JPH0982446A - Superconductive connecting method for superconductive wire - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide superconductive connectability for a Bi series oxide superconductive wire with another while good superconductive characteristics of the wire are maintained by allowing a brazing material containing Bi and a specific alloy to exist in the connecting region, and melting it and solidifying in the non- oxidative atmosphere. SOLUTION: A Bi series oxide superconductor 1 crystallized through a heat treatment is coupled with a tape-form oxide superconductive wire 2 consisting of a sheath material 6 such as silver, and in the coupling part a brazing material 4 of Bi-Sn alloy is arranged, and the part is enclosed with a reinforcement 3 consisting of a silver tape, and its outside is covered with a protection cover 5. From a nozzle 7 nitrogen gas is fed so that the inside of the cover 5 is put in a non-oxidative atmosphere, and the connecting region is heated to 300 deg.C along with melting of the brazing material 4 and then cooled to the room temp. to cause solidification. According to this procedure which includes heat treatment in non-oxidative atmosphere using a Bi-containing brazing material having a high critical current density, the material concerned and the brazing material are prevented from oxidating to suppress formation of a on- superconductive oxide, and it is possible to establish good superconductive characteristics.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an improvement in a method of connecting a Bi-based oxide superconducting wire and a metal-based superconducting wire, or a Bi-based oxide superconducting wire while maintaining a good superconducting connection state. is there.

[0002]

2. Description of the Related Art Oxide superconductors are characterized by far higher superconducting transition temperature (hereinafter referred to as Tc) and upper critical magnetic field (hereinafter referred to as Hc ₂ ) than conventional metal-based superconductors. Therefore, it is expected to be applied to various fields. Among the oxide superconductors, the Bi-based oxide superconductors have a Tc of about 80 K and a low Tc phase 110
There is a high Tc phase of about K and Hc ₂ is 1 in both cases.
It is expected to exceed 00T. The former (low Tc phase)
Has a molar ratio of Bi, Sr, Ca, Cu of approximately 2: 2 :.
Since it is 1: 2, it is generally called a Bi-2212 phase, and the latter (high Tc phase) has a molar ratio of about 2: 2.
Since it is 2: 2: 3 (however, a part of Bi is replaced by Pb), it is generally called a Bi-2223 phase.

Oxide superconducting wires are produced using these oxide superconductors, which are generally produced by the following method. First, Bi ₂ O ₃ , SrCO ₃ , CaCO ₃ , Cu
Raw material powders such as O are weighed, pulverized and mixed, heat-treated and calcined. Next, it is filled in a silver pipe, a silver billet or the like, and then drawn and rolled to form a tape. Furthermore,
The formed wire is heat-treated to crystallize the oxide superconductor in a highly oriented state. By applying such a manufacturing method, a good superconducting wire having a high critical current (hereinafter referred to as Ic) can be manufactured.

As described above, Hc ₂ of the oxide superconductor greatly exceeds that of the metal superconductor. Therefore,
When the superconducting magnet is composed of the oxide superconductor, it becomes possible to generate a strong magnetic field that cannot be generated by the conventional metal-based superconducting magnet. An example of applying such characteristics is a high-resolution NMR analyzer that exerts great power in determining the molecular structure of a polymer compound such as protein. In this device, the stronger the magnetic field applied to the sample is, The amount of information obtained will increase, and the molecular structure will be determined in more detail.

A feature of the superconductor is that the permanent current induced in the superconducting loop has a small temporal variation and does not generate heat. Conventionally, as a superconductor, a metal-based superconductor material has been applied to various devices that require a highly accurate and stable magnetic field, such as an MRI medical diagnostic device and a high resolution NMR analyzer. In these cases, in order to effectively utilize the above-mentioned features, the superconducting wires used in the device are connected to each other while maintaining the superconducting state, so that a permanent current flows in a loop. In recent years, attempts have been made to insert an oxide superconducting coil into a metal-based superconducting coil to generate a higher magnetic field. In order to operate these superconducting coils in the persistent current mode, it is necessary to connect the oxide superconductors or the oxide superconductor and the metallized superconductor in a superconducting state.

When the metal-based superconducting coil and the oxide superconducting coil are independently operated in the persistent current mode, the oxide superconducting wire may be superconductingly connected to the oxide superconducting wire of the same kind, so that it is relatively easy. Expected to be connectable. However, when adopting such a configuration, it is necessary to install a dedicated permanent current switch for each superconducting coil.

When operating the superconducting coil in the permanent current mode, when increasing or decreasing the generated magnetic field to a predetermined value, a current is supplied to the circuit in which the DC power source and the superconducting coil are connected in series. Need to be flushed. Further, when a current is made to flow up to a predetermined current value, or when a superconducting current is passed at a predetermined current value in the permanent current mode, it is necessary to flow the superconducting current in a circuit formed of a superconductor and not including a DC power source. . A special switch used to make such a switch is a persistent current switch.

In such a persistent current switch, when the current is applied to the DC power supply (OFF state), the electrical resistance of the permanent current switch is set higher than the internal resistance of the DC power supply, and the current is applied to the permanent current switch. In the (ON state), the resistance must be zero in the superconducting state as well as in the persistent current switch. Since it is necessary to switch the electric resistance in this way, it is difficult to manufacture a persistent current switch, and in particular, a persistent current switch corresponding to an oxide superconductor has not yet been developed.

On the other hand, in the case of a metallic superconducting wire, a permanent current switch is mounted in a commercially available superconducting magnet such as a superconducting magnet for an NMR analyzer, which is technically established at a high level. Has been done.

If the metal-based superconducting wire and the oxide superconducting wire can be superconductingly connected, each superconducting coil is connected in series and forms one closed loop, so that one permanent current switch should be arranged. . That is, by using the persistent current switch for the metal-based superconducting coil, which has been technically established, it is not necessary to develop an oxide superconducting persistent current switch which is technically difficult to manufacture. Moreover, by adopting such a configuration, it becomes possible to use a persistent current switch having high reliability.

As described above, the techniques for superconducting connecting oxide superconducting wires to each other or connecting oxide superconducting wires to metal-based superconducting wires have important meanings, and the establishment of such techniques is desired. Is the reality.

[0012]

Since the superconducting connection has such an importance, various methods have been proposed for connecting oxide superconductors to each other.
(1) A method in which oxide superconductors are adhered to each other and then heated and fired (for example, JP-A 1-23379), (2) A method in which the connection portion is surrounded by a reinforcing material or a sleeve, and the oxide superconducting powder is sintered therein. (For example, JP-A-63-269468 and JP-A-1-17384) are known.

However, in these connection methods (1) and (2), since the connection parts are both formed of a sintered body produced by a solid-phase reaction, the orientation and adhesion of the superconductor crystal are poor,
There is a drawback in that the Ic of the connection portion is greatly reduced as compared with the Ic of other portions.

On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 3-78879 discloses a method of joining oxide superconductors or an oxide superconductor and a normal conductor by (3) brazing an oxide superconductor and a member to be joined. A method has been proposed in which the transition metal oxide is interposed between the brazing material and the oxide superconductor, and the bonding is basically performed. However, with this method,
Since the transition metal oxide, which is a non-superconducting oxide, exists in the current path, the normal conducting current flows instead of the superconducting flowing current, and a finite voltage is generated.
In this method, an In alloy (99.5% In-0.5% Nb) added with Nb is used as a brazing material in the example, but since this brazing material has a low Tc of about 5K,
There is a drawback that the critical current density (hereinafter referred to as Jc) at the liquid helium temperature becomes extremely low. Therefore, even if the superconducting coil manufactured by this method is used, the operation in the persistent current mode cannot be expected.

Further, Japanese Patent Application Laid-Open No. 4-301388 proposes a method of superconductingly connecting an oxide superconducting wire and a metallic superconducting wire. In this method, (4) the bonding length of the oxide superconducting wire and the metal-based superconducting wire is 10 cm or more, and the brazing material (solder material) is used for bonding.
However, this method also has the following problems. That is, in this method, as pointed out in JP-A-3-78879, oxygen in the atmosphere or oxygen in the oxide superconductor oxidizes the surface of the brazing material to form a non-superconducting oxide. . Therefore, even if the superconducting coil manufactured by this method is used, it is difficult to operate in the persistent current mode.

The present invention has been made under these circumstances, and its purpose is to provide a Bi-based oxide superconducting wire or a metal-based superconducting wire while maintaining good superconducting properties. It is an object of the present invention to provide a method useful for manufacturing a superconducting coil which can be connected to a wire and can be operated in a persistent current mode.

[0017]

Means for Solving the Problems The present invention, which has achieved the above object, means that when a Bi-based oxide superconducting wire and a metal-based superconducting wire or Bi-based oxide superconducting wires are superconductingly connected, a connection region is formed. , Bi is present, the brazing material is melted in a non-oxidizing atmosphere, and then solidified to connect the superconducting wire to be connected to each other. Examples of the brazing material used in the above method include an alloy containing at least one of Sn, Nb, and Pb in addition to Bi, an alloy containing In in the alloy, and the like.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is constructed as described above, but in short, a brazing material containing Bi is present in the connection region, and the brazing material is melted in a non-oxidizing atmosphere and then solidified. Then, it has been found that a good superconducting connection can be achieved without causing the problems as in the prior art, and the present invention has been completed.

As the brazing material used in the present invention, in addition to Bi, an alloy containing any one or more of Sn, Nb and Pb, and
If necessary, these are alloys containing In, etc., but these brazing materials are melted at a relatively low temperature of about 300 ° C. or lower to become a liquid state. Therefore, as compared with the methods (1) and (2) in which the superconducting connection is made by utilizing the solid-phase reaction, the adhesion between the superconductors is improved, and good superconducting connection can be achieved.
In the present invention, it is necessary to use a brazing material containing at least Bi, for the following reason.

The superconducting transition temperature Tc of the brazing material containing Bi is
It is about 9K, which is lower than that of the oxide superconductor. But
And braze materials such as Pb-Sn alloy and In-Pb alloy
It shows a high Tc. Also, J at liquid helium temperature
c is also a brazing material containing Bi, Pb-Sn alloy or In-P
10 times higher than brazing materials such as b alloys^Four A / cm ² 
It shows the value of the degree. This is because Bi is a superconductor crystal
Pinning center that deposits inside and traps magnetic flux
Therefore, it is considered that a high Jc is exhibited.

Regarding the content of Bi in the brazing material,
It is preferably about 10 to 60% by weight. That is, when the Bi content is less than 10% by weight, the above-mentioned effects are not exhibited and 60
When it exceeds the weight%, Tc decreases.

Generally, when Jc is low, a method of increasing the length of the connecting portion, widening the area, and increasing Jc is adopted. However, since a brazing material containing Bi has a higher Jc than brazing materials such as conventional Pb-Sn alloys and In-Pb alloys, such brazing materials are melted and solidified in a non-oxidizing atmosphere. By
It is possible to secure a sufficiently high Jc without lengthening the connecting portion.

According to the method of the present invention, the problems shown in the above methods (3) and (4) do not occur. That is, the above (3),
In the method of (4), the surface of the inclusions was oxidized and the superconducting property was deteriorated.However, the oxygen in the atmosphere that causes this oxidation is allowed to have an allowable concentration by performing heat treatment in a non-oxidizing atmosphere. Will be reduced to: Specifically, the oxygen partial pressure during the heat treatment is preferably 0.05 atm or less. Also, the oxygen remaining in the molten state of the inclusions may oxidize the inclusion alloy,
Regarding this, In (0.3 eV) having a low electron affinity
Is added to the brazing material and absorbed in the In, it is possible to prevent the brazing material from oxidizing and forming a non-superconducting oxide at the interface with the oxide superconductor. From this point of view, in the present invention, the brazing material used is I
It is preferable to use those containing n. Further, in order to exert such effects, the content of In added as necessary is preferably about 1 to 5% by weight.

Conventionally, there was a problem that the oxide superconductor reacts with oxygen to form a non-superconductor phase when the inclusions solidify, but the present invention can solve such a problem for the following reason. . When a metal and an oxide are mixed, it is mainly determined whether oxygen existing in the oxide stays in the oxide or reacts with the mixed metal to form another oxide. Determined by the electron affinity of the metal. A metal having a negative electron affinity and a larger value has a higher tendency to bond with oxygen to become a positive ion and form an oxide.
The constituent metal elements of the Bi-based oxide superconductor are Bi, Pb,
Sr, Ca, Cu, and their electron affinity is Sr.
And Ca show negative values, Bi and Pb are 1.1 eV, Cu
Is 1.2 eV. Sn and Nb which may be added to the brazing material containing Bi are 1.2 eV and 1.0 e, respectively.
V. From these facts, Sr and Ca have a very strong tendency to maintain a positive ion state. Also B
Since i, Pb, Cu, Sn, and Nb have almost the same electron affinity value, when the metal and the positive ion are mixed, the positive ion state is not positively replaced.
It is kept in almost the initial state. From the above, since these elements try to maintain the state of positive ions in the Bi-based oxide superconductor, they deprive the oxygen of the Bi-based oxide superconductor and the brazing material containing Bi is oxidized. It does not form non-superconducting oxides.

By the above operation, the oxide superconducting wires or the oxide superconducting wire and the metal-based superconducting wire are
The superconducting connection is made while maintaining a good superconducting state.
Even if the connection length at this time is 5 cm or less, 10
A connection resistance of ^-11 Ω or less can be realized.

The present invention will be described in more detail with reference to the following examples, but the following examples are not intended to limit the present invention, and any modification of the design of the present invention can be made without departing from the spirit of the preceding and the following. It is included in the technical scope.

[0027]

【Example】

Example 1 FIG. 1 is a schematic view showing an example of the structure for carrying out the method of the present invention, in which 1 is an oxide superconductor, 2 is an oxide superconducting wire, 3 is a reinforcing material, and 4 is a brazing material. 5 is a heat insulating cover, 6 is a sheath material, and 7 is a nozzle.

The oxide superconducting wires 2 were connected to each other in the order shown below according to the structure shown in FIG. First, a single core (Bi-22
12) A tape-shaped oxide superconducting wire 2 comprising an oxide superconductor 1 (thickness: 70 μm) and a sheath material 6 made of pure silver.
Two pieces (thickness: 200 μm, width 8: mm) were prepared, a brazing material 4 made of a Bi—Sn alloy was arranged in the connecting portion, and the connecting portion was covered with a silver tape as the reinforcing body 3. At this time, the length of the superconducting connection region was 5 cm. Further, the outside is covered with a heat insulating cover 5, and a nozzle 7 is inserted into the heat insulating cover 5 to flow a nitrogen gas to make the inside of the heat insulating cover 5 a non-oxidizing atmosphere and keep the oxygen partial pressure of the connection region at 0.05 atm or less. YSZ (yttrium-stabilized zirconia)
It confirmed using the oxygen sensor.

Then, the surface of the silver tape which is the reinforcing member 3 was covered with an electric insulator, and a heater for heating was wound. This heating heater heats the connection area to 300 ° C.
After holding for a minute to melt the brazing material 4, it was cooled to room temperature and solidified.

As a result of measuring Ic (4.2K, 2T) across the connecting region with a standard of 10 ^-11 Ω, Ic (of the oxide superconducting wire 2 of (Bi-2212) oxide not including the connecting region was found. = 100 A), the same value was obtained.

In the same manner as above, a single core (Bi-222
3) The oxide superconducting wire 2 was superconductingly connected, and Ic (4.2K, 2T) was measured in the same manner as above. As a result, the I of the (Bi-2223) oxide superconducting wire 2 not including the connection region
The same value as c (= 80 A) was obtained.

The inventors of the present invention performed the same measurement using a silver alloy to which Ni or Mg was added in a small amount for the purpose of increasing the strength, instead of the pure silver sheath material 6, and found that the connection region was crystallized. Since it has no effect, the same effect was obtained. Further, even if argon gas was introduced into the connection region instead of nitrogen gas, the oxygen partial pressure could be controlled, and the same effect as above was obtained.

Comparative Example 1 In the same manner as in Example 1, a tape-shaped oxide superconducting wire comprising a single core (Bi-2212) oxide superconductor 1 (thickness: 70 μm) and a sheath material 6 made of pure silver. 2 (thickness: 20
0 μm, width 8: mm), and prepare Pb-
A brazing material 4 made of Sn alloy was arranged and the connecting portion was covered with a silver tape as the reinforcing body 3. At this time, the length of the superconducting connection region was 5 cm.

After that, the surface of the silver tape as the reinforcing member 3 was covered with an electric insulator, and a heater for heating was wound. With this heater for heating, the connecting portion was heated to 200 ° C. in the atmosphere and held for 10 minutes to melt the brazing material, and then cooled to room temperature to be solidified.

Then, in the same manner as in Example 1, Ic (4.2
As a result of measuring (K, 2T), the connection area is not included (Bi
-21212) Ic of oxide superconducting wire 2 (= 100 A)
Only 50A, which is half the value of, was obtained.

Example 2 FIG. 2 is a schematic diagram showing another example of the structure for carrying out the method of the present invention, the basic structure of which is similar to that of FIG. 1, and the corresponding parts are the same. The duplicate description is avoided by adding the reference numeral of.

The superconducting wire rods 2 were connected to each other in the order shown below according to the configuration shown in FIG. As shown in FIG. 2, a 50-core (Bi-2212) oxide superconductor 1 (thickness: 10 μm) which has already been heat-treated and crystallized.
m) and a flat oxide superconducting wire 2 (thickness: 500 μm (0.5 mm), width: 2) composed of a sheath material 6 made of sterling silver.
mm) were prepared, and superconducting connection was performed in the same manner as in Example 1.

Then, in the same manner as in Example 1, Ic (4.2
As a result of measuring K, 2T), Ic (= 7) of 50-core (Bi-2212) oxide superconducting wire 2 which does not include a connection region.
The same value as 0A) was obtained.

Example 3 In the same manner as in Example 1, a tape-shaped oxide superconducting wire comprising a single-core (Bi-2212) oxide superconductor 1 (thickness: 70 μm) and a sheath material 6 made of pure silver. 2 (thickness: 20
0 μm, width 8: mm), and prepare Pb-
A brazing material 4 made of an Sn-In alloy was arranged and the connecting portion was covered with a silver tape which was the reinforcing body 3. At this time, the length of the superconducting connection portion was 5 cm.

After that, superconducting connection was performed in the same manner as in Example 1. Then, in the same manner as in Example 1, Ic (4.2
As a result of measuring (K, 2T), the connection area is not included (Bi
-21212) Ic of oxide superconducting wire 2 (= 100 A)
A value of Ic (= 120 A) of more than 1.0 was obtained, which was further improved as compared with Example 1.

Example 4 FIG. 3 is a schematic diagram showing still another example of the structure for carrying out the method of the present invention. In this figure, the oxide superconducting wire 2 and the metal-based superconductor 8 are superconductingly connected. The same reference numerals are attached to the portions corresponding to those in FIGS. In the order shown below, the superconducting connection between the oxide superconducting wire 2 and the metal superconductor 7 was performed according to the configuration shown in FIG.

Multi-core (Nb-Ti) metal-based superconducting wire 8
The superconducting filaments embedded in the are exposed in advance to increase the connection area, and a brazing material made of a Bi—Sn alloy is previously coated on this portion, and the metallic superconducting wire 8 and the single core (Bi-2212 ) A brazing material 4 made of a Bi—Sn alloy was arranged in a portion to which the oxide superconducting wire 2 was connected, and the connecting portion was covered with a silver tape as the reinforcing body 3. At this time, the length of the superconducting connection portion was 5 cm. However, in the oxide superconducting wire 2, it is difficult to expose the oxide superconductor 1 from the viewpoint of insufficient strength.
Only the superconducting filament of the metal-based superconducting wire 8 was exposed.

After that, superconducting connection was performed in the same manner as in Example 1. Then, in the same manner as in Example 1, Ic (4.2
As a result of measuring K, 2T), Ic (= 100) of the single-core (Bi-2212) oxide superconducting wire 2 and the multi-core (Nb-Ti) metal-based superconducting wire 7 which do not include a connection region.
The same value as in A) was obtained.

Comparative Example 2 In the same manner as in Example 4, a superconducting filament embedded in a multifilamentary (Nb-Ti) metal-based superconducting wire 8 was prepared.
A brazing material made of a Pb-Sn alloy is coated in advance, and the metal-based superconducting wire 8 and a single core (Bi-
2212) In the region where the oxide superconducting wire 2 is connected, P
A brazing material 4 made of a b-Sn alloy was arranged, and the connection area was covered with a silver tape which was the reinforcing body 3. At this time, the length of the superconducting connection region was 4 cm.

After that, superconducting connection was performed in the same manner as in Example 1. Then, in the same manner as in Example 1, Ic (4.2
As a result of measuring K, 2T), Ic (= 100) of the single-core (Bi-2212) oxide superconducting wire 2 and the multi-core (Nb-Ti) metal-based superconducting wire 7 which do not include a connection region.
Only half the value of A) was obtained.

Example 5 In the same manner as in Example 4, the superconducting filament embedded in the multifilamentary (Nb-Ti) metal superconducting wire 8 was
A brazing material made of a Bi-Pb-In alloy is coated in advance, and a region where the metal-based superconducting wire 8 and the single-core (Bi-2212) oxide superconducting wire 2 are connected to each other is provided in the Bi-Sn-In alloy. The brazing material 4 consisting of was placed and the connecting portion was covered with the silver tape which was the reinforcing body 3. At this time, the length of the superconducting connection region was 5 cm.

After that, superconducting connection was performed in the same manner as in Example 1. Then, in the same manner as in Example 1, Ic (4.2
As a result of measuring K, 2T), Ic (= 100) of the single-core (Bi-2212) oxide superconducting wire 2 and the multi-core (Nb-Ti) metal-based superconducting wire 8 which do not include a connection region.
A value of Ic (= 120 A) exceeding A) was obtained, which was a further improvement over Example 4.

Example 6 In Example 5, multicore (Nb-Ti) metal-based superconductivity
Wire 8 is made of multi-core (Nb _Three Sn) Placed on metallic superconducting wire 8
Replacement, superconducting filament embedded in it
In addition, a brazing material composed of Bi-Nb-In alloy was previously
The metal-based superconducting wire 8 and the single core
(Bi-2212) oxide superconducting wire 2
The brazing material 4 made of Bi-Nb-In alloy is arranged in the portion
Then, the connection portion was covered with a silver tape which was the reinforcing body 3. this
At this time, the length of the superconducting connection portion was 5 cm.

After that, superconducting connection was performed in the same manner as in Example 1. Then, in the same manner as in Example 1, Ic (4.2
As a result of measuring K, 2T), Ic (= 100) of the single-core (Bi-2212) oxide superconducting wire 2 and the multi-core (Nb-Ti) metal-based superconducting wire 8 which do not include a connection region.
Values of Ic (= 120 A) above A) were obtained.

[0050]

As described above, according to the present invention, when superconducting oxide superconducting wires or between an oxide superconducting wire and a metal superconducting wire, the critical current is equal to or higher than the critical current of each connected superconducting wire. Superconducting connections with current have become possible. Therefore, by applying the present invention, it is possible to operate in the persistent current mode even in the superconducting coil in which the oxide superconducting wire rods or the oxide superconducting wire rods and the metal-based superconducting wire rods are superconductingly connected, and the superconducting magnet for generating a super strong magnetic field It is also extremely advantageous in manufacturing.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an example of the configuration when carrying out the method of the present invention.

FIG. 2 is a schematic view showing another example of the structure when carrying out the method of the present invention.

FIG. 3 is a schematic view showing still another configuration example when carrying out the method of the present invention.

[Explanation of symbols]

 1 Oxide Superconductor 2 Oxide Superconducting Wire 3 Reinforcing Material 4 Brazing Material 5 Insulation Cover 5 6 Sheath Material 7 Nozzle 8 Metallic Superconducting Wire

Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI Technical display location C04B 37/02 ZAA C04B 37/02 ZAAB H01F 6/06 ZAA H01R 43/02 ZAAA H01R 43/02 ZAA H01F 5 / 08 ZAAE

Claims (3)

[Claims] 1. When a Bi-based oxide superconducting wire and a metal-based superconducting wire or Bi-based oxide superconducting wires are superconductingly connected to each other, a brazing material containing Bi is present in the connection region, and the brazing material is non-oxidized. A superconducting connection method for a superconducting wire, which comprises melting the material in a strong atmosphere and then solidifying it to connect the superconducting wire to be connected. 2. The brazing material comprises Sn, Nb,
The method according to claim 1, which is an alloy containing any one or more of Pb. 3. The method according to claim 1, wherein the brazing material is an alloy further containing In.