EP1789977A1

EP1789977A1 - Varistor comprising an insulating layer produced from a loaded base glass

Info

Publication number: EP1789977A1
Application number: EP05789564A
Authority: EP
Inventors: Thomas Jost; Andreas Schriener; Harald Reisinger; Gerd Klemen
Original assignee: Epcos AG
Current assignee: TDK Electronics AG
Priority date: 2004-09-15
Filing date: 2005-09-15
Publication date: 2007-05-30
Anticipated expiration: 2025-09-15
Also published as: WO2006029610A1; JP4755648B2; US8130071B2; JP2008513982A; US20080030296A1; DE102004044648A1; EP1789977B1

Abstract

The invention relates to a varistor which comprises a ceramic base (1) whose surface is at least partially provided with an insulating layer (2). Said layer is composed of a base glass and a filler, said filler containing 3Al2O32SiO2.

Description

description

VARISTOR WITH AN INSULATING LAYER OF BASIC GLASS WITH FILLER

The invention relates to a varistor.

Zinc oxide (ZnO) energy-generating resistors are non-linear voltage-dependent resistance bodies, which surround ceramic sintered bodies based on zinc oxide as a resistance element. In the case of varistors, the electrical resistance decreases sharply above a response voltage with increasing voltage. Due to this electrical behavior varistors for the protection of electrical equipment and devices against Ü. used voltages and surges. The varistor is thereby switched in parallel to the protective electrical system and limited by its current-voltage characteristic, the maximum occurring at the electrical system voltage. For electrically contacting the varistors, electrodes are applied to the front sides of the cylindrical main body of the varistors.

Overvoltages and voltage peaks can be roughly subdivided over a time into lightning overvoltage (time range: microseconds), switching overvoltages (time range: milliseconds) and temporary overvoltages (time range: seconds). In particular, overvoltages in the microsecond range can reach very high voltage peaks. These very fast and high voltage peaks not only load the zinc oxide ceramic of the varistor very strongly, but it also comes without suitable countermeasures to an electric Über¬ shock on the outside or surface of the varistor. From US 5,294,909 a zinc oxide varistor is known, in which the lateral surface of the ceramic base body is provided with a layer of high resistance. The crystallized glass composition for the wetting of the ceramic base body has lead oxide (PbO) as the main component and is enriched with the components ZnO, B ₂ O ₃ , SiO ₂ , MoO ₃ , WO ₃ , TiO ₂ and NiO in order to obtain the crystallinity and to promote the insulating property of the layer. The addition of larger amounts of PbO to the insulating layer increases its thermal expansion coefficient, with the addition of larger amounts of ZnO permitting crystallization of the glass composition of the layer. In contrast, the addition of larger amounts of B ₂ O ₃ leads to a reduction in the crystallization of the layer, in particular when the weight fraction of the layer of B ₂ O ₃ exceeds 15%. Furthermore, the increase in the amount of SiO ₂ leads to a reduction in the crystallization, the thermal expansion coefficient being simultaneously increased.

In the application, arresters consisting of varistors are exposed to environmental influences such as moisture and chemical pollutants over long periods of time (lifetime> 30 years). There is the danger that these environmental influences lead to a reduction of the ZnO ceramic of the varistor and change the current-voltage characteristic. The protective function against environmental influences takes over the wrapping.

It is the object of the present invention to specify means for increasing the flashover resistance of a varistor. Another object is to provide means with which the ceramic of a varistor can be protected from environmental influences. A varistor is proposed which has a ceramic base body whose surface is at least partially provided with an insulating layer which is composed of a base glass and a filler, the filler containing 3Al ₂ O ₃ 2SiO ₂ .

With the stated composition, a high electrical insulation capacity is given, which is jointly responsible for a good Über¬ impact resistance of the varistor.

The insulating layer is also harmless in terms of its environmental compatibility, since it does not have to contain lead. Advantageously, the layer is free of lead.

It is preferred that the layer has a filler content of 5 to 40%. With this proportion of filler it is achieved that the thermal expansion coefficient of the insulating layer is reduced in order to avoid cracking of the layer. In particular, it can be achieved with a proportion of filler in this range that the thermal expansion coefficient of the layer is lower than that of the ceramic base body of the varistor.

It is favorable if zinc oxide accounts for 30 to 50% by weight of the base glass.

In addition, a varistor is provided which has a ceramic base body, wherein a layer which contains material-strengthening fibers is applied to at least a portion of the ceramic base body.

With the material-strengthening fibers, the layer is given a high strength, so that the layer at elevated mechanical see or thermal stresses do not rupture or auf¬ bursts.

Preferably, the layer at least partially hermetically seals the ceramic base body to the outside, so that the necessary to ignite the electrical component or the ceramic body oxygen can not penetrate to the hot ignition source of the varistor or the ceramic body. In the absence of this oxygen, the varistor can not come to ignition even with considerable overvoltage.

It is a further advantage of the high-strength layer that the escape of harmful materials of the ceramic Grund¬ body is avoided to the outside. The poisoning potential is thus lowered for a user.

In addition, thermal insulation of the electrical component with respect to the environment is ensured by means of the layer, so that burning of a user upon contact with the varistor is impeded and thus the potential for danger is reduced.

It is preferred that the layer comprises refractory or at least flame retardant materials. If, despite the high layer strength, for example under extreme pressure or temperature conditions, the electrical component or the ceramic base body are ignited, the flame-retardant materials of the layer can slow down the spread of the Bren¬ nens.

Protection against external flaming is likewise achieved with such a refractory layer. The risk of ignition of the entire electrical component, or the Propagation of the firing to an arrangement of a plurality of electrical components can advantageously be reduced with this measure.

According to one embodiment, the material-strengthening fibers are added to the mullite mixture. This results in an insulating layer with a high flashover and material strength. In addition, if flame retardant materials are additionally added to the mullite mixture, the refractoriness of the varistor or of the insulating layer can also be increased.

The invention will be explained in more detail with reference to the following embodiments and figures. Showing:

FIG. 1 shows a varistor which is provided with metallization on the front side and with an insulating layer on its lateral surface,

FIG. 2 shows a graph for illustrating the failure rate of varistors with and without an insulating layer that has a 3Al ₂ O ₃ 2SiO ₂ insulating layer at different current loads,

FIG. 3 shows a varistor with a fiber-containing outer layer,

FIG. 4 shows a varistor according to FIG. 3 with frontally applied contact bodies,

FIG. 5 shows an electrical component with a plurality of internal electrodes and a fiber-containing outer layer. Load cases in the sense of a direct lightning strike are standardized as 4/10 μs tests in the IEC standard 60099-4 ver¬ anchored. The 4/10 μs test has a rise time to the peak current of 4 μs, with a cooldown of 50% of the peak value 10 μs. For arresters of the 1OkA and 20kA class, the load with two impulses with a peak current of 100 kA each is prescribed, without resulting in a flashover on the arrester or varistor. Loads according to the 4/10 test are referred to below in this document as impulse loads.

FIG. 1 shows a varistor with a ceramic base body 1, the surface of which is provided with an insulating layer 2 and whose end faces are provided with metallizations or electrodes 3. In particular, the lateral surface of the base body 1 is provided with the insulating layer. For the insulating layer, a composite glaze consisting of a base glass and a filler is proposed. The base glass contains 30 to 50% ZnO, 30 to 40% B ₂ O ₃ , 0 to 10% CuO and 0 to 10% P ₂ O ₅ . As a filler of the mixture, mullite (3Al ₂ O ₃ 2SiO ₂ ) in the range of 5 to 40% is used. The filler is added in powder form (grain size 0 to 200 microns) of the glass layer or the glaze.

During glass penetration, the base glass or the glass frit melts, runs down and forms a glass-like coating of the varistor. For the filler grains contained, the temperature of the glass burn used is far below the melting point of the filler grains, which is why they do not melt and can be embedded unchanged in the base glass. A filler content of between 5 and 40% has proven to be advantageous for the composite glaze or the insulating layer.

The application of the insulating layer can be carried out, for example, by the following steps:

1) Mix the base glass frit with the filler mullite, water and a binder.

2) Application of the resulting paste by means of spraying or paste printing technology.

3) baking the glass paste at 600 to 680 ⁰ C, which hereby at the same time er¬ a tempering step for the varistor and the long-term stability of the ceramic is improved.

In order not to influence the current-voltage characteristic of varistors or only slightly, the temperature must not be too high during the production step of the coating of the ZnO ceramic. Therefore, only glasses with low melting points should be used. Glasses with a low melting point and good insulation properties for energy varistors, however, have in the past only been realized by glasses containing glasses or bismuth based glasses, whereby lead-containing glasses can not meet environmental protection requirements and bismuth-based glasses are expensive due to the high bismuth raw material costs he is. Organic paints, on the other hand, represent a cost-effective possibility of encasement, but are fraught with weaknesses in relation to the desired long-term stability of energy varistors. An important point for the pulse strength of cladding or insulating layers is the thermal shock resistance. At a pulse load, the temperature of the Energievaristors can rise within microseconds by up to 150 ⁰ C. If the coefficient of thermal expansion of the coating is greater than that of the ceramic, this load increasingly leads to crack formation in the coating and thus to poor pulse strength. Low-melting glasses consistently have too high a thermal expansion coefficient compared to a zinc oxide ceramic, so that the pulse strength thus remains unsatisfactory.

By contrast, the admixture of filler with a very low coefficient of thermal expansion into the base glass leads to a lower coefficient of thermal expansion of the insulating layer. By adding the filler mullite so the thermal expansion coefficient of the glaze ver¬ kleinert. By optimizing the expansion coefficient of the composite glaze, it can be adapted to approximately the value of the expansion coefficients of the zinc oxide ceramic of the varistor.

The following table shows the thermal expansion coefficient of varistor ceramic and a composite glaze for different temperatures.

The values T, αl, α2 in each case represent the temperature, the expansion coefficient of the varistor ceramic and the expansion coefficient of the insulating layer or composite glaze.

The varistor can be designed as a multilayer varistor with integrated internal electrodes, in which case the contact bodies are preferably arranged on the side surface of the basic body. Each contact body is contacted with one end of an inner electrode of an inner electrode set, see also FIG. 5.

FIG. 2 is a graphical representation of the failure rate of varistors with and without an insulating layer provided with mullite with increasing current impulse load. The ver ^¬ Tikale axis represents the cumulated failure rate of Varisto¬ ren percentage represents the horizontal axis, however, the applied on the varistors pulse current in amperes. The dark bars show the behavior of varistors provided with a mullite-containing insulating layer. It can clearly be seen how the failure rates of such varistors begin to increase only at a relatively high value of 110 kA (kilo-ampere), especially when this pulse is applied in short time periods in succession. On the other hand, the failure rate of varistors without an insulating layer having a multilayer already increases at 90 kA. The weight fraction of mullite is the insulating layer of the the gray bars represented Varistors is 20%. Energy varistors with a height of 44 mm and a diameter of 43.5 mm were used.

A mullite-containing composite glaze therefore has a design optimized thermal expansion coefficient. The glaze also has a very good mechanical strength, which also has a positive effect on the pulse strength. Thus, the bending tensile strength at 20% by weight of mullite is 78 MPa.

The present composite glaze advantageously also protects the ceramic due to the glassy fusion zuver¬ casual from environmental influences. It is also non-toxic and unobjectionable in the sense of environmental compatibility, since in particular it can also be composed of lead-free. Likewise, the composite glaze does not have to contain bismuth, so that it is substantially less expensive than currently used alternatives. The filler mullite used has a low thermi¬ rule expansion coefficient in the range of 40 * 10 ^-7 (K ^"1) and a high melting point> 1800 ° C. The ho¬ hen melting point, it is ensured that during the Ein¬ firing the glaze no or at least only a very ge rings chemical and / or physical conversion of Füll¬ material takes place.

FIG. 3 shows a varistor whose surface is at least partially provided with an insulating layer 2, which contains fiber composite materials 4. The fiber composites are preferably added to the previously described mullite mixture. The layer preferably hermetically seals an inner region of the ceramic base body to the outside. By means of the fiber composite materials, a substantial increase in the strength of the insulating sheath 2 of the transistor is achieved. As a result, the sheath can withstand high loads, such as, for example, a thermally induced expansion of the ceramic base body, without it forming cracks or openings. The thermally induced expansion of the ceramic body can be triggered, for example, by applying an increased operating voltage, which can locally lead to melting of the varistor ceramic with explosive leakage of ceramic material and various reaction products and thus to ignition of the varistor enclosure. As a result, this can lead to the ignition of entire devices or Anlagen¬ parts in which the varistor is used. By means of the layer containing fibers, it is avoided that the possibly harmful materials released from the ceramic base body escape to the outside, or that the oxygen necessary for the ignition penetrates into the interior region of the ceramic base body.

An increased strength of the Varistorumhüllung 2 is achieved with the addition of fibrous reinforcing materials unterschied¬ Licher length of organic and inorganic nature, as well as with the addition of organic and inorganic Matrixelemen¬ th or composites.

Aramid fiber is preferred as the fiber 4 of organic nature. Fiber of inorganic nature is preferably glass fibers, carbon fibers or mineral wool. These have the advantage that they have a flame-retardant effect.

Suitable organic matrix elements or composite materials are silicone resins, phenolic resins or epoxy resins. As inorganic Matrix elements are preferably used hydraulically setting Ke¬ ceramics and cements.

Glass fiber shreds 4 having a length of 0.2 mm in different mixing ratios are preferably mixed with a silicone resin paint formulation or phenolic resin paint formulation to form a dippable or sprayable mixture which can be applied to the ceramic base body. The application of the sheath 2 can take place in several shifts until the required sheath thickness is achieved. In this case, 3 to 7, in particular 5 dives are vor¬ given to reach a coating thickness of between 7 and 9 mm, since it has been found that this thickness gives a particularly good strength, but only a ver¬ relatively short production time required is.

After a hardening process characterized by a temperature increase, for example by passing the varistor through an oven, the casing 2 enriched with the additives is brought to the desired high strength.

FIG. 4 shows a varistor 1, which is provided with contact bodies 3 at the end. It is preferred that the application of the envelope 2 takes place prior to the firing of the contact bodies, so that the layer applied on the front sides of the varistor is softened by the extremely high temperature during the burning-in of the contact body and pushed away or removed in an anvil , Thus, the contact bodies 3 each have an outwardly directed, free surface, which can be contacted with another contact body. However, it is also possible to restrain the contact bodies 3 on the end faces of the ceramic basic body 1. Subsequently, for example, after the curing process, the coating is deposited by means of an etching process from the places where no coating is desired, in particular above the Kon¬ contact body and then immerse the varistor in a coating mass or liquid.

FIG. 5 shows a multilayer varistor with a ceramic main body 1, in the interior of which internal electrodes 5 are arranged, which are each connected at one end to a contact body or metallization 3 applied to the top or side surface of the ceramic main body. The multilayer varistor has a mullite-containing outer layer 3 according to the preceding exemplary embodiments, which may be enriched with material-strengthening fibers. Thus, a multilayer varistor is provided, which can not be set on fire by means of a high-strength, preferably flame-retardant sheath 2, even with accidental or accidental overvoltages, or at least only with difficulty. As described with reference to FIG. 4, it is preferred that the metallizations 3 be free of cladding materials.

LIST OF REFERENCE NUMBERS

1 ceramic body of a varistor

2 insulating layer

3 metallization

4 fiber

5 internal electrodes

Claims

claims

A varistor comprising a ceramic base body (1) whose surface is at least partially covered by an insulating layer (2) which is composed of a base glass and a filler, wherein the filler is 3Al ₂ O ₃ 2SiO ₂ ent ¬ stops.

2. Varistor according to claim 1, wherein the layer (2) has a filler weight fraction of 5 to 40%.

3. Varistor according to one of claims 1 or 2, wherein the base glass has a ZnO weight fraction of 30 to 50% auf¬.

4. Varistor according to one of the preceding claims, wherein the base glass has a B ₂ θ ₃ weight fraction of 30 to 40%.

5. Varistor according to one of the preceding claims, wherein the base glass has a CuO weight fraction of up to 10%.

6. Varistor according to one of the preceding claims, wherein the base glass has a P ₂ θ ₅ weight fraction of up to 10%.

7. Varistor according to one of the preceding claims, in which the end faces of the ceramic base body (1) are provided with metallizations (3).

8. Varistor according to one of the preceding claims, wherein the varistor is a Vielschichtvaristor.

9. Varistor according to one of the preceding claims, wherein the insulating layer (2) contains material-strengthening fibers (4).

10.Varistor according to claim 9, wherein the fibers (4) are orga¬ African and / or inorganic.

11.Varistor according to one of claims 9 or 10, wherein the insulating layer (2) contains an organic composite (4).

12.Varistor according to one of claims 9 or 10, wherein the layer (2) contains an inorganic composite (4).

13.Varistor according to one of the preceding claims, wherein the layer (2) is solidifiable by increasing the temperature.