Entropic effects in carbon nanotubes-templated crystallization of Poly(3-alkyl thiophenes,P3HT,P3OT)

A significant increase in polymer crystallinity is reported in composites of carbon-nanotubes (CNT) and Poly(3-octylthiophene-2,5-diyl), P3HT and Poly(3-octylthiophene-2,5-diyl), P3OT; Differential scanning calorimetry (DSC) reveal an increase from about 40% crystallinity of the native P3HT to ∼62% in composites containing 25 wt% MWNT. A similar behavior is observed in P3OT with ∼68% crystallinity, a double crystallization peak and higher melting temperature than the native polymers. The effect is unique to CNT and is not induced by fullerenes or graphene layers. High-resolution transmission electron microscopy, (HRTEM) of CNT-polymer dispersions reveal chains stacked upon the CNT in an elongated, stretched conformation. Following a detailed molecular study by Bernardi et al. and the HRTEM observations the DSC results are attributed to a CNT-mediated entropic effect: due to their intrinsic, 1D cylindrical shape the CNT impose an increased conjugation length on chains adsorbed and stacked upon dispersed CNT. Crystallization thus commences from a heterogeneous mixture of native chains and chains with a longer persistence length (higher effective rigidity) and consequentially a lower effective height of the entropic barrier for crystallization. The findings offer a new insight into the origins of CNT-induced polymer nucleation.     

Numerical modeling of damage evolution of DP steels on the basis of X-ray tomography measurements

In order to couple the damage evolution and the stress state of DP steel grades, a new advanced GTN (Gurson-Tvergaard-Needleman) model was developed and implemented into a finite element code. This model is an extension of the original one. It takes into account the plastic anisotropy and the mixed (isotropic + kinematic) hardening of the matrix. Two different methods to compute the void volume fraction were developed and used within the constitutive equations. The first method is new and allows the accurate modeling of the observations of damage initiation and growth in DP steels measured using high-resolution X-ray absorption tomography ( [Bouaziz et al., 2008] and [Maire et al., 2008]). The second method is classic and assumes the additive decomposition of the total void volume fraction into a nucleation and a growth part. A parametric study is carried out to assess the effect of the kinematic hardening on some mechanical parameters such as the equivalent plastic strain, the triaxiality and the porosity. The numerical predictions are favorably compared to the experimental results.     

Phenomenologically modeling the formation and evolution of the solid electrolyte interface on the graphite electrode for lithium-ion batteries

The formation and evolution of the solid electrolyte interface (SEI) film during the first electrochemical intercalation of lithium into graphite were modeled as a special precipitation process including a nucleation phase of the SEI film's solid deposition, and followed by a growth phase involving the precipitation of new solids on previously formed solid nuclei. It was shown that the solid species can nucleate in the electrolyte solution, directly on the graphite surface, or adjacent to an already present particle on the graphite surface when precipitating from the electrolyte solution. Within the framework of classical nucleation theory (CNT), we can qualitatively explain the origin of the two-layer structure of SEI films, which consists of a thin, compact polycrystalline or heteromicrophase layer rich in inorganic species (e.g., LiF, Li₂O) close to the electrode, and a thicker porous and amorphous layer composed mainly of organic compounds (e.g., ROLi, ROCO₂Li) that is farther from the graphite.     

Cooperative arrangement of self-assembled Ge dots on pre-grown Si mesas

In this work, we report on the study of one-dimensional (1-D) and two-dimensional (2-D) cooperative arrangements of self-assembled Ge dots grown on patterned Si (001) substrates. Selective epitaxial growth (SEG) of Si mesas was first performed to form Si mesas before Ge deposition. Self-assembled growths of Ge dots with variable Ge amount have been carried out. It is found that the cooperative arrangements of Ge dots are dependent on Ge amount deposited. The cooperative arrangements is attributed to the self-regulation of the dot size and position, which is driven by the minimization of the total free energy including strain energy, surface energy and deformation repulsive energy. This cooperative arrangement promoted by the energetically preferential nucleation on Si mesas allows us to control the placement of the self-assembled dots at the specific positions.     

马氏体相变的弹性波促发形核

使用Gleeble-1500型热模拟机，对30CrMnSiMoV超高强度钢在奥氏体状态下预应变。而后分三种工艺淬火：预应变结束后不卸载立即喷雾冷却；预应变结束后卸载，并于960℃保温5min后再喷雾冷却；预应变后不卸载，重新加热至1050℃，并保温5min，发生再结晶后再喷雾冷却。经对其马氏体组织进行分析发现：处于释放状态的弹性应变能会促发马氏体相变均匀形核；而塑性应变对马氏体的生长有限制作用，可间接影响马氏体的相变形核。两者的恰当配合，可显著细化马氏体，使其板条平均宽度接近纳米量级水平（平均宽度为120nm）。在此基础上，提出了一种马氏体的软模弹性波动形核机制。可解释所获得的实验结果。     

A molecular dynamics study of bubble nucleation in liquid oxygen with impurities

The present study investigates bubble nucleation in liquid oxygen with dissolved impurities (nitrogen or helium molecules) using molecular dynamics simulations. When the mole fraction of impurities is 0.05, there is a fundamental difference in the bubble nucleation mechanism between the two dissolved impurities cases; vaporization in the homogeneous bulk makes a bubble in the case of a nitrogen‐dissolved liquid while phase separation of impurities and liquid molecules makes a nucleus in the case of a helium‐dissolved liquid. Fluctuations can cause local voids, which in turn can grow to be bubbles, and this effect is stronger in the case of a helium‐dissolved liquid with a lower mole fraction (0.01) than in the case of a nitrogen‐dissolved liquid with a higher mole fraction (0.05). From these results, we conclude that helium molecules have a much stronger action to raise the bubble formation pressure compared with nitrogen. In this paper, the kinetically‐defined critical nucleus, which is a very important factor in quantitatively evaluating the nucleation mechanism, is also estimated through the calculation of the size change rate of each nucleus. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(7): 514–526, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20082     

Investigations on nucleation thermodynamical parameters of NdBa2Cu3O7-δ (Nd123) crystallization by high temperature solution growth

Investigations on nucleation thermodynamical parameters are very essential for the successful growth of good quality single crystals from high temperature solution. A theoretical estimation of the nucleation thermodynamical parameters like interfacial energy between the solid Nd123 and its flux BaO-CuO, metastable zone-width, Gibbs free energy, critical energy barrier for nucleation and critical nucleation radius have been calculated from the knowledge of solubility data and by applying the classical nucleation theory. Results are discussed to understand the growth kinetics of Nd123 crystals. Paper presented at the 5th IUMRS ICA98, October 1998, Bangalore     

Probabilistic modeling of fatigue related microstructural parameters in aluminum alloys

This paper presents the results of probabilistic modeling of the fatigue related microstructural parameters in unclad 2024-T351 aluminum sheets. The statistical distributions of the constituent particle size, which were obtained from metallographic measurements from polished surfaces, were determined by graphical goodness-of-fit tests. The distributions of the crack-nucleating particle sizes were determined using the data measured from various fatigue fracture surfaces. Initially, an extreme value theory based model was investigated to correlate the overall particle distribution with its fatigue subsets. Furthermore, a new Monte Carlo simulation was developed to determine the fatigue subsets using the microstructural parameters such as particle size, grain size, and grain orientation distributions, in association with qualitative criteria on fatigue crack nucleation and growth mechanisms.     

Undercooling and Nucleation Studies on Pure Refractory Metals by Drop Tube Processing: Comments on the Limit to Crystalline Nucleation of Cubic and Compact Metals

A graphic interpretation for homogeneous nucleation is discussed in the framework of the classical theory of nucleation. If the value of two-thirds of the melting temperature may be regarded as an ultimate limit for liquid undercooling, the physical limit is shown to be an intrinsic property of the element under consideration, as verified by undercooling experiments performed on refractory metals under ultrahigh vacuum conditions. The graphic interpretation also suggests elements for new experiments, since the likelihood of heterogeneous nucleation remains. Some of them could be relevant to a nucleation mechanism through metastable polymorphism as proposed for Zr.     

Equations for nucleation of hydrogen gas pores during solidification of aluminium seven weight percent silicon alloy

A statistical prediction model has been applied to the nucleation rate as a function of temperature for an Al7Si alloy based upon observations of pore formation during solidification in an XTGS. A normalised Gaussian function was developed ((1), (2), (3) and (4)) which may be used to describe pore nucleation in mathematical simulations of porosity formation during solidification.