Range Of Metastability Research Articles

Deactivation of Pressure-Induced Amorphization in Silicalite SiO2 by Insertion of Guest Species

The incorporation of carbon dioxide or argon stabilizes the structure of the microporous silica polymorph silicalite well beyond the stability range of tetrahedrally coordinated SiO(2) and, in fact, beyond even the metastability range of low-pressure silica polymorphs such as quartz and cristobalite at room temperature. The bulk modulus of silicalite strongly increases as a result of the incorporation of CO(2) or Ar and is equivalent to that of quartz. The insertion of these species deactivates the normal compression and pressure-induced amorphization mechanisms in this material, impeding the softening of low-energy vibrations, amorphization, and the eventual increase in silicon coordination up to at least 25 GPa.

Quasi-isothermal temperature-modulated differential scanning calorimetry (TMDSC) for the separation of reversible and irreversible thermodynamic changes in glass transition and melting ranges of flexible macromolecules

Abstract With standard differential scanning calorimetry (DSC), it is possible to derive calorimetric data for equilibrium or metastable samples. The introduction of temperature-modulated DSC (TMDSC) permits in its quasi-isothermal (non-scanning) mode (TMDC), long-time apparent heat capacity measurements of high precision (±1 %). For flexible molecules, heat capacity measurements from the various calorimetric methods could be combined in the ATHAS Data Bank, which now contains experimental data for over 200 materials. These data were linked to the vibrational and large-amplitude motion of the constituent atoms and molecules, to provide a base for the judgement of the thermal analyses, extending outside the range of equilibrium or metastability with an error of only 2-5 %. The TMDC together with DSC is now able to quantitatively assess the reversibility of thermal processes. A sufficient number of systems have been analyzed in this fashion to develop better understanding of macro-, micro-, and nanophases of flexible macromolecules. The new concepts discussed are: (1) multiple glass transitions due to possible rigid-amorphous fractions (RAFs) and glass transitions within crystals, both observed in semicrystalline macromolecules, and (2) locally reversibly melting on the surface of chain-folded crystals. The locally reversible melting decreases with crystal perfection and also disappears when the chains become rigid.

New Lennard-Jones metastable phase

Despite the extensive use of the Lennard-Jones potential in atomistic simulations, we present a metastable phase not reported so far for the Lennard-Jones solid. This phase having space group I4¯3d arises from distortion of the body-centered cubic network. In contrast to the body-centered cubic phase, the I4¯3d phase possesses a noticeable range of metastability.

Energy release associated with a first-order phase transition in a rotating neutron star core

We calculate energy release associated with a first order phase transition at the center of a rotating neutron star. The results are based on precise numerical 2-D calculations, in which both the polytropic equations of state (EOS) as well as realistic EOS of the normal phase are used. Presented results are obtained for a broad range of metastability of initial configuration and size of the new superdense phase core in the final configuration. For small radii of the superdense phase core analytical expressions for the energy release are obtained. For a fixed overpressure dP (the relative excess of central pressure of collapsing metastable star over the pressure of equilibrium first-order phase transition) the energy release remarkably does not depend on the stellar angular momentum and coincides with that for nonrotating stars with the same dP. The energy release is proportional to dP^2.5 for small dPs, when sufficiently precise brute force 2-D numerical calculations are out of question. For higher dPs, results of 1-D calculations of energy release for non-rotating stars are shown to reproduce, with very high precision, the exact 2-D results for rotating stars.

Elucidating Quantitative Stability/Flexibility Relationships Within Thioredoxin and its Fragments Using a Distance Constraint Model

Numerous quantitative stability/flexibility relationships, within Escherichia coli thioredoxin (Trx) and its fragments are determined using a minimal distance constraint model (DCM). A one-dimensional free energy landscape as a function of global flexibility reveals Trx to fold in a low-barrier two-state process, with a voluminous transition state. Near the folding transition temperature, the native free energy basin is markedly skewed to allow partial unfolded forms. Under native conditions the skewed shape is lost, and the protein forms a compact structure with some flexibility. Predictions on ten Trx fragments are generally consistent with experimental observations that they are disordered, and that complementary fragments reconstitute. A hierarchical unfolding pathway is uncovered using an exhaustive computational procedure of breaking interfacial cross-linking hydrogen bonds that span over a series of fragment dissociations. The unfolding pathway leads to a stable core structure (residues 22-90), predicted to act as a kinetic trap. Direct connection between degree of rigidity within molecular structure and non-additivity of free energy is demonstrated using a thermodynamic cycle involving fragments and their hierarchical unfolding pathway. Additionally, the model provides insight about molecular cooperativity within Trx in its native state, and about intermediate states populating the folding/unfolding pathways. Native state cooperativity correlation plots highlight several flexibly correlated regions, giving insight into the catalytic mechanism that facilitates access to the active site disulfide bond. Residual native cooperativity correlations are present in the core substructure, suggesting that Trx can function when it is partly unfolded. This natively disordered kinetic trap, interpreted as a molten globule, has a wide temperature range of metastability, and it is identified as the "slow intermediate state" observed in kinetic experiments. These computational results are found to be in overall agreement with a large array of experimental data.

Solid Nitrogen at Extreme Conditions of High Pressure and Temperature

AbstractFor Abstract see ChemInform Abstract in Full Text.

Aqueous nonelectrolyte solutions — Part XX: Formula of structure I methane hydrate, congruent dissociation melting point, and formula of the metastable hydrate

Sixteen new measurements of high precision for structure I methane hydrate with water between 31.93 and 47.39 °C are shown to be metastable and exhibit higher methane pressures than found by earlier workers. Comparison of earlier measurements between 26.7 and 47.2 °C permit positive identification of the structure II and the structure I hydrates. Forty-nine equilibrium constants Kp(h1[Formula: see text]l1g) for dissociation of structure I methane hydrate into water and methane, 32 between –0.29 and 26.7 °C for the stable hydrate and 17 between 31.93 and 47.39 °C for the metastable hydrate, are best represented by a three-parameter thermodynamic equation, which indicates a standard error (SE) of 0.63% on a single Kp(h1[Formula: see text]l1g) determination. The congruent dissociation melting point C(h1l1gxm) of metastable structure I methane hydrate is at 47.41 °C with SE 0.02 °C and at pressure 505 MPa. The congruent equilibrium constant Kp(h1[Formula: see text]l1g) is 102.3 MPa with SE 0.2 MPa. ΔH°t(h1[Formula: see text]l1g) is 62 281 J mol–1 with SE 184 J mol–1, and the congruent formula is CH4·5.750H2O with SE 0.059H2O. At the congruent point, ΔV(h1[Formula: see text]l1g) is zero within experimental precision, and its estimate is 1.3 with SE 1.6 cm3 mol–1. The stability range of structure I methane hydrate with water extends from quadruple point Q(s1h1l1g) at –0.29 °C up to quadruple point Q(h1h2l1g) at 26.7 °C, and its metastability range with water extends from 26.7 °C up to the congruent dissociation melting point C(h1l1gxm) at 47.41 °C. Key words: methane hydrate, clathrate structure I, metastability range, dissociation equilibrium constant, formula, congruent melting point, metastability of structure I hydrate.

On the penetration depth of a strong field into superconductors

The field dependence of the magnetic penetration depth over the entire range of stability and metastability of the Meissner state was determined within the framework of the Ginzburg-Landau theory. A simple interpolation formula is suggested.

Aqueous nonelectrolyte solutions. Part XIX. Congruent dissociation melting point and the formula of structure II methane hydrate

Twenty-four equilibrium pressures, P(h2l1g), of structure II methane hydrate h2 with water l1 between 27.0 and 46.9°C are well represented by a four-parameter equation, which indicates a standard error (SE) of 1.95% on a single pressure measurement. Forty equilibrium constants Kp(h2[Formula: see text]l1g) for dissociation of structure II methane hydrate into water and methane between 27.0 and 47.7°C and at pressures up to 784 MPa at 45.0°C are best represented by a three-parameter thermodynamic equation, which indicates an SE 1.25% on a single Kp(h2[Formula: see text]l1g) determination. The congruent dissociation melting point C(h2l1gxm) of structure II methane hydrate is at 47.71°C with SE 0.03°C and at pressure 533 MPa with SE 5 MPa. The congruent Kp(h2[Formula: see text]l1g) is 102.9 with SE 0.3 MPa, ΔH°t(h2[Formula: see text]l1g) is 61 531 with SE 244 J mol–1, and the congruent formula is CH4·5.670H2O with SE 0.061H2O. At congruent point ΔV(h2[Formula: see text]l1g) = 0 and its estimate is 1.0 with SE 1.6 cm3 mol–1. Stability range of structure II methane hydrate with water extends from quadruple point Q(h1h2l1g) at 26.7°C and 55.5 MPa up to quadruple point Q(h2h3l1g) at 47.3°C and 620 MPa. The metastability range of structure I methane hydrate with water is discussed.Key words: methane hydrate, clathrate structure II, stability range, dissociation equilibrium constant, formula, congruent melting point, metastability of structure I hydrate.

Modeling anisotropic chemical wave patterns in the NO+H2 reaction on a Rh(110) surface

A realistic mathematical model describing the formation of anisotropic chemical wave patterns in the NO+H2 reaction on a Rh(110) surface is presented. For the point model describing the local reaction kinetics a bifurcation analysis has been conducted. In order to take into account the state-dependent anisotropy of surface diffusion site-blocking effects through coadsorbates for the diffusing species were introduced. The spatially distributed model reproduces well the experimentally determined excitability and bistability range and the existence ranges for the different types of chemical wave patterns: elliptical and rectangular target patterns, travelling wave fragments, and varying front geometries in the range of double metastability were modeled. In addition, the dependencies of the pulse/front velocity on the hydrogen partial pressure and temperature were simulated.

Modelling collective opinion formation by means of active Brownian particles

The concept of active Brownian particles is used to model a collective opinion formation process. It is assumed that individuals in community create a two-component communication field that influences the change of opinions of other persons and/or can induce their migration. The communication field is described by a reaction-diffusion equation, the opinion change of the individuals is given by a master equation, while the migration is described by a set of Langevin equations, coupled by the communication field. In the mean-field limit holding for fast communication we derive a critical population size, above which the community separates into a majority and a minority with opposite opinions. The existence of external support (e.g. from mass media) changes the ratio between minority and majority, until above a critical external support the supported subpopulation exists always as a majority. Spatial effects lead to two critical ``social'' temperatures, between which the community exists in a metastable state, thus fluctuations below a certain critical wave number may result in a spatial opinion separation. The range of metastability is particularly determined by a parameter characterizing the individual response to the communication field. In our discussion, we draw analogies to phase transitions in physical systems.

Metastability range of the Bose condensate in $\mathsf{^7}$ Li-fermion mixtures at T = 0

We evaluate the ground state of a mixture of bosons and spin-polarized fermions in the case of attractive boson-boson interactions, using a variational Ansatz for the Bose condensate wave function and the Thomas-Fermi approximation for the fermions in the mean field of the condensate. Within this approximation we show that the presence of the fermions tends to restrict the metastability range of the condensate, irrespectively of the sign of the boson-fermion interactions. Numerical illustrations are reported for mixtures of 7 Li atoms with fermions having the 6 Li mass.

New statistical mechanical treatment of systems near surfaces. IV. Surface and surface-induced capillary transitions in polymer solutions

We apply a recently developed analytic but approximate method to study surface and surface-induced capillary transitions in a solution of polydisperse linear and/or branched polymers confined between two infinite but identical surfaces. We use an equilibrium polymerization model where various densities in the system are controlled by the corresponding activities. The bulk region, i.e., the central region between the surfaces may or may not be in the bulk equilibrium state (see the text). We find a line of first-order transitions, commonly known as a prewetting transition line, passing through the phase separation point in the bulk equilibrium state, at which the surface undergoes a first-order transition. For a certain range around the bulk equilibrium transition point, the bulk region undergoes a stable-metastable transition. This presents us with the possibility of being able to prepare the system in a bulk metastable state, no matter how far apart the two surfaces are. This range is found to be identical to the range of metastability for the bulk equilibrium state. Outside of this range, the transition line behaves like a prewetting transition line and, as usual, terminates at surface critical points, one on each side of the above range.

Range of metastability of tetragonal zirconia in some rare earth doped zirconia fibers

Studies on the thermal treatment of amorphous zirconia (gel, precipitate, etc.) have shown that its conversion to crystalline forms usually starts around 400±500 8C [1±4], and that the precise temperature of conversion may vary with the composition (e.g. the type and amount of cationic dopants, hydroxyl content). It has been reported by various workers [2± 4] that the product of this initial crystallization is a metastable tetragonal (t) phase, not the thermodynamically stable monoclinic (m) phase. Further, it has been shown that this metastable t-phase persists, even at higher temperatures within the stability range of the m-phase before ®nally converting to the latter. Thus, for an undoped zirconia gel, crystallization mainly to the t-phase was observed at 400 8C, and t! m conversion at 1000 8C [4]. Generally speaking, this follows Ostwald's step rule [5]. In addition, the apparent stabilization of t-ZrO2 at low temperatures has been explained to be a function of structural similarities of an amorphous phase with the t-phase [3], crystallite size [6], effect of strain [5] etc., apart from the presence of ionic impurities (as mentioned previously). Naskar and Ganguli [7] recently studied the crystallization behavior of rare earth (RE) doped zirconia gel ®bers that were obtained by spinning doped zirconium acetate sols. They showed (Fig. 1) that the metastable t-phase obtained from ZrO2RE2O3 gel ®bers yields partially or fully to the mphase at higher temperatures as a function of the size of the cationic dopant (2 mol % rare earth oxide in each case). Fig. 1 indicates that although all the dopant cations yielding t-ZrO2 were larger in size than Zr4‡ (La . Pr . Nd . Sm . Gd . Dy . Zr), the larger cations among them yielded the phase with progressively shorter thermal ` stability'' ranges. On

The role of substrate quality on misfit dislocation formation in pseudomorphic high electron mobility transistor structures

We examined the role of substrate quality on the epitaxial microstructure and performance of pseudomorphic InGaAs/AlGaAs/GaAs high electron mobility transistors (HEMTs). High resolution x-ray diffraction, high resolution x-ray topography, and transmission electron microscopy indicate that, for a given channel layer thickness, the misfit dislocation density is always lower for HEMT structures grown on substrates having lower threading dislocation densities. Furthermore, the onset of misfit dislocation formation occurs at higher channel layer thicknesses for HEMTs grown on substrates having a lower threading dislocation density when compared to those grown on substrates with a higher threading dislocation density. However, the ratio of the density of misfit dislocations to threading dislocations is greater than one on the low dislocation density substrates, which demonstrates that other misfit dislocation nucleation sources (i.e., surface particulates) are significant when there is an insufficient density of threading dislocations. The Hall conductivity measurements show that the performance of HEMT structures improves with higher substrate quality, showing that the range of epitaxial layer metastability increases with improved substrate quality.

Gold-bearing arsenian pyrite and marcasite and arsenopyrite from Carlin Trend gold deposits and laboratory synthesis

Invisible gold in natural and synthetic arsenian pyrite and marcasite correlates with anomalous As content and Fe deficiency, and high contents of invisible gold in most natural and all synthetic arsenopyrite correlate with excess As and Fe deficiency. As-rich, Fe-deficient arsenopyrite synthesized hydrothermally contains up to 3.0 wt% Au uniformly distributed in growth zones of light backscattered electron contrast. At the Deep Star gold deposit, Carlin Trend, Nevada, the sulfide compositions apparently span the full range of metastability from FeSz to near FeAsS (40 at% S); arsenian pyrite contains up to 0.37 wt% Au, but arsenopyrite has excess S and is relatively Au poor. Observed minimum Fe contents are 29.1 at% in arsenian pyrite and marcasite from the Deep Star deposit and 31.3 at% in synthetic arsenopyrite. We suggest that invisible gold in arsenian pyrite and marcasite and arsenopyrite from sediment-hosted gold deposits represents Au removed from ore fluids by chemisorption at As-rich, Fe-deficient surface sites and incorporated into the solids in metastable solid solution. However, the oxidation state of invisible gold (Aua, AU1+) remains uncertain because the chemisorption process is intrinsically nonsystematic in terms of crystal-chemical parameters and does not result in definitive atomic substitution trends.

Observation of hysteresis at the spin-flop transition induced by a pulsed magnetic field

We present the results of diffraction research on the spin-flop transition in α-Fe 2O 3 and Cr 2O 3 performed on the SNIM-2 spectrometer of the IBR-2 pulsed reactor using a pulsed magnetic field. The field pulses had a sinusoidal form with a time duration of 400–1000 μs. The hysteresis dependences of the scattering intensity on the field value were obtained. In the case of hematite, with a change in amplitude of the magnetic pulses, the hysteresis loop takes its utmost shape and width, which do not change with further increases in amplitude. The width of the utmost loop is almost one order less than the theoretical range of metastability of the phase states. With decreasing temperature the width of the utmost loop decreases. In the case of Cr 2O 3, hysteresis is observed at magnetic field amplitudes which do not reach the critical value for phase transition. This testifies to a delay of the turning process of sublattice magnetization in the initial phase state.

A theoretical analysis of the chilling susceptibility of hypoeutectic FeC alloys

In this work an analytical solution of general validity is proposed to explain the chill tendency of cast iron of different compositions. A balance of the various heat contributions during the solidification process is employed in arriving at an expression for the chilling equivalent E. It is found that E can be directly related to the modulus of the casting and it includes the effects of (a) nucleation and growth of pro-eutectic as well as eutectic phases, (b) the range of metastability, and (c) the pouring temperatures. Moreover, the predictions of the present analysis are in good agreement with the experimental data of chill penetrations found in wedge shaped castings. Quantitatively, this analysis predicts that small sulphur additions impede the growth of graphite eutectic cells while raising the degree of undercooling in cast iron.

Bogolyubov-Tyablikov-like approximation for nematic liquid crystals: Bridge between continuum and mean-field theories.

A procedure analogous to the Bogolyubov-Tyablikov approximation is proposed in order to bridge between the continuum theory and the Maier-Saupe model of nematic liquid crystals. The equation obtained for the order parameter, which results in a first-order transition, reduces to the Maier-Saupe equation in the limit of infinite-range interactions. A prescription for the evaluation of the free energy in the framework of the Bogolyubov-Tyablikov treatment is presented, and the resulting Maxwell construction is used to determine the first-order transition temperature. The transition temperature, the range of metastability, and the value of the order parameter at the transition are all considerably lower than in Maier-Saupe theory.

Kinetic aspects of the formation of the ordered phase in stiff‐chain helical polyamino acids

AbstractThe helical polyamino acids exhibit a two‐component temperature‐composition phase diagram that corresponds well in overall appearance to the Flory prediction for rigid, impenetrable rods. The phase diagram may be characterized as a narrow, typically 1–2%, biphasic region separating the disordered (isotropic) state from the ordered (liquid–crystalline) state, connected to a wide biphasic region in which nearly pure polymer is in equilibrium with nearly pure solvent. Initial studies have been undertaken to deduce the kinetics of formation of the ordered phase, both when the thermodynamically stable state is the ordered state, as well as when it is in an ordered‐disordered biphasic state. The time course of the formation of the ordered phase at different temperatures is reminiscent of analogous studies on polymer crystallization, describable by a nucleation and growth mechanism. Formation of the ordered phase when the final state is the narow biphasic state appears also to follow a nucleation‐growth mechanism. However, when the final state is the wide biphasic region, the morphology of the final state is that of an optically clear, self‐supporting gel, even at very low concentrations (as low as 0.03 wt %). The reasons for and nature of the gel state are believed to be a kinetic phenomenon. The shape of the phase diagram is such that the temperature range of thermodynamic metastability is small and phase separation occurs in the thermodynamically unstable region. In the unstable region the zero activation‐energy spinodal decomposition mechanism dominates the kinetics of phase separation, leading to bicontinuous interconnected phases that have little possibility or driving force to rearrange further. The polymer network structure is proposed to be a bulk thermodynamic phase, characterized by bundles or sheets of polymer between branch points, rather than individual chains as in the usual polymer network. Rheological, electron microscope, and light‐scattering data are presented that support this view. The possibility of other stiff‐chain polymers forming a network structure for similar reasons is discussed.