Melting Kinetics Research Articles

Nanosized n-eicosane as phase change materials: Phase behaviors and phase transition kinetics

Through confined in nanopores, the model compound of normal eicosane (n-C20H42, C20) shows tunable melting temperature and enthalpy change, which provides a series of nanosized phase change materials with various melting points. The phase behaviors of C20 in the pores of controlled pore glasses (CPGs) and SBA-15 are estimated using differential scanning calorimetry. The melting point of C20 decreases with the decreasing pore size, following a relation by the Gibbs–Thomson equation. Maximum decreases of C20 in the pores of CPGs and SBA-15 are about 7 and 13 K, respectively, which are the melting point widows for thermal energy storage formed by the nanoconfinement. Kinetics of melting of the bulk and nanoconfined C20 reveals reduced effective activation energy in the pores. The nanosized C20 is a good example to obtain PCMs with adjustable thermal properties.

Interior Melting of Rapidly Heated Gold Nanoparticles.

Normal melting invariably starts from surfaces or interfaces due to the weaker bonding constraints in these regions. However, we show that melting can be initiated from the interior of gold nanoparticles with high heating rates. We find that melting starts from the surface with the formation of a premelting layer, as usual, but that the premelting layer does not extend to the interior under certain conditions. Instead, liquid nucleation occurs in the core of the nanoparticle. This unexpected interior melting is connected to the slower melting kinetics, which is related to heat transfer near the premelted surface. The required conditions for interior melting are a suitable size of the nanoparticle and a sufficiently fast heating rate. The present results point to a novel melting regime in nanoparticles. We note that the time scales are now accessible using ultrafast tools such as X-ray lasers that can probe dynamical structure changes, suggesting opportunities for experiments.

Thermal energy storage in a confined cylindrical heat source filled with phase change materials

Thermal energy storage in the phase change materials (PCMs) around a confined heated cylindrical heat source has been investigated numerically. This work aims to explore three major objectives namely; evolution of melting front around the cylindrical heat source, quantification of overall rate of heat transfer and identifying the factors to maximize the energy storage. The enthalpy-porosity formulation is utilized to model melting of PCMs. The flow and energy equations with relevant boundary conditions have been solved using finite element method for range of parameters as Rayleigh number (102≤Ra≤106), Prandtl number (102≤Pr≤103), Stefan number (0.01≤Ste≤0.5) and aspect ratio (0.1≤AR≤0.5). The evolution of melting front and melt fraction, streamlines, isotherms, Nusselt number and thermal energy storage are reported as a function of Rayleigh number (Ra), Prandtl number (Pr), Stefan number (Ste), and aspect ratio (AR). Stefan number and the aspect ratio have shown to be a positive influence on the amount of thermal energy absorbed during the melting of PCMs. At low Rayleigh and Stefan number, melting kinetics is shown to be sluggish. All else being equal, the energy storage exhibits three distinct regimes depending upon the Fourier number namely, the lag phase, the exponential phase and stationary phase. For process design calculations, the gross engineering parameters like Nusselt number and steady-state melt fraction are often required, and therefore a predictive correlation for the value of the average Nusselt number is proposed to facilitate the estimation of average Nusselt number at intermediate values of dimensionless numbers for a given practical application.

Convective Heat Transfer During Melting in a Solar LHTES

Melting combined with natural convection in a shell and Latent Thermal Energy Storage (LHTES) tube driven by a solar collector was analyzed numerically in the present work. This work's particularity lies in the fact that the HTF temperature varies at each moment following the solar irradiance curve. A program (UDF) has been developed and integrated into Ansys to meet this requirement. The use of this coupling strategy allows obtaining realistic unsteady LHTES results. Several numerical investigations were carried out to analyze the effect of the heat sources' power on the accumulator's performance. The obtained results show that natural convection considerably influences the heat transfer as well as the melting kinetics of the Phase Change Material (PCM). Besides, the results show that increasing the heat transfer fluid's thermal load can increase the melting rate of the PCM and the stored energy and reduce the entire melting time.

The Narrow Thickness Distribution of Lamellae of Poly(butylene succinate) Formed at Low Melt Supercooling

Crystallization of poly(butylene succinate) (PBS) at 100 °C, about 30 K below the equilibrium melting temperature, allowed the preparation of crystals, which were analyzed regarding their zero-entropy-production melting temperature. Irreversible melting occurs in a rather narrow temperature window of only around 8 K, between 101 and 109 °C, revealing a narrow distribution of the thickness of isothermally formed lamellae and a rather low thickening/stabilization factor of less than 1.4. Quasi-isothermal temperature-modulated differential scanning calorimetry suggests significant reversible melting and crystallization during and after crystallization, proving the existence of a large fraction of crystalline phase being at the stability limit at the crystallization temperature. Heating of crystals formed at 100 °C to above their zero-entropy-production melting temperature, followed by isothermal annealing, permitted the analysis of the kinetics of irreversible melting, yielding superheating-dependent rate constants of the order of magnitude of 10² s–¹ 5–10 K above the zero-entropy-production melting temperature. The advanced analysis of the melting behavior of polymer crystals isothermally grown at low melt supercooling allows one to draw conclusions about their (inherently) low thermodynamic stability.

Melting kinetics, ultra-drawability and microstructure of nascent ultra-high molecular weight polyethylene powder

For the production of high-performance polyethylene fibers and tapes by ultra drawing, using solid-state processing of disentangled nascent ultra-high molecular weight polyethylene (UHMWPE), the maximum draw ratio is an important design parameter, as it determines the maximum degree of chain alignment and, therewith, the properties of the final product. It would, therefore, be advantageous to have a fast scanning method to estimate the maximum drawability of reactor powders. In the present work, the melting behavior of nascent UHMWPE reactor powder was studied by differential scanning calorimetry, followed by isoconversional analysis to evaluate the apparent activation energy barrier of melting for the different UHMWPE grades of different level of disentanglement. Powder samples were solid-sate processed into tapes at elevated temperature. Ultra drawing of these tapes at the optimum drawing temperature allowed for the evaluation of the maximum draw ratio (λmax) that is determined by the entanglement density. The morphology of the lamellar structure of the nascent powder was studied by small-angle X-ray scattering and by scanning transmission electron microscopy analyses. A correlation between the melting kinetics, the microstructure and the maximum draw ratio was observed for all grades, suggesting that the apparent activation energy of melting can be used as a screening method to estimate the maximum draw ratio.

Kinetics of DNA Melting and Annealing Under Small Tension

Kinetics of DNA Melting and Annealing Under Small Tension

Reactive Sintering of Ground Tire Rubber (GTR) Modified by a Trans-Polyoctenamer Rubber and Curing Additives.

The proposed method of ground tire rubber (GTR) utilization involves the application of trans-polyoctenamer rubber (TOR), a commercially available waste rubber modifier. The idea was to investigate the influence of various curing additives (sulfur, N-cyclohexyl-2-benzothiazole sulfenamide (CBS), dibenzothiazole disulfide (MBTS) and di-(2-ethyl)hexylphosphorylpolysulfide (SDT)) on curing characteristics, physico-mechanical, thermal, acoustic properties as well as the morphology of modified GTR, in order to evaluate the possibility of reclaiming GTR and the co-cross-linking between applied components. The results showed that the presence of the modifier without the addition of curing additives hinders the physico-mechanical properties of revulcanized GTR. The addition of SDT, CBS, MBTS and sulfur change the melting kinetics of TOR, indicating partial degradation and/or co-cross-linking between components. In the studied conditions, the best mechanical properties were obtained by the samples cured with sulfur. The morphology analysis, combined with the physico-mechanical results, indicated that when the surface of the GTR is more developed, obtained by the addition of TOR, the properties of the GTR improve.

Digital metal printing by electrohydrodynamic ejection and in-flight melting of microparticles

Metal additive manufacturing (AM) will impact many industries, including those producting of advanced structural, propulsion, electronic, and thermal systems. Many established metal AM processes involve layer-by-layer deposition and selective binding or melting of metal powders. However, these processes do not easily permit multi-material printing or printing directly onto non-planar surfaces. By contrast, AM techniques that deposit metal directly are typically low resolution (e.g., directed energy deposition), or cannot achieve bulk metal properties (e.g., ink-based methods). Here, we present a high resolution direct metal printing method that potentially addresses these limitations. Individual metal microparticles are electrohydrodynamically ejected on-demand from a nozzle-confined meniscus and laser-melted in-flight before landing and solidifying on a substrate. We demonstrate printing of solder and platinum particles ranging in size from 30 to 150 µm and explore the process parameter space as limited by the ejection conditions and the kinetics of melting, impact, and solidification. Our experiments demonstrate process compatibility with a wide range of powder feedstock materials, and tunability of the solidified morphology via the laser processing parameters. • Developed a direct printing method by on-demand depositing discrete metal microdroplets. • Enable multi-material printing by decoupling particle ejection and melting processes. • Revealed in-flight melting conditions for various metal microparticles. • Experimentally and numerically analyzed the molten droplet impingement and solidification.

Numerical Simulation of Melting Kinetics of Metal Particles during Tapping with Argon-Bottom Stirring

Molten steel is alloyed during tapping from the melting furnace to the argon-bottom stirred ladle. The metallic additions thrown to the ladle during the ladle filling time are at room temperature. The melting rates or kinetics of sinking-metals, like nickel, are simulated through a multiphase Euler–Lagrangian mathematical model during this operation. The melting rate of a metallic particle depends on its trajectory within regions of the melt with high or low turbulence levels, delaying or speeding up their melting process. At low steel levels in the ladle, the melting rates are higher on the opposite side of the plume zone induced by the bottom gas stirring. This effect is due to its deviation after the impact of the impinging jet on the ladle bottom. The higher melting kinetics are located on both sides at high steel levels due to the more extensive recirculation flows formed in taller baths. Making the additions above the eye of the argon plume spout increases the melting rate of nickel particles. The increase of the superheat makes the heat flux more significant from the melt to the particle, increasing its melting rate. At higher superheats, the melting kinetics become less dependent on the fluid dynamics of the melt.

Coupling of dynamic ductile damage and melting in shock-induced micro-spalling: Modeling and applications

Micro-spalling is a dynamic fragmentation process that is coupled with shock-induced overheating and melting. It has been observed in series of shock experiments and in several modern industrial applications such as inertial confinement fusion and laser-shock surface micromachining. Modeling micro-spalling is beyond the scope of traditional damage mechanics which ignores solid–liquid transformation.Here, for the first time, we develop a continuum model of micro-spalling by coupling hydro-elastic–plastic-damage mechanics and high-pressure melting kinetics. A two-scale framework is proposed for modeling mechanical responses of partially melted materials in shock-induced micro-spalling. Temperature and pressure dependence of shock-induced melting rate is formulated. In modeling dynamic ductile damage, temperature and (partial) melting effects are involved.The model is implemented into a finite element code and used as a predictive tool to simulate plate impact spalling experiments on aluminum. The simulations reveal typical characteristics of micro-spalling resulted from coupling effects of dynamic damage and melting, concerning evolutions of damage and phase distributions and thermodynamic paths. The simulations are in good agreement with previous experiments and molecular dynamics simulations. Particularly, the calculated free surface velocity profiles and spall strengths are quantitatively consistent with experimental measurements in a wide range of initial temperature up to the melting point. These results indicate that the present model is a reasonable uniform description of spalling process, covering transition from classical solid spalling to melting-accomplished micro-spalling.

Impact of 5-formylcytosine on the melting kinetics of DNA by 1H NMR chemical exchange.

5-Formylcytosine (5fC) is a chemically edited, naturally occurring nucleobase which appears in the context of modified DNA strands. The understanding of the impact of 5fC on dsDNA physical properties is to date limited. In this work, we applied temperature-dependent 1H Chemical Exchange Saturation Transfer (CEST) NMR experiments to non-invasively and site-specifically measure the thermodynamic and kinetic influence of formylated cytosine nucleobase on the melting process involving dsDNA. Incorporation of 5fC within symmetrically positioned CpG sites destabilizes the whole dsDNA structure—as witnessed from the ∼2°C decrease in the melting temperature and 5–10 kJ mol−1 decrease in ΔG°—and affects the kinetic rates of association and dissociation. We observed an up to ∼5-fold enhancement of the dsDNA dissociation and an up to ∼3-fold reduction in ssDNA association rate constants, over multiple temperatures and for several proton reporters. Eyring and van’t Hoff analysis proved that the destabilization is not localized, instead all base-pairs are affected and the transition states resembles the single-stranded conformation. These results advance our knowledge about the role of 5fC as a semi-permanent epigenetic modification and assist in the understanding of its interactions with reader proteins.

Bloating mechanism in lightweight aggregates: Effect of processing variables and properties of the vitreous phase

The firing behaviour of five waste glasses was studied to define key parameters able to predict bloating performances in industrial lightweight aggregates production. Bloating was investigated by hot-stage microscopy and laboratory kiln experiments, also with SiC as expanding agent. In-situ and ex-situ results were contrasted to shed light on the scale up process. Both macro- and microstructure of aggregates are related to the melt properties and crystallization phenomena. A phenomenological model for SiC-induced expansion was proposed. The glass chemical composition controls melt viscosity and kinetics of expansion, while the increase of aggregate size fosters the extent of bloating.

Effect of multi-step annealing above the glass transition temperature on the crystallization and melting kinetics of semicrystalline polymers

Fast scanning calorimetry (FSC) measurements were carried out for multi-step, isothermal, crystallized poly (butylene terephthalate) to quantify the effect of single-step to triple-step annealing above Tg (40 °C interval below the previous annealing temperature) on the crystallization/melting kinetics. In the case of double-step annealing, two endothermic peaks were observed on the FSC curves due to the melting of the crystal formed during the 1st step (base crystal; BC) and 2nd step (thermal history crystal; THC). The crystallization rate of the THC decreased due to the spatial restriction of the pre-existing BC. The melting kinetics of the THC were not affected by the time of 1st step crystallization. If the sample was re-heated to the 1st step temperature (Tc1) after multiple annealing, the thermal history of the previous annealing was completely erased. Moreover, in the re-annealing process at Tc1, secondary crystallization showed the same behavior as that expected from the result of single-step annealing.

Effect of Silicon Concentration on Melting Behavior of Scraps in Hot Metal

To clarify the effects of Si concentration, temperature, and time on melting of the silicon steel, three silicon steels with different Si concentrations were used to conduct an experimental investigation of the melting of scrap cylinders under natural convection. Thermodynamics and kinetics of scrap cylinder melting were revealed and analyzed based on the experimental results. Carbon diffusion between the cylinders and hot metal during melting was modeled using Thermo-Calc 2017b software to evaluate the mass-transfer coefficients. Results showed that a higher Si concentration and lower melting temperature led to slower melting of the silicon steel scrap cylinder. The mass-transfer coefficient of C during the melting decreased with an increase of Si concentration. At 1623 K (1350 °C), the mass-transfer coefficients were 1.322 × 10−4, 0.436 × 10−4, and 0.142 × 10−4 m s−1 for low (0.30 mass pct), medium (1.58 mass pct), and high (3.21 mass pct) Si concentrations in the scrap cylinders, respectively; at a melting temperature of 1723 K (1450 °C), the values of the respective mass-transfer coefficients were 1.370 × 10−4, 0.613 × 10−4, and 0.289 × 10−4 m s−1.

Nano-mesh superstructure in single-walled carbon nanotube/polyethylene nanocomposites, and its impact on rheological, thermal and mechanical properties

The mesh formation of single-walled carbon nanotube (SWCNT) superstructure, i.e. nano-mesh, in low- and high-density polyethylene (LDPE and HDPE) matrices was observed by using plasma etching technique and SEM analysis. Nano-mesh was established by two mechanisms: i) alignment of SWCNT bundles (forming network backbone) and ii) shish-kebab induced crystallization (forming network net). The network was formed at low SWCNT content due to the high aspect ratio of nano-filler, which additionally contributed to alignment. Established nano-mesh superstructure caused confinement effect disrupting crystallization and melting kinetics, which directly affected mechanical properties. Although mechanical properties of LDPE and HDPE nanocomposites profoundly improved, i.e. Young’s modulus increased for 230% and 30%, respectively, at only 1 wt% of SWCNTs, HDPE nanocomposites showed visible confinement that reduced effective reinforcement. It seems that HDPE nanocomposites cannot achieve full reinforcing potential since confinement effect restricts the crystal growth, which gradually reduces effective reinforcement in semi-dilute regime.

New insights into the memory effect on the crystallization behavior of poly(phenylene sulfide)

In this article, the melt and crystallization kinetics of poly(phenylene sulfide) (PPS) powder was studied by differential scanning calorimetry (DSC), fourier transform infrared spectroscopy (FTIR) and polarized optical microscopy (POM). It is found that the folded-chain crystals of PPS (melting point at 281 °C) could be erased at about 350 °C, while the equilibrium melting point of extended-chain crystals (melting point at 295 °C) is 370 °C. Although the residual nuclei could be eliminated completely at 370 °C, the spherulite growth rate of PPS during crystallization keeps accelerating with the increasing melting relaxation time, resulting from the transformation of the lamellae and the amorphous cluster to an ordered structure with low entanglement density. Furthermore, due to the restricted chain mobility for the semi-flexible PPS, this unique memory effect would survive in the whole manufacturing process, resulting in the varying results from different researches and the significant influence on the processability and performance of PPS products.

Melting Kinetics of Nascent Poly(tetrafluoroethylene) Powder.

The melting behavior of nascent poly(tetrafluoroethylene) (PTFE) was investigated by way of differential scanning calorimetry (DSC). It is well known that the melting temperature of nascent PTFE is about C, but reduces to C for once molten material. In this study, the melting temperature of nascent PTFE crystals was found to strongly depend on heating rate, decreasing considerably for slow heating rates. In addition, during isothermal experiments in the temperature range of C, delayed melting of PTFE was observed, with complete melting only occurring after up to several hours. The melting kinetics of nascent PTFE were analyzed by means of the isoconversional methodology, and an apparent activation energy of melting, dependent on the conversion, was determined. The compensation effect was utilized in order to derive the pre-exponential factor of the kinetic model. The numerical reconstruction of the kinetic model was compared with literature models and an Avrami-Erofeev model was identified as best fit of the experimental data. The predictions of the kinetic model were in good agreement with the observed time-dependent melting of nascent PTFE during isothermal and constant heating-rate experiments.

Heterogeneous melting kinetics in polycrystalline aluminum.

The heterogeneous melting kinetics of polycrystalline aluminum is investigated by a theoretical model which represents the overall melting rate as a functional of the Weibull grain-size-distribution. It is found that the melting process is strongly affected by the mean-grain-diameter, but is insensitive to the shape parameter of the Weibull distribution. The temperature-time-transformation (TTT) diagrams are calculated to probe dependence of the characteristic timescale of melting on the overheating temperature and the mean-grain-diameter. The model predicts that the heterogeneous melting time of polycrystalline aluminum exponentially depends on temperature in high temperature range and the exponent constant is an intrinsic material constant independent of the mean-grain-diameter. Comparisons between TTT diagrams of heterogeneous melting and homogenous melting are also provided.

Poly(ethylene terephthalate) Powder-A Versatile Material for Additive Manufacturing.

The 3D printing of articles by the effect of a directed laser beam on a plastic powder is a demanding process, and unlike injection molding, very few polymers work well enough with it. Recently, we reported that poly(ethylene terephthalate) (PET) powder has intrinsically good properties for 3D printing. Basic mechanical properties were shown earlier and it was demonstrated that unfused but heat-exposed PET powder does not degrade quickly allowing good re-use potential. In this work, we conducted a detailed comparison of the mechanical properties of PET and polyamide 12 from different build orientations. PET powders with two different molecular weights were used. With the high molecular weight powder, the processing parameters were optimized, and the printed bars showed little difference between the different orientations, which means there is low anisotropy in mechanical properties of built parts. Based on processing experience of the first powder, the second powder with a lower molecular weight was also very printable and complex parts were made with ease from the initial printing trials; since the process parameters were not optimized then, lower mechanical properties were obtained. While the intrinsic material properties of PET (melting and re-crystallization kinetics) are not the best for injection molding, PET is eminently suitable for powder bed fusion.