JPS63242964A - Alumina ceramics and manufacture - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to alumina ceramics and a method for manufacturing the same, and particularly aims to improve bending strength without causing deterioration of ultraviolet transmittance. (Er.
asable and programmable R
It is particularly suitable for use as an ultraviolet-transmitting material, such as in packages for ead-only memories.

(Prior art) [In EFROM, since it is necessary to erase the contents of the semiconductor memory and rewrite the information by irradiation with ultraviolet rays, the package must have at least the central area of the upper surface (generally called a window). It is required to be transparent, and conventional window materials for such EPROM packages have been disclosed in Japanese Patent Publication No. 39-240 and Japanese Patent Publication No. 47-51801 due to their good thermal conductivity. The disclosed translucent alumina ceramics have been used.

(Problem to be solved by the invention) Tamade's translucent alumina ceramics has traditionally been used mainly as arc tubes for high-pressure metal discharge lamps, so we focused on its high transmittance in the visible light region. Therefore, MgO etc. are added as a sintering aid, and the temperature is heated to 1750℃.
The crystal grain size was made as large as possible by sintering at higher temperatures.

However, when the crystal grain size was increased, the problem remained that the mechanical strength was extremely low (about 2500 kgf/cm'').

Here, the window material for the EPROM baggage is IC.
Excellent mechanical strength, especially bending strength, is required for applications in thin-walled load systems such as cards and for plastic packaging.

The present invention advantageously solves the above-mentioned problems, and aims to propose an alumina ceramic having significantly improved bending strength without reducing ultraviolet transmittance, together with an advantageous manufacturing method thereof. .

(Means for Solving the Problems) That is, the present invention is an alumina ceramic having a purity of 99.90 wtχ (hereinafter simply expressed in %) or more and a crystal grain size of 8 μm or less.

In addition, the present invention provides 7 g of aluminum oxide powder with a purity of 99.9% or more and a specific surface area of 3 to 10 m obtained by thermally decomposing aluminum ammonium sulfate, and 0.1% or less as a sintering aid. After adding MgO and a molding aid, it is molded into a predetermined shape, and then heated at 1450 to 1600°C.
This is a method for producing alumina ceramics, which comprises firing in a non-oxidizing atmosphere.

In this invention, the purity of alumina ceramics is 99%.
．． The reason why it is limited to 9% or more is because if the purity is less than 99.9%, not only the ultraviolet transmittance will decrease but also the bending strength will decrease.

In addition, the crystal grain size is set to 8 μm or less because the crystal grain size is 8 μm or less.
This is because if it exceeds m, sufficient bending strength cannot be obtained even if the purity is 99.9% or more.

By satisfying the above appropriate ranges for both purity and crystal grain size, the bending strength can be increased to 5000 kgf/cm.
” or more and thickness: Wavelength at 0.2mo+: 25
It is possible to obtain alumina ceramics having an ultraviolet transmittance of 40% or more and having excellent bending strength and ultraviolet transmittance.

Next, a manufacturing method according to the present invention will be explained.

In this invention, aluminum ammonium sulfate is used as a raw material for aluminum oxide powder (hereinafter referred to as alumina powder).The reason for this is that it not only has high purity and a suitable specific surface area, but also can be used in the firing process as described below. This is because it is possible to easily obtain alumina powder that can be sintered at a much lower temperature than the firing temperature of alumina ceramics.

The specific surface area of the alumina powder obtained here is 3 m”7g.
If it is less than 10m", sintering will not only proceed poorly, but also bores will occur, resulting in low ultraviolet transmittance and bending strength. On the other hand, if it exceeds 10m"7g, sintering will progress too much. Although the ultraviolet transmittance improves, the bending strength deteriorates, so the specific surface area of alumina powder should be 3 to 10 mg/g.
limited to the range of

However, the specific surface area of such alumina powder is greatly influenced by the treatment conditions during thermal decomposition of ammonium aluminum sulfate, particularly the treatment temperature and time.

The preferred processing conditions for obtaining alumina powder with an appropriate specific surface area are as follows.

Crystals of aluminum ammonium sulfate salt highly purified by recrystallization treatment are filled into a high-purity alumina pod, and after being heated and pyrolyzed in an electric furnace, the crystals are heated to 1100 to 13
By raising the temperature to 00 DEG C. and maintaining it for at least 30 minutes to 5 hours, alumina powder in the α-crystalline phase with a specific surface area of 3 to 10 m'' and 7 g is obtained.

It is well known that alumina powder undergoes phase transformation, but
If the treatment temperature is lower than 1100° C., alumina in the γ-crystalline phase will be mixed, and during sintering to obtain an alumina product, the γ will convert to the α phase, making it difficult to control the sintering and making it impossible to obtain the desired product.

Moreover, at temperatures above 1300° C., a reaction occurs between the powder particles, forming pseudoparticles, which inhibits easy sinterability.

Further, since temperature alone causes a drastic change in the specific surface area, it is desirable to control by adding a time factor, and in order to obtain a uniform powder, it is preferable to hold the temperature for at least 30 minutes or more. However, it is industrially undesirable to treat for more than 5 hours.

Next, a sintering aid and a forming aid are added to the alumina powder obtained, and then it is formed into a predetermined shape.For example, extrusion is used to form tubes and rods, while other methods are used. Press molding is suitable for plate-shaped materials such as window materials for EPROM packages.

The most suitable sintering aid is MgO. It is known that MgO is used as a grain growth inhibitor in the production of translucent alumina ceramics, but in the present invention, it is preferable to use MgO in an amount of 0.1% or less, rather than using only easily sinterable alumina powder. It was found that by adding MgO in the range of 0.03 to 0.08%, a product with no internal bores and extremely uniform crystal grains could be obtained. This is because Mg0 is added as an aqueous solution of magnesium nitrate which thermally decomposes to MgO in the pre-firing process, so the MgO component is distributed extremely uniformly, and the sintering reaction occurs outside the molded body during the main firing. It is presumed that this is because it has the effect of suppressing the progress of winding up the bore starting from the surface.

In addition, the type of molding aid is determined depending on the molding method; for example, in the case of press molding, polyvinyl alcohol is advantageously suitable as a binder and polyethylene glycol as a lubricant. The amount is preferably about 1.5 to 3.0%.

In addition, all such molding aids evaporate or decompose during firing, which will be described later, and are ultimately volatilized from the sintered body, so they do not remain in the product.

Thereafter, the obtained molded body is pre-fired in the air to decompose and remove the forming aid, and then the product is produced by main firing, but this main firing must be carried out in a non-oxidizing atmosphere. be. This is because, in an atmosphere other than non-oxidizing, there is a strong possibility that sintering will proceed by entraining the bore, resulting in a decrease in strength and ultraviolet transmittance. Here, the non-oxidizing atmosphere includes not only a gas atmosphere such as hydrogen gas or ammonia decomposition gas, but also a vacuum atmosphere. Note that when using a gas atmosphere as the non-oxidizing atmosphere,
It is preferable that the dew point is about -15 to 5°C. This is because when the dew point is higher than 5°C, sintering progresses slowly;
This is because when the temperature is lower than 15°C, the reducing power becomes strong and a low-order oxide of aluminum oxide is generated, which is likely to volatilize and form a bore, and high-purity gas with a low dew point is expensive.

In addition, if the firing temperature is lower than 1450°C, sintering will not proceed sufficiently and the degree of bonding of grain boundaries will be weak, making it impossible to obtain satisfactory bending strength and ultraviolet transmittance.
If the temperature exceeds 00°C, the crystal grain size becomes too large, resulting in a decrease in bending strength, so the firing temperature is 1450 to 1600°C.
It is necessary to carry out the test within a temperature range of

The firing time is preferably about 0.5 to 5 hours, although it varies depending on the size of the product, desired strength (particle size), and set temperature.

(Function) The reason why excellent bending strength can be obtained according to the present invention without causing a decrease in ultraviolet transmittance is considered to be as follows.

In other words, alumina powder obtained by thermally decomposing aluminum ammonium sulfate is easily sinterable, can be easily densified to 98% or more of the theoretical density at 1600°C or lower, and has uniform grains with a crystal grain size of 8 μm or less. This is because it is easy to obtain alumina ceramics with an appropriate specific surface area for structural alumina ceramics.

(Example) Ammonium aluminum sulfate was thermally decomposed under various conditions shown in Table 1 to obtain alumina powder.

Table 1 shows the purity and specific surface area of the obtained alumina powder.

For comparison, Table 1 also shows cases where aluminum carbonate and aluminum hydroxide were used as raw materials.

Table 1 Next, magnesium nitrate was added to each of the obtained powders to give a concentration of 0.07% as MgO, and 2.5% of polyvinyl alcohol and polyethylene glycol were added as molding aids, and then an appropriate amount of water was added. After that, it was granulated with a spray dryer to obtain a powder with an average particle size of 70 to 90 μm. I11 square, thickness 0.25mm
It was press-formed into a board.

Next, the obtained molded bodies were fired under various conditions shown in Table 2.

Table 2 also shows the results of examining the bending strength, ultraviolet transmittance, density, and average particle size of the alumina ceramics thus obtained.

Table 2 Rate l Bending strength is JIS-R16014 point bending method Umbrella 2
As is clear from the same table with no MgO added, when alumina powder with a comparative area below the lower limit of this invention is used as a raw material (N117
) did not have good bending strength or ultraviolet transmittance. Further, Nα8 whose firing temperature exceeded the upper limit had a large particle size and good ultraviolet transmittance, but was inferior in bending strength.

On the other hand, both Nα9 and Nα10 whose firing temperature was below the lower limit not only had low bending strength but also extremely poor ultraviolet transmittance.

Moreover, No. 2 did not contain MgO as a sintering aid.

No. 13 could hardly be said to have sufficient bending strength and ultraviolet transmittance.

When alumina powder with too large a specific surface area was used as a raw material (Nα14), the particles became too large, and although the ultraviolet transmittance was good, the bending strength was poor.

Furthermore, when aluminum carbonate and aluminum hydroxide were used as raw materials (Nα15 and 16), only low values of bending strength and ultraviolet transmittance were obtained, respectively.

On the other hand, Nos. 1 to 6 that satisfy the requirements of this invention
In all cases, high bending strength and good ultraviolet transmittance were obtained.

Note that Nα11 and Nα12 are reference examples in which the dew point of the atmospheric gas deviates from the lower and upper limits of the preferred range, respectively.
In such cases, either bending strength or ultraviolet transmittance.
Good values could only be obtained for this method.

(Effects of the Invention) Thus, according to the present invention, mechanical strength, especially bending strength, can be significantly improved without reducing ultraviolet transmittance, which is advantageous.

Claims (1)

[Claims] 1. Purity: 99.90wt% or more and crystal grain size of 8μ
Alumina ceramics with a thickness of less than m. 2. The bending strength is 5000 kgf/cm^2 or more, and
The alumina ceramic according to claim 1, which has an ultraviolet transmittance of 40% or more at a wavelength of 2537 Å at a thickness of 0.2 mm. 3. Purity obtained by thermally decomposing ammonium aluminum sulfate: 99.9 wt% or more and specific surface area of 3 to 10 m^
2/g of aluminum oxide powder and 0.2/g of aluminum oxide powder as a sintering aid.
A method for producing alumina ceramics, which comprises adding 1 wt% or less of MgO and a forming aid, forming into a predetermined shape, and then firing in a non-oxidizing atmosphere at 1,450 to 1,600°C. 4. The method according to claim 3, wherein the non-oxidizing atmosphere is a gas atmosphere with a dew point in the range of -15 to 5°C.