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- ItemOpen AccessA multi parametric measurement and control system implemented on flexible substrates with printing technologies
Τμήμα Φυσικής (ΔΔ)Μπαρμπάκος, Δημήτριος; Barmpakos, DimitriosIn this doctoral dissertation, materials for development of electronic devices via printing processes were evaluated. More specifically, two silver-based inks with different sintering requirements (chemical and thermal sintering), a conductive polymer (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)), an ethyl-cellulose stabilized graphene dispersion in cyclohexanone – terpineol and a water-based functionalized reduced graphene oxide ink were used for developing humidity, temperature and strain sensors on paper and polyimide substrate. Also, electrical contact methods were evaluated; pre-patterned copper tracks on polyimide were interfaced with inkjet-printed graphene and silver nanoparticle structures via direct over-printing and commercial connectors. A comprehensive literature review was performed for identifying possible material – substrate combinations that could lead to cost-effective, repeatable production of sensors at a mass scale via inkjet printing. Two resistive output humidity sensors, one with Ag-nanoparticle based electrodes and one with PEDOT:PSS on paper substrate were developed, demonstrating the usage of paper substrate as a humidity sensing film with a mean resistance change of one order of magnitude per 10 %rH in the range of 0 to 30 %rH and a mean resistance change of one order of magnitude per 20 %rH in the range 30 – 90 %rH. Ag nanoparticle and PEDOT:PSS-based temperature sensors on paper, and graphene and f-rGO temperature sensors on polyimide substrate were developed and evaluated for their electrical characteristics and their Thermal Coefficients of Resistance (TCR) were extracted as 9.389×10-4 oC-1, -0.0139 oC-1, -1.94x10-3 oC-1 and -1.64x10-2 oC-1, respectively. Graphene-based materials exhibited good thermal cycling endurance and a mean response time of 2.47 s (graphene) and 2.94 s (f-rGO). Ag nanoparticle and PEDOT:PSS-based strain sensors were evaluated for creating a complete sensing platform alongside the aforementioned sensors; gauge factors of GFAgTensile = 0.4259 and GFPEDOT:PSSTensile = 0.1422 for tensile strain and GFAgCompress = -0.1572 and PEDOT:PSS GFPEDOT:PSSCompress = 0.1448 for compressive experiments. Also, long-term mechanical strain sensing was evaluated with a mechanical XY precision stage; samples were bended for 1000 cycles and resistance was measured in parallel. Ag nanoparticle-based strain sensor outperformed PEDOT:PSS in terms of mechanical endurance. 2D thermal flow sensors were studied and two approaches for fabrication were investigated: firstly, a 2D thermal flow sensor consisting of discrete SMT platinum elements which acted both as micro-heaters and sensing elements, was developed on prelaminated polyimide with copper tracks. Capabilities to detect flow amplitude with two modes of operation (namely, constant current and constant temperature) and angle of attack were assessed via a custom measurement setup. Transfer of technology to a fully printed device was performed via screen printing of active components consisting of activated carbon and BaTiO3 which acted as heaters and sensing elements on PET substrate. The same modes of operation were evaluated and characteristics regarding flow detection capabilities were extracted. For the SMT based flexible device on polyimide, an artificial neural network was also developed for accompanying and correcting the measurements offline. Both flow sensors were capable of detecting flows in the range of 0 to 25 SLPM, while constant temperature mode was evidently more sensitive (12.6 mW/(m/s) versus -2.80 mW/(m/s) for constant current mode) for the SMT sensor. Different methods for electrical interfacing between printed lines and traditional Cu-based electronics on polyimide was also performed. It was concluded that depending on the application, commercial mechanical connectors such as Amphenol FPC and Clincher can provide adequate electrical interfacing. Also, as also evaluated by finite element analysis, the engagement points and measurement method are of major importance in electrical field formation and current density.
- ItemOpen AccessApplication of a high-resolution weather model in the area of the western Gulf of Corinth for the tropospheric correction of interferometric synthetic aperture radar (InSAR) observations
Τμήμα Φυσικής (ΔΔ)Ρουκουνάκης, Νικόλαος; Αργυρίου, Αθανάσιος; Briole, Pierre; Κατσάμπαλος, Κωνσταντίνος; Morel, Laurent; Doubre, Cécile; Lott, François; Jacquemoud, Stéphane; Roukounakis, NikolaosSpace geodesy techniques (SAR interferometry and GNSS) have recently emerged as an important tool for mapping regional surface deformations due to tectonic movements. A limiting factor to this technique is the effect of the troposphere, as horizontal and vertical surface velocities are of the order of a few mm yr-1, and high accuracy (to mm level) is essential. The troposphere introduces a path delay in the microwave signal, which, in the case of GNSS Precise Point Positioning (PPP), can nowadays be successfully removed with the use of specialized mapping functions. Moreover, tropospheric stratification and short wavelength spatial turbulences produce an additive noise to the low amplitude ground deformations calculated by the (multitemporal) InSAR methodology. InSAR atmospheric phase delay corrections are much more challenging, as opposed to GNSS PPP, due to the single pass geometry and the gridded nature of the acquired data. Several methods have been proposed, including Global Navigation Satellite System (GNSS) zenithal delay estimations, satellite multispectral imagery analysis, and empirical phase/topography estimations. These methods have their limitations, as they rely either on local data assimilation, which is rarely available, or on empirical estimations which are difficult in situations where deformation and topography are correlated. Thus, the precise knowledge of the tropospheric parameters along the propagation medium is extremely useful for the estimation and minimization of atmospheric phase delay, so that the remaining signal represents the deformation mostly due to tectonic or other geophysical processes. In this context, the current PhD Thesis aims to investigate the extent to which a high-resolution weather model, such as WRF, can produce detailed tropospheric delay maps of the required accuracy, by coupling its output (in terms of Zenith Total Delay or ZTD) with the vertical delay component in GNSS measurements. The model initially is operated with varying parameterization in order to demonstrate the best possible configuration for our study, with GNSS measurements providing a benchmark of real atmospheric conditions. In the next phase, the two datasets (predicted and observed) are compared and statistically evaluated for a period of one year, in order to investigate the extent to which meteorological parameters that affect ZTD, can be simulated accurately by the model under different weather conditions. Finally, a novel methodology is tested, in which ZTD maps produced from WRF and validated with GNSS measurements in the first phase of the experiment are used as a correction method to eliminate the tropospheric effect from selected InSAR interferograms. Results show that a high-resolution weather model which is fine-tuned at the local scale can provide a valuable tool for the tropospheric correction of InSAR remote sensing data.
- ItemOpen AccessBehavior of stable isotopes (D – 18O) of H2O in atmospheric processes
Τμήμα Φυσικής (ΔΔ)Σαλαμαλίκης, Βασίλειος; Αργυρίου, Αθανάσιος; Καζαντζίδης, Ανδρέας; Ντότσικα, Ελισάβετ; Ζαγγανά, Ελένη; Κασσωμένος, Παύλος; Κιουτσιούκης, Ιωάννης; Holmgren, Karin; Salamalikis, VasileiosThe use of environmental tracers, tracers that naturally exist in the Earth system not due to any anthropogenic activity, may offer direct insights into the mechanisms and interactions taking place within the components of the global hydrological cycle. Among environmental tracers, stable water isotopic species due to their conservative essence have been extensively used in order to seek information about several meteorological, hydrological, climatic, etc. disciplines at different spatial and temporal scales. The isotopic applications may extend from meteorological studies as the recognition of the main moisture transport pathways of the air parcels responsible for precipitation over a certain area to anthropological and archaeological applications as the reconstruction of past human dietary patterns and the migration of past populations. The isotopic applications increased nowadays since the isotope derived information from climate archives such speleothems, tree rings, lake sediments, corals, human and animal bones etc. are excellent proxies for the past climate reconstructions. Thus isotopic tracers can be applied in order to obtain information about past climate patterns and to further improve our thoughts for planning and studying the current climate behavior and also the future shifts. Tracing the water cycle requires knowledge of the isotopic patterns for various water reservoirs such as precipitation, lakes, rivers and groundwater. Precipitation is the key component of the global hydrological cycle since it links the atmosphere with the soil-plant continuum. Thus the accurate understanding of the isotopic patterns of precipitation and water vapor is the principal step for water resources studies and planning. This thesis focuses on the behavior of the stable water isotopologues (18O and 2H) in various atmospheric processes under the view of water vapor and precipitation patterns. Chapter 1 provides an overview of the basic isotopic methodology, including the theory of the different fractionation processes that control the distribution of the stable water species in phase changes, the isotopic models for the theoretical simulation of the stable isotopic content of water vapor and precipitation; finally the main isotopic effects that modulate the global isotope precipitation patterns are briefly discussed. Chapter 2 focuses on the isotopic variability of water vapor in a specific location of the Eastern Mediterranean, namely Patras, Greece and its dependence to different air mass trajectories. This study is divided in two parts. The first part examines the temporal variability and the ‘seasonal’ and ‘temperature’ effects of the water vapor isotopes while in the second part a moisture source analysis combined with isotopic tracers is presented. The temperature dependence is more distinct in the mean monthly δv for temperatures below 18 oC, resulting to temperature gradients equal to 0.43‰/oC and 2.6‰/oC for 18O and deuterium respectively. The mean monthly isotopic distribution of water vapor in Patras follows the isotopic variability of a typical coastal station where the maximum of the isotopic composition is reached in June. The Vapor Evaporation Line (VEL) in Patras is δ2H = 6.8*δ18Ο-3.5, indicating evaporation at high humidity conditions (90 - 95%). The isotopic composition is strongly affected by westerly air masses but also by air masses coming from Eastern Europe. More depleted isotopic signatures are obtained for Medium Moving trajectories while the deuterium excess especially for air masses with terrestrial source history becomes greater than 18‰. In general higher d-excess values are observed for air parcels having crossed the middle of the Mediterranean basin. Chapter 3 examines the sub-cloud evaporation effect on precipitation in terms of its isotopic composition using three different isotopic approaches namely that of Meteoric Water Lines (MWLs), an isotope-evaporation model and a numerical isotope-evaporation model. The overall analysis is performed using isotopic data for various stations located in the Northern Hemisphere. The lowest MWL slopes are obtained for Madrid, Spain (αLMWL = 7.33) and Rehovot, Israel (αLMWL = 7.35) while in the greatest deviation from the equilibrium state is detected in Rehovot. Then an isotope-evaporation model is evaluated for the calculation of the isotopic enrichment in the falling raindrops induced by the sub-cloud evaporation effect. Both δ18O and δ2H show a linear relationship with the evaporated fraction with slopes ranging between 0.25‰/% and 0.28‰/% for 18O while for 2H the Δδ/EF values are around 0.9‰/%. The isotopic enrichment is highly correlated with the raindrop diameter and relative humidity conditions where Δδ increases with decreasing relative humidity and decreasing raindrop size. Temperature also contributes to the sub-cloud evaporation effect leading to higher isotopic enrichments for higher temperatures. However the temperature dependence is more distinguishable for smaller raindrops. Then, the vertical variation of the isotopic composition of falling raindrops and the corresponding isotopic enrichment is investigated through a numerical isotope-evaporation model. The isotopic composition becomes enriched in comparison to the initial isotopic composition at the cloud base level when the raindrops travel through drier and warm atmospheres, leading to Δδ18Ο up to 20‰ depending on the raindrop size and the initial meteorological conditions. For near saturated atmospheric conditions (RH = 95%) the isotopic composition does not vary significantly, indicating the absence of sub-cloud evaporation. Chapter 4 discusses the periodic patterns and the ‘temperature effect’ of δ18O in precipitation for various stations located around Central Europe, using periodic and wavelet models. The seasonal distribution of δ18O follows the temporal variability of air temperature providing seasonal amplitudes ranging from 0.94 to 4.47‰; the monthly isotopic maximum is observed in July. The Morlet Wavelet Transform reveals the main periodicity at 12-months where the wavelet power is mainly concentrated. Stations with limited seasonal isotopic effect depict a complex isotopic fingerprint that cannot be examined solely by the seasonality effect. In the case of the temperature dependencies on δ18O the isotope-temperature slope ranges from 0.11 to 0.47‰/oC with steeper values observed at the southernmost stations of the study area. Apart from the linear regression analysis the isotope-temperature slope is investigated under a spectral point of view. High coherencies are detected at the periodicity of 12-months. Generally the slope fluctuates around a mean value but in certain cases (sites with low seasonal effect) abrupt slope changes are derived and the slope becomes strongly unstable. The time-frequency slope is calculated at the annual periodicity mode ranging from 0.45 to 0.83‰/oC with higher values at stations that show a more distinguishable seasonal isotopic behavior. Vienna, Austria, depicts the most complicated coherency spectrum. Apart from the 12-month periodicity mode, inter-annual (bi-annual and decadal) periodicities are also represented with a decadal slope (1.9‰/oC) almost stable over the entire time period. In Chapter 5 the spatial patterns of the isotopic composition of precipitation in a global scale are briefly discussed. Global gridded isotopic (δ18O and δ2H) surfaces of precipitation through the Köppen-Geiger climate partitioning system (Köppen-Geiger, K-G precipitation ‘isoscapes’) are generated using a hybrid regression/geostatistical approach. The K-G ‘isoscapes’ capture the main isotopic effects as revealed by the Rayleigh distillation process with depleted values at higher latitudes (i.e. North America), at more inland locations (i.e. Siberia) and at high elevated areas (Andes, Alps and the Tibetan Plateau). However the isotopic grids fail to reproduce the isotopic depletion when moving from lower mid-latitudes to the equatorial regions since both the isotopic models and the spatially resolved residuals are unable to explain the isotopic composition in those regions where the high precipitation amounts and the strongly humid conditions are responsible for the observed isotopic depletions. The spatial isotopic uncertainty is quantified in terms of regression and interpolation uncertainties; high uncertainties are due to the isotopic regression models. The mean uncertainty values are equal to ±1.53‰ and ±13.1‰ for δ18O and δ2H respectively. Low uncertainties are associated to adequately sampled regions while high values are obtained for high-elevated regions and in areas with absence of isotopic measurements. The high correlations between the K-G ‘isoscapes’ and GNIP observation reveal the successful isotopic representation of the gridding procedure. Furthermore the generated isotopic grids are compared against the isotopic products from iso-GCMs. Significant spatial isotopic differences among the K-G ‘isoscapes’ and ECHAM, MIROC free models are investigated with negative differences in the Northern-Western and the Central-Eastern parts of Africa. On the other side positive differences are depicted in southern Iran, Pakistan and north India for both isotopic species and iso-GCMs.
- ItemOpen AccessCategorization of electromagnetic radiation scatterers by means of synthetic aperture radar (SAR) images
Τμήμα Φυσικής (ΔΔ)Κουρούπης, Γεώργιος; Αναστασόπουλος, Βασίλης; Αναστασόπουλος, Βασίλης; Οικονόμου, Γεώργιος; Βλάσσης, Σπυρίδων; Υφαντής, Απόστολος; Αναστασόπουλος, Δημήτριος; Σπηλιόπουλος, Νικόλαος; Παναγιωτάκης, Γεώργιος; Kouroupis, GeorgiosRadar image analysis is the cutting edge technology in Remote Sensing for observing the Earth’s surface. Radars as passive sensors acquire the backscattered solar illumination from the Earth’s surface and as such they are subject to atmospheric conditions. On the other hand, radars can be active sensors that transmit polarized electromagnetic waves towards the Earth and then receive the echoes that are backscattered. Such an active radar system is independent of solar illumination, allowing day and night imaging. Nowadays radar systems are capable of providing very high resolution images of the Earth’s surface by means of Synthetic Aperture Radar (SAR) systems. SAR imaging is ideal to monitor dynamic processes on the Earth’s surface in a reliable, continuous and global way. A SAR system operates in the microwave region of the electromagnetic spectrum making it, also, independent of any weather effects, smoke, fog and other related phenomena. Given the potential of such high resolution images, SAR imaging constitute a vast and active research area with applications in landslides observation, earthquake monitoring, maritime security, search and rescue missions, automatic target recognition and many more. A modern SAR system can transmit electromagnetic waves in two different polarizations, namely horizontal and vertical, and can receive their backscattered echoes in the same two polarizations. In this way, four combinations of transmitting and receiving signals are created. Each of the transmitting-receiving signal combination constitutes a single image of the same geographical scene, while the four images altogether constitute a fully-polarimetric image. The fully-polarimetric image expresses the polarimetric properties of the Earth’s scene that is depicted on it. In this Ph.D. thesis the polarimetric nature of targets that are depicted in a SAR image has been extensively studied. In this context, a target is considered as a single or a group of pixels that form a unique and distinctive object while a complex target is considered the one that has a complex physical structure, such as ships, buildings, plants and etc. Radar polarimetry studies the way in which a radar signal interacts with a real target aiming to deduce its physical and geometric properties. It is long known that when a transmitted electromagnetic wave is being scattered from a target its polarimetric properties are being subject to changes. These changes are directly linked to the scatterer physical and electrical properties. Several researchers such as J. Huynen, E. Luneburg, S. R. Cloude, W.L. Cameron, R. Touzi and more have advanced the theoretical framework of polarimetric decompositions, theories that constitute the extraction of meaningful physical properties from a target. Polarimetric approaches are mostly divided into two categories, in Coherent Target Decompositions (CTDs) and in Incoherent Target Decompositions (ICTDs). Each approach presents its own advantages and disadvantages with respect to scatterer representation and classification to elemental scattering mechanisms. However, it is observed that, under certain conditions, different CTDs assign different physical properties to a single scatterer which seems rather irrational. The physical properties of a scatterer cannot change with respect to different representation models. The classification of scatterer by means of a CTD must always be the same, regardless the additional information that a different representation model may provide. In this thesis the polarimetric nature of the elemental scatterers, the relationship between the two most famous CTDs, the Huynen- and Cameron CTD, is examined and a unifying approach is presented. Additionally, this study is focused on the evaluation of the interferometric information of a SAR system from a polarimetric point of view. Interferometric SAR (InSAR) imaging has the means for generating digital elevation models (DEMs) and ground deformation maps that present the relative deformation of the Earth’s surface between acquisitions. Differential Ιnterferometry (DInSAR) is a technique derived from classic Interferometry in order to produce interferograms from which the topographic contribution has been eliminated. Even so, DInSAR inherits many difficulties of InSAR with the main being the temporal and spatial decorrelation. However, single pixels that are coherent over long time intervals and over wide look-angle variations are able to overcome the aforementioned obstacles. In this thesis the polarimetric nature of the so-called persistent scatterers is evaluated and their use as an auxiliary tool for preprocessing a scene is assessed. Furthermore, a novel CFAR ship detector that exploits the scattering mechanism of a scatterer instead of its amplitude information is developed. Ships are depicted by pixels that present strong echoes on a background with relatively low backscattering power formed by the surface of the sea, making ship detection feasible. The vast majority of the literature regards the development of CFAR ship detectors using only the backscattering amplitude information from single polarimetric images. Under this framework, one must take under consideration several factors regarding sea-ship distinction with the sea-states, the local weather phenomena and the combination of sea and Kelvin wakes being of the highest importance. However, these detection approaches would fail to perform when the radar signals present backscattering power of the same order. Therefore, the polarimetric nature of the radar signals must also be examined. That being said, there has been made only a small effort on fully-polarimetric ship detection and there is still a significant lack of a relative systematic and automatic experimental work. In this thesis our aim is to connect polarimetric decompositions with stochastic methods to achieve an automatic and systematic ship detection procedure based on the geometric properties of the scene scatterers. A novel ship detection scheme based on first order Markov models along with Cameron’s decomposition elemental scatterers is introduced. Cameron’s symmetric scatterers will be used as the quantized structural elements of the scene. Nonetheless, the scene scatterers distribution is owed solely to the local topography of the scene and the random position of the ships. Thus, in order to overcome the stochastic nature of the composition of the scene the first order Markov chain model is utilized. First-order Markov chains are adequate to describe the stochastic nature of the polarimetric properties of a scene making the detection procedure feasible. In this way, the dependence of the CFAR detectors to power of the radar signals is overcome. Achieved ship detection performance is high for sea monitoring.
- ItemOpen AccessCosmological aspects of unified theories
Τμήμα Φυσικής (ΔΔ)Λυμπέρης, Ανδρέας; Lymperis, AndreasIn this thesis, we study cosmological aspects of some well-known dark energy models, and extending our investigation into Unified Theories. We explore the existence of geodesically complete singularities in quintessence and scalar-tensor quintessence models using analytical expressions for the scale factor and the scalar field. Next, using the Tsallis entropy and applying the first law of thermodynamics we construct several cosmological scenarios. The sequence of dark energy span and, depending on the value of the parameter of dark energy equation of state, during the evolution, experience the phantom-divide crossing and can be quintessence or phantom-like. Finally, we revisit inflation with non-canonical scalar fields by applying deformed steepness exponential potentials. A significant advantage of this approach is that free parameters, such as the non-canonicality exponent do not need to acquire unnatural values. The resulting scenario can lead to inflationary observables in remarkable agreement with observations.
- ItemOpen AccessDesign of fractional-order analog integrated circuits
Τμήμα Φυσικής (ΔΔ)Τσιριμώκου, Γεωργία; Ψυχαλίνος, Κωνσταντίνος; Ψυχαλίνος, Κωνσταντίνος; Αλεξίου, Γεώργιος; Καλύβας, Γρηγόριος; Οικονόμου, Γεώργιος; Φωτόπουλος, Σπυρίδων; Βλάσσης, Σπυρίδων; Μπακάλης, Δημήτριος; Tsirimokou, GeorgiaThis Ph.D. dissertation deals with the design of low-voltage analog fractional-order integrated circuits. Fractional-order systems are an emerging area of multidisciplinary research labeled even as the “21st century systems”. Electronic engineers are very interested in applying the concept of fractional calculus. It is motivated mainly by the interdisciplinary nature of this research and possibility to obtain qualitatively new circuit solutions that can provide characteristics not available at integer-order systems. For example, the capability for precise control of the slope of frequency characteristics in fractional-order filters in comparison with the corresponding integer-order filters is an attractive feature. Fractional-order impedance circuits are also very promising in modeling electrical properties of biological materials, tissues or cells. Such element, which exhibits fractional-order impedance properties is known as fractance device. However, there are no commercial fractance devices that make use of the advantages of s^α, integer-order approximations have to be used. For this purpose, the 2nd-order approximation of Continued Fraction Expansion is utilized in order to present a systematic way for describing the design equations of fractional-order generalized transfer functions. To this direction, the design and realization of fractional-order analog integrated circuits offering: (i) capability for on-chip implementation, (ii) capability for low-voltage operation, and (iii) electronic adjustment of their characteristics, is introduced for the first time in the literature. Moreover, they are resistorless realizations, and only grounded capacitors are employed. As a first step, fractional-order differentiator/integrator topologies are introduced, which are able to fulfill the following benefits: (i) capability of being realized using the same topology, (ii) the frequency characteristics as well as the fractional-order α are able to be easily electronically tuned, and (iii) they are fully integratable topologies. Furthermore, fractional-order generalized filters are realized, offering the following characteristics (i) capability of realizing different families of filters (i.e. Butterworth, Chebyshev, etc) using the same topology (ii) capability of realizing different types of filters classified through the form of frequency response (i.e. lowpass, highpass, bandpass, etc) using the same topology. All the above frequency characteristics as well as the fractional-order are able to be easily electronically tuned offering design flexibility and programmability. Also, sampled-data fractional-order filters are also realized for the first time in the literature.The main active cells that are employed are current mirrors, non-linear transconductance cells (known as S, C cells), and Operational Transconductance Amplifiers (OTAs). As a result, the designer has only to choose the appropriate values of the dc bias currents in order to realize the desired transfer function, and, therefore, the proposed schemes offer attractive features.In addition, fully integrated fractional-order (capacitor and inductor) emulators, offering the following attractive benefits: electronic tuning of the impedance, the order, and the bandwidth of operation, are fabricated using the AMS 0.35um CMOS process the efficiency of which is proofed through experimental results.Finally, applications of fractional-order circuits are presented proofing the nessecity of fractional-order calculus, especially when compared with the corresponding integer-order counterparts. Thus, a pre-processing stage suitable for the implementation of the Pan-Tompkins algorithm for detecting the QRS complexes of a noisy electrocardiogram is realized. Also, a fully tunable implementation of the Cole-Cole model used for the modeling of biological tissues is realized. A simple non-impedance based measuring technique for super-capacitors is introduced and this is very important taking into account that the characterization of the parameters of fractional -order circuits is an important procedure, which in general requires an expensive equipment.
- ItemOpen AccessDevelopment and characterization of solar energy conversion devices based on nanocomposite semiconductors
Τμήμα Φυσικής (ΔΔ)Συγκρίδου, Δήμητρα; Βιτωράτος, Ευάγγελος; Βιτωράτος, Ευάγγελος; Σταθάτος, Ηλίας; Πομώνη, Αικατερίνη; Κροντηράς, Χριστόφορος; Λευθεριώτης, Γεώργιος; Παλίλης, Λεωνίδας; Πουλόπουλος, Παναγιώτης; Sygkridou, DimitraThis doctoral dissertation presents the study of the development and characterization of solar energy conversion devices based on nanocomposite semiconductors. The PhD research is focused on third generation solar cells, namely in dye−sensitized solar cells (DSSCs) and perovskite solar cells (PSCs). The innovation of this study is detected on the use of novel materials by well−known methods in order to intervene in the structural and morphological properties of the photoelectrode, the investigation of newly synthesized sensitizers and hybrid organic/inorganic materials employed in quasi−solid state electrolytes for their jellification, alternative counter electrodes to replace platinum and composite anodes for efficient electron transport to enhance the electrical characteristics of the final devices. Additionally the upscaling of transparent strip−shaped DSSC of various lengths (1 cm to 45 cm) was carried out by inkjet printing the electrodes to ensure the reproducibility and the accuracy of the research findings and the obtained results are evaluated. The study regarding DSSCs is limited on TiO2 solar cells and is divided into individual sections each one confining in one of the components of a DSSC. Firstly, the research is concentrated on the photoanode and specifically on different approaches to modify the films to optimize the performance of the corresponding devices. The methods that were tested were the incorporation of a small amount of carbonaceous materials (carbon black powder (CBP) or multi−walled carbon nanotubes (MWCNTs)), the surface treatment of the TiO2 photoanode films by soaking them in a TiO2 solution and the testing of different TiO2 precursor materials or surfactants. The use of template free P25−TiO2 solutions modified with CBP or MWCNTs showed that the incorporation of MWCNTs improved the overall performance of the DSSCs, in both sintering temperatures examined (100oC and 500oC) with the highest efficiency recorded for the devices employing P25−TiO2 electrodes with 0.1 wt% of MWCNTs. However, the CBP modified P25−TiO2 films resulted in poorer performing devices for all cases tested. The DSSCs with the post−treated P25−TiO2 films, where the anodes were immersed in two TiO2 solutions used for the first time (Titanium(IV) (triethanolaminato)isopropoxide and Titanium(IV) bis(ammonium lactate)dihydroxide solution) followed by annealing at 300oC and 500oC, demonstrated improved short−circuit current density promoting their overall efficiency up to 26−30% compared with solar cells with untreated electrodes. Among the titanium precursor materials used to fabricate transparent TiO2 films for DSSC, the solar cells with the Titanium(IV) butoxide photoanodes displayed the best electrical parameters and an 11.2% higher power conversion efficiency compared with the solar cells with the Titanium(IV) isopropoxide anode. In contrast, the last modification method tested using the surfactant Dioctyl sulfosuccinate sodium salt (AOT) in the TiO2 solution which could be easily removed by rinsing without damaging the film, didn’t reach to the desired results and the attempt to simplify the fabrication of TiO2 films through this procedure was given up. Regarding the dye complexes tested as sensitizers in quasi−solid state DSSCs, initially six newly synthesized ruthenium dye complexes with different pyridine and bipyridine side groups were studied. The DSSCs that were sensitized with the dyes having two bipyridines (bpy-bpy) in their structure demonstrated almost the same performance as the solar cells that were sensitized with the commercially available dye D907 which has a similar structure as the new complexes. In particular, the solar cell sensitized with the dye CS28 displayed a slightly better power conversion efficiency than the one corresponding to the DSSC sensitized with D907 (3.28% and 3.26% respectively). Subsequently, two new triphenylamine based organic dyes with or without the additional electron donating hexyloxy groups, having a benzimidazole derivative as π−bridge were studied. The DSSCs sensitized with the dye without the additional hexyloxy groups (MZ−341) were more efficient compared with the ones sensitized with the dye with the hexyloxy groups (MZ−235) because of the higher absorbance of the former on the TiO2 photoelectrode. Finally, a water based natural dye solution was prepared by extracting the phycoerythrin pigment from red algae and was used as a sensitizer for DSSCs. The best results were obtained for an acidic dye solution (pH=3) and for a temperature of 35oC during the sensitization of the anode. In the electrolyte solution only a minor alteration was examined regarding the hybrid material used for the gradual solidification of the electrolyte. All five hybrid organic/inorganic materials that were synthesized with polypropylene or polyethylene of different oligomer chain length as organic sub−phase demonstrated good thermal stability up to 150oC with ED600−ICS slightly standing out. Nevertheless no obvious differences were detected in the electrical parameters of the DSSCs that employed these hybrid materials in their electrolyte solution. As far as the alternative counter electrodes tested to replace platinum is concerned, DSSCs with the nickel doped CoS2 and the polypyrrole counter electrode displayed better electrical characteristics compared with the solar cells with platinum. In fact in both cases an enhanced short−circuit current density was recorded improving the solar cells’ overall efficiency. Proceeding to the manufacture of large area transparent DSSCs with electrodes fabricated by inkjet printing, the main variations observed at these strip−shaped DSSC of various lengths (1 cm−45 cm) were at the JSC and the FF values which decreased as the length increased. Even so, the construction of strip−shaped DSSCs of intermediate length (20 cm−25 cm) connected in order to form a solar module in an almost square form could be a viable solution providing satisfactory results. Finally, the PSCs with the TiO2−In2O3 binary electron transport layer which was used as a scaffold in order for the mixed halide perovskite (CH3NH3PbI3-xClx) to infiltrate exhibited a maximum power conversion efficiency of 12.86% which improved the efficiency of the PSCs with the pristine TiO2 over 28%. This is attributed to the better interfacial connection between TiO2−In2O3/CH3NH3PbI3-xClx, the efficient electron transport due to optimized alignment of the energy levels of the mixed oxides and the perovskite, as well as the reduction of the recombination processes. Baring in mind that all manufacturing processes for the perovskite solar cells were carried out in ambient conditions renders these results quite impressive. Overall, it is presumed that the obtained research results, their interpretation and the final conclusions derived from this work will give an insight for future research prospects on third generation solar cells.
- ItemOpen AccessDevelopment of drug delivery composite systems based on stimuli responsive segmented polymers
Τμήμα Φυσικής (ΔΔ)(2013-12-06) Popescu, Maria-Teodora; Τσιτσιλιάνης, Κωνσταντίνος; Αντιμησιάρη, Σοφία; Στάικος, Γεώργιος; Μπόκιας, Γεώργιος; Ιατρού, Ερμής; Αυγερόπουλος, Απόστολος; Βαμβακάκη, Μαρία; Τσιτσιλιάνης, ΚωνσταντίνοςΗ ληψη φαρμακου με τη μεσολαβηση ενος φορεα έχει αναδειχθεί ως μια ισχυρη μεθολογια θεραπειας ποικιλων παθολογιων. Για να βελτιωθει ο θεραπευτικος δεικτης τοσο των παραδοσιακων οσο και των πρωτοπορων φαρμακων, νανοσωματιδια σχεδιαστηκαν αριστοποιωντας το μεγεθος και τα επιφανειακα χαρακτηριστικα, ωστε να βελτιωσουν την αποτελεσματικοτητα των φαρμακων σε συγκεκριμενους ιστους, τον ελεγχο πανω στην κινητικη της αποδεσμευσης, την προστασια του δραστικου παραγοντα ή συνδυασμο ολων των παραπανω. Παραγοντες αντιθεσης μπορουν επισης να ενσωματωθουν σε νανοσωματιδια για διαγνωστικους σκοπους ογκων. Η μοναδικη παθοφυσιολογια μεμονομενων ογκων, οπως η αυξημενη τους διαπερατοτητα, το φαινομενο παρακρατησης και το οξινο μικροπεριβαλλον, επιτρεπει την παθητικη συσσωρευση νανοσωματιδιων στους ογκους με τη χρηση ενδοφλεβιων ενεσεων. Για τη θεραπευτικη αριστοποιηση, διεγειρο-εξαρτωμενοι νανοφορεις εχουν αναδειχθει ως καινοτομες προγραμματιζομενες πλατφορμες παραδοσης φαρμακων, οπου το ιδιο το συστημα παραδοσης γινεται ενεργος μετοχος, παρα παθητικο οχημα, επιτρεποντας την εξειδικευμενη αποκριση του νανοσωματιδιου σε παθολογικες “ενεργοποιησεις”. Η αποτελεσματικοτητα εξυπνων συστηματων παραδοσης, βασισμενων σε νανοσωματιδια, ενισχυεται απο χαρακτηριστικα τους οπως η μειωμενη συχνοτητα δοσεων, ο βελτιωμενος δεικτης ασφαλειας και η θεραπευτικη τους αποτελεσματικοτητα. Επιπλεον της συστημικης χορηγησης, η τοπικη αποδεσμευση φαρμακων μπορει να επιτευχθει χρησιμοποιωντας μακροσκοπικες θηκες φαρμακου κοντα στο στοχο. Αναμεσα σε ποικιλα συστηματα που μελετηθηκαν για αυτην την προσεγγιση, in situ συντιθεμενα βιουλικα, ανταποκρινομενα σε περιβαλλοντικα ερεθισματα, εχουν κερδισει σημαντικη προσοχη, λογω του μη διεισδυτικου χαρακτηρα τους, της μειωσης των παρενεργειων σχετιζομενων με τη συστημικη χορηγηση και του καλυτερου ελεγχου πανω στη βιοδιασπορας τους. Με σκοπο την περαιτερω αριστοποιηση του φαρμακοκινητικου προφιλ των μακροσκοπικων αποθηκων, συνθετα συστηματα ισως αποδειχθουν αποτελεσματικοτερα για τον ελεγχο της απελευθερωσης του φαρμακου. Αυτη η διατριβη εστιαζει στην αναπτυξη νανοδομημενων συνθετων συστηματων για εφαρμογες στην απελευθερωση φαρμακων, με ξεχωριστη εμφαση στο σχηματισμο πολυπλοκων νανοφορεων που θα αποτελεσουν στεγαστρα για την ελεγχομενη απελευθερωση μιας ποικιλιας θεραπευτικων παραγοντων. Καταρχην ενα βιοσυμβατο, διττα pH-ελεγχομενο poly(2-vinyl pyridine)-b-poly(acrylic acid)-b-poly(n-butylmethacrylate) (P2VP-PAA-PnBMA) triblock terpolymer μελετηθηκε ως ενας ενεσιμος πηκτωματοποιητης. Αυτο το τριπολυμερες εμφανιζει μια αποτομη pH-επαγομενη μεταβαση φασεων σε τιμες πανω απο 5 και σχηματιζει μια ισχυρη υδρογελη κατω απο φυσιολογικες συνθηκες. Μελετηθηκε η ενσωματωση νανοφορεων λιποσωματων, που φερουν ενα υδροφιλο παραγοντα μεσα σε αυτην την ενεσιμη υδρογελη. Η ενθυλακωση των PC/Chol λιποσωματιων τα οποια φερουν καλσείνη, ως προτυπο υδροφιλικο παραγοντα, επεδειξε αριστο ελεγχο στην απελευθερωση του φαρμακου, σε χρονικη διαρκεια εφαμιλλη των θεραπειων, το οποιο θα μπορουσε περαιτερω να διαμορφωθει ρυθμιζοντας την συγκεντρωση του πηκτωματοποιητη. Συμπληρωματικα σε αυτο, η ερευνα εστιαστηκε στην αναπτυξη καινοτομων συνθετων συστηματων μεσω της επεκτασης της φυσης των συστατικων μερων προς την αυξημενη ανθεκτικοτητα του κυριου φορεα, τη βιοσυμβατοτητα και την αποκριση σε ερεθισματα σε ενα στενοτερο πλαισιο, καταλληλοτερο για εφαρμογες in vivo. Εχοντας υποψη τα εγγενη πλεονεκτηματα των πολυμερικων κυστιδιων εναντι των λιπιδικων αναλογων τους, εξευρενησαμε την αυτοοργανωση μιας σειρας αμφιφιλικων πενταμπλοκ τριπολυμερων και των τριμπλοκ συμπολυμερων τους, προδρομων για τον σχηματισμο των πολυμεροσωματων (polymersomes). Το πολυμερες αποτελειτο απο ενα pH-ευαισθητο poly(2-vinylpyridine) (P2VP) κεντρικο μπλοκ, φεροντας και στις δυο ακρες υδροφιλικο poly(ethyleneoxide) (PEO) (PEO-P2VP-PEO τριμπλοκ συμπολυμερες) και προστατευομενο στο ακρο με υδροφοβικη poly(ε-caprolactone) (PεCL), (PCL-PEO-P2VP-PEO-PCL, πενταμπλοκ τριπολυμερες). Επιπροσθετως, η μερικη μετατροπη του 2VP κεντρικου μπλοκ σε τεταρτοταγες συντελεστηκε για την εισαγωγη μονιμων κατιονικων ομαδων μεσα στο πενταμπλοκ τριπολυμερες. Αυτα τα συστηματα επεδειξαν μια πλουσια παλεττα μορφολογιων, σε φυσιολογικες συνθηκες, υπαγορευομενες απο τη μεθοδο προετοιμασιας. Πολυμερικα κυστιδια εληφθησαν αποκλειστικα μεσω της μεθοδου ενυδατωσης λεπτου υμενιου (μεμβρανης) και επεδειξαν διαφορετικες διαμορφωσεις μεμβρανης συμφωνες με την τοπολογια και τη φυση του συστατικου πολυμερους. Ειδικοτερα, το τριμπλοκ συμπολυμερες σχηματισε μια μονης στοιβαδας P2VP μεμβρανη περιβαλλωμενη απο αλυσιδες PEO στις υδατικες τους εσωτερικες και εξωτερικες επιφανειες ενω το πενταμπλοκ τριπολυμερες σχηματισε νεες, σε σχημα λουλουδιου, μορφολογιες μεμβρανης, αποτελουμενες απο αυτοοργανωμενα PCL ακρα αλυσιδων και P2VP υδροφοβικη μπλοκ στοιβαδα, ενωνοντας τους βροχους των αλυσιδων PEO. Επιπλεον, τα τεταρτοταγη τριπολυμερη κυστιδια επεδειξαν την παρουσια θετικα φορτισμενων τεταρτοταγων 2VP ομαδων στην εξωτερικη τους επιφανεια. Η δυνατοτητα των πενταμπλοκ κυστιδιων ως φορεων ληψης φαρμακων μελετηθηκε με την ενκαψυλιωση καλσεινης, ενα προτυπο υδροφιλικο παραγοντα, σε πολυμεροσωματα και τα αποτελεσματα της αποδεσμευσης της καταγραφηκαν και συγκριθηκαν για διαφορετικα πολυμεροσωματα και σε διαφορετικες τιμες pH καταλληλες για εφαρμογες in vivo. Οι σχηματισμοι νανοσωματιδιων εδειξαν σημαντικες διαφορες στο προφιλ απελευθερωσης, με ελεγχωμενη και παρατεταμενη αποδεσμευση για τα πενταμπλοκ κυστιδια σε σχεση με τα αντιστοιχα τριμπλοκ, δειχνοντας το πλεονεκτημα της πενταμπλοκ αρχιτεκτονικης εναντι της συμβατικης αναλογης τριμπλοκ αρχιτεκτονικης. Επιπροσθετως, ταχυτερο προφιλ αποδεσμευσης εληφθη με μειωση του pH, ως αποτελεσμα της πρωτονιωσης του δομικου στοιχειου, ενισχυοντας ετσι την διαπερατοτητα της μεμβρανης. Εφαρμοζοντας μια τεχνικη ανταλλαγης διαλυτων, νανοσωματιδια μικυλλιων εληφθησαν με crew-cut ή διαμερισματοποιημενη μορφολογια, ρυθμιζοντας το βαθμο ιονισμου των 2VP ομαδων (π.χ. μερικη μετατροπη σε τεταρτοταγη). Τα πενταμπλοκ τριπολυμερη μικκυλια μελετηθηκαν περαιτερω ως παραγοντες πολυτροπικης απεικονισης και φορεις φαρμακων, με ενσωματωση χρυσου (Au) στον μικυλλιακο P2VP πυρηνα για να κατασκευαστουν υβριδικα Au@πολυμερο νανοσυνθετικα υλικα. Ποικιλα κολλοειδη μορφολογικα νανοσωματιδια Au οπως νανοσωματιδια Au@core-corona και νανοσωματιδια Au@core-Au@corona συντεθηκαν πανω στο αντιστοιχο εκμαγειο μικκυλιων του πενταμπλοκ συμπολυμερους. Τα πενταμπλοκ μικυλλιακα συγκροτηματα και νανοσωματιδια Au@πενταμπλοκ μπορουν να περικλεισουν τον ελαχιστα υδατοδιαλυτο παραγοντα, Tamoxifen, στους υδροφοβικους μικκυλιακους τους πυρηνες. Απο την ελεγχωμενη απελευθερωση φαρμακου απο τα νανοσωματιδια, προεκυψε ενα ανωτερο προφιλ αποδεσμευσης για τα υβριδικα νανοσωματιδια Au@πολυμερες, σε σχεση με τα αντιστοιχα στοιχειωδη αμφιφιλικα πενταμπλοκ πολυμερικα μικκυλια. Μολις αποδειχθηκε η χαμηλη διαπερατοτητα των πενταμπλοκ τριπολυμερων κυστιδιων στα υδροφιλικα φαρμακα παραλληλα με την ικανοτητα για επιτυχη ενσωματωση παραγοντων αντιθεσης (Au) και διατηρηση της αποδεσμευσης των υδροφοβικων φαρμακων, επικεντρωθηκαμε στη περαιτερω προετοιμασια των συνθετων νανοσωματιδιων χρυσου (AuNPs) πανω στα πενταμπλοκ τριπολυμερη κυστιδια PCL-PEO-P2VP-PEO-PCL για την πολυτροπικη ληψη φαρμακων. Για αυτο το λογο, υδατοδιαχεομενα προενεργοποιημενα AuNPs που φερουν υδροφοβικο μοριο (Nile Red), ενσωματωθηκαν στο υδροφιλικη υδατικη κοιλοτητα των κυστιδιων και η διττη απελευθερωση των AuNP και των NR παρακολουθηθηκε σε συνθηκες pH που προσωμοιαζουν τις φυσιολογικα και κακοηθη περιβαλλοντα. Ως πειραμα ελεγχου, το NR ενσωματωθηκε μεσα στα πενταμπλοκ κυστιδια και οι κινητικες της παραδοσης εκτιμηθηκαν. Η συνθετη ληψη του υδροφοβικου φορτιου απο αυτα τα νανοσωματιδια χρυσου σε σχηματισμους πολυμεροσωματων, εδειξε βελτιωμενη και ελεγχομενη κινητικη σε σχεση με τους μεμονωμενους νανοφορεις (στοιχειωδη AuNPs και ή πολυμεροσωματιδια ελεγχου), σε συνδυασμο με την απελευθερωση των AuNPs, η οποια μπορουσε περαιτερω να ρυθμιστει με τροποποιηση του pH ή χημικης τροποποιησης των δομικων συστατικων της μεμβρανης. Αφοτου αποδειχθηκαν τα πλεονεκτηματα των πενταμπλοκ τριπολυμερων κυστιδιων PCL-PEO-P2VP-PEO-PCL, σε ορους ανθεκτικοτητας και ευελικτης διαμορφωσης της διαπερατοτητας της μεμβρανης (pH, Τεταρτοταγοποιηση), η εστιαση της μελετης αυτης προχωρησε προς την αναπτυξη καινοτομων, αποκρινομενων σε ερεθισματα, πλεγματων υδρογελης για την αναπτυξη συνθετων συστηματων. Για αυτο μελετησαμε ενα βιοσυμβατο τριμπλοκσυμπολυπεπτιδιο ως ενεσιμο πηκτωματοποιητη. Το συμπολυμερες παρασκευαστηκε απο ενα κεντρικο pH-ελεγχομενο μπλοκ πολυγλουταμινικου οξεος PGA, πλαισιωμενο απο δυο μικρα υδροφοβικεα μπλοκ πολυαλανινης (PAla), (PAla-PGA-PAla). Αυτο το συστημα επεδειξε pΗ-επαγομενη μεταβαση απο μικρα ινιδια σε σφαιρικους σχηματισμους με αυξηση του pΗ, σαν αποτελεσμα της αποπρωτονιωσης του μπλοκ PGA και της αλλαγης διαμορφωσης απο α-ελικα σε τυχαια σπειρα. Περαιτερω, μια αυξηση της ιοντικης ισχυος σε φυσιολογικο pΗ (PBS pH 7.4, 0.15M NaCl), προκαλεσε ενδομοριακη αναδιπλωση των τελικων ομαδων του PΑla μεσω Η-δεσμων, με το σχηματισμο πλεγμενων ινιδιακων νανοταινιων. Αυτα τα ινιδια που εληφθησαν σε χαμηλη συγκεντρωση μπορουν περαιτερω, με αυξηση της συγκεντρωσης του πολυπεπτιδιου, να οργανωθουν σε υπερμοριακες ταινιες περιπλεγμενων ινων, οι οποιες μπορουν να αποδομηθουν και να αναδιοργανωθουν με χρηση θερμανσης/ψυξης μεσα σε ενα δικτυο περιπλεγμενων υπερινων. Περαιτερω αυτοσυντηρουμενες υδρογελες εληφθησαν πανω απο μια κρισιμη συγκεντρωση πηκτωματος 4.5wt%, χαρακτηριζομενες απο υψηλη ελαστικοτητα (G’~106 Pa) και υψηλη ανακαμψη απο διατμητικη λεπτυνση. Η περαιτερω αυξηση της υδροφοβικοτητας των ακραιων μπλοκ μελετηθηκε με συζευξη ακραιων αμινοομαδων αλυσιδων με βενζαλδευδη (Bz) μεσω μιας αντιδρασης βασης Schiff. Αυτη η χημικη τροποποιηση προικιζει τα συστηματα ωστε να ειναι pΗ-ευαισθητα, εντος μιας κλιμακας pΗ σχετικης με τις in vivo εφαρμογες, χαρη στο δυναμικο χαρακτηρα του ιμινικου δεσμου που επιτρεπει συζευξη/αποσυνδεση της βενζαλδευδης αλλαζοντας το pΗ απο 7.4 σε 6.5. Η παρουσια της βενζαλδευδης συζευγμενης στα ακρα αλυσιδων του PAla-PGA-PAla τριμπλοκ συμπολυμεροπεπτιδιου ειχε ως συνεπεια την ενισχυμενη τοποθετηση των υπερινων σε παχεις σωρους που αναστελλουν την ικανοτητα του συστηνατος για πηκτωματωση. Αντιθετα, διασπαση της βενζαλδευδης με μειωση του pΗ στο 6.5 ειχε ως αποτελεσμα τη μερικη ανακτηση της υδρογελης με χρηση της θερμοκρασιας. Η μεταβαση απο διαλυμα σε γελη ηταν επισης αντιστρεψιμη μεσω ξευξης/αποσυνδεσης του βενζοικου-ιμινικου συνδετη, ενεργοποιουμενη απο μια μικρη αλλαγη στο pΗ εντος του φυσιολογικου φασματος. Ενα τετοιος καινοτομος, διττα αποκρινομενος, τριμπλοκ συμπολυμεροπεπτιδικος σχηματισμος βασιζομενος στο ιμιδικο δυναμικο ομοιοπολικο δεσμο ερευνηθηκε σαν ενα ικριωμα για την ενθυλακωση των κυστιδικων πενταμπλοκ νανοφορεων, ενισχυμενων με ενα υδροφιλο καθετηρα, την καλσεινη, στο υδατικη τους κοιλοτητα. Οι σχηματισμοι πολυμεροσωματιδιων/πολυπεπτιδιων που παρασκευαστηκαν σε φυσιολογικο pΗ επεδειξαν σημαντικα ταχυτερο προφιλ αποδεσμευσης σε σχεση με τη συνθετη υδρογελη σε pΗ 6.5. Επιπλεον και οι δυο σχηματισμοι (διαλυμα και υδρογελη) επεδειξαν ενισχυμενη κατακρατηση φαρμακου συγκρινομενοι με τους στοιχειωδεις νανοφορεις πολυμεροσωματων. Σε ολο το ευρος αυτης της εργασιας, παρατηρηθηκε ενα μεγαλο ευρος συνθετων σχηματισμων ληψης φαρμακων που εσωκλειουν τα πλεονεκτηματα της βιοσυμβατοτητας, της ανθεκτικοτητας, των ρυθμιζομενων επιφανειακων χαρακτηριστικων, της καλυψης, της αποκρισης σε ερεθισματα και των πολυτροπικων θεραπευτικων-διαγνωστικων δυνατοτητων με υψηλη δυναμικη για αριστοποιηση θεραπειων.
- ItemOpen AccessDigital equalization techniques for the compensation of nonlinear impairments in long-haul coherent optical communication systems
Τμήμα Φυσικής (ΔΔ)Βγενοπούλου, Βασιλική; Οικονόμου, Γεώργιος; Ρούδας, Ιωάννης; Μπερμπερίδης, Κωνσταντίνος; Jaouen, Yves; Καμαλάκης, Θωμάς; Μαδαμόπουλος, Νικόλαος; Συγκλέτος, Στυλιανός; Οικονόμου, Γεώργιος; Vgenopoulou, VassilikiIn 2008, the commercialization by Nortel of coherent optical communications digital transceivers operating at 40 Gb/s using Differential Phase Shift Keying (DPSK) revolutionized the field of fiber-optic communications because it provided superior spectral efficiency compared to the state-of-the-art binary intensity-modulation direct-detection receivers. A precursor to this development was the introduction in the early 2000 of differential phase shift keying in conjunction with a direct-detection using a delay interferometer. The advantages of latter modulation formats were the following: a) it allowed more efficient bandwidth utilization due to its superior spectral efficiency compared to on-off keying; b) it was more resilient to fiber transmission impairments (i.e., Chromatic Dispersion (CD), Polarization Mode Dispersion (PMD), fiber nonlinearities) due to its more compact spectrum and constant envelope compared to on-off keying. Nonetheless, fiber-optic communication systems using DPSK were severely degraded by nonlinear phase noise. One way to quantify this system degradation is to accurately derive the nonlinear phase noise statistics. In previously-published articles the characteristic function of the nonlinear phase noise was calculated analytically and the Probability Density Function (PDF) of the nonlinear phase noise was calculated numerically using the Fast Fourier Transform (FFT). In this thesis, we derive an analytical expression of the PDF using the method of steepest descent in a fiber-optic communication system employing M-DPSK with direct detection. The accuracy of our approximation is by far superior to the numerical method providing great accuracy at the PDF tails from where the error probability can be determined. Nowadays, high-speed, high-spectral efficiency, coherent optical communication systems use advanced modulation formats, such as 16 and 64 Quadrature Amplitude Modulation (QAM) in order to achieve 400 Gb/s and 1 Tb/s per channel. Major transmission impairments e.g., CD, PMD and Self-Phase Modulation (SPM) can be compensated using electronic equalization. Fiber nonlinearities such as nonlinear phase noise, and inter-channel four wave mixing are not compensated and can severely limit the maximum transmission reach. Over the past few years, Space Division Multiplexing (SDM) utilizing multicore or Few- Mode Fibers (FMFs) have been studied intensively as a potential candidate for increasing link capacity beyond 1 Tb/s. It is anticipated that the deployment of SDM along with the Multiple-Input Multiple-Output (MIMO) nonlinear equalizers can further extend link capacity. Digital nonlinear equalization techniques fall into two categories: a) Pre-compensation techniques at the optical transmitter that render the signal propagation more resilient to fiber nonlinearities and b) post-compensation techniques at the optical receiver, based on Digital Signal Processing (DSP), applied to the distorted signal after propagation. The latter category includes a fully numerical method called Digital Backpropagation (DBP) and a semi-analytical method based on Volterra series. These methods have been applied to the equalization of nonlinear distortion in optical super-channels either on a channel-by-channel or on a multi-channel basis. Recently-published results have revealed that a multi-channel equalization scheme, using 80 steps-per-span DBP, provides up to 3.8 dB Q^2- factor improvement after ~ 3,200 km of transmission reach. However, this impressive performance is achieved at the expense of computational complexity since many samples per symbol are required. Considering the potential of nonlinear equalizers in the field of SDM coherent optical communication systems, previous publications have shown that linear impairments such as Differential Mode Group Delay (DMGD) and mode coupling, can be almost completely eliminated by linear MIMO-DSP techniques. Nonetheless, inter-modal nonlinearities can still limit the maximum reach in these systems. Even though nonlinear equalization techniques have been studied for dual-polarization standard single-mode fibers and six-mode fibers with two modes, to the best of our knowledge, no scalable MIMO beyond 2×2 has been presented until now for nonlinearity mitigation. In this dissertation, we study the performance of Volterra-based equalizers when applied in channel-by-channel and in multi-channel equalization in single-mode fiber systems. In addition, we study Volterra-based MIMO nonlinear equalizers, beyond 2×2, which, to the best of our knowledge, is done for the first time. More specifically, first, we compare, both experimentally and by simulation, the performance of a nonlinear equalizer using 3rd-order inverse Volterra series transfer function and a nonlinear equalizer based on DBP-Split-Step Fourier (SSF) method in a coherent optical communication system using 400 Gb/s coherent 16QAM Orthogonal Frequency Division Multiplexing (OFDM) super-channels. Both studies show a ~ 0.3 dB Q^2-factor improvement, compared to the linear equalization case, in the presence of inter-channel nonlinearities, generated by adjacent channels carrying either single-carrier or multi-carrier modulation formats. Then, the computational complexity of the nonlinear algorithms is estimated and is shown that, in all cases, the multi-step-per-span DBP-SSF is slightly superior to the other nonlinear equalizer but at the vast expense of computational complexity. Second, we conduct a comparison study between the performance of a 3rd -order inverse Volterra series transfer function nonlinear equalizer and the performance of the single-step-per span and multistep-per-span DBP equalizer performing in multi-channel equalization after more than 1,000 km of a single-mode fiber link. Until now, many studies have shown that the Volterra-based nonlinear equalizers, which are essentially implemented with a single-step-per-span, are inferior compared to the single-step-per span DBP in single-channel equalization. On the contrary, we show that the inverse Volterra nonlinear equalizer performs similarly to the highly-complex DBP-SSF equalizer even with 40 samples per symbol in the case of multi-channel equalization. We demonstrate this in simulations of a 400 Gb/s, dual-polarization, 16QAM, quasi-Nyquist, multiplexed OFDM super-channel, after 1,600 km transmission distance. Finally, we develop a MIMO 3rd-order inverse Volterra series transfer function nonlinear equalizer for a long-haul, FMF coherent optical communication system. Even though DBP technique has prevailed as the method of choice for nonlinear compensation in single-mode fiber systems, it has been shown that the single-step-per span inverse Volterra series transfer function nonlinear equalizer exhibits similar performance to much more complex multi-step-per-span DBP in multi-channel equalization. Our aim is to discover, via extensive numerical simulations, the maximum number of propagating modes for which the 3rd-order inverse Volterra series transfer function MIMO nonlinear equalizer can still provide decent compensation (i.e., at least 1 dB Q^2-factor improvement compared to linear equalization) of the nonlinear effects. Our results show that a ~ 1 dB Q^2-factor improvement can be achieved for 16QAM spatial super-channels of total capacity 6×32 Gbaud after propagation through 1,040 km of an FMF link. Our study is completed with the evaluation of the computational complexity of the nonlinear equalizers under study.
- ItemOpen AccessDigital image watermarking for copyright protection
Τμήμα Φυσικής (ΔΔ)(2010-08-06T07:18:56Z) Gilani, Syed Asif Mahmood; Σκόδρας, Αθανάσιος; Σκόδρας, Αθανάσιος; Αναστασόπουλος, Βασίλης; Οικονόμου, Γεώργιος-
- ItemOpen AccessGeneralized entanglement in quantum information theory : optimization free measures and faithful extraction protocols
Τμήμα Φυσικής (ΔΔ)Κόλλας, Νικόλαος; Αναστόπουλος, Χάρης; Αναστόπουλος, Χάρης; Τερζής, Ανδρέας; Σουρλάς, Δημήτριος; Γκίκας, Δημήτρης; Πάχος, Γιάννης; Έλληνας, Δημοσθένης; Λουκόπουλος, Βασίλειος; Kollas, NikolaosBy studying generalizations of entanglement to quantum resource theories,we will provide a continuous family of optimization free measures to quantify the resource in a system, for any theory whose set of free states is given by the image of an idempotent and unital quantum operation. With the help of these measures we will argue for the correct definition of a coarse grained projective quantum measurement. The question of extraction of quantum coherence from one system to another in a faithful way is also studied. We construct two novel protocols, one able to extract only a small amount of coherence each time and the other a larger amount and apply these to a reservoir in a coherent and vacuum squeezed state of a harmonic oscillator. By studying extraction to qubits it is shown that for a single extraction from both reservoirs the stronger protocol outperforms a previous method known as catalytic coherence.
- ItemOpen AccessGround deformation observed in the western Corinth rift (Greece) by means of SAR interferometry
Τμήμα Φυσικής (ΔΔ)(2014-04-30) Ηλίας, Παναγιώτης; Αναστασόπουλος, Βασίλειος; Briole, Pierre; Θεοδωρίδης, Σέργιος; Ροντογιάννης, Αθανάσιος; Παρχαρίδης, Ισαάκ; Σώκος, Ευθύμιος; Στείρος, Στάθης; Elias, PanagiotisThe rift of Corinth (Greece) has been long identified as a site of major importance in Europe due to its intense tectonic activity. It is one of the world’s most rapidly extending continental regions and it has one of the highest seismicity rates in the Euro-Mediterranean region. The GPS studies conducted since 1990 indicate a north–south extension rate across the rift of ~1.5 cm year-1 around its western termination. Geological evidences show that the south coast of the rift is uplifting whereas the north part is subsiding. The western termination of the rift in the Patras broader area, with many active faults lying very close and inside the city of Patras, presents major scientific and socio-economic importance. Recent seismicity has affected this end of the rift with the Movri (Achaia) earthquake in june 2008 and a seismic swarm around Efpalio (Fokida) in January 2010. Additionally the presence of a plurality of geophysical phenomena and morphological features renders this area and the Gulf of Corinth generally, as natural laboratory, a place of international initiatives as the Corinth Rift Laboratory and a case study for the EO Supersites initiative. Seismic and geodetic ground measurements from permanent networks (since 2000) and measuring campaigns (since 1990) have been (and are) performed. Moreover dense SAR data are available since 1992 and the ERS1 mission. Motivated by the lack of precise and extended mapping of the vertical deformation of the area and by the limitations of the GPS network (in terms of density of points) we investigate, model and interpret a large set of SAR interferometry data completed by the GPS data. The SAR interferometry data are very powerful for measuring vertical motions, for mapping localized deformations across faults or other features and for mapping and modeling the co-seismic deformation produced by earthquakes. We processed ascending and ascending acquisitions of ASAR/ENVISAT in the period between 2002-2010, to produce Persistent Scatterrers and Small Baseline Subsets deformation rates maps. These products have been combined together but also constrained with a number of GPS observations in order to extract the precise Up-Down and East-West deformation components field. The methodology chain performed globally well over the area (despite the vegetation cover and slopes) and provides accurate and robust results in many areas. We verified the agreement between GPS and the InSAR deformation field rates. We also compared them with remote sensing and ground observations of independent studies. We focused in specific case studies and presented the deformation rates along cross sections inside the city of Patras, around the Rion-Antirion bridge, around the urban areas of Psathopyrgos, Aigion, Sellianitika, Nafpaktos, Ακratas, the island of Trizonia, and the river deltas of Psathopyrgos, Sellianitika, Aigion, Mornos, Marathias and Akrata. Significant ground deformation is observed within the city of Patras itself, due not only to urban subsidence often seen elsewhere, but also to the motion of shallow structures likely to be induced by deep tectonic movements at the junction of the right lateral strike-slip fault linked to the Movri and penetrating inland between Rio and Patras (trans-tensional fault of Rio Patras), and the Psathopirgos normal fault at the entrance of the Corinth Rift. The Rio-Patras fault is a transition, oblique, structure, connecting the strike-slip zone to the south and the extentional area to the east. The Aigion fault appears very active with uplift rate of about 2mm/an, the highest rate across the Corinth Rift in the sample period, this uprising damping in the three kilometers separating this fault from the West Helike fault to the south. The observed discontinuities of the deformation field are not always correlated with seismic activity at the same place in the sampled period. The Temeni-Valimitika delta, east of Aigion, is the only delta of the area not subsiding (at least at its bigger part). We think that this is because it is located on the footwall of the Aigion fault with the delta compaction/subsidence balanced by the tectonic uplift. All the other deltas are subsiding due to the compaction of their sediments, and in the big ones it is possible to observe a linear increasing rate as approaching their coastal borders. The 2008 and 2010 seismic events occurred in the study area are modeled by inversion of their sources parameters using a model of dislocation in an homogenous elastic half-space constrained by the seismic, the GPS and the SAR interferometry data. At the broad scale, most of our studied tectonic features are pieces of a (diffuse) triple junction between micro-plates at the boundary between the rift of Corinth to the east and the termination of the Hellenic arc to the west. We briefly investigated and discuss the Trikonida and Aitoliko valley deformation field in the northwest of the triple junction area. Finally for the sake of completeness and in order to assess the capabilities of the space geodesy we presented some inferred discontinuities occurred by landslides and some by unclear origin and requiring further investigations.
- ItemOpen Access
- ItemOpen AccessIn-situ, ταχεία και μη-διαταρακτική διαγνωστική διαδικασιών καύσης και των προϊόντων με φασματοσκοπία πλάσματος επαγόμενο από λέιζερ (LIBS)
Τμήμα Φυσικής (ΔΔ)(2014-08-19) Κοτζαγιάννη, Μαρία; Κουρής, Στυλιανός; Άγγλος, Δημήτριος; Βάινος, Νικόλαος; Γεώργας, Αθανάσιος; Κούτσελας, Ιωάννης; Μαστοράκος, Επαμεινώνδας; Χατζηαποστόλου, Αντώνιος; Kotzagianni, MariaΤα τελευταία χρόνια, η φασματοσκοπία πλάσματος επαγόμενο από λέιζερ (LIBS) έχει προσελκύσει μεγάλο ερευνητικό ενδιαφέρον καθώς αποτελεί μία πειραματικά απλή και αποτελεσματική τεχνική, η οποία παρέχει τη δυνατότητα λήψης μετρήσεων για απευθείας ποιοτική και ποσοτική στοιχειακή ανάλυση. Η τεχνική LIBS στηρίζεται στη δημιουργία σπινθήρα/πλάσματος μέσω ισχυρά εστιασμένης δέσμης λέιζερ στην επιφάνεια ή στο εσωτερικό του δείγματος, στην ακόλουθη διέγερση και ατομοποίηση των στοιχείων του στόχου και στην τελική καταγραφή και φασματοσκοπική ανάλυση της εκπεμπόμενης ακτινοβολίας του πλάσματος. Λόγω των πολλών πλεονεκτημάτων που συγκεντρώνει η τεχνική, το LIBS έχει προταθεί για πληθώρα πρακτικών, τεχνικών και τεχνολογικών εφαρμογών σε ένα ευρύ φάσμα ερευνητικών πεδίων. Από την άλλη μεριά, στον τομέα της καύσης, η ποσότητα καυσίμου σε ένα εύφλεκτο μίγμα είναι αντικείμενο μείζονος σημασίας καθώς επηρεάζει σημαντικά την απόδοση των χημικών διεργασιών και την παραγωγή και εκπομπή ρύπων. Επομένως, δημιουργείται η ανάγκη ανάπτυξης μίας γρήγορης και μη παρεμβατικής διαγνωστικής τεχνικής για τη μέτρηση της περιεκτικότητας του καυσίμου τοπικά στη φλόγα με καλή τόσο χωρική όσο και χρονική ανάλυση. Στα πλαίσια της παρούσας διδακτορικής διατριβής, η τεχνική LIBS η οποία συγκεντρώνει όλα αυτά τα πλεονεκτήματα χρησιμοποιήθηκε για αυτό το σκοπό. Κατά τη διάρκεια των πειραμάτων, χρησιμοποιήθηκαν πηγές λέιζερ διάρκειας παλμών ns και fs, ενώ τα συστήματα καύσης που μελετήθηκαν ήταν φλόγες υδρογονανθράκων-αέρα, στρωτής και τυρβώδους ροής, απλής και συνθετότερης γεωμετρίας. Από τα LIBS φάσματα φλογών διαφορετικής σύστασης, προέκυψε λοιπόν ότι υπάρχει μία ισχυρή εξάρτηση μεταξύ των εντάσεων διαφόρων φασματικών γραμμών με το λόγο ισοδυναμίας. Επομένως, μέσω της συσχέτισης αυτής μπορεί να επιτευχθεί με μεγάλη ακρίβεια τόσο η μέτρηση της περιεκτικότητα σε καύσιμο φλογών άγνωστης σύστασης όπως επίσης και η μέτρηση της κατανομής του καυσίμου τοπικά μέσα σε όλη την έκταση της φλόγας παρέχοντας σημαντικές πληροφορίες για την δομή της. Τέλος, εφαρμόστηκε μία παραπλήσια διαγνωστική τεχνική, κατά την οποία η διηλεκτρική κατάρρευση του μέσου ήταν αποτέλεσμα ενός ηλεκτρικού σπινθήρα: electrical Spark Induced Breakdown Spectroscopy (SIBS) όπου και πραγματοποιήθηκε η συγκριτική μελέτη της ακτινοβολίας του πλάσματος επαγόμενο μέσω οπτικής και ηλεκτρικής διέγερσης.
- ItemOpen AccessInterdisciplinary applications of fractional-order circuits
Τμήμα Φυσικής (ΔΔ)Καπουλέα, Σταυρούλα; Kapoulea, StavroulaThe aim of this Ph.D. dissertation is to demonstrate the interdisciplinary nature of fractional calculus and the innovative advantages that fractional-order circuits can offer in a wide range of scientific fields. The design of such circuits for diverse filtering, control, and biological/biomedical applications is the core of this work. The main objective is to bridge the gap between theory and practical applications and, consequently, to prove the practical value of this type of circuits in numerous branches of science and technology. Different types of non-integer order transfer functions are studied, including: (i) fractional-order transfer functions, which are based on fractional-order Laplace operator(s), (ii) power-law transfer functions, which are formed through the exponentiation of the whole initial function to a fractional power, and (iii) double-order transfer functions, derived from the combination of the two previous types. Even though the power-law and double-order function types have been utilized for describing controllers and impedances, it is the first time in the literature that power-law and double-order filters are introduced, and this is an important contribution of this Thesis. As the exploitation of fractional calculus in the branch of engineering is one of the recently developed research disciplines, there is no commercial production of fundamental elements that could directly build a fractional-order circuit. This condition generates a requirement for development of alternative methods, in order to approximate the fractional behavior and realize the corresponding circuitry. Towards this purpose, a thorough research on the provided approximation techniques, that are able to approach different forms of fractional-order functions, is performed with the aim of presenting and comparing the available tools. Subsequently, an extensive reference on various types of structures, capable of implementing the function derived from the approximation process, is made, including passive and active configurations. The target is to achieve the most effective combination of applied approximation and utilized structure, in order to assure the best option for each case study. The research pivots on three principal fields: (I) filtering, (II) control systems, and (III) biology/biomedicine. A methodical approach to the mathematical framework of the case under consideration is initially performed, in order to extract the critical characteristics. Then, depending on the type of the application, the specifications and the performance requirements that need to be fulfilled, the proposed procedure for the development of the mathematical functions into the corresponding implementable circuits, is presented step by step. The final stage includes simulation and/or experimental verification of the process and the derived circuitry, supported by suitable equipment that is available in the laboratory. The theoretical analysis, as well as the approximation processes, for each case study, are performed using the MATLAB software. The performance evaluation of the proposed circuits is based on simulation results obtained using the OrCAD PSpice simulator or the Cadence software and the Design Kit provided by the Austria Mikro Systeme (AMS) CMOS 0.35 μm technology process, or on experimental results obtained using an Anadigm AN231K04 Field-Programmable Analog Array development board.
- ItemOpen AccessLaser induced breakdown spectroscopy assisted by machine learning for the classification/authentication of food products : the case of olive oil
Τμήμα Φυσικής (ΔΔ)(2023-01-16) Στέφας, Δημήτριος; Stefas, DimitriosSince the development of the laser, the scientific landscape has altered dramatically, resulting in novel experimental methodologies and a variety of technological spinoffs. One field-deployable analytical approach is Laser-Induced Breakdown Spectroscopy (LIBS), sometimes termed Laser-Induced Plasma Spectroscopy (LIPS). LIBS employs a high-energy pulsed laser to produce a plasma that vaporizes a sample. Spectral characteristics produced by excited species, i.e., atoms, ions and molecules, are utilized to gather quantitative and qualitative analytical information about the sample. Due to the fact that the optical emission from the plasma comprises the spectral signatures of all the elements present in the sample material, the elemental composition of the investigated sample may be rapidly determined by observing its LIBS spectra. LIBS offers the remarkable capability to perform multielement real-time analysis, which is not achievable with other traditional techniques. However, due to its relatively low sensitivity, the detection of trace elements remains difficult, which is a key disadvantage of this approach. The first record of a laser produced plasma was reported almost immediately after the invention of the laser, while within the last three decades a considerable number of LIBS-related applications have been proposed and realized. Moreover, statistical approaches for analyzing LIBS spectra are being improved, and commercialized LIBS instruments are available, while theoretical and computational models of plasma formation and expansion have been thoroughly examined through experiments. In the past decade, chemometric and machine learning tools for analyzing LIBS spectroscopic data have reignited scientific interest in LIBS-related applications, because of the huge datasets with thousands of variables provided in extremely quick acquisition times, compared to other spectroscopic techniques. An emerging and challenging application is the analysis of foodstuff, mainly as a quality assurance method. In this thesis the application of LIBS, assisted by machine learning, to the analysis of olive oils is investigated. The classification of different olive oils is performed based either on their geographical or cultivar origins. Different machine learning algorithms are tested and the analysis of the LIBS data provides insight into the spectral features that are most important for the successful classification. Moreover, a direct comparison of LIBS with absorption spectroscopy is performed, and the subsequent fusion of the different spectroscopic data is performed to enhance the classification accuracy. In that spirit, this work demonstrates the enhanced capabilities of LIBS for the analysis of foodstuff, as a tool proposed for the quality assessment of olive oil products. Chapter 1 of the present thesis provides an overview of the current state-of-the-art concerning the application of Laser Induced Breakdown Spectroscopy for the analysis of foodstuff, in general. Special emphasis is given to several foods of interest, such as olive oil, honey and milk. The principles of LIBS technique are discussed and described in detail. A summary of the machine learning methods used in LIBS analyses is given, with emphasis on the validation of the predictive models, as well. Then, specific food science applications of LIBS are presented thoroughly. In Chapter 2 LIBS spectra from a total of 139 extra virgin and virgin olive oil samples (EVOOs and VOOs) are classified based on the samples’ geographical origins. Different machine learning algorithms are employed. These are Linear Discriminant Analysis (LDA), Extremely Randomized Classification Trees (ERTC) and eXtreme Gradient Boosting (XGBoost) and their classification performance is assessed. Additionally, the spectral features’ importance on the classification was calculated and the most important ones were identified. In Chapter 3 a comparative study between LIBS and UV-Vis absorption spectroscopy is presented and performed, regarding the classification of olive oils based on their geographical origins. Both LIBS and absorption spectra were initially preprocessed by means of Principal Component Analysis (PCA) and were subsequently used for the construction of predictive models, employing LDA and Support Vector Machines (SVM). Following, in Chapter 4, extra virgin olive oils are discriminated based on their cultivar origin. In continuation to Chapter 3, LIBS and absorption spectra of the samples are classified by employing LDA and Gradient Boosting algorithms and the subsequent fusion of the two different origins spectroscopic data, i.e., the emission and the absorption spectra, is proposed as an efficient strategy for predicting the cultivar origin of olive oils.
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- ItemOpen AccessManufacturing and experimental investigation of green composite materials
Τμήμα Φυσικής (ΔΔ)(2014-04-30) Κουτσομητοπούλου, Αναστασία; Παπανικολάου, Γιώργιος; Παπανικολάου, Γεώργιος; Κωστόπουλος, Βασίλειος; Μαυραντζάς, Βλάσης; Γαλιώτης, Κωνσταντίνος; Βογιατζής, Γεώργιος; Αγγελόπουλος, Γεώργιος; Ψαρράς, Γεώργιος; Koutsomitopoulou, AnastasiaThe aim of the present thesis is to explore sustainable low cost environmentally friendly composite materials. It is a step by step experimental research. Firstly, taking under consideration the so far commercial available non-organic materials used as reinforcement and the petroleum based resins used as matrices, composite materials were fabricated and mechanically characterized. Different components in micro- and nano- scale were combined. Afterwards, the non-organic materials used as reinforcements were substituted by different types of non conventional natural-based fillers. The fillers (corn starch and olive pit granules) were in powder form, derived from agricultural local resources and additionally flax fabric used to produce laminated composites. All the semi-green epoxy composites were characterized by means of three-point bending testing. Moreover, the manufactured composites were induced in several sources of damage and their residual properties were extensively investigated. More precisely, the effect of the strain-rate and low velocity impact as well as of thermal fatigue, on the mechanical properties of the olive pit and the flax fabric reinforced resin was studied. Since, conventional and semi-green composite materials were fabricated and experimentally investigated, the final objective of the present thesis was to produce novel green composites materials by substituting the petroleum-based epoxy resin with a biodegradable derived from natural resources biopolyester. In order to accomplish this target, polylactic acid (PLA) was combined with olive pits in powder form at different concentrations. Olive pits, is almost unknown non-traditional filler to composites, obtained during the oil extraction process. It is a raw material characterized by its low cost and its abundance, since it consists a waste product of the olive oil industry. In order to successfully accomplish this part of research, experiments were taken place in France at the CMGD (Centre des Matériaux de Grande Diffusion) Institute of the École Nationale Supérieure des Mines d’ Alés, under the guidance of Prof. A. Bergeret within the framework of research cooperation with the main supervisor of this thesis, Prof. G. Papanicolaou. The most important feature of the present green composites is their satisfactory mechanical and thermal performance in combination with their complete biodegradability. The PLA/olive pit composites could be applied to various components with moderate strength such as automotive interiors, interior building applications, durable goods, serviceware and food packaging material The aim of this part of the study was to investigate the effect of three types of olive pit powder at different weights fractions on the physical and mechanical properties of polylactide (PLA) matrix composites. For the preparation of the powder, two different grinding procedures were applied, producing three types of olive pit powder. Various measurements were accomplished to determine characteristics such as the density and the size distribution and the shape of the powder. Different PLA/ olive pits powder composites were manufactured by extrusion and injection molding. A comparative study between the different composites was made in order to investigate the matrix-filler interactions, occurring between the PLA and olive pit granules and their overall physical, mechanical and thermomechanical properties were investigated by means of TGA, FT-IR, DSC, SEM, flexural and uni-axial tensile testing. Finally, theoretical predictive models were applied in most of the composite materials manufactured in the present work. These models making use of minimal number of experimental results can satisfactorily predict the residual properties of damaged materials, irrespectively of the type of the material investigated and the damage source. Namely, the Modulus Predictive Model (ΜPM), the Residual Properties Model (RPM) and the Residual Strength after Impact Model (RSIM), have been successfully applied. A big number of interesting conclusions have been derived from the present work. However, a general conclusion is that a totally green composite with useful properties and applications is a promising target for the humanity and the planet survivability.
- ItemOpen AccessMulti-data rate receiver for high-speed serial interfaces
Τμήμα Φυσικής (ΔΔ)Τσιμπός, Ανδρέας; Tsimpos, AndreasThe subject of this thesis is the design and verification of the Receiver end, of such a high-speed serial transceiver, suitable for mobile electronic devices (smartphones, tablets etc.), as well as the development of novel design techniques and circuit architectures capable of offering improved performance in several critical subsystems of a HSSI Receiver. At first, the timing noise (Jitter) sources encountered in serial transmission links are studied and analyzed. A comprehensive method for the calculation of the jitter noise frequency components and the behavioral modeling of jitter noise sources, is proposed. In addition, a Verilog-AMS based HSSI jitter compliant generator, capable of generating random data sequences distorted by Jitter is implemented. The proposed Jitter generator can be easily parameterized in order to be compliant with the specifications of any serial data transmission standard while it can be utilized for the time efficient performance verification of any HSSI Receivers. Afterwards, the research deals with jitter elimination techniques, related to jitter which affect the transmitted signal due to transmission medium (channel) limited bandwidth. Also, a multi-rate programmable continuous time linear channel equalizer (CTLE), is proposed. The proposed equalizer, by exploiting a coarse small-signal transfer function tuning methodology, which is based on proper re-adjustments of its sub-circuits biasing characteristics, makes feasible its operation in the wide range of frequencies dictated by M-PHY standard. In parallel, it is able to offer optimized power consumption, depending on the input data rate. Continuing, the study focuses on high precision multiphase clock generation techniques. The generation of multiphase clocks is one of the most important operations, which the timing and synchronization circuits incorporated in HSSI transceivers must be able to accomplish. Two novel Phase Interpolator topologies are proposed, which offer the advantages of high accuracy, multi-rate operation, low power consumption as well as the ability to be easily reformed in order to support higher phase resolution. The ability of the proposed circuits to operate seamlessly in real world HSSI applications, while offering competitive performance characteristics, as compared to already existing topologies in the literature, is demonstrated through the design of a clock and data recovery (CDR) loop which is, perhaps, the most critical component of an HSSI Receiver. Finally, a GHz range clock frequency multiplier, is proposed. The proposed frequency multiplier, based on a multiphase combination technique, is capable of realizing clock frequency multiplications, with an integer multiplication step. By exploiting the same operation principle, the proposed circuit is also able to perform fractional frequency multiplications, through simple reconfiguration of the phase interpolator circuit. All circuits proposed in this thesis have been designed according to the specifications implied by MPHY ver. 3.0 standard, which is one of the most popular HSSI protocols for mobile electronic devices. In this way, their ability to be properly adopted in real-world serial data transmission systems is demonstrated.
- ItemOpen AccessNovel CMOS analog integrated circuits for implementing approximants of the fractional-order Laplacian operator
Τμήμα Φυσικής (ΔΔ)Μπερτσιάς, Παναγιώτης; Bertsias, PanagiotisThis Ph.D. Thesis deals with the design of novel CMOS analog integrated circuits, which have been derived through approximation methods of the fractional-order Laplacian operator's implementation. Main attributes of the proposed topologies are the circuit simplicity, due to the small number of the employed MOS transistors, the reduced power consumption, the limited silicon area and the electronic tuning of their characteristics through appropriate dc bias currents. In recent years, fractional calculus has been applied to circuit design, rendering it one of its most interesting interdisciplinary fields. For the implementation of fractional-order circuits, except for the direct, but commercially unavailable, way of using fractional-order capacitors in the standard topologies, an alternative and, simultaneously, efficient method is the approximation of their behavior through passive or active components. The former implies the utilization of appropriately configured RC networks, easily derived, but not electronically programmable, while the latter favors the electronic adjustment at the expense of the increased active component count. Thus, the main objective of the Thesis is to overcome the existing obstacles through two primary axes. The first one includes the implementation of novel simple structures, using implemented fractional-order capacitors or approximated RC networks, and the realization of capacitorless multi-feedback topologies for performing low-order approximations. The second axis comprises the development of new systematic methods for decomposing the initial higher-order approximated transfer functions into a product or sum of simple lower-order active filter terms. The practical value of the proposed concepts is validated through application examples of various scientific fields. The performance of the presented structures is evaluated through simulation results that have been derived, using the Cadence software and the Design Kit provided by the Austria Mikro Systeme (AMS) CMOS 0.35μm technology process.