Method of manufacturing a high temperature superconductor with improved transport properties

A method of preparing a high temperature superconductor. A method of preparing a superconductor includes providing a powdered high temperature superconductor and a nanophase paramagnetic material. These components are combined to form a solid compacted mass with the paramagnetic material disposed on the grain boundaries of the polycrystaline high temperature superconductor.

The present invention is concerned generally with an article of manufacture
 and a method of manufacturing high temperature superconductors having
 improved transport properties. More particularly, the invention is
 concerned with an article and method of manufacture of high temperature
 superconductors having improved transport critical current density, or
 intergrain J.sub.c.
 Development of useful high temperature superconductors has been difficult
 since the discovery of such superconductors. In order to be commercially
 useful, it is important to improve the transport critical current density
 of these superconductors. Grain boundaries in polycrystalline forms of
 these superconductors act as weak links, and thus the grain boundaries can
 drastically diminish current transport across the boundaries which
 degrades transport properties.
 OBJECTS OF THE INVENTION
 It is therefore an object of the invention to provide an improved form of
 high temperature superconductor.
 It is another object of the invention to provide a novel article and method
 of manufacture of high temperature superconductor with improved transport
 properties.
 It is a further object of the invention to provide an improved article and
 method of manufacture of a high temperature superconductor with improved
 intergrain critical current density properties.
 It is still another object of the invention to provide a novel article and
 method of manufacture of a high temperature superconductor having
 nanophase paramagnetic particles disposed in the grain boundaries to
 diminish weak transport links.
 These objects and other advantages and features of the invention will be
 readily apparent from the following description of the preferred
 embodiments, taken in conjunction with the accompanying drawings described
 hereinbelow.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
 The intergrain critical current density, J.sub.c, in a high temperature
 superconductor (hereinafter, "HTSC") can be improved by addition of
 ultrafine grained phases, such as nanophase paramagnetic particles and
 most preferably superparamagnetic particles (herein "paramagnetic" shall
 include both paramagnetic and superparamagnetic particles). Normally,
 application of a magnetic field causes drastic diminution of J.sub.c which
 is usually quite large within each HTSC grain. In order to overcome the
 weak links created by grain boundaries in a polycrystalline HTSC, small
 size paramagnetic particles can be disposed on the grain boundaries to
 channel the magnetic flux thereby enabling improved current flow at
 selected locations along the grain boundary.
 While not limiting the invention, it is believed that use of ultrafine
 grained (such as nanophase size particles) paramagnetic particles cause
 concentration of applied magnetic field lines in the vicinity of the
 paramagnetic particles and leads to reduced magnetic field strength
 elsewhere along the HTSC grain boundaries. Consequently, enhanced currents
 can be transported in those areas along the grain boundaries having lesser
 magnetic field density.
 As shown in FIG. 1, various amounts of nanophase Cr.sub.2 O.sub.3 particles
 were added to conventional superconducting stoichiometries of 123 YBaCu
 oxide. The method used to manufacture the nanophase particles is described
 in U.S. Pat. No. 5,128,081 which is incorporated by reference herein. The
 most preferred size of Cr.sub.2 O.sub.3 particles was determined to be
 about 10-500 .ANG. in diameter. As can further be noted in FIG. 1, the
 amount of ultra-fine grained Cr.sub.2 O.sub.3 has an optimum effect
 between 0 and 0.3% Cr.sub.2 O.sub.3 for the 123 YBaCu oxide system. It is
 believed that too many Cr.sub.2 O.sub.3 particles can result in too many
 paramagnetic centers which does not effectively concentrate the magnetic
 flux. This arrangement can thus have substantially the same effect as
 having no paramagnetic centers at all. However, there is clearly a
 beneficial effect with 0.1% Cr.sub.2 O.sub.3 with substantially improved
 J.sub.c over the range of at least 10-50 gauss field. This same beneficial
 result can be observed in FIG. 2 wherein the ratio of J.sub.c (H)/J.sub.c
 (H=0) is plotted for 123 YBaCu oxide polycrystalline specimens with and
 without the nanophase Cr.sub.2 O.sub.3 additions (about 0.1% by weight).
 In other forms of the invention, various paramagnetic materials (such as
 iron oxides) can be made in a nanophase size range, added to the HTSC
 material and the J.sub.c increased by minimizing the effect of the
 magnetic field on intergrain transport properties. In other embodiments
 using other HTSC materials, such as BSCCO, the HTSC material can likewise
 be combined with a paramagnetic particle dispersion in the manner
 described herein. Such systems can use the basic concept of the
 microscopic concentration of magnetic fields by such paramagnetic
 particles dispersed along grain boundaries of the HTSC material. The
 following nonlimiting example illustrates various details of a preferred
 method of the invention.
 EXAMPLE
 YBCO (123) powders can be prepared generally in accordance with the methods
 set forth in U.S. Pat. No. 5,086,034 which is incorporated by reference
 herein. In a preferred method, 121.78 grams of powdered Y.sub.2 O.sub.3,
 422.92 grams of powdered BaCO.sub.3, and 255.32 grams of powdered CuO were
 mixed together and wet milled for .apprxeq.15 hours in methanol in
 polyethylene jars containing ZrO.sub.2 grinding media. The resultant
 slurry was pan dried and screened through a 30 mesh sieve. The screened
 powder was placed in dense alumina pans and inserted into a long tube
 furnace. A vacuum was established in the furnace of about 2.66 Pa (0.02 mm
 Hg), and oxygen gas was introduced into the furnace and adjusted to about
 266 Pa (2 mm Hg) at a flow rate of about 2 liters/minute. The furnace
 temperature was increased at a rate of .apprxeq.100.degree. C./hour to
 .apprxeq.625.degree. C. when CO.sub.2 evolution began. At 625.degree. C.
 the heating rate was reduced to .apprxeq.20.degree. C./hour to maintain
 the CO.sub.2 level to no greater than 2.66 Pa (0.02 mm Hg). Heating was
 continued to 800.degree. C. The temperature and pressure were held at
 800.degree. C. for about four hours until CO.sub.2 evolution ceased and
 formation of the ceramic was complete. The material was then cooled to
 about 600.degree. C., the vacuum was discontinued and the oxygen pressure
 increased to 1.times.10.sup.5 Pa (760 mm Hg, atmospheric pressure).
 Cooling was continued to 450.degree. C. and that temperature was held for
 about 10 hours. After cooling to room temperature, the powder was crushed
 in a mechanical grinder, and the particle size was .apprxeq.5 microns. The
 powder was then characterized by XRD and DTA (Differential Thermal
 Analysis) and determined to be YBCO (123).
 The mixing of YBCO and Cr.sub.2 O.sub.3 (nanophase material) was performed
 as follows: About 0.187 grams of nanophase Cr.sub.2 O.sub.3 was placed in
 a clean beaker and .apprxeq.200 ml of methanol was added and mixed with
 the components in an ultrasonic mixer device (with a microtip) for
 .apprxeq.5 minutes. We then added 35.534 gams of YBCO Powders (prepared as
 above) and again mixed in the ultrasonic mixer for .apprxeq.5 minutes. The
 mixed slurry was dried on a hot plate in a Pyrex tray. The dried paste was
 again dried in an oven maintained at .apprxeq.150.degree. C. overnight.
 The overnight dried powder was crushed in a mortar and pestle. Pellets
 (.apprxeq.0.5 inch diameter) were pressed from this powder. For control
 experiments YBCO powders without any Cr.sub.2 O.sub.3 were also pressed.
 These pressed pellets were sintered in oxygen at ambient pressure at
 950.degree. C. for 10 hours. During cooling they were held at
 .apprxeq.450.degree. C. for 60 hours for oxygenation and then cooled to
 room temperature.
 Thin specimen slices were cut from the sintered pellets using a slow speed
 diamond saw. Four probe DC measurements were made to determine the J.sub.c
 values in zero and applied external fields. FIG. 1 shows the J.sub.c vs.
 field for YBCO with 0%, 0.1%, and 0.3% (weight) Cr.sub.2 O.sub.3. FIG. 2
 shows the J.sub.c (in field)/J.sub.c (zero field) vs applied field for a
 YBCO+0.1 wt. % Cr.sub.2 O.sub.3 sample.
 The above embodiments illustrate various forms of the invention but
 variations and modifications of these embodiments are encompassed within
 the scope of the following claims.