Superconductive material

The disclosed superconductive material has a characteristic in accordance with which electrical resistance disappears at a temperature of at least more than the boiling point of 20.3.degree. K. (-252.7.degree. C.) of liquid hydrogen and relates to La-Ba-Cu-O series superconductive material. Said superconductive material consists essentially of a composition having the formula EQU (La.sub.1-x M.sub.x).sub.2 CuO.sub.4-x/2 wherein, M=Ba or Ba(Sr, Ca) and x=0.04.about.0.20 as a main body, wherein the material has a K.sub.2 NiF.sub.4 crystal structure.

BACKGROUND OF THE INVENTION 
(a) Field of the Invention 
The present invention relates to a superconductive material with an 
electrical resistance that disappears at a low temperature temperature of 
which is more than the boiling point of 20.3.degree. K. (-252.7.degree. 
C.) of liquid hydrogen. More specifically, the present invention relates 
to La-Ba-Cu-O series superconductive material. 
(b) Description of the Related Art 
Hitherto known superconductors cannot avoid cooling by cryogenic liquid 
helium (boiling point: 4.2.degree. K.), and the vast popularization has 
therefore been prevented by the expensive cooling cost and the uneven 
distribution of helium in resources. 
A confirmed superconductive material having the highest critical 
temperature is Nb.sub.3 Ga. Its critical temperature T.sub.c (i.e. 
transition starting temperature) is 23.6.degree. K., and the material 
cannot reach a usable level under cooling of liquid hydrogen (i.e. boiling 
point: 20.3.degree. K.) or liquid neon (i.e. boiling point: 27.1.degree. 
K.). 
In general, in order to confirm a certain material as superconductive, it 
is necessary to certify that (1) the material structure is definite, (2) 
the material has a stability for superconductivity and the test result is 
reproducible, (3) an electrical resistance is rapidly lowered from a 
certain temperature (i.e. superconductivity transition starting 
temperature) together with the temperature drop within a range of several 
degrees, and (4) the material shows the Meissner effect (i.e. complete 
diamagnetism) characteristic of a superconductive phenomenon. 
Thereto, no material satisfying any one of the above conditions can be 
found, and there has not been developed any practically usable 
superconductive material at a temperature higher than the boiling point of 
liquid hydrogen. 
SUMMARY OF THE INVENTION 
The invention aims to provide practically usable superconductive material 
showing the superconductivity by electrical resistance disappearing at a 
temperature of more than the above-described well-known superconductive 
material (e.g., Nb.sub.3 Ge), for example, at the temperature of the 
boiling point of liquid hydrogen (20.3.degree. K.) or liquid neon 
(27.1.degree. K.). 
The present inventors have strenuously studied and clarified the 
superconductive material actually having superconductivity among the 
hitherto synthesized lanthanum-barium-copper-oxygen series oxides such as 
a mixed body of oxides having each kind of crystal structure, further 
confirmed the structure thereof, and found a superconductive material 
wherein the transition starting temperature is 30.degree. K., which is 
more than 25.degree. K. where electrical resistance disappears and it is 
practically usable at a temperature of liquid hydrogen. Thus, the 
inventors have attained the present invention. 
The present invention provides a superconductive material with as a main 
body of a composition having the formula 
EQU (La.sub.1-x M.sub.x).sub.2 CuO.sub.4-x/2 
wherein M represents Ba and X is a numerical value of 0.04.about.0.20, 
preferably 0.05.about.0.15, and having a K.sub.2 NiF.sub.4 type crystal 
structure, and showing superconductivity by disappearing electrical 
resistance under cooling at a temperature of more than the boiling point 
of 20.3.degree. K. of liquid hydrogen. In the superconductive material 
according to the invention, when x in the above formula is less than 0.04, 
the superconductivity of the material obtained disappears, and when x is 
more than 0.20, the superconductivity is unfavorably deteriorated. 
Almost all copper exists in the bivalent condition, and as a result that La 
is trivalent and M is bivalent, a theoretical mol ratio of oxygen to 
copper becomes 4-X/2, but there is a case that a part of copper becomes 
trivalent according to a sintering temperature or atmosphere at the time 
of crystallizing said composition, and in such a case, the total mol ratio 
of oxygen to copper is slightly higher than 4-X/2, but the present 
invention does not exclude such a case. That is, it is preferable to 
occupy almost all portions by the composition of (La.sub.1-x 
M.sub.x).sub.2 CuO.sub.4-x/2. 
Even if high temperature sintering is carried out under an oxygen 
atmosphere, formation of trivalent copper is not so large, but usually 
less than 10%. 
Further, a part of M may preferably contain any component other than barium 
(Ba), such as that substituted by strontium (Sr) or calcium (Ca). 
A "main body" mentioned in the present invention means that the condition 
is occupied almost all portion by said composition, and the crystal 
structure is substantially the same and no bad influence is given to 
achievement of the object of the invention, any metal other than said 
composition, such as strontium (Sr) or calcium (Ca) can be coexistent. A 
blending ratio enabling to substitute for barium (Ba) is up to 50% for 
strontium (Sr) and up to 30% for calcium (Ca).

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The superconductive material consisting of a composition of said formula as 
a main body and having a K.sub.2 NiF.sub.4 type crystal structure starts 
superconductive transition at a temperature (at least more than 20.degree. 
K.) higher than that of the hitherto known superconductive material. 
The invention will be explained by referring to examples in detail, but the 
invention is not limited to these examples. 
EXAMPLE 1 
In said formula, X is 0.075; a lanthanum-barium-copper-oxygen composition 
was prepared. 
The calculated amounts of each powder of special grade reagent of La.sub.2 
O.sub.3, BaCO.sub.3 and CuO were mixed in an agate mortar, reacted at 
1,100.degree. C. in a crucible and further reacted in the air for 10 
hours, and ground, and further reacted in the air atmosphere at 
1,100.degree. C. for 10 hours. 
Thereafter, the mixed powder was ground again, pressed with a pressure of 
about 1,000 kg/cm.sup.2 to make pellets, and sintered in a furnace at a 
temperature of 1,000.degree. C. for 5 hours. 
It was confirmed by X-ray diffraction that each sintered material had a 
K.sub.2 NiF.sub.4 type crystal structure. 
The superconductive transition temperature of the specimen was examined by 
electrical resistance measurement, and as a result, it was found that the 
present specimen causes superconductive transition at 35.degree. K. to 
lower electrical resistance, and the condition of electric resistance 
completely disappearing begins at less than 25.degree. K. 
FIG. 1 is a graph showing the result after the temperature change of 
resistivity of the specimen (La.sub.0.925 Ba.sub.0.075).sub.2 CuO.sub.4 
according to the invention. The abscissa shows an absolute temperature T 
(.degree.K.) and the ordinate shows an electrical resistivity 
.rho.(10.sup.-3 .OMEGA..cm). In FIG. 1, "." show the use platinum for 
electrode, and ".DELTA." show the use gold therefor. It is confirmed from 
FIG. 1 that the data of ".DELTA." shows that the superconductive 
transition starting temperature is 35.degree. K. and the electrical 
resistance become zero at the absolute temperature of 25.degree. K., and 
the present specimen is a superconductor. 
EXAMPLE 2 
The calculated amounts of each powder of special grade reagent of La.sub.2 
(CO.sub.3).sub.2, BaCO.sub.3 and CuO were prepared a 
lanthanum-barium-copper-oxygen composition which X is 0.05 and 0.15 in the 
above formula by the same method as in Example 1. The composition was 
fired in the same manner as in Example 1. 
It was confirmed by X-ray diffraction that each sintered material has a 
K.sub.2 NiF.sub.4 type crystal structure. 
The superconductivity transition starting temperature T.sub.c and the 
transition temperature width .DELTA.T.sub.c of each sintered material were 
measured and the result is shown in Table 1. 
TABLE 1 
______________________________________ 
Superconductivity 
transition Transition 
starting temperature 
X temperature T.sub.c 
width .DELTA.T.sub.c 
______________________________________ 
0.05 17.degree. K. 
4.degree. K. 
0.15 20.degree. K. 
5.degree. K. 
______________________________________ 
The superconductive material according to the present invention has the 
following advantages. (1) Because the superconductive material according 
to the present invention has a high critical temperature, cooling is far 
easier than the prior material. (2) For the above reason, the 
superconductive material according to the present invention can widely be 
applied to large-sized electronic apparatuses. (3) Even when the material 
is heated at a high temperature of about 1,100.degree. C. in the air, the 
material is stable and freely usable in the manufacture of superconductive 
wire and electronics elements to a great extent. (4) Since the 
superconductive material according to the present invention is of a 
ceramics superconductor, its electric, magnetic and mechanical properties 
are considered to be different from those of the prior metallic 
superconductors, and as a result, when it is applied as Josephson elements 
and superconductive quantum interference elements, these properties are 
flexibly increased. (5) This material can be used under cooling at the 
temperature of the boiling point: 20.3.degree. K. of liquid hydrogen. 
EXPERIMENT I 
The inventors conducted the following experiment to prove the occurrence of 
high T.sub.c superconductivity in the oxide composed of La, Ba and Cu 
cations from measurements of magnetic susceptibility. 
The measurements were made using a SQUID (superconducting quantum 
interference device) magnetometer (SHE, model 805) on polycrystalline 
powder specimens at low magnetic fields below 10 Oe to investigate the 
Meissner effect associated with superconductivity. The temperatures were 
calibrated using standard superconductors Pb, Nb and Nb.sub.3 Sn. 
The starting materials were prepared by two methods: one was a mixing of 
La.sub.2 O.sub.3, BaCO.sub.3 and CuO and the other a coprecipitation from 
solutions of La-, Ba- and Cu-acetates with oxalic acid in appropriate 
cation ratios, La:Ba:Cu=(1-x):x:1, in both cases. The starting material 
with the composition of x=0.15 was reacted at 900.degree. C. in air. The 
analysis of X-ray powder diffraction indicated mainly perovskite 
structure, probably based on (LaBa)CuO.sub.3, mixed with layer-type 
perovskite, possibly (LaBa).sub.2 CuO.sub.4, and a small amount of other 
unidentified phases. 
The sintered powder was then reduced for 30 minutes at 900.degree. C. in an 
Ar-O.sub.2 atmosphere. As compared with the initial powder, the amount of 
the perovskite phase was considerably reduced relative to the layer-type 
phase, but the annealed sample is still a mixture of these dominant 
phases. 
Temperature dependences of the susceptibility on typical samples are shown 
in FIGS. 2 and 3 a large magnetic susceptibility of more than (-3 to 
-10).times.10.sup.-4 emu/g was observed in both samples. The diamagnetic 
response increases with decreasing temperature, still increasing even at 
5.degree. K. The magnitude of diamagnetic susceptibility corresponds to 
about 10% of the perfect diamagnetism (X=-1/4.pi.) for the sample shown in 
FIG. 2. Therefore, about 10% of the total volume of the sample is 
considered to be in the superconducting state at 5.degree. K. This 
indicates that the observed superconductivity is probably bulk in nature, 
although the superconducting state is not formed over the whole specimen. 
For the sample shown in FIG. 3, the onset of superconductivity is observed 
at still higher temperatures. Even at 29 K the diamagnetic susceptibility 
is significantly larger than ordinary diamagnetic contributions, such as a 
core diamagnetism or a Landau-Peierls diamagnetism. The result is 
reproducible, exhibiting exactly the same magnetic response after several 
heat-cycles. 
The broad transition observed is probably due to nonhomogeneity of the 
sample, as frequently observed in other oxide superconductors. Thus the 
result of the magnetic susceptibility indicates that a high-T.sub.c 
superconducting state is certainly realized in a part of the sample. 
Efforts are under way to identify the superconducting phase and to 
synthesize the material composed of a single phase, as well as to 
investigate the dependences of T.sub.c on various parameters, Ba 
composition, oxygen deficiency and annealing conditions. 
EXPERIMENT II 
In the preceding Experiment I, we observed high-T.sub.c superconductivity 
with T.sub.c near 30.degree. K. in the La-Ba-Cu oxides. This material was 
suggested by Bednorz and Muller to be a high T.sub.c superconductor as a 
result of their resistivity measurement. In both cases, the starting 
material was prepared to have a composition (La.Ba)CuO.sub.3 and the 
products were annealed in low oxygen pressure. The samples were apparently 
composed of more than two phases; dominantly the perovskite and the 
layer-pervoskite like (K.sub.2 NiF.sub.4 -type) phase and the annealing of 
"as-prepared" powders in the cation ratios (La.Ba):Cu=1:1 in a reduced 
atmosphere results in a remarkable increase in the fraction of the K.sub.2 
NiF.sub.4 -type phase. The superconducting phase, however, has not yet 
been specified. 
In this Experiment H, the inventors conducted the following experiment to 
present evidence that the K.sub.2 NiF.sub.4 -type phase (La.Ba).sub.2 
CuO.sub.4-y, is responsible for the high-T.sub.c superconductivity. We 
prepared the samples in the cation ratios (La.Ba):Cu=2:1 which were 
annealed at various oxygen pressures. The volume fraction of the 
superconducting phase was estimated by measuring the diamagnetic 
susceptibility. It was found that the increase in the fraction of K.sub.2 
NiF.sub.4 -type phase estimated from X-ray analysis corresponded to the 
increase in the volume fraction of the superconducting phase. This 
suggests that the high-T.sub.c superconductivity may be realized in the 
K.sub.2 NiF.sub.4 -type phase, probably (La.Ba).sub.2 CuO.sub.4-y. 
In order to confirm the occurrence of superconductivity in the K.sub.2 
NiF.sub.4 -type phase, the inventors have synthesized a single phase 
(La.Ba).sub.2 CuO.sub.4-y. The powdered specimen was prepared by reacting 
the mixture of La.sub.2 O.sub.3, BaCO.sub.3 and CuO in cation ratios 
La:Ba:Cu=2(1-x):2x:1 (x=0, 0.05, 0,075, 0.10 and 0.15) at 1,100.degree. C. 
in air for 24 hours. The X-ray powder diffraction pattern of the prepared 
sample is shown in FIG. 4, which indicates that almost a single phase with 
K.sub.2 NiF.sub.4 -type structure is synthesized. No trace of the 
perovskite phase nor any other phase is observed. 
The result of the magnetic susceptibility measurement on the K.sub.2 
NiF.sub.4 -type single phase with x=0.075 is shown in FIG. 5. A rather 
steep superconducting transition as compared with the previous result is 
seen at around 29.degree. K. About 30% of the total volume is estimated to 
be in the superconducting state at 5.degree. K., that is much larger than 
the previous result. Superconductivity was observed also for the samples 
with x=0.05, 0.10 and 0.15 but not for x=0. The effect of the annealing in 
the reduced atmosphere was examined, and it was found that the annealing 
results in the disappearance of superconductivity. 
Inventors have also made a resistivity measurement on the sintered pellet 
of the powder with x=0.075 as shown in FIG. 6. The measurement was made by 
the usual four-probe method with gold-evaporated electrodes. The 
resistivity gradually decreases with lowering temperature and then drops 
abruptly at around 32.degree. K. The resistivity below 22.degree. K. is 
smaller than the limit of our instrumental resolution. Combined with the 
magnetic susceptibility data, it can be said that the sample is actually 
superconducting. 
We conclude from these results that high-T.sub.c superconductivity is 
realized in the K.sub.2 NiF.sub.4 -type phase, (La.Ba).sub.2 CuO.sub.4-y. 
In this structure, the substitution of La with Ba will lead to the 
Cu-mixed-valence state, Cu.sup.2+ -Cu.sup.3+. On the other hand, the 
reduction of the oxidation degree will bring Cu.sup.3+ back to Cu.sup.2+, 
destroying the mixed-valence state. Thus, the mixed-valence seems to play 
an important role in the present high-T.sub.c superconductor as in the 
case of LiTi.sub.2 O.sub.4 and BaPb.sub.1-x Bi.sub.x O.sub.3 which is 
known in the prior art. 
The broad transition and the incomplete Meissner effect are probably due to 
the inhomogeneity in the sample, such as the fluctuation of the La/Ba 
composition or the oxygen deficiency. We concluded in Experiment II that 
attempts should be continued to improve the homogeneity as well as to 
obtain single crystals. 
EXPERIMENT III 
We have succeeded in synthesizing material of a single K.sub.2 NiF.sub.4 
-type phase which exhibited a sharp superconducting transition near 
30.degree. K. in both susceptibility and resistivity measurement. Notably 
the resistivity was substantially zero below 22.degree. K. 
Here we report the result of resistivity measurements in details. The dc 
resistivity measurements were performed on hot-pressed specimens of 
(La.sub.1-x Ba.sub.x).sub.2 CuO.sub.4 in the usual four-probe method. The 
starting material was prepared in the same procedure as described in the 
preceding Experiment II. A sintered pellet was hot pressed at 
1,000.degree. C. under a pressure of 300 kbar for 3 hours. The specimens 
used for the resistivity measurements were cut into rectangular shape, 
typically 1.times.2.times.10 mm.sup.3. The electrodes were attached on 
these specimens in two different ways; evaporation of gold films or 
application of platinum paste followed by heat treatment at 1,100.degree. 
C. in O.sub.2 atmosphere. 
FIG. 7 shows the temperature dependence of two samples cut from the same 
hot-pressed ingot with Ba composition x=0.075 but with different 
electrodes. Temperature was monitored by a calibrated germanium 
thermometer (Lake Shore Model No. GR-200A-1000) as well as by an Au(Fe)-Ag 
thermocouple. They were attached in the close vicinity of the samples. The 
temperature was stabilized by making a balance between heating power and 
the pressure of helium exchange gas. 
As is seen more clearly in FIG. 1, both samples exhibit a sharp drop in the 
resistivity around 30.degree. K., the onset of the drop being at 
temperature as high as 35.degree. K. for the sample with Au electrodes. 
These data were taken with current density j=0.13 A/cm.sup.2. The 
resistivity was found to be sensitive to the current density in the 
temperature range between the onset and 20.degree. K. This is due either 
to the percolative conduction in the granular specimens or to the 
Joule-heating at the contacts with the electrodes where the contact 
resistance was as high as 20 .OMEGA. in the case of Au electrodes. The 
low-T tail of the superconducting transition seen in both samples might 
arise from such effects. Nevertheless, the "zero-resistance" state was 
observed, within our instrumental resolution, below 25.degree. K. in both 
samples with the above current density. This temperature already exceeds 
the highest onset T.sub.c in Nb.sub.3 Ge (23.7.degree. K.). 
The normal-state resistivity is higher, by a factor of about 4 at 
50.degree. K., for the sample with gold electrodes than for the one with 
Pt-paste electrodes, showing a gradual increase below 90.degree. K. Such 
increase was also observed more dominantly in the samples of Bednorz and 
Muller and in our samples in the earlier stage. At present it is not clear 
whether this arises from the disorder-effect, i.e. the effect of 
inhomogeneity in the sample, such as the presence of secondary phases or 
resistive barriers on the surface of each grain. It cannot be ruled out 
either that it is intrinsic in origin, since the relevant electronic 
states in this material might be strongly localized Cu-3d-derived 
orbitals. 
The sample with Pt electrodes, on the other hand, does not show a low-T 
increase in resistivity. Certainly the heat-treatment on attaching 
Pt-paste electrodes affected the property of the sample. In this case the 
normal-state resistivity increases almost linearly, like usual metals, as 
the temperature is raised. The residual resistivity ratio is about 3, as 
defined by R(300K)/R(40K). The T-linear term is also seen in the sample 
with Au electrodes where it seems to compete with the low-T resistivity 
rise, giving rise to a minimum at around 90.degree. K. The presence of an 
obvious T-linear term is in contrast to the case of BaPb.sub.1-x Bi.sub.x 
O.sub.3. In the latter material the resistivity is either almost 
T-independent or decreasing with increase of T over a wide temperature 
region, so that the mean free path of the conduction electrons is 
estimated to be of the order of lattice spacing. Therefore, it is 
estimated that the mean free path of La-Ba-Cu oxide might be longer as 
compared with that in BaPb.sub.1-x BixO.sub.3. 
So far none of the oxide materials with K.sub.2 NiF.sub.4 -type structure 
have not shown a truly metallic behavior over a wide temperature range. 
La.sub.2 CuO.sub.4 was once considered to be metallic. However, the recent 
result indicates a semiconducting T-dependence of the resistivity below 
300.degree. K. Thus, as far as we know, (La.sub.1-x Ba.sub.x).sub.2 
CuO.sub.4 is the first truly metallic material with K.sub.2 NiF.sub.4 
-type structure as shown in FIG. 8. 
We are now interested in the effects of magnetic field on the 
superconductivity of the present hight-T.sub.c material. It was already 
confirmed by the magnetic susceptibility measurements that this material 
is a type II-superconductor, the magnetization varying linearly with field 
and showing a maximum at around 100 Oe at 4.5.degree. K. As a preliminary 
work, we have measured resistivity by applying a field up to 60 kOe. The 
superconducting state was not destroyed even at 60 kOe for the 
temperatures lower than 10.degree. K. Measurements of the critical field 
using high magnetic field of 400 kOe are now under way. 
Although the invention has been described with a certain degree of 
particularity, it is understood that the present disclosure has been made 
only by way of example and that numerous changes in details of 
construction and the combination and arrangement of parts may be resorted 
to without departing from the scope of the invention as hereinafter 
claimed.