Method for manufacturing a multilayer varistor, use of a metal paste for forming a metal layer, green body for manufacturing a multilayer varistor, and multilayer varistor

Abstract

The present invention relates to a method for manufacturing a multilayer varistor (1), preparing a metal paste (103) containing silver and nickel, wherein the mass ratio of nickel to the metal in the metal paste (103) is at most 25%; applying the metal paste (103) onto a ceramic green film (102); applying a further ceramic green film (102) onto the metal paste (103) to produce a sandwich structure; firing the green film (102) together with the applied metal paste (103), wherein the green film (102) is converted into a ceramic layer (2) and the metal paste (103) is converted into an internal electrode (3).

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a multilayer varistor, a green body for manufacturing a multilayer varistor, the use of a suitable metal paste, and a multilayer varistor.

Background Art

[0002] In multilayer varistor elements, typically, a zinc oxide-based ceramic having internal electrodes is used. Since the internal electrodes have a melting point high enough to obtain the desired electronic properties by sintering in the varistor-ceramic, they preferably contain a silver-palladium alloy. Examples of such varistors and electrode compositions are disclosed, for example, in publication EP 3300087 A1, CN 106782956 A, or DE 112019003625 T5.

[0003] In order to eliminate expensive noble metals such as palladium, the internal electrodes of current multilayer varistors are partially made of silver, a relatively inexpensive noble metal. In addition to zinc oxide, a high proportion of bismuth(III) oxide, for example, is added to the varistor ceramic in order to enable the necessary densification and particle growth of the ceramic even at a low sintering temperature below the melting point of silver [see German Patent Publication DE 102015120640 A1 and non-patent literature Bernik et al.: Ceramics Silikaty 62(1), 8 - 14(2018)].

[0004] However, the silver electrodes are attacked by the melt of bismuth(III) oxide during sintering, and silver is partially oxidized to become Ag2(I)BiO3 or Ag(II)BiO3, respectively. Also, the higher the sintering temperature, the stronger the attack on the silver electrodes by oxidation. Bi2O3 melts between 817 °C and 824 °C [see non-patent literature: Sadecka et al.: Journal of Materials Science, 2017, 52(10), pp.5503 - 5510 - reference].

[0005] The oxidation of silver causes thinning and disappearance of the electrodes.

[0006] In an alternative approach, attempts are being made to completely replace the precious metal of the varistor electrode. For example, Chinese Patent Publication Gazette CN 104658727 A discloses an electrode made of pure nickel. However, in this case, sintering is only possible in a protective atmosphere under oxygen exclusion, and no varistor active ceramic is produced. Therefore, this technology is not used in the commercial field. SUMMARY OF THE INVENTION

[0007] Accordingly, an object of the present invention is to provide an alternative method for manufacturing an improved multilayer varistor.

[0008] The present invention relates to a method for manufacturing a multilayer varistor. This method includes at least the steps described below, and these steps are preferably executed in the order shown.

[0009] In one step, a metal paste containing silver and nickel is prepared. The mass ratio of nickel to the metal in the metal paste exceeds 0% and is at most 25%, preferably at least 0.15% and at most 20%. Here and below, the percentages always indicate mass ratios.

[0010] In particular, in order to prepare the metal paste, a silver phase and a nickel phase can be mixed. The silver phase contains or consists of silver (Ag), and the nickel phase contains or consists of nickel (Ni). Silver and nickel are, for example, in the solid form of powder or granules, or in the fluid form of a melt or vapor. The method of mixing silver and nickel is not particularly limited. In addition to silver and nickel, the paste may contain other metals, as well as organic and inorganic additives or adjuvants. By mixing, a uniform distribution of nickel in silver is achieved. Nickel exists in the form of fine particles.

[0011] By mixing the silver phase and the nickel phase, a single-phase or at least two-phase metal paste is obtained. In the metallic phase, nickel dissolves up to a mass ratio of 0, 15%, and an AgNi alloy is formed. Furthermore, the added nickel is present in a separate nickel phase.

[0012] Therefore, the proportion of nickel metal in the metal in the metal paste is preferably 0.50% or more, preferably 1.0% or more or 3.0% or more. The proportion of nickel is preferably selected to a height such that a nickel phase mainly containing nickel is formed in the metal paste in addition to the silver phase.

[0013] If the proportion of nickel metal exceeds 20%, it may damage the varistor. Therefore, the maximum proportion of nickel in the metal paste is preferably less than 20%, preferably less than 19% or less than 17.5%.

[0014] In a further step, the metal paste is applied or coated (aufgetragen) onto the ceramic green film. The ceramic green film can comprise any ceramic material in an unsintered state. In particular, the ceramic material comprises various metal oxides forming the raw material of the ceramic, as well as an organic binder and auxiliary agents, and optionally additional doping agents.

[0015] For example, the metal paste can be screen-printed onto the green film. Next, by adding an organic solvent and auxiliary agents, the target viscosity of the metal paste suitable for screen printing is set.

[0016] In a further step, a further ceramic green film is applied to produce a sandwich-like structure. In particular, for example, to compensate for shrinkage during sintering, the green film or a further green film can also be applied next to the metal paste. However, the metal paste is exposed at least on one side (an einer Seite) to the surroundings.

[0017] Also, processes such as decarburization and debinding can be carried out before the sintering step.

[0018] In a further step, the green film is sintered in a common process step together with the applied metal paste, the green film is converted into a ceramic layer, and the metal paste is converted into an internal electrode.

[0019] Nickel is preferentially diffused into and oxidized in the transition layer of the ceramic layer being formed during sintering. The transition layer is adjacent to the internal electrode being formed. Thereby, a transition layer is formed in which a nickel oxide phase is formed adjacent to the internal electrode. Ni is oxidized, for example, to nickel(II) oxide (NiO).

[0020] For example, the nickel oxide phase can be formed in the form of a plurality of crystallites or as an aggregated crystal film.

[0021] In a preferred embodiment, the thickness of the transition layer is 5 μm or less, more preferably 3 μm or less.

[0022] Optionally, a laminate can be formed from a plurality of green films to which the metal paste has been applied to each green film or selected green films, and the entire laminate can be sintered together in a single process step. Next, the green films are pressed against each other before sintering to bond the green films. Thereafter, before sintering, individual components of a specific dimension can be cut from the laminate, which is also referred to as "cutting". Thus, hereinafter, the laminate can represent both the uncut laminate and the components cut from a larger laminate.

[0023] Also, steps such as decarbonization and debinding can be performed before sintering.

[0024] To achieve sufficient particle growth and a desirable particle boundary structure in the ceramic, sintering is carried out, for example, in an atmosphere or an oxygen-enriched atmosphere having a substantial oxygen ratio (wesentlichem Sauerstoffanteil).

[0025] Each layer containing the metal paste can further include a ceramic green film or a part of the green film.

[0026] The outer layers of the laminate in the stacking direction are preferably each formed by one ceramic green film.

[0027] The metal paste is preferably exposed to the environment on at least one side (einer Seite) of the laminate or component. Thus, the metal paste forms a region of the outer surface of the laminate on at least one side. An additional metal layer can in particular be applied to the outer surface of the laminate after sintering and can contact the metal paste inside the laminate. The outer metal layer can have a composition different from that of the metal paste.

[0028] The metal paste can have different compositions in each layer. The metal paste is preferably always of the same composition in each layer.

[0029] The described method provides a multilayer varistor having an electrode containing silver and can avoid the addition of further noble metals such as palladium, and thus the varistor can be manufactured more economically.

[0030] Furthermore, nickel in the metal paste protects silver from oxidation attack during sintering of the ceramic, thus reducing the consumption of the electrode due to the chemical reaction between silver and the ceramic and optimizing the use of silver material.

[0031] In one embodiment, sintering is performed at a temperature exceeding 900 °C, preferably exceeding 940 °C, more preferably exceeding 950 °C. The maximum achievable temperature is lower than the melting temperature of silver. The melting temperature of silver is 962 °C under normal conditions. When silver melts, it is heated above the melting temperature, so individual droplets of silver are formed, and the structure of the electrode deteriorates and changes.

[0032] Due to the protective effect of nickel, the ceramic and metal paste can be heated below the melting point of silver and sintered in an oxygen atmosphere. As a result, it becomes possible to favorably form varistor ceramics having a corresponding particle size and a corresponding grain boundary structure. Grain boundaries form the substantial electrical resistance of the ceramic material. In particular, large particles with clearly formed grain boundaries can be generated.

[0033] Therefore, varistor characteristics can be improved by raising the peak temperature during sintering. In particular, the electrical characteristics of the varistor can be improved. For example, by raising the temperature during sintering, a varistor with a lower varistor voltage can be obtained. The varistor voltage is defined as the voltage that needs to be applied to the varistor to generate a current of 1 milliampere (1 mA).

[0034] In one embodiment, silver represents the maximum mass ratio of all components to the metal paste. In particular, silver has the largest mass ratio among all metals in the metal paste.

[0035] In one embodiment, the metal paste contains only metals of silver and nickel in metallic form.

[0036] The mass ratio of nickel to the metal in the metal paste is preferably at least 0.15% and at most 20%.

[0037] Preferably, the metal ratio of the metal paste is 0.15% - 20% nickel and 80% - 99.85% silver.

[0038] In one embodiment, the metal paste consists of silver, nickel, and further non-metallic inorganic and organic components. The use of further precious metals other than silver can be avoided. The further components are, for example, organic binders or fillers for adjusting shrinkage or enhancing adhesion.

[0039] In one embodiment, the ceramic green film contains ZnO and bismuth(III) oxide (Bi2O3). Thereby, a ceramic having electrical properties advantageous for use as a varistor, such as a high threshold resistance or a low varistor voltage, can be formed.

[0040] In one embodiment, the ceramic green film consists of at least 90% by mass of ZnO and Bi2O3, or ZnO, Bi2O3, and antimony(III) oxide (Sb2O3). Thereby, a ceramic having desired ceramic properties can be formed. In order to appropriately adjust the particle growth and particle structure, preferably, the ratio of bismuth Bi to antimony Sb in the ceramic is greater than 1:1.

[0041] The ceramic green film can also contain organic or inorganic binders, solvents, plasticizers, and further additives.

[0042] In one embodiment, during sintering, nickel diffuses at the boundary of the internal electrode with respect to the ceramic layer or within the ceramic layer formed adjacent to the internal electrode. These layers adjacent to the internal electrode are defined as protective layers. After sintering, the ceramic in the protective layer is doped with nickel. Furthermore, a separate nickel phase may be formed in the protective layer.

[0043] Therefore, an advantage of this method is that the ceramic is doped with nickel during sintering.

[0044] In the protective layer, advantageously, at least a part of Ni is oxidized to, for example, nickel(II) oxide (NiO), and advantageously, a nickel oxide phase is formed in the transition layer.

[0045] Bi2O3 in the formed protective layer is preferably partially reduced to bismuth(II) oxide (BiO). This nickel oxide can form, for example, nickel zinc spinel. Therefore, the closer to the formed internal electrode, preferably a higher proportion of Bi2O3 is reduced.

[0046] After that, the nickel oxide phase in the transition layer forms a barrier adjacent to the internal electrode, through which further Bi2O3 cannot diffuse to the internal electrode, thus avoiding the oxidation of silver.

[0047] Nickel in the internal electrode has multiple protective effects. Nickel is preferentially oxidized over silver. Nickel is a base metal, and thus, when nickel diffuses into the ceramic, a protective layer is formed around the internal electrode during sintering, and Bi2O3 is reduced. Also, within the transition layer adjacent to the internal electrode, when nickel oxidizes, a barrier of nickel oxide is formed around the internal electrode.

[0048] Following the described method steps, in further embodiments, additional optional method steps can be performed.

[0049] An example of such a method step is to press the laminate to stably bond the layers to each other. The pressing is performed before sintering.

[0050] After pressing, the laminate can be divided (also referred to as cutting) into a plurality of parts having predetermined dimensions.

[0051] In any subsequent step, organic substances in the green film or metal paste, such as binders and auxiliaries, can be burned out by heating.

[0052] After these steps, sintering is performed as described above.

[0053] After sintering, an external contact can be applied to the multilayer varistor to electrically connect the formed internal electrodes. The external contact can be obtained, for example, by dipping the output surface of the internal electrode into a metal paste and then baking the paste.

[0054] The present invention further relates to a multilayer varistor designed to be manufacturable by the method described above. However, the varistor is not limited to being manufactured by the above method.

[0055] The present invention also relates to a green body for manufacturing a multilayer varistor. The green body can have all the features described above with respect to the method, and vice versa.

[0056] In particular, the green body includes at least two ceramic green films and a metal layer arranged in a sandwich structure between the at least two ceramic green films.

[0057] The metal layer can include a metal paste containing silver and nickel, and the mass ratio of nickel to the metal in the metal paste is at most 25%, preferably at least 0.15% and at most 20%.

[0058] In addition to the metal paste, a ceramic green film surrounding the metal paste on a plurality of sides (Seiten) can also be provided within the metal layer. The metal paste is exposed on the periphery in at least one direction.

[0059] In a preferred embodiment, the metal paste consists of only silver and nickel as metals and non-metallic organic and / or inorganic components. That is, the metal paste in metallic form contains only silver and nickel as metals.

[0060] In a preferred embodiment, at least 90% by mass of the ceramic green film consists of ZnO and Bi2O3, or ZnO, Bi2O3 and Sb2O3.

[0061] The present invention further relates to the use of a metal paste containing silver and nickel to form a metal layer containing silver in or on a ceramic containing Bi2O3, wherein the mass ratio of nickel to the metal of the metal paste is at most 25%, preferably at least 0.15% and at most 20%.

[0062] The metal paste and the ceramic can have all of the above characteristics, and vice versa.

[0063] In particular, in this way, a lead-containing ceramic can be replaced with a bismuth oxide-containing ceramic and can be brought into contact with a silver electrode. This is because bismuth oxide hardly erodes the silver in the formed metal layer during sintering due to the nickel ratio in the metal paste.

[0064] In an embodiment, the metal paste can consist of silver, nickel, and further non-metallic organic and / or inorganic components.

[0065] Since nickel diffuses into the ceramic during the manufacturing process, the formed metal layer preferably contains only silver as the metal.

[0066] The ceramic consists of at least 90% by mass of ZnO and Bi2O3, or of ZnO, Bi2O3 and Sb2O3.

[0067] The present invention also relates to a multilayer varistor. The varistor can be designed in the same manner as the above-described embodiments. The ceramic layer, internal electrodes, and other elements of the varistor can have the same characteristics as described above.

[0068] The varistor is preferably manufactured by the above-described method.

[0069] The varistor includes at least two ceramic layers and internal electrodes containing silver arranged in a sandwich structure between the two ceramic layers.

[0070] Inside the ceramic layer, one transition layer is formed adjacent to each internal electrode, and a nickel oxide phase is formed in the transition layer. The nickel oxide phase is preferably formed between the internal electrode and the remaining ceramic layer.

[0071] Preferably, the nickel oxide phase is formed as a continuous film. Particularly preferably, the nickel oxide phase is formed as a continuous film separating the internal electrode from the remaining ceramic, thereby forming a continuous barrier between the internal electrode and the remaining ceramic.

[0072] In an alternative embodiment, the nickel oxide phase is formed in the form of a plurality of discontinuous microcrystals.

[0073] Also, the nickel oxide phase can be formed outside the transition layer or inside the internal electrode.

[0074] The transition layer preferably has a thickness of at most 5 μm.

[0075] In one embodiment, a protective layer is formed in each ceramic layer adjacent to the internal electrode, and bismuth oxide mainly exists in a reduced form in the protective layer.

[0076] Advantageously, as approaching the internal electrode, more bismuth oxide exists in the reduced form of BiO.

[0077] Advantageously, the protective layer also has an increased nickel doping compared to the remaining part of the ceramic. The protective layer is adjacent to the internal electrode and includes the transition layer defined above.

[0078] For example, the thickness of the transition layer is 40 μm. In particular, the thickness of the transition layer is preferably at most 10 times the thickness of the internal electrode.

[0079] In one embodiment, at least 90% by mass of the ceramic layer outside the protective layer consists of zinc oxide and bismuth oxide, or consists of zinc oxide, bismuth oxide, and antimony oxide. Specific examples of zinc oxide include ZnO, specific examples of bismuth oxide include BiO and Bi2O3, and specific examples of antimony oxide include Sb2O3.

[0080] In addition, the ceramic layer also contains NiO, and NiO is formed by the oxidation of nickel from the metal paste during sintering, at least in the transition layer adjacent to the internal electrode.

[0081] In one embodiment, the mass ratio of elemental nickel to the total mass of elemental nickel and silver in the multilayer varistor is at most 25%.

Brief Description of the Drawings

[0082] Hereinafter, embodiments will be described with reference to the drawings. The present invention is not limited to the following embodiments.

[0083]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Embodiments for Carrying Out the Invention

[0084] Figure 1 shows a first embodiment of a varistor 1 according to the present invention. This is a multilayer varistor in which a ceramic layer 2 and a layer having internal electrodes 3 therebetween are alternately laminated. The internal electrodes 3 are surrounded by ceramic and are exposed (liegen...frei) on each one side of the outer surface of the varistor 1 (an jeweils einer Seite).

[0085] In this embodiment, the internal electrode 3a is exposed every other one in the lamination direction on the first side surface, and the internal electrode 3b therebetween in the lamination direction is exposed on the second side surface opposite to the first surface.

[0086] The internal electrode preferably contains only silver as the metal.

[0087] This ceramic is a zinc-bismuth ceramic containing ZnO and Bi2O3. Further, this ceramic can contain further metal oxides, particularly zinc oxide, bismuth oxide and antimony oxide, as well as a doping agent.

[0088] To manufacture the varistor 1, a ceramic green film containing the aforementioned metal oxides in a predetermined composition is provided. The green film is printed, for example, by screen printing using a metal paste. The metal paste contains silver and nickel. The ratio of nickel to the metal in the metal paste is between 0.15% and 20%. By adding an organic solvent and an auxiliary agent, the target viscosity of the metal paste suitable for screen printing is set.

[0089] In a subsequent step, the printed green film and the unprinted green film are stacked on top of each other with high positional accuracy in a defined order to realize the described varistor layer structure.

[0090] Subsequently, the laminate is mechanically pressed to stably bond the layers to each other. By cutting at the specified positions, parts having a defined size are manufactured from the laminate. An exemplary varistor component 101 in the green state is shown in FIG. 2. The varistor component 101 includes a laminate of green ceramic layers 102 and a layer of metal paste 103.

[0091] In a subsequent step, the organic binders and auxiliary materials present in the ceramic green film and the metal paste are burned out (ausgebrannt) in order to ensure the strength required in the aforementioned method steps. For this purpose, the parts are heated in an oven for a sufficient time.

[0092] After the organic components have been burned out, the component is sintered. Due to the protective effect of the nickel proportion in the metal paste, the sintering temperature at the peak (in der Spitze) can be heated to a temperature slightly lower than the melting temperature of silver. The sintering temperature is maintained for a sufficiently long time, for example, for 180 minutes.

[0093] Subsequently, the sintered varistor component is electrically contacted. External electrodes are applied in order to be able to electrically contact the component. These are obtained, for example, by dipping the output surface of the internal electrode into silver paste and then firing (Einbrennen) the silver paste. For example, the firing is carried out at a temperature of about 650 °C to 700 °C.

[0094] FIG. 3 shows a varistor 11 according to the prior art having a normal silver electrode 13 without nickel addition. Since the prices of other precious metals such as palladium are high, preferably silver is used for the internal electrodes of the varistor.

[0095] On the one hand, in order to configure appropriate electrical properties of the varistor, a sufficiently high sintering temperature is required to appropriately adjust the particle growth and particle boundary structure. However, silver has a relatively low sintering temperature compared to other noble metals.

[0096] Thereafter, with reference to FIGS. 10 and 11, the desired electrical properties will be described.

[0097] For a multilayer varistor, a zinc oxide ceramic having good varistor properties is usually used.

[0098] As in the example according to the present invention in FIG. 3, when a pure silver electrode is used, a zinc oxide ceramic of a normal composition is not suitable as a varistor ceramic. At a temperature lower than the melting point of Ag, such a zinc oxide ceramic cannot form an appropriate structure. Therefore, for example, even at a temperature lower than the melting temperature of Ag, in order to enable the formation of a particle boundary structure and particle growth to a desired degree, the proportion of bismuth oxide is increased and added to the ceramic composition.

[0099] However, the disadvantage of adding a large amount of bismuth(III) oxide Bi2O3 is its reactivity, and a part of the silver material is oxidized during sintering. In particular, near the outer surface of the varistor 11, and thus in the outer region 14 of the internal electrode 13 that is in contact with oxygen from the sintering atmosphere, Ag is partially or completely oxidized, and as a result, the electrical contact of the electrode is impaired.

[0100] FIG. 3 shows a REM microscopic image of a cross section of the multilayer varistor 11. The internal electrode 13 made of pure silver is significantly thinned in the outer region 14 due to oxidation attack.

[0101] Furthermore, the silver diffused into the ceramic deteriorates the insulating properties of the ceramic.

[0102] FIG. 4 shows how the addition of nickel exerts a protective effect on the internal electrode 23, which preferably consists of Ag.

[0103] Nickel is a base metal more base than silver. Therefore, nickel in Bi2O3 is preferentially oxidized. Thus, silver remains in the reduced metallic form.

[0104] Furthermore, due to the diffusion of nickel into the ceramic, the partial oxidation of nickel, and the partial reduction of bismuth oxide, a layer containing nickel oxide NiO and bismuth(II) oxide BiO is formed, in particular, in the region adjacent to the internal electrode of the ceramic layer 22. In the REM microscope image of FIG. 4 of the corresponding varistor 21 of the present invention, the protective layer 25 formed in this way can be seen. For example, NiO reacts with ZnO to form Ni-Zn spinel, making it even more difficult for Bi2O3 to access the internal electrode.

[0105] In FIG. 4, the bright spots in the ceramic indicate the Bi2O3 phase. Adjacent to the internal electrode 23, these bright spots are no longer seen within the protective layer 25. Therefore, after a sufficient amount of nickel has been oxidized, Bi2O3 can no longer reach the electrode, and thus no further nickel or silver is oxidized. Instead, dark spots are seen in the protective layer, suggesting the formation of the BiO phase. The protective layer 25 has, for example, a thickness about 10 times that of the internal electrode 23.

[0106] FIG. 5 is an enlarged image by a REM microscope. In the nickel oxide phase, it is recognized as fine-grained microcrystals 24 of the internal electrode 23 within the internal electrode 23 and in the transition layer adjacent to the internal electrode 23. The microcrystals 24 are surrounded by circles and marked, forming a diffusion barrier between the ceramic and the internal electrode.

[0107] FIG. 6 shows the distribution of the concentration of elemental silver in the varistor captured by EDX, that is, energy-dispersive X-ray spectroscopy, within the cross section shown in FIG. 5. FIG. 7 shows the distribution of elemental oxygen. FIG. 8 shows the distribution of elemental zinc, which is displayed brightly for high concentrations and darkly for low concentrations.

[0108] Since there is no oxygen in the silver layer and no silver in the oxygen-rich phase, it can be seen that silver cannot be oxidized. The silver layer of the internal electrode is protected from attack by oxidation.

[0109] Also, it can be seen that the ceramic zinc oxide phase, i.e., the region where zinc and oxygen are present, does not reach the silver layer shown in FIG. 6.

[0110] FIG. 9 shows the EDX concentration distribution of the element nickel. FIG. 9 shows that a nickel-rich phase is formed between the zinc-rich region and the silver-rich region. FIG. 7 shows that the nickel-rich phase is also rich in oxygen. This is the nickel oxide microcrystal 24 shown in FIG. 5.

[0111] In this way, since the electrode is protected from oxidation attack on silver or nickel, the electrode can be made narrow in the stacking direction. Specifically, it can be made as thin as at least 6 μm, more preferably 5 μm, and even more preferably 4 μm. Thereby, the metal, especially the silver material, for forming the electrode can be saved. Therefore, external electrical contact is ensured.

[0112] FIGS. 10 and 11 show the electrical characteristic curves, i.e., the current-voltage characteristics, of various varistor components.

[0113] In the double logarithmic graph, the x-axis represents the current A and the y-axis represents the voltage V.

[0114] FIG. 10 compares the characteristic curve of a nickel-free varistor having a silver electrode with the characteristic curve of the electrode of a varistor in which the nickel ratio in the metal paste is 15% (or 17.2 vol%) with respect to the total mass of silver and nickel in the metal paste of the green body. The characteristic of the nickel-free varistor is the upper graph and the characteristic curve of the silver-nickel varistor is the lower graph. The two electrodes were sintered at 900 °C respectively.

[0115] In particular, the breakdown voltage, i.e., the voltage at which the varistor conducts electricity thereafter, and the non-linearity of the curve, which is a measure of the quality of the varistor, are considered.

[0116] The higher the non-linearity, the better the switching between the conductive state and the non-conductive state. The following curves show that similar characteristics can be achieved by adding nickel, but there are no problems regarding the consumption of the materials and external contacts described above with reference to FIGS. 2 and 3.

[0117] FIG. 11 shows, in addition to the characteristic curve of the nickel-free varistor having a silver electrode from FIG. 10, i.e., the uppermost curve in the figure, and the characteristic curve of the varistor with a nickel ratio of 15% described above, i.e., the lowermost curve in the figure, five further characteristic curves.

[0118] All of the other five characteristic curves were measured using the electrodes of varistors in which the nickel content in the metal paste is 2.6% (3% by volume) with respect to the total mass of Ni and Ag in the metal paste of the green body.

[0119] Various varistors each containing a nickel ratio of 2.6% were sintered at all different temperatures. Among the above curves, the uppermost curve corresponds to the electrode sintered at 900 °C, the next curve corresponds to the electrode sintered at 920 °C, the next curve corresponds to the electrode sintered at 940 °C, the next curve corresponds to the electrode sintered at 950 °C, and the lowermost curve corresponds to the electrode sintered at 960 °C. The two further characteristic curves in FIG. 10 are related to the electrodes sintered at 900 °C, respectively.

[0120] FIG. 11 shows that when the nickel ratio is decreased, the non-linearity of the curve approaches that of the pure silver electrode curve. In particular, the non-linearity of the curve can be further improved.

[0121] A higher sintering temperature can significantly reduce the varistor voltage, i.e., the voltage required to obtain a current of 1 mA. In this example, the varistor voltage of the silver electrode in the nickel-free varistor is 82 volts, while the varistor voltage of the Ag-Ni varistor sintered at 960 °C with 2.6% Ni is 55 volts. This is a 33% reduction in the varistor voltage.

[0122] This significantly expands the possibilities for setting the characteristic curve via the sintering temperature. When using a pure silver electrode, the same high sintering temperature is not possible as it would cause destruction of the silver electrode.

Explanation of symbols

[0123] 1, 11, 21 Sintered varistor 2, 22 Ceramic layers 3, 13, 23 Internal electrodes 3a Left - hand side internal electrode 3b Right - hand side internal electrode 14 Outer region 24 NiO microcrystals 25 Protective layers 101 Varistor in the green state 102 Ceramic green layers 103 Metal paste

Claims

1. A method for manufacturing a multilayer varistor (1), comprising the step of preparing a metal paste (103) containing silver and nickel, wherein the mass ratio of nickel to the metal in the metal paste (103) is at most 25%, the step of applying the metal paste (103) onto a ceramic green film (102), the step of applying a further ceramic green film (102) onto the metal paste (103) to produce a sandwich structure, and the step of sintering together the metal paste (103) onto which the ceramic green film (102) has been applied, converting the ceramic green film (102) into a ceramic layer (2) and the metal paste (103) into an internal electrode (3). Method.

2. During the sintering, nickel diffuses into the layer of the ceramic layer (2) formed adjacent to the internal electrode (3) formed and is oxidized there, thus forming a nickel oxide phase in a transition layer (24) adjacent to the internal electrode (3). The method according to claim 1.

3. The sintering is carried out at a temperature exceeding 900 °C. The method according to claim 1 or 2.

4. The sintering is carried out at a temperature exceeding 940 °C or 950 °C. The method according to claim 3.

5. The metal paste (103) in metallic form contains only metallic silver and nickel. The method according to any one of claims 1 to 4.

6. The transition layer (24) has a thickness of at most 5 μm. The method according to any one of claims 1 to 5.

7. The ceramic green film (102) has a mass ratio of at least 90% of ZnO and Bi ２ O ３ , or ZnO, Bi ２ O ３ and Sb ２ O ３ consisting of, The method according to any one of claims 1 to 6.

8. During the sintering, nickel diffuses into the protective layer (25) of the ceramic layer (2) to be formed, adjacent to the internal electrode (3) to be formed. The method according to any one of claims 1 to 7.

9. Bi ２ O ３ is partially reduced in the protective layer (25) to form BiO. The method according to claim 8.

10. As approaching the internal electrode (3) to be formed, a higher proportion of Bi ２ O ３ in the protective layer (25) is reduced. The method according to claim 9.

11. The sintering is performed in an atmosphere having a considerable oxygen content. The method according to any one of claims 1 to 10.

12. The sintering is performed under ambient air. The method according to claim 11.

13. The mass ratio of nickel in the metal of the metal paste (103) is at least 0.15% and at most 20%. The method according to any one of claims 1 to 12.

14. A green body (101) for manufacturing a multilayer varistor (1), comprising two ceramic green films (102) and a metal layer, The metal layer is disposed within a sandwich structure between the two ceramic green films (102), and includes a metal paste (103) containing silver and nickel. The metal paste (103) and the metal paste (103) having a mass ratio of nickel to the metal in the metal paste (103) of at most 25% are provided. Green body.

15. The metal paste (103) in the form of metal contains only metallic silver and nickel. The green body according to claim 14.

16. The mass ratio of nickel in the metal of the metal paste (103) is at least 0.15% and at most 20%. The green body according to claim 14 or 15.

17. The ceramic green film (102) has a mass ratio of at least 90% of ZnO and Bi ２ O ３ or ZnO, Bi ２ O ３ and Sb ２ O ３ consisting of The green body according to any one of claims 14 to 16.

18. Bi ２ O ３ Use of a metal paste (103) containing silver and nickel for forming a metal layer containing silver on or in a ceramic containing Bi The mass ratio of nickel in the metal paste (103) is at most 25%. Use of the metal paste.

19. The metal paste (103) in the form of metal contains only metallic silver and nickel. Use of the metal paste according to claim 18.

20. The formed metal layer contains only silver as the metal. Use of the metal paste according to claim 18.

21. The ceramic has a mass ratio of at least 90% of ZnO and Bi ２ O ３ , or ZnO, Bi ２ O ３ and Sb ２ O ３ consisting of Use of the metal paste according to any one of claims 18 to 20.

22. A multilayer varistor (1) comprising at least two ceramic layers (2) and an internal electrode (3) containing silver disposed between the at least two ceramic layers (2), Transition layers (24) are respectively formed in the ceramic layers (2) adjacent to the internal electrode (3), and a nickel oxide phase is formed in the transition layers (24). Multilayer varistor.

23. The nickel oxide phase is formed between the internal electrode (3) and the remaining ceramic layer (2). Multilayer varistor according to claim 22.

24. The nickel oxide phase is in the form of a continuous film. Multilayer varistor according to claim 22 or 23.

25. The nickel oxide phase is in the form of a plurality of discontinuous microcrystals. Multilayer varistor according to claim 22 or 23.

26. The transition layer (24) has a maximum thickness of 5 μm. Multilayer varistor according to any one of claims 22 to 25.

27. The internal electrode (3) contains only silver as a metal. Multilayer varistor according to any one of claims 22 to 26.

28. A protective layer (25) is formed in each of the ceramic layers (2, 22) adjacent to the internal electrode (3), and bismuth oxide mainly exists in a reduced form in the protective layer (25). The multilayer varistor according to any one of claims 22 to 27.

29. Within the protective layer (25), as approaching the internal electrode (3), bismuth oxide increases and exists in the form of reduced BiO. The multilayer varistor according to claim 28.

30. At least 90% by mass of the ceramic layer (2) outside the protective layer (25) consists of zinc oxide and bismuth oxide, or zinc oxide, bismuth oxide, and antimony oxide. The multilayer varistor according to claim 28 or 29.

31. The mass ratio of nickel in the elements of the multilayer varistor (1) to the total mass of nickel and silver in the elements is at most 25%. The multilayer varistor according to any one of claims 22 to 30.

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