Ceramic material, varistor, and method for producing the ceramic material and the varistor

Abstract

In an embodiment a ceramic material includes ZnO as main constituent, Y as a first additive, second additives including at least one compound containing a metal element, wherein the metal element is selected from the group consisting of Bi, Cr, Co, Mn, Ni and Sb, Si.sup.4+ as a first dopant and second dopants having at least one compound containing a metal cation from Al.sup.3+, B.sup.3+, or Ba.sup.2+, wherein a corresponds to a molar proportion of Bi calculated as Bi.sub.2O.sub.3, b corresponds to a molar proportion of Y calculated as Y.sub.2O.sub.3, c corresponds to a molar proportion of Al calculated as Al.sub.2O.sub.3, d corresponds to a molar proportion of Ba calculated as BaO, e corresponds to a molar proportion of B calculated as B.sub.2O.sub.3, f corresponds to a molar proportion of Si calculated as SiO.sub.2, g corresponds to a molar proportion of Ni calculated as NiO, h corresponds to a molar proportion of Co calculated as Co.sub.3O.sub.4, i corresponds to a molar proportion of Cr calculated as Cr.sub.2O.sub.3, j corresponds to a molar proportion of Sb calculated as Sb.sub.2O.sub.3, and k corresponds to a molar proportion of Mn calculated as Mn.sub.3O.sub.4.

Claims

1. A ceramic material comprising: ZnO as main constituent; Y as a first additive; second additives comprising Cr and optionally at least one additional compound containing a metal element, wherein the metal element is selected from the group consisting of Bi, Co, Mn, Ni and Sb; Si.sup.4+ as a first dopant; and second dopants comprising Ba.sup.2+ and optionally at least one additional compound containing a metal cation from Al.sup.3+ or B.sup.3+, wherein a corresponds to a molar proportion of Bi calculated as Bi.sub.2O.sub.3, b corresponds to a molar proportion of Y calculated as Y.sub.2O.sub.3, c corresponds to a molar proportion of Al calculated as Al.sub.2O.sub.3, d corresponds to a molar proportion of Ba calculated as BaO, e corresponds to a molar proportion of B calculated as B.sub.2O.sub.3, f corresponds to a molar proportion of Si calculated as SiO.sub.2, g corresponds to a molar proportion of Ni calculated as NiO, h corresponds to a molar proportion of Co calculated as Co.sub.3O.sub.4, i corresponds to a molar proportion of Cr calculated as Cr.sub.2O.sub.3, j corresponds to a molar proportion of Sb calculated as Sb.sub.2O.sub.3, and k corresponds to a molar proportion of Mn calculated as Mn.sub.3O.sub.4, wherein the molar proportions mentioned above are based on 100 mol% of ZnO, wherein:
0.5 mol %≤b≤3.0 mol %,
0 mol %<d<0.1 mol %, and
0 mol %<f<0.1 mol %,
0.0 mol %<i≤0.3 mol %, wherein:
0.1 mol %≤a≤0.99 mol % when Bi is present,
0 mol %<c<0.1 mol % when Al.sup.3+ is present,
0 mol %<e<0.1 mol % when B.sup.3+ is present,
0.7 mol %≤g≤1.5 mol % when Ni is present,
0.3 mol %≤h≤0.8 mol % when Co is present,
1.1 mol %≤j≤1.9 mol % when Sb is present, and
0.2 mol %≤k≤0.4 mol % when Mn is present.

2. The ceramic material as claimed in claim 1, wherein at least one of the additives is selected from the group consisting of metal oxides, metal carbonates, metal acetates, metal nitrates and mixtures thereof.

3. The ceramic material as claimed in claim 1, wherein at least one of the dopants is selected from the group consisting of metal nitrides, metal nitrates, metal acetates, metal hydroxides, metal oxides and mixtures thereof.

4. The ceramic material as claimed in claim 1, wherein the ceramic material is sintered at a sintering temperature of not exceeding 1010° C.

5. A method for producing the ceramic material as claimed in claim 1, the method comprising: producing a first suspension containing a first portion of the additives and the dopants; adding ZnO to the first suspension; producing a colloidal suspension containing at least one further compound selected from the additives; mixing the first suspension and the colloidal suspension thereby forming a resulting suspension; drying the resulting suspension to form the ceramic material; and burning off volatile constituents from the ceramic material.

6. The method as claimed in claim 5, wherein producing the colloidal suspension comprises converting an initial charge of the at least one further compound selected from the additives in form of a solution to the colloidal suspension by precipitation by a precipitant.

7. The method as claimed in claim 5, further comprising: producing pellets from the burnt ceramic material; pressing the pellets; burning off organic binders; sintering the pressed pellets to provide a ceramic body; and surface grinding the sintered ceramic body.

8. The method as claimed in claim 7, wherein producing the pellets comprises: forming a ceramic mass by blending the burnt ceramic material; drying the ceramic mass; and sieving the dried ceramic mass.

9. The method as claimed in claim 5, further comprising: processing the burnt ceramic material to provide a ceramic film; printing the ceramic film with first and second inner electrodes; stacking a plurality of ceramic films; pressing the stacked ceramic films; stamping a ceramic component out of the pressed and stacked ceramic films; debindering the ceramic component; sintering the ceramic component at not more than 1010° C.; and applying outer electrodes to the ceramic component.

10. The method as claimed in claim 9, wherein processing the burnt ceramic material to provide the ceramic film comprises: suspending the ceramic material; introducing auxiliaries for film drawing; and drawing the ceramic film.

11. The method as claimed in claim 9, wherein the first and second inner electrodes comprises Ag, Pd or an alloy of Ag and Pd.

12. A varistor comprising: a ceramic body containing a sintered ceramic material according to claim 1.

13. The varistor as claimed in claim 12, wherein the varistor has a specific varistor voltage of ≥1000 V/mm.

14. A method for producing the varistor as claimed in claim 12, the method comprising: forming and sintering the ceramic body from the ceramic material, wherein the ceramic material is sintered at not more than 1010° C.; applying outer contacts to the sintered ceramic body; and baking the outer contacts into the ceramic body.

15. A ceramic material comprising: ZnO as main constituent; Y as a first additive; second additives comprising Cr and optionally at least one additional compound containing a metal element, wherein the metal element is selected from the group consisting of Bi, Co, Mn, Ni and Sb; Si.sup.4+ as a first dopant; and second dopants comprising Ba.sup.2+ and optionally at least one additional compound containing a metal cation from Al.sup.3+or B.sup.3+.

16. The ceramic material as claimed in claim 15, wherein at least one of the additives is selected from the group consisting of metal oxides, metal carbonates, metal acetates, metal nitrates and mixtures thereof.

17. The ceramic material as claimed in claim 15, wherein at least one of the dopants is selected from the group consisting of metal nitrides, metal nitrates, metal acetates, metal hydroxides, metal oxides and mixtures thereof.

18. The ceramic material as claimed in claim 15, wherein the ceramic material is sintered at a sintering temperature of not exceeding 1010° C.

19. A varistor comprising: a ceramic body containing a sintered ceramic material according to claim 15.

20. The varistor as claimed in claim 19, wherein the varistor has a specific varistor voltage of ≥1000 V/mm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) There follows a detailed description of the invention with reference to working examples and accompanying figures.

(2) FIG. 1 shows a top view (A) and a side view (B) of a working example of a monolithic varistor containing a ceramic body in cylindrical form;

(3) FIG. 2 shows a schematic cross section of an embodiment of a multilayer varistor containing a ceramic body; and

(4) FIG. 3 shows an image of the microstructure of a ceramic produced according to embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(5) Identical elements, similar elements or those that appear to be the same are given the same reference numerals in the figures. The figures and the size ratios in the figures are not true to scale.

(6) FIG. 1 shows a top view (A) and a side view (B) of one embodiment of a monolithic varistor containing a ceramic body 1 in cylindrical form that has been produced from ceramic material according to embodiments. For the production of the ceramic material, a suspension was first produced from a first portion of the additives and the dopants. For this purpose, oxides or other suitable oxide-convertible starting compounds of Bi (0.58 mol %, calculated as B.sub.2O.sub.3 and based on 100 mol % of ZnO), Cr (0.0 mol %≤Cr≤0.3 mol %, calculated as Cr.sub.2O.sub.3 and based on 100 mol % of ZnO), Co (0.3 mol %≤Co≤0.8 mol %, calculated as Co.sub.3O.sub.4 and based on 100 mol % of ZnO), Mn (0.2 mol %≤Mn≤0.4 mol %, calculated as Mn.sub.3O.sub.4 and based on 100 mol % of ZnO), Ni (0.7 mol %≤Ni≤1.5 mol %, calculated as NiO and based on 100 mol % of ZnO) and Sb (1.1 mol %≤Sb≤1.9 mol % calculated as Sb.sub.3O.sub.2 and based on 100 mol % of ZnO) were weighed out, mixed with water and ground by means of a horizontal stirred ball mill with ZrO.sub.2 grinding bodies. Subsequently, the dopants Al (0 mol %≤Al<0.1 mol % calculated as Al.sub.2O.sub.3 and based on 100 mol % of ZnO), Ba (0 mol %≤Ba<0.1 mol % calculated as BaO and based on 100 mol % of ZnO), B (0 mol %≤B<0.1 mol % calculated as B.sub.2O.sub.3 based on 100 mol % of ZnO) and Si (0 mol %≤Si<0.1 mol % calculated as SiO.sub.2 and based on 100 mol % of ZnO) were added in a water-soluble form. Subsequently, the main ZnO component, for example, in powder form, was stirred into the first suspension and grinding was continued until a median grain size of d(50%)<0.7 m was attained.

(7) In a second step, an aqueous solution of yttrium acetate containing 1.03 mol % of yttrium (calculated as Y.sub.2O.sub.3 and based on 100 mol % of ZnO) was prepared. The yttrium present in the solution was precipitated as yttrium hydroxide by means of ammonium carbonate in excess while stirring, which formed a colloidal suspension. In order to allow the reaction to run to completion, the colloidal suspension was stirred for a further 30 min after the addition of the precipitant.

(8) Subsequently, the first suspension containing the first portion of the additives, the dopants and the main ZnO component was mixed with the colloidal suspension and converted to a homogeneous resulting suspension by means of a homogenizer working by the rotor-stator principle for 30 min.

(9) The resulting suspension thus obtained was dried by means of a suitable method, for example, spray drying, sieved and freed of volatile constituents, such as ammonium residues and acetate residues, in an oven at 500° C., to obtain the ceramic material.

(10) For the production of the ceramic body 1, the ceramic material was blended with water and organic binders and then dried. The dried mass was sieved with a sieve of mesh size 180 μm in order to obtain free-flowing and compressible pellets that were pressed on a laboratory press to give a cylinder having a diameter of 15.5 mm and a thickness of 1.3 mm. The pressed cylinder was freed of temporary binders in a decarburization oven at 450° C. and then sintered at 1010° C. for 3 hours.

(11) The sintered component was surface ground to a thickness of 0.35 mm. The ground surfaces were printed centrally with a silver paste that was then baked into the ceramic body at 750° C. in order to obtain the outer contacts 2 of the varistor. The monolithic varistor thus obtained shows a high specific varistor voltage of 1528 V/mm.

(12) FIG. 2 shows, in schematic cross section, an embodiment of a multilayer varistor wherein the ceramic body 10 has been produced from a ceramic material according to embodiments, wherein the ceramic material has been produced in the same way as described for FIG. 1. For the production of the ceramic body 10 of the multilayer varistor, the ceramic material was processed to a ceramic film in a first step. For this purpose, the ceramic material was suspended in a solvent and provided with auxiliaries for film drawing. By means of the doctor blade method, a ceramic film was produced on a laboratory drawing machine in such a way that a layer thickness of 47 μm was achieved after the sintering. Subsequently, for production of first and second inner electrodes 20 and 30 of the multilayer varistor, the ceramic film was printed with an Ag/Pd inner electrode metallization. In a further step, a multitude of ceramic films were stacked one on top of another in such a way as to obtain an alternating sequence of the first and second inner electrodes 20 and 30. The film stack obtained was compressed and a component of type 1210 was punched out. Subsequently, the component was debindered and sintered at 1010° C. For attachment of the first and second inner electrodes 20 and 30 to the outer electrodes 20′ and 30′, the ends of the sintered component were dipped into an Ag paste and baked, by means of which the component can be contacted. Then the first inner electrodes 20 are connected to the outer electrodes 20′ and the second inner electrodes 30 to the outer electrodes 30′. The varistor thus obtained has a specific varistor voltage of 1468 V/mm.

(13) FIG. 3 shows an image of the microstructure of an embodiment of a ceramic body that has been produced from the ceramic material according to embodiments, for example, a ceramic body produced by the process described with reference to FIG. 1.

(14) In the microstructure, it is possible to distinguish three phases. ZnO grains B constitute the main component. In addition, an yttrium- and bismuth-rich phase A and an antimony-rich phase C occur. The addition of the yttrium component as a colloidal solution generated a very homogeneous distribution of the yttrium- and bismuth-rich phase A in the densely sintered ceramic, which uniformly and effectively limited the grain growth of the ZnO. This is clearly manifested by the average grain size of the ZnO grains B, which is within a range between 1 μm and 3 μm. The small amount of additives in conjunction with the homogeneous distribution of the yttrium- and bismuth-rich phase A and of the antimony-rich phase C in a ceramic body according to embodiments assures good contact between the individual ZnO grains B. This leads to improved volume efficiency of the ceramic body containing the ceramic material that has been produced by one of the processes described above, by means of which it is possible, for a given input voltage, to use smaller varistors or, for the same active volume, to achieve a higher input voltage. As a result, the ceramic body consisting of the novel ceramic material meets the demands with regard to further miniaturization and enhanced performance of varistors.