Method for producing a contact layer

Abstract

The invention relates to a method for producing a contact layer (2) on a contact electrode (1) of a vacuum interrupter (100), a layer of contact-layer printing paste (300) being printed onto a front face (7) of the contact electrode (1) by way of screen printing in order to form the contact layer (2).

Description

[0001] Description

[0002] Method for producing a contact layer

[0003] The present invention relates to a method for producing a contact layer and a switching contact of a vacuum interrupter.

[0004] In vacuum interrupters, the most expensive material is usually the contact material used for the contact layer of the switching contacts, which is also simply referred to as "contact material": mixtures of copper and chromium (CuCr), tungsten carbide and silver (WC-Ag) or tungsten and copper (WCu) are often used. Contact plates are usually manufactured from these composite materials using sintering or melting techniques as separate components. These contact plates are soldered onto the current-carrying contact electrodes of the vacuum interrupter, where they form the contact layer. Since the contact plates are manufactured as separate components, the contact material must have a minimum thickness of a few millimeters.

[0005] In many applications, particularly with diffuse arcs and few switching cycles, contact erosion is so low that only a smaller thickness of contact material is really required. In these cases, a contact material thickness of less than 1 mm is often sufficient. It therefore makes sense to only apply the contact material to the contact electrodes in the layer thickness actually required. The difficulty, however, lies in applying the contact material in an economical process as a dense, temperature-resistant, crack-free and sufficiently smooth layer and to bond it securely to the substrate, i.e. the contact electrodes.

[0006] Known processes by which contact materials can be applied as a contact layer directly onto the contact electrodes are powder metallurgy processes, see e.g. EP1844486A1 (ABB Technology AG [CH]) October 17, 2007, and cold gas spraying, see e.g. W02022 / 130604A1 (Mitsubishi Electric Corporation [JP]; Osaka University [JP]) June 23, 2022. However, both processes are complex and therefore relatively expensive.

[0007] There is therefore a need for an improved process by which contact materials can be applied as a contact layer directly onto contact electrodes.

[0008] This object is achieved according to the invention by a method having the features specified in claim 1 and a switching contact having the features specified in claim 9.

[0009] The method according to the invention is a method for producing a contact layer on a contact electrode of a vacuum interrupter. A vacuum interrupter generally has an evacuated switching chamber enclosed by a housing, in which a stationary switching contact and a switching contact movable in the axial direction are arranged. In a closing process, the movable switching contact can be brought into contact with the stationary switching contact by means of an axial movement and, in an opening process, can be brought into distance from the stationary switching contact by means of an opposite axial movement. The two switching contacts each have a current-carrying contact electrode, on the mutually facing end faces of which a contact layer is arranged, which contact each other when the vacuum interrupter is in the current-conducting state and the two switching contacts are pressed against one another.The contact electrode is made of an electrically conductive material, particularly a metallic material such as copper, a copper alloy, or stainless steel. This process is characterized by the fact that a layer of contact layer printing paste, which serves to form the contact layer, is printed onto the end face of the contact electrode using a screen printing process. The term "screen printing" also refers to stencil printing, a printing process similar to screen printing in which a stencil is used without a supporting screen.

[0010] Screen printing is used to coat the contact electrode with the contact layer printing paste. The desired shape of the contact layer, e.g., a circular area, is reproduced on a screen or stencil, and the contact layer printing paste is then applied to the contact electrode, which serves as the substrate. A multi-layer contact layer can be achieved with repeated printing.

[0011] The object is also achieved according to the invention by a switching contact of a vacuum interrupter which has a contact layer which was produced by means of a screen printing process.

[0012] The invention is based on the discovery that screen printing is suitable for producing a contact layer on a contact electrode at high throughput and at low cost. Screen printing technology is highly developed and predestined for parallel production processes. Compared to screen printing, the previously used processes of sintering and cold spraying are considerably more complex, expensive, and slower.

[0013] In addition, screen printing technology offers a great deal of freedom with regard to the size and shape of the contact layer. Screen printing technology is also significantly more flexible than previously used methods for producing contact layers: it makes it easy to apply multiple layers or gradient layers and to vary the thickness of the individual layers. A gradient layer is defined by a gradual change in the chemical composition; this avoids a direct interface between two chemically very different materials. Furthermore, with screen printing technology, no further processing of the layers is necessary after the final sintering process. Advantageous embodiments and developments of the invention are specified in the dependent claims.

[0014] According to a preferred embodiment of the invention, before the layer of contact layer printing paste is printed, at least one layer of intermediate layer printing paste is printed onto the end face of the contact electrode using a screen printing process to form at least one intermediate layer arranged between the end face of the contact electrode and the contact layer. To ensure good adhesion of the contact layer to the contact electrode and to adapt the thermal properties of the various materials, one or more printed intermediate layers can be applied to the contact electrode. A multi-layer intermediate layer can be achieved with repeated printing.

[0015] To produce interlayer printing paste, powder of materials intended for forming the interlayer is mixed in defined ratios with other paste components, usually a binder and a solvent. Dispersants, thixotropic agents, and plasticizers can also be used in the interlayer printing paste. Both organic and aqueous interlayer printing pastes are used. For example, polyvinyl butyral can be used as a binder, terpineol as a solvent, diisononyl adipate as a plasticizer, and a castor oil derivative as a thixotropic agent.

[0016] Screen printing is used to coat the contact electrode with the interlayer printing paste. The desired shape of the intermediate layer, e.g. a circular area, is reproduced on a screen or stencil, and with the help of this, the interlayer printing paste is applied to the contact electrode, which acts as the substrate. A multi-layer intermediate layer can be achieved with repeated printing strokes. It is possible to gradually change the composition of several intermediate layers printed on top of one another in order to create so-called gradient layers (also known as "gradient structures"). For example, when changing from a first intermediate layer to an adjacent second intermediate layer, the resulting gradient is reduced.The ratio between a conductive metal (Cu, Ag) and a hard material is changed in such a way that the end result is an n-layer structure in which the first layer (= first intermediate layer) consists predominantly of conductive metal and the last layer (= n-th intermediate layer) consists predominantly of hard material. The number n of intermediate layers is not limited, where n is an integer greater than zero.

[0017] According to a preferred embodiment of the invention, after printing a layer of intermediate layer printing paste, this layer is dried before a further layer of printing paste, be it an intermediate layer printing paste or a contact layer printing paste, is printed onto the layer of intermediate layer printing paste.

[0018] According to a preferred embodiment of the invention, after the layer of contact layer printing paste has been printed, the one or more layers of printing paste printed onto the contact electrode are sintered. In this case, all layers of printing paste printed onto the contact electrode, including any one or more layers of intermediate layer printing paste and the layer of contact layer printing paste, are subjected to a thermal treatment. A temperature is selected for sintering that is, on the one hand, high enough to create a dense structure of the material, and, on the other hand, low enough that melting processes do not occur.Depending on the substrate (= contact electrode), either vacuum, high vacuum, or a protective atmosphere (Ar, N2, H2, or a mixture of these) can be selected as the sintering atmosphere. According to a preferred embodiment of the invention, the contact layer printing paste is produced by mixing particles of a contact material with a binder to form a printing paste. The contact material comprises a highly electrically conductive material and / or a wear-resistant hard material. A highly electrically conductive material is defined as a material with an electrical conductivity of at least 10. 2 S / m, preferably at least 10 6S / m; in particular, metals are used as electrically conductive materials. An example of a wear-resistant hard material is tungsten, tungsten carbide, and chromium, or a material containing one or more of the aforementioned materials. It is possible to mix powder of materials intended for forming the contact layer in defined ratios with other paste components, usually a binder and a solvent.

[0019] According to a preferred embodiment of the invention, multiple layers of contact layer printing paste are applied one above the other. It is possible for the compositions of the multiple contact layers printed one above the other to change gradually (gradient layers). When applying multiple layers of contact layer printing paste, it is also possible for this layer to be dried after printing a layer of contact layer printing paste before printing another layer of contact layer printing paste.

[0020] In addition, dispersants, thixotropic agents and plasticizers can also be used in the contact layer printing paste. Both organic and aqueous contact layer printing paste are used. For example, polyvinyl butyral can be used as a binder, terpineol as a solvent, diisononyl adipate as a plasticizer and a castor oil derivative as a thixotropic agent. According to this preferred embodiment of the invention, the contact material is mixed with a binder to form a contact layer printing paste, this contact layer printing paste is applied to the contact electrode (= substrate) using a screen printing process and finally the contact material is firmly bonded to the contact electrode in a sintering process, wherein the binder preferably burns out without leaving any residue.

[0021] According to a preferred embodiment of the invention, the contact material comprises a material pairing of the following: copper-chromium, tungsten-copper, tungsten carbide-silver, tungsten carbide-copper. Composite materials made of electrically highly conductive materials, preferably metal such as in particular copper and silver, and erosion-resistant hard materials, in particular tungsten, tungsten carbide and chromium, can be used as the contact layer. Preferred material pairings are in particular copper-chromium, tungsten-copper, tungsten carbide-silver and tungsten carbide-copper. The particle size of the metal powder (copper and / or silver) is between 2 and 20 pm, preferably 5 to 10 pm. The hard material (tungsten carbide, tungsten, chromium) has particle sizes between 2 and 40 pm, preferably the particle size is between 7 and 15 pm.

[0022] According to a preferred embodiment of the invention, the contact layer has a thickness of 100 to 1500 μm. The contact layer can be between 100 and 1500 μm thick, preferably between 200 and 1000 μm. However, thinner or thicker contact layers are also possible, depending on the requirements of a specific embodiment.

[0023] According to a preferred embodiment of the invention, the intermediate layer printing paste is produced by mixing particles of stainless steel, copper, silver, a silver-stainless steel alloy or a copper-stainless steel alloy with a binder to form a printing paste. Suitable printed intermediate layers are particularly stainless steel, copper, silver and alloys of silver-stainless steel and copper-stainless steel. The preferred stainless steel is 1. 4404 (316L). The particle size of powders is between 2 and 20 μm, preferably 5 to 10 μm for copper and silver and 5 to 30 μm, preferably 8 to 15 μm for stainless steel powder. It is possible to use several intermediate layers in order to match parameters such as thermal expansion, shrinkage behavior, etc. of the contact layer and the contact electrode to one another in several steps.

[0024] According to a preferred embodiment of the invention, the one or more intermediate layers each have a thickness of 10 to 100 gm. An intermediate layer can be between 10 and 100 gm thick, preferably between 20 and 70 gm. However, thinner or thicker intermediate layers are also possible, depending on the requirements of a specific embodiment.

[0025] According to a preferred embodiment of the invention, the surface of the contact electrode has a depressed region into which the at least one layer of printing pastes, comprising intermediate layer and contact layer printing pastes, is printed. At the edges of the printed layers, heating during sintering can cause the printed layers to run. This can lead to undesired structures, e.g. sharp edges. To avoid this, the contact electrode can have a slight depression into which the intermediate layer and / or contact layer printing pastes are printed. This depression is designed in such a way that sharp edges, which can arise if the edges of the printed layers run, are avoided.

[0026] The depth of the depressed region, measured relative to a non-depressed region, is preferably smaller than the layer thickness of the layers printed on the contact electrode. The non-depressed region preferably surrounds the depressed region in a ring-like manner.

[0027] The invention is explained below using several exemplary embodiments with the aid of the accompanying drawings. Each of these shows, schematically and not to scale, Fig. 1 shows a longitudinal section through a vacuum interrupter;

[0028] Fig. 2A to Fig. 2C

[0029] Process steps for producing a contact layer and an intermediate layer on a contact electrode;

[0030] Fig. 3 shows a longitudinal section of a switching contact with an intermediate layer;

[0031] Fig. 4A to Fig. 4F

[0032] Process steps for producing a contact layer and four intermediate layers on a contact electrode;

[0033] Fig. 5 shows a longitudinal section of a switching contact with four intermediate layers;

[0034] Fig. 6 is a longitudinal section of a switching contact illustrating the alignment of the edges of the printed layers;

[0035] Fig. 7 shows a longitudinal section of a switching contact with a lowered area of ​​the contact electrode; and

[0036] Fig. 8 is a plan view of the end face of a contact electrode with a depressed area.

[0037] Fig. 1 shows a longitudinal section of a vacuum interrupter 100 known in the prior art, comprising an evacuated switching chamber 6 enclosed by a housing 50, in which a stationary switching contact 30, also referred to as a fixed contact for short, and an axially movable switching contact 40, also referred to as a moving contact for short, are arranged. The stationary contact 30 is located at one end of a stationary contact rod 10, also referred to as a fixed contact rod for short, which is led out of the vacuum interrupter 100 in a vacuum-tight manner through a first metallic cover 70, e.g., by soldering the fixed contact rod 10 and the cover 70. The moving contact 40 is located at one end of an axially movable contact rod 9, also referred to as a moving contact rod for short, which is guided in a displaceable and rotationally secure manner by means of a bearing 130 which is fixed to a second cover 80 and is led out of the vacuum interrupter 100 through the second cover 80.By means of the moving contact rod 9, the moving contact 40 can be brought into contact with the fixed contact 30 during a closing operation and can be spaced apart from the fixed contact 30 during an opening operation. The covers 70, 80, together with an insulating cylinder 60 arranged between them, which can be made of ceramic material, form the vacuum-tight housing 50 of the vacuum interrupter 100.

[0038] The passage of the moving contact rod 90 through the second cover 80 is held vacuum-tight by means of a metallic bellows 120, the first end of which is connected to an inner circumference of a circular through-opening 170 which is arranged in a cover base 160 of the second cover 80, and the second end of which is connected to a projection 110 of the moving contact rod 9, referred to as a bellows cap, e.g. by soldered connections. The bearing 130 comprises a perforated disk-shaped bearing flange 140 and a tubular guide part 150 which is attached concentrically to the bearing flange 140 and connected to it; the bearing 130 can be made in one piece. The bearing 130 can be made of plastic. The bearing flange 140 is centered and fixed to the cover base 160.

[0039] Fig. 2A to Fig. 2C show method steps for producing a contact layer and a single intermediate layer on a contact electrode. Fig. 2A shows a longitudinal section through a cylindrical contact electrode 1 which is fastened with a first end face to one end of a contact rod 10, e.g. by means of a soldered connection. In a first method step, a layer 200 of intermediate layer printing paste is applied in disc form by means of screen printing to an end face of the contact electrode 1 which is applied by the contact rod 10, so that the arrangement shown in Fig. 2B is produced. This is followed by a drying step for drying the layer 200 of intermediate layer printing paste.

[0040] In a process step following the drying step, a layer 300 of contact layer printing paste is applied in a disc-like manner to the dried layer 200 of intermediate layer printing paste by means of screen printing, so that the arrangement shown in Fig. 2C is obtained.

[0041] This is followed by a sintering step in which the two layers 200, 300 printed on the contact electrode 1 are sintered.

[0042] Fig. 3 shows the switching contact 30 after sintering. The switching contact 30 has a contact electrode 1 which has a disk-shaped intermediate layer 2 on one end face of the contact electrode 1 and a disk-shaped contact layer 3 on an end face of the intermediate layer 2 facing away from the contact electrode 1. The end face 11 of the contact layer 3 facing away from the intermediate layer 2 is the face which comes into contact with a corresponding contact layer of a second switching contact 40, as shown in Fig. 1, in the vacuum of a switching chamber 6.

[0043] Fig. 4A to Fig. 4F show method steps for producing a contact layer and four intermediate layers on a contact electrode. Fig. 4A shows a longitudinal section through a cylindrical contact electrode 1 which is fastened with a first end face to one end of a contact rod 10, e.g. by means of a soldered connection. In a first document, a first layer 200a of intermediate layer printing paste is applied in disc form by means of screen printing to an end face of the contact electrode 1 applied by the contact rod 10, so that the arrangement shown in Fig. 4B is produced. This is followed by a first drying step for drying the first layer 200a of intermediate layer printing paste.In a second printing step, a second layer 200b of interlayer printing paste is applied in a disc-shaped manner by screen printing onto a front side of the first layer 200a of interlayer printing paste applied by the contact rod 10, resulting in the arrangement shown in Fig. 4C. This is followed by a second drying step for drying the second layer 200b of interlayer printing paste.

[0044] In a third document, a third layer 200c of interlayer printing paste is applied in a disc-shaped manner by screen printing onto a front side of the second layer 200b of interlayer printing paste applied by the contact rod 10, resulting in the arrangement shown in Fig. 4D. This is followed by a third drying step for drying the third layer 200c of interlayer printing paste.

[0045] In a fourth document, a fourth layer 200d of interlayer printing paste is applied in a disc-shaped manner by screen printing onto a front side of the third layer 200c of interlayer printing paste applied by the contact rod 10, resulting in the arrangement shown in Fig. 4E. This is followed by a fourth drying step for drying the fourth layer 200d of interlayer printing paste.

[0046] In a process step following the fourth drying step, a layer 300 of contact layer printing paste is applied in a disc-like manner to the dried fourth layer 200d of intermediate layer printing paste by means of screen printing, so that the arrangement shown in Fig. 4 F is obtained.

[0047] This is followed by a sintering step in which the five layers 200a, 200b, 200c, 200d, 300 printed on the contact electrode 1 are sintered. Fig. 5 shows the switching contact 30 after sintering. The switching contact 30 has a contact electrode 1 which has four disk-shaped intermediate layers 2a, 2b, 2c, 2d on one end face of the contact electrode 1 and a disk-shaped contact layer 3 on an end face of the fourth intermediate layer 2d facing away from the contact electrode 1. The end face 11 of the contact layer 3 facing away from the fourth intermediate layer 2d is the face which comes into contact with a corresponding contact layer of a second switching contact 40, as shown in Fig. 1, in the vacuum of a switching chamber 6.

[0048] Fig. 6 shows a section through a switch contact, illustrating the running of the edges of the printed layers 2, 3. The use of screen printing as a method for printing the layers of printing paste leads to a running of the layers, namely the intermediate layer 2 and the contact layer 3, at their edge. A run-out 4 of the printed layers 2, 3 forms on the contact electrode 1, which does not occur with other known methods for applying contact material, sintering and cold gas spraying.

[0049] Fig. 7 shows a configuration of the contact electrode 1 to limit the runout of the printed layers shown in Fig. 6. For this purpose, a depressed region 13 is formed in a central region of the end face 7 of the contact electrode 1, which is separated from an annular raised region 12 by an annular wall 5.

[0050] Fig. 8 shows a plan view of the end face 7 of a contact electrode 1 with a central depressed region 13, a raised region 12 surrounding the central depressed region 13 in a ring shape, and an annular wall 5 between the depressed region 13 and the raised region 12. The screen printing of the printing pastes 200, 300 takes place only in the depressed region 13. The runout 4 of the printed layers 2, 3 occurring during sintering is caught by the wall 5 and spatially confined.

Claims

Patent claims 1. Method for producing a contact layer (2) on a contact electrode (1) of a vacuum interrupter (100), wherein a layer of contact layer printing paste (300) for forming the contact layer (2) is printed onto an end face (7) of the contact electrode (1) by means of a screen printing process.

2. Method according to claim 1, wherein, before the layer (300) of contact layer printing paste is printed, at least one layer (200) of intermediate layer printing paste is printed onto the end face (7) of the contact electrode (1) by means of a screen printing process to form at least one intermediate layer (2) arranged between the end face (7) of the contact electrode (1) and the contact layer (3).

3. The method according to claim 2, wherein after printing a layer (200) of intermediate layer printing paste, this layer (200) is dried before a further layer (200, 300) of printing paste is printed onto the layer (200) of intermediate layer printing paste.

4. Method according to one of the preceding claims, wherein after printing the layer (300) of contact layer printing paste, the one or more layers (200, 300) of printing paste printed on the contact electrode are sintered.

5. Method according to one of the preceding claims, wherein the contact layer printing paste is produced by mixing particles of a contact material comprising an electrically highly conductive material and / or an erosion-resistant hard material with a binder to form a printing paste.

6. The method according to claim 5, wherein the contact material comprises one of the following material pairs: copper-chromium, tungsten-copper, tungsten carbide-silver, tungsten carbide-copper.

7. A method according to any one of the preceding claims, wherein the interlayer printing paste is prepared by mixing particles of stainless steel, copper, silver, a silver-stainless steel alloy or a copper-stainless steel alloy with a binder to form a printing paste.

8. Method according to one of the preceding claims, wherein the surface of the contact electrode has a depressed region (5) into which the at least one layer (2, 3) of printing pastes is printed.

9. Switching contact (30, 40) of a vacuum interrupter (100), comprising a contact layer (3) produced by means of a screen printing process.

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