Inhomogeneous Structure Research Articles

Формирование на поверхности трековых мембран гидрофобных покрытий методом магнетронного распыления полимеров в вакууме

The surface properties and chemical structure of nanoscale coatings deposited on the poly(ethylene terephthalate) track-etched membranes surface by magnetron sputter deposition of ultra-high molecular weight polyethylene and polytetrafluoroethylene in vacuum are studied. It is shown that the application of coatings leads to hydrophobization of membrane surface, the degree of which depends on the type of polymer used for sputtering and the thickness of the coating. It is established that the use of this modification method leads to smoothing of structural inhomogeneities of the membrane surface layer. This result is explained by the deposition of coatings in the pore channels at a certain depth from the entrance and the overlap of pores on the surface of modified membranes. Besides, the deposition of coatings on the track-etched membrane surface leads to a change in the pore shape. The diameter of the pores on the untreated side of the membranes remains unchanged and decreases significantly on the modified side. The pores of the membranes acquire an asymmetric (conical) shape in this case. The study of the chemical structure of coatings by X-ray photoelectron spectroscopy showed that they contain oxygen-containing functional groups due to the oxidation of the polymer matrix. The developed composite membranes can be used in the membrane distillation processes for the desalination of seawater.

The effect of partial delignification on the stress–strain relationship in transverse compression

Due to very significant improvements in the mechanical performance of solid wood, densification after partial or full delignification has received renewed attention. While studies predominantly focus on isolated cases, e.g., one wood species densified in one anatomical direction, systematic investigations of the effect of wood structure on the densification process are seldom performed. The present study compares the deformation patterns of one representative softwood (spruce) and hardwood (poplar), respectively, in transverse compression. In terms of variables, the native and partially delignified state, wet and dry conditions, and three different orientations of wood grain are considered. It shows that the structural inhomogeneity of spruce wood, with low-density earlywood and high-density latewood, governs its densification pattern and hinders defect-free densification. Contrarily, diffuse-porous poplar is structurally more homogeneous, which together with more efficient softening by delignification leads to favorable densification behavior. Overall, delignification led to a significant softening of both wood species in transverse compression, which greatly enhanced the softening effect of high wood moisture content.

Measurements of parameters of structures of C-C composites to ensure the computation of change in FV shape and aerodynamic characteristics

The paper provides a designed under the supervision and with the participation of authors of the paper complex of methods of accounting for structural inhomogeneities of elements of a Thermal Protection System (TPS) made of C-C composites used to estimate ablated shapes of high-speed Flight Vehicles (FVs) and corresponding methods of measuring and processing of the needed structural parameters of C-C composites. During the mass ablation along a flight trajectory reinforcing elements and a graphitized matrix ablate differently, and there appears primary large-scale roughness on the surface. Moreover, pores, shells, and other fine-grained inhomogeneities outcrop forming the secondary finely divided roughness. Roughness impacts heat exchange, and summary equivalent roughness parameters are used to estimate FVs ablated shapes. A C-C composite graphitation degree that is defined with the X-ray diffraction study also influences an ablation rate. The density of materials is linearly related to their porosity, and for a number of currently in use and promising materials the density distributions were studied using computer tomography as well as the distribution of pores, shells, and other fine-grained inhomogeneities over volume of TPS elements was examined with electron microscopes. X-ray diffraction studies of the C-C composites were also carried out. Comparative computations of ablated shapes and aerodynamic characteristics of a model cylindrical FV with applied TPS elements of the examined C-C composites were performed with the use of obtained results of tomography, electron microscopic and X-ray diffraction studies.

Corrosion behavior of Ni62Nb33Zr5 bulk metallic glasses after annealing and cryogenic treatments

The microstructural evolution and corrosion behavior of Ni62Nb33Zr5 bulk metallic glasses (BMGs) after annealing treatment (AT) at different crystallization temperatures and cryogenic treatment (CT) at −100 ℃ are experimentally investigated. Appropriate AT and CT can both improve the thermal stability and comprehensive corrosion resistance of as-cast BMG in 3.5 wt.% NaCl solution. The annealed and cryo-treated BMGs exhibit one more finite diffusion layer loop in the electrical equivalent circuit than the as-cast BMG, indicating the complexity of the corrosion behavior. Superior corrosion resistance is obtained in the cryo-treated BMG because the high degree of amorphization caused by CT reduces the structural inhomogeneity. Lower Cd and higher Rd values are obtained for the cryo-treated BMG, revealing the formation of a more stable passive film. Among the annealed BMGs, the fully crystallized sample exhibits a higher anti-corrosion performance owing to the existence of Nb-rich oxides in the crystallization products. The passive film is found to be composed mainly of Nb2O5 and ZrO2, demonstrating that Nb and Zr are conducive to reacting with oxygen to form a passive film. Based on the goal of maintaining a fully amorphous phase, appropriate CT causing structural homogeneity of the BMG is a simple and effective means to improve the comprehensive corrosion resistance.

Эффект Мейсснера и структурная неоднородность в ВТСП YBa-=SUB=-2-=/SUB=-Cu-=SUB=-3-=/SUB=-O-=SUB=-6.92-=/SUB=-

For optimally doped high-temperature superconductors YBa2Cu3O6.92 with various degrees of structural inhomogeneity, the influence of this inhomogeneity on the manifestation of the Meissner effect is considered. The measurement results indicate that for the samples under study, for which almost identical superconducting transition Tс = 92K temperatures were realized, the Meissner effect values turned out to be different and correlated with the degree of structural inhomogeneity, which is also characteristic of other magnetic properties.

Gastric Trichobezoar Mimicking a Large Abdominal Tumour: Case report

Background: A bezoar is a collection of indigestible substances that, after swallowing, form a mass in the gastrointestinal tract, most often in the stomach. A trichobezoar is a mass made of ingested, undigested hairs and food remains, while ingestion of hair is called trichophagia. This is an extremely rare condition, with 90% of cases occurring among women, out of which 80% are under the age of 30. The diagnosis is based on clinical examination, findings obtained through radiological modalities (ultrasound and computed tomography) and endoscopy. Objective: The aim of this article is to present the case of a 17-year-old female patient who visited the doctor due to abdominal pain and a visible mass in the projection of the stomach. Case presentation: The patient, accompanied by her parents, and following the instructions of a general practitioner who suspected the presence of a tumour in the abdomen, reported to the surgical clinic. The US examination revealed the presence of a heteroechoic mass in the stomach area, but it could not be determined with certainty where it belonged (Samsung Medison V8 ultrasound system). The CT scans revealed the presence of an inhomogeneous structure in the stomach area occupying the entire volume of the stomach. The mentioned structure included the presence of internal air particles and marginal imbibition of the orally applied contrast agent. The described structure did not infiltrate the stomach wall and, based on the CT scan, the diagnosis established was a bezoar. Under general anaesthesia, a laparotomy with a gastrotomy was performed, and a hard hook shaped mass composed of hair and remains of undigested food was removed from the stomach. Conclusion: A mobile, palpable mass located in the abdomen of younger female patients accompanied by abdominal symptoms, especially in patients where there is information about hair pulling and swallowing, may suggest trichobezoar. The golden standard for trichobezoar presentation is upper endoscopy. If unavailable, the diagnosis is established by a CT scan, preferably with oral application of a contrast agent. Treatment of larger trichobezoars is surgical, while endoscopic and laparoscopic removal is possible only in the case of small trichobezoars.

Peculiarities of Formation of Flat Inhomogeneous Structures in Nanoscale Magnetic Films

Peculiarities of Formation of Flat Inhomogeneous Structures in Nanoscale Magnetic Films

Quantifying nematic order in the evaporation-driven self-assembly of halloysite nanotubes: nematic islands and the critical aspect ratio.

Halloysite nanotubes (HNTs) are naturally occurring clay minerals found in the Earth's crust that typically exist in the form of high aspect-ratio nanometer-long rods. Here, we investigate the evaporation-driven self-assembly process of HNTs and show that a highly polydisperse collection of HNTs self-sort into a spatially inhomogeneous structure, displaying a systematic variation in the resulting nematic order. Through detailed quantification using the nematic order parameter S and nematic correlation functions, we show the existence of well-defined isotropic-nematic transitions in the emerging structures. We also show that the onset of these transitions gives rise to the formation of nematic islands, a phase resembling ordered nematic domains surrounded by an isotropic phase, which grow in size with S. Detailed image analysis indicates a strong correlation between local S and the local aspect ratio, L/D, with nematic order possible only for rods with L/D ≥ 6.5 ± 1. Finally, we conclude that the observed phenomena directly result from aspect ratio-based sorting in our system. Altogether, our results provide a unique method of tuning the local microscopic structure in self-assembled HNTs using L/D as an external parameter.

Features of ultrasonic visualization of layered objects

Pulsed acoustic microscopy (100–200 MHz) was used to study layers of pyrolytic graphite on a glass substrate with a thickness comparable to wavelength of the probing sound. Mechanism of natural amplification of acoustic contrast in visualization process of such structures is shown. Images interpretation of the inhomogeneous structure of the layer-substrate contact is supported by a quantitative assessment of acoustic impedances.

KINETIC INSTABILITY IN AN ENSEMBLE OF DISLOCATIONS UNDER PLASTIC DEFORMATION OF METALS

Based on a system of evolutionary equations for an ensemble of edge dislocations, the problem associated with the development of instability of a homogeneous distribution of dislocations is considered. Despite the large number of theoretical studies on this topic, most of them are considered within the framework of reaction-diffusion phenomenological models, which contradicts the mathematical structure of the initial evolutionary equations, which are hyperbolic equations. For the initial system of hyperbolic equations, the case is considered when the process of plastic deformation develops along a given axis. The elastic interaction of dislocations is neglected in order to reveal the instability features of the homogeneous state of the system due to the local kinetics of dislocations. The stability analysis of the homogeneous state was carried out for a system of dislocations of two types differing in mobility. As a result of linearization of the initial system, a system of homogeneous linear algebraic equations is obtained. From the condition of the existence of its nontrivial solutions, a dispersion equation is found, using which, on the basis of the Raut – Hurwitz criterion, the range of parameters of the initial system at which kinetic instability is realized is established. In the instability region, an expression is obtained for the increment of the unstable mode, which determines the behavior of the system beyond the bifurcation point, and an expression for the characteristic scale of the inhomogeneous structure. The region of system parameters where the diffusion dynamics of the system is realized is found. It is shown that the Turing instability for the total dislocation density is realized in this parameter range. Within the framework of the initial system of hyperbolic equations, the dynamics of both total and excess dislocation densities is significant. It is the excess density that leads to disorientation in the formation of heterogeneous dislocation structures. This is a fundamental point, since at the advanced stage of plastic deformation, the dislocation structure is basically a system of disoriented cells and ragged sub-boundaries.

Features of Dynamics and Instability of Plasma Jets Expanding into an External Magnetic Field in Laboratory Experiments with Compact Coaxial Plasma Generators on a Large-Scale “Krot” Stand

Structural instabilities that develop during pulsed injection of dense plasma jets into vacuum in the presence of an external quasi-homogeneous magnetic field are studied by high-speed photography using ICCD cameras. The experiments are carried out in the chamber of the “Krot” stand, which has record-breaking dimensions in its class of installations (diameter—3 m, length of the working section—10 m), and makes it possible to study plasma dynamics by various diagnostic methods at scales of more than 1 m both along the magnetic field and in the direction transverse to the magnetic field. During injection along the magnetic field, a transverse collimation of the flow of ionized matter and the development of a flute instability of the plasma boundary are observed, which, at the late stages of expansion, leads to the plasma leaving the injection region in the form of several jets across the field. During transverse injection, the formation of a collimated flow, a “plasma sheet,” is observed, in which, as the plasma moves across the field, inhomogeneous structures develop in the direction of injection.

LES flamelet modeling of hydrogen combustion considering preferential diffusion effect

A flamelet-generated manifold (FGM) method that explicitly considers the preferential diffusion effect, referred to as FGM-PD method, is employed for large-eddy simulations (LESs) of a lean-premixed H2/air low-swirl lifted flame, and the validity is examined by comparing with the experiment. First, the applicability of the FGM-PD method is investigated by one-dimensional numerical simulations of planar laminar premixed H2/air flames. Next, LESs of a lean-premixed H2/air low-swirl lifted flame are performed employing the FGM-PD and conventional FGM methods. Results of the one-dimensional numerical simulations show the importance of considering preferential diffusion to accurately predict species concentrations near the flame front. The FGM-PD method accurately predicts this, and therefore, reproduces the laminar burning velocity and spatial distributions of temperature and mixture fraction. Three-dimensional LES results confirm that the prediction accuracy of the velocities near the flame front is improved by employing this FGM-PD method. Additionally, the OH mass fraction distribution predicted by the FGM-PD method exhibits the inhomogeneous finger-like structure, which has been observed in previous experiments. This inhomogeneity of OH mass fraction distribution, which corresponds to that of the reaction rate, predicted by the FGM-PD method, strongly affects the flame front structure.

Temperature profile and thermal piston component of photoacoustic response calculated by the fractional dual-phase-lag heat conduction theory

We present the temperature distribution predictions for photothermal systems by considering an extension of dual-phase lag. It is an extension of the GCE-II and GCE-III models with a fractional dual-phase lag from kinetic relaxation time. Solving the one-dimensional problem considering a planar and periodic excitation, we obtained the temperature distribution and the Photoacoustic (PA) signal for the transmission setup. We also analyze the effects of fractional order derivatives and kinetic relaxation time. It is shown that the derived models have promising results that could be used to explain the experimentally observed behavior of PA signals measured on thin films with an inhomogeneous internal structure.

Experimental Study on the Tridacna squamosa Shell: Distinctive Structure and Mechanical Behavior.

Tridacna squamosa, Lamarck, 1819 (Bivalvia Cardiida Cardiidae, known as the fluted giant clam) is one of the largest-sized bivalve shells, which is equipped with a strong and tough bioceramic shell to effectively protect itself from the attack of predators. To better understand the mechanical defense mechanism, the relationship between the microstructure, composition, and mechanical properties of the Tridacna squamosa shell was investigated. We find that the Tridacna squamosa shell is composed of aragonite CaCO3 and a small portion of organic matter, which are well-arranged, assembling a multiscale, inhomogeneous, and anisotropic structure. Three levels of microstructure units are identified, including the smallest aragonite rods, medium sheets, and block-like lamellae. Such multiscale structures are the main contributor to creating abundant fracture surfaces much larger than the case for single mineral components, leading to multiple toughening mechanisms observed in Vickers indentation experiments, such as pulled-out of mineral platelet and crack deflection. The material inhomogeneity in the cross-sectional direction indicates that the material is stronger at the inner layer than that at the outer layer, which also facilitates an effective defense against the predator attack. This study may provide insights into the design of biomaterials with the desired mechanical properties.

СВОЙСТВА ПОКРЫТИЙ ПОСЛЕ МАГНИТНО-ЭЛЕКТРИЧЕСКОГО УПРОЧНЕНИЯ И ЛАЗЕРНОЙ ОБРАБОТКИ

One of the methods of restoration and hardening of worn surfaces of the working bodies of agricultural machines is the application of coatings and (or) hardening heat treatment. Magnetic-electric hardening (MEH) and laser methods of hardening are analyzed. Subsequent processing by a continuous laser of coatings formed after MEA was studied to eliminate such defects as high surface roughness and the likelihood of microcracks. from two-component alloyed powders based on Fe-5% V and Fe-Ti iron, FBKh-6-2 alloy on a special installation. The subsequent laser treatment of the coatings was carried out on a CO2 laser with a power of 1 kW. The change in the intensity of the process of magnetic-electric hardening and the transfer of powder material was experimentally established. The mechanisms of thermal phenomena that ensure the melting of particles of a ferromagnetic powder during magnetic-electric hardening, the formation of a coating and structural transformations in the surface layer are considered. It has been established that when modeling thermal processes from a fast-moving source, it is advisable to use a relation that describes the process of heat propagation from a point source moving fast along a half-space with an adiabatic boundary. It has been established using experimental studies that laser treatment of coatings formed after MEA significantly reduces the gradient of structural inhomogeneity, leads to the complete disappearance of microcracks, reduces the average thickness variation of coatings up to 1.2 times and the volumetric porosity of coatings up to 3 times. reduction of microstresses from 0.74% to 0.1%.

A Review of the Continuum Theory-Based Stress and Drag Models in Gas-Solid Flows

The continuum theory-based models, which include solid stress models and gas-solid drag models, are required for the modeling of gas-solid flows in the framework of the Eulerian–Eulerian method. The interactions among particles are characterized by their diverse behaviors at different flow regimes, including kinetic motion, particle–particle collision and enduring friction. It is difficult to describe the particle behaviors at various regimes by mathematical methods accurately. Therefore, it is very important to develop proper solid stress models that can capture the inherent characteristics of the flow behaviors. In addition, the gas-solid fluidization system is a typical heterogeneous system, which exhibits locally inhomogeneous structures such as bubbles or particle clusters with different shapes and sizes. Due to these inhomogeneous characteristics, the gas-solid drag model has become one of the key challenges in the simulation of gas-solid flows. Various forms of constitutive relations for solid stress models and gas-solid drag models have been reported in the literature. In this paper, we reviewed the solid stress models crossing various flow regimes and drag models in both micro- and mesoscales, which provide a useful reference for model selection in simulating gas-solid flows.

Probing primordial black holes with anisotropies in stochastic gravitational-wave background

Primordial black holes, if considered to constitute a significant fraction of cold dark matter, trace the inhomogeneous large-scale structure of the Universe. Consequently, the stochastic gravitational-wave background, originating from incoherent superposition of unresolved signals emitted by primordial black hole binaries, is expected to display anisotropies across the sky. In this work, we investigate the angular correlations of such anisotropies for the first time and demonstrate their difference from the analogous signal produced by astrophysical black hole binaries. We carefully evaluate the associated uncertainties due to shot-noise and cosmic variance, and demonstrate that the studied signal in the low-frequency regime can be differentiated from the signal of astrophysical origin. Our results are particularly promising in the stellar mass-range, where the identification of the merger origin has been particularly challenging.

Stochastic (re)constructions of non-stationary material structures: Using ensemble averaged correlation functions and non-uniform phase distributions

Depending on the spatial distribution of mass or phases within the studied materials, they can be classified into two distinct property types: stationary and non-stationary. Stochastic reconstruction with simulated annealing and correlation functions is a powerful technique to produce material structural images from limited input data, but they usually deal with statistically homogeneous images only. In this work we show that the alteration of the initial phase distributions before starting annealing can produce non-stationary structures. The degree of such non-stationarity depends on numerous parameters of the annealing, including maximum correlation length, the choice of correlation function weights, annealing cooling schedule, in a convoluted manner—this makes it impossible to segregate the influence of each parameter. In general, we have shown that it is possible to choose such parameters in order to suppress the inhomogeneity of the resulting replicas. The degree of stationarity of the stochastic reconstructions had a significant influence on the physical properties of the reconstructed binary structures—computed full permeability tensors showed different degree of anisotropy and off-diagonal terms values depending on the reconstruction parameters. The proposed approach to produce non-stationary structures from ensemble averaged set of correlation functions opens numerous ways to attack theoretical and practical problems with natural and artificial materials with statistically inhomogeneous structure.

SIMULATION OF INTERIM FORGING OF DEPOSITED PRODUCTS IN ANSYS MECHANICAL APDL (IMPLICIT ANALYSIS)

The paper considers the problem of simulating the interim forging during additive manufac-turing. Wire-arc additive technologies are associated with formation of technological residual stress fields, porosity, inhomogeneous structure and anisotropy, as well as unwanted defects, such as cracks, delamination or warping of the part. Interlayer hardening via forging both com-pensates such disadvantages and improves the mechanical properties of structures. Mathemat-ical modeling is one of main methods of studying these processes. There are a lot of publica-tions on modeling the formation of fields of residual stresses and heat shrinkage deformations in products obtained using additive technologies, including the method of wire welding. The work aims at checking the adequacy of using ANSYS Mechanical APDL for numerical modeling of interim metal forming processes. In this work, the Johnson – Cook viscoplastic model from Ex-plicit Dynamics was adapted to the capabilities of ANSYS Mechanical APDL for three materials: Amg6, 12X18H10T, VT6. As a physical model in ANSYS Mechanical APDL, a multilinear iso-tropic MISO plasticity model is chosen, which, unlike the Johnson – Cook model, does not take into account the effect of strain rate on the elastic-plastic behavior of the material. The values of the material constants for the MISO model are identified. The adequacy of replacing the non-stationary statement with a quasi-static one is proved, due to a slight loss of accuracy. A three-dimensional model of bar forging for three types of materials was built and implemented, its identification and verification were carried out by comparing with the results of a full-scale exper-iment. A good agreement between the calculated data and experiment is shown. Based on the data obtained, a conclusion was made about the admissibility of using the implicit solver ANSYS Mechanical APDL for calculating the processes of interim forging of the deposited products with acceptable accuracy.

The method of fictitious discrete models in calculations of composite bodies

As is known, the calculation of the static strength of elastic composite bodies (CB) is reduced to finding the maximum equivalent stresses for these bodies. The finite element method (FEM) is widely used for the analysis of the stress state of CB. The basic discrete models (BM), which take into account the inhomogeneous structure of bodies in the framework of a micro-approach, have a high dimension. To reduce the dimension of discrete models, multigrid finite elements (MgFE) are effectively used. However, there are BM CB (for example, BM bodies with a micro-homogeneous structure), which have such a high dimension that the implementation of FEM for such BM using MgFE, due to limited computer resources, is difficult. To solve this problem, it is proposed to use fictitious discrete models whose dimensions are less than the dimension of the BM CB. In this paper, we propose a method of fictitious discrete models (MFDM) for calculating the strength of elastic bodies with an inhomogeneous, micro-homogeneous regular structure. The proposed method is implemented using FEM with the use of MgFE and adjusted strength conditions that take into account the error of approximate solutions. The method is based on the position that the solutions that meet the BM CB differ little from the exact ones. The calculation of CB according to MFDM is reduced to the construction and calculation of the strength of fictitious discrete models (FM), which have the following properties. The FM reflects: the shape, characteristic dimensions, attachment, loading and type of the inhomogeneous structure of the CB, and the distribution of elastic modulus corresponding to the BM CB. The FM dimension is less than the BM dimension of the CB. The sequence consisting of FM converges to BM, i.e. the limiting FM coincides with BM. The convergence of such a sequence ensures uniform convergence of the maximum equivalent voltages of the FM to the maximum equivalent voltage of the BM. Two types of FM are considered. The first type of FM consists of scaled discrete models, the second type consists of FM with variable characteristic dimensions. Calculations show that the implementation of FEM for FM using MgFE leads to a large saving of computer resources, which allows the use of MFDM for bodies with a micro-homogeneous regular structure. The calculation of the strength of CB according to MFDM requires times less computer memory than a similar calculation using BM CB, and does not contain a procedure for grinding BM. The use of adjusted strength conditions allows us to use approximate solutions with a large error in the calculations of CB for strength, which leads to an increase in the efficiency of MFDM. The given example of calculating the strength of a beam with an inhomogeneous regular fibrous structure according to MFDM shows its high efficiency.