Melting Kinetics Research Articles

Melting in Microgravity

The Isothermal Dendritic Growth Experiment constituted a series of three NASA-supported microgravity experiments (USMP-2, -3, and -4), which flew aboard the space shuttle Columbia. These space flight experiments grew and recorded dendrites in the absence of gravity-induced convective heat transfer. USMP-4, for the first time, allowed streaming of near-real-time video data. Using 30-fps video data, we studied both freezing and melting sequences for pivalic acid (PVA) at different supercoolings. We report on the melting process of a PVA dendritic mushy zone, observed for the first time under convection-free conditions. Conduction-limited melting processes are of importance in orbital melting of materials, meteoritic genesis, mushy-zone evolution, and in fusion weld pools where the length scales for thermal buoyancy are highly restricted. Microgravity video data show that PVA dendrites melt into fragments that shrink at accelerating rates to extinction. The melting paths of individual fragments follow a characteristic time dependence for the diminishing length scales within the mushy zone. The theoretical melting kinetics against which the experimental observations are compared is based on the conduction-limited quasi-static process of melting under shape-preserving conditions. Good agreement between theory and experiment was found for the melting of a selected needle-shaped prolate spheroidal PVA crystal fragment with an aspect ratio of C/A = 12.

High-Speed Calorimetry for the Study of the Kinetics of (De)vitrification, Crystallization, and Melting of Macromolecules

This paper reports on the characteristics and use of a new mode of measurement: High Performance DSC (HPer DSC), which represents a major step forward in high-speed calorimetry, as compared to standard DSC. It facilitates the study of the kinetics and metastability of macromolecular systems, especially the analysis of rate-dependent phenomena in real time. Controlled and constant scan rates at hundreds of degrees per minute and combinations thereof both in cooling and in heating are possible. Heats of transition, heat capacities, temperature-dependent crystallinities, etc. can be established at the extreme rates applied. Examples of the utilization of HPer DSC are given for polymers with respect to the effective hindrance of crystallization and cold crystallization, avoidance of recrystallization and the rate dependency of vitrification and devitrification. Low, milligram-scale sample masses, even down to the microgram level, are utilized. The short measuring times also provide the high throughput needed in combinatorial chemistry.

Isothermal and nonisothermal melt‐crystallization kinetics of syndiotactic polystyrene

AbstractAnalyses of the isothermal and nonisothermal melt kinetics for syndiotactic polystyrene have been performed with differential scanning calorimetry, and several kinetic analyses have been used to describe the crystallization process. The regime II→III transition, at a crystallization temperature of 239°, is found. The values of the nucleation parameter Kg for regimes II and III are estimated. The lateral‐surface free energy, σ = 3.24 erg cm−2, the fold‐surface free energy, σe = 52.3 ± 4.2 erg cm−2, and the average work of chain folding, q = 4.49 ± 0.38 kcal/mol, are determined with the (040) plane assumed to be the growth plane. The observed crystallization characteristics of syndiotactic polystyrene are compared with those of isotactic polystyrene. The activation energies of isothermal and nonisothermal melt crystallization are determined to be ΔE = −830.7 kJ/mol and ΔE = −315.9 kJ/mol, respectively. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2528–2538, 2002

Influence of Copper on Decarburization Kinetics of Stainless Steel Melt at High Temperatures.

A kinetic study on the decarburization reaction of the Fe-Cr-Cu-C melts has been carried out at 1 873 K focusing on the effect of copper on the reaction rates. The decarburization rate of the Fe-C melt was preliminarily measured at 1 873 K. The result of the present work exhibits a good agreement with other studies. The addition of chromium to the Fe-C melt decreases the interfacial reaction rate. It is considered that the effect of chromium on a retard of the interfacial reaction would have a relationship to the surface adsorption of chromium. The addition of copper to the Fe-Cr-C melt decreases the decarburization reaction rate. The delay of the interfacial reaction by copper at high concentrations of carbon could be responsible for the decrease in the overall reaction rate. The effect of copper on the reaction rate could be quantified by the following equation: k Fe-Cr-Cu =kFeCr/1+0.09[wt%Cr] From the results of the dependence of the interfacial rates on temperature for each alloy, it could be proposed that the presence of chromium on the reaction surface in the Fe-Cr-C melt would delay the reaction rate, which was originated from the additional reaction steps. However, it is suggested that copper would delay the decarburization rate by blocking a part of the reaction sites in the Fe-Cr-Cu-C melt.

Superheating of the melting kinetics in polymer crystals: a possible nucleation mechanism

Melting kinetics of polymer crystals has been examined experimentally by calorimetric methods utilizing the combination of a conventional differential scanning calorimetry of heat flux type (CDSC-HF) and a temperature-modulated DSC (TMDSC). The superheating effect in the kinetics has been discussed based on a modeling of the melting kinetics. For low-density polyethylene and linear polyethylene, the melting rate showed nearly linear dependence on the degree of superheating, which indicates the kinetics controlled by heat diffusion or by surface kinetics on rough interface. For isotactic polypropylene, poly(ethylene terephthalate) and poly(ϵ-caprolactone), the dependence is non-linear and close to the limiting case of exponential dependence, which indicates nucleation-controlled kinetics of melting. A possible mechanism of the activation process in the melting kinetics has been discussed in consideration of the specific feature of polymer crystals far from its most stable state. The consistency of the results of CDSC-HF and TMDSC has been confirmed by this analysis with a calibration of peak temperature for the instrumental thermal delay in CDSC-HF.

Kinetics of reversible surface crystallization and melting in poly(ethylene oxide): Effect of crystal thickness observed in the dynamic heat capacity

Poly(ethylene oxide) (PEO) in the semi-crystalline state shows a reversible surface crystallization and melting; a temperature decrease leads to a certain crystal thickening, a temperature increase reversely to an expansion of the amorphous intercrystallite layers. Dynamic calorimetry provides a means to investigate the kinetics of the process. The structural rearrangement in the region of the crystalline-amorphous interface can only be accomplished if the chains can slide through the crystallites. One therefore expects the associated time to change with the crystallite thickness. Variations of the crystal thickness of PEO can be achieved by choosing different crystallization temperatures. We studied the effect of the crystal thickness employing temperature-modulated differential scanning calorimetry and heat wave spectroscopy, and by carrying out small-angle X-ray scattering experiments for the structural characterization. The effect of the crystal thickness is clearly observed. Results indicate that the sliding diffusion through the crystallites takes place by helical jumps of whole stems. Data yield the activation energy per unit length of the stems.

The effect of a magnetic field on the kinetics of melting of ionic crystals

The kinetics of melting of an ionic crystal treated in a pulsed magnetic field (H ∼ 106 A/m) was studied by a computer-aided DTA technique. Significant changes in the nonequilibrium thermodynamic parameters of transient processes during melting and in the enthalpy of melting were observed for KCl crystals, which is evidence that the magnetic field induces a special nonequilibrium state in these nonmagnetic crystals.

Effect of chromium on the kinetics of the contact melting and the graphite-to-diamond transformation in the Co–Fe–C system

We have studied the effect of chromium concentration in alloys of the Co–Fe system on their interaction with graphite at p, T parameters of thermodynamic stability of diamond. It has been found that addition of chromium to the alloys stabilizes the Me 3C-type carbide, as a result, the structure of a layer of a contact melting at the alloy–graphite interface is identical to a horizontal section of the metastable phase diagram of the Fe–C system. It is shown that addition of chromium to the Co–Fe–C system lowers the eutectic melting temperature by 80–100 K. In this case, the coefficient of carbon diffusion in the melt increases by ∼20–30%. An increased surface activity of a Cr-containing melt with respect to graphite is noted, which is the reason of intensive intrusion of the melt deep into a graphite layer along the grain boundaries. As a result, the number of nucleating diamond crystals and the degree of the graphite→diamond transformation increases.

Evidence for carbon monoxide binding to sickle cell polymers during melting

The melting of sickle cell hemoglobin (HbS) polymers was induced by rapid dilution using a stopped-flow apparatus. The kinetics of polymer melting were monitored using light scattering. Polymer melting in the absence of any hemoglobin ligand was compared to melting when the diluting buffer was saturated with carbon monoxide (CO). In this way the role of CO in polymer melting could be assessed. The data were analyzed using models that assumed that melting occurs only at the ends of polymers. It was further assumed that CO could only bind to HbS in the solution phase. However, our data could not be fitted to this model, where CO cannot bind directly to the polymer. Thus, CO probably binds directly to the polymers during our melting experiments. This result is discussed in terms of oxygen induced polymer melting and polymerization processes in sickle cell disease

Thermodynamics and kinetics of shear-induced melting of a thin layer of lubricant confined between solids

Using Landau’s phenomenological theory of phase transitions, the shear-induced melting of a thin layer of substance confined between two crystalline surfaces is considered. The kinetics of melting and solidification is considered for static and alternating loads. The possibility of two consecutive “melting” (solid-to-liquid) transitions is discussed. As a result of the first transition, modulation of the microscopic density becomes zero only in the direction of shear (partial melting) and as a result of the second, it also disappears in the normal direction (complete melting).

Force-induced melting of a short DNA double helix.

The dynamic behaviour of DNA is of fundamental importance to many cellular processes. One principal characteristic, central to transcription and replication, is the ability of the duplex to "melt". It has recently been shown that dynamic force spectroscopy provides information about the energetics of biomolecular dissociation. We have employed this technique to investigate the unbinding of single dodecanucleotide molecules. To separate the duplex to single-stranded DNA, forces ranging from 17 to 40 pN were required over a range of loading rates. Interpretation of the dependence of melting force on loading rate revealed that the energy barrier to rupture is between 9 and 13 kcal mol-1 in height and situated 0.58 nm from an intermediate structural state. Thermal melting studies show that, prior to dissociation, the oligonucleotide underwent a transition which required between 7 and 11 kcal mol-1 in energy. Through combined dynamic force spectroscopy and thermal melting studies we show the derivation of an energy landscape to dissociate a 12-mer duplex. Until very recently, this type of information was only accessible by computational analysis. Additionally, the force spectroscopy data allow an estimation of the kinetics of duplex formation and melting.

A modeling of the irreversible melting kinetics of polymer crystals responding to temperature modulation with retardation of melting rate coefficient

An extension of the modeling proposed previously has been examined for the irreversible melting kinetics of polymer crystals on heating with response to temperature modulation. The previous modeling has been successful in the explanation of the frequency dependence of the apparent heat capacity obtained with temperature modulated DSC in the melting region of poly(ethylene terephthalate), polyethylene and poly(caprolactam). In the present work this modeling was extended to explain an unusual behavior reported by Schawe et al. for poly(ϵ-caprolactone) and syndiotactic polypropylene, in which the latent heat gave a substractive effect to the real part of the apparent heat capacity. A retardation of the melting rate coefficient in response to temperature change has been considered. The retardation implies an activation process in the melting kinetics of polymer crystals.

Periodically Modulated Driving Force Applied with TMDSC to the Crystallization and Melting Kinetics of Ice Crystals Confined in a Porous Silica Gel

The application of a periodically modulated driving force has been examined in the melting and crystallization kinetics of ice crystals confined in a porous media. The kinetic response of transformation gives the real and imaginary parts of the ‘apparent’ heat capacity obtained with a temperature modulated differential scanning calorimetry (TMDSC). Based on a modelling of the kinetics, the detailed examination of the frequency dispersion and its dependence on underlying heating/cooling rate enables us to evaluate the transformation rate and the dependence of the rate coefficient on the driving force, i.e. the degree of supercooling or superheating. The experimental results indicate that the transformation processes are limited by heat diffusion from the growth interface of each crystallite to surroundings.

Investigation of mass transfer in the complex system Te-Tl by the contact-melting method

An investigation of the kinetics of contact melting of the complex system Te—Tl under unsteady diffusion conditions is made. The growth of the contact interlayer obeys a parabolic law. The interdiffusion coefficient \(\tilde D\) is calculated in the temperature range 260–300°C. It has been found that \(\tilde D\) virtually does not change within the temperature range 260–280°C. The effect of decrease in the temperature of contact melting relative to the equilibrium eutectic is established. A relationship between the superheating of a melt of the stable eutectic and the magnitude of its supercooling is found.

Infiltration kinetics model of liquid metal into a fibrous preform in centrifugal accelerating field

The infiltration kinetics of the metal melt into a fibrous preform in centrifugal accelerating field is analyzed on the basis of Da rcy's law and the assumption that the fibrous preform is treated as “bundle of capillaries”. The critical rotating speed is analyzed with the established mo del. The influences of the metal melt mass,the rotating speed of the equipmen t,the casting height, the original outer radius of the metal melt and the fibrou s volume fraction in fibrous preform on infilatration are studied. The results show that the critical rotating speed is dependent on critical pressure, castin g height, metal melt mass and the character of fibrous preform. With the incr ease in the metal melt mass, rotating speed of the equipment and original outer radius of the metal melt, or the decrease in casting height and fibrous volume f raction in fibrous of the metal melt,or the decrease in casting height and fibro us volume fraction in fibrous preform,infiltration of metal melt for fibrous pre form becomes easier.

Melting and superheating of low-dimensional materials

The state-of-art of melting and superheating for low-dimensional materials is reviewed. Irregular size dependence of melting kinetics was found for ultrafine particles. Substantial melting point elevation (superheating) was observed in both nanoparticles and thin films with an epitaxial confinement. These findings might significantly advance our understanding of the nature of melting of solids providing the relevant theoretical and computer simulation investigations are stimulated.

Laser Thermal Induced Crystallization for 20 nm Device Structures

ABSTRACTThe melt kinetics of shallow junction formation by laser thermal processes has been studied using transient conductance measurements. The melt and solidification dynamics of 20 nm amorphous layers were measured and shown to follow behaviors predicated by deeper melts, including explosive crystallization and interface bounce back. The effects of surface barrier oxides and metal absorber layers, required for CMOS process integration, were examined and shown to be nearly negligible. Quantitative evaluation of a device process window by these measurements was in good agreement with sheet resistance results. Finally, the effect of the buried oxide in SOI structures was investigated. Solidification velocities in such structures were reduced by a factor of three as compared with bulk silicon.

Critical Autoignition Conditions for Heterogeneous Condensed Systems in the Presence of Phase Transformations

The critical ignition conditions for a single particle which interacts with a melting medium to form an intermetallic compound on its surface were investigated using the model of a quasistationary melting front. For linear and parabolic drag laws, three characteristic regions, which transform with change of Biot's criterion, are distinguished on the diagram of critical parameters. It is shown that for small values of this criterion, the conditions of self‐heating of the particle are identical to the ignition conditions in a medium with constant temperature. For values of Biot's criterion greater than unity, the critical conditions are substantially affected by the parameters determining the melting kinetics. The characteristic features of self‐heating of a single particle can determine the qualitative pattern of self‐heating of the heterogeneous system as a whole.

Reversing melting and crystallization of indium as a function of temperature modulation

The melting and crystallization of indium has been reinvestigated with the heater-controlled Mettler–Toledo differential scanning calorimeter DSC 820. The heat-flow rate during melting and crystallization is largely determined by instrumental characteristics and often does not permit the evaluation of kinetic parameters of the phase transition. We found that the observed reversing character of the melting depends strongly on the modulation parameters. For quasi-isothermal, temperature-modulated calorimetry with a symmetric sawtooth, the degree of reversibility decreases with increasing amplitude of temperature modulation. At a lower average temperature of modulation relative to the melting temperature, the partial melting process can be fully reversing, while at higher average temperatures, the melting becomes less reversing due to instrumental time restrictions for the completion of melting. With an appropriately modified, nonsymmetric sawtooth, however, the phase transition can be made fully reversing, even in the later stages of the melting process (as long as crystal nuclei remain). The influence of sample mass, sample position within the pan, and type of pan are analyzed, and the conditions for the study of the kinetics of crystallization of nucleated crystals close to the melting temperature, and by implication, also the melting kinetics, are suggested.

N-Dodecane melting studied by the combined use of different calorimetric modes

The solid–liquid phase transition of n-dodecane has been studied with a modulated adiabatic scanning calorimeter (MASC). The well-known features of melting — such as the existence of a pre-melting region and the dependence of the melting curve on the scanning rate — were observed, together with completely new features which, to the best of our knowledge, have never been reported before, such as the presence of a small secondary, non-reproducible peak in the melting curve. This last result was obtained working in the scanning operation mode, at rates lower than 3.3×10 −4 K s −1. The complex melting process occurring in the sample was studied inside the melting temperature interval, using the temperature step scanning and the adiabatic-like enthalpy step operation mode of MASC. A very slow rate of attainment of equilibrium was observed when the liquid and solid phases were co-existing. Temperature modulation, in both the continuous scanning mode and the quasi-isothermal step-scanning mode, was also used to obtain a clearer characterisation of the melting kinetics. The melting transition of n-dodecane is discussed in the light of the results obtained by applying to one and the same sample different operation modes. The transition enthalpy values measured are compared to the data in the literature. The two peaks observed in the melting curve of n-dodecane are analysed and possible explanations for the second peak are discussed.