Nanoparticle synthesis has drawn great attention in the last decades. The study of crystal growth mechanisms and optimization of the existing methods lead to the increasing accessibility of nanomaterials, such as gold nanotriangles which have great potential in the fields of plasmonics and catalysis. To form such structures, a careful balance of reaction parameters has to be maintained. Herein, a novel synthesis of gold nanotriangles from seeds derived with a micromixer, which provides a highly efficient mixing and simple parameter control is reported. The impact of the implemented reactor on the primary seed characteristics is investigated. The following growth steps are studied to reveal the phenomena affecting the shape yield. The use of microfluidic seeds led to the formation of well-defined triangles with a narrower size distribution compared to the entirely conventional batch synthesis. A shortened two-step procedure for the formation of triangles directly from primary seeds, granting an express but robust synthesis is further described. Moreover, the need for a thorough study of seed crystallinity depending on the synthesis conditions, which - together with additional parameter optimization - will bring a new perspective to the use of micromixers which are promising for scaling up nanomaterial production is highlighted.

Upon exposure to oxygen, NiTi forms a Ti-rich surface oxide layer, and mass balance requires that a Ni-enriched zone forms below the oxide. Both the surface oxide layer and the Ni-enriched zone are discussed to affect key properties of NiTi as material for minimally invasive medical application or for actuators, e.g., the release of Ni and the resistance to initiation and propagation of cracks when exploiting the shape memory effect/pseudoelasticity. However, owing to the small extension of the Ni-enriched zone of a few nanometers, little is known about its crystallinity, phase and evolution of composition. We study the formation of the surface layers during annealing at similar to 500 degrees C for up to 10 min, as routinely applied for shape-setting. With an approach employing nano beam electron diffraction in a transmission electron microscope and an in-house software package for phase analysis it is shown for the first time that phase transformations in the Ni-enriched zone, specifically from NiTi to Ni4Ti3, occur already after 2 min of annealing, much earlier than documented in the literature. Furthermore, the phase transformation to the thermodynamically stable Ni3Ti in the Ni-enriched zone is shown to be mediated by metastable Ni4Ti3. In contrast to Ni4Ti3 precipitates in NiTi bulk, the morphology of the crystalline phases is granular. Considering the swift formation of Ni-rich phases below the surface oxide and the observation of a band-like diffraction contrast in the TEM images originating from crystal interfaces, a phase transformation mechanism in the Ni-enriched zone is suggested consisting of short range order rearrangements of atoms.

Electromagnetic levitation is a method for contain-erless high-temperature treatment of metal, semiconductor, and alloy samples. This method is widely used to investigate the thermophysical and thermochemical properties of liquid melts, as well as their crystallization kinetics. An alternating electro-magnetic field induces an induction current inside a sample, resulting in a Lorentz force opposing the gravitational force. The Lorentz force lifts the sample, which is heated and melts in a levitation chamber due to the current flowing through it. In this paper, we present an analytical model of the sample levitation process, considering the structure of the electromagnetic levitator coil and options for its optimization for experiments. The kinetics of high-speed solidification of undercooled droplets in the chamber of the electromagnetic levitator is analyzed.

Niobium carbide (NbC) catalytic films were deposited on vanadium (V) foils by magnetron sputtering to prepare the NbC/V composite membranes for high temperature hydrogen separation. It is found that the sputtering power and substrate bias largely affect the structure and catalytic activity of the NbC films, and thus the hydrogen permeability of the NbC/V composite membranes. Optimized conditions for membrane preparation were obtained, which generated a pronounced high hydrogen permeability, particularly similar to 5.85 x 10(-8) mol H-2 m(-1) S-1 Pa-0.5 at 650 degrees C. This is 1.9 times that of pure palladium (Pd). The high hydrogen permeability of the NbC/V membranes can be attributed to the high catalytic activity of the NbC films towards hydrogen dissociation, as the cubic NbC phase in the films exhibits preferential growth along the catalytically dominated crystalline plane and a relatively high percentage. This work demonstrates that the NbC/V system shows high potential as a new non-precious metal membrane for hydrogen separation and purification at high temperatures. (C) 2021 Elsevier B.V. All rights reserved.

Layers of aligned dyes are key to photo-driven charge separation in dye sensitized solar cells, but cannot be exploited as rectifying membranes in photocatalysis to separate half-cells because they are not sufficiently stable. While impressive work on the fabrication of stable noncovalent membranes has been recently demonstrated, these membranes are inherently suffering from non-uniform orientation of the constituting dyes. To stabilize layers made from uniformly assembled and aligned dyes, they can be covalently cross-linked via functional groups or via chromophores at the expense of their optical properties. Here stable membranes from established dyes are reported that do not need to be elaborately functionalized nor do their chromophores need to be destroyed. These membranes are free-standing, although being only non-covalently linked. To enable uniform dye-alignment, Langmuir layers made from linear, water-insoluble dyes are used. That water-soluble charge transfer dyes adsorb onto and intercalate into the Langmuir layer from the aqueous subphase, thus yielding free-standing, molecularly thin membranes are demonstrated. The developed bifacial layers consist almost entirely of π-conjugated units and thus can conduct charges and can be further engineered for optoelectronic and photocatalytic applications.

Understanding phenomena that occur during gradient annealing and initial transient of Bridgman-type directional solidification processes is essential for producing high-performance materials with specific properties. An experiment with alternating long time gradient annealing and directional solidification periods was performed on the International Space Station, using a near-peritectic transparent TRIS-NPG alloy. It transpired that accumulation of solute ahead of the solid/liquid interface continued to progress and that steady-state growth conditions were never achieved. The results demonstrate that (i) liquid being squeezed out from the mush during the long time gradient annealing period disables the formation of a flat interface; (ii) a thermal bias caused a slightly tilted planar solidification front; and (iii) growth of the metastable pro-peritectic α-phase led to the formation of a supersaturated solid that solidified with an intriguing low growth rate.

In this work, a multi-phase cellular automaton (CA) model is extended for the quantitative simulation of peritectic phase transition. First, the effects of cooling rate/supersaturation and temperature on the peritectic transformation kinetics in Fe-C alloys are investigated by utilizing the present CA model. The CA simulations show that supersaturations in the parent phases (liquid and δ-ferrite) increase the L/γ interface growth velocity remarkably, but tinily for the δ/γ interface migration velocity. There exists a transition supersaturation for isothermal transformations, at which the growth rates of the two interfaces are equal. The transition supersaturation is found to increase with decreasing temperature. Microstructural evolution at different cooling rates during peritectic transformation is simulated using the experimental conditions. At low cooling rates, the δ/γ interface propagates at a higher velocity than the L/γ interface. At high cooling rates, however, the γ-phase grows more into the L-phase with a cellular morphology. Then, the proposed CA model is applied to simulate the microstructural evolution during peritectic reaction. It is observed that the γ-phase propagates along the L/δ interface and finally encircles the δ-phase. Meanwhile, the intervenient γ-phase grows in thickness through peritectic transformation. The CA simulations are compared reasonably well with the experimental data and analytical calculations.

The spatial and compositional complexity of 3D structures employed in today's nanotechnologies has developed to a level at which the requirements for process development and control can no longer fully be met by existing metrology techniques. For instance, buried parts in stratified nanostructures, which are often crucial for device functionality, can only be probed in a destructive manner in few locations as many existing nondestructive techniques only probe the objects surfaces. Here, it is demonstrated that grazing exit X-ray fluorescence can simultaneously characterize an ensemble of regularly ordered nanostructures simultaneously with respect to their dimensional properties and their elemental composition. This technique is nondestructive and compatible to typically sized test fields, allowing the same array of structures to be studied by other techniques. For crucial parameters, the technique provides sub-nm discrimination capabilities and it does not require access-limited large-scale research facilities as it is compatible to laboratory-scale instrumentation.

Experiments were carried out onboard the International Space Station using the electromagnetic levitation (EML) facility on dendritic growth in Al-35 at%Ni under primary solidification of the NiAl(B2)-phase. The “velocity versus undercooling” relationship shows an anomalous behavior. At low undercoolings, $$\Delta T<250$$ K, the velocity unexpectedly decreases with increasing undercooling. For higher undercoolings, $$\varDelta T>250$$ K, the crystal velocity increases with increasing undercooling following a thermodynamically consistent trend. For such anomalous behavior, a special mechanism was identified through the different crystals having scales and dendritic morphology. The scaled front consists of numerous nuclei ahead of the crystal–liquid interface that occurs in the undercooling range featuring the negative slope for the crystal growth velocity at $$\varDelta T<250$$ K. This unusual behavior is confirmed experimentally and explained theoretically using analytical solutions for the crystal growth front with the polydisperse ensemble of crystals nucleating ahead of it.

This study evaluates the effect of plasma surface functionalization of reduced graphene oxide particles on the processing characteristics and homogeneity of dispersion of a bisphenol A-(epichlorhydrin) epoxy matrix and amine-based hardener with varying weight fractions from 0.00 to 1.50 wt%. It was observed that amine-functionalized reduced graphene oxide leads to a more drastic viscosity increase of up to 18-fold of the uncured suspensions and that its presence influences the conversion rates of the curing reaction. Optical microscopy of thin sections and transmission electron microscopy analysis showed that a more homogeneous dispersion of the particles could be achieved especially at higher weight fractions by using an appropriate surface functionalization. This knowledge can be used to define suitable processing conditions for epoxies with amine-based hardeners depending on the loading and functionalization of graphene-related particles.

Microstructure of Al-40 wt%Si samples solidified in electromagnetic levitation furnace is studied at high melt undercooling. Primary Si with feathery and dendritic structures is observed. As this takes place, single Si crystals either contain secondary dendrite arms or represent faceted structures. Our experiments show that at a certain undercooling, there exists the microstructural transition zone of faceted to non-faceted growth. Also, we analyze the shape of dendritic crystals solidifying from liquid Si as well as from hypereutectic Al-Si melts at high growth undercoolings. The shapes of dendrite tips grown at undercoolings >100 K along the surface of levitated Al-40 wt%Si droplets are compared with pure Si dendrite tips from the literature. The dendrite tips are digitized and superimposed with theoretical shape function recently derived by stitching the Ivantsov and Brener solutions. We show that experimental and theoretical dendrite tips are in good agreement for Si and Al-Si samples.

A theory of stable dendrite growth in an undercooled binary melt is developed for the case of intense convection. Conductive heat and mass transfer boundary conditions are replaced by convective conditions, where the flux of heat (or solute) is proportional to the temperature or concentration difference between the surface of the dendrite and far from it. The marginal mode of perturbation wavelengths is calculated using the linear morphological stability analysis. Combining this analysis with the solvability theory, we have derived a selection criterion that represents the first condition to define a combination of dendrite tip velocity and tip diameter. The second condition—the undercooling balance—is derived for intense convection. The theory under consideration determines the dendrite tip velocity and tip diameter for low undercooling. This convective theory is combined with the classical theory of dendritic growth (conductive boundary conditions), which is valid for moderate and high undercooling. Thus, the entire range of melt undercooling is covered. Our results are in good agreement with experiments on Al–Ge crystallization.

Structure formation during solidification of a Pd-Ni-Cu-P melt is studied. It is demonstrated that changes in the heat transfer conditions lead to a nonlinear change in the characteristics of the structure. The article presents the regimes of cooling the samples and the results of their structure and composition studies. It is found that a decrease in the cooling rate of the alloy leads to an increase in the size, proportion and composition of nanoinclusions in an amorphous matrix. X-ray diffraction method, electron probe microanalysis, transmission microscopy and scanning calorimetry are used for samples characterization. This article is part of the theme issue 'Transport phenomena in complex systems (part 2)'.

Microstructural evolution in the presence of liquid film migration (LFM) is simulated for Al-Cu alloys using a cellular automaton (CA) model. Simulations are performed for the microstructural evolution and concentration distribution in an Al-4 wt.%Cu alloy with initially equiaxed grain structures holding in a temperature gradient. A slight deviation from local equilibrium, estimated from experimental data, is considered to be the driving force for LFM. The direction of LFM is triggered by concentration fluctuations setting a concentration gradient as a further driving force. The simulation successfully reproduces the experimentally observed microstructures generated by LFM accompanied by a particle free zone behind the liquid film. The solid concentration in the particle free zone is found to be the equilibrium solid concentration. The simulated concentration profile across the migrating liquid film agrees well with experimental measurements. The simulated grain structure becomes coarser and highly elongated after holding in the temperature gradient. The results reveal that the increase in transversal grain width is mainly controlled by LFM, while the grain elongation in longitudinal direction is attributed to both LFM and temperature gradient zone melting. The solid concentration decreases from the initial (supersaturated) composition to the local equilibrium solid concentration corresponding to the local temperature. This article is part of the theme issue 'Transport phenomena in complex systems (part 2)'.

The thermodynamic description of the fcc phase in the Al-Cu system has been revised, allowing for the prediction of metastable fcc/liquid phase equilibria to undercoolings of ΔT = 421 K below the eutectic temperature. Hypoeutectic Al-Cu alloys that are prone to pronounced microsegregation were solidified containerlessly in electromagnetic levitation. Solidus and liquidus concentrations were experimentally determined from highly undercooled samples employing energy-dispersive X-ray analysis. Solid concentrations at a rapidly propagating solid/liquid interface were additionally calculated using a sharp interface model that considers all undercoolings and is based on solvability theory. Modelling results (front velocity versus undercooling) were also corroborated by in situ observation with a high-speed camera. A newly established thermodynamic description of the fcc phase in Al-Cu is compatible with existing CALPHAD-type databases. Inconsistencies of previous descriptions such as a miscibility gap between Al-fcc and Cu-fcc on the Al-rich side, an unrealistic curvature of the solidus line in the same composition range or an azeotropic point near the melting point of Cu, are amended in the new description. The procedure to establish the description of phase equilibria at high undercoolings can be transferred to other alloy systems and is of a general nature. This article is part of the theme issue 'Transport phenomena in complex systems (part 2)'.

Porous platinum is a frequently used catalyst material in electrosynthesis and a robust broadband absorber in thermoelectrics. Pore size distribution and localization determine its properties by a large extent. However, the pore formation mechanism during the growth of the material remains unclear. In this work we elucidate the mechanism underlying electrochemical growth of nanoporous platinum layers and its control by ionic concentration and current density during electrolysis. The electrode kinetics and reduction steps of PtCl4 on platinum electrodes are investigated by cyclic voltammetry and impedance measurements. Cyclic voltammograms show three reduction steps: two steps relate to the platinum cation reduction, and one step relates to the hydrogen reduction. Hydrogen is not involved in the reduction of PtCl4, however it enables the formation of nanopores in the layers. These findings contribute to the understanding of electrochemical growth of nanoporous platinum layers in isopropanol with thickness of 100 nm to 500 nm.

A temperature gradient evaluation method for swift determination of the Curie temperature T-C based on thermal conductivity and thermal diffusivity as functions of temperature is presented. In an imposed one-dimensional temperature gradient, temperature profiles along the axis of a cylindrical sample are recorded in steady-state and transient states using a photon detector. Transient temperature profiles are evaluated by an inverse numerical method, yielding the temperature dependent thermal diffusivity with a distinct change in slope at T-C. The slope of the steady-state temperature profile adjusts according to the local thermal conductivity and changes abruptly at T-C. The high-resolution determination of the temperature dependent thermal properties allows the identification of T-C by two independent evaluations during a single experimental run, solely relying on data of thermal conductivity and thermal diffusivity. The obtained T-C excellently agrees with literature values gained by conventional methods.

The developed model of diffusion‐limited and diffusionless solidification of a eutectic alloy describes the relation “undercooling (ΔT)‐velocity (V)‐interlamellar spacing (λ)” for two cases. Namely, when the solidification front velocity V is smaller than the solute diffusion speed in bulk liquid VD, V < VD, the model predicts a regime of eutectic solidification similarly to known classical models. If the solidification front velocity V is higher than the diffusion speed, V > VD, the solidification is mainly controlled by kinetic and thermal undercoolings. New expressions for the solute distribution coefficient and slope of the liquidus lines are supplied. The influence of the model parameters on the growth kinetics during eutectic solidification is discussed. Model predictions are compared with experimental data for the solidification of an Fe–B alloy with eutectic composition. Computational results show that the model agrees well with experimental data especially for low and high undercoolings, extending the undercooling range that can be covered by sharp interface modeling.

The influence of natural oxide layers at interfaces in diffusion couples is studied. Concentration distributions at (Ni)/NiAl interfaces and the formation of the Ni3Al phase at 650 degrees C are characterized using scanning electron microscopy (SEM) and aberration corrected scanning transmission electron microscopy (STEM) with EnergyDispersive X-ray spectroscopy (EDX) with unprecedented spatial resolution. Employing the common practice of chemical etching with 10 vol% HCl does not remove the oxide layers irrevocably - they are detected at the Kirkendall plane after diffusion annealing of (Ni)/NiAl diffusion couples, if high resolution microscopy is employed. The oxide layer at the Kirkendall plane is of a thickness of similar to 5 nm, comparable to that of the combined natural oxide layers on the (Ni) and NiAl surfaces. The oxide acts as a diffusion barrier, retarding or locally suppressing growth of Ni3Al into the (Ni) phase. For comparison, oxide-free Kirkendall planes were obtained by subjecting the joining surfaces of (Ni) and NiAl to gentle grinding with low contact pressure in a glove-box in purified Ar atmosphere before assembling the diffusion couple. After diffusion annealing, the Ni3Al layers are distinctly thicker and of a more homogeneous layer thickness as compared to samples with residual oxides. It is shown that evaluating interdiffusion coefficients in oxide-free diffusion couples yields higher values as compared to the conventionally assembled counterparts.

Understanding the rapid solidification behavior characteristics, nucleation undercooling, and nucleation mechanism is important for modifying the microstructures and properties of metal alloys. In order to investigate the rapid solidification behavior in-situ, accurate measurements of nucleation undercooling and cooling rate are required in most rapid solidification processes, e.g., in additive manufacturing (AM). In this study, differential fast scanning calorimetry (DFSC) was applied to investigate the nucleation kinetics in a single micro-sized Al-20Si (mass%) particle under a controlled cooling rate of 5000 K/s. The nucleation rates of primary Si and secondary α-Al phases were calculated by a statistical analysis of 300 identical melting/solidification experiments. Applying a model based on the classical nucleation theory (CNT) together with available thermodynamic data, two different heterogeneous nucleation mechanisms of primary Si and secondary α-Al were proposed, i.e., surface heterogeneous nucleation for primary Si and interface heterogenous nucleation for secondary α-Al. The present study introduces a practical method for a detailed investigation of rapid solidification behavior of metal particles to distinguish surface and interface nucleation.