JP2017149621A - Honeycomb structure and method for producing honeycomb structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a honeycomb structure including an oxidation prevention layer which can adjust the degree of softening of silicate glass.SOLUTION: The honeycomb structure in which a surface of the substrate of a silicon carbide ceramic sintered body formed in a honeycomb structure including plural cells sectioned by partitions that extend in a single direction and that are disposed in rows is coated with an oxidation prevention layer has a configuration in which the oxidation prevention layer includes a silicate glass phase and a crystal phase of silicon dioxide. In the configuration, the rate of cristobalite included in the crystal phase to silicon carbide can be set at 3 mass% to 28 mass%.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a honeycomb structure suitable for use as a heat storage body, and a method for manufacturing the honeycomb structure.

  As a ceramic heat storage body used in a heat storage burner (regenerative burner) or the like, balls made of alumina have been frequently used (for example, see Patent Document 1). However, when the heat storage body recovers heat from the heated gas and when the recovered heat is applied to the unheated gas, there is a problem that the pressure loss is large because the gas flows through the gaps between the balls. . In addition to the small surface area of the ball, the center portion does not contribute to heat exchange, so that there is a problem that heat exchange is insufficient.

  On the other hand, a technique of using a ceramic honeycomb structure such as alumina, cordierite, or mullite as a heat storage body has also been proposed (see, for example, Patent Document 2). The honeycomb structure includes cells partitioned by a large number of partition walls, and the cells extend in a single direction. Therefore, there is an advantage that the pressure loss accompanying the gas flow is small. In addition, the honeycomb structure has an advantage that the surface area is much larger than that of the ball.

  However, ceramics such as alumina, cordierite, and mullite have low thermal conductivity. Therefore, it cannot be said that it is suitable as a material that alternately repeats heat storage and heat dissipation, and has a problem in terms of heat exchange efficiency. Further, many partition walls that make the surface area of the honeycomb structure very large are very thin with a thickness of less than 1 mm. Therefore, the heat storage body having a honeycomb structure has a problem that cracks and cracks are likely to occur due to a temperature change caused by repeated heat storage and heat release, that is, a thermal shock resistance is low.

  In view of this, the present applicant has previously proposed a heat storage body in which a honeycomb structure base is a silicon carbide ceramic sintered body (see Patent Document 3). Silicon carbide is a material having high thermal conductivity among ceramics. Specifically, the thermal conductivity of alumina, cordierite, and mullite is 9-30 W / m · K, 0.6 W / m · K, and 1.5 W / m · K, respectively. The thermal conductivity of silicon carbide is as high as 75 to 130 W / m · K. Therefore, the heat storage body using the silicon carbide based ceramic sintered body as a base has high heat exchange efficiency.

  In addition, the thermal expansion coefficient of silicon carbide is as low as 0.4 to 0.6% at a temperature of 1000 ° C. This is about 1/2 of the thermal expansion coefficient of alumina. That is, since silicon carbide has high thermal conductivity and low thermal expansion coefficient, it has excellent thermal shock resistance. Therefore, a heat storage body using a silicon carbide based ceramic sintered body as a base is suitable as a heat storage body that continues to undergo temperature changes associated with repeated heat storage and heat dissipation.

  However, there is a problem that silicon carbide is oxidized when it is used at a high temperature in an atmosphere containing oxygen. In view of this, the technique of Patent Document 3 employs means for forming an anti-oxidation layer of silicate glass on the surface of the base body, which is a silicon carbide ceramic sintered body. This silicate glass layer prevents the silicon carbide and oxygen on the substrate from coming into contact with each other, so that the oxidation of silicon carbide is effectively suppressed.

  In addition, as a result of the study by the present applicant, it has been found that the thermal shock resistance can be further improved by providing an anti-oxidation layer of silicate glass, compared with silicon carbide having a high thermal shock resistance. This is thought to be due to the fact that silicate glass is softened and plastically deformed at high temperatures, so that crack extension is suppressed and brittle fracture of the silicon carbide ceramic sintered body is suppressed. .

  However, the property that silicate glass softens at high temperatures is an advantage as described above, but when the degree of softening is large, foreign matter such as dust adheres to clog the cells of the honeycomb structure, and heat There was a risk of reducing the efficiency of the exchange. Moreover, there existed a possibility that the malfunction that heat storage bodies or a heat storage body and a casing might adhere via a softened silicate type glass might arise.

JP 2003-343829 A JP 2003-287379 A Japanese Patent No. 5709007

  Therefore, in view of the above circumstances, the present invention has an object to provide a honeycomb structure including an antioxidant layer capable of adjusting the degree of softening of the silicate glass and a method for manufacturing the honeycomb structure. .

In order to solve the above problems, the honeycomb structure according to the present invention is
“Honeycomb in which the surface of a silicon carbide based ceramic sintered body formed in a honeycomb structure having a plurality of cells partitioned by partition walls extending in a single direction is coated with an antioxidant layer A structure,
The antioxidant layer includes a silicate glass phase and a silicon dioxide crystal phase.

The honeycomb structure of the present configuration can be manufactured by the following manufacturing method of the honeycomb structure (hereinafter sometimes simply referred to as “manufacturing method”). That is,
“An antioxidant containing silicon dioxide is applied to the surface of a silicon carbide based ceramic sintered body formed in a honeycomb structure having a plurality of cells partitioned by partition walls extending in a single direction. Coat,
By the heat treatment, the antioxidant is made silicate glass and a part of silicon dioxide is crystallized.

  The silicon dioxide crystals do not melt at a temperature at which the silicate glass softens. Therefore, in forming an antioxidant layer from an antioxidant containing silicon dioxide, a portion of silicon dioxide is crystallized, and silicon dioxide crystals are precipitated in the silicate glass phase, thereby reducing the total amount of silicon dioxide. Compared with the case of using silicate glass, the degree of softening of the antioxidant layer at a high temperature is reduced. Thereby, the adjustment which reduces the grade which an antioxidant layer softens under high temperature is attained, maintaining the effect | action which a thermal shock resistance is improved by the softening of a silicate type glass. In other words, there is provided a honeycomb structure in which the antioxidant layer that suppresses the oxidation of silicon carbide on the base body is adjusted to the extent that it is softened at a high temperature without impairing the effect of further improving the thermal shock resistance of silicon carbide. be able to.

The honeycomb structure according to the present invention has, in addition to the above-described configuration, “the crystal phase includes cristobalite,
The ratio of cristobalite to silicon carbide is 3% to 28% by weight.

  Silicon dioxide is polymorphic, and there are three types of crystal structures at 1 atm, quartz, tridymite, and cristobalite, each of which has a high temperature type (β type) and a low temperature type (α type). As will be described in detail later, the entire honeycomb structure in which the surface of the substrate is coated with an antioxidant layer has excellent thermal shock resistance by adjusting the ratio of cristobalite to silicon carbide to 3% by mass to 28% by mass. A honeycomb structure can be obtained.

  As described above, as an effect of the present invention, it is possible to provide a honeycomb structure including an antioxidant layer that can adjust the degree of softening of the silicate glass, and a method for manufacturing the honeycomb structure.

2 is an X-ray diffraction pattern showing a crystal phase of Example 1. FIG. 4 is an X-ray diffraction pattern showing a crystal phase of Example 4. (A) The microscope observation image of Example 2, and (b) The microscope observation image of Example 4. It is a graph which shows the change of the expansion coefficient with respect to the temperature of the crystal | crystallization of silicon dioxide.

  Hereinafter, a honeycomb structure according to an embodiment of the present invention and a manufacturing method thereof will be described. The honeycomb structure of the present embodiment has a configuration in which the surface of the silicon carbide ceramic sintered body is covered with an antioxidant layer. The substrate is formed in a honeycomb structure, that is, a structure including a plurality of cells partitioned by partition walls extending in a single direction. The antioxidant layer includes a silicate glass phase and a silicon dioxide crystal phase.

  The honeycomb structure having such a structure is obtained by coating the surface of the silicon carbide ceramic sintered body formed in the honeycomb structure with an antioxidant containing silicon dioxide, and using the silicate glass as the antioxidant. And a heat treatment for crystallizing a part of silicon dioxide.

  More specifically, the coating process of the antioxidant includes a process of applying or spraying the antioxidant to the surface of the substrate, a process of immersing the substrate in the antioxidant, or a process of impregnating the substrate with the antioxidant. can do.

  The antioxidant containing silicon dioxide becomes a silicate glass phase by heat treatment after being melted by heating and then cooled to a temperature lower than the glass transition point. Here, in addition to silicon dioxide, the antioxidant can contain sodium oxide, potassium oxide, potassium carbonate, boric acid and the like. With these components, the softening point, strength, and viscosity of the produced silicate glass can be adjusted.

  The heat treatment for crystallizing a part of silicon dioxide contained in the antioxidant is a treatment for decreasing the cooling rate during cooling, or a temperature between the glass transition point and the glass softening point, particularly close to the glass softening point. It can be set as the process hold | maintained at temperature for a long time. Further, in these treatments, a substance serving as a crystal nucleus can be contained in the antioxidant. Or the component which accelerates | stimulates the production | generation of a crystal | crystallization can be contained in antioxidant. For example, aluminum oxide is said to promote cristobalite crystal formation, and calcium oxide is said to promote tridymite crystal formation.

  Next, using the honeycomb structures of Examples 1 to 6, the results of examining the influence of the presence of the silicon dioxide crystal phase on the softening of the antioxidant layer and the thermal shock resistance of the honeycomb structure will be shown.

For the samples of Examples 1 to 6, substrates prepared under the same conditions were used. This substrate is formed from silicon carbide as an aggregate, and a raw material containing a silicon source and a carbon source that reacts and produces silicon carbide. After firing in a non-oxidizing atmosphere, excess carbon is removed in an oxidizing atmosphere. It has been removed by heating. The substrate contains a slight excess of silicon (single substance) in addition to silicon carbide. The honeycomb structure of the substrate had a partition wall thickness of 0.65 mm, a cell density of 50 cells / inch ² , and an outer shape of a cube of 50 mm × 50 mm × 50 mm. The pores are formed in the disappearance trace of the carbonaceous material used as the carbon source, and the partition walls are porous.

  The above substrates were impregnated with antioxidants at different ratios. In the impregnation treatment, first, the substrate is accommodated in a container that can be sealed, and the air in the container is sucked by a vacuum pump or the like. Thereby, the open pores of the porous substrate are degassed. Next, the slurry of the antioxidant is introduced into the sealed container in which the substrate is accommodated through a pipe or hose with an on-off valve. As a result, the outer surface of the substrate is covered with the antioxidant, and the antioxidant penetrates into the degassed open pores.

  Thereafter, excess antioxidant was removed from the substrate. For example, excess antioxidant can be removed by continuously applying vibration to the substrate or by blowing compressed air onto the substrate.

  Here, as the antioxidant, the samples of Examples 1 to 5 except Example 6 were impregnated with an antioxidant having the same composition. The sample of Example 6 was impregnated with an antioxidant to which 5% by mass of alumina was added as a component for promoting cristobalite crystal formation.

  The heat treatment was performed on the samples of Examples 1 to 6 at the heating temperature, the heating time, and the temperature lowering rate described later.

  Each sample after the heat treatment was quantified by a wet chemical analysis method. At the time of quantification, the silicon dioxide content (total amount including glass phase and crystal phase) is calculated from Si detected from the solution melted in hydrofluoric acid and detected from the remainder not melted in hydrofluoric acid. Si was calculated as a silicon compound (including silicon carbide) other than silicon dioxide. The results are shown in Table 1.

  Moreover, about each sample after heat processing, the X-ray-diffraction pattern was measured and the crystal phase identification and fixed_quantity | quantitative_assay by the Rietveld method were performed. Here, to measure the X-ray diffraction pattern, an X-ray diffractometer (manufactured by Rigaku, Ultimate III) is used, and a copper tube, a voltage of 40 kV, a current of 40 mA, a step width of 0.02 degrees, 2 It measured on the conditions of degree / min.

  The crystal phase of silicon dioxide observed in the X-ray diffraction pattern of each sample was cristobalite in Example 1, and cristobalite and tridymite in the other Examples 2 to 6. In the X-ray diffraction pattern of any sample, in addition to the silicon dioxide crystal phase and the silicon carbide crystal phase, peaks of other crystal phases containing silicon such as simple silicon were observed. As an example, the X-ray diffraction patterns of Example 2 and Example 4 are shown in FIGS. 1 and 2, respectively.

  The sum of silicon carbide quantified from the X-ray diffraction pattern and a crystalline phase containing other silicon that is neither silicon carbide nor silicon dioxide is a silicon compound other than silicon dioxide quantified by wet chemical analysis and the subject of quantification. Are the same. Table 2 shows the composition in which silicon compounds other than silicon dioxide by wet chemical analysis are divided into silicon carbide and other silicon compounds based on this relationship.

  In addition, based on the above relationship, the silicon dioxide crystal phase determined from the X-ray diffraction pattern is compared with the silicon dioxide content (total amount including the glass phase and crystal phase) determined by the wet chemical analysis method. The glass phase of silicon was quantified. Table 3 shows the quantitative results of cristobalite and tridymite as the glassy phase of silicon dioxide and the crystalline phase of silicon dioxide in each sample together with the heat treatment conditions.

  Table 3 shows the results of calculating the ratio of the silicon dioxide crystal phase to the sum of the silicon dioxide glass phase and the silicon dioxide crystal phase (cristobalite and tridymite). When this calculation result is compared between Example 1 and Example 2 in which only the heating temperature is different under the heat treatment conditions, the higher the heating temperature (closer to the glass softening point), the higher the generation rate of the crystalline phase of silicon dioxide. ing. In addition, when Example 2 and Example 3, and Example 4 and Example 5 in which only the heating time is different under the heat treatment conditions are compared, the longer the heating time, the higher the generation rate of the crystalline phase of silicon dioxide. . Furthermore, when Example 2 and Example 4, and Example 3 and Example 5 which differ only in the temperature-fall rate in heat processing conditions are contrasted respectively, the one where the temperature-fall rate is small has increased the production | generation ratio of the crystal phase of silicon dioxide. . In addition, when Example 5 and Example 6 which differ only in the presence or absence of addition of alumina to the antioxidant are compared, the generation rate of the silicon dioxide crystal phase is increased by adding alumina, and cristobalite It can be seen that a lot is generated.

  For each sample, the surface covered with the antioxidant layer was observed with a digital microscope (microscope), and crystals considered to be silicon dioxide were observed in any sample. As an example, the observation image of Example 2 is shown in FIG. 3A, and the observation image of Example 4 is shown in FIG. In Example 2, cristobalite crystals were clearly observed, and in Example 4, both cristobalite crystals (circled range C) and tridymite crystals (circled area T) were clearly observed. .

  As described above, with respect to the samples of Examples 1 to 6 each including the glass phase of silicon dioxide and the crystal phase of silicon dioxide in the antioxidant layer, adhesion due to softening of the antioxidant layer and thermal shock resistance are as follows: Evaluated.

<Adhesion due to softening of antioxidant layer>
Each sample was held in an electric furnace at a temperature of 1000 ° C. for 30 minutes, then taken out of the furnace, and 5 g of calcium sulfate powder was sprayed on the surface (antioxidation layer) of the sample. Calcium sulfate powder is like dust. After cooling the sample to room temperature, the powder was blown off with compressed air, and then the mass of the sample was measured. The increase compared with the mass before the test is the mass of the calcium sulfate powder attached to the sample. When the adhesion amount is 0.5 g or more per sample, it is evaluated as “x” because there is much adhesion due to softening of the antioxidant layer, and when 0.05 to 0.5 g, the adhesion is slightly high. The case of 0.05 g or less was evaluated as “◯” because there was almost no adhesion.

<Heat shock resistance>
Each sample was held in an electric furnace at a temperature of 1000 ° C. for 30 minutes, then taken out of the furnace, dropped in water and rapidly cooled. The presence or absence of occurrence of cracks or cracks was visually observed, the case where cracks or cracks occurred in one test was evaluated as “x”, and the case where cracks or cracks occurred in two or three repeated tests was evaluated as “ Evaluated as “△”, evaluated as “◯” when cracks or cracks occurred in 4 to 5 repeated tests, and evaluated as “◎” when neither cracks nor cracks occurred in 5 repeated tests did.

  Table 4 shows the evaluation results of adhesion due to softening of the antioxidant layer and thermal shock resistance. For comparison, the results of a similar evaluation test for the sample of Comparative Example 4 are also shown in Table 4. Comparative Example 4 is the same sample as Example 4 except for the temperature lowering rate under heat treatment conditions, the temperature lowering rate is as large as 300 ° C./h, and the antioxidant layer is composed only of the glass phase.

  As can be seen from Table 4, by precipitating the silicon dioxide crystal phase in addition to the glass phase in the antioxidant layer, softening of the antioxidant layer when the honeycomb structure is used at a high temperature is suppressed, and accompanying softening It was confirmed that adhesion could be prevented. In combination with the results shown in Table 3, the ratio of the silicon dioxide crystal phase to the sum of the silicon dioxide glass phase and the silicon dioxide crystal phase is in the range of at least 20 mass% to 61 mass% (Example 1 6), adhesion due to softening can be suppressed.

  Table 4 also shows the calculated values of the ratio (mass%) of cristobalite to silicon carbide. In Example 6 in which the proportion of cristobalite is 28% by mass, the thermal shock resistance is evaluated as “◯”, but in Example 3 in which the proportion of cristobalite is 31% by mass, the evaluation of the thermal shock resistance is reduced to “Δ”. doing. From this, it was considered that when the proportion of cristobalite increases, the thermal shock resistance of the honeycomb structure having the antioxidant layer decreases. This is because, as shown in FIG. 4, among quartz, tridymite, and cristobalite, which is a crystal structure of silicon dioxide at 1 atm, cristobalite has a large coefficient of thermal expansion in the entire temperature range from room temperature to 1000 ° C. It was thought that the thermal shock resistance decreased due to the generation of stress due to the difference in thermal expansion coefficient between the expansion and compression accompanying the temperature change and the silicon carbide of the substrate.

  In addition, as is apparent from FIG. 4, the thermal expansion coefficient of cristobalite changes greatly when it transitions between the high temperature type and the low temperature type. From this, it was considered that when a large amount of cristobalite is contained as a silicon dioxide crystal, the thermal shock resistance is reduced by the stress generated with the temperature change. Here, it is preferable that the ratio of cristobalite to silicon carbide is at least 3% by mass to 28% by mass as the range in which the thermal shock resistance evaluation is confirmed to be “◯” or “「 ”. As a range in which the evaluation of the property is confirmed to be “◎”, it is more preferable that the ratio of cristobalite to silicon carbide is at least 14 mass% to 17 mass%.

  FIG. 4 is quoted from “Ceramic Chemistry” written by the Ceramic Industry Board of Education, published by the Ceramic Association of Japan, June 30, 1982, page 137.

  As described above, according to the present embodiment, the surface of the substrate, which is a silicon carbide ceramic sintered body having a honeycomb structure, is coated with the oxidation preventing layer, and the oxidation preventing layer is formed of silicon dioxide crystals in the silicate glass phase. By setting it as the layer which has precipitated, adhesion by the softening of the antioxidant layer at the time of using at high temperature can be suppressed.

  In addition, the degree of softening of the antioxidant layer should be adjusted by the crystalline phase without impairing the thermal shock resistance inherent to the silicon carbide of the substrate and the effect of further improving the thermal shock resistance due to the silicate glass phase. Can do. In particular, by setting the ratio of cristobalite to silicon carbide within the above predetermined range, a honeycomb structure having excellent thermal shock resistance and suppressed adhesion due to softening can be manufactured.

  The present invention has been described with reference to the preferred embodiments. However, the present invention is not limited to the above-described embodiments, and various improvements can be made without departing from the scope of the present invention as described below. And design changes are possible.

  For example, the honeycomb structure of the present invention is suitable for use as a heat storage body, but the application is not limited to a heat storage body for a heat storage burner, and for other devices such as a heat collector of a solar thermal power generation apparatus. It can also be used as a heat storage body. In addition, the honeycomb structure of the present invention can be applied to a heat exchange body that is not limited to the use as a heat storage body, and that exchanges heat between media flowing through adjacent cells via partition walls.

Claims (3)

Honeycomb structure in which the surface of a silicon carbide ceramic sintered body formed in a honeycomb structure having a plurality of cells partitioned by partition walls extending in a single direction is coated with an antioxidant layer Body,
The honeycomb structure according to claim 1, wherein the antioxidant layer includes a silicate glass phase and a silicon dioxide crystal phase. The crystalline phase comprises cristobalite;
The honeycomb structure according to claim 1, wherein the ratio of cristobalite to silicon carbide is 3 mass% to 28 mass%. The surface of a silicon carbide ceramic sintered body formed in a honeycomb structure having a plurality of cells partitioned by partition walls extending in a single direction is coated with an antioxidant containing silicon dioxide. And
A method for manufacturing a honeycomb structure, characterized in that, by heat treatment, the antioxidant is made into silicate glass and a part of silicon dioxide is crystallized.