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Title: Synthese en karakterisatie van cupraten met een Cu(III) gehalte als mogelijke referentiematerialen voor de analyse van supergeleiders
Authors: De Backer, An 
Advisors: Van Poucke, Lucien
Mullens, Jules
Yperman, Jan
Issue Date: 1995
Abstract: In ceramic high-Tc superconductors the oxygen content plays a crucial role for the properties of the material. The controversy that exists is whether Cu(III) or 0(1) is present. Many analysis techniques to determine the oxygen stoichiometry, or more specifically the Cu(III) or 0(1) content, are described in literature. To optimize these methods of analysis there is a need for a good standard reference material which means that also compounds with a high Cu(III) content are necessary. About ten years ago especially complexes of Cu(III) (in solution) were known but in this work we were in search of solid cupr(III)ates. As oxides they resemble more the high Tc superconductors. The synthesis of Ba(Cu02)2, NaCu02, KCu02 and LiCu02 has been examined and several parameters were changed to obtain an optimal Cu(III) content. It was found out that in the case of sodium the best results are obtained: by mixing of Na2o2 and CuO which gives after 96 hours of heating at 450°C in a continuous oxygen stream a Cu(III) content of 96-97%. These cuprates were not intensively studied before. So this work can contribute to a more extensive knowledge of the synthesis, the composition and the properties of these powders. About the composition we know that the products consist of the desired cupr(III)ate, CuO, adsorbed water and carbonate. This last component gives some trouble in a few analysis methods so we looked for a procedure to avoid CO~-. Our conclusion is that the use of a copperoxalate precursor for CuO gives products with considerably less co;-. The stability as a function of time shows us that NaCu02 is the most stable cupr(III)ate of those we have examined. When temperature is raised successive reductions of Cu(III) to Cu(II), and possibly to Cu(I) and metallic Cu can take place, depending on the atmosphere in which the warming up is happening. The more oxygen in the atmosphere the longer a material can keep the higher oxidation state of copper. Comparison of the different alkalicuprates in oxygen atmosphere shows that NaCu02_x is again the most stable compound. Oxygen content determination can be done by iodometry, the method that is automated in this work. Optimization of the reproducibility was necessary and could be realized mainly by using a magnetic way of stirring instead of the original mechanic way. Another important thing in automating a method is the fact that one has to take care that the equipment is very well cleaned and dried between two measurements. Voltammetry, in which Cu(III) is specifically determined, shows a good agreement for NaCu02_x with iodometry. Besides photometry points out that there is no peroxidic oxygen present in solution. Other potentiometric titrations, the Fe2+/Cr2027- and TiCliCr2027- method, didn't give good results. There was a negative error caused by loss of oxygen. Gasvolumetric measurements give a higher Cu(III) content than iodometry but there exists the problem of carbonate resulting in CO2 that gives positive errors. The general conclusion can be that NaCu02_x is a good standard reference material for cu(III) in methods like iodometry, voltammetry, temperature programmed reduction, spectroscopic techniques XPS, XANES, ... The use of this material in H2/Ar reduction in TGA and gasvolumetry gives problems because of the carbonate present. By optimizing the synthesis of NaCu02 from the oxalate precursor there might be a solution for this problem.
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Category: T1
Type: Theses and Dissertations
Appears in Collections:PhD theses
Research publications

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