EP0649150A1 - Composite material - Google Patents

Abstract

The composite material has a filler and a matrix embedding the filler. At least one physical variable produces in this composite material at least two non-linear changes of one material property or at least one non-linear change in each case of one of at least two material properties by acting on the filler and/or the matrix. The filler predominantly contains a component with particles of a core-shell structure and/or of a granular microstructure. However, filler with particles of a granular microstructure does not contain any further filler component with electrically conductive particles whose electrical conductivity is higher than the electrical conductivity of the particles of a granular microstructure in the effect of an electric field leading to a non-linear change of the electrical conductivity of the composite material. The composite material is simple to produce and, by suitable selection of the filler and of the matrix, can be easily adapted in its material properties to a specified set of requirements. <IMAGE>

Description

TECHNICAL AREA

The invention is based on a composite material with a filler and a matrix embedding the filler, in which at least one physical variable by acting on the filler and / or the matrix has at least two nonlinear changes in a material property or at least one nonlinear change in each case one of at least two Material properties.

STATE OF THE ART

An electrical resistance is known from EP 0 548 606 A2. 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 locally or through the entire resistance body above a predetermined limit value of the voltage percolating paths that derive 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.

SUMMARY OF THE INVENTION

The invention, as specified in claim 1, is based on the object of specifying a composite material of the type mentioned at the outset, which is simple to manufacture and can be easily adapted to a given requirement profile with regard to its material properties by suitable selection of the filler and the matrix.

The composite material according to the invention is characterized in that it can be easily adapted to a requirement profile which comprises at least two material properties with non-linear behavior by suitable selection of filler and matrix.

The filler and / or the matrix can react to an external physical variable by means of a structural change, for example a phase transition from solid to liquid, which causes a nonlinear change in a material property, for example the electrical conductivity. A nonlinear change in a material property can, however, also be brought about by the action of the external physical quantities, for example an electrical field, without changing the structure.

In the following, a matrix is referred to as active if it undergoes a structural change when one or more physical variables act, which leads to a non-linear change in a material property of the composite material. A matrix is said to be passive if it does not undergo any structural change when one or more physical variables are involved and thus does not cause a non-linear change in a material property of the composite material.

A polymer, for example a thermoplastic and / or thermoset and / or an elastomer, is generally provided as the matrix. 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 filled with deformable particles such as hollow spheres, thermoplastics), or electric fields (piezoelectric polymers such as polyvinylidene fluoride) Structural changes responded.

The filler should contain particles of a core-shell structure or of a granular structure with average particle sizes of typically up to a few 100 μ. However, if the filler has a component with particles of granular structure, the composite should not contain a filler component with electrically conductive particles whose electrical conductivity is higher than the electrical conductivity of the particles of granular structure upon exposure to a non-linear change in the electrical conductivity of the Composite material leading electrical field.

The shells of the particles of core-shell structure are advantageously made of insulating material, whereas the cores of these particles preferably consist of electrically conductive and / or 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 when the material of the cores 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 V₂O₃ or doped BaTiO₃ and the insulating shells VO₂, V₂O₅, TiO₂, BaO, BaS or BaSO₄. The aforementioned advantageous effects can also be achieved with cores made of doped or undoped semiconducting material, such as in particular ZnO, SiC, Si, TiO₂ or SnO₂.

Have cores of the particles of electrically conductive material, such as in particular TiC, TiB₂, BaTi, SrTi, V₂O₃, Al, Cu, Sn, Ti or Zn and the shells of the particles are formed by a material with a high dielectric constant, which is non-linear from an external physical Size depends, preferably one Ferroelectric or an antiferroelectric, there is a composite material which can be used as a dielectric.

If the matrix of such a filler is formed by an elastomeric and therefore pressure-active polymer, and the shells contain a bismuthate such as, in particular, BaW _1/3 Bi₂₃O₃, a niobate such as, in particular, PbFe _0.5 Nb _0.5 0₃, and a scandate such as, in particular, PbW _{1 / 3} Sc _2/3 O₃, a stannate such as in particular SrSnO₃, a tantalate such as in particular PbFe _0.5 Ta _0.5 O₃, a titanate such as in particular BaTiO₃ or SrTiO₃, a zirconate such as in particular PbZrO₃, a manganite such as in particular PbW _1/3 Mn _2/3 O₃, a rhenite such as in particular BaMn _0.5 Re _0.5 O₃, a tellurite such as in particular BaMn _0.5 Te _0.5 0₃, a tungsten (VI) oxide such as in particular PbMg _0.5 W _0.5 0₃ or a gallium (VI) oxide such as in particular PbW _1/3 Ga _2/3 O₃, alone or in a mixture, two nonlinear changes in the dielectric constant and thus also the loss factor are achieved with such a composite with pressure and temperature changes. 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. It is preferable to provide cores made of V₂O₃ and / or BaTiO₃, each in doped form, and shells made of electrically highly conductive material, such as TiB₂ or TiC, in such a composite material. 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, TiO₂ or ZnO, and / or BaTiO₃, SrTiO₃, InSb, GaAs or Si. Such composites exhibit two non-linear oppositely directed changes in temperature changes electrical conductivity 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 Varistoren, in denen als Widerstandskörper gemäss vier Ausführungsbeispielen der Erfindung ausgebildete Verbundwerkstoffe vorgesehen sind,

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 des ersten Varistors,

Fig. 4

ein Diagramm, in dem die Dielektrizitätskonstante des Verbundwerkstoffs des ersten Varistors in Abhängigkeit vom Füllstoffanteil des Verbundwerkstoffs angegeben ist,

Fig. 5

ein Diagramm, in dem der Verlustfaktor des Verbundwerkstoffs des ersten Varistors in Abhängigkeit vom Füllstoffanteil des Verbundwerkstoffs angegeben ist,

Fig. 6

ein Diagramm, in dem die Dielektrizitätskonstante eines Kondensators in Funktion der Temperatur angegeben ist, wobei das Dielektrikum des Kondensators von dem im ersten Varistor vorgesehenen Verbundwerkstoff gebildet ist,

Fig. 7

ein Diagramm, in dem die Dielektrizitätskonstante eines Kondensators in Funktion der Temperatur angegeben ist, wobei der Kondensator als Dielektrikum einen gemäss einer weiteren Ausführungsform der Erfindung ausgebildeten Verbundwerkstoff enthält, und

Fig.8

eine Temperatur-Widerstands-Kennlinie eines PTC-Widerstandes, dessen Widerstandskörper aus einem gemäss einer weiteren Ausführungsform der Erfindung gebildeten Verbundwerkstoff besteht.

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 varistors, in which composite materials designed as resistance bodies according to four exemplary embodiments of the invention are provided,

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 of the first varistor,

Fig. 4

1 shows a diagram in which the dielectric constant of the composite material of the first varistor is given as a function of the filler content of the composite material,

Fig. 5

1 shows a diagram in which the loss factor of the composite material of the first varistor is given as a function of the filler content of the composite material,

Fig. 6

2 shows a diagram in which the dielectric constant of a capacitor is given as a function of the temperature, the dielectric of the capacitor being formed by the composite material provided in the first varistor,

Fig. 7

a diagram in which the dielectric constant of a capacitor is given as a function of temperature, the capacitor containing a composite material formed according to a further embodiment of the invention, and

Fig. 8

a temperature-resistance characteristic of a PTC resistor, the resistance body consists of a composite material formed according to a further embodiment of the invention.

WAYS OF CARRYING OUT THE INVENTION

In a first embodiment of the composite material according to the invention - as is known from the manufacture of the varistor - was first made from a suspension or a solution of zinc oxide and dopants based on several elements, such as Bi, Sb, Mn, Co, Al, ... , produced by spray drying a granulate with particle diameters between 3 and 300 microns. The granulate was sintered into a powder at temperatures of approx. 1200 ° C. The powder particles are essentially spherical and each consist of a large number of grains which adjoin one another in the manner of the casing sections of a football casing. Each of the grains of a powder particle consists of ZnO, which is known in a known manner with Bi, Sb, Mn, and / or other elements is doped and conducts electrical current well. There are electrically insulating grain boundaries between adjacent grains, which become electrically conductive when a voltage of about 3 volts is applied. Depending on the selection of the dopants and the type of manufacturing process, powder particles can be produced which are electrically conductive when voltages between 3 and 200 volts are applied and are electrically non-conductive below this voltage. The powder particles therefore have a non-linear behavior with respect to an external electrical field, primarily determined by the grain boundaries. Instead of being spherical, the powder particles can also have a needle or plate shape and, depending on the production conditions, can be compact or hollow.

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

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 I _c achieved that an inadmissible heating of the varistor is avoided.

It can be seen from FIG. 2 that the non-linear behavior of the varistor is improved with increasing filler fraction ff [volume percent]. Sufficiently good non-linear behavior with regard to the external tension E is achieved with filler fractions of approx. 30 to 50 percent by volume. With these filler fractions, overheating of the varistor is also reliably avoided by activating the polymer matrix.

From Fig. 3 it can be seen that a varistor with the composite material described above can also be used as an NTC or PTC element. When heated, the specific resistance R of the composite material non-linearly decreases at temperatures T between 20 and 80 ° C. in order to increase non-linearly again at temperatures between 110 and 130 ° C. The first change in resistance is caused by the semiconducting zinc oxide of the filler and the second change in resistance by the polymer matrix active at approx. 110 to 130 ° C.

Due to the capacitive effect caused by the grain boundaries of the individual powder particles (space charges), the composite material provided in the varistor can also be used as the dielectric of a capacitor. The sizes of the dielectric constants and the loss factor tan δ of the composite material as a function of the filler fraction ff [volume percent] can be seen from FIGS. 4 and 5. It can be seen from these figures that with filler contents between 25 and 50 percent by volume, good dielectric properties are achieved for many capacitor applications. When the temperature rises, the dielectric constant and the loss factor are increased non-linearly. From FIG. 6, this is based on the temperature response of the dielectric constant ε of a composite material with a filler component of 25 percent by volume. The same applies to the loss factor of this composite material.

In a further embodiment of the composite material according to the invention, a ferro- or antiferroelectric material, for example barium titanate, is provided as the filler and a duromer based on epoxy is provided as the polymer matrix. With this composite material, the matrix behaves passively when heated. As can be seen from FIG. 7, the dielectric constant ε of the composite material rises non-linearly above a temperature of approximately 60 ° C. This leads to a non-linear change in capacitance of a capacitor provided with such a composite material as a dielectric. In addition, an additional non-linear change in the dielectric constant occurs when high voltage is applied.

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 V₂O₃, and shells made of an oxide, such as in particular VO₂ or V₂O₅. 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 V₂O₃ and shells made of VO₂-containing filler 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 a predetermined limit power has recorded and is heated to a limit temperature determining a PTC effect, the previously electrically conductive V₂O₃ 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 BaTiO₃ instead of the cores made of V₂O₃. The shells are advantageously formed from BaO, BaS, BaSO₄, V₂O₃, VO₂ or TiO₂. Since BaTiO₃ causes a much stronger PTC effect than V₂O₃ at a given limit temperature due to a structural change, such a varistor limits the power considerably more than the previously described varistor. This can be seen from its characteristic curve from FIG. 1, designated by reference number III.

Similar self-protection with similar varistor behavior can be achieved if the cores, which are surrounded by an insulating shell, such as e.g. Si, SiC, SnO₂, TiO₂ or ZnO contain. By using a matrix made of an active polymer, for example a thermoplastic, such as polyethylene, self-protection can be achieved in such a varistor as in the two varistors described above with cores made of V₂O₃ and BaTiO₃ by a corresponding to the varistor with the characteristic I of the polymer matrix PTC transition can be improved considerably. This can be seen from his characteristic curve from FIG. 1 provided with the reference symbol IV.

In another exemplary embodiment, the composite material according to the invention contains particles of a core-shell structure embedded in a polymer matrix with cores made of electrically highly conductive material, for example of a barium-titanium, Strontium titanium or titanium base alloy, and shells made of insulating material with a high dielectric constant, such as undoped barium titanate or strontium titanate. In contrast to a composite material with particles of solid material and with a high dielectric constant, this composite material concentrates the electrical field extremely strongly into the shells when an external voltage is applied. If the temperature changes, this leads to a particularly strong non-linear change in the dielectric constant. Due to a structural change in the shells of the filler, a further non-linear change in the dielectric constant of the composite material also occurs when a high voltage is applied.

In a further exemplary embodiment, the composite material according to the invention 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 highly conductive material, such as TiB₂, TiC or a metal. The cores preferably contain V₂O₃ or BaTiO₃, 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.

It can be seen from FIG. 8 that this PTC effect considerably increases the resistivity of the PTC element. The current carried by the resistor is now limited considerably. This happens very quickly because of the rapid heating of the electrically conductive particles. The slowly warming polymer only reaches its softening temperature after a certain time, expands and interrupts the current paths with a non-linear increase in the specific resistance of the PTC element.

Claims (17)

Composite material with a filler and with a matrix embedding the filler, in which at least one physical variable causes at least two nonlinear changes in a material property or a nonlinear change in each case one of two material properties by acting on the filler and / or the matrix, characterized in that the Filler mainly contains a component with particles of a core-shell structure and / or of a granular structure, but a filler with particles of a granular structure does not contain any further filler components with electrically conductive particles whose electrical conductivity is higher than the electrical conductivity of the particles of a granular structure upon exposure to an electrical field that leads to a non-linear change in the electrical conductivity of the composite material. Composite material according to claim 1, characterized in that the shells of the particles of core-shell structure consist of insulating material and the cores of the particles consist of electrically conductive and / or electrically semiconductive material. Composite material according to claim 2, characterized in that the shells of the particles of a chalcogenide, such as in particular an oxide or sulfide, are formed by a nitride, phosphide and / or sulfate. Composite material according to one of claims 2 or 3, characterized in that the insulating shells are dimensioned such that the electrical conductivity of the composite material changes non-linearly at a predetermined value of an electrical field acting in the composite material. Composite material according to claim 4, characterized in that the cores contain doped V₂O₃ or doped BaTiO₃ and the insulating shells VO₂, V₂O₅, TiO₂, BaO, BaS or BaSO₄. Composite material according to claim 4, characterized in that the cores of the filler contain doped or undoped semiconducting material, such as in particular ZnO, SiC, Si, TiO₂ or SnO₂. Composite material according to one of claims 5 or 6, characterized in that the matrix is formed by a polymer which undergoes a structural change when the at least one physical variable acts. Composite material according to claim 2, characterized in that the cores of the particles have electrically conductive material, such as in particular TiC, TiB₂, BaTi, SrTi, V₂O₃, Al, Cu, Sn, Ti or Zn, and that the shells of the particles of one material are formed with a high dielectric constant, which depends nonlinearly on the at least one physical variable. Composite material according to claim 8, characterized in that the material with high dielectric constant is ferroelectric or antiferroelectric, and that the matrix is formed by a polymer which undergoes a structural change when the at least one physical variable acts. Composite material according to claim 9, characterized in that the matrix is formed from an elastomeric polymer, and that the shells bismuth, niobate, scandate, stannate, tantalate, titanate, zirconate, manganite, rhenite, tellurite, tungsten (VI) oxide or gallium ( VI) oxide alone or in a mixture. Composite material according to claim 9, characterized in that the matrix is formed from a piezoelectric polymer, in particular polyvinylidene fluoride, and that the shells bismuth, niobate, scandate, stannate, tantalate, titanate, zirconate, manganite, rhenite, tellurite, tungsten (VI) oxide or gallium (VI) oxide alone or in a mixture. Composite material according to claim 1, characterized in that 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 the at least one physical variable acts. Composite material according to claim 12, characterized in that the matrix is formed by a polymer which undergoes a structural change when the at least one physical variable acts, and that the cores contain V₂O₃ and / or BaTiO₃, each doped. Composite material according to claim 1, characterized in that the particles of granular 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. Composite material according to claim 14, characterized in that the particles are ferroelectric or antiferroelectric and in particular doped or undoped bismuth, niobate, scandate, stannate, tantalate, titanate, zirconate, manganite, rhenite, tellurite, tungsten (VI) oxide or gallium (VI) oxide alone or in a mixture. Composite material according to claim 14, characterized in that the particles consist of doped metal oxide or carbide, such as SiC, TiO₂ or ZnO, and / or BaTiO₃, SrTiO₃, InSb, GaAs or Si. Composite material according to one of claims 14 to 16, characterized in that the matrix is formed by a polymer which undergoes a structural change when the at least one physical variable acts.

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