Vilnius Gediminas Technical University
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Influence of Hot Carrier and Thermal Components on Photovoltage Formation across the p–n Junction
No full textIn the present work we reveal the existence of the hot carrier photovoltage induced across a p–n junction in addition to the classical carrier generation-induced and thermalization-caused photovoltages. On the basis of the solution of the differential equation of the first-order linear time-invariant system, we propose a model enabling to disclose the pure value of each photovoltage component. The hot carrier photovoltage is fast since it is determined by the free carrier energy relaxation time (which is of the order of 10−12 s), while the thermal one, being conditioned by the junction temperature change, is relatively slow; and both of them have a sign opposite to that of the electron-hole pair generation-induced component. Simultaneous coexistence of the components is evidenced experimentally in GaAs p–n junction exposed to pulsed 1.06 μm laser light. The work is remarkable in two ways: first, it shows that creation of conditions unfavorable for the rise of hot carrier photovoltage might improve the efficiency of a single junction solar cell, and second, it should inspire the photovoltaic society to revise the Shockley–Queisser limit by taking into account the damaging impact of the hot carrier photovoltage.This article belongs to the Section Applied PhysicsThis work was in part supported by the Research Council of Lithuania (grant No. 01.2.2-LMT-K-718-01-0050)
Investigation and Improvement of Bond Performance of Synthetic Macro-Fibres in Concrete
No full textStrength and stiffness are the key parameters characterising the bond performance of fibres in concrete. However, a straightforward procedure for estimating the bond parameters of a synthetic macro-fibre does not exist. This study employs pull-out tests to investigate the bond behaviour of synthetic macro-fibres. Two types of macro-fibres available in the market were investigated. A gripping system was developed to protect the fibres from local damage. The experimental campaign consisted of two stages. At the first stage, 32 concrete specimens were manufactured for performing 96 pull-out tests (three fibre samples were embedded in each cube perpendicular to the top surface and two sides). Two types of macro-fibres with either 10 or 20 mm embedment length were tested. The obtained load–displacement diagrams from pull-out tests demonstrate that the bond performance (characterised by the strength and deformation modulus) of the “top” fibres is almost 20% weaker than fibres positioned to the side surfaces. At the second stage, one type of macro-fibre was chosen for further experimentation of the feasibility of improving the bond performance through the use of colloidal silica or steel micro-fibres. This investigation stage employed an additional 36 concrete specimens. The use of steel micro-fibres was found to be an efficient alternative. The success of this solution requires a suitable proportioning of the concrete.This article belongs to the Special Issue Advanced Composites: From Materials Characterization to Structural Application (Second Volume)This project has received funding from the European Regional Development Fund (Project
No. 01.2.2-LMT-K-718-03-0010) under grant agreement with the Research Council of Lithuania (LMTLT). The APC
was funded by Vilnius Tech
Universiteto ekonominio tvarumo modeliavimas
Full text linkThe doctoral dissertation examines university activity at sustainable economic development. The analysis of scientific literature reveals that the sustainable development of universities is mostly explored by scientific works at the macro level. The quality of higher education is another topic that is widely discussed by researchers. There is a lack of scientific works that examine the possibility for universities to strive for sustainable development at the institutional level. This dissertation addresses the need to create a model that would help universities to move towards sustainable economic development. The research object is the economic sustainability of the university. The aim of the dissertation is to create a model of university economic sustainability and to assess the impact of areas of the university activity on the economic sustainability of the university. The main objectives of the thesis are as follows: to analyse scientific literature on the practical implementation of the concept of sustainable development at universities; to examine the relationship between the quality management of university activities and the concepts of sustainable development from a theoretical perspective; to exemine the complexity of the university's economic sustainability by analyzing the university processes and the diversity of stakeholdres surrounding the university and to perform the economic substantiation of the complexity of the university's activities; to assess the relationship between the quality of the study process at the university and the concept of sustainable development to design the theoretical model for the comprehensive assessment of the study process at the university to make an economic validation of university’s activities; to develop an algorithm for the comprehensive quality assessment system for the study process at the university with reference to theoretical simulation using the Analytical Hierarchy Process (AHP) method, and to verify the suitability of the comprehensive quality assessment model for the study process at the university regarding the examples of Lithuanian and foreign universities.
This dissertation is composed of an introduction, three chapters, general conclusions, a references list, a list of scientific publications, and appendices. The first chapter explores the scientific background in the field of the theoretical concept of university economic sustainability. The second chapter explains the comprehensiveness of university activities and composes the model of comprehensive assessment of the university-level higher education study process. The third chapter discusses the empirical research and findings at Lithuanian and foreign universities.
Five articles on the dissertation’s subject were published: one in scientific journal included in the publications of the Clarivate Analytics database, three in a publications of other international databases, one in peer-reviewed international conference proceedings. The findings of the doctoral thesis were presented in two international scientific conferences.Dissertatio
Kogeneracinės jėgainės šilumos akumuliacinės talpos veikimo režimų tyrimai
No full textSummary of doctoral dissertationSummary of doctoral dissertatio
Research and assessment of the total carbon content and carbon dioxide emissions from soils of different land-use purpose
No full textDisertacijoje nagrinėjamas bendrosios anglies kiekio dirvožemyje ir iš dirvožemio išskiriamo anglies dioksido dydžio sąryšis. Pagrindiniai trimo objektai yra bendrosios anglies kiekis ir anglies dioksido emisijos Neries regioninio parko, Paluknio, Užpalių ir Pailiepio vietovėse.
Darbe sprendžiami uždaviniai: bendrosios anglies kiekio ir anglies dioksido emisijos įvertinimas skirtingos žemėnaudos dirvožemiuose; anglies dioksido išsiskyrimo iš dirvožemio dėsningumai ir sąsaja su bendrosios anglies kiekiu. Trečias uždavinys – modeliavimu įvertinti bendrosios anglies pokyčius ir anglies dioksido emisijos prognozinius kiekius bei priklausomybė nuo klimato rodiklių.
Disertaciją sudaro įvadas, šeši skyriai, rezultatų apibendrinimas, literatūros sąrašas ir autoriaus publikacijų disertacijos tema sąrašas.
Įvadiniame skyriuje aptariama tiriama problema, darbo aktualumas, aprašomas tyrimų objektas, formuluojamas darbo tikslas bei uždaviniai, aprašoma tyrimų metodika, darbo mokslinis naujumas, darbo rezultatų praktinė reikšmė, ginamieji teiginiai. Įvado pabaigoje pristatomos disertacijos tema autoriaus skelbtos publikacijos ir konferencijų pranešimai bei disertacijos struktūra.
Pirmasis skyrius skirtas literatūros analizei. Analizuojama informacija apie dirvodaros energetiką, medžiagų balansą, organinės medžiagos susidarymo ir skaidymosi keliai. Įvertinama, kas nulemia dirvožemio dujų sorbcines savybes ir anglies dioksido emisijos kiekį. Aptariami modeliai, kurie taikomi bendrosios anglies kiekiui prognozuoti ir anglies dioksido emisijos pokyčiams įvertinti. Skyriaus pabaigoje formuluojamos išvados.
Antrajame skyriuje pateikti bendrosios anglies tyrimai, atlikti Neries regioniniame parke, Paluknio, Užpalių ir Pailiepio vietovėse, taip pat atliktų tyrimų metodika ir suformuluotos išvados.
Trečiajame analizuojami bendrosios anglies kiekio pH, sunkiųjų metalų ir dirvožemio tyrimai.
Ketvirtajame skyriuje pateikti anglies dioksido išskyrimo iš dirvožemio tyrimai atlikti Neries regioniniame parke.
Penktajame pateikiami bendrosios anglies kiekio ir anglies dioksido emisijos prognoziniai modeliavimo rezultatai, atlikti DNDC modeliu. Šeštame skyriuje pateikiamas išradimo patobulinimas, skirtas dirvožemio ėminiams.
Disertacijos tema yra paskelbti 5 moksliniai straipsniai: du – mokslo žurnale „ISI Web of Science“ duomenų bazėje; vienas – mokslo žurnale, įtrauktame į Thomson ISI sąrašą; vienas – mokslo žurnale, referuojam-ame tarptautinėse duomenų bazėse; vienas – tarptautinės ir respublikinės konferencijos medžiagoje.Daktaro disertacij
The investigation of electromagnetic processes in electromagnetic launchers using colossal magnetoresistance sensors
No full textSummary of doctoral dissertationSummary of doctoral dissertatio
The Reinforced Spun Concrete Poles under Physical Salt Attack and Temperature: A Case Study of the Effectiveness of Chemical Admixtures
No full textThe present paper focused on the investigation of the effectiveness of using various chemical admixtures and their effect on the strength and deformability of the reinforced spun concrete members—the supporting poles of the overhead power transmission lines—under the unfavorable long-term combined action of the aggressive salt-saturated groundwater and the temperature changes. According to the long-term experimental program, 96 prismatic spun concrete specimens were subjected to multi-cycle (25-50-75 cycles) processing under the combined aggressive environmental conditions. It has been found that chemical admixtures which decrease the initial water-cement ratio produce a considerable positive effect on the mechanical properties of spun concrete used in hot and arid climates and exposed to physical salt attack (PSA). Superplasticizers decrease the initial water-cement ratio the most, and, due to a unique concrete compaction method used, they produce the most homogeneous and dense concrete structure. They can be recommended as most effective in increasing the durability of spun concrete used under the above-mentioned aggressive environmental conditions.Kliukas, R.; Jaras, A.; Lukoševičienė, O. The Reinforced Spun Concrete Poles under Physical Salt Attack and Temperature: A Case Study of the Effectiveness of Chemical Admixtures. Materials 2020, 13, 5111
A Novel CRITIC-Fuzzy FUCOM-DEA-Fuzzy MARCOS Model for Safety Evaluation of Road Sections Based on Geometric Parameters of Road
No full textTrends of globalization very often cause the emergence of phenomena that asymmetrically affect the overall sustainability of the transport system. In order to predict certain situations and potentially be able to manage the transport system, it is necessary to manage risk situations and traffic safety in a timely manner. This study has conducted an investigation which implies defining the level of safety of a total of nine sections of two-lane roads. The main aim of the paper is to create a new multiphase model consisting of CRITIC (The CRiteria Importance Through Intercriteria Correlation), Fuzzy FUCOM (Full Consistency Method), DEA (Data Envelopment Analysis), and Fuzzy MARCOS (Measurement Alternatives and Ranking according to the COmpromise Solution) methods for determining the level of traffic safety on road sections under the conditions of uncertainty. In order for the created model to be adequately applied, eight parameters were created, and they were classified through four inputs and four outputs. To calculate the significance of the inputs, the CRITIC method based on the symmetric correlation matrix was used, and taking into account the nature of the outputs, the Fuzzy FUCOM method based on averaged values using the fuzzy Bonferroni Mean (BM) operator was applied to determine their weights. To determine the degree of safety, the DEA model was created. After that, the Fuzzy MARCOS method was used in order to determine the final ranking of the remaining five sections of the road network. Finally, the verification of results was performed through three phases of Sensitivity Analysis (SA).This article belongs to the Special Issue Sustainability Assessment using Uncertain Decision Making Approaches in the Era of Globalizatio
Development of energy efficient photovoltaic microinverter
No full textDarbe siūlomi sprendimai ir atliekami tyrimai skirti fotovoltinio mikroinverterio naudingumo koeficiento didinimui bei jo tiekiamos srovės į elektros tinklą netiesinių iškraipymų mažinimui. Sukurtas ir eksperimentiškai ištirtas energetiškai efektyvus vienos pakopos fotovoltinis tinklo mikroinverteris. Sukurtas valdymo algoritmas ir jį įgyvendinantis valdiklis tiekiamos į elektros tinklą mikroinverterio srovės valdymui.
Disertaciją sudaro įvadas, trys skyriai, bendrosios išvados, naudotos literatūros ir autoriaus publikacijų disertacijos tema sąrašai.
Įvadiniame skyriuje aptariama tiriamoji problema, darbo aktualumas, aprašomas tyrimų objektas, formuluojamas darbo tikslas bei uždaviniai, aprašoma tyrimų metodika, darbo mokslinis naujumas, darbo rezultatų praktinė reikšmė, ginamieji teiginiai. Įvado pabaigoje pristatomos disertacijos tema autoriaus paskelbtos publikacijos ir pranešimai konferencijose bei disertacijos struktūra.
Pirmajame skyriuje atlikta fotovoltinių inverterių savybių mokslinių tyrimų analizė. Išanalizuoti fotovoltinių mikroinverterių veikimo principai, aptarti jų privalumai ir trūkumai, išanalizuotos mikroinverterio išėjimo srovės valdymo sistemos. Apžvelgti mikroinverteriuse naudojami nuolatinės įtampos keitikliai. Suformuluoti disertacijoje uždaviniai.
Antrajame skyriuje pateikti sukurtų nuolatinės įtampos grįžtamojo (angl. Flyback) keitiklio su alternatyvia aktyviąja viršįtampio gesinimo grandine ir dviejų raktų grįžtamojo keitiklio eksperimentiniai naudingumo koeficiento tyrimų rezultatai. Jie palyginti su klasikinio nuolatinės įtampos grįžtamojo keitiklio tyrimų rezultatais. Pasiūlytas naujas vienos pakopos fotovoltinis mikroinverteris, sukurtas naudojant du dviejų raktų grįžtamuosius nuolatinės įtampos keitiklius. Atlikti mikroinverterio naudingumo koeficiento ir išėjimo įtampos bei srovės tyrimai.
Trečiajame skyriuje pasiūlytas naujas proporcinis integralinis (PI) valdiklis su keičiamu laike proporciniu koeficientu, ištirta mikroinverterio srovės valdymo sistema su sukurtu valdikliu. Gauti mikroinverterio išėjimo srovės spektrai, įvertinti ir palyginti srovės netiesiniai iškraipymai. Ištirta mikroinverterio elementų parametrų įtaka naudingumo koeficientui. Tyrimai atlikti eksperimentiškai bei modeliuojant. Sukurto mikroinverterio naudingumo koeficiento padidinimui, patobulintas jame naudojamas grįžtamasis transformatorius.
Pagrindiniai disertacijos rezultatai paskelbti 7 mokslinėse publikacijose – 5 iš jų atspausdinti recenzuojamuose mokslo žurnaluose, 2 konferencijų leidiniuose. Rezultatai viešinti 10 mokslinių konferencijų.Daktaro disertacij
The Efficiency of Utilisation of High-strength Steel in Tubular Profiles
No full textThe use of high-strength steel (HSS) is a current trend of the construction industry. Tubular profiles are widely used in various structural applications because of their high stiffness-to-weight ratio, exceptional resistance to torsion, and aesthetic appearance. However, the increase of the strength for the same elastic modulus of the material and geometry of tubular profiles is often not proportional to the rise of the load-bearing capacity of the structural element. The obtained experimental results support the above inference. The study was based on the flexural test results of two groups of HSS and normal-strength steel (NSS) tubular specimens with a 100 × 100 × 4 mm (height × width × thickness) cross-section. Numerical (finite element) simulation results demonstrated that the shape of the cross-section influenced the efficiency of utilisation of HSS. The relationship between the relative increase of the load-bearing capacity of the beam specimen and the corresponding change of the steel strength determined the utilisation efficiency.This article belongs to the Special Issue Advanced Composites: From Materials Characterization to Structural ApplicationThe European Social Fund provided a financial support for this research within the framework of the project “Development of Competences of Scientists, other Researchers and Students through Practical Research Activities” (project no. 09.3.3-LMT-K-712)