JPH0761892B2 - Cordierite honeycomb structure - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cordierite honeycomb structure catalyst carrier, and more particularly to a honeycomb structure having low expansion and excellent thermal shock resistance, which is used as a catalyst carrier for purifying automobile exhaust gas.

[0002]

2. Description of the Related Art With the progress of industrial technology in recent years, there has been an increasing demand for materials having excellent heat resistance and thermal shock resistance. Thermal shock resistance is one of the important characteristics especially in the ceramic honeycomb catalyst carrier used in the automobile exhaust gas purifying apparatus, and it causes sudden heat generation and engine start / stop due to the catalytic reaction of unburned hydrocarbons and carbon monoxide in the exhaust gas. It is required to have high thermal shock resistance to withstand the thermal stress caused by the temperature difference generated in the honeycomb structure due to the temperature change caused by rapid heating and quenching during the installation. The demand is strong with driving.

This thermal shock resistance is represented by the difference between the rapid heating and quenching endurance temperature, and it has been found that the endurance temperature difference is inversely proportional to the coefficient of thermal expansion among the characteristics of the honeycomb. The smaller the coefficient of thermal expansion is, the more the endurance temperature is. It is known that the difference is large, and that the honeycomb structure has a large contribution particularly in the direction perpendicular to the flow path (axis in FIG. 4B).

[0004]

Conventionally, it is known that cordierite ceramics exhibit a low expansion property, and are disclosed in, for example, US Pat. No. 3,885,977 (corresponding Japanese application: Japanese Patent Laid-Open No. 50-75611). 25-10 as has been
Coefficient of thermal expansion between 00 ° C is 11 × 10 ^{-7 in} at least one direction
Cordierite ceramics with an orientation of less than / ° C are shown, and the planar orientation due to the plate-like clay and laminated clay is described as the cause of this orientation. The temperature was between 25 and 1000 ℃.
Compositions having a low expansion of 56 × 10 ⁻⁶ / ° C. are disclosed.

On the other hand, as a characteristic of the system using silica here, as shown in the examples, the coefficient of thermal expansion of the A-axis is shown.
0.62-0.78 × 10 ^-6 / ° C, B-axis thermal expansion coefficient is 1.01-
It was as large as 1.08 × 10 ⁻⁶ / ° C., and there was a problem that the expansion of the B-axis thermal expansion coefficient, which substantially contributes to thermal shock resistance, could not be achieved.

Further, in US Pat. No. 3,950,175 (JP-A-50-75612), a part or the whole amount of talc or clay in a raw material is silica such as pyroferrite, kyanite, quartz or fused silica. Or at least 20% of 10μ by replacing with silica alumina source material
It is disclosed that a cordierite-based porous ceramic having pores with a diameter larger than m can be obtained.

On the other hand, among these, a composition using fused silica as a silica raw material and having a large number of atmospheric holes of 10 μm or more is disclosed, but there is no description about reduction in expansion, and reduction in B-axis thermal expansion coefficient is achieved. could not.

Further, in Japanese Patent Publication No. 57-28390, a talc average particle size of 5 to 150 μm is used.
It is disclosed that a low expansion of 1.6 × 10 ⁻⁶ / ° C. or less can be obtained at 00 ° C., but 0.9 × 10 ⁻⁶ / ° C. at 25 to 1000 ° C.
There is no description of a composition exhibiting a low expansion of less than, and the thermal expansion coefficient in the A-axis and B-axis directions could not be further reduced.

Furthermore, Japanese Patent Application No. 61-61, which is the prior application of the inventor.
-183904 shows a combination of high-purity amorphous silica and fine-grained alumina based on the use of fine-grained talc of 5 μm or less for the purpose of densification with porosity of 30% or less. A low expansion of less than 0.3 × 10 ^-6 / ° C between 0 ° C is not obtained. In the present invention, the coefficient of thermal expansion in the A-axis and B-axis directions is further reduced in the range where the porosity exceeds 30% and 42% or less.

The object of the present invention is to solve the above problems,
It is an object of the present invention to provide a cordierite honeycomb structure excellent in heat resistance and thermal shock resistance by lowering the A-axis and B-axis coefficient of thermal expansion of a conventional cordierite honeycomb structure.

[0011]

The cordierite honeycomb structure of the present invention has a chemical composition of main components of SiO ₂ 42 to 56% by weight, Al ₂ O ₃ 30 to 45% by weight, and MgO 12 to 16% by weight. , The main crystal phase is cordierite, and the coefficient of thermal expansion between 40 and 800 ℃ in the flow direction of the honeycomb structure (axis of Fig. 4A) is 0.3 × 10 ^-6 / ℃ or less, Direction (axis of Fig. 4B) between 40 and 800 ℃ has a coefficient of thermal expansion of 0.5 × 10 ^-6 /
C. or less, the main structure structure is composed of cordierite crystal aggregates (domains) and pores, and microcracks are developed along the C-axis direction of the cordierite crystals in the domain structure. Is. The structure of the honeycomb structure also contains a small amount of glass phase and sub-crystal phase.

[0012]

In the above-mentioned constitution, the low expansion can be achieved because the use of high-purity amorphous silica makes the reaction system greatly different from the cordierite-forming reaction system of talc, kaolin, and alumina. Shift to the high temperature side, and the preferred cordierite crystal orientation, that is, the maximum diameter of the cordierite crystal with the C-axis crystallization direction aligned in the same direction is 20 μm.
This is because it is possible to obtain more than m domains. Furthermore, the average length of the cordierite crystal in the C-axis direction is 1 to 5
A microstructure is obtained in which the automorphic cordierite crystal having a C-axis / A-axis aspect ratio of 1.5% or more of the cordierite crystal of 80% or more in μm is significantly developed.

That is, by forming domains and pores in which the C-axis crystallizing directions of the cordierite crystal are aligned in the same direction as the main structure structure of the honeycomb structure, microcracks are generated in the C-axis direction of the cordierite crystal in the domain structure. It becomes possible to progress along with, and it becomes easier to absorb the thermal expansion in the A-axis and B-axis directions of the cordierite crystal that expands positively.
It is considered that the contribution of the microcracks to the low expansion becomes large, and the honeycomb structure has a low expansion. Further, by setting the domain preferably to have a maximum diameter of 20 μm or more, microcracks can be further propagated in the C-axis direction of the cordierite crystal. These are C of cordierite crystals
The average length in the axial direction is 1 to 5 μm, and 80% or more of the cordierite crystal has a C-axis / A-axis aspect ratio of 1.5 or more, which is a significantly developed automorphic cordierite crystal.

Furthermore, as a feature of this microstructure, the amount of microcracks does not differ greatly from that of talc, kaolin, and alumina-based cordierite materials, but the microcracks tend to fall along the C-axis direction of the cordierite crystal in the domain structure. Since the thermal expansion in the A-axis and B-axis directions of the cordierite crystal that expands positively is absorbed, the contribution of microcracks to the expansion becomes large, and the honeycomb structure has a low expansion. It is thought to be to do.

For low expansion, the chemical composition of the honeycomb structure is 42 to 56% by weight, preferably 47 to 53% by weight of SiO ₂ , and Al.
30-45 wt% preferably 32-38 wt% in ₂ O _3, 12 to 16 wt% in MgO preferably preferably set to 12.5 to 15 wt%, the component for example TiO ₂ is inevitably mixed , CaO
, KNaO and Fe ₂ O ₃ may be contained in a total amount of 2.5 wt% or less.

It is preferable that the crystal phase substantially consists of cordierite crystals, and the amount of cordierite crystals is 90% by weight or more, and mullite and spinel (including sapphirine) as other contained crystals are contained.

If the porosity of the catalyst carrier is less than 30%, the catalyst supporting conditions will deteriorate, and if it exceeds 42%, the strength will decrease and the thermal shock resistance after supporting the catalyst will deteriorate.
Over 42% is preferable, but it is set by changing the carrying conditions and the canning method.

The coefficient of thermal expansion is 0.3 × 10 ^-6 / ° C in the A-axis direction.
And the B-axis direction exceeds 0.5 × 10 ^-6 / ° C, the thermal shock resistance deteriorates, so the A-axis direction is 0.3 × 10 ^-6 / ° C.
It was limited to the following and 0.5 × 10 ⁻⁶ / ° C. or less in the B-axis direction. The coefficient of thermal expansion in the A-axis direction is more preferably 0.2 × 10 ⁻⁶ / ° C. or less.

Regarding the talc particle size, if the average particle size is less than 5 μm, the coefficient of thermal expansion increases and the porosity decreases, and if the average particle size exceeds 100 μm, both the coefficient of thermal expansion and the porosity increase, so the average particle size 5 Limited to ~ 100 μm. The average particle size is preferably 7 to 50 μm.

The particle size of silica is limited to 15 μm or less because the coefficient of thermal expansion in the B-axis direction and the porosity increase when the average particle size exceeds 15 μm. If the type of silica is crystalline silica, the coefficient of thermal expansion increases, the thermal shock resistance deteriorates, and the porosity also increases, so amorphous silica is used.

The alumina particle size is limited to 2 μm or less because the coefficient of thermal expansion increases when the average particle size exceeds 2 μm. As this alumina, Na ₂ O amount of 0.
It is preferable to use 12% by weight or less of low soda alumina because it enables a lower expansion.

The kaolin particle size is preferably 2 μm or less in average particle size, and the use of talc particles having an average particle size of 1/3 or less is preferable because the orientation of cordierite crystals is promoted and low expansion can be achieved.

Aluminum hydroxide used as the alumina raw material is preferable because it has an average particle size of 2 μm or less because it promotes cordierite crystal orientation and is very effective in reducing expansion.

Amorphous silica is preferably used in an amount of 8 to 20% by weight because it is most effective in reducing expansion.

[0025]

EXAMPLES The present invention will be described in more detail with reference to Examples and Comparative Examples.

Example 1 The raw materials having the chemical analysis values and particle sizes shown in Table 1 were used as N in Tables 2 to 4.
o. 1 to No. 36 were mixed, and after adding methyl cellulose, kneading was performed to obtain an extrudable kneaded clay.

Next, the kneaded material of each batch was subjected to a known extrusion molding method to have a rib thickness of 152 μm, a square cell shape with 62 cells per square centimeter, a diameter of 93 mm and a height of 10.
It was molded into a 0 mm cylindrical honeycomb structure. After the honeycomb structure was dried, it was fired at the maximum temperature shown in Tables 2 to 4, and the characteristics of the sintered body were evaluated such as the coefficient of thermal expansion of A and B axes, the porosity, the amount of cordierite crystals, and the thermal shock resistance. did. The evaluation results are also shown in Tables 2 to 4.

From the above results, FIG. 1 shows the relationship between the thermal expansion coefficient in the A-axis direction and the thermal shock resistance temperature, and FIG. 2 shows the relationship between the thermal expansion coefficient in the B-axis direction and the thermal shock resistance temperature.
In comparison with the relationship between the average particle size of talc and the coefficient of thermal expansion in the A and B axis directions in batches No. 1 to No. 7, the average particle size of talc shown in FIG. And the coefficient of thermal expansion.

The average particle size of the raw materials in Table 1 is talc.
(A), (B), and (C) were measured by a dry separation method using a JIS standard sieve, and the others were measured by an X-ray sedimentation method using a cedigraph manufactured by Micromeritics.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[Table 3]

[0033]

[Table 4]

From the results shown in Tables 2 to 4, the average particle size is 5 to 10
Test Nos. 2 to 6, 9 to 14, 16, 18 and 20 to 35 using talc of 0 μm, alumina having an average particle diameter of 2 μm or less, and high-purity amorphous silica having an average particle diameter of 15 μm or less are the present invention. It was found that the thermal expansion coefficients of the A-axis and the B-axis defined by 1) were satisfied.

Sample No. having a talc particle size outside the scope of the present invention was used.
Sample Nos. 1 and 7, alumina having a particle size outside the scope of the present invention, Sample No. 15 having a silica grain size outside the scope of the present invention, and Samples Nos. 17 and 19 using crystalline silica were respectively the A axis and the A axis specified in the present invention. It was also found that the coefficient of thermal expansion of the B axis was not satisfied.

Further, it is known from FIGS. 1 and 2 that the thermal shock resistance temperature is inversely proportional to the coefficient of thermal expansion, and the correlation is remarkable between the coefficient of thermal expansion of the B axis and that from FIG. Although talc having the same particle size as that of Japanese Examined Patent Publication No. 57-28390 is used, it was found that the thermal expansion coefficient can be made extremely small by using high-purity amorphous silica in combination with fine-grained alumina.

Example 2 Several kinds of samples shown in Tables 2 to 4 were used in Example 1
Prepared in the same manner as above, the minimum domain major axis of each sample, the average length of the cordierite crystals, the crystal ratio of the aspect ratio of 1.5 or more, the I ratio of the cordierite crystals on the honeycomb wall surface (parallel surface in the honeycomb extrusion direction) [ I (110) / {I (110) +
I (002)}] was obtained. The results are shown in Table 5.

In Table 5, the minimum domain major axis was determined from the minimum domain major axis that can be confirmed from the SEM photograph of each sample. In addition, for the average length of cordierite crystals and the amount of crystals with an aspect ratio of 1.5 or more, cordierite crystals were randomly selected from the SEM photographs of each sample, and the length and width of each crystal were measured and the aspect ratio was calculated. I asked.

[0039]

[Table 5]

From the results shown in Table 5, in some of the samples of the present invention, the minimum domain major axis is 20 μm or more, the average length of cordierite crystals is 1 to 5 μm, and the crystal amount ratio of aspect ratio 1.5 or more is 80% or more. It was found that the above ranges were present, and these ranges were found to be preferable ranges in the present invention. Furthermore, it was found that the I ratio of the honeycomb wall surface is preferably 0.78 or more.

5 (a) and 5 (b) are SEM photographs of Test No. 32 (invention) at 50 times and 2000 times, respectively, and FIGS. 6 (a) and 6 (b).
Shows SEM photographs of Test No. 36 (reference example) at 50 times and 2000 times. Further, FIG. 7 shows a diagram for explaining each region of the SEM photograph shown in FIG.

From FIGS. 5 (a), (b) and FIG. 7, the sample No. 32 of the present invention has a long columnar cordierite self-extending in the C-axis direction and an average length of 3.5 μm. It can be seen that the shape crystals are very well developed and form domains of 20 μm or longer. In addition, crystals with an aspect ratio of 1.5 or more
It accounts for 85%, and many microcracks are along the crystal C-axis direction in the domain. 50x SEM shown in Fig. 5 (a)
Enlarged in the photo, you can see the automorphic crystals and domains. In addition, the major axis of the large domain has a major axis of 100 μm or more, which makes it difficult to confirm by SEM.

On the other hand, the samples shown in FIGS. 6 (a) and 6 (b)
In the reference example of No. 36, cordierite automorphic crystals were not observed in most parts, and the average length of the automorphic crystals that could be confirmed was 0.8 μm. Therefore, the formation of domains is relatively small (major axis is 10 μm or more) in only a small part. The 2000x photograph shown in Fig. 6 (b) shows the area where the automorphic crystals were relatively developed, but again, there were few crystals with an aspect ratio of 1.5 or more, and only 30% was observed as a whole. Although microcracks are also present, their relationship with cordierite crystals is not clear.

Further, FIGS. 8 (a) and 8 (b) show SEM photographs of Test No. 32 (invention) in the same visual field at room temperature and 800 ° C. By comparing Figures 8 (a) and 8 (b), it was confirmed that the microcracks that were open at room temperature were almost completely closed at 800 ℃, which means that the microcracks contribute to the low expansion of cordierite honeycomb. It shows that it is contributing.

Furthermore, test No. 32 (invention) is shown in FIG.
And the thermal expansion hysteresis curves up to 1200 ° C of No. 36 (reference example) are shown. From Fig. 9, the maximum hysteresis amount of test No. 32 (maximum difference in thermal expansion coefficient at the same temperature between expansion curve during heating and contraction curve during cooling) was 0.086%, and maximum hysteresis amount in test No. 36 was 0.068%. is there. The maximum hysteresis amount is considered to represent the amount of microcracks and the contribution to low expansion, and No. 32 and No. 36 show no significant difference in the amount of microcracks in the microstructure observation. Therefore, the effect of microcracks on low expansion is No. 32.
It is shown that is larger.

[0046]

As is apparent from the above detailed description, according to the present invention, microcracks are propagated along the C-axis direction of cordierite crystal by forming the honeycomb structure structure into domains and pores. It is possible to make the thermal expansion absorption of the microcracks act in the A-axis and B-axis directions of the cordierite crystal that positively expands.
Coefficient of thermal expansion between ˜800 ° C. A-axis: 0.3 × 10 ⁻⁶ / ° C. or less, B-axis: 0.5 × 10 ⁻⁶ / ° C. or less, the honeycomb structure is excellent in heat resistance and thermal shock resistance. Therefore, the present invention is extremely useful industrially, and is particularly useful for forming a manifold of an automobile exhaust gas purifying apparatus, which is required to have high heat resistance and thermal shock resistance, and a ceramic catalyst carrier for high-speed operation.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the coefficient of thermal expansion of the A axis and the thermal shock resistance temperature.

FIG. 2 is a graph showing the relationship between the coefficient of thermal expansion of the B axis and the thermal shock resistance temperature.

FIG. 3 is a graph showing the relationship between the average particle size of talc and the coefficient of thermal expansion.

FIG. 4 is a perspective view showing an example of a honeycomb structure.

FIG. 5: 50 × and 2000 showing the structure of the crystal of test No. 32
It is a double SEM photograph.

FIG. 6: 50 × and 2000 showing the structure of the crystal of test No. 36
It is a double SEM photograph.

FIG. 7 is a diagram for explaining each region of the SEM photograph shown in FIG. 5 (a).

[Figure 8] Normal temperature and 800 in the same field of view of test No. 32
It is a SEM photograph which shows the structure of the crystal of (degreeC).

FIG. 9 is a diagram showing the thermal expansion hysteresis curves of Test Nos. 32 and 36 up to 1200 ° C.

Claims (6)

[Claims] 1. A honeycomb structure having a chemical composition of main components of SiO ₂ 42 to 56% by weight, Al ₂ O ₃ 30 to 45% by weight, MgO 12 to 16% by weight, and a main crystal phase of cordierite. The coefficient of thermal expansion between 40 and 800 ℃ in the flow direction of the honeycomb structure is 0.3.
× 10 ^-6 / ℃ or less, the coefficient of thermal expansion between 40 ~ 800 ℃ in the direction perpendicular to the flow path is 0.5 × 10 ^-6 / ℃ or less, the main structure texture is cordierite crystal aggregate (domain), A cordierite honeycomb structure comprising pores, wherein microcracks extend along the C-axis direction of the cordierite crystal in the domain structure. 2. The cordierite honeycomb structure according to claim 1, which has a porosity of more than 30% and 42% or less. 3. The cordierite honeycomb structure according to claim 1, wherein the coefficient of thermal expansion between 40 and 800 ° C. in the flow direction of the honeycomb structure is 0.2 × 10 ⁻⁶ / ° C. or less. 4. The cordierite honeycomb structure according to claim 1, wherein the maximum domain diameter is 20 μm or more. 5. A cordierite crystal having an average length in the C-axis direction of 1 to 5 μm and 80% or more of cordierite crystal C.
The cordierite honeycomb structure according to claim 1, wherein an axial / A axial length ratio (aspect ratio) is 1.5 or more. 6. A cordierite crystal I ratio I = I (110) / {I (110) + on a honeycomb wall surface (a plane parallel to the honeycomb extrusion direction).
The cordierite honeycomb structure according to claim 1, wherein I (002)} is 0.78 or more.