JP2649813B2 - Method for producing silicon nitride whiskers from rice husk - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing silicon nitride whiskers from rice hulls.

Reinforcing materials for composite materials such as FRM (fiber reinforced metal) and FRC (fiber reinforced ceramic), which are attracting attention as next-generation structural materials, include whiskers, polycrystalline, amorphous short fibers, and continuous fibers. Is considered. Among them, whiskers are a single crystal having a large shape ratio (length / diameter), and have high strength as compared with other fibers, and are therefore particularly promising materials.

Whiskers that are currently being studied for practical use include silicon nitride (Si ₃ N ₄ ), silicon carbide (SiC),
And the like potassium titanate _{(K 2 O · 6TiO 2)} . In particular, silicon nitride has attracted attention as a reinforcing material for composite materials used at high temperatures because it has excellent high-temperature strength and thermal shock resistance and exhibits a tensile strength close to a theoretical value.

Conventional technology and its problems As a method for producing silicon nitride whiskers, a solid phase method,
A liquid phase method and a gas phase method are known, and a solid phase method and a gas phase method are industrially employed.

The solid phase method, or contacting the N ₂ gas to the SiO ₂ powder, SiO ₂
The N ₂ gas or the N ₂ —NH ₃ mixed gas is brought into contact with the —C—fluoride-based mixture to obtain a silicon nitride whisker. However, the silicon nitride whiskers obtained by this solid phase method are not uniform in shape, have relatively large diameters, and have large variations.

The gas phase method includes (a) a method of bringing NH ₃ gas into contact with silicon chloride, (b) a method of reacting monosilane (SiH ₄ ) with an NH ₃ gas, (c) silicon diimide [｛Si (NH) ₂ ｝ _x ] is thermally decomposed in an N ₂ gas atmosphere or an NH ₃ gas atmosphere. However, these methods include:
In addition to the common drawback that the raw materials used are expensive, (a) chlorine is mixed into the product, and (b) monosilane is oxidized in the air intensely. And it is difficult to treat waste gas. (C)
Thus, there is a problem that the imide remains in the product, so that the smell of NH ₃ remains.

Means for Solving the Problems The present inventor has been thinking in view of the state of the art as described above. In the process, we focused on rice husks, which are currently produced in Japan at about 3 million tons (1986). That is, the rice hulls contain about 15 to 20% by weight of SiO _2, although there is a slight change depending on the production area, and occupy about 95% of the inorganic components of the rice husks. However, most of the rice husk is disposed of as agricultural waste, and only a portion is used as fuel. Therefore, if the above-mentioned problems of the prior art can be solved and this rice husk can be effectively used, a new silicon resource can be found. As a result of further research based on such a new idea, the present inventors have finally completed the present invention. That is, the present invention provides the following method: "(a) treating dried rice hulls with an inorganic acid, filtering and washing solids to obtain an acid-treated rice hull; and (b) A) heat-treating the obtained acid-treated rice husks in the presence of an organic iron compound and in the flow of N ₂ gas to grow silicon nitride whiskers. The rice husk used as a starting material in the present invention may be used as it is, but it is preferable that it be washed and dried in advance because it usually contains contaminants.

The rice hulls, which have been washed and dried as required, are treated with a dilute solution of an inorganic acid. As the inorganic acid, nitric acid, hydrochloric acid, sulfuric acid and the like are used.The concentration of the solution is not particularly limited, but in the case of nitric acid and hydrochloric acid, it is about 0.5 to 2 N, and in the case of sulfuric acid, it is about 0.1 to 0.5 N. Is preferred. The acid treatment may be performed under conditions (temperature and time) capable of removing components other than SiO ₂ (organic substances, P ₂ O ₅ , K ₂ O, CaO, etc.),
Although not particularly limited, it is preferable to carry out the reaction at about 80 to 100 ° C. for about 1 to 3 hours.
Next, the treatment liquid is filtered, and the remaining solid content is washed with water and dried to obtain an acid-treated product. Washing and drying methods, etc.,
There is no particular limitation, and conditions may be employed such that the acid is removed and moisture is removed to such an extent that the reaction is not hindered during the subsequent reaction.

The acid-treated product thus obtained is placed in a reactor, brought into contact with N ₂ gas in the presence of an organic iron compound placed on a carbon plate, and kept at a predetermined temperature. Examples of the organic iron compound include ferrocene [Fe (C ₅ H ₅ ) ₂ ], tetracarbonylbisdiiron [Fe (CO) ₄ (C ₂ H ₅ ) ₂ ], bis (η-indenyl) iron (II) [Fe (C ₉ H ₇ ) ₂ ], methyl ferrocene [Fe (C ₅ H ₅ ) (C ₅
H ₄ CH ₃ )], formylferrocene [Fe (C ₅ H ₅ ) · (C ₅ H ₄ C
HO)], ferrocene monocarboxylic acid _{[Fe (C 5 H 5)} (C 5 H 4
COOH)], methyl ferrocene monocarboxylate [Fe (C
_{5 H 5) (C 5 H} 4 COOCH 3)] and the like. The organic iron compound is an essential component in the present invention for obtaining high-quality silicon nitride whiskers having a long size and a uniform diameter, and dissolves gas phase species such as SiO, C, and N ₂ during the reaction, These become supersaturated and VLS (vapor, liquid, soli)
d) It is estimated that silicon nitride whiskers are grown by the mechanism.

_{SiO 2 + C → SiO + CO} 3SiO + 3CO + 2N 2 → si 3 N 4 + 3CO 2 In the case where in the absence of an organic iron compound is subjected to heat treatment for viewing Karasan treated is short dimension, slightly large diameter, irregular low quality Is obtained.

The flow rate of N ₂ gas is 1 per unit sectional area (cm ² ) of the reactor.
It is preferable to be about 0 to 40 ml / min. If the flow rate of N ₂ gas is less than 10 ml / min, whisker growth will be insufficient, while if it exceeds 40 ml / min, many defects will occur.
Whiskers of irregular diameter tend to be formed. The heat treatment is preferably performed at about 1350 to 1500 ° C. When the heat treatment temperature is lower than 1350 ° C., whiskers are hardly formed, while when the heat treatment temperature is higher than 1500 ° C., SiC is generated. The heat treatment time depends on the desired size of the whisker,
It varies depending on the size of the reaction apparatus, the reaction temperature and the like, and is not particularly limited, but is usually about 8 to 10 hours.

After the end of the nitriding reaction, in order to prevent oxidation, the system is cooled under a flow of N ₂ gas until the temperature reaches about 900 ° C., and then cooled to room temperature.

The silicon nitride whiskers thus obtained consist essentially of α-Si ₃ N ₄ and usually have an average purity (as Si ₃ N ₄ ) of from 97 to
About 98% or more, length about 100-2000 μm and diameter 0.1-0.2.
Those having a diameter in the range of about 5 μm and obtained under the same reaction conditions have relatively uniform lengths and diameters.

According to the present invention, the following effects are achieved.

(A) Since rice husks conventionally disposed of as agricultural waste are used as a silicon source, the cost of raw materials is greatly reduced.

(B) The treatment of rice hulls with an inorganic acid results in the formation of SiO ₂
Since other components are removed in advance, high-purity silicon nitride whiskers can be obtained.

(C) The manufacturing equipment may have a simple structure.

(D) By using the organic iron compound, a long silicon nitride whisker can be obtained.

(E) The obtained silicon nitride whiskers have few defects and uniform length and diameter.

EXAMPLES Examples are shown below to further clarify features of the present invention.

Example 1 The present invention was implemented using an apparatus whose outline is shown as a sectional view in FIG.

The apparatus shown in FIG. 1 includes a carbon tube (3) inserted into an alumina tube (1), a thermocouple (5), and a heating element (not shown) provided around the alumina tube (1). ing. Inside the carbon pipe (3), a carbon container (15) having perforated wall plates (11) and (13) perpendicular to the N ₂ gas flow direction (7) and (9), and a horizontal carbon plate (17) ) And a perforated carbon plate (19).

The rice hulls were placed on a 9-mesh wire net, washed thoroughly with water from above, and dried at 60 ° C. for 24 hours. Next, 230 ml of 1N HNO ₃ was added to 10 g of the dried rice husks, and the mixture was heated at 90 ° C. for 3 hours.
For 24 hours to obtain acid-treated rice hulls.

1 g of the acid-treated rice hulls thus obtained was placed in the carbon container (15), and 0.1 g of ferrocene was placed in an area of 1.
It was evenly spread on an 8 × 10 cm ² carbon plate (17). In this state, the N ₂ gas is supplied through the unit cross-sectional area (c
The temperature in the carbon container (15) was lowered by 5 while flowing in the direction of arrows (7) to (9) at a rate of 32 ml / min per m ² ).
The temperature was raised at a rate of ° C / min, and a nitriding reaction was performed at 1400 ° C for 10 hours.

After the nitridation reaction, the temperature in the carbon container (15) is 90
The N ₂ gas was continued to flow until the temperature dropped to 0 ° C, and then allowed to cool naturally to room temperature in air.

Thus, a reaction product was formed mainly on the outside of the perforated wall plate (13) and on the carbon plate (17). A small amount of carbon was mixed in the obtained product.
White silicon nitride whiskers having a size of about 2000 μm and a diameter of about 0.1 to 0.3 μm were obtained.

The obtained silicon nitride whiskers were analyzed by X-ray diffraction.
It was confirmed to be α-Si ₃ N ₄ .

A small amount of silicon nitride whiskers (α-Si ₃ N ₄ ) were also formed in the carbon container (15), and this whisker had a length of 100 μm or less, a diameter of about 0.2 to 2 μm, and a diameter of about 0.2 to 2 μm. It was slightly irregular.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an outline of a manufacturing apparatus used in Example 1 of the present application. (1) Alumina tube (3) Carbon tube (5) Thermocouple (7) Flow direction of N ₂ gas (9) Flow direction of N ₂ gas (11) Perforated Wall plate (13): Perforated wall plate (15): Carbon container (17): Horizontal carbon plate (19): Perforated carbon plate

Claims (1)

(57) [Claims] (A) treating dried rice hulls with an inorganic acid, filtering and washing solids to obtain acid-treated rice hulls; and (b) treating the obtained acid-treated rice hulls with an organic iron compound. A process of growing silicon nitride whiskers by heat-treating in the presence of N ₂ and flowing N ₂ gas to produce silicon nitride whiskers using rice husk as a raw material.