JPH03215400A - Needle-crystalline aluminum oxide and its preparation - Google Patents

Abstract

PURPOSE:To simply prepare highly pure needle-crystalline aluminum oxide having a large aspect ratio and having the long axial direction thereof in the C axial direction of alpha-Al2O3 by heating needle crystal aluminum borate at a specific temperature in an atmosphere containing steam. CONSTITUTION:Needle crystal aluminum borate is heated at a temperature of 800-1500 deg.C in a steam-containing atmosphere and subsequently the treated product is preferably washed with an acid or alkali to provide needle crystal aluminum oxide having a long diameter/short diameter ratio of >=2 and comprising alpha-Al2O3 having the long axial direction of the granule in the C axial direction. Since the thermal treatment of the needle crystal aluminum borate is performed in the steam-containing atmosphere and the treated product is further washed with the acid or alkali, impurities in the treated product can substantially perfectly removed. The prepared highly pure and highly strong needle crystal aluminum oxide is suitable as a reinforcing material for plastics, ceramic, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a highly pure acicular aluminum oxide that can be used, for example, in fiber-reinforced metals, plastics, ceramics, etc., and a method for producing the same.

(Prior Art) Aluminum oxide (A1203) is the most common ceramic material and is used in many fields including structural parts.

In addition, whiskers, which are one of the inorganic materials with needle-like crystals, are being applied in various fields as fiber-reinforcing materials for the purpose of improving the mechanical properties of materials.

Conventionally, SiC whiskers have been commonly used as acicular crystal inorganic materials, but SiC whiskers are very effective in themselves, so their uses are limited.

Therefore, there has recently been a desire to develop acicular crystal materials made of cheaper oxides, especially aluminum oxide, and it is expected that the field of application thereof will be expanded.

(Problems to be Solved by the Invention) However, a method for producing aluminum oxide whiskers has not yet been established and is not commercially available either domestically or internationally. In addition, there are reports that some whiskers have been experimentally produced by chemical vapor deposition, but it is difficult to control the manufacturing conditions and very few high-quality products have been obtained. Even if a fiber reinforced material was produced, almost no effect could be expected from its addition.

Therefore, the present inventors proposed that acicular aluminum oxide can be easily obtained by first using acicular aluminum borate as a precursor and heat-treating it at a temperature of 800 to 1500°C. .

However, in the above method, a large amount of impurities such as boric acid tends to remain in the heat-treated product, resulting in a problem in terms of purity.

(Objective of the Invention) An object of the present invention is to provide a highly pure acicular aluminum oxide and a novel method for easily producing the same.

? Means for Solving the Problem) As a result of research into the above-mentioned problem, the inventors of the present invention have developed a method that does not include water vapor in the atmosphere when heat-treating the acicular crystal substance of aluminum borate at 800 to 1500°C. Impurities are almost completely removed by treating the aluminum oxide after heat treatment with acid or alkali, and as a result, the aspect ratio expressed by the major axis/minor axis is 2 or more and the aluminum oxide is long. It has been found that highly pure acicular aluminum oxide whose axis direction is the α-A1zOiOC axis direction is produced.

The present invention will be explained in detail below.

The acicular aluminum borate used in the present invention is generally 9A1■03.28z03 or 3A1z03.
- It is represented by the chemical formula of 2BzOi, and has an average aspect ratio of more than 1, particularly 10-100. This acicular aluminum borate is Al
It can be easily obtained by heating and melting a mixture of z03 and 8203 or a mixture prepared by adding a flux that promotes crystal growth to this mixture, and then slowly cooling the mixture.

According to the invention, the aluminum borate is dispersed in a suitable liquid, for example a solvent such as alcohol or water, before the heat treatment. At this time, an organic polymer may be added to the dispersion if necessary. By adding this organic polymer, the acicular particles of aluminum borate can be entangled with the molecular chains and mesh molecules of the polymer, thereby preventing the acicular particles from coming into contact with each other. A commercially available dispersant may also be added during this mixing, but since dispersants usually contain metal elements, the amount used should be kept to a minimum in order to obtain highly pure acicular aluminum oxide. It is better.

Next, the dispersion liquid in which aluminum borate is dispersed is heat-treated at a temperature of 800 to 1500°C, after the organic medium is volatilized if desired. The atmosphere at this time can be an inert gas, reducing gas, or oxidizing gas atmosphere, but
According to the present invention, it is important to include water vapor in the atmosphere at this time. This water vapor accelerates the release of boron in aluminum borate. Although the reason is not clear, it is thought that boron reacts with water vapor and evaporates as hydroxide. In addition, water vapor is 1 in the atmosphere.
It is desirable that the pressure be present at a pressure of 0 to 8QQtorr. This heat treatment releases the boron in the aluminum borate, ultimately producing acicular aluminum oxide without agglomerated particles.

The temperature during heat treatment was set in the above range because if the temperature is lower than 800°C, boron removal will be incomplete and no aluminum oxide will be produced, and if it exceeds 1500°C, acicular aluminum oxide will not be produced. However, this is because the crystals tend to aggregate, making it difficult to handle when used as a raw material. Further, after the heat treatment is completed, it is preferable to cool it as soon as possible. The appropriate cooling rate during cooling is 200° C./hour or more, preferably 400° C./hour or more. This is because if the cooling rate is too slow, the once separated aluminum oxide and boron oxide will react again to produce aluminum borate.

Further, according to the present invention, the removal of boron can be further promoted by making carbon present during the heat treatment. The carbon source at this time may be carbon powder or a substance that can generate carbon by heat treatment, such as a phenol resin, mixed into aluminum borate, or heat treatment may be performed in a carbon pot.

According to the present invention, by washing the heat-treated product obtained by the above method with acid or alkali, the remaining amorphous phase can be dissolved and removed, thereby further increasing the purity of the acicular aluminum oxide. be able to.

The amorphous phase in the heat-treated product is composed of boron oxide produced by decomposition of aluminum borate and other impurities, but when acid cleaning is performed, an acid with a concentration of several percent, such as hydrofluoric acid, 60 using hydrochloric acid, sulfuric acid, nitric acid, etc.
Most of the non-quality phases are dissolved and removed by washing at ~100°C for about 1 to 12 hours. Further, in the case of alkaline cleaning, the cleaning may be carried out in the same manner as above using sodium hydroxide or the like.

In the acicular aluminum oxide obtained by such heat treatment, the amount of boron oxide, which is a main impurity, can be reduced to 5% by weight or less in terms of boron. Furthermore, it can be seen from the XwA diffraction chart in FIG. 1 that it is highly pure aluminum oxide and that its crystals are of the α type. In addition, the particle shape has an asvect ratio expressed by major axis/minor axis of 2, as is clear from the electron microscope shown in Figure 2.
Above, in particular, 15 or more is ensured. Further, when such crystals were analyzed using a transmission electron microscope and an electron beam diffraction method, it was revealed that the long axis direction of the particles was the C-axis direction.

The invention will now be explained with the following examples.

(Example) Predetermined amounts of boron oxide and aluminum hydroxide were thoroughly mixed with a solvent, placed in a crucible, and heated at 1300° C. in the atmosphere. After maintaining this state for 30 minutes, it was slowly cooled to room temperature.

The obtained solid was washed with water and then treated with acid to form acicular crystals with an average aspect ratio of 50. 0.・Aluminum borate represented by 2BZO3 was obtained.

Next, alcohol was added as a solvent to 50 g of this aluminum borate, and after mixing well, A was added as a carrier gas.
14 using r, air, and N2! The treatment was carried out at a flow rate of /win at the temperature and time shown in Table 1.

The obtained treated product was subjected to X-ray diffraction measurement, the detected crystal phase was identified, and the average aspect ratio was measured from the electron micrograph. Furthermore, the amount of boron as an impurity was determined by ICP analysis.

The results are shown in Table 1.

(Left below) ? According to Table 1, the amount of boron is 4% by weight in dry Ar gas only (1), but when water vapor is added to 50t
By adding orr, the amount of boron was reduced to 2% by weight. As the partial pressure of water vapor increases, the amount of boron can be reduced, and at 500 Torr or more, it can be kept below the measurement limit.

Other gases include air and N! Even when using Ar gas, the same results as Ar gas were obtained.

Regarding the heat treatment temperature, it was confirmed that aluminum oxide was formed at a temperature of 800°C or higher; however, (2) aluminum oxide aggregated strongly when the temperature exceeded 600°C, so a temperature of about 1300°C was appropriate.

In addition, as a result of analyzing the sample of the present invention by electron beam diffraction, a diffraction pattern with an interplanar spacing of 2.12 people in the longitudinal direction of needle-like crystals was observed, which corresponds to the (
006) plane, and it was found that the longitudinal direction of the acicular crystals was the C axis of α181■08. (Example 2) Aluminum borate obtained in the same manner as in Example 1 was
Heat treatment was performed in a vacuum at 0° C. for 1 hour. The obtained powder was subjected to X-ray diffraction measurement and identified as α-AI,
A diffraction peak of 03 was observed. In addition, this powder was
When observed with M (transmission electron microscope), a glass phase consisting of acicular aluminum oxide and light elements was observed, and the average aspect ratio expressed by major axis/minor axis was 15 from the electron microscope.

Next, this powder was washed with acid or alkali under various conditions shown in Table 2. After washing, the powder was thoroughly rinsed with distilled water and then dried to prepare a sample for analysis.

The amount of residual boron in each of the obtained samples was determined by ICP analysis, and the amounts are shown in Table 2.

The results are shown in Table 2. (Margin below) No. 2 table * indicates a reference sample.

As is clear from Table 2, the sample only subjected to heat treatment (Nct
In l), the amount of boron is 5% by weight, while the concentration of boron is 1
By washing with % hydrofluoric acid, it was possible to reduce the amount to 1.5% by weight. By increasing the concentration further or increasing the temperature, it was possible to reduce the amount of boron to below 1% by weight. In addition, hydrochloric acid, sulfuric acid, and nitric acid were tested, and although they were not as effective as hydrofluoric acid, they were able to reduce boron. In addition, using sodium hydroxide as an alkaline cleaning method was also able to reduce the amount of boron. In addition, as a result of analysis by electron beam diffraction, all of the samples prepared according to the present invention showed that, as in Example 1, the direction of the long axis of the acicular crystal grains was the C axis of α-Alibi.

(Example 3) In Table 1 prepared in Example 1, a sample of 11 kL4 was treated with hydrofluoric acid at a concentration of 10% at a temperature of 80'C for 6 hours. As a result, the amount of boron in all samples was below the measurement limit, and impurities could be almost completely removed.

In addition, the X-ray diffraction chart of the obtained sample was
A second electron micrograph is shown in the figure and to show the grain structure.
Shown in the figure.

(Effects of the Invention) As described in detail above, according to the present invention, highly pure acicular aluminum oxide can be produced by simple processing, and therefore can be produced stably from an industrial perspective.

Due to its high purity, this acicular aluminum oxide has high strength, and is particularly effective in strengthening materials when combined with metals, plastics, or ceramics. It can be applied in various fields.

[Brief explanation of drawings]

FIG. 1 is an X-ray diffraction chart of the acicular aluminum oxide of the present invention, and FIG. 2 is an electron micrograph showing the particle structure of the acicular aluminum oxide prepared according to the present invention.

Claims (3)

[Claims] (1) Acicular crystalline aluminum oxide consisting of α-Al_2O_3 having a long axis/breadth axis ratio of 2 or more and the long axis direction of the particles being the C-axis direction. (2) A method for producing acicular aluminum oxide, which comprises heat-treating acicular aluminum borate at a temperature of 800 to 1500°C in an atmosphere containing water vapor. (3) Acicular aluminum borate at 800-1500℃
1. A method for producing acicular aluminum oxide, which comprises heat-treating the product at a temperature of , and then washing the treated product with acid or alkali. (4) Production of acicular aluminum oxide, which is characterized by heat-treating acicular aluminum borate at a temperature of 800 to 1500°C in an atmosphere containing water vapor, and then washing the treated product with acid or alkali. Method.