Incipient Nucleation Research Articles

Design of alkali activated foamy binders from Sicilian volcanic precursors

In the present study, porous inorganic polymers were synthetized by alkaline activation of volcanic ash and ‘ghiara’ paleosoil using Al swarf and Al commercial powder as pore inducing agent. The Alkali Activated Foams (AAFs) were characterized in terms of mineralogy, macro- and microstructure, by synchrotron X-ray diffraction, electron microscopy, infrared spectroscopy, and X-ray computed microtomography. The obtained porous matrices show different microstructures; the size and extent of the pores vary according to the type, size and amount of the foaming agent and to the viscosity of the pastes. Addition of Al swarf to the volcanic ash based slurry generates expansion of the paste, whereas ghiara based one do not expand due to a lower viscosity of the slurry. Moreover, results indicate that the Al addition procedure during the synthesis affects the pore characteristics: Al powder added directly to the volcanic ash-based slurry triggers an incipient pore nucleation, whereas AAFs prepared adding Al powder to the solid mix do not develop an extended pore network. Results of this research show that porosity can be efficiently tailored in volcanic based AAFs by regulating the amount and type of Al.

Revisiting Homochiral versus Heterochiral Interactions through a Long Detective Story of a Useful Azobis-Nitrile and Puzzling Racemate.

This paper documents and reinvestigates the solid-state and crystal structures of 4,4'-azobis-4-cyanopentanoic acid (ACPA), a water-soluble azobis-nitrile of immense utility as a radical initiator in living polymerizations and a labile mechanophore that can be embedded within long polymer chains to undergo selective scission under mechanical activation. Surprisingly, for such applications, both the commercially available reagent and their derivatives are used as "single initiators" when this azonitrile is actually a mixture of stereoisomers. Although the racemate and meso compounds were identified more than half a century ago and their enantiomers were separated by classical resolution, there have been confusing narratives dealing with their characterization, the existence of a conglomeratic phase, and fractional crystallization. Our results report on the X-ray crystal structures of all stereoisomers for the first time, along with further details on enantiodiscrimination and the always intriguing arguments accounting for the stability of homochiral versus heterochiral crystal aggregates. To this end, metadynamic (MTD) simulations on stereoisomer molecular aggregates were performed to capture the incipient nucleation events at the picosecond time scale. This analysis sheds light on the driving homochiral aggregation of ACPA enantiomers.

Surface Adatom Diffusion-Assisted Dislocation Nucleation in Metal Nanowires.

We employ a hybrid diffusion- and nucleation-based kinetic Monte Carlo model to elucidate the significant impact of adatom diffusion on incipient surface dislocation nucleation in metal nanowires. We reveal a stress-regulated diffusion mechanism that promotes preferential accumulation of diffusing adatoms near nucleation sites, which explains the experimental observations of strong temperature but weak strain-rate dependence as well as temperature-dependent scatter of the nucleation strength. Furthermore, the model demonstrates that a decreasing rate of adatom diffusion with an increasing strain rate will lead to stress-controlled nucleation being the dominant nucleation mechanism at higher strain rates. Overall, our model offers new mechanistic insights into how surface adatom diffusion directly impacts the incipient defect nucleation process and resulting mechanical properties of metal nanowires.

Threshold Identification and Damage Characterization of Woven GF/CF Composites under Low-Velocity Impact

The Delamination Threshold Load (DTL) is a key parameter representing damage resistance of a laminate and is normally identified by locating a sudden drop in the impact force-time history for the laminate made of unidirectional layers. For the woven composite, however, their failure mechanisms appear different and the current literature is not providing any clear procedure regarding the identification of the delamination initiation, as well as the evolution of the failure mechanisms associated with it. In this paper, experimental data have been collected using woven glass and carbon fiber composites. The results are analyzed in terms of force-time and force-displacement curves. While delamination and other damages were clearly observed using ultrasonic scans, the analysis of the results does not reveal any trend changes of the curves that can be associated with the incipient nucleation of delamination. A preliminary discussion regarding the nature of the mechanisms through which the delamination propagates in woven composite and a justification for the absence of a sudden change of the stiffness have been presented. It raises a question on the existence of DTL for woven composites under low velocity impact.

Microscopic mechanisms behind nucleate boiling heat transfer enhancement on large-aspect-ratio concave nanostructured surfaces for two-phase thermal management

• Aspect-ratio effects of nano-cavities on nanoscale boiling are quantitatively illustrated. • The incipient nucleation time and temperature are significantly enhanced for deeper cavities. • An optimal cavity depth for maximal nucleate boiling enhancement is obtained. • Thermal energy accumulation efficiency of liquid at the initial nucleation site are elaborated. • Microscopic mechanisms are elucidated thoroughly by molecular dynamics simulations. Enhancing boiling heat transfer and elaborating underlying mechanisms are of great significance to assist in improving the efficiency of energy systems and solving thermal management bottlenecks in advanced electronics. Numerous experimental studies have proved nanostructured surfaces enable boiling heat transfer to be enhanced strikingly. Nevertheless, it is a great challenge to perform more in-depth research due to the limitation of the observation accuracy of conventional experimental approaches. To optimize designs of nanostructured boiling surfaces and fully reveal the microscopic mechanisms, the aspect-ratio effects of rectangular nano-cavities on nucleate boiling heat transfer are investigated systematically by molecular dynamics simulations in this study. The nano-cavities have the same width of 3 nm and different depths of 2 nm to 8 nm, corresponding to aspect ratios from 2:3 to 8:3. The results illustrate that, compared with the plain surface, concave nanostructured surfaces can cause a significant heat accumulation effect, resulting in remarkable reinforcements in absorbed heat flux and bubble nucleation. The nucleate boiling heat transfer enhancements originating from large-aspect-ratio nano-cavities are fully verified by the notable reductions in the incipient nucleation time and boiling initiation temperature. Attractively, for cavities with the same width, an optimal depth for maximal nucleate boiling enhancement can be obtained, which is 7 nm in the present study. In this case, the incipient nucleation time is about eight times shorter than that for the plain substrate and the decrease in boiling initiation temperature is up to 53 K. Moreover, a novel and straightforward insight into the microscopic enhanced mechanism has been proposed. It is elucidated thoroughly from the perspective of the thermal energy accumulation of liquid at the initial nucleation site by taking into account the heat absorbed from the solid wall and the heat transferred to the adjacent cryogenic liquid comprehensively. These analysis and simulation results are in great agreement. This study provides significant guidance for state-of-the-art two-phase thermal management applications.

The Competition Between Deformation Twinning and Dislocation Slip in Deformed Face-Centered Cubic Metals

The competition between deformation twinning and dislocation slip underpins the evolution of mesoscale plasticity in face-centered cubic materials. While competition between these mechanisms is known to be related to the critical features of the generalized planar fault energy landscape, a physical theory that tracks competition over extended plasticity has yet to emerge. Here, we report a methodology to predict the mesoscale evolution of this competition in deformed crystals. Our approach implements kinetic Monte Carlo simulations to examine fault structure evolution in face-centered cubic metals using intrinsic material parameters as inputs. These results are leveraged to derive an analytical model for the evolution of the fault fraction, fault densities and partitioning of plastic strains among deformation mechanisms. In addition, we define a competition parameter that measures the tendencies for deformation twinning and dislocation slip. In contrast to previous ‘twinnability’ parameters, our derivation considers deformation history when examining mechanism competition. This contribution therefore extends the reach of deformation twinning theory beyond incipient nucleation events. These products find direct applications in work hardening and crystal plasticity models, which have previously relied on phenomenological relations to predict the mesoscale evolution of deformation twin microstructures.

Molecular dynamics study on enhanced nucleate boiling heat transfer on nanostructured surfaces with rectangular cavities

Nanostructured surfaces have been proved to bring remarkable enhancements in nucleate boiling heat transfer, which are particularly attractive in thermal energy fields. To fully understand the effects of nano-cavities on nucleate boiling and elucidate the underlying enhanced mechanisms, a comparative molecular dynamics study on nucleate pool boiling heat transfer of liquid argon over the plain copper substrate and nanostructured substrates with different rectangular cavities is performed. The nano-cavities have the same depth of 5 nm and different widths of 3 nm, 5 nm and 8 nm. The bubble dynamics behavior on various surfaces is observed based on simulation snapshots. The results manifest that the rectangular nano-cavity can significantly reduce time and wall superheat required for the onset of nucleate boiling, as well as delay the transition from nucleation boiling regime to film boiling regime. The incipient nucleation time tin can be reduced to 990 ps from 5600 ps. Additionally, compared with the plain substrate, the rectangular nano-cavity can result in a striking decrease in boiling initiation temperature, which is up to 59 K. The underlying enhanced mechanisms are well elucidated based on the structural feature of the rectangular nano-cavity and simulation results. The liquid inside the rectangular cavity can obtain additional thermal energy from sidewalls, leading to a significant local heat accumulation effect and the heat transfer efficiency reinforcement. It is found there is a coupling enhancement effect of heat accumulation when the width of rectangular cavity is smaller. Consequently, the 3 nm wide nano-cavity can achieve maximum enhancement. These findings provide crucial evidence at the nanoscale to verify that nano-cavity can significantly enhance nucleate boiling not only by reducing nucleation time but also by decreasing the boiling initiation temperature. This study is of importance to promote further insights into the enhanced mechanism of nucleate boiling at the nanoscale and provide guidance for the performance improvement in boiling surfaces for advanced thermal energy systems.

Molecular Insight into Bubble Nucleation on the Surface with Wettability Transition at Controlled Temperatures.

A surface with a smart wettability transition has recently been proposed to enhance the boiling heat transfer in either macro- or microscale systems. This work explores the mechanisms of bubble nucleation on surfaces with wettability transitions at controlled temperatures by molecular simulations. The results of the interaction energy at the interface and potential energy distribution of water molecules show that the nanostructure promotes nucleation over the copper surface and causes lower absolute potential energy to provide fixed nucleation sites for the initial generation of the bubble nucleus and shortens the incipient nucleation time, as compared to the mixed-wettability or hydrophilic nanostructure surface. An investigation on more nanostructured surfaces shows that a surface (F) with a wettability transition temperature of 620.0 K has the shortest average incipient nucleation time at 1672 ps with a wall temperature of 634.3 K. The surface with tunable wettability has also a high interfacial thermal conductance at low superheats, but it may not promote the critical heat flux at high superheats. The heat-transfer performance of the smart surface is better than the plate, the hydrophobic nanostructure, and the mixed-wettability surfaces, while it is lower than the hydrophilic nanostructure surface. This proposes a new method and provides insight for promoting bubble nucleation on a surface with temperature-dependent wettability.

Investigation of water bubble nucleation by using molecular dynamics simulation

Generation of a bubble nucleus is the initial stage of nucleate boiling and an essential part of the nucleate boiling theory. In the present study, the molecular dynamics simulation method is conducted to study the bubble nucleation of water on a grooved copper substrate with hydrophilicity. An idea based on the competition between molecular kinetic energy and molecular potential restriction, corresponding to the motivity and resistance for bubble nucleation at the nanoscale, is extended to identify the intrinsic regime of bubble nucleation of water. If a group of water molecules obtain enough kinetic energy such that their potential restriction is broken and become activated molecules at some time, the bubble nucleus is taken shape. Simulation results of bubble nucleation behaviors on the substrate with different sizes of grooves indicate that this idea works well in clarifying the intrinsic regimes of bubble nucleation and the nucleation difference between different cases. In addition, this idea can be used to obtain the incipient nucleation position, nucleation time, and the volume of the bubble nucleus without any subjective threshold.

Effect of interstitial oxygen and nitrogen on incipient plasticity of NbTiZrHf high-entropy alloys

In this work, instrumented nanoindentation was employed to investigate the effect of interstitial oxygen or nitrogen addition on the incipient plasticity and dislocation nucleation in a body-centered cubic NbTiZrHf high-entropy alloy (HEA) at loading rates of 10–1000 µN/s. We conducted quantitative statistical analysis and density functional theory (DFT) calculations to identify the role of interstitial oxygen/nitrogen during the onset of plasticity. Synchrotron X-ray diffraction and transmission electron microscopy were also performed to confirm that the oxygen/nitrogen atoms were indeed present as interstitial solutes. These interstitial solutes could increase the critical shear stress required to initiate plasticity, and nitrogen yielded a larger hardening effect than oxygen. The activation volumes were evaluated to be about 2–3 atomic volumes, indicating cooperative migration of multiple atoms during the dislocation nucleation, and neither oxygen nor nitrogen appeared to significantly affect this activation process. Hardness tests were also carried out and the result demonstrated that the enhancement of the critical shear stress for incipient plasticity was not caused by the traditional solid-solution strengthening mechanism. DFT calculations revealed that oxygen/nitrogen interstitials induced local charge transfer and improved the lattice cohesion, which was probably responsible for the enhanced pop-in load/stress in the current interstitially alloyed HEAs.

Study on the effect of foreign particle on bubble nucleation by using molecular dynamics simulation

To promote the engineering application of additive in enhancing nucleate boiling, the effect of foreign particles on bubble nucleation is investigated by the molecular dynamics simulation method. Some foreign atoms are added into the pure argon system with a proportion of 5%. The liquid is heated by a hydrophilic smooth platinum substrate to achieve bubble nucleation. The results show that foreign atoms have significant impacts on bubble nucleation. Compared with the pure liquid system, the bubble nucleation efficiencies in the aspects of the incipient nucleation time and temperature are promoted by the foreign atom with a smaller energy parameter than argon atom but decreased with the increasing energy parameter. Then, the intrinsic regime of the difference between pure liquid and mixed liquid in bubble nucleation is fully illustrated based on the competition between atomic potential energy and atomic kinetic energy, which are the restriction and impetus for bubble nucleation, respectively. Furthermore, based on the influencing regime of foreign atoms in bubble nucleation, a further application of foreign atoms in controlling the nucleation position is developed.

Kinetic Model of Incipient Hydride Formation in Zr Clad under Dynamic Oxide Growth Conditions.

The formation of elongated zirconium hydride platelets during corrosion of nuclear fuel clad is linked to its premature failure due to embrittlement and delayed hydride cracking. Despite their importance, however, most existing models of hydride nucleation and growth in Zr alloys are phenomenological and lack sufficient physical detail to become predictive under the variety of conditions found in nuclear reactors during operation. Moreover, most models ignore the dynamic nature of clad oxidation, which requires that hydrogen transport and precipitation be considered in a scenario where the oxide layer is continuously growing at the expense of the metal substrate. In this paper, we perform simulations of hydride formation in Zr clads with a moving oxide/metal boundary using a stochastic kinetic diffusion/reaction model parameterized with state-of-the-art defect and solute energetics. Our model uses the solutions of the hydrogen diffusion problem across an increasingly-coarse oxide layer to define boundary conditions for the kinetic simulations of hydrogen penetration, precipitation, and dissolution in the metal clad. Our method captures the spatial dependence of the problem by discretizing all spatial derivatives using a stochastic finite difference scheme. Our results include hydride number densities and size distributions along the radial coordinate of the clad for the first 1.6 h of evolution, providing a quantitative picture of hydride incipient nucleation and growth under clad service conditions.

Nucleate boiling on nanostructured surfaces using molecular dynamics simulations

Nanostructures have significant impacts on incipient nucleation time and onset temperature during nucleate boiling. This study compares three types of surfaces, namely smooth, concave structured, and convex structured to investigate the mechanism of nucleate boiling via molecular dynamics simulations. The results show that nanostructured surfaces enhance bubble nucleation compared to smooth surfaces, provide fixed nucleation sites for initial embryo generation and shorten incipient nucleation time. Furthermore, nucleate boiling occurs more quickly on concave nanostructured surfaces than on convex ones regardless of the nanostructure size. However, with decreasing nanostructure size, the difference between them gradually decreases. A further analysis of the onset temperature is performed under various surface conditions. The onset temperature of nucleate boiling on the large concave nanostructured surface is much lower than on large convex nanostructured surface, owing to the stronger heat accumulation effect of the cavity. However, the onset temperature on the small concave nanostructured surface is higher, because liquid atoms are strongly absorbed into the small cavity and crystallized with the hydrophilic surface. The results improve the understanding of nanoscale nucleate boiling and provide a reference for the design of microelectronic devices.

A theoretical and experimental study of typical heterogeneous ice nucleation process on auto windshield under nocturnal radiative cooling and subfreezing conditions

In the present study, we theoretically and experimentally investigated a typical and practical heterogeneous ice nucleation process on auto windshield under nocturnal radiative cooling and subfreezing conditions. During the experiment, a general class A passenger car was placed outdoors in a subfreezing environment (i.e., 44.14° N, 126.45° E) during nights in January, with a purpose to produce a nocturnal radiative cooling condition for supercooling the windshield to trigger the ice nucleation. The results revealed a distinctive three-stage frosting process: (I) inhomogeneous occurrence of incipient ice nucleation; (II) small-sized ice crystals grew into large-sized ones primarily based on the incipient nucleation sites, together with a continual formation of new nucleation sites and a merging between large-sized and small-sized crystals; (III) large-sized ice crystals coalesced into large-scale groups, tending to make a full ice coverage. Unlike the classical three-period frosting process, the present study exhibited an absence of the drop-wise condensation but with a presence of dendritic crystals nearly parallel to the supercooled surface. Besides, the incipient ice nucleation was first initiated on the upper part of the windshield, which then propagated downward with a further heat loss of the windshield due to the continual nocturnal radiation. Meanwhile, the probable presence of surface roughness or contamination was much more likely to trigger an earlier initiation of incipient heterogeneous ice nucleation. Additionally, the arrival of saturation point, rather than the nominal supersaturation state (i.e., the condensation nucleation point), could be expected to approximately mark the initiation of icing under a practical engineering condition. Higher relative humidity tended to induce an earlier occurrence of ice nucleation and produce a larger surface coverage.

The race within supercooled liquids-Relaxation versus crystallization.

Can any liquid be cooled down below its melting point to an isentropic (Kauzmann) temperature without vitrifying or crystallizing? This long-standing question concerning the ultimate fate of supercooled liquids is one of the key problems in condensed matter physics and materials science. In this article, we used a plethora of thermodynamic and kinetic data and well established theoretical models to estimate the kinetic spinodal temperature, TKS (the temperature where the average time for the first critical crystalline nucleus to appear becomes equal to the average relaxation time of a supercooled liquid), and the Kauzmann temperature, TK, for two substances. We focused our attention on selected compositions of the two most important oxide glass-forming systems: a borate and a silicate-which show measurable homogeneous crystal nucleation in laboratory time scales-as proxies of these families of glass-formers. For both materials, we found that the TKS are significantly higher than the predicted TK. Therefore, at ambient pressure, at deep supercoolings before approaching TK, crystallization wins the race over structural relaxation. Hence, the temperature of entropy catastrophe predicted by Kauzmann cannot be reached for the studied substances; it is averted by incipient crystal nucleation. Our finding that TKS > TK for two real glasses corroborate the results of computer simulations for a pressurized silica glass.

Start-up characteristics of MEMS-based micro oscillating heat pipe with and without bubble nucleation

The startup characteristics of micro oscillating heat pipe (micro-OHP) have been experimentally investigated using a high-speed CCD camera in conjunction with the temperature measurement. The micro-OHP was fabricated on a silicon wafer by the MEMS technology, having trapezoidal channels with a hydraulic diameter of 357 μm. HFE-7100 was used as the working fluid with volumetric filling ratios ranging from 31% to 72%. The effects of filling ratio and heating power input on the start-up process were presented. Two different start-up behaviors, so-called start-up with and without bubble nucleation (STWBN/STWOBN), subject to different heat input levels were observed, and the underlying mechanisms were elaborately analyzed. The occurrence of STWBN or STWOBN depends mostly upon the spatial distribution of slugs/plugs in the micro-OHP. For the STWOBN mode, the fluid movement could be triggered by intense liquid film evaporation without nucleation with evaporator temperature less than 80 °C. However, the STWBN mode is sometimes indispensable to excite the two-phase oscillations in the micro-OHP due to the non-desirable slug/plug distribution, leading to much higher evaporator temperatures even greater than 140 °C to initiate the bubble generation. To predict the startup temperature at the STWBN mode, a theoretical model to describe the wall superheat required for incipient bubble nucleation in microchannels was propounded and largely agreed with the present experimental data. Micro-OHPs worked at the STWBN mode may greatly hinder their applications in electronics cooling, and artificial cavities fabricated or embedded on silicon channel wall surfaces are proposed to facilitate bubble nucleation and then the startup at low temperatures.

Revealing the Initial Chemistry of Soot Nanoparticle Formation by ReaxFF Molecular Dynamics Simulations

This work presents long time ReaxFF MD simulations of fuel-rich combustion for up to 10 ns to explore the initial mechanism of soot nanoparticle formation. A 24-component rocket propellant 1 (RP-1) model based on the major components of RP-1 fuel was employed. Simulations were performed by GPU-accelerated code GMD-Reax, and reactions therein were revealed with the aid of VARxMD. Simulated evolution of physical and chemical properties of the largest molecule exhibits the overall structural transitions of three stages for incipient ring formation, nucleation, and graphitization from fuel molecules to the formation of a single soot nanoparticle. The incipient ring formation takes place in stage 1 by large ring generation from activated aliphatic polyyne-like chains, ring number increase from internal bridging between carbon atoms of large rings, and consequent formation of PAH-like molecules with aliphatic side chains. Nucleation of a nanoparticle in stage 2 is the result of coalescence of PAH-like molecules...

Synchrotron tomographic quantification of strain and fracture during simulated thermal maturation of an organic‐rich shale, UK Kimmeridge Clay

AbstractAnalyzing the development of fracture networks in shale is important to understand both hydrocarbon migration pathways within and from source rocks and the effectiveness of hydraulic stimulation upon shale reservoirs. Here we use time‐resolved synchrotron X‐ray tomography to quantify in four dimensions (3‐D plus time) the development of fractures during the accelerated maturation of an organic‐rich mudstone (the UK Kimmeridge Clay), with the aim of determining the nature and timing of crack initiation. Electron microscopy (EM, both scanning backscattered and energy dispersive) was used to correlatively characterize the microstructure of the sample preheating and postheating. The tomographic data were analyzed by using digital volume correlation (DVC) to measure the three‐dimensional displacements between subsequent time/heating steps allowing the strain fields surrounding each crack to be calculated, enabling crack opening modes to be determined. Quantification of the strain eigenvectors just before crack propagation suggests that the main mode driving crack initiation is the opening displacement perpendicular to the bedding, mode I. Further, detailed investigation of the DVC measured strain evolution revealed the complex interaction of the laminar clay matrix and the maximum principal strain on incipient crack nucleation. Full field DVC also allowed accurate calculation of the coefficients of thermal expansion (8 × 10−5/°C perpendicular and 6.2 × 10−5/°C parallel to the bedding plane). These results demonstrate how correlative imaging (using synchrotron tomography, DVC, and EM) can be used to elucidate the influence of shale microstructure on its anisotropic mechanical behavior.

Multiscale simulation of incipient plasticity and dislocation nucleation on nickel film during tilted flat-ended nanoindentation

Multiscale simulations of the tilted flat-ended nanoindentation with different tilted angles (from 5° ∼ 30°) on the (−110) surface of nickel crystal were studied using the QC method. The model of the indentation is a flat-end indenter inclined by an angle ɛ driven into a half-plane vertically. Load-displacement responses, initial stages of the plasticity deformations and dislocation emissions for nickel film at different inclined angles were obtained and analyzed as well. An energy criterion was successfully proposed to analyze the critical load for the first dislocation emission beneath the edge of the indenter. Simulation results agree well with analytical ones. An elastic model based on the contact theory and the Peierls-Nabarro dislocation model were combined to analyze when and where the dislocation will be emitted beneath the lower surface of an inclined indenter. Results indicate that the key parameter is the ratio of the contact half-width to the position of the slip plane. This parameter shows the range in which a dislocation will probably be emitted. This mechanism explains the simulation results well. This work is of value for understanding the mechanism of dislocation emissions of FCC crystals under tilted flat-ended nanoindentation while providing approaches to predicting when the first dislocation will be emitted and where subsequent dislocations will probably be emitted.

Incipient plasticity and dislocation nucleation in body-centered cubic chromium

In this study, we conducted nanoindentation tests to explore the incipient plasticity (or onset of yielding) in a fully annealed coarse-grained Cr at loading rates between 0.02 and 2mN/s and temperatures ranging from 295 to 423K. The maximum shear stress at the first indentation pop-in were evaluated and found comparable to the theoretical shear strength for the yielding of dislocation-free Cr. The activation volume and activation energy for the onset of yielding were also evaluated using Schuh׳s statistical model, and further compared with that anticipated for homogeneous and heterogeneous dislocation nucleation processes. We concluded that heterogeneous nucleation was the favorite mechanism and the type of point defect that induced dislocation nucleation appeared to be interstitials rather than vacancies in the body-centered cubic Cr.