JPS62293702A - Manufacture of ceramic zinc oxide varistor material - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention [Field of Industrial Application] This invention uses zinc oxide as a base and adds cobalt and/or manganese and/or antimony and/or bismuth, and in some cases The present invention relates to a method for producing and using a ceramic varistor material to which chromium and/or aluminum and/or barium are supplementarily added.

[Prior Art] A varistor is an electrical component that has a high resistance characteristic at a value below the cut-off voltage (cart/off electric field strength x thickness) and suddenly becomes low in resistance when this cart/off voltage is reached. be. Varistors are used as overvoltage protection elements and are used not only for low-voltage installations, hermetically sealed semiconductor electronic circuits, but also for high-voltage installations, for example as lightning arresters.

In particular, it is known to make varistors from ceramic materials based on zinc oxide to which cobalt and/or manganese and antimony and bismuth are added as additional elements. Other additive elements that may be used include chromium, aluminum, barium, etc. A powder mixture is made from the oxides of the elements used, and a molded body is made from this mixture by pressing, printing, film stretching, etc.
It is known to sinter this compact into a ceramic material. If appropriate, the mixture of raw materials is first subjected to a displacement process, optionally followed by comminution, but the displacement process can also proceed partially or entirely during the sintering period.

Apart from the selection of the elements used as raw materials, the type of grain growth that occurs or is obtained during sintering plays an important role in the properties and quality of the finished varistor. The sintering temperature, ie the temperature during the holding period of the sintering process, is very important for this purpose. If the sintering temperature is increased,
It is known that the grain size of the finished ceramic varistor material increases. For said materials made according to the above description, in particular 8
A minimum sintering temperature above 00°C is required.

Although ultrafine-grained varistor materials are of particular interest due to their high cut-off voltages, this varistor material is known to require relatively low sintering temperatures, and although sintering begins, the varistor Requires such low temperatures that the effectiveness is significantly reduced or not achieved at all.

[Problems to be Solved by the Invention] The present invention provides improved varistor characteristic values, a higher response gradient and/or a higher breakdown field strength compared to the state of the art and, in particular, aims to create a varistor material with a high cut-off voltage based on zinc oxide. Furthermore, the invention aims to facilitate the manufacturing method with regard to the need for accurate maintenance of manufacturing parameters. Furthermore, the invention aims at producing such a varistor material in such a way that it is particularly suitable for special applications such as, for example, nλ-thickness varistors, multilayer varistors, compact high-voltage varistors, etc.

[Means for solving the problem] This purpose is to select raw materials that can dissolve individual elements contained in the varistor material, and to create a mixture of raw materials consisting of these molten substances in the manufacturing method of ceramic varistor materials. This is achieved by preparing a common solution as a material, subjecting this common solution to continuous processing according to the spray reaction method, and subsequently performing molding and sintering.

The invention is based on the finding that, despite many years of development, significant improvements can still be achieved by means of special adjustment means. According to the invention, even when using low sintering temperatures (below 900°C, especially below 850°C), excellent electrical values (especially high nonlinear coefficient
) and fine-grained properties are obtained.

This invention provides a uniform ceramic varistor material. There are also glass frits or varistor materials embedded within a glass compound. Additionally, such materials have poor electrical values.

[Example] Next, the present invention will be described in detail with reference to drawings showing an example of the manufacturing method based on the present invention and three product examples using the varistor material manufactured based on this method.

Figure 6 shows the conventionally used varistor ceramic Zno9.
+Coo, o+Mno, ozBio, o3Sbo, o3
0 shows a diagram of the varistor characteristics (relationship between electric field strength and current density), and also shows the sintering temperature from 1100°C to 800°C in the sintering process for 2 hours and 600°C, respectively. Figure 2 shows the properties of six sintered examples for a holding time of 20 hours. The formulation of raw materials used for the barista samples was based on conventional techniques. As can clearly be seen from the figure, only sintering at 1000° C./2 h (curve 3) and 900° C./2 h (curve 4) gives values that can be used in practice. In particular, as shown in Figure 6, 800
When sintering at ℃, no varistor effect is obtained regardless of the time.

In contrast to the results shown in FIG. 6, FIG. 5 shows the significant advance achieved by the present invention. Again, the varistor material is the same, and very good varistor properties are obtained even down from 1100° C./2h to 800° C./2h, as shown by the four curves 4 in FIG. aoo
It is characterized by the fact that excellent varistor characteristics can be obtained at a very low sintering temperature of 'c' (curve 4).
At 800° C. the corresponding curve shown in FIG. 6 does not even show any sign of a varistor effect.

Tables 1 and 2 show the corresponding results, Table 1 for the inventive adjustment in terms of sintering temperature, electric field strength, nonlinear coefficient, and Table 2 for the conventional adjustment.

Table 1 “Zno9+Coo, o+Mno, ozBio, o:+
Sbo, o30 "Invention Clothes 2" Zn+1.91cOO, O+Mno, ozBio,
o3sbo, 030"Conventional Figure 1 shows the preparation process according to the invention. According to the invention, zinc acetate, cobalt acetate, manganese acetate, antimony acetate and bismuth nitrate are added to the previously discussed zinc oxide varistor ceramic. Zinc,
Instead of the acetates of cobalt, antimony and manganese, the corresponding nitrates of these elements can also be used. The solubility of the raw materials considered here is important. Water is the preferred solvent for zinc-based, cobalt-based, and manganese-based raw materials.

Hot acetic acid is particularly suitable for antimony acetate. For bismuth nitrate, it is recommended to use a mixed range of water and nitric acid as the solvent. Additionally, these solutions can be stored for virtually any period of time.

The individual raw materials mentioned above are mixed in the amounts required in each case for the above-mentioned varistor ceramic, and the mixture is then a solution. This mixed solution is further processed in a spray reaction method known per se.

This reactive atomization has already been described for piezoelectric ceramic materials in DE 34 07 059 and DE 3 409 815.

For this reaction spray, an apparatus shown in FIG. 7 described in German Patent Application No. 3407059 can be used. In FIG. 7, 100 indicates a heatable chamber, and inside 109 of this chamber the components of the supplied solution 107 are reacted. Solution 107 using an ultrasonic atomizer
can be atomized into fog 108. 116 and 1161
indicates a gas supply pipe. Arrows 111 indicate the flow. 11
0 indicates reaction-separated zinc oxide, and the additive substance, which is also present in the solution 107, is very uniformly dispersed within the particles as well as inside the particles.

Details of this process, which is known per se, can be found in Ellen Ivers-Tiff6e.
)'s printed matter ``Proxy 1 of Porous Piezoelectric Ceramic from Chemically Prepared Raw Materials (Production
of Porous PiezoelectricCe
ramic from Chemically
Prepared Raw Materials
) J, Ferro Electrics
-electrics), 1986.

An important feature of reactive spraying is that the mixed solution is sprayed into a hot chamber via an ultrasonic atomizer. The prevailing high temperatures not only lead to drying, but also promote the mutual reaction of the components of the mixed solution. The reaction spray product, with which a very high homogeneity of the distribution of the additive material in the zinc oxide generated during this reaction spray is obtained, is separated off, for example in a cyclone, and removed, for example, continuously from the device. In this way, a correspondingly modified material is obtained, which is processed into shaped bodies, optionally with the addition of a binder. However, this step can optionally also be preceded by a further auxiliary hexaconversion process at temperatures below 700° C. and optionally followed by re-grinding in a flour mill.

The shaping is carried out according to the desired shape of the varistor, for example a plate or a block is pressed. Advantageously, the layers on the substrate are produced by printing. Advantageously, the self-supporting film is produced by film stretching. Printing and film stretching are methods used in particular in thick film varistors and multilayer varistors. For both of these areas, the process also involves the application of electrodes, which can later be used, for example, between the substrate and the varistor thick film applied on this substrate or between the individual varistors of a multilayer varistor. Between the layers,
In addition, it should be mentioned that for conventional thick-film varistors and multilayer varistors platinum, palladium or correspondingly high-temperature resistant electrode materials were absolutely necessary. On the other hand, in the present invention, relatively base metals such as silver, copper, etc. can also be used instead. This means that with this invention, the subsequent sintering can be done at a temperature of less than 900°C, even less than 800°C, and despite this, very good varistor properties (see Figure 5) can be guaranteed. That's why.

The object obtained by molding is also called an elementary body.

The subsequent sintering of the element body takes place, for example, in a continuous furnace and at the temperatures previously mentioned in FIG. . According to the invention it is even possible to make do with temperatures of 700°C to 800'C. This very last-mentioned low temperature is particularly advantageous for thick-film varistors (varistor layers coated on a substrate made of, for example, alumina, with electrodes made of, for example, silver, copper, etc., between the substrate and the varistor layer). . This is because the thermal strain and thermal load of the material can each be kept to a minimum without suffering any damage in terms of the properties of the varistor effect.

Instead of striving to work at as low a sintering temperature as possible, the present invention provides a sintering temperature of 900°C to 1100'C, especially 1000
In particular, varistor ceramics sintered according to curves 1 and 2 shown in FIG. 5 (sintered according to the curves 1 and 2 in FIG. It was surprisingly confirmed that the particles were extremely fine (compared to other materials). In this invention, it has been determined that various high sintering temperatures and sintering times have very little effect on the fineness, ie, do not result in coarse-grained ceramics.

That is, as shown in the curve in Figure 5, with various high sintering temperatures, various high cut-off electric field strengths (if the thickness of the varistor is appropriate, this value is the cut-off voltage of the varistor) are obtained. .

It basically holds that increasing the sintering temperature results in a reduction in the porosity and thus an increase in the density of the ceramic varistor material according to the invention. Reducing the porosity of the material is particularly important for high voltage varistors. This is because the dielectric breakdown strength of the material is significantly related to this property. However, in materials based on the conventional state of the art, high sintering temperatures inevitably result in a coarse-grained structure, and since the function of the varistor effect is a characteristic of individual particles, according to the conventional technology, high voltage varistors ( This creates a need for a relatively thick layer (measured between the electrodes).

Despite the high sintering temperatures of the ceramic varistor material according to the invention, the fine-grained structure allows the varistors to be produced with correspondingly small thicknesses. This results in a clear saving in the materials used or to be processed.

FIG. 2 shows a cross-sectional view of the thick H-tavaristor l seen from the side.

2 is a small substrate made of, for example, alumina; 3 is a layer made of sintered ceramic varistor material provided on this small substrate; 4 and 5 are flat electrode layers; electrode layer 4 is shown in the figure; In all cases, an electrode layer is provided between the sub-substrate and the varistor layer 3 as shown in FIG. This electrode layer 4 is already present during the sintering process and must be able to withstand the effects occurring in the sintering process.

In this invention, a low sintering temperature of 800° C. or less is possible, and silver, copper, etc. can be used for the electrode 4.

FIG. 3 shows a side cross-sectional view of a portion of a multilayer varistor 11, which has superimposed layers 13 of varistor material. Also provided between these layers 13 is one electrode layer 14 (corresponding to the electrode layer 4). For manufacturing the multilayer varistor 11, the electrode layer 14
The as-yet-unsintered films 13, which have been coated with a layer of, for example, silver paste, copper paste, etc., are laminated onto one another and the entire laminate is sintered.

FIG. 4 shows in a perspective view two high-voltage varistors with a disc 23 of varistor material. 24 indicates an electrode layer provided on the upper surface of the disk 21.

A corresponding electrode layer is applied as a counter-electrode on the lower surface of this disc 21.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the manufacturing method based on this invention, and FIGS.
/ Figures 5 and 6 are partial cross-sectional views or perspective views of varistors as examples of different uses of helistor materials, and are diagrams showing examples of the characteristics of varistors made by the manufacturing method based on this invention and the conventional manufacturing method, respectively. The figure shown, FIG. 7, is a cross-sectional view of an example of an apparatus used for reactive spraying. ■...Raw material, m...Solution of raw material, ■・Φ・Mixed solution, ■...Reaction spray, ■...Substitution reaction, ■
...Molding and sintering. FIGI AC; C2H502 I02 IG 3 IG 4

Claims (1)

[Claims] 1) Adding cobalt and/or manganese and/or antimony and/or bismuth to a zinc oxide base,
A method for producing a ceramic varistor material supplemented with chromium and/or aluminum and/or barium in some cases, in which a compound of zinc and a compound of additional elements are mixed together in a predetermined proportion, optionally carrying out a displacement reaction, optionally followed by grinding,
Then, in the method of forming, optionally applying electrodes, and finally sintering, the soluble raw materials (I) of the individual elements contained in the varistor material are selected, and these molten substances (III) are selected. ) is prepared as a mixture (IV) of raw materials, and this common solution is subjected to the spray reaction method (
A process for producing a ceramic zinc oxide varistor material, characterized in that the process is continued according to VI), followed by shaping and sintering (VIII). 2) The manufacturing method according to claim 1, characterized in that zinc acetate is used as a raw material. 3) The manufacturing method according to claim 1, characterized in that zinc nitrate is used as a raw material. 4) Claim 1, characterized in that the sintering (VIII) is carried out at a temperature of 700°C to less than 900°C.
The manufacturing method according to any one of Items 1 to 3. 5) Claim 1, characterized in that the sintering (VIII) is carried out at a temperature of 700°C to less than 830°C.
The manufacturing method according to any one of Items 1 to 3. 6) Any one of claims 1 to 5, characterized in that an electrode made of a relatively base metal is applied onto the element body of the varistor material already before sintering (VIII). The manufacturing method described in. 7) Claim 1, characterized in that after the reaction spraying (VI) and before the shaping (VIII), optionally still an auxiliary displacement reaction (VII) is carried out at a temperature below 700°C. The manufacturing method according to any one of Items 6 to 6.