Investigation of the Mechanisms of Flash Sintering in Ceramic Materials

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Fundamental Limit of Nanophotonic Light-trapping in Solar Cells

Establishing the fundamental limit of nanophotonic light-trapping schemes is of paramount importance and is becoming increasingly urgent for current solar cell research. The standard theory of light trapping demonstrated that absorption enhancement in a medium cannot exceed a factor of 4n^2/ sin^2(\theta), where n is the refractive index of the active layer, and \theta is the angle of the emission cone in the medium surrounding the cell. This theory, however, is not applicable in the nanophotonic regime. Here we develop a statistical temporal coupled-mode theory of light trapping based on a rigorous electromagnetic approach. Our theory reveals that the standard limit can be substantially surpassed when optical modes in the active layer are confined to deep-subwavelength scale, opening new avenues for highly efficient next-generation solar cells

Η ιδιαίτερη μεταχείριση των κληρικών από νομοκανονική άποψη

Είναι αλήθεια οτι υπάρχει η εντύπωση σε μεγάλο μέρος των πολιτών πως οι θρησκευτικοί λειτουργοί της Ορθόδοξης Εκκλησίας είναι προνομιούχοι στην Ελλάδα. Ωστόσο, κατά κύριο λόγο τα προνόμια των κληρικών είναι δυσμενή. Διαχρονικά οι ιερείς στερούνται θεμελιωδών δικαιωμάτων που απολαμβάνουν ακώλυτα οι υπόλοιποι πολίτες. Δεν μπορούν να εκφράσσουν τη γνώμη τους ελεύθερα σε δημόσιο βήμα, οφείλουν να λάβουν άδεια από τον επίσκοπο ακόμα και για να μεταβούν σε άλλη εκκλησιαστική επαρχία ή το εξωτερικό, υποχρεούνται να φορούν αποκλειστικά τα άμφιά τους και άλλα πολλά. Αυτή η περιστολή των ατομικών δικαιωμάτων λαμβάνει χώρα μέσω των νόμων του κράτους αλλά και των ιερών κανόνων. Μάλιστα όπου ο περιορισμός αυτός θεμελιώνεται στο κανονικό δίκαιο έχει κριθεί οτι είναι κατ΄ αρχήν σύννομος γιατί η αρχή της αυτοοργάνωσης των θρησκευτικών κοινοτήτων έχει προβάδισμα έναντι της κατοχύρωσης δικαιωμάτων για όλους τους πολίτες από τη Πολιτεία. Οι τομείς του δικαίου στους οποίους εντοπίζεται ιδιαίτερη μεταχείριση των κληρικών είναι το Συνταγματικό δίκαιο και το δικαιο ατομικών δικαιωμάτων, το υπαλληλικό, το ιδιωτικό, το ποινικό και το δικονομικό. Στις μέρες μας, γίνεται προσπάθεια για πληρέστερη κατοχύρωση των δικαιωμάτων των πολιτών. Φυσικά, ένα ιδιαίτερο καθεστώς, όπως είναι αυτό που ισχύει για τους κληρικούς, δεν απορρίπτεται a priori χωρίς να έχει αξιολογηθεί. Στην αξιολόγηση αυτή σταθμίζεται από τη μία η ανάγκη για προστασία των δικαιωμάτων με βάση την αρχη της ισότητας και από την άλλη η εξίσου σημαντική διασφάλιση της θρησκευτικής ελευθερίας που εν προκειμένω μεταφράζεται στη δυνατότητα αυτοοργάνωσης μιας θρησκευτικής κοινότητας. Η εργασία αυτή επομένως έχει σκοπό την ανάδειξη των περιπτώσεων που παρατηρείται ειδική αντιμετώπιση των κληρικών μέσα από τους νόμους και τους ιερούς κανόνες, την αξιολόγησή τους επί τη βάσει της αρχής της αναλογικότητας αλλά και της θρησκευτικής ελευθερίας, την παρουσίαση των εξελίξεων διαχρονικά στα ζητήματα που τίθενται και την παράθεση νομολογίας επ' αυτών.It is true that there is an impression among many citizens that the religious ministers of the Orthodox Church are privileged in Greece. However, mainly the privileges of the clergy are unfavorable. Priests have long been deprived of fundamental rights enjoyed by other citizens. They can not express their opinion freely in public, they have to get permission from the bishop even to go to another ecclesiastical province or abroad, they are obliged to wear only their vestments and much more. This restriction of individual rights takes place through the laws of the state as well as the sacred rules. In fact, if this restriction is based on laws, it has been judged to be legal in principle because the principle of self-organization of religious communities takes precedence over the guarantee of rights for all citizens by the State. The areas of law in which special treatment of clergy is found are constitutional law and civil rights law, civil law, private law, criminal law and procedural law. Nowadays, an effort is being made to fully secure the rights of citizens. Of course, a special status quo, such as that which applies to clergy, is not rejected a priori without being evaluated. This assessment weighs on the one hand the need to protect rights based on the principle of equality and on the other the equally important guarantee of religious freedom which in this case translates into the possibility of self-organization of a religious community. This work therefore aims to highlight the cases where there is a special treatment of the clergy through laws and sacred rules, their evaluation on the basis of the principle of proportionality but also religious freedom, the presentation of developments over time in the issues raised and the citation of case law on them

Nanoscale stacking fault-assisted room temperature plasticity in flash-sintered TiO2.

Ceramic materials have been widely used for structural applications. However, most ceramics have rather limited plasticity at low temperatures and fracture well before the onset of plastic yielding. The brittle nature of ceramics arises from the lack of dislocation activity and the need for high stress to nucleate dislocations. Here, we have investigated the deformability of TiO2 prepared by a flash-sintering technique. Our in situ studies show that the flash-sintered TiO2 can be compressed to ~10% strain under room temperature without noticeable crack formation. The room temperature plasticity in flash-sintered TiO2 is attributed to the formation of nanoscale stacking faults and nanotwins, which may be assisted by the high-density preexisting defects and oxygen vacancies introduced by the flash-sintering process. Distinct deformation behaviors have been observed in flash-sintered TiO2 deformed at different testing temperatures, ranging from room temperature to 600°C. Potential mechanisms that may render ductile ceramic materials are discussed

Anisotropic lattice expansion determined during flash sintering of BiFeO3 by in-situ energy-dispersive X-ray diffraction

BiFeO3 has a Curie temperature (TC) of 825 °C, making it difficult to sinter using conventional methods while maintaining the purity of the material, as unavoidably secondary phases appear at temperatures above Tc. Flash sintering is a relatively new technique that saves time and energy compared to other sintering methods. BiFeO3 was flash sintered at 500 °C to achieve 90% densification. In-situ energy dispersive X-ray diffraction (EDXRD) revealed that the material did not undergo any phase transformation, having been sintered well below the TC. Interestingly, anisotropic lattice expansion in the material was observed when the sample was exposed to the electric field.U.S. Office of Naval Research (ONR) N00014-10-1- 042, N00014-17-1-2087, Sub 4104-78982U.S. Department of Energy DE-AC02-06CH1135

Field-induced mass transport phenomena in flash sintered high temperature ceramics explored by in situ SEM and TEM

Flash sintering has attracted significant attention lately as its remarkable rapid densification process at low sintering temperature leads to the retention of fine grains and enhanced dielectric properties. However, the underlying mechanism of flash sintering and mechanical behaviors of flash-sintered ceramics remain poorly understood. Here, we report the microstructure of flash-sintered yttria-stabilized zirconia (YSZ) and TiO2 by transmission electron microscope (TEM) and their high temperature in-situ micropillar compression studies inside a scanning electron microscope (SEM). Our studies on flash-sintered YSZ show that YSZ exhibits high inelastic strain (~ 8%) primarily due to phase transformation toughening below 400°C. At higher temperatures, crack nucleation and propagation are significantly retarded and prominent plasticity arises mainly from dislocation activities. The holding time and current density limit after the onset of flash for flash-sintered TiO2 significantly affect the microstructure and mechanical behavior. High dislocation density and stacking faults have been observed in the flash-sintered TiO2 under TEM. The presence of high-density defects generated during flash sintering plays a major role in the overall microstructure and mechanical behavior of ceramics

Fundamental limit of light trapping in grating structures

We use a rigorous electromagnetic approach to analyze the fundamental limit of light-trapping enhancement in grating structures. This limit can exceed the bulk limit of 4n^2, but has significant angular dependency. We explicitly show that 2D gratings provide more enhancement than 1D gratings. We also show the effects of the grating profile's symmetry on the absorption enhancement limit. Numerical simulations are applied to support the theory. Our findings provide general guidance for the design of grating structures for light-trapping solar cells.Comment: 15 pages, 8 figure

Dramatic reduction of surface recombination by in-situ surface passivation of silicon nanowires

Nanowires have unique optical properties [1-4] and are considered as important building blocks for energy harvesting applications such as solar cells. [2, 5-8] However, due to their large surface-to-volume ratios, the recombination of charge carriers through surface states reduces the carrier diffusion lengths in nanowires a few orders of magnitude,[9] often resulting in the low efficiency (a few percent or less) of nanowire-based solar cells. [7, 8, 10, 11] Reducing the recombination by surface passivation is crucial for the realization of high performance nanosized optoelectronic devices, but remains largely unexplored. [7, 12-14] Here we show that a thin layer of amorphous silicon (a-Si) coated on a single-crystalline silicon nanowire (sc-SiNW), forming a core-shell structure in-situ in the vapor-liquid-solid (VLS) process, reduces the surface recombination nearly two orders of magnitude. Under illumination of modulated light, we measure a greater than 90-fold improvement in the photosensitivity of individual core-shell nanowires, compared to regular nanowires without shell. Simulations of the optical absorption of the nanowires indicate that the strong absorption of the a-Si shell contributes to this effect, but we conclude that the effect is mainly due to the enhanced carrier lifetime by surface passivation