Method for producing a multilayer varistor, use of a metal paste for forming metal layers, green body for producing a multilayer varistor, and multilayer varistor

Abstract

The present invention relates to a method for producing a multilayer varistor (1), having the steps of: preparing a metal paste (103) comprising silver and nickel, wherein the mass proportion of nickel in the metals of the metal paste (103) is at most 25%; applying the metal paste (103) to a ceramic green foil (102); applying a further ceramic green foil (102) to the metal paste (103) to produce a sandwich-like structure; and sintering the green foils (102) with the applied metal paste (103), wherein the green foils (102) are converted to ceramic layers (2) and the metal paste (103) is converted to an inner electrode (3).

Description

[0001] Description

[0002] Method for producing a multilayer varistor, use of a metal paste for forming metal layers, green body for producing a multilayer varistor and multilayer varistor

[0003] The present invention relates to a method for producing a multilayer varistor, a green body for producing a multilayer varistor, the use of a suitable metal paste and a multilayer varistor.

[0004] Multilayer varistor elements typically use zinc oxide-based ceramics with internal electrodes. The internal electrodes preferably comprise silver-palladium alloys, as these have a sufficiently high melting point to allow the desired electronic properties to be achieved in the varistor ceramic through sintering. Examples of such varistors and electrode compositions are disclosed, for example, in patent applications EP 3 300 087 A1, CN 106782956 A, or in the German publication DE 11 2019 003 625 T5.

[0005] To avoid the use of more expensive precious metals such as palladium, internal electrodes in current multilayer varistors are sometimes manufactured with the comparatively less expensive precious metal silver. In addition to zinc oxide, a high proportion of bismuth (III) oxide, for example, is then added to the varistor ceramic to enable the required densification and grain growth of the ceramic even at low sintering temperatures below the melting point of silver [see German patent application DE 10 2015 120 640 Al and the non-patent literature Bernik et al.: Ceramics-Silikäty 62 (1), 8-14 (2018)].

[0006] During sintering, however, the silver electrode is attacked by the bismuth (III) oxide melt, and the silver is partially oxidized to Ag2(I)BiO2 or Ag(II)BiO2. The higher the sintering temperature, the greater the oxidation attack on the silver electrode. Bi2O3 melts between 817 °C and 824 °C [see the non-patent literature Sadecka et al.: Journal of Materials Science, 2017, 52 (10), pp. 5503-5510].

[0007] The oxidation of the silver leads to the thinning or disappearance of the electrode.

[0008] Alternative approaches attempt to completely replace the precious metals used in varistor electrodes. For example, Chinese patent application CN 104658727 A discloses electrodes made of pure nickel. However, sintering is then only possible in a protective atmosphere with the exclusion of oxygen, which does not produce varistor-active ceramic. For this reason, this technology is not used commercially.

[0009] An object of the present application is therefore to provide an alternative process for the production of improved multilayer vari-interferences.

[0010] The present invention relates to a method for producing a multilayer varistor. The method comprises at least the steps described below, which are preferably carried out in the specified order. In one step, a metal paste comprising silver and nickel is prepared. The mass fraction of nickel in the metals of the metal paste is more than 0% and at most 25%, and preferably at least 0.15% and at most 20%. The percentages here and below always refer to mass fractions.

[0011] In particular, a silver phase and a nickel phase can be mixed to prepare the metal paste. The silver phase contains or consists of silver (Ag) and the nickel phase contains or consists of nickel (Ni). Silver and nickel are present, for example, in solid form as a powder or granulate or in liquid form as a melt or vapor. The process for mixing silver and nickel should not be particularly limited. In addition to silver and nickel, the paste can comprise other metals as well as organic and inorganic additives or auxiliaries. Mixing ensures that the nickel is evenly distributed in the silver. The nickel is in the form of fine particles.

[0012] Mixing the silver and nickel phases creates a single-phase or at least two-phase metal paste. Nickel can be dissolved in the metal phase in a mass fraction of up to 0.15%, forming an Ag-Ni alloy. Any nickel added above this concentration exists in a separate nickel phase.

[0013] The proportion of nickel metal in the metals in the metal paste is therefore preferably at least 0.50%, more preferably at least 1.0% or 3.0%. The proportion of nickel is preferably selected to be high enough that, in addition to the silver phase, a nickel phase which predominantly contains nickel is formed in the metal paste. A proportion of nickel metal above 20% can lead to damage to the varistor. The maximum proportion of nickel in a metal paste is therefore 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 a ceramic green sheet. The ceramic green sheet can contain any ceramic material in the unsintered state. The ceramic material includes, in particular, various metal oxides, which form the starting materials of the ceramic, as well as organic binders and auxiliary materials, and possibly additional dopants.

[0015] The metal paste can be screen-printed onto the green foils, for example. By adding organic solvents and additives, a target viscosity suitable for screen printing is then achieved in the metal paste.

[0016] In a subsequent step, another ceramic green foil is applied to the metal paste to create a sandwich-like structure. In particular, the green foil or another green foil 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 environment.

[0017] Furthermore, steps such as decarburization and debinding can be performed prior to the sintering step. In a subsequent step, the green foils are sintered with the applied metal paste in a single process step, converting the green foils into ceramic layers and the metal paste into an internal electrode.

[0018] During sintering, nickel is preferentially dissolved in transition layers of the forming ceramic layers and oxidized there. The transition layers border the forming internal electrode. Thus, transition layers are formed adjacent to the internal electrode, in which a nickel oxide phase is formed. The Ni is oxidized, for example, to nickel(II) oxide (NiO).

[0019] The nickel oxide phase can, for example, be formed in the form of several crystallites or as a coherent crystalline film.

[0020] In a preferred embodiment, the thickness of the transition layer is not more than 5 pm, more preferably not more than 3 pm.

[0021] Optionally, a stack can be formed from several green sheets, with metal paste applied to each or selected green sheets, and the entire stack can be sintered together in a single process step. Before sintering, the green sheets are then preferably pressed together to bond the green sheets. Subsequently, individual components of defined dimensions can be cut from the stack before sintering, a process referred to as "cutting." In the following, the term "stack" can therefore refer to both an uncut stack and a component cut out of a larger stack.

[0022] Furthermore, steps such as decarburization and debinding can take place before sintering.

[0023] Sintering takes place in an atmosphere with a significant oxygen content, for example in ambient air or in an oxygen-enriched atmosphere, in order to achieve sufficient grain growth and the desired grain boundary structure in the ceramic.

[0024] Each of the layers containing a metal paste may additionally comprise a ceramic green sheet or a section of a green sheet.

[0025] Outer layers of the stack in a stacking direction are preferably formed by a ceramic green foil each.

[0026] The metal paste is preferably exposed to the environment on at least one side of the stack or component. The metal paste thus forms a portion of an outer surface of the stack on at least one side. Additional metal layers can be applied to the outer surface of the stack, particularly after sintering, and can be in contact with the metal paste inside the stack. The outer metal layers can differ in composition from the composition of the metal paste.

[0027] The metal paste can have a different composition in each layer. Preferably, the metal paste always has the same composition in each layer. The described method provides a multilayer varistor having electrodes containing silver, eliminating the need for other precious metals such as palladium, thus allowing the varistor to be manufactured more cost-effectively.

[0028] Furthermore, the nickel in the metal paste protects the silver from oxidative attacks during sintering of the ceramic, so that shrinkage of the electrode due to chemical reaction of the silver with the ceramic can be reduced and the use of silver material can be optimized.

[0029] 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 operating temperature is below the melting point of silver. The melting point of silver under normal conditions is 962 °C. If the silver melts because it is heated above its melting point, individual silver droplets form, and the structure of the electrode is adversely altered.

[0030] Due to the protective effect of the nickel, the ceramic and the metal paste can be heated to below the melting point of silver and sintered in an oxygen atmosphere, making it easy to form a varistor ceramic with the required grain sizes and a corresponding grain boundary structure. The grain boundaries form the essential electrical resistances in the ceramic material. In particular, larger grains with clearly defined grain boundaries can be produced. By increasing the peak temperature during sintering, the varistor properties can be improved. In particular, the electrical properties of the varistor can be improved. For example, by increasing the temperature during sintering, a varistor with a lower varistor voltage can be obtained.The varistor voltage is defined as the voltage that must be applied to a varistor to produce an electrical current of one milliampere (1 mA).

[0031] In one embodiment, silver makes up the largest mass fraction of all components in the metal paste. In particular, silver makes up the largest mass fraction of all metals in the metal paste.

[0032] In one embodiment, the metal paste in the form of metals comprises exclusively the metals silver and nickel.

[0033] The mass fraction of nickel in the metals of the metal paste is preferably at least 0.15% and at most 20%.

[0034] The metal content of the metal paste preferably consists of 0.15% to 20% nickel and 80% to 99.85% silver.

[0035] In one embodiment, the metal paste consists of silver, nickel, and other non-metallic inorganic and organic components. The use of other precious metals besides silver can be omitted. The other components are, for example, organic binders or fillers, for example, to accommodate shrinkage or to increase adhesion. In one embodiment, the ceramic green films comprise ZnO and bismuth (111) oxide (Bi2Os). These can be used to form ceramics that have advantageous electrical properties for use in a varistor, such as a high threshold resistance or a low varistor voltage.

[0036] In one embodiment, the ceramic green sheets consist of at least 90% by mass of ZnO and Bi2Os or of ZnO, Bi2Os, and antimony (III) oxide (Sb2Os). This allows the formation of ceramics with desirable ceramic properties. The ratio of bismuth (Bi) to antimony (Sb) in the ceramic is preferably greater than 1:1 to suit grain growth and grain structure.

[0037] In addition, the ceramic green film can also contain organic or inorganic binders, solvents, plasticizers and other additives.

[0038] In one embodiment, during sintering, nickel diffuses to the boundary between the inner electrode and the ceramic layers, or into the layers of the forming ceramic layers that border the forming inner electrode. These layers bordering the inner electrode are defined as the protective layer. After sintering, the ceramic in the protective layer is doped with nickel. Furthermore, a separate nickel phase can form in the protective layer.

[0039] One advantage of the present process is the doping of the ceramic with nickel during sintering. In the protective layer, at least a portion of the Ni is preferably oxidized, for example, to nickel (II) oxide (NiO), and preferably forms the nickel oxide phase in the transition layer.

[0040] The Bi2O3 in the forming protective layer is preferentially partially reduced to bismuth(II) oxide (BiO). The nickel oxide can then form, for example, nickel-zinc spinels. With increasing proximity to the forming internal electrode, a higher proportion of Bi2O3 is therefore preferentially reduced.

[0041] The nickel oxide phase in the transition layer then forms a barrier adjacent to the inner electrode through which no further Bi2Os can diffuse to the inner electrode and thus oxidation of the silver can be avoided.

[0042] The nickel in the inner electrode thus provides multiple protective effects. On the one hand, nickel is preferentially oxidized before silver because it is less noble, and thus, the diffusion of Ni into the ceramic during sintering creates a protective layer around the inner electrode in which Bi2Os is reduced. Furthermore, the oxidation of the nickel creates a barrier of nickel oxide around the inner electrode in a transition layer adjacent to the inner electrode.

[0043] The described process steps can be followed by further optional process steps in further versions.

[0044] An example of such a process step is the

[0045] Pressing the stacked layers together to securely bond them together. Pressing is performed before sintering.

[0046] Following the pressing process, the stack can be cut into several components with defined dimensions in a further step (also called cutting).

[0047] In a subsequent optional step, the organic components, such as binding agents and additives, in the green foils or metal paste can be burned out by heating.

[0048] After these steps, the described sintering takes place.

[0049] Following sintering, external contacts can be applied to the multilayer varistor, which electrically connect the formed internal electrodes. The external contacts can be achieved, for example, by immersing the exit surfaces of the internal electrodes in a metal paste and then firing the paste.

[0050] The invention further relates to a multilayer varistor designed to be manufactured using the method described above. However, the varistor should not be limited to manufacture using the aforementioned method.

[0051] The invention further relates to a green body for producing a multilayer varistor. The green body can have all the features previously described with regard to the method, and vice versa.

[0052] In particular, the green body comprises at least two ceramic green films and a metal layer arranged in a sandwich structure between the at least two ceramic green films.

[0053] The metal layer may comprise a metal paste comprising silver and nickel, wherein the mass fraction of nickel in the metals of the metal paste is a maximum of 25%, and preferably at least 0.15% and a maximum of 20%.

[0054] In addition to the metal paste, the metal layer may also contain a ceramic green film that surrounds the metal paste on several sides. The metal paste is exposed to the environment in at least one direction.

[0055] In a preferred embodiment, the metal paste consists exclusively of the metals silver and nickel and of non-metallic organic and / or inorganic components. The metal paste thus comprises exclusively the metals silver and nickel in the form of metals.

[0056] In a preferred embodiment, the ceramic green sheets consist of at least 90% by mass of ZnO and Bi2O3 or of ZnO, Bi2O3 and Sb2O3.

[0057] The present invention further relates to the use of a metal paste comprising silver and nickel for forming metal layers comprising silver in or on a Bi2O3-containing ceramic, wherein the mass fraction of nickel in the metals of the metal paste is a maximum of 25% and preferably at least 0.15% and a maximum of 20%.

[0058] The metal paste and the ceramic can exhibit all of the previously described features, and vice versa. In particular, lead-containing ceramics can be replaced by bismuth oxide-containing ceramics and contacted with silver electrodes, since the bismuth oxide, due to the nickel content in the metal paste, hardly or not at all attacks the silver in the forming metal layer during sintering.

[0059] In embodiments, the metal paste can consist of the metals silver, nickel and other non-metallic organic and / or inorganic components.

[0060] The metal layer formed preferably comprises only silver as the metal, since the nickel is dissolved into the ceramic during the manufacturing process.

[0061] The ceramic can consist of at least 90% by mass of ZnO and Bi2O3 or of ZnO, Bi2O3 and Sb2O5.

[0062] The present invention further relates to a multilayer varistor. The varistor can be designed analogously to the previously mentioned embodiments. The ceramic layers, internal electrodes, and other elements of the varistor can have the same features as previously described.

[0063] The varistor can preferably be manufactured by the method described above.

[0064] The varistor comprises at least two ceramic layers, as well as an inner electrode arranged in a sandwich structure between the two ceramic layers and comprising silver. Adjacent to the inner electrode, a transition layer is formed in each of the ceramic layers, in which a nickel oxide phase is formed. The nickel oxide phase is preferably formed between the inner electrode and the remaining ceramic layers.

[0065] The nickel oxide phase is preferably formed as a continuous film. The nickel oxide phase is particularly preferably formed as a continuous film that separates the inner electrode from the remaining ceramic, thus forming a continuous barrier between the inner electrode and the remaining ceramic.

[0066] In an alternative embodiment, the nickel oxide phase is formed in the form of several non-connected crystallites.

[0067] Furthermore, a nickel oxide phase can also be formed outside the transition layer or inside the inner electrode.

[0068] The transition layer preferably has a maximum thickness of 5 pm.

[0069] In one embodiment, a protective layer is formed in each of the ceramic layers adjacent to the inner electrode, in which bismuth oxides are predominantly present in the reduced form BiO.

[0070] Bismuth oxides are preferentially located with increasing proximity to the

[0071] The inner electrode is present increasingly in the reduced form BiO. Preferably, the protective layer furthermore has a higher doping with nickel compared to the remaining ceramic. The protective layer borders the inner electrode and comprises the transition layer defined above.

[0072] The thickness of the transition layer is, for example, 40 pm. In particular, the thickness of the transition layer is preferably not more than ten times the thickness of the inner electrode.

[0073] In one embodiment, the ceramic layers outside the protective layers consist of at least 90% by mass of zinc oxide and bismuth oxides or of zinc oxide, bismuth oxides, and antimony oxides. Zinc oxides include, in particular, ZnO, bismuth oxides include, in particular, BiO and Bi2O2, and antimony oxides include, in particular, Sb2O3.

[0074] Furthermore, NiO is also contained in the ceramic layers, which is formed at least in a transition layer adjacent to the inner electrode during sintering by the oxidation of the nickel from the metal paste.

[0075] In one embodiment, the mass fraction of elemental nickel in relation to the sum of the total masses of elemental nickel and silver in the multilayer varistor is a maximum of 25%.

[0076] In the following, exemplary embodiments of the invention are explained in more detail with reference to figures. The invention is not limited to the following exemplary embodiments.

[0077] The figures show: Figure 1: Example of a multilayer varistor in cross section.

[0078] Figure 2 : Multilayer varistor before sintering in cross section .

[0079] Figure 3: SEM micrograph of the silver electrode of a varistor not according to the invention.

[0080] Figure 4: Light microscope image of the inner electrode of a varistor according to the invention.

[0081] Figure 5: Enlarged SEM microscope image of the inner electrode of a varistor according to the invention.

[0082] Figure 6 : EDX concentration distribution of the element silver in the section from Figure 5 .

[0083] Figure 7 : EDX concentration distribution of the element oxygen in the section from Figure 5 .

[0084] Figure 8 : EDX concentration distribution of the element zinc in the section from Figure 5 .

[0085] Figure 9 : EDX concentration distribution of the element nickel in the section from Figure 5 .

[0086] Figure 10 : Characteristic varistor curves of a silver electrode and an Ag-Ni electrode .

[0087] Figure 11: Characteristic varistor core lines of a silver electrode and various Ag-Ni electrodes. Figure 1 shows a first exemplary embodiment of the varistor 1 according to the invention. It is a multilayer varistor in which ceramic layers 2 and intermediate layers with internal electrodes 3 are alternately stacked. The internal electrodes 3 are surrounded by ceramic and are exposed on one side of the outer surface of the varistor 1.

[0088] In the present embodiment, each second internal electrode 3a is exposed on a first side in the stacking direction and the internal electrode 3b located therebetween in the stacking direction is exposed on a second side opposite the first side.

[0089] The internal electrodes preferably comprise exclusively silver as the metal.

[0090] The ceramic is a zinc-bismuth ceramic containing ZnO and Bi2O3. The ceramic may also contain other metal oxides, particularly zinc oxides, bismuth oxides, and antimony oxides, as well as dopants.

[0091] To produce the varistor 1, ceramic green foils containing the aforementioned metal oxides in a predetermined composition are provided. The green foils are printed with a metal paste, for example by screen printing. The metal paste contains silver and nickel. In the example, the proportion of nickel in the metals in the metal paste is between 0.15% and 20%. By adding organic solvents and auxiliary materials, a target viscosity of the metal paste is set that is suitable for screen printing. In a subsequent step, printed and unprinted green foils are then stacked on top of one another in a defined sequence and with high positioning accuracy in order to realize the described layer structure of the varistor.

[0092] The stack is then mechanically pressed to securely bond the layers together. Components with defined sizes are then produced from the stack by cutting at predetermined positions. An exemplary varistor component 101 in the green state is shown in Figure 2. The varistor component 101 comprises a stack of green ceramic layers 102 and layers with metal paste 103.

[0093] In a subsequent step, the organic binders and additives present in the ceramic green sheets and metal paste, which ensure the necessary strength in the aforementioned process steps, are burned out. For this purpose, the components are heated in a furnace for a sufficiently long time.

[0094] After the organic components have been burned out, the components are sintered. Due to the protective effect of the nickel content in the metal paste, the sintering temperature can be raised to just below the melting point of the silver. The sintering temperature is maintained for a sufficiently long time, for example, over 180 minutes.

[0095] The sintered varistor components are then electrically contacted. To establish electrical contact between the components, external electrodes are applied. These can be achieved, for example, by immersing the exit surfaces of the internal electrodes in a silver paste and then firing the silver paste. Firing takes place, for example, at a temperature of approximately 650 to 700 °C.

[0096] Figure 3 shows a varistor 11 not according to the invention with a conventional silver electrode 13, without added nickel. Due to the high prices of other precious metals such as palladium, silver is preferred for the internal electrodes in varistors.

[0097] On the other hand, sufficiently high sintering temperatures are necessary to develop suitable electrical properties of the varistor, in order to adjust grain growth and grain boundary structures. However, compared to other precious metals, silver has a comparatively low sintering temperature.

[0098] The desired electrical properties are then discussed with reference to Figures 10 and 11 .

[0099] Zinc oxide ceramics, which have good varistor properties, are commonly used in multilayer varistors.

[0100] When using pure silver electrodes, as in the non-inventive example shown in Figure 3, a zinc oxide ceramic of conventional composition is unsuitable as a varistor ceramic. Below the melting point of Ag, the appropriate structure could not be produced with such a zinc oxide ceramic. Therefore, for example, an increased proportion of bismuth oxide is added to the ceramic composition to enable the formation of grain boundary structures and grain growth to the desired extent even at temperatures below the melting point of Ag.

[0101] The disadvantage of adding a lot of bismuth ( 111 ) oxide Bi2Os is its reactivity , which leads to an oxidation of part of the silver material during sintering .

[0102] In particular, in an outer region 14 of the internal electrodes 13, which are close to the outer surfaces of the varistor 11 and are therefore in contact with oxygen from the sintering atmosphere, the Ag is completely oxidized in sections, which affects the electrical contact of the electrodes.

[0103] Figure 3 shows an SEM micrograph of a section through a multilayer varistor 11. The inner electrode 13, made of pure silver, is severely thinned in the outer region 14 due to oxidative attack.

[0104] The silver embedded in the ceramic further impairs the insulation properties of the ceramic.

[0105] Figure 4 shows how the nickel additive provides a protective effect for the inner electrode 23, which preferably consists of Ag.

[0106] Nickel is a less noble metal than silver. Therefore, nickel is preferentially oxidized by Bi2O3. The silver thus remains in its reduced metallic form.

[0107] Furthermore, due to the diffusion of the nickel into the ceramic, the partial oxidation of the nickel, and the partial reduction of the bismuth oxide, a layer comprising nickel oxide NiO and bismuth(II) oxide BiO is formed, primarily in the regions of the ceramic layers 22 adjacent to the inner electrode. This protective layer 25 formed in this way can be seen in the SEM micrograph through a corresponding varistor 21 according to the invention in Figure 4. The NiO can, for example, react further with ZnO to form Ni-Zn spinels, which further impede access of the Bi2O3 to the inner electrode.

[0108] In Figure 4, the bright spots in the ceramic indicate the Bi2O3 phase. Adjacent to the inner electrode 23, none of these bright spots are visible in the protective layer 25. This means that after a sufficient amount of nickel has been oxidized, Bi2Os no longer reaches the electrode, and thus no further nickel or silver can be oxidized. Instead, dark spots can be seen in the protective layer, indicating the formation of a BiO phase. The protective layer 25, for example, is approximately ten times thicker than the inner electrode 23.

[0109] Figure 5 shows an enlarged view of the SEM micrograph, in which the nickel oxide phases can be seen as fine-grained crystallites 24 of the inner electrode 23 and in a transition layer adjacent to the inner electrode 23. The crystallites 24 are circled and form a diffusion barrier between the ceramic and the inner electrode.

[0110] Figure 6 shows the concentration distribution of the element silver in the varistor, determined by EDX (energy-dispersive X-ray spectroscopy), within the section shown in Figure 5. Figure 7 shows the distribution of the element oxygen in a similar manner. Figure 8 shows the distribution of the element zinc in a similar manner. A high concentration is shown in light, a low concentration in dark.

[0111] It can be seen that there is no oxygen in the silver layer and no silver in the oxygen-free phase, so the silver cannot be oxidized. The silver layer of the inner electrode is thus protected from oxidative attack.

[0112] Furthermore, it can be seen that the ceramic zinc oxide phase, i.e. the area in which zinc and oxygen are present, does not reach the silver layer shown in Figure 6.

[0113] Figure 9 shows the EDX concentration distribution of the element nickel. Figure 9 shows that a nickel-rich phase is formed between the zinc-rich and the silver-rich regions. Figure 7 shows that the nickel-rich phase is also rich in oxygen. These are the nickel oxide crystallites 24 shown in Figure 5.

[0114] Since the electrodes are thus protected from oxidative attack on the silver or nickel, the electrodes can be designed to be narrower in the stacking direction, namely down to at least 6 pm, preferably 5 pm, and even more preferably 4 pm. This allows for savings in metal, and especially silver, material for forming the electrodes. Electrical contact to the outside is thus ensured.

[0115] Figures 10 and 11 show electrical characteristics, namely current-voltage characteristics, of various varistor components. In the double-logarithmic plots, the x-axis represents the current in A, and the y-axis represents the voltage in V.

[0116] Figure 10 compares the characteristic curve of a nickel-free varistor with a silver electrode with the characteristic curve of the electrode of a varistor with a nickel content in the metal paste of 15% (or 17.2 volume percent), based on the sum of the masses of silver and nickel in the metal paste of the green body. The characteristic curve of the nickel-free varistor is shown at the top of the diagram, and that of the Ag-nickel varistor is shown at the bottom. Both electrodes were sintered at 900 °C.

[0117] In particular, the breakdown voltage, i.e. the voltage above which the varistor conducts electrically, and the non-linearity of the curves, which is a measure of the quality of the varistor, are considered.

[0118] Higher nonlinearity indicates better switching between electrically conductive and electrically non-conductive states. The curve below shows that a similar characteristic can be achieved by adding nickel, without, however, encountering the problems previously discussed in relation to Figures 2 and 3 regarding material consumption and external contacting.

[0119] In Figure 11, in addition to the characteristic curve of a nickel-free varistor with a silver electrode (the top curve in the diagram), and the characteristic curve of the previously described varistor with a 15% nickel content (the bottom curve in the diagram), five further characteristic curves are shown from Figure 10. The five further characteristic curves were all measured with electrodes of varistors containing a nickel content of 2.6% (3% by volume) in the metal paste, based on the total mass of Ni and Ag in the metal paste of the green body.

[0120] The various varistors, each with a 2.6% nickel content, were all sintered at different temperatures. The top curve corresponds to an electrode sintered at 900 °C, the next to an electrode sintered at 920 °C, the next to an electrode sintered at 940 °C, the next to an electrode sintered at 950 °C, and the bottom curve to an electrode sintered at 960 °C. The two remaining curves in Figure 10, as described, correspond to electrodes that were each sintered at 900 °C.

[0121] Figure 11 shows that reducing the nickel content leads to a more approximate nonlinearity of the curves, similar to that of the pure silver electrode. In particular, the nonlinearity of the curves can be further improved.

[0122] By increasing the sintering temperature, the varistor voltage—the voltage required to achieve a current flow of 1 mA—can be significantly reduced. In the 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 Ag-Ni varistor with 2.6% Ni, sintered at 960°C, is 55 volts. This represents a 33% reduction in the varistor voltage. This significantly expands the possibilities for adjusting the characteristic curves via the sintering temperature. Similarly high sintering temperatures are not possible when using pure silver electrodes, as this would destroy the silver electrode.

[0123] List of reference symbols

[0124] 1,11,21 sintered varistor 2,22 ceramic layers

[0125] 3,13,23 internal electrodes

[0126] 3a Left-side internal electrodes

[0127] 3b right-side internal electrodes

[0128] 14 outer area 24 NiO crystallites

[0129] 25 protective layers

[0130] 101 Varistor in green state

[0131] 102 ceramic green layers 103 metal paste

Claims

Patent claims 1. Method for manufacturing a multilayer varistor (1) comprising the steps: Preparing a metal paste (103) comprising silver and nickel, wherein the mass fraction of nickel in the metals of the metal paste (103) is a maximum of 25%, Applying the metal paste (103) to a ceramic green film (102) , Applying another ceramic green film (102) to the metal paste (103) to create a sandwich-like structure and Sintering of the green foils (102) with the applied metal paste (103), wherein the green foils (102) are transformed into ceramic layers (2) and the metal paste (103) into an internal electrode (3).

2. Method according to claim 1, wherein during sintering nickel is diffused into layers of the forming ceramic layers (2) adjacent to the forming inner electrode (3) and oxidized there, so that a nickel oxide phase is formed in transition layers (24) adjacent to the inner electrode (3).

3. Method according to claim 1 or 2, wherein the sintering is carried out at a temperature above 900 °C.

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

5. Method according to any one of claims 1 to 4, wherein the metal paste (103) in the form of metals comprises exclusively the metals silver and nickel.

6. Method according to any one of claims 1 to 5, wherein the transition layer (24) has a maximum thickness of 5 pm.

7. Method according to any one of claims 1 to 6, wherein the ceramic green films (102) consist of at least 90% by mass of ZnO and Bi2O3 or of ZnO, Bi2O3 and Sb2Os .

8. Method according to any one of claims 1 to 7, wherein during sintering nickel is deposited into protective layers (25) adjacent to the forming inner electrode (3) of the forming Ceramic layers (2) diffused.

9. Method according to claim 8, wherein Bi2Os in the protective layers (25) is partially reduced to BiO.

10. Method according to claim 9, wherein with increasing proximity to the forming inner electrode (3) a higher proportion of the Bi2C>3 in the protective layers (25) is reduced.

11. Method according to any one of claims 1 to 10, wherein the sintering takes place in an atmosphere with a substantial oxygen content.

12. The method of claim 11, wherein the sintering takes place under ambient air.

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

14. Green body (101) for the production of a multilayer varistor (1) comprising two ceramic green sheets (102) and a metal layer arranged in a sandwich structure between the two ceramic green sheets (102) and comprising a metal paste (103) containing silver and nickel, wherein the mass fraction of nickel in the metals of the metal paste (103) is a maximum of 25%.

15. Green body (101) according to claim 14, wherein the metal paste (103) in the form of metals contains exclusively the metals silver and nickel.

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

17. Green body (101) according to one of claims 14 to 16, wherein the ceramic green films (102) consist of at least 90% by mass of ZnO and Bi2O3 or of ZnO, Bi2O3 and Sb2O3.

18. Use of a metal paste (103) containing silver and nickel for the formation of silver-containing metal layers in or on a Bi2O3-containing ceramic, wherein the mass fraction of nickel in the metal paste (103) is a maximum of 25%.

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

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

21. Use of a metal paste (103) according to claims 18 to 20, wherein the ceramic consists of at least 90% by mass of ZnO and Bi2O3 or of ZnO, Bi2O3 and Sb2Os.

22. Multilayer varistor (1) comprising at least two ceramic layers (2) and an inner electrode (3) which 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 ceramic layer (2) adjacent to the inner electrode (3), in which a nickel oxide phase is formed.

23. Multilayer varistor (1) according to claim 22, wherein the nickel oxide phase is formed between the inner electrode (3) and the other ceramic layers (2).

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

25. Multilayer varistor (1) according to claim 22 or 23, wherein the nickel oxide phase is formed in the form of several non-connected crystallites.

26. Multilayer varistor (1) according to any one of claims 22 to 25, wherein the transition layer (24) has a maximum thickness of 5 pm.

27. Multilayer varistor (1) according to one of claims 22 to 26, wherein the inner electrode (3) contains only silver as the metal.

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

29. Multilayer varistor (1, 21) according to claim 28, wherein in the protective layers (25) bismuth oxides are increasingly present in the reduced form BiO with increasing proximity to the inner electrode (3).

30. Multilayer varistor (1) according to one of claims 28 or 29, wherein the ceramic layers (2) outside the protective layers (25) constitute a mass fraction of at least 90% consist of zinc oxide and bismuth oxides or of zinc oxide, bismuth oxides and antimony oxides.

31. Multilayer varistor (1, 21) according to one of claims 22 to 30, wherein the mass fraction of elemental nickel in relation to the sum of the total masses of elemental nickel and silver in the multilayer varistor (1) is a maximum of 25%.

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