JPH0437028B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a cordierite honeycomb structure, and more specifically, a highly airtight cordierite honeycomb structure for a rotating heat storage type or heat transfer type heat exchanger having excellent thermal shock resistance, airtightness, and heat resistance. The present invention relates to a cordierite honeycomb structure and a method for manufacturing the same. (Prior Art) With the progress of industrial technology in recent years, there has been an increasing demand for materials with excellent heat resistance and thermal shock resistance. The thermal shock resistance of ceramics is influenced by the material's properties such as coefficient of thermal expansion, thermal conductivity, strength, modulus of elasticity, and Boisson's ratio, as well as the size and shape of the product, as well as heating,
It is also influenced by the cooling state, ie the rate of heat transfer. Among these factors that affect thermal shock resistance, it is known that the coefficient of thermal expansion has a particularly large contribution rate, and in particular, when the rate of heat transfer is high, it is largely influenced only by the coefficient of thermal expansion. The development of low expansion materials with excellent thermal shock resistance is strongly desired. Conventionally, as a relatively low expansion ceramic material,
Cordierite is known, but in general, cordierite is difficult to sinter to a high density, especially from room temperature.
Cordierite base material exhibits low thermal expansion coefficient of 2.0×10 ^-6 /℃ or less up to 800℃, which contains extremely small amount of impurities such as calcia, alkali, potash, and soda that can act as fluxing agents. Since it is necessary to limit the glass phase to a very small amount, the glass phase becomes porous. Therefore, when such cordierite ceramics is applied to a rotating regenerative heat exchanger with a honeycomb structure, for example, the pores on the surface of the partition wall that form the through holes of the honeycomb structure due to its large open porosity.
In particular, fluid leakage occurs between the heating fluid and the heat recovery fluid through the communication holes, which has a serious disadvantage that the heat exchange efficiency and the efficiency of the entire system in which the heat exchanger is used are reduced. For these reasons, there has been a strong desire for a low-expansion, highly airtight cordierite honeycomb structure that has excellent thermal shock resistance. It has been known that cordierite ceramics exhibit low expansion properties, for example, as described in U.S. Patent No.
Specification No. 3885977 (corresponding Japanese application: Japanese Patent Application Laid-open No. 1989-
As disclosed in Publication No. 75611), 25~
Thermal expansion coefficient between 1000℃ in at least one direction
Cordierite ceramics with an orientation smaller than 11×10 ^-7 /°C are known, and the planar orientation caused by plate clay and laminated clay is described as the cause of this orientation. Unlike the present invention, coarse talc of 10-20μm was used, and clay was also used.
It covers a wide range of ~10 μm, and there is no mention of pores at all. Furthermore, U.S. Patent No. 3950175
No. 50-75612), at least 20% of the talc or clay in the raw material is replaced by a silica or silica-alumina source raw material such as pyroferite, kyanite, quartz, or fused silica. It has been disclosed that cordierite porous ceramics having pores with a diameter larger than 10 μm can be obtained, but unlike the present invention, pores with a diameter of 5 μm or more can be obtained.
There is no description of increasing airtightness by suppressing the pore volume to 0.04 cc/g or less using fine talc with an average particle size of 7 μm or less. Furthermore, although JP-A-53-82822 describes a positive correlation between the average talc particle size and the average pore size ^of cordierite ceramics, In order to have a coefficient of thermal expansion below ℃, the average particle size of talc must be 10~
It is necessary to make coarse grains of 50 μm, and it cannot be easily analogized to the low expansion and high airtight cordierite ceramics shown in the present invention. Furthermore, in JP-A No. 59-122899, the porosity is 20~20.
A highly airtight cordierite rotary regenerative heat exchanger is disclosed, which is characterized in that the open pores on the surface of the partition wall forming the through holes of a honeycomb structure mainly composed of 45% cordierite are sealed with a filling material. However, by using fine talc with an average particle size of 7 μm or less and fine kaolin with an average particle size of 2 μm or less in the cordierite raw material as in the present invention, the volume of pores with a diameter of 5 μm or more is suppressed to 0.04 cc/g or less, improving airtightness. There is no mention of increasing it. Also, JP-A-59-122899
The manufacturing method of No. 1 requires a step of supporting the filler material as a slurry on the fired cordierite honeycomb and then re-firing it, so compared to the present invention, it is more likely to cause cell clogging, resulting in higher costs. There is. (Problems to be Solved by the Invention) The purpose of the present invention is to provide a highly airtight cordierite honeycomb structure that achieves both airtightness and low expansion, which are the problems of conventional cordierite ceramics. do. (Means for solving the problem) The present inventors used talc with an average particle size of 7 μm or less and kaolin with an average particle size of 2 μm or less and 1/3 or less of the talc average particle size. The total pore volume with a diameter of 5 μm or more is 0.04
cc/g or less, porosity is 30% or less to make it substantially airtight, and the coefficient of thermal expansion between 40 and 800℃ is 1.0×
10 ^-6 /℃ or less. Setting the average particle size of talc to 7 μm or less is effective in reducing the pore size. Furthermore, using kaolin with a diameter of 2 μm or less is effective in suppressing porosity. Furthermore, the combination of talc with an average particle size of 7 μm or less and kaolin with an average particle size of 1/3 or less of the talc average particle size promotes the orientation of cordierite crystals in the cell walls of the cordierite honeycomb structure. This contributes to low expansion. The particle sizes of talc and kaolin were determined from the average particle size based on the blended weight ratio of raw and calcined products. The present inventors also found that the chemical composition of the honeycomb structure is 42 to 56% by weight of _SiO2 , preferably 47 to 56% by weight.
53 wt%, 30-45 wt% _{in Al2O5} preferably 32
~38% by weight, preferably 12-16% by weight in MgO
It has been found that 12.5 to 15% by weight is suitable. The honeycomb structure may contain other unavoidable components such as TiO ₂ , CaO, KNaO, Fe ₂ O ₃ in a total amount of 2.5% by weight or less, but P ₂ O ₅
is preferably less than 0.1%. The reason for limiting these values is to make the main component of the crystalline phase the cordierite phase and to remove the high expansion glass phase generated from impurities. Furthermore, the present inventors have determined that when firing a dried honeycomb structure body, the temperature is raised at an average rate of 20 to 60 °C/h in the temperature range of 1100 to 1350 °C just before reaching the firing temperature. After that, it was found that it is preferable to perform the firing at a firing temperature of 1350 to 1440°C. If the heating rate is less than 20°C/h, the coefficient of thermal expansion will be large, and if it is higher than 60°C/h, the pore diameter during firing will be large and the airtightness will be poor. A suitable range is usually 30 to 50°C/h. Moreover, by firing at a temperature of 1350 to 1440°C, the main component of the crystal phase can be made into a cordierite phase. The cell density of the honeycomb structure is 62 cells/cm ² (400
The reason why the rib thickness is set to 203 μm (8 mil) or ^less is to make the honeycomb structure a heat exchanger with high heat exchange efficiency and low pressure loss. There is no particular limitation on the shape of the cells, but they can be of various shapes, such as triangular, square, rectangular, hexagonal, and other shapes with fins. The total pore volume of pores with a diameter of 5 μm or more is 0.04 cc/
The reason for limiting the amount to less than 0.026 cc/g is that, as shown in Figure 1, fluid leakage through the thin walls of such a honeycomb structure is mainly limited to 0.026 cc/g or less.
This is because the pores are larger than m. In order to satisfy this condition, the porosity is preferably at least 30% or less. In the present invention, when talc and kaolin are made into fine particles,
It also includes the use of calcined talc and calcined kaolin, which are effective in suppressing the occurrence of cracks in honeycomb structures due to shrinkage during drying and firing. If the calcining temperature of talc or kaolin is increased, it will increase the porosity and the coefficient of thermal expansion, so when using a calcined product, it is preferable to keep the calcining temperature as low as possible. The effects of the present invention cannot be obtained unless fine particles having a particle size similar to that of the raw raw material are used. Other cordierite forming raw materials, such as alumina source raw materials such as alumina and aluminum hydroxide, and silica source raw materials such as silica and silica sand, that have been conventionally used can be used, but the amount of impurities such as alkali in the chemical composition is appropriate. It is necessary to optimize the particle size, such as cutting of coarse particles, depending on the rib thickness of the honeycomb structure to be manufactured. Therefore, in the chemical composition of the present invention, SiO ₂ is preferably 42 to 56% by weight, preferably 47 to 53% by weight, Al ₂ O ₃ is 30 to 45% by weight, preferably 32 to 38% by weight, and MgO is preferably 12 to 16% by weight. talc with an average particle size of 7 μm or less, kaolin with an average particle size of 2 μm or less and 1/3 of the average particle size of talc, and other cordierite forming raw materials are mixed so that the amount is 12.5 to 15% by weight. A plasticizer and a binder are added to this mixture to make a plasticized deformable batch, and this plasticized batch is molded by extrusion molding and then dried. In the temperature range of °C, the temperature is raised at an average heating rate of 20 to 60 °C/hour, and then fired at a firing temperature of 1350 to 1440 °C. A cordierite honeycomb structure with a total pore volume of 5 μm or more pores of 0.04 cc/g or less, a porosity of 30%, and a thermal expansion coefficient of 1.0 × 10 ^-6 /°C or less between 40 and 800°C. It is manufactured. Firing is usually carried out for 0.5 to 12 hours. The main component of the honeycomb structure is 90% by weight or more of cordierite. Other crystals are mainly mullite and spinel (including saphirin), each of which accounts for 2.5% by weight or less. The total pore volume of the honeycomb structure is 0.04 for pores larger than 5 μm.
When the pressure is below cc/g, especially below 0.026cc/g,
When the leakage rate is 1.4 Kg/cm ² , the leakage rate is 100 g/sec·m ² or less, especially 50 g/sec·m ² or less, making it substantially airtight, making it suitable for use as a heat exchanger. (Function) The present invention has a honeycomb structure in which the total pore volume of pores of 5 μm or more is 0.04 cc/g or less and the porosity is 30%.
Since it is small, the amount of leakage is small and the heat exchange efficiency is high. Also, the thermal expansion coefficient is 1.0× at 40 to 800℃
It has high thermal shock resistance as it is small at less than 10 ^-6 /℃.
In addition, since the rib thickness can be made as thin as 203 μm (8 mil) or less, pressure loss is small, and high-density cells can be achieved, making it possible to obtain a ceramic heat exchanger with high heat exchange efficiency. Furthermore, since the pore diameter is small and the porosity is small, the strength of the honeycomb structure is higher than that of conventional honeycomb structures, and the walls can be made thinner. The present invention will be described in more detail below with reference to Examples and Comparative Examples. Example 1 The raw materials having the chemical analysis values and particle sizes shown in Table 1 below were mixed into batches No. 1 to No. 30 in Table 2 according to the mixing ratio, talc particle size, and kaolin particle size shown in Table 2.
4.5 parts by weight of methyl cellulose and added water were added to 100 parts by weight, and the mixture was kneaded to obtain an extrudable clay. The raw material used here was one that had passed through a sieve with a mesh size of 63 μm. Next, each batch of clay was molded into ribs with a thickness of 120μ by a known extrusion method.
A cylindrical honeycomb structure with a diameter of 93 mm and a height of 100 mm, having a rectangular cell structure with 167 cells per square centimeter and a short side/long side = 1/1.73 was molded. After drying, the honeycomb structures of each batch were fired under the firing conditions shown in Table 2, and the characteristics of the sintered bodies were the coefficient of thermal expansion at 40°C to 800°C, porosity, and total pore size of 5 μm or more. The volume, leakage amount, amount of cordierite crystals, and thermal shock resistance were evaluated. The evaluation results are also shown in Table 3. In addition, P ₂ O ₅ was less than 0.1% in the chemical composition of the sintered bodies of all batches. Example 2 The batch No. 4 in Table 2 was extruded using a die with a different cell structure in the same manner as in Example 1, and fired to form a batch with a diameter of 93 mm having the cell structure shown in Table 3.
mm, height 100mm cylindrical honeycomb structure No.31~No.
40 were manufactured. The pressure drop, leakage amount, and coefficient of thermal expansion (CTE) of each honeycomb structure were evaluated. The evaluation results are also shown in Table 3.

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[Table] *3 Mercury intrusion method *4 Pressurized air pressure 1.4Kg/cm ² , test piece shape 75mm
Measured using a honeycomb structure with a diameter of 60 mm and no outer wall. The measurement method is US DOE/NASA/0008-12, NASACR-
165139 “Ceramic Regenerator System Development Program - Final Report”
According to P213.
*5 Quantitative value based on X-ray diffraction ZnO internal standard *6 Endurance temperature when placed in an electric furnace, held for 30 minutes, and taken out at room temperature *7 No. 18 fired product has glass powder and cordierite powder supported at 20% Open pore adhesion treatment is carried out on

[Table] As an actual example, FIGS. 4 and 5 show an example of a rotary regenerative heat exchanger composed of a plurality of cordierite matrix segments. (Effects of the Invention) As shown in Tables 2 and 3, according to the present invention, the total pore volume of pores of 5 μm or more is 0.04 cc/g.
In the following, a honeycomb structure having a coefficient of thermal expansion of 1.0×10 ⁻⁶ /° C. or less, excellent thermal shock resistance, extremely small amount of leakage, and excellent heat exchange efficiency can be obtained. Therefore, the present invention is extremely useful industrially.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between 5μm pore volume and test piece leakage amount, including batches in Table 2 and other experimental examples, and Figure 2 is a diagram showing the relationship between talc A in Table 1.
Figure 3 is a diagram showing the particle size distribution curves of kaolins A to E in Table 1. Figures 4 and 5 are composed of a plurality of cordierite matrix segments. FIG. 2 is a diagram showing a rotating regenerative heat exchanger.

Claims (1)

[Claims] 1. Chemical composition: 42 to 56% by weight of SiO ₂ and 30% by weight of Al ₂ O ₃
~45% by weight, talc with an average particle size of 7 μm or less, with an average particle size of 2 μm so that MgO is 12-16% by weight
Kaolin and other cordierite-forming raw materials having an average particle size of 1/3 or less of the average particle size of talc are mixed, and a plasticizer and a binder are added to this mixture to make a plasticized deformable batch. The plasticized batch is molded by extrusion and then dried, and the dried product is heated at 1100 to 1350℃ just before reaching the firing temperature.
A method for producing a cordierite honeycomb structure, characterized in that the temperature is raised at an average temperature increase rate of 20 to 60°C/hour in the temperature range of 20 to 60°C, and then fired at a firing temperature of 1350 to 1440°C. 2. The manufacturing method according to claim 1, using talc having an average particle diameter of 5 μm or less. 3. The manufacturing method according to claim 1, using kaolin having an average particle diameter of 1 μm or less.