Gas separation – Specific media material – Ceramic or sintered
Patent
1994-03-11
1995-12-12
Chiesa, Richard L.
Gas separation
Specific media material
Ceramic or sintered
55301, 55DIG5, 55DIG30, 60311, 96146, 96153, 264 295, 264 60, 264DIG48, B01D 3920, C01B 3136
Patent
active
054745870
DESCRIPTION:
BRIEF SUMMARY
CROSS REFERENCE TO RELATED APPLICATION
This application is a national phase of PCT/DE 92/00753 filed 5 Sep. 1992 and based, in turn, upon German national application P 41 30 630.9 of 14 Sep. 1991 under the International Convention.
FIELD OF THE INVENTION
The present invention relates to a process for producing porous, fluid-permeable moldings of silicon carbide and to a diesel-soot filtering element as made by the process.
BACKGROUND OF THE INVENTION
Ceramic filtering media of ceramic carbide and a process for making same are known, for example, from the German Patent 33 05 529. Such filter bodies can, because of their chemical stability and their to high temperatures and to temperature resistance fluctuation be used as hot gas filters, through-flow electrodes or catalyst carriers.
For these and also for other applications it is, however, advantageous for such fluid-permeable silicon carbide bodies to be electrically heatable with low voltages, especially voltages below 50 volts, so that they can serve both as filters and simultaneously as heating elements.
For this purpose, a silicon carbide body has been provided, for example, in German Patent 33 05 529 with a high electrical resistance of 20 k.OMEGA.cm for use as a heating element. It is indeed also known that the electrical conductivity of silicon carbide can be varied over a wide range by suitable doping with aluminum, boron, nitrogen, etc.
Thus silicon carbide structural elements are doped with nitrogen by annealing them in a nitrogen atmosphere at 1900.degree. C. for several hours. This is, however, an especially slow process and, in any event, the electrical resistance of such doped silicon carbide bodies is always too high for the use of such bodies as heating elements in the low voltage range.
OBJECTS OF THE INVENTION
It is, therefore, the object of the invention to provide a process of the aforedescribed type for making porous fluid-permeable moldings of silicon carbide, which because of a reduced electrical resistance are electrically heatable and thus can also be used as heating elements.
SUMMARY OF THE INVENTION
The object set forth for the invention is attained in with the invention, in that the molding is subjected after a coking process to tempering at 1400.degree. C. to 2000.degree. C. and the resulting siliconizing step is carried out with formation of the .beta. silicon carbide simultaneously with doping with nitrogen. The crux of the invention thus is that the doping is not effected on the finished siliconized body but on coked material during the siliconizing step. As a result, the final body has an electrical resistance imparted thereto which is drastically reduced in spite of the high porosity and the doping is effected in a shorter time than when the doping process is carried out on the finished siliconized body.
The doping which is effected simultaneously with the siliconizing can be carried out by providing the nitrogen in the form of an appropriate compound, for example, as silicon nitrite, which is added to the powder mixture or by impregnating the coked material with a nitrogen containing substance as, for example, aluminum nitrite. Both inventive modes of the process have the effect that during the tempering of the moldings to 1400.degree. C. to 2000.degree. C., nitrogen liberated at these temperatures from the nitrogen-containing compound effects a doping; an optionally doping is also effected with aluminum. A further possibility of doping with simultaneous siliconizing resides in the heating of the moldings to a temperature of 1400.degree. to 2000.degree. C. under a nitrogen or nitrogen-containing atmosphere.
To achieve a uniform electrical heatability of the molding, it should be intrinsically homogeneous. For this purpose, the powder mixture is initially slurried in a liquid in which the binder is partly or fully soluble and which also contains the further components and which is introduced into another liquid with which the solvent is miscible and in which the binder however is insoluble or only slightly sol
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Dias Francisco J.
Koch Freimut J.
Chiesa Richard L.
Dubno Herbert
Forschungszentrum Julich GmbH
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