Μετάβαση από τα δίκτυα χαλκού σε δίκτυα πρόσβασης νέας γενιάς

Η μελλοντική εξέλιξη τόσο των ενσύρματων όσο και των ασύρματων αστικών δικτύων βασίζεται κατά κύριο λόγο στα δίκτυα πρόσβασης νέας γενιάς. Η παρούσα εργασία περιγράφει αρχικά τα ενσύρματα δίκτυα της τεχνολογίας FTTx, την παρούσα μορφή τους και τους περιορισμούς τους. Ακολούθως, περιγράφονται οι μελλοντικές εξελίξεις στο συγκεκριμένο τομέα της τεχνολογίας, εστιάζοντας κυρίως στα δίκτυα VDSL2 και την τεχνολογία LTE, τα οποία θα έχουν σημαντική μελλοντική επίδραση στην εξέλιξη των δικτύων πρόσβασης νέας γενιάς. Οι τάσεις των δικτύων νέας γενιάς (NGA) βασίζονται κυρίως στα Παθητικά Οπτικά Δίκτυα, τα οποία αναλύονται σε TDMA PON και WDM PON, κατηγορίες που αναλύονται ανάλογα με τις χρονοθυρίδες και το μήκος κύματος αντίστοιχα. Στη συνέχεια, γίνεται αναφορά των ασύρματων δικτύων πρόσβασης νέας γενιάς, εξετάζοντας οι σύγχρονες τεχνολογίες όπως τα WLAN και τα Wifi, τα LMDS δίκτυα καθώς και τα δίκτυα δορυφορικά ευρυζωνικά δίκτυα. Επίσης γίνεται αναφορά και σε άλλες καινούριες τεχνολογίες, όπως τα Δίκτυα Νέας Γενιάς (NGN) αλλά και στα κυψελωτά δίκτυα, ξεκινώντας από το GSM, φτάνοντας μέχρι το LTE Advanced. Τέλος, αναφέροντας τα Δίκτυα Νέας Γενιάς δίνεται έμφαση στη σημασία των Πράσινων Δικτύων και πώς αυτή αυτά παρέχονται στους χρήστες και επηρεάζει την καθημερινότητα και ποιότητα ζωής τους σε βάθος χρόνου. Εν κατακλείδι, τα NGA θα πρέπει να είναι σε θέση να υπερνικήσουν διάφορες προκλήσεις, όπως η παροχή διαφορετικών ευζωνικών υπηρεσιών καθώς και η διευκόλυνση της διαλειτουργικότητας των διαφορετικών τεχνολογιών. Επιπλέον, τα NGA θα πρέπει να παρέχουν μεγαλύτερο εύρος ζώνης με όσο το δυνατόν μεγαλύτερη μείωση του κόστους των υφιστάμενων υπηρεσιών. Παράλληλα, θα πρέπει να αποτελούν τη βάση των ασύρματων και ενσύρματων δικτύων πρόσβασης (WiFi και WLAN και FTTx), κάτι το οποίο αποτελεί ένα σχετικό ζήτημα για τη σύγκλιση μεταξύ των ενσύρματων και των ασύρματων τεχνολογιών. Με βάση τη μελέτη που εκπονήθηκε, στα πλαίσια αυτής της εργασίας, σε έργα που υποστηρίζονται από ελληνικούς παρόχους η εφαρμογή των FTTx υπάρχει σε αρκετές περιοχές κατά κύριο λόγο σε επίπεδο FTTC. Αναμένουμε σύντομα, να φτάσει και σε επίπεδο FTTH, όπου η υπηρεσία και το κόστος θα είναι προσιτά πλέον σε κάθε οικιακό πελάτη.The future development of both wired and wireless urban networks is primarily based on NGA networks. Firstly, this first describes the wired networks with FTTx technology, in their present form and their limitations. After that, future developments in this field of technology are described, focusing especially on VDSL2 networks and LTE technology, which will have a significant future impact on the development of NGA networks. The trends of the new generation access (NGA) networks are mainly based on Passive Optical Networks, which are categorized in TDMA PON and WDM PON, and are being characterized according to the slots and wavelength respectively. Then, there is made a reference of wireless NGA networks by examining modern technologies such as WLAN and Wifi, the LMDS networks and satellite broadband networks. Reference is also made to other new technologies such as Next Generation Networks (NGN) and to cellular networks, starting from the GSM, reaching to the LTE Advanced. Finally, referring to the New Generation Networks, is emphasized the importance of the Green Network and how it shall be provided to users and affect their daily lives and their quality of life over time. In conclusion, the NGA should be able to overcome various challenges, such as the provision of different broadband services and facilitating interoperability of different technologies. In addition, the NGA should provide more bandwidth and at the same time the greatest possible reduction of the cost of existing services. Also, they should be the basis of wireless and wireline access networks (WiFi and WLAN and FTTx), something which is a relevant issue for the convergence between wireline and wireless technologies. Based on the study conducted as part of this work, projects supported by Greek providers with implementation of FTTx exist in many areas mainly as application of FTTC. We expect soon to arrive in FTTH level, where the service and the cost will be more accessible in every household customer

Layer-by-Layer Assembly of Clay-Carbon Nanotube Hybrid Superstructures

Much of the research effort concerning layered materials is directed toward their use as building blocks for the development of hybrid nanostructures with well-defined dimensions and behavior. Here, we report the fabrication through layer-by-layer deposition and intercalation chemistry of a new type of clay-based hybrid film, where functionalized carbon nanotubes are sandwiched between nanometer-sized smectite clay platelets. Single-walled carbon nanotubes (SWCNTs) were covalently functionalized in a single step with phenol groups, via 1,3-dipolar cycloaddition, to allow for stable dispersion in polar solvents. For the production of hybrid thin films, a bottom-up approach combining self-assembly with Langmuir-Schaefer deposition was applied. Smectite clay nanoplatelets act as a structure-directing interface and reaction media for grafting functionalized carbon nanotubes in a bidimensional array, allowing for a controllable layer-by-layer growth at a nanoscale. Hybrid clay/SWCNT multilayer films deposited on various substrates were characterized by X-ray reflectivity, Raman, and X-ray photoelectron spectroscopies, as well as atomic force microscopy

Enhancement of endogenous midbrain neurogenesis by microneurotrophin BNN-20 after neural progenitor grafting in a mouse model of nigral degeneration

We have previously shown the neuroprotective and pro-neurogenic activity of microneurotrophin BNN-20 in the substantia nigra of the “weaver” mouse, a model of progressive nigrostriatal degeneration. Here, we extended our investigation in two clinically-relevant ways. First, we assessed the effects of BNN-20 on human induced pluripotent stem cell-derived neural progenitor cells and neurons derived from healthy and parkinsonian donors. Second, we assessed if BNN-20 can boost the outcome of mouse neural progenitor cell intranigral transplantations in weaver mice, at late stages of degeneration. We found that BNN-20 has limited direct effects on cultured human induced pluripotent stem cell-derived neural progenitor cells, marginally enhancing their differentiation towards neurons and partially reversing the pathological phenotype of dopaminergic neurons generated from parkinsonian donors. In agreement, we found no effects of BNN-20 on the mouse neural progenitor cells grafted in the substantia nigra of weaver mice. However, the graft strongly induced an endogenous neurogenic response throughout the midbrain, which was significantly enhanced by the administration of microneurotrophin BNN-20. Our results provide straightforward evidence of the existence of an endogenous midbrain neurogenic system that can be specifically strengthened by BNN-20. Interestingly, the lack of major similar activity on cultured human induced pluripotent stem cell-derived neural progenitors and their progeny reveals the in vivo specificity of the aforementioned pro-neurogenic effect

From Homogeneous to Heterogenized Molecular Catalysts for H2 Production by Formic Acid Dehydrogenation: Mechanistic Aspects, Role of Additives, and Co-Catalysts

H2 production via dehydrogenation of formic acid (HCOOH, FA), sodium formate (HCOONa, SF), or their mixtures, at near-ambient conditions, T < 100 °C, P = 1 bar, is intensively pursued, in the context of the most economically and environmentally eligible technologies. Herein we discuss molecular catalysts (ML), consisting of a metal center (M, e.g., Ru, Ir, Fe, Co) and an appropriate ligand (L), which exemplify highly efficient Turnover Numbers (TONs) and Turnover Frequencies (TOFs) in H2 production from FA/SF. Typically, many of these ML catalysts require the presence of a cofactor that promotes their optimal cycling. Thus, we distinguish the concept of such cofactors in additives vs. co-catalysts: When used at high concentrations, that is stoichiometric amounts vs. the substrate (HCOONa, SF), the cofactors are sacrificial additives. In contrast, co-catalysts are used at much lower concentrations, that is at stoichiometric amount vs. the catalyst. The first part of the present review article discusses the mechanistic key steps and key controversies in the literature, taking into account theoretical modeling data. Then, in the second part, the role of additives and co-catalysts as well as the role of the solvent and the eventual inhibitory role of H2O are discussed in connection to the main mechanistic steps. For completeness, photons used as activators of ML catalysts are also discussed in the context of co-catalysts. In the third part, we discuss examples of promising hybrid nanocatalysts, consisting of a molecular catalyst ML attached on the surface of a nanoparticle. In the same context, we discuss nanoparticulate co-catalysts and hybrid co-catalysts, consisting of catalyst attached on the surface of a nanoparticle, and their role in the performance of molecular catalysts ML

Atomic {Pdn+-X} States at Nanointerfaces: Implications in Energy-Related Catalysis

Palladium is among the most versatile noble-metal atoms that, when dispersed on solid supports, can be stabilized in 0, +1, +2, +3 redox states. Moreover, despite its noble-metal character, Pd shows a considerable degree of chemical reactivity. In Pd Nanoparticles (NPs), atomic {Pdn+-X} states, where n = 0, 1, 2, 3, and X = atom or hydride, can play key roles in catalytic processes. Pd-oxygen moieties can be stabilized at nanointerfaces of Pd in contact with metal-oxides. These {Pdn+-X}s can be either isolated Pd atoms dispersed on the support, or, more interestingly, atomic states of Pd occurring on the Pd NPs. The present review focuses on the role of such {Pdn+-X} states in catalytic processes related to energy storage or energy conversion, with specific focus on photocatalysis, H2 production reaction (HRR), oxygen reduction reaction (ORR), and water-splitting. Synthesis of atomic {Pdn+-X} states and their detection methodology is among the current challenges. Herein, the chemistry of {Pdn+-X} states on Pd- [metal oxide] interfaces, methods of detection, and identification are discussed. The implication of {Pdn+-X} in transient catalytic intermediates is reviewed. Finally, the role of {Pdn+-X} in photo electrocatalytic processes is critically discussed

Lattice Defects Engineering in W-, Zr-doped BiVO4 by Flame Spray Pyrolysis: Enhancing Photocatalytic O2 Evolution

A flame spray pyrolysis (FSP) method has been developed, for controlled doping of BiVO4 nanoparticles with W and Zr in tandem with the oxygen vacancies (Vo) of the BiVO4 lattice. Based on XPS and Raman data, we show that the nanolattice of W-BiVO4 and Zr-BiO4 can be controlled to achieve optimal O2 evolution from H2O photocatalysis. A synergistic effect is found between the W- and Zr-doping level in correlation with the Vo-concentration. FSP- made W-BiVO4 show optimal photocatalytic O2-production from H2O, up to 1020 μmol/(g × h) for 5%W-BiVO4, while the best performing Zr-doped achieved 970 μmol/(g × h) for 5%Zr-BiVO4. Higher W-or Zr-doping resulted in deterioration in photocatalytic O2-production from H2O. Thus, engineering of FSP-made BiVO4 nanoparticles by precise control of the lattice and doping-level, allows significant enhancement of the photocatalytic O2-evolution efficiency. Technology-wise, the present work demonstrates that flame spray pyrolysis as an inherently scalable technology, allows precise control of the BiVO4 nanolattice, to achieve significant improvement of its photocatalytic efficiency

Stabilization of Phenolic Radicals on Graphene Oxide:An XPS and EPR Study

A graphene oxide-gallic acid hybrid material was synthesized by the immobilization of gallic acid (3,4,5-trihydroxobenzoic acid) on graphene oxide. The grafting was achieved via the formation of amide bonds between the amine groups on the organofunctionalized graphite oxide surface and the carboxyl groups of the gallic acid molecules. The EPR signal of the gallic acid radicals in this hybrid material remained almost unaltered over at least 500 days, with less than 3% signal decay over that period, pointing to the truly remarkable stability of these radicals. The produced material was characterized by Fourier transform infrared, X-ray photoelectron, and electron paramagnetic resonance spectroscopies as well as by thermogravimetric analysis and the Kaiser test. The stability of the radicals in the material was studied in powder form and in aqueous solution vs pH. We demonstrate that in the graphene oxide-gallic acid hybrid material a radical is favorably stabilized on the ring-O while the oxidation of the second OH is precluded, and this results in long-term stabilization of the gallic acid radicals in solid hybrid material. Thus, in applications where it will be used under O-2-free and humidity-free conditions, the graphene oxide-gallic acid hybrid material is a reliable spintronics scaffold

Atomic {Pdⁿ⁺-X} States at Nanointerfaces: Implications in Energy-Related Catalysis

Palladium is among the most versatile noble-metal atoms that, when dispersed on solid supports, can be stabilized in 0, +1, +2, +3 redox states. Moreover, despite its noble-metal character, Pd shows a considerable degree of chemical reactivity. In Pd Nanoparticles (NPs), atomic {Pdn+-X} states, where n = 0, 1, 2, 3, and X = atom or hydride, can play key roles in catalytic processes. Pd-oxygen moieties can be stabilized at nanointerfaces of Pd in contact with metal-oxides. These {Pdn+-X}s can be either isolated Pd atoms dispersed on the support, or, more interestingly, atomic states of Pd occurring on the Pd NPs. The present review focuses on the role of such {Pdn+-X} states in catalytic processes related to energy storage or energy conversion, with specific focus on photocatalysis, H2 production reaction (HRR), oxygen reduction reaction (ORR), and water-splitting. Synthesis of atomic {Pdn+-X} states and their detection methodology is among the current challenges. Herein, the chemistry of {Pdn+-X} states on Pd- [metal oxide] interfaces, methods of detection, and identification are discussed. The implication of {Pdn+-X} in transient catalytic intermediates is reviewed. Finally, the role of {Pdn+-X} in photo electrocatalytic processes is critically discussed

Efficient Low-Temperature H₂ Production from HCOOH/HCOO^– by [Pd⁰@SiO₂‑Gallic Acid] Nanohybrids: Catalysis and the Underlying Thermodynamics and Mechanism

Hybrid Pd⁰-based nanoparticles have been synthesized in aqueous solution by two routes: (a) reduction of Pd ions by gallic acid (GA) producing Pd⁰-GA and (b) Pd⁰ formed on SiO₂-GA nanohybrids where GA was covalently grafted on SiO₂ nanoparticles (Pd⁰@­SiO₂-GA). In both protocols, Pd⁰ nanoparticles were formed <i>in situ,</i> under alkaline pH, via reduction of Pd²⁺ ions by GA radicals formed by atmospheric O₂. XRD and TEM data show that the Pd⁰@­SiO₂-GA consists of 6.5 nm Pd⁰ nanoparticles finely dispersed on the SiO₂-GA nanosupport, whereas Pd⁰-GA consists of aggregated 12 nm Pd⁰ nanoparticles. The two families of Pd⁰ nanohybrids have been studied for catalytic H₂ production from formic acid/​sodium formate in aqueous solution at near ambient temperatures 40–80 °C. Pd⁰@­SiO₂-GA achieves H₂ production from NaCOOH/​HCOOH at 19 mL/min per mg of Pd. This outperforms by a factor of 400% the H₂ production by (Pd⁰-GA) particles, as well as all Pd⁰-SiO₂ catalysts, so far reported in the literature. The Pd⁰@­SiO₂-GA catalyst faces a significantly lower activation barrier (<i>E</i>_a = 42 kJ/mol) compared to <i>E</i>_a = 54 kJ/mol for Pd⁰-GA. A physicochemical mechanism is discussed which entails the involvement of CO₂/​HCO₃^–, as well as an active cocatalytic effect of gallic acid as proton shuttle. The results reveal that the SiO₂-GA matrix plays a dual role: (i) GA moieties capped by Pd⁰ nanoparticles impose a fine dispersion of the Pd⁰ nanocatalysts on the surface, and (ii) surface-grafted GA moieties <i>not capped</i> by Pd⁰ provide cocatalytic agents that promote the HCOOH deprotonation. From the engineering point of view, the superior H₂ production rate of the Pd⁰@­SiO₂-GA system is due to two factors: (i) the lower thermodynamic barrier, which is due to the cocatalytic GA moieties not capped by Pd⁰ particles, and (ii) fine dispersion of the Pd⁰ nanoparticles on the SiO₂ surface optimizes the kinetics of the reaction