Papers by Keyword: Pores

Abstract: To prepare bulk single-crystal REBCO superconductors by the new single-direction growth method (SDMG: Single-Direction Melt-Growth), it is necessary to produce a large-area seed of high quality, for example, based on EuBCO. Since the samples prepared by the SDMG method copy the structure of the seed, for the production of large-area seeds it is necessary to optimize the time-temperature regime in order to grow seeds with a suitable structure and composition and minimize structural defects (limiting the amount of subgrains and others). A higher growth rate was used in comparison with the standard growth rates used to produce EuBCO seeds of larger dimensions. The increased growth rate in the crystal growth window reduces the outflow of the melt from the sample, and thus it is possible to achieve a single-crystal sample in the entire volume of the precursor. The samples were produced at different growth rates: 1; 2; 3 and 5 °C/h. The microstructure of the samples was studied by polarized light microscopy and scanning electron microscopy. The size and distribution of Eu211 particles in the sample volume and the subgrain structure were studied on the fabricated samples.

Abstract: In current concrete practice, several types of air entraining admixtures are used. These admixtures aim to improve the durability of concrete in aggressive exposure conditions caused by negative temperatures in combination with salts. The current article attempts to describe the types actually used, namely, synthetic tensides and natural resins and to also compare their effect on the durability of physical and mechanical parameters. With these admixtures, air entrained concrete with different amounts of air content in the concrete are designed. Furthermore, the article describes methods for determining the parameters of air entrained concrete, as well as their advantages, disadvantages and their evaluation.

Abstract: Hydrogel spheres made of sodium polyacrylate with free absorption of water can increase in volume by 100-1000 times due to the violation of the balance of forces of attraction and repulsion between the links of the gel. The addition of water will shift the balance in favor of the repulsive forces. This property will allow them to be used in cement systems as an active expanding aggregate, which will compact the structure, reduce the water-cement ratio during the formation of the cement paste structure, and then give water to the hardening cement stone. The hydrogel sphere itself will decrease in volume to its initial state and will leave the pore in its place. The resulting material can be considered aerated concrete with compacted inter-pore partitions. The advantage of this method over the traditional method of self-compacting masses is that there is no need to supply thermal energy to activate the expansion of the active aggregate, for example, by passing an electric current through the hardening mass. In this article, the properties of the hydrogel are investigated, the dynamics of the expansion of hydrogel spheres in water at different temperatures is determined.

Abstract: New porous films based on polyanionic cellulose with AlOOH nanoparticles have been developed. The morphology of the films has been studied by electron microscopy: the size of the formed pores is 1000-500 microns; the total surface porosity of the films is 30%. Using infrared microscopy, it was shown that during the formation of porous films, their chemical composition remains unchanged. Differential scanning calorimetry was used to determine the threshold for thermal destruction of porous films: 306 С. The possibility of using the obtained materials as antifriction coatings when filling the pores with solid lubricant MoS₂ is considered. It is shown that for a steel sample protected by a porous coating with MoS₂, the friction coefficient decreases by 50% compared to the friction coefficient for a steel surface under a load of up to 450 MPa.

Abstract: The article presents the results of the impact of laser welding parameters on defect occurrence in the welded seam of the Ni-based heat-resistant alloy Kh45VMTYuBR applied for the manufacture of gas-turbine engines. On the basis of the research the authors conduct the analysis of dimensions as well as the number of pores and micro-cracks in the welded seam. The paper provides the recommendations on the selection of the laser welding mode for the heat-resistant alloy to reduce defect occurrence in a welding seam.

Abstract: The goal of this work was to investigate effect of heating time during welding on mechanical properties of High Performance Polyamide thermoplastic composite material PPAGF40. Studies showed that welded joints with shorter heating time from 6.1 s to 6.7 s had a higher amount of pores and burn mark defects compared to the welded joints with longer heating time. Cross-section analysis of welded samples revealed tendency, that samples with higher heating time from 7.4 s to 11.5 s possessed more homogenous weld seam and higher resistance to burst pressure, respectively from 5.1 bar to 7.5 bar, while the resistance to pressure values of shortest heating time range, respectively are varying from 3.3 bar to 3.9 bar. The weld seam defects like pores and burn marks weakened the welded joint. 470 W of laser power energy input influenced fast melt-down of a cover and a housing, which caused material partial evaporation and later on formation of pores inside weld seam. By using lower 450 W of laser power and scanning speed of 1900 – 1700 mm/s, we can observe more homogenous weld seam. The cross-section of the samples were examined by using optical microscope. Mechanical resistance to pressure was performed in order to evaluate the performance of welded joints of different heating time of PPAGF40.

Abstract: A novel method to obtain porous three-dimensional chitosan-based matrices has been developed. The structure is characterized by an interconnected system of pores, with controlled diameter by varying the concentration of chitosan and the nature of the solvent. The material is biodegradable, biocompatible, with high adhesion to fibroblasts and promotes its proliferation.

Abstract: Theoretical principles of the formation of highly organized porous structure of cellular concrete are developed, based on model concepts of the dynamics of the expanding gas cavity in the liquid phase as a single control cell. The peculiarities of controlling the formation of cellular structure of aerated concrete based on the balance of forces in a three-phase disperse system on the model "gas pore - molding mixture" are revealed and a coalescing-aggregate scheme for porosity formation of the aerated concrete mixture is proposed. It is shown that, in accordance with the refined Rayleigh-Plesset equation, the determining factor in the formation of the cellular structure of aerated concrete is the pressure over the mixture to be poroused, the effect of the porosity being achieved by reducing the external pressure to the vacuum level. The division of pores by size in anaerated concrete mixture is proposed. The maximum pore size is determined by the capillary Laplace constant. The prospects of aerated concrete technology are associated with a decrease in the maximum and average size of cellular pores, as well as methods for eliminating pores of air entrainment and segmented pores. Reducing of the size of the pores will be reflected in the decrease of the Bond quantity and in the increase of the importance of capillary forces in the formation of the porous structure of aerated concrete. The concepts of the types of cellular structures are developed, depending on the average density and their boundaries for cellular concrete are established.

Abstract: The microstructure and pores of the consolidated powder materials are important to determine the final properties of the pieces produced. Therefore, the control of their volume fractions and morphologies represent actual technological challenges. Besides the quantification of the phase and pores volume fractions there are important features related with the morphological aspects and the phase boundary structure that reveals the mechanisms of properties improvements, which are crucial for development of materials obtained by powder metallurgy techniques. In this paper, stereological measurements of pores present in the material stainless steel (316L) are obtained by combining serial sectioning technique and optical microscopy (OM). Dilatometric experiments were carried out in order to simulate heat treatment for sintering the material. The 3D microstructures are obtained by images reconstruction using numerical pixels treatment and stereological results of the porosities are presented.

Abstract: The Nd:YAG laser welding process of AZ31B alloys was performed by using the six-axis robot in this work. The microstructure characterization of AZ31B auto-welded joints was studied by using scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The laser welding process resulted in the formation of equiaxed grains in the center of the fusion zone (FZ) and columnar grains near the FZ boundary, meanwhile some eutectic β-Mg17Al12 particles were observed in the microstructure. No clear heat affected zone (HAZ) was observed in the welded AZ31B joint. Furthermore, some pores were observed in the base material (BM) and FZ.