Processes of forming Ag doped conductive crystalline bismuth mixed alkaline earth copper oxide films

A process is disclosed of promoting the growth of crystalline bismuth mixed alkaline earth copper oxide grains in forming a conductive film by incorporating silver in the bismuth mixed alkaline earth copper oxide prior to sintering.

FIELD OF THE INVENTION 
The invention relates to conductive articles and processes for their 
formation. Specifically, the invention relates to a process of forming a 
conductive crystalline bismuth mixed alkaline earth copper oxide film on a 
substrate and to the conductive article produced. 
BACKGROUND OF THE INVENTION 
The term "superconductivity" is applied to the phenomenon of immeasurably 
low electrical resistance exhibited by materials. Until recently 
superconductivity had been reproducibly demonstrated only at temperatures 
near absolute zero. As a material capable of exhibiting superconductivity 
is cooled, a temperature is reached at which resistivity begins to 
decrease (conductivity begins to increase) markedly as a function of 
further decrease in temperature. This is referred to as the 
superconducting onset transition temperature or, in the context of 
superconductivity investigations, simply as the onset critical temperature 
(T.sub.c). T.sub.c provides a conveniently identified and generally 
accepted reference point for marking the onset of superconductivity and 
providing temperature rankings of superconductivity in differing 
materials. The highest temperature at which superconductivity (i.e., zero 
resistance) can be measured in a material is referred to as the onset 
superconductivity temperature (T.sub.o). 
H. Maeda, Y. Tanaka, M. Fukutomi, and Y. Asano, "A New High T.sub.c 
Superconductor Without a Rare Earth Element", Japanese Journal of Applied 
Physics, Vol. 27, No. 2, pp. L209 and L210, first reported that at least 
one compound of bismuth, strontium, calcium, copper, and oxygen had been 
found to be super-conducting. 
One of the difficulties that has arisen in attempting to form films of 
bismuth mixed alkaline earth copper oxides that are superconducting is 
that high sintering temperatures are required to form the crystalline 
grains of bismuth mixed alkaline earth copper oxide responsible for 
superconductivity. It is the necessity of sintering at high temperatures 
that has set the formation of superconducting films in a class apart from 
the preparation of bulk superconducting materials. In conductive film 
fabrication substrate interaction with the film during sintering must be 
taken into account. The greater the thermal driving force (a function of 
the time and temperature of sintering) required to form the crystalline 
grains of the film, the greater is the risk of degradation due to unwanted 
interaction between the conductive film and substrate. 
S. Jin et al, Appl. Phys. Lett. 52(19), 9 May 1988, pp. 1628-1630, reports 
the incorporation of silver in bismuth strontium calcium copper oxides. 
RELATED PATENT APPLICATIONS 
Agostinelli, Paz-Pujalt, Mehrotra, and Hung U.S. Ser. No. 172,926, filed 
Mar. 25, 1988, now abandoned in favor of U.S. Ser. No. 214,976, filed July 
5, 1988, now abandoned in favor of U.S. Ser. No. 359,306, filed May 31, 
1989, now U.S. Pat. No. 4,950,643 commonly assigned, discloses the 
successful formation of superconductive films using bismuth mixed alkaline 
earth copper oxides. The partial substitution of barium for strontium and 
the partial substitution of antimony for bismuth are disclosed. 
Strom U.S. Ser. No. 291,921, filed Dec. 29, 1988, titled SUPERCONDUCTING 
THICK FILMS FOR HYBRID CIRCUITRY APPLICATIONS, now abandoned in favor of 
U.S. Ser. No. 556,520, filed July 20, 1990, commonly assigned, discloses 
the successful formation of BSCCO-2212 superconducting thick films. The 
partial substitution of barium for strontium and the partial substitution 
of lead and/or antimony for bismuth are disclosed. 
Mir et al U.S. Ser. No. 308,297, filed Feb. 9, 1989, now U.S. Pat. No. 
4,988,674, commonly assigned, titled ELECTRICALLY CONDUCTIVE ARTICLES AND 
PROCESSES FOR THEIR FABRICATION, discloses the formation of a flexible 
superconductive film by providing a release layer comprised of gold and/or 
silver on a substrate, forming a conductive cuprate layer on the 
substrate, bonding a flexible organic film to the conductive cuprate 
layer, and stripping the conductive cuprate layer and organic film from 
the substrate. 
Lelental and Romanofsky U.S. Ser. No. 347,604, concurrently filed and 
commonly assigned, titled PROCESSES OF FORMING CONDUCTIVE FILMS AND 
ARTICLES SO PRODUCED, now abandoned in favor of U.S. Ser. No. 546,458, 
filed June 29, 1990, discloses the formation of bismuth mixed alkaline 
earth copper oxide conductive films in the presence of a stoichiometric 
excess of bismuth or bismuth in combination with lead. 
Agostinelli, Hung, Lelental, and Romanofsky U.S. Ser. No. 347,600, 
concurrently filed and commonly assigned, titled PROCESSES OF FORMING 
CONDUCTIVE FILMS AND ARTICLES SO PRODUCED, discloses the formation of 
bismuth mixed alkaline earth copper oxide conductive films in the presence 
of lead. 
SUMMARY OF THE INVENTION 
In one aspect this invention is directed to a process of forming on a 
substrate a conductive film containing grains of crystalline bismuth mixed 
alkaline earth copper oxide comprising locating a bismuth mixed alkaline 
earth copper oxide on the substrate and sintering at least a portion of 
the mixed alkaline earth copper oxide to form a conductive crystalline 
grain structure. The process is characterized in that silver is 
incorporated in the bismuth mixed alkaline earth copper oxide as a grain 
growth promoting agent prior to sintering. 
In another aspect the invention is directed to an article comprised of a 
substrate bearing a conductive film containing grains of crystalline 
bismuth mixed alkaline earth copper oxide. The article is characterized in 
that silver is present in the bismuth mixed alkaline earth copper oxide. 
DESCRIPTION OF PREFERRED EMBODIMENTS 
Except as otherwise noted, the term "conductive" is used to mean 
electrically conductive and all ratios and percentages throughout this 
disclosure are on an atomic basis. 
The invention relates to a compositionally altered preparation of a bismuth 
mixed alkaline earth copper oxide film on a substrate and to articles 
produced by the process. It has been discovered that silver acts as a 
grain growth promoting agent during the sintering step of forming a 
conductive bismuth mixed alkaline earth copper oxide film. 
As demonstrated in the Examples below, providing silver in bismuth mixed 
alkaline earth copper oxide prior to sintering results in the formation of 
larger crystalline grain sizes for a given sintering time and temperature. 
Thus, the presence of silver in the mixed oxide acts as a grain growth 
promoting agent. The silver can be used to either produce larger 
crystalline grain sizes in the conductive film employing a given sintering 
time and temperature or to decrease the sintering time and/or temperature 
required to produce crystalline grains of a selected size range. 
At silver concentrations of from 1 to 20 percent, based on copper, the 
presence of silver allows advantages to be realized in any one or a 
combination of four different ways: 
(a) Larger crystalline grain sizes can be produced as compared to those 
produced in a comparable film lacking silver; 
(b) The same crystalline grain sizes can be produced in films containing 
and lacking silver, where the film containing silver is sintered for a 
shorter time period; 
(c) The same crystalline grain sizes can be produced in films containing 
and lacking silver, where the film containing silver is sintered at a 
lower temperature; and 
(d) Higher onset transition temperatures and superconductivity can be 
achieved than in the absence of silver. 
Silver concentrations can be varied over the range of from 1 to 20 percent, 
preferably 5 to 10 percent, based on copper. The full advantage of the 
silver grain growth promoting agent is generally achieved at 
concentrations of 10 percent or less, based on copper. The silver is 
believed to be present in the conductive film in its zero valent 
(metallic) form. Concentrations of silver of up to 20 percent, based on 
copper, assure adequate silver for grain growth promotion while limiting 
the formation of a separate silver phase. 
In one form of the invention the bismuth mixed alkaline earth copper oxide 
is conventionally formulated for sintering, except that silver is added in 
the proportions indicated. The silver can be added in any convenient 
finely divided metallic form. Alternatively, the silver can be added as an 
oxide or as any other convenient silver precursor. When added as a 
precursor the same heating step prior to sintering converts the silver 
precursor to silver and also converts other metal oxide precursors to 
metal oxides. However, unlike the other metals, which are converted from 
metal oxide precursors to metal oxides, silver precursors are converted to 
metallic silver during heating. 
The mixed alkaline earth oxides can be selected and employed in any 
convenient proportion in relation to copper conventionally employed in 
forming bismuth mixed alkaline earth copper oxide crystalline conductive 
films. Typically the mixed alkaline earth oxides are mixtures of strontium 
and calcium oxides, although from 0 to 10 percent barium, based on barium 
and strontium combined, can be substituted for strontium. Strontium is to 
a degree interchangeable with calcium in the higher onset transition 
crystalline phase. More than half of either strontium or calcium can 
readily be replaced by the other. 
It is recognized that lead can be added as an oxide or oxide precursor to 
the metal oxide mixture present before sintering. Lead can take the place 
of up to 50 percent (preferably up to 40 percent) of the bismuth. Lelental 
and Romanofsky cited above discloses the formation of bismuth mixed 
alkaline earth copper oxide conductive films in the presence of a 
stoichiometric excess of bismuth or bismuth in combination with lead. 
Agostinelli, Hung, Lelental, and Romanofsky cited above discloses the 
formation of bismuth mixed alkaline earth copper oxide conductive films in 
the presence of lead. Lead acts as a grain growth promoter and produces 
optimum conductive films at somewhat lower temperatures (&gt;885.degree. C.) 
than bismuth alone. 
Since lead is an optional ingredient, its minimum concentration can be 
zero; however, when the advantages of lead incorporation are sought, it is 
preferred that lead be present in a minimum concentration of at least 1 
percent (preferably at least 5 percent) based on bismuth and lead 
combined. As employed herein references to bismuth mixed alkaline earth 
copper oxides are to be understood to include lead in the useful 
proportions indicated above, unless otherwise indicated. 
The proportions in which the various metal oxides or oxide precursors are 
brought together prior to sintering is dictated by the crystalline phases 
sought to be formed. It is known that bismuth strontium calcium copper 
oxides are capable of forming at least two clearly distinguishable 
superconductive phases. A superconducting phase having the lower onset 
transition temperature (T.sub.c =85.degree. K.) is also identified by a 
crystal cell 30.7.ANG. c axis and is generally accepted to satisfy the 
theoretical stoichiometry Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8 
(hereinafter also referred to as BSCCO-2212). A superconducting phase 
having a higher onset transition temperature (T.sub.c at least 110.degree. 
K.) is also identified by a crystal cell 37.ANG. c axis and is generally 
accepted to satisfy the theoretical stoichiometry Bi.sub.2 Sr.sub.2 
Ca.sub.2 Cu.sub.3 O.sub.10 (hereinafter also referred to as BSCCO-2223). 
The simplest approach to forming metal oxide mixtures satisfying the 
requirements of the invention is to provide the metal oxides or oxide 
precursors in the metal ratios conforming to the ideal stoichiometry of 
the higher or lower onset transition phases (or any desired mixture of 
these phases) or within a plus or minus variance for any individual metal 
(taking copper as a reference) of no more than 20 percent, preferably no 
more than 10 percent, taking the known interchangeabilities of the 
alkaline earth metals into account. 
For example, in seeking to prepare a superconductive film according to the 
invention approximating the properties of BSCCO-2212, a very simple 
mixture of metal oxides or oxide precursors, where silver can be as an 
oxide, oxide precursor, or metal, can satisfy the following metal 
relationship: 
EQU Bi.sub.2 Sr.sub.2 Ca(CuAg.sub.x).sub.2 (I) 
where 
x 0.01 to 0.2 (preferably 0.05 to 0.1). 
Taking into account the variances discussed above, it is appreciated that a 
more general range of proportions can satisfy the following metal 
relationship: 
EQU M.sub.a IIA.sub.b (CuAg.sub.x).sub.2 (II) 
where 
M is bismuth and from 0 to 50 percent (preferably 0 to 40 percent) lead, 
based on bismuth and lead, and 
a is from 1.6 (preferably 1.8 and optimally &gt;2.0) to 6 (preferably 4); 
IIA is a combination of the alkaline earth elements A and Ca in a ratio of 
2.5:0.5 to 1.0:1.5; 
A is strontium and from 0 to 10 percent barium, based on strontium and 
barium; 
b is from 2.4 to 3.6 (preferably 2.7 to 3.3); and 
x is from 0.01 to 0.2 (preferably 0.05 to 0.1). 
In seeking to prepare a superconductive film according to the invention 
approximating the properties of BSCCO-2223, a very simple proportion of 
metal components can satisfy the following metal relationship: 
EQU Bi.sub.2 Sr.sub.2 Ca.sub.2 (CuAg.sub.x).sub.3 (III) 
where x is as previously defined. Taking into account the variances 
discussed above, it is appreciated that a more general range of workable 
proportions can satisfy the following metal relationship: 
EQU M.sub.a IIA.sub.b (CuAg.sub.x).sub.3 (IV) 
where 
M is as previously defined; 
a is from 1.6 (preferably 2.0 and optimally &gt;3.0) to 9 (preferably 6); 
IIA is a combination of the alkaline earth elements A and Ca in a ratio of 
3:1 to 1:3; 
A is strontium and from 0 to 10 percent barium, based on strontium and 
barium; 
b is 3.2 (preferably 3.6) to 4.8 (preferably 4.4); and 
x is as previously defined. 
Even larger stoichiometric excesses of bismuth and alkaline earth metals 
can be employed. Because of their low cost larger excesses of the alkaline 
earth metals are not objectionable, except to the extent that separate 
phases tend to reduce the proportion of the more desirable conductive 
phase or phases. Stoichiometric excesses of bismuth are to some extent 
volatilized during sintering. It is therefore apparent that relationships 
III and IV can be combined into a general relationship V below, 
particularly in forming mixed crystalline conductive phases: 
EQU M.sub.a IIA.sub.b (CuAg.sub.x).sub.c (V) 
where 
M, IIA, and x are as previously defined; 
a is from 1.6 to 9; 
b is from 2.4 to 4.8; and 
c is from 2 to 3. 
Any conventional technique heretofore taught for forming a mixture of 
bismuth, alkaline earth, and copper oxides on a substrate for 
crystallization can be employed to form silver doped bismuth mixed 
alkaline earth copper oxide (hereinafter also referred to as M-IIA-C-O-Ag) 
layers of this invention. For example, the metal oxide deposition 
procedure of Yoshitake, T. Satoh, Y. Kubo, and H. Igarashi, "Preparation 
of Thin Films by Coevaporation and Phase Identification in Bi-Sr-Ca-Cu-O 
System", Japanese Journal of Applied Physics, Vol. 27, No. 6, June 1988, 
pp. L1089-L1091, cited above and here incorporated by reference, can be 
employed. 
The present invention is fully applicable to the techniques of forming 
thick (&gt;5 .mu.m) film M-IIA-C-O superconductive articles disclosed by 
Strom cited above. Only the compositional adjustments discussed above are 
required to employ the Strom procedures. 
Preferred procedures for preparing Ag doped M-IIA-C-O layers is similar to 
that set out in Mir et al European Patent Application 0 290 357, published 
Nov. 9, 1988, here incorporated by reference, except that Ag doped 
M-IIA-C-O is substituted for rare earth alkaline earth copper oxide 
(therein also referred to as RAC) and firing and cooling conditions are 
modified as described below. 
These preferred procedures for forming Ag doped M-IIA-C-O layers are 
similar, except for the compositional improvements noted above, to the 
procedures described in Agostinelli et al U.S. Pat. No. 4,950,643, cited 
above, for forming heavy pnictide mixed alkaline earth cooper oxide 
(therein also referred to as ) layers. To form a precursor layer onto a 
selected substrate is coated a solution consisting essentially of a 
volatilizable film forming solvent and metal-ligand compounds of each of 
the metals M, IIA, and Cu containing at least one thermally decomposable 
ligand. Ag is preferably also incorporated via a metal-ligand compound, 
the same ligands known to be useful with the other metals also being 
useful with silver. The solvent and ligands are removed from the substrate 
by heating in the presence of oxygen to form an initial mixture of the 
metals in the form of an oxide or an oxide intermediate (e.g., a 
carbonate). However, normally the silver-ligand compound is converted 
directly to silver metal on heating. Heating to a high temperature is then 
undertaken to complete conversion of any intermediates to oxides and to 
convert the resulting Ag doped M-IIA-C-O layer to the desired conductive 
crystalline form. 
The first step toward conductive article formation is, of course, substrate 
selection. Preferred substrates are those inert toward or at least 
minimally interactive with the M-IIA-C-O layer. It is generally preferred 
to select substrates from among materials which exhibit the same or a 
similar crystalline form, such as a perovskite crystalline form. Lanthanum 
gallate (LaGaO.sub.3), lanthanum aluminate (LaA10.sub.3), and potassium 
tantalate (KTaO.sub.3) are examples of perovskite crystalline substrates. 
Alkaline earth oxides and zirconia constitute particularly preferred 
classes of substrates. They are in general relatively inert, refractory 
materials which exhibit limited interaction with the M-IIA-C-O layers 
during their formation. Magnesium oxide and strontium titanate (a 
perovskite) are examples of readily available alkaline earth oxide 
substrate materials. As employed herein the term "alkaline earth oxide 
substrate" is employed to indicate substrates that are comprised of 
alkaline earth oxides. The lower firing temperatures made possible by the 
inclusion of lead also make the use of polycrystalline and monocrystalline 
alumina attractive. 
It is recognized that the selection of substrates can be broadened by 
employing barrier layers and/or repeating the film forming sequence so 
that earlier formed layers in effect serve as a substrate for later formed 
M-IIA-C-O layers. Both of these variations are taught by Mir et al EP 0 
290 357, cited above. Other barrier layer arrangements, such as those 
disclosed by Agostinelli et al U.S. Ser. No. 85,047, filed Aug. 13, 1987, 
and Hung et al U.S. Ser. No. 153,699, filed Feb. 3, 1988, both commonly 
assigned, are also specifically contemplated. For example, in employing 
silicon as a substrate it is specifically contemplated to employ zirconia, 
alkaline earth oxides such as magnesia, and noble metals such as gold as 
barrier layer materials. A preferred combination is provided by a silicon 
substrate and a triad of barrier layers, the barrier layer nearest the 
silicon support being silica, the barrier layer farthest from the silicon 
support being a Group 4 metal oxide (preferably zirconia), and the 
interposed barrier layer being a mixture of silica and zirconia. 
To form the precursor layer a solution of a film forming solvent, a bismuth 
compound, optionally a silver compound, at least two alkaline earth metal 
compounds, and a copper compound is prepared. Each of the compounds 
consists of metal ion and one or more volatilizable ligands. Most useful 
metal-ligand compounds (e.g., metalorganic compounds) thermally decompose 
to form metal oxides. Some metal compounds, in particular some alkaline 
earth organic compounds, can form metal carbonates on decomposition, which 
can then be converted to oxides during heating to crystallization 
temperatures. A ligand oxygen atom bonded directly to a metal other than 
silver is often retained with the metal in the Ag doped M-IIA-C-O layer, 
although other ligand oxygen atoms are generally removed. Typically the 
ligands and their component atoms other than oxygen are outgassed at 
temperatures of less than 600.degree. C. On the other hand, to avoid loss 
of materials before or during initial coating of the metal-ligand 
compounds, it is preferred that the ligands exhibit limited, if any, 
volatility at ambient temperatures. Metal-ligand compounds having any 
significant volatility below their decomposition temperature are 
preferably avoided. 
Metalorganic (including metallo-organic and organometallic) compounds, such 
as metal alkyls, alkoxides, .beta.-diketone derivatives, and metal salts 
of organic acids-e.g., carboxylic acids, constitute preferred metal-ligand 
compounds for preparing precursor coatings. The number of carbon atoms 
in the organic ligand can vary over a wide range, but is typically limited 
to less than 30 carbon atoms to avoid unnecessarily reducing the 
proportion of metal ions present. Carboxylate ligands are particularly 
advantageous in promoting metal-ligand solubility. While very simple 
organic ligands, such as oxalate and acetate ligands, can be employed in 
one or more metal-ligands compounds, depending upon the film forming 
solvent and other metal-ligand compound choices, it is generally preferred 
to choose organic ligands containing at least 4 carbon atoms. The reason 
for this is to avoid crystallization of the metal-ligand compound and to 
improve solubility. When heating is begun to remove the film forming 
solvent and ligands, the solvent usually readily evaporates at 
temperatures well below those required to remove the ligands. This results 
in leaving the metal-ligand compounds on the substrate surface. When the 
ligands have few carbon atoms or, in some instances, linear carbon atom 
chains, crystallization of the metal-ligand compounds occurs. In extreme 
cases crystallization is observed at room temperatures. This works against 
the molecular level uniformity of heavy pnictide, alkaline earth, and 
copper sought by solution coating. Choosing organic ligands exhibiting 4 
or more carbon atoms, preferably at least 6 carbon atoms, and, preferably, 
ligands containing branched carbon atom chains, reduces molecular spatial 
symmetries sufficiently to avoid crystallization. Optimally organic 
ligands contain from about 6 to 20 carbon atoms. 
Instead of increasing the molecular bulk or modifying the chain 
configuration of organic ligands to avoid any propensity toward 
metalorganic compound crystallization on solvent removal, another 
technique which can be employed is to incorporate in the film forming 
solvent a separate compound to act as a film promoting agent, such as a 
higher molecular weight branched chain organic compound. This can, for 
example, take the form of a branched chain hydrocarbon or substituted 
hydrocarbon, such as a terpene having from about 10 to 30 carbon atoms. 
The film forming solvents can be chosen from a wide range of volatilizable 
liquids. The primary function of the solvent is to provide a liquid phase 
permitting molecular level intermixing of the metalorganic compounds 
chosen. The liquid is also chosen for its ability to cover the substrate 
uniformly. Thus, an optimum film forming solvent selection is in part 
determined by the substrate chosen. Generally more desirable film forming 
properties are observed with more viscous solvents and those which more 
readily wet the substrate alone, or with an incorporated wetting agent, 
such as a surfactant, present. 
It is appreciated that a wide variety of ligands, film promoting agents, 
and film forming solvents are available and can be collectively present in 
a virtually limitless array of composition choices. 
Exemplary preferred organic ligands for metal organic compounds include 
metal 2-ethylhexanoates, naphthenates, neodecanoates, butoxides, 
isopropoxides, rosinates (e.g., abietates), cyclohexanebutyrates, and 
acetylacetonates, where the metal can be any of Ag, M, IIA, or Cu to be 
incorporated in the Ag doped M-IIA-Cu-O layer. Exemplary preferred film 
forming agents include 2-ethylhexanoic acid, rosin (e.g., abietic acid), 
ethyl lactate, 2-ethoxyethyl acetate, and pinene. Exemplary preferred film 
forming solvents include toluene, 2-ethylhexanoic acid, n-butyl acetate, 
ethyl lactate, propanol, pinene, and mineral spirits. 
As previously noted, the metal-ligand compounds are incorporated in the 
film forming solvent in the proportion desired in the final crystalline Ag 
doped M-IIA-C-O layer. The bismuth, lead, alkaline earth, copper, and 
silver can each be reacted with the same ligand forming compound or with 
different ligand forming compounds. The metal-ligand compounds can be 
incorporated in the film forming solvent in any convenient concentration 
up to their saturation limit at ambient temperature. Generally a 
concentration is chosen which provides the desired crystalline Ag doped 
M-IIA-Cu-O layer thickness for the process sequence. Where the geometry of 
the substrate permits, uniformity and thickness of the metal-ligand 
coating can be controlled by spinning the substrate after coating around 
an axis normal to the surface of the substrate which has been coated. A 
significant advantage of spin coating is that the thickness of the coating 
at the conclusion of spinning is determined by the contact angle and 
viscosity of the coating composition and the rate and time of spinning, 
all of which can be precisely controlled. Differences in the amount of the 
coating composition applied to the substrate are not reflected in the 
thickness of the final coating. Centrifugal forces generated by spinning 
cause excess material to be rejected peripherally from the article. 
It is preferred to employ firing temperatures ranging from 800.degree. C. 
(optimally at least 825.degree. C.) to 910.degree. C. (optimally 
885.degree. C. or less). Sintering temperatures must be high enough to 
produce the crystalline conductive phase or phases being sought. At the 
same time sintering temperatures must be maintained below temperatures at 
which the metals separate into different phases. Generally Ag doped 
M-IIA-C-O conductive films can be formed by sintering in the same time and 
temperature ranges conventionally employed in the absence of silver. 
The duration of sintering can be very short, where the object is to form 
the lower onset transition (T.sub.c =85.degree. K.) phase. Sintering times 
as short as 1 minute (preferably 5 minutes) are effective forming this 
phase. When the lower onset transition phase is being formed, there is no 
advantage to extending sintering times beyond about 20 minutes. However, 
when the higher onset transition phase (T.sub.c at least 110.degree. K.) 
is being formed, extended sintering times are contemplated. For maximum 
conversion to the higher onset transition phase sintering times times 
range from 2 to 200 hours (optimally from 6 to 100 hours). If mixed phases 
are acceptable any sintering time from 1 minute to 200 hours can be 
employed. 
Ag doped M-IIA-C-O layer crystallization can be undertaken in any 
convenient oxidizing atmosphere, including oxygen or oxygen contained in a 
convenient carrier, such as argon or air. The crystalline Ag doped 
M-IIA-C-O layers show little response to variances in cooling and/or 
annealing following crystal formation.

EXAMPLES 
The invention can be better appreciated by reference to the following 
specific embodiments of the invention: 
Precursors 
The following individual metallo-organic precursors, prepared as described, 
were employed: 
Bi-Pl. Bismuth 2-ethylhexanoate 
20.0 g Excess Bi.sub.2 O.sub.3 
25.0 g 2-Ethylhexanoic acid 
Mix the solid bismuth oxide with 2-ethylhexanoic acid and heat to 
approximately 120.degree. C. Add a few drops of ammonium hydroxide (30% 
NH.sub.3 /H.sub.2 O) in order to speed up the reaction. After refluxing 
for 4 hours, filter, dry with sieves, and concentrate the liquid phase. 
Analysis showed 17.7% Bi by weight. 
Ca-Pl. Calcium 2-ethylhexanoate 
Calcium carbonate was treated with excess of 2-ethylhexanoic acid and 
xylene as needed at 120.degree. C. for 18 hours. The mixture was then 
filtered and dried with molecular sieves. This was followed by 
concentration and filtration of the solution. Analysis showed a residue of 
4.58% CaO (3.27% Ca) by weight. 
Sr-Pl. Strontium cyclohexanebutyrate 
This compound is available commercially in high quality from Eastman 
Chemicals. It is supplied with an assay indicating a strontium 
concentration of 19.4% by weight for the lot which was used. 
Cu-Pl. Copper 2-ethylhexanoate 
The Cu precursor was prepared by reacting copper acetate with 
2-ethylhexanoic acid as follows: 
React copper acetate (available from Baker, as 1766-1, 31.8% Cu by weight) 
with 2-ethylhexanoic acid by mixing 2.0 g Cu Acetate into 8.0 g 
2-ethylhexanoic acid, and heating to the boiling point for 5 minutes in an 
open vessel. Add back enough 2-ethylhexanoic acid to cancel the weight 
lost during reaction to return to 10.0 g total solution. At this point a 
stable Cu precursor solution having 6.36% Cu by weight is produced. 
BSCC-Pl. Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8+x precursor 
A precursor solution for Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8+x was 
prepared as described below: 
2.0 g of Bi-Pl was mixed with 1.43 g of Ca-Pl and 2.32 g of Cu-Pl. These 
three solutions were miscible and formed a stable solution. To this were 
added 1.05 g Sr-Pl followed by heating at the boiling point to get all of 
the powder into solution. Some reaction had also likely occurred during 
this step (exchanges of carboxylate ligands). The result was a stable blue 
solution. To this solution was added 0.5 g rosin (available from Eastman 
Chemicals as Rosin 2315) followed by gentle heating to get all of the 
rosin in solution. The solution was filtered to remove any particulate 
contaminants. The result was a stable solution having excellent 
rheological properties. 
Ag-Pl. Silver 2-Ethylhexanoate 
A commercial silver 2-ethylhexanoate (source: Strem) was combined with 
solvents as follows: To 0.5 g Ag 2-ethylhexanoate was mixed 4.7 g cineole 
and 1.6 g toluene. The mixture was heated until no solid remained. The 
solution contained a calculated silver concentration of 3.5 percent by 
weight, based on total weight. 
Coating Comparisons 
Precursors Ag-Pl and BSCC-1 were combined in various proportions to produce 
a variety of coatings differing in the percentage of silver. The 
precursors were spin coated onto the {100} surface of monocrystalline 
magnesia substrates at 5000 rpm for 20 seconds. The coated substrates were 
heated to 550.degree. C. on a hot plate. The coated substrate samples were 
sintered to form the crystalline grains responsible for superconductivity. 
Sintering was conducted in a furnace at 855.degree. C. for 5 minutes in 
air, with the samples being rapidly cooled. The final film thickness was 
about 1.5 .mu.m. 
Control 1 
In this coating only BSCC-Pl was employed--i.e., no Ag was introduced into 
the film. The processed film had a room temperature sheet resistance of 
approximately 25 ohm/square and low temperature four point resistance 
measurement revealed an onset transition temperature (T.sub.c) above 
100.degree. K. with superconductivity (zero resistance) being attained 
75.degree.-78.degree. K. The film was polycrystalline with grain sizes 
being in the range of about 1 .mu.m. X-ray diffraction analysis showed a 
highly c-axis oriented BSCCO-2212 crystal structure. 
EXAMPLE 2 
In this coating 5 percent silver, based on copper, was present. The film 
was polycrystalline with grain sizes being in the range of about 3 to 6 
.mu.m. This was significantly greater than the corresponding value of the 
undoped superconducting film. X-ray diffraction analysis indicated the 
presence of the oriented superconducting oxide phase. The resistivity 
retained a clearly metallic behavior down to the superconducting 
temperature (T.sub.o) at 82.degree. K. 
EXAMPLE 3 
In this coating 10 percent silver, based on copper, was present. The film 
was polycrystalline with grain sizes being in the range of about 4 to 8 
.mu.m. X-ray diffraction analysis confirmed the formation of the 
preferentially oriented orthorhombic superconducting phase. Peaks 
corresponding to pure silver were also seen. The oxide film shows a sharp 
transition to superconductivity with a midpoint transition temperature of 
90.degree. K. and zero resistivity (T.sub.o) at 85.degree. K. 
The advantages of silver doping were still observed when the proportion of 
silver was increased, but no further improvement in performance was 
observed. 
The invention has been described in detail with particular reference to 
preferred embodiments thereof, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.