Substrate for Ceramic Superconductor With Improved Barrier

FIG. 1 is a perspective view of a ceramic superconductor coated wire;

FIG. 2 is an enlarged cross-sectional view of the coated wire as seen along the line 2--2 in FIG. 1;

FIG. 3 is a perspective view of a ceramic superconductor coated plate or ribbon;

FIG. 4 is a cross-sectional view of the coated plate as seen along the line 4--4 in FIG. 3;

FIG. 5 is a perspective view of a ceramic superconductor coated hollow cylinder;

FIG. 6 is a cross-sectional view of the coated hollow cylinder as seen along the line 6--6 in FIG. 5;

FIG. 7 is a perspective view of an alternate configuration for the substrate of the present invention with a portion shown in cross-section for clarity; and

FIG. 8 is a cross-sectional view of an alternate

embodiment of the substrate of the present invention as would be seen along the line 2--2 in FIG. 1.

Referring initially to FIG. 1, it will be seen that the present invention can be configured as a wire, generally designated 10. In FIG. 2, it will be seen that the wire 10 essentially comprises a substrate 12, an interaction zone 14 and a layer 16 of ceramic superconducting material. Further, it is to be appreciated that interaction zone 14 itself substrate 12 and an interface layer 20 which is formed between oxide layer 18 and ceramic superconductor layer 16. When the present invention is configured as wire 10, substrate 12 is essentially an elongated cylindrical-shaped base member and interaction zone 14 is established as a layer on the surface of substrate 12 in accordance with procedures to be subsequently disclosed. Ceramic superconductor layer 16 completely covers interaction zone 14 to establish a wire 10 in which substrate 12, interaction zone 14 and ceramic layer 16 are substantially concentric. As will be appreciated by the skilled artisan, the cooperation of these components of wire 10 is crucial to its efficacy. Accordingly, each component must be considered in light of its effect on the combination.

Substrate 12 needs to be selected with certain characteristics in mind. First, substrate 12 must be able to provide an effective mechanical support for ceramic superconductor layer 16. Second, it should not chemically interact with the ceramic superconductor layer 16 in any way which will "poison" the ceramic layer 16 and thereby reduce its efficacy Further, for certain applications, it may be desirable for the substrate to be nonmagnetic at the very low operating temperatures of the ceramic superconductor. Of primary interest in this regard are the stainless steels. It has been determined, however, that in order for stainless steels to maintain their nonmagnetic behavior at cryogenic temperatures, the fcc nonmagnetic austentite (6) phase must remain stable with respect to a transformation to the ferromagnetic bcc martensite (.alpha.') phase. The temperature at which martensite first forms on cooling is referred to as M.sub.s. A relationship for M.sub.s has been determined: M.sub.s (K)=1578-61.1Ni-41.7Cr-36.1Mo-27.8Si-1667(C+N) where the elemental symbols represent the weight percent of the alloying elements in solution with iron in the austentite phase. Thus, variations in the content of an alloy's constituent elements in accordance with the above relationship for M.sub.s can effectively lower the magnetic properties of the resultant alloy. Further, with such variations and the addition of aluminum (Al) it is possible to obtain essentially nonmagnetic alumina formers. Also, the magnetic susceptibility of alloys appears to scale with the Ni content. Therefore preferred alloys are those with M.sub.s <0 K and with minimal nickel content.

With regard to the need that substrate 12 provide an effective support for ceramic layer 16, it will be appreciated by the skilled artisan that most metallic substances can be configured to satisfy this requirement. Due to the typically fragile nature of ceramic layer 16, however, it is desirable that substrate 12 be able to impose axial compressive stresses on ceramic layer 16 in addition to providing structural support for the layer 16. This particular capability is necessary because it is known that ceramics are generally able to withstand greater compressive stresses than tensile stresses before they crack. It happens that, since ceramic layer 16 is sintered onto substrate 12 at much higher temperatures than the operating temperatures of wire 10, the desired situation between substrate 12 and layer 16 can be created if substrate 12 is selected to have a slightly higher coefficient of thermal expansion than that for ceramic layer 16. With this relationship, substrate 12 will impose a force on ceramic layer 16 at the operating temperatures of wire 10 which will urge ceramic layer 16 toward an axially compressive state.

Importantly, some mechanism is required for wire 10 which chemically isolates substrate 12 from ceramic superconductor layer 16. This is needed so that the ceramic layer 16 is not poisoned and the substrate 12 is not compromised. In accordance with the present invention, this isolation is accomplished by interaction zone 14. As stated above, interaction zone 14 has a protective oxide layer 18. Preferably, this protective oxide layer 18 can be formed on the surface of substrate 12 by heating substrate 12 to temperatures in the range of 800.degree. C. to 900.degree. C. and maintaining this temperature for approximately one to ten hours. It has been determined, however, that shorter time periods in the range of approximately one tenth (0.1) to two (2) hours are also effective for forming the oxide layer 18. This, of course, presupposes that substrate 12 contains appropriate oxide formers which will establish the protective oxide layer 18 of interaction zone 14. Preferably, such an oxide former comes from the group which includes aluminum (Al), silicon (Si), beryllium (Be) and manganese (Mn) since wires which contain these elements are commercially available. Also, these elements are particularly good because it is known that they will migrate to the surface 18 of substrate 12 more quickly than other constituent elements in the substrate 12. The result is that an oxide of these elements is formed as a layer on substrate 12. As shown in FIG. 2, this oxide layer 18 will form the part of interaction zone 14 which is adjacent substrate 12.

Several commercially available products, which are set forth below in Table 1 exhibit the desirable characteristics for substrate 12 to varying degrees.

Specifically, from the products listed in Table 1, Hoskins 875 is preferred because it generates an oxide layer 18 of alumina (Al.sub.2 O.sub.3) as part of interaction zone 14 which is particularly effective for inhibiting the diffusion of other constituent elements from substrate 12. Equally important, as will become more apparent after further disclosure, the alumina layer is also very effective for inhibiting the diffusion of constituent elements from ceramic superconductor layer 16 into substrate 12. While Hoskins 875 has been suggested here as a preferred substrate 12, this is not done with the suggestion that the other listed products are less effective. Indeed, the efficacy of any particular substrate will depend on its intended application.

Preferably, ceramic layer 16 comprises a well known superconductor having constituent elements which include a rare earth (RE) together with barium (Ba), copper (Cu) and oxygen (O). Typically, this superconductor is designated RE Ba.sub.2 Cu.sub.3 O.sub.7-x and is familiarly referred to as 1-2-3. It is to be understood, however, that other known superconductors can be used without departing from the spirit or intent of the present invention. For example, a bismuth (Bi) or thallium (Tl) based superconductor can also be used.

As shown in FIG. 3, the ceramic superconducting layer 16 may also be bonded to a substrate 22 which is configured as a ribbon, a plate or some other structure having a flat surface. Additionally, as shown in FIG. 7, a substrate 40 can be used which is a substantially cylindrical-shaped tube. Further, as shown in FIG. 4, substrate 22 can have superconductor layers 16 bonded on either side thereof. More specifically, FIG. 4 shows that substrate 22 has an interaction zone 14 which includes an oxide layer 18 and an interface layer 20. Another oxide layer and interface layer are in an interaction zone 14 which is formed on its opposite side. The ceramic superconductor layers 16 are bonded respectively thereon in a structure that is similar to that for layer 16 and substrate 12 of wire 10. Thus, regardless whether substrate 12 or 22 or 40 is used, the general relationships between the substrates 12 or 22 or 40, interaction zones 14 and the ceramic superconductor layers 16 will remain structurally and chemically the same.

In order to appreciate the relative dimensions of the present invention, for a typical superconductor the depth 24 of ceramic superconductor layer 16 is on the order of approximately seventy-five (75) microns whereas the depth 26 of substrate 22 is on the order of approximately one hundred fifty (150) microns. The depth of interaction zone 14 therebetween will typically be on the order of five (5) to ten (10) microns and oxide layer 18, within this interaction zone, is approximately one (1) to four (4) microns thick. It is noted that these same relative dimensions are also applicable for wire 10 as shown in FIG. 2. Specifically, the diameter of substrate 12 will be on the order of seventy-five (75) microns, the thickness of interaction zone 14 will be on the order of five (5) to ten (10) microns and the thickness of ceramic superconductor layer 16 on the order of seventy-five (75) microns.

It is of interest that during the sintering of ceramic superconductor layer 16, there will be some reaction between the superconducting material in layer 16 and the oxide layer 18 which creates interface layer 20 between layer 16 and the substrate 12. Specifically, in the case where Hoskins 875 is used as substrates 12, 22, 40, a very small amount of predominantly barium (Ba) and to a lesser extent copper (Cu) will be depleted from the 1-2-3 material during sintering of layer 16 and will react with the alumina (Al.sub.2 O.sub.3) of oxide layer 18 to form an interface layer 20 which may contain barium aluminate (BaAl.sub.2 O.sub.4). In most cases this interaction is relatively minor and has minimal, if any, effect on the efficacy of the wire 10. Importantly, however, the interface layer 20 should be relatively small compared to the ceramic superconductor layer 16. This is so primarily in order to minimize the depletion of barium (Ba) from ceramic layer 16.

As stated above, it may be desirable to sinter the ceramic superconductor at temperatures in the range of 970.degree. C. to 1030.degree. C. in order to increase intergranular contact which increases the critical current density (J.sub.c) for the ceramic. Unfortunately, at temperatures above approximately 1000.degree. C. in some substrate materials, such as Hoskins 875, the interdiffusion between substrate 12 and ceramic 16 is significantly increased. It has been determined, however, that precoating substrate 12 with a compound that can form an additional oxide which cooperates with oxides formed from the substrate 12 to create an interface zone 14 can inhibit interdiffusion at the elevated temperatures.

In FIG. 8, a superconductor wire 10 which results from the use of a precoated substrate 12 is shown. Specifically, beginning at the center of wire 10, it will be seen that the interface zone 14 between substrate 12 and ceramic superconductor layer 16 comprises oxide layer 18, precoat oxide layer 42 and interface layer 20. Preferably, precoat oxide layer 42 is yttrium oxide which results from the heating of yttrium hydroxide. The yttrium hydroxide is formed by the hydrolysis of the precursor, yttrium alkoxide. It will be appreciated, however, that rare earth oxides can also be used to form the precoat layer 42. Further, zirconium oxide can also be used. It is also to be appreciated that precoat 42 can be applied using vapor deposition techniques such as sputtering or plasma spraying in addition to solution techniques such as mentioned above.

FIG. 5 shows that the invention is also applicable in situations where a substrate 28 is configured as a hollow cylinder. With such a substrate it is possible to coat the inner surface 30 of substrate 28 as shown in FIG. 6. It will be easily appreciated by the skilled artisan that the exterior surface of a substrate 30 could be coated in a like manner similar to that shown for substrate 22 in FIG. 4.

FIG. 7 shows a configuration for the wire 10 wherein substrate 40 is a tube. The ceramic superconductor layer 16 fills the lumen of tube 40 and the interface zone 14 is formed on the inside wall of substrate 40 between the substrate 40 and ceramic superconductor 16. Though not shown in FIG. 7, it is to be appreciated that a precoat oxide layer similar to that disclosed above for layer 42 could also be used with this configuration.

While substrate configurations in the form of a wire (FIG. 1), a plate or ribbon (FIG. 2), a tube (FIG. 7) and a hollow cylinder (FIG. 5) have been shown for the present invention, these configurations are intended to be only exemplary. It is to be appreciated by the skilled artisan that substrates of various configurations are suitable for use with the present invention and are determined only by the specific requirements of the application.

In the contemplation of the present invention, the ceramic superconductor layer 16 can be bonded to a substrate in any of several ways all known in the pertinent art. For purposes of discussion, a substrate 12 will be considered with a view toward the manufacture of a superconductor wire 10.

Preliminarily, if a material such as Hoskins 875 is to be used for substrate 12 and ceramic sintering is to be accomplished at temperatures above 1000.degree. C., it is necessary to precoat substrate 12. This is done by placing a composition such as yttrium alkoxide on the surface of substrate 12 wherever ceramic superconductor layer 16 is to be disposed. Substrate 12 with the yttrium alkoxide coating is then fired to approximately 400.degree. C. to form an yttrium oxide precoat oxide layer 42 on the substrate 12. If the superconductor material is to be deposited on substrate 12 by electrophoresis, temperatures for forming the precoat 42 can't be too high or an electrically insulating barrier will be formed which will not allow electrophoresis to be performed. Otherwise, higher temperatures may be preferred.

It is to be appreciated that the actual coating of a superconducting material such as 1-2-3 onto substrate 12 is accomplished by covering the entire surface of substrate 12 over the precoat oxide layer 42 if such a layer is used. This can be done with a solution in which superconducting material is dissolved. Further, grains of superconducting material can be deposited directly onto the substrate 12 in any of several ways known in the pertinent art, such as by slurry coating or by electrophoresis. The coated substrate 12 is then placed into an oxidizing environment and heated to a temperature in the range of 800.degree. C. to 900.degree. C. for the purpose of forming oxide layer 14. As mentioned earlier, it happens that in this particular environment, the protective oxide formers in substrate 12 will migrate to the surface 18 of substrate 12 much more rapidly than other constituents of the substrate 12. In the oxidizing environment, these oxide formers then establish the oxide layer 18 which is useful for subsequently inhibiting the interdiffusion of respective constituent elements between substrate 12 and ceramic superconductor layer 16. During this process, it is preferred that the temperature of substrate 12 in the oxidizing environment be maintained in the range 800.degree. C. to 900.degree. C. for approximately one tenth (0.1) hour to two (2) hours. The oxidized substrate 12 and grains of superconducting material on oxide layer 18 may then be elevated in temperature into the range of approximately 900.degree. C. to 980.degree. C. to sinter the grains of superconducting material. This sintering process requires maintenance of temperatures in the range of approximately 900.degree. C. to 980.degree. C. for approximately four hours. As suggested above, the sintering process may be accomplished at elevated temperatures in the range of 970.degree. C. to 1030.degree. C. for approximately one (1) to fifteen (15) minutes. During this sintering process, the superconducting elements react with oxide layer 18, and precoat oxide layer 42 if used, to create interface layer 20 on which ceramic layer 16 is being formed. A superconductor wire 10 has thus been formed. It is interesting to note that, prior to the sintering step, the unsintered grains of superconducting material present a porous region through which oxygen can penetrate to contact substrate 12 during the forming of oxide layer 18. This is so regardless whether grains of superconducting material have been deposited directly onto the surface of substrate 12 or have been coated onto the surface of substrate 12 in solution.

In the case where superconducting material is to be vapor deposited onto substrate 12, it may be necessary that substrate 12 be preoxidized. This is done by placing an uncoated, or a precoated, substrate 12 in an oxidizing environment at temperatures in the range of approximately 800.degree. C. to 900.degree. C. As before, these temperatures are maintained for approximately one tenth (0.1) to ten (10) hours. This will cause the oxide layer 18 to form on the surface of substrate 12 and this preoxidized substrate 12 may then be subjected to any process well known in the pertinent art, such as by sputtering, evaporation, or chemical vapor deposition, which will vapor deposit superconducting material onto oxide layer 18.

Further protection of the superconductors fabricated in accordance with the present invention can be obtained by coating the conductor with silver (Ag) or nickel (Ni). Additionally, this silver or nickel coating (not shown) will be helpful in applications where it is necessary to facilitate electrical contact with the ceramic superconductor layer 16.

For the particular substrate 40 shown in FIG. 7, the procedure is substantially the same. There is, however, the initial requirement that the grains of 1-2-3 superconducting material be placed within the lumen of substrate 40 prior to the sintering process. This can be done in any manner well known in the pertinent art such as by filling the lumen with grains of superconductor material and then swaging, rolling or drawing the substrate 40. Oxygen necessary for the oxidation of substrate 40 prior to the actual sintering process and for the oxidation of the superconductor after sintering can be provided in several ways. For example, tubular substrate 40 can be formed with slits 44 which will provide access for the oxygen into the lumen of substrate 40. Alternatively, tubular substrate 40 can be oxidized before the grains of superconductor material are placed in the lumen. Further, silver oxide (Ag.sub.2 O), when mixed with grains of superconductor material and placed in the lumen of the tube, is known to release oxygen at temperatures between 200.degree. C. and 500.degree. C. and it may therefore be used to create the protective oxide layer 18.

Under any of the above-described procedures, it is necessary that the step of forming the oxide layer on substrate 12 be continued for a period of time which is sufficient to allow stabilization of the oxide on surface 18 of substrate 12. The examples given above indicate this procedure should be continued for approximately one tenth (0.1) to ten (10) hours. If oxide stabilization does not occur, the very purpose of the oxide layer will be compromised. This is crucial because it is of utmost importance to the present invention that oxide layer 14 be established between substrate 12 and ceramic superconductor layer 16 for the purpose of inhibiting interdiffusion of constituent elements between substrate 12 and ceramic superconductor layer 16.

While the particular substrate for supporting a ceramic superconductor as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as defined in the appended claims.