Metastable Liquid Phase Research Articles

Abnormal resistivity behavior of Co–Cu alloys and responsible metastable liquid phase separation

The electrical resistivity of Co-Cu alloys with Cu content of 10, 20, 30, 40, 50, 60, 70, 80, and 90 at.% was measured in the temperature range from 1400 to 2100K. The experiments were performed during of heating and subsequent cooling of the samples. For alloys with Cu content of 30, 40, 60, 70, 80, and 90 at.%, a bend in the cooling curve specific to the liquid-liquid phase separation (LLPS) process was observed. LLPS is shown when the melt is undercooled to the binodal temperature T∗. The phase transition model of LLPS was proposed for to theoretically determine the temperatures T∗. The LLPS model demonstrates the possibility of percolation phenomena occurring in Cu-Co heterogeneous melts rich in cobalt. The LLPS model shows good agreement with experimental data and the abnormal resistivity behavior of Co–Cu alloys.

Influence of Carbon Content on Element Diffusion in Silicon Carbide-Based TRISO Composite Fuel

The coating layers of Tri-structural Isotropic Particles (TRISO) serve to protect the kernel and act as barriers to fission products. Sintering aids in the silicon carbide matrix variably react with TRISO coating layers, leading to the destruction of the coating layers. Investigating how carbon content affects element diffusion in silicon carbide-based TRISO composite fuel is of great significance for predicting reactor safety. In this study, silicon carbide-based TRISO composite fuels with different carbon contents were prepared by adding varying amounts of phenolic resin to the silicon carbide matrix. X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) were employed to characterize the phase composition, morphology, and microstructure of the composite fuels. The elemental content in each coating layer of TRISO was quantified using Energy-Dispersive X-ray Spectroscopy (EDS). The results demonstrated that the addition of phenolic resin promoted the uniform distribution of sintering aids in the silicon carbide matrix. The atomic percentage (at.%) of aluminum (Al) in the pyrolytic carbon layer of the TRISO particles reached its lowest value of 0.55% when the phenolic resin addition was 1%. This is because the addition of phenolic resin caused the Al and silicon (Si) in the matrix to preferentially react with the carbon in the phenolic resin to form a metastable liquid phase, rather than preferentially consuming the pyrolytic carbon in the outer coating layer of the TRISO particles. The findings suggest that carbon addition through phenolic resin incorporation can effectively mitigate the deleterious reactions between the TRISO coating layers and sintering aids, thereby enhancing the durability and safety of silicon carbide-based TRISO composite fuels.

Metastable phase separation and crystalline orientation feature of electromagnetic levitation processed CoCrCuFeNi high entropy alloy

The metastable liquid phase separation and rapid solidification kinetics of CoCrCuFeNi high entropy alloy (HEA) has been investigated by electromagnetic levitation (EML) technique. The maximum liquid undercooling attained 452 K (0.27TL), and the critical undercooling to initiate phase separation was determined as 172 K. By modulating the degree of alloy undercooling, the overall macrosegragation pattern achieved a transition from homogeneous dendrites into dispersed structures and finally into core-shell structures. Both microstructure and electron backscatter diffraction (EBSD) characterizations revealed that liquid undercooling played a crucial role in the modulation of dendritic anisotropy correlated with crystalline orientations. Coarse high entropy face-centered cubic (HEF) dendrites were produced with 〈100〉 preferred orientation at small undercoolings. Once undercooling exceeded the threshold of 172 K, the previously uniform liquid alloy was separated into Cu-depleted and Cu-rich zones. Numerous equiaxed HEF grains of relatively random orientation together with some twin crystals were displayed in Cu-depleted zone, whereas in Cu-rich zone HEF phase solidified into many slender dendrites. Theoretical analyses indicated that equiaxed grains were produced in the regime of thermal diffusion-controlled dendritic growth with actual solidifying velocity up to 42.7 m s−1. Furthermore, physical properties examinations demonstrated that large liquid undercoolings enhanced the alloy microhardness and low temperature saturated magnetization but reduced its electrical resistivities.

Complex oiling-out behavior of procaine with stable and metastable liquid phases.

During the crystallization of a solute from solvent(s), spontaneous liquid-liquid phase separation (LLPS) might occur, under certain conditions. This phenomenon, colloquially referred to as "oiling-out" in the pharmaceutical industry, often leads to undesired outcomes, including undesired particle properties, encrustation, ineffective impurity rejection, and excessively long process time. Therefore, it is critical to understand the thermodynamic driving force and phase boundaries of this phenomenon, such that rational strategies can be developed to avoid oiling-out or minimize its negative impact. In this study, we systematically evaluated the oiling-out behavior of procaine, a low melting point drug, in the solvent systems heptane, and ethanol-heptane as a function of temperature and solvent composition. In the procaine-heptane binary system, we observed a region where the LLPS is metastable with respect to crystallization, which is most commonly observed in the crystallization of modern active pharmaceutical ingredients (APIs); however, we also identified a region of the phase diagram where the LLPS is stable with respect to crystallization, and therefore will persist indefinitely. In the procaine-ethanol-heptane ternary system we identified five different regions, including a homogeneous liquid (L) region, two solid-liquid (SLI and SLII) regions, a liquid-liquid (LILII) region, and a solid-liquid-liquid (SLILII) region. The binary and ternary phase diagrams were also predicted using a state-of-the-art thermodynamic model: the SAFT-γ-Mie equation of state, and the results were compared with experimental data. Our findings highlight the complexity of oiling-out behavior. This work also represents a combined modeling and experimental platform to identify phase boundaries that will enable rational selection of strategies to crystallize active pharmaceutical ingredients with oiling-out risks.

Extension of the delayed equilibrium model to flashing flows of organic fluids in converging-diverging nozzles

Flashing flows of organic fluids find application in various energy systems, but the available modelling approaches rely on semi-empirical correlations calibrated for other fluids and operating conditions. This paper extends the state-of-the-art one-dimensional models developed for water to flashing of organic fluids, using R134a as a reference. First, the delayed equilibrium model combined with Grönnerud's friction correlation is identified as the most appropriate approach, reducing the error on mass flow rate with respect to experimental results from 48.3 %, obtained with the homogeneous equilibrium model, to 10.3 %. Then, the traditionally adopted incompressible flow assumption for the metastable liquid phase is replaced by a dedicated thermodynamic model, preventing the appearance of unphysical metastable phase temperatures. Moreover, the delayed equilibrium model is tailored to R134a by tuning the semi-empirical coefficients of the constitutive law. The proposed modifications to the state-of-the-art delayed equilibrium model result in an improvement in the prediction of the pressure profiles and the mass flow rate for flashing flows of R134a, with a decrease in the error on mass flow rate from 10.3 % to 6.1 % with respect to the original formulation.

Case study for the formation of a surface alloy on cemented tungsten carbide using ultrashort MHz to GHz burst-pulses

The authors of this paper report on the generation and utilization of high-frequency combined MHz- and GHz-bursts consisting of ultrashort laser pulses in order to form a surface alloy on the composite material cemented tungsten carbide. The resulting topographies, ablation depths and chemical compositions of the irradiated material surfaces were characterized with respect to the modification of the surface of a composite material whose elements tungsten carbide (WC) and cobalt (Co) permit the formation of a surface alloy only in a common meta-stable liquid phase. The results of this case study demonstrate that using high-frequency bursts of ultrashort laser pulses in the investigated parameter range, the formation of a surface alloy is possible without a tendency to change the chemical composition of WC and Co, and the depth of re-solidified material depends on the combination of MHz and GHz burst-pulses.

Spinodal liquid phase separation enabling dissimilar resistance spot welding of immiscible iron and copper alloy system

The heat un-balance and the immiscibility of the Cu and Fe alloy system are the key challenges during Cu/Fe resistance spot welding. Utilising electrodes of different geometries enabled the formation of a mixed Cu–Fe fusion zone. It is identified that despite the immiscibility of Cu and Fe in the solid-state, metastable liquid phase separation enables a metallurgical joining between Cu and Fe. The liquid phase spinodal decomposition led to the formation of an ultra-fine interconnected network of Cu-rich and Fe-rich phases in the fusion zone. The higher hardness of the fusion zone compared to the base metals, the susceptibility to solidification cracking, and the electrode indentation in the Cu sheet govern the mechanical properties of the Cu/Fe resistance spot welds.

Patterned crystal growth and heat wave generation in hydrogels

The crystallization of metastable liquid phase change materials releases stored energy as latent heat upon nucleation and may therefore provide a triggerable means of activating downstream processes that respond to changes in temperature. In this work, we describe a strategy for controlling the fast, exothermic crystallization of sodium acetate from a metastable aqueous solution into trihydrate crystals within a polyacrylamide hydrogel whose polymerization state has been patterned using photomasks. A comprehensive experimental study of crystal shapes, crystal growth front velocities and evolving thermal profiles showed that rapid growth of long needle-like crystals through unpolymerized solutions produced peak temperatures of up to 45˚C, while slower-crystallizing polymerized solutions produced polycrystalline composites and peaked at 30˚C due to lower rates of heat release relative to dissipation in these regions. This temperature difference in the propagating heat waves, which we describe using a proposed analytical model, enables the use of this strategy to selectively activate thermoresponsive processes in predefined areas.

Numerical modeling of single droplet flash boiling behavior of e-fuels considering internal and external vaporization

In recent years, significant efforts have been made to develop e-fuels from renewable electricity and carbon sources for enabling highly efficient and advanced propulsion systems. Compared to conventional fuels, such fuels can have very different thermo-physical properties depending on their molecular structure. Particularly, fuels with high vapor pressures are highly susceptible to flash boiling depending on boundary conditions, which can significantly alter the spray formation and mixing behavior. Thus, it becomes imperative to develop a fundamental understanding of the underlying physics associated with the flash boiling of these fuels in a single droplet configuration. In this work, oxymethylene ethers (OMEx) are chosen as a generic example to study the flashing behavior of newly developed e-fuels. This study employs the Lagrangian Particle Tracking (LPT) approach considering both internal and external vaporization of flash boiling single droplets. The internal vaporization model includes several sub-models that compute bubble number density, bubble growth rate, and droplet bursting criterion. External vaporization is modeled considering heat transfer from the droplet interior to the droplet surface and from the surrounding gas to the droplet surface. The study reveals that the formation and subsequent growth of vapor bubble nuclei is the primary source causing the transition of the metastable liquid phase into the stable state. We found that for moderate to high superheating degree, the bubble growth characteristics indicate three distinct growth phases: (1) surface tension-controlled, (2) transition, and (3) inertia-controlled, whereas, for low superheating degree, only two of these were present, namely (1) surface tension-controlled, and (2) transition phase. It is also observed that the chain length of OMEx has significant impact on bubble dynamics. OME4 is found to have a larger critical nucleus, a longer time delay in bubble growth, and a slower growth rate compared with dimethyl ether (DME). Furthermore, a quantitative analysis shows that droplets burst earlier with increasing superheating degree. In addition, it is found that the system pressure has a negligible influence on the initiation of the bursting process, except when the superheating degree is very low.

Emergence of dynamical disorder and phase metastability in carbon nanobowls

The condensed phases of the carbon-based nanobowl corannulene are herein investigated in the temperature range 200–573 K, combining differential scanning calorimetry, synchrotron X-ray diffraction, and quasielastic neutron-scattering. For the first time, we identify the presence of a well-defined thermal event at 382 K, a figure well below a melting point of 540 K. Contrary to naïve expectation, detailed analysis of the neutron-scattering data below and above this pre-melting transition signals the emergence of correlated stochastic dynamics within both thermodynamically stable solid phases of the material. We find that these are progressively responsible for the suppression of molecular and supramolecular order over mesoscopic length scales, and are associated with the formation of high-symmetry rotor-like states exhibiting localized stochastic motions. Upon cooling from the melt, we have also discovered a robust hysteresis associated with the existence of hitherto-unknown metastable liquid (deep-supercooled) and disordered-solid phases. This behaviour is markedly different from that observed in the quintessential Buckminsterfullerene C60 or other chemically substituted fullerene adducts studied to date at this level of detail. These results evince new and yet-to-tapped opportunities for the use of the stable and metastable phases of carbon-based nanobowls in novel applications exploiting the emergence of dynamical disorder at the nanoscale.

Structure of levitated Si–Ge melts studied by high-energy x-ray diffraction in combination with reverse Monte Carlo simulations

The short-range order in liquid Si, Ge and binary Si x –Ge1−x alloys (x = 0.25, 0.50, 0.75) was studied by x-ray diffraction and reverse Monte Carlo simulations. Experiments were performed in the normal and supercooled liquid states by using the containerless technique of aerodynamic levitation with CO2 laser heating, enabling deeper supercooling of liquid Si and Si–Ge alloys than previously reported. The local atomic structure of liquid Si and Ge resembles the β-tin structure. The first coordination numbers of about 6 for all compositions are found to be independent of temperature indicating the supercooled liquids studied retain this high-density liquid (HDL) structure. However, there is evidence of developing local tetrahedral ordering, as manifested by a shoulder on the right side of the first peak in S(Q) which becomes more prominent with increasing supercooling. This result is potentially indicative of a continuous transition from the stable HDL β-tin (high pressure) phase, towards a metastable low-density liquid phase, reminiscent of the diamond (ambient pressure) structure.

In Situ Monitoring of a Quasi Solid-State Diffusion-Driven Cocrystallization in the Diphenylamine–Benzophenone System

Diphenylamine reacts with benzophenone in the solid state through the formation of a eutectic liquid intermediate, from which the final product, in the form of a hydrogen-bonded cocrystal, is finally formed. By a simple experiment, the phase diagram of the benzophenone/diphenylamine system is effectively divided into two parts, which enabled monitoring of the overall process from the side of each reactant separately. Accompanied by isothermal and nonisothermal spectroscopic monitoring of the system, this provides additional details of the mechanistic pathway of the process of cocrystal formation through a metastable liquid intermediate phase.

Phase Behavior and Polymorphism of Saturated and Unsaturated Phytosterol Esters.

This study investigated how the physicochemical characteristics of phytosterol esters are influenced by the chain length and degree of unsaturation of the fatty acid ester moiety. Saturated and unsaturated phytosterol esters (PEs) were synthesized by the esterification of different types of fatty acids (stearic, palmitic, lauric, oleic, and linoleic acid) to β-sitosterol. The non-isothermal crystallization and melting behavior of the pure PEs were analyzed. It was proven by X-ray diffraction that saturated β-sitosteryl esters and β-sitosteryl oleate formed a bilayer crystal structure. The lamellar spacings of the bilayer structure decreased with decreasing fatty acid chain length and with an increasing degree in unsaturation. The degree of unsaturation of the fatty acid chain of the β-sitosteryl esters also influenced the type of subcell packing of the fatty acid moieties in the bilayer structure, whether or not a metastable or stable liquid crystalline phase was formed during cooling. Furthermore, it was found that the melting temperature and enthalpy of the β-sitosteryl esters increased with an increasing fatty acid chain length while they decreased with an increasing degree of unsaturation. The microscopic analyses demonstrated that β-sitosteryl oleate formed much smaller spherulites than their saturated β-sitosteryl analogues.

Evolution of phases and their thermal stability in Ge–Sn nanofilms: a comprehensive in situ TEM investigation

The group IV Ge–Sn system is a promising candidate for a variety of electronic and photonic applications due to its tunable bandgap and compatibility with Si-based technology. However, the peculiarities of the components interaction, phases evolution and, especially, their thermal stability are still far from a brimming comprehension. In the course of in situ TEM heating experiments, we demonstrate the high potential of using nanosized, layered Sn/amorphous-Ge films for the synthesis of Ge–Sn solid solutions with an enhanced Sn content. Particularly, we observe the formation of highly uniform, diamond structured Ge1-xSnx solid solution with Sn content of 33 at% by metal-induced crystallization (MIC) at 90 °C. The Ge0.67Sn0.33 alloy is stable in the range of 20–150 °C, whereat additional heating leads to partial Sn segregation and subsequent melting of the Sn-rich phase. The formation of the metastable and stable Sn-rich liquid phases, which was observed at 90 and 150–190 °C, respectively, has a key role in the system thermal stability. The temperatures and the observed complex interphase interactions are discussed in the frame of the size effects in nanoscaled structures.

Nucleation and Growth of Amino Acid and Peptide Supramolecular Polymers through Liquid-Liquid Phase Separation.

The transition of peptides and proteins from the solution phase into fibrillar structures is a general phenomenon encountered in functional and aberrant biology and is increasingly exploited in soft materials science. However, the fundamental molecular events underpinning the early stages of their assembly and subsequent growth have remained challenging to elucidate. Here, we show that liquid-liquid phase separation into solute-rich and solute-poor phases is a fundamental step leading to the nucleation of supramolecular nanofibrils from molecular building blocks, including peptides and even amphiphilic amino acids. The solute-rich liquid droplets act as nucleation sites, allowing the formation of thermodynamically favorable nanofibrils following Ostwald's step rule. The transition from solution to liquid droplets is entropy driven while the transition from liquid droplets to nanofibrils is mediated by enthalpic interactions and characterized by structural reorganization. These findings shed light on how the nucleation barrier toward the formation of solid phases can be lowered through a kinetic mechanism which proceeds through a metastable liquid phase.

Nucleation and Growth of Amino Acid and Peptide Supramolecular Polymers through Liquid–Liquid Phase Separation

AbstractThe transition of peptides and proteins from the solution phase into fibrillar structures is a general phenomenon encountered in functional and aberrant biology and is increasingly exploited in soft materials science. However, the fundamental molecular events underpinning the early stages of their assembly and subsequent growth have remained challenging to elucidate. Here, we show that liquid–liquid phase separation into solute‐rich and solute‐poor phases is a fundamental step leading to the nucleation of supramolecular nanofibrils from molecular building blocks, including peptides and even amphiphilic amino acids. The solute‐rich liquid droplets act as nucleation sites, allowing the formation of thermodynamically favorable nanofibrils following Ostwald's step rule. The transition from solution to liquid droplets is entropy driven while the transition from liquid droplets to nanofibrils is mediated by enthalpic interactions and characterized by structural reorganization. These findings shed light on how the nucleation barrier toward the formation of solid phases can be lowered through a kinetic mechanism which proceeds through a metastable liquid phase.

Microstructural evolution and magnetic property of a rapidly solidified ternary Fe37.5Cu37.5Sn25 peritectic-type alloy

The liquid Fe37.5Cu37.5Sn25 peritectic-type alloy was rapidly solidified by glass fluxing, drop tube, and melt spinning techniques. The solidification structures consisted of three phases, including the αFe solid solution and the Cu3Sn and Cu6Sn5 intermetallic compounds under different conditions. In this work, the bulk undercoding ΔT equals the liquidus temperature TL minus the nucleation temperature TN of the primary solid phase. During the bulk undercooling process, a maximum liquid undercooling of 272 K (0.18TL) was achieved. Metastable liquid phase separation was induced if the undercooling exceeded 177 K. Within a moderate undercooling range from 8 to 177 K, the solidified structures showed coarse dendrites, and the dendritic growth velocity increased to 41.5 mm/s following a power relation. As the drop tube technique provided a containerless state, a maximum surface cooling rate of 3.5 × 104 K/s was achieved. The alloy droplets displayed metastable liquid phase separation even at the largest droplet diameter of 944 μm. The cooling rate and thermal Marangoni migration were found to greatly influence the phase-separated patterns of the alloy droplets. Under the melt spinning condition, the microstructures of the Fe37.5Cu37.5Sn25 alloy ribbons were significantly refined and displayed soft magnetic characteristics. Furthermore, the experimental results demonstrated that the grain size of the αFe phase affected the coercivity of the alloy ribbons.

Liquid phase separation in undercooled Cu-Co alloys under the influence of static magnetic fields.

Undercooling of Cu-based alloys often induces metastable liquid phase separation followed by rapid solidification of separated liquids. The rapid solidification can help freeze in the morphology of a higher-melting liquid and eases difficulties in studies of liquid phase separation kinetics. In the present work, the influence of static magnetic fields on liquid phase separation in bulk Cu84Co16 composition was investigated. Inductively melted samples were glass-fluxed, undercooled and solidified under uniform and non-uniform magnetic fields generated by a superconducting magnet. Solidification microstructure of the phase-separated samples was examined using an optical microscope. The imposition of the magnetic fields, both uniform and non-uniform, altered the morphology, segregation pattern and size distribution of Co-rich droplets due to liquid phase separation. The imposition of the non-uniform magnetic fields with positive and negative gradients brought about segregation of the Co-rich droplets at the top and the bottom side of the samples, respectively. Such influence of the static magnetic fields is interpreted by assuming intensification of convective flow and Kelvin force-controlled migration of the Co-rich droplets. This article is part of the theme issue 'Heterogeneous materials: metastable and non-ergodic internal structures'.

Melting process and the size depression of the eutectic temperature in Ag/Ge and Ge/Ag/Ge layered films

Melting temperature is one of the principal parameters of phase equilibria thermodynamics and its value affects the majority of materials properties. Despite previous studies, the fundamental mechanisms of interphase interactions at melting in two-component nanomaterials are under consideration. We address these issues in a comprehensive experimental study of phase transformations in nanosized Ag-Ge eutectic system by means of in situ TEM techniques. The Ge/Ag/Ge and Ag/Ge layered films with thin Ag layer (1–50 nm) on top or in-between amorphous Ge films have been used as a model of a binary system of variable size. Morphology, crystalline structure and composition of Ag-Ge films have been examined in situ and ex situ by advanced (S)TEM techniques, the eutectic temperature (TE) and the Ge crystallization temperature have been measured as a function of Ag film thickness. A few hundred degrees lowering of TE with the film thickness reduction has been registered. Complex changes of the Ge/Ag/Ge films morphology below the eutectic temperature driven by Ag-mediated crystallization of amorphous Ge have been observed and related to the formation of metastable (liquid and/or Ag-Ge hcp) phases at the metal-semiconductor interface at 250 °C already.

A study on the micro-evolution of mechanical property and microstructures in (Cu-30Fe)-2X alloys with the addition of minor alloying elements

The morphology of the phase and the grain size generally have a significant influence on the mechanical properties of the alloy. (Cu-30Fe)-2X alloys also significantly have been affected by the morphology and solidification process of the phase. Compared with the Cu-30Fe alloy, which has a dendritic Fe phase due to the separation of meta-stable liquid phases, the addition of Si causes the formation of a spherical Fe primary phase with a fine size, and enhancement of the volume fraction of the secondary separated Cu phase in the formed Fe phase and primary α-Fe phase. Moreover, it leads to an implement of yield strength that is about 35% greater than a Cu-30Fe alloy. The addition of Nb and V resulted in the formation of coarse Fe units, which resulted in a decrease in the yield strength due to the concentration of stress. These results will be used as useful data for industrial applications aimed at controlling microstructures and improving mechanical properties in alloy designs.