Nucleation Thermodynamics inside Micro/nanocavity

A thermodynamic approach at the nanometer scale was performed for the heterogeneous nucleation inside nanocavity, and an analytical expression of the critical energy of nucleation was evaluated considering a rough ball nucleus nucleating inside nanocavity. Compared with the case of the nucleation locating on planar or convex substrate, the critical energy of nucleation inside the concave substrate is the smallest. Based on the thermodynamic and kinetic analyses, at low supersaturation, the smaller the curvature radius of cavity and/or the smaller the contact angle, the smaller the critical energy of nucleation, and the larger the nucleation rate. At high supersaturation, the nucleation rate increases with increasing the contact angle and/or increasing the curvature radius of cavity. In this way, at the low supersaturation, the heterogeneous nucleation rate is larger than the homogeneous one, as the nucleation rate is mainly determined by the heterogeneous nucleation. At the high supersaturation, the heterogeneous nucleation rate is smaller than the homogeneous one, as the nucleation rate is mainly determined by the homogeneous nucleation.     

用Monte Carlo法模拟纯铜的静态再结晶

利用Monte Carlo(MC)法进行了纯铜静态再结晶的二维模拟，通过监测存储能量的变化历程发现，当初始存储能量较低的时候，静态再结晶是以非均匀形核（晶界形核）的方式进行的，当初始存储能量充分高的时候，静态再结晶是以均匀形核（晶内形核）的方式进行的。初始存储能量较高的时候，能量释放的速率要比初始存储能量较低的时候的速率高，确定了区分均匀形核和非均匀形核的临界应变。     

Pit morphology characterization and corrosion fatigue crack nucleation analysis based on energy principle

Pit always changes its shape and size during corrosion fatigue. The actual morphology of pit is an outcome of the interaction between the variation in the elastic energy, surface energy and electrochemical energy stored in the cyclically stressed solid. In this paper, a two‐variable semi‐elliptical model is proposed to depict the pit's growing morphology. The critical condition for corrosion fatigue crack nucleation is deduced according to dislocation theory, and the influences of some important factors on critical pit size and crack nucleation life are discussed.     

Prediction of fatigue crack nucleation life in polycrystalline AA7075‐T651 using energy approach

A crystal plasticity (CP) simulation and an energy‐based model is presented to predict the fatigue nucleation onset for polycrystalline AA 7075‐T651. Different microstructure morphology and grain sizes are employed in the simulations. Using a simple method, statistically stored dislocation (SSD) and geometrically necessary dislocation (GND) as decoupled with crystal plasticity model are estimated using a double round‐notch specimen test data, and CP simulation. The dislocation density parameter approximated from plastic energy density, stored energy density, elastic energy and accumulated slip validated with double hole experimental data. Sensitivity analysis is performed with respect to different microstructures and dislocation density parameters. Roughly, maximum 30% difference between experimental nucleation life and the simulated one is observed. The simulated predictions are in fair agreement with test data. The proposed strategy is suitable to study the scatter of fatigue nucleation life.     

铁基合金中马氏体形核动力学探讨

根据马氏体相变的经典形核理论,重新估算了铁基合工中马氏体均匀形核激活能,研究了激活能与界面能和相变驱动力的关系。结果表明,热激活形核具有很低的形核激活能,热激活能形核动力学与马氏体形核动力学实验研究结果一致,在动力学研究的基础上,提出了马氏体相变的形核机制,在形核机制中,除了热激活菜核,还考虑了应力对马氏体形核的作用,特别是在低温时由于急应内应力对形核的贡献。     

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     

Model calculations of flux-tube nucleation in thin-film type I superconductors

In the current-induced resistive state, multiquantum flux tubes are nucleated at the edge of thin-film strips of type I superconductors. The nucleation process can be theoretically described by an extended Gibbs free-energy barrier model. Within this model the size of the nucleated flux tubes is obtained by minimizing the total energy of the flux tube. The dependence of the flux-tube size on the nucleation rate and the transport current is calculated including the influence of the preceding flux tubes and the positive domain wall energy of type I superconductors. The interaction with the preceding flux tubes generates a steplike decrease of the flux-tube size with increasing nucleation frequency, whereas the wall energy establishes a minimum value of the flux-tube size. These calculations are in good agreement with recent experiments on the nucleation process of multiquantum flux tubes.Financial support provided by the Deutsche Forschungsgemeinschaft.     

CVD金刚石薄膜二次形核机制的研究

根据实验观察分析提出了金刚石二次形核机制 ,此机制认为二次形核很容易在( 10 0 )晶面上和晶界上形成。通过比较 ( 10 0 )面的二次形核和形成新生长台阶的系统自由能差 ,可知当气氛中的碳氢基团浓度较大时 ,粘附在基底的碳氢基团发生堆集 ,如果堆集碳氢基团高度尺寸较大时将形成二次晶核。也对晶界二次形核的系统自由能差进行了推导 ,结果表明晶界二次形核是自发的 ,将导致体系自由能的下降。     

Effect of transition elements (Ti, V, Cr, Mn, Co, Ni) on thermal stability of Fe-B metallic glasses part I: Activation energy for crystallization

An investigation of the activation energy E_c for crystallization in iron-boron-based metallic glasses submitted to partial crystallization has been undertaken. The hypothesis of quenched-in nuclei is put forward when E_c remains unchanged. The contribution of activation energy for nucleation is evaluated when both nucleation and crystal growth act.     

Nucleation kinetics of paramagnetic and diamagnetic metal melts under a high magnetic field

The influence of a high magnetic field (HMF) on the nucleation kinetics of paramagnetic aluminum and diamagnetic zinc melts has been investigated by differential thermal analysis (DTA).It is found that the application of an HMF increases the undercooling of pure aluminum and pure zinc at the same heatingcooling rates.Moreover,the quantitative analysis of activation energy calculated from the DTA results using the Kissinger method manifests that the HMF reduces the activation energy of pure aluminum and pure zinc.Regardless of magnetism,the nucleation frequency under an HMF is higher than that without an HMF.Furthermore,the increase in undercooling under the HMF is mainly attributed to the increase of the contact angle,calculated by the functional relationship between the cooling rate and undercooling.This result is consistent with the increase of the calculated nucleation work for pure aluminum and pure zinc.Additionally,the increase in undercooling caused by the HMF is partly ascribed to the magnetic field-induced suppression of thermal convection in the undercooled melt.     

Solid-liquid interfacial energy on crystallization in Fe-Si-B alloy glass

Isothermal crystallization on ageing is investigated in Fe-10at% Si-15at% B alloy glass. Based on homogeneous nucleation theory, the solid-liquid interfacial energy is estimated by use of the nucleation frequency of crystallization. The energy obtained is quite close to that estimated by extending Turnbull's assumption for alloys.     

Focused Nanosecond Pulsed Laser-Induced Homogeneous Nucleation in Undercooled Oxide Melts

Induced homogeneous nucleation of undercooling melt is a fundamental challenge with exciting implications for composite ceramic and metal alloy materials. Nucleation can be induced externally, but physical mechanical properties reduce due to the introduction of impurities. Herein, a novel focused nanosecond pulsed laser-induced nucleation approach is demonstrated for oxide ceramic melts at undercooled state. Using a self-made aerodynamic levitation setup, the pulsed laser-induced nucleation is controlled by optimizing the pulse energy density at different undercooled temperatures. Exploiting this approach, nucleation and growth are observed from surface of undercooled alumina (Al₂O₃) and neodymium yttrium aluminum garnet (Nd:YAG) melts with high-speed camera. The undercooling sample experiences a nucleate–solidify–recalescence–remelting–recalescence–solidify process. The correlation between nucleation time and undercooling temperature confirms that the undercooled melt is nucleated homogeneously by the pulsed laser. The special microstructural of samples agrees well with the prediction of two-step nucleation theory. Therefore, an assumption for the nucleation mechanism with the high-speed images and the microstructure analysis is proposed. This proposed method may provide a different perspective for the classical nucleation theory of homogeneous nucleation.     

A stored energy criterion for fatigue crack nucleation in polycrystals

A series of microstructurally-differing, large-grained, notched, polycrystal BCC ferritic steel bend test samples have been analysed to extract the experimentally observed sites of fatigue crack nucleation together with the numbers of cycles to cause crack nucleation. The samples have been modelled with explicit representation of both grain morphologies and crystallographic orientations using crystal plasticity which has enabled a detailed assessment to be made of key microstructure-level quantities such as accumulated slip, slip rate, and densities of both statistically stored and geometrically necessary dislocations local to the experimentally observed sites of crack nucleation. These quantities when considered independently have not been found to correlate with experimentally observed cycles to nucleation.A new criterion for fatigue crack nucleation has been introduced in which a critical stored energy density, G_c, is argued to be necessary in order for crack nucleation. The rate of stored energy density determined at the sites of crack nucleation has been shown to correlate well with experimental measurement of cycles to nucleation, and the number of cycles to cause fatigue crack nucleation, for the samples for which such measurements are available, is well predicted. The criterion enables prediction of cycles to crack nucleation for all of the experimental samples and has been shown to demarcate correctly the crack nucleation lives observed over the range of differing experimental microstructures.     

Sequences of precipitate nucleation

Solid-solid nucleation theory, as recently extended to faceted nuclei, is applied to several problems in understanding experimentally observed sequences of precipitate nucleation involving transtion phases during isothermal ageing. Transition precipitates form due to a low interfacial energy resulting from a greater structural similarity to the matrix than the equilibrium phase or succeeding transition phases and in spite of a usually smaller driving force, ¦ΔG _v ¦, for nucleation than any of these competing phases. Transition phases formed after the first one are predicted to nucleate preferentially at the interphase boundaries of their predecessors, also on the basis of interfacial energy, rather than ΔG _v considerations. Most examples of precipitation sequences cited from the literature are in agreement with this prediction; the few exceptions found are noted and discussed. The preference of equilibrium phases for nucleating at high-energy grain boundaries and of transition precipitates for nucleation at dislocations and other lower energy sites is explained on the following basis. The high-energy boundaries reduce the net interfacial free energy needed for nucleation of either phase so much that the higher ¦ΔG _v ¦ for the equilibrium phase becomes dominant. At the lower energy sites, the smaller reduction in ΔG ^*, the free energy of activation for nucleation, provided makes the lower interfacial energy of the transition phases the dominant factor in causing such phases to nucleate more rapidly.     

Diffusion-induced step decoration of Co on Ag(001)

We have investigated submonolayers of Co deposited on a Ag(001) surface with ultra low energy ion deposition and a deposition energy ranging from 5 to 30 eV. The sites of nucleation, the island densities and heights were determined with scanning tunneling microscopy. Next to randomly distributed islands on the terraces observed for every deposition energy, preferential nucleation on the upper side of the Ag steps was observed only for 5 and 15 eV. We found that for increasing characteristic length between the islands on a terrace, the number of islands per unit step length increases. This implies that step decoration is related to enhanced diffusion of Co atoms on the Ag surface during nucleation.     

Determination of the strain generated in InAs/InP quantum wires: prediction of nucleation sites

The compositional distribution in a self-assembled InAs(P) quantum wire grown by molecular beam epitaxy on an InP(001) substrate has been determined by electron energy loss spectrum imaging. We have determined the strain and stress fields generated in and around this wire capped with a 5?nm InP layer by finite element calculations using as input the compositional map experimentally obtained. Preferential sites for nucleation of wires grown on the surface of this InP capping layer are predicted, based on chemical potential minimization, from the determined strain and stress fields on this surface. The determined preferential sites for wire nucleation agree with their experimentally measured locations. The method used in this paper, which combines electron energy loss spectroscopy, high-resolution Z contrast imaging, and elastic theory finite element calculations, is believed to be a valuable technique of wide applicability for predicting the preferential nucleation sites of epitaxial self-assembled nano-objects.     

A mechanism of grain nucleation during relaxation of the latent energy of junction disclinations in the course of plastic deformation

A model describing the nucleation of a new grain at the boundary of an existing grain during relaxation of the latent energy of junction disclinations formed in the course of plastic deformation is considered. This nucleation mechanism is shown to be energetically favorable. The dependence of the equilibrium size and misorientation of a nucleus on its shape is analyzed.     

Nucleation and growth of sputtered and evaporated trigonal selenium films

Critical nucleation temperature experiments, scanning electron microscopy and light microscopy were used to study the nucleation and growth of trigonal Se films deposited by evaporation and sputtering on both gold and anodized aluminum substrates. The major nucleation and growth processes were shown to be the condensation of Se₂ molecules followed by their polymerization into Se_n chains, where n can be very large. In sputtering, these chains have a higher surface mobility, due in part to their higher residual kinetic energy, and thus they can form large trigonal Se crystallites. The high incident kinetic energies also allow the Se to break through the surface contamination of the substrate. This is important since the Se chain mobility should also be dependent on the surface free energy and on the nucleation site density of the substrate. Very adherent large-grained trigonal Se films were grown by sputtering on Au substrates. It was shown that nucleation sites for the large trigonal Se crystallites were not created by the incident flux but are inherent on the substrate. The high energy incident flux did, however, increase nucleation probability for the initial Se₂ condensation. For sputtering on anodized aluminum substrates the combination of low surface energy and high nucleation site density decreased chain mobility and thus prevented formation of large crystallites.     

Vortex-Loops and Solid Nucleation in Superfluid 4He and 3He

We propose a new model for the nature of the nucleation of solid from the superfluid phases of ⁴He and ³He. Unique to the superfluid phases the solid nucleation involves an extremely fast solidification front. We depart from the usual quasi-static treatment of solid nucleation by proposing that the nucleation of a solid seed is helped by the simultaneous nucleation of vortex-loops in the superfluid around it. It is the composite entity which is nucleated out of the over-pressurized liquid. This occurs when the local release of pressure creates a velocity field in the superfluid which in turn facilitates the nucleation of vortex-loops. The kinetic energy gain of this process balances the surface tension, as the solid surface is quickly covered by many vortex-loops (hairy snow-ball). We show that this scenario gives good agreement with many experiments on heterogeneous nucleation, where the energy barrier is found to differ with the classical theory of homogeneous nucleation by 8 orders of magnitude. We propose several experiments that could show the involvement of vortices with solid nucleation.