Plastic and nonmetallic article shaping or treating: processes – Outside of mold sintering or vitrifying of shaped inorganic... – Producing microporous article
Reexamination Certificate
2001-08-24
2003-05-20
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Outside of mold sintering or vitrifying of shaped inorganic...
Producing microporous article
C264S642000
Reexamination Certificate
active
06565797
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a silicon nitride filter suitable for removing dust, etc. contained in a high temperature exhaust gas, and a method for its production.
BACKGROUND ART
Heretofore, a cordierite type ceramic filter or silicon carbide type ceramic filter has been proposed as a filter to remove dust, etc. contained in a high temperature exhaust gas. However, the cordierite type ceramic filter is not necessarily adequate from the viewpoint of heat resistance and corrosion resistance although it is excellent in thermal shock resistance, and the silicon carbide type ceramic filter is not necessarily adequate with respect to the thermal shock resistance, although it is excellent in heat resistance and corrosion resistance.
Particularly when the ceramic filter is one intended for arresting diesel particulates (hereinafter referred to simply as particulates) discharged from a diesel engine (hereinafter referred to simply as an engine), it has been likely with the above-mentioned cordierite type filter or the silicon carbide type filter that the particulates arrested by the filter will locally burn to cause a melting loss, thus presenting a fatal damage to the ceramic filter. Further, the particulates contain a sulfur content and a phosphor content, whereby acid resistance is required, but the cordierite type filter used to be not necessarily adequate with respect to the acid resistance.
On the other hand, silicon nitride has excellent characteristics with respect to heat resistance, thermal shock resistance, corrosion resistance, acid resistance, mechanical strength, etc., and is expected to be useful as a filter for dust arresting or dust removing in a high temperature or corrosive environment. Especially, silicon nitride is excellent in heat resistance, thermal shock resistance, acid resistance and mechanical strength, and is thus considered to be a material suitable for a filter for particulates.
As a method for producing such a silicon nitride filter, several have been proposed.
For example, JP-A-6-256069 proposes a method of firing a green body comprising silicon nitride particles, clay and an oxide. Further, JP-A-7-187845, JP-A-8-59364 and JP-A-6-24859 propose methods of using as starting materials a mixture comprising silicon nitride particles and an organic silicon compound, a mixture comprising silicon nitride particles and a polysilazane and a mixture comprising silicon nitride particles and a synthetic resin foam, respectively. However, such methods of using silicon nitride particles as starting materials have had a problem that as compared with a method of using metal silicon particles as the starting material and converting it to silicon nitride by direct nitriding, pores with pore diameters of at most 1 &mgr;m are little, whereby the Young's modulus is high, the thermal shock resistance tends to be poor, the production cost tends to be problematic since the silicon nitride particles are relatively expensive.
On the other hand, as a method of employing metal silicon particles, JP-A-1-188479 proposes a production method to obtain a porous product having a nitriding ratio of the metal silicon particles of at most 50%, by using as a starting material a mixture comprising metal silicon particles and silicon nitride particles. However, in this method, the nitriding ratio of the metal silicon particles is at most 50%, whereby there will be a substantial amount of silicon metal remaining in the silicon nitride sintered body in the form of metal silicon without being nitrided, whereby there will be a problem such that excellent heat resistance or corrosion resistance of silicon nitride will be impaired.
Further, by the method of using metal silicon particles, sintering of the formed silicon nitride particles is usually not sufficient, whereby the mechanical strength of the porous body thereby obtained, tends to be inadequate.
DISCLOSURE OF THE INVENTION
The present invention provides a method for producing a silicon nitride filter, which comprises heat-treating in nitrogen a green body comprising from 40 to 90 mass % (hereinafter referred to simply as %) of metal silicon particles having an average particle diameter of from 1 to 200 &mgr;m and from 10 to 60% of a pore-forming agent, provided that the total amount of the metal silicon particles and the pore-forming agent is at least 90%, to form a porous product made substantially of silicon nitride.
Another invention of the present invention provides a silicon nitride filter characterized in that the porosity is from 40 to 70%, and the cumulative pore volume of pores with diameters of at most 1 &mgr;m is from 1 to 15 vol % of the total pore volume.
BEST MODE FOR CARRYING OUT THE INVENTION
In the method for producing a silicon nitride filter of the present invention, a green body is used which comprises from 10 to 60% of a pore-forming agent and from 40 to 90% of metal silicon particles having an average particle diameter of from 1 to 200 &mgr;m, provided that the total amount of the metal silicon particles and the pore-forming agent is at least 90%.
If the pore-forming agent is less than 10%, the proportion of pores to perform a filter function tends to be inadequate, and if the pore-forming agent exceeds 60%, no adequate strength tends to be obtained, although the porosity of the filter becomes large. Further, if the average particle diameter of the metal silicon particles is less than 1 &mgr;m, the amount of moisture or oxygen adsorbed from outside air during e.g. preparation of the green body tends to increase, and when heat treated, the metal silicon particles tend to be oxidized before being nitrided, whereby the amount of silicon dioxide formed, tends to be too large. Further, if the average particle diameter of the metal silicon particles exceeds 200 &mgr;m, metal silicon particles not nitrided tend to remain in the interior of the sintered body even after the heat treatment, whereby the properties as the silicon nitride filter tend to deteriorate. If the metal silicon particles are less than 40%, the merits of using metal silicon particles, i.e. the merit of using the direct nitriding reaction of metal silicon, will not be utilized. On the other hand, if the content of the metal silicon particles exceeds 90%, the content of the pore-forming agent tends to be small, whereby the porosity cannot be made large. The purity of the metal silicon particles may suitably be selected depending upon the purpose and application.
In this specification, the pore-forming agent is not particularly limited so long as it forms pores. The pore-forming agent may, for example, be one which flies upon e.g. decomposition during heat treatment, to form pores (hereinafter referred to as a flying-type pore-forming agent) or oxide ceramic hollow particles.
The heat-treating conditions are preferably two-step heat treatment in a nitrogen atmosphere, which is divided into a first step suitable for nitriding metal silicon particles and a second step suitable for sintering silicon nitride particles as formed nitride.
As the heat-treating conditions of the first step, it is preferred to maintain in a nitrogen atmosphere from 1000 to 1400° C. for from 4 to 24 hours. If the temperature is lower than 1000° C., nitriding of metal silicon particles tends to hardly take place. On the other hand, if the temperature exceeds 1400° C., the metal silicon particles will melt in the vicinity of the melting point (1410° C.) of metal silicon, whereby the shape cannot be maintained, such being undesirable. If the temperature maintaining time is less than 4 hours, nitriding of metal silicon particles tends to be inadequate, such being undesirable. Further, if the temperature maintaining time exceeds 24 hours, the nitriding reaction will no more substantially proceed, and the operation cost increases, such being undesirable.
As heat-treating conditions of the second step, it is preferred to maintain in a nitrogen atmosphere at from 1450 to 1800° C. for from 1 to 12 hours. If the temperature is lower than 1450° C., sintering of the silic
Arai Kanji
Miyakawa Naomichi
Shinohara Nobuhiro
Takahashi Hideo
Asahi Glass Company Limited
Fiorilla Christopher A.
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