EP0189087A1 - Voltage-dependent electric resistance (varistor) - Google Patents

Abstract

A voltage-dependent resistor or varistor is composed of a monolithic ceramic body made up of a plurality of layers of varistor material containing zinc oxide, alternating with layers of precious metal serving as coatings on the layers and which are alternatingly electrically connected to separate locations on the exterior surfaces of the body. The porosity of the layers of varistor material does not exceed 5%; the proportion of bismuth is at most 1 mol %; and the precious metal coatings include 50-80% by weight of palladium.

Description

The invention relates to a voltage-dependent electrical resistance (varistor) consisting of a ceramic, monolithic body made of a plurality of layers of 20 to 350 µm thick made of varistor material with grain sizes of 7 to 20 µm based on zinc oxide (Zn0) with up to 6 mol%. with additions of oxides of one or more of the metals Bi, Sb, Co, Ni, Cr, Mn, Mg, B, Al, Ba and as a coating serving, at most 10 µm thick noble metal layers that alternate with the varistor material layers and alternate to different locations of the Lateral surfaces of the body guided and there are electrically conductive and contacted with other metal layers.

It is therefore a varistor that is manufactured using multilayer technology.

Such a varistor is described in the publication "Advances in Ceramics" (American Ceram. Society, Columbus) 1981, Vol. 1, pages 349 to 358. The average grain size is given there as 10 µm. The response voltage per grain boundary is 2 to 3 V. The data for the thickness of the varistor material layers are 20 to 200 μm, the properties being measured on varistors with a layer thickness of 40 μm or 150 μm with 20 layers stacked one above the other. The non-linearity coefficient α is given as 20 to 30, while the varistor voltage, measured at 1 mA, is given as 4 to 40 volts.

Fired-in silver electrodes are specified as metal layers for contacting the coatings arranged alternately in the monolithic body on its surface. There is no further information about the material of the linings inside the monolithic body. There is also no information about the porosity of the material.

In the publication "Journal of Applied Physics", 54 (5) May 1983, pages 2764 to 2772, which also, like the publication discussed above, deals with low-voltage varistors, on page 2765, left column, with reference to the above published publication, varistors in multi-layer technology would have reduced current density in the event of overvoltages and high capacitance without solving the basic problem of grain size distribution.

Pages 44 to 63 of the SIEMENS brochure "Noble gas-filled surge arresters; metal oxide varistors SIOV" from November 1984 explain in detail the theoretical basics of metal oxide varistors based on zinc oxide, and also the definitions for the individual terms contain.

I₂ ist dabei ein Ampere, I₁ = 1 mA, U₂ ist die bei 1 A gemessene Spannung und U_l ist die bei 1 mA gemessene Spannung.So on page 48 the non-linearity exponent α is given with

I ₂ is an ampere, I ₁ = 1 mA, U ₂ is the voltage measured at 1 A and U _l is the voltage measured at 1 mA.

The voltage that is measured at 1 mA is defined as the varistor voltage on page 52. The varistor voltage is used to classify varistors.

Varistors for low voltages, so-called low-voltage varistors, which are manufactured using conventional technology, have grain sizes of approximately 100 .mu.m and even larger in order to keep the number of grain boundaries between the coatings low. However, such a coarsely crystallizing material leads to the problem that the grain size distribution scatters strongly and thus the steepness of the I-U characteristic (non-linearity coefficient a) drops sharply.

Low-voltage varistors manufactured in this way cannot generally be used to protect against higher voltages because the heat generated in the ceramic body cannot be dissipated.

The present invention has for its object to improve a voltage-dependent electrical resistance (varistor) of the type mentioned in that the range of the varistor voltage is expanded so that in this way and from the same material, varistors with different varistor voltages can be produced that the The amount of the palladium usually used for such components is reduced and, last but not least, improved heat dissipation results.

• a) die Porosität der Varistormaterialschichten des Keramikkörpers 5 % nicht übersteigt,
• b) der Anteil an Wismuth (Bi, gerechnet als Bi₂0₃) im Varistormaterial 0,4 bis höchstens 1 Mol% (entsprechend 2 bis 5 Gew%) beträgt und
• c) die Beläge aus 50 bis 80 Gew% Silber (Ag) und 50 bis 20 Gew% Palladium (Pd) bestehen.

To achieve this object, the voltage-dependent electrical resistance of the type specified at the outset is characterized in that

• a) the porosity of the varistor material layers of the ceramic body does not exceed 5%,
• b) the proportion of bismuth (Bi, calculated as Bi ₂ 0 ₃ ) in the varistor material is 0.4 to at most 1 mol% (corresponding to 2 to 5% by weight) and
• c) the coatings consist of 50 to 80% by weight of silver (Ag) and 50 to 20% by weight of palladium (Pd).

The low porosity, which should preferably be less than 1%, ensures that the metal of the internal electrodes cannot penetrate into pores, as a result of which a shortened electrode path is created, which leads to an early flashover (short circuit) in the event of impulse loading. The reduction in the bismuth content from usually more than 2 mol% to at most 1 mol% and preferably 0.6 mol% has the effect that on the one hand the grain growth is reduced and thus the grain size distribution is evened out, and on the other hand that the reaction of the coatings with the ceramic material in the Sintering temperature is avoided, which avoids alloying of the palladium with the consequence of the island formation of the deposits.

The toppings preferably consist of 70% by weight of silver and 30% by weight of palladium.

It is also advantageous if the ceramic body consists of varistor material layers, the thickness of which is in the range from 35 μm to 350 μm, with thicker layers resulting in higher varistor voltages in the range from 4 volts to 350 volts.

• Zn0 94,6 (87,3), Bi₂0₃ 0,6 (3,2), Sb₂0₃ 1,6 (5,1), Co₃0₄ 0,4 (1,1), Ni0 1,3 (1,1), Cr₂0₃ 0,6 (1,1), MnC0₃ 0,8 (1,02), Mg0 0,06 (0,003), B203 0,033 (0,05), A1₂0₃ 0,002 (0,017) und BaC0₃ 0,005 (0,001).

The varistor body is preferably 1 to 10 mm long, 1 to 3.6 mm wide and 0.5 to 3 mm thick, the thickness always being less than the smallest length or width. The preferred composition of the varistor material is as follows (data in mol% and in brackets in% by weight):

• Zn0 94.6 (87.3), Bi ₂ 0 ₃ 0.6 (3.2), Sb ₂ 0 ₃ 1.6 (5.1), Co ₃ 0 ₄ 0.4 (1.1), Ni0 1.3 (1.1), Cr ₂ 0 ₃ 0.6 (1.1), MnC0 ₃ 0.8 (1.02), Mg0 0.06 (0.003), B203 0.033 (0.05), A1 ₂ 0 ₃ 0.002 (0.017) and BaC0 ₃ 0.005 (0.001).

The low bismuth content enables sintering temperatures of up to 1,150 ° C, which means that thin layers and a corresponding number of layers can be used to produce varistors with a varistor voltage down to 4 V.

These varistors are manufactured using multilayer technology in the same way as is known, for example, for multilayer ceramic capacitors. In this context, reference is made to the U.S. Patents 2,736,080 and 3,235,939 and to DE-PS 1 282 119.

From the starting material, which has an average grain size of about 1 μm due to fine grinding, organic binder materials (e.g. polymethylacrylates, methyl cellulose, polyvinyl alcohol) and solvents (e.g. water, ethyl methyl ketone) as well as plasticizers (phthalates, esters) are used - as is per se is known - a slip is produced, which is then drawn out into a very thin film using conventional technologies (calendering, stripping technology - doctor blade). A sample of the inner coverings made from the specified silver-palladium compound is applied to pieces of approximately the size of postcards onto the foils thus produced, after which a corresponding number of such postcard-sized foils are stacked on top of one another in such a way that the alternating displacement of the coverings in the finished product Body results. Finally, after a pressing process, the layer varistor is separated from the stack in its raw form and - after passing through a tempering and binder burnout cycle customary in multilayer technology - sintered at temperatures up to 1150 ° C.

As stated, this method is well known, although variations in the specific production method are also known.

• Fig. 1 schematisch einen Schichtvaristor,
• Fig. 2 ein U-I-Diagramm, das die Verbesserung durch die Erfindung darstellt,
• Fig. 3 ein Diagramm zu Vergleich mit dem Stand der Technik,
• Fig. 4 ein Diagramm über die Abhängigkeit der Varistorspannung von der Sintertemperatur und
• Fig. 5 ein Diagramm über die Abhängigkeit des Schutzpegels von der Sintertemperatur.

The invention is explained in more detail below with reference to the figures. Show it

• 1 schematically shows a layer varistor,
• 2 is a UI diagram illustrating the improvement of the invention;
• 3 is a diagram for comparison with the prior art,
• Fig. 4 is a diagram of the dependence of the varistor voltage on the sintering temperature and
• Fig. 5 is a diagram of the dependence of the protection level on the sintering temperature.

In Fig. 1, the varistor body 1 is shown schematically, which consists of layers 2 of varistor material. The coverings 3 and 4 alternate with varistor material layers 2, the coverings 3 in the present case being guided to the right outer surface 5 and the coverings 4 to the left outer surface 6 of the ceramic body. As a result of the sintering process, the ceramic body 1 consists of a monolithic block, in the interior of which the coatings 3 and 4 are arranged. It is also possible that the pads 3 and 4 protrude on the same side of the monolithic block, where at the ends to be contacted then end alternately at different points in this surface side and are contacted there with opposite poles.

The pads 3 and 4 are connected to each other with opposite poles and electrically conductive. In the present case, opposite-pole means that the coatings 3 on the surface 5 with a further metal layer 7, e.g. made of silver or another solderable metal, which is connected to a pole of the voltage source or the circuit, while the coatings 4 on the surface side 6 by the further metal layer 8, also made of silver or the like, connected to each other The opposite pole of the voltage source or the circuit is connected.

Reference number 9 denotes the thickness of the layers 2 made of varistor material.

A prerequisite for the operation of the varistor is that the distances 10 and 11 between the top layer 3 and the bottom layer 4 and the surface 14 or the surface 15 and the distances 12 of the layers 3 to the metal layer 8 and the distances 13 of the metal layers 4 to the metal layer 7 are each greater than the thickness 9 of the layers 2 made of varistor material. In order to achieve the necessary size of the distances 10 and 11, there are for example varistor material layers 2 ′ which do not contain any coatings 3 or 4. In FIG. 1, this is due to the boundary lines 16 and 17 between the top layer 2 provided with a covering 3 and the layer 2 ′ without a covering or between the lower layer 2 provided with the covering 4 and the layer 2 without a covering 'shown.

The varistor according to the invention can be provided with power supply wires 18 and 19, which are soldered to the metal layers 7 or 8 or attached in some other way.

If the varistor according to the invention is to be used as a chip by placing it on and attaching it to contact points on printed conductor tracks, then instead of the power supply wires there can be contact surfaces which, in the present example, through the extensions 20 and 21 of the metal layer 7 on the surfaces 14 and 15, and are represented by extensions 22 and 23 of the metal layer 8 on the surfaces 14 and 15.

For use in printed circuits in which the contact points are arranged in a grid dimension (simple or even multiple of 2.5 mm), the grid spacing 24 between the power supply wires 18 and 19 is to be determined, as is known per se for such components.

On the other hand, the necessary grid spacing 25 between the extensions 20 and 22 or 21 and 23 can be determined by appropriate choice of dimensions.

It can be seen from Fig. 1 that the coverings 3 and 4 in their embodiment of the invention in a thickness of up to 5 µm and preferably 2 µm ensure good dissipation of the heat generated in the interior of the monolithic block, since they are by using relatively more silver than palladium can be made thicker than is possible for pure palladium layers because of the production and because of the relatively high costs for pure palladium.

The UI diagram shown in FIG. 2 shows one of the advantages of the present invention, which consists in the fact that the small amount of bismuth in the varistor material and the possible use of silver in larger amounts for the coatings 3 and 4 compared to the palladium make alloying away of the metal of the coverings and thus an island formation which deteriorates the properties does not occur.

This is expressed in FIG. 2 by the fact that in conventional varistors the curve 26 suddenly rises steeply at current intensities in the upper region, while the curve 27 for varistors according to the invention has a considerably reduced increase in this upper region.

The island formation, which is caused by alloying away the linings (migration), causes the terminal voltage to rise sharply at high currents, because the series resistance of the linings increases sharply due to this island formation.

3 shows a U-I diagram in which a varistor of the present invention (curve 30) is compared with known varistors (curves 28 and 29). 3 is taken from FIG. 2 of the publication "Advances in Ceramics" mentioned at the beginning.

Curve 28 applies to varistors that consist of 20 varistor material layers each 40 μm thick, while curve 29 applies to known varistors with 20 varistor material layers each 150 μm thick.

Curve 30 applies to varistors of the present invention made of 50 layers each 30 µm thick.

It can be seen from FIG. 3 that the known varistors result in a strongly increasing terminal voltage of up to 100 volts even at 10 amperes, while such an increase does not take place in curve 30. It would occur as curve 30 'shows if the rules of the present invention are not followed.

Despite the high number of 50 layers, which significantly increases the stability of the varistor, the heat dissipation from the body with coatings of 70% silver and 30% palladium, each with a thickness of 2.0 pm, is sufficiently large to ensure the functionality of the varistor To ensure even at high currents or voltages.

The diagram according to FIG. 4 shows the varistor voltage as a function of the sintering temperature at a sintering time of one hour for varistors which consist of 10 layers, with different layer thicknesses being present.

The varistor voltage is given in volts on the ordinate and the sintering temperature t in ° C on the abscissa. Here too, the coatings consist of 70% silver and 30% palladium and are 2 µm thick.

Curve 31 applies to varistors consisting of 10 layers with a layer thickness of 165 μm each. Curve 32 applies to varistors consisting of 10 layers with a layer thickness of 77 µm each.

Curve 33 applies to varistors consisting of 10 layers with a thickness of 37 μm each, and curve 34 applies to varistors composed of 10 layers with a layer thickness of 23 μm each.

It can be seen from FIG. 4 that decreasing varistor voltages can be achieved with decreasing layer thickness and increasing sintering temperature.

Due to the relatively high sintering temperature of up to 1080 ° C, a very high density, i.e. achieved a very low porosity of the layers of ceramic material, which significantly improves the electrical properties of the varistors. The increased sintering temperature is possible due to the low bismuth content.

5 shows curves which illustrate the protection level as a function of the sintering temperature.

The protection level of varistors is the terminal voltage that occurs at a varistor when a current pulse of the indicated current intensity occurs.

The terminal voltage in volts is plotted on the ordinate, while the sintering temperature t _{s is given} in "C on the abscissa.

Four pairs of curves 35, 36, 37 and 38 are indicated, specifically for layer thicknesses in the sintered state of 165, 77, 37 and 23 µm. The upper curve applies to a current of 10 A and the lower curve applies to a current of 5 A.

It can also be seen from the diagram according to FIG. 5 that with decreasing layer thickness and increasing sintering temperature, decreasing values of the terminal voltage are achieved.

The specified varistor material ensures a dielectric strength of 300 V / mm, which ensures sufficient steepness (non-linearity exponent o () even with thin layers.

In the case of coarsely crystalline material with a dielectric strength of less than 150 V / mm, problems arise due to too few grains with a large scatter in the diameter, as explained in the above-mentioned publication "Journal of Applied Physics".

• Bezugszeichenliste
• 7 Patentansprüche
• 1 Zusammenfassung
• 5 Figuren

Investments:

• Reference symbol list
• 7 claims
• 1. Summary
• 5 figures

Reference symbol list

• 1 ceramic, monolithic body
• 2 layers of ceramic varistor material with coverings 3, 4
• 2 'layers of ceramic varistor material without coatings 3 coatings of metal layers arranged in the body 1
• 4 coverings made of metal layers arranged in the body 1
• 5 side surface of the body 1
• 6 side surface of the body 1
• 7 Additional metal layer on the side surface 5
• 8 Additional metal layer on the side surface 6
• 9 thicknesses of layers 2
• 10 Distance between the top surface 3 and surface 14
• 11 Distance between the underlay 4 and surface 15
• 12 distance of the pads 3 to the metal layer 8
• 13 distance of the pads 4 to the metal layer 7
• 14 surface of the body 1
• 15 surface of the body 1
• 16 boundary line between upper layer 2 with coating 3 and layer 2 'without coating
• 17 boundary line between lower layer 2 with coating 4 and layer 2 'without coating
• 18 power supply wire, attached to the metal layer 7
• 19 power supply wire, attached to the metal layer 8
• 20 extension of the metal layer 7 to the surface 14
• 21 Extension of the metal layer 7 to the surface 15
• 22 Extension of the metal layer 8 to the surface 14
• 23 Extension of the metal layer 8 to the surface 15
• 24 pitch between the power supply wires 18 and 19
• 25 pitch between extensions 20 and 22 or 21 and 23
• 26 U-I curve for known varistors
• 27 U-I curve for varistors according to the invention
• 28 U-I curve for known varistors of 20 layers, each 40 µm thick
• 29 U-I curve for known varistors of 20 layers, each 150 µm thick
• 30 U-I curve for varistors according to the invention of 50 layers, each 30 µm thick
• 30 'U-I curve shape, if not according to the invention
• 31 Vt _s curve for varistors consisting of 10 layers, each 165 µm thick
• 32 Vt _s curve for varistors from 10 layers, each 77 µm thick
• 33 Vt _s curve for varistors from 10 layers, each 37 µm thick
• 34 vt _s curve for varistors made of 10 layers, each 23 µm thick
• 35 Vt _s curve for varistors consisting of 10 layers, each 165 µm thick
• 36 Vt _s curve for varistors from 10 layers, each 77 µm thick
• 37 Vt _s curve for varistors from 10 layers, each 37 µm thick
• 38 Vt _s curve for varistors from 10 layers, each 23 µm thick

Claims (7)

1.Voltage-dependent, electrical resistance (varistor) consisting of a ceramic, monolithic body (1) made of a variety of 20 to 350 µm thick layers (2) made of varistor material with grain sizes of 7 to 22 µm on the basis of zinc oxide with up to 6 Mol% of additions of oxides of one or more of the metals Bi, Sb, Co, Ni, Cr, Mn, Mg, 8, Al, Ba and serving as coatings (3, 4), at most 10 µm thick noble metal layers, with the varistor material layers (2) alternate and alternate to different locations of the side surfaces (5, 6) of the body (1) and there are electrically conductive and opposite poles are contacted with further metal layers (7, 8), characterized in that a) the porosity of the varistor material layers (2) of the ceramic body (1) does not exceed 5% b) the proportion of bismuth (Bi, calculated as Bi ₂ 0 ₃ ) in the varistor material is 0.4 to at most 1 mol% (corresponding to 2 to 5% by weight) and c) the coatings consist of 50 to 80% by weight of silver (Ag) and 50 to 20% by weight of palladium (Pd). 2. Voltage-dependent electrical resistance according to claim 1, characterized in that the porosity of the varistor material layers (2) is less than 1%. 3. Voltage-dependent electrical resistance according to claim 1 or 2, characterized g + h - net in that the bismuth fraction is 0.6 mol% (corresponding to 3.2% by weight Bi ₂ 0 ₃ ). 4. Voltage-dependent electrical resistance according to claim 1, 2 or 3, characterized in that the coatings (3, 4) consist of 70 wt% Ag and 30 wt% Pd. 5. Voltage-dependent electrical resistance according to one of claims 1 to 4, characterized in that the ceramic body (1) consists of varistor material layers (2) whose thickness (9) is in the range from 20 µm to 350 µm, with thicker layers ( 2) result in higher varistor voltages in the range from 4 volts to 350 volts. 6. Voltage-dependent electrical resistance according to one of claims 1 to 5, characterized in that the varistor body (1) is preferably 1 to 10 mm long, 1 to 3.6 mm wide and 0.5 to 3 mm thick, the Thickness (9) is always less than the smallest length or width. 7. Voltage-dependent electrical resistance according to one of claims 1 to 6, characterized in that the varistor material is composed as follows (data in mol% and in brackets in% by weight): Zn0 94.6 (87.3), Bi ₂ 0 ₃ 0.6 (3.2), Sb ₂ 0 ₃ 1.6 (5.1), Co ₃ O ₄ 0.4 (1.1), Ni0 1.3 (1.1), Cr ₂ O ₃ 0.6 (1.1), MnCO ₃ 0.8 (1.02), MgO 0.06 (0.003), B203 0.033 (0.05), A1 ₂ 0 ₃ 0.002 (0.017), and BaC0 ₃ 0.005 (0.001).

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