Um die anderen Arten von Veröffentlichungen zu diesem Thema anzuzeigen, folgen Sie diesem Link: Growth of single crystals.
Dissertationen zum Thema „Growth of single crystals“
Geben Sie eine Quelle nach APA, MLA, Chicago, Harvard und anderen Zitierweisen an
Machen Sie sich mit Top-50 Dissertationen für die Forschung zum Thema "Growth of single crystals" bekannt.
Neben jedem Werk im Literaturverzeichnis ist die Option "Zur Bibliographie hinzufügen" verfügbar. Nutzen Sie sie, wird Ihre bibliographische Angabe des gewählten Werkes nach der nötigen Zitierweise (APA, MLA, Harvard, Chicago, Vancouver usw.) automatisch gestaltet.
Sie können auch den vollen Text der wissenschaftlichen Publikation im PDF-Format herunterladen und eine Online-Annotation der Arbeit lesen, wenn die relevanten Parameter in den Metadaten verfügbar sind.
Sehen Sie die Dissertationen für verschiedene Spezialgebieten durch und erstellen Sie Ihre Bibliographie auf korrekte Weise.
1
Deniz, Derya. „Growth And Characterization Of Inse Single Crystals“. Master's thesis, METU, 2004. http://etd.lib.metu.edu.tr/upload/12605213/index.pdf.
Der volle Inhalt der QuelleAnnotation:
In this study, InSe single crystals were grown from the melt using conventional Bridgman-Stockbarger system. The grown crystals were implanted by N-ions to investigate the doping effect. the stoichiometry and the structural features were examined by scanning electron microscope and X-ray diffraction method, respectively. We have observed that the ingot was stoichiometric and the structure was hexagonal. Temperature dependent conductivity and Hall effect measurements were carried out to investigate the electrical properties of as-grown, as-implanted and annealed samples within the temperature range of 80-400 K. To investigate the annealing effect on both the absorption and photoluminescence (PL) spectra, absorption and PL measurements were performed at room temperature.
N-implantation reduced the resistivity order from 103 to 101 (Ω
-cm). We have used temperature dependent conductivity and Hall effect measurements to analyze the dominant scattering mechanisms. Hall measurements showed that all the samples had n-type conduction. Absorption measurements showed that InSe had direct band gap. It was observed that annealing had almost no effect an both room temperature absorption and PL spectra of the samples.
2
Constantinidis, G. „Growth and characterisation of single CuInSe2̲ crystals“. Thesis, University of Salford, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.381646.
Der volle Inhalt der Quelle
3
Brown, Stephen James. „The Czochrlaski growth and characterisation of single crystals of lead molybdate“. Thesis, University of Cambridge, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.364392.
Der volle Inhalt der Quelle
4
Möckel, Robert. „Growth and properties of GdCa4O(BO3)3 single crystals“. Doctoral thesis, Technische Universitaet Bergakademie Freiberg Universitaetsbibliothek "Georgius Agricola", 2012. http://nbn-resolving.de/urn:nbn:de:bsz:105-qucosa-90095.
Der volle Inhalt der QuelleAnnotation:
In der vorliegenden Arbeit wird die Einkristallzüchtung nach dem Czochralskiverfahren von GdCa4O(BO3)3 (GdCOB) beschrieben. Aus insgesamt 18 Zuchtversuchen, bei denen auch die Ziehgeschwindigkeit zwischen 1 und 3mm/h variiert wurde, wurden erfolgreich nahezu perfekte Einkristalle gewonnen. In einigen Kristallen traten jedoch auch Risse oder Einschlüsse auf. Diese enthielten neben Iridium vom Tiegelmaterial auch andere Phasen des Gd2O3–B2O3–CaO-Systems, sowie P und Yb, deren Herkunft unklar ist. Als Hauptziehrichtung wurde die kristallographische b-Achse gewählt, ergänzt durch einige Experimente in der c-Richtung. In den drei kristallographischen Hauptrichtungen wurden die thermischen Ausdehnungskoeffizienten von GdCOB bestimmt. Diese können in zwei nahezu lineare Bereiche unterteilt werden: von Zimmertemperatur bis etwa 850° C und von 850 bis 1200° C, wobei die Koeffizienten im Hochtemperaturbereich deutlich höher sind (unter 850° C: alpha_a=11.1, alpha_b=8.6, alpha_c=13.3 10^-6/K, oberhalb 850° C: alpha_a=14.1, alpha_b=11.7, alpha_c=17.8 10^-6/K). Daraus ergibt sich, dass ein Phasenübergang höherer Ordnung vorliegen muss. Als mögliche Ursache wurde mittels HT-Raman Spektroskopie ein Ordnungs-Unordnungs-Übergang identifiziert, während dessen die BO3-Gruppen in der Struktur leicht rotieren. Weitere Untersuchungen mittels thermodynamischer Methoden führten zu schwachen, aber eindeutigen Signalen, die diesem Effekt ebenfalls zuzuordnen sind. Obwohl das Material ein vielversprechender Kandidat für piezoelektrische Anwendungen im Hochtemperaturbereich ist, wurde dieser Effekt bisher unzureichend beschrieben. Dieses Verhalten, kombiniert mit den anisotropen thermischen Ausdehnungskoeffizienten, könnte eine der Ursachen für das Vorkommen von Rissen in den Kristallen während der Synthese darstellen. Spektroskopische Untersuchungen ergaben einen großen Transparenzbereich von 340 bis 2500nm (29 400–4000 cm^-1), was für optische Anwendungen von großer Bedeutung istIn a series of 18 growth experiments, GdCa4O(BO3)3 (GdCOB) single crystals were successfully grown by the Czochralski method. They have a well-ordered structure, as revealed by single crystal structure analysis. Although the main growth direction was along the crystallographic b-axis, some experiments were conducted using the cdirection. Pulling velocities were varied between 1 and 3mm/h. Except for a few crystals with cracks or elongated "silk-like" inclusions consisting of multiphase impurities, most of the obtained crystals are of good quality. Those inclusions contain iridium, deriving from the crucible, P and Yb with unclear source, and other phases from the system Gd2O3–B2O3–CaO. Thermal expansion coefficients of GdCOB were determined in the directions of the crystallographic axes and found to be approximately linear in two temperature ranges: from 25° C to around 850° C, and from 850 to 1200° C, with the latter range showing significantly higher coefficients (below 850° C: alpha_a=11.1, alpha_b=8.6, alpha_c=13.3 10^-6/K, and above 850° C: alpha_a=14.1, alpha_b=11.7, alpha_c=17.8 x10^-6/K). This sudden increase of thermal expansion coefficients indicates a phase transition of higher order. An order-disorder transition in form of the rotation of BO3-triangles in the structure was made tentatively responsible for this transition, as revealed by HT-Raman spectroscopy. This transition was also detected by DSC-methods but appeared to result in very weak effects. Although the material is thought to represent a promising candidate for high temperature piezoelectric applications (noncentrosymmetric space group Cm), this effect of change in specification has not been described and it is unknown whether it has influence on the piezoelectric properties. Furthermore, this characteristic behaviour in combination with anisotropic coefficients may be the reason for the development of cracks during cooling of crystals, making the growth difficult. Spectroscopic investigation revealed a wide transparency range from 340 to 2500nm (29 400–4000 cm^-1) of GdCOB, which is a very important property for optical applications
5
Burnett, Timothy Laurence. „Growth and charaterisation of bismuth ferrite lead titanate single crystals“. Thesis, University of Leeds, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.487709.
Der volle Inhalt der QuelleAnnotation:
Bismuth ferrite lead titanate (BFPT) shows potential in high temperature piezoelectric and ferroelectric applications as well in the emerging field of multiferroic research. Single crystals of (BFPT) have been grown for the first time using the flux growth method.
6
Stoica, Laura Andreea. „Relaxor-PbTiO3 single crystals and polycrystals : processing, growth and characterisation“. Thesis, University of Leeds, 2016. http://etheses.whiterose.ac.uk/16259/.
Der volle Inhalt der QuelleAnnotation:
Acoustic transducers operate using polycrystalline lead zirconate titanate (PZT) since the 1950s’. Recently, relaxor-lead titanate (relaxor-PT) single crystals have been developed and exhibit up to 650% higher piezoelectric charge coefficient and up to 50% higher electromechanical coupling factor, compared with PZT. Transducers built with relaxor-PT crystals show increased bandwidth, lower power consumption and increased sensitivity. Despite the significant advantages over PZT, a growth method for relaxor-PT single crystals that is both economical and able to produce homogeneous, highly dense crystals is yet to be found. Furthermore, one of the most important relaxor-PT solid solution is Mn modified Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3- PbTiO3 due to its thermal stability and low electrical and mechanical losses compared with other relaxor-PT solid solutions. However, the behaviour of Mn in this compound is not fully understood and control of properties, such as the mechanical quality factor, is difficult. The aim of this study was to compare Bridgman and Solid State techniques for growth of single crystalline Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3. Subsequent to this, several Mn modified compositions were designed and investigated to determine the mechanisms by which mechanical and electrical losses are lowered upon Mn incorporation. Bridgman experiments have shown that a 20 mm ø x 60 mm length and highly dense single crystal can be obtained, with (011) the natural growth direction. Seeded Bridgman was also investigated as a method of controlling the orientation of the grown crystal, but was proven challenging due to nucleation of several crystallites. Solid State experiments, which involve attaching a single crystal seed to a polycrystalline matrix and promoting boundary migration of the seed into the matrix, showed that crystal growth is encouraged when a Pb-based interlayer exists in between the seed and matrix. An epitaxially deposited, uniform thin film as interlayer was found particularly beneficial. More research is needed to determine conditions for growth of a single crystal of useful size by Solid State. Analysis of several relaxorPT compositions modified with the same amount of Mn revealed that alterations of the relaxor-PT formulation affects behaviour of the multi-valent Mn which in turn in- fluences electromechanical properties. These findings are of use to the industrial and scientific communities. Crystal growth results indicate Bridgman as suitable method if growth of crystals is desired in a short time frame, whilst Solid State results provide the basis of a new approach for growing relaxor-PT crystals. The study of Mn modified compositions provides new insights into the role of Mn substitution for manipulating the electrical and mechanical properties of complex, relaxor-PT solid solutions.
7
Begum, Salma. „Measurement of Ionic Concentration of Nematic Liquid Crystals and Single Crystal Growth of Organic Semiconductors“. Kent State University / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=kent1543277105262258.
Der volle Inhalt der Quelle
8
Bacon, Neil J. „Laboratory studies of the growth, sublimation, and light-scattering properties of single levitated ice particles /“. Thesis, Connect to this title online; UW restricted, 2001. http://hdl.handle.net/1773/9735.
Der volle Inhalt der Quelle
9
Glacki, Alexander. „VGF growth of 4” GaAs single crystals with traveling magnetic fields“. Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät, 2014. http://dx.doi.org/10.18452/17028.
Der volle Inhalt der QuelleAnnotation:
Im Rahmen der vorliegenden Arbeit wurden Si-dotierte und undotierte 4” VGF-GaAs Einkristalle unter dem Einfluss von Wandermagnetfeldern (TMF) gezüchtet. Die für den Prozess benötigte Wärme und das Wandermagnetfeld wurden simultan mithilfe der kombinierten Regelung von Gleich- und Wechselströmen in einem KRISTMAG Heizer-Magnet-Modul (HMM) erzeugt. Alle Züchtungsexperimente wurden in einer kommerziellen VGF-Anlage mit eingebautem Eintiegel-HMM und in einer neu entwickelten VGF-Anlage mit Multitiegel-HMM durchgeführt. Der Einfluss der durch die Lorentzkräfte angetriebenen Schmelze auf die Form der fest-flüssig Phasengrenze wurde innerhalb einer TMF-Parameterstudie analysiert. Im Vergleich mit Referenzkristallen, welche ohne TMF gezüchtet wurden, zeigte sich, dass die Durchbiegung der Phasengrenze durch die Anwendung eines geeigneten Doppelfrequenz-TMF um etwa 30% verringert und der Kontaktwinkel am Tiegel um etwa 10% vergrößert werden kann. Zudem wurden Synergieeffekte von TMF-Anwendung und den Ansätzen zur Prozessintensivierung - Scale-Up, Speed-Up und Numbering-Up - für die Verbesserung der Prozesseffizienz erfolgreichnachgewiesen. Es wurden gleichzeitig zwei 4” VGF-GaAs:Si Einkristalle unter dem Einfluss eines TMF in einer Multitiegel-Anlage gezüchtet. In Kristallen, welche ohne oder mit zu starkem TMF gezüchtet wurden, waren Wachstumsstreifen sichtbar. Wurde die magnetische Flussdichte des TMF an den Kristallisationsverlauf angepasst, konnten nahezu keine Mikroinhomogenitäten detektiert werden. Die Länge der Kristallfacetten stabilisierte sich durch den Einsatz der Wandermagnetfelder. Zusätzlich konnten die Versetzungsdichten innerhalb der Kristalle durch Optimierung des thermischen Aufbaus und der Phasengrenzform signifikant reduziert werden. Mithilfe eines dem Züchtungsverlaufes angepassten Doppelfrequenz-TMF sowie der Nutzung eines BN-Suszeptors, wurde eine durchschnittliche EPD von 100 cm-2 in einem GaAs:Si Kristall erzielt.Within the framework of this thesis Si-doped and undoped 4” VGF-GaAs single crystals were grown under the influence of traveling magnetic fields (TMF). A KRISTMAG heater-magnet module (HMM) was used for the efficient simultaneous generation of heat and TMF during the process through a combination of DC and AC control. Growth experiments were carried out in a commercial VGF growth setup equipped with a single-crucible HMM and a newly designed VGF setup with a multi-crucible HMM. The impact of the Lorentz force driven melt flow on the shape of the solid-liquid interface was analyzed in a TMF parameter study on frequency, phase shift, and current. With the application of suitable double-frequency TMF during growth, the interface deflection was reduced by about 30% and crucible contact angles increased within the order of 10%, compared to reference crystals grown without TMF. Synergy effects of TMF application on process intensification approaches scale-up, speed-up, and numbering-up were successfully shown. Two 4” VGF-GaAs:Si single crystals were simultaneously grown under the influence of a TMF in the multi-crucible HMM. With TMF application changing structural and electronic properties as well as micro- and macrosegregation were investigated on Si-doped VGF-GaAs single crystals. Striations were observed in crystals grown without or too strong TMF. Almost no micro-inhomogeneities were detected when the magnetic flux densities of the TMF were matched to the progression of solidification. Facets lengths in the crystal cone were found to be more stable with applied TMF. Further, the combined optimization of the conventional thermal setup and a reduction of the interface deflection with TMF application significantly reduced dislocation densities inside the crystals. An average EPD value around 100 cm-2 was obtained for GaAs:Si growth with a growth-matched double-frequency TMF and applied BN susceptor in the single-crucible VGF setup.
10
Tavakoli, Mohammad Hossein. „Numerical analysis of seeding process during Czochralski growth of oxide single crystals“. [S.l.] : [s.n.], 2006. http://se6.kobv.de:8000/btu/volltexte/2006/14.
Der volle Inhalt der Quelle
11
Dennis, Roger James. „Mechanistic modelling of deformation and void growth behaviour in superalloy single crystals“. Thesis, Imperial College London, 2000. http://hdl.handle.net/10044/1/11302.
Der volle Inhalt der QuelleAnnotation:
In this work the constitutive behaviour and the influence of casting defects on the failure of Ni-base single crystal (SC) superalloy components is investigated. It is well known that the presence of casting-related porosities can lead to the nucleation of microcracks under both creep and fatigue conditions and thus, ultimately produce component failure. A rate dependent crystallographic formulation is introduced to describe the inelastic deformation behaviour of the latest generation of single crystal superalloys. The evolution of the current dislocation and obstacle network is described through appropriate slip resistance and internal or back stress variables for each slip system. Good correlations are obtained between experimental data and numerical predictions within the 750◦C .- 950◦C temperature range and for < 001 > and < 111 > crystallographic orientations. The formulation is then numerically implemented into the FE method and used to investigate the deformation of a representative material volume containing a spherical void of approximately 20 micrometers diameter. The functional dependence of the void growth rates in terms of material anisotropy, stress state, temperature and interaction with a free surface is determined. It is shown that the rate of growth of casting defects in an infinite single crystal medium is strongly dependent on the applied triaxiality and relative orientation between the crystallographic axes and the applied stresses. Furthermore, it has been found that, for the acceleration of the defect growth rate as the result of its proximity to a free-surface to be non-negligible, the void needs to be within two diameters of the free surface. Based on the above results, a framework is proposed to describe the growth of embedded casting defects within superalloy single crystals under a given applied multiaxial stress state. The framework provides an explicit link between the mesoscopic (at the level of the voids) and the macroscopic length scales. A number of blunt notch bar creep specimens and thermo-mechanical cyclic stress-strain specimens were tested to validate both the rate dependent crystallographic formulation and the micro-mechanics void growth model. In addition microstructural analysis of the blunt notch bars provided necessary data for the development of a micro-crack initiation criterion. A life assessment methodology combining these models is developed and applied in the analysis of an actual gas turbine blade. It is expected that the understanding of defect growth kinetics and the corresponding damage accumulation will be beneficial in the design and life prediction of superalloy components.
12
Nakamura, Daisuke. „Bulk growth and extended-defect analysis of high-quality SiC single crystals“. 京都大学 (Kyoto University), 2008. http://hdl.handle.net/2433/136293.
Der volle Inhalt der Quelle
13
Tonn, Justus [Verfasser], und Arne [Akademischer Betreuer] Cröll. „Growth and characterisation of lead iodide single crystals = Züchtung und Charakterisierung von Bleiiodideinkristallen“. Freiburg : Universität, 2012. http://d-nb.info/1122592302/34.
Der volle Inhalt der Quelle
14
Djamin, Martin. „Growth and characterisation of CuInâ†xGaâ†1â†-â†xSeâ†2 single crystals and device fabrication“. Thesis, University of Salford, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.386451.
Der volle Inhalt der Quelle
15
Menzel, Robert [Verfasser], und Matthias [Akademischer Betreuer] Bickermann. „Growth Conditions for Large Diameter FZ Si Single Crystals / Robert Menzel. Betreuer: Matthias Bickermann“. Berlin : Universitätsbibliothek der Technischen Universität Berlin, 2013. http://d-nb.info/1032693339/34.
Der volle Inhalt der Quelle
16
Henderson, M. B. „Fatigue crack growth in single crystal superalloys“. Thesis, University of Oxford, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.314993.
Der volle Inhalt der Quelle
17
McComber, Kevin A. „Single-crystal germanium growth on amorphous silicon“. Thesis, Massachusetts Institute of Technology, 2011. http://hdl.handle.net/1721.1/69792.
Der volle Inhalt der QuelleAnnotation:
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2011.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 130-136).
The integration of photonics with electronics has emerged as a leading platform for microprocessor technology and the continuation of Moore's Law. As electronic device dimensions shrink, electronic signals encounter crippling delays and heating issues such that signal transduction across large on-chip distances becomes increasingly more difficult. However, these issues may be mitigated by the use of photonic interconnects combined with electronic devices in electronic-photonic integrated circuits (EPICs). The electronics in proposed EPIC designs perform the logic operations and short-distance signal transmission, while photonic devices serve to transmit signals over longer lengths. However, the photonic devices are large compared to electronic devices, and thus the two types of devices would ideally exist on separate levels of the microprocessor stack in order to maximize the amount of silicon substrate available for electronic device fabrication. A CMOS-compatible back-end process for the fabrication of photonic devices is necessary to realize such a three-dimensional EPIC. Back-end processing is limited in thermal budget and does not present a single-crystal substrate for epitaxial growth, however, so high-quality crystal fabrication methods currently used for photonic device fabrication are not possible in back-end processing. This thesis presents a method for the fabrication of high-quality germanium single crystals using CMOS-compatible back-end processing. Initial work on the ultra-high vacuum chemical vapor deposition of polycrystalline germanium on amorphous silicon is presented. The deposition can be successfully performed by using a pre-growth hydrofluoric acid dip and by limiting the thickness of the amorphous silicon layer to less than 120 nm. Films deposited at temperatures of 350° C, 450° C, and 550° C show (110) texture, though the texture is most prevalent in growths at 450° C. Poly-Ge grown at 4500 C is successfully doped n-type in situ, and the grain size of as-grown material is enhanced by lateral growth over a barrier. Structures are fabricated for the growth of Ge confined in one dimension. The growths show faceting across large areas, in contrast to as-deposited poly-Ge, corresponding to enhanced grain sizes. Growth confinement is shown to reduce the defect density as the poly-Ge grows. When coalesced into a continuous film, the material grown from 1 D confinement exhibits a lower carrier density and lower trap density than as-deposited poly-Ge, indicating improved material quality. We measure an increased grain size from as-deposited poly-Ge to Ge grown from ID confinement. Single-crystal germanium is grown at 450° C from confinement in two dimensions. Such growths exhibit faceting across the entire crystal as well as the presence of E3 boundaries ({111} twins), with many growths showing no other boundaries. These twins mediate the growth of the crystal, as they serve as the points for heterogeneous surface nucleation of adatom clusters. The twins can form after the crystal nucleates and are strongly preferred in order to obtain appreciable crystal growth rates. We model the growths from the confining channels in order to find the optimum channel geometry for large, uniform, single-crystal growths that consistently emerge from the channel. The growths from 2D confinement show lower trap density than those from 1 D confinement, indicating a further enhancement of the crystal quality due to the increased confinement. This method of single-crystal growth from an amorphous substrate is extensible to any materials system in which selective non-epitaxial deposition is possible.
by Kevin A. McComber.
Ph.D.
18
Hoffman, Timothy B. „Optimization and characterization of bulk hexagonal boron nitride single crystals grown by the nickel-chromium flux method“. Diss., Kansas State University, 2016. http://hdl.handle.net/2097/32797.
Der volle Inhalt der QuelleAnnotation:
Doctor of PhilosophyDepartment of Chemical Engineering
James H. Edgar
Hexagonal boron nitride (hBN) is a wide bandgap III-V semiconductor that has seen new interest due to the development of other III-V LED devices and the advent of graphene and other 2-D materials. For device applications, high quality, low defect density materials are needed. Several applications for hBN crystals are being investigated, including as a neutron detector and interference-less infrared-absorbing material. Isotopically enriched crystals were utilized for enhanced propagation of phonon modes. These applications exploit the unique physical, electronic and nanophotonics applications for bulk hBN crystals. In this study, bulk hBN crystals were grown by the flux method using a molten Ni-Cr solvent at high temperatures (1500°C) and atmospheric pressures. The effects of growth parameters, source materials, and gas environment on the crystals size, morphology and purity were established and controlled, and the reliability of the process was greatly improved. Single-crystal domains exceeding 1mm in width and 200μm in thickness were produced and transferred to handle substrates for analysis. Grain size dependence with respect to dwell temperature, cooling rate and cooling temperature were analyzed and modeled using response surface morphology. Most significantly, crystal grain width was predicted to increase linearly with dwell temperature, with single-crystal domains exceeding 2mm in at 1700°C. Isotopically enriched ¹⁰B and ¹¹B hBN crystal were produced using a Ni-Cr-B flux method, and their properties investigated. ¹⁰B concentration was evaluated using SIMS and correlated to the shift in the Raman peak of the E[subscript 2g] mode. Crystals with enrichment of 99% ¹⁰B and >99% ¹¹B were achieved, with corresponding Raman shift peaks at 1392.0 cm⁻¹ and 1356.6 cm⁻¹, respectively. Peak FWHM also decreased as isotopic enrichment approached 100%, with widths as low as 3.5 cm⁻¹ achieved, compared to 8.0 cm⁻¹ for natural abundance samples. Defect selective etching was performed using a molten NaOH-KOH etchant at 425°C-525°C, to quantify the quality of the crystals. Three etch pit shapes were identified and etch pit width was investigated as a function of temperature. Etch pit density and etch pit activation energy was estimated at 5×10⁷ cm⁻² and 60 kJ/mol, respectively. Screw and mixed-type dislocations were identified using diffraction-contrast TEM imaging.
19
Zhang, Xi. „Vapor Phase Growth of ZnO Single Crystals/Thin Films and Attempts for p-type Doping“. Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2014. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-142870.
Der volle Inhalt der QuelleAnnotation:
The growth of ZnO single crystals and ZnO thin films on Si substrates by an open-system vapor phase method was studied in this thesis. The as-grown ZnO single crystals were investigated by means of photoluminescence (PL). Two unique emissions were observed in virgin and hydrogenated crystals. The up-to-now attempts for the p-type doping of ZnO were summarized and our doping studies were performed using nitrogen and antimony. The seed-free and open-system vapor phase method is a simple and low cost approach to grow good quality ZnO single crystals. The growth parameters, including flow rates of N2, H2, O2, and growth temperatures, have various influences on the crystal growth, and also on the optical properties of the as grown crystals. The as-grown crystals are c-axis oriented needle crystals, and the crystals typically have a maximum length of 40 mm and a maximum diameter of 1 mm. The needle-shaped crystals are n-type with main donors due to Al, Ga, and In impurities, as determined from the PL spectra. Two unidentified PL emission lines (P1 at 3.3643 eV and P2 at 3.3462 eV) are observed in our vapor phase grown ZnO single crystals. P1 is attributed to the recombination of an exciton bound to a shallow donor，which has a binding energy of 42.2 meV. Hydrogenation of the as-grown ZnO single crystal leads to the appearance of the P2 line and a great reduction of the P1 line. Subsequent isochronal annealing in the ambient atmosphere leads to gradual reduction of P2 and the reappearance of P1. The PL measurements indicate that hydrogen is involved in the defect origins of the P2 line. ZnO thin films were deposited on Si substrates by the vapor phase method. Three different types of configurations with alternative source materials and oxidizers were employed and compared. It is demonstrated that, methods with lower growth temperatures are easier to deposit homogenous ZnO films on Si substrate. Donor-acceptor-pair (DAP) transition at 3.245 eV and its phonon replicas were observed in the PL spectra of the thin films, which are grown by the hydrogen-free vapor phase method. The appearance of DAP transition indicates the presence of acceptor in the films. The long-standing challenge of p-type doping in ZnO is mainly attributed to the low valence band maximum (VBM) at the absolute energy scale, the spontaneous formation of compensating defects and the lack of appropriate acceptors with small ionization energy. Two attempts for the p-type doping of ZnO were performed by nitrogen diffusion into ZnO single crystals from plasma after the growth or by in-situ doping antimony during the growth of ZnO films. No hole conductivity could however be achieved in our doped samplesIn dieser Arbeit wurde das Wachstum von ZnO-Einkristallen und Dünnfilmschichten auf Si durch chemische Gasphasenabscheidung in einem offenen System untersucht. Die hergestellten ZnO-Einkristalle wurden mit Photolumineszenzmessungen (PL) untersucht. Es konnten sowohl in unbehandelten als auch in mit Wasserstoff behandelten Proben zwei charakteristische Linien beobachtet werden. Sowohl die bisherigen Versuche zur p-Typ Dotierung von ZnO als auch die in dieser Arbeit durchgeführten Versuche mit Stickstoff und Antimon werden zusammengefasst und präsentiert. Die Keimkristall-freie Gasphasenabscheidung (CVD) in offenen Systemen ist eine einfache und kostengünstige Methode zur Herstellung von qualitativ hochwertigen ZnO-Einkristallen. Die Wachstumsparameter, einschließlich der Flussraten von N2, H2 und O2 sowie der Wachstumstemperatur beeinflussen das Kristallwachstum sowie die optischen Eigenschaften der hergestellten Kristalle. Die hergestellten Kristalle wachsen typischerweise als entlang der c-Achse orientierte Nadeln mit Längen von bis zu 40 mm und Durchmessern von bis zu 1 mm. Die nadelförmigen Kristalle besitzen eine n-Typ Dotierung, welche hauptsächlich durch Verunreinigung mit Al, Ga und In verursacht wird. Zwei bisher nicht identifizierte PL-Linien (P1 bei 3,3643 eV und P2 bei 3,3462 eV) wurden in den hergestellten Kristallen beobachtet. P1 wird der Rekombination von Exzitonen an flachen Donatoren mit einer Bindungsenergie von 42,2 meV zugeordnet. Eine Wasserstoffbehandlung der hergestellten Kristalle führt zum Erscheinen der P2-Linie und einer starken Unterdrückung der P1-Linie. Anschließende isochronische Temperung in Luft führt zu einer schrittweisen Reduzierung der Intensität der P2-Linie und zu einer Verstärkung der P1-Linie. Photolumineszenzmessungen weisen auf eine Korrelation von P2 mit Wasserstoff hin. Zusätzlich wurden mit der CVD-Methode dünne ZnO-Schichten auf Si-Substraten abgeschieden. Drei unterschiedliche Konfigurationen mit verschiedenen Ausgangsmaterialien (ZnO-Pulver bw. Zn-Pulver) und verschiedenen Oxidationsmitteln (O2 bzw. Wasser) wurden untersucht und verglichen. Es wird gezeigt, dass mit den Konfigurationen mit geringerer Wachstumstemperatur am einfachsten homogene ZnO-Schichten auf Si abgeschieden werden können. Ein Donator-Akzeptor-Paar-Übergang (DAP) bei 3,245 eV und die dazugehörigen Phononenrepliken wurden in den Schichten beobachtet, welche in einer Wasserstoff-freien Konfiguration abgeschieden wurden. Diese DAP-Übergänge sind ein Hinweis auf die Anwesenheit von Akzeptoren. Die seit langem bestehende Herausforderung der p-Typ-Dotierung von ZnO hat ihre Wurzeln hauptsächlich in dem niedrig liegenden Valenzbandmaximum (VBM) auf der absoluten Energieskala, der spontanen Bildung von kompensierenden Defekten sowie dem Mangel an geeigneten Akzeptoren mit geringer Ionisierungsenergie. Zwei Versuche zur p-Typ-Dotierung von ZnO durch Behandlung der Kristalle mit N-Plasma bzw. durch in-situ Dotierung mit Sb während des Kristallwachstums wurden durchgeführt. Allerdings konnte damit keine nachweisbare Löcherleitung in den behandelten Proben erreicht werden
20
Imanieh, Mohsen. „Growth and characterisation of CuInSeâ†2 and CuGAâ†XInâ†1-â†XSeâ†2 single crystals“. Thesis, University of Salford, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.253047.
Der volle Inhalt der Quelle
21
Nguyen, Thai Thu Hien. „Influence of crystallisation environment on the nucleation and growth of single crystals of (RS)-ibuprofen“. Thesis, University of Leeds, 2013. http://etheses.whiterose.ac.uk/5771/.
Der volle Inhalt der QuelleAnnotation:
The crystal growth of ibuprofen as a function of crystallisation environment is presented. Measurement of solubility as a function of solvent (ethanol 95%, ethyl acetate, acetonitrile and toluene) and temperature using UV/Vis spectroscopy and gravimetric analysis reveals less than ideal behaviour, consistent with strong solute-solute interactions, particularly in the case of acetonitrile. Examination of the nucleation kinetics reveals a comparatively small metastable zone width (MSZW) (ΔT = 10.0 - 13.9oC) with interfacial tensions in the range 2.56 – 5.52.10-3 (J/m2). The growth rates for the {001} and {011} faces of spontaneously nucleated single crystals are precisely measured in-situ using optical microscopy revealing that their growth rates increase with increasing supersaturation to different extents, depending on the solvent type with concomitant impact on the crystal habit. For similar supersaturations, the growth rate of {011} is lowest in ethanol and highest in acetonitrile, whilst that of {001} is lowest in toluene and highest in ethanol. The aspect ratios, as a function of supersaturation, are generally higher at the 15 mL than the 0.5 mL crystalliser scale size. The data is consistent with a 2-D Birth and Spread growth model for both surfaces and scale sizes. This is in good agreement with the calculated surface entropy α factor. The measured growth rate dispersion for these faces is much less for these spontaneously nucleated crystals in stagnant, supersaturated solutions when compared to the literature data obtained for a stirred batch crystalliser. Additional higher index re-entrant faces are found at high levels of supersaturation consistent with their alignment either parallel to the {012} or {112} face. The growth rate in the non-agitated jacketed vessel is found to be slightly higher than that for the agitated reactors with both being in quite in a good agreement with literature data suggesting that these results on single crystals will have utility in representative the growth rates of a population of crystals in an agitated crystallizer. Observations are is rationalised using molecular and crystallographic modelling revealing e.g. that polar protic solvents inhibit the growth rate of faces containing available hydrogen bonding binding sites, such as carboxylic acid groups. Molecular conformational searching reveals the molecular structure in the solid-state to be close to that of molecules minimum intra-molecular energy. Conformer population analysis as a function of solvent reveals no significant solvent effect on the low energy conformer distribution indicative a low likelihood of polymorphic behaviour for this compound in good agreement with literature.
22
Lin, Chenting. „Single crystal growth and characterization of BSO (Bi12SiO20)“. Thesis, Massachusetts Institute of Technology, 1994. http://hdl.handle.net/1721.1/11647.
Der volle Inhalt der Quelle
23
SIBILIA, MIRTA. „Organic semiconducting single crystal growth on naostructured matrices“. Doctoral thesis, Università degli Studi di Trieste, 2017. http://hdl.handle.net/11368/2908140.
Der volle Inhalt der QuelleAnnotation:
In recent years, the development of the organic electronics has led to the employment of organic materials as the basis for many electronic devices, such as organic light emitting diodes, organic field effect transistors, organic solar cells and radiation sensors. As regards radiation sensors, the studies reported in literature mainly refer to devices based on thin film organic semiconductors, which, however, present problems due to instability, degradation and low reproducibility. Organic single crystals overcame most of the major limitations inherent to thin film-based detectors.
In this experimental work, a starting plethora of commercially available compounds has been crystallized and evaluated in terms of both overall crystallizability and applicability as direct X-ray detectors. This first step of work allow to select only those structures capable to provide single-crystals with good morphological properties and electronic properties suitable for X-ray detections.
A careful screening of some thermodynamic variables affecting the growth has been carried out and the obtained results suggest applicability of the present approach to achieve the control of size, quality and crystal habit. Indeed, single crystals with dimensional and morphological properties suitable to the application as X-ray detector have been prepared.
Single-crystals have been physically attached to Au electrodes deposited on a thin film of polyethylennaphthalate – via the functionalization of the electrode with different self-assembled monolayers (SAMs), chemically and structurally similar to the molecule constituting the selected crystal. In particular, two of the explored SAMs showed very promising results in the first qualitative adhesion tests.
24
Wu, Peitsen. „Development of a novel growth method for AｌN bulk single crystals using elemental aluminum and nitrogen gas“. 京都大学 (Kyoto University), 2015. http://hdl.handle.net/2433/202716.
Der volle Inhalt der Quelle
25
Coad, S. „Neutron scattering on single crystals of Cuâ†1â†-â†x(Zn/Ni)â†xGeOâ†3 and other exotic low dimensional systems“. Thesis, University of Warwick, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.245935.
Der volle Inhalt der Quelle
26
Srivastava, Ankit. „Void Growth and Collapse in a Creeping Single Crystal“. Thesis, University of North Texas, 2011. https://digital.library.unt.edu/ark:/67531/metadc84281/.
Der volle Inhalt der QuelleAnnotation:
Aircraft engine components can be subjected to a large number of thermo-mechanical loading cycles and to long dwell times at high temperatures. In particular, the understanding of creep in single crystal superalloy turbine blades is of importance for designing more reliable and fuel efficient aircraft engines. Creep tests on single crystal superalloy specimens have shown greater creep strain rates for thinner specimens than predicted by current theories. Therefore, it is necessary to develop a more predictive description of creep processes in these materials for them to be used effectively. Experimental observations have shown that the crystals have an initial porosity and that the progressive growth of these voids plays a major role in limiting creep life. In order to understand void growth under creep in single crystals, we have analyzed the creep response of three dimensional unit cells with a single spherical void under different types of isothermal creep loading. The growth behavior of the void is simulated using a three dimensional rate dependent crystal plasticity constitutive relation in a quasi-static finite element analysis. The aim of the present work is to analyze the effect of stress traixiality and Lode parameter on void growth under both constant true stress and constant engineering stress isothermal creep loading.
27
Rutkowska, Agnieszka. „Growth and electrochemical characterisation of single-walled carbon nanotubes on single crystal quartz“. Thesis, University of Warwick, 2010. http://wrap.warwick.ac.uk/3923/.
Der volle Inhalt der QuelleAnnotation:
Single walled carbon nanotubes (SWNTs) have unique structural and electronic properties which drive their use in many different fields of modern technology. In electrochemistry, SWNTs have been shown to have outstanding electrochemical characteristic enabling their application in trace level electroanalysis amongst other areas. The SWNT electrochemical activity has been described as originating solely from sidewall structural defects, oxidised open ends and post-growth metal nanoparticles (NPs). However, recent studies have demonstrated the electrochemical activity of pristine and defect-free SWNTs, grown on insulating substrates using catalysed chemical vapour deposition (cCVD) method, suggesting sidewalls support electron transfer (ET). In this thesis cCVD is employed to grow SWNTs of different geometrical arrangements on single crystal stable temperature (ST)-cut quartz substrates for the development of novel nanoelectrodes (NEs) and optically transparent electrodes (OTE). Majority of the work concentrates on cCVD and electrochemical studies of horizontally aligned SWNTs on ST-cut quartz. The quality of SWNT horizontal arrangement is shown to strongly depend on the crystallographic structure of the ST-cut quartz substrate. Perfectly aligned SWNTs are utilised as (i) one dimensional (1D) templates for electrodeposition of silver NPs, and (ii) NEs for the fundamental assessment of SWNT electrochemical behaviour. Formation of uniform silver nanowires (NWs) on most of the SWNTs is possible at high deposition overpotentials and times. Uniformity of the NWs indicates high structural quality of the horizontally aligned SWNTs. Nernstian behaviour of SWNT NEs for simple outer-sphere redox couples is shown in localised voltammetric measurements with microcapillaries, 25 - 50 μm in diameter serving as probes, filled with the redox active electrolyte solution and the reference electrode (RE). For the first time measurements at single SWNTs in the positive and negative potential window are performed, revealing a strong dependence of SWNT electrochemical activity on SWNT structural and electronic properties. Finally, careful tuning of the cCVD parameters enables growth of interconnected SWNT networks on ST-cut quartz, with complete surface coverage and no sign of alignment. These ultrathin mats are utilised as novel OT disk ultramicroelectrodes (OT-UMEs) with complete transparency in the ultraviolet and visible (UV-Vis) range, metallic behaviour and improved electrochemical usability in comparison to conventional solid UMEs.
28
Nunes, Benjamin P. (Benjamin Paul) 1976. „Edge-defined film-fed growth of single-crystal piezoelectrics“. Thesis, Massachusetts Institute of Technology, 2001. http://hdl.handle.net/1721.1/17530.
Der volle Inhalt der QuelleAnnotation:
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2001.Includes bibliographical references (leaves 97-99).
Many transducer technologies would benefit tremendously from the development of shaped, oriented single-crystals, of a high-strain, piezoelectric material. Recently, unusually high electrostrictive and piezoelectric actuation has been observed in polycrystals and flux-grown <100> single-crystals of ... Using seeded, Edgedefined Film-fed Growth (EFG) and the related Stepanov Technique (ST), low-hysteresis, highstrain, <100> and <111> oriented, single-crystals of BNBZT can be grown in rod and fiber form, with direct applications in active fiber composites and related devices. For this work, <100> and <111> oriented, single-crystal rods and fibers were grown via ST and EFG. Fibers, 260-700[mu]m in diameter and over 1.0 meter long, were grown using a custom built EFG machine and a capillary-shaper; rods, 2-3mm in diameter, up to 110mm long were grown using a floating-shaper. In all cases, strontium titanate (STO) was found to be an effective seed crystal. <111> oriented tetragonal crystals generated low hysteresis actuation consistent with a polarization rotation mechanism [14], but with only modest strains: ... <100> oriented tetragonal BNBZT generated high strains up to ... with hysteresis consistent with 90° domain switching. Electromechanical actuation and crystal structure in this system appear to be strongly affected by deviations from stoichiometry (B-site vacancies). Barium segregation and bismuth vaporization can also compromise electromechanical performance. Hypotheses are posed to explain the low actuation seen from <111> oriented ferroelectrics, and the effects of cation deficiencies on phase-stability. Cracks, pores, and other growth challenges encountered in ST and EFG growth of BNBZT are described.
by Benjamin P. Nunes.
S.M.
29
Rittenhouse, Tilghman L. (Tilghman Lee) 1972. „Single crystal growth and characterization of silicon germanium alloys“. Thesis, Massachusetts Institute of Technology, 1999. http://hdl.handle.net/1721.1/85267.
Der volle Inhalt der Quelle
30
Lauque, Olivier. „Effects of abrasive waterjet erosion on single crystal silicon“. Thesis, Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/16782.
Der volle Inhalt der Quelle
31
Lempidaki, Dimitra. „An investigation of crack growth phenomena in single crystal superalloys“. Thesis, Imperial College London, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.430750.
Der volle Inhalt der Quelle
32
Yang, Wen Yang, und 楊文揚. „Properties and growth of C60 single crystals“. Thesis, 1995. http://ndltd.ncl.edu.tw/handle/37445153866967226919.
Der volle Inhalt der Quelle
33
Thakare, Amol G. „Numerical Simulations Of Void Growth In Ductile Single Crystals“. Thesis, 2008. https://etd.iisc.ac.in/handle/2005/854.
Der volle Inhalt der QuelleAnnotation:
The failure mechanism in ductile materials involves void nucleation, their growth and subsequent coalescence to form the fracture surface. The voids are generated due to fracture or debonding of second phase particles or at slip band intersections. The triaxial stress field prevailing around a crack tip and in the necking region strongly influences the growth of these voids. In the initial stages of deformation, these microscale voids are often sufficiently small so that they exist entirely within a single grain of a polycrystalline material. Further, single crystals are used in high technology applications like turbine blades. This motivates the need to study void growth in a single crystal while investigating ductile fracture. Thus, the objectives of this work are to analyze the interaction between a notch tip and void as well as the growth and coalescence of a periodic array of voids under different states of stress in ductile FCC single crystals.
First, the growth of a cylindrical void ahead of a notch tip in ductile FCC single crystals is studied. To this end, 2D plane strain finite element simulations are carried out under mode I, small scale yielding conditions, neglecting elastic anisotropy. In most of these computations, the orientation of the FCC single crystal is chosen so that notch lies in the (010) plane, with notch front along the [101] direction and potential crack growth along [101]. This orientation has been frequently observed in experimental studies on fracture of FCC single crystals. Three equivalent slip systems are considered which are deduced by combining three pairs of 3D conjugate slip systems producing only in-plane deformation. Attention is focused on the effects of crystal hardening, ratio of void diameter to spacing from the notch on plastic flow localization in the ligament connecting the notch and the void as well as their growth. The results show strong interaction between slip shear bands emanating from the notch and angular sectors of single slip forming around the void leading to intense plastic strain development in the ligament. However, the ductile fracture processes are retarded by increase in hardening of the single crystal and decrease in ratio of void diameter to spacing from the notch. In order to examine the effect of crystal orientation, computations are performed with an orientation wherein the three effective slip systems are rotated about the normal to the plane of deformation. A strong influence of crystal orientation on near-tip void growth and plastic slip band development is observed. Further, in order to study the synergistic, cooperative growth of multiple voids ahead of the notchtip, an analysis is performed by considering a series of voids located ahead of the tip. It is found that enhanced void growth occurs at higher load levels as compared to the single void model.
Next, the growth and coalescence of a periodic array of cylindrical voids in a FCC single crystal is analyzed under different stress states by employing a 2D plane strain, unit cell approach. The orientation of the crystal studied here considers [101] and [010] crystal directions along the minor and major principal stress directions, respectively. Three equivalent slip systems, similar to those in the notch and void simulations are taken into account. Fringe contours of plastic slip and evolution of macroscopic hydrostatic stress and void volume fraction are examined. A criterion for unstable void growth which leads to onset of void coalescence is established. The effects of various stress triaxialities, initial void volume fraction and hardening on void growth and coalescence is assessed. It is observed that plastic slip activity around the void intensifies with increase in stress triaxiality. The macroscopic hydrostatic stress increases with deformation, reaches a peak value and subsequently decreases rapidly. An increase in stress triaxiality enhances the macroscopic hydrostatic stress sustained by the unit cell and promotes void coalescence. The stress triaxiality also has a profound effect on the shape of the void profile. The values of critical void volume fraction and critical strain, which mark onset of void coalescence, decrease within crease in stress triaxiality. However, the onset of void coalescence is delayed by increase in hardening and decrease initial void volume fraction.
34
Thakare, Amol G. „Numerical Simulations Of Void Growth In Ductile Single Crystals“. Thesis, 2008. http://hdl.handle.net/2005/854.
Der volle Inhalt der QuelleAnnotation:
The failure mechanism in ductile materials involves void nucleation, their growth and subsequent coalescence to form the fracture surface. The voids are generated due to fracture or debonding of second phase particles or at slip band intersections. The triaxial stress field prevailing around a crack tip and in the necking region strongly influences the growth of these voids. In the initial stages of deformation, these microscale voids are often sufficiently small so that they exist entirely within a single grain of a polycrystalline material. Further, single crystals are used in high technology applications like turbine blades. This motivates the need to study void growth in a single crystal while investigating ductile fracture. Thus, the objectives of this work are to analyze the interaction between a notch tip and void as well as the growth and coalescence of a periodic array of voids under different states of stress in ductile FCC single crystals.
First, the growth of a cylindrical void ahead of a notch tip in ductile FCC single crystals is studied. To this end, 2D plane strain finite element simulations are carried out under mode I, small scale yielding conditions, neglecting elastic anisotropy. In most of these computations, the orientation of the FCC single crystal is chosen so that notch lies in the (010) plane, with notch front along the [101] direction and potential crack growth along [101]. This orientation has been frequently observed in experimental studies on fracture of FCC single crystals. Three equivalent slip systems are considered which are deduced by combining three pairs of 3D conjugate slip systems producing only in-plane deformation. Attention is focused on the effects of crystal hardening, ratio of void diameter to spacing from the notch on plastic flow localization in the ligament connecting the notch and the void as well as their growth. The results show strong interaction between slip shear bands emanating from the notch and angular sectors of single slip forming around the void leading to intense plastic strain development in the ligament. However, the ductile fracture processes are retarded by increase in hardening of the single crystal and decrease in ratio of void diameter to spacing from the notch. In order to examine the effect of crystal orientation, computations are performed with an orientation wherein the three effective slip systems are rotated about the normal to the plane of deformation. A strong influence of crystal orientation on near-tip void growth and plastic slip band development is observed. Further, in order to study the synergistic, cooperative growth of multiple voids ahead of the notchtip, an analysis is performed by considering a series of voids located ahead of the tip. It is found that enhanced void growth occurs at higher load levels as compared to the single void model.
Next, the growth and coalescence of a periodic array of cylindrical voids in a FCC single crystal is analyzed under different stress states by employing a 2D plane strain, unit cell approach. The orientation of the crystal studied here considers [101] and [010] crystal directions along the minor and major principal stress directions, respectively. Three equivalent slip systems, similar to those in the notch and void simulations are taken into account. Fringe contours of plastic slip and evolution of macroscopic hydrostatic stress and void volume fraction are examined. A criterion for unstable void growth which leads to onset of void coalescence is established. The effects of various stress triaxialities, initial void volume fraction and hardening on void growth and coalescence is assessed. It is observed that plastic slip activity around the void intensifies with increase in stress triaxiality. The macroscopic hydrostatic stress increases with deformation, reaches a peak value and subsequently decreases rapidly. An increase in stress triaxiality enhances the macroscopic hydrostatic stress sustained by the unit cell and promotes void coalescence. The stress triaxiality also has a profound effect on the shape of the void profile. The values of critical void volume fraction and critical strain, which mark onset of void coalescence, decrease within crease in stress triaxiality. However, the onset of void coalescence is delayed by increase in hardening and decrease initial void volume fraction.
35
Gorzkowski, Edward P. „The effect of liquid phase chemistry on growth of lead magnesium niobate-lead titanate single crystals by seeded polycrystal conversion /“. Diss., 2004. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3127523.
Der volle Inhalt der Quelle
36
Annamalai, N. „Energy and Mass Transfer during Hydrothermal Crystal Growth of Quartz single Crystals“. Thesis, 1990. http://cgcri.csircentral.net/2905/.
Der volle Inhalt der Quelle
37
Padmanabhan, B. „Growth, Transport, Magnetic And Thermal Studies On Single Crystals Of Pr1-xPbxMnO3“. Thesis, 2007. https://etd.iisc.ac.in/handle/2005/534.
Der volle Inhalt der QuelleAnnotation:
Mixed valence manganites with the perovskite structure R1-xAxMnO3 (where R = La, Nd, Pr and A = Ba, Ca, Sr, Pb) have been a popular subject of contemporary research because of their interesting physical properties such as competing magnetic orders, metal-insulator transitions and colossal magnetoresistance. A complex interplay between structure, electronic and magnetic properties results in rich phase diagrams involving various metallic, insulating and magnetic phases. A review of the literature related to rare-earth managnites clearly reveals that the systems with Pb as a divalent dopant are relatively less explored. This may be due to the volatile nature of lead based compounds which are used as precursors for preparing these systems. This has motivated us to take up research on Pb doped rare earth manganites.
This thesis is divided into eight chapters. The first introductory chapter gives a brief review of the work on manganites which have already been reported in the literature following which the motivation for carying out the present investigation is given. The second chapter deals with technical details of various instruments used in the present reasearch work.
The third chapter is related to growth of single crystals, their preliminary characterization, magnetization and resistivity studies. Single crystals of Pr1-xPbxMnO3 are grown by flux technique for different compositions. Crystals are characterized by energy dispersive x-ray analysis (EDAX) and inductively coupled plasma atomic emission spectroscopy (ICPAES) for compositional analysis. Magnetization and resistivity studies are carried out on Pr1-xPbxMnO3 for three compositions viz. x = 0.2, 0.23 and 0.3. The magnetization vs. temperature plots show that all the three compositions undergo a transition from paramagnetic to ferromagnetic state. The magnetization in the low temperature ferromagnetic region obeys Bloch`s law. The susceptibility in the paramagnetic region is fitted to Curie Weiss law. Deviation of susceptibilty from Curie Weiss law, a feature observed in all the three crystals has been attributed to formation of ferromagnetic clusters at ~ 250 K. The cluster formation has its implications on all other properties in the temperature range from TC to 250 K where TC is the magnetic transition temperature.
Resistivity measurements are carried out on the same three compositions. The x = 0.2 and 0.23 compositions undergo a transition from paramagnetic insulating to ferromagnetic insulating phases. The x = 0.3 composition shows a metal – insulator transition at nearly 35 K above TC.
Chapter 4 describes the critical behaviour of Pr1-xPbxMnO3 for two compositions, viz. x = 0.23 and 0.3. For critical studies, magnetization vs. field measurements are carried out in the temperature range TC ± 10 K. Using modified Arrott plots and Kouvel-Fisher method the critical exponents and precise value of TC are obtained. The x = 0.23 composition shows results which indicate a conventional second order phase transition shown by a 3D Heisenberg ferromagnet. It also obeys the universal scaling behaviour. However, the x = 0.3 composition shows deviation from this behaviour. Hence to probe further into the nature of magnetic transition of this compound the effective critical exponents are calculated as a function of reduced temperature ε (=(T-TC)/TC). Based on the behaviour of effective exponents the nature of the transition in the x = 0.3 composition is described in detail. The unconventional ordering is attributed to presence of possible magnetic frustration in the system.
In chapter 5 the resistivity and magnetoresistance behaviour of the x = 0.23 and 0.3 crystals are discussed. Initially the nature of plots of temperature and field variation of resistivity are described for both the cases. Detailed measurements are carried out at the magnetic transition region. The analysis is carried out in terms of critical scattering behaviour at the transition region. The zero field resistivity is analyzed in terms of theory of Fisher and Langer, while the magnetoresistance is fitted to scaling theory at the critical region developed by Balberg and Helman. It is seen that the x = 0.23 crystal shows a critical behaviour in resistivity for zero field as well as in magnetoresistance close to TC.
However, the behaviour of the x = 0.3 composition is more complex. A simpler critical scattering theory alone cannot explain its large negative magnetoresistance.
Chapter 6 contains the EPR studies on the x = 0.23 and 0.3 compositions. Analysis is carried out in the paramagnetic region. The EPR signals are fitted to a modified Dysonian equation. The intensity, linewidth, and asymmetry parameter are obtained as a function of temperature from fitting. The parameters are obtained till 210 K for both compositions. The intensity is fitted to a Curie Weiss law. The linewidth shows a “bottleneck” mechanism and is proportional to conductivity. Hence it is fitted to activated behaviour. In addition, a secondary signal develops at low fields from 240 K and is present till 200 K in both the compositions. This is explained by means of phase separation.
In chapter 7 the specific heat of the x = 0.23 and 0.3 compositions are discussed. The measurements are carried out from 2 to 300 K in zero field and also in the presence of 3 Tesla magnetic field. The analysis is carried out in two separate sections. The first section deals with the low temperature analysis from 2 to 80 K where apart from the usual lattice, electron and magnetic terms, presence of Schottky anomaly is also discussed. The Schottky peak occurs at a relatively higher temperature of around 40 K. Due to presence of higher order lattice terms the Schottky effect is not easily discernible. It is extracted only from fitting. In the second section, the specific heat associated with ferromagnetic – paramagnetic transition is extracted. The lattice term in the entire temperature range from 10 to 300 K except at the transition region is fitted to Einstein function. The magnetic specific heat is obtained by subtracting the Einstein specific heat from the total specific heat. The change in entropy due to magnetic transition is also calculated for both compositions.
In chapter 8 the general conclusions derived from the work presented in this thesis are summarized along with the scope for future work in this system.
38
Padmanabhan, B. „Growth, Transport, Magnetic And Thermal Studies On Single Crystals Of Pr1-xPbxMnO3“. Thesis, 2007. http://hdl.handle.net/2005/534.
Der volle Inhalt der QuelleAnnotation:
Mixed valence manganites with the perovskite structure R1-xAxMnO3 (where R = La, Nd, Pr and A = Ba, Ca, Sr, Pb) have been a popular subject of contemporary research because of their interesting physical properties such as competing magnetic orders, metal-insulator transitions and colossal magnetoresistance. A complex interplay between structure, electronic and magnetic properties results in rich phase diagrams involving various metallic, insulating and magnetic phases. A review of the literature related to rare-earth managnites clearly reveals that the systems with Pb as a divalent dopant are relatively less explored. This may be due to the volatile nature of lead based compounds which are used as precursors for preparing these systems. This has motivated us to take up research on Pb doped rare earth manganites.
This thesis is divided into eight chapters. The first introductory chapter gives a brief review of the work on manganites which have already been reported in the literature following which the motivation for carying out the present investigation is given. The second chapter deals with technical details of various instruments used in the present reasearch work.
The third chapter is related to growth of single crystals, their preliminary characterization, magnetization and resistivity studies. Single crystals of Pr1-xPbxMnO3 are grown by flux technique for different compositions. Crystals are characterized by energy dispersive x-ray analysis (EDAX) and inductively coupled plasma atomic emission spectroscopy (ICPAES) for compositional analysis. Magnetization and resistivity studies are carried out on Pr1-xPbxMnO3 for three compositions viz. x = 0.2, 0.23 and 0.3. The magnetization vs. temperature plots show that all the three compositions undergo a transition from paramagnetic to ferromagnetic state. The magnetization in the low temperature ferromagnetic region obeys Bloch`s law. The susceptibility in the paramagnetic region is fitted to Curie Weiss law. Deviation of susceptibilty from Curie Weiss law, a feature observed in all the three crystals has been attributed to formation of ferromagnetic clusters at ~ 250 K. The cluster formation has its implications on all other properties in the temperature range from TC to 250 K where TC is the magnetic transition temperature.
Resistivity measurements are carried out on the same three compositions. The x = 0.2 and 0.23 compositions undergo a transition from paramagnetic insulating to ferromagnetic insulating phases. The x = 0.3 composition shows a metal – insulator transition at nearly 35 K above TC.
Chapter 4 describes the critical behaviour of Pr1-xPbxMnO3 for two compositions, viz. x = 0.23 and 0.3. For critical studies, magnetization vs. field measurements are carried out in the temperature range TC ± 10 K. Using modified Arrott plots and Kouvel-Fisher method the critical exponents and precise value of TC are obtained. The x = 0.23 composition shows results which indicate a conventional second order phase transition shown by a 3D Heisenberg ferromagnet. It also obeys the universal scaling behaviour. However, the x = 0.3 composition shows deviation from this behaviour. Hence to probe further into the nature of magnetic transition of this compound the effective critical exponents are calculated as a function of reduced temperature ε (=(T-TC)/TC). Based on the behaviour of effective exponents the nature of the transition in the x = 0.3 composition is described in detail. The unconventional ordering is attributed to presence of possible magnetic frustration in the system.
In chapter 5 the resistivity and magnetoresistance behaviour of the x = 0.23 and 0.3 crystals are discussed. Initially the nature of plots of temperature and field variation of resistivity are described for both the cases. Detailed measurements are carried out at the magnetic transition region. The analysis is carried out in terms of critical scattering behaviour at the transition region. The zero field resistivity is analyzed in terms of theory of Fisher and Langer, while the magnetoresistance is fitted to scaling theory at the critical region developed by Balberg and Helman. It is seen that the x = 0.23 crystal shows a critical behaviour in resistivity for zero field as well as in magnetoresistance close to TC.
However, the behaviour of the x = 0.3 composition is more complex. A simpler critical scattering theory alone cannot explain its large negative magnetoresistance.
Chapter 6 contains the EPR studies on the x = 0.23 and 0.3 compositions. Analysis is carried out in the paramagnetic region. The EPR signals are fitted to a modified Dysonian equation. The intensity, linewidth, and asymmetry parameter are obtained as a function of temperature from fitting. The parameters are obtained till 210 K for both compositions. The intensity is fitted to a Curie Weiss law. The linewidth shows a “bottleneck” mechanism and is proportional to conductivity. Hence it is fitted to activated behaviour. In addition, a secondary signal develops at low fields from 240 K and is present till 200 K in both the compositions. This is explained by means of phase separation.
In chapter 7 the specific heat of the x = 0.23 and 0.3 compositions are discussed. The measurements are carried out from 2 to 300 K in zero field and also in the presence of 3 Tesla magnetic field. The analysis is carried out in two separate sections. The first section deals with the low temperature analysis from 2 to 80 K where apart from the usual lattice, electron and magnetic terms, presence of Schottky anomaly is also discussed. The Schottky peak occurs at a relatively higher temperature of around 40 K. Due to presence of higher order lattice terms the Schottky effect is not easily discernible. It is extracted only from fitting. In the second section, the specific heat associated with ferromagnetic – paramagnetic transition is extracted. The lattice term in the entire temperature range from 10 to 300 K except at the transition region is fitted to Einstein function. The magnetic specific heat is obtained by subtracting the Einstein specific heat from the total specific heat. The change in entropy due to magnetic transition is also calculated for both compositions.
In chapter 8 the general conclusions derived from the work presented in this thesis are summarized along with the scope for future work in this system.
39
Huang, Jeng-Kuang, und 黃政光. „Growth and Characterization of Ruthenium Dichalcogenides Single Crystals“. Thesis, 1995. http://ndltd.ncl.edu.tw/handle/61876477432319047673.
Der volle Inhalt der Quelle
40
Zhumekenov, Ayan A. „Halide Perovskite Single Crystals: Design, Growth, and Characterization“. Diss., 2020. http://hdl.handle.net/10754/664934.
Der volle Inhalt der QuelleAnnotation:
Halide perovskites have recently emerged as the state-of-the-art semiconductors with the unique combination of outstanding optoelectronic properties and facile solution synthesis. Within only a decade of research, they have witnessed a remarkable success in photovoltaics and shown great potential for applications in light-emitting devices, photodetectors, and high-energy sensors. Yet, the majority of current perovskite-based devices still rely on polycrystalline thin films which, as will be discussed in Chapter 2, exhibit inferior charge transport characteristics and increased tendency to chemical degradation compared to their single-crystalline analogues. In this regard, unburdened from the effects of grain boundaries, single crystals demonstrate the upper limits of semiconductor performance. Their study is, thus, important from both fundamental and practical aspects, which present the major objectives of this dissertation.
In Chapter 3, we study the intrinsic charge transport and recombination characteristics of single crystals of formamidinium lead halide perovskites. While, in Chapter 4, we investigate the mechanistic origins of rapid synthesis of halide perovskite single crystals by inverse temperature crystallization. Understanding the nucleation and growth mechanisms of halide perovskites enables us to design strategies toward integrating their single crystals into device applications. Namely, in Chapters 5 and 6, we demonstrate crystal engineering approaches for tailoring
the thicknesses and facets of halide perovskite single crystals to make them suitable for, respectively, vertical and planar architecture optoelectronic devices. The findings of this dissertation are expected to benefit future studies on fundamental characterization of halide perovskites, as well as motivate researchers
to develop perovskite-based optoelectronic devices with better crystallinity, performance and stability.
41
WEN, CHIN CHEN, und 陳敬文. „Growth,Structure and Characteristic of Langasite Single Crystals“. Thesis, 2007. http://ndltd.ncl.edu.tw/handle/04247219812279177897.
Der volle Inhalt der QuelleAnnotation:
碩士中華技術學院
電子工程研究所碩士班
95
In this research paper, we use Russian VNIISIMS Corporation's Kristall-3M crystal grower, designed with Czochralski Method , and a high quality of La3Ga5SiO14 (LGS) single crystal has been made first at Taiwan. The LGS compound has high piezoelectric coupling coefficient , temperature compensation and low acoustic loss. In the process of crystal growing, we filled the argon and 3% oxygen. The rotational speed of pulling rod is 6 rpm, and the crystal grow with the speed 2.1 ~ 2.6 mm/hr. The length of ingot is 85 mm, and it’s diameter is 35 mm, it’s weight is 540 g. We use X-ray powder diffraction and the Raman scattering to find some characteristic of the crystal. As Compared with X diffraction's data (ICDD), it shows that the crystal structure is effected obviously by heat field condition, growth rate and annealing. The Raman spectrum obtained from the [0001] direction shows that the LGS single crystal exist some dislocation. Keywords：Czochralski Method , Langasite
42
Sekhon, Mandeep. „Numerical simulation of growth of silicon germanium single crystals“. Thesis, 2015. http://hdl.handle.net/1828/5999.
Der volle Inhalt der QuelleAnnotation:
SixGe1-x is a promising alloy semiconductor material that is gaining importance in the semiconductor industry primarily due to the fact that silicon and germanium form a binary isomorphous system and hence its properties can be adapted to suit the needs of a particular application. Liquid phase diffusion (LPD) is a solution growth technique which has been successfully used to grow single crystals of SixGe1-x. The first part of this thesis discusses the development of a fixed grid solver to simulate the LPD growth under zero gravity condition. Initial melting is modeled in order to compute the shape of the initial growth interface along with temperature and concentration distribution. This information is then used by the solidification solver which in turn predicts the onset of solidification, evolution of the growth interface, and temperature and concentration fields as the solidification proceeds. The results are compared with the previous numerical study conducted using the dynamic grid approach as well as with the earth based experimental results. The predicted results are found to be in good qualitative agreement although certain noticeable differences are also observed owing to the absence of convective effects in the fixed grid model. The second part investigates the effects of crucible translation on the LPD technique using the dynamic grid approach. The case of constant pulling is examined first and compared with the available experimental results. Then a dynamic pulling profile obtained as a part of simulation process is used to achieve the goal of nearly uniform composition crystal. The effect of crucible translation on the interface shape, growth rate, and on the transport process is investigated. Finally, the effect of magnetic field on the LPD growth is examined.Graduate
43
Fu, Cheng-Kuang, und 傅正光. „Growth of Bi2Sr2CuO6 single crystals by slowing cooling method“. Thesis, 1996. http://ndltd.ncl.edu.tw/handle/46366222317624002956.
Der volle Inhalt der QuelleAnnotation:
碩士國立中央大學
化學工程學系
84
In order to understand the object of the reason on the resulting superconductivity and study the fundamental of superconductivity, we must find the superconducting single crystals to research. Because it has a characteristic of the lower critical temperature, Bi2+xSr2-xCuO6+δ(2201) superconducting single crystals were suitable for the research. How to find the crystals of high quality is the main purpose of the paper.We try to thirty-eight kinds of composition according to phase diagram. Single crystals of Bi2+xSr2-xCuO6+δ phase have grown by slowing cooling in an alumina crucible. We separated the crystals from crucible by cleavage. In addition, we have finished the development about the standard of the SEM/ EDS.The result showed that we have gotten the some superconducting crystals if use the initial composition of Bi0.33Sr0.33Cu0.33Oy. However, the crystal*s sizes were not big. If we use the initial composition of Bi0.35Sr0.35Cu0.30Oy, we would get the high quality crystals. Some crystals became superconducting after O2-annealing at 750℃for 100 h.
44
LIN, SHUI-SHENG, und 林水盛. „Growth and characterization of RuSe2 and RuS2 single crystals“. Thesis, 1991. http://ndltd.ncl.edu.tw/handle/11215833214125644416.
Der volle Inhalt der Quelle
45
Hu, Rong-Zhang, und 胡榮章. „Growth and characterization of MgO doped LiNbO3 single crystals“. Thesis, 1991. http://ndltd.ncl.edu.tw/handle/31415271115076826958.
Der volle Inhalt der Quelle
46
Lai, Xiang-Ru, und 賴相儒. „Crystal growth and characterization of Mo1-xCrxSe2 (0 ≤ x ≤ 0.2) and Cr2Se3 Single Crystals“. Thesis, 2018. http://ndltd.ncl.edu.tw/handle/k4vss4.
Der volle Inhalt der QuelleAnnotation:
碩士國立臺灣科技大學
應用科技研究所
106
In recent years, transition metal di chalcogenides (TMDCs) has received more attentions as potential materials for application as optoelectronic devices, green catalyst, hydrogen energy and also in solar cells. We have synthesized and grown the layered single crystals of chromium doped molybdenum di selenides Mo1-xCrxSe2 (0≤ x ≤0.2) series and also Cr2Se3 crystal using chemical vapor transport (CVT) method using Iodine as transport agent. Crystal structures of Mo1-xCrxSe2 series and Cr2Se3 are well identified by XRD measurement. An enhanced crystal quality and enlarged lattice constant of Mo1-xCrxSe2 series as a result of gradual increase of chromium amount were characterized by high resolution transmission electron microscope (HRTEM). The first order E1g and A1g modes of these sample crystals were clearly detected by Raman spectroscopy which allowed shift to lower wavenumber as the chromium concentration increased in the series. Investigation on the various optical and photochemical properties such as photo-transmission, photoconductivity on both samples were carried out. The results showed good optical responsivity due to incorporation of chromium in the Mo1-xCrxSe2. The free excitonic features EA1, EA2 and EB of the chromium doped Mo1-xCrxSe2 were observed by thermoreflectance (TR) spectroscopy. Metallic behavior of Cr2Se3 crystal was confirmed by four-point resistivity measurement, where the resistivity increases with temperature increased between 20 and 300K. Mo1-xCrxSe2 series displayed n-type semiconductor behavior by hot probe method owing to Cr6+. Thus, Mo1-xCrxSe2 explicated photo-catalyst nature of these sample crystals showed better degradation of methylene blue (MB) because of redox property of Cr3+/Cr6+ and reduced band gap of Mo1-xCrxSe2. Therefore, the tunable photo-catalyst nature of Mo1-xCrxSe2 series renders the material a suitable candidate for green energy application.
47
Wu, Chien-Cheng, und 吳建澄. „Growth and Characterizations of Nd-doped Lithium Tantalate Single Crystals“. Thesis, 2011. http://ndltd.ncl.edu.tw/handle/51711215861463148918.
Der volle Inhalt der QuelleAnnotation:
碩士國立臺灣大學
化學工程學研究所
99
Congruent 0.2 and 0.5 mol% Nd-doped LiTaO3 single crystals were grown by the conventional Czochralski technique. Near-stoichiometric lithium tantalate wafers were also produced from these crystals by the vapor-transport equilibration (VTE) process. The crystals were characterized by the measurements of composition (by inductively coupled plasma atomic emission spectroscopy (ICP-AES), UV absorption edge, Curie temperature and thermal diffusivity. From the ICP-AES study, we found that Li/Ta ratio was increased by the VTE treatment, while Nd/Ta ratio was found to decrease with the crystal length. The blue shift in absorption edge was observed in the VTE treated crystals as compared with the congruent ones. The Curie temperature decreased with the increasing Nd concentration, while thermal conductivity was found to increase after the VTE treatment. FTIR was also conducted to investigate how impurity and Li/Ta affect the OH- vibration mode. And there are two blue shift absorption bands in 0.5mol%Nd:SLT in which we use a Ta vancancy model to explain the phenomenon.
48
Chiou, Shiou-Jen, und 邱秀珍. „The Growth of ZnSe Single Crystals by Physical Vapor Transport“. Thesis, 1993. http://ndltd.ncl.edu.tw/handle/02745036050926186201.
Der volle Inhalt der QuelleAnnotation:
碩士國立交通大學
材料科學(工程)研究所
81
ZnSe has been of interest as one of the primary candidates for blue light emitting diode application for long, because of its efficiency in direct band to band transitions (2.7eV at 300K). High quality ZnSe single crystals are required for applications in optoelectronic devices. Physical vapor transport method enables us to obtain ZnSe crystal without heavy twins, and it is less vulnerable to contamination compared with bulk growth techniques. In this study, special care has been taken to purify the starting materials and to adjust stochiometry. Three bake-out processes for the source materials at 1043±3℃ were considered in correlation with the growth results. Single crystals were conical tip of 30°. Dislocation etch pits were revealed by etching in boiling 50% NaOH solution. Observation with optical and scanning electron microscopies (SEM) indicated that the grown facets were {110} planes. Crystals were grown preferentially along [111]B direction. The etch pit density (EPD) was measured on the as-grown {110} facets. Double crystal x-ray diffraction materials purified with method A is superior than those from the other two methods.
49
Peng, Wei. „Single Crystals of Organolead Halide Perovskites: Growth, Characterization, and Applications“. Diss., 2017. http://hdl.handle.net/10754/623284.
Der volle Inhalt der QuelleAnnotation:
With the soaring advancement of organolead halide perovskite solar cells rising from a power conversion efficiency of merely 3% to more than 22% shortly in five years, researchers’ interests on this big material family have been greatly spurred. So far, both in-depth studies on the fundamental properties of organolead halide perovskites and their extended applications
such as photodetectors, light emitting diodes, and lasing have been intensively reported. The great successes have been ascribed to various superior properties of organolead halide hybrid perovskites such as long carrier lifetimes, high carrier mobility, and solution-processable high quality thin films, as will be discussed in Chapter 1. Notably, most of these studies have been
limited to their polycrystalline thin films. Single crystals, as a counter form of polycrystals, have no grain boundaries and higher crystallinity, and thus less defects. These characteristics gift single crystals with superior optical, electrical, and mechanical properties, which will be discussed in Chapter 2. For example, organolead halide perovskite single crystals have been reported with much longer carrier lifetimes and higher carrier mobilities, which are especially intriguing for optoelectronic applications. Besides their superior optoelectronic properties, organolead halide perovskites have shown large composition versatility, especially their organic components, which can be controlled to effectively adjust their crystal structures and further fundamental properties. Single crystals are an ideal platform for such composition-structure-property study since a uniform structure with homogeneous compositions and
without distraction from grain boundaries as well as excess defects can provide unambiguously information of material properties. As a major part of work of this dissertation, explorative work on the composition-structure-property study of organic-cation-alloyed organolead halide perovskites using their single crystals will be discussed in Chapter 3 and 4.
Despite their outstanding charge transport characteristics, organolead halide perovskite single crystals grown by hitherto reported crystallization methods are not suitable for most optoelectronic devices due to their small aspect ratios and free standing growth. As the other major part of work of this dissertation, explorative work on growing organolead halide perovskite monocrystalline films and further their application in solar cells will be discussed in Chapter 5.
50
Ghosh, Nilotpal. „Growth, Structure And Physical Properties Of Lead Doped Rare Earth Manganite Single Crystals“. Thesis, 2004. https://etd.iisc.ac.in/handle/2005/1324.
Der volle Inhalt der Quelle