Melting Kinetics Research Articles

A hybrid mesoscale-continuum approach to understand and predict melting kinetics of Al powders during laser processing

Abstract Laser interaction with metallic powders during additive manufacturing (AM) leads to fast heating and cooling rates that can affect the quality of the final products due to the formation of defects. One of the first steps towards predicting microstructures generated during AM, therefore, requires an accurate understanding of the laser energy deposition mechanisms that determine the melting kinetics at the level of individual powders. The critical challenge, however, is the availability of computational methods that can model the laser energy absorption, heat transfer, and the related microstructure evolution in individual metal powders at the length and time scales of AM. This manuscript demonstrates the capability of a novel scale-bridging methodology that combines the mesoscale quasi-coarse-grained dynamics (QCGD) simulations with a continuum two-temperature model (TTM) to account for the atomistic mechanisms of laser energy deposition and microstructure evolution and predict the kinetics of melting of individual powders at the experimental time and length scales. The scale-bridging capability of the hybrid QCGD-TTM simulations is demonstrated here by investigating the laser-induced microstructure evolution in aluminum powders with various sizes ranging from 200 nm to 20 µm. The analysis of the evolution of temperature, pressure, phase fraction, and melt fronts suggests the melting mechanism is heterogeneous due to the interaction with a laser, and the melting time is observed to decrease exponentially as the laser intensity increases. The solid-liquid interface velocity can be quantified to identify correlations with interface temperatures, and the predicted values satisfy the theoretically reported limits of crystal stability of metals against homogeneous melting. In addition, the pre-melting is found at the grain boundaries of 20 µm polycrystalline aluminum powder, while a minute contribution to melting is observed. This manuscript demonstrates the capability of the QCGD-TTM method to capture laser-powder interaction and allow the investigation of the kinetics of laser melting.

Preliminary study on smelting micro-carbon ferromanganese alloy by electro silicon thermal one-step process

Abstract In this paper, the related thermodynamic analysis of the one-step electro silicon thermal smelting process of micro-C-ferroalloy was carried out, and the effects of smelting temperature (T), slag alkalinity (R), silicon ratio (KSi) and holding time (t) on the appearance quality of the alloy, the Mn content, the Si content of the alloy and the Mn element recovery rate were studied. It is concluded that under the conditions of a smelting temperature of 1400°C, the basicity of slag R is 1.2, the silicon addition coefficient KSi is 1.0, the holding time is 60 min, the alloy appearance quality is good, Mn content of the alloy is high, Si content is low, and Mn yield is relatively high. Under laboratory conditions, the Mn yield of the alloy obtained by this method is low, and the thermodynamic equilibrium and reaction kinetics of the alloy melt and slag system of this method still need to be further studied to provide a theoretical basis for the expansion test.

Estimation of the Thickness of Ice Melting Front by Studying the Kinetics of Ice Ball Melting in Air

Estimation of the Thickness of Ice Melting Front by Studying the Kinetics of Ice Ball Melting in Air

Simulation of melt kinetics of in-situ molding of thermoplastic CF/PEEK composites

To reveal the interaction mechanism between the composites and the laser during the laser in-situ forming process of carbon fiber composites, the temperature change history and phase transition process on the surface of CF/PEEK composites under the action of flat-top beams with different moving directions were simulated by using COMSOL Multiphysics finite element simulation software. The results show that, due to the anisotropy of CF/PEEK composites, the transverse spread of the temperature field is always larger than the longitudinal spread, and when the moving angle is less than 45°, the surface temperature and the surface heat-affected zone of CF/PEEK composites increase with the increasing of the moving angle. The surface temperature and the area of heat affected zone are maximum when the angle of movement is 45°. As the angle of movement increases from 45° to 90°, the area and temperature of the heat-affected zone on the surface of the composite decreases with the increase in the angle of movement. The area of the melting zone is consistent with the distribution of the temperature field on the composite surface. The transverse broadening of the melting zone is always larger than the longitudinal broadening. The area of the melting zone is largest when the laser spot moves at an angle of 45°, and the area of the melting zone is smallest when the spot moves at an angle of 90°.

Shape compensation for carbon fiber thermoplastic composite stamp forming

Shape compensation of tooling for stamp forming of carbon fiber thermoplastic laminated composites is described as a process wherein the virtual twin, FORM3D is utilized to determine the tooling geometry required to produce the desired part geometry in a single step process. FORM3D is a physics-based virtual twin that incorporates anisotropic phenomena including transient heat transfer, thermoviscoelasticity, thermoelastic and crystallization shrinkage, and polymer crystallization and melting kinetics. This procedure for determining the compensated tool geometry is illustrated for a composite laminate constructed of AS4/PEKK prepreg, supplied by Solvay Thermoplastic Composites, for a quasi-isotropic layup of variable thickness and double curvature: major radius of curvature of 1556.7 mm and minor radius of curvature of 75.7 mm. The resulting, compensated shape was shown to agree with the desired geometry within ±0.25 mm.

Modelling the Melting Kinetics of Polyetheretherketone Depending on Thermal History: Application to Additive Manufacturing.

Recent techniques for forming thermoplastics, such as welding, automated fibre placement or additive manufacturing, generate successive rapid heating and cooling cycles that cause the partial melting of crystals during the process. The melting of an interface is essential to guarantee a good molecular diffusion across the welded parts. Nevertheless, no model can correctly predict the melting kinetics and consequently the evolution of the crystalline degree during the layers' deposition process. The purpose of this paper was to define the melting kinetics depending on the crystallization conditions for polyetheretherketone (PEEK). Firstly, a non-isothermal crystallization model was proposed over a wide range of cooling rates from 0.1 K.s-1 to 150 K.s-1. Experimental results have highlighted a dual-mode behaviour of melting and demonstrated the dependence of melting temperatures on crystallization conditions. Based on these observations, a model was developed to predict the melting behaviour after non-isothermal crystallization. The melting model revealed that after high cooling rates, primary and secondary crystals melt separately at low temperatures, while after slow cooling rates, both structures melt simultaneously at higher temperatures. Finally, the melting model was applied to the FFF thermal cycle to illustrate the influence of process parameters on the melting kinetics during deposition.

Inverted nucleation for photoinduced nonequilibrium melting.

Ultrafast photoinduced melting provides an essential platform for studying nonequilibrium phase transitions by linking the kinetics of electron dynamics to ionic motions. Knowledge of dynamic balance in their energetics is essential to understanding how the ionic reaction is influenced by femtosecond photoexcited electrons with notable time lag depending on reaction mechanisms. Here, by directly imaging fluctuating density distributions and evaluating the ionic pressure and Gibbs free energy from two-temperature molecular dynamics that verified experimental results, we uncovered that transient ionic pressure, triggered by photoexcited electrons, controls the overall melting kinetics. In particular, ultrafast nonequilibrium melting can be described by the reverse nucleation process with voids as nucleation seeds. The strongly driven solid-to-liquid transition of metallic gold is successfully explained by void nucleation facilitated by photoexcited electron-initiated ionic pressure, establishing a solid knowledge base for understanding ultrafast nonequilibrium kinetics.

Visualization of intracellular ice formation and growth in mouse oocytes

BACKGROUND: Characterization of intracellular ice formation (IIF) in oocytes during the freezing and thawing processes will contribute to optimizing their cryopreservation. However, the observation of the ice formation process in oocytes is limited by the spatiotemporal resolution of the cryomicroscope systems. OBJECTIVE: To observe the intracellular icing of oocytes during cooling and rewarming, and to study the mechanism of formation and growth of intracellular ice in oocytes. MATERIALS AND METHODS: Mouse oocytes were frozen at different cooling rates to induce intracellular ice formation using a cryomicroscopy system consisting of a microscope equipped with a cryogenic cold stage, an automatic cooling system, a temperature control system, and a high-speed camera. The growth patterns of intracellular ice in oocytes were analyzed from the images recorded. Finally, the growth rate of intracellular ice formation in oocytes was calculated using an automatic intracellular ice tracking method.RESULTS: The IIF temperature decreased gradually with the increase in cooling rate. Initiation sites of IIF could be classified into three categories: marginal type, internal type and coexisting type. There was a strong predominance for ice crystal initiation site in the oocytes, with up to 80% of the initiation sites located in the marginal region. The intracellular ice growth modes of darkening and twitching cells were characterized by “spreading” and “clustering”, respectively. In addition, twitching cells started to recrystallize during rewarming, while darkening cells did not. The instantaneous maximal growth rate of ice crystals in twitching cells was about 10 times higher than that in darkening cells. CONCLUSION: By visualising the growth of ice crystals in mouse oocytes during cooling and rewarming, we obtained valuable information on the kinetics of ice formation and melting in these cells. This information can help us understand how ice formation and melting affect the viability and quality of oocytes after cryopreservation.

Characteristics and mechanism of slagging in a 500 t/d MSW incinerator

With the promotion of classified collection of municipal solid waste (MSW), the heating value of waste sent to incinerators has significantly increased, which affects slagging and ash deposition. Thoroughly studying the slag characteristics in incinerators is important for determining its formation conditions and processes. In this study, slag samples from the furnace and superheaters of a 500 t/d incinerator were comprehensively studied. The slag characteristics were measured using X-ray diffraction (XRD), X-ray fluorescence (XRF), and field-emission high-resolution scanning electron microscopy (SEM); the chemical thermodynamic equilibrium calculations were carried out using FactSage; and the melting kinetics were analysed using differential scanning calorimetry (DSC). It was found that under the high-temperature environment of the furnace, CaO and Fe2O3/MgO/Al2O3 react with SiO2 to form low-melting-point eutectic crystals, which are in a molten or semi-molten state and easily adhere to the inner wall of the furnace to form slag that is difficult to remove. CaSO4 does not melt in the furnace but moves with the flue gas and is mainly deposited on the surfaces of the third- and second-stage superheaters. Na2SO4 and K3Na(SO4)2 are in a gaseous state under high-temperature conditions in the furnace, and then transform into a liquid or solid state and condense on the surface of the superheaters. The ash melting process in the furnace can be described using zero-order kinetics. These research results provide a theoretical foundation for the detailed prediction of slagging and ash deposition in waste incinerators.

The Isothermal Melting Kinetics of Ultra-High Molecular Weight Polyethylene Crystals.

The isothermal melting behaviors of ultra-high molecular weight polyethylene (UHMWPE) with different entangled states (i.e., nascent and melt-crystallized samples) are studied. For two kinds of UHMWPE samples, the result shows that the relative content of survived crystals (Xs) exponentially decreases with time and reaches a constant value. It is suggested that such a melting behavior is related to the observed nonlinear growth of crystals induced by the kinetically rejected entanglements accumulated at the growth front. Additionally, the exponential decay of Xs with time provides a characteristic melting time (τ) for the melting process. Compared to the melt-crystallized UHMWPE, the τ value of nascent UHMWPE is generally longer even in a higher temperature range, which is mainly because the former has a larger entanglement density difference. Furthermore, these observations demonstrate that UHMWPEs with different entangled states have an analogous melting mechanism since they exhibit a similar melting activation energy (≈1300kJ mol-1).

Enhancing paraffin wax crystallization and investigating thermo-physical properties of ethylene-propylene-diene monomer/paraffin wax/silica phase change system: The influence of silica nanoparticles

Nowadays, phase change systems play a crucial role in thermal energy storage applications. However, key challenges persist in these systems, namely achieving a higher phase change enthalpy while minimizing leakage of the phase change materials (PCMs). This study focuses on investigating the thermo-physical properties of a phase change composite comprising Paraffin Wax (PW) and silica nanoparticles. The objective is to enhance the thermal efficiency of an ethylene-propylene-diene monomer (EPDM) rubber/PW PCM/silica composite by utilizing silica nanoparticles as nucleating agents to promote the crystallinity of PW PCM. The results demonstrate that the incorporation of 0.4 wt% silica nanoparticles maximizes the crystallinity of PW PCM, coinciding with the threshold for particle aggregation within the PW PCM matrix. Furthermore, a lower crosslink density in the rubber matrix facilitates a higher loading of PW PCM (197 %) and mitigates leakage (1.36 %). Thermo-physical analysis of the composite materials reveals that the presence of silica nanoparticles significantly reduces the melting kinetics and enhances the crystallinity of PW PCM. Moreover, the inclusion of silica nanoparticles elevates the energy storage capacity of the EPDM/PW PCM/silica composite by augmenting the heat capacity and substantially reducing the melting and freezing kinetics of PW PCM.

Numerical simulation and analysis of heat transfer and melting rate of nano-enhanced PCM composite embedded in a concentrator photovoltaic system

The primary objective of this research is to explore the impact of nanoparticles-infused PCM (phase change material) in the context of nano-PCM-PV technology (PV: photovoltaic panel). Computational investigations were conducted to evaluate the effectiveness of RT25HC, a paraffin wax PCM, in conjunction with various nanoparticles (MgO, TiO2, ZnO and CuO) in solar panel cooling. The study also considers the influence of PV module inclination angles (β: 0 to 90°). High-resolution numerical simulations using the ANSYS-Fluent CFD platform were employed. The findings highlight the intricate relationship between nanoparticle addition, melting kinetics, electrical efficiency and PCM enthalpy. Specifically, the melting rate of RT25HC PCM exhibits notable differences between horizontal (β = 0°) and inclined panels (β ≠ 0°), with a melting fraction of 12 % at t = 2 h for β = 0° compared to 40–44 % for β ≠ 0°. Nanoparticles maintain a higher electrical efficiency (11.6 %) for horizontal panels, whereas inclined panels (β ≠ 0°) experience declining electrical efficiency (7.5 % at t = 8 h). For β = 0°, the PCM system effectively maintains the surface panel's temperature constant at 44 °C for a long time (up to 8 h). However, when nano-PCM systems are introduced, this temperature stabilizes at a slightly lower value of 40 °C, representing a 4 °C reduction attributed to the presence of nanoparticles. Conversely, for β ≠ 0, the surface panel's temperature stabilizes at 44 °C for up to t = 2 h, then experiences a sharp increase, ultimately reaching 120 °C at t = 8 h, regardless of the type or absence of nanomaterials. Overall, the most effective application of nano-PCM for cooling the PV panel was predicted at a horizontal orientation of the panel, regardless of the nanomaterial type.

Entropic barrier theory of polymer melting

AbstractWe present a theory of melting kinetics of semicrystalline polymers at temperatures above the equilibrium melting temperature, by accounting for conformational entropy of chains during melting. We have derived free energy landscapes for escape of individual chains from a lamella into the amorphous phase as a function of the characteristics of the initial lamella, such as the lamellar thickness, number of chain folds, fold‐ and lateral‐surface free energies, and mean energy of a monomer inside the lamella. We show that melting of lamellae is always accompanied by a free energy barrier which is entirely entropic in origin. In terms of the parameters characterizing the lamellae and the extent of superheating, closed‐form formulas are presented for the equilibrium melting temperature, driving force for crystallization, free energy barrier height, average expulsion time of a single chain from a lamella, and the melting velocity of lamellae. The present entropic barrier theory predicts that the dependence of melting velocity on superheating is nonlinear and non‐Arrhenius, in qualitative agreement with experimental observations reported in the literature. The derived formulas open an opportunity to further explore the role of various molecular features of semicrystalline polymers on their melting kinetics.

Wetting and pressureless infiltration behavior of Cu/316L system at 1100°C

It is still one of the great challenges to fully understand the coupling of reactive infiltration kinetics and wetting mechanism in the pressureless infiltration processes. We studied the wetting and pressureless infiltration behavior of Cu on 316L substrate with different pore sizes and porosity at 1100 °C under high vacuum by using the sessile drop method. The initial contact angle of Cu/316L system is ∼ 26 °, and rapid spreading (in ∼1.35 s) occurs under the action of capillary force due to the dissolutive wetting mechanism. The passivated film (Cr2O3) on the surface of 316L stainless-steel removed by self-cleaning reaction and interface reaction accompanying CrO gas phase release. Further, the obstacles of wetting and infiltration were eliminated. The reactive infiltration kinetics of Cu/316L cannot be predicted by the Washburn equation, preferring to an exponential variation. The infiltration kinetics is consistent with the wetting kinetics of melt on the smooth substrate surface, that is, the infiltration kinetics is limited by the dissolution kinetics.

Z-shaped dejamming phase diagram of colloidal gels

For physically gelled colloidal suspensions, there are two routes to transform the gel from solid to liquid. One is to raise the temperature, and the other is to increase the shear deformation. In this investigation, we found that the phase boundary of this solid-to-liquid transformation exhibits a surprising Z-shaped curve in the strain-temperature plane. This nonmonotonic feature in phase transition appears to be present in various nanoparticle-filled colloidal gels with significant differences in chemical composition, filler type, structure, particle shape, average diameter, and particle size distribution. By applying the Kraus model to the breakage and restoration of filler networks and comparing our findings to nonequilibrium glassy behavior, we found that this nonmonotonic phenomenon can be theoretically predicted by combining the glassy melting kinetics of filler networks at high temperatures with the viscosity-retarded dissociation between particles at low temperatures.

Direct Imaging of Surface Melting on a Single Sn Nanoparticle.

Despite previous studies, understanding the fundamental mechanism of melting metal nanoparticles remains one of the major scientific challenges of nanoscience. Herein, the kinetics of melting of a single Sn nanoparticle was investigated using in situ transmission electron microscopy heating techniques with a temperature step of up to 0.5 °C. We revealed the surface premelting effect and assessed the density of the surface overlayer on a tin particle of 47 nm size using a synergetic combination of high-resolution scanning transmission electron microscopy imaging and low electron energy loss spectral imaging. Few-monolayer-thick disordered phase nucleated at the surface of the Sn particle at a temperature ∼25 °C below the melting point and grew (up to a thickness of ∼4.5 nm) into the solid core with increasing temperature until the whole particle became liquid. We revealed that the disordered overlayer was not liquid but quasi-liquid with a density intermediate between that of solid and liquid Sn.

High precision control of laser energy for laser-matter interaction studies.

Precise, highly reproducible control of the laser energy is required for high confidence laser-matter interaction research such as in dynamic compression science and high energy density physics. The energy must be adjustable without affecting the pulse shape (time varying intensity) or beam smoothness. We have developed a convenient two-stage energy tuning method for a nominal 100J, 351nm (UV) laser. The energy is adjusted in 10J (10%) increments by operating the laser at full energy and inserting a beam splitter in the laser output. As the splitter is located after the final frequency tripling optics, the UV pulse shape is unchanged. The energy is varied by substituting a splitter of different reflectivity. For finer 3J (3%) increments, the infrared pulse is attenuated inside the laser before the final amplifier. This requires modest tuning to preserve the pulse shape. The demonstrated variation in shot-to-shot reproducibility is less than +/-2.5J (5% of the full energy), irrespective of the laser output energy. These approaches can be adapted to most ∼100J class lasers. We describe these techniques and show two examples where they have elucidated the underlying physics in laser shock compression experiments. One used only the beam splitters to establish the pressure for melting in iron. The other combined both techniques to finely increment the peak stress (∼2GPa steps) in germanium to precisely determine the onset and completion of melting-including the melting kinetics. These unambiguous results would not be possible without the developments described here.

The kinetics of the ice-water interface from abinitio machine learning simulations.

Molecular simulations employing empirical force fields have provided valuable knowledge about the ice growth process in the past decade. The development of novel computational techniques allows us to study this process, which requires long simulations of relatively large systems, with abinitio accuracy. In this work, we use a neural-network potential for water trained on the revised Perdew-Burke-Ernzerhof functional to describe the kinetics of the ice-water interface. We study both ice melting and growth processes. Our results for the ice growth rate are in reasonable agreement with previous experiments and simulations. We find that the kinetics of ice melting presents a different behavior (monotonic) than that of ice growth (non-monotonic). In particular, a maximum ice growth rate of 6.5 Å/ns is found at 14K of supercooling. The effect of the surface structure is explored by investigating the basal and primary and secondary prismatic facets. We use the Wilson-Frenkel relation to explain these results in terms of the mobility of molecules and the thermodynamic driving force. Moreover, we study the effect of pressure by complementing the standard isobar with simulations at a negative pressure (-1000bar) and at a high pressure (2000bar). We find that prismatic facets grow faster than the basal one and that pressure does not play an important role when the speed of the interface is considered as a function of the difference between the melting temperature and the actual one, i.e., to the degree of either supercooling or overheating.

Controllability of luminescence wavelength from GeSn wires fabricated by laser-induced local liquid phase crystallization on quartz substrates

We examined the effects of the laser scan speed and power on the Sn fraction and crystallinity of GeSn wires of 1 μm width and 1 mm length fabricated by laser-induced local liquid phase crystallization on quartz substrates. The Sn fraction increased from 1% to 3.5% with an increasing scan speed from 5 to 100 μm s−1, corresponding to a luminescence wavelength of 1770–2070 nm. This result can be interpreted as the scan speed dependence of the non-equilibrium degree during crystal growth. The increase in the laser power reduced the Sn fraction and caused a blue shift in the luminescence wavelength. We discuss these phenomena based on the growth kinetics of zone melting.

Real-time (nanoseconds) determination of liquid phase growth during shock-induced melting

Melting of solids is a fundamental natural phenomenon whose pressure dependence has been of interest for nearly a century. However, the temporal evolution of the molten phase under pressure has eluded measurements because of experimental challenges. By using the shock front as a fiducial, we investigated the time-dependent growth of the molten phase in shock-compressed germanium. In situ x-ray diffraction measurements at different times (1 to 6nanoseconds) behind the shock front quantified the real-time growth of the liquid phase at several peak stresses. These results show that the characteristic time for melting in shock-compressed germanium decreases from ~7.2nanoseconds at 35gigapascals to less than 1nanosecond at 42gigapascals. Our melting kinetics results suggest the need to consider heterogeneous nucleation as a mechanism for shock-induced melting and provide an approach to measuring melting kinetics in shock-compressed solids.