Face-centered Cubic Crystals Research Articles

Preparing of CuCo2O4 compound by Sol-gel method and studying its structural properties

تم في هذا البحث تحضير مركب كوبالتيت النحاس CuCo2O4 بطريقة الـ Sol-Gel انطلاقا من ملح كبريتات الكوبالت ونترات النحاس. تم استخدام مركب عضوي هو البكتين كعامل تثبيت أعطى استقرارعالي لجملة الهيدروكسيدات أثناء التحضير. تم حرق العينة المحضرة عند درجات حرارة مختلفة ضمن المجال (400-1000°C) لتحديد درجة الحرارة الأفضل للحصول على البلورات المطلوبة من المركّب CuCo2O4. درست الخواص التركيبية للأكسيد المحضّر باستخدام تقنية انعراج الأشعة السينية (XRD) وجهاز التحليل الحراري التفاضلي (DTA)، ومطيافية تحت الأحمر (IR)، والمجهر الالكتروني الماسح (SEM). تم تحديد درجة حرارة الاصطناع المثلى عند الدرجة 600°C. بينت دراسة مخططات انعراج الأشعة السينية أن المركب يتبلور وفق بنية بلورية مكعبية متمركزة الوجوه FCC من نمط السباينل ومجموعة تناظر فراغية S.G هي Fd3m. حُسبت ثوابت الشبكة وحجم التبلور وعدد الصيغ للمركّب المحضّر وكانت 8.044 Å ، 520.49Å3 ، 8 على الترتيب. تبين أن حجم الحبيبات للمركّب هو 17.4nm. تطابقت الحسابات التجريبية لـ d مع قيم البطاقة المرجعية بنسبة تطابق 99.5% كحد أدنى. حسبت الكثافة النظرية والتجريبية للمركّب المحضّر وكانت النتائج متقاربة. أظهرت منحنيات التحليل الحراري التفاضلي إلى وجود خمس آثار حرارية أهمها الأثر الحراري الناشر عند الدرجة 390 والأثر الحراري الماص عند الدرجة 740°C الللذان يشيران إلى بدء تشكل المركب وبدء تفككه على الترتيب. يؤكد مخطط الطيف تحت الأحمر (IR) الحصول على المركب المطلوب من خلال القمم العائدة لاهتزازات الروابط (Co-O) و (Cu-O).

Characterization of Kinetics-Controlled Morphologies in the Growth of Silver Crystals from a Primary Lead Melt

Silver, a precious metal, can be recovered as a by-product of the processing of non-ferrous metals such as lead. In this work, silver crystals grown from the controlled cooling of a 10% silver–90% lead melt have been examined to quantify crystal morphologies developed under industrial conditions. X-ray tomography (XCT) is adapted to quantify the size and morphology of silver crystal structures grown from the Ag-Pb melt. The examination utilized high X-ray energies and small sample sizes to mitigate attenuation and enhance image quality. Examination of single crystal dendrites under high magnification demonstrates that silver crystals, even those grown under commercial conditions, yield a Face-Centered Cubic (FCC) crystalline lattice, which could be important for the practical extension of this work to the commercial production of Ag nano-crystals and crystalline supra-molecular structures. The crystals observed are composed of multiple twinned euhedral grains in a variety of dendritic to acicular arrangements, yielding a substantial heterogeneity of crystalline forms. XCT data were used to generate size and shape descriptors for the individual crystals. The results were compared to an equivalent set of descriptors generated from laser sizing examination of a sample of unconsolidated crystals from the same experimental run. The correspondence to within 9% of the crystal equivalent diameters determined independently by the XCT and laser sizing demonstrates a favorable outcome in particle sizing as achieved by visual inspection of XCT results. XCT examination of crystal assemblages identifies small octahedral crystals and larger triangular platelets. The structures expected for FCC crystals grown at thermodynamically controlled conditions are not observed in our systems, suggesting the possibility of the first crystal nuclei form at such conditions, but their growth transition to kinetically controlled mechanisms occurs as their size increases above a threshold cutoff. Based on literature observations, this size threshold is much smaller than the resolution of the XCT instrumentation employed herein. Our characterization data are in fact consistent with thermodynamics/kinetics—and then kinetics-controlled mechanisms—as the crystal size increases. This observation is important because the systems considered here are representative of commercial processes. As such, this work extends prior crystal growth concepts, which were explored in aqueous systems often probed by electrodeposition.

Exploring the growth kinetics of novel and diverse morphologies in nickel oxide nanostructures

This study focuses on the synthesis and characterization of nickel oxide (NiO) nanostructures with diverse morphologies, including nanoflowers, nanocapsules, and nanosnakes, achieved through a cost-effective hydrothermal method. Structural analysis using X-ray diffraction (XRD) confirmed the face-centered cubic crystal (FCC) structure of the NiO nanostructures, with accurate fitting of diffraction peaks achieved through Rietveld refinement within the Fm-3 m space group. The Ni-O bond lengths were determined as 2.0789 Å for nanocapsules, 2.1039 Å for nanoflowers, and 2.0824 Å for nanosnakes based on the 1D profile. The calculated average crystallite size, microstrain, and crystallite size using the Scherrer formula ranged from approximately 6 nm to 20 nm. The study investigates the formation mechanisms underlying the observed diverse morphologies of NiO nanostructures, which were further confirmed by field emission scanning electron microscopy (FESEM). Additionally, the electrochemical properties of the synthesized NiO nanostructures were evaluated using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS). Notably, the nanoflowers demonstrated a higher specific capacitance compared to the nanocapsules and nanosnakes. This comprehensive investigation provides valuable insights into the growth kinetics, structural properties, and electrochemical behavior of novel NiO nanostructures, offering potential applications in energy storage.

The influence of engineering strain rates on the atomic structure, crack propagation and cavitation formation in Al based metallic glass

To enhance the toughness of metallic glasses (MGs), considerable scientific efforts have been made to investigate the physical mechanism of fracture behavior of MGs in past few years. However, many unresolved puzzles remain about the fracture mechanisms of MGs at present. In this paper, Molecular Dynamics (MD) simulations have been used to investigate the tensile fracture behavior of Al based MGs under nine different Engineering strain rates. Our results show that the higher the Engineering strain rate is, the longer the period of plastic deformation, the shorter the crack length. We have observed that Al amorphous with notch gradually crystallizes during tensile fracture, however, there are differences in the degree of crystallization and atomic arrangement as well as micro-structure in different regions with different Engineering strain rates. In the process of tensile fracture of Al based MGs, the crack propagation mainly along the hexagonal-close-packed (HCP) grain boundaries between face-centered-cubic (FCC) crystal blocks. As the Engineering strain increases, numerous HCP atoms located at grain boundaries gradually migrate into the interior of FCC crystal blocks, the velocity of crack propagation will also increase and finally induce the brittle fracture of Al based MGs. Meanwhile, we effectively demonstrated the existence of HCP and FCC atoms in Al based MGs during tensile deformation by analyzing the bond-orientational order parameters ql. The analysis of thermodynamics (entropy) showed that there is a positive correlation between the Engineering strain rate and the strain of maximum entropy when the Engineering strain rate exceeds 0.002 ps−1. The formation of cavitation ahead of crack could be attributed to the migrate of atoms from the higher entropy cavity region to the lower entropy crystallization region. This study enriches the traditional tensile deformation theories and provides a new understanding for the physical origins of cavitation in the crack tip of MGs.

Effect of Dislocation Character on the CRSS

The yield strength of a crystalline structural material is a fundamental mechanical property predominantly governed by the critical stress for dislocation slip. This Critical Resolved Shear Stress (CRSS) is strongly influenced by the character of the dislocation (e.g., screw, edge, or mixed) as shown in previous experimental studies. Existing analytical approaches for CRSS prediction assume an atomic row description of the slip plane and do not account for Wigner-Seitz (WS) cell area at each discrete lattice site. Further, inadequate consideration of the material's elastic anisotropy and the presumed dislocation “core-width” level precludes correct CRSS determination. This study proposes a predictive model applied to Face Centered Cubic (FCC) materials addressing these shortcomings in predicting glide stress of a dissociated dislocation. The core-width is rigorously determined from the minimization of total energy comprised of continuum strain energy (ESTRAIN) and atomistic misfit energy (EMISFIT) of the dislocation's core. The ESTRAIN is obtained from dislocation strain-fields calculated using the fully-anisotropic Eshelby-Stroh formalism. The EMISFIT is determined from the Generalized Stacking Fault Energy (GSFE) landscape of the slip plane. Previous EMISFIT calculations are restricted to slipped rows in ‘simple’ cubic lattices which do not represent the slip-planes in FCC crystals. The developed model is used to predict CRSS for a wide range of metallic materials correcting the overprediction of experimental CRSS levels. The results unveiled the remarkable dependence of CRSS on the dislocation character, revealing the non-trivial dependence on GSFE parameters. Thus, this study addresses a major void in structure-property prediction for structural materials.

Polymorph Stability and Free Energy of Crystallization of Freely-Jointed Polymers of Hard Spheres.

The free energy of crystallization of monomeric hard spheres as well as their thermodynamically stable polymorph have been known for several decades. In this work, we present semianalytical calculations of the free energy of crystallization of freely-jointed polymers of hard spheres as well as of the free energy difference between the hexagonal closed packed (HCP) and face-centered cubic (FCC) polymorphs. The phase transition (crystallization) is driven by an increase in translational entropy that is larger than the loss of conformational entropy of chains in the crystal with respect to chains in the initial amorphous phase. The conformational entropic advantage of the HCP polymer crystal over the FCC one is found to be ΔschHCP-FCC≈0.331×10-5k per monomer (expressed in terms of Boltzmann's constant k). This slight conformational entropic advantage of the HCP crystal of chains is by far insufficient to compensate for the larger translational entropic advantage of the FCC crystal, which is predicted to be the stable one. The calculated overall thermodynamic advantage of the FCC over the HCP polymorph is supported by a recent Monte Carlo (MC) simulation on a very large system of 54 chains of 1000 hard sphere monomers. Semianalytical calculations using results from this MC simulation yield in addition a value of the total crystallization entropy for linear, fully flexible, athermal polymers of Δs≈0.93k per monomer.

Biogenic Pd-nanoparticles from Lantana trifolia seeds extract: Synthesis, characterization, and catalytic reduction of textile dyes

In the present work, the microwave-assisted biogenic synthesis of Palladium Nanoparticles (PdNPs) employing different concentrations of Lantana trifolia seeds extract as a reducing and stabilizing agent in accordance with the concept of environmentally friendly approach. The synthesized PdNPs were characterized using several analytical techniques to examine their morphology, microstructure, crystallinity, optical, and surface functional modification properties. The formation of PdNPs in the reaction solution was evidenced by a visible black color and broad continuous absorption spectra in the UV–Vis absorption spectra. The FTIR spectra were revealed that the phytochemicals were interacted with Pd metal in nanoparticle formation. The prepared PdNPs have a spherical shape and polydispersity nature with an average size of particles is 10 ± 2 nm. In addition, the synthesized PdNPs were well stabilized (zeta potential = −25.5 ± 2.8 mV) by the phytochemicals in seed extract and a face-centered cubic crystal (FCC) structure. The biosynthesized PdNPs were further investigated as a nanocatalyst in the catalytic reduction of Crystal Violet (CV) and Methyl Orange (MO) dyes in the presence of NaBH4 at room temperature. For CV and MO dyes reduction, the homogenous catalytic performance of PdNPs resulted in practically > 98 % of reduction with kinetic rate constant values of 0.393 min−1 (10 min) and 0.232 min−1 (18 min), respectively. Moreover, there is no significant loss in catalytic activity was observed after recovery and reusability of the biosynthesized PdNPs.

Polymorphism and Perfection in Crystallization of Hard Sphere Polymers

We present results on polymorphism and perfection, as observed in the spontaneous crystallization of freely jointed polymers of hard spheres, obtained in an unprecedentedly long Monte Carlo (MC) simulation on a system of 54 chains of 1000 monomers. Starting from a purely amorphous configuration, after an initial dominance of the hexagonal closed packed (HCP) polymorph and a transitory random hexagonal close packed (rHCP) morphology, the system crystallizes in a final, stable, face centered cubic (FCC) crystal of very high perfection. An analysis of chain conformational characteristics, of the spatial distribution of monomers and of the volume accessible to them shows that the phase transition is caused by an increase in translational entropy that is larger than the loss of conformational entropy of the chains in the crystal, compared to the amorphous state. In spite of the significant local re-arrangements, as reflected in the bending and torsion angle distributions, the average chain size remains unaltered during crystallization. Polymers in the crystal adopt ideal random walk statistics as their great length renders local conformational details, imposed by the geometry of the FCC crystal, irrelevant.

Analysis of monotonic and cyclic crack tip plasticity for a stationary crack tip in a FCC crystal

The fields produced at the tip of a Mode I crack in a ductile single crystal have been previously investigated using theoretical, experimental, and numerical methods. Limitations in the material model complexity used in theoretical approaches and difficulties in experimentally measuring in situ crack tip fields invite consideration of numerical crystal plasticity. Prior crystal plasticity analyses of crack tip plasticity have leveraged simple phenomenological constitutive forms and investigated limited loading scenarios. The current work extends prior approaches by using a recently developed face-centered cubic (FCC) crystal plasticity model that considers dislocation substructures and complex back stress evolutions during cyclic loading in concert with a finite element model of a stationary crack tip. The model is used to i) more thoroughly understand the conditions where theoretical analyses remain valid in the context of dislocation interactions and substructure development and ii) explore potential fatigue crack growth driving forces for microstructurally small and physically small cracks. Specific observations are made regarding the observed formation of alternating bands of forward and reverse shear, unique to the present model with dislocation substructure, as well as the influence of ratcheting strain on the cyclic irreversibility of slip and associated correlative fatigue crack growth relations.

Point defects in crystals of charged colloids.

Charged colloidal particles-on both the nano and micron scales-have been instrumental in enhancing our understanding of both atomic and colloidal crystals. These systems can be straightforwardly realized in the lab and tuned to self-assemble into body-centered-cubic (BCC) and face-centered-cubic (FCC) crystals. While these crystals will always exhibit a finite number of point defects, including vacancies and interstitials-which can dramatically impact their material properties-their existence is usually ignored in scientific studies. Here, we use computer simulations and free-energy calculations to characterize vacancies and interstitials in FCC and BCC crystals of point-Yukawa particles. We show that, in the BCC phase, defects are surprisingly more common than in the FCC phase, and the interstitials manifest as so-called crowdions: an exotic one-dimensional defect proposed to exist in atomic BCC crystals. Our results open the door to directly observe these elusive defects in the lab.

Description of plane strain deformation of FCC crystals by a gradient theory of crystal plasticity

A higher-order gradient crystal plasticity theory applicable to three-dimensional problems is reduced to a plane strain version to be used for the analysis of the deformation of face-centered cubic (FCC) crystals at a particular orientation that the out-of-plane plastic deformation cancels out. Plane strain conditions have been frequently quoted in the literature on theoretical crack problems and experimental studies on FCC crystals since effective information on the fundamental deformation behavior of such materials under multiaxial states of stress and strain is obtained. The present plane strain theory is formulated with planar pseudo slip systems that are contractions of the crystallographic slip systems 111110 of FCC crystals. The resultant theory is purely two-dimensional, but it involves the nature of FCC crystal deformation, i.e., effects of latent hardening, cross slip, and backstresses (equivalently, internal stresses with the opposite sign) associated with the distribution of the edge and screw components of the geometrically necessary dislocations.

Ideal and limiting strength of a Lennard-Jones crystal at temperatures lower than the melting line endpoint temperature: molecular dynamics simulation

ABSTRACT The conditions of stability of a stretched face-centered cubic (FCC) crystal against infinitesimal and finite changes in the state parameters have been considered at temperatures lower than that of the endpoint of the melting line [V.G. Baidakov, S.P. Protsenko. Phys. Rev. Lett. 95 015701 (2005)]. Molecular dynamics simulation has been used to calculate effective isothermal elastic moduli, average lifetimes, the work of formation of a critical nucleus and the nucleation rate close to the boundary of the limiting stretching of a Lennard-Jones FCC crystal. States in which the isothermal bulk elastic modulus K is equal to zero, with the shear moduli and being nonzero, have been achieved. It is shown that at K < 0 the crystal retains thermodynamic stability. The kinetics of the phase decay of a Lennard-Jones crystal beyond to the line K = 0 has been investigated. It has been found that the decay proceeds by the nucleation mechanism, through the formation and growth of new-phase nuclei. As distinct from small stretchings, at the boundary of limiting ones the nucleus has a form close to spherical.

Crystal plasticity based study to understand the interaction of hydrogen, defects and loading in austenitic stainless-steel single crystals

A crystal plasticity-based finite element study is performed to understand hydrogen effects on void growth in single crystals of austenitic stainless steel. The model assumes plastic deformation is driven primarily by dislocation motion and captures the influence of hydrogen. Hydrogen effects are incorporated by assuming agreement with the hydrogen enhanced localised plasticity (HELP) mechanism. Despite experimental evidence, hydrogen effect on face centred cubic (FCC) crystals has hitherto not been considered in a numerical void growth model for a wide range of stress states. For the first time, the influence of hydrogen on void growth for different Lode parameters at single crystalline levels is investigated for a range of stress triaxialities in FCC crystals. Hydrogen was found to increase equivalent stresses and hardening responses for various stress triaxialities and Lode parameters. Hydrogen also induces higher void growth response at different stress states, and this was more pronounced at high stress triaxialities.

Viscoplastic effects on plane strain fracture of FCC and BCC single crystals

In the present work, simulations of mode I crack growth and plastic yielding at a crack tip in a plane strain crystal are considered. The theory of crystal plasticity is used and the analyses are made for small strains and rotations. The crack tip is considered as if it is inserted in a metallic grain (single crystal). Two types of crystals are simulated: face-centered cubic (FCC) and body-centered cubic (BCC). The crack growth zone is modeled by cohesive interfaces whose separation process is described by a cohesive zone model. The material and the cohesive surfaces have independent constitutive relations and fracture is an outcome of the deformation process. The cohesive tractions are implemented within the finite element method model through the principle of virtual work. A finite element mesh with rectangular bilinear elements is combined with cohesive elements to model the structure. BCC structures showed a greater toughness than FCC structures in the orientation tested, considering all other properties the same. Viscous properties influence on fracture—crack growth characteristics such as crack extension and time for the beginning of the growth process—were also explored and direct comparisons between the two types of crystal were made. In these cases, FCC crystals showed a greater rate dependency than BCC.

DNA Base Pair Stacking Crystallization of Gold Colloids

We describe a DNA base pair (bp) stacking driven 3D crystallization of 70-80 nm gold nanospheres (Au NSs) into a large-area, face-centered-cubic (FCC) lattice. Although great advances have been achieved over the past decade, DNA nanoparticle (NP) crystallization has relied solely on the base complementary binding. This limits the accessible crystal size particularly for the larger and heavier Au NPs (>50 nm). In this work, we argue that the use of DNA bp-stacking (so-called blunt-end stacking) instead of complementary binding can widen the scope of controlled interparticle interactions used to assemble larger Au colloids into a larger-area crystal. Through the optimization of the melting transition, relatively large Au NSs (e.g., 75 nm) with nearly ideal roundness can be crystallized into FCC crystals with the area of up to approximately 1400 μm2. A strong metallodielectric stopband is experimentally observed in the visible range, confirming the high quality of our self-assembled Au colloidal crystals.

Molecular Dynamics Simulations of Ti Crystallization with Solid–Liquid Configuration Method

The computation models were created with the solid-liquid configuration method. The molecular dynamics simulations were performed to exhibit the crystallization process of liquid pure Ti to hexagonal close packed (HCP) and face-centered cubic (FCC) structure crystals. The results showed that both HCP and FCC crystallizations start from the solid–liquid interfaces and develop toward the middle. The system energy sharply falls at the beginning of crystallization, then is reduced slowly, and finally has a sudden down jump at the end of crystallization. The first peak of RDF is increased significantly with time and some new secondary peaks occur, which is consistent with configuration evolution. The HCP crystal has a longer crystallization process but lower stable energy than the FCC crystal.

Effect of dislocation channeling on void growth to coalescence in FCC crystals

The effect of dislocation channeling - a heterogeneous deformation mode at the grain scale observed in irradiated, quenched or heavily cold-worked materials - on void growth to coalescence in Face-Centered-Cubic (FCC) crystals is investigated experimentally. Solution Annealed 304L austenitic stainless steel is used as a model FCC material, in a reference state or irradiated with protons in order to trigger dislocation channeling deformation mode. Micrometric cylindrical voids drilled using Focused Ion Beam (FIB) technique at the grain scale in thin tensile samples subjected to uniaxial stress loading conditions allows a detailed description of void growth and coalescence. Compared to the reference state where plasticity appears homogeneous at the void scale, dislocation channels strongly interact with voids at the irradiated state, especially at low applied strain where characteristic localization patterns are observed and described. As applied strain increases, deformation becomes more and more homogeneous at the void scale through gradual activation of secondary channels. Numerical simulations based on FCC crystal plasticity constitutive equations are performed and compared to experiments. The main experimental features are recovered by the numerical simulations, but discrepancies remain for both reference and irradiated states. Ductile fracture modeling in materials exhibiting dislocation channeling is finally discussed based on these experimental and numerical results.

Transformations of body-centered cubic crystals composed of hard or soft spheres to liquids or face-centered cubic crystals.

The monodispersed hard-sphere system is one of the simplest models for the study of phase transitions. Despite intensive studies of crystallization and melting of hard-sphere face-centered cubic (FCC) crystals, the phase transformations of hard-sphere body-centered cubic (BCC) crystals have not been explored because hard spheres cannot form a stable BCC lattice. In fact, unstable BCC hard-sphere crystals and their related phase transformations can be experimentally achieved. Here, we measured the kinetics of the melting and solid-solid transformations of BCC hard-sphere crystals at various volume fractions via molecular dynamics simulations. When the volume fraction ϕ < 0.494, the system melts catastrophically. At ϕ > 0.545, the BCC crystal transforms to a metastable polycrystal consisting of FCC and hexagonal close-packed (HCP) domains, which is different from those crystallized from supercooled liquids, and then slowly equilibrates toward the FCC crystal. At 0.494 < ϕ < 0.545, the BCC crystal transforms to an intermediate-order metastable state consisting of BCC and non-crystal particles without FCC and HCP symmetries and then equilibrates toward the coexistence of the FCC crystal and liquid. We further studied the melting and BCC-FCC transitions of crystals composed of soft spheres with potential u(r) = ϵ(r/σ)-n . The unstable BCC crystals at n = 12, 9, 8 exhibit similar melting and BCC-FCC transitions as hard-sphere BCC crystals, while the metastable BCC crystals at n = 5, 6, 7 melt quickly at low densities but take very long time for the BCC-FCC transition at high densities. We also estimate the BCC-FCC interfacial energy and critical nucleus size. These results cast light on the melting and solid-solid transformations of atomic BCC crystals, which exist widely in nature.

Cross Stacking Faults in Zr(Fe,Cr)2 Face-Centered Cubic Laves Phase Nanoparticle

Stacking faults (SFs) are mostly related to the glide of partial dislocations. Cross stacking faults (CSFs) in Zr(Fe,Cr)2 face-centered cubic (FCC) Laves phase nanoparticle (NP) were investigated by transmission electron microscopy (TEM). Our results reveal that CSFs originate from the phase boundary by ⅙ Shockley partial dislocations on crossed close-packed planes simultaneously. More importantly, the driving force of CSFs was further discussed. That is, when the loading direction is parallel or close to directions, the CSFs will be induced in FCC crystals. Knowledge about our investigations will provide a fundamental understanding of the CSFs in FCC crystals.

Dislocation Networks and the Microstructural Origin of Strain Hardening

When metals plastically deform, the density of line defects called dislocations increases and the microstructure is continuously refined, leading to the strain hardening behavior. Using discrete dislocation dynamics simulations, we demonstrate the fundamental role of junction formation in connecting dislocation microstructure evolution and strain hardening in face-centered cubic (fcc) Cu. The dislocation network formed consists of line segments whose lengths closely follow an exponential distribution. This exponential distribution is a consequence of junction formation, which can be modeled as a one-dimensional Poisson process. According to the exponential distribution, two non-dimensional parameters control microstructure evolution, with the hardening rate dictated by the rate of stable junction formation. Among the types of junctions in fcc crystals, we find that glissile junctions make the dominant contribution to strain hardening.