Nucleation Characteristics Research Articles

Effect of Gas Atmosphere in the Heating Stage on Limiting Nucleation of Graphene on Copper Foils by Low Pressure Chemical Vapor Deposition

AbstractTo investigate the effect of gas atmosphere in the heating stage of graphene fabrication on graphene nucleation, hydrogen and nitrogen are introduced into the furnace respectively in the heating stage. The results show that graphene growing on copper foils heated in nitrogen atmosphere and in hydrogen atmosphere present different nucleation characteristics. The analysis implies that the gas atmosphere in the heating stage affects the graphene nucleation and growth by changing the morphology of the copper surface and the inherent carbon concentration in the substrate, and this change actually results from the interaction of the copper surface and the gas atmosphere. The difference in size and quality of the fabricated graphene domains also indicates nitrogen atmosphere in the heating stage favors fabricating large‐sized graphene domains with good quality. Thus, substituting hydrogen for nitrogen in the heating stage would be a simpler and easier way to depress graphene nucleation as well as improve the graphene quality.

Unified scaling law for rate factor of crystallization kinetics

Features of the crystallization kinetics define directly the rate characteristics: the crystal nucleation rate, the crystal growth rate and the so-called kinetic rate factor known also as the attachment rate (of particles to the surface of a crystalline nucleus). We show that the kinetic rate factor as function of the reduced temperature follows a unified scaled power law. This scenario is confirmed by our simulation results for model atomistic systems (crystallizing volumetric liquids and liquid thin film) and by available experimental data for crystallizing polymers. We find that the exponent of this unified scaling law is associated with a measure of the glass-forming ability of a system. The results of the present study extend the idea of a unified description of the rate characteristics of the crystal nucleation and growth kinetics by means of the scaling relations.

Pressure Induced Structural Changes of Proteins Affecting the Ice Nucleation Temperature of Pork Loins.

This study investigated the effects of pressure-mediated protein changes on the ice nucleation temperature of pork loins. To variate chemical state of meat proteins, pork loin was pressurized at varying pressure levels (100–500 MPa) for 3 min, and moisture content, expressible moisture (EM) and differential scanning calorimetry (DSC) were analyzed. Although, all treatments showed similar moisture content, EM and degree of protein unfolding of pork loin showed different features as of 300 MPa. At moderate pressure treatments (100–200 MPa), all protein fractions were detected in DSC experiments, and pork loin had lower EM than control (p<0.05). Meanwhile, myosin and actin of pork loin treated at greater than 300 MPa were completely unfolded, and the treatments showed high EM compared to control (p<0.05). Unfolding of meat proteins was a factor suppressing ice nucleation, and the ice nucleation temperature tended to decrease with increasing applied pressure level. The ice nucleation characteristics of pressurized pork loin exhibited a potential application in freezing storage of pressurized meat with less tissue damage comparing to freeze fresh meat, and further exploration regarding the quality change after freezing of fresh and pressurized meat was warranted.

Investigation of NbC/TiC Heterogeneous Nucleation Interface by First-Principles and Experimental Methods

The mechanism of the heterogeneous nucleation of NbC on TiC precipitates was investigated systematically in this paper. The interfacial properties of NbC (100)/TiC (100), NbC (110)/TiC (110), and NbC (111)/TiC (111) interfaces were studied by first-principles calculations. The results showed that the NbC (111)/TiC (111) interface with the Nb–C bond is the most stable one, and the stability of interfaces with the C–Ti, Nb–Ti, and C–C bonds decreases in turn. The interface of the Nb/C-terminated and Third Layer (TL) stacking sequence (NCTL) has the largest adhesion work (10.15 J/m2) and the smallest equilibrium interface spacing (1.290 Å). In the range of low niobium (Nb) chemical potential and high carbon (C) chemical potential, the nucleation of NbC on TiC precipitates takes precedence over the epitaxy growth in the coherent relationship of [ 1 1 ¯ 0 ] ( 111 ) NbC / / [ 1 1 ¯ 0 ] ( 111 ) TiC , while the nucleation of NbC on TiC precipitates is prior to the epitaxy growth in the coherent relationship of [ 001 ] ( 100 ) NbC / / [ 001 ] ( 100 ) TiC in the range of high niobium (Nb) chemical potential and low carbon (C) chemical potential. Besides, the characteristics of heterogeneous nucleation precipitates in Nb–Ti microalloyed steels were analyzed by transmission electron microscopy (TEM). The orientation relationship between the (Ti, Nb) C and (Nb, Ti) C precipitates follows [ 1 1 ¯ 0 ] ( 111 ) ( Nb , TiC ) / / [ 1 1 ¯ 0 ] ( 111 ) ( Ti , Nb ) C , which is consistent with the calculated result.

Bubble nucleation, micro-explosion and residue formation in superheated jatropha oil droplet: The phenomena of vapor plume and vapor cloud

The evaporation and secondary atomization of droplets in cylinder has been confirmed to be one of major factors determining combustion efficiency. Experiments were carried out to elucidate the bubble nucleation and micro-explosion characteristics of superheated jatropha oil (JO) droplet during evaporation process at 873, 973, 1073 K and atmospheric pressure. A single droplet was placed at the intersection of two quartz fiber and introduced rapidly into a high temperature chamber. Several interesting features such as bubble nucleation, bubble growth and coalescence, puffing, vapor jetting, droplet jetting, surface pit, floating raft and micro-explosion were observed. Two different types of micro-explosion, namely global and local micro-explosion were identified. It was also found that chemical reactions occurred during the evaporation process of JO droplet. Bubbles in JO droplet were produced partly by superheating of low boiling point fatty acid and partly by pyrolysis of long-chain fatty acids. There were four modes of bubble nucleation in JO droplet: surface pit nucleation, particle-droplet interface nucleation, fiber-droplet interface nucleation, and floating raft interface nucleation. Surface pit nucleation and floating raft interface nucleation were the main mode. The evaporation residue formed at the end of evaporation, which was caused by the polymerization and aromatization reactions. Most importantly, vapor plume and vapor cloud induced by evaporation have been discovered for the first time. Vapor loud was very similar to the liquid state in the dilute spray region. These were mainly due to the non-isothermal condensation caused by Stefan outflow.

In Situ Observation of Acicular Ferrite Nucleation and Growth at Different Cooling Rate in Ti-Zr Deoxidized Steel

The effects of cooling rate on acicular ferrite (AF) nucleation, growth, and inclusion characteristics in Ti-Zr deoxidation steel were studied by utilizing the high temperature confocal laser scanning microscope (HT-CLSM). The results indicated that with the increase of cooling rate, the ferrite start nucleation temperature decreased, and the difference of first nucleation temperature between AF and ferrite side plate (FSP) reduced. When the cooling rate increased to 10.0 °C/s, AF and FSP simultaneously nucleated at 564.5 °C. In addition, the AF actual growth rate rose with the increase of cooling rate and reached 30.13 µm/s at 10.0 °C/s cooling rate. The AF ratio in microstructure increased first and then decreased with the cooling rate increase and was up to the maximum 45.83 pct at 1.0 °C/s cooling rate. For inclusion characteristics, cooling rates had no obvious effect on inclusion types, but had a great influence on inclusions size distribution. With the cooling rate increase, the inclusion average diameter reduced, and diminished to 1.39 µm at 10.0 °C/s cooling rate. Finally, the AF nucleation on the Ti-Zr-Mn-O-S + TiN inclusion could be explained by the low lattice misfit between ferrite and TiN that precipitated on the Ti-Zr-Mn-O-S inclusion surface.

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.

Electrodeposition of Indium on Glassy Carbon from Tetrabutylammonium Chloride Containing Solutions

The effectiveness of tetrabutylammonium chloride (TBACh) as inhibition additive of dendritic growth of indium has been investigated by means of cyclic voltammetry and chronoamperometry methods. The rotating disk electrode (RDE) method allowed the calculation of the diffusion coefficient of In3+ ions using the Levich equation, at 25 °C is 4.41 × 10–6 cm2/s. Diffusion coefficient of indium ions determined by chronoamperometry using the Cottrell law (6.63 × 10–6 cm2/s) is in consistent with the value calculated by the Levich equation. The addition of tetrabutylammonium ions to the electrolyte reduces the diffusion coefficient and inhibits the cathodic process by increasing the activation energy from 10.5 kJ/mol to 20.7 kJ/mol. The indium nucleation and growth on glassy carbon in chloride solutions was studied by single potentiostatic pulse techniques. The nucleation mechanism was evaluated by analyzing the influence of different TBACh ion concentration and applied potentials. The electrocrystallization mechanisms were determined by fitting the experimental non-dimensional current transients on the basis nucleation and growth model developed by Scharifker-Hills. The type of nucleation corresponding to the progressive three-dimensional nucleation with diffusion control is determined. Based on theoretical models of 3D multiple nucleation from the potentiostatic current transients were calculated nucleation characteristics, such as the stationary nucleation rate, saturation nucleus density and the average grains radius of indium deposits. The leveling action of TBACh on the electrodeposition of indium at concentration of 10-4 M was found.

Bainite growth retardation due to mechanical stabilisation of austenite

This study shows the first ever attempt to calculate bainite kinetics based on characteristics of nucleation as well as displacive growth. A model for bainite kinetics is proposed whereby in conjunction to nucleation the displacive growth of laths is described by a growth factor which is assumed to vary linearly with the net driving force and thereby ensures consistency with the thermodynamic growth criterion. Furthermore, the growth factor is supposed to have an inverse dependence on the elastic strain energy to account for retardation of the growth due to mechanical stabilisation. When a bainitic lath is formed the surrounding austenite is work hardened due to the plastic accommodation of the shape change associated with the transformation. The dislocation strengthening is assumed to mechanically stabilize the austenite against subsequent transformation to bainite and/or martensite. Geometrical constraints are also assumed to contribute to the mechanical stabilisation of untransformed austenite in partially bainitic microstructures. The overall degree of mechanical stabilisation has been quantified for low-Si steels by measuring the decrease in start temperature of the second phase martensite after partial bainitic transformations. Based on these experiments an empirical equation has been developed to describe the decrease in the growth factor with progressing transformation. Subsequently, the model has been validated using numerous fraction curves of 10 different low-Si steels. Fits have been optimized by adjusting only the autocatalysis factor for each holding temperature. Owing to the growth factor the model accurately describes the strong retardation in kinetics in the second stage of transformation.

High-temperature nanoindentation size effect in fluorite material

Micromechanical loading events such as pop-in and indentation size effect (ISE) are well understood at room temperature and have been widely accepted as fundamental factors in the study of material mechanics at the microscale. Experimentally it has been observed that such phenomena can be greatly affected by temperature, but quantitative studies of temperature effects are rarely conducted. This paper presents the effects of high-temperature on the pop-in load and ISE of calcium fluoride (CaF2) single crystal material through in-situ high-temperature nanoindentation experiments and explains the localized dislocation events at the atomic level with molecular dynamic (MD) calculations. Our experimental results show that an increase in temperature leads to a reduction of the pop-in load, the hardness of the material, and the material dependent length scale during plasticity (defined in the Nix-Gao relation). Our computed results show that the differences in dislocation nucleation characteristics of CaF2 at various temperatures are the main reasons for the changes in its material properties. Such an experimental-computational study provides a comprehensive analysis of the relationships between temperature and dislocation mechanisms during nanoindentation.

Experimental investigation of progressive cracking processes in granite under uniaxial loading using digital imaging and AE techniques

For a full understanding of the cracking processes in granite, uniaxial compression tests are conducted on prismatic specimens with pre-existing flaws, and digital imaging and AE techniques are applied to monitor real-time rupture behaviors. The digital image correlation (DIC) method is used to detect the process zone nucleation characteristics in granite. A novel approach for representing AE data in terms of the inter-event time (IET) function F(τ) is employed to analyze fracture-related AE event rate characteristics. Based on the coupled analyses of the acousto-optic-mechanical characteristics, the complete cracking processes of granite are classified into six levels, which are distinguishable by five characteristic stresses. Process zone nucleation, which represents the clustering of micro-cracks, is incorporated into the classification for the first time. Process zone nucleation is visualized as a white patch and is either linear or diffuse. By DIC analysis, the maximum principal strain field reliably predicts a linear white patch, and the maximum shear strain field implies a diffuse white patch. The evolutionary rules of the IET function F(τ) strongly support the new classification of cracking levels in granite. For the identification of stress thresholds and cracking levels, a combined acousto-optic-mechanical methodology is established, in which the role of the IET function F(τ) is highlighted.

Study of the Microstructure and Ring Element Segregation Zone of Spray Deposited SiCp/7055Al.

Composites of 7055 aluminum (Al) matrix reinforced with SiC particles were prepared using the spray deposition method. The volume fraction of the phase reinforced with SiC particles was 17%. The effect of the introduction of SiC particles on the deposited microstructure and properties of the composites was studied in order to facilitate the follow-up study. The structure and element enrichment zone of spray-deposited SiCp/7055 Al matrix composites were studied by Optical Microscope (OM), X-ray diffraction (XRD), Scanning Electronic Microscopy (SEM) and Transmission electron microscopy (TEM). The results show that the reinforcement phases of the SiC particles were uniformly distributed on the macro and micro levels, and a few SiC particles were segregated into annular closed regions. C and Si on the surface of SiC particles diffused to the Al matrix. The distribution of the two elements was gradient weakening with SiC particles as the center, and the enrichment zones of Si, Mg and Cu formed in the middle of the closed annular area of a few SiC particles. The enrichment zones were mainly composed of alpha-Al, SiC, Al2CuMg, Al2Cu and MgZn2. AlCu and AlMgCu phase precipitate on the surface of the SiC particles, beside the particle boundary, and had the characteristics of preferred nucleation. They tended to grow at the edges and corners of SiC particles. It was observed that the formation of nanoparticles in the alloy had a pinning effect on dislocations. The different cooling rates of the SiC particles and the Al matrix led to different aluminum liquid particle sizes, ranging from 20 to 150 μm. In the region surrounded by SiC particles, the phenomenon of large particles extruding small particles was widespread. Tearing edges and cracks continued to propagate around the SiC particles, increasing their propagation journey and delaying the fracture of the materials.

Investigation on heterogeneous nucleation substrate of Y2O3 as NbC in hypereutectic Fe–Cr–C hardfacing coating by experiment and first-principles calculation

Three kinds of hypereutectic Fe–Cr–C hardfacing coatings were designed to research the refinement effect and heterogeneous nucleation mechanism of NbC on nano-Y2O3 in the coatings. The microstructure and elemental maps of Fe–Cr–C–Nb–Y2O3 coating were investigated by field emission scanning electron microscope. The phase structures of the coating and heterogeneous nucleation were researched by transmission electron microscope (TEM). The lattice mismatch between Y2O3 and NbC was computed by Bramfitt’s two-dimensional lattice mismatch theory. The adhesive strength, interface energy and electronic properties of Y2O3(111)/NbC(110) interfaces were calculated by a first-principles method. The bonding characteristics of heterogeneous nucleation interfaces were analyzed by charge density difference, electron localization function and Bader charge population. The experimental results show that the primary M7C3 can be refined by NbC, when Nb additive was added into the Fe–Cr–C coating. When nano-Y2O3 and Nb additive were added into the coating simultaneously, the primary M7C3 can be further refined. The SEM results show that NbC can be effective heterogeneous nucleation substrate of the primary M7C3 and there is a compound containing rare earth (RE) element Y in NbC particle. The TEM results verified the RE compound in NbC particle is Y2O3, and they combine together tightly. The calculation results indicate that the two-dimensional lattice mismatch of Y2O3(111)/NbC(110) interface is 6.8%, which reveals that Y2O3 meets the geometric condition to act as a medium-effective heterogeneous nucleation substrate of NbC. In all interface models, the adhesive strength of O-Bridge model is the largest (Wad = 9.75 J/m2) and its stability is the best (γ = − 0.87 J/m2). The major bond type of O-terminated interfaces is covalent one. Actually, the interfacial energy of O-terminated interface is negative, which reveals that their formation process is a process of energy reduction and structure stabilization. It means Y2O3 and NbC can form stable interface structure. It can be predicted that the heterogeneous nucleation occurs preferably on the O-terminated interfaces. Therefore, Y2O3 can serve as an effective heterogeneous nucleation substrate of NbC to refine it.

Shape-Selective Electroless Plating within Expanding Template Pores: Etching-Assisted Deposition of Spiky Nickel Nanotube Networks.

Nano-objects are favored structures for applications such as catalysis and sensing. Although they already provide a large surface-to-volume ratio, this ratio can be further increased by shape-selective plating of the nanostructure surfaces. This process combines the conformity of autocatalytic deposition with the defined nucleation and growth characteristics of colloidal nanoparticle syntheses. However, many aspects of such reactions are still not fully understood. In this study, we investigate in detail the growth of spiky nickel nanotubes in polycarbonate template membranes. One distinctive feature of our synthesis is the simultaneous growth of nanospikes on both the inside and outside of nanotubes while the tubes are still embedded in the polymer. This is achieved by combining the plating process with locally enhanced in situ etching of the poylmer template, for which we propose a theory. Electron microscopy investigations reveal twinning defects as the driving force for the growth of crystalline nanospikes. Deposit crystallinity is ensured by the reducing agent hydrazine. Iminodiacetic acid is not only used as a complexing agent during synthesis but apparently also acts as a capping agent and limits random nucleation on the spike facets. Finally, we apply our synthesis to templates with interconnected pores to obtain free-standing spiky nickel nanotube networks, demonstrating its ability to homogeneously coat substrates with extended inner surfaces and to operate in nanoscale confinement.

Experimental study of wall nucleation characteristics in flow boiling under subatmospheric pressures in a vertical square channel

Experiments have been conducted in a vertical square channel to investigate wall nucleation characteristics in subcooled boiling flows under subatmospheric pressure. The objective of this work is to provide a unique dataset to evaluate the scalability of existing models of nucleation characteristics for flow boiling conditions below atmospheric pressure as the performance of these models has not been investigated under these conditions. The experimental data is presented as a parametric study of the effects of important dimensionless groups on nucleation characteristics, specifically boiling number, Jakob number, and density ratio. Bubble departure diameters and departure frequencies are measured and compared to existing models. Some existing models are shown to predict departure diameter well but mainly overpredict departure frequency at low pressure.

Bainite Growth Retardation Due to Mechanical Stabilisation of Austenite

This study shows the first ever attempt to calculate bainite kinetics based on characteristics of nucleation as well as displacive growth. A model for bainite kinetics is proposed whereby in conjunction to nucleation the displacive growth of laths is described by a growth factor which is assumed to vary linearly with the net driving force and thereby ensures consistency with the thermodynamic growth criterion. Furthermore, the growth factor is supposed to have an inverse dependence on the elastic strain energy to account for retardation of the growth due to mechanical stabilisation. When a bainitic lath is formed the surrounding austenite is work hardened due to the plastic accommodation of the shape change associated with the transformation. The dislocation strengthening is assumed to mechanically stabilize the austenite against subsequent transformation to bainite and/or martensite. Geometrical constraints are also assumed to contribute to the mechanical stabilisation of untransformed austenite in partially bainitic microstructures. The overall degree of mechanical stabilisation has been quantified for low-Si steels by measuring the decrease in start temperature of the second phase martensite after partial bainitic transformations. Based on these experiments an empirical equation has been developed to describe the decrease in the growth factor with progressing transformation. Subsequently, the model has been validated using numerous fraction curves of 10 different low-Si steels. Fits have been optimized by adjusting only the autocatalysis factor for each holding temperature. Owing to the growth factor the model accurately describes the strong retardation in kinetics in the second stage of transformation.

Computational fluid dynamics simulation of the supersonic steam ejector using different condensation model

This paper addresses the non-equilibrium condensation (NEC) in supersonic steam ejector under the assumptions of no slip velocity between the droplets and vapor phase and homogenous nucleation. The experimental data carried out by Moore has been used to verify the numerical results. It is illustrated that the maximum value of the flow mach number of the NEC model is lower than that of the equilibrium condensation model, and NEC model increases the ejector?s entrainment ratio in comparison equilibrium condesation model. When using the NEC model, the nucleation characteristics such as subcooling degree, nucleation rate could be obtained in ejector flow field.

Моделирование нуклеации в бинарных сплавах на основе метода функционала плотности свободной энергии

In this study, the approach for nucleation rate calculation is developed on the basis of free energy density functional approach. The nucleation rate and characteristics of critical nucleus are calculated from results of composition field simulation, derived by the solution of deterministic or stochastic Cahn-Hilliard equation.

Phase Separation in Electrodeposited Ag-Pd Alloy Films from Acidic Nitrate Bath

The electrochemical deposition of Ag-Pd from acidic nitrate solutions has been studied. Under suitable conditions, phase separation in the form of solid solutions with different compositions is observed at the micron scale. The phase separation is not generated by electrochemical instabilities, but instead is due to a wide difference in the intrinsic reduction kinetics and the distinct nucleation characteristics. At UPD conditions Ag-Pd films could be grown via underpotential codeposition, in close agreement to thermodynamic predictions. At intermediate overpotentials, phase separation occurs due to local concentration gradients generated by mass transfer limitation for Ag and large nucleation barrier for Pd. Under mass transfer limitations, the growth is uniform and the composition corresponds to the prediction based on the ion diffusivities.

First laboratory results of supersaturated water nucleation characteristics at low temperatures

Researchers report nucleation rates and trends in nucleation cluster size of extremely supersaturated water vapor.