Patent Number: 
Section: description

To prepare the precursor composition from which the HTS compound for the body article will be prepared, any of the three commonly employed techniques may be used, namely the solid state, the coprecipitation, and the sol-gel techniques. A description of these techniques may be found in Proceeding on the Symposium on High Temperature Superconducting Materials. University of North Carolina. Chapel Hill. N.C., W. E. Hatfield and J. H. Miller, Jr. (Eds.), M. Dekker, New York (1988). The solid-state technique is preferred for purposes of its simplicity. To prepare the HTS presursor composition, oxygen-rich compounds of the desired metal components, such as oxides, nitrates, or carbonates, are intimately mixed in the amounts appropriate to supply the metal atoms in the ratios desired for an HTS compound. The formula to which the HTS compound is to be prepared will depend upon whether the dopant is to be externally or internally incorporated. Wherein the Li or B dopant is to be externally incorporated, for a 123 HTS compound, oxygen rich compounds of the desired metal components are used in amounts appropriate to supply the metal atoms in the ratio of L:M:Cu of 1:2:3; for a bismuth or thallium HTS compound the ratio of T:Mxe2x80x2:Ca:Cu is 2:2:n:n+1. A dopant compound containing Li or B is added in the amount desired and the compounds are intimately mixed, formed into a body of the desired shape and then sintered or melt texturized to convert the mixture into a HTS compound of the formula L1M2Cu3O7 or T2Mxe2x80x22CanCun+1O6+2n, as the case may be. The Li or B dopant is intimately distributed throughout the body externally of the unit cell of the HTS compound with the dopant being primarily located at grain boundary locations. In the case of external doping it is preferred to employ an amount of the dopant (D) which provides for an atomic ratio of D:Cu which is equal to or less than 0.5, more preferably less than 0.4, and most preferably less than 0.3. It is most preferred to utilize a dopant that is isotopically enriched in 6Li or 10B isotopes. Wherein the Li or B dopant (xe2x80x9cIDxe2x80x9d) is to be internally incorporated within the HTS compound oxygen-rich compounds of the desired HTS metals and the dopant compound are intimately mixed in the amounts appropriate to supply atoms to provide a final HTS compound of the formula: (L+M)3-zDzCu3O6+d wherein: L is yttrium, lanthanum, neodymium, samarium, europium, gadolinium, dysprosium, holmium, erbium, thulium, ytterbium, or lutetium, or mixtures thereof including mixtures with scandium, cerium, praseodymium, terbium; M is barium, strontium or mixtures thereof; D is lithium or boron; xe2x80x9czxe2x80x9d is greater than zero and less than or equal to 0.3; xe2x80x9cdxe2x80x9d is from about 0.7 to about 1.0; and the ratio of L::M is from about 0.35 to about 0.6 provided that L does not exceed one in number and M does not exceed two in number; or T2Mxe2x80x22Can(Cu1-zxe2x80x2Dzxe2x80x2)n+1O6+2n  wherein T is bismuth and Mxe2x80x2 is strontium or T is thallium and Mxe2x80x2 is barium; xe2x80x9cnxe2x80x9d is 1 to 3; D is lithium or boron; and xe2x80x9czxe2x80x9d is greater than zero and less than or equal to 0.5. The intimately mixed compounds are formed into a body of the desired shape and then sintered or melt texturized to convert the mixture into the internally doped HTS compound. The Li or B dopant atoms are incorporated within the unit cells of the HTS compound. In the case of a 123 HTS compound the dopant atom is located at a L and/or the M atomic occupation sites within the unit cell of a 123 HTS compound. In the case of a bismuth or thallium HTS compound the dopant atom is located at a Cu atom occupation site within the unit cell of the HTS composition. The preferred method for incorporation of the dopant is that of internal incorporation. In this regard, with respect to a 123 HTS compound, it is preferred to incorporate the dopant in an atomic amount relative to copper to provide for a ratio of 6Li:Cu of from about 1xc3x9710xe2x88x927 to about 2xc3x9710xe2x88x921, more preferably from about 1xc3x9710xe2x88x926 to about 1xc3x9710xe2x88x921, and most preferably from about 1xc3x9710xe2x88x925 to about 5xc3x9710xe2x88x922. With regard to the above amounts of dopant it is preferred that the L:M ratio be from about 0.35 to about 0.6, more preferably from about 0.45 to about 0.55 provided that L does not exceed 1 and M does not exceed 2. Most preferably, the L:M ratio is 0.5. Preferably the total amount of lithium should not exceed a quantity which provides for a ratio of Li:Cu of about 1xc3x9710xe2x88x921 (i.e. Zxe2x89xa60.3). The desired loading of 6Li:Cu can be achieved at lower total loading of lithium by use of a lithium reagent which is enriched in the 6Li isotope. Likewise, with respect to boron, its total quantity should not exceed that which provides a ratio of B:Cu of 7xc3x9710xe2x88x921. Use of a boron reagent which is enriched in the 10B isotope allows a lower total loading of boron into the HTS compound. With respect to the internal incorporation of a dopant within a bismuth or a thallium HTS composition it is preferred to incorporate the dopant in an atomic amount relative to copper to provide for a ratio of 10B:Cu or 6Li:Cu of from about 2.5xc3x9710xe2x88x928 to about 6xc3x9710xe2x88x921; more preferably from about 1xc3x9710xe2x88x926 to about 1xc3x9710xe2x88x921; and most preferably from about 1xc3x9710xe2x88x924 to about 1xc3x9710xe2x88x923. Preferably the total loading of dopant relative to copper should not exceed about 1.0. The starting precursor 123 composition may be prepared by a solid state reaction procedure wherein L2O3, M(NO3)2, MCO3 or M(OH)2 and CuO powders of reagent grade are intimately ground and mixed in a weight percent proportion to provide a mixed powder wherein the appropriate ratio of the rare earth (L) to alkaline earth (M) to copper metal atoms. The Li or B dopants may be added to and intimately mixed with the powder reagents before sintering. The precursor composition may also be prepared by the coprecipitation of (nitrate) salts of L, M and Cu in the appropriate metal constituent ratios. If desired, the precursor composition may be prepared by a sol-gel technique. When prepared by a coprecipitation or sol-gen technique it is preferred to add and intimately admix the Li or B dopant subsequently. The precursor composition is preferably prepared as a powdered productxe2x80x94i.e, one the powder constituents of which are not yet formed into a particular coherent shape. In this event, it is necessary to add and intimately admix the Li or B dopant with such powder prior to forming such Li or B doped powdered precursor composition into the body form desired before subjecting such shaped body to sintering or melt texturizing. However, the precursor composition is prepared, the method of this invention is operative on a preformed body comprised of an oxide composition wherein the L:M:Cu:D metals are intimately admixed. The precursor composition is reacted, i.e., sintered or melt texturized, to convert its constituent components into a 123 HTS compound. Those Li compounds which are powders and are suitable for incorporation into the precursor composition include LiOH, LiO2, LiH, Li2CO3 and Li2C2 with Li2CO3 being preferred. Those B compounds which are suitable include B2O3. A compound of lithium can be employed wherein the isotopes of lithium are present in their natural abundance, namely 6Li comprises about 7.5 weight % and 7Li comprises about 92.5 weight % of the lithium content. Since 7Li does not undergo a nuclear reaction by thermal neutrons its presence within an HTS body would constitute a useless substitute. Accordingly, a compound which is enriched in the 6Li isotope is preferred for use in order to minimize the amount of 7Li which is introduced into the HTS body. Compounds of lithium which are enriched in the 6Li isotope permit a greater loading of the reactable 6Li to be achieved which not only reduces the level at which unreactable 7Li contaminant is introduced into the HTS body but also permits the body to be effectively irradiated in a shorter period of time which provides for less residual radio-activity. For example, with regards to a composition prepared with a Li dopant of natural abundance (7.56 wt. % 6Li) of the formula Y0.93Ba1.92Li0.15Cu3O7, a composition of equivalent 6Li content may be prepared with a lithium reagent enriched to 25 wt. % 6Li which is of the formula Y0.9777Ba1.9777Li0.0466Cu3)7; or with a 50 wt. % 6Li enriched reagent, to a formula of Y0.9879Ba1.9879Li0.0242Cu3O7; or with a 100 wt. % 6Li reagent, to a formula of Y0.994Ba1.993Li0.013Cu3O7; each of the above formulas yield an equal number of reached 6Li atoms for exposure to the same thermal neutron fluence. The minimum quantity of 6Li which may be effectively utilized is in part a function of the maximum thermal neutron fluence to which the HTS body will be permitted to be exposed. For purposes of minimizing exposure of the HTS body to the thermal neutron flux and in order to not overly extend the cost of irridation treatment, it is preferred that 6Li be incorporated into the HTS body in an amount which provides for an atomic ratio of 6Li to copper which is about 1xc3x9710xe2x88x927 or greater. These same considerations apply wherein a boron compound is used as a dopant. The 10B isotope which is reactive to thermal neutrons occurs in a natural abundance of about 19.8 weight % with the non-reactive 11B isotope comprising the balance of about 80.2 weight %. However the doped HTS precursor composition may be prepared, and however or into what particular body article form that precursor composition may be shaped, it is such preshaped precursor composition body article which is uniformly doped with Li or B and then reacted to form the HTS compound which is the starting body article to which the operation of this process applies. Such Li or B doped precursor composition is preferably first pre-sintered at a temperature of from about 900-960xc2x0 C. until its basic unit cell composition comprises L1M2Cu3O6+xcex4 or T2Mxe2x80x22CanCun+1O6+2n, as the case may be. The crystalline symmetry of a 123 HTS precursor compound may basically be tetragonal i.e., A=B=C and hence nonsuperconducting. Preferably, following the sintering treatment a 123 HTX compound body is slowly cooled in the presence of oxygen to insure a xcex4=about 0.7 to about 1.0, for an oxygen content of from about L2M2Cu3O6.7 to about L1M2Cu3O7 to insure such compound is superconducting at a Tcxe2x89xa777xc2x0 K. The overall processing can be viewed as comprising the overall steps of: (1) synthesis of a 6Li or 10B doped HTS powder precursor, (2) sintering of the HTS precursor into a 6Li or 10B doped HTS compound of a predetermined body form, (3) preferably, melt-textured growth of the compound into a body of highly aligned grains of HTS compound doped with Li or B, (4) oxygenation of a 123 HTS compound to that of an orthorhombic crystalline symmetry and (5) irradiating the body with thermal neutrons. To minimize the content of radioactive reaction products which may be imparted to the body by the neutrons it is preferred to use a doping level of 6Li or 10B which permits the optimum Jc level to be obtained at relatively low fluence levels. Accordingly, whatever may be the Li or B source, that is whether the source is natural abundance Li or B, or whether the source is a 6Li or 10B enriched compound, it is preferred to incorporate 6Li to a level which provides a ratio of 6Li:Cu of at least about 0.0001, and more preferably of at least about 0.001. Since 10B has a larger thermal neutron cross-section than 6Li, when 10B is used as the dopant, it may be incorporated in lesser quantities than 6Li while permitting the same density of induced defects pinning centers to be activated. Accordingly, when 10B is used as the dopant it is preferred to incorporate it in amounts which provide for a ratio of 10B:Cu of at least about 2.5xc3x9710xe2x88x925 more preferably of at least about 0.001. When 6Li or 10B is incorporated with the HTS compound body in the quantities as above described, the quantity of radioactive isotopes produced by the neutron flux is reduced since the level of irradiation exposure (i.e., fluence) needed to optimize the Jc of the body is reduced. The only radioactive isotope of long life concern produced by the thermal neutrons is Ba133 which results from the nuclear reaction of Ba132 (barn 8.5) which occurs in a natural abundance of 0.19 wt. %. Ba133 has a half-life of 7.2 years. A sample of externally doped YBa2Cu3O7 or of internally doped (YBa)3-xDxCu3O7 should be allowed to decay for about 30 days after irradiation after which any 90Y content (Txc2xd=2.7 days) has decayed to a safe level. The sample will also be at a safe level of Ba133 since it is present in minor amounts only due to the low abundance of Ba132 and low reaction cross-section. The degree to which the body article is irradiated depends in part upon its content of the reactive dopant, either 6Li or 10B. With a Y0.93Ba1.92Li0.15Cu3O7 (naturally abundant Li) HTS body composition the resulting Jc of the body has been observed to increase progressively with irradiation at least up to that point wherein about 2xc3x9710xe2x88x929 atoms of 6Li have undergone reaction relative to each copper atom present in the material. Accordingly, a doped body should be exposed to a thermal neutron fluence sufficient to provide a ratio of reacted 6Li atom (6Li*) to copper atom of 6Li*:cu of from at least about 5xc3x9710xe2x88x928, and more preferably at least from about 5xc3x9710xe2x88x927. Wherein the HTS composition is one of bismuth or thallium the sample should be exposed to a neutron fluence sufficient to react at least about 2xc3x9710xe2x88x929 dopant atoms per copper atom, and perefrably at least about 1xc3x9710xe2x88x926 dopant atoms per copper atom. For some applications it may be desired to have a body composed of an HTS material wherein some areas within the body have a higher Jc than other areas. Such applications may be those wherein a field profile control for trapping magnetic fields is needed for the design of a levitation bearing and like devices. The process of this invention is well suited to the formation of a HTS body article which is internally or externally patterned to possess regions of higher Jc adjacent to regions of lower Jc. This may be readily accomplished by imparting to the HTS body a patterned doping with the 6Li or 10B dopant following which the so-patterned body is irradiated with thermal neutrons as previously described. The effects of thermal neutron induced reaction products on the superconducting properties of melt-texture Bixe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O doped with different isotopes with large thermal neutron induced nuclear reaction cross sections and large Q-values is illustrated below. Samples of Bi2Sr1.8Ca1Cu1.2Li0.8O8 (naturally abundant Li) had been preformed to disc shape by melt texturing were sealed in fused quartz tubes under about 0.8 atm. of oxygen to avoid oxygen losses during irradiation. The neutron irradiation was performed in pile of the reactor at Texas AandM University. The thermal neutron flux was 1.8xc3x971013/cm2.sec and the fast neutron flux was less than one percent. The pile temperature during the irradiation was about 65xc2x0 C. The samples were irradiated to thermal neutron fluences from 3xc3x971016 /cm2 to 3xc3x971018/cm2 at reactor temperature. The influence of the irradiation on the critical current density Jc, upper critical magnetic field, transition temperature Tc , and transition width was thereafter determined. The critical transition temperature was measured by four-point probe and magnetization measurements were performed at a SQUID system to study the effect of this kind of thermal neutron irradiation on flux pinning and the critical magnetization current. The measurements, illustrated by FIGS. 1 and 2, show the Tc does not change significantly, but Jc increased by a factor of 6.2. This enhancement of Jc relative to unirradiated samples demonstrates that artificially created defects are induced in the sample which act as strong flux pinning centers. The effects of thermal neutron induced reaction products on the superconducting properties of sintered Y0.93Ba1.92Li0.15Cu3O7 (naturally abundant Li) was examined. The Li doped YBCO composition was prepared by intimately admixing in appropriate quantities of Y2O3, BaCO3, CuO and Li2CO3. The Li2CO3 reagent was one with natural abundant lithium. The powder mixture was preformed to disc shape and sintered. Disc samples were sealed in fused quartz tubes under about 0.8 atmospheres of oxygen. These discs were then irradiated in the pile of a reactor for various periods of time to expose each to a different cumulative level of thermal neutron fluence. The influence of the irradiation on the critical current density Jc, upper critical magnetic field, transition temperature Tc, and transition width was thereafter determined. The critical transition temperature was measured by four-point probe and magnetization measurements were performed at a VSM system to study the effect of this kind of thermal neutron irradiation on flux pinning and the critical magnetization current. Magnetic measurements on disc samples subjected to different levels of neutron fluence, as shown in FIG. 3, show an enhancement in the Jc of each relative to preirradiation Jc of the YBCO material. Radiation to a fluence level of about 0.75xc3x971017/cm2 enhanced the Jc by a factor of about 12. Continued exposure to higher fluence levels continued to enhance the resulting Jc of the sample to at least a fluence level of 1018/cm2 without any observable adverse effect upon the Tc properties of the sample. FIG. 4 shows the magnetization hystereisis loop of a YBCO sample having a Li doping level of 5 atomic % relative to atomic copper after irradiation to a neutron fluence level of 1018/cm2 in comparison to an unirradiated sample of similar composition. The invention has been described with reference to its preferred embodiments. One skilled in the art may appreciate from this description changes or variations which may be made which do not depart from the scope or spirit of the invention described above and claimed hereafter.