JPH0518781B2 - - 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. The present invention relates to a lightweight refractory material that can withstand repeated heating and cooling and is suitable for heating element supports, wall construction materials, and the like. [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, it is desirable to use lightweight materials with as low a specific heat as possible. desired. 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 refractory has a high silica content, when used as a firing aid for ceramic semi-finished products that easily react with silica, it causes a chemical reaction that not only causes the ceramic products to fuse or denature, but also causes the refractory itself to However, deterioration in strength and durability is unavoidable. Furthermore, when used in a high-temperature reducing atmosphere or inert atmosphere, silica tends to volatilize, resulting in a significant decrease in strength. Furthermore, silica usually remains in the form of cristobalite, which undergoes rapid volume changes at around 200°C, so there is room for improvement in products containing this in terms of spalling resistance (durability against rapid heating and cooling). be. [Problems to be Solved by the Invention] The object of the present invention is to create a lightweight refractory that does not have the above-mentioned drawbacks, that is, to be chemically inert under any conditions of use, and to have a high degree of heat resistance and durability. The object of the present invention is to provide a lightweight refractory material and a method for producing the same. [Means for solving the problem] The lightweight refractory provided by the present invention has a length of 2000 μm.
The following high alumina short fibers or a mixture of the short fibers and an alumina refractory powder are bonded together with an alumina binder to form a porous molded body with a bulk specific gravity of 0.5 to 1.5, with no free silica. It is substantially free of. The present invention also provides a particularly advantageous method for producing the lightweight refractories of the present invention, namely polycrystalline alumina short fibers having a length of 20 to 2000μ or adding an equal or less amount of alumina refractory powder to the short fibers. 2 to 30% by weight of an alumina binder was added and mixed, the resulting mixture was molded, and then heated for 1400 to 1600 hours until the presence of free silica originating from polycrystalline alumina staple fibers was no longer observed. The present invention provides a manufacturing method characterized by firing at ℃. First, to explain the above manufacturing method, when compared with the conventional method mentioned above, the feature of this manufacturing method is that polycrystalline alumina fibers are cut into fine pieces, made into something close to powder, and this is used as the main raw material. That, and
By not using too many silicic acid raw materials, no silica other than the small amount of silica derived from the polycrystalline alumina fibers is prevented. The main raw material, polycrystalline alumina short fibers of 2000μ or less, is prepared by cutting polycrystalline alumina fibers using a suitable wet or dry grinder. This fiber exhibits properties similar to powder,
Mixes very easily and uniformly with other powder raw materials. Fibers with a fiber length exceeding 2000μ, especially when the fiber diameter is small, tend to tangle during the raw material mixing process, making it difficult for the refractory powder and binder to enter into the tangles. Therefore, it tends to be inferior in terms of strength and durability. However, if the fiber length is too short, 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 to 500 microns, with an average of about 200 microns. The thickness of the fibers is not particularly limited, but is preferably in the range of about 1 to 5 microns. As the alumina refractory powder, it is desirable to use high-purity alumina powder such as calcined alumina powder, fused alumina powder, and aluminum hydroxide. The amount of alumina powder used is at most the same amount as the alumina fiber. Use of excessive amounts will make the product dense and have poor insulation and durability. Suitable alumina binders include colloidal alumina, aluminum sludge (aluminum hydroxide gel produced by alumite treatment), and aluminum hydroxide gel obtained by reacting aluminum sulfate with an alkali. It is desirable that the amount of this binder used (in terms of Al ₂ O ₃ ) is 2 to 30% by weight based on the mixture of alumina fiber and refractory powder. There are similar disadvantages. 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. After drying the obtained molded body, it is fired at about 1400 to 1600°C until substantially no free silica is observed to harden the binder. In this process, the silica contained in the alumina short fibers (usually around 1 to 5% by weight) reacts with the surrounding alumina and turns into mullite, but the remaining alumina remains in the state of corundum. It is stabilized and remains in the product without substantial change in fibrous morphology. Insufficient calcinations that leave free silica in the form of cristobalite exhibit the drawbacks of silica-containing products as described above. Disappearance of free silica can be confirmed by conventional powder X-ray diffraction method. In the lightweight refractory of the present invention obtained as described above, corundum fibers (or corundum fibers and alumina refractory powder) having a fiber shape of 2000 μm or less, although short, are bonded by alumina at their contact points. 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 does not substantially contain free silica, which is undesirable due to its reactivity and volatility, and is mainly composed of extremely finely cut alumina short fibers. Due to the special composition of the ingredients, it has a wider usable range than conventional silica-containing alumina lightweight refractories, and also exhibits excellent high-temperature durability. And even if the bulk specific gravity is less than about 1.0, it is easy to obtain a material that shows sufficient strength for practical use.
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 with a pulper to have a fiber length of about 50 to 500μ. Refractory powder: Sintered alumina Binder: Colloidal alumina However, in Comparative Examples 4 and 5, colloidal silica was used. Of the above raw materials, alumina fibers and refractory powder are first dispersed, then a binder is added and stirred, followed by suction dehydration molding. The obtained molded body is dried with hot air and then fired at 1300 to 1500°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. Average crack occurrence temperature: Prepare a test piece cut into a plate shape of 100 mm x 100 mm x 4 mm. The five sheets were stacked, placing cubic alumina sintered bodies of 6 mm on each side as spacers in the four corners between the sheets, and above and below them,
Plates (dummy) similar to the test pieces are placed one by one in the same manner. The test pieces assembled as described above were placed in an electric furnace at 300°C.
Heat for 40 minutes. Thereafter, it is taken out of the furnace, left to cool, and then visually inspected for the presence or absence of cracks. Raise the temperature of the electric furnace by 50℃ and repeat the same test. Repeat this while increasing the temperature by 50°C until cracks occur in all test pieces, and calculate the average crack occurrence temperature T _av (°C) using the following formula:
seek. T _av = 1/5 _o 〓 ^{i = 0} x _i・T _i where x _i is the number of test pieces that cracked at temperature T _i ℃ (300 + 50i), and n is the total number of test pieces that cracked.

[Table] This is the number of heating cycles until cracks occur. Further, X-ray diffraction charts showing the crystal structures of the products of Example 1 and Comparative Example 4 are shown in FIGS. 1 and 2. Furthermore, each product of Example 1 and Comparative Example 4 was tested in a reducing atmosphere (75% hydrogen + 25% nitrogen).
A test was conducted in which the material was heated to 1500°C for 5 hours. The physical properties after heating were as follows. Example 1 Comparative Example 4 Bulk specific gravity 1.05 1.12 Bending strength (Kgf/cm ² ) 102 45

[Brief explanation of the drawing]

FIG. 1: An X-ray diffraction chart showing the crystal structure of the refractory according to Example 1. FIG. 2: An X-ray diffraction chart showing the crystal structure of the refractory according to Comparative Example 4.

Claims (1)

[Claims] 1 High alumina short fibers with a length of 2000μ or less or a mixture of the short fibers and an alumina refractory powder are mutually bonded by an alumina binder to form a porous structure with a bulk specific gravity of 0.5 to 1.5. forming a quality molded body,
A lightweight refractory characterized by not containing free silica. 2. The lightweight refractory according to claim 1, wherein the content of the alumina refractory powder is not higher than the content of the high alumina short fibers. 3 Add and mix 2 to 30% by weight of an alumina binder to a mixture of polycrystalline alumina short fibers with a length of 20 to 2000μ or the above short fibers and an equal amount or less of alumina refractory powder, The resulting mixture was molded and then molded until the presence of free silica originating from the polycrystalline alumina staple fibers was no longer observed.
A method for producing lightweight refractories characterized by firing at 1400-1600℃.