Author: MARTYNAS ŠAPUROV
Dissertation title: Development of Smooth Asymmetric Reactive Power Compensator
Fields of science: Electrical and Electronics Engineering (T001)
Scientific supervisor: dr. Algirdas Baškys
Defence of the dissertation: September 28, 2020
SUMMARY:
The aim of the performed research is development of reactive power compensators for the smooth and asymmetric reactive power compensation in the low voltage utility grid. The cascaded inverter has been modified and adapted for the smooth compensation of reactive power. The topologies of the conventional and cascaded inverter-based reactive power compensators with the individual inverters for the every phase for the asymmetric compensation of reactive power have been proposed. The shunt-connected compensator for the smooth and asymmetric compensation of reactive power in the low voltage utility grid has been developed. The investigation of developed reactive power compensators was performed using simulation and experimentally. The dissertation consists of an introduction, three chapters, general conclusions, references and the list of author's publications on the topic of the dissertation. The research problem, the relevance of the work, the object of research are presented in the introductory chapter. The aim and objectives of the work, the research methodology, scientific novelty of the work, the practical significance of the work results, defended statements are presented as well. At the end of the introduction, the author's publications and conference papers on the topic of the dissertation and the structure of the dissertation are given. The first chapter analyzes the reactive power compensation techniques and compensator types. The inverter-based reactive power compensators and shuntconnected reactive power compensators are analyzed. The conclusions are drawn out and the tasks of the dissertation are formulated. The conventional and cascaded inverter-based reactive power compensators are analyzed in the second chapter. The topologies of reactive power compensators with the individual ordinary inverters and cascaded inverters for the every phase for the asymmetric compensation of reactive power are proposed. The Matlab/Simulink models of compensators were created and investigation of compensators using simulation was performed. In the third chapter the developed shunt-connected compensator for smooth asymmetric compensation of reactive power in low voltage utility grid is presented. The investigation of compensator was performed experimentally using developed experimental test bench. The main results of the dissertation have been published in 4 scientific publications – 3 of them have been published in peer-reviewed scientific journals, 1 in conference proceedings. The results were presented at 5 scientific conferences.
Author: EDVARDAS BIELSKIS
Dissertation title: Development of energy efficient photovoltaic microinverter
Fields of science: Electrical and Electronics Engineering (T001)
Scientific supervisor: dr. Algirdas Baškys
Defence of the dissertation: September 28, 2020
SUMMARY:
The aim of the performed research is to improve the efficiency of the photovoltaic grid tied microinverter and reduce the nonlinear distortions of the microinverter current supplied to the electricity network. An energy-efficient single-stage photovoltaic microinverter has been developed and investigated. The control algorithm and the controller has been developed for the tracking of sinus shape of the microinverter output current. The investigation was performed using simulation and experimentally. The dissertation consists of an introduction, three chapters, general conclusions, references and the list of author's publications on the topic of the dissertation. The research problem, the relevance of the work, the object of research are presented in the introductory chapter. The aim and objectives of the work, the research methodology, scientific novelty of the work, the practical significance of the work results, defended statements are presented as well. At the end of the introduction, the author's publications and conference papers on the topic of the dissertation and the structure of the dissertation are given. In the first chapter, the research analysis of the properties of photovoltaic inverters is performed. The principles of operation of photovoltaic microinverters are analyzed, their advantages and disadvantages are discussed, microinverter output current control systems are analyzed. DC-DC (DC – direct current) voltage converters used in microinverters are reviewed. The tasks of the dissertation are formulated. The experimental investigation results of efficiency of a developed DC-DC voltage flyback converter with an alternative active surge suppression circuit and a two-switch DC-DC flyback converter are presented in the second chapter. A new single stage grid tied photovoltaic microinverter based on couple of two-switch DC-DC flyback converters has been proposed. The investigation of proposed microinverter efficiency was performed. In the third chapter a new PI controller with a time-varying proportional constant is proposed and a microinverter output current sinus shape tracking system with a developed controller was investigated. The nonlinear distortions of the current were evaluated. The influence of parameters of the microinverter elements on the microinverter efficiency was investigated. The flyback transformer used in the microinverter has been upgraded. The main results of the dissertation have been published in 7 scientific publications – 5 of them have been published in peer-reviewed scientific journals, 2 in conference proceedings. The results were presented at 10 scientific conferences.
Author: AGNĖ MIKALAUSKAITĖ
Dissertation title: Investigation on the surface decoration of magnetic iron oxide nanoparticles with gold nanocomposites
Fields of science: Chemistry (N003)
Scientific supervisor: dr. Arūnas Jagminas
Defence of the dissertation: November 27, 2020
SUMMARY:
A simple method for coating the surface of magnetic Nps with gold using vitamin C as a reducing agent capable of to binding with cobalt ferrite was developed. This study describes a novel and efficient way for covering of various cobalt ferrite nanoparticles with gold shell without nucleation and growth of the separate gold crystals using methionine as a reducer of gold species and stabilizing agent of Nps. Formation of CoFe2O4@Au NPs was confirmed by XRD, TEM, EDX, high-resolution TEM studies, variation of NPs size, and FTIR spectra. FTIR spectral analysis showed different adsorption behavior of dopamine hydrochloride on the surface of CoFe2O4 and CoFe2O4 @ Au NPs. Gold cluster synthesis for the first time using a branched-chain amino acid food supplement was optimized. The obtained gold clusters characterized by intense red photoluminescence, peaked in a vicinity of 670–710 nm, a relatively long lifetime of several microseconds, and high quantum yield of approximately 7%, similar to ones synthesized with bovine serum albumin.
Author: AUDRIUS DRABAVIČIUS
Dissertation title: Formation of chalcogenide solar cell absorber layers using electrochemical deposition of precursors
Fields of science: Chemistry (N003)
Scientific supervisor: habil. dr. Remigijus Juškėnas
Defence of the dissertation: November 27, 2020
SUMMARY:
The main focus of this dissertation is on the copper-zinc-tin sulfide thin films, their synthesis, and research. Thin films formed by sulfurizing/selenizing electrochemically deposited Cu-Sn and Cu-Sn-Zn precursor layers were investigated in the work. Different methods of electrochemical deposition of the precursor were tested and compared, including the electrochemical deposition of metals using a unique electrochemical cell produced by a 3D printer. Also, the conditions of sulfurization/selenization of the precursor layers were investigated. The phase composition and photoelectrochemical activity of the formed CTSe layers were investigated. The formed CZTS absorber layers were characterized using SEM, TEM, EDX, Raman spectroscopy, photoelectrochemical analysis, XRD, and photoluminescence analysis methods. Significant attention in the dissertation was given to determine the phase composition and the defects formed in the kesterite layer. For the first time, electrochemically formed CZTS layers were investigated by the XRD method using Cu Kα1 radiation. The kesterite crystalline structure order/disorder levels determined by Raman spectroscopy and by RSD methods were compared.
Author: DOMAS JOKUBAUSKIS
Dissertation title: Development and applications of compact spectroscopic terahertz imaging systems using principles of optical beam engineering
Fields of science: Physics (N002)
Scientific supervisor: prof. habil. dr. Gintaras Valušis
Defence of the dissertation: December 10, 2020
SUMMARY:
The dissertation objective was to investigate and improve terahertz (THz) imaging techniques allowing for inspection of hidden objects without harmful X-rays. The work was carried out as follows. Firstly, an efficient and cost-effective compact homodyne THz imaging scheme was developed using reliable bow-tie diodes as an alternative to heterodyne THz imaging technique for low absorbing object identification using paper sheets for phase shifting at 0.3 THz and 0.6 THz frequencies. Secondly, effect of molecular beam epitaxy growth conditions on InGaAs/InP layers dedicated for bow-tie diodes fabrication was investigated and optimal growth regime with respect to InGaAs diodes parameters was determined. Furthermore, silicon diffractive optics-based Fibonacci bifocal THz lens was designed, fabricated and investigated. Additionally, the designed bifocal THz lens was applied for THz imaging, demonstrating simultaneous multi-focal THz imaging. Moreover, diffractive THz lens allowing Bessel THz imaging was developed with extended focal depth up to 20 mm suitable for imaging of objects hidden in post packages. Additionally, purely silicon optics based Bessel THz imaging was demonstrated and deconvolution algorithms were applied for enhancement of the contrast. Finally, THz beam manipulation using spatial filtering methods was investigated and applied for THz imaging revealing its advantages in comparison with other beam engineering methods.
Author: ROKAS NAUJALIS
Dissertation title: Star Clusters in Galaxies. Multicolour HST photometry
Fields of science: Physics (N002)
Scientific supervisor: prof. dr. Vladas Vansevičius
Defence of the dissertation: December 11, 2020
SUMMARY:
Star clusters are composed of stars with similar age and chemical composition. This makes them a powerful tool for studying star formation histories of galaxies. We studied star clusters in two types of galaxies. So far, only one cluster has been known in the irregular dwarf galaxy Leo A, and we have discovered four more low-mass young clusters. In the Andromeda galaxy, cluster aperture photometry is strongly affected by the large number of bright field stars. To reduce their influence, we have developed a new method of adaptive aperture photometry. Using it, we performed aperture photometry of 1181 clusters on CCD mosaics taken from the Hubble Space Telescope archive and demonstrated that our results are more consistent with theoretical models than previously published data. We also modeled the collapse of molecular clouds at different ambient pressures and showed that in high-pressure environments, the efficiency of star formation is higher and newly formed clusters are more strongly bound by gravity.
Author: MONIKA KIRSNYTĖ
Dissertation title: Semiconductor polymeric composites formation by bioinspired in situ polymerization process with non-conductive substrates
Fields of science: Chemistry (N003)
Scientific supervisor: dr. Arūnas Stirkė
Defence of the dissertation: December 11, 2020
SUMMARY:
In recent decades, polymers have been extensively studied for their specific tunable intrinsic electrical conductivity properties and various application possibilities in radar-absorbent materials, smart textiles, flexible electronics, biosensors or energy harvesting, or energy-storing materials. Therefore, there is a need to produce and investigate various semiconductive composites. This thesis presents semiconductor composites of polymer origin by bioinspired in situ polymerization process by utilizing non-conductive substrates. Firstly, a new biogenic polypyrrole (PPy) microsphere synthesis method with Streptomyces spp. bacteria were applied and analysed. We tried to mimic the polymerization, as mentioned earlier, results by a bioinspired microemulsion polymerization process in microfluidic systems. Also, we implement a novel PPy in situ polymerization process on non-conductive surfaces, including various textiles, which leads to conductive composite formation. Composites dopant incorporation into an adhesive matrix or pyrrole aqueous solution results in dependencies for electrical conductivity and shielding effectiveness parameters to evaluate PPy composites efficiency for electromagnetic wave absorbent materials.
Author: JONAS KLIMANTAVIČIUS
Dissertation title: Semiconductor polymeric composites formation by bioinspired in situ polymerization process with non-conductive substrates
Fields of science: Material Engineering (T008)
Scientific supervisor: prof. habil. dr. Nerija Žurauskienė
Defence of the dissertation: December 18, 2020
SUMMARY:
The magnetoresistive properties of lanthanum manganites are attracting considerable interest of research and applications. It was demonstrated that manganite La1-xSrxMnO3 (LSMO) films exhibiting colossal magnetoresistance (CMR) effect can be successfully used for the development of CMR-B-scalar sensors operating at room temperatures and measuring the absolute magnitude of magnetic flux density in very small volumes. This thesis is focused on the investigation of magnetoresistive properties of the LSMO films to use them for pulsed magnetic field measurements at room and higher temperatures. The study presents the investigation of LSMO films grown on glass ceramics by pulsed injection MOCVD method, and demonstrates possibility to tune their magnetoresistive properties in a wide range of temperatures and magnetic fields by changing film thickness and deposition temperature. Further, it is shown that Mn excess in LSMO films could significantly increase their metal-insulator transition temperature and thus increase the magnetoresistance values at higher than room temperatures. Also, the dependence of the magnetoresistive properties of the LSMO films grown on different substrates (AT-cut quartz, Al2O3 and Si/SiO2-1000) is investigated and analyzed in the thesis. Finally, the successful application of CMR-B-scalar sensors, produced using LSMO films with Mn excess on Al2O3 substrate, for the investigation of magnetic field dynamics during Magnetic Pulse Welding (MPW) of metals is presented. The possibility to use these sensors for the analysis of MPW processes and nondestructive evaluation of welding quality is demonstrated.