Homogeneous Crystals Research Articles

Effect of density change at crystallization on a one-dimensional heat balance equation at solid–liquid interface

In this paper, we study how we should treat a steady-state interface between homogeneous crystal and melt phases, where steady temperature gradients are present at both sides. In particular, keeping the geometry of Czochralski method in mind, we study a correction for the heat balance equation at the interface between the two phases. We show that a netagive term proportional to the third power of the pulling velocity, which is caused by the density difference between the two phases, is added to the heat balance equation.

Probing van der Waals interactions at two-dimensional heterointerfaces.

Two-dimensional (2D) heterostructures assembled via van der Waals (vdW) interactions have sparked immense interest in fields from physics1,2 to electronics3,4. Understanding the vdW interaction at these heterointerfaces is critical for the sophisticated construction and manipulation of these 2D heterostructures. However, previous experimental research has mainly focused on the interlayer interactions in homogeneous graphite crystals5,6 and the interactions between graphene and substrates7. Theoretically, although a variety of vdW methods have been incorporated in density functional theory to probe the interactions of homogeneous vdW crystals, the reliability of these vdW methods in 2D heterostructures remains to be verified. Here, we show, by contact-splitting transfer of graphite from hexagonal boron nitride (BN) to molybdenum disulfide (MoS2), that graphite experiences a stronger vdW interaction with MoS2 than with boron nitride. Quantitative measurements using a graphite-wrapped atomic force microscope tip show that the critical adhesion pressures between BN and graphite and MoS2 and graphite are respectively 0.953 and 1.028 times that between graphite and graphite. The results are consistent with the prediction based on Lifshitz theory, implying an important role of material dielectric function in the vdW interactions at heterointerfaces. These findings offer us more freedom in the construction of 2D heterostructures, and a technique to disassemble 2D heterostructures is demonstrated.

Crystallization of cross-linked polyethylene by molecular dynamics simulation

The crystallization of polymers is not, despite its importance in science and engineering, entirely understood due to the challenge of tracking the behavior of individual polymer chains in the crystallizing melt. However, increasing computational resources have brought the crystallization process within reach of molecular simulations, and several groups have published simulations of crystal nucleation and growth in polymers. Yet, these studies have focused on linear polymer chains, and no results have been reported on cross-linked polymers, which are common in everyday applications. Here, we perform molecular dynamics simulations of the homogeneous crystallization of cross-linked polyethylene at high undercooling. Large cross-link densities cause the crystallization to slow down and reduce the final degree of crystallization. As expected, cross-links are rejected from the crystals into the amorphous phase. We observe that at all cross-link densities the intercrystalline amorphous phase is characterized by a single distribution of free segment lengths (amorphous segments with no cross-links). The findings provide a basis for detailed computational studies of semi-crystalline cross-linked polymer systems.

Improving collective variables: The case of crystallization.

Several enhanced sampling methods, such as umbrella sampling or metadynamics, rely on the identification of an appropriate set of collective variables. Recently two methods have been proposed to alleviate the task of determining efficient collective variables. One is based on linear discriminant analysis; the other is based on a variational approach to conformational dynamics and uses time-lagged independent component analysis. In this paper, we compare the performance of these two approaches in the study of the homogeneous crystallization of two simple metals. We focus on Na and Al and search for the most efficient collective variables that can be expressed as a linear combination of X-ray diffraction peak intensities. We find that the performances of the two methods are very similar. Wherever the different metastable states are well-separated, the method based on linear discriminant analysis, based on its harmonic version, is to be preferred because simpler to implement and less computationally demanding. The variational approach, however, has the potential to discover the existence of different metastable states.

Epitaxial Growth, Morphology, and Temperature Stability of Quartz-Like Germanium Dioxide Crystals

The conditions and mechanisms of epitaxial growth of quartz-like α-GeO2 crystals on quartz substrates using an evaporative-recirculation method are considered. Relatively homogenous α-GeO2 crystals weighing up to 200 g are grown at a growth rate of up to 0.3 mm/day. It is established that molecular adhesion (cohesion) at the boundary between the quartz substrate and the overgrown layer of α-GeO2 cannot prevent its transition to a stable poorly soluble rutile-like phase. This makes it impossible to grow high-germanium quartz single crystals industrially using a mixture of quartz and quartz-like α-GeO2 as a batch. However, this process can be implemented if other more soluble germanium-containing compounds, such as quartz-like Si-containing germanium-oxide, are found.

Sulfur isotope analysis of microcrystalline iron sulfides using secondary ion mass spectrometry imaging: Extracting local paleo-environmental information from modern and ancient sediments.

Sulfur isotope ratio measurements of bulk sulfide from marine sediments have often been used to reconstruct environmental conditions associated with their formation. In situ microscale spot analyses by secondary ion mass spectrometry (SIMS) and laser ablation multiple-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) have been utilized for the same purpose. However, these techniques are often not suitable for studying small (≤10 μm) grains or for detecting intra-grain variability. Here, we present a method for the physical extraction (using lithium polytungstate heavy liquid) and subsequent sulfur isotope analysis (using SIMS; CAMECA IMS 7f-GEO) of microcrystalline iron sulfides. SIMS sulfur isotope ratio measurements were made via Cs+ bombardment of raster squares with sides of 20-130 μm, using an electron multiplier (EM) detector to collect counts of 32 S- and 34 S- for each pixel (128 × 128 pixel grids) for between 20 and 960 cycles. The extraction procedure did not discernibly alter pyrite grain-size distributions. The apparent inter-grain variability in 34 S/32 S in 1-4 μm-sized pyrite and marcasite fragments from isotopically homogeneous hydrothermal crystals was ~ ±2‰ (1σ), comparable with the standard error of the mean for individual measurements (≤ ±2‰, 1σ). In contrast, grain-specific 34 S/32 S ratios in modern and ancient sedimentary pyrites and marcasites can have inter- and intra-grain variability >60‰. The distributions of intra-sample isotopic variability are consistent with bulk 34 S/32 S values. SIMS analyses of isolated iron sulfide grains yielded distributions that are isotopically representative of bulk 34 S/32 S values. Populations of iron sulfide grains from sedimentary samples record the evolution of the S-isotopic composition of pore water sulfide in their S-isotopic compositions. These data allow past local environmental conditions to be inferred.

A Comparison of Heterogeneous/Homogeneous Crystallization for Phosphate Recovery from Biosolids

In this study, a fluidized bed reactor was used to compare heterogeneous and homogeneous crystallization for recovering magnesium ammonia phosphate (MAP) from the dewatering filtrate at a municipal water resource recovery facility. Investigating the factors affecting crystallization revealed that pH exerted a greater effect than the Mg/P molar ratio. The results of a heterogeneous crystallization experiment showed that removal efficiency of phosphate (RP%) and crystal efficiency of phosphate (CP%) were 68% and 66%, respectively, at a PO4 -P concentration of 200 mg/L, pH of 8.0, and Mg/P molar ratio of 1.0 for 3 h. The reaction rate of heterogeneous crystallization was markedly higher than that of homogeneous crystallization. Those pellets produced through the heterogeneous crystallization exhibited a dense surface (particle size 0.5-1.0 mm; water content 8.7%). Through scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), and X-ray diffraction (XRD) examination, the crystallized patterns of the needle-shaped pellets were identified as MgNH4 PO4 ·6H2 O. This article is protected by copyright. All rights reserved.

Growth of homogeneous mixed A1-xBx bulk crystals: Analytical prediction, phase-field modeling and experiment comparison

An analytical model is proposed to predict the growth behavior of homogeneous mixed A1-xBx bulk crystals by the traveling liquidus-zone (TLZ) method. The solidification and melting rates at the cold and hot solid/liquid (S/L) interfaces are determined by the solute conservation equations, coupled with the non-linear concentration distribution in the liquid zone. A novel time-dependent critical translation rate is derived, which can completely suppress the composition segregation in the grown crystal with an arbitrary liquid zone width. The proposed model is validated by the thin interface phase-field model for the TLZ-grown Si0.5Ge0.5 crystal, regarding the growth kinetics and concentration distributions in the crystal and liquid phase. A good agreement between the analytical predictions and phase-field simulations is achieved. The growth kinetics and composition profiles in the grown crystals predicted by the present analytical model agree well with the experimental measurements reported in the literature. It is found that after an initial transient, the non-linear liquid concentration distribution reaches the quasi-stationary condition. The average crystal growth rate increases with increasing temperature gradient. The proposed critical translation rate can precisely match the time-dependent solidification rate at the cold S/L interface, leading to a completely homogeneous TLZ-grown Si0.5Ge0.5 bulk crystal for an arbitrary initial liquid zone width. It is demonstrated that the present analytical model with the non-linear liquid concentration distribution is applicable to predict the growth behavior of homogeneous mixed crystals with a wide region of temperature gradient, translation rate and initial liquid zone width.

Removal of iron as oxyhydroxide (FeOOH) from aqueous solution by fluidized-bed homogeneous crystallization

Abstract In this study, iron is removed from aqueous solution using a fluidized-bed crystallizer. Homogeneous oxyhydroxide pellets (FeOOH) were recovered at various value of pH, surface loading (L), and initial ferrous concentration to assess the treatability of iron-containing water. The optimal conditions for treating initial ferrous concentrations of 100–300 mg/L were L = 0.6 kg/m2/h (with 29 m/h of upflow velocity), a bed expansion of 50% and pH = 6.5 yielding the total iron removal (TR) of more than 99% and a corresponding iron crystallization ratio (CR) of 85%. X-ray diffractometry (XRD) revealed that the pellets consisted of a mixture of goethite (α-FeOOH) and lepidocrocite (γ-FeOOH). Scanning electron microscopy (SEM) yielded images of pellets of up to 1 mm in diameter, which were assembled from FeOOH flakes with thicknessess of 0.2–0.4 µm.

Electronic structure and exciton shifts in Sb-doped MoS2 monolayer

The effective manipulation of excitons is important for the realization of exciton-based devices and circuits, and doping is considered a good strategy to achieve this. While studies have shown that 2D semiconductors are ideal for excitonic devices, preparation of homogenous substitutional foreign-atom-doped 2D crystals is still difficult. Here we report the preparation of homogenous monolayer Sb-doped MoS2 single crystals via a facile chemical vapor deposition method. A and B excitons are observed in the Sb-doped MoS2 monolayer by reflection magnetic circular dichroism spectrum measurements. More important, compared with monolayer MoS2, the peak positions of two excitons show obvious shifts. Meanwhile, the degeneration of A exciton is also observed in the monolayer Sb-doped MoS2 crystal using photoluminescence spectroscopy, which is ascribed to the impurity energy levels within the band-gap, confirmed by density function theory. Our study opens a door to developing the doping of 2D layered transition metal dichalcogenides with group-V dopants, which is helpful for the fundamental study of the physical and chemical properties of transition metal dichalcogenides.

Metallurgical Structure of A356 Alloy Solidified by Mechanical Stirring

The study investigated the effects of mechanical stirring before solidification on the metallurgical structure of hypoeutectic aluminum-silicon A356. A series of stirring trials were conducted in the present study. Emphasis were placed on the morphological changes of the primary phase, which was subjected to different levels of stirring at various values of the rod material and its diameter, insertion temperature and rotation speed. It was found that when the rod was made of the same material as the molten metal, it acted as a nucleation site to generate numerous nucleated primary crystals, which separated from the rod surface continuously into the molten metal with the rotation of the stirring rod, resulting in the refinement and spheroidization of the primary crystals. The ideal semisolid slurry with homogeneous spherical and fine primary crystals could be obtained by optimizing rod insertion temperature, rotation speed and its diameter, which is a key factor in semi-solid forming.

Heterogeneous versus homogeneous crystal nucleation of hard spheres.

Hard-sphere model systems are well-suited in both experiment and simulations to investigate fundamental aspects of the crystallization of fluids. In experiments on colloidal models of hard-sphere fluids, the fluid is unavoidably in contact with the walls of the sample cell, where heterogeneous crystallization may take place. In this work we use simulations to investigate the competition between homogeneous and heterogeneous crystallization. We report simulations of wall-induced nucleation for different confining walls. Combining the results of these simulations with earlier studies of homogeneous crystallization allows us to assess the competition between homogeneous and heterogeneous nucleation as a function of the wall type, fluid density and the system size. On flat walls, heterogeneous nucleation will typically overwhelm homogeneous nucleation. Even for surfaces randomly coated with spheres with a diameter that was some three times larger than that of the fluid spheres - as has been used in some experiments - heterogeneous nucleation is likely to be dominant for volume fractions smaller than ∼0.535. Only for a disordered coating that has the same structure as the liquid did we find that nucleation was likely to occur in the bulk. Hence, such coatings might be used to suppress heterogeneous nucleation in experiments. Finally, we report the apparent homogeneous nucleation rate taking into account the formation of crystallites both in the bulk and at the walls. We find that the apparent overall nucleation rates coincide with those reported in "homogeneous nucleation" experiments. This suggests that heterogeneous nucleation at the walls could partly explain the large discrepancies found between experimental measurements and simulation estimates of the homogeneous nucleation rate.

The Effect of Ion-Beam Bombardment on Solution-Processed Nickel Oxide Films Used for Liquid Crystal Alignment

We induced a homogeneous liquid crystal (LC) alignment state on ion-beam (IB)-irradiated NiO films fabricated via solution-processing. A topographical analysis was performed using field emission-scanning electron microscopy of the films after various annealing temperatures. After IB irradiation, the surface of the NiO films was rough irrespective of annealing temperature. IB-irradiated NiO films exhibited slightly reduced transparency in the visible region compared to as-deposited NiO film. Moreover, the IB-irradiated NiO films fabricated at the lowest annealing temperature of 100°C exhibited uniform homogeneous LC alignment caused by microgroove-like topographic modification and chemical modification of the NiO surface by the IB irradiation. The pretilt angle of the LC molecules on IB-irradiated NiO film annealed at 100, 200, 300, and 400°C were on average 0.02 0.05, 0.43, and 0.05o, respectively, making IB-irradiated NiO film eminently suitable for LC alignment.

Euhedral crystals of Na2TiGeO5with switchable translucency and reorientable microstructure

Crystal twinning in euhedral crystals reveals anomalous optical properties.

Effect of supercooling accompanying the freezing process on ice crystals and the quality of frozen strawberry tissue

Degradation of agricultural product quality occurs hugely during the freezing process, since ice crystal formation with volume expansion and increased osmotic pressure between cell membranes damages their microstructure. Recently, it is studied a freezing accompanied supercooled state for the entire food product, which induces finer and more homogenous ice crystals and improved the quality of soy bean gels after thawing. In this study, texture, drip loss, and microstructural changes were measured in frozen-thawed strawberries prepared by freezing with supercooling. In addition, ice crystals were observed directly using synchrotron X-ray computed tomography in supercooled frozen strawberries. The results showed, although supercooled freezing has the potential to form finer ice crystals in strawberry tissues, the quality of frozen strawberries was not improved compared with conventional slow freezing. It assumed that a higher and momently ice nucleation caused by supercooling was promoted a drastic water migration and huge degradation on strawberry cells by osmotic pressure in the frozen strawberries.

Correcting symmetry imperfections in linear multipole traps.

Multipole radio-frequency (RF) traps are central to collisional experiments in cryogenic environments. They also offer possibilities to generate new type of ion crystals topologies and in particular the potential to create infinite 1D/2D structures: ion rings and ion tubes. However, multipole traps have also been shown to be very sensitive to geometrical misalignment of the trap rods, leading to additional local trapping minima. The present work proposes a method to correct non-ideal potentials, by modifying the applied RF amplitudes for each trap rod. This approach is discussed for the octupole trap, leading to the restitution of the ideal Mexican-Hat-like pseudo-potential, expected in multipole traps. The goodness of the compensation method is quantified in terms of the choice of the diagnosis area, the residual trapping potential variations, the required adaptation of the applied RF voltage amplitudes, and the impact on the trapped ion structures. Experimental implementation for macroscopic multipole traps is also discussed, in order to propose a diagnostic method with respect to the resolution and stability of the trap drive. Using the proposed compensation technique, we discuss the feasibility of generating a homogeneous ion ring crystal, which is a measure of quality for the obtained potential well.

The effect of process parameters on the amorphous citrate sol-gel synthesis of Cu-doped Ca12Al14O33

Ca12Al14O33 (C12A7) is a highly functional material due to its ability to act as an anionic source and its conversion from a wide band-gap insulator to a conductive oxide material as a function of processing. Cationic doping, on the order of 1 mol%, of C12A7 has been shown to have profound effects on the physical properties of the material. One such cation, Cu, increases the catalytic efficiency and drastically alters the electronic landscape. The amorphous citrate sol-gel synthesis technique is widely used to synthesize doped C12A7 materials as it promises homogeneous cationic dispersion and a fine and controlled grain size. This promise relies on the formation of a homogeneous citrate resin of cationic citrate species, homogeneous decomposition of cation citrate species, and homogeneous crystallization of the desired phase. In this report, theoretical complex diagrams are calculated to determine the optimal pH for citrate resin formation in the Cu-C12A7 system. The resin decomposition and crystallization kinetics are characterized through high-temperature X-ray diffraction and thermogravimetric analysis. The optimal pH range for the citrate resin homogeneity was theoretically determined to be a range from 3 to 6, and experimental characterization shows that raising the pH from < 1 to 4 leads to Cu-C12A7 phase purity, reduces grain size by 30%, and inhibits CaCO3 formation to lower the synthesis temperature from ∼900 °C to 840 °C. This report highlights the importance of sol-gel solution chemistry and the corroboration of XRD analysis with microstructural analysis

Crystalline composition and morphology in isotactic polypropylene pipe under combining effects of rotation extrusion and fibril β‐nucleating Agent

Polypropylene (PP) pipe is extraordinarily important in modern construction and transportation engineering owing its predominant advantages over metal such as good dimensional stability, low cost, and light weight. However, low toughness and hoop strength always stand in the way of the wide application. In this study, two kinds of β‐nucleating agents (NAs) (WBG‐II and NAB‐83) were added in isotactic polypropylene and thereafter, the crystalline composition, morphology, and alignment were investigated under the combination effects of rotation extrusion and NA. The experimental results showed that both NAB‐83 and WBG‐II could promote numerous β crystals, but dot‐like NAB‐83 could only induce randomly distributed isotropic spherulites. On the contrary, by adjusting the temperature gradient along the extrusion to rotation section, WBG‐II first dissolved into PP melts and then self‐assembled into fiber‐like nuclei before the formation of PP crystals. Compared to homogeneous crystallization, fibrous WBG‐II nuclei lowered the energy barrier for PP crystals; therefore, epitaxial β lamellae grew on these fibrous NAs and stacked into β‐form hybrid shish kebabs. Meanwhile, the alignment of these highly oriented structures deflected from the axis driven by the helical movement caused by die rotation. Consequently, PP pipe with outstanding hoop tensile performance as well as excellent toughness was prepared. J. VINYL ADDIT. TECHNOL., 25:E195–E202, 2019. © 2018 Society of Plastics Engineers

Non‐Joulian Magnetostriction and Non‐Joulian Magnetism

Complementing volume‐conserving Joule magnetostriction (JM), the non‐Joulian magnetostriction (NJM) phenomenon has recently been discovered whose hallmark is change in volume of the crystals in magnetic fields. This article reviews the unique set of non‐Joulian properties that are so non‐conforming relative to conventional magnets that they merit the designation of a distinctly new class of (functional) magnets. This review distils key aspects of non‐Joulian magneto‐elasticity and magnetism, including the sign of volume change (contracting or expanding); the orientation dependence of volume change that is key to realizing large volume changes (remarkably, this volume‐anisotropy coexists with a seemingly isotropic magnetization response for all crystal axes); and a digital coercivity landscape unlike that seen in conventional magnets. NJM lay undiscovered for nearly two decades because prior literature assumed, without experimental foundation, that iron‐based alloys obeyed volume‐conserving JM. Measurements now show that volume change is inherent across wide composition ranges, occurring in both quenched, metastable, and inexorably slow‐cooled, equilibrated crystals. While not essential, quenching promotes long‐range periodicity of magneto‐elastic gradients with enhanced NJM. Another objective is to “frame” Joulian‐to‐non‐Joulian behavior within a single framework of magnetically responsive, wave‐like elastic medium in which approach to homogeneity yields the limiting case of JM; in fact, gradient magnet‐elasticity is indispensable to describing non‐Joulian magnetism. A consequential auxiliary finding is the large differences in intrinsic magneto‐elastic constants for seemingly equivalent crystal axes, consistent with gradient magneto‐elasticity. Consequentially, a sizeable literature that previously characterized these crystals using volume‐conserving assumption and framework of phenomenological magnetostriction theory of homogeneous cubic crystals is rendered moot, and unreliable for high‐fidelity applications.

Optimization of the Growth Technology for Crystals with Garnet Structure Based on X-Ray Diffraction Data

High efficiency of the methods of double-crystal X-ray diffractometry (DCXRD) and topography for improving the growth technology of highly homogeneous crystals has been demonstrated on the example of gadolinium gallium garnet (GGG) single crystals. The main types of structural defects observed in Czochralski-grown GGG crystals are found to be macroscopic inhomogeneity of composition distribution, caused by the facet effect manifestation; microinhomogeneous distribution of impurity and main components of the composition in striations; dislocations; and second-phase inclusions. The relationship between the type and density of newly formed defects and the technological conditions for crystal growth are considered. Optimization of the composition of crystals and their growth technology made it possible to obtain high-quality dislocation-free crystals of GGG and complex-substituted garnets on its basis for magneto-optical and microwave devices, elements of solid-state lasers, and other applications.