Ternary system of a ceramic superconductor Y-Ba-Cu-O

Abstract

Many studies have been reported on high-T~ superconducting oxides in the past 4 years. A pioneering discovery of the 40 K class La-Ba(Sr) -Cu-O [1, 2] system was followed by the attainment of superconductivity above 90 K in the Y B a C u O system [3]. Recently two new superconductor compounds with T~ above 100 K were discovered, B i S r C a C u O by Maeda et al. [4] and T a C a B a C u O by Shing and Herman [5,6]. Up to this point, many researchers have devoted much of their efforts to finding compounds of higher T~. There are many important parameters that are currently being studied in high-T~ superconducting materials. Among these it is important to know how much deviation from the stoichiometric formula may be allowed without losing the superconducting characteristics. This applies to the easiest way of producing these compounds, which is by sintering the compound powder, where the distribution of the different elements is not very uniform. In this letter we present the superconducting systems made of mixtures of Y203, BaCO 3 and CuO with different initial compositions, in which additions up to 200% above the 1-2-3 formula of one of the constituents gives samples with high-To superconductivity. Varying the starting composition, samples were prepared as follows. Appropriate amounts of Y203 (purity 99.9%), BaCO 3 (99.9%) and CuO (99.9%) were mixed in an agate mortar and then calcined at 900 °C for 12 h in air atmosphere. The mixtures were pulverized and pressed into pellets 1.25cm in diameter. Then the pellets were sintered at 900 °C for 24 h and were allowed to cool down inside the oven. It is not necessary to form pellets in order to obtain a superconductor sample, but pellets allow better manipulation in performing measurements. Two methods were used for testing superconductivity: the Meissner effect (all samples were tested by this method) and resistivity measurements using the four-probe method (only those samples with 0, 50, 75 and 100% CuO in excess were tested). The percentage in excess or deficiency of each element in the sample was measured with respect to the total amount of Y203, BaCO3 and CuO necessary to obtain the 1-2-3 stoichiometric compound Y1Ba2Cu307-x • The d.c. resistivity measurements were made employing the usual four-probe method. We selected a small sample (7 mm in diameter) of each