JP2549976B2 - Heat-resistant mullite sintered body - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-resistant mullite sintered body having excellent durability at high temperatures.

[0002]

BACKGROUND OF THE INVENTION Mullite ceramics have been used as porcelain for science and chemistry, refractories, etc. for a long time. It has come to the spotlight as a structural material.

[0003] Conventional mullite ceramics are produced by reaction firing from natural raw materials such as kaolin and alumina. In this case, however, impurities such as alkali metals and alkaline earth metals are considerably present in the raw materials, and mullite is also used. Since silica is used in excess of the composition of No. 1, a large amount of glass phase is formed in the sintered body, and its thermal property is 1
It was inferior at 300 ° C or higher.

Recently, a mullite-based sintered body having a small glass phase has been produced by using a mullite raw material powder synthesized so that the Al ₂ O ₃ / SiO ₂ ratio becomes a mullite composition ratio (for example, , JP-A-61-286264), a sintered body excellent in high temperature strength is obtained as compared with the conventional mullite sintered body. However, this new mullite sintered body also has insufficient durability at high temperatures from a practical viewpoint, and as a heat resistant structural material, improvement in heat resistance at high temperatures of about 1500 ° C. or higher is demanded. ing.

[0005]

In view of the problems of the prior art as described above, the present inventor has earnestly studied to obtain a heat-resistant mullite sintered body having excellent durability at high temperature. As a result, mainly the mullite crystals obtained by adjusting the Al ₂ O ₃ / SiO ₂ ratio within a certain range and appropriately adjusting the density of the sintered body, the crystal grain size of the mullite crystal, and the bending strain amount of the sintered body. The sintered body made of is excellent in heat resistance, thermal shock resistance, durability and the like.
It has been found that a mullite fired body having particularly excellent durability can be obtained when rO ₂ is not substantially present, and the present invention has been completed here.

That is, the present invention provides the following heat-resistant mullite sintered body.

1. (1) Al ₂ O ₃ / SiO ₂ (weight ratio) is 71 / 29-
78/22 and (2) average crystal grain size of 1 to 50 μm
And (3) the relative density to the theoretical density is 85 to 94.
%, (4) 1600 ° C., load 10 kgf / cm ²
The amount of bending strain in 5% or less per hour,
(5) A heat-resistant mullite sintered body, which is mainly composed of mullite crystals.

2. _2. The heat-resistant mullite sintered body according to item 1, which is substantially free of ZrO ₂ .

The requirements to be satisfied by the heat-resistant mullite sintered body of the present invention will be described in detail below.

(A) The component ratio of Al ₂ O ₃ and SiO ₂ is Al ₂ O ₃ / SiO ₂ (weight ratio) = 71/29 to 7
Within the range of 8/22.

When the ratio of Al ₂ O ₃ and SiO ₂ is within this range, the durability of the sintered body becomes particularly high. SiO ₂
However, if it exceeds this ratio, SiO ₂ or an amorphous phase is likely to be generated in the sintering step, and the strength at room temperature is improved, but the durability is lowered, which is not preferable. On the other hand, Al ₂
When O ₃ becomes too much, a large amount of alumina precipitates,
Further, the mullite crystals change from acicular to granular and become crystals with a small aspect ratio, and the thermal shock resistance, durability, etc. are reduced. Al ₂ O ₃ / SiO ₂ ratio is 72/28 to 77/2
It is preferably 3 and more preferably 73/27 to 76/24.

(B) The average particle size of the mullite crystals in the sintered body is 1 to 50 μm.

When the average grain size of the mullite crystals is less than 1 μm, the durability of the sintered body is not improved and the average grain size is 5
If it exceeds 0 μm, the strength decreases, which is not preferable. The average particle size of the mullite crystals is preferably 2 to 30 μm. The particle size distribution of mullite crystals is 0.5 μm.
The following crystals are preferably 30% by volume or less, and such a sintered body can exhibit desired excellent performance. When many crystals having a grain size of 0.5 μm or less are continuously present, creep is likely to occur from this portion at high temperature, which is not preferable.

(C) The relative density to the theoretical density is 85 to
The range is 94%.

When the relative density is 85 to 94%, 6 to 15% of vacancies are present in the mullite sintered body, and these vacancies inhibit grain boundary diffusion and suppress plastic deformation. Therefore, durability can be improved. Further, the decrease in mechanical strength due to the holes is relatively small. Voids are preferably present mainly at the grain boundaries. The conventional heat-resistant mullite sintered body has been sintered to have a relative density of 95% or more to improve mechanical and thermal characteristics. In this case, heat resistance and durability at high temperature are improved. In order to densify the material which is not sufficient and is difficult to sinter, it is necessary to use a fine raw material powder or to fire at a higher temperature, which causes a problem of high cost. Also, the relative density is 85
If it is less than%, the mechanical properties, corrosion resistance, etc. are inferior. The relative density is preferably 88 to 94%.

(D) 1600 ° C., load 10 kgf / cm
The bending strain amount in ² is 5% or less per hour.

In the present invention, the bending strain amount is 1600 ° C.
A load of 10 kgf / cm ² was applied to the sample in the furnace held at, and after a steady strain was obtained in 4-point bending, measurement was performed for 5 hours, and the average value per hour was calculated by the following formula.

Strain amount (%) = 6tδ ² ÷ {(L + 2m)
(Lm)} × 100 L: Length of lower span, m: Length of upper span, t: Length of sample, δ: Deformation amount According to the study of the present inventors, high temperature of mullite sintered body The properties cannot be estimated from the behavior at low temperatures, and in order to have good heat resistance, thermal shock resistance, durability, etc. at high temperatures, the mullite sintered body at 1600 ° C. obtained by the above method should be used. It has been found that the bending strain amount needs to be 5% or less, and if it exceeds 5%, the life is shortened when used for a long time at a high temperature of about 1500 ° C. or higher. this is,
It is considered that the mullite sintered body causes viscous flow at around 1550 ° C. and exhibits different behavior at around 1600 ° C. even though the properties at 1500 ° C. or less are the same. The bending strain amount is preferably 3% or less.

(E) The sintered body is mainly composed of mullite crystals.

The sintered body needs to consist mainly of mullite crystals, and it is preferable that the mullite crystals are present in the sintered body in an amount of about 70% by volume or more. Alumina phase is 1
It may be contained up to about 5% by volume, and this does not significantly impair the physical properties of the sintered body. In addition, SiC,
Even if Cr ₂ O ₃ or the like is contained up to about 40% by weight, the physical properties of the sintered body are not adversely affected and the toughness and strength can be improved. It is preferable that the content of each of the alkali metal oxide and CaO is 0.1% by weight or less. The glass phase of the sintered body is preferably about 5% by volume or less.

The mullite sintered body of the present invention is ZrO 2.
₂ is preferably substantially free of. As a conventional method for producing a mullite sintered body, a method in which a trace amount of ZrO ₂ is mixed is generally adopted.
When O ₂ is present, ZrO ₂ existing at the grain boundaries of the sintered body at a high temperature has the effect of promoting the sintering of mullite, but at the temperature of 1500 ° C. or higher, it affects the plastic deformation of mullite, It has been found that it deteriorates high temperature properties such as high temperature strength and creep resistance. Therefore, ZrO ₂ is preferably mixed in an amount of force majeure of less than about 0.1% by weight, more preferably less than about 0.05% by weight, and not more than the analytical limit in the chemical analysis by the usual gravimetric method. Is most desirable.

To produce the mullite sintered body of the present invention,
A powder raw material in which the ratio of Al ₂ O ₃ and SiO ₂ is within a predetermined range, and the main component consisting of Al ₂ O ₃ and SiO ₂ is previously mullitized by calcination is molded by an ordinary method, Baking is desirable. Such powder raw material can be prepared by various known methods using a liquid raw material, and the manufacturing method is not particularly limited. In addition, Al and Si
It may be synthesized by a solid phase method or a melting method from a compound containing, and further, a sintered body may be obtained from these raw materials by direct reaction sintering.

In the case of the coprecipitation method or the sol-gel method, which is the most general method for obtaining a mullite powder raw material, for example, A corresponding to the Al / Si ratio in the mullite compact sintered body is used.
A liquid raw material containing a 1-compound and a Si compound is prepared, and each compound is coprecipitated or gelled and then dried to obtain a powder. Next, the mullite generation temperature of 900 to 1500
After calcination at ℃, crushed, preferably with an average particle size of 5μm
The following powder raw material for sintering may be used. Coarse mullite particles having a particle size of about 50 μm or less may be added to the powder thus obtained.

The raw material powder produced as described above is
The sintered body of the present invention is formed by a conventional method such as CIP, mechanical press, extrusion, or cast molding, and is fired at about 1600 to 1800 ° C.

In the production of the sintered body of the present invention, Zr
O ₂ , alkali metal oxide, alkaline earth metal oxide,
If a large amount of Fe ₂ O ₃ , TiO ₂ or the like is mixed, the bending strain amount of the sintered body tends to be large. Therefore, in order to obtain the above-mentioned predetermined bending strain amount, it is necessary to adopt a material or process that does not mix these as much as possible. Is preferred.

[0026]

EFFECTS OF THE INVENTION The mullite sintered body of the present invention is excellent in high temperature durability, heat resistance, thermal shock resistance, high temperature strength, thermal stability, and the like, and has excellent characteristics even at high temperatures exceeding 1500 ° C. Therefore, it is highly useful as a roller for high temperature, a belt, a member for heat treatment such as a container, a jig for measuring high temperature characteristics, a core tube and the like.

[0027]

EXAMPLES The present invention will be described in more detail with reference to the following examples.

Example 1 A powder was produced by a sol-gel method from a liquid raw material containing an Al compound and a Si compound whose weight ratios as Al ₂ O ₃ and SiO ₂ were the ratios shown in Table 1, and dried. After that, it is calcined at 1380 ° C for 2 hours to synthesize mullite,
It was ground to an average particle size of 1.5 μm. To this, a water-soluble acrylic polymer was added as a binder, and a spray dryer was used to obtain granules for molding having a size of about 60 μm. Using this, a mullite sintered body was obtained by molding at a molding pressure of 1.5 ton / cm ² by the CIP method and firing at the temperature shown in Table 1 for 3 hours. The measurement of the amount of strain is as follows: width 5 mm, thickness 2 mm, length 14
Using a 0 mm sample, L = 10 in the above calculation formula
It was performed by applying a load of 10 kgf / cm ² at cm and m = 3 cm. The characteristics of these sintered bodies are shown in Table 1.

[0029]

[Table 1]

Example 2 80 parts by weight of the mullite crushed powder used in Sample No. 1 in Table 1 of Example 1 had an Al ₂ O ₃ / SiO ₂ ratio of 76/24 and had an average particle size of 40 μm. After adding 20 parts by weight of mullite particles, further pulverize and disperse for 12 hours, and after drying, add water and a binder to prepare a kneaded material, forming pressure 50 k
It was extruded at g / cm ² and fired at the temperature shown in Table 2 for 3 hours to obtain a mullite sintered body. Table 2 shows the characteristics of these sintered bodies.

[0031]

[Table 2]

Example 3 A powder was produced by a sol-gel method from a liquid raw material containing an Al compound and a Si compound in a weight ratio of Al ₂ O ₃ and SiO ₂ shown in Table 3, and dried. Then, it was calcined at 1380 ° C. for 2 hours to synthesize mullite. Then, except for sample No. 12, the amount of Zr shown in Table 3
O ₂ was added and about 60 μm of granules for molding were obtained by a spray dryer. Using this, molding was carried out at a molding pressure of 1.5 ton / cm ² by the CIP method, and at a temperature shown in Table 3
It was fired for a time to obtain a mullite sintered body. Table 3 shows the characteristics of these sintered bodies.

[0033]

[Table 3]

Test Example 1 An outer diameter of 30 mm and an inner diameter of 24 were obtained using the samples of the above-described examples.
Manufacture a tube with a length of 1 mm and a temperature of 1600
℃, put in an electric furnace with an effective width of 40 cm in the furnace, while rotating at 2 rpm, apply a load of 2 kg to the tube,
After running for an hour, the bending amount of the tube was measured. When the tube bent 10 mm on the way, the tube was stopped at that time and the time was measured.

After the completion of the measurement, the tube was quickly pulled out from the furnace and placed on a brick at room temperature to examine whether it would be damaged by thermal shock. The results are shown in Table 4.

[0036]

[Table 4]

From the above results, sample numbers 1 to 5, 10
The mullite sintered body of the present invention of No. 13 has heat resistance at high temperature,
It was found that the durability was excellent and the thermal shock resistance was also good.

[0038]

Claims (2)

(57) [Claims] (1) Al ₂ O ₃ / SiO ₂ (weight ratio) is 71/29 to 78/22, (2) average crystal grain size is 1 to 50 μm, and (3) relative to the theoretical density. Relative density is 85-94%, (4) 1600 ° C, load 10k
(5) A heat-resistant mullite sintered body, which has a bending strain amount of 5% or less per hour in gf / cm ² and is mainly composed of mullite crystals. _2. The heat-resistant mullite sintered body according to claim 1, which is substantially free of ZrO ₂ .