Electrodeposition Process for Forming Superconducting Ceramics

The present invention relates generally to superconducting ceramics and processes for forming thin films of superconducting ceramics. More particularly, the present invention is directed to a electrodeposition process where an alternating current is utilized in conjunction with a direct electrical current to deposit superconducting ceramics onto a wide variety of substrates.

2. Description of Related Art

There has been a great deal of effort expended in designing and developing suitable processes for making superconducting ceramics. The most desirable of these processes are those which are simple, efficient and are capable of producing a ceramic superconductor with a relatively high superconducting transition temperature (T.sub.c). Processes which employ deposition as part of the superconductor fabrication procedure have been proposed due to the simplicity, reproducability and coating quality which can be obtained using electrodeposition techniques.

The deposition processes used to form superconducting ceramics typically involve electroplating a mixture of the desired metals onto a suitable surface to form a film of superconducting precursor metals which is then annealed at high temperature to form the superconducting oxide. (Exemplary electrodeposition procedures are set forth in the following U.S. Pat. Nos.: 4,870,051; 4,879,270; 4,939,308; 5, 120,707; and 5,162,295.)

Although many of the deposition procedures which have been developed are well suited for their intended purpose, there still is a continuing need to develop further deposition procedures that are simple, efficient and are capable of producing superconducting ceramics which have relatively high superconducting transition temperatures.

In accordance with the present invention, a deposition process which is based upon electrodeposition is provided for producing superconducting ceramics. The process is simple, efficient and is capable of producing superconducting ceramics having relatively high superconducting transition temperatures.

The method of the present invention involves providing a working electrode which has a surface upon which superconducting precursor metals are deposited. The working electrode, along with a counter-electrode, is emersed in a solution which comprises a mixture of superconducting precursor metal ions. The metal ions are electrodeposited onto the surface of the working electrode to form a film thereon which includes a mixture of the superconducting precursor metals in the proportions which are required for formation of a superconducting ceramic.

As a feature of the present invention, electrodeposition of the superconducting precursor metal ions onto the working electrode surface is accomplished by applying a direct current voltage between the working electrode and the counterelectrode and, in addition, an alternating current is simultaneously applied between the working electrode and counterelectrode, The combination of applying a direct current voltage in combination with an alternating current was found to produce a film of superconducting precursor metals which is particularly well suited for further fabrication into superconducting ceramics.

The film of superconducting precursor metals which is deposited in accordance with the present invention is then oxidized by annealing at high temperature to produce a wide variety of superconducting ceramics. The present invention is particularly well-suited for producing superconducting ceramics such as TIBaCaCuO, YBaCuO and BiSrCaCuO. This method may also be used in producing any of the other known superconductor oxides.

As a feature of the present invention, the superconducting ceramic maybe doped with silver by including silver ion in the electrolyte. Further, metals such as thallium can be electrodeposited during the initial deposition of metal precursors. Alternatively, the thallium or other metal may be incorporated into the film from a vapor source during the oxidation/annealing step.

The improved electrodeposition method of the present invention provides a process which has considerable practical potential, particularly in the fabrication of large non-planar devices. The improved deposition method is a low-cost process which has the ability to deposit films of ceramic superconductors on substrates of any shape, including wires and tapes. The annealing time required to oxidize the deposited precursors in accordance with the present invention is decreased because mixing of the deposited metal ions occurs at an atomic level. This results in the production of submicron sized particles which are produced in the deposited film and are especially well suited for oxidation to form superconducting ceramics.

The above discussed and many other features and attendant advantages of the present invention will become apparent as the invention becomes better understood by reference to the following detailed description when taken in conjunction with the accompanying drawings.

FIG. 1 sets forth an X-ray diffraction scan of a superconducting film prepared in accordance with the present invention on a silver substrate.

FIG. 2 is a graph showing the resistance versus temperature for a TIBaCaCuO (TBCCO) film prepared in accordance with the present invention.

FIG. 3 is a representative I-B curve of a film prepared in accordance with the present invention measured at 77.degree. K in perpendicular magnetic field with values from 0 to 0.3T.

FIG. 4 sets forth the I.sub.c versus H.sub.t data for a film prepared in accordance with the present invention measured at 77.degree. K

The process of the present invention first involves electrodeposition of superconducting precursor metal ions onto a desired substrate surface to form a film of superconducting precursor metals in proportions which are sufficient to be oxidized into a superconducting ceramic. After electrodeposition, the deposited film is then oxidized/annealed at high temperature to form the final superconducting ceramic.

The electrodeposition is accomplished by applying fixed direct current voltage onto which is applied an alternating current. As will be described in detail below, the application of an alternating current frequency between the electrodeposition deposition electrodes is an important aspect of the present invention which enhances the overall fabrication process and resulting superconducting ceramic.

In the present invention, a working electrode is provided onto which the superconducting precursor metals are to be deposited. One or more counterelectrodes are utilized. Preferably, the working electrode will be silver or other suitable material providing a desirable substrate for the final superconducting film. The shape of the electrode can be in virtually any form, including wires, tapes and any other desired shape onto which the superconducting film is to be fabricated. The counterelectrodes are preferably made from platinum or other inert material.

The electrolyte bath can be any of the known electrolyte baths which have been used to deposit superconducting metal ions onto working electrodes. The metal ions will include La, Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Pb, Hg, Ag, Tm Yb, Lu, Th, Ba, Bi, TI, Sr, Ca and Cu. The particular concentrations of the metal precursor ions are selected depending upon the desired superconducting ceramic. The particular metal ions and their concentrations within the electrolyte are either well known or may be determined with a small amount of routine experimentation. The relative concentrations of the superconducting precursor metal ions are chosen in proportions which are sufficient to insure that subsequent oxidation of the deposited film results in the formation of a superconducting ceramic. The superconducting precursor metal ions are preferably added to the electrolyte as salts. Nitrate salts are preferred, however, other conventional metal salts have been used and are suitable. The preferred electrolyte is dimethyl sulfoxide (DMSO). Other conventional electrolytes may be utilized, including mixture of water and DMSO, dimethyl formamide (DMF), mixture of water and DMF. The amount of water in the mixtures with DMSO and DMF can range from 1 volume percent to 50 volume percent. When a mixture of water and DMSO or DMF is used, it is preferred that the amount of water in the mixture be on the order of 10 to 20 volume percent.

Once the solution of electrolyte and desired metal ions has been prepared, the working electrode and counterelectrodes are emersed into the solution. As an important aspect of the present invention, a direct current having a voltage of between 2 to 10 volts is applied between the working electrode and the counterelectrode. Voltages on the order of 4 to 6 volts are preferred. In addition to applying a direct current voltage, an alternating current is applied between the working and counterelectrodes. The frequency of the alternating current is between about 5 KHz to 1 00 KHz. Frequencies on the order of 15 to 20 KHz are preferred. Power supplies which are capable of providing constant voltage output while simultaneously applying an alternating current are commercially available. It is preferred that the voltage setting of the power supply is not affected when varying the current frequency and, equally important, the frequency should not be affected when varying the voltage. Preferably, the current pulse train AC signal is a square wave train with on/off intervals increasing or decreasing with varying frequency. A preferred exemplary power source is available from Team Specialty Products Corporation (Albuquerque, N.M.). This power source is capable of providing a constant 6 amp output between a counterelectrode and working electrode with the alternating current variable between 0 and 18.2 KHz. The alternating current is independent of the voltage output which is variable from 0 to 20 volts. The power supply is stable and not easily affected by power line fluctuations. The power supply voltage setting is not affected when varying the current frequency and the frequency is not affected when varying the voltage. The current pulse train for this power supply is a square wave train with on/off intervals increasing or decreasing with varying frequency (on command).

The particular voltage and alternating current frequency which is applied between the counter and working electrodes will vary depending upon the particular metal ions being deposited and the final superconducting ceramic desired. It is a matter of routine experimentation to vary both voltage and frequency within the ranges set forth above to establish applied voltages and frequencies which optimize electrodeposition deposition and superconductor ceramic production.

Typically, the electrodeposition is conducted at room temperature. However, if desired, the temperature can be raised to as high as 100.degree. C. Other electrodeposition parameters are conventional and well known to those skilled in the art. The time required to electrodeposition deposit films of desired thicknesses will vary depending upon a number of different parameters. The amount of time required for depositing a particular thickness of material is easily established by routine experimentation. If desired, a relatively thick film may be deposited and then scraped from the surface to form precursor powder.

Once the film is deposited as set forth above, the next step in the method of the present invention involves oxidation of the film of superconducting precursor metals to form the superconducting ceramic. This oxidation/annealing step is conducted in the same manner as the oxidation/annealing step set forth in the previously described group of patents Typically, annealing will take place at temperatures on the order of 600.degree. to 1200.degree. C in an oxygen containing atmosphere. The annealing can take place in air, if desired. In certain situations, it is desirable to introduce one or more of the metals required for superconductor ceramic formation during the annealing process. For example, thallium may be deposited from the vapor phase during the annealing step, instead of during the initial deposition process. Further, the superconducting precursor metal film may be doped with small amounts of silver on the order of between about 0.1 percent and about 4.0 percent by weight. This doping is accomplished by adding a silver salt, such as Ag NO.sub.3 to the electrolyte.

Examples of practice are as follows:

a) Electrodeposition of superconductor precursor metal film

A electrodeposition solution of dimethylsulfoxide (DMSO) was prepared having the following salt concentrations of metal ions: 7.59 gm Ba(NO.sub.3).sub.2, 3.64 gm Ca(NO.sub.3).sub.2 .cndot.4H.sub.2 O, 2.98 gm CU(NO.sub.3).sub.2 .cndot.6H.sub.2 O and 0.2 gm AgNO.sub.3 in 390 ml of DMSO.

The electrodeposition was performed in a closed-cell configuration at 24.degree. C. The electrolyte solution was purged through a commercial diffuse bubbler with an initial high flow (30 cc/min) of the N.sub.2 gas for 30 minutes prior to an actual electrodeposition deposition, thereby removing most of the atmospheric contaminants, followed by a low flow (8.5 cc/min) purge during the electrodeposition deposition process to prevent recontamination. The working electrode was commercial-grade flexible 0.125-mm thick Ag foil (99.9% pure). The applied direct current voltage ranged from 4 to 6 volts and the applied alternating current frequency range was 15 to 18.2 KHz. All samples were deposited in a vertical cell, where the electrodes (working and counter) were suspended vertically from the top of the cell. All chemicals were of Analar or Puratronic-grade purity and were used as received. A conventional two-electrode cell was employed, where the counter electrode was a Pt gauze. Two opposed Pt gauze counter electrodes on either side of the Ag foil working electrode were employed so that a properly stoichiometric film was deposited on both sides of the Ag substrate. A number of film depositions were performed under a controlled atmosphere (i.e. purged with N.sub.2 gas) and with minimal stirring, thus assuring the process occurred in the diffusion-limited regime.

b) Annealing of precursor BCCO

The deposited films were processed using a two-zone flowing anneal which employs a separate TI source zone and a sample zone within which O.sub.2 is at an ambient level. The annealing process consisted of inserting the sample and heating to 860.degree. C. while the TI source was kept initially at a low temperature of 690.degree. C. for 10 minutes and then increased to 725.degree. C. and held for 45 minutes. The sample and the thallium oxide boat were cooled at a rate of 5.degree. C./minute at the end of the process. The X-ray diffraction (2.crclbar. vs intensity) of one of the annealed films is shown in FIG. 1 indicates a mixture of 1223 (TIBa.sub.2 Ca.sub.2 Cu.sub.3 O.sub.x), 2223 (TI.sub.2 Ba.sub.2 Ca.sub.2 Cu.sub.30 .sub.x) and 2212 (TI.sub.2 Ba.sub.2 Ca.sub.1 Cu.sub.2 O.sub.x) phases.

The samples were evaluated for their superconducting properties by resistance versus temperature (R-T) measurements and four-probe transport (critical current) measurements in He vapor. Four Ag contacts were made on one side of the sample by evaporating Ag in vacuum. Ag wires were soldered to the Ag contacts using In solder. The sample was inserted slowly into the precooled chamber of the squid-magnet that is capable of producing maximum fields of 5.5T. The magnetic field was applied perpendicular to the plane of the substrate. X-ray diffraction data were obtained on a Rigaku Rotaflex Model Ru 200 instrument using CuKa radiation. The X-ray peak of Ag that was usually present due to the Ag contacts or the Ag foil and was used as a reference for the X-ray diffraction analysis.

The resistive transition for the film (shown in FIG. 2) is fairly sharp with an onset temperature of 118.degree. K. and zero-resistance temperature of 110.degree. K. The transport measurements of the film are shown in FIG. 3 (I vs V), and FIG.4 (I.sub.c vs H.sub.T), The transport was measured at 77.degree. K. under a magnetic field up to 0.3T. The field dependence for the TI-superconductor film was impressive even for the field oriented perpendicular to the ab plane.

The electrodeposition deposition process described in Example 1 is carried out except that TI is included in the DMSO electrolyte in a concentration of 1 gm TI NO.sub.3. The electrodeposited TI is annealed with the other deposited metals under favorable ambient conditions for a time and temperature sufficient to form the superconducting ceramic.

7.666 gm Y(NO.sub.3).sub.3 .cndot.6H.sub.2 O, 10.427 gm Ba(NO.sub.3).sub.2, and 5.915 gm CU(NO.sub.3).sub.2 .cndot.6H.sub.2 O is dissolved in 300 ml dimethyl sulfoxide (DMSO) solution. A YBaCuO precursor film is then deposited onto a silver foil electrode in the same manner as Example 1. The precursor film is then annealed under favorable ambient conditions for a time and temperature sufficient to form a superconducting ceramic.

7.666 gm Y(N.sub.03).sub.6 .cndot.6H.sub.2 O, 10.427 gm Ba(N03).sub.2, and 5.915 gm Cu(NO.sub.3).sub.2 .cndot.6H.sub.2 O is dissolved in 300 ml DMSO solution. To this solution 30 ml water is added. A YBaCuO precursor film is then deposited onto a silver foil electrode in the same manner as Example 1. The precursor film is then annealed under favorable ambient conditions for a time and temperature sufficient to form a superconducting ceramic.

7.706 gm Ba(NO.sub.3)2, 3.543 gm Ca(NO3)2.4H20, 3.558 gm CU(NO3)2* 6H20, and 0.204 gm AgNO.sub.3 is dissolved in 390 ml DMSO solution. A BaCaCuAgO precursor film is then deposited onto a silver foil electrode in the same manner as Example 1. The precursor film is then annealed under favorable ambient conditions for a time and temperature sufficient to form a superconducting ceramic. EXAMPLE 6--BaCaCUAgO

7.706 gm Ba(NO.sub.3).sub.2, 3.543 gm Ca(NO.sub.3).sub.2 .cndot.4H.sub.2 O, 3.558 gm CU(NO.sub.3).sub.2 .cndot.6H.sub.2 O, and 0.204 gm AgNO3 is dissolved in 390 ml DMSO solution. To this solution 30 ml water is added. A BaCaCuAgO precursor film is then deposited onto a silver foil electrode in the same manner as Example 1. The precursor film is then annealed under favorable ambient conditions for a time and temperature sufficient to form a superconducting ceramic.

7.706 gm Ba(NO.sub.3).sub.2, 3.567 gm Ca(NO.sub.3).sub.2 .cndot.4H.sub.2 O, and 3.555 gm Cu(NO.sub.3).sub.2 .cndot.6H.sub.2 O is dissolved in 200 ml DMSO solution. A BaCaCuO precursor film is then deposited onto a silver foil electrode in the same manner as Example 1. The precursor film is then annealed under favorable ambient conditions for a time and temperature sufficient to form a superconducting ceramic.

7.706 gm Ba(NO.sub.3).sub.2 .cndot.3.567 gm Ca(NO.sub.3).sub.2 .cndot.4H.sub.2 O, 3.555 gm Cu(NO.sub.3).sub.2 .cndot.6H.sub.2 O is dissolved in 200 ml DMSO solution. To this solution 30 ml water is added. A BaCaCuO precursor film is then deposited onto a silver foil electrode in the same manner as Example 1. The precursor film is then annealed under favorable ambient conditions for a time and temperature sufficient to form a superconducting ceramic.

7.706 gm Ba(NO.sub.3).sub.2 .cndot.3.974 gm Ca(NO.sub.3).sub.2 .cndot.4H.sub.2 O, 3.552 gm CU(NO.sub.3).sub.2 .cndot.6H.sub.2 O, 0.882 gm Bi(NO.sub.3).sub.3 .cndot.5H.sub.2 O, 1.015 gm Sr(NO.sub.3).sub.2, and 1.158 gm Pb(NO.sub.3).sub.2 is dissolved in 300 ml DMSO solution. A BaCaCuBiSrPbO precursor film is then deposited onto a silver foil electrode in the same manner as Example 1. The precursor film is then annealed under favorable ambient conditions for a time and temperature sufficient to form a superconducting ceramic.

7,705 gm Ba(NO.sub.3).sub.2 3.974 gm Ca(NO.sub.3).sub.2 .cndot.4H.sub.2 O, 3,553 gm Cu(NO.sub.3).sub.2 .cndot.6H.sub.2 O, 0.882 gm Bi(NO.sub.3).sub.3 .cndot.5H.sub.2 O, 1.015 gm Sr (NO.sub.3).sub.2, and 1,158 gm Pb(NO.sub.3).sub.2 is dissolved in 300 ml DMSO solution. To this solution 30 ml water is added, A BaCaCuBiSrPbO precursor film is then deposited onto a silver foil electrode in the same manner as Example 1, The precursor film is then annealed under favorable ambient conditions for a time and temperature sufficient to form a superconducting ceramic.

1.423 gm Pb(NO.sub.3).sub.2, 1.957 gm Bi(NO.sub.3).sub.3).sub.3 .cndot.5H.sub.2 O, 9.870 gm Sr(NO.sub.3).sub.2, 5.037 gm Ca(NO.sub.3).sub.2 .cndot.4H.sub.2 O, and 4.054 gm CU(NO.sub.3).sub.2 .cndot.6H.sub.2 O, is dissolved in 300 ml DMSO solution. A PbBiSrCaCuO precursor film is then deposited onto a silver foil electrode in the same manner as Example 1. The precursor film is then annealed under favorable ambient conditions for a time and temperature sufficient to form a superconducting ceramic.

1.423 gm Pb(NO.sub.3).sub.2, 1.957 gm Bi(NO.sub.3).sub.3 .cndot.5H.sub.2 O, 9.870 gm Sr(NO.sub.3).sub.2, 5.037 gm Ca(NO.sub.3).sub.2 .cndot.4H.sub.2 O, and 4.054 gm CU(NO.sub.3).sub.2 .cndot.6H.sub.2 O, is dissolved in 300 ml DMSO solution. To this solution 30 ml water is added. A PbBiSrCaCuO precursor film is then deposited onto a silver foil electrode in the same manner as Example 1. The precursor film is then annealed under favorable ambient conditions for a time and temperature sufficient to form a superconducting ceramic.

2.212 gm Bi(NO.sub.3).sub.3 .cndot.5H.sub.2 O, 11.608 gm Sr(NO.sub.3).sub.2, 5.595 gm Ca(NO.sub.3).sub.2 .cndot.4H.sub.2 O, and 4.094 gm Cu(NO.sub.3).sub.2 .cndot.6H.sub.2 O, is dissolved in 300 ml DMSO solution. A PbBiSrCaCuO precursor film is then deposited onto a silver foil electrode in the same manner as Example 1. The precursor film is then annealed under favorable ambient conditions for a time and temperature sufficient to form a superconducting ceramic.

2.212 gm Bi(NO.sub.3).sub.3 .cndot.5H.sub.2 O, 11.608 gm Sr(NO.sub.3).sub.2, 5.595 gm Ca(NO.sub.3).sub.2 .cndot.4H.sub.2 O, and 4.094 gm Cu(NO.sub.3).sub.2 .cndot.6H.sub.2 O, is dissolved in 300 ml DMSO solution. To this solution 30 ml water is added. A PbBiSrCaCuO precursor film is then deposited onto a silver foil electrode in the same manner as Example 1. The precursor film is then annealed under favorable ambient conditions for a time and temperature sufficient to form a superconducting ceramic.

2.027 gm TINO.sub.3, 2.000 gm Bi(NO.sub.3).sub.3 .cndot.5H.sub.2 O, 14.206 gm Sr(NO.sub.3).sub.2, 9.413 gm Ba(NO.sub.03).sub.3, 6.217 gm Ca(NO.sub.3).sub.2 .cndot.4H.sub.2 O, 5.061 gm Cu(NO.sub.3).sub.2 .cndot.6H.sub.2 O, and 0.204 gm AgNO.sub.3 is dissolved in 300 ml DMSO solution. A PbBiSrCaCuO precursor film is then deposited onto a silver foil electrode in the same manner as Example 1. The precursor film is then annealed under favorable ambient conditions for a time and temperature sufficient to form a superconducting ceramic.

2.027 gm TINO.sub.3, 2.000 gm Bi(NO.sub.3).sub.3,5H.sub.2 O, 14.206 gm Sr(NO.sub.3).sub.2, 9.413 gm Ba(NO.sub.3).sub.2, 6.21 7 gm Ca(NO.sub.3).sub.2 .cndot.4H.sub.2 O, 5.061 gm Cu(NO.sub.3).sub.2 .cndot.6H.sub.2 O and 0.204 gm AgNO.sub.3 is dissolved in 300 ml DMSO solution. To this solution 30 ml water is added. A PbBiSrCaCuO precursor film is then deposited onto a silver foil electrode in the same manner as Example 1. The precursor film is then annealed under favorable ambient conditions for a time and temperature sufficient to form a superconducting ceramic.

Having thus described exemplary embodiments of the present invention, it should be noted by those skilled in the art that the disclosures herein are exemplary only and that various other alternations, adaptations and modifications may be made within the scope of the present invention. Accordingly, the present invention is not limited to the specific embodiments as illustrated herein.