Metastable Amorphous Phase Research Articles

Solid-State Forms of Zoledronic Acid: Polymorphism in Hydrates

Solid-state forms of zoledronic acid, a nitrogen-containing bisphosphonate used for treatment of bone diseases are studied using different experimental techniques (DSC, TG, and XRD). Two degrees of hydration have been obtained, containing one and three water molecules per zoledronic acid molecule. The crystal structure of the trihydrated form is reported. Two different anhydrated forms have been obtained when the hydrated ones were heated. Besides, during the dehydration process, an amount of metastable amorphous phase appears, as a function of the dehydration rate. The stability of the obtained crystalline forms is examined under high humidity and a different trihydrated form was obtained, setting clear that the same degree of hydration (trihydrated) can be obtained in two different crystalline forms, and then very different thermal behaviors have been observed.

Analysis of Lithium Insertion/Deinsertion in a Silicon Electrode Particle at Room Temperature

The dependence of the open-circuit potential on the state of charge in lithium insertion electrodes is usually measured at equilibrium conditions. For the modeling of lithium–silicon electrodes at room temperature, the use of a pseudo-thermodynamic potential vs composition curve based on metastable amorphous phase transitions with path dependence is proposed. Volume changes during lithium insertion/deinsertion in a single silicon electrode particle under potentiodynamic control are modeled and compared with experiments to provide justification for the same. Only if asymmetric transfer coefficients and sluggish kinetics are experimentally observed can kinetic hysteresis be reasoned for the potential gap in Li–Si system. The particle model enables one to analyze the influence of diffusion in the solid phase, particle size, and kinetic parameters without interference from other components in a practical porous electrode. Concentration profiles within the electrode particle under galvanostatic control are investigated. Sluggish kinetics is established from cyclic voltammograms at different scan rates. This work stresses the need for accurate experimental determination of kinetic parameters (and thus the exchange current density) in silicon nanoparticles. This model and knowledge thereof can be used in the cell-sandwich model for the design of lithium-ion cells with composite silicon negative electrodes.

Amorphous Precursors in the Nucleation of Clathrate Hydrates

The nucleation and growth of clathrate hydrates of a hydrophobic guest comparable to methane or carbon dioxide are studied by molecular dynamics simulations of two-phase systems. The crystallization proceeds in two steps: First, the guest molecules concentrate in "blobs", amorphous clusters involving multiple guest molecules in water-mediated configurations. These blobs are in dynamic equilibrium with the dilute solution and give birth to the clathrate cages that eventually transform it into an amorphous clathrate nucleus. In a second step, the amorphous clathrate transforms into crystalline clathrate. At low temperatures, the system can be arrested in the metastable amorphous clathrate phase for times sufficiently long for it to appear as an intermediate in the crystallization of clathrates. The "blob mechanism" unveiled in this work synthesizes elements of the labile cluster and local structuring hypotheses of clathrate nucleation and bears strong analogies to the two-step mechanisms of crystallization of proteins and colloids.

Functional biomimetic analogs help remineralize apatite-depleted demineralized resin-infiltrated dentin via a bottom–up approach

Natural biominerals are formed through metastable amorphous precursor phases via a bottom–up, nanoparticle-mediated mineralization mechanism. Using an acid-etched human dentin model to create a layer of completely demineralized collagen matrix, a bio-inspired mineralization scheme has been developed based on the use of dual biomimetic analogs. These analogs help to sequester fluidic amorphous calcium phosphate nanoprecursors and function as templates for guiding homogeneous apatite nucleation within the collagen fibrils. By adopting this scheme for remineralizing adhesive resin-bonded, completely demineralized dentin, we have been able to redeposit intrafibrillar and extrafibrillar apatites in completely demineralized collagen matrices that are imperfectly infiltrated by resins. This study utilizes a spectrum of completely and partially demineralized dentin collagen matrices to further validate the necessity for using a biomimetic analog-containing medium for remineralizing resin-infiltrated partially demineralized collagen matrices in which remnant seed crystallites are present. In control specimens in which biomimetic analogs are absent from the remineralization medium, remineralization could only be seen in partially demineralized collagen matrices, probably by epitaxial growth via a top–down crystallization approach. Conversely, in the presence of biomimetic analogs in the remineralization medium, intrafibrillar remineralization of completely demineralized collagen matrices via a bottom–up crystallization mechanism can additionally be identified. The latter is characterized by the transition of intrafibrillar minerals from an inchoate state of continuously braided microfibrillar electron-dense amorphous strands to discrete nanocrystals, and ultimately into larger crystalline platelets within the collagen fibrils. Biomimetic remineralization via dual biomimetic analogs has the potential to be translated into a functional delivery system for salvaging failing resin–dentin bonds.

Rapid solidification of cryolite and cryolite–alumina melts

AbstractRapid solidification processing (with a cooling rate in the interval 10 5 -10 6 K s −1 ) was used to prepare deeply undercooled cryolite-alumina melts. These samples were analyzed by XRD, infrared, and Raman spectroscopy. Besides cryolite, the amorphous phase and a low amount of ι-Al 2 O 3 were detected. Annealing of the quenched sample revealed the transformation of metastable amorphous phases into different products depending on the annealing conditions. The results obtained showed that all of the elements (Na, Al, O, and F) are probably present in the amorphous parts of the quenched samples.Graphical Abstract[IMAGE]

Atomistic origins of the phase transition mechanism in Ge2Sb2Te5

The fast and reversible phase transition mechanism between crystalline and amorphous phases of Ge2Sb2Te5 has been in debate for several years. Through employing first-principles density functional theory calculations, we identify a direct structural link between the metastable crystalline and amorphous phases. The phase transition is driven by the displacement of Ge atoms along the rocksalt [111] direction from stable octahedron to high energy unstable tetrahedron sites close to the intrinsic vacancy regions, which generates a high energy intermediate phase between metastable and amorphous phases. Due to the instability of Ge at the tetrahedron sites, the Ge atoms naturally shift away from those sites, giving rise to the formation of local-ordered fourfold motifs and the long-range structural disorder. Intrinsic vacancies, which originate from Sb2Te3, lower the energy barrier for Ge displacements, and hence, their distribution plays an important role in the phase transition. The high energy intermediate configuration can be obtained experimentally by applying an intense laser beam, which overcomes the thermodynamic barrier from the octahedron to tetrahedron sites. The high figure of merit of Ge2Sb2Te5 is achieved from the optimal combination of intrinsic vacancies provided by Sb2Te3 and the instability of the tetrahedron sites provided by GeTe.

Atomic-Scale Structural Evolution upon Crystallite Nucleation and Growth in Amorphous Fe78B13Si9

The structural evolution upon nucleation and growth of nanocrystals was investigated for Fe78B13Si9 alloy. Upon the onset of α-Fe nanocrystallization, free volumes dominantly surrounded by Fe atoms are developed in the intergranular amorphous phase. Along with the growth of Fe-based nanocrystallites, free volumes in the intergranular amorphous phase enlarge and Fe atoms in the surroundings of the free volumes get partially replaced by B and Si. The results suggest that nanocrystallization is associated with a local structural relaxation in the metastable intergranular amorphous phase.

Devitrification of Fe-Based Amorphous Metal SAM 1651 and the Effect of Heat-Treatment on Corrosion Behavior

Devitrification of amorphous alloys is a process in which thermodynamically preferable crystalline phases form from the metastable amorphous phase. The corrosion behavior of the crystalline phases and the remaining amorphous material is likely to be different from that of the fully amorphous material and could have detrimental effects on the corrosion resistance of the devitrified material. The effect of heat-treatment on the corrosion behavior of an Fe-based bulk metallic glass [structural amorphous metal (SAM) 1651] was examined, which revealed a degradation in the corrosion resistance of SAM 1651 upon annealing at . However, the partially devitrified material still exhibited good corrosion resistance even in the highly aggressive environments of 6 M HCl. The preferential corrosion sites of nanoscale in both fully amorphous and partially devitrified materials were identified with transmission electron microscope analysis. The corrosion resistance of both fully amorphous and partially devitrified materials was explained in terms of chemical and structural characteristics of the alloys.

Structure, phase composition and magnetic properties of Fe–Si–C system due to mechanical activation of Fe–Si alloy in organic liquids

Fineness, structure, phase composition, and magnetic properties of the powders produced by mechanical milling of Fe 80Si 20 in liquid hydrocarbon environment (heptane, heptane with oleic acid additive) in planetary ball mill have been studied. Under milling the alloys are saturated with the products of organic liquid decomposition – C, O, H, and then metastable amorphous and carbide phases form. The particle size decreases down to 0.1–0.2 μm. Isochronous (775 K, 1 h) annealing leads to the formation of the Fe 8Si 2C iron silicon carbide, as well as Fe 3Si and C – for milling in heptane, and Fe 3C and SiO 2 – for milling with oleic acid additive. The magnetic properties of the powders depend both on the milling time and milling environment, and on the annealing temperature. An increase in the coercivity from 13 up to 226 Oe correlates with the amount of the synthesized Fe 8Si 2C. It is ferromagnetic ( T c = 788 K) and temperature-resistant up to 870 K.

Model for the Particle Size, Overpotential, and Strain Dependence of Phase Transition Pathways in Storage Electrodes: Application to Nanoscale Olivines

In the drive toward improved electrical energy storage for applications ranging from wireless devices to electric vehicles to grid stabilization, nanoscale materials are of growing interest as ion storage electrodes. Nanoscale olivines based on LiMPO4 (M = Fe, Mn, Co, Ni) are one class of compounds for which recent experimental developments reveal very different phase transition and solid-solubility behavior compared to larger particles. The olivines may be an exemplar for generalized behavior for which metastable crystalline or amorphous phases are produced under the large driving forces incurred during electrochemical reactions. Here we use a diffuse-interface thermodynamic model to assess the conditions under which amorphous phase transitions may occur in nanoscale LiMPO4 particles. There are three central conclusions. First, assuming as with similar solids that the amorphous phase has the lower surface energy, it is found that an initially crystalline phase may undergo amorphization during cycling when the particle size is below a critical value. Second, the effect of applied electrical overpotentials on the phase stability is evaluated for the first time, and is found to strongly influence the phase transition pathways of small particles. Third, the tendency to amorphize is significantly affected by the magnitude of the misfit strain between the lithiated and delithiated crystalline phases. It is shown that there exists a critical misfit strain above which the preferred transformation pathway is amorphization, regardless of the particle size. We use these results to interpret experimentally observed behavior of olivines, including data that up to now have been unexplained.

Phase transformation behaviors of SiO2 doped Ge2Sb2Te5 films for application in phase change random access memory

The improvement in the phase change characteristics of Ge2Sb2Te5 (GST) films for phase change random access memory applications was investigated by doping the GST films with SiO2 using cosputtering at room temperature. As the sputtering power of SiO2 increased from 0to150W, the activation energy for crystallization increased from 2.1±0.2to3.1±0.15eV. SiO2 inhibited the crystallization of the amorphous GST films, which improved the long term stability of the metastable amorphous phase. The melting point decreased with increasing concentration of SiO2, which reduced the power consumption as well as the reset current.

Matrix Macromolecules in Hard Tissues Control the Nucleation and Hierarchical Assembly of Hydroxyapatite

Biogenic minerals found in teeth and bones are synthesized by precise cell-mediated mechanisms. They have superior mechanical properties due to their complex architecture. Control over biomineral properties can be accomplished by regulation of particle size, shape, crystal orientation, and polymorphic structure. In many organisms, biogenic minerals are assembled using a transient amorphous mineral phase. Here we report that organic constituents of bones and teeth, namely type I collagen and dentin matrix protein 1 (DMP1), are effective crystal modulators. They control nucleation of calcium phosphate polymorphs and the assembly of hierarchically ordered crystalline composite material. Both full-length recombinant DMP1 and post-translationally modified native DMP1 were able to nucleate hydroxyapatite (HAP) in the presence of type I collagen. However, the N-terminal domain of DMP1 (amino acid residues 1-334) inhibited HAP formation and stabilized the amorphous phase that was formed. During the nucleation and growth process, the initially formed metastable amorphous calcium phosphate phase transformed into thermodynamically stable crystalline hydroxyapatite in a precisely controlled manner. The organic matrix-mediated controlled transformation of amorphous calcium phosphate into crystalline HAP was confirmed by x-ray diffraction, selected area electron diffraction pattern, Raman spectroscopy, and elemental analysis. The mechanical properties of the protein-mediated HAP crystals were also determined as they reflect the material structure. Such understanding of biomolecule controls on biomineralization promises new insights into the controlled synthesis of crystalline structures.

Characterization of TiCr(C,N)/amorphous carbon coatings synthesized by a cathodic arc deposition process

Ternary TiCrN and nanocomposite TiCr(C,N)/amorphous carbon (a-C) coatings with different carbon contents (0–26.6 at.%) were synthesized by cathodic arc evaporation with plasma enhanced duct equipment. The structural, chemical, and mechanical properties of the deposited films were studied by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and nanoindentation measurement. The atomic content ratios of carbon/(Ti + Cr) and carbon/nitrogen increased with increasing C 2H 2 flow rate. A nanocomposite structure of coexisting metastable hard TiCr(C,N) crystallites and amorphous carbon phases was found in the TiCr(C,N)/a-C coatings, those possessed smaller crystallite sizes than the ternary TiCrN film. XPS analyses revealed the concentration of a-C increased with increasing carbon content from 8.9 at.% to 26.6 at.%. Exceeding the metastable solubility range of carbon within the TiCrN lattice, the carbon formed a-C phase in the deposited coatings. The nanocomposite TiCr(C,N)/a-C coatings exhibited higher hardness value of 29–31 GPa than the deposited TiCrN coating (26 ± 1 GPa). It has been found that the structural and mechanical properties of the films were correlated with the carbon content in the TiCr(C,N)/a-C coatings.

A quantitative DSC analysis of the metastable phase behavior of the sucrose–water system

The phase behavior of the sucrose–water system is investigated using a quantitative calorimetric analysis. On the basis of a thermodynamic discussion of the melting behavior of the crystalline water component, the composition of the metastable amorphous phase and the so-called maximum freeze-concentrated composition were determined. This metastable phase can be described in a first approximation as an association complex consisting of one molecule of sucrose and five molecules of water which are held together by hydrogen bonding. The critical mass fraction of sucrose is thus 0.792. Furthermore, the origin of the different transitions in the aqueous phase is discussed. By analyzing the ice melting, an analytical relationship of the water activity coefficient is derived from the experimental data.

Structure and properties of ion-plasma deposited Ni-C films in a metastable state

A modified ion-plasma sputtering technique was used to produce metastable states, in particular, amorphous (from the standpoint of X-ray diffraction) and nanocrystaline phases in nickel-carbon film alloys in a wide composition range from 7 to 61.4 at. % C. Short-range order parameters of amorphous phases and lattice parameters of metastable phases were determined; the sequential formation and transformation of intermediate phases during the transition of the Ni-C alloys into the equilibrium state are studied. Electrical properties of the deposited alloys in different states were measured; the results obtained are explained.

Rapid compression induced solidification of two amorphous phases of poly(ethylene terephthalate)

Melts of poly(ethylene terephthalate) were solidified by rapid compression to 2 GPa within 20 ms and by a series of comparative processes including natural cooling, slow compressing and rapid cooling, respectively. By combining XRD and differential scanning calorimetry data of the recovered samples, it is made clear that rapid compression induces two kinds of amorphous phases. One is relatively stable and can also be formed in the slow compression and the cooling processes. Another is metastable and transforms to crystalline phase at 371 K. This metastable amorphous phase cannot be obtained by slow compression or natural cooling, and its crystallization temperature is remarkably different from that of the metastable amorphous phase formed in the rapid cooling sample.

Tunable Linear Thermal Expansion Coefficient of Amorphous Zirconium Tungstate

Zirconium tungstate (α‐ZrW2O8) exhibits isotropic negative thermal expansion over a wide range of temperatures. Under high pressure it transforms to a metastable amorphous phase with positive thermal expansion coefficient. α‐ZrW2O8 can be fully recovered by annealing the amorphous phase at temperatures slightly above 600°C, at room pressure. In this work, we describe the dependency of the linear thermal expansion coefficient of amorphous ZrW2O8 with the annealing temperature, showing that it can be continuously tuned, in two stages, from about 7.5° to −9°C−1. These two stages are related to the relaxation of the amorphous phase and the onset of recrystallization, respectively.

Dependence of magnetic properties on ferromagnetic layer thickness in trilayer Co/Ge/Co films with granular semiconducting spacer

We have investigated the magnetic properties of trilayer films of Co–Ge–Co. At a fixed thickness of germanium of 3.5 nm, the formation and distribution of metastable amorphous and cubic phases depends on the thickness of the ferromagnetic layer. The portion of the stable hexagonal phase is affected, too. Possible mechanisms for forming the observed magnetic structure are discussed.

Local atomic ordering and nanoscale phase separation in a Pd-Ni-P bulk metallic glass

The local structure in a ${\mathrm{Pd}}_{40}{\mathrm{Ni}}_{40}{\mathrm{P}}_{20}$ bulk metallic glass was examined using a spherical-aberration-corrected high resolution TEM. Fcc-Pd(Ni) type nanoclusters and local compound (phosphide)-like nanoclusters with sizes of $1--2\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ embedded in a dense-randomly-packed amorphous matrix were clearly observed under an appropriate imaging condition. However, three-dimensional atom-probe elemental mapping revealed there is virtually no nanoscale compositional difference between the nanoclusters and amorphous matrix beyond the statistical error range. A very small interfacial energy between the nanophase and the matrix is able to form a metastable amorphous phase with a structural fluctuation.

SiO[sub 2] Incorporation Effects in Ge[sub 2]Sb[sub 2]Te[sub 5] Films Prepared by Magnetron Sputtering for Phase Change Random Access Memory Devices

The phase change characteristics of Ge 2 Sb 2 Te 5 (GST) films for phase change random access memory devices were improved by incorporating SiO 2 into the GST film using cosputtering at room temperature. Isochronal annealing showed an increased resistivity of the crystallized GST films in proportion to the incorporated quantity of SiO 2 which leads to a decrease in the writing current. SiO 2 also inhibits crystallization of the amorphous GST film which can improve the long term stability of the metastable amorphous phase.