EP0649150B1 - Composite material - Google Patents

Description

TECHNICAL AREA

The invention is based on an electrical Resistance according to the preambles of claims 1, 6 and 11.

STATE OF THE ART

An electrical resistor of the aforementioned type is known from EP-A2-0 548 606.
This resistor contains a resistance body made of a composite material with a polymer as a matrix. An electrically conductive powder, such as carbon black, and a powdery varistor material, for example based on a spray granulate, are embedded in the polymer matrix as fillers. With this resistor, the electrically conductive powder forms current paths through the resistor body in normal operation. The resistance body heats up intensely above a certain value of the current. The polymer matrix expands greatly and thus separates the particles of the electrically conductive filler that form the current path. The electricity is interrupted. If the voltage at or locally in the resistance body rises too much, the particles of the varistor material form percolating paths above a predetermined limit value of the voltage locally or through the entire resistance body, which dissipate the undesirably high voltage. However, two different fillers are required for the functions of current interruption and voltage limitation described above and caused by a non-linear behavior of the composite material with regard to the current or the applied voltage. This is undesirable for some applications and may lead to difficulties in the manufacture of the composite.

Another electrical resistance of the aforementioned type is described in DE-A-37 07 503. This resistance has one Resistor body made of a PTC composite material with a Polymer matrix and two powders embedded in the matrix, from which one is made of particles with an electrical Conductivity of at least 100 S / m, in particular carbon black, Silver, gold or nickel, and the other of particles with an electrical conductivity of at most 0.1 S / m and the best possible thermal conductivity, such as in particular Silicon, selenium or SiC is formed.

SUMMARY OF THE INVENTION

The invention, as specified in the claims, lies based on the task of resistance of the aforementioned Specify which is easy to manufacture and by appropriate choice of filler and matrix with regard to its material properties easily to a given Requirements profile can be adjusted.

The resistance according to the invention is characterized in that that it is easy by appropriate selection of filler and matrix can be adapted to a specific requirement profile. This requirement profile can already be used for the State-of-the-art PTC behavior, where the resistance is above a transition temperature increases its resistivity nonlinearly and in doing so limits a current carried by him, also a previous one existing varistor behavior after performing the PTC transition include. Such an electrical resistance is then a Varistor with self-protection against excessive heating.

The requirement profile can be in addition to the aforementioned PTC transition include another PTC transition. The resistance is then a PTC resistor in which the temperature of the Current is gradually limited.

The filler and / or the matrix can be compared to one external physical size due to a structural change, for example a phase transition from solid to liquid, react by a nonlinear change in a material property, for example the electrical conductivity, is caused. A nonlinear change in one Material properties can also be influenced by the external physical quantities, for example an electrical one Field without causing structural changes.

In the following, a matrix is called active if it when one or more physical quantities act, one Undergoes structural change which leads to a nonlinear Change in a material property of the composite material leads. A matrix is said to be passive if it Impact of one or more physical quantities none Undergoes structural change and therefore not a non-linear one Change in a material property of the composite material evokes.

A polymer, for example a thermoplastic and / or thermoset and / or an elastomer, is generally provided as the matrix. If appropriate, however, an inorganic material, for example glass, ceramic, for example based on ZrO ₂ , quartz, geopolymer and / or metal, can also be provided as the matrix. The matrix is predominantly made up of solids, but may also be liquid if necessary. The matrix can be passive, but is generally selected to actively respond to temperature changes (polyethylene), pressure (elastomers or thermoplastics filled with deformable particles such as hollow spheres, thermoplastics), or electric fields (piezoelectric polymers such as polyvinylidene fluoride) Structural changes responded.

The filler should have particles of core-shell structure or of granular structure with average particle sizes of typically contain up to some 100 µ. If the filler a component with particles of granular structure However, the composite material should not have a filler component contain with electrically conductive particles whose electrical conductivity is higher than electrical conductivity the particles of granular structure upon exposure one to a nonlinear change in electrical Conductivity of the composite leading electrical Field.

The shells of the particles of core-shell structure are with Advantage from insulating material, whereas the core of this Particles preferably made of electrically conductive and / or exist electrically semiconducting material.

If the shells of these particles consist of a chalcogenide, such as, in particular, an oxide or sulfide, a nitride, phosphide and / or sulfate, they should be dimensioned such that the electrical conductivity of the composite material is non-linear for a given value of an electrical field acting in the composite material changes. If the particles are then in a passive matrix formed by a thermoplastic or thermosetting polymer, the electrical conductivity of this composite material can change twice nonlinearly when an appropriate electric field is selected. A first of these nonlinear changes causes a voltage limitation, a second a current, power or energy limitation. If, on the other hand, the particles are in an active matrix formed by a thermoplastic or thermoset or elastomeric polymer, then a third non-linear change in the conductivity of the composite material can also be achieved, which serves the additional self-protection of the composite material against excessive power consumption and thus against overheating . The cores can advantageously contain doped V ₂ O ₃ or doped BaTiO ₃ and the insulating shells VO ₂ , V ₂ O ₅ , TiO ₂ , BaO, BaS or BaSO ₄ . The aforementioned advantageous effects can also be achieved with cores made of doped or undoped semiconducting material, such as in particular ZnO, SiC, Si, TiO ₂ or SnO ₂ .

If the cores of the particles have electrically conductive material, such as in particular TiC, TiB ₂ , BaTi, SrTi, V ₂ O ₃ , Al, Cu, Sn, Ti or Zn, and the shells of the particles are formed from a material with a high dielectric constant, which is non-linear depends on an external physical size, preferably a ferroelectric or an antiferroelectric, so there is a composite material which can be used as a dielectric.

In the case of such a filler, the matrix is formed by an elastomeric and therefore pressure-active polymer, and the shells contain a bismuthate such as, in particular, BaW _1/3 Bi ₂₃ O ₃ , a niobate such as, in particular, PbFe _0.5 Nb _0.5 0 ₃ , and a scandate such as in particular PbW _1/3 Sc _2/3 O ₃ , a stannate such as in particular SrSnO ₃ , a tantalate such as in particular PbFe _0.5 Ta _0.5 O ₃ , a titanate such as in particular BaTiO ₃ or SrTiO ₃ , a zirconate such as in particular PbZrO ₃ , a manganite such as in particular PbW _1/3 Mn _2/3 O ₃ , a rhenite such as in particular BaMn _0.5 Re _0.5 O ₃ , a tellurite such as in particular BaMn _0.5 Te _0.5 0 ₃ , a tungsten ( VI) oxide such as in particular PbMg _0.5 W _0.5 0 ₃ or a gallium (VI) oxide such as in particular PbW _1/3 Ga _2/3 O ₃ , alone or in a mixture, are used in such a composite in printing and Temperature changes two nonlinear changes in the dielectric constant and thus the loss factor is achieved. These two changes favor the use of such a composite material as a dielectric of a pressure and temperature-dependent capacitance.

If, on the other hand, the matrix in such a filler is formed by a piezoelectric polymer, in particular polyvinylidene fluoride, and the shells contain bismuth, niobate, scandate, stannate, tantalate, titanate, zirconate, manganite, rhenite, tellurite, tungsten (VI) oxide or gallium (VI ) oxide, alone or in a mixture, two nonlinear changes in the dielectric constant are produced in such a composite material when the electric field strength and the temperature change. This composite material can therefore be used as a dielectric of a voltage and temperature-dependent capacitance. The same also applies to a composite material with a corresponding filler, but with a matrix formed by an active thermoplastic or thermosetting polymer. If the composite contains a filler in which both the cores and the shells of the particles of core-shell structure are formed from electrically conductive material, the cores and / or the shells undergoing a structural change when exposed to temperature, such composite material can be used as a PTC resistor. In such a composite material, it is preferable to provide cores made of V ₂ O ₃ and / or BaTiO ₃ , each in doped form, and shells made of electrically highly conductive material, such as TiB ₂ or TiC. The shells should have a thickness such that the reduced electrical conductivity of the cores when there is a change in structure causes an increase in the electrical resistance of the composite material, for example a doubling. In this way, a reduction in a current conducted through the PTC resistor, for example a halving, can be achieved very quickly when a limit temperature is reached. If an active matrix, for example a thermoplastic or thermosetting polymer, is additionally provided, then the slower heating of the polymer then further limits the already reduced current.

The particles of granular structure provided in the filler as an alternative or optionally together with the particles of core-shell structure are formed either by crushing a sintered ceramic or a polycrystalline semiconductor or by spray drying a suspension or solution and calcining or sintering the spray-dried particles. These particles can be ferroelectric or antiferroelectric and are primarily bismuth, niobate, scandate, stannate, tantalate, titanate, zirconate, manganite, rhenite, tellurite, tungsten (VI) oxide or gallium (VI) oxide, alone or in a mixture and also doped or undoped. The particles can also consist of doped metal oxide or carbide, such as SiC, TiO ₂ or ZnO, and / or BaTiO ₃ , SrTiO ₃ , InSb, GaAs or Si. Such composites exhibit two non-linear, oppositely directed changes in the electrical conductivity when the temperature changes and can be used as a combined NTC and PTC resistance element. If the particles with a granular structure are embedded in an active matrix, two non-linear changes in the electrical conductivity occur, one of which has a voltage-limiting effect and the other has a current-, power- or energy-limiting effect.

DESCRIPTION OF THE DRAWINGS

Fig. 1

die Strom-Spannungs-Kennlinien von vier jeweils als Varistor ausgeführten Ausführungsbeispielen des Widerstands nach der Erfindung,

Fig. 2

einen Teilabschnitt der Strom-Spannungs-Kennlinie eines ersten der in Fig.1 angegebenen vier Varistoren sowie Teilabschnitte der Strom-Spannungs-Kennlinien weiterer Varistoren, welche sich vom ersten Varistor lediglich durch die Höhe des Füllstoffanteils unterscheiden,

Fig. 3

eine Temperatur-Widerstands-Kennlinie -Kennlinie des ersten Varistors, und

Fig.4

eine Temperatur-Widerstands-Kennlinie eines als PTC-Widerstand ausgeführten Widerstands nach der Erfindung.

Preferred exemplary embodiments of the invention and the further advantages achievable therewith are explained in more detail below with reference to drawings. Here shows:

Fig. 1

the current-voltage characteristics of four embodiments of the resistor according to the invention, each designed as a varistor,

Fig. 2

a partial section of the current-voltage characteristic curve of a first of the four varistors shown in FIG. 1 and partial sections of the current-voltage characteristic curves of further varistors, which differ from the first varistor only in the amount of the filler fraction,

Fig. 3

a temperature-resistance characteristic curve of the first varistor, and

Fig. 4

a temperature-resistance characteristic of a resistor designed as a PTC resistor according to the invention.

WAYS OF CARRYING OUT THE INVENTION

In a first embodiment of the resistor according to the Invention was - as known from the manufacture of varistors is - initially from a suspension or a solution of zinc oxide and dopants based on several elements, such as Bi, Sb, Mn, Co, Al, ..., by spray drying a granulate with particle diameters generated between 3 and 300 microns. The granules was sintered into a powder at temperatures of approx. 1200 ° C. The powder particles are essentially spherical trained and each consist of a variety of Grains, which are in the manner of the casing sections of a football casing adjoin. Each of the grains of a powder particle consists of ZnO, which is known to contain Bi, Sb, Mn, and / or further elements and electrical current leads well. Are between adjacent grains electrically insulating grain boundaries, which when a Voltage of about 3 volts become electrically conductive. Depending on Leave selection of dopants and type of manufacturing process powder particles are thus produced, which are present when Voltages between 3 and 200 volts electrically conductive and are electrically non-conductive below this voltage. The Powder particles therefore have an external electrical Field nonlinear, primarily due to the grain boundaries certain behavior. Instead of spherical shape, the Powder particles also have a needle or plate shape and can be compact or hollow depending on the manufacturing conditions be trained.

25, 30, 35, 40 and 45 parts of the aforementioned powder was each mixed intensively with polyethylene and through Hot press composite materials with a polyethylene matrix and with filler proportions of 25, 30, 35, 40 and 45 percent by volume produced.

A varistor containing 25 parts by volume of doped ZnO has the current-voltage characteristic I shown in FIG. 1. Below a critical current intensity I _c , the varistor behaves essentially like a conventional varistor based on a sintered ceramic and has a highly non-linear dependence of the current I it carries on the applied voltage E. The current is conducted in percolating paths formed by powder particles. Above the critical current I _c , the polymer matrix is heated to temperatures higher than the melting temperature of polyethylene. The polymer matrix expands and breaks the current-carrying paths. The varistor now goes back to a high-resistance state and blocks the current. By activating the matrix above the critical current intensity I _c it is achieved that inadmissible heating of the varistor is avoided.

From Fig. 2 it can be seen that with increasing proportion of filler ff [volume percent] the nonlinear behavior of the varistor is improved. Sufficiently good non-linear behavior regarding the external tension E is filled with filler 30 to 50 percent by volume. With these filler proportions is also activated by activating the polymer matrix safely avoided overheating of the varistor.

From Fig. 3 it can be seen that a varistor with the previously Composite described as an NTC or PTC element can be used. When heated, it decreases at temperatures T between 20 and 80 ° C the specific Resistance R of the composite is nonlinear to Temperatures between 110 and 130 ° C non-linear again increase. Here, the first change in resistance by the semi-conductive zinc oxide of the filler and the second Resistance change due to the active at approx. 110 to 130 ° C Polymer matrix.

In another embodiment, particles of shell-core structure are used as fillers. One of these fillers contains cores made of conductive material, such as in particular V ₂ O ₃ , and shells made of an oxide, such as especially VO ₂ or V ₂ O ₅ . If such fillers with a volume fraction of typically 20 to 50 percent by volume are embedded in a passive matrix, for example a thermoset based on epoxy, then such a composite material can advantageously be used as a resistance body of a varistor. The current-voltage characteristic of a varistor with a resistance body based on an epoxy matrix and a core made of filler containing V ₂ O ₃ and shells made of VO ₂ is shown in FIG. 1 and identified by the reference symbol II. From this characteristic curve it can be seen that the current carried by the varistor increases non-linearly above a specified limit voltage and thus limits the voltage present. Although this limitation is significantly lower than that of the varistor based on polymer and ZnO (characteristic I), it is completely sufficient for many applications, especially in the low-voltage range. As soon as the varistor has consumed a predetermined limit power and is heated to a limit temperature that determines a PTC effect, the previously electrically conductive V ₂ O _{3 changes} its structure and forms a non-conductive phase. This limits the power implemented in the varistor non-linearly. Due to the second non-linear change in the characteristic curve, self-protection against excessive power consumption is achieved in accordance with the varistor with characteristic curve I.

Self-protection can be improved if the filler contains cores made of doped BaTiO ₃ instead of the cores made of V ₂ O ₃ . The shells are advantageously formed from BaO, BaS, BaSO ₄ , V ₂ O ₃ , VO ₂ or TiO ₂ . Since BaTiO ₃ at a predetermined limit temperature due to a change in structure a substantially stronger PTC effect such a varistor causes as V ₂ O _3, limits the performance significantly stronger than the varistor described above. This can be seen from its characteristic curve from FIG. 1, designated by the reference symbol III.

Similar self-protection with similar varistor behavior can be achieved if the cores surrounded by an insulating shell contain semiconducting material, such as Si, SiC, SnO ₂ , TiO ₂ or ZnO. By using a matrix made of an active polymer, for example a thermoplastic, such as polyethylene, with such a varistor, as with the two varistors described above, with cores made of V ₂ O ₃ and BaTiO _3, self-protection can be provided by a characteristic I corresponding to the varistor PTC transition caused by the polymer matrix can be improved considerably. This can be seen from his characteristic curve from FIG. 1 provided with the reference symbol IV.

In a further exemplary embodiment, the composite material is used as a resistance body of a PTC resistor. The composite material contains an active polymer, such as preferably polyethylene, and a filler with a core-shell structure. Both the cores and the shells are made of electrically conductive material. The material is selected in such a way that the cores and / or the shells undergo a structural change when one or more physical variables are involved. The shells are preferably made of a material with good electrical conductivity, such as TiB ₂ , TiC or a metal. The cores preferably contain V ₂ O ₃ or BaTiO ₃ , each in doped form. When such a PTC resistor is heated by a current, the contact points of the individual filler particles in the current path and thus the filler particles are first heated. Above a material-specific transition temperature, the structure of the cores changes and their resistivity increases considerably due to a PTC effect in a non-linear manner.

From Fig. 4 it can be seen that this PTC effect specific resistance of the PTC element increased considerably. The current carried by the resistor is now quite significant limited. This is due to the rapid warming of the conductive particles very quickly. That slows down heating polymer only reaches after a certain time its softening temperature, expands and interrupts while increasing the specific resistance of the PTC element the current paths.

Claims (12)

1. Electrical resistor having a resistance body made of a filler and a polymer matrix in which the filler is embedded, the resistivity of which resistor increases nonlinearly above a transition temperature owing to a PTC transition at which current paths formed by the filler are interrupted by the polymer matrix, characterized in that the filler predominantly comprises particles of core-shell structure having cores of electrically conducting and/or electrically semiconducting materials and shells of insulating material and/or particles of grainy textured structure containing electrically conducting grains and containing electrically insulating grain boundaries, in a manner such that, below the transition temperature, the resistor behaves like a varistor and a current conducted by the resistor on applying a voltage increases nonlinearly.
2. Resistor according to Claim 1, characterized in that the cores contain Si, SiC, SnO₂, TiO₂ or ZnO.
3. Resistor according to Claim 1, characterized in that the particles of grainy textured structure are formed either by comminuting a sintered ceramic or a polycrystalline semiconductor, or by spray-drying a suspension or solution and calcining or sintering the spray-dried particles.
4. Resistor according to Claim 3, characterized in that the particles are composed of doped metal oxide or metal carbide, such as SiC, TiO₂ or ZnO and/or BaTiO₃, SrTiO₃, InSb, GaAs or Si.
5. Resistor according to one of Claims 1 to 4, characterized in that the polymer is polyethylene.
6. Electrical resistor having a resistance body made of a filler and a polymer matrix in which the filler is embedded, the resistivity of which resistor increases nonlinearly above a transition temperature owing to a PTC transition, characterized in that the filler predominantly comprises particles of core-shell structure having cores of electrically conducting and/or electrically semiconducting material and shells of insulating material, in a manner such that, below the transition temperature, the resistor behaves like a varistor and a current conducted by the resistor on applying a voltage increases nonlinearly, and in that the PTC transition is due to a structural change in the material of the cores.
7. Resistor according to Claim 6, characterized in that the shells of the particles are formed by a chalcogenide such as, in particular, an oxide or a sulphide, by a nitride, phosphide and/or sulphate.
8. Resistor according to Claim 7, characterized in that the cores contain doped V₂O₃ or doped BaTiO₃ and the insulating shells contain VO₂, V₂O₅, TiO₂, BaO, BaS or BaSO₄.
9. Resistor according to Claim 8, characterized in that the cores of the filler contain doped or undoped semiconducting material such as, in particular, ZnO, SiC, Si, TiO₂ or SnO₂.
10. Resistor according to one of Claims 6 to 9, characterized in that the polymer is a thermosetting material.
11. Electrical resistor having a resistance body made of a filler and a polymer matrix in which the filler is embedded, the resistivity of which resistor increases nonlinearly above a first transition temperature owing to a first PTC transition at which current paths formed by the filler are interrupted by the polymer matrix, characterized in that the filler predominantly comprises particles of core-shell structure having cores and shells of electrically conducting material, in a manner such that, at a second transition temperature situated below the first transition temperature, the resistivity increases nonlinearly owing to a second PTC transition due to a change in the structure of the cores.
12. Resistor according to Claim 11, characterized in that the material of the cores contains V₂O₃ or BaTiO₃, in doped form in each case, and the material of the shells contains TiB₂, TiC and/or a metal.

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