DE2445659A1 - METAL OXYDE VARISTOR WITH A PASSIVATING COATING - Google Patents

Description

Coating

The present invention relates to metal oxide varistors and a passivating coating to improve stability and electrical properties of metal oxide varistors.

In general, the current flowing between two spaced points is directly proportional to the potential difference between these points. For most known substances, the current conduction through these substances is equal to the applied potential difference divided by a constant which is defined by Ohm's law as the resistance of the substance in question . Bs there

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however, some substances that exhibit a non-linear resistance behavior show «, Some components, such as metal oxide varistors, make use of these substances and require the aid of the following equation (I), to indicate a quantitative relationship between current and voltage;

(1) I =

where V is the voltage applied to the element, I is the current flowing through the element, C is a constant and cn. a Exponent is greater than 1. Since the value of c * ^ is the degree of Determines the non-linearity of the element, it is in general desired that CX. is relatively high. CA is made according to calculated by the following equation (2);

(2) ^1ο ®ιο ^ ^{_2/1! 5} '

1 n «r (V / V)

IQg ₁₀ IV ₂ 'l'

where V ₁ and V _{p are} the voltages of the element at the given currents I ₁ and I ₀ .

J. &

At very low and very high voltages, metal oxide varistors give way from the behavior expressed by Equation 1 and approach a linear resistance behavior. For a very wide usable voltage range, however, metal oxide varistors correspond to the equation (1).

The values of C and c *. can be varied over a wide range

man

be done by / dxe varistor composition and manufacturing process changes. Another useful varistor property is the varistor voltage, which is called the voltage over the element is defined at which a given stream passes through the element flows «It is common to use the varistor voltage at

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to measure a current of one milliampere and the following reference to varistor voltages is intended to refer to the voltage measured in this way. Another useful varistor property is leakage current (im English called "leakage current"). This is the constant direct current that flows through the element when it is halfway through is exposed to the varistor voltage. High leakage current causes wasted energy and, if high enough, can cause heat-up due to the flow of current of the element, which in turn leads to a higher current. This is how high leakage current can do it Let element get out of control thermally. The aforementioned relationships are known.

Metal oxide varistors are usually manufactured as follows: A large number of additives are mixed with a powdered metal oxide, usually zinc oxide. It is customary to use k to 12 additives, which together make up only a small proportion of the end product, for example less than 5 to 10 mol%. In some cases the additives comprise less than 1 Mo1%. The types and amounts of additives employed will vary with the properties desired in the varistor. In an extensive literature, metal oxide varistors with various additive combinations are described (sidie, for example, US Pat. No. 3,663,558). A portion of the mixture of metal oxide and additive is then pressed into a body of a desired shape and size. This body is then sintered in a known manner for an appropriate time at a suitable temperature. Sintering causes the necessary reactions between the additives and the metal oxide and causes the mixture to melt into a coherent pellet. After sintering, metallic contacts are applied and leads are attached to the contacts

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attached. Finally, the element is generally encapsulated.

Problems that arise in the manufacture of varistors The known processes include poor stability and leakage current that result in the Make element unsuitable for certain applications. Various varistor compositions have been studied to control leakage current, however none of these compositions were heretofore entirely acceptable. Passivating coatings are sometimes used used to increase the stability of the element and reduce its leakage current. However, the previously known coatings are not entirely successful and, moreover, the components of the coating sometimes have an adverse effect on the electrical properties of the element.

It is therefore desirable to have a passivating coating for metal oxide varistors, which is completely compatible with the stability of the element and this elevated; as well as reducing the leakage current and thereby creating significantly improved elements.

To achieve these desirable results, in one aspect the present invention provides a metal oxide varistor comprising a body consisting essentially of a metal oxide and a relatively small amount of a variety of additives and carries a passivating coating, this coating at least one ge of the additives.

The invention is characterized by a metal oxide varistor which comprises a body which is essentially composed of a metal oxide and a small one

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Percentage of a large number of preselected additives. The metal oxide and the additives are mixed together and then a portion of the mixture is compressed into a body of the desired shape and size. The pressed one The body is then sintered in a known manner to form the varistor body. Then you bring on the Body applies a coating and heats the whole thing again, as described in more detail below. Metal contacts are then applied and lead wires attached attached to the contact. Then the element can be encapsulated.

A feature of the coating according to the invention is that it is a passivating coating and that it is stable of the element significantly improved. To create a coating that is compatible with the varistor body, the preselected additives form a major part of the coating. In this way, the tolerability of Coating and varistor ensured.

The preselected additives can be reacted with one another beforehand to form a reaction product and the reaction product can then be ground and mixed with the metal oxide before pressing and sintering the pellet. It has been found that such a technique provides a varistor which has very useful electrical properties, high stability and a very uniform composition. A detailed explanation of the method for producing a varistor by converting the additives beforehand is given in the German patent application with the title, filed on the same day. Contain "varistor for low voltage and process for its production", for which the priority of U.S. patent application Ser. No. 4 oil is taken from 131 · 27 September 1973 in claim. The coating can comprise the reaction product. It can also include a carrier

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sen, as will be explained in more detail below, to the adhesion of the reaction product to the surface of the body to strengthen.

Another feature of the coating of the invention it is that it is free of silver and other monovalent ions which tend to break down during heating of the coating to diffuse quickly into the varistor pellet. It is beneficial to avoid such ion diffusion, as it causes high leakage current and other effects on varistor characteristics that are difficult to predict are. Furthermore, the coating can also be produced without silicon, as explained in more detail below will. It was found that the inclusion of silicon in the coating for a varistor pellet was the Varistor voltage of the treated element increased. Therefore, if you want to make varistors for low voltage, it is preferred to use a silicon-free coating.

Another feature of the invention is that the coating the adhesion of the contacts is strengthened. A preferred method of reinforcement is to surface to provide the varistor body with a contour. For example, solid particulate material can be incorporated into the Coating can be applied, and the heating of the coating can be carried out at a temperature in which the solid particulate material is not melts.

Oa metal contacts, if, according to such known methods, such as the application and heating of silver paste or the use of metal flame spraying, the contour of the Adjust substrate surface exactly, which in the present Pal-Ie is the surface of the varistor pellets., It is clear that through the use of a contoured pellet surface

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substantial increase in the adhesion of the contact is realized.

Of course, other methods can also be used to contour the surface of the pellet Mistake. For example, you can apply a coating that shrinks when it hardens. The shrinking coating leads for the formation of hoists that have a contouring effect cause on the surface.

Since coatings are often applied to varistor bodies produced by known methods, the varistor according to the invention can only be produced with low levels This entails extra costs, as the varistor manufacturers already have the coating equipment.

The present invention thus creates a coating for metal oxide varistors which acts as a passive coating and thus improves the stability of the element. The coating is completely compatible with the varistor body and since the coating is free from monovalent ions _r it does not impair the electrical properties of the varistor. Furthermore, the coating for elements for low voltage can be made silicon-free. Finally, the coating strengthens the adhesion of the contacts to the varistor body and thus prevents failure of the element due to detached, torn or broken contacts. "

The invention is explained in more detail below with reference to the drawing *

Show in detail:.

Fig. 1 is a cross section of a preferred metal oxide. Varistors and

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2U5659

-ö-

Fig. 2 is a detail "y ^re cross-section of a portion of the varistor shown in Figure 1.

In Figure 1, a metal oxide varistor 10 is shown, which has a sintered body 11, which is essentially is composed of a metal oxide and a large number of preselected additives. The body 11 can be produced by known methods or by an improved technique explained in more detail below. The body is surrounded by a passivating coating 12, and two metallic contacts 13 lie on the coating on the two main surface areas of the body and 14. Two lead wires 15 and 16 are attached to contacts 13 and 14; e.g. through conductive connections, such as soldering points 17 and l8.

Part of the varistor 10 of FIG. 1 is shown in FIG. Specifically, in FIG. 2, a corner of the body 11 is pictured. In the enlarged Figure 2 is the pass si vie -> rende coating 12 To see more clearly. It can be seen in particular that it consists of a coating material, is embedded in the solid particulate material. Some of the grains 19 of solid particulate material extend across the surface of the coating 12, while other grains such as grain 20 are completely encapsulated by the coating. The grains 19 and 20 are part of a system promoting the adhesion of the contacts which is present in the coating 12 The surface of the coating 12 is much more roughened and contoured due to the presence of the particles 19 and 20 than if these particles were not present. It it has been found that contacts applied by conventional methods such as applying a silver paste or by metal flame spraying to closely conform to the surface irregularities. The contact 13 is therefore

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adhere more permanently to the irregular surface than it would on a smooth surface. Integration the solid particulate material 19 and 20 increases the surface irregularities by at least an order of magnitude Reinforcement of the adhesion of the contacts is obtained. To give the coating good passivating properties and making them perfectly compatible with the Pellet.11 includes that. Coating 12 preferably at least some of the preselected additives. E.g. It has been found that an effective passivating coating can be created using the above Reaction product used. This reaction product, which is in the above-mentioned patent application from the same day is described in more detail, can through thorough Mixing the preselected additives in the absence of the metal oxide and the subsequent reaction of the additives with one another. For example, the Additives are heated and cooled and in this way fused into a solid body. The body will then ground to form the reaction product. The metal oxide is then mixed with the reaction product and pressed and sintered in the usual way.

A varistor with excellent electrical properties can be made from 98 mol% zinc oxide, 0.5 mol 1% bismuth oxide, 0.5 mol cobalt oxide, 0.5 mol 1% manganese oxide, and 0.5 mol 56 titanium oxide. According to the process disclosed in the above patent application, the oxides of bismuth, cobalt, manganese and titanium are thoroughly mixed with one another in equal molar amounts, then heated and cooled to form a crystalline solid. The crystalline solid is ground to form the reaction product. Zinc oxide and the reaction product are thoroughly mixed together, and part of the mixture is pressed and sintered to form the varistor body 11.

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For the specific composition of a varistor, as described above, passivating coatings can be made from any of the following grusid materials:

1) eat g of reaction product

2) 10.5 g of reaction product + 5J g of Bi ₀ O

3) 14.5 g of reaction products 2.6 g Η ,, ΒΟ

4) 10.5 g reaction product + 5 _S 7 g Bi ₀ O + 1.9 g H ₀ BO "

5) 10.5 g reaction product + 5.7 g ^Bi ₂ ° 3 ^{+ 1} ^ S H_B0

+ 3) 6 g ^Sb 2 ° 3

6) 50 g reaction product ψ 30 g Bi_0 "+ 15 g H ₀ B0_

d j yy

+ 7g SiO ₂ .

Of the öbig @; a ÄÄätzea hardly, the approach No. 6 for varistors Bffiittl © r @ r raad Eaohear voltage, while ea ojrsiräsiseht melm saag, this Ässatz, which contains silicon, for the Harst © lluaag of Yariastoren for low voltage to avoid dia Silisinias di © Spauuaung a varistor increases. Next siad all © © © a big Ämsäts® would aeigasa free of silver and other monovalent ions, u ± @ while described DE® subsequently heating the coating to quickly ren through the Varistorlcörper 11 to Diffundi @. Such a diffusion of monovalent ions could affect the stability of the varistor 10 and affect the electrical properties of the element in a manner difficult to predict. Another patent application filed on the same day © with the title "Silver-free passivating coating for varistors" is described, for which the priority of the US patent application Serial No. 4θ1 323 of 27 »Sept. 1973 in A» «prttcSi ge» © iSBi © is is ο

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Are compounds of bismuth, boron, antimony and silicon used, they are carriers and serve to strengthen the adhesion of the solid particulate material 19 and 20 on body 11. To apply the passivating coating, any of the above is used Approaches selected and mixed with a diluent to facilitate handling. One for the above Approaches usable diluent is composed of 135 g of N-butyl acetate, 20 g of ethyl cellulose and 15 g of butyl carbitol.

The application of the coating can be carried out as follows: After mixing with the diluent, the coating is liquid and it is poured over the sintered bodies 11 and these are immersed in the coating. The diluent is then evaporated at a relatively low temperature. The elements are then heated to a temperature in the range from 600 to 1,000 ^° C. for a preselected time. The temperature and time selected are intended to ensure that the carrier is melted and so increases the adhesion of the particulate material 19 and 20 to the body 11. However, the particulate material and 20 should not melt in the process, so that a body 11 with the coating 12 and the irregular surface is obtained, as shown in FIG. After heating, the metal icons 13 and Ik are applied in the usual way, as already mentioned by applying a silver paste or flame spraying, the contacts 13 and 14 will adapt closely to the irregular surface of the body 11 and thus adhere firmly to it.

If for any reason it should be desired, the carrier can also be used with various solid particulate materials. Another suitable festival

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Particulate matter would be grains of aluminum oxide.

The present invention therefore creates a varistor with a passivating coating, which reduces the leakage current, the stability of the element improved and the adhesion of the contacts strengthened. Studies have shown that the adhesion the contacts are improved by at least one order of magnitude and the leakage current is reduced by several orders of magnitude has been.

Many modifications and variations can be made within the scope of the present invention. For example, other carriers and diluents can be used. In addition, it should be pointed out that when using approach 1 for the coating, only the reaction product and diluent are applied to the body. The coating therefore consists only of reaction product. If certain carriers are used, these are essentially completely absorbed in the outer part of the varistor body, but they help the adhesion of the particulate material, since it is still embedded in it, even if only partially «

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Claims (7)

Claims I.) Metal oxide varistor with a body that is essentially consists of a metal oxide and a relatively small amount of a variety of additives and carries a passivating coating, thereby marked that the coating comprises at least some of the additives. 2. varistor according to claim 1, characterized ge Ind ei ch η et _s that the coating comprises a reaction product that at least some of the additives is obtained by the mutual reaction. 3. Varistor according to claim 2, characterized in that the coating continues a carrier for enhancing the adhesion between the body and the reaction product. k. Varistor according to claim 3? characterized in that the carrier comprises a compound selected from the compounds of bismuth, boron, antimony and silicon, 5 «varistor according to claim 3, characterized in that that the additives comprise the oxides of vismuth, cobalt, manganese and titanium, wherein the metal oxide is zinc oxide. 6. varistor according to one of claims 1, 2, 3 or 5, characterized in that the coating is free of silicon ions. 5098H / 087A 7. A method for producing a varistor according to any one of claims 1 to 6, characterized through the following stages; Creating a body that is essentially made up of one Metal oxide and a relatively small proportion of a reaction product obtained by Melting together a large number of preselected additives, Applying a passivating coating to the body, the coating comprising the reaction product and
Heating the body. 8, procedure according to AELsprueln 7? thereby g e kenras © i ehsae t,. that heating the body takes place at a temperature sufficient to the carrier to ^ © hiaSlaea, which however is not so hooh, to melt the reaction product. 9 · A method according to claim 8 © $ or _t characterized in that metallic contacts applied to the body 509814/0874