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Defended Dissertations in 2011


Andrius Devižis
Author: Andrius Devižis
Dissertation title: Charge carrier transport in conjugated polymer films revealed by ultrafast optical probing
Fields of science: Physical sciences, Physics (02P)
Scientific supervisor: Habil. Dr. Vidmantas Gulbinas
Defence of the dissertation: 2011-01-18
 
Summary
Conjugated polymers are promising candidates for applications in all kinds of organic optoelectronic devices: OLEDs, organic field-effect transistors (OFETs) and organic photovoltaic cells. Charge carrier mobility is one of the crucial parameters that determine applicability of organic semiconductors. Despite record braking bulk mobilities above 10 cm2/Vs in single cystalline organic materials, polymers have characteristic mobilities of the order of 10-2 ÷ 10-6 cm2/Vs. Mobility values obtained for thick samples might not be sufficient to fully describe charge transport on the nano scale, such as in organic devices. Functional layers of these devices usually have a thickness of tens of nanometers. It is a challenge to probe charge transport on short distance and ultrafast time scale due to the lack of an appropriate experimental method. A novel experimental method for this purpose was developed in the course of this thesis in the laboratory of Molecular compound physics (Institute of Physics). It is based on time-resolved electric field-induced second harmonic generation (TREFISH).
The main goal of this work was to investigate transport features of photogenerated electrical charge by means of TREFISH technique in n-conjugated polymers. The main attention was focused on transport dynamics of the photogenerated charge carriers. Several tasks have been completed to achieve this goal. TREFISH measurement setup was implemented in the laboratory of Molecular compounds physics, and applicability of the method has been verified. Measurements were performed on three different model polymers: methyl substituted ladder- type poly(para-phenylene) (MeLPPP), poly(fluorene-co-benzothiadiazole) (F8BT) and poly(spirobifluorene-co-benzothiadiazole) (PSF-BT), having different morphological and chemical structure. Relaxation of the excited state and generation of the charge carriers in a new co-polymer PSF-BT have been determined as well. These findings provided complementary information for determination of charge transport properties from TREFISH measurement data. Temperature dependences were investigated for the examination of temperature-activated transport concept. Monte-Carlo computer simulations within the framework of Gaussian disorder model were performed to interpret motion of the electric charge carrier on short time and length scales.
It has been found that motion of photogenerated charge carriers in π-conjugated polymer films experiences rapid dynamics after excitation. Different time domains of charge transport were distinguished. Initial fast transport of photogenerated charge carriers corresponds to the carrier motion along the single polymer chain or conjugated segment of the polymer chain. Slowest carrier motion phase is well described by the stochastic drift, which is attributed to interchain  jumps and determines the macroscopic equilibrium mobility. Thus, the equilibrium mobility value is not applicable to the transport on nanometer scale up to tens of nanometers. In addition,  transport of photogenerated charge carriers initially is temperature-independent in PSF-BT polymer. Turnover from temperature-independent to temperature-activated carrier drift takes place on the time scale of about hundred of picoseconds and at the average drift distance of several nanometers. This phenomenon is related to relaxation of the carrier energy within density of states (DOS) and intrachain carrier motion.
Dynamic behavior of the photogenerated charge carrier transport in π -conjugated polymers has been experimentally examined in detail for the first time. These findings provide  knowledge about charge transport mechanisms in polymers. Also, they may give guidelines for the designers of organic electronic devices.
Andrius Garbaras
Author: Andrius Garbaras
Dissertation title: Study of aerosol particle origin and dispersion by isotope ratio mass spectrometry
Fields of science: Physical sciences, Physics (02P)
Scientific supervisor: Prof. Dr. Habil. Vidmantas Remeikis
Defence of the dissertation: 2011-07-04
 
Summary
Objective and tasks af the work
The objective of this work was to investigate the origin, sources and formation of organic compounds and black carbon in atmospheric aerosol particles and to develop new research methods.
To attain this objective the following tasks have been formulated:
  • Development of the identification method for the aerosol origin and
  • source by investigating the carbon isotope mass ratios.
  • Illustration of possibilities of the carbon isotope ratio method by identifying the aerosol particle origin during the air mass long-range transport at the Preila Environmental pollution research station.
  • Investigation of variation of the black carbon amount in aerosol particles in the diesel engine exhaust depending on the engine working parameters and fuel composition.
  • Investigation of the carbon isotope ratio variation in natural aerosol depending on the aerosol particle size distribution.
  • Determination of the partial contribution of natural and anthropogenic particles in aerosol by investigation of stable carbon and radiocarbon isotope ratio changes during continental and marine air mass transport.
Novelty of the work
For the first time the partial contribution of natural marine origin arc anthropogenic aerosol particles during marine and continental air mass transport by measuring δ13C variation with the isotope ratio mass spectrometer and radiocarbon (14C) concentrations with the accelerator mass spectrometer in atmospheric aerosol particle samples has been determined.
By employing a multi-stage impactor and a stable isotope mass spectrometer, for the first time the influence of natural aerosol particle sources has been evaluate aerosol particles of different size.
By investigating the isotope ratio changes in black carbon emitted by the transport (powered by diesel fuel), the influence of biodiesel fuel supplements on the black carbon concentration in exhaust has been evaluated for the first time.
 
Conclusions
  • The carbon isotope ratio in anthropogenic origin aerosol particles corresponds to the isotope ratio of burnt fuel (biomass and fossil fuel), and this parameter is suitable for identification of the aerosol particle source. In the aerosol particles from the environment in the size range up to 1 µm, vegetative organic carbon is prevailing. The largest organic and elemental carbon ratio value (OA/EA= 13.1) was observed in the accumulation mode, in the particle size interval from 0.18 µm to 0.32 µm. Such value is typical of background localities. δ13C values of elemental carbon in the accumulation mode varied from -22,9 ‰ to -26,3 ‰, while organic carbon δ I3C values were about -28 ‰. From the carbon isotope ratio values it has been determined that the elemental carbon precursors in aerosol particles were from burning products, while the source of organic carbon was volatile organic compounds from vegetation.
  • In the aerosol particles larger than 1 µm the carbonate component is observed. In the range of all sizes the carbon isotope ratio of vegetative organic origin component does not change. By combining aerosol mass and isotope ratio mass spectrometry methods it has been determined that in aerosol particles of the accumulation mode (in the size interval from 0.056 to 1 µm) from 80 % to 90 % of carbon is of organic (vegetative) origin.
  • The measured δI3C values in diesel fuel and diesel fuel with 10% and 40 % of ethanol varied from -29 ‰ to -30,7 ‰. It has been determined that the isotope fractionation between fuel and aerosol particles was 1.3 ‰and did not depend on the engine load and revolutions per minute, when diesel fuel with 40 % of ethanol was used. Such fractionation factor between this type of fuel and aerosol particles indicates that ethanol did not participate in the aerosol particle formation.
  • The method of identifying the aerosol particles from marine, biomass and continental fossil fuel burning products has been developed and experimentally realized. It has been shown that in clean air over the North Eastern Atlantic Ocean the marine biogenic submicron carbonaceous particle source dominates (80 %). In continental air masses on average there are 30 % of marine aerosol particles, 40 % from fossil fuel and 30 % from biomass burning aerosol particles.
Marijus Brikas
Author: Marijus Brikas
Dissertation title: Microprocessing of silicon and metals with high-pulse-repetition-rate picoseconds lasers
Fields of science: Technological sciences, Material engineering (08T), Laser technology (T 165)
Scientific supervisor: Dr. Gediminas Račiukaitis
Defence of the dissertation: 2011-03-23
 
Summary
The scientific tasks of this work
During preparation of this thesis, the high-pulse-repetition-rate lasers were developed.
Suitable niches for applications of such lasers had to be found. Thus, tasks of this work were formulated carrying out on-demand and fundamental research:
  • to investigate the impact of microprocessing with picosecond pulses for silicon;
  • to investigate accumulation effects processing silicon and metals with high- repetition-rate lasers;
  • to find out optimal focusing conditions to maximize the energetic ablation efficiency;
  • to determine the experimental procedures for optimization of processing parameters;
  • to investigate the opportunities of nanoparticle generation by laser ablation in a liquid medium.
Statements to be defended
  • The maximal ablation rate of a material using the fixed pulse energy can be achieved by selecting optimal focusing conditions, when laser fluence at the center of the Gaussian beam is 7.4 times higher than the ablation threshold of the material.
  • During laser cutting of silicon in the air, doping of the laser-cut surface with carbon takes place at a depth of up to 5 µm from carbon dioxide in the ambient, and the resulting silicon carbide has influence on the surface quality of the cut.
  • Heat abstraction from the workpiece, during laser cutting of stents from Nitinol, limits the applicable average laser power and the effective cutting speed.
  • Picosecond laser ablation of silver and gold in the liquid generated a stable colloidal nanoparticle solution with a narrow size distribution.
Conclusions
  • The ablation threshold decreases when several pulses are applied due to incubation of defects. Accumulation effects are stronger for picosecond than nanosecond pulses and this can be caused by stronger shockwaves for shorter pulses.
  • The volumetric ablation efficiency non-linearly depends on the laser pulse energy The model which allows determination of optimal beam focusing conditions for maximum evaporation rate at given pulse energies, was developed. The maximal ablation rate of the material was achieved when laser fluence in the center of the Gaussian beam was 7.4 times higher than the ablation threshold of the material.
  • Optimal focusing conditions limit the precision of machining due to a large spot size in focus. Therefore, the process parameters have to be intelligently controlled for fast machining and effective energy usage, combining rough and fast machining with accurate and slow one.
  • It has experimentally been defined that the energetic ablation efficiency at high pulse energies is limited due to plasma shielding. The increase of laser pulse energy is not an effective way to increase productivity. The laser beam split and parallel processing can be applied for efficient application of high pulse energy lasers in microfabrication.
  • Empirical relationships were found between process parameters and machining performance of silicon for laser percussion drilling and cutting.
  • The contact of silicon with air molecules during the ablation process in the presence of laser radiation and laser-ignited plasma initiated thermo-chemical reactions. The thermal gradient stipulated diffusion of surface-adsorbed ions, leading to doping of silicon with carbon. Silicon carbide type bonds were formed below the surface and could be the reason for “channeling” at the wafer cut. Shielding gas of nitrogen can prevent carbonization of the surface.
  • Picosecond lasers were used for cutting the electrostatic micro-positioning actuator from silicon and forming 3D structures in instrumental steel and nitinol.
  • Laser-machined surface quality depends on process parameters and the minimum value of Ra = 130 nm is achieved for a copper sample.
  • Heat abstraction from the workpiece, during laser cutting of stents from nitinol, limits the potential use of the average laser power and the effective cutting speed.
  • The picosecond laser can effectively generate nanoparticles of a narrow size distribution in liquids The silver nanoparticle generation rate in water was 8.6 µg/s at 5.5 W average laser power and 50 kHz repetition rate. Gold nanoparticles were generated at 5.9 µg/s rate in n-hexan medium. Nanoparticles produced by laser ablation are stable for a long time.
Mikhail Grishin
Author: Mihail Grishin
Dissertation title: Dynamic of continuously pumped regenerative laser amplifiers
Fields of science: Physical sciences, Physics (02P)
Scientific supervisor: Prof. Habil. Dr. Vidmantas Gulbinas
Defence of the dissertation: 2011-06-23 
 
Summary
This thesis presents a detailed study of continuously pumped regenerative amplifiers based on long-relaxation-time laser media. The goal of the research was to develop a general pattern of complex dynamics peculiar to such systems at high repetition rates and to find a way to improve performance characteristics affected by instabilities. Basic parameters of the optimally coupled regenerative amplifier operating in stable regime were derived in analytical form. They include optimum initial and final gains, the maximum output pulse energy, the power dissipation, the multi-pass 5-integral and the roundtrip number providing the maximum output energy. A comprehensive pattern of existing dynamic regimes (stable, quasi-periodic and chaotic) was represented in space of controlling parameters. It has been found that the space of unstable operation decreases as the seed pulse energy increases. A method of stability diagrams, which forms a systematic approach to the optimization of regenerative amplification dynamics and in particular allows one to determine the seed pulse level sufficient to maintain the operation stable, has been developed. Performance characteristics were determined in the critical range of repetition rates, where instabilities are pronounced at the most and analytical solutions are unavailable. The experiments, carried out using the diode pumped Nd:YVO4 laser system, exhibited a good agreement with theoretical inferences. It has been demonstrated that increase in the seed pulse energy reduces the critical range of repetition rates and thus allows the system capabilities to be exploited to full extent.
Mindaugas Gedvilas
Author: Mindaugas Gedvilas
Dissertation title: Self-organization in thin metal films under laser irradiation
Fields of science: Technological sciences, Material engineering (08T), Laser technology (T 165)
Scientific supervisor: Dr. Gediminas Račiukaitis
Defence of the dissertation: 2011-11-08
 
Summary
The present PhD thesis is the experimental and theoretical analysis of ripple formation in the thin chromium film on a glass substrate under irradiation with a sequence of partially overlapping laser pulses. The beam of a nanosecond laser tightly focused to a line was applied for the back-side ablation of the thin chromium film on the glass substrate. The stripe-like area ablated with a single laser pulse had cylindrical ridges of the melted metal. The partially overlapping pulses formed a complicated structure made of the metal remaining from the ridges. Regular structures, ripples, were formed when laser fluence was slightly above the single-pulse removal threshold and the shift between pulses was less than half width of the stripe ablated with a single laser pulse. The regular ripples were located periodically with the period of 2.5 - 4 µm. Ripples were orientated perpendicularly to the long axis of the beam spot and their length increased with every shifted pulse. Formation of regular diffraction gratings was experimentally implemented by using the above-mentioned technique. The grating period changed linearly with the laser fluence and it decreased with increasing the shift between pulses. Different models of the ripple formation in the thin metal film were considered, and the Plateau-Rayleigh instability of the cylindrical ridge formation during laser ablation appeared to be the most probable process responsible for initiation of the ripple formation. The Marangoni convection of the molten metal from hot areas to cold ones was the stabilizing process of the steady ripple formation. The possible ripple formation was investigated in different metal films on the glass substrate: aluminum, copper, gold and silver. Diverse behavior of the films depending on the metal, the shift between pulses and laser fluence was observed. Strong film adhesion to the glass substrate was important for ridge formation. The cylindrical ridge formation during laser ablation was found to be essential for initiation of the ripple formation.
Nerijus Slavinskis
Author: Nerijus Slavinskis
Dissertation title: Short pulse Q-switched longitudinally diode pumped solid state minilasers: generation, characterization and application
Fields of science: Technology sciences, Material engineering (08T), Laser technology (T 165)
Scientific supervisor: Prof. Habil Dr. Aleksandr Dementjev
Defence of the dissertation: 2011-06-28
 
Summary
Main objectives:
  • To develop a mathematical traveling wave model of generated pulses of actively and passively Q-switched longitudinally diode pumped solid state lasers, perform numerical modelling and show advantages of this model compared with the usually used point laser model.
  • To estimate causes of timing jitter of indicated types of lasers and to measure timing jitter values of generated pulses of created experimental minilasers when pulsed pumping is used, to perform timing jitter numerical modeling and suggest its reduction possibilities.
  • To demonstrate practical application possibilities of created minilasers, for this purpose making effective pulse compression by stimulated Brillouin scattering and measuring nonlinear refractive indexes of optical materials by the Z-scan method, and also to show that parameters of generated pulses satisfy the requirements for portable laser cleaning and LIBS diagnostic systems.
Practical value of this work:
The proposed timing jitter reduction method in a passively Q-switched laser using sharp triggered start up and release of modulated diode pumping can be easily realized experimentally. It allows one substantially to expand application possibilities of simple and robust passive Q-switch lasers.
Showed that given short (~ 1 ns) pulses of a diode pumped Q-switched Nd:YAG minilaser and using easily available CC14 liquid it is experimentally possible to achieve pulses shorter than ~ 60 ps with 50 % energy efficiency whose duration and amplitude stability and timing jitter parameters are 3-4 times better, while duration of compressed Stokes pulses are almost two times shorter than in a standard stimulated Brillouin scattering compression experiments.
Created short pulse minilasers, especially with the effective pulse compression, easily meet the criteria for Z-scan, portable laser cleaning and LIBS diagnostic systems. Thus, it is hope for expansion of these lasers applications in the processes of diagnostics and technology.
 
Main results and conclusions
  • For the numerical analysis of the laser generation dynamics a more detailed traveling wave model has been developed. This model includes the level splitting due to the Stark effect, Boltzman factors of population densities, relaxation times from the excited lasing levels, direct pumping possibility and other parameters of active and passive elements including their distribution in the resonator.
  • The developed software allows control of time and spatial distributions of forward and backward traveling waves of pump and laser intensities in the resonator, time and spatial changes of the level population densities in the active element and the saturable absorber. Performed calculations have shown that generation dynamics highly depend on the optical element sequence inside the resonator, the shapes of the pulses traveling in opposite directions are different and their temporal modulation when fast Q- switching is used has the period shorter than the resonator round-trip time.
  • The CW pumping modulation of passively Q-switched lasers by triggered high enough power rectangular pulses with their steeply start up and automated cancellation after the laser pulse is generated allows one to synchronize pulse generation moments with external events and to reduce their timing jitter by two orders of magnitude. Such pumping modulation can be easily realized experimentally and allows one to significantly extend application possibilities of these simple and robust passively Q-switched lasers.
  • The method of second order moments adapted for the data processing obtained by the moving knife method allows one to find beam widths of the general astigmatic beam in directions of principal ellipse axes and ellipse orientation angle in the laboratory coordinate system. This modified moving knife method can be successfully used for measurements of beam widths and propagation ratios defined by second order moments in the spectral ranges where multielement detectors like CCD do not exists.
  • The effective short pulse compression is realized by the new method, when in the medium with electrostriction nonlinearity the picosecond pulse is generated not from the level of spontaneous-scattering noise, but from the seed signal of the same carrier frequency and duration, which is counter propagating to the focused pump pulse. This allows us using easily available CC14 liquid to compress short (~ 1 ns) pulses of the diode pumped Nd:YAG laser with 50 % energy efficiency up to pulses shorter than 60 ps, whose duration, amplitude stability and timing jitter parameters are 3-4 times better, while duration of compressed pulses is almost two times shorter than that of compressed Stokes pulses in standard stimulated Brillouin scattering experiments.
  • Using Z-scan method for the small nonlinear refractive index measurements of a fused silica good compatibility between the measured values (γ2 ≅ 2.6.10-16 cm2/W) is obtained only when during the processing of experimental data the temporal and spatial parameters of the used (compressed by stimulated Brillouin scattering, single and multiple mode pulses of minilaser) pulses are properly (through the coefficients appropriately calculated from experimental data) taken into account.
  • Created short pulse actively Q-switched minilasers (with negative feedback) provide possibility to synchronize the pulses of such minilasers allowing one to perform perspective LIBS technique with sequence of two pulses. Thus, created short pulse minilasers, especially with the effective pulse compression, easily meet the criteria for Z-scan, portable laser cleaning and LIBS diagnostic systems. Thus, there is hope for expansion of these lasers in the processes of diagnostics and technology.
Marius Treideris
Author: Marius Treideris
Dissertation title: Formation and investigation of hybryd nanostructures
Fields of science: Technological sciences, Material engineering (08T)
Scientific supervisor: Assoc. Prof. Habil. Dr. Irena Šimkienė
Defence of the dissertation: 2011-08-26
 
Summary
 
Aim of the work
The aim of this work is to develop the well-controlled chemical and electrochemical methods for the formation of materials with nanometer-sized elements on porous dielectric or semiconductor basis, to characterize such materials by their physical properties and to propose their practical use.
 
Scientific novelty
The formation technology of morphology-controlled porous silicon layers was designed by using the 1.2 µm light illuminating the etched surface or the bottom side of the substrate. The technology for the formation of organic-inorganic hybrid nanostructures on the basis of porous Si was developed. The main regularities in the interaction between Si surface and porphyrin were revealed.
The dependence of the morphology of electrochemically formed porous GaP on technological parameters was determined. The GaP nanorods were formed for the first time by using electrochemical etching in nitric acid with organic solvent.
Iron/ iron oxide nanoparticles and clusters in Si02 thin-layer matrix were formal and the iron charge state was found to depend critically on the essential features of precursor and the annealing atmosphere.
 
General conclutions
  • Si n-type porous layer morphology depends not only on technological parameters, but also on the illumination. Illuminating the surface of the incandensend lamp (λ max = 1.2 µm, 600 lux), pores oriented perpendicular to the surface are formed. Increasing light intensity (1200 lux), the pores are branched, intertwined, and they form an irregular structure. Illuminating the sample from the substrate backside, the pores are uniform in depth, they are equal to 2.5 to 3 µm in size. Such structural differences are because of holes generated at various illumination conditions. When the etched surface is illuminated, the holes are generated in the walls between the pores and the pores became branched. While illuminating the backside of the substrates, the holes are generated uniformly at the bottom of sample and are moved towards the surface by the electric field. Then, the holes reach the bottom of pores, oxidize Si atoms. In this case, the pores grow continuously with the diameter practically unchanged.
  • Deposited from aqueous solution, the self-organization of porphyrins is different on various substrates. On a hydrophilic surface, a continuous layer of smooth J-aggregates or FeTPPS dimers is formed. On the partially hydrophobic c-Si substrates with negatively surface, the adsorbed positively charged aggregates with the oriented structure are formed. Iron porphyrins on a partially hydrophobic surface formed the rings of monomers and nanoparticles of dimers inside the ring.
  • Pore size and morphology of por-GaP, formed by electrochemical etching technique, depend essentially on the type of electrolyte. When sulphuric acid and hydrofluoric acid were used at low current density, the structures typical for these electrolytes were observed. Structures etched in sulphuric acid, hydrofluoric acid and nitric acid with high current densities have been obtained for the first time. The fine structure in reflection spectra of por-GaP correlate with particular features in the morphology of the samples. For the first time, GaP nanorods were formed by electrochemical etching. A prototype of gas sensor on the basis of por-GaP/ITO was tested.
  • The sol-gel technology for the formation of controlled-porosity silicon oxide layers on crystalline silicon substrates was designed. The chemical procedure for insertion of iron and its compounds into porous Si02 layers was investigated. The method for iron and iron oxide formation in dielectric matrix by varying the annealing conditions was developed. It was found that forming the hybrid structures, on the one hand, the dielectric medium porosity can be controlled by changing the precursor composition and technological regimes. On the other hand, the formation of Fe/Fe-0 nanostructures and clusters can be controlled by both precursor composition and annealing procedure.
Nina Prokopčiuk
Author: Nina Prokopčiuk
Dissertation title: Application of probabilistic methods for ionizing radiation dose assessment
Fields of science: Physical sciences, Physics (02P)
Scientific supervisor: Dr. Tatjana Nedveckaitė
Defence of the dissertation: 2011-11-25
 
Summary
The main aim of the doctoral dissertation
The aim of this work is to determine, characterize and assess the probable impact of ionizing radiation on the public health and environment (including flora and fauna) in the environment of nuclear power engineering objects (in case of the Maišiagala radioactive waste disposal facility - by evaluating a possible impact on humans and in case of Ignalina NPP cooling pond Lake Drūkšiai - by evaluating a possible impact on freshwater ecosystem biota) as well as to determine whether this activity after evaluating its character and impact on the environment meets the standards valid in the Republic of Lithuania or in the European Union and is permissible at a selected site at present or in the future.
 
Practical importance of the dissertation
Referring to the long-term observation data of radionuclide migration in the environment of the Maišiagala repository and by applying probabilistic methods, a significant human exposure decrease has been determined after installation of additional protective barriers. It has been determined that the Ignalina NPP cooling pond, Lake Drūkšiai, from the point of view of biota radiation protection (background exposure significantly exceeds the anthropogenic one, conservative risk factors make up about 10% of the screening value proposed in the European Union) could be used for the further development of nuclear power engineering taking into account a possible impact of temperature, chemical contamination and other factors. The annual effective dose human exposure due to consumption of fish from the INPP cooling pond, Lake Drūkšiai, is of the order of some µSv. Based on these data it can be stated that amateur fishing in Lake Drūkšiai from the human radiation protection point of view is possible.
 
Conclusions 
  • Based on the evaluation results of human exposure by applying the probabilistic uncertainty analysis methods introduced in the RESRAD-OFFSITE program and taking into consideration the selected scenarios and parameter uncertainties, it has been determined that in the environment of the Maišiagala repository after installation of additional protective barriers, the annual effective human exposure dose is significantly lower as compared to the annual limited dose of 0.2 µSv determined in the environment of nuclear power engineering objects and 95th percentile does not exceed the exposure of 1 mSv per year regulated in the main human radiation protection standards in the Republic of Lithuania.
  • In case of the near-surface Maišiagala repository of radioactive waste, after evaluation of the radionuclide migration and human exposure uncertainties by applying the correlation and regression analysis introduced in the RESRAD-OFFSITE program it has been determined that the saturated zone horizontal lateral dispersivity, the hydraulic gradient and hydraulic conductivity of saturated zone, the precipitation amount and water consumption have the largest impact on the human annual effective doses.
  • The analysis of the obtained data has shown that out of all radionuclides taken for storage in the environment of the Maišiagala repository 3H and l4C are regarded as dangerous (the largest exposure is related to drinking-water) together with 36C1 (the largest exposure due to consumption of local food).
  • The exposure dose rate of reference organisms of Lake Drūkšiai, the INPP cooling pond, biota (evaluated using the ERICA code) due to INPP discharges and water pathway radionuclide migration from a hypothetic Stabatiškės radioactive waste disposal facility shows that the dose rate due to the impact of ionizing radiation of anthropogenic radionuclides does not exceed the screening value of 10 µGy/hour recommended in the European Union.
  • Among reference biota organisms, the benthonic organisms (Insectum larvae, Crustacea, Bivalvia mollusca, Gastropoda) and vascular hydrophytes (Tracheophytes) of Lake Drūkšiai experience the largest exposure.
  • The analysis of the obtained data has shown that out of all radionuclides in Lake Drūkšiai bottom sediments after termination of the Ignalina NPP operation, from the biota total exposure point of view 60Co and l37Cs are regarded as dangerous, and the internal exposure dose rate value is also mostly caused by 90Sr activity concentration in bottom sediments.
  • The internal annual effective exposure dose of adults due to consumption of fish from the INPP cooling pond. Lake Drūkšiai, is of the order of several µSv.
  • It has been determined that the Ignalina NPP cooling pond, Lake Drūkšiai, in case of necessity and from the point of view of biota radiation protection (natural background exposure significantly exceeds the anthropogenic one, and conservative risk factors make up about 10 % of the regulated limited value of risk factors proposed in the European Union) can be used for further development of nuclear power engineering taking into account a possible impact of temperature, chemical contamination and other factors.
Tatjana Gric
Author: Tatjana Gric
Dissertation title: Electromagnetic field and dispersion characteristic calculations of open waveguides made of absorptive materials
Fields of science: Physical sciences, Physics (02P), Condensed matter: Electronic structure; Electrical, Magnetic and optical properties; Superconductors; Magnetic resonance; Relaxation; Spectroscopy (P260)
Scientific supervisor: Prof. Dr. Habil. Liudmila Nickelson
Defence of the dissertation: 2011-11-25
 
Summary
 
The object of the research
The object of the research is the waveguide made of absorptive materials.
The aim of the work - to find the peculiarities of the electrodynamical charac­teristics and to define the areas of their applications in the microwave devices.
 
Practical value
The obtained research results can be useful for creating of the microwave elec­tronic devices, for example, filters, modulators, phase shifters, absorbers. The created algorithms are useful in order to investigate the absorptive material waveguides of the complicated cross-shapes.
 
General conclusions
  • The open waveguides on the basis of absorptive materials with differ­ent shapes of the cross-section can be investigated using the created computer algorithms. We are able to calculate the dispersion characteristic and 2D and 3D electromagnetic field distributions. These results can be very useful constructing the waveguide device.
  • The created computer algorithms were used investigating the wave­guides made of the new materials, i. e. metamaterials, SiC. The got re­sults are in good agreement with the well known electrodynamical laws. By this way the created computer software could be used inves­tigating waveguides made of different materials.
  • The circular metamaterial waveguides (r = 2 mm, r = 2.5 mm) could be used as filters. However the losses of the first higher mode propa­gating in the waveguide with the smaller radius change in an irregular manner. By this reason the main mode could be modulated in the smaller waveguides.
  • The circular SiC waveguide (r = 2.5 mm) could be used as modulator. The wave phase could be modulated changing the temperature of the waveguide.
  • Changing the dimensions of the periodical waveguide, i.e. the dimen­sions of the air holes and their dimensions, we can reach the suitable electrodynamical characteristics of the waveguide, i.e. the dispersion characteristics and electromagnetic field distributions.
The patent
Gric, T.; Nickelson, L. & Asmontas, S. 2010-11-25, Waveguide Modulator. Lietuvos Respublikos patentas Nr. LT 5710. Application of Invention 2010-040 is given in Offi­cial bulletin of the state patent bureau of the republic of Lithuania 2010/11, ISSN 1648- 9985.
Audrius Pašiškevičius
Author: Audrius Pašiškevičius
Dissertation title: Synthesis of the vanadium oxide compounds and investigation by X-ray photoelectron spectroscopy method
Fields of science: Physical sciences, Physics (02P)
Scientific supervisor: Habil. Dr. Vladimiras Bondarenka
Defence of the dissertation: 2011-01-25
 
Summary
Aim and tasks of the work. The aim of dissertation is to synthesize the thin films of vanadium oxide, and vanadium molecular xerogels and bronzes using sol-gel technology and to study the chemical composition of these films using X- ray photoelectron spectroscopy (XPS) method. The following tasks were formulated:
  • To synthesize vanadium pentoxide xerogel and thin film using sol-gel technology method.
  • Using sol-gel technology method synthesize β-phase of vanadium- sodium xerogel and bronze, vanadium-ammonium oxide xerogels, vanadium-hydroquinone and vanadium-titanium-hydroquinone molecular oxide’s xerogels and bronzes.
  • Determine the binding energies of mixed valence vanadium ions in various hydrated compounds using XPS method.
  • To investigate the possibilities on practical application of hydrated vanadium compounds.
Scientific novelty. The thin films of vanadium pentoxide, β-phase of vanadium-sodium oxide xerogel bronze, molecular oxide bronzes of vanadium- hydroquinone, vanadium-titanium-hydroquinone and hydrated compounds of vanadium-ammonium was synthesized using sol-gel technology for the first time. The valence states of metals in mentioned compounds are determined.
Practical value. It is shown that it is possible to produce the thin films of vanadium oxide bronzes using simple methods. The possibility to use vanadium- ammonium oxide hydrated compounds as materials for producing the ammonium sensors is shown in the dissertation.
General Conclusions
The molecular oxide gels of vanadium-sodium, vanadium-hydroquinone and oxide bronzes of vanadium-hydroquinone are synthesized using sol-gel synthesis method for the first time and the XPS spectra of these compounds are analyzed. It is determined that:
  • The binding energy of V 2p3/2 peak is shifted to lower energy region about 1.3 eV of V4+ ions in V205based xerogels compared to the main V5+ peak.
  • The vanadium ions are in V5+ and V4+ states with reduction ratio of about 0.04 independent on the x value and the molybdenum ions are in Mo6+ states in (NH4)2-xHxV9Mo3O>31±δ.nH20 (x = 0, 1) gels. The position of the N 1s peak indicates that the nitrogen is connected with hydrogen in NH4 ions.
  • In V1.67Ti0.33O4.85.nH20/2HQ and V1.67Ti0.33O4.85.nH20/HQ (HQ — hydroquinone) xerogels the ions of titanium are in stable 4+ states independent of the hydroquinone intercalation degree (one or two HQ). Vanadium ions in all cases (one or two HQ) are in V3+, V4+ and V5+ valence states. The increase in quantity of hydroquinone leads to the growth of relative concentration of V3+ and V4+ ions.
  • The sol-gel method allows embed the molecules of ammonia into the matrix of vanadium pentoxide and that allows use the obtained oxide xerogels of vanadium-ammonium as initial material for manufacturing the ammonia gas sensors.
  • Using the sol-gel technological method it is possible to produce vanadium oxide xerogels intercalated with metal ions (in our case - sodium) as well as molecular xerogels (hydroquinone, ammonium) without much difficulty. The thin films of vanadium pentoxide. vanadium-sodium oxide bronze, vanadium-hydroquinone and vanadium-titanium-hydroquinone oxide bronzes can be prepared from xerogels heating them up to 580 K.