JP2805010B2 - Method for forming electrode of oxide superconductor - Google Patents

Description

The present invention relates to a method for forming an electrode on an oxide superconductor.

[Prior Art] When an electrode is formed on a superconductor and power is supplied to the superconductor through the electrode, if the resistance of the junction between the electrode and the superconductor is large, loss occurs at the junction due to energization. Heat may be generated and superconductivity may be lost. In general, it is not preferable that the joint between the electrode and the superconductor has nonlinear characteristics.

Therefore, when an electrode is formed on a superconductor, an ohmic contact is formed at the junction between the electrode and the superconductor to make the voltage-current characteristics of the junction linear and to reduce the resistance as much as possible. Need to be smaller.

Recently, YBa ₂ Cu ₃ O _7-δ (abbreviated as YBCO), Bi
An oxide superconductor such as (Pb) SrCaCuO has attracted attention as a material exhibiting superconductivity at high temperatures. Currently, three methods have been attempted as a method of forming an electrode on this type of oxide superconductor.

The first method is a method using a silver paste. In this method, an electrode is formed at a predetermined position with a silver paste in a step in which the oxide superconductor is pre-baked, and then the oxide superconductor is applied in an oxygen atmosphere. The electrode is fired when firing.

The second method is a method of forming an electrode from a thin film of Ag, Pt or Au. In this method, Ag, Pt or Au is deposited on the electrode forming surface of the oxide superconductor to form a thin film of any of these, and then the oxide superconductor on which the thin film is formed is heat-treated in an oxygen atmosphere. .

The third method is a method of directly soldering a lead wire to an oxide superconductor while applying ultrasonic waves.

[Problems to be Solved by the Invention] According to the first method, an ohmic contact can be formed at a junction between the electrode and the oxide superconductor to reduce the resistance of the contact.

However, since this method requires a step of attaching a silver paste to the oxide superconductor and a step of firing at a high temperature, there is a problem that the number of steps is increased and workability is poor. In addition, since it is difficult to form various patterns with high precision by using silver paste, the first method involves forming electrodes by controlling the shape and dimensions accurately and forming electrodes in minute parts. There is also a problem that it is difficult.

Next, the second method has a problem that the cost required for forming an electrode is increased because an expensive apparatus is required for performing cumbersome vapor deposition and many steps are required. Further, since only a planar electrode can be formed by this method, it is necessary to bond a lead wire to the electrode when connecting the electrode to another portion, which is troublesome.

Further, the third method (ultrasonic soldering method) can be performed relatively easily, but this method has a problem that the resistance value of the joint between the electrode and the oxide superconductor cannot be sufficiently reduced. is there. The resistance of the junction between the electrode and the superconductor is preferably at least 10 ⁻⁵ Ωcm ² or less, but the ultrasonic soldering method can usually reduce the resistance only to about 10 ⁻¹ Ωcm ² .

An object of the present invention is to provide a method for forming an electrode of an oxide superconductor, which can easily and accurately form an electrode on the oxide superconductor.

It is another object of the present invention to provide a method for forming an electrode of an oxide superconductor, which can easily form both a planar electrode and a linear electrode.

[Means for Solving the Problems] According to the method of the present invention, a contact element for power application facing the oxide superconductor is provided, and a normal conductor is provided between the power application contact and the oxide superconductor. A pulse current is applied to the contact portion between the electrode material and the oxide superconductor through the power supply contact while the contact for voltage application is pressed to the oxide superconductor side while sandwiching the electrode material made of I do. This causes instantaneous heat generation at the contact portion to join the electrode material to the oxide superconductor.

Depending on the oxide superconductor, an ohmic contact may not be formed at the junction between the electrode and the oxide superconductor when the electrodes are joined as described above.

In this case, the heat treatment is performed by placing the oxide superconductor to which the electrodes are joined by the above method in an oxygen atmosphere.

The power application contact may be any as long as it can conduct electricity to the contact portion between the electrode material and the oxide superconductor in a state where the electrode is pressed against the oxide superconductor. A plurality of contacts are provided on the same surface side of the oxide superconductor so as to form a plurality of pairs, and the pair of power application contacts forming the pair are opposed to the oxide superconductor. An electrode material is sandwiched between the pair of charging contacts and the oxide superconductor, a voltage is applied between the pair of charging contacts, and one of the charging contacts → Electrode material →
Contact part between electrode material and oxide superconductor → oxide superconductor → contact part between oxide superconductor and electrode material → path of the other power contact and one power contact → Current flows through the path of the electrode material → the other power application contact.

Also, the pair of power application contacts are opposed to each other with the oxide superconductor in between, and in the path of one power application contact → electrode material → oxide superconductor → the other power application contact A current may flow.

The electrode material is preferably formed of any of silver, gold and silver.

[Operation] As described above, the electrode material made of a normal conductor is sandwiched between the contact for electricity application and the oxide superconductor, and the contact for electricity application is pressed toward the oxide superconductor. In this state, a pulse current is applied to the contact portion between the electrode material and the oxide superconductor through the power application contact, and heat is generated in the vicinity of the contact portion between the electrode material and the oxide superconductor by the current application. In addition, the electrode material can be bonded to the oxide superconductor. In this case, energization may be performed by applying a pulsed voltage to the contact for power application, and the joining of the electrode materials is completed in a very short time.

When energization is performed for a long time when energizing the contact portion between the electrode material and the oxide superconductor and joining them, if the energization is performed for a long time, the superconductor is heated more than necessary, and the The contact portion becomes a normal conductor and loses its function as a superconductor.
In the present invention, since the current flowing through the contact portion between the electrode material and the oxide superconductor is made into a pulse current to instantaneously generate heat at the contact portion, the joining is performed without impairing the function of the oxide superconductor. It can be performed. Further, when the current flowing through the contact portion between the electrode material and the oxide superconductor has a pulse waveform as in the present invention, it is possible to prevent the electrode material from being heated and melted more than necessary and lost.

The state of the joint between the electrode material and the oxide superconductor is not clear, but the electrode material and the oxide superconductor are partially sintered at the interface, or a part of the electrode material is Is presumed to be in a state of melting or softening and biting into fine pores distributed on the surface of the oxide superconductor.

Depending on the oxide superconductor, a low-resistance ohmic contact may not be formed at the junction between the electrode material and the oxide superconductor only by energization.
In this case, an ohmic contact can be formed by arranging the oxide superconductor to which the electrode material is bonded in an oxygen atmosphere and performing heat treatment. An example of such an oxide superconductor requiring heat treatment in an oxygen atmosphere is YBCO.

Ohmic contact cannot be obtained at the stage when the electrode material is joined to the oxide superconductor by energization because oxygen is lost from the crystal when the oxide superconductor is heated and the electrode material and the oxide superconductor are lost. It is considered that the crystal structure of the oxide superconductor changes from an orthorhombic system to a tetragonal system near the junction with the conductor. Even if the ohmic contact cannot be obtained at the stage of joining the electrode materials, if heat treatment is performed in an oxygen atmosphere thereafter, a low-resistance ohmic contact can be obtained because oxygen is absorbed in the oxide superconductor crystal. It is presumed that the crystal structure near the junction between the electrode material and the oxide superconductor is changed from a tetragonal system to an orthorhombic system.

Embodiment An embodiment of the present invention will be described below with reference to the accompanying drawings.

The method according to the present invention can be carried out using a known spot welding apparatus or an apparatus having the same principle as the apparatus.

FIG. 1 schematically shows the structure of the apparatus used in the embodiment of the present invention. In this example, a plate-shaped oxide superconductor 2 is arranged on a base plate 1, and a pair of power-supply members formed in a rod shape on an electrode forming surface (upper surface in this example) 2a of the oxide superconductor. The contacts 3 and 4 are arranged in parallel and face each other. The power application contacts 3 and 4 are arranged at a predetermined distance d in a direction parallel to the oxide superconductor 2 and the electrode forming surface 2a so as to be movable in a direction perpendicular to the electrode forming surface 2a. Keep it.

The power application contacts 3 and 4 are connected to the spot welding power supply 7 through the wirings 5 and 6, and a pulse-like voltage as shown in FIG. To get.

In forming the electrodes, first, an electrode material 8 made of silver, gold, platinum, or the like is sandwiched between the tips of the power application contacts 3, 4 and the oxide superconductor 2, and is not shown. The power application contacts 3 and 4 are pressed against the oxide superconductor 2 by a pressing device.
In this state, a pulsed voltage is applied from the power supply 7 to the power application contacts 3 and 4, and the power supply 7 → the contact 3 → the electrode material 8 is applied.
A current flows through the path of the oxide superconductor 2 → the electrode material 8 → the contact 4 → the power supply 7 and the power supply 7 → the contact 3 → the electrode material 8 → the contact 4 → the power supply 7. As a result, heat is generated in the vicinity of the contact portion between the electrode material 8 and the oxide superconductor 2, and the electrode material 8 is joined to the oxide superconductor 2.

In an embodiment, the critical temperature T _C as the oxide superconductor 2 is 93
Using YBa ₂ Cu ₃ O _7-δ of K, diameter 50μ as electrode material 8
m silver wire was used. The contacts 3 and 4 for power application are made of a copper-chromium alloy, and the distance d between the contacts 3 and 4 is 0.2.
5 mm. While pressing the contacts 3 and 4 toward the oxide superconductor ^{2 with} a pressing force of 70 kg / cm ^2, a peak value of 1 V and a pulse width t were applied between the welding power supply 7 and the contact 3 and 4 for power application. When a pulse voltage of 30 msec was applied, the electrode material 8 could be joined to the oxide superconductor 2. The area of the joint is 2 × 10 ^-4
It was cm ^2.

For the sample in which the electrode material 8 was bonded to the oxide superconductor 2 in this manner, the electrode material 8 and the oxide superconductor 2
When the voltage-current characteristics of the junction between the two were measured, the results shown in FIG. 4 were obtained. In FIG. 4, a curve a shows the characteristic at room temperature, and a curve b shows the characteristic at 80 K lower than the critical temperature T _C (= 93 K) of the oxide superconductor 2. These results show that the voltage-current characteristics of the junction between the electrode material 8 and the oxide superconductor 2 are non-linear, and that no ohmic contact is formed.

FIG. 5 shows the result of measuring the temperature change of the contact resistance at the junction between the electrode material and the oxide superconductor with respect to the above sample, and the contact resistance decreases as the temperature increases. I have. This indicates that the junction exhibits the behavior of the semiconductor.

Therefore, the electrode material 8 was heat-treated by placing the bonded oxide superconductor 2 in a furnace in an oxygen atmosphere. In the experiment, samples A, B and C of YBCO to which a silver wire was bonded by the above method were prepared.

Next, Samples A, B and C were each heat-treated for 1 hour in a 100% oxygen atmosphere at different heat treatment temperatures. The pattern of the temperature change in this heat treatment is as shown in FIG. First, the temperature is raised from room temperature to a predetermined heat treatment temperature X ° C at a rate of 6 ° C per minute. After maintaining the temperature X ° C. for one hour, the temperature is lowered to room temperature at a rate of 1 ° C. per minute, and the heat treatment is completed.

In the experiment, the heat treatment temperature X of samples A, B and C was set to 400 ° C., 500 ° C. and 600 ° C., respectively, and the state of the junction was examined. As a result, no ohmic contact was formed by the heat treatment at 400 ° C. It was clarified that a complete ohmic contact was formed by the heat treatment at 600C and 600C.

For the samples A to C after the heat treatment, the contact resistance of each joint was measured by a well-known three-terminal method, and the temperature change was obtained. As a result, the results shown in FIG. 7 were obtained. That is, when the heat treatment temperature is set to 400 ° C., a characteristic (behavior of the semiconductor) is shown in a curve a of FIG. 7 in which the contact resistance of the junction decreases with an increase in temperature in a region of 100 K or more. This means that no ohmic contact has yet been formed at the junction. Curves b and c show the case where the heat treatment temperature is 500 ° C. and 600 ° C., respectively, and in these cases, the characteristic that the contact resistance of the joint increases in all regions with the rise in temperature is obtained. These characteristics indicate that an ohmic contact is formed at the junction.

As an example, FIG. 8 shows the results of measuring the voltage-current characteristics of the junction of Sample B that was heat-treated at 500 ° C. In FIG. 8, a straight line a shows the characteristic at room temperature, and a straight line b shows the characteristic at 80K. In either case, the current-convection pressure characteristics of the junction are linear characteristics,
This means that an ohmic contact is formed at the junction. The contact resistance at 80K is 1.2 × 10
^-7 Ωcm ² .

The following table summarizes the relationship between the heat treatment temperature, the state of the joint, and the magnitude of the contact resistance for the samples A to C.

In this table, "non ohmic" indicates that no ohmic contact is formed at the junction between the electrode material and the oxide superconductor, and "ohmic" indicates that an ohmic contact is formed at the same junction. .

From the above table, it is possible to form a ohmic contact at the joint by applying a heat treatment at a temperature of 500 ° C. or more in an oxygen atmosphere after contacting the silver wire with the YBCO and generating heat by energization. It can be seen that the resistance can be reduced to 10 ⁻⁶ Ωcm ² or less.

From the above, it can be seen that even when an ohmic contact is not formed at the stage when the electrode material is bonded to the oxide superconductor, the ohmic contact can be formed by performing the heat treatment in an oxygen atmosphere. This is because the oxide superconductor in the vicinity of the junction where oxygen is lost due to heat generated at the time of joining and transition from orthorhombic to tetragonal, absorbs oxygen by heat treatment in an oxygen atmosphere, and becomes orthorhombic again. This is probably due to dislocation to the crystal. It is considered that the reason why the heat treatment at 500 ° C. or higher is required in the case of YBCO is that YBCO easily absorbs oxygen at 500 ° C. or higher.

In the above embodiment, the pair of power application contacts 3 and 4 are arranged side by side on the same surface of the oxide superconductor 2, and an electrode material is placed between these contacts and the oxide superconductor. As shown in FIG. 3, at least one pair of power application contacts 3 and 4 are arranged to face each other with the oxide superconductor 2 interposed therebetween, as shown in FIG. , 4 may be interposed between the oxide superconductor 2 and the electrode material 8. Also in this case, by applying a pulse-like voltage between the contacts 3 and 4, heat is generated at the contact portion between the electrode material 8 and the oxide superconductor 2 to join them.

In the above embodiment, a linear electrode material is used, but a planar electrode may be formed using a foil electrode material.

In each of the above embodiments, each power-applying contact is formed in a rod shape, but the shape of the power-applying contact can be appropriately changed according to the shape of the electrode material.

When a planar electrode material is used, a plurality of pairs of power application contacts may be provided as needed to increase the number of junctions.

As shown in FIG. 3, when the pair of power application contacts are arranged on both sides of the oxide superconductor, the contact 4 that directly contacts the oxide superconductor can be formed in a plate shape. .

When the contacts 3 and 4 are arranged as shown in FIG. 1, a pulse voltage may be applied between the base plate 1 and the contact 3 and between the base plate 1 and the contact 4 respectively. Good.

[Effects of the Invention] As described above, according to the present invention, an electrode material made of a normal conductor is sandwiched between a charging contact and an oxide superconductor to oxidize the charging contact. When a pulse current is applied to the contact portion between the electrode material and the oxide superconductor through the power application contact in a state where the pressure is applied to the material superconductor side, heat is generated only for a short time at the contact portion, and the electrode material is heated. Can be easily bonded to the oxide superconductor without damaging the superconductivity with the oxide superconductor and without damaging the electrode material. I can do it.

In particular, according to the second aspect of the present invention, a low-resistance ohmic contact can be formed even with an oxide superconductor that easily transitions from an orthorhombic system to a tetragonal system when heat is applied, such as YBCO. There is an advantage that an electrode can be formed.

Further, in the present invention, since both a linear electrode and a planar electrode can be formed, an electrode suitable for the intended use can be formed. Especially when it is necessary to wire the oxide superconductor to other parts, by using a linear electrode material, the electrode itself can be used as wiring,
The man-hour can be reduced.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing the configuration of an apparatus for carrying out the method of the present invention, FIG. 2 is a wave diagram showing a waveform of a pulse voltage applied between contacts of the apparatus of FIG. FIG. 3 is a configuration diagram showing another configuration example of an apparatus for carrying out the method of the present invention, and FIGS. 4 and 5 respectively show a voltage vs. voltage of a junction at the stage when the electrode material is joined to the oxide superconductor. FIG. 6 is a diagram showing a temperature characteristic of a current characteristic and a contact resistance, FIG. 6 is a diagram showing a temperature change pattern of a heat treatment in an embodiment of the present invention, and FIG. Diagram, 8th
The figure is a diagram showing voltage-current characteristics of the connection part after the heat treatment. DESCRIPTION OF SYMBOLS 1 ... Base plate, 2 ... Oxide superconductor, 3, 4 ... Contact for power application, 7 ... Power supply device for spot welding, 8 ... Electrode material.

──────────────────────────────────────────────────続 き Continued on the front page (72) Kuraichi Ogawa 4-3-1-16 Maikodai, Tarumizu-ku, Kobe-shi, Hyogo (72) Ryuharu Aoyama 2-1-1 Tagawa, Yodogawa-ku, Osaka-shi, Osaka Co., Ltd. (56) References JP-A-58-87884 (JP, A) JP-A-60-166276 (JP, A) JP-A-62-202876 (JP, A) (58) Fields studied (Int. . ^6, DB name) H01L 39/00 ZAA H01L 39/22 - 39/24 ZAA C04B 37/02 H01L 21/60

Claims (5)

(57) [Claims] 1. A method for forming an electrode on an oxide superconductor, comprising the steps of: providing a contact for power application facing the oxide superconductor, and providing a contact between the power application contact and the oxide superconductor. An electrode material made of a normal conductor is sandwiched between the electrodes, and the contact between the electrode material and the oxide superconductor is passed through the contact for applying power while the contact for applying power is pressed toward the oxide superconductor. A method of forming an electrode of an oxide superconductor, comprising: applying a pulse current to a portion to generate heat near the contact portion to join the electrode material to the oxide superconductor. 2. A method for forming an electrode on an oxide superconductor, comprising: providing a power application contact facing the oxide superconductor; and providing a contact between the power application contact and the oxide superconductor. An electrode material made of a normal conductor is sandwiched, and the electrode material is brought into contact with the oxide superconductor through the power application contact with the power application contact pressed against the oxide superconductor. The electrode material is joined to the oxide superconductor by generating heat near the contact portion by applying a pulse current to the portion, and then the oxide superconductor to which the electrode material is joined is heat-treated in an oxygen atmosphere. A method for forming an electrode of an oxide superconductor, comprising: 3. A plurality of pairs of power application contacts are provided so as to form a pair, and the pair of power application contacts are arranged on the same surface of the oxide superconductor. 3. The method for forming an electrode of an oxide superconductor according to claim 1, wherein the electrode material is sandwiched between an electrical contact and an oxide superconductor. 4. A plurality of power application contacts are provided so as to form a plurality of pairs, and the pair of power application contacts are opposed to each other with the oxide superconductor therebetween. 3. The method for forming an electrode of an oxide superconductor according to claim 1, wherein an electrode material is sandwiched between the contact and the oxide superconductor. 5. The method for forming an electrode of an oxide superconductor according to claim 1, wherein said electrode material is made of silver, gold or platinum.