JPH0518780B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to lightweight refractories with high heat resistance, and more specifically relates to various ceramic products such as ceramic electronic components (ceramic capacitors, alumina substrates, ferrite elements, thermistors, varistors). etc.), ceramic sliding materials, general ceramics, etc., firing aids such as saggers and stands used to support the object to be fired during the firing process, and heat shields for various kilns. This invention relates to a lightweight refractory material suitable for heating element supports and the like that can withstand repeated heating and cooling. [Prior Art] The above-mentioned firing aids and kiln constituent materials must have a high degree of heat resistance that can withstand repeated high-temperature heating and cooling, and mechanical strength appropriate to the intended use. In order to reduce the thermal energy consumed by these devices and shorten the time required for heating and cooling, thereby reducing energy costs and improving productivity, they are lightweight and have a small heat capacity per volume as much as possible. It is hoped that In particular, in the case of baking aids, it is strongly desired that they be lightweight in order to facilitate transportation and other handling. As one of the lightweight refractories to meet such demands, Japanese Patent Application Laid-Open No. 59-88378 describes a powder of refractory materials such as alumina and mullite with a weight of 90 to 50 wt.
% and refractory fibers such as alumina and mullite 10
0.5 to 10 parts by weight of amorphous silica is added to 100 parts by weight of aggregate consisting of ~50 wt%, and then molded.
Then, a refractory obtained by firing at 1450 to 1600°C is described. However, since this lightweight refractory is made of high alumina and has a large coefficient of thermal expansion, there is room for improvement in terms of spalling resistance. [Problems to be Solved by the Invention] The object of the present invention is to provide a lightweight refractory that does not have the above-mentioned drawbacks, that is, a lightweight refractory that is lightweight and has high strength, yet exhibits high heat resistance and durability, and its production. It is about providing law. [Means for solving the problem] The lightweight refractory provided by the present invention has a length of 2000 μm.
It is a porous molded body made of the following alumina short fibers, alumina continuous fibers with a length of 1 to 40 mm, and refractory powder mutually bonded by mullite, and the crystal structure is mainly composed of mullite and corundum. and the amount of mullite is 12 mol % or more of the total amount of mullite and corundum, and it is characterized in that it does not substantially contain free silica. The present invention also provides a particularly advantageous method for producing the lightweight refractories of the present invention, namely polycrystalline alumina short fibers cut to a length of 20 to 2000 μm and polycrystalline short fibers cut to a length of 1 to 40 mm. 10 to 30% by weight of amorphous silica powder is added to a mixture of high-quality alumina continuous fibers or a refractory powder of an equal amount or less, and the resulting mixture is molded, and then the presence of cristobalite is recognized. 1400 ~ until gone
The present invention provides a manufacturing method characterized by firing at 1600°C. First, to explain the above manufacturing method, in the firing process after forming the raw material mixture, the surface layer of the polycrystalline alumina fiber is mullite (as opposed to the amorphous silica powder or the crystalline silica/cristobalite produced therefrom). 3Al ₂ O ₃ .2SiO ₂ ), and this mullite acts as a binder to form a strong molded body. The unreacted fiber core remains as alumina (corundum) and maintains its fiber shape.
Compared to the conventional method described above, this manufacturing method is characterized by cutting a portion (preferably most) of the polycrystalline alumina fibers into fine pieces, preventing the fibers from becoming tangled in the raw material mixing process and separating them from other powders. This is achieved by increasing the blending ratio of fibers and amorphous silica while ensuring uniform mixing with raw materials (particularly amorphous silica). A high fiber ratio helps improve product strength, and increasing the blending ratio of amorphous silica to completely react with alumina (that is, mullite) lowers the alumina ratio in the product. It helps improve product heat resistance. Polycrystalline alumina short fibers of 2000μ or less, which are part of the raw material, are prepared by cutting polycrystalline alumina fibers using a suitable wet or dry grinder. Although alumina fibers with a length of 1 to 40 mm or more are used, the length of this ultra-shortened fiber group must be 2000 μm or less. When the fibers are made short in this way, the fibers as a whole exhibit properties similar to those of powder, and mix well with other powder raw materials. Fibers with a fiber length exceeding 2000μ, especially when the fiber diameter is small, tend to get tangled during the raw material mixing process, making it difficult for other powder raw materials to enter therein. Therefore, if the blended amount of amorphous silica is large, the amorphous silica (or its cristobalite in the firing process) and polycrystalline alumina fibers will not react uniformly, and some of the silica will remain unreacted. It remains in the product as cristobalite and deteriorates the durability of the product (cristobalite itself has good heat resistance, but stable crystal forms differ in the high temperature region and low temperature region, which border around 250℃). , a volume change occurs during the process of dislocation between these two crystal forms, so if cristobalite exists in a refractory, it causes cracks to occur due to heating and cooling.) However, if the fiber length is too small, it will be difficult to obtain a product with low specific gravity and excellent strength and durability, so it is desirable to set the lower limit to about 20μ. A particularly preferred fiber length is about 50~
500μ, average approximately 200μ. Another polycrystalline alumina continuous fiber is
Length 2 mm or more, preferably 1 to 40 mm, optimally 2 mm
~20mm. Since this fiber is mainly used for product reinforcement, a small amount (preferably about 0.5 to 10% by weight of the total amount of all polycrystalline alumina fibers and refractory powder) should be uniformly dispersed. It is necessary to pay attention to the following.
The preferred length is 2 to 20 mm. Fibers that are too long or excessive amounts of fibers make the product have a non-uniform structure, which tends to deteriorate the physical properties. Furthermore, this fiber is
The length may be just different from the polycrystalline alumina short fiber used at 2000μ or less, but
Slightly thick fibers with a diameter of 5 to 20 μm are preferable because they are less likely to get tangled during the molding material mixing process and serve as cores for agglomeration of the powder raw materials, improving moldability. Particularly suitable as this fiber are continuous fibers cut into pieces, such as chopped strands (but weakly converged so that they can be easily opened). Those prepared from continuous fibers have uniform fiber diameters and uniform lengths both within and between single fibers, so it is possible to achieve a large reinforcing effect with a small amount by using those with optimal properties. Polycrystalline alumina fibers include 95% Al ₂ O ₃ and SiO ₂ 5 with α-alumina as the main crystal phase.
%, with mullite as the crystal phase
Al ₂ O ₃ 72%, SiO ₂ 5%, Al ₂ O ₃ 80 with both α-alumina and mullite as crystal phases
%, about 20% SiO ₂ , and Al ₂ O ₃ −SiO ₂ −
Includes B ₂ O ₃ types. The amorphous silica powder accounts for 10 to 30% by weight of the total amount of polycrystalline alumina fibers and refractory powder.
If the blending ratio of amorphous silica is less than 10% by weight,
Although there is little risk of free silica remaining after firing,
The product ends up being high alumina. On the other hand, if the blending ratio of amorphous silica exceeds 30% by weight,
Even if the polycrystalline alumina fibers are cut to achieve uniform mixing and sufficient firing is performed, some of the amorphous silica remains unreacted in the form of cristobalite, causing poor heat resistance. easy to become The blending ratio of amorphous silica is such that the total Al ₂ O ₃ /SiO ₂ weight ratio is 70/30 to 90/2 when alumina or mullite introduced in the form of refractory powder is also considered.
It is desirable to make it 10. As the amorphous silica, for example, silica sol can be used. As the refractory powder, it is desirable to use high-purity crystalline powder such as alumina powder, mullite powder, and aluminumlite powder in a range of 5 to 100 parts by weight per 100 parts by weight of polycrystalline alumina fibers, but in addition, , cordierite, syamoto, fireclay, kaolin, etc. may be used in combination up to about 5% by weight. These raw materials are mixed in the above-mentioned ratio, and an appropriate amount of water is added before and after mixing to make the whole into a wet state or a slurry state. Next, the raw material mixture is molded by a conventional dehydration molding method, and the molding is preferably carried out under conditions such that the bulk specific gravity of the final product is about 0.5 to 1.5. When the obtained molded body is dried and fired at approximately 1400 to 1600°C, the polycrystalline alumina fibers maintain their fibrous form and, as mentioned above, part of the surface layer becomes amorphous silica or amorphous silica. It reacts with the cristobalite and becomes mullite, but the remaining part is stabilized as corundum. This reaction takes about 1 to 10 hours to completely consume the cristobalite, and when the firing process is carried out, the product has a crystal structure consisting mostly of corundum and mullite, of which mullite accounts for about 12 to 90 mol%. is obtained. Insufficient firing that leaves cristobalite exhibits the aforementioned drawbacks of cristobalite-containing products. Disappearance of cristobalite can be confirmed by ordinary powder X-ray diffraction method. In the lightweight refractory of the present invention obtained as described above, corundum fibers having a fiber shape of 2000μ or less, although they are short, corundum fibers having a longer reinforcing effect, and refractory powder are present at their contact points. It is joined by mullite,
It has a large amount of microscopic voids. The standard and preferred porosity is about 50-80%, so that the refractory has a bulk specific gravity of 0.5-1.5 and exhibits a heat capacity per unit volume of about 1/3 of that of dense alumina-based refractories. . The lightweight refractory of the present invention can be used as it is, or if necessary, can be subjected to cutting processing or a heat-resistant surface coating (for example, zirconia coating).
It can be used as a firing aid or a kiln component as described above. [Effects of the Invention] The lightweight refractory according to the present invention has a large amount of mullite, which has better thermal properties than alumina, and contains substantially no free silica. ) Due to the high efficiency of reinforcement with two types of polycrystalline alumina fibers, the material can withstand high temperatures far superior to conventional high-alumina lightweight refractories, even though it is obtained using polycrystalline alumina fibers as a raw material. Shows durability.
Moreover, even if the bulk specific gravity is about 1.0 or less, it is possible to obtain a material that shows sufficient strength for practical use, and it has the advantage of being easy to cut. [Example] The present invention will be described below with reference to Examples and Comparative Examples. The raw materials used in each example are as follows. Polycrystalline alumina fiber untreated product: Fiber diameter 3μ, average fiber length approximately 50mm, Al ₂ O ₃ 95%, SiO ₂ 5%. Ultra-shortened product: The above-mentioned untreated product is opened and cut using a pulper to have a fiber length of approximately 50 to 500μ. Reinforcing fiber: Al ₂ O ₃ −B ₂ O ₃ −SiO ₂ polycrystalline alumina fiber (Al ₂ O ₃ 68%, SiO ₂ 27
%, _{B2O3 5} %, single fiber fiber diameter 15μ,
Chopped strand product with a length of 10 mm; however, Comparative Example 8 is 1 mm, Comparative Example 9 is 30 mm) Refractory powder: Sintered alumina Amorphous silica: Silica sol Among the above raw materials, first is polycrystalline alumina fiber. and refractory powder are dispersed in water (the chopped strand reinforcing fibers are opened to single fibers), then amorphous silica is added and stirred,
Suction dehydration molding. The obtained molded body is dried with hot air and then fired at 1500 to 1600°C for 3 hours. Table 1 shows the results of experiments conducted by variously changing the mixing ratio of raw materials and processing conditions in the above manufacturing method. The test method for the characteristic values shown in the table is as follows. Bending strength: For a test piece with a thickness of 6 mm, width of 25 mm, and length of 75 mm, span of 50 mm, loading rate of 0.2
mm/min, measured at room temperature or 1400℃. Spalling resistance: thickness 6mm, width 200mm, length 200mm
mm test piece at 600℃, 700℃ or 800℃
Place it in a furnace and heat it for 1 hour, then air cool it and check for cracks. If there is no crack, repeat the same operation for up to 30
Repeat until cycle. shown in the table

【table】

[Table] The numerical values are the number of heating cycles that led to cracks. Further, an X-ray diffraction chart showing the crystal structure of the product of Example 1 is shown in FIG.

[Brief explanation of the drawing]

FIG. 1: X-ray diffraction chart showing the crystal structure of the refractory according to Example 1.

Claims (1)

[Claims] 1. Short alumina fibers with a length of 2000μ or less,
It is a porous molded body made of continuous alumina fibers with a length of 1 to 40 mm and refractory powder bonded to each other by mullite, and the crystal structure is mainly composed of mullite and corundum, and the amount of mullite is equal to that of mullite. of total amount with corundum
A lightweight refractory characterized by having a content of 12 mol% or more and substantially free of free silica. 2. The lightweight refractory according to claim 1, having a bulk specific gravity of 0.5 to 1.5. 3. 10 to 30 weight of polycrystalline alumina short fibers with a length of 20 to 2000μ, polycrystalline alumina continuous fibers with a length of 1 to 40 mm, or a mixture of these and an equal or less amount of refractory powder. % of amorphous silica powder is mixed, the resulting mixture is molded, and then fired at 1400 to 1600°C until the presence of cristobalite is no longer recognized. 4. The manufacturing method according to claim 3, in which powdered alumina, mullite, or aluminumite is used as the refractory powder. 5 Polycrystalline alumina continuous fibers are added in an amount of 0.5 to 0.5 of the total amount of all polycrystalline alumina fibers and refractory powder.
The manufacturing method according to claim 3, wherein 10% by weight is used. 6 Length 2~ as polycrystalline alumina continuous fiber
20 mm and a diameter of 5 to 20 μm. The manufacturing method according to claim 3. 7. The manufacturing method according to claim 3, in which cut continuous fibers are used as the polycrystalline alumina continuous fibers.