No description available.

Thesis (M.S.)--University of Florida, 2003. / Title from title page of source document. Includes vita. Includes bibliographical references.

No description available.

No description available.

The structural chemistry and magnetic properties of a number of manganese containing hexagonal perovskites have been studied by X-ray diffraction, neutron diffraction and magnetometry. Trends in the magnetic properties are investigated as a function of the hexagonal stacking sequence and manganese oxidation state. The synthesis of the series of BaMnO_3-x hexagonal perovskites is discussed. By varying both the partial pressure of oxygen and the firing temperature, stacking sequences with a range of hexagonal to cubic layer ratios can be synthesised. Factors which increase the structural tolerance factor are found to increase the proportion of hexagonal layers in the stacking sequence. The crystallographic properties of the BaMnO_3-x compounds are discussed, and the oxide vacancies are found to be exclusively located in the face-sharing hexagonal (h) layers, particularly those which are adjacent to two apex-sharing cubic (c) layers. The preferential localisation of oxide vacancies in chc sites can be used to rationalise the observed stacking sequences, as well as the limiting stoichiometries of BaMnO_3-x structures. The magnetic behaviour of the BaMnO_3-x phases is investigated, both individually and as a function of the stacking sequence. A strong direct exchange interaction between face-sharing cations couples all magnetic moments antiferromagnetically above ambient temperature. A slightly weaker interaction (utilising the 180° superexchange pathways between apex-sharing MnOe octahedra) results in three-dimensional antiferromagnetic order at a temperature in the range 230 ≤ T (K) ≤ 280. The strength of this interaction is found to be dependent on the length of the face-sharing chains. A third interaction occurs at T ~ 45K, believed to represent a canting transition. Low temperature neutron diffraction data reveal that the magnetic moments order in a simple antiferromagnetic manner. The magnitude of the ordered moment shows a dependence on the length of the face sharing chains, where phases with shorter chains have a larger ordered moment. Long-range magnetic order is disrupted by charge disorder in the mixed Mn(III)/Mn(IV) system 4H-BaMnO_{2.6 5(1)}. The hexagonal BaMn_1-zTi_z O_{3- x} and BaMn_1-zZr_z0_3-x systems were also studied. Synthesis under argon atmospheres allows a range of new phases to be accessed. These include a novel manganese-zirconium phase, 6H-BaMn_0.8Zr_0.2O_2.81(1), and BaMn_o.55Ti<sub>0.45,/sub>O_3-x, which adopts the rare 15R' stacking sequence. The BaMn_1-z Ti_zO_3-x system confirms that although the tolerance factor controls the proportion of cubic layers, it does not control the arrangement of these layers. This is controlled by the size of the B cations and the proportion of oxide vacancies. Partial cation order occurs in 6H-BaMn_o.8Zr_0.2O_2.81(1), which can be rationalised on the basis of the size difference between manganese and zirconium ions. Topotactic reductions using binary hydrides are carried out in order to decouple the manganese oxidation state from the adopted stacking sequence. Using hydride reductions, 4H-Ba_o.5Sr_o.5MnO_3-x can be topotactically reduced to 4H-Ba_o.5Sr_o.5MnO_2.o2(1). This increases the strength of the 180° superexchange interaction, and antiferromagnetic order is achieved above ambient temperature. Using the same techniques, 4H-BaMnO_3-x can be reduced to 4H-BaMnO_2.47(1) or 4H-BaMnO_2.o6(1). These phases undergo a distortion to an orthorhombic unit cell.

Thesis (MSc.(Metallurgy)--University of Pretoria, 2008. / Includes bibliographical references.

Thesis (M. Sc.)--Carleton University, 2002. / Includes bibliographical references (p. 116-117). Also available in electronic format on the Internet.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18591号 / 工博第3952号 / 新制||工||1607(附属図書館) / 31491 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 河合 潤, 教授 酒井 明, 教授 伊藤 秋男 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Indiana University-Purdue University Indianapolis (IUPUI) / The innovation and investigation of catalysts in proton exchange membrane fuel cells are included in this thesis. In the first part of this work, stability of the catalyst support of PEMFC catalyst is investigated. Nanoscale platinum particles were loaded on two different kinds of carbon supports, nano graphene sheets and functionalized carbon black/graphene hybrid were developed by the liquid phase reaction. The crystal structure of two kinds of catalysts was characterized by X-ray diffractometer (XRD). The morphology and particle size were characterized by scanning electron microscope (SEM) and transmission electron microscope (TEM). Pt loading was measured by thermal gravimetric analysis (TGA). The Brunauer, Emmett and Teller (BET) method was applied to test the surface area of the catalysts. The electrochemical surface area (ECSA) and mass activity during oxygen reduction reaction (ORR) process for two kinds of catalyst were tested by cyclic voltammetry method under different conditions. The stability of the catalysts were tested by accelerated durability test (ADT). The results show that although the mass activity of Pt/graphene is much lower, the stability of it is much better than that of the commercial catalyst. After adding functionalized carbon black (FCB) as spacer, the stability of the catalyst is preserved and at the meantime, the mass activity becomes higher than 20% Pt/XC72 catalyst. The lower mass activity of both catalysts are due to the limitation of the electrolyte diffusion into the carbon support because of the aggregation nature of graphene nano-sheets. After introducing functional carbon black as spacer, the mass activity and ECSA increased dramatically which proved that FCB can be applied to prevent the restacking of graphene and hence solved the diffusion problem. In the meantime, the durability was still keeping the same as Pt/graphene catalyst. In the second part of the work, the restacking problem was solved by introducing FCB as spacers between functionalized graphene nanosheets. The same measurement was applied to test the electrochemical performance of Pt/FCB/FG catalyst. The new catalyst showed a higher mass activity compared to Pt/graphene catalyst which meant the restacking problem was partially solved. The durability of the Pt/FCB/FG catalyst was still excellent.