JP2003137671A - Mullite-based porous body and method of producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily improve resistance to erosion, creep resistance and spalling resistance at a low cost. SOLUTION: A blend of an alumina powder and a silicon carbide powder is formed, and the formed body is fired at 1,550 to 1,700 deg.C under an oxidizing atmosphere. Thereby, activated silica formed by the oxidation reaction is reacted with the alumina powder, and primary particles 2, 2a, etc., comprising a mullite crystal are formed, and at the same time, the primary particles 2 and secondary particles 3, 3a, etc., formed by the coagulation of the primary particles 2 are strongly bonded through bonding phases 4 formed from the activated silica.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a porous body made of mullite ceramics, which is useful as a refractory material such as a jig for firing and a member for constructing a firing furnace, and a method for producing the same.

[0002]

_2. Description of the Related Art Conventionally, mullite ceramics (3Al ₂
O ₃ · 2SiO ₂ ) porous bodies are widely used as firing jigs such as sheaths and setters for firing (during firing) ceramic products, hearth plates, fire block constructions, etc. There is. As a method for producing a mullite porous body, alumina (Al ₂ O) containing an alkaline component such as soda having a lower melting point than ceramics as a sintering aid is used.
₃ ) Alumina powder is mixed with powder and silica (SiO ₂ ) powder, and the mixture is molded and then fired at a temperature of about 1600 ° C. There is a general method in which the mullite crystals are sintered together with the silica powder by reacting with the silica powder. However, the firing jig and the like manufactured by this method were used for rapid firing in which the product was fired at a high temperature in a short time because the sintering aid once liquefied was precipitated as a glass phase at the grain boundaries. In this case, the creep resistance and spalling resistance are insufficient and there is a risk of damage, and when used in a jig for firing electronic parts containing highly erodible components such as ferrite, piezoelectric elements, and multilayer capacitors, The highly erosive component contained in electronic parts has a problem that the glass phase is corroded, the mechanical strength is remarkably lowered, and cracks and delamination occur. For products (electronic parts),
Since the chemical components in the product are consumed by the erosion reaction at the time of baking, the chemical composition of the product fluctuates, resulting in deterioration of quality. Therefore, a mixture of ultra-high-purity alumina powder and silica powder that does not contain a sintering aid is molded and then fired at a high temperature of 1800 ° C. or higher, so that a mullite porous material having almost no glass phase at grain boundaries is formed. The technology to gain the body was developed. However, the use of expensive ultra-high purity raw materials and 1800 ° C
Since it must be fired at the above high temperature, not only the cost of raw materials, fuel, etc. will increase, but also a great thermal load will be applied to the firing furnace, which will shorten the service life of the firing furnace. There was a task to be done. In addition, although a method of using high-purity and easily-sinterable alumina powder and silica powder synthesized by the sol-gel method is also partially adopted, it is an inexpensive general popular product because more expensive synthetic raw materials are used. Was not suitable for the production of, and its use was limited to a very limited number of special purposes.

[0003]

DISCLOSURE OF THE INVENTION The present invention provides erosion resistance,
(EN) Provided are a mullite porous body having improved creep resistance and spalling resistance, and a method for producing the mullite porous body simply and inexpensively.

[0004]

SUMMARY OF THE INVENTION In view of the above-mentioned problems, the present invention is directed to molding a compound of alumina powder and silicon carbide powder,
By firing in the range of 1550 ° C. to 1700 ° C. in an oxidizing atmosphere, activated silica produced by the oxidation reaction is reacted with alumina powder to produce primary particles composed of mullite crystals, and the primary particles and the primary particles. The above-mentioned problem is solved by firmly binding the secondary particles aggregated with each other through the binding phase formed of activated silica.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram showing the microstructure of a mullite porous material according to the present invention, and FIG. 2 is a partially enlarged view of FIG. In the porous body 1 according to the present invention, a plurality of mullite crystals (primary particles) 2, 2a ... Are combined to form an aggregate (secondary particle) 3,
3a ..., and innumerable aggregates 3, 3a ... Are three-dimensionally combined to form a mosaic-like microstructure. The mullite crystals 2, 2a ... And the aggregates 3, 3a. Silica is bonded via the bonding phase 4 of the main component. In addition, the secondary particles 3,
The small particles 5 between the primary particles 2 and 2a in 3a.
5a ... are present, and small pores 5, 5a ... Are present between the secondary particles 3, 3a.
Larger atmospheric holes 6, 6a ... Are present. 3 is a SEM (scanning electron microscope) photograph of the mullite porous body 1 of the present invention, and FIG. 4 is an SE of the conventional mullite porous body.
It is an M photograph.

Next, a method for producing the mullite porous material of the present invention will be described. First, alumina (Al ₂ O ₃ ) powder and silicon carbide (SiC) powder are blended, and the blend is molded into a desired shape by an appropriate method such as pressure molding. The alumina powder and the silicon carbide powder do not have to be ultra-high purity with extremely few impurities, and it is not necessary to include a sintering aid having a melting point lower than that of ceramics such as alumina, silicon carbide, and mullite. The ideal mixing ratio of alumina powder and silicon carbide powder is 3: 2 (80:20 by weight), that is, the theoretical molar ratio of alumina to silica in mullite ceramics (3Al ₂ O ₃ .2SiO ₂ ). But
When the blending amount of the silicon carbide powder exceeds the theoretical molar ratio, free silica that deteriorates the spalling resistance of the porous body 1 is generated. Therefore, the blending ratio of the silicon carbide is preferably the theoretical molar ratio or less. Although some silica powder may be added to the mixture, in this case, the total amount of the silicon carbide powder and the silica powder is adjusted so as not to exceed the theoretical molar ratio of silica in the mullite ceramics. Next, the molded body is fired in an oxidizing atmosphere to form the porous body 1. Firing temperature is 1
When the temperature is lower than 550 ° C, unreacted substances are present due to insufficient mullite synthesis reaction, and when the temperature is higher than 1700 ° C, the mullite crystals (primary particles) 2, 2a ... grow excessively and the creep resistance decreases. Therefore, the firing temperature is 1550 ° C to 1
A range of 700 ° C is desirable.

The present invention will be described in more detail based on the following examples. First, powder raw materials of alumina, silicon carbide, fused silica, and silica were mixed in the proportions shown in Table 1, and each compound was wet-milled with a ball mill to give a particle size of 5 to 1 for each compound.
It was adjusted to 0 μ.

[0008]

[Table 1]

Next, an appropriate amount of binder is added to each dried formulation (for example, 4% C per 100 g of dry powder of each formulation).
MC aqueous solution (about 5 g) was added and kneaded, and the kneaded product was pressure-molded at a pressure of 30 MPa. Finally, each molded body was fired under an oxidizing atmosphere under the condition that the maximum temperature shown in Table 1 was maintained for 4 hours to prepare test bodies 1 to 4. The produced crystals were all mullite crystals of test bodies 1 to 4, and the bending strength, apparent specific gravity, bulk specific gravity, porosity, water absorption coefficient, thermal expansion coefficient and thermal conductivity were as shown in Table 1.

Further, the creep resistance, spalling resistance and chemical resistance (acid resistance, alkali resistance) of the test bodies 1 to 4 were evaluated by the following methods. As the evaluation method of the creep resistance, the dimensions of the test bodies 1 to 4 are width × length × thickness = 20 × 18.
Processed to 0 x 4 mm, and test specimens 1 to 4 with fulcrum spacing 15
Horizontally supported by a 0 mm supporting tool, and a weight of 100 g is placed at the center between the fulcrums with a size of 20 × 30 × 50 mm and a weight of 100 g.
In this state, the temperature was raised to 1450 ° C. at a temperature rising rate of 200 ° C./Hr, the maximum temperature was maintained for 24 hours, and then naturally cooled, and the bending amount (mm) of each of the test bodies 1 to 4 was measured with a gap gauge.

As an evaluation method of the spalling resistance, the dimensions of the test bodies 1 to 4 are width × length × thickness = 100 × 100 ×
After processing to 4 mm, the test pieces 1 to 4 were put into a furnace set to a predetermined temperature, held for 30 minutes, and then taken out of the furnace.
After natural cooling, cracks, observe the presence of crack defects, if no defects are found, raise the furnace temperature and perform the same operation as above, repeat this cycle until defects occur, The temperature immediately before the occurrence of defects was taken as the spalling resistance temperature.

As a method for evaluating chemical resistance (acid resistance, alkali resistance), the dimensions of the test pieces 1 to 4 are width × length × thickness =
After processing into 10 × 10 × 4 mm and measuring the dry weight of these test bodies 1 to 4, 10% HCl aqueous solution or 10% N
The entire test bodies 1 to 4 were completely immersed in the aOH aqueous solution, heated to 90 ° C. in this state and kept for 24 hours, and then washed with water,
After drying, the dry weight was measured again, and the weight loss per unit area was calculated by the following formula. Re = (W ₁ −W ₂ ) / A Re: Weight loss per unit area (mg / cm ² ) W ₁ : Dry weight (mg) of test pieces 1 to 4 before immersion in an acid or alkaline aqueous solution W ₂ : Acid Alternatively, the dry weight (mg) of the test bodies 1 to 4 after being immersed in the alkaline aqueous solution A: the surface area (cm ² ) of the test bodies 1 to 4 As a comparison object, a dense body of 99% alumina, and alumina 9
A 5% porous body and a mullite porous body prepared by the conventional method were evaluated by the same method, and the results are shown in Table 2 together with the evaluation results of the test bodies 1 to 4.

[0013]

[Table 2]

As shown in Table 2, the porous body 1 (test pieces 1 to 4) of the present invention has all the creep resistance, the spalling resistance, and the chemical resistance (acid resistance, alkali resistance). Good results were obtained in comparison with the comparative products.

The reason why this result was obtained is considered as follows. In general, regardless of whether it is a dense body or a porous body, the mechanical strength and creep resistance of a ceramic sintered body are proportional to the size of the crystal (primary particles, that is, raw material powder) that constitutes the sintered body. The larger the value, the higher the mechanical strength and creep resistance of the sintered body, but in order to completely sinter a larger raw material powder, it is necessary to increase the diameter of the raw material powder and raise the firing temperature. However, there was a practical limit in the past.

On the other hand, the porous body 1 of the present invention comprises silica activated by being oxidized and activated during firing in an oxidizing atmosphere (silicon, silicon monoxide, and dioxide generated during the process of oxidizing silicon carbide into silicon dioxide). Radicals of silicon having extremely high reactivity due to excess or deficiency of electrons in atoms. Thus, radical-type silicas derived from silicon carbide are referred to as activated silica in the present specification) and alumina. By reacting with the powder to generate mullite crystals, the mullite crystals immediately after generation are also in the active state, and the crystals in the active state (primary particles 2, 2a ...) Are strongly bound to each other by the reaction between the solids. Further, the primary particles 2, 2a ...
3a ... Are formed, and the secondary particles 3, 3a ... Are strongly bonded to each other to form a mosaic microstructure. Further, a very small part of the activated silica is not crystallized, and silica is precipitated at the grain boundary as a binder phase 4 containing silica as a main component.
The bonds between 2a ... And the secondary particles 3, 3a. Therefore, the porous body 1 of the present invention behaves as if the secondary particles 3, 3a ... In which the primary particles 2, 2a ... Are tightly bound are like one large crystal, resulting in extremely high mechanical strength. It is considered to exhibit creep resistance. or,
The binder phase 4 containing silica as a main component has a melting point lower than that of ceramics, as compared with a glass phase made of a sintering aid such as soda.
Highly erosion resistant, not attacked by acid or alkali.

The spalling resistance (R) of the ceramic sintered body is expressed by the following equation. R = (thermal conductivity × bending strength) / (thermal expansion coefficient × elastic modulus) Here, the thermal conductivity and the thermal expansion coefficient are unique constants depending on the type of ceramics. Therefore, in order to improve the spalling resistance of the sintered body, it is necessary to increase the bending strength (mechanical strength) and decrease the elastic modulus.
The elastic modulus is inversely proportional to the pore diameter of the sintered body, and the larger the pore diameter, the smaller the elastic modulus. That is, in the porous body 1 of the present invention, since the primary particles 2, 2a ... And the secondary particles 3, 3a ... Are strongly bonded, the bending strength (mechanical strength) is large, and as described above, Since it has a mosaic-like microstructure and has small pores 5 and 5a existing between the primary particles 2 and 2a between the secondary particles 3 and 3a. , The elastic modulus becomes small. Therefore, since the bending strength (mechanical strength) of the porous body 1 is large and the elastic modulus is small, it is considered that the porous body 1 exhibits excellent spalling resistance.

[0018]

In summary, according to the present invention, since the mullite crystals (primary particles) 2, 2a ... And their aggregates (secondary particles) 3, 3a ... Are bonded through the bonding phase 4 containing silica as a main component, The bond between the particles 2, 2a ... And the secondary particles 3, 3a.
In addition, between the secondary particles 3 and 3a, there are the small pores 5 and 5a existing between the primary particles 2 and 2a, which are larger than the atmospheric pores 6 and 6a. It has spalling resistance and is most suitable for various refractories used for rapid firing. Further, the binder phase 4 containing silica as a main component is not corroded by an acid or an alkali and has an extremely excellent corrosion resistance. Therefore, if the firing jig formed of the porous body 1 of the present invention is used, an electronic component Even if a product containing highly erosive components such as
Not only is it not eroded at all, it can be used repeatedly, and the composition of the product can be stabilized to improve product quality.

Since a mixture of alumina powder and silicon carbide powder is molded and then fired in an oxidizing atmosphere in the range of 1550 ° C. to 1700 ° C., activated silica activated in the oxidation reaction process is formed. By reacting with alumina powder, mullite primary particles 2, 2a ...
The primary particles 2, 2a ... And the secondary particles 3, 3a ... In which the primary particles 2, 2a ... Are agglomerated are firmly bound by the binder phase 4 formed of activated silica, and the extremely excellent creep resistance and the scratch resistance are obtained. The porous body 1 having a poling property can be obtained. Furthermore,
Since relatively inexpensive raw materials are used and firing is carried out in a temperature range that is practically reasonable, raw material costs and fuel costs can be kept low, and since the firing furnace is not burdened and the firing furnace is not damaged, Because running costs can be reduced,
Product cost can be reduced.

Since silica powder is added to the composition, it is possible to further reduce the cost of the product by using a cheaper raw material such as silica stone.

Since the blending amount of the silicon carbide powder or the total blending amount of the silicon carbide powder and the silica powder is set to be equal to or less than the theoretical molar ratio of silica in the mullite ceramics, the spalling resistance of the porous body 1 is adversely affected. The production of free silica can be suppressed, and the durability of the porous body 1 can be further enhanced, which is a great practical effect.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a microstructure of a mullite porous material according to the present invention.

FIG. 2 is a partially enlarged view of FIG.

FIG. 3 is a scanning electron micrograph of a mullite porous body 1 of the present invention.

FIG. 4 is a scanning electron micrograph of a conventional mullite porous body.

[Explanation of symbols]

1 Porous body 2, 2a ... Primary particles 3, 3a ... Secondary particles 4 bonded phase

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 4G019 GA02 GA04                 4G030 AA36 AA37 AA47 BA23 BA25                       BA33 CA01 CA05 CA09 GA11                       GA14 GA22 GA25 GA27 HA01                 4K051 AA07 BE00 BE03

Claims (4)

[Claims] 1. A mullite porous body, characterized in that mullite crystals and aggregates thereof are bonded through a bonding phase containing silica as a main component. 2. A mullite porous body characterized in that it is formed by molding a mixture of alumina powder and silicon carbide powder and then firing the mixture in an oxidizing atmosphere at a temperature in the range of 1550 ° C. to 1700 ° C. Production method. 3. The method for producing a mullite porous body according to claim 2, wherein silica powder is added to the composition. 4. The method according to claim 2, wherein the blending amount of the silicon carbide powder or the total blending amount of the silicon carbide powder and the silica powder is set to be equal to or less than the theoretical molar ratio of silica in the mullite ceramics. A method for producing a mullite porous body.