Metastable Melt Research Articles

Phase separation in Cu 46Zr 47− xAl 7Gd x metallic glasses

The influence of Gd addition on phase separation of rapidly quenched Cu 46Zr 47− x Al 7Gd x metallic glasses ( x = 2,5,7) was studied. For low Gd content ( x = 2), a homogeneous glass is obtained for the as-quenched state. Annealing leads to cluster formation by nucleation and growth prior to crystallization. For the Gd contents x = 5 and 7, early stages of spinodal decomposition are observed in the as-quenched glasses. Further annealing increases the amplitude of the compositional fluctuations prior to crystallization. Atom probe tomography gives evidence of the presence of Gd-enriched clusters of 2–5 nm size for the Cu 46Zr 42Al 7Gd 5 glass. The structure formation as a function of the Gd content is essentially determined by the composition dependence of the miscibility gap of the metastable undercooled melt. Early stages of spinodal decomposition or almost homogeneous glassy states are obtained if the critical temperature of liquid–liquid phase separation is close or below to the glass transition temperature.

Hambergite-rich melt inclusions in morganite crystals from the Muiane pegmatite, Mozambique and some remarks on the paragenesis of hambergite

Raman spectroscopic studies of daughter crystals of hambergite [Be2BO3(OH, F)] in primary melt and secondary fluid inclusions in morganite crystals from the Muiane pegmatite, Mozambique, show that the inclusions have extremely high beryllium concentrations, corresponding to as much as 10.6% (g/g) in melt inclusions and 1.25% (g/g) BeO in fluid inclusions. These melt and fluid inclusions were trapped at about 610°C and 277°C, respectively. We propose two possible mechanisms for the formation of the hambergite crystals: (i) direct crystallization from a boron- and beryllium-rich pegmatite-forming melt or (ii) these are daughter crystals produced by the retrograde reaction of the boron-rich inclusion fluid with the beryl host, after release of boric acid from the primary trapped metastable volatile-rich silicate melt during cooling and recrystallization. Although we favor the second option, either case demonstrate the extent to which Be maybe concentrated in a boron-rich fluid at relatively high temperatures, and in which species of Be maybe transported. One important constraint on the stability of the hambergite paragenesis is temperature; at temperatures of ≥650°C (at 2 kbar) hambergite is not stable and converts to bromellite [BeO].

On the decrease in the temperature of contact melting in metal systems with intermetallic compounds

A sufficient condition for the manifestation of the ΔT effect is proposed on the basis of an analysis of published data and the results obtained by the authors. An attempt is made to justify the non-mandatory delay of critical nuclei of intermetallic compounds for the purpose of explaining metastable contact melting. The low-temperature character of contact melting under such conditions is determined by the nanocontact melting of contacting micro-(nano) protrusions of pure component and intermetallic compound.

Phase formation in contact of dissimilar metals

Formation and growth of intermediate phases in contact of the crystalline samples forming a two-component eutectic system is considered. It is shown that during the competition to a growing liquid phase the intermediate solid phases cannot grow by diffusion. The alternative is formation of metastable areas of a liquid phase.Measurements of liquid layers extent in Pb-Bi and In-Bi systems have allowed to define the composition of liquid on interface where formation of metastable liquid is possible. The results show that the concentration interval of a liquid layer corresponds to a stable constitution diagram. In order to explain the experimental results the hypothesis according to which the intermediate solid phases are formed as a result of precipitation from metastable melt is considered.The experimental confirmation of formation and crystallization of a metastable liquid is the fact that intergrowth of the samples forming system with an intermetallic phase at temperatures below the temperature of fusion of the most low-melting eutectic is observed.The possibility of the processes concerned with the occurrence of metastable areas of a liquid is showed by means of computer imitation.

High Pressure Differential Scanning Calorimetry Investigations on the Pressure Dependence of the Melting of Paracetamol Polymorphs I and II

A high pressure differential scanning calorimeter (HP‐DSC) has been used to investigate the pressure dependence of the melting of the monoclinic (Form I) and orthorhombic (Form II) polymorphs of paracetamol (acetaminophen). DSC scans obtained at ambient pressure show that the stable monoclinic form melts at 442K while the metastable orthorhombic form melts at 430K. HP‐DSC scans obtained for pressures up to about 450MPa show that the melting temperatures of both Forms I and II increase with increasing pressure, but the latter more rapidly than the former. This results in a cross‐over at about 250MPa, where the two forms have approximately the same melting temperature, while at higher pressures Form II becomes the more stable phase. Although no solid–solid transitions have been observed, the coordinates of the I–II–liquid triple point have been found experimentally (p = 258.7MPa and T = 489.6 K) for the first time, and confirm those predicted by Espeau et al. from a topological p–T diagram based on theoretical arguments and experimental data at ambient pressure. © 2007 Wiley‐Liss, Inc. and the American Pharmacists Association J Pharm Sci 96: 2784–2794, 2007

Density of molten sulfur in the 334–508 K range

As the density of elemental sulfur was determined on slowly cooling from 508 K, it was unexpected for the metastable molten state to be maintained to temperatures as low as 334 K, thus expanding by about 20 K the T-range within which experimental data become available. In the 334–430 K range, the density ρ of the metastable molecular melt was fitted to ρ (g cm −3) = 2.18835–0.00098187 T (K), subsequently leading to volumic expansion coefficients α V( T) = −(1/ ρ( T))( ∂ρ/ ∂T) P of 5.28 × 10 −4 K −1 at 334 K and 5.56 × 10 −4 K −1 at 430 K.

Metastable extensions of phase equilibrium lines and singular points of simple substance

The thermodynamic properties of crystal, liquid, and gas in the stable and metastable states have been determined by molecular dynamics simulation of a system of 2048 Lennard-Jones particles. The spinodals of a superheated crystal, a superheated liquid, and a supersaturated vapor have been approximated; the spinodal for a supercooled liquid turns out to be nonexistent. The liquid-vapor, liquid-crystal, and crystal-vapor equilibrium curves and their extensions beyond the triple point have been calculated. It has been shown that, as distinct from the metastable extension of the saturation curve, which terminates at the zero isotherm, the metastable melting and sublimation curves terminate at, respectively, the stretched liquid and superheated crystal spinodals. The properties of the critical end points of metastable equilibrium of extended phases are considered.

Molecular dynamics simulation of cavitation in a lead melt at negative pressures

The molecular dynamics method was used to simulate cavitation in a metastable lead melt and determine the stability limits. States at temperatures below critical (T < 0.5T c) and large negative pressures were considered. Interatomic interactions were described by the realistic embedded atom potential. The kinetic boundary of liquid phase stability was shown to be different from the spinodal. The kinetics and dynamics of cavitation were studied. The pressure dependences of cavitation frequencies were obtained over the temperature range 700–2700 K. The results of molecular dynamics calculations were compared with estimates based on classical nucleation theory.

Self-organization of spatially inhomogeneous structures during volume crystallization of polydisperse systems

The kinetic instability arising during crystallization of metastable supercooled binary melts (produced by applying laser radiation to respective binary compounds) and culminating in the formation of ordered structures is studied. The nonlinear self-organization of inhomogeneous structures in a polydisperse system is statistically simulated using (i) a self-consistent set of kinetic equations for the cluster distributions in space and size and (ii) balance equations for the temperature and concentration of dissolved impurities. Allowance is made for cluster growth and evaporation, dependence of the phase transition temperature on the curvature of the cluster surface and impurity concentration, diffusion mobility of the clusters, and their withdrawal from the nucleation zone. The domain of the kinetic instability and the scale of the resulting inhomogeneous structures are determined.

Polymer crystallization from the metastable melt: The formation mechanism of spherulites

One of the most popular crystalline morphologies is a spherulite. An evidence is reported that the spherulite is crystallized through a dense packing state of small particles appearing in a droplet, which is caused by the primary phase separation of the melt in the metastable region of a phase diagram proposed by Olmsted et al. [Olmsted PD, Poon WCK, McLeish TCB, Terrill NJ, Ryan AJ. Phys Rev Lett 1998; 81: 373]. According to this phase diagram, the crystallization from the metastable state causes the nucleation and growth (N & G) of nematic domains, here named droplets, in the isotropic matrix. As a next step, the secondary phase separation of spinodal decomposition (SD) type into smectic and amorphous domains occurs inside the droplet where entanglements are excluded from the smectic to the amorphous domain; then such an SD structure turns into a densely packing structure of many small particles owing to surface tension. At this final stage of the induction period a long period peak of small-angle X-ray scattering (SAXS), so-called SAXS before WAXS, appears, which may be due to the average distance between these small particles. Furthermore, it is considered that crystalline lamellae are formed by radial and azimuthal fusion of these small particles inside the droplet, resulting in a spherulite. Such a type of crystallization occurs most commonly when flexible polymers are crystallized under the usual conditions. This tentative concept of spherulitic growth, which is completely different from a theory by Keith and Padden [Keith HD, Padden FJ. J Appl Phys 1963; 34: 2409], would give a new insight into problems of spherulites.

Microscopic investigations of homogeneous nucleation in charged sphere suspensions

We studied the homogeneous nucleation kinetics of an aqueous suspension of charged colloidal spheres under de-ionized conditions. Samples of equilibrium crystalline structure were shear molten and the metastable melt left to solidify after cessation of shear. At low particle number densities n, corresponding to low metastability of the melt, nucleation was monitored directly via video microscopy. We determined the nucleation rates gamma(t) by counting the number of newly appearing crystals in the observation volume per unit time. Using a suitable discrete adaptation of Avrami's [J. Chem. Phys. 7, 1003 (1939); ibid.8, 212 (1940); ibid.9, 177 (1941)] model for solidification via homogeneous nucleation and subsequent growth, we calculate the remaining free volume VF(t) to obtain the rate densities J(t) = gamma(t)/VF(t). We observe J(t) to rise steeply, display a plateau at a maximum rate density Jmax, and to decrease again. With increased n the plateau duration shrinks while Jmax increases. At low to moderate number densities fully solidified samples were analyzed by microscopy to obtain the grain-size distribution and the average crystallite size angle brackets(L). Under the assumption of stationarity, we obtained the nucleation rate density J(Avr), which increased strongly with increasing n. Interestingly, J(Avr) agrees quantitatively to Jmax and to J(Avr) as obtained previously from scattering data taken on the same sample at large n. Thus, by combination of different methods, reliable nucleation rate densities are now available over roughly one order of magnitude in n and eight orders of magnitude in J.

Isostatic Ultra-High-Pressure Effects on Supercooled Melts in Colloidal Triglyceride Dispersions

Colloidal triglyceride dispersions can form metastable supercooled melts that need tedious tempering processes to be transformed into a crystalline state. We investigated the possibility of transforming the supercooled melts into crystals in a short time by treatment with isostatic high pressure. Nanosized supercooled melts of triglycerides in aqueous dispersions (prepared by high-pressure homogenization) were exposed to isostatic ultrahigh pressure for short periods to initialize crystallization processes. The dispersions were analyzed with different appropriate measuring methods such as differential scanning calorimetry, nuclear magnetic resonance, X-ray scattering, and electron paramagnetic resonance. The resulting particle sizes were estimated by photon correlation spectroscopy and transmission electron microscopy. The results of differential scanning calorimetry, X-ray, and nuclear magnetic resonance experiments show the induction of triglyceride particle crystallization by high-pressure treatment. Electron paramagnetic resonance spectroscopy shows that the triglyceride crystallization coincides with relocation of the lipophilic probe into a more polar environment. Transmission electron microscopy micrographs show the transformation of supercooled liquid particles into crystallized anisotropic particles. Crystal transformation in nanoscaled systems can be delayed for months, depending on the materials and the composite. It is shown that isostatic high-pressure treatment can be a valuable method to induce, accelerate, and control crystallization processes in specific colloidal triglyceride dispersions.

Phase-field modeling on morphological landscape of isotactic polystyrene single crystals

Spatio-temporal growth of isotactic polystyrene single crystals during isothermal crystallization has been investigated theoretically based on the phase field model by solving temporal evolution of a nonconserved phase order parameter coupled with a heat conduction equation. In the description of the total free energy, an asymmetric double-well local free energy density has been adopted to represent the metastable melt and the stable solid crystal. Unlike the small molecule systems, polymer crystallization rarely reaches thermodynamic equilibrium; most polymer crystals are kinetically stabilized in some metastable states. To capture various metastable polymer crystals, the phase field crystal order parameter at the solidification potential has been treated to be supercooling dependent such that it can assume an intermediate value between zero (melt) and unity (perfect crystal), reflecting imperfect polycrystalline nature of polymer crystals. Two-dimensional simulations exhibit various single crystal morphologies of isotactic polystyrene crystals such as faceted hexagonal patterns transforming to nonfaceted snowflakes with increasing supercooling. Of particular interest is that heat liberation from the crystallizing front influences the curvature of the crystal-melt interface, leading to directional growth of lamellar tips and side branches. The landscape of these morphological textures has been established as a function of anisotropy of surface energy and supercooling. With increasing supercooling and decreasing anisotropy, the hexagonal single crystal transforms to the dense lamellar branching morphology in conformity with the experimental findings.

Enthalpic and entropic origins of nucleation barriers during polymer crystallization: the Hoffman–Lauritzen theory and beyond

In the search for a greater understanding of polymer crystallization, numerous experimental observations with regards to microscopic structures and macroscopic properties have been reported in the past half-century. There are generally two types of experimental results to provide information about the mechanisms of polymer crystal growth, i.e. molecular dynamic/scattering and structural/morphological. Since we cannot follow the trajectory of individual chain molecules when they undergo the transition from liquid to solid state during the crystallization process, structural/morphological analysis of polymer crystals reveal information recorded during this process. Namely, the final structure and morphology of polymer crystals have atomic, stem and global chain conformation information embedded in them during crystallization which provides evidence which can be used to deduce molecular aspects of the polymer crystallization process. It is commonly understood that polymer crystallization, from the thermodynamic perspective, is a first-order transition involving the relaxation of a metastable undercooled melt towards the equilibrium state which is rarely reached in polymer crystals. This process is controlled by a free energy barrier. A molecular model is required to describe the landscape of the free energy barriers and to provide an analytical explanation concerning and predictions about polymer crystallization. The Hoffman–Lauritzen (HL) theory, which was put forward more than 40 years ago, was one of the first analytical theories to illustrate how polymers crystallize. Since then, modifications to the HL theory and suggestions for new approaches have been reported, but the core physical picture of the HL theory has largely remained intact. This article consists of four major parts: (1) we will compare the crystallization of small molecules and long chain molecules, and the relationship between crystallization and crystal habits. The diversity of crystalline structures and morphologies of semi-crystalline polymers must be taken into account when studying the crystallization mechanism of polymers (2) this article also serves as a brief review of the HL theory and its importance in our understanding of polymer crystallization (3) we have tried to answer the question: what is the nucleation barrier? Specifically, we will illustrate that the nucleation barrier in polymer crystallization consists of both enthalpic and entropic components as deduced from experimental results. This barrier, from our perspective, consists of selection processes taking place in different length- and time-scales (4) finally, there is a brief discussion on what issues remain, in particular, in the areas of undercooled liquid structures and the initial stages of crystallization.

Melting investigation of the systemBaF2–BaO–½Y2O3–CuOx–H2O

In order to understand low-temperature melting during the‘BaF2 process’, equilibria in the quaternary Ba, Y, Cu//O, F reciprocalsystem have been investigated using a compositional model which can berepresented as a trigonal prism. This prism is comprised of three tetrahedra:BaF2–Y F3–CuF2–CuOx, BaF2–Y F3––CuOx,and BaF2–BaO––CuOx. Systematic differential thermal analysis (DTA) studies of compositionsspaced along compositional vectors extending from the fluoride endof the prism to the oxide end gave evidence of low-melting liquids(<600 °C) near the fluorine-richregion in the BaF2–Y F3–CuF2–CuOx tetrahedron. Inthe intermediate BaF2–Y F3––CuOx tetrahedron, a low-temperature DTA peak(550–570 °C) was also identified; this has been shown to be due to a reversiblephase transformation in crystalline YOF. In the oxide-richBaF2–BaO––CuOx tetrahedron, where, in the presence of water vapour, the principal defluorinationprocess is generally thought to occur, the lowest melting temperature observed was815 °C atpO2 = 20 Paand pH2O = 2.1 kPa. However, published observations on films undergoing theBaF2 process have suggested the presence of an amorphous phase thought to be a liquid at . Based on our results, the low-melting liquids reported in theliterature do not appear to exist as a stable liquid phase in theBaF2–BaO––CuOx–H2O system under the conditions of our experiments. Rather, the formation of low-meltingliquid in Ba–Y–Cu–O–F films at may require (a) relatively fluorine-rich compositions, (b) metastable melting, (c) formationof hydroxide- or hydroxyfluoride-based liquids, or (d) some combination thereof.

Thermal characterization of poly(lactide-co-glycolide) microspheres containing bupivacaine base polymorphs

Solid-state chemistry of bupivacaine base, obtained by precipitation from bupivacaine hydrochloride solutions with ammonia, was investigated. Two bupivacaine base polymorphs (Form I: Tm=105.6±0.0°C; Form II: Tm=97.6±0.2°C) were isolated depending on the precipitation conditions and characterised by thermal analysis and FTIR spectroscopy. No weight loss was evidenced by TG confirming that no solvate formation had occurred. Biodegradable poly(lactide-co-glycolides) microspheres containing bupivacaine base were prepared by spray-drying. In the microspheres bupivacaine base was present as the metastable low melting crystal form independently of the bupivacaine base / poly(lactide-co-glycolides) ratio.

Calorimetry of liquids in the system Na2O-Fe2O3-SiO2

Transposed-temperature, drop-calorimetry measurements were performed in the systems Na2SiO3-Fe2O3-SiO2 and Na2SiO3-NaFeSi2O6 at 1373 K to investigate thermodynamic properties of Fe3+-bearing silicate liquids. The results confirm previously measured enthalpy of Na2SiO3 and SiO2 melts. No significant heats of mixing were observed in the Na2SiO3-SiO2 liquids or in peralkaline liquids in the system Na2SiO3-Fe2O3. The enthalpy of mixing of peralkaline liquids in the system Na2SiO3-Fe2O3-SiO2 was less than the standard deviation of drop calorimetry (approx. ±10 kJ/mol). From calorimetric data of Na2SiO3-Fe2O3 liquids extrapolated to hematite composition and published heat capacity, the enthalpy of fusion of hematite is estimated to be 83.3 ± 10.5 kJ/mol at 1373 K and 133.4 ± 10.5 kJ/mol at its melting point 1895 K. The Fe3+/Fe2+ ratio of the liquid was estimated from thermodynamic calculations using the fusion enthalpy of hematite and published data of FeO and O2, and reproduced measured Fe3+/Fe2+ ratios in natural silicate liquids and in CaO-Al2O3-Fe2O3-FeO-SiO2 liquids. No significant heats of mixing were observed in the Na2SiO3-NaFeSi2O6 liquids. The enthalpy of formation of NaFeSi2O6 (acmite) at 1bar and 298 K calculated from calorimetric data is −2546.9 ± 17 kJ/mol. The enthalpy and entropy of fusion of acmite are estimated to be 70.5 ± 9.4 kJ/mol and 51.3 ± 6.8 J/K-mol, respectively, at metastable congruent melting point, 1373 K. The entropy of fusion of acmite is similar to that of NaAlSi2O6 (jadeite), indicating that Fe3+ and Al3+ have analogous structural roles in pyroxene melts.

Metastable States in Bismuth‐Containing Oxide Systems

AbstractFor Abstract see ChemInform Abstract in Full Text.

Melting of a peritectic phase

The reverse of a peritectic reaction belongs to the class of eutectic and eutectoid reactions and the same theory is now applied after modification for the fact that the predominating diffusion occurs within one of the growing phases, here the liquid one. The complications caused by the metastable congruent melting point are examined.

Thermodynamics of crystalline nanonucleus formation from liquid phase

The energy of crystal nucleation from liquid phase was considered, with the following two stages taken into account: (1) the formation of metastable supercooled melt (solution), containing pre-nuclei with intermediate amorphous (quasicrystalline) structure, and (2) the transformation of amorphous clusters into solid crystalline nuclei having different structures. With growth of a nucleus the nucleation energy profile manifests 2–3 maxima corresponding to these stages, and the kinetics of the non-stationary nucleation has five characteristic variations.