Melt Structure Research Articles

Influence of chain entanglements and melt memory effect on the crystallization behavior of polyphenylene sulfide

Polyphenylene sulfide (PPS) has a prospective application in high-frequency communication due to its excellent dielectric properties. However, owing to the strong melt memory effect, the crystalline structure of PPS is very sensitive to the processing conditions and difficult to be controlled. To date, the melt memory effect of polymer cannot be detected directly and quantitatively. In this work, four PPS samples were successively prepared via polymerization, extrusion and dissolution treatment. The Fourier infrared transform spectroscopy (FTIR), high-temperature gel permeation chromatography (HT-GPC) and dynamic rheological tests disclose that these four PPS samples have analogous molecular structure and only differ in entanglement density. The molecular weight between entanglements (Me) decreases gradually from 1188 to 354 g/mol. Furthermore, differential scanning calorimetry (DSC) and polarized optical microscopy (POM) measurements were carried out to study the effect of entanglement density on the crystallization kinetics of PPS. It is found that entanglement density is a key indicator to characterize the melt memory effect of PPS semi-quantitatively, and the residual local ordered structures in PPS melt is proportional to Me. In addition, the crystallization kinetics of PPS is determined by the cooperation of melt memory effect (nucleation) and entanglement density (growth). Accordingly, the relationship of “chain entanglements” → “melt memory effect” → “crystallization behavior” of PPS is established, constructing a roadmap for the precise control of crystalline structures of PPS products during practical processing.

Melt structure of calcium aluminate-based non-reactive mold flux: Molecular dynamics simulation and spectroscopic experimental verification

In this study, a combination of molecular dynamics (MD) simulations and spectroscopic experiments was used to investigate the effect of alkali metal oxides (Na2O and Li2O) on the structure of calcium aluminate-based non-reactive mold flux. The results of the MD simulations show that Al-O bonding forms the basis of the slag structure. The increase of Y2O (Y represents Na or Li) promotes the formation of AlIV in the melt and improves the stability of the Al-O structure, while the polymerization of the melt and the complexity of the network structure show a decreasing trend. Raman spectroscopy results show that the characteristic peaks move to lower wave numbers with the increase of Y2O, and the polymerization degree and complexity of the network structure of the slag also decrease. MAS-NMR results show that there are multiple coordination states of Al3+ in the mold flux. With the increase of Y2O content, the content of AlIV and [AlO3F] in the mold flux increases, indicating that the Al-O network structure becomes more stable. The experimental results are in agreement with the MD simulation results. The findings of this work provide a detailed study for a deeper understanding of the structure and properties of non-reactive mold flux.

Structural evolution and mechanical properties of SiO2-Al2O3-MgO-B2O3 glasses with partial substitution of MgO by MgF2

SiO2-Al2O3-MgO-B2O3-MgF2 glasses were prepared using a high-temperature melting method, and their amorphous nature was confirmed through X-ray diffraction (XRD). X-ray photoelectron spectroscopy (XPS) and magic angle spinning nuclear magnetic resonance (MAS-NMR) were used to investigated structural changes in the glass. It is found that substituting MgO with MgF2 disrupted the Si-O and Al-O networks in the high-temperature glass melt structure, reducing the degree of glass network polymerisation and viscosity. During cooling and annealing, moderate amounts of fluoride ions (F−) promoted the linkage of a partially fragmented glass network by converting [BO3] and [AlO5] into [BO4] and [AlO4]. An increased F− content improved the mechanical properties of the glass, including flexural strength, Young's modulus, and hardness, which peaked at 157.50 MPa, 95.21 GPa, and 838.57 HV with 2 mol% MgF2 substitution. These findings advance our understanding of the behavior of MgF2-substituted glass.

OpenFOAM modeling of beryllium melt motion and splashing from first wall in ITER

Beryllium (Be) is a material which will be used as a plasma facing component in ITER due to its unique properties of high thermal conductivity, low density, and high strength. However, under extreme conditions of high temperature and pressure, Be can melt at the surface of tiles and molten droplets can be ejected into the reactor leading to disruption of fusion plasma. The pressure, mass density, velocity of Be vapor, and variations of temperature at the melt layer interface can influence the splashing of Be melt. The Computational Fluid Dynamics (CFD) model based on the OpenFOAM toolbox, a free open source CFD software package, was developed to treat the coupled flow of liquid Be metal and its vapor. The vapor-melt interface is modeled using the volume of fluid (VOF) approach implemented in the interCondensatingEvaporatingFoam solver that solves the continuity, momentum, heat conduction, and VOF equations. This CFD model is capable to predict the hydrodynamic effects of Be vapor on the melt layer motion, splashing, non-linear growth of melt waves, and ejection of molten droplets. The modeling accounts for the effects of thermal, viscous, gravitational, and surface tension forces at the vapor-melt interface. In this research, we used the interCondensatingEvaporatingFoam solver to simulate the effects of Be phase change and the development of melt motion with the droplets ejected from the surface. The CFD model accounts for inter-phase change between Be liquid and Be vapor. The evaporation model was validated against the Stefan phase-change problems. The influence of heat and mass transfer across the vapor-melt interface on melt layer stability is also investigated. The results provide an understanding of how the rate of phase change affects the development of melt structures and waves at the vapor-melt interface.

Electrochemical and Spectroscopic Study on Electrochemical Processing of Silicon in Fluoride Melt

For sustainable space utilization on the Moon and the Mars, it is essential to establish in-situ resources and energy utilization systems in space. Because of the large amount of silica in the regolith which composed of metallic oxides on the Moon and the Mars, the technology development for the covering the silica to silicon and oxygen is indispensable. The silicon can be used as a key material for silicon photovoltaics. Against this background, the authors have studied the electrochemical reduction of silica to silicon in fluoride melt. We propose a one-step process to obtain a silicon film on the cathode by electrolysis using molten salts containing silica. To determine the feasibility of the proposed process, it is necessary to understand the effects of the physicochemical properties such as the coordination structure of Si ions in the fluoride melt since the difference in the melt structure affects the nucleation and growth mechanisms of the silicon electrodeposition. However, there are few reports on the dissolved state of silicon ions in fluoride melt. The author thus clarified the relationship of the electrochemical behavior of coordination structure of Si ions in fluoride melts composed of LiF, KF, and NaF.The coordination structure of Si ions in the fluoride melt was investigated by high-temperature Raman spectroscopy and quantum chemical calculations based on the density functional theory (DFT). The vibrational modes of the Si ions were detected through high-temperature Raman spectroscopy and the Raman bands were simulated by DFT calculations. We clarified the interaction between Si ions and cation and anion species such as Li+, Na+, K+, O2−, and F−. In addition, the electrochemical behavior of Si ions was investigated by electrochemical measurements. The effect of O2− ions on the dissolution of SiO2 and the electrodeposition process of Si in fluoride molten salts revealed a significant increase in the reduction current of Si ions as well as in the film thickness and current efficiency of deposited Si. The molten fluoride system, which can fabricate Si films from SiO2 at low temperatures, has the potential to develop environmentally friendly and sustainable electrolytic systems.

Structure and Dynamics of a Polybutadiene Melt Confined between Alumina Substrates

Structure and Dynamics of a Polybutadiene Melt Confined between Alumina Substrates

Reprocessible nanohybrid shish-kebab superstructures of poly (lactic acid) crystallites evolving in quiescent melt: Soft epitaxy nucleation in correlation with mechanics

Poly (butylene terephthalate) (PBT) nanofibrils (NFs) were developed in-situ by implementing a rheologically tuned melt spinning process on poly (lactic acid) (PLA)/PBT blends. In the absence of any flow fields, melt crystallization of fabricated PLA/PBT in-situ nanofibrillar composites (NFCs) resulted in orthogonal patterning of PLA nanodomain crystallites around single or double PBT NFs. Geometric confinement (soft epitaxy) was the nucleation mechanism of PLA chains at the interface of isotropic distributed PBT NFs followed by the formation of reprocessible 2D nanohybrid shish-kebab (NHSK) superstructures. By increasing the PBT NFs content and their bundling, a fan-shaped transcrystalline structure was formed due to seeding or epitaxial nucleation on the bundle of PBT NFs owning a less curved interface. The nucleation and growth mechanism of crystallites structures which were influenced by temperature and PBT NF content have been discussed thermodynamically and kinetically by parallel investigations on microscopic observations and X-ray diffraction analysis. The evolution of different crystalline structures in quiescent melt in the forms of spherulites, NHSK, or fan-shaped transcrystallinity induces a local orientation in the isotropic NFCs. Mechanical propertie improvements of NFCs were compared in amorphous and crystalline states with tailored NHSK superstructures.

The magmatic-hydrothermal transition in P-rich pegmatitic melts: Crystal-melt-fluid interactions recorded by phosphate minerals

Rare-element pegmatites are the crystallization product of melts strongly enriched in incompatible elements. Some of these elements, e.g., Li, B, P, F, and H2O, function as melt structure modifiers, allowing such melts to reach unusually low crystallization temperatures and regulating the saturation of aqueous-carbonic fluids from the melt. However, the timing of fluid saturation is highly debated, and the processes that mark the magmatic-hydrothermal transition in rare-element pegmatites are still elusive. The crystallization sequence of phosphate minerals can be used to assess the timing and nature of fluid-related processes in pegmatitic systems. In the Buranga pegmatite, western Rwanda, magmatic phosphates are replaced by (hydrous) alkali-rich phosphates and later by strongly hydrated phosphates as the dike cools. This reaction series indicates a disequilibrium in the crystal-melt-fluid system and records complex fluid-rock interactions during dike consolidation. This contribution compares these interactions with the fluid record preserved in fluid inclusions in the main phosphates of the crystallization sequence. Three major fluid inclusion types are observed, aqueous-carbonic (mostly-two-phase, L-V), carbonic-aqueous (two-phase, V-L, or three-phase, L-L-V), and crystal-rich inclusions (multiphase, L-V-Solids). Crystal-rich fluid inclusions in trolleite [Al4(PO4)3(OH)3], the last primary phosphate to precipitate from the melt, show a paragenesis of solid phases similar to the secondary phosphates observed in the dike. LA-ICPMS analyses of individual fluid inclusions show high concentrations of Na and K, with some content of Li, Rb, Cs, B, Fe, and Mn in the fluid. A similar composition is present in all inclusion types within the same mineral. Crystal-rich fluid inclusions demonstrate that during the magmatic-hydrothermal transition, primary phosphates react with the fluid in the presence of melt to generate secondary phosphates. No evidence of external fluid sources has been observed during the magmatic-hydrothermal transition. The fluid inclusion record shows that the fluid salinity is kept constant due to fluid-mineral interactions and corroborates the hypothesis that fluid-soluble elements are internally remobilized in the melt-crystal-fluid system during the magmatic-hydrothermal stage. These interpretations have implications for the general evolution of P-rich pegmatitic melts and the mineralization of Nb and Ta in these systems.

Analysis of the structure and viscosity of iron melts containing titanium at various concentration

This study aims to investigate the correlation between viscosity and liquid structure in Fe-4.5 wt%C-xTi melts. As the temperature decreases, Free-Fe and A-Fe species undergo a transformation into B-Fe, resulting in a significant reduction in free volume. This leads to the aggregation and growth of existing clusters as well as the formation of new clusters, thereby increasing both the number and average volume of atomic clusters. Consequently, the viscosity of the melt experiences a substantial increase as the temperature decreases. In the high-temperature liquid phase region, an elevated titanium content enhances the abundance of titanium-centered clusters within the melt, promoting the stable growth of atomic clusters. Additionally, the increased presence of titanium facilitates the binding of a greater number of carbon atoms and enhances the occupation of free space by iron atoms. As a combined effect, the viscosity of the molten iron rises with an increase in titanium content. At temperatures below the liquidus temperature, a higher titanium content decreases the precipitation temperature of graphite. Consequently, melts with a greater titanium content exhibit a pronounced increase in viscosity at a lower temperature threshold. The primary factor influencing viscosity is the mass fraction of graphite precipitation, and the impact of titanium content diminishes as the titanium content increases.

In-situ observation and crystallization kinetics during the solidification of Cr-containing stainless steel slags

For a sustainable recycling process of Cr-containing stainless steel slags, high chromium recovery through selective crystallization could be achieved when thermophysical properties are controlled based on the crystallization kinetics. The in-situ observation and crystallization kinetics of synthesized Cr-containing slags has been studied. The crystallization characteristics in the CaO–SiO2–Al2O3–MgO–Cr2O3 slag system were investigated by using CLSM (confocal laser scanning microscopy) and DSC (differential scanning calorimetry). Analysis of crystallization kinetics was divided into spinel phase and calcium silicate phase. The growth mechanism of primary spinel phase has been demonstrated through CSD (crystal size distribution) theory with varying temperatures and cooling rates. The crystallization kinetic analysis of calcium silicate phase was performed using the modified Avrami equation and the combined Avrami and Ozawa equation. The Friedman differential isoconversional method was applied for determining the apparent activation energy during non-isothermal crystallization. Observations of crystalline phase morphologies and the precipitated phase was identified by using SEM-EDS (scanning electron microscopy-energy dispersive spectroscopy) and XRD (X-ray diffraction) analyses. The change in the crystallization characteristics were compared with the slag melt structures analyzing by Raman spectroscopy.

Quasi-stable structures in equilibrium dense bismuth melt: Experimental and first principles theoretical studies

Near the melting temperature, equilibrium bismuth melt is characterized by structural features that are absent in equilibrium monatomic simple liquids. In the present work, the structure of bismuth melt is studied by X-ray diffraction experiments and quantum chemical calculations. The presence of quasi-stable structures in the melt has been found, the lifetime of which exceeds the structural relaxation time of this melt. It is shown that these structures are characterized by a low degree of ordering and spatial localisation. It was found that up to 50% of the atoms in the melt can be involved in the formation of these structures. The elementary structural units of these structures are triplets of regular geometry with the characteristic lengths 3.25 Å and 4.7 Å as well as with the characteristic angles 45∘ and 90∘. The characteristic lengths of these triplets are fully consistent with correlation lengths associated with the short-range order in bismuth melt.

The impact of oxide impurity on the structure of FLiNaK and FLiNaK–CeF3 melts: A simulation study

Oxide impurities affect the performance of the molten mixtures applicable in molten salt reactors. To reveal the underlying reasons of changes in physicochemical properties, the in-depth investigations on microscopic structure of melts are demanded. In this study, the FLiNaK and FLiNaK–CeF3 molten mixtures with small addition of Li2O were simulated within the ab initio theory to obtain the local structure patterns. The emphasis is placed on oxygen-based groupings since these are expected to be the most stable. In order to estimate the stability, the calculation of ion pair lifetimes was performed. It is shown that the most probable ion groupings existing in the FLiNaK–Li2O melt are [OLi5K], [OLi5K2] and [OLi4NaK2], while the FLiNaK–Li2O–CeF3 melt contain [OCe2Li2], [OCe2Li2K], [OCe2Li3] and [OCe3Li] units predominantly. Within the groupings, the most stable ion pairs are O-Li or O-Ce, depending on the melt composition.

Influence of BaO and MgO on viscosity, crystalline phase, and structure of CaO–Al2O3–10%SiO2–20%CaF2-based system

In the current study, the low SiO2 mold flux for the continuous casting of the transformation-induced plasticity steel was studied to avoid the interface reaction between the dissolved aluminum in the molten steel and silica in the mold flux. The viscometer, Raman spectra, and X-ray disproportion were employed to study the viscosity, melt structure, and crystalline phase of the mold flux, respectively. The viscosity of the mold flux decreased with the replacement of MgO by BaO due to the balance of the melt structure and the precipitation phase. The aluminosilicate and silicate structures were depolymerized to simpler units with the replacement of MgO by BaO in the mold flux since a preference that Ba2+ tended to act as a charge compensator. The viscosity of the mold flux roughly increased with the addition of the total BaO and MgO content due to the formation of the MgAl2O4 phase in the pure liquid phase. The substantial precipitation of the MgAl2O4 phase in the liquid melt caused a sharp increase in the viscosity of the mold flux, which should be avoided by lowering the MgO content to less than 3 wt%.

Effects of PMMT on microstructure and macrosegregation of large-sized 6061 alloy billet

The pulsed magnetic field melt treatment (PMMT) was applied in the direct chill (DC) casting of 6061 alloy billets with a diameter of 430 mm. The results indicated that the billet solidification structure was refined and homogenised. The grain size was reduced by 20.4% in the billet centre. Meanwhile, the PMMT can reduce the addition of grain refiner. The negative macrosegregation in the billet centre was reduced significantly. This can be attributed to the melt structure being changed by the energy of the pulsed magnetic field, which leads to the temperature of the melt variation and increased nucleation rate. This study may offer a promising method for the high-quality large-sized 6061 alloy billets preparation with less grain refiner.

Al-26 and O-18 tracer diffusion in a titania-coated sodium aluminosilicate glass

Although TiO2 is a common nucleating agent in the production of aluminosilicate glass-ceramics, its role as a catalyst for the nucleation of the functional crystal remains unclear. Therefore, the Ti, Al and O mobility was studied in situ using 26Al and 18O tracer diffusion and Ti chemical diffusion experiments in an aluminosilicate melt of albite composition. The albite glass serves as a surrogate for lithium and magnesium aluminosilicate glass-ceramic compositions, as it allows to study atomic mobilities free of crystallisation artefacts. The depth profiles showed that Al mobility increased significantly and became comparable to that of oxygen when Ti simultaneously diffused into the glass. In the absence of Ti, the diffusivity of Al was two orders of magnitude lower than that of oxygen. The co-diffusion coefficients of Ti and Al seem to confirm a proposed preferential structural linkage between Al and Ti species in the network structure of aluminosilicate melts.

Structure and properties of CoCrFeNiMnBe high-entropy alloy films obtained by melt quenching

In this study nanostructured films of the Co16.67Cr16.67Fe16.67Mn16.67Ni16.67Be16.67 high-entropy alloy have been synthesized by the method of melt quenching. The structure and physical properties of melt quenched films and as-cast specimens of identical composition were investigated. The XRD patterns allowed us to establish that the investigated alloy in the as-cast state has a multiphase structure in which there are the FCC phase with lattice parameter a = 0.3598 nm, BCC phase (a = 0.2872 nm), and B2-type intermetallic compounds BeNi(Co) (a = 0.2616 nm). At the same time, the melt quenched sample has a structure consisting of the FCC phase (a = 0.3579 nm), the BeNi(Co) phase (a = 0.2610 nm) and the minor BCC phase. Both as-cast specimens and melt quenched films reveal a typical ferromagnetic behavior and exhibit semi-hard magnetic properties and high mechanical characteristics.

Exploration of the amphoteric transition of Al2O3 on melt structure and viscosity of silicate slag

The amphoteric oxide Al2O3 has different behaviors in different chemical environments. In an acidic environment, Al3+ breaks the network structure while in an alkaline environment, Al3+ forms a three-dimensional network like Si4+. In this paper, the amphoteric effect of Al2O3 on the structure and viscosity of a five-oxide silicate system (SiO2–Al2O3–FeO–CaO–MgO) is studied. The viscosities with different Al2O3 contents were experimented on an inclined plane and compared by the flow length of liquid slags. The study shows that the melt viscosity increases and then decreases with increasing Al2O3 content. As Al2O3 increases, the coordination number of the basic cations increases, changing from a network destruction ion to a charge balance ion. And the coordination number of Al3+ has a higher shift from four to five. The detailed reaction pathways of the structural units are revealed through the variation of oxygen types. The addition of Al2O3 leads to the replacement of [SiO4]4- by [AlO4]5-, with a decrease in the contents of Si–O–Si and Si–O-M while an increase of Si–O–Al, Al–O–Al and TO. Therefore, the network strength increases first and then decreases which is consistent with the tendency of viscosity.

Effect of residual carbon on coal ash melting characteristics in reducing atmosphere

The melting behavior of coal ash at high temperatures directly affects slag discharge in gasifiers. Residual carbon is a key factor that influences the melting and transformation process of coal ash, affecting its melting characteristics, mineral composition, phase transformation, and melt structure. This study examines how residual carbon affects the melting process through mineral transformation, melt structure, phase transformation, atmosphere reaction, dissolution, and collapse of the physical structure of slag. The type and amount of residual carbon have different effects on the melting process. Changes in minerals result in complex changes in the phase transformation process. Residual carbon deprives some Fe and Si elements of the liquid phase, forming new aggregated minerals that compete with others. Residual carbon disrupts the equilibrium and eutectic structure of the system, dissociates the Si-O structure, and reduces the Fe-O structure, forming Fe0 or Fe2+. Free iron atoms form melt structures, such as Fe3C and FeSi.

General Theoretical Concepts of the Liquid State from the Point of View of Statistical Thermodynamics

t. The article presents General theoretical concepts of the liquid state from the point of view of statistical thermodynamics, as well as computer modeling of the dynamic properties of the liquid state. It is shown that in melts, due to the high mobility of its particles and the absence of a long-range order (as in a crystal lattice), the nearest surroundings of different atoms or molecules can differ quite significantly both in the number of neighbors and in their spatial location. In this regard, the correlation function was calculated using the molecular dynamics method and the radial distribution function. A machine experiment for modeling the structure of melts, as well as for constructing the geometry of cluster structures, is characterized. Specific algorithms for their implementation are considered. Cyclic BornKarman boundary conditions are introduced, the base cell is surrounded by its own replicas, and each such replica exactly repeats the microstates of the base lattice. A General algorithm for constructing Voronoi polyhedra using computer programs is given

Atomic structure and physical properties of peridotite glasses at 1 bar

Earth’s mantle, whose bulk composition is broadly peridotitic, likely experienced periods of extensive melting in its early history that formed magma oceans and led to its differentiation and formation of an atmosphere. However, the physical behaviour of magma oceans is poorly understood, as the high liquidus temperatures and rapid quench rates required to preserve peridotite liquids as glasses have so far limited their investigation. In order to better characterize the atomic structure and estimate the physical properties of such glasses, we examined the Raman spectra of quenched peridotite melts, equilibrated at 1900 °C ± 50 °C at ambient pressure under different oxygen fugacities (fO2), from 1.9 log units below to 6.0 log units above the Iron-Wüstite buffer. Fitting the spectra with Gaussian components assigned to different molecular entities (Q-species) permits extraction of the mean state of polymerisation of the glass. We find that the proportions of Q1 (0.36–0.32), Q2 (0.50–0.43), and Q3 (0.16–0.23) vary with Fe3+/FeTOT (FeTOT = Fe2+ + Fe3+), where increasing Fe3+/FeTOT produces an increase in Q3 at the expense of Q2 at near-constant Q1. To account for the offset between Raman-derived NBO/T (2.06–2.27) with those determined by assuming Fe2+ exists entirely as a network modifier and Fe3+ a network former (2.10–2.44), ∼2/3 of the ferric iron and ∼90% of the ferrous iron in peridotite glasses must behave as network modifiers. We employ a deep neural network model, trained to predict alkali and alkaline-earth aluminosilicate melts properties, to observe how small variations in the atomic structure of peridotite-like melts affect their viscosity. For Fe-free peridotite-like melts, the model yields a viscosity of ∼ −1.75 log Pa s at 2000 °C, similar to experimental determinations for iron-bearing peridotite melts. The model predicts that changes in the peridotite melt atomic structure with Fe3+/FeTOT yield variations in melt viscosity lower than 0.1 log Pa s, barely affecting the Rayleigh number. Therefore, at the high temperatures typical of magma oceans, at least at 1 bar, small changes in melt structure from variations in oxidation state are unlikely to affect magma ocean fluid dynamics.