1. Field of the Invention
The present invention relates to a method of manufacturing an air-tight ceramic container and a method of manufacturing a vacuum interrupter using the air-tight ceramic container.
2. Description of the Prior Art
A ceramic is an insulative material having a good heat-resistance and hence is used as a material for various electrical parts. On example is an air-tight container used in an electrical part such as a vacuum interrupter. Such an air-tight container is used in such a manner that its interior is maintained in a vacuum state or filled with an inert gas. Therefore, in order to maintain such an internal atmosphere, an air-tight property must be strictly maintained.
As shown in FIG. 1A, a conventional airtight ceramic container has a structure in which opening end faces of ceramic tubular member 1 are shielded by metal cover members 2. In manufacturing such an air-tight ceramic container, the opening end faces of ceramic tubular member 1 is metallized, then metal cover members 2 is bonded thereon by brazing. In this conventional method, since a coefficient of thermal expansion of ceramic tubular member 1 differs from that of each metal cover member 2, a thermal stress is generated at a bonding portion when the portion is cooled after being heated upon brazing. When the thermal stress is large, cracks are generated in ceramic tubular member 1, and no sufficient air-tight property can be obtained. Therefore, the following techniques have been proposed to reduce the thermal stress and prevent generation of cracks.
According to a first technique, a metal having a small coefficient of thermal expansion such as Mo or W or an alloy having a small coefficient of thermal expansion such as invar or Kovar is used for the metal cover member.
According to a second technique, an end portion of each metal cover member 2 is bent as shown in FIG. 1A, and its end face is bonded to the end face of ceramic tubular member 1 (endface-bonding), thereby reducing a bonding area. Since the magnitude of thermal stress generated at the bonding portion is proportional to a bonding area between the two members, this endface-bonding contributes to reduce the thermal stress. In order to obtain a sufficient bonding strength and air-tight property at the bonding portion in the endface-bonding, brazing filler metal layer 8 must be spread like an unfolded fan from the end portion of metal cover member 2 toward the end face of ceramic tubular member 1, as shown in FIG. 1B.
Metallization used in manufacturing the above air-tight ceramic container will be described below. Conventional metallizing methods are as follows
(1) A powder mainly consisting of Mo or W is applied on a surface of a ceramic member and heated up to 1,400.degree.0 to 1,700.degree. C. in a reducing atmosphere to react it with the ceramic material, thereby performing metallization. Ni or the like is plated on a metallized layer as needed. In this method, a treatment must be performed at a very high temperature for metallizing, resulting in complex manufacturing steps.
(2) Au or Pt is placed on a surface of ceramic member and heated while a pressure is applied thereto, thereby performing metallization. In this method, since an expensive noble metal is used, manufacturing cost is increased. In addition, since a high pressure is required to increase a contact property, it is not preferable to apply this method to electronic parts not allowing deformation.
(3) An active metal such as Ti or Zr and a transition metal such as Ni or Cu are placed on a ceramic base material and heated at a temperature higher than the melting point of the alloy of these metals, thereby performing metallization (Japanese Patent Disclosure (Kokai) No. 56-163093). This method utilizes the fact that when an active metal such as Ti or Zr is used to form an alloy with a transition metal such as Cu or Ni, these alloys have a melting point lower by several hundreds .degree.C. than that of either of the metals in their eutectic compositions. In this method, since the active metal wets the ceramic material, metallization can be performed substantially without pressurization. In addition, it is possible to metallize the surface of ceramic member with a strong bond by an effect of the active metal.
In order to manufacture an air-tight ceramic container using any of above metallizing methods (1) to (3), the end faces of a ceramic tubular member are metallized and then metal cover members are brazed to the tubular member. That is, since metallizing of ceramic tubular member, and brazing of the metal cover members must be independently performed, manufacturing steps are undesirably complicated. Therefore, a method has been proposed in which metal cover members are brazed to the end faces of a ceramic tubular member without independent metallizing step, thereby manufacturing an air-tight ceramic container.
The following one-step bonding method (Japanese Patent Disclosure (Kokai) No. 59-32628) has been proposed as a method of bonding a ceramic member with a metal member without independent metallizing step. In this method, a brazing filler metal having a low melting point (especially an Ag-brazing filler metal) which contains Ti and/or Zr as the active metal is used. Alternatively, a foil of the above active metal and the Ag-brazing filler metal are stacked, and the stacked body is inserted and heated between the ceramic and metal members. Since this one-step bonding method does not require independent metallizing step, manufacturing steps can be simplified.
However, in the above one-step bonding method, a preferred bonding structure shown in FIG. 1B cannot be obtained. When a bonding area between the ceramic and metal members is sufficiently large, a substantially sufficient bonding property can be obtained by the one-step bonding method. However, if the bonding area is small, a sufficient bonding property with a preferred bonding structure of FIG. 1B is not attained. Therefore, this method is not suitable to manufacture the air-tight ceramic container described above. That is, even when brazing metal foil 3 larger than the end face of metal cover member 2 is used for brazing as shown in FIG. 1C, a filler metal layer is formed only immediately below the end face of metal cover member 2, as shown in FIG. 1D, because wettability of the ceramic surface with the melted filler metal is not sufficient. As a result, a clearance is easily produced in a bonding portion, and metal cover member 2 is removed even with a small external force.
A conventional vacuum interrupter using the above air-tight ceramic container will be described below.
FIG. 2 shows a structure of the conventional vacuum interrupter. In FIG. 2, reference numeral 1 denotes a ceramic tubular member. Metal cover members 2a and 2b are air-tightly bonded to both opening end faces of ceramic tubular member 1 by brazing with Ag-filler metal layer 8a and 8b, thereby constituting a vacuum container whose interior is maintained in a vacuum. In this vacuum container, fixed and movable conductor rods 5a and 5b are provided to oppose each other and extend through metal cover members 2a and 2b, respectively. As shown in FIG. 2, fixed conductor rod 5a is fixed to metal cover member 2a, and movable conductor rod 5b is movable along its axial direction. A pair of contact members 3a and 3b are placed at opposing end portions of conductor rods 5a and 5b, respectively. Contact member 3a is a fixed contact member, and contact member 3b is a movable contact member. Contact member 3b is brazed to movable conductor rod 5b either directly or through an electrode (not shown). The other end of fixed conductor rod 5a constitutes fixed terminal 4a, and the other end of movable conductor rod 5b constitutes movable terminal 4b. Therefore, when movable conductor rod 5b moves in the axial direction, contact members 3a and 3b are opened/closed. Bellows 7 is mounted on movable conductor rod 5b so that movable member 5b can move in the axial direction while a vacuum air-tight state in the container is maintained by bellows 7. A metal arc-shield (not shown) is provided on bellows 7 to prevent bellows 7 from being covered with arc vapor. In addition, metal arc-shield 6 is provided to cover contact members 3a and 3b, thereby preventing ceramic tubular member 1 from being covered with the arc vapor. Therefore, an evaporated contact member material is prevented from being adhered on an inner surface of ceramic tubular member 1 to cause short-circuiting.
In the above vacuum interrupter, the arc-shield 6 must be fixed at a predetermined position in the vacuum container. For this purpose, projection 1' is formed in ceramic tubular member 1 as shown in FIG. 2. Projection 1' is formed to engage with recess 6' provided to arc-shield 6, thereby preventing removal or movement of arc-shield 6. Alternately, a recess may be formed in ceramic tubular member 1 and engaged with a projection formed on arc-shield 6. This fixing method does not require metallization when arc-shield 6 is mounted on ceramic tubular member 1 and therefore is economically advantageous. However, since a gap is inevitably produced between ceramic tubular member 1 and arc-shield 6, arc-shield 6 is vibrated or moved when the vacuum valve is vibrated. Furthermore, when a trouble occurs in recess 6', arc-shield 6 may be removed from a predetermined mounting portion, thereby degrading withstand voltage and blocking characteristics.
As a second method of fixing arc-shield 6 to ceramic tubular member 1, an inner surface of ceramic tubular member 1 is metallized and then arc-shield 6 is brazed to the inner surface. In this method, any of above methods (1) to (3) can be used as the metallizing method. According to this method, arc-shield 6 can be prevented from being removed or moved. However, with any of methods (1) to (3), problems caused by metallization occur as described above. That is, in method (1), complex steps such as a high-temperature treatment are required. In method (2), not only an economical problem is posed but also productivity is degraded because a tool for obtaining a sufficient pressure occupies a predetermined space in a brazing furnace. In method (3), it is difficult to obtain a desired bonding strength.
FIG. 3 shows a third method of mounting arc-shield 6 on ceramic tubular member 1. That is, ceramic tubular member 1 in FIG. 2 is divided into two ceramic members 1a and 1b, and metallization is performed to opposing end faces 9a and 9b. Then, a flange formed on arc-shield 6 is inserted and air-tightly shielded between end faces 9a and 9b. Also in this case, any of above methods (1) to (3) is used as the metallizing method. Therefore, since metallization is used, the same drawbacks as in the above two methods are present. Furthermore, this method is economically disadvantageous because the number of metallizing portions is increased. In addition, since the number of portions to be air-tightly shielded is increased, this method is disadvantageous in terms of reliability for maintaining the air-tight property.