• Hybrid reinforced concrete with controlled volume deformations for hydrotechnical facilities

    pg(s) 224-227

    The development of hydration processes in cement concrete is associated with the release of significant amounts of heat, which leads to a significant exothermic temperature increasing in the concrete body, especially valid for massive structures. This creates a temperature gradient from the inside of the array to the surface. The latter may be re lated to the development of unacceptable internal stresses, often leading to cracking, defects and reducing the durability of the facility. The report presents the individual stages of the search for an optimal technical solution for the construction of a specific hydro-technical facility – a massive hybrid reinforced trapezoidal wall of a water catchment gorge, in the area of Stara Zagora town. In the context of specified geometric dimensions of the facility, the specific features of the exothermic increase of the temperature in the concrete body as a function of the hydration processes are discussed. A reasonable choice of a specific type of cement is proposed in order to limit the amount of heat released – slag cement CEM III-A 42.5 N with a specific heat of hydration up to 280 J/g. A specific concrete mix design has been proposed, providing a balanced increase in temperature within acceptable limits, preventing cracking, in two possible scenarios – winter and summer outdoor temperature conditions. An additional advantage of the mix is the inclusion in the recipe of fiber-reinforcement and high-tech chemical admixtures – deep internal crystallization and shrinkage-compensating one. Specific calculation data for the kinetics of temperature increasing are presented, and the obtained values are critically evaluated in terms of guaranteed cracks eliminating in the structure.

  • Monitoring the change of corrosion resistance of HCT600X+Z galvanized steel after plastic strain

    pg(s) 221-223

    This work deals with corrosion measurements of galvanized steel HCT600X+Z subjected to progressively increasing plastic strain. The corrosion resistance measurements were carried out by non-destructive EIS (electrochemical impedance spectroscopy) test and destructive LP (linear polarization) test. As a result of the work, the measured data were analyzed and the results were processed using Nyquist diagram and Tafel analysis.

  • Study of age hardened MS1 material after the abrasive water jet application

    pg(s) 217-220

    This contribution deals with the study of cut surface after the abrasive water jet application on the material Maraging Steel MS-1, prepared in the form of 3D printing method Direct Metal Laser Sintering. The aim of the study is to point out the morphology of the cut plane under the use of various technological parameters, like feed rate of machining and abrasive mass flow at the constant cut pressure. For the track morphology monitoring after the abrasive water jet application, scanning electron microscope SEM MIRA 3, f. Tescan, was used. For the identification of observed particles stabbed in the cut track, chemical composition EDX analysis was used.

  • Determination of the quality of renovation layers in tribological conditions

    pg(s) 213-216

    Molds designed for high-pressure casting of aluminum are exposed to very intense thermal, mechanical but also chemical stress during their operation. This stress leads to a synergistic effect of a combination of high-temperature corrosion processes in molten metals, under real conditions associated with mechanical wear. High-temperature corrosion in the environment of liquid metals occurs in the foundry industry, when casting molten metal most often into steel molds. Repair of worn parts of molds by welding, which can be performed even after their irreversible surface degradation, is a very efficient, cost-effective and envi-ronmentally acceptable form of their maintenance, while the chemical and physical properties are welded layers if they exceed the properties of the original material.

  • Foam ceramic blocks with low thermal conductivity suitable for the construction of roads and urban square pavements and non-load-bearing partition walls

    pg(s) 186-188

    Lightweight porous ceramic has a number of advantages over the tightly sintered one depending on the field of application. As a building material with low thermal conductivity and good sound insulation properties, foam ceramic is a suitable material for the production of thermal and sound insulation panels, partition walls, as well as for layers laid under asphalt to protect road and urban square pavements from freezing. As a filling, foamed ceramic materials are also added during the production of lightweight concrete used in construction and architecture and in the manufacture of decorative and non-structural insulation elements. In the present study we consider the characteristics of marl clay, found around the village of Lovets near the town of Shumen, with coal as a foaming additive, and the technological regulation for the manufacture of foam ceramic blocks.

  • Pattern steel: production technology and fracture surface evaluation

    pg(s) 183-185

    Presented article deals with the description of the technology of pattern steel production – forging welding, evaluation of fracture surfaces and mechanical tests of examined samples (resulting steel and its individual components). The damascus steel (pattern steel) was prepared by forge welding from a packet consist of a few hundred layers. Marbling was created by folding layers of 2 components: hard and tough steel of different thickness. For evaluation of mechanical properties was applied: Charpy impact test. The evaluation of the fracture surfaces of individual samples was performed by scanning electron microscopy (SEM). The values of mechanical properties of pattern steel themselves are in the range of values of mechanical properties of individual components. The fracture surfaces of the materials have a characteristic relief (in relation to the mechanical properties of the individual components).

  • Properties of polymer composites with long fibers

    pg(s) 179-182

    The paper is focused on composite materials with a polymer thermosetting matrix reinforced with long (continuous) fibers. These materials offer their specific properties that can be used in various areas of industry. From this point of view, polymer mat rix composites are, due to their mechanical properties and low weight, the most promising material for use in various application solutions. However, their disadvantage is the high price, which is higher than the price of conventionally used materials. The method of production of components from composite materials itself requires precisely specified technological procedures, which differs slightly from the traditionally used methods of component production, e.g. of metal materials.

  • Strain controlled fatigue of 40X steel

    pg(s) 146-149

    The fatigue strength of 40X steel at 25 oC was determined following ASTM E606M standard for strain-controlled tests. The studied experimental dependence was modelled with a power function as well as with the functions and coefficients described in the standard. The material expresses plastic properties for cyclic loadings above 0.2% strain amplitude.

  • Mesoporous transition metal oxide composites for effective remediation of air from VOCs

    pg(s) 142-145

    This study is aimed at the preparation of mesoporous Cu-Ti-M oxide composites (M=Ce, Zr, Sn) by combination of template assisted hydrothermal technique with the conventional impregnation or novel “chemisorption -hydrolysis” techniques. The obtained materials are characterized by nitrogen physisorption, XRD, SEM, FTIR, HRTEM, XPS, TPR and tested as catalysts for ethyl acetate oxidation, as a probe VOCs molecule. The regulation of the microstructure, surface and redox properties and the related catalytic behavior of the composites by changing of the metal oxide dopant and preparation technique used is discussed in details.

  • Investigation of Heat-Treated Steels Using the Magnetic Noise Method

    pg(s) 138-141

    This paper presents the results of an investigation conducted on samples of high-quality heat-treated carbon steel using the magnetic noise method. The objective of the work was to find the proper informative parameters for identifying the obtained s tructures with different hardness. To this end, samples were prepared using quenching and tempering heat treatment at different temperatures , from 150°C to 500°C. After microstructural analysis and hardness measurements, the samples were investigated using the magnetic noise method. The signals were visualized and analyzed using a digital oscilloscope and the Multi Instrument 3.8 software program. The RMS informative parameters and the registration time of the magnetic noise signals were used to identify the obtained microstructures with different characteristics and hardness.

  • Monolayer FeSe Superconductor on Si(001): Electronic Structure Calculations

    pg(s) 111-113

    Iron-based unconventional superconductors have attracted intense interest after the critical temperature of FeSe was enhanced by more than one order of magnitude. This was achieved for FeSe placed on top of an insulating oxide substrate with its thickness reduced to the nanometer limit. There are numerous indications of the critical importance of specific features of the FeSe electronic structure in the vicinity of the Fermi surface. Here, we explore how the FeSe band topology changes when located on a Si(001) surface, by first-principles calculations based on the density functional theory. We determine which interface arrangement is preferred and what is the optimal distance between FeSe and Si. Our calculations reveal interesting effects of Si proximity on the FeSe band structure. Bands corresponding to hole pockets at the Γ point in NM FeSe are generally pushed down below the Fermi level. We explain these changes by a redistribution of electrons between different Fe orbitals rather than charge transfer to/from Si.

  • Temperature regime in obtaining BaTiO3-BaSnO3 system and study of dielectric properties

    pg(s) 108-110

    A system based on BaTiO3 – BaSnO3 (BT-BS) has been developed in search of highly efficient lead-free piezoelectric ceramics. The powder samples were mixed and homogenized in a ratio of 50%: 50% and 15%: 85%. From the compositions thus obtained, tablets with the presence of a PVA plasticizer were formed, which were further annealed in a furnace with a corundum backfill. The temperature regime is in two steps. Drying time up to 800oC with a delay of 1h 30min. and additional heating to 1150°C with one hour delay. The results were characterized by XRD analysis, the relative dielectric constant of the two samples was determined.