Small Supercoolings Research Articles

METHOD OF REDUCE SUPERCOOLING PHENOMENON OF PHASE CHANGE MATERIAL

In this research, the author proposed a supercooling phenomenon reduction PCM that suppresses the decrease in latent heat by installing a PCM with a small supercooling degree on the heat transfer surface side of the PCM with a large supercooling degree. As a result, it was shown that the supercooling phenomenon reduction PCM can reduce the supercooling degree by forming and growing solidified crystals even when the temperature is higher than the supercooling release temperature. Furthermore, the calculation results for predicting the cooling and heating characteristics of the supercooling phenomenon reduction PCM were in good agreement with the experimental values.

Paste-like recyclable Ga liquid metal phase change composites loaded with miscible Ga2O3 particles for transient cooling of portable electronics

• Miscible Ga 2 O 3 particles are introduced into Ga liquid metal to tailor thermophysical properties. • Phase change composites show low supercooling and low thermal interfacial resistance. • Paste-like composites are applied for transient cooling of USB flash disks. • Reusability and recyclability of liquid metal composites are demonstrated. With the increasing power density and continuously added functionalities, normal operation of portable electronic devices such as USB flash disks and smart phones calls for facile and efficient thermal management. Ga-based liquid metals have attracted intensive research attention for thermal management applications because of their high thermal conductivity and low toxicity, but as a type of phase change materials they still suffer from large supercooling and leakage issues. Herein, this work reports direct incorporation of high-loading miscible Ga 2 O 3 particles within Ga liquid metals to prepare non-leakage phase change composites for transient heat absorption. By loading 16.7 wt% of Ga 2 O 3 particles, paste-like liquid metal composites with a high thermal conductivity (∼21 W/m·K), a low interfacial thermal resistance (∼3.5 mm 2 ·K·W −1 ), a small supercooling degree (∼8.8 °C) and a large volumetric latent heat (∼308 J/cm 3 ) were obtained. These advantageous features enable application of the prepared liquid metal phase change composites for high-performance temporary heat absorption of intermittent-use electronic devices such as USB flash disks. Moreover, the attached phase change composites can be easily peeled off from electronic devices, and Ga liquid metals and Ga 2 O 3 particles can be readily recycled by immersing the composites within acidic or basic solutions. The facile direct hand-mixing fabrication process, broadly tailorable supercooling degree and form-stability properties, reusability and recyclability would make the prepared liquid metal composites suitable for phase change-based thermal management applications.

Fabrication and characteristics of eutectic hydrated salts/fumed silica composite as form-stable phase change materials for thermal energy storage

In this research, a novel form-stable composite phase change material (PCM) for building envelopes was prepared and evaluated. Calcium chloride hexahydrate (CCH) and magnesium nitrate hexahydrate (MNH) were dissolved and mixed uniformly to obtain the binary eutectic hydrated salt (BEHS) with adjustable phase change temperature. With the increase of MNH content, the phase change temperature and latent heat of BEHS gradually decreased. The BEHS with the onset temperature and peak temperature of 21.26 °C and 27.51 °C and corresponding latent heat of 142.4 J/g was selected as the PCM and combined with the support material SiO 2 to obtain BEHS/SiO 2 composite PCMs，where the minimum SiO 2 content to prevent BEHS from leaking was 25 wt%. The characterization results of SEM, XRD and FT-IR indicated that BEHS was successfully filled into the pore structure of SiO 2 , and there was good compatibility between them. By adding 2.5 wt% strontium chloride hexahydrate as a nucleating agent, the supercooling degree of BEHS/SiO 2 composite PCMs was eliminated. The onset temperature and peak temperature of BEHS/SiO 2 composite PCMs were measured to be 17.85 °C and 27.28 °C, respectively, and the corresponding latent heat was 97.3 J/g. A 300 melting and solidification cycles test proved that it possessed good thermal reliability. Moreover, the numerical simulation was performed to evaluate the effect of BEHS/SiO 2 composite PCMs on the thermal performance and energy performance of the building. The obtained results showed that composite PCMs could effectively reduce room temperature fluctuations and energy consumption. • A new type of eutectic hydrated salts/fumed silica composite PCM is prepared. • The thermal properties of eutectic salts are affected by the content of MNH. • The composite PCM has a small supercooling degree and good thermal reliability. • The composite PCM improves the thermal and energy performance of the building.

Fabrication and thermal properties of CaCl2·6H2O–CO(NH2)2/SiO2 as room-temperature shape-stable composite PCM for building thermal insulation

The combination of room-temperature phase change materials (PCMs) and building materials can reduce the energy consumption of heating equipment and improve indoor thermal comfort. In this paper, the novel CaCl2·6H2O–CO(NH2)2/SiO2 room-temperature shape-stable composite PCM was fabricated by melt-blending method with CaCl2·6H2O as the basic PCM, CO(NH2)2 as the phase change temperature regulator and hydrophilic fumed silica (SiO2) as the carrier. The temperature-regulating effect of CO(NH2)2 on CaCl2·6H2O and the shape-stabilizing effect of SiO2 on CaCl2·6H2O–CO(NH2)2 PCM were emphatically analyzed. The results revealed that the obtained composite PCM with 6% CO(NH2)2 and 20% SiO2 possessed a favorable form-stability, relatively high enthalpy (114.3 J g-1) and appropriate phase change temperature (21.03 °C) which decreased by 7.81 °C compared with pure CaCl2·6H2O (28.84 °C). Furthermore, its small supercooling degree (0.259 °C), low thermal conductivity (0.3667 W·m-1·K-1) as well as the excellent cyclic reliability and thermal stability indicated that the composite PCM has good thermodynamic properties, enabling its application for indoor building thermal insulation and even some other areas.

Droplet re-icing characteristics on a superhydrophobic surface

Water icing is a natural phase change phenomenon which happens frequently in nature and industry and has negative effects on a variety of applications. Deicing is essential for iced surfaces, but even for a nanoengineered superhydrophobic surface, deicing may be incomplete with many adherent unmelted ice droplets which have potential for reicing. Here, we focused on the reicing characteristics of droplets on a solid superhydrophobic surface, which has lacked attention in previous studies. Our results show that the nucleation and ice crystal growth characteristics of a reicing droplet are quite different from those of a first-time icing droplet. During reicing, secondary nucleation due to fluid shear always occurs first on the edges of unmelted ice, accompanied by fast-growing ice crystals that can trigger heterogeneous nucleation when in contact with the solid surface. The reicing takes place under very small supercooling (less than 0.5 °C), and the superhydrophobic surface does not play a key role, meaning that any current icephobic surfaces lose their features, which poses great challenges for anti-icing. In addition, because of the small supercooling, no recalescence phenomenon appears during reicing and the droplet remains transparent instead of clouding. Owing to the unmelted ice floating on the top of the droplet, the droplet shape after reicing is also distinguishing from that after normal icing, but the pointy tip formation during reicing and normal icing shows a uniformity. These results shall deepen the understanding of the anti-icing and deicing physics.

Al/Al2O3 core/shell microencapsulated phase change material for high-temperature applications

The purpose of this study was to prepare an Al/Al2O3 core–shell microencapsulated phase-change material (MEPCM) for a high-temperature thermal energy storage (TES) system. Al (melting temperature: 660 °C) was selected as a raw material for use as a phase change material (PCM). The MEPCM was prepared in two steps: (1) the formation of an AlOOH precursor shell on the PCM microspheres via boehmite treatment in boiled distilled water and (2) heat oxidation treatment in an O2 atmosphere to form a stable Al2O3 shell. The effects of the heat oxidation temperature on the shell morphology, shell structure, heat capacity, and cyclic durability of the prepared MEPCMs were examined. The resultant MEPCM was a core–shell-type structure composed of a stable α-Al2O3 shell and an Al core. The melting temperature of the MEPCMs was close to that of pure Al, and small supercooling of approximately 10 K was observed. Although the increase of the heat oxidation temperature reduced the heat-storage capacity of the MEPCMs (273–301 J g−1) as larger amounts of Al were consumed to form α-Al2O3, the repetition durability of the developed specimens was improved by increasing the heat-treatment temperature. In conclusion, the Al-based MEPCM is a potential candidate for high-temperature TES applications owing to its high melting temperature, high thermal storage capacity, and small degree of supercooling.

The A-B transition in superfluid helium-3 under confinement in a thin slab geometry

The influence of confinement on the phases of superfluid helium-3 is studied using the torsional pendulum method. We focus on the transition between the A and B phases, where the A phase is stabilized by confinement and a spatially modulated stripe phase is predicted at the A–B phase boundary. Here we discuss results from superfluid helium-3 contained in a single 1.08-μm-thick nanofluidic cavity incorporated into a high-precision torsion pendulum, and map the phase diagram between 0.1 and 5.6 bar. We observe only small supercooling of the A phase, in comparison to bulk or when confined in aerogel, with evidence for a non-monotonic pressure dependence. This suggests that an intrinsic B-phase nucleation mechanism operates under confinement. Both the phase diagram and the relative superfluid fraction of the A and B phases, show that strong coupling is present at all pressures, with implications for the stability of the stripe phase.

The microscopic origin of the extreme glass-forming ability of Albite and B2O3

Understanding the conditions that favour crystallisation and vitrification has been a longstanding scientific endeavour. Here we demonstrate that the extremely high glass-forming ability of unseeded supercooled Na2O·Al2O3·6SiO2 (Albite) and B2O3—known for decades as “crystallisation anomaly”—is caused by insufficient crystal nucleation. The predicted temperatures of the maximum homogeneous nucleation rates are located well below their glass transition temperatures (Tg), in a region of very high viscosity, which leads to extremely long nucleation time-lags and low nucleation rates. This behaviour is due to the remarkably small supercoolings where the glass transition occurs for these liquids, which correspond to a very small driving force for crystallisation at and above the Tg, where crystallisation is normally observed. This meagre nucleation ability is caused by the significant difference in the structures of the supercooled liquids and their isochemical crystals. These findings elucidate the cause behind the crystallisation anomaly, and could be used for the design of other oxide glasses that are extremely stable against crystallisation.

Orientation Control and Crystallization in a Soft Confined Phase Separated Block Copolymer

Microdomain orientation and crystallization were examined in a crystalline–amorphous diblock copolymer, hydrogenated poly(high-1,4-butadiene)-b-poly(high-3,4-isoprene) (E/MB), which forms cylinders of the crystallizable block (polyethylene, E). Atomic force microscopy (AFM) was used to locally control the orientation of the E cylinders. The orientation process and subsequent crystallization behavior were investigated in situ as a function of temperature by AFM. Fully confined crystallization was observed within the range 25–50 °C, with templated and breakout crystallization observed at higher crystallization temperature. The growth rate of templated crystallization along and perpendicular to the existing microdomain structure was measured, and the ratio between these rates was found to increase rapidly with decreasing temperature with a change from ∼4.8 at 100 °C to ∼8 at 97 °C. Two maxima in the degree of orientation of the crystallized regions were found: one at relatively small supercoolings (e.g., 95 ...

Polymorph selection and nucleation pathway in the crystallization of Hertzian spheres.

The crystallization process of Hertzian spheres is studied by means of molecular dynamics simulations in an NPT ensemble where the total number of particles N, the pressure P, and the temperature T are kept constant. It has been observed that the bond orientational ordering rather than the translational ordering (density) plays a primary role. The crystal polymorphs are determined by the state points. Under the conditions of small supercooling, the system is likely to be nucleated into crystals that have a preference for the metastable bcc structure, which can be regarded as a manifestation of the Alexander-McTague mechanism. In contrast, small nuclei are found to have a preference for fcc symmetry under conditions of a high degree of supercooling. Prestructured precursors that act as seeds and wet on the nuclei during nucleation always have a high degree of bcc-like ordering, despite different state points. The results above may provide a clue to the understanding of the crystallization process in core-softened particles.

Zinc-Nucleated D2 and H2 Crystal Formation from Their Liquids

Calorimetric measurements at University of Rochester Laboratory for Laser Energetics of D2 crystallization from the melt indicate that zinc can act as a heterogeneous nucleation seed with suppressed supercooling. We further studied this effect for a variety of zinc substrates using the optical-access cryogenic sample cell at Lawrence Livermore National Laboratory. Small supercoolings are observed, some as low as 5 mK, but results depend on the zinc history and sample preparation. In general, thin samples prepared by physical vapor deposition were not effective in nucleating crystal formation. Larger (several-millimeter) granules showed greater supercooling suppression, depending on surface modification and granule size. Surfaces of these granules are morphologically varied and not uniform. Scanning electron microscope images were not able to correlate any particular surface feature with enhanced nucleation. Application of classical nucleation theory to the observed variation of supercooling level with granule size is consistent with nucleation features with sizes <100 nm and with wetting angles of a few degrees.

Fast crystal growth in glass-forming liquids

In liquids of high glass-forming ability, in which crystal growth rates are low, the rates can be measured over the full range of supercooling from the liquidus temperature down to the glass transition. For systems of low glass-forming ability, growth rates are readily measured at small supercooling and at very large supercooling around the glass-transition temperature, but it is difficult to acquire data over the full range of intermediate supercooling, especially at the maximum in growth rate. Data at intermediate supercoolings are however of considerable interest for understanding glass formation in such systems as pure metals and chalcogenides for phase-change data storage. We will review the methods emerging for making such measurements, and will note that the fragility of the liquid (including possible crossover from ‘fragile’ to ‘strong’ liquid behaviour on cooling) is an important part of understanding fast crystal growth. We also note that there are deficiencies in existing theories of fast crystal growth.

A method of promoting single crystal yield during melt growth of semiconductors by directional solidification

For certain semiconductors with important applications, the existing unseeded bulk directional solidification crystal growth technique from the melt usually results in poor quality multi-crystalline ingots which causes the low yield of the commercial growth process. The multi-grained crystal growth is mainly caused by the large supercool of the melt, which not only results in a large section of ingot solidifying uncontrollably under spontaneous nucleation but also prohibits the ideal growth condition that small single crystal nuclei form at the very tip of the ampoule and grow into large single grains. To promote nucleation under the condition of small supercooling, a method was employed to induce nucleation by mechanical perturbation at a critical time during growth. The technique was applied to the bulk crystal growth process of Cd1−xZnxTe ingots. The comparison between the crystalline quality of the crystals grown with and without the mechanically induced nucleation shows that the yield of single crystalline can been vastly improved with the application of the technique.

The Influence of the Effect of Solute on the Thermodynamic Driving Force on Grain Refinement of Al Alloys

Grain refinement is known to be strongly affected by the solute in cast alloys. Addition of some solute can reduce grain size considerably while others have a limited effect. This is usually attributed to the constitutional supercooling which is quantified by the growth restriction factor, Q. However, one factor that has not been considered is whether different solutes have differing effects on the thermodynamic driving force for solidification. This paper reveals that addition of solute reduces the driving force for solidification for a given undercooling, and that for a particular Q value, it is reduced more substantially when adding eutectic-forming solutes than peritectic-forming elements. Therefore, compared with the eutectic-forming solutes, addition of peritectic-forming solutes into Al alloys not only possesses a higher initial nucleation rate resulted from the larger thermodynamic driving force for solidification, but also promotes nucleation within the constitutionally supercooled zone during growth. As subsequent nucleation can occur at smaller constitutional supercoolings for peritectic-forming elements, a smaller grain size is thus produced. The very small constitutional supercooling required to trigger subsequent nucleation in alloys containing Ti is considered as a major contributor to its extraordinary grain refining efficiency in cast Al alloys even without the deliberate addition of inoculants.

Experimental study of single CO2 and mixed CO2 + TBAB hydrate formation and dissociation in oil-in-water emulsion

In secondary refrigeration applications, phase change material (PCM) slurry could be used as secondary refrigerant in order to combine a more efficient secondary loop and fluids with higher energy storage capacity. In such systems, energy is stored mainly during a phase change process of the storage material and a carrier fluid should be used as a continuous phase. The obtained system is called Phase Change Slurry (PCS). It has a higher energy density than single-phase secondary refrigerants due to both sensible and latent heat capacities of the materials.The purpose of this paper is to seek an appropriate secondary refrigerant for cold production and refrigeration applications between 273 and 293 K. Such PCS should have a high melting enthalpy and good flowing capacities. Hence, two types of PCS systems were combined: oil in water emulsion and CO2/CO2 + tetra-n-butylammonium bromide (TBAB) hydrate slurry. A DTA device was used in which the phase change temperatures and the corresponding enthalpies for many PCS systems were measured. A new system was thus developed that consists of oil in water emulsion/tetra-n-butylammonium bromide/carbon dioxide. The total heat capacity of the system was high, up to 142 kJ kg−1. It was higher than many other oil-in-water emulsions in the literature. Furthermore, the final system fits the requirements: It is stable, has a small supercooling degree and the phase changes occur under moderate thermodynamic conditions.

Mathematical modeling of solidification process near the inner core boundary of the Earth

Radially symmetric analytic solutions of the heat and mass transfer equations governing convection in the Earth’s fluid core are found in terms of deviations from the adiabatic reference state. We demonstrate that an increase of the convective velocity leads to a decrease of the light constituent mass fraction and specific entropy. Where fluid is rising/descending, convective motions decrease/increase the mass fraction and entropy at the inner core boundary (ICB). The influence of convective motions on the thermal fluxes at the core mantle boundary is studied. On the basis of exact solutions we demonstrate that the liquid is supercooled near the ICB. An important point is that an increase in the convective velocity directed to the ICB increases the constitutional supercooling. We show that the anelastic model (AM) can be used only at small supercoolings near the ICB. The most probable solidification scenario “constitutional supercooling and morphological instability” should be described by a mushy layer theory near the ICB and by the AM in the rest region of the fluid outer core. On the basis of dendritic theory and selection mechanisms of crystal growth the dendrite tip radius and interdendritic spacing in the mushy layer at the ICB are determined in the presence of convection.

One-dimensional solidification of supercooled melts

In this paper a one-phase supercooled Stefan problem, with a nonlinear relation between the phase change temperature and front velocity, is analysed. The model with the standard linear approximation, valid for small supercooling, is first examined asymptotically. The nonlinear case is more difficult to analyse and only two simple asymptotic results are found. Then, we apply an accurate heat balance integral method to make further progress. Finally, we compare the results found against numerical solutions. The results show that for large supercooling the linear model may be highly inaccurate and even qualitatively incorrect. Similarly as the Stefan number β→1+ the classic Neumann solution which exists down to β=1 is far from the linear and nonlinear supercooled solutions and can significantly overpredict the solidification rate.

Crystallization and Morphology of Autophobic Dewetted Poly(ε‐caprolactone)‐b‐poly(L‐lactide) Diblock Copolymer Ultrathin Films

AbstractWe have investigated the crystallization and morphological behaviors of poly(ε‐caprolactone)‐b‐poly(L‐lactide) (PCL‐b‐PLLA) in its autophobic dewetted ultrathin films (∼11 nm) using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The autophobic dewetting process creates a well defined film geometry containing an extremely thin wetting layer (∼4.5 nm) with densely distributed micrometer droplets atop, which restricts the primary nucleation process to occurring only in the droplets. In addition to the normally encountered flat‐on lamellae, the growth of edge‐on lamellae in such a thin wetting layer has been observed on both of two crystallization paths. In thermal crystallization, flat‐on lamellae are favored at small supercoolings while edge‐on lamellae appear at very large supercoolings both in the droplets and the wetting layer. For cold crystallization, the edge‐on lamellae can form easily in the droplets and grow into the wetting layer even at very small supercoolings. These observations are explained on the basis that the nucleation and lamellar orientation are strongly affected by the film geometry, the crystallization paths, and the applied supercoolings.

Quark-hadron phase transitions in the viscous early universe

Based on hot big bang theory, the cosmological matter is conjectured to undergo QCD phase transition(s) to hadrons, when the universe was about $1-10 \mu$s old. In the present work, we study the quark-hadron phase transition, by taking into account the effect of the bulk viscosity. We analyze the evolution of the quantities relevant for the physical description of the early universe, namely, the energy density $\rho$, temperature $T$, Hubble parameter $H$ and scale factor $a$ before, during and after the phase transition. To study the cosmological dynamics and the time evolution we use both analytical and numerical methods. By assuming that the phase transition may be described by an effective nucleation theory (prompt {\it first-order} phase transition), we also consider the case where the universe evolved through a mixed phase with a small initial supercooling and monotonically growing hadronic bubbles. The numerical estimation of the cosmological parameters, $a$ and $H$ for instance, makes it clear that the time evolution varies from phase to phase. As the QCD era turns to be fairly accessible in the high-energy experiments and the lattice QCD simulations, the QCD equation of state is very well defined. In light of this, we introduce a systematic study of the {\it cross-over} quark-hadron phase transition and an estimation for the time evolution of Hubble parameter.

Nucleation Effects of Nucleobases on the Crystallization Kinetics of Poly(L‐lactide)

AbstractThe effects of nucleobases, especially uracil, on the nonisothermal and isothermal crystallization, melting behavior, spherulite morphology, and crystalline structure of bio‐based and biodegradable PLLA are studied. The melt‐ and cold‐crystallization rates of PLLA increase with increasing uracil loading. The melting behavior of nonisothermally melt‐ and cold‐crystallized PLLAs depends on the uracil content. The isothermal crystallization kinetics is analyzed based on an Avrami model. The incorporation of uracil changes the t1/2/Tc profile of PLLA due to the more distinct heterogeneous nucleation effects at small supercooling. The crystalline structure of PLLA is not affected by uracil presence. The nucleation density increases and the spherulite size decreases by uracil incorporation. magnified image