Directional Solidification Of Eutectics Research Articles

Bulk nanocomposite made of ZnO lamellae embedded in the ZnWO4 matrix: growth from the melt

Zinc oxide (ZnO) nanostructures exhibiting high exciton binding energy and efficient radiative recombination, even at the room temperature, are of increasing interest due to their prospective exploitation in optoelectronic and laser applications. However, attempts to synthesize well-ordered structures through simple and fast process have faced many difficulties. Here, we demonstrate a novel manufacturing method of ZnO lamellae embedded in a crystalline wide band gap dielectric matrix of the zinc tungstate, ZnWO4. The manufacturing method is based on a directional solidification of a eutectic composite, directly from the melt, resulting in a nanostructured bulk material. Electron microscopy studies revealed clear phase separation between the ZnO and ZnWO4 phases, and cathodoluminescence confirmed exciton emission at room temperature and thus high quality and crystallinity of the ZnO lamellae, without defect emission. Hence, utilization of directional solidification of eutectics may enable cost-efficient manufacturing of bulk nanostructured ZnO composites and their use in optical devices.Graphical abstract

Selective surface-enhanced Raman scattering in a bulk nanoplasmonic Bi2O3-Ag eutectic composite

Abstract A recent surge of interest in surface-enhanced Raman scattering (SERS) has stimulated the search for new systems that can be utilized to fabricate high-performance optical devices. However, the two-dimensional design of the vast majority of SERS-based assemblies has significantly hindered their real-life applicability, motivating the development of three-dimensional volumetric materials. Here, we report selective SERS observed in a volumetric Bi2O3-Ag eutectic composite obtained by the micro-pulling-down method utilizing directional solidification of eutectics. The enhancement of the Raman signal originates from the localized surface plasmon resonance, LSPR, resulting from silver nanoparticles embedded in the composite. The plasmonic origin of the enhancement is confirmed by characteristic features, such as (i) an enhancement magnitude >103, (ii) the correspondence between the Raman bands’ intensity upon excitation by different wavelengths and the localized surface plasmon resonance (LSPR) intensity, and (iii) the occurrence of overtones, which are absent in the as-grown material that does not exhibit LSPR. The examined Bi2O3-Ag eutectic-based composite is obtained by directional solidification using a simple crystal growth technique. It is the first case of a bulk SERS-active material fabricated by crystal growth techniques, which opens new perspectives towards scalable three-dimensional optical elements with tunable properties based on Raman scattering.

Formation of Faceted Spirals during Directional Eutectic Solidification

Formation of Faceted Spirals during Directional Eutectic Solidification

Progress in modeling solidification in molten salt coolants

Molten salts have been proposed as heat carrier media in the nuclear and concentrating solar power plants. Due to their high melting temperature, solidification of the salts is expected to occur during routine and accidental scenarios. Furthermore, passive safety systems based on the solidification of these salts are being studied. The following article presents new developments in the modeling of eutectic molten salts by means of a multiphase, multicomponent, phase-field model. Besides, an application of this methodology for the eutectic solidification process of the ternary system LiF–KF–NaF is presented. The model predictions are compared with a newly developed semi-analytical solution for directional eutectic solidification at stable growth rate. A good qualitative agreement is obtained between the two approaches. The results obtained with the phase-field model are then used for calculating the homogenized properties of the solid phase distribution. These properties can then be included in a mixture macroscale model, more suitable for industrial applications.

Phase-field study on the effects of process and material parameters on the tilt angle during directional solidification of ternary eutectics

In the directional solidification of molten alloys, the configuration of growing phases plays a vital role in the properties of the resulting materials. During directional eutectic solidification, the growth direction and the velocity can be controlled by an imprinted temperature gradient. For this kind of process, different instabilities can occur. One observed instability is tilted growth of the solid phases against the direction of the imprinted temperature gradient. In ternary eutectics this phenomenon can occur in systems with isotropic phases. To investigate the effects of the different process and material parameters, simulation studies with a thermodynamically consistent phase-field model based on the grand potential approach, are conducted. An idealized system with different isotropic interfacial energies, diffusion coefficients, lamellar spacings, solidification velocities and slopes of the imposed temperature gradient is systematically studied. The effects on tilting are analyzed and discussed. To quantitatively determine the tilt angle automatized, an unsupervised measurement method is developed. Based on the results of the measurements, a fitting function is derived to predict the tilt angles.

Evolution of Columnar-Dendritic Microstructure during Upward Directional Solidification of Eutectic and Off-Eutectic Pb-Sn Alloys

Upward directional solidification experiments were carried out on various Pb-Sn alloys using water cooled copper chills. Different combinations of growth rate V and composition Co were used to investigate their effect on microstructure evolution during solidification. The effect of solidification velocity and composition on the structure formation is analyzed. It is observed that the hypoeutectic Pb-Sn alloys exhibit columnar dendritic structure with α phase rich region. In case of hypereutectic alloys the columnar dendritic structure is with β phase rich region. In both the cases dendritic structure was found to surround by the eutectic structure, a typical result of non equilibrium cooling. The eutectic composition exhibits complete eutectic structure aligned in the direction of solidification.

How Far Are We from Making Metamaterials by Self‐Organization? The Microstructure of Highly Anisotropic Particles with an SRR‐Like Geometry

AbstractMetamaterials offer new unusual electromagnetic properties, which have already been demonstrated, and many postulated new functionalities are yet to be realized. Currently, however, metamaterials are mostly limited by narrow band behavior, high losses, and limitation in making genuinely 3D materials. In order to overcome these problems an overlap between metamaterial concepts and materials science is necessary. Engineered self‐organization is presented as a future approach to metamaterial manufacturing. Using directional solidification of eutectics, the first experimental realization of self‐organized particles with a split‐ring resonator‐like cross section is demonstrated. This unusual morphology/microstructure of the eutectic composite has a fractal character. With the use of TEM and XRD the clear influence of the atomic crystal arrangement on the microstructure geometry is presented. The materials obtained present very high anisotropy and can be obtained in large pieces. Metallodielectric structures can be created by etching and filling the space with metal. The next steps in the development of self‐organized materials exhibiting unusual properties are discussed.

Oriented Microstructures of Polystyrene-b-poly(l-lactide) Thin Films Induced by Crystallizable Solvents

Polystyrene-b-poly(l-lactide) (PS−PLLA) diblock copolymers with high and low molecular weights were synthesized to form strongly and weakly segregated thin-film morphologies, respectively (i.e., ordered lamellar and disordered textures). Large-sized, well-oriented lamellar microstructures of PS−PLLA thin films were produced for both samples by using the crystallizable solvents: benzoic acid (BA) and hexamethylbenzene (HMB). In strongly segregated (high Mn) PS−PLLA, the oriented microstructures were obtained through the PS−PLLA microphase separation due to directional crystallization of crystallizable solvents at the eutectic point (directional eutectic solidification) regardless of the PLLA crystallization. Lattice matching between crystalline substrates and PLLA was found to be nonessential for inducing microstructure orientation although it may improve its orientation order. In weakly segregated (low Mn) PS−PLLA, the oriented microstructures were formed through the directional crystallization of PLLA o...

Channeled Ni-YSZ Cermets Produced from NiO-YSZ Eutectics by Laser Melting

NiO-YSZ was produced by directional solidification of eutectics (DSE) using laser melting. This technique produces highly structured material consisting of NiO and YSZ lamellae with variable interlamellar spacing depending on growth conditions. The microstrocture is homogeneous and the material mechanically stable during thennochemical reduction. Complete reduction of the NiO produces a porous cermet alternating lamellae of Ni and YSZ. The coefficient of thermal expansion of the cermet is that of the YSZ skeleton. Reaction kinetics at different temperatures during the reduction process was studied by gravimetric methods and SEM experiments, showing that the reduction process seems to be controlled by the O 2- diffusion through the YSZ phase. TEM experiments reveal that the low-energy interfaces formed during the eutectic growth are maintained after reduction. These interfaces are expected to prevent particle coarsening and give a long-term stability to the anode during operation at high temperatures. Resistivity values and pore distribution present no degradation after 300 hr at 900°C under H 2 /N 2 atmosphere. This stability is an improvement for its use as a functional layer in solid-oxide fuel cell (SOFC) anodes.

Self assemble silicide architectures by directional solidification

Directional solidification of eutectics has self assembly characteristics that can fabricate two dimensional periodic arrays of two or more phases. The ordering of the phases can be utilized as a “lithographic technique” to produce porous structures for fabrication of miniature devices. Directional solidification by the Bridgman technique was applied to the eutectic systems Si–TiSi 2 and Si–YSi 2. Patterned growth of TiSi 2 rods occur in the silicon matrix during solidification. Micro-channel structure of pillar arrays of TiSi 2 was produced by silicon dissolution by KOH. Average TiSi 2 rod diameter was 2.5 μm with 99% of the population falling in the 2–3 μm range. Colony formation could not be suppressed in the Si–TiSi 2 system. Patterned growth is not observed in Si–YSi 2 system. Eutectic and hyper-eutectic Si–YSi 2 compositions formed anomalous eutectic microstructure. Microstructure shows dependence upon the solidification rate.

Growth morphology of the NiAl–V in situ composites

Intermetallic matrix composites (IMCs) are considered candidate materials for high-temperature applications provided that the proper combination of properties can be developed. Among a large number of intermetallics, a particular interest has been shown in the NiAl-based composites. Attractive properties of this compound include higher melting temperature, better oxidation resistance, and lower density when compared to conventional superalloys. However, insufficient low-temperature fracture resistance is a major drawback. Stringent requirements for chemical and mechanical compatibility between the matrix and the reinforcing phase have recently renewed interest in directional solidification of the eutectic alloys. Microcomposite structures generated by directional solidification of eutectics offer many unique microstructural advantages, including thermodynamic stability up to the melting temperature, directional alignment and fine dispersion of component phases. Although a large number of directionally solidified eutectic systems have been investigated so far, little information on the eutectics in NiAl–V system has been reported. The present paper is concerned with the growth morphology of the NiAl–V in situ composites. Samples prepared by arc melting were processed in a directional solidification furnace with induction heating. The main objective was to analyze the solidification microstructure and morphology regarding the growth conditions and composition. Sample characterization, beside the optical and scanning electron microscopy (SEM), included the use of differential thermal analysis (DTA), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD) technique.

Phase field modeling of directional eutectic solidification and comparison with experiments

We develop a phase-field model of eutectic solidification by combining the “interface field” concept [Steinbach and Pezzola, Physica D, 1999, 134, 385] and thin interface model[Kim et al., Phys. Rev. E, 1999, 60, 7186]. The model was applied to generate various pattern instabilities in directional thin eutectic solidification of CBr4—C2Cl6 alloys. Numerical calculation was performed under real experimental conditions without any fitting parameters. Depending on the lamellar spacing and alloy composition, various patterns including regular lamella, oscillatory, and tilted patterns appear at the same instability domains observed in experiments.

Microdomain patterns from directional eutectic solidification and epitaxy

Creating a regular surface pattern on the nanometre scale is important for many technological applications, such as the periodic arrays constructed by optical microlithography that are used as separation media in electrophoresis, and island structures used for high-density magnetic recording devices. Block copolymer patterns can also be used for lithography on length scales below 30 nanometres (refs 3-5). But for such polymers to prove useful for thin-film technologies, chemically patterned surfaces need to be made substantially defect-free over large areas, and with tailored domain orientation and periodicity. So far, control over domain orientation has been achieved by several routes, using electric fields, temperature gradients, patterned substrates and neutral confining surfaces. Here we describe an extremely fast process that leads the formation of two-dimensional periodic thin films having large area and uniform thickness, and which possess vertically aligned cylindrical domains each containing precisely one crystalline lamella. The process involves rapid solidification of a semicrystalline block copolymer from a crystallizable solvent between glass substrates using directional solidification and epitaxy. The film is both chemically and structurally periodic, thereby providing new opportunities for more selective and versatile nanopatterned surfaces.

Organisation of carbon and boron nitride layers in mixed nanoparticles and nanotubes synthesised by arc discharge

bonds attests for the presence of ordered BN domains and of carbon domains; (ii) the elemental profiles show that BN layers and carbon layers are immiscible with a radial organisation into two to five domains; and (iii) the sets of layers at free surfaces – including the inner surfaces of tubes – are always made of carbon. The origin of this chemical organisation, which is most likely obtained during the growth, is discussed. For the hafnium-boride metallic particles coated by C/BN envelopes, a model based on the solidification from the outside to the inside of isolated liquid-like droplets is proposed: the carbon phase solidifies first according to theoretical phase diagrams, and forms the outer shells. For the tubes, a directional eutectic solidification process is shown to account for the observed C/BN/C sequence, in a vapour–liquid–solid scheme, with an hafnium-rich liquid-like particle at the tip of the tube.

Processing, Microstructures, and Properties of Cr-Cr3Si, Nb-Nb3Si, and V-V3Si Eutectics

Abstract The present paper examines the potential of refractory metal-A15 silicide composites as structural materials for high temperature applications. Three eutectic systems are considered, Cr-Cr3Si, Nb-Nb3Si, and V-V3Si, since they all have melting points above 1700°C and densities lower than Ni-based superalloys. Eutectic compositions were selected because directional solidification of eutectics can be used to generate aligned composite microstructures. The present paper describes processing, microstructures and properties of directional ly solidified Cr-Cr3Si, Nb-Nb3Si, and V-V3Si. Based on microstructural and chemical analyses, some phase diagram modifications are suggested. The orientation relationships between the refractory metal and A15-silicide phases were characterized using transmission electron microscopy. In the Cr-Cr3Si eutectic a strong orientation relationship was observed; there was excellent lattice matching of the (110) in Cr and (210) in Cr3Si. Room temperature and elevated temperature microhard-ness and fracture toughness are also reported; these indicate good strength retention in Nb-Nb3Si at elevated temperatures.

Similarity equation in non-axisymmetric lamellar eutectic growth

The authors extend their previous derivation (1991) of similarity laws in directional solidification of eutectics to general non-axisymmetric growth patterns. This involves mathematical subtleties which are not encountered in the symmetric case. The result explains the observation that numerical solutions describing tilted eutectic growth share the basic similarity property of axisymmetric solutions. They find additional scaling relations of the form lambda approximately V-1/2g(G/V), e.g. for the wavelength lambda of a tilted pattern at fixed tilt angle phi (V is the pulling velocity, G the temperature gradient). Discussing the question of universality of the scaling function g for different distinguished wavelengths, the authors are led to the prediction that the transition to a parity-broken state takes place at roughly twice the selected wavelength of the symmetric pattern for sufficiently large velocities and that the ratio of these wavelengths increases with decreasing velocity.

Cold-crucible directional solidification of refractory metal-silicide eutectics

Microcomposite structures generated by directional solidification of eutectics offer many unique microstructural advantages, including thermodynamic stability, directional alignment, and a fine dispersion of component phases. In an effort to improve the performance of advanced turbine engines, a Czochralski crystal growth method in conjunction with cold-crucible induction melting has been developed for the production of high-temperature eutectic composites. Cold-crucible directional solidification has been successfully applied to the growth of refractory metal-silicide eutectics into directional microcomposites. The eutectics studied include Cr-Cr3Si,Nb-Nb3Si,and V-V3Si, which have melting temperatures ofl,705°C, 1,880°C, and 1,870°C, respectively.

Directional solidification of eutectics: New prospects for refractory oxide cermaics (review)

The directional solidification of the eutectic in the refractory oxide systems results in the formation of a plate-shaped or fibrous oriented composite structure. The theoretical fundamentals of the method of directionally solidified eutectics, developed on the basis of the metallic systems, can also be applied extensively to refractory oxide DSEs so that their structure and composition can be controlled in the required direction.