TECHNICAL FIELD This invention relates to superconducting composites and superconducting bearings using such composites.

BACKGROUND ART Superconducting composites are well known. See, for example, DeReggi et al. U.S. Patent No. 4,954,481, which teaches a composite matrix of superconducting powder and acrylic thermoplastic. The composite materials comprise from about twenty to about eighty percent weight superconductor powder.

Superconducting bearings can be constructed from such superconducting composites. Gyorgi et al. U.S. Patent No. 4,797,386 discloses a journal bearing having a cylindrical magnet, magnetized axially and disposed within a hollow cylinder made of superconducting material. Agarwala U.S. Patent No. 4,892,863 discloses a thrust bearing having a superconductor disk and a rotating permanent magnet coaxially above it. A Type II superconducting material is preferred for the disk.

DISCLOSURE OF THE INVENTION A superconducting composite comprises a superconductor powder and an acrylic thermoplastic blended between 1:1 and 3:1 by volume. The optimum volume of superconductor powder is between fifty five percent and sixty percent. This composite can be machined into a hollow cylinder for a journal bearing, or it can be machined into a disk for a thrust bearing.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a flowchart of a method for formulating the superconducting composite of the present invention;

Fig. 2 is a plot of particle size versus bearing load capacity;

Fig. 3 is a plot of volume percentage of superconductor in the composite versus bearing load capacity;

Figs. 4 and 5 are perspective views of thrust and journal bearings fabricated from the superconducting composite described in connection with Fig. 1; and

Figs. 6 and 7 are plots of radial and axial load capacities for the thrust bearing shown in Fig. 4.

BEST MODE FOR CARRYING OUT THE INVENTION Referring now to Fig. 1, a superconducting ceramic precursor material, such as YBa_Cu ₃ o ₇ __. powder, is first melted at temperatures in the range 1300°C to 1400°C for 5-10 minutes. The melt is then rapidly quenched to solidify the material in a homogeneous subatomic lattice. This may best be accomplished by pouring the melted material over copper plates that are cooled to liquid nitrogen temperatures (77°K). The quenched YBa_Cu ₃ 0 ₇ __- material is a black, brittle amorphous solid. This solid is then vacuum dried to remove any moisture that has condensed on the surface.

Next, the amorphous solid is ground to a fine powder with an average particle size which is preferably not greater than 175 microns. The fine powder is then pressed into bulk shapes such as disks or cylinders.

The bulk shapes are melt textured in a heat treating process. The bulk shape is melted into a liquid; then the liquid is cooled slowly. This slow cooling promotes the growth of relatively long grains, i.e., grains having a length greater than 1 mm. This process can also be adapted to promote the growth of uniformly sized grains having any desired length up to slightly greater than 1cm. Thus, as discussed, below an optimal grain size which can be then ground to a preferred

particle size, such as 700 microns, can be obtained by a adjusting the combination of initial melt temperature and cooling rate factors.

The melt textured solid superconductor is then ground to a fine powder, having a particle size distribution between 20 microns and 1 mm in size. The optimal size of the resulting ground particles, is believed to be in the range of between 600 to 800 microns. The superconductor powder is then sized by sieving through gradated screens.

The heat treated and sized superconductor powder is next annealed in a second heat treatment carried out in an oxygen atmosphere. It is preferred to anneal the sized powder, as opposed to the bulk textured material, because of the higher surface area of the ground powder.

The sized, annealed superconductor powder is subsequently mixed with a polymer such as an acrylic thermoplastic in the ratios 1:1 to 3:1. The sized superconducting powders may be selected to have an approximately uniform particle size, or alternatively, a selected gradation, combination, or distribution of particle sizes may be optimal.

The acrylic thermoplastic, such as methyl methacrylate, is selected as having a resistance to degradation, either by cracking or decomposing, which it maintains when cooled below the critical temperature τ _c of the selected superconductor material. The superconductor-acrylic composite mixture is ball milled with zirconia balls for 5 to 20 minutes to promote uniform wetting of the superconductor particles and insure a homogeneous distribution of acrylic and superconductor.

The mixture is placed or injected into a mold of a press, and compacted under approximately 28kN of pressure at a temperature of about 180°C for approximately nine minutes. The sample is allowed to cool for three minutes and is then removed from the press. The result is a

two-phase composite structure that combines the flux-pinning properties of the superconductor with the toughness and flexibility of a polymeric material. However, since the superconductor material is not continuous through the sample, the composite will not exhibit the zero resistance to electrical current property of a pure superconductor.

The resulting composite structures are easily machined, if required, and can withstand cryogenic temperatures without brittle fracture. It should be noted that the final molding and pressing of the composite material can be carried out to produce a bulk material having a gross net shape in any injection moldable configuration. Fig. 2 is a plot of particle size on the X-axis and bearing load capacity on the Y-axis for the YBa_.Cu ₃ 0 ₇ superconductor composite described above. The data for the graph was generated using 70% by volume of superconductor and 30% by volume acrylic thermoplastic. The graph suggests an increased load capacity as a function of particle size occurs when the particle size increases, particularly for particle sizes in the range of between about 600 and 800 microns.

Fig. 3 is a plot of volume percentage of YBa ₂ Cu ₃ 0 ₇ superconductor in the composite on the

X-axis and bearing load capacity on the Y-axis for the superconductor composite described above. The data for this graph was generated using a mean particle size of 425 microns. The graph indicates that the bearing load capacity increases as the volume percentage of superconductor material is increased to between fifty five to sixty percent. Surprisingly however, the bearing load capacity actually decreases when the percentage of superconductor material is further increased. Thus, for the YBa ₂ Cu ₃ 0 ₇ superconductor based composite, the preferred volume percentage of superconductor is in the

range of between about fifty to seventy percent, and optimally between about fifty five to sixty percent. It is currently believed that this is the preferred range required to minimize the amount of acrylic thermoplastic yet still hold the superconductor particles together. Samples made with lower amounts of acrylic do not hold together as well, tending to deteriorate with time.

The phenomena of a maximum load bearing capacity for the superconductor composite may result from two factors. First, a pure type II superconductor exhibits magnetic flux pinning, which is the result of a magnetic field being induced into the superconductor. The superconductor resists change or displacement of this induced magnetic field. Further, repulsive forces arise from the interaction of the pinned magnetic field with the remaining external magnetic field. The superconductor particles in the superconductor composite also exhibit flux pinning properties. A portion of the magneti" ^r lux can be visualized as penetrating the composites' ac./lic matrix along the magnetically invisible acrylic thermoplastic between the discreet superconductor particles held in the acrylic matrix. The magnetic flux traversing through the acrylic matrix is pinned in at least partially pinned in the superconductor particles. The interaction of this phenomena between and about the superconducting particles in the acrylic matrix changes the nature of the magnetic field internal to the composite, potentially increasing the flux-pinning property as viewed from a position external to the superconductor composite.

The second factor concerns the magnetic fields established in the superconductor particles to support the flux pining magnetic fields. In the composite superconductor, the acrylic matrix separates the superconducting particles. Thus, the magnetic fields are constrained in the individual superconducting particles,

polarizing the individual particles with the axes of polarity being all generally in alignment along the magnetic field lines of the inducing magnetic field.

Journal and thrust bearings can be constructed from the superconducting composite described above. When the Type II superconductor is cooled below its critical temperature T _c (77 ^β K for YBa_Cu ₃ 0 ₇ _ _x ) , its flux-pinning properties gives the bearing a measure of stability lacking in conventional passive magnetic bearings and bearings made of Type I superconductors.

Referring now to the- thrust bearing shown in Fig. 4, a rare earth cylindrical magnet 20, magnetized axially, is positioned near a surface of a superconductor composite disk 22. When the disk 22 is chilled below the critical temperature T _e of the superconductor, the magnet 20 is levitated above the surface of the disk 22. When the magnet 20 is oriented as shown, it can spin on its axis with very little resistance for essentially any orientation of the magnetic axis except parallel to the plane of the surface of the disk 22, and it exhibits a resistance to both axial and radial applied forces. This is attributed to flux pinning within the superconductor composite disk 22. The magnetic field pinned in the disk 22 is symmetrical and constant for any angular position of the magnet 20 about its magnetic axis. Thus, rotation of the magnet 20 about its magnetic axis does not alter the magnetic flux that is pinned in the superconducting material of the disk 22. However, translation of the magnet 20 in either the radial or axial direction, or a change in orientation of the magnetic axis, changes the distribution of magnetic flux pinned in the disk 22. Because the Type II superconductor resists any change in a pinned magnetic flux, there arises a resisting force which counters the translation or change in orientation. An exemplary thrust bearing has an 18 Mega-Gauss Oersted (MGO) , samarium-cobalt magnet 20 and a disk 22 that is 0.6

cm thick and 1.8 cm in diameter. The gap between the magnet 20 and disk 22 ranges between 0.025 and 0.05 centimeters.

For a 0.25 inch, 3000 MGO magnet at 0.01 inch gap, a plot of the radial (perpendicular to shaft axis) load capacity is shown in Fig. 6, and a plot of the axial (parallel to shaft axis) thrust load capacity is shown in Fig. 7. The axial stiffness was measured at 0.34 kgf/cm ² per bearing for a 0.05 cm. gap. For the journal bearing shown in Fig. 5, a rare earth cylindrical magnet 20', magnetized axially, is placed inside a hollow cylinder 22' made of the superconducting composite described above. The cylinder 22' is chilled below the superconductor's critical temperature T _e . The magnet 20* levitates axially within the superconductor composite cylinder 22' . The cylindrical magnet 20' can be easily made to spin on its axis with very little resistance. In addition, the magnet 20' exhibits a resistance to both axial and radial applied forces.

An exemplary journal bearing has an 18 Mega-Gauss

Oersted (MGO), samarium-cobalt magnet 20' and a cylinder

22' that is 1.2 cm thick and 2.5 cm long with a 1.8 cm outer diameter. Clearance between the magnet 20' and cylinder 22' is 0.12 cm.

It will be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. All such modifications are intended to be included within the scope of the invention as defined in the appended claims.