DE1446978A1 - Refractory oxidation-resistant materials and methods for their manufacture - Google Patents

Description

Heat-resistant, oxidation-resistant materials and processes for their Manufacture The present invention relates to novel silicide based materials and methods of making them.

In the drawings, FIG. 1 shows, in the form of a schematic diagram, the production of an electrical heating element by infiltration and FIG. 2 is a diagram showing the individual work steps involved in assembling two bodies of different composition produced by infiltration. Figures 3 to 6 different types of electrical elements. _ In dam #, diagram of fig. 7 shows the specific electrical resistance as a function of temperature in elements of different composition. The diagram in FIG. 8 shows how the amount of absorbed alloy depends on the amount of alloy used for the infiltration of porous shaped bodies. Chemical composition of the materials According to the invention, the silicide material consists of a metal silicide and silicon carbide, the latter making up 3o to 9o vol.% And the metal silioid 10 to 7o vol.% Of the solid material. According to a further feature of the invention, the properties of the heat-resistant material can be modified by adding up to 20% by volume of one or more oxides. The oxides can be in the form of an oxide mixture or solid solution or chemical compounds of oxides. The oxydin mixture can contain oxygen compounds of at least one of the metals Al, Be, Oe, Qr, Hf, Mg, Tip Si, Zr, Th, Y and other "rare earth metals. Of these metals are primarily Si, Zr, Be, Al , Th. And Ce in consideration.

The presence of an oxide component, for example quartz glass, in bodies of silicon carbide and refractory silicide is essential Affect the physical properties of these bodies. To a practical one important material of this type is obtained when porous bodies made of silicon carbide are heated in air together with a silicide, forming quartz glass will. The latter forms a film on the surface of the body and seals partly the pores in this body.

It was to be assumed that such a body, for example made of silicon arbide and molybdenum silicide, which is filled with quartz glass, must be very resistant to oxidation will. This is the case at both moderate and high temperatures, below the prerequisite that the temperature is kept constant. Will body which Quartz glass contained, repeated sudden changes in temperature or in the air exposed to very high temperatures, for example over 15,000 ", they suffer as was found, an oxidation, which is probably comparatively slight, turns out to be but in certain cases it can prove harmful. Become electrical resistance elements If exposed to high stresses at high temperatures, it already causes a low oxidation an increase in the electrical resistance, which is in the detrimental to the passage of time. Practical tests have shown that this small increase in drag is fairly constant regardless of the Amount of existing Quartz glass. Only if all of the quartz glass is on the surface of the element, i.e. if there are no forums of the body are filled with quartz glass, there is no increase in the resistance to Completely, even if he was at the same temperature for several thousand hours was in operation until 1550o. :, it can be assumed that the inability of the quartz glass Keeping the oxygen out of the air is related to the diffusion of oxygen in hot quartz glass compared to that in SiO or MoSi2 is considerable. The present invention therefore relates to operations for the production of bodies which are completely pore-free or at least free of continuous Pores are and consist only of E-Mliciumearbid and heat-resistant silicide and accordingly after heating in air, for example to 15,000, exclusively Contains quartz glass in the form of a very thin outer film.

The ratio between the silicide and silicon contents of the refractory Materials are essential in terms of the properties desired. Since both silicides and bilicium carbide have excellent heat resistance have, it is to be expected that all materials, which these two main components will have good heat resistance. This is actually true the case. It was found that even small amounts of silicide reduce the resistance to oxidation of silicon carbide significantly improve at temperatures above 13.o0. The invention therefore includes materials with a silicon carbide content of 3o to 9o vol.%.

The metal silicide consists of up to 90% by weight of one or more metals from the group of W, Mo, Cr, Ta, Nb, V, Hf, Zr or Ti together with up to 30% by weight of one or more elements from the group of metals Al, Be, Ca, Ce, Co, Cu, Mg, Fe, Mn, Ni and from the elements C and B. In the metal silicide used, the silicon content should normally be high in order to make the silicide resistant to oxidation. It was found that the silicon content of the mixed silicon in question should be at least 10% by weight. A lower overall content is not sufficient to form a sufficient protective layer of SiO2 on the grain surfaces. It has also been found that the silicon content should not exceed 70 % by weight, because at higher silicon contents the melting point becomes so low that the mixed silicide is unusable for practical purposes. It follows that the melting point of the metal silicide in the equilibrium state should be considerably higher than the temperature to which the finished product is to be exposed in operation.

The invention includes all combinations of the abovementioned silicides with silicon carbide and, depending on requirements, with one or more oxides9 As before Said above, MoSi2 is the heat-resistant one because of the particularly good results Result in materials which contain MoSi2 in combination with silicon carbide, of special interest. So it should be the metal silicide 1o to 7o vol.%, The Silicon carbide 3o to 9o vol.% And the oxide form 0 to 20 vol.% Of the sintered body.

2. The infiltration process for the production of the materials - The present invention also relates to a process for the production of silicide bodies, which consist of a silicon carbide framework with a pore-filling structure. metallic binding component and are characterized by a very low porosity.

Non-porous bodies made of silicon carbide and a metal silicide can be produced by powder metallurgy by pressure sintering or cold pressing. In the first case, bodies with up to 60% silicon carbide can be produced, while the second method works satisfactorily only with low silicon carbide contents. The production of bodies with more than 60% SiC has not been carried out without considerable difficulties. The method of the present invention aims to avoid serve difficulties and enables the production of heat-resistant and oxidation-resistant articles having a 15 vol.% Not exceeding total porosity which up to 90 vol.% Silieiumoarbid, calculated on the solid substance ,. together with a metal silicide (A). In accordance with the invention, a porous body is first formed which essentially consists of one or more of the following substances, namely hexagonal SiC, cubic SiO, graphite, amorphous carbon and a carbon-containing, earbonable material This alloy has a percentage by weight of silicon which exceeds that of the refractory alloy. The porous body with the powder surrounding it is now in an earbonizing atmosphere at a temperature above the melting temperature of the powder, but below the decomposition temperature of the SiG heated, and for a time that is sufficient that parts of the molten powder penetrate substantially into all pores of at least one part of the porous starting body that adheres together, with the remainder of this powder on the outside of the body interspersed with it allows a loosely adhering porous cake to be formed, consisting of SiO particles formed in situ and embedded in a sintered residue of the carbonized alloy powder; this loosely adhering porous cake is then removed from the surface of the body which is permeated with the powder.

The alloys and binders treated according to the invention (A, B and a) can be of various types. To make the finished product a good resistance to oxidation at high temperatures, i.e. above 1200o, preferably above 14000, the metallic binder (A) must be such temperatures should also be resistant to oxidation, which is a disadvantage regarding the use of heat-resistant silicides, borides, aluminides and titanides of refractory metals and metalloids means. The invention comprises in the first place The production of silicon carbide bodies or products with a metallic line Silicon binder, preferably MoSi2. The addition of boron to heat-resistant bilicides is of interest in some ways, for example because borides are common have very high melting points. An interesting combination is that of Silicon with boron alone. The invention accordingly also encompasses products made of silicon arbid with a binder (A) made of silicon boride.

3. The framework body used for the infiltration The silicon carbide used for the production of the products or bodies according to the invention can be produced in various ways. It is possible, for example, to use X -, - silicon carbide, which can have a grain size of a few lvhkron up to several millimeters differs from the above-mentioned type in that it has a cubic crystal structure.: It is generally expedient that the grain sizes of the silicon carbide vary within wide limits, as a result of which a low porosity of the finished product is obtained. In the field of ceramic technology It is known that mixing different mandrel fractions of silicon carbide results in a reduction in porosity, but the exact proportions are a function of both the desired grain size interval and the binder used, so that a general rule cannot be given 778 are e.g. for powder mixtures, which are made exclusively from Silie iumcarbid exist without any binder, the laws that have to be adhered to so that a minimum porosity is obtained. For example, a mixture of 38% coarse, 20% medium and 42% fine grain is used, in whichever the largest grain size in each fraction has a diameter that is two to three times as large as that of the next finer fraction; in this case a density of 2.59 / 0m3 is obtained, which corresponds to a total porosity of 19%. According to the invention it was found that it is expedient to at least essentially comply with this rule and at the same time replace part of the fine fraction with the binder (B), while the coarse and medium fractions consist exclusively of silicon carbide. If jr @ -SiC is used in combination with QG --SiG, a0 it is preferably contained entirely in the 42 ö which are formed by the fine fraction. If the specific weights of the additives differ from that of the SiC, please note that the ratios 38-2o-42 relate to percentages by volume, so that you have to convert to percentages by weight.

So, for example, a shaped body made of 31% by weight of a metallic binder (B) with a specific weight of 8, o and 69% by weight SiC (specific weight 3.1) are produced, the following mixture is used to achieve a minimum porosity: 31% by weight metal powder (1o Yikron) 15 vol. # O 22% by weight SiG, fine (lo ") = 27% by volume -` 42% by volume 16 wt.% SiG, medium (25 ") = 2o vol.% 31 wt.% SiC, coarse (70 ") = 38 vol.j (The volume percent are the theoretical values for a non-porous body).

According to the invention it is also possible to use the silicon carbide at least partially in situ in the body, for example by having a Carbon skeleton silicating gases or the action of molten silicon by exposing a silicon framework to carbon-containing gases treated. It is also possible to use a carbon and Silicon go out, which is pressed and then sintered, -wherein silicon carbide is formed. The framework body can either only be made of carbon in some form or consist of a mixture of carbon and silicon carbide, it is also possible to combine this formation of SiO with the infiltration process, so that only a single heat treatment is necessary.

The substances used as a binder (B) for the production of the Porous starting bodies made of silicon carbide can be of different types and in different Way to be introduced. One way is to get rid of a powder mixture starts out, on the one hand from Siliciumoarbid or in Siliciumoarbid.umwandelbaren Materials, on the other hand, consists of a metal or an alloy, on what the mixture is sintered until it has the desired porosity. Instead of of a metal, compounds can also be used which under the effect of high Temperatures or suitable gases are converted into a metal, for example an oxide or a mixture of oxides, which in a hydrogen atmosphere at high Temperature is reduced to a metal.

On the other hand, the binders (8) can also be oxides that are difficult to reduce, capable of reacting with metals at high temperatures. such a thing Oxide is e.g. SiO2, which reacts with Si at 18,000 to form SiO, the latter volatilized, the invention accordingly also includes the cases in which the Silicon carbide was bonded to Si 02 and interspersed with a silicon alloy (C) is.

However, there is no need to use a binder at all the production, to use the porous starting body, in which case the latter consists exclusively of SiO in the form of a recrystallized framework.

The porous starting body, which in the sense of the invention is to be subjected to the infiltration process, must be sufficiently heat-resistant so that it does not suffer any damage at the temperature of this process. If pure silicon is used, this temperature is at least 18oo0, regardless of the fact that silicon already melts at 142o0. It can be assumed that this required excess temperature of about 4000 is related to the fact that penetration only takes place when the thin silicon dioxide film, which easily forms on molten bilium, is removed. As already mentioned, this is the case when silicon reacts with silicon dioxide to form bilium monoxide, which evaporates at 180 °. The production of the initial porous molded body can be carried out by various methods known per se, which is why details are only given for a few different compositions in the exemplary embodiments described below. The composition and porosity of the starting body can vary within wide limits, but in general the pore volume should be 15 to 60% of the volume of the starting body, the open pores should have a maximum diameter of 50 microns. This restriction on the pore size is obtained by using a mixture which preferably contains fine-grained fractions of both silicon carbide and the binder; the grain size should be up to 0.1 mm. It is important that the components are intimately mixed. Pore spaces of more than 50 microns do not result in a good end product, since it is necessary that the reaction of the penetrated alloy (C) with the binder (B), which is enclosed in the starting body, is complete that the finished body contains undesirable substances that are not sufficiently resistant to oxidation.

With reference to the closed and open pores, it should be said in this context that the open porosity is determined by measuring the water absorption. by the conventional methods used in the ceramic field. Ze total percent porosMt (p) is calculated from the observed (d) and the theoretical density (D), which the starting body would have according to the mixing rule if it were not porous, according to the following formula The difference between the total porosity and the open forosity is referred to here as the closed porosity. Since the determination of D requires knowledge of the composition and the latter cannot always be precisely determined, it will often be necessary to determine the various porosities by microscopic means.

The content of the binder (B) in the starting body can be low and for example 1Vol.%. However, it should preferably not be below 5% by volume and not. are above 40% by volume, calculated on the solid substance. Contains the binder (B) SiO2, it should be noted that the reaction with the penetrated into the forums Alloy (C) is accompanied by gas formation which lasts for four short periods of time Infiltration: process takes place, which is why difficulties are encountered with particularly high Si02 = contents The SiO2 content of the porous starting body should preferably occur between 2 and 1o vol.%.

4. P'aeken of the framework in silicon powder If silicon-containing alloys are used for the infiltration process, as provided by the invention, it has surprisingly been shown that the infiltration process is favored if the alloy powder is very fine-grained. If you use a powder with a of more than 6o meshes, difficulties arise in separating the dem subject body of. the excess adhering molten alloy material, resulting in a rough surface of the body. Better results are obtained with grain sizes below 100 microns. Powders finer than 10 microns provided pellets with a smooth surface without any adhering molten material. To carry out the infiltration, the starting bodies can be packed in loose alloy powder or in porous masses made of this, which have a porosity of at least 30 % by volume. The porosity should preferably be between [0] and 60% by volume.

The packing of the starting bodies made of bilicium carbide in the alloy powder can be done in different ways. Should the service strength of the 3ilieiumcarbidpressling be insufficient so that a powder layer is applied to this mechanically it is more even when producing straight bars, for example Thickness possible to insert the cold-formed rod centrally into a paper tube, whereupon the space between paper and stick with the. Alloy powder filled in. Iin in general, however, it is possible to apply such a silicon carbide body to the molded silicon carbide body To give strength that it can be coated with the metal powder, for example according to iz @ gen ie one of the used in the manufacture of coated welding electrodes Procedure. In this case it is advisable to use an organic or inorganic, to use temporary binders, for example water glass, bentonite7 paraffin, plastic or the like

The above-mentioned method of wrapping the rod of silicon carbide coated with the alloy powder in paper offers advantages that were not foreseen especially in mass production. j # s has shown that the rods with their paper sleeves can be tightly packed without affecting the infiltration process. This is apparently due to the fact that the paper forms a thin-walled tube of silicon carbide during and after its combustion, which is able to effectively keep the powdered alloy away by the latter being completely absorbed by a single silicon carbide rod without it being on the surface of this rod or attached to any of the other bars. The rods with their paper sleeves are preferably arranged horizontally, parallel and close to one another, so that they touch each other along straight lines. : g sharp is in this way, for example, possible to arrange seven rods in such a way that their centers form the corners of a regular hexagon which lies in a plane at right angles to the rod shafts, with the seventh rod forming the center of the hexagon. To carry out the infiltration process, you can from the seven rods can easily be inserted into a carbon or graphite tube, which has the advantage that the furnace equipment is better utilized than in the case when the rods are individually subjected to the heat treatment. Practical tests have also shown that the simultaneous annealing of several rods results in an improvement in the quality of the second product, because random fluctuations in furnace temperature and atmosphere are more easily compensated for when the furnace is filled with the bars.

The above-described method according to the invention is suitable for 'the production of straight bars. Other shapes, such as hairpin Resistance elements can be created by combining straight bars with curved ones 2 table elements made from other materials. consist of MoSi2, for example, obtain. This can be achieved by butt welding, creating elements each desired shape can be produced.

Although paper wrapping is particularly beneficial for those prominent described embodiment of the inventive method is suitable, but it is also indicated when the silicon arbide rod is in a separate sintering process was recrystallized. If it was only pressed or dried, it gives it the paper .. still has the strength required for the infiltration process.

Instead of paper, other materials can also be used which, like this one, cannot be completely burned to carbon. For example, thin covers made of plastic material can be used. The covering can also be applied by spraying or tepid. It is also possible to coat a paper or a plastic oil with a layer of the alloy powder required for the infiltration before they are placed around the silicon carbide rod. Finally, in mass production it is also possible to manufacture the coated rod in a single operation by extruding bilicium carbide, an alloy powder and a shell with the aid of a forming die.

After the infiltration process, it may be advisable to carry out a further auxiliary treatment in order to stabilize the reaction product (A) formed or in order to: achieve the most complete reaction possible between the binder (B) and the layer (C) used for pore filling. The infiltration process as such takes place at great speed, i.e. in a few seconds, because of the heat capacity of the object whose pores are to be filled and possibly because of the heat capacity of the furnace or crucible used for the process , the duration of the .3 heating to the high temperature should generally be long enough so that a subsequent heat treatment, which in certain cases is superfluous, is unnecessary. In some cases, however, it is desirable to subject the bodies in the air to a heat treatment at a temperature which is lower than the temperature during the infiltration process in order to prevent oxidation of the metallic product formed (A.) to effect. This can give rise to the formation of a vitreous coating made of quartz glass, which fills all remaining pores and acts as an outer protective layer, which also improves the corrosion resistance.

The method according to the invention is explained in more detail in the following exemplary embodiments. Example 1 See a powdered mixture of 60 lines of silicon carbide, one part of bentonite, 36 parts of molybdenum and ¢ parts of silicon (parts by weight) was mixed with an appropriate amount of water and extruded into 1/2-inch rods. The 3entonite was added in order to obtain sufficient processing strength for the sintering process. Green, hexagonal silicon carbide with a grain size of 0.15 mm and finer was used, while the diolybdenum powder and the silicon powder had a grain size of less than 10 microns. The rods were dried and sintered in hydrogen gas at 15,000 for 15 minutes to give a flexural strength of 120 kg / Mm 2. Because plolybdenum. and silicon were present in an atomic ratio of 2.7 a 1, it can be assumed that the compound Mo 3si has predominantly formed in the porous starting body. the porosity of the starting body was now 40; lo; they were treated with molten silicon (C) at 185o0 by placing the rods together with the coarse silicon powder in an elongated, covered graphite crucible which was heated by induction heating. By determining the increase in weight, it was established that the sintered body had absorbed an amount of silicon which accounted for 30 to 40% of the pore volume actually present. Finally, the body was annealed in the air at 1550 for 100 hours. The microscopic examination showed that after the oxidizing annealing the body was composed of two phases, namely of hexagonal silicon carbide and molybdenum disilicide (A). It should be noted that after the oxidizing glow inside the body not the slightest traces of @oxidation products could be observed. The penetration of the SiC and Mosi2 particles is complete; the boundaries between the phases were line-sharp, at least at 40o linear magnification. Based on the composition of the body and to be volume-weight is its density calculated to 9o% des_theoretischen fertes, which corresponds to an overall porosity of 1 o Vol.%. During the oxidizing annealing process, the body was covered with a superficially adhering skin, which mainly consists of silicon oxide in the form of a gas-tight glass, which increases the resistance of the material to oxidation at high temperatures.

Example 2i A mixture of 30 % by weight of o (, -SiC with a grain size finer than 0.1 mm and 70% by weight of molybdenum silicide was first molded and pre-sintered in hydrogen gas. The silicide contained 55 g of Mo and 45% of Si; his Particles had a grain size between 3 and 8. The body was then sintered in the same furnace, first in dry hydrogen gas at a temperature of up to 1650 ° and immediately afterwards in carbon monoxide at 1700 ° for 30 minutes with the carbon monoxide d and thereby forms / # - silicon urbi d in, si tu together with a hdolyb dänsili ei d of the composition Ido 5äi3. Below 165o0 the silicide should not come into contact with carbon monoxide because this gives rise to the formation of evidence The silicon penetration was carried out as in Example 1. The advantage of this process is that the 4-silicon formed umearbid is able to bring about a uniform mechanical bond between the grains of α (, silicon carbide. This gives a body which has a continuous, three-dimensional framework made of bilicium carbide, which gives the product high mechanical strength even at high temperatures.

Example 31 In a porous starting molded body made of recrystallized silicon carbide, the density is 2.06 g / cm3, corresponding to a total porosity of 36% by volume. The impregnation is carried out with an alloy (C) consisting of 55% Si and 45% boron. The weight of the absorbi th alloy (C) is 28 i of the weight of the porous body). the. metallic binder (A) has the composition SiB3. Oxidation tests in pure oxygen at 14,000 showed a total weight increase of only 0.054% over 15 hours.

Example 4 :.

Hexagonal silicon carbide is mixed as in Example 1 with equal amounts of Fe: rrotitan (30% Ti) with a grain size of 10 microns and quickly sintered at 13000 in hydrogen gas, followed by an alloy containing 5.5% Si and 45% Ti the impregnation takes place. As soon as the increase in weight of the starting body is 95, the metallic binder (A) has the following composition after impregnation: 25% iron, 40% titanium and 35% silicon. Example 5: The production method gemäse Example 4 can be carried out in such a manner that -the in the finished product, a metallic binder (A) Composition (Ti, Cr) B2 + 2o% by weight is introduced S1 and in the manner that. an initial molded body made of $ 1G with (Ti, Cr) B2 as a binder is impregnated with pure silicon. Example 6: The method according to Example 1 is carried out with a porous starting body which, in addition to bilicium carbide, contains molybdenum silicoaluminide as binder (B) with the composition: 83% Mo, 14% gi, 3% A1. After impregnation with an alloy (C) of 50 Si and 50% Al, the end product contains a binder (A) with the composition: 65% Mo, 23% Si and 12% @ Al.

Example 7 Black SiC with a grain size of less than 0.5 mm is mixed with 20% by weight of titanium sponge with a maximum particle size of 20 microns. mixed. By sintering the mixture in hydrogen gas at 165o0, starting moldings are produced and these are impregnated with a ' silicon-boron alloy of the composition BiB3 at 1900 @ ned. If the amount of 911ioium triboride absorbed is 2/3 of the Gewiohtea - of the titanium sponge, so the metallic binder (A) in the end product the composition Ti ZSiB3. Examples 8 Never 111 pulve @. si arte Four tubes of re-crystallized Sia were placed in molybdenum silioids (finer than 1o microns) - einge- with various Silioiumgehalten embeds and erhitzt- gesohloosenen in a graphite crucible 15-minute at 21oo0. The dimensions of the tubes were outside diameter 12.3 mm, inside diameter 490 mm, length 50 mm; after the infiltration process, they showed no change. The result of the verses is summarized in the table below. At si e1 No. 8 9 10 11 Oi content of the bilioid in the powder 58% 5o% 40% 37% after infiltration 50% 43% 33% 32% Pipe weight before infiltration 1190 g 1190 g 1190 g 1190 8 "after infiltration 18.7 g 19.8 g 19.7 g 2ä, 1 2 Special el. Resist, in ohm # mm2 / m before infiltration 17oo 170o 17o0 1700 after infiltration (2o0) 4.5 5.5 67 870 " before infiltration (15o00) 15 17 3o 2oo .Be it should be noted that the coefficient of the spec. electrical resistance in Examples 8 and 9 is positive, which opens up interesting possibilities regarding the use of these materials as electrical resistance elements for heating purposes in an oxidizing atmosphere. Since in all examples the porosity has been reduced from 30% to less than 5% by volume, the resistance to corrosion is particularly important if the composition of the Eilioida after the infiltration process is chosen so that it corresponds to the formula Mosi2, or that there is a slight decrease in silicon, e.g. corresponding to 34 to 36% Si. 5. Influence of the pores present in the starting body. Extensive studies have shown that the size of the pores within the starting molded body, db by di e dili oiumhaltige alloy should be completed, a kritsöhen of influence on the properties' of the end products produced exerts. If a starting body with "a durohsohnittliohen Yorendurohmesser of more than about 10 microns, for example 100 microns, is impregnated with a molybdenum-silicon alloy and subjected to a hi + zebehand-. Jung that the finished body contains. MoSi2 in its pores and aua If the body is rapidly cooled, this MoSi2 will burst into a large number of small crystal grains, each of which has an average partial ohmic diameter of about 10 microns the Rekri st alli si augmentation of Moß12, ie than about loooo more, the small crystal particles grow again to form larger particles together, which in turn zerplatsen on cooling. This lreaheinungen affect vED sahiedene properties of Pertigprodükte, so the electrical resistivity and the mechanical strength The influence on the specific resistance is particularly noticeable and manifests itself in a great increase in it when the body is cooled. This resistance can be a few hundred enlargement percentage and also be accompanied by a delay check inter alia is manifested, the further enlarged spez.Widerstand.während several days after cooling to room temperature check. Bodies with such properties are obviously unusable for use as electrical resistance materials, for example. The unfavorable effect associated with the @cooling is probably related to the differences in the thermal expansion, coefficients of the SiCI and the metallic component. A reduction in the grain size and pore size would not bring about a favorable solution, since the infiltration of the fine] pores would be made more difficult because they are largely closed . The total porosity of the body would be higher than that which is obtained when larger pores are filled. According to a further embodiment variant of the present invention, it is ensured that the described unfavorable effect can be avoided.

The difficulties discussed above are overcome in that the pore-filling metallic component consists of at least two different phases which have an average grain size of less than 20 microns, the grain size preferably being about 10 microns. This prevents the metallic phase from bursting when bodies which contain metal particles in only one phase are cooled. The presence of at least two different phases has the effect that further substantial grain growth is inhibited at high temperatures, which also helps to avoid cracking on cooling.

The phase composition of the metallic components can be influenced in various ways. A deg consists in choosing the composition of the impregnation alloy in such a way that the desired phases are supplied in the final product. Furthermore, the impregnated body can also be subjected to a heat treatment in an atmosphere of controlled composition, in such a way that new phases are formed in the metallic component If the atmosphere is charburizing, for example, any excess silicon that forms bonds in the grain between the bilicide grains or within the silicide mass is converted in situ into SiG can also be used to Monitor grain growth.

Bodies in which the metallic component consists entirely or in part of MoEi2 are of particular interest. According to the invention ».-. a metallic component which is less exposed to bursting and subsequent grain growth, obtained by adding one or more of the following elements to the impregnation alloy: W, Cr, T a. Nb, Y, Ti, Zr, Hf, B, Mn, Fe, Co and Ni. Bi-e metallic components then consist of at least two phases, one of which is preferably a normal tetragonal MoSi2 and the other or the other is gel silicicides. If Cr is added, the metallic component consists of two phases, namely 1. tetragonal MoS1.2 and 2. a hexagonal mixed silicide (Mo, Cr) Si2. In this case. The MoSi2 may dissolve a small amount of the Grei2, but it still retains its characteristic tetragonal MoS12 structure. It is advisable to keep the addition of chromium or other similar additives in small amounts, namely between 1 and 15%, preferably around 5% by weight.

Will infiltration or any subsequent heat treatment carried out in a carburizing atmosphere, as already said above, some of the excess silicon in seats is converted into SiO. Such silicon carbide crystals have a size of 1 to, lo microns and are inside the metallic component educated. They influence the quality of the end product in a very special way. -The carburization is expedient in CD for 10 to 60 minutes at 170 to 19ooo executed.

6. Incomplete pore filling In the embodiment of the method according to the invention described above, the infiltration is expediently carried out so that the porous starting body is embedded in a powdered alloy and then heated in order to melt the powder. The pores are thereby filled with melted -mass as long as stock is available at this. ]) aThis enables local impregnation to be carried out in, `areas, where this is desired. With this method electrical resistance elements can be manufactured, which have different specific resistance values in their different parts without. that it would be necessary to measure the cross-section of the porous starting kdrpe m ' to change. It is also possible to regulate the menu ge dee 'Zülhoeateriale with reference to the porosity of the starting grain. adjust pers occasional forositätsveränderungen to a Bark product with a constant and reproducible content of Mäaliisoher component to get. Incomplete impregnation results in . depending on the composition. @ ttn @ tetsung and temperature of the alloys, different results. In certain cases there is an even distribution of the le- Government and: erbleibenden pores. In other cases it will a superficial layer of the porous starting body up to a well i mmed depth completely filled, while in deeper Sohiohten @ a remarkable absorption of the alloy does not follows. Accordingly, the present invention is not based on materials bere $ rän "o which are completely impregnated. Is only a thin one Oohioht completely filled out, will be valuable technical proprietary. Aohaften received, while at the same time considerable savings of alloy. To make the superficial layer one effective Bohntz against corrosive. .Arrudes from fees or liquid It is difficult to give this layer such a thickness $ u give the .ndestena-twice the diameter of the largest Bilioi = carbide partohen. which is enclosed in the body Bind, 1) 1 e , Sohiohtdiake still definitely at least loo microns, preferably at least o # 5 mm. In other. Cases can the thickness can be considerably greater, for example - if it is to brick crucibles, capsules, muffles and the like. Is in which the Bohiohten. never filled pores advantageously with thicknesses of 2 to 5 mm. In big. 8 are the amounts of absorbed alloy tion - as a function of the amount of alloy powder used - shows. 21- it ta tsahlen refer to-_the weight of the porous Outer body. In 7ig. 7 is the specific resistance at different different temperatures for bars (A to F), which with different 'Amounts of alloy impregnated are entered. These bars were all derived from SAO and had a grain size of 325 meshes. The Atab G corresponds to the stick 7, but it was made from a mixture of 8o Yo SiC, 325 mesh, and 2o 81C, 8oo mesh see made. _ ° At epi ele 12 to 238 The following table gives a list of 12 experiments; wel- che the-different ways through the execution of the objective ir- Illustrate finding. Each experiment was carried out as follows Rods made of recrystallized silicon umo arbid with a porosity of about 30% were placed in a graphite crucible, with one Silicide powder layer separating the rods from the crucible. The rod- diameter was 12.5 mm; the ELtab had a central hole of 4 mm. The powder coating had a thickness of 3.0 mm. Of the Graphite crucible was quickly heated to 2,000 and at this temperature held for 15 minutes. After cooling, the following became Data of the impregnated rods determined = specific weight, apez. resistance at 2o0 and weight gain and change in specific resistance after oxidation at 15oo0. The silicide powder composed of 4 parts MoOi2, grain size lo IvLikron, and a part of silicon powder, grain size below 43 microns, and also from a further addition of 5% by weight of an element from the group: Qr, f, 0o, Ta, Zr, W, B, Ni, Fe, Al and Mn. Ver Such 23 was a blind test without this separate addition: a) Example No. 12 13 14 15 16 17 b) Powder additive Cr V Co Ta Zr-W e) Weight before infiltration 13.41 12.32 12.24 12.7 14.5 12.36 d) Weight after infiltration 2o, 91 23.76 23.84 22.9 24.5 22.11 e) volume, em3 5.9 6.1 6.2 5.9 6.4 5.9 f) Specific weight before (d1) 2.27 2, o2 1.97 2.15 2.26 2.1o g) Specific weight according to (d2) 3.54 3.88 3.84 3.88 3.82 3.75 h) d2 - d1 - 1.27 1.86 1.87 1.73 1.56 1.65 i) Oxidation,% weight gain, oo6, o14, o2o, o22, olo, o22 j) Specific resistance to oxidation 53 169 53 145 43 72 k) After% ydatian werend 2o h 32 1o8 38 llo 24 72 at 2o C. ohm # mm / m 1) After oxidation for 4o h 3o 116 53 93 33 82 at 2o C ohm. mm / m m) After ogdation during 6o h 32 117 52 92 36 82 at 2o C ohm # mm / m n) After Ogdati on about 30 days at 2o 0 ohm # mm / m 3o 117 63 88 38 83 p) Porosity before infiltration (p) 29 37 38 1/2 33 29 34 1/2 q) (d2 - d1) / p 4.4 5, o 4.9 5.3 5.4 4.8 r) water absorption after 195 1.8 '195 o, 8 2.1 4.2 infiltration a) Example No. 18 19 2 o 21 22 23 b) Additive powder B Ni Ire Al Mn. - c) Weight before infiltration 12, o7 12, o1 11.48 11.65 13.87 ll, o d) Weight after infiltration 22.49 23.84 2o, 83 22.48 22.35 1998 e) Volume, cm3 6; o 6.2 5.5 5.4 5.9 5.3 f) Specific weight before (d1) 2, o1 1.94 2, o8 2, l6 2.35 2.07- g) Spec. Weight according to (d2) 3974 3.84 3.78 4.16 3.78 3.72 h) d2 - d1 1.73 1.9o 1.7o 2.0o 1.43 1.65 i) Oxidation,% weight increase me, o11, o37, o48; o17, oll. 9010 j} Specific resistance before 19 19 1o3 29 29 5, .5 oxidation After oxidation during 2 2o h bet.2o 0 ohm s mm / m 34 14 99 33o 29 82 L) Wait after oxidation 40h at 2o 0 ohm # mm / m 49 16 146 3 0 5 33 -108 1) Wait after oxidation 60h at 2o 0 ohm. mm / m 56 14 146 .348 38 85 1} After oxidation Z during 69 16 145 339 41 260 3o days 2 at 2o 0 ohm. mm / m @) Pmosity before infiltration (p) 37 39 1/2 35 32 1/2 26 1/2 35 . @ (d2 - d1) / p 4.7 4.8 4.9 6.1 5.4 4.7 Water absorption after 197 2.5 not determined 4.0 infiltration In line i of the table is the weight gain per hour after Glowing indicated for 40 hours in air at 150 °. In the Lines _k bims m is the specific resistance in ohm # mm 2 / m after the Glowing and 20, 40 and 60 hours listed. The line n is the q t $. resistance after annealing for 60 hours and after = subsequent storage for three days at room temperature to take,. Line q, brings the values of (d2 - d1) / p, ie the ratio .of . absorbed alloy to the porosity of the starting body. If the pores are completely filled, this ratio is oo a meal for the ape $ .weight of the alloy. -This value fluctuates in the twelve Vernunhen between 4.4 and 6.1, which is a good approximation with the specific weights of the base alloy, 4.7 g / cm3, and den Noßi2, 6.2 8 / 0m.3, stands. As in connection with the Description of the embodiment of the invention explained first mentioned, the bilioium content of the alloy falls, according to Garburie tion and: formation of 50% cubic BiG in situ in the powder to about 37% in the final product. For a 100% pore. .long, the value 4.7 of the above ratio means that no garburization occurred, while the value of 6.2 shows that ' the above-mentioned excess amount of silicon is converted into 8i0 became. Versnah 23 shows that the specific resistance of the impregnated ned body (5.5 ohms) during the 20-hour glow is increased (up to 82 ohms). With continued glow are the changes are irregular and it is difficult to reproduce to maintain bable values, especially when the po @ ben cool down quickly. Dien. Is a consequence of this, then regarding the annealed samples the change in the specific resistance is subject to a delay are. In: Experiment 23, the specific resistance changes before 85 ohms in 26o ohm simply by storing the sample for three days Room temperature. Examination of the samples under the microscope after glowing for 60 hours * shows very clearly that the pores contained in the filled pores have burst into a large number of small parts with an approximate diameter of 10 microns. The original particle size was about 100 to zoo microns.

The embodiment of the invention described above makes it possible to obtain a very fine-grained structure of the alloy in the end product, without annealing of the product, which is accompanied by irregular changes in the specific resistance. This result can be achieved by adding certain metals. These form misehatlicides with the molybdenum silicide, which trigger the formation of a secondary phase. This causes the metal silicide to settle. composed of two phases, one of which is MoSi. and the other is the mixed silicide. It can be seen from the table that the extra metallic additives, albeit in different degrees, have the effect that irregularities in the change in the specific resistance are avoided.

An addition of 1 to 15%, preferably 5%, of chromium is advantageous (Experiment 12) in order to maintain or improve the oxidation resistance, whereby at the same time the changes with regard to the sepz.ffideratand after the glow do not matter.

7. Use of unsintered starting bodies for the infiltration . Surprisingly, investigations have shown that the production of pressed bodies according to the method of the invention can be significantly simplified if the production of the porous starting body and the infiltration process take place in two combined annealing stages, the part one and the same furnace process.

The annealing required for the method according to the invention in the production of the starting body and in the impregnation process must be carried out at very high temperatures. These annealing operations make pine considerable part of the necessary for the production of the body, or. Pressed products according to the invention from costs to be expended: As was surprisingly found, it is possible to carry out both the production of the porous starting body and the impregnation process in a single annealing operation. This particular embodiment of the invention consists essentially in the fact that a pulverized mixture, which preferably consists exclusively of silicon carbide and temporary binders, is formed, then embedded in a powder of a silicon-containing alloy and finally heated in such a way that the bilium carbide particles sinter together and at the same time or before the pores of the initial body formed are penetrated with the silicon-containing alloy, recrystalli si eren.

In this embodiment of the invention, the annealing is preferred carried out in an atmosphere containing carbon monoxide, which is consequently incomplete Combustion of the graphite or carbon is formed from which the annealing furnace tube In this case, a reaction occurs between during the infiltration the invading alloy and the carbonaceous atmosphere that leads to the formation of silicon carbide in situ: leads.

In the last-described embodiment, no less than three different processes take place during a single annealing process, namely 1. sintering of the bilicium carbide parts in a porous starting body, 2. penetration with the silicon-containing alloy and 3. partial formation of 8 silicon parts are located within the penetrated alloy. This process thus enables a considerable reduction in manufacturing costs and at the same time results in products with more beneficial properties. The latter is surprising and is the result of the fact that the process completely avoids the difficulties which occur when heating or cooling the furnace and which result in the control of the protective atmosphere being inadequate. A particular problem in this context is the harmful formation of oxygen, which contains secondary products. This formation occurs when solid silicon-containing alloys are carburized in CO at temperatures below about 16,500. It does not occur when following the recommendations of this invention.

Example -`'4: Green silicon carbide with a grain size of 800 meshes was mixed with 3% water glass (380 Bb) and pressed to form a rod measuring 100 x 6 x 5.5 mm. The weight of the rod after drying was 6.0 g. The dried rod was embedded in a surrounding layer of uniform thickness of a powder mixture which contained 80 parts by weight of UIoSi2, 20 parts by weight of silicon and 5 parts by weight of chromium powder. The particle size of the molybdenum disilicide was less than 10 microns, while those of silicon and chromium powder were less than 50 microns. The rod with the powder layer (7g) was placed in a graphite crucible and heated at 2,000 for 15 minutes. After annealing, the bar had increased in weight by 4.9 g, that is, by 82 Ω. A photomicrograph of the impregnated body showed at 500 times linear magnification that the penetration between silicon carbide and alloy is particularly good and that no pores at all can be detected. Since the silicon carbide was not graded in terms of its grain size, but rather had a uniform grain size of 10 to 15 microns, the pores of the starting body - and thus the silicide particles - were relatively large; they had a maximum diameter of about 15 fvdkron.

zei spi e1 25 : Green silicon carbide with a grain size of 800 mesh was used as the starting material, which was comminuted by wet grinding in a hard metal mill for 72 hours to a maximum grain size of about 8 microns. Otherwise, the manufacturing process as described in Example 24 was followed and gave a body of the same appearance but with considerably smaller silicide particles. Example 26: The production of an electrical resistance element according to the invention is described in detail below. This element can also be used in an oxidizing atmosphere at a temperature of up to 1550o, without aging. The element is produced as a unitary body, but, as will be explained below, is composed of a central annealing zone 21 and two .n.d zones 22a and 22b, which have a lower specific resistance, whereby the provision of special cooling devices for the end zones is unnecessary .

The production is divided into the following process steps, of which each will be treated individually. Reference is made to FIG. 1 of the drawings taken: preparation of a mass for the extrusion of silicon carbide. Extrusion and drying the silicon carbide rods. Preparation of the alloy powder.

Pack the silicon carbide rods in alloy powder. Infiltration.

Final oxidative sintering and verification.

In addition, for further explanation of the during the impregnation The processes in progress also dealt with the material balance.

Preparation of a mass for the extrusion of silicon carbide. Green, coarsely ground bilicium carbide was further ground and sieved until the entire amount passed through a 325 mesh sieve. The grinding took place in a Kallermü # .e. The powder was treated for a few hours in a kneader with an aqueous solution of an organic glue based on gelulose ester, which bears the brand name MODOCCZZ M and is manufactured by the Swedish company Mo. & Doms jö Aktiebolaget. The amount of dry glue was 3.5% of the weight of the silicon arbi ds; 8 liters of water were added to every 1 kg of glue. The mixture was made at 50o so that the content gradually increased as the mixing progressed. was decreased. As soon as this was reduced to about 8% of the weight of the 1vIasse, the latter had a consistency suitable for further treatment. The mixing was now interrupted and the mass transferred to a vacuum mill, in which it was subjected to a pressure of 20 MM / H9 and pressed into a cylinder with a diameter of 50 mm.

Pressing and drying of the silicon arbide rods.

The pressed cylinders were placed in a piston press and pressed into long rods with a diameter of 8 mm. . The rods were cut into lengths of ¢ 0o mm. which corresponds to the dimensions of the finished resistor element. The moist rods were dried in a climatic chamber at ¢ o0 and then had such a strength that they could be subjected to further treatment without any loss of substance. The density of the bar was 2924 g / cm3, of which about 3.5 is accounted for by glue, while the remainder, that is about 2.17 g / cm3, is silicon carbide. From this it is calculated that the silicon arbide framework of the dried rods contains about 35% pores. The purpose of the impregnation that now follows is to fill all or a substantial part of these pores with an alloy of the composition MoSi2. The glue is added during the impregnation and leaves a small amount of carbon, which, however, is converted into silicon material.

Preparation of the alloy powder.

Molybdenum disilicide, produced in a known manner by an exothermic reaction, is comminuted and ground in gasoline in a hard metal mill. After 96 hours of grinding, the desired grain size was achieved, ie the entire material had a grain size of less than 10 microns. 80 parts by weight of the disilicide powder were with 2o parts by weight of pure silicon (99%) with a grain size of less than 325 meshes and with 5 parts by weight chromium powder, equally if the grain size is less than 325, mixed. After all of the interest had been drawn off, it was pulverized Mass mixed in the dry state. Pack the silicon carbide rods in Lef "ion powder. The dry ätabe were, as in fig. 1 explained with the powder surround. Each rod had tongs of 400 mm and was Coated with the palver in a way that the central zone 5 had a length of zoo-mm and an oversize 6 of 1.5 mm, while two .Y @ .d zones 7 loo mm long and a coating 8 of approximately 2 mm thick. The amount of powder for the different zones were carefully weighed, taking into account that the thinner layer 8o #o and the thicker layer 12o; #, of the court of the uncoated rod in the relevant zone. .3 in the case of mass production it is. Advantageous, the desired fillers amount to be pressed into tubular pieces, which after drying be pressed against the staff. -gis is then easy to get an exact ab- To obtain an estimate of the amount of powder available for each zone. If the rod is provided with the powder layer, it becomes a layer thick paper 9, with which it is ready-made for the infiltration tration process is. Infiltration . The rooms, which are layered and wrapped in paper, are delts of three rods each introduced into a graphite tube 1o; the mutual hesitation of the bars is from r'ig. 1 can be seen. The graphite tubes had 50 mm pliers and a 4 mm hand thickness. Before each impregnation, about 10 g of garphite powder was Brought to the inside of the graphite tubes, which then turned some eels were, zs next: carbon becomes in the infiltration process consumed, which is otherwise taken from the inside of the uraphite tubes men would have to be. If you add graphite as described used, the same pipe can last practically for an unlimited period of time unless it breaks accidentally. On the other hand, if no additional graphite powder is used, the impregnation process will be seriously disturbed after the first use of a new graphite tube. The graphite tubes with the three inserted in them. coated rods are then placed in a graphite tube furnace. Preferably, the three tubes are placed side by side in parallel in the furnace, similar to what is the case for the three rods. The furnace then contains nine bars. However, a larger number of tubes can also be accommodated in the furnace - for example seven tubes in close packing, so that 21 rods are produced in each firing stage.

The graphite tubes are placed horizontally in the furnace, which then at the ends: is provided with graphite plugs. JQiese, the ends don't have to be perfect Close tightly, because the carbon oxide formed has to be drawn off freely. It is not Special protective gas required, but it has proven to be useful in mass production proven to conduct carbon monoxide through the furnace. About overheating of those parts to avoid the coated rods, which are directly on the stove pipe, it is advisable that the inner graphite tubes do not pierce the furnace tube directly but with the help of vron spacers 'on' the outer earths of the pipe inside one A distance of about 5 mm from the landing of the stove pipe must be kept. -This Zat a length of 100 mm and a sand thickness of 5 mm in the annealing 3one, which -a Has a length of 5oo mm. The two end zones each have a length of 25o mm and a wall thickness of 10 mm, Der) fen is cooled by means of a cooling jacket with water.

) The stovepipe is connected to a transformer of 50 kVA. the The temperature is raised to 2,000 in 5 minutes and kept constant for 15 minutes. The temperature measurement is carried out by reading through the opening in a stopper at the end of the furnace tube, as a result of the development of smoke and gas during the glowing process it is; chirderig to control the temperature closely. fear-observation ler Adjustment of the transformer from one attempted to another can the required accuracy in terms of temperature even in mass production secured. The reading is so imprecise that the actual temperature during the impregnation can vary between 1000 and 21000. When the oven has cooled down When it is finished, which takes about 60 minutes, the tubes are removed, the rods with a Brush cleaned and ready for another treatment.

: The amount of powder available for impregnation has a significant influence on the properties of the impregnated product. The more ver is added, the more alloy is absorbed. The alloy is evenly distributed in the porous carbide body and is therefore not concentrated on the surface of the same if the amount of powder was not sufficient for complete impregnation. The relationship between the amount of powder used and the amount of alloy absorbed is shown in fig. 8 It has been found that the absorption in relation to the amount of powder is up to a sharp. Limit increases. If more powder is added, the amount of alloy absorbed remains constant since the pores are then practically completely filled. The excess of the ulver remains as a porous slag on the outside of the im-. The whole aggregate is surrounded by a thin tube made of bilicium carbide, which separates each rod from neighboring rods. This carbid tube is created by the reaction of the silicon on the paper into which the rods were wrapped. As a result of the formation of the carbide tube, it is possible to burn several rods at the same time in the manner described above, without any loss of accuracy with regard to the amount of absorbed alloy, final oxidizing sintering and testing.

The cleaned rods contain continuous pores, at least in their central annealing zones, which have to be closed before the rods can be used for their intended purpose. The rods are first placed in ovens and left in the air for ten hundred hours Heated from 150o ° to 15500. The weight of the annealing zone of the rods @ becomes then increased, due to the formation of quartz glass, which the: koren closes and also a protective layer on the surface of the J # lementes. Another holvre is that - @r dider- stand is increased. . At 15.o0 the resistance of the central -tral annealing zone after a heating time of -24 hours from 19o to 21o ohm / mm2 / tn, but remains then with continued heat essentially constant. The resistance of the outer zones increased by the same treatment from 22 to 25 ears. After this When heated in air, the resistance of the element under the -be- controlled in high temperature conditions. Such elements whose The resistance value is within predetermined limits Metallizing provided with contacts and are then: ready to use. Ls is also possible directly in connection with the oxidation To carry out control by passing an electric current through lets the element step through: Material expenditure . The reactions going on during the impregnation are on best by determining the cost of materials clearly. Uru the To avoid complicating calculations, the following is the impregnation nation of such bars, which weave through the end zones in correspond to your obex-described element and are therefore complete have filled pores. To achieve great accuracy: the `read included the impregnation of three 2 bars of 40o mm each Length. Before impregnation, the bar weight was 122 g. The bars 144 g of alloy powder were packed. After the reaction had the weight of the bars increases to 205 g, while the back remaining powder residue was 32 g. This rest was. analyzed and Contains 35.2% Mo, 3.6% Cr, 47.5% Sii'11.8% C, reads impurities The impregnated rods would have the following composition 16y8 '% Mo. 215 % Cr, 59.0 % Si, 2o. o% C, reads contamination o As far as the impregnation process is concerned, the following material balance sheet, with all figures the total weight in g indicate: Inserted -Verunrei-. Total - Contaminants received - Gesi Material approvals Products approvals Mo Cr S i C Mio Or Si C dried 83, 36 3 122 arbid rods iegierungs- 68 7 68 1 144 _ powder 'on the 8 8 atmosphere more absorbed _well ens toff it would be, of which 35 5 123 4o 2 2o fermented filled turden 'ulverrest 11 1 15 4 1 3 Losses 22 1 13 1 ; Steam and smoke). _total 68 7 151 44 4 274 68 7 151 44 4 21 The total volume of the original rods including the pores is 56 em3, which means the volume weight was 2.16. From this It pays that 32% or 18 cm3 of: ferment for the impregnation To be available. After the infiltration, 15 g of newly generated silicon carbide formed and 68 g of an elish silicide found, its Composition, regardless of a small excess of silicon can be written as (iioo 8Cro 2) Si2. The volume of this pore-filling lumbar substance amounts to a total of 16 cm3o. From this it can be calculated that the remaining pores form 2 cm3 or 3 Volo; ö. Of interest is determine, Ä.; 32 about 1o j @ of the silicon carbide of the impregnated grain pers is formed in situ by carburizing the silicon. 'From the 8 absorbed carbon is etixla half for this dildlan,! r des SiC consumed within the body,; ^ ;, lzrend the second iialte for the Formation of SiC in the powder residue, and consumes outside the body will. The latter educate; de.s SiO is very important, because it is a 13- dingurig for the formation of a smooth surface on the impregnated nerten product is. Once the carburization of the remaining powder is disturbed, a kind of Ridge, and it is difficult to mechanically- fully- lig to remove. But it has been found that it is possible to remove this burr by repeatedly burning at 2ooö0 in Co gas. The burr is then converted into loose powder, which is essentially union is silicon carbide. Does the atmosphere contain sufficient Amount of carbon monoxide, so has the amount of alloy powder linen influence on the formation of the ridge, and it can do more than that- 2 times the required amount of f-powder can be used, namely without the slightest disadvantage. Only 50 o of the added molybdenum is used while the rest is lost. An essential result is obtained, if Carbon monoxide as protective gas - during the entire impregnation vorganöes used Prdo .us can also be useful, the oxy- dation des legz_erun "#spulv (# - rs. during the areas described above to be excluded. The oxidation leads to the formation of SiO2 Consequence which escapes in the form of SiO during the impregnation. #If also the silicon losses from the economic side seen are not of great importance, it is for him Process important that the. Silicon content uri-ger careful "tiOntrol- le is held because it is only possible through this, _ a constant To obtain composition, which in some cases for the @ nd- rproduct is sold. .3ci game 27 ' : in deaa previous -oeis?) iel was the production of an aider-- Statidselementes described in which the annealing zone emn higher Had resistance than the parts of the wind, due to the fact that his pores are unvo-! lst-zndi1; # have been filled in: After the remaining lcjzen pores of the annealing zone with resin glass during the final the oxidizing S_n: - ;; eration were filled out, this was in li'igo 3 -Showed 1 line received as i # 7 final product. But there are also others Ways to create different resistances in the impregnated material. Fig. 4 shows another element of constant thickness and manufactured by means of a special joining process. This is explained in FIG.

A porous, dried rod 11a, Fig. 2, which form the annealing zone is made in the same manner as shown in Fig. 1, with from Silicon carbide with a particle size! 325 stitches and glue are assumed. another porous, dried rod llb is produced in the same way, however, from a mixture of equal silicon arbide (8oo meshes.) and MoSi2 with a particle size of 1o Mkron and glue. Both bars are as if from the nigur can be seen, sanded and with a lübel 12b and one in the adapted Bore 12a provided.

The grinding of the dried mass can be carried out very easily. The dowel and the wall of the bore are moistened with a glue solution and rubbed against each other for about a minute until a good connection is established. The bonding layer 14 between the two rods now consists of a uniform material of high viscosity. If this has been dried, so there is a homogeneous transition 15 between the rods. The opposite ends of the annealing zone are provided in the same way with a connecting part; the rod, which is composed of three parts, is impregnated as indicated in FIG The end product is shown in Fig. 4, in which, however, a pair of outer connections 16 made of MoSi2 have been firmly connected to the inner connection parts 17 by welding is mgöoeh because the connecting parts 17 contain about 6o Vol4 MoSi, and only 40 Vol% o SiG. A On the other hand, the material of the annealing zone 18, which has a rigid recrystallized structure, cannot be welded to the material of the connecting parts or to MoSi2. .die Fig Q 5 and S show two hairpin örmi ge elements g which .in essentially the same way as the elements according to the. Pig. 3 and 4 are produced. In FIG. 5, the annealing zone is 24 bent, and their indene are connected with connecting parts-25-, ndeno In Figo, 6 the curved rod 19 consists of 1Vioi2, while di.e inner and outer connecting parts 2o according to the one described above Method with the annealing zone forming part 21 were connected. Example 28 :. _ Another way of producing cold ends in electrical ones Heating elements emerge from the following description. An extruded rod of the same composition as in Example 26 is given, is dried and the mean ab- cut with a carbon-containing carbonic fluid like farfurol. The composite rod will then packed in alloy powder as shown in Fig. 1. That plastic matter is used in an early stage of the impregnation Process earbonized, and that in the pores of the original body Carbon formed in sitaz completely after impregnation. Converted to Siü .-- The amount of silicon alloy in the finished Rod will therefore be lower in the middle section than in den -.Lndeno The greater consumption of silicon in the middle section has to be compensated_ * ground by the fact that you are around the ttättelbereich Around: a allo -Uses ung spulver which has a higher silicon Retained than that used for impregnating the ends Alloy powder.

Claims (1)

P ate nt claims 1. Process for the production of essentially dense bodies, consisting of silicon carbide and at least 10 percent by weight, but not more than 70 percent by weight, of silicon. containing, refractory alloy, characterized in that a porous body is first formed, which consists essentially of one or more substances from the group of hexagonal silicon carbide, cubic silicon carbide, graphite, amorphous carbon and a carbon-containing, carbonizable material, then this starting body brings into intimate contact with a powdered alloy enveloping it, which contains silicon in a percentage by weight which exceeds the percentage by weight of silicon of the refractory alloy, then the porous starting body and the powder enveloping it in a carbonizing atmosphere at a temperature above the melting temperature of the powder, but below the decomposition; -s one lets the rest of the powder on the outside of the impregnated body form a loosely adhering, porous cake, consisting of silicon carbide particles formed in situ, which is embedded in a sintered stretched residue of carbonized alloy powder, and that finally this loosely adhering, porous Cake removed from the surface of the impregnated, dense body. 2. Method of making substantially dense Bodies, 'consisting of: silicon carbide, silica and a refractory control with at least 10% wt and not more than 70, ° 1 Gewo silicon, whereby the silica acid 1 to 20% vol., the = decoration 15 to 70% vol -.- and the silicon carbide is 30 to 85 c / o vol. of the body, characterized in that one first has a porous Body shapes. which consists essentially of one or more "Substances from the group of hegagonal silicon carbide" cubic silicon carbide, graphite amorphous carbon as well as from a carbon-containing, earbonizing available material, then this starting body in intimate contact with a pulverized one enveloping him Alloy brings silicon in a weight percent set containing the Bilicium weight percent of the refractory alloy exceeds, then the porous Starting body and the powder surrounding it in one carbonizing atmosphere at a temperature above half of the melting temperature of the powder powder but below the decomposition temperature of the silicon carbide and during heated for a time sufficient to allow part of the melted powder in part of the pores at least of an adhering part of the starting body penetrates, leaving the remaining pores. stay open and 1. make up to 20% of the volume of the impregnated body, . that the nest of the 2ulv = ors at. the outside of the impregnated body a loosely: adherent, porous Can form chewing, consisting of in situ; educated Silicon carbide particles in a sintered residue of carbonized alloy powder is embedded, and that you finally get this loosely clinging. Porous copper from the surface of the impregnated, dense body removed, whereupon the porous impregnated body is heated in an oxidizing atmosphere at 'k4000 to' t6000 and for a time sufficient for the formation of silica in substantially all of the pores of the impregnated body. 3. Method of making substantially dense bodies , consisting of silicon carbide and a refractory alloy containing at least 10% by weight but not more than 70% by weight of silicon, characterized in that a porous starting body is first formed from recrystallized silicon carbide, this starting body in intimate contact with a surrounding it powdered alloy that brings silicon in one. Contains percentage by weight, which exceeds the percentage by weight of bilicium of the refractory alloy, thereupon the starting liquid and the powder coating it in a carbonizing atmosphere at a temperature above the melting temperature of the powder, but below the decomposition temperature of the silicon arbide and heated for a time which is sufficient that substantially all pores in at least one adhering part of the starting body are filled by a part of the molten powder, so that the rest of the powder - on the outside of the impregnated body - is allowed to form a loosely adhering, porous cake, consisting of silicon carbide particles formed in situ, which in a sintered back made of carbonized alloy powder is embedded? and that finally the loosely adhering, porous cake is removed from the surface of the impregnated, dense body. 4. A method for the production of substantially dense bodies, consisting of SiC and MoSi2, characterized in that one is first a. porous body forr, c, which consists essentially of one or more substances from the group of hexagonal SiC, cubic SiC. Graphite, amorphous carbon as well as a carbon-containing, carbonizable material consists' this starting body then by sheathing with a pulverized: "of 47 to 80% by weight of silicon and 53 to 20% by weight of molybdenum existing alloy brings the starting body into intimate contact with the Powder enveloping it is heated in a carbonizing atmosphere at a temperature above the melting temperature of the powder but below the decomposition temperature of the SiO for a sufficient time that essentially all pores of the starting body are filled with a part of the molten alloy and the alloy within the pore spaces the starting body is carburized with in situ formation of SiC particles and formation of a higher-melting alloy, essentially corresponding to the formula MoSi2, so that the rest of the powder is allowed to form a loosely adhering, porous cake on the outside of the impregnated body, existing of silicon carbide particles formed in seats, which is embedded in a sintered residue of carbonized alloy powder and that finally the loosely adhering, porous cake is removed from the surface of the impregnated, dense body. 59 -erfahren according to claim 1, characterized in that one for the. Forming the porous starting body SiC particles, a water-soluble cellulose glue and water mixes, this mixture is kneaded into a plastic mass, this is pressed into a rod-shaped form and the molding is dried. whereby a form-retaining training. corridor body with 95 to 70% goals is obtained. 6. A method for producing an electrical heating element according to claim 5, which element consists of a central heating zone and two adjacent colder zones, the electrical resistance of which is lower than that of the central zone, characterized in that; From a first mass of SiC a rod forming a heating zone part and from a second mass of SIC and silicide two rods forming cold zone parts are pressed and the ends of the heating zone part are each connected to one end of the cold zone parts, whereupon the entire, assembled rod is covered with the powdered alloy and is heated and impregnated therewith, 7. A method for producing an electrical heating element according to claim 5, which element consists of a central heating zone and two adjacent colder zones, the electrical resistance of which is lower than that of the central zone, characterized in that one consists of a made of SiG measures a rod pressed, dried and impregnated a central part of the rod with a liquid, carbon-containing, carbonizable material, then heated the rod to decompose the carbonizable material and to form carbon in the pores of the central rod part, whereupon man the gan zen rod covered with the powdered alloy, heated and placed in order to transfer the carbon formed in the central part III Silieiuiiicarbid, whereby a middle -i (i zzorie is obtained, whose el.ekiri "3clier tiä_der- st: a @ c -ri@ssc1.i ;; t as derjens Irishman of the bciiachbax ° I, tai <silrl: -oxieri @@ f - x,? l = tc B.-More heat-resistant and more oxidation-resistant, essentially: dense body, consisting. made of S0, a fire-proof one Alloy and an oxide component, thereby marked net that- .the SiO forms a framework and -the. alloy and the oxide component is the pore space of this framework fill in, with the body made of 30 to 90% vol. SiG, up to 20% vol. oxide component and 10 to 70% vol. silicon Alloy is composed and that alloy from up to 90% by weight of one or more metals from he Group - yV, Mo, 0r, Ta, No, V, Hf =, Zr. Ti. And up to 30% wt one or more elements from the group Al ,. Be, Ga ;, Ce. Co, Cu, Mg, Fe, Mn, Ni, 0, B, as well as from at least- 10% wt. but not more than 70 wt. Si exists during the oxide component is made up of oxygen compounds or several @ elements from the group Al, Be, 0e, 0rl Hf9 Mg, Ti, -ts ZrP Th, -Y and other rare earth metals together- sets, and that-the grain size-of the alloy in diameter- cut is not higher: than 10 microns and.the particles. - size of the SiO is between 60 and 1200 mesh. ' Body according to claim 8, consisting essentially of Si.C and MoSi2, characterized in that it is rod-shaped and consists of at least two sections that form one piece, one of which has a higher percentage of SC and has a lower percentage of MoSi2 than - the other sections. 10. Body according to claim 89, characterized in that it is formed from SiO and SiB3,