JPH03265600A - Production of mullite whisker - Google Patents

Abstract

PURPOSE:To obtain high purity mullite whiskers with no stuck particles at a low cost by converting a mullite forming sol obtd. by mixing a boehmite sol with a silica sol into gel with HF, drying and firing the gel. CONSTITUTION:A proper amt. of an acid is added to an aq. dispersion of boehmite under heating to >=80 deg.C to prepare a boehmite sol by peptization and this sol is mixed with a silica sol in 1.37-1.76 molar ratio of Al2O3 to SiO2 to obtain a mullite forming sol. This sol is converted into gel by adding HF in 0.01-0.1 molar ratio of HF to 3Al2O3.2SiO2 and the gel is dried at <=120 deg.C, pulverized and fired by heating to 1,000 deg.C in an open system and from 1,000 deg.C to 1,200-1,600 deg.C in a hermetically sealed system.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a method for producing mullite whiskers, which are used, for example, as fillers for whisker-reinforced composite ceramics and polymers.

[Conventional technology]

Mullite is 3^120.・2Si02(to1□Os/S
iO2 molar ratio = 1.5) is aluminum silicate, but J. varnish pasta (J, ^, Pa5k)
Research paper (Ceramic International
l, 9(4), 107-113 (1983))
According to ^1□Ch/SiO2 (molar ratio) is in the range of 1.5 to 2.87 to form a solid solution with alumina. Mullite has traditionally been used in chemical industrial porcelain and refractories, but in recent years, mullite has been used for its low coefficient of thermal expansion, low dielectric constant, and excellent mechanical properties such as stability at high temperatures and thermal shock resistance. Therefore, it can be used as a ceramic for electronic materials (IC substrate or package) that surpasses alumina, and
It is seen as a promising material for high-temperature structural ceramics (high-temperature conveyance rollers, firing tools) that surpasses silicon carbide. However, mullite ceramics have a problem of inferior fracture toughness. As a means to solve these problems, a method has been proposed in which fibrous or whisker-like reinforcing materials, such as carbon fibers or silicon carbide whiskers, are incorporated into mullite ceramics. Also, a method of incorporating alumina whiskers or mullite whiskers is also being considered. Conventionally, a gas phase reaction method has been proposed as a method for producing mullite whiskers. Furthermore, as shown in Japanese Patent Application Laid-open No. 1-212299, a method has been proposed in which 1 to 30 at. There is.

[Problem to be solved by the invention]

By the way, the above-mentioned gas phase reaction method has disadvantages in that the growth rate of mullite whiskers is slow, it takes time, and furthermore, the production cost is high because the raw material gas is expensive. Also, ^IF proposed in Japanese Unexamined Patent Publication No. 1-212299. In the method of adding IF, since it is a solid particle, it is difficult to mix it uniformly with the mullite raw material, and whiskers are only partially obtained. Furthermore,
The obtained mullite is ^120 U = 73.7 mol%^1
□0. Theoretical composition of mullite (mullite ^12
There was also a drawback that there was a considerable deviation from the value (0s = 60 mol%). The first object of the present invention is to provide a method for producing high-quality mullite whiskers without particle sticking. A second object of the present invention is to provide a method for producing mullite whiskers with high purity close to the theoretical composition. A third object of the present invention is to provide a method for manufacturing mullite whiskers at low cost.

[Means to solve the problem]

The purpose of the present invention is to provide a mullite sol preparation step in which a mullite composition sol is prepared by mixing a boehmite sol and a silica sol, a gelling step in which HF is added to the mullite composition sol to gel it, and this gelling step. This is achieved by a method for producing mullite whiskers, which is characterized by comprising a drying step of drying the obtained gel, and a firing step of burning the dried product obtained in this drying step. In the above method for producing mullite whiskers, the mullite composition sol has a molar ratio of ^hoz/5iOz of 1.
It is preferable that the boehmite sol and the silica sol are mixed so that the ratio is in the range of 37 to 1.76, and
3^120.・2SiOz (molar ratio) is 0.01 to 0.
It is preferable that HF is added so that the temperature becomes 1, and it is preferable that the temperature is lower than 120°C in the drying process, and in the firing process, the temperature is in an open system up to 1000°C, and the temperature from 1000°C to 1200°C is It is preferable that the firing is carried out in a closed system up to a firing temperature of 1600°C. Here, it is preferable that the mullite composition sol has a ^1zOs/5iO2 (molar ratio) of 1.37 to 1.76.
．． If the value is smaller than 37, cristobalite (Si(
h) tends to precipitate, and conversely, if the value is larger than 1.76, corundum (α-^1□0.) tends to precipitate. Therefore, in order to produce high-purity mullite whiskers, it is desirable that ^120z/5iOi (molar ratio) is 1.37 to 1.76. When producing a mullite composition sol, it is preferable to use a boehmite sol, and this boehmite sol is prepared by heating an aqueous dispersion of boehmite to 80°C or higher and adding an inorganic acid such as nitric acid or hydrochloric acid or an acetic acid or formic acid. It is obtained by peptizing it by adding an appropriate amount of organic acid. A boehmite sol can also be obtained by peptizing γ-alumina, which has a spinel-type defect structure and has high activity, with the above-mentioned acid. Further, as the silica sol, colloidal silica (silica sol) or silica fine particles having high reaction activity can be used. Colloidal silica is a colloidal solution in which ultrafine silica particles are dispersed in water, and examples of silica particles include white carbon produced by a wet method and fumed silica produced by a dry method. Further, HF was selected because HF is an aqueous solution, which enables uniform mixing in a wet process. Then, the amount added is HF/3^120.・2SiOz
The reason why it is preferable to set the molar ratio to 0.01 to 0.1 is because if it is less than 0.01, there will be no addition effect, and if it is more than 0.1, fixed particles other than whiskers will be formed. This is because there was a tendency to The reason why it is preferable to dry the gel at a temperature lower than 120°C is that if the gel is dried at a temperature higher than 120°C, HF will evaporate, and the reaction [M−○−OH+HF−+M−0−F×H20 (M is AI , Si) becomes insufficient, making it difficult to obtain mullite whiskers. Then, by finely pulverizing the mullite composition gel produced as described above using a pulverizing device such as a ball mill or stirring mill while taking care to avoid contamination, high-purity mullite powder can be obtained. The obtained product is fired, and the reason why it is preferable to perform this firing process in an open system up to 1000°C is to dehydrate the hydrate, and it is preferable to perform it in a closed system from 1000°C to the firing temperature. The reason why it is preferable to carry out this process is to allow Ftj, which is decomposed during firing, to sufficiently contact the ^1□03.SiO2 source.

【Example】

First, alumina sol and silica sol, which serve as starting sol raw materials, were prepared in the following manner. Alumina sol (boehmite sol) is commercially available boehmite powder (Catapal B^120 from Vista Chemical Company).
73.0 wt%) was added to ion-exchanged water together with nitric acid, and the mixture was heated at 80° C. under normal pressure for 3 hours. The silica sol was commercially available colloidal silica powder (Nippon Silica Co., Ltd., N1psil E220A 5i02 content 92.
4 wt%) was added to ion-exchanged water together with nitric acid, and the pH was adjusted to 3 or less under normal pressure. Furthermore, the above alumina sol and silica sol are mixed with ^1□O
A mullite composition sol was prepared by mixing at x/SiO2 (molar ratio) in the range of 1.37 to 1.76. Next, 47% hydrofluoric acid was added to the mullite composition sol using HF/
3^120.・2Si02 (molar ratio) is 0.008 to 0
．． Add in a range of 15 to gel, and the f & 90 to 1
It was dried at 30°C for 48 hours and ground in a ball mill. The gel powder thus obtained was heated to 600°C in an open system.
After heat treatment for 30 minutes at a temperature of
It was baked for 1 hour at a temperature range of 150 to 1650°C. The composition of the product obtained as above is X! ! The product was analyzed by diffraction and the morphology of the product was observed by electron microscopy, and the results are shown in Table 1. Further, FIGS. 1 and 2 show electron micrographs and electron diffraction patterns of mullite whiskers obtained in the above example. As is clear from these photos, the product is <OO1>
It is a single-crystal whisker that has grown in the direction, and the aspect ratio (length/diameter ratio) is as large as 10 or more, indicating that it is only a uniform whisker. [Effects of the invention] ■ Wet process using HF Since you will be hired,
Mixing becomes uniform, particles are not stuck to the whiskers, and high-quality whiskers are obtained. ■ Since the reaction is based on the sol-gel method, highly pure mullite with a composition close to the theoretical composition can be obtained. ■ Mullite whiskers can be obtained at low cost because inexpensive raw materials are used and the manufacturing process is simple.

[Brief explanation of drawings]

FIG. 1 is an electron beam photograph of a mullite whisker produced according to the present invention, and FIG. 2 is an electron diffraction pattern of a mullite whisker produced according to the present invention.

Claims (5)

[Claims] (1) A mullite sol preparation step in which a mullite composition sol is prepared by mixing boehmite sol and silica sol, a gelling step in which HF is added to the mullite composition sol to gel it, and a gel obtained in this gelling step. A method for producing mullite whiskers, comprising a drying step of drying and a firing step of firing the dried product obtained in the drying step. (2) Al_2O_3/SiO_2 of mullite composition sol
2. The method for producing mullite whiskers according to claim 1, wherein boehmite sol and silica sol are mixed so that the molar ratio is in the range of 1.37 to 1.76. (3) The method for producing mullite whiskers according to claim 1, wherein HF is added so that HF/3Al_2O_3·2SiO_2 (molar ratio) is 0.01 to 0.1. (4) The method for producing mullite whiskers according to claim 1, wherein the drying step is performed at a temperature lower than 120°C. (5) In the firing process, temperatures up to 1000°C are open system;
The method for producing mullite whiskers according to claim 1, wherein the firing is carried out in a closed system from 1000°C to a firing temperature of 1200 to 1600°C.