DE2112396C2 - Process for the production of products from silicon nitride with the aid of a dispersed organic binder - Google Patents

Description

The invention relates to a process for the production of products made of silicon nitride according to FIG Art.

One method of this type is described in Trans.-Brlgt. Ceram. Soc. 60 (1961), page 607, paragraph 2, became known. A wax emulsion is then added to the clay powder as a pearled organic binder in order to ensure good flowability during pressing. Before the Nitridleren the emulsion is burnt by combustion in air, which at first ignition requires ^M.

The addition of wax during the molding and sintering of ceramic powders as a pressing aid and to improve the green strength is also according to the publication of the US Atomic Energy Comlsslon Oakrldge, Tenn. UNITED STATES. I-3O-52-885-W 14 808, July 1945: "The Preparation of Crucibles from Nitrides", pp. 11 to 13, by LS. Foster known. Thereby wax is introduced into a hot benzene solution, which was added to the excretion of the wax, after first the Solution was mixed with the nitrides. After the wax has separated out, the supernatant liquid is decanted and the solid substance is dried as well pressed through a sieve. The articles of nitride are enveloped by the wax excreted by the ether. A relatively dense body is thus obtained, which is suitable for subsequent nitriding not suitable. Before the product is fired, it must first be heated to allow binding agent and Lubricants are burned out. Cracked products can also form if the products are under high pressure, which prevents them from escaping in the vaporous state. Thus, the The above-mentioned products are the prerequisite for a first later onset of nitriding, in which the Metal must remain free of oxygen and impurities, does not meet.

To reduce the risk of breakage in products made of silicon nitride, it was according to DE-OS 19 42 460 known to apply layers of fibrous material alternately with powdered silicon on a substrate and then to carry out nitriding in a vacuum-tight oven at high temperature. Included thin, compact layers are created in which it is not important to create porosity.

According to GB-PS 9 27 921 and Ceramic Age, Feb. 1960, page 28, dissolved polymethacrylates were as easily baked-out binders known in ceramic masses. Here as well as in the case of solutions other than Binders, such as when using Sugar in solution according to GB-PS 9 51 681, but when the solvent evaporates, a film layer forms on the surfaces of the particles of the pressed length, air can also be trapped under this film. Already when the solvent is removed, but at the latest when the binder is baked out, it can occur in the In the area of trapped air, there will be a swelling, which will damage the compact. Furthermore the silicon particles cannot be kept free of oxygen to the required extent, to later experience a thorough nitriding.

Corresponding phenomena occur in a procedure according to the DE-PS named as the earlier right 1 94 95 087. An organic binder is used here in the form of a solution with the proviso that the solvent is evaporated after the molding has been produced.

Proceeding from the prior art described in the introduction, the invention is based on the object the products made of silicon nitride, the To improve dimensional accuracy and mechanical property values. An oxidation of the silicium like her Is unavoidable due to the binding agent burning out, should be avoided. The generated »green« After the binding agent has baked out, the compact should have sufficient porosity to enable the nitrogen-containing atmosphere to act evenly during the development phase.

This problem is solved according to the proposal of the identification number of claim 1, for which the dependent claims 2 to 8 provide advantageous further developments.

Thus, according to the invention, a defined dispersion is used that is guaranteed. that the individual slllclumtellchen by plastic particles from one-

others are kept at a distance. This occurs not only when the dispersant is removed before the compact is manufactured, but also when the dispersant is still present in the compact after the compact has been manufactured. A coherent pore volume is retained, so that any trapped air can escape without difficulty during the final removal of the binding agent, without damaging the compact by drifting. In the same way, the open pore volume enables the polymeric material to be removed during baking.

The dispersant is expediently removed only after the object has been shaped. Trapped air can be effectively reduced by the fact that after the removal of the Dispersant a further pressing process is connected, although a compaction of the object occurs, but the open pore size is retained. to A particularly economical process flow is achieved if the treatment of the objects in a furnace that contains a nitrogenous Has atmosphere, and In soften both the polymer When heated, material is removed and the silicon is converted to silicon nitride.

The maintenance of the pore volume as a prerequisite for a final, thorough cleaning can still be ensured according to a further proposal of the invention that the silicon and fibrous, heat-resistant material was added to the existing mixture of the dispersion of the polymeric material will. One does not arrive at a compact structure, as it was known, but at one The structure retained in its porosity high strength. Both silicon nitride fibers and aluminum foil fibers can be used as fibrous, heat-resistant material be used.

According to a first embodiment of the invention, 1 kg of fine-grained Slllclumpulver (Felnkörnlgkeltsgrad = 3 μm) is brought into an agitator mixer with a dispersion of acrylic latex in 150 cm 'of water. The acrylic latex consists of 249 cm 'of a product known as "Acronal 4D" (Römpp "Chemielexikon", 6th edition, Volume I, rare 48/49). To avoid foaming during mixing, 5 cm ^{J of} a suitable antifoam agent is added. The entire mixture is then mixed thoroughly in the agitator mixer for the period of 6 hours and then dried in an air drum dryer to produce a powder. The powder is pressed into flat strips under a pressure of about 70 mPa and then sintered in a reducing atmosphere of split ammonia for 2 hours at 450 ° C. and treated for a further 2 hours at 1150 ° C. in order to remove the polymeric acrylate material and to produce a Slllclumband . The clipboard is then activated. By heating it in a gas-emitting atmosphere of 10% hydrogen and 90% nitrogen for a tent by 16 hours at 135o ° C and for 7 hours at 1450 ⁰ C; the end product then consists of a sheet of silicate nitride with a modulus of rupture of 246 mPa. In the embodiment it is advantageous that the dispersant was removed from the mixture consisting of silicon and the polymeric material before the mixture was shaped into an object, so that a powder was obtained in this way which could then be pressed into the object.

In a modification of the first embodiment of the invention, 1 kg of clay powder with an average particle size of 3 μm for a period of 6 hours is ^{mixed in an agitator mixer in an aqueous dispersion of 416 cm 3} of acrylic latex. During the first 4 hours of the mixture, it is heated to the middle of the floor! to drive out and to produce a powder, which is then processed in a conventional powder mill to a flat strip material at a speed of about 3 m / min at a rolling pressure of about 14 mPa. The resulting ribbon is then sintered to refine the acrylic material and form porous silicon. A two-zone furnace with an atmosphere of split ammonia is used, the temperatures in the zones being 450 ° C and 900 ° C. In the 450 ° C zone, the tape will last for an hour? held and then held in the zone at 900 ° C for a further hour. The porous silicon is then nitrided in a separate furnace, where it is exposed to a gassing atmosphere of 90% nitrogen and 10% hydrogen for a period of 16 hours

Μ 1350 ° C and exposed for 7 hours at 1450 ° C.

The powder produced according to this modification form can advantageously be converted into corrugated tape process by treating it with appropriately shaped rollers. Selective catfish can also be used for the flat, subject the rolled strip to further shaping, for example to produce a corrugated strip before the tape is heated to remove the acrylic material.

According to a second embodiment of the invention, 1 kg of fine-grain silicon powder (degree of granularity 3 μm) is mixed with 4 ¹ J cm ^{1 of} acrylic latex in a mixer for 15 minutes until a dough is formed. The dough produced is then shaped by hand on a base made of polyethylene and passed under a doctor blade to form a flat product from the dough, which is then ^{dried in air at 80 °} C. for 3 hours in order to remove the aqueous dispersant from the mixture of the Remove silicon with polymeric acrylic. The Flachmsterlal is then

* o Baked in an oven at 450 ° C for half an hour and at 900 ° C for another half hour in an atmosphere consisting of cracked ammonia to remove the polymeric material from the flat material and to form porous silicon. The porous membrane can be treated in this state and can be cleaned by means of conventional reaction sintering processes. Example catfish can be the Nltridlerung by heating the porous Slllciums as described above for 16 hours at 135o ⁰ C and run for 7 hours at 1450 ° C, wherein the final product is in a mechanically fixed Slllclum-Nltrld sheet. In one example, the final product had a modulus of rupture of 280 + 35 mPa. In this second exemplary embodiment, it is advantageous that the mixture of silicon and the polymeric material, starting from a dough, is shaped into the desired catfish before the dispersant is removed from the mixture.

In a third embodiment according to the invention, 20 g of silicate-Nltrld fibers and 1 l of water are added to the mixture consisting of silicon and acrylic latex according to the above example; the slurry formed in this way is mixed using a rotating disc at high speed in order to obtain a proper distribution of the fibers in the mixture without damaging it. The mixture is then filtered and used to form a dough which is of the consistency required to be shaped; he

is then used to remove the dispersant and of the polymeric material and nitrided in a similar manner as described above.

In a modification of the third embodiment The silicon nitride fibers of this example are replaced by aluminum silicate fibers in the form of staples or Bulk goods replaced. The aluminum silicate fibers are made with the silicon and the acrylic latex dispersion mixed, a vibration mixer being used in the case of bulk goods. The mix will then processed into a dough which, after molding, is baked into the desired objects, to remove the dispersant and polymeric material; eventually the silicon becomes in some suitable catfish nitrided.

According to a fourth embodiment, 1 kg of silicon powder is mixed with 3 l of a two percent mixture of aluminum-silicate fiber in water using a disk rotating at high speed. The aluminum silicate fibers are those known as Fiberfrax (Römpp Chemielexikon, 6th edition, Volume II, page 2071). The mixture is then filtered and placed in a tubular mixer along with an aqueous dispersion of 416 cm ^{3 of} the acrylic latex, whereupon the material is heated during the mixing process to remove the aqueous dispersant and produce a powder which is then rolled in one Two-zone furnace is sintered to remove the polymeric material and finally nitrided in a separate furnace.

According to a fifth embodiment, the dough-like mixture is made of silicon and an acrylic latex dispersion prepared together with Siliclum-Nitrld-Fibers or Aluminium-Stlikat-Fibers and after shaping of the object dried to remove the dispersant. Before baking for removal of the polymeric material, the article is then placed in a mold under a pressure of up to about 79 mPa pressed. After the polymeric material has been removed, the article is hydrated in the manner described.

The tolerances of the finished silicon nitride product can be determined after the silicon has been formed Keep dough to the intended object shape within very narrow limits, because baking with dough Removal of the polymeric material only leads to a shrinkage of 0.1% and no further dimensional changes occur during the nitriding of silicon. You can also use the traditional mix a body has been molded, make further sculptures on the molded body after the dispersant was removed.

In the case of sintering for the removal of the acrylic material and nitriding in a nltridicsand atmosphere in the same furnace, the mixture of silicon powder and acrylic latex, which has been shaped into a form, is charged directly into the nitriding furnace and brought to a temperature in a gas-emitting atmosphere at a rate of 400 ° C / hour heated of 1250 ⁰ C, is held at which time the temperature for constant for 3 hours, remove the acrylic material. The oven temperature is then increased to 1350 ⁰ C for 16 hours and at 145 ° ⁰ C for 7 hours to complete the Nitrldierung perform.

Claims (8)

Patent claims: 1. Process for the manufacture of products from silicon nitride, silicon being in powder form with a dispersing, organic binder in a with silicon non-reactive liquid is first mixed, then the product is shaped and then the organic binder is removed by heating and the product is In a nitrogen-containing atmosphere The sphere is heated until the silicon is converted into silicon nitride, characterized in that that as an organic binder a poly-acrylic ester In In the form of an aqueous, plasticizer-free dispersion is used, from which after done by heating Removal of the dispersant, the polymeric material is removed in a reducing atmosphere. 2. The method according to claim 1, characterized in that the dispersant consists of the mixture consisting of silicon and polymeric material is removed before the article is molded. 3. The method according to claim I, characterized in that the dispersant consists of the mixture consisting of silicon and polymeric material is removed after the mixture has been molded into the article. 4. The method according to claim 3, characterized in that the object after removal of the Dispersant is pressed to further remove the trapped air. 5. The method according to claims 1 to 4, characterized in that the object is heated in a nitrogen-containing atmosphere in an oven with the proviso that the polymeric material is removed and converting the silicon to silicon nitride in the same furnace. 6. The method according to claims 1 to 3, characterized in that fibrous, heat-resistant material is added to the mixture consisting of silicon and the dispersion of the polymeric material. *O 7. The method according to claim 6, characterized in that the fibrous, heat-resistant material consists of Slllclumnltrld fibers. 8. The method according to claim 6, characterized in that the fibrous, heat-resistant material < ⁵ consists of Alumlnlumslllkat fibers.