The following specification describes the invention and the manner in which it is to be performed:

FIELD OF THE INVENTION

The present invention relates to an improved process for joining of tubes of (Bi,Pb)-2223 oxide superconductor.

More particularly, present invention relates to an improved process for the formation of superconducting joint which is capable of carrying stably the critical current with nearly a negligible loss.

BACKGROUND OF THE INVENTION

Due to the rapid advancements in the methods based on the joining of particularly High temperature superconducting (HTS) materials like: Rare earth (RE) based (e.g.YiBa ₂ C0 ₃ 07- x), bismuth based systems [e.g. Bi ₂ Sr ₂ CaiCu ₂ 0s _+x {abbreviated as Bi- 2212},(Bi,Pb) ₂ Sr ₂ CaiCu ₂ 0 _8+x {abbreviated as (Bi,Pb)-2212}, (Bi,Pb) ₂ Sr ₂ Ca ₂ Cu ₃ Oio _+x {abbreviated as (Bi,Pb)-2223,(Bi,Pb) ₂ Sr ₂ Ca ₂ Cu ₃ Oio _+x ({abbreviated as (Bi,Pb)-2223}] and MgB ₂ with a superconducting joint, it has been possible to have large size superconducting components/products so that they can be used in large-scale applications like: magnetic resonance imaging (MRI) magnet systems, superconducting magnetic separators, superconducting magnet energy storage systems (SMES), nuclear magnetic resonance (NMR) systems, superconducting transmission cables, freezer cooling superconducting magnets, nuclear fission reactor magnets, high energy particle accelerators, large superconducting magnet systems, superconducting generators and motors , magnetic shields, current leads (CLs) etc. However, in literature most of the research that has been carried out involves a superconducting joint method for the metal clad tapes/wires and small bulk samples etc.

For example, for joining tapes (m-Km) of (Bi,Pb)-2223 ,Bi-2212/(Bi,Pb)-2212, YBCO and MgB ₂ etc, the various methods include hot isopressing joining, overlapping fusing, laser / spot welding, soldering, brazing, lap/ butt/ ultrasonic/ laminate joining, etc. [U.S. Pat: 5,004,722, Japan. J. Appl. Physics, vol.34, p.4770 (1995); U.S. Patent, 6,133,814; U.S. Pat: 6,159,905, U.S. Patent, 6,753,748, EP 19930102579, Appl. Supercond, vol3, no.4 p207 (1995), Supercond. Sci. Technol.Vol, 13, p.237 (2000); U.S. Pat: 6,194,226, U.S. Patent Nos: 6,561,412 and 7,001,870; Using these techniques, joint tapes as long as of length 32.2 kilometers of Bi-2223 system have been produced by K hkura et al. (IEEE Trans.Appl.Superconductivity,Vol. l8,no.2, pp 356-359, June 2008.

Whereas for joining small (mm-cm) bare samples (pellets, disks, bars, crystals etc.) the various methods for all the Bi-2223/(Bi,Pb)-2223, Bi-2212/(Bi,Pb)-2212, YBCO and MgB ₂ materials used are: solid state diffusion bonding, melt/-solidifying processes, autogeneous/gas flame welding, C0 ₂ laser welding, microwave joining, liquid phase aided joining, laser irradiation, reactive liquid infiltration (RLI), use of superconducting paste and modifications of these methods etc. [Y.Mutoh et al. Japanese J. Appl. Phys.Vol29, No.8, p LI 432 (Augl990); Laid-open No. HeiH 6-211588; J.Electrochemical Society, Vol 136, No.2, p 582- 583 (Feb 1989); S.Haseyama et al, Physica C, 354, p437 (2001); C. Vipulanand and W.Lu, IEEE Tran. Appl. Supercon. Vol. 13, No.2, June (2003); B.Bozzo et al. Supercond.Sci.Technol.Vol.18, pl227 (2005); J.Am.Ceram.Soc, Vol 79, p 885 (1996); J.Cai et al. Supercond. Sci. Technol. volume 5, p599 (1992); C.H.Kao et al. Chinese J.Physics, Vol 34, p325 (1996); N. Sakai et al. Physica C, Vol. 426-431, p515 (2005), D. Isfort et al, Physica C, Vol 390, p341 (2003); X.Clhaud et al. Supercon.Sci.Technol. Vol.19, S590 (2006); G.Giunchi et al, IEEE Trans. Appl. Supercond. Vol.20, no.3, pl524 (2010)]. Using these techniques, joint bare bulk samples with reasonably good superconducting properties of size as large of 4 cm size have been produced.

On the other hand, the technology for joining large size (not less than cm range) bare bulk superconductors like bare rods, hollow pipes/cylinders and tubes etc. which is rapidly becoming important and more challenging is still under development. The main difficulty is due to (i) unsuitability of the methods used for joining tapes and small bare samples etc.

(which have a smaller joining area), in joining of large tubes/rods, and (ii) difficulty in making a good homogeneous superconducting joint over a larger area between such large samples with maximum percentage retention of critical current (I _c ).

However, introduced below are the methods known so far for joining of large size samples like rods, hollow pipes, and tubes:

US 5,244,876 patent disclosed an autogeneous welding technique to join a pair of preformed silver sheathed Bi-2212 superconducting rods obtained from melt casting method. This method comprises of heating end faces of the two silver sheathed superconducting rods to bright- red heat and bringing close together with a gap and filling this gap with molten material of the same superconducting rod followed by recovery heating of the joint region at 815 °C for 12 hours in an oven. The joint zone between the two round rods was superconducting and the long joined rod sample obtained from this method has dimensions: length = 600 mm and diameter =12 mm. However, the drawback of this method is that the joint is generally resistive, because the joint parts thereof are produced through melting of the preformed superconducting phase, that a full recovery of damaged superconducting phase is hard to procure.

Journal "T. Kasuga et al. (J.Am.Ceram. Soc, vol. 79,No.40, 9885, 1996)" disclosed a direct joining method (i.e. without use of any intervening material as disclosed in the above patent) using flame welding for joining a pair of bare Bi-2212 melt cast preheated rods. They further extended this method for joining Bi-2212 pipes also. According to this method, a high temperature flame melts two edge portions of the glass-ceramics (i) rods with same (2 mm) diameter and (ii) tubes with different diameters (15 mm and 10 mm) to be joined, and the molten faces are placed quickly against each other. Subsequently the joined portion is annealed sufficiently with a soft flame of 900°C-950°C for 5 min., resulting in a unified joined product with no cracks. Finally the joined product is reheated to make the joined portion superconducting. The joint made according to this method has almost the same J _c (i.e. ~ 100% critical current retention) as that of the original glass-ceramics and the joined tube thus obtained is as long as ~95mm. However, drawback of this method is that the devices are complex when Bi system bulk materials were obtained with the melt cast process.

Journal "M.S.Tseluevskii et al.(IEEE Trans. Appl. Superconductivity, Vol 7, 2087,1997)" disclosed a different approach for joining two parts of a sintered Bi-2212 tubes. In this method, the Bi-2212 tube divided into two parts was spliced by sandwiching between them some amount of the starting mixture of the raw materials having the same composition as that employed in preparing the tube. To form superconducting phase in the splice short time interval of the pre-melting were chosen somewhat lower than the melting temperature (880°- 900°C) of phase Bi-2212 with subsequent isothermal annealing for three days at much low temperature (780°-820°C). Following the thermal treatment of the system consisting of BSCCO tubes and a small amount of the starting mixture sandwiched between them, the space between the parts of the tube appeared to be filled with superconducting material, which held them sufficiently tight. Drawback of this method is that a decrease in critical temperature (T _c °) of the joined part from 86 K to 81 K and the joined portion do contain some impurities phases. Further no measurements on critical current density (J _c ) were disclosed. The drawbacks of all the above melt-processed methods for making joints between silver sheathed and or bare rods/pipes/tubes are that it requires process temperature close to the melting point of the HTS. Therefore these are expensive and also the HTS can become weak or slag at these extreme temperatures. Moreover these methods are limited to a system that is compatible with a partial melting stage like Bi-2212 and RE -123. In the case of a system like (Bi, Pb)-2223, it is difficult to apply the melt processed method as it incongruently [R.Flukiger et al. Bismuth-Based High Temperature Superconductors, Ed. H.Maeda and K. Togano, p 319 (1996)]. Further, since the (Bi,Pb)-2223 material has been identified as the most suitable HTS for power supply of systems at cryogenic temperatures due to their higher J _c , low toxicity and low thermal conductivity [P. Hermann, Hand book of Applied Superconductivity ed B.Seeber (Bristol :institute of Physics Publishing),p 801-44,1998], therefore, a different method needs to be applied.

WO 2008/093354, disclosed a method of joining a pair of bare (Bi,Pb)-2223 tubes using a combination of superconducting paste (joining material) and a common bush for clubbing the tubes together. This process involves the preparation of a partially preformed superconducting material, followed by cold isostatic pressing (CIP) of the powder of partially preformed superconducting material into tube shape of different dimensions and further provided thermal deposition of a silver layer at both ends of the tube . The process further involves the lapping of one of the end faces of a pair of said tubes to be joined. These lapped end faces of both the tubes clubbed together on a common bush are coated with a superconducting paste. The paste used is made by mixing partially preformed (Bi,Pb)-2223 superconducting powder may be (i) in organic formulation: polyvinyl butryal (binder), cyclohexane (solvent) and fish oil (dispersant), or (ii) in air drying silver paint, or (iii) in isoamylacetate and fish oil etc. and the bush is made of silver. Then these coated end faces are closed pressed together to form a joint. This joint portion and the end portions of the tubes are wrapped with a perforated silver foil followed by deposition of another layer of silver. Finally, the assembly of this joint portion and the tubes is heat treated in air for 100 to 150 hours and at temperatures from 830°C to 850°C. Then reporting that the joint made according to this process is superconducting and the length of the joined pair can be as long as 640 mm [(inner diameter) ID of 10.1 mm, (outer diameter) OD of 12.4 mm]; and it can be as long as 440 mm [ID = 28.6 mm, OD=31.2 mm]. The joint is able to carry 71 % to 91% of the critical current as that of the unitary tube.

The drawback of this reference is that the percentage critical current retention of the joint tube pair made according to this method is not more than 91%. The possible reason for this may be due to: (i) use of common silver bush which is metallic, therefore, in case of a weak joint, the current can also pass locally through it and there will be some loss in the current through the joint, (ii) inadequate quality of the organic formulation used for making superconducting paste which may not be binding superconducting particles so densely and also leave the residual phases; thereby leads to pores and impurities at the joint to make a weak superconducting joint, (iii) coating of the superconducting paste on the end faces only probably generated only a limited area for diffusion bonding i.e. a limited superconducting path area and (iv) probability of disturbance of the physically joined portion due pressure exerted at the joint portion when another layer of silver was spray deposited by a spray gun, thereon resulting into gap/pores and thus making the joint weak.

In the case of a good joint, the joined tube pair can not be shunted by silver bush, and therefore, current will flow through the joint without loss. If it is not a good joint then DC current can also pass locally through the silver bush and therefore, there will be a loss in the current flowing through the joint.

From the hitherto known art as described above, it is clear that, it would be desirable to provide improvements and modification to the above method of joining a pair of bare bulk (Bi, Pb)-2223 superconducting tubes such as that the desired transport critical current degradation is negligible or minimized at the joint and thereby overcoming the shortcomings associated with the prior art.

Other alternatives like (i) change of material from metallic silver to superconducting for the common bush so that even if the joint is weak, the current can flow through the superconducting bush and also reduces the cost as the cost of silver is almost double the cost of the superconducting material,(ii) better organic formulation to improve the quality of the superconducting pastes in order to make a good superconducting joint, (iii) applying superconducting paste not only at the end faces of the tubes but also at (a) end inner surface portions of the tube/outer surface of the bush to fill the gaps between the tube and the bush and (b) at the end outer surface of the tube to fill the gaps between perforated silver foil and the tube in order to increase the area for diffusion bonding to improve continuity of the superconducting path through the joint portion; (iv) omitting the step of deposition of a silver layer on the perforated silver foil wrapping the physically joined portion, to prevent the physically joined portion from getting disturbed etc. are required to make the superconducting joint with minimum critical current degradation. OBJECTIVES OF THE INVENTION

The main object of the invention is to provide an improved process for joining oxide superconducting tubes together with an improved superconducting joint which obviates the drawbacks mentioned above.

Another object of the present invention is to provide an improved process for joining (Bi, Pb)-2223 superconducting tubes that will result in with minimum impurities/ pores in the joined portion.

Yet another object of the present invention is to provide a process, wherein the joint can carry improved percentage critical current retention more than 91% of that of the unitary tube. Yet another object of the present invention is to use a common bush made of superconducting material rather than made of metallic silver in order to improve the superconducting path through the joint.

Yet another object of the present invention is to modify the superconducting paste using a novel inexpensive environmentally safe binding agent lignin in the form of lignin powder in combination with boiled organic linseed oil - the richest source of lignin to avoid other additional solvents, dispersants etc. in order to improve binding between superconducting particles and to make the joint with minimum residual phases.

Yet another object of the present invention is to polish the end faces, inner surface of the end portions of the tube to be joined and outer surface of the superconducting bush to expose a fresh surface and immediately apply modified superconducting paste on the said polished surfaces to prevent all the fresh exposed surfaces from contamination of air and moisture for a better superconducting joint.

Yet another object of the present invention is to apply modified superconducting paste on the silver layer of the end portions of the tubes to be joined to improve the physical contact between the tubes and the perforated silver sheet.

Yet another object of the present invention is to apply modified superconducting paste by extrusion method to avoid the formation of voids etc.

Yet another object of the present invention is to skip the step of deposition of silver layer on the perforated silver sheet wrapping the physically joined portion to prevent the probability of the physically joined portion being getting disturbed.

SUMMARY OF THE INVENTION

Accordingly, present invention provides an improved process for making a joint between (Bi,Pb)-2223 oxide superconducting tubes with improved superconducting properties and the said process comprising the steps of: (i) cold isostatic pressing (CIP) of calcined spray dried powder of (Bi,Pb)-2223 system at a pressure in the range of 200-300 MPa to obtain a tube of length in the range of 430 to 500 mm, outer diameter in the range of 50 to 150 mm and wall thickness in the range of 2 to 10 mm; (ii) sintering the tube as obtained in step (i) at a temperature in the range of 830 to 850°C in air for 60 to 100 hours; (iii) grinding sintered tubes as obtained in step (ii) to obtain (Bi,Pb)-2223 superconducting powder; (iv) cold isostatic pressing a part of (Bi,Pb)-2223 powder as obtained in step (iii) at a pressure in the range of 300 to 400 MPa into tubes having outer diameter (OD) in the range of 10.6-12.4 mm, inner diameter (ID) in the range of 9.3-10. lmm and length in the range of 100 to 320mm to obtain (Bi, Pb)-2223 superconducting tubes; (v) mixing (Bi, Pb)-2223 powder as obtained in step (iii) in lignin powder and boiled organic linseed oil to obtain (Bi,Pb)-2223 superconducting paste; (vi)polishing 10 to 20 mm long outer surface at both end portions of the (Bi, Pb)-2223 superconducting tubes as obtained in step (iv) with a subsequent thermal spray deposition of metal silver layer on the polished surface; (vii)wrapping a perforated silver sheet on the said silver layer of one of the end portions of two tubes as obtained in step (vi) followed by depositing another silver layer to form end current contacts; (viii) polishing other end face and other end inner surface of the tubes as obtained in step (vii) and immediately applying a (Bi,Pb)-2223 superconducting paste as obtained in step (v) on the said polished end faces and inner surfaces; (ix) polishing both end face and both end inner surface of the tube as obtained in step (vi) and immediately applying a (Bi,Pb)-2223 superconducting paste as obtained in step (v) on the said polished end faces and inner surfaces; (x) for joining two tubes, arranging two tubes of step (viii) in such a way that the said coated end faces are in close contact to each other by inserting a common bush inside the contacting silver deposited end portions of the tube pair with electrical current contacts are outside; (xi) for joining more than two tubes, arrange tubes of step (ix) in the middle with tubes of step (viii) on either side in such a way that the said coated end faces are in close contact to each other by inserting a common bush inside the contacting silver deposited end portions of the middle tubes and the end portion with electrical current contacts are outside; (xii) physically contacting the said coated end faces and applying superconducting paste on the said physically joined portion and silver layer deposited end portions of tubes of step (x) and step (xi) and subsequently wrapping the said physically joined portion with a perforated silver sheet to ensure that the end faces are properly joined together; (xiii) sintering the above said combination of the said joint portion and the tubes of step (viii) at a temperature in the range of 830 to 850°C in air for a time period of 100-150 hours to obtain the superconducting joint between said tubes.

In another aspect of the present invention, the organic vehicle to make (Bi,Pb)-2223 superconducting paste is a mixture of lignin powder and boiled organic linseed oil.

In yet another aspect of the present invention, (Bi, Pb)-2223 superconducting paste is applied by painting brush or by extrusion method preferably by extrusion method.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents the joint of the invention between a pair of tubes of dimensions of length of up to about 320 mm, outer diameter up to 12.4 mm and wall thickness up to 2 mm according to the present invention: numeral (1) is an oxide superconducting tube; numeral (2) is a polished outer surface; numeral (3) is a silver layer; numeral (4) is polished end face; numeral (5) is a polished inner surface; numeral (6) is a superconducting bush; numeral (7) is a superconducting paste; and numeral (8) is a perforated silver sheet.

FIG. 1(a) represents the joints designated as A and B of the invention between tubes more than two.

FIG. 2 represents a schematic drawing of the set up for measuring the voltage and current through each of the joined tubes on the one hand and through joint regions (A, B) for finding critical current (I _c ) according to the method of the invention on the other hand.

DETAILED DESCRIPTION OF THE INVENTION

While the invention is susceptible to various modifications and alternative forms, specific aspect thereof has been shown by way of example and will be described in detail below. It should be understood, however that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the invention.

The Applicants would like to mention that the examples are mentioned to show only those specific details that are pertinent to understanding the aspects of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having benefit of the description herein.

The terms "comprises", "comprising", or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a process that comprises a list of components does not include only those components but may include other components not expressly listed or inherent to such process. In other words, one or more elements in a system or process proceeded by "comprises... a" does not, without more constraints, preclude the existence of other elements or additional elements in the system or process.

In the following detailed description of the aspects of the invention, reference is made to the accompanying drawings that form part hereof and in which are shown by way of illustration specific aspects in which the invention may be practiced. The aspects are described in sufficient details to enable those skilled in the art to practice the invention, and it is to be understood that other aspects may be utilized and that charges may be made without departing from the scope of the present invention. The following description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.

Accordingly, present invention provides an improved process for making a joint between (Bi,Pb)-2223 oxide superconducting tubes with improved superconducting properties and the said process comprising the steps of: (i) cold isostatic pressing (CIP) of calcined spray dried powder of (Bi,Pb)-2223 system at a pressure in the range of 200-300 MPa to obtain a tube of length in the range of 430 to 500 mm, outer diameter in the range of 50 to 150 mm and wall thickness in the range of 2 to 10 mm; (ii) sintering the tube as obtained in step (i) at a temperature in the range of 830 to 850°C in air for 60 to 100 hours; (iii) grinding sintered tubes as obtained in step (ii) to obtain (Bi,Pb)-2223 superconducting powder; (iv) cold isostatic pressing a part of (Bi,Pb)-2223 powder as obtained in step (iii) at a pressure in the range of 300 to 400 MPa into tubes having outer diameter (OD) in the range of 10.6-12.4 mm, inner diameter (ID) in the range of 9.3-10. lmm and length in the range of 100 to 320mm to obtain (Bi, Pb)-2223 superconducting tubes; (v) mixing (Bi, Pb)-2223 powder as obtained in step (iii) in lignin powder and boiled organic linseed oil to obtain (Bi,Pb)-2223 superconducting paste; (vi)polishing 10 to 20 mm long outer surface at both end portions of the (Bi, Pb)-2223 superconducting tubes as obtained in step (iv) with a subsequent thermal spray deposition of metal silver layer on the polished surface; (vii)wrapping a perforated silver sheet on the said silver layer of one of the end portions of two tubes as obtained in step (vi) followed by depositing another silver layer to form end current contacts; (viii) polishing other end face and other end inner surface of the tubes as obtained in step (vii) and immediately applying a (Bi,Pb)-2223 superconducting paste as obtained in step (v) on the said polished end faces and inner surfaces; (ix) polishing both end face and both end inner surface of the tube as obtained in step (vi) and immediately applying a (Bi,Pb)-2223 superconducting paste as obtained in step (v) on the said polished end faces and inner surfaces; (x) for joining two tubes, arranging two tubes of step (viii) in such a way that the said coated end faces are in close contact to each other by inserting a common bush inside the contacting silver deposited end portions of the tube pair with electrical current contacts are outside; (xi) for joining more than two tubes, arrange tubes of step (ix) in the middle with tubes of step (viii) on either side in such a way that the said coated end faces are in close contact to each other by inserting a common bush inside the contacting silver deposited end portions of the middle tubes and the end portion with electrical current contacts are outside; (xii) physically contacting the said coated end faces and applying superconducting paste on the said physically joined portion and silver layer deposited end portions of tubes of step (x) and step (xi) and subsequently wrapping the said physically joined portion with a perforated silver sheet to ensure that the end faces are properly joined together; (xiii) sintering the above said combination of the said joint portion and the tubes of step (viii) at a temperature in the range of 830 to 850°C in air for a time period of 100-150 hours to obtain the superconducting joint between said tubes.

In another embodiment of the present invention, the (Bi,Pb)-2223 system having molar ratio of Bi : Pb : Sr : Ca : Cu : Ag, in the range of Bi 0.35-3.05 : Pb 0. 35-3.05: Sr 0.35-3.05 :Ca 0.35-

3.05 · Cu Ο.35-3.Ο5 : Ag 0.35-3.05.

In yet another embodiment of the present invention, the (Bi,Pb)-2223 system having molar ratio of Bi ₁ . ₈₄ Pbo. 3 ₅ Sr ₁ . ₉₁ Ca ₂ .o ₅ Cu3.o ₅ Ag _1-2 .

In another embodiment of the present invention, the organic vehicle to make (Bi,Pb)-2223 superconducting paste is a mixture of lignin powder and boiled organic linseed oil.

In yet another embodiment of the present invention, weight ratio of superconducting powder, lignin powder and organic linseed oil is in the range of 5 :0.002:0.9 to 5 :0.004: 1.1.

In yet another embodiment of the present invention, in steps (viii), (ix), (x), (xi) and (xii) the (Bi, Pb)-2223 superconducting paste is applied by painting brush or by extrusion method, preferable by extrusion method

In yet another embodiment of the present invention, the common bush used is of (Bi, Pb)- 2223 superconducting material.

In yet another embodiment of the present invention, critical current retention (I _c ) percentage is in the range of 94 to 100%.

In still another embodiment of the present invention, the boiled organic linseed oil in the paste is used so that it penetrates along with the lignin powder particles and the superconducting powder particles deeper and faster into pores of the joining portions of the as CIP processed tubes to be joined and bush as common carrier.

In yet another embodiment of the present invention, the superconducting joint shows reproducibility in the range of 70% to 80%.

In still another embodiment of the present invention, the tubes, the common bush and the paste comprises of the same material that is starting (Bi, Pb)-2223 superconducting powder. The tubes, the common bush and the paste obtained are respectively referred to as (Bi, Pb)- 2223 tubes, (Bi, Pb)-2223 superconducting bush and (Bi, Pb)-2223 superconducting paste. In yet another embodiment of the present invention, the outer diameter of the (Bi, Pb)-2223 bush is in the range of 9.2 mm to 10.0 mm.

In yet another embodiment of the present invention, the length of the (Bi, Pb)-2223 bush is in the range of 15 mm to 30 mm.

In still another embodiment of the present invention, the step of deposition of silver on the perforated silver sheet wrapping the physically joined portion is omitted.

In yet another embodiment of the present invention, the length of the tube may be in the range of 100-320 mm, the outer diameter may be in the range of 10.6-12.4 mm and the wall thickness may be in the range of l-2mm.

In yet another embodiment of the present invention, the number of joints may be one or more. In yet another embodiment of the present invention, the number of tubes may be two or more.

Present invention provides an improved process for making a joint between superconducting tubes with improved superconducting joint from an improved superconducting paste and superconducting carrier bush particularly to (Bi Pb) ₂ Sr ₂ Ca ₂ Cu ₃ Oio+ _X : Ag [Bi, Pb)- 2223 :Ag] system. Ag (1.2 M%>- optimized value) is used as an additive.

To improve the superconducting joint connectivity between tubes to be joined, the method included the steps:

(i) Replacement of silver bush with a superconducting bush in order to (i) increase superconducting path through the component tubes, (ii) decrease the thermal losses due to several orders of magnitude lower thermal conductivity of the Bi-2223 system (0.8W/m ) as compared to that of the silver (51 lOW/m ) at operative temperature of

20 K and (iii) reduce the material cost to nearly half.

(ii) Modification of the joining superconducting paste material in order to improve binding between the superconducting particles with minimum by-products at the joint, superconducting paste may obtained by dispersing the superconducting powder in an organic vehicle comprising of not yet used an outstanding binder such as lignin powder and boiled organic linseed oil.

For making superconducting slurry/paste, so far used is an organic vehicle, i.e. a mixture of synthetic organic polymer binders such as polyvinyl butryal (PVB), polyisobutyl methacrylate, polyvinyl acetate/alcohol, polyethylene glycol, ethyl cellulose, acrylic resin, polyisobutyl methacrylate etc. in combination with dispersants (fish oil, triglycerides, sordiatantrioleate etc) and solvents (cyclohexane, ethanol, xylene, butyl acetate, butyl phthalate, terpineol etc.). It has been found generally that with the use of a combination of binder, dispersant and solvent, after the heat treatments, residual products are formed, which play a role of in reducing J _c .

Further among all these binders, PVB is the most commonly used. However, it has drawbacks of getting deteriorated upon water exposure; it requires solvents dispersants etc. and it costs around Rs.400-500/Kg. Therefore, there is a need for choosing a material which alone can serve the properties of a binder, solvent, dispersant etc. and can bind the superconducting particles strongly with minimum residuals and is less costly at the same time.

There is a very well known versatile natural non-toxic, fibrous and harmless organic polymer outstanding binder called as lignin. It is found in all living plants and has been used enormously ranging from medicinal to varying industrial applications. For example, in industrial applications as a (i) binder in oil painting, wooden articles and animal feed pellets etc. [S.Tongfu et al. Front. Optoelctron. China, vol.2, 244-247 (2009); P.Subramanian,US AA 61L900FI; B.Baurhoo et al. Animal Feed Sci. and TechnoLvol 144, issues 3-4,pp 175- 184, July2008]; (ii) dispersant in high performance cement applications, carbon block slurries, clay products, dyes, cement, floor tiles etc.; [A.Mathial and D.G.Kubat; European J.Wood and wood products, Vol 52, no. l, pp 9-18, DOI: 10,1007/BF02615010, B.Fross/US Patent 4,450,106; D.Feldman et al. J.Appl.Poly.Sci.,vol 89, pp 2000-2010 May (2003)]; (iii) additive to lead-acid storage battery, oil field applications and agricultural chemicals, paper/cloth, strong and stiff polymer composites [N.Hira et al, J.Power Source, vol 158, pp 1106-1109, Aug (2008); N.A.El-Wakil, J.Appl.Polymer Science, vol.113, pp 793-801 (2009); W.Thielemann and R.P.Wool, Polymer composites, pp 697-705, 2005]/carbon fibers [N.Reddy et al. Macromolecular Materials and Engng. Vol. 292, issue 4, pp 458-466 (Aprill, 2007); S.Kubo and J.F.Kalda, J.Polymers and the Enviornment, vol 13, pp97-104 (2005)] etc. It is surprising that despite having several advantages and producing worldwide several thousand of patents covering uses of lignin in any form, it has not been used so far in making of any of the superconducting ( whether conventional or unconventional ) products or paste or slurry etc. Some of its established advantages can be exploited. For example: (i) it is an outstanding binder and binds fine particles into a dense mass when the temperature is under 400°C, thereby minimizing pores at the joint and also disappear automatically if the temperature is between 400°C and 500°C, thereby leaving minimum residuals,i.e what remains is the purer material; (ii) it has unique versatile nature i.e. alone it can serve the role of dispersant, solvent, adhesive, and surfactants, therefore, does not require any additional material; (iii) it is fibrous in nature, therefore can strengthen the brittle materials like oxide superconductors; (iv) it has a unique feature of soaking deep into visible and microscopic pores (this is a feature that can not be reproduced with any other binder), therefore, can help in filling the pores along with superconducting particles;(v) it is a low cost (Rs.40-50/Kg) material. Thus, a unique combination of gluing and deep soaking properties makes lignin an outstanding economical material for use in making joints between superconductors.

Therefore, for making of superconducting paste, an attractive possibility is to use lignin as a bonding agent so that the joint area can be dense, strong and also free of any by-products after the high temperature treatment.

It is known that lignin has always been used in different forms depending upon the application. For example, in dietary applications, it has been used in the form of organic linseeds/linseed oil (richest natural source of lignin). On the other hand, for industrial applications it has been used as a binder/solvent/dispersant in forms like: boiled linseed oil, lignin powder, lignin chemically modified (lignosulfonates, kraft lignin etc.). Since chemically modified lignin is hydrophilic in nature, therefore, not suitable for oxide superconductors. Moreover, it also contains inorganic contents which are difficult to remove and are likely to be in the joined portion even after sintering, therefore, these are not suitable. Therefore, in order to avoid chemicals, lignin powder and/or boiled organic linseed oil could be better choices. Since, using lignin powder alone, formation of paste is not possible, and using boiled organic linseed oil alone there is a possibility of lignin being destroyed, therefore, a combination of both can be a better option. Further, boiled organic linseed oil has the capability of penetrating deeper and faster along with the binder particles and the superconducting particles into pores of the body where ever applied, thereby is expected to make a stronger joint.

Furthermore, hydrophobic and dust repellent nature of both the lignin powder as well as boiled linseed oil prevents the particles to be bonded and the surfaces to be joined from the attack of water molecules and dust particles thereby can help in forming a better joint. It is to mention here that, in literature, use of either lignin powder or boiled organic linseed oil have been reported so far, however, a combination of lignin powder and boiled organic linseed oil has not yet reported. (iii) Use of extrusion method for applying superconducting paste on the polished end faces and inner/outer surfaces of the physically joined portions in order to avoid pores/bubbles. It is to note that for joining superconductors using superconducting paste, so far spraying method for coating Bi-2223 paste has been disclosed [S.Haseyama et al. Physica C Superconductivity Vol 356, issu.1-2, vol7, PP 23-30 (Jan 2001)], the use of extrusion method for applying superconducting paste in order to make superconducting joints has not yet disclosed in the prior art methods,

(iv) Omission of the step of thermal spray deposition of another layer of silver on the perforated silver foil wrapping the joint portion in order to prevent probability of removal of the applied superconducting paste from the surface of the joined portion and from the end faces of the tubes to be joined due to pressure exerted on the joint portion.

To improve the superconducting properties of such a joint, the improved method included cold isostatic pressing of the starting homogeneous (Bi,Pb)-2223 powder obtained by grinding of the sintered tube (at a temperature range of 830°-850°C in air for a time period of 60-100 hours in air) of length in the range of 430 to 500 mm, outer diameter in the range of 50 to 150 mm and wall thickness in the range of 2 to 10 mm made from cold isostatic pressing of calcined spray dried powder from a group of bismuth based oxide superconductor (Bii. ₈ 4 Pbo. 35 Sr 1.91 Ca ₂ .os Cu 3.05 Ag _{1 -2} ) into tubes (at a pressure in the range of 300 to 400 MPa) having end faces to be joined;

The said (Bi, Pb)-2223 superconducting powder is used for making (i) tubes to be joined, (ii) paste and (iii) common bush. The tubes thus obtained from (Bi, Pb)-2223 powder hereafter are called as (Bi, Pb)-2223 tubes, the paste thus obtained from (Bi, Pb)-2223 powder hereafter is called as (Bi, Pb)-2223 paste and the common bush thus obtained from (Bi, Pb)- 2223 powder hereafter is called as (Bi, Pb)-2223 bush.

Cold isostatic pressing the said homogeneous (Bi,Pb)-2223 powder material in tubes at a pressure of 300 to 400 MPa. Polishing both end outer surface portions of the said final tubes; depositing metal silver layer on the said polished end portions; next taking two such tubes; wrapping one of the said end portion with a perforated silver sheet and further depositing a silver layer by a metal spray gun to form end electrical current contacts. Polishing (i) end faces/ inner surfaces of the other end portions of these two tubes having end electrical contacts, (ii) both end faces/inner surfaces of the tubes without end electrical current contacts and (iii) outer surface of the common bush in order to improve mechanical contact between the end faces to be joined as well as between tubes and the carrier bush and subsequently applying (Bi, Pb)-2223 paste made by mixing (Bi, Pb)-2223 powder with lignin powder and boiled organic linseed oil on all the said polished surfaces to improve physical contact area.

Clubbing the said tubes longitudinally end to end on a common bush so that the polished end faces are in contact with each other and the end portions with electrical current contacts are outside. It is these polished end faces where the joint is formed. Applying the said (Bi, Pb)- 2223 paste on the said polished end faces. These coated end faces are put in contact touching each other, reapplying the (Bi, Pb)-2223 paste on the outer silver layer coated surface of the physically joint portion to improve the physical joint connectivity. Subsequently wrapping the joined portion coated with the (Bi, Pb)-2223 paste and the joined region with a perforated silver sheet. Then finally co-sintering the overall assembly of these physically joint portions and the tubes at temperatures from 835 °C to 850 °C in air for 100 to 150 hours. The joint has made on tubes of length up to 320 mm, OD up to 12.4 mm and wall thickness up to 2 mm. The joint made has critical current (I _c ) at least 94% of the critical current of the individual tubes. Further, the joint is capable of carrying a continuous current of not less than 330A at 77 K in self-field.

This invention addresses an improved process that can be utilized to develop strongly joined tubes of size (L=100-320 mm, OD= 12.1-12.4 mm and wall thickness =1-2 mm) with high current-carrying capacity desirable for various applications such as superconducting magnets, fault current limiters, current leads, and the like. The joint can carry continuous transport current not less than 330 A which is equal to that of the individual component tube (330A) at 77K in self-field. This is an improved method of joining oxide superconductor components because the present invention provides a joint between bodies which are hollow and that can account for the improved superconductivity with nearly negligible degradation in critical current (I _c ).

The novelty of the improved process of the present invention for joining (Bi, Pb)-2223 oxide superconducting tubes with improved superconducting properties from replaced conducting silver bush by a superconducting bush, improved superconducting paste by using a not yet used natural novel binder (lignin) and extrusion method of applying superconducting paste lies in the improvement of superconducting joint, such as increase in percentage retention of transport critical current through the joint. Moreover, using a low cost binder lignin, replacing silver bush by a superconducting bush and omitting the step of deposition of silver on the wrapped perforated silver sheet makes this method more economical in comparison to the prior art method.

The novelty of the process of the present invention for joining (Bi, Pb)-2223 oxide superconducting tubes and improved joining method for the preparation thereof has been achieved by the non-obvious inventive steps of:

(i) replacing the conducting silver carrier bush by superconducting bush in order to improve superconducting path through the joint.

(ii) modification of the superconducting paste by using an outstanding natural binder: lignin which has not been used so far in the case of any superconductor (from conventional to unconventional).

(iii) using extrusion method for applying superconducting paste to make a joint with minimum pores/voids.

(iv) omitting the step of deposition of silver layer on the perforated silver sheet wrapping the physically joined portions in order to prevent the physically joined portion from the probability of being getting disturbed.

Thus the novel characteristics have been achieved by the non obvious inventive steps of (i) replacing the conducting silver bush by superconducting bush, (ii) modifying the superconducting paste formed from lignin binder alone rather than using a combination of organic binder, dispersant, solvent etc and by following by simple/minimum steps as detailed herein above.

This joining technique may be applied widely to superconducting appliances for example, long conductors, nuclear magnet resonance (NMR) systems, medicinal MRI systems, superconducting power reserving systems, magnetic separation units, single crystal drawing- out systems in magnetic fields, freezer cooling superconducting magnet systems and the like and also to the application of high field magnet systems, where long length current leads are required for instance to connect a power source or other conventional equipments like accelerators etc. to the superconducting components.

The process of making joint between tubes in accordance with the present invention is illustrated in Figure 1, wherein reference numeral 1 is a (Bi, Pb)-2223 tube; numeral 2 designates outer surface; numeral 3 is a silver layer; numeral 4 is polished end face; numeral 5 designates inner surface; numeral 6 is (Bi, Pb)-2223 carrier bush; numeral 7 is (Bi, Pb)- 2223 paste and numeral 8 is a perforated silver sheet. Fig. 1(a) illustrates the process of making joints more than one between tubes more than two.

The critical current (I _c ) and voltage of the component tube superconductor and of the joined tubes prepared by the process of the present invention described herein above were measured by four-terminal method as illustrated in Fig. 2 for joints A and B of the drawings accompanying this specification. Critical current (I _c ) was determined using 1 V/cm criterion.

For this twelve terminals were made on the superconductor portion designated with numeral (1). All these terminals were of silver. Two outer (designated as I) on end portions of the tubes were current terminals. Two inner terminals (designated as Vc) and ten middle (designated as Va, Vb) were voltage terminals. In four-probe method, for voltage taps, air- drying silver paste was used and connecting copper wires were soldered directly to the superconductor. Vc terminals are soldered close the current contact and measures voltage of the joined tube assembly. Va and Vb terminals measure voltages across the joint and component tube respectively. Accuracy of the measurements was about ±10%. The measurements were at a sample temperature of 77 K and in self- field (0T).

EXAMPLES

The following examples are given by way of illustration and therefore, should not be construed to limit the scope of the present invention.

EXAMPLE 1

A starting powder of (Bi, Pb)-2223 was obtained by grinding a sintered (at a temperature of 835°C in air for 80 hours) tube (length= 430 mm, outer diameter = 50 mm and inner diameter = 47 mm) made from cold isostatic pressing (at a pressure of 300 MPa) of calcined spray dried powder of Bii. ₈ 4 Pbo.35 Sr 1.91 Ca 2.05 Cu 3.05 Ag _1-2 system.

Next, a part of this staring powder was cold-isostatically pressed at 400 MPa into tubes of length (L) = 122 mm, outer diameter (OD) = 12.4 mm and inner diameter (ID) = 10.1 mm. Thereafter, these (Bi, Pb)-2223 tubes having end faces obtained in this manner were used for making joints between them.

Another part of this powder was used for making (Bi, Pb)-2223 superconducting paste.

15 mm long outer surface (2) was polished at both end portions of these superconducting tubes (1) with a subsequent thermal spray deposition of metal silver layer (3). To form current contacts (I), a perforated silver sheet was wrapped round the silver layer of one of the ends of the tube and was subsequently deposited with another silver layer. An end face (4) and 15mm long inner surface (5) of these superconducting tubes (1) were polished to expose fresh surfaces.

A pair of such tubes was so clubbed together by inserting silver bush (6) of size: L = 15 mm, O.D.= 10.0 mm and I.D.= 6.0 mm inside the end portions that the polished end faces of the superconductors (1) as shown in Fig. l were in close contact with a gap and the current contacts (I) are outside. The current contacts are shown in Fig.2.

A modified (Bi, Pb)-2223 paste (7) consisting of 5 gm of the (Bi, Pb)-2223 powder in 0.003gm of lignin powder and 0.06 ml ( equivalent to 0.9 gm) of boiled organic linseed oil was applied immediately on the said polished end faces (4) as well as on end inner surfaces (5) by using a painting brush. The said end faces coated with this modified paste were then brought into physical contact so that their coated end faces touch each other.

Next, the modified paste was also applied on the silver layer (3) and on the physically joined portion as shown in Fig. 1 and then was subsequently wrapped with a perforated silver sheet (8) followed by metal silver spray deposited layer (not shown in Fig. l).

Finally, the assembly of the physically joined a pair of tubes and the joint portion having length of 244 mm was sintered in an electric muffle furnace at 835°C for 100 firs in air to obtain joined tube pair. The joint portion was checked for superconductivity and found that the joint of this tube pair exhibited a transport current of 311 A, which was lower than that of the component/individual component tube itself (330 A) at 77 K, 0T and the percentage critical current (I _c ) retention of the joined tube pair was 94 %. This lowering of transport current may probably be due to the presence of pores/voids at the joint portion as a result applying superconducting paste by painting brush.

However, the joined pair obtained according to this example using modified superconducting paste has transmission performance, which is enhanced by about 3% over that of the prior art method.

The obtained results are shown in Table- 1. EXAMPLE 2

The (Bi, Pb)-2223 tubes (1) of length (L) = 122 mm, outer diameter (OD) = 12.4 mm and inner diameter (ID) = 10.1 mm as shown in Fig. 1 to be joined together were prepared in the same manner as explained in Example -1. The modified (Bi, Pb)-2223 superconducting paste was prepared in the same manner as explained in Example- 1.

To minimize the formation of voids, pores etc. while applying the modified superconducting paste with the use of a painting brush as used in Example- 1, another way was applied. That is, the modified superconducting paste was applied by using a well known extrusion method. 15 mm long outer surface (2) was polished at both end portions of these superconducting tubes (1) with a subsequent thermal spray deposition of a metal silver layer (3). To form current contacts (I), a perforated silver sheet was wrapped round the silver layer of one of the ends of the tube and was subsequently deposited with another silver layer. An end face (4) and 15mm long inner surface (5) of these superconducting tubes (1) were polished to expose fresh surfaces.

A pair of such tubes was so clubbed together by inserting silver bush (6) of size:L = 15 mm, O.D.= 10.0 mm and I.D.= 6.0 mm inside the end portions that the polished end faces of the superconductors (1) were in close contact with a gap as shown in Fig. l and the current contacts (I) are outside. The current contacts are shown in Fig.2.

A modified (Bi, Pb)-2223 paste (7) consisting of 5 gm of the (Bi, Pb)-2223 powder in 0.003 gm of lignin powder and 0.06ml ( equivalent to 0.90 gm) of boiled organic linseed oil was immediately applied on the said polished end faces (4) as well as on end inner surfaces (5) by extrusion method. The said end faces coated with this modified paste were then brought into physical contact so that their coated end faces touch each other.

Next, the modified paste was also applied on the silver layer (3) and on the physically joined portion and then was subsequently wrapped with a perforated silver sheet (8) followed by metal silver spray deposited layer.

Finally, the assembly of the physically joined a pair of superconducting tubes and the joint portion having length of 244 mm was sintered in an electric muffle furnace at 835°C for 100 hrs in air to obtain joined tube pair. The joint portion between two hollow tubes was checked for superconductivity and found that the joint of this tube pair exhibited a transport current of 317 A, which was lower than that of the individual component itself (330 A) at 77 K, 0T and the percentage critical current retention of the joined tube pair was 96%.

The joined pair obtained according to this example has transmission performance, which is enhanced by about 2% over that of the former example-l .This enhancement in the percentage critical current retention suggests that the extrusion method used for applying superconducting paste has reduced the pores/voids etc. at the joint.

The obtained results are shown in Table- 1. EXAMPLE 3

The (Bi, Pb)-2223 tubes (1) as shown in Fig. l to be joined together were prepared in the same manner as explained in Example -1.

The modified (Bi, Pb)-2223 paste was prepared in the same manner as explained in Example- 1.

The modified (Bi, Pb)-2223 paste was applied by extrusion method in same manner as explained in Example-2.

To prevent the joint from being getting disturbed, the step of metal silver spray deposition at the physically joint portion wrapped with perforated silver sheet was omitted.

15 mm long outer surface (2) was polished at both end portions of these superconducting tubes (1) with a subsequent deposition of a silver layer (3). To form current contacts (I), a perforated silver sheet was wrapped round the silver layer of one of the ends of the tube and was subsequently deposited with another silver layer. An end face (4) and 15mm long inner surface (5) of these superconducting tubes (1) were polished to expose fresh surfaces.

A pair of such tubes was so clubbed together by inserting silver bush (6) of size: L = 15 mm, O.D.= 10.0 mm and I.D.= 6.0 mm inside the end portions that the polished end faces of the superconductors (1) were in close contact with a gap as shown in Fig. l and the current contacts (I) are outside. The current contacts are shown in Fig.2.

A modified (Bi, Pb)-2223 paste (7) consisting of 5 gm of the (Bi, Pb)-2223 powder in 0.003 gm of lignin powder and 0.06 ml ( equivalent to 0.90 gm) of boiled organic linseed oil was immediately applied on the said polished end faces (4) as well as on end inner surfaces (5) by extrusion method. The said end faces coated with this modified paste were then brought into physical contact so that their coated end faces touch each other.

Next, the modified paste was also applied on the silver layer (3) and on the physically joined portion and then was subsequently wrapped with a perforated silver sheet (8) as shown in Fig. l

Finally, the assembly of the physically joined a pair of superconducting tubes and the joint portion having length of 244 mm was sintered in an electric muffle furnace at 835°C for 100 hrs in air to obtain joined tube pair. The joint portion between two hollow tubes was checked for superconductivity and found that the joint of this tube pair exhibited a transport current of 324 A, which was slightly lower than that of the individual component itself (330 A) at 77 K, 0T and the percentage critical current retention of the joined tube pair was 98%.

The joined pair obtained according to this example has transmission performance, which is enhanced by about 2% over that of the former example-2. This enhancement in the percentage critical current retention suggests that omitting the step of deposition of silver layer at the physically joint portion wrapped with perforated silver sheet probably prevented the joint from getting disturbed.

The obtained results are shown in Table- 1

EXAMPLE 4

The (Bi, Pb)-2223 tubes (1) as shown in Fig. l to be joined together were prepared in the same manner as explained in Example -1.

The modified (Bi, Pb)-2223 paste was prepared in the same manner as explained in Example- 1.

The modified (Bi, Pb)-2223 paste was applied by the extrusion method in same manner as explained in Example-2.

The step of metal silver spray deposited layer on the physically joined portion wrapped with a perforated sheet was omitted in the same manner as in Example-3.

To further improve superconducting path through the joint another way was applied. That is, a superconducting bush of (Bi, Pb)-2223 (6) rather than the silver bush was used. (Bi, Pb)- 2223 bush was made so that even if the joint if the joint is weak, the current can flow through (Bi, Pb)-2223 bush. This bush was made by cold isostatic pressing of the (Bi, Pb)-2223 powder at a pressure of 400MPa. Then the surface of this bush was polished and subsequently coated with a layer of the modified (Bi, Pb)-2223 paste (7) consisting of 5 gm of the powder of the same (Bi, Pb)-2223 powder in 0.003 gm of lignin powder and 0.06 ml ( equivalent to 0.90 gm) of boiled organic linseed oil.

15 mm long outer surface (2) was polished at both end portions of these tubes (1) with a subsequent deposition of a silver layer (3). To form current contacts (I) a perforated silver sheet was wrapped round the silver layer of one of the ends of the tube and was subsequently deposited with another silver layer. An end face (4) and 15 mm long inner surface (5) of these superconducting tubes (1) were polished to expose fresh surfaces.

A pair of such tubes was so clubbed together by inserting (Bi, Pb)-2223 bush (6) of size: L = 15 mm, O.D.= 10.0 mm and I.D.= 6.0 mm inside the end portions that the polished end faces of the superconductors (1) were in close contact with a gap as shown in Fig. l and the current contacts (I) are outside. The current contacts are shown in Fig.2.

A modified (Bi, Pb)-2223 paste (7) consisting of 5 gm of the (Bi, Pb)-2223 powder in 0.003 gm of lignin powder and 0.06ml ( equivalent to 0.90 gm) of boiled organic linseed oil was immediately applied on the said polished end faces (4) as well as on end inner surfaces (5) by extrusion method. The said end faces coated with this modified paste were then brought into physical contact so that their coated end faces touch each other.

Next, the modified paste was also applied on the silver layer (3) and on the physically joined portion and then was subsequently wrapped with a perforated silver sheet (8) as shown in Fig. l

Finally, the assembly of the physically joined a pair of superconducting tubes and the joint portion having length of 244 mm was sintered in an electric muffle furnace at 835°C for 100 hours in air to obtain joined tube pair. The joint portion between two hollow tubes was checked for superconductivity and found that the joint of this tube pair exhibited a transport current of 330 A, which was the same as that of the individual component itself (330 A) at 77 K, 0T and the percentage critical current retention of the joined tube pair was 100%.

The joined pair obtained according to this example has transmission performance, which is enhanced by about 2% over that of the former example-3. This enhancement in the percentage critical current retention suggests that using superconducting bush in place of metallic silver bush has improved the superconducting path.

The obtained results are shown in Table- 1.

EXAMPLE 5

In order to make joint between longer tubes, the (Bi, Pb)-2223 tubes (1) having length (L) = 320 mm (than in all the above mentioned examples), outer diameter (OD) = 12.4 mm and inner diameter (ID) = 10.1 mm as shown in Fig. l to be joined together were prepared in the same manner as explained in example -1.

The modified (Bi, Pb)-2223 paste was prepared in the same manner as explained in example- 1.

The modified (Bi, Pb)-2223 paste was applied by the extrusion method in same manner as Explained in Example-2.

Step of metal silver spray deposited layer on the perforated silver sheet was omitted in the same manner as explained in Example-3.

The (Bi, Pb)-2223 bush was prepared and was coated with a layer of the (Bi,Pb)-2223 superconducting paste in the same manner as explained in Example-4.

20 mm long outer surface (2) was polished at both end portions of these superconducting tubes (1) with a subsequent deposition of a silver layer (3). To form current contacts (I), a perforated silver sheet was wrapped round the silver layer of one of the ends of the tube and was subsequently deposited with another silver layer. An end face (4) and 20 mm long inner surface (5) of these superconducting tubes (1) were polished to expose fresh surfaces.

A pair of such tubes was so clubbed together by inserting (Bi, Pb)-2223 bush (6) of size: L = 30 mm, O.D.= 10.0 mm and I.D.= 6.0 mm inside the end portions that the polished end faces of the superconductors (1) were in close contact with a gap as shown in Fig. l and the current contacts (I) are outside. The current contacts are shown in Fig.2.

A modified (Bi, Pb)-2223 paste (7) consisting of 5 gm of the (Bi, Pb)-2223 powder in 0.003 gm of lignin powder and 0.06ml ( equivalent to 0.90 gm) of boiled organic linseed oil was immediately applied on the said polished end faces (4) as well as on end inner surfaces (5) by extrusion method. The said end faces coated with this modified paste were then brought into physical contact so that their coated end faces touch each other.

Next, the modified paste was also applied on the silver layer (3) and on the physically joined portion and then was subsequently wrapped with a perforated silver sheet (8) as shown in Fig. l .

Finally, the assembly of the physically joined a pair of superconducting tubes and the joint portion having length of 640 mm was sintered in an electric muffle furnace at 835°C for 100 hours in air to obtain joined tube pair. The joint portion between two hollow tubes was checked for superconductivity and found that the joint of this tube pair exhibited a transport current of 330 A, which was the same as that of the individual component itself (330 A) at 77 K, 0T and the percentage critical current retention of the joined tube pair was 100%.

The joined pair obtained according to this example has transmission performance, which is the same as that of the former example-4.

The obtained results are shown in Table- 1. EXAMPLE 6

In order to join more than two tubes, the (Bi, Pb)-2223 tubes (1) having length (L) = 320 mm, outer diameter (OD) = 12.4 mm and inner diameter (ID) = 10.1 mm as shown in Fig. l to be joined together were prepared in the same manner as explained in example -1.

The modified (Bi, Pb)-2223 paste (7) was prepared in the same manner as explained in Example- 1.

The modified (Bi, Pb)-2223 paste was applied by the extrusion method in same manner as explained in Example-2.

Step of metal silver spray deposited layer was omitted in the same manner as explained in Example-2. The (Bi, Pb)-2223 bush (6) was prepared and was coated with a layer of the (Bi,Pb)-2223 superconducting paste in the same manner as explained in Example-4.

Three (Bi, Pb)-2223 tubes were taken. First polished 20mm long (2) both end outer surface portions of such tubes with a subsequent deposition of a silver layer (3). Next to form current contacts (I) as shown in Fig.2, two such tubes were taken and perforated silver sheet was wrapped round the silver (3) of one of the ends of these tubes with a subsequent deposition of another silver layer. The end faces (4) and 20 mm long inner surfaces (5) of the other end portion of these two tubes with current contacts and both end faces of the third tube without current contacts were polished to expose a fresh surface.

Clubbing the said three tubes longitudinally together with tube without current contents in the middle and with tubes having current contents on either side by inserting (Bi, Pb)-2223 bush (6) of size: L = 30 mm, O.D.= 10.0 mm and I.D.= 6.0 mm inside both the end portions of the middle tube in such a way that the polished end faces (4) of the superconductors (1) were in close contact with a gap as shown in Fig. la and the current contacts (I) are outside. The current contacts are shown in Fig.2.

A modified (Bi, Pb)-2223 paste (7) consisting of 5 gm of the (Bi, Pb)-2223 powder in 0.003 gm of lignin powder and 0.06 ml (equivalent to 0.90 gm) of boiled organic linseed oil was immediately applied at the said polished end faces (4), which were then brought into physical contact so that their coated end faces touch each other.

Next, the modified paste was also applied on the silver layer (3) and on the physically joined portions designated as A and B and then was subsequently wrapped with a perforated silver sheet (8) as shown in Fig. la.

Finally, the resulting joint tubes and both the joint portions A and B having length of 960 mm were sintered in an electric muffle furnace at 835°C for 100 hours in air to obtain the joint tube assembly. In order to check whether the joints between the three hollow tubes were now superconducting, both the joints: A and B of this joined tube triplet were tested for superconductivity and found that both the joints A as well as joint B of this tube triplet exhibited a transport current of 330 A, which was the same as that of the individual component itself (330 A) at 77 K, 0T and the percentage critical current retention of the joined tube triplet was 100%.

The obtained results are shown in Table- 1.

In Table- 1 below is given the collated data from the above noted examples. The comparative data shows the critical current of the component tube, critical current of the joined tubes and percentage of retention at 77 K in self-field of various (Bi, Pb)-2223 tubes and also clearly highlights the resultant novelty of superconducting joint between second sintered hollow tubes due to non-obvious inventive steps of using a (Bi, Pb)-2223 superconducting bush in place of silver bush to bring the ends of the tubes in close contact and in making the joint with a modified (Bi, Pb)-2223 paste using a natural not yet used binder lignin.

Table-1: Results obtained and experiment conditions from example 1 to 6

ADVANTAGES OF THE INVENTION

i. It leads to the formation of a superconducting joint which can carry critical current same as that of the individual tube, i.e. the improving the percentage critical current retention from 71-91% to 94-100% of the joined tube.

ii. Use of superconducting carrier bush instead of metallic silver bush improves superconducting path through the joint, reduces total cost due to reduction in the (i) thermal conductivity by several orders of magnitude [i.e. from 5110 W/m of silver to 0.8 W/mK of (Bi,Pb)-2223] and (ii) material cost (reduced nearly to half) makes this process much more economical with improved superconducting joint.

iii. Use of an outstanding not yet used binder lignin powder in combination with boiled organic linseed oil in making modified superconducting paste that serves to penetrate deep inside the pores/voids bind the superconducting particles homogeneously and densely facilitates the diffusion bonding.

iv. Use of lignin as a binder which is much cheaper than that of the other binders also makes this process more economical.

v. Apart from end faces, applying the modified superconducting paste at the end faces (i) end inner surfaces of the tubes outer surface of the bush and end outer surfaces of the tubes fills the in between gaps and enhances the interface diffusion bonding area makes the entire joint stronger and superconducting.

vi. Use of extrusion method for applying superconducting paste makes the physical joint portion with minimum pores/voids etc.

vii. It does not require the additional step of deposition of silver layer on the perforated silver sheet wrapping the physically joined portions, therefore making the method more economical (i.e. by reducing the number of steps and saving the cost of silver).

viii. Improved quality of the superconducting joint between hollow oxide tube superconductors by using (i) replacing silver bush with a superconducting bush (ii) modified joining superconducting paste with the use of lignin which alone has the versatile property of glue/dispersant/solvent, and (iii) omitting of thermal deposition of silver layer on the perforated silver sheet wrapping the joint portions, can be obtained.