Sealed glass coating of high temperature ceramic superconductors

A method and article of manufacture of a lead oxide based glass coating on a high temperature superconductor. The method includes preparing a dispersion of glass powders in a solution, applying the dispersion to the superconductor, drying the dispersion before applying another coating and heating the glass powder dispersion at temperatures below oxygen diffusion onset and above the glass melting point to form a continuous glass coating on the superconductor to establish compressive stresses which enhance the fracture strength of the superconductor.

The present invention is concerned generally with a method and coating for 
high temperature ceramic superconductors. More particularly, the invention 
is concerned with a method and glass coating applied to a ceramic 
superconductor at low temperatures to avoid chemical reaction with the 
superconductor material. The coated superconductor acts to enhance 
fracture strength and provide a moisture and environmental barrier for the 
superconductor. The methodology was tested successfully using one of the 
most reactive ceramic superconductors, YBa.sub.2 Cu.sub.3 O.sub.x. 
The high temperature ceramic superconductors have demonstrated the 
potential for many technological applications. However, as a consequence 
of being ceramic materials, physical properties which are unrelated to 
electrical conductivity make their applicability to practical problems 
questionable. For example, one of the primary deficiencies in metal oxide 
superconductors as applied to electrical power systems (for use in motors, 
power transmission and power storage) is the extremely brittle mechanical 
characteristics and sensitivity to flaws. Conventional solutions to this 
problem include: (a) manufacture of components with larger dimensions than 
needed for electrical purposes to minimize the chances of fracture during 
use, (b) encasing the ceramic superconductor components in surrounding 
protective housings, (c) forming a precursor metal structure having the 
desired final geometry and then performing complex oxidation treatments to 
achieve the final ceramic composition; this method avoids applying 
stresses to the ceramic, such as would occur in direct formation of a coil 
of ceramic superconductor wire; and (d) use of microstructural 
modifications, such as metal particles and high strength whisker phases 
dispersed in the ceramic superconductor. These methods all suffer from a 
variety of disadvantages associated with providing additional components 
and the necessity of performing complex chemical processing steps. 
It is, therefore, an object of the invention to provide an improved method 
and coating for a ceramic superconductor. 
It is another object of the invention to provide a novel method and glass 
coating for a ceramic superconductor. 
It is a further object of the invention to provide an improved method and 
sealed glass coating for a ceramic superconductor wherein the glass 
coating is applied at low softening temperatures. 
It is yet another object of the invention to provide a novel method and 
glass coating for ceramic superconductors which does not cause a chemical 
reaction and provides a passivating covering of the superconductor. 
It is an additional object of the invention to provide an improved method 
and glass coating for ceramic superconductors resulting in substantially 
enhanced fracture strength for the superconductor. 
It is still a further object of the invention to provide a method of 
applying a fireable glass dispersion to a ceramic superconductor allowing 
formation of a substantially complete glass coating on a ceramic 
superconductor.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
A YBa.sub.2 Cu.sub.3 O.sub.x ceramic superconductor was selected for 
treatment in order to test one of the most reactive forms of the ceramic 
superconductors. For example, this class of superconductor is known to be 
much more sensitive to chemical effects than bismuth or thallium based 
ceramic superconductors. Therefore, the method and coatings were developed 
such that chemical reactions are minimized and a uniform coating achieved 
on the ceramic superconductor. Glass powders are dispersed in a medium, 
such as an organic solution or other suitably viscous (and chemically 
nonreactive) medium, which allows suspension of the glass powder. 
Subsequently, after applying the glass powder containing solution the 
dispersal medium can be removed, leaving a glass powder on the 
superconductor. 
In order to achieve the desired increase in fracture toughness for the 
ceramic superconductor, it is highly preferred to have a compressive 
residual stress exerted on the surface of the ceramic superconductor. This 
can be achieved by selecting glass compositions which have coefficients of 
thermal expansion which are slightly less than the ceramic superconductor. 
Such glasses can, for example, include lead oxide (80%-95%), boron oxide 
(10%-20%), silicon dioxide (0.5%-1.5%), zinc oxide (0%-10%) and tin oxide 
(0%-5%). 
In a preferred form of the invention, a stable dispersion of glass powders 
are therefore suspended in an organic solution which does not react with 
the ceramic superconductor. The powders in suspension can be applied to 
ceramic superconductor wires by dipping the wire into the glass/solution 
mixture. In other embodiments, the glass powder solution can be sprayed or 
painted onto the superconductor surfaces. After applying the glass powder 
solution to the superconductor, the volatile constituents can be easily 
removed by mild heating, such as at 100.degree. C., between coatings being 
applied. 
In order to form a bonded glass coating which is smooth and continuous and 
establishes a compressive residual stress on the superconductor surface, 
the dried coating is fired at temperatures of about 400.degree. C. in an 
oxygen containing atmosphere for roughly 0.5-2 hours. Under these general 
conditions, oxygen loss from the superconductor is limited due to the slow 
diffusion of oxygen at such low temperature. Consequently, the fundamental 
superconducting properties will not be affected by unwanted chemical 
modifications of the superconductor. The superconducting transition 
temperature and sharpness of the superconducting transition were 
unaffected for the coated superconductor processed in accordance with the 
invention. 
The resulting coated superconductors underwent fracture strength testing, 
and the level of strength increased substantially. Upon achieving a 
thickness of about 0.4 mm, the level of fracture strength was increased 
about four times the level of the uncoated ceramic superconductor. 
The following nonlimiting example set forth procedures and results for 
coatings on YBa.sub.2 Cu.sub.3 O.sub.x. 
EXAMPLE I 
A Ferro Glass 2760 was obtained in powder form from Ferro Corp., Cleveland, 
Ohio. This glass and other examples described hereinafter have at least 
lead oxide (80-95%), boron oxide (10-20%), and silicon oxide (0.5-1.5%). 
Some also have zinc oxide (0-10%) and tin oxide (0-5%). The high lead 
content insures low softening points and high thermal expansion 
coefficients. This particular 2760 glass primarily is a lead oxide-zinc 
oxide-boron oxide-silicon oxide glass of approximately 44 micron mean 
particle size. These powders were dispersed in isopropyl alcohol with 
50-80% glass powder by weight. The Glass 2760 has a softening point of 
370.degree. C. and a thermal expansion coefficient of 11.3.times.10.sup.-6 
.degree. C..sup.-1 compared to about 15.times.10.sup.-6 .degree. C..sup.-1 
for YBa.sub.2 Cu.sub.3 O.sub.x. The powders were applied to YBa.sub.2 
Cu.sub.3 O.sub.x wire (about 1000 microns wire diameter) by dipping in the 
glass powder solution and dried quickly at about 100.degree. C. between or 
after being dipped. The dipped wire was then fired at about 400.degree. C. 
in an oxygen atmosphere for about 0.5-2 hours to bond the coatings to the 
YBa.sub.2 Cu.sub.3 O.sub.x wire and form a continuous, smooth coating 
thereon. A number of different thickness coatings were obtained (depending 
on the number of times the wire was dipped or otherwise applied), and the 
resulting increase in fracture strength can be seen in FIG. 5. The 
fracture tests were based on a four-point bending method with the inner 
load span 13 mm and the outer load span 25 mm. 
EXAMPLE II 
Substantially the same procedure was followed as for Example I, but a 
Coming Glass 7567 was used as the starting glass powder. The 7567 is 
primarily a lead oxide-zinc oxide-boron oxide-silicon oxide of 
approximately 44 microns mean particle size. These powders were dispersed 
in xylene with 4-20% by weight rubber cement in 80-96% xylene and 50-80% 
by weight glass powder. The 7567 has a softening point of about 
358.degree. C. and a thermal expansion coefficient of 12.0.times.10.sup.-6 
.degree. C..sup.-1 compared to about 15.times.10.sup.-6 .degree. C..sup.-1 
for YBa.sub.2 Cu.sub.3 O.sub.x. The 7567 was then fired at about 
400.degree.-410.degree. C., and the resulting effect of the coatings can 
be seen in FIG. 5. 
EXAMPLE III 
Substantially the same procedure was followed as for Example I, but a Ferro 
Glass 4000 was used as the starting glass powders. The 4000 is primarily a 
lead oxide-boron oxide-silicon oxide glass of 44 micron mean particle 
size. These powders were dispersed in isopropyl alcohol with 50-80% glass 
powder by weight. The 4000 glass has a softening point of about 
360.degree. C. and a coefficient of thermal expansion of 
8.1.times.10.sup.-6 .degree. C..sup.-1 compared to about 
15.times.10.sup.-6 .degree. C..sup.-1 YBa.sub.2 Cu.sub.3 O.sub.x. 
The invention therefore allows achievement of an increase of about four 
times the fracture strength of an uncoated ceramic superconductor without 
affecting the superconducting properties (the transition temperature 
remained the same and the transition was sharp). This approach does not 
degrade the chemical structure of highly reactive YBa.sub.2 Cu.sub.3 
O.sub.x, and thus the technique is equally applicable to other ceramic 
superconductors. The glass coating further acts as an environmental 
barrier and thermal barrier to heat loss when needed (such as for 
temperature sensors). 
While preferred embodiments of the invention have been shown and described, 
it will be clear to those skilled in the art that various changes and 
modifications can be made without departing from the invention in its 
broader aspects as set forth in the claims provided hereinafter.