Suppression Of Nucleation Research Articles

Cu-alloying effect on crystallization kinetics of Ti41Zr25Be28Fe6 bulk metallic glass

Compared with Ti41Zr25Be28Fe6 bulk metallic glass, (Ti41Zr25Be28Fe6)93Cu7 glassy alloy possesses a much narrower supercooled liquid region but the glass-forming ability is dramatically improved. Isochronal and isothermal differential scanning calorimetry measurements were adopted to investigate Cu-alloying effect on the crystallization transformation kinetics of Ti41Zr25Be28Fe6 glassy alloy. It is found that Cu alloying increases the activation energies of Ti41Zr25Be28Fe6 glassy alloy for glass transition and crystallization in continuous heating. Moreover, the isothermal activation energy of (Ti41Zr25Be28Fe6)93Cu7 glassy alloy increases as process of crystallization transformation, while Cu-free alloy exhibits a contrary tendency. The addition of Cu also decreases the Avrami exponent of the base alloy, resulting in the suppression of crystal nucleation and growth.

Control of Na-EMT Zeolite Synthesis by Organic Additives

Organic additives were used to control the formation of EMT-type zeolite in a sodium-rich initial system. Triethanolamine was employed as a nucleation suppression agent able to complex the aluminum during the synthesis of the EMT-type zeolite. The commonly used tetramethylammonium (TMA) chloride and tetraethylammonium (TEA) chloride as structure directing agents in zeolite synthesis were also employed in this study. The triethanolamine has the most pronounced effect on the crystal growth process of the EMT-type zeolite. The use of triethanolamine resulted in the formation of large crystals (100 nm) with the framework composition different from the counterpart obtained in the organic-free system. Therefore, the function of triethanolamine is attributed to the immobilization of Al in the initial gel and thus partial suppression of zeolite nucleation. The immobilization of a part of Al resulted in the EMT zeolite crystals with a higher Si/Al ratio (1.4). In contrast, the TMA and TEA organic additives have a ...

Probing the structure of a liquid metal during vitrification

Using aerodynamic levitation, vitrification of a ternary Zr–Cu–Al alloy was observed in-situ by high energy synchrotron radiation X-ray diffraction in the temperature range from above the liquidus Tliq to well below the glass transition temperature Tg. The evolution of the atomic structure was studied using pair distribution functions (PDF) and molecular dynamic (MD) simulations. Vitrification was rendered possible due to the enhanced stability of the undercooled Zr–Cu melt after Al addition. Results indicate three regimes in the liquid alloy’s structural pathway to vitrification. Short (SRO) and medium range order (MRO) develop significantly during cooling the liquid phase to the glassy state. The rate of structural rearrangements is enhanced in the super-cooled liquid between Tliq-140K and Tg. The populations of atomic clusters with icosahedral local symmetry become predominant as Tg is approached and facilitate vitrification and suppression of crystal nucleation and growth. The scenario of a possible fragile to strong transition in the super-cooled liquid is discussed.

Interfacial mixing of nickel vanadium multilayers induced by cold rolling

During the cold rolling of multilayers of Ni and V with an average composition Ni70V30, Ni30V70 and Ni57V43, deformation induces phase transformation and an interfacial mixing. After repeated cold rolling at room temperature, X-ray diffraction (XRD) and scanning transmission electron microscope (STEM) results demonstrate that between pure Ni and V layers a metastable fcc solid solution phase forms in Ni70V30, a metastable bcc solid solution phase forms in Ni30V70 and metastable fcc and bcc solid solution phases form in Ni57V43 with suppression of nucleation of intermetallic phases. At an intermediate stage of the reaction when both pure Ni and V still remain, a smoothly varying composition profile was determined from the electron energy loss spectrum (EELS) across the multilayers. The composition dependent effective interdiffusion coefficient that was calculated based on the obtained profile was found to increase monotonically from 1.2×10−15cm2/s to 1.6×10−13cm2/s with increase of Ni mole fraction. Compared to the stored energy due to dislocation and interfaces, the excess chemical free energy from the interfacial mixing is the largest portion of total stored energy from deformation, which represents a form of mechanochemical transduction.

Large scale bi-layer graphene by suppression of nucleation from a solid precursor

Nucleation was controlled and suppressed by two-way carrier gas insertion and continuous bilayer graphene was synthesized from a botanical derivative, camphor.

Microstructure, optical and FTIR studies of Ni, Cu co-doped ZnO nanoparticles by co-precipitation method

Zn0.96−xCu0.04NixO (0⩽x⩽0.04) nanoparticles were synthesized by co-precipitation method. The X-ray diffraction pattern showed the crystalline nature of prepared nanoparticles with hexagonal wurtzite structure. The average crystal size is decreased from 27 to 22.7nm when Ni concentration is increased from 0% to 2% due to the suppression of nucleation and subsequent growth of ZnO by Ni-doping. The increased crystal size from 22.7 to 25.8nm (ΔD∼3.1nm) by Ni-doping from 2% to 4% is due to the creation of distortion centers and Zn/Ni interstitials. The cell parameters and volume of the lattice showed solubility limit at 2% of Ni doping. The energy dispersive X-ray spectra confirmed the presence of Cu and Ni in Zn–O. The optical absorption spectra showed that the absorption was increased up to Ni=2% due to the creation of carrier concentration by Ni-doping and decreased beyond 2% due to the presence of more defects and interstitials in the Zn–Ni–Cu–O lattice. The observed red shift of energy gap from 3.65eV (Ni=0%) to 3.59eV (Ni=2%, ΔEg≈0.06eV) is explained by sp–d exchange interactions between the band electrons and the localized d-electrons of the Ni2+ ions. The blue shift of energy gap from 3.59eV (Ni=2%) to 3.67eV (Ni=4%, ΔEg≈0.08eV) is explained by Burstein–Moss effect. Presence of chemical bonding was confirmed by FTIR spectra.

Effect of nucleation undercooling on the kinetics and mechanism of the peritectic phase transition in steel

The mechanism and kinetics of the peritectic phase transition in steel were studied by using a concentric solidification technique in a high-temperature laser-scanning confocal microscope. Strong solute diffusion fields are established during solidification of the primary delta phase, which leads to suppression of nucleation of the austenite phase, and hence the extent to which a steel can be cooled to a temperature below the equilibrium peritectic temperature before the peritectic reaction will occur. The magnitude of these diffusion fields was also determined as a function of carbon content, alloying additions, cooling rate and the fraction of primary delta phase that solidified before the peritectic reaction occurred. Depending on the extent of undercooling required to nucleate the first austenite platelet on the liquid/delta-ferrite interface, three different modes of the solid-state peritectic transformation of delta to austenite have been observed: a diffusion-controlled phase transformation that occurred by the progression of a planar interface; a diffusion-controlled transformation with cellular/dendritic morphology; and a massive-type of transformation. Under certain circumstances, a changeover between these modes has also been observed. The underpinning thermodynamic considerations that lead to these different modes of transformation are discussed and a new explanation is provided for the occurrence of a massive transformation of delta-ferrite to austenite. The relevance to and implications of these new findings to the continuous casting of steel of near-peritectic steel composition are also discussed.

The complex response of Arctic aerosol to sea-ice retreat

Abstract. Loss of summertime Arctic sea ice will lead to a large increase in the emission of aerosols and precursor gases from the ocean surface. It has been suggested that these enhanced emissions will exert substantial aerosol radiative forcings, dominated by the indirect effect of aerosol on clouds. Here, we investigate the potential for these indirect forcings using a global aerosol microphysics model evaluated against aerosol observations from the Arctic Summer Cloud Ocean Study (ASCOS) campaign to examine the response of Arctic cloud condensation nuclei (CCN) to sea-ice retreat. In response to a complete loss of summer ice, we find that north of 70° N emission fluxes of sea salt, marine primary organic aerosol (OA) and dimethyl sulfide increase by a factor of ~ 10, ~ 4 and ~ 15 respectively. However, the CCN response is weak, with negative changes over the central Arctic Ocean. The weak response is due to the efficient scavenging of aerosol by extensive drizzling stratocumulus clouds. In the scavenging-dominated Arctic environment, the production of condensable vapour from oxidation of dimethyl sulfide grows particles to sizes where they can be scavenged. This loss is not sufficiently compensated by new particle formation, due to the suppression of nucleation by the large condensation sink resulting from sea-salt and primary OA emissions. Thus, our results suggest that increased aerosol emissions will not cause a climate feedback through changes in cloud microphysical and radiative properties.

Development of a Plane Strain Tensile Geometry to Assess Shear Fracture in Dual Phase Steels

A geometrically modified sample capable of generating a triaxial stress state when tested on a standard uniaxial tensile frame was developed to replicate shear fractures observed during stretch bend tests and industrial sheet stamping operations. Seven commercially produced dual phase (DP) steels were tested using the geometrically modified sample, and the modified sample successfully produced shear fractures on a unique shear plane for all steels. For each steel, void densities were determined, based on metallographic analyses, as a function of imposed displacement. Microstructural properties of ferrite and martensite grain size, martensite volume fraction (MVF), retained austenite content, Vickers hardness, average nanoindentation hardness, average ferrite and martensite constituent hardness, and tensile properties were obtained in order to evaluate potential correlations with void data. A linear correlation was observed between Vickers hardness and the average nanoindentation hardness, verifying the ability of nanoindentation to produce data consistent with more traditional hardness measurement techniques. A linear relationship was observed between the number of voids present at 90% failure displacement and the martensite/ferrite hardness ratio, indicating that a decrease in relative hardness difference in a microstructure can suppress void formation, and potentially extend formability limits. The void population appeared independent of MVF, grain size, and tensile properties suggesting that constituent hardness may be a dominant parameter when considering suppression of void nucleation in DP steels.

Effect of defect imbalance on void swelling distributions produced in pure iron irradiated with 3.5 MeV self-ions

Ion irradiation has been widely used to simulate neutron-induced radiation damage. There are a number of features of ion-induced damage that differ from neutron-induced damage, however, and these differences require investigation before ion data can be confidently used to predict behavior arising from neutron bombardment. In this study 3.5MeV self-ion irradiation of pure iron was used to study the influence on void swelling of the depth-dependent defect imbalance between vacancies and interstitials that arises from various surface effects, forward scattering of displaced atoms, and especially the injected interstitial effect. It was observed that the depth dependence of void swelling does not follow the behavior anticipated from the depth dependence of the damage rate. Void nucleation and growth develop first in the lower-dose, near-surface region, and then moves to progressively deeper and higher-damage depths during continued irradiation. This indicates a strong initial suppression of void nucleation in the peak damage region that is eventually overcome with continued irradiation. Using the Boltzmann transport equation method, this phenomenon is shown to be due to depth-dependent defect imbalances created under ion irradiation. These findings demonstrate that void swelling does not depend solely on the local dose level and that this sensitivity of swelling to depth must be considered in extraction and interpretation of ion-induced swelling data.

Effects of ALPS on Mechanical Property in Bainitic Steels

Aggregate of bainite laths with almost parallel slip systems between neighboring bainite laths, hereafter referred to as ALPS, has great effect on the improvement of mechanical properties in steels. Elongation increases remarkably with increasing the number of bainite laths within an ALPS. When a bainite lath begins to deform, the neighboring bainite lath also easily deforms to relax the deformation strain, because of good parallelism between their slip systems. Under the cooperative deformation of bainite laths, the area of interface between neighboring bainite laths increases during deformation. The increase in the area of interface between neighboring bainite laths suppresses localization of dislocations at the interface, that is, dislocation density per unit area of the interface between neighboring bainite laths hardly increases, resulting in the suppression of nucleation of cracks at the interface between neighboring bainite laths. Ductile fracture would occur along the boundary between ALPSs. It could be suggested that larger ALPS and/or ALPS consisting of large number of bainite laths induce larger elongation in steels.On the other hand, it has been reported that tensile strength increases in proportion to inverse of square root of d, the d being the average size of bainite laths [1]. In order to form fine bainite lath, dislocation network instead of inclusion in austenite was utilized as nucleation site for bainite lath. Great barriers to be overcome exist for the improvement of both strength and toughness. An idea for the improvement of both strength and toughness is shown in this study.

Kinetics of the Carbon Anode in Molten Salt

Efficiency in the C/CO3 2-/O2 system (725 °C) is related to the distribution of current in the porous anode. Efficiency below 50% results when oxidation occurs within matrix at low local rates, producing CO and CO2 by a 2-e- net reaction. Efficiency of 75% HHV requires concentration of current on a polarized exterior surface and produces CO2 product. This picture is supported by segmentation of the I-V curve into two distinct "regimes" separated by an inflection. We present the following model: as applied current is raised, CO2 accumulates within micropores forming supersaturated levels of CO2. This raises the anode potential by 100-250 mV--enough to promote pure CO2 production. Concentration increases until (1) overall polarization drives CO2 production to the exterior surface, or, (2) bubbles nucleate and block the pores, shifting the reaction to the accessible exterior. Support for this interpretation is found in suppression of bubble nucleation by surface tension effects leading to high supersaturation; in comparing amplitudes (0.1-0.25 V) of I/V hysteresis loops with the overpotential required for 4 e- pathways; and in the magnitude of limiting current for the porous anode.

Fluxing of Pd–Si–Cu bulk metallic glass and the role of cooling rate and purification

Boron oxide fluxing represents an interesting processing technique for attaining a simultaneous improvement in many properties of bulk metallic glasses (BMGs). This concerns glass-forming ability (i.e. a more than three times larger critical casting thickness), in combination with a significant improvement of thermal and mechanical characteristics. In particular, a better understanding of the flux treatment can result in further enhancement of toughness and ductility, which is crucially needed for structural applications. This study investigates the effects of fluxing of Pd–Si–Cu BMGs. By studying four differently processed materials (i.e. cast, water-quenched, fluxed-cast and fluxed alloys), the influences of applied cooling rate and purification associated with flux treatment can be decoupled. The better glass-forming ability is linked to the reduction of heterogeneities, whereas the thermal characteristics depend on both parameters. The interplay of slower quenching rates and the suppression of heterogeneous nucleation is reflected in an extended supercooled liquid range, improving the formability of the BMG. Better ductility is also obtained and can be explained in terms of purification, which is correlated with a decrease in oxygen concentration. In summary, the properties of fluxed specimens are improved not only by the reduction of heterogeneous nucleation, but also via modified cooling rates during fluxing. This study illustrates the importance of adequate processing of BMG-forming liquids to achieve improved properties and casting thicknesses.

Acceleration or Suppression of α-Phase Precipitation Using Isothermal ω Phase in Ti-20 at.pct Nb Alloy

Homogeneous precipitation of a fine α phase in the β matrix of Ti alloys is a promising method for obtaining a highly strengthened Ti-based alloy. Isothermal ω particles are known to be the nucleation sites for fine α-phase precipitation, but an understanding of the kinetics of α-phase formation on isothermal ω particles is still lacking. This study aimed to reveal the effect of isothermal ω particles on α-phase precipitation onset time. Two-step isothermal aging of a Ti-20 at.pct Nb alloy after solid solution treatment at 1273 K (1000 °C) was carried out. The first step of the aging at 633 K (360 °C) involved the formation of isothermal ω particles in the β matrix. This was followed by a second aging step at 673 K, 723 K, and 773 K (400 °C, 450 °C, and 500 °C) for α-phase precipitation. Suppression of α-phase nucleation on the isothermal ω particles occurred at 673 K (400 °C), whereas acceleration of α-phase nucleation on the isothermal ω particles was observed at 723 K and 773 K (450 °C and 500 °C). Thermodynamic stability of the isothermal ω particles and solute partitioning were controlling factors for the α-phase precipitation kinetics.

Micron‐Sized Polymer Particles by Membrane Emulsification

SummaryDispersions of micron‐sized styrene droplets in water were produced by straight through microchannel arrays. The monomer droplets have been polymerized by suspension polymerization using an initiator system, which was only oil soluble. A significant amount of submicron particles was formed by secondary nucleation. The side reaction was emulsion polymerization, which was confirmed by the molecular weight distribution of the reaction product. Suppression of secondary nucleation was only partially possible by using the water soluble inhibitor NaNO2. Increasing the monomer to water ratio before polymerization resulted in less emulsion polymerization. Molecular weight was found to be strongly dependent on the initiator concentration as well as on the inhibitor concentration. It was demonstrated that polymer particles with a diameter of about 10 μm can be produced using emulsification of the monomer by microchannel arrays prior to polymerization.

Crystallization in a sheared colloidal suspension

We study numerically the crystallization process in a supersaturated suspension of repulsive colloidal particles driven by simple shear flow. The effect of the shear flow on crystallization is two-fold: while it suppresses the initial nucleation, once a large enough critical nucleus has formed its growth is enhanced by the shear flow. Combining both effects implies an optimal strain rate at which the overall crystallization rate has a maximum. To gain insight into the underlying mechanisms, we employ a discrete state model describing the transitions between the local structural configurations around single particles. We observe a time-scale separation between these transitions and the overall progress of the crystallization allowing for an effective Markovian description. By using this model, we demonstrate that the suppression of nucleation is due to the inhibition of a pre-structured liquid.

Structural Analysis of Carbon-Added Na–Ga Melts in Na Flux GaN Growth by First-Principles Calculation

We investigated the fundamentals of the effect of C addition on Na flux GaN growth by first-principles calculation. We simulated C-added Na–Ga melts using molecular dynamics (MD) simulations to examine the local melt structure around a N atom. We also calculated C–N bond energy using constrained MD simulations. Results show that a N atom bonded to a C atom and there were no Ga atoms around the N atom because C–N bond energy was larger than Ga–N bond energy. This is the reason for the suppression of heterogeneous nucleation by C addition. It was also found that the C–N bond energy was affected by surrounding Ga atoms and that the C–N atomic distance increased with the Ga coordination number around the N atom.

Covalently bonded polystyrene/SiO2 microspheres via emulsion polymerisation stabilised solely by surface active Pickering stabiliser

Covalently bonded polystyrene/SiO2 microspheres were synthesised via Pickering emulsion polymerisation using methacryloxypropyltrimethoxysilane modified SiO2 as a sole stabiliser. The effect of polymerisation condition on morphologies of composites was investigated, and a formation mechanism of microspheres with different morphologies was proposed. Three strategies, increasing amount of stabiliser, adding inhibitor in continuous phase and employing cross-linking agent, were employed to regulate the morphology of products. It was concluded that suppression of nucleation in continuous phase was critical for the formation of dispersive microspheres with high sphericity and smooth surface.

Preparation of nanostructured CeCO3OH particles from aqueous solutions and gels containing biological polymers and their thermal conversion to CeO2

Nanostructured CeCO3OH particles were prepared from aqueous solutions and gels containing CeCl3 and biological polymers (gelatin and agar) by the addition of (NH4)2CO3 solutions. The morphologies of the CeCO3OH products depended on the kind and concentration of the biological polymers. Sheaf- and cocoon-like CeCO3OH particles of ca. 2 μm in size consisting of fine particles of ca.10 nm in diameter were obtained from the precursor solutions containing gelatin. On the other hand, CeCO3OH precipitates with inhomogeneous size and shape were obtained from the precursor solutions and gels containing agar. Such morphological variation of CeCO3OH particles was thought to be due to the suppression of nucleation and growth of CeCO3OH crystals by the biological polymers. The CeCO3OH particles thus obtained were converted to CeO2 particles by the heat treatment at 600 °C, and the morphology as well as nanostructure remained during the heat treatment.

Heterogeneous Nucleation and Growth of Lithium Electrodeposits on Negative Electrodes

By starting from fundamental principles, the heterogeneous nucleation and growth of electrodeposited anode materials is analyzed. Thermodynamically, we show that an overpotential-controlled critical radius has to be overcome in order for dendrite formation to become energetically favorable. Kinetically, surface tension and overpotential driving forces define a critical kinetic radius above which an isolated embryo will grow and below which it will shrink. As a result, five regimes of behavior are identified: nucleation suppression regime, long incubation time regime, short incubation time regime, early growth regime, and late growth regime. In the nucleation suppression regime, embryos are thermodynamically unstable and unable to persist. For small overpotentials, below a critical overpotential, 2η○, and between the thermodynamic and kinetic critical radius, a metastable regime exists where the local electrochemically enabled Gibbs-Thomson interactions control the coarsening of the embryos, thus defining the long incubation time regime. In addition, very broad nuclei size distributions are favored. For large overpotentials, above 2η○, a short incubation time regime develops as a result of the small energy barrier and large galvanostatic driving forces. In addition, very narrow size distributions of nuclei are favored. In the early growth regime, thermodynamically and kinetically favored nuclei grow to reach an asymptotic growth velocity. Finally, in the late growth regime, morphological instabilities and localized electric fields dominate the morphology and microstructural evolution of the deposit.