Method for producing multi-layer varistor, use of metal paste for forming metal layer, green body for producing multi-layer varistor, and multi-layer varistor

Abstract

The invention relates to a method for producing a multi-layer varistor (1), comprising the following steps: preparing a metal paste (103) containing silver and nickel, the mass proportion of nickel in the metal of the metal paste (103) being at most 25%, applying the metal paste (103) to a ceramic green film (102), applying another ceramic green film (102) to the metal paste (103) to produce a sandwich structure, and sintering the green film (102) with the applied metal paste (103), wherein the green film (102) is converted into the ceramic layer (2) and the metal paste (103) is converted into the inner electrode (3).

Description

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

[0002] Zinc oxide-based ceramics with internal electrodes are often used for multilayer varistor elements. The internal electrodes preferably contain silver-palladium alloys, since these alloys have a sufficiently high melting point to set the desired electronic properties in the varistor ceramic by sintering. Examples of such varistors and electrode compositions are disclosed, for example, in patent applications EP 3300087 A1, CN106782956 A or German publication DE 112019003625 T5.

[0003] In order to avoid the use of more expensive noble metals such as palladium, the internal electrodes in current multilayer varistors are sometimes made of the relatively cheaper noble metal silver. In addition to zinc oxide, for example, a high proportion of bismuth (III) oxide is added to the varistor ceramic in order to achieve the required densification of the ceramic and the required grain growth even at low sintering temperatures below the melting point of silver [see German patent application DE 102015120640 A1 and non-patent literature Bernik et al.: Ceramics-Silikáty 62 (1), 8–14 (2018)].

[0004] However, during the sintering process, the silver electrode is corroded by the bismuth (III) oxide melt and the silver is partially oxidized to Ag2(I)BiO3 or Ag(II)BiO3. The higher the sintering temperature, the more severe the corrosion of the silver electrode due to oxidation. Bi2O3 melts at 817°C to 824°C [see non-patent literature Sadecka et al.: Journal of Materials Science, 2017, 52(10), pp. 5503-5510].

[0005] Oxidation of silver results in thinning or loss of electrodes.

[0006] In alternative approaches, attempts are made to completely replace the noble metals of the varistor electrodes. For example, electrodes made of pure nickel are disclosed in the Chinese patent application CN104658727 A. However, sintering can only be carried out in a protective atmosphere with the exclusion of oxygen, in which case no varistor-active ceramic is produced. Therefore, this technology has not yet been used in the commercial field.

[0007] It is therefore an object of the present application to provide an alternative method for producing an improved multilayer varistor.

[0008] The present invention relates to a method for producing a multilayer varistor. The method comprises at least the following steps, which are preferably performed in the order given.

[0009] In one step, a metal paste containing silver and nickel is prepared. The mass proportion of nickel in the metal of the metal paste is greater than 0% and a maximum of 25%, preferably at least 0.15% and a maximum of 20%. The percentage data here and below are always based on mass proportions.

[0010] In particular, a silver phase and a nickel phase can be mixed to prepare a metal paste. The silver phase contains silver (Ag) or consists of silver (Ag), and the nickel phase contains nickel (Ni) or consists of nickel (Ni). Silver and nickel are present, for example, in the solid form of powder or particles or in the liquid form of a melt or in the form of vapor. The method of mixing silver and nickel should not be particularly limited. In addition to silver and nickel, the paste can also contain other metals and organic and inorganic additives or auxiliaries. A uniform distribution of nickel in the silver is achieved by mixing. Nickel is present in the form of fine particles here.

[0011] By mixing the silver phase and the nickel phase, a single-phase or at least two-phase metal paste is formed. The mass proportion of nickel dissolved in the metal phase can be up to 0.15%, thereby forming an Ag-Ni alloy. In addition, the nickel added is present in a separate nickel phase.

[0012] Therefore, the nickel metal proportion in the metal in the metal paste is preferably at least 0.50%, more preferably at least 1.0% or 3.0%. The nickel proportion is preferably selected to be high enough so that in addition to the silver phase, a nickel phase mainly containing nickel is formed in the metal paste.

[0013] A nickel metal proportion exceeding 20% ​​may result in damage to the varistor. Therefore, the maximum proportion of nickel in the metal paste is preferably less than 20%, more preferably less than 19% or less than 17.5%.

[0014] In a further step, the metal paste is applied to the ceramic green sheet. The ceramic green sheet can contain any ceramic material in the unsintered state. The ceramic material contains in particular various metal oxides of the starting materials forming the ceramic as well as organic binders and additives and possibly further dopants.

[0015] For example, the metal paste can be printed onto the green film by screen printing. By adding organic solvents and auxiliaries, a target viscosity suitable for screen printing is set in the metal paste.

[0016] In a further step, another ceramic green film is applied to the metal paste to produce a sandwich structure. In particular, the green film or another green film can also be applied next to the metal paste, for example to compensate for shrinkage during sintering. However, at least one side of the metal paste is exposed to the surrounding environment.

[0017] Furthermore, steps such as decarburization and debinding may be performed before the sintering step.

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

[0019] During the sintering process, nickel preferably diffuses into the transition layer of the formed ceramic layer and is oxidized there. The transition layer is adjacent to the formed internal electrode. Thus, a transition layer is formed adjacent to the internal electrode, in which a nickel oxide phase is formed. For example, Ni is oxidized to nickel (II) oxide (NiO).

[0020] The nickel oxide phase may be formed, for example, in the form of a plurality of crystallites or as a continuous crystalline film.

[0021] In a preferred embodiment, the thickness of the transition layer is not greater than 5 μm, more preferably not greater than 3 μm.

[0022] Optionally, a stack can be formed from a plurality of green films, wherein a metal paste is applied to each green film or selected green films and the entire stack is sintered together in a single process step. Prior to sintering, the green films are preferably pressed together in this case to bond the green films. Individual components of a specific size can then be cut out of the stack before sintering, which is referred to as "cutting". Therefore, in the following, the term "stack" refers both to an uncut stack and to components cut out of a larger stack.

[0023] Furthermore, steps such as decarburization and debinding may be performed prior to sintering.

[0024] Sintering is carried out in an atmosphere with a significant oxygen proportion, for example in ambient air or in an oxygen-rich atmosphere, in order to achieve sufficient grain growth and the desired grain boundary structure in the ceramic.

[0025] Each layer containing a metal paste may additionally contain a ceramic green film or a portion of a green film.

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

[0027] The metal paste is preferably exposed to the surrounding environment on at least one side of the stack or assembly. Thus, the metal paste forms part of the outer surface of the stack on at least one side. In particular, after sintering, a further metal layer can be applied to the outer surface of the stack and in contact with the metal paste inside the stack. The composition of the outer metal layer can be different from the composition of the metal paste.

[0028] The metal paste may have a different composition in each layer. The metal paste preferably has the same composition throughout each layer.

[0029] The method provides a multilayer varistor having silver-containing electrodes, wherein the addition of further noble metals such as palladium is not necessary, so that the varistor can be produced more cost-effectively.

[0030] In addition, the nickel in the metal paste protects the silver from oxidative attack during the sintering of the ceramic, thereby reducing the electrode loss due to the chemical reaction of silver with the ceramic and optimizing the material usage of the silver.

[0031] In one embodiment, sintering is carried out at a temperature above 900° C., preferably above 940° C. or more preferably above 950° C. The maximum possible implementation temperature is below the melting temperature of silver. The melting temperature of silver under normal conditions is 962° C. If silver melts by being heated above its melting temperature, separate silver droplets will be formed and the structure of the electrode will be adversely changed.

[0032] Due to the protective effect of nickel, the ceramic and metal paste can be heated to below the melting point of silver and sintered in an oxygen atmosphere, making the formation of varistor ceramics with the corresponding desired grain size and corresponding grain boundary structure easy to achieve. The grain boundaries form the main resistance in the ceramic material. In particular, larger grains with clearly formed grain boundaries can be produced.

[0033] By increasing the peak temperature during the sintering process, the performance of the varistor can thus be improved. In particular, the electrical properties of the varistor can be improved. For example, by increasing the temperature during the sintering process, a varistor with a lower varistor voltage is obtained. The varistor voltage is defined as the voltage that must be applied to the varistor to generate a current at the level of one milliampere (1 mA).

[0034] In one embodiment, silver accounts for the largest mass proportion of all components in the metal paste. In particular, silver accounts for the largest mass proportion of all metals in the metal paste.

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

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

[0037] The metal proportions of the metal paste preferably consist of 0.15% to 20% nickel and 80% to 99.85% silver.

[0038] In one embodiment, the metal paste consists of silver, nickel and other non-metallic inorganic and organic components. No other precious metals are required in addition to silver. Other components are, for example, organic binders or fillers, for example, to adjust shrinkage or increase adhesion.

[0039] In one embodiment, the ceramic green film contains ZnO and bismuth (III) oxide (Bi 2 O 3 ). This makes it possible to form a ceramic having advantageous electrical properties for a varistor, such as a high threshold resistance or a low varistor voltage.

[0040] In one embodiment, at least 90% by weight of the ceramic green film consists of ZnO and Bi2O3 or of ZnO, Bi2O3 and antimony (III) oxide (Sb2O3). Thus, a ceramic with the desired ceramic properties can be formed. The ratio of bismuth Bi to antimony Sb in the ceramic is preferably greater than 1:1 in order to suitably set the grain growth and grain structure.

[0041] Furthermore, the ceramic green film may contain, for example, organic or inorganic binders, solvents, plasticizers and other additives.

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

[0043] The advantage of the method is therefore that the ceramic is doped with nickel during the sintering process.

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

[0045] The Bi2O3 in the protective layer formed is preferably partially reduced to bismuth(II) oxide (BiO). The nickel oxide can then form, for example, nickel-zinc spinel. As one approaches the inner electrode formed, the higher proportion of Bi2O3 is therefore preferably reduced.

[0046] At this time, the nickel oxide phase in the transition layer forms a barrier layer adjacent to the internal electrode, and no more Bi2O3 can diffuse into the internal electrode through the barrier layer, so the oxidation of silver can be avoided.

[0047] The nickel in the inner electrode thus has multiple protective effects. On the one hand, nickel is preferably oxidized before silver because it is less noble, so that a protective layer is formed around the inner electrode by diffusion of Ni into the ceramic during the sintering process, in which Bi2O3 is reduced. In addition, due to the oxidation of nickel, a barrier layer made of nickel oxide is formed around the inner electrode in the transition layer adjacent to the inner electrode.

[0048] In another embodiment, the method steps may be followed by further optional method steps.

[0049] An example of such a method step is pressing the stacked layers in order to stably bond the layers to one another. Pressing is performed before sintering.

[0050] After pressing, the stack can be cut into components of specific dimensions in a further step (also called trimming).

[0051] In a subsequent optional step, organic substances such as binders and additives in the green film or metal paste may be burned out by heating.

[0052] After these steps, the sintering is carried out.

[0053] After sintering, external contacts can be applied to the multilayer varistor, which contacts electrically connect the formed internal electrodes. For example, the external contacts can be realized by dipping the exit faces of the internal electrodes into a metal paste and then baking the paste.

[0054] The invention also relates to a multilayer varistor which is designed such that it can be manufactured using the above method. However, the varistor should not be limited to being manufactured by the aforementioned method.

[0055] The invention also relates to a green body for producing a multilayer varistor. The green body can have all the features previously described in connection with the method, and vice versa.

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

[0057] The metal layer may comprise a metal paste containing silver and nickel, wherein the mass proportion of nickel in the metal of the metal paste is maximum 25%, preferably at least 0.15% and maximum 20%.

[0058] In addition to the metal paste, a ceramic green film can also be arranged in the metal layer, which surrounds the metal paste from multiple sides. The metal paste is exposed to the surrounding environment in at least one direction.

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

[0060] In a preferred embodiment, the ceramic green film consists in a mass proportion of at least 90 mass % of ZnO and Bi 2 O 3 or of ZnO, Bi 2 O 3 and Sb 2 O 3 .

[0061] The invention also relates to the use of a metal paste comprising silver and nickel for forming a silver-containing metal layer in or on a B2O3-containing ceramic, wherein the mass proportion of nickel in the metal of the metal paste is a maximum of 25%, preferably at least 0.15% and a maximum of 20%.

[0062] Metal pastes and ceramics can have all of the above characteristics, and vice versa.

[0063] In particular, for example, lead-containing ceramics can be replaced by bismuth-oxide-containing ceramics and contacted with silver electrodes, since, due to the nickel proportion in the metal paste, the bismuth oxide does not or hardly attacks the silver in the metal layer formed during sintering.

[0064] In embodiments, the metal paste may be composed of metallic silver, nickel, and other non-metallic organic and / or inorganic components.

[0065] The metallic layer formed preferably contains only silver as metal, since nickel diffuses into the ceramic during the production process.

[0066] At least 90% by weight of the ceramic may consist of ZnO and Bi 2 O 3 or of ZnO, Bi 2 O 3 and Sb 2 O 3 .

[0067] The present invention also relates to a multilayer varistor. The varistor can be designed similarly to the previously mentioned embodiments. The ceramic layers, internal electrodes and other components of the varistor can have the same features as described above.

[0068] The varistor can preferably be manufactured by the above-mentioned method.

[0069] The varistor includes at least two ceramic layers and an internal electrode which is arranged between the two ceramic layers in a sandwich structure and contains silver.

[0070] Adjacent to the internal electrodes, transition layers are formed in each of the ceramic layers, in which a nickel oxide phase is formed. The nickel oxide phase is preferably formed between the internal electrode and the rest of the ceramic layer.

[0071] The nickel oxide phase is preferably formed as a continuous film. Particularly preferably, the nickel oxide phase is formed as a continuous film which separates the internal electrode from the remaining ceramic and thus forms a coherent barrier layer between the internal electrode and the remaining ceramic.

[0072] In an alternative embodiment, the nickel oxide phase is formed as a plurality of discontinuous crystallites.

[0073] In addition, the nickel oxide phase may also be formed outside the transition layer or inside the inner electrode.

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

[0075] In one embodiment, a protective layer is respectively formed in the ceramic layer adjacent to the internal electrode, in which protective layer the bismuth oxide is present predominantly in reduced form BiO.

[0076] Preferably, the bismuth oxide is present increasingly in the reduced form BiO as one gets closer to the inner electrode.

[0077] The protective layer preferably also has an increased nickel doping compared to the remaining ceramic.The protective layer adjoins the internal electrode and comprises the transition layer defined above.

[0078] The thickness of the transition layer is, for example, 40 μm. In particular, the thickness of the transition layer is preferably not greater than ten times the thickness of the internal electrode.

[0079] In one embodiment, at least 90% of the weight proportion of the ceramic layer outside the protective layer consists of zinc oxide and bismuth oxide or zinc oxide, bismuth oxide and antimony oxide. Zinc oxide includes ZnO in particular, bismuth oxide includes BiO and Bi2O3 in particular, and antimony oxide includes Sb2O3 in particular.

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

[0081] In one embodiment, the mass proportion of the element nickel in the multilayer varistor is a maximum of 25%, relative to the sum of the total mass of the element nickel and silver.

[0082] The embodiments of the present invention are explained in more detail below with reference to the accompanying drawings. The present invention is not limited to the following embodiments.

[0083] The attached figure shows:

[0084] Figure 1 : Cross section of an embodiment of a multilayer varistor.

[0085] Figure 2 : Cross section of the multilayer varistor before sintering.

[0086] Figure 3 : SEM microscope image of the silver electrode of the varistor not according to the present invention.

[0087] Figure 4 : Optical microscope image of the inner electrode of the varistor according to the present invention.

[0088] Figure 5 : A magnified SEM microscope image of the inner electrode of the varistor according to the present invention.

[0089] Figure 6 : Figure 5 EDX concentration distribution of silver element in a local area.

[0090] Figure 7 : Figure 5 EDX concentration distribution of oxygen element in the local area.

[0091] Figure 8 : Figure 5EDX concentration distribution of zinc element in the local area.

[0092] Fig. 9 : Figure 5 EDX concentration distribution of nickel element in a local area.

[0093] Fig.10 : Unique varistor characteristic lines of silver electrode and Ag-Ni electrode.

[0094] Fig.11 : Unique varistor characteristic lines of silver electrodes and various Ag-Ni electrodes.

[0095] Figure 1 A first embodiment of a varistor 1 according to the invention is shown. It is a multilayer varistor in which ceramic layers 2 and intermediate layers with internal electrodes 3 are stacked alternately. The internal electrodes 3 are surrounded by the ceramic and are exposed on each side of the outer surface of the varistor 1.

[0096] In the present embodiment, each second internal electrode 3 a is exposed on a first side in the stacking direction, and the internal electrode 3 b located therebetween is exposed in each case on a second side opposite to the first side in the stacking direction.

[0097] The internal electrode preferably contains only silver as metal.

[0098] The ceramic is a zinc-bismuth ceramic containing ZnO and B2O3. In addition, the ceramic may contain other metal oxides, in particular zinc oxide, bismuth oxide and antimony oxide, and dopants.

[0099] In order to manufacture the varistor 1, a ceramic green film containing the metal oxide of a predetermined composition is provided. For example, a metal paste is printed on the green film by screen printing. The metal paste contains silver and nickel. In this embodiment, the nickel ratio in the metal of the metal paste is 0.15% to 20%. By adding an organic solvent and an auxiliary agent, a target viscosity of the metal paste suitable for screen printing is set.

[0100] The printed and unprinted green films are then stacked on top of one another in a specific sequence and with high positional accuracy in a subsequent step in order to achieve the described layer structure of the varistor.

[0101] The stack is then mechanically pressed to bond the layers stably to one another. Components of specific dimensions are then produced from the stack by cutting at predetermined locations. Figure 2 An exemplary varistor component 101 in a green state is shown in FIG. The varistor component 101 comprises a stack of a green ceramic layer 102 and a layer having a metal paste 103 .

[0102] In a subsequent step, the organic binders and additives present in the ceramic green film and the metal paste are burned out to ensure the necessary strength in the above method steps. To this end, the assembly is heated in a furnace for a sufficiently long time.

[0103] After the organic components are burned out, the assembly is sintered. Due to the protective effect of the nickel component in the metal paste, the peak sintering temperature can be heated to slightly below the melting temperature of silver. The sintering temperature is maintained for a sufficiently long time, for example, more than 180 minutes.

[0104] The sintered varistor component is then electrically contacted. In order to be able to make electrical contact with the component, external electrodes are applied. This is achieved, for example, by dipping the exit surface of the internal electrode into a silver paste and subsequently baking the silver paste. The baking is carried out, for example, at a temperature of about 650 to 700° C.

[0105] Figure 3 A varistor 11 not according to the invention is shown, which has ordinary silver electrodes 13, without nickel addition. Silver is preferably used for the internal electrodes in varistors due to the high price of other noble metals such as palladium.

[0106] On the other hand, in order to form suitable electrical properties of the varistor, a sufficiently high sintering temperature is required to properly set the grain growth and grain boundary structure. However, compared with other noble metals, the sintering temperature of silver is relatively low.

[0107] Subsequent reference Fig.10 and 11 Discuss the desired electrical properties.

[0108] Zinc oxide ceramics with good varistor properties are often used in multilayer varistors.

[0109] When using pure silver electrodes, such as Figure 3 In the non-inventive embodiments, zinc oxide ceramics of conventional composition are not suitable as varistor ceramics. Below the melting point of Ag, a suitable structure cannot be produced by such zinc oxide ceramics. Therefore, for example, an increased proportion of bismuth oxide is added to the ceramic composition to enable the formation of a grain boundary structure and a desired degree of grain growth even at temperatures below the melting temperature of Ag.

[0110] However, a disadvantage when adding a large amount of bismuth (III) oxide Bi2O3 is its reactivity, which causes part of the silver material to be oxidized during the sintering process. In particular, in the outer region 14 of the internal electrode 13 close to the outer surface of the varistor 11 and therefore in contact with oxygen from the sintering atmosphere, Ag is locally completely oxidized, thereby causing the electrical contact of the electrode to be impaired.

[0111] Figure 3The SEM microscope image shows a section through the multilayer varistor 11. The inner electrode 13 made of pure silver is severely diluted in the outer region 14 due to oxidative attack.

[0112] Silver diffused into the ceramic further degrades the insulating properties of the ceramic.

[0113] Figure 4 3 shows how the nickel additive exerts a protective effect on the internal electrode 23 which preferably consists of Ag.

[0114] Nickel is a less noble metal than silver. Therefore, nickel is preferentially oxidized by Bi2O3. Silver remains in its reduced metallic form.

[0115] 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 of the ceramic layer 22 adjacent to the internal electrode. Figure 4 In the SEM microscope image through the corresponding varistor 21 according to the invention, the protective layer 25 formed in this way can be seen. For example, NiO can further react with ZnO to form Ni-Zn spinel, which further makes it difficult for Bi2O3 to enter the inner electrode.

[0116] exist Figure 4 In the figure, the bright spots in the ceramic show the Bi2O3 phase. Adjacent to the internal electrode 23, these bright spots can no longer be seen in the protective layer 25, so that after a sufficient amount of nickel has been oxidized, the Bi2O3 no longer reaches the electrode and thus cannot oxidize more nickel or silver. Instead, black spots can be seen in the protective layer, which indicates the formation of the BiO phase. The thickness of the protective layer 25 is, for example, about ten times the thickness of the internal electrode 23.

[0117] Figure 5 , a magnified view of the SEM microscope image can be seen in FIG. 1 , in which the nickel oxide phase can be seen as fine-grained crystallites 24 of the internal electrode 23 located in the internal electrode 23 and in the adjacent transition layer near the internal electrode 23. The crystallites 24 are marked with circles and form a diffusion barrier between the ceramic and the internal electrode.

[0118] Figure 6 It is shown in Figure 5 The section shown shows the concentration distribution of the silver element in the varistor measured by EDX (ie, energy dispersive X-ray spectroscopy). Figure 7 The distribution of the oxygen element is shown in a similar manner. Figure 8 The distribution of zinc is shown in a similar manner, with high concentrations shown as bright and low concentrations as dark.

[0119] It can be seen that there is no oxygen in the silver layer and no silver in the oxygen-rich phase, so the silver cannot be oxidized. Therefore, the silver layer of the internal electrode is protected from oxidation corrosion.

[0120] Furthermore, it can be seen that the ceramic ZnO phase, i.e. the region where zinc and oxygen are present, does not extend to the Figure 6 Silver layer shown.

[0121] Fig. 9 The EDX concentration distribution of the nickel element is shown. Fig. 9 It is shown that the nickel-rich phase forms between the zinc-rich region and the silver-rich region. Figure 7 shows that the nickel-rich phase is also rich in oxygen. These are Figure 5 Nickel oxide crystallites 24 are shown in FIG.

[0122] Since the electrodes are thus protected from oxidative attack by silver or nickel, the electrodes can be made narrower in the stacking direction, i.e. at least 6 μm, more preferably 5 μm, even more preferably as thin as 4 μm. Thus, metal material, in particular silver material, used to form the electrodes can be saved. This ensures electrical contact with the outside.

[0123] Fig.10 and 11 The electrical characteristic lines, ie the current-voltage characteristics, of various varistor components are shown.

[0124] In a double logarithmic graph, the x-axis shows the current in A and the y-axis shows the voltage in V.

[0125] Fig.10 The characteristic curve of a nickel-free varistor with silver electrodes is compared with the characteristic curve of an electrode of a varistor with a nickel content of 15% (based on the sum of the masses of silver and nickel in the green metal paste) (or 17.2% by volume) in the metal paste. The characteristic curve of the nickel-free varistor can be seen above the graph and the characteristic curve of the silver-nickel varistor below. Both electrodes were sintered at 900° C.

[0126] In particular, the breakdown voltage (ie the voltage above which the varistor conducts) and the nonlinearity of the curve (ie a measure of the quality of the varistor) are to be taken into account.

[0127] Higher nonlinearity indicates better switching between the conducting and non-conducting states. The curves shown below show that similar properties can be achieved by incorporating nickel, but without the effects of the previous reference Figure 2 and 3 Issues concerning material consumption and external contacts were discussed.

[0128] In addition to from Fig.10In addition to the characteristic line of the nickel-free varistor with silver electrodes (i.e. the uppermost curve in the diagram) and the characteristic line of the varistor containing 15% nickel (i.e. the lowermost curve in the diagram) described above, Fig.11 Five additional characteristic lines are shown in .

[0129] The five further characteristic lines were each measured using electrodes of a varistor having a nickel content of 2.6% in the metal paste (based on the total mass of Ni and Ag in the green metal paste) (3% by volume).

[0130] Various varistors each having a nickel content of 2.6% were sintered at different temperatures. The top curve mentioned belongs to an electrode sintered at 900°C, the next one to an electrode sintered at 920°C, the next one to an electrode sintered at 940°C, the next one to an electrode sintered at 950°C, and the bottom one to an electrode sintered at 960°C. As mentioned above, Fig.10 The other two characteristic lines in belong to electrodes sintered at 900° C. in each case.

[0131] from Fig.11 As can be seen from the graph, reducing the nickel ratio leads to a nonlinearity of the curve that is close to the curve of a pure silver electrode. In particular, the nonlinearity of the curve can be further improved.

[0132] By increasing the sintering temperature, the varistor voltage, i.e., the voltage required to achieve a current of 1 mA, can be significantly reduced. In this example, the varistor voltage of the silver electrode in the varistor without nickel is 82 volts, while the varistor voltage of the electrode in the silver-nickel varistor containing 2.6% nickel (sintered at 960°C) is 55 volts. The varistor voltage is reduced by 33%.

[0133] This significantly expands the possible ways of setting the characteristic line via the sintering temperature. When using pure silver electrodes, similarly high sintering temperatures are not feasible since this would lead to damage of the silver electrodes.

[0134] Reference numerals list

[0135] 1, 11, 21 Sintered varistor

[0136] 2.22 Ceramic Layer

[0137] 3, 13, 23 inner electrodes

[0138] 3a Left inner electrode

[0139] 3b Right inner electrode

[0140] 14 External Area

[0141] 24 NiO microcrystals

[0142] 25 protective layer

[0143] 101 Varistor in green state

[0144] 102 Raw ceramic layer

[0145] 103 Metal paste.

Claims

1. A method for producing a multilayer varistor (1), comprising the following steps: A metal paste (103) containing silver and nickel is prepared, wherein the mass proportion of nickel in the metal of the metal paste (103) is a maximum of 25%, Applying a metal paste (103) onto the ceramic green film (102), applying another ceramic green film (102) to the metal paste (103) to produce a sandwich structure, and The green film (102) is sintered 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).

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

3. The method according to claim 1 or 2, wherein the sintering is performed at a temperature above 900°C.

4. The method according to claim 3, wherein the sintering is performed at a temperature higher than 940°C or higher than 950°C.

5. The method according to any one of claims 1 to 4, wherein the metal paste (103) contains only metallic silver and nickel in metallic form. 6 . The method according to claim 1 , wherein the transition layer ( 24 ) has a thickness of at most 5 μm.

7. The method according to any one of claims 1 to 6, wherein at least 90% by weight of the ceramic green film (102) consists of ZnO and Bi2O3 or of ZnO, Bi2O3 and Sb2O3.

8. The method according to claim 1, wherein during the sintering process nickel diffuses into a protective layer (25) of the formed ceramic layer (2) adjacent to the formed internal electrode (3).

9. The method according to claim 8, wherein Bi2O3 is partially reduced to BiO in the protective layer (25).

10. The method according to claim 9, wherein the higher proportion of Bi2O3 in the protective layer (25) decreases as one approaches the formed inner electrode (3).

11. The method according to any one of claims 1 to 10, wherein the sintering is carried out in an atmosphere having a significant oxygen proportion.

12. The method of claim 11, wherein the sintering is performed under ambient air.

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

14. A green body (101) for producing a multilayer varistor (1), comprising two ceramic green films (102) and a metal layer arranged in a sandwich structure between the two ceramic green films (102) and having a metal paste (103) containing silver and nickel, wherein the mass proportion of nickel in the metal of the metal paste (103) is a maximum of 25%.

15. The green body (101) according to claim 14, wherein the metal paste (103) contains only metallic silver and nickel in metallic form.

16. The green body (101) according to claim 14 or 15, wherein the mass proportion of nickel in the metal of the metal paste (103) is at least 0.15% and at most 20%.

17. The green body (101) according to any one of claims 14 to 16, wherein at least 90% by weight of the ceramic green film (102) consists of ZnO and Bi2O3 or of ZnO, Bi2O3 and Sb2O3.

18. Use of a metal paste (103) containing silver and nickel for forming a silver-containing metal layer in or on a Bi2O3-containing ceramic, wherein the mass proportion of nickel in the metal paste (103) is at most 25%.

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

20. Use of a metal paste (103) according to claim 18 or 19, wherein the formed metal layer contains only silver as metal.

21. Use of a metal paste (103) according to claims 18 to 20, wherein a mass proportion of at least 90 mass % of the ceramic consists of ZnO and Bi2O3 or of ZnO, Bi2O3 and Sb2O3.

22. A multilayer varistor (1) comprising at least two ceramic layers (2) and an internal electrode (3), wherein the internal electrode (3) is arranged in a sandwich structure between the at least two ceramic layers (2) and contains silver, wherein a transition layer (24) is formed in each of the ceramic layers (2) adjacent to the internal electrode (3), and a nickel oxide phase is formed in the transition layer (24).

23. The multilayer varistor (1) according to claim 22, wherein the nickel oxide phase is formed between the internal electrode (3) and the rest of the ceramic layer (2).

24. The multilayer varistor (1) according to claim 22 or 23, wherein the nickel oxide phase is formed in the form of a continuous film.

25. The multilayer varistor (1) according to claim 22 or 23, wherein the nickel oxide phase is formed in the form of a plurality of discontinuous crystallites. 26 . The multilayer varistor ( 1 ) according to claim 22 , wherein the transition layer ( 24 ) has a thickness of at most 5 μm. 27 . The multilayer varistor ( 1 ) according to claim 22 , wherein the internal electrode ( 3 ) contains only silver as metal.

28. The multilayer varistor (1, 21) according to any one of claims 22 to 27, wherein a protective layer (25) is formed in each of the ceramic layers (2, 22) adjacent to the internal electrode (3), in which protective layer (25) bismuth oxide is present predominantly in reduced form BiO.

29. The multilayer varistor (1, 21) according to claim 28, wherein in the protective layer (25) bismuth oxide is present increasingly in reduced form BiO as one approaches closer to the internal electrode (3).

30. The multilayer varistor (1) according to claim 28, wherein at least 90% by weight 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.

31. The multilayer varistor (1, 21) according to claim 22, wherein the mass proportion of the element nickel in the multilayer varistor (1) is a maximum of 25% relative to the sum of the total mass of the element nickel and silver.

Images