KR960000064B1 - PREPARATION OF HEXAGONAL PLATE Ñß-ALUMINA SINGLE CRYSTAL POWDER BY GAS-STALEREATION - Google Patents

Abstract

The title single crystalline powder is prepared by reacting a material containing aluminum element with a fluorine based compound at 1,000-1,300 deg.C for 1-10 hours. The obtained powder has 1-10 microns particle size and the ratio of diameter to thickness ratio of 5-15 and is useful to reinforce composite material (ex. ceramic, metallic or polymer matrix composite). The aluminum raw material is selected from bohemite, bayerite, diaspore or gamma-alumina, and the fluorine based compound is selected from AlF, NH4F or HF.

Description

Method for preparing hexagonal alpha-alumina single crystal powder by vapor phase reaction

1 is a SEM photograph of the hexagonal plate-shaped alpha alumina powder of the present invention obtained by using an AlF 2 catalyst,

2 is a result of XRD analysis of hexagonal alpha alumina powder of FIG.

3 is a schematic diagram of an experimental apparatus for producing hexagonal plate-shaped alpha alumina powder using an HF catalyst,

4 is a schematic diagram of an experimental apparatus for producing hexagonal plate-shaped alpha alumina powder using NH₄F or NH₄HF2 catalyst.

* Explanation of symbols for main parts of the drawings

1,1 ′: Aluminum raw material powder 2,2 ′: Electric heating element

3,3 ′: catalyst material 4,4 ′: ammonia water

5,5 ′: carrier gas

The present invention relates to a method for producing hexagonal plate-shaped alpha alumina single crystal powder. More specifically, the present invention relates to a method for producing a hexagonal plate-shaped alumina single crystal powder from an aluminum element-containing material and a fluorine compound by gas phase reaction, and its use as a composite structural reinforcing material.

Ceramic, which is mainly used in the field of structural materials, has a disadvantage of being easily broken due to low toughness. In order to increase toughness, a method of manufacturing a composite by mixing a secondary material with a matrix material is generally used. , Fibers and tubular materials are known. However, whiskers and fibers are too expensive to be manufactured by the methods that have been developed so far, and in particular, whiskers are detrimental to the human body due to their shape characteristics, and thus, there is considerable difficulty in practical use. On the other hand, plate-shaped single crystal powder has a relatively low production cost, relatively simple process has a relatively advantageous advantage in mass production, and thus, many studies on the composite production using the same have been reported. Carmen Niscbik et al., J. Am. Cream. Soc., 74 [10] 2464-68 (1991) and Australian Patent No.

Among the plate materials, in particular hexagonal plate alpha alumina has attracted much attention as a secondary material of the composite because it is chemically very stable and excellent mechanical properties. In the case of single crystal of alpha alumina, the bending strength is about 600 MPa at room temperature, and the Vickers degree value has a very high value of 18 to 23 GPa. Thus, alumina single crystals are very useful as secondary materials of composite materials because of their excellent mechanical properties.

The method of growing plate-shaped alumina single crystals has been studied since the early 1900s, and the Bernoulli method, the Chokrasky method, the hydrothermal method, and the ablation method are known. Among these, the flux method has the advantage that the device is simple and can be grown at a relatively low temperature.

The preparation of the alumina single crystal on the hexagonal plate by the flux method began with the growth of the single crystal using the cryolite mineral as a flux. Cryolite is a fluorine-based compound obtainable in nature is known to play a role in inhibiting the growth of the [0001] direction in the alumina crystal system and lowering the transition temperature to alpha alumina. Keiji Daimon et al., In his paper [Keiji Daimon and Etsuro Kato, J. Crystal Growth, 75, 348 (1986)] precipitated aluminum sulfate crystals from an aqueous aluminum sulfate solution to form a powder, followed by 25wt% AlF₃ and It has been announced that it is possible to prepare a hexagonal plate-shaped alumina powder having a size of 20 to 30 μm by heating several hours at 900 to 1200 ° C., which are cited in the paper [P. Hartman, J. Crystal Growth, 49, 166 (1980)]. Therefore, AlF₃ could be used as a mineralizer to inhibit the growth of the alumina in the C-axis direction and to help crystal growth, thereby obtaining hexagonal alumina crystals.

According to French Patent No. 2441584, aluminum hydroxide is used as a raw material of alumina, and it is reacted at 1200 to 1450 ° C using cryolite as flux, and the diameter is 16 to 250 μm and the thickness-to-diameter formation ratio is about 3 to 7 A relatively large alpha alumina plate-like single crystal powder having was prepared.

According to French Patent No. 262075, tubular alumina powder was prepared by using boehmite, transitional alumina, and gamma alumina, which is a transitional alumina, as a flux and using LiF₂ and AlF₃ as a flux.

According to French Patent No. 2319580, an alpha alumina hexagonal plate-like powder having an average particle diameter of about 6 to 8 μm was prepared by hydrothermal method.

On the other hand, Japanese Patent No. 60-54916 discloses hexagonal plate-shaped alpha alumina particles having a shape ratio of 5 to 15 through heat treatment at 800 to 1300 ° C by forming a precipitate from a liquid phase reaction between an aqueous solution of aluminum sulfate and sodium carbonate. It describes a method of making a process.

However, these prior arts have some problems. For example, in the case of the fluxing method, the alpha aluminum must be soluble at high temperature, so it is difficult to select the flux, and in order to obtain the final product after the completion of the reaction, an acid treatment or washing to separate the flux and the product is required, so a continuous process is performed. There are many difficulties to do this. In addition, the hydrothermal method has the advantage that the plate-like powder can be produced at a low temperature, but the high pressure is maintained during the reaction, so there are various restrictions in industrialization. In addition, in the solution method for producing a plate-like powder through the reaction of the liquid phase, the process is complicated, the production cost increases, and there is a problem that the harmful gas generated during heat treatment must be disposed of.

Therefore, the present inventors have studied to improve the above problems, according to the gas phase reaction of the fluorine-based compound and the element containing aluminum element, the continuous process is possible, and the catalyst gas used in the reaction can be recovered and reused, and the gas Since the catalyst in the state is used, it is not necessary to post-treatment to remove the flux after the reaction as in the flux method, and thus the manufacturing cost can be lowered, and the alpha-alumina hexagonal plate powder with high purity can be obtained, thus completing the present invention. It became.

That is, the present invention relates to a method for producing alpha-alumina single crystal powder on hexagonal plate by vapor-reacting an alumina element-containing material and a fluorine-based catalyst and its use as a composite reinforcing material.

In the present invention, amorphous aluminum, aluminum oxide, or transitional alumina may be used as the aluminum element-containing material. Among them, aluminum hydroxide before forming alpha-phase alumina, specifically boehmite (A100H), gibbsite (Al (OH) ₃) ), Bayerite (Al (OH) 3) or Diaspore (Al00H) and transitional alumina, such as gamma alumina.

In the present invention, the fluorine-based compound (catalyst) used to prepare the hexagonal alumina powder includes AlF3, NH, F, NH₄HF2, HF, and the like. Generally, a ratio of 0.15 moles or more of fluorine atoms per mole of aluminum atoms present in the aluminum element-containing material. Used as. Since the catalyst material added to the reaction is always mixed in an excess amount, it is possible to recover the excess catalyst gas and to allow a continuous process to lower the manufacturing cost. The catalyst gas serves to inhibit the growth of the alpha alumina in the [0001] direction.

The alumina hexagonal plate powder of the present invention is prepared through the gas phase reaction of the aluminum-containing material and the fluorine catalyst. That is, the fluorine-based catalyst in a gaseous state is reacted with an aluminum-containing material at about 1000 ° C. or more to prepare a tubular single crystal powder. If the reaction is expressed by chemical reaction formula is as follows.

Here, n represents hydration water and XF represents a fluorine compound.

When the transition alumina or aluminum hydroxide is used as the aluminum element-containing material, it is generally known that the transition to the alpha phase occurs at 1200 ° C. or higher, but in the reaction of the present invention, the transition temperature is about 200 ° C. due to the influence of the fluorine-based catalyst gas. Can be lowered. Therefore, alpha phase alumina single crystal can be obtained by making it react at 1000 degreeC.

The hexagonal plate-shaped alumina single crystal powder produced by the present invention is several microns in size, has a regular hexagonal shape, and reinforces the composite having ceramic, metal, polymer, etc. as a matrix material by using the excellent mechanical and chemical properties described above. Can be used as a material.

According to the present invention, a method for preparing hexagonal alumina single crystal powder is specifically as follows.

First, a mixture of hydroxide alumina powder or transition alumina powder as an aluminum-containing material and a material which can be used as a catalyst are mixed uniformly so that the molar ratio of aluminum atoms to fluorine atoms of the aluminum raw material and the fluorine compound is 1: 0.15 or more. The mixed powder is placed in an alumina crucible and subjected to heat treatment at a temperature of 1000 to 1300 ° C. for at least 1 hour, preferably at 1000 ° C. for 1 to 10 hours to prepare a hexagonal plate-shaped alumina single crystal powder having a diameter-to-thickness ratio of about 10.

In the above method, when NH₄F and NH₄HF₂ are used as catalysts, the catalysts are thermally decomposed into the gaseous phase NH3 and HF at a temperature lower than the reaction temperature. Thus, as shown in FIG. 4, the catalysts are thermally decomposed in advance at a temperature of 200 to 300 ° C. And then into a reactor with a carrier gas, for example air, nitrogen or argon.

In addition, in the case of using a gaseous or liquid catalyst at room temperature, as shown in FIG. 3, a substance having a sufficient vapor pressure at room temperature, such as HF, for example, is directly introduced into the reactor together with a carrier gas. For example, HF solution can be controlled by raising the temperature, such as by heating the bath, to control the vapor pressure of the catalyst gas, the gas generated at this time is introduced into the reactor with the carrier gas.

On the other hand, the heat treatment time depends on the heat treatment temperature, in the case where the heat treatment temperature is 1000 ℃ or less, for example 900 ℃, the alpha phase and all abnormalities coexist and the heat treatment at least 10 hours to obtain the alpha phase hexagonal plate-like powder. However, at a temperature of 1000 ° C. or more, if only one hour or more treatment is performed, it does not affect the phase change or the aspect ratio very much. In addition, the phenomenon that the formation ratio decreases as the heat treatment temperature is increased was confirmed by the analysis of SEM pictures.

The invention is further illustrated by the following examples, which are not intended to limit the invention.

Example 1

The alumina secondary butoxide is hydrated in deionized water of 80 ° C. or higher to obtain an alumina sol solution. This solution was dried to obtain an alumina gel powder having a size of 3 μm.

The powder was uniformly mixed with 5 mol% AlF 3, placed in an alumina crucible, and heated in an electric furnace at 1000 ° C. for 1 hour to prepare a hexagonal powder. The crystal state of this powder was confirmed by SEM, which is shown in FIG. In addition, X-ray diffraction analysis was carried out to confirm the phase of the powder prepared, and the results are shown in FIG. As can be seen in Figure 2 it can be seen that only the alpha phase is present in the alumina powder prepared according to the present invention. In addition, the particle size distribution was confirmed using a Malvern Laser Particle Size Analyzer to determine the particle size of the powder, the average particle diameter was 7μm.

Example 2

Alumina gel powder and HF gas catalyst prepared in the same manner as in Example 1 were used, and a hexagonal plate powder was prepared at the same temperature and time as in Example 1 using an experimental apparatus as shown in FIG.

Examples 3 and 4

Alumina gel powder prepared in the same direction as in Example 1 and NH₄F and NH₄HF₂ catalysts were used, and hexagonal plate powders were prepared at the same temperature and time as in Example 1 using an experimental apparatus as shown in FIG. It was.

Examples 5 and 6

Gibsite, which is aluminum hydroxide, and gamma alumina, which is a transition alumina, were used as aluminum raw materials, respectively, to prepare alpha-alumina powder on a hexagonal plate at the same temperature and time as in Example 1. X-ray diffraction analysis was performed to confirm the phase of the prepared powder. As a result, it was confirmed that the phase was the alpha alumina phase as shown in FIG. In addition, it was confirmed by optical microscope observation that a hexagonal powder was formed.

Example 7

The alumina gel powder prepared in the same manner as in Example 1 was dry mixed with AlF 3 and then placed in an alumina crucible in an electric furnace, and the heat treatment time was 1, 3, 5, 7 and 10 hours as shown in the following table. Alumina powder was prepared by conversion to 900, 1000, 1100 and 1200 ° C.

The shape and phase of the powder after heat treatment were confirmed by SEM and X-ray diffraction analysis. The results are shown in the following table and it was confirmed that all samples treated at 1000 ° C. for 1 hour or longer had a thin phase of alpha phase and regular hexagon.

TABLE 1

N: The powder on the hexagonal plate is not formed.

P: A part of alumina powder is formed in hexagonal plate shape.

A: The whole alumina powder is formed in the hexagonal plate shape of alpha phase.

Claims (9)

A method for producing a hexagonal plate-shaped alpha alumina single crystal powder, characterized by reacting an aluminum element-containing material and a fluorine compound at 1000 to 1300 ° C. for 1 to 10 hours. The method of claim 1, wherein the aluminum element-containing material is boehmite, gibbsite, bayerite, diaspore or gamma alumina. The method of claim 1, wherein the fluorine-based compound is AlF₃, NH₄F, NH₄HF₂ or HF. The method according to claim 1, wherein the aluminum element-containing material and the fluorine compound are reacted so that the molar ratio of aluminum atoms to fluorine atoms is 1: 0.15 or more. The method according to claim 3, wherein the fluorine-based compound is NH₄F or NH₄HF2, and a corresponding gas obtained by thermal decomposition of NH₄F or NH₄HF2 in advance at a temperature of 200 to 300 ° C is introduced into the reactant together with a carrier gas. . The method according to claim 3, wherein the fluorine compound is HF, and the HF is introduced into the reaction system together with a carrier gas in a gaseous state. The method according to claim 1, wherein the powder produced has a size of 1 to 10 m and a diameter to thickness ratio of 5 to 15. A hexagonal plate-shaped alpha aluminum single crystal powder prepared according to the method of claim 1 for use as a reinforcing material in a composite. The powder of claim 8, wherein the composite comprises a ceramic, metal or polymer matrix.