A high-strength silicide phase in a stainless steel alloy designed for wear-resistant applications

Wear- and corrosion-resistant hardfacing steels rely on carbon for strengthening. Here, the authors report an ultra-high strength silicide phase that could lead to a new class of silicide-strengthened stainless steels and alternative coatings for nuclear applications

Analysis of the pore of the unusual major intrinsic protein channel, yeast Fps1p

Fps1p is a glycerol efflux channel from Saccharomyces cerevisiae. In this atypical major intrinsic protein neither of the signature NPA motifs of the family, which are part of the pore, is preserved. To understand the functional consequences of this feature, we analyzed the pseudo-NPA motifs of Fps1p by site-directed mutagenesis and assayed the resultant mutant proteins in vivo. In addition, we took advantage of the fact that the closest bacterial homolog of Fps1p, Escherichia coli GlpF, can be functionally expressed in yeast, thus enabling the analysis in yeast cells of mutations that make this typical major intrinsic protein more similar to Fps1p. We observed that mutations made in Fps1p to "restore" the signature NPA motifs did not substantially affect channel function. In contrast, when GlpF was mutated to resemble Fps1p, all mutants had reduced activity compared with wild type. We rationalized these data by constructing models of one GlpF mutant and of the transmembrane core of Fps1p. Our model predicts that the pore of Fps1p is more flexible than that of GlpF. We discuss the fact that this may accommodate the divergent NPA motifs of Fps1p and that the different pore structures of Fps1p and GlpF may reflect the physiological roles of the two glycerol facilitators

Crystal structures' determination using electron diffraction and high resolution transmission electron microscopy

The aim of this doctoral dissertation is the structural investigation of: a) the 6H-SiC semiconductor b) the Ni-Te system c) the Cu₂₋δSe material using a combination of automatic electron tomography (ADT) and precession technique. While the structure of 6H-SiC was already known, in this thesis it was the first time it was made possible to refine semiconductor’s structure using the combination of automatic electron tomography and precession technique. The results are in agreement with what was already known from x-rays methods, with a relatively low error factor (R=13.7%), validating ADT as a powerful and reliable technique for structure determination. The Ni-Te system is a two-solid-phase mixture. In this material nanodomains of a modulated unknown phase Ni₁₊ₓTe (phase-β) were found inside the Ni₁Te₁ matrix (phase-α). The nanodomains have a size of few tens of nanometers, appearing as a small minority in volume embedded in the matrix. It has been found that the structure of phase-β has a monoclinic spacegroup (Pm). This structure is a superstructure of a hypothetical hexagonal structure, which has never been observed. The Cu₂₋δSe material undergoes a phase transition from an ordered room temperature superstructure (phase-β) to a disordered fcc one (phase-α). Part of this thesis is an effort to find which tetrahedral and octahedral sites were occupied in its semioccupied layers. The applied method, using film instead of a CCD camera, and the huge number of Cu and Se atoms in the cell, disallowing the crystallographic software to run properly, operated beyond its reliability limits and gave untrustworthy results.Στην παρούσα διδακτορική διατριβή έγινε μελέτη του ημιαγωγού 6H-SiC, του συστήματος Ni-Te και του υλικού Cu₂₋δSe με τη συνδυαστική χρήση αυτοματοποιημένης ηλεκτρονικής τομογραφίας (ADT) και μετάπτωσης της ηλεκτρονικής δέσμης (precession technique). Πιο αναλυτικά, μπορεί η δομή του ημιαγωγού 6H-SiC να είναι ευρέως γνωστή, όμως είναι η πρώτη φορά που μέσα από τη συνδυαστική χρήση (ADT) και μετάπτωσης της ηλεκτρονικής δέσμης κατέστη εφικτή η επιβεβαίωση δομής ημιαγωγού. Την ίδια στιγμή το σφάλμα απόκλισης (R=13.7%) είναι το χαμηλότερο, που έχει παρατηρηθεί μέχρι στιγμής κατά τη χρήση των τεχνικών αυτών. Το σύστημα Ni-Te αποτελείται από δύο φάσεις τις -α και -β. Η φάση-β παρατηρήθηκε ότι εμφανίζεται σε νάνο-περιοχές μεγέθους μέχρι 70nm. Με τη χρήση της ADT, σε συνδυασμό με την τεχνική μετάπτωσης της ηλεκτρονικής δέσμης, παρατηρήθηκε και επιβεβαιώθηκε ότι η κυρίαρχη φάση είναι η γνωστή Ni-Te με δομή τύπου NiAs (φάση-α). Επίσης, βρέθηκε ότι η στοιχειομετρία της δεύτερης φάσης (φάση-β) είναι Ni₅Te₄ ενώ η ομάδα συμμετρίας της ότι είναι η Pm. Διαπιστώθηκε ακόμη ότι στη δομή της φάσης-β ευνοείται η κατάληψη των οκταεδρικών αρχικά θέσεων και ορισμένων τετραεδρικών στη συνέχεια. Το γεγονός αυτό έχει ως αποτέλεσμα τη δημιουργία μεικτής δομής όχι μόνο εξαιτίας της συμμετοχής δυο ειδών ατόμων (άτομα Νi και Te) αλλά και από την άποψη των πληρούμενων πολυέδρων (οκτάεδρα και τετράεδρα). Το υλικό Cu₂₋δSe (0 ≤ δ ≤ 0.25) είναι ένα κράμα ιοντικού - ηλεκτρονικού αγωγού. Σε υψηλές θερμοκρασίες έχει μία κυβική δομή τύπου αντιφθορίτη (α-φάση) και θερμοκρασία δωματίου εμφανίζει κανονική υπερδομή γνωστή ως β-φάση. Στην παρούσα διδακτορική διατριβή έγινε προσπάθεια να διευκρινιστεί ποιες ακριβώς θέσεις καταλαμβάνονται στις μερικώς κατειλημμένες οκταεδρικές και τετραεδρικές στρώσεις. Η όλη μέθοδος α) χρήση film αντί για CCD κάμερα για τη λήψη των εικόνων περίθλασης και β) ο μεγάλος αριθμός ατόμων Cu και Se που υπήρχαν στην κυψελίδα και επηρέασαν την ομαλή λειτουργία των τα κρυσταλλογραφικών προγραμμάτων) λειτούργησε στα όριά της και δεν έδωσε αξιόπιστα αποτελέσματα

UNDERWATER 3D MODELING: IMAGE ENHANCEMENT AND POINT CLOUD FILTERING

This paper examines the results of image enhancement and point cloud filtering on the visual and geometric quality of 3D models for the representation of underwater features. Specifically it evaluates the combination of effects from the manual editing of images’ radiometry (captured at shallow depths) and the selection of parameters for point cloud definition and mesh building (processed in 3D modeling software). Such datasets, are usually collected by divers, handled by scientists and used for geovisualization purposes. In the presented study, have been created 3D models from three sets of images (seafloor, part of a wreck and a small boat's wreck) captured at three different depths (3.5m, 10m and 14m respectively). Four models have been created from the first dataset (seafloor) in order to evaluate the results from the application of image enhancement techniques and point cloud filtering. The main process for this preliminary study included a) the definition of parameters for the point cloud filtering and the creation of a reference model, b) the radiometric editing of images, followed by the creation of three improved models and c) the assessment of results by comparing the visual and the geometric quality of improved models versus the reference one. Finally, the selected technique is tested on two other data sets in order to examine its appropriateness for different depths (at 10m and 14m) and different objects (part of a wreck and a small boat's wreck) in the context of an ongoing research in the Laboratory of Photogrammetry and Remote Sensing

Extended -spectrum Beta- lactamases in Salmonella isolates

We study the resistance of salmonelle to beta lattamine