Browsing by Author "Ivanova V."
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Item Assessment of Physical-Chemical Characteristics of Surface Water from Key Sites of the Mesta River: State and Environmental Implications(2018-06-12) Georgieva S.; Gartsiyanova K.; Ivanova V.; Vladimirova L.The anthropogenic source pollution of the Mesta River was assessed during the period 2011 and 2016 in terms of pH, conductivity, chemical oxygen demand (COD), anions and heavy metals in key sites of the Mesta river pointed as: S1 (the Mesta river before the Iztok river); S2 (the Iztok river before the Mesta river); S3 - the river Mesta after the Iztok river); S4 - the river Mesta at Momina Klisura, near Bukovo) and S5 - the river Mesta before the Greece border, after the Matnitsa river. The application of multivariate clustur analysis (CA) for the interpretation of a large and complex data matrix obtained during a monitoring program of surface water in Mesta river is presented in this study. The dataset consists of analytical results from a 6-yrs survey conducted in selected points of the river system. The physical-chemical characterization in the water samples were made in accordance with the Directive 2000/60/EU-Water Framework Directive (WFD) and its equivalent criteria transposed into the Water Law (WL) in Bulgaria. In water, concentrations (mg L-1) during 2016 of NO2 - (0.006 to 0.052), NO3 -(0.01 to 1.33) and total contents in μg L-1 of Cu (<0.002), Pb (<0.003), Co(<0.002), Ni(<0.003), S(<0.050) and Zn(<0.02), pH (5.60 to 8.00), and electrical conductivity (0.12 to 48.60 mS.cm-1) were agreed with environmental standards except cadmium Cd (>0.15 μg L-1) and PO4 3-(0.15 mg L-1). During analyzed period the cadmium concentration was much higher than recommended limit only. In this sense, it was possible to demonstrate relatively good quality of river water even with numbers of industrial and touristic activities in the analyzed area but also to consider a future concept on cadmium sources and their eliminated.Item COMPARATIVE EVALUATION OF METHODS FOR QUANTIFICATION OF MERCURY AT TRACE LEVEL IN AQUATIC BIOTA SAMPLES AS A BIO-INDICATOR(2022-01-01) Ivanova V.; Oster C.; Surleva A.; Fisicaro P.This study presented detailed evaluation of analytical characteristics of four advanced methods for total mercury quantification in aquatic biota samples. The method comparison as a validation strategy to ensure reliable and comparable results according to the ISO 17025 was presented. An analytical performance of the advanced mercury analyzer was compared with isotope dilution mass spectrometric measurements. Certified reference materials for mussel, oyster and scallop tissue were used during methods evaluation. The methods were then applied to unknown samples, such as Gammarus and Sea Bass samples. The highest values obtained for total mercury content in these samples was below maximum permissible limits: (0.222 ± 0.030) and (0.724 ± 0.022) mg kg-1. This comparative study could be of interest to laboratories for environmental monitoring or quality control of fishery products, as well as to other industriesItem Design of a Multifunctional Operating Station Based on Augmented Reality (MOSAR)(2021-03-01) Ivanova V.; Vasilev P.; Stoianov I.; Andreev R.; Boneva A.Design principles of a novel Multifunctional Operation Station (MOS) using Augmented Reality (AR) technology (MOSAR) are proposed in this paper. AR-based design allows more ergonomic remote instrument control in real time in contrast to classical instrument-centered interfaces. Another advantage is its hierarchical software structure including multiple programming interpreters. The MOSAR approach is illustrated with a remote surgical operating station that controls intelligent surgical instruments. The implementation of the Operation Station (MOS) is based on the multiplatform open-source library Tcl/Tk, and an AR extension has been developed on a Unity platform, using Vuforia SDK.Item Effect of expanded vermiculite and expanded perlite as pore forming additives on the physical properties and thermal conductivity of porous clay bricks(2018-01-01) Georgiev A.; Yoleva A.; Djambazov S.; Dimitrov D.; Ivanova V.The way to achieve better energy efficiency in buildings is to improve their thermal insulation properties. This can be done by increasing the porosity of the clay bricks by introducing pore-forming additives to the clay or production of perforated bricks. In general, thermal conductivity of the bulk clay bricks is about 1.0 W/mK depending on their raw materials, processing, firing temperatures and fired densities. These values can be reduced by addition of various inorganic and organic pore-forming agents into the brick raw material mixtures before firing. This paper presents the effect of adding of expanded vermiculite and expanded perlite in quantity of 0, 3, 5 and 8 mass % on the physical properties and thermal conductivity of clay porous bricks fired at 900oC for 2 hours. Expanded vermiculite and expanded perlite were selected as pore former as they have very low density and good thermal insulation properties. The results show an increasing of water absorption and apparent porosity compared t? fired pure clay bricks and lower thermal conductivity of the fired at 900oC products with expanded vermiculite and expanded perlite. At small amounts of pore former, the properties of the fired clay materials slightly changed and more pronounced is the influence of vermiculite. Expanded vermiculite and expanded perlite in amount of 8 mass % can be used as a pore forming agents in bricks production to improve thermal conductivity of clay bricks and keep acceptable the compressive strength. Keywords: expanded vermiculite, expanded perlite, porous clay bricks, porosity, thermal conductivity.Item Ensuring the Quality of the Analytical Process in a Research Laboratory(2024-04-01) Surleva A.; Angelova L.; Ilieva D.; Ivanova V.; Surleva O.; Chavdarova K.Featured Application: The approaches for estimation of the analytical behavior of measuring methods could be applied in a research laboratory aiming at the characterization of new materials or a new method for analysis. The limitations of well-known approaches are described, and the alternatives are discussed. This paper discusses approaches for verification of methods of measurements of chemical and physical characteristics of specific samples. The limitations of well-known approaches are discussed. Some examples of alternatives are given to demonstrate specific issues encountered in the research laboratory analyzing new materials or characterizing new properties of materials. Application of sequential procedure using lower quantities of samples and reagents is presented. A standard addition to solid samples is discussed. The approach of control charts for estimation of method uncertainty for determination of plant available phosphorus is presented. The method comparison is applied as an approach to verification of alkaline reactivity by inductively coupled plasma–optical emission spectroscopy (ICP-OES) measurement, as well as density of newly synthesized chalcogenide glass materials. The presented examples demonstrated that alternative approaches are needed in order to verify the methods applied due to the great variety of activities and corresponding tasks in a research laboratory.Item EXPLORING THE STRUCTURAL AND ELECTRONIC CHARACTERISTICS OF AMORPHOUS Ge – Te - In MATERIAL THROUGH AB INITIO METHODS(2024-01-01) Zaidan A.; Ivanova V.; Petkov P.; Bancheva-Koleva P.This study employs density functional theory (DFT) and molecular dynamics to meticulously investigate the structural and electronic properties of ternary chalcogenide compounds, specifically (GeTe4)1-x Inx, and (GeTe5)1-x Inx across a range of compositions ( x = 0, 5, 10, 15, 20 at %). Utilizing the local density approximation within the framework of first-principles calculations, we comprehensively analyze the pair correlation function, static structural factor, electronic density of states, and electronic band gap energy. Our results reveal a notable decrease in the energy band gap of Germanium-Tellurium with the incorporation of Indium atoms. The structural changes observed in the Ge-Te matrix with Indium doping, as evidenced by the changes in the pair correlation function and static structure factor, are consistent with and supportive of the observed decrease in the band gap energy. This phenomenon is primarily attributed to the significant contribution of Indium atoms to the conduction band edge, offering new insights into the material’s electronic behaviour.Item Glass Formation and Properties of Multicomponent Glasses of the As2Se3-Ag2Te-GeTe System(2024-01-01) Karadashka I.; Ivanova V.; Jordanov V.; Karadjova V.Chalcogenide alloys of As2Se3-Ag2Te-GeTe were synthesized using the melt-quenching technique. By the visual and XRD analyses, the state of obtaining alloys was proven (glass, crystalline, glass + crystalline), and the glass formation region in the system was established. The thermal characteristics of some samples were determined—temperatures of glass transition (Tg); crystallization (Tcr); and melting (Tm). The basic physicochemical parameters, such as density (d) and Vickers microhardness (HV), were measured. Compactness (C), as well as some thermomechanical characteristics, such as module of elasticity (E), volume (Vh), and formation energy (Eh) of micro-voids, were calculated, and the influence of the composition on these characteristics was investigated. The addition of silver telluride resulted in a decrease in Tg and HV values and an increase in d and Vh values. No thermochemical effects of crystallization or melting were detected in some of the alloys. The obtained results were in agreement with the available literature data for similar systems.Item Improvement of Stevia rebaudiana Bertoni In Vitro Propagation and Steviol Glycoside Content Using Aminoacid Silver Nanofibers(2022-10-01) Sichanova M.; Geneva M.; Petrova M.; Miladinova-Georgieva K.; Kirova E.; Nedev T.; Tsekova D.; Iwanov I.; Dochev K.; Ivanova V.; Trendafilova A.The food industry is interested in replacing artificial sweeteners with natural sugars that possess zero calories and carbohydrates and do not cause spikes in blood sugar levels. The steviosides leaves, synthesized at Stevia rebaudiana Bertoni, are 300 times sweeter than common table sugar. Stevia propagation is limited due to the poor viability of the seeds, the long time and low germination rate, and the poor rooting ability of vegetative cuttings. Because of this, an alternative biotechnological method for its reproduction is being studied, such as multiple shoot production through direct organogenesis using nanofibers, formed from a derivative of amino acid valine as a carrier of the biologically active agent silver atoms/particles (NF-1%Ag and NF-2%Ag). The stevia explants were cultured on a medium containing NF-1%Ag and NF-2%Ag at concentrations of 1, 10, 50, and 100 mg L−1. The NF-1%Ag and NF-2%Ag treatment caused hormetic effects on stevia plantlets. At low concentrations of from 1 to 50 mg L−1 of nanofibers, the stimulation of plant growth was observed, with the maximum effect being observed at 50 mg L−1 nanofibers. However, at the higher dose of 100 mg L−1, inhibition of the values of parameters characterizing plant growth was recorded. The presence of nanofibers in the medium stimulates stevia root formatting.Item Influence of the Abiotic Elicitors Ag Salts of Aspartic Acid Derivatives, Self-Organized in Nanofibers with Monomeric and Dimeric Molecular Structures, on the Antioxidant Activity and Stevioside Content in Micropropagated Stevia rebaudiana Bert.(2023-10-01) Sichanova M.; Geneva M.; Petrova M.; Miladinova-Georgieva K.; Kirova E.; Nedev T.; Tsekova D.; Ivanova V.; Trendafilova A.The use of nanomaterials in biotechnology for the in vitro propagation of medical plants and the accumulation of certain biologically active metabolites is becoming an efficient strategy. This study aimed to evaluate the influence of the concentration (0, 1, 10, 50, and 100 mg L−1) of two types of nanofibers on the growth characteristics, the antioxidant status, and the production of steviol glycosides in micropropagated Stevia rebaudiana Bert. plantlets. The nanofibers were synthesized by aspartic acid derivatives (L-Asp) Ag salts self-organized into nanofibers with two different molecular structures: monomeric, containing one residue of L-Asp with one hydrophilic head which bonds one Ag ion (NF1-Ag salt); and dimeric, containing two residues of L-Asp with two hydrophilic heads which bond two Ag ions (NF2-Ag salt). An increase in the shoots from the explants’ number and length, biomass accumulation, and micropropagation rate was achieved in the plants treated with the NF1-Ag salt in concentrations from 1 to 50 mg L−1 after 30 days of in vitro proliferation compared to the NF2-Ag salt. In contrast, the plants grown on MS media supplemented with NF2-Ag salt exhibited an increase in the level of stevioside, rebaudioside A, and mono- (CQA) and dicaffeoylquinic (DCQA) acids as compared to the NF1-Ag salt.Item Interaction of KLAKLAK-NH2 and Analogs with Biomimetic Membrane Models(2024-03-01) Vitkova V.; Antonova K.; Petkov O.; Stoyanova-Ivanova A.; Jaber S.; Ivanova V.; Naydenova E.; Danalev D.Background: Specifically designed peptide mimetics offer higher selectivity regarding their toxicity to mammalian cells. In addition to the α-helix conformation, the specific activity is related to the peptide’s ability to penetrate the cell membrane. The alterations in lipid membrane properties were addressed in the presence of the peptide KLAKLAK-NH2 and analogs containing β-alanine, strengthening the antibacterial activity and/or naphtalimide with proven anticancer properties. Methods: The molecular interactions of the peptide mimetics with POPC bilayers were studied using FTIR-ATR spectroscopy. The thermal shape fluctuation analysis of quasispherical unilamellar vesicles was applied to probe the membrane bending elasticity. The impedance characteristics of bilayer lipid membranes were measured using fast Fourier-transform electrochemical impedance spectroscopy. Results: A lateral peptide association with the membrane is reported for β-alanine-containing peptides. The most pronounced membrane softening is found for the NphtG-KLβAKLβAK-NH2 analog containing both active groups that corroborate with the indications for 1,8-naphthalimide penetration in the lipid hydrophobic area obtained from the FTIR-ATR spectra analysis. The β-alanine substitution induces strong membrane-rigidifying properties even at very low concentrations of both β-alanine-containing peptides. Conclusions: The reported results are expected to advance the progress in tailoring the pharmacokinetic properties of antimicrobial peptides with strengthened stability towards enzymatic degradation. The investigation of the nonspecific interactions of peptides with model lipid membranes is featured as a useful tool to assess the antitumor and antimicrobial potential of new peptide mimetics.Item Optical properties of chalcogenide Ge-Te-In thin films(2012-01-01) Zaidan A.; Ivanova V.; Petkov P.Thin films of the chalcogenide (GeTe4)1-xIn x with various compositions (x 0, 5, 10, 15, 20 at %) were deposited under vacuum on glass substrates by thermal evaporation. The optical transmission and reflection spectra of the films at normal incidence were investigated in the spectral range from 800 to 2600 nm. Using the transmission spectra, the optical constants (refractive index (n) and extinction coefficient (k)) were calculated based on Swanepoel's method. The optical band gap (E gopt) was also estimated using Tauc's extrapolation procedure. © Published under licence by IOP Publishing Ltd.Item OPTICAL PROPERTIES OF ELECTROSPRAY DEPOSITED PAZO POLYMER FILMS DOPED WITH GeTe4-Cu CHALCOGENIDE PARTICLES(2022-01-01) Stoilova A.; Lilova V.; Ivanova V.; Trifonova Y.; Dimov D.Composite films based on the azopolymer poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt] doped with GeTe4-Cu chalcogenide particles have been prepared through electrospray deposition. Polarization microscopy was applied to study the films morphology. The transmittance coefficient (T), the reflectance coefficient (R), the refractive index (n), the extinction coefficient (k), the optical absorption coefficient (α) and the optical band gap (Formula presented) of the composite films and of the pure azo polymer film were determined. A significant change in the optical properties of the composite films in comparison to the non-doped azo polymer film has been observed.Item Physical Vapor Deposition of Indium-Doped GeTe: Analyzing the Evaporation Process and Kinetics(2024-08-01) Zaidan A.; Ivanova V.; Petkov P.Chalcogenide glasses have broad applications in the mid-infrared optoelectronics field and as phase-change materials (PCMs) due to their unique properties. Chalcogenide glasses can have crystalline and amorphous phases, making them suitable as PCMs for reversible optical or electrical recording. This study provides an in-depth analysis of the evaporation kinetics of indium-doped chalcogenides, GeTe4 and GeTe5, using the physical vapor deposition technique on glass substrates. Our approach involved a detailed examination of the evaporation process under controlled temperature conditions, allowing precise measurement of rate changes and energy dynamics. This study revealed a significant and exponential increase in the evaporation rate of GeTe4 and GeTe5 with the introduction of indium, which was particularly noticeable at higher temperatures. This increase in evaporation rate with indium doping suggests a more complex interplay of materials at the molecular level than previously understood. Furthermore, our findings indicate that the addition of indium affects the evaporation rate and elevates the energy requirements for the evaporation process, providing new insights into the thermal dynamics of these materials. This study’s outcomes contribute significantly to understanding deposition processes, paving the way for optimized manufacturing techniques that could lead to more efficient and higher-performing optoelectronic devices and memory storage solutions.Item Physico-chemical characterization of new tellurium based chalcogenide materials(2018-01-01) Trifonova Y.; Ivanova V.; Lilova V.; Vasileva D.; Boteva T.Bulk samples of Ge-Te-Cu system are synthesized by melt-quenched technique into a water/ice mixture. The synthesis is carried out at a constant heating rate of 5.10-2 K s-1 reaching the final temperature of 1373 K. Some physico-chemical properties of the synthesized bulk samples are defined. The density of the latter undergoes various changes with Cu content increase most likely due to the gradual substitution of tellurium atoms with copper leading to structural changes in the samples. The dependencies of the bulk samples compactness, molar volume and free volume on their composition follow that of the density. A change in the structure of GeTe4-Cu leading to extreme physico-chemical properties is observed in accordance with the theory of Thorphe for transitions in the structure of covalent chalcogenide glasses at an average coordination number 2,4. It proceeds at Zglass = 2,46 corresponding to a content of 10 mol % of Cu.Item Se-BASED CHALCOGENIDE GLASSES AS HOLOGRAPHIC MEDIA(2020-01-01) Trifonova Y.; Stoilova A.; Ivanova V.; Lilova V.; Petkov P.The present paper reports on the effect of the addition of IIIA metal (Ga or In) to Se-based chalcogenide thin films on the diffraction efficiency. The holographic scalar recording is accomplished using standard interferometric configuration with argon laser with a wavelength of 448 nm, spatial frequency of 2,2.10s m−1 and diameter of the spot 2 mm. The diffraction efficiency relation to the glassy system composition and the time of illumination at constant intensity of a laser beam is investigated. The results are interpreted in terms of the electronic structure in disordered systems.Item Structural investigation of tellurium based thin films(2018-01-01) Ivanova V.; Trifonova Y.; Lilova V.; Mikli V.; Stoyanova-Ivanova A.Tellurium based materials possess a low temperature phase change transition. Sometimes they cannot be obtained in an amorphous state under normal conditions. Thus they can be used as an active element in phase change memory devices. Our study is dedicated to the synthesis of tellurium based materials using the systems (GeTe3)100-xInx, x = 5 mol %, 10 mol %, 15 mol % and 20 mol % and (GeTe4)100-xInx, x = 5 mol %, 10 mol %, 15 mol % and 20 mol %, the deposition of thin films on their ground and the investigation of their morphology, topology and structure aiming to obtain new knowledge for evaluation of the possibility for application of these materials in phase change memory devices.Item Unified Modelling Language Application for Laparoscopic Instrument Design(2024-01-01) Ivanova V.; Boneva A.; Vasilev P.Laparoscopic surgery is a very popular medical intervention for the diagnosis and treatment of some abdominal problems and diseases. Compared with open surgery, laparoscopic procedures reduce patient trauma and recovery time. Still, at the same time, the surgeon’s dexterity is reduced as a result of the operation specificity and instrument construction. Correct behaviour of every device and instrument during its activities is an important condition for the proper operation of the whole system. The main purpose of this work is to model the operating behaviour of an instrument-organ interaction in an environment which is similar to the real one. The ultimate target of this study is focused on the development of a functionally operating model of a laparoscopic executive instrument for robots with improved engineering characteristics. To achieve the goals, the following main tasks are decided: i) Unified Modelling Language is applied to demonstrate the operating behaviour of a device in real-time. UseCase diagram and 3 Activity diagrams have been developed; ii) an original model of an instrument with 4 degrees of freedom for robot-assisted surgery is designed. In contrast to EndoWrist technology created by Intuitive Surgical Incorporation, USA for DaVinci instruments (with 3 orthogonal rotations), we offered other construction decisions. The designed instrument provides a kinematic structure with a combination of perpendicular and parallel rotations ((Formula presented)) which avoids additional rolls and allows obtaining the optimal working area of this instrument. This study is a continuation of previous work in the surgical robotics area.Item Validation of Ion Chromatographic Method for Determination of Standard Inorganic Anions in Treated and Untreated Drinking Water(2018-06-12) Ivanova V.; Surleva A.; Koleva B.An ion chromatographic method for determination of fluoride, chloride, nitrate and sulphate in untreated and treated drinking waters was described. An automated 850 IC Professional, Metrohm system equipped with conductivity detector and Metrosep A Supp 7-250 (250 x 4 mm) column was used. The validation of the method was performed for simultaneous determination of all studied analytes and the results have showed that the validated method fits the requirements of the current water legislation. The main analytical characteristics were estimated for each of studied analytes: limits of detection, limits of quantification, working and linear ranges, repeatability and intermediate precision, recovery. The trueness of the method was estimated by analysis of certified reference material for soft drinking water. Recovery test was performed on spiked drinking water samples. An uncertainty was estimated. The method was applied for analysis of drinking waters before and after chlorination.