Slovak University of Technology in Bratislava
  • Bratislava, Slovakia, Slovakia
Recent publications
This work deals with the mechanical properties and microstructure of additively manufactured (AM) 316L stainless steels strengthened with Al2O3 nanoparticles. At first, the critical processing parameters of laser powder bed fusion (LPBF) were optimized using Taguchi analysis in terms of laser power (180–210 W), scan speed (90–270 mm/s), hatch space (200–300 µm), and layer thickness (20–40 µm) to obtain materials with the highest density. Based on the modeling predictions and experimental findings, energy density was determined as a critical factor in controlling the integrity of deposited structures and their cracking behavior. The microstructural features and mechanical properties of the produced nanocomposites were characterized afterward. Grain structural aspects and crystallographic textural evolution across different sections of the printed nanocomposites, along with the distribution of reinforcing alumina nanoparticles, were studied using optical microscopy (OM), channeling contrast field emission-scanning electron microscopy (FE-SEM) imaging, electron backscattering diffraction (EBSD), and transmission electron microscopy (TEM) analyses. The fabricated materials' mechanical properties were also evaluated using indentation micro-hardness testing across the printed cubic structures. Trends revealed a considerable grain structural refinement down to the nanometric scale by incorporation and uniform distribution of alumina nanoparticles depending on the selective laser melting (SLM) processing parameters. This consequently improved the hardness of the materials up to around 270 Vickers. The role of unstable Al2O3 nanoparticles would directly strengthen the stainless steel metal matrix by generating new in-situ agents during LPBF. Moreover, the crucial indirect influence was discovered in controlling the involved solidification mechanisms and subsequent microstructural features.
Sugeno-like operators are binary operations based generalization of Sugeno integral and are still defined on real valued fuzzy measures. This work discusses the aggregation methods in the situation where both inputs and fuzzy measures are attached with numerical uncertainties. That is, when an input vector and a fuzzy measure are given, each of the entries of the vector and the measure value on each subset can be attached with numerical uncertainty degrees. To fulfill this meaningful aim of performing Sugeno-like aggregation with numerical uncertainties, it needs two parts of work. We firstly formally define basic uncertain vector and basic uncertain fuzzy measure. Then we discuss the methods to construct or adjust basic uncertain vector and basic uncertain fuzzy measure in two situations, the general fuzzy measure situation and the probability measure only situation, respectively. With uncertain input and uncertain fuzzy measure, we then accordingly propose the corresponding restricted Sugeno-like operator, for which some different logic restrictions are analyzed. All the proposals also have full consistency for they can immediately degenerates into Sugeno-like operator when the attached uncertainties disappear.
This study defines interval type basic uncertain information and BUI type basic uncertain information, which are two extensions of basic uncertain information and can model more types of uncertainties and uncertainty involved evaluation problems. Under rules based and linguistic decision making environments, we analyze and provide some classification rules with real values, interval values, BUI granules, interval type basic uncertain information and BUI type basic uncertain information. Using the proposed new concepts and decision making methods, an illustrative example in preference and uncertainty involved multi-source evaluation is presented.
Basic uncertain information is a newly proposed normative formulation to express and model uncertain information. This study further generalizes this concept by introducing the concept of refined interval of discourse in which the true value is known to be included. Hence, we define some new definitions of relative basic uncertain information, relative certainty/uncertainty degree and comprehensive certainty/uncertainty with some related measurements and analysis. With the introduced uncertain data type, we define two corresponding aggregation operators, namely, the relative basic uncertain information valued weighted arithmetic mean operator and the interval induced relative basic uncertain information valued ordered weight averaging operator. An application of the proposed concepts and methods in multi-agents evaluation is provided.
Rao algorithms that include three algorithms are very simple and parameter-less algorithms with effective and desirable performance. This paper modifies these three algorithms, merges them, and establishes a powerful group algorithm. In the first optimization step, the suggested algorithm is tested on 30 standard CEC2014 functions with 50 dimensions to compare it with main algorithms, several well-known algorithms, and modified versions of RAO algorithm. It becomes evident in the first test that the suggested optimizer is effective, and reliable for optimization of real-parameter functions, and it has shown its superiority to original RAO algorithm and several modern and modified versions of RAO algorithms for most of the test functions and achieved more acceptable results than them. Moreover, the suggested algorithm benefits a faster convergence characteristic than original RAO algorithms. The proposed Colonial Competitive RAO (CCRAO) has been applied on five popular engineering problems and its results have been compared with those of recent papers. According to the results, CCRAO is an effective, robust, and reliable optimizer for engineering design problems and can contain all useful features of RAO algorithms altogether. CCRAO has succeeded to converge to the best solution for these engineering problems and surpasses most of the other algorithms.
When exposed to fluid flow, elastically supported multiple square cylinders may experience either one or a combination of vortex-induced vibration, galloping, and wake-induced vibration phenomena. Due to these high-amplitude instabilities, it is necessary to utilize vibration control devices. The present paper studies the suppression of flow-induced vibration (FIV) of tandem-arranged square-section cylinders, which can oscillate independently in the streamwise and transverse directions at low Reynolds numbers. The vibration reduction is achieved by directly applying opposing forces using an active FIV control system based on the intelligent proportional-integral-derivative controller. In the present study, the fluid flow equations are calculated through the finite volume technique, by which the aerodynamic forces are attained. Next, based on the computed excitation forces, the motion equations of cylinders are solved within a user-defined function code. The numerical results show that the controller successfully suppresses the vibrations of the front and rear cylinders by a maximum of 94% and 92% at Re = 80 and by 98% and 97% at Re = 100. At Re = 230 and 250, the controller successfully diminishes the oscillation of the front cylinder by a maximum of 79% and 80%, respectively. The significant intensity of the front cylinder's wake makes the active control system unable to capably affect the vibrations of the downstream cylinder at Re = 230 and 250.
This work presents the direct crude glycerol use by high cell-density fermentation of Escherichia coli expressing recombinant glycoside hydrolase that cleaves steryl glucosides, one of the main biodiesel contaminants. Optimized fermentation with crude glycerol reached the biomass concentration of 30 g/l with the 0.285 U/mgDCW specific activity, which is comparable with the pure glycerol production. The expression of the inclusion bodies was eliminated by the increase of the induction temperature to 40 °C. The produced enzyme was able to hydrolyze 300 ppm of steryl glucosides, the major biodiesel contaminants, and reached 100 % conversion. This model approach of waste carbon recycling (crude glycerol) for biorefinery-beneficial recombinant enzyme production is in line with the sustainable circular bioeconomy concept.
At the DREAMS (DREsden AMS) facility we are implementing a so-called Super-SIMS (SIMS = Secondary Ion Mass Spectrometry) device, which combines the micron-scale spatial resolution of a commercial SIMS (CAMECA IMS 7f-auto) with the very high selectivity through molecule suppression by AMS. We have demonstrated high transmission for major element ions including silicon, fluorine and iodine, however the lack of well characterized calibration materials makes a true quantification of trace and ultra-trace elements contents difficult. Measurements of P in Si show the linearity of the instrument’s relative sensitivity factor over more than three orders of magnitude, and measurements of the isotopic ratio of bromine in ZnS document the reliability of our approach. The goal of the DREAMS Super-SIMS project is to provide quantitative concentration data of ultra-trace elements in geological samples in the context of resource technology.
Photovoltaic solar-based façade concepts are considered one of the promising representatives in the overall energy-saving campaign. The presented study aims at the simulation approach and its validation relative to experimental measurements of a double-skin building-integrated photovoltaic (BiPV) concept coupled with phase change material (PCM) in climate-responsive façade design. A comparative study of the thermo-responsive reactions and electricity production of two BiPV façades (with and without PCM layer) was conducted using the building energy simulation (BES) method to reveal the complexity of building performance predictions. An empirical validation of the BES tool working under the EnergyPlus computational engine is conducted in this connection. The consistency between the simulation results and the experimental data obtained via calorimetry and dynamic outdoor tests is comprehensively investigated. The current zonal modelling approach of the BES method is suitable when predicting the thermo-responsive capabilities of a PCM-based BiPV façade. Accordingly, the best agreement is found in the PCM data based on Triple-layer calorimetry (3LC). Using PCM in a BiPV system can increase the maximum peak electricity production from 4.3 to 4.8 % obtained experimentally with a 10–14 K decrease in PV panel operating temperature. In contrast, a difference, from 3.8 to 5.4 %, is observed with the equivalent one-diode model between the simulation results for solar panels based on copper‑indium‑gallium-selenide (CIGS) technology. Hence, the performance prediction of PV electrical conversion efficiency is calibrated using a semiconductor band gap at a value of 1.4 eV.
The paper focuses on the analysis and comparison of tube wall thickness after cold drawing. The tube wall thickness obtained by the experiment was compared with the wall thickness obtained using finite element method (FEM) simulation in the simulation software DEFORM-3D. For the experiment the tube sinking technology was performed and all tubes were made of steel E235. Tubes were the outer diameter of Ø14, Ø16, Ø18 mm and the tube wall thickness was 1 and 2 mm. All tubes were drawn by single-pass tube sinking technology to the final diameter of Ø12 mm. Tube sinking process is a tube drawing technology through a drawing die without the use of a mandrel. Tube sinking is used as a final drawing operation, especially in the production of precision tubes of smaller diameter. The resulting comparison showed that the tube wall thickness obtained by the simulation in the DEFORM software very well matches with tube wall thickness obtained in the experiments. Based on the results, it can be stated that in the future it will be possible to replace some of the real experiments with FEM simulation in the DEFORM software.
Mycotoxins induce oxidative stress, hypoxia, and cause immunosuppressive effects. Moreover, emerging evidence show that mycotoxins have a potential of inducing cellular senescence, which are involved in their immunomodulatory effects. Mycotoxins upregulate the expression of senescence markers γ-H2AX, senescence-associated β-galactosidase, p53, p16, and senescence-associated secretory phenotype (SASP) inflammatory factors. Moreover, mycotoxins cause senescence-associated cell cycle arrest by diminishing cyclin D1 and Cdk4 pathways, as well as increasing the expression of p53, p21, and CDK6. Mycotoxins may induce cellular senescence by activating reactive oxygen species (ROS)-induced oxidative stress. In addition, hypoxia acts as a double-edged sword on cell senescence; it could both act as the stress-induced senescence and also hinder the onset of cellular senescence. The SASP inflammatory factors have the ability to induce an immunosuppressive environment, while mycotoxins directly cause immunosuppression. Therefore, there is a potential relationship between mycotoxins and cellular senescence that synergistically cause immunosuppression. However, most of the current studies have involved the effect of mycotoxins on cell cycle arrest, but only limited in-depth research has been carried out to link the occurrence of this condition (cell cycle arrest) with cellular senescence.
To ascertain quercetin’s and rooperol’s potency of H-atom donation to CH3OO• and HOO•, thermodynamics, kinetics and tunnelling, three forms of chemical reaction control, were theoretically examined. In lipid media, H-atom donation from quercetin’s catecholic OH groups via the proton-coupled electron transfer (PCET) mechanism, is more relevant than from C-ring enolic moiety. Amongst rooperol’s two catecholic moieties, H-atom donation from A-ring OH groups is favored. Allylic hydrogens of rooperol are poorly abstractable via the hydrogen atom transfer (HAT) mechanism. Kinetic analysis including tunnelling enables a more reliable prediction of the H-atom donation potency of quercetin and rooperol, avoiding the pitfalls of a solely thermodynamic approach. Obtained results contradict the increasing number of misleading statements about the high impact of C–H bond breaking on polyphenols’ antioxidant potency. In an aqueous environment at pH = 7.4, the 3-O− phenoxide anion of quercetin and rooperol’s 4′-O− phenoxide anion are preferred sites for CH3OO• and HOO• inactivation via the single electron transfer (SET) mechanism.
In the presented work, poly(3-hydroxybutyrate)-PHB-based composites for 3D printing as bio-sourced and biodegradable alternatives to synthetic plastics are characterized. The PHB matrix was modified by polylactide (PLA) and plasticized by tributyl citrate. Kaolin particles were used as a filler. The mathematical method “Design of Experiment” (DoE) was used to create a matrix of samples for further evaluation. Firstly, the optimal printing temperature of the first and upper layers was determined. Secondly, the 3D printed samples were tested with regards to the warping during the 3D printing. Testing specimens were prepared using the determined optimal printing conditions to measure the tensile properties, impact strength, and heat deflection temperature (HDT) of the samples. The results describe the effect of adding individual components (PHB, PLA, plasticizer, and filler) in the prepared composite sample on the resulting material properties. Two composite samples were prepared based on the theoretical results of DoE (one with the maximum printability and one with the maximum HDT) to compare them with the real data measured. The tests of these two composite samples showed 25% lower warping and 8.9% higher HDT than was expected by the theory.
Silicon carbide (SiC) has been envisioned as an almost ideal material for power electronic devices; however, device reliability is still a great challenge. Here we investigate the reliability of commercial 1.2-kV 4H-SiC MOSFETs under repetitive unclamped inductive switching (UIS). The stress invoked degradation of the device characteristics, including the output and transfer characteristics, drain leakage current, and capacitance characteristics. Besides the shift of steady-state electrical characteristics, a significant change in switching times points out the charge trapping phenomenon. Transient capacitance spectroscopy was applied to investigate charge traps in the virgin device as well as after UIS stress. The intrinsic traps due to metal impurities or Z1,2 transitions were recognized in the virgin device. The UIS stress caused suppression of the second stage of the Z1,2 transition, and only the first stage, Z10, was observed. Hence, the UIS stress is causing the reduction of multiple charging of carbon vacancies in SiC-based devices.
Accurate estimation of the longitudinal dispersion coefficient (LDC) is essential for modeling the pollution status in rivers. This research investigates the capabilities of machine-learning methods such as multi-layer perceptron (MLP), multi-layer perceptron trained with particle swarm optimization (MLP-PSO), multi-layer perceptron trained with Stochastic gradient descent deep learning (MLP-SGD) and different regressions including linear and non-linear regressions (LR and NLR) methods for determining the LDC of pollution in natural rivers and evaluates the accuracy of these methods in comparison with real measured data. Furthermore, the correlation coefficient (CC), root mean squared error (RMSE) and Willmott’s Index (WI) were implemented to evaluate the accuracies of the mentioned methods. Comparison of the results showed the superiority of the MLP-SGD model with CC of 0.923, RMSE of 281.4 and WI of 0.954, which indicates the undeniable accuracy and quality of the deep-learning model that can be used as a powerful model for LDC simulation. Also due to the acceptable performance of the PSO algorithm in the hybridization of the MLP model, the use of PSO algorithms is recommended to train machine-learning techniques for LDC estimation.
With the increasing number of devices connected to the Internet of Things, expectations from its technologies are getting higher. One of the most promising technologies for IoT networks is LoRa. It enables transmission over long distances with minimum power consumption. However, the current solution for network optimization, adaptive data rate, is only able to set the configuration for the stationary environment, where the conditions in the network do not change very often. Adaptive data rate is also not able to deal with mobile nodes. This results in a high number of collisions in a harsh environment, which leads to higher power consumption. Some of the devices have only a very limited power supply and need to prolong the battery lifetime. The latest research has shown that using reinforcement learning techniques, especially algorithms for a multi-armed bandit problem, leads to a better power efficiency and higher packet delivery ratio. In this paper, we briefly introduce energy-wise LoRa@FIIT protocol. We list and briefly describe different algorithms for communication parameters selection and propose a network testbed for simulations. Furthermore, we present results from performed simulations and compare results of two popular multi-armed bandit algorithms, Thompson sampling and upper confidence bound. The results show that the first one provides about 20% higher packet delivery ratio, while the second one provides a slightly lower power consumption.
Research infrastructures play a key role in technological development supporting science, technology and innovation. For the scientific community, but also for the creation of state science policy, it is necessary to have an overview of the research infrastructures that are available. Information is required on what infrastructures are available, who manages them and what services they provide. The evidence of research infrastructures is an integral part of the R&D information system. Its development at national level follows from EU activities in this area. The software module for research infrastructure should meet the requirements of CRIS system. It should contain well-defined workflow and user interface. The second requirement for created data model has been to achieve the maximal level of CERIF compatibility, including linking with main CRIS entities. The article presents brief situation overview in Slovakia and the experience with the implementation of the Map of research infrastructure, and with its integration to the information system SK CRIS.
A divergent strategy for natural polyketides synthesis has been designed. This synthetic route allowed chemical alterations leading to all stereoisomers of the natural agropyrenol 1, sordarial 2, and heterocornol B 4. Key steps involve desymmetrization of divinylcarbinol using asymmetric Sharpless epoxidation and Heck coupling of an easily available aromatic partner and prepared chiral alkene. The versatility of the synthetic method was demonstrated on the preparation of heterocornol A 3 and sordariol 5. The absolute and relative configurations of prepared natural compounds 2·1/3C6H12 and 4 were confirmed and assigned by single-crystal X-ray analysis.
A cadastre is a system of major importance for the economy and for management strategies in support of sustainable development. Thus, its modernisation process (especially in the case of buildings) is extremely important. This study compared the results from the cadastre moderni-sation process of Poland and Slovakia, that is, countries with a different way of building development and different historical cadastral traditions. It was certain that in countries with dispersed development-such as Poland-the modernisation process could significantly change the cadas-tre's picture. However, the analysis of the number of buildings in the cadastre after modernisation revealed a change of 3.048% for scattered development and only 6% for compact development. Thus, the urgent need to perform retrofits in countries with a scattered pattern of development was demonstrated. In addition, a comparative analysis proved that excessively frequent changes in the law cause the cadastre base to lose its validity. The Polish building cadastre has become a victim of such frequent changes in the law. It happened that just after the cadastre had been modernised at a high financial cost, the data on buildings collected in the cadastre became outdated as a result of a change in the law. Research highlighted that frequent changes and inconsistencies in the law result in a state of affairs in which activities that should be systemic and technical instead become activities that depend on political aspects.
Magnetically Soft Materials The cover image shows a supersilent grid transformer (inaudible to a human ear) with the core produced from a zero‐magnetostriction, magnetically soft AlCoNiFeCu2 ferromagnetic high‐entropy alloy, as described in article number 2201535 by Jože Luzar, Janez Dolinšek, and colleagues. Supersilence is a consequence of the material's decomposition into three phases, which creates a specific nanostructure that averages the magnetic anisotropy and the magnetostriction to zero.
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4,157 members
Martin Weis
  • Institute of Electronics and Photonics
Mikulas Huba
  • Institute of Automotive Mechatronics
Vazovova 5, 812 43, Bratislava, Slovakia, Slovakia
Head of institution
prof. Ing. Miroslav Fikar, DrSc.
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