A polycrystalline Y-Ba-Cu-O superconductor doped with sufficient .sup.235 U and/or .sup.239 Pu atoms is irradiated with thermal neutrons to produce from about 2.5.times.10.sup.14 to about 50.times.10.sup.14 fission events per cubic centimeter of superconductor.

This invention relates to a polycrystalline Y--Ba--Cu--0 superconductor 
material which has been doped with fissionable material and irradiated to 
improve its superconductive properties. 
This invention is related to U.S. Pat. No. 3,310,395 to Swartz et al., 
assigned to the assignee hereof and incorporated herein by reference. 
The discovery of ceramic superconductors with high critical temperatures 
has greatly improved the prospect of widespread practical application of 
superconductivity. Unfortunately, there are still major hurdles to be 
overcome before such prospects can be realized. In particular the 
critical-current densities of sintered, polycrystalline high-temperature 
superconductors are much too low, especially in useful magnetic fields at 
higher temperatures. It appears that the intergranular superconductive 
coupling in polycrystalline materials is very weak and that flux pinning 
within grains is low, leading to very rapid flux creep and low 
critical-current densities even in single grains. 
The pinning of flux within bulk type-II superconductors is caused by 
imperfections in the lattice. For traditional practical low-temperature 
superconductors such as Nb--Ti or Nb.sub.3 Sn, such imperfections or 
pinning centers include dislocations, grain boundaries, and 
non-superconducting inclusions or voids. For the newer, high-temperature 
ceramic superconductors it is not clear what the naturally occurring weak 
pinning centers are. The traditional methods of introducing lattice 
defects, such as cold work and grain-boundary control through heat 
treatment, appear not to be useful for the ceramic materials; they are 
brittle and their grain boundaries are so weak that they increase the 
movement of flux instead of decreasing it. Another approach that has been 
effective in the older, low-temperature superconductors is the 
introduction of pinning centers by radiation damage. 
In one embodiment of the present invention, fission-fragment damage is 
introduced by doping Y--Ba--Cu--O with natural uranium and exposing the 
material to thermal neutrons, inducing fission of the uranium -235 
component. The special opportunity that is envisioned in inducing fission 
is that the fraction of dispersed, isolated point defects is minimized 
relative to clumps of displaced atoms and disordered material, which are 
expected to be more effective pinning centers. Thus, greater flux pinning 
should be created before the total damage begins to degrade the critical 
temperature seriously.

Briefly stated, the present process comprises providing a matrix-forming 
powder of oxides or precursor therefor wherein the oxide composition 
corresponds to the matrix composition comprised of YBa.sub.2 Cu.sub.3 
O.sub.7.times.y where y ranges from zero to about 1, providing an oxide 
additive or precursor therefor selected from the group consisting of 
natural uranium dioxide, uranium-235 dioxide, plutonium -239 dioxide and 
mixtures thereof, all of the uranium component of said additive and all of 
the plutonium-239 component of said additive providing dopant atoms, said 
natural uranium dioxide providing dopant atoms comprised of natural 
uranium which includes the uranium-235 isotope, forming a mixture of said 
matrix-forming powder and said additive or precursor therefor, heating 
said mixture to a reaction temperature ranging from greater than about 
800.degree. C to below the melting point of said oxides to produce a 
reaction product comprised of said matrix composition and said dopant 
atoms wherein said dopant atoms being substituted in said matrix 
composition in an amount ranging to their solubility limit therein, said 
precursors decomposing below said reaction temperature producing said 
oxides, comminuting said reaction product to produce a sinterable powder, 
forming said sinterable powder into a compact, sintering said compact in 
an oxidizing atmosphere at a temperature ranging from about 900.degree. C. 
to below the melting point of said sinterable powder producing a sintered 
polycrystalline body having an open porosity ranging from zero to about 
20% by volume of said body, cooling said body in an oxidizing atmosphere 
at a rate which produces a superconductive body and irradiating said 
superconductive body with thermal neutrons causing a sufficient number of 
said uranium-235 and/or plutonium-239 dopant atoms to undergo fission to 
produce from about 2.5.times.10.sup.14 to about 50.times.10.sup.14 fission 
events per cubic centimeter of the resulting irradiated superconductive 
body, said uranium-235 and/or plutonium-239 dopant atoms being present in 
an amount sufficient to produce said fission events. 
In the present process, there is no loss, or no significant loss, of dopant 
atoms prior to irradiation of the superconductive body. Therefore, the 
number of dopant atoms in the mixture comprised of the matrix-forming 
powder and additive is the same as, or not significantly different from, 
the number of dopant atoms in the resulting sinterable powder, or in the 
resulting sintered body, prior to irradiation. 
Generally, in carrying out the present process, yttrium oxide, barium 
carbonate, and copper oxide are used to provide the matrix-forming powder. 
They are formulated to have a metal oxide composition which corresponds to 
the composition YBa.sub.2 Cu.sub.3 O.sub.7-y, wherein y ranges from zero 
to about 1, frequently from zero to about 0.7. 
The present additive powder is an oxide, or precursor therefor, selected 
from the group consisting of natural uranium dioxide (UO.sub.2), 
uranium-235 dioxide (.sup.235 UO.sub.2), plutonium-239 dioxide (.sup.239 
PuO.sub.2) and mixtures thereof. The particular amount of additive is 
determined empirically. The additive should be used at least in an amount 
which provides sufficient uranium-235 and/or plutonium-239 dopant atoms to 
produce in the present process from about 2.5.times.10.sup.14 to about 
50.times.10.sup.14 fission events per cubic centimeter of the resulting 
irradiated superconductive body. 
Natural uranium dioxide powder, which is derived from powdered uranium ore, 
provides dopant atoms comprised of natural uranium which includes three 
natural radioactive isotopes. These are (% by weight of total natural 
uranium atoms) U-234 (0.006%), U-235 (0.7%) and U-238 (99%). Uranium-235 
(.sup.235 U) is a readily fissionable isotope, and it is this isotope of 
uranium which is caused to undergo fission herein. 
Generally, natural uranium dioxide powder is used in an amount ranging from 
about 0.04% to about 0.2% by weight of the mixture comprised of the 
matrix-forming powder and additive. Generally, it is used in an amount 
which provides dopant atoms, i.e. its entire uranium component including 
isotopes thereof, ranging from about 150 to about 380 atomic parts, i.e. 
dopant atoms, per million atomic parts of the total amount of the mixture 
of matrix-forming powder and additive. Generally, for optimum results, the 
natural uranium dioxide is used in an amount which provides within about 
.+-.5% of the solubility limit of the dopant atoms in the sintered 
superconductive body, before irradiation thereof, which is about 200 
atomic parts of dopant atoms, per million atomic parts the mixture of 
matrix-forming powder and additive. 
Uranium-235 dioxide is preferred since it provides only U-235 dopant atoms 
thereby providing a larger effect, i.e. more fission events, with lesser 
irradiation. Generally, uranium-235 dioxide is used in an amount ranging 
from about 3 parts per million to about 2000 parts per million, preferably 
between 6 and 800 parts per million, by weight of the mixuture of 
matrix-forming powder and additive. Generally, the .sup.235 UO.sub.2 
powder is used in an amount which provides from about 0.6 to about 380 
atomic .sup.235 U parts per million atomic parts of the mixture of 
matrix-forming powder and additive. For optimum results, the .sup.235 
UO.sub.2 powder provides within about .+-.5% of the solubility limit of 
the .sup.235 U atoms in the sintered body which is about 150 atomic 
.sup.235 U parts per million atomic parts the mixture of matrix-forming 
powder and additive. 
Generally, .sup.239 PuO.sub.2 ranges from about 2 parts per million to 
about 0.15%, preferably about between about 5 and 600 parts per million, 
by weight of the mixture of matrix-forming powder and additive. Generally, 
the .sup.239 PuO.sub.2 powder provides from about 0.4 to about 380 atomic 
.sup.239 Pu parts per million atomic parts of the mixture of 
matrix-forming powder and additive. For optimum results, the .sup.239 
PuO.sub.2 powder provides within about .+-.5% of the solubility limit of 
.sup.239 Pu atoms in the sintered body. 
In one embodiment, the additive is comprised of a mixture formed by adding 
.sup.235 UO.sub.2 to natural uranium dioxide, i.e. enriched uranium 
dioxide, wherein .sup.235 UO .sub.2 comprises from about 1% to about 93% 
by weight of the additive, or wherein the .sup.235 U atoms comprise from 
about 1% to about 93% by weight of the dopant atoms. 
If desired, a particulate inorganic precursor of the reactant oxides can be 
used. The precursor should decompose completely to form the oxide and 
by-product gas or gases leaving no contaminants in the reacted mass. 
Barium carbonate is a useful precursor for barium oxide. The precursor 
should be used in an amount sufficient to produce the respective oxide in 
the required amount. 
The oxides or precursors therefor should be of a size which allows the 
reaction product to be produced. Generally, these powders are used in the 
particle size range in which they are available commercially, which 
ordinarily ranges from submicron up to about 100 microns. The powders 
should be free of large, hard aggregates, i.e. significantly above 100 
microns in size, which might survive the mixing process and prevent 
sufficient reactant contact for satisfactory reaction rates. 
The matrix-forming powders and additive are admixed to form a mixture which 
preferably is uniform or substantially uniform in order to produce a 
reaction product which preferably is uniform or substantially uniform. 
Mixing of the powders can be carried out by a number of conventional 
techniques such as, for example, ball milling. 
The mixture of matrix-forming powder and additive is reacted to produce the 
present reaction product. The mixture is reacted in an oxidizing 
atmosphere generally at a temperature ranging from greater than about 
800.degree. C. to below the melting point of the metal oxides. Frequently, 
reaction temperature ranges from about 850.degree. C. to 1000.degree. C. 
or from about 900.degree. C. to 950.degree. C. Reaction time is determined 
empirically. Generally, the reaction product is cooled in an oxidizing 
atmosphere to about room temperature. Generally, the oxidizing atmosphere, 
i.e. the atmosphere for carrying out the reaction as well as for cooling 
the reaction product, is comprised of at least about 1% by volume of 
oxygen and the remainder of the atmosphere is a gas which has no 
significant deleterious effect on the reaction product. Representative of 
such gases is nitrogen or a noble gas such as argon or helium. Preferably, 
the oxidizing atmosphere is comprised of oxygen or air. Generally, the 
oxidizing atmosphere is at about atmospheric pressure. 
The reaction product is comprised of the matrix composition YBa.sub.2 
Cu.sub.3 O.sub.7-Y, where y ranges from zero to about 1, frequently from 
about zero to about 0.7, and the present dopant atoms which are 
distributed in the matrix composition. The dopant atoms are dissolved in 
the matrix composition in an amount ranging to their solubility limit 
therein. 
The reaction product is comminuted to produce the desired sinterable 
powder. Comminution can be carried out in a conventional manner such as, 
for example, by milling. Generally, the sinterable powder has an average 
particle radius ranging from submicron to about 10 microns, frequently 
from about 0.1 micron to about 5 microns, or from about 0.2 micron to 
about 4 microns. Average particle size can be determined by conventional 
techniques. 
A number of conventional procedures can be used to form the sinterable 
powder into a compact. For example, the sinterable powder can be extruded, 
injection molded, die-pressed, slip cast or tape cast to produce the 
compact of desired shape. 
Lubricants, dispersants, binders, or similar form-promoting materials, 
useful in producing the compact can be admixed with the sinterable powder. 
Such materials are well-known in the art and can be used in a conventional 
manner with the particular amount thereof being determined empirically. 
Generally, they are organic preferably of the type which evaporate or 
decompose on heating at relatively low temperatures, preferably below 
500.degree. C., leaving no residue or no significant residue. The 
form-promoting material should have no significant deleterious effect in 
the present process. 
The compact should have a density at least sufficient to produce the 
present sintered body. Preferably, it has a density of at least about 45% 
of its theoretical density to promote densification during sintering. 
Sintering of the compact is carried out in an oxidizing atmosphere which 
generally is about atmospheric pressure. The oxidizing atmosphere should 
be at least sufficiently oxidizing to produce a sintered body wherein the 
O, i.e. oxygen, component has a value of at least about 6.0. Generally, 
the sintering, i.e. firing, atmosphere contains at least about 1% by 
volume of oxygen and the remainder of the atmosphere should be a gas which 
has no significant deleterious effect on the sintered product. 
Representative of such gases is nitrogen or a noble gas such as argon or 
helium. Most preferably, the sintering atmosphere is comprised of oxygen. 
Sintering is carried out at a temperature ranging from about 900.degree. C. 
to below the melting temperature of the sinterable powder. Generally, 
sintering temperature ranges from about 900.degree. C. to about 
1000.degree. C., and typically it ranges from about 950.degree. C. to 
about 975.degree. C. The particular sintering temperature is determined 
empirically and depends largely on particle size, density of the compact 
and final density desired in the sintered product. Generally, higher 
sintering temperatures produce sintered bodies of higher density and 
larger grain size. 
Sintering time can vary and is determined empirically. Longer sintering 
times generally produce sintered bodies with larger grains. Generally, 
sintering time ranges from about two hours to eight hours. 
The sintered body is cooled in an oxidizing atmosphere generally at about 
atmospheric pressure at a rate which produces the present superconductive 
body. The cooling schedule can vary and is determined empirically. 
Generally, the cooling oxidizing atmosphere contains at least about 20% by 
volume of oxygen and the remaining gas should have no significant 
deleterious effect on the superconductive product. Preferably, the 
oxidizing atmosphere is air but more preferably it is oxygen. 
Specifically, during the cooling procedure, generally at a temperature 
ranging from about 700.degree. C. to about 400.degree. C., the sintered 
body should be cooled at a rate sufficient to produce the orthorhombic 
crystal structure in an amount at least sufficient to produce the 
superconductive body. Generally, in this temperature range of about 
700.degree. C. to about 400.degree. C., additional oxygen is incorporated 
into the body. Sufficient oxygen should be incorporated in the body to 
permit formation of the required orthorhombic crystal structure. 
Cooling of the body from about 400.degree. C. can be at a more rapid rate, 
but not so fast as to fracture the body by thermal shock. The body is 
usually cooled to room temperature, i.e. from about 15.degree. C. to about 
30.degree. C. The present process has no significant effect on the amounts 
of the other, i.e. non-oxygen, components of the body. 
The sintered body and the resulting superconductive body have the same 
density or porosity. The body may have some closed porosity and generally 
has open porosity. Preferably, pores are small, preferably less than one 
micron, and sufficiently distributed in the body so that they have no 
significantly deleterious effect on mechanical properties. Porosity can be 
determined by standard metallographic techniques, such as, for example, 
optically examining a polished cross-section of the body. 
By closed porosity, it is meant herein closed pores or voids in the 
sintered body, i.e. pores not open to the surface of the body and 
therefore not in contact with the ambient atmosphere. Generally, closed 
porosity ranges from 0 to about 10%, preferably it is less than about 5%, 
or less than about 1% by volume of the body. 
By open porosity, it is meant herein pores or voids which are open to the 
surface of the sintered body, thereby making the interior surfaces 
accessible to the ambient atmosphere. 
The sintered body should have sufficient surface area to permit production 
of the superconductive body and this is determined empirically. 
Specifically, the sintered body, during cooling thereof in an oxidizing 
atmosphere, should have at least sufficient surface area for contact with 
oxygen to allow production of the superconductive body. Generally, a 
portion of the surface area of the sintered body is provided by its open 
porosity. For a very thin body, open porosity may not be needed. 
Generally, the superconductive body has an open porosity ranging from 0 to 
about 20%, frequently from about 2% to about 20%, or from about 5% to 
about 15%, by volume of the body. 
Preferably, the total porosity of the sintered body is not greater than 
about 20%. 
Generally, the superconductive sintered body, before and after irradiation, 
is comprised of grains which are substantially equiaxed. Average grain 
size is at least about 1 micron and can range widely depending largely on 
the size of the sinterable powder and sintering conditions. Generally, 
average grain size can range from about 1 micron to about 100 microns, 
frequently from about 5 microns to about 15 microns. 
The superconductive body contains the orthorhombic crystal structure in an 
amount at least sufficient to give the desired superconductivity. 
Generally, the presence of the orthorhombic phase can be determined by 
x-ray diffraction analysis, transmission electron microscopy, or polarized 
light microscopy. The superconductive body is polycrystalline. 
Superconductivity of the sintered body, before and after irradiation, can 
be determined by conventional techniques. For example, it can be 
demonstrated by magnetic flux exclusion, the Meissner effect. Generally, 
the present superconductive body, before and after irradiation thereof, 
has a zero resistance transition temperature, i.e. a temperature below 
which there is no electrical resistance, greater than about 77K, 
preferably at least about 85K, and most preferably higher than about 90K. 
In carrying out the present process, the superconductive body is irradiated 
with thermal neutrons to cause a sufficient number of uranium-235 dopant 
atoms, or plutonium-239 dopant atoms, or a mixture thereof to undergo 
fission to produce from about 2.5.times.10.sup.14 to about 
50.times.10.sup.14 fission events per cubic centimeter of the body. By 
fission events, it is meant herein the energetic process of division of a 
fissionable nucleus that has absorbed a neutron into primarily two heavy 
nuclei, each comprising substantially half of the mass of the original 
fissionable nucleus, and the motion of these fission fragments until they 
come to rest. The fission products are known and can be identified and 
counted by a mass spectrometer. It is the production and motion of these 
fission fragments which causes radiation damage in the grains of the body 
resulting in a significant improvement in certain superconductive 
properties. Generally, the production of less than about 
2.5.times.10.sup.14 fission events per cubic centimeter of the body may 
not have a significant effect on its properties. The production of more 
than about 50.times.10.sup.14 fission events per cubic centimeter of the 
body may have a significant deleterious effect thereon, such as, for 
example, lowering its zero resistance transition, i.e. critical, 
temperature substantially. Preferably, irradiation is carried out to 
produce from about 6.times.10.sup.14 to about 12.times.10.sup.14 fission 
events per cubic centimeter of the body. 
The present irradiation of the superconductive body with thermal neutrons 
can be carried out in a known manner. The radiation dose should be 
sufficient to produce the present fission events in the body and should 
not have a significantly deleterious effect thereon. The particular 
radiation dose is calculated from the number of .sup.235 U and/or .sup.239 
Pu dopant atoms in the body, the fission cross-sections of the .sup.235 U 
and/or .sup.239 Pu, and the particular superconductive properties desired. 
Generally, the minimum radiation dose ranges from about 1.times.10.sup.13 
to about 1.times.10.sup.17 thermal neutrons per square centimeter of the 
body. Generally, a radiation dose of at least about 1.times.10.sup.13 
thermal neutrons per square centimeter of the body is required when the 
dopant atoms are comprised of .sup.235 U or .sup.239 Pu. Generally, a 
radiation dose of at least about 1.times.10.sup.17 thermal neutrons per 
square centimeter of the body is required when the dopant atoms are 
comprised of natural uranium. 
Generally, irradiation is carried out as close to room temperature as the 
neutron source that is used permits. Irradiation is carried out in an 
atmosphere which can be at about atmospheric pressure or a partial vacuum 
which has no significant deleterious effect on the superconductive body 
which can be determined empirically. Preferably, irradiation is carried 
out in an atmosphere of oxygen or an oxygen-enriched atmosphere at about 
atmospheric pressure or a partial vacuum if the sample has a tendency to 
lose oxygen. 
Generally, the present process lowers the zero resistance transition 
temperature of the superconductive body less than 30%, preferably less 
than 20%, and more preferably it does not change it, or does not change it 
significantly. 
The present process produces a superconductive polycrystalline body having 
a significantly higher magnetic hysteresis at a temperature at which it is 
superconductive above 5K. Generally, the magnetic hysteresis of the body 
is at least about 10%, and frequently more than 100%, higher at a 
temperature at which it is superconductive above 5K. 
By a temperature at which the body is superconductive, it is meant herein a 
temperature at which it has no electrical resistance. 
Also, the present process produces a polycrystalline body comprised of 
grains wherein the individual grains have a significantly higher critical 
current density at a temperature above 5K at which the body is 
superconductive. Generally, the intra-grain critical current density is at 
least about 10%, and frequently more than 100%, higher at a temperature at 
which the body is superconductive above 5K. 
Since magnetic hysteresis is a measure of critical current density, it can 
be inferred that the present process produces a superconductive 
polycrystalline body having a significantly increased critical current 
density, generally at least about 10% higher, at temperatures above 5K at 
which the body is superconductive. 
Briefly stated, the present superconductive polycrystalline body is 
comprised of the composition YBa.sub.2 Cu.sub.3 O.sub.7--y, where y ranges 
from zero to about 0.3, and dopant atoms selected from the group 
consisting of natural uranium, .sup.235 U, .sup.39 Pu, and mixtures 
thereof, said natural uranium atoms ranging from a detectable amount to 
less than about 380 atomic parts per million atomic parts of said body, 
said .sup.235 U atoms ranging from a detectable amount to less than about 
380 atomic parts per million atomic parts of said body, said .sup.239 Pu 
atoms ranging from a detectable amount to less than about 380 atomic parts 
per million atomic parts of said body, said dopant atoms being substituted 
in said composition in an amount ranging to their solubility limit 
therein, said body containing from about 2.5.times.10.sup.14 to about 
50.times.10.sup.14 fission events per cubic centimeter of said body. 
The present irradiated superconductive body contains at least a detectable 
amount of dopant atoms, i.e. an amount detectable by a mass spectrometer. 
In one embodiment, the dopant atoms in the present irradiated 
superconductive body are comprised of natural uranium ranging from greater 
than about 150 atomic parts per million atomic parts of the body to within 
about .+-.5% of the solubility limit of the atoms in the body. 
In another embodiment, the dopant atoms in the present irradiated body are 
comprised of .sup.235 U atoms ranging from about 1 atomic part per million 
atomic parts of the body to within about .+-.5% of their solubility limit 
in the body. 
In another embodiment, the dopant atoms in the irradiated body are 
comprised of .sup.239 Pu atoms ranging from about 1 atomic part per 
million atomic parts of the body to within about .+-.5% of their 
solubility limit in the body. 
The superconductive body produced by the present process is useful as a 
conductor for magnets, motors, generators, and power transmission lines. 
The invention is further illustrated by the following examples: 
EXAMPLE 1 
Three powder compositions were prepared. Powder A was formulated to give 
YBa.sub.2 Cu.sub.3 O.sub.7. Powder B was formulated to give YBa.sub.2 
Cu.sub.3 O.sub.7 and included UO.sub.2 in an amount of 0.08% by weight of 
the total amount of Powder B (150 atomic ppm U). Powder C was formulated 
to give YBa.sub.2 Cu.sub.3 O.sub.7 and included UO.sub.2 in an amount of 
0.2% by weight of the total amount of Powder C (380 atomic ppm U). Natural 
uranium dioxide was used. 
The powders were prepared in the same manner. The required amounts of 
yttrium oxide, copper oxide, barium carbonate and uranium dioxide (none 
for Powder A) were ball milled in methanol with zirconia balls for four 
hours at room temperature. After drying for four hours under heat lamps, 
the powders were screened -28 mesh. 
Each resulting screened powder was calcined in air at about atmospheric 
pressure in an alumina tray at 955.degree. C for 24 hours. After 
calcining, each powder was quite friable. 
Each resulting powder was milled at room temperature for two hours in 
heptane using a few drops of a wetting agent sold under the trademark 
Hamposyl-0. Each powder was then dried under nitrogen at 50.degree. C. 
X-ray diffraction analysis of each of the powders showed only the YBa.sub.2 
Cu.sub.3 O.sub.7 phase. No identification of the microstructural location 
of the uranium was possible by x-rays. 
Each powder was then die pressed at 12,000 psi. The resulting compacts were 
sintered in an atmosphere of oxygen at about atmospheric pressure. The 
heating rate was .RTM..degree. C. per hour to 955.degree. C. where the 
samples were held for 8 hours. Cooling was also done in an oxygen 
atmosphere at about atmospheric pressure. Cooling to 400.degree. C. took 4 
hours, and the samples were held at 400.degree. C. for 2 hours before 
cooling to room temperature. 
Sintered body A (formed from Powder A) had a density of 5.62 g/cm.sup.3. 
Sintered body B (formed from Powder B) had a density of 5.45 g/cm.sup.3. 
Sintered body C (formed from Powder C) had a density of 5.66 g/cm.sup.3. 
It was determined that sintered bodies A, B, and C were superconductive and 
had a zero resistance transition temperature greater than 77K. 
EXAMPLE 2 
Bars about 3.5mm .times. 3.5mm .times. 5mm were cut from the sintered 
bodies produced in Example 1 for irradiation and testing. 
The samples were sealed in fused quartz tubes under 1/2 atmosphere of 
oxygen, and were exposed to thermal neutrons (ratio of &lt;1 MeV to &gt;1 MeV, 
1700:1) in Port V-11 of the Vertical Irradiation Thimble of the Brookhaven 
High Flux Beam Reactor. The nominal background temperature during 
irradiation was 60.degree. C. the highest temperature was less than 
100.degree. C. The samples were exposed to nominal fluences of 
2.times.10.sup.15, 4.times.10.sup.17, 1.2.times.10.sup.18, and 
5.times.10.sup.18 thermal neutrons per cm.sup.2, values later determined 
to be 2.09 (.+-.1.18).times.10.sup.15, 4.29 (.+-.1.13).times.10.sup.17, 
1.23 (.+-.1.05).times.10.sup.18, and 3.99 (.+-.1.13).times.10.sup.18 by 
fission track counting in glass dosimeters as disclosed in the article by 
R.L. Fleischer, P.B. Price, and R.M. Walker, Nucl. Sci. Eng. 22, 153 
(1965). 
Early radioactivity was nearly equal for samples of the same size, 
regardless of uranium content. Later, the dominant fission decay shows the 
expected 2.5:1 activity ratio expected for samples with 380 and 150 ppm of 
uranium. 
Fission fragments for .sup.235 U have a double-peaked mass and energy 
distribution with an average of mass of 117 and energy of 84 MeV. The 
maximum internal concentration of fission events of 5.1.times.10.sup.14 
/cm.sup.3 was designed to approach (within a factor of two) that known to 
have a large effect on the critical current of V.sub.3 Si at 1 to 3 Tesla, 
as disclosed in the article by C.P. Bean, R.L. Fleischer, P.S. Swartz, and 
H.R. Hart,Jr., J. Appl. Phys. 37, 2218 (1966). The concentration is given 
by 31.times.10.sup.-8 .phi.c, where .phi. is the neutron fluence in 
cm.sup.-2 and c, the uranium concentration, is in atom fraction/10.sup.6. 
Given that the sum of the ranges of the two fragments from a fission event 
is 16 .mu.m, the maximum internal dose may also be thought of as 
4.times.10.sup.11 fission fragments/cm.sup.2 
MEASUREMENT OF FLUX PINNING BY MAGNETIC HYSTERESIS 
The measurements of the change in bulk flux pinning upon fission-fragment 
irradiation were carried out. Two potential measures of flux pinning are 
magnetic hysteresis and transport critical current. For polycrystalline 
samples the transport critical-current density is strongly dependent on 
the usually very poor intergrain coupling. Therefore, magnetic hysteresis 
was chosen for the measurement. The primary results are presented as 
magnetic hysteresis (emu/cm.sup.3) at a field of 1 Tesla measured as a 
function of temperature (4 K to 77 K). 
In order to measure the magnetic hysteresis loops, a Princeton Applied 
Research vibrating-sample magnetometer was used with a 3 Tesla 
electromagnet. The current in the electromagnet was programmed with a 
triangular wave form in order to generate the magnetization-hysteresis 
curve; saturation of the iron in the electromagnet resulted in a nonlinear 
field sweep. The field was measured by a Hall probe placed in the gap of 
the magnet. An ac signal proportional to the magnetization was detected by 
pick-up coils arranged to cancel out common-mode noise. The output of the 
coils was amplified by a lock-in amplifier using a 100 ms filter time 
constant. The outputs of the Hall probe (field) and lock-in 
(magnetization) were fed to an x-y recorder. Calibration of the 
magnetization scale was done by measuring the saturation flux from a disk 
of pure nickel of known weight. The sample was mounted with the long axis 
perpendicular to the magnetic field. Demagnetization effects are 
negligible (&lt;1.5%) for the magnetizations observed at 1 Tesla. 
A Janis helium cryostat, a Lake Shore temperature controller, and a 
carbon-glass resistance thermometer were used to control the temperature 
in the range 4 K to 77 K. At each temperature, after the temperature was 
stabilized, the sample was run through one .+-.2.5 Tesla field cycle 
before recording the magnetization-hysteresis curve. A constant sweep rate 
of current corresponding to a 1000 second period was used. 
Magnetization-hysteresis curves at 35 K for irradiated (4.times.10.sup.18 
/cm.sup.2) and unirradiated samples (150 atomic ppm U) are shown in FIG. 
1. The arrows at 1 Tesla indicate the definition of hysteresis, .DELTA.M, 
used in the following discussions; .DELTA.M is the full difference in 
magnetization. The data are the average of the .DELTA.M's obtained at +1 
Tesla and at -1 Tesla. In order to determine the sensitivity of the 
hysteresis to the rate of field sweep, .DELTA.M at 1 Tesla was measured 
for field sweep periods ranging from 20 seconds to 2000 seconds for the 
samples and temperature shown in Figure 1. The hysteresis at .+-.1 Tesla 
was found to decrease linearly with the logarithm of the period, with a 
total change of 19% for the unirradiated sample and 12% for the irradiated 
sample, over this 100-times range in sweep period. 
In FIG. 2 are the results of hysteresis measurements displayed for samples 
containing 150 atomic ppm of uranium in both the unirradiated and 
irradiated (1.23.times.10.sup.18 /cm.sup.2) states and an irradiated 
sample with no added uranium. Strong, temperature-dependent enhancement 
was observed. The enhancements in flux pinning are 3.7 times at 4.5 K, 20 
times at 63 K, and 8.3 times at 77 K. Note that flux pinning is decreasing 
rapidly at liquid nitrogen temperature. 
The temperature dependence of the hysteresis indicated that not only the 
magnitude of the hysteresis has increased upon radiation; the pinning 
energy of the fission- fragment-induced pinning centers was larger than 
that of the pinning centers in the unirradiated samples. 
Data for the other irradiations are presented in Table I for 4.5, 63, and 
77 K. 
TABLE I 
______________________________________ 
Magnetic Hysteresis at 1 Tesla (emu/cc) 
U-Con- Thermal Neutron Fluence 
centration 4.3 .times. 
1.23 .times. 
4.0 .times. 
and Temp. 
0 10.sup.17 cm.sup.2 
10.sup.18 /cm.sup.2 
10.sup.18 /cm.sup.2 
______________________________________ 
0 ppm Sample A Sample A1 Sample A2 
______________________________________ 
4.5K 128 120 141 
63K 1.15 1.16 1.85 
77K 0.39 0.35 0.46 
______________________________________ 
150 ppm Sample B Sample B1 Sample B2 
Sample B3 
______________________________________ 
4.5K 122 143 450 303 
63K 1.52 6.31 30.5 20.7 
77K 0.52 0.90 4.32 2.56 
______________________________________ 
380 ppm Sample C Sample C1 Sample B2 
Sample C3 
______________________________________ 
4.5K 132 136 260 339 
63K 1.54 6.36 14.9 18.6 
77K 0.60 1.16 1.77 2.02 
______________________________________ 
Table I shows that the hysteresis for the undoped samples was unchanged by 
a thermal neutron fluence of 4.3.times.10.sup.17 /cm.sup.2 and increased 
by 10 to 60% for 1.2.times.10.sup.18 /cm.sup.2. These increases were 3.7%, 
3.1%, and 4.2% of those produced in the 150 ppm-uranium sample. The 
hysteresis values for the 150 ppm and 380 ppm samples were enhanced 
relative to both the undoped samples and the unirradiated doped samples, 
with a saturation of the enhancement occurring between 1.2.times.10.sup.18 
and 4.times.10.sup.18 /cm.sup.2. 
The enhancement was essentially the same for the two concentrations of 
uranium, actually larger for the 150 ppm sample at the intermediate 
fluence of 1.2.times.10.sup.18 /cm.sup.2. 
CRITICAL TEMPERATURES 
Measurements of critical temperature were made for the unirradiated and 
irradiated (4.times.10.sup.18 /cm.sup.2) samples using a frequency-shift 
ac susceptibility technique disclosed in an article by A.L. Schawlow and 
G.E. Devlin, Phys. Rev. 113, 120 (1959). 
The results are shown in FIGS. 3 and 4. The irradiation lowered the onset 
temperatures only slightly, from 91 to 90 K and from 91.5 to 89 K for the 
150 and 380 atomic ppm samples respectively. However, the 5% to 95% widths 
were increased upon irradiation from 4 K to 7 K and 12 K for the 150 and 
380 ppm samples, respectively. 
The decrease in the transition temperature of the irradiated samples also 
reflects the higher radiation damage in the sample of higher nominal 
uranium content. 
MICROSCOPIC CONSIDERATIONS 
Transmission electron microscopy on a 380 ppm U-containing sample exposed 
to thermal neutron irradiation of 2.5.times.10.sup.15 /cm.sup.2 showed 
planar faults and localized regions of high dislocation density that are 
not commonly observed in the undoped sintered compacts. No evidence of 
fission-fragment tracks was seen. 
Examination by transmission electron microscopy and electron microprobe 
analysis showed that the uranium was inhomogeneously distributed, with 
some locally concentrated in phases other than YBa.sub.2 Cu.sub.3 O.sub.7, 
for example in inclusions of Ba--Cu--U--O or Ba--Y--U--O. There was no 
indication that the uranium was located preferentially on grain 
boundaries. The distribution of the most abundant uranium-bearing 
inclusions was on a fine enough scale that all of the sample could be 
reached by fission fragments, given the 8 .mu.m average range of fission 
fragments expected for Y--Ba--Cu--O. 
Light microscopy and image analysis were used to determine the size and 
shape distributions of the grains of the sintered compacts for the 150 and 
380 ppm U-containing samples. The average ratios of the maximum diameter 
to the minimum diameter were found to be 1.8 and 1.9 for the 150 and 380 
ppm samples respectively. The volume weighted average equivalent diameters 
were 11.7 and 13.9 .mu.m, respectively. Finally, the volume fractions of 
pores were found to be 12 and 6%, respectively. In addition, the image 
analysis was used to determine the effective size and shape of the grains 
for use in calculating the intragranular critical-current density from the 
magnetic hysteresis by means of the critical-state model, as described in 
the next section. 
INTERPRETATION OF MAGNETIC HYSTERESIS: INTRAGRANULAR 
CRITICAL-CURRENT DENSITIES 
The critical-state model, as disclosed in the articles by C.P. Bean, Phys. 
Rev. Lett. 8, 250 (1962); C.P. Bean, Rev. Mod. Phys. 36, 31 (1964), 
together with an appropriate measurement of magnetic hysteresis, allows 
one to determine, for a homogeneous material, the product of the 
critical-current density and the dimension of the sample, J.sub.c D. 
Unfortunately, for an inhomogeneous material such as a sintered compact 
the application of the critical-state model is not straightforward. At one 
extreme the current may flow as if the sample were homogeneous, yielding 
J.sub.c,s D.sub.s, where D.sub.s is the dimension of the sample. At the 
other extreme, the grains (or even smaller entities) may be essentially 
isolated as far as supercurrents are concerned; in this case the product 
obtained is J.sub.c,g D.sub.g, where D.sub.g is the dimension of the 
grain. lntermediate situations can occur, in which a portion of the super 
current flows throughout the sample and another portion is restricted to 
the grains. The critical-state model does not allow one to determine 
separately the applicable sizes of the regions and the appropriate J.sub.c 
's. There is thus an essential ambiguity in the interpretation of magnetic 
hysteresis by means of the critical-state model. 
It is possible in principle to determine the applicable D and thus J.sub.c 
by a destructive experiment in which the sample is ground to powder and 
the magnetic hysteresis is followed as the size of the powder is 
decreased. The magnetic hysteresis remains unchanged as long as the 
dimension of the sample or powder is greater than the effective D; it 
decreases with size as the powder is ground into sizes smaller than D. 
Such experiments, as disclosed in the articles by M. Suenaga, A. Ghosh, T. 
Asano, R.L. Sabatini, and A.R. Moodenbaugh, High Temperature 
Superconductors, Mater.Res.Soc.Symp.Proc., Vol. 99, edited by M.B. 
Brodsky, R.C. Dynes, K. Kitazawa, and H.L. Tuller (Materials Research 
Society, Pittsburgh, 1988), p. 247; and K. Itoh, H. Wada, T. Kuroda, Y. 
Kaieda, 0. Odawara, and T. Oie, Cryogenics 28, 575 (1988); have shown that 
for polycrystalline sintered compacts the effective size is not the sample 
size, but is closer to the (much smaller) grain size. This result is to be 
expected if the intergrain superconductive coupling is very weak. In the 
present analysis the assumption is made that, at 1 Tesla, the currents are 
restricted to the grains; the measured, appropriately averaged grain size 
is used for D and the current densities thus calculated are described as 
intragranular critical-current densities. 
The critical-state model yields particularly simple expressions for the 
hysteresis for crystals of simple cross-section when certain conditions 
are met: The applied field is large compared with H.sub.c1 ; the field 
variation across the sample is small enough that J.sub.c varies little 
across the sample; and the field has been swept through a sufficiently 
large excursion that critical currents have been induced in the same sense 
throughout the sample. These conditions are met at 1 Tesla in the present 
experiment. Such results are given below for two cross sections: 
EQU Circular, diameter D:.DELTA.M = J.sub.c D/30 
EQU Rectangular, S.sub.2 &gt;S.sub.1 :.DELTA.M= J.sub.c (S.sub.1 /20)[1-(S.sub.1 
13S.sub.2)] 
Examination of many grains in light micrographs led to the choice of a 
slightly more complex cross-section, a split circle joined by a 
rectangular mid-section, for the analysis of the magnetic hysteresis. The 
image analysis of the grains in the light micrograph yielded D.sub.max and 
the area of the exposed face of each grain. These parameters were used to 
determine for each grain its maximum and equivalent minimum dimensions, 
D.sub.max and D.sub.1. For this cross section the critical-state model 
yields: 
EQU .DELTA.M= J.sub.c .times.(D.sub.1 /20 ).times.[1-(1- .pi./6 )D.sub.1 
/D.sub.max ]/[ 1-(1- .pi./4 )D.sub.1 /D.sub.max ] 
The contribution of each grain to the magnetic hysteresis was Calculated 
and summed for several hundred grains to give the factor relating the 
magnetic hysteresis to the critical-current density. Such analyses were 
performed for the 150 and 380 ppm U-containing samples, but not for the 
undoped sample. For convenience of discussion and presentation the factor 
obtained can be related to an effective diameter using the expression 
given above for a circular cross section. The effective diameters are 9.0 
and 10.5 .mu.m, respectively. It should be noted that these effective 
diameters differ from the volume-weighted values given above; the 
weighting differs for the magnetic analysis. 
The intragranular critical-current densities calculated are shown in FIG. 2 
on the right-hand scale. The discussion in the section, Measurement of 
Flux Pinning By Magnetic Hysteresis, applies to the critical-current 
densities as well, since the magnetic hysteresis and critical-current 
densities are related by a constant factor for each sample. Caution must 
be exercised in considering these current densities, for the use of the 
dimensions of the grains in the critical-state model is, as mentioned 
above, an oversimplification of a complex situation. If the dimensions of 
the sample had been used instead of the grain size, the critical-current 
densities calculated would have been smaller by a factor of 400. The 
important point is that, whether measured by magnetic hysteresis or by a 
derived critical-current density, fission-fragment irradiation of 
Y--Ba--Cu--O leads to a significant enhancement of flux pinning. 
One special merit of fission events is a greater localization of damage 
into clumps of disorder - as opposed to dispersed point defects, such as 
produced for example by electron or gamma-ray irradiation. Because 
transmission electron microscopy examination found no tracks, the damage 
present is not from ionization, but from the 5% of the fission energy that 
goes directly into atomic collisions. 
Even so, the expected damage is more localized than that from other forms 
of radiation damage. 
TABLE II 
______________________________________ 
Estimated Atomic Displacements per Particle 
Approximate Number of 
Particle Atomic Displacements 
______________________________________ 
1 MeV electron 1 
0.0025 eV neutron 
10 
1 MeV neutron 2,000 
1 pair fission fragments 
200,000 
______________________________________ 
Table II indicates the relative damage effectiveness of fission fragments 
under the assumption that each 50 eV expended in atomic collisions can on 
the average displace an atom. In addition, the fission damage is localized 
to within &lt;10 .mu.m of the fissionable atom, whereas the mean free path 
for a 1 MeV neutron in YBa.sub.2 Cu.sub.3 O.sub.7 is 3.1 cm and its energy 
will be lost through a series of collisions, not in a single event. 
The effects seen here are clearly caused by fission and not primarily by 
the neutron irradiation. Effects were present at thermal neutron doses as 
low as 4.times.10.sup.17 /cm.sup.2 which included a fast component of only 
2.4 x 1014/cm2. Thermal neutrons, via capture and gamma-ray emission can 
produce displacements as disclosed in the article by R.M. Walker, J. NucI. 
Mat. 2, 147 (1960). 
In this case, the number is uncertain, but not necessarily small, relative 
to fission damage, but it is more uniformly dispersed. Using Walker's 
first order formula and existing data as disclosed in the article by E. 
Troubetzkoy and H. Goldstein, NucIeonics 18 (11), 171 (1960); it was 
calculated that less than 5.3% of the displacements could be from thermal 
neutrons. If .sup.235 U were used in place of natural uranium, fission 
would be enhanced relative to thermal 
neutron effects by 139 times.