JPH0585499B2 - - Google Patents

Description

[Detailed description of the invention]

[0001]

TECHNICAL FIELD The present invention relates to a mullite sintered body that can be used at high temperatures and has excellent durability.

[0002]

[Prior art and its problems] Mullite porcelain has been used for a long time as ceramics for physics and chemistry, rollers for roller hearth kilns that perform heat treatment, and the like.

[0003] For example, roller hearth kilns have the advantage of greatly improving thermal efficiency and shortening heat treatment time because the heat capacity of the members required to move the object to be fired is extremely small.
It has become popular in recent years.

[0004] Currently, rollers made of stainless steel, heat-resistant alloys, ceramics, mullite porcelain, alumina, etc. are used as rollers for roller hearth kilns. However, with metal rollers, 1000
℃ or less, ceramic or porcelain rollers can only be used in furnaces with temperatures below 1250℃, and alumina rollers are only used in extremely small test furnaces, and there is a big limit to the heat treatment temperature. Furthermore, these rollers have restrictions on the load that can be applied to each roller.
Because there is a limit to the firing weight per volume, tiles,
It is currently only used for heat treatment of lightweight materials that are fired at low temperatures, such as tableware.

[0005] However, with the future spread of ceramic products, especially so-called advanced ceramics, it is thought that the importance of energy saving and speeding up of heat treatment processes will increase. For this reason, rollers for roller hearth kilns must have high mechanical strength, excellent corrosion resistance against evaporated matter from heat sources and heat-treated materials, high toughness at high temperatures, and resistance to creep. What is desired is a material that satisfies the requirements of resistance to breakage, high thermal shock resistance, excellent durability, and low heat dissipation.

[0006] However, conventional mullite porcelain has not been able to fully meet these demands.

【0007】

[Means for solving the problem] The present inventor
In view of the shortcomings of the conventional technology, as a result of extensive research, we have developed a specific technology that has high mechanical strength, excellent corrosion resistance, has a flexible structure and is resistant to creep rupture at high temperatures, has high thermal shock resistance, and has excellent durability. We discovered that sintered mullite satisfies this requirement.
The present invention was completed based on this knowledge.

[0008] That is, the present invention provides i) Al ₂ O ₃ and SiO ₂
Total amount of 97.0% by weight or more,) CaO 0.2% by weight or less,) Total amount of Na ₂ O and K ₂ O 0.2% by weight or less,
and) _ZrO2 0.05-2.8% by weight, _Al2
O ₃ /SiO ₂ (molar ratio) = 1.30 to 1.85, bulk density 2.96
It pertains to a mullite sintered body mainly composed of mullite crystals with a particle size of g/cm ³ or more.

[0009] The sintered body mainly composed of mullite crystals in the present invention is not limited to a sintered body consisting only of mullite solid solution, but has a small amount of amorphous matrix present in the grain boundaries and alumina crystals (α- Al ₂
It is sufficient that the sintered body contains 10% by volume or less of O ₃ ) of the mullite crystals, and specifically satisfies the following conditions.

[0010] a) Al ₂ O ₃ /SiO ₂ (molar ratio) is 1.30 ~
It is within the range of 1.85. When Al ₂ O ₃ /SiO ₂ is less than 1.30, the amount of mullite crystal phase decreases and SiO ₂
Or, since the amorphous phase increases, high-temperature creep and corrosion resistance decrease, resulting in poor durability. On the other hand, when Al ₂ O ₃ /SiO ₂ exceeds 1.85, the amount of Al ₂ O ₃ crystal phase increases in addition to the mullite crystal phase, resulting in a decrease in high-temperature strength, toughness, and durability, making it unsuitable. It is.

b) The total amount of Al ₂ O ₃ and SiO ₂ is 97.0% by weight or more. Although it is desirable that the total amount of Al ₂ O ₃ and SiO ₂ be large, it is not necessarily economical. In addition, components such as TiO ₂ , Cr ₂ O ₃ and Y ₂ O ₃ cause a slight deterioration in the high-temperature properties of the sintered body, but they have effects such as promoting sintering and improving toughness. Therefore, components other than Al ₂ O ₃ and SiO ₂ containing these components are permissible as long as they are less than 3%. But Al ₂ O ₃
If the total amount of SiO ₂ and SiO 2 is less than 97% by weight, it is not preferable because corrosion resistance to flying evaporates from the heat source or the object to be treated decreases, and high-temperature creep decreases.

[0012] c) The content of CaO is 0.2% by weight or less. If the content of CaO increases, it is thought that a CaO- _Al2O3 - _{SiO2 -} based compound will be formed, which is not preferable because creep rupture increases at high temperatures. For this reason
The content of CaO needs to be 0.2% by weight or less, preferably 0.1% by weight or less, and more preferably 0.05% by weight or less.

[0013] d) The total content of Na ₂ O and K ₂ O is
The content shall be 0.2% by weight or less. If the content of Na ₂ O and K ₂ O increases, Na ₂ O or/and K ₂ O-Al ₂ O ₃ -SiO _2- based compounds will be formed, resulting in a decrease in quality and life. 0.2% by weight or less, preferably 0.1% by weight
The following shall apply.

[0014] e) The content of ZrO ₂ is 0.05 to 2.8% by weight. ZrO ₂ has the effect of promoting sintering of mullite sintered bodies, and by containing a certain amount of mullite,
Sinterability is improved, and it is possible to prevent a decrease in corrosion resistance and mechanical strength that occurs when the bulk density is low. This sintering promoting effect is presumed to be due to ZrO ₂ being present at the grain boundaries of mullite crystals during the sintering process and promoting mutual diffusion of mullite at the grain boundaries. At the same time,
The presence of ZrO ₂ has the effect of suppressing the segregation of the amorphous matrix that occurs at the mullite grain boundaries, and contributes to improving the toughness and creep properties of the mullite sintered body at high temperatures. However, on the other hand,
ZrO ₂ also has drawbacks such as reduced durability due to contamination by flying vapors from the heat source or the material to be heat treated at high temperatures, so it is not preferable to have too much ZrO ₂ .

[0015] If the content of ZrO ₂ is less than 0.05% by weight, the above-mentioned effects will be reduced, and if it exceeds 2.8% by weight, corrosion resistance, durability, etc. will be greatly reduced, which is not preferable. More preferably, the content of _ZrO2 is 0.1
~2.5% by weight.

[0016] f) The bulk density is 2.96 g/cm ³ or more. If the bulk density is less than 2.96 g/cm ³ , the corrosion resistance and mechanical strength will decrease, so it is necessary to set the bulk density to 2.96 g/cm ³ or more.
Preferably 3.00g/ ^cm3 or more, more preferably 3.05
g/ ^cm3 or more.

[0017] The mullite sintered body of the present invention is produced, for example, as follows.

[0018] As a starting material, Al ₂ O ₃ /SiO ₂ =
1.30-1.85, _Na2O , _K2O , CaO and _ZrO2
It is desirable to use a raw material that contains only a specified amount of the active ingredient or that only a specified amount remains after firing, and to use a powder in which the active ingredients are uniformly distributed. Also,
ZrO ₂ may be mixed as abrasion powder using ZrO ₂ media during pulverization. These are not limited to those in which mullite crystals are formed in advance, and may be a mixture of alumina and silica or a compound of alumina and/or silicate in which mullite crystals are formed in the firing process. The main starting materials that can be used in the present invention include, for example, a mixture of easily sinterable low soda alumina with a specific surface area of 2 to 20 m ² /g and high-purity silica or quartz, and this mixture is sintered at 1300 to 1450°C. Examples include raw materials prepared by synthesizing mullite, and raw materials prepared from Al and Si compound solutions by coprecipitation methods, roasting methods, etc.

[0019] In the present invention, first, the raw material has an average particle size (measured by sedimentation method or light transmission method based on Stokes' law) of usually 2 μm or less, preferably 1.5 μm.
The particles are ground and dispersed to a particle size of 0.5 to 1.0 μm or less, preferably 0.5 to 1.0 μm. Specific surface area 2 by BET method
It is preferable to set it as 15 m ^<2> /g. It is efficient to carry out this grinding and dispersion process in a wet manner.
Ball milling may be performed using a grinding media made of agate, alumina, zirconia, etc. using a mill lined with rubber, resin, alumina, etc. As a crusher, a ball mill, a vibration mill, an attrition mill, a centrifugal mill, etc. can be used, and crushing and dispersion can be performed simultaneously. When using fine powder raw materials of about 2 μm or less, the pulverization step can be omitted, but the raw materials must be sufficiently dispersed.
It is necessary to use a raw material whose components are distributed as uniformly as possible.

[0020] The raw material that has been pulverized and/or dispersed is then molded into a desired shape in a molding step. As the molding method, methods used for ordinary ceramic products can be applied, and for example, the following method is used for molding. In CIP (hydrostatic pressing), the above raw materials
A granulated powder (mainly by spray drying method) to which a few percent of a molding aid such as PVA is added is prepared and molded. The molding pressure is preferably about 0.5 to 2 tons/cm ² . In the case of extrusion molding, a pasty molding aid with high caking properties such as methylcellulose, dextrin, and PVA is kneaded with the raw material to be processed, and then molded using an extruder. In the case of the cast molding method, the raw material to be treated is in the form of a slurry, and the viscosity is 30~30.
It is adjusted to 3000 cp, molded using a mold such as a plaster mold, and dried after molding to form a molded body. These molded bodies are made into shapes with additional dimensions larger than desired dimensions based on firing shrinkage in the firing process.

[0021] Next, the molded body is heated to 1500 ~
It is fired at a temperature of 1750℃. The firing temperature depends on the type of raw material, the components of the molded body, the firing activity of the molded body, etc., but in the case of pressureless sintering, it is preferably 1550 to 1750°C, more preferably 1550 to 1700°C.
℃. If the sintering temperature is lower than 1500°C, the desired density cannot be obtained, and if the sintering temperature is higher than 1750°C, it is not economical and the crystal grain size becomes too coarse, leading to a decrease in strength and density. This is undesirable because deformation tends to occur during the firing process.

[0022] When obtaining a roller for a roller hearth kiln using the mullite sintered body of the present invention, its dimensions depend on the width of the kiln, the shape of the material to be heat treated, the weight per area, the operating temperature, etc. It depends, but the outer diameter is 15~50mm, outer diameter/inner diameter = 1.2~
1.5, length/outer diameter = about 25 to 80 is preferable. Although the inner surface is not necessarily limited to being circular, it is desirable that the outer surface be a perfect circle with an accuracy within ±0.5 mm.

[0023]

[Effects of the Invention] The mullite sintered body of the present invention has the following excellent properties. 1 Generally considered to be high-temperature, high-strength ceramics.
Compared to Si ₃ N ₄ , SiC, etc., it is stable in a high temperature open atmosphere. 2 Low thermal conductivity, less heat dissipation, economical, and high thermal efficiency. 3.It exhibits high durability against contamination by flying vapors from heat sources and heat-treated materials at high temperatures, and can be used for long periods of time. 4. Has thermal shock resistance that can withstand rapid temperature changes. It has a lower elastic modulus than ceramics such as 5Si ₃ N ₄ , SiC, and Al ₂ O ₃ and deforms appropriately, especially at high temperatures.
It is a flexible structure that is less susceptible to stress concentration. 6 Mullite crystals are acicular with a high aspect ratio, forming a network structure, having high toughness and being resistant to creep rupture.

[0024] The mullite sintered body of the present invention has the above-mentioned excellent properties, has excellent durability, and can withstand high loads at high temperatures, so it can be used as a heat-resistant member, especially,
It is useful as a roller for roller hearth kilns, etc.

【0025】

[Examples] The present invention will be explained in more detail based on Examples below.

【0026】

[Example 1] Average particle size 0.7 μm and specific surface area 8 m ² /
g 99.95% _Al2O3 and 99.9% _SiO2 with average _particle size 1.5μm
and 3 kg of Al ₂ O ₃ /SiO ₂ = 1.34.
Raw materials synthesized into mullite by firing at 1400℃,
Add 1% CMC, wet-mill and disperse for 48 hours using a 91% Al ₂ O ₃ ball mill and reinforced zirconia grinding media to obtain an average particle size of 0.7 μm, and then spray dry to form a molding powder. Obtained 2.5Kg.

[0027] This powder was formed into a pipe shape by CIP at a pressure of 1.5 tons/cm ² and fired at 1680°C for 3 hours.

【0028】

[Example 2] Using the same raw materials as in Example 1, Al ₂
After blending 3 kg so that O ₃ /SiO ₂ =1.47 and further adding 0.14% of precipitated calcium carbonate in terms of CaO, the mixture was calcined at 1650° C. for 3 hours in the same manner as in Example 1.

[0029]

[Example 3] Using the same raw materials as in Example 1, Al ₂
After blending 3 kg so that O ₃ /SiO ₂ = 1.62 and adding 2.5% by weight of 99.9% ZrO ₂ fine powder with a specific surface area of 10 m ² /g and an average particle size of 0.4 μm, a ball mill made of reinforced zirconia and a grinding medium were used. The powder was wet-pulverized and dispersed for 48 hours to give an average particle size of 0.55 μm. The subsequent steps were the same as in Example 1, and firing was performed at 1630°C for 5 hours.

【0030】

[Example 4] Al ₂ O ₃ /SiO ₂ = 1.80, Al ₂ O ₃
Electrofused mullite fine powder 30 with a total amount of _SiO2 99.2% by weight
Weight% and Al ₂ O ₃ and SiO _{2 of Example} 1
After adding 1.5% by weight of 99.0% ZrO ₂ fine powder with an average particle size of 0.7 μm to the raw material consisting of 70% by weight of SiO ₂ = 1.55, it was wet-processed in a resin mill.
Using 99.5% Al ₂ O ₃ media, by vibration,
The mixture was ground and dispersed to an average particle size of 1.1 μm in 12 hours.

[0031] A small amount of surfactant was added to this slurry, the viscosity was adjusted to 250 cp, and a pressure of 2 Kg/cm ² was applied to cast it into a plaster mold, and after drying, it was baked at 1650° C. for 3 hours.

[0032]

[Example 5] Al− with Al ₂ O ₃ /SiO ₂ =1.80 synthesized and calcined from a solution of Al and Si compounds by thermal decomposition method
Si spinel raw material powder with 0.3% Y ₂ O ₃ and feldspar [main components (Na, K) ₂ O・Al ₂ O ₃・6SiO ₂ ] was added to Na ₂ O + K ₂
Addition and blending so that O is 0.1% by weight, and the average particle size
After adding 0.5 wt% of 0.6 μm 99.9% _ZrO2 fine powder,
Grinded to an average particle size of 0.6 μm in the same manner as in Example 3,
It was treated and molded in the same manner and fired at 1590°C for 6 hours.

【0033】

[Example 6] Precipitated calcium carbonate was added to a raw material consisting of 60% by weight of powder obtained by mixing 40% by weight of mullite and 99.9% alumina SiO ₂ in Example 4 so that Al ₂ O ₃ /SiO ₂ = 1.80 was mixed with precipitated calcium carbonate in terms of CaO. 0.05% by weight and 2.0% by weight in terms of ZrO ₂ of zirconium hydroxide were added, and the mixture was fired in the same manner as in Example 4 at a firing temperature of 1720°C for 2 hours.

【0034】

[Example 7] A mullite powder of Al ₂ O ₃ /SiO ₂ = 1.47 synthesized and calcined from a solution of Al and Si compounds by a thermal decomposition method was used.
A fired body was produced in the same manner as in Example 5 by adding 1.0% by weight of ZrO ₂ fine powder.

【0035】

[Comparative example 1] Al ₂ O ₃ /SiO ₂ = 1.18, grinding time
A fired body was produced in the same manner as in Example 1, except that the heating time was 15 hours.

【0036】

[Comparative Example 2] Mullite of Example 4, 99.6% alumina powder, kaolin (main component Al ₂ O ₃・SiO ₂・2H ₂
O) and feldspar to give Al ₂ O ₃ /SiO ₂ = 1.27, and wet-pulverized for 48 hours using a 91% Al ₂ O ₃ ball mill and crushing media, then dehydrated and dried. After adding a thickener and kneading to form a clay and extrusion molding,
It was baked at ℃ for 3 hours.

【0037】

[Comparative Example 3] ZrO ₂ 1.5% by weight, MgO 1.3% by weight
The powder was pulverized in the same manner as in Example 1, except that 0.3% by weight of Fe ₂ O ₃ was added, and then molded and fired in the same manner as in Comparative Example 2.

【0038】

[Comparative Example 4] Al ₂ O ₃ /SiO ₂ = 1.47, feldspar
A fired body was produced in the same manner as in Example 1, except that the total amount of Na ₂ O and K ₂ O was 0.24% by weight.

【0039】

[Comparative Example 5] Al ₂ O ₃ /SiO ₂ = by changing the blending ratio
1.78, and a fired body was produced in the same manner as in Comparative Example 2 except that the amount of ZrO ₂ added was 3.5% by weight.

【0040】

[Comparative Example 6] 97% mullite used in Example 4
The powder was ground for 120 hours using an Al ₂ O ₃ mill and grinding media, and thereafter a fired body was produced in the same manner as in Example 1.

[0041]

[Test Example] Accelerated life tests and durability tests were conducted on the pipe-shaped fired bodies produced in the above Examples and Comparative Examples. The composition and bulk density of the fired body are shown in Table 1, and the test results are shown in Table 2. The test method is as shown below. ○ Accelerated life test: A pipe with an outer diameter of 21 mm, an inner diameter of 16 mm, and a length of 450 mm is used as a sample. Support points are set at a distance of 300 mm between the supports, and the temperature inside the furnace is maintained at 1400°C or 1460°C, with both ends located outside the furnace. A sample was set up so that the pipe would break, a load was applied to the center of the pipe, and the time required for the pipe to break was determined. Note that the pipe was rotated half a rotation every 5 minutes.
Table 2 shows the load in terms of stress per unit area, which is based on the following calculation formula.

[0042] Stress per unit area (Kgf/cm ² )=
8wl/(D ⁴ − d ⁴ )π/D W: load, l: span length, D: outer diameter, d: inner diameter This test condition is based on the load applied to one roller in a practical kiln, which is 3 to 6 It is estimated that the load will be twice as long, and the life of a practical kiln is expected to be more than 10 ⁴ times longer. Therefore, if the lifespan is more than 100 minutes,
It is believed that it enables use at higher temperatures or higher loads. ○Durability test: Inside of a heavy oil-burning ceramics firing furnace during operation
A pipe sample with an outer diameter of 21 mm, an inner diameter of 16 mm, and a length of 150 mm was inserted into an atmosphere at 1500°C, held for one month, then taken out indoors, and its bending strength was measured based on the previous formula (span 100 mm) and tested. The intensity is expressed as a percentage of the previous intensity.

【0043】

[Table 1] ■■■ Turtle shell [0027] ■■■

【0044】

[Table 2] ■■■ Turtle shell [0028] ■■■ From Table 2, rollers for roller hearth kilns using the mullite sintered body of the present invention can withstand use at high temperatures and high loads, and even after long-term use. It is clear that it has high strength.

Claims (1)

[Claims] [Claim 1] i Total amount of Al ₂ O ₃ and SiO ₂ 97.0
% by weight or more, CaO 0.2% by weight or less, total amount of Na ₂ O and K ₂ O 0.2% by weight or less, and ZrO ₂ 0.05-2.8% by weight, Al ₂ O ₃ /SiO ₂ (molar ratio) = 1.30- 1.85, a mullite sintered body mainly composed of mullite crystals with a bulk density of 2.96 g/cm ³ or more.