JP3185960B2 - Method for producing porous aluminum titanate sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a porous aluminum titanate sintered body. The porous aluminum titanate sintered body of the present invention can be used for diesel particulate filters and the like.

[0002]

2. Description of the Related Art The exhaust gas of a diesel engine contains soot-like fine particles called diesel particulates, which is one of the causes of air pollution. Therefore, a filter for removing diesel particulates (hereinafter referred to as D
PF) is being developed.

The function of the DPF is to not only capture diesel particulates, but also to burn and remove the captured diesel particulates to withstand repeated use. Therefore, the DPF is generally made into a honeycomb shape made of porous ceramics, and captures the diesel particulates in the pores. When the pressure loss increases due to clogging, the diesel particulates captured by heating with a heater or a burner are used. It is configured so that it can be regenerated by burning off.

The combustion start temperature of diesel particulates is about 600 ° C., but when the amount of accumulation is large, the combustion temperature during regeneration is as high as 1100 ° C. to 1300 ° C.
Therefore, a thermal stress is generated in the DPF during regeneration due to a steep temperature gradient, which may cause cracks or cracks. For this reason, DPF requires high heat resistance of 1300 ° C. or higher and high thermal shock resistance. Examples of such a material for DPF include cordierite and silicon carbide.

In order to use these ceramics as DPF, it is necessary to form a porous body having pores capable of capturing diesel particulates. As a method of manufacturing a porous ceramic sintered body, for example, Japanese Patent Application Laid-Open
Japanese Patent No. 208870 discloses a method of molding and mixing a mixed powder of a granular material made of graphite or carbon and a ceramic powder such as alumina.
A production method is described in which a powder and a granule are burned by firing to obtain a porous ceramic sintered body. Japanese Patent Laid-Open No. Hei 2
No. 69367 describes a method for producing a porous ceramic sintered body by forming a compact from an organic powder such as plastic beads and a ceramic powder such as silicon carbide, calcining and sintering it. ing.

[0006]

When a DPF as a porous sintered body is formed from each ceramic material and tested, cordierite has excellent thermal shock resistance because of its low thermal expansion. Cracks may occur or melt away at 1100-1300 ° C.
It became clear that there was a problem in reproducibility.

Although silicon carbide is excellent in heat resistance and strength, its thermal expansion coefficient is as high as 10 to 20 times that of cordierite, and it may be broken by thermal stress. Therefore, it is considered that the DPF is divided to reduce the thermal stress.
In this case, there is a problem in the sealing property. Further, since silicon carbide has a high thermal conductivity and heat dissipation by a heater heating becomes large and is insufficient, it is necessary to use a burner heating which requires a large amount of regeneration energy.

Therefore, in the present invention, aluminum titanate is used as a new material. The aluminum titanate sintered body is a material having a small coefficient of thermal expansion like cordierite. However, the reason why the coefficient of thermal expansion is small has not been clarified so far, and a technique for forming a porous material has not been established. The present invention has been made in view of such circumstances, and has elucidated the reason for the low thermal expansion of the aluminum titanate sintered body, and has disclosed a porous aluminum titanate sintered body capable of maximizing its effect. The purpose is to do.

[0009]

According to the study of the present inventors, it has been clarified that the low thermal expansion of aluminum titanate has a great relationship with the microcracks present in the grain boundaries of the sintered body. Was. That is, the crystal of aluminum titanate has a large thermal anisotropy, and the a-axis and the b-axis have a positive coefficient of thermal expansion, while the c-axis has a negative coefficient of thermal expansion. Therefore, at the time of cooling from high temperature to room temperature during firing, microcracks are introduced into grains or grain boundaries due to thermal anisotropy of crystal axes.

[0010] Here, when the aluminum titanate sintered body is heated, even if the crystal particles thermally expand, the expansion is absorbed by the closure of the microcracks. Therefore, the sintered body has a small apparent thermal expansion coefficient. Generally, the thermal shock resistance (R) is expressed by the following equation. The thermal shock resistance (R) increases as the coefficient of thermal expansion (α) decreases. R = σ (1-ν) / Eα (σ: strength, ν: Poisson's ratio, E: Young's modulus) That is, by adjusting the degree of porosity and optimally controlling the degree of generation of microcracks, thermal expansion An aluminum titanate sintered body having an extremely small coefficient can be obtained, and a highly durable DPF free from cracking even when abruptly increasing in temperature during regeneration can be manufactured.

Therefore, in the present invention, the thermal expansion coefficient of the obtained porous sintered body can be made extremely small by optimally controlling the particle size of the aluminum titanate powder, the particle size of the combustible powder, and the amount added. It was done.

[0012]

[0013]

The method for producing a porous aluminum titanate sintered body of the present invention comprises the steps of: preparing a coarse powder of aluminum titanate having an average particle size of 5 to 50 μm and a fine powder of aluminum titanate having an average particle size of 3 μm or more and less than 5 μm. 30% by weight or less
The average particle size bimodal powder mixed in an amount from 40 to 120
A mixed powder to which 10 to 30 % by weight of a flammable powder having a thickness of 10 μm is added is formed, and then fired in an oxidizing atmosphere at a temperature of 1550 ° C. or less.

The most significant feature of the present invention resides in that a bimodal powder is used as the aluminum titanate powder.
This bimodal powder is composed of a coarse powder having a large particle size and a fine powder having a small particle size. Among them, the average particle size of the coarse powder is 5 to 50 μm, and the average particle size of the fine powder is 3 μm or more and less than 5 μm. If the average particle size of the coarse powder is smaller than 5 μm, it does not make sense to use the bimodal powder, and the pore size of the porous body becomes small. Also 50μ
When it is larger than m, a problem occurs in terms of strength.

When the average particle size of the fine powder is smaller than 3 μm, the sinterability is improved, the pore size is reduced, and
If it is larger than m, it does not make sense to use the bimodal powder, and the strength of the sintered body is not sufficiently improved and the coefficient of thermal expansion is also increased. Fine powder in bimodal powder
It is desirable to add it in the range of 30 % by weight or less. If it is added in excess of 30 % by weight, the coefficient of thermal expansion increases and the porosity also decreases.

As the combustible powder, plastic powder and
Organic powder such as wood flour, or graphite or carb
Carbon black, etc.
You. Among them, use carbon powder whose particle size can be easily adjusted.
Is preferred. Average particle size of flammable powder is 40-120μ
m. When the average particle size is smaller than 40 μm,
If the pore diameter is too small and exceeds 120 μm, it will be fired
However, the porosity is reduced due to a large shrinkage factor. Special
Is preferably from 100 to 100 μm. Also combustible powder
The amount of the powder added is 10 to 30% by weight based on the total amount of the mixed powder.
You. Obtained when the amount of flammable powder is less than 10% by weight
The porosity of the sintered body is too small, exceeding 30% by weight
If porosity becomes too large when added, sintering becomes difficult
There is a case. Particularly desirable is 20 to 30% by weight.
Another method for producing a porous aluminum titanate sintered body according to the present invention comprises the steps of: adding an aluminum titanate powder having an average particle size of 1 to 50 μm to a flammable powder 15 having an average particle size of 10 to 40 μm;
After forming a mixed powder to which -20% by weight is added and firing at 1400-1600 ° C. in a non-oxidizing atmosphere, sintering of aluminum titanate does not proceed further in an oxidizing atmosphere 6
A heat treatment is performed at 00 to 1000 ° C. to burn the combustible powder.

The most important feature of the present invention is that after firing in a non-oxidizing atmosphere, heat treatment is further performed in an oxidizing atmosphere at a temperature range in which sintering of aluminum titanate does not proceed to burn the combustible powder. . The average particle size of the aluminum titanate powder used here is 1 to 50 μm.
is there. Particle size of aluminum titanate powder is smaller than 1 μm
At the same time, the pore size of the sintered body became too small and DPF was used.
The pressure loss in the case increases. In addition, the particle size is from 50 μm
If it becomes large, the porosity becomes too large and the strength is poor.
There is a case. Particularly desirable is a range of 10 to 20 μm.
You.

The average particle size of the combustible powder is 10 to 40 μm.
m, the amount added is 15 to 20% by weight. Average particle size is 1
If the particle size is less than 0 μm or the amount added is less than 15 % by weight, the porosity becomes small.
If it exceeds, the porosity increases and the strength decreases. In addition, the sintering temperature can use the range of 1400-1600 degreeC.

[0020]

In the method for producing a porous aluminum titanate sintered body according to the present invention, the particle size of the aluminum titanate powder, the particle size and the amount of the flammable powder are set in the ranges described in claim 1, and the firing is performed. It is possible to optimally control the pores and microcracks in the aggregate. Thereby, the porosity is 30 to
A porous sintered body having a small thermal expansion coefficient of 50% can be manufactured.

[0021] Then claim aluminum titanate powder
If a bimodal powder is used in the range described in 1, the sintering is locally promoted at the grain boundary via the fine powder, and the strength is improved. Further, it is considered that microcracks increase with this, and a sintered body having a smaller thermal expansion coefficient can be obtained. More sintered in a non-oxidizing atmosphere as in claim 2, wherein, combustible powder is aluminum titanate powder is sintered in a state remaining as it is. Thereafter, by performing a heat treatment in an oxidizing atmosphere, the combustible powder burns to form pores. Therefore, in this manufacturing method, shrinkage is prevented by the presence of the flammable powder at the time of sintering, so that pores can be formed in accordance with the particle diameter of the flammable powder, and the control of pores is extremely easy. Further, compared with the case of firing in an oxidizing atmosphere, the same porosity can be obtained with a smaller amount of combustible powder.

[0022]

The present invention will be described more specifically with reference examples and examples. Reference Example 1 Aluminum titanate was synthesized by heating a raw material consisting of 6 wt% of SiO ₂ , ₂ wt% of Fe ₂ O ₃ , and the balance of Al ₂ TiO ₅ (Al ₂ O ₃ + T ₅ O ₂ ) at 1500 ° C. This was pulverized to prepare raw material powders having various average particle sizes shown in Table 1. Next, 20% by weight of artificial graphite powder having an average particle size of 40 μm was mixed with each powder.
As the mixed powder, a raw material powder, a graphite powder, water and a dispersant were charged into a ball mill using a rubber ball containing an iron core in a polyethylene pot, and mixed for 24 hours by a wet method. The average particle size of each powder was measured using a laser light scattering type particle size distribution meter.

After drying each of the mixed powders, a binder was added and the mixture was granulated, and molded into a mold at a pressure of 500 kg / cm ² to form a 5 × 5
× 50 mm test piece shaped compacts were each produced. Then, each of the compacts is placed in the atmosphere at 1400 to 155.
The specimen was fired at a predetermined temperature in the range of 0 ° C. for 4 hours to produce a test piece as an aluminum titanate sintered body. The porosity, the average thermal expansion coefficient at 40 to 1000 ° C., and the four-point bending strength of each of the obtained test pieces were measured, and the results are shown in Table 1.

[0024]

[Table 1]

From Table 1, it can be seen that the particle diameter of the raw material powder of aluminum titanate is larger and the porosity is higher as the firing temperature is lower. The porosity which is desirable as the DPF is 30 to 50%, and the sintering temperature satisfying the porosity is 1500 ° C. or less when the raw material powder particle size is 20 μm.
1450 ° C or less at １３13 μm, 140 at 3 to 7 μm
It was 0 ° C. When the particle diameter is 7 μm or less, closed pores are also present due to sintering. In order to maintain the porosity required for DPF, if the raw material powder is 20 μm, the temperature is 1500 ° C.
Hereinafter, if the raw material powder is 10 to 13 μm, 1400 to 1
450 ° C is suitable.

Looking at the coefficient of thermal expansion, the particle diameter is 20 μm.
Of raw material powder of 1.0 × 10 even at low temperature of about 1400 ° C
A low coefficient of thermal expansion of ^-6 / ° C or less can be maintained. However, when a raw material having a particle size of 3 μm is fired at a low temperature, the coefficient of thermal expansion becomes as large as 3 to 4 × 10 ⁻⁶ / ° C. this is,
It is considered that the larger the particle size, the more the microcracks tend to increase, and as a result, the lower the coefficient of thermal expansion.

Regarding the strength, the smaller the particle diameter of the raw material powder, the higher the strength, and the higher the firing temperature, the higher the particle diameter of 7 μm or more. However, 3 μm
With the particle size of, the strength decreases conversely as the firing temperature increases. This is presumably because when the particle size is small, sintering proceeds even at a low temperature to increase the strength, but at a high temperature, microcracks are generated along with the grain growth, and the decrease in strength due to the microcracks is presumed. ( Reference Example 2 ) A mixed powder obtained by mixing a carbon powder having an average particle diameter of 40 μm with the aluminum titanate powder having an average particle diameter of 20 μm in Reference Example 1 in the range of 10 to 50% by weight shown in Table 2 was used.
A molded body was formed in the same manner as in Reference Example 1 . Each compact was fired in the air at 1450-1550 ° C. for 4 hours, and the porosity of each sintered compact was measured.

[0028]

[Table 2] According to Table 2, when the carbon powder content is 10% by weight or less, the porosity becomes 20% or less, and the pressure loss is large for use as a DPF, which is not preferable. Also add 3 carbon powders
When added in excess of 0% by weight, the porosity does not reach 50% due to shrinkage during firing. (Example 1) coarse powder of the same average particle size 20μm as in Reference Example 1, Reference
As shown in Table 3, a fine powder having an average particle diameter of 3 μm was mixed with a bimodal aluminum titanate powder having an average particle diameter of 3 μm as shown in Table 3 to obtain an average particle diameter of 40 μm.
Each compact was produced in the same manner as in Reference Example 1 , using a mixed powder to which 20% by weight of the carbon powder was added. The calcined each 4 hours at 1450 ° C. and 1550 ° C., for each obtained test pieces Reference Example 1
The porosity, the average coefficient of thermal expansion, and the four-point bending strength were measured in the same manner as described above. Table 3 shows the results.

[0029]

[Table 3] As shown in Table 3, at both 1450 ° C. and 1550 ° C. , the strength was improved by adding the fine powder. This is presumed to be due to local promotion of sintering at the grain boundary via the fine powder mixed in the coarse powder.

Regarding the average coefficient of thermal expansion, no matter what amount of the fine powder added, the one sintered at 1550 ° C. shows a smaller value. Also, as the amount of the fine powder added increases, the coefficient of thermal expansion decreases, and
It is lowest at about 0% by weight, but increases again at 50% by weight. This is considered to be due to the following mechanism.

That is, the grain growth is locally promoted by the addition of the fine powder, and the thermal expansion coefficient decreases due to the increase of microcracks accompanying the local growth. However, when the amount of the fine powder added further increases, it is considered that when the amount of the fine powder becomes too large, the increase in the particle size of the fired body is suppressed, and the generation of microcracks is suppressed. Therefore, when a bimodal powder is used, it is desirable that the amount of the fine powder added is about 30% by weight.

The porosity is higher by about 5% when fired at 1450 ° C. than when fired at 1550 ° C. The porosity decreases as the amount of the fine powder added increases. Therefore, in order to obtain a porosity of 30% or more, the amount of the fine powder added when firing at 1450 ° C. is set to 50% by weight or less, and when firing at 1550 ° C. , 30% is added.
% By weight or less. (Example 2) coarse powder of the same average particle size 20μm as in Reference Example 1, Reference
20 to 40% by weight of a carbon powder having an average particle size of 40 to 120 μm shown in Table 4 was mixed with a bimodal powder of aluminum titanate to which 30% by weight of a fine powder having an average particle size of 3 μm was added as in Example 1. a mixed powder of reference
Each molded body was produced in the same manner as in Example 1 . Then, each was fired at 1550 ° C. for 4 hours in the air, and the porosity and the porosity of each of the obtained test pieces were determined in the same manner as in Reference Example 1 .
The average coefficient of thermal expansion and the four-point bending strength were measured. Table 4 shows the results.

[0033]

[Table 4] Table 4 shows that not only the porosity and strength but also the coefficient of thermal expansion can be controlled by changing the particle size and the amount of carbon powder added. (Example 3) coarse powder of the same average particle size 20μm as in Reference Example 1, Reference
As shown in Table 5, 5 to 30% by weight of a carbon powder having an average particle diameter of 40 μm was mixed with a bimodal powder of aluminum titanate to which 30% by weight of a fine powder having an average particle diameter of 3 μm was added as in Example 1. Reference Example 1
Each molded body was produced in the same manner as described above. This molded body is placed in a vacuum furnace of 0.01 torr at 1400 to 1600 ° C.
For 4 hours.

Next, at 600 to 1000 ° C.
For 2 hours to burn off carbon powder remaining in the sintered body. Reference for each specimen obtained
The porosity and the four-point bending strength were measured in the same manner as in Example 1, and the shrinkage was calculated from the volume measurements of the compact and the sintered body before and after sintering. Table 5 shows the results.

[0035]

[Table 5] (Example 4) coarse powder of the same average particle size 20μm as in Reference Example 1, Reference
As shown in Table 6, 5 to 30% by weight of a carbon powder having an average particle size of 40 μm was mixed with a bimodal powder of aluminum titanate to which 30% by weight of a fine powder having an average particle size of 3 μm was added as in Example 1. Reference Example 1
Each molded body was produced in the same manner as described above. The molded body was placed in an atmosphere of argon gas or nitrogen gas at 1400 to 16
It was baked at 00 ° C. for 4 hours.

Next, at 600 to 1000 ° C. in the atmosphere.
For 2 hours to burn off carbon powder remaining in the sintered body. Reference for each specimen obtained
The porosity and the four-point bending strength were measured in the same manner as in Example 1, and the shrinkage was calculated from the volume measurements of the compact and the sintered body before and after sintering. Table 6 shows the results.

[0037]

[Table 6] From Tables 5 and 6, the shrinkage rate was 4% or less in each of the sintered bodies, and no large shrinkage was observed as in the case of firing in the air. This is because carbon powder does not burn during sintering under reduced pressure or in an atmosphere of argon gas or nitrogen gas, so that contraction is prevented by resistance of the carbon powder. During the heat treatment in the atmosphere, the carbon powder is burned and removed but the temperature is low, so that further sintering is prevented.

That is, it can be seen that the porosity of the sintered body can be almost controlled by the amount of the carbon powder added.

[0039]

According to the method for producing a porous aluminum titanate sintered body of the present invention, a sintered body having an optimum porosity, low thermal expansion and excellent strength is easily and stably produced. can do. Since the obtained sintered body has a lower thermal expansion property than cordierite, by applying this manufacturing method to the manufacture of DPF, a DPF excellent in durability and free from breakage or erosion during regeneration is manufactured. be able to. Furthermore, since it withstands a regeneration temperature of 1000 ° C., the regeneration efficiency is extremely high, and the pore diameter distribution is relatively uniform, so that pressure loss can be easily reduced, which contributes to downsizing and weight reduction.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-311360 (JP, A) JP-A-3-208870 (JP, A) JP-A-63-201073 (JP, A) JP-A-4- 187578 (JP, A) JP-A-3-215375 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C04B 38/00-38/10 C04B 35/46

Claims (2)

(57) [Claims] An aluminum titanate having an average particle size of 5 to 50 μm.
Powder having a mean particle size of 3 μm or more and less than 5 μm.
Fine powder of aluminum titanate in an amount of 30% by weight or less
The average particle size of the mixed bimodal powder is 40 to 120 μm.
Powder mixture containing 10 to 30% by weight of flammable powder
And then fired in an oxidizing atmosphere at a temperature of 1550 ° C or less.
A method for producing a porous aluminum titanate sintered body, characterized by comprising: 2. An aluminum titanate having an average particle size of 1 to 50 μm.
Powder having an average particle size of 10 to 40 microns
Molding mixed powder with 5-20% by weight added, non-oxidizing
After firing at 1400-1600 ° C. in an atmosphere,
Sintering of aluminum titanate does not progress in the oxidizing atmosphere
Heat treatment at 600-1000 ° C. to burn the combustible powder
Method for producing a porous aluminum titanate sintered body characterized by that.