KR100328945B1 - Method for preparing silica radome by direct coagulation casting process - Google Patents

Abstract

PURPOSE: A method for preparing a silica radome using the solidification is provided, to reduce the amount of the impurities and to increase the sintering temperature by employing the granulated fumed silica of high purity and improved adsorbing velocity and the super microparticle silica particle obtained by the hydrolysis of a silicon-based alkoxide as a source material and to solidify the sol by controlling the pH, thereby enhancing the sintering efficiency and the mechanical strength of the silica radome. CONSTITUTION: The method comprises the steps of mixing the fumed silica with a specific surface area of 20-300 m2/g and the hydrolyzed silicon-based alkoxide in the ratio of 99:1 to 70:30 by weight; mixing them with distilled water to make the content of the total silica be 20-80% to the distilled water and dispersing it to prepare a silica slurry; adding a strong base containing ammonium ions to the silica slurry to adjust pH to be 9.5-13.5, maintaining the pH for 24 hours at a room temperature, and defoaming it; adding a pH lowering agent to the solution to adjust pH to be 7.5-9.5 to solidify the solution; and drying, calcining and sintering the molded one to prepare a silica radome. Preferably the strong base is tetramethylammonium hydroxide or tetraethylammonium hydroxide; and the pH lowering agent is methyl formate.

Description

Method for preparing silica radome by coagulation method {Method for preparing silica radome by direct coagulation casting process}

The present invention relates to a method for producing a silica radome using a coagulation method, and more particularly, a silica radome which can be used for a high-speed missile flying as a raw material of a hydrolyzate of fumed silica and silicon-based alkoxide as a coagulation method. It relates to a method for molding.

Generally, a radome is a cover for an external radar antenna such as an aircraft, and is a structural material that protects an external radar antenna.

In order for such a radome to effectively protect the radar antenna, the interference of the antenna's electrical behavior must be very small.

Especially in the case of radome used in the super high speed missile radar having the supersonic speed of Mach 3 to Mach 5, the mechanical and electrical properties in the high temperature range of 500-800 ° C and the erosion by rain and sand, which are harsh environmental conditions Immunity should also be considered.

Radomes made of plastic operate normally in supersonic speeds below Mach 3 but not in missiles flying at higher speeds than Mach 3.

Therefore, missiles flying at Mach 3 or higher speeds should use radome made of ceramic material.

Conventionally, alumina (Al ₂ O ₃ ), slip-casted fused silica, and cordierite have been used as ceramic radome materials for use in high-speed missiles. .

Alumina has a low thermal shock resistance, but a large dielectric constant has a disadvantage of being subjected to a lot of heat shock and high dielectric loss when used at high temperatures.

In the case of cordierite, there is an intermediate characteristic between the properties of alumina and that of slip cast molten silica. Therefore, when used at a temperature of 600 ℃ or more has a disadvantage that the loss of tanδ is more than 0.01, the dielectric loss is relatively high.

Molten silica manufactured by slip casting has excellent thermal shock resistance and has a very low dielectric constant, so it is widely used as a radome for high speed missiles. However, slip cast molten silica has a disadvantage of being vulnerable to erosion by rain and sand, which are environmental conditions.

The slip cast molten silica is weak to rain and sand erosion because of the low mechanical strength of the molten silica.

The low mechanical strength of the molten silica melts because the thermal expansion coefficient does not coincide in the cooling process after firing because the molten silica is crystallized due to impurities present in the molten silica when the molten silica is molded and calcined at a high temperature of 1150-1200 ° C. Since the silica is destroyed, it is calcined at a low temperature. By this low temperature baking, the molten silica has a low density microstructure, and as a result, there is a problem of weakening rain and sand erosion.

Conventional methods for producing silica radome include melt-casting, slip-casting, iso-static pressing or injection molding.

These processes take the form of first producing a shaped body of desired shape, and then sintering at a constant temperature after the appropriate after-treatment.

In the case of the slip casting method, in the gypsum mold that is generally used, alkali-based impurities are adsorbed from the gypsum to the molded surface by direct contact with the surface of the gypsum. The molded body is destroyed by crystallization, and when the molded layer is manufactured by the gypsum mold, the thickness of the molded layer of the molded article increases, so that the diffusion path of moisture is longer, and thus, a process time is long when a thick textured body is desired.

In addition, it is difficult to precisely control the thickness of the ground layer so that the molded body itself or the sintered body after firing should be mechanically processed to have a desired dimension.The mechanical processing after firing requires a separate process and takes additional time, so it is economical. There is a problem that the fall, the molded article produced by the slip casting method is generally weak mechanical strength and the risk of breakage during processing.

In the hydrostatic molding method, compared to the slip casting method, the strength of the molded body is improved, and thus the workability is excellent. This is large and there is a problem that causes a non-uniform phenomenon of sintering when firing.

In the case of the injection molding method, due to the nature of the process, the amount of organic matter added as a binder is higher than that of other processes, and thus, it is difficult to remove these organic matters after molding, and in particular, many toxic substances are discharged when the organic matter is removed.

The sol-gel method is a method that can improve the disadvantages of the conventional molding method as described above. In the case of the sol-gel method, since the surface of the molded body depends on the surface of the mold to be employed, the surface of the molded body is also as smooth as the same, so that post-molding is hardly required after molding, and the thick molded body can be easily manufactured.

In addition, the use of wet dispersion has the advantage that there is almost no organic substance added with uniform pore distribution and does not cause contamination.

On the other hand, in order to apply the sol-gel method to the molten silica, the particle size of the powder should be small, but there is a problem in that the gelation does not occur easily because there is a limit in terms of reducing the particle size of the powder.

On the other hand, in the general sol-gel method, the gelation reaction, which is a sol-to-gel conversion reaction, is caused by a condensation polymerization reaction, which is an interaction of hydroxyl groups by hydration of an oxide surface, or a salt containing fluorine or other salts used as a gel accelerator. The gel produced by the ion exchange reaction by addition shows thixotropy behavior.

That is, even after the gelation reaction is completed and solidified, there is a characteristic of recovering fluidity again when mechanical stress is applied from the outside in the initial stage of drying without a decrease in moisture.

Due to this property, the strength of the wet gel is relatively weak, which has a problem in that it can be easily destroyed in the manufacture of a relatively large dry gel.

In order to solve the above problems, the present invention provides a mixture of ultra-fine silica particles obtained through hydrolysis of silicon-based alkoxide and high-purity fumed silica, which has been improved in the growth rate through granulation by aqueous dispersion and secondary heat treatment. The purpose of the present invention is to provide a method for producing a silicon radome having excellent sinterability and excellent mechanical properties by increasing the firing temperature by reducing the total amount of impurities and solidifying the sol by adjusting the pH.

In order to achieve the above object, the present invention provides a mass ratio of fumed silica and silicon alkoxide hydrolyzate having a specific surface area of 20-300 m ² / g in the range of 99: 1 to 70:30 of fumed silica: silicone alkoxide hydrolyzate. Silica slurry was prepared by mixing and dispersing distilled water so that the total silica content was 20-80% with respect to distilled water, and then adding a strong base containing ammonium ions to the silica slurry to have a pH of 9.5 or more and less than 13.5. After adjusting to and maintaining at room temperature for 24 hours, and then defoaming, and after the pH lowering agent was added to coagulate by adjusting the pH to more than 7.5 to less than 9.5 to dry, calcined and sintered to form a silica radome Silica radome production method using a coagulation method characterized in that.

Hereinafter, the present invention will be described in detail.

In the present invention, direct coagulation casting is used as a method for improving the disadvantages of the sol-gel method, and the coagulation method is dispersed by mutual repulsion using surface charge of solid particles in an oxide sol, and then acid or base is added again. When the pH of the sol is removed to the iso-electric point region of the oxide to remove the mutual repulsion by the electrostatic force, the particles are strongly adhered to each other by the van der Waals forces between the particles.

In the present invention, a hydrolyzate of silicon-based alkoxide and fumed silica are used as a raw material for producing silica radome instead of molten silica having a large amount of impurities.

Silicon alkoxides usable in the present invention include TEOS [tetraethylor thosilicate, Si (OC ₂ H ₅ ) ₄ ], TMOS [tetramethylorthosilicate, Si (OCH ₃ ) ₄ )], and the like.

The hydrolyzate of the silicon-based alkoxide is prepared by mixing the silicon-based alkoxide with distilled water to undergo hydrolysis while vigorously stirring at room temperature, and then gelling it by maintaining the temperature at 80 ° C. for at least 4 hours when the hydrolysis is completed. At this time, the vessel in which the reaction is proceeding is sealed so that there is no loss of water, and the hydrolysis is completed by stirring for about 24 hours.

The silica gel obtained in the above process is a structure in which several to several tens of silica particles are connected.

As a next step, fumed silica, which is a fine commercial silica powder, is mixed with distilled water.

Fumed silica that can be used in the present invention is silicon dioxide, a specific surface area of 50 m ² / g and an average particle size of 40 nm Aerosil OX-50 (Germany Old products, hereinafter referred to as "OX-50", and the like.

When the fumed silica soluble slurry and the hydrolyzate of silicon alkoxide are mixed in a blender, the mixture has some fluidity. Ball milling to increase the mixing and dispersing of the mixture results in a high mixing and dispersing degree, resulting in a very low viscosity and well dispersed silica sol.

At this time, the fumed silica: silicone alkoxide hydrolyzate is mixed in a mass ratio of 99: 1 to 70:30.

The mixed silica powder sol obtained above is heated to 110 ° C. and maintained for at least 48 hours to form a completely dried silica mass.

The dried silica mass is made into granules of about 0.5mm through dry grinding process, and then calcined for 1-5 hours at 400-1200 ℃ in air to form hard silica granules.

At this time, the specific surface area of the powder is hardly reduced compared to the specific surface area of primary silicasol.

The heat treated silica granules are mixed with secondary distilled water and then mixed in a blender to make the silica granules have some fluidity, and ball milling to obtain a slurry having a very low viscosity.

The content of silica in the silica sol is preferably 20-80% of the distilled water.

The optimum dispersion by ball milling is determined by measuring the average particle size change and particle size distribution of the mill according to the ball milling time. In general, the average particle size should be less than 1㎛ and more preferably less than 0.5㎛. The particle size distribution of the pulverized product is preferably radian type.

Finally, a strong base containing ammonium (NH ₃ ) ions is added to the prepared silica sol to adjust the pH of the silica sol to more than 9.5 to less than 13.5. Examples of strong bases containing ammonium (NH ₃ ) ions that can adjust the pH from 9.5 to less than 13.5 include tetramethylammonium hydroxide (hereinafter referred to as "TMAH") and tetraethylammonium hydroxide (Tetra). ethylammoniumhydroxide (hereinafter referred to as " TEAH "), and the addition amount of the strong base is preferably 2.0% of the silica content.

The pH of the silica sol is lowered to more than 7.5 to less than 9.5 by adding a pH lowering agent, a coagulant, to the silica sol from which bubbles are removed. Examples of the pH lowering agent include methyl formate and the like. The pH lowering agent is added while slowly stirring 1.2-1.4 equivalents in the equivalence ratio to the added base, and the sufficiently stirred silica sol is poured into the cone mold. After injection into the mold, the mold is sealed to prevent evaporation of moisture.

The mold uses a hard and water-prone mold, such as polypropylene mold, polystyrene mold, Teflon mold, wax mold, and the like.

As time passes, the pH of the silikisol injected into the mold begins to drop sharply and rapidly solidifies when the pH reaches 7.5-9.5. The time to completion of the coagulation reaction is influenced by the amount of solids and the amount of coagulation accelerator, but it is generally preferred to be within 5 to 10 minutes. Increasing the temperature shortens the solidification reaction time.

After solidification is completed for 48 hours, the silica gel is removed from the mold and dried at room temperature. Although there is a difference depending on the thickness of the molded body, if it is kept at room temperature for 14 days or more, 98% of moisture is removed. The first drying gel is dried again using a drier. The temperature is increased to 4 ° C. per hour to 110 ° C. and maintained at 110 ° C. for 48 hours to sufficiently remove the adsorbed water on the surface of the silica particles.

The dried gel is heated up to 900 ℃ 100 hours per hour and maintained at 900 ℃ for more than 20 hours. The calcination process at 900 ° C. completely removes the structural water of the silica, since the residual hydroxide (OH) group acts as a factor for crystallizing the silica during firing at a high temperature.

After sintering at 1100-1200 ° C, firing at 1100-1200 ° C within 30 minutes, and cooling to room temperature, the production of silica radome is completed.

Hereinafter, the present invention will be described in more detail with reference to the following examples.

(Example)

The hydrolyzate of the silicon-based alkoxide was mixed with TEOS, which is a silicon-based alkoxide, in a mass ratio of 1: 4, hydrolyzed at room temperature for 24 hours with vigorous stirring, and then again heated to 80 ° C and maintained at 80 ° C for 4 hours. By gelation. At this time, the container in which the reaction is progressing is sealed.

Fumed silica is a commercially available low cost aerosil OX-50 (Aero-sil OX-50 from Degusa, Germany), and aerosil OX-50 is mixed in distilled water in a mass ratio of 1: 5.

The fumed silica-soluble slurry: TEOS hydrolyzate was mixed in a mass ratio of 6: 5, then mixed in a high speed blender for 10 minutes, and ball milled for 8 hours using a quartz jar and a ball.

The prepared slurry is a silica sol having a uniformly dispersed mixed silica powder having an average particle size of 0.5 μm in which the ultrafine particles having a unit size and the particles having an average particle size of 40 nm are strongly bonded to each other obtained through hydrolysis of TEOS.

After ball milling, the mixed silica sol is heated up to 110 ° C and dried for 48 hours to make silica lumps. Makes solid silica granules.

The heat-treated powder is mixed with secondary distilled water at a mass ratio of 1: 1, mixed in a high speed blender for 10 minutes, and ball milled for 8 hours using a quartz jar and a ball to obtain a slurry having a very low viscosity.

After ball milling, TMAH was added 2.0% to silica sol in silica sol, stirred for 1 hour, maintained at room temperature for 24 hours, and defoaming. After the defoaming operation, 1.3 equivalents of methyl formate were added to the silica sol in an equivalence ratio with TMAH slowly stirring.

After being sufficiently stirred, silica sol was injected into a cone-shaped polypropylene mold, followed by coagulation for 7 minutes, and maintained for 24 hours. The silica molded body was removed from the mold and placed on a soft sponge.

The obtained molded product is maintained at room temperature for 14 days, dried, and then again heated to a temperature increase rate of 4 ° C. per hour to 110 ° C. and then dried at 110 ° C. for 48 hours.

The dried gel was calcined while maintaining the temperature at 100 ° C. to 900 ° C. for 20 hours at 900 ° C., followed by sintering at 1150 ° C. for 2 hours, and calcining at 1200 ° C. for 30 minutes. After sintering and firing, cool to room temperature by furnace cooling.

As a result of analyzing the sintered compact obtained by the above process by XRD (X-Ray Diffraction Meter), the amount of cristobalite as a crystal phase is less than 1%, and the relative density of the sintered compact is 94%.

As described above, the present invention uses the hydrolyzate of silicon-based alkoxide and fumed silica as raw materials to lower the content of impurities to improve sinterability, to delay the crystallization of the gel molded body at firing temperature by removing impurities, and then to add strong alkali. By adding a pH lowering agent as a coagulant to solidify the sol to prepare a molded body, there is an effect that can be produced a silicon radome excellent in sinterability and excellent mechanical properties.

In addition, there is an effect that can minimize the post-processing of the sintered body by using a precision mold mechanically mirror surface processing.

Therefore, the silica radome produced by the above method has a high mechanical strength, unlike the conventional silica radome has a strong characteristic against erosion by rain and sand.

Claims (7)

The mass ratio of fumed silica and silicon alkoxide hydrolyzate having a specific surface area of 20-300 m ² / g is in the range of 99: 1 to 70:30 and the content of total silica is distilled water. Silica slurry was prepared by mixing and dispersing in distilled water so as to belong to 20-80% with respect to the silica slurry, and then adding a strong base containing ammonium ion to the silica slurry to adjust the pH to 9.5 to less than 13.5 and maintained at room temperature for 24 hours. After the defoaming, and then adding a pH lowering agent to coagulation by adjusting to pH 7.5 or more and less than 9.5, the formed silica molded body is dried, calcined, and sintered to produce a silica radome using a coagulation method. Manufacturing method. The method of claim 1, wherein the strong base is one of tetramethylammonium hydroxide (TMAH) and tetraethylammonium hydroxide (TEAH). The method for producing silica radome using the coagulation method according to claim 1, wherein the addition amount of the strong base is 2.0% of the silica content. The method for preparing silica radome using the coagulation method according to claim 1, wherein methyl formate is used as the pH lowering agent. The method for preparing silica radome using the coagulation method according to claim 1, wherein the addition amount of the pH lowering agent is 1.2-1.4 in an equivalent ratio to the addition amount of the strong base. The method for producing silica radome using the coagulation method according to claim 1, wherein the coagulation time is 5-10 minutes. The hydrolyzate of the silicon-based alkoxide is mixed with distilled water adjusted to pH 1.0-2.5 by adding an acid aqueous solution, hydrolyzed with vigorous stirring at room temperature, and then again heated to 80 when hydrolysis is completed. A method for producing silica radome using a coagulation method, characterized in that the product is heated to ℃ and kept at 80 ℃ for 4 hours to gelate.