Multilayer Varistor and Method for Manufacturing a Multilayer Varistor

Abstract

In an embodiment a method for manufacturing a multilayer varistor includes providing a first ceramic powder for producing a first ceramic material and at least one second ceramic powder for producing a second ceramic material, wherein the ceramic powders differ from each other in concentration of monovalent elements X.sup.+ by 50 ppm≤Δc(X.sup.+)≤5000 ppm, wherein X.sup.+=(Li.sup.+, Na.sup.+, K.sup.+ or Ag.sup.+), and wherein Δc denotes a maximum concentration difference occurring between an active region and a near-surface region of the multilayer varistor, slicking of the ceramic powders and forming of green films, partially printing of a part of the green films with a metal paste to form inner electrodes, stacking printed and unprinted green films, laminating, decarbonizing and sintering the green films and applying outer electrodes.

Claims

1.-32. (canceled)

33. A method for manufacturing a multilayer varistor, the method comprising: providing a first ceramic powder for producing a first ceramic material and at least one second ceramic powder for producing a second ceramic material, wherein the ceramic powders differ from each other in concentration of monovalent elements X.sup.+ by 50 ppm≤Δc(X.sup.+)≤5000 ppm, wherein X.sup.+=(Li.sup.+, Na.sup.+, K.sup.+ or Ag.sup.+), and wherein Δc denotes a maximum concentration difference occurring between an active region and a near-surface region of the multilayer varistor; slicking of the ceramic powders and forming of green films; partially printing of a part of the green films with a metal paste to form inner electrodes; stacking printed and unprinted green films; laminating, decarbonizing and sintering the green films; and applying outer electrodes.

34. The method according to claim 33, wherein partially printing comprises partially printing those green films with the metal paste which have a lower concentration of monovalent elements X.sup.+ than remaining green films.

35. The method according to claim 33, wherein the green films are stacked such that the second ceramic material forms a cover layer of the multilayer varistor.

36. The method according to claim 33, wherein the ceramic powders comprise ZnO as a main component.

37. The method according to claim 33, wherein the ceramic materials comprise a varistor forming oxide or a rare earth oxide and further oxides.

38. The method according to claim 33, wherein the ceramic materials are additionally doped with Pr, La or Y.

39. The method according to claim 33, wherein the ceramic materials differ in a potassium content and a lanthanum content in a ppm range.

40. The method according to claim 33, wherein the second ceramic material arranged in the near-surface region is doped with 1000 ppm potassium.

41. The method according to claim 40, wherein the second ceramic material is additionally doped with 1000 ppm La.

42. The method according to claim 41, wherein the lanthanum doped second ceramic material has a reduced stray capacitance compared to the second ceramic material only doped with potassium.

43. The method according to claim 33, wherein the first ceramic material has the lowest concentration of monovalent elements X.sup.+, and wherein the second ceramic material has the highest concentration of monovalent elements X.sup.+.

44. The method according to claim 33, further comprising providing a third ceramic powder for producing a third ceramic material, wherein a concentration of monovalent elements X.sup.+ in the third ceramic powder is lower than the concentration of monovalent elements X.sup.+ in the second ceramic powder but higher than the concentration of monovalent elements X.sup.+ in the first ceramic powder.

45. The method according to claim 33, wherein the green films are stacked such that the multilayer varistor has a defined concentration gradient of the monovalent elements X.sup.+, and wherein a concentration decreases starting from the second ceramic material to the first ceramic material.

46. A multilayer varistor comprising: a ceramic body having a plurality of inner electrodes, an active region and a near-surface region, at least one first ceramic material and at least one second ceramic material, wherein the ceramic materials differ from each other in a concentration of monovalent elements X.sup.+ by a maximum of 50 ppm≤Δc(X.sup.+)≤5000 ppm, wherein X.sup.+=(Li.sup.+, Na.sup.+, K.sup.+ or Ag.sup.+), and wherein Δc denotes a maximum concentration difference occurring between the active region and the near-surface region.

47. The multilayer varistor according to claim 46, wherein the first ceramic material is arranged in the active region, and wherein the second ceramic material forms an insulating cover layer of the ceramic body.

48. The multilayer varistor according to claim 46, wherein the ceramic materials comprise a varistor forming oxide or a rare earth oxide and further oxides.

49. The multilayer varistor according to claim 48, wherein the ceramic materials are additionally doped with Pr, La or Y.

50. The multilayer varistor according to claim 46, wherein the second ceramic material is doped with 1000 ppm potassium.

51. The multilayer varistor according to claim 50, wherein the second ceramic material is additionally doped with 1000 ppm La.

52. The multilayer varistor according to claim 51, wherein the lanthanum doped second ceramic material has a reduced stray capacitance compared to the second ceramic material only doped with potassium.

53. The multilayer varistor according to claim 46, wherein the ceramic body comprises at least three ceramic materials, and wherein a third ceramic material is arranged between the first ceramic material and the second ceramic material.

54. The multilayer varistor according to claim 53, wherein the third ceramic material has an medium concentration of monovalent elements X.sup.+.

55. The multilayer varistor according to claim 53, wherein a relative permittivity εr of the second ceramic material and a relative permittivity εr of the third ceramic material is lower than a relative permittivity εr of the first ceramic material.

56. The multilayer varistor according to claim 46, wherein the highest concentration of monovalent elements X.sup.+ is present in the near-surface region, and wherein the lowest concentration of monovalent elements X.sup.+ is present in the active region.

57. The multilayer varistor according to claim 46, wherein the first ceramic material has the lowest concentration of monovalent elements X.sup.+, and wherein the second ceramic material has the highest concentration of monovalent elements X.sup.+.

58. The multilayer varistor according to claim 46, wherein the ceramic materials differ chemically from each other by ≤1%.

59. The multilayer varistor according to claim 46, wherein relative permittivities εr of the first and second ceramic materials differ from each other by ≥ a factor 5.

60. The multilayer varistor according to claim 46, wherein the concentration of monovalent elements X.sup.+ in the active region is <100 ppm.

61. The multilayer varistor according to claim 46, wherein the concentration of monovalent elements X.sup.+ decreases gradually starting from the near-surface region towards the active region.

62. The multilayer varistor according to claim 46, wherein a thickness of the second ceramic material and/or a third ceramic material is/are adapted to a diffusion behavior of the monovalent elements.

63. The multilayer varistor according to claim 46, wherein the ceramic materials are based on ZnO.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0054] The drawings described below are not to be taken as true to scale. Rather, individual dimensions may be enlarged, reduced or even distorted for better representation.

[0055] Elements which resemble each other or which perform the same function are designated with the same reference signs.

[0056] FIG. 1 shows a sectional view of a multilayer varistor according to a first embodiment

[0057] FIG. 2 shows a sectional view of a multilayer varistor according to a further embodiment; and

[0058] FIG. 3 shows a sectional view of a multilayer varistor according to a third embodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0059] FIG. 1 shows a first embodiment of a multilayer varistor 1. The multilayer varistor 1 has a ceramic body 2. A plurality of inner electrodes 5 are formed in the ceramic body 2. In FIG. 1, only two inner electrodes 5 are shown. Of course, the multilayer varistor 1 can have more than two inner electrodes 5. The inner electrodes 5 comprise silver, palladium, platinum or an alloy of these metals.

[0060] In this embodiment, the inner electrodes 5 are arranged alternately and overlap in an inner region of the multilayer varistor 1. The overlapping region forms an active region 3 of the multilayer varistor 1.

[0061] The multilayer varistor 1 further has a near-surface region 4. The near-surface region 4 exhibits only minimal electrical conductivity. The near-surface region 4 adjoins a top side 1a and a bottom side 1b of the multilayer varistor 1, as can be seen from FIG. 1. The near-surface region 4 comprises a cover layer or an insulation region of the multilayer varistor 1.

[0062] In this embodiment, the multilayer varistor 1 further has two outer electrodes 9. However, the multilayer varistor 1 may have more than two outer electrodes 9. The outer electrodes 9 are electrically connected to the inner electrodes 5 for electrically contacting the multilayer varistor 1. The outer electrodes 9 are formed on side surfaces of the multilayer varistor 1. Furthermore, the outer electrodes 9 are also formed on portions of the bottom side 1b and the top side 1a of the multilayer varistor 1.

[0063] According to the embodiment shown, the outer electrodes are formed in a single layer.

[0064] Alternatively, the outer electrodes 9 may also have a multilayer structure (not explicitly shown). Preferably, in this case, the respective outer electrode 9 has a first or inner layer for contacting the inner electrodes 9. The first layer preferably has 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 that enables soldering of the multilayer varistor 1 to circuit boards. The third layer preferably has tin. In this embodiment, the varistor 1 must be provided with a protective layer (preferably glass) before electroplating. In particular, in this case, a further protective layer (electroplating protection, for example glass) is applied (not explicitly shown) to the upper side is and the lower side 1b (i.e. 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.

[0065] In the embodiment according to FIG. 1, the ceramic body 2 comprises two ceramic materials or varistor ceramics 6, 7.

[0066] A first or primary ceramic material 6 is formed in an inner region of the multilayer varistor 1. In particular, the active region 3 has the first ceramic material 6. A second or modified ceramic material 7 is formed in an edge region of the multilayer varistor 1. In particular, the second ceramic material is arranged in the near-surface region 4 and thus substantially in the inactive region. However, in addition to the second ceramic material 7, the inactive region also comprises a portion of the first ceramic material 6, as shown in FIG. 1.

[0067] The ceramic materials 6, 7 comprise ZnO. In particular, ZnO is the main component of the ceramic materials 6, 7. Furthermore, the ceramic materials 6, 7 may comprise a varistor forming oxide such as bismuth oxide or a rare earth oxide (e.g. praseodymoxide) as well as other oxides which improve the varistor properties.

[0068] The ceramic materials 6, 7 are chemically approximately identical. In particular, the ceramic materials 6, 7 chemically match ≥99%. However, the ceramic materials 6, 7 have different dielectric constant εo*εr or relative permittivity εr. In particular, the dielectric constants εo*εr or relative permittivity εr of the ceramic materials 6, 7 differ from each other by a factor≥5. Here, 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.

[0069] This is achieved by the fact that the ceramic materials 6, 7 differ from each other in the concentration of monovalent elements X.sup.+ (X.sup.+ stands for Li.sup.+, Na.sup.+, K.sup.+ or Ag.sup.+).

[0070] For example, the ceramic materials differ from each other by a maximum of 50 ppm<Δc (X.sup.+)<5000 ppm. Here, 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.

[0071] The monovalent elements Li.sup.+, Na.sup.+, K.sup.+, Ag.sup.+ act as “acceptor doping” in the semiconducting ZnO. Therefore, the above doping can be applied to all ZnO based varistor ceramics (regardless of the formulation).

[0072] 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.

[0073] It is advantageous if the concentration X.sup.+ in the active region 3 (concentration of monovalent elements in the first ceramic material 6) is at a low level (X.sup.+<100 ppm). In other words, in the active region 3 the concentration of monovalent elements X.sup.+ is much lower than in the inactive region or near-surface region 4.

[0074] A low concentration of monovalent elements X.sup.+ is associated with a large (or greater) dielectric constant. Consequently, the active region 3 has a higher dielectric constant/relative permittivity than the near-surface region 4. An increase in the concentration of monovalent elements X.sup.+ causes a decrease in the dielectric constant. Overall, a significant decrease in the dielectric constant is obtained even with small amounts of monovalent elements added.

[0075] In summary, the two ceramic materials 6, 7 are combined in such a way that the highest concentration of monovalent elements X.sup.+ is present in the near-surface region 4 and the lowest concentration in the active region 3. The second ceramic material 7 thus serves as an insulating cover layer with acceptor doping and low relative permittivity. Starting from the near-surface region 4, the concentration gradually decreases toward the active region 3 (concentration gradient). This significantly reduces the parasitic capacitance/stray capacitance of the multilayer varistor 1.

[0076] Since the ceramic materials 6, 7 are chemically nearly identical, no mechanical (cracks, bending) and chemical (reaction, diffusion zones) problems occur during sintering of the ceramic.

[0077] FIG. 2 shows a second embodiment of a multilayer varistor 1. With regard to the design and arrangement of the inner electrodes 5 and outer electrodes 9, reference is made to the description in connection with FIG. 1.

[0078] In contrast to the multilayer varistor shown in FIG. 1, the multilayer varistor in this embodiment has three ceramic materials/varistor ceramics 6, 7, 8 with different concentrations of monovalent elements X.sup.+. The first or primary ceramic material 6 is arranged in the active region 3—as already described in connection with FIG. 1. The second and third ceramic materials (modified ceramic materials) 7, 8 are arranged in the near-surface region 4 the surface. The third ceramic material 8 is arranged between the first and second ceramic materials 6, 7.

[0079] The first ceramic material 6 has a low concentration of monovalent elements. Thus, the first ceramic material 6 exhibits a high dielectric constant. The second ceramic material 7 exhibits a higher concentration of monovalent elements than the first ceramic material 6. The concentration of monovalent elements in the third ceramic material 8 is intermediate between that of the first ceramic material 6 and the second ceramic material 7. In particular, the first ceramic material 6 exhibits the lowest concentration of monovalent elements and the second ceramic material 7 exhibits the highest concentration of monovalent elements. The third ceramic material 8 has a medium concentration. Thus, a concentration gradient is created.

[0080] The concentration of the 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 insulation zone with acceptor doping and low relative permittivity.

[0081] FIG. 3 shows a third embodiment of a multilayer varistor 1. With respect to the design and arrangement of the outer electrodes 9, reference is made to the description in connection with FIG. 1. In contrast to the embodiments shown in FIGS. 1 and 2, the inner electrodes 5 in this embodiment are arranged in a tip-to-tip position. The region between the tips of the inner electrodes 5 forms the active region 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.

[0082] Analogous to the multilayer varistor described in connection with FIG. 2, the multilayer varistor 1 in this embodiment example has three ceramic materials 6, 7, 8 with different concentrations of monovalent elements X.sup.+.

[0083] 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. The thicknesses of the cover layers (second and third ceramic materials 7, 8) are selected so that as little diffusion as possible of the acceptors into the active region 3 occurs. The thickness of the cover layers is understood to be a respective extension of the second ceramic material 7 and the third ceramic material 8 perpendicular to a main extension of the multilayer varistor 1.

[0084] Overall, the concentration of the 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 an insulating cover layer with acceptor doping and low relative permittivity. With regard to the further features of the ceramic materials 6, 7, 8, reference is made to the description of FIG. 2.

[0085] The particular advantage is that the electrical properties of the modified varistor ceramics 7, 8 (second and third ceramic materials 7, 8) are very different from those of the original varistor ceramics (first or primary ceramic material 6) without any significant chemical differences between the materials. Therefore, the materials are otherwise nearly identical and can be processed without problems.

[0086] In the following, a method for manufacturing a multilayer varistor 1, in particular a multilayer varistor according to one of the above embodiments, is described. The method comprises the following steps:

[0087] A) In a first step, ceramic powders are provided from individual components. In this process, a first ceramic powder is provided for forming the first ceramic material (primary ceramic material) 6. A second ceramic powder is further provided to form the second ceramic material (modified ceramic material) 7. In one embodiment, a third ceramic powder may also be provided to form the third ceramic material (modified ceramic material) 8 (see FIGS. 2 and 3). The ceramic powders are chemically 99% identical. The ceramic powders essentially have ZnO as the base material.

[0088] Table 1 shows a possible composition of the base material of the ceramic powders. Of course, other compositions are also conceivable, with ZnO being the main constituent of the ceramic material in each case.

TABLE-US-00001 TABLE 1 Composition of the base material of the ceramic powders. Amount [mol Element] Main constituent Zn (ZnO)  94.0% Doping element [-oxide] Al (Al.sub.2O.sub.3)  400 ppm Ca (CaO)  150 ppm Co (Co.sub.3O.sub.4)  3.50% Cr (Cr.sub.2O.sub.3) 1000 ppm K (K.sub.2O) <100 ppm *) Pr (Pr.sub.6O.sub.11) 4900 ppm Y (Y.sub.2O.sub.3) 1.825% *) Cross impurities and input by process: typically 1-10 ppm potassium.

[0089] However, the ceramic powders differ in the concentration of monovalent elements X.sup.+. In particular, the ceramic powders differ in the concentration X.sup.+ by 50 ppm≤Δc(X.sup.+)≤5000 ppm.

[0090] In this case, the first or primary ceramic powder has the lowest concentration of acceptors/monovalent elements. Preferably, the concentration of monovalent elements X.sup.+ 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 an intermediate/medium concentration of acceptors/monovalent elements.

[0091] 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 binder and dispersant and then drawn into green films. The films are cut to size.

[0092] In a further step C), a part of the green films is partially printed with a metal paste (preferably silver and/or palladium) to form the inner electrodes 5. Only those green films are partially printed with the metal paste which are later arranged in the active region 3. In other words, only the green films made of the first ceramic powder are printed with the metal paste.

[0093] Optionally, another metal paste (preferably silver and/or palladium) can be printed on a part of the green films to form protective electrodes 10 (see FIG. 3). Preferably, this metal paste is printed on the green films with the lowest and/or the medium concentration of monovalent elements (FIG. 3).

[0094] In a further step D), printed and unprinted green films are stacked. Stacking is carried out in such a way that the final multilayer varistor 1 has a defined concentration gradient of monovalent elements X.sup.+, with the concentration decreasing from the second ceramic material 7 via the third ceramic material 8 (FIGS. 2 and 3) to the first ceramic material 6.

[0095] In a further step, the green films are laminated, decarburized and sintered. The sintering temperature is preferably 1100° C.

[0096] In a final step, external electrodes 9 are applied.

[0097] The method produces a multilayer varistor 1 which has a very low stray capacitance and thus a low capacitance.

[0098] An advantage is that the manufacturing process 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, 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 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, only have a small concentration difference (concentration gradient), diffusion of the same into the active volume even during sintering can be neglected. Therefore, the cover layers can be dimensioned with sufficiently high thicknesses, which enhances the shielding effect.

[0099] To characterize the cover layers, modifications (variations with modified doping according to Table 2 below) were produced in a previous test procedure starting from the base material (see Table 1) and their relative permittivity determined. The powder mixtures were ground, evaporated and decarburized. The decarburized powders were granulated with organic binder and pressed into disks (15 mm diameter, 1 mm height). The discs were sintered and ground to 0.3 mm height. Finally, the disks were circularly (5 mm diameter) imprinted on both sides with silver paste, and baked.

[0100] The capacitances of the disks were measured at 1V and 1 kHz (see Table 2). Using the formula for the capacitance of the plate capacitor, the dielectric constant or relative permittivity of the ceramic could be determined: εr=(C*d)/(A*εo).

TABLE-US-00002 TABLE 2 Results of the base material and the modified varistor ceramics. Addition of Sintering Relative Composition X.sup.+ (X = potassium) temperature permittivity Base material Without addition 1100° C. 80 (=reference) Base material 100 ppm K 1100° C. 73 Base material 1000 ppm K 1100° C. 54 Base material + 1000 ppm K 1100° C. 10 1000 ppm La Base material 5000 ppm K 1100° C. 9.4

[0101] The characterization test procedure provided possible compositions with reduced relative permittivity suitable for testing the invention on the multilayer varistor.

[0102] Finally, the following is a brief summary of the testing of the invention.

[0103] Three ceramic powders were prepared, which differed only in potassium and lanthanum content in the ppm range (see Table 2). The main constituent of all powders was zinc oxide (see Table 1).

[0104] The composition of the first ceramic powder corresponded to that of 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.

[0105] The powder mixtures thus prepared were milled, spray-dried and decarburized. The decarburized powders were slurried with organic binder and dispersant and drawn into films. The films were cut to size, printed with palladium paste, stacked, and cut into multilayer components.

[0106] The simplest design (see FIG. 1) of a 1206 ML varistor with 2 inner electrodes (120 micrometer electrode gap and 0.8 mm.sup.2 overlap area) was chosen for testing. With the three types of ceramic foils, three types of components were produced.

[0107] The first type of components consisted of the base material throughout (=the reference type). The second type of components consisted in the core of the base material with a cover layer of the second ceramic (with increased potassium concentration). The third type of component consisted of a core of the base material with a cover layer of the third ceramic (with increased potassium concentration and lanthanum-doped).

[0108] The components produced in this way were sintered at 1100° C. in each case. Micrographs showed that the cover layers were flawlessly (no cracks, etc.) sintered with the core layer. Finally, the components were metallized with outer electrodes of a layer of silver and baked.

[0109] The capacitances of the components were measured at 1 V and 1 MHz. The first type of components (reference type) had a capacitance of 17.7±3.1 pF. The second type of components (cover layer with increased potassium concentration) showed a capacitance of 13.2±1.3 pF. This corresponds to a 25% reduction in capacitance. The third type of components (cover layer with increased potassium concentration and lanthanum doped) showed a capacitance of 11.1±2.4 pF. This corresponds to a reduction in capacitance of 37%. It could thus be shown that even the simplest way of applying embodiments of the invention leads to a significant reduction in the total capacitance of the multilayer varistor.

[0110] The current/voltage characteristics of the components were measured with increasing static current levels in the range of 10 nA to 1 mA. The first type of components (reference type) showed a varistor voltage at 1 mA of 2159±144 V mm.sup.−1. The second type of components showed a varistor voltage at 1 mA of 2210±172 V mm.sup.−1. This corresponds to a change in varistor voltage of only 2%. The third type of components showed a varistor voltage at 1 mA of 2273±183 V mm.sup.−1. This corresponds to a change in varistor voltage of 5%.

[0111] Thus, it can be seen that the varistor voltage (U.sub.v@1 mA) is hardly affected by the application of the cover layers/modified varistor ceramics. It can be concluded that the active volume of the varistor was not affected or even damaged by the cover layers.

[0112] The description of the objects disclosed herein is not limited to the individual specific embodiments. Rather, the features of the individual embodiments may be combined with one another in any desired manner—to the extent that this is technically feasible.