SE435173B - PROCEDURE FOR PREPARING A HIGH DENSITY CERAMIC SILICONE CARBID MATERIAL - Google Patents

Description

7935707- * 1 10 15 20 25 30 35 2 da product has a breaking modulus of 26.1 kPa at room temperature and 33.8 kPa at 371 ° C. A recent progress has been made in the use of boron as a density increasing additive, the amount of addition usually being in the range of from about 0.3 to about 3.0% by weight, based on the powder. The boron additive may be in the form of elemental boron or in the form of a boron-containing compound, for example boron carbide. Examples of silicon carbide powders containing boron can be found in U.S. Patents 3,852,099, 3,954,483 and 3,968,194.

It has now been discovered that a high density increase can be achieved when sintering a silicon carbide-containing powder containing beryllium as a density-increasing aid, if the sintering is carried out in a beryllium-containing atmosphere.

By carrying out the sintering operation in an atmosphere containing beryllium, the amount of beryllium which would normally leave the powder compact will be reduced, and the sintered ceramic product has a more accurate composition; and is less porous than sintered products, which are generated when simply adding beryllium to the powder as an additive. Beryllium can be added to the furnace atmosphere by introducing beryllium compounds into the sintering chamber, which generate a significant vapor pressure in the sintering temperature range. Such compounds may conveniently be introduced into the sintering chamber in the form of a solution or slurry of the beryllium compound and by applying the solution or slurry to the inside of the chamber. Suitably, acetone may be used as a carrier, but other carriers, such as water or other available liquids, may be used, the arm purpose of the liquids being to enable good distribution of the beryllium material on the walls of the sintering chamber. A beryllium atmosphere may conveniently be provided with by means of a coating or protective mixture, a powder composition containing a neryllium additive, for example a mixture of silicon carbide and beryllium carbide. When using such a mixture, the article to be sintered is placed in the cover or protective mixture and the article present in the mixture is subjected to sintering conditions.

Alternatively, beryllium can be added to the furnace atmosphere by using a beryllium compound for itself in the sintering chamber or by using furnace components, containers, crucibles and the like which contain a significant amount of beryllium. Crucibles, which are repeatedly used in the manufacture of sintered silicon carbide articles of the present invention, can build up or accumulate a beryllium concentration. The beryllium content of such crucibles can be monitored by conventional analytical methods, such as emission spectroscopy, to determine the amount of beryllium in the crucible and to determine whether additional beryllium is required to provide the beryllium atmosphere in connection with the practice of the invention.

The silicon carbide powder used as a starting material, which contains from about 0.5 to about 5.0% by weight of excess carbon, is mixed with finely divided beryllium or a beryllium-containing compound. Preferably, the particle size of both components is less than 5 μm and preferably less than 2 μm.

Exceptionally good distribution is achieved when the components have a mürke grain size of more than 1.0 μm. In order to achieve an increase in density, the beryllium or the beryllium-containing additive should be used in such an amount that between about 0.03 and about 1.5% by weight, calculated on the powder, is beryllium. The use of less than about 0.03% by weight of beryllium has been found not to lead to any significant increase in the density of the sintered product. If more than about 1.5% by weight of beryllium is added, this has been found to be detrimental to the increase in density.

A shield density of at least 75% by weight, calculated on the theoretical maximum density, is required for most purposes, and shield densities of at least 85%, calculated on the theoretical maximum density, are often required. Sintered products with densities of 85% by weight, calculated on the theoretical maximum density, can be obtained in practicing the method of the present invention.

The beryllium additive used in connection with the present invention can be used alone or mixed with other density-increasing aids, the most common of which are boron in the form of elemental boron or boron-containing compounds.

Lives in amounts between about 0.10 and about 1.5% by weight are valuable; 7995707-1 10 15 20 25 30 35 4 however, densities of over 90%, calculated on the theoretical maximum density, have been achievable when the boron additive is mixed in amounts of from about 0.1 to about 0.3% by weight. In general, such mixtures, when ready for sintering, contain from about 0.03% to about 1.5% by weight of beryllium and a total of between about 0.03% and about 3.0% by weight of density-increasing aids.

The bisel carbide starting material is preferably a powder which has a particle size of less than 1 μm and which has a specific surface area of more than 8.0 m 2 / g and contains from about 0.5 to about 5.0% by weight of excess carbon. In general, powder compositions having specific surfaces between about 5 and about 20 m 2 / g have been found to be excellently useful. the excess carbon can be introduced, for example, during the manufacturing process, by subsequent addition of carbon or carbonaceous material or as a binder before sintering; The beryllium or beryllium-containing additive found as valuable in the starting materials usually has a particle size below 50 μm and preferably below 10 μm. In order to facilitate an even distribution of the beryllium or the beryllium-containing additive in the silicon carbide powder, iron sizes of less than 5 .mu.m are excellent for achieving a homogeneous mixture which is useful in sintering. Other additives can be used but are not necessary to promote the increase in density or the increase in density during the sintering process. Preferably, the sintering is performed in an inert atmosphere; Such gases as argon or helium, which are inert to silicon carbide in the sintering temperature range, are very suitable for use. A reducing atmosphere can also be utilized.

The present invention utilizes a beryllium-containing atmosphere during the sintering process. The use of beryllium in the sintering atmosphere gives a marked improvement when the beryllium partial pressure in the atmosphere during sintering is equal to or higher than the equilibrium bed pressure of the beryllium contained in the silicon carbide powder compact. When the beryllium partial pressure in the sintering atmosphere is the same as or greater than the beryllium partial pressure in the article to be sintered, no loss of volatile beryllium will occur during the sintering process. The now remaining beryllium in the article serves as an aid in the search for density. At sintering temperatures, the beryllium partial pressure in the atmosphere is usually at least 10 atm and preferably at least 10_3 atm.

Silicon carbide powders which contain beryllium or beryllium-containing compounds as density enhancing aids usually contain beryllium in amounts between about 0.03 and about 1.5% by weight and preferably from about 0.04 to about 1.5% by weight. The final sintered material usually contains approximately the same percentage of beryllium.

It has been found that sintering in a beryllium-containing atmosphere does not appear to significantly alter the amount of beryllium in the final product. The beryllium-containing atmosphere serves to inhibit the release of beryllium from the powder compact during the sintering operation without adding any significant amount of beryllium to the product.

In pressure-free sintering, a silicon carbide powder containing from about 0.5 to about 5.0% by weight of excess carbon is mixed, thus to form a homogeneous mixture with beryllium or a beryllium-containing additive to a total amount of about 0.03 to about 1. 5% by weight of beryllium must be included. The homogeneous mixture is then formed into a compact.

Suitable additives to increase the flowability and binding of the particles can be mixed into the starting material mixture.

The compact is then subjected to a sintering in an inert or reducing atmosphere in which the beryllium partial pressure is equal to or greater than the equilibrium vapor pressure of the beryllium contained in the silicon carbide powder compact at a temperature between about 1950 and about 2300 ° C for a sufficient time. to achieve a silicon carbide product, which has a density above 75%, calculated on the theoretical maximum density. More specifically, a silicon carbide powder, which has a specific surface area of about 11 m 2 / g and contains about 2% by weight of excess carbon, can be mixed with between about 0.04 and about 1% by weight of beryllium, suitably added in the form of Be 2 C, or in ele- 7935707-1 10 l5 20 25 30 35 6 mental form. The resulting mixture is compressed to a density of about 1.76 g / cm 3. Binders can be used to increase the flowability of the powder or to increase the raw strength of the compact. The compression body is then subjected to sintering, preferably in an inert atmosphere, in which the beryllium partial pressure during the sintering process is about 10 ° C at a temperature of about 20 ° C for a time of about 30 atm or higher. The sintering operation is usually carried out. After cooling, the sintered product typically has a density above 85%, calculated on the theoretical maximum density. In the following, the invention will be further elucidated with some non-limiting exemplary embodiments. If nothing else. given in the description and claims, the temperatures are given in degrees Celsium and all concentrations in parts by weight and percentage by weight. * jr Example 1 'Control test A silicon carbide powder having the following specifications was used as starting material. The silicon carbide powder had a specific surface area of over 8.0 m2 / g and had the following analysis: * Oxygen <0.8% by weight; iron <0.2 wt% ä aluminum <0.4 wt% nickel <0.1 wt% titanium <0.1 wt% tungsten <0.5 wt% E free silicon <0.4 wt% silicon carbide> 97, 5% by weight A composition comprising 95% silicon carbide powder of the above composition was mixed in acetone with 5% phenolic resin (known as RESIN No 8121, a product of Varcum Chemical Company, USA). The slurry consisted of about 1 part by weight of mixture and about 1 part by weight of acetone. The slurry was mixed for about 30 minutes, and the acetone was then allowed to evaporate. The resulting powder mixture was pressed into cylindrical compacts with a diameter of 12.7 mm 10 l5 20 25 30 35 79os7o1 ~ 1 7 and a weight of about 1.5 g each. The compacts typically had a density of about 1.76 g / cm 3.

The compacts prepared in the manner described above were placed in a graphite crucible, which was covered and fired into a graphite tube furnace with resistance heating elements and having a temperature of 2080 ° C in the hot zone of the furnace, the furnace containing an argon atmosphere. After the compacts had passed through the furnace tube, the bodies had been sintered to a bulk density of 1.83 g / cm 3, ie about 57% of the theoretical maximum density.

Examples 2 and 3 Beryllium addition Six mixtures having the powder composition given in Example 1 were prepared except that varying amounts of beryllium were added in the form of beryllium carbide having a particle size below 10 μm, this beryllium being added to each mixture. The composition of the mixtures is given in Table 1. Four cylindrical compacts, which had a diameter of 12.7 mm and weighed about 1.5 g each, were pressed from each mixture in the manner given in Example 1. These compacts were divided into two batches, A and B, with two compacts from each mixture being included in each batch.

The compacts from batch A were fired in a graphite tube furnace with resistance heating elements in accordance with Example 1. The compacts from batch B were fired in the same manner as the compacts from batch A except that a cover or protective mixture was used to surround the compacts in the crucible. This mixture was in the form of a powder having the composition 97.5% silicon carbide, 2.0% carbon and 0.5% beryllium in the form of beryllium carbide. The purpose of this cover or protective mixture was to increase the amount of beryllium in the atmosphere around the compacts. The bulk density of the bodies before and after sintering was measured and is given in Table 1. The compacts included in batch A, which were burned in crucibles without any beryllium-containing atmosphere, had a density of 68.5-75.4% in sintered state, calculated on theoretical the maximum density. The batches contained in batch B, which were fired in crucibles containing a beryllium-containing atmosphere, had a density of 68.8-93.5% in the sintered or fired state, calculated on theoretical terms. the maximum density.

Example 4 Additive Mixtures A silicon carbide powder, the composition of which was similar to the composition of Example 1, was prepared and divided into batches.

Various amounts of finely divided boron carbide and beryllium carbide were added separately to each batch to obtain powder compositions, which are given in Table 2. The different batches were then mixed with a carbon feedstock and pressed into compacts in the same manner as in Example 1.

Two compacts from each batch were placed in a crucible of the same composition, covered with a crucible lid and placed in a graphite boat with a diameter of 10 mm and a length of 483 mm. The compacts were sintered by the boat with the compacts being driven through a graphite tube furnace, which had a resistance heating element and which contained an argon atmosphere during the heating. The compacts had a residence time of 30 minutes via 120 ° C. The results are given in Table 2. Thus, in the case of mixture No. 8, in the case of mixture No. 8, a silicon carbide starting material containing 0.10% boron, 0.10% beryllium , 2.0% carbon and 97.80% silicon carbide, pressed to a soluble E density of 1.72 g / cm3 or 53.6%, calculated on the theoretical ¿maximum density. After firing, the density had increased to 2.98 g / cm 3 or 92.8%, based on the theoretical maximum density of silicon carbide.

A control sample containing only 0.5% boron and no beryllium was prepared as above. The control sample was sintered in the same manner as above except that beryllium and boron were not present in the sintering atmosphere.

After firing and sintering, the control sample was found to have a bulk density of 79.0%, calculated on the theoretical maximum density. 7965707-1 w.Nm wm. ~ Ss.Hm @@. H ° w. @ M o. ~ Oo.HQ ~ .o MH o. @ M wo.m @ .mm m> .H ~ m. ~ M ø. ~ ~ mo o ~ .o Nä m. ~ m ~ Hm ~. «m« ~ .H ow.> m o. ~ oH.oo ~ .o HH H.wm mH.n @ .mm -.H ßw.> m o. ~ mo.oo ~ .o OH H. «m ~ o.« o.mm ° ß.H ~ m. ~ @ ø. ~ wm, o oH.ø 0 w. ~ m @@ . ~ ~. ~ m H ~. ~ @ w. ~ m o. ~ oH, ° oH.ow w. @ w _ m @. ~ @ .mm ~ ß.H ßw.> m o. ~ ~ oo oH .w N umuwmcuw fi ëwxm fl Eu \ w umummcwuwñwxmñ Eu \ m NNNN un Mmmuouomu> mwm xwwumuomu> m N n omm U om m wdwc umummøm fl wmuuømm uwuwwcm fl umwumm Hm fl uwum fl mflflm mm m mm mm. ~. ~ ~. «M« ~ .H o @ .H o. ~ O «~ @@ m m.mw w @. ~ M.« Mm ~ .H «.mß ~«. ~ M, «mm> .H ° wo °. ~ Q ~. ~ Mm ~. ~ W ° w. ~ M. «M mß.H« .m> ~ «, ~ ~.« M «~ .H o« .o ø. ~ Q @. ~ M «m. ~ M mm. ~ W.« M @ ~. ~ ~. «> W ~. ~ H.mm -.H o ~. ° o. ~ Ow. ~ @ M ~. ~ m @ m. ~ m.mm æß.H Ho ~ n ~. ~ n.mm w ~ .H oH.o o. ~ om.ßm N n.mm oo.m ~. «m« ~ .H o. Hß -. ~ @. «M @ ~. ~ Mo.o o. ~ Mm. ~ M H umuwmcwwm fi Eo \ w uwu fi mdmww fl Eo \ w uwu fi wømww fl Bu \ w uou fi mumvwâ Eu \ w NNN nu |« ~ ms xm fi u m: Hume xm fi u. m | fi x ~ s ßm fl u m | «x ~ e xw fi w M mm U u fi m w = fl = IUHONU> fl N IUHOUQ AND N IUHOUU> MN IQVONU bd N .IUGN fi m Uwu fi w fi w fl UNHUÉHW Hüumm fi w fl UNUHWS HUn-.mw fiflfl Vdhuß fl m Umwwm fi w fl UNUM-m fl H fl lmhwumåmwdmwuß H fl HunmH

Claims (7)

79'95707 * -1 10 15 20 25 10 PATENT REQUIREMENTS 1. A method of sintering silicon carbide powder containing beryllium or a beryllium-containing compound as a density enhancing aid for the production of a high density silicon carbide material, provided that the sintering is carried out in a beryllium-containing atmosphere in which the partial pressure of the beryllium ring is partial pressure. equal to or greater than the equilibrium characteristic of the bervllium in the powder. A method according to claim 1, wherein the partial pressure of the beryllium is brought to be at least 10-4 atm. 3. A method according to claim 1 or 2, characterized in that the silicon carbide powder is brought to contain beryllium in an amount of between about 0.03 and about 1.5% by weight. 4. A method according to claim 1, 2 or 3, characterized in that the sintering atmosphere comprises an inert gas. 5. A method according to any one of claims 1-4, characterized in that the beryllium is introduced into the atmosphere in the form of beryllium carbide. 6. A method according to any one of claims 1-5, characterized in that the silicon carbide powder is also brought to contain boron or a boron-containing compound as a density-increasing aid in an amount of up to about 1.5% by weight of boron. 7. The method of claim 6, wherein boron is added in an amount of between about 0.1 and about 0.3% by weight. 10 7905707-1 SUMMARY A sinterable silicon carbide mixture is described, which is prepared by mixing finely divided silicon carbide, which contains between about 0.5 and about 5.0% by weight. excess carbon, and a finely divided, beryllium-containing additive, the amount of beryllium in the mixture being brought to be between about 0.03 and about 1.5% by weight, calculated on the weight of the powder. A high density ceramic silicon carbide product is prepared from the powder mixture by pressureless sintering, the article being initially formed and then sintered in a beryllium-containing atmosphere at a temperature of about 1950-2300 ° C.