Spontaneous Amorphization Research Articles

The as-deposited structure of co-sputtered Cu–Ta alloys, studied by X-ray diffraction and molecular dynamics simulations

In this study a direct comparison between lattice spacings predicted by molecular dynamics (MD) simulations and values measured from sputtered Cu–Ta films by X-ray diffraction (XRD) is reported. The study spans the entire composition range between pure Cu and pure Ta and takes into account all the phases documented for the Cu–Ta system in the literature (i.e. α-Cu, α-Ta, β-Ta as well as X-ray amorphous structures). For both the experiments and the MD simulations the results are compared to the literature and discrepancies are critically discussed. A direct comparison between simulation and experiments shows that the MD simulations reproduce the interatomic distances observed in the experiments accurately over most of the composition range, with the exception of alloys with 15–30at.% Ta content where the MD simulations show spontaneous amorphization whereas the experiments suggest that Ta-rich particles are formed in face-centered cubic solid-solution phase.

Amorphization of oxides under irradiation with fast neutrons

A variant of the solid-state radiation amorphization as a result of accumulation of the critical concentration of defects in the crystal has been considered using the example of oxides with the garnet and perovskite structures irradiated by fast neutrons. It has been shown that such defects can be antisite defects, the formation of which leads to considerable static displacements from the equilibrium sites of nearest ions and, consequently, to the loss of stability of the crystalline structure. The dependences of the root-mean-square displacements of oxygen ions on the concentration of the antisite defects are constructed based on the analysis of the experimental data. It has been established that the so-called critical concentrations of antisite defects, at which the spontaneous amorphization occurs, differ for oxides with the garnet and perovskite structures. As the criterion of the spontaneous radiation amorphization, it is proposed to consider the critical static displacement of the ions, which is identical for studied oxides and equal to ∼0.28 A, or ∼0.14 in fractions of interatomic distances, which is close to the well-known Lindemann melting criterion.

Mechanism of Solid State Amorphization of Glucose upon Milling

Crystalline α-glucose is known to amorphize upon milling at -15 °C while it remains structurally invariant upon milling at room temperature. We have taken advantage of this behavior to compare the microstructural evolutions of the material in both conditions in order to identify the essential microstructural features which drive the amorphization process upon milling. The investigations have been performed by differential scanning calorimetry and by powder X-ray diffraction. The results indicate that two different amorphization mechanisms occur upon milling: an amorphization at the surface of crystallites due to the mechanical shocks and a spontaneous amorphization of the crystallites as they reach a critical size, which is close to 200 Å in the particular case of α-glucose.

Study of the solid-state amorphization of (GaSb)1 − x Ge x semiconductors by real-time neutron diffraction and electron microscopy

The spontaneous amorphization of high-pressure quenched phases of the GaSb-Ge system has been studied by neutron diffraction while slowly heating the phases at atmospheric pressure. The sequence of changes in the structural parameters of the initial crystalline phase and the final amorphous phase is established. The behavior of the phases and the correlation in the structural features of the phase transitions and anomalous thermal effects exhibit signs of the inhomogeneous model of solid-state amorphization.

Spontaneous Crystalline-to-Amorphous Phase Transformation of Organic or Medicinal Compounds in the Presence of Porous Media, Part 1: Thermodynamics of Spontaneous Amorphization

Spontaneous crystalline-to-amorphous phase transformation of organic or medicinal molecules in the presence of mesoporous materials has been observed, for which pathway was suggested to be via the vapor phase, that is, sublimation of the crystalline molecules followed by adsorption on the porous media. The objective of this paper is to rigorously evaluate this amorphization pathway and to study the thermodynamics of spontaneous amorphization. Mesoporous silicon dioxide (SiO(2)) was used as a model system. Physical mixtures of SiO(2) and crystalline compounds were prepared and stored at 0% relative humidity (RH) and 40 °C. Loss of crystallinity of the model compounds was confirmed using powder X-ray diffraction and polarized light microscopy. Adsorption chamber was set up, in which naphthalene and SiO(2) were stored, without physical contact, under reduced pressure at 0% RH and 40 °C. Data confirmed that the rate and extent of sublimation and adsorption of naphthalene were significant for amorphization to occur on a pharmaceutically relevant timescale. Furthermore, a thermodynamic model has been developed to explain spontaneous amorphization. This unique phase transformation phenomenon can be a simple and effective method to improve the aqueous solubility and bioavailability of poorly soluble drug molecules.

Highly efficient amorphization of drugs by the participation of molecular complex

Spontaneous amorphization of ibuprofen (IB) by the coexistence of its solvent vapor and polymeric matrix, hydroxypropyl methylcellulose (HPMC), were elucidated by combining X-ray diffractometry, mid- and near-IR spectroscopy, and thermal analyses, together with the stability test of the non-crystalline IB by prolonged exposure to water vapor. The non-crystalline state of IB was achieved by the recombination of hydrogen bonds from intracrystalline ones to intermolecular ones between IB and HPMC. Coexistence of the solvent molecular species, either methanol or ethanol, significantly increased the stability of the non-crystalline states of IB. Since IB globules of 20 – 50 nm were observed by TEM, IB in a non-crystalline state cannot be regarded as a perfect molecular dispersion. We therefore attributed the apparent stability increase to two aspects, i.e., (1) molecular interaction between IB and HPMC by abridging ethanol or methanol molecules via both hydrogen bonds and hydrophobic effects, and (2) retardation of nucleation and growth by suppression of IB molecular mobility by IB-HPMC network structure.

Rapid amorphization of molecular crystals by absorption of solvent molecules in the presence of hydrophilic matrices

Two organic molecular crystalline species, ibuprophen (IB) and indomethacine (IM) were subjected to methanol absorption in the presence of hydrophilic organic matrix, hydroxypropyl methylcellulose (HPMC). While spraying of 8–10% methanol or water on the drug–matrix mixture decreased the subsequent milling time for amorphization, absorption of methanol in a closed container caused spontaneous amorphization of IB was observed to give a nanocomposites with macroscopic agglomerates up to 250 μm after methanol absorption for overnight. Gentle mechanical homogenization under saturated methanol vapor with a newly developed apparatus, a tandem rotation mill (TRM), brought about homogeneous grains of IB-HPMC nanocomposites with the average particle size, 30 μm. We observed amorphous particles of IB in 60 nm regime dispersed in HPMC matrix under a transmission electron microscope (TEM). In the case of IM, mechanical homogenization with TRM was indispensable to obtain similar nanocomposites with HPMC.

Theory of spontaneous amorphization of metastable crystalline phases

Some metastable crystalline alloys show spontaneous amorphization and subsequent re-crystallization during transformation. This means that the amorphization is the result of competitive transformation kinetics in which both the metastable amorphous and stable crystalline phases are formed in parallel. However, the amorphous phase formation rate is much larger than that of the crystalline phase, so that in the first stage of transformation the resulting phase is amorphous. In the paper a theoretical description of the kinetics of these two processes is presented, taking into account the competitive formation of crystalline and amorphous phases on the boundaries of grains of the initial phase. The equations of nucleation and growth of the competitive phases are solved. The general approach is applied to the amorphization transformation kinetics in Cd–Sb alloys. The numerical analysis of equations of kinetics and the comparison with experimental results are carried out for Cd 43Sb 57.

Crystal stability limits at positive and negative pressures, and crystal-to-glass transitions.

The direct crystal-to-glass transformation, i.e., spontaneous amorphization, which was first observed by thermal annealing of stishovite ${\mathrm{SiO}}_{2}$ at ambient pressure, has now been observed as an isothermal phenomenon during both compression and decompression of initially stable crystals. While counterintuitive, and dependent on kinetically controlled metastable events, the phenomenon is of broad interest and potential importance in materials science and geophysics. In this paper we use a combination of molecular dynamics simulations and analyses of laboratory data to explore the metastable crystal ranges, including the negative pressure range, for key compounds such as the ices, silicas, and alkaline earth perovskites. Our focus is on the establishment of phenomenological patterns rather than on specific metastability-terminating mechanicsms. We find that a simple quadratic law, P-${\mathit{P}}_{\mathit{s}}$\ensuremath{\sim}(V-${\mathit{V}}_{\mathit{s}}$${)}^{2}$ (where ${\mathit{P}}_{\mathit{s}}$ and ${\mathit{V}}_{\mathit{s}}$ are the values of the pressure P and volume V on the spinodal), well approximates the equations of state over much of the metastable and even the stable range---and implies the existence of an isochoric boundary line for stability to isotropic density fluctuations. We delineate the conditions under which amorphization occurs, usually substantially before the stability limit is reached. (c) 1995 The American Physical Society

The T-P phase diagrams of amorphous GaSb, InSb and InAs

A thermodynamic two-level model is developed and applied for description of metastable phase equilibria in amorphous GaSb, InSb and InAs alloys. The metastable T-P phase diagrams are calculated for these alloys, which include the lines of the semiconductor-to-metal transitions as well as the boundaries of complete thermodynamic instability of the semiconducting or metallic phases. The calculated phase transitions are of first-order, and the transition lines terminate at critical points which are in the region of negative pressures. It is shown that a tendency to spontaneous amorphization of a metastable crystalline high-pressure phase produced by quenching under pressure, the amorphization parameters and a semiconductor-to-metal transition in the amorphous state are conditioned by the interrelation between the positions of the lines in the actual equilibrium (stable) and calculated metastable phase diagrams.

Role of Interface in Ion Mixing or Thermal Annealing Induced Amorphization in Multilayers in Some Immiscible Systems

AbstractSix binary metal systems were selected to study the possibility of forming amorphous alloys by ion mixing or thermal annealing in multilayered Films, i.e. the Ta-Cu, Zr-Nb, Zr-Ta, Y-Zr, Y-Mo and Y-Ta systems, featuring positive heats of formation (ΔHf) ranging from +3 to +40 kJ/mol. Firstly, the interfacial free energy consisting of a chemical and an elastic terms was calculated and added to the energetic state of the multilayers. It was found that the excess interfacial free energy increased with increasing the fraction of the interfacial atoms in the multilayers, and could raise the multilayers to an energy level intersecting with or being higher than thai of the amorphous phase possessing a typical convex shape. It is therefore possible to produce amorphous alloys in such systems, if the multilayered filins included enough fraction of the interfacial atoms. The multilayered samples were then designed and prepared accordingly and both ion mixing and thermal annealing under appropriate conditions resulted in the formation of a number of new amorphous alloys, confirming the above prediction based on the interfacial free energy concern. It is noted that the success of synthesising amorphous alloys by solid-state reaction in the immiscible systems develops a new glass forming technique, namely interface-generated spontaneous amorphization, which has a great potential to produce new and relatively thick amorphous films, e.g. a Ta72Cu28 amorphous film of 800 nm thick was obtained.

On the Roles of Temperature and Interfaces in Irradiation and Thermally Induced Solid State Amorphization

ABSTRACTThe roles of irradiation temperature and interfaces (free surfaces and grain boundaries) in irradiation- and thermally- induced amorphization of ceramics (coesite, apatite, olivines and spinels) have been studied by transmission electron microscopy (TEM). The irradiations were performed with 1.5 MeV Kr+, 200 keV and 1 MeV electrons over a wide temperature range (20-700 K). The critical amorphization dose at which amorphization is complete, Dc, increased with increasing irradiation temperature for most materials except coesite (a high pressure polymorph of Si02) which showed a decreasing Dc with increasing temperature under 1 MeV electron irradiation. Although amorphization may occur directly within a displacement cascade or by cascade overlap, this study shows that free surfaces and grain boundaries are favorable sites for nucleation of amorphous volumes. Once the amorphous volume is formed at interfaces, it may grow rapidly under continued irradiation. Coesite which has a glass transition temperature higher than its melting temperature underwent spontaneous amorphization during thermal annealing at 1200 K. This thermally-induced amorphization also started at free surface and grain boundaries and propagated into the interior of the crystal. The interface-mediated amorphization is analogous to the process of thermodynamic melting.

Short range order in amorphous Zn 41Sb 59 obtained by thermopressure processing

The short range order of amorphous Zn 41Sb 59 obtained by spontaneous amorphization of the metastable metallic high-pressure phase has been investigated. The structure factor was determined by time-of-flight neutron diffraction on the pulsed neutron source of LINAC ‘Fakel’ in the range of momentum transfer up to 17 Å −1 and the radial distribution function was calculated. It is shown that the structure of the amorphous phase is similar to that of the crystalline semiconductor phase ZnSb.

Spontaneous Amorphization of High-Pressure Phases in Binary Alloys of B-Elements

Common factors are analyzed for a special case of alloy amorphisation in binary systems of B-elements (s, p-elements of IIB-VB subgroups of the periodic table) after high pressure action. The alloys having a semiconductor phase in the initial state change under pressure to a new crystal state-a high-pressure metal phase, which is retained by quenching down to low temperature at atmospheric pressure. With increasing temperature the quenched phase loses stability and the alloy becomes amorphous. There exists a range of alloy compositions favourable for amorphization, corresponding to a electron concentration of 3.7 to 4.0 electrons per atom. [Russian Text Ignored].

Thermobaric quenching—A new method for producing metastable states

An alternative method of obtaining amorphous state by spontaneous amorphization of high-pressure metastable phases (HPP) after thermobaric quenching is discussed. Amorphous states in the ZnSb, CdSb, ZnCdSb, GaSb and AlGe systems have been obtained. All the initial crystalline HPPs are superconducting metal phases, and the resulting amorphous phases are semiconductors. The amorphization proceeds at temperatures −70 to +70 °C, doess not violate the compactness of the samples, is accompanied by a considerable volume increase and by large exothermal heat evolution.

Structure and properties of silicon dioxide thermal films I. SiO2 films of up to 50 nm thickness

The growth kinetics and the structure of oxides, as well as the macrostresses and density of the fixed positive charge are studied in SiO2 films up to 50 nm thick, obtained by thermal oxidation in dry oxygen at T = 1373 K. At a certain stage of oxidation, growth kinetics anomalies and changes in macrostresses and the density of the fixed positive charge due to structural transformations in the oxide (spontaneous amorphization effect) are observed. The obtained results are explained on the basis of the physical double-zone model of the silicon heterogeneous oxidation reaction. [Russian Text Ignored].