Abstract:
A process for producing a ceramic member for bonding, which comprises: a first step of preparing a lower layer paste with the use of a first mixture comprising nickel, tungsten and molybdenum, applying the lower layer paste to a surface of a ceramic base which is a sintered ceramic, and drying the resultant coating layer to form a lower layer after the applying; a second step of preparing an upper layer paste with the use of a second mixture comprising one of nickel and nickel oxide and at least one of copper, copper oxide, manganese and manganese oxide, applying the upper layer paste to the lower layer, and drying the resultant coating layer to form an upper layer after the applying; and a third step of heating the lower layer and the upper layer to bake the lower layer and the upper layer.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a member required to have bonding strength, airtightness and other properties as in the case of bonding a metal to a ceramic. More particularly, the invention relates to a process for producing a ceramic member for bonding, a ceramic member for bonding, a bonded object, a vacuum switch, and a vacuum vessel. 
     BACKGROUND OF THE INVENTION 
     The molybdenum-manganese method (Mo—Mn method; Telefunken method) has conventionally been known as a method for metallizing a surface of a ceramic base. 
     This Mo—Mn method comprises applying a metallizing ink prepared by adding a bonding aid such as Mn powder, Ti powder and glass ingredient (SiO 2 ) to a powder of high-melting metal such as tungsten and molybdenum and mixing the powder mixture with an organic binder to make a paste to a ceramic base, and then baking the ink layer (baking method). 
     SUMMARY OF THE INVENTION 
     The technique of the related art described above necessitates a baking temperature as high as from 1,300 to 1,500° C. for metallization and, hence, has had a problem that the sintering cost regarding furnace structure, utilities, expendable heat-resistant materials, etc. is high. 
     Another problem is that the ceramic itself deforms in the high-temperature baking, resulting in a product which does not satisfy dimensional accuracy. 
     Although a measure in overcoming the problem described above may be to bake a metallizing ink having a conventional composition at a temperature lower than 1,300° C., this low-temperature baking poses a problem that a sufficient bonding strength cannot be obtained. 
     Furthermore, in the case where a metallic layer formed by metallization by the Mo—Mn method is to be bonded to another metallic member or the like by brazing, it is necessary to improve the wettability thereof by a brazing material so as to obtain satisfactory bonding. Although it is hence inevitable to conduct post-treatments such as nickel plating and subsequent sintering, there has been a problem that these post-treatments make the production process complicated. 
     Further, in recent years, a technique has been proposed which comprises applying a tungsten paste to a ceramic base to form a lower layer (sublayer), applying a nickel paste to the lower layer, baking the coated material, and then subjecting the material directly to brazing for the purpose of reducing the number of steps (see “Electronic Ceramics”, December 1991). However, this technique leaves something to be desired because it requires a baking temperature as high as not lower than 1,250° C., adding to the sintering cost and deteriorating the dimensional accuracy of the resulting ceramic product. 
     The invention has been worked out in order to solve the aforementioned problems. An aim of the invention is to provide a process for producing a ceramic member for bonding which enables low-temperature sintering and simplified production process and gives a high dimensional accuracy, and a ceramic member for bonding, a bonded object, a vacuum switch and a vacuum vessel produced thereby. 
     (1) In order to accomplish the aforementioned aim of the invention, the first essence of the invention lies in a process for producing a ceramic member for bonding, which comprises: 
     A first step of applying a lower layer paste prepared from a first mixture comprising nickel, tungsten and molybdenum to the surface of a ceramic base which is a sintered ceramic, and then drying the coating layer to form a lower layer; 
     A second step of applying an upper layer paste prepared from a second mixture of nickel or nickel oxide and at least one of copper, copper oxide, manganese and manganese oxide to the lower layer, and then drying the coating layer to form an upper layer; and 
     A third step of heating and baking the lower layer and the upper layer. 
     In the invention, a mechanism of bonding of a Mo—W—Ni layer which is a lower layer to a ceramic base is formed. For the aforementioned baking (i.e., sintering the lower layer and the upper layer by calcining), the addition of Nickel causes the acceleration of sintering of Molybdenum particles, enabling low temperature sintering. Further, since the lower layer paste comprises tungsten incorporated therein, the sintering rate of Molybdenum particles can be relaxed to accomplish fair bonding over a wide temperature range. 
     Moreover, in the invention, a mechanism of bonding of a Ni—(Cu, Mn) layer which is an upper layer to a Mo—W—Ni layer which is a lower layer is formed. For the aforementioned baking, the incorporation of copper or manganese in the upper layer causes the drop of melting point of nickel, making it possible to form a dense alloy layer on the lower layer. In this arrangement, fair brazing can be effected even without nickel plating and subsequent sintering. Further, since the upper layer is an alloyed layer, the excessive diffusion of nickel in the lower layer containing molybdenum can be lessened, making it possible to prevent strength drop due to oversintering of molybdenum. 
     In the invention, the aforementioned action allows omission of nickel plating and sintering therefor, which has heretofore been required, making it possible to drastically simplify the production process and hence drastically reduce the production cost. Further, the lower layer and the upper layer can be sintered at a temperature as low as not higher than 1,200° C., the sintering cost regarding furnace structure, utilities, expendable heat-resistant materials, etc. can be reduced. Moreover, the low temperature sintering makes it difficult for the ceramic itself to deform and hence makes it possible to obtain a high dimensional accuracy. 
     The lower layer paste can be prepared by mixing a first mixture containing a nickel powder, a tungsten powder and a molybdenum powder with an organic binder. The upper layer paste can be prepared by mixing a second mixture containing a nickel powder or nickel oxide powder with at least one of a copper powder, copper oxide powder, manganese powder and manganese oxide powder or a second mixture containing a nickel-copper alloy powder or nickel-manganese alloy powder with an organic binder. 
     When baking at the third step is effected in a humidifying reducing atmosphere such as H 2  and H 2 /N 2 , particularly at a temperature of from 1,080° C. to 1,200° C., the resulting product has a high bonding strength and airtightness to advantage. 
     (2) The second essence of the invention lies in the aforementioned process for producing a ceramic member for bonding, wherein the first mixture comprises nickel, tungsten and molybdenum incorporated therein in an amount of from 1% to 10% by weight, from 20% to 69% by weight and from 30% to 69% by weight, respectively. 
     In the invention, since the content of nickel in the first mixture is not lower than 1% by weight, nickel reacts with molybdenum, which is a high melting metal, to accelerate sintering of the lower layer (metallic layer). In this manner, sintering can be thoroughly effected even at low temperatures. Further, since the content of nickel in the first mixture is not greater than 10% by weight, oversintering of molybdenum can be prevented, making it possible to prevent the lack of strength of bonding between the ceramic base and the metallic layer. 
     Moreover, since the content of tungsten in the first mixture is not lower than 20% by weight, the temperature range within which a high strength metallic layer can be formed is wide. Further, since the content of tungsten in the first mixture is not greater than 69% by weight, the incorporation of molybdenum and nickel exerts an effect of preventing undersintering at low temperatures. 
     Further, since the content of molybdenum in the first mixture is not lower than 30% by weight, a firm metallic layer can be formed at low temperatures. Moreover, since the content of molybdenum in the first mixture is not greater than 69% by weight, the incorporation of tungsten and nickel can exert a good effect. 
     Examples of materials other than the metallic ingredients of the lower layer paste include organic binder. 
     (3) The third essence of the invention lies in the process for producing a ceramic member for bonding according to the first or second essence of the invention, wherein the first mixture further comprises a silicon oxide component incorporated therein in an amount of from 2% to 15% by weight. 
     In the invention, since the first mixture comprises a silicon oxide (SiO 2 ) ingredient incorporated therein in an amount of from 2% to 15% by weight, the resulting airtightness can be improved. 
     (4) The fourth essence of the invention lies in the process for producing a ceramic member for bonding according to any one of the first to third essences of the invention, wherein the second mixture comprises nickel or nickel oxide and at least one of copper, copper oxide, manganese and manganese oxide incorporated therein in an amount of from 35% to 75% by weight and from 25% to 65% by weight, respectively. 
     In the invention, since the content of nickel or nickel oxide in the second mixture is from 35% to 75% by weight, the resulting ceramic member for bonding exhibits an enhanced bonding strength and an excellent airtightness. 
     Further, since the content of at least one of copper, copper oxide, manganese and manganese oxide in the second mixture is not lower than 25% by weight, the resulting ceramic member for bonding exhibits an enhanced brazability and bonding strength. Moreover, since the content of at least one of copper, copper oxide, manganese and manganese oxide in the second mixture is not greater than 65% by weight, the lack of strength between the ceramic base and the metallic layer due to penetration of these ingredients into the metallic layer can be prevented. 
     Examples of materials other than the metal and metal oxide ingredients of the upper layer include organic binder. 
     When the content of the silicon oxide ingredient in the second mixture is from 2% to 10% by weight, the resulting ceramic member for bonding exhibits an even higher airtightness. 
     (5) The fifth essence of the invention lies in a ceramic member for bonding, comprising: 
     A metallic layer which is a lower layer comprising nickel, tungsten and molybdenum provided on the surface of a ceramic base which is a sintered ceramic; and 
     An alloy layer which is an upper layer comprising nickel and at least one of copper and manganese provided on the surface of the metallic layer with or without an interlayer interposed therebetween. 
     In the invention, since the incorporation of nickel allows the acceleration of sintering of molybdenum particles in the lower layer, the lower layer can be sintered at low temperatures. Further, the incorporation of tungsten allows the relaxation of the sintering rate of molybdenum particles, making it possible to provide an excellent bonding over a wide temperature range. 
     Moreover, the incorporation of copper and manganese in addition to nickel in the upper layer causes the drop of melting point of nickel, making it possible to form a dense alloy layer on the upper layer. In this arrangement, fair brazing can be effected without nickel plating or the like. Further, since the upper layer is an alloy layer, excessive diffusion of nickel in the lower layer, which comprises molybdenum incorporated therein, can be lessened, making it possible to prevent the drop of strength due to oversintering of molybdenum. 
     In the invention, the aforementioned action allows the omission of nickel plating and subsequent sintering, which have heretofore been required, making it possible to drastically simplify the working procedure and hence drastically reduce the production cost. Further, the composition of the invention can be sintered at low temperatures, making it possible to reduce the sintering cost regarding furnace structure, utilities, expendable heat-resistant materials, etc. Moreover, the low temperature sintering makes it difficult for the ceramic to deform, thereby giving a high dimensional accuracy. In addition, sintering can be thoroughly effected at low temperatures, a high bonding strength can be secured. 
     While the alloy layer which is an upper layer maybe formed directly on the metallic layer which is a lower layer, an interlayer having a different constitution from that of these layers may be formed interposed therebetween. 
     (6) The sixth essence of the invention lies in the ceramic member for bonding according to the fifth essence of the invention, wherein the metallic layer comprises nickel, tungsten and molybdenum in an amount of from 0.7% to 8% by weight, from 15% to 75% by weight and from 20% to 80% by weight, respectively. 
     In the invention, since the content of nickel in the metallic layer is not smaller than 0.7% by weight, sintering can be thoroughly effected at low temperatures. Further, since the content of nickel in the first mixture is not greater than 10% by weight, oversintering of molybdenum can be prevented, making it possible to prevent the lack of strength of bonding between the ceramic base and the metallic layer. 
     Further, since the content of tungsten is not lower than 15% by weight, the temperature range within which a firm metallic layer can be formed is wide. Moreover, since the content of tungsten is not greater than 75% by weight, the incorporation of molybdenum and nickel can exert a good effect. 
     Further, the metallic layer comprises molybdenum incorporated therein in an amount of from 20% to 80% by weight and thus forms a firm metallic layer. 
     (7) The seventh essence of the invention lies in the ceramic member for bonding according to the fifth or sixth essence of the invention, wherein the metallic layer further comprises a silicon oxide component incorporated therein in an amount of from 3% to 18% by weight as calculated in terms of oxide. 
     In the invention, since the metallic layer comprises a silicon oxide component incorporated therein in an amount of from 3% to 18% by weight, the adhesion between the ceramic member and the metallic layer is extremely high, enhancing airtightness. 
     (8) The eighth essence of the invention lies in the ceramic member for bonding according to any one of the fifth to seventh essences of the invention, wherein the alloy layer comprises nickel, copper and manganese incorporated therein in an amount of from 10% to 75% by weight, from 20% to 85% by weight and from 5% to 40% by weight, respectively. 
     In the invention, the alloy layer comprises nickel in an amount of from 10% to 75% by weight and thus exhibits a bonding strength with the metallic layer and a high airtightness. 
     Further, since the content of copper is not smaller than 20% by weight, the resulting alloy layer exhibits an excellent brazability and a high strength. Moreover, since the content of copper is not smaller than 85% by weight, it contributes to the enhancement of the strength of bonding of the ceramic base to the metallic layer. 
     When the aforementioned alloy layer further comprises a silicon oxide component incorporated therein in an amount of from 0.05 to 1.0 parts by weight as calculated in terms of oxide, the adhesion between the ceramic member and the metallic layer is extremely high to enhance the airtightness. 
     (9) The ninth essence of the invention lies in the ceramic member for bonding according to any one of the fifth to eighth essences of the invention, wherein the interlayer provided interposed between the metallic layer which is a lower layer and the alloy layer which is an upper layer is an interlayer formed by a nickel-molybdenum alloy. 
     The invention constitutes exemplification of the ingredients of the interlayer. This interlayer may be formed under some sintering conditions. In the case where the interlayer exists, there occurs little change in properties such as bonding strength. 
     (10) The tenth essence of the invention lies in a bonded object comprising a metallic member bonded to a ceramic member for bonding according to any one of the fifth to ninth essences of the invention with at least the metallic layer and the alloy layer provided interposed therebetween. 
     In the invention, a ceramic member for bonding and a metallic member are bonded to each other with the aforementioned metallic layer and alloy layer provided interposed therebetween. In some detail, a metallic member is bonded with, e.g., a brazing material to the alloy layer of a ceramic member for bonding prepared by forming a metallic layer on the surface of a ceramic base, and then forming an alloy layer thereon. The aforementioned interlayer may be provided interposed between the metallic layer and the alloy layer. 
     Accordingly, nickel plating (on the surface of the metallic layer) and subsequent sintering as conventionally effected are not necessary, making it possible to braze the metallic member directly to the alloy layer. This reduces the number of production steps and hence the production cost. Further, this bonded object has a high bonding strength and a high dimensional accuracy. 
     (11) The eleventh essence of the invention lies in a bonded object comprising another ceramic member for bonding bonded to a ceramic member for bonding according to any one of the fifth to ninth essences of the invention with at least the metallic layer and the alloy layer provided interposed therebetween. 
     In the invention, a ceramic member for bonding and another ceramic member for bonding are bonded to each other with the aforementioned metallic layer and alloy layer provided interposed therebetween. The aforementioned interlayer may be provided interposed between the metallic layer and the alloy layer. 
     An example of such an arrangement is a bonded object having two ceramic members for bonding comprising a metallic layer and an alloy layer formed thereon bonded to each other at the alloy layer thereof with a brazing material. 
     Accordingly, as in the tenth essence of the invention, nickel plating and subsequent sintering as conventionally effected are not necessary, making it possible to braze the metallic member directly to the alloy layer. This reduces the number of production steps and hence the production cost. Further, this bonded object has a high bonding strength and a high dimensional accuracy. 
     (12) The twelfth essence of the invention lies in a vacuum switch comprising a bonded object according to the tenth or eleventh essence of the invention. 
     The invention concerns a vacuum switch comprising the aforementioned bonded object. An example of this vacuum switch is an electric circuit switch comprising a ceramic insulating valve suitable particularly for switching of high-voltage large-current electricity. 
     (13) The thirteenth essence of the invention lies in a vacuum vessel comprising a bonded object according to the tenth or eleventh essence of the invention. 
     The invention concerns a vacuum vessel (e.g., insulating valve) used in the aforementioned vacuum switch, etc. A vacuum switch (electrical circuit switch) can be fabricated by disposing electrodes and other necessary members in this vacuum vessel. 
     In the case where there is contained the aforementioned silicon oxide component, the proportion of other ingredients of the various essences of the invention (metals and oxide thereof) may be properly adjusted within their predetermined range such that the sum of the content of all these ingredients is 100% by weight depending on the content of the silicon oxide component. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a sectional view illustrating the bonded object of Example 1 with its essential part shown broken. 
     FIG. 2 is a slant view illustrating the bonded object of Example 1. 
     FIG. 3 is a view illustrating a method for measuring the bonding strength of the bonded object of Example 1. 
     FIG. 4 is a sectional view illustrating the bonded object of Example 2 with its essential part shown broken. 
     FIG. 5 is a partially cutaway view illustrating the vacuum switch of Example 3. 
     FIG. 6 is a sectional view illustrating the vacuum switch of Example 3 with its essential part shown broken. 
     FIG. 7 is a sectional view illustrating the vacuum switch of Example 4 with its essential part shown broken. 
     FIG. 8 is a sectional view illustrating an important part of a bonded object shown in Comparative Example 1. 
     FIG. 9 is a slant view illustrating a bonded object obtained in Comparative Example 1. 
     FIG. 10 is a view illustrating a method of measuring the bonding strength of the bonded object obtained in Comparative Example 1. 
     FIG. 11 is a sectional view illustrating an important part of a bonded object shown in Comparative Example 2. 
     FIG. 12 is a partially cutaway view illustrating the vacuum switch shown in Comparative Example 3. 
     FIG. 13 is a sectional view illustrating an important part of the vacuum switch shown in Comparative Example 3. 
     FIG. 14 is a sectional view illustrating an important part of the vacuum switch shown in Comparative Example 4. 
     FIG. 15 is a sectional view illustrating an important part of a bonded object shown in Comparative Example 5. 
     FIG. 16 is a slant view illustrating a bonded object obtained in Comparative Example 5. 
     FIG. 17 is a view illustrating a method of measuring the bonding strength of the bonded object obtained in Comparative Example 5. 
     FIG. 18 is a sectional view illustrating an important part of a bonded object shown in Comparative Example 6. 
     FIG. 19 is a partially cutaway view illustrating the vacuum switch shown in Comparative Example 7. 
     FIG. 20 is a sectional view illustrating an important part of the vacuum switch shown in Comparative Example 7. 
     FIG. 21 is a sectional view illustrating an important part of the vacuum switch shown in Comparative Example 8. 
    
    
     DESCRIPTION OF THE REFERENCE NUMERALS AND SIGNS 
       1  . . . Ceramic member for bonding 
       3  . . . Metallic member 
       5 ,  35 ,  162 ,  219 ,  221  . . . Brazing material 
       7 ,  7 ′,  37 ,  37 ′,  70 ,  370  . . . Bonded object 
       9  . . . Ceramic base 
       11 ,  171 ,  211 ,  213  . . . Metallic layer 
       13  . . . Deposit layer formed by metallization 
       313 ,  373 ,  315 ,  317  . . . Alloy layer 
       31  . . . First ceramic member for bonding 
       33  . . . Second ceramic member for bonding 
       39  . . . First ceramic base 
       41  . . . First metallic layer formed by metallization 
       343  . . . First alloy layer 
       45  . . . Second ceramic base 
       47  . . . Second metallic layer formed by metallization 
       349  . . . Second alloy layer 
       161 ,  207  . . . Arc shield 
       101  . . . Insulating valve 
       100 ,  100 ′,  100 ″,  200 ,  200 ′,  200 ″ . . . Vacuum switch (high-load switch) 
       201  . . . Upper insulating valve 
       203  . . . Lower insulating valve 
       205  . . . Connecting member 
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments (examples) of the process for producing a ceramic member for bonding, the ceramic member for bonding, the bonded object, the vacuum switch and the vacuum vessel of the invention will be described in connection with the drawings. 
     EXAMPLE 1 
     A bonded object ceramic member for bonding and a metallic member will be described hereinafter by way of example. 
     a) As diagrammatically shown in FIG. 1, a ceramic member for bonding  1  and a metallic member  3  are bonded to each other with a brazing material  5  to form a bonded object  7 ′ in the present example. 
     More specifically, the ceramic member for bonding  1  comprises a metallic layer (lower layer)  11  formed on a ceramic base  9  by metallization. An alloy layer (upper layer)  313  is formed on the metallic layer  11 . The alloy layer  313  and the metallic member  3  are bonded to each other with the brazing material  5 , whereby the ceramic member for bonding  1  and the metallic member  3  are integrated. 
     In particular, in the present example, the metallic layer  11  comprises nickel in an amount of from 0.5% to 8% by weight, tungsten in an amount of from 15% to 75% by weight, molybdenum in an amount of from 18% to 81% by weight and SiO 2  in an amount of from 1.4% to 19% by weight as calculated in terms of oxide. Further, the alloy layer  313  comprises nickel and copper (or manganese) in an amount of from 12% to 76% by weight and from 19% to 87% by weight (or from 2% to 42%), respectively. 
     Accordingly, as obedient from the experiment example described later, the bonded object  7 ′ of the present example exhibits a high strength of bonding between the ceramic base  9  and the metallic layer  11  and hence an excellent airtightness. 
     Further, since brazing can be effected with the metallic member  3  on the alloy layer  313  without nickel plating and subsequent sintering as conventionally effected, the production process can be drastically simplified. 
     b) A process for producing a ring-shaped test piece as an example of this bonded object  7 ′ will be described hereinafter together with a process for producing a ceramic member for bonding  1 . 
     (1) Firstly, a second mixture was prepared from materials selected from nickel powder, copper powder, manganese powder, molybdenum powder, nickel-copper alloy powder and nickel-manganese alloy powder set forth in Table 1 below. The second mixture powder (having a concentration of, e.g., 87% by weight) was pulverized and mixed with an organic binder (having a concentration of, e.g., 13% by weight) such as ethyl cellulose to prepare a lower layer paste (first metallizing ink). 
     The first mixture comprises nickel, tungsten and molybdenum incorporated therein as metallic ingredients in an amount of from 0.5% to 10% by weight, from 20% to 70% by weight and from 20% to 70% by weight, respectively. 
     (2) Similarly, a second mixture was prepared from materials selected from nickel powder, copper powder, molybdenum powder, nickel-copper alloy powder and nickel-manganese alloy powder set forth in Table 1 below. The second mixture powder (having a concentration of, e.g., 87% by weight) was pulverized and mixed with an organic binder (having a concentration of, e.g., 13% by weight) such as ethyl cellulose to prepare an upper layer paste (second metallizing ink). 
     The second mixture comprises nickel or nickel oxide and at least one of copper, copper oxide and manganese oxide as a metal or metal oxide ingredient in an amount of from 30% to 80% by weight and from 20% to 70% by weight, respectively. 
     (3) Subsequently, the aforementioned lower layer paste was applied to a ceramic base  9  (cylindrical test piece having a thickness of 5 mm, an outer diameter of 5 mm and an inner diameter of 8.5 mm) made of alumina (having an alumina concentration of, e.g., 92% by weight) which is a sintered ceramic to a thickness of from about 10 μm to 20 μm, and then dried to form a lower layer (which later becomes a metallic layer  11 ). 
     (4) Subsequently, the aforementioned upper layer paste was applied to the entire surface of the lower layer to a thickness of from about 10 μm to 20 μm, and then dried to form an upper layer (which later becomes an alloy layer  313 ). 
     (5) Subsequently, the ceramic base  9  having the aforementioned upper layer and lower layer formed thereon was placed in a furnace where it was then sintered at a temperature of from 1,050 to 1,250° C. as shown in Table 2 below in an atmosphere of H 2 /N 2  (1:1) forming gas having a wetter temperature of 40° C. As a result, a ceramic member for bonding  1  was obtained which consisted of the ceramic base  9  and a metallic layer  11  and an alloy layer  313  formed thereon by metallization. 
     (6) Subsequently, the ceramic member for bonding  1  and a metallic member  3  made of Kovar (Fe—Ni—Co) were brazed together. 
     Specifically, a foil of a solver brazing material (BAg-8) was disposed interposed between the alloy layer  313  and the metallic member  3  (e.g., a Kovar disk having a thickness of 1 mm and an outer diameter φ of 16 mm). This assemblage was heated at a predetermined brazing temperature and then cooled to braze the ceramic member for bonding  1  and the metallic member  3  together. Thus, a bonded object  7 ′ was completed. 
     In some detail, using the first and second mixtures (i.e., lower layer paste and upper layer paste) having different compositions as set forth in Table 1 below, ring-shaped test pieces (samples of the invention comprising tungsten incorporated in the lower layer and nickel ingredient and copper or manganese ingredient incorporated in the upper layer) were produced as bonded objects  7 ′ to be tested as shown in FIG.  2  through the steps (1) to (6) described above. 
     Comparative Sample Nos. 22 to 25 (samples free of copper and manganese in the upper layer and free of tungsten in the lower layer) were further produced using pastes having different compositions. Table 1 shows the composition of the first and second mixtures. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                   
                 Lower layer paste 
                 Upper layer paste 
                   
               
               
                 Sample 
                 (for metallic layer) 
                 (for alloy layer) 
               
             
          
           
               
                 No. 
                 Mo 
                 W 
                 Ni 
                 SiO 2   
                 Ni 
                 Cu 
                 Mn 
                 Mo 
                 Remarks 
               
               
                   
               
             
          
           
               
                 1 
                 20 
                 70 
                 5 
                 5 
                 50 
                 50 
                 — 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 first essence 
               
               
                 2 
                 29 
                 70 
                 1 
                 0 
                 50 
                 50 
                 — 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 first essence 
               
               
                 3 
                 30 
                 69 
                 1 
                 0 
                 50 
                 50 
                 — 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 second essence 
               
               
                 4 
                 30 
                 60 
                 5 
                 5 
                 50 
                 50 
                 — 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 second essence 
               
               
                 5 
                 69 
                 20 
                 5 
                 6 
                 50 
                 50 
                 — 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 second essence 
               
               
                 6 
                 69 
                 18 
                 6 
                 7 
                 50 
                 50 
                 — 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 first essence 
               
               
                 7 
                 70 
                 20 
                 5 
                 5 
                 50 
                 50 
                 — 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 first essence 
               
               
                 8 
                 44.5 
                 50 
                 0.5 
                 5 
                 50 
                 50 
                 — 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 first essence 
               
               
                 9 
                 44 
                 50 
                 1 
                 5 
                 50 
                 50 
                 — 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 second essecne 
               
               
                 10 
                 35 
                 50 
                 10 
                 5 
                 50 
                 50 
                 — 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 second essence 
               
               
                 11 
                 48 
                 50 
                 1 
                 1 
                 50 
                 50 
                 — 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 second essence 
               
               
                 12 
                 40 
                 35 
                 10 
                 15 
                 50 
                 50 
                 — 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 second essence 
               
               
                 13 
                 35 
                 35 
                 10 
                 20 
                 50 
                 50 
                 — 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 second essence 
               
               
                 14 
                 45 
                 45 
                 5 
                 5 
                 30 
                 70 
                 — 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 first essence 
               
               
                 15 
                 45 
                 45 
                 5 
                 5 
                 40 
                 60 
                 — 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 second essence 
               
               
                 16 
                 45 
                 45 
                 5 
                 5 
                 70 
                 30 
                 — 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 second essence 
               
               
                 17 
                 45 
                 45 
                 5 
                 5 
                 80 
                 20 
                 — 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 first essence 
               
               
                 18 
                 45 
                 45 
                 5 
                 5 
                 30 
                 — 
                 70 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 first essence 
               
               
                 19 
                 45 
                 45 
                 5 
                 5 
                 40 
                 — 
                 60 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 second essence 
               
               
                 20 
                 45 
                 45 
                 5 
                 5 
                 70 
                 — 
                 30 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 second essence 
               
               
                 21 
                 45 
                 45 
                 5 
                 5 
                 80 
                 — 
                 20 
                 — 
                 Example of the 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 first essence 
               
               
                 22 
                 45 
                 45 
                 5 
                 5 
                 30 
                 — 
                 — 
                 70 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 23 
                 45 
                 45 
                 5 
                 5 
                 50 
                 — 
                 — 
                 50 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 24 
                 45 
                 45 
                 5 
                 5 
                 70 
                 — 
                 — 
                 30 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                 25 
                 90 
                 — 
                 5 
                 5 
                 50 
                 50 
                 — 
                 — 
                 Comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Example 
               
               
                   
               
             
          
         
       
     
     c) The ceramic members for bonding  1  thus prepared were each polished on the section thereof. The section of the ceramic members  1  for bonding were each then quantitatively analyzed for the components of the metallic layer  11  and the alloy layer  313 . In some detail, the quantitative analysis was conducted using an electron probe microanalyzer (accelerating voltage: 20 kv; spot diameter: 5 μm). The results are set forth in Table 2 below. 
     With respect to each sample, the analysis was made on five points so as to reduce influences of segregation and the five found values obtained for each component were averaged. In Table 2 below, the content of silicon (% by weight) is represented in term of oxide. The other constituents of each layer are glass ingredients such as Al 2 O 3 , MgO and CaO diffused from the ceramic base. 
     
       
         
               
               
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
             
             
               
                   
                   
               
               
                   
                 Found value after sintering (average) [wt-%] 
                   
               
             
          
           
               
                 Sample 
                 Lower layer (metallic layer) 
                 Upper layer (alloy layer) 
                   
               
             
          
           
               
                 No. 
                 Mo 
                 W 
                 Ni 
                 SiO 2   
                 Ni 
                 Cu 
                 Mn 
                 Mo 
                 Remarks 
               
               
                   
               
             
          
           
               
                 1 
                 18.3 
                 75.6 
                 1.5 
                 4.1 
                 33.5 
                 64.4 
                 — 
                 0.6 
                 Example of the first essence 
               
               
                 2 
                 20.1 
                 76.1 
                 0.8 
                 2.2 
                 34.4 
                 62.2 
                 — 
                 0.7 
                 Example of the first essence 
               
               
                 3 
                 22.8 
                 74.3 
                 0.7 
                 1.5 
                 35.1 
                 63.8 
                 — 
                 0.6 
                 Example of the second essence 
               
               
                 4 
                 22.7 
                 73.1 
                 0.8 
                 3.0 
                 17.0 
                 79.3 
                 — 
                 0.1 
                 Example of the second essence 
               
               
                 5 
                 76.2 
                 15.7 
                 3.2 
                 2.5 
                 13.2 
                 80.9 
                 — 
                 0.6 
                 Example of the second essence 
               
               
                 6 
                 77.0 
                 14.5 
                 3.4 
                 3.1 
                 21.6 
                 74.5 
                 — 
                 1.8 
                 Example of the first essence 
               
               
                 7 
                 81.3 
                 15.0 
                 1.9 
                 1.7 
                 31.0 
                 64.5 
                 — 
                 1.7 
                 Example of the first essence 
               
               
                 8 
                 58.6 
                 33.4 
                 0.5 
                 3.8 
                 35.1 
                 62.1 
                 — 
                 1.9 
                 Example of the first essence 
               
               
                 9 
                 52.1 
                 40.0 
                 1.4 
                 4.0 
                 34.2 
                 62.9 
                 — 
                 1.7 
                 Example of the second essence 
               
               
                 10 
                 58.5 
                 28.5 
                 7.8 
                 3.9 
                 37.5 
                 59.8 
                 — 
                 2.0 
                 Example of the second essence 
               
               
                 11 
                 60.1 
                 33.0 
                 1.8 
                 2.7 
                 26.2 
                 69.3 
                 — 
                 2.5 
                 Example of the second essence 
               
               
                 12 
                 52.4 
                 24.5 
                 4.4 
                 16.0 
                 27.8 
                 69.8 
                 — 
                 1.2 
                 Example of the second essence 
               
               
                 13 
                 46.1 
                 27.8 
                 4.8 
                 19.0 
                 39.9 
                 57.9 
                 — 
                 1.7 
                 Example of the second essence 
               
               
                 14 
                 62.4 
                 24.6 
                 5.6 
                 3.9 
                 12.0 
                 87.0 
                 — 
                 0.8 
                 Example of the first essence 
               
               
                 15 
                 60.3 
                 31.6 
                 3.4 
                 2.9 
                 12.7 
                 81.1 
                 — 
                 0.5 
                 Example of the second essence 
               
               
                 16 
                 61.0 
                 30.0 
                 1.6 
                 4.7 
                 57.8 
                 35.3 
                 — 
                 4.8 
                 Example of the second essence 
               
               
                 17 
                 57.3 
                 31.8 
                 7.1 
                 1.4 
                 72.0 
                 18.5 
                 — 
                 7.7 
                 Example of the first essence 
               
               
                 18 
                 55.0 
                 30.7 
                 4.3 
                 5.0 
                 31.4 
                 — 
                 42.3 
                 22.1 
                 Example of the first essence 
               
               
                 19 
                 59.2 
                 30.0 
                 4.4 
                 3.6 
                 40.0 
                 — 
                 31.7 
                 23.2 
                 Example of the second essence 
               
               
                 20 
                 57.2 
                 31.0 
                 6.3 
                 2.8 
                 68.2 
                 — 
                 6.1 
                 23.5 
                 Example of the second essence 
               
               
                 21 
                 60.0 
                 24.8 
                 6.7 
                 3.8 
                 76.5 
                 — 
                 2.3 
                 20.2 
                 Example of the first essence 
               
               
                 22 
                 61.7 
                 24.1 
                 7.0 
                 2.3 
                 27.5 
                 — 
                 — 
                 65.8 
                 Comparative Example 
               
               
                 23 
                 64.8 
                 20.0 
                 7.1 
                 2.5 
                 44.0 
                 — 
                 — 
                 50.6 
                 Comparative Example 
               
               
                 24 
                 60.3 
                 27.9 
                 6.4 
                 3.0 
                 60.9 
                 — 
                 — 
                 33.3 
                 Comparative Example 
               
               
                 25 
                 91.4 
                 — 
                 2.8 
                 3.0 
                 45.3 
                 45.8 
                 — 
                 4.0 
                 Comparative Example 
               
               
                   
               
             
          
         
       
     
     d) Subsequently, the bonded objects  7 ′ produced by the process described above were examine for bonding strength. 
     In some detail, as shown in FIG. 3, each bonded object  7 ′ was placed in such an arrangement that the metallic member  3  faced downward and the lower side of a peripheral part of the ceramic base  9  was supported on a cylindrical iron pedestal  21 . A cylindrical punching rod  25  made of stainless steel was inserted from the upper side into the central through-hole  23  of the ceramic base  9  held in that state. The punching rod  25  was then moved downward as viewed in the drawing at a rate of 0.5 mm/min. 
     The strength (breaking strength) developed when the metallic member  3  separated from the ceramic base  9  was measured with a load cell (not shown) disposed over the punching rod  25 . This strength was taken as brazing strength. With respect to each sample, the brazing strengths for the respective sintering temperatures are set forth in Table 3 below. 
     In the column “Evaluation of bonding strength and obtained data” in Table 3, “⊚” indicates that the sample had a bonding strength of not lower than 20 MPa, “◯” indicates that the sample had a bonding strength of from not lower than 15 MPa to lower than 20 MPa, “Δ” indicates that the sample had abonding strength of from not lower than 10 MPa to lower than 15 MPa, and “X” indicates that the sample had a bonding strength of lower than 10 MPa. In addition, bonding strength data of each of the samples are shown in parentheses “( )” at the column “Evaluation of bonding strength and obtained data”. 
     
       
         
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                   
                 Evaluation of bonding strength and obtained data [MPa] 
               
               
                 Sample 
                 Sintering temperature [° C.] 
               
             
          
           
               
                 No. 
                 1,050 
                 1,100 
                 1,150 
                 1,200 
                 1,250 
               
               
                   
               
             
          
           
               
                 1 
                 X (2.8) 
                 Δk (12.5) 
                 ⊚k (21.1) 
                 ◯k (16.9) 
                 X (1.0) 
               
               
                 2 
                 X (1.2) 
                 Δ (14.1) 
                 ⊚k (20.5) 
                 ◯k (15.3) 
                 X (3.8) 
               
               
                 3 
                 X (3.9) 
                 ◯ (18.5) 
                 ⊚k (22.8) 
                 ◯k (16.6) 
                 X (5.9) 
               
               
                 4 
                 X (4.8) 
                 ◯k (19.1) 
                 ⊚k (24.3) 
                 ◯k (17.4) 
                 X (4.9) 
               
               
                 5 
                 X (2.9) 
                 ⊚k (21.5) 
                 ⊚k (23.7) 
                 ◯k (17.4) 
                 X (5.0) 
               
               
                 6 
                 X (2.0) 
                 ⊚k (21.8) 
                 ⊚k (20.3) 
                 X (8.7) 
                 X (1.1) 
               
               
                 7 
                 X (4.7) 
                 ⊚k (20.4) 
                 ⊚k (20.2) 
                 X (7.5) 
                 X (1.2) 
               
               
                 8 
                 X (1.7) 
                 Δ (11.2) 
                 ◯k (17.4) 
                 ◯k (15.8) 
                 X (6.8) 
               
               
                 9 
                 X (1.5) 
                 ◯k (17.3) 
                 ◯k (19.4) 
                 ◯k (16.7) 
                 X (6.9) 
               
               
                 10 
                 Δ (10.9) 
                 ⊚k (21.8) 
                 ⊚k (21.5) 
                 ◯k (16.7) 
                 X (1.2) 
               
               
                 11 
                 X (3.9) 
                 ◯ (15.6) 
                 ◯ (18.7) 
                 ⊚k (20.8) 
                 ◯ (15.2) 
               
               
                 12 
                 Δ (11.2) 
                 ⊚k (22.2) 
                 ◯k (17.9) 
                 Δk (13.3) 
                 X (2.9) 
               
               
                 13 
                 Δk (11.2) 
                 ⊚k (22.0) 
                 ⊚k (21.4) 
                 Δ (13.4) 
                 X (2.5) 
               
               
                 14 
                 X (3.0) 
                 X (8.9) 
                 ⊚k (18.3) 
                 ◯k (15.4) 
                 Δ (10.7) 
               
               
                 15 
                 X (4.2) 
                 ⊚k (22.8) 
                 ⊚k (23.5) 
                 ◯k (16.9) 
                 Δ (11.3) 
               
               
                 16 
                 Δ (12.5) 
                 ⊚k (21.5) 
                 ⊚k (22.9) 
                 ◯k (15.3) 
                 Δ (11.0) 
               
               
                 17 
                 Δ (13.1) 
                 ◯k (17.2) 
                 ◯k (19.0) 
                 X (9.2) 
                 X (7.4) 
               
               
                 18 
                 X (3.0) 
                 Δk (14.8) 
                 ◯k (18.1) 
                 ◯ (15.1) 
                 X (6.0) 
               
               
                 19 
                 X (4.3) 
                 ◯k (18.4) 
                 ⊚k (21.2) 
                 ◯k (16.0) 
                 X (8.2) 
               
               
                 20 
                 X (5.3) 
                 ⊚k (20.5) 
                 ◯k (19.1) 
                 ◯k (15.5) 
                 X (7.9) 
               
               
                 21 
                 X (2.9) 
                 ◯k (15.4) 
                 ◯k (17.2) 
                 X (8.3) 
                 X (4.0) 
               
               
                 22 
                 X (2.0) 
                 X (7.8) 
                 Δk (13.9) 
                 Δ (11.3) 
                 X (2.3) 
               
               
                 23 
                 X (1.9) 
                 X (6.9) 
                 ◯k (17.1) 
                 Δ (10.7) 
                 X (2.0) 
               
               
                 24 
                 X (2.5) 
                 Δ (12.2) 
                 ◯k (19.0) 
                 X (8.3) 
                 X (1.9) 
               
               
                 25 
                 X (4.9) 
                 ⊚k (20.5) 
                 ◯ (18.9) 
                 X (7.4) 
                 X (1.3) 
               
               
                   
               
             
          
         
       
     
     As can be seen in Table 3 above, Sample Nos. 1 to 21, which are within the scope of the invention, advantageously have a high brazing strength because the metallic layers formed by metallization have sufficiently sintered even through low-temperature sintering. There also is an advantage that since the metallic layers in those samples can be sufficiently sintered at low temperatures, the ceramic members for bonding can retain high dimensional accuracy as compared with the case where sintering is conducted at high temperatures. 
     It is also found that since the samples of the invention comprise tungsten incorporated in the lower layer, sintering can be conducted within a wide temperature range, making it possible to provide the sintered products obtained within the temperature range with high bonding properties. 
     In particular, Sample Nos. 3 to 5, 9 to 13, 15, 16, 19 and 20, which satisfy the requirements of the second and sixth essences of the invention, have a high bonding strength and thus are more desirable. 
     On the contrary, Sample Nos. 22 to 25, which are comparative, disadvantageously have a low brazing strength. 
     d) Subsequently, the samples of bonded object  7 ′ produced by the process described above were each examined for airtightness. 
     In some detail, the air was evacuated from one side of the bonded object  7 ′ shown in FIG. 3 (not higher than 1×10 −6  Pa). The other side of the bonded object  7 ′ was filled with helium. In this arrangement, the bonded object  7 ′ was then examined for leakage of helium. 
     In Table 3 above, the symbol “k” indicates that the sample had no leakage of helium. 
     As can be seen in Table 3 above, among Sample Nos. 1 to 21 of the invention, Sample Nos. 1, 4, 5, 9, 10, 12, 13, 16, 19 and 20 exhibit an excellent airtightness. 
     EXAMPLE 2 
     Example 2 will be described hereinafter. However, the description of the same points as in Example 1 will be omitted. 
     In this example, a bonded object comprising two ceramic members bonded to each will be described as an embodiment. 
     a) As shown diagrammatically in FIG. 4, in the present example, a first ceramic member  31  made of alumina is bonded to a second ceramic member for bonding  33  made of alumina similar to the first ceramic member  31  made of alumina to form a bonded object  37 ′. 
     More specifically, the first ceramic member for bonding  31  comprises a first ceramic base  39  and a first metallic layer  41  formed thereon by metallization. A first alloy layer  343  is formed on this first metallic layer  41 . On the other hand, the second ceramic member for bonding  33  comprises a second ceramic base  45  and a second metallic layer  47  formed thereon by metallization. A second alloy layer  349  is formed on this second metallic layer  47 . 
     The first alloy layer  343  and the second alloy layer  349  are bonded to each other with a brazing material  35  to integrate the first ceramic member  31  for bonding and the second ceramic member  33  for bonding into one. 
     b) A process for producing the bonded object  37 ′ will be described hereinafter together with processes for producing first ceramic member  31  for bonding and second ceramic member  33  for bonding. 
     (1) Using particulate upper layer paste ingredients set forth in Table 1 above, upper layer pastes for respective samples were produced in the same manner as in Example 1 (hereinafter, the points on which explanation is omitted here are the same as in Example 1). 
     (2) Subsequently, the lower layer pastes thus produced were each applied to a surface of a first ceramic base  39  and to a surface of a second ceramic base  45 , and then dried to form a lower layer. 
     (3) Subsequently, the upper layer pastes thus produced were each applied to a surface of the lower layer of the first ceramic base  39  and the second ceramic base  45 , and then dried to form an upper layer. 
     (4) Subsequently, the first and second ceramic bases  39  and  45  having the aforementioned upper layer and lower layer formed thereon, respectively, were each placed in a furnace where they were then sintered at a temperature of from 1,050 to 1,250° C. As a result, first and second ceramic members for bonding  31  and  33  were obtained having first and second alloy layers  343  and  349  deposited on the first and second metallic layers  41  and  47 , respectively. 
     (5) Subsequently, a silver brazing material  35  was interposed between the two alloy layers  343  and  349  to braze the ceramic members for bonding  31  and  33  together. Thus, the ceramic members for bonding  31  and  33  were integrated to each other to complete a bonded object  37 ′. 
     The bonded object  37 ′ of the present example exhibits a high bonding strength and hence an excellent airtightness as in Example 1. 
     EXAMPLE 3 
     Example 3 will be described hereinafter. However, the description of the same points as in Examples 1 and 2 will be omitted. 
     The present example describes application of a bonded object comprising a ceramic member for bonding and a metallic member, such as that shown in Example 1, to a vacuum switch as an embodiment. 
     This vacuum switch is a high-load switch which comprises a vacuum vessel having built-in electrodes and other components and is suitable for the switching of high-voltage large-current electricity. 
     More specifically, as shown in FIG. 5, this high-load vacuum switch  100 ″ comprises an insulating valve  101 , first and second end covers  102  and  103  attached to the insulating valve  101  so as to cover the respective ends thereof, a fixed electrode  104  attached to the first endcover  102  and projecting into the insulating valve  101 , and a movable electrode  105  disposed so as to be freely slidable through the second end cover  103 . The fixed electrode  104  and the movable electrode  105  constitute a contact point  106 . 
     The insulating valve  101  is a sintered ceramic which has an alumina content of 92% by weight and has a nearly cylindrical shape having an inner diameter of 80 mm, a wall thickness of about 5 mm, and a length of 100 mm. The insulating valve  101  comprises a straight barrel part  110  having a constant inner diameter and a ridge part  112  projecting inward from the inner circumferential wall  111  and extending throughout the circumference along a circle located intermediately between both ends of the barrel part  110 . The insulating valve  101  further has a glaze layer  115  on the outer circumferential surface thereof. 
     The first and second end covers  102  and  103  are constituted of disk-shaped Kovar (Fe—Ni—Co) plates respectively having in the centers thereof holes  121  and  132  for securing the fixed electrode  104  and a guide  131  thereto. This guide  131  has been disposed so that the movable shaft  151  of the movable electrode  105  is readily slidable. 
     The fixed electrode  104  consists of a fixed shaft  141 , as one end thereof, which is firmly fitted to the hole  121  and a disk-shaped electrode  142 , as the other end thereof, which projects into the insulating valve  101 . 
     The movable electrode  105  consists of the movable shaft  151 , as a rear end part thereof, which slides within the guide  131  and an electrode  152 , as a front end part thereof, which comes into contact with the electrode  142  of the fixed electrode  104 . This movable electrode  105  is equipped with a metallic bellows  153  extending from that part of the movable shaft  151  which is near the electrode  152  to the second end cover  103 . The bellows  153  enables the electrode  105  to perform its switching function while maintaining a vacuum. 
     The metallic bellows  153  is surrounded by a bellows cover  154  so as to be prevented from coming into direct contact with a metal vapor generating from the electrodes  142  and  152  (i.e., contacts  143  and  155  disposed on the front ends thereof) upon current switching. 
     The contact point  106  has such a structure that fusion bonding caused by a vacuum arc generated is less apt to occur. This has been accomplished by employing a high-melting tungsten-based metal sinter as the material of the contacts  143  and  155  where the electrodes  142  and  152  contact with each other. 
     Furthermore, an arc shield  161  is disposed so as to surround the contact point  106 . This arc shield  161  has been bonded by brazing to the ridge part  112  of the insulating valve  101  in order to prevent the metal vapor from depositing on the inner circumferential wall  111  of the insulating valve  101  to reduce insulation. 
     Namely, like the bonded object of Example 1, the high-load switch  100 ″ of this Example comprises the insulating valve  101 , which is a ceramic member for bonding, and the arc shield  161 , which is a metallic member, bonded by brazing with a brazing material  162  to the ridge part  112  of the insulating valve  101 . 
     FIG. 6 diagrammatically illustrates an important part of the switch  100 ″. Specifically, the ridge part  112  of the insulating valve  101  has a metallic layer  171  formed on the top thereof by low-temperature metallization in the same manner as in Example 1. This metallic layer  171  is coated with an alloy layer  373 . This alloy layer  373  has been bonded to the arc shield  161  by brazing with the brazing material  162 . 
     Thus, the insulating valve  101  equipped with the arc shield  161  (and hence the high-load switch  100 ″) can be produced at low cost, and high dimensional accuracy and high bonding strength can be realized. 
     EXAMPLE 4 
     Example 4 will be described hereinafter. However, the description of the same points as in Example 3 will be omitted. 
     The present example describes application of a bonded object comprising a ceramic member for bonding and a metallic member to a vacuum switch as in Example 3. However, this vacuum switch differs from that of Example 3 in the structures of the arc shield and the insulating valve. 
     An important part of the vacuum switch is diagrammatically shown in FIG.  7 . This vacuum switch (high-load switch)  200 ″ has a connecting member  205  made of oxygen-free copper interposed by brazing between an upper insulating valve  201  and a lower insulating valve  203 , and further has an arc shield  207  bonded by brazing to the front end of the connecting member  205 . 
     In particular, in the part (fixing part  209 ) where the upper insulating valve  201  and lower insulating valve  203  are fixed to the connecting member  205 , the upper and lower insulating valves  201  and  203  respectively have metallic layers  211  and  213  formed thereon by metallization in the same manner as in Example 1. These metallic layers  211  and  213  are coated with alloy layers  315  and  317 . 
     These alloy layers  315  and  317  have been bonded to the connecting member  205  with brazing materials  219  and  221 , respectively. Thus, the two insulating valves  201  and  203  have been united with the connecting member  205 . 
     The two insulating valves  201  and  203  have, on the outer circumferential surfaces thereof, glaze layers  223  and  225 , respectively, which are the same as the glaze layer in Example 3. 
     The present example brings about the same effects as Example 3. 
     The invention should not be construed as being limited to the Examples give above in any way. It is a matter of course that the invention can be practiced in various modes unless these modes depart from the spirit of the invention. 
     For example, while Examples 1 and 2 have been described with reference to the case where a metallic layer which is a lower layer and an alloy layer which is an upper layer are bonded directly to each other, an interlayer having a constitution different from these layers (e.g., nickel-molybdenum alloy layer) may be formed interposed therebetween. 
     Comparative Example 1 
     A bonded object comprising a ceramic member for bonding and a metallic member is described as an embodiment. 
     a) FIG. 8 diagrammatically shows this embodiment, which is a bonded object  7  comprising a ceramic member for bonding  1  and a metallic member  3  bonded thereto with a brazing material  5 . 
     More specifically, the ceramic member for bonding  1  comprises a ceramic base  9  and a metallic layer  11  formed thereon by metallization. A deposit layer (plating layer)  13  has been formed by plating on the metallic layer  11 . The deposit layer  13  and the metallic member  3  have been bonded to each other with the brazing material  5 , whereby the ceramic member for bonding  1  and the metallic member  3  have been united with each other. 
     b) A process for producing a ring-shaped test piece as an example of this bonded object will be explained below together with a process for producing a ceramic member for bonding. 
     (1) First, particulate metallizing-ink ingredients shown in Table 2-1 which will be given later were pulverized and mixed together, and this powder mixture (e.g., 87% by weight) was mixed with an organic binder (e.g., 13% by weight) such as, e.g., ethyl cellulose to obtain a paste. Thus, a metallizing ink was produced. 
     (2) The metallizing ink thus produced was applied to a surface of an alumina-based ceramic base  9  (e.g., 92% by weight alumina) which was a sintered ceramic (e.g., a ring-shaped test piece having a thickness of 5 mm, an outer diameter of 30 mm, and an inner diameter of 8.5 mm) in a thickness of about from 10 to 20 μm. 
     (3) Subsequently, the ceramic base 9 coated with the metallizing ink was placed in a furnace and sintered (metallized) at the temperatures of from 1,150 to 1,350° C. shown in Table 2-2 below in an atmosphere of H 2 /N 2  (1:1) forming gas having a wetter temperature of 50° C. Thus, a ceramic member for bonding  1  was obtained which consisted of the ceramic base  9  and a metallic layer  11  formed thereon by metallization. 
     (4) The surface of the metallic layer  11  (metallized side) was electroplated with nickel to form a deposit layer  13 . This deposit layer  13  was sintered (sintered) in an H 2  atmosphere at a temperature of 830° C. 
     (5) Thereafter, the ceramic member for bonding  1  and a metallic member  3  made of Kovar (Fe—Ni—Co) were brazed together. 
     Specifically, a foil of a silver brazing material (BAg-8)  5  was interposed between the deposit layer  13  and the metallic member  3  (e.g., a Kovar disk having a thickness of 1 mm and an outer diameter of 16 mm). This assemblage was heated at a given brazing temperature and then cooled to braze the ceramic member for bonding  1  and the metallic member  3  together. Thus, a bonded object  7  was completed. 
     Using the metallizing inks having different compositions as shown in Table 2-1 below, ring-shaped test pieces (samples) Nos. 2-2 to 2-17 as shown in FIG. 9 were produced, as bonded objects  7  to be tested, through the steps (1) to (5) described above. Sample No. 2-1 as a comparative example was further produced using a metallizing ink having a different composition (not containing nickel). 
     
       
         
               
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                                                      TABLE 2-1 
               
             
             
               
                   
               
               
                 Sample 
                 Content in metallizing ink [wt %] 
                   
               
             
          
           
               
                 No. 
                 Mo 
                 Ni 
                 Mn 
                 Ti 
                 SiO 2   
                 Total 
                 Remarks 
               
               
                   
               
             
          
           
               
                           2-1 
                 91.5 
                 0.0 
                 7.3 
                 1.2 
                 0.0 
                 100.0 
                 comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 example 
               
               
                 2-2 
                 81.0 
                 0.5 
                 7.3 
                 1.2 
                 10.0 
                 100.0 
                 comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 example 
               
               
                 2-3 
                 80.5 
                 1.0 
                 7.3 
                 1.2 
                 10.0 
                 100.0 
                 comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 example 
               
               
                 2-4 
                 76.5 
                 5.0 
                 7.3 
                 1.2 
                 10.0 
                 100.0 
                 comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 example 
               
               
                 2-5 
                 71.5 
                 10.0 
                 7.3 
                 1.2 
                 10.0 
                 100.0 
                 comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 example 
               
               
                 2-6 
                 68.5 
                 13.0 
                 7.3 
                 1.2 
                 10.0 
                 100.0 
                 comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 example 
               
               
                 2-7 
                 85.5 
                 5.0 
                 7.3 
                 1.2 
                 1.0 
                 100.0 
                 comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 example 
               
               
                 2-8 
                 71.5 
                 5.0 
                 7.3 
                 1.2 
                 15.0 
                 100.0 
                 comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 example 
               
               
                 2-9 
                 66.9 
                 5.0 
                 7.3 
                 1.2 
                 20.0 
                 100.0 
                 comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 example 
               
               
                 2-10 
                 76.5:W 
                 5.0 
                 7.3 
                 1.2 
                 10.0 
                 100.0 
                 comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 example 
               
               
                 2-11 
                 81.3 
                 5.0 
                 2.5 
                 1.2 
                 10.0 
                 100.0 
                 comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 example 
               
               
                 2-12 
                 68.8 
                 5.0 
                 15.0 
                 1.2 
                 10.0 
                 100.0 
                 comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 example 
               
               
                 2-13 
                 77.5 
                 5.0 
                 7.3 
                 0.2 
                 10.0 
                 100.0 
                 comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 example 
               
               
                 2-14 
                 84.7 
                 5.0 
                 7.3 
                 3.0 
                 10.0 
                 100.0 
                 comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 example 
               
               
                 2-15 
                 97.0 
                 3.0 
                 0.0 
                 0.0 
                 0.0 
                 100.0 
                 comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 example 
               
               
                 2-16 
                 83.5 
                 5.0 
                 7.3 
                 1.2 
                 3.0 
                 100.0 
                 comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 example 
               
               
                 2-17 
                 82.5 
                 5.0 
                 1.0 
                 1.2 
                 10.0 
                 100.0 
                 comparative 
               
               
                   
                   
                   
                   
                   
                   
                   
                 example 
               
               
                   
               
             
          
         
       
     
     Furthermore, during the production process described above, the metallic layer  11  of each ceramic member for bonding  1  produced was quantitatively analyzed to determine the contents of components. This quantitative analysis was made with an electron probe microanalyzer (accelerating voltage, 20 kV; spot diameter, 5 μm). The results obtained are shown in Table 2-2 below. 
     With respect to each sample, the analysis was made on five points so as to reduce influences of segregation and the five found values obtained for each component were averaged. In Table 2-2, the contents of silicon, aluminum, calcium, and magnesium are given in terms of oxide amount. 
     
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
             
           
               
                                                      TABLE 2-2 
               
             
             
               
                   
               
               
                 Sample 
                 Found value after sintering (average) [wt %] 
               
             
          
           
               
                 No. 
                 Mo 
                 Ni 
                 Mn 
                 Ti 
                 SiO 2   
                 Al 2 O 3   
                 CaO 
                 MgO 
                 Total 
               
               
                   
               
             
          
           
               
                 2-1 
                 77.8 
                 0.0 
                 2.4 
                 0.6 
                 11.3 
                 6.5 
                 1.0 
                 0.4 
                 100.0 
               
               
                 2-2 
                 75.7 
                 0.4 
                 2.6 
                 0.7 
                 12.3 
                 6.8 
                 1.0 
                 0.5 
                 100.0 
               
               
                 2-3 
                 75.4 
                 0.6 
                 2.7 
                 0.7 
                 12.2 
                 6.8 
                 1.1 
                 0.5 
                 100.0 
               
               
                 2-4 
                 78.6 
                 2.2 
                 2.6 
                 0.7 
                 9.1 
                 5.7 
                 1.1 
                 0.0 
                 100.0 
               
               
                 2-5 
                 70.0 
                 7.6 
                 2.0 
                 1.4 
                 11.2 
                 6.5 
                 1.1 
                 0.2 
                 100.0 
               
               
                 2-6 
                 69.1 
                 9.8 
                 1.9 
                 1.3 
                 10.5 
                 6.2 
                 1.0 
                 0.2 
                 100.0 
               
               
                 2-7 
                 79.4 
                 2.3 
                 2.7 
                 0.7 
                 8.4 
                 5.4 
                 0.9 
                 0.2 
                 100.0 
               
               
                 2-8 
                 72.0 
                 2.2 
                 1.7 
                 0.5 
                 16.3 
                 6.2 
                 1.0 
                 0.1 
                 100.0 
               
               
                 2-9 
                 67.5 
                 2.0 
                 1.6 
                 0.4 
                 21.0 
                 6.4 
                 1.0 
                 0.0 
                 100.0 
               
               
                 2-10 
                 78.8 
                 2.1 
                 2.5 
                 0.7 
                 9.0 
                 5.8 
                 1.1 
                 0.0 
                 100.0 
               
               
                 2-11 
                 77.9 
                 2.2 
                 1.4 
                 0.7 
                 10.8 
                 5.9 
                 1.0 
                 0.1 
                 100.0 
               
               
                 2-12 
                 75.3 
                 2.3 
                 3.0 
                 0.7 
                 11.2 
                 6.2 
                 1.1 
                 0.2 
                 100.0 
               
               
                 2-13 
                 76.2 
                 2.1 
                 2.5 
                 0.1 
                 11.5 
                 6.4 
                 1.1 
                 0.1 
                 100.0 
               
               
                 2-14 
                 75.6 
                 2.8 
                 2.6 
                 2.1 
                 10.2 
                 5.7 
                 1.0 
                 0.0 
                 100.0 
               
               
                 2-15 
                 84.5 
                 2.6 
                 0.0 
                 0.3 
                 6.8 
                 5.0 
                 0.7 
                 0.1 
                 100.0 
               
               
                 2-16 
                 78.4 
                 2.4 
                 2.8 
                 0.6 
                 8.8 
                 5.8 
                 1.0 
                 0.2 
                 100.0 
               
               
                 2-17 
                 78.2 
                 2.3 
                 0.5 
                 0.6 
                 11.1 
                 6.1 
                 1.0 
                 0.2 
                 100.0 
               
               
                   
               
             
          
         
       
     
     In producing sample No. 2-10, tungsten was used in place of molybdenum. 
     c) The bonded object samples produced by the process described above were examined for bonding strength. 
     This examination was conducted in the following manner. As shown in FIG. 10, each bonded object  7  was placed so that the metallic member  3  faced downward and that the lower side of a peripheral part of the ceramic base  9  was supported on a cylindrical iron pedestal  21 . A cylindrical punching rod  25  made of stainless steel was inserted from the upper side into the central through-hole  23  of the ceramic base  9  held in that state. The punching rod  25  was then moved downward at a rate of 0.5 mm/min. 
     The strength (breaking strength) at the time when the metallic member  3  separated from the ceramic base  9  was measured with a load cell (not shown) disposed over the punching rod  25 . This strength was taken as brazing strength. With respect to each sample, the brazing strengths for the respective sintering temperatures are shown in the following Table 2-3. 
     
       
         
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                                                      TABLE 2-3 
               
             
             
               
                   
               
               
                 Sam- 
                   
                   
               
               
                 ple 
                 Brazing strength [MPa] 
                  Evalu- 
               
             
          
           
               
                 No. 
                 1150° C. 
                 1200° C. 
                 1250° C. 
                 1300° C. 
                 1350° C. 
                 ation 
               
               
                   
               
             
          
           
               
                 2-1 
                 4.9 
                 8.0 
                 9.7 
                 16.7 
                 18.6 
                 X 
               
               
                 2-2 
                 6.9 
                 12.7 
                 11.8 
                 6.9 
                 4.9 
                 Δ 
               
               
                 2-3 
                 11.8 
                 17.6 
                 14.7 
                 7.8 
                 3.9 
                 ◯ 
               
               
                 2-4 
                 17.6 
                 19.6 
                 15.7 
                 6.9 
                 3.9 
                 ◯ 
               
               
                 2-5 
                 17.6 
                 18.6 
                 14.7 
                 6.9 
                 2.9 
                 ◯ 
               
               
                 2-6 
                 12.7 
                 12.7 
                 10.8 
                 5.9 
                 2.9 
                 Δ 
               
               
                 2-7 
                 9.8 
                 15.4 
                 13.3 
                 4.9 
                 3.9 
                 Δ 
               
               
                 2-8 
                 14.7 
                 17.6 
                 15.7 
                 6.9 
                 3.9 
                 ◯ 
               
               
                 2-9 
                 10.8 
                 14.7 
                 11.8 
                 3.9 
                 2.9 
                 Δ 
               
               
                 2-10 
                 16.9 
                 19.1 
                 16.0 
                 5.9 
                 2.9 
                 ◯ 
               
               
                 2-11 
                 15.3 
                 14.1 
                 10.9 
                 5.9 
                 3.9 
                 Δ 
               
               
                 2-12 
                 12.8 
                 14.0 
                 11.0 
                 6.9 
                 3.9 
                 Δ 
               
               
                 2-13 
                 13.7 
                 15.6 
                 11.8 
                 5.7 
                 4.2 
                 Δ 
               
               
                 2-14 
                 15.2 
                 14.5 
                 10.2 
                 6.0 
                 4.3 
                 Δ 
               
               
                 2-15 
                 10.3 
                 14.6 
                 15.7 
                 6.9 
                 3.9 
                 Δ 
               
               
                 2-16 
                 12.8 
                 17.2 
                 15.6 
                 5.9 
                 3.9 
                 ◯ 
               
               
                 2-17 
                 14.5 
                 13.6 
                 10.1 
                 5.7 
                 2.9 
                 Δ 
               
               
                   
               
             
          
         
       
     
     In the column “Evaluation” in Table 2-3, ◯ indicates that the sample had a peak not lower than 17 MPa in the range of from 1,150 to 1,250° C., Δ indicates that the sample had a peak of from 10 to 17 MPa in that temperature range, and X indicates that the sample had no peak of 10 MPa or higher in that temperature range. 
     Comparative Example 2 
     In Comparative Example 2, explanations on the same points as in Comparative Example 1 are omitted. 
     In this Comparative Example, a bonded object comprising two ceramic members bonded to each other is described as an embodiment. 
     a) FIG. 11 diagrammatically shows this embodiment, which is a bonded object  37  comprising a first ceramic member for bonding  31  made of alumina and, bonded thereto with a brazing material  35 , a second ceramic member for bonding  33  made of alumina and similar to the first ceramic member  31 . 
     More specifically, the first ceramic member for bonding  31  comprises a first ceramic base  39  and a first metallic layer  41  formed thereon by metallization. A first deposit layer  43  has been formed on this first metallic layer  41  by nickel plating. On the other hand, the second ceramic member for bonding  33  comprises a second ceramic base  45  and a second metallic layer  47  formed thereon by metallization. A second deposit layer  49  has been formed on this second metallic layer  47  by nickel plating. The first ceramic member for bonding  31  and the second ceramic member for bonding  33  have been united with each other by bonding the first deposit layer  43  to the second deposit layer  49  with the brazing material  35 . 
     b) A process for producing a ring-shaped test piece as an example of this bonded object will be explained below together with a process for producing ceramic members for bonding. 
     (1) Using particulate metallizing-ink ingredients shown in Table 2-1 given above, metallizing inks for respective samples were produced in the same manner as in Comparative Example 1 (hereinafter, the points on which explanation is omitted here are the same as in Comparative Example 1). 
     (2) Each of the metallizing inks thus produced was applied to a surface of a first ceramic base  39  and to a surface of a second ceramic base  45 . 
     (3) Subsequently, the first and second ceramic bases  39  and  45  coated with the metallizing ink were placed in a furnace and sintered at temperatures in the range of from 1,150 to 1, 350° C. to obtain first and second ceramic members for bonding  31  and  33 . 
     (4) The surfaces of the first and second metallic layers  41  and  47  thus formed by metallization were nickel-plated to form first and second deposit layers  43  and  49 . 
     (5) Thereafter, a silver brazing material  35  was interposed between the two deposit layers  43  and  49  to braze the ceramic members for bonding  31  and  33  together. Thus, the ceramic members  31  and  33  were united with each other to complete a bonded object  37 . 
     c) The bonded object samples (test pieces of sample Nos. 2-1 to 2-17) produced by the process described above were examined for bonding strength by the same method as in Comparative Example 1. 
     The results obtained are shown in Table 2-4 below. 
     
       
         
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                                                      TABLE 2-3 
               
             
             
               
                   
               
               
                 Sam- 
                   
                   
               
               
                 ple 
                 Brazing strength [MPa] 
                  Evalu- 
               
             
          
           
               
                 No. 
                 1150° C. 
                 1200° C. 
                 1250° C. 
                 1300° C. 
                 1350° C. 
                 ation 
               
               
                   
               
             
          
           
               
                 2-1 
                 16.6 
                 28.5 
                 37.8 
                 58.8 
                 63.7 
                 X 
               
               
                 2-2 
                 24.5 
                 47.0 
                 38.0 
                 26.5 
                 20.6 
                 Δ 
               
               
                 2-3 
                 47.0 
                 63.7 
                 60.8 
                 32.3 
                 14.7 
                 ◯ 
               
               
                 2-4 
                 60.8 
                 66.6 
                 63.7 
                 27.4 
                 15.7 
                 ◯ 
               
               
                 2-5 
                 62.7 
                 63.0 
                 60.8 
                 26.5 
                 9.8 
                 ◯ 
               
               
                 2-6 
                 46.8 
                 44.0 
                 34.1 
                 22.5 
                 11.8 
                 Δ 
               
               
                 2-7 
                 32.3 
                 52.2 
                 46.1 
                 20.6 
                 15.7 
                 Δ 
               
               
                 2-8 
                 56.8 
                 62.7 
                 62.7 
                 27.4 
                 14.7 
                 ◯ 
               
               
                 2-9 
                 41.1 
                 44.7 
                 35.1 
                 16.7 
                 10.8 
                 Δ 
               
               
                 2-10 
                 58.9 
                 64.3 
                 63.5 
                 25.1 
                 13.3 
                 ◯ 
               
               
                 2-11 
                 59.4 
                 59.6 
                 45.3 
                 23.8 
                 11.2 
                 Δ 
               
               
                 2-12 
                 44.3 
                 48.7 
                 40.2 
                 26.2 
                 10.4 
                 Δ 
               
               
                 2-13 
                 58.8 
                 58.6 
                 48.4 
                 28.4 
                 8.4 
                 Δ 
               
               
                 2-14 
                 58.2 
                 56.1 
                 51.2 
                 21.4 
                 10.1 
                 Δ 
               
               
                 2-15 
                 40.1 
                 48.7 
                 53.8 
                 26.2 
                 10.2 
                 Δ 
               
               
                 2-16 
                 51.2 
                 62.4 
                 58.4 
                 27.8 
                 11.3 
                 ◯ 
               
               
                 2-17 
                 56.3 
                 54.1 
                 49.6 
                 21.8 
                 9.7 
                 Δ 
               
               
                   
               
             
          
         
       
     
     In the column “Evaluation” in Table 2-4, ◯ indicates that the sample had a peak not lower than 60 MPa in the range of from 1,150 to 1,250° C., Δ indicates that the sample had a peak of from 40 to 60 MPa in that temperature range, and X indicates that the sample had no peak of 40 MPa or higher in that temperature range. 
     Comparative Example 3 
     In Comparative Example 3, explanations on the same points as in Comparative Examples 1 and 2 are omitted. 
     This Comparative Example describes application of a bonded object comprising a ceramic member for bonding and a metallic member, such as that shown in Comparative Example 1, to a vacuum switch as an embodiment. 
     This vacuum switch is a high-load switch which comprises a vacuum vessel having built-in electrodes and other components and is suitable for the switching of high-voltage large-current electricity. 
     The vacuum switch is explained by FIG. 12 in the same manner as the above-described explanations by FIG. 5 in EXAMPLE 3 except that the reference sign  100 ″ is replaced with  100 . 
     Namely, like the bonded object of Comparative Example 1, the high-load switch  100  of this Comparative Example comprises the insulating valve  101 , which is a ceramic member for bonding, and the arc shield  161 , which is a metallic member, bonded by brazing with a brazing material  162  to the ridge part  112  of the insulating valve  101 . 
     FIG. 13 diagrammatically illustrates an important part of the switch  100 . Specifically, the ridge part  112  of the insulating valve  101  has a metallic layer  171  formed on the top thereof by low-temperature metallization in the same manner as in Comparative Example 1. This metallic layer  171  is coated with a deposit layer  173  formed by nickel plating. This deposit layer  173  has been bonded to the arc shield  161  by brazing with the brazing material  162 . 
     Comparative Example 4 
     In Comparative Example 4, explanations on the same points as in Comparative Example 3 are omitted. 
     This Comparative Example describes application of a bonded object comprising a ceramic member for bonding and a metallic member to a vacuum switch as in Comparative Example 3. However, this vacuum switch differs from that of Comparative Example 3 in the structures of the arc shield and the insulating valve. 
     The vacuum switch is explained by FIG. 14 in the same manner as the above-described explanations by FIG. 7 in EXAMPLE 4 except that the reference sign  200 ″,  315  and  317  are replaced with  200 ,  215  and  217 , respectively. 
     The two insulating valves  201  and  203  have, on the outer circumferential surfaces thereof, glaze layers  223  and  225 , respectively, which are the same as the glaze layer in Comparative Example 3. 
     This Comparative Example brings about the same effects as Comparative Example 3. 
     Comparative Example 5 
     A bonded object comprising a ceramic member for bonding and a metallic member is described as a comparative example. 
     The bonded object is explained by FIG. 15 in the same manner as the above-described explanations by FIG. 8 in COMPARATIVE EXAMPLE 1 except that the reference sign  7  and  13  are replaced with  70  and  313 , respectively. 
     In particular, in this comparative example, the layer formed by metallization  11  comprises from 71 to 88% by weight molybdenum, from 0.7 to 5.5% by weight nickel, and from 3 to 18% by weight SiO 2  in terms of silicon oxide amount, while the alloy layer  313  comprises from 36 to 61.3% by weight nickel and from 33 to 60% by weight copper (or from 2 to 30% by weight manganese). 
     b) A process for producing a ring-shaped test piece as an example of this bonded object  70  will be explained below together with a process for producing a ceramic member for bonding  1 . 
     (1) First, a first paste (first metallizing ink) was produced by preparing a first mixture consisting of molybdenum particles, nickel particles, and SiO 2  particles in a proportion shown in Table 2-5 below, pulverizing and mixing the first mixture of particles, and mixing the first mixture (e.g., 87% by weight) with an organic binder (e.g., 13% by weight) such as, e.g., Ethyl cellulose. 
     (2) A second paste (second metallizing ink) was produced in the same manner by preparing a second mixture consisting of two or more members selected from nickel particles, copper particles, manganese particles, molybdenum particles, nickel-copper alloy particles, and nickel-manganese alloy particles as shown in Table 2-5 below, pulverizing and mixing the second mixture of particles, and mixing the second mixture (e.g., 87% by weight) with an organic binder (e.g., 13% by weight) such as, e.g., Ethyl cellulose. 
     (3) The first paste was applied to an alumina-based ceramic base 9 (e.g., 92% by weight alumina) which was a sintered ceramic (e.g., a ring-shaped test piece having a thickness of 5 mm, an outer diameter of 30 mm, and an inner diameter of 8.5 mm) in a thickness of about from 10 to 20 μm. The coating layer was dried to form a first layer (which will become a metallic layer  11  through metallization). 
     (4) Subsequently, the second paste was applied to the whole surface of the first layer in a thickness of about from 10 to 20 μm, and the coating layer was dried to form a second layer (which will become an alloy layer  313 ). 
     (5) The ceramic base  9  thus coated with the first layer and the second layer was placed in a furnace and sintered at the temperatures of from 1,050 to 1,200° C. shown in Table 2-7 in an atmosphere of H 2 /N 2  (1:1) forming gas having a wetter temperature of 50° C. Thus, a ceramic member for bonding  1  was obtained which consisted of the ceramic base  9 , a metallic layer  11  formed thereon by metallization, and an alloy layer  313  formed thereon. 
     (6) Thereafter, the ceramic member for bonding  1  and a metallic member  3  made of Kovar (Fe—Ni—Co) were brazed together. 
     Specifically, a foil of a silver brazing material (BAg-8)  5  was interposed between the alloy layer  313  and the metallic member  3  (e.g., a Kovar disk having a thickness of 1 mm and an outer diameter of 16 mm). This assemblage was heated at a given brazing temperature and then cooled to braze the ceramic member for bonding  1  and the metallic member  3  together. Thus, a bonded object  70  was completed. 
     Using the first and second pastes having different compositions as shown in Table 5 below, ring-shaped test pieces (samples) Nos. 2-101 to 2-110 and 2-115 to 2-120 as shown in FIG. 16 were produced, as bonded objects  70  to be tested, through the steps (1) to (6) described above. 
     Samples Nos.2-111 to 2-114 as comparative examples were further produced using pastes differing in those first and second pastes in composition. 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
                                                      TABLE 2-5 
               
             
             
               
                   
                   
               
               
                   
                   
                 First paste 
                   
               
               
                   
                   
                 (for layer formed 
                 Second paste 
               
               
                   
                 Sample 
                 by metallization) 
                 (for alloy layer) 
               
             
          
           
               
                   
                 No. 
                 Mo 
                 Ni 
                 SiO 2   
                 Ni 
                 Cu 
                 Mn 
                 Mo 
               
               
                   
                   
               
             
          
           
               
                 Compara- 
                 2-101 
                 92 
                 3 
                 5 
                 30 
                 70 
                 — 
                 — 
               
               
                 tive 
                 2-102 
                 92 
                 3 
                 5 
                 40 
                 60 
                 — 
                 — 
               
               
                 Example 
                 2-103 
                 92 
                 3 
                 5 
                 50 
                 50 
                 — 
                 — 
               
               
                   
                 2-104 
                 92 
                 3 
                 5 
                 70 
                 30 
                 — 
                 — 
               
               
                   
                 2-105 
                 92 
                 3 
                 5 
                 80 
                 20 
                 — 
                 — 
               
               
                   
                 2-106 
                 92 
                 3 
                 5 
                 30 
                 — 
                 70 
                 — 
               
               
                   
                 2-107 
                 92 
                 3 
                 5 
                 40 
                 — 
                 60 
                 — 
               
               
                   
                 2-108 
                 92 
                 3 
                 5 
                 50 
                 — 
                 50 
                 — 
               
               
                   
                 2-109 
                 92 
                 3 
                 5 
                 70 
                 — 
                 30 
                 — 
               
               
                   
                 2-110 
                 92 
                 3 
                 5 
                 80 
                 — 
                 20 
                 — 
               
               
                   
                 2-111 
                 92 
                 3 
                 5 
                 30 
                 — 
                 — 
                 70 
               
               
                   
                 2-112 
                 92 
                 3 
                 5 
                 50 
                 — 
                 — 
                 50 
               
               
                   
                 2-113 
                 92 
                 3 
                 5 
                 70 
                 — 
                 — 
                 30 
               
               
                   
                 2-114 
                 92 
                 3 
                 5 
                 100 
                 — 
                 — 
                 — 
               
               
                   
                 2-115 
                 94.5 
                 0.5 
                 5 
                 50 
                 50 
                 — 
                 — 
               
               
                   
                 2-116 
                 94 
                 1 
                 5 
                 50 
                 50 
                 — 
                 — 
               
               
                   
                 2-117 
                 85 
                 10 
                 5 
                 50 
                 50 
                 — 
                 — 
               
               
                   
                 2-118 
                 98 
                 1 
                 1 
                 50 
                 50 
                 — 
                 — 
               
               
                   
                 2-119 
                 65 
                 10 
                 25 
                 50 
                 50 
                 — 
                 — 
               
               
                   
                 2-120 
                 70 
                 10 
                 20 
                 50 
                 50 
                 — 
                 — 
               
               
                   
               
             
          
         
       
     
     Furthermore, a section of each ceramic member for bonding  1  produced was polished, and the layer formed by metallization  11  and the alloy layer  313  were quantitatively analyzed to determine the contents of components. This quantitative analysis was made with an electron probe microanalyzer (accelerating voltage, 20 kV; spot diameter, 5 μm). The results obtained are shown in Table 2-6 below. 
     With respect to each sample, the analysis was made on five points so as to reduce influences of segregation and the five found values for each component were averaged. In Table 2-6, the content of silicon is given in terms of oxide amount. In each layer, the remainder is accounted for by glass ingredients, such as Al 2 O 3 , MgO, and CaO, which have come into the layer from the ceramic base by diffusion. 
     
       
         
               
               
             
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                   
                                                      Table 2-6 
               
             
             
               
                   
                   
               
               
                   
                 Found value after sintering (average) [wt %] 
               
             
          
           
               
                   
                   
                 Layer formed 
                   
               
               
                   
                   
                 by metallization 
                 Alloy layer 
               
               
                   
                 Sample 
                 (lower layer) 
                 (upper layer) 
               
             
          
           
               
                   
                 No. 
                 Mo 
                 Ni 
                 SiO 2   
                 Ni 
                 Cu 
                 Mn 
                 Mo 
               
               
                   
                   
               
             
          
           
               
                 Compara- 
                 2-101 
                 91.3 
                 0.5 
                 4.7 
                 29.2 
                 68.3 
                 — 
                 1.0 
               
               
                 tive 
                 2-102 
                 85.8 
                 0.9 
                 5.8 
                 37.7 
                 56.3 
                 — 
                 1.4 
               
               
                 Example 
                 2-103 
                 90.9 
                 2.2 
                 3.1 
                 45.1 
                 46.2 
                 — 
                 3.8 
               
               
                   
                 2-104 
                 90.2 
                 2.8 
                 3.0 
                 58.1 
                 35.4 
                 — 
                 4.3 
               
               
                   
                 2-105 
                 89.7 
                 3.2 
                 4.2 
                 61.4 
                 30.5 
                 — 
                 5.1 
               
               
                   
                 2-106 
                 89.0 
                 1.2 
                 4.3 
                 35.3 
                 — 
                 34.8 
                 21.1 
               
               
                   
                 2-107 
                 89.4 
                 1.3 
                 4.0 
                 43.3 
                 — 
                 26.1 
                 18.5 
               
               
                   
                 2-108 
                 87.5 
                 1.6 
                 4.2 
                 52.3 
                 — 
                 19.7 
                 19.7 
               
               
                   
                 2-109 
                 88.0 
                 1.5 
                 4.6 
                 61.1 
                 — 
                  2.5 
                 30.5 
               
               
                   
                 2-110 
                 85.3 
                 2.9 
                 4.7 
                 80.4 
                 — 
                  0.9 
                 9.7 
               
               
                   
                 2-111 
                 87.4 
                 2.7 
                 4.8 
                 25.3 
                 — 
                 — 
                 67.8 
               
               
                   
                 2-112 
                 87.5 
                 3.2 
                 3.7 
                 43.3 
                 — 
                 — 
                 51.5 
               
               
                   
                 2-113 
                 87.0 
                 4.0 
                 3.9 
                 60.8 
                 — 
                 — 
                 33.5 
               
               
                   
                 2-114 
                 86.8 
                 4.3 
                 3.8 
                 89.1 
                 — 
                 — 
                 3.5 
               
               
                   
                 2-115 
                 88.8 
                 0.7 
                 4.8 
                 46.2 
                 47.3 
                 — 
                 4.0 
               
               
                   
                 2-116 
                 86.7 
                 1.4 
                 4.2 
                 46.7 
                 47.1 
                 — 
                 4.0 
               
               
                   
                 2-117 
                 84.7 
                 5.3 
                 3.2 
                 47.2 
                 47.0 
                 — 
                 3.7 
               
               
                   
                 2-118 
                 93.3 
                 1.3 
                 3.3 
                 45.3 
                 46.4 
                 — 
                 4.2 
               
               
                   
                 2-119 
                 70.1 
                 4.1 
                 19.2 
                 44.8 
                 46.8 
                 — 
                 2.8 
               
               
                   
                 2-120 
                 71.3 
                 4.5 
                 16.7 
                 46.5 
                 47.2 
                 — 
                 3.5 
               
               
                   
               
             
          
         
       
     
     c) The samples of the bonded object  70  produced by the process described above were examined for bonding strength. 
     This examination was conducted in the same manner as in COMPARATIVE EXAMPLE 1 except that FIG. 10 is replaced with FIG.  17  and the reference sign  7  is replaced with  70  in the description. With respect to each sample, the evaluation of the brazing strengths (bonding strengths) for the respective sintering temperatures are shown in the following Table 2-7. 
     In the column “Evaluation of bonding strength and obtained data” in Table 2-7, ◯ indicates that the sample had a bonding strength higher than 17 MPa in the range of from 1,050 to 1, 200° C., Δ indicates that the sample had a bonding strength of from 11 to 17 MPa in that temperature range, and X indicates that the sample had a bonding strength lower than 11 MPa in that temperature range. In addition, bonding strength data of each of the samples are shown in parentheses “( )” at the column “Evaluation of bonding strength and obtained data” in table 2-7. 
     
       
         
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
                                                      TABLE 2-7 
               
             
             
               
                   
                   
               
               
                   
                 Evaluation of bonding strength and obtained 
               
               
                   
                 data [MPa] 
               
               
                   
                 Sintering temperature [° C.] 
               
             
          
           
               
                   
                 1050 
                 1100 
                 1150 
                 1200 
               
               
                   
                   
               
             
          
           
               
                 Compara- 
                 2-101 
                 X (2.0) 
                 Δ (14.2) 
                 ◯ (17.3) 
                 X (5.4) 
               
               
                 tive 
                 2-102 
                 X (2.5) 
                 ◯k (19.8) 
                 ◯k (17.6) 
                 X (5.8) 
               
               
                 Example 
                 2-103 
                 X (5.7) 
                 ◯k (18.5) 
                 ◯k (19.3) 
                 X (6.2) 
               
               
                   
                 2-104 
                 Δ (11.5) 
                 ◯k (17.8) 
                 ◯k (18.7) 
                 X (6.0) 
               
               
                   
                 2-105 
                 X (6.0) 
                 ◯k (17.4) 
                 Δ (16.4) 
                 X (6.1) 
               
               
                   
                 2-106 
                 X (2.0) 
                 ◯ (17.2) 
                 X (10.0) 
                 X (5.3) 
               
               
                   
                 2-107 
                 X (1.8) 
                 ◯k (18.4) 
                 ◯ (18.4) 
                 X (5.7) 
               
               
                   
                 2-108 
                 X (2.2) 
                 ◯k (19.0) 
                 ◯k (18.8) 
                 X (5.5) 
               
               
                   
                 2-109 
                 X (3.1) 
                 ◯k (17.5) 
                 ◯k (18.1) 
                 X (5.0) 
               
               
                   
                 2-110 
                 X (5.4) 
                 ◯ (17.1) 
                 Δ (15.3) 
                 X (4.7) 
               
               
                   
                 2-111 
                 X (1.9) 
                 X (3.8) 
                 Δ (11.3) 
                 X (4.1) 
               
               
                   
                 2-112 
                 X (2.0) 
                 X (6.0) 
                 Δ (14.1) 
                 Δ (11.2) 
               
               
                   
                 2-113 
                 X (2.2) 
                 X (7.6) 
                 Δ (12.8) 
                 X (4.0) 
               
               
                   
                 2-114 
                 X (1.7) 
                 X (8.4) 
                 Δ (13.2) 
                 X (4.2) 
               
               
                   
                 2-115 
                 X (1.0) 
                 Δ (15.5) 
                 ◯k (17.4) 
                 Δ (12.8) 
               
               
                   
                 2-116 
                 X (1.2) 
                 ◯k (18.1) 
                 ◯k (18.4) 
                 X (5.7) 
               
               
                   
                 2-117 
                 X (7.8) 
                 ◯k (17.5) 
                 ◯k (17.3) 
                 X (6.0) 
               
               
                   
                 2-118 
                 X (2.1) 
                 Δk (15.3) 
                 ◯k (17.2) 
                 Δ (12.2) 
               
               
                   
                 2-119 
                 Δ (11.5) 
                 ◯k (17.7) 
                 Δk (14.4) 
                 X (5.7) 
               
               
                   
                 2-120 
                 Δ (13.5) 
                 ◯k (18.1) 
                 ◯k (17.3) 
                 X (5.7) 
               
               
                   
               
             
          
         
       
     
     d) Subsequently, the samples of the bonded object  70  produced by the process described above were examined for airtightness. 
     Specifically, one side of the bonded object  70  shown in FIG. 15 was made vacuum (1×10 −8  Torr or lower) and the other side was filled with helium to examine the bonded object  70  for helium leakage. 
     The results obtained are shown in Table 2-7 by means of symbol “k”. 
     Comparative Example 6 
     In Comparative Example 6, explanations on the same points as in Comparative Example 5 are omitted. 
     In this Comparative Example, a bonded object comprising two ceramic members bonded to each other is described as an embodiment. 
     a) FIG. 18 diagrammatically shows this embodiment, which is a bonded object  370  comprising a first ceramic member for bonding  31  made of alumina and, bonded thereto with a brazing material  35 , a second ceramic member for bonding  33  made of alumina and similar to the first ceramic member  31 . 
     More specifically, the first ceramic member for bonding  31  comprises a first ceramic base  39 , a first metallic layer  41  formed thereon by metallization, and a first alloy layer  343  formed on this first metallic layer  41 . On the other hand, the second ceramic member for bonding  33  comprises a second ceramic base  45 , a second metallic layer  47  formed thereon by metallization, and a second alloy layer  349  formed on this second metallic layer  47 . 
     The first ceramic member for bonding  31  and the second ceramic member for bonding  33  have been united with each other by bonding the first alloy layer  343  to the second alloy layer  349  with the brazing material  35 . 
     b) A process for producing this bonded object  370  will be explained below together with a process for producing the first and second ceramic members for bonding  31  and  33 . 
     (1) Using particulate ingredients shown in Table 2-5 as first-paste and second-paste ingredients, a first paste and a second paste for each sample were produced in the same manner as in Comparative Example 5 (hereinafter, the points on which explanation is omitted here are the same as in Comparative Example 5). 
     (2) The first paste was applied to a surface of each of first and second ceramic bases  39  and  45  and then dried to form respective first layers. 
     (3) Subsequently, the second paste was applied to the surface of the first layer of each of the first and second ceramic bases  39  and  45  and then dried to form respective second layers. 
     (4) Thereafter, the first and second ceramic bases  39  and  45  each coated with the first layer and the second layer were placed in a furnace and sintered at temperatures in the range of from 1,050 to 1,200° C. Thus, a first ceramic member for bonding  31  having a first layer formed by metallization  41  and a first alloy layer  343  superposed thereon and a second ceramic member for bonding  33  having a second layer formed by metallization  47  and a second alloy layer  349  superposed thereon were obtained. 
     (5) Subsequently, a silver brazing material  35  was interposed between the two alloy layers  343  and  349  to braze the ceramic members for bonding  31  and  33  together. Thus, the ceramic members  31  and  33  were united with each other to complete a bonded object  370 . 
     This bonded object  370  shown as an embodiment has high bonding strength and excellent air tightness like the bonded object shown in Comparative Example 5. 
     Comparative Example 7 
     In Comparative Example 7, explanations on the same points as in Comparative Examples 5 and 6 are omitted. 
     This Comparative Example describes application of a bonded object comprising a ceramic member for bonding and a metallic member, such as that shown in Comparative Example 5, to a vacuum switch as an embodiment. 
     This vacuum switch is a high-load switch which comprises a vacuum vessel having built-in electrodes and other components and is suitable for the switching of high-voltage large-current electricity. 
     The vacuum switch is explained by FIG. 19 in the same manner as the above-described explanations by FIG. 5 in EXAMPLE 3 except that the reference sign  100 ″ is replaced with  100 ′. 
     Namely, like the bonded object of Example 5, the high-load switch  100 ′ of this Example comprises the insulating valve  101 , which is a ceramic member for bonding, and the arc shield  161 , which is a metallic member, bonded by brazing with a brazing material  162  to the ridge part  112  of the insulating valve  101 . 
     FIG. 20 diagrammatically illustrates an important part of the switch  100 ′. Specifically, the ridge part  112  of the insulating valve  101  has a metallic layer  171  formed on the top thereof by low-temperature metallization in the same manner as in Comparative Example 5. This layer formed by metallization  171  is coated with an alloy layer  373 . This alloy layer  373  has been bonded to the arc shield  161  by brazing with the brazing material  162 . 
     Thus, the insulating valve  101  equipped with the arc shield  161  (and hence the high-load switch  100 ′) can be produced at low cost, and high dimensional accuracy and high bonding strength can be realized. 
     Comparative Example 8 
     In Comparative Example 8, explanations on the same points as in Comparative Example 7 are omitted. 
     This Comparative Example describes application of a bonded object comprising a ceramic member for bonding and a metallic member to a vacuum switch as in Comparative Example 7. However, this vacuum switch differs from that of Comparative Example 7 in the structures of the arc shield and the insulating valve. 
     The vacuum switch is explained by FIG. 21 in the same manner as the above-described explanations by FIG. 7 in EXAMPLE 4 except that the reference sign  200 ″ is replaced with  200 ′. 
     The two insulating valves  201  and  203  have, on the outer circumferential surfaces thereof, glaze layers  223  and  225 , respectively, which are the same as the glaze layer in Comparative Example 7. 
     This Comparative Example brings about the same effects as Comparative Example 7.