Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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APPLICATION FOR PATENT
Inventor: CHING-WU CHU
Title: SUPERCONDUCTIVITY IN AN OXIDE
COMPOUND SYSTEM WITHOUT RARE EARTH
Background of the Invention
This invention relates to superconducting
compositions, i.e., compositions offering no electrical
resistance at a temperature below a critical temperature;
to processes for their production and to methods for their
use.
Superconductivity was discovered in 1911.
Historically, the first observed and most distinctive
property of a superconductive material is the near total
loss of electrical resistance by the material when at or
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below a critical temperature that is a characteristic of
the material. This critical temperature is referred to as
the superconducting transition temperature of the
material, Tc.
The history of research into the superconductivity of
specific materials began with the discovery in 1911 that
mercury superconducts at a transition temperature of about
4 K. In the late 1920's, NbC was found to superconduct
at a higher temperature, namely up to about 10.5 K.
Thereafter other compounds and alloys of Nb were examined
and various Nb compositions were discovered with
progressively, but only slightly higher, superconducting
transition temperatures. In the early 1940's NbN was
observed with a transition temperature of about 14 K;
Nb3Sn was reported in the early 1950's; Nb3(Al-Ge) was
reported in the late 1960's; and Nb3Ge was reported in the
early 1970's to have a transition temperature of up to
21 K. Careful optimization of Nb3Ge thin films led to an
increase of the critical temperature for such material up
to 23.3 K. While this work led to progress the maximum
temperature at which superconductivity could occur was
raised to only 23.3 K since research started
three-quarters of a century ago. The existing theories
explained the superconductivity of these materials, but
did not predict superconductivity of higher than 40 K.
Significant progress in finding materials which
superconduct at higher transition temperatures than that
of Nb3Ge thin films was not made until 1986.
In 1986, specially prepared coprecipitated and heat
treated mixtures of lanthanum, barium, copper and oxygen,
that have an abrupt decrease in resistivity "reminiscent
of the onset of percolative superconductivity" were
reported by J.G. Bednorz and R.A. Muller, "Possible High
Tc Superconductivity In The Ba-La-Cu-O System,"
Z.Phys.B.-Condensed Matter, 64, pp. 189-193 (1986). Under
atmospheric pressure conditions, the abrupt change in
resistivity for these compositions -- i.e., that
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temperature at which a portion of the material begins to
show properties reminiscent of percolative superconductivity
-- were reported to approach the 30 K range. The authors
refer to this phenomenon as a "possible" case of
superconductivity. The compositions reported by Bednorz et
al to have such properties at a temperature as high as 30 K
comprise La5_xBaxCu5O5(3_Y) where X = 0.75 to 1 and Y > 0.
Superconductivity is a potentially very useful
phenomenon. It reduces heat losses to zero in electrical
power transmission, magnets, leviated monorail trains and
many other modern devices. However, superconductivity of a
material occurs only at very low temperatures. Originally,
and until the inventions outlined herein, liquid helium was
the required coolant to provide the conditions necessary for
superconductivity to occur.
It would be desirable to produce a superconducting
composition that has a transition temperature which exceeds
those of superconducting compositions previously described.
It would be particularly desirable to develop a
superconducting composition that has a TC of 77 K or
higher. Such a composition would enable the use of liquid
nitrogen instead of liquid helium to cool the
superconducting equipment and would dramatically decrease
the cost of operating and insulating superconducting
equipment and material.
The prior art teaches a mixed phase oxide prepared
according to a nominal formulation of Y1.2Bao.8Cu0Y ( "y" is a
number from 2 to 4) that superconducts at a temperature of
80 K or greater. The mixed phase oxide can comprise a
green and black phase with the black phase being the phase
responsible for the high temperature (i.e., TC = 77 K or
greater) superconduction and being of the formula YBa2Cu3O6+a
(a is a number between 0.1 to 1.0).
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A high temperature superconducting composition of the
general formula LM2A30 6+a may be prepared wherein "L" is
scandium, yttrium, a rare earth element (atomic numbers 51
to 71) or mixtures thereof; "M" is barium, strontium,
calcium, magnesium, mercury or mixtures thereof; "A" is
copper, bismuth, titanium, tungsten, zirconium, tantalum,
niobium, vanadium or mixtures thereof; and "M" is preferably
barium or strontium and "A" is preferably copper.
Confirmation by other workers in the field of species
LBa2Cu3O6+a as a high temperature superconductive composition
has been widespread since disclosure of the high temperature
superconducting mixed phase oxide prepared to the nominal
formula Y,.2Bao.8CuOY.
The objective of a superconducting composition having
TC of 77 K or higher has been achieved with the LM2A3O6+a
compositions as mentioned above. It is, however, still
desirable to develop other compositions which do not require
the presence of a rare earth element and which will
superconduct at a temperature of 77 K or higher. The prior
art acknowledges the possibility of such materials in the
context of the general formula [L1_XMx]aAbOY wherein the value
of "x" may be 1.0 maximum, in which case the rare earth
element "L" is removed and the general formula becomes
MaAb0Y.
Michel et al has recently described the observance
of superconductivity in a Bi2Sr2Cu2O7 composition up to a
TC of 22 K. The TC of such composition was reported to
be highly sensitive to impurities. Additionally, a brief
news report of an observation of superconductivity in a
Bi-Sr-Ca-Cu-O system between 75 and 120 K recently
appeared in Japan Economic News on January 22, 1988, but
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no details were given concerning the composition, processing
or structure of the material.
Summary of the Invention
In accordance with one aspect of the invention there is
provided a material which is superconductive at a
temperature of 77 K or higher, said material comprising a
multiphase oxide of nominal composition M*aA*bOy wherein M*
is a mixture of divalent alkaline earth metals selected from
the group consisting of Ba, Sr, Ca and Mg wherein the ratio
of the alkaline earth metal of larger atomic radius to the
alkaline earth metal of smaller atomic radius is from about
1:1 to about 3:1; A* is a mixture of Cu and a trivalent
metal selected from the group consisting of Bi and Ti
wherein the molar ratio of Cu to said trivalent metal is
from about 1:1 to about 3:1; "a" is 1 to 2; "b" is 1; and
"y" is 2 to 4.
In accordance with another aspect of the invention
there is provided a material which is superconductive at a
temperature of 77 K or higher, said material comprising a
multiphase oxide of nominal composition BiCaSrCuO2y wherein
"y" is 2 to 4 and having a sufficient quantity of a
crystalline phase composition of a formula Bi2CaSr2Cu2Og
wherein "g" is a value from about 8 to about 9 which
provides said crystalline phase composition with zero
electrical resistance at a temperature of 77 K or higher to
cause the material to exhibit zero electrical resistance at
a temperature of 77 K or higher.
In accordance with yet another aspect of the invention
there is provided an oxide composition of nominal formula
TdM*eCufOg wherein "T" is Bi or TI; "M*" is a mixture of
divalent alkaline earth metals selected from the group
consisting of Ba, Sr, Ca and Mg wherein the ratio of the
alkaline earth metal of larger atomic radius to the alkaline
earth metal of smaller atomic radius is from about 1 to
about 3; "d" is a number from about 1 to about 3; "e" is a
number from about 1 to about 6; "f" is a number from about 1
to about 6; and "g" is a number from about 0.5 (3d + 2e +
2f) to about 0.5 (3d + 2e + 3f) that provides the oxide
composition with zero electrical resistance at a temperature
of 77 K or higher.
In accordance with yet another aspect of the invention
there is provided a crystalline phase composition comprising
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cations of Bi, Ca, Sr, and Cu approximating the ratio of
2:1:2:2 for Bi:Ca:Sr:Cu and which exhibits zero electrical
resistance at a temperature of 77 K or higher.
In accordance with yet another aspect of the invention
there is provided a superconducting oxide composition of
nominal formula TaM*eCufO9 wherein "T" is Bi or TI ; "M*" is a
mixture of divalent alkaline earth metals selected from the
group consisting of Ba, Sr, Ca and Mg wherein the ratio of
the alkaline earth metal of larger atomic radius (M') to the
alkaline earth metal of smaller atomic radius (MS) is from
about 1 to about 3; "d" is a number from about 1 to about 3;
"e" is a number from about 1 to abut 6; "f" is a number from
about 1 to about 6; and "g" is a number from about 0.5
(3d + 2e + 2f) to about 0.5 (3d + 2e + 3f) that provides the
oxide composition with zero electrical resistance at a
temperature of 77 K or higher, wherein said composition is
made by a process comprising the steps of: compressing a
mixture of solid powdered compounds comprising: (a) T203,
(b) MLCO3 or MLO, (c) M8C03 or MSO, and (d) CuO in proportions
appropriate to yield said formula; heating the compressed
powder mixture to a temperature of from about 800 C to about
910 C fro a time sufficient to complete the solid state
reaction; and quenching said reacted compressed mixture to
room temperature.
More specifically, an embodiment of the present
invention describes a superconducting composition comprising
a metal oxide of the formula TaM*eCufOg wherein "T" is a
trivalent transition metal such as Bi, Tl, In, Sb, or
mixtures thereof; "M*" is a mixture of alkaline earth metals
such as Sr and Ca, Sr and Mg, and Ca and Mg in ratio of the
alkaline earth metal of larger atomic radius (ML) to the
alkaline earth metal of smaller atomic radius (MS) of from
about 1:1 to about 3:1; "d" is a number from about 1 to
about 3; "e" is a number from about 1 to about 6; "f" is a
number from about 1 to about 6; "g" is a number between from
about (3d + 2e + 2f)/2 to about (3d + 2e + 3f)/2 that
provides the metal oxide with zero electrical resistance at
a temperature of 77 K or higher. Preferably "T" is bismuth;
"M*" is Ca and Sr at a ratio of 1:2; "d" is 2; "e" is 3; "f"
is 2 and "g" is a number between about 8 to about 9.
The trivalent element "T" and the selection of an
alkaline earth metal pair in appropriate ratio in view of
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their atomic radius to correspond the alkaline earth metal
pair to the atomic radius size of the trivalent element "T"
employed, is crucial to obtaining a metal oxide composition
that will crystallize to a form favorable to high
temperature superconduction. A crystalline form in which
Cu-atoms are in planar configuration is required for high
Tc. The crystalline form that provides for high T, is a
perovskite related structure with substantial deviations
from the ideal perovskite arrangement of metal atoms. Hence
in the high Tc species of Bi2Ca1Sr2Cu2Og (g = 8 to 9) bismuth
appears to be concentrated in layers similar to the Bi2O2
slabs of phases such as BaBi4Ti4O15 discussed by B.
Aurivailius, Arkiv Kemi 1, 499 (1950). For the species
Bi2CaSr2Cu2Og (g = 8 to 9) the weak electron density
associated with
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every fourth layer of the crystalline structure suggests
interlayers region of weak bonding. In the new structure
that provides Tc > 77 K, copper-oxygen layers appear to be
continuous over hundreds of unit cells.
Brief Description of the Drawings
Fig. 1 illustrates DTA, TGA, and DTG results for
BCSCO-a and -b in air with compositions Bi:Ca:Sr:Cu being
1:1:1:1 and 1:1:1:2. The scan speed for temperature is
20 /min. TM is the meling point.
Fig. 2 illustrates the temperature dependence of
resistance for Bi-Ca-Sr-Cu oxide superconductor
compositions prepared of nominal formula 1:1:1:1
(BCSCO-a); 1:1:1:2 (BCSCO-b); and 1:1:1:3 (BCSCO-c).
Fig. 3 illustrates the temperature dependence of
magnetization for the Bi-Ca-Sr-Cu oxide superconductor
species BCSCO-a and BCSCO-c.
Fig. 4 illustrates selected-area electron diffraction
pattern of the Bi-Ca-Sr-Cu oxide superconductor showing
hk0 diffraction spots. Strong spots correspond to
0
2.7 x 2.7 A subcell, while superlattice reflections along
0
a' and b' indicate spacings of 5.4 and 27.2 A
respectively.
Fig. 5 illustrates selected-area electron diffraction
pattern of the 002 diffraction row, which is streaked but
show's a strong 15.4 A periodicity. The 004 difraction is
0
indicated. Also illustrated is a high-resolution image
0
taken parallel to the layers shows the 15.4 A spacing,
0
with subspacings of 3.8 A. The contrast of these layers
differs, suggesting a possible ABAC-ABAC type stacking of
perovskite units. A structural defect (arrowed) may
correspond to a Bi-free region of Ca-Sr-Cu perovskite.
Fig. 6 illustrates the selected-area OkQ electron
diffraction pattern as characterized by an A-centered
0
27 x 31 A lattice. Also illustrated is a high-resolution
image of the (100) plane revealing numerous stacking
faults and defects as well as the A-centered layered
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structure. Though most of the structure appears to be
orthogonal, locally inclined blocks may indicate a fine-
scale twinning or may represent "monoclinic" regions of
related but different structure.
Fig. 7 illustrates X-ray results for BCSCO-b
synthesized at different temperatures: a - 820 C,
b - 864 C, c - 880 C, d - the superconducting phase. Curve
e is for BCSCO-c with composition ratio of 1:22:14:6.2
synthesized at 850 C.
Fig. 8 illustrates resistance (R) vs. temperature (T)
for BCSCO-b synthesized at different temperatures:
a - 820 C, b - 864 C, c - 880 C. Curve d is for BCSCO-c.
Fig. 9 illustrates magnetization (M) vs. T for BCSCO-b
synthesized at different temperatures: a - 820 C,
b - 864 C, c - 880 C. Curve d is for BCSCO-c.
Fig. 10 illustrates R - T for BCSCO-b in different
magnetic fields.
Detailed Description of the Preferred Embodiments
It is known to make superconducting compositions of
the formula [L1-XMX]aAbOY wherein "L" is scandium, yttrium,
lanthanum, cerium, praseodymium, neodymium, samarium,
europium, gadolinium, terbium, dysprosium, holminum, erbium,
thulium, ytterbium, or lutetium, and preferably "L" is
yttrium, lanthanum, neodynium, samarium, europium,
gadolinium, erbium or lutetium; "A" is copper, bismuth,
titanium, tungsten, zirconium, tantalum, niobium, vanadium
or mixtures thereof and "A" preferably is copper; "M" is
barium, strontium, calcium, magnesium, mercury or mixtures
thereof and "M" is preferably barium or strontium; and "a"
is 1 to 2; "b" is 1; and "y" is about 2 to about 4; "x" is
0.01 to 1.0 maximum and "x" is preferably from about 0.60 to
about 0.90 and most preferably from about 0.65 to about 0.80
and when "a" is 2 "x" is preferably from about 0.01 to about
0.5 and most preferably from about 0.07 to about 0.5.
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Production of an oxide complex of Y-Ba-Cu-O wherein
x = 0.4 and a = 2 produced a multiphase oxide material of
nominal formula Y1.2Bao.8CuOy which exhibited zero
electrical resistance at a temperature of 77 K or greater.
The phase within that material determined to be the
crystalline phase composition responsible for such high
temperature superconduction was isolated and identified to
be YBa2Cu3O6+8; wherein 8 is a value between 0.1 to 1.0
that provides the crystalline phase composition with zero
electrical resistance at a temperature of 77 K or higher.
Accordingly, a new class of compostions of formula
LM2A3O6+0; preferably LM2Cu3O6+8; was disclosed which had
zero electrical resistance at a temperature of 77 K or
higher.
Another species of materials within the formula
[L1-xMx]aAb0y has been found which has zero electrical
resistance at a temperature of 77 K or higher. The
species comprises that class of compositions wherein "x"
equals 1, to yield a formula of
M*aA*bOy
wherein the M* constituent comprises a mixture of divalent
alkaline earth metals and the A* constituent comprises a
mixture of copper with at least one other "A", preferably
bismuth. In a preferred composition the M* constituent is
a 1:1 mixture of Ca and Sr, the A* constituent is a 1:1
mixture of Cu and Bi and "a" is 1. Accordingly, an oxide
material prepared to a nominal formula of
(Cao=sSro=5)1(Cuo=sBio=5)iOy yields a multiphase material
which exhibits zero electrical resistance at a temperature
of 77 K or higher. The material does not contain a rare
earth metal. In this regard it is believed that bismuth,
a trivalent element, serves a similar function to that of
a trivalent rare earth with regards to creating a
perovskite related crystalline form favorable to the
occurrance of high temperature (i.e., Tc >_77 K)
superconduction. Hence, for convenience the nominal
formulation may be rewritten as follows:
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Bi1CaiSrlCu1O2y (1:1:1:1)
It has been found that the production of such high
temperature superconducting material may be facilitated by
employing copper in an excess up to about six times the
amount required to produce a material of the 1:1:1:1 nominal
formulation described above. A material produced to a
nominal composition of:
BilCalSriCu3Oh (1:1:1:3)
wherein "h" is a number between 6.5 to 8.0, is a multiphase
material which exhibits zero electrical resistance at 77 K
or higher. In the sense of the ratio of trivalent
constituent to alkaline earth constituent to copper, the
1:1:1:3 nominal composition is analogous to the LMCu3O6+a
class of high temperature superconductor materials.
As before noted, whether prepared as a 1:1:1:1 or a
1:1:1:3 nominal composition, or even as a (1:1:1:2)
Bi1Ca1Sr1Cu2O,, where "j" is between 5.5 and 6.5, each high
temperature superconducting material comprises a multiphase
oxide.
Examination of the multiphase oxide material reveals
at least four distinct phase compositions. The nominal
composition of that phase determined to be the phase
responsible for the high temperature superconduction has
been determined to be as follows:
Bi2Ca1Sr2Cu2O8+a (2:1:2:2)
where a is a value between 0.1 to 1.0 that provides the
phase composition with zero electrical resistance at a
temperature of 77 K or higher.
Trivalent metals ("T") other than bismuth may be
employed in the production of a high temperature
superconductive oxide material of the formula M*aA*bOY.
Desirably such other trivalent metals should have an atomic
radius no smaller than 1.5 A and no larger than 2.1 A.
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The trivalent element "T" and the selection of an
alkaline earth metal pair in appropriate ratio in view of
their atomic radius to correspond the alkaline earth metal
pair to the atomic radius size of the trivalent element
"T" employed, is crucial to obtaining a metal oxide
composition in which a phase will crystallize to a form
favorable to high temperature superconduction. A
crystalline form in which Cu-atoms are in planar
configuration is required for high Tc. The crystalline
form that provides for high Tc is a perovskite related
structure with substantial deviations from the ideal
perovskite arrangement of metal atoms. Hence in the high
Tc bismuth species, bismuth appears to be concentrated in
layers similar to the Bi2O2 slabs of phases such as
BaBi4Ti4O15 discussed by B. Aurivilius, Arkiv Kemi 1, 499
(1950). For the species Bi2CaSr2Cu2Og (g = 8 to 9) the
weak electron density associated with every fourth layer
of the crystalline structure suggests interlayers region
of weak bonding. High resolution transmission electron
microscopy (TEM) images show that the compound has a
four-layer structure and that the bonding between every
fourth layer is weak. In the new structure that provides
a Tc of 77 K or greater, copper-oxygen layers appear to be
continuous over hundreds of unit cells.
For convenience the phase composition within a
multiphase material prepared with a nominal composition
Bi:Ca:Sr:Cu of 1:1:1:1; 1:1:1:2; or 1:1:1:3 may be
represented as a metal oxide of the formula
TdM*eCuf0g
wherein "T" is a trivalent transition metal such as Bi,
Al, Ba, T1, In, Sb, or mixtures thereof; "M*" is a mixture
of alkaline earth metals such as Sr and Ca, Ba and Sr, Ba
and Ca, Sr and Mg, and Ca and Mg in a ratio of the
alkaline earth metal of larger atomic radius (ML) to the
alkaline earth metal of smaller atomic radius (Ms) of from
about 1:1 to about 1:3; "d" is a number from about 1 to
about 3; "e" is a number from about 1 to about 6; "f" is a
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from about (3d + 2e + 2f)/2 to about (3d + 2e + 3f)/2 that
provides the metal oxide with zero electrical resistance
at a temperature of 77 K or higher. Preferably "T" is
bismuth; "M*" is Ca and Sr at ratio of 1:2; "d" is 2; "e"
is 3; "f" is 2 and "g" is a number between about 8 to
about 9.
A method for making such TdM*eCuf0g containing
superconductive composition oxide materials, includes the
following steps, and for convenience is referred to as the
compressed powder reaction method. Selected amounts of
solid powdered compounds containing T, ML, Ms, A, and 0
are thoroughly mixed preferably by selecting appropriate
amounts of T203, MLCO3, MsCO3 (or MLO and MSO)and AO. The
thoroughly mixed powder mixture is compressed into pellets
which are thereafter reacted at a temperature between
about 800 C and about 910 C, preferably about 850 C to
about 890 C, for a time sufficient to complete the solid
state reaction. Thereafter the reacted pellets are
rapidly quenched to room temperature. Mixing is
preferably accomplished by an intensive mixer such as a
jar mill or more preferably a ball mill. Pelletization of
the oxide mixture is carried out at an applied pressure of
from about 100 to about 30,000 psi and preferably at an
applied pressure of from about 100 to about 500 psi, most
preferably at about 500 psi. Reaction of the pelletized
mixture may be conducted in air for about 5 minutes to
about 24 hours, and most preferably in a reduced oxygen
atmosphere of about 2000 p for about 5 to about 30 minutes
preferably for about 5 to about 15 minutes. Following the
completion of the reaction step the reacted pellet
composition is rapidly quenched to room temperature in
air.
Sample preparation parameters can affect the
electronic and magnetic properties of the TdM*eCufOg class
of oxide compounds drastically. It has been observed that
the formation conditions for TdM*eCufOg for different
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"T's" are different. The reaction time, the reaction
temperature, the quenching rate, the reaction atmosphere
and the compositions are all inter-related. For instance,
oxide complexes within this class can be made insulating,
partially superconducting or completely superconducting by
varying the reaction temperature and the quenching rate
while keeping the compositions unchanged. The reaction
temperature can be reduced by increasing the "d"
parameter, reducing the "f" parameter, increasing the "T"
component with greater atomic radius or doping the
composition with monovalent alkaline elements.
Generally wherein the reaction atmosphere is a
reduced oxygen atmosphere of about 2000 p the reaction may
be conducted at a lower temperature than where the
reaction is carried out under atmospheric conditions.
Under a reduced oxygen atmosphere of about 2000 p the
reaction temperature required to produce an oxide complex
having superconducting properties is from about 800 to
about 950 C and preferably from about 820 to about 910 C.
For a reaction under atmospheric conditions the
temperature required to produce superconducting properties
is from about 800 C to about 910 C preferably from about
850 C to about 890 C. For either type of reaction
atmosphere higher temperatures, up to the melting point of
the lowest melting component of the starting materials in
eutectic admixture, could be employed; however it is
sometimes preferred to use such higher reaction
temperatures since they may tend to promote the formation
of the oxide complex compared to that optimum attainable
by use of lower reaction temperatures. The optimum
reaction temperature is dependent upon the elemental
composition of the oxide complex being prepared and the
optimum reaction temperature for a particular oxide
complex may be established without undue experimentation.
Reactions carried out at temperatures significantly lower
than as discussed above generally result in an oxide
complex that has only insulating or semiconducting
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electrical properties rather than superconducting
properties.
The reaction atmosphere employed also influences the
time of reaction to completion. Generally, reaction under
a reduced oxygen atmosphere of about 2000 p requires a
significantly shorter reaction, on the order of about 3 to
45 minutes for gram size reactions, compared to an
atmospheric reaction, which generally requires from about
5 minutes to 8 hours for gram size reactions. A similar
trend would be expected for larger scale reactions,
although the optimum reaction time for such larger scale
reaction would have to be determined by observation. One
method for determination of the completion of reaction is
to monitor samples by X-ray diffraction for depletion of
diffraction peaks that correspond to the starting material
and growth to maximum intensity of diffraction peaks which
correspond to the desired TdM*eCuf09 phase. The optimum
reaction time is dependant upon the elemental composition
of the oxide complex being prepared and the reaction
temperature and may be established by observation without
undue experimentation. Optimum superconducting properties
are obtained by timing the reaction to that point wherein
the maximum amount of starting materials have been
converted to the desired TdM*eCufO9 phase.
When the reaction has proceeded to the point of
maximum attainable TdM*eCuf09 phase content, it is
desirable to then rapidly quench the reaction material to
room temperature. This generally produces a narrower
temperature transition range between Tco and Tc for the
i
oxide complex so produced and also terminates any side
reaction that may occur which would otherwise convert the
TdM*eCuf09 phase content to an inferior superconducting
phase structure. For material produced under atmospheric
conditions rapid quenching is conveniently obtained by
immediately transferring the reacted material from the
heated reaction vessel to a heat sink. For gram
quantities of material an aluminum plate adequately
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functions as a suitable heat sink for rapid quenching.
Wherein the reacted material has been prepared in a
reduced oxygen atmosphere, upon completion of the reaction
the sample may be rapidly quenched by passing oxygen at
ambient temperature over the reacted sample.
The superconducting compositions of the present
invention have the potential for being used in a wide
variety of applications. For example, when used in a wire
or conductor form, they may be used in electrical power
transmission, energy storage, controlled fusion reaction,
electricity generation, mass transportation and magnets.
In a thin film form, they may be used in ultra-sensitive
detectors and in ultra-fast computers. In addition, they
may be used in a superconducting-magnetic-superconducting
multi-layer form for use in ultra-sensitive ultra-fast-
electromagnetic micro devices.
The following discussion of Bi-Ca-Sr-Cu-oxide systems
is representative of the TdM*eCuf09 oxide complexes and
methods of producing the oxide complexes of the invention.
Generally, the standard 4-probe technique was used to
measure resistivity, and an inductance bridge was employed
for ac magnetic susceptibility X-determination and a
magentometer was used for dc magnetization measurements.
The temperature was measured using the Au+0.07%Fe-chromel,
and chromel-alumel thermocouples in the absence of a
magnetic field, and a carbon-glass thermometer in the
presence of a field. The latter was calibrated against
the former without a field. Magnetic fields up to 6T were
generated by a superconducting magnet.
Three Bi-Ca-Sr-Cu-O (hereafter BCSCO) samples were
synthesized by the described solid-state reaction
techniques from appropriate amounts of Bi2O3i CuO, SrCO3,
and CaCO3. The BCSCO samples were prepared according to a
nominal composition of 1:1:1:1 for BCSCO-a; 1:1:1:2 for
BCSCO-b, and 1:1:1:3 for BCSCO-c. The starting
ingredients used were Bi2O3, (99-99.999%), Bi(NO3)3.5H2O
(99.99%), CaCO3 (99-99.995%), SrCO3 (99-99.999%) and CuO
CA 01341621 2011-09-06
X341621
-15-
(99-99.999%). The initial powder materials of appropriate
amounts were thoroughly mixed. The mixture was then
heated and cooled in air. The heat treatment conditions
were different for samples BCSCO-a and -b. The DTA, TGA,
and DTG results are shown in Fig. 1 for BCSCO-a and -b,
with compositions of Bi:Ca:Sr:Cu = 1:1:1:1 and 1:1:1:2,
respectively. Three reaction peaks at T1, T2, and T3 are
are clearly evident below melting. This observation is
typical for all compositions examined, provided that the
Sr/Ca ratio is greater than 0.7. The increase of Cu tends
to change T1, T2, and T3 somewhat and to enhance the
temperature difference between T3 and melting. After the
complete reaction of BCSCO, heating to or cooling from the
melting temperature did not result in any weight loss, in
contrast to the 90K LM2Cu3O6+0 superconductors. This
suggests possible greater chemical stabilities of BCSCO
than LM2Cu3O6+0 superconductors.
Bar samples were cut from sintered BCSCO pellets. A
four-lead method was employed for the resistance
measurements and a PAR Model M155 vibrating sample
magnetometer was used for investigating magnetization.
The temperature dependence of resistance (R) for these
samples and magnetization (M) for BCSCO-a and -c appear in
Figures 2 and 3, respectively.
Only one superconducting transition was observed in
BCSCO-a, but two occur in BCSCO-b and BCSCO-c (Figure 2).
The magnetization measurements represented in Figure 3
show that 12% of the Meissner effect in these samples is
associated with a 115K transition. The total Meissner
effect of the samples is about 40% compared to a Pb sample
of similar size. The only partial Meissner effect may be
attributed to the multiphase nature and/or low flux
trapping of the BCSCO sample. Greater Meissner effects
(up to 60%) have been detected in other BCSCO samples; in
some specimens as much as two-thirds of the effect is
associated with the 115K transition.
X-ray powder diffraction patterns of the Cu-rich
BCSCO-c sample revealed a substantial amount of unreacted
CA 01341621 2011-09-06
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copper oxide, which was not present in BCSCO-a or -b.
Samples were examined by optical and electron microscopy,
x-ray powder and single-crystal diffraction, and electron
microanalysis. Powder x-ray diffration patterns were made
on a Rigaku DMAX-3B automated diffractometer.
Single-crystal x-ray diffraction was performed on a Rigaku
automated four-circle diffractometer with monochromated
rotating anode Mo source. Electron microanalyses were
obtained on a JSM 35 scanning electron microscope,
operated at 20 KV and 0.01 pm spot size. Standards
included pure Bi and Cu metal, Sr-bearing glass and a
diopside jadeite pyroxene. A Phillips EM420
65- 35
transmission electron microscope with EDAX, Si-Li
detector, and a Princeton Gamma-Tech 4 data analysis
system was employed. TEM samples were mounted on
holey-carbon Be-mesh grids to avoid Cu-contaimination
during qualitative analysis.
The BCSCO samples comprised at least four phases, two
phases were alkaline earth copper oxides, another phase
was a bismuth alkaline earth oxide, and the four phase was
the superconductor phase. Elongated subhedral to euhedral
crystals of a tranparent, birefringent, pleochroic (red to
colorless) phase was prominent in grain mounts of all
three of the BCSCO (a,b, and c) samples examined with a
polarizing microscope. Needle-like crystals up to 100 Iim
long, though abundant in some samples, constitute a small
fraction (probably less than 5%) of the sample volume.
Electron microanalysis of five different crystals of this
insulating phase yields an average composition of
(Ca Sr )2(Cu Bi )03. Some or all of the bismuth
0.92 0.08 0.96 0.3
may represent background secondary scattering from
adjacent Bi-rich phases; therefore, this crystalline phase
may be bismuth free. A 90 x 10 x 5 pim crystal was
selected for single-crystal x-ray diffraction study and
was found to have orthohombic symmetry with a = 12.234,
0
b = 3.777 and c = 3.257 A. These values are almost
identical to those reported for stoichiometric Ca2CuO3 by
*trade mark
CA 01341621 2011-09-06
-17- 34162
Teske and Mtlller-Buschbaum Z. Anorg. Alle. Chem., 379, 234
(1970).
A second bismuth-poor phase, distinguished in grain
mounts as black, opaque, elongated euhedral to subhedral
grains up to 60 pm long, was found in samples BCSCO-b and
BCSCO-c to have a composition of approximately
(Cao.6Sro.4)Cu1.7503 based on microanalysis of four grains.
Approximately 0.03 atoms of bismuth were also detected per
one alkaline earth cation. Three crystals of this phase
were examined by single-crystal x-ray techniques. Though
the stoichiometry is similar to CaCu203r the unit cell and
structure differ from those reported by Teske and
Moller-Buschbaum, Z. Anorg. Alle. Chem., 370, 134 (1969)
for the pure calcium copper oxide. Standard indexing
procedures yielded an F-centered orthorhombic unit cell-
o
with a = 11.328, b = 12.774 and c = 3.896 A for all three
crystals. Striations characteristic of twinning and peak
splittings were observed for these grains, however, and it
is likely that the symmetry is monoclinic.
A third minor phase, distinguished by a composition
enriched in bismuth and lacking in copper, was observed by
electron microanalysis in sample BCSCO-a. The
composition, based on analyses of three different equant
grains and normalized to one Bi, is (Ca Sr )BiO
0.45 0.39 3-a
Two poorly-resolved electron diffraction patterns were
obtained for this phase; one revealed an orthogonal net
0 0
approximately 10.5 x 3.0 A and the other shows a 9.5 A
spacing with faint intermediate supercell reflections,
0
possibly characteristic of a 19.0 A repeat. The
structure of this phase may be related to strontium
bismuth oxide Sro=9Bi1.102=55, a tetragonal phase with
0
a = 13.329 and c = 4.257 A which was described by
Guillermo et al Rev. Chim. Miner., 15, 153 (1978). That
0
phase displayed characteristic spacings of 9.30 A (110 and
0
3.06 A (301).
The fourth phase, which is the superconducting
compound, was abundant in all three samples. It was
CA 01341621 2011-09-06
_18- 1341621 distinguished by a layered structure probably related to
the class of layered bismuth compounds described by
Aurivillus Arkiv. Kemi, 1, 463 and 499 (1950). These
structures incorporate both perovskite-type layers and
unusual Bi202 layers of bismuth in distorted four
coordination. Transmission electron microscopy (TEM)
revealed that the grains possessed a perfect basal
(hereafter termed 001) cleavage, similar to that of clay
minerals. The structure, therefore, must consist of a
layered atomic arrangement with planes of weak bonding.
Electron diffraction in the TEM of the (001) layers is
facilitated by the tendency of almost every grain to lie
flat on the holey carbon film. Numerous (hkO) electron
diffraction patterns were obtained (Figure 4), all of
0
which show a prominent perovskite-like 2.7 x 2.7 A subcell
0
and a distinctive 5.41 x 27.2 A supercell. A few grains,
lying near the edge of the grid, were found to be oriented
with (001) layers perpendicular to the grid. From these
0
crystallites a stacking periodicity of 15.39 A was
observed (Figure 5). High-resolution images of these
grains clearly revealed a four-layer structure with a 3.86
0
A subcell (Figure 5). The image contrast of these layers
suggests a possible ABAC-ABAC repeat pattern.
A single grain oriented with a perpendicular to the
gird yielded an electron density pattern corresponding to
0 0
a 5.44 x 30.78 A centered lattice with a 5-repeat (27.2 A)
superlattice parallel to b (Figure 6). High-resolution
images of this plane showed a "brick-like pattern
characteristic of the A-centered structure, as well as
numerous stacking faults and other defects. Many of these
defects are probably associated with interfaces between
perovskite and the Bi202 modules, both of which display
0
approximately 3.8 A layer spacings.
The TEM initial cell parameters were used to index 26
lines in the complex x-ray powder pattern (Table 1). Note
0
that the moderately strong 001 line at 15.4 A provides a
useful marker for this phase. Refined orthorhombic
CA 01341621 2011-09-06
13 R21
-19- 4 1
lattice constants based on the powder diffraction lines
were a = 5.410 0.003.b = 5.439 0.005 (x 5), and c =
0
30.78 0.03 A. These parameters are related to the simple
0
cubic perovskite cell (a cube with a = 3.85 A) by the
ratios ,/2 x 5V2 x 8.
The superconductor phase occurs in fine-grained
masses of black, opague flattened crystallites. The'
average diameter of these thin plates is less than 5 pm
and the thickness is usually less than 0.1 pm. Polished
scanning electron microscopy (SEM) mounts revealed that
this phase forms a matrix of randomly-oriented
interlocking flakes in which the other single-crystal
phases "float." This texture implies that the
superconducting phase crystallized last, at a lower
temperature than the other phases. The average
composition of the superconductor was determined from 15
analyses from three different samples. The ratio of
cations approximates 2:1:2:2 for Bi:Ca:Sr:Cu. The average
formula may be represented more precisely as
Bi2(Sr 0 56 Ca 0 39 Bi 0 05 ) 3 Cu2O8+8. Note that bismuth can
form a solid solution with strontium and calcium in some
structures. Analysis totals, based on oxides and assuming
divalent copper and trivalent bismuth, totaled
approximately 97%. It is likely, therefore, that some of
the copper or bismuth are in higher oxidation states.
Considerable variability of composition is observed
from grain to grain. The ratio of total (Cu + Bi) to
total (Ca + Sr) is relatively constant at 1:00:0.68 0.02.
The ratio of Cu to Bi, however, ranges from 40:60 to
49:51, with most compositions closer to the latter value.
The Sr:Ca ratio ranges from 38:30 to 43:25 with an average
value of 40:28. Some of these variations may result from
microscopic inclusions of other phases or from secondary
scattering from adjacent grains. It seems likely,
however, that the superconducting phase has a variable
composition with Bi-Ca-Sr solid solution. Furthermore,
the presence of numerous defects and stacking faults
provides an additional mechanism for incorporating
CA 01341621 2011-09-06
-20- 1 3 4 1 6 2 1
composition variation. These variations may account for
the differences between the superconductivity behavior
displayed by the three samples.
The new structure high temperature superconductor
Bi2CaSr2Cu20g (g is 8 to 9) is closely related to the 22 K
superconducting phase described by Michel et al, Z. Phys.,
B.68, 421 (1987) which has an approximate formula
0
Bi2Sr2Cu2O7 and unit cell parameters 5.32 x 26.6 x 48.8 A.
Both the Michel material and the new phase structure
display a distinctive b superlattice behavior, both have a
prominent layered habit, and both possess long c-axis
repeats. The new phase structure appears to be distinct,
however, based on the different c-axis repeat and
differences in the x-ray powder diffraction patterns.
A detailed description of the complex perovskite
related structure of the new material will not be possible
without improved powder diffration data, coupled with
high-resolution imaging of the lattice by TEM. Such
studies are now in progress. Nevertheless, several
structural inferences can be drawn from the compositional,
powder X-ray and TEM data already obtained. A
one-dimensional electron density profile is calculated
based on the observed powder diffraction intensities of
001 reflections. This analysis indicates a pronounced
layering of cations in (001), with substantial deviations
from the ideal perovskite arrangement of metal atoms.
Bismuth appears to be concentrated in layers, perhaps
similar to the Bi202 slabs of phases such as BaBi4Ti4015.
Weak electron density associated with every fourth layer
of the structure suggests interlayer regions of weak
bonding. It may be speculated that copper and oxygen
adopt the planar configuration common to the other known
high-temperature oxide superconductors, but there is not
yet any direct evidence to support this supposition.
Superstructures parallel to the a and b axes may be
the result of cation and oxygen ordering. Note, that
electron diffraction patterns reveal extensive streaking
CA 01341621 2011-09-06
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along c and b, as well as spot splitting, twinning,
stacking faults and other defects that will likely disrupt
most linear structures, if the latter exist. If these
features affect superconductivity then annealing
conditions and oxygen content should have an important
effect on the temperature and sharpness of the
superconducting transitions. The numerous structural
defects provide a mechanism for incorporating
compositional variations, as well as for generating
numerous closely-related homologous structures with
slightly larger or smaller superstructure.
In spite of the extremely fine-grained texture and
high density of defects, the copper-oxygen layers appear
to be continuous over hundreds of unit cells. This
layered structure may possess one significant advantage in
terms of processing and applications. Fabrication of the
superconductor by compression into planes yield
superconductor components with enhanced properties (e.g.,
critical current) in a specific plane. Similarly,
fabrication by rolling could produce wires more flexible
than those of ordinary ceramic materials.
Table II below gives the data obtained by a powder
x-ray analysis of the superconducting Bi-Ca-Sr-Cu oxide
phase. Patterns were obtained with filtered Cu radiation.
Pure silicon (NBS Standard References material 640) was
used as an internal standard.
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TABLE II
Powder diffraction pattern for the Bi-Ca-Sr-Cu oxide
0
superconductor, based on a subcell 5.410 x 4.439 x 30.78 A
h k 1 dohs dcacl I/10
0 0 2 15.66 15.39 12
0 1 1 5.361 5.359 5
0 0 8 3.857 3.847 23
1 0 6 3.736 3.723 3
1 1 3 3.593 3.594 41
1 1 5 3.257 3.256 100
0 0 10 3.085 3.080 24
1 1 7 2.890 2.891 62
2 0 0 2.709 2.705 91
2 0 2 2.668 2.664 19
0 0 12 2.556 2.565 18
2 1 1 2.413 2.415 5
2 0 8 2.214 2.213 4
0 0 14 2.200 2.199 3
2 0 10 2.033 2.032 26
1 1 13 2.013 2.015 11
2 1 9 1.978 1.977 4
2 2 0 1.915 1.918 31
2 2 4 1.862 1.862 9
1 1 15 1.809 1.809 13
0 3 3 1.787 1.786 3
3 1 3 1.687 1.689 5
3 1 5 1.648 1.649 13
3 0 8 1.634 1.633 7
3 1 7 1.595 1.595 13
3 1 9 1.531 1.531 10
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Figure 4 illustrates selected-area electron
diffraction pattern of the Bi-Ca-Sr-Cu oxide
superconductor showing hk0 diffraction spots. Strong
0
spots correspond to 2.7 x 2.7A subcell, while superlattice
reflections along a and b indicate spacings of 5.4 and
0
27.1 A respectively.
Figure 5 illustrates selected-area electron
diffraction pattern of the 00k diffraction row, which is
0
streaked but shows a strong 15.4 A periodicity. The 004
diffraction is indicated.
Figure 5 also illustrates a high-resolution image
0
taken parallel to the layers shows the 15.4 A(001)
0
spacing, with subspacings of 3.8 A. The contrast of these
layers differs, suggesting a possible ABAC-ABAC type
stacking of perovskite units. A structural defect
(arrowed) may correspond to a Bi-free region of CaSr-Cu
perovskite.
Figure 6 illustrates the selected-area Ok2 electron
diffraction pattern is characterized by an A-centered 27 x
0
31 A lattice.
Figure 6 also illustrates a high-resolution image of
the (100) plane reveals numerous stacking faults and
defects as well as the A-centered layered structure.
Though most of the structure appears to be orthogonal,
locally inclined blocks may indicate a fine-scale twinning
or may represent "monoclinic" regions of related but
different structure.
BCSCO is a quinary superconducting compound system,
consisting of the trivalent Bi. Structural data shows
that the 90K-transition in BCSCO is associated with the
Bi2CaSr2Cu2O8+0 (2:1:2:2) phase. A single phase BCSCO
sample with a TC-120K has yet to be obtained for detailed
physical characterization.
X-ray powder diffraction was carried out on more than
45 BCSCO samples of different compositions prepared under
different conditions. By comparing the X-ray data with
the electrical and magnetic results, the diffraction
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pattern for the superconducting phase was isolated as
shown in Fig. 7. No difference has been detected yet
within the resolution limits of the instrument used
between samples with Tc's between 60K and 120K. Results
for BCSCO-b synthesized at various temperatures slightly
higher than T1, T2, and T3 are also displayed in Figure 7.
It is clear that the superconducting phase starts to grow
slowly at temperatures above T1, then rapidly above T1.
The basic pattern of the superconducting phase appears in
samples for Cu to Bi ratio varying from -0.8 to 6.0 in the
fashion mentioned above. For example, when the insulating
Ca2CuO3 was doped with only a few percent of Bi and Sr,
the sample became completely superconducting resistively
below 60K and the superconducing phase appeared in the
X-ray diffraction pattern. A Cu-rich sample BCSCO-c (with
a composition ration of 1:22:14:6.2) synthesized at 850 C
was also studied. The X-ray data showed that a great
majority of the sample belonged to the superconducting
phase, in spite of the samll amount of Bi. Synthesis in a
pure oxygen atmosphere was found to be detrimental to the
superconducting properties of BCSCO, probably due to the
conversion of Bi+3 to Bi+S. It even made the sample
paramagnetic.
Typical resistance (R) and magnetization (M) results
are shown respectively in Figs. 8 and 9 for BCSCO-b,
synthesized at different temperatures. It is clear that
both the Tc and the Meissner effect grow as the synthesis
temperature increases. The 120K transition occurs only
when the sample is synthesized above -850K, although lower
Tc happens in samples prepared at lower temperatures.
However, the superconducting properties deteriorate when
the sample is heated above the melting point. In these
two figures, results for BCSCO-c are also shown. A trace
of the 120K-transition is evident although the bulk of the
sample is superconducting at -85K. It should be noted
that the porosity of the samples can be rather different.
This is particularly true for samples displaying a large
CA 01341621 2011-09-06
-25- 1 3 4 1 6 2 1
transition at -120K, i.e., it is very fluffy. Therefore
no effort was made to extract resistivity or to calculate
M per volume.
The magnetic field effect of R for a BCSCO-b sample
is shown in Fig. 10. The transition is clearly suppressed
to lower temperature and broadened in the presence of
magnetic field. The broadening is similar to that
observed in LBa2Cu3O6+0 polycrystalline compounds due to
the weak coupling between the highly anisotropic structure
of the compounds. The upper critical field at 0 K for the
120K transition is estimated to be -187T.
From the above R-and M-results, samples with a single
superconducting transition at -90K or below can be
obtained rather easily. However, no sample has exhibited
only one complete transition at -120K. In view of the-
large fraction of the Meissner effect associated with this
120K transition, the failure to achieve a complete
resistive transition at this high temperature suggests
that the 120K Tc material must be enclosed by the 90K Tc
material. This is in strong contrast to the LBa2Cu3O6+M
compounds or even the low Tc BCSCO superconductors. On
the other hand, the appearance of two resistive
superconducting transitions is similar to the A-15
superconducting compounds. The drastic difference between
the morphologies of the high Tc BCSCO compounds and the
22K Bi2Sr2Cu2O7 compound mentioned earlier indicate that
there exists a difference between their growth processes.
Careful studies on the reaction kinetics and the phase
diagram of BCSCO is needed to achieve single transition
sample with Tc-120K. High temperature annealing for more
than 24 hours has not enhanced the signal size.
The structure study showes that the BCSCO material
with compositions 2:1:2:2 superconduct at -90K have a
four-layer structure, although the exact atomic
arrangement is yet to be determined. Numerous structural
defects have been observed in microcrystalites of this
compound and have been suggested to be responsible for the
CA 01341621 2011-09-06
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existence of a large homogeneity range in BCSCO. The high
Tc phase, i.e. -120K, may have a homologous structure
closely related to the 2:1:2:2 phase. This is consistent
with the observation of similar X-ray diffraction patterns
of samples with Tc ranging from 40 to 120K. Single
crystals with a Tc-90K have been obtained and
investigated.
The Tc of BCSCO compositions depends sensitively on
the synthesis temperature but not on the Cu to Bi ratio.
High Tc phase appears only for samples prepared at
temperatures above 850 C but below melting. The present
study also demonstrates that there exists a large
homogeneity range for the superconducting phase to form,
preferably in a Cu-rich environment. This raises
questions concerning the essential role of planar
configuration of Cu-ions in BCSCO. The 120K-transition
phase may possess a homologous structure of the newly
identified 2:1:2:2 90K phase. Even higher Tc may be
achievable in this system with increasing complexity in
crystal chemistry.
