Multi-layer varistor and method for manufacturing a multi-layer varistor

Description

[0001] The present invention relates to a multilayer varistor. The present invention further relates to a method for manufacturing a multilayer varistor.

[0002] Multilayer varistors are used as effective protective elements against temporary overvoltages (such as ESD - Electrostatic Discharge). In rapidly developing communication technology, there is an increasing need for protective elements to safeguard sensitive electronics. Due to the high frequencies involved in signal transmission and the fact that these protective elements are integrated directly into the lines, their capacitance must be kept as low as possible. Otherwise, interference and signal loss will occur within the signal lines.

[0003] Reducing the capacitance of a multilayer varistor presents considerable difficulties. While the active area (overlap area) and thus the capacitance can be reduced through design modifications, this proportionally reduces the leakage current and the protective effect. From a material perspective, 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 the barrier layers between the ZnO grains. The series and parallel connection of the individual barrier layers determines the capacitance—but also the breakdown voltage of the active region. Since the breakdown voltage is predetermined in the component design, it also determines the capacitance of the active region. The Dk of the ZnO ceramic is largely coupled to the breakdown voltage and therefore cannot be used as a degree of freedom to reduce the capacitance.

[0004] The capacitance of a varistor is determined not only by the capacitance of the active volume (the ceramic between the inner electrodes) but also by the stray capacitance of the ceramic component outside the active volume (surface layers and insulation zone). As the active area of ​​the component decreases, the proportion of stray capacitance to the total capacitance increases, thus limiting the effect achievable with a design featuring minimal electrode overlap. Therefore, to efficiently reduce the capacitance of a varistor, it is necessary to minimize this stray capacitance as much as possible.

[0005] Various methods are known for reducing conductivity and stray capacitance in the area outside the inner electrodes, but these are either ineffective or have other disadvantages. The simplest way to achieve this is to vitrify the surface of the multilayer varistor after sintering. This glass layer has the additional advantage of chemically insulating the ceramic, thus increasing the component's durability. Therefore, the additional use of this method can be beneficial even when other methods are employed. However, since the glass layer is very thin, its effectiveness is limited, and the use of other methods or a combination of methods is advantageous.

[0006] In document DE 100 26 258 B4, a bismuth-containing topcoat is applied as electroplating protection instead of a protective glass layer; this topcoat can be sintered together with the varistor ceramic. The chemical composition of the topcoat differs significantly from that of the ZnO ceramic, resulting in a detrimental diffusion and reaction zone during sintering. The influence of the topcoat on the dielectric constant is not discussed.

[0007] Document JP 3735151 B2 describes a process in which the outermost regions of the ceramic are chemically modified after sintering. During an additional heat treatment, lithium or sodium diffuses 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 thus be significantly reduced. The disadvantage of this method is that this subsequent modification is quite complex. Furthermore, another heat treatment would be required to apply an additional glass layer to the outside, which is extremely difficult due to the high diffusion rates of sodium and lithium.

[0008] Document JP H-113809 A describes a multilayer varistor consisting of an insulating substrate with a low dielectric constant, onto which the actual varistor ceramic is laminated. The substrate itself is also produced layer by layer from a ceramic with a low dielectric constant. A disadvantage of this multilayer varistor is its complex manufacturing process: instead of a single ceramic, two different ceramics with significantly different properties are required. This can be achieved through different chemical compositions, which results in a weak bond between the substrate and the varistor ceramic.

[0009] Document DE 10 2018 116 221 A1 describes a multilayer varistor consisting of two chemically very different materials that differ in their ZnO grain size after the sintering process. The aim of this multilayer varistor design is to keep the current flow within the component away from thermomechanical weak points and thus increase the impulse resistance of the protective element. The effect of the chemically very different materials, which are both used in the active area, on the capacitance of the multilayer varistor is not the focus of this document.

[0010] Document DE 10 2017 105 673 A1 describes the combination of two different ZnO ceramics to increase the impulse withstand capability of the device. The two materials must be bonded to the electrodes to demonstrate the effect. The effects in near-surface areas and the impact on capacitance are not addressed.

[0011] Document DE 103 50 343 Al describes a multilayer chip varistor with a stack of resistive layers and inner electrode layers. The ion intensity ratio of the alkali metal A and zinc (Zn) (A / Zn) is 0.001 < (A / Zn) < 500 in a region from the surface of the element body to a depth of (0.9 x I).

[0012] Document EP 3 300 087 Al describes a multilayer varistor (MLV) with a ceramic body and internal electrodes.

[0013] In the production of the varistor, a step of high-temperature diffusion of low-valent alkali metal ions onto the MLV sintered body takes place by immersing the MLV sintered body in an alkali metal ion solution with a concentration of 5-80%, preferably 40-80%.

[0014] Document US 2014 / 167909 Al describes a chip varistor with a varistor body that has a first element body section exhibiting a non-linear voltage-current characteristic and a second element body section in which an electric current is more likely to flow than in the first element body. The increased resistance region is achieved primarily through the diffusion of alkali metals.

[0015] This means that the known methods have clear disadvantages, or their effectiveness in reducing scattering capacity is not given.

[0016] The object of the present invention is to describe a multilayer varistor and a method for manufacturing a multilayer varistor which solve the problems stated above.

[0017] This problem is solved by a multilayer varistor and a method for manufacturing a multilayer varistor according to the independent claims.

[0018] One aspect is described as a multilayer varistor. The multilayer varistor has a ceramic body. The ceramic body has multiple layers. A large number of internal electrodes are formed within the ceramic body. The internal electrodes contain, for example, silver, palladium, platinum, or an alloy of these metals.

[0019] The ceramic body has an active region. The ceramic body also has an inactive region. The active region refers to the areas between the different internal electrodes of differing polarity, which are crucial for the current flow between them. In contrast, the areas in the ceramic body of the multilayer varistor that do not (or do not significantly) contribute to the current flow between the differently contacted internal electrodes are referred to as the inactive region.

[0020] The ceramic body has a near-surface region. This near-surface region borders both a top and a bottom surface of the multilayer varistor. The near-surface region exhibits only minimal electrical conductivity and is essentially electrically insulating. It comprises a cover layer and / or an insulating zone of the multilayer varistor.

[0021] The ceramic body comprises at least one primary ceramic material. Preferably, the multilayer varistor comprises exactly one primary ceramic material. The ceramic body comprises at least one second modified ceramic material. The main component of both ceramic materials is zinc oxide (ZnO). In particular, both ceramic materials are based on ZnO.

[0022] The first and second ceramic materials differ in the concentration of monovalent elements X+ and elements with a stable oxidation state of +I, respectively. X+ is selected from Li+, Na+, K+, or Ag+. Preferably, the monovalent elements have a low diffusion coefficient. The multilayer varistor is preferably fabricated by a process that will be described in detail later.

[0023] The second, or modified, ceramic material is doped with monovalent elements. For example, the second ceramic material is doped with potassium oxide. The first, or primary, ceramic material can be free of monovalent doping. Alternatively, the first ceramic material can also be slightly doped with monovalent elements.

[0024] The dopants that differentiate the ceramic materials are present in low concentrations. While the electrical properties of the second / modified ceramic material differ significantly from those of the first / primary ceramic material due to doping with monovalent elements, there is no significant chemical difference between the two materials. In particular, the two materials are otherwise nearly identical.

[0025] Doping with monovalent elements, even in small amounts, significantly reduces the dielectric constant. Consequently, the second, or modified, ceramic material has a lower dielectric constant than the first, or primary, ceramic material. This allows for the creation of a multilayer varistor with reduced stray capacitance and, consequently, a reduced overall capacitance.

[0026] According to one embodiment, the highest concentration of monovalent elements X+ is found in the near-surface region. The lowest concentration of monovalent elements X+ is found in the active region. Consequently, the concentration of monovalent elements decreases from the surface towards the interior / active region of the multilayer varistor. Accordingly, the dielectric constant increases from the surface towards the interior of the multilayer varistor. This reduces the stray capacitance of the varistor. The overall capacitance of the varistor is therefore effectively reduced.

[0027] According to one embodiment, the ceramic materials differ chemically by ≤ 1%. In other words, the ceramic materials are chemically nearly identical. Therefore, both materials can be processed together exceptionally well. For example, the layers of the modified materials can be sintered together without defects. This results in a particularly reliable multilayer varistor.

[0028] According to one embodiment, the dielectric constants εr of the first and second ceramic materials differ by a factor of ≥ 5. Therefore, the stray capacitance of the varistor can be significantly reduced in a simple way by only a small amount of monovalent doping.

[0029] According to one embodiment, the first / primary ceramic material is arranged in the active region. 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 bottom surfaces of the multilayer varistor. Consequently, the multilayer varistor has an insulating cover layer or cladding with a low dielectric constant. The stray capacitance of the multilayer varistor is thus significantly reduced in a simple manner compared to conventional multilayer varistors.

[0030] According to one embodiment, the ceramic materials differ in the concentration of monovalent elements X+ by a maximum of 50 ppm ≤ Δc(X+) ≤ 5000 ppm. Δc denotes the maximum concentration difference that occurs between the active region and the near-surface region.

[0031] In other words, the concentration of acceptors in the second ceramic material is at most 50 ppm to 5000 ppm higher than in the first ceramic material. Preferably, the ceramic materials of the multilayer varistor differ from each other by 100 ppm ≤ Δc(X +< ) ≤ 1000 ppm.

[0032] The concentration of monovalent elements X+< in the active region is preferably < 100 ppm, preferably < 50 ppm. The first ceramic material is therefore almost free of monovalent elements. The proportion of monovalent elements is primarily due to their diffusion from the second ceramic material during the fabrication of the multilayer varistor.

[0033] Since the monovalent elements in which the two ceramic materials differ exhibit only a small concentration difference (concentration gradient), their diffusion into the active region can be neglected, even during sintering. Therefore, the cover layers (second or modified ceramic material) can be dimensioned with sufficiently large thicknesses, thereby enhancing the shielding effect.

[0034] According to the invention, the ceramic body comprises at least three ceramic materials. In particular, the ceramic body comprises the first / primary ceramic material, the second / modified ceramic material, and a third / modified ceramic material. However, the ceramic body can also comprise more than three ceramic materials. For example, the ceramic body can also comprise a fourth or modified ceramic material.

[0035] The third ceramic material is positioned between the first and second ceramic materials. It is located in the inactive region, specifically near the surface of the multilayer varistor. This third ceramic material forms a near-surface insulation zone. The three ceramic materials differ chemically by ≤ 1%.

[0036] The three ceramic materials differ in their concentration of monovalent elements. The first ceramic material (active area) has the lowest concentration of monovalent elements. The second ceramic material (outer insulating layer) has the highest concentration of monovalent elements. The third material (near-surface insulation zone) has a concentration of monovalent elements that lies between that of the first and second ceramic materials.

[0037] In particular, the concentration of monovalent elements X +< decreases gradually from the near-surface region towards the active region (concentration gradient). This effectively reduces local chemical differences.

[0038] According to one embodiment, the thickness of the second and / or third ceramic material is adapted to the diffusion behavior of the monovalent element. In particular, the thickness is chosen such that as little diffusion as possible occurs from the acceptors into the active region. The thickness of the cover layers is thus adapted to the diffusion coefficient of the monovalent element. Specifically, the thickness decreases with increasing diffusion coefficient. Due to the reduced diffusion, a defined concentration gradient of monovalent elements arises, and consequently, a defined gradient of the electrical properties, especially the dielectric constant.

[0039] The thickness of the second and third ceramic materials is determined by the overall height of the component and its internal structure. The design principle is that the effectiveness increases with the higher the proportion of the second and third ceramic materials in the inactive surface layers. However, this also increases the risk of the single-valent element diffusing into the active area during sintering. For example, a safety margin of 100 µm may be advisable. In other words, after the last printed laminate, another 100 µm of the first ceramic material remains as a "diffusion buffer." However, a smaller safety margin is also conceivable. Alternatively, the second and third ceramic materials can be applied directly after the last printed layer.

[0040] According to another aspect, a method for producing a multilayer varistor is described. Preferably, the multilayer varistor described above is produced by the method. All properties disclosed with respect to the multilayer varistor or the method are also disclosed with respect 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 method comprises the following steps: A) Providing a first or primary ceramic powder for the production of a first ceramic material. Providing at least one second or modified ceramic powder for the production of a second ceramic material.

[0041] The ceramic powders consist essentially of ZnO. The second ceramic powder is doped, in particular with a small amount of monovalent elements X⁺<, for example Li⁺<, Na⁺<, K⁺<, or Ag⁺<. The first ceramic powder may be free of monovalent doping or may contain a small amount of monovalent elements. 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 coefficient.

[0042] For example, doping with potassium (e.g., K₂O, KC₄H₅O₆, or K₂Co₃) can be used. The latter, in particular, is characterized by low losses during sintering due to its high melting point and high decomposition temperature. Alternatively, lithium or sodium can also be used as dopants. Sodium and lithium are hardly, if at all, susceptible to peroxide formation in air, and their melting points are very high. This helps to keep losses during sintering to a minimum.

[0043] The dopant is present only in a low concentration. The ceramic powders differ in the concentration of monovalent elements X+ by 50 ppm ≤ Δc(X+) ≤ 5000 ppm. Δc represents the maximum concentration difference that occurs between an active region and a near-surface region of the finished multilayer varistor.

[0044] In an alternative embodiment, a third ceramic powder can be provided to produce a third ceramic material. In this case, 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. The third ceramic powder therefore has an average concentration of monovalent elements. B) Sliding the ceramic powders in a solvent and drawing or forming green films. C) Partially printing some of the green films with a metal paste, for example, silver and / or palladium, to form internal electrodes. Specifically, those green films with a lower concentration of monovalent elements X +< than the other green films are partially printed with metal paste. In particular, those green films with the lowest concentration of monovalent elements are printed, i.e., the green films produced from the first ceramic powder.

[0045] Furthermore, additional green films with the lowest or medium concentration of monovalent elements can be printed with metal paste to form Faraday or protective electrodes.

[0046] D) Stacking of printed and unprinted green films. The green films are stacked in such a way that the second ceramic material forms a top layer of the multilayer varistor. If a third ceramic material is present, the green films are stacked in such a way that the green films made of the third ceramic material are positioned between the green films made of the first and the third ceramic material.

[0047] The green films are stacked in such a way as to create a defined concentration gradient of monovalent elements X +<, with the concentration decreasing from the second ceramic material (top layer) to the first ceramic material (active area). E) Laminating, decarburizing, and sintering the green foils. Preferably, the green foils are sintered at 1100°C. F) Applying external electrodes for electrical contacting the multilayer varistor. The external electrodes can be single-layer (CN type) or multi-layer. In the case of a three-layer external electrode, an additional nickel layer and a solderable tin layer would be applied during electroplating. Before electroplating, the component must be provided with a protective layer (vitrification).

[0048] The modified ceramic materials are of particular importance in this process. Since the modified ceramic materials are to be produced 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 crucial that the mechanical and thermal properties of the materials are well-matched. At the same time, the electrical properties must be adapted to the significantly different requirements.

[0049] This paper utilizes the concept of a cover layer or encapsulation with a low dielectric constant to reduce the capacitance of a multilayer varistor. Previous solutions require complex manufacturing processes and / or additional process steps, or are unsuitable for reducing stray capacitance. Diffusion of lithium into the finished sintered component presents a particular challenge. This necessitates the use of highly concentrated lithium compounds (e.g., Li₂CO₃) to achieve sufficient penetration depth; however, this also carries the risk of the lithium penetrating into the active volume and compromising the component's functionality.

[0050] If, on the other hand, a ceramic with a significantly different chemical composition is used as the cover layer, the increased manufacturing effort is compounded by the disadvantage of minimal bonding between the cover layer and the varistor ceramic. The mechanical properties (modulus of elasticity, strength, thermal expansion, etc.) differ considerably, as a sufficient difference in electrical properties is necessary. This negatively impacts the mechanical stability of the entire component.

[0051] These disadvantages are effectively circumvented by the method described above and the resulting multilayer varistor.

[0052] The drawings described below are not to be considered as being to scale. Rather, individual dimensions may be enlarged, reduced, or distorted for better illustration.

[0053] Elements that are identical or that perform the same function are designated with the same reference symbols.

[0054] They show: Figure 1 is a sectional view of a multilayer varistor according to a first embodiment, Figure 2 is a sectional view of a multilayer varistor according to a further embodiment, Figure 3 is a sectional view of a multilayer varistor according to a third embodiment.

[0055] The Figure 1 Figure 1 shows a first embodiment of a multilayer varistor 1. The multilayer varistor 1 has a ceramic body 2. A plurality of internal electrodes 5 are formed in the ceramic body 2. Figure 1 Only two internal electrodes 5 are shown. Of course, the multilayer varistor 1 can have more than two internal electrodes 5. The internal electrodes 5 are made of silver, palladium, platinum, or an alloy of these metals.

[0056] In this embodiment, the inner electrodes 5 are arranged alternately and overlap in an inner region of the multilayer varistor 1. The overlap region forms an active region 3 of the multilayer varistor 1.

[0057] The multilayer varistor 1 further comprises a near-surface region 4. The near-surface region 4 exhibits only minimal electrical conductivity. The near-surface region 4 borders a top surface 1a and a bottom surface 1b of the multilayer varistor 1, as shown in the Figure 1 can be extracted. The near-surface area 4 has a cover layer or an insulating area of ​​the multilayer varistor 1.

[0058] In this embodiment, the multilayer varistor 1 further comprises two external electrodes 9. However, the multilayer varistor 1 can also have more than two external electrodes 9. The external electrodes 9 are electrically connected to the internal electrodes 5 for electrical contact between the multilayer varistor 1 and the internal electrodes 5. The external electrodes 9 are formed on the side surfaces of the multilayer varistor 1. Furthermore, the external electrodes 9 are also formed on parts of the underside 1b and the top surface 1a of the multilayer varistor 1.

[0059] According to the illustrated embodiment, the outer electrodes are constructed in a single layer.

[0060] Alternatively, the outer electrodes 9 can also be multilayered (not explicitly shown). Preferably, in this case, each outer electrode 9 has a first or inner layer for contacting the inner electrodes 9. The first layer preferably consists of silver. Each outer electrode 9 has a second or middle layer as a diffusion barrier. The second layer preferably consists of nickel. Each outer electrode 9 has a third or outer layer that enables the multilayer varistor 1 to be soldered onto circuit boards. The third layer preferably consists of tin. In this embodiment, the varistor 1 must be provided with a protective layer (preferably glass) before electroplating.In this case, a further protective layer (galvanic coating, for example glass) is applied to the top surface 1a and the bottom surface 1b (i.e., over the second ceramic material 7 described below) (not explicitly shown). This glass layer chemically insulates the ceramic body 2 and thus increases the durability of the varistor 1.

[0061] The ceramic body 2 has, in the embodiment shown, Figure 1 two ceramic materials or varistor ceramics 6, 7.

[0062] A first or primary ceramic material 6 is formed in an inner region of the multilayer varistor 1. In particular, the active region 3 contains the first ceramic material 6. A second or modified ceramic material 7 is formed in a marginal region of the multilayer varistor 1. In particular, the second ceramic material is located in the near-surface region 4 and thus essentially in the inactive region. In addition to the second ceramic material 7, the inactive region also contains a portion of the first ceramic material 6, as shown in the figure. Figure 1 as is evident.

[0063] 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), as well as other oxides that improve the varistor properties.

[0064] The ceramic materials 6 and 7 are chemically nearly identical. In particular, they are chemically identical to ≥ 99%. However, the ceramic materials 6 and 7 have different dielectric constants ε₀ * ε₀r and dielectric constants ε₀r. Specifically, the dielectric constants ε₀ * ε₀r and dielectric constants ε₀r of the ceramic materials 6 and 7 differ by a factor ≥ 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 near-surface region 4.

[0065] This is achieved by the ceramic materials 6, 7 differing from each other in the concentration of monovalent elements X +< (X +< stands for Li +< , Na +< , K +< or Ag +< ).

[0066] For example, the ceramic materials differ from each other by a maximum of 50 ppm < Δc (X +< ) < 5000 ppm. Δc denotes the maximum concentration difference that occurs between the active region 3 and the near-surface region 4. Preferably, the concentration of monovalent elements in the near-surface region 4 is 100 ppm to 1000 ppm higher than in the active region 3.

[0067] The monovalent elements Li+, Na+, K+, Ag+ act as "acceptor doping" in semiconducting ZnO. Therefore, the doping described above can be applied to all ZnO-based varistor ceramics (regardless of the formulation).

[0068] Overall, the ceramic materials 6, 7 must be doped with acceptors exhibiting relatively low diffusion coefficients. Furthermore, the dopants that differentiate the ceramic materials 6, 7 must be present in low concentrations.

[0069] 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 region 3 is significantly lower than in the inactive region or the near-surface region 4.

[0070] A low concentration of monovalent elements X+ is associated with a large (or higher) dielectric constant. Consequently, the active region 3 has a higher dielectric constant than the near-surface region 4. An increase in the concentration of monovalent elements X+ causes a decrease in the dielectric constant. Overall, a significant reduction in the dielectric constant is achieved even with small additional amounts of monovalent elements.

[0071] In summary, the two ceramic materials 6, 7 are combined in such a way as to achieve the highest concentration of monovalent

[0072] Elements X +< are present in the near-surface region 4 and the lowest concentration is in the active region 3. The second ceramic material 7 thus serves as an insulating cover layer with acceptor doping and a low dielectric constant. Starting from the near-surface region 4, the concentration decreases gradually towards the active region 3 (concentration gradient). This significantly reduces the parasitic capacitance / scattering capacitance of the multilayer varistor 1.

[0073] Since the ceramic materials 6, 7 are chemically almost identical, no mechanical (cracks, bending) or chemical (reaction, diffusion zones) problems occur during the sintering of the ceramic.

[0074] The Figure 2Figure 1 shows a second embodiment of a multilayer varistor 1. Regarding the design and arrangement of the inner electrodes 5 and outer electrodes 9, reference is made to the description in connection with Figure 1. Figure 1 referred.

[0075] In contrast to the one in Figure 1 The multilayer varistor shown in this embodiment comprises three ceramic materials / varistor ceramics 6, 7, 8 with different concentrations of monovalent elements X +<. The first or primary ceramic material 6 is – as already mentioned in connection with Figure 1 As described, the first ceramic material (modified ceramic materials) is located in the active region 3. The second and third ceramic materials (modified ceramic materials) 7, 8 are located in the near-surface region 4. The third ceramic material 8 is located between the first and second ceramic materials 6, 7.

[0076] The first ceramic material 6 has a low concentration of monovalent elements. Therefore, 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 lies between that 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. The third ceramic material 8 has a medium concentration. This creates a concentration gradient.

[0077] The concentration of acceptors in the second and third ceramic materials 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 or insulating zone with acceptor doping and a low dielectric constant.

[0078] The Figure 3 Figure 1 shows a third embodiment of a multilayer varistor 1. With regard to the design and arrangement of the external electrodes 9, reference is made to the description in connection with… Figure 1 referred to. In contrast to those in the Figures 1 and 2In the illustrated embodiments, the inner electrodes 5 are arranged in a tip-to-tip position. The area between the tips of the inner electrodes 5 forms the active region 3 of the multilayer varistor 1. Additionally, the multilayer varistor 1 has metallic protective or Faraday electrodes 10, which enhance the protective function of the multilayer varistor 1 against electrostatic discharges.

[0079] Analogous to the one in connection with Figure 2 The multilayer varistor described above has three ceramic materials 6, 7, 8 with different concentrations of monovalent elements X +< in this embodiment.

[0080] The Faraday electrodes 10 help to prevent diffusion between the ceramic materials 6, 7, 8. Due to the reduced diffusion, a defined concentration gradient is created, and consequently, a defined gradient of the electrical properties, especially the dielectric constant, is formed. The thicknesses of the cover layers (second and third ceramic materials 7, 8) are chosen to minimize diffusion of the acceptors into the active region 3. The thickness of the cover layers is defined as the respective extent of the second ceramic material 7 and the third ceramic material 8 perpendicular to a main extent of the multilayer varistor 1.

[0081] Overall, the concentration of acceptors in the second and third ceramic materials 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 insulating cover layers with acceptor doping and low dielectric constant. For further details regarding the design characteristics of the ceramic materials 6, 7, 8, please refer to the description in [reference to be added]. Figure 2 referred.

[0082] The particular advantage of this invention is that the electrical properties of the modified varistor ceramics 7, 8 (second and third ceramic materials 7, 8) differ significantly from those of the original varistor ceramics (first and primary ceramic materials 6) without any significant chemical differences between the materials. Therefore, the materials are otherwise almost identical and can be processed without any problems.

[0083] The following describes a method for manufacturing a multilayer varistor 1, in particular a multilayer varistor according to one of the embodiments above. The method comprises the following steps: A) In a first step, ceramic powders are provided from individual components. A first ceramic powder is provided to form the first ceramic material (primary ceramic material) 6. A second ceramic powder is also provided to form the second ceramic material (modified ceramic material) 7. In an embodiment, a third ceramic powder can also be provided to form the third ceramic material (modified ceramic material) 8 (see Figures 2 and 3The ceramic powders are chemically ≥ 99% identical. The ceramic powders consist primarily of ZnO as their base material. Table 1 shows a possible composition of the base material of the ceramic powders. Of course, other compositions are also conceivable, in each case with ZnO being the main component of the ceramic material. Table 1: Composition of the base material of the ceramic powders. *) Cross-contamination and process input: typically 1-10 ppm potassium Main component Amount [mol of element] Zn (ZnO) 94,0 % Doping element [-oxide] Amount [mol of element] Al (Al 2 0 3 ) 400 ppm Ca (CaO) 150 ppm Co (Co 3 O 4 ) 3,50 % Cr (Cr 2 O 3 ) 1000 ppm K (K 2 O) < 100 ppm *) Pr (Pr 6 O 11 ) 4900 ppm Y (Y 2 O 3 ) 1,825 %

[0084] 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 ≤ Δc(X +< ) ≤ 5000 ppm.

[0085] The first or primary ceramic powder has the lowest concentration of acceptors / monovalent elements. Preferably, the concentration of monovalent elements X + < in the first ceramic powder is < 100 ppm. The second ceramic powder has the highest concentration of acceptors / monovalent elements. The third ceramic powder has a medium / intermediate concentration of acceptors / monovalent elements.

[0086] In a second step (B), green films are formed from the ceramic powders. For this, the powders are first ground, spray-dried, and decarburized. The decarburized powders are mixed with an organic binder and dispersant and then drawn into green films. The films are then cut to size.

[0087] In a further step C), a portion of the green foils is partially printed with a metal paste (preferably silver and / or palladium) to form the inner electrodes 5. Only those green foils that will later be located in the active area 3 are partially printed with the metal paste. In other words, only the green foils produced from the first ceramic powder are printed with the metal paste.

[0088] Optionally, a further metal paste (preferably silver and / or palladium) can be printed onto some of the green foils to form protective electrodes 10 (see Figure 3). Preferably, this metal paste is printed onto the green foils with the lowest and / or medium concentration of monovalent elements ( Figure 3 ).

[0089] In a further step D), printed and unprinted green films are stacked. The stacking is carried out in such a way that the final multilayer varistor 1 has a defined concentration gradient of monovalent elements X +<, with the concentration increasing from the second ceramic material 7 through the third ceramic material 8 ( Figures 2 and 3 ) decreases down to the first ceramic material 6.

[0090] In a further step, the green films are laminated, decarburized, and sintered. The sintering temperature is preferably 1100°C.

[0091] In a final step, external electrodes are applied.

[0092] The process produces a multilayer varistor 1 which has a very low stray capacitance and therefore a low capacitance.

[0093] One advantage of this invention is that its production is very simple. 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 in chemical composition. Therefore, the powder, slurry, and film properties of the materials are very similar and can be processed in the same way. The same applies to the processing of the films into laminates and the final assembly of the components (cutting, decarburizing, sintering). Since the elements, such as potassium, in which the materials differ, exhibit only a small concentration difference (concentration gradient), their diffusion into the active volume can be neglected, even during sintering.Therefore, the top layers can be dimensioned with sufficiently high thicknesses, which increases the shielding effect.

[0094] To characterize the surface layers, modifications (variations with altered doping according to Table 2 below) were produced in a prior test procedure based on the base material (see Table 1), and their dielectric constants were determined. The powder mixtures were milled, evaporated, and decarburized. The decarburized powders were granulated with an organic binder and pressed into discs (15 mm diameter, 1 mm height). The discs were sintered and ground to a thickness of 0.3 mm. Finally, the discs were printed on both sides with a circular pattern (5 mm diameter) of silver paste and fired.

[0095] The capacitances of the disks were measured at 1 V and 1 kHz (see Table 2). Using the formula for the capacitance of a parallel-plate capacitor, the dielectric constant or dielectric number of the ceramic could be determined: εr = (C * d) / (A * ε0). Table 2: Results of the base material and the modified varistor ceramics composition Addition of X+ (X=Potassium) Sintering temperature dielectric constant Basic material (=reference) Without addition 1100° C 80 Basic material 100 ppm K 1100° C 73 Basic material 1000 ppm K 1100° C 54 Base material + 1000 ppm La 1000 ppm K 1100° C 10 Basic material 5000 ppm K 1100° C 9, 4

[0096] The characterization test procedure provided possible compositions with reduced dielectric constants that were suitable for testing the invention on the multilayer varistor.

[0097] Finally, the testing of the invention is briefly summarized below.

[0098] Three ceramic powders were produced, differing only in their potassium and lanthanum content in the ppm range (see Table 2). The main component of all powders was zinc oxide (see Table 1).

[0099] The first ceramic powder had the same composition as the base material (see Table 1). The second ceramic powder was additionally doped with 1000 ppm potassium. The third ceramic powder was additionally doped with 1000 ppm potassium and 1000 ppm lanthanum.

[0100] The resulting powder mixtures were milled, spray-dried, and decarburized. The decarburized powders were mixed with an organic binder and dispersant and drawn into films. The films were cut to size, printed with palladium paste, stacked, and cut into multilayer components.

[0101] The simplest design was used for testing (see Figure 1 A 1206 ML varistor with two internal electrodes (120 micrometer electrode spacing and 0.8 mm² overlap area) was selected. Three types of components were produced using the three types of ceramic foils.

[0102] The first type of component consisted entirely of the base material (= the reference type). The second type of component consisted of the base material core with a top layer of the second ceramic (with an increased potassium concentration). The third type of component consisted of the base material core with a top layer of the third ceramic (with an increased potassium concentration and lanthanum-doped).

[0103] The components produced in this way were each sintered at 1100°C. Micrographs showed that the surface layers were flawlessly fused to the core layer (no cracks, etc.). Finally, the components were metallized with outer electrodes made of a single layer of silver and baked.

[0104] The capacitances of the components were measured at 1 V and 1 MHz. The first type of component (reference type) exhibited a capacitance of 17.7 ± 3.1 pF. The second type of component (surface layer with increased potassium concentration) exhibited a capacitance of 13.2 ± 1.3 pF. This corresponds to a reduction in capacitance of 25%. The third type of component (surface layer with increased potassium concentration and lanthanum-doped) exhibited a capacitance of 11.1 ± 2.4 pF. This corresponds to a reduction in capacitance of 37%. Thus, it was demonstrated that even the simplest application of the invention leads to a significant reduction in the overall capacitance of the multilayer varistor.

[0105] The current / voltage characteristic of the components was measured with increasing static currents in the range of 10 nA to 1 mA. The first type of component (reference type) exhibited a varistor voltage of 2159 ± 144 V / mm at 1 mA. The second type of component exhibited a varistor voltage of 2210 ± 172 V / mm at 1 mA. This corresponds to a change in the varistor voltage of only 2%. The third type of component exhibited a varistor voltage of 2273 ± 183 V / mm at 1 mA. This corresponds to a change in the varistor voltage of 5%.

[0106] It is evident that the varistor voltage (Uv @ 1 mA) is hardly affected by the application of the coating layers / modified varistor ceramics. From this, it can be concluded that the active volume of the varistor was neither affected nor damaged by the coating layers.

[0107] The description of the items listed here is not limited to the individual specific embodiments. Rather, the features of the individual embodiments can be combined with one another as desired – insofar as this is technically feasible. Reference symbol list

[0108] 1 Multilayer varistor 1a Top side 1b Bottom side 2 Ceramic body 3 Active area 4 Near-surface area 5 Internal electrodes 6 First ceramic material 7 Second ceramic material 8 Third ceramic material 9 Outer electrode 10 Protective electrode

Claims

1. Process for producing a multi-layer varistor (1) comprising the steps of: A) Providing a first ceramic powder for producing a first ceramic material (6) and at least a second ceramic powder for producing a second ceramic material (7), wherein the ceramic powders differ from one another in their concentration of monovalent elements X+ by 50 ppm ≤ Δc (X+) ≤ 5000 ppm, wherein X+ = (Li+, Na+, K+ or Ag+) and wherein Δc denotes the maximum concentration difference occurring between an active region (3) and a near-surface region (4) of the multi-layer varistor (1); B) Slurrying the ceramic powders and forming green films; C) Partially printing of part of the green films with a metal paste for forming internal electrodes (5); D) Stacking printed and unprinted green films; E) Laminating, decarbonizing and sintering the green films; F) Applying external electrodes (10), wherein step D) comprises stacking the green films in such a way that the second ceramic material (7) forms a top layer of the multi-layer varistor (1).

2. Process according to claim 1, wherein step C) comprises partially printing with metal paste those green films which have a lower concentration of monovalent elements X+ than the other green films.

3. Process according to either of claims 1 or 2, wherein the ceramic powders have ZnO as the primary constituent.

4. Process according to any one of the preceding claims, wherein the ceramic materials (6, 7) comprise a varistor-forming oxide or a rare earth oxide and further oxides which improve the varistor properties.

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

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

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

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

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

10. Process 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+.

11. Process according to any one of claims 1 to 10, wherein step A) comprises providing a third ceramic powder for producing a third ceramic material (8), wherein 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.

12. Process according to any one of claims 1 to 11, wherein the green films in step D) are stacked such that the multi-layer varistor (1) has a defined concentration gradient of monovalent elements X+, wherein the concentration decreases from the second ceramic material (7) to the first ceramic material (6).

13. Multi-layer varistor (1) comprising a ceramic body (2) having a multiplicity of internal electrodes (5), wherein the ceramic body (2) has an active region (3) and a near-surface region (4) and wherein the ceramic body (2) comprises at least one first ceramic material (6) and at least one second ceramic material (7), wherein the ceramic materials (6, 7) differ from one another in a concentration of monovalent elements X+ by not more than 50 ppm ≤ Δc(X+) ≤ 5000 ppm, wherein X+ = (Li+, Na+, K+ or Ag+) and wherein Δc is the maximum concentration difference occurring between the active region (3) and the near-surface region (4), wherein the ceramic body (2) comprises at least three ceramic materials (6, 7, 8) and wherein the third ceramic material (8) is arranged between the first ceramic material (6) and the second ceramic material (7), wherein the first ceramic material (6) is arranged in the active region (3) and wherein the second ceramic material (7) forms an insulating top layer of the ceramic body (2).

14. Multi-layer varistor (1) according to claim 13, wherein the ceramic materials (6, 7) comprise a varistor-forming oxide or a rare earth oxide and further oxides which improve the varistor properties.

15. Multi-layer varistor (1) according to claim 14, wherein the ceramic materials (6, 7) are additionally doped with Pr, La or Y.

16. Multi-layer varistor (1) according to any one of claims 13 to 15, wherein the second ceramic material (7) is doped with 1000 ppm of potassium.

17. Multi-layer varistor (1) according to claim 16, wherein the second ceramic material (7) is additionally doped with 1000 ppm of La.

18. Multi-layer varistor (1) according to claim 16 and claim 17, wherein the lanthanum-doped second ceramic material (7) has a reduced stray capacitance compared to the second ceramic material (7) doped only with potassium.

19. Multi-layer varistor (1) according to any one of claims 13 to 18, wherein the highest concentration of monovalent elements X+ is present in the near-surface region (4) and wherein the lowest concentration of monovalent elements X+ is present in the active region (3).

20. Multi-layer varistor (1) according to any one of claims 13 to 19, 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+.

21. Multi-layer varistor (1) according to claim 20, wherein the third ceramic material (8) has an intermediate concentration of monovalent elements X+.

22. Multi-layer varistor (1) according to any one of claims 13 to 21, wherein the ceramic materials (6, 7, 8) differ from one another chemically by ≤ 1%.

23. Multi-layer varistor (1) according to any one of claims 13 to 22, wherein the dielectric numbers εr of the first and second ceramic material (6, 7) differ from one another by ≥ factor 5.

24. Multi-layer varistor (1) according to any one of claims 13 to 23, wherein a dielectric number εr of the second ceramic material (7) and the third ceramic material (8) is lower than a dielectric number εr of the first ceramic material (6).

25. Multi-layer varistor (1) according to any one of claims 13 to 24, wherein the concentration of monovalent elements X+ in the active region (3) is < 100 ppm.

26. Multi-layer varistor (1) according to any one of claims 13 to 25, wherein the concentration of monovalent elements X+ gradually decreases from the near-surface region (4) in the direction of the active region (3).

27. Multi-layer varistor (1) according to any one of claims 13 to 26, wherein a thickness of the second and / or of the third ceramic material (7, 8) is adapted to a diffusion behaviour of the monovalent element.

28. Multi-layer varistor (1) according to any one of claims 13 to 27, wherein the ceramic materials (6, 7, 8) are based on ZnO.

29. Multi-layer varistor (1) according to any one of claims 13 to 28, wherein the multi-layer varistor (1) is produced by a process according to any one of claims 1 to 12.