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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Pressure and doping effects on the anomalous phase transition in ternary superconductor Bi2Rh3Se2

Chen, Ching-Yuan 23 July 2012 (has links)
Bi2Rh3Se2 have been known as a charge-density-wave (CDW) superconductor, where the superconducting critical temperature Tc and the CDW phase transition are about 0.7 K and 250 K, respectively. Since there has no definite proof that the anomaly at around 250 K comes from charge-density-wave, we wished to provide another evidence to study whether the superconductor had the properties of CDW by electric resistivity measurements applied different pressures. Bi2Rh3Se2 was prepared by using the solid state reaction method and heating in the quartz tube. After the sample was synthesized, the quality was identified by XRD, MPMS, and specific heat probe. With the confirmation of the above-mentioned measurements, we can determine the sample¡¦s quality is good. Furthermore, temperature-dependent resistivity (2-340 K) under pressure (up to 22.23 kbar) on the ternary superconductor Bi2Rh3Se2 are performed to study the possible coexistence of CDW and superconductivity. Interestingly, the resistive anomaly occurred at Ts~250 K, is shifted to higher temperature with increasing pressure. This experimental finding is not consistent with a traditional CDW transition. Moreover, the temperature-dependent Transmission Electron Microscopy (TEM) electron diffraction is evident a structural phase transition from space group ¡§C1 2/m 1¡¨ (Ts > 250 K) to ¡§P1 2/m 1¡¨ (Ts < 250 K). Finally, We do the Co doping to make sure the effects of chemical pressure on this phase transition. The results are opposite to imposed by physical pressure that the transition is shift to lower temperature with more Co inside the sample.
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2

Hochtemperaturinduzierte Mikrostrukturänderungen und Phasenübergänge in nanokristallinen, metastabilen und defektbehafteten Aluminiumoxiden

Thümmler, Martin 03 December 2024 (has links)
Within the collaborative research center SFB 920 “Multifunctional Filters for Metal Melt Filtration”, the thermally induced formation of metastable aluminum oxides and related microstructural changes were investigated. It was confirmed that the γ-Al₂O₃ phase possesses a defective spinel structure containing Al vacancies that preserve the stoichiometry of this phase. The presence of vacancies fragments apparently the γ-Al₂O₃ crystallites into nanocrystalline domains, which are separated by non-conservative antiphase boundaries (APBs) of the type {100} ¼<110>. These APBs form a 3D network that is randomly distributed over all crystallographically equivalent lattice planes. This phenomenon causes a starlike (and hkl-dependent) broadening of the reciprocal lattice points that correspond to the aluminum sublattice. It was shown that the extent of the broadening of the reciprocal lattice points can be predicted by employing the phase shift factors. With increasing degree of the APBs ordering, the initial streaks representing the broadened reflections start to split, forming superstructure reflections. This superstructure of γ-Al₂O₃ is commonly known as δ-Al₂O₃. Between the ordered APBs, the crystal structure of δ-Al₂O₃ is closely related to the crystal structure of monoclinic θ-Al₂O₃. The phase transition of γ-Al₂O₃/δ-Al₂O₃ to θ-Al₂O₃ proceeds via migration of just three Al³⁺ cations to the neighboring tetrahedral and octahedral sites in the cubic close packed (ccp) oxygen sublattice. The general migration vector is ⅛<111> (γ-Al₂O₃). Diffraction effects associated with different intermediate states can be explained by an improper long-range ordering of equivalent APBs or certain Al³⁺ cations and the local formation of θ-Al₂O₃ within the δ-Al₂O₃ superstructure. The formation of θ-Al₂O₃ is accompanied by an increase of the occupancy of the tetrahedral sites in the oxygen sublattice by the Al³⁺ cations. In surrounding local γ-Al₂O₃ domains, however, some cations migrate from the tetrahedral to the octahedral sites. Thus, the local formation of θ-Al₂O₃ is nearly invisible for the ²⁷Al 1D magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Still, it was recognized by the 2D multiple quantum (MQ) MAS NMR spectroscopy. A continuous formation of the θ-Al₂O₃ domains was confirmed by the Raman spectroscopy, X-ray diffraction (XRD) and selected area electron diffraction (SAED). The proposed microstructure and transformation models helped to explain the thermal stabilization of the metastable alumina phases by Si-doping. For investigation of the thermally induced phase transitions in metastable alumina phases, boehmite (γ-AlO(OH)) was chosen as the starting compound. However, the metastable alumina phases were also observed in endogenous inclusions present in solidified steel melts. For identification of these phases, a procedure for reconstruction of spherical Kikuchi maps from recorded EBSD patterns was developed.
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