EP0992042B1 - Non-linear resistance with varistor behaviour and method for the production thereof - Google Patents

Abstract

The nonlinear resistor has varistor behaviour and has a matrix and a filler in powder form which is embedded in the matrix. The filler contains sintered varistor granules with predominantly spherical particles of doped metal oxide. These particles are made up of crystalline grains separated from one another by grain boundaries. The filler also contains electrically conductive particles, which cover at most a part of the surfaces of the spherical particles, and/or the varistor granules contain two fractions of particles with different sizes, of which the particles in the first fraction have larger diameters than the particles in the second fraction and are arranged essentially in the form of close sphere packing and the particles in the second fraction fill the interstices formed by the sphere packing.The resistor can be produced straightforwardly and cost-effectively and is distinguished by a high nonlinearity coefficient, which is desired for a good protection characteristic, and by a high power acceptance.

Description

TECHNICAL AREA

The invention is based on a non-linear resistor Varistor behavior according to the preamble of claim 1. This resistor contains a matrix and a powdery filler embedded in the matrix. The filler contains a sintered varistor granules with predominantly spherical particles of doped metal oxide. The particles are made of crystalline, constructed by grain boundaries separate grains. Opposite Comparable acting resistors based on a sintered ceramic consuming sintering processes are much simpler, such Composite resistors made relatively simple and in great variety of shapes become. The invention also relates to a method for producing this Resistance.

STATE OF THE ART

A resistor of the aforementioned type is described in R.Strümpler, P.Kluge-Weiss and F.Greuter "Smart Varistor Composites", Proceedings of the 8th CIMTECH World Ceramic Congress and Forum on New Materials, Symposium VI (Florence, June 29 - July 4, 1994). This resistor consists of a polymer filled with a powder. The powder used is a granulate which has been produced by sintering a spray-dried varistor powder on the basis of a zinc oxide doped with oxides of Bi, Sb, Mn, Co, Al and / or other metals. This granule has ball-shaped spherical particles with varistor behavior, which are composed of crystalline grains separated by grain boundaries. The diameters of these particles are up to 300 microns. By changing the dopants and the sintering conditions, the electrical properties of the sintered granules, such as the nonlinearity coefficient α _B or the breakdown field strength U _B [V / mm], can be adjusted over a wide range. With the same starting materials, such a resistor has a higher nonlinearity coefficient and a higher breakdown field strength as the filler content decreases. However, it has been shown that then when limiting a voltage, the capacity for energy is relatively low.

In WO 97/26693 is a composite material based on a polymeric matrix and of a powder embedded in this matrix. As a powder is a Granules are also used, which also by sintering a spray-dried Varistor powder based on one with oxides of Bi, Sb, Mn, Co, Al and / or other metals doped zinc oxide was produced. This granulate has Art a football shaped, spherical particles with varistor behavior, which composed of crystalline, separated by grain boundaries grains are. The particles have a diameter of at most 125 microns and have a Size distribution following a Gaussian distribution. This material is in Cable connections and cable terminations used and forms there voltage-controlling layers.

In US 4,726,991, US 4,992,333, 5,068,634 and US 5,294,374 voltage limiting resistors made of a polymer and a powdery filling material based on conductive and / or semiconducting Particles indicated. These resistors become over-voltage protection achieved by dielectric breakdown of the polymer. Since this is relatively high Temperatures may occur, the surge should not be reversible and the energy absorption capacity is relatively low.

US-A-5,669,381 describes a voltage limiting non-linear Resistance of a polymeric matrix 25 (according to Figure 2), in the three fractions of electrically conductive and / or semiconducting particles 21, 22, 23 with Diameters in the 100 μm range, in the μm range and in the submicron range and possibly also Isolierstoffteilchen 24 are embedded. The Nonlinearity is achieved in this resistance by the matrix and the optionally provided Isolierstoffteilchen the electrically conductive and electrically semiconducting particles at low stress to form a high ohmic resistance separates from each other. When a Voltage impulse collapses the isolating insulation and becomes the Voltage pulse limited. The fact that the filler is tightly packed, be according to column 2, lines 6 ff achieved good electrical properties.

PRESENTATION OF THE INVENTION

The invention, as indicated in the claims, the object is underlying to provide a resistor of the type mentioned, which is despite a good non-linearity coefficient for a good protection characteristic characterized by a high power consumption, and at the same time a procedure too create, with such a resistance in a particularly advantageous manner can be produced.

By selecting a suitable filler in the resistance after the Invention achieves electrical properties that are based on a varistor a pottery come relatively close. It is essential that either one suitably structured conductive filler filler is provided and / or that a Varistor granules is used, which is a particularly high packing density allows. It can then with one from the injection, the extrusion or Giessharztechnik known technology in a relatively simple manner Resistors me varistor behavior are produced, which is characterized by a good Protective characteristic and a high power consumption distinguished. From It is particularly advantageous here that by a suitable choice of Starting components and easy to set process parameters Varistors can be produced, which in terms of their shape and their physical properties a broad spectrum and in particular a relatively high energy absorption or switching capacity exhibit.

The non-linear resistor according to the invention can advantageously be used as field-controlling Element in Kabetgamituren or as overvoltage protection element (varistor) be used. He can work in both the low and the medium and High voltage technology can be used and can because of its simple Manufacturing and further processing readily a complex geometry exhibit. If necessary, he can, for example as protection and / or Control element, by casting directly to an electrical apparatus, For example, a circuit breaker, be molded or as a thin Paint coating are applied. Furthermore, he can be screen printed in Hybrid methods are used for integrated circuits.

In the method according to the invention, the addition of the varistor particles additionally provided in the filler electrically conductive particles before Merging of filler and matrix material with the varistor particles their surfaces connected. When merging, the electric Conductive particles with great certainty not from the surfaces of Loosen varistor particles so that resistors produced by this method excellent electrical properties, in particular extremely stable current-voltage characteristics, exhibit.

Particularly good electrical properties are achieved, if still existing, loosely electrically conductive particles before mainly by mixing and infiltration caused merging with the matrix material, such as through Washing, sieving or air classification, are removed from the filler.

At the same time is achieved by the inventive method that the electric conductive particles evenly distributed over the surfaces of varistor particles and make an atomic bond with the varistor material. The Contact effect of the filler is so much improved and it is enough a relatively small proportion of electrically conductive particles in the filler, around resistors with excellent electrical properties, such as especially a large current carrying capacity to get.

WAY FOR CARRYING OUT THE INVENTION

Non-linear resistors with varistor behavior formed as varistor composites were prepared by mixing polymeric material with a filler. Such mixing methods are well known in the art and need not be further explained. The polymers may be thermosets, in particular epoxy or polyester resins, polyurethanes or silicones, or else thermoplastics, for example HDPE, PEEK or ETFE. Instead of the polymer may also be a gel (eg silicone gel), a liquid (eg silicone oil, polybutane, ester oil, fats), a gas (air, nitrogen, SF ₆ , ...), a gas mixture and / or a glass occur.

All polymers of liquid components, such as epoxy resins, were premixed and poured in vacuum over the filler, leaving an infiltration took place. The infiltrated samples were partially spun afterwards, e.g. in a centrifuge for 1/2 - 1 h at 2000 revolutions. It could be so desired high filling levels of up to 60% can be achieved.

Thermoplastic samples were prepared by mixing the filler together with the polymer, e.g. ETFE, premixed and then at elevated temperature, for example, 280 ° C, at pressures of several, typically 5 to 50, bars into a mold pressed.

The filler used here contained varistor particles of doped metal oxide having a predominantly spherical structure, wherein the particles of crystalline, by Grain boundaries of separate grains were constructed. The filler was prepared as follows:

In a conventional spray-drying process, a varistor mixture consisting of commercially available ZnO doped with oxides of Bi, Sb, Mn and Co and with Ni, Al, Si and / or one or more further metals was added as an aqueous suspension or solution processed approximately spherical particles having granules. The granules were sintered in a chamber furnace, for example on a ZnO-coated Al ₂ O ₃ plate, a Pt film or a ZnO ceramic, or optionally also in a rotary kiln. The heating times during sintering were up to 300 ° / h, typically eg 50 ° C / h or 80 ° C / h. The sintering temperature was between 900 ° C and 1320 ° C. The holding times during sintering were between 3h and 72h. After sintering, the mixture was cooled at a rate between 50 ° C / h and 300 ° C / h.

The Varistorgranulat thus prepared was subsequently in a Vibrating device or separated by light mechanical rubbing. By Seven were from the separated granules then granule fractions with Particle sizes between 90 and 160 microns, 32 and 63 microns and less than 32 microns produced.

Varistor granules of the various fractions were determined in certain Weight ratios mixed together. Some of these mixtures and some of the fractions became a metal powder with geometrically anisotropic, in particular flaky, electrically conductive particles with a thickness to length ratio of typically 1/5 to 1/100 mixed, z. B. Ni flakes whose length was on average less than 60 microns. The length the metal particle was chosen in each case to be smaller on average was the radius of an average sized particle of the coarse (90 - 160 μm) varistor granules. This and a small proportion, typically 0.05 to 5 volume percent of the varistor granules, was the formation of avoided metallic conductive percolation paths in the mixture.

The starting components of the filler generally became several Hours premixed in a turbo mixer. Was one of the starting components the metal powder, so put its particles to the surfaces the spherical Varistorteilchen, so that particularly low-resistance contacts created between the individual Varistorteilchen. In addition fall smaller particles into the interior of a small percentage as a hollow sphere trained Varistorteilchen and help so Stromführungsengpässe too Reduce.

As a metallic filler are also fine platelets, easily deformable, soft Particles and / or short fibers conceivable. An advantage is a metallic filler with Particles which are in the range of the highest processing temperatures melt, preferably in the contact points of the Varistorteilchen accumulate and lead there to an improved local contact.

Further, as a metallic filler and fine powder, such as on the basis of Silver, copper, aluminum, gold, indium and their alloys, or conductive Oxides, borides, carbides with particle diameters preferably between 1 and 20 μm are used. The particles of these powders can easily be formed spherical.

Before combining the matrix material and filler should be in the filler contained electrically conductive particles with the varistor particles at the Surfaces are connected. It can then be applied to a matrix material on the Base of a polymer, such as an epoxy resin, the content of conductive small particles and a lower value of 0.05 Percent by volume.

Such a surface connection may be advantageous by a heat treatment be achieved. After mixing the varistor particles and the electric Although conductive particles adhere these particles well on the surfaces the varistor particle. It has been shown, however, that in the following Merging, preferably mixing and infiltrating, with the Matrix material, such as a polymer, a gel or an oil, such as on the base of a silicone, the electrically conductive particles partly on the Float matrix material and then the dielectric strength of a thus substantially impaired resistance. By with the heat treatment initiated processes, in particular Diffusion processes, however, the electrically conductive particles become solid with the Surface connected. During subsequent merging (mixing, Infiltrate) with matrix material is a floating of the electrically conductive Particles on the matrix material avoided. Also in other mixed and Compounding steps can not lead to a redistribution of electrical conductive particles come. Optionally in the heat-treated filler Existing loose particles may be mixed with the prior art before merging Matrix material preferably removed by washing, sieving or air classification become. The temperatures required for the heat treatment are in essentially determined by the material of the electrically conductive particles. For Silver has a treatment time of about 3 h one Heat treatment temperature of about 400 ° C proved sufficient. higher Temperatures (up to 900 ° C) are possible, but then you have to pay attention be that the electrical properties of varistor particles are not too strong to change. Such changes could be made, for example, by a reaction of the Material of the electrically conductive particles with the bismuth phase of Varistor particles occur.

Especially low harmful reactions occur when as electrical conductive particles low melting fine solder particles are used, and if the surface compound produced by adhesion is optionally is still tempered at low temperatures.

Good surface connections are also obtained by Varistor particle-containing powder in a metal-containing solution or dispersion is dispersed, and that by wet-chemical precipitation of the disperse solution or dispersion or by electrochemical or galvanic deposition the Surface connection is generated. By subsequent heat treatment This connection can still be strengthened.

Also by dispersion of a Varistorpartikel containing powder in one metal-containing solution or dispersion, and by subsequent reactive Spray drying or spray pyrolysis of the disperse solution or dispersion can solid surface connections between the varistor particles and the electrically conductive particles are produced. Likewise is one Surface coating from the gas phase possible, as with advantage by Sputtering, vapor deposition or spraying, for example in a fluidized bed or in a varistorgranulat- and gas-containing powder flow is achieved.

An advantageous surface coating is also by Reibkontaktierung reached. In this case, the varistor granules or at least a part thereof and / or the electrically conductive particles in a mixer friction body of the Material added to the electrically conductive particles and / or it contains the Lining of the mixer Material of electrically conductive particles. alternative The surface coating can also by introducing the Varistorgranulats and reaches the electrically conductive particle in a mechano-fusion system as described by Hosokawa Micron Europe B.V., 2003 RT Haarlem, Holland.

Optionally, for example, if the matrix contains a silicone, it is of Advantage, at least a portion of the varistor granules and / or the electric conductive particles to be provided with an adhesion promoter. The adhesion of the Filler in the matrix is then optimized. Such adhesion agents are in the generally applied in the form of a thin layer on the filler. Suitable adhesion promoters are, for example, silanes, titanates, zirconates, aluminates and / or chelates. In this case, the electrically conductive particles can also be added to the bonding agent and thus in economically particular advantageously be used in the same order process.

Resistance bodies were produced from which trial resistances having a volume of a few mm ³ to several dm ^{3 were} realized by sawing, grinding and attaching two electrodes, for example by coating with a metal such as gold or aluminum. In addition, specimens were also produced in which the electrodes were cast directly when cast with a casting resin, such as an epoxy or a silicone.

The following table shows the compositions of four of these sample resistors, where D is the diameter of the particles of the varistor granules. resistance polymer filler 1 50% by volume of epoxy 50% by volume varistor granules, D = 90-160 μm 2 45% by volume of epoxy 48% by volume varistor granules, D = 90 - 160 μm 7% by volume varistor granules, D = 32-63 μm 3 50% by volume of epoxy 47.5% by volume varistor granulate, D = 90 - 160 μm 2.5 vol% Ni flakes 4 45% by volume of epoxy 48% by volume varistor granules, D = 90 - 160 μm 5.5% by volume varistor granulate, D = 32-63 μm 1.5 vol% Ni flakes

All of these resistors were made from the same starting polymer and the same coarse starting granules (D = 90 - 160 microns) made.

The resistor 1 was state of the art.

In contrast to the resistor 1, the resistor 2 had a higher Filler density as well as an additional 15% by volume of the coarse Starting granules amount of the previously described, feinkömigen Varistor granules (D = 32 - 63 microns) on.

In contrast to the resistors 1 and 2, the resistor 3 had a 5 vol% on the filler amount of electrically conductive Ni flakes.

In contrast to the resistors 1 to 3, the resistor 4 had both a about 10 vol% of the filler amount of the fine granular Varistorgranulats as well as about 3% by volume amount of electrically conductive Ni flakes.

As can be seen from the following table, the breakdown field strength U _B [V / mm], the nonlinearity coefficient α _B and the maximum absorbed power P [J / cm ³ ] were determined on these four resistors.

To determine U _B and α, a variable DC voltage was applied to the resistors and the resistors were exposed to electric field strengths between about 5 and about 500 [V / mm]. Depending on the prevailing field strength, the current density J [A / cm ² ] flowing in each of the resistors was determined. The determined values of U and J determined the current-voltage characteristics of the resistors. From each of the characteristic curves, the breakdown field strength U _{B of} the assigned resistor was determined at a current density of 1.3 × 10 ^-4 [A / cm ² ]. For each of the resistances, α _B was taken from the slope of the tangent to the associated current-voltage characteristic twice logarithmically in the point determined by the breakdown field strength U _B.

P was determined from current pulse experiments in which the resistors in a tester were exposed to several 8/20 μs current pulses with current density amplitudes up to 1 [kA / cm ² ] at electrical field strengths up to 800 [V / mm]. sample U _B [V / mm] α _B P [J / cm ³ ] 1 321 16.7 23.8 2 239 28.8 38.2 3 150.8 24.7 74.6 4 176.1 20.6 109.6

From this table it can be seen that the resistors 2 to 4 are distinguished from the prior art resistor (resistor 1) both by a greater nonlinearity coefficient α _B and by an increased power consumption P and this with simultaneously low breakdown field strength. This is, on the one hand, a consequence of the improved contacting of the individual varistor particles with one another by the additionally electrically conductive particles contained in the mixture and, secondly, a consequence of a particularly high density of varistor particles. This high density is caused by a varistor granules with two fractions of particles of different sizes, of which the particles of the first fraction have larger diameter than the particles of the second fraction and are arranged substantially in the form of a dense sphere packing and the particles of the second fraction fill in the gaps formed by the ball packing.

The diameters of the particles of the first fraction are preferably between about 40 and about 200 microns. To achieve a high density, it is particularly favorable when the diameter of the particles of the second fraction about 10 to about 50% of Diameter of the particles of the first fraction, and if the proportion of second fraction about 5 to about 30 percent by volume of the fraction of the first fraction is.

It has been shown that an improved energy intake is achieved when at least one further fraction of predominantly spherically formed Particles is provided whose diameter is about 10 to about 50% of the diameter the particles of the second fraction and, for example, particles smaller than 32 have μm. The energy intake and / or other properties can additionally be improved by special stoichiometric Compositions and by certain structures of the individual fractions, by selecting suitable electrically conductive particles and by application predetermined conditions in the production of the fractions, in particular during sintering.

Claims (19)

1. Nonlinear resistor with varistor behaviour, containing a matrix and a filler in powder form which is embedded in the matrix, in which the filler is composed of sintered varistor granules with predominantly spherical particles of doped metal oxide, which particles are made of crystalline grains separated from one another by grain boundaries, characterized in that the filler also contains electrically conductive particles, which cover at most a part of the surfaces of the spherical particles.
2. Resistor according to Claim 1, characterized in that the electrically conductive particles provided in the filler make up from about 0.05 to about 5% by volume of the filler.
3. Resistor according to one of Claims 1 or 2, characterized in that the electrically conductive particles are of geometrically anisotropic design.
4. Resistor according to Claim 3, characterized in that at least a portion of the electrically conductive particles is in wafer and/or flake form and these wafers and/or flakes have a thickness to height ratio of from about 1/5 to 1/100.
5. Resistor according to Claim 4, characterized in that the length of the wafers and/or flakes is on average less than the radius of the particles in the first fraction of the varistor granules.
6. Resistor according to Claim 3, characterized in that at least a portion of the electrically conductive particles is formed by short fibres.
7. Resistor according to one of Claims 1 to 6, characterized in that at least a portion of the varistor granules and/or the electrically conductive particles is provided with an adhesion promoter.
8. Resistor according to one of Claims 1 to 7, characterized in that the varistor granules contain at least two fractions of particles with different sizes, of which the particles in the first fraction have larger diameters than the particles in the second fraction and are arranged essentially in the form of close sphere packing and the particles in the second fraction fill the interstices formed by the sphere packing.
9. Resistor according to Claim 8, characterized in that the diameters of the particles in the second fraction are from about 10 to about 50% of the diameters of the particles in the first fraction.
10. Resistor according to Claim 9, characterized in that the diameters of the particles in the first fraction are from about 40 to about 200 µm.
11. Resistor according to one of Claims 8 to 10, characterized in that the quantity of the second fraction is from about 5 to about 30% by volume of the amount of the first fraction.
12. Resistor according to one of Claims 8 to 11, characterized in that at least one further fraction of predominantly spherically formed particles is present, whose diameters are from about 10 to about 50% of the diameters of the particles in the second fraction.
13. Process for the production of a resistor according to Claim 1, in which the filler in powder form which contains the varistor particles and electrically conductive particles, is combined with a material forming the matrix, characterized in that before the combination the electrically conductive particles contained in the filler are bonded to the varistor particles on their surfaces.
14. Process according to Claim 13, characterized in that electrically conductive particles are combined by mixing with a powder which contains the varistor particles and in that the mixture formed in this way is heat treated at temperatures at which the surface bond is formed.
15. Process according to Claim 14, characterized in that solder particles are used as electrically conductive particles.
16. Process according to Claim 14 or 15, characterized in that electrically conductive particles that are not surface-bound are removed from the heat-treated mixture, preferably by washing, screening or air separation.
17. Process according to Claim 13, characterized in that a powder which contains varistor particles is dispersed in a metal-containing solution or dispersion, and in that by wet chemical precipitation of the disperse solution or dispersion or by electrolytic or electrochemical deposition, the electrically conductive particles bonded to the surfaces of the varistor particles are produced as a precipitation or deposition product.
18. Process according to Claim 17, characterized in that the precipitation product is heat treated.
19. Process according to Claim 13, characterized in that a powder which comntains varistor particles is dispersed in a metal-containing solution or dispersion, and in that the electrically conductive particles bonded to the surfaces of the varistor particles are produced by reactive spray drying or spray pyrolysis of the disperse solution or dispersion.