High Supercooling Research Articles

Crystallization and Component Properties of Polyamide 12 at Processing‐Relevant Cooling Conditions

For semicrystalline thermoplastics, the temperature‐time behavior during cooling of the melt can significantly affect the formation of crystalline structures (e.g., spherulite sizes, the degree of crystallization, crystal modifications, etc.) and hence, the resulting global component properties. In this paper, the crystallization of polyamide 12 at different cooling rates as well as process‐based temperature‐time profiles during cooling were investigated analytically with fast scanning calorimetry. Furthermore, based on analytical results, foils were extruded at different cooling conditions and tested mechanically and tribologically. The analysis reveals differences of the crystal melting behavior for different cooling conditions which is likely to originate in two different polymorphs of polyamide 12, γ, and γ′. Furthermore, mechanical investigations carried out with the extruded foils show that parts produced at high supercooling have less stiffness than parts produced at low supercooling, probably due to differences in the crystalline structures. Regarding tribological parameters, no clear differences could be measured. POLYM. ENG. SCI., 57:450–457, 2017. © 2016 Society of Plastics Engineers

Supercooling of Hydrogen on Template Materials to Deterministically Seed Ignition-Quality Solid Fuel Layers

We explored templating effects of various materials for hydrogen (H2 and D2) solidification by measuring the degree of supercooling required for liquid hydrogen to solidify below each triple point. The results show high supercooling (>100 mK) for most metallic, covalent, and ionic solids, and low supercooling (<100 mK) for van der Waals (vdW) solids. We attribute the low supercooling of vdW solids to the weak interaction of the substrate and hydrogen. Highly ordered pyrolytic graphite showed the lowest supercooling among materials that are solid at room temperature, but did not exhibit a templating effect within a fill-tube and capsule assembly.

Overwintering biology and limits of cold tolerance in larvae of pistachio twig borer, Kermania pistaciella.

Pistachio twig borer, Kermania pistaciella is an important pest of pistachio trees. It has an univoltine life-cycle and its larvae tunnel and feed inside pistachio twigs for almost 10 months each year. The last larval instars overwinter inside the twigs. Survival/mortality associated with low temperatures during overwintering stage is currently unknown. We found that overwintering larvae of the Rafsanjan (Iran) population of K. pistaciella rely on maintaining a stably high supercooling capacity throughout the cold season. Their supercooling points (SCPs) ranged between -19.4 and -22.7°C from October to February. Larvae were able to survive 24 h exposures to -15°C anytime during the cold season. During December and January, larvae were undergoing quiescence type of dormancy caused probably by low ambient temperatures and/or changes in host tree physiology (tree dormancy). Larvae attain highest cold tolerance (high survival at -20°C) during dormancy, which offers them sufficient protection against geographically and ecologically relevant cold spells. High cold tolerance during dormancy was not associated with accumulation of any low-molecular mass cryoprotective substances. The SCP sets the limit of cold tolerance in pistachio twig borer, meaning that high mortality of overwintering populations can be expected only in the regions or years where or when the temperatures fall below the average larval SCP (i.e., below -20°C). Partial mortality can be expected also when temperatures repeatedly drop close to the SCP on a diurnal basis.

Kinetics of isothermal and non-isothermal crystallization of poly(vinylidene fluoride) by fast scanning calorimetry

Crystallization from melt of poly(vinylidene fluoride) was studied by thin film chip calorimetry at cooling rates from 500 to 100,000 Ks−1 and isothermally down to 76 °C. At ca. 70 °C, for cooling rates higher than 2000 Ks−1, there appears a change in crystallization from high temperature α phase to low temperature β phase. The amorphous state is preserved at cooling rate 100,000 Ks−1. Analysis of the crystallization kinetics with Ziabicki model reveals maximum of the steady-state crystallization rate of β phase as 2200 s−1 at 22 °C, and the highest crystallization rate of α phase as 200 s−1 at 70 °C. Approximation of the temperature dependent steady-state crystallization rate with the Turnbull and Fisher nucleation model results in the equilibrium melting temperatures 227 and 173 °C for the α and β phase, respectively, and in the energy barrier for short-distance transport, ED, as 70–80 kJ mol−1 at high supercooling.

Atomic mobility in the overheated amorphous GeTe compound for phase change memories

AbstractAbstractauthoren Phase change memories rest on the ability of some chalcogenide alloys to undergo a fast and reversible transition between the crystalline and amorphous phases upon Joule heating. The fast crystallization is due to a high nucleation rate and a large crystal growth velocity which are actually possible thanks to the fragility of the supercooled liquid that allows for the persistence of a high atomic mobility at high supercooling where the thermodynamical driving force for crystallization is also high. Since crystallization in the devices occurs by rapidly heating the amorphous phase, hysteretic effects might arise with a different diffusion coefficient and viscosity on heating than on cooling. In this work, we have quantified these hysteretic effects in the phase change compound GeTe by means of molecular dynamics simulations. The atomic mobility in the overheated amorphous phase is lower than in supercooled liquid at the same temperature and the viscosity is consequently higher. Still, the simulations of the overheated amorphous phase reveal a breakdown of the Stokes–Einstein relation between the diffusion coefficient and the viscosity, similarly to what we found previously in the supercooled liquid. Evidences are provided that the breakdown is due to the emergence of dynamical heterogeneities at high supercooling.

Two processes of α-phase formation in polypropylene at high supercooling

The non-isothermal and isothermal crystallization of different types of commercial polypropylenes (PP) are investigated by fast scanning calorimetry and wide-angle X-ray scattering. Typically, PP shows two different crystallization processes: below 55°C the mesophase formation and at higher temperatures the α-phase formation. Here we report the first observation of an additional crystallization process in some PP-types. It is observed between 45°C and 75°C, and is understood as a consequence of an additional nucleation of α-phase crystals.

Behavior of Surface-Functionalized Multiwall Carbon Nanotube Nanofluids during Phase Change from Liquid Water to Solid Ice

Multiwall carbon nanotube (MWCNT) nanofluids have been shown to enhance the crystallization process of water to ice. While the beneficial effects of MWCNTs on phase change processes are well-documented, little work has been conducted to investigate the behavior of MWCNTs during and after exposure to freezing conditions. In this work, the crystallization morphology of water droplets containing surface-functionalized hydrophilic MWCNTs was evaluated at three driving force temperatures and two concentrations of nanofluid. At low supercoolings, the MWCNTs are completely expelled from the crystal matrix due to slow solidification rates. At high supercoolings, the MWCNTs are embedded in the solid droplet within air volumes and interdendritic regions as a result of rapid crystallization speeds. The results show that the dispersion of MWCNTs within the solid ice matrix itself was not achieved at these levels of supercooling. Under all conditions, freezing of the colloidal system results in destabilization of the ...

Simultaneous Synchrotron WAXD and Fast Scanning (Chip) Calorimetry: On the (Isothermal) Crystallization of HDPE and PA11 at High Supercoolings and Cooling Rates up to 200 °C s(-1).

An experimental setup, making use of a Flash DSC 1 prototype, is presented in which materials can be studied simultaneously by fast scanning calorimetry (FSC) and synchrotron wide angle X-ray diffraction (WAXD). Accumulation of multiple, identical measurements results in high quality, millisecond WAXD patterns. Patterns at every degree during the crystallization and melting of high density polyethylene at FSC typical scanning rates from 20 up to 200 °C s(-1) are discussed in terms of the temperature and scanning rate dependent material crystallinities and crystal densities. Interestingly, the combined approach reveals FSC thermal lag issues, for which can be corrected. For polyamide 11, isothermal solidification at high supercooling yields a mesomorphic phase in less than a second, whereas at very low supercooling crystals are obtained. At intermediate supercooling, mixtures of mesomorphic and crystalline material are generated at a ratio proportional to the supercooling. This ratio is constant over the isothermal solidification time.

The experimental exploration of sodium chloride solution on thermal behavior of phase change materials

Phase change materials (PCMs) are the effective substances for thermal energy storage. Unfortunately, various problems such as high supercooling degree, low crystal growth rate and poor thermal conductivity greatly hinder the large-scale utilization of PCMs. The present study focuses on improving the crystallization and decreasing supercooling degree by adding various proportions of NaCl/NaCl solutions into the n-octadecane-based PCMs for thermal energy storage. The experimental results show that 20wt% NaCl solutions have the greatest effect on the thermal performance of PCMs. The supercooling degree has been minimized up to 6°C with the addition of NaCl. It can not only promote crystallization under 1wt% addition rate, but also enhance latent heat storage performance. Such observations have been verified by the kinetics of crystallization. The researches on supercooling could advance the application of PCMs on solar energy.

Towards modelling of initial and final stages of supercooled water solidification

It is well known from experimental studies (e.g., Jung et al., 2012, [13]), that solidification of supercooled water occurs in two stages: the initial rapid, recalescent stage of crystallization and the final slower, quasi-isothermal stage. To date, it is not well understood how these stages can be modelled. In the present study it is demonstrated that both freezing stages of supercooled water can be mathematically modelled by utilizing exclusively the minimal (pertinent to the heat-diffusion description) model of solidification: the first unstable stage is physically well described by the dendritic-growth-approach, whereas the second stage of the freezing process is qualitatively and quantitatively defined by a stable planar solidification. The computational model, introduced initially in [7,31], has been appropriately upgraded with respect to the high-fidelity discretization of the thermal-energy equation as well as the normal-to-the-interface temperature gradient determination. At supercooling degrees ΔT higher than ≈5 K (corresponding to high supercooling range), the previously obtained numerical results related to the unstable freezing exhibit a distinct deviation from available experimental data. Possible reasons for this departure can be either the non-consideration of the thermal interaction between the growing needle-like dendrites or neglect of the kinetic effects influencing the growth at such high supercooling conditions. Two-dimensional computations concerning the growth of a needle-like dendrite surrounded by an array of needles indicate that they do not considerably influence the steady-state tip velocity of an isolated needle at higher supercooling. Therefore, the thermal energy equation has been extended through an appropriately derived term mimicking the kinetic undercooling phenomenon. The relevant computational validation results in a very good qualitative and quantitative agreement with experiments in the high-supercooling range as well, illustrating the model's predictive capability of describing both stages of crystallization: the first rapid, dendritic growth (pertinent to unstable freezing) and the second planar growth stage (corresponding to the stable freezing).

The Gibbs free energy difference between a supercooled melt and the crystalline phase of polymers

The temperature dependence of Gibbs free energy difference between the supercooled liquid and the crystalline phase, ΔGc, plays a fundamental role for the description of crystallization processes especially at high supercooling as occurring with many technical processes. In the literature, many different approximations for ΔGc(T) can be found. A test of these models with polymer data from the ATHAS database shows that none of them can be used as a general model. We present a new model for ΔGc(T) of polymers, which was successfully tested on ten different polymers. The result indicates that our approach might be generally valid for all polymers in the temperature range between the equilibrium melting temperature and the glass transition temperature.

Temperature Dependency of Nucleation Efficiency of Carbon Nanotubes in PET and PBT

The crystallization behavior of polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) filled with different amount of multi-walled carbon nanotubes (MWCNT) has been experimentally determined in the entire temperature range between glass transition and melting temperature. Differential fast scanning chip calorimetry, realizing cooling and heating rates of several 1 000 K · s−1, have been applied under isothermal and non-isothermal conditions. The MWCNTs act as heterogeneous nuclei for both polymers. Increasing filler content speeds up crystallization at low supercooling, where heterogeneous nucleation dominates, for more than one order of magnitude. At high supercooling (crystallization temperature below 120 °C for PET and 70 °C for PBT) crystallization kinetics is independent on MWCNT concentration. Here crystallization via homogeneous nucleation is always determining the overall crystallization kinetics and addition of nucleating agents is not effective anymore. Depending on the processing conditions these observations may be important for structure formation in PET and PBT composites.

Density of heterogeneous and homogeneous crystal nuclei in poly (butylene terephthalate)

Quantitative analysis of the nucleation density of poly (butylene terephthalate) using microscopy yielded values of 106 and 1015 nuclei mm−3 for the cases of heterogeneous and homogenous nucleation on crystallization at low and high supercooling of the melt, respectively. Fast scanning chip calorimetry revealed that the largely different nucleation densities cause two crystallization-rate maxima at 130 and 70°C, with characteristic minimum crystallization times of the high- and low-temperature crystallization processes being about 1 and 0.1s, respectively. It has furthermore been excluded by X-ray scattering that the low-temperature crystallization-rate maximum is due to the formation of a different crystal polymorph; independent of the supercooling there is observed formation of α-crystals. The crystallization study has been completed by analysis of the non-isothermal crystallization behavior, identifying a minimum critical cooling rate of about 10Ks−1 to avoid completion of the high-temperature crystallization process and to initiate crystallization at low temperature. For complete vitrification, the PBT melt needs to be cooled faster than 200Ks−1.

Structural Features of Electrodeposited Metals as a Result of Ultra-Rapid Solidification of a Highly Supercooled Liquid Metal Phase

With a view to validating the existence of the phenomenon of phase formation via a liquid state stage in metals being electrodeposited, experiments were carried out to reveal electrodeposited metal's structural features typical of metals produced by solidification at ultra-high rates under high supercooling, and to establish consistent changes in metal structural characteristics with increasing degree of supercooling in electrodeposition. Following experimental facts in favour of the existence of the phenomenon in point were found: (1) occurrence of spherulites and pentagonal quasicrystals in electrodeposit layers adjoining the cathode, the occurrence being typical of metals produced by ultra-rapid solidification of a highly supercooled liquid metal phase; (2) emergence of the electrodeposited metals solely in a spherulitic form when transition of spherulitic growth to drusy growth with increasing deposit thickness was prevented; and (3) regular change in characteristics of point, linear and planar defects in the crystalline lattice with increasing degree of supercooling in electrodeposition of metals.

Crystallization kinetics of polyamide 66 at processing-relevant cooling conditions and high supercooling

Processing of polyamide 66 (PA 66) by injection molding includes cooling of the melt at rates between 100 and 103K/s and its solidification at high supercooling. The kinetics of crystallization at such conditions is unknown and has been evaluated in this work using fast scanning chip calorimetry. Slow cooling of the melt of PA 66 leads to formation of crystals, with the maximum crystallinity being about 30%. It has been found by analysis of the crystallinity as a function of the cooling rate that crystallization is suppressed on cooling faster than about 300K/s. Isothermal analysis of the crystallization rate revealed a bimodal temperature dependence, with maxima obtained at about 165 and 110°C. It is suggested that the observation of two distinct crystallization-rate maxima is related to a change of the nucleation mechanism on temperature variation or the formation of different crystal polymorphs exhibiting different growth rate. The findings provide a fundamental step towards accurately predicting the solidification behavior, the skin–core morphology, and properties of injection moldings of PA 66.

Phase-change characteristics and thermal performance of form-stable n-alkanes/silica composite phase change materials fabricated by sodium silicate precursor

A series of n-alkanes/silica composites as form-stable phase change materials (PCMs) were synthesized in a sol–gel process using sodium silicate precursor. The chemical compositions and structures of the synthesized composites were characterized by Fourier transform infrared spectroscopy. Scanning electric micrographs show an irregularly spherical morphology of the n-alkanes/silica composites, and transmission electric micrographs confirm that the n-alkanes have been well encapsulated by silica. These n-alkanes/silica composites keep a good sharp stability due to the support of silica wall even if the n-alkanes are in molten state. The differential scanning calorimetric analysis indicates that the phase change behaviors and characteristics of the n-alkanes/silica composites strongly depend on the carbon atom number in n-alkanes, and meanwhile, the encapsulated n-alkanes have a high thermal storage capability. The investigation on thermal performance demonstrated that the n-alkanes/silica composites achieved a high thermal conductivity, low supercooling, and good work reliability as a result of the encapsulation of n-alkanes with highly thermal conductive inorganic silica. Moreover, the thermal stability of the composites was also improved due to the protection of silica wall toward the encapsulated n-alkanes. It is anticipative that, owing to the easy availability and low cost of sodium silicate, the synthetic technology developed by this work has a high feasibility in the industrial manufacture of the form-stable PCMs.

In situ synchrotron radiation X-ray scattering study on the effect of a stearic sucrose ester on polymorphic behavior of a new sunflower oil variety

The effect of the stearic sucrose ester (SE) S-170 on crystallization behavior and polymorphism of two stearins obtained from a new variety of high stearic high oleic sunflower oil was studied by pulsed nuclear magnetic resonance (p-NMR), small (SAXS) and wide (WAXS) angle X-ray scattering using synchrotron light and differential scanning calorimetry (DSC). p-NMR studies showed that there is always a crystallization temperature below which SE S-170 accelerated crystallization and above which SE S-170 delayed nucleation and growth. The effect of SE S-170 strongly depended on supercooling. It was efficient as a seed for high supercooling (low crystallization temperatures) but this efficiency diminished at low supercooling (temperatures close to the melting point) when few crystals were formed. SAXS and WAXS results demonstrated that depending on crystallization temperature SE S-170 promoted crystallization of α and β forms with more polymorphic similarity and inhibited occurrence of β′ forms especially the β′2 polymorph. However, in some conditions SE S-170 favored crystallization of β′1 polymorph. DSC experiments showed that SE S-170 significantly diminished total melting enthalpies when the effect was a delay in crystallization. For other conditions no significant differences were found in melting temperatures or total melting enthalpies. When stearins were stored at 25°C, crystallization in the β2 form was promoted. Depending on crystallization temperature, polymorphic forms β′1 and β2 may be obtained as the main polymorphic forms. This is very relevant from the technological point of view. Depending on the application, SE S-170 may help obtain the required polymorphic form: β′1 form for spreads and β2 polymorph for chocolate production.

Effect of High Heat Input on Toughness and Microstructure of Coarse Grain Heat Affected Zone in Zr Bearing Low Carbon Steel

Microstructure evolution and impact toughness of simulated coarse grained heat affected zone (CGHAZ) in Zr bearing low carbon steel have been investigated in this study. Thermal simulator was used to simulate microstructure evolution of CGHAZ with high heat input welding thermal cycle at 1400°C peak temperature. Microstructure of CGHAZ consisted of high volume fraction of AF inside grain and GBF at prior austenite grain boundaries. Prior austenite grain size of CGHAZ increases with heat input increasing. Excellent impact toughness (more than 100 J) of CGHAZ with heat input of 1000 kJ/cm was obtained in this experiment. Impact toughness of CGHAZ with 400 kJ/cm (230 J) is the highest, because austenite grain size of CGHAZ with 400 kJ/cm favors the development of AF inside grain. Impact toughness is not only related with high angle boundaries but also with effective grain size. High supercooling of CGHAZ provided driving force for the AF transformation during welding thermal cycle, increasing the number of AF.

Kinetics of nucleation and crystallization of poly(ε-caprolactone) – Multiwalled carbon nanotube composites

The nucleation efficiency of multiwall carbon nanotubes (MWCNT) in poly(ε-caprolactone) (PCL), as an example, was tested for a wide range of temperatures and cooling rates and compared to the efficiency of homogeneously formed nuclei. The temperature range below the maximum of crystallization rate is generally not accessible for non-isothermal cooling experiments because the sample becomes amorphous at the needed cooling rates. Isothermal experiments after fast quenches extend the temperature range down to and below the glass transition. The employed differential fast scanning calorimeter (DFSC) allows cooling at rates up to 100,000K/s and precise adjustment and control of isothermal conditions in the time range from 10−4 to 104s and longer. As shown in previous work, heterogeneous crystal nucleation dominates at low supercooling, revealing a significant dependence of crystallization rate on MWCNT concentration. Nevertheless, no saturation of the nucleation activity at a MWCNT loading of 0.2–0.5wt% as seen in slow DSC experiments was observed at the much higher cooling rates employed here. At high supercooling, where homogeneous nucleation is prevalent, the addition of MWCNT neither enhances nor reduces the crystallization rate. At the temperature of maximum homogeneous nucleation rate, formation of homogeneous nuclei always dominates crystallization.

Influence of processing conditions on polymer crystallization measured by fast scanning DSC

The structural formation of polymers during processing significantly influences the mechanical properties and the temperature stability of polymer products. The analysis of structural formation by conventional thermal analysis techniques is limited because of the relatively low scanning rates. Thus, reorganization during heating changes the initial structure, and the applicable cooling rates are not representative for the applied cooling rates during production, i.e., crystallization at high supercooling cannot be investigated. To overcome these limitations, chip calorimeters with very high scanning rates have been developed. The fast scanning Flash DSC 1 based on MEMS chip-sensors allows for scanning rates up to 40,000 K s−1. In this paper, we discuss some basic concepts of chip calorimetry in general. We then study the influence of additives and molecular modifications on the structural formation at technically relevant cooling rates. This information is crucial to adapt polymer formulation and processing conditions to specific product requirements.