Distribution Of Nucleation Sites Research Articles

Flow boiling characteristics in bi-porous minichannel heat sink sintered with copper woven tape

Micro/minichannel heat sinks with functional structure surface provide a potential solution for the thermal management of increasing heat flux electronic devices. In this study, a novel bi-porous minichannel heat sink (BPM) sintered with copper woven tape is proposed to enhance the flow boiling heat transfer performance. On the surface of the copper woven tape there are two kinds of pore structures, comprising cavities formed by copper strands and crevices formed by copper wires. The flow boiling performances, including heat transfer performance, pressure drop and flow instability at different mass fluxes of G = 158, 253, 348 kg/m2•s, were experimentally investigated for BPM. Besides, a comparison study with a conventional rectangular minichannel with plain surface (PSM) was also conducted. The bubble behaviors observed by high-speed camera were investigated and analyzed. It is found that the onset of nucleate boiling of BPM is triggered at smaller wall superheat than that of PSM, benefiting from the larger quantity and more uniform distribution of nucleation sites generated by the novel bi-porous structure. The heat transfer coefficient of BPM is larger than that of PSM whether at low or high heat flux. Due to the uneven surface of the copper woven tape, the pressure drop of BPM increases more significantly with increasing heat flux, and is notably dependent on the mass flux and vapor quality. The flow instability is more moderate at a given mass flux owing to the more uniform bubble size and bubble distribution. This study shows that BPM is of high heat transfer performance and offers relatively stable operation condition, which is promising to solve thermal problem for high heat flux application.

Explosive Volcanic Eruptions and Spinodal Decomposition: A Different Approach to Deciphering the Tiny Bubble Paradox

AbstractBubbles in magmas drive explosive volcanic eruptions. The spatial distribution of bubble nucleation sites in an ascending, decompressing, and supersaturating magma is one of the primary controls on ash morphologies and volcanic hazards. The mechanism of bubble formation is important because it ultimately determines the spatial distribution of bubbles in the fragmenting magma. The initial nucleation of bubbles in a homogeneous magma is problematical because excessive surface tension pressure in very small, nascent bubbles should drive exsolved volatiles back into the melt. This thermodynamic barrier to bubble viability confounds understanding of homogeneous bubble nucleation, yet very small bubbles form, grow, and ultimately drive explosive volcanic eruptions. We refer to this as “the tiny bubble paradox.” Classical nucleation theory typically explains bubble formation and growth, but we propose that a spectrum of bubble‐forming mechanisms may include both homogeneous nucleation and spinodal decomposition (the spontaneous unmixing of phases by uphill diffusion) as end‐member processes. As spinodal decomposition progresses, regularly sized and regularly spaced quasi‐spherical zones form with increasingly high concentration of dissolved water at the centers. Bubble formation occurs as the concentration of water in the interior of the water‐rich zones approaches 100% and the concentration of melt approaches zero. The presence of a broad, diffuse, concentration gradient of water rather than a narrow water‐melt interface means that there is no surface, per se, for surface tension to arise. This is the crux of the solution of the tiny bubble paradox.

Fully Doctor-bladed efficient perovskite solar cells in ambient condition via composition engineering

Abstract It is very meaningful to develop large-scale, low-cost technology for fabricating efficient perovskite solar cells (PSCs) to accelerate their commercialization. Doctor-blading is one of important scalable technologies for processing PSCs, but the power conversion efficiencies (PCEs) of fully doctor-bladed PSCs, including electron transport layer, perovskite layer and hole transport layer, are still lag far behind the PSCs fabricated via conventional spin-coating technology, especially fabricated in ambient condition. Herein, highly efficient planar heterojunction PSCs with a structure of ITO/SnO2/FAxMA(1-x)PbIyBr(3-y)/Spiro-OMeTAD/Ag are achieved by fully doctor-blading technique in ambient condition, in which high-quality perovskite films with low trap-density are fabricated via two-step sequential deposition with a low temperature process by simultaneously introducing composition engineering and additive-doping technology. Organic cation is added into the PbI2 precursor to reduce the uneven distribution of nucleation sites in the perovskite films during doctor-blading process and promote the uniform growth of perovskite grain. Moreover, 2,3,5,6-tetrafluoro-7,7,8,8-tetra-cyanoquinodimethane (F4-TCNQ) acted as the doping additive is employed into perovskite, resulting in healing the perovskite grain boundary and reducing trap-density accordingly. As a result, the doctor-bladed PSCs fabricated in ambient condition exhibit the champion PCE of 18% and a stabilized efficiency of 17.7%. Furthermore, PSCs fabricated via fully doctor-blading in ambient condition achieve the PCE of 17.0% with negligible hysteresis. This work provides an important strategy for scalable fabrication of efficient PSCs in ambient condition and potentially accelerates the commercialization.

Normalized Lithium Growth from the Nucleation Stage for Dendrite-Free Lithium Metal Anodes.

Inducing uniform deposition of lithium from the stage of metal crystallization nucleation is of vital importance to achieve dendrite-free lithium anodes. Herein, using experiments and simulation, homogenization of Li nucleation and normalization of Li growth can be achieved on PNIPAM polymer brushes with lithiophilic functional groups modified Cu substrates. The lithiophilic functional groups of amide O can homogenize ion mass transfer and induce the uniform distribution of Li nucleation sites. What is more, the ultra-small space between each brush can act as the channels for Li transportation and normalization growth. Owing to the synergistic effect of homogenization and normalization of electrodeposited Li, the obtained planar columnar Li anode exhibits excellent cycle stability at an ultra-high current density of 20 mA cm-2 .

Normalized Lithium Growth from the Nucleation Stage for Dendrite‐Free Lithium Metal Anodes

AbstractInducing uniform deposition of lithium from the stage of metal crystallization nucleation is of vital importance to achieve dendrite‐free lithium anodes. Herein, using experiments and simulation, homogenization of Li nucleation and normalization of Li growth can be achieved on PNIPAM polymer brushes with lithiophilic functional groups modified Cu substrates. The lithiophilic functional groups of amide O can homogenize ion mass transfer and induce the uniform distribution of Li nucleation sites. What is more, the ultra‐small space between each brush can act as the channels for Li transportation and normalization growth. Owing to the synergistic effect of homogenization and normalization of electrodeposited Li, the obtained planar columnar Li anode exhibits excellent cycle stability at an ultra‐high current density of 20 mA cm−2.

Simulation of Drop-Size Distribution During Dropwise and Jumping Drop Condensation on a Vertical Surface: Implications for Heat Transfer Modeling.

Accurate models for condensation heat transfer are necessary to improve condenser design. Drop-size distribution is an important aspect of heat transfer modeling that is difficult to measure for small drop sizes. The present work uses a numerical simulation of condensation which incorporates the possibility of coalescence and coalescence-induced jumping over a range of drop sizes. Results of the simulation are compared with previous theoretical models and the impact of the assumptions used in those models is explored. In particular, previous drop-size distribution models may predict heat transfer rates less accurately for high contact angles and for coalescence-induced jumping since coalescence occurs over a range of drop sizes and does not always result in departure. The influence of various input parameters (nucleation site distribution approach, nucleation site density, contact angle, maximum drop size, heat transfer modeling to individual drops, and minimum jumping size) on the drop-size distribution and overall heat transfer rate is explored. Assignment of the nucleation site spatial distribution and heat transfer model affect both the drop-size distribution and predicted overall heat transfer rate. Results from the simulation suggest that, when the contact angle is large (as on superhydrophobic surfaces) and no coalescence-induced jumping occurs, the heat transfer may not be as sensitive to the maximum drop-size as previously supposed. Furthermore, this work suggests that when coalescence-induced jumping occurs, reducing the maximum drop size may not always increase heat transfer since drops similar in size to those removed by coalescence-induced jumping can contribute significantly to the overall heat transfer rate.

Enhanced Environmental Scanning Electron Microscopy Using Phase Reconstruction and Its Application in Condensation.

Environmental scanning electron microscopy (ESEM) is a broadly utilized nanoscale inspection technique capable of imaging wet or insulating samples. It extends the application of conventional scanning electron microscopy (SEM) and has been extensively used to study the behavior of liquid, polymer, and biomaterials by allowing for a gaseous environment. However, the presence of gas in the chamber can severely degrade the image resolution and contrast. This typically limits the ESEM operating pressure below 1000 Pa. The dynamic interactions, which require even-higher sensitivity and resolution, are particularly challenging to resolve at high-pressure conditions. Here, we present an enhanced ESEM technique using phase reconstruction to extend the limits of the ESEM operating pressure while improving the image quality, which is useful for sensing weak scattering from transparent or nanoscale samples. We applied this method to investigate the dynamics of condensing droplets, as an example case, which is of fundamental importance and has many industrial applications. We visualized dynamic processes such as single-droplet growth and droplet coalescence where the operating pressure range was extended from 1000 to 2500 Pa. Moreover, we detected the distribution of nucleation sites on the nanostructured surfaces. Such nanoscale sensing has been challenging previously due to the limitation of resolution and sensitivity. Our work provides a simple approach for high-performance ESEM imaging at high-pressure conditions without changes to the hardware and can be widely applied to investigate a broad range of static and dynamic processes.

Electron‐Transport‐Layer‐Assisted Crystallization of Perovskite Films for High‐Efficiency Planar Heterojunction Solar Cells

AbstractCrystal engineering of CH3NH3PbI3 perovskite materials through template‐directed nucleation and growth on PbI2 nuclei dispersed in a polar fullerene (C60 pyrrolidine tris‐acid, CPTA) electron transport layer (ETL) (CPTA:PbI2) is proposed as a route for controlling crystallization kinetics and grain sizes. Chemical analysis of the CPTA:PbI2 template confirms that CPTA carboxylic acid groups can form a monodentate or bidentate chelate with Pb(II), resulting in a lower nucleation barrier that promotes rapid formation of the tetragonal perovskite phase. Moreover, it is demonstrated that a uniform CH3NH3PbI3 film with highly crystalline and large domain sizes can be realized by increasing the spacing between nuclei to retard perovskite crystal growth via careful control of the preferred nucleation site distribution in the CPTA:PbI2 layer. The improved perovskite morphology possesses a long photoluminescence lifetime and efficient photocarrier transport/separation properties to eliminate the hysteresis effect. The corresponding planar heterojunction photovoltaic yields a high power conversion efficiency (PCE) of 20.20%, with a high fill factor (FF) of 81.13%. The average PCE and FF values for 30 devices are 19.03% ± 0.57% and 78.67% ± 2.13%, respectively. The results indicate that this ETL template‐assisted crystallization strategy can be applied to other organometal halide perovskite‐based systems.

Shock responses of nanoporous aluminum by molecular dynamics simulations

We present systematic investigations examining the shock responses of nanoporous aluminum (np-Al) by nonequilibrium molecular dynamics simulations. The dislocation nucleation sites are observed to be concentrated in the low latitude region near the equator of the spherical void surfaces. We propose a continuum wave reflection theory and a resolved shear stress model to explain the distribution of dislocation nucleation sites. The simulations reveals two mechanisms of void collapse: the plasticity mechanism and the internal jetting mechanism. The plasticity mechanism, which leads to transverse collapse of voids, prevails under relatively weaker shocks; the internal jetting mechanism, which leads to longitudinal filling of the void vacuum, plays a more significant role as the shock intensity increases. In addition, we observed that the temperature rises while the pressure drops for tens of picoseconds in the shocked np-Al. This thermodynamic phenomenon is different from that observed in single-crystal Al, where the temperature and pressure immediately reach the Hugoniot constants. The influences of void collapse on spall fracture of np-Al are studied. Under the same loading velocity, the spall strength of np-Al is observed to be lower than that of single-crystal Al; however, the spall resistance is higher in np-Al than in single-crystal Al. This resistance is explained by the combined influences of thermal dissipation and stress attenuation during shock wave propagation in np-Al.

Fabrication of few-layer graphene film based field effect transistor and its application for trace-detection of herbicide atrazine

We describe the fabrication of highly sensitive graphene-based field effect transistor (FET) enzymatic biosensor for trace-detection of atrazine. The few-layers graphene films were prepared on polycrystalline copper foils by atmospheric pressure chemical vapor deposition method using an argon/hydrogen/methane mixture. The characteristics of graphene films were investigated by scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. The results indicated low uniformity of graphene layers, which is probably induced by heterogeneous distribution of graphene nucleation sites on the Cu surface. The pesticide detection is accomplished through the measurement of the drain-source current variations of the FET sensor upon the urea enzymatic hydrolysis reaction. The obtained biosensor is able to detect atrazine with a sensitivity of 56 μA/logCATZ in range between 2 × 10−4 and 20 ppb and has a limit of detection as low as 0.05 ppt. The elaboration of such highly sensitive biosensors will provide better biosensing performances for the detection of biochemical targets.

Misfit Dislocation Guided Topographic and Conduction Patterning in Complex Oxide Epitaxial Thin Films

Interfacial dissimilarity has emerged in recent years as the cornerstone of emergent interfacial phenomena, while enabling the control of electrical transport and magnetic behavior of complex oxide epitaxial films. As a step further toward the lateral miniaturization of functional nanostructures, this work uncovers the role of misfit dislocations in creating periodic surface strain patterns that can be efficiently used to control the spatial modulation of mass transport phenomena and bandwidth‐dependent properties on a ≈20 nm length scale. The spontaneous formation of surface strain‐relief patterns in La0.7Sr0.3MnO3/LaAlO3 films results in lateral periodic modulations of the surface chemical potential and tetragonal distortion, controlling the spatial distribution of preferential nucleation sites and the bandwidth of the epilayer, respectively. These results provide insights into the spontaneous formation of strain‐driven ordered surface patterns, topographic and functional, during the growth of complex oxide heterostructures on lengths scales far below the limits achievable through top‐down approaches.

Self-Organization of Microscale Condensate for Delayed Flooding of Nanostructured Superhydrophobic Surfaces.

Superhydrophobic surfaces enhance condensation by inhibiting the formation of an insulating liquid layer. While this produces efficient heat transfer at low supersaturations, superhydrophobicity has been shown to break down at increased supersaturations. As heat transfer increases, the random distribution and high density of nucleation sites produces pinned droplets, which lead to uncontrollable flooding. In this work, engineered variations in wettability are used to promote the self-organization of microscale droplets, which is shown to effectively delay flooding. Virus-templated superhydrophobic surfaces are patterned with an array of superhydrophilic islands designed to minimize surface adhesion while promoting spatial order. By use of optical and electron microscopy, the surfaces are optimized and characterized during condensation. Mixed wettability imparts spatial order not only through preferential nucleation but more importantly through the self-organization of coalescing droplets at high supersaturations. The self-organization of microscale droplets (diameters of <25 μm) is shown to effectively delay flooding and govern the global wetting behavior of larger droplets (diameters of >1 mm) on the surface. As heat transfer increases, the surfaces transition from jumping-mode to shedding-mode removal with no flooding. This demonstrates the ability to engineer surfaces to resist flooding and can act as the basis for developing robust superhydrophobic surfaces for condensation applications.

Modeling on the solidification structure of Fe-Ni-based alloys using cellular automaton method

In the present work, the solidification structure of the Fe-Ni-based alloy under vacuum condition using Cellular Automaton Method has been established with the commercial software ProCAST. Based on a three-dimensional (3D) cellular automata method, the nucleation on the ingot surface and in the bulk liquid has been simulated using two distributions of nucleation sites. The predicted temperature profiles of the mold and the solidification structure of the ingot were compared with the measurement, showing a reasonable agreement. The validated model was then used to study the effects of undercooling, casting temperature, mold initial temperature and filling rate on the solidification structure of the ingot. The results show that, the undercooling is a critical parameter on the solidification. Meanwhile, a low filling rate and casting temperature can refine the grains and extend the equiaxed crystal fraction, but the initial temperature of the mold has little effect on the final solidification structure.

Nucleation and Growth Behavior of Electrodeposited Lithium in Ionic Liquid

Metallic lithium is a promising anode candidate for rechargeable battery; however, the dendrite formation during charging should be suppressed to improve safety and cycleability. Many researches have been reported from the viewpoint of the control of the surface film on lithium metal1. Ionic liquids nowadays attract much attention from electrochemists, some of which have excellent stability at highly cathodic states (such as Li/Li+). Our group has focused on their unique features and utilized these ionic liquids as electrolyte with lithium metal anodes. In the present study, in order to determine the condition for the non-dendritic lithium deposition, we investigated the electrodeposition behavior during nucleation and growth state in ionic liquids based on overpotential and Li-ion diffusion. 10 wt.% of Li[Tf2N] was added to [C3mpip][Tf2N], [C4mpyr][Tf2N], and [N6,1,1,1][Tf2N] as the electrolytes. Lithium metal was electrodeposited on nickel substrates at current densities ranging from 5 to 200 μA cm-2 for total charge amount of 0.1 and 3 C cm-2. The morphology of electrodeposited lithium was observed by SEM. For the nucleation state, we focused on the case with charge amount of 0.1 C cm-2. The deposits were better distributed when the current density was increased (Figure 1). Additionally, the deposits were better distributed when the electrolyte viscosity was increased. In these conditions, an increased overpotential was confirmed. This is believed to lead to a wide distribution of nucleation sites due to decreased critical radii of the nuclei2. On the contrary, for the growth state, we focused on the case with the charge amount of 3 C cm-2. More deposits were more dendritic when the current density was increased (Figure 2) and also when the electrolyte viscosity was increased. This result can be rationalized by considering that the lithium ion concentration extremely decreased at the electrode surface, since Li-ion diffusion is slower than the decrement in the concentration of Li-ion at the electrode surface3. These tendency is common for noble-metal electrodeposition. The reason why these tendency is also observed for less noble metals such as lithium is considered to be that the ionic liquid used in this study is very stable at the highly cathodic state and therefore less effect of native film formation on the deposition. 1. D. Aurbach, E. Zinigrad, Y. Cohen, H. Teller , Solid State Ionics, 148, 405 (2002). 2. H. Sano, H. Sakaebe, H. Senoh, H. Matsumoto, J. Electrochem. Soc., Submitted. 3. H. Sano, H. Sakaebe, H. Senoh, H. Matsumoto, J. Electrochem. Soc., 161, A1236 (2014). Figure 1

CHF prediction model research for flat and hypervapotron heating surface in subcooled flow nucleate boiling

Based on fundamental nucleation boiling theory, nucleation site distribution and hot spot model, a theoretical critical heat flux (CHF) prediction model in subcooled flow boiling is presented in this paper. With the increase of heat flux, nucleation site density will finally approach the limit of site population. When the critical number of nucleation sites is reached in an area, it will cut off the supply of liquid for the central site, and then CHF emerges. In this paper, an improved hot spot model is proposed, which uses statistical bubble departure diameter and standard square shape. CHF prediction can be reasonably achieved based on fully developed boiling heat transfer curve, nucleation site density and distribution. Some related experimental data are referred and R134a subcooled flow boiling data on flat and hypervapotron test section are reported. Comparison results showed a good prediction of CHF.

Role of machining defects and residual stress on the AISI 304 fatigue crack nucleation

AbstractMachining defects as rebuilt material and dislodgement were often induced by cutting of difficult to machining AISI 304 stainless steel. Their density increases with a decreasing of cutting speed. The effect of these defects on surface topography and residual stress was evaluated by roughness and X‐ray diffraction measurements coupled with numerical simulation. The role of the rebuilt material on the distribution of fatigue crack nucleation sites was investigated by scanning electron microscope examination of post fatigue samples loaded at different imposed strain amplitudes. The association of machining defects and fatigue crack nucleation sites was attributed to the contribution of additional tensile residual stresses induced by rebuilt material rather than local stress concentration. Moreover, the fatigue crack coalescence is promoted by increasing the rebuilt material density. When the machining defect density increases from 5 to 60 particles/mm2, the fatigue life decreases from −22% to −65% with respect to the electropolished surface.

Molecular dynamics study of the micro-spallation of single crystal tin

The micro-spallation of single crystal tin is studied using the molecular dynamics method. Firstly, the Hugoniot curves of tin are obtained and compared with experimental data. Secondly, the dynamical evolutionary processes of micro-spallation are investigated under various impact velocities via non-equilibrium molecular dynamics. The simulated results reasonably represent the ejection phenomena that damage, and the liquid fragments that accompany, the reflection of a strong shock wave at the free surface of the target. Our atomic-scale analysis demonstrates that the micro-mechanism of micro-spallation is attributed to melting and cavitation, i.e., nucleation, growth and coalescence of voids. Also, the primary difference between classical spallation and micro-spallation lies in the amount of voids and the distribution of void nucleation sites. The void volume distribution is quantitatively described by a power function. The relation between the formation of voids and thermal dissipation is also qualitatively discussed.

Dendritic Growth in Mg-Based Alloys: Phase-Field Simulations and Experimental Verification by X-ray Synchrotron Tomography

Changes in polycrystalline dendritic growth patterns during solidification result in a variety of solidified dendritic structures and morphologies. These microstructural changes are induced by a variety of effects such as the random distribution of nucleation sites and orientations, the interaction of growing individual dendritic grains, and effects of solid-liquid interfacial energy anisotropy. Here, we have studied the formation of the complicated and diverse dendrite morphologies both experimentally, by electron backscatter diffraction and by X-ray tomography; and numerically by three-dimensional phase-field simulations. Three binary magnesium alloys were considered in this study: Mg-Al, Mg-Zn, and Mg-Sn alloys. We show that the solidification microstructure can be attributed to the following factors: The interaction of the growing dendrites, the anisotropy of the growth, and the distribution and initial random orientations of nucleation sites.

Coupling of Crystal Plasticity Finite Element and Phase Field Methods for the Prediction of SRX Kinetics after Hot Working

The conventional approach to model static recrystallization (SRX) kinetics is to use the well‐known Kolmogorov–Johnson–Mehl–Avrami (KJMA) equation. However, the validity of KJMA kinetics depends on whether the real recrystallization process follows the assumptions made in the derivation of the KJMA kinetics, which are a random distribution of nucleation sites, isotropic growth and a constant growth velocity. When these assumptions are violated the KJMA kinetics loose their physical meaning and turn into an empirical formula with limited predictive capability. In such cases, phase field (PF) method can be an alternative to the KJMA model, since it is based on irreversible thermodynamics. An effective strategy to apply the PF method to static recrystallization problems is coupling it to a deformation model though which the prior deformation process is simulated. For that purpose crystal plasticity finite element method (CPFEM) can be employed. In this work, an example of a weak coupling strategy was applied to a steel grade of commercial interest (X60Mn23 TWIP steel) for which KJMA equation is invalid. Deformation of a Voronoi type representative volume element was modeled using a Crystal plasticity finite element formulation after determining the material parameters of a strain hardening model from macroscopic experiments. Afterwards, the deformed structure was transferred on to a finite difference grid with a dedicated mapping scheme, which was then used as input for phase field simulation. Finally, the simulation results were compared with the results of stress relaxation tests. It was found that although the grain morphology of the simulation differed from the experimental results, the coupled simulation strategy was able to mimic the main features of the static recrystallization (SRX) kinetics.

Directed Actin Assembly and Contractility

The organization of actin filaments into higher-ordered structures governs eukaryotic cell shape and movement. Global actin network size and architecture is maintained in a dynamic steady-state through regulated assembly and disassembly. We have developed a micropatterning method that enables the spatial control of actin nucleation sites for in vitro assays (Reymann, Nat Mat, 2010). These actin templates were used to evaluate the response of oriented actin structures to myosin-induced contractility. We determine that myosins selectively contract and disassemble anti-parallel actin structures while parallel actin bundles remain unaffected. In addition, the local distribution of nucleation sites and the resulting orientation of actin filaments regulate the scalability of the contraction process. This “orientation selection” mechanism for selective contraction and disassembly reveals how the dynamics of the cellular actin cytoskeleton is spatially controlled by actomyosin contractility. Further application of the micropatterning method will be presented in particular recent data on the reconstitution of a lamellipodium-type of actin organization and the fabrication of three-dimensional electrical connections by means of directed actin self-organization.