Dip coating of YBCO precursor films on substrates

Abstract

A method for providing a superconductive coating on, for instance, three dimensional substrates by dip coating is disclosed. The dip coating formulation includes a vehicle of terpineol, butoxyethyl acetate and mixed with YBCO, a vehicle precursor material, to form an ink having a viscosity ranging from 2100 cPs to about 2500 cPs. After the substrate is dipped in the dip coating ink, it is dried and sintered. The formulation is prepared by dissolving the binder in the terpineol and butoxyethyl acetate solvents to create a vehicle. The unreacted YBCO precursor material is then milled with the vehicle to create an ink a dip ink formulation having the aforenoted viscosity.

Description

FIELD OF THE INVENTION

[0001] The present invention relates generally to superconducting materials, and more particularly to methods of manufacturing structures coated with high-temperature superconducting materials. Still more specifically, the present invention relates to the dip coating of high-temperature superconducting coatings on substrates using unreacted YBCO precursors materials and a subsequent melt processing technique.

BACKGROUND OF THE DISCLOSURE

[0002] The discovery that certain ceramic materials exhibit superconductivity at above liquid nitrogen temperatures has stimulated intensive research. Once such ceramic material is YBa2Cu3O6+x where x ranges from 0 to 1 or “YBCO.” Many uses for such materials have been suggested and attempted, including, for example, devices operating with microwave or radio frequency signals such as antennas, magnetic resonance imaging pickup coils, resonators, and the like. Optimal performance of such devices may depend upon having the lowest possible surface resistance.

[0003] Low-surface resistance high-temperature superconducting materials have been successfully fabricated in the form of thin films of ceramic. Such films typically have a thickness on the order of 0.5 μm and are formed by depositing the ceramic material or its precursors on the surface of a planar, single crystal substrates using techniques such as co-evaporation, sputtering, laser ablation, and molecular beam epitaxy. The disadvantages of these techniques are discussed in U.S. Pat. Nos. 5,789,347 and 6,119,025 which disclose a “melt processing” process.

[0004] The melt texture process of the '347 and '025 patents involves heating a film that contains YBCO starting materials or precursor materials on a zirconia ceramic substrate at a temperature above 1015° C. in pure oxygen. The film is applied by doctor blading. The heat treatment is fast and relatively simple, but it cannot be used on metallic substrates due to the extreme temperatures (>1015° C.) required to generate the YBCO in the film. The typical surface resistance of the flat films produced by the melt texture process of the '347 and '025 patents are about 0.1 milliohms while the surface resistance of small diameter curved surfaces, e.g., 1-3 mm diameter, is somewhat higher, about 0.3 milliohms. The coatings disclosed in the '347 and '025 patents are applied by screen printing, painting, doctor blading or spin coating.

[0005] U.S. Pat. Nos. 5,340,797 and 5,527,765 disclose a “reactive texture” process which involves forming films on metallic substrates from compounds containing constituents of YBCO. The substrate and films are then heated to near 900° C. which results in a decomposition of the compounds containing constituents of YBCO and the crystallization of YBCO or the substrate. Substrates are typically stainless steel or INCONEL™ (a.k.a. PYROMET™) which require thick silver plating before the application of the YBCO film. The heat treatment requires multiple gas changes including a warm-up in carbon dioxide. The dwell is typically performed in a 2 Torr oxygen atmosphere, but it is claimed to work in higher oxygen concentrations all the way up to pure oxygen. The process is very sensitive and can be difficult to control.

[0006] U.S. Pat. No. 5,856,277 discloses a “surface texture” process which is a way to alter the surface of a bulk pellet of YBCO. The top layer of the resulting structure is typically much thicker than the film produced in the melt texture, surface texture and reactive texture processes discussed above.

[0007] The melt process, surface texture and reactive texture processes all utilize some degree of melting and recrystallization. The YBCO grain size in the surface texture process of the '277 patent is typically somewhat smaller than that of the melt texture and reactive texture processes, but the surface resistance is about the same as in the other two texturing methods.

[0008] Conventional sinter processes use the same substrates and temperatures as the reactive texture process of the '797 and '765 patents but such conventional sinter processes use only phase-pure YBCO and do not involve melting any portion of the film. There is a single gas change at the end of the dwell time at maximum temperature when oxygen concentration is switched from a 1 % oxygen atmosphere to a pure oxygen atmosphere. Conventional sinter processes are typically easy to perform but result in films with a resistivity that is significantly higher than that obtained by the melt process, reactive texture and surface texture processes. However, the surface resistance provided by the conventional sinter processes is superior to that of ordinary conductors such as copper or silver, even at 77° K. Unlike the melt texture, reactive texture and surface texture processes, the YBCO grains produced by the conventional sintering processes are microscopic and randomly oriented, thus resulting in higher surface resistance.

[0009] The '347, '025, '797, '765 and '277 patents are all owned by the assignee of the present application and the disclosures of said patents are incorporated herein by reference.

[0010] To date, a reactive dip coating process has not been developed which provides a superconductive coating with a satisfactory resistance that can be applied by a dip coating the substrate. The development of a dip coating technique would greatly facilitate the fabrication of substrates coated with a superconducting material thereby lowering the cost of products with superconducting coatings.

[0011] The creation of a suitable dip coating ink formulation is particularly problematic because the ink must not have a rapid evaporation rate and the resultant coating must be strong enough for subsequent handling. Currently, there are no available inks which have a suitably high viscosity rate for a sufficiently thick or strong coating in combination with a low evaporation rate. Further, a dip coating ink formulation must be able to stay in suspension so that the solids do not settle out during a production shift, such as an eight hour period.

SUMMARY OF THE DISCLOSURE

[0012] The present invention satisfies the aforenoted need by providing a formulation for dip coating an unreacted superconducting precursor coating on a substrate. The formulation comprises terpineol, butoxyethyl acetate, one or more binders and unreacted YBa2Cu3O6+x precursor materials.

[0013] In a further refinement, the present invention provides a formulation for dip coating a superconducting coating on a substrate that comprises:

[0014] a vehicle comprising from about 47 wt % to about 49 wt % terpineol, from about 47 wt % to about 49 wt % butoxyethyl acetate, from about 2 wt % to about 4 wt % binder, and the vehicle is mixed with unreacted YBa2Cu3O6+x precursor material so that the formulation comprises from about 71 wt % to about 73 wt % unreacted YBa2Cu3O6+x precursor material, and from about 27 wt % to about 29 wt % vehicle.

[0015] In another refinement, the present invention provides a method for applying a superconducting coating onto a substrate which comprises providing a dip coating formulation that comprises unreacted YBa2Cu3O6+x precursor materials and a vehicle that comprises terpineol, butoxyethyl acetate and binder, dipping the substrate in the dip coating formulation, removing the substrate from the dip coating formulation, drying the substrate and melt processing the substrate.

[0016] In a further refinement, the present invention provides a method for applying a superconducting coating onto a substrate by dip coating. The method comprises providing a substrate having a first thickness, providing a vehicle that comprises from about 47 wt % to about 49 wt % terpineol, from about 47 wt % to about 49 wt % butoxyethyl acetate and from about 2 wt % to about 4 wt % of a binder. The method further comprises mixing the vehicle with unreacted YBa2Cu3O6+x precursor powder to provide a formulation comprising from about 71 wt % to about 73 wt % unreacted YBa2Cu3O6+x precursor powder and from about 27 wt % to about 29 wt % vehicle followed by dipping the substrate in the dip coating formulation to form a coating thereon that has a second thickness, removing the substrate from the dip coating formulation, drying the substrate and melt processing the substrate.

[0017] In a further refinement, the method comprises measuring the thickness of the coating after the drying step and, if the coating thickness is unsatisfactory, removing the coating and starting the process again.

[0018] In yet another refinement, the method comprises measuring the thickness of the coating after the melt processing step and, if the thickness of the coating is unsatisfactory, removing the coating from the substrate and starting the process again.

[0019] In a further refinement, the vehicle viscosity is controlled to arrange from about 40 cPs to about 65 cPs at about 100−1.

[0020] In a further refinement, the vehicle viscosity is about 50 cPs. In another refinement, the dip coating formulation has a viscosity ranging from about 2100 cPs to about 2500 cPs at about 20−1.

[0021] In a further refinement, the dip coating formulation has a viscosity of about 2400 cPs at about 20−1.

[0022] In a further refinement, the resultant coating has a preferred thickness ranging from about 170 μm to about 300 μm.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

[0023] The formulation for dip coating substrates, including three dimensional substrates and other substrates, includes a vehicle mixed with unreacted YBCO precursor powder so that the formulation comprises from about 71 wt % to about 73 wt % unreacted YBCO precursor powder and from about 27 wt % to about 29 wt % of a vehicle. The vehicle comprises from about 47 wt % to about 49 wt % terpineol, from about 47 wt % to about 49 wt % butoxyethyl acetate and from about 2 wt % to about 4 wt % of a binder. The unreacted YBCO precursors include Y2O3, BaCO3 and CuO. The terpineol and butoxyethyl acetate serve as solvents. The terpineol is preferably alpha-terpineol and the butoxyethyl acetate is preferably 2-butoxyethyl acetate. The preferred binders are acryloid, more preferably B-67™ M acryloid and cellulose, more preferably T-200™ cellulose. Preferably, the vehicle and the dip coating formulation are free of dispersants as they are deemed unnecessary. The disclosed process and formulation are especially adaptable for use on yttria (partially stabilized) zirconia substrates.

[0024] One preferred formulation is as follows:

Preferred Weight %
Vehicle
Alpha-terpineol                  48.72
2-Butoxyethyl acetate (a.k.a. “BCA”) 48.72
B-67 ™ acryloid (a.k.a. “paraloid”) 1.28
T-200 ™ ethylcellulose           1.28
Dip Coating of Ink Formulation
unreacted YBa2Cu3O6+x Precursor (a.k.a. 72
“YBCO precursor”)
Vehicle                          28

[0025] Generally, the solvents content control the viscosity of the dip coating ink or formulation. Accordingly, when alpha-terpineol is chosen as a solvent, if too much alpha-terpineol is provided, the ink formulation can be too thin, resulting in a film that is too thin. If an insufficient amount of alpha-terpineol is provided, the ink formulation can be too viscous resulting in a film that is too thick. Similarly, butoxyethyl acetate is chosen as a solvent, if too much butoxyethyl acetate is provided, the ink formulation can be too thin, resulting in a film that is too thin. If an insufficient amount of butoxyethyl acetate is provided, the ink formulation can be too viscous resulting in a film that is too thick.

[0026] If the binder or binders are present in too great of an amount, the resulting ink formulation is too viscous and the resulting film can be too thin. If the binder or binders are present in an insufficient amount, the unfired film is too weak resulting in poor adhesion to the substrate.

[0027] Accordingly, when T-200™ ethylcellulose is chosen as a binder, if the T-200™ ethylcellulose is present in too great of an amount, the resulting ink formulation is too viscous and the resulting film can be too thin. If the T-200™ ethylcellulose is present in an insufficient amount, the unfired film is too weak resulting in poor adhesion to the substrate.

[0028] Accordingly, when B-67 ™ acryloid is chosen as a binder, if the B-67™ acryloid is present in too great of an amount, the resulting ink formulation is too viscous and the resulting film can be too thin. If the B-67™ acryloid is present in an insufficient amount, the unfired film is too weak resulting in poor adhesion to the substrate.

[0029] Similarly, if too much vehicle is added to the dip coating formulation, the resultant ink or formulation is too thin and the viscosity can be unsatisfactorily low, thereby resulting in a coating that is too thin. If the vehicle is added in an insufficient amount, the resultant formulation or ink is too thick, resulting in a coating that can be unacceptably thick.

[0030] If the YBCO precursor is present in too great of an amount, the resultant ink formulation can be too viscous resulting in an unfired film that is weak. If the unreacted YBCO precursor is present in an insufficient amount, the ink can be too thin or have an insufficient viscosity resulting in a fired film that is too thin.

[0031] Combinations of other solvents in addition to alpha-terpineol and butoxyethyl may also be utilized. Binders other than B-67™ acryloid and T-200™ ethylcellulose may also be utilized.

[0032] In creating the vehicle, the solids, i.e., the B-67™ acryloid and T-200™ ethylcellulose are dissolved in the alpha-terpineol and 2-butoxyethyl acetate. Then, the YBCO is mixed with the resulting vehicle to produce an ink. A substrate, such as yttria (partially stabilized zirconia) substrate, is then dipped into the dip ink formulation, removed and dried. The drying process can be carried out a temperature of about 90° C. During the drying process, the substrate can be rotated. Finally, the substrate is melt processed. The melt processing is carried out by heating the substrate at a rate of about 300° C. per hour to a temperature of about 1050° C. and holding the substrate at that first temperature for about six minutes. The heating and holding steps are preferably carried out in a pure oxygen atmosphere. The substrate is then cooled at a rate of about 120° C. per hour to a temperature of about 300° C. in a pure oxygen atmosphere followed by further cooling at a faster rate of about 300° C. per hour to room temperature, again in a pure oxygen atmosphere. Variations of the melt processing procedure disclosed in U.S. Pat. Nos. 5,789,347 and 6,119,205 may also be employed.

[0033] A preferred viscosity range for the vehicle is from about 40 cPs to about 65 cPs at about 100 s−1, preferably about 50 cPs 100 s−1. The viscosity of the resulting dip coating formulation or ink preferably ranges from about 2100 cPs to about 2500 cPs at 20 s−1, preferably about 2400 cPs at 20 s−1. The viscosity values are taken with a BROOKFIELD™ viscometer.

[0034] The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications would be obvious to those skilled in the art.

Claims

1. A formulation for dip coating a superconducting coating onto a substrate, the formulation comprising: from about 71 wt % to about 73 wt % unreacted YBa2Cu3O6+x precursor powder and from about 27 wt % to about 29 wt % vehicle, the vehicle comprising from about 47 wt % to about 49 wt % terpineol, from about 47 wt % to about 49 wt % butoxyethyl acetate, and from about 2 wt % to about 4 wt % of a binder.

2. The dip coating formulation of claim 1 wherein the terpineol is further characterized as being alpha-terpineol.

3. The dip coating formulation of claim 1 wherein the butoxyethyl acetate is further characterized as being 2-butoxyethyl acetate.

4. The dip coating of claim 1 wherein the binder comprises acryloid and cellulose.

5. The dip coating formulation of claim 4 wherein the acryloid is further characterized as being B-67™ acryloid and is present in the vehicle in an amount ranging from about 1 wt % to about 2 wt %.

6. The dip coating formulation of claim 4 wherein the cellulose is further characterized as being T-200™ cellulose and is present in the vehicle in an amount ranging from about 1 wt % to about 2 wt %.

7. The dip coating formulation of claim 1 wherein the formulation is substantially free of dispersant.

8. The dip coating formulation of claim 1 wherein the vehicle comprises about 48.7 wt % terpineol.

9. The dip coating formulation of claim 3 wherein the vehicle comprises about 48.7 wt % 2-butoxyethyl acetate.

10. The dip coating formulation of claim 5 wherein the vehicle comprises about 1.3 wt % B-67™ acryloid.

11. The dip coating formulation of claim 6 wherein the vehicle comprises 1.3 wt % T-200 ™ cellulose.

12. The dip coating formulation of claim 1 wherein the terpineol is further characterized as being alpha-terpineol and is present in the vehicle in an amount of about 48.72 wt %.

13. The dip coating formulation of claim 1 wherein the butoxyethyl acetate is further characterized as being 2-butoxyethyl acetate and is present in the vehicle in an amount of about 48.72 wt %.

14. The dip coating formulation of claim 4 wherein the acryloid is further characterized as being B-67™ acryloid and is present in the vehicle in an amount of about 1.28 wt %.

15. The dip coating formulation of claim 4 wherein the cellulose is further characterized as being T-200™ cellulose and is present in the vehicle in an amount of about 1.28 wt %.

16. The dip coating formulation of claim 4 wherein the binder is present in the vehicle in an amount ranging from about 2 wt % to about 3 wt %.

17. The dip coating formulation of claim 4 wherein the binder is present in the vehicle in an amount of about 2.56 wt %.

18. The dip coating formulation of claim 1 wherein the vehicle has a viscosity ranging from about 40 cPs to about 65 cPs at 100 s−1.

19. The dip coating formulation of claim 1 wherein the vehicle has a viscosity of about 50 cPs at 100 s−1.

20. The dip coating formulation of claim 1 wherein the formulation has a viscosity ranging from about 2100 cPs to about 2500 cPs at 20 s−1.

21. The dip coating formulation of claim 1 wherein the formulation has a viscosity of about 2400 cPs at 20 s−1.

22. A method for applying a superconducting coating onto a substrate by dip coating, the method comprising: providing a substrate with a first thickness; providing a vehicle comprising from about 47 wt % to about 49 wt % terpineol, from about 47 wt % to about 49 wt % butoxyethyl acetate, and from about 2 wt % to about 4 wt % of a binder; mixing the vehicle with unreacted YBa2Cu3O6+x, precursor powder to provide a dip coating formulation comprising from about 71 wt % to about 73 wt % unreacted YBa2Cu3O6+x precursor powder and from about 27 wt % to about 29 wt % vehicle; dipping the substrate in the dip coating formulation to form a coating thereon having a second thickness thereon; removing the substrate from the dip coating formulation; drying the substrate; and melt processing the substrate.

23. The method of claim 22 further comprising measuring the thickness of the coating after the drying step.

24. The method of claim 22 further comprising measuring the thickness of the coating after the melt processing step.

25. The method of claim 22 further comprising measuring the first thickness of the substrate before the dipping step and measuring the second thickness of the coating after the drying step and, if the second thickness of the coating is unsatisfactory, removing the coating from the substrate and performing the method again.

26. The method of claim 22 further comprising measuring the first thickness of the substrate before the dipping step and measuring the second thickness of the coating after the melt processing step and, if the thickness of the coating is unsatisfactory, removing the coating from the substrate and performing the method again.

27. The method of claim 22 further comprising measuring the first thickness of the substrate before the dipping step, measuring the second thickness of the coating after the drying step, measuring the second thickness of the coating after the melt processing step and, if the thickness of the coating is unsatisfactory after either the drying or melt processing steps, removing the coating from the substrate and performing the method again.

28. The method of claim 22 wherein the terpineol is further characterized as being alpha-terpineol.

29. The method of claim 22 wherein the butoxyethyl acetate is further characterized as being 2-butoxyethyl acetate.

30. The dip coating of claim 22 wherein the binder comprises acryloid and cellulose.

31. The method of claim 30 wherein the acryloid is further characterized as being B-67™ acryloid and is present in the vehicle in an amount ranging from about 1 wt % to about 2 wt %.

32. The method of claim 30 wherein the cellulose is further characterized as being T-200™ cellulose and is present in the vehicle in an amount ranging from about 1 wt % to about 2 wt %.

33. The method of claim 22 wherein the formulation is substantially free of dispersant.

34. The method of claim 22 wherein the vehicle comprises about 48.7 wt % terpineol.

35. The method of claim 30 wherein the vehicle comprises about 48.7 wt % 2-butoxyethyl acetate.

36. The method of claim 31 wherein the vehicle comprises about 1.3 wt % 1B-67™ acryloid.

37. The method of claim 32 wherein the vehicle comprises 1.3 wt % T-200 cellulose.

38. The method of claim 22 wherein the terpineol is further characterized as being alpha-terpineol and is present in the vehicle in an amount of about 48.72 wt %.

39. The method of claim 22 wherein the butoxyethyl acetate is further characterized as being 2-butoxyethyl acetate and is present in the vehicle in an amount of about 48.72 wt %.

40. The method of claim 31 wherein the acryloid is further characterized as being B-67™ acryloid and is present in the vehicle in an amount of about 1.28 wt %.

41. The method of claim 32 wherein the cellulose is further characterized as being T-200™ cellulose and is present in the vehicle in an amount of about 1.28 wt %.

42. The method of claim 30 wherein the binder is present in the vehicle in an amount ranging from about 2 wt % to about 3 wt %.

43. The method of claim 30 wherein the binder is present in the vehicle in an amount of about 2.56 wt %.

44. The method of claim 22 wherein the vehicle has a viscosity ranging from about 40 to about 65 cPs at 100 s−l.

45. The method of claim 22 wherein the vehicle has a viscosity of about 50 cPs at 100 s−1.

46. The method of claim 22 wherein the formulation has a viscosity ranging from about 2100 cPs to about 2500 cPs at 20 s−1.

47. The method of claim 22 wherein the formulation has a viscosity of about 2400 cPs at 20 s−1.

48. A formulation for dip coating a superconducting coating onto a substrate, the formulation consisting essentially of: about 72 wt % unreacted YBa2Cu3O6+x precursor powder and about 28 wt % vehicle, the vehicle consisting essentially of from about 47 wt % to about 49 wt % alpha-terpineol, from about 47 wt % to about 49 wt % 2-butoxyethyl acetate, from about 1 wt % to about 2 wt % B-67™ acryloid, and from about 1 wt % to about 2 wt % T-200™ cellulose.

49. The dip coating formulation of claim 48 wherein the formulation is substantially free of dispersant.

50. The dip coating formulation of claim 48 wherein the vehicle comprises about 48.7 wt % alpha-terpineol.

51. The dip coating formulation of claim 48 wherein the vehicle comprises about 48.7 wt % 2-butoxyethyl acetate.

52. The dip coating formulation of claim 48 wherein the vehicle comprises about 1.3 wt % B-67™ acryloid.

53. The dip coating formulation of claim 48 wherein the vehicle comprises 1.3 wt % T-200™ cellulose.

54. The dip coating formulation of claim 48 wherein the vehicle has a viscosity ranging from about 40 to about 65 cPs at 100 s−1.

55. The dip coating formulation of claim 48 wherein the vehicle has a viscosity of about 50 cPs at 100 s−1.

56. The dip coating formulation of claim 48 wherein the formulation has a viscosity ranging from about 2100 cPs to about 2500 cPs at 20 s−1.

57. The dip coating formulation of claim 48 wherein the formulation has a viscosity of about 2400 cPs at 20 s−1.

58. A method for applying a superconducting coating onto a substrate by dip coating, the method comprising: providing a substrate with a first thickness; providing a vehicle consisting essentially of from about 47 wt % to about 49 wt % alpha-terpineol, from about 47 wt % to about 49 wt % 2-butoxyethyl acetate, from about 1 wt % to about 2 wt % B-67™ acryloid, and from about 1 wt % to about 2 wt % T-200™ cellulose; mixing the vehicle with unreacted YBa2Cu3O6+x precursor powder to provide a dip coating formulation comprising about 72 wt % unreacted YBa2Cu3O6+x precursor powder and about 28 wt % vehicle and a viscosity of about 2400 cPs at 20 s−1; dipping the substrate in the dip coating formulation form a coating thereon having a second thickness; removing the substrate from the dip coating formulation; drying the substrate; and melt processing the substrate.