(Hg,Cu)Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.y oxide superconductor and method of preparing the same

Provided is an Hg-Ba-Ca-Cu-O oxide superconductor having a high superconductivity transition temperature Tc and a method which can prepare the same in excellent reproducibility. This oxide superconductor consists essentially of Hg, Ba, Ca, Cu and O, and is expressed in a chemical formula (Hg.sub.1-X Cu.sub.X)Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.Y, wherein X=0.05 to 0.7 and Y=8 to 8.75. A method of preparing the oxide superconductor comprises a step of mixing raw materials of Hg, Ba, Ca and Cu with each other so that (Hg+Ba):Ca:Cu=b:1:C and Hg:Ba=(1-a):a, wherein 0.625.ltoreq.a.ltoreq.0.714, 1.ltoreq.b.ltoreq.3 and 1.667.ltoreq.c.ltoreq.3.444, in mole ratio, and compression-molding the mixture, and a step of heat treating a compact obtained by the compression molding. This oxide superconductor has a superconductivity transition temperature Tc of 134 K., which is the highest at present.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an oxide superconductor and a method of 
preparing the same, and more particularly, it relates to an oxide 
superconductor having a high superconductivity transition temperature 
(Tc=134 K.) exceeding the liquid nitrogen temperature and a method of 
preparing the same. 
2. Description of the Background Art 
A superconductor has unique properties of its own such as zero electrical 
resistance, complete diamagnetism and a Josephson effect. Due to such 
properties, wide applications of superconductors are expected in relation 
to power transportation, a generator, nuclear fusion plasma confinement, a 
magnetic levitation train, a magnetic shield, a high-speed computer and 
the like. 
In 1986, Bednorz and Muller discovered an oxide superconductor (La.sub.1-X 
Ba.sub.X).sub.2 CuO.sub.4 having a high superconductivity transition 
temperature Tc of about 30 K. Following this discovery, there have been 
reported oxide superconductors having high superconductivity transition 
temperatures Tc such as YBa.sub.2 Cu.sub.3 O.sub.7 (Tc=90 K.), 
Bi-Sr-Ca-Cu-O (Tc=110 K.) and Tl-Ba-Ca-Cu-O (Tc=125 K.). Many researches 
are now being made in relation to methods of preparing such 
superconductors, physical properties and applications thereof and the 
like. 
In the process of such researches, a superconductor having a 
superconductivity transition temperature Tc of 133.5 K. has recently been 
confirmed in relation to Hg-Ba-Ca-Cu-O ceramics (A. Schilling et al., 
Nature 363 (1993) 56). 
As to the superconductor exhibiting a superconductivity transition 
temperature Tc of 133.5 K., it is inferred from observation of its lattice 
image with a transmission electron microscopy that a substance expressed 
in a chemical formula HgBa.sub.2 Ca.sub.2 Cu.sub.3 O.sub.Y (Y: arbitrary 
number) having such a crystal structure that eight atomic layers of 
HgO.sub.Z -BaO-CuO.sub.2 -Ca-CuO.sub.2 -Ca-CuO.sub.2 -BaO are stacked 
exhibits the high superconductivity transition temperature Tc. However, no 
crystal structure of a superconducting crystal phase has been clarified 
with respect to this superconductor. 
A. Schilling et al. further reported that they prepared the superconductor 
by preparing Ba.sub.2 CaCu.sub.2 O.sub.Y as a precursor, mixing the same 
with HgO, vacuum-sealing the mixture in a silica tube, introducing the 
silica tube into a steel container and firing the same at 800.degree. C. 
for 5 hours. However, they clarified no mixing ratio of the precursor to 
HgO. Further, they reported that the silica tube which was introduced into 
the steel container was cracked after firing, and it was difficult to 
reproduce their experiment. 
Although the superconductor having a superconductivity transition 
temperature Tc of 133.5 K. was confirmed in the aforementioned manner, it 
has generally been extremely difficult to reproduce equivalent 
superconductivity since there has been established no method of preparing 
the same. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an Hg-Ba-Ca-Cu-O oxide 
superconductor which exhibits a superconductivity transition temperature 
Tc of 134 K. 
Another object of the present invention is to provide a method which can 
prepare an Hg-Ba-Ca-Cu-O oxide superconductor exhibiting a 
superconductivity transition temperature Tc of 134 K. in excellent 
reproducibility. 
According to an aspect of the present invention, provided is an oxide 
superconductor which consists essentially of Hg, Ba, Ca, Cu and O, and is 
expressed in a chemical formula (Hg.sub.1-X Cu.sub.X)Ba.sub.2 Ca.sub.2 
Cu.sub.3 O.sub.Y, wherein X=0.05 to 0.7 and Y=8 to 8.75. 
According to the present invention, the oxide superconductor has such a 
crystal structure that eight atomic layers of (Hg, Cu)O.sub.Z 
-BaO-CuO.sub.2 -Ca-CuO.sub.2 -Ca-CuO.sub.2 -BaO, wherein Z=0 to 0.75, are 
stacked. 
According to another aspect of the present invention, a method of preparing 
an oxide superconductor which consists essentially of Hg, Ba, Ca, Cu and 
O, and is expressed in a chemical formula (Hg.sub.1-X Cu.sub.X)Ba.sub.2 
Ca.sub.2 Cu.sub.3 O.sub.Y, wherein X=0.05 to 0.7 and Y=8 to 8.75 is 
provided. 
According to this method, raw materials of Hg, Ba, Ca and Cu are first 
mixed with each other so that (Hg+Ba):Ca:Cu=b:1:C and Hg:Ba=(1-a):a, 
wherein 0.625.ltoreq.a.ltoreq.0.714, 1.ltoreq.b.ltoreq.3 and 
1.667.ltoreq.c.ltoreq.3.444, in mole ratio, and the mixture is 
compression-molded. Then, a compact obtained by such compression molding 
is heat treated. 
Preferably, the compact is heat treated in a closed container. 
Preferably, the compact is heat treated at a temperature of 640.degree. to 
780.degree. C. 
Preferably, the compact is further annealed after the heat treatment. 
According to the present invention, it is possible to obtain an oxide 
superconductor exhibiting a superconductivity transition temperature Tc of 
134 K., which is the highest at present (as of June 1993). 
According to the present invention, it is possible to prepare the oxide 
superconductor exhibiting the highest superconductivity transition 
temperature Tc of 134 K. in excellent reproducibility. 
The foregoing and other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.