EP4205148A1

EP4205148A1 - Multi-layer varistor and method for producing a multi-layer varistor

Info

Publication number: EP4205148A1
Application number: EP21749584.5A
Authority: EP
Inventors: Hermann GRÜNBICHLER; Jaromir Kotzurek; Franz Rinner
Original assignee: TDK Electronics AG
Current assignee: TDK Electronics AG
Priority date: 2020-08-26
Filing date: 2021-07-26
Publication date: 2023-07-05
Also published as: JP2022552069A; DE102020122299B3; WO2022042971A1; US11901100B2; CN114521274A; US20220406493A1; JP2024045288A

Abstract

The invention relates to a multi-layer varistor (1) comprising a ceramic body (2) having a large number of inner electrodes (5), the ceramic body (2) comprising an active region (3) and a near-surface region (4), and the ceramic body (2) comprising at least one first ceramic material (6) and at least one second ceramic material (7), the ceramic materials (6, 7) differing in a concentration of monovalent elements X⁺, where X⁺ = Li⁺, Na⁺, K⁺ or Ag⁺. The invention further relates to a method for producing a multi-layer varistor (1).

Description

description

Multilayer varistor and method for producing a multilayer varistor

The present invention relates to a multilayer varistor. The present invention also relates to a method for producing a multilayer varistor.

multi-layered resp. Multilayer varistors are used as effective protective elements against temporary overvoltages (such as ESD - "Electrostatic Discharge"; electrostatic discharge) . In the rapidly developing communication technology, there is an increasing need for protective elements that protect sensitive electronics . Due to the high frequencies during signal transmission and due to the installation of these protective elements directly in the lines, the capacitances of these components must be kept as low as possible, otherwise interference and losses will occur within the signal-carrying lines.

Reducing the capacitance of a multilayer varistor presents considerable difficulties. Although the active area (overlap area) and thus the capacitance can be reduced structurally, the leakage current and the protective effect are reduced proportionally as a result. From the point of view of the material, a material with a lower dielectric constant (DK) would be desirable. The material used for multilayer varistors consists of doped zinc oxide (ZnO). The DK of this ceramic is dominated by barrier layers between the ZnO grains. The series and parallel connection of the individual barrier layers results in the capacity - but also the Breakdown voltage of the active area. Since the breakdown voltage is specified in the design of the component, this also results in the capacitance of the active area. The DK of the ZnO ceramic is largely coupled with the breakdown voltage and therefore cannot be used as a degree of freedom to lower the capacitance.

In addition to the capacitance of the active volume (the ceramic between the inner electrodes), the stray capacitance of the ceramic component outside the active volume (cover layers and isolation zone) also contributes to the capacitance of a varistor. When the active area in the component is reduced, the proportion of the stray capacitance in the total capacitance increases more and more and thus limits the achievable effect by a design with a minimal overlapping area of the electrodes. Therefore, in order to efficiently reduce the capacitance of a varistor, it is necessary to reduce this stray capacitance as much as possible.

Various methods are known for reducing the conductivity and for reducing the stray capacitance in the area outside the internal electrodes, but these are either not very effective or have other disadvantages. The simplest way to achieve this is to glaze the surface of the multilayer varistor after sintering. This glass layer has the additional advantage that it chemically insulates the ceramic and thus increases the durability of the component. Therefore, the additional use of this method can also be useful when other methods are used. However, since the glass layer is very thin, the effectiveness of this method is limited and the use of other methods or the combination with other methods is advantageous . In document DE 100 26 258 B4, instead of the protective glass layer, a cover layer containing bismuth is applied as electroplating protection, which can be sintered together with the varistor ceramic. The chemical composition of the top layer is very different from that of the ZnO ceramic, resulting in a disadvantageous diffusion and reaction zone during sintering. An influence on the dielectric constant by the cover layer is not discussed.

The document JP 3735151 B2 describes a method in which the outermost areas of the ceramic are chemically modified after sintering. During an additional heat treatment, lithium or sodium is diffused into the surface of the ceramic body. Doping with acceptors reduces the leakage current and the dielectric constant of the outermost layer. The capacitance of the multilayer varistor can be significantly reduced in this way. The disadvantage of this method is that this subsequent modification requires considerable effort. Further heat treatment would also be required to apply an additional layer of glass on the outside, which is extremely difficult due to the high diffusion rates of sodium and lithium.

The document JP H-113809 A describes a multilayer varistor consisting of an insulating support layer with a low dielectric constant, onto which the actual varistor ceramic is laminated. The carrier layer itself is also made from a ceramic with a low dielectric constant using the layer process. A disadvantage of this multilayer varistor is the costly production of the same: instead of a uniform ceramic, two different ceramics with very different properties are required. This can be due to a different marriage Mixed composition can be achieved, resulting in a weak bond between the carrier layer and the varistor ceramic.

The document DE 10 2018 116 221 A1 describes a multi-layer varistor that consists of two chemically very different materials that differ in the ZnO grain size after the sintering process. The aim of this construction of a multilayer varistor is to keep the current flow in the component away from thermo-mechanical weak points and thus increase the impulse strength of the protective element. The focus here is not on the effect of the chemically very different materials, which are both used in the active area, for example, on the capacitance of the multilayer varistor.

The document DE 10 2017 105 673 A1 describes the combination of two different ZnO ceramics in order to increase the pulse strength of the component. The two materials must be bonded to the electrodes in order to show the effect. The effects in near-surface areas as well as the impact on capacity are not addressed.

The known methods therefore have clear disadvantages or they are not effective in reducing the stray capacitance.

The object of the present invention is to describe a multilayer varistor and a method for producing a multilayer varistor which solve the above problems. This object is achieved by a multilayer varistor and a method for producing a multilayer varistor according to the independent claims.

According to one aspect, a multilayer varistor is described. The multilayer varistor has a ceramic body. The ceramic body has a plurality of layers. A plurality of internal electrodes are formed in the ceramic body. The inner electrodes have, for example, silver, palladium, platinum or an alloy of these metals.

The ceramic body has an active area. The ceramic body also has an inactive area. The areas between the different inner electrodes of different polarity, which are decisive for the current flow between the same, are to be understood as the active area. In contrast, the areas in the ceramic body of the multilayer varistor that do not (or not significantly) contribute to the flow of current between the differently contacted internal electrodes are referred to as the inactive area.

The ceramic body has an area close to the surface. The area close to the surface in each case borders on an upper side and an underside of the multilayer varistor. The area close to the surface only has minimal electrical conductivity. The area close to the surface is designed to be essentially electrically insulating. The area close to the surface comprises a cover layer and/or an isolation zone of the multilayer varistor.

The ceramic body has at least a first or primary ceramic material on . Preferably, the multi-layer varistor has exactly a first or primary ceramic material on . the Ceramic body at least one second or modified ceramic material. The main component of the two ceramic materials is zinc oxide (ZnO). In particular, the two ceramic materials are based on ZnO.

The first and the second ceramic material differ in a concentration of monovalent elements X ⁺ or elements with a stable oxidation state +1. X+ is selected from Li ⁺ , Na ⁺ , K ⁺ or Ag ⁺ . The monovalent elements preferably have a low diffusion constant. Preferably, the multilayer varistor is manufactured by a method which will be described later in detail.

The second or modified ceramic material is doped with the monovalent elements. For example, the second ceramic material is doped with potassium oxide. The first or primary ceramic material may be free from doping with monovalent elements. Alternatively, however, the first ceramic material can also be slightly doped with monovalent elements.

The dopants that distinguish the ceramic materials occur in low concentrations. Due to the doping with monovalent elements, the electrical properties of the second/modified ceramic material differ greatly from those of the first/primary ceramic material. However, chemically there is no significant difference between the ceramic materials. Notably, the two materials are otherwise nearly identical.

Doping with monovalent elements causes a significant reduction in the dielectric constant, even in small amounts. Consequently, the second or modified

Ceramic material has a lower dielectric constant as the first or primary ceramic material. A multilayer varistor with a reduced stray capacitance and consequently a reduced total capacitance can thus be provided.

According to one exemplary embodiment, the highest concentration of monovalent elements X ⁺ is in the area close to the surface. The lowest concentration of monovalent elements X ⁺ is in the active region. The concentration of monovalent elements consequently decreases, starting from the surface towards the inner area/active area of the multilayer varistor. Accordingly, the value for the dielectric constant increases, starting from the surface towards the interior of the multilayer varistor. This reduces the stray capacitance of the varistor. The total capacitance of the varistor is consequently effectively reduced.

According to one exemplary embodiment, the ceramic materials differ chemically from one another by <1%. In other words, the ceramic materials are nearly identical chemically. This means that both materials can be excellently processed together. For example, the layers of modified materials can be sintered together without defects. A particularly reliable multilayer varistor is thus made available.

According to one exemplary embodiment, the dielectric constants sr of the first and second ceramic material differ from one another by a factor of >5. The stray capacitance of the varistor can consequently be significantly reduced in a simple manner by the only slight doping with monovalent elements. According to one exemplary embodiment, the first/primary ceramic material is arranged in the active area. The second/modified ceramic material forms an insulating cover layer of the ceramic body. In particular, the second ceramic material is arranged on the top and on the bottom of the multilayer varistor. Consequently, the multi-layer varistor has an insulating cover layer or low-dielectric cladding on . The stray capacitance of the multilayer varistor is thus significantly reduced in comparison to conventional multilayer varistors.

According to one exemplary embodiment, the ceramic materials differ from one another in the concentration of monovalent elements X ⁺ by a maximum of 50 ppm<Ac(X ⁺ )<5000 ppm. Ac denotes the maximum concentration difference that occurs between the active area and the area near the surface.

In other words, the concentration of acceptors in the second ceramic material is at most between 50 ppm and 5000 ppm higher than in the first ceramic material. The ceramic materials of the multilayer varistor preferably differ from one another by 100 ppm<Ac(X ⁺ )<1000 ppm.

The concentration of monovalent elements X ⁺ in the active area is preferably <100 ppm, preferably <50 ppm. The first ceramic material is consequently almost free from monovalent elements. The proportion of monovalent elements is due in particular to their diffusion from the second ceramic material during production of the multilayer varistor.

Since the monovalent elements in which the two ceramic materials differ, only a small concentrate!- ion difference (concentration gradient), diffusion thereof into the active region can be neglected even during sintering. The cover layers (second or modified ceramic material) can therefore be dimensioned with sufficiently high thicknesses, as a result of which the shielding effect is enhanced.

According to one embodiment, the ceramic body has at least three ceramic materials. In particular, the ceramic body has the first/primary ceramic material, the second/modified ceramic material and a third/modified ceramic material. However, the ceramic body can also have more than three ceramic materials. For example, the ceramic body can also have a fourth or modified ceramic material.

The third ceramic material is interposed between the first ceramic material and the second ceramic material. The third ceramic material is arranged in the inactive area and in particular in the area close to the surface of the multilayer varistor. The third ceramic material forms a near-surface insulation zone. The three ceramic materials differ chemically by < 1%.

The three ceramic materials differ in the concentration of monovalent elements. The first ceramic material (active area) has the lowest concentration of monovalent elements. The second ceramic material (outer insulating cover layer) has the highest concentration of monovalent elements. The third material (near-surface isolation zone) has a concentration of monovalent elements that is between that of the first and second ceramic materials. In particular, the concentration of monovalents decreases

Elements X ⁺ starting from the near-surface area in the direction of the active area gradually (concentration gradient). In this way, local chemical differences can be effectively reduced.

According to one exemplary embodiment, a thickness of the second and/or the third ceramic material is adapted to a diffusion behavior of the monovalent element. In particular, the thickness is chosen such that there is as little diffusion of the acceptors into the active region as possible. The thickness of the cover layers is thus adapted to the diffusion constant of the monovalent element. In particular, the thickness reduces as the diffusion constant increases. Due to the reduced diffusion, a defined concentration gradient of monovalent elements occurs and, associated with this, a defined gradient of the electrical properties, above all the dielectric constant.

The thickness of the second and third ceramic material is based on the overall height of the component and its internal structure. The design principle is that the effectiveness increases the higher the proportion of the second and third ceramic material in the inactive cover layers. On the other hand, this increases the risk that the monovalent element can diffuse into the active area during sintering. For example, a safety distance of 100 pm can be useful. In other words, after the last printed laminate, there are still another 100 μm from the first ceramic material as “diffusion buffer”. But also a smaller one

Safety distance is conceivable. Alternatively, you can after the last printed layer continue directly with the second and third ceramic material.

According to a further aspect, a method for producing a multilayer varistor is described. The multilayer varistor described above is preferably produced by the method. All properties that are disclosed in relation to the multilayer varistor or the method are also disclosed correspondingly in relation to the respective other aspect and vice versa, even if the respective property is not explicitly mentioned in the context of the respective aspect. The procedure consists of the following steps :

A) providing a first or primary ceramic powder for producing a first ceramic material. Providing at least a second or modi fi ed ceramic powder for the production of a second ceramic material.

The ceramic powders essentially contain ZnO. The second ceramic powder is doped, in particular slightly doped, with monovalent elements X ⁺ , for example Li ⁺ , Na ⁺ , K ⁺ or Ag ⁺ . The first ceramic powder may be free from monovalent element doping or may have a small amount of monovalent element doping. In particular, the concentration of monovalent elements in the first ceramic powder is many times lower than the concentration of monovalent elements in the second ceramic powder. The dopant has a low diffusion constant.

For example, a doping with potassium (for example K2O, KC4H5O6 or K2C03) can be present. In particular, the latter is characterized in that - due to a high Melting point and a high decomposition temperature - few losses occur during sintering. As an alternative to this, for example Li or Na can also be used as doping. Na and Li are hardly or not at all susceptible to peroxide formation in air and the melting points of the metals are very high. The losses during sintering can thus be kept low.

The dopant only occurs in a low concentration. The ceramic powders differ in the concentration of monovalent elements X ⁺ by 50 ppm < Ac (X ⁺ ) < 5000 ppm. Ac denotes the maximum concentration difference that occurs between an active area and an area close to the surface of the finished multilayer varistor.

In an alternative embodiment, a third ceramic powder can additionally be provided for the production of a third ceramic material. Here, the concentration of monovalent elements X ⁺ in the third ceramic powder is lower than in the second ceramic powder but higher than in the first ceramic powder. The third ceramic powder therefore has an average concentration of monovalent elements.

B) Slurrying of the ceramic powder in a solvent and film drawing or formation of green films.

C) Partial printing of part of the green foils with a metal paste, for example silver and/or palladium, to form internal electrodes. In this case, those green tapes are partially printed with metal paste which have a lower concentration of monovalent elements X ⁺ than the other green tapes. In particular, those Printed green tapes with the lowest concentration of monovalent elements, ie the green tapes, which are made from the first ceramic powder.

Furthermore, other green tapes with the lowest or medium concentration of monovalent elements can be printed with metal paste to form Faraday or guard electrodes .

D) Stacking of printed and unprinted green films. The green films are stacked in such a way that the second ceramic material forms a cover layer of the multilayer varistor. If a third ceramic material is present, the green sheets are stacked in such a way that the green sheets made of the third ceramic material are arranged between the green sheets made of the first and the third ceramic material.

The green sheets are in particular stacked in such a way that a defined concentration gradient of monovalent elements X ⁺ is formed, with the concentration decreasing starting from the second ceramic material (cover layer) down to the first ceramic material (active area).

E) Lamination, decarburization and sintering of the green films. The green sheets are preferably sintered at 1100° C.

F) Application of external electrodes for electrical contacting of the multi-layer varistor. The outer electrodes can be single-layered (CN type) or multi-layered.

In the case of a three-layer outer electrode, an additional Ni and a solderable Sn layer would then be applied in an electroplating shop. Before electroplating, the component must be provided with a protective layer (glazing). The modified ceramic materials are of particular importance in the process. Since the modified ceramic materials are to be manufactured using the same process as the primary ceramic material, and the various ceramic materials are to be processed together in the stacking, laminating and sintering steps, it is important that the mechanical and thermal properties of the materials are well matched are . At the same time, the electrical properties have to be adapted to the very different requirements.

In the present case, the concept of a top layer or Low-dielectric cladding used to reduce the capacitance of a multilayer varistor. Previous solutions require complex methods and/or additional process steps for the production thereof or are unsuitable for reducing the stray capacitance. The diffusion of lithium into the finished sintered component poses a particular challenge. It is necessary to work with highly concentrated lithium compounds (e.g. L12CO3) in order to achieve a sufficient penetration depth; on the other hand, there is a risk that the lithium will penetrate into the active volume and endanger the functionality of the component.

If, on the other hand, a ceramic that differs greatly in its chemical composition is used as the cover layer, there is the disadvantage of the minimal bond between the cover layer and the varistor ceramic in addition to the expense involved in production. The mechanical properties (modulus of elasticity, strength, thermal expansion, etc.) sometimes differ greatly from one another, since a sufficient difference in the electrical properties is necessary. is agile . This negatively affects the mechanical stability of the entire component.

These disadvantages are effectively circumvented by the method described above and the resulting multilayer varistor.

The drawings described below are not to be taken as true to scale. Rather, individual dimensions can be enlarged, reduced or also distorted for better representation.

Elements that are the same or that take on the same function are denoted by the same reference symbols.

Show it :

FIG. 1 shows a sectional view of a multilayer varistor according to a first exemplary embodiment,

FIG. 2 shows a sectional view of a multilayer varistor according to a further exemplary embodiment,

FIG. 3 shows a sectional view of a multilayer varistor according to the third exemplary embodiment.

FIG. 1 shows a first embodiment of a multilayer varistor 1 . The multilayer varistor 1 has a ceramic body 2 . A large number of internal electrodes 5 are formed in the ceramic body 2 . In Figure 1 are only two internal electrodes 5 shown. Of course, the multilayer varistor 1 can have more than two internal electrodes 5 . The inner electrodes 5 have silver, palladium, platinum or an alloy of these metals.

In this exemplary embodiment, the inner electrodes 5 are arranged alternately and overlap in an inner area of the multilayer varistor 1 . The overlapping area forms an active area 3 of the multilayer varistor 1 .

The multilayer varistor 1 also has a region 4 close to the surface. The region 4 close to the surface has only minimal electrical conductivity. The region 4 close to the surface borders on an upper side 1a and an underside 1b of the multilayer varistor 1, as can be seen in FIG. The near-surface area 4 has a top layer or. an isolation area of the multi-layer varistor 1 .

In this exemplary embodiment, the multilayer varistor 1 also has two outer electrodes 9 . However, the multilayer varistor 1 can also have more than two outer electrodes 9 . The outer electrodes 9 are electrically connected to the inner electrodes 5 for electrical contacting of the multilayer varistor 1 . The external electrodes 9 are formed on side faces of the multilayer varistor 1 . Furthermore, the external electrodes 9 are also formed on parts of the bottom 1b and the top 1a of the multilayer varistor 1 .

According to the exemplary embodiment shown, the outer electrodes are constructed in one layer. As an alternative to this, the outer electrodes 9 can also have a multilayer structure (not shown explicitly). In this case, the respective outer electrode 9 preferably has a first or inner layer for contacting the inner electrodes 9 . The first layer preferably comprises silver. The respective outer electrode 9 has a second or middle layer as a diffusion barrier. The second layer preferably comprises nickel. The respective outer electrode 9 has a third or outer layer, which enables the multilayer varistor 1 to be soldered to printed circuit boards. The third layer preferably comprises tin. In this exemplary embodiment, the varistor 1 must be provided with a protective layer (preferably glass) before electroplating. In this case, in particular, a further protective layer (electroplating protection, for example glass) is applied (not explicitly shown) to the upper side 1a and the lower side 1b (ie over the second ceramic material 7 described below). This glass layer chemically insulates the ceramic body 2 and thus increases the durability of the varistor 1 .

In the exemplary embodiment according to FIG. 1, the ceramic body 2 has two ceramic materials or Varistor ceramics 6 , 7 on .

A first or primary ceramic material 6 is formed in an inner portion of multilayer varistor 1 . In particular, the active area 3 has the first ceramic material 6 . A second or Modified ceramic material 7 is formed in an edge area of multilayer varistor 1 . In particular, the second ceramic material is arranged in the area 4 close to the surface and thus essentially in the inactive area. In addition to the second ceramic material 7, the inaccurate tive area but also a part of the first ceramic material 6, as can be seen from Figure 1.

The ceramic materials 6, 7 contain ZnO. In particular, ZnO is the main component of the ceramic materials 6, 7. Furthermore, the ceramic materials 6, 7 may contain a varistor-forming oxide such as bismuth oxide or a rare earth oxide (e.g. praseodymium oxide) and other oxides which improve the varistor properties.

The ceramic materials 6, 7 are chemically almost identical. In particular, the ceramic materials 6, 7 correspond chemically to >99%. However, the ceramic materials 6, 7 have a different dielectric constant εo*εr or dielectric constant s _r . In particular, the dielectric constants εo*εr or dielectric constants s _r of the ceramic materials 6, 7 differ from one another by a factor of >5. The dielectric constant of the first ceramic material 6 - and thus in the active region 3 - is greater than the dielectric constant of the second ceramic material 7 - and thus in the region 4 near the surface.

This is achieved in that the ceramic materials 6, 7 differ in the concentration of monovalent elements X ⁺ (X ⁺ stands for Li ⁺ , Na ⁺ , K ⁺ or Ag ⁺ ).

For example, the ceramic materials differ from each other by a maximum of 50 ppm < Ac (X ⁺ ) < 5000 ppm. Ac denotes the maximum concentration difference that occurs between the active area 3 and the area 4 near the surface. Preferably the concentration is monovalent Elements in the near-surface area 4 are 100 ppm to 1000 ppm higher than in the active area 3 .

The monovalent elements Li ⁺ , Na ⁺ , K ⁺ , Ag ⁺ act as "acceptor doping" in the semiconducting ZnO . Therefore, the above doping can be applied to all ZnO-based varistor ceramics (regardless of the recipe).

Overall, the ceramic materials 6, 7 must be doped with acceptors that have relatively low diffusion constants. Furthermore, the dopants in which the ceramic materials 6, 7 differ must occur in low concentrations.

It is advantageous if the concentration X ⁺ in the active region 3 (concentration of monovalent elements in the first ceramic material 6) is at a low level (X ⁺ <100 ppm). In other words, the concentration of monovalent elements X ⁺ in the active area 3 is significantly lower than in the inactive area or . in the near-surface area 4 .

A low concentration of monovalent elements X ⁺ is associated with a large (or larger) dielectric constant. Consequently, the active region 3 has a higher dielectric constant/dielectric constant than the region 4 close to the surface. An increase in the concentration of monovalent elements X ⁺ causes the dielectric constant to decrease. Overall, a significant reduction in the dielectric constant is achieved even with small amounts of monovalent elements added.

In summary, the two ceramic materials 6, 7 are combined in such a way that the highest concentration of monovalent Elements X ⁺ in the near-surface region 4 and the lowest concentration in the active region 3 is present. The second ceramic material 7 thus serves as an insulating cover layer with acceptor doping and a low dielectric constant. Starting from the area 4 close to the surface, the concentration decreases step by step in the direction of the active area 3 (concentration gradient). This significantly reduces the parasitic capacitance/stray capacitance of the multilayer varistor 1 .

Since the ceramic materials 6, 7 are chemically almost identical, there are no mechanical (cracks, bending) and chemical (reaction, diffusion zones) problems during the sintering of the ceramic.

FIG. 2 shows a second embodiment of a multilayer varistor 1 . With regard to the design and arrangement of the internal electrodes 5 and external electrodes 9, reference is made to the description in connection with FIG.

In contrast to the multilayer varistor shown in FIG. 1, the multilayer varistor in this exemplary embodiment has three ceramic materials/varistor ceramics 6, 7, 8 with different concentrations of monovalent elements X ⁺ . The first or As already described in connection with FIG. 1, the primary ceramic material 6 is arranged in the active area 3 . The second and third ceramic material (modified ceramic materials) 7 , 8 is arranged in the region 4 near the surface. The third ceramic material 8 is arranged between the first and the second ceramic material 6 , 7 .

The first ceramic material 6 has a low concentration of monovalent elements. The first ceramic material 6 has a high dielectric constant. The second ceramic material 7 has a higher concentration of monovalent elements than the first ceramic material 6 . The concentration of monovalent elements in the third ceramic material 8 is between those of the first ceramic material 6 and the second ceramic material 7 . In particular, the first ceramic material 6 has the lowest concentration of monovalent elements and the second ceramic material 7 has the highest concentration of monovalent elements. The third ceramic material 8 has a medium concentration. This creates a concentration gradient.

The concentration of the acceptors in the second and third ceramic material 7 , 8 is for example between 50 ppm and 5000 ppm higher than in the active ceramic layer (first or primary ceramic material 6 ). The second and third ceramic materials 7, 8 serve as an insulating cover layer and. Isolation zone with acceptor doping and low dielectric constant.

FIG. 3 shows a third embodiment of a multilayer varistor 1 . With regard to the design and arrangement of the outer electrodes 9, reference is made to the description in connection with FIG. In contrast to the embodiments shown in FIGS. 1 and 2, the internal electrodes 5 in this exemplary embodiment are arranged in a tip-to-tip position. The area between the tips of the internal electrodes 5 forms the active area 3 of the multilayer varistor 1 . In addition, the multilayer varistor 1 has metallic protective or Faraday electrodes 10, which increase the protective function of the multilayer varistor 1 against electrostatic discharges. Analogously to the multilayer varistor described in connection with FIG. 2, the multilayer varistor 1 in this exemplary embodiment has three ceramic materials 6, 7, 8 with different concentrations of monovalent elements X ⁺ .

The Faraday electrodes 10 contribute to preventing the diffusion between the ceramic materials 6,7,8. Due to the reduced diffusion, a defined concentration gradient arises and, associated with this, a defined gradient of the electrical properties, above all the dielectric constant. The thicknesses of the cover layers (second and third ceramic material 7, 8) are chosen so that the least possible diffusion of the acceptors into the active region 3 occurs. A respective extension of the second ceramic material 7 or of the third ceramic material 8 perpendicular to a main extension of the multilayer varistor 1 is understood as the thickness of the cover layers.

Overall, the concentration of the acceptors in the second and third ceramic material 7, 8 is between 50 ppm and 5000 ppm (preferably between 100 ppm and 1000 ppm) higher than in the active ceramic layer (first ceramic material 6). The second and third ceramic materials 7, 8 serve as an insulating cover layer with acceptor doping and a low dielectric constant. With regard to the further design features of the ceramic materials 6, 7, 8, reference is made to the description of FIG.

The particular advantage of this invention is that the electrical properties of the modified varistor ceramic 7, 8 (second or third ceramic material 7, 8) differ greatly from those of the original varistor ceramic (first or primary res ceramic material 6 ) differ without the materials being chemically significantly different from each other . Therefore the materials are otherwise almost identical and can be processed without any problems.

A method for producing a multilayer varistor 1 , in particular a multilayer varistor according to one of the above exemplary embodiments, is described below. The procedure consists of the following steps :

A) In a first step, ceramic powders made up of individual components are provided. A first ceramic powder is provided to form the first ceramic material (primary ceramic material) 6 . A second ceramic powder for forming the second ceramic material (modified ceramic material) 7 is also provided. In one exemplary embodiment, a third ceramic powder can also be provided for forming the third ceramic material (modified ceramic material) 8 (see FIGS. 2 and 3). The ceramic powders are chemically 1 99 % identical. The ceramic powders essentially contain ZnO as the base material. Table 1 shows a possible composition of the base material of the ceramic powder. Of course, other compositions are also conceivable, with ZnO being the main component of the ceramic material in each case.

Table 1: Composition of the base material of the ceramic powder. * ) Cross-contamination and entry through process: typically 1- 10 ppm potassium

However, the ceramic powders differ in the concentration of monovalent elements X ⁺ . In particular, the ceramic powders differ in the concentration X ⁺ by 50 ppm<Ac(X+)<5000 ppm.

The first or Primary ceramic powder has the lowest concentration of acceptors/monovalent elements. The concentration of monovalent elements X ⁺ in the first ceramic powder is preferably <100 ppm. The second ceramic powder has the highest concentration of acceptors/monovalent elements. The third ceramic powder has an intermediate/intermediate concentration of acceptors/monovalent elements.

In a second step B), green films are formed from the ceramic powders. For this purpose, the powders are first ground, spray-dried and decarburized. The decarburized powders are slurried with organic binders and dispersants and then drawn into green sheets. The foils are cut to size.

In a further step C), part of the green films is partially printed with a metal paste (preferably silver and/or palladium) to form the internal electrodes 5 . In this case, only those green films are partially printed with the metal paste which will later be used in the active rich 3 are arranged. In other words, only the green sheets that are made from the first ceramic powder are printed with the metal paste.

Optionally, a further metal paste (preferably silver and/or palladium) can also be printed onto part of the green foils in order to form protective electrodes 10 (see FIG. 3). Preferably, this metal paste is on the green sheets with the lowest and / or the. average concentration of monovalent elements (FIG. 3).

In a further step D), the stacking of printed and unprinted green films takes place. The stacking takes place in such a way that the final multilayer varistor 1 has a defined concentration gradient of monovalent elements X ⁺ , the concentration decreasing starting from the second ceramic material 7 via the third ceramic material 8 ( FIGS. 2 and 3 ) to the first ceramic material 6 .

In a further step, the green films are laminated, decarburized and sintered. The sintering temperature is preferably 1100° C.

In a last step, external electrodes 9 are applied.

The method produces a multilayer varistor 1 which has a very low stray capacitance and therefore a low capacitance.

An advantage of this invention is that the production involves very little effort. The modified varistor ceramic (second or third ceramic material 7, 8) is treated in production in the same way as the original/primary varistor ceramic (first ceramic material 6), since the materials differ only slightly chemically. Therefore, the powder, slip and foil properties of the materials are very similar and can be processed in the same way. The same applies to the processing of the foils into laminates and the finishing of the components (cutting, decarburization, sintering). Since the elements, such as potassium, in which the materials differ from each other have only a small difference in concentration (concentration gradient), diffusion thereof into the active volume can be neglected even during sintering. Therefore, the cover layers can be dimensioned with sufficiently high thicknesses, which increases the shielding effect.

In order to characterize the cover layers, modifications (variations with altered doping according to Table 2 below) were produced in a previous test method starting from the base material (see Table 1) and their dielectric constant was determined. For this purpose, the powder mixtures were each ground, evaporated and decarburized. The decarburized powders were granulated with an organic binder and pressed into disks (15 mm in diameter, 1 mm in height). The disks were sintered and ground to a height of 0.3 mm. Finally, the disks were printed with silver paste in a circle (5 mm diameter) on both sides and burned in.

The capacities of the disks were measured at 1 V and 1 kHz (see Table 2). The dielectric constant or dielectric constant of the ceramic could be determined using the formula for the capacitance of the plate capacitor: s _r = (C

*d) / (A*so) •

Table 2: Results of the base material and the modified varistor ceramics

With the characterization test method, possible compositions with a reduced dielectric constant were provided which were suitable for testing the invention on the multilayer varistor.

Finally, the testing of the invention is briefly summarized below.

Three ceramic powders were produced, which differed only in the potassium and lanthanum content in the ppm range (see Table 2). The main component of all powders was zinc oxide (see Table 1).

The composition of the first ceramic powder corresponded to the base material (see Table 1). The second ceramic powder was additionally doped with 1,000 ppm of potassium. The third ceramic powder was additionally doped with 1000 ppm potassium and 1000 ppm lanthanum.

The powder mixtures produced in this way were ground, spray-dried and decarburized. The decarburized powders were ganic binder and dispersant and drawn into films. The foils were cut to size, printed with palladium paste, stacked and cut into multilayer components.

The simplest design (see FIG. 1) of a 1206 ML varistor with 2 internal electrodes (120 micron electrode spacing and 0.8 mm ² overlap area) was selected for testing. Three types of devices were produced with the three types of ceramic sheets.

The first type of components consisted consistently of the base material (= the reference type). The core of the second type of component consisted of the base material with a covering layer of the second ceramic (with increased potassium concentration). The core of the third type of component consisted of the base material with a covering layer of the third ceramic (with increased potassium concentration and lanthanum-doped).

The components produced in this way were each sintered at 1100°C. The micrographs showed that the cover layers were sintered with the core layer without any defects (no cracks, etc.). Finally, the components with outer electrodes made of a layer of silver were metallized and burned in.

The capacitances of the components were measured at 1 V and 1 MHz. The first type of component (reference type) had a capacitance of 17.713.1 pF. The second type of device (top layer with increased potassium concentration) had a capacitance of 13.211.3 pF. This corresponds to a reduction in capacity of 25%. The third type of device (cap layer with increased potassium concentration and doped with lanthanum) had a capacitance from 11.1±2.4 pF to . This corresponds to a reduction in capacity of 37%. It was thus possible to show that even the simplest type of application of the invention leads to a significant reduction in the overall capacitance of the multilayer varistor.

The current/voltage characteristic of the components was measured with increasing static currents in the range from 10 nA to 1 mA. The first type of component (reference type) showed a varistor voltage at 1 mA of 21591144 V with ¹ . The second type of device had a varistor voltage at 1 mA of 22101172 V mnr ¹ . This corresponds to a change in the varistor voltage of only 2%. The third type of device had a varistor voltage at 1 mA of 22731183 V mnr ¹ . This corresponds to a 5% change in the varistor voltage.

It is thus evident that the varistor voltage (U _v @ lmA) is hardly influenced by the use of the cover layers/modified varistor ceramics. From this it can be concluded that the active volume of the varistor was not influenced or even damaged by the cover layers.

The description of the objects specified here is not limited to the individual specific embodiments. On the contrary, the features of the individual embodiments can be combined with one another as desired, as far as this is technically feasible. reference character list

1 multilayer varistor la upper side 1b lower side

2 ceramic bodies

3 Active area

4 Near-surface area

5 inner electrodes 6 first ceramic material

7 Second ceramic material

8 Third ceramic material

9 outer electrode

10 guard electrode

Claims

patent claims

1. A method for producing a multilayer varistor (1) having the following steps:

A) providing a first ceramic powder for producing a first ceramic material (6) and at least one second ceramic powder for producing a second ceramic material (7), the concentration of monovalent elements X ⁺ in the ceramic powders being 50 ppm <Ac (X ⁺ ) < 5000 ppm differ from each other, where X ⁺ = (Li ⁺ , Na ⁺ , K ⁺ or Ag ⁺ ) , and where Ac denotes the maximum concentration difference between an active area (3) and an area close to the surface (4) of the multilayer varistor (1 ) occurs;

B) slurring of the ceramic powder and formation of green films;

C) partial printing of a portion of the green sheets with a metal paste to form internal electrodes (5);

D) Stacking of printed and unprinted green sheets;

E) Lamination, decarburization and sintering of the green sheets;

F) Application of external electrodes (10).

2. The method according to claim 1, wherein in step C) those green sheets are partially printed with metal paste, which have a lower concentration of monovalent elements X ⁺ than the other green sheets.

3. The method according to claim 1 or 2, wherein the green sheets are stacked in step D) such that the second ceramic material (7) forms a cover layer of the multilayer varistor (1).

4. The method according to any one of claims 1 to 3, wherein the ceramic powders have ZnO as a main component.

5. The method according to any one of the preceding claims, wherein the ceramic materials (6, 7) have a varistor-forming oxide or a rare earth oxide and other oxides which improve the varistor properties.

6. The method according to any one of the preceding claims, wherein the ceramic materials (6, 7) are additionally doped with Pr, La or Y.

7. The method according to any one of the preceding claims, wherein the ceramic materials (6, 7) differ in the potassium and lanthanum content in the ppm range.

8. The method according to any one of the preceding claims, wherein the in the near-surface region (4) arranged second ceramic material (7) is doped with 1000 ppm of potassium.

9. The method according to claim 8, wherein the second ceramic material (7) is additionally doped with 1000 ppm La.

10. The method according to claim 8 and claim 9, wherein the lanthanum-doped second ceramic material (7) has a reduced stray capacitance compared to the only potassium-doped second ceramic material (7).

11. The method according to any one of the preceding claims, wherein the first ceramic material (6) has the lowest concentration of monovalent elements X ⁺ and wherein the second ceramic material (7) has the highest concentration of monovalent elements X ⁺ .

12. The method according to any one of claims 1 to 11, wherein in step A) a third ceramic powder for the production of a third ceramic material (8) is provided, the concentration of monovalent elements X ⁺ in the third ceramic powder being smaller than in the second ceramic powder but greater than im first ceramic powder is.

13. The method according to any one of claims 1 to 12, wherein the green sheets are stacked in step D) such that the multilayer varistor (1) has a defined concentration gradient of monovalent elements X ⁺ , the concentration starting from the second ceramic material (7) to to the first ceramic material (6) decreases.

14. Multilayer varistor (1) having a ceramic body (2) with a plurality of internal electrodes (5), wherein the ceramic body (2) has an active area (3) and a near-surface area (4), and wherein the ceramic body (2) has at least a first ceramic material (6) and at least one second ceramic material (7), the ceramic materials (6, 7) differing from one another in a concentration of monovalent elements X ⁺ by a maximum of 50 ppm < Ac (X ⁺ ) < 5000 ppm, wherein X ⁺ = (Li ⁺ , Na ⁺ , K ⁺ or Ag+) , and where Ac denotes the maximum concentration difference that occurs between the active region (3) and the near-surface region (4).

15. Multilayer varistor (1) according to claim 14, wherein the first ceramic material (6) is arranged in the active region (3) and wherein the second ceramic material (7) forms an insulating cover layer of the ceramic body (2).

16. Multilayer varistor (1) according to claim 14 or 15, wherein the ceramic materials (6, 7) have a varistor-forming oxide or a rare earth oxide and other oxides which improve the varistor properties.

17. Multilayer varistor (1) according to claim 16, wherein the ceramic materials (6, 7) are additionally doped with Pr, La or Y.

18. Multilayer varistor (1) according to any one of claims 14 to 17, wherein the second ceramic material (7) is doped with 1000 ppm of potassium.

19. Multilayer varistor (1) according to claim 18, wherein the second ceramic material (7) is additionally doped with 1000 ppm La.

20. Multilayer varistor (1) according to claim 18 and claim 19, wherein the lanthanum-doped second ceramic material (7) has a reduced stray capacitance compared to the second ceramic material (7) doped only with potassium.

21. Multilayer varistor (1) according to one of claims 14 to 20, wherein the ceramic body (2) has at least three ceramic materials (6, 7, 8), the third ceramic material (8) between the first ceramic material (6) and the second ceramic material (7) is arranged.

22. Multi-layer varistor (1) according to any one of claims 14 to

21, with the highest concentration of monovalent elements X+ being in the near-surface region (4) and the lowest concentration of monovalent elements X ⁺ being in the active region (3).

23. Multi-layer varistor (1) according to any one of claims 14 to

22, wherein the first ceramic material (6) has the lowest concentration of monovalent elements X ⁺ and wherein the second ceramic material (7) has the highest concentration of monovalent elements X ⁺ .

24. Multilayer varistor (1) according to claim 21 and claim

23, wherein the third ceramic material (8) has an intermediate concentration of monovalent elements X ⁺ .

25. Multi-layer varistor (1) according to any one of claims 14 to

24, wherein the ceramic materials (6, 7, 8) differ chemically from one another by <1%.

26. Multi-layer varistor (1) according to any one of claims 14 to

25, wherein the dielectric constants sr of the first and second ceramic material (6, 7) differ by a factor of 5 from one another.

27. Multilayer varistor (1) according to claim 21, wherein a dielectric constant er of the second ceramic material (7) and the third ceramic material (8) is lower than a dielectric constant sr of the first ceramic material (6).

28. Multi-layer varistor (1) according to any one of claims 14 to

27, the concentration of monovalent elements X+ in the active region (3) being <100 ppm.

29. Multi-layer varistor (1) according to any one of claims 14 to

28, the concentration of monovalent elements X+ gradually decreasing, starting from the region close to the surface (4) in the direction of the active region (3).

30. Multi-layer varistor (1) according to any one of claims 14 to

29, wherein a thickness of the second and/or the third ceramic material (7, 8) is adapted to a diffusion behavior of the monovalent element.

31. Multi-layer varistor (1) according to any one of claims 14 to

30, wherein the ceramic materials (6, 7, 8) are based on ZnO.

32. Multi-layer varistor (1) according to any one of claims 14 to

31, wherein the multilayer varistor (1) is produced by a method according to any one of claims 1 to 13.