U.S. patent application number 10/541462 was filed with the patent office on 2006-03-09 for sintered ceramic compact and ceramic filter.
This patent application is currently assigned to Ibiden Co., Ltd.. Invention is credited to Kazushige Ohno, Hiroki Sato.
Application Number | 20060051556 10/541462 |
Document ID | / |
Family ID | 34309306 |
Filed Date | 2006-03-09 |
United States Patent
Application |
20060051556 |
Kind Code |
A1 |
Ohno; Kazushige ; et
al. |
March 9, 2006 |
Sintered ceramic compact and ceramic filter
Abstract
The present invention provides for a ceramic sintered body and a
ceramic filter having a long-term stability which can prevent
cracks from occurring due to the breakage of ceramic particles when
thermal stress is applied in regeneration process and the like, and
can prevent catalyst carried from deteriorating when regeneration
treatment is conducted repeatedly. The invention is a ceramic
sintered body comprising ceramic coarse particles and porous
bonding layers existing between the ceramic coarse particles to
connect the particles and comprising ceramic fine particles having
an average particle size smaller than that of the ceramic coarse
particles and/or the aggregates thereof, and a ceramic filter
prepared by using the ceramic sintered body.
Inventors: |
Ohno; Kazushige; (Gifu,
JP) ; Sato; Hiroki; (Gifu, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Ibiden Co., Ltd.
1, Kanda-cho 2-chome, Ogak-shi
Gifu
JP
503-0917
|
Family ID: |
34309306 |
Appl. No.: |
10/541462 |
Filed: |
September 13, 2004 |
PCT Filed: |
September 13, 2004 |
PCT NO: |
PCT/JP04/13705 |
371 Date: |
July 6, 2005 |
Current U.S.
Class: |
428/116 ;
428/117; 501/87; 501/88; 501/89 |
Current CPC
Class: |
C04B 2235/77 20130101;
C04B 2235/5427 20130101; C04B 38/00 20130101; C04B 2237/365
20130101; C04B 2235/5436 20130101; C04B 35/80 20130101; C04B
2235/3418 20130101; C04B 35/6263 20130101; C04B 2111/00793
20130101; C04B 37/005 20130101; C04B 35/626 20130101; C04B 2235/526
20130101; C04B 35/6303 20130101; C04B 2235/402 20130101; Y10T
428/24157 20150115; F01N 2450/28 20130101; C04B 2235/5224 20130101;
Y10T 428/24149 20150115; C04B 35/565 20130101; C04B 2235/404
20130101; C04B 2235/80 20130101; C04B 2235/428 20130101; Y10T
428/249986 20150401; C04B 35/62655 20130101; Y10T 428/249953
20150401; B01D 46/2418 20130101; C04B 2235/5228 20130101; C04B
2235/5472 20130101; B32B 2315/02 20130101; C04B 35/6316 20130101;
C04B 2235/383 20130101; C04B 2235/405 20130101; Y10T 428/249978
20150401; C04B 35/806 20130101; C04B 2235/656 20130101; C04B
2237/083 20130101; C04B 38/00 20130101; C04B 35/565 20130101 |
Class at
Publication: |
428/116 ;
428/117; 501/087; 501/088; 501/089 |
International
Class: |
C04B 35/56 20060101
C04B035/56; B32B 3/12 20060101 B32B003/12 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 12, 2003 |
JP |
2003-361229 |
Claims
1. A ceramic sintered body comprising ceramic coarse particles and
bonding layers existing between the ceramic coarse particles to
connect the particles and including ceramic fine particles having a
mean particle size smaller than that of the ceramic coarse
particles.
2. The ceramic sintered body according to claim 1, wherein the
ceramic coarse particles are single-crystal.
3. The ceramic sintered body according to claim 1, wherein the
bonding layer is formed with ceramic fine particles having an
average particle size smaller than the ceramic coarse particles,
and/or a sintered body of aggregates thereof.
4. The ceramic sintered body according to claim 1, wherein the
bonding layer is a brittle body having strength lower than that of
the ceramic coarse particles.
5. The ceramic sintered body according to claim 3, wherein the
bonding layer is a polycrystalline body comprising a plurality of
ceramic fine particles.
6. The ceramic sintered body according to claim 5, wherein the
ceramic fine particles are formed by sintering with the grain
boundary remained.
7. The ceramic sintered body according to claim 1, wherein the
bonding layer contains at least one sintering aid selected from
iron, aluminium, nickel, titanium, chromium and oxide.
8. The ceramic sintered body according to claim 7, wherein a
content of the sintering aid is higher than that contained in the
ceramic coarse particles.
9. The ceramic sintered body according to claim 1, wherein the
ceramic coarse particles and the bonding layers are formed by
silicon carbide.
10. The ceramic sintered body according to claim 1, wherein a ratio
of an average particle size of the ceramic coarse particle to the
ceramic fine particles is 15:1.about.200:1.
11. The ceramic sintered body according to claim 1, wherein a ratio
of total weight of the ceramic coarse particles to the ceramic fine
particles is 1:1.about.9:1.
12. The ceramic sintered body according to claim 1, wherein the
ceramic sintered body is porous.
13. A ceramic filter with a honeycomb structure comprising a
pillar-shaped porous ceramic member or a combination of a plurality
of the pillar-shaped porous ceramic members in which a plurality of
cells as a gas passageway are arranged side by side in a
longitudinal direction through cell walls and either one end
portions of these cells are plugged, wherein the filter itself is
formed by a ceramic sintered body comprising ceramic coarse
particles and a bonding layer existing between the ceramic coarse
particles to connect the particles and including ceramic fine
particles having an average particle size smaller than that of the
ceramic coarse particles.
14. The ceramic filter according to claim 13, wherein the ceramic
coarse particles are single-crystal.
15. The ceramic filter according to claim 13, wherein the bonding
layer is formed by ceramic fine particles having an average
particle size smaller than that of the ceramic coarse particles,
and/or a sintering body of aggregates thereof.
16. The ceramic filter according to claim 13, wherein the bonding
layer is brittle body having a strength lower than the ceramic
coarse particles.
17. The ceramic filter according to claim 15, wherein the bonding
layer is a polycrystalline body comprising a plurality of ceramic
fine particles.
18. The ceramic filter according to claim 17, wherein the ceramic
fine particles are formed by sintering with the grain boundary
remained.
19. The ceramic filter according to claim 13, wherein the bonding
layer contains at least one sintering aid selected from iron,
aluminium, nickel, titanium, chromium, and oxide.
20. The ceramic filter according to claim 19, wherein the content
of the sintering aid is higher than that contained in the ceramic
coarse particles.
21. The ceramic filter according to claim 13, wherein the ceramic
coarse particles and the bonding layer are formed by silicon
carbide.
22. The ceramic filter according to claim 13, wherein a ratio of an
average particle size of the ceramic coarse particles to the
ceramic fine particles is 15:1.about.200:1.
23. The ceramic filter according to claim 13, wherein a ratio of
total weight of the ceramic coarse particles to the ceramic fine
particles is 1:1.about.9:1.
24. The ceramic filter according to claim 13, wherein the ceramic
sintered body is porous.
Description
RELATED APPLICATION
[0001] This application is an application claiming a priority right
based on Japanese Patent Application No. 2003-361229 filed on Sep.
12, 2003.
TECHNICAL FIELD
[0002] This invention relates to a ceramic sintered body and a
ceramic filter produced by using the ceramic sintered body, and
more particularly to a ceramic filter used for removing
particulates discharged from an internal combustion engine such as
a diesel engine or the like. Moreover, a catalyst can be carried on
the ceramic filter.
BACKGROUND ART
[0003] It has been pointed out that the exhaust gas discharged from
internal combustion engines in vehicles such as buses or trucks,
construction machines and the like contains a large number of fine
particulates, and causes a harmful effect on environment and the
human body. Therefore, it has been required to remove and purify
the particulates. In order to fulfill such requirement, a filter
for purifying the exhaust gas, for example, a filter with a
honeycomb structure comprising porous ceramics has been
developed.
[0004] FIG. 6 is an example of the conventional ceramic filter with
a honeycomb structure. The conventional filter is constituted with
a cylindrical-shaped honeycomb structural body 30 formed by
arranging a plurality of cells 31 as an exhaust gas path side by
side in the longitudinal direction through cell walls 33.
[0005] As shown in FIG. 6(b), the cells 31 are plugged at either
one end portions of inlet side or outlet side for the exhausted gas
with plugging materials 32, in which the exhaust gas flown into
certain cells 31 passes through the cell walls 33 separating these
cells 31 and flows out from another cells 31.
[0006] When such a ceramic structural body 30 is placed in an
exhaust path of an internal-combustion engine, particulates in the
exhaust gas discharged from the internal-combustion engine are
caught by the cell walls 33 when passing through the honeycomb
structural body 30, and as a result, the purification of the
exhaust gas is conducted.
[0007] As a filter material for such a honeycomb structural body
may have hitherto been used oxides such as cordierite and the like,
carbides and the like. Among them, silicon carbide has advantages
that it is excellent in the thermal conductivity, heat resistance,
mechanical properties, chemical resistance and the like.
[0008] For instance, JP-A-S60-264365 discloses a porous silicon
carbide sintered body having a three-dimensional net-like structure
mainly composed of plate crystals with an average aspect ratio of
2-50.
[0009] JP-A-H04-187578 discloses a method of producing a porous
silicon carbide sintered body by mixing .alpha.-type silicon
carbide powder having a mean particle size of 0.3-50 .mu.m and
.beta.-type silicon carbide powder having a mean particle size of
0.1-1.0 .mu.m to form raw powder and firing the raw powder.
[0010] JP-A-H05-139861 discloses a method of producing a
.beta.-type porous silicon carbide sintered body by mixing silicon
carbide powder having a mean particle size of 0.5-100 .mu.m and
.beta.-type polycrystalline silicon carbide powder having a mean
particle size of 0.1-5 .mu.m to prepare raw powder and firing the
raw powder.
[0011] JP-A-H06-1822282 discloses a method of producing a catalyst
carrier by shaping and firing silicon carbide powder having a
specific surface area of 0.1-5 m.sup.2/gr and impurity components
of 1.0-5%.
[0012] JP-A-H09-202671 discloses a method of producing a silicon
carbide honeycomb filter by mixing .alpha.-type silicon carbide
powder having a mean particle size of 0.3-50 .mu.m, .beta.-type
silicon carbide powder having a mean particle size of 0.1-1.0 .mu.m
and the like to form raw powder and firing it.
[0013] JP-A-2000-16872 discloses a method of producing a porous
silicon carbide sintered body by mixing .alpha.-type silicon
carbide powder having a mean particle size of 5-100 .mu.m and
.alpha.-type or .beta.-type silicon carbide powder having a mean
particle size of 0.1-1.0 .mu.m to form a mixture and firing it.
[0014] JP-A-2001-97776 discloses a porous silicon carbide sintered
body and the like wherein silicon carbide crystal particles
constituting a porous structure are connected to each other through
neck portions and the neck portion smoothly curves.
[0015] Generally, a filter for purifying the exhaust gas is
subjected to a regeneration treatment in order to burn and remove
particulates after catching a certain amount of particulates.
However, when a filter made of silicon carbide is subjected to the
regeneration treatment, large cracks may be generated in the filter
itself due to thermal stress generated in the regeneration
treatment. The filter having the cracks has a problem that the
exhaust gas leaks out from the cracks and the catching of the
particulates becomes incomplete after a long-term use of the
filter. Such cracks can occur across silicon carbide particles and
cause breakage of the filter.
[0016] Further, JP-A-2002-201082 discloses a porous honeycomb
structural body for a filter including fire-resistant particles
such as silicon carbide particles and the like, and metallic
silicon. In such a honeycomb structural body, a catalyst is carried
which acts to lower activate energy for the combustion of
particulates or conversion harmful gas components such as CO, HC,
NO.sub.x and the like. Further, as the degree of dispersion to the
honeycomb structural body becomes higher, the reaction site to the
particulates and the harmful gas components increases and the
activity also increases. At a high temperature, however, the
specific surface area of the catalyst carrier used for increasing
the dispersion degree of the catalyst such as alumina and the like
decreases and the sintering of the catalyst itself is caused.
Consequently, it is known that the dispersion degree gets worse.
Besides, in such a honeycomb structural body, the thermal
conductivity is low as compared with the honeycomb structural body
made only of silicon carbide, so that when the same amount of
particulates is burnt at the regeneration treatment, the activity
of the catalyst carried may decrease because heat from burning
portions of particulates on surfaces of cells and the like is hard
to disperse and the temperature of the burning portions becomes
extremely high. Therefore, in the honeycomb structural body
including heat-resistant particles and metallic silicon,
visible-size cracks may be generated at the regeneration
treatment.
[0017] It is an object of the invention to solve the above problems
inherent to the conventional techniques, and to provide a ceramic
sintered body having a long-term stability which can prevent cracks
from occurring due to the breakage of ceramic particles when
thermal stress is applied and catalyst carried from deteriorating
when thermal stress is repeatedly applied, and a ceramic filter
produced by using the ceramic sintered body.
DISCLOSURE OF THE INVENTION
[0018] The invention is a ceramic sintered body comprising ceramic
coarse particles and a bonding layer existing between the ceramic
coarse particles to connect the particles and including ceramic
fine particles having a mean particle size smaller than that of the
ceramic coarse particles.
[0019] In this invention, the ceramic coarse particles are
single-crystal.
[0020] Also, in the invention, the bonding layer is formed with
ceramic fine particles having a mean particle size smaller than
that of the ceramic coarse particles and/or a sintered body of
aggregates thereof, or is a brittle body having a strength lower
than that of the ceramic coarse particles, or is a polycrystal body
comprising a plurality of the ceramic fine particles, and the
ceramic fine particles are formed by sintering with the grain
boundary remained, and contain at least one sintering aid selected
from iron, aluminium, nickel, titanium, chromium, oxide, and
further, the content of the sintering aids is higher than that in
the ceramic coarse particles.
[0021] In the ceramic sintered body according to the invention, an
average particle size ratio of the ceramic coarse particles to the
ceramic fine particles is 15:1.about.1:200 and a ratio of the total
weights of the ceramic coarse particles to the ceramic fine
particles is 1:1.about.1:9.
[0022] Next, the invention proposes a honeycomb-structural ceramic
filter comprising a pillar-shaped porous ceramic member or a
combination of a plurality of the pillar-shaped porous ceramic
members in which a plurality of cells as a gas passageway are
arranged side by side in a longitudinal direction through cell
walls and either one end portions of these cells are plugged,
characterized in the filter itself is formed by a ceramic sintered
body comprising ceramic coarse particles and a bonding layer
existing between the ceramic coarse particles to connect the
particles and including ceramic fine particles having a mean
particle size smaller than that of the ceramic coarse
particles.
[0023] In the ceramic filter according to the invention, the
concrete structures of the ceramic coarse particles and the bonding
layer are the same as described in the ceramic sintered body, and
therefore detailed explanation is omitted.
[0024] As ceramic used in the invention may be mentioned, for
example, alumina, zirconia, mullite, silica, cordierite and the
like.
[0025] As the nitride ceramics may be mentioned, for example,
aluminium nitride, silicon nitride, boron nitride, titanium nitride
and the like.
[0026] As the carbide ceramics may be mentioned, for example,
silicon carbide, zirconium carbide, titanium carbide, tantalum
carbide, tungsten carbide and the like.
[0027] These ceramics may be used alone or in a combination of two
or more.
[0028] In the invention, the ceramic sintered body is preferable to
use silicon carbide as the ceramic coarse particle and the bonding
layer. Further, the ceramic is preferable to show two peaks of
particle size according to a particle size distribution of
particles (vertical axis: number of particles, horizontal axis:
particle size) and have an average particle size of not less than
30 .mu.m.
[0029] Further, the ceramic sintered body is preferable to be a
porous body.
[0030] Hereinafter, as the ceramic sintered body according to the
invention, there is explained a case of mainly using silicon
carbide. The ceramic sintered body may be referred to as silicon
carbide sintered body, the coarse particles comprising silicon
carbide may be referred to as silicon carbide coarse particles, and
the ceramic fine particles may be referred to as silicon carbide
fine particles in the following explanation.
[0031] Further, the honeycomb structural body as the feature of the
structure of the ceramic filter according to the invention is a
pillar-shaped body formed by arranging a plurality of cells as an
exhaust gas passageway side by side in the longitudinal direction
through cell walls. There are both one-piece type and aggregate
type honeycomb structural bodies. Hereinafter, the one-piece type
honeycomb structural body having an integrated structure is formed
independently as a whole, while the aggregate type honeycomb
structural body has a structure that a plurality of ceramic
sintered bodies (units) are united through sealing material
layers.
[0032] The ceramic filter of the invention is preferable to be
formed by using the above aggregate type honeycomb structural
bodies using the silicon carbide sintered bodies.
[0033] In the ceramic filter having the aggregate type honeycomb
structure, it is preferable that the sealing material layers are
formed not only between the units but also on the outer peripheral
surface and that an adhesive having adhesion function is used as
the sealing material layer.
[0034] The ceramic sintered body of the invention having the
structure explained above, that is, the silicon carbide sintered
body is characterized in that mean particle size ratio of the
silicon carbide coarse particles to the silicon carbide fine
particles is adjusted to be 15:1.about.200:1 and the ratio of the
total weights thereof is adjusted to be 1:1.about.9:1 and that the
bonding layer comprising silicon carbide fine particles and/or a
polycrystalline body made of the silicon carbide fine particle
group is interposed between the silicon carbide coarse particles,
whereby the bonding layer can develop the function of mitigating
the aforementioned thermal shock and efficiently prevent cracks
from occuring in the sintered body.
[0035] Further, in the silicon carbide sintered body, by adjusting
a mean particle diameter of the silicon carbide coarse particle to
not less than 30 .mu.m, the number of the bonding layers is
decreased, while the number of the silicon carbide fine particles
per the bonding layer is increased, so that it is possible to
sufficiently ensure the thickness of the layer comprising the
polycrystalline bodies constituted with the silicon carbide fine
particles and/or the silicon carbide fine particle group (bonding
layer) to effectively act on the mitigation of the thermal shock as
described above. Moreover, the bonding layer means bonding portions
wherein the silicon carbide coarse particles are connected to each
other through the polycrystalline body comprising at least one
silicon carbide fine particle and/or a group thereof.
[0036] In the ceramic filter according to the invention produced by
using the silicon carbide sintered body, the whole of the honeycomb
structural body can be enclosed and compressed by an action of the
sealing material layer, and it becomes possible to efficiently
prevent minute cracks generated by impact, thermal stress and the
like from growing to be a visible size and silicon carbide
particles from shedding accompanied with the occurrence of
cracks.
[0037] In the aggregate type ceramic filter integrated by using the
silicon carbide sintered bodies and combining a plurality of the
honeycomb bodies prepared by these sintered bodies through the
sealing material layer, there are advantages in reducing thermal
stress and improving heat resistance by said sealing material
layer, and adjusting the size freely by increasing and decreasing
the number of the honeycomb structural bodies, whereby it becomes
possible to catch particulates and the like in the exhaust gas more
efficiently by the cell walls separating the cells.
[0038] Further, by utilizing the ceramic filter of the invention as
the exhaust gas purifying apparatus for vehicles, it is possible to
catch particulates in the exhaust gas completely for long periods,
reduce the deterioration of a catalyst when it is carried, prevent
the breakage of the filter since minute cracks generated by impact,
thermal stress or the like do not grow to be a visible size,
prevent silicon carbide particles from shedding accompanied with
the occurrence of cracks, improve its heat resistance, and adjust
the size freely.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1(a) is a perspective view schematically showing an
embodiment of the one-piece type honeycomb structural body using
the silicon carbide sintered body according to the invention, and
FIG. 1(b) is a section view shown by an arrow A-A in FIG. 1(a).
[0040] FIG. 2 is a SEM photograph of the silicon carbide sintered
body according to the invention showing an embodiment of a state of
bonding silicon carbide coarse particles and silicon carbide fine
particles (Example 10).
[0041] FIG. 3 is a SEM photograph of the silicon carbide sintered
body according to the invention showing an another embodiment of a
state of bonding silicon carbide coarse particles and silicon
carbide fine particles (Reference Example 1).
[0042] FIG. 4 is a TEM photograph of the silicon carbide sintered
body according to the invention showing a crystal condition of a
cross section of the combined state in Referenced Example 1.
[0043] FIG. 5(a) is a FE-SEM photograph (2000 magnification) of a
cross section of the bonded state in Reference Example 1, and FIG.
5(b) and FIG. 5(c) are X-ray spectral figures of elemental analysis
at positions A and B in FIG. 5(a), respectively.
[0044] FIG. 6 is a perspective view schematically showing an
embodiment of the aggregate type honeycomb structural body using
the silicon carbide sintered body according to the invention.
[0045] FIG. 7 is a section view illustrating an embodiment of an
exhaust gas purifying apparatus for vehicles equipped with the
ceramic filter according to the invention.
[0046] FIG. 8(a) is a perspective view illustrating an embodiment
of the conventional honeycomb structural bodies, and FIG. 8(b) is a
section view along a line B-B in FIG. 8(a).
[0047] FIG. 9 is a SEM photograph of the silicon carbide sintered
body in Comparative Example 7.
BEST MODE FOR CARRYING OUT THE INVENTION
[0048] As a desirable embodiment of the ceramic sintered body
having a honeycomb structural body, a ceramic filter 20 having
one-piece type honeycomb structure using ceramic carbide
(hereinafter referred to as one-piece type honeycomb filter) is
shown in FIG. 1. The one-piece type honeycomb filter 20 is a
square-pillar shaped porous body having a plurality of cells 21
arranged side by side in its longitudinal direction through cell
walls 23. These cells 21 are plugged at either one end portions of
inlet side or outlet side of the exhaust gas with a sealing
materials 22 to function the cell walls 23 separating the cell 21
as a filter. That is, the exhaust gas flown into one cell 21 always
passes the cell wall 23 and thereafter flows out of another cell
21.
[0049] The one-piece type honeycomb filter 20 itself, especially
the cell wall 23 is formed by porous sintered bodies constituted
with ceramic carbide coarse particles 101 each having a large
particle size and a bonding layer comprising ceramic carbide fine
particles 102 each having a small particle size and/or aggregates
thereof. That is, as shown in FIG. 2 and FIG. 3, the one-piece type
honeycomb filter has a structure that silicon carbide coarse
particles 101 are connected to each other through polycrystal body
103 (bonding layer) made of silicon carbide fine particles 102
and/or the aggregates formed by a group of the silicon carbide fine
particles 102.
[0050] In such a one-piece type honeycomb filter 20, thermal stress
generated in the regeneration treatment and the like is mitigated
by the role of the bonding layer comprising the silicon carbide
fine particle 102 and/or the polycrystal body 103 as described
above. Although the mechanism of the mitigation is not clear, it
can be considered as follows: For example, if a fine crack 104
which can be hardly observed until SEM and the like as shown in
FIG. 3 is generated in the bonding layer comprising the polycrystal
body 103, this crack is transmitted to the silicon carbide coarse
particle 101 as a skeleton particle to prevent the crack from
growing to be a visible large size. It is considered that, in the
bonding layer formed by the polycrystal body 103 comprising the
silicon carbide fine particles 102 and/or the aggregates thereof,
the silicon carbide fine particles 102 complicatedly bind together
in random directions. Therefore, in the one-piece type honeycomb
structural body 20, particulates in the exhaust gas can be surly
caught continuously after the regeneration treatment or the
like.
[0051] Further, the bonding layer formed by the polycrystal body
103 comprising groups of the silicon carbide fine particles 102
shows a property as a ceramic joint body having adhesion and
bonding function or a ceramic brittle body that has smaller
strength and is easily broken as compared with the ceramic coarse
particles. The ceramic joint body or the ceramic brittle body is
formed by aggregating the silicon carbide fine particles 102 to a
polycrystalline condition with keeping the particle form.
Therefore, it can be distinguished from an aggregate formed by
fusing the silicon carbide fine particles 102 without maintaining
the particle form as observed by using the transmission electronic
microscope (TEM).
[0052] Furthermore, it is desirable that a metal such as iron,
aluminium, nickel, titanium, chromium or the like, or a metal oxide
thereof is included in the interface of the fine particles forming
the bonding layer.
[0053] They are considered to work as a sintering aid for ceramic,
and further have an action for mitigating the stress. In other
words, the metal is easy to be melted because the melting point is
iron: 1540.degree. C., aluminium: 660.degree. C., nickel:
1450.degree. C., titanium: 1660.degree. C., and chromium:
1860.degree. C., while the melting point of ceramic is
approximately 2000.degree. C. As a result, in the case of using as
a filter, at the high temperature where the biggest thermal stress
is exerted, that is, when the ceramic brings about the thermal
expansion and the like, the metal is melted to be like an elastic
body to mitigate the stress between ceramic particles or form gaps
between the ceramic particles, whereby the compression force
between the ceramic particles is mitigated, and hence the stress
can be mitigated.
[0054] Further, when the fine particle is especially carbide
ceramics or nitride ceramics, it is preferable that grain boundary
is an oxide ceramic (e.g. silica). It is considered that since
oxide ceramics has lower thermal conductivity than carbide ceramics
or nitride ceramics, heat insulation effect partly appears compared
to the bonding layer made only of carbide ceramics or nitride
ceramics, and rapid temperature hardly occurs to mitigate thermal
stress. The oxide ceramics sometimes work as a substance for
inhibiting the firing.
[0055] In the invention, the silicon carbide coarse particles 101
as a skeleton particle of the ceramic sintered body have a mean
particle size larger than that of the silicon carbide fine
particles 102. The preferable lower limit is 30 .mu.m and the
preferable upper limit is 70 .mu.m. When the mean particle size is
less than 30 .mu.m, the number of the bonding layers increases to
make the thickness too thin and hence stress can not be
sufficiently mitigated in the bonding layer. While, when the mean
particle size exceeds 70 .mu.m, the number of the bonding layers
decreases and it is difficult to form the bonding layer thickly,
and as a result, the strength of the honeycomb structural body 20
lowers and the figures can not be maintained. In addition, when it
exceeds 70 .mu.m, defective forming may be caused in the forming
and production process.
[0056] In the invention, the silicon carbide fine particles 102
constituting the bonding layer of the ceramic sintered body have a
mean particle size smaller than the silicon carbide coarse
particles 101. The preferable lower limit is 0.1 .mu.m and the
preferable upper limit is 2.0 .mu.m. When the means particle size
is less than 0.1 .mu.m, it is considered that the bonding layer is
incorporated into the coarse particles and the bonding layer can
not easily be formed since the sintering of the silicon carbide
fine particles is promoted. Further, the cost for producing the
silicon carbide fine particles 102 increases to bring about cost
up. On the other hand, when the mean particle size exceeds 2.0
.mu.m, it is difficult to form the bonding layer by the silicon
carbide fine particles 102 and stress cannot be effectively
mitigated in the bonding layer.
[0057] The preferable lower limit of the mean particle size ratio
of the silicon carbide coarse particles 101 to the silicon carbide
fine particles 102 (mean particle size of the silicon carbide
coarse particles 101/mean particle size of the silicon carbide fine
particles 102) is 15 times, and the preferable upper limit is 200
times. When the ratio is less than 15 times, the formation of
bonding portion by the silicon carbide fine particles 102 is
difficult and stress can not be sufficiently mitigated in the
bonding layer. While when the ration exceeds 200 times, the
strength of the one-piece type honeycomb structural body 20
extremely lowers to easily break down due to vibration during the
production or use when being mounted on vehicles and the like.
[0058] The preferable lower limit of the ratio of the total weights
of the silicon carbide coarse particles 101 to the silicon carbide
fine particles 102 (total weight of the silicon carbide coarse
particles 101/total weight of the silicon carbide fine particles
102) is 1 times, and the preferable upper limit is 9 times. When
the ratio is less than 1 times, since the rate of silicon carbide
fine particles 102 is high, aggregated portion of the silicon
carbide fine particles 102 is formed in addition to the bonding
layer and thereby densified to hardly be porous. Also, since
thermal stress is concentrated into the portion, it is considered
that the one-piece type honeycomb structural body 20 is easily
broken. On the other hand, when the ratio exceeds 9 times, since
the rate of silicon carbide fine particles 102 is low, it is
difficult to form the bonding layer by the silicon carbide fine
particles 102 and stress can not be sufficiently mitigated in the
bonding layer.
[0059] As the sealing material 22 for plugging the end portions of
the cells 31, it is desirable to use the same porous ceramic as the
cell walls 23. Because the adhesion strength between the both
becomes high and also the porosity of the sealing material 22 is
adjusted likewise the cell walls 23, whereby the thermal expansion
coefficient of the cell walls 23 can be matched with the thermal
expansion coefficient of the sealing material 22. As a result, it
is made possible to prevent the formation of a gap between the
sealing material 22 and the cell walls 23 due to thermal stress in
the production or use, and the occurrence of cracks in the sealing
material 22 or the cell wall 23 contacted therewith.
[0060] The one-piece type honeycomb filter 20 can be carried with a
catalyst for decreasing activation energy for the combustion of the
particulates or converting harmful components such as CO, HC,
NO.sub.x or the like in the exhaust gas. That is, the one-piece
type honeycomb filter 20 carried on surfaces of the cell walls 23
and the like with the catalyst works not only as a filter for
catching particulates in the exhaust gas, but also as a catalyst
converter for converting CO, HC, NO.sub.x or the like contained in
the exhaust gas.
[0061] The one-piece type honeycomb filter 20 uses mainly the
silicon carbide coarse particles 101 and silicon carbide fine
particles 102 as a starting material and shows a high thermal
conductivity. Therefore, the maximum temperature inside the filter
in the regeneration treatment does not rise as compared to the
conventional honeycomb structural body formed by joining silicon
carbide particles inferior in thermal conductivity through metallic
silicon, and the activity of the catalyst is not lowered.
[0062] As the catalyst carried in the honeycomb filter 20 may be
used any ones which can lower the activation energy for the
combustion of particulates or conversion harmful components such as
CO, HC, NO.sub.x and the like in the exhaust gas. For example, a
noble metal such as platinum, palladium, rhodium or the like can be
used. Particularly, a so-called three-way catalyst consisting of
platinum, palladium and rhodium is preferable. In addition to the
noble metal, it is desirable to use an alkali metal (Group 1 of the
Periodic Table), an alkaline earth metal (Group 2 of the Periodic
Table), a rare earth element (Group 3 of the Periodic Table), a
transition metal element and the like.
[0063] The catalyst may be carried on the surfaces of pores in the
honeycomb filter 20, or may be uniformly carried on the cell walls
23 at a given thickness. Also, the catalyst may be uniformly
carried on the surfaces of cell walls 23 and/or pores or unevenly
carried at certain side. In particular, it is desirable to carry
the catalyst on the surfaces of cell walls 23 or on the surfaces of
pores in the vicinity thereof in the cells 21 at inflow side. It is
more desirable to carry the catalyst on the both surfaces, because
the catalyst can easily contact with the particulates to conduct
combustion of the particulates efficiently.
[0064] When the catalyst is applied to the honeycomb filter 20, it
is desirable to coat the surface of the honeycomb structural body
with a support material such as alumina or the like prior to the
application of the catalyst. Because, it is possible to make the
specific surface area of the honeycomb structural body large to
enhance the dispersivity of the catalyst and increase the reaction
site of the catalyst. Also, the support material can prevent the
sintering of catalyst metal and thereby the heat resistance of the
catalyst can be improved to decrease the pressure loss.
[0065] The one-piece type honeycomb structural body carried with
the catalysts functions as the same exhaust gas purifying apparatus
as the known DPF (diesel particulate filter) equipped the with
catalyst. Meanwhile, detailed explanation for the case when the
one-piece type honeycomb structural body according to the invention
functions as a catalyst-carrying body is omitted here.
[0066] The one-piece type honeycomb structural body 20 shown in
FIG. 1 is square-pillar shaped. However, the shape of the one-piece
type honeycomb structural body according to the invention is not
especially limited as far as it is pillar-shaped body, and mention
may be made of, for example, a pillar-shaped body which cross
section is polygonal, circular, or ellipsoidal as a shape of a
section perpendicular to a longitudinal direction.
[0067] The porosity of the silicon carbide honeycomb structural
body constituting the one-piece type honeycomb filter 20 is not
particularly limited, but the lower limit is desired to be 30% and
the upper limit is desired to be 80%. When the porosity of the
structural body is less than 30%, the honeycomb filter 20 may be
clogged easily, while when it exceeds 80%, the strength of the
one-piece type honeycomb filter 20 lowers to be broken easily.
Moreover, the porosity can be measured by the well-known methods,
such as mercury injection method, Archimedes method, measurement
through scanning electron microscope (SEM) and the like.
[0068] The average pore diameter of the one-piece type honeycomb
filter 20 is preferably not more than 5 .mu.m, while the upper
limit is not more than 50 .mu.m. When it is less than 5 .mu.m, the
particulates easily cause clogging, while when it exceeds 50 .mu.m,
the particulates can pass through the pores and the catching
efficiency of the particulates lowers to impede the functioning as
a filter.
[0069] Although the illustration is omitted, in the one-piece type
honeycomb filter according to the invention, it is desirable to
form a sealing material layer on an outer peripheral surface
thereof.
[0070] Such a one-piece type honeycomb filter may have a sealing
material layer formed on an outer peripheral surface thereof.
Because, when the sealing material layer is formed on the outer
peripheral surface of the honeycomb structural body, it is
effective to bundle the one-piece type honeycomb filter through the
sealing material layer, whereby there can be prevented fine cracks
growing to be visible size due to impact, further thermal stress or
the like and silicon carbide powders from shedding accompanied with
the occurrence of the cracks.
[0071] As a material constituting the sealing material layer can be
used, for example, a sealing material consisting of an inorganic
binder, an organic binder, inorganic fibers and/or inorganic
particle, and the like.
[0072] As the inorganic binder, mention may be made of, for
example, silica sol, alumina sol and the like. They may be used
alone or in a combination of two or more. Among the inorganic
binders, silica sol is desirable.
[0073] As the organic binder, mention may be made of, for example,
polyvinyl alcohol, methyl cellulose, ethyl cellulose, carboxymethyl
cellulose and the like. They may be used alone or in a combination
of two or more. Among the organic binders, carboxymethyl cellulose
is desirable.
[0074] As the inorganic fiber, mention may be made of, for example,
ceramic fibers such as silica-alumina, mullite, alumina, silica and
the like. They may be used alone or in a combination of two or
more. Among the inorganic fibers, silica-alumina fiber is
desirable.
[0075] As the inorganic particles, mention may be made of, for
example, carbides, nitrides and the like. Concretely, there may be
mentioned inorganic powder or whisker consisting of silicon
carbide, silicon nitride, boron nitride and the like. They may be
used alone or in a combination of two or more. Among the inorganic
particles, silicon carbide having an excellent thermal conductivity
is desirable.
[0076] Next, as the ceramic filter according to the invention,
mention may be made of an aggregate type ceramic filter constituted
by bundling a plurality of ceramic filters through adhesive sealing
material layers, in addition to the one-piece type ceramic filter
consisting only of one ceramic filter as mentioned above.
[0077] Such an aggregate type ceramic filter is a preferable
embodiment in that the sealing material layer can mitigate thermal
stress to improve the heat resistance of the filter, the number of
the ceramic structural bodies can be increased and decreased as a
unit to freely adjust the size. Moreover, the one-piece type
honeycomb filter has the same filter function as the aggregate type
honeycomb filter.
[0078] FIG. 4 shows another embodiment of the invention and is a
perspective view showing an aggregate type honeycomb filter
constituting by bundling a plurality of units of ceramic(silicon
carbide) sintered bodies of honeycomb structural bodies through
sealing material layers. As shown in the figure, the aggregate type
honeycomb filter 10 is used as an exhaust gas purification filter
and formed by bundling a plurality of the above units with
honeycomb structure through sealing material layers 14 in a
cylindrical shape to constitute a honeycomb block 15 and further
coating another sealing material layers 13 around the honeycomb
blocks 15 in order to prevent leakage of the exhaust gas.
[0079] In the aggregate type honeycomb filter 10, the sealing
material layer 14 is inserted between ceramic honeycomb structural
body units 20 and functions as an adhesive bundling and adhering a
plurality of the ceramic honeycomb structural body units. On the
other hand, it is preferable that the sealing material layer 13 is
formed so as to enclose an outer peripheral surfaces of the
honeycomb block 15 as an aggregate body of the units, and functions
as a sealing material for preventing leakage of the exhaust gas
passing through the cells from the outer peripheral surface of the
honeycomb block 15 when the aggregate type honeycomb filter 10 is
disposed in an exhaust path of an internal combustion engine, and
be made from a material hardly permeating gas as compared with the
ceramic sintered body-itself.
[0080] In the aggregate type honeycomb filter 10, the sealing
material layers 13 and 14 may be made of the same material, or
different materials. Further, when the sealing material layers 13
and 14 are made of the same material, the compounding ratio of the
materials can be the same or different.
[0081] However, the sealing material layer 14 may be made of a
porous material capable of flowing the exhaust gas, but it is
preferable to be made of a densified material. On the other hand,
the sealing material layer 13 is preferable to be made of a
densified material. Because, the sealing material layer 13 is used
for the purpose of preventing the leakage of the exhaust gas from
the outer peripheral surface of the honeycomb block 15 when the
aggregate type honeycomb filter 10 is disposed in an exhaust path
of an internal combustion engine.
[0082] As a material constituting the sealing material layer 13 and
the sealing material layer 14 can be used, for example, material
prepared by compounding the above-mentioned inorganic binder,
organic binder, inorganic fibers and/or inorganic particles.
[0083] The aggregate type honeycomb filter 10 may have a
cylindrical shape, but as far as it is pillar-shaped, the cross
section vertical to a longitudinal direction can be, for example,
polygonal, circular, or ellipsoidal.
[0084] The aggregate type honeycomb filter 10 can be prepared by
bundling a plurality of honeycomb structural body units and then
forming the outer peripheral surface into a polygonal, circular,
ellipsoidal shape or the like, or by previously working the shape
of cross section of the honeycomb structural body units, and then
bundling them through an adhesive so as to make the shape of cross
section vertical to a longitudinal direction polygonal, circular,
ellipsoidal or the like. For example, the aggregated type honeycomb
filter may be a cylindrical-shaped aggregate type honeycomb filter
prepared by bundling 4 pillar-shaped one-piece type honeycomb
structural body wherein the shape of cross section vertical to a
longitudinal direction polygonal is fan-shape formed by dividing a
circle in quarters.
[0085] Next, an example of the method of manufacturing the
honeycomb filter using the silicon carbide sintered body according
to the invention is explained.
[0086] When the honeycomb structural body is a one-piece type
honeycomb filter constituted by a single silicon carbide sintered
body (honeycomb structural body unit) as a whole, a starting
material paste consisting mainly of the aforementioned silicon
carbide coarse particles and silicon carbide fine particles is
extrusion-molded to form a silicon carbide green shaped body having
substantially the same shape as a desired one-piece type honeycomb
filter.
[0087] The starting material paste is not especially limited, but
it is desirable to use materials wherein the porosity of the
ceramic member after the production is made 30-80%, and there can
be used, for example, one obtained by adding a binder, a dispersion
medium and the like to the aforementioned silicon carbide coarse
particles and silicon carbide fine particles.
[0088] As the binder can be used, for example, methyl cellulose,
carboxy methyl cellulose, hydroxy ethyl cellulose, polyethylene
glycol, phenol resin, epoxy resin and the like.
[0089] The compounding amount of the binder is usually desirable to
be about 1-20 parts by weight per 100 parts by weight of the
silicon carbide particle.
[0090] As the dispersion medium can be used, for example, an
organic solvent such as benzene or the like, an alcohol such as
methanol or the like, water and so on. The dispersion medium is
compounded in a proper amount for making the viscosity of the
starting material paste within a certain range.
[0091] The silicon carbide powder, binder and dispersing medium are
mixed in an attritor or the like, sufficiently kneaded by means of
a kneader or the like and then extrusion molded.
[0092] The raw material paste may be added with a material
obstructing firing and/or a sintering aids advancing firing. The
average particle size, particle size distribution, and blending
quantity of the material obstructing firing and the sintering aids
are adjusted depending on the average particle size, particle size
distribution, and blending quantity of the silicon carbide fine
particles, whereby the ceramic shaped body of the ceramic honeycomb
structural body after firing can be made to have a structure that
the silicon carbide coarse particles are bonded with each other
through the silicon carbide fine particles and/or the bonding layer
comprising of polycrystal body formed by the silicon carbide
particles.
[0093] Also, a shaping assistant may be added to the starting
material paste, if necessary. As the shaping assistant can be used,
for example, ethylene glycol, dextrin, aliphatic acid soap,
polyvinyl alcohol and the like.
[0094] To the starting material paste may be added balloons of
hollow microspheres composed mainly of oxide ceramic, spherical
acryl particles, hole-forming agent such as graphite or the like,
if necessary.
[0095] As the balloon can be used, for example, alumina balloon,
glass microballoon, silas balloon, fly ash balloon (FA balloon),
mullite balloon and the like. Among them, fly ash balloon is
desirable.
[0096] Then, the ceramic shaped body is dried by using a microwave
drying machine, a hot-air drying machine, a dielectric drying
machine, a reduced-pressure drying machine, a vacuum drying
machine, a freeze drying machine and the like to form a dried body,
and thereafter the dried body is subjected to a plugging treatment
in which given cells are filled with a sealing material paste as a
sealing material and clogged at either end portions thereof.
[0097] The sealing material paste is not particularly limited, but
it is desirable to use materials in which a porosity of a sealing
material produced after a post-process is made within 30.about.80%.
For example, the same material as in the aforementioned starting
material paste can be used. The starting material paste is
desirable to be a material prepared by adding a lubricant, a
solvent, a dispersant, and a binder to the ceramic particles. The
reason is that the settling of the silicon carbide particles in the
sealing material paste in the plugging treatment can be
prevented.
[0098] Next, the dried body of the silicon carbide shaped body
filled with the sealing material paste is degreased and fired under
given conditions to form a silicon carbide one-piece type honeycomb
filter comprised of a porous sintered body as a whole.
[0099] As the conditions for decreasing the dried body may be used
conditions conventionally applied in the production of filters
comprising porous ceramics.
[0100] The conditions for firing the dried body are decided
depending on the average particle size, particle size distribution,
and blending quantity of the silicon carbide coarse particles, the
silicon carbide fine particles, the material obstructing firing,
the sintering aid advancing firing and the like, whereby the
honeycomb structural body after the firing can have a structure
that the silicon carbide coarse particles bond with each other
through the bonding layer of polycrystal body formed by the silicon
carbide fine particles and/or an aggregate body of the silicon
carbide particles. Concretely, conditions of 1800-2200.degree. C.
and 3 hours or the like may be used.
[0101] When a catalyst is supported on the one-piece type honeycomb
filter, it is desirable to form am alumina film having a high
specific surface area on the ceramic fired body obtained by firing
and provide a catalyst such as platinum or the like on the surface
of the alumina film.
[0102] As a method for forming the alumina film on the surface of
the silicon carbide sintered body, there are, for example, a method
wherein a solution of a metal compound containing aluminum such as
Al(NO.sub.3).sub.3 or the like is impregnated in the silicon
carbide sintered body and heated, a method wherein a solution
containing alumina powders is impregnated in the silicon carbide
sintered body and heated, and the like.
[0103] As a method for providing a co-catalyst or the like on the
alumina film may be used, for example, a method wherein a solution
of a metal compound containing a rare-earth element such as Ce
(NO.sub.3).sub.3 or the like is impregnated in the silicon carbide
sintered body and heated.
[0104] As a method for providing a catalyst on the alumina film,
may be used, for example, a method wherein a solution of
dinitrodiamine platinum nitrate
([Pt(NH.sub.3).sub.2(NO.sub.2).sub.2]HNO.sub.3) is impregnated in
the ceramic fired body and heated.
[0105] Further, when the honeycomb structural body is an aggregate
type honeycomb filter 10 constituted by bundling a plurality of the
honeycomb units through the sealing material 14 as shown in FIG. 6,
a step of applying a sealing material paste forming a sealing
material layer 14 on the side of a honeycomb structural body unit
at a uniform thickness and laminating other honeycomb structural
body unit is successively repeated to prepare a laminate of
square-pillar shaped aggregate type honeycomb structural body
having a given size.
[0106] As the material constituting the sealing material paste is
already explained and the explanation thereof is omitted here.
[0107] Next, the laminate of the honeycomb structural body units
(aggregated body) is heated to dry and solidify the sealing
material paste layer to thereby form a sealing material layer 14,
and thereafter an outer peripheral portion thereof is cut into a
shape as shown in FIG. 6 with a diamond cutter or the like to
prepare a honeycomb block 15.
[0108] A sealing material layer 13 is formed on the outer periphery
of the honeycomb block 15 with the above sealing material paste,
whereby there can be produced an aggregate type honeycomb filter 10
constituted by bundling a plurality of the honeycomb structural
body units through the sealing material layers 14.
[0109] As an application of the honeycomb filter using the ceramic
sintered body according to the invention, it is desirable to use in
an exhaust gas purification apparatus for vehicles. FIG. 7 is a
diagrammatically section view illustrating an embodiment of the
exhaust gas purifying apparatus for vehicles equipped with the
honeycomb structural body.
[0110] As shown in FIG. 7, the exhaust gas purifying apparatus 600
is mainly constituted with a honeycomb filter 60, a casing 630
covering the outside of the honeycomb filter 60, a keep seal
material 620 disposed between the honeycomb filter 60 and the
casing 630, and a heating means 610 arranged at an exhaust gas
flowing side of the honeycomb filter 60. To an end portion of the
casing 630 introducing the exhaust gas is connected an inlet pipe
640 connected to an internal-combustion engine such as an engine or
the like, and the other end portion of the casing 630 is connected
with a discharge pipe 650 connected to an exterior. Moreover, an
arrow in FIG. 7 shows a flow of the exhaust gas.
[0111] In FIG. 7, the honeycomb filter 60 may be the one-piece type
honeycomb filter 20 shown in FIG. 1 or the aggregate type honeycomb
filter 10 shown in FIG. 6.
[0112] In the exhaust gas purification apparatus 600 having such a
construction, the exhaust gas discharged from the
internal-combustion engine such as engine or the like is introduced
into the casing 630 through the inlet pipe 640 and flown into the
honeycomb filter 60 from the cells open at a flowing side, passed
cell walls to conduct purification by catching particulates with
the cell walls and then discharged through the discharge pipe 650
to an exterior.
[0113] In the exhaust gas purification apparatus 600, when a large
number of the particulates is stored on the cell walls of the
honeycomb filter 60 to raise pressure loss, the regeneration
treatment of the honeycomb filter 60 is conducted.
[0114] In the regeneration treatment, a gas heated by using the
heating means 610 is flown into the inside of cells of the
honeycomb filter 60, whereby the honeycomb filter 60 is heated and
the particulates stored on the cell walls are combusted and
removed. Also, the particulates can be combusted and removed by
using a post injection system.
EXAMPLES
[0115] Hereinafter, examples of the invention will be concretely
described with reference to the drawings. However, the invention is
not limited to the examples only.
Example 1 of the Present Invention
[0116] (1) A starting material paste is prepared by wet-mixing 70%
by weight of .alpha.-type silicon carbide powder having an average
particle size of 30 .mu.m (silicon carbide coarse particles) with
30% by weight of .alpha.-type silicon powder having an average
particle size of 0.5 .mu.m (silicon carbide fine particles) and
adding and kneading with 15 parts by weight of an organic binder
(methyl cellulose) and 20 parts by weight of water based on 100
parts by weight of the resulting mixed powder. The silicon carbide
fine particles are prepared by previously acid-cleaning with nitric
acid, hydrofluoric acid, hydrochloric acid or the like, and then
adding 0.7 parts by weight of iron powder having an average
particle diameter of 0.1 .mu.m and particle distribution within
.+-.10% of the average particle size based on 100 parts by weight
of silicon carbide fine particles.
[0117] Next, small amounts of a lubricant and a plasticizer are
added to the starting material paste, mixed and kneaded, and
thereafter extrusion molded to form a ceramic shaped body having a
similar shape of the cross section as shown FIG. 1(a). Next, the
ceramic shaped body is dried by a microwave drier to form a ceramic
dried body, thereafter a paste having the same composition as that
of the ceramic shaped body is filled in predetermined cells and
again dried by the drier. Then, the dried body is degreased at
400.degree. C. and fired at 1900.degree. C. in an aragon atmosphere
under an atmospheric pressure for 3 hours to obtain a honeycomb
filter 20 comprising the silicon carbide sintered body as shown in
FIG. 1 having a porosity of 50%, an average pore size of 12 .mu.m,
a size of 34 mm.times.34 mm.times.150 mm, 324 pieces of cells, and
0.4 mm thick of a cell wall 23.
[0118] (2) 16 filter units (4 units.times.4 units) with honeycomb
structure are connected through a heat-resistant sealing material
paste containing 30 wt % of alumina fibers of 0.2 mm in fiber
length, 21 wt % of silicon carbide particles of 0.6 .mu.m in
average particle size, 15 wt % of silica sol, 5.6 wt % of
carboxymethyl cellulose and 28.4 wt % of water and then cut with a
diamond cutter to prepare a cylindrical ceramic block 15 having a
diameter of 144 mm.times.a length of 150 mm. In this case, the
thickness of the sealing material layer 14 for connecting one-piece
type honeycomb filter units is adjusted to be 1.0 mm.
[0119] Next, a sealing material paste is prepared by mixing and
kneading 23.3 wt % of ceramic fiber made of alumina silicate (shot
content: 3%, fiber length: 0.1-100 mm) as an inorganic fiber, 30.2
wt % of silicon carbide powder having a average particle size of
0.3 .mu.m as an inorganic particle, 7 wt % of silica sol (SiO.sub.2
content in sol: 30 wt %) as an inorganic binder, 0.5 wt % of
carboxymethyl cellulose as an organic binder and 39 wt % of
water.
[0120] Then, a sealing material paste layer of 1.0 mm in thickness
is formed around the outer peripheral portion of the honeycomb
block 15 by using the sealing material paste. And, the sealing
material paste layer is dried at 120.degree. C. to prepare a
cylindrical aggregate type honeycomb filter 10 having cylindrical
shape as shown in FIG. 6 and mainly comprising of silicon carbide
sintered bodies.
[0121] (3) Al(NO.sub.3).sub.3 is poured in 1.3-butanediol solution
and stirred at 60.degree. C. for 5 hours to prepare 1.3-butanediol
solution containing 30 wt % of Al(NO.sub.3).sub.3. The aggregate
type honeycomb structural body 10 is immersed into the
1,3-butanediol solution, then heated at 150.degree. C. for 2 hours
and at 400.degree. C. for 2 hours, further immersed in water of
80.degree. C. for 2 hours, and heated at 700.degree. C. for 8 hours
to form an alumina layer on the surface of the aggregate type
honeycomb filter 10.
[0122] Ce (NO.sub.3).sub.3 is poured in ethylene glycol and stirred
at 90.degree. C. for 5 hours to prepare ethylene glycol solution
containing 6 wt % of Ce (NO.sub.3).sub.3. The aggregate type
honeycomb filter 10 having an alumina layer on the surface thereof
is immersed into the ethylene glycol solution, and then heated at
150.degree. C. for 2 hours and at 650.degree. C. in nitrogen
atmosphere for 2 hours to form catalyst layers comprising rare
earth oxide-containing alumina layers on the surface of the
aggregate type honeycomb filter 10.
[0123] Dinitrodiamine platinum nitrate ([Pt(NH.sub.3).sub.2
(NO.sub.2).sub.2]HNO.sub.3 having platinum concentration of 4.53 wt
% is diluted with a distilled water, and the aggregate type
honeycomb filter 10 having rare earth oxide-containing alumina
layers on the surface thereof is immersed in the prepared solution
to attach Pt of 2 g/L to the surface, and thereafter heated at
110.degree. C. for 2 hours and at 500.degree. C. in nitrogen
atmosphere for 1 hour to carry a platinum catalyst having an
average particle size of 2 nm on the surface of the aggregate type
honeycomb filter 10.
Examples 2.about.11 of the Invention
[0124] A cylindrical-shaped aggregate type honeycomb filter 10
mainly comprising silicon carbide sintered bodies and carrying a
platinum catalyst is prepared in the same manner as in Example 1
except for changing the average particle sizes of silicon carbide
coarse particles and silicon carbide fine particles used in the
preparation of starting material paste, the compounding ratio of
the silicon carbide coarse particles and the silicon carbide fine
particles used in the preparation of starting material paste, and
the firing temperature when a ceramic dried body is fired to
produce a one-piece type honeycomb filter 20 as shown in Table 1
below.
Reference Example 1
[0125] 70% by weight of .alpha.-type silicon carbide powder having
an average particle size of 30 .mu.m (silicon carbide coarse
particles) and 30% by weight of .alpha.-type silicon carbide powder
having an average particle size of 0.5 .mu.m (silicon carbide fine
particles) are wet-mixed, and to 100 parts by weight of the
obtained mixture are added and kneaded 15 parts by weight of an
organic binder (methyl cellulose) and 20 parts by weight of water
to prepare a starting material paste. The above silicon carbide
fine particles are prepared by previously acid-cleaning with nitric
acid, and then adding 0.7 parts by weight of iron powder having an
average particle diameter of 0.1 .mu.m and particle distribution
within .+-.10% of the average particle diameter based on 100 parts
by weight of silicon carbide fine particles.
[0126] Next, small amounts of a lubricant and a plasticizer are
added to the starting material paste, further mixed and kneaded,
and thereafter extrusion-molded to form a ceramic shaped body
having a similar shape of the cross section as shown in FIG. 6.
Then, the ceramic shaped body is dried by a microwave drier to form
a ceramic dried body and thereafter a paste having the same
composition as that of the ceramic shaped body is filled in
predetermined cells and again dried by the drier. Then, the dried
body is degreased at 400.degree. C. and fired at 1900.degree. C. in
an aragon atmosphere under an atmospheric pressure for 3 hours to
obtain a cylindrical-shaped honeycomb filter 30 comprising the
silicon carbide sintered body shown in FIG. 6 having a porosity of
50%, an average pore size of 12 .mu.m, 144 mm in diameter.times.150
mm in length, and cell wall 23 of 0.4 mm thick.
(2) To the honeycomb filter 30 prepared in aforementioned (1) is
provided a catalyst in the same manner as (3) in Example 1 of the
invention.
Comparative Examples 1.about.9
[0127] A cylindrical-shaped aggregate type honeycomb filter 10
mainly comprising silicon carbide sintered bodies and carrying a
platinum catalyst is prepared in the same manner as in Example 1
except for changing the average particle sizes of silicon carbide
coarse particles and silicon carbide fine particles used in the
preparation of starting material paste, the compounding ratio of
the silicon carbide coarse particles and the silicon carbide fine
particles used in the preparation of starting material paste, the
firing temperature when a ceramic dried body is fired to produce a
one-piece type honeycomb filter 20, and particle distribution of
iron powder added to silicon carbide fine particle as shown in
Table 1 below.
[0128] In Comparative Example 2, small amounts of a lubricant and a
plasticizer are added to starting material paste, further mixed and
kneaded, and thereafter extrusion-molded to form a ceramic shaped
body having a similar shape of the cross section as shown FIG.
1(a), but subsequent steps are not carried out due to defect of the
obtained shaped body.
[0129] Further, the honeycomb structural body produced in
Comparative Example 5 is weak in strength due to imperfect
firing.
Comparative Example 10
[0130] (1) 70% by weight of .alpha.-type silicon carbide powder
having an average particle size of 32.6 .mu.m and 30% by weight of
metallic silicon having an average particle size of 4.0 .mu.m are
wet-mixed, and to 100 parts by weight of the obtained mixture are
added and kneaded 6 parts by weight of an organic binder (methyl
cellulose), 2.5 parts by weight of surface-active agent, and 24
parts by weight of water to prepare a starting material paste.
[0131] Next, the starting material paste is extrusion-molded to
prepare a ceramic shaped body having a similar shape of the cross
section as shown FIG. 1(a). Next, the ceramic shaped body is dried
by using a microwave drier to form a ceramic dried body and
thereafter a paste having the same composition as that of the
ceramic shaped body is filled in predetermined cells and again
dried by the drier. Then, the dried body is degreased at
550.degree. C. in an oxidizing atmosphere and fired at 1600.degree.
C. in an aragon atmosphere under an atmospheric pressure for 3
hours to obtain a one-piece type honeycomb filter 20 comprising
silicon carbide-metallic silicon sintered body as shown in FIG.
1(a) having a porosity of 50%, an average pore size of 20 .mu.m, a
size of 34 mm.times.34 mm.times.150 mm, 324 pieces of cells, and a
cell wall 23 of 0.4 mm thick.
[0132] (2) A cylindrical-shaped aggregate type honeycomb filter 10
mainly comprising silicon carbide-metallic silicon sintered bodies
and carrying a platinum catalyst is produced in the same manner as
Example 1 (2) and (3) except for using the one-piece type honeycomb
filter 20 produced in aforementioned (1).
(Evaluation Test)
(1) Connecting State of Silicon Carbide Particles
[0133] Each honeycomb filter according to Examples, Reference
Example, and Comparative Examples is observed through SEM at a
magnification of 2000 within a range of 10 mm.times.10 mm to
examine whether each silicon carbide coarse particle are connected
through more than one silicon carbide fine particles, and/or
polycrystalline body formed by silicon carbide particles, that is,
presence or absence of connecting portions. The results are shown
in Table 1. The SEM photograph of Example 10 at 2000 magnification
is shown in FIG. 2. The SEM photograph of Comparative Example 7 at
5000 magnification is shown in FIG. 9.
(2) Presence or Absence of Cracks in Regeneration Treatment
[0134] An exhaust gas purifying apparatus as shown FIG. 6 is
produced and disposed in an exhaust path of an engine by using the
honeycomb structural body according to Examples, Reference Example,
and Comparative Examples. The engine is driven at a driving state
of a revolution number of 3000 rpm and a torque of 50 Nm for given
period of time, and thereafter regeneration treatment (post
injection system) is repeated 100 times to observe if a crack is
generated in the honeycomb structural body through visual
observation and SEM observation. The results are shown in Table
1.
[0135] The SEM photograph at 5000 magnification of Reference
Example 1 after this test is shown in FIG. 3. Moreover, the TEM
photograph of Example 1 before this test is shown in FIG. 4. As a
result of conducting qualitative analysis and element mapping, it
turned out that a lower-left portion is a single-crystal silicon
carbide of coarse particle 101 and silicon carbide of fine
particles 102 is polycrystallized to form a bonding layer 103 at a
right-upper portion. It also turned out to be a case 105 (white
portion) wherein iron is contained between fine particles.
[0136] FIG. 5(a) shows a result of measuring the same Example
through FE-SEM at 2000 magnification, and FIGS. 5(b) and (c) are
X-ray spectral figures of results of conducting qualitative
analysis to element analysis at each position of A and B in FIG.
5(a) using X-ray. As shown in these figures, coarse particles 101
can be observed in A, and iron, aluminium, nickel, titanium, and
chromium can be observed in B.
(3) An Average Particle Size of Platinum Catalyst After
Regeneration Treatment
[0137] In the honeycomb structural body according to each of
Examples, Reference Example, and Comparative Examples after the
evaluation test (2), a platinum catalyst is observed through a
transmission electron microscope (TEM) to obtain an average
particle size. The results are shown in Table 1. TABLE-US-00001
TABLE 1 particle size average particle compounding distribution to
an presence average size(.mu.m) ratio of ratio(coarse average
particle firing or absence presence or particle size coarse fine
average particle:fine size of iron temperature of bonding absence
of of platinum particle particle particle size particle) powder (%)
(.degree. C.) layer crack (nm) Example 1 30 0.5 60 7:3 10 1900
presence absence 15 Example 2 40 0.5 80 7:3 10 2000 presence
absence 15 Example 3 50 0.5 100 7:3 10 2050 presence absence 15
Example 4 60 0.5 120 7:3 10 2100 presence absence 15 Example 5 70
0.5 140 7:3 10 2150 presence absence 15 Example 6 40 0.5 80 5:5 10
2000 presence absence 15 Example 7 40 0.5 80 9:1 10 2000 presence
absence 15 Example 8 40 0.5 80 7:3 10 1800 presence absence 15
Example 9 40 0.5 80 7:3 10 2200 presence absence 15 Example 10 30
2.0 15 7:3 10 1900 presence absence 15 Example 11 40 0.2 200 7:3 10
1900 presence absence 15 Reference 30 0.5 60 7:3 10 1900 presence
presence 15 Example 1 (fine crack) Comparative 25 0.5 50 7:3 50
1900 absence presence 15 Example 1 Comparative 80 0.5 160 7:3 50 --
-- -- -- Example 2 Comparative 40 0.5 80 10:0 -- 2000 absence
presence 15 Example 3 Comparative 40 0.5 80 4:6 50 2000 absence
presence 15 Example 4 Comparative 40 0.5 80 7:3 50 1600 -- -- --
Example 5 Comparative 40 0.5 80 7:3 50 2300 absence presence 15
Example 6 Comparative 11 0.5 22 7:3 50 2200 absence presence 15
Example 7 Comparative 20 2.0 10 7:3 50 1900 absence presence 15
Example 8 Comparative 50 0.2 250 7:3 50 2000 absence presence 15
Example 9 Comparative 32.6 4.0 8 7:3 -- 1600 presence absence 50
Example 10
[0138] As shown in Table 1, in the honeycomb filter having the
bonding layer according to Examples 1-11, visible cracks are not
generated even if the regeneration treatment is conducted
repeatedly. While, in the honeycomb filter according to Reference
Example 1, fine cracks can be observed through SEM observation as
the outer peripheral surface is not fastened with the sealing
material.
[0139] Further, in the honeycomb structural body formed by
sintering silicon carbide particles only, the average particle
diameter of platinum catalyst after repeatedly conducting
regeneration treatment is smaller and the activity of the platinum
catalyst is higher as compared with the honeycomb structural body
formed by sintering silicon carbide and metallic silicon
(Comparative Example 10).
Conventional Example
[0140] A porous silicon carbide sintered body disclosed in
JP-A-S60-264365 is characterized in that three-dimensional net-like
structure is constituted by plate crystals of silicon carbide and
is entirely different from the ceramic sintered body according to
the invention constituting porous structure by silicon carbide
particles having different particle sizes.
[0141] Further, JP-A-H4-187578 discloses a method of manufacturing
a sintered body comprising .alpha.-type silicon carbide powder
having a large particle size and .beta.-type silicon carbide powder
having small particle size wherein the sintered body can be formed
by grain-growing the .beta.-type silicon carbide without
grain-growing the high-temperature and stable .alpha.-type silicon
carbide.
[0142] Also in JP-A-H5-139861, JP-A-H9-202671 and JP-A-2000-16872,
the manufacturing method for sintered body comprising similar
silicon carbide is disclosed.
[0143] In each of these conventional examples, however, volume
diffusion between silicon carbide coarse particles and grain
boundary diffusion are sufficiently developed. In addition, they
relate to a technique for constructing the same crystal structure
by uniting silicon carbide coarse particles and silicon carbide
fine particles through phase-transition of unstable .beta.-type
silicon carbide powder to .alpha.-type silicon carbide powder,
which is different from the bonding layer of the present
invention.
[0144] A porous silicon carbide sintered body disclosed in
JP-A-2001-97776 is characterized in that the neck portion curves
gently and the firing is carried out at a high temperature so as to
make the neck portion smooth, so that the silicon carbide fine
particles do not retain the form of particle after firing and the
bonding layer is not formed by polycrystalline body, hence, the
sintered body is different from the ceramic sintered body of the
invention.
INDUSTRIAL APPLICABILITY
[0145] The application of the ceramic sintered body is not
particularly limited, but is useful, for example, in the
semiconductor manufacturing field for substrates for ceramic
heater, probe card, wafer prober and the like, and/or in the filed
of substrate for inspection apparatus, substrate for integrated
circuits, substrate for liquid crystal display, and ceramic
filter.
* * * * *