Patent Application
20100173785
The present invention is directed toward a method for wet-chemical production of a textured HTSC on a carrier without the use of fluorine-containing solvents.
Textured HTSCs on a carrier are also known as thin-film HTSCs and differ from the conventional multifilament HTSCs that are produced according to the “powder in the tube” method and from HTSCs that are produced by pre-pressing and subsequent sintering of HTSC powder especially in such a way that the superconducting cross-sectional area of the thin-film HTSC is substantially smaller. That is why it is essential for thin-film HTSCs that the HTSC layer on the carrier has the highest possible critical current density. The critical current density depends very essentially on the fact that the HTSC does not have any warpage or the like on the carrier, i.e. the HTSC layer, and thus has the most even texture, which means it is textured. The texture of an HTSC layer can be determined by means of X-ray diffractometry.
In the wet-chemical production of thin-film HTSCs, the HTSC layer must be deposited as textured as possible on the carrier. This is influenced, among other things, by the composition of the precursor solution. Typically, trifluoroacetic acid (TFA) is used in the production of the HTSC precursor solution along with at least one organic salt and/or one organic solvent and/or one organic complexing agent. If no trifluoroacetic acid (TFA) is added to the HTSC precursor solution, barium carbonate is obtained during the later heat treatment. Barium carbonate is chemically very stable; consequently, barium bonded as carbonate is no longer available for the formation of the REBa2Cu3Ox superconductor and obstructs current transport at the grain boundaries. If solvents with trifluoroacetic acid are used for the salts, barium fluoride is obtained instead of barium carbonate. Barium fluoride will react during heat treatment with water vapor to barium oxide and hydrofluoric acid. The problem is that the water vapor will at first diffuse into the HTSC precursor layer and the hydrofluoric acid needs to diffuse out of the layer. That is why only comparatively thin layers can be grown. Moreover, pores are obtained in the HTSC layer by the diffusion. It is a further disadvantage in the use of TFA that the obtained hydrofluoric acid is very poisonous and is still caustic when diluted.
It is generally understood that the problems in connection with hydrofluoric acid can be reduced by utilizing a multiple coating process in which a fluorine-free layer is applied to the carrier and, subsequently, a fluorine-containing (i.e. TFA-based) precursor solution is applied to the carrier to form the uppermost layer. The fluorine contained in the uppermost layer is sufficient, so that no relevant quantities of barium carbonate will be produced in the lower fluorine-free layer during the heat treatment.
Although methods for producing HTSC powders in which no TFA is used are generally known, it is not possible with any of these “TFA-free” methods to deposit a textured HTSC layer on a carrier.
The invention relates to a method for wet-chemical production of a textured high-temperature superconductor (HTSC) on a preferably strip-like carrier. For this purpose, an HTSC precursor solution is produced at first by dissolving rare earth (RE), barium (Ba) and copper (Cu) salts. The HTSC precursor solution is then applied to a preferably textured carrier and is preferably surface-dried. An amorphous layer is produced during drying from the salts. In the subsequent heat treatment, the salts are converted into a textured cuprate superconductor. In one embodiment, the process allows the formation of an HTSC precursor solution that is free from fluorine.
The present invention is directed toward a method for the wet-chemical production of a textured HTSC layer on a carrier. A high temperature superconductor (HTSC) solution is prepared by dissolving rare earth (RE), barium (Ba) and copper (Cu) salts. The rare earth elements include rare earth metals such as yttrium and neodymium. The precursor solution is applied to a carrier to form an HTSC precursor layer, and then the precursor layer is heat treated. During the heat treatment, the HTSC precursor layer is heated to a first temperature that is greater than the decomposition temperature Tz of the salt anions to produce barium carbonate and copper oxide. The HTSC precursor layer is thereafter heated to a second temperature Ts at which the barium carbonate and the copper oxide partially melt in a binary reaction, but which lies beneath the temperature at which RE2BaCuOx is formed. The HTSC precursor layer may be surface dried prior to heating to temperature Tz.
Thus, during the heat treatment of the precursor layer, the precursor layer is heated to a first temperature to cause the decomposition of the anions of the salts. In this process, barium carbonate, copper oxide and an oxide of the used rare earths (e.g. yttrium oxide (Y2O3)) are obtained. Thereafter, the precursor is heated to a second temperature Ts, with the temperature falling within a range at which the precursor layer will melt at least partially, but which is below the temperature in which non-superconducting REBaCuOx (e.g. Y2BaCuO5) is obtained. Specifically, at the second temperature Ts, an at least partial melt is obtained in a binary reaction from barium carbonate and copper oxide.
Preferably, temperature Ts is chosen in such a way that a complete barium carbonate/copper oxide melt is obtained. The temperature Ts depends on the atmosphere in the used furnace, especially its oxygen content, the used rare earth metal and the stoichiometry of the metals in the melt. By way of example, if a nitrogen atmosphere is in the furnace, Yttrium (RE=Y) is used as the rare earth metal. If the ratio of Y:Ba:Cu is 1:2:3, then Ts is approximately 780° C. (partial melt from approximately 620° C., formation of Y2BaCuOx from approximately 825° C., corresponding approximately to 620° C.≦Ts≦825° C.). By way of further example, if air is in the furnace (with RE=Y, Y:Ba:Cu as 1:2:3), then Ts is approximately 900° C. (e.g., 780° C.≦Ts≦915° C.). If there is pure oxygen in the furnace (with RE=Y, Y:Ba:Cu as 1:2:3), then Ts is approximately 926° C. (e.g., 810° C.≦Ts≦938° C.). The temperatures must be chosen slightly higher when using neodymium as a rare earth metal (i.e., RE=Nd).
As a result of the described choice of temperature, it is possible to omit TFA and other fluorine-containing additives in the production of the HTSC precursor solution. That is, the HTSC precursor solution can be free from TFA (and thus fluorine) because the BaCO3 is unstable in the melt in the presence of CuO and disintegrates into a barium oxide (BaO) and carbon dioxide (CO2). This is surprising because it has been generally assumed that BaCO3 will only disintegrate at 1450° C.
The CO2 obtained from the disintegration of BaCO3 will outgas and the remaining BaO will react with CuO and the rare earth oxide (e.g. Y2O3) into REBa2Cu3Ox (e.g. YBa2Cu3Ox, which is also known as YBCO). The REBa2Cu3Ox (e.g. YBCO) will crystallize out on the carrier, with a texture transfer occurring from the uppermost (buffer) layer of the carrier to the REBa2Cu3Ox layer.
Water is preferably used as a solvent for producing the HTSC precursor solution. The water can be acidified with acetic acid.
An amine may be added to the HTSC precursor solution. The amine includes, but is not limited, to, Triethanolamine (TEA) and/or ethylene diamine tetraacetate (EDTA). TEA and EDTA stabilize the ions in the precursor solution as complexing agents during the surface drying of the HTSC precursor solution.
Nitrates, tartrates, citrates, and/or acetates may be used as the salts (i.e., the RE, BA, and Cu salts) in the production of the HTSC precursor solution. These salts are easy to handle and the anions of these salts can be decomposed by the application of heat. The salts must preferably be dosed in the production of the HTSC precursor solution in such a way that RE:Ba:Cu is about 1:1.5-2.5:2.5-4, preferably about 1:1.5-2:3-3.5.
Oxidic nanoparticles may further be added to the HTSC precursor solution to form so-called pinning centers for magnetic flux lines in the produced HTSC. In this manner, the critical current density of the HTSC can thus be increased. The same effect can be achieved by adding further metal salts.
In addition, the HTSC precursor solution may further include an organic salt, an organic solvent, an organic complexing agent, or combinations thereof.
The precursor solution preferably possesses a pH-value of between 3 and 9, and preferably possesses a pH of about 6.75.
Once the HTSC precursor solution has been applied to the carrier, its viscosity will be preferably set to approximately 1 mPas to approximately 30 mPas, and preferably to approximately 10 mPas.
During the heat treatment, the temperature at which barium oxide and copper oxide form a melt but no RE2BaCu3Ox is obtained is maintained for at least 25 minutes. Thereafter, the HTSC can be cooled to a temperature Ta of between approximately 380° C. and approximately 550° C., and preferably approximately 400° C. This temperature is may be kept substantially constant for at least 20 minutes.
While the HTSC precursor reacts at a temperature Ts into REBa2Cu3Ox, the atmosphere in the used furnace preferably has an oxygen content of less than 20%, especially preferably less than 5%. Preferably, the HTSC is subjected to loading with oxygen of an atmosphere of at least 50%, preferably 100% oxygen, only when it cools off, e.g., when the furnace has cooled off to temperature Ta.
The carrier may be a metal strip with at least one ceramic buffer layer (e.g., a buffer layer made of lanthanum zirconate, cerium oxide or strontium titanate), or may be a purely ceramic carrier, e.g., made of strontium titanate or lanthanum aluminate. The carrier is preferably textured, i.e. it has a textured surface on which the HTSC is deposited, with a texture transfer being made from the carrier to the HTSC.
It is understood that several HTSC precursor solutions can be applied to the carrier and can then be heat-treated. Similarly, a further HTSC layer can be applied to an HTSC layer (e.g., at least two HTSC precursor layers may be applied successively on the carrier).
The invention is explained again on the basis of the following examples and
Y, Ba, and Cu acetate (Y:Ba:Cu as 1:2:3) were dissolved in water laced with acetic acid by cooking in return flow. Triethanolamine (TEA) was added to the thus produced HTSC precursor solution as a complexing agent. The ratio of the sum total of the metals (Y, Ba, Cu) to TEA was 1:2.5. The pH-value was set to approximately 6.75 by adding ammonia (approx. 25% by weight). The HTSC precursor solution was clear and blue. It was vaporized to a viscosity of approximately 9 mPas at approximately 60° C. and applied thereafter to a cleaned and degreased carrier made of polished (100) SrTiO3 by immersion of the carrier into the HTSC precursor solution (drawing out speed approx. 50 mm/min). The carrier with the HTSC precursor solution was surface-dried for approx. 1 h at 60° C. The HTSC precursor solution was then gel-like. The subsequent heat treatment occurred in a furnace. The furnace with the HTSC precursor samples was heated with 10° C./min from room temperature to Ts=815° C. (800° C. to 835° C. were possible). An N2 atmosphere with an O2 content of approximately 200 ppm was in the furnace. After approx. 150 minutes, the furnace temperature was set to approximately 525° C. and the furnace atmosphere was exchanged for O2. The furnace was cooled to approximately 400° C. and this temperature was held for approximately 30 minutes. After approx. 5 hours, the furnace was switched off and the HTSC samples were removed after cooling to room temperature. The characterization of the samples resulted in Tc=92K with ΔTc=5K and Jc=1.87 MA/cm2.
An HTSC precursor solution was produced according to example 1, applied to a carrier and surface-dried. The subsequent heat treatment occurred in an analogous fashion, but the furnace with the HTSC precursor samples contained an N2 atmosphere with an O2 content of 1000 ppm. The furnace was heated with 10° C./min from room temperature to Ts of =838° C. (810° C. to approx. 890° C. were possible). The furnace temperature was reduced to approx. 525° C. 150 minutes after reaching Ts and the furnace atmosphere was exchanged for O2. The furnace was cooled to approx. 400° C. and this temperature was held for approx. 5 hours. The furnace was then switched off and the HTSC samples were removed after cooling to room temperature. The characterization of the samples resulted in Tc=92K with ΔTc=2K and Jc=1.28 MA/cm2.
Y, Ba and Cu acetate (Y:Ba:Cu as 1:2:3) were dissolved in water with 15% by volume of acetic acid by cooking in return flow (1 h, 90° C.). Thereafter, the pH value of the HTSC precursor solution was set to 6 by adding approx. 25% by weight of ammonia. TEA was added as a metal complexing agent, with the ratio of the sum total of the metal ions (Y, Ba, Cu) to TEA being 1:0.5. The HTSC precursor solution was clear, blue and had a pH of 6.2. It was subsequently applied to a degreased carrier made of polished (100) SrTiO3 by immersion of the carrier in the HTSC precursor solution (drawing out speed approx. 50 mm/min). The HTSC precursor solution on the carrier was surface-dried for approx. 1 hour at 60° C. The subsequent heat treatment occurred in a furnace with N2 atmosphere and 200 ppm O2. The furnace was heated with the sample by approx. 10° C. min−1 to 815° C. The final temperature was held approx. 150 min. Thereafter the furnace was cooled to a temperature of 525° C. and the atmosphere was exchanged for O2. The furnace was cooled to approx. 400° C. and this temperature was held for approx. 5 hours. The furnace was then switched off and the HTSC samples were removed after cooling to room temperature. The characterization of the samples resulted in Tc=92K with ΔTc=4K and Jc=0.89 MA/cm2.
An HTSC precursor solution was produced according to example 3, but ethylene diamine tetraacetate (EDTA) was added instead of TEA, with the ratio of the sum total of the metal ions (Y, Ba, Cu) to EDTA being 1:0.5. The precursor solution was applied as in example 3 to an SrTiO3 carrier and heat-treated. The characterization of the samples resulted in Tc=92K with ΔTc=4K and Jc=1.07 MA/cm2.
Diluted aqueous citric acid (1.8M) were added under stirring at air to an aqueous solution of Nd, Ba and Cu nitrate (Nd:Ba:Cu as 1:2:3) with a total concentration of 0.6M. The ph value was set to 6 with an aqueous ammonia solution in order to prevent the precipitation of the metal hydroxides and a recrystallization during the subsequent steps. The HTSC precursor solution was applied to a polished (100) and degreased SrTiO3 carrier by immersion of the same (drawing out speed approx. 170 mm/min). The HTSC precursor solution on the carrier was subjected to 60° C. for several hours in a dust-free furnace until the consistency of the HTSC precursor solution or layer was gel-like. This was followed by a heat treatment for producing an HTSC. For this purpose, the carrier was heated in a furnace with 1° C./min to firstly 500° C. and then with 5° C./min to 940° C. An argon (Ar) atmosphere with 1% O2 was in the furnace. Subsequently, the obtained NdBa2Cu3Ox layer was annealed at 450° C. for 5 hours under an O2 atmosphere and thus charged with oxygen. The characterization of the samples resulted in Tc=89K with ΔTc=10K and Jc=0.3 MA/cm2. An examination of the HTSC layer with an atomic force microscope (AFM) showed a layer thickness of approx. 1 μm. The comparatively slow heating to 500° C. at first and then 940° C. was necessary so that no fissures would form in the HTSC precursor layer.
As in example 1, an HTSC precursor solution was produced, applied, surface-dried and heat-treated. The sample was heated up to 850° C. No YBCO was obtained in this test, but non-superconducting Y2BaCuO5 which was identified by means of scanning electron microscopy on the basis of its characteristic needle-like structure.
As in example 1, an HTSC precursor solution was produced, applied to a carrier, surface-dried and heat-treated. There was an N2 atmosphere in the furnace with 1000 ppm O2 and the sample was heated up to 880° C. No YBCO was obtained in this test, but non-superconducting Y2BaCuO5 which was identified by means of scanning electron microscopy on the basis of its characteristic needle-like structure.
Thus, above described process provides a method for the wet chemical production of an HTSC layer on a carrier, wherein an HTSC precursor solution comprising no trifluoroacetate may be utilized if the same is heated to a temperature Ts during the heat treatment of the HTSC precursor, wherein the remaining substances of the HTSC precursor solution form at least a partial melt, which is below the temperature at which RE2BaCuOx is formed, and which is deposited from the liquid phase while forming a peritectic. In this manner, an HTSC layer may be formed form a precursor solution that contains no fluorine.
While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102008004818.6 | Jan 2008 | DE | national |
This application is a continuation of International Application No. PCT/EP2009/000214, filed on Jan. 15, 2009, entitled “A Wet-Chemical Method for Producing a High-Temperature Superconductor,” which claims priority under 35 U.S.C. §119 to Application No. DE 102008004818.6 filed on Jan. 17, 2008, entitled “A Wet-Chemical Method for Producing a High-Temperature Superconductor,” the entire contents of which are hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2009/000214 | Jan 2009 | US |
| Child | 12726421 | US |
