IE912809A1

IE912809A1 - Process for producing moldings from silicon-infiltrated¹silicon carbide

Info

Publication number: IE912809A1
Application number: IE280991A
Authority: IE
Original assignee: Hoechst Ceram Tec Ag
Priority date: 1990-08-09
Filing date: 1991-08-08
Publication date: 1992-02-12
Also published as: DE59100483D1; EP0470622A3; EP0470622B1; EP0470622A2; FI913757A; NO913097L; FI913757A0; ES2046832T3; ATE95808T1; DE4025235C1; DK0470622T3; JPH06211585A; NO913097D0

Abstract

For the production of mouldings of silicon-infiltrated silicon carbide, in particular of pipes, a mixture of silicon carbide powder, organic binder and optionally carbon is moulded to give a cylindrical elongated green compact, the binder of the green compact is removed in a nonoxidising atmosphere by carbonisation at about 1000 DEG C and the porous blank obtained is siliconised by the action of liquid silicon at temperatures of at least 1400 DEG C. During the siliconisation, the blank rests on a porous SiSiC support which is in contact with liquid silicon. The SiSiC support has a straight channel with an angular cross-section and recesses in which the cylindrical blank to be siliconised is placed, and the SiSiC support has downward-projecting side walls whose lower surface is in contact with the liquid silicon. <IMAGE>

Description

Description Process for producing moldings from silicon-infiltrated silicon carbide The invention relates to a process for silicizing porous, 5 elongate, cylindrical moldings of silicon carbide/carbon, using a porous carrier which is composed of siliconinfiltrated silicon carbide and has a channel for receiving the molding.

In the infiltration of silicon carbide moldings with silicon, zones of particularly high silicon content, so-called silicized tracks, and in addition not fully silicized components frequently form. Both these phenomena cause waste.

According to the process of EP 0,134,254, the infiltra15 tion of the silicon carbide/carbon molding takes place via a porous silicon carbide plate which is provided with a coating of boron nitride, silicon carbide and carbon. Underneath the silicon carbide plate and, if appropriate, to the side thereof, there is (before the furnace is heated up) lumpy elemental silicon. It has been found, however, that the use of plate-shaped SiSiC infiltration aids has disadvantages. When the silicon, which is located between the firing plates and the silicon carbide plates used as infiltration aids, has melted, the plates sink due to gravity through the silicon (which has a lower density) and displace the latter. Due to the surface tension, silicon facings of up to 5 mm in height can form on the firing plate. If, after the silicon carbide plates used as infiltration aids have sunk through, a higher facing were to form, this would cause the silicon to run off the plate (Figures la and b) . Since the quantity of silicon provided on the firing plate is such that it is just sufficient to infiltrate completely the components located thereon and also to compensate for evaporation losses, the run-off of silicon leads to incomplete infiltration of at least some components. A precautionary extra provision of silicon on the firing plate is ruled out, since otherwise the components are joined after cooling so firmly to the infiltration aid that detachment without damage is impossible. If the silicon runs from one firing plate to a plate located below, it will cause sticking on the latter.

As mentioned above, the silicon level rises due to sinking of the SiSiC plates into the molten silicon. This can have the result that, when too low plates are used, the components come into direct contact with the molten silicon. This leads to inhomogeneous infiltration and to the occurrence of stresses in the component, which frequently manifests itself by formation of cracks. The resulting cracks are filled with silicon and are visible as silicized tracks in the component. It might be possible to prevent this effect, on the one hand, by a reduction in the quantity of silicon per firing plate or by increasing the thickness of the silicon carbide plates. However, both possibilities adversely affect the economics of the process.

The process described, using SiSiC plates, is also used for silicizing cylindrical tubes and rods.

Because of the small thickness of the porous carrier plate used and the small distance from the molten silicon, however, it was difficult to achieve homogeneous silicization. This problem, however, can be solved at least partially by arranging the carrier plate on an SiSiC container with molten silicon (German Offenlegungsschrift 3,719,606), so that it is no longer in direct contact with the molten silicon.

Furthermore, a carrier plate also has the disadvantage that the tubular object to be silicized will, after completion of the silicization, stick over its entire length to the substrate. - 3 It was therefore the object to provide a process, in which easy detachment of the tubular objects is possible after the infiltration process, without the objects being damaged by flaking. The invention is based on the realization that the contact surfaces between the tubular object to be silicized and the carrier plate (= infiltration aid) should be as small as possible.

A process for silicizing porous elongate cylindrical moldings of silicon carbide/carbon has now been found, in which a mixture of silicon carbide powder, organic binder and, if appropriate, carbon is molded to give a cylindrical green compact, the binder of the green compact is removed by carbonization at about 1000 °C in a non-oxidizing atmosphere and the resulting blank is silicized by the action of molten elemental silicon at temperatures of at least 1400’C, the blank resting during silicizing on a porous SiSiC carrier, whose lower part is in contact with molten silicon, and the assembly of SiSiC carrier and resulting cylindrical SiSiC molding being cooled after completion of the silicization, wherein the SiSiC carrier has a straight channel with an angular cross-section and recesses, into which the blank to be silicized is placed, and the SiSiC carrier has downwardprotruding side walls, whose undersides are in contact with the molten silicon. The cylindrical blank can be rod-shaped or tubular.

An SiSiC carrier which can be used according to the invention is shown in Figure 2. This is a prism of 11shaped cross-section. The arms 1 should, at their underside, dip into molten silicon (not drawn). The angle 2 serves to receive the blanks. Figure 2a shows the section A-B of Figure 2b. Figure 2b shows a plan view of the carrier with the perforations 3.

As a first approximation, the quantity of the silicon used should just correspond to the quantity absorbed on silicizing. The rate at which the silicon migrates upwards through the M-shaped prism (= infiltration aid) can be controlled by the size and number of the pores filled with silicon. The lower the density of the infiltration aid, the greater is the surface area fraction of the silicon and hence the effective capillary area. The greater this area, the higher is also the migration rate. The arithmetical relationship between the density of the infiltration aid and the surface area fraction of the silicon is illustrated in the table which follows.

Density of the infiltration Surface area fraction of aid the silicon or effective capillary area g/cm3 % 2.50 80.7 2.60 69.3 2.70 58.0 2.80 46.6 2.90 35.2 3.00 23.9 3.10 12.5 This effect has already been described (for the silicization of a green compact) in Special Ceramics 5, 1970, in connection with Figure 6.

A reduction in the pore diameter brakes the flow of silicon, and an increase accelerates the latter. The magnitude of the pore radii depends on the silicon carbide grain sizes employed in the preparation of the firing aids. Coarse grain sizes (e.g. F 230) give large pores, and fine grain sizes (e.g. F 1200) give small pores. By means of a suitable selection of the silicon carbide grain sizes, the pore radii distributions can thus be adjusted within a wide range, if required.

Claims

1. Patent claims 1. 1. 2.

2. 3.

3. 4.

4. A process for silicizing a porous elongate cylindrical molding of silicon carbide/carbon, in which a mixture of silicon carbide powder, organic binder and, if appropriate, carbon is molded to give a cylindrical green compact, the binder of the green compact is removed by carbonization at about 1000C In a non-oxidizing atmosphere and the blank is silicized by the action of molten silicon at temperatures of at least 1400’C, the blank resting during silicizing on a porous SiSiC carrier, whose lower part is in contact with molten silicon, and the assembly of SiSiC carrier and resulting cylindrical SiSiC molding being cooled after completion of the silicization, wherein the SiSiC carrier has a straight channel with an angular cross-section and recesses, into which the cylindrical blank to be silicized is placed, and the SiSiC carrier has downward-protruding side walls, whose undersides are in contact with the molten silicon. The process as claimed in claim 1, wherein the cylindrical blank to be silicized is a tube. A process as claimed in claim 1, substantially as hereinbefore described with reference to the accompanying drawings. A porous elongate cylindrical molding of silicon carbide/carbon, whenever treated by a process claimed in a preceding claim.