Heterogeneous Nucleation Characteristics Research Articles

Heterogeneous nucleation condensation of methane gas on the wall-A molecular dynamics study

Methane is condensed in a low-temperature heat exchanger, a critical step in the liquefaction of natural gas. Currently, the microscopic mechanism regarding the heterogeneous condensation of methane on the heat exchanger wall remains unclear. This lack of understanding is of scientific importance in advancing the development of natural gas liquefaction processes. Therefore, molecular dynamics simulation is used in this paper to study the process of methane heterogeneous nucleation and core growth during the early stage of condensation, and analyze the influence mechanism of wall energy and cold source temperature on nucleation kinetics. The results show that the high-energy wall can enhance the interaction between cold wall atoms and methane molecules, facilitate heat transfer, accrete methane molecules to condense and adsorb on the wall to form a core, thus increasing the condensation rate of methane. Whereas, the low-temperature cold source promotes the condensation nucleation and growth process by increasing the supersaturation of methane. This study investigates the process and kinetic characteristics of heterogeneous nucleation of methane from a microscale perspective, providing guidance for the development of natural gas liquefaction in low-temperature heat exchangers, with the aim of enhancing the diversity and reliability of energy supply.

Density functional theory investigation on the stability, adhesion strength, tensile properties and fracture behavior of γ-Fe/Cu heterogeneous nucleation interface

To theoretically study the stability and the heterogeneous nucleation characteristics of γ-Fe/Cu interface in steel/copper bimetallic, the six surface models and seven interface models were investigated through density functional theory. Besides, the interface stability, adhesion energy, electronic properties, tensile process and fracture behavior of three kinds typical Fe/Cu interface model were calculated by first-principles calculations. Simultaneously, the mechanism on heterogeneous nucleation of pure Fe on Cu-based alloy was revealed. The results indicate that the bramfitt’s lattice mismatches of all the three Fe/Cu interfaces were both 4.11%, and all the Cu surfaces and the Fe surfaces were trends to converge, when the number of layers were 9 and 7, respectively. Among three kinds Fe/Cu interfaces with HCP structure, the Fe (110)/Cu (110) interface shown higher strength of metallic bonds and stable ionic properties. When the strain is 37%, the maximum value of the tensile stress is about 20.3GPa and the electron density between Fe and Cu atoms drop to disappear. The calculated adhesion work of Fe (110)/Cu (110) interface with HCP structure is 1.88 J/m2, is higher than that between Cu/Cu melt (1.78 J/m2), which proved that the Fe particles can promote heterogeneous nucleation of Cu atoms and then account for the grain refinement of Cu-based alloy.

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.

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.

The degeneration of the heterogeneous nucleation for the large precipitates (Ti,Nb,V)(C,N) in ESR H13+Nb tool steel with low-MgO slag

The characteristics of the large heterogeneous precipitates (Ti,Nb,V)(C,N) with oxide nucleus in H13+Nb tool steel produced through ESR process were studied, and compared with the ones produced without the remelting process. ESR process with slag 50wt% CaF2 -35wt% Al2 O3 -10wt% CaO-5%wt% MgO has a negative effect on controlling the primary carbonitride for the larger amount of ones with size >5 μ m, which is partly due to the different heterogeneous nucleation characteristics. 100% of the nucleant oxide in the NON-ESR samples is Al2 O3 -MgO spinel with stoichiometric composition and the size is mostly located in 0.5 ~ 1.5μ m, which shows a strong selectivity on the nucleant inclusions. However, the trend is not obvious for the ESR samples in which the average value of Mg/Al atomic ratio in nucleant spinel is just 0.135. The behavior of heterogeneous nucleation on spinel with low Mg/Al ratio in ESR sample is closer to that on alumina, which leads to the degeneration of the heterogeneous nucleation. It is a part of reason for the increment of the precipitates size. The thermodynamic analyses show that it should keep the total content of Mg in liquid steel larger than 4 ppm if the oxide is located in the stoichiometric spinel phase region.

Thermodynamics and Characteristics of Heterogeneous Nucleation on Fractal Surfaces

Clarifying heterogeneous nucleation process on fractal surfaces is an important issue to understand its thermodynamic principles and forward the engineering aspects. We developed a thermodynamic model to capture the process of forming a crystal or drop embryo on fractal surfaces and the corresponding characteristic parameters. We showed that the differences between the critical size of the embryos on the fractal surfaces and those on the flat surfaces are negligible for the hydrophobic nuclei, but become significant for the hydrophilic nuclei. We reported the available domains of forming the critical embryos and obtained the shape function for the first time. The results also recover the data reported in the literature. Finally, we present the possible mechanisms of vanishing of the energy barriers against forming the critical embryos on the fractal surfaces as the apparent contact angle is relatively small. The obtained results may help to engineer the nucleation process by designing substrate surfaces i...

The characteristics of heterogeneous nucleation on concave surfaces and implications for directed nucleation or surface activity by surface nanopatterning

The characteristics of heterogeneous nucleation on concave spherical surfaces were analysed using a novel analytical approach and compared with nucleation on both convex and planar surfaces. The complex expressions of ∂ΔG/∂r (ΔG: free energy change and r: embryo radius) for nucleation on concave spherical surfaces can be reduced to the simple form for nucleation on planar surfaces by introducing an appropriate pseudo-contact angle, providing a new fundamental link. The advantages of heterogeneous nucleation on a concave spherical surface of radius R over a planar surface occur primarily when 2R<10r⁎ (r*: nucleus radius) and diminish rapidly when 2R>10r*. This is similar to nucleation on convex spherical surfaces, where the disadvantages of nucleation over a planar surface occur primarily when 2R<10r* and diminish rapidly when 2R>10r*. The substrate size 10r* thus provides an approximate borderline that distinguishes between curved and planar surfaces for nucleation. The advantages or disadvantages of nucleation over a planar surface are most outstanding when the concave (advantageous) or convex (disadvantageous) surface shows a specific contact angle with the nucleus. The nanoscale nature of the threshold size 10r* presents a clear fundamental support to surface nanopatterning for directed nucleation. A minimum depth is required for a nanoscale crater to be a favourable nucleation site.

Nucleation of pentacene thin films on silicon dioxide modified with hexamethyldisilazane

Nucleation and growth of pentacene on silicon dioxide surfaces modified with hexamethyldisilazane, HN[Si(CH3)3]2, has been examined using supersonic molecular beam techniques and atomic force microscopy. Similar to growth on clean SiO2 surfaces, the rate of deposition at a fixed incident flux decreases with increasing kinetic energy of incident pentacene, indicative of trapping mediated adsorption. Unlike clean, unmodified SiO2 surfaces, however, growth on the modified surface exhibits characteristics of heterogeneous nucleation, where the maximum island density is independent of deposition rate. Further, islands in the submonolayer regime are two molecules high, unlike the one molecule high islands observed on clean SiO2.

Morphology and thermal characteristics of nano-sized Pb–Sn inclusions in Al

Nanoembedded aluminum alloys with bimetallic dispersoids of Sn and Pb of compositions Sn82–Pb18, Sn64–Pb36, and Sn54–Pb46were synthesized by rapid solidification. The two phases, face-centered-cubic Pb and tetragonal Sn solid-solution, coexist in all the particles. The crystallographic relation between the two phases and the matrix depends upon the solidification pathways adopted by the particles. For Al–(Sn82–Pb18), we report a new orientation relation given by [011]Al//[010]Sn and (o11)A1//(101)Sn. Pb exhibits a cube-on-cube orientation with Al in few particles, while in others no orientation relationship could be observed. In contrast, Pb in Sn64–Pb36and Sn54–Pb46particles always exhibits cube-on-cube orientation with the matrix. Sn does not show any orientation relationship with Al or Pb in these cases. Differential scanning calorimetry studies revealed melting at eutectic temperature for all compositions, although solidification pathways are different. Attempts were made to correlate these with the melting and heterogeneous nucleation characteristics.

Experimental investigation of calcium silicate hydrate (C-S-H) nucleation

Due to the importance of calcium silicate hydrate (C-S-H) in cement chemistry, its nucleation mode and parameters influencing it were investigated. It has been observed that the C-S-H nucleation follows the general laws governing the nucleation. The degree of supersaturation has been found to be the main parameter controlling homogeneous nucleation rates. The lime concentration in solution, well known to be the most important parameter determining the kinetic, morphological and structural features of C-S-H, also controls the nucleation characteristics of heterogeneous nucleation, i.e. during hydration of cement. The correlation between heterogeneous nucleation of C-S-H and possible final mechanical properties of cement paste has been well established.

Elastic model of a dislocation center for martensite nucleation

The possibility of spontaneous nucleation of a crystal of new phase when the original structure is metastable is usually connected with the catalyzing effect of defects playing the role of nucleation centers. In the case of the {gamma}{r_arrow}{alpha} martensite transformation in iron alloys, even individual dislocations can act as such defects, based on analysis of long-range elastic fields of isolated linear dislocations in a linearly elastic anisotropic continuum, the authors established the existence of a correlation between the geometric characteristics of the elastically deformed state in the vicinity of 60-degree and 30-degree dislocations and the structure and morphological characteristics of {alpha}-martensite observed in massive iron alloy samples. These results suggest that the dislocation affects the pathway of the martensite reaction and allows the authors to say that the specific characteristics of heterogeneous nucleation of new phase for the martensite mechanism of the {gamma}{r_arrow}{alpha} transformation involves singling out a single structural rearrangement variant which is suitable from the standpoint of adapation of the transforming lattice to the characteristic features of the elastically deformed state created by the dislocation. The possibilities for such adaption are limited by the crystallography of the transformation and the reactions of the surrounding austenite occurring when regularmore » connections exist with the morphological characteristics of the martensite crystal, and are not necessarily compatible with the individual features of the elastic field of each dislocation. Considering this, the authors can introduce the concept of a dislocation center for nucleation of a martensite crystal about the region of the dislocation where conditions are realized which are favorable for the formation of a nucleus of martensite crystal of a certain shape and orientation, and they can develop an elastic model corresponding to this concept.« less

Heterogeneous nucleation characteristics of mixed vapor of benzene and cyclohexane on a cold substrate.

The heterogeneous phase transition of mixed benzene and cydohexane vapors on a cold copper substrate was observed by slowly cooling the substrate under reduced pressure conditions. In the conversion of the vapor to the condensed phase, vapor-to-liquid or vapor-to-solid phase transitions occurred depending on the mixing ratio of both vapors. By measuring the critical vapor pressure and temperature, corresponding critical supersaturation ratios were determined for various mixing ratios of both vapors. The critical pressure for the formation of solid benzene from mixed vapor agreed closely with that of pure vapor having higher composition. The critical pressure for the formation of solid cyclohexane from mixed vapor decreased from 6 to 9% compared with that from pure vapor. Liquid condensate was formed at critical supersaturation ratios of almost unity. A phase diagram was constructed by solving a non-linear simultaneous equation derived from vapor-liquid equilibrium and critical pressure-temperature relationships. It agreed in a practical sense with the observed results.

The crystallography of heterogeneous nucleation in a Ni-45 wt% Cr alloy

Abstract A detailed transmission electron microscopy study has been made of the crystallographic characteristics of heterogeneous nucleation at grain boundaries, twin boundaries and dislocations in a Ni-45 wt% Cr alloy. Nucleation at incoherent twin boundaries and grain boundaries was observed to be easier than at coherent twin boundaries or dislocations. In all cases a unique orientation relationship, Kurdjumov-Sachs (K-S) was developed between the precipitates and matrix. Nucleation at grain boundaries was favoured if the plane of the grain boundary lay close to (121)f, the low energy habit plane of the precipitate. Growth of the precipitate generally occurred into the grain with the irrational orientation relationship, but at coherent twins or grain boundaries which had a near twin orientation relationship the precipitates grew into both grains. At (121) incoherent twin steps, the nucleus formed with a (121)f habit plane parallel to the twin step and grew into the matrix with which it kept a K-S relationship. Nucleation at dislocations was favoured when the dislocation line lay nearly parallel to the invariant line of the lath-shaped precipitates and the Burgers vector of the dislocation was closet to the maximum misfit vector of the precipitate.

Direct Copper Precipitation from a Loaded Chelating Extractant by Pressure Hydrogen Stripping

The kinetics of direct copper precipitation from a loaded chelating extractant (Kelex 100) using hydrogen in an autoclave (pressure hydrogen stripping) was studied. Copper, in powder form, was found to precipitate rapidly from loaded Kelex 100/decanol/kerosene solvents by reaction with hydrogen at pressures between 520 and 4000 kPa and temperatures from 443 to 488 K. The overall process has heterogeneous nucleation characteristics. Nuclei are provided through slow thermal dissociation of the copper chelate. The freshly produced metallic copper, acting autocatalytically, accelerates the precipitation kinetics. In addition to temperature and pressure, the effects of seeding, agitation, copper concentration, ligand concentration, and copper chelate age were investigated. Some physical and chemical properties of the powder product were also determined.