WO1995028365A1

WO1995028365A1 - Process for producing porous, current-conducting moulded bodies made of silicon carbide and moulded bodies thus produced

Info

Publication number: WO1995028365A1
Application number: PCT/DE1995/000540
Authority: WO
Inventors: Freimut J. Koch; Francisco J. Dias; Marian Kampel; Hubertus Nickel
Original assignee: Forschungszentrum Jülich GmbH
Priority date: 1994-04-19
Filing date: 1995-04-15
Publication date: 1995-10-26
Also published as: DE4413127C1

Abstract

A process is disclosed for producing porous, current-conducting moulded bodies made of silicon carbide. For that purpose, a granulate with grains in a size range from 0.2 to 10 mm or a powder mixture are used. To form the granulate, silicon, carbon and/or α-silicon carbide are used as starting materials, besides a coking organic binder. A green compact is formed therewith and coked in an inert gas atmosphere or in a vacuum by heating up to a temperature in a range from 600 to 1000 °C. The green compact obtained by coking is heated up to a temperature from 1600 to 2000 °C, causing silicon to react with carbon and form β-silicon carbide. Components may thus be produced with more or less sharply defined partial regions, layers or zones, whose arrangement in the component determines its temperature profile when current flows therethrough. These areas have a lower or higher specific resistance, as required. To form these moulded bodies, the powder mixtures and/or granulates used to form the various layers and/or regions are composed of coking organic binder, and in addition either of silicon, of silicon and carbon, of silicon, carbon and α-silicon carbide, or only of α-silicon carbide, and of further doping substances required to achieve the desired electric conductivity.

Description

Process for the production of porous, flowable shaped bodies made of silicon carbide and shaped bodies

The invention relates to a process for the production of porous, flowable shaped bodies made of silicon carbide, in which a granular fraction from the grain size range of 0.2-10 mm or from mixed powder, in the formation of which as starting materials, besides a coking material organic binder silicon, carbon and / or α-silicon carbide are used, a green body is formed, which is coked in an inert gas atmosphere or in a vacuum by heating to a temperature in the range of 600 - 1000 ° C and in which arises after the coking process ¬ the shaped body is heated to a temperature of 1600 - 2000 ° C, the silicon reacting with carbon to ^ silicon carbide. It also relates to a flowable molded body

Silicon carbide, electrically heatable via electrical contacts, with a porosity of 35 - 75%.

In order that a ceramic component can be used as a resistance heating element at a given operating voltage and operating temperature, either its total electrical resistance must be adjusted according to its geometric shape or this must be taken into account taking into account the specific electrical the state of the material.

If, for example, it is a highly porous SiC material as in the case of a flow heating element (DFH) or diesel soot filter, the component must have a relatively thick wall due to the low strength of the material, so that the component is mechanically sufficient both in handling and in use is stable. Such a diesel soot filter is known from DE-OS 41 30 630.

Components of this type, since they have a uniform specific electrical resistance and since the entire mass of the component has to be heated, require a great deal of energy and time to reach the operating temperature. For the use of such a tubular component as a diesel soot filter with periodic regeneration, this fact is particularly disadvantageous because, on the one hand, the outer surface of the filter occupied by soot during the regeneration, because of the cooling effect of the inflowing engine exhaust gas, only slowly increases the ignition temperature of the deposited soot is sufficient and, on the other hand, the filtered exhaust gas is unnecessarily overheated, since the internal temperature of the filter is much higher. Reversing this temperature profile would significantly reduce these disadvantages.

It is therefore an object of the invention to provide a method which enables the production of components of the type described at the outset which have more or less sharply delimited partial areas, layers or zones, the arrangement of which in the component has its temperature profile during the passage of current determine. Depending on the requirements, the areas should have a lower or higher specific one, regardless of their porosity Exhibit resistance.

This object is achieved according to the invention by the procedural measures according to claim 1.

Subregions or layers with different specific electrical resistance can be formed solely by using different mixed powders or granules for the individual subregions without using dopants. For example, a partial area with a relatively high electrical resistance is obtained if a mixed powder consisting of a small amount of binder is used in a stoichiometric ratio of silicon and as much α-silicon carbide as possible. Depending on the specific electrical resistance to be achieved, the starting powder or granulate suitable for this purpose is assumed.

An additional possibility of forming regions or layers which differ with regard to the electrical properties is obtained by using the dopants. The effect of the dopants with regard to the desired electrical properties is basically linked to the formation of β-silicon carbide.

The dopants are mixed in a suitable amount with the starting materials for the mixed powders or granules. Both electron donors such as antimony (Sb), phosphorus and tin as well as electron receptors such as aluminum, beryllium and boron can be used elementarily or chemically bonded as dopants. The partial areas of the shaped bodies made of these predoped materials exhibit after the thermal treatment tion, depending on the type of doping, the type and amount of additives and - as mentioned above - depending on the starting materials, different specific electrical resistances.

If a relatively low specific electrical resistance is to be achieved, then less than 70% α-silicon carbide is to be used in the mixed powder and / or granulate provided for the corresponding sub-area. In this case, it is also recommended that the preform formed after the coking process be heated under nitrogen and / or nitrogen-containing atmosphere.

If the siliconization is carried out under argon, high resistance values at room temperature are obtained for materials that have been doped with beryllium, boron and aluminum.

* ^• > temperature (> 10 ^{* J} Ohm x cm) achieved, while for example the resistance value of Sb-doped material is less than 10 Ohm x cm. If, on the other hand, the siliconization is carried out under nitrogen or nitrogenous argon, an additional N-doping of the material takes place, as a result of which the above-mentioned resistance values can be reduced to a few 10-potencies.

It is advantageous if the nitrogen content in the sintering atmosphere is at least 20% to form a layer and / or an area with the best possible electrical conductivity.

In order to obtain a shaped body which is to be used as a filter, it is advisable to proceed according to a procedure known from DE-OS 41 30 630 when producing the mixed powder or granules. When using this procedure, the filler grains coated with the binder and thereby get a so-called CM material (CM = Coat Mix):

To produce the mixed powder, a liquid, e.g. Alcohol, the binder, which is wholly or partly soluble in the liquid, and the other components of the mixed powder form a suspension. This is then added to another liquid with which the solvent is miscible, but in which the binder is not or only sparingly soluble, whereupon the grains coated with the binder settle out as a muddy mass. The grains are freed from the supernatant liquid by decanting and then dried.

It is recommended to inject the slurry in a mixing chamber into the separating liquid by means of a mixing nozzle so that the grains are evenly coated with the binder.

The proportion of binder in the sludge used should be 5 to 30% by weight of the dry weight.

Starting powder with grain sizes in the range of 0.5 - 50 μm is used for the mixed powders. During the mixing process, these agglomerate up to 200 μ.

Different predoped mixed powders or granules can be processed in layers or zones to form a shaped body which, after siliconization, has layers or zones with different resistance values. When an electrical voltage is applied, temperature fields can thus be generated in a targeted manner within such a shaped body. To form a shaped body with a surface layer that at least partially covers it, the surface layer is formed by spraying a mixed powder slurry provided therefor onto the green body.

The object on which the invention is based is thus achieved by a flowable shaped body of the type described at the outset, which is characterized by layers and / or subregions with different specific electrical resistance, which differ by doping elements, in particular their type and amount. These layers can also have different porosities.

If, for example, a layer is to be heated to an operating temperature of 700 ° C. in the low-voltage range, the geometry (thickness and length of the layer) and the specific electrical resistance must be selected accordingly when using the method according to the invention.

In the event that the flow-through body is to be used as a filter element, for example as a diesel soot filter, and is to be regenerated via the electrical contacts, a flow-through body which has a surface layer is particularly suitable. which has a higher specific electrical conductivity than the part of the body covered by the surface layer.

The specific electrical resistance of the surface layer of this filter body is 10 ^{- *} ^ up to 10 ohm x cm. If the body through which flow can flow is tubular, it can be used, for example, as a diesel soot filter in such a way that the exhaust gas of a diesel engine carrying the soot particles is applied to it from the outside. With a correspondingly selected porosity, these particles only penetrate into the surface of the filter body by about 0.5-2 mm. For regeneration of the filter, ie for the combustion of the soot particles collected by the filter body, it is therefore sufficient to heat only a surface layer corresponding to this thickness. The thickness of the surface layer and its specific electrical resistance is therefore chosen such that the area of the filter into which the soot particles penetrate can be heated. In other words: the surface layer has a maximum thickness of 2 mm.

The thickness of the surface layer can be kept small if the surface layer is also fine-pored, so that the soot particles are stopped in it. In order to obtain a high gas permeability of the filter, the partial area of the filter which is covered by the surface layer is chosen to be coarse-pored.

An expedient further development of the flowable molded body as a diesel soot filter consists in that the wall of the body has a higher specific electrical conductivity at both ends in the area of the electrical contacts attached for electrical heatability compared to at least one continuous partial area lying between the contacts. This prevents undesired heating of the ends of the body during the regeneration phase (when the current flows). Rather, only the thin surface layer is heated to the temperature required for the combustion of the soot particles. Shaped bodies are shown in the drawing, which are explained in more detail in connection with the exemplary embodiments described below. Experimental diagrams are also shown, which are also explained below.

Show it:

FIG. 1 molded body with a doped area lying between two non-predoped areas; FIG. 2 molded articles with a planar arrangement of the partial areas; FIG. 3 longitudinal section through a tubular filter element; FIG. 4 diagram: specific ohmic resistance of shaped bodies or partial areas of shaped bodies as a function of different dopings;

FIG. 5 diagram: specific ohmic resistance of moldings or partial moldings depending on the nitrogen content of the sintering atmosphere.

Example 1

Production of a molded body with a doped partial area lying between two undoped areas according to FIG. 1.

To produce the granules to be doped only with nitrogen, 200 g of novolak resin were dissolved in 1 liter of ethanol and by adding 397 g of Si and 70 g of electrographite (particle diameter of the filler material used). rialien between 0.5 mm and 50 microns) produced a slurry, which was sprayed with water according to the above-mentioned coat-mix method (DE-OS 41 30 630), the water-insoluble binder resin separating from the filler particles. The powder coated in this way was separated from the liquid by filtration, agglomerated with a tumble mixer and dried. The fraction of the agglomerates with a grain diameter of 0.5-1 mm was separated by sieving. Another granulate was produced as described above, but an additional 10.7 g of aluminum oxide were added to the filler mixture as a predoping agent.

For the production of green bodies, an area about 12 mm wide was cut off by foil in a die with a base area of 15 × 70 mm. Material without pre-doping was filled in on the outside and the aluminum oxide-containing granules were filled in on the inside. Figure 1 illustrates this using a sketch.

After the films had been removed, a green body was produced by die pressing at 130 ° C. and a pressing pressure of 300 hPa.

This green compact was coked by heating to 850 ° C. under argon, the binder resin phase being converted to carbon with loss of weight and dimensional changes.

The body was then under nitrogen

P 41 30 630.9-45 heated to 1900 ° C. in a resistance-heated oven, the proportions of free carbon and free silicon being converted to silicon carbide without the body experiencing any further dimensional changes. The electrical resistance of the individual zones of the body could be determined using the four-point method. It was 0.007 Ωcm at both ends; the phase predicted with Al in the middle of the body had a specific electrical resistance of 0.014 Ωcm.

Example 2

Production of a shaped body with a planar arrangement of the partial areas according to FIG. 2.

According to embodiment 1, two granules were produced, 6.6 g of antimony and 7.2 g of boron carbide being added to one of the starting filler mixtures as the predoping agent.

The granulate fractions 0.2-0.4 mm were further used in the case of the Sb-predoped material, and the fraction 1.0-1.6 mm was used for the boron-predoped material.

A body (base area of the die 15 × 70 mm) was produced from these granules with the aid of a molding die according to DE 39 25 844 C2, said body consisting of a 2 mm thick layer of the pre-doped granulate and an approximately 8 now thick layer of the material with boron predoping. The test body was coked as described in Example 1 and siliconized at 1900 ° C. under N ₂ . A sketch of this body is shown in Figure 2.

The length of the siliconized body was 65.3 mm after removal of a narrow strip, the width 12.9 mm and the height 9.7 mm. The height of the Sb-doped layer was 2 mm. The total resistance of the body was with R = 0.182 Ω determined.

To determine the resistance of the individual layers, the Sb-doped part was separated from the narrow strip. The specific resistance of the material doped with boron could be determined using the four-point method with 9 = 0.209 Ωcm. Based on the total body, this results in a resistance of 1.378 Ω. The specific resistance of the Sb-predoped layer could thus be determined by calculation to be 0.0083 Ωcm.

Example 3

According to embodiment 1, 200 g of novolak resin, 234 g of silicon and 232 g of silicon carbide were used according to the coat-mix method. with the addition of 7.2 g of boron carbide as a preliminary dopant, a starting powder was prepared, which was also granulated. From the granulate fraction 1.0-1.6 mm of this material, together with the Sb-predoped graralate used in embodiment 2, a planar material combination according to embodiment 2 was produced, coked and siliconized under an N2 atmosphere.

The specific electrical resistances for the boron-doped layer were determined to be 0.0083 Ωcm.

Example 4

Production of a filter element according to FIG. 3. The granules used in embodiment 3 were processed into a tubular diesel soot filter, carbonized and siliconized under a nitrogen atmosphere.

The 1 mm thick "glow and filter layer" as well as the tube ends made of Sb-pre-doped material had a specific electrical resistance of 0.0083 Ωcm, the 5 mm thick boron-pre-doped, α-SiC-containing "support layer" of 0.520 Ωcm on.

Test results according to FIG. 4

The diagram reproduced here shows the specific ohmic resistance of differently doped, finished molded bodies or molded body parts at different operating temperatures.

These are moldings which, after their production, did not contain more than 30% of α-silicon carbide.

The doping element is indicated on the respective curve in the diagram. The dosage was 1 wt.

It can be seen from the diagram that doping with boron or aluminum leads to a higher resistance than an undoped shaped body has.

Doping with antimony leads to a lower specific electrical resistance.

A further reduction in the specific electrical resistance was achieved in the case of moldings which had been sintered in a nitrogen-containing atmosphere. The doping with the above-mentioned elements enables different specific electrical see setting resistors. The lowest specific electrical resistance was achieved in the case of a shaped body whose starting material was doped with antimony and which had been sintered in a nitrogen atmosphere.

Test results according to FIG. 5

The diagram shows that a nitrogen content of approx. 20% in the sintering atmosphere is sufficient to obtain the lowest possible ohmic resistance. Lower levels of nitrogen in the sinter atmosphere give the possibility of setting higher specific ohmic resistances.

The moldings used in the investigation had an α-silicon carbide content of not more than 30% after their production.

Claims

Patent claims

1. Process for the production of porous, flow-through shaped bodies made of silicon carbide, in which a granular fraction from the grain size range of 0.2-10 mm or from mixed powder, in the formation of which as starting materials, besides a coking organic binder or silicon , Carbon and / or α-silicon carbide are used, a green body is formed, which is coked in an inert gas atmosphere or in a vacuum by heating to a temperature in the range from 600-1000 ° C. and in which the shaped body formed after the coking process is at a temperature is heated from 1600 - 2000 ° C, the silicon reacting with carbon to ß-silicon carbide, characterized in that in order to achieve a shaped body consisting of layers and / or partial areas of different specific electrical conductivity during the formation of the green body for corresponding layers and / or Areas of mixed powder and / or granules are used are made from the coking organic binder and additionally optionally from silicon or silicon and carbon or silicon and carbon and α-silicon carbide or only α-silicon carbide and optionally additionally with dopants to achieve the desired electrical Conductivity have been formed.

2. The method of claim 1, d a d u r c h g e k e n n z e i c h n e t that electron donors as dopants such

Antimony, phosphorus and tin can be used.

3. Vei drive according to claim 1, d a d u r c h g e k e n n z e i c h n e t that electron receptors such as aluminum, beryllium and boron are used as dopants.

4. The method according to any one of claims 1 to 3, that the dopants are added to the mixed powder and / or granules in an elementary or chemically bonded manner.

5. Method according to one of the preceding

Claims that d e r c h g e k e n n z e i c h n e t that less than 70% α-silicon carbide is used in the mixed powder or granules.

6. The method of claim 5, d a d u r c h g e k e n n z e i c h n e t that the heating of the pre-body formed after the coking process takes place under nitrogen or nitrogen-containing atmosphere.

7. The method according to claim 6, characterized in that to form a layer and / or an area with the best possible electrical Leitfä- The nitrogen content in the sintered atmosphere is at least 20%.

8. The method according to any one of the preceding claims, d a d u r c h g e k e n e z e i c h n e t that to form a molded body with a at least partially covering this surface layer, the surface layer by spraying a powder or provided

Granulate slurry is formed on the green body.

9. Flowable body made of silicon carbide, can be heated electrically and regenerated via electrical contacts, with a porosity of 35-75%, suitable layers and / or partial areas with different specific electrical resistance, which are characterized by doping elements, in particular their

Type and quantity.

10. Flowable body according to claim 9, d a d u r c h g e k e n e z e i c h n e t that layers and / or partial areas have differing porosity.

11. Flowable body according to claim 9 or 10, g e k e n n z e i c h n e t d u r c h a surface layer which has a higher specific electrical conductivity than the part of the body covered by the surface layer.

12. flowable body according to claim 11, characterized in that the specific electrical resistance of the surface layer 10 is up to 10 ohms x cm.

13. Flowable body according to claim 11 or 12, so that the surface layer is fine-pored, the sub-area of the molded body is coarse-pored.

14. Flowable body according to one of claims 9 to 12, d a d u r c h g e k e n n z e i c h n e t that the body is tubular.

15. Flow-through body according to one of claims 11 to 14, characterized in that the wall of the body at both ends in the region of the electrical contacts attached for electrical heatability has a higher specific electrical area than at least one continuous partial region lying between the contacts Has conductivity.