Vacuum switch contact materials and the manufacturing methods

Abstract: In manufacturing the contact materials which satisfy all the criteria required for use in vacuum switches as to chopping current, circuit breaking performance, withstand voltage, welding separation force and wear, Cu powder and Ta₂O₅ powder are mixed, and compressed while heating below the melting point of Cu in a nonoxidizing atmosphere. Or a green compact is first manufactured from Ta₂O₅ powder or a mixture of Ta₂O₅ and Cu powder, and molten Cu is made to infiltrate into the compact. Preparing a green compact from a mixture of Cu powder and Ta₂O₅ powder, it may be sintered at a temperature below the melting point of Cu, re-pressed, and resintered at 400 - 900 o C in a nonoxidizing atmosphere.

Description: BACKGROUND OF THE INVENTION This invention relates to a contact material for vacuum switch, and the method of its manufactures. The characteristics required of a vacuum switch contact material are superior circuit breaking performance, high withstand voltage, a small chopping current, low welding separation force (which means a force required for pulling apart both contacts melted together by means of current) and low wear. A material which satisfies all these characteristics has however still not been developed, and some of them have to be sacrificed depending on the intended application under present conditions. Cu-Cr contact materials, for example, have excellent circuit breaking performance and withstand voltage, and so they are used mainly for vacuum circuit breakers, however as they have a high chopping current, they are not very suitable for motor switches. Also, they have a rather high welding separation force, so that considerable force has to be exerted on the circuit breaker side. Ag-WC contact materials, on the other hand, have a small chopping current and are therefore used in vacuum switches for motors, but they have inferior circuit breaking performance and are not very suitable as circuit breakers. The characteristics of contacts therefore depend on the materials of which they consist, but their characteristics may also vary according to the method of their manufacture. For example, Cu-Cr contact materials manufactured by the sintering method show peak circuit breaking performance when the Cr content is near 25% by weight, but if manufactured by the infiltration method, they show peak performance when the Cr content is near 45% by weight. Conventional vacuum switch contact materials, therefore, were not satisfactory with respect to all characteristics, and some characteristics had to be sacrificed in order to fit a specific application. A contact material was therefore desired which could offer some improvement of characteristics even if only slight. The inventors also discovered contact materials with excellent circuit breaking performance and anti-weld property, which consists of Cu (main component), a secondary component such as Mo, and metal oxides such as Ta₂O₅, and already filed a patent claim for them (Japanese Patent Application Laid-Open No. 1984-215621). Even these materials were however unsatisfactory as a stable, low chopping current could not be obtained. This was probably due to the effect of the secondary material, and some improvement was desired. SUMMARY OF THE INVENTION This invention was conceived to eliminate the above problems, to provide a contact material with a low chopping current, excellent circuit breaking performance and withstand voltage, low welding separation force (which means a force required for pulling apart both contacts melted together by means of current) and low wear, and a method for manufacturing this material. Aims of this invention is to provide: a vacuum switch contact material consisting of Cu, and a tantalum oxide given by the formula Ta x O y , where x = 1 - 2, and y = 1 - 5; a method of manufacturing a vacuum switch contact material, wherein Cu powder and Ta₂O₅ powder are mixed together, and the powder mixture obtained is compressed while being heated at a temperature below the melting point of Cu in a nonoxidizing atmosphere; a method of manufacturing a vacuum switch contact material, wherein a green compact is first manufactured from Ta₂O₅ powder or a mixture of Ta₂O₅ and Cu powder, and by raising the pressure of the furnace atmosphere while depositing the compact in a molten Cu in a nonoxidizing atmosphere, molten Cu is made to infiltrate into the voids in the compact; and a method of manufacturing a vacuum switch contact material, wherein a green compact is first manufactured from a mixture of Cu powder and Ta₂O₅ powder, the compact is sintered at a temperature below the melting point of Cu in a nonoxidizing atmosphere, the sintered compact obtained is re-pressed, and then resintered at a temperature of 400 - 900°C in a nonoxidizing atmosphere. As a result, the contact material of this invention has all the properties required of a vacuum switch, e.g. suitable circuit breaking performance, withstand voltage, chopping current, welding separation force and wear, and a material with these excellent properties can moreover be manufactured by the method of this invention. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a flowchart showing the major steps in the manufacture of the contact material of Embodiment 1. Fig. 2 is a flowchart showing the major steps in the manufacture of the contact material of Embodiment 2. Fig. 3 is a flowchart showing the major steps in the manufacture of the contact material of Embodiment 3. Fig. 4 is a graph showing the circuit breaking performance of the contact materials in Embodiments 1 - 3. Fig. 5 is a graph showing the chopping currents of the contact materials in Embodiments 1 - 3. Fig. 6 is a graph showing the probability of discharge occurring in the contact materials of Embodiments 1 - 3. DETAILED DESCRIPTION OF THE EMBODIMENTS The contact material in this invention, which consists essentially of Cu and Ta x O y (where x = 1 - 2 and y = 1 - 5), satisfies all the criteria required for use in vacuum switches as to chopping current, circuit breaking performance, withstand voltage, welding separation force (which means a force required for pulling apart both contacts melted together by means of current) and wear. X-ray diffraction analysis shows that apart from Ta₂O₅, compounds such as TaO₂ and Ta₂O₃ are also present as the above-mentioned Ta x O y . The proportions of Cu and Ta x O y in the contact material of this invention vary according to the manufacturing method. However, from the viewpoint of circuit-breaking performance the proportion by volume should be 98/2 - 25/75, from the viewpoint of chopping current it should be 60/40 - 20/80, and from the viewpoint of withstand voltage it should be 95/5 - 20/80, specifically 60/40 - 25/75 is preferable. When the contact material is manufactured by Method 1 to be described later, in order to give it improved circuit breaking performance compared to conventional Cu-Cr contacts containing 25% Cr by weight, the proportion of Cu/Ta x O y should be 98/2 - 45/55 (these proportions are represented by volume unless otherwise specified); to keep the chopping current below 1 A after the switch has operated 10,000 times on a load voltage of 600 A, the proportion should be 60/40 - 20/80; and to obtain a contact material which, while excelling in the above two points, has a welding separation force of not more than 10 kg f with low wear, the proportion should be 60/40 - 45/55. Contact materials manufactured by Method 1 have excellent electrical performance such as high circuit breaking performance and a small chopping current, and they do not demand an excessive equipment outlay. When the contact material is manufactured by Method 2 to be described later, in order to give it improved circuit breaking performance compared to conventional Cu- Cr contacts containing 25% parts by weight of Cr, the proportion Cu/Ta x O y should be 65/35 - 25/75; to keep the chopping current below 1 A after the switch has operated 10,000 times on a load current of 600 A, the proportion should be 60/40 - 20/80; and to obtain a contact material which, while excelling in the above 2 points, has a welding separation force of not more than 10 kgf with low wear, the proportion should be 60/40 - 25/75. Contact materials manufactured by Method 2 are excellent in that since they can be made to contain a high proportion of Ta₂O₅, the compositional proportion can be chosen so as to fully satisfy the requirements of high circuit breaking performance and a small chopping current. When the contact material is manufactured by Method 3 to be described later, in order to give it improved circuit breaking performance compared to conventional Cu-Cr contacts containing 25% Cr by weight, the proportion of Cu/Ta x O y should be 95/5 - 55/45; to keep the chopping current below 1 A after the switch has opened and closed 10,000 times on a load current of 600 A, the proportion should be 60/40 - 50/50; and to obtain a contact material which, while excelling in the above 2 points, has a welding separation force of not more than 10 kg f with low wear, the proportion should be 60/40 - 55/45. Contact materials manufactured by Method 3 have slightly inferior performance compared to those manufactured by other methods, but the method has the excellent advantage that it requires no new equipment, and conventional equipment may be used by established manufacturers in the field. Methods of manufacturing the contact material of this invention will now be described. As Cu and Ta₂O₅, the components of the contact material of this invention, have very poor mutual wettability, a satisfactory material cannot be obtained by conventional manufacturing methods. They can, however, be obtained by the three methods described below. In Method 1, as shown in Fig. 1, Cu powder and Ta₂O₅ powder are first mixed together 1, the powder mixture obtained is packed into a carbon die or other device 2, and the mixture is compressed while heating it at a temperature below the melting point of Cu in a nonoxidizing atmosphere 3. The above-mentioned Cu powder should preferably have a purity of not less than 99%, and a particle diameter no greater than 70 µm. It is also preferable that the Ta₂O₅ powder should have a purity of not less than 99%, and a particle diameter no greater than 40 µm. Instead of Ta₂O₅ powder, other tantalum oxide powders may be used, but as they are thermally unstable compared to Ta₂O₅ and are more difficult to procure, Ta₂O₅ is to be preferred. Cu/Ta₂O₅, the mixing proportion of Cu powder and Ta₂O₅ powder is 99/1 - 20/80 by volume, but taking electrical properties into consideration, it is preferable that this proportion is 60/40 - 45/55. If the proportion of Ta₂O₅ powder exceeds 80% by volume, the pressure used in the pressurizing step described below has to be very high, and bigger and more costly equipment tends to be required. The mixing of Cu powder and Ta₂O₅ may be carried out by the usual method. The above-mentioned nonoxidizing atmosphere prevents oxidation of the Cu powder, and promotes the sintering process. It may be a reducing atmosphere such as hydrogen, an inert gas atmosphere such as argon or nitrogen, or a vacuum of about 10⁻³ - 10⁻⁵ torr. Of these, hydrogen or a vacuum is to be preferred from the viewpoint that it reduces the surface of the Cu powder and promotes sintering. The temperature used in the heating must be below the melting point of Cu (1083°C) to prevent blow-out of molten Cu through the gaps in the carbon die, and it should preferably be no higher than 1080°C. If it is too low, however, an excessive pressure has to be used in the pressurizing step, and a very long time is required for the pressurization. In practice, therefore, a temperature of not less than 900°C and preferably not less than 1050°C is desirable. There are no special restrictions on the method used to achieve the pressurization, however a load of not less than 200 kg/cm² is preferable from the viewpoint of reducing the the percentage of voids and promoting sintering. If the load is increased, the pressurization time can be shortened, but the pressurizing mechanism and die then have to be made larger. In practice, therefore, it is preferable to use a pressure no greater than 500 kg/cm². The time required for pressurization is from 30 min. - 3 hours as it is preferable to achieve the density of a sintered compact of not less than 99%. The time should be regulated appropriately such that if Ta₂O₅ is present in large quantity as a constituent the time is lengthened, and if the pressurizing load is high the time is shortened. If however a density of 99% cannot be achieved even if 3 hours is exceeded, a very long time would be required unless the pressurizing load were increased, and this is impractical from a production viewpoint. In Method 2, as shown in Fig. 2, Ta₂O₅ powder is taken or Ta₂O₅ powder and Cu powder are mixed together 4, and used to make a green compact 5. While the compact is placed in a molten Cu 6, the pressure of the ambient nonoxidizing atmosphere is then increased such that the molten Cu infiltrates into the voids in the compact 7. The Ta₂O₅ powder and Cu powder used in Method 2 are the same as those used in the Method 1. In Method 2, Ta₂O₅ powder may be the sole constituent of the compact, but if the percentage of voids in the compact is high (higher than about 65%), Cu powder may be used together with Ta₂O₅ powder and the apparent proportion of Ta₂O₅ in the compact may be decreased. In this case, the proportion of Cu powder in the mixture should be no greater than 35% by volume, and preferably no greater than 20% by volume. The reason for the concurrent use of Cu powder is that if Ta₂O₅ is used alone, the compact collapses during handling when it contains 65% or more voids. If, for example, Cu and Ta₂O₅ each represent 50% by volume and there are 50% voids in the compact, it would be expected that the final composition will be 75% Cu by volume, and 25% Ta₂O₅ by volume. In practice, however, the compact is disintegrated in the molten Cu as will be described later, and a container is needed to hold it. If also the proportion of Cu powder is increased, the casting pressure tends to be greater. In practice, the lower limit for the proportion of Ta₂O₅ in the final composition is 30% by volume, and it is found by experiment that the proportion of Cu powder should preferably be not less than 35% by volume. The compact consists of Ta₂O₅ powder or a mixture of Ta₂O₅ powder and Cu powder, and it is formed by the usual methods in such a way that the percentage of voids in it is no greater than about 65%. If the percentage of voids is greater than 65%, the compact easily collapses and it is difficult to manufacture a contact material from it. The compact obtained is then coated with, for example, Cu powder, placed in a crucible and heated in a nonoxidizing atmosphere such that the Cu powder melts. After the compact has been coated with molten Cu, the pressure of the gas atmosphere is raised to 100 - 2000 atm. and held for 30 min. - 1 hour so that the molten Cu penetrates into the holes in the compact. The heating temperature mentioned above should preferably be over the melting point of Cu so as to infiltrate the molten Cu into the voids in the compact. The nonoxidizing atmosphere may be the same as used in Method 1, but in order to extract the interstitial gas in the contact material, it is preferable to maintain a vacuum when melting the Cu. It should be noted that if hydrogen is used at high temperature and pressure, it makes the pressure container brittle, and it is therefore preferable to use a mixture of argon and hydrogen, for example. Also, the pressure of the gas atmosphere during the pressurizing step should be not less than 100 atm., and it should preferably be not less than 100 times the pressure when the Cu has melted before pressurization in order to reduce the volume of interstitial gas in the contact material to 1/100 or less of its original volume. After the Cu has infiltrated into the compact, it is cooled. When the Cu has solidified, the gas pressure is returned to normal pressure, and the contact material of this invention is thereby obtained. In Method 3, as shown in Fig. 3, Cu powder and Ta₂O₅ powder are first mixed together 9 and a green compact is manufactured the same as Method 1, 10. The compact obtained is then sintered in a nonoxidizing atmosphere below the melting point of Cu 11 and after re-pressing the sintered compact by hot working or cold working 12, it is resintered at 400 - 900°C in a nonoxidizing atmosphere 13. The Cu powder and Ta₂O₅ powder used in Method 3 are the same as those used in Method 1 described above. Cu/Ta₂O₅, the mixing proportion of Cu powder to Ta₂O₅ powder is 99/1 - 40/60 by volume, but taking electrical properties into consideration, it is preferable that this proportion is 60/40 - 55/45. If the percentage of Ta₂O₅ powder exceeds 60% by volume, the pressure used in the repressurizing step described below has to be very high, and in practice, the desired density cannot be achieved due to insufficient pressure. The sintering of the compact manufactured from the powder mixture in the same way as in Method 1 is carried out below the melting point of Cu, preferably at a temperature no higher than 1080°C and no lower than 1050°C. If the sintering temperature is higher than the melting point of Cu and the Cu melts, it flows out of the compact as Cu and Ta₂O₅ have very poor mutual wettability. The sintering time should preferably be 3 - 5 hours in order that sintering can be carried out at normal pressure. Further, the nonoxidizing atmosphere used may be the same as in Method 1. As the sintered compact obtained contains from 5 to 30% by volume of voids (the percentage of voids increases with an increasing proportion of Ta₂O₅), it is re-pressed to reduce these voids. There is no special restriction on the method used to carry out this re-pression, but it is preferable to carry it out at a temperature in the range of from ambient temperature to 400°C. At room temperature, the pressure may for example be 7000 Kg/cm² or higher, and should preferably be 3000 Kg/cm² or higher at 400°C. The temperature during the resintering process may be 400 - 900°C, and should preferably be in the region of 800°C. If resintering is carried out at a temperature above 900°C, the proportion of voids tends to increase again, and if it is carried out below 400°C, cracks tend to appear during subsequent mechanical processing to manufacture electrodes. The resintering time should preferably be 3 - 5 hours to eliminate stress and improve cohesive strength of Cu. Resintering and re-pression can be repeated any number of times. The contact material of this invention and its method of manufacture will now be described more concretely with reference to specific examples. Embodiment 1 Cu powder (particle diameter no greater than 70 µm purity not less than 99%), and Ta₂O₅ powder (particle diameter no greater than 40 m, purity not less than 99%), were weighed out in volumetric ratios of 99/1, 98/2, 95/5, 90/10, 85/15, 80/20, 70/30, 60/40, 50/50, 40/60, 30/70 and 20/80. After mixing in a ball mill, the mixture was packed into a carbon die, a temperature of 1050°C maintained under vacuum, and the mixture pressed for 1 hour under a load of 200 kg/cm². The mixture was then cooled in this state to 800°C, and the pressing load was released. After cooling, the sintered compact was removed from the die, fashioned into desired electrode shapes by mechanical processing, and incorporated into vacuum switches. The circuit breaking performance, chopping current and withstand voltage were then examined by the following methods. The results are shown in Figs. 4 - 6. (Circuit Breaking Performance) In the circuit breaking test, the current was increased gradually from 5 kA. The final value of current at which circuit breaking occurred successfully was taken as the circuit breaking performance and compared to the performance of a contact material consisting only of Cu. In Fig. 4, d shows the performance of a Cu-Cr contact containing 25% by weight of Cr for comparison purposes. (Chopping Current) The chopping current was measured initially, and then after 1000, 3000, 6000 and 10,000 on-load switching operations with a load current of 600 A. Measurements were suspended any time the chopping current exceeded 1 A. The data in Fig. 5 are averages for contact materials in Embodiments 1 - 3 including Ta₂O₅ with the proportions shown in Fig. 5. (Withstand Voltage) The contact was closed on a load of 1000 A, and opened on no load, and then a high voltage (30 kV) was applied with the contact open, and the presence or absence of discharge was checked. This cycle was repeated 1000 times, and the probability of discharge occurring was examined. In Fig. 6, d and e respectively show the performance of a Cu-Cr contact containing 25% by weight of Cr, and a WC-Ag contact with 50% by volume of Ag. Contacts with a low probability of discharge have an excellent withstand voltage. The wear of the contact was also examined when the chopping current was measured. It was found that for contacts containing not less than 30% by volume of Ta₂O₅, the wear was no greater than 0.1 mm even after 10000 on-load switching operations, i.e. the contacts suffered very little wear. For welding separation force, a current of 12.5 kA was passed for 3 sec, and the separation force measured by a tensile tester. It was found that the force was 30 kg f for a material containing 5% by volume of Ta₂O₅, but no greater than 10 kg f when the proportion of Ta₂O₅ was 10% by volume or more. In practice, weld was not observed most of the contacts and it was evidenced that the material causes very little weld. Embodiment 2 The same mixtures of Cu powder and Ta₂O₅ powder were used as in Embodiment 1. In the case of contact materials wherein Ta₂O₅ accounted for not less than 50% of the final composition by volume, the powder was packed into a die such that the proportion of voids was respectively 55%, 45%, 32% and 30%. As described later, molten Cu infiltrates into these voids, but as some sintering of the Ta₂O₅ takes place in practice, the ratio Cu/Ta₂O₅ in the final composition becomes respectively 50/50, 40/60, 30/70 and 20/80. For materials wherein on the other hand Ta₂O₅ accounted for not more than 35% by volume of the final composition, a mixture of Cu powder and Ta₂O₅ powder was used so that the compact would not break during handling. A mixed powder where the ratio Cu/Ta₂O₅ was 35/65, was packed into a die such that the percentage of voids was respectively 61% and 53%. In these cases also, as some sintering of the Ta₂O₅ takes place as described above, the ratio Cu/Ta₂O₅ in the final composition becomes respectively 70/30 and 65/35. For materials wherein Ta₂O₅ in the final composition accounts for 35 - 50% by volume, Cu powder may or may not be used as desired. In the case described here, Cu powder was used. A mixed powder where the ratio Cu/Ta₂O₅ was 20/80 was packed into a die such that the percentage of voids was 56%. The ratio Cu/Ta₂O₅ in the final composition of this material was 60/40. The compact obtained was so placed in a crucible that it was covered with Cu powder, and heated to 1200°C in vacuum so as to cover it with molten Cu. Argon gas was then introduced, and its pressure raised to 100 atm. and maintained for 1 hour so that the molten Cu press-infiltrated into the voids of the compact. The compact was cooled to the solidification point of the molten Cu, and the ambient pressure was restored to atmospheric. After cooling, a piece containing Cu/Ta₂O₅ was extracted by mechanically shaving from an infiltrated lump coated with Cu, an electrode was manufactured as in Embodiment 1, and the electrode incorporated into a vacuum switch. The circuit breaking performance, chopping current and withstand voltage were then examined, and the results are shown in Figs. 4 - 6. The wear and welding separation force of the contact were also examined, and very similar results to those of Embodiment 1 were obtained. Embodiment 3 As in Embodiment 1, Cu powder and Ta₂O₅ powder were weighed out in the proportions of 99/1, 98/2, 95/5, 90/10/, 85/15, 80/20, 70/30, 60/40, 50/50, 40/60 and 30/70. After mixing in a ball mill, the mixture was packed into a die, and a compact was formed by applying a pressure of not less than 3000 Kg/cm². The compact obtained was sintered in an atmosphere of hydrogen at 1050°C for 3 hours to give a sintered compact with 5 - 30% of voids by volume. The higher the Ta₂O₅ content, the higher the percentage of voids. The sintered compact was then put into a metal mold, re-pressed at ambient temperature under a pressure of 7000 Kg/cm², and resintered in an atmosphere of hydrogen at 800°C for three hours. From this sintered compact, electrodes were manufactured as in Embodiment 1, and incorporated into vacuum switches. The circuit breaking performance, chopping current and withstand voltage were then examined, and the results are shown in Figs. 4 - 6. When the wear and welding separation force of the contacts were also examined, very similar results to Embodiment 1 were obtained. The results obtained in Embodiments 1 - 3 which are shown in Figs. 4 - 6 will now be discussed. From Fig. 4, the peak performance of Embodiment 1a was 4.5 times that of Cu, 1.7 times that of Cu with 25% Cr by weight; the peak performance of Embodiment 2b was 4.6 times that of Cu and 1.8 times that of Cu with 25% Cr by weight; the peak performance of Embodiment 3c was 3.8 times that of Cu and 1.5 times that of Cu with 25% Cr by weight. As the conventional Cu-Cr contact with 25% Cr by weight is fully adequate in practice from the viewpoint of circuit-breaking performance, it is preferable that the Ta₂O₅ content of the contact material of this invention is within a range where the material has superior circuit breaking performance to the conventional material, i.e. above line d in Fig. 4. Further, the difference in the peak heights of a and c in the figure is due to the fact that whereas the density of the contact material in Embodiment 1 is not less than 99% of the theoretical ratio, the density of the material in Embodiment 3 is only about 96% of this ratio. Further, the difference in the positions of the peaks on a and b is due to differences in the structure of the contact material. In Embodiment 2, as described above, molten Cu is press-infiltrated into the voids of the compact, and so Cu is probably distributed more continuously compared to the case of materials manufactured by other methods even where the Ta₂O₅ content is high. Compacts manufactured by the method of Embodiment 2b with a high percentage of voids collapse very easily, and are difficult to manufacture. It is therefore preferable that the compact contains not less than 35% by volume of Ta₂O₅. But the compacts of Ta₂O₅ containing less voids were formed by sintering in air or oxygen to reduce the percentage of voids. From Fig. 5, it is seen that for compacts containing 5% by volume of Ta₂O₅, the chopping current was approx. 1 A even in the initial state (state when no on-load switching operations have been carried out), and that after 1000 on-load switching operations, it increased to as much as 35 A. As the proportion of Ta₂O₅ increased, however, the chopping current in the initial state declined, and at 40% or more by volume of Ta₂O₅, it fell to 0.55 A. As the Ta₂O₅ increased further, the increasing rate of the chopping current lowered even when the number of on-load switching operations increased; at 40% or more by volume of Ta₂O₅, the chopping current was not more than 1 A even after 10000 on-load switching operations, which is a really outstanding performance. The chopping currents shown in Fig. 5 are average values. For materials containing a low proportion of Ta₂O₅ the maximum value of chopping current was also high, while for those with a high proportion of Ta₂O₅ the maximum value was low and stable. In the case of materials with 5% by volume of Ta₂O₅, for example, the maximum value of chopping current was 3 A even in the initial state, and it increased to 4 A after 1000 on-load switching operations. In the case of materials with 40% by volume of Ta₂O₅, on the other hand, the maximum value of chopping current was 0.7 A in the initial state, and it was not more than 1.2 A and was stable after 10000 on-load switching operations. Unlike the case of circuit breaking performance, chopping current did not show much variation according to the manufacturing methods, showing that it had little dependence on this factor. The reason why the maximum value of chopping current depends on the Ta₂O₅ content but not on the manufacturing method as described above, is probably that the distribution of Ta₂O₅, the Ta₂O₅ content and the value of the current are relatively small. In other words, as the current used for on-load switching and the current used for the measurement of the arc are small, i.e., within a range of several 10 - several 100 A, the arc is small, and the point on the electrode surface where the arc is generated is extremely small. If therefore Ta₂O₅ is present where the arc is generated, the chopping current will be low, but if it is generated on a Cu part, the chopping current will be large. For this reason, if the Ta₂O₅ content is high and it is uniformly distributed, the chopping current will be small. Further, although the surface layers of the electrode are gradually worn down by on-load switching operations, the chopping current will not increase sharply by on-load switching providing there is sufficient Ta₂O₅ in the contact. In the case of circuit breaking performance, on the other hand, as there is a heavy current arc as much as 12.5 kA. Consequently the entire surface of the contact is exposed to the arc, and the physical properties of the contact as a whole such as its electrical and thermal conductance become important. These properties often depend on the structure of the contact. In the case of contact materials manufactured in Embodiment 2 where molten Cu is made to penetrate into the compact, for example, Cu is continuously distributed throughout the structure. As a result, the electrical and thermal conductance are high, and the circuit breaking performance of the material is excellent even if it contains a large proportion of Ta₂O₅. From Fig. 6, it is seen that when the contact material of this invention contains 5 - 17% by volume of Ta₂O₅, it is more difficult to cause a discharge than in the case of conventional Cu-Cr contacts with 25% Cr by weight, and it also has an excellent withstand voltage. Further, it is seen that when the material contains 5 - 80% of Ta₂O₅, it is more difficult to cause a discharge than in the case of conventional Ag-WC contacts with 50% by volume of WC, and it has excellent performance. For contacts with 5% by volume of Ta₂O₅, some weld tends to occur albeit slight when the load is connected. This probably causes projection of Cu on the contact surface and hence easier discharge. In the case of contacts of pure Cu, considerable weld occurred when the load was connected, and measurements were not possible. When the contact materials obtained in Embodiments 1 - 3 were analyzed by X-ray diffraction, it was found that apart from Cu and Ta₂O₅, they also contained compounds of the type of TaO₂ and Ta₂O₃. From the above, it was thus established that the contact material of this invention has superior circuit breaking performance to conventional Cu-Cr contacts with 25% Cr by weight when the Ta₂O₅ content was within the range 2 - 75% by volume. The chopping current was no greater than 1 A when the Ta₂O₅ content was 40% or more by volume (the same level as that of conventional Ag-WC contacts with 50% by volume of WC). The withstand voltage was higher than that of conventional Cu-Cr contacts with 25% Cr by weight when the Ta₂O₅ content was 5 - 17% by volume, and higher than that of conventional Ag-WC contacts with 50% by volume of WC when it was 5 - 80% by volume. The contact material of this invention was also found to have excellent wear and welding separation force properties.

Claims: A vacuum switch contact material consisting of Cu, and a tantalum oxide given by the formula Ta x O y , where x = 1 - 2, and y = 1 - 5. A contact material as claimed in claim 1, wherein x and y in said formula are 2 and 5 respectively. A contact material as claimed in claim 1 or 2 which is formed from Cu and tantalum oxide each having a purity of at least 99%. A contact material as claimed in claim 1 or 2 which includes Ta₂O₅ and components of the type TaO₂ and Ta₂O₃. A method of manufacturing a vacuum switch contact material, comprising the steps of: mixing Cu powder and Ta₂O₅ powder together; packing said powder mixture into a carbon die or other devices; and compressing said mixture while heating at a temperature below the melting point of Cu in a nonoxidizing atmosphere. A method as claimed in claim 5, wherein the time required for said pressurisation is so adjusted within the range of 30 minutes to 3 hours that the density of the sintered compact is not less than 99%. A method as claimed in claim 5 or claim 6, wherein said heating temperature is 1050 - 1080°C. A method of manufacturing a vacuum switch contact material, comprising the steps of: preparing Ta₂O₅ powder or a mixture of Ta₂O₅ powder Cu powder; packing said powder or powder mixture into a die or other devices; compressing said powder or mixture to manufacture a green compact; placing said compact in a crucible so that the compact is covered with Cu powder; heating said compact in a nonoxidizing atmosphere such that said Cu powder melts; raising the pressure of said furnace atmosphere so as to infiltrate said molten Cu into the voids in said compact; cooling it to the solidification point of the molten Cu; and restoring the ambient pressure to atmospheric. A method as claimed in claim 8, wherein the mixing proportion of Cu powder to Ta₂O₅ powder by volume is 60/40 - 25/75. A method as claimed in claim 8 or claim 9 wherein the percentage of voids in said compact is no greater than about 65%. A method as claimed in any one of claims 8, 9, or 10, wherein the pressure of the gas atmosphere is raised to 100-200 atm. and held for 30-60 minutes. A method of manufacturing a vacuum switch contact material, comprising the steps of: mixing Cu powder and Ta₂O₅ powder together; packing said powder mixture into a die or other devices; compressing said mixture to manufacture a green compact; sintering said green compact at a temperature below the melting point of Cu in a nonoxidizing atmosphere; re-pressing said sintered compact by hot working or cold working; and re-sintering said compact in a non-oxidising atmosphere. A method as claimed in claim 12, wherein a load used in said pressurisation lies between the range of about 200 kg/cm² to 500 kg/cm². A method as claimed in claim 12 or 13, wherein the required time for said pressurisation is so adjusted within the range of 30 minutes to 3 hours that the density of sintered compact is not less than 99%. A method as claimed in any one of claims 12, 13, 14 wherein the sintering time is 3-5 hours. A method as claimed in any one of claims 10 to 13 wherein said nonoxidizing atmosphere is hydrogen, or a vacuum of about 10⁻³ - 10⁻⁵ torr. A method as claimed in any one of claims 10 to 14, wherein said re-sintering and re-pressing are repeated one or more times.

Inventors: NAYA EIZO C O MITSUBISHI DENKI, OKUMURA MITSUHIRO C O MITSUBIS, NAYA, EIZO, C/O MITSUBISHI DENKI K.K., OKUMURA, MITSUHIRO, C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1990
Main classification: H01H1
IPC codes: H01H33, H01H1, C22C32