DE938536C - Refractory items made of silicon carbide - Google Patents

Description

This invention relates to refractories and bonded Silicon carbide composite articles.

In particular, it relates to refractory forms of a composite nature made from bonded silicon carbide, which not only have good thermal conductivity and strength at high temperatures, but are also particularly resistant to oxidation and to strong and sudden exposure to heat, and the objects with which the refractory material comes into contact at high temperatures, do not stain or change color.

Another object of the present invention is to provide an improved article from to produce bonded silicon carbide, which withstands heavy loads at high temperatures.

Another object of the invention is to provide a bonded silicon carbide article to produce, in which the binder and the silicon carbide have a fire resistance that is superior to that of conventional binders such as glass, porcelain and the like.

Further features and advantages of the present invention will become apparent from the description that follows emerged.

According to the invention, objects are produced from bonded silicon carbide, the main mass of which consists of granular silicon carbide with silicon nitride as a binder, the certain added glaze-forming Contains components such as iron-manganese compounds, etc. The main mass of the item

is firmly related to a surface part consisting of granular silicon carbide with silicon nitride as Binder consists, the surface part in the is essentially free of the glaze-forming components of the inner part. The nitride bond in the The bulk of the object is obtained by using a metal alloy such as ferro-silicon or ferro-manganese silicon, and the nitride bond in the "surface part obtained by using silicon, ίο The silicon is converted to silicon nitride when the object is fired, mostly also the entire alloy at the same time.

Since the invention is particularly useful in the manufacture of lining bricks from cemented silicon carbide - consisting of a main and a surface part - is suitable, needs the outer part • of course not to cover the entire surface of the object. It can e.g. B. a compound Lining tiles are made in such a way that only one of the plate surfaces of the Brick, namely the one that comes into contact with the objects in the kiln, with a surface layer is provided and thus receives a non-staining coating, whereas the lower or opposite Side of the brick can consist of the main mass. In other words, it can be a compound Lining bricks are made, one or both of which are plate sides with a layer of Silicon carbide particles bonded by silicon nitride as a binder, is coated and its main mass of silicon carbide, connected by a Silicon alloy nitride such as ferro-manganese silicon or ferro-silicon. In the same way In the case of a tube or other shaped object, either the inner and / or outer curved one Surface be coated with silicon carbide and silicon nitride as binders and the main mass of it Silicon carbide connected by the nitride of a silicon alloy of the aforementioned type. In order to make the invention easier to understand, reference is made to the figures of the drawing, which represent an embodiment of the present invention:

Fig. Ι represents a perspective drawing of a lining tile of silicon carbide made according to the invention;

Fig. 2 is a sectional view taken along line 2-2 of Fig. 1, and

Fig. 3 is a view similar to that of Fig. 2 showing a modified embodiment of one according to the invention produced lining brick shows in which only one surface of the brick with a non-staining Surface coating is provided.

To further illustrate the invention, the following examples are given. They show the kind and show how the composite articles of silicon carbide with a nitride as Binders can be produced according to the invention.

example 1

A lining tile with a surface area of about 35.6 X 38.1 cm and about 2.5 cm thick, as in Figures 1 and 2 of the drawing was made using the following compounds:

Composition A

Silicon carbide with a grain size corresponding to a clear mesh size between 1.54 and 0.48 mm

Silicon carbide with a grain size corresponding to ■ a clear mesh size

"of 0.177 mm and finer

- Sihciumpulver with a grain size corresponding to a clear mesh size of 0.074 mm and finer

Dry lignon ... ·. ·

Bentonite gel .....,

Composition B

Silicon carbide with a grain size corresponding to a clear mesh size between 1.54 and 0.48 mm

Silicon carbide with a grain size corresponding to a clear mesh size of 0.177 ^{mn: i un (} l finer

Ferromanganese silicon powder with a grain size corresponding to a clear mesh size of 0.074 mm and finer '.

Silicon powder with a grain size corresponding to a clear mesh size of 0.074 mm

Dry lignon

Bentonite gel.

Parts by weight

48

16

5 6

Parts by weight

48

10

IO

4 5

The Ferromangansilicmm alloy used above contains about 48 ° / ,, silicon, 2o ° / ₀ manganese and the balance mostly iron. Ferromanganese silicon, ferro silicon or alloys of other compositions can be used with satisfactory results, or a mixture of powdered silicon can be used together with powdered iron or with powdered iron and manganese. It is desirable, however, that the silicon content is at least 50% of the entire metal-containing components and / ₀ usually 75 to o, 8 ° from the total content of the crude approach - is · - calculated on dry weight; the remaining alloy metals are 2 to 25%.

Composition A is used to produce surfaces 4 and 5 in the manner described below Lining tile, as shown in Figs. 1 and 2, used and the. Composition B for making of the inner part 6 of the brick.

The ingredients of compositions A and B with the exception of the bentonite gel are used for the purpose of good Mixture first dry for 15 minutes in a roller barrel and then mixed in for another 20 minutes wet mixed in a kneading mixer. Each mixture must be so moist that it settles with the bentonite gel Mix well, which is made from 4 parts of water and one part of dry bentonite powder. That Bentonite gel is used in composition A to

take the finely divided silicon powder from the mixture and in Composition B for the inclusion of the finely divided ferromanganese silicon powder, as this Metal powder is otherwise very flaky and extremely difficult to handle. The powders will be equal = distributed moderately and uniformly within the mixture, allowing a well-mixed approach with appropriate Consistency for shaping is created. First, a lining tile according to Fig. Ι

ίο and 2, in which a sufficient amount of Composition A is placed in the mold so that a layer of this material approximately 0.476 cm thick is formed on the bottom of the mold when it is pressed. The material is smoothed and evenly distributed over the bottom plate of the mold and, if desired, subjected to slight pressure so that the material lies firmly in the mold and prevents it from slipping during the subsequent addition of the next layers of material. A sufficient amount of Composition B is then placed in the mold over the lightly pressed material of Composition A and pressed down slightly so that a layer of Composition B is approximately 1.27 cm thick. This material is then distributed evenly over the space of the mold and, if desired, gently pressed down again, after which a second layer of Composition A is applied under slight pressure in an amount that a layer thickness of about 0.476 cm is created. After the last layer of the material has been evenly distributed, the upper plate of the mold is placed, the mold is placed in a hydraulic press and the brick is pressed ^{by a pressure of over 350 kg / cm 2.} During the last pressing, the three layers are pressed together and form a firmly coherent body in which the composition B forms the core with a thickness of about 1.27 cm, which is covered on both sides with a surface layer of the composition A, the respective thickness of about 1.27 cm. The resulting object is then removed from the mold, dried and placed in a muffle furnace. The normal atmosphere in the furnace is replaced by a continuous stream of nitrogen (purity 99.7 ° / ₀ ), the temperature of the furnace chamber is gradually brought to 1400 ^{0 over} the course of several hours and kept that high for 12 hours with further introduction of nitrogen. The furnace is then allowed to cool in a stream of nitrogen to room temperature or to a temperature for further processing of the brick, which is then removed from the furnace. Such bricks are very suitable for use in the oven for firing white goods, etc., in which the heating takes place relatively quickly. The bricks produced in this way have the advantage not only of not staining the goods with which the brick comes into contact, but also of withstanding the rapid changes in temperatures in such furnaces.

Example 2

According to Fig. 3 is a brick of the type shown there in the same way as above for a brick according to Fig. ι and 2 had been described, only that initially not the composition A is brought into the form, but instead the amount of composition B is increased accordingly, to compensate for the missing lower layer of composition A, so that the pressed brick a Thickness of about 2.5 cm, although it consists of only two different layers. Definitely The thicknesses of the various layers of material can vary considerably from those above Example given dimensions vary without departing from the scope of the invention. You depend on it depends on what properties are required of the brick, especially overall thickness, non-staining Properties and resistance to sudden * heat exposure. All of this depends on that final use of the item.

It is interesting to note that there is no change in volume of the article on firing. When weighing the molded and dried articles before and after firing, a Weight increase of about 6 to 8% found due to the content of silicon and silicon alloy in calculated for the object and reduced by the amount of the loss of volatile substances Indicate weight gain, which requires an almost complete rearrangement of at least the silicon and a part, if not all alloy metals, proves to nitrides in the end product.

The breaking strength of the articles prepared according to the composition and procedure of Example 1 was of silicon carbide at 1350 ⁰ 469 kg / cm ^2. This strength is approximately twice as high as that of highly bonded, difficult-to-melt objects made of silicon carbide with the usual glass- or porcelain-like binders that are on the market.

The combined amounts of silicon and ferromanganese silicon or ferrosilicon, as used in the main mass of the refractory articles according to the invention, can be between 4 and. 40 ° / ₀ are the dry weight of the crude mixture. The amount of silicon in the raw batch from which the surface layer of the article is made can also be 4 to 40% of the dry weight of the batch. However, it is usually desirable to use about 8 to 40% of the metallic nitride-forming binders for each part of the article in order for the articles to have sufficient thermal conductivity, high strength in heat, and resistance to sudden exposure to high degrees of heat.

The metals of the ferro-manganese silicon and ferrous silicon alloys In addition to silicon, glaze-forming constituents are described here. This expression was applied to the alloy metals as found in Composition B of Example 1. In this composition, the alloys mentioned serve as a source for the nitride bond and form a certain amount of glassy material or glaze on the surface when they are used Objects that come into contact with it stain or discolor, whereas the composition A of Example 1, in which the nitride binder

obtained from powdered silicon, among the same Firing conditions did not cause glazing. It can be assumed that the presence of these additional Metal constituents either as such or in conjunction with small amounts of silicon or others in the granular silicon carbide present impurities the resistance of the obtained Objects against bursting or jumping in the event of sudden exposure to heat are significantly increased. Simultaneously ίο keeps the restriction of these so-called glaze-forming Components on the inner part of the mass the surface of the object of undesirable vitreous components free or almost free, so that the objects coming into contact with it not may become blotchy or discolored.

Very good results will be obtained when using it

of commercially available Sihciummetall, which is more suitable Fineness is ground, achieved. The analysis of a commercially available silicon, which according to the invention Use well Hess, yielded the following in addition to silicon. Impurities:

Iron: 0.87%

Chromium 0.21%

Aluminum 0.60 ° / ₀

Calcium ° .34 ° / o

To achieve within Ernes useful period of time with the use of commercially available silicon satisfactory conversion of the silicon to silicon nitride, the silicon should be so fine that it passes through a sieve having a mesh width of 0.074 mm ^{un (i} finer, a particle size of 70 Even faster nitride formation is achieved when the silicon has a fineness of about -10 to 20 microns and less.Satisfactory conversion of the silicon to silicon nitride has also been achieved with pure silicon (99.8% Si), although It has been found that nitride formation takes longer when using pure silicon than when objects of the same size and shape are made from commercially available silicon of the same fineness It has also been found that the rate of conversion of pure silicon to silicon nitride is increased by adding a small amount of iron powder, approximately ^x / ₄ to 1 percent by weight, which corresponds approximately to the amount of iron found in commercial silicon. The more rapid conversion of commercial silicon to silicon nitride compared to pure silicon is therefore attributable to the small amounts of iron that are usually found as an impurity in commercial silicon. - Such small amounts of iron also do not cause any noticeable appearance of glassy material in the end product.

If nitrogen with a purity of 99.7 ° / ₀ in the above examples for use

recommended, the same results can be obtained with other non-oxidizing nitrogenous gases. For example, commercially available hydrogen cooling can with an approximate content of 93% nitrogen and 7 ° / ₀ hydrogen or ammonia gas may be used in the same manner instead of nitrogen.

Even if a number of non-oxidizing, nitrogenous gases were mentioned here as the source of nitrogen can be used for the formation of the nitride in the reaction space through which the nitrogen-containing gas is continuously being withdrawn. found that nitride formation also occurred other way can be effectively done by making sure that the object surrounding and penetrating atmosphere has a non-oxidizing character and is sufficient Nitrogen is supplied for nitride formation. For example, over-firing the molds after the Invention the mass embedded in a suitable mixture of coke and sand and at the above Temperatures to be fired. The silicon of the surface and at least the silicon of the alloy in the main mass are converted into nitride by the nitrogen in the air, which forms the embedding layer penetrates and reacts with the silicon of the objects embedded therein. The coke the Embedding layer combines with the oxygen in the air and forms carbon monoxide before the free one Oxygen reaches the objects to be burned, so that the gases that penetrate the object in the consist essentially of a mixture of nitrogen and carbon monoxide. Under these conditions Almost all silicon combines with nitrogen to form silicon nitride. There must be enough coke in the mixture must be taken care of so that there is an excess of carbon and carbon monoxide forms as carbon dioxide and no free oxygen remains. The embedding material must be present in sufficient quantity to react in the manner described To ensure the supply of acid-free nitrogen for each object during firing, each must single shape of adjoining objects far apart due to the embedding material in between be separated, otherwise there will be insufficient nitride formation within a useful time.

The best results, that is a highly effective and thorough conversion of the silicon to Sihciumnitrid were achieved when the formation of the nitride at temperatures slightly below the melting point of silicon (1420 ^0), for example. B. at 1350 to 1400 °, was carried out. Sufficient formation of nitride took place even at around 1300 °. Furthermore, during the nitride formation and especially after this has taken place for a while, the temperature can be increased above the melting point of the silicon in order to ensure complete conversion of the silicon to silicon nitride.

The articles produced according to the invention can be shaped by any known method, e.g. B. by compression molding, machine tamping, hand tamping, shaking, vibration tamping, press hammer flat and corner tamping, etc.

Of course, the products of the present invention in their various modifications

Cations are not restricted to specific areas of use, such as from the previous examples could be inferred. The products can be made in any desired shape. she are therefore not only suitable for many uses that require refractory materials be e.g. B. bricks, blocks, lining bricks, muffles, furnace linings and special shapes for use in and around kiln and others

ίο high temperature apparatus but are also used for many high temperature applications such as jet engine combustion chambers, liners for exhaust tubes, rocket combustion chambers and exhaust nozzles, turbine blades, Stator blades, lens molds, electrical fuses, etc. are suitable. They are also used for making of laboratory equipment such as incinerator, crucible, burner holder, etc. It's going ok from the fact that the forming the surface

ao composition can extend over the entire exterior of the object or only over certain ones Parts of the same in which no glaze or no glaze-forming material is desired.

Claims (9)

Patent claims: 1. Refractory articles made of bonded silicon carbide, characterized in that you Core made of granular silicon carbide, which is made up of iron and manganese compounds Silicon nitride is bonded, and its surface consists of a layer of silicon carbide, which is bound by the same material as the core, but the iron and manganese compounds Contains only in such a small amount that no glaze forms on the surface of the objects. 2. Refractory articles according to claim 1, characterized in that they are made of granular Silicon carbide and silicon nitride exist as binders and only the inner one Part contains iron and manganese compounds in quantities that are able to form a glaze. 3. Refractory articles according to claim 1, characterized in that they consist of a surface layer and a main mass firmly connected to this consist of silicon carbide cemented by silicon nitride, the Main mass contains iron. 4. Refractory articles according to claim 3, characterized in that the main mass Contains 2 to 25% iron and manganese compounds. 5. Fire-resistant articles according to claim 1, characterized in that the main mass of cemented with 4 to 40% Ferromangansiliciumnitrid SiKciumcarbidkörnern and the surface layer of cemented with 4 to '40 ° / ₀ is silicon nitride silicon carbide grains. 6. Refractory articles according to claim 1, characterized in that they consist of a surface layer of silicon carbide grains and 4 to 40% silicon nitride as a binder and one The main mass consists of silicon carbide grains cemented by 4 to 40% ferrosilicon nitride are. 7. Refractory articles according to claim 1, characterized in that they consist of a surface layer of silicon carbide grains and 4 to 4 <D% silicon nitride as a binder and one The main mass consists of silicon carbide grains, which are made up of 4 to 40% iron and manganese compounds containing silicon nitride are cemented as a binder. 8. Refractory articles according to claim 1, characterized in that with lining bricks the main mass of silicon carbide, connected by silicon nitride, with glaze-forming Components consists, at least one flat side of this brick with a layer of in is coated in the same way connected, granular silicon carbide, in which the binder is free of the glaze-forming constituents contained in the main mass. 9. Refractory articles according to claims 1 and 8, characterized in that in the case of cladding bricks made entirely of silicon carbide grains are composed of silicon nitride as a binder, at least one flat side contains so little iron and manganese that no glaze can form on the surface. 1 sheet of drawings I 509 637 1.56