Browsing by Author "Atanasova S."

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    Depth Dependence of Friction in Self-Lubricating Cu-Based Materials for Highly Demanding Applications
    (2025-09-01) Tabakova P.; Petrova A.P.; Kolev H.; Hodjaoglu F.; Andreeva R.; Zahariev I.; Avdeev G.; Atanasova S.; Grigorov K.
    The selection of materials with high mechanical and tribological properties defines current trends in modern materials science. In this paper we investigate the use of alloying elements, such as phosphorus (P), manganese (Mn), tin (Sn), antimony (Sb) and lead (Pb), in order to improve tribological characteristics in Cu-based self-lubricating materials. Using structural (XRD, OM) and chemical (EDX, XRF, XPS) analyses we observed distinctive elemental segregation and diffusion towards the surface under mechanical pressure, driven by elevated temperatures. Additionally, we perform GDOES in order to prove the dynamics of element migration towards the surface under working conditions. The latter is very important, because new phases, such as Pb0.8Sb0.2, SnPb2O4, Pb2O, Cu3P, etc. are formed and located near the surface, which improves the tribology in situ. The role of these phases is diverse and differentiated, such as directly lowering the friction coefficient (FC) and as a consequence lowering the temperature, which under operating conditions is over 700 °C. Alloying elements also lead to improved hardness. It was found that the friction coefficient in vacuum dropped as much as 40 times and the wear at load 40 N dropped 4 times in compared the pure copper sample.
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    Development and validation of HPLC-DAD methodology for simultaneous qualitative and quantitative determination of thirteen substances with a steroid structure
    (2023-01-01) Zaharieva Z.; Atanasova S.; Danalev D.; Jaber S.; Foteva T.; Nemska V.; Tanev D.; Georgieva N.
    Using supplements in different sports is a common practice for many athletes. Unfortunately, the growth of this market has entailed some speculations, such as the addition of excessive doses of potentially toxic ingredients. Sometimes the doses listed on the package do not correspond to the real content. Some nutritional supplements on sale may contain undeclared ingredients. Some of those supplements are prohibited substances according to different regulations. Herein, a simple HPLC/DAD procedure that is easy to apply in conventional laboratory practice was developed for the simultaneous determination of 13 substances with steroid structure in nutritional supplements for sport: testosterone, testosterone propionate, testosterone enanthate, methyltestosterone, nandrolone, nandrolone propionate, nandrolone decanoate, methandienone, androstenedione, trenbolone, trenbolone acetate, trenbolone enanthate and boldenone undecylenate. The methodology includes gradient elution with mobile phase A: MeOH:ddH2O (55:45) and mobile phase B: 100% MeOH in a standard HPLC system containing a Halo 90 Å, C18 (150 x 4.6 mm, 2.7 µm) column, flow rate 0.6 mL/min, UV wavelength of 254 nm, temperature of 40 °C and 20 μL injection volume. The developed methodology was validated according to the corresponding official documents. The key parameters used for the selection of the optimal HPLC conditions were the ability of the mobile phase and solvents to be used with both an HPLC/MS and a GC/MS chromatographic system. The obtained total run time, the reproducibility of the retention times, the separation of all peaks and peak characteristics meet all requirements.