High temperature superconductor system comprising Tl2 Ba2 CuO+δ
This is a continuation-in-part of U.S. patent application Ser. No. 144,114, filed on Jan. 15, 1988 now U.S. Pat. No. 4,994,432. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to high temperature superconductors and methods of making same. 
     Recently, there has been much work done on the utilization of ternary oxides containing rare earth elements. barium, and copper for superconductivity above the temperature of liquid nitrogen. To date, these systems typically require four elements: a rare earth; an alkaline earth; copper; and oxygen. Superconductor systems based on rare earth elements have not been entirely satisfactory. The use of rare earth elements in the production of superconductors increases the cost of the resultant superconductors because these rare earth elements are in relatively short supply and are expensive. Furthermore, these systems typically exhibit limited transition temperatures at or below 93 K. Partial substitutions of elements for the member(s) in these systems have not produced significantly higher temperature superconductors. 
     Typical high temperature superconductive systems based on rare earth elements also suffer the disadvantage of not being producible in an expedient manner. Therefore, typical methods for producing these high temperature superconductive systems do not provide low cost processing and manufacturability. 
     Accordingly, there is a need for a new superconductive system and process for making same. 
     SUMMARY OF THE INVENTION 
     The present invention provides a new high temperature superconductive system and method of making same. 
     The new high temperature superconductive system is free of rare earth elements and is unique to date among high temperature superconductive systems. To this end, the present invention provides a high temperature superconductive system containing thallium (Tl) and having the following general formula: 
     
         Tl--R--X--Y 
    
     wherein: 
     R is a metallic element; 
     X is a metallic element; and 
     Y is a non-metallic element. 
     Preferably: R is an element chosen from Group 2A; X is an element chosen from Group 1B; and Y is an element chosen from Group 6A. 
     In a preferred embodiment, R is barium (Ba), X is copper (Cu), and Y is oxygen (0). 
     In a preferred embodiment, the superconductor of the new superconductive system of the present invention has the following approximate stoichiometry: 
     
         T1Ba.sub.a Cu.sub.b O.sub.c 
    
     wherein: 
     a is greater than 0.2 and less than 5; 
     b is greater than or equal to 0.2 and less than 15: and 
     c is greater than a+b and less than 2+a+b. 
     In a preferred embodiment, a superconducting phase is created having the approximate composition Tl 2  Ba 2  CuO 6+ δ  and a tetragonal unit cell of approximately 5.4×5.4×23.5 A 3 . 
     A method of producing the high temperature superconductor of the present invention is also provided. The method of the present invention allows the superconductor to be prepared at temperatures of approximately 850° to about 950° C. in flowing oxygen. Accordingly, the method of the present invention allows the superconductor to be formed at relatively low temperatures. Furthermore, the method of the present invention allows the superconductor to be produced rapidly, in approximately 30 minutes. 
     Additional advantages and features of the present invention are described in, and will be apparent from, the detailed description of the presently preferred embodiments and from the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates variations in the electrical resistance vis-a-vis temperature of two Tl-Ba-Cu-O superconductor samples constructed in accordance with the present invention. 
     FIG. 2 illustrates the electrical resistance vis-a-vis temperature of a Tl-Ba-Cu-O sample constructed in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
     The present invention provides a new high temperature superconductor and method of making same. As used herein, the term &#34;high temperature&#34; refers to a temperature above the boiling point of liquid nitrogen. The present invention also provides a new superconductive system that may produce higher temperature, even room temperature, superconductors by further elemental substitution and variation of preparation procedures. 
     As set forth in the background of the invention, the high temperature superconductors that have been proposed to date require four elements: a rare earth; an alkaline earth; copper; and oxygen. The inventors of the present invention have discovered a superconductive system that does not require the presence of a rare earth element. To this end, the present invention provides a superconductive system based on thallium (Tl). The superconductor constructed in accordance with the present invention is superconducting with a critical temperature of at least 85 K with indications of smaller superconductive phase at 270 K. Furthermore, the inventors have found that the new high temperature superconductor can be rapidly produced at relatively low temperatures. 
     The superconductor system of the present invention has the following general formula: 
     
         Tl--R--X--Y 
    
     wherein: 
     R is a metallic element; 
     X is another metallic element; and 
     Y is a non-metallic element. 
     Preferably, R is an element chosen from Group 2A, X is an element chosen from Group 1B, and Y is an element chosen from Group 6A. 
     In a preferred embodiment, R is barium (Ba), X is copper (Cu), and Y is oxygen (O). 
     In a preferred embodiment, a superconductor constructed from the new superconductive system of the present invention has the following approximate stoichiometry: 
     
         TlBa.sub.a Cu.sub.b O.sub.c 
    
     wherein: 
     a is greater than 0.2 and less than 5; 
     b is greater than or equal to 0.2 and less than 15; and 
     c is greater than a+b and less than 2+a+b. 
     An 85 K superconducting phase in a T1-Ba-Cu-O system made pursuant to Example 1 below was identified by electron microprobe analysis and powder x-ray diffraction. The phase was found to have an approximate composition of Tl 2  Ba 2  CuO 6+ δ  and a tetragonal or pseudotetragonal unit cell of approximately 5.4×5.4×23.5 A 3 . The 5.4 A a-axis and b-axis dimension may be reduced by the usual square root of 2 factor. The phase had a copper-oxygen monolayer. 
     By way of example and not limitation, examples of the new high temperature Tl-Ba-Cu-O superconductor of the present invention, and method of making same, will now be given. 
     EXAMPLE 1 
     A. The following reagents were utilized: 
     1. T1 2  O 3 , 
     2. BaCO 3 , 
     3. CuO. 
     B. The following procedure was followed: 
     1. A mixture of a one molar portion of BaCO 3  and a three molar portion of CuO was ground with an agate mortar and pestle, heated in air at approximately 925° C. for more than 24 hours (with several intermediate grindings) to obtain a uniform black powder having the nominal formula BaCu 3  O 4 . 
     2. The resulting BaCu 3  O 4  powder was mixed with Tl 2  O 3  to a molar ratio of 1:0.5. The mixture was completely ground and then pressed into a pellet. 
     3. A tube furnace was heated to a temperature of approximately 880° to about 900° C. in flowing oxygen. 
     4. The pellet was then placed in the tube furnace for 2-5 minutes while the temperature and oxygen flow were maintained. 
     5. After the pellet was slightly melted it was taken out of the furnace and quenched in air until it reached room temperature. 
     By visual inspection, it was determined that the Tl 2  O 3  had been partially volatilized as black smoke, part had become a liquid, and part had reacted with the Ba-Cu oxide to form a partially melted black porous material which is multiphase and superconductive. The samples prepared by this procedure had an onset temperature of about 96 K, a midpoint of about 85 K, and a zero resistance temperature of about 81 K. 
     FIG. 1 illustrates resistance versus temperature dependences down to the boiling point of liquid nitrogen (77 K) for two samples made pursuant to the procedure of this example. The samples were measured in a simple liquid nitrogen Dewar. Quantitative magnetic examinations of Meissner effect flux expulsion confirmed that the sharp drop of resistance originated from the superconductivity of these samples. 
     EXAMPLE 2 
     A. The following reagents were utilized: 
     1. Tl 2  O 3 , 
     2. BaCO 3 , 
     3. CuO. 
     B. The following procedure was followed: 
     1. A mixture of a one molar portion of BaCO 3  and a three molar portion of CuO was ground with an agate mortar and pestle. The ground mixture was heated in air at approximately 925° C. for more than 24 hours with several intermediate grindings to obtain a uniform black powder having a nominal composition BaCu 3  O 4 . 
     2. The resulting BaCu 3  O 4  powder was mixed with Tl 2  O 3  in a molar ratio of 1:1, completely ground, and pressed into a pellet. 
     3. A tube furnace was heated to approximately 950° C. in flowing oxygen. 
     4. The pellet was then placed in the tube furnace for 2-5 minutes while maintaining the temperature and oxygen flow. 
     5. After the pellet had completely melted, it was taken out of the furnace and quenched in air until it reached room temperature. 
     The melted sample had visible crystalline grains. FIG. 2 illustrates the electrical resistance versus temperature for a superconductor constructed pursuant to this example. As illustrated, resistance-temperature dependence showed a sharp drop of resistance at about 270 K, which suggests a very high temperature superconducting phase in this sample. 
     EXAMPLE 3 
     A. The following reagents were utilized: 
     1. Tl 2  O 3 , 
     2. BaCO 3 , 
     3. CuO. 
     B. The following procedure was followed: 
     1. A mixture of a two molar portion of BaCO 3  and a three molar portion of CuO was ground with an agate mortar and pestle, heated in air at approximately 925° C. for more than 24 hours (with several intermediate grindings) to obtain a uniform black powder having the nominal composition Ba 2  Cu 3  O 5 . 
     2. The resultant Ba 2  Cu 3  O 5  powder was mixed with Tl 2  O 3  to a molar ratio of 1:0.75, completely ground, and pressed into a pellet. 
     3. A tube furnace was heated to a temperature of approximately 880° to about 900° C. in flowing oxygen. 
     4. The pellet was placed in the tube furnace for 2-5 minutes while maintaining the temperature and oxygen flow. 
     5. After the pellet had slightly melted it was taken out of the furnace and quenched in air until it reached room temperature. 
     The samples prepared by this procedure have been found to be superconductive and have an onset temperature of about 96 K, a midpoint of about 85 K, and a zero resistance temperature of about 81 K. 
     EXAMPLE 4 
     A. The following reagents were utilized: 
     1. Tl 2  O 3 , 
     2. BaCO 3 , 
     3. CuO. 
     B. The following procedure was followed: 
     1. Appropriate amounts of Tl 2  O 3 , BaCO 3  and CuO with a nominal composition of TlBaCu 3  O 5 .5+x were mixed and ground, and pressed into a pellet. 
     2. A tube furnace was heated to a temperature of approximately 900° to about 950° C. in flowing oxygen. 
     3. The pellet was placed in the tube furnace for 2-5 minutes while the temperature and oxygen flow were maintained. 
     4. After the pellet had slightly melted, the pellet was taken out from the furnace and was quenched in air to room temperature. 
     The heated partially melted sample exhibited superconductive properties. The onset temperature (sharp resistance decrease) for this sample was about 90 K. 
     EXAMPLE 5. 
     A. The following reagents were utilized: 
     1. Tl 2  O 3 , 
     2. BaCO 3 , 
     3. CuO. 
     B. The same procedure was followed as set forth in Example 4 above except that the nominal composition of the sample was Tl 0 .5 BaCu 3  O 4 .8+x. The heated (partially melted) sample was superconductive. The onset temperature (sharp resistance decrease) for this sample was about 90 K. 
     EXAMPLE 6 
     A. The following reagents were utilized: 
     1. Tl 2  O 3 , 
     2. BaCO 3 , 
     3. CuO. 
     B. The same procedure was followed as set forth in Example 4 above except that the nominal composition of the sample was Tl 2  BaCu 3  O 7+x . The heated (partially melted) sample was superconductive. The onset temperature (sharp resistance decrease) for this sample was about 90 K. 
     EXAMPLE 7 
     A. The following reagents were utilized: 
     1. Tl 2  O 3 , 
     2. BaCO 3 , 
     3. CuO. 
     B. The same procedure was followed as set forth in Example 4 above except that the nominal composition of the sample was Tl 5  BaCu 3  O 11 .5+X. The heated (partially melted) sample was superconductive. The onset temperature (sharp resistance decrease) for this sample was about 90 K. 
     EXAMPLE 8 
     A. The following reagents were utilized: 
     1. Tl 2  O 3 , 
     2. BaCO 3 , 
     3. CuO. 
     B. The same procedure was followed as set forth in Example 4 above except that the nominal composition of the sample was TlBa 2  Cu 3  O 6 .5+X. The heated sample had partially melted and was superconductive. The onset temperature (sharp resistance decrease) for this sample was about 90 K. 
     EXAMPLE 9 
     A. The following reagents were utilized: 
     1. Tl 2  O 3 , 
     2. BaCO 3 , 
     3. CuO. 
     B. The following procedure was followed: 
     1. A mixture of a one molar portion of BaCO 3  and a three molar portion of CuO was ground with an agate mortar and pestle, then heated in air at approximately 925° C. for 2 hours, and reground. 
     2. The resulting BaCu 3  O 4  powder was put into a quartz boat, and lightly pressed with an agate pestle. 
     3. Tl 2  O 3  crystalline powder having approximately half the weight of the BaCu 3  O 4  powder was put on the BaCu 3  O 4 , and heated in a tube furnace in air at a temperature of approximately 900° to about 950° C. for about 30 minutes. 
     4. After almost all of the Tl 2  O 3  had disappeared (by being volatized, melted and/or reacted), the heated sample was taken from the furnace and quenched in air to room temperature. 
     By visual inspection, it was determined that the Tl 2  O 3  had partially volatilized as black smoke, part of the Tl 2  O 3  had become a liquid, and part had reacted with Ba-Cu oxide to form a black porous material which is multiphase and superconductive. This superconductor material had a transition temperature of 78 K (the transition temperature specified here represents the midpoint of the 90% resistance and 10% resistance points in the transition region). 
     It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention and without diminishing its attendant advantages. It is therefore intended that such changes and modifications be covered by the appended claims.