Εστίαση ακτίνων Χ χρησιμοποιώντας Microchannel plates

89 σ.Η οπτική συμπεριφορά των MicroChannel Plates (MCP) "lobster-eye" γεωμετρίας σε ακτίνες Χ έχει περιγραφεί συχνά στη βιβλιογραφία. Ερευνήσαμε τις εστιακές ιδιότητες ενός MCP ακτινικής γεωμετρίας, τετραγωνικών οπών χρησιμοποιώντας μια πηγή ακτίνων Χ. Μια τέτοια οπτική συσκευή λειτουργεί με ολικές ανακλάσεις των ακτίνων Χ στα τοιχώματα των διαύλων του, οδηγώντας, έτσι, την ακτινοβολία σε συγκεκριμένο εστιακό σημείο. Κάποια βασικά χαρακτηριστικά των MCPs χάρη στα οποία βρίσκουνε πολλές εφαρμογές όπως σε τηλεσκόπια ακτίων Χ είναι το σχετικά μικρό κόστος κατασκευής σε σύγκριση με άλλες συσκευές εστίασης ακτίνων Χ, το μικρό τους βάρος αλλά και το μεγάλο εύρος του φακού. Στην συγκεκριμένη εργασία ερευνήσαμε την αποδοτικότητα και τις ιδιότητες εστίασης ακτίνων Χ ενός κυκλικού μικροδιαυλικού επίπεδου πλακιδίου (MCP), τετραγωνικών διαύλων, κατασκευασμένο από την Photonis SAS. Χρησιμοποιήσαμε ακτίνες Χ ενέργειας 8,05 keV παραγόμενες από ένα δισκίο χαλκού.The optical behavior of MicroChannel Plates (MCP) "lobster-eye" geometry in X-Rays has often been described in the literature. We investigated the properties of a radial geometry MCP having square channels using an X-ray source. Such an optical element operates with total reflection X-ray on the walls of the channels, leading, the radiation to a specific focal point. Some key features of MCPs due to which they are used in many applications, such as X-ray telescopes, are the relatively low manufacturing cost compared with other X-ray focusing elements, their light weight and the wide range of the lens. In this paper we investigated the efficiency and the X-ray focusing properties of a flat circular microchannel plate (MCP) having square channels constructed from Photonis SAS. We used an X-Ray source emmiting energy at 8,05 keV produced by a copper tablet.Γεώργιος Κ. Παπαγεωργίο

Catalytic Hydrogenation and Hydrodeoxygenation of Furfural over Pt(111): A Model System for the Rational Design and Operation of Practical Biomass Conversion Catalysts

Furfural is a key bioderived platform chemical whose reactivity under hydrogen atmospheres affords diverse chemical intermediates. Here, temperature-programmed reaction spectrometry and complementary scanning tunneling microscopy (STM) are employed to investigate furfural adsorption and reactivity over a Pt(111) model catalyst. Furfural decarbonylation to furan is highly sensitive to reaction conditions, in particular, surface crowding and associated changes in the adsorption geometry: furfural adopts a planar geometry on clean Pt(111) at low coverage, tilting at higher coverage to form a densely packed furfural adlayer. This switch in adsorption geometry strongly influences product selectivity. STM reveals the formation of hydrogen-bonded networks for planar furfural, which favor decarbonylation on clean Pt(111) and hydrogenolysis in the presence of coadsorbed hydrogen. Preadsorbed hydrogen promotes furfural hydrogenation to furfuryl alcohol and its subsequent hydrogenolysis to methyl furan, while suppressing residual surface carbon. Furfural chemistry over Pt is markedly different from that over Pd, with weaker adsorption over the former affording a simpler product distribution than the latter; Pd catalyzes a wider range of chemistry, including ring-opening to form propene. Insight into the role of molecular orientation in controlling product selectivity will guide the design and operation of more selective and stable Pt catalysts for furfural hydrogenation

Towards reviving post-Olympic Athens as a cultural destination

This paper examines the effects of global change on the status and qualities of the Greek national capital, Athens, focusing on how they affect the development of cultural tourism in the city. Although Athens constituted one of the most significant destinations for Greek tourism in the past, in recent years it started to weaken. Athens is characterised by a series of problems, among them are the degradation of its environment and quality of life and traffic congestion. However, in terms of tourism development, the Olympic Games helped in re-imaging the city and upgrading its infrastructure. This study based on semi-structured interviews with top officials reveals how global change has affected Athens’ socio-cultural/economic status, identity and image. Despite the tourism policy/planning responses to global changes, Athens’ tourism continues to decline leaving unexplored potential such as its rich cultural heritage, new multicultural identity and the New Acropolis Museum. The paper suggests that cultural elements of capital cities must be multidimensional including a variety of attractions and amenities. The use of cultural heritage assets needs to be in line with global developments in order for cities to effectively leverage heritage for cultural tourism

Deprotection, tethering and activation of a one-legged metalloporphyrin on a chemically active metal surface: NEXAFS, synchrotron XPS and STM study of [SAc]P- Mn(III)Cl on Ag(100)

Abstract The structural and reactive properties of the acetyl-protected "one-legged" manganese porphyrin [SAc]P-Mn(III)Cl on Ag(100) have been studied by NEXAFS, synchrotron XPS and STM. Spontaneous surface-mediated de-protection occurs at 300 K accompanied by spreading of the resulting thio-tethered porphyrin across the metal surface. Loss of the axial chlorine ligand occurs at 498 K, without any de-metallation of the macrocycle, leaving the Mn center in a low co-ordination state. At low coverages the macrocycle is markedly tilted towards the silver surface, as is the phenyl group that forms part of the tethering "leg". In the monolayer region a striking transition occurs whereby the molecule rolls over, preserving the tilt angle of the phenyl group, strongly increasing that of the 2 macrocycle, decreasing the apparent height of the molecule and decreasing its footprint, thus enabling closer packing. These findings are in marked contrast with those previously reported for the corresponding more rigidly bound four-legged porphyrin [JACS 2009[JACS , 126, 1910 suggesting that the physico-chemical properties and potential applications of these versatile systems should be strongly dependent on the mode of tethering to the surface

Direct Silicon Heterostructures With Methylammonium Lead Iodide Perovskite for Photovoltaic Applications

We investigated the formation of photovoltaic (PV) devices using direct n-Si/MAPI (methylammonium lead tri-iodide) two-sided heterojunctions for the first time (as a possible alternative to two-terminal tandem devices) in which charge might be generated and collected from both the Si and MAPI. Test structures were used to establish that the n-Si/MAPI junction was photoactive and that spiro-OMeTAD acted as a “pinhole blocking” layer in n-Si/MAPI devices. Two-terminal “substrate” geometry devices comprising Al/n-Si/MAPI/spiro-OMeTAD/Au were fabricated and the effects of changing the thickness of the semitransparent gold electrode and the silicon resistivity were investigated. External quantum efficiency and capacitance–voltage measurements determined that the junction was one-sided in the silicon—and that the majority of the photocurrent was generated in the silicon, with there being a sharp cutoff in photoresponse above the MAPI bandgap. Construction of band diagrams indicated the presence of an upward valence band spike of up to 0.5 eV at the n-Si/MAPI interface that could impede carrier flow. Evidence for hole accumulation at this feature was seen in both Kelvin-probe transients and from unusual features in both current–voltage and capacitance–voltage measurements. The devices achieved a hysteresis-free best power conversion efficiency of 2.08%, V OC 0.46 V, J SC 11.77 mA/cm2, and FF 38.4%, demonstrating for the first time that it is possible to create a heterojunction PV device directly between the MAPI and n-Si. Further prospects for two-sided n-Si/MAPI heterojunctions are also discussed