JP3130987B2 - Alumina ceramics and manufacturing method thereof - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alumina ceramic having high corrosion resistance and high creep resistance.

[0002]

2. Description of the Related Art Fine ceramics have recently been used as various structural parts because of their superior wear resistance, corrosion resistance and heat resistance compared to metals. It is widely used for pump parts and medical equipment parts. This alumina ceramic contains Al ₂ O ₃ as a main component and contains SiO ₂ , MgO, CaO and the like as a sintering aid. For example, 99% by weight of an Al ₂ O ₃ raw material having a particle diameter of several μm is used. After granulating a ceramic raw material consisting of SiO ₂ , MgO, CaO, etc., the remainder is formed into a predetermined shape,
By baking at 00 to 1600 ° C., alumina ceramics having an average crystal particle diameter of 20 to 40 μm could be obtained.

Further, Japanese Patent Publication No. 63-66795,
As disclosed in JP-A-3-51244 and the like,_Two
O_ThreeThe content is 99.6 to 99.9% by weight, and the balance is S
iO _TwoHigh-purity alumina core made of MgO, CaO, etc.
Lamix is also used, and such high-purity aluminum
Ceramics have high wear resistance and low impurity content
In order to prevent contamination of the equipment with impurities.
Great advantage over metal materials, chemical reactions occur
It is also used for parts where For example, nuclear energy
Has recently been actively used for enrichment of uranium
It has come to be.

[0004] Uranium enrichment methods include gas diffusion methods (separation of uranium 235 and 238 using hexaporous uranium molecules using a porous membrane) and centrifugal separation method (hexafuuranium is centrifuged at a high speed by a centrifuge). ) Or a chemical method under research and development (separation by dissolving uranium in acid and utilizing the difference in the redox reaction between uranium 235 and 238),
A molecular laser method (irradiation of laser light to hexafluorouranium to separate 235 from which fluorine was removed and 238 as it is) and an atomic laser method (irradiation of a specific wavelength of laser to uranium 2
35 is ionized and separated by applying an electric field).
Among these, the chemical method is highly efficient, but it is difficult to put it into practical use because the plant becomes quite large. The gas diffusion method has been used for the longest time, but has a low efficiency, and is being converted to another method. Furthermore, although the centrifugal separation method is currently most popular, it is still superior to the gas diffusion method but still has low efficiency, and like the gas diffusion method, the concentration operation must be performed in order to raise uranium to the required concentration. It needs to be repeated. Therefore, practical use of a highly efficient laser concentration method has been a problem.

The atomic laser method, which is one of the laser enrichment methods, irradiates uranium with a laser beam having a specific wavelength that is absorbed only by uranium 235, ionizes only uranium 235, generates an electric field, and recovers the electric field. That is. Here, as a laser beam, a copper vapor laser (CVL) is mainly used as a dye laser whose wavelength can be changed. In this copper vapor laser, neon gas is pulse-discharged inside a cylindrical ceramic tube, and the inside of the tube is raised to 1400 to 1600 ° C. by the discharge to dissolve and evaporate high-purity copper to generate a laser beam. ing.

The characteristics required of this ceramic tube are that it does not deform (high-temperature creep) during use at high temperatures, and that it has high purity in order to minimize the influence of impurities on electric discharge. Therefore, an Al ₂ O ₃ content of 9
High-purity alumina ceramics of about 9.9% by weight have been used.

[0007]

However, such a high-purity alumina ceramic having a high Al ₂ O ₃ content has a very small content of SiO ₂ , MgO, CaO, etc., which are sintering aids. It was hard to tie.
Therefore, in order to improve this difficulty in sintering, an alumina raw material has been developed by a chemical synthesis method such as a coprecipitation method or an alkoxide method, and sintering in a practical temperature range (1300 to 1700 ° C.) has become possible. High-purity alumina ceramic tubes required for the atomic laser method were manufactured using the raw materials. .

However, this material has a very small primary material particle size of 0.4 to 0.5 μm in order to improve sinterability, and the resulting alumina ceramic has a crystal particle size of 0.4 to 0.5 μm. It is as small as 2 μm or less, further 1 μm or less, and as a result, there is a problem that high-temperature creep easily occurs. The ceramic tube used for the above atomic laser method has a diameter of 100 m.
m, a length of about 3 m, which is easily deformed by its own weight because it is used at a high temperature, and becomes unusable after several uses, thus hindering the practical use of the atomic laser method.

Also, such alumina ceramics having a small crystal particle diameter have poor corrosion resistance, and have a problem when used in fields such as chemical pumps and medical equipment.

[0010]

In view of the above, the present invention provides an A
When the l ₂ O ₃ content is 99.8% by weight or more, and the average crystal grain size of the sintered body is 2 μm or more, the creep resistance and corrosion resistance at a creep rate of 2.5 × 10 ⁻³ / hour or less at 1400 ° C. It is intended to obtain high-purity alumina ceramics.

In the present invention, the content of Al ₂ O ₃ is 99.8% by weight or more because the alumina content is 9% by weight.
If the amount is less than 9.8% by weight, the creep resistance and the corrosion resistance are reduced, and the amount of impurities is increased. Preferably, the amount is 99.9% by weight or more. Also, Al ₂ O ₃
In addition, the sintering aid contains at least one kind of SiO ₂ , MgO, or CaO of 0.2% by weight or less, preferably 0.1% by weight or less.

Further, as described above, in such high-purity alumina ceramics, it is necessary to reduce the particle size of the primary raw material, so that the crystal particle size of the sintered body tends to be small. It has been found that by increasing the crystal grain size of such high-purity alumina ceramics, corrosion resistance and creep resistance can be particularly improved. In the present invention, the average crystal grain size of the sintered body is set to 2 μm or more because the average crystal grain size is 2 μm.
If it is smaller than m, the creep resistance and corrosion resistance will be deteriorated, and preferably 5 μm or more. On the other hand, when the average crystal particle diameter is increased, the bending strength of the alumina ceramic tends to decrease. For use as a normal structural member, the average crystal particle diameter of the sintered body is preferably 40 μm or less, more preferably 30 μm or less.

Here, the mechanism by which the alumina ceramics of the present invention can enhance corrosion resistance and creep resistance will be described. That is, usually alumina ceramics are Al ₂ O
_{It is} composed of _three crystals and a vitreous matrix layer made of a sintering aid such as SiO ₂ , CaO, MgO. In a chemical or the like, the glass layer is eroded and eluted, leading to the dropout of Al ₂ O ₃ particles. Therefore, in order to increase the corrosion resistance of alumina ceramics, the glass layer may be reduced as much as possible.

On the other hand, when a constant load is continuously applied to the alumina ceramics for a long time, grain boundary sliding gradually occurs in the Al ₂ O ₃ crystal and creep occurs. Therefore, in order to increase the creep resistance, the glass layer that causes grain boundary sliding should be reduced as much as possible. Also,
This grain boundary slip is also related to the particle size of the Al ₂ O ₃ crystal,
The larger the particle size, the less likely each crystal is to shift,
They found that grain boundary slip was small and creep resistance could be increased.

Therefore, the present invention provides an Al ₂ O ₃ content of 9
9.8% by weight or more, and the content of the sintering aid for forming the glass layer is reduced to 0.2% by weight or less.
The firing temperature is increased to grow Al ₂ O ₃ crystals to increase the crystal particle diameter, and in the process, Al ₂ O 3
_The glass layer is further reduced by forming a solid solution in the _three crystals. Thus, the alumina ceramics of the present invention has a glass layer as small as possible,
As a result, corrosion resistance can be improved. In addition, creep resistance can be increased by reducing the number of glass layers and increasing the average crystal grain size of the sintered body to 2 μm or more.

Further, the alumina ceramics of the present invention comprises:
With such a configuration, the creep rate at 1400 ° C. is set to 2.5 × 10 ⁻³ / Hr or less, but if the creep rate is higher than 2.5 × 10 ⁻³ / Hr, for example, uranium enrichment This is because when used for a ceramic tube, the life is extremely shortened. Here, the creep rate is the amount of strain of the test piece per hour in a state where a constant stress is applied to the ceramic test piece using a four-point bending tester, and the calculation formula is: ε = ｛6d / (L −l) · (L + 2l)｝ · D ε: strain amount d: sample width L: lower jig span l: upper jig span D: strain amount ε obtained by measuring deflection amount divided by time . Also,
In the present invention, the stress is 1.5 kg / mm ² and d = 4.0.
mm, L = 30 mm, and l = 10 mm.

As described above, the content of Al ₂ O ₃ is 9
When the content is increased to 9.8% by weight or more and the sintering aid is decreased, sinterability is poor, and it is difficult to obtain a complete sintered body. Therefore, in the present invention, a fine primary raw material having an average particle diameter of 1 μm or less is prepared by a chemical synthesis method such as a coprecipitation method or an alkoxide method, and a predetermined binder is added to the raw material, mixed, granulated, and pressed. And the like to form a predetermined shape. Thereafter, at a maximum firing temperature of 1500 ° C. or higher, preferably 1600 ° C. or higher, the firing time from the start of the temperature increase to the end of the temperature decrease is 30 hours or more, and the average crystal particle diameter of the sintered body is 2
What is necessary is just to bake until it becomes more than micrometer. At this time, since the particle diameter of the primary raw material is small, the activation energy of the particles is high, and the sinterability can be improved. Further, since the particle diameter of the primary raw material is small, the crystal particle diameter of the sintered body tends to be small. However, by increasing the firing temperature or lengthening the firing time, the Al ₂ O ₃ crystal grows. It should be done.

[0018]

Embodiments of the present invention will be described below.

Experimental Example 1 First, an experiment was conducted to examine the relationship between the average crystal grain size of alumina ceramics and creep resistance. Al ₂ O ₃
The content is 99.95% by weight, and the remaining 0.05% by weight is S
It is composed of iO ₂ , MgO, CaO, etc., and has an average particle size of 0.
A binder such as PVA or PEG was added to the primary raw material of 4 to 0.5 μm and granulated by a spray drier. This was applied at a molding pressure of 1 ton / cm ^{2 according to} JIS R1601.
After press-forming into a test piece shape based on the above, using an electric furnace, in an oxidizing atmosphere, the maximum firing temperature 1300-17
The sintering was performed at 00 ° C. for a sintering time of 40 hours from the start of the temperature increase to the end of the temperature decrease. Average crystal particle diameter of the obtained sintered body,
Table 1 shows the measurement results of the bending strength and the creep rate.
FIG. 1 shows the relationship between the firing temperature and the average crystal particle diameter, and FIG. 2 shows the relationship between the average crystal particle diameter and the creep rate.

The average crystal particle diameter is obtained by analyzing the surface of the sintered body with an image processing device (Luzex). As for the creep rate, the strain amount of the test piece per hour in a state where a constant stress was applied to the ceramic test piece using the four-point bending tester as described above was obtained.

[0021]

[Table 1]

From the results in Table 1 and FIG.
The average crystal particle diameter of the sintered body can be increased as the firing temperature is increased, and the average crystal particle diameter can be 2 μm or more at a firing temperature of 1500 ° C. or higher, and the average crystal particle diameter can be increased at a firing temperature of 1600 ° C. or higher. It can be seen that the thickness can be set to 5 μm or more.

Further, from the results shown in Table 1 and FIG. 2, it can be seen that the larger the crystal particle diameter, the lower the creep speed and the higher the creep resistance. Here, the characteristics required for the ceramic tube used in uranium enrichment by the atomic laser method are desirably that the creep rate at 1400 ° C. is 2.5 × 10 ⁻³ / hour or less. It can be understood that this required property is satisfied when the average crystal particle diameter of is 2 μm or more.

Of course, the higher the creep resistance, the better.
With the increase in crystal grain size, the bending strength decreases,
In selecting the crystal grain size, it is necessary to determine it in consideration of the required bending strength. Actually, in order to be used as a structural member, the flexural strength needs to be 10 kg / mm ² or more, and for that purpose, the average crystal grain size needs to be 40 μm or less, preferably 30 μm or less.

In this experimental example, the average crystal particle diameter was increased by increasing the firing temperature. However, the average crystal particle diameter can be increased by increasing the firing time.

Experimental Example 2 Next, an experiment was conducted to examine the relationship between the Al ₂ O ₃ content and the creep rate. The creep rate when the alumina content was changed was examined in the same manner as in Experimental Example 1. The results are as shown in Table 2.

[0027]

[Table 2]

The results clearly show that as the Al ₂ O ₃ content increases, the creep speed decreases, and the creep resistance improves. In order to reduce the creep rate at 1400 ° C. to 2.5 × 10 ⁻³ / hour or less, which is a required characteristic of the ceramic tube used for uranium enrichment as described above, Al ₂ O
It is also understood that the content of ₃ should be 99.8% by weight or more.

As described above, the Al ₂ O ₃ content is closely related to the creep resistance. This is because those having many components other than Al ₂ O ₃ such as SiO ₂ , MgO and CaO can easily obtain a sintered body having a large crystal grain size because they act as a sintering aid. Is usually generated as a glass phase at the Al ₂ O ₃ crystal grain boundary, so that it is likely to be softened at high temperatures and to deteriorate the creep resistance. Also, A
Since ceramics with a high l ₂ O ₃ content have few impurities, when used as a ceramic tube for uranium enrichment, there is also an effect that unnecessary discharge phenomena can be prevented.

EXPERIMENTAL EXAMPLE 3 Further, an experiment for examining the relationship between the Al ₂ O ₃ content and the corrosion resistance was conducted. An Al ₂ O ₃ raw material having an average particle diameter of 1 μm or less is used as a primary raw material, and the Al ₂ O ₃ content is variously changed to form a predetermined shape. Then, the temperature is raised at a maximum firing temperature of 1400 to 1700 ° C. Firing was performed for 60 hours from the start to the end of cooling to obtain a test piece of alumina ceramics. The test piece thus obtained was placed in each chemical kept at 95 ° C. for 10 hours × 1.
After soaking for 0 cycles for a total of 100 hours, the weight loss was measured. The results are as shown in Table 3.

[0031]

[Table 3]

The results clearly show that those having an Al ₂ O ₃ content of 99.5% by weight or less have a large weight loss due to chemicals. In addition, even if the Al ₂ O ₃ content is 99.9% by weight, the one having an average crystal particle diameter of 2 μm or less is
The weight loss was large and the corrosion resistance was poor. This is because the solid solution of the sintering aid in the Al ₂ O ₃ crystal accompanying the crystal growth is small,
This is because many glass layers were present at the grain boundaries of the Al ₂ O ₃ crystal.

On the other hand, in Examples of the present invention in which the content of Al ₂ O _{3 was} 99.8% by weight or more and the average crystal particle diameter was 2 μm or more, the weight loss due to chemicals was extremely small and the corrosion resistance was excellent. Understand. Therefore, it can be seen that in order to enhance the corrosion resistance, the Al ₂ O ₃ content should be 99.8% by weight or more and the average crystal grain size of the sintered body should be 2 μm or more.

Experimental Example 4 Next, the ceramic tube for enriching uranium was formed using the alumina ceramics of the present invention and the comparative example. The ceramic tube was formed into a cylindrical shape having a diameter of 100 mm and a length of 3 m, and was formed of various alumina ceramics shown in Table 4, and used as a uranium enrichment ceramic tube by an atomic laser method. The respective times until the ceramic tube was deformed and became unusable were compared (99.5% by weight of Al ₂ O ₃ and an average crystal particle diameter of 10 μm).
m was 1 and the ratio was 1). The results are as shown in Table 4.

[0035]

[Table 4]

The results clearly show that the comparative example Al
_{When the} content of ₂ O ₃ was as low as 99.5% by weight or when the average crystal particle size was as small as 1 μm, the time until the sample became unusable was short. On the other hand, the examples of the present invention can be used for a long time and showed excellent results. Therefore, as a uranium enrichment ceramic tube, Al ₂ O
_{3 The} content is 99.8% by weight or more and the average crystal particle size is 2
If the alumina ceramic of the present invention having a size of μm or more is used,
It can be seen that particularly excellent effects are achieved.

[0037]

As described above, according to the present invention, the content of Al ₂ O ₃ in alumina ceramics is 99.8% by weight or more, and the average crystal grain size of the sintered body is 2 μm.
m or more and the creep rate at 1400 ° C.
By setting it to 5 × 10 ⁻³ / hour or less, corrosion resistance and creep resistance can be extremely increased. Such alumina ceramics have an Al ₂ O ₃ content of 9%.
After forming a primary material having a mean particle diameter of 1 μm or less into a predetermined shape at a rate of 9.8 wt% or more, the firing time from the start of temperature rise to the end of temperature drop is 30 hours or more at a maximum firing temperature of 1500 ° C. or more. Can be easily obtained by baking so that is not less than 2 μm.

The alumina ceramics of the present invention thus obtained can be applied to chemical pumps and medical equipment by utilizing excellent corrosion resistance, or used for enriching uranium by utilizing excellent creep resistance. If used for a load cell or a high-temperature jig other than the ceramic tube, it can be favorably used for a long time.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a firing temperature and an average crystal particle diameter of a sintered body in alumina ceramics.

FIG. 2 is a graph showing a relationship between an average crystal particle diameter and a creep rate in alumina ceramics.

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Takeshi Inoku, 10-10 Kawai, Gamo-cho, Gamo-gun, Shiga Prefecture Examiner Hitoshi Misaki in the Shiga Gamo Plant of Kyocera Corporation (56) References JP-A-54-87716 ( JP, A) JP-A-60-16861 (JP, A) JP-A-61-118905 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C04B 35/10 C04B 35/64

Claims (2)

(57) [Claims] 1. The sintered body has an Al ₂ O ₃ content of 99.8% by weight or more, an average crystal grain size of 2 μm or more, and 1400 ° C.
Alumina ceramics having a creep rate of 2.5 × 10 ⁻³ / hour or less. 2. After a primary raw material having an Al ₂ O ₃ content of 99.8% by weight or more and an average particle diameter of 1 μm or less is formed into a predetermined shape, the maximum firing temperature is 1500 ° C. or more, and from the start of temperature rise to the end of temperature fall. The method for producing alumina ceramics according to claim 1 , wherein the sintering is carried out so that the average crystal particle diameter of the sintered body is 2 µm or more, with the sintering time being 30 hours or more.