F.J. Gotor

Synthesis of YBa2Cu3O7−x by sol-gel route formation of YBaCuO oxycarbonate intermediate

Abstract High- T c superconducting powders of the Y-Ba-Cu-O system are prepared by a solution-polyacrylamide gel using citric acid as a complexing agent. This method provides an easy way to prepare reactive YBaCuO powders. However, sol-gel synthesis of this oxide involves the formation of intermediate phases which impedes the obtaining of the pure phase at low temperature. An intermediate oxycarbonate phase forms between 800 and 850°C in flowing oxygen. From powder X-ray diffraction, thermal analysis and IR spectroscopy, it is concluded that the intermediate oxycarbonate has an average tetragonal structure — SG P4/ mmm — similar to the parent oxide with a stoichiometry close to YBa 2 Cu 2.95 (CO 3 ) 0.35 O 6.6 . The carbonate group is located in the center of the basal CuO square. This compound has superconducting properties. A pure 123 phase is obtained when the sol-gel precursor is annealed at 925°C in O 2 .

Carbon in YBa2Cu3O7−x: origin and effects

Abstract Residual carbon content in powders, sintered ceramics and textured samples was determined. Its source was shown to be essentialy the starting material precursors. Carbon content may be reduced to below 600 ppm if no liquid phase is formed during precursor processing. The case of samples processed by solidification from a liquid is important. In samples textured on MgO substrate, carbon is supplied by this substrate. In this case, liquid phase plays an active role on the carbon retention in the YBa2Cu3O7−x structure during crystallisation. Magnetic susceptibility behaviour of textured samples was analysed and discussed in relation to the presence of carbon. A qualitative correlation between the superconducting transition width and the carbon content was established. A sharp superconducting transition at 91.5 K was achieved when carbon content was less than 600 ppm.

Copper deficiency in YBa2Cu3O7−x ceramics, textured and single crystals

Abstract Copper deficiency is studied in YBa 2 Cu 3 O 7− x phase. We compare polycrystals formed by sintering with textured and single crystals. The experimental method uses electron probe micro-analysis and X-ray structural determinations on single crystals. We show that YBa 2 Cu 3 O 7− x phase accommodates Cu deficiency up to several percent. Copper vacancies are located at the Cu(1) site (in the chains). Copper deficiency is evidenced when a liquid phase is involved in the chemical process. The implications of copper vacancies on the physical properties has probably to be reconsidered in future.

About C in YBaCuO powders and textured ceramics

Abstract Carbon is found to be present in fully oxygenated YBaCuO powders, it is combined with oxygen as a CO 3 2− ions, either in insertion (z=0 or in substitution Cu(1). C is also found in ceramics or textures. Liquid possibly plays a role in C retention. C is detrimental to superconducting properties.

Analysis of infrared-visible-near-ultraviolet reflectivity of conducting and superconducting oxides

Abstract We analyse infrared-visible-near-ultraviolet spectra of oxide materials in the semiconducting or metallic phase. We show that the response of laser-ablated YBa 2 Cu 3 O 7 film can be fitted by a modified Drude term without mid-infrared absorptions. Mid-infrared and visible absorptions are discussed for textured YBaCuO, BaBiO 3 and Pr 2 NiO 4+δ as well as indication of their polaronic origin.

Grain growth, sintering and weak links in YBCO ceramics: advantage of sol-gel process

Abstract YBCO ceramics have presently very low transport critical current capacities. One of the objectives of the present work is to identify some parameters on which to play to improve it.

Microstructure of textured YBa2Cu3O7-δ prepared by the melted zone in magnetic field: texturation mechanism

Abstract A zone melting set-up with a magnetic field perpendicular to the temperature gradient is used in order to optimize (a, b) plane alignment and thus critical current densities. Large samples with high Jc values in magnetic fields at 77K are obtained. These studies are carried out by optical microscopy, analytical transmission electron microscopy and X ray diffraction pole figures. Our results indicate that crystallite misorientation within a single domain does not exceed 10°. The application of a magnetic field of 5 T imposes the c-axis orientation during grain growth. Indeed, we observe an intergrain dispersion much smaller than 40° with respect to the magnetic field direction. The 123 matrix contains large amounts of unoriented 211 inclusions which are of different size.

Paramagnetic Cu2+ centers in oxycarbonate phase YBa2Cu2.95(CO3)0.35O6.6 probed by EPR and magnetic susceptibility

Abstract Room temperature EPR and magnetic susceptibility studies are reported for the YBa 2 Cu 2.95 (CO 3 ) 0.35 O 6.6 phase, an intermediate phase in the synthesis of YBa 2 Cu 3 O 7−δ by a sol-gel route. EPR measurements have revealed spectra of Cu 2+ ions in an orthorhombic local-crystal symmetry which are attributed to copper ions in a modified Cu(1) site. The presence of CO 2− 3 groups in the structure, surrounding the Cu(1) atoms, is at the origin of the EPR signal. The values of the g -tensor for this signal are as follows: g x = 2.045(5), g y = 2.117(5) and g z = 2.228(5). The d.c. magnetic susceptibility has shown a paramagnetic term superimposed on the diamagnetic contribution of the superconducting regions. The temperature dependence of the paramagnetic response is described by a perfect Curie law plus a constant term. The molar Curie constant is 1.1 × 10 −2 cm 3 K/mol Cu. The free spin concentration calculated from this Curie constant agrees well with the intensity of the EPR signal, suggesting it emanates from isolated Cu 2+ paramagnetic centers.

Micro and nano structure of 123/211 interfaces in MTG ceramics

Abstract We have studied by analytical TEM and HREM the microstructure and the nanostructure of 123 matrix/211 inclusion interfaces in order to determine their cristallographic relationship and their chemistry. Three types of relaxed interfaces have been characterized called A1, A2, and A3. These are related to interfacial extended defect formation and to texturation mechanism.

Intermediate steps in YBa2Cu3O7?x synthesis by sol-gel method: YBaCuO oxycarbonate: Code: DP23

High Tc-superconducting powders of the Y-Ba-Cu-O system are prepared by a solution-polyacrylamide gel using citric acid as a complexing agent. This method provides an easy way to prepare reactive YBaCuO powders by sol-gel synthesis. However this synthesis involves intermediate phases formation which impedes the obtention of the pure phase at low temperature. An intermediate oxycarbonate phase forms between 800° and 850°C in flowing oxygen. From powder X-ray diffraction, thermal analysis and IR spectroscopy, it is concluded that the intermediate oxycarbonate has an average tetragonal structure—SG P4/mmm—similar to that of the parent oxide with a stoichiometry close to YBa2Cu2.95(CO3)0.35O6.6. The carbonate group is located in the center of the basal CuO square. This compound has superconducting properties. A pure 123 phase is obtained when the xerogel precursor is annealed at 925°C in O2 or at 800°C in Ar, then in O2. The grain growth and microstructure development of YBaCuO have also been investigated and compared using two different powders, i.e. sol-gel route and commercial powder from Hoechst.