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1.
Characterization of Mg low-index surfaces by first-principles
Miha Gunde, 2017, magistrsko delo

Opis: In this work, three surfaces, namely, Mg(0001), Mg(10 ̄1 0) and Mg(11 ̄2 0) are investigated by means of Density Functional Theory and Generalized Gradient Approximation first-principle calculations. The surfaces have been characterized in terms of their interlayer distances, surface energies, Projected Density of States and Surface Core Level Shifts. Contrary to what was found in previous studies based on Density Functional Theory-Local Density Approximation, the use of Generalized Gradient Approximation exchange correlation functional unravels the oscillatory behavior of both interlayer distances and Surface Core Level Shifts of Mg(10 ̄1 0). Moreover, the Projected Density Of States of un-reconstructed Mg(11 ̄2 0) exhibit the typical quantisation of electronic levels associated to a 2D-confinement of a 3D nearly-free electron gas, as observed in nano-wires, and a Peierls instability along the unconfined direction. Different possible reconstructions on Mg(11 ̄2 0) have then been investigated. In particular, it is found that the surface energy of (2X1) and (3X1) reconstructions is almost degenerate and lower than the surface energy of the unreconstructed Mg(11 ̄2 0) surface. Because of the quasi-degeneracy of surface energies, Mg(11 ̄2 0) surface should be composed by a combination of all the quasi degenerate phases.
Ključne besede: Density Functional Theory, first-principle calculations, Mg surface, Surface Core-Level Shift, surface relaxation, surface reconstruction
Objavljeno: 04.07.2017; Ogledov: 299; Prenosov: 318
.pdf Polno besedilo (1,77 MB)

2.
SiPM as light detector in Cherenkov telescopes cameras at CTA
Aleš Bogovič, 2017, magistrsko delo

Opis: This master thesis describes characterization of Silicon photomultipliers (SiPM) for the upgrade of large size telescope (LST) camera at Cherenkov telescope array (CTA). Upon the construction, CTA will be the largest and most sensitive facility for detection of very-high energy (VHE, > 30 GeV) gamma-rays ever. It will use three different sizes of telescopes, where small and medium telescopes camera will be SiPM based from the beginning, but LST camera is planned to get SiPM based camera with frst upgrade. Camera upgrade is among others also being developed at University of Padovas Department of Physics and Astronomy (INFN). Due to limitations of their laboratory characterization of prototypes is done at Institute Jooef Stefan in Ljubljana, where I measured breakdown voltage of and determined photon detection effciency depending on angle of photon arrival. This information is important for determining which SiPM is best for CTA telescopes.
Ključne besede: Silicon photomultiplier, SiPM, CTA
Objavljeno: 03.05.2017; Ogledov: 474; Prenosov: 5
.pdf Polno besedilo (14,05 MB)

3.
Characterization of Nd2Fe14B sintered magnets and effect of addition TbF and DyF by Grain Boundary Diffusion Process
Jan Ferjančič, 2016, magistrsko delo

Opis: Sintrani Nd2Fe14B magneti tretje generacije so večinoma uporabljeni v avtomobilski in transportni industriji. Sintranim magnetom s strukturo Nd2Fe14B so dodane težke redke zemlje (TRZ, e.g. Dy in Tb). Ta vrsta magnetov je uporabljena v električnih pomožnih sistemih (EPS), ki vrtijo volanski drog. Zahtevani lastnosti magnetov v EPS motorjih (v MAHLE Letriki d.o.o.) sta remanenca (Br) 1240-1350 mT in koercitivnost (Hcj) ≥1592 kA/m. Dodajanje TRZ omogoča, da magnet ohranja svojo magnetnost tudi pri visokih temperaturah, kar pa občutno poveča ceno magneta. V zadnjih letih so proizvajalci magnetov razvili novo metodo, tako imenovan postopek interkristalne difuzije Tb in Dy. Difuzija prične med termično obdelavo končanega magneta prevlečenim s slojem TRZ. Med termično obdelavo TRZ difundirajo s površine v notranjost magneta prek kristalne strukture Nd in med zrna Fe, rezultat je visoka prostorska razpršenost TRZ. Interkristalna difuzija omogoča, da lahko ima magnet z manjšo vsebnostjo TRZ enako ali boljšo koercitivnost in remanenco v primerjavi s standardnim postopkom. Cilj naloge je karakterizacija magnetov, z osredotočenostjo na opis mikrostrukturne homogenosti, homogenosti kristalnih območij in homogenost posameznih elementov v magnetni strukturi. S ciljem, da se definira posebnost interkristalnega difuzijskega postopka. Analiza magnetov v prerezu je bila izvedena z vrstičnim elektronskim mikroskopom (SEM) v povezavi z energijsko disperzijsko rentgensko spektroskopijo (EDX), s katero smo določali lokalne elemente. Z opazovanjem mikrostrukture smo želeli potrditi homogenost celotne magnetne strukture, kljub nekaterim nehomogenostim pridobljenih iz drugih člankov. Opazili smo razlike v homogenosti med različnimi magneti le pri standardnem postopku. Kljub vsemu povprečna vrednost komponent in prostorska razporeditev elementov potrjuje naše predpostavke.
Ključne besede: NdFeB magneti, interkristalna difuzija TRZ, SEM in EDX
Objavljeno: 29.07.2016; Ogledov: 1358; Prenosov: 79
.pdf Polno besedilo (4,17 MB)

4.
Properties of Null Hypersurfaces
Hovhannes Demirtshyan, 2016, magistrsko delo

Opis: The aim of this thesis is to investigate the effects that a lightlike singular hypersurface can have on a congruence of timelike (spacelike) geodesics and to extend the existing theory to the case of null geodesics. The introduction discusses the applications of singular hypersurfaces for the description of physical phenomena, their major classfications and includes a short discussion of the two theoretical approaches that exist to study singular hypersurfaces. The second chapter contains detailed description of these approaches. The theoretical frameworks for both cases of lightlike and timelike (spacelike) hypersurfaces are developed. This chapter also discusses the application of these theories to the case when the hypersurface contains a plane fronted lightlike signal. The final chapter starts with a discussion of the effects that a lightlike singular hypersurface can have on a congruence of timelike (spacelike) geodesics. A new approach to these calculations is presented together with an extension of the theory to the case of a congruence of null geodesics. At the end of the chapter a concrete example and its similarities with the case of timelike geodesics is discussed. In conclusion, the thesis suggests a new mathematical framework for describing a congruence of null geodesics crossing a singular null hypersurface. The results may be applied in experimental physics to detect impulsive signals which are located in singular null hypersurfaces and to this end there is a discussion of the properties and possibilities for a detector of impulsive lightlike signals, which include gravitational waves.
Ključne besede: singular hypersurface, impulsive signal, gravitational wave, null geodesic, timelike (spacelike) geodesic
Objavljeno: 16.05.2016; Ogledov: 1230; Prenosov: 56
.pdf Polno besedilo (475,54 KB)

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