DE2501894A1 - ELECTRICAL RESISTOR ELEMENT AND METHOD OF ITS MANUFACTURING - Google Patents

Description

PATENT LAWYERS:
Dipl.-Ing. W. COHAUSZ Dipl.-Ing. W. FLORACK Dipl.-Ing. R. KNAUF Dr.-Ing., Dipl.-Wirtsch.-Ing. A. GERBER ■ Dipl.-Ing. HB COHAUSZ

The Carborundum Company January 17, 1975

1625 Buffalo Avenue
Niagara Palls, NY / USA

Electrical resistance element and method for its Manufacturing

The invention relates to an electrical resistance element, in particular for igniting combustible gases, and to a method for its manufacture.

A material with suitable specific electrical resistance and high resistance at high temperatures and in a corrosive environment there are numerous electrical and thermal application possibilities, e.g. the use of Heating elements. The use is of particular interest of such a material for ignition devices for flammable gases, In such an application, the relationship between electrical resistance and temperature is important, which is established by the Temperature resistance characteristic is reproduced. A temperature-resistance curve is used to avoid electrical short circuits desirable, which is flat or rising at higher temperatures.

For safety reasons and because of the shortage of gaseous,

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There is a need for low power electrical igniters that the pilot lights in fuels Can replace gas stoves and other gas appliances. It has been found that the gas consumption of constantly burning pilot lights makes up a significant proportion of the energy consumed by a gas appliance. In addition, pilot flames produce not inconsiderable Amounts of nitrous gases, which are the main constituent of smog. In closed living spaces, the emissions of this Constantly burning pilot flames may lead to the maximum permissible values for nitrous gases being exceeded.

It is already known that two ignition devices are used in burners To use fired silicon carbide, as these are extremely high operating temperatures without decomposition or material deterioration endure. In addition, they are corrosion-resistant and against the destructive influences of the combustion products essentially insensitive. Such SiC ignition devices have hitherto been made from self-bonded silicon carbide such as the thermistor coil igniter according to US-PS 3,467,812 and the ignition device US Pat. No. 3,372,305. However, such ignition devices could not fully satisfactory because of their fragility, high cost, required voltage and power consumption.

The invention is based on the object of the disadvantages to eliminate known ignition devices and to specify a resistance element suitable as an ignition device, which is more reliable, is easily reproducible and physically strong, with which high temperatures can be reached quickly and repeatedly and that can be operated at lower voltages (6 to 220 volts) with lower power consumption, its react-

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tion time, ie the period of time until the ignition temperature of common fuel gases is reached, should be less than 30 seconds and its power consumption, if possible, less than 100 watts, preferably less than 35 watts. The resistance element should be resistant to a rapid transition from room temperature to 1000 ^° C. and higher and be able to withstand rapid changes between these extreme temperatures over a long period of use.

According to the invention, this object is achieved by an electrical resistance element dissolved, which consists of doped siliconized graphite and a reaction time of less than 30 seconds and has a power consumption of less than 100 W.

The method for producing the electrical resistance element is that one formed from a graphite tape Element and a siliconizing mixture of a silicon source, carbon and a carbonaceous binder is heated in the presence of a doping element to a temperature at which free silicon is obtained, to substantially all of the carbon present in silicon carbide is converted.

Advantageous further developments of the invention are set out in the subclaims specified. ■

The invention is described in more detail with the aid of the drawings. It demonstrate·

1 shows a front view of a resistance element suitable as an ignition device according to the invention;

Fig. 2 is an elevational view of another resistive element suitable as an ignition device according to the invention;

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Figure 2a is a side elevation of the resistor element of Figure 2; and

Fig. 3 Temperature-resistance characteristics of two examples of Resistance elements according to the invention.

Silicon carbide has long been used as a high temperature refractory material known and has also been used for high temperature electrical heating elements. According to the invention, a Resistance element by siliconizing a structure made of discontinuous Lamellar graphite produced. This siliconization does not simply mean the formation of a silicon carbide surface layer on the substrate, but the complete conversion of the entire graphite substrate into a silicon carbide with predetermined properties. The electrical resistance element created in this way is characterized by a very rapid reaction, great versatility and ease of manufacture.

The siliconizing of carbon substrates is already known and is being used for example, in U.S. Pat. No. 2,4,1326. In this Trap, however, silicon carbide is formed in situ by a carbon body with a continuous skeletal structure of the Exposure to elemental silicon at a temperature above the melting point of silicon. The educated Silicon carbide therefore retains a skeletal and reticulate structure.

As is well known, graphites consist of hexagonal layers Arrays or networks of carbon atoms. These layer planes of hexagonally arranged carbon atoms are essentially flat and oriented in such a way that the individual layers are essentially parallel to one another at the same distance.

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Graphite can therefore be regarded as carbon with a lamellar structure, ie with a structure made up of layers or lamellae made up of interconnected carbon atoms. In the present invention, a sheet is used as the starting material which consists essentially of graphite, which is preferably free of any binding agents. Such a flexible, binder-free graphite sheet can be produced by pressing or compressing expanded graphite particles with a certain force. After pressing, the expanded graphite particles retain the shape produced by the pressing. The density and thickness of the sheet material can be influenced by changing the pressing pressure. The density of the sheet material should expediently be in the range from 0.64 to 1.60 g / cm 2, preferably 0.96 to 1.28 g / cm 2. Such sheet material can be made to have a uniform thickness ranging from 0.0025 to about 12.7 µm. Ribbons of the desired size and shape can easily be cut from this sheet material ("sheet" here means a flat, narrow strip of sheet material with a thickness of 12.7 mm or less, preferably less than 1.27 mm.) Resistance elements can be produced from strips shaped in this way by siliconizing according to the disclosure below. However, it has been found that a thickness of less than 2.54 mm, preferably less than 1.27 mm, is most suitable for siliconizing. It should be noted that there appears to be a relationship between thickness, density, and siliconizability, since the ease of siliconization decreases with increasing thickness and density. Therefore, if the thickness of the tape is increased, the density can be reduced to compensate. It should also be noted that when such a sheet material is siliconized, an increase in thickness of about 60 to 10% takes place.

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The flexible graphite sheet material produced by the above method can be modified in various ways. For example, impregnating agents can be used in its formation or additives, such as dopants, incorporate or mix with the expanded graphite and the mixture to form the desired sheet material. One or both surfaces of the soft, flexible graphite sheet material can also be used emboss or otherwise provide a pattern. The flexible graphite material produced in this way can be processed by winding, rolling or otherwise processed into solid and hollow bodies or products of any desired shape. Thin sheets 0.1-0.4 mm thick can be bonded together to form a thicker sheet material. It was found, however, that with laminated sheets of more than 1.3 mm thickness a separation of the individual layers often occurs, when these leaves are silicated.

It is known from US Pat. No. 3,440,061 that highly oriented graphites can be treated in such a way that the space between the superposed carbon layers or lamellae is considerable is expanded so that a pronounced expansion takes place in a direction perpendicular to the layers. That way you can an expanded or puffed graphite structure is formed, in which the lamellar character is essentially retained. By expanding graphite characters to a final thickness that is is at least 80 times or more the original thickness, a graphite material is obtained which is without use a binding agent in a cohesive or one-piece sheet structure. e.g. sheets, sheets, strips, ribbons or the like, can be transferred. In addition, the flexible graphite sheet material thus formed can be cut into narrow strips that are used for the production of woven or braided graphite textiles or by rolling, winding or otherwise

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Processing methods formed into solid or hollow bodies can be.

After such a flexible graphite sheet has dried and finished, the material used to make the final electrical resistance element can be cut to any desired shape. As indicated above, it is possible Manufacture moldings of almost any desired shape or dimension.

The graphite feedstock is then placed in a graphite crucible or another suitable container and covered with a siliconizing mixture. The amount of siliconizing mixture is about 20 times the weight of the green body to be treated, but at least 4 times the weight of the Green body. The siliconization mixture consists of a silicon source in such a concentration that the proportion of silicon 65 to 91 wt .- ^ of the mixture makes up 4 to 25 $ carbon graphite and $ 1 to $ 15 of a carbonaceous binder. It is essential that the siliconizing mixture contains enough silicon to remove all of the carbon present in silicon carbide convict. Suitable sources of silicon are elemental silicon, silicon dioxide and silicon nitride. It is also desirable that in the silage mixture free graphite carbon is present so that on the silicon carbide body formed can form a loose coating of a silicon carbide framework that allows the body to adhere to the siliconizing mixture so that a homogeneous body with a minimum of silicon inclusions is obtained. The proportions of silicon and carbon are chosen so that, taking into account those present in the graphite material to be siliconized Amount of carbon a stoichiometric ratio

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gives. The siliconization mixture can also contain essential doping elements contained in small proportions; however, these can also be introduced in other ways, as described below will.

The lamellar graphite body and the siliconizing mixture are then * heated to siliconizing temperature. If the siliconizing is carried out in a vacuum, it has been found that a temperature of 14-50 to 1850 ⁰ C is sufficient for a treatment time of 60 minutes, if a tube furnace at atmospheric pressure and an inert or doping atmosphere are used, a temperature of 1800 to 2200 ⁰ C is appropriate. The minimum temperature required is in any case a temperature above the temperature at which free silicon is obtained from the siliconizing mixture.

The graphite body and siliconizing mixture are heated until essentially all of the carbon present is heated is converted into silicon carbide. The body and the mixture are then allowed to cool to room temperature and the body is cleaned Body by removing the excess siliconizing mixture. The silicon carbide thus formed has suitable properties for Use as a heating element or ignition device, especially if the siliconizing mixture contained a doping element. But it is also possible, the temperature-resistance characteristic of this body by incorporating doping elements in the Modify body during a second burn. Other modifications can also result from a second brand or Result in reheating process. For example, elements that have been fired twice can be heated to significantly higher temperatures will. The second firing can be done by heating the silicon carbide body to a temperature above the previously indicated

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applied siliconization temperature. For example, the second firing may be by heating at 18oo and 2200 ⁰ C in vacuum or at 2000 are executed to 2500 ⁰ C in an inert or dopant atmosphere. As can be clearly seen, the second firing merely consists of additional heating above the original siliconization temperature, without the need for intermediate cooling. The heating can therefore consist of a gradual increase in the temperature to the siliconization temperature and then a further increase in temperature to the afterburning temperature. The second firing or post-firing leads to the removal of excess silicon from the silicon carbide body and is also likely to result in grain enlargement and phase changes in the silicon carbide. As stated above, it is possible to carry out the doping of the silicon carbide body by introducing a doping element during siliconization - either in the siliconization mixture or in the atmosphere used. Alternatively, the doping can also take place during afterburning, for example by afterburning in a nitrogen atmosphere. After the second firing, the structure is allowed to cool to room temperature and then cleaned of excess silicon and other adhering material. The structure can then be provided with electrical contacts in a suitable manner, for example by coating with an electricity-conducting material such as aluminum. Such coatings are applied to the outer surfaces of the connection parts of the electrical resistance element made of silicon carbide in the vicinity of the end parts and form the connections for the resistance body. In addition to electrical conductivity, the selected metal should have properties such that it is compatible with the silicon carbide material. The coatings can be sprayed onto the resistor body or in any other form. can be applied in a customary manner. Suitable materials

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are carbonyl nickel, tungsten, aluminum, gold, silver, etc. Suitable application methods are electro-atomization, coating in a molten bath, deposition from a slurry, painting and vapor deposition. A preferred contact material is tungsten, which is advantageously applied by electro-sputtering will.

Doping the silicon carbide coating is required to to achieve the desired resistivity. The doping element can be either η-type or p-type. The influence of afterburning and doping is illustrated in Fig. j5, by graphing the relationship between resistance and temperature for various silicon carbide elements. From this it can be seen that a positive temperature-resistance characteristic curve by using correct doping elements and / or Treatment conditions can be achieved. As can be seen, elements made from essentially pure silicon carbide have resistors of about 10 ohm.cm or higher. The presence of a doping element such as phosphorus, nitrogen, boron or aluminum is necessary to lower the body's resistance to a usable level. Of course, other suitable Doping elements are used. Such doped silicon carbide bodies are suitable depending on the special properties caused by the doping element selected and the concentration of the doping element for use as semiconductors, thermistors, varistors or resistors.

Suitable compounds can be used for doping during afterburning, in the siliconization mixture or in the siliconization atmosphere be used. For example, nitrogen can be supplied as a gas or as a decomposition product of acetonitrile will. The elements or compounds come as fixed sources

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fertilize the aluminum, boron and phosphorus in question. Such Substances can be incorporated into the silicon carbide in a concentration of 10 to 10% by weight.

Mixtures of doping elements can be used to achieve certain purposes. For example, a mixed doping of boron and nitrogen can be used to achieve a temperature-resistance characteristic, the slope of which is ^{negative at temperatures below 1000 °} C. and positive ^{above 1000 ° C.} Localized doping can also be used to produce preferred zones of low resistance.

Examples of resistance elements according to the invention, which are considered suitable for ignition devices, are shown in FIGS. In Figure 1, the resistor body 1 has a relatively narrow high-temperature zone 2 and wider contact zones 3, which are formed by the legs. The thickness of such a body can expediently be in the order of magnitude of approximately 0.25 mm. The particular shape represents a suitable combination of surfaces in which the legs 3 of the body are kept relatively cool, while the resistance of the high-temperature zone 2 is high enough to limit the current strength. The Figuren2 and 2a show an alternative form of an ignition device, wherein the film element 1 of silicon carbide has a hairpin-shaped form, the legs carry 3 electrical contacts%, the conducting of an electricity base material such as graphite or silicon carbide are produced. A suitable ceramic binder can be used to connect the silicon carbide film to the base part. The high temperature zone of the elements of Figures 2 and 2a is that in cross section

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diminished zone 2. It is convenient to use the high temperature zone to limit in order to keep the current strength required for heating as low as possible. This can be achieved by that the surface of the silicon carbide in the high temperature zone relative to that formed by the legs is cooler Zone diminished. When properly trained, has a resistance element the desired lower power consumption, the required temperature in the high temperature zone and relatively cool contact points. The part of the resistor elements shown with a reduced cross-section works as a high-temperature zone, since this zone must carry the same electrical current as the legs or cooler parts of the element.

3 shows the temperature-resistance characteristics of two examples of elements according to the invention. Curve 1 is derived from a doped electrical resistance element of the invention has been siliconized invention, while curve 2 is one of a resistance element, the siliconized both as has been post-baked in nitrogen atmosphere at 2400 ⁰ C. The different electrical properties that can be achieved with the method of the invention should be clearly reflected in this figure.

The invention is illustrated further by means of the following examples.

example 1

A sheet of pliable, substantially binder-free graphite as described above and available from Union Carbide Company under the trademark GRAFOIL was cut to the desired shape prior to siliconization. The piece

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had the shape shown in Fig. 2 with a in cross section reduced middle section. The width of the piece was on Leg part 6.35 mm and on the reduced cross-section High temperature zone 3.2 mm; the length was 51 mm. Thickness of the band 0.38 mm. The siliconizing mixture had the following composition :

Silicon-75 wt -.%

Graphite 14 wt .- ^

Synthetic resin binder 11 wt .- ^

Aluminum ^ l wt -.%

The binder consisted of a phenolic resin sold under the name Varcum 8121, B-I78 from Varcum Ine. is available. The workpiece was placed on a layer of the siliconizing mixture in a graphite crucible and covered with further siliconizing mixture. The crucible and contents were then heated ^{to about 650 °} C. for about 1/2 hour at a vacuum of 10 mbar in a customary vacuum oven. The siliconized workpiece, which now essentially consisted of silicon carbide, was allowed to cool to room temperature in a vacuum and cleaned after the negative pressure had been equalized. The resistor element thus obtained had a density of about $ 95 of theoretical density, a porosity of about $ 1 to $ 5, and contained less than $ 10 free silicon and less than \% free carbon. With the help of clamps, the element was connected to a power source and tested as an ignition device for common fuel gas. The initial voltage of 18.28 V rose at 1300 ⁰ C to 22.79 V, and the current was 3.4 A. time-to-ignition: 6 seconds.

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Example 2

A workpiece similar to that of Example 1 was shown in the following siliconizing mixture is used:

Silicon-77 wt -.%

Graphite 12 wt -.%

Corn syrup 11 wt -.%

Aluminum <1 wt -.%

The corn syrup used as a binding agent is commercially available as KARO syrup. The element was heated in a tube furnace about 14 to 15 minutes in an argon atmosphere to a temperature of 2000 ⁰ C, allowed to cool and post-baked for 15 minutes in argon at ⁰ C 25OO. The element was in turn connected to a power source with clamps. The electrical properties of the entire element made it appear suitable as an ignition device. The initial voltage of 38.3 V rose at a temperature of 1070 ⁰ C to 29.9 V, and the current was 4.8 A. period of time until the ignition time: 29 seconds.

example

A number of resistance elements of the form shown in FIG. 1 were produced from a strip of discontinuous, binder-free lamellar graphite with a thickness of 0.38 mm. Electrical contacts were applied to each of the elements either by flame spraying aluminum and nickel or by exothermic flame spraying a mixture of 80 ^ nickel, \ K% chromium and 6% aluminum. The various treatment conditions and the influence of various doping elements are shown in Table I.

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Table I. Element classification conditions

A. vacuum B. Tube furnace,
Argon atmosphere C. Tube furnace,
Nitrogen atmosphere D. Tube furnace,
Argon atmosphere E. Tube furnace,
Nitrogen atmosphere P. Vacuum, no doping G Tube furnace,
Argon atmosphere

Afterburning conditions
no
no

no

Tube furnace,
Nitrogen atmosphere

Tube furnace,
Nitrogen atmosphere

vacuum

Tube furnace,
Argon atmosphere

Resistance at room temperature (ohms)

8 18

230 130

OO CO -P--

Example 4

Following the procedure of Example 2, a resistor element of the shape shown in Fig. 1 was prepared. After the second firing in argon at 23OO ⁰ C, the element was postbaked again, this time an additional 15 minutes at 2400 ⁰ C in a nitrogen atmosphere. Clamps were attached to the base of the element and the element was connected to a power source. During the test, the element showed the following properties: At 1000 ^° C., the voltage was 23.8 V and the current was 1.42 A; at 1200 ^° C. and 32.3 V, a current strength of 1.53 A was measured. The power consumption of the element was thus at 1000 ⁰ C 33.8 W and 1200 W. ⁰ C 49.4

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Claims (20)

Expectations Electrical resistance element, in particular for ignition combustible gases, characterized in that it consists of doped siliconized graphite and a reaction time of less than 30 seconds and a power consumption of less than 100 Watbhat. 2. Electrical resistance element according to claim 1, characterized in that that the graphite is a ribbon of discontinuous flake graphite. J. Electrical resistance element according to claim 2, characterized in that that the thickness of the tape is about 0.25 to about 2.5 mm. 4. Electrical resistance element according to one of the claims 1 to 3, characterized in that it consists essentially of silicon carbide and less than 1.0 wt -% of a doping element is selected from the group aluminum, boron, nitrogen and phosphorus.. " 5. Resistance element according to one of claims 1 to 4, characterized characterized in that it has a zone the resistance of which is higher than the remainder of the element. 6. Use of the electrical resistance element according to one of claims l to 5 as an ignition device for combustible gases. 7. Process for the production of an electrical resistance element according to claim 1, characterized in that an element formed from a graphite strip and a siliconizing element 28 484 U / Be - 2 - 50 983 0/0724 Op Mixture of a silicon source, carbon and one carbonaceous binder in the presence of a doping element is heated to a temperature at which free silicon is obtained until substantially all carbon present is converted to silicon carbide. 8. The method according to claim 7, characterized in that silicon is used as the silicon source. 9. The method of claim 7 or 8, characterized in that the heating is carried out in an inert gas atmosphere at a temperature of about l800 to about 2200 ⁰ C. 10. The method according to claim 7 or 8, characterized in that the heating is carried out in a vacuum. 11. The method according to any one of claims 7 to 10, characterized in that the silicided element is further heated to a temperature above l800 ⁰ C. 12. The method according to claim 11, characterized in that the siliconized element is further heated to a temperature above 2000 ° C. 13. The method according to any one of claims 7 to 12, characterized in that that a siliconizing mixture is used which contains the doping element. 14. The method according to any one of claims 1 to 12, characterized in, that a graphite tape is used which contains the coloring element. 509830/0 15. The method according to claim l4, characterized in that a graphite tape is used, which is the doping element Contains only in selected zones. 16. The method according to any one of claims 7 to 15, characterized in that a doping element from the group Aluminum, boron, nitrogen and phosphorus are used. 17. The method according to any one of claims 7 to l6, characterized in that that the further heating is carried out in an atmosphere containing the doping element. 18. The method according to any one of claims 1 to l6, characterized in that that the further heating is carried out in vacuo. 19. The method according to any one of claims 7 to 16, characterized in that that the heating is carried out in an inert gas atmosphere. 20. The method according to claim 17, characterized in that the further heating in a nitrogen-containing atmosphere is performed. 509830/0724