Abstract
In an embodiment a multilayer varistor includes a ceramic body made from a varistor material, wherein the ceramic body includes a plurality of inner electrodes, first regions and second regions, wherein the varistor material in the first regions has a first average grain size D.sub.A, wherein the varistor material in the second regions has a second average grain size D.sub.B, and wherein D.sub.A<D.sub.B.
Claims
1. A multilayer varistor comprising: a ceramic body made from a varistor material, wherein the ceramic body comprises a plurality of inner electrodes, first regions and second regions, wherein the first regions are arranged in active zones of the varistor, and the second regions are arranged in inactive zones of the varistor, wherein the varistor material in the first regions has a first average grain size D.sub.A, wherein the varistor material in the second regions has a second average grain size D.sub.B, and wherein D.sub.A<D.sub.B.
2. The multilayer varistor according to claim 1, wherein the first regions have an average grain size D.sub.A<3 μm and the second regions have an average grain size D.sub.B>3 μm.
3. The multilayer varistor according to claim 1, wherein the first regions have an average grain size D.sub.A<0.9 μm and the second regions have an average grain size D.sub.B>0.9 μm.
4. The multilayer varistor according to claim 1, wherein each region comprises at least one partial layer or an areal region of a partial layer of the ceramic body.
5. The multilayer varistor according to claim 1, wherein the active zones are formed in the regions around ends of differently contacted first and second inner electrodes, and wherein the second regions are formed in the further active zones and the inactive zones.
6. The multilayer varistor according to claim 5, wherein a plurality of varistors in the ceramic body are in serial interconnection with one another.
7. The multilayer varistor according to claim 1, wherein ends of the differently contacted first and second inner electrodes of the multilayer varistor each frontally face each another, and wherein the first regions are formed in the active zone between the differently contacted first and second inner electrodes, and the second regions are formed in the inactive zones.
8. A module comprising: a plurality of combined multilayer varistors according to claim 1, wherein a volume region containing inner electrodes comprises the first regions and volume regions containing no inner electrodes comprise the second regions.
9. A module comprising: a plurality of combined multilayer varistors according to claim 1, wherein the ceramic body has internal contacts and external contacts configured to be connected to further components, wherein a volume region contains inner electrodes and volume regions bordering the external contacts comprises the first regions, and wherein volume regions containing no inner electrodes and do not border the external contacts comprise the second regions.
10. The module according to claim 9, wherein the first regions have an average grain size D.sub.A<3 μm and the second regions have an average grain size D.sub.B>3 μm.
11. The module according to claim 9, wherein the first regions have an average grain size D.sub.A<0.9 μm and the second regions have an average grain size D.sub.B>0.9 μm.
12. A module comprising: a plurality of combined multilayer varistors comprising a ceramic main body made from a varistor material, wherein the ceramic body comprises a plurality of inner electrodes, first regions and second regions, wherein the varistor material in the first regions has a first average grain size D.sub.A, wherein the varistor material in the second regions has a second average grain size D.sub.B, wherein D.sub.A<D.sub.B, wherein the ceramic body has internal contacts and external contacts configured to be connected to further components, wherein a volume region contains inner electrodes and volume regions bordering the external contacts comprises the first regions, and wherein volume regions containing no inner electrodes and do not border the external contacts comprise the second regions.
13. The module according to claim 12, wherein the first regions have an average grain size D.sub.A<3 μm and the second regions have an average grain size D.sub.B>3 μm.
14. The module according to claim 12, wherein the first regions have an average grain size D.sub.A<0.9 μm and the second regions have an average grain size D.sub.B>0.9 μm.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) The invention is described in more detail hereinafter with reference to exemplary embodiments and associated figures.
(2) FIG. 1 shows in a schematic cross section an embodiment of a multilayer varistor having a reduced average grain size in the active zones.
(3) FIG. 2 shows in a schematic cross section an embodiment of a multilayer varistor having a reduced average grain size in the active zones in the region around the ends of the inner electrodes.
(4) FIG. 3 shows in a schematic cross section an embodiment of a multilayer varistor having serially connected varistors and a reduced average grain size in the active zones in the region around the ends of the inner electrodes.
(5) FIG. 4 shows in a schematic cross section an embodiment of a multilayer varistor having oppositely contacted, mutually confronting ends of the inner electrodes and a reduced average grain size in the active zone between the oppositely contacted inner electrodes.
(6) FIG. 5 shows in a schematic cross section an embodiment of a multilayer varistor having a reduced average grain size in the inactive zones.
(7) FIG. 6 shows in a schematic cross section and a plan view an embodiment of a multilayer varistor module having a reduced average grain size in the volume region containing inner electrodes.
(8) FIG. 7 shows in a schematic cross section an embodiment of a multilayer varistor module having contacts for further components and having a reduced average grain size in the volume regions containing inner electrodes and in the volume regions bordering on the external contacts.
(9) Identical elements, similar elements or apparently similar elements in the figures have been given the same reference symbols. The figures and the size ratios in the figures are not to scale. The regions in FIGS. 1 to 7 shown with shading are regions with relatively small average grain size, while unshaded regions are regions with relatively greater average grain size.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
(10) FIG. 1 shows in a schematic cross section an embodiment of a multilayer varistor which comprises a ceramic body, with active zones 3 between first and second, differently contacted inner electrodes 1 and 2 comprising first regions A, and inactive zones 4 comprising second regions B. The first regions A here have an average grain size of <3 μm, and the second regions B have an average grain size of >3 μm. As a result of the reduced average grain size in the active zones it becomes possible to achieve higher threshold voltages for given volumes of the active zones. Moreover, it becomes possible to reduce the volume of the active zone for a given threshold voltage, so achieving further miniaturization of the multilayer varistor. It becomes possible, furthermore, for a given threshold voltage and given active volume, to increase the number of inner electrodes in the active volume, with consequently better diversion of occurring electrical currents. As a result, the current robustness of the multilayer varistor is improved.
(11) FIG. 2 shows in a schematic cross section an embodiment of a multilayer varistor which comprises a ceramic body, where active zones 3′ around the regions of the ends of the differently contacted first and second inner electrodes 1 and 2 comprise the first regions A, and the rest of the active zones 3, and the inactive zones 4, comprise the second regions B. The first regions A here have an average grain size of <3 μm, and the second regions (B) have an average grain size of >3 μm. As a result of the reduced grain size in the active zones 3′ around the regions of the ends of the differently contacted first and second inner electrodes 1 and 2, the current density is evened out along these electrodes and local heating thereof is prevented. As a result, consequently, of a reduced mechanical load on the ceramic body, the stability of the multilayer varistor is improved.
(12) FIG. 3 shows in a schematic cross section an embodiment of a multilayer varistor which comprises a ceramic body, comprising two serially connected varistors, where the active zones 3′ around the regions of the ends of the connecting inner electrode 12 comprise the first regions A, and the rest of the active zones 3, and the inactive zones 4, comprise the second regions B. The first regions A here have an average grain size of <3 μm, and the second regions B have an average grain size of >3 μm. As a result of the reduced grain size in the active zones 3′ around the regions of the ends of the connecting inner electrode 12, the current density in these zones is reduced and local heating of the inner electrodes is prevented. Since this results in a reduced mechanical load on the ceramic body, the stability of the multilayer varistor is improved.
(13) FIG. 4 shows in a schematic cross section an embodiment of a multilayer varistor comprising a ceramic body in which the differently contacted first and second inner electrodes 1 and 2 in a layer plane face each other frontally, wherein the active zone 3 between the differently contacted first and second inner electrodes 1 and 2 comprises the first regions A, and the inactive zones 4 comprises the second regions B. Here, the first regions A have an average grain size of <3 μm, and the second regions B have an average grain size of >3 μm. As a result of the reduced grain size in the active zone 3, the threshold voltage of the multilayer varistor is increased, and current density at the ends of the differently contacted first and second inner electrodes 1 and 2 is optimized, thereby improving the stability and the varistor properties of the multilayer varistor.
(14) FIG. 5 shows in a schematic cross section an embodiment of a multilayer varistor which comprises a ceramic body, where the active zones 3 between the differently contacted first and second inner electrodes 1 and 2 comprise the second regions B, and the inactive zones 4 comprise the first regions A. Here, the first regions A have an average grain size of <3 μm, and the second regions B have an average grain size of >3 μm. As a result of the reduced average grain size in the inactive zones 4, the electrical insulation resistance of these zones is increased.
(15) FIG. 6 shows in a plane view A and a schematic cross section B an embodiment of a multilayer varistor module which comprises a ceramic body, in which a first and a second varistor according to embodiments are combined and arranged at a defined distance d from one another. Here, the first varistor according embodiments comprises the differently contacted first and second inner electrodes 1 and 2, and the second varistor according to embodiments comprises the differently contacted third and fourth inner electrodes 6 and 7. A volume region 5 containing inner electrodes comprising the inner electrodes 1, 2, 6 and 7 comprises the first regions A, and the volume regions 8 which do not contain any inner electrodes comprise the second regions B. Here, the first regions A have an average grain size of <3 μm, and the second regions B have an average grain size of >3 μm. As a result of the reduced average grain size, especially in the regions of the distance d between inner electrodes of the first and second varistors, the insulation resistance in these regions is increased. As a result, a mutual negative influence of the first and second varistors on each other as a result of unwanted voltage breakdowns over the distance d is prevented.
(16) FIG. 7 shows in a schematic cross section A an embodiment of a multilayer varistor module which comprises a ceramic body, which combines the first and the second varistors according to embodiments, arranged at a defined distance d from one another. Moreover, the ceramic body of the multilayer varistor module comprises internal contacts 10 and external contacts 11 and 14, by which further components (not shown) can be mounted on the module. In addition, the volume region 5 containing inner electrodes, and the volume regions 9 which border on the external contacts 11 and 14, contain the first regions A with an average grain size of <3 μm. The volume regions 13 which contain no inner electrodes and do not border on the contacts 11 and 14 contain the second regions B with an average grain size of >3 μm. As a result of the average grain size in the volume regions 5 and 9, reduced relative to the volume regions 13, there is first an increase in the insulation resistance at the distance d between, for example, the second inner electrode 2 of the first varistor and the fourth inner electrode 7 of the second varistor, but also the insulation resistance between the second inner electrode 2 of the first varistor and the fourth inner electrode 7 of the second varistor and the external contacts 11 and 14 is improved. This prevents negative interaction between the varistors and, for example, a power semiconductor, such as an LED.
(17) The invention is not restricted to the exemplary embodiments by the description on the basis of said exemplary embodiments. Rather, the invention encompasses any new feature and also any combination of features, which in particular comprises any combination of features in the patent claims and any combination of features in the exemplary embodiments, even if this feature or this combination itself is not explicitly specified in the patent claims or exemplary embodiments.
