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The main emphasis has been the search for new high critical temperature superconducting oxides. Efforts have been focused essentially on both fluorination and oxidation by an electrochemical method of oxide deriving from lanthanum 2 copper oxide 4. It has been shown that at low temperature (less than 200 C for the fluorination process and room temperature for the electrochemical process) oxides can be made into superconductors by inserting either fluorine or oxygen atoms into the lattice of the oxides. Interesting structural transitions have been studied. Using a chlorination treatment of lanthanum 2 copper oxide 4 at low temperature it has been shown that there is an oxidation of the oxide similar to results obtained by treating the material under high oxygen pressure. A phase diagram for a superoxygenated system, such as lanthanum 2 copper oxide (4+delta), shows clearly the occurrence of 2 different orthorhombic phases. The influence of fluorination on thermal conductivity, X-ray diffraction, specific heat, magnetic susceptibility, electrical resistivity and thermoelectric power has been investigated.

Studies have been made on the effect of oxygen nonstoichiometry upon superconductivity in bismuth cuprate and lead cuprate using X-ray diffraction techniques and high resolution electron microscopy (HREM) for understanding the structures and magnetic and electrical measurements for physical characterization. A series of new superconductors of formulation lead(1-x) A(x) strontium 2 (calcium(1-y) yttrium(y)) copper 2 oxygen (7-delta) were isolated for A equal to copper magnesium, zinc, scandium, cadmium and indium with critical temperatures ranging from 35 K to 70 K. The microstructure of the different samples was studied and several kinds of extended defects were found.

This research has involved gas solid reactions of high critical temperature superconductors, particularly of the bismuth family, preparations of thin films of superconductors by several methods including laser ablation deposition (LAD) and the analysis of transport properties especially the Hall effect. The process of making thin films of reproducible quality by laser ablation has been mastered. Efforts are being made to manufacture thin film devices from yttrium barium 2 copper 3 oxygen 7 and bismuth 2 strontium 2 calcium(n-1) copper n (n = 2, 3) systems. An instrument has been developed to analyze gas solid reactions.

This research was firstly to obtain good ceramics of yttrium barium 2 copper 3 oxygen(7-y) with a favourable microstructure for obtaining high critical densities, secondly to study the effect of iron doping on the high temperature susceptibility of yttrium barium 2 copper 3 oxygen(7-y) and thirdly to propose a novel method of processing melt cast bismuth strontium calcium copper oxide (BSCCO) rods. In the latter case, for bismuth based compounds, bulk samples were prepared as rods by a melt casting process. The application of an electric field to the rods under certain conditions produced a well defined hot zone which travelled along the rod from the positive to the negative electrode. This is a possible alternative processing route for producing BSCCO materials.
Research on new superconducting materials will be guided by the essential structural and chemical features that have emerged from study of the well-known high temperature superconductors
(La, Sr)2CuO 4-delta and YBA2Cu3O7-delta. Namely an anisotropic structure with low dimensional elements (Cu-O planes), a transition element which is formally in a mixed valence state, one configuration having S as 1/2 (Cu 2+), and proximity to an antiferromagnetic ground state or a metal-insulator transition.
Preliminary characterization of new materials will involve chemical studies using the scanning electron microprobe and Auger microscopy, structural studies by X-ray diffraction and search for signs of superconductivity by susceptibility and magnetization measurements. As new phases are isolated, their structure and physical properties will be more fully characterized.


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Cours de la Libération 351

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