Superconductor-coated carbon fiber composites

Superconducting composites are made from ceramic-type superconductors coated onto a low resistivity carbon fiber selected from those high strength fibers which have an ultrahigh modulus and high thermal conductivity. Flexible conductors of several different structures made from such composites are described as well as other useful forms of the composites.

BACKGROUND OF THE INVENTION 
This invention relates to a superconductor-carbon fiber composite 
comprising a high strength, ultrahigh modulus, high thermal conductivity 
carbon fiber which is coated with a ceramic-type superconductor. More 
particularly, this invention relates to superconducting, 
super-conductor-carbon fiber composites comprised of a high strength, 
ultrahigh modulus, high thermal conductivity, low resistivity carbon fiber 
which is coated with an adhering layer of a ceramic-type superconductor 
such as a rare earth (R.E.), Ba, Cu, oxide-type superconductor 
(1-2-3,superconductor), which composite is capable of achieving 
significant current densities at high magnetic field strengths under 
superconducting conditions. The term carbon fiber as used herein includes 
both a carbon monofilament as well as a bundle of monofilaments (a yarn). 
Recently, a number of published reports have appeared which describe 
superconducting ceramic-type materials composed of a combination of rare 
earth (e.g. yttrium) oxide, barium oxide and copper oxide which have 
significantly higher superconduction transition temperatures than earlier 
materials such as Nb/Ti alloys, niobium carbonitride and the like. 
Superconducting transition temperatures above 77.degree. K. (the boiling 
point of liquid nitrogen) are commonly found for these materials, and even 
higher transition temperatures are considered possible based upon recent 
revisions to existing theories explaining superconducting behavior. The 
economic advantage that these new superconductors could have over 
previously existing lower superconducting-transition-temperature 
superconductors is large enough that many new uses for superconductors now 
can be devised and present uses enormously improved. However, because 
these new mixed-oxide superconductors are brittle, ceramic-like materials, 
they do not lend themselves easily to fabrication in the form of high 
strength, wire-type geometries, a requirement for many important uses to 
which superconductors have been put in the past. These uses largely 
revolve about strong field magnets used in high energy physics, traffic 
engineering, etc. 
One way of fabricating a brittle superconducting material in wire-like form 
is set forth in an article by K. Brennfleck et al. entitled "Chemical 
Vapor Deposition of Superconducting Niobium Carbonitride Films on Carbon 
Fibers" which was published in Proceedings of the 7th Conference on 
Chemical Vapor Deposition, Electrochemical Society (1979) at p. 300. This 
article describes depositing a niobium carbonitride layer directly onto a 
THORNEL.RTM. 400 multifilament yarn by chemical vapor deposition (CVD) to 
form a superconducting composite. However, the Brennfleck et al. 
composites employ a low thermal conductivity, more reactive carbon fiber 
and the structure shown in the photomicrographs accompanying the article a 
poor physical structure. Additional aspects of niobium carbonitride-carbon 
fiber based superconducting composites are taught in U.S. Pat. Nos. 
4,299,861; 4,581,289; and 4,657,776. Recently, ultrahigh modulus, high 
thermal conductivity carbon fibers of low resistivity have become 
available which will perform most, if not all, the stabilization required 
for carbon fiber superconducting composite operation. See, for example, 
U.S. Pat. No. 4,005,183 Singer granted to Union Carbide. Thus, the need 
for the outermost copper coating used in the previous literature for 
stabilization is either reduced or eliminated resulting in simpler and 
more economical devices. 
The usefulness of an intermediate carbide or oxide layer between a carbon 
fiber and a niobium carbonitride superconductor layer to improve adhesion 
of the superconductor is taught in U.S. Pat. No. 4,585,696. Such a layer 
depends upon its intermediate (to the fiber and superconductor) 
coefficient of expansion to achieve its adhesive effect. 
The new mixed-oxide ceramic-type superconductors are different in physical 
properties than the Brennfleck et al. niobium carbonitride material and 
these differences lead to different considerations for fabricating the 
superconductor into wire-like form. For example, the niobium compound has 
a cubic crystal structure and its critical current and critical fields are 
isotropic, i.e., the same along each of its three crystallographic axes. 
The new 1-2-3 superconductors on the other hand show a much smaller 
critical current and critical field along the c crystallographic axis than 
along the a and b crystallographic axes. Thus, it may be important to 
align the a b planes of the 1-2-3 superconductor microcrystals as 
completely as possible parallel to the fiber axis for maximum 
effectiveness when made in a superconducting device. 
Now it has been found that ceramic-type superconductors such as the 
recently discovered R.E., Ba, Cu oxide-type superconductors can be formed 
on low resistivity, high thermal conductivity, high strength, ultrahigh 
modulus carbon fibers in adhering layers by several different techniques 
to yield useful superconducting composites. Additionally, it is possible 
that at least some preferred orientation of the superconductor 
microcrystals on the fiber can be produced, which composites can be formed 
into strong, flexible conductors capable of exhibiting substantial 
critical currents and critical magnetic fields under superconducting 
conditions. 
SUMMARY OF THE INVENTION 
Described herein is a superconducting composite comprising a low 
resistivity, high strength, ultrahigh modulus carbon fiber, said fiber 
coated with an adhering layer of a ceramic-type superconductor. 
Also described is a superconducting composite comprising a low resistivity, 
high strength, ultrahigh modulus carbon fiber exhibiting high thermal 
conductivity, said fiber coated with an adhering layer of superconducting 
mixed oxide having a transition temperature above 77.degree. K., which 
mixed oxide is of formula A.sub.1 B.sub.2 Cu.sub.3 O.sub.7-x' wherein A 
is one or more elements selected from the group consisting of yttrium, 
lanthanum and the lanthanides, B is one or more Group IIA elements, and x 
is a number between 0 and 1.

DETAILED DESCRIPTION OF THE INVENTION 
The type of carbon fiber useful for the invention described herein is a 
fiber made from pitch, polyacrylonitrile (PAN), rayon and the like. Such 
fibers can be made by extrusion through a spinnaret of melted pitch, "PAN 
dope" or "rayon dope." The useful fibers, in tape, sheet or tube form, can 
contain one to several thousand or more individual monofilaments per 
bundle. Typical of such yarns are THORNEL.RTM. pitch, PAN-based, and 
rayon-based fibers which are supplied as continuous length, high strength 
bundles consisting of varying number of fibers, twisted or untwisted. The 
ultrahigh modulus, pitch-based, high thermal conductivity carbon fibers 
are preferred since beside the ultrahigh modulus and high thermal 
conductivity, the higher temperature method of their preparation makes 
them more graphitic and hence oriented, and more resistant to oxidative 
attack. However, fibers from PAN or rayon sources could also be preferred 
if their properties could match those made of pitch. It is preferred to 
use untwisted carbon fiber yarns for the purposes of this invention. 
The carbon fiber useful herein is a low resistivity fiber. Fibers made from 
the feeds described above are of low resistivity, but typically the lower 
the resistivity the better suited the fiber for this invention. Any 
improvement on their resistivity such as by doping with SbF.sub.5 and the 
like is desirable as long as strength, modulus and thermal conductivity do 
not particularly suffer. Resistivities of less than about 3 .mu.ohm-m are 
typical for these fibers, but fibers of higher resistivity (less than 20 
.mu.ohm-m) may be used. Preferred are fibers having a resistivity of less 
than about 1.5 .mu.ohm-m. Thickness of the carbon fiber useful herein is 
dictated to some extent by the use to which the superconducting composite 
is to be put, but it should be thick enough to avoid easy fiber breakage 
and not so thick as to preclude the fiber showing the flexibility normally 
expected of an electrical conductor, if the end result is used for that 
purpose. Length of the fiber typically depends upon the end use of the 
composite and the method of making the composite. The tensile strength of 
the carbon fiber should be generally above about 150 ksi and, more 
preferably, above about 300 ksi. Most preferably, it is above about 350 
ksi. Such high strength fibers give composites which, when used to wind 
superconducting magnets, can help withstand the stress produced by the 
high fields produced in high magnetic field superconducting magnets. A 
thermal conductivity at least about 20 percent as great as copper (390 
watts/m/.degree. C. at 20.degree. C.) is preferred, and a value of at 
least that of copper is more preferred. Most preferred, is a fiber having 
a thermal conductivity at least twice that of copper. A modulus of at 
least 20 Msi is useful, but a modulus of at least about 80 Msi and, more 
preferably, of at least about 100 Msi is preferred. 
Pitch-based fibers are preferred here as they can be more inert during 
laying down of the superconductor layer, have high strength, ultrahigh 
modulus, and have higher thermal conductivity and lower resistivity. Also, 
the orientation of the basel planes of the graphitic microcrystals 
composing the pitch-based fiber surface is more parallel to the fiber 
length. Most preferred are the pitch-based grades of THORNEL.RTM. fibers 
such as P-100, P-120, P-130. Other carbon fibers with strengths, moduli 
and thermal conductivities similar to the THORNEL.RTM. pitch-based grades 
are also preferred. 
The superconductors useful in the invention described herein are 
ceramic-type superconductors. They include niobium carbonitride and oxide, 
the La.sub.2 CuO.sub.4 materials and, importantly, the 
recently-discovered, so-called, 1-2-3 superconductors. These latter 
materials are of general formula A.sub.1 B.sub.2 Cu.sub.3 O.sub.7-x' where 
A is one or more elements selected from yttrium, lanthanum or a 
lanthanide, B is one or more Group IIA metal such as calcium, strontium or 
barium, etc., and x runs between 0 and 1. These materials can be made by 
heating mixtures of, for example, an yttrium compound, a barium compound, 
a copper (II) compound in the proportions given by the general formula 
above and annealing in an oxygen-containing or releasing atmosphere. A 
typical compound is YBa.sub.2 Cu.sub.3 O.sub.6.93. They have crystal 
structures which are based upon the Perovskite structure and 
superconducting transition temperatures between about 90.degree. and about 
98.degree. K., although it is likely that certain members of the family 
will show higher superconducting transition temperatures. Also, there is 
some indication that the copper or oxygen portion of these superconductors 
can be in part or completely replaced by another element, and it is meant 
to cover such compounds within the description of the invention contained 
herein. 
The type of low resistivity, pitch-based carbon fiber preferred in this 
invention is not only the ultrahigh modulus, high strength type but, 
particularly, the high thermal conductivity type. The high thermal 
conductivity feature is highly desirable as it allows a quick and even 
distribution of temperature when using the superconducting composite, and 
it is particularly useful for adiabatic stabilization where the composite 
is used for high field magnet purposes. A thermal conductivity at least as 
great copper (390 watts/m/.degree. C. at 20.degree. C.) is preferred, and 
a value of at least twice that of copper is more preferred. Most 
preferred, is a fiber having a thermal conductivity at least three times 
that of copper. The pitch-based fibers are particularly preferred herein 
because in part they have an ultrahigh modulus. The stiffness obtained by 
such an ultrahigh modulus is particularly advantageous if an electrical 
conductor to be made from the composites taught. By ultrahigh modulus is 
meant moduli of at least about 80 Msi and, more preferably, of at least 
about 100 Msi. 
In general, at temperatures below about 600.degree. C. the coefficient of 
thermal expansion (CTE) of the carbon fiber portion of the inventive 
composite is negative in the axial direction of the fiber and not far from 
zero in the radial direction. The solid, ceramic-type superconductors 
which are to be coated on the fiber generally have an overall positive 
CTE. Thus, during laydown of the superconductor on the fiber or where the 
fiber/superconductor composites are temperature cycled, the superconductor 
coating may crack, loosen, or peel leading to physical property 
degradation of the composite. To overcome this difficulty the 
superconductor coating can be laid down over a buffer layer deposited on 
the fiber prior to laying down the superconductor layer. The buffer layer 
must be compressible and thick enough to accommodate the contraction of 
the superconductor layer. This compressible layer may also help to orient 
the superconductor microcrystals making up the coating such that the 
planes of lowest electrical resistivity align themselves as fully as 
possible along the fiber axis. This latter orientation, however obtained, 
is believed important for obtaining maximum current density along the 
fiber axis. This compressible layer should also be as inert to the 
superconductor used as possible. Thickness of the compressive layer is 
determined by the compressible layer porosity, carbon fiber CTE and 
superconductor CTE as may be understood by one skilled in the art, but is 
generally about 100 Angstroms to about 10 microns thick, more particularly 
about 0.5 micron to about 2 microns thick. One such way of laying down a 
compressible graphitic layer on a carbon fiber is taught in U.S. Pat. No. 
3,799,790, the contents of which are incorporated herein by reference. 
This patent teaches the vapor deposition of a mixture of pyrolytic carbon 
and aluminum oxide and heating the result to remove the aluminum oxide 
leaving behind a porous, compressible, highly-graphitic layer. As may be 
recognized by one skilled in the art other methods of matching fiber and 
superconductor CTEs can be used. However, the '790 patent is the preferred 
method of matching CTEs for this invention. 
Some, if not all, of the solid superconductors coated on the carbon fiber 
to make the composite of the instant invention may react at high 
temperature with the surface of the carbon fiber, even when using pitch 
fibers of lowered reactivity, which are the preferred materials. Such a 
graphitic compressible layer as described above can serve as a protective 
layer as well as accommodating the different CTEs of fiber and 
superconductor. This reaction can take place during the laying down of the 
solid superconductor coating, as above described, or during the high 
temperature annealing (densification) process normally required in 
converting the superconductor layer into its superconducting form. Such 
reaction or corrosion can degrade the properties of the superconducting 
composite to a marked degree. It is therefore useful in certain instances 
to lay down a thin, electrically conducting coating over the carbon fiber 
before laying down the superconductor layer but generally after laying 
down the compressible layer, if used. Such metals as copper, silver (if 
the annealing temperature is not too high), gold, transition element 
carbides and nitrides, and the like can be of service and be both 
conductive and protective. This layer, if used, is desirably quite thin, 
on the order of tens of Angstroms thick and less than about 1000 Angstroms 
thick. 
To coat the surface of the carbon fiber a number of different methods may 
be used. Desirably, one should make the coating reasonably uniform in 
thickness over the length of the fiber and, very importantly, continuous 
over the entire fiber surface. For maximum current density and other 
beneficial effects, care should be taken that the fiber is either 
continuously covered or essentially continuously covered. Solid 
superconductor layer thicknesses of 100 Angstroms or more, more preferably 
about 5000 to about 50,000 Angstroms thick, are desirable to insure an 
adequate electrical path for use of the composite as a current carrying 
device. Too thick a layer of the solid superconductor on the fiber can 
adversely affect the desired fiber flexibility and hence conductor 
flexibility and is to be avoided except for those uses where a stiff fiber 
conductor made from the instant composite can be tolerated. Too thin a 
layer can adversely affect the current density. Where two dimensional 
carbon fibers are employed (woven and non-woven fabrics, etc.), 
flexibility of the composite is not so important. Such two dimensional 
carbon fiber geometries are useful, for example, for making 
superconducting composites, used for electrical and magnetic shielding, 
and conducting tubular conductors which are designed to carry the coolant 
internally. 
Deposition techniques for use herein can be quite varied but obviously some 
are more suitable for complete fiber coverage than others. For example, a 
simple technique is to solution coat the carbon fiber by making up a 
solution, aqueous or non-aqueous, containing the proper amounts of 
compounds of the elements which are to make up the particular 
superconductor chosen. Alternatively, the superconductor can be prepared 
by a dry method and then dissolved in an aqueous oxidizing acid solution, 
for example, nitric acid. Either solution may then be applied to the 
carbon fiber, used with or without a compressive layer and/or an outer 
enrobing layer, by running the fiber through the solution. The fiber is 
then heated, usually in an appropriate atmosphere to produce the 
superconductor in a thin coating on the fiber by annealing the 
superconductor layer to achieve the correct stoichiometry, densification 
and crystal form. Chemical vapor deposition techniques are particularly 
useful for the purpose of coating the carbon fiber since the fiber, which 
has a low electrical resistivity, can be electrically heated. Volatile 
compounds used to make the superconductor can be then decomposed and 
deposited by contact with the hot carbon fiber. Halides, organometallic 
compounds and other volatile compounds can be used for this purpose. 
Electroplating can be also a particularly good method of putting a metal 
layer down on the fiber. 
Other potentially useful deposition techniques embrace the following: 
1. sol-gels and soaps 
2. sputtering followed by oxidation 
3. electron beam evaporation followed by oxidation 
4. liquid phase epitaxy 
5. laser induced deposition 
Care should be taken however, that the method chosen is able to completely 
cover the surface to be coated--a very desirable condition for all the 
layers described herein. A final adhering metal or alloy coating of the 
composite is desirable to protect it from decomposition by air, moisture, 
etc., and also to provide additional electrical and thermal stabilization. 
It can also serve as a suitable surface to which electrical connections 
can be made; for example, connection of the composite to a power source. 
Such conducting materials as gold, silver, copper, aluminum, solder, and 
the like, can be used for this purpose. Alternatively, the coating can be 
of material which is able to be coated at the conductor ends with a 
solderable material. Such coatings should be relatively thin, about a few 
hundreds up to about a few thousands of Angstroms thick and are best laid 
down in a continuous coating by CVD, sputtering, electroplating, etc., as 
detailed above. Vapor deposition and electroplating methods are preferred. 
In FIG. 1, a cross-sectional view of a composite of the instant invention 
is shown. The inner ring 1 is the carbon fiber, the middle ring 2 is the 
superconductor layer, and 4 represents a thin enrobing outer layer. In 
FIG. 2, the same composite is shown except that an intermediate ring 3, a 
compressive layer, has been added. 
Low resistivity, pitch-based carbon fiber as a substrate for 
superconductors is not only excellent because of the high strength, high 
thermal conductivity, inertness and ultrahigh modulus advantages but also 
because it lends itself to continuous industrial production. For example, 
carbon yarn could be payed off a supply spool and into a reactor where 
electrical contacts on the yarn cause it to be locally heated and where a 
pyrolytic C/Al.sub.2 O.sub.3 layer is applied. From this first heated 
zone, the yarn could travel in a continuous fashion through a second 
chamber where, again, the yarn is heated by its own electrical resistance 
to a temperature sufficient to expel the aluminum and oxygen and 
graphitize the low density compressive carbon layer left behind. The yarn 
could then enter a third chamber where it is similarly heated and coated 
with the protective layer if required, initially, and then with the 
desired superconductor. As the yarn is moved along, other chambers could 
be placed in the line to adjust the stoichiometry of the superconducting 
layer or to anneal it in order to optimize its superconducting properties. 
A following chamber could then apply an outer coating to the yarn, 
possibly using electroplating, before it is wound on a spool as a finished 
product. 
The following Examples will serve to illustrate certain specific 
embodiments of the herein disclosed invention. These Examples should not, 
however, be construed as limiting the scope of the novel invention 
contained herein as there are many variations which may be made thereon 
without departing from the spirit of the disclosed invention, as those of 
skill in the art will recognize. 
ILLUSTRATIVE EXAMPLES 
EXAMPLE 1 
A ceramic-type superconductor is made by intimately mixing and grinding 
yttrium nitrate, copper (II) nitrate and barium hydroxide in the proper 
proportions followed by heating the mixture in an inert container in air 
at about 900.degree. C. The solidified black mass is reground and reheated 
several times, and its superconducting transition temperature, tested by 
measuring its resistivity, found to be about 93.degree. K. 
A portion of the 1-2-3 superconductor made above is dissolved in an 
oxidizing inorganic acid and a short length of a pitch-based, high 
strength THORNEL.RTM. ultrahigh modulus, high thermal conductivity, low 
resistivity fiber dipped into the dark solution of superconductor. The 
fiber is dried and annealed in a nitrogen atmosphere by heating briefly at 
900.degree. C. followed by a long anneal in flowing oxygen at about 
500.degree. C., followed by a slow cool. A resistivity versus temperature 
test of the yarn shows that its resistivity dropped precipitously at about 
83.degree. K., indicating the composite is superconducting at that 
temperature. 
The THORNEL.RTM. fiber used in this Example is made by Amoco Performance 
Products, Inc., Ridgefield, Conn. 06877 and is a strong, ultra high 
tensile, pitch-based yarn with the following average properties: strength 
350-400 ksi; modulus&gt;about 130 Msi; density about 2.2 g/cc; CTE about 1.9 
ppm/.degree. C.; thermal conductivity 1100-1200 W/mK and electrical 
resistivity about 1.1 .mu.ohm-m. The yarn is supplied as a continuous roll 
with a nominal fiber count of about 2000, each filament of an effective 
diameter about 10 microns. Other details of such fibers are to be found in 
U.S. Pat. No. 4,005,183 and European Published Application 85-200687.3, 
both of which are incorporated herein by reference. 
EXAMPLE 2 
The THORNEL.RTM. fiber of Example 1 is coated with an approximately 1 
micron thick coating of aluminum oxide/pyrolytic carbon by the method of 
U.S. Pat. No. 3,799,790 and the aluminum oxide removed by vacuum 
evaporation at about 2200.degree. C. leaving a porous, low density, highly 
graphitic coating on the fiber. The latter is then dipped in the solution 
of superconductor made up as in Example 1 and dried and annealed in the 
same way. When its resistivity as a function of temperature is measured, 
the test showed that the composite's resistivity dropped precipitously at 
about 83.degree. K. indicating the composite is superconducting at that 
temperature.