JPH0148231B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention particularly relates to a method for producing porous mullite porcelain, which has low thermal conductivity and excellent thermal shock resistance. Low thermal conductivity is required as a high-temperature insulation material.
Ceramic fibers in the 0.0001 Cal/cm・sec・℃ class are known, but since fibers with irregular shapes and low mechanical strength are inconvenient for practical use, ceramics with regular shapes and low thermal conductivity are desired. has been done. In addition, thermal shock resistance is required for high-temperature structural materials including the above-mentioned high-temperature insulation materials, and since the thermal shock resistance is approximately proportional to thermal conductivity, ceramics with low thermal conductivity have high thermal shock resistance. It was considered difficult to assign gender. The present invention provides a mesh-like porous mullite porcelain that has a low thermal conductivity approximately equal to that of the above-mentioned ceramic fiber, and a thermal shock resistance equivalent to that of cordierite or β-spodumene porcelain, which is said to have the highest value among ceramics. That is. Example Metal silicon (98% purity, 150 mesh, Mitsuwa Chemical) 200g Alumina (99.9% purity, Taimicron AG, Daimei Chemical) 1092g Methyl ethyl ketone (solvent) 720g Dibutyl phthalate (plasticizer) 80g Ionet S-20 (dispersion) After mixing and pulverizing 8 g or more of the mixture for 24 hours in an alumina porcelain ball mill with an internal volume of 3, 120 g of polyvinyl butyral was added as a binder and mixing was continued for another 24 hours. The obtained slurry was formed into a sheet on a polyester film by a doctor blade method, and a green sheet with a thickness of 4 mm was obtained by air drying for 15 hours. This 4 mm thick sheet is fired under various conditions (1
Table 1 shows the properties of the samples obtained after holding the ceramic composition for a long time, together with the general properties of other ceramic compositions.

【table】

[Table] The temperature difference when sharpness occurred was measured.
2. Standard values are given for the properties of mullite, cordierite, and β-spodium porcelain.
As is clear from the table above, Samples Nos. 1 to 6 and Nos. 11 to 16 produced according to the present invention actually have a mullite structure with a porous structure with a porosity of 55% or more, and the porous structure is Samples No. 2 and No. 14 listed as representative examples
As shown in Figures 1 and 2, which are micrographs (×5000) of mullite, needle-like crystals are formed spatially in a network shape, and the high heat resistance inherent to mullite is comparable to that of ceramic fiber. It has low thermal conductivity (insulation) and high mechanical strength. Therefore, in addition to the above characteristics, high-temperature insulation materials such as furnace materials, burner nozzles, arc insulation plates, etc., have thermal shock resistance equivalent to low-expansion porcelain such as cordierite and β-spodium, so they can be used from the outside. The compressed air is preheated to a high temperature before being introduced into the combustion chamber,
After fuel is injected and burned in the combustion chamber to operate the turbine, a portion of the thermal energy of the high-temperature exhaust gas is recovered into the compressed air introduced into the combustion chamber and released into the atmosphere.Especially when the temperature difference is large. Bioceramics is highly effective as a material for various heat exchangers, including the honeycomb structure type of gas turbines, and its mesh structure, which has a porosity of up to 92%, is used as a furnace material and as a biochemical reaction catalyst carrier. Although it is effective for a wide range of applications such as adsorption sheets for liquid chromatography, sample No. 7 and the like have hydrogen gas and ammonia decomposition gas dew points exceeding 10°C even if the starting materials are the same.
No. 17 exhibited a granular crystal structure and exhibited various characteristic values approaching those of the mullite porcelain cited as a reference example. In the present invention, metallic silicon is used as the SiO ₂ source that reacts with Al ₂ O ₃ to produce mullite, and the reason for firing in a reducing atmosphere with a water vapor dew point of 10°C or less is that the starting material metallic silicon is Oxidized to SiO and _SiO2 by water vapor in the firing atmosphere,
The latter SiO ₂ reacts with Al ₂ O ₃ to form mullite, but if the dew point is lowered, the SiO ₂ concentration is low.
Since it reacts with Al ₂ O ₃ , the reaction takes time to produce mullite with a network crystal structure, and the SiO in equilibrium with the SiO ₂ becomes SiO as SiO ₂ reacts with Al ₂ O ₃ and decreases. ₂ and reacts with free Al ₂ O ₃ sequentially to produce mullite with a network crystal structure, and it has been experimentally confirmed that this reaction can be obtained by limiting the water vapor dew point to 10°C or less. This is because the. Therefore, the lower limit of water vapor dew point is -5 from the point of view of mass production.
It is about ℃. In the previous table, metallic silicon and alumina were used as starting materials and were weighed to obtain the theoretical composition of mullite, but CaO and MgO were used as sintering accelerators according to the usual method.
Even if free Al ₂ O ₃ or SiO ₂ is present in the final product, it is allowed as long as it is within 8% of the mullite. In addition, metallic silicon is used as a starting material and SiO ₂ source.
Although Si cannot be moved, aluminum compounds such as alumina sol and aluminum hydroxide can be used as the Al ₂ O ₃ source, and other reducing gases such as a mixed gas of carbon monoxide and nitrogen gas can be used for the firing gas. Good too. Furthermore, in the example, the powder of the starting material was made into a slurry, and green sheets were manufactured using the doctor plate method. A molding method can be selected depending on the purpose, such as press molding performed by granulation.

[Brief explanation of drawings]

Figure 1 is an electron micrograph (magnification: 5000x) of the crystal structure of porous mullite porcelain according to the present invention, and Figure 2 is an electron micrograph (magnification: 5000x) of the crystal structure of porous mullite porcelain according to another embodiment of the present invention. times).

Claims (1)

[Claims] 1. A mesh-like porous mullite porcelain characterized by firing a compact mainly composed of mixed powder of metallic silicon and aluminum compound in a reducing atmosphere such as hydrogen gas or ammonia decomposition gas with a dew point of 10°C or less. manufacturing method.