Energy, vol. 40, pp. 256-274, (2015) [3] K. Zeng, and D. Zhang. “Recent progress in alkaline water electrolysis for hydrogen production and applications.” Progress in Energy and Combustion Science, vol. 36, pp. 307-326, (2010) [4] N. Mattern, T. Gemming, J. Thomas, G. Goerigk, H. Franz, and J. Eckert. “Phase Separation in Ni-Nb-Y metallic glasses.” Journal of Alloys and Compounds, vol. 495, pp. 299-304, (2010) [5] A. Gerbert, N. Mattern, U. Kuhn, J. Eckert and L. Schultz. “Electrode characteristics of two-phase glass-forming Ni-Nb-Y alloys.” Intermetallics, vol. 15, pp. 1183-1189, (2007) Figure 1

Thermal and plastic behavior of nanoglasses

Abstract

X-ray study of lithium disilicate glass: High pressure densification and polyamorphism

We have investigated the structural changes induced by high-pressure in lithium disilicate glass with stoichiometric composition Li2O.2SiO2 (LS2). Using toroidal type high-pressure chambers, glass samples were processed at 2.5GPa, 4GPa, 6GPa and 7.7GPa at room temperature. Synchrotron X-ray diffraction measurements were used to obtain the radial distribution functions (RDF) in order to follow the structural changes induced after processing at high pressure. Compared to a pristine sample, the main change observed for the samples processed up to 6GPa was associated to the distortion of the SiO4 tetrahedral structure. However, for the sample processed at 7.7GPa, a drastic change in the RDF indicated the formation of a distinct amorphous phase. The observed polyamorphism is important for a better understanding of the nucleation kinetics of crystalline phases under high pressure.

Structural analysis of pressure-amorphized zirconium tungstate

The negative thermal-expansion material zirconium tungstate, when subjected to a moderate pressure of 2 GPa, turns amorphous irreversibly; the amorphous state being $\ensuremath{\sim}26\mathrm{%}$ more dense than the crystalline phase $\ensuremath{\alpha}\text{-Zr}{({\text{WO}}_{4})}_{2}$. Structure of pressure-amorphized $\text{Zr}{({\text{WO}}_{4})}_{2}$ is investigated using synchrotron x-ray diffraction. Structure factors $S(q)$ of samples recovered from different pressures are obtained. The first peak in $S(q)$ for 10.5 GPa-treated sample is found to occur at a slightly larger $q$ than that recovered from 5.5 GPa, suggesting that the amorphous samples recovered from higher pressure could be more dense. The pair-correlation functions $g(r)$, obtained by Fourier transforming $S(q)$, suggests that the further densification of amorphous phase occurs via contraction of second and third neighbor shells representing O-O and W-W and W-Zr distances rather than first neighbor shell. In contrast to earlier speculations, the present results do not show evidence for an increase in the coordination number and consequent existence of distinct amorphous phases in samples recovered from pressures up to 10.5 GPa.

Hierarchies of networked phases induced by multiple liquid–liquid critical points

Nanoparticles and colloids functionalized by four single strands of DNA can be thought of as designed analogs to tetrahedral network-forming atoms and molecules, with a difference that the attached DNA strands allow for control of the length scale of bonding relative to the core size. We explore the behavior of an experimentally realized model for nanoparticles functionalized by four single strands of DNA (a tetramer), and show that this single-component model exhibits a rich phase diagram with at least three critical points and four thermodynamically distinct amorphous phases. We demonstrate that the additional critical points are part of the Ising universality class, like the ordinary liquid-gas critical point. The dense phases consist of a hierarchy of interpenetrating networks, reminiscent of a woven cloth. Thus, bonding specificity of DNA provides an effective route to generate new nano-networked materials with polyamorphic behavior. The concept of network interpenetration helps to explain the generation of multiple liquid phases in single-component systems, suggested to occur in some atomic and molecular network-forming fluids, including water and silica.

Dynamic mechanical properties of random ethylene/1-octene copolymers prepared by rapid cooling

The dynamic mechanical properties of a well-characterized series of homogeneous ethylene/1-octene copolymers with different random hexyl branch contents and prepared using different cooling conditions have been examined using dynamic mechanical analysis (DMA). It was confirmed that the relaxation behavior of copolymers varied continuously with the branch content: the magnitude of the β relaxation increased with branch content while the intensity of the α relaxation decreased with the branch content; both relaxation temperatures decreasing with increasing branch level in the copolymers. Copolymers prepared at different cooling conditions were further examined and strikingly continuous changes were found for the first time. The β relaxation was shown to correlate to the amorphous region, while the α 1 and α 2 relaxations can be clearly differentiated for some samples and are assumed to be associated with the interlamellar slip and intra-crystalline c-shear processes respectively. With increasing cooling rate, the relative intensity of α 1 relaxation to α 2 relaxation was found to decrease while the β relaxation did not change. The most informative data is determined from deconvolution of tan δ spectra. In higher crystallinity polymers the α 1 and α 2 relaxations are closely related in activation energy but have different temperature locations. For lower crystallinity systems, where the α 1 relaxation cannot be observed, the α 2 and β relaxations are closely linked, with activation energies approaching one another. These results show very clearly that, although the observed relaxation data can be separated through deconvolution into three separate peaks, the behaviors are closely linked. Presumably, this a clear reflection of the role of tie molecules in binding phases together and in influencing dynamic mechanical behavior. A clear change of behavior has also been observed in the β relaxation when a distinct amorphous phase exists outside of the spherulites, confirming the general belief that the crystalline phase influences the amorphous phase when it is confined within a spherulite. Again, this behavior is reflecting the role of tie molecules in binding together the nanocomposite structure of a spherulite.

In situBrillouin Spectroscopy of a Pressure-Induced Apparent Second-Order Transition in a Silicate Glass

Brillouin scattering measurements of a silicate glass, carried out at high pressures in the diamond anvil cell, show a dramatic increase in the pressure dependence of longitudinal velocity, and a discontinuity in the compressibility of the glass at about 6 GPa. While a first-order phase transition has been documented under pressure within amorphous ice, we demonstrate that an apparent second-order transition to a new, structurally distinct amorphous phase can occur via the abrupt onset of a new compressional mechanism, which may be triggered by a shift in polymerization of the glass or an onset of a change in coordination of silicon, within pressurized amorphous silicates.

White luminescence from Si+ and C+ ion-implanted SiO2 films

The microstructural and optical analysis of SiO2 layers emitting white luminescence is reported. These structures have been synthesized by sequential Si+ and C+ ion implantation and high-temperature annealing. Their white emission results from the presence of up to three bands in the photoluminescence (PL) spectra, covering the whole visible spectral range. The microstructural characterization reveals the presence of a complex multilayer structure: Si nanocrystals are only observed outside the main C-implanted peak region, with a lower density closer to the surface, being also smaller in size. This lack of uniformity in their density has been related to the inhibiting role of C in their growth dynamics. These nanocrystals are responsible for the band appearing in the red region of the PL spectrum. The analysis of the thermal evolution of the red PL band and its behavior after hydrogenation shows that carbon implantation also prevents the formation of well passivated Si/SiO2 interfaces. On the other hand, the PL bands appearing at higher energies show the existence of two different characteristics as a function of the implanted dose. For excess atomic concentrations below or equal to 10%, the spectra show a PL band in the blue region. At higher doses, two bands dominate the green–blue spectral region. The evolution of these bands with the implanted dose and annealing time suggests that they are related to the formation of carbon-rich precipitates in the implanted region. Moreover, PL versus depth measurements provide a direct correlation of the green band with the carbon-implanted profile. These PL bands have been assigned to two distinct amorphous phases, with a composition close to elemental graphitic carbon or stoichiometric SiC.

Role of nitrogen related complexes in the formation of defects in silicon

Defect size and density distributions were obtained as a function of depth in nitrogen doped CZ silicon (N-CZ) following Hi–Lo–Hi and Lo–Hi annealing, using an oxygen precipitate profiler. The defects were also delineated by Wright etching and Nomarski optical microscopy on both cleaved and bevel polished samples. In addition to the enhanced precipitation and absence of voids previously reported for N-CZ Si, an unexpected mode of precipitation has been found near the annealed wafer surface, just above the traditional denuded zone. This oxynitride precipitate is discussed with regard to N-related complex interactions and point defect supersaturations/injection. High resolution transmission electron microscopy revealed that most precipitates have an octahedral shape with two distinct amorphous phases, which reflect a transition from an initial phase containing both N and O to one with primarily O, as verified with Z-contrast TEM and electron energy loss spectroscopy.

Spatially Resolved Crystalline and Amorphous Components of Polymeric Materials by Carbon-13 Chemical Shift Imaging Performed with Radio-Frequency Field Gradients

It is shown that chemical shift imaging can be easily adapted to high-resolution solid-state experiments by means of radio-frequency field gradient technology. (The gradient is delivered by a two-turn flat coil, the axis of which coinciding with the rotor axis.) The resulting two-dimensional diagram involves chemical shift information in one dimension and spatial information (along the rotation axis) in the other dimension. Different procedures (physical filters) have been considered in order to enhance specific properties of the crystalline and amorphous components in high-density polyethylene (hdpe). Two of them rely on differences in cross-polarization or in cross-polarization inversion processes, while a third one is based on carbon-13 longitudinal relaxation times. The latter is shown to be especially efficient for effectively obtaining separate images of the amorphous and crystalline components and ultimately unraveling their distribution within the sample. In particular, it is demonstrated that two structurally distinct amorphous phases are spatially separated in the sample under investigation.

Liquid−Liquid Immiscibility in Single-Component Network-Forming Fluids: Model Calculations and Implications for Polyamorphism in Water

Analytical calculations and Monte Carlo simulations of a lattice model of a tetravalent network-forming fluid show liquid−liquid immiscibility at low temperatures. Three types of phase behavior are found to occur: vapor−liquid equilibrium, vapor−liquid equilibrium plus liquid−liquid immiscibility with an upper critical temperature, and vapor−liquid equilibrium plus closed-loop liquid−liquid immiscibility. The coexisting liquids differ appreciably in density, structure, and molecular dynamics. The low-density liquid displays more structured, open networks, and slower translational and network-restructuring dynamics than the high-density liquid. The model provides useful insights into the molecular basis of low-temperature immiscibility in network-forming fluids, and its relation to the unusual transition between distinct amorphous phases in vitreous water.

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 H 2O and SiO 2. 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.

Evidence for two distinct amorphous phases in (Zr 0.667Ni 0.333) 1− xB x alloys

The density, electrical transport, and superconducting properties of amorphous (Zr 0.667Ni 0.333) 1− x B x where 0 ⩽ x ⩽ 0.25 have been measured. The results, together with the pair correlation function and the deduced density of states at the Fermi surface, suggest that a composition dependent structural transition occurs from one type of amorphous structure to another near x ∼ 0.05.

DTA studies on the glycerol—water system

A modified duPont 900 DTA cell was used to study the complex phase transitions in the glycerol—water system at relatively rapid rates of heating and cooling and under carefully monitored thermal history conditions. This system is of particular interest in the cryoprotection of red blood cells. The separation of crystals and a constant-composition amorphous phase from water-rich systems tends to distort the glass transition ( T g) vs. composition curves unless special precautions are exercised. Extrapolation of T g to zero glycerol content gave a hypothetical second-order transition in “amorphous” ice of −123 ±1°C at a heating rate of 30°C/min. It is suggested that the glycerol—water system may exhibit the phenomena of vitreous polymorphs wherein two distinct amorphous phases coexist in metastable equilibrium in the solid state. The observed second-order transition in one of these phases occurred near, and in some cases slightly above, T g for pure glycerol. Limited low temperature X-ray diffraction studies on 43.7 wt.% glycerol showed the existence of two crystalline forms, one of which could only be formed by rapid cooling from the liquid phase. Neither form gave diffraction patterns corresponding to known forms of ice.