Ceramic material, varistor and methods of preparing the ceramic material and the varistor

Abstract

A ceramic material, a varistor and methods for forming a ceramic material and a varistor are disclosed. In an embodiment, a ceramic material includes ZnO as a main component and additives selected from the group consisting of an Al.sup.3+-containing solution, a Ba.sup.2+-containing solution, and at least one compound containing a metal element, wherein the metal element is selected from the group consisting of Bi, Sb, Co, Mn, Ni, Y, and Cr.

Claims

1. A ceramic material comprising: ZnO as a main component; and additives comprising an Al.sup.3+-containing solution, a Ba.sup.2+-containing solution, a compound comprising Bi, a compound comprising Sb, a compound comprising Co, a compound comprising Mn, a compound comprising Ni, and a compound comprising Y, wherein c.sub.1 is an equivalent content of Co in Co.sub.3O.sub.4, m is an equivalent content of Mn in Mn.sub.3O.sub.4, s is an equivalent content of Sb in Sb.sub.2O.sub.3, a is an content of Al.sub.3+, y is an equivalent content of Y in Y.sub.2O.sub.3, b.sub.1 is an equivalent content of Bi in Bi.sub.2O.sub.3, n is an equivalent content of Ni in NiO, and b.sub.2 is an content of Ba.sub.2+, and wherein 0.40 mol %≤b.sub.1≤0.55 mol %, 1.10 mol %≤s≤1.90 mol %, 0.50 mol %≤c.sub.1≤0.80 mol %, 0.20 mol %≤m≤0.30 mol %, 0.70 mol %≤n≤1.20 mol %, 0.25 mol %≤y≤0.45 mol %, 0.003 mol %≤a≤0.006 mol %, and 0.005 mol %≤b.sub.2≤0.015 mol %.

2. The ceramic material according to claim 1, wherein a content of the additives in the ceramic material is ≤5 mol %.

3. The ceramic material according to claim 1, wherein at least one compound is selected from the group consisting of metal oxides, metal carbonates, metal acetates, metal nitrides and mixtures thereof.

4. The ceramic material according to claim 1, wherein the compounds are Bi.sub.2O.sub.3, Sb.sub.2O.sub.3, Co.sub.3O.sub.4, Mn.sub.3O.sub.4, NiO, and Y.sub.2O.sub.3.

5. The ceramic material according to claim 1, wherein the Al.sup.3+-containing solution and the Ba.sup.2+-containing solution are solutions selected from the group consisting of nitrides, acetates, hydrates, and mixtures thereof.

6. The ceramic material according to claim 1, wherein the ceramic material has a sintering temperature of between 1020° C. inclusive and 1060° C. inclusive.

7. The ceramic material according to claim 1, wherein the compound comprising Mn is Mn.sub.3O.sub.4.

8. The ceramic material according to claim 1, wherein the compound comprising Ni is NiO.

9. The ceramic material according to claim 1, wherein the compound comprising Sb is Sb.sub.2O.sub.3.

10. The ceramic material according to claim 1, wherein the compound comprising Bi is Bi.sub.2O.sub.3.

11. The ceramic material according to claim 1, further comprising a compound comprising Cr, wherein c.sub.2 is an equivalent content of Cr in Cr.sub.2O.sub.3, and wherein 0.00 mol %≤c.sub.2≤0.10 mol %.

12. The ceramic material according to claim 11, wherein (c.sub.1+5c.sub.2+2s+4y−m−250a)(1−z)/b.sub.1=F, and wherein 0.27≤F≤0.43 and z is a content of ZnO.

13. The ceramic material according to claim 11, wherein the compound comprising Cr is Cr.sub.2O.sub.3.

14. The ceramic material according to claim 11, wherein the compound comprising Co is Co.sub.3O.sub.4.

15. A method for forming the ceramic material according to claim 1, the method comprising: weighing, mixing and ball-milling a first part of additives; adding the ZnO, and a second part of additives; forming a homogeneous slurry; and spray-drying the slurry to form a granule of the ceramic material.

16. The method according to claim 15, wherein the first part of additives is at least one compound containing a metal element, wherein the metal element is selected from the group consisting of Bi, Sb, Co, Mn, Ni, Y, and Cr, and wherein the second part of additives is at least one of the Al.sub.3+-containing solution or the Ba.sub.2+-containing solution.

17. A method for forming a varistor, the method comprising: forming a ceramic body comprising the ceramic material formed according to claim 15; and applying electrode layers on the ceramic body, wherein the ceramic material is sintered at a temperature of between 1020° C. inclusive and 1060° C. inclusive to form the ceramic body.

18. The method according to claim 17, wherein forming the ceramic body further comprises: dry-pressing the granule of the ceramic material; and debindering the ceramic material.

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

20. The varistor according to claim 19, wherein the varistor has a varistor gradient E.sub.1mA of between 480 V/mm inclusive and 640 V/mm inclusive.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the ceramic material, the varistor and the methods of preparing the ceramic material and the varistor are further explained in the following by examples and figures.

(2) FIG. 1 shows a cross-section of a varistor;

(3) FIG. 2 shows the dependence of varistor gradient E.sub.1 mA on the composition factor F;

(4) FIG. 3 shows a picture of the microstructure of one embodiment of the ceramic made of the ceramic material; and

(5) FIG. 4 shows the dependence of several electric properties on the content of Ba.sup.2+.

(6) Equal, similar or apparently equal elements have the same numbers or symbols in the figures. The figures and the proportions of elements in the figures are not drawn to scale. Rather, several elements may be presented disproportionately large for a better presentation and/or a better understanding.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

(7) FIG. 1 shows the cross-section of a varistor according to one embodiment. It contains the ceramic body 10, electrode layers 20 and isolation layers 30. For the preparation of the varistor first the ceramic body 10 is formed.

(8) For this, solid state additive raw material as Bi.sub.2O.sub.3, Sb.sub.2O.sub.3, Y.sub.2O.sub.3, Co.sub.3O.sub.4, Cr.sub.2O.sub.3, Mn.sub.3O.sub.4 and NiO (or other types of oxides, carbonates, acetates, or nitrides with the equivalent quantity of metal elements) are weighed, mixed and ball-milled in water to get the desired particle size distribution. The main component ZnO is then introduced in form of a powder together with the Al.sup.3+- and Ba.sup.2+-solution (in terms of nitrides, acetates or hydrates) into the system. Additional water and some organics (e.g., binder, dispersing agent, defoaming agent) are further introduced to form a homogeneous slurry of desirable viscosity, density, or solid content. A granule of desired diameter and size distribution, packing density, flowability and pressability is then produced by spray-dry method out of the slurry.

(9) The size of the further formed cylinder-shaped green parts containing the granules of ceramic material depends on the further characterization method: for example, for electric characterization disk-shaped green parts of 15.6 mm in diameter and 1.8 mm in thickness are dry-pressed from the granule, followed by debindering in air at about 500° to remove the organic components. The discs are then sintered at 1040° C. for three hours to get a dense ceramic body. The top and bottom surfaces are metallized, for example, with Ag by sputtering.

(10) For energy varistors as shown in FIG. 1, the side surface of the ceramic part is coated with a layer of high isolative glaze by spraying and tempering to form the isolation layer 30. The top and bottom surfaces are ground to remove the contaminated glaze and to get desired height and surface quality. The top and bottom surfaces are fully metallized, for example, with Al by Schoop-process.

(11) In the following, several examples for ceramic materials and ceramics made thereof are shown.

(12) The samples E01 to E43 described in table 1 explain the correlation between the contents of additives and varistor gradient E.sub.1 mA under defined process conditions. Within the specified range of each component, including ZnO and other additives, the varistor gradient strongly depends on the relative content of the most relevant additive components Bi.sub.2O.sub.3, Sb.sub.2O.sub.3, Y.sub.2O.sub.3, Co.sub.3O.sub.4, Cr.sub.2O.sub.3, Mn.sub.3O.sub.4, NiO and Al.sup.3+. The dependence can be expressed by a linear correlation between E.sub.1 mA and the composition factor F, wherein F is the function of the content of Co.sub.3O.sub.4 (c.sub.1), Mn.sub.3O.sub.4 (m), Sb.sub.2O.sub.3 (s), Cr.sub.2O.sub.3 (c.sub.2), Al.sup.3+ (a), Y.sub.2O.sub.3 (y), Bi.sub.2O.sub.3 (b.sub.1), and ZnO (z):
F=(c.sub.1+5c.sub.2+2s+4y−m−250a)(1−z)/b.sub.1

(13) To get a desired varistor gradient, e.g., E.sub.1mA is between 480 V/mm inclusive and 640 V/mm inclusive, the content of the additives should be adjusted so that the factor F is between 0.26, preferably 0.27 inclusive and 0.43 inclusive.

(14) The samples E01 to E43 are prepared as mentioned with respect to FIG. 1, wherein disk-shaped green parts of 15.6 mm in diameter and 1.8 mm in thickness are dry-pressed from the granule, followed by debindering in air at about 500° C. to remove the organic components. The disks are then sintered at 1040° C. for three hours to get dense ceramic bodies.

(15) For the characterization of the samples, the discs are metallized at the top and bottom surface with Ag by sputtering. The electric properties of the metallized parts are characterized and then normalized to an energy varistor of 125 mm in diameter and 18 mm in height for a fair comparison.

(16) The varistor gradient E.sub.1 mA is measured with a low DC current, which gives a current density of 1 mA over a ceramic disc of 125 mm in diameter (or about 10 μA/cm.sup.2). The clamping gradient E.sub.10 kA is measured with 8/20 μs discharging wave, and gives a current density of 10 kA over a ceramic disc of 125 mm in diameter (or about 100 A/cm.sup.2). The steepness s7 is equal to E.sub.10kA/E.sub.1mA. The leakage current density at room temperature and at 170°, J.sub.S, is measured under DC field of 0.75 E.sub.1mA.

(17) Table 1 shows the composition of each example E01 to E43, wherein the contents of ZnO and the additives are given in mol %, the factor F and the varistor gradient E.sub.1mA. It can be seen that, for a factor F which is between 0.27 and 0.43 the varistor gradient is in the desirable range between 480 V/mm and 640 V/mm. If factor F is bigger or smaller than 0.43 and 0.27, respectively, the desirable range of the varistor gradient cannot be achieved (examples E03, E06 to E08, E18, E23 and E24).

(18) TABLE-US-00001 TABLE 1 Examples ZnO Bi.sub.2O.sub.3 Co.sub.3O.sub.4 Mn.sub.3O.sub.4 Sb.sub.2O.sub.3 Cr.sub.2O.sub.3 NiO Ba.sup.2+ Al.sup.2+ Y.sub.2O.sub.3 F E.sub.1 mA (V/mm) E01 95.5 0.47 0.51 0.27 1.81 0.093 0.97 0.0086 0.0059 0.31 0.39 611 E02 95.4 0.47 0.61 0.27 1.82 0.093 0.97 0.0086 0.0059 0.31 0.41 621 E03 95.1 0.48 0.51 0.27 2.20 0.094 0.98 0.0087 0.0059 0.31 0.50 668 E04 95.5 0.47 0.51 0.27 1.81 0.093 0.97 0.0086 0.0059 0.31 0.39 606 E05 95.5 0.47 0.51 0.27 1.81 0.093 0.97 0.0086 0.0041 0.31 0.43 642 E06 95.4 0.47 0.61 0.27 1.82 0.093 0.97 0.0086 0.0041 0.31 0.45 682 E07 95.3 0.47 0.71 0.27 1.82 0.093 0.97 0.0087 0.0041 0.31 0.47 689 E08 95.2 0.48 0.82 0.27 1.82 0.093 0.97 0.0087 0.0041 0.31 0.49 698 E09 95.9 0.47 0.50 0.27 1.44 0.092 0.98 0.0085 0.0041 0.31 0.33 572 E10 95.7 0.47 0.71 0.27 1.44 0.092 0.96 0.0086 0.0041 0.31 0.36 575 E11 95.7 0.47 0.71 0.27 1.44 0.092 0.96 0.0086 0.0029 0.31 0.39 625 E12 95.9 0.47 0.70 0.27 1.19 0.092 0.95 0.0085 0.0041 0.42 0.35 556 E13 95.5 0.57 0.71 0.27 1.46 0.092 0.96 0.0086 0.0041 0.43 0.36 604 E14 95.8 0.47 0.71 0.19 1.44 0.092 0.96 0.0086 0.0041 0.31 0.36 597 E15 95.8 0.47 0.71 0.27 1.44 0.000 0.96 0.0086 0.0041 0.31 0.31 568 E16 96.0 0.47 0.71 0.27 1.44 0.092 0.67 0.0086 0.0041 0.31 0.34 581 E17 96.1 0.47 0.70 0.27 1.19 0.000 0.95 0.0085 0.0029 0.31 0.28 491 E18 96.4 0.46 0.70 0.26 0.95 0.000 0.95 0.0085 0.0016 0.31 0.25 452 E19 95.9 0.47 0.71 0.19 1.44 0.000 0.96 0.0086 0.0041 0.31 0.32 554 E20 96.0 0.47 0.71 0.11 1.44 0.000 0.96 0.0085 0.0041 0.31 0.32 538 E21 96.3 0.47 0.70 0.11 1.19 0.000 0.95 0.0085 0.0040 0.31 0.26 471 E22 96.3 0.47 0.70 0.11 1.19 0.000 0.95 0.0085 0.0029 0.31 0.28 504 E23 97.1 0.32 0.48 0.18 0.99 0.000 0.66 0.0084 0.0040 0.21 0.19 437 E24 94.5 0.62 0.94 0.38 1.92 0.000 1.28 0.0088 0.0042 0.41 0.45 656 E25 95.8 0.47 0.71 0.27 1.44 0.000 0.96 0.0060 0.0041 0.31 0.31 554 E26 95.8 0.47 0.71 0.27 1.44 0.000 0.96 0.0086 0.0041 0.31 0.31 552 E27 95.8 0.47 0.71 0.27 1.44 0.000 0.96 0.0086 0.0041 0.31 0.31 543 E28 95.7 0.47 0.81 0.27 1.44 0.000 0.96 0.0086 0.0041 0.31 0.33 553 E29 95.9 0.47 0.71 0.27 1.38 0.000 0.96 0.0088 0.0029 0.31 0.32 560 E30 98.0 0.47 0.71 0.27 1.32 0.000 0.96 0.0085 0.0029 0.31 0.31 537 E31 95.9 0.47 0.61 0.27 1.32 0.000 0.96 0.0086 0.0029 0.31 0.33 544 E32 95.9 0.47 0.71 0.19 1.44 0.000 0.95 0.0103 0.0041 0.31 0.32 660 E33 95.1 0.43 0.70 0.19 1.19 0.000 0.95 0.0085 0.0040 0.42 0.32 560 E34 96.1 0.47 0.70 0.19 1.19 0.000 0.95 0.0085 0.0040 0.42 0.30 534 E35 96.0 0.50 0.70 0.19 1.19 0.000 0.95 0.0085 0.0041 0.42 0.28 512 E36 96.1 0.43 0.70 0.19 1.19 0.000 0.95 0.0085 0.0040 0.38 0.31 540 E37 96.1 0.47 0.70 0.19 1.19 0.000 0.95 0.0102 0.0040 0.42 0.30 545 E38 96.0 0.47 0.70 0.27 1.19 0.000 0.95 0.0085 0.0041 0.42 0.30 524 E39 96.1 0.47 0.60 0.27 1.19 0.000 0.95 0.0085 0.0040 0.42 0.29 521 E40 96.2 0.46 0.50 0.26 1.19 0.000 0.95 0.0085 0.0040 0.42 0.27 514 E41 96.2 0.46 0.60 0.19 1.19 0.000 0.95 0.0085 0.0040 0.42 0.29 531 E42 96.1 0.46 0.60 0.26 1.10 0.000 0.95 0.0085 0.0040 0.46 0.28 503 E43 96.2 0.46 0.60 0.26 1.01 0.000 0.95 0.0084 0.0040 0.50 0.27 485

(19) The dependence of the varistor gradient on the composition factor F is also shown in FIG. 2 where the correlation between E.sub.1 mA and F can be clearly seen.

(20) The expression of F reflects the effectiveness of a respective component affecting the varistor gradient which is actually determined by grain size and the grain boundary potential formed during sintering. The grain growth is mainly controlled by the formation and distribution of the secondary phases, e.g., spinel phase Zn.sub.7Sb.sub.2O.sub.12, and Y—Bi-rich phase as can be seen in FIG. 3. In FIG. 3 the microstructure of the exemplary sample E15 ceramic is shown. A shows a Y—Bi-rich phase, B shows a Bi-rich liquid phase and C shows a spinel phase Zn.sub.7Sb.sub.2O.sub.12.

(21) A spinel phase is characterized with a grain size of 2 to 4 m, while the Y—Bi-rich phase has a much smaller size of sub-micron in diameter. As a consequence, the introduction of Y.sub.2O.sub.3 brings about double effectiveness in comparison with that of Sb.sub.2O.sub.3 in achieving the desired varistor gradient. The formation of spinel phase consumes much ZnO and is not favourable for high volume efficiency. By introducing 1 mol % Sb.sub.2O.sub.3, about 7 mol % of the ZnO would be taken by the spinel phase and the effective varistor volume fraction (ZnO grains) has to be reduced by so much. In contrast, the Y—Bi-rich phase grains have a small content of ZnO and have minor influence on the volume efficiency. Accordingly, ultra-high gradient varistor ceramics with reduced Sb.sub.2O.sub.3 content (e.g., 1.1 mol % to 1.2 mol %) can be achieved by collaboratively varying the contents of other additives so that the factor F is within a certain scale (e.g., the compositions of E12, E22 and E33 to E42).

(22) Samples E44 to E46 are listed in Table 2 together with samples E15 and E25, and show the influence of Ba.sup.2+-content on the high temperature leakage current J.sub.S and steepness characteristic s7.

(23) TABLE-US-00002 TABLE 2 E.sub.1mA J.sub.s Examples ZnO Bi.sub.2O.sub.3 Co.sub.3O.sub.4 Mn.sub.3O.sub.4 Sb.sub.2O.sub.3 Cr.sub.2O.sub.3 NiO Ba.sup.2+ Al.sup.2+ Y.sub.2O.sub.3 (V/mm) S7 (A/cm.sup.2) E15 95.8 0.47 0.71 0.27 1.44 0.000 0.96 0.0086 0.0041 0.31 568 1.57 1.91 10.sup.−5 E25 95.8 0.47 0.71 0.27 1.44 0.000 0.96 0.0060 0.0041 0.31 554 1.61 2.07 10.sup.−5 E44 95.8 0.47 0.71 0.27 1.44 0.000 0.96 0.0000 0.0041 0.31 492 1.61 3.45 10.sup.−5 E45 95.8 0.47 0.71 0.27 1.44 0.000 0.96 0.0103 0.0041 0.31 524 1.61 1.88 10.sup.−5 E46 95.8 0.47 0.71 0.27 1.44 0.000 0.96 0.0172 0.0041 0.31 531 1.62 1.70 10.sup.−5

(24) The compositions listed in Table 2 all have the same amount of additives except the content of Ba.sup.2+. The contents of ZnO and the additives are given in mol %. The preparation process and electric characterization are the same as described, for examples, E01 to E43. The basic electric properties as a function of the Ba.sup.2+-content b.sub.2 are plotted in FIG. 4. Here, the content of Ba.sup.2+ b.sub.2 is plotted against the high temperature leakage current J.sub.S, the steepness s7 and the varistor gradient E.sub.1mA. The circles in FIG. 4 are the respective measured values of the samples, the boxes are statistic representations of the scattering (e.g., median, mean, maximum, minimum etc. value, depending on the definition in the plotting). Apparently, a trace amount as low as 0.0060% of Ba.sup.2+ can effectively reduce the high temperature leakage current density J.sub.S, which is crucial for the energy capability in device operation. A further increase of b.sub.2 leads to even lower leakage current, but the steepness s7 gets worse when the b.sub.2 exceeds 0.0150%.

(25) The samples E47 to E49, listed in Table 3 together with samples E10, E15 and E19, are prepared out of the same compositions as E10, E15 and E19 respectively except that the dimensions of the green parts before debindering and sintering are 150 mm in diameter and 25 mm in height. Thus, in Table 3, the sample number (examples), the composition, the diameter D and the thickness T are listed together with the electric properties varistor gradient E.sub.1mA, steepness s7, high temperature leakage current J.sub.S and high temperature power loss P.sub.cov.

(26) TABLE-US-00003 TABLE 3 Examples Composition D (mm) T (mm) E.sub.1mA (V/mm) S7 J.sub.s (A/cm.sup.2) P.sub.cov (W) E10 E10 13 1.5 575 1.60 1.97 10.sup.−5 E47 E10 125 18 548 1.50 20.6 E15 E15 13 1.5 568 1.57 1.91 10.sup.−5 E48 E15 125 18 545 1.48 14.5 E19 E19 13 1.5 554 1.60 1.54 10.sup.−5 E49 E19 125 18 512 1.51 12.9

(27) The debindering and sintering conditions are the same as the small discs in examples E01 to E46. A layer of glass material is then sprayed on the side surface of the sample, followed by tempering at 510° C. to get a dense and highly isolative glaze coating. The parts are then ground on both main sides to desired thickness, e.g., 18 mm. Aluminum metallization is provided on the top and bottom surface for electric contacting, for example, by Schoop-process. The basic electric properties are characterized (the high temperature power loss P.sub.COV instead of J.sub.S is measured at 190° C. under a 50 Hz-AC field of E.sub.10kA/2.75 in amplitude) and compared with a small disc of examples E10, E15 and E19. Basically, the varistor gradient E.sub.1mA of the small discs could be well reproduced with a slight offset of about 5%, while the steepness s7 drifts to lower values due to the size effect. Lower P.sub.COV could be expected for materials of lower J.sub.s as both arise from the same physical effect (high temperature resistance).

(28) So it could be shown that the ZnO-based ceramic materials can be used for metal oxide varistors used in gas-isolated arrestors. The ultra-high varistor gradient of these materials is essential for miniaturization and design simplification of the arrestor devices.

(29) The scope of protection of the invention is not limited to the examples given herein above. The invention is embodied in each novel characteristic and each combination of characteristics, which particularly includes every combination of any features which are stated in the claims, even if this feature or this combination of features is not explicitly stated in the claims or in the examples.