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Item Change in pH and conductivity during the rinsing and the biosorption of copper ions onto pumpkin peel(Univerzitet u Zenici, Fakultet inženjerstva i prirodnih nauka, 2025) Marković, Marina; Gorgievski, Milan; Marković, Miljan; Grekulović, Vesna; Štrbac, Nada; Zdravković, Milica; Božinović, KristinaThe changes in pH and conductivity during the rinsing of the pumpkin peel, and the biosorption of Cu2+ ions, were the subject of this work. The obtained data showed that the pH value of the solutions increased during the rinsing of the biosorbent, as a result of the transfer of H+ ions from the aqueous phase into the structure of the pumpkin peel. An increase in the conductivity value was observed in the initial period of rinsing the pumpkin peel, followed by a decrease. The increase in conductivity in the initial phase is contributed to the self-leaching of the alkali and alkaline earth metal ions from the structure of the pumpkin peel, which were transferred into the aqueous phase. The further decrease in conductivity is a result of the dilution of the aqueous phase. The pH value decreased during the biosorption of Cu2+ ions, as hydrogen ions were transferred from the pumpkin peel structure into the solution, and then exchanged with Cu2+ ions. The conductivity value increased during the biosorption process, with a rapid increase in the initial period of 5 minutes, due to the transfer of alkali and alkaline earth metal ions into the solution.Item DTA-TGA and thermodynamic studies of walnut shells used for the biosorption of Cu2+ ions from synthetic solutions(Fakultet Tehničkih nauka, Kosovska Mitrovica, 2025) Marković, Marina; Gorgievski, Milan; Marković, Miljan; Štrbac, Nada; Grekulović, Vesna; Zdravković, Milica; Milošević, NemanjaBiosorption is a simple, economical and environmentally friendly process of removing pollutants by binding them to biological material [1]. Biological materials that can be used to remove or recover organic and inorganic substances from aqueous solutions include living or dead microorganisms and their components, seaweed, plant material, industrial and agricultural waste and natural residues [2]. In this paper, the thermodynamic analysis of the biosorption of copper ions on walnut shells, as well as the DTA-TGA analysis of walnut shells, are presented. Thermogravimetry was used to study the thermal degradation characteristics of walnut shells. The sample was heated in an inert atmosphere from 20 to 900 oC, using a simultaneous DSC-TGA device SDT Q600 (TA Instruments). The TGA curve showed two stages of weight loss during heating. In the interval from 20 to 150 oC, a weight loss of 6.64% was recorded, which was due to the evaporation of moisture from the sample. Further weight loss of 69.05% was observed in the temperature range from 150 to 900 oC, which was due to the decomposition of hemicellulose, cellulose and lignin present in the biosorbent structure [3]. The DTA curve showed that the process was accompanied by an endothermic peak at a temperature of 279.81 oC. The total weight loss was 75.69%. Thermodynamic studies are conducted to investigate the influence of temperature on the biosorption process and to obtain information about the feasibility, spontaneity and nature of the process [4]. Therefore, thermodynamic parameters such as the change in Gibbs free energy (ΔG0), the change in enthalpy (ΔH0) and the change in entropy (ΔS0) were determined. In order to determine the thermodynamic parameters, 0.5 g of walnut shells were brought into contact with 50 mL of a synthetic solution of copper ions with a concentration of 0.2 g L-1, at different temperatures (25 oC, 35 oC and 40 oC). The obtained results for the change in Gibbs free energy (ΔG0 ) were negative at all three temperatures, indicating the feasibility of the process and spontaneous nature of the biosorption. The obtained positive value of enthalpy (ΔH0 = 1.12 kJ mol-1) indicates that the biosorption reaction is endothermic and consequently consumes energy. The negative value of entropy (ΔS0 = -6.26 J mol-1 K-1) indicates that there is less randomness at the interface between solid and solution.Item Al-Sn alloys as composite phase change materials for thermal energy storage: microstructural and thermal characterization(Fakultet Tehničkih nauka, Kosovska Mitrovica, 2025) Manasijević, Dragan; Balanović, Ljubiša; Marković, Ivana; Gorgievski, Milan; Stamenković, Uroš; Kovacević, AvramAl–Sn alloys are well known for their excellent resistance to wear and corrosion, as well as their solid mechanical performance, making them popular choices for bearing applications [1,2]. More recently, however, they have attracted attention as potential composite phase change materials (C-PCMs) for thermal energy storage (TES) systems. To determine their suitability for such applications, a thorough investigation of their thermal characteristics is essential. In this study, Al–Sn alloys containing 11.7, 22.4, 32.8, 41.1, and 53.4 atomic percent Sn were prepared by melting high-purity metals. Continuous stirring of the melt was employed to prevent segregation, followed by casting into stainless steel molds. The resulting ingots displayed uniform microstructures, free of cracks and pores. Thermal diffusivity of the solid Al–Sn samples was measured between 25°C and 150°C using the light flash method. Density at room temperature was determined via the Archimedes principle, while specific heat capacities across different temperatures were calculated using the CALPHAD (Calculation of Phase Diagrams) approach. Thermal conductivity values were then derived using an appropriate conversion relationship. Additionally, phase transition temperatures and the associated thermal effects were analyzed by differential scanning calorimetry (DSC). The study mapped how thermal conductivity changes with both alloy composition and temperature, and also assessed how latent heat of fusion varies across the compositions. Microstructural and phase composition analyses were performed using scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS). The findings offer valuable insights into the thermal behavior and microstructural characteristics of Al–Sn alloys, contributing to the development of advanced PCMs for thermal energy storage applications.Item Kinetic and DTA-TGA analysis of Cu2+ biosorption on hazelnut shells(Fakultet Tehničkih nauka, Kosovska Mitrovica, 2025) Gorgievski, Milan; Marković, Miljan; Štrbać, Nada; Manasijević, Dragan; Grekulović, Vesna; Jovanović, Dalibor; Marković, MarinaBiosorption can be defined as the ability of biomaterials to accumulate and concentrate heavy metals from even the most dilute aqueous solutions. The biosorption process involves a solid phase (biological material) and a liquid phase (solvent, usually water) containing the dissolved metal. [1]. In this work, hazelnut shells were used as biosorbents for the biosorption of Cu2+ ions from synthetic solutions. This paper presents the results of kinetic and DTA-TGA analysis of the biosorption of Cu2+ ions on hazelnut shells. A pseudo-first order kinetic model, a pseudo-second order kinetic model, an intraparticle diffusion kinetic model and an Elovich kinetic model were used to describe the kinetics of biosorption of Cu2+ ions with hazelnut shells. The change in the adsorption capacity of copper ions with time was determined by contacting a series of solutions of copper ions with an initial concentration of 0.2 g dm-3 with 0.5 g of the sample for different contact times. Biosorption was terminated after 90 minutes, assuming that this time was long enough to establish equilibrium in the system [2]. It can be seen from Fig. 1a that at the beginning of the process (in the first 10 minutes) the adsorption capacity increases sharply with the contact time. The sudden increase in capacity in the first 10 minutes of the process is due to the large number of active sites available on the surface of the adsorbent. Thereafter, until the end of the experiment, a slight increase in adsorption capacity can be observed, which is due to the decrease in available active sites and the decrease in the concentration of copper ions in the solution [3]. The pseudo-second order kinetic model, with the correlation coefficient R2 = 0.961, showed the best agreement with the experimental data. Based on this model it can be concluded that chemisorption is a possible binding mechanism of copper ions on the surface of hazelnut shells [4].Item Microstructural and thermal properties of the Al-Cu eutectic alloy(Fakultet tehničkih nauka, Kosovska Mitrovici, 2025) Manasijević, Dragan; Balanović, Ljubiša; Cimpoesu, Nicanor; Markovic, Ivana; Gorgievski, Milan; Stamenković, Uroš; Stepanović, AleksandraUnderstanding thermal characteristics such as thermal conductivity, specific heat capacity, and latent heat of fusion is crucial when developing phase change materials (PCMs) for latent heat energy storage (LHES) systems [1]. Among metal-based PCMs, aluminum-based eutectic alloys have emerged as some of the most extensively studied due to their favorable thermal and mechanical properties [2].In this study, the Al–33.6 mass% Cu eutectic alloy was investigated in terms of its microstructure, thermal diffusivity, thermal conductivity, specific heat, and latent heat of melting. Techniques including scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), differential scanning calorimetry (DSC), and the light flash method were employed. Analysis revealed that the alloy’s microstructure contains both fine and coarse (Al)+Al₂Cu eutectic phases. Measurements showed that specific heat, thermal diffusivity, and thermal conductivity all increase with temperature over the range of 25–400 °C. At room temperature, the alloy exhibits a thermal conductivity of 134.3 W·m⁻¹·K⁻¹. The latent heat of fusion was determined to be 319.5 J·g⁻¹. Overall, the findings highlight the strong potential of the Al–Cu eutectic alloy as a candidate material for use in phase change thermal energy storage applications.Item Thermal properties of eutectic Sn-0.7Cu alloy reinforced with graphene nanosheets produced by powder metallurgy technique(Fakultet Tehničkih nauka, Kosovska Mitrovica, 2025) Nedeljković, Milan; Mladenović, Srba; Ćosović, Vladan; Marković, Ivana; Petrović, Jasmina; Stamenković, Uroš; Mitrović, Milijana; Kovačević, AvramIn this research, the effect of graphene nanosheets (GNS) on the thermal conductivity of Sn-0.7Cu-xGNS (x= 0, 0.02, 0.04, 0.06, 0.08 and 0.1 wt.%) nanocomposite materials was investigated. The GNS were successfully incorporated into the matrix alloy using the powder metallurgy technique, which includes mixing and mechanical alloying, cold compacting and sintering. Thermal diffusivity was measured at room temperature in an inert protective atmosphere using the xenon-flash method. The Sn-0.7Cu matrix alloy has a thermal conductivity of 53 Wm⁻¹K⁻¹ [1], while GNS exhibit a significantly higher value of 5000 Wm⁻¹K⁻¹ [2]. Therefore, it can be predicted that adding GNS will enhance thermal conductivity. However, the situation becomes more complex with composite materials [3]. The obtained value for thermal conductivity of the matrix alloy is 28.42 Wm⁻¹K⁻¹, while the composite with a 0.08 %GNS has the highest thermal conductivity of 35.29 Wm⁻¹K⁻¹. However, as the GNS content further increases, the thermal conductivity decreases. The decrease in thermal conductivity can be attributed to the agglomeration of GNS along the grain boundaries. This agglomeration leads to the accumulation of dislocations, which distort the matrix lattice, increasing the probability of electron scattering. As a result, the number of effective electrons (as heat carriers) in the crystal is reduced, which raises resistance and decreases thermal conductivity. The results indicated that the incorporated GNS enhanced the thermal conductivity of the matrix alloy to a certain extent.Item Analysis and statistical modeling of copper ions biosorption onto calcined chicken eggshell(Serbian Chemical Society, 2025) Marković, Miljan; Gorgievski, Milan; Štrbac, Nada; Grekulović, Vesna; Zdravković, Milica; Marković, Marina; Stanković, DaliborThe study on the possible use of calcined chicken eggshells as a biosorbent for copper ions removal from aqueous solutions, as well as some comparisons between raw and calcined eggshells, are presented in this paper. SEM-EDS and FTIR analysis of the calcined chicken eggshell samples were performed. In addition, the DTA-TGA analysis on raw chicken eggshells was performed. The influence of various process parameters, such as solution pH, stirring rate, biosorbent mass and Cu2+ concentration, was investigated. The kinetic analysis using four different empirical kinetic models was performed. The equilibrium analysis was done using the Langmuir, Freundlich and Temkin isotherm models. The process was optimized using the response surface methodology based on the Box–Behnken design (RSM-BBD). The obtained results are compared to our previous study on raw eggshells as a biosorbent for Cu2+ removal, in order to determine the justification for biosorbent modification (i.e., the calcination of raw eggshells).Item Effect of heat treatment on the mechanical and structural properties of X155CrVMo12-1 steel(Univerzitet u Zenici, Fakultet inženjerstva i prirodnih nauka, 2025) Stamenković, Uroš; Marković, Ivana; Mladenović, Srba; Manasijević, Dragan; Milanović, Petar; Nedeljković, MilanThis paper investigates the effects of oil quenching and tempering at various temperatures on the mechanical and structural properties of X155CrVMo12-1 tool steel. The steel specimens were austenitized at 1030 °C for half an hour, followed by quenching and tempering at different temperatures. Mechanical properties were assessed by measuring the hardness of the samples after each heat treatment. Using the conversion formula, the tensile strength values were calculated based on the measured hardness. Structural changes were analyzed using optical microscopy. The results indicate that the highest hardness values were achieved after the quenching process. However, an increase in tempering temperature led to a decrease in hardness. Optical microscopy revealed that quenching caused the appearance of martensite in the microstructure, while tempering at various temperatures caused alterations in the martensite structure.Item Shifting from Physical to Virtual Classroom in Accounting Education: A Study of Students’ Experiences During the COVID-19 Pandemic in Serbia(Springer Nature, 2024) Fedajev, Aleksandra; Jovanović, Dejan; Veličković, MilicaThe COVID-19 pandemic onset has caused significant changes in all areas of human activities, with changes in education being among the most evident. The change in educational systems was particularly challenging for developing countries like Serbia, where most higher education institutions were still in the early stages of implementing information and communication technology (ICT) in education. The pandemic forced higher education institutions in Serbia to shift from traditional to distance learning to enable continuous teaching while implementing pandemic prevention measures. The purpose of this study was to explain how two variables—technological environment and educational environment during online courses—influence students’ perceived satisfaction with online courses and their ability to acquire additional professional skills. The statistical analysis in the study is based on surveying 180 bachelor students attending accounting education in distance learning mode. Study findings are used to define some recommendations for accounting educators in Serbia for updating future learning practices based on lessons learned during the pandemic. The derived recommendations may be useful for similar disciplines and other developing countries.Item Journal of Mining and Metallurgy, 60 A (1) (2024)(Univeristy of Belgrade, Technical Faculty in Bor, 2024) Bogdanović, Grozdanka; Sokolović, Jovica; Cocić, Mira; Stojković, Pavle; Balanović, KatarinaItem Survey on star partial order in indefinite inner product spaces(Matematički institut SANU, 2022) Stanišev, IvanaThis is a short survey on the star partial order for matrices which is considered in the spaces with indefinite metric. The specific geometry of these spaces effects even on the definition of a star partial order, demanding the existence of the Moore--Penrose inverse of a certain matrix. A characterization of this partial order is also given. Some of the interesting properties are generalized and illustrated by the appropriate examples.Item Quasihyponormal and Strongly Quasihyponormal Matrices in Inner Product Spaces(International Linear Algebra Society, 2012) Radojević, Ivana; Đorđević, DraganThe notion of quasihyponormal and strongly quasihyponormal matrices is introduced in spaces equipped with possibly degenerate indefinite inner product, based on the works that studied hyponormal and strongly hyponormal matrices in these spaces. Generalizations of some results known for normal and hyponormal matrices are established.Item New results for EP matrices in indefinite inner product spaces(Springer, 2014) Radojević, IvanaIn this paper we study J-EP matrices, as a generalization of EP-matrices in indefinite inner product spaces, with respect to indefinite matrix product. We give some properties concerning EP and J-EP matrices and find connection between them. Also, we present some results for reverse order law for Moore-Penrose inverse in indefinite setting. Finally, we deal with the star partial ordering and improve some results given in the “EP matrices in indefinite inner product spaces” (2012), by relaxing some conditions.Item Moore-Penrose Inverse in Indefinite Inner Product Spaces(University of Niš, Faculty of Sciences and Mathematics, 2017) Radojević, Ivana; Đorđević, DraganWe present the definition and some properties for the Moore-Penrose inverse in possibly degenerate indefinite inner product spaces. The extensions of appropriate results, given for matrices in Euclidean and nondegenerate indefinite inner product spaces are established. All this is done by using the concept of linear relations.Item An ADAM-based approach to unveiling entrepreneurial ecosystems in selected European countries(Emerald, 2025) Popović, Gabrijela; Fedajev, Aleksandra; Mitić, Petar; Meidutė-Kavaliauskienė, IevaPurpose This study aims to integrate the resource-based view (RBV) with other theories that consider external factors necessary to respond successfully to dynamic and uncertain entrepreneurial business conditions. Design/methodology/approach The paper introduces an multi-criteria decision-making (MCDM) approach, utilizing the axial-distance-based aggregated measurement (ADAM) method with weights determined by the preference selection index (PSI) method, to rank eight European countries based on the Global Entrepreneurship Monitor (GEM) data. Additionally, the paper extends the existing entrepreneurial ecosystem taxonomy (EET), offering an additional classification. Findings The performed analysis emphasizes the importance and necessity of involving different dimensions of EE in assessing the countries' entrepreneurship performance, which facilitates creating adequate policy measures. Research limitations/implications The crucial limitations are assessments based only on the GEM data from a particular period, possibly leading to a certain bias. Future research should involve data from various resources to increase the results' reliability. Originality/value The ranking results and country classification obtained using the ADAM-based approach and two distinct taxonomies served as the basis for formulating tailored policy recommendations, aiming to formulate tailored policy implications for increasing the number of new entrepreneurs and improving innovativeness, sustainability and internationalization of existing entrepreneurs for each group of countries.Item Neuro-fuzzy prediction model of occupational injuries in mining(Taylor & Francis, 2024) Ivaz, Jelena; Petrović, Dejan; Stojadinović, Saša; Stojković, Pavle; Petrović, Sanja; Zlatanović, DraganObjectives. This study investigates the possibility of developing a unique model for predicting work-related injuries in Serbian underground coal mines using neural networks and fuzzy logic theory. Accidents are common due to the unique nature of underground mineral extraction involving people, machinery and limited workplaces. Methods. A universal model for predicting occupational accidents takes into account influential factors such as organizational aspects, personal and collective protective equipment, on-the-job training and leadership factors. The selected networks achieved a prediction accuracy of >90%. Results. The study successfully identifies potential risks and critical worker groups leading to injuries. The sensitivity analysis provides insights for targeted safety measures and improved organizational practices. Conclusion. This data-driven approach makes a valuable contribution to safety in the mining industry. Implementation of the predictive model can reduce injuries and machine damage, and improve worker well-being.Item Western Balkan countries’ innovation as determinant of their future growth and development(Taylor & Francis, 2024) Fedajev, Aleksandra; Panić, Marija; Živković, ŽivanThe Western Balkan (WB) countries (Albania, Bosnia & Herzegovina, Kosovo, Montenegro, North Macedonia, and Serbia) have been trying to become members of the EU for a long time, but they are increasingly moving away from the EU average in terms of development level. The question is whether these countries will be able to catch up with the EU countries in the near future and how they will do it. The reason for such results should be searched into insufficient innovation in these countries. This research aims to perform a comparative analysis of the capabilities and efficiency of the National Innovation System (NIS) in WB and CEE countries using the entropy-based PROMETHEE method for ranking. The rankings are performed using the two most reputable 2022 reports in this area: Global Innovation Index (GII) and European Innovation Scoreboard (EIS). For summarizing the results, the Borda rule is applied. The ranking results for both NIS capabilities and NIS efficiency indicate that WB countries are at the bottom of the rankings, having considerable disadvantages compared to CEE countries. Certain recommendations and guidelines for policymakers in these countries for improving innovation in the future have been defined according to practices from identified benchmarking countries.Item Exploring the Nexus of Distance Learning Satisfaction: Perspectives from Accounting Students in Serbian Public Universities During the Pandemic(Springer Nature, 2025) Fedajev, Aleksandra; Jovanović, Dejan; Janković-Perić, Marina; Radulescu, MagdalenaAmid the COVID-19 pandemic, teachers globally transitioned to distance learning, presenting significant challenges, particularly in developing countries. In that sense, the study investigates the usefulness and satisfaction (US) of distance learning (DL) among accounting students in Serbian public universities during the COVID-19 pandemic. The research focuses on three key factors affecting perceived US derived from existing literature, including teaching process quality (TPQ), technical qualities of distance learning platforms (TQ), and compatibility with social and pandemic conditions (CSPC). The developed theoretical model is predominantly based on the user satisfaction model. Data from 373 surveyed students, gathered through a validated questionnaire, underwent analysis using the partial least squares structural equation model (PLS-SEM). Results reveal that CSPC and TPQ significantly impact the US, whereas TQ has a minor and statistically insignificant effect. The R2 value indicates these three constructs explain a significant portion of the variance for the US, with moderate effects of CSPC and TPQ and a small effect of TQ, indicated by f2 values. Moreover, the model is found to be significantly predictive, according to the Q2 value. Besides theoretical implications, the study suggests important practical implications for university management, emphasizing regular student surveys, continuous improvement of DL systems, and effective and continuous teacher training as the vital measures for enhancing teaching quality. Improving TPQ and TQ also impacts the Serbian economy by fostering workforce development, enhancing digital transformation, stimulating telecommunication industry growth, driving digital information sector development, attracting investment and innovation, boosting global competitiveness, and promoting lifelong learning.Item Star partial order in indefinite inner product spaces(Taylor & Francis, 2022) Stanišev, IvanaWe define the star partial order for matrices in spaces with an indefinite inner product. We also give a characterization of that order in terms of matrices and their Moore-Penrose inverses. Finally, some interesting properties are shown.Item Characterization of the Cu-Ge-In: microstructural, mechanical, electrical properties, scheil and laver simulation(Polish Academy of Sciences, 2025) Đorđević, Aleksandar; Zečević, MIlena; Minić, Duško; Kolarević, Milan; Balanović, Ljubiša; Manasijević, Dragan; Petrović, MilenkoBased on application of Cu-based alloys and special application of Ge-based alloys it is from huge interest to study properties of the Cu-Ge-X alloys. In this paper selected system is Cu-Ge-In. This system was previously studied by our group. In this paper results are focused on electrical and mechanical properties. Experimental tests were performed on 12 ternary alloys. Six different experimental techniques were used to test the ternary alloys. The microstructure was tested using light optical microscopy (LOM) and scanning electron microscopy (SEM). The composition of the phases and the composition of the alloys were examined by energy dispersive spectroscopy (EDS). X-ray diffractometric analysis (XRD) was used to determine the phases. Properties such as hardness and electrical conductivity tests were performed. Those properties were used for calculation and modeling those properties along all composition ranges. Isothermal section at 25°C were predicted. Calculated isothermal section and were compared with results of the EDS and XRD test. Good agreement of calculated and experimental result has been reached. Best results of electrical conductivity and hardness give alloys with composition Cu80.93Ge9.86In9.21.