Fracture Toughness for Copper Oxide Superconductors

Bulk ceramic copper oxide superconductors can experience large stresses in many applications. A useful design criterion for the mechanical properties of this type of superconductor is the ability to withstand a strain of 0.2%. The high-temperature superconductor YBa.sub.2 Cu.sub.3 O.sub.x (123) is generally weak because of its low fracture toughness and the presence of relatively large flaws and thus, in general, does not have the desired flexibility. The fracture toughness (K.sub.IC) of 123 is generally only =1-2 Mpa(m)e.sup.0.5. Fracture toughness is related to fracture strength (.sigma..sub.f) by

where Y is a geometric factor and c is the size of the critical flaw. Therefore, if the flaw population remains constant, increases in K.sub.IC should impart proportional increases in strength. The flexibility of a brittle wire or tape is directly related to its strength by

where r is the radius of the wire, p is the bending radius, and E is the Young's modulus. Thus, even in the absence of ductility, significant increases in K.sub.IC may help allow for fabrication of robust, flexible wires and tapes. This invention is applicable generally to the ceramic copper oxide superconductors, including the 123 superconductor and the superconductors having various rare earth substitutions for yittrium. In addition, the invention pertains to other presently known ceramic copper oxide superconductors, namely Bi-Sr-Ca-Cu-O.sub.x (several different phases are known) and Tl-Ba-Ca-Cu-O.sub.x (several different phases are known). When we speak of copper oxide superconductors we mean these three systems.

K.sub.IC of ceramics can be enhanced by several means. Some mechanisms, such as promotion of a tortuous crack path, lead to only modest improvements. Others, such as microcrack toughening, can be effective, but would be of little use to 123 superconductor or the other ceramic copper oxide superconductors owing to a concomitant degradation of electrical properties. The mechanisms that show the most promise for toughening high-temperature superconductors involve addition of second-phase particles. It has been shown that addition of 15 to 20 volume percent Ag particles nearly doubles K.sub.IC and can improve strength. The improvements are due to the ductility of the Ag and to a favorable residual stress state in which the brittle ceramic is in compression and the ductile Ag is in tension.

It has been found that dispensing an agent throughout a ceramic copper oxide superconductor, if the agent is properly chosen and coated, improves the toughness and strength of the superconductor without degrading the superconducting properties. The dispersed agents are ZrO.sub.2, Al.sub.2 O.sub.3, MgO, Al.sub.2 O.sub.3.SiO.sub.2 (mullite) and SiO.sub.2. The preferred agents are ZrO.sub.2, Al.sub.2 O.sub.3 and MgO. The agents commercially are available in various physical forms, such as particles, whiskers, platelets and continuous fibers. When ZrO.sub.2 particles are used, the tetragonal ZrO.sub.2 must be employed, whereas ZrO.sub.2 whiskers and continuous fibers need not be tetragonal. Al.sub.2 O.sub.3 is available in whiskers, fibers and platelets and any such form or forms is acceptable. MgO is available in whiskers and continuous fibers, either of which is satisfactory as well as mixtures of both forms. Al.sub.2 O.sub.3.SiO.sub.2 is available in continuous fibers, whiskers and platelets, while SiO.sub.2 is available in continuous fibers and whiskers.

Generally, it has been found that the agent should be present in a concentration of at least about 10 mole percent of the superconductor and not more than about 30 mole percent. The preferred concentration is about 20 mole percent. Various compounds may be used to protect the agent and prevent deleterious chemical reaction with the superconductor. The coating compounds are selected from the phase diagram for each of the superconductor systems. The selected compounds have the properties of being entirely chemically compatible with the respective superconductor. No chemical potential exists between the superconductor and the compound and there is no thermodynamic driving force for interdiffusion. In addition, the compounds listed have been found to be less reactive with foreign oxide additives (e.g., ZrO.sub.2 or MgO) than would be the superconductor itself. The coating compounds are Y.sub.2 BaCuO.sub.5 (otherwise known as 211), Ca.sub.2 CuO.sub.3, Sr.sub.2 CuO.sub.3, (Ca,Sr).sub.1 CuO.sub.2 and CaCuO.sub.2. Specifically, the 211 compound is used to protect agents dispersed in the YBa.sub.2 Cu.sub.3 O.sub.x superconductor. Ca.sub.2 CuO.sub.3 or Sr.sub.2 CuO.sub.3 or (Ca, Sr).sub.1 CuO.sub.2 or mixtures thereof are used to protect agents dispersed in Bi-Sr-Ca-Cu-O.sub.x ceramic copper oxide superconductors, while Ca.sub.2 CuO.sub.3 or CaCuO.sub.2 or (Ca, Sr).sub.1 CuO.sub.2 mixtures thereof are used to protect agents dispersed in Tl-Ba-Ca-Cu-O.sub.x ceramic oxide superconductors.

This invention is based on the finding that tetragonal ZrO.sub.2 particles can be coated with Y.sub.2 BaCuO.sub.5 (211) and that these coated particles increase the K.sub.IC of 123 when dispersed in the superconductor and prevent the ZrO.sub.2 from chemically reacting with the 123 material. These experiments in which .apprxeq.10 mole percent 211-coated ZrO.sub.2 was added to 123 produced an increase in K.sub.IC from 1.5.+-.0.5MPa(m).sup.0.5 t to 2.6.+-.0.4MPa(m).sup.0.5.

However, the 123 superconductor and ZrO.sub.2 will chemically react: as a consequence, 123 decomposes into 211, CuO, and BaCuO.sub.2, and ultimately BaZrO.sub.3 is formed. This reaction is undesirable because the amount of superconducting material is decreased and because intact ZrO.sub.2 is needed for the improvement in K.sub.IC of the host 123. The 211 phase is more refractory than is 123 and is chemically compatible with 123. The 211 phase acts as an effective barrier layer that limits deleterious interaction between the 123 and ZrO.sub.2. The invention, generally, is providing an agent for addition to a copper oxide superconductor which strengthens and toughens the superconductor and coating the agent with a component which is chemically inert to the superconductor to prevent degradation of the superconductor when the agent is added thereto.

Accordingly, it is an object of the invention to provide a material which can be added to a ceramic copper oxide superconductor that is chemically inert thereto and which strengthens and toughens the superconductor.

Another object of the invention is to provide stronger and tougher ceramic oxide superconductors and methods of making same.

Yet another object of the invention is to provide a ceramic copper oxide superconductor having dispersed therethrough an oxide-based strengthening and toughening agent selected from the class consisting of ZrO.sub.2, Al.sub.2 O.sub.3, MgO, Al.sub.2 O.sub.3 -SiO.sub.2, SiO.sub.2 and mixtures thereof present in a amount in the range of from about 10 mole percent to about 30 mole percent, the agent being present as a discrete phase coated with a compound chemically inert to the superconductor, the compound being selected from the class consisting of Y.sub.2 BaCuO.sub.5, Ca.sub.2 CuO.sub.3, Sr.sub.2 CuO.sub.3, (Ca, Sr).sub.1 CuO.sub.2 and CaCuO.sub.2.

Still another object of the invention is to provide a ceramic copper oxide superconductor selected from the class consisting of YBa.sub.2 Cu.sub.3 O.sub.x, Bi-Sr-Ca-Cu-O.sub.2 and Tl-Ba-Ca-Cu-O.sub.x having dispersed therethrough an oxide-based strengthening and toughening agent selected from the class consisting of ZrO.sub.2, Al.sub.2 O.sub.3, MgO and mixtures thereof present in the range of from about 10 mole percent to about 30 mole percent, the agent being present as a discrete phase coated with a compound chemically inert to the superconductor, the compound being selected from the class consisting of Y.sub.2 BaCuO.sub.5, Ca.sub.2 CuO.sub.3, Sr.sub.2 CuO.sub.3, (Ca, Sr)1CuCO.sub.2 and CaCuO.sub.2.

A final object of the invention is to provide a method of toughening a ceramic copper oxide superconductor comprising forming a solution of the salts of a chemical compound selected from the group consisting of Y.sub.2 BaCuO.sub.5, Ca.sub.2 CuO.sub.3, Sr.sub.2 CuO.sub.3 (Ca, Sr).sub.1 CuO.sub.2 and CaCuO.sub.2 in a solvent wherein the salts are present in ratios to produce one of the group, introducing particulates of an agent in particle, whisker, platelet or continuous fiber form into the solution, the agent being selected from the class consisting of ZrO.sub.2 whiskers or continuous fibers, tetragonal ZrO.sub.2 particles, Al.sub.2 O.sub.3, MgO, Al.sub.2 O.sub.3 -SiO.sub.2, SiO.sub.2 and mixtures thereof, adjusting the pH of the solution until the agent or agents are uniformly dispersed in the solution forming a sol, spray drying the sol and calcining to form the agent particulates coated with one of the group of compounds, thereafter mixing the coated agent particulates with a powder of a ceramic copper oxide superconductor wherein the coated agent particulates are present in the range of from about lo mole percent to about 30 mole percent, and thereafter forming the mixture into a solid ceramic copper oxide superconductor having a theoretical density of not less than about 85%.

The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.

For the purpose of facilitating an understanding of the invention, there is illustrated in the accompanying drawings a preferred embodiment thereof, from an inspection of which, when considered in connection with the following description, the invention, its construction and operation, and many of its advantages should be readily understood and appreciated.

FIG. 1 is a SEM micrograph of spray-dried coatings on ZrO.sub.2 particles;

FIG. 2 is a flow chart of process for making 211 coatings on tetragonal ZrO.sub.2 particles;

FIGS. 3(a)-(c) are SEM photographs of fracture surfaces: (a) 123 bar that was sintered in O.sub.2 at 930.degree. C. for 6 hours and was .apprxeq.80% dense, (b) 123 bar that was sintered in O.sub.2 at 990.degree. for 6 hours, and (c) 123/211/20 mole percent ZrO.sub.2 bar that was sintered in O.sub.2 at 990.degree. C. for 6 hours.

Phase-pure, orthorhombic 123 was synthesized by solid-state reaction of reagent-grade Y.sub.2 O.sub.3, BaCO.sub.3, and CuO powders. Synthesis was accomplished in 4 hours at 800.degree. C by exposure to pure O.sub.2 of reduced total pressure. Part of the 123 powder was mixed by mortar and pestle with .apprxeq.10 or 20 mole percent tetragonal ZrO.sub.2 particles (composition 97 mole percent ZrO.sub.2, 3 mole percent Y.sub.2 O.sub.3 ; average particle size .apprxeq.0.8 .mu.m) that had been coated with 211. The remaining pure 123 and the 123 with 211-ZrO.sub.2 additions were used to make bars for testing. The 211 solution for coating the ZrO.sub.2 was prepared from metal acetate salts dissolved in distilled water. The aqueous solution was heated to assist in dissolving the solids and ammonium hydroxide was added to adjust the pH and stabilize the solution. If base was not used, the acetate salts precipitated and homogeneity was lost.

The as-received ZrO.sub.2 powder contained agglomerates that were 10 to 50 .mu.m in diameter. The ZrO.sub.2 powder was ball milled and dried to break up the agglomerates prior to use. Spray drying of the slurry of ZrO.sub.2 and 211precursor solution produced a fine-particle-size powder. Little reagglomeration of the coated particles occurred during spray drying (FIG. 1).

The coated ZrO.sub.2 powder was calcined in air at 950.degree. C. for 4 hours, which converted the precursor metal salts to 211. FIG. 2 shows a flow chart of the process from sol to a 211-coated ZrO.sub.2 powder. X-ray diffraction analysis indicated that ZrO.sub.2 and 211 were present and that some BaZrO.sub.3 was also produced. The BaZrO.sub.3 was probably at the 211/ZrO.sub.2 interface, and thus was separated from the 123. Irrespective of its proximity to the 123, BaZrO.sub.3 is much less reactive with 123 than is ZrO.sub.2.

Bar specimens were cold pressed from the 123 and 123/211/ZrO.sub.2 powders. The average bar dimensions before firing were about 50 mm.times.7 mm.times.3 mm. The bars were sintered for 3-6 hours in flowing O.sub.2 at temperatures from 950.degree. to 990.degree. C. The densities of the bars were calculated from the masses and measured dimensions. Some of the bars that were sintered at 990.degree. C. were annealed in Oz at 425.degree.-450.degree. C. for 10 hours to convert the tetragonal 123 to the superconducting orthorhombic phase. Resistivity versus temperature curves were obtained for these bars. In addition, critical current densities were obtained at 77K with zero applied magnetic field. A standard 4-probe method and a criterion of 1 .mu.V/cm were used.

Fracture toughness values were determined for the remaining bars by the single-edge notched beam method. Notches were cut normal to a 3 mm.times.50 mm face (into the 7-mm dimension) and were about 3 mm deep. Four bars of each composition were loaded separately in three-point bending at a rate of 1.27 mm/min. K.sub.IC was calculated from

where P is the load, L is the support span (19 mm for these tests), Y is a constant related to the specimen geometry, C is the notch depth, B is the specimen width (.apprxeq.3 mm), and W is the specimen height (.apprxeq.7 mm). Some of the bars were also tested in the unnotched condition in 3-point bending to determine strength. The fracture surfaces of several bars were examined by scanning electron microscopy (SEM).

In addition to the acetate salts dissolved in water used in the method disclosed above, other suitable solvents include ethanol, methoxy ethanol and ethylene glycol (when nitrate salts are used). Salts other than acetates are acceptable, such as hydroxides or nitrates. The alkoxides such as isopropoxide of the ceramic superconductors may be used for preparing same and also used for the salts of the agents.

It was found that additions of 10 mole percent tetragonal ZrO.sub.2 to 123 improved K.sub.IC only if the 123 superconductor material was at least 80% dense. For bars fired at 930.degree.-950.degree. C., the densities were 85-90% of theoretical and K.sub.IC increased from 1.2-1.5 MPa(m).sup.0.5 for pure 123 to .apprxeq.2.5 MPa(m).sup.0.5 for the 123+10% ZrO.sub.2.

The 211 coatings allow for high sintering temperatures without excessive reaction between 123 and ZrO.sub.2. Sintering in O.sub.2 at 900.degree. C. produced 123/211ZrO.sub.2 composites that were 85-90% dense and 123 bars that were .apprxeq.93% dense. A summary of the K.sub.IC data is shown in Table 1.

The first observation is that higher temperatures produced denser tougher 123 superconductor. SEM revealed that significant grain growth accompanied the additional densification, see FIGS. 3(a)-(b). The K.sub.IC tests produced primarily intergranular fracture paths. For such fracture, it is known for ceramics that K.sub.IC increases with grain size because the crack path becomes more tortuous.

ZrO.sub.2 additions improved K.sub.IC, but only modest improvement was observed for .apprxeq.10 mole percent additions. 20 mole percent ZrO.sub.2 improved the K.sub.IC by nearly 50% (Table 1). It should be noted that for specimens of constant density, the 10 mole percent ZrO.sub.2 additions also increased K.sub.IC by .apprxeq.50%.

The average strengths of the 123 ceramic oxide superconductor and 123/211 ZrO.sub.2 bars were all about 60 MPa, despite the differences in K.sub.IC. The composite bars possessed a finer grain size (FIGS. 3a-c), which should, if anything, lead to increased strength because of a reduced flaw size. Examination of the fracture surfaces revealed, however, that regions surrounding some of the 211-coated ZrO.sub.2 particles were poorly sintered. These porous regions were larger than the grain size and constituted large flaws. It is assumed that failure initiated in these regions. Thus, an increased K.sub.IC was compensated for by an increased flaw size, with a net result of no change in strength.

All of the 123 ceramic superconductors and 123/211/ZrO.sub.2 bars that were measured exhibited a T.sub.c onset of .apprxeq.90K. At 77K in zero applied field, the J.sub.c values of the bars were .apprxeq.200-300 A/cm.sup.2. No significant differences in J.sub.c were observed among the specimens. The average J.sub.c values were quite low and were in part attributable to the large grain sizes of the specimens. It is believed that stresses generated during cooling induce microcracking, which leads to lower J.sub.c values. The magnitude of these stresses is proportional to grain size.

A J.sub.c of 200 A/cm.sub.2 is not sufficient for most applications, but may be adequate for some that require only modest current densities. For those applications, mechanical reliability is likely to be important.

While there has been disclosed what is considered to be the preferred embodiment of the present invention, it is understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.