Compositions – Electrically conductive or emissive compositions – Metal compound containing
Reexamination Certificate
2003-05-12
2004-11-02
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Metal compound containing
C501S123000, C117S947000
Reexamination Certificate
active
06811726
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to high critical temperature, ceramic, oxide superconductors comprising one transition metal, one metal of Group 2 of the Periodic Table and one metal of Group 1 of the Periodic Table. A key distinctive feature of the products of this invention is the hexagonal crystal symmetry of their structure that comprises highly covalent oxide chains containing the transition metal. The chains are parallel to the c axis. More specifically the superconductors of this invention comprise the transition metals nickel or cobalt. They may be prepared in powder form, in polycrystalline compacts, in dense polycrystalline aggregates and in single crystals.
The invention also relates to processes to prepare the superconductors in each of the above mentioned forms.
In another specific embodiment the invention relates to precursors of the superconductors and their preparation.
Superconductors are useful materials that find applications in magnetic, electric and electronic applications such as high flux electromagnets, magnetic instrumentation, transmission lines, levitation phenomena, storage of electrical energy, etc.
2 Description of the Previously Published Art
2.1 High Critical Temperature Ceramic Oxide Superconductors.
Superconductivity is the property of materials that exhibit zero electrical resistance when cooled to or below a temperature called the critical temperature (T
c
). It was discovered by H. Kamerlingh Onnes in 1911 using the extremely low temperature of liquid helium. For many decades superconductivity remained a laboratory curiosity with no extensive practical applications because of the very low temperatures required to achieve it in metals, metallic alloys and binary metallic compounds.
In the 1970's superconductivity was observed in perovskite metal oxides structures. By 1975 A. W. Sleight and coworkers found that in BaPbO
3
(barium plumbate) which is a perovskite-type oxide, 5 to 30% substitution of Bi for Pb induces superconductivity. These findings did not receive much attention from the scientific community possibly because of their low T
c
.
In 1986 a major breakthrough was achieved by T. G. Bednorz and K. A. Müller (
Z. Phys
. 1986, B64, 189) with the discovery of a complex ceramic oxide that becomes a superconductor at about 30 K. The material was a complex oxide of lanthanum, barium and copper, with perovskite-related symmetry (tetragonal with space group l4/mmm) and having a composition of
La
1.85
Ba
0.15
CuO
4
Formula 1
This impressive result was immediately followed up by much research throughout the world, and by 1987 the isostructural superconductor La
1.85
Sr
0.15
CuO
4
with a T
c
of about 37 K was prepared by J. M. Tarascon et al. (
Science
, 1987, 235, 1373). Also that year C. W. Chu et al (
Phys. Rev. Lett
. 58, 1891-1894 (1987)) prepared the superconductor YBa
2
Cu
3
O
7
(called 1-2-3 because of the atomic ratios of the metals) with a T
c
of about 93 K, which is higher than the boiling point of liquid nitrogen. This development was both a major scientific and a technological breakthrough because superconductivity was achieved for the first time using a practical and readily available coolant that opened a wide field of applications.
During the last 15 years, scientists have made many variations of, and advances over the original material, with increases in T
c
to about 128 K (Tl
2
Ca
2
Ba
2
Cu
3
O
10
).
In order to clearly describe materials and avoid confusion, the specific meaning of certain terms used in this application, will be defined next.
The term “parent” is used to refer to oxide compounds consisting chemically of a transition metal oxide and an ionic metal oxide such as for example La
2
CuO
4
.
The term “main cation” refers to the ionic metal cation in the parent material such as for example La
3+
.
The term “doping” and related terms such as “dopant”, “doped”, etc., refer to the replacement in the crystal structure, on a one for one atomic basis, of part of the main cation, by cations of different but fix valence for example Ba
2+
in La
1.85
Ba
0.15
CuO
4
.
The new material discovered by Bednorz and Müller lead to the new superconductor class referred to as high-T
c
ceramic, oxide superconductors. The series of complex oxides prepared in their work may be visualized as descendants from the parent oxide lanthanum cuprate (La
2
CuO
4
) after doping with barium. The series may be represented by the variable formula:
La
(2−x)
Ba
x
CuO
4
Formula 2
in which x ranges from about 0.05 to about 0.25. In this range the doping, either with Ba or Sr does not affect the tetragonal symmetry (space group l4/mmm) of the parent lanthanum cuprate, although it slightly changes the unit cell dimensions. The most significant discovery was that starting in the range of about x=0.05 to about x=0.10 the doped materials became low T
c
superconductors. As x increased so did T
c
up to x=0.15. At this point the T
c
became about 30 K. Beyond x=0.15 the materials, while remaining superconductors, decreased in T
c
.
Detailed studies of the crystal structure revealed that all the superconductors of the barium series, as well as the isostructural members of the strontium series consist of alternating charged layers of opposite sign. One type of layer is a covalent square planar array with a composition of [CuO
4/2
] or [CuO
2
] located next to parallel ionic layers with a composition of [La
(2−x)
Ba
x
O
2
]. The average charge density of the ionic layer is readily calculable from the composition and valence of its constituent ions. It is +(2−x) per [La
(2−x)
Ba
x
O
2
]. For the [CuO
2
] covalent layer the average charge density must become −(2−x) in order to maintain overall crystal neutrality.
It is interesting to note that the charge density of the [CuO
2
] covalent layer goes from −2 for the parent material La
2
CuO
4
to −(2−x) for any of the doped members of the series and that the [CuO
2
] layer undergoes oxidation as x increases. In this progression the parent material undergoes a transition from insulator to superconductors. The resulting T
c
's are in some unknown manner a function of the degree of oxidation of the covalent [CuO
2
] plane.
Work to better understand these observations lead to some fundamental questions regarding the oxidation of the [CuO
2
] layers.
Does it result in the oxidation of Cu
2+
to Cu
3+
or in the oxidation of O
2−
to O
1−
to form electron holes?
Is it possible that both elements undergo oxidation and that there exist an equilibrium between the four possible valences? (Cu
2+
, Cu
3+
, O
2−
and O
1−
).
In the latter case, which one is the predominant oxidized element, Cu
3+
or O
1−
?
This matter was studied using Hall effect measurements and other techniques that showed that the great majority of the electrical carriers are O
1−
holes.
Regarding electrical conductivity, the present accepted view is that the holes move in the two-dimensional [CuO
2
] layers.
These data are in sharp contrast with the behavior of earlier metallic superconductors in which electrons carried the current in three, not two dimensions.
On another matter, the negative results of hundreds of studies done all over the world over the last 15 years or so using transition metals other than copper lead to the empirical “inference” that only ceramic materials comprising copper oxide yield high T
c
superconductors.
Another interesting point worth noting on the work of the last 15 years or so is that during that period no high-T
c
, “one-dimensional” ceramic oxide superconductor has been reported.
From a theoretical point of view, the fundamental reasons for the cuprates to become high-T
c
superconductors are not as yet fully understood. The BCS (Bardeen, Cooper and Schrieffer) the
Cabic Edward J.
Kopec Mark
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