Amorphous Solid Phase Research Articles

Atomistic insight into structure and properties of oxide films formed upon oxidation of Al–Mg alloys – reactive molecular dynamics study

Oxidation phenomena on metal surfaces can be advanced using atomistic modeling for the study of the structure and properties of the growing oxide films. Molecular dynamics investigations were performed to study thermal oxidation of single-crystal and polycrystalline Al–Mg alloys with low Mg content of up to 2.5 at. %. Structure, topological atom network and surface topography of the oxide films grown on Al–Mg surfaces at 300, 475 and 663 K are determined. Mg-content and temperature dependent oxide films developed on Al-Mg alloy substrates with two-phase oxide film formation as also confirmed experimentally. It is related with a gradual long-range order formation above 475 K for (Al,Mg)-oxide films, for which solid amorphous phase predominates over crystalline phase. The 1.12-nm-thick oxide films grown at 300 K are fully amorphous and build up from (Al, Mg)-oxide which the Al2O3 dominates. Increase of Mg coefficient confirms faster Mg diffusion to oxide/alloy interface at high oxidation temperature and the formation of MgO-like network. The presence of planar crystal defect (grain boundary, GB) into Al-Mg alloy substrate governs the kinetics of oxide film growth and its structure evolution. GB also compensate internal stresses during oxide film growth. Obtained results are in good agreement with experimental observations.

Metastable phases of Ag–Si: amorphous Si and Ag-nodule mediated bonding

Metastable phases such as supersaturated solid solutions, supercooling, and amorphous phases are well-known in metallurgy. They are often composed in non-equilibrium states and can be transformed into a stable phase by overcoming an energy barrier with driving forces. Particularly, it has been widely used for material strengthening and heterogeneous nucleation of precipitates in solids is mainly induced by heat treatments for supersaturated solid solutions. However, little is known about the metastable phases of the Ag–Si alloy, although it is a well-known simple binary eutectic alloy. Here, we show that the metastable phases composed of amorphous Si and supersaturated Ag solid solution are induced by the eutectic reaction under rapid cooling of Ag–Si. Furthermore, the solute Si in the Ag matrix reacts with oxygen to precipitate Ag by-products, which grow as nodules. The Ag nodules have high crystallinity and robust interfacial structures, and the nodule growth leads to the formation of cross-links between the Ag–Si particles. We also demonstrate the Ag nodule-mediated bonding where the rapidly cooled Ag–Si ribbon is directly used as a bonding medium, indicating the possibility of using it as a high-temperature bonding material with low-temperature processes.

The key role in the structure and properties of a novel CrNiTiMo high-entropy alloys films prepared by magnetron sputtering: Bias voltage

In this study, CrNiTiMo high-entropy alloys films were prepared on 304 stainless steel and silicon wafer substrates by direct current magnetron sputtering and the effects of bias voltages (0 to −200 V) on microstructure, chemical composition, hardness and corrosion resistance were investigated. The results showed that the surface of the CrNiTiMo HEAs films exhibited equiaxed nanoparticles and are composed of uniformly distributed Cr, Ni, Ti, and Mo elements. Meanwhile, the films developed a columnar structure and formed a biphasic structure with body-centered cubic solid solution and amorphous phases. The roughness decreased and then increased with increasing bias voltage, and the film deposited at −50 V had the lowest roughness (1.02 ± 01 nm), which also had a maximum hardness of approximately 9.52 ± 0.2 GPa and a maximum elastic modulus of approximately 187 ± 6 GPa. In addition, all films showed better corrosion resistance compared to 304 stainless steel substrates at 3.5 wt% NaCl solution. The corrosion current density (icorr) of the films decreased and then increased with increasing bias voltages and reached the lowest icorr of about 2.49 × 10−8A/cm2, indicating that the films deposited at a bias voltage of −50 V had the best corrosion resistance. These was attributed to the formation of the passivation films mainly composed of TiO2 and Cr2O3 and the improvement of the surface quality of the film as well as the refinement of the columnar structure and particles.

Unravelling the Role of Structural Factors in the Luminescence Properties of Persulfurated Benzenes.

Room temperature phosphorescence rarely occurs from pure organic molecules, especially in the solid-state. A strategy for the design of highly emissive organic phosphors is still hard to predict, thus impeding the development of new functional materials with the desired optical properties. Herein, we analyze a family of alkyl and aryl-substituted persulfurated benzenes, the latter representing a class of organic solid-state triplet emitters able to show very high emission quantum yield at room temperature. In this work, we correlate structural parameters with the photophysical properties observed in different experimental conditions (diluted solution, crystalline and amorphous phase at RT and low temperature). Our results, corroborated by a detailed computational analysis, indicate a close relationship between the luminescence properties and i) the nature of the substituents on the persulfurated core, ii) the adopted conformations in the solid state, both in crystalline and amorphous phases. These factors contribute to characterize the lowest-energy lying excited-state, its deactivation pathways, the phosphorescence lifetime and quantum yield. These findings provide a useful roadmap for the development of highly performing purely organic solid-state emitters based on the persulfurated benzene platform.

High‐Toughness Hydrated Polymer Electrolytes for Advanced Structural Supercapacitors

AbstractStructural supercapacitors that simultaneously bear mechanical loads and store electrical energy have exciting potential for enhancing the efficiency of various mobile systems. However, a significant hurdle in developing practical structural supercapacitors is the inherent trade‐off between their mechanical properties and electrochemical capabilities, particularly within their electrolytes. This study demonstrates a tough polymer electrolyte with enhanced multifunctionality made through the controlled hydration of a solid polymer electrolyte with poly(lactic acid) (PLA) and lithium salts. Characterization via differential scanning calorimetry, X‐ray diffraction, and Fourier transform infrared spectroscopy confirms the consistent amorphous solid solution phase in varying salt concentrations, whether dried or hydrated. Electrochemical tests and tensile tests are performed to evaluate the ionic conductivity and mechanical properties of these electrolytes. The results indicate that the strategic incorporation of water in the polymer electrolyte significantly enhances the ionic conductivity while preserving its mechanical properties. A specific composition demonstrated a remarkable increase in ionic conductivity (3.11 µS cm−1) coupled with superior toughness (15.4 MJ m−3), significantly surpassing the base polymer. These findings open new horizons for integrating electrochemical functionality into structural polymers without compromising their mechanical properties. Additionally, the paper reports the successful fabrication and testing of structural supercapacitor prototypes combining carbon fibers with fabricated electrolytes, showcasing their potential for diverse applications.

Detrital Input Sustains Diatom Production off a Glaciated Arctic Coast

AbstractIn the Arctic and subarctic oceans, the relatively low supply of silicon (compared to other nutrients) can make it limiting for the growth of diatoms, a fundamental building block of the oceanic food web. Glaciers release large quantities of dissolved silicon and dissolvable solid amorphous silica phases into high‐latitude estuaries (fjords), but the role of these glacially‐derived silica phases in sustaining diatom growth in the coastal and open‐water sectors remains unknown. Here we show how stable and radiogenic silicon isotopes can be used together to address this question, using southwest Greenland as a case study. This study finds enhanced levels of detrital (i.e., mineral) amorphous silica, likely glacially‐sourced, sustaining a large portion of diatom growth observed off the coast, revealing how the phytoplankton community can function during high‐meltwater periods.

Influence of structural Fe content in clay minerals on selenite redox reactions: Kinetics and structural transformations

Redox reactions control the environmental fate of selenium, an element of concern due to its small gap between beneficial and detrimental effects on human health and due to the longevity of the radionuclide 79Se produced in nuclear reactors. Fe-bearing clay minerals are major redox-active ingredients of Earth’s critical zone and constitute an important component of the barrier in (radioactive and other) waste repositories. Here we systematically investigate selenite (Se(IV)O32−) sorption and reduction by Fe-bearing clay minerals of the smectite group, using batch experiments and X-ray absorption fine structure (XAFS) spectroscopy to elucidate reaction mechanisms and kinetics up to 720 or 3600 h. We found a linear relationship between structural Fe(II) content and selenite reduction rate. Selenite reduction depends also on redox potential and pH, with pH 7 showing a slower reduction rate than at pH 5. Selenite first sorbs to clay edge sites by forming an inner-sphere sorption complex, before being reduced to Se(0). The first redox product was amorphous (red) Se(0), which gradually transformed into trigonal (grey) Se(0), indicative of a kinetically hindered conversion of an amorphous (or less crystalline) solid phase into the thermodynamically stable and more crystalline form of the solid. Despite an Fe2+aq concentration of up to 6·10−4 M in the clay suspension, and their thermodynamic prediction for the clays at lower Eh, we did not observe formation of iron selenides. These insights into the selenite reduction mechanisms by Fe-bearing clays provides valuable information for the development of effective approaches for selenite immobilization.

Rationality of two-phase coexistence with no element segregation in a CuZr-based amorphous alloy composite

Here, the structure features of Cu62Zr34.4Al3Nb0.6 amorphous alloy composites (AACs) are studied by combining experimental characterization with first-principles calculation. Element segregation, which is common in conventional alloy materials, does not occur in two phases (crystalline and amorphous phase) of the AACs due to complex solid solution. The lattice parameters, electron distribution, charge transfer and electrical potential, which are directly related to atomic bonding and lattice parameters of different structures, are obtained by calculating the various types of structural models, including interstitial solid solution, substitutional solid solution, B2 and amorphous phase. Finally, the formation and stability of solid solution in crystalline phase and amorphous phase are elucidated by first-principles calculation. The work has important guiding significance for understanding the two-phase structure and solidification mechanism in AACs materials.

The slow molecular mobility in the amorphous solid and supercooled liquid phases of brucine, an anti-cancer drug. A study by differential scanning calorimetry (DSC), thermostimulated currents (TSC) and dielectric relaxation spectroscopy (DRS)

In this work, we explore in detail the molecular mobility in amorphous brucine, i.e. in the glassy state and in the supercooled liquid immediately above the glass transition, where viscosity is high and where a spatially heterogeneous dynamic is detected. Brucine is an alkaloid used in ancient times by Chinese medicine in the treatment of cancer, it can be obtained in the amorphous form by cooling from the melt and the high stability exhibited makes it a good candidate for being use in novel formulations.Combining thermostimulated depolarization currents (TSDC), and dielectric relaxation spectroscopy (DRS) different types of motions have been probed. In the glassy state, two secondary relaxations γ and β were characterized, the latter with features compatible with a Johari-Goldstein mobility. The cooperative α relaxation was clearly identified, and by combining also with results taken from differential scanning calorimetry (DSC), it was possible to determine parameters such as the glass transition temperature, and fragility index which indicate a moderately fragile glass former. In addition, the determination of the βJG relaxation times and activation energy, both above and below Tg, were a new result of the current investigation.Additionally, TSDC measurements, although with some uncertainty, reveal the existence of a mobility above Tg. On the other hand, in the supercooled liquid region, the analysis of the conductivity obtained by DRS, shows a breakdown of the Debye-Stokes-Einstein relation pointing to a heterogeneous dynamic that could be in the origin of the observed mobility by TSDC.

Interface formation and defect elimination mechanism of T2/304L interface during electromagnetic pulse welding process

Experimental and numerical methods are carried out to analyze the interface formation and defect elimination mechanism during electromagnetic pulse welding T2 to 304L tube by changing the inner tube morphology from original scheme to stage scheme, ramp stage, and location stage. When the original scheme is operated with 13mm lap length, the morphology of interface is divided into four zones containing end unwelded zone, first welded zone, central unwelded zone, and second welded zone due to the different impact velocity and angle during the initial impact process, parallel impact process, and final impact process. The parallel impact leads to the formation of the central unwelded zone, containing (Cu, Fe) solid solution, oxides, nano grains, and amorphous phase. By providing a suitable impact angle, the ramp scheme (α = 7°) and location scheme (L = 6.5 mm) can eliminate the defect in the central unwelded zone.

Short‐Range Atomic Topology of Ab Initio Generated Amorphous PdSi Alloys

AbstractSince the pioneering efforts of Duwez and co‐workers in 1965, when a solid amorphous phase of palladium‐silicon (a‐PdSi) was obtained in the vicinity of the eutectic concentration, much work has been done. However, some points related to the atomic structures remain to be systematized. In this work, 8 amorphous Pd100‐cSic alloys (c = 2.5, 5, 10, 13.34, 15, 17.5, 20, and 22 at%) are generated by molecular dynamics ab initio simulations; the short‐range structure is analyzed using several correlation functions, like Pair Distribution Functions, reduced Pair Distribution Functions, Plane Angle Distribution Functions. Other related properties, like nearest‐neighbors and some Frank–Kasper polyhedra are reported. The generated samples correctly reproduce the scarce experimental pair correlation functions that are reported in the literature. An unexpected outcome is the appearance of structural changes in the neighborhood of Pd86.66Si13.34.

The Aqueous Chemistry of Zirconium as a Basis for Better Understanding the Formation of Zirconium Conversion Coatings: Updated Thermodynamic Data

This work tackles the aqueous chemistry of Zr, aiming to contribute to a better understanding of Zr conversion coatings as one of the important contemporary means of corrosion protection. Equilibrium predominance diagrams based on experimentally confirmed Zr–OH and Zr–F aqueous species concerning Zr amorphous solid phase, along with an updated Zr E−pH (Pourbaix) diagram, are constructed. Since ZrO2+ existence had been conclusively disproven in both the aqueous and solid states, we chose to depict mononuclear species with ZrOH3+ and polynuclear with Zr4(OH)8 8+. The formation of the Zr solid phase is assumed to include Zr tetrameric species, Zr4(OH)8 8+, as a fundamental building block thereof. The role of F– and ZrF6 2– ions in Zr conversion baths and subsequently formed coatings is described. The introduction of ZrF6 2– anions keeps Zr solvated in the form of a complex, thus preventing the early onset of hydrolysis. The conversion of Zr species and the coating formation mechanism are further discussed from electrochemical and sol-gel perspectives, aiming to give a foundation for future predictions and rationalisation of Zr conversion coating formation, with the possibility of extensions to various bath additives.

Structural Approach to Understanding the Formation of Amorphous Metal Hydroxides.

This study investigates the formation of amorphous tetravalent metal hydroxides, M(OH)4, based on the structural analysis by small- and wide-angle X-ray scattering (SWAXS) and on the electrical potential charge near the surface of M(OH)4 particles. The amorphous zirconium hydroxide solid phases that aged in NaCl and CaCl2 solutions at 25 °C exhibited a hierarchical structure consisting of primary particles of a few nanometers in size and their aggregates more than 100 nm in size. The SWAXS profiles suggested that the size of the primary particles depends on the ionic strength and electrolytes in the sample solutions. The smaller size of the primary particles observed in solutions with higher ionic strength can be explained by the thinner electrical double layer. Additionally, we focused on the ζ potentials of M(OH)4 suspensions in NaCl, NaNO3, and CaCl2 solutions. With the aid of reference systems of metal oxides, MO2, it was found that the ζ potentials were well interpreted by a traditional surface ionization and complexation model, and the size distributions of large aggregates were explained by the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory with the ζ potential values. The present study suggests the formation mechanism of amorphous metal hydroxides through a combination of structural analysis and investigation of electrical potentials.

The formation of structural disorder in FeNi-based alloys – Theoretical approach

• Positive values of enthalpy of formation of amorphous phase in Fe-Ni system confirmed on the basis of Miedema’s model. • Insignificant influence of Co doping on the enthalpies of formation in FeNi system. • Promotion of amorphous phase and solid solution formation ability depending on Cu substitution. • Cu as promising candidate when aiming destabilization of FeNi crystalline structure. FeNi-based alloys are known to crystallize in bcc or fcc-structures depending on their composition. In many slowly cooled objects, like meteorites, L 1 0 -type phase (tetrataenite) with its characteristic layered Fe/Ni structure has been found. Nevertheless, the synthesis of L 1 0 FeNi phase in industrial amount in a bulk form has not been possible, mainly due to sluggish diffusion and low order/disorder temperature of this phase. We present results of the calculations of formation enthalpies for solid solution and amorphous phase in Fe-Ni system. Additionally, we performed calculations for Co and Cu substitutions (found in meteoritic matter). Our calculations confirm poor glass forming ability of FeNi system and limited influence of Co in this aspect. In contrast, Cu is a promising candidate when aiming at destabilization of FeNi solid solution, as a starting point in L 1 0 phase formation. Formation enthalpy for some Cu-substituted compositions reaches highly negative values. It is expected that at least local chemical segregation should be facilitated by the presence of Cu atoms (attributed mainly to different values of Cu-Co and Cu-Ni interfacial enthalpies).

Photo-induced preparation of Ce-doped terbium aluminum garnet and its luminescent properties

Ce(III) activated terbium aluminum garnet (Tb3Al5O12:Ce, TAG:Ce) powder as a luminescent phosphor was synthesized by the photo-induced method. The irradiation of an aqueous solution containing formate anions and soluble salts of both metal ions by UV light led to the precipitation of an amorphous solid phase. Subsequent heat treatment at 1200 °C resulted in the crystallization of the TAG:Ce phase-pure garnet structure. The structure and morphology of the products were investigated by X-ray powder diffraction (XRPD) and by scanning electron microscopy (SEM). The particle size, evaluated by transmission electron microscopy (TEM), ranges from 50 to 150 nm. The manufactured Ce-doped TAG features intensive luminescence corresponding to Ce(III) 5d–4f transitions at 559 nm. Due to its high density, chemical stability, non-toxic character, and excellent scintillation efficiency, the application of Ce-doped TAG nano-powder in photodynamic therapy and preparation of optical ceramics was suggested.

Tungsten combustion in impact initiated W–Al composite based on W(Al) super-saturated solid solution

Element W can effectively improve the density of energetic structural materials. However, W is an inert element and does not combust in air. To change the reaction characteristics of W, 60 at.% Al was introduced into W through mechanical alloying. XRD analysis shows that after 50 h of ball milling, the diffraction peak of Al completely disappears and W(Al60) super-saturated solid solution powder is obtained. Further observation by HAADF and HRTEM reveals that the W(Al60) super-saturated solid solution powder is a mixture of solid solution and amorphous phase. Based on the good thermal stability of W(Al60) alloy powder below 1000 °C, W(Al60)–Al composite was synthesized by hot pressing process. Impact initiation experiments suggest that the W(Al60)–Al composite has excellent reaction characteristics, and multiple types of tungsten oxides are detected in the reaction products, showing that the modified W is combustible in air. Due to the combustion of tungsten, the energy release rate of the W(Al60)–Al composite at speed of 1362 m/s reaches 2.71 kJ/g.

Observation of banded spherulite in a pure compound by rhythmic growth

Banded spherulitic growth of crystals is observed in some materials with spherically symmetric growth front and periodic radial variation of birefringence. This variation of birefringence in quasi-two-dimensional geometry produces concentric interference color bands when viewed through crossed polarizers. In most materials, the banded spherulites are found to be formed by radially oriented periodically twisted fibrillar crystallites. Here, we report the formation of banded spherulites due to the rhythmic growth of concentric crystallite-rich and crystallite-poor bands for a pure compound consisting of strongly polar rodlike molecules. The compound exhibits coexistence of untwisted fibrillar crystallites and an amorphous phase in its most stable solid state. On sufficient supercooling of the sample from its melting point, the banded spherulites are formed with a periodic variation of composition of untwisted radially aligned fibrillar crystallites and an amorphous solid phase. We have developed a time-dependent Ginzburg-Landau model to account for the observed banded spherulitic growth.

Adding a New Dimension to the Amorphous Solid Dispersion Phase Diagram: Studying Dissolution Kinetics of Crystalline Drugs in a Polymer Matrix Using Temperature Dependent XRPD and DSC

Hot-melt-extrusion (HME) is an enabling technology used for poorly soluble active pharmaceutical ingredients (APIs) to increase the bioavailability by embedding the drug in a water soluble and often amorphous carrier such as a polymer. Knowledge of the most critical factors impacting the dissolution rate of crystalline API in the polymer during manufacturing will provide useful insight for process improvement. In this study, crystalline APIs (Acetaminophen, APAP and Indomethacin, IMC) were analyzed in a polymeric matrix (Copovidone, PVP-VA64) via X-Ray Powder Diffraction (XRPD) and Differential Scanning Calorimetry (DSC) to follow the dissolution process under various conditions in a down-scaled static laboratory system. The combination of in-situ XRPD measurements and a kinetic model based on DSC data proved to be a suitable tool to investigate the dissolution process and can be applied to various APIs and polymers to avoid residual crystallinity and thermal degradation. Thus, the temperature-composition phase diagram in a thermodynamic equilibrium is augmented by the kinetic component as new dimension. The obtained findings set the foundation for investigating the dissolution kinetics and enable the transition from a static to a dynamic system.

Ensemble-based machine learning models for phase prediction in high entropy alloys

High entropy alloys (HEAs), which are multi-component alloys having constituent elements in equi-atomic or near equi-atomic ratios, are receiving immense attention owing to their remarkable mechanical and physical properties. These unusual properties depend on one or more of the phases that these alloy systems constitute, namely solid solution (SS), intermetallic compound (IM), and amorphous (AM) phases. Therefore, phase prediction is crucial in selecting appropriate elements that lead to the formation of a HEA with desirable properties. In this work, machine learning (ML) models are used on a set of design parameters using multi-classification for HEA phase prediction. The ML models comprised ensemble-based (Random Forest and Stacked ensemble) and support vector machine (SVM) methods. To predict solid solution phase formation, researchers used atomic size difference and the parameter Ω=TmΔSmixΔHmix where Tm is average melting point, ΔSmix is entropy of mixing and ΔHmix is mixing enthalpy. The features used in this study are Ω, atomic size difference, average electron concentration, electronegativity difference, average melting point, mean atomic radius, mixing enthalpy, the entropy of mixing, and bulk modulus. The training dataset comprising of 601 as cast alloys is used with cross-validation for test data, and the phases SS, IM, AM, SS + IM, and IM + AM are predicted. The phase prediction accuracies are calculated using one-vs-rest, and precision-recall curves are plotted to determine the model performance. For phases AM, SS, and IM, the stacked ensemble displayed better accuracies when compared to SVM and Random Forest. The findings indicate that the stacked ensemble, which combines weak learners and meta-models, provides accuracy comparable to the neural network model accuracy reported in the literature. These findings provide insights to researchers and practitioners in selecting features and ML models while designing HEAs.

Amorphization, mechano-crystallization, and crystallization kinetics of mechanically alloyed AlFeCuZnTi high-entropy alloys

Equiatomic and non-equiatomic AlFeCuZnTi high-entropy alloys (HEAs) were prepared by high energy ball milling of elemental powders. For the equiatomic HEA with chemical composition at the boundary of solid solution and amorphous phase regions, an amorphous phase was obtained at moderate milling times, which was mechano-crystallized into a BCC solid solution by continued milling. For the non-equiatomic HEA with chemical composition well within the solid solution region, the same BCC solid solution was obtained during mechanical alloying. The crystallization kinetics of the amorphous HEA was studied by differential scanning calorimetry (DSC) thermal analysis. The crystallization activation energy (calculated based on the Kissinger equation) was correlated to the mean self-diffusion activation energy of the constituting elements, which was related to the multi-component nature of HEAs.