GB1586384A

GB1586384A - Electrically conductive composition and process for its productiuon

Info

Publication number: GB1586384A
Application number: GB1808878A
Authority: GB
Original assignee: Preh GmbH
Current assignee: Preh GmbH
Priority date: 1977-05-07
Filing date: 1978-05-05
Publication date: 1981-03-18
Also published as: FR2389984A1; DE2720615C2; AT364029B; JPS6128201B2; BR7802826A; JPS5416696A; ATA333078A; FR2389984B1; DE2720615A1

Description

(54) ELECTRICALLY CONDUCTIVE COMPOSITION AND PROCESS FOR ITS PRODUCTION (71) We, WALTER PREH GESELLSCHAFT MIT BESCHRANKTER HAFTUNG, a German Federal Republic body corporate, the sole responsible partner in Preh, Electrofeinmechanische Werke, Jakob Preh, Nachf. GmbH & Co, formerly known as Preb, Elektrofeinmechanische Werke. Jakob Preh Nachf., of Schweinfurter Strasse 5, Bad Neustadt/Saale, Germany, (fed rep), do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to an electrically conductive composition comprising a mixture of an electrically conductive component in the form of particles in an electrically nonconducting, curable polymer, and a process for its production.

Compositions according to the invention may be employed for the production of electrical resistors. In addition. like most conductive compositions. it may be used as a screening coating. for example for the purpose of earthing receptacles.

More particularly in the use of electrical resistors, the requirement exists to make the temperature coefficient of the composition as low as possible and constant over a wide temperature range. The temperature coefficient is generally ascertained bv dividing the change of the resistance value. calculated on the value at room temperature. by the resistance value at room temperature and the temperature difference. The temperature coefficient plays a part more particularly in the case of resistance values with small tolerances. Therefore, a low and constant temperature coefficient is an important requirement especially for precision resistors.

It is already known to produce compositions in the form of layers for electrical resistors by the so-called organic thick-film technique. In this process of production. electrically conductive particles such as. for example. lampblack. graphite. carbon fibre. silver. nickel.

chromium and metal allovs or metal oxides are so embedded in an organic. electrically insulating. binding polymer such as. for example, polyethylene or epoxy or phenol resins.

that after curing an electrically conducting matrix is formed. the electrical conductivitv of the laver being determined itirci nlin bv the filling concentration and the arrangement and the electrical properties of the particles admixed with the polymer.

In the case of layer compositions containing carbon particles. the temperature coefficient depends upon the temperature. In the case of metal-metal oxide containing laver compositions. the temperature coefficient may also be influenced bv the composition of ihe layer. it being independent of the resistance value. In the case of carbon containing layer resistors. the attainable electrical conductivity is limited towards low ohmic values by the relativelv low conductivity of the carbon particles admixed in the form of graphite. lamp black or carbon fibre: such carbon containing layer resistors have a high negative temperature coefficient.

Especiallv when base and hence relativelv cheap. metals are employed in the compositions. the long-term electrical stabilitv is often jeopardised by redox processes on the surface. Generally speaking. resistors having a positive temperature coefficient are obtained.

It is also known to produce so-called cermet resistors bv the inorganic thick-film technique. In such cases. glass of a kind having a low melting point is employed as the electrically non-conductin and simultaneously binding component. There are preferably used as the electrically conducting matrix. high-grade and consequently osid:tion-resistant metals or oxides thereof, such as silver, platinum, ruthenium and palladium. The resistivity and the temperature coefficient can be varied by mixing a number of pastes having differing electrical conductivity, the conductivity of a paste depending upon the specific conductivity of the noble metal or oxide admixed with the glass frit and upon the mixing proportion of the said metal or oxide.

When lamp black or graphite is used in the form of electrically conductive particles in an electrically insulating polymer. a number of disadvantages arise. Since, as already mentioned, the electrical conductivity of the composition depends inert alia upon the filling concentration of the particles, different masses having different packing densities must be kept ready to provide a wide range of electrical resistance values. However, different packing densities give different rheological properties to the composition. Also, the different packing density results in a different contraction behaviour during curing of layer-form compositions.The varying surface tension from layer to layer, due to the varying properties of the lamp black and the graphite, also contributes to a poor reproducibility of resistance values from batch to batch, especially when the screen printing method is adopted for the production of the resistance layer.

It is already known, for producing differing resistance values with constant packing density of the particles in the polymer, to make the particles constituting the electrically conductive component from a refractory inorganic oxide material. to the surface of which there is applied a layer of a carbon-containing pyropolymer. The electrically conductive component amounts to 10 - 95% by weight calculated on the final composition of the mixture and the particle size is less than 20 Ktm. However, differing conductivities of such particles can only be produced by variation of the layer thickness of the pyrolytic carbon with which the individual refractory particles are coated.The range of low conductances which are necessary for high ohmic resistance arrangements can be obtained by intensive reduction to a few mono-lavers of the carbon-containing pyropolymer coating. However, the high resistance values thereby obtained are accompanied by an increasing impairment of the behaviour of the temperature coefficient. One explanation for this is that the "grain boundaries'' of the carbon layers have onlv a relativelv weakly pronounced continuity. As the layer thickness decreases. these contact points become increasingly important. Since the stray resistance of the contact points at the boundary layers is highly temperature-sensitive.

this condition is macroscopicallv expressed in a temperature coefficient of the resistance which rapidly deteriorates with decreasing thickness of the carbon layer. Therefore. thicker layers of material having a higher resistivity, but the same resistance per square, have lower temperature coefficients of resistance.

Desirable conductive compositions are therefore those in which. for a constant packing density of the electrically conductive component in the binder. a varietv of resistance values can be produced. and in which the temperature coefficient is minimised.

In accordance with the present invention there is provided an electrically conductive composition comprising a mixture of (a) an electrically non-conductive. curable polymer and (b) à particulate electrically conductive component which comprises pyrolytic carbon doped and/or coated with an element selected from groups Ill to VIII of the periodic system.

The invention also provides a process for producing such a composition which comprises (i) pyrolysing a carbon containing compound to form the pyrolytic carbon. (ii) doping and/or coating the pyrolvtic carbon with the said element to form the particulate electrically conductive component. and (iii) mixing the electrically conductive component and an electrically non-conductive. curable polymer to form the desired composition.The electrically conductive component. which may also be termed semiconducting material. is thus obtained bv pvrolysis of a carbon-containing compound and is doped andiol coated with one or more elements of the Illrd - VII Ith groups of the periodic system. The doping and/or coating step (ii) may be carried out simultaneously with the pyrolysis step (i).

Alternatively steps (i) and (ii) may be successive.

In one embodiment of the invention. the pyrolytic carbon is obtained by pyrolysis of a gaseous or liquid hydrocarbon. such as an aliphatic or aromatic hvdi-ocaihon or mixture thereof.

It is alternatively possible to obtain the pyrolytic carbon by pyrolysis of a pulverulent carbon-containing organic material such as dextrose. glucose. starch or coal-tar pitch. or by pyrolysis of a heterocyclic compound.

In these embodiments. the pyrolysis is preferably carried out at a temperature of 6í)í)- 1 ( í)í) C.

Plcfclnblv the pyrolytic carbon is tieatctl with the doping and/or coating element from the gas phase bv the applicition of heat to a compound of the clement.

The elements employed for doping and/or coating may be. for example. boron. silicon, gciinnium or phosphorus. lii addition. it is possible to use for this purpose metals such as, for example, aluminium, titanium, zirconium, vanadium, chromium, tungsten, iron, cobalt, nickel or molybdenum. The semiconducting material preferably has an. electrical conductivity of from 1 x 10-8 to 1 (ohm-L cm-').

In addition to the electrically conductive component, substances having a high electrical loss factor and a high dielectric constant, for example a refractory material, may be admixed with the polymer. The substances are ground to extemely fine particles If the pyrolysis step of the process, preferably carried out in the temperature range of 600 - 16()0 C, takes place in the presence of refractory material, the carbon is deposited on the latter with the doping or coating element and/or as a compound in the form of a coating which is preferably at least a monolayer.thick. The resultant electrical conductivity of such particles depends upon the pyrolysis temperature and upon the quantitative ratio of the carbon-containing compound to be pyrolysed and the doping substance in the pyrolysis mixture.Since most elements of llIrd-VIllth groups of the periodic system are deposited in the form of non-conductors, -the conductivity of the particles can thus be controlled by means of these doping substances.

The low temperature coefficients which can be obtained with the compositions of the invention are probably due to a number of factors. Elements of IIlrd-VIIlth groups of the periodic system promote the dehydrogenation or.graphitising process which proceeds during the pyrolysis of the hydrocarbon. The influence of the "contact points" between the carbon grain boundaries is reduced by greater layer thicknesses.

Cross-linkages formed between adjacent carbon atom layers by doping elements also cannot be excluded. All these factors contribute to the temperature stability of resistors produced from such compositions.

As refractory material having a high electrical loss -factor and a high relative dielectric constant, there may be employed for example barium titanate, titanium oxide, silicon oxide, aluminium oxide, iron oxide, silicon carbide, iron carbide, iron silicide, chromium silicide or mixtures thereof.

Of course, the electrically conductive component may contain a mixture of different semiconducting materials, each of which has a different conductivity.

Also. in a further embodiment of the invention, the electrically conductive component i.e. the semiconducting material is heat-treated in vacuo or in a nitrogen or inert gas atmosphere at a temperature of from 8000 ,to 1600"C.

The process of the invention may include the additional step of curing the composition. It has been found that the curing of resistance layers packed with pyrolytic carbon doped with elements of the IIIrd -Vlllth groups of the periodic system may be more uniformly carried out in a microwave field. The effectiveness of the evolution of heat produced by microwaves is enhanced by the presence of dielectric pigments or dielectric sites in the carbon itself. These may be for example aluminum oxides, titanium oxides, aluminium phosphate, silicon dioxide, silicon carbide and aluminium nitride.

Such dielectrically highly active materials incorporated in the polymer matrix very rapidly develop heat in the microwave field. Each particle of material may be regarded as a heating element. With a uniform distribution of these particles in the polymer matrix, a rapid and uniform curing of the polymer binder takes place.

The curing is preferably carried out with microwaves in the frequency range from 2400 to 6000 Mhz, more especially at 2450 MHz.

The following Examples illustrate the invention.

Example 1: 100 g of refractory material in the form of extremely small titanium oxide particles of size < 5 ttm and having a surface area of 15 m2/g-were heated in the presence of a gas mixture consisting of 45cue r/c of propane (as carbon containing compound). 5% of boron trichloride (as compound of the doping element) and 50cur of hydrogen at a temperature of l()()()0C.

The electrically conductive component comprising boron-doped pyrolytic carbon thus produced was extremelyfinely ground in.a ball mill and dispersed in a bead mill in a proportion of 55% by weight of the conductive component to 45% of solid epoxy binder component. being an electrically non-conductive, curable polymer.

The composition. in the form-of a paste -suitable for screen printing. which was thus obtained was pressed on to a hard paper substrate by screen printing and cured for 11/2 minutes in a microwave oven operating art a power of 4() W/cm-.

The resistance arrangement thus obtained had a resistance' value of X40 KQ/C and a temperature coefficient of - 2()() ppm/ C.

E.rzess7lple 11.

l(lt) g of extremely small aluminium oxide particles of size < 5 m and having a surface area of 8 m2/g were heated with a gas mixture consisting of 30% of acetvlene (as carbon containing compound), 3% of titanium tetrachloride (as compound of the doping element) and 67% of argon at a temperature of 900 C. The electrically conductive component comprising titanium doped pyrolytic carbon obtained was extremely finely ground in a ball mill with 30 ml of ethanol, then combined with epoxy binder and pressed on to a hard paper substrate by screen printing as indicated in Example 1. The arrangement obtained was cured in a circulating-air furnace at 1800C for 30 minutes.

The resistance value obtained was 500 K8/n, with a temperature coefficient of - 350 ppmPC.

Example 111: 100 g of the highly active dielectric material aluminium phosphate having a surface area of 17 m2/g and an extremely small particle size of < 5 llm were heated with a gas mixture consisting of 80% of nitrogen and 20% of cyclohexane (carbon containing compound) at a temperature of tA()() C for 25 minutes.

The temperature was thereafter raised to 900 C and the treatment was continued for a period of 5 minutes with 5% of silicon tetrachloride (as doping element compound) in hydrogen.

The electrically conductive component thus obtained was worked up into a screen printing paste with epoxy binder as in Example I and pressed on to hard paper, and cured for 2 minutes in a microwave oven operating at a power of 25 W/cm-.

The resistance arrangement had a resistance value of 120 KQ/C and a temperature coefficient of - 300 ppm/ C.

WHAT WE CLAIM IS: 1. An electrically conductive composition comprising a mixture of (a) an electrically non-conductive. curable polymer and (b) a particulate electrically conductive component which comprises pryolytic carbon doped and/or coated with an element selected from groups III to Vill of the periodic system.

2. A composition according to claim 1 in which the pyrolytic carbon is the pyrolysis product of a gaseous or liquid hydrocarbon or hydrocarbon mixture.

3. A composition according to claim 2 in which the hydrocarbon is propane. acetylene or cyclohexane.

4. A composition according to claim I in which the pyrolytic carbon is the pyrolysis product of a heterocvclic compound.

5. A composition according to claim 1 in which the pyrolytic carbon is the pyrolysis product of a pulverulent organic material.

6. A composition according to claim 5 in which the organic material is dextrose, glucose, starch or coal-tar pitch.

7. A composition according to any one of the preceding claims wherein the element with which the pyrolytic carbon is doped and/or coated is boron. silicon. germanium or phosphorus.

8. A composition according to any one of claims I to 6 wherein the element with which the pyrolytic carbon is doped and/or coated is a metal.

9. A composition according to claim 8 wherein the metal is aluminium, titanium.

zirconium, vanadium. chromium. tungsten. iron. cobalt. nickel or molybdenum.

1(). A composition according to any one of the preceding claims wherein the pvrolytic carbon is treated with the doping and/or coating element from the gas phase by the application of heat to a compound of the element.

11. A composition according to any one of the preceding claims wherein the electrically conductive component has an electrical conductivity at ambient temperature of from l x I() to I ohm-' ohm em 12. A composition according to any one of the preceding claims wherein the electrically conductive component comprises more than one species of doped and/or coated pvrolytic carbon of different electrical conductivity.

13. A composition according to any one of the preceding claims wherein the mixture additionally includes a pirticulate substance having a high electrical loss factor and a high relative dielectric constant.

14. A composition according to claim 13 wherein the substance is a refrictory material.

15. A composition according to claim 14 wherein the refractory material is barium titanate . titanium oxide. sil icon oxide. aluminium oxide. iron oxide. silicon carbide. iron carbide. iron silicidc. chromium silicide or mixtures thereof.

16. A composition according to claim 14 or 15 wherein the pyrolytic carbon doped and/or coated with the said element is in the form of a deposit of at least a mono layer on the surface of the particulate refractory matciial.

17. A composition according to ins one of the preceding claims wherein the mixture additionally includes a highly active dielectric material which enhances the hcating cffect of

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Claims

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containing compound), 3% of titanium tetrachloride (as compound of the doping element) and 67% of argon at a temperature of 900 C. The electrically conductive component comprising titanium doped pyrolytic carbon obtained was extremely finely ground in a ball mill with 30 ml of ethanol, then combined with epoxy binder and pressed on to a hard paper substrate by screen printing as indicated in Example 1. The arrangement obtained was cured in a circulating-air furnace at 1800C for 30 minutes.

The resistance value obtained was 500 K8/n, with a temperature coefficient of - 350 ppmPC.

Example 111:

100 g of the highly active dielectric material aluminium phosphate having a surface area of 17 m2/g and an extremely small particle size of < 5 llm were heated with a gas mixture consisting of 80% of nitrogen and 20% of cyclohexane (carbon containing compound) at a temperature of tA()() C for 25 minutes.

The temperature was thereafter raised to 900 C and the treatment was continued for a period of 5 minutes with 5% of silicon tetrachloride (as doping element compound) in hydrogen.

The electrically conductive component thus obtained was worked up into a screen printing paste with epoxy binder as in Example I and pressed on to hard paper, and cured for 2 minutes in a microwave oven operating at a power of 25 W/cm-.

The resistance arrangement had a resistance value of 120 KQ/C and a temperature coefficient of - 300 ppm/ C.

WHAT WE CLAIM IS: 1. An electrically conductive composition comprising a mixture of (a) an electrically non-conductive. curable polymer and (b) a particulate electrically conductive component which comprises pryolytic carbon doped and/or coated with an element selected from groups III to Vill of the periodic system.
2. A composition according to claim 1 in which the pyrolytic carbon is the pyrolysis product of a gaseous or liquid hydrocarbon or hydrocarbon mixture.
3. A composition according to claim 2 in which the hydrocarbon is propane. acetylene or cyclohexane.
4. A composition according to claim I in which the pyrolytic carbon is the pyrolysis product of a heterocvclic compound.
5. A composition according to claim 1 in which the pyrolytic carbon is the pyrolysis product of a pulverulent organic material.
6. A composition according to claim 5 in which the organic material is dextrose, glucose, starch or coal-tar pitch.
7. A composition according to any one of the preceding claims wherein the element with which the pyrolytic carbon is doped and/or coated is boron. silicon. germanium or phosphorus.
8. A composition according to any one of claims I to 6 wherein the element with which the pyrolytic carbon is doped and/or coated is a metal.
9. A composition according to claim 8 wherein the metal is aluminium, titanium.
zirconium, vanadium. chromium. tungsten. iron. cobalt. nickel or molybdenum.

1(). A composition according to any one of the preceding claims wherein the pvrolytic carbon is treated with the doping and/or coating element from the gas phase by the application of heat to a compound of the element.
11. A composition according to any one of the preceding claims wherein the electrically conductive component has an electrical conductivity at ambient temperature of from l x I() to I ohm-' ohm em
12. A composition according to any one of the preceding claims wherein the electrically conductive component comprises more than one species of doped and/or coated pvrolytic carbon of different electrical conductivity.
13. A composition according to any one of the preceding claims wherein the mixture additionally includes a pirticulate substance having a high electrical loss factor and a high relative dielectric constant.
14. A composition according to claim 13 wherein the substance is a refrictory material.
15. A composition according to claim 14 wherein the refractory material is barium titanate . titanium oxide. sil icon oxide. aluminium oxide. iron oxide. silicon carbide. iron carbide. iron silicidc. chromium silicide or mixtures thereof.
16. A composition according to claim 14 or 15 wherein the pyrolytic carbon doped and/or coated with the said element is in the form of a deposit of at least a mono layer on the surface of the particulate refractory matciial.
17. A composition according to ins one of the preceding claims wherein the mixture additionally includes a highly active dielectric material which enhances the hcating cffect of

incident microwave radiation.
18. A composition according to claim 17 wherein the highly active dielectric material is selected from aluminium oxides, titanium oxides, aluminium phosphate, silicon dioxide, silicon carbide and aluminium nitride,
19. A composition according to any one of the preceding claims wherein the polymer is an epoxy resin.
20. A composition according to claim 1 substantially as described in any one of the Examples.
21. A composition according to claim 1 substantially as hereinbefore described.
22. An electrical resistor comprising a composition according to any one of the preceding claims in cured form.
23. A process for producing an electrically conductive composition comprising a mixture of (a) an electrically non-conductive, curable polymer and (b) a particulate electrically conductive component which comprises pyrolytic carbon doped and/or coated with an element selected from groups III to VIII of the periodic system, which process comprises (i) pyrolysing a carbon containing compound to form the pyrolytic carbon, (ii) doping and/or coating the pyrolytic carbon with the said element to form the particulate electrically conductive component, and (iii) mixing the electrically conductive component and an electrically non-conductive, curable polymer to form the desired composition.
24. A process according to claim 23 wherein step (ii) is carried out from the gas phase by the application of heat to a compound of the element.
25. A process according to claim 23 or 24 wherein steps (i) and (ii) are carried out simultaneously.
26. A process according to claim 23 or 24 wherein step (ii) is carried out subsequent to step (i).
27. A process according to any one of claims 23 to 26 wherein the pyrolysis of the carbon containing compound is carried out at a temperature of from 600 to 16000C.
28. A process according to any one of claims 23 to 27 wherein steps (i) and (ii) are carried out in the presence of a particulate substance having a high electrical loss factor and a high relative dielectric constant.
29. A process according to claim 28 wherein the substance is a refractory material.
30. A process according to claim 29 wherein the pyrolytic carbon doped and/or coated with said element is deposited in at least a mono layer on the surface of the particulate refractory material.
31. A process accordng to claim 28 29 or 30 wherein the electrically conductive component is heat treated in vacllo or in a nitrogen or inert gas atmosphere at a temperature of from 8()() to 1600"C.
32. A process according to any one of claims 23 to 31 which includes the additional step of curing the composition.
33. A process according to claim 32 wherein curing is carried out by subjecting the composition to microwave radiation.
34. A process according to claim 33 wherein the microwave radiation has a frequency of from 240() to 6()()() MHz.
35. A process according to claim 34 wherein the microwave radiation has a frequency of 2450 MHz.
36. A process according to claim 23 substantially as described in any one of the Examples.
37. A process according to claim 23 substantially as hereinbefore described.
38. A composition or cured composition whenever produced by the process according to any one of claims 23 to 37.