Orientation Of Crystallites Research Articles

Microstructural evolution and texture analysis of magnesium phosphate cement

AbstractThree‐dimensional quantitative image analysis from synchrotron X‐ray microcomputed tomography indicated a coarsening of the microstructure of magnesium potassium phosphate cements driven by crystallization of K‐struvite from the first amorphous product. Porosity and pore surface area increased because of the progressive build‐up of a network of elongated/tabular crystal domains, with density higher than the amorphous. The known increase in strength with time is thought to occur thanks to the overwhelming contribution of a developing interlocked lath‐shaped microstructure. Combined X‐ray and neutron diffraction texture analysis indicated that at least a fraction of K‐struvite nucleates at the surface of MgO grains, suggesting the intervention of more than one crystallization mechanism. The detected weak texture, compatible with a nearly random orientation of crystallites, and the isotropic pore fabric, are beneficial with respect to crack propagation.

Study of the phase composition of fine-grained tungsten carbide based ceramic materials by x-ray phase analysis

The results of X-ray diffraction analysis of qualitative and quantitative phase composition of powder and solid polycrysral samples of the system a-WC/W 2 C are presented. Powder (nanopowder) samples were obtained by plasmochemical synthesis from tungsten oxide and hydrocarbon. High-density fine-grained structure of ceramics was formed by electropulse plasma sintering (spark plasma sintering) of initial industrial a-WC powders (AlfaAesar). Experimental data were obtained on a Shimadzu XRD-7000 X-ray diffractometer (CuKa, λ = 1.54 A). The optimal modes of X-ray shooting of the ceramic samples based on tungsten carbide are presented. In the absence of reference standards for initial crystalline phases (WC and W2C), quantitative phase analysis was carried out using the reference intensity ratio (RIR) or corundum number (instead of the method of calibration curve). The required quantitative ratios were calculated using the structural parameters of corundum a-Al 2 O 3 and phases a-WC, W 2 C. The results of determining the repeatability of the intensity values were used to select the optimal exposure time and width of the receiving slit of the detector of diffracted X-rays. Study of the effect of the quality of sample preparation on the sensitivity of qualitative X-ray phase analysis showed the feasibility of grinding of the surface of solid ceramic samples and their pre-polishing with a diamond paste (grain size not less than 5/3 pm). The accuracy of quantitative phase analysis of powder and sintered ceramic samples was evaluated. The layer-by-layer phase analysis of the ceramic preform revealed a preferential crystallite orientation (texture) in the surface layer and the absence of the impurity phase W 2 C. The sensitivity of X-ray phase analysis to the content of a-W 2 C phase in nanopowders of tungsten monocarbide has been estimated.

Characteristics of the Phase Transition in Vanadium Dioxide Films Obtained via Chemical Vapor Deposition

Changes in the characteristics of the semiconductor–metal phase transition in VO2 polycrystalline films obtained via chemical vapor deposition from vanadyl acetylacetonate vapor and oxygen with the temperature of synthesis varying from 373 to 473°C and the oxygen pressure varying from 25 to 90 sccm were studied. The critical temperature of the phase transition and the width of the hysteresis are calculated from the temperature dependences of the films’ coefficients of reflection. These dependences are determined using a special spectrophotometric system during the thermal cycling of the samples from room temperature to 100°C. The relationship between the phase transition characteristics and the phase composition and texture of the films is established. It is found that the critical temperature and the narrowest hysteresis correspond to films containing single monoclinic phase VO2(M1) with the preferential orientation (texture) of crystallites in the (011) plane.

Varying steps in hysteresis loops of Co square nano-frames

Abstract Magnetic nanostructures are investigated nowadays for diverse applications as well as in basic research. They often show unexpected magnetization reversal processes which are not only of high interest for understanding magnetization reversal processes on the nano-scale, but also due to their possible use in magneto-electronic devices, spintronics, neuromorphic computing and other areas. Square nano-frames prepared from iron, e.g., belong to the magnetic nano-particles exhibiting additional stable states at remanence which can be used for quaternary storage devices. The reason for this study is that nano-particles prepared from permalloy or nickel with their very low magneto-crystalline anisotropy do usually not show steps in the hysteresis loop and corresponding stable intermediate state, while the magneto-crystalline anisotropy of cobalt often dominates over the shape-anisotropy, resulting in a large number of steps which are not always stable. Here we report on experimental investigations by magneto-optic Kerr effect (MOKE) and micromagnetic simulations of square Co nano-frames, exhibiting steps along the slopes of the hysteresis loops. The positions and numbers of these steps varied during measurements without intentionally changing the experimental setup. Micromagnetic simulations were carried out to explain the experimental findings which could be attributed to different Co crystallite orientations, resulting in a modified magneto-crystalline anisotropy and thus different hysteresis loops for smallest variations of the laser beam position on the samples during MOKE measurements.

Influence of Annealing and Biaxial Expansion on the Properties of Poly(l-Lactic Acid) Medical Tubing.

Poly-l-lactic acid (PLLA) is one of the most common bioabsorbable materials in the medical device field. However, its use in load-bearing applications is limited due to its inferior mechanical properties when compared to many of the competing metal-based permanent and bioabsorbable materials. The objective of this study was to directly compare the influence of both annealing and biaxial expansion processes to improve the material properties of PLLA. Results showed that both annealing and biaxial expansion led to an overall increase in crystallinity and that the crystallites formed during both processes were in the α’ and α forms. 2D-WAXS patterns showed that the preferred orientation of crystallites formed during annealing was parallel to the circumferential direction. While biaxial expansion resulted in orientation in both axial and circumferential directions, with relatively equal sized crystals in both directions, Da (112 Å) and Dc (97 Å). The expansion process had the most profound effect on mechanical performance, with a 65% increase in Young’s modulus, a 45% increase in maximum tensile stress and an 18-fold increase in strain at maximum load. These results indicate that biaxially expanding PLLA at a temperature above Tcc is possible, due to the high strain rates associated with stretch blow moulding.

Selective growth of SiC nanowires in interlaminar matrix for improving in-plane strengths of laminated Carbon/Carbon composites

β-SiC nanowires (SiCNWs) were selectively grown in the interlaminar matrix with a volume fraction of 0.65% by applying a pyrocarbon coating on carbon fibers, which realizes the proper reinforcement of C/C composites. The thickness of the pyrocarbon is optimized to 0.5 μm based on the analysis of in-situ fiber strengths with the fracture mirror method. The pyrocarbon coating increased the in-situ fiber strength by ∼7% and prevent brittle fracture of the composites. Compared with C/C, the interlaminar shear and flexural strength of SiCNW-C/C (10.06 MPa and 162.44 MPa) increase by 158% and 57%. Incorporating SiCNWs changes the crystallite orientations and refines the crystallite size of pyrocarbon matrix. The functions of SiCNWs vary with their loading density. When SiCNWs are sufficient in the matrix, they help reinforcing and improving the critical failure stress of the matrix. When their density decreases to a certain degree, SiCNWs help changing the crystallite orientations of pyrocarbon and toughening the matrix.

Prediction of Bone Quality of Remodeling Trabeculae Using Multi-Scale Stress Analyses with a Homogenization Technique Reflecting Material Anisotropy

High stress in osteoporotic bone that leads to fracture is related to time-dependent changes of bone quality, which is defined by the trabecular structure and the material anisotropy derived from biological apatite (BA) crystallite orientation, with loss of bone mass. To assess and estimate the relativity of variation with time of bone quality, bone mass and fracture risk for the same subject in the future, prediction of remodeling and change in stress distribution in bone is necessary. We established a computational framework with multi-scale stress analyses and remodeling simulations. Then, using healthy and osteoporotic pig’s trabeculae, stress analyses of their remodeled tissue considering BA orientation were performed. The homogenized stiffness in the direction which had adapted to the mechanical environment was high even though bone mass decreased. The relationship between the present percentage of the high stress and the decrease of bone density was not proportional in the osteoporotic remodeling process, but it increased more when the bone mass decreased. These findings suggest that future fracture risk could be detected in more detail by time-dependent variance of bone quality, and the relationship between bone mass, bone quality, and fracture risk could be potentially elucidated by our computational frameworks.

Comparative morphological and crystallographic analysis of copper powders obtained under different electrolysis conditions

Abstract Production of copper powders by the potentiostatic electrolysis under different hydrogen evolution conditions was investigated. Copper powders were characterized by the scanning electron microscope (SEM), X-ray diffraction (XRD), particle size distribution (PSD), and by the determination of the specific surface area (SSA) of the formed powders. Depending on quantity of hydrogen generated during electrolysis, the two types of particles were formed: dendrites and cauliflower-like particles. The dendrites were formed without, while cauliflower-like particles with the quantity of evolved hydrogen enough to achieve strong effect on hydrodynamic conditions in the near-electrode layer. Although macro structure of the particles was very different, they showed similar micro structure. Namely, both types of the particles consisted of small agglomerates of approximately spherical Cu grains at the micro level. The existence of the spherical morphology was just responsible for random orientation of Cu crystallites in both types of particles. The SSA of cauliflower-like particles was more than two times larger than that of the dendrites, while their size was considerably smaller than that of the dendritic particles. In this way, the useful benefit of Cu powder formation in the conditions of vigorous hydrogen evolution is shown.

The microstructure and texture of Gilsocarbon graphite

The material properties of nuclear graphite are defined by the as-manufactured graphite texture and their evolution due to irradiation and oxidation in reactors. Thus an understanding of the graphite microstructure and texture is essential in simulating and the predicting material property changes in nuclear graphite. The texture of nuclear graphite involves the preferred orientation of graphite crystallites and the domain structure. This study developed a technique based on combined micro-Raman spectroscopy mapping and atomic force microscope, and demonstrate the application of this technique to texture characterization of GCMB Gilsocarbon graphite. A crystallite orientation analysis approach was proposed to describe the local arrangement of graphite crystallites. For comparison, the texture characterization results are present for Gilsonite coke filler, and different parts of binder phase, respectively. The domain orientation within Gilsonite coke filler is irregular, but statistically follows the spherical particles. The major binder phase shows less ordered texture, however, certain parts of the binder phase shows very ordered planar crystallite orientation, and even larger lateral crystal size than that of filler particle. The clear description of texture provides the bases for future simulations and investigation of the properties and irradiation performance of Gilsocarbon graphite.

The Orientation of Strain-Induced Crystallites in Uniaxially-Strained, Thin and Wide Bands Made from Natural Rubber

Vulcanized natural rubber (unfilled and filled with 20 phr carbon black) is strained. We suppress the macroscopic formation of fiber symmetry by choosing strip-shaped samples ("pure-shear geometry") and investigate the orientation of the resulting crystallites by two-dimensional wide-angle X-ray diffraction (WAXD), additionally rotating the sample tape about the straining direction. Indications of a directed reinforcing effect of the strain-induced crystallization (SIC) in the thin strip are found. In the filled material fewer crystallites are oriented and the orientation distribution of the oriented crystallites is less perfect. The results confirm, that it is important for the evaluation of crystallinity under deformation to check, whether fiber symmetry can be assumed. This has consequences in particular on the quantitative interpretation of space-resolved scanning experiments in the vicinity of crack tips. Furthermore it raises the question, whether there is an asymmetric reinforcing effect of the SIC in the vicinity of crack tips inside natural rubber.

Tensile Stress Generation on Crystallization of Polymer Networks

When crystallized under constant strain, conventional semicrystalline networks exhibit a loss in tension due to directionally preferred crystallization of chains along the strain axis. This study shows that a dual-cured, semicrystalline polymer network with programmed chain bias can generate ∼400 kPa of tensile stress upon crystallization. Constant strain experiments were conducted to monitor stress in configurationally biased networks as they were cooled and crystallized. Time resolved wide-angle X-ray scattering was used to study crystallization kinetics and the evolution of crystallite orientation. In configurationally biased networks without external load, initial crystallization along the internal chain bias direction is rapidly followed by crystal growth in the orthogonal direction. Under external loads, crystallization first occurs along the external load direction followed by the configurational chain bias direction. Calorimetry measurements indicate that two distinct crystal populations form upon...

Structure, optical properties and resistance to laser radiation of thin barium disilicide films grown on silicon

Polycrystalline and oriented films of barium disilicide (BaSi2) with a thickness of up to 100 nm were formed on silicon (111) substrates by high-temperature (800 ° C) solid-phase (single-stage and two-stage) annealing. The single phase of barium disilicide films and their semiconductor nature have been proven to be below 1.25 eV according to X-ray and optical spectroscopic methods. Two preferential orientations of the BaSi2 crystallites were detected and their orientation was determined in the films formed by two-stage annealing. According to the calculations of the parameters of the crystal structure of BaSi2 films, a compression of the unit cell volume from 2.7% to 5.13% was found, depending on the cooling time to room temperature. The stability of the films to laser radiation was studied by registering the Raman spectra with a variable power of laser radiation. The maximum power density of the laser beam (3⋅109 W/m2), which does not lead to the beginning of the destruction of these films, wasdetermined.

The microstructures and mechanical properties of ultra-high-strength PAN-based carbon fibers during graphitization under a constant stretching

Commercial ultra-high-strength PAN-based carbon fibers (T1000G) were heat-treated at the temperature range of 2300–2600 °C under a constant stretching of 600 cN. After continuous high-temperature graphitization treatment, microstructures, mechanical properties and thermal stability of the carbon fibers were investigated. The results show that the T1000G carbon fibers present the similar round shape with a smooth surface before and after graphitization, indicating the carbon fibers are fabricated by dry–wet spinning. In comparison, the commercial high-strength and high-modulus PAN-based carbon fibers (M40JB and M55JB) present elliptical shapes with ridges and grooves on the surface, indicating the carbon fibers are fabricated by wet spinning. After graphitization treatment from 2300 to 2600 °C under a constant stretching of 600 cN, the Young’s modulus of the T1000G carbon fibers increases from about 436 to 484 GPa, and their tensile strength decreases from about 5.26 to 4.45 GPa. The increase in Young’s modulus of the graphitized T1000G carbon fibers is attributed to the increase in the crystallite sizes and the preferred orientation of graphite crystallites along the fiber longitudinal direction under a constant stretching condition. In comparison with the M40JB and the M55JB carbon fibers, the graphitized T1000G carbon fibers are easier to be oxidized, which can be contributed to the formation of more micropores and defects during the graphitization process, thus leading to the decrease in the tensile strength.

Optical Characterization of Cesium Lead Bromide Perovskites

CsPbBr3 and Cs4PbBr6 perovskite powders have been synthesized through a relatively simple low-temperature and low-cost method. Nanocrystalline films have also been deposited from solutions with four different molar compositions of binary salt precursors. Optical absorption, emission and excitation spectra have been performed in the UV-visible spectral range while X-ray diffraction (XRD) has been recorded to characterize the nanocrystal morphology for the different molar compositions. A preferential orientation of crystallites along the (024) crystalline plane has been observed as a function of the different deposition conditions in films growth. All the crystals show an absorption edge around 530 nm; Tauc plots of the absorption returned bandgaps ranging from 2.29 to 2.35 eV characteristic of CsPbBr3 phase. We attribute the UV absorption band peaked at 324 nm to the fundamental band-to-band transition for Cs4PbBr6. It was observed that the samples with the most ordered Cs4PbBr6 crystals exhibited the most intense emission of light, with a bright green emission at 520 nm, which are however due to the luminescence of the inclusion of CsPbBr3 nanoclusters into the Cs4PbBr6. The latter shows instead an intense UV emission. Differently, the pure CsPbBr3 powder did not show any intense fluorescent emission. The excitation spectra of the green fluorescent emission in all samples closely resemble the CsPbBr3 absorption with the peculiar dip around 324 nm as expected from density of state calculations reported in the literature.

Preparation of a high surface area zirconium oxide for fuel cell application

Stable and high surface area zirconium oxide nanoparticles have been synthesised by means of the hydrothermal method. The Brunauer–Emmett–Teller results show that a high surface area of 543 m2/g was obtained in the hydrothermal process, having a high porosity in nanometre range. The hydrothermal method was applied at 120 °C by using an autoclave with a Teflon liner at an ambient pressure for 48 h. High-resolution scanning electron microscopy shows the different morphologies of zirconia nanoparticles, which could be categorised as one-dimensional and zero-dimensional, as they had a high crystallite orientation, which was also confirmed by the X-ray diffraction (XRD). The mixture of two types of cubic phases in one sample was obtained from XRD and confirmed by the zirconia nanostructure, showing the stable phase of fluorite, which has full cubic symmetry (Im-3m), and also an Arkelite zirconia nanostructure, showing the stable phase of fluorite, which has full cubic symmetry (Fm-3m). The XRD results also show the different structure orientations of face-centred cubic and body-centred cubic in one sample.

Sound Velocities and Elastic Moduli of Phases I and V of Silicon at High Pressures

Pressure dependences of longitudinal sound velocities, in the two phases Si‐I and Si‐V of silicon having, respectively, cubic‐diamond and primitive‐hexagonal structures, are measured using the technique of picosecond laser ultrasonics adapted to samples compressed in a diamond anvil cell (DAC). For the Si‐I phase, stable at atmospheric pressure, the longitudinal sound velocity along the 〈100〉 direction is obtained in a single crystal up to 9 GPa. In the case of the Si‐V phase, the average sound velocity for an isotropic polycrystalline sample VL(avg) is measured for the first time between 18 and 27 GPa. Above this pressure, preferred orientation of the hexagonal crystallites of Si‐V, with their c‐axes parallel to the compressional direction in the DAC and to the acoustic pulse propagation direction, is progressively developing thus precluding further VL(avg) measurement. The experimental single crystal elastic constants C11(P) and C12(P) of Si‐I, and the shear modulus of polycrystalline Si‐V deduced here are in a very good agreement with our first principles calculations and with the earlier results reported for Si‐I.

Effect of crystallinity of PAN-based carbon fiber surfaces on the formation characteristics of silicon carbide coating

Silicon carbide (SiC) coating was synthesized on PAN-based carbon fiber surfaces by reaction of silicon monoxide (SiO) with carbon fibers. The effect of the crystallinity of carbon fiber surfaces on the formation characteristics of SiC coating was discussed. As the crystallinity of carbon fiber surfaces increases, the thickness of SiC coatings decreases from 341 nm to 128 nm and the surface of SiC coating becomes dense. The microstructure of coated fibers shows that the growth orientation of SiC crystallites is consistent with the orientation of graphite crystallites. It is proposed that amorphous carbon and graphite crystallites in carbon fibers would transform into amorphous SiC and continuous SiC crystallites, respectively.

The Effect of Organic Acids in the Presence of Cyclic Lactams on Kinetics of Electrodeposition and Morphology of Nickel Coatings Modified with Polymer

Using stationary electrolysis, the effect of the structure and concentration of some organic acids on kinetic parameters of electrodeposition of nickel from sulfate electrolytes containing cyclic lactams, such as e-caprolactam (CL) and N-methylpyrrolidone (MP), microstructure, mean grain size, and crystallite orientation has been studied. Integrated analysis of the data of polarization, adsorption, and spectral investigations has shown that enhancement of adsorption of amino acids in the presence of lactams leads not only to an increase in the polarization of the process, but also to significant changes of surface morphology. According to X-ray phase analysis and SEM data, the surface becomes smoother in the presence of amino acids, the grain size decreases, and a predominant orientation of crystallites appears in some cases. The formation of metal–polymer sharp-edged grains in the coatings prepared in the presence of CL and glycine has resulted in the formation of continuous smoothed layer and appearance of mirror shine of the deposits.

Why some carbons may or may not graphitize? The point of view of thermodynamics

Not all carbons graphitize in equal measure. Some will develop a structure which approaches the one of perfect graphite (graphitizable carbons) upon heat treatment, while others will not (non-graphitizable carbons). The present work develops a phenomenological model for the conceptual understanding of graphitizability (capacity to graphitize). To support this model, a mathematical formalism, inspired from thermodynamics, is proposed to calculate the Ultimate Graphitizability (ηg) of some graphitizable and non-graphitizable carbon materials. ηg is the average interlayer spacing (d002) of a graphenic carbon following graphitization at ∼3400K. ηg can be estimated assuming a topological graphitization mechanism operating between 1700K and 3400K. Two independent variables define ηg: d002(Tα) and d002(Tβ). Tα and Tβ are arbitrarily selected temperatures between 1700K and 2550K (the graphitization threshold). In order to better understand the parameters affecting d002(Tα) and d002(Tβ), new carbonization/graphitization experimental results are presented. These suggest that d002(Tα) and d002(Tβ) are correlated to the oxygen/hydrogen composition ratio and the relative mesoscale crystallite orientation of some graphitizable carbons following the end of primary carbonization.

Preparation of cadmium sulfide nanoparticles and mediation thereof across poly(hydroxybutyrate) nanocomposite

Herein, we used chemical precipitation method to synthesize cadmium sulfide (CdS) nanoparticles (NPs) with different concentrations of CdNO3. The CdS NPs of size 105 nm and the corresponding zeta potential of 42.2 mV were achieved at CdNO3 concentration of 1 mM. X-ray diffraction (XRD) analysis revealed W-type hexagonal geometry of CdS NPs. The prepared CdS NPs were introduced into poly(hydroxybutyrate) (PHB) matrix to fabricate CdS/PHB composite nanomaterial. The physicochemical characterization showed successful synthesis of a novel nanocomposite. The XRD analysis showed that CdS/PHB nanocomposite exhibited hexagonal crystal structure, i.e., crystallite orientation of nanocomposite was majorly controlled by CdS NPs. Scanning electron micrographs of CdS NPs and CdS/PHB nanocomposite revealed spherical morphology of CdS NPs and their successful placement into PHB matrix with some random aggregations. The results were further confirmed using transmission electron microscopy (TEM) and a high-resolution TEM. Thermogravimetric analysis showed improved thermal characteristics of nanocomposites after incorporation of CdS NPs into PHB matrix.