JP3013999B2 - Corrosion-resistant aluminum nitride sintered compact for molten aluminum and method for producing the same - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a corrosion-resistant aluminum nitride sintered compact which is used as a member of a device for handling molten aluminum and aluminum alloy (hereinafter, simply referred to as “molten metal treatment device”). It is.

2. Description of the Related Art A molten metal processing apparatus includes a protective tube for an immersion heater, a gas injection tube for degassing, a molten metal stirring member, a mold for continuous casting, a crucible, a melting furnace and a holding furnace, and the like. In recent years, ceramic sintered compacts have been applied.

This type of ceramic sintered compact is molded in a desired shape and sintered, and a sintered block arranged in a desired shape and sealed is used in various forms together with various accessory members. In contact with the high-temperature aluminum melt.

In that case, it is necessary to have various properties corresponding to the purpose of use, but it is also necessary to have corrosion resistance that includes the seizure of the molten metal and the pollution of the molten metal in common with the molten aluminum. You.

For example, when the molten metal is baked on the furnace wall in a smelting furnace or the like, if another type of aluminum alloy is to be smelted, the baked metal causes contamination of the newly poured molten metal. In addition, accurate temperature data cannot be obtained if the thermometer is seized on the protective tube. Therefore, if machining is performed to remove seizure, cracks may occur at that time, which may cause damage. In addition, when reacting with the molten metal, not only its surface is corroded, but also corrosion products are eluted, which causes contamination of the molten metal. As is clear from these, when the ceramic sintered compact does not have corrosion resistance, the maintenance work of the apparatus becomes complicated, the life of the apparatus is shortened, and the molten metal is contaminated by a reaction product with the molten aluminum. Inconvenience will occur.

Generally, ceramic sintered compacts have been converted from those based on natural materials to those based on synthetic ceramics. For example, silicon nitride-based materials, silicon carbide-based materials, or composite materials obtained by adding boron nitride or aluminum nitride to the above materials have been proposed, and can be found in JP-A-61-205671. Further, as a method for improving these, a method in which an aluminum nitride layer is formed on a surface layer in contact with a molten metal is proposed in Japanese Patent Application Laid-Open No. 63-238950.

Furthermore, Japanese Patent Application Laid-Open No. 63-288966 and the like have proposed aluminum nitride products for molten copper metal.

2. Problems of the Related Art As described above, attempts have been made to apply an aluminum nitride sintered body to molten aluminum, but deterioration with time has been observed, and contamination of the high-purity molten aluminum has occurred.

In addition, when the cause was examined, it was found that by improving the wettability and the reactivity with the molten aluminum, deterioration over time could be prevented and the corrosion resistance could be improved.

Then, when the contact angle of the aluminum nitride sintered body with the molten aluminum was examined, it was found that the corrosion resistance could be improved if the contact angle was 110 degrees or more, but it was found that there was variation depending on the manufacturing conditions.

In other words, a method for producing an aluminum nitride sintered body includes a method of adding a sintering aid to aluminum nitride powder and sintering in a nitrogen atmosphere, or a method of sintering under pressure by a hot press without using a sintering aid. Hot press sintering is widely used.

In these sintered bodies, for example, a grain boundary phase of a compound of yttrium (Y) -aluminum (Al) -oxygen (O) or a compound such as sialon or aluminum nitride polytype exists, or, for example, yttrium It has been found that there is a surface layer such as a mixture of yttrium nitride and aluminum nitride due to migration of the compound to the surface and reduction nitridation.

If these are present, they react with the molten aluminum and cause the contamination of the molten metal eluted in the molten metal, or an aluminum oxide layer having a higher wettability than aluminum nitride is formed, and the molten aluminum is easily attached or seized. Was. Therefore, it has been found that a solution to these problems can stably improve the corrosion resistance of aluminum nitride.

The present invention provides a corrosion-resistant aluminum nitride sintered compact for molten aluminum, comprising a sintered aluminum nitride having a contact angle to molten aluminum of 110 ° or more and having at least a surface layer without a grain boundary phase and a surface layer. It is.

Here, the contact angle for molten aluminum was determined by Naoto Yoshimi et al., Journal of the Japan Institute of Metals, Vol. 52, No. 12, (1988), p. 1179-
The measurement was carried out according to the measurement method described in the article of "Wetting of molten A on MgO surface" published on page 1186.

That is, a sample cut from an aluminum nitride sintered body (30
mmφ) surface was polished with silicon carbide water-resistant abrasive paper, mirror-finished with _a surface roughness R _a <0.3 μm using _a diamond buff, ultrasonically cleaned in acetone, and mounted in a measuring furnace. .

In order to make the measurement atmosphere 97 vol% of helium and 3 vol% of hydrogen, oxygen and water contained in the gas are removed by passing through a sponge titanium furnace heated to 800 ° C. and a liquid nitrogen trap to obtain a measurement gas.

The aluminum melt used for the measurement is prepared by melting 70 mg of pure 99.99% pure aluminum pieces in a high-purity alumina tube that has the same atmosphere as the measurement atmosphere, removing oxides on the surface with an aqueous solution of hydrogen fluoride, and maintaining the temperature at 1100 ° C. And placed in a vacuum furnace.

Then, the inside of the measuring furnace was evacuated to about 10 ^-5 Torr,
After baking for one minute, the above-described measurement gas was introduced so that the furnace pressure became 1.15 atm. After the temperature of the aluminum nitride sintered body of the sample reached 1100 ° C, molten aluminum was dropped from the 1 mmφ hole formed at the tip of the above-mentioned alumina tube, and the aluminum nitride sintered body was placed on the aluminum nitride sintered body with a 35 mm camera with a macro telephoto lens. The droplet was photographed.

The contact angle was measured by projecting the photographed film to a magnification of 50 times with a universal projector equipped with a rotation angle scale.

When the contact angle was 90 degrees or more, the contact angle was calculated using the table of Bashforth and Adams, and when the contact angle was 90 degrees or less, it was read directly.

If the contact angle with the molten aluminum by the above measurement method is 110 degrees or more, preferably 120 to 130 degrees, the corrosion resistance to the molten aluminum is at a practical level and the seizure resistance to the molten metal is also good. But the contact angle is
If the temperature is less than 110 degrees, the contamination of the molten metal due to the elution of the cation impurities and the roughening of the aluminum nitride surface due to the reaction between the surface layer and the molten aluminum occur, and it is not possible to appropriately prevent the bite and seizure of the molten metal.

Furthermore, the surface of the aluminum nitride sintered compact, particularly at least the surface layer which is the contact surface with the molten metal, has a grain boundary phase and a surface layer,
Preferably, at least a depth of 100 μm or more from the surface does not exist, thereby preventing deterioration of corrosion resistance with time. In this case, the surface layer may be directly measured. However, as a simple method, if the oxygen concentration in the surface layer is 0.05% by weight or less and the total cationic impurity concentration is 0.3% by weight or less, the “grain boundary” in the present invention is used. It can be determined that "the state where the phase and the surface layer are substantially absent".

When firing in a nitriding gas reducing atmosphere in which a carbon gas is present as described later, the concentration gradient in which oxygen and all cationic impurities are concentrated on the surface layer side because the grain boundary phase diffuses from the center to the surface side. Occurs. In that case, when there is no surface layer and a grain boundary phase is present, the carbon concentration is often 0.2% by weight or more and the metal ion impurity concentration is often 0.5% by weight or more. When there is no grain boundary phase and a surface layer exists, the oxygen concentration is often 0.05% by weight or more and the total cation impurity concentration is often 0.5% by weight or more. Depending on the firing conditions, there may be intermediate states between these.

The following means can be used as a specific means for removing and excluding the grain boundary phase and the surface layer.

That is, the purity and the amount of the raw aluminum nitride powder and the sintering aid are adjusted so that the oxygen content in the sintered aluminum nitride sintered compact is 0.01 to 0.1% by weight, The total adjustment is made in consideration of the sintering conditions so that the compound element to be used (including the compound element and expressed by the amount of the element) is 0.001 to 0.3% by weight.

In this case, "oxygen" as the oxygen content includes those derived from the oxygen component contained as an impurity of the aluminum nitride powder itself and the oxygen component mixed in from the sintering aid. The cationic impurities include those derived from metal impurities contained as impurities in the aluminum nitride powder itself and metal impurities caused by the metal compound added as a sintering aid.

Therefore, it can be easily dealt with by limiting the specification of the sintering aid, but if the sintering aid is insufficient, sintering does not occur sufficiently or oxygen is dissolved in the aluminum nitride crystal and aluminum nitride polytype Etc. are generated. In addition, when the amount of addition cannot be reduced below a certain level due to the effect of use, volatilization and removal can be performed by setting the firing time to a sufficiently long time. Alternatively, when the oxygen content in the raw material aluminum nitride powder is large, by increasing the amount of the sintering aid and performing firing for a long time,
Mutual components can be reduced.

As the sintering aid, a calcium compound and an yttrium compound are applied, and the latter is more preferable.

As an yttrium compound, yttrium oxide (Y ₂
O ₃ ), yttrium fluoride (YF ₃ ), and yttrium nitrate (Y (NO ₃ ) ₃ .6H ₂ O) are applied. When these compounds are applied, firing is performed in a nitrogen gas atmosphere in the presence of carbon gas. As a result, during firing, the Y-A-O compound moves along the grain boundaries and easily diffuses to the surface, is reduced in the vicinity of the surface, becomes yttrium nitride, and volatilizes in the sintering furnace. Even if it is formed on the surface, it can be easily hydrolyzed and removed by immersing it in water in a later step, so that it is optimal.

An organic binder, for example, stearic acid, polyvinyl butyral, etc., was added to the adjusted aluminum nitride powder of 0.1 to 2 μm, kneaded with a nylon ball or nylon-coated ball mill, and further granulated to 200 to 400 μm. Or a hydrostatic press to form a desired molten metal processing apparatus to obtain a green molded body.

The green molded body, after degreasing as usual, is subjected to a normal pressure sintering method, at a level corresponding to the vapor pressure corresponding to the firing temperature, for example,
Sintering and firing at 1700 to 2000 ° C. for 5 to 50 hours in a nitrogen gas reducing atmosphere containing about 7.0 × 10 ^{−8 to} 8.0 × 10 ⁻⁶ mmHg of carbon gas. Thereby, the grain boundary phase can be removed. Hereinafter, an example of the manufacturing method will be described.

That is, the green compact is placed in a container with a lid made of boron nitride or aluminum nitride, and first heated and sintered at 1700 to 1900 ° C. for 1 to 10 hours in a nitrogen gas atmosphere furnace of 0.9 to 1.1 atm. Oxygen in the compact is trapped in the grain boundary phase to generate a grain boundary phase. Next, the obtained one-stage sintered body is placed in a carbon container or on a grill made of carbon plate, or placed on a grill made of aluminum nitride plate in a furnace in which the furnace wall material of the firing furnace is made of carbon or graphite. Under a nitrogen gas reducing atmosphere where carbon gas is present,
Bake at 50-2000 ° C for 5-40 hours. As a result, the grain boundary phase generated during the one-step sintering moves from the center to the surface and is reduced and nitrided to produce yttrium nitride, part of which is volatilized in a nitrogen gas atmosphere furnace. Some yttrium nitride is generated as a new surface layer in the aluminum nitride sintered body.

After the produced yttrium nitride is carried out of the sintering furnace, it is easily hydrolyzed by leaving it in the air or a humidified atmosphere, or by immersing it in water, converted to yttrium oxide and easily from the sintered surface. Removed.

Therefore, the aluminum nitride sintered compact after the sintering process is removed from the sintering furnace when the temperature is lowered to 200 ° C. or less, and then, for example, immersed in a water tank for 10 to 30 hours, and washed to remove the new surface layer. I can do it.

Examples Hereinafter, Examples and Comparative Examples will be shown to show the superiority of the sintered compact according to the present invention.

Example 1 Aluminum nitride powder (Nikko Thermal Top ES6 manufactured by Nippon Light Metal Co., Ltd .: oxygen content 1.6% by weight, cation impurity 0.03% by weight, average particle size 0.8 μm, BET specific surface area 5.5 m)
² / g) 100 parts by weight, 5 parts by weight of yttrium oxide powder (purity 99.9% by weight, average particle diameter 0.4 μm, BET specific surface area 22 m ² / g) as a sintering aid, 1 part by weight of stearic acid, polyvinyl Butyral (3 parts by weight) was kneaded with n-butyl alcohol as a solvent using a nylon ball / nylon pot for 24 hours, and the obtained slurry was air-dried to give a lump.

The mass is then crushed to a powder of 200-400 μm,
Vertical 100 mm, horizontal 200mm by uniaxial pressing at a pressure of 0 kg / cm ^2,
A 10 mm-thick tile-shaped green molded body was obtained.

After raising the temperature of this green molded body from room temperature to 500 ° C. at a rate of 10 ° C./hour in air, the green molded body is heated at 500 ° C. for 10 hours, degreased, and housed in a sealed boron nitride container. Put into a graphite heater furnace (atmosphere sintering furnace made by Nippon Light Metal Co., Ltd .: trade name "Nikafu") and supply 1 atmosphere of nitrogen gas at 1.5 /
While flowing at a flow rate of 1 minute, it was held at 1800 ° C. for 2 hours and sintered.

Next, the sintered body is placed on an aluminum nitride plate grill and fired at 1900 ° C. for 30 hours in a graphite heater furnace in a nitrogen gas reducing atmosphere (carbon gas is generated from the furnace wall material) in the presence of carbon gas. Processing was performed. After the temperature was lowered to 200 ° C., it was taken out of the graphite heater furnace, left in a room, and after hydrolyzing the yttrium nitride formed on the surface, it was cleaned by blowing air with an air gun.

The sample for measurement described above was cut out from the obtained aluminum nitride sintered body, and the composition was analyzed, and the contact angle was measured at the start of measurement and after 3 hours by the contact angle measurement method described above.

The sample whose contact angle was measured after 3 hours was immersed in a resin for microscopic observation, cured, cut, and the cut side was mirror-polished with diamond powder, and the polished surface was observed with a polarizing microscope (× 700). The thickness of the reaction layer formed at the interface between the aluminum nitride sintered body and the metallic aluminum was determined.

 Table 1 summarizes the above measurement results.

Example 2 Aluminum Nitride Powder (Nikko Thermal Top ST, trade name, manufactured by Nippon Light Metal Co., Ltd .; oxygen content 0.8% by weight, cation impurity 0.03% by weight, average particle size 2.5 μm, BET specific surface area 2.4 m ² /
g) Except for adding 3 parts by weight of yttrium oxide to 100 parts by weight (the same as in Example 1), kneading, molding, degreasing, two-stage sintering and hydrolysis in exactly the same manner as in Example 1. Processed. The same measurement as in Example 1 was performed on the obtained sintered body, and the results are shown in Table 1.

Example 3 Aluminum Nitride Powder (trade name: Nikko Thermal Top HD, manufactured by Nippon Light Metal Co., Ltd .; oxygen content: 1.0% by weight, cation impurity: 0.03% by weight, average particle size: 2.4 μm, BET specific surface area: 3.0 m ² /
g) Two-stage sintering and hydrolysis were carried out in exactly the same manner as in Example 1 except that 3 parts by weight of yttrium oxide (the same as in Example 1) was added to 100 parts by weight. Table 1 shows the measurement results of the characteristics.

Comparative Examples 1 to 3 In Examples 1, 2, and 3, the sintering process was simply performed in the first stage.
The characteristics were measured for a comparative example in which only sintering at 1800 ° C. for 2 hours was performed, and the calcination treatment and the hydrolysis treatment in the nitrogen gas reducing atmosphere where the second stage carbon gas was present were not performed. As shown in Table 1, it is shown that the presence of the grain boundary phase causes deterioration with time of the corrosion resistance.

Examples 4 to 6 Using the powders prepared in Examples 1, 2, and 3, a green molded body molded and degreased in the same manner as in Example 1 was placed in a carbon container (a carbon gas was generated from the carbon container during heating). 20 at 1850 ° C in a graphite heater furnace in a nitrogen gas atmosphere
A one-stage sintering / firing treatment for holding for a time was performed. Then, it was left in the atmosphere for 48 hours to hydrolyze the yttrium nitride formed on the surface, and was cleaned by air blowing.

Table 1 shows the results of measuring the characteristics of the obtained sintered body in the same manner as in Example 1.

Comparative Examples 4 and 5 Comparative examples were performed under the same conditions except that the sintering and firing conditions in the graphite heater furnace were changed to a short-time treatment of 1850 ° C. for 2 hours in Examples 4 and 5. Similar characteristics were measured. The results are shown in Table 1. As shown in Table 1, the corrosion resistance is low due to insufficient removal of the grain boundary phase, and the contamination of the molten metal is also caused.

Comparative Example 6 Example 1 was repeated except that 1.5 parts by weight of yttrium oxide powder (the same as in Example 1) was added to 100 parts by weight of aluminum nitride powder (of the same type as that used in Example 3). Kneading, molding, and degreasing were performed under the same conditions.

The green compact was placed in a carbon container and sintered and fired at 1850 ° C. for 10 hours in a graphite heater furnace in a nitrogen gas atmosphere. After the yttrium nitride formed on the surface of the aluminum nitride sintered body was hydrolyzed and removed, the characteristics of the obtained sintered body were measured. However, deterioration of the corrosion resistance was observed because the firing effect was still insufficient.

Comparative Example 7 In Example 1, without adding the sintering aid yttrium oxide, the mixture was treated under the same conditions until kneading, molding, and degreasing, and the obtained green molded body was stored in a sealed container made of boron nitride. The sintering and firing treatment was performed at 1950 ° C. for 5 hours in a graphite heater furnace in a nitrogen gas atmosphere. The results of measuring the properties of the obtained sintered body are shown in Table 1. As a result, no contamination of the molten metal occurred, but seizure of the molten metal was observed.

Example 7 4 parts by weight of yttrium oxide, 1 part by weight of stearic acid, 1 part by weight of dibutyl phthalate, and 1 part by weight of polyvinyl butyral were added to 100 parts by weight of aluminum nitride powder (the same one used in Example 2). Using ethanol as a solvent, ball mill mixing was performed for 24 hours using a nylon ball and a nylon pot.

Further adjust the viscosity of the slurry to 300 centipoise by adding ethanol and use a nitrogen gas circulation type spray dryer.
Granulated to 50-70 μm. 1ton / c of granulated material by rubber press
molded m ^2, and the processed into the crucible shape of 130mmφ × 150mm depth, followed by heated at a rate of 5 ° C. / time to 500 ° C. from room temperature, was subjected to degreasing treatment for holding at 500 ° C. 20 hours.

The molded body was housed in a container made of boron nitride and fired at 1800 ° C. for 5 hours in a graphite heater furnace in a nitrogen atmosphere.
Next, a crucible compact was formed on an aluminum nitride plate grill, and calcined at 1850 ° C. for 6 hours in a graphite heater furnace in a nitrogen gas atmosphere.

The obtained sintered body was immersed in water for 50 hours to hydrolyze the surface yttrium nitride, washed with water, and cleaned.

Vacuum melting casting was performed using the obtained aluminum nitride crucible. Ultra-high-purity aluminum having a purity of 99.999% by weight was melted in a crucible by heating to 780 ° C. in a vacuum of 1 to 3 × 10 ⁻⁴ Torr.

Although the degree of vacuum decreased due to the release of gas in aluminum, the temperature was maintained for 6 hours while maintaining 1 to 3 × 10 ⁻³ Torr, and then cast into a mold to analyze impurities of aluminum. The same operation was repeated 60 times, but no biting of the aluminum metal into the crucible surface and no cracks on the crucible surface occurred. Further, no impurities considered to have eluted from the aluminum nitride sintered body were detected in the cast aluminum.

Effect of the Invention The present invention relates to an aluminum nitride sintered compact having excellent corrosion resistance to molten aluminum, having a contact angle with molten aluminum of 110 ° or more, and at least a surface layer having a grain boundary phase and a surface layer. 1) The molten aluminum and aluminum alloy are less likely to wet the aluminum nitride sintered compact, the penetration of the molten metal into the sintered compact is eliminated, and the seizure of the molten metal is prevented. The erosion that has passed through is also reduced.

That is, when the seizure of the molten metal occurs, a cleaning operation such as polishing of the molten metal contact surface is required for each production lot of the molten metal, and the operation management becomes complicated.

In addition, if infiltration occurs, cracks due to heat cycles of heating and cooling during operation are induced, which is not preferable.

2) The reactivity of the surface of the aluminum nitride sintered compact is lost, the elution from the surface layer is prevented, the contamination of the molten metal is eliminated, and the thermal conductivity is reduced due to the formation of an aluminum oxide film due to the reaction with the molten metal, and the surface is secondary. This prevents activation and deterioration of corrosion resistance.

Today, high-purity aluminum materials and alloy materials are required for various applications, and it is extremely important to reduce the cause of molten metal contamination from the molten metal processing equipment.
This problem is to be solved.

And the like can exhibit extremely remarkable practical merits.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-51-40332 (JP, A) JP-A-1-160873 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C04B 35/58 104

Claims (3)

(57) [Claims] 1. A corrosion-resistant aluminum nitride for molten aluminum, comprising a sintered aluminum nitride having a contact angle with molten aluminum of 110 degrees or more and having no grain boundary phase and no surface layer at least in a surface layer in contact with the molten metal. Sintered compact. 2. The corrosion-resistant aluminum nitride for molten aluminum according to claim 1, wherein the oxygen concentration in the surface layer of the aluminum nitride sintered body is 0.05% by weight or less and the total cation impurity concentration is 0.3% by weight or less. Sintered compact. 3. A green compact obtained by adding yttrium oxide as a sintering aid to aluminum nitride powder, kneading it in the presence of an organic binder, granulating and forming a green compact, In a nitrogen gas reducing atmosphere in which a carbon gas equivalent to pressure exists, baking is performed at 1700 to 2000 ° C. for 5 to 50 hours, and the contact angle to molten aluminum is 110 ° or less,
A method for producing a corrosion-resistant aluminum nitride sintered compact for molten aluminum, comprising obtaining an aluminum nitride sintered body having no grain boundary phase and no surface layer at least in a surface layer in contact with the molten metal.