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Polymer crystal-melt interfaces and nucleation in polyethylene  [PDF]
Scott T. Milner
Physics , 2010,
Abstract: Kinetic barriers cause polymers to crystallize incompletely, into nanoscale lamellae interleaved with amorphous regions. As a result, crystalline polymers are full of crystal-melt interfaces, which dominate their physical properties. The longstanding theoretical challenge to understand these interfaces has new relevance, because of accumulating evidence that polymer crystals often nucleate via a metastable, partially ordered "rotator" phase. To test this idea requires a theory of the bulk and interfacial free energies of the critical nucleus. We present a new approach to the crystal-melt interface, which represents the amorphous region as a grafted brush of loops in a self-consistent pressure field. We combine this theory with estimates of bulk free energy differences, to calculate nucleation barriers and rates via rotator versus crystal nuclei for polyethylene. We find rotator-phase nucleation is indeed favored throughout the temperature range where nucleation is observed. Our methods can be extended to other polymers.
Pronounced ductility in CuZrAl ternary bulk metallic glass composites with optimized microstructure through melt adjustment
Zengqian Liu,Ran Li,Gang Liu,Kaikai Song
AIP Advances , 2012, DOI: 10.1063/1.4754853
Abstract: Microstructures and mechanical properties of as-cast Cu47.5Zr47.5Al5 bulk metallic glass composites are optimized by appropriate remelting treatment of master alloys. With increasing remelting time, the alloys exhibit homogenized size and distribution of in situ formed B2 CuZr crystals. Pronounced tensile ductility of ~13.6% and work-hardening ability are obtained for the composite with optimized microstructure. The effect of remelting treatment is attributed to the suppressed heterogeneous nucleation and growth of the crystalline phase from undercooled liquid, which may originate from the dissolution of oxides and nitrides as well as from the micro-scale homogenization of the melt.
Electric field induced nucleation: An alternative pathway to metallic hydrogen  [PDF]
M. Nardone,V. G. Karpov
Physics , 2011,
Abstract: Electric field induced nucleation is introduced as a possible mechanism to realize a metallic phase of hydrogen. Analytical expressions are derived for the nucleation probabilities of both thermal and quantum nucleation in terms of material parameters, temperature, and the applied field. Our results show that the insulator-metal transition can be driven by an electric field within a reasonable temperature range and at much lower pressures than the current paradigm of P > 400 GPa. Both static and oscillating fields are considered and practical implementations are discussed.
Plasmonic mediated nucleation of resonant nano-cavities in metallic layers  [PDF]
V. G. Karpov,M. Nardone,A. V. Subashiev
Physics , 2012, DOI: 10.1063/1.4737654
Abstract: We predict plasmonic mediated nucleation of pancake shaped resonant nano-cavities in metallic layers that are penetrable to laser fields. The underlying physics is that the cavity provides a narrow plasmonic resonance that maximizes its polarizability in an external field. The resonance yields a significant energy gain making the formation of such cavities highly favorable. Possible implications include nano-optics and generation of the dielectric bits in conductive films that underlie the existing optical recording phase change technology.
TOPICAL REVIEW - Texturing by cooling a metallic melt in a magnetic field
Robert F Tournier and Eric Beaugnon
Science and Technology of Advanced Materials , 2009,
Abstract: Processing in a magnetic field leads to the texturing of materials along an easy-magnetization axis when a minimum anisotropy energy exists at the processing temperature; the magnetic field can be applied to a particle assembly embedded into a liquid, or to a solid at a high diffusion temperature close to the melting temperature or between the liquidus and the solidus temperatures in a region of partial melting. It has been shown in many experiments that texturing is easy to achieve in congruent and noncongruent compounds by applying the field above the melting temperature Tm or above the liquidus temperature of alloys. Texturing from a melt is successful when the overheating temperature is just a few degrees above Tm and fails when the processing time above Tm is too long or when the overheating temperature is too high; these observations indicate the presence of unmelted crystals above Tm with a size depending on these two variables that act as growth nuclei. A recent model that predicts the existence of unmelted crystals above the melting temperature is used to calculate their radius in a bismuth melt.
Nucleation of a stable solid from melt in the presence of multiple metastable intermediate phases: Wetting, Ostwald step rule and vanishing polymorphs  [PDF]
Mantu Santra,Rakesh S. Singh,Biman Bagchi
Physics , 2013,
Abstract: In many systems, nucleation of a stable solid may occur in the presence of other (often more than one) metastable phases. These may be polymorphic solids or even liquid phases. In such cases, nucleation of the solid phase from the melt may be facilitated by the metastable phase because the latter can "wet" the interface between the parent and the daughter phases, even though there may be no signature of the existence of metastable phase in the thermodynamic properties of the parent liquid and the stable solid phase. Straightforward application of classical nucleation theory (CNT) is flawed here as it overestimates the nucleation barrier since surface tension is overestimated (by neglecting the metastable phases of intermediate order) while the thermodynamic free energy gap between daughter and parent phases remains unchanged. In this work we discuss a density functional theory (DFT) based statistical mechanical approach to explore and quantify such facilitation. We construct a simple order parameter dependent free energy surface that we then use in DFT to calculate (i) the order parameter profile, (ii) the overall nucleation free energy barrier and (iii) the surface tension between the parent liquid and the metastable solid and also parent liquid and stable solid phases. The theory indeed finds that the nucleation free energy barrier can decrease significantly in the presence of wetting. This approach can provide a microscopic explanation of Ostwald step rule and the well-known phenomenon of "disappearing polymorphs" that depends on temperature and other thermodynamic conditions. Theory reveals a diverse scenario for phase transformation kinetics some of which may be explored via modern nanoscopic synthetic methods.
Inferring snowpack ripening and melt-out from distributed measurements of near-surface ground temperatures  [PDF]
M.-O. Schmid,S. Gubler,J. Fiddes,S. Gruber
The Cryosphere , 2012, DOI: 10.5194/tc-6-1127-2012
Abstract: Seasonal snow cover and its melt regime are heterogeneous both in time and space. Describing and modelling this variability is important because it affects diverse phenomena such as runoff, ground temperatures or slope movements. This study presents the derivation of melting characteristics based on spatial clusters of ground surface temperature (GST) measurements. Results are based on data from Switzerland where ground surface temperatures were measured with miniature loggers (iButtons) at 40 locations referred to as footprints. At each footprint, up to ten iButtons have been distributed randomly over an area of 10 m × 10 m, placed a few cm below the ground surface. Footprints span elevations of 2100–3300 m a.s.l. and slope angles of 0–55°, as well as diverse slope expositions and types of surface cover and ground material. Based on two years of temperature data, the basal ripening date and the melt-out date are determined for each iButton, aggregated to the footprint level and further analysed. The melt-out date could be derived for nearly all iButtons; the ripening date could be extracted for only approximately half of them because its detection based on GST requires ground freezing below the snowpack. The variability within a footprint is often considerable and one to three weeks difference between melting or ripening of the points in one footprint is not uncommon. The correlation of mean annual ground surface temperatures, ripening date and melt-out date is moderate, suggesting that these metrics are useful for model evaluation.
Nucleation of stable cylinders from a metastable lamellar phase in a diblock copolymer melt  [PDF]
Robert A. Wickham,An-Chang Shi,Zhen-Gang Wang
Physics , 2003, DOI: 10.1063/1.1572461
Abstract: The nucleation of a droplet of stable cylinder phase from a metastable lamellar phase is examined within the single-mode approximation to the Brazovskii model for diblock copolymer melts. By employing a variational ansatz for the droplet interfacial profile, an analytic expression for the interfacial free-energy of an interface of arbitrary orientation between cylinders and lamellae is found. The interfacial free-energy is anisotropic, and is lower when the cylinder axis is perpendicular to the interface than when the cylinders lie along the interface. Consequently, the droplet shape computed via the Wulff construction is lens-like, being flattened along the axis of the cylinders. The size of the critical droplet and the nucleation barrier are determined within classical nucleation theory. Near the lamellar/cylinder phase boundary, where classical nucleation theory is applicable, critical droplets of size 30--400 cylinders across with aspect ratios of 4--10 and nucleation barriers of 30--40 k_B T are typically found. The general trend is to larger critical droplets, higher aspect ratios and smaller nucleation barriers as the mean-field critical point is approached.
Inferring snow pack ripening and melt out from distributed ground surface temperature measurements  [PDF]
M.-O. Schmid,S. Gubler,J. Fiddes,S. Gruber
The Cryosphere Discussions , 2012, DOI: 10.5194/tcd-6-563-2012
Abstract: The seasonal snow cover and its melting are heterogeneous both in space and time. Describing and modelling this variability are important because it affects divers phenomena such as runoff, ground temperatures or slope movements. This study investigates the derivation of melting characteristics based on spatial clusters of temperature measurements. Results are based on data from Switzerland where ground surface temperatures were measured with miniature loggers (iButtons) at 40 locations, referred to as footprints. At each footprint, ten iButtons have been distributed randomly few cm below the ground surface over an area of 10 m × 10 m. Footprints span elevations of 2100–3300 m a.s.l. and slope angles of 0–55°, as well as diverse slope expositions and types of surface cover and ground material. Based on two years of temperature data, the basal ripening date and the melt-out date are determined for each iButton, aggregated to the footprint level and further analysed. The date of melt out could be derived for nearly all iButtons, the ripening date could be extracted for only approximately half of them because it requires ground freezing below the snow pack. The variability within a footprint is often considerable and one to three weeks difference between melting or ripening of the points in one footprint is not uncommon. The correlation of mean annual ground surface temperatures, ripening date and melt-out date is moderate, making them useful intuitive complementary measured for model evaluation.
Modeling of heat transfer in the cooling wheel in the melt-spinning process  [PDF]
B. Karpe,B. Kosec,M. Bizjak
Journal of Achievements in Materials and Manufacturing Engineering , 2011,
Abstract: Purpose: In the case of continuous casting of metal ribbons with the melt-spinning process on the industrial scale, larger quantity of melt could lead to a slow excessive warming of the chilling wheel, which would further lead to solidification of a ribbon at non-uniform conditions and increased wearing of the wheel. Primary goal of our work was to determine to what extent the release of heat during contact of the melt/ribbon on the circumferential surface of the chilling wheel affect its surface temperature rise, and inversely how much elevated temperature of the chill wheel surface affects on metal ribbon cooling rate and its solidification velocity.Design/methodology/approach: On the basis of developed mathematical model, a computer program was made and used for analyses of heat transfer in the melt-spinning process.Findings: The calculations show that contact resistance between metal melt and chilling wheel has a great influence on melt/ribbon cooling and chill wheel heating rate, and must not be neglected in numerical calculations, even if its value is very low. In the case of continuous casting, significant “long term” surface temperature increase may take place, if the wheel is not internally cooled. But inner cooling is effective only if wheel casing thickness is properly chosen.Research limitations/implications: Influence of process parameters and chill wheel cooling mode on cooling and solidifying rate over ribbon thickness are outlined.Practical implications: Directions for the chill wheel cooling system design are indicated.Originality/value: New method for determining contact resistance through variable heat transfer coefficient is introduced which takes into account physical properties of the casting material, process parameters and contact time/length between metal melt/ribbon and substrate and enables cooling rate prediction before the experiment execution. In the case of continuous casting, heat balance of the melt-spinning process is calculated and influence of the chill wheel cooling mode on cooling rate of metallic ribbon is analyzed.
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