CA1153710A

CA1153710A - Ceramic honeycomb filter

Info

Publication number: CA1153710A
Application number: CA000379296A
Authority: CA
Inventors: Noboru Higuchi; Teruo Yano; Masahiro Ohnishi
Original assignee: NGK Insulators Ltd
Current assignee: NGK Insulators Ltd
Priority date: 1980-08-28
Filing date: 1981-06-08
Publication date: 1983-09-13
Also published as: JPS5742316A; US4364760A; JPH0253083B2

Abstract

Abstract of the Disclosure The disclosed ceramic honeycomb filter comprises a ceramic honeycomb structural body having a multiplicity of parallel channels extending therethrough, selected channels being sealed at one ends thereof while the remainder of the channels being sealed at opposite ends thereof in such a manner that, as dust-containing gas flows therethrough from said one ends to said opposite ends, the gas passes through walls between adjacent channels where the dust particles are collected.
The ceramic honeycomb structural body has a coefficient of thermal expansion .alpha.A of 0 to 8.0x10-6/°C for 40°C to 800°C, and the sealing material of the channels at said ends thereof has a coefficient of thermal expansion .alpha.B
which is in the following range for 40°C to 800°C.

.alpha.A-.alpha.B ?3.5x10-6/°C, preferably .alpha.A-.alpha.B ?2.5x10-6/°C.

Description

1~L53~

This invention relates to a ceramic hone~comb filter Eor removing floating fine particles ~rom exhaust gas.
Various conventional filters have been used for removing floating particles from gas; for instance, a filter with filler such as adsorbing material or the like arranged therein, steel wool, ceramic fiber, and other porous materials. In the case of filters to remove fine particles, means for making the filtering meshes small are used in order to increase removing efficiency thereof~
but such means result in a ~isadvantage that the presswre loss therein becomes large when a high filtration rate is required. Moreover, the structure of the conventional filters is limited to comparatively simple ones, e.g., sheet-formed, plate-forrned, or cylindrical filter, and filters of corrugated struc-ture, double cylindrical form, or lamination type are used in order to make ~heir filter-ing areas large. However, it has been difficult to remarkably increase the filtering areas of the conventional filters.
On the other hand~ a honeycomb structural body has widely been used as a support of catalyst to clean automobile exhaust gas, a heat exchanger or the like.
The honeycomb structural body has noticea`ble advantages in that fluid pressure loss therethrough is low because a multiplicity of parallel channels of certain configuration are uniformly disposed therein, that -the surface area per unit volume thereof is large, and that partition walls between the adjacent channels can be rnade thin -to allow easy heat-up thereof. The actwal application of the

2 ~ ;

~537~

honeycomb struct~re, however, has been limited to heat exchangers and catalyst supports.
It has been proposed to make a ceramic honeycomb filter by sealing selected channels of a ceramic honeycomb structure at one ends thereof while sealing -the remainder of the channels at the opposite ends thereof in such a manner that dust-containing gas flows therethrough from said one ends to the opposite ends. Since every channel which is open at said one end is closed at the said opposite end, the gas has to pass through the partition walls. Namely, the partition walls which are porous serve as filtering means and filter the floating fine particles from the gas flowing therethrough.
Accordingly, sealing materials used in the proposed ceramic honeycomb filter should tightly adhere to the partition wall, so as to prevent the dust-containing gas from passing through -the ceramic ho~eycomb filter without being filtered at the aforementioned partition walls. In case of using the ceramic honeycomb filter at a high temperature, both the sealing material and the honeycomb structural body must have high heat resistances.
More particularly, it is necessary to select proper sealing material from ceramic materials which hardly form liquid phase even at a temperature for firing the ceramic honeycomb structural body. With the ceramic honeycomb filters which have been proposed heretofore, when the cera~ic honeycomb structural body with the sealing material applied thereto is fired and cooled, the partition walls tend to be cracked or the applied sealing material portions ~ tend to shrink excessively to produce gaps between the 37~

partition walls and the sealing material, which gaps tend to cause the sealing material to be separated from the honeycomb structural body. In short, the ceramic honeycolab filters of the prior art have shortcomings in -that the difference of shrinkages be-tween the partition walls and the sealing material during cooling tend to cawse cracks of the partition walls and separation of th~ sealing material from the ceramic honeycomb structural body Due -to such shortcomings, the use of -the ceramic honeycomb filter has been limited heretofore.
Therefore, an object of the present invention is to obviate the aforesaid shortcomings of the prior art. In studying the ceramic honeycomb filters having a multiplicity of parallel channels whose ends are selec-tively plugged by sealing material, the inventors noted the effects of the relationship of the coefficients of thermal expansion between -the ceramic honeycomb structural body and the sealing material on the bondage without apparent liquid phase, at the contact portions between the partition walls and the sealing material. Whereby, the inven-tors have found the following relationship in selecting proper sealing matieral for the ceramic honeycomb filters. More particularly, in the ceramic honeycomb filter according to the present invention, the ceramic honeycomb structural body has a coefficient of thermal expansion ~A of 0 to 8.0x10-6/C for 40C to 800C, and the sealing material of the channels at said ends thereof has a coefficient of thermal expansion ~B which is in a range of ~A-~B ~3-5X10-6/C for 40C to 800OC.
In a preferable embodilaent of the invention, ~LS3~

the aforesaid relationship of the coefficients of thermal expansion between the ceramic honeycomb structural body and the sealing material is in a range of ~A-~B ~2~5xlo-~/oc for 40C to 800C.
- Methods for producing ceramic honeycomb structural bodies include various processes, for example a paper dipping process wherein an organic porous sheet is impregnated with a ceramic slurry and a honeycomb s~ructural body is formed and the formed body is fired, a pipe binding process wherein a large number of given shaped pipes are bound, an extrusion process wherein a batch raw material is extruded through a die provided with a large number of given shaped slits and a press process.
If a method for producing a ceramic honeycomb struct~lral body is explained in more detail with respect to an extrusion process, to fine powders of a ceramic raw material, such as alumina, silica~ mullite, silicon carbide, silicon nitride, cordierite or the like are added an organic binder and a plasticizer, the mixture is kneaded to prepare a composition consisting mainly of ceramic raw material, this composition is extruded through a die provided with a large number of slits, which forms a given shape, for example a polygon such as a triangle, tetragon, hexagon and the like, circle, oval, etc. in the cross section of the channels to form a monolith structure, and the shaped structure is dried and fired to obtain a porous ceramic honeycomb structural body.
The ceramic honeycomb structural body thus produced has a thin wall thickness, so that the s~lrface area per -unit volume of the ceramic honeycomb structural ~5~ 0 body is remarkably large. Thus, the ceramic honeycomb structural body has a multiplicity of paralled channels extending therethrough with partition walls consisting of porous material. When certain channels of a ceramic honeycomb structural body are sealed at one ends thereof and the remaining channels thereof are sealed at the opposite ends, a ceramic honeycomb filter is formed, which filter uses the partition walls of the channels for filtering. Whereby, a very large effective filtering area per unit volume is achieved. As a result, it is made possible to make the ceramic filter small in size and to minimize the pressure loss in the filter.
For a better understanding of the inven-tion, reference is taken to the accompanying clrawings, in which:
Fig. 1 is a front view showing one embodiment of a ceramic honeycomb fil-ter according to the present invention;
Fig. 2 is a side view of Fig. 1, with a part thereof cut away;
Fig. 3 is an enlarged schematic view of adjacent channels in the filter according to the invention;
Fig. 4 is an enlarged cross-sectional view of a sealed portion of a channel, showing the conditions in which sealing material adheres to the wall of the channel;
Figs. S to 8 are schematic illustrations of possible defects which may be caused in the vicinity of the sealed portion of the channel; and Fig. 9 is a graph showing the result of tests made on different samples of the ceramic honeycomb filter of the present invention.

i3~

Throughout the different views of the drawin~s, the numeral 1 is a ceramic hone~comb strwctur~l body, the numeral 2 is a channel, the numeral 3 ls a partition wall between adjacent channels, the numeral 4 is a plug made of sealing material, the numeral S is a pore, the numeral 6 is a crack, and the nwmeral 7 is a gap.
Referring to Fig. l through Fig. 3, the ceramic honeycomb filter according to ~he present inven-tion uses a ceramic honeycomb structural body l having a multiplicity of parallel channels 2 extending therethro-ugh defined by hole partition walls 3 and uniEormly disposed therein.
A part of the channels, for instance, one half of the entire channels, are sealed at one ends thereof by plugs 4 made of sealing materials, while the remainder of the channels are sealed at the opposite ends thereof by similar plugs 4. In the example of Fig. 2 and Fig. 3, about one hal~ of the channels are sealed at the leEt-hand ends thereof by the plugs 4, and the remaining half of the channels are sealed at the right-hand ends thereof by the plugs 4. ~hen the left-hand end of -the ceramic honeycomb structural body is disposed on the inlet of dust-containing gas with the channels extending along the flowing direction of the gas, the gas enters into the channels having openings 2a at the left-hand ends thereof.
The right-hand ends of the channels with the aforementioned - left-hand end openings 2a are closed by the plugs ~, so that the gas passes -through the partition walls 3 between adjacent channels, so that the gas leaves the filter at the right-hand end openings 2b of the adjacent channels.
Thus, the partition walls 3 act as filtering members to ~5~7~C~

collect floating dust particles from the gas flowing therethrough.
Accordingly, the sealing materials formi-ng the plug 4 must tightly adhere to the partition wa:Lls 3, so as to prevent the dust-containing gas from leaving the ceramic honeycomb structwral body 1 without passing through the partition walls 3 acting as the fil-tering members. When the ceramic honeycomb ~ilter is used at a high temperature, both the honeycomb structural body and the sealing material forming the plugs should have high heat resistances. To this end, the sealing material wi-th a high heat resistance showld pre~erably be selected from suitable ceramic materials which hardly melt a~ such a high temperature as that for firing the ceramic honeycomb structural body.
The sealing material ha~ing the aforementioned high heat resistance adheres to the wall of the channels not mainly by melting reactions bu-t by a combined mechanism of physical fitting of the sealing materal into the channel 2 and the physical fitting of the sealing material into pores 5 on the hole partition wall 3, as shown in Fig. 4.
When the ceramic honeycomb structural body 1 with plugs ~
are fired and then cooled, the ceramic honeycomb structural body of the prior art e~perienced cracks 6 generated on the partition walls 3, as shown in Figs. 5 and 6, or gaps 7 generated between the partition walls 3 and -the plugs ~
made of the sealing material, as shown in Figs. 7 and ~.
The cracks 6 and the gaps 7 are due to unmatch of shrinkage between the partition walls and the plugs. The cracks 6 and the gaps 7 cause the plugs ~ to be separated from the llS37~0 ceramic honeycomb sLructural body l, which separation deteriorates the filtering effects to a great extent.
The present invention eliminates the risk of the aforementioned cracks 6 and the gaps 7 in the ceramic honeycomb filter, by selecting proper coefficients of thermal expansion for both the ceramic honeycomb structural body 1 and the sealin~ material for -the plugs 4.
The inventors have carried out a series of tests by using systematic combinations of ceramic honeycomb structural bodies consisting of vario~ls materials having gradually varying coefficients of thermal expansion in a range of 0 to 8.0x10-6/C for 40C to 800C and sealing materials consisting of ceramic material having gradually varying coefficients of thermal expansion similar to those of the aforementioned ceramic honeycomb structural bodies. Each of the afore~entioned combinations was formed into a ceramic honeycomb filter through a process to be described later in Example, and the conditions oE
the joint portion 'between the sealing material and the honeycom'b structural body after firing was observed.
f'able 2 and Fig. 9 show the result of the observation.
As can be seen from the table and the figure~ when coefficien-t o~' thermal expansion of the sealing material was smaller than that of the ceramic honeycomb structural body and the difference therebetween exceeded 3.5xla-6/
the partition walls frequently produced crac~s.
On the contrary, when the coefficient of thermal expansion of the sealing material was larger than that of the ceramic honeycomb structural body and the difference therebetween exceeded 3.5xlo-6/oc9 gaps were produced 115;37~

between the partition walls and the sealing material, and sometimes plugs formed of the sealing material fell out from the honeycomb structural bodies. Moreover, to check the performance of the thus obtained ceramic honeycomb filters, the sound ceramic honeycomb filters after the aforementi~ned observation where heat treated at 1,200C
for 6 hours. As a result, it was found that satisfactory combinations which were free from said deects of cracks and gaps had differences of coefficients of thermal e~pansion between the ceramic honeycomb structural body and the sealing material not greater than 3.5xlo~6/
preferably not greater than 2.5x10~6/C, in absolute values.
Once the a-foresaid rela-tionship of the coefficients of thermal expansion between the honeycomb structural body and the sealing rnaterial is given, even if the firing temperature of the ceramic honeycomb structwral body is varied and even if the heat-treating temperature of the ceramic honeycomb filter after sealing ~0 the sealing material is varied, as long as the coefficients of thermal expansion of the ceramic materials and the firing temperat~res thereof are known beforehand, it is possible to determine the propriety of a combination of these materials without actually producing a ceramic honeycomb fi~ter from such materials. ~urthermore, when the material of the sealing material is different from that of the ceramic honeycomb structural body for the reason of the workability of sealing operation, limitation on the porosity of the sealing material or the like, the aforementioned range of difference of the coe~ficients of ~5~7~

thermal expansion will prvvide an index for determining the propriety of the sealing material.
The invention wil] now be described in further detail by referring to Example. The materials used in the Example were selected so as to provide the gradual variation of the coefficients of thermal expansion, and the present invention is not limited to the materials used in the Example.
Example Mixture of starting materials were prepared by using nine ceramic materials having different coefficients of thermal expansion as shown in Table 1. A monolithic honeycomb body was made from each of the mixtwres by extrusion, and the extruded honeycomb body was dried and fired at 1,400C for 6 hours to produce a ceramic honeycomb structural body for forming a ceramic honeycomb filter.
The ceramic honeycomb structural body after firing was 120 mm in diameter and 150 mm in length, and the parti-tion wall between adjacent channels thereof was 0,30 mm in thickness. Each ceramic honeycomb body ~hus produced had about 200 channels per square inch of its cross section.

~S37~

Table l Coefficient of Ceramic material thermal expansion xl0~6 (1/C) (40-800C) Zirconia spinel 8.0 (ZrO2-MgO AQ2O3) Clay bond silicon carbide 7.3 Mullite-zircon 5 2 (3A~203 2sio2 -Zr2 SiO2 ) Mullite (3AQ203 2si2 ) 4.7 Zircon (ZrO2 SiO2) 3.5 Cordierite-mullite (2MgO- 2AQ203 SSiO2 -3AQ203 2SiO2) 2.0 Non-oriented cordierite 1 6 (2MgO-2A~2 03 ~ 5si2 ) _ ¦
Oriented cordierite (~MgO-2A~203 5SiO2) 0.7 Mullite aluminum ti-tanate (3AQ203 2sio2 -A~203 Ti2 ) U . O

In Table 1, the non-orientated cordierite was prepared by extending and firing the powder o fired cordierite ceramics once fired, while the oriented cordierite was prepared by extruding and firing the powder of raw materials to produce cordierite upon firing.
The difference of the coefficients of thermal expansion between the oriented and non-oriented cordierites was due to the presence or absence of orientation of the cordierite crystals in the ceramic honeycomb structural body.
Separately, eight kinds o ceramic materials shown in Table 1~ excluding the orien~ed cordierite, were dried and ground so as to ob-tain powder with a grain si~e ~537~l0 of less than 1~9 ~m ~100 mesh). Paste-like sealing materials were ~ormed by adding polyvinyl alcohol and water as a bonding agent to the ceramic material powder thus obtained.
On the other hand, a sheet of paper was adhered to one end surface of -the ceramic honeycomb structural body where the channels thereof opened, and a plurality of holes were bored on the paper by a needle at the positions of selected channels to be sealed, so as to form a checker-flag pattern on the paper in the case of the illustrated embodiment. The aforesaid paste-like sealing material was charged into the holes at the bored portions of the paper and dried. Those channels which had not been closed by the paste-like sealing material were closéd at the opposite end surface of the ceramic honeycomb structural body The sealed structural body was then fired at 1,400C. The paper was burnt up during the firing and a desired ceramic hone~comb filter was - obtained. To check all the combinations of the ceramic honeycomb structural bodies and the sealing materials of the Example, 72 different samples of the ceramic honeycomb filter were formed.
As to the samples taken out of furnace~ after the firing, the conditions of the sealed portions of the channels were chekced by observation. Sound samples, which had no cracks in the partition wall~ and no gaps between the partition walls and the plugs formed of the sealing material, were further heat treated at 1,200C
for 6 hours, and the conditions of the sealed portions were again checked by observation.

1~5~7~

Table 2 and Fig. 9 show the reswlt of s-uch observations. In Fig. 9, the cross mark x shows presence of defects after the firing for sealing, the triangular mark ~ shows presence of defects a:Eter the heat treatmen-t at 1,200C despite being sound after the firi.ng for sealing, and the circular mark o shows absence of any defects after both the firing at 1,400C and heating at 1,200C.
It was concluded from the result of the observa-tions that, when the coefficient of thermal expansion of the ceramic honeycomb structural body at 40C to 800C
was in the range of 0 to 8.0x10~6/C and the difference of the coefficien-ts of thermal expansion between the ceramic honeycomb structural body and the sealing material for 40C to 800C was in the range of 3.5Xl0-6/Oc in absolute value, more preferably 2.5x10-6/C in absolute value, then sound ceramic honeycomb filters were obtained.

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As apparent from the above Example, in the ceramic honeycomb fil~er according to the invention the sealing material tightly seals the channels and the sealing materials con-tains no ingredients subject to melting reaction, so that the ceramic honeycomb structural body is free from denaturing and d~terioration of its heat resistance. As a result, a ceramic honeycomb filter having a high removing efficiency and a high heat resistance is provided, which filter is very effective in removing fine soot particles from high-temperature exhaust gas of Diesel engines and o-ther internal combustion engines.
In this case, carbon dust and other fine particles collected in the filtering portion can be burnt into carbon monoxide (CO), hydrocarbons (HCn) or nitrogen oxides (NOx) which can be purified by a separate three way catalyst means, so that the ceramic honeycomb filter according to the invention does not require any cleaning and is continuously usable for a long period of time.
Thus, the present invention contributes greatly to development of the industry.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are de-fined follows:-

1. A ceramic honeycomb filter comprising a ceramic honeycomb structural body with a multiplicity of parallel channels extending therethrough said channels being selectively sealed by sealant, said ceramic honeycomb structural body having a coefficient of thermal expansion .alpha.A of 0 to 8.0x10-6/°C for 40°C to 800°C, said sealing material having a coefficient of thermal expansion .alpha.B for 40°C to 800°C in a range satisfying a relationship of .alpha.A-.alpha.B ?3.5x10-6/°C.

2. A ceramic honeycomb filter as set forth in claim 1, wherein the coefficients of thermal expansion .alpha.A
and .alpha.B satisfy a relationship of .alpha.A-.alpha.B ?2.5x10-6/°C.

3. A ceramic honeycomb filter as set forth in one of claims 1 and 2, wherein said ceramic honeycomb structural body is made of cordierite ceramic material, and said sealing material consists of at least one ceramic material.
selected from the group consisting of cordierite ceramic material and mullite ceramic material.

4. A ceramic honeycomb filter as set forth in one of claims 1 and 2, wherein said ceramic honeycomb structural body is made of mullite ceramic material, and said sealing material consists of at least one ceramic material selected from the group consisting of cordierite ceramic material and mullite ceramic material.