WO2001069695A1

WO2001069695A1 - Load controlled rolling of superconducting tape

Info

Publication number: WO2001069695A1
Application number: PCT/DK2001/000170
Authority: WO
Inventors: Hengning Wu
Original assignee: Nordic Superconductor Technologies A/S
Priority date: 2000-03-14
Filing date: 2001-03-14
Publication date: 2001-09-20
Also published as: AU2001242300A1

Abstract

The present invention relates to a method for obtaining an increased density of a material. More specifically, the invention relates to a method of increasing the critical current density of a superconducting material, especially of a Bi-based high temperature oxide superconducting material or precursor powder, the superconducting material being surrounded by a metal matrix thereby forming a length of conductor. According to the invention a method for obtaining an increased critical current density of a superconducting material is provided, the method comprising at least two intermediate rolling steps of subjecting the length of conductor to a rolling process in which a predetermined rolling load is set, each intermediate rolling step taking place between two heat treatment processes, wherein at least one intermediate rolling process takes place at a lower load than the previous intermediate rolling process.

Description

Load controlled rolling of superconducting tape

The present invention relates to methods for obtaining an increased density of a material. More specifically, the invention relates to a method of increasing the critical current density of a superconducting material, especially of a Bi-based high temperature cκiαe superconducting material or precursor powder by load-contrclled roiling, this increasing the density of the material. Such a method is especially useful when producing a Bi-based oxide superconducting tape in which a plurality of filaments of the oxide superconductor is surrounded by a metal matrix, the method providing a superconducting tape with improved critical current density. Presently pre- ferred superconducting materials for use in a method according to the present invention are materials belonging to the (BiPb) -Sr-Ca-Cu-0 group of materials.

Background of the invention

A known manufacturing method for superconducting conductors is the oxide-powder in tube method (referred to as Powder-In-Tube or PIT technology, , in which an oxide precursor powder is introduced into a metal tuoe, typically a silver or silver alloy tube, which by diameter reduction is manufactured into a wire by means of mechanical deformation, such as drawing, swaging or rolling. The wire is subsequently cut into a plurality of sections and arranged in an outer metal tube, typically a silver or silver alloy tube, which is subjected to further mechanical deformation to form a multifilamentary wire .

The multifilamentary wire is then rolled or pressed into a tape, which is subsequently sintered to bring the oxide powder into the superconducting state and to increase the critical current density of the tape. By tape is meant a length, which has a width greater than its thickness.

In order to increase tne critical current density, the sintering process is repeateo with intermediate mechanical deformation between the sintering steps to improve tne texture and density of tne superconducting material. Tne intermediate mechanical deformation can be achieved by pressing or rolling. When the pressing or reding ae- formation is performeα, crac.ing may be caused m the superconducting core. These cracks are to a certain degree healed m the subsequent heat treatment; however, large cracks not healed m the heat treatment will reduce the critical current density.

Therefore, there will be a trade-off between a Higher final density and a more severe crack formation. Generally speaking, pressing tends to cause less cracking ana a higher effective pressure can thus be used to increase tne density of the oxide cere while high pressure during rolling will decrease the critical current density αue to crack formation (G. Grasso, A. Jeremie, and R. Fluκιger, Supercond. Sci. Technol . , 8, 327, 1995).

However, the simple pressing method is not suitable to produce superconducting tapes m long lengths. Cne way to adapt the pressing method for production of long length tapes is the continuous periodic pressing m which a special profile die is used for pressing (F. Marti, G. Grass, Y. B. Huang, M. Dhalie, G. Witz, R. Passeπni, E. Giannini, E. Bellmgeπ, E. ^'alker, R. Flukiger, Progress in Improving Jc of Long 3ι (2223) Tapes by Periodic Pressing, Advances in Superconductivity XI/2, Koshitαka Ta]ima eds, Springer Tokyo, 939, 1995) . But rolling is the preferred method for long length tapes due to its high productivity. Using rclls of a larger diameter can alleviate the crack formation in the transverse direction of the tape and improve the critical current αεnsity (T. Hikata and K. Sato, AS Patent 5,246,917, 1993). An eccentric rolling methoα r.as also been developed to simulate the pressing condition (L. Kcpera, P. Kovac, and I. Husek, New rolling technique for texturing of Bi

Ag tapes, Superconα. Sci . Technol., 11, 433, 1998^ . In addition to the deformation method, the proper deformation parameters are essential for high critica-- c rrent density.

In the above-referreo disclosure by Grasso et al . it is further described tr.at a maximum critical current J can be achieved by a rolling process m which a given optimum pressure is exerted on the tape during a number of deformation steps between tne heat treatments.

Summary of the invention

Accordingly, an ob_jective of the present invention is to provide improved metnods suitable for use m tne produc^¬ tion of an oxide superconducting length of tape, in order to obtain a high critical current density in a rolling process .

In a first aspect, a method for obtaining an increased critical current density of a superconducting material is provided, the method comprising at least two intermediate rolling steps of sub ecting the length of conductor to a rolling process m v.nich a predetermined rolling load is set, each intermediate rolling step taking place between two heat treatment processes, wherein at least one intermediate rolling process takes place at a lower load than the previous intermediate rolling process. This aspect of the invention is based on the realisation that an optimum rolling load should be determined for each intermediate rolling, this m contrast to for example Grasso et al . (se above) which discloses that an op- timum critical current is achieved by a number of defor^¬ mation steps at the same pressure.

A problem with this known method is that the optimum pressure (or load) is determined after the tape has oeer finished, i.e. after a number of intermediate rolling steps and corresponding heat treatments . As heat treatment is a very slow process, which for each treatment ma_. take from 2-6 weeks, it takes a very long time to determine the optimum load to be used during the rolling steps .

Thus, m a further aspect of the invention, a method is provided for determining an optimum load for a single intermediate rolling, where a lower load will not give the necessary densification and a higher load will cause excess cracking.

More specifically, a method is provided comprising the steps of determining a first load range for which a mc- notonous, substantially linear increase m reduction ra^¬ tio takes place with increasing load, and a second load range for which the monotonous, substantially linear increase m reduction ratio no longer takes place with increasing load, determining the transition point repre- sented by the first data point m the second load range which deviates from the monotonous, substantially linear increase m reduction ratio m the first load range, ano determining an optimum load corresponding to the transition point. Preferably the method comprises a further step of determining an optimum load range around the optimum load. Based en the above, an optimum load for an intermediate rolling can be determined and the production of the "intermediate" tapes can commence without awaiting the test results for the finished superconducting tape. Inis is m contrast to the method disclosed by Grasso et al . in which only the finally achieved critical current is measured, as measuring the "intermediate" critical current cannot be used for determining a single optimum mterme- diate rolling load, i.e. a tape wifn a hign critical current after a first intermediate rolling does not necessary lead tc a finished tape navmg the highest possible critical current.

Furtner, load has also shown to be a more suitable parameter for process control than the traditionally used reduction control since the change of thickness, i.e. the reduction, is very small compared to the thickness of the tape. However, the change of load is large which allows for easy process setting.

In yet another aspect, the optimum load ranges for a first rolling and a subsequent second roiling are provided for a superconducting tape. Especially, the pres- sure for the first rolling should be the highest with a reduced pressure for the subsequent intermediate roiling. In a preferred embodiment, the pressure for tne first intermediate rolling should be around 1,1 -1,4 G? and the pressure for the second intermediate roiling should be around 0.5 - 0.7 GP

In the present application the term "superconducting" is used to denote both the precursor powder material and the subsequent actually superconducting material. Tne invention will now oe described m further detail with reference to the accompanying drawings, wherein:

Figs, la - lh is a schematic illustration of the Powder- In-Tube (PIT) process,

Fig. 2 shows a graphic representation of a first reduc^¬ tion ratio-to-rolling load relation for a given tape,

Fig. 3 shows a graphic representation of a second reduction ratio-to-rollmg load re_atιon for a given tape,

Fig. 4 snows a first grapπic representation of a cr-_t±cal current-to-rollmg load relation for a given tape, ana

Fig. 5 shows a second graphic representation of a critical current-to-rollmg load relation for a given tape.

First the principle steps for a process of producing a length of a superconducting -.ength of tape wιl_ oe described with reference to figs, la through lh.

First a silver or silver alloy tube is filled witn a superconducting precursor powαer, typically a metal oxide (fig. la), and then the tube is drawn into a single filament wire (fig. lb), which suosequently is cut into sections and packed into a multifilamentary wire (fig. lc) . The multifilamentary wire is then drawn to a small- diameter wire (fig. Id), _Λr.ιch is subsequently roiled into a thin tape, m whicn eacn of the single po^ er con^¬ taining filaments is transformed into a flattened structure resembling the overall appearance of the rolled tape. Thereafter the tape is neat treated m a furnace to obtain the superconducting properties of the precursor and to heal the cracks caused by tne roiling process (fig. If). After the first neat treatment the tape is subjected to an intermediate rolling (fig. Ig) after which it is sub ect to a further heat treatment fig. lh) to heal cracks formed m the deformation process during the intermediate rolling. The last two steps correspond- mg to figs. Ig and In may be repeated several times.

Next the problem of octainmg a high densification of tne superconducting material yet avoiding the formation of excess cracking will oe discussed with reference to fig. 2.

Fig. 2 shows the relation between the reduction ratio and the load (measured as the roll separating force during a first intermediate re --ling. As can be seen frcr the dia- gram, with an increase m load, there is a range of monotonous increase m reduction ratio. Further, _n a relatively well-defined range, the increase is substantially linear, this being tne "linear" range. However, as is clear from the figure, the monotonous increase m reduc- tion ratio neither takes place at very low load levels, nor after a load of a certain height. As can also be seen m the figure, from loads of a given value the relation between the reduction ratio and the load becomes very irregular. Experience r.as shown that within this range of load, excess cracking of the superconducting material takes place, however, corresponding to an increased re^¬ duction ratio m the range of monotonous increase m reduction ratio, increased densification takes place. The problem is therefore, to roll the length of tape corre- spondmg to the maximum within the monotonous!_., increasing range. In the past this has traditionally been achieved by determining the maximum reduction ratio and then to control the rolling process accordingly, i.e. if the optimal reduction rate had been found to be 3.5 then this was the value according to which the rolling process was controlled. However, as tne typical tape thickness is 0.2 mm with a variation of up to 0.01 mm (or 5-), it is very dιffιcu_t to control tne optimum rolling condition with the reduc- tion ratio as a controlling parameter. This is also ι_- lustrated by tne figure, for example if the actual roi_- mg process ta<es place at a reduction ratio of 9.5 , as compared to tne set value of 8.5 c, then rolling takes place far within the load range for which severe crackmg may take place.

In contrast, _-oad is a more suitable arameter for process control since the change of load is large as compared with the change m reduction ratio, tnis both m aosoiute and relative terms, this allowing for easy process control.

In the following a method is described by which the present invention can be turned into practise. For a given specimen, i.e. a tape to be rolled, tests are performed by which the reduction ratio ( %) is plotted against the load (in ton) for a number of different load values, a data point representing a reduction ratio for a give" load. The ob ect of these tests are to determine a number of data points for the load range for which monotonous, substantially linear increase m reduction ratio takes place, as well to determine a number of data points for the load range for which the relationship between reduction ratio ano load becomes irregular or the reduction ration no longer increases with increased load. The nur- ber of data points needed to achieve a useful graphic representation, i.e. a curve, of the reduction ratio versus load relation may vary, however, as such a curve is typical when materials are tested deformation tests, it would be a routine experiment for the skilled person to achieve a "typical" curve as shown m fig. 2. From the graphic representation a range can be approximately outlined by two parallel lines corresponding to a range of monotonous, substantially linear increase in re- duction ratio against load, the range enclosing the test values within the linear range. Again, the typical test values will not represent a perfect linear relationship, but in practise the skilled person will readily identify those values on the basis of which the range should be determined, i.e. obviously erroneous data points should be neglected. The load for the first data point that de^¬ viates from the "linear" behaviour, i.e. falls outside the range between the drawn lines, is defined as the transition point. The optimum, roiling load is, according to the invention, around the transition point as schematically shown by the rectangular range in fig. 2.

In the shown embodiment a range has been outlined by two parallel lines with a separation of about 11 in reduction ratio. The optimum load determined from fig. 2 would be approximately 3.0 tons. The range for the optimum rolling load around the transition point is here within -30! to +30 of the transition point load value, and preferably in the -10^:;= to +10 range of the transition point load value. Although the I_: is lower for lower load, the scat^¬ tering is less and this can be an advantage for some applications. In the present context the term, "around" may also imply a range which just includes the optimum load, e.g. from 0 to +101.

Indeed, the linear range could also be defined using a straight curve fitted corresponding to the data points in the linear range. In the same way a (non-linear) curve could be fitted for the non-linear range. The transition point could then be determined using, for example, the standard deviation value calculated for the linear range, for example as the point where the non-linear curve deviates from the calculated standard deviation value.

In tne past, when more than one intermediate roiling were used in the manufacturing of a superconducting tape, the first and the subsequent rolling procedures were performed with the same set value for the reduction ratio (or load as in Grasso et al^', . However, the present inventors have found that the relation between the reduction ratio and the load (measured as the roll separating force) during a second (or subsequent) intermediate rolling is very different from the proceeding rolling.

To illustrate this, reference is made to fig. 3 depicting the relation between the reduction ratio and the load during the second intermediate rolling, i.e. after treatment in the furnace, of the same specimen used for determining the relation shown it fig. 2. In this case, the first data point outside the "linear" range is far away from the parallel lines, however, it is readily recognised by the skilled person that more data points are needed in this range to determine a more precise load value for the transition point. The optimum loao determined from fig. 3 would be approximately 1.25 tons. What is essential is the recognition that the optimum load for the second intermediate rolling is much smaller than the corresponding value during the first intermediate roll- inσ .

Example 1

Reference is now made to fig. 4. A superconducting precursor powder (BiPb) _Sr_;Ca;Cu-_;0_.. was filled into a silver tube of 20 mm in outer dia .eter and 17 mm in inner diame- ter, which was drawn into a wire of 2.2 mm in cuter diameter. Then the tube was cut into 37 sections of egual length and tne sections were filled into a silver alloy tube of 20 mm m c_ter diameter and 17 mm m inner diameter, which in turn Λ~S drawn into 1.03 mm m outer diameter and then rolleo into a tape of 0.22 mm tnicKness.

Thereafter, tne tape _Λas heat treated at 800 to 84C"C and then gradually cooled. Then the tape was subjected to a first intermediate rolling with an optimised load of 3 tons after which tne tape was subjected to a second heat treatment within tne same temperature range. After the second heat treatment the tape was cut into a number of lengths which were tnen subjected to a second intermediate rolling m whicn the load for different lengths were gradually changed from 0 to 3 tons; more specifically, the length were roiled at loads of 0, 0.5, 1.0, 1.5, 2.0 and 3.0 tons. Thereafter, the samples were subjected to a third heat treatment. The critical currents (m Amperes) for the samples at ^~~ K were measured and are shown fig. 4. The trend suggests that a sample treated m the optimised load range will give the highest critical currents and less scattering, i.e. when treated at a load value the range from 1.0 to 1.5 ton as illustrated fig. 4 and as also found in fig. 3.

Example 2

Reference is now made to fig. 5. A tape was prepared in a similar way as Example 1. The following three different loads were used for the first and second intermediate rolling.

(A) 1.0 ton

(B) 3.0 tons

(C) 5.0 tons Therefore we have nine combinations AA, AB, AC, BA, 3B, BC, CA, CB, CC, where the first and second letters represent the loads for the first and second intermediate rolling respectively. Again the optimum condition EA gave the highest critical current, i.e. a load for the first intermediate rolling of 3.0 tons as illustrated in fig. 2, and a load for the second intermediate rolling of 1.0 ton as illustrated in fig. 3. The average rolling pressure can be calculated by

P R'Δh - b

where P is the load, R ' is the roll radius after adjust^¬ ment for the elastic flattening, Δh is the absolute re^¬ duction in thickness, and b the average width of thϊ tape. Corresponding to the load ranges illustrated i: figs. 2 and 3, the corresponding pressure for the firs: intermediate rolling should be around 1.1-1.4 GPa, an: the pressure for the second intermediate rolling shoul: be around 0.5-0.7 GPa.

When the optimised load range has been determined for . specific sample under specific condition, the corresponding pressure range can be calculated. However, it has also been found that this calculated pressure can be use: to calculate a new optimised load range, for example whe: the width of the tape or the diameter of the rollers are changed, this saving a new determination of the optimise: load range.

Claims

CLAIMS :

1. A method for obtaining an increased critical current density of a superconducting material, said su- perconductmg material being surrounded by a metal matrix therεc_- forming a length of conductor, tne method comprising at least two intermediate rolling steps of subjecting the length of conductor to a rolling process wnich a predetermined rolling load is set, each mter- mediate rolling step taking place between two heat treatment processes, characterized _n that at least one intermediate rolling process takes place at a lower load than the previous intermediate rolling process.

2. A method as defined m claim 1, characterized in that for each intermediate rolling process an optimum loao range is determined for the length of conductor, whicn length is subsequently rolled with a load set within the determined optimum _oad range.

3. A method as defined :laιm 2, characterized in that _-t comprises at least one load setting step which an optimum rolling load is determined prior to an intermediate rolling, the load setting step comprising the steps of: determining a plurality of data points each representing a thickness reduction ratio for the length of conductor for a given load, such that the data points represent a first load range for which a monotonous, sub- stantially linear increase in reduction ratio takes place with increasing load, and a second load range for which the monotonous, substantially linear increase m reduction ratio no longer takes place with increasing load, determining the transition point represented by the first data point in the second load range which deviates from the monotonous, substantially linear increase in reduction ratio in tne first load range, determining an optimum load corresponding to the transition point, determining an optimum load range around the optimum load, rolling the length of conductor with a ioao set within the determined optimum load range.

4. A method as defined in claim 3, characterized in that the data points are plotted in a graphic form, and that a linear range for which the monotonous, substan- tially linear increase in reduction ratio takes place with increasing load is outlined by two parallel lines, the range enclosing the data points within the linear range; and that the load for the first data point that de- viates from the linear range between the drawn lines, is defined as the transition point.

5. A method as defined in claim 3 cr 4, character- ized in that the optimum load range for each intermediate rolling process is from -30% to +30% of the transition point load value, and preferably in the -101 to -10* range of the transition point load value.

6. A method as defined in any of the previous claims, characterized in that at least a first and a second intermediate rolling are conducted, the load range for the first intermediate rolling being set corresponding to a pressure in the range from 1.1 to 1.4 GPa, the load range for the second intermediate rolling being set corresponding to a pressure m the range from 0.5 to 0.7 GPa.

7. A metnod for determining an optimum rolling load prior to an intermediate rolling of a superconducting material, said superconducting material being surrounded by a metal matrix thereby forming a length of conductor, comprising the steps of: determining a plurality of data points eacn representing a thickness reduction ratio for the length of conductor for a given load, such that the data points represent a first load range for which a monotonous, substantially linear increase reduction ratio takes place with increasing load, and a second load range for which the monotonous, substantially linear increase reduction ratio no longer takes place with increasing load, determining the transition point represented by the first data point the second load range which devi- ates from the monotonous, substantially linear increase reduction ratio m the first load range, determining an optimum load corresponding to the transition point, and preferably determining an optimum load range around the optimum load.

8. A method as defined m claim 7, characterized m that the data points are plotted m a graphic form, and that a linear range for which the monotonous, substan- tially linear increase m reduction ratio takes place with increasing load is outlined by two parallel lines, the range enclosing the data points within the linear range; and tnat the load for the first data point that deviates from tne linear range between the drawn lines, is defined as the transition point.

9. A method as defined m any of the previous claims, characterized m that the superconducting material or precursor powder is a Bi-based metal oxide, ano in that the metal matrix is silver or a silver alley.