S.C. Chang

A comparison of the properties of Bi-2223 precursor powders synthesized by various methods

Abstract We report on the study of the (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O x precursor powders prepared by coprecipitation, citrate gel, and spray pyrolysis. All powders had the same starting nominal compositions, and were calcined under the same conditions. The properties of the precursor powders are compared. It is found that the phase compositions of these precursor powders are not identical, and the powder prepared by spray pyrolysis seems to be particularly different. Moreover, the observation of the Bi-2223 phase formation reveals that the spray pyrolysis powder transforms to Bi-2223 far faster than the other powders.

Effect of Pb doping in high-Tc Bi2Sr2CaCu2Oy superconductors studied by X-ray absorption near-edge structure spectroscopy

Abstract An investigation of the variation of hole concentration within the CuO 2 planes using X-ray absorption near-edge structure spectroscopy with synchrotron radiation in the (Bi 2− x Pb x )Sr 2 CaCu 2 O y (0⩽ x ⩽0.3) series of high- T c superconductors synthesized by solid-state reaction in air is presented. Pb is found to have the valence of +2 in the samples of this series. Moreover, the hole concentration increases with increasing Pb content. The results lead us to conclude that the substitution of Pb 2+ in the Bi 3+ sites of the (Bi,Pb)-2212 system causes the compounds to be in the over-doped state. This effect results in the compounds having a high critical current density.

Hydroxyapatite Ceramics from Hydrothermally Prepared Powders

Hydroxyapatite (Ca{sub 5}(PO{sub 4}){sub 3}(OH)) is an effective material for artificial human bone production. Hydroxyapatite powders were hydrothermally produced in this work by reacting Ca(OH){sub 2} with Na{sub 3}PO{sub 4}{center_dot}12H{sub 2}O in an autoclave at various temperature and for various times. The particle size of hydroxyapatite was observed to be very fine, uniform, around 50 nm, as well as independent of reaction time. The hydroxyapatite powders were compacted and sintered at various temperatures for 2 hrs. The density, grain size, and hardness of the hydroxyapatite ceramics were measured and compared with those of the hydroxyapatite ceramics produced by the powders from the commercial source. The hydroxyapatite ceramics from the hydrothermal powders were found to have a higher density, smaller grain size, and higher hardness. After the hydroxyapatite ceramics were dipped in a simulated biological body liquid for 10 days, the density and hardness of the hydroxyapatite ceramics from the hydrothermal powders were less deteriorated than those of the hydroxyapatite ceramics from the commercial powder.

Tuning Pb content in High-Tc Bi2Sr2CaCu2Oy superconductors studied by X-ray absorption near-edge structure spectroscopy

The variation of hole concentration within the CuO2 planes via tuning Pb content in (Bi2−xPbx)Sr2CaCu2Oy (0.2≤x≤0.6) has been investigated by X-ray absorption near-edge structure (XANES) spectroscopy using synchrotron radiation. The profile of XANES spectra indicated that Pb can be incorporated into Bi-2212 phase under nitrogen atmosphere around x=0.6. However, after annealing the nitrogen-sintered samples in oxygen, Pb solubility is found to be decreased only up to x=0.2 in (Bi2−xPbx)Sr2CaCu2Oy, showing that compounds with high Pb content are more stable under oxygen-free atmosphere.

Effect of secondary phases in the precursor powders on the transformation to the (Bi,Pb)-2223 phase

The (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 2 system [also know as (Bi,Pb)-2223] has shown promise for use in superconducting tapes and wires due to its high transition temperature and high critical current density. The tapes and wires are usually fabricated by the powder-in-tube method, after which they are subjected to thermal and mechanical treatments. Depending on the nature of the heat treatment, phase transformations occur in the powder and it is of paramount importance to understand the response of the powders to different processing conditions. In this study, we have synthesized a precursor powder of the nominal composition Bi 1.8 Pb 0.33 Sr 1.87 Ca 2 Cu 3 O y by spray pyrolysis. The powder was then calcined under controlled conditions between 760 and 800°C in an atmosphere between 0 and 21% O 2 for 2 to 24 hours with controlled heating and cooling, such that different amounts of Pb are incorporated into the majority phase. The resultant powders were then converted to the (Bi,Pb)-2223 phase with suitable heat treatments. Using powder Xray diffraction and magnetization measurements, the volume fractions of the secondary phases in the precursors were estimated and the effect of the phases on the conversion to the (Bi,Pb)-2223 phase was studied. We show that the ratio of the intensity of the (020) and (115) reflections of the (Bi,Pb)-2212 phase can be used to tune the precursor powders for optimal conversion. While a lower fraction of the secondary phases such as CaO and CuO is desirable in the precursor powders, a higher volume fraction of Ca 2 PbO 4 seems to help in the rapid conversion to the (Bi,Pb)-2223 phase.