JPS5939704A - Method for synthesizing silicon nitride - Google Patents

Abstract

PURPOSE:To manufacture alpha-type silicon nitride crystals as a starting material for a sintered body while preventing conversion into beta-type crystals due to hot spots by providing specified physical properties to amorphous silicon nitride powder and heat treating the powder under specified conditions. CONSTITUTION:Amorphous silicon nitride powder having 0.1-0.5 bulk specific gravity is filled on a tray-shaped calcining plate by 2-10kg/m<2> basing on the area of the available heating surface of the plate and 10-40kg/m<2> basing on the area of a projected plane of the plate, and the powder is calcined at 1,450- 1,750 deg.C temp. of the heating surface of 0.1-4hr in a nonoxidizing atmosphere. The amount of fine powder to be treated is increased without increasing the size of the apparatus, hot spots which are liable to cause conversion into beta-type crystals are not formed, and alpha-type silicon nitride crystals as a starting material for a sintered body are obtd. The hot spot phenomenon is easily avoided by making the vertical section of the calcining plate uneven.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for synthesizing silicon nitride, and particularly to the firing method for obtaining silicon nitride by firing an amorphous silicon nitride.

Silicon nitride sintered bodies have recently attracted attention from various quarters as a variety of heat-resistant materials, and their high corrosion resistance has led to a wide range of uses.

Among these uses, blades for gas turbines,
It is said to be particularly useful for radiant tubes in high-temperature furnaces because of its excellent properties.

Various methods have been proposed for producing silicon nitride, but among these, the method of causing a gas phase reaction between silicon tetrachloride and ammonia is known as the most popular method. In this method, silicon tetrachloride and ammonia are generally added in the amount necessary for the reaction in the absence of oxygen at a concentration of 590 to 15
A gas phase reaction is carried out at 00° C. to synthesize alpha crystals of silicon nitride in one step, and this synthesized product is immediately used as a raw material for a sintered body.

However, according to studies conducted by the present inventors, such a one-stage synthesis method has a low conversion rate to α-crystalline silicon nitride, which is considered to be preferable as a sintering raw material. It was found that compounds were mixed in, which inhibited sinterability.

In view of this, the present inventor divided the synthesis reaction into two stages,
As a first step, silicon tetrachloride and ammonia are reacted at a specific ratio for a specific time and at a specific temperature to form amorphous silicon.
The compound is actively synthesized, and then as a subsequent reaction, it is subjected to a specific period of time in a nitrogen or ammonia stream.
It was discovered that α-crystalline silicon nitride with good sintering properties could be obtained in high yield by maintaining it at a specific temperature, and a patent application was filed in 1973.
It was proposed as No. 3-4'1712.

However, in this series of methods, there is a problem in which β crystals, which are a crystal form undesirable as a raw material powder for a sintered body, are formed in part, depending on the means of carrying out the reaction in the latter stage. That is, in small-scale production of silicon nitride raw material powder, the reaction format can be arbitrarily selected from a relatively wide variety, and the formation of β crystals can be suppressed relatively easily.

However, as the scale of production increases, the reaction format is constrained by thermal energy, operability, equipment costs, etc., and it becomes difficult to suppress the production of β crystals.

In the latter stage reaction as a large-scale production method, generally the obtained silicon nitride is charged into a reaction vessel and heated from the outside of the vessel to α-crystallize it.

Here, silicon nitride as a sintering raw material is desirably as finely powdered as possible in order to improve the sinterability of the sintered body. For this reason, it is desirable that the silicon nitride obtained in the first stage is also in the form of fine powder. However, if the silicon nitride obtained in the previous step is in the form of a fine powder, the bulk specific gravity will be small and it will have heat insulating properties and will also generate a large amount of heat when it transforms into α crystals. For this reason, the so-called hot spot phenomenon occurs in which the fine powder becomes extremely hot near the middle layer of the layer filled in the container.
This part tends to become β crystallized.

In view of these points, the present inventor has developed various methods for the purpose of finding a processing means for the latter stage that can increase the amount of fine powder that can be processed without increasing the size of the device as much as possible, and that does not cause hot spots. As a result of research and consideration, it has been found that the above object can be achieved by imparting specific physical properties to the powder to be treated and heat-treating the powder under specific conditions.

Thus, in the present invention, amorphous silicon nitride powder having a bulk specific gravity of 0.1 to 0.5 is placed on a shelf-shaped fired plate at a rate of 2 to 10 Ky/i with respect to the actual heat transfer area of the plate. , and silicon nitride, which is filled with 10 to 40 Kq/m' to the projected area of the shelf surface and fired in a non-oxidizing atmosphere at a heat transfer surface temperature of 1450 to 1750°C for 01 to 4 hours. To provide a method for the synthesis of

In the present invention, the shelf-shaped fired plate is not just a flat plate, but its vertical cross-sectional shape has various uneven shapes. For example, the shapes are Koji J, l Hatake f, Hehe F, ] Tatami F.

υv, W, to the whore! It is possible to employ other appropriate uneven shapes such as ridges, ridges, etc., and combinations thereof. The reason for adopting these shapes is to prevent more than a certain amount of the nitride powder to be fired to be filled into one recess, thereby avoiding the hot spot phenomenon. That is, in the present invention, if the total projected area of the shelf board is determined to be a size, an appropriate shape is selected from among the above-mentioned cross-sectional shapes in order to prevent the generation of hot spots.

In the present invention, the shelf board surface constitutes a heat transfer surface no matter what uneven shape it has, and the shelf board surface that is in direct contact with the nitride powder to be fired is the actual heat transfer area according to the present invention. . Usually, the actual heat transfer area is a recess.

In the present invention, the amorphous silicon nitride powder to be fired must have a bulk specific gravity of o, i to o5. If the bulk specific gravity is less than the above range, industrial production efficiency will be poor, and conversely if it exceeds the above range, α
Both are unsuitable because the amount of heat generated per unit volume during the transition to crystals becomes too large and hot spots are likely to occur.

Further, the amount of the powder supplied to the actual heat transfer surface of the shelf plate is 2 to 10
9/7 is required. If the amount is less than the above range, industrial production efficiency will be poor, and if it exceeds the above range, hot spots are likely to occur due to insufficient heat removal due to α crystal transition. Therefore, both are inappropriate.

Furthermore, in the present invention, it is necessary to fill the powder at 10 to 40 kg/m' with respect to the projected area of the shelf surface. If the filling amount is less than the above range, industrial production efficiency will be low, and conversely, if it exceeds the above range, no matter how specific the shape of the shelf board and firing conditions are, it will essentially become a hot spot. Both of these methods are unsuitable, since the object of the present invention cannot be achieved.

Thus, in the present invention, the powder is fired for 01 to 4 hours in a non-oxidizing atmosphere at a temperature of the heat transfer surface of the shelf plate of 1450 to 1750° C., thereby obtaining an α-crystal powder serving as a raw material for the sintered body. Note that, in general, the smaller the amount of powder to be fired packed into the shelf board, the shorter the firing time tends to be.

As the material of the shelf board used in the present invention, graphite, silicon carbide sintered body, silicon nitride sintered body, alumina, etc. can be appropriately adopted, for example.

Note that as the non-oxidizing atmosphere, an atmosphere of nitrogen, hydrogen, argon, helium, etc. can be appropriately employed.

Next, the present invention will be explained using examples.

Example 1 51014 and NHa were reacted using an externally heated flow-through reactor consisting of a musciitic empty column reaction tube with an inner diameter of 70 mm and a length of 1.5 m.
The reaction was carried out at 1000℃ (supplied gas molar ratio NH3/5
il14=1.2, reaction time 15θθC), particle size 0.
An amorphous silicon nitride powder having a bulk density of 5 to 2 μm and a bulk specific gravity of 0.25 was obtained. Next, the projected area is 0.408 m2 (80 crn
x51crn), width 3cn+ as a vertical section.

Height] Actual heat transfer area 1.664 with 8 grooves of 0 cIn
- Fill a graphite shelf board with the above powder 6.7 to +7 almost evenly on the shelf board, and make 15% of the projected area of the board.
4Kq/m, 3 for the actual heat transfer area. s K9/-) Baked at 1550°C x 2 (electrically heated) in a nitrogen atmosphere,
Crystallized silicon nitride powder was obtained.

The obtained powder had a particle size of 0.3 to 2 μm and an α crystal content (=”, a, J′ x 1o o ) of 90 μm.

Examples 2 to 4 Using the same reactor and firing furnace as in Example 1, Example 1
Table 1 shows the physical properties of the product obtained as a result of the same treatment.

Comparative Example The same amorphous silicon compound powder as in Example 1 was prepared with a size of 8.
0m x 5'1m x thickness 4C1n graphite shelf board (flat plate without grooves) was filled with the above powders 6.3 and 9 evenly so that the projected area of the board was 15.4 kg /
rr? , the actual heat transfer area is also 15.4 Ky/m
), 1550° C. xz & firing (electrical heating) was performed in a nitrogen atmosphere to obtain crystallized silicon nitride powder. The obtained powder had a particle size of 0.4 to 2 μm and an α crystal content of 56%.

Comparative Example The same amorphous silicon desalination powder as in Example 1 was prepared with a size of 8.
A graphite shelf board (flat plate without grooves) measuring 0 crn x 51 crn x 4 cIn thickness was filled with 6.3 kg of the above powder evenly, and the projected area of the board was 15.4 kg.
/ m", 15.4Kg/'m for the actual heat transfer area
), and fired (electrically heated) at 1550° C. twice in a nitrogen atmosphere to obtain crystallized silicon nitride powder. The obtained powder had a particle size of 04 to 2 μm and an α crystal content of 56%.

Claims (1)

[Claims] (1) Amorphous silicon nitride powder having a bulk specific gravity of 0.1 to 0.5 is placed on a shelf-shaped fired plate at a rate of 2 to 10 kg/m' relative to the actual heat transfer area of the plate. , and 10 to 40 Kg/n? with respect to the projected area of the shelf surface. 1. A method for synthesizing silicon nitride, which comprises filling the silicon nitride and firing it in a non-oxidizing atmosphere at a heat transfer surface temperature of 1450 to 1750° C. for 0.1 to 4 hours.