METHOD FOR PRODUCING AN ELECTRODE MATERIAL AND ELECTRODE MATERIAL

Abstract

A method for producing an electrode material (1), comprising: forming a molded article by molding a powder of a solid solution of chromium and at least one kind of heat-resistant element selected from Mo, W, Ta, Nb, V and Zr; filling and molding a Cr powder around an outer periphery of the molded body, thereby forming an integrally molded body; and infiltrating the integrally formed body with a conductive element selected from Cu, Ag and an alloy of Cu and Ag.

Description

Field of the InventionThe present invention relates to an electrode material for use in a vacuum switch and so forth. More particularly, the present invention relates to a method for producing an electrode material in which a heavy current interrupting capability and a capacitor switching capability are required, and the electrode material.Prior ArtAn electrode material for use for an electrode of a vacuum switch (VS), etc. is required to satisfy the following characteristics: (1) high interruption capacity; (2) high withstand voltage; (3) low contact resistance; (4) high welding resistance; (5) low contact wear; (6) low interruption current; (7) good workability; and (8) high mechanical strength.Since some of the above-mentioned features balance each other, there is no electrode material satisfying all of the above-mentioned features. Electrode materials are therefore suitably selected depending on the applications of breakers such as those for heavy current interruption or high withstand voltage. It has been an important question how to develop an electrode material having different properties.In recent years, the conditions of using vacuum switches have become severe and, at the same time, the range of applications of vacuum switches in capacitor circuits has been increasing. In a capacitor circuit, a voltage is applied between electrodes which is two or three times as high as the usual voltage. For this reason, it is considered that contact surfaces of the electrodes suffer considerable damage by the arc generated at the time of the current interruption or current switching operation, thereby easily causing re-ignition of the arc. Accordingly, there has been an increasing demand for a contact material having withstand voltage and current cut capabilities superior to those of conventional Cu-Cr electrode materials.As a method for manufacturing Cu-Cr electrodes having superior electrical characteristics such as current interrupting capability and withstand voltage capability, an electrode manufacturing method is known in which a Cu powder as a base material is mixed with a Cr powder for improving electrical characteristics and a powder of a heat resistant element (such as molybdenum (Mo), tungsten (W), niobium (Nb), tantalum (Ta), vanadium (V), zirconium (Zr), or the like) for micronizing the Cr particles, followed by press molding the mixed powder in a mold and sintering the molded body (see, for example, Patent Documents 1 and 2).More specifically, a Cu-Cr electrode material is prepared using a Cr powder having a particle size of 200 to 300 μm as a starting material; and a heat-resistant element is added to the Cu-Cr electrode material to enable micronization of the Cr powder by a microstructure method, that is, to promote alloying of Cr and the heat-resistant element and to improve deposition of Cr-X fine particles (wherein X is the heat-resistant element) in the Cu base material phase. Thereby, the electrode has a composition in which Cr particles of 20 to 60 μm in diameter are uniformly dispersed in the Cu base material phase in the form of binding the heat resistant element therein.In order to improve the electrical characteristics such as current interrupting capability and withstand voltage capabilities of the above electrode material, it is necessary to increase the contents of Cr and the heat-resistant element in the Cu base material phase, and to finely and uniformly disperse the particles of Cr and the solid solution of Cr and the heat-resistant element in the Cu base material phase.As a result of extensive studies, the present inventors have invented an electrode material of a Cu-Cr heat resistant element (for example, Mo) system (see, for example, Patent Documents 3 to 5). This electrode material combines uniform dispersion of fine Cr-containing particles with uniform dispersion of fine Cu structures as a high conductive component and exhibits superior heavy current interrupting and withstand voltage capabilities.In general, contact materials for use in breakers or the like must be stabilized in withstand voltage capability by a voltage forming treatment in which fine projections or adhering foreign matters are struck on contact surfaces between contacts, or by a current forming treatment in which contact surfaces are melted by arc.However, the electrode material of a Cu-Cr heat resistant element (for example, Mo) system has a higher surface hardness and a higher melting point than the conventional Cu-Cr electrode materials. Therefore, there is a possibility that the energy required for stabilizing the withstand voltage capability becomes high. There is also a possibility that deposits inside the vacuum switch generated by the stabilizing treatments become a factor destabilizing withstand voltage capability. In addition, the electrode material of a Cu-Cr heat resistant element system (for example, Mo) has the same current application capability as the conventional Cu-Cr electrode materials, so that a smaller electrode diameter cannot be obtained and a shortening of the time required for the forming treatment cannot be expected by reducing the contact area.Patent Document 12 describes manufacturing a contact plate for an electrode having a Cu-Cr-Mo inner part and a Cu-Cr outer part by a method including filling the outer region of a metal mold with Cu-Cr powder, filling the inner region of the metal mold with Cu-Cr-Mo powder, a subsequent one-time press molding, and a subsequent sintering process. Patent document 13 describes press forming, sintering and subsequent Cu infiltration of a Mo-Cr powder. Patent document 14 describes press molding and subsequent sintering of a mixture of Cu powder, Cr powder and Mo powder.Documents from the Prior ArtPatent DocumentsPatent Document 1: Japanese Patent Application Laid-Open No. 2012-7203Patent Document 2: Japanese Patent Laid-Open No. 2002-180150Patent Document 3: Japanese Patent No. 5861807Patent Document 4: Japanese Patent No. 5880789Patent Document 5: Japanese Patent No. 5904308Patent Document 6: Japanese Patent Laid-Open No. 2016-065281Patent Document 7: Japanese Patent Application Laid-Open No. 2012-133988Patent Document 8: Japanese Patent Laid-Open No. H 05-047275Patent Document 9: Japanese Patent Laid-Open No. S63-266720Patent Document 10: Japanese Patent Laid-Open No. 2015-078435Patent Document 11: Japanese Patent Application Laid-Open No. 2010-277962Patent Document 12: US 2015 / 0200059 A1Patent Document 13: US 2016 / 0369373 A1Patent Document 14: US 2016 / 0332231 A1SUMMARY OF THE INVENTIONIt is an object of the present invention to provide a method for producing an electrode material having superior current interruption and withstand voltage capabilities.According to an aspect of the present invention for achieving the above object, there is provided a manufacturing method of an electrode material, which comprises: forming a molded body by molding a powder of a solid solution of Cr and at least one kind of heat-resistant element selected from Mo, W, Ta, Nb, V and Zr; filling and molding a powder of Cr around the outer periphery of the molded body, thereby forming an integrally molded body; and infiltrating the integrally molded body with a conductive element selected from Cu, Ag and an alloy of Cu and Ag.According to another aspect of the present invention for achieving the above object, there is provided a manufacturing method for an electrode material as described above, wherein the manufacturing method further comprises sintering the integrally molded body, and wherein, in the infiltrating, the sintered integrally molded body is infiltrated with the conductive member.According to still another aspect of the present invention for achieving the above object, there is provided a manufacturing method of an electrode material as described above, wherein the manufacturing method further comprises sintering the molded body, and wherein, in filling and forming, the integrally molded body is obtained by filling and forming the powder of Cr around the sintered molded body.According to still another aspect of the present invention for achieving the above object, there is provided a manufacturing method of an electrode material as described above, wherein in the X-ray diffraction measurement of the solid solution powder, either a peak to be attributed to Cr or a peak to be attributed to the heat-resistant element has disappeared.According to an aspect of the present invention for achieving the above object, there is provided an electrode material comprising: a central part having good current interrupting capability; and an outer circumferential part disposed on an outer periphery of the central part, the central part having a mixed metal composition in which solid solution particles are uniformly dispersed in a Cu phase, the solid solution particles being formed of a solid solution of Cr and at least one kind of heat-resistant element selected from Mo, W, Ta, Nb, V and Zr, the mixed metal composition containing, in terms of a weight ratio with respect to the mixed metal composition, 20 to 70% of Cu, 1.5 to 64% of Cr and 6 to 76% of the heat-resistant element, wherein the balance is inevitable impurities, the particles of the solid solution in the mixed metal composition have an average particle diameter of 20 μm or smaller and are uniformly dispersed in the Cu phase with a dispersion state index of 1.0 or lower, and the outer circumferential part contains 75 wt % to 90 wt % of Cr based on a weight of the outer circumferential part, the balance being Cu.According to the present invention, it is possible to obtain the electrode material having superior interrupting and withstand voltage capabilities.Brief Description of the DrawingsFIG. 1 is a schematic view of an electrode material according to an embodiment of the present invention. FIG. 2 is a flowchart of a manufacturing method of an electrode material according to an embodiment of the present invention. FIG. 3 is a schematic cross-sectional view of a vacuum switch having an electrode contact formed of the electrode material according to the embodiment of the present invention. FIG. 4 is a reflected electron image of a boundary portion between two regions of the electrode material (at magnification: 50 times). FIG. 5 is a reflected electron image of a boundary portion between two regions of the electrode material (at 500 magnifications). Fig. 6(a) is a schematic view of an examination sample; and Fig. 6(b) is a pictorial view of the examination sample before and after a tensile strength test. FIG. 7 is a diagram showing details of the electrode materials according to Examples 1 to 9 and Reference Examples 1 and 2. FIG. 8 is a pictorial view of a conventional electrode material (CuCr electrode) during a 33 kA interruption.DESCRIPTION OF THE EMBODIMENTSA manufacturing method of an electrode material and an electrode material according to embodiments of the present invention will be described in detail below with reference to the drawings. In the following description of the embodiments, unless otherwise specified, an average particle diameter, a median diameter d50, a volume-related relative particle amount, and similar average values are measured by a laser diffraction particle size analyzer (available from CILAS Inc. under the trade name of CILAS 1090L).Referring to FIG. 1, an electrode material 1 manufactured by an electrode material manufacturing method according to an embodiment of the present invention includes a cylindrical columnar center part 2 and an outer circumferential part 3 disposed on an outer periphery of the center part 2. For example, the central portion 2 is a Cu-Cr heat resistant element system portion having superior heavy current cut and capacitor switching capabilities; and the outer peripheral portion 3 is a Cu-Cr system portion having superior withstand voltage capability.The center part 2 is formed by, for example, forming a skeleton from a solid solution of chromium (Cr) and a heat-resistant element, and infiltrating the skeleton with a conductive element such as copper (Cu), silver (Ag), or Cu-Ag alloy. Preferably, the center part 2 is formed using an electrode material disclosed in Patent Documents 3 to 5 and so forth. In the following, the elements forming the central part 2 will be explained in detail.The heat-resistant element may be a single element or a combination of elements selected from molybdenum (Mo), tungsten (W), tantalum (Ta), niobium (Nb), vanadium (V), zirconium (Zr), beryllium (Be), hafnium (Hf), iridium (Ir), platinum (Pt), titanium (Ti), silicon (Si), rhodium (Rh), ruthenium (Ru), and the like. Particularly preferred are Mo, W, Ta, Nb, V and Zr, each of which has a pronounced micronizing effect on Cr particles. In the case where the heat-resistant element is used in the form of a powder, the powder of the heat-resistant element has an average particle diameter of, for example, 2 to 20 μm, preferably 2 to 10 μm, so that the Cr-containing particles (for example, particles of a solid solution of the heat-resistant element and Cr) are formed finely and uniformly dispersed in the electrode material. The content of the heat-resistant element in the center part 2 is generally 6 to 76 wt %, preferably 32 to 68 wt %, based on the weight of the center part 2. in this content range, the center part 2 achieves improved withstand voltage and current cut capabilities without impairing mechanical strength and workability.The content of Cr in the central part 2 is generally 1.5 to 64 wt %, preferably 4 to 15 wt %, based on the weight of the central part 2. in this content range, the central part 2 achieves improved withstand voltage and current cut capabilities without impairing mechanical strength and workability. If Cr is used in the form of a powder, the Cr powder has a particle size of, for example, -48 mesh (i.e., a particle diameter of less than 300 μm), -100 mesh (i.e., a particle diameter of less than 150 μm), more preferably -325 mesh (i.e., a particle diameter of less than 45 μm), so that the center part 2 having superior withstand voltage and current interrupting capabilities can be formed. In particular, the use of the Cr powder having a particle size of -100 mesh results in a reduction in the amount of Cr remaining, which may be a factor that increases the particle diameter of Cu infiltrated into the electrode material.The proportion of the conductive member (such as Cu, Ag or Cu-Ag alloy) in the central part 2 is generally 20 to 70 % by weight, preferably 25 to 60 % by weight, based on the weight of the central part 2. in this proportion range, the central part 2 achieves reduced contact resistance without impairing withstand voltage and current interrupting capabilities. Since the proportion of the conductive member in the central part 2 is determined by the operation of infiltrating the conductive member, the sum of the proportions of the heat-resistant member, the Cr and the conductive member does not exceed 100% by weight based on the weight of the central part 2.The outer peripheral part 3 is formed by, for example, molding a powder of Cr and infiltrating the resultant molded body with a conductive member such as Cu. There is no particular limitation on the particle diameter of Cr as a constituent element of the outer peripheral part 3. The content of Cr in the outer peripheral part 3 is generally 60% by weight or more, preferably 75% by weight to 90% by weight, based on the weight of the outer peripheral part 3.Referring to the flowchart of FIG. 2, the manufacturing method of an electrode material according to an embodiment of the present invention will be explained in detail below. Although the following explanation is given using Mo as an example of the heat-resistant element and Cu as the conductive element, the same applies to the case of using another heat-resistant element and another conductive element.In the mixing step S 1, a heat-resistant element powder (for example, Mo powder) and a Cr powder are mixed. It is preferable to mix the Mo powder and the Cr powder such that the weight ratio of Mo to Cr is 1 or more to 1, more preferably 3 or more to 1, still more preferably 9 or more to 1. In this range of the weight ratio, the center part 2 can be formed with superior withstand voltage and current interrupting capabilities.In the preliminary sintering step S 2, the mixed powder (hereinafter, simply referred to as the "mixed powder") obtained by mixing the Mo powder and the Cr powder in the mixing step S 1 is placed in a container that can neither react with Mo nor with Cr (such as a container made of alumina), and then subjected to preliminary sintering at a certain temperature (for example, 1250° C. to 1500° C.) under a non-oxidizing atmosphere (for example, a hydrogen atmosphere or a vacuum atmosphere). By the preliminary sintering, a MoCr solid solution is obtained wherein Mo and Cr are mutually dissolved and diffused. In the preliminary sintering step S 2, the preliminary sintering is not necessarily performed until all of Mo and Cr are transferred to the solid solution. However, the use of the preliminarily sintered body in which either one or both of the X-ray diffraction peaks (XRD) attributed to the elements Mo and Cr have completely disappeared (that is, one of Mo and Cr has completely dissolved in the other element) contributes to higher withstand voltage capability of the center part 2. For this reason, it is preferable that: if the Mo powder is mixed in a large amount, the sintering temperature and the duration of the preliminary sintering step S 2 are set such that at least the peak to be assigned to Cr has disappeared in the MoCr solid solution spectrum measured by X-ray diffraction; and if the Cr powder is mixed in a large amount, the sintering temperature and the duration of the preliminary sintering step S 2 are set such that at least the peak to be assigned to Mo has disappeared in the MoCr solid solution spectrum measured by X-ray diffraction.In the preliminary sintering step S 2, the mixed powder may be subjected to press forming (pressing treatment) before the preliminary sintering. By the press forming, mutual diffusion of Mo and Cr can be promoted, so that the duration of preliminary sintering is shortened and the temperature of preliminary sintering is lowered. There is no particular limitation on the pressure applied in press forming. The press forming pressure is preferably 0.1 t / cm 2 or lower. When the press forming pressure on the mixed powder is very high, the provisional sintered body may become hard and thus difficult to pulverize in the subsequent pulverization step S 3.In the pulverization step S 3, a powder of the MoCr solid solution (hereinafter also referred to as "MoCr powder") is obtained by pulverizing the MoCr solid solution with a pulverizer (such as a planetary ball mill). Although it is preferable to perform pulverization in pulverization step S 3 under a non-oxidizing atmosphere, pulverization may be performed in the air. The pulverization conditions are set to allow pulverization of the particles (secondary particles) when the particles of the MoCr solid solution are joined together. The longer the pulverization time, the smaller the average particle diameter of the particles of the MoCr solid solution. By setting the pulverization conditions so that the volume-related relative particle amount of particles of 30 μm diameter or smaller (preferably particles of 20 μm diameter or smaller) in the MoCr powder reaches 50% or more, the center part 2 in which MoCr particles (i.e., particles formed by mutual dissolution and diffusion of Mo and Cr) and Cu structures are uniformly dispersed can be obtained.In the forming step S 4, the MoCr powder is subjected to forming. For example, the forming is performed by press-molding the MoCr powder at a pressure of 2 t / cm 2.In the main sintering step S 5, the simple molded body of the MoCr powder is subjected to main sintering, thereby forming a MoCr sintered body (MoCr skeleton). For example, the main sintering is performed by sintering the molded body of the MoCr powder at 1150° C. for 2 hours under a vacuum atmosphere. The main sintering step S 5 is a step in which the denser MoCr sintered body is formed by the MoCr particles deforming and joining. It is preferable to perform sintering of the MoCr powder at a temperature higher than or equal to the temperature condition of the later Cu infiltration step S 7. For example, the sintering temperature is preferably set to 1150° C. or higher. This is because, when the main sintering is performed at a temperature lower than the infiltration temperature, a gas contained in the MoCr sintered body is again evolved during the Cu infiltration and remains in the resultant Cu infiltrated body. The presence of such a gas becomes a factor affecting withstand voltage and current interrupting capabilities. In the main sintering step S 5, therefore, the sintering temperature is set to be higher than or equal to the Cu infiltration temperature and lower than or equal to the melting point of Cr. The sintering temperature is preferably in the range from 1150 to 1500° C. In this temperature range, the MoCr particles can be tightly packed and sufficiently degassed. The main sintering step S 5 is not necessarily performed. The outer peripheral part forming step S 6 and the Cu infiltration step S 7 may be performed on the molded body obtained in the molding step S 4 or the sintered body (MoCr solid solution) obtained in the preliminary sintering step S 2.In the outer peripheral part forming step S 6, a Cr powder is filled and press-formed (for example, at a pressure of 3 t / cm 2). to the outer periphery of the MoCr sintered body obtained in the main sintering step S 5. Then, the integrally molded body is sintered at 1150° C. for 2 hours under a vacuum atmosphere, for example, and thereby processed into a base material body of MoCr phase and Cr phase (porous composite sintered body). In the outer peripheral part forming step S 6, the sintering is not necessarily performed. The subsequent Cu infiltration step S 7 may be performed on the integrally molded body without sintering.In the Cu infiltration step S 7, the base material body (porous composite sintered body) is infiltrated with Cu. For example, the MoCr sintered body is infiltrated with Cu by placing a Cu plate material on the MoCr sintered body and maintaining them under a non-oxidizing atmosphere at a temperature higher than or equal to the melting point of Cu for a certain time (for example, at 1150° C. for 2 hours).A vacuum switch can be constructed by using the electrode material manufactured by the manufacturing method for the electrode material according to the embodiment of the present invention (hereinafter, also simply referred to as the "electrode material according to the embodiment of the present invention"). Referring to FIG. 3, the vacuum switch 4 using the electrode material according to the embodiment of the present invention includes a vacuum container 5, a fixed electrode 6, a movable electrode 7, and a main shield 13.The vacuum container 5 has an insulating tube 8 which is closed at both open ends thereof by a fixed-side end plate 9 and a movable-side end plate 10, respectively.The fixed electrode 6 is fixed in a state of passing through the end plate 9 on the fixed side. One end of the fixed electrode 6 is fixed at a position opposite and opposite to one end of the movable electrode 7 inside the vacuum container 5. An electrode contact 11 formed of the electrode material according to the embodiment of the present invention is disposed at an end portion of the fixed electrode 6 that is opposite and opposite to the movable electrode 7.The movable electrode 7 is provided by the movable-side end plate 10 so as to be coaxial with the fixed electrode 6. By a switching means, not shown, for opening / closing the fixed electrode 6 and the movable electrode 7, the movable electrode 7 can be axially moved. An electrode contact 11 is also disposed at an end portion of the movable electrode 7 which is opposite to and opposed to the fixed electrode 6. In addition, between the movable electrode 7 and the movable-side end plate 10, a bellows 12 is disposed to allow the fixed electrode 6 and the movable electrode 7 to be opened / closed by vertically moving the movable electrode 7 while maintaining the vacuum inside the vacuum container 5.The main shield 13 is disposed so as to cover a contact part of the electrode contact of the fixed electrode 6 and the electrode contact of the movable electrode 7, and protect the insulating tube 8 from the arc generated between the fixed electrode 6 and the movable electrode 7.[Example 1]An electrode material according to Example 1 was prepared in accordance with the flow chart of Fig. 2. In the following explanation, forming step S 4 to Cu infiltration step S 7 will be explained in detail. (The same applies to the other examples.) As methods for producing a MoCr fine powder, those described in Reference Examples 1 and 2 mentioned below are known. However, the production method of the MoCr fine powder is not limited to those described in Reference Examples 1 and 2 mentioned below.The electrode material according to Example 1 was an electrode material prepared by sintering the integrally molded body in the outer peripheral part forming step S 6, without sintering the molded body (that is, without performing the main sintering step S 5), and infiltrating the resultant base material body with Cu.A MoCr fine powder of 5.7 μm median diameter (MoCr weight ratio: Mo:Cr=9:1) was molded with a pressing pressure of 3 t / cm 2 to form a molded body having a diameter φ of 40 mm and a length L of 24 mm. A Cr powder (median diameter: 64 μm) was filled on an outer periphery of the molded body and molded with a pressing pressure of 3 t / cm 2 to form an integrally molded body having a diameter φ of 80 mm and a length L of 24 mm. The integrally molded body was sintered at 1150° C. for 1.5 hours under a vacuum atmosphere, and thereby processed into a base material body (porous composite sintered body). The base material body thus formed was infiltrated with Cu by placing a Cu plate material on the base material body and holding the base material body and the Cu plate material together in a vacuum furnace at 1150° C. for 2 hours. Thus, the electrode material of Example 1 was obtained. Subsequently, the electrode material was machined to remove excess Cu remaining after the Cu infiltration and allow the central part (CuCrMo region) and the outer peripheral part (CuCr region) to be exposed on a surface of the electrode material. The electrical conductivity of the electrode material according to Example 1 was measured on both sides. It was confirmed that the electric conductivity of the central part of the electrode material was 36% IACS, and the electric conductivity of the outer circumferential part of the electrode material was 21% IACS.Reflected electron images of a boundary portion between the central and outer peripheral parts of the electrode material according to claim 1 are shown in Figs. 4 and 5. It can be seen from FIG. 4 that the central part and the outer, encircling part were connected to one another without large cavities being present at the seam of the central part and of the outer, encircling part. It is apparent from FIG. 5 that the Cr particles were more firmly bonded to the MoCr particles in the boundary portion. It was assumed that the weight ratio of Mo and Cr in the MoCr region of the boundary portion was about 1:1 (the weight ratio of Mo and Cr in a portion of the electrode material outside the boundary portion was 9:1). In addition, it was assumed that Cr particles that advanced to the boundary portion were those of Cr that were dissolved in Cu during Cu infiltration and diffused to the MoCr region but were not dissolved in the Mo-Cr particles. Although it is assumed that Mo diffused into the Cr phase simultaneously with the diffusion of Cr into the MoCr region, such diffused Mo was very small and thus not recognizable. In this way, the boundary portion was formed with a boundary layer in which MoCr and Cr were mutually dissolved and diffused, so that the connection between the central part and the outer circumferential part was strong.Herein, a tensile strength test was performed using a test sample 14 as shown in FIG. 6( a) to compare the bonding strength of the electrode material of Example 1 with that of a CuCr material (as the electrode material of Comparative Example 1 mentioned below) currently used as the contact material of a vacuum switch in terms of the tensile strength. The tensile strength can be regarded as an index of breaking or deformation of the electrode at each switching operation of the vacuum switch. Therefore, it is estimated that the electrode material is usable as a contact material of a vacuum switch when it has a maximum tensile strength higher than or equal to that of the currently used CuCr material.The test sample was prepared by machining the electrode material according to Example 1 so that the joint of the electrode material was located at a central portion 14 aof the test sample 14. The maximum tensile strength of the test sample was measured with a precision universal tester at a speed of 1 mm / min. The appearances of the test sample of the electrode material of Example 1 before and after the tensile strength test are shown in FIG. 6( b). The maximum tensile strength of the test sample of the electrode material according to Comparative Example 1 was also measured in the same manner as Example 1. As a result of comparing the test results, it was confirmed that the maximum tensile strength of the electrode material according to Example 1 (i.e., the strength of the joint of the central part and the outer circumferential part) was 1.4 times as large as that of the electrode material according to Comparative Example 1. These measurement results are shown in FIG. 7 as a relative value to the maximum tensile strength of the electrode material according to Comparative Example 1, respectively.[Example 2]An electrode material according to Example 2 was an electrode material prepared by infiltrating the base material body with Cu without sintering the molded body and without sintering the integrally molded body.A MoCr fine powder of 5.7 μm median diameter (MoCr weight ratio: Mo:Cr=9:1) was molded with a pressing pressure of 3 t / cm 2 to form a molded body having a diameter φ of 40 mm and a length L of 24 mm. A Cr powder (median diameter: 64 μm) was filled on an outer periphery of the molded body and molded with a pressing pressure of 3 t / cm 2 to form an integrally molded body having a diameter φ of 80 mm and a length L of 24 mm. The integrally molded body thus formed was infiltrated with Cu by placing a Cu plate material on the integrally molded body and holding the integrally molded body and the Cu plate material together in a vacuum furnace at 1150° C. for 2 hours. Thus, the electrode material of Example 2 was obtained. The tensile strength and electric conductivity of the electrode material of Example 2 were measured and confirmed to be the same as those of the electrode material of Example 1. In other words, it was confirmed that the electrode material of Example 2 had sufficient strength to resist repeated mechanical impacts over a long period of time by switching operations of a vacuum switch.[Example 3]An electrode material according to Example 3 was an electrode material which was prepared by sintering the integrally molded body without sintering the molded body, and infiltrating the resultant base material body with Cu. In this example, the particle diameter of the Cr powder used as the raw material for the outer peripheral part was different from that of the electrode material of Example 1.A MoCr fine powder of 5.79 μm in media diameter (MoCr weight ratio: Mo:Cr=9:1) was molded with a pressing pressure of 3 t / cm 2 to form a molded body having a diameter φ of 40 mm and a length L of 24 mm. A Cr powder (median diameter: 39 μm) was filled on an outer periphery of the molded body and molded with a pressing pressure of 3 t / cm 2 to form an integrally molded body having a diameter φ of 80 mm and a length L of 24 mm. The integrally molded body was sintered at 1150° C. for 1.5 hours under a vacuum atmosphere, and thereby processed into a base material body (porous composite sintered body). The base material body thus formed was infiltrated with Cu by placing a Cu plate material on the base material body and holding the base material body and the Cu plate material together in a vacuum furnace at 1150° C. for 2 hours. Thus, the electrode material of Example 3 was obtained. The tensile strength and the electric conductivity of the electrode material of Example 3 were measured, and their values were confirmed to be the same as those of the electrode material of Example 1.[Example 4]An electrode material according to Example 4 was an electrode material produced in the same manner as the electrode material according to Example 3 except that the molding pressures for the molded body and the integrally molded body were changed.A MoCr powder of 5.7 μm median diameter (MoCr weight ratio: Mo:Cr=9:1) was molded with a pressing pressure of 2 t / cm 2 to form a molded body having a diameter φ of 40 mm and a length L of 24 mm. A Cr powder (median diameter: 39 μm) was filled on an outer periphery of the molded body and molded with a pressing pressure of 2 t / cm 2 to form an integrally molded body having a diameter φ of 80 mm and a length L of 24 mm. The integrally molded body was sintered for 1.5 hours at 1150° C. under a vacuum atmosphere, and thus processed into a base material body (porous composite sintered body) by sintering. The base material body thus formed was infiltrated with Cu by placing a Cu plate material on the base material body and holding the base material body and the Cu plate material together in a vacuum furnace at 1150° C. for 2 hours. Thus, the electrode material of Example 4 was obtained. The tensile strength and electric conductivity of the electrode material of Example 4 were measured, and their values were confirmed to be the same as those of the electrode material of Example 1.As mentioned above, by forming the integrally molded body, the electrode material having superior interrupting and withstanding voltage capabilities was obtained even when the pressing pressures for the molded body and the integrally molded body were changed.[Example 5]An electrode material according to Example 5 was an electrode material prepared by sintering the molded body but without sintering the integrally molded body and infiltrating the resultant base material body with Cu.A MoCr fine powder of 5.7 μm median diameter (MoCr weight ratio: Mo:Cr=9:1) was molded with a pressing pressure of 3 t / cm 2 to form a molded body having a diameter φ of 40 mm and a length L of 24 mm. The molded body was sintered by holding at 1150° C. for 1.5 hours under a vacuum atmosphere. A Cr powder (median diameter: 64 μm) was filled on an outer periphery of the sintered body and molded with a pressing pressure of 3 t / cm 2 to form an integrally molded body having a diameter φ of 80 mm and a length L of 24 mm. The integrally molded body thus formed was infiltrated with Cu by placing a Cu plate material on the integrally molded body and holding the integrally molded body and the Cu plate material together in a vacuum furnace at 1150° C. for 2 hours. Thus, the electrode material of Example 5 was obtained. The tensile strength and electric conductivity of the electrode material of Example 5 were measured, and their values were confirmed to be the same as those of the electrode material of Example 1.As mentioned above, by forming the integrally molded body, the electrode material having superior interrupting and withstanding voltage capabilities was obtained even when the molded body (center part) was subjected to sintering.[Example 6]An electrode material according to Example 6 was an electrode material obtained by sintering the molded body, sintering the integrally molded body, and then infiltrating the resultant base material body with Cu.A MoCr fine powder of 5.7 μm median diameter (MoCr weight ratio: Mo:Cr=9:1) was molded with a pressing pressure of 3 t / cm 2 to form a molded body having a diameter φ of 40 mm and a length L of 24 mm. The molded body was sintered by being maintained at 1150° C. for 1.5 hours under a vacuum atmosphere. A Cr powder (median diameter: 64 μm) was filled on an outer periphery of the sintered body and molded with a pressing pressure of 3 t / cm 2 to form an integrally molded body having a diameter φ of 80 mm and a length L of 24 mm. The integrally molded body was sintered at 1150° C. for 1.5 hours under a vacuum atmosphere, and thereby processed into a base material body (porous composite sintered body). The base material body thus formed was infiltrated with Cu by placing a Cu plate material on the base material body and holding the base material body and the Cu plate material together in a vacuum furnace at 1150° C. for 2 hours. Thus, the electrode material of Example 6 was obtained. The tensile strength of the electrode material of Example 6 was measured and confirmed to be the same as that of a conventional electrode material. In addition, the electric conductivity of the electrode material according to Example 6 was measured, and it was confirmed that its value corresponded to that of the electrode material according to Example 1.As mentioned above, the electrode material having superior interrupting and withstanding voltage capabilities was obtained even when the molded body and the integrally molded body were each subjected to sintering.[Example 7]An electrode material according to Example 7 was an electrode material prepared by sintering the integrally molded body without sintering the molded body and infiltrating the resultant base material body with Cu. Here, the electrode material according to Example 7 was characterized in that the central part was formed with a large area.A MoCr fine powder of 5.7 μm median diameter (MoCr weight ratio: Mo:Cr=9:1) was molded with a pressing pressure of 3 t / cm 2 to form a molded body having a diameter φ of 63 mm and a length L of 24 mm. A Cr powder (median diameter: 64 μm) was filled on an outer periphery of the molded body and molded with a pressing pressure of 3 t / cm 2 to form an integrally molded body having a diameter φ of 80 mm and a length L of 24 mm. The integrally molded body was sintered at 1150° C. for 1.5 hours under a vacuum atmosphere, and thereby processed into a base material body (porous composite sintered body). The base material body thus formed was infiltrated with Cu by placing a Cu plate material on the base material body and holding the base material body and the Cu plate material together in a vacuum furnace at 1150° C. for 2 hours. Thus, the electrode material of Example 7 was obtained. The tensile strength and electric conductivity of the electrode material of Example 7 were measured, and their values were confirmed to be the same as those of the electrode material of Example 1.As mentioned above, the electrode material having superior interrupting and withstanding voltage capabilities was easily obtained even when the central portion of the integrally molded body was increased in diameter.[Example 8]An electrode material according to Example 8 was an electrode material prepared by sintering the integrally molded body without sintering the molded body and infiltrating the resultant base material body with Cu. In this example, the Mo:Cr weight ratio and the median diameter of the MoCr powder used as the starting material were different from those in the other examples. For preparing the electrode material of Example 8 (and for preparing the electrode material of Example 9 mentioned below), a MoCr fine powder was prepared using a Cr powder of 18 μm median diameter. Even under the same heating conditions of the MoCr solid solution powder, due to the formation of Cr residue particles and secondary particles (as aggregates), the particle diameters of the MoCr solid solution powder become large, resulting in deterioration of the dispersibility of the solid solution particles in the electrode material, because the content of Cr in the MoCr solid solution powder increases. In other words, it becomes difficult to pulverize the MoCr solid solution powder, so that the median diameter of the MoCr solid solution powder tends to become large as the content of Cr in the MoCr solid solution powder increases. For this reason, a Cr powder having a relatively small particle diameter is used for preparing a powder of a MoCr solid solution having a relatively high Cr content, for example, in the range of Mo:Cr=1:3 to 3:1 to enable fine dispersion of the CuCrMo structures.The MoCr fine powder of 7.1 μm median diameter (MoCr weight ratio: Mo:Cr=3:1) was prepared. The MoCr fine powder was molded at a pressing pressure of 3 t / cm 2 to form a molded body having a diameter φ of 40 mm and a length L of 24 mm. A Cr powder (median diameter: 64 μm) was filled on an outer periphery of the molded body and molded with a pressing pressure of 3 t / cm 2 to form an integrally molded body having a diameter φ of 80 mm and a length L of 24 mm. The integrally molded body was sintered at 1150° C. for 1.5 hours under a vacuum atmosphere, and thereby processed into a base material body (porous composite sintered body). The base material body thus formed was infiltrated with Cu by placing a Cu plate material on the base material body and holding the base material body and the Cu plate material together in a vacuum furnace at 1150° C. for 2 hours. Thus, the electrode material of Example 8 was obtained. The electrical conductivity of the electrode material according to Example 8 was measured on both sides. It was confirmed that the electric conductivity of the central part (CuCrMo region) of the electrode material was 30% IACS, and the electric conductivity of the outer circumferential part (CuCr region) of the electrode material was 21% IACS.As mentioned above, the electrode material having superior interrupting and withstand voltage capabilities was obtained even when the mixing ratio to Mo:Cr=3:1 was changed.[Example 9]An electrode material according to Example 9 was an electrode material prepared by sintering the integrally molded body without sintering the molded body and infiltrating the resultant base material body with Cu. In this example, the Mo:Cr weight ratio and the median diameter of the MoCr powder used as the starting material were different from those in the other examples.A MoCr fine powder of 23.7 μm median diameter (MoCr weight ratio: Mo:Cr=1:1) was prepared and molded with a pressing pressure of 3 t / cm 2 to form a molded body having a diameter φ of 40 mm and a length L of 24 mm. A Cr powder (median diameter: 64 μm) was filled on an outer periphery of the molded body and molded with a pressing pressure of 3 t / cm 2 to form an integrally molded body having a diameter φ of 80 mm and a length L of 24 mm. The integrally molded body was sintered at 1150° C. for 1.5 hours under a vacuum atmosphere, and thereby processed into a base material body (porous composite sintered body). The base material body thus formed was infiltrated with Cu by placing a Cu plate material on the base material body and holding the base material body and the Cu plate material together in a vacuum furnace at 1150° C. for 2 hours. The electrical conductivity of the electrode material according to Example 9 was measured on both sides. It was confirmed that the electric conductivity of the central part (CuCrMo region) of the electrode material was 29% IACS, and the electric conductivity of the outer circumferential part (CuCr region) of the electrode material was 22% IACS.As mentioned above, the electrode material having superior interrupting and withstand voltage capabilities was obtained even when the mixing ratio to Mo:Cr=1:1 was changed.[Reference Example 1]An electrode material according to Reference Example 1 was an electrode material having no outer peripheral part (CuCr region). For preparing the electrode material of Reference Example 1, a Mo powder having a particle size of 2.8 to 3.7 μm was used. When the particle size distribution of the Mo powder was measured with a laser diffraction particle size analyzer, it was determined that the median diameter d50of the Mo powder was 5.1 μm (d10=3.1 μm, d90=8.8 μm). Also, a Cr powder of -325 mesh was used (screen opening size: 45 μm).First, the Mo powder and the Cr powder were mixed in a weight ratio of 9:1. The mixed powder was subjected to heating and pulverization, thereby forming a MoCr powder. The median diameter of the heated MoCr powder was 5.7 μm (measured by a laser diffraction particle size analyzer). The MoCr powder was molded. The resulting molded body was sintered. The sintered body was subjected to HIP treatment and then infiltrated with Cu. Thus, the electrode material of Reference Example 1 was obtained. The electrode material according to Reference Example 1 had a composition of Cu:Cr:Mo=25:7.5:67.5 (weight ratio).[Reference Example 2]An electrode material according to Reference Example 2 was an electrode material having no outer peripheral part (CuCr region). Starting materials used for preparing the electrode material according to Reference Example 2 were a Mo powder having a particle size of 2.8 to 3.7 μm and a Cr powder having a median diameter of 20 μm (each measured by a laser diffraction particle size analyzer).The Mo powder and the Cr powder were mixed in a weight ratio of 3:1. The mixed powder was subjected to heating and pulverization, thereby forming a MoCr powder. The MoCr powder was molded (pressing pressure 3.6 t / cm 2). The resulting molded body was sintered. The sintered body was infiltrated with Cu. Thus, the electrode material of Reference Example 2 was obtained. The electrode material according to Reference Example 2 had a composition of Cu:Cr:Mo=50:12.5:37.5.[Comparative Example 1]An electrode material according to Comparative Example 1 was a conventional CuCr electrode material containing 50 wt% of Cu and 50 wt% of Cr.The electrode material according to Comparative Example 1 was produced by molding a Cr powder, sintering the molded body, and infiltrating the resultant base material body with Cu.The electrode materials of Reference Examples 1 and 2 and the electrode material of Example 1 were formed to have the same diameter, respectively installed in vacuum switches, and subjected to current forming. The number of current shaping treatments performed on the vacuum switch using the electrode material according to Reference Example 1 to achieve a set target voltage was 1.5 times or more as high as that of the vacuum switch using the electrode material according to Reference Example 2. the current value required for current shaping of the vacuum switch using the electrode material according to Reference Example 1 was 1.2 times or more as large as the current value required for current shaping of the vacuum switch using the electrode material according to Reference Example 2. In addition, the vacuum switch having the electrode material according to Reference Example 2 was unstable in withstand voltage due to the occurrence of deposits in the vacuum switch during current forming.The vacuum switch with the electrode material according to Example 1 was subjected to the same number of current forming treatments as the vacuum switch with the electrode material according to Reference Example 2. before and after the current forming, the contact resistance of the electrode material decreased by 10%. As is apparent from this result, the contact resistance of the surface of the electrode material of Example 1 was lowered by interrupting a large current, so that the electrode material had good resistance to welding caused by the contact resistance.In addition, electrode contacts were respectively formed from the electrode materials according to Comparative Example 1 and Example 1 and incorporated into vacuum switches. Tables 1 and 2 show the results of surface roughness measurements of the electrode contacts in the vacuum switches after a plurality of breaking operations. The measurement results of Comparative Example 1 are shown in Table 1; and the measurement results of Example 1 are shown in Table 2. Table 1 Table 1RoughnessRaRaRzRaRaRzMeasured value1,124,750,451,981,134,630,512,320,954,10,542,371,074,430,462,111,054,140,482,17Mean Value1,064,410,492,19Table 2Table 2RoughnessRaRaRzRaRaRzMeasured value0,72,670,542,290,752,850,542,340,762,840,622,560,722,690,572,40,762,90,562,5Mean Value0,742,790,572,42As is apparent from a comparison of Tables 1 and 2, the surface roughness of the electrode material according to Example 1, particularly the surface roughness of the central part of the electrode material, was smaller than that of the electrode material according to Comparative Example 1, and therefore it is considered that the factor of increase in contact resistance in the electrode material according to Example 1 was reduced to a smaller extent than in the electrode material according to Comparative Example 1.The vacuum switches having the electrode contacts formed of the electrode materials according to Reference Example 2 and Example 1 were also tested by a capacitor switching test (72 kV, 20 MVA, TRV 72.5 kV / √3×1.4×2√2, interruption current 160 A) and by an interruption test (interruption current 25 kArm, interruption current phase angle 40 to 250 degrees, TRV 132 kVpeak (0.75 kV / μs)).As shown in FIG. 7, both the vacuum switch having the electrode material of Reference Example 2 and the vacuum switch having the electrode material of Example 1 exhibited a good result in the disconnection test (that is, exhibited a disconnection range as specified by standards). The vacuum switch with the electrode material according to Example 1 exhibited a reignition probability of 0% in the capacitor switching test, and exhibited superior capacitor switching capability to the vacuum switch with the electrode material according to Reference Example 2.As described above, by the manufacturing method of the electrode material according to the embodiment of the present invention, the electrode material having superior interruption and withstand voltage capabilities is obtained. The electrode material can be obtained with superior capacitor switching capability. The electrode material can also be obtained with superior current-supplying capability. Number and energy cost of surface forming treatments required for the electrode material are reduced by forming around the outer periphery of the center part the CuCr region having high withstand voltage in which not only MoCr particles but also Cu structures are finely dispersed. Therefore, the electrode material can prevent deposits from forming inside the vacuum switch by the surface formation treatment of the switch contact, and thus achieve superior break-off and capacitor switching capabilities.Since the integrally molded body in which the Cr powder is filled and molded around the MoCr powder central part is infiltrated with Cu, the connection of the central part and the outer circumferential part is enhanced by the phenomenon of dissolution and diffusion of Cr into MoCr by the Cu infiltration. In other words, the joining strength of the center part and the outer peripheral part is improved because Cr is slightly dissolved from the outer peripheral part in Cu and diffuses from Cu into the MoCr particles of the center part.In addition, the shrinkage rate of the central part (molded body) during sintering (or Cu infiltration) is reduced by forming the central part (molded body) from the powder of MoCr solid solution. On the other hand, the molded body of the Cr powder shrinks during sintering (or Cu infiltration). Therefore, the mutual diffusion of elements at the boundary between the central part and the outer peripheral part is promoted by the shrinkage of the outer peripheral part during the sintering (or the Cu infiltration) of the integrally molded body, whereby the connection of the central part and the outer peripheral part can be further enhanced.The present inventors have previously developed an electrode material having superior interruption and withstand voltage capabilities as disclosed in Patent Documents 3 to 5. This electrode material has a structure in which Cu structures are finely dispersed, so that it is difficult to melt a surface of the electrode material by surface forming. In contrast, the electrode material according to the embodiment of the present invention has a structure in which the CuCr region having high withstand voltage is formed as the outer circumferential part. Therefore, the number of current forming treatments required for the electrode material according to the embodiment of the present invention is substantially reduced, so that not only the power cost of current forming is reduced, but also deposits are prevented from forming inside the vacuum switch by current forming.A conventional electrode material (CuCr electrode material) causes current interruption during a forming treatment because the arc generated over the entire electrode material converges to the central part of the electrode material as shown in FIG. 8. This leads to the problem that by locally heating the central part of the electrode material, contaminating elements (such as Cu, Cr and so forth) are released from the surface of the electrode material into the interior of the vacuum switch.During such current shaping treatment, a CuCr surface phase in which fine Cr particles are dispersed is formed on a surface of the CuCr electrode material. Since the CuCr surface phase has higher withstand voltage than a CuCr main material of the electrode, withstand voltage capability of the electrode material is improved by the current shaping treatment. Herein, the CuCr surface phase is formed from the central part of the electrode material so that the surface of the electrode material after the forming treatment is covered with the CuCr surface phase.On the other hand, a CuCrMo electrode material has a structure in which high-melting MoCr particles and Cu structures are finely dispersed, so that it is difficult to melt a surface of the electrode material and it is difficult to form a finely dispersed surface phase on the surface of the electrode material. Thus, until a set target voltage is reached, a large number of current shaping treatments are required. As a result, a large amount of energy is required for the current shaping treatments. In addition, there is a possibility that, by releasing impurity elements (such as Cu, Cr, and so on) from the surface of the electrode material into the vacuum switch, the withstand voltage performance of the vacuum switch becomes unstable when a large number of current forming treatments are performed.In the electrode material according to the embodiment of the present invention, the outer peripheral part has a lower melting point than the central part, so that it is easy to form the fine dispersed CuCr surface phase (containing Mo derived from the central part). Therefore, the electrode material can be provided with a fixed target voltage and reduced contact resistance by performing the same or similar number of current forming treatments as in a conventional CuCr electrode material. During current forming, a surface phase having high withstand voltage is formed on the surface of the electrode material. This surface phase forms from the central part of the electrode material and spreads along a radial direction of the electrode material such that the surface of the electrode material is covered with the surface phase. In the central part, the surface phase is composed primarily of MoCr or fine CuCrMo structures on the CuCrMo main material. In the outer, circumferential part, the surface phase is composed primarily of MoCr, Cr or CuCrMo structures on the CuCr main material. It is considered that, by the current forming, the withstand voltage capability of the entire electrode material can be improved because both of the above-mentioned surface phases have higher hardness and withstand voltage than a material in the bulk of the electrode. The electrode material according to the embodiment of the present invention has high hardness and high withstand voltage, and achieves good capacitor switching capability. Since not only the center electrode but also the surface phase formed by current forming (in particular, the surface phase formed on the surface of the outer peripheral part) have high hardness, the surface of the electrode material can be prevented from being roughened by inrush current. The electrode material according to the embodiment of the present invention is therefore suitable for use in a capacitor circuit in which a voltage two or three times as high as the usual is applied between the electrodes at the time of interruption of a weak current, and in which roughening of the electrode surface may be caused by inrush current.The electrode material according to the embodiment of the present invention as a whole maintains its energization capability because both the central part and the outer circumferential part contain the same high-conductive member (for example, Cu) as the main arc component. The very long time required for stabilizing the withstand voltage capability can be shortened by reducing the area of the central part (for example, the MoCr body) on the surface of the electrode material. The central portion of the electrode material has high heat resistance and is difficult to melt and therefore exhibits improved resistance to local heating caused by convergence of an arc in a current interruption operation.Conventionally, an electrode for a capacitor circuit is constructed by providing a large-diameter SUS electrode portion to secure withstand voltage, and then disposing a small-diameter electrode contact made of CuCrMo material on the electrode portion. When the electrode contact is formed in this manner, there arises a problem that as the contact area is reduced, the interruption current becomes very small. Some devices have been introduced to increase the contact area to improve the interrupting capability. However, the increase in the contact area may result in deterioration of the capacitor switching capability.In order to improve the capacitor switching capability and the heavy current interrupting capability, modifications have been required to ensure the energization capability. As a modification of the structure of the vacuum switch electrode, a composite contact material whose composition changes in a radial direction has already been known (see, for example, Patent Documents 8 to 10). However, the composite contact material faces the problem that as unevenness is generated in the main arc component by a plurality of heavy current switching operations, the contact resistance increases. In addition, due to the complexity of the electrode structure and the manufacturing method, the composite contact material is not suitable for mass production as a product for vacuum applications.The electrode material according to the embodiment of the present invention has good capacitor switching capability, which eliminates the need for an SUS electrode portion for securing withstand voltage. Even if the electrode material is increased in diameter for a larger contact area, the amount of energy required for the stabilization treatment of the electrode material is kept low. The electrode material according to the embodiment of the present invention thus achieves good capacitor switching capability. Accordingly, the vacuum switch with the electrode material according to the embodiment of the present invention has a substantially reduced electrode diameter and a very greatly reduced cost, as compared with a conventional vacuum switch (for example, having a contact of 20 to 30 mm in diameter and an SUS electrode portion of 100 mm in diameter).The optimum area ratio of the central portion and the circumferential portion at the surface of the electrode material is varied depending on the electrode structure, the coil shape, the arc diffusion state and the like. Therefore, the optimum area ratio of the central part and the peripheral part is set arbitrarily in accordance with the electrode structure, the arc diffusion state and the like. Since the easy-to-melt area (i.e., the area in which the collision energy of the ions is large) is determined by the magnetic flux density between the electrodes, the optimum area ratio of the central part and the outer circumferential part is set according to the distribution of the magnetic flux density.Although the electrode material and the manufacturing method for the electrode material according to the embodiments of the present invention have been described above with reference to the specific examples, it should be understood that: the present invention is not limited to the above-mentioned specific embodiments; various modifications and changes of the embodiments are possible within the range that does not affect the features of the present invention; and that these modifications and changes are included within the scope of the present invention.For example, the central part may be made of the electrode material disclosed in Patent Documents 3 to 5, so that fine and uniform dispersion of Cr-containing particles as well as fine and uniform dispersion of high-conductivity Cr structures is possible. In this case, the central part having superior withstand voltage and current interrupting capabilities can be formed by increasing the content of the heat resistant element in the central part.The average particle diameter of the fine particles (i.e., the particles of the solid solution of the heat-resistant element and Cr) in the center part is preferably controlled to be 20 μm or smaller, more preferably 15 μm or smaller, as determined according to the Fullerman equation. The center part can be formed with superior withstand voltage and current cut capabilities by controlling the volume-based relative particle amount of particles having a diameter of 30 μm or smaller in the MoCr powder to be 50% or more. In addition, the center part having superior current interruption and withstand voltage capabilities can be formed by controlling the dispersion state index CV of the fine particles of the center part in which the heat-resistant element and Cr are mutually dissolved and diffused (that is, the particles of the solid solution of the heat-resistant element and Cr) to be 2.0 or less, preferably 1.0 or less, as determined based on the average value and the standard deviation of the distances between the mass centers of the fine particles.The central part may be formed by sintering a mixed powder of a powder of a heat-resistant element (for example, Mo powder) and a Cr powder and infiltrating the resultant sintered body with Cu. In this case, the capacitor switching capability of the electrode material decreases, so that the electrode material may not be usable for capacitor circuit applications in terms of its performance. However, in terms of interrupting capability and withstand voltage capability, the electrode material is superior to conventional CuCr electrode materials and is therefore applicable to any purposes other than capacitor circuit applications.The central portion can be formed with superior withstand voltage and current interrupting capabilities by increasing the content of the heat resistant element in the central portion as mentioned above. The higher the proportion of the heat-resistant element in the central part is, the more the withstand voltage capability of the central part is improved. However, it may become difficult to infiltrate the central part with Cu when the central part contains only the heat-resistant element (i.e., does not contain Cr). Therefore, the weight ratio of the heat-resistant element to Cr in the solid solution powder as the raw material for the center part is preferably set to be 1 or more of the heat-resistant element to 1 of Cr, more preferably 3 or more of the heat-resistant element to 1 of Cr, even more preferably 9 or more of the heat-resistant element to 1 of Cr, in order for the electrode material to achieve superior withstand voltage capability.Since the electrode material (particularly, the central part) according to the embodiment of the present invention is produced by an infiltration method, the filling degree of the electrode material becomes 95% or higher, so that less roughening of the surface of the contact occurs due to the arc in current interruption or current switching operations. In other words, the electrode material achieves superior withstand voltage capability without generating small protrusions and recesses on the surface due to the presence of pores. By infiltrating Cu into the pores of the porous base material body, the electrode material exhibits a higher mechanical strength and a higher hardness than those of an electrode material produced by a sintering method, thereby achieving superior withstand voltage and capacitor switching capabilities.The powder of the MoCr solid solution is not limited to those prepared by the method described in the above-mentioned embodiments. A powder of a MoCr solid solution prepared by any known method (such as a jet milling method or atomization method) can be used.Although in the above-mentioned embodiments, both the molded body and the integrally molded body are formed by a pressing machine, the formation of the molded body and / or the integrally molded body is not limited to such a molding method. The molded body and the integrally molded body may be formed by any known method. By performing HIP treatment after the main sintering and before the Cu infiltration, the filling degree of the MoCr sintered body can be increased to improve the withstand voltage capability of the electrode material.The pressing pressure in forming the center part may be different from the pressing pressure in forming the integrally molded body. For example, electrode materials having superior withstand voltage capability were obtained even by setting the press forming pressure for the center part to 3 t / cm 2 while changing the press forming pressure for the integrally molded body to 2.5 t / cm 2 or 2 t / cm 2 in Example 8. In this case, the electric conductivity of the outer peripheral part improved with the decreasing pressing pressure for the integrally molded body (specifically, 22% IACS at 3 t / cm 2, 23 % IACS at 2.5 t / cm 2 and 24% IACS at 2 t / cm 2).

Claims

A method for producing an electrode material (1) comprising: forming a molded body by molding a powder of a solid solution of chromium and at least one kind of a heat resistant element selected from Mo, W, Ta, Nb, V and Zr; filling and molding a Cr powder around an outer periphery of the molded body, thereby forming an integrally molded body; and infiltrating the integrally molded body with a conductive element selected from Cu, Ag and an alloy of Cu and Ag.The method for manufacturing the electrode material (1) according to claim 1, further comprising: sintering the integrally molded body, wherein upon infiltrating, the sintered integrally molded body is infiltrated with the conductive member.The method for producing the electrode material (1) according to claim 1 or 2, further comprising: sintering the molded body, wherein in the filling and forming, the integrally molded body is obtained by filling and molding the Cr powder around the sintered molded body.The method for producing the electrode material (1) according to any one of claims 1 to 3, wherein in the X-ray diffraction measurement of the powder of the solid solution, either a peak to be attributed to Cr or a peak to be attributed to the heat-resistant element has disappeared.An electrode material (1) comprising: a central part (2) having good current interrupting capability; and an outer circumferential part (3) disposed on an outer periphery of the central part (2), the central part (2) having a mixed metal composition in which solid solution particles are uniformly dispersed in a Cu phase, wherein the solid solution particles are formed of a solid solution of Cr and at least one kind of heat resistant element selected from Mo, W, Ta, Nb, V and Zr, wherein the mixed metal composition comprises, in terms of weight ratio with respect to the mixed metal composition, 20 to 70% of Cu, 1.5 to 64% of Cr and 6 to 76% of the heat resistant element, the balance being unavoidable impurities, wherein the particles of the solid solution in the mixed metal composition have an average particle diameter of 20 μm or smaller and are uniformly dispersed in the Cu phase with a dispersion state index of 1.0 or lower as determined based on an average value and a standard deviation of distances between the centers of mass of the particles of the solid solution dispersed in the Cu phase, and wherein the outer circumferential part (3) comprises 75 wt % to 90 wt % of Cr based on a weight of the outer circumferential part (3), the balance being Cu.

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