• Magnetism in 2D van der Waals halide VI3

    pg(s) 84-85

    Magnetic van der Waals (vdW) materials composed of two-dimensional (2D) layers bonded to one another through weak interactions exhibit promising potentials for high-tech magnetic, magneto-electric, and magneto-optic applications in nanostructures. Due to their intrinsic magnetocrystalline anisotropy, several vdW magnets could be thinned down to nanoscale thickness, while still maintaining magnetism. Prominent examples of such materials are transition metal trihalides, in particular CrI3, a first atomically thin ferromagnet, realized in 2017.
    Recently, VI3 has been found to belong among 2D ferromagnets at temperatures below 50K. It is a semiconductor undergoing a subtle structural phase transition at 78K. Furthermore, its magnetic anisotropy exhibits rather unusual features. We have studied its properties by first principles calculations and reproduced the unusual magnetic anisotropy. Its properties have been linked to lattice distortions present at some of its low temperature phases.

  • Assessment of toxic gas emissions from flammable building thermal insulation materials upon fire conditions

    pg(s) 79-83

    Smoke generated during a fire is a serious danger to people as a result of reduced air visibility, high temperature, toxicity and reduced oxygen content. In regard to victims, the inhalation of a mixture of toxic gases, which are formed as products of combustion, plays a key role. In the conditions of modern construction, and in order to improve energy efficiency in the buildings’ construction, various insulating materials are used. Along with the advantages of good insulation, there are also dangers from the behavior of insulating materials in an event of fire. The purpose of the present study is to assess the hazards and to study the composition of the gas mixture released during the combustion of various thermal insulating materials: glass wool, stone wool; expanded polystyrene; extruded polystyrene.

  • Post welding residual stress in P91 alloy steel

    pg(s) 73-78

    The chromium-molybdenum steel pipe P91 is widely used in the power plants construction, because they can withstand higher temperatures and operating pressures, in order to increase operating efficiency and output. This material has a complex microstructure which is extremely vulnerable to convert into Austenite form near the temperature for heat treatment. During the welding procedure should be very carefully, because otherwise the residual stress installed post welding will reach high values. A series of experimental studies was carried out, which has shown the development in the magnitude of transverse and longitudinal residual stresses and the values in both directions were determined. For but welded joints the residual stresses in the three direction were measured and the results are reported here. For measuring was used the hole drilling method through “measuring balls” or electrical strain gauge. Except the experiments the finite element method was used to determinate the value and the distribution of the residual stresses. The experimental and FEM results were compared between them and no discrepancy was found.

  • Investigation of solders surface tension in temperature range from melting point up to 673К

    pg(s) 69-72

    Solders are fusible metal alloys, used in industry to create permanent bond between metal surfaces. In order to achieve this, solders need to be heated above their melting point and used in liquid phase. Low melting temperature is essential from technological point of view, as well as for soldered components safety. Typical solders have Lead (Pb) аs а base component, having melting temperature of 600.6 K. Adding up to 60% of Tin (Sn) to the alloy, reduces melting temperature down to 456-461 K in average. Since 2006, RoHS regulation enforce industrial use of Lead-Free solders, typically having much higher melting temperature. However, Pb:Sn solders with up to 40% Sn still have their industrial applications, usually used for soldering Cu and Zn coated pipes while the 60% Sn containing solders are used for soldering of
    electrical cables.
    The achieving of these goals requires knowledge of liquid phase surface tension. Because of the difficulties, related to such measurement, the available data in literature are limited. The current report presents an equipment for measurement of the surface tension, based on the Wilhelmy plate method as well as the applied measurement procedure. The Wilhelmy method has relatively good stability. The contact angle also can be considered zero for the examined samples therefore correction coefficients are not required for the measurement. The results from measurement of Pb:Sn in ratio 40:60 and also in ratio 60:40 show that the oxidation in excess of Pb lowers the surface tension while for the case with excess of Sn, the oxidation lead to increase of the surface tension if compare with the surface tension obtained for the same solders but under non-oxidizing conditions. This could be attributed to formation in excess of PbO and SnO on the liquid surface of the corresponding solder

  • Preparation and characterization of BaTi0.89 Sn0.11O3 and Ba0.89Sn0.11TiO3

    pg(s) 67-68

    In recent years, interest in the application and use of materials for supercapacitors for electric vehicles has grown significantly. The advantage of capacitor ceramics over other dielectric materials for producing supercapacitors is its environmental friendliness and high economic efficiency. This paper presents the results of a study of capacitor ceramics doped with Sn at different locations of the modifier (tin) in BaSnTiO3 and BaTiSnO3 crystal lattice. The influence of modifiers in low-temperature sol-gel synthesis was studied. The samples were annealed at 1000°C. The resulting phases were identified by X-ray phase analysis. Microscopic analysis was also performed.

  • High-temperature layered composite with a metal matrix, reinforced with single-crystal sapphire fibers

    pg(s) 62-66

    Heat-resistant composites with a layered niobium-based matrix reinforced with single-crystal sapphire fibers have been obtained. The fibers were grown from the melt by the modified Stepanov method. The composites were prepared by solid-phase technology by diffusion welding of multilayer packages of foils of the Nb–0.1% C and aluminum alloy, interlaid with sapphire fibers in layers of a suspension mixture of Nb powder. The production of fibers, their structure and strength testing procedure are described. The formation of multilayer packets, the structure and results of bending strength tests of composites are presented

  • Polypropylene-Polycarbonate composites with graphene oxide nanosheets: synthesis and characterization

    pg(s) 56-61

    Herein the effect of graphene oxide nanosheets (GOSs) on the thermo-mechanical stabilities of polypropylene (PP)/polycarbonate (PC), polymer blend nanocomposites (PNCs) fabricated via melt compounding has been explored. The comparison of properties of pure blend with PNCs, pointed out whether the mixing sequences have any effect on thermo-mechanical stabilities of PNCs. The PP/PC/GO nanocomposite exhibited an excellent increment in tensile strength and melting temperature as compared to the neat blend. In contrast, the PC/GO/PP nanocomposite, prepared by mixing the GOSs with PC first and then PC/GO with PP, has lower thermo-mechanical stability than PP/PC/GO. Interestingly, PP/GO/PC nanocomposite prepared by mixing the GOSs with PP first and then PP/GO with PC, showed the highest improvement in thermo-mechanical stability as compared to pure blend, PP/PC/GO, and PC/GO/PP. We attribute this trend of stabilities for PNCs due to the different extent of GOSs distributions within the polymer matrix. For PP/PC/GO, the GOSs were effectively dispersed in the PP phase due to low viscosity. In the case of PC/GO/PP, only small amount of GOSs was localized in PP and most of GOSs remain in PC phase. However, in case of PP/GO/PC, almost all GOSs were dispersed homogeneously in both phases resulting in a distinct increment of thermo-mechanical stabilities.

  • Preparation and study of new class SeO2 based glass obtained at high oxygen pressure

    pg(s) 38-41

    In recent years, scientists have turned their attention to the synthesis of a new class of amorphous materials – glass with the participation of SeO2 (selenite glass), and to the study of their structure. Among the established centers for systematic research on these materials are the Institute of Metal Science, Equipment and Technologies with Center for Hydro- and Aerodynamics “Acad. A. Balevski” at the Bulgarian Academy of Sciences (IMSETHC – BAS) – Sofia, Bulgaria and the Department of Silicate Technology at the University of Chemical Technology and Metallurgy (UCTM) – Sofia, Bulgaria. These types of glass have specific properties such as high permeability in the visible and the near infrared regions of the spectrum, high refractive index, and low melting point. On the other hand, the amorphous materials in this class of can serve as model compositions. The obtained structural information can be used to form generalizations about the vitreous state and the possibility to modify the structure and properties of glass in order to improve the technological parameters.

  • Influence of the pressing force on the strength properties of sintered materials based on water-dispersed iron powders alloyed with copper

    pg(s) 34-37

    In this publication we study the influence of the pressing force on the strength characteristics of sintered materials based on water – dispersed iron powders alloyed with copper. Three brands of iron powders were studied – AHC 100.29, ASC 100.29 and ABC 100.30, to which 2 and 4% electrolytic copper were added. After pressing with a force of 300 ÷ 800 MPa, they were sintered at a temperature of 1150 ° C for 1 h. Experiments were performed to determine three strength characteristics – tensile strength, yield strength, elongation. In determining the mechanical characteristics in order to prevent the influence of porosity, five measurements were made for each type of samples, and in the graphical interpretation of the results the arithmetic mean values were used.

  • Modeling of the process of ultrasonic flaw detection of materials with internal microdefects of different origin

    pg(s) 32-33

    Miniaturization of technical products is one of the driving forces for the development of high-tech systems. The article considers the most well-known method of determining microdefects of microwelded joints – the method of ultrasonic flaw detection. It is known that microdefects weaken the cross-section of the micro-seam, reduce its strength and are stress concentrators. A dynamic computer model of ultrasound propagation in microwelded welds of metal containing defects of micrometric size of various shapes filled with gas (most often, air) was created. For ultrasonic control, oscillations with a frequency of 0.5 MHz and a direct transducer emitting longitudinal waves perpendicular to the contact surface into the welded joint were used. On the basis of results of computer modeling the regularity of sensitivity of measurement of microdefects from parameters of the ultrasonic microdefectoscope which with a high degree correlates with experimentally received data is revealed.

  • Glass forming ability and crystallization behaviour of amorphous and nanosized rapidly solidified (Al75Cu17Mg8)100-xZnx alloys

    pg(s) 366-369

    The rapidly solidified (Al75Cu17Mg8)100-xZnx( x=0,1,2,3 at. %) alloys were obtained by melting in an induction furnace and then rapidly quenched by the planar flow casting (PFC) method in instalations, created at the IMSTCA-BAS. By TEM and X-ray analises were obtained data, that the microstructure of the alloys is an amorphous matrix with nanosized particles with dimensions 16÷.90 nm. The obtained amorphous alloys have relatively good glass forming ability. With increasing content of Zn the amount of the amorphous phase and glass transition temperature Tg also increase

  • Structure and composition of innovative porous composite material

    pg(s) 362-365

    The subject of this article is a study on the composition and structure of innovative Porous Composite Material (PCM). The latter is obtained from powders of glass waste and heat-treated rice husks. The PCM is non-flammable, non-combustible and has high heat and sound insulation properties. The structural elements made of it are lighter and stronger than the ones made of foam glass. It is expected that PCM is to find wide application in construction and as an absorbent of heavy metals and oil from polluted water