Title of Invention	"AN IMPROVED PROCESS FOR PREPARATION OF MULTILAYERED SILVER SHEATHED BISMUTH BASED SUPERCONDUCTING TAPES WITH HIGH CRITICAL CURRENT DENSITIES"
Abstract	An improved process for the preparation of multilayered silver sheathed bismuth based superconducting tapes with high critical current densities. In the present invention multilayered tapes are fabricated from individual monolayer tapes having a high degree of grain orientation and densification. Such monolayer tapes with thickness 0.09-0.2 mm are stacked and folded by a thin silver sheet of thickness in the range 0.05-0.1 mm and are further rolled and heat treated to obtain critical current densities comparable to those of the monolayer tapes. Further, the method allows use of a higher superconductor to silver ratio.

Title of Invention

"AN IMPROVED PROCESS FOR PREPARATION OF MULTILAYERED SILVER SHEATHED BISMUTH BASED SUPERCONDUCTING TAPES WITH HIGH CRITICAL CURRENT DENSITIES"

Abstract

An improved process for the preparation of multilayered silver sheathed bismuth based superconducting tapes with high critical current densities. In the present invention multilayered tapes are fabricated from individual monolayer tapes having a high degree of grain orientation and densification. Such monolayer tapes with thickness 0.09-0.2 mm are stacked and folded by a thin silver sheet of thickness in the range 0.05-0.1 mm and are further rolled and heat treated to obtain critical current densities comparable to those of the monolayer tapes. Further, the method allows use of a higher superconductor to silver ratio.

Full Text	This invention relates to an improved process for the preparation of multilayered silver sheathed bismuth based superconducting tapes with high critical current densities. The multilayered silver sheathed bismuth based superconducting tapes with high critical current densities prepared by the process of the present invention will find application in power transmission, high field magnets for MRI, scientific equipments, particle accelerators, mineral separators and electric motors. It has been widely accepted that bismuth based high temperature superconductors are suitable for processing superconducting tapes with high critical current densities at 77 K by powder-in-tube (PIT) technique. Reference may be made to our copending Patent Application No. 2370 / DEL / 95 and A.N. Iyer et al. , ; Proceedings of the Symposium on Processing of long lengths of superconductors held in Pittsburg, Pennsylvania, USA, p.13 (1994); T. Hikata et al. presented at the International Workshop on superconductivity, Kyoto, Japan (1994); G. Grasso et al. physica C 241, 45 (1995)]. Prototype coils and magnets have been developed by a few research groups using monofilamentary tapes with generated fields of several tesia [ p. Haldar et al. Proceedings of the Symposium on processing of long length of Superconductors held at Pittsburg, Pennsylvania, USA, p-23 (1994) ] . World wide efforts are now being concentrated on development of multifilamentary tapes/wires by different methods with high critical current densities. The following methods are currently used far fabrication of muitifi1aimentary tapes. i. Wire clrawing - Rebundling - Wire drawing method [S. X. Dou et al. Proceedings of the Symposium on processing of long lengths of superconductors held at Pittsburg, Pennsylvania, USA, p.31 (1994); K. Kwasnitz et al. Physxca C, 233, 423 (1994)] In this method the superconducting powder is initially packed in a high purity silver tube of appropriate dimensions. After sealing the ends the tube is groave--rDl 1 ed/drawn into wires. The wires are cut into D ieces and are rebundled in anoth¬er silver tube or in a silver billet in which holes are drilled in a desired pattern. This tuba 'billet bundled wi th wires having the superconductor core inside is further groove-rol1ed/drawn into wi res» These wires are subsequently flat~rolled and heat treated to obtain multifilamentary tapes. 2- Jelly-Rol1 Process [C.H. Kato et al. Superconductor Science and Technology 7, 470 (1994); K.liatsuzaki et al . Japanese Journal of Applied Phys¬ics, 33, L308 (1994)]. Here a layer of superconducting material is deposited on a silver sheet. The deposited sheet is rolled to form a 'jelly-roll' configuration. The jelly-rolled si silver/superconductor composite is subsequently flat—rolled and heat treated to yield muitilayered silver sheathed superconducting tapes, 3. Accordion Folding Method EL. Martini et al. Superconductor Science and Technology, 7, 24(1994)] In this method a silver sheet is folded in the form af an accordion with uniform spacing. The spaces between the silver layers are then filled with the superconductor powder- The silver/superconductor layered structure is then uniaxially pressed to form a rectanoular billet. It is then longitudinally cut width-wise so "as to get strips of the layered composite. These strips are subsequently rolled and heat treated to obtain multilayered silver sheathed superconducting tapes. The main drawback of ail the hitherto known methods for making multifilamentary tapes is the limitation of obtaining high critical current density (Jc). It has been found that the Jc values of the multifi1amentary- t-zpes prepared by the above meth¬ods are considerably lower compared to monofi1 amentary tapes prepared by powder in tube (PIT) method. For example, Jc values greater than 60,000 A/cm2 at 77 K. and self field has been report¬ed for monofilamentary silver sheathed bismuth based supercon-ducting tapes whereas the best result of Jc for multifi1 amentary tapes prepared by the well accepted existing technique, i.e. by wire drawing - rebundling - wir3 drawing method is IS,500 A/cm2 CS.X. Dou et al. Proceedings of the Symposium on Processing of long lengths of superconductors held at Pittsburg, Pennsylvania, USA, p.31 (1994); Y. Yamada et al. Proceedings of the 5th Inter¬national Symposium on Superconductivity, p.717 (1993); fi. Ueyama et al. Japanese Journal of Applied Physics, 30, L1384 (1994)]. This is because the degree of grain alignment and densification of the individual superconducting layers in a multilayered tape are significantly lower than those achieved in a monofi1amentary tape . In the fabrication of monafilamentary tapes each tape is separately rolled down to an overall thickness of around 0.09 mm in different stages tuith intemediate heat treatment. The proc¬ess imparts a very high degree of grain aligment to the bismuth based superconductor core which consists of platy grains. On the other hand, in the hitherto known processes for multilayered tapes, the rol1xng operation is done combinedly for all the 1 ayers right from the beginning. Here;, the load that can be applied during rolling is limid by the deformation characteris¬tics of the soft silver matri layers provided between the alter¬nate layers of the superconducto" core. Therefore, the degree of grain alignment and the densificttion of the individual supercon-ducting layers of the multilayered tapers are considerably lower than that of the monolayer tape prepared by PIT method. The main objec t of the present invention is to provide a process for the preparation of multilayered silver sheathed bismuth based superconducting tapes with high critical current densities. It is also the object of the present invention to reduce the silver to superconductor ratio in the fabricated tapes. The critical current density of mono or multifi1amentary tapes ca.n be improved significantly by increasing the degree of grain alignment along with the rolling direction of the tapes (in the a-b plane) and by increasing the densification of the super¬conducting layer. In all the hitherto known processes for fabri¬cation of multilayered tapes hardly any afetempt has been made to impart increased grain orientation or densification of the individual layers of the superconductor before these are rolled together. As a result, the grain orientation and densification of the individual superconductor layers fabricated by the known processes are considerably lower compared to those of a monolayer tape prepared by repeated rolling and heat treatment. In the present invention multilayered tapes are fabricated from individual monolayer tapes having a high degree of grain orientation and densification. Such monolayer tapes with thickness 0.09-0.2 mm are stacked and folded by a thin silver sheet of thickness in the range 0.05-0.1 mm and are further rolled and heat treated to obtain critical current densities comparable to those of the monolayer tapes. Further, the method allows use of a higher superconductor to silver ratio. Consequently the process of the present invention not only provides multilayered tapes with increased "engineering Jc" but also saves a good amount of high purity silver. Accordingly the present invention provides An improved process for the preparation of multilayered silver sheathed bismuth based superconducting tapes with high critical current densities which comprises (a) packing a highly reactive ( Bi.Pb) - Sr-Ca-Cu-O precursor powder free from carbon in high purity seamless silver tubes, (b) groove-rolling and annealing the silver tubes to form silver sheathed wires with thickness in the range of 0.8-2mm, (c) flat-rolling and annealing the wires sheathed (Bi,Pb)-Sr-Ca-Cu-O tapes with thickness in the range of 0.3-0.5 mnn, (d) re-flat-rolling and heat treating the tapes at a temperature in the range 810-840°C in an oxidizing atmosphere for a period in the 100 - 150 h to obtain silver sheathed monolayer superconducting tapes with a thickness in the range 0.08-0.2 mm and having a high degree of grain orientation and densification, (e) stacking five to twenty numbers of the said monolayer tapes and folding them together by using high purity silver sheets having a thickness in the range 0.05 to 0.1 mm. (f) annealing and flat-rolling the folded structure to form multilayered tapes to a thickness in the range 0.25 to 1.5 mm, (g) heat treating the multilayered tapes at a temperature in the range of 810-840°C in an oxidizing atmosphere for a period in the range 50-150 h to obtain desired multilayered silver sheathed bismuth based superconducting tapes with high critical current densities. According to a feature of the process of other present invention the cation stoichiometry of the precursor powder used is Bi : Pb : Sr: Ca : Cu = 1.5 - 1.9 : 0.3 - 0.5 : 1.8 - 2.3 : 2 - 2.5 : 2.5 - 3.8. According to another feature of the present invention the purity of the silver tubes and sheets used is 99.99%-According to yet another feature of the present invention the oxtydising atmosphere used may be such as air, low/reduced oxygen partial pressure. The details of the process of the present invention is described in the examples provided below which are given by way of i1lus¬tration only and should not be ronstrued to limit the scope of the present invention. Example-1 Monolayer silver sheathed bismuth based superconductino tapes with a thickness in the range 0.l2 to 0.l5 mm was initially prepared by PIT method by using a highly reactive precursor powder of bismuth based suoerconductor with a cation ratio Bi : Fb : Sr : Ca : Cu = 1.8 : 0.4 : 2 : 2.2 : 3 [The method has been described in our Patent No.2370/DEL/95 dated 21.12.19953 Monolayer tapes heat treated partially at a temperature in the range 830-83S°C in air for a period of 150 h were cut into pieces of 5 cm on length. Five numbers of these pieces were stacked one over the other and folded together using high purity silver sheet of thickness 0.09 mm. The folded composite structure wi th a thickness of around 0.9 mm was intermittently annealed (5000C, 30 min) and flat-rolled in three steps to a thickness of about 0.45 mm. This was subsequently heat treated in air at a temperature in the range 830-838°C for a period of 100 h to obtain multilay¬ered silver sheathed bismuth based superconducting tapes with critical current density of 15,300 A/cm2 at 77 K and self- field. Example 2 Monolayer tapes with a thickness in the range 0.14-0.17 mm and heat treated partially at a temperature in the ranpe 816- B22°C for a period of 150 h in air atmosphere havino 10*/. oxygen and the balance nitrogen were cut into pieces of 4 cm in length. Five numbers of these pieces yiere stacked and folded together using a high purity silver sheet of thickness 0.1 mm- The folded structure with a thickness of around 1 mm was intermittently anneal ed (500°C, 30 min) and fl, at-rolled in three steps to a thickness of 0.55 mm. This was subsequently heat treated at a temperature in the range 816-822°C in an atmosphere having 10% oxygen and the balance nitrogen for a period of 100 h to yield maltilayered silver sheathed bismuth based superconducting tapes with a critical current dencaity of 17,300 A/cm2 at 77 K and self field . Example 3 Monolayer tapes with a thickness in the range 0.12-0.15 mm and partially heat treated at a temperature in the range 816- o 822 C in an atmosphere having 10% oxygen and the balance nitrogen for a period of 10 h were cut into pieces of 5 cm in length. Five numbers of these pieces were stacked and folded together using high purity silver sheet of thickness 0.09 mm. The folded structure with a thickness of around 0.9 mm was intermittently annealed (500°C, 30 min) and flat-rolled in three steps to a thickness of -0-. 45 .mm. This was subseaueatly heat traated at a temperature in the range S16-822°C in an atmosphere having 10% oxygen and the balance nitrogen for a period of 150 h to yield multilayered silver sheathed bismuth based superconducting tapes with a critical current density of 16,000 A/cm2 at 77 K and self field. Example 4 Monolayer tapes wi th a thickness In the range 0.14-0.17 mm and partially heat treated at a temperature in the range 830- o B38 C in air for a period of 100 h were cut into pieces of 4 cm length. Five numbers of these pieces Were stacked and folded together using high puri ty silver sheet of thickness 0.1 mm. The folded structure wi th a thickness of around 1 mm was intermit- tently anneal ed (500°C , 30 min) and f lat-rol led in three steps to a thickness of 0.55 mm. This was subsequently heat treated at a temperature In the range 830-838°C in air for a period of 150 h to yield multilayered silver sheathed bismuth based superconduct¬ing tapes with a critical current density of 17,000 A/cm2 at 77 K and self field. The main advantages of the method of the present inven¬tion are: (1) The method enables production of multilayered tapes with high critical current densities comparable to monolayer tapes as it allows use of monolayer tapes having a high degree of grain orientation and densification as the starting material. (2) The method of production can be made continuous by using long length monolayer tapes and obtain multilayered tapes in long length far direct application. (3) The method allaws uae of a higher superconductor to silver ratio compared to the known processes. This not only increases the "engineering Jc" of the overall tape but also saves a good amount of high purity silver. We Claim: 1. An improved process for the preparation of multilayered silver sheathed bismuth based superconducting tapes with high critical current densities which comprises (a) packing a highly reactive ( Bi.Pb) - Sr-Ca-Cu-O precursor powder free from carbon in high purity seamless silver tubes, (b) groove-rolling and annealing the silver tubes to form silver sheathed wires with thickness in the range of 0-8-2mm, (c) flat-rolling and annealing the wires sheathed (Bi,Pb)-Sr-Ca-Cu-0 tapes with thickness in the range of 0.3-0.5 mm, (d) re-flat-rolling and heat treating the tapes at a temperature in the range 810-840°C in an oxidizing atmosphere for a period in the 100 - 150 h to obtain silver sheathed monolayer superconducting tapes with a thickness in the range 0.08-0.2 mm and having a high degree of grain orientation and densification, (e) stacking five to twenty numbers of the said monolayer tapes and folding them together by using high purity silver sheets having a thickness in the range 0.05 to 0.1 mm. (f) annealing and flat-rolling the folded structure to form muitilayered tapes to a thickness in the range 0,25 to 1,5 mm, (g) heat treating the muitilayered tapes at a temperature in the range of 810-840°C in an oxidizing atmosphere for a period in the range 50-150 h to obtain desired multilayered silver sheathed bismuth based superconducting tapes with high critical current densities. 2. An improved process as claimed in claim 1 wherein the starting powder used is having cation stolchiometry ranging Bi; Pb : Sr: Ca : Cu = 1.5 -1.9 :0.3 - 0.5 : 1.8 - 2.3 : 2 - 2.5 : 2.5 ~ 3.8. 3. An improved process as claimed in claims 1 and 2 wherein the purity of the silver tubes and sheets used is 99.99%. 4. An improved process as claimed in claims 1 - 3 wherein the oxidizing atmosphere used is such as air, low / reduced oxygen partial pressure. 5. An improved process for preparation of multilayered silver sheathed bismuth based superconducting tapes with high critical current densities substantially as herein described with reference to the examples.

Full Text

This invention relates to an improved process for the preparation of multilayered silver sheathed bismuth based superconducting tapes with high critical current densities.
The multilayered silver sheathed bismuth based superconducting tapes with high critical current densities prepared by the process of the present invention will find application in power transmission, high field magnets for MRI, scientific equipments, particle accelerators, mineral separators and electric motors.
It has been widely accepted that bismuth based high temperature superconductors are suitable for processing superconducting tapes with high critical current densities at 77 K by powder-in-tube (PIT) technique. Reference may be made to our copending Patent Application No. 2370 / DEL / 95 and A.N. Iyer et al. , ; Proceedings of the Symposium on Processing of long lengths of superconductors held in Pittsburg, Pennsylvania, USA, p.13 (1994); T. Hikata et al. presented at the International Workshop on superconductivity, Kyoto, Japan (1994); G. Grasso et al. physica C 241, 45 (1995)]. Prototype coils and magnets have been developed by a few research groups using monofilamentary tapes with generated fields of several tesia [ p. Haldar et al. Proceedings of the Symposium on processing of long length of Superconductors held at Pittsburg, Pennsylvania, USA, p-23 (1994) ] . World wide efforts are now being concentrated on development of multifilamentary tapes/wires by different methods with high critical current densities. The following methods are currently
used far fabrication of muitifi1aimentary tapes.
i. Wire clrawing - Rebundling - Wire drawing method [S. X. Dou et al. Proceedings of the Symposium on processing of long lengths of superconductors held at Pittsburg, Pennsylvania, USA, p.31 (1994); K. Kwasnitz et al. Physxca C, 233, 423 (1994)]
In this method the superconducting powder is initially packed in a high purity silver tube of appropriate dimensions. After sealing the ends the tube is groave--rDl 1 ed/drawn into wires. The wires are cut into D ieces and are rebundled in anoth¬er silver tube or in a silver billet in which holes are drilled in a desired pattern. This tuba 'billet bundled wi th wires having the superconductor core inside is further groove-rol1ed/drawn into wi res» These wires are subsequently flat~rolled and heat treated to obtain multifilamentary tapes.
2- Jelly-Rol1 Process
[C.H. Kato et al. Superconductor Science and Technology 7,
470 (1994); K.liatsuzaki et al . Japanese Journal of Applied Phys¬ics, 33, L308 (1994)].
Here a layer of superconducting material is deposited on a silver sheet. The deposited sheet is rolled to form a 'jelly-roll' configuration. The jelly-rolled si silver/superconductor composite is subsequently flat—rolled and heat treated to yield muitilayered silver sheathed superconducting tapes,
3. Accordion Folding Method
EL. Martini et al. Superconductor Science and Technology, 7,
24(1994)]
In this method a silver sheet is folded in the form af an accordion with uniform spacing. The spaces between the silver layers are then filled with the superconductor powder- The silver/superconductor layered structure is then uniaxially pressed to form a rectanoular billet. It is then longitudinally cut width-wise so "as to get strips of the layered composite. These strips are subsequently rolled and heat treated to obtain multilayered silver sheathed superconducting tapes.
The main drawback of ail the hitherto known methods for making multifilamentary tapes is the limitation of obtaining high critical current density (Jc). It has been found that the Jc values of the multifi1amentary- t-zpes prepared by the above meth¬ods are considerably lower compared to monofi1 amentary tapes prepared by powder in tube (PIT) method. For example, Jc values greater than 60,000 A/cm2 at 77 K. and self field has been report¬ed for monofilamentary silver sheathed bismuth based supercon-ducting tapes whereas the best result of Jc for multifi1 amentary tapes prepared by the well accepted existing technique, i.e. by wire drawing - rebundling - wir3 drawing method is IS,500 A/cm2 CS.X. Dou et al. Proceedings of the Symposium on Processing of long lengths of superconductors held at Pittsburg, Pennsylvania, USA, p.31 (1994); Y. Yamada et al. Proceedings of the 5th Inter¬national Symposium on Superconductivity, p.717 (1993); fi. Ueyama et al. Japanese Journal of Applied Physics, 30, L1384 (1994)]. This is because the degree of grain alignment and densification of the individual superconducting layers in a multilayered tape are significantly lower than those achieved in a monofi1amentary
tape . In the fabrication of monafilamentary tapes each tape is separately rolled down to an overall thickness of around 0.09 mm
in different stages tuith intemediate heat treatment. The proc¬ess imparts a very high degree of grain aligment to the bismuth based superconductor core which consists of platy grains. On the other hand, in the hitherto known processes for multilayered tapes, the rol1xng operation is done combinedly for all the 1 ayers right from the beginning. Here;, the load that can be applied during rolling is limid by the deformation characteris¬tics of the soft silver matri layers provided between the alter¬nate layers of the superconducto" core. Therefore, the degree of grain alignment and the densificttion of the individual supercon-ducting layers of the multilayered tapers are considerably lower than that of the monolayer tape prepared by PIT method.
The main objec t of the present invention is to provide a process for the preparation of multilayered silver sheathed bismuth based superconducting tapes with high critical current densities. It is also the object of the present invention to reduce the silver to superconductor ratio in the fabricated tapes.
The critical current density of mono or multifi1amentary tapes ca.n be improved significantly by increasing the degree of grain alignment along with the rolling direction of the tapes (in the a-b plane) and by increasing the densification of the super¬conducting layer. In all the hitherto known processes for fabri¬cation of multilayered tapes hardly any afetempt has been made to
impart increased grain orientation or densification of the individual layers of the superconductor before these are rolled together. As a result, the grain orientation and densification of the individual superconductor layers fabricated by the known processes are considerably lower compared to those of a monolayer tape prepared by repeated rolling and heat treatment. In the present invention multilayered tapes are fabricated from individual monolayer tapes having a high degree of grain orientation and densification. Such monolayer tapes with thickness 0.09-0.2 mm are stacked and folded by a thin silver sheet of thickness in the range 0.05-0.1 mm and are further rolled and heat treated to obtain critical current densities comparable to those of the monolayer tapes. Further, the method allows use of a higher superconductor to silver ratio. Consequently the process of the present invention not only provides multilayered tapes with increased "engineering Jc" but also saves a good amount of high purity silver.
Accordingly the present invention provides An improved process for the preparation of multilayered silver sheathed bismuth based superconducting tapes with high critical current densities which comprises
(a) packing a highly reactive ( Bi.Pb) - Sr-Ca-Cu-O precursor powder free from carbon in high purity seamless silver tubes,
(b) groove-rolling and annealing the silver tubes to form silver sheathed wires with thickness in the range of 0.8-2mm,
(c) flat-rolling and annealing the wires sheathed (Bi,Pb)-Sr-Ca-Cu-O tapes with thickness in the range of 0.3-0.5 mnn,
(d) re-flat-rolling and heat treating the tapes at a temperature in the range 810-840°C in an oxidizing atmosphere for a period in the 100 - 150 h to obtain silver sheathed monolayer superconducting tapes with a thickness in the range 0.08-0.2 mm and having a high degree of grain orientation and densification,
(e) stacking five to twenty numbers of the said monolayer tapes and folding them together by using high purity silver sheets having a thickness in the range 0.05 to 0.1 mm.
(f) annealing and flat-rolling the folded structure to form multilayered tapes to a thickness in the range 0.25 to 1.5 mm,
(g) heat treating the multilayered tapes at a temperature in the range of 810-840°C in an oxidizing atmosphere for a period in the range 50-150 h to obtain desired multilayered silver sheathed bismuth based superconducting tapes with high critical current densities.
According to a feature of the process of other present invention the cation stoichiometry of the precursor powder used is Bi : Pb : Sr: Ca : Cu = 1.5 - 1.9 : 0.3 - 0.5 : 1.8 - 2.3 : 2 - 2.5 : 2.5 - 3.8.
According to another feature of the present invention the purity of the silver tubes and sheets used is 99.99%-According to yet another feature of the present invention the oxtydising atmosphere used may be such as air, low/reduced oxygen partial pressure.
The details of the process of the present invention is described in the examples provided below which are given by way of i1lus¬tration only and should not be ronstrued to limit the scope of the present invention.
Example-1
Monolayer silver sheathed bismuth based superconductino
tapes with a thickness in the range 0.l2 to 0.l5 mm was initially
prepared by PIT method by using a highly reactive precursor
powder of bismuth based suoerconductor with a cation ratio Bi :
Fb : Sr : Ca : Cu = 1.8 : 0.4 : 2 : 2.2 : 3 [The method has been
described in our Patent No.2370/DEL/95 dated 21.12.19953
Monolayer tapes heat treated partially at a temperature in the
range 830-83S°C in air for a period of 150 h were cut into pieces
of 5 cm on length. Five numbers of these pieces were stacked one
over the other and folded together using high purity silver sheet
of thickness 0.09 mm. The folded composite structure wi th a
thickness of around 0.9 mm was intermittently annealed (5000C, 30
min) and flat-rolled in three steps to a thickness of about 0.45
mm. This was subsequently heat treated in air at a temperature
in the range 830-838°C for a period of 100 h to obtain multilay¬ered silver sheathed bismuth based superconducting tapes with critical current density of 15,300 A/cm2 at 77 K and self-
field.
Example 2
Monolayer tapes with a thickness in the range 0.14-0.17
mm and heat treated partially at a temperature in the ranpe 816-
B22°C for a period of 150 h in air atmosphere havino 10*/. oxygen
and the balance nitrogen were cut into pieces of 4 cm in length.
Five numbers of these pieces yiere stacked and folded together
using a high purity silver sheet of thickness 0.1 mm- The folded
structure with a thickness of around 1 mm was intermittently
anneal ed (500°C, 30 min) and fl, at-rolled in three steps to a
thickness of 0.55 mm. This was subsequently heat treated at a
temperature in the range 816-822°C in an atmosphere having 10%
oxygen and the balance nitrogen for a period of 100 h to yield
maltilayered silver sheathed bismuth based superconducting tapes
with a critical current dencaity of 17,300 A/cm2 at 77 K and
self field .
Example 3
Monolayer tapes with a thickness in the range 0.12-0.15
mm and partially heat treated at a temperature in the range 816-
o 822 C in an atmosphere having 10% oxygen and the balance nitrogen
for a period of 10 h were cut into pieces of 5 cm in length.
Five numbers of these pieces were stacked and folded together
using high purity silver sheet of thickness 0.09 mm. The folded
structure with a thickness of around 0.9 mm was intermittently
annealed (500°C, 30 min) and flat-rolled in three steps to a
thickness of -0-. 45 .mm. This was subseaueatly heat traated at a
temperature in the range S16-822°C in an atmosphere having 10%
oxygen and the balance nitrogen for a period of 150 h to yield multilayered silver sheathed bismuth based superconducting tapes with a critical current density of 16,000 A/cm2 at 77 K and self field.
Example 4
Monolayer tapes wi th a thickness In the range 0.14-0.17
mm and partially heat treated at a temperature in the range 830-
o B38 C in air for a period of 100 h were cut into pieces of 4 cm
length. Five numbers of these pieces Were stacked and folded
together using high puri ty silver sheet of thickness 0.1 mm. The
folded structure wi th a thickness of around 1 mm was intermit-
tently anneal ed (500°C , 30 min) and f lat-rol led in three steps to
a thickness of 0.55 mm. This was subsequently heat treated at a
temperature In the range 830-838°C in air for a period of 150 h
to yield multilayered silver sheathed bismuth based superconduct¬ing tapes with a critical current density of 17,000 A/cm2 at 77 K and self field.
The main advantages of the method of the present inven¬tion are:
(1) The method enables production of multilayered tapes with high
critical current densities comparable to monolayer tapes as it
allows use of monolayer tapes having a high degree of grain orientation and densification as the starting material.
(2) The method of production can be made continuous by using long
length monolayer tapes and obtain multilayered tapes in long

length far direct application.
(3) The method allaws uae of a higher superconductor to silver
ratio compared to the known processes. This not only increases the "engineering Jc" of the overall tape but also saves a good amount of high purity silver.

We Claim:
1. An improved process for the preparation of multilayered silver sheathed bismuth based superconducting tapes with high critical current densities which comprises
(a) packing a highly reactive ( Bi.Pb) - Sr-Ca-Cu-O precursor powder free from carbon in high purity seamless silver tubes,
(b) groove-rolling and annealing the silver tubes to form silver sheathed wires with thickness in the range of 0-8-2mm,
(c) flat-rolling and annealing the wires sheathed (Bi,Pb)-Sr-Ca-Cu-0 tapes with thickness in the range of 0.3-0.5 mm,
(d) re-flat-rolling and heat treating the tapes at a temperature in the range 810-840°C in an oxidizing atmosphere for a period in the 100 - 150 h to obtain silver sheathed monolayer superconducting tapes with a thickness in the range 0.08-0.2 mm and having a high degree of grain orientation and densification,
(e) stacking five to twenty numbers of the said monolayer tapes and folding them together by using high purity silver sheets having a thickness in the range 0.05 to 0.1 mm.
(f) annealing and flat-rolling the folded structure to form muitilayered tapes to a thickness in the range 0,25 to 1,5 mm,
(g) heat treating the muitilayered tapes at a temperature in the range of 810-840°C in an oxidizing atmosphere for a period in the range 50-150 h to
obtain desired multilayered silver sheathed bismuth based superconducting tapes with high critical current densities.
2. An improved process as claimed in claim 1 wherein the starting powder used is having cation stolchiometry ranging Bi; Pb : Sr: Ca : Cu = 1.5 -1.9 :0.3 - 0.5 : 1.8 - 2.3 : 2 - 2.5 : 2.5 ~ 3.8.
3. An improved process as claimed in claims 1 and 2 wherein the purity of the silver tubes and sheets used is 99.99%.
4. An improved process as claimed in claims 1 - 3 wherein the oxidizing atmosphere used is such as air, low / reduced oxygen partial pressure.
5. An improved process for preparation of multilayered silver sheathed bismuth based superconducting tapes with high critical current densities substantially as herein described with reference to the examples.

Documents:

259-del-1997-abstract.pdf

259-DEL-1997-Claims.pdf

259-del-1997-complete specification (granted).pdf

259-del-1997-correspondence-others.pdf

259-del-1997-correspondence-po.pdf

259-del-1997-description (complete).pdf

259-del-1997-form-1.pdf

259-del-1997-form-19.pdf

259-del-1997-form-2.pdf

259-del-1997-form-3.pdf

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Patent Number

194297

Indian Patent Application Number

259/DEL/1997

PG Journal Number

41/2004

Publication Date

09-Oct-2004

Grant Date

03-Feb-2006

Date of Filing

31-Jan-1997

Name of Patentee

COUNCIL OF SCIENTIFIC & INDUSTRIAL RESEARCH

Applicant Address

RAFI MARG, NEW DELHI-110001, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	PERUMAL GURUSWAMY	RRL (CSIR), THIRUVANANTHAPURAM, INDIA.
2	UPENDRAN SYAMAPRASAD	RRL (CSIR), THIRUVANANTHAPURAM, INDIA.
3	MADHAVAN SANKARA SARMA	RRL (CSIR), THIRUVANANTHAPURAM, INDIA.
4	KRISHNA GOPAKUMAR WARRIER	RRL (CSIR), THIRUVANANTHAPURAM, INDIA.
5	ALATHOOR DAMODARAN DAMODARAN	RRL (CSIR), THIRUVANANTHAPURAM, INDIA.

PCT International Classification Number

H01L 39/24

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA