JPH0513906B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a silicon nitride hollow body. Here, hollow bodies include cylindrical bodies,
A shape that has a part that leads to the outside.

Among silicon nitride Si ₃ N ₄ products, those called reaction sintered bodies are usually made by applying nitrogen gas to a molded body of Si powder or a molded body of (Si + Si ₃ N ₄ ) powder mixture while nitriding it. Manufactured by sintering. This kind of product has thermal shock resistance, hardness,
It has excellent electrical insulation and chemical stability at high temperatures, and has almost no shrinkage during reaction sintering.
Because it has the advantage of achieving high dimensional accuracy,
Widely used in applications such as fireproof materials, wear-resistant materials, corrosion-resistant materials, and insulation materials.

As is common with ceramics, materials for applications that require wear resistance, heat resistance, heat insulation, etc. may not need to be solid; rather, hollow materials may be used in order to reduce weight. In many cases, it is preferable.

However, as a hollow ceramic molded body,
Until now, only a few hollow spheres (balloons) of glass or aluminium have been known, and their shapes are limited, so they cannot be used for any structural parts. With respect to silicon nitride, hollow bodies are not known and their manufacture has never been attempted.

The inventor discovered that silicon nitride has heat resistance, heat insulation, wear resistance,
Focusing on the fact that silicon nitride has many uses that take advantage of its corrosion resistance, and that parts for these uses can often be hollow bodies, we conducted research with the intention of providing silicon nitride hollow bodies, and developed the present invention. I've reached it.

The silicon nitride hollow body has a structure in which the surface layer portion is made of a silicon nitride reaction sintered body and the interior is hollow. This structure is acceptable, and sometimes preferred, for applications where only surface shape and properties are of concern.

In order to manufacture a silicon nitride hollow body having such a structure, the phenomenon that when a molded body made of fine silicon powder is sintered, the porosity of the surface layer is higher than that of the inside is utilized.

That is, the method for producing a silicon nitride hollow body of the present invention involves molding silicon powder with an average particle size of 15 μm or less, and pre-sintering the molded body to produce a pre-sintered body with a higher porosity in the surface layer than in the interior. and make it into a pre-sintered body.
Nitrogen is applied at a temperature of 1100°C or higher, but lower depending on the melting point of silicon, to convert the surface layer into silicon nitride by nitriding, and then heated to a temperature higher than the melting point of silicon to remove any unreacted silicon remaining inside. Consists of melting and draining.

More specifically, if a pre-sintered body of silicon powder with a surface layer density of 93% or less and an internal density of 95% or more is nitrided, the surface layer will almost completely convert to silicon nitride. On the other hand, the interior remains mostly Si because it is difficult for nitrogen gas to penetrate.

It is necessary that the average particle size of the Si powder be 15 μm or less in order to obtain a pre-sintered body in which the porosity of the surface layer portion and the interior portion are clearly different.

In carrying out the present invention, sintering is promoted according to the technology previously invented and disclosed by the present inventor together with collaborators, or the present inventor's subsequent research results,
It is recommended that an appropriate amount of either one, preferably both, of the substance that promotes nitriding be converted into the raw material Si powder.

Substances that promote sintering include carbon, boron, and aluminum, and boron is particularly effective. Boron is added as an element or a compound in an amount of 0.15 to 5.0% by weight to the Si powder.

Substances that promote nitriding and also contribute to improving sinterability are Fe, Co, Ni, Cr, Mo, Mn,
W, Ti, Zr, Ta, Nb, V, Mg, Ca, Cu, Zn
and Sn, one or more elements selected from these or their compounds as the above elements (in the case of two or more types, the total amount) 0.05 ~
Add 2.0% by weight.

The atmosphere for preliminary sintering may be an inert gas as is commonly used, but a vacuum or H ₂ atmosphere tends to more easily form a layer with a lower density on the surface layer.

The sintering temperature and time are selected so that the density is 95% or more in the interior and 93% or less in the surface layer, as described above. The specific conditions may vary depending on the difference in sinterability due to the particle size of the Si powder and the sintering promotion effect of additives, so they may be determined experimentally depending on each case.

In the present invention, the nitriding of the Si pre-sintered body can be carried out in the same manner as in the production of conventional silicon nitride reaction sintered bodies.

That is, it is generally heated to a temperature of 1100 to 1500°C under a nitrogen gas atmosphere at atmospheric pressure. As the atmospheric gas, in addition to N ₂ , NH ₃ or a mixed gas of N ₂ or NH ₃ with N ₂ or an inert gas can be used. The pressure may be in any range from increased pressure to atmospheric pressure to reduced pressure, but nitriding tends to proceed more quickly when the pressure is slightly reduced.

Remains unreacted inside the nitrided sintered body
Si melts and flows out by heating it above its melting point, ie, 1414°C. generated on the surface layer
Since Si ₃ N ₄ usually has 10 to 20% pores, molten Si can flow out even without providing any pores. Of course, depending on the purpose, holes may be made to speed up the outflow.

The atmosphere at this time may be an atmosphere of oxidizing gas, inert gas, _N2 , _H2 , or under vacuum. It has been experienced that an oxidizing atmosphere results in poorer wetting of Si and Si ₃ N ₄ and easier outflow through the pores. When trying to cause Si to flow out through the surface Si ₃ N ₄ layer, it is better to avoid an oxidizing atmosphere and improve wetting of Si and Si ₃ N ₄ to facilitate the flow.

According to the present invention, a hollow silicon nitride reaction sintered body can be directly produced. Conventionally, it has been necessary to use tedious methods such as preparing a hollow molded body and sintering and nitriding it, or in some cases, manufacturing it in two or more parts and joining them together. However, all these problems will be resolved.

If necessary, the hollow body can be filled with other materials such as various metals, inorganic compounds, or organic compounds, especially synthetic resins, etc., to increase its strength or give it other properties and use it as a composite body. You can also do it.

Example: Put Si fine powder with an average particle size of 0.11μ into a mold, and
It was pressed at a pressure of Kg/cm ² to preform into a cylinder with a diameter of 30 mm and a height of 30 mm, and this was further formed into a cylinder using a rubber press under a pressure of 2000 Kg/cm ² .

The molded body was heated at 1375℃ in a vacuum of 1×10 ^-3 Torr.
Preliminary sintering was performed by heating for 2 hours.

The pre-sintered body had a density of approximately 70% in the surface layer with a thickness of 2.5 to 3.0 mm, and the interior was highly dense with almost no pores.

In (N ₂ + 5% H ₂ ) gas, this pre-sintered body was heated at 1350°C x 20 hours → 1380°C x 60 hours → 1410°C x
It was nitrided by heating for 20 hours.

A sintered body with Si ₃ N ₄ on the surface and unreacted Si on the inside,
Heated to 1500℃ for 5 hours in Ar atmosphere to remove the internal Si.
was melted and allowed to flow through the surface layer.

The resulting hollow silicon nitride reaction sintered body has a diameter of approximately
It is a hollow cylinder of 24 mm x height 25 mm, and the thickness of the ₄ Si ₃ N layers is
The diameter was 2.5 to 3.0 mm, and the density was 77%.

Example 2 Fe ₂ O ₃ powder 1.0% in Si fine powder with an average particle size of 0.18μ
and 0.5% B were mixed, molded in the same manner as in Example 1, and pre-sintered at 1385° C. for 3 hours in an Ar atmosphere.

This sintered body was nitrided under the same conditions as in Example 1, and then heated in the air at 1500° C. for 5 hours to completely melt and flow out unreacted Si.

A hollow cylinder with external dimensions of approximately 24 mm in diameter and 25 mm in height was obtained, consisting of walls of 1.5-2.0 mm thickness.

Claims (1)

[Scope of Claims] 1. Molding silicon powder with an average particle size of 15μ or less, pre-sintering the molded body to create a pre-sintered body with a higher porosity in the surface layer than the inside, and producing a pre-sintered body. to
The surface layer is converted to silicon nitride by nitriding with nitrogen at a temperature of 1100°C or higher but lower than the melting point of silicon, and then heated to a temperature higher than the melting point of silicon to remove any unreacted material remaining inside. A method for manufacturing a silicon nitride hollow body, which comprises melting and flowing silicon. 2. The manufacturing method according to claim 1, which is carried out by adding a substance that promotes sintering and/or a substance that promotes nitriding to the raw silicon powder.