Crystal Growth Direction Research Articles

A self-standing three-dimensional covalent organic framework film

Covalent crystals such as diamonds are a class of fascinating materials that are challenging to fabricate in the form of thin films. This is because spatial kinetic control of bond formation is required to create covalently bonded crystal films. Directional crystal growth is commonly achieved by chemical vapor deposition, an approach that is hampered by technical complexity and associated high cost. Here we report on a liquid-liquid interfacial approach based on physical-organic considerations to synthesize an ultrathin covalent crystal film. By distributing reactants into separate phases using hydrophobicity, the chemical reaction is confined to an interface that orients the crystal growth. A molecular-smooth interface combined with in-plane isotropic conditions enables the synthesis of films on a centimeter size scale with a uniform thickness of 13 nm. The film exhibits considerable mechanical robustness enabling a free-standing length of 37 µm, as well as a clearly anisotropic chemical structure and crystal lattice alignment.

Lightweight chitosan-derived carbon/rGO aerogels loaded with hollow Co1-xNixO nanocubes for superior electromagnetic wave absorption and heat insulation

Designing lightweight electromagnetic (EMW) absorption materials with an ultra-broad effective absorption bandwidth (EAB), strong reflection loss (RL), and low filling level remains a challenging. In this study, an anisotropic chitosan-derived carbon/reduced graphene oxide (rGO) aerogel loaded with hollow ZIF-67 nanocubes (CG3) was constructed via unidirectional freeze-drying, heat treatment, and in situ ZIF-67 growth, followed by chemical etching. The interconnected oriented cell walls, hollow ZIF-67 nanocubes, and heterogeneous carbon/rGO interfaces enhanced EMW absorption as a result perfect impedance matching, multiple scattering, and strong dielectric polarizations. Thus, an outstanding EMW absorption performance with an ultra-broad EAB of 9.5 GHz and a minimal RL (RLmin) of −53.3 dB was achieved for the CG3 aerogel with an ultra-low filling ratio (5.5 wt%). Further, to integrate strong EMW absorption and heat insulation abilities, the hollow ZIF-67 nanocubes loaded on the carbon/rGO aerogel were converted into hollow Co1-xNixO nanocubes (CG4) via a cation exchange reaction and annealing treatment in air. Owing to the oriented pore structure, the CG4 aerogel exhibited obviously anisotropic EMW absorption and heat insulation. When the CG4 aerogel was cut along the longitudinal section parallel to the direction of ice crystal growth (CG4-L), a tubelike porous structure was revealed, and this endowed the aerogel with the highest EMW absorption (EAB = 8.7 GHz, RLmin = -52.3 dB) and thermal insulation (thermal conductivity = 0.057 W m−1 K−1) performance because of the extended EMW transmission and thermal transfer pathways. Thus, this work offers an important design strategy for obtaining high-performance thermally insulating, EMW-absorbing composites.

Shape control of Au nanostructures using peptides for biotechnological applications.

Metallic gold (Au) nanostructures have attracted attentions in various fields of materials science and electrical science in terms of catalysts, sensing systems, photonic devices, and drug delivery systems because of their characteristic physical, chemical, and biocompatible properties. Recently, Au nanostructures with near-infrared light absorbing properties have shown potential for applications such as biological imaging and thermotherapy in biotechnological fields. However, fabrication of Au nanostructures with complex shapes often requires the use of highly biotoxic substances such as surfactants and reducing agents. Peptides are promising compounds for controlling the shape of Au nanostructures by mineralization with several advantages for this purpose. In this highlight, we focus on the shapes with respect to the fabrication of Au nanostructures using biocompatible peptides. We classify the peptides that form Au nanostructures into three broad categories: those that bind Au ions, those that reduce Au ions, and those that control the direction of Au crystal growth. Then, we briefly summarize the correlations between peptide sequences and their roles, and propose future strategies for fabricating Au nanostructures using peptides for biotechnological applications.

Growth of highly aligned ZnO nanorod arrays on zinc plates: Morphological and structural characterization

In this work, uniform and highly aligned ZnO nanorod arrays were simply synthesized on zinc plates by hydrothermal method using zinc–ammonia complex solution. Effects of ZnO seed layer formation, Zn(NO3)2 concentration and NH3/Zn(NO3)2 mole ratio (R) in the reaction medium on the morphology and crystal structure of nanorods were investigated in detail. ZnO nanorods grown on zinc surfaces were highly crystalline and had a hexagonal wurtzite structure, as revealed by XRD analysis. In addition, the dominant crystal growth direction was in c-axis, indicating the verticality of the nanorods. SEM images showed that one-dimensional (1D) ZnO nanorods with high verticality and number density were synthesized on seeded plates whereas randomly oriented arrays were grown on non-seeded surfaces. Increase in the Zn2+ concentration changed the top ends of the rods from tapering to hexagonal ends; and also led to an increase in the average size and verticality of the nanorods. The increase in R value (or i.e., amount of NH3) caused the rods to misalign on the surface and decrease in size. The size of well-aligned nanorods can be adjusted from 50 to 260 nm in diameter and 0.11 to 6 m in length by changing the reaction parameters, implying their large potential to be used in photocatalytic and also in electronic applications.

Large-size high-quality CdSe-OPO component for far IR laser output prepared by directional crystal growth technique

Φ40 × 100 mm3 CdSe crystal was obtained using a capsuled Mo-crucible method combined with a seed directional technique.

A novel approach to improve the macro-segregation defect and mechanical properties of Al–Mg–Mn aluminum alloys during twin-roll continuous casting

In this paper, the microstructure and properties of Al–Mg–Mn alloy slabs with different thicknesses by twin-roll continuous casting were investigated. Under the twin-roll continuous casting process, the alloy melt conducts directional crystallization due to the near-horizontal heat transfer direction. In this way, the growth direction of the columnar crystal is changed to improve the shape and size of the sump and effectively eliminate the central segregation defect. Additionally, the improvement of segregation defects was more significant with the increase in thickness. As low angle grain boundary content and solution degree increased, the electrical conductivity decreased, the hardness increased, and the phenomenon of hard-soft-hard core-shell was significantly improved.

Effect of Tempering Process on Microstructure and Properties of Resistance Spot-Welded Joints of δ-TRIP Steel

In this paper, a medium-frequency inverter spot welder was used for resistance spot-welding experiments on 980 MPa grade cold-rolled δ-TRIP(Transformation-induced Plasticity) steel. The effects of the tempering process on the morphology, microstructure, element distribution, and properties of spot-welded joints were studied by Scanning Electron Microscope (SEM), Transmission Electron Microscopy (TEM), and Electron-Probe MicroAnalysis (EPMA). The microstructure of the nugget zone obtained by single-pulse process was δ ferrite, lath martensite, and twin martensite. After adding tempering under the single-pulse process, the microstructure was δ ferrite and lath martensite. However, the morphology of the microstructure was still dendritic, which remained unchanged. The tensile shear failure of spot-welded joints under the two processes was an interface failure, and the fractures were cleavage fractures. After adding tempering, the interface fracture surface presents two kinds of fracture characteristics: the outer cracks’ growth direction was consistent with the columnar crystal growth direction, and the inner crystal cracks occurred in the nugget core and finally grew along the columnar grain boundary. Due to the significant hardness difference between δ ferrite (283 HV) and martensite (533 HV), the low-strength δ ferrite phase was the main channel of crack propagation. After adding tempering, the hardness distribution of the spot-welded joints was more uniform and the tensile shear force increased (7.4 kN→8.5 kN).

3D Orientation Imaging of Polymer Chains with Polarization-Controlled Coherent Raman Microscopy

Despite the ubiquity of three-dimensional (3D) anisotropic materials, their 3D molecular alignment cannot be measured using conventional two-dimensional (2D) polarization imaging. Here, we present images of the 3D angles of molecular orientations with submicrometer spatial resolution acquired through polarization-controlled coherent anti-Stokes Raman scattering microscopy. The hyperspectral Raman data of a polyethylene (PE) film were converted into images, showing the polymer chains' 3D angles and order parameters. The 3D orientation images of PE chains in ring-banded spherulites show that the azimuthal angles of the chains are perpendicular to the crystal growth direction, while the out-of-plane angles display limited-range oscillations synchronous with ring banding. The prevailing crystal growth model of fully twisting lamellae is inconsistent with the observed restricted oscillations of the out-of-plane direction, which are unobservable through conventional 2D projected imaging. This high-resolution, label-free, quantitative imaging of 3D molecular orientation can become a standard measurement tool for the microscopic structures of complex synthetic and biological materials.

Multi-scale simulation of deformation and recrystallization of austenitic stainless steels with solidified columnar crystal structures

Columnar grain structures (CCS) often distinct in steel ingots, which have to be refined and homogenized during forging. In this investigation, simulation of deformation and dynamic recrystallization of austenitic stainless steels with CCSs were carried out by macroscopic, mesoscopic (microscopic) and nanoscale simulation techniques. (1) Using molecular dynamics simulation method, the nano-CGSs model with different loading directions was simulated. The results show that the deformation stresses are anisotropic with variation of angles between the loading direction and the columnar crystal growth direction. The higher stresses present at the 0° and 90° angles due to higher dislocation density; However, the lower stresses present at the angles from 30° to 60° due to higher stacking faults and twins. (2) The cellular automata (CA) fractal rules were proposed to simulate nucleation and grain growth of dynamic recrystallization by introducing weighted variables considering Σ3 twin nucleation rate for the twinning-promoted recrystallization. The CA method of nucleation at primary columnar crystal boundaries, secondary dendrites and deformation bands was proposed to simulate the joint nucleation. (3) The coupling simulation of macroscopic thermal parameters by finite element method, solidification of columnar structures and hot deformation dynamic recrystallization by CA was realized.

Control of perovskite film crystallization and growth direction to target homogeneous monolithic structures

Getting performant organo-metal halide perovskite films for various application remains challenging. Here, we show the behavior of solvent and perovskite elements for four different perovskites families and nine different initial precursor solution systems in the case of the most popular preparation process which includes an anti-solvent dripping-assisted spin coating of a precursor solution and a subsequent thermal annealing. We show how the initial solution composition affects, first, the film formed by spin coating and anti-solvent dripping and, second, the processes occurring upon thermal annealing, including crystal domain evolution and the grain growth mechanism. We propose a universal typology which distinguishes three types for the growth direction of perovskite crystals: downward (Type I), upward (Type II) and lateral (Type III). The latter results in large, monolithic grains and we show that this mode must be targeted for the preparation of efficient perovskite light absorber thin films of solar cells.

Combined Electrodialysis and Peptide-Directed Struvite Recovery System

Recovery of N and P as struvite (MgNH4PO4 6H2O) has gained interest in recent years as both a flow assurance measure and as a sustainable P source for fertilizer. Electrochemically precipitated struvite offers advantages compared to conventional precipitation methods by reducing the need for expensive additives and has the potential for coupling with renewable energy sources. However, high energy inputs to overcome the solution resistance of most wastewaters limit the feasibility of the technology. In this work, a coupled electrodialysis-struvite recovery system is presented to concentrate the wastewater (producing a separate desalinated stream) while producing struvite in a single unit operation. Furthermore, a common dental peptide was explored as a method to control and increase growth of struvite crystals at neutral pH. Cysteine-capped peptide was attached to a gold mesh substrate in a range of loadings (5-27ug/cm2). Chronoamperometry was conducted by first placing the peptide-loaded mesh in the struvite reactor filled with synthetic wastewater at pH 7.1 and then applying a range of potentials (0.5-0.99V) to the magnesium anode. White precipitates were observed and characterized as struvite by XRD. Struvite crystal morphology was investigated by SEM. We report that a peptide loading of 19 ug/cm2 led to a nearly 14% increase in struvite precipitated compared to bare Au with no peptide attached. Furthermore, longer, dendritic struvite crystals formed in the presence of peptide compared to orthorhombic crystals which formed without peptide. These results indicate that the peptide alters local supersaturation, leading to an increase in directional crystal growth. By demonstrating the utility of peptides for increasing struvite precipitation at neutral pH, our findings improve the viability of electrochemical struvite precipitation for a wider range of wastewater compositions and highlights how peptides can modulate crystal growth.

PTFE Crystal Growth in Composites: A Phase-Field Model Simulation Study.

We investigated, via a phase-field model simulation, the effects of a matrix’s properties and a filler’s characters on the polytetrafluoroethylene (PTFE) crystal growth process in composites under various supercooling degrees. The results show that the supercooling degree has a deciding influence on the crystal growth process. The intrinsic properties of PTFE polymer, such as anisotropic strength and phase transition latent heat, affect the growth rate, orientation, and interfacial integrity of the crystal trunk and the branching of the PTFE crystal growth process. The factors of the PTFE crystallization process, such as anisotropic strength and phase translation interface thickness, affect the uniformity and crystallization degree of the PTFE crystal. In the composites, the biphasic interface induces the crystal growth direction via the polymer chain segment migration rate, of which the degree depends on the shapes of the filler and the PTFE crystal nucleus. According to the results, choosing the low molecular weight PTFE and mixture filler with various particle sizes and surface curvatures as the raw materials of PTFE-based composites improves the crystallization of the PTFE matrix.

Fabrication of Discontinuous Dendritic CH3NH3PbBr3 Perovskite Microdisk Arrays for Microlasers

AbstractMethylammonium (CH3NH2, MA) lead halide single crystals have attracted attention due to their excellent properties, such as tunable bandgap, high absorption efficiency, long carrier diffusion length, low non‐radiative recombination rate, high gain efficiency, and ultra‐low trap state density. However, excellent methylammonium lead bromine (MAPbBr3) microdisk laser arrays reported at present mainly rely on nanoimprint or inkjet printing technology, which highly depends on the equipment. Herein, an approach to prepare high‐quality MAPbBr3 microdisk arrays is developed in which well‐aligned microdisk arrays grow via partial dissolution and recrystallization of dendrites induced by Marangoni flow in a solution self‐assembly growth process. On the same dendrite, the growth direction of the microdisk crystals is highly consistent, and the crystal size distribution is uniform. Besides, the MAPbBr3 microdisks show excellent laser performance with a low threshold of 43.3 µJ cm−2 and a high Q factor (Q = 2072). The small volume and high‐quality square MAPbBr3 perovskite microdisks have great potential in the application of integrated photoelectric lasers.

Effect of controlled operating pressure on physical optical and electrical properties of AZO thin film by DC magnetron sputtering technique

The study aimed to research the controlled operating pressure in the range of 1.10 × 10–2 − 2.60 × 10–3 mbar with the adjustment position of plate valve (40 – 100%) using DC pulsed magnetron sputtering system. The AZO films were prepared onto n-type silicon (100) and glass slide substrates. A comparison, all samples were assigned 400 nm of AZO film thickness and then the prepared AZO films were investigated by field-emission scanning electron microscope (FE-SEM) for surface morphologies, X-ray diffraction (XRD) for crystallinity, UV-Vis-NIR spectrophotometry for optical property, Four-point probe and Hall effect instrument for electrical properties, respectively. The results from FE-SEM image showed that the AZO film was columnar structure. The XRD patterns of AZO films demonstrated the crystal growth direction which preferred AZO growth was indicated the (polycrystalline) hexagonal wurtzite structure. For the optical property, the transmittance (%T) clearly showed transparent film, indicating good average value which was 81.54 – 83.28% in visible region (380 to 780 nm of wavelength). Furthermore, the sheet resistivity, high mobility and carrier concentration displayed 91.48 Ohm sq−1, 3.40 cm2 Vs–1 and 7.00 × 1020 cm–3, respectively. These results were decided that the position of plate valve of 85% with operating pressure of 2.70 × 10–3 mbar was the optimal condition. Finally, the excellent electrical and good optical properties of AZO film is mainly applied in several application for optoelectronic device and solar cell.

Effect of Substrate Surfaces for Crystal Growth of a Photochromic Diarylethene by Sublimation

The state of the substrate surface is one of the key factors in varying morphologies of crystals produced by the sublimation method of some organic compounds. In this work, we succeeded in separately preparing different morphologies of rodlike crystals of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene (1a), which are classified into the hollow crystal and the feather-like crystal, by sublimation to glass substrates with the hydrophilic and hydrophobic surfaces, respectively. To clarify the crystal growth process on each surface of the glass substrate, we investigated miller indices of crystal faces attached to the substrate surface in the early stage of sublimation and crystal growth directions of these rodlike crystals from these crystal faces by X-ray diffraction measurements and polarizing microscopic observation. As a result, it was revealed that the heterojunction between the crystal faces produced in the early and following stages of sublimation leads to two different crystal morphologies. Moreover, it was confirmed that the heterojunctions occur in a specific orientation between these crystal faces because the lattice points in these crystal faces are in good agreement. Finally, we have demonstrated photomechanical behaviors of the hollow and feather-like crystals.

Perspectives of Double Feeding of the Melt Method on the Example of the Analysis of the InAs(1-x)–GaAs(x) Solid Solution Crystals Growth

Using the example of InAs-GaAs system, the potential of double feeding of the melt method for growing single crystals of semiconductor solid solutions is shown. The concentration profiles of InAs-GaAs solid solutions crystals for certain modes of the considered method are calculated in Pfann approximation. In addition, under the same conditions, InAs-GaAs crystals were grown and their concentration profiles were determined. The results obtained give good agreement between the experimental and calculated data. It is shown that mathematical modeling of InAs-GaAs crystals growth by setting the crystallization and melt-feeding rates with constituent components makes it possible to obtain the required distribution of the main components in the direction of crystal growth, including uniform distribution, in the entire series of continuous InAs-GaAs solid solutions. The double feeding of the melt method is shown to be promising for obtaining single crystals of semiconductor solid solutions with a given concentration profile.

Effect of beam oscillation on the mechanical properties of fiber-laser-welded thin-plate X2CrTiNb18 ferritic stainless-steel joints

This study adopted fiber laser welding (FLW) and oscillating fiber laser welding (OFLW) to weld 1.2 mm-thick X2CrTiNb18 ferritic stainless steel (FSS) in the form of square butt joint. The microstructure and mechanical properties of two types of joints were characterized and compared. The results showed that beam oscillation had a marginal effect on the unidirectional tension properties of the joints. However, the plasticity and fracture toughness of the joint were markedly increased. The beam oscillation changed the morphology and microstructure of the weld. It transformed the outer profile of the weld cross-section into an inverted trapezoid, and formed an equiaxed grain zone in the weld. Beam oscillation decreased the angle between the growth direction of columnar crystals around the equiaxed grains and the weld centerline, which improved the deformation coordination of the grains. The plasticity and fracture toughness of X2CrTiNb18-FSS joints were also improved accordingly.

Development of analysis method for hot cracking considering mechanical and metallurgical factors

Since welding hot cracking is one of the most harmful defects which may largely reduce the strength of welded joints and structures, it is very important to study the prediction and prevention of its generation. Hot cracking is caused by both mechanical and metallurgical factors. In this research, authors proposed a new computational method for evaluating the occurrence of hot cracking through considering both mechanical and metallurgical behavior. As the metallurgical behavior, strain due to solidification shrinkage and mechanical resistance of molten metal were modelled using the solidification ratio as a parameter. The mechanical behavior, an index for generation of hot cracking using the increment of plastic strain is proposed. In addition, authors proposed a simplified prediction method using the temperature gradient for the direction of columnar crystal growth, which affects the location of hot cracking, and is used to evaluate the associate angle of the columnar crystals. The developed method was applied to butt welding. Through the computation using proposed method, it was found that the predicted direction of columnar crystal growth by the proposed method is in good agreement with the experimental results. In addition, the generation of hot cracking in the same butt welding was predicted using the proposed method and the obtained cracking positions agreed well with those observed by experiment. The Influence of the strain of solidification shrinkage on hot cracking was investigated in relation to the surrounding constraints, the temperature gradient and solidification mode, which are presumed to be highly related to the occurrence of hot cracking. From the above results, it can be concluded that the proposed method can analyze the effects of mechanical and metallurgical factors on hot cracking.

Crystallographic Variability in Additive Manufacturing

The crystallographic textures produced during additive manufacturing can be understood, predicted, and manipulated by varying the grain nucleation and growth processes. The resultant textures are primarily dictated by the melt pool geometry, which defines the local thermal gradient and thus the preferred crystal growth directions, as well as the scan strategy, which controls the propagation of grain orientations into subsequent layers. This texture can be diluted through heterogeneous nucleation of new grain orientations, which can occur through a variety of mechanisms. This ability to control the texture during additive manufacturing can enable the location-specific control of properties as a function of position in the build.

Numerical Analysis of Dislocation Density of c ‐, a ‐, m ‐axes Grown Al 2 O 3 Crystals during Annealing Process

A numerical study of dislocation density is carried out by 3D and time-dependent analysis using the Alexander–Haasen model to estimate growth direction dependence of plastic deformation of Al2O3 single crystals during the annealing process. The direction of crystal growth is set to c-, a-, and m-axes. The calculated results show distribution of dislocation density in the crystal grown in the c-axis has a sixfold symmetry while those in the crystals grown in the a- and m-axes show asymmetric distribution. Dislocation densities in the a- and m-axes Al2O3 crystals are larger than that in the c-axis crystal.