Browsing by Author "HO, JS"

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MICROSTRUCTURE, THERMOELECTRIC-POWER AND MAGNETIC IRREVERSIBILITY OF COPRECIPITATED YBA2CU4O8 POWDERS
(1992-03-01)
LIU, RS
;
ZHENG, DN
;
OBERTELLI, SD
;
HO, JS
;
CHEN, TM
The details of microstructure of triethylammonium oxalate coprecipitated YBa2Cu4O8 powders (T(c) = 83 K), prepared under one atmosphere of oxygen, were investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS) and selected area electron diffraction (SAED). The coprecipitated powders are characterized by a submicron particle size and homogenous chemical composition of Y 1.0(3)Ba2.0(1)Cu4O8 as determined by SEM and EDS, respectively. The electron diffraction spots along the c* direction of the coprecipitated YBa2Cu4O8 microcrystals have extended streaks corresponding to a defect stacking fault. The YBa2Cu4O8 material, as prepared, is under-doped as compared to (Y0.9Ca0.1)Ba2Cu4O8 (T(c) = 88 K) as evidenced by thermoelectric power measurements. The observed magnetic irreversibility line for YBa2Cu4O8 shifts to lower temperatures as compared to that for YBa2Cu3O7; this may arise from the reduced structure dimensionality of YBa2Cu4O8 as compared to that of YBa2Cu3O7. The upper critical field [H(c)2(0)] and the Ginzburg-Landau coherence length (zeta) of the YBa2Cu4O8 material are determined to be 117 T and 17 angstrom, respectively.
SYNTHESIS OF A 90 K Y2BA4CU7O15-X SUPERCONDUCTOR UNDER AMBIENT-PRESSURE BY TRIETHYLAMMONIUMOXALATE COPRECIPITATION
(1993-10-01)
CHEN, TM
;
HO, JS
;
LIU, RS
;
KOO, HS
;
應用化學系
A 90 K superconducting Y2Ba4Cu7O15-x (247) powder with fine and homogeneous grain size was prepared by oxalate/triethylamine co-precipitation of a stoichiometric amount of mixed cations of yttrium, barium and copper from their aqueous solution of nitrate salts. Characterization results of the 247 superconductor by X-ray diffraction, electrical resistivity and magnetization measurements are reported. The intragrain critical current density (J(c)) of the as-prepared 247 superconductor was derived from magnetic hysteresis to be 8.5 X 10(6) A/cm2 at 5 K and 1 T. This new route may have considerable potential in the large-scale preparation of Y2Ba4Cu7O15-x.