Patent Application
20100257829
1. Field of the Invention
The present invention relates to a honeycomb structure composed of ceramics. The present invention also relates to a honeycomb structure that captures particulates contained in a fluid such as an exhaust gas generated by internal combustion engines or boilers, and to a purifying apparatus using the same.
2. Description of the Related Art
For the purpose of capturing particulates containing carbon as a main component, that are contained in an exhaust gas generated by internal combustion engines (particularly particulates in an exhaust gas of diesel engines (diesel particulates)), a honeycomb structure composed of ceramics has hitherto been used.
This honeycomb structure has plural flow paths which are formed by being separated with a lattice-shaped partition wail composed of porous ceramics. Also, each flow path is alternately plugged in either one end or the other end of the honeycomb structure. Therefore, when the exhaust gas is introduced through an inlet of a filter and then discharged through an outlet, particulates in the exhaust gas is captured by the partition wall.
There are now required a honeycomb structure that is excellent in heat resistance and thermal shock resistance and is less likely to undergo thermal decomposition, and also exhibits stable mechanical properties even when subjected to a heat treatment, and a purifying apparatus using the same.
Patent Document 1: International Publication No. WO 2005/005019 pamphlet
The honeycomb structure according to one aspect of the present invention is made from a ceramic body including a crystal of MgTi2O5—Al2TiO5.
Also, the honeycomb structure according to another aspect of the present invention has plural flow paths therein which are formed by being separated with a partition wall composed of ceramics, the flow paths extending from one end to the other end of the honeycomb structure and being alternately plugged in the one end and the other end by plugged portions. The partition wall includes a crystal of MgTi2O5—Al2TiO5.
Also, the honeycomb structure according to yet another aspect of the present invention has plural flow paths therein which are formed by being separated with a partition wall composed of ceramics, the flow paths extending from one end to the other end of the honeycomb structure and being alternately plugged in the one end and the other end by plugged portions, the partition wall extending in a monoaxial direction. The partition wall includes a crystal of MgTi2O5—Al2TiO5 and, a change ratio CR between before and after a heat treatment of the partition wall at a temperature of 1,200° C. for 2 hours is 20 or less, the change ratio CR represented by an equation (1) shown below:
C
R=(|Ca−Cb/Ca)×100 (1)
where
Ca: a compression failure strength (MPa) in the monoaxial direction before the heat treatment; and
Cb: a compression failure strength (MPa) in the monoaxial direction after the heat treatment.
Also, the honeycomb structure according to further aspect of the present invention has plural flow paths therein which are formed by being separated with a partition wall composed of ceramics, the flow paths extending from one end to the other end of the honeycomb structure and being alternately plugged in the one end and the other end by plugged portions. The partition wall includes a crystal of MgTi2O5—Al2TiO5 and a proportion PR represented by an equation (2) shown below is 84 or less, where a porosity of the plugged portion is Ps and a porosity of the partition wall is Pw.
P
R=(|Pw−Ps/Pw)×100 (2)
The purifying apparatus according to one aspect of the present invention includes the honeycomb structure described above, and a casing that accommodates the honeycomb structure and has an inlet port and an outlet port, wherein a fluid introduced through the inlet port of the casing is passed through the honeycomb structure and then discharged through the outlet port of the casing.
The honeycomb structure and the purifying apparatus are excellent in heat resistance and thermal shock resistance. Also, magnesium titanate (MgTi2O5) suppresses decomposition of aluminum titanate (Al2TiO5) at a high temperature. Furthermore, since MgTi2O5—Al2TiO5 is stoichiometric, a crystal is less likely to undergo mechanical strain as compared with a non-stoichiometric crystal. Therefore, even when subjected to a heat treatment, less change in mechanical properties arises before and after the heat treatment.
As shown in
When one end (
As shown in
The partition wall 4 includes a crystal of stoichiometric magnesium aluminum titanate (MgTi2O5—Al2TiO5). Herein, magnesium aluminum titanate is a solid solution and has a main peak at 28 of 25.5° to 26.5° in an X-ray diffraction chart.
Since the partition wall 4 is composed of a solid solution of stoichiometric magnesium aluminum titanate, magnesium titanate (MgTi2O5) suppresses decomposition of aluminum titanate (Al2TiO5) at a high temperature. Since both magnesium titanate and aluminum titanate are stoichiometric, mechanical strain generated in a crystal is suppressed as compared with a non-stoichiometric crystal. Therefore, even before and after the heat treatment, mechanical properties of the partition wall 4 are stable and the mechanical strength does not drastically decrease after the heat treatment.
The partition wall 4 may contain, as a minor component, oxides such as titanium oxide (TiO2), potassium oxide (K2O), sodium oxide (Na2O), magnesium oxide (MgO) and aluminum oxide (Al2O3). Of these minor components, the content of potassium oxide (K2O) is preferably 0.2% by mass or less and that of sodium oxide (Na2O) is preferably 0.9% by mass or less so as to obtain a preferred porous honeycomb structure. Each content of potassium oxide (K2O) and sodium oxide (Na2O) may be determined by X-ray fluorescence spectrometry or inductively coupled plasma (ICP) emission spectrometry. Specifically, with respect to potassium oxide (K2O) and sodium oxide (Na2O), each content of a metal element K or Na may be measured and expressed in terms of the oxide.
The composition of the crystal of the stoichiometric magnesium aluminum titanate (MgTi2O5—Al2TiO5) crystal and that of the minor component may be identified by an X-ray diffraction method. Also, the proportion of each component in the partition wall 4 may be determined by X-ray fluorescence spectrometry or inductively coupled plasma (ICP) emission, spectrometry.
It is particularly preferred that the content of aluminum titanate (Al2TiO5) is from 60 to 70% by mass, and that of magnesium titanate (MgTi2O5) is from 16 to 26% by mass, balance being iron oxide (Fe2O3), based on 100% by mass of the components that constitute the partition wall 4.
Usually, aluminum titanate (Al2TiO5) is thermally decomposed at a temperature within a range from 850 to 1,280° C. Therefore, when the partition wall 4 is composed of ceramics containing aluminum titanate (Al2TiO5), the partition wall requires thermal shock resistance enough to prevent breakage even when thermal decomposition of the compound arises. Since the honeycomb structure 1 of the present embodiment contains magnesium titanate, thermal decomposition is suppressed, thus making it possible to attain a change ratio CR of the compression failure strength in which the value of an equation (1) shown below is low. In other words, the change ratio CR between before and after a heat treatment of the partition wall at a temperature of 1,200° C. for 2 hours is 20 or less, the change ratio CR represented by the equation (1) shown below:
C
R=(|Ca−Cb|/Ca)×100 (1)
where
Ca: a compression failure strength (MPa) in the monoaxial direction (the direction A in
Cb: a compression failure strength (MPa) in the monoaxial direction after the heat treatment.
The compression failure strengths Ca/Cb are measured, for example, in accordance to JASO M 505-87, and the change ratio CR may be determined by the equation (1). As the measuring sample, a cubic sample measuring 10 mm in each side obtained by hollowing from each honeycomb structure 1 may be used.
When the change ratio CR is 20 or less, decomposition of aluminum titanate (Al2TiO5) at a high temperature may be reduced, thus making it possible to maintain thermal shock resistance at a high level even when repeatedly used. Therefore, even when regeneration (honeycomb structure is converted into a regeneratable state by removing particulates captured by the partition wall 4 through incineration or back washing) of the honeycomb structure 1 is repeated, the honeycomb structure is not easily broken.
In the honeycomb structure 1 of the present embodiment, a proportion PR as a porosity defined by an equation (2) shown below is 84 or less, where a porosity of the plugged portion 3 is Ps and a porosity of the partition wall 4 is Pw. Therefore, in the heat treatment that is performed for regeneration, the generation of cracks and melt loss in the boundaries between the partition wall 4 and the plugged portion 30 may be reduced.
P
R=(|Pw−Ps|/Pw)×100 (2)
When the proportion PR of the porosity represented by the equation (2) is 84 or less, a difference in shrinkage percentage between the partition wall 4 and the plugged portion 3 in the sintering step decreases. As a result, since stress concentration in the boundaries between the partition wall 4 and the plugged portion 3 is reduced, the thermal shock resistance of the honeycomb structure 1 is improved. Thus, the generation of cracks and melt loss in the boundaries between the partition wall 4 and the plugged portion 3 in the regeneration may be reduced. By adjusting the shrinkage percentage of the plugged portion 3 to be equivalent to that of the partition wall 4 in the firing step in advance, the generation of cracks in the boundaries between the partition wall 4 and the plugged portion 3 after firing is suppressed even when the plugged portion 3 and the partition wall 4 are integrally fired.
It is particularly preferred that the porosity (Pw) of the partition wall 4 is preferably adjusted to 30% or more and 60% or less, and the porosity (Ps) of the plugged portion 3 is preferably adjusted to 9.6% or more and 60% or less. These porosities (Pw), (Ps) may be measured using a mercury injection method.
When an input end face of the partition wall 4 is observed from planar view, the shape of the open portion of larger flow paths 2 may be, for example, an octagon shape having an area larger than that of the plugged portion as shown in
Also, when the input end face of the partition wall 4 is observed from planar view, the shape of the open portion of larger flow paths 2 may be, for example, a tetragon shape with a corner portion having an arc shape, having an area larger than that of the plugged portion as shown in
With such a configuration, as shown in
It is particularly preferred that the hydraulic diameter of larger flow paths 2 in a tetragon shape with a corner portion having an arc shape is 1.55 times or more and 1.95 times or less that of smaller flow paths 2 having an area smaller than that of the above larger flow paths in
The method for producing a honeycomb structure 1 will be described below.
First, a mixed raw material is prepared. In order to obtain a honeycomb structure in which a partition wall 4 includes a crystal of stoichiometric magnesium aluminum titanate (MgTi2O5—Al2TiO5), it is necessary to control each mean particle size of titanium oxide (TiO2), aluminum oxide (Al2O3) and magnesium oxide (MgO) as raw materials.
Titanium oxide (TiO2) having a mean particle size of 1 to 10 μm and aluminum oxide (Al2O3) having a mean particle size of 1 to 10 μm are prepared. Then, 100 parts by mass of a component composed of titanium oxide (TiO2) and aluminum oxide (Al2O3) at a molar ratio of titanium oxide (TiO2) and aluminum oxide (Al2O3)(TiO2:Al2O3=40 to 60:60 to 40) is mixed with 1 to 10 parts by mass of magnesium oxide (MgO) having a mean particle size of 1 to 10 μm and 1 to 10 parts by mass of silicon dioxide (SiO2) to obtain a mixed raw material.
Herein, either an oxide having a spinel structure containing Mg, or an Mg-containing compound that is converted into MgO by firing may be used in place of magnesium oxide (MgO).
The change ratio (CO of the compression failure strength in the honeycomb structure is influenced by the content of silicon dioxide (SiO2). In other words, the higher the content of silicon dioxide (SiO2), the change ratio (CR) becomes lower, whereas, the lower the content of silicon dioxide (SiO2), the change ratio (CR) becomes higher. From such a point of view, the content of silicon dioxide (SiO2) in the mixed raw material is preferably adjusted within a range from 3 to 10 parts by mass so as to obtain a honeycomb structure in which the change ratio (CR) is 20 or less.
To the mixed raw material obtained as described above, a predetermined amount of a pore-forming agent such as graphite, starch or a resin powder is added and, furthermore, a plasticizer, a thickener, a lubricant and water are added, followed by mixing using a universal mixer, a rotary mill or a type V mixer to obtain a mixture. This mixture is kneaded using a three-roll mill or a kneader to obtain a plasticized kneaded mixture.
Using a die, the resultant plasticized kneaded mixture is molded by an extrusion molding machine. The die to be used is a die that has an inner diameter that determine an outer diameter of a green compact is, for example, from 100 to 250 mm, and also has a slit for forming a partition wall 4 of a honeycomb structure 1. The kneaded mixture is charged in the extrusion molding machine mounted with this die, and then formed under pressure into a green compact having a honeycomb shape. Then, the green compact is dried and cut to a predetermined length.
The flow paths 2 of the green compact are alternately plugged in the one end or the other end. Some of plural flow paths 2 are selectively subjected to masking. At this time, the flow paths 2 to be masked are selected so that the flow paths 2 to be plugged are disposed in a checkered pattern. The masked output end face (the symbol OF in
The green compact is then fired. The green compact is fired by maintaining at a temperature within a range from 1,250° C. to 1,700° C. for 0.5 hour to 5 hours using a firing furnace such as an electric furnace or a gas furnace.
Regarding the honeycomb structure 1 thus obtained, since magnesium titanate (MgTi2O5) and aluminum titanate (Al2TiO5) are stoichiometric, a crystal is less likely to undergo mechanical strain and it is possible to reduce a change in mechanical properties of the partition wall 4 before and after the heat treatment.
The honeycomb structure 1 thus obtained may efficiently capture particulates in a fluid over a long period of time.
The case of using the honeycomb structure 1 of the present embodiment as a filter that captures diesel particulates in the exhaust gas of diesel engines (not shown) will be described below. As shown in
When a diesel engine (not shown) runs and the exhaust gas (EG) is introduced into the casing 7 through the exhaust pipe 9, the exhaust gas (EG) is introduced into the flow paths 2 that are not plugged by the plugged portion 3a from the input end face (IF) of the honeycomb structure 1. In the flow paths 2 into which the exhaust gas (EG) has been introduced, the exhaust gas (EG) is prevented from flowing out since the end portion of the output end face (OF) is plugged by the plugged portion 3b. The exhaust gas (EG) that is prevented from flowing out passes through the porous partition wall 4 and is discharged through the adjacent flow paths 2 in which the output end face (OF) is not plugged by the plugged portion 3b. In the partition wall 4, diesel particulates contained in the exhaust gas (EG) are captured in pores therein. In other words, purified air is introduced into the adjacent flow paths 2. Since the end portion of the input end face (IF) side of the adjacent flow paths 2 is plugged, purified gas is not mixed with the exhaust gas (EG). In such a manner, the exhaust gas (EG) that has been introduced into the honeycomb structure 1 of the purifying apparatus 10 is purified into a state free from the diesel particulates and is discharged through the output end face (OF) to the outside.
In such a purifying apparatus 10, the honeycomb structure 1 of the present embodiment may be preferably used as a filter, and thus diesel particulates may be efficiently captured over a long period of time.
While an example using the exhaust gas as a fluid was described in the present embodiment, a liquid may also be used as the fluid. For example, tap water or sewerage may be used as the fluid, and also the purifying apparatus of the present embodiment may be applied for filtration of the liquid.
First, 100 parts by mass of a component composed of titanium oxide X, titanium oxide Y and aluminum oxide at a molar ratio of 20:20:60 shown in Table 1 was mixed with 5 parts by mass of magnesium oxide (MgO) and 5 parts by mass of silicon dioxide (SiO2) to obtain a mixed raw material.
To the mixed raw material thus obtained, a predetermined amount of graphite as a pore-forming agent was added. Furthermore, a plasticizer, a thickener, a lubricant and water were added, followed by mixing using a rotary mill to obtain a slurry. The slurry obtained by the rotary mill was kneaded using a kneader to obtain a plasticized kneaded mixture.
Next, the resultant plasticized kneaded mixture was charged in an extrusion molding machine mounted with a die that has an inner diameter that determine an outer diameter of a green compact is 250 mm, and also has a slit for forming a partition wall 4 of a honeycomb structure 1. The kneaded mixture was then formed under pressure into a green compact having a honeycomb shape. Then, the green compact was dried and cut to a predetermined length.
Some of flow paths 2 were subjected to masking so that the flow paths are disposed in a checkered pattern. The output end face (OF) was dipped in the slurry. Pins with a tip having a flat shape coated with a water-repelling resin were inserted into flow paths 2 from the input end face (IF), followed by drying at a normal temperature. Thus, plugged portions 3b were formed on the outlet side of the green compact. The pins were removed and the same operation as described above was carried out on the input side (IF) to form the plugged portions 3 on the input side of the green compact. In both plugged portions 3a, 3b, the same mixed raw material as that used to form the partition wall 4 was used.
The green compact was then fired at a temperature of 1,500° C. for 4 hours using an electric furnace to obtain a honeycomb structure.
The composition of ceramics that constitute the partition wall 4 of the resultant honeycomb structure was identified using an X-ray diffraction method. The composition formulas are shown in Table 1.
As the measuring sample, twenty cubic samples (excluding the plugged portions 3a, 3b) each measuring 10 mm in each side were made by hollowing from each honeycomb structure 1. The compression failure strength in a monoaxial direction A of each sample was measured, and then the mean value and standard deviation of the measured values were calculated. The compression failure strength in the monoaxial direction A was measured in accordance with JASO M 505-87. The results are shown in Table 1.
As is apparent from Table 1, all samples Nos. 2 to 4, in which at least one of magnesium titanate and aluminum titanate that constitutes the partition wall 4 is non-stoichiometric, exhibited the mean value of the compression failure strength of 3.82 MPa or less, and standard deviation of 0.6 MPa or more.
In contrast, sample No. 1, in which magnesium titanate and aluminum titanate are solid-soluted in a stoichiometric ratio, exhibited the mean value of the compression failure strength of 4.32 MPa, that was higher than those of samples Nos. 2 to 4. Furthermore, standard deviation of sample No. 1 was 0.4 MPa and was smaller than those of samples Nos. 2 to 4. As is apparent from these results, the honeycomb structure containing stoichiometric magnesium titanate and aluminum titanate, like sample No. 1, is a honeycomb structure that is excellent in mechanical properties of the partition wall 4 and exhibits less variation in mechanical properties.
Next, 100 parts by mass of a component composed of titanium oxide X, titanium oxide Y and aluminum oxide at a molar ratio of 20:20:60 (each having the same mean particle size of 5 μm) shown in Table 2 was mixed with 5 parts by mass of magnesium oxide (MgO) having a mean particle size of 5 μm and 5 parts by mass of silicon dioxide (SiO2) to obtain a mixed raw material.
Then, a honeycomb structure was produced in the same manner as in Example 1.
The composition of ceramics that constitute the plugged portion 3 of the resultant honeycomb structure was identified using an X-ray diffraction method. The composition formulas are shown in Table 2.
In order to observe the state of bonding between the plugged portion 3 and the partition wall 4, light was irradiated from the direction of an axis A using a fiber scope. Light does not permeate through the end face when there is no gap between the plugged portion 3 and the partition wail 4, while light permeates through the end face when there a gap between the plugged portion 3 and the partition wall 4. The honeycomb structure in which light permeation through the end face was partially confirmed was rated “confirmed”, whereas, the honeycomb structure in which light permeation through the end face was not confirmed was rated “unconfirmed”. The results are shown in Table 2.
As is apparent from Table 2, in samples Nos. 6 to 8 in which both magnesium titanate and aluminum titanate that constitute the plugged portion 3 are non-stoichiometric, a gap was confirmed. In contrast, in sample No. 5 in which both magnesium titanate and aluminum titanate that constitute the plugged portion 3 are stoichiometric, a gap was not confirmed. As is apparent from these results, the honeycomb structure 1 containing stoichiometric magnesium titanate and aluminum titanate, like sample No. 5, is a honeycomb structure having high capturing efficiency.
First, 100 parts by mass of a component composed of titanium oxide and aluminum oxide at a molar ratio TiO2:Al2O3 of 40:60 was mixed with 5 parts by mass of magnesium oxide (MgO) and silicon dioxide (SiO2) (the content is shown in Table 3) to obtain a mixed raw material.
A honeycomb structure was produced from the resultant mixed raw material in the same manner as in Example 1. The compression failure strength in the direction of an axis A of a cubic sample measuring 10 mm in each side obtained by hollowing from each honeycomb structure was measured. This measured value is shown in Table 0.3 as Ca.
Also, the honeycomb structure was subjected to a heat treatment at a temperature of 1,200° C. for 2 hours. The compression failure strength in the direction of an axis A of a cubic sample measuring 10 mm in each side obtained by hollowing from each honeycomb structure 1 was measured. This measured value is shown in Table 3 as Cb. The change ratio (CR) of the compression failure strength was calculated from the compression failure strengths Ca, Cb in the monoaxial direction (the direction an axis A).
Another honeycomb structure obtained in the same manner was placed in an electric furnace and then allowed to stand at a constant temperature for 1 hour. Then, the honeycomb structure was taken out in the atmosphere at room temperature (24° C.). A difference in temperature between the constant temperature at which cracks were confirmed in the honeycomb structure and room temperature (24° C.) is shown in Table 3 as a thermal shock-resistant temperature.
As is apparent from Table 3, as compared with sample No. 12 (number of regeneratable times: 738 times) in which the change ratio (CR) of the compression failure strength exceeds 20, samples Nos. 9 to 11, in which the change ratio (CR) of the compression failure strength is 20 or less, exhibited higher thermal shock-resistant temperature and also exhibited large number of regeneratable times of 755 times or more. As is apparent from these results, the honeycomb structure 1 in which the change ratio (CR) of the compression failure strength is 20 or less, like sample No. 12, is a honeycomb structure that is excellent in thermal shock resistance.
TiO2 having a mean particle size of 5 μm and Al2O3 having a mean particle size of 5 μm were prepared. 100 parts by mass of a component composed of titanium oxide and aluminum oxide at a molar ratio TiO2:Al2O3 of 40:60 was mixed with 5 parts by mass of magnesium oxide (MgO) having a mean particle size of 5 and 5 parts by mass of silicon dioxide (SiO2) to obtain a mixed raw material.
Then, a honeycomb structure was produced in the same manner as in Example 1. The porosity of the plugged portions 3a, 3b was adjusted by the volume ratio of graphite as a pore-forming agent. The volume ratio of graphite is shown in Table 4.
The resultant honeycomb structure was placed in an electric furnace and then allowed to stand at a constant temperature for 1 hour. Then, the honeycomb structure was taken out in the atmosphere at room temperature (24° C.) A difference in temperature between the constant temperature at which cracks were confirmed in the honeycomb structure and room temperature (24° C.) is shown in Table 4 as a thermal shock-resistant temperature.
Also, the porosities (Ps), (Pw) of the plugged portion 3 and the partition wall 4 were measured by a mercury injection method, and the measured values and the proportion (PR) calculated by the measured values are shown in Table 4.
As is apparent from Table 4, as compared with sample No. 13 (thermal shock-resistant temperature: 720° C.) in which the proportion (PR) exceeds 84, samples Nos. 14 to 17, in which the proportion (PR) is 84 or less, exhibited higher thermal shock-resistant temperature of 760° C. or higher. As is apparent from these results, the honeycomb structural bodies 1 having a porosity PR of 84 or less, like samples Nos. 14 to 17, are honeycomb structural bodies that are excellent in thermal shock resistance.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007-253767 | Sep 2007 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2008/067481 | 9/26/2008 | WO | 00 | 6/8/2010 |
