KR100328943B1 - Method for preparing silica radome by slip casting process - Google Patents

Abstract

PURPOSE: A method for preparing a silica radome using the slip casting is provided, to reduce the amount of the impurities contained in silica and to inhibit the crystallization of silica due to the impurities by employing the fumed silica of high purity and the hydrolyzed silicon-based alkoxide, thereby increasing the sintering temperature and enhancing 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 with the hydrolyzed silicon-based alkoxide in the ratio of 90:1 to 70:30 by weight; mixing them with distilled water to make the ratio of the total silica to distilled water be 30:70 to 70:30 by weight and dispersing them to prepare a silica slurry; casting the silica slurry to a gypsum mold to form the adsorbed layer on the surface of the gypsum mold; and drying, calcining and sintering the molded one to prepare a silica radome.

Description

Method for preparing silica radome using slip casting method {Method for preparing silica radome by slip casting process}

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

The slip casting method is a non-pressing powder molding method used for molding ceramics and the like, and is used to produce a small amount of a product having a complicated shape.

In the slip casting method, a slip of a liquid suspended in water with ceramic powder is poured into a mold of a plaster material coated with a release agent to absorb the moisture of the slip into the gypsum. After absorbing the moisture in the gypsum, the mold is removed and dried to obtain a molded product in which the powder is solidified. When the product is completely dried and sintered, the product is completed.

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 for ultra high speed missile radars with supersonic speeds of Mach 3 to Mach 5, mechanical and electrical properties in the high temperature region of 500-800 ° C and resistance to rain and sand erosion, which are harsh environmental conditions Consideration should also be given.

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.

In order to exclude the influence of impurities in the molten silica, a method of dispersing fine amorphous fumed silica powder containing little impurities in water and using it as a slurry for slip casting has been proposed.

However, in the above method, when the fumed silica powder is too fine to start to form a ground layer on the surface of the slip casting mold during slip casting, extremely fine pores are formed in the ground layer, and the diffusion of moisture into the mold is rapidly reduced. There is a problem that occurs slowing down too much.

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. It is an object of the present invention to provide a method for producing silica radome which can suppress the crystallization of silica due to impurities by reducing the total amount of impurities to improve the firing temperature and increase the mechanical strength of the silica radome.

According to the present invention for achieving the above object, 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. Silica slurry was prepared by mixing and dispersing the total silica and distilled water in distilled water so as to be in the range of 30:70 to 70:30, and then casting the slurry on a gypsum mold to form a ground layer, and then drying the formed silica molded body. Silica radome production method using the slip casting method, characterized in that for producing, calcination, sintering silica radome.

Hereinafter, the present invention will be described in detail.

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.

Examples of the silicon alkoxide that can be used in the present invention include TEOS [tetraethylorthosilicate, 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, hydrolyzing with vigorous 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 based on silicon dioxide (silica dioxide), the specific surface area of 50㎡ / g, the average particle size of 40nm Aerosil OX-50 (Aero-sil OX-50: Germany de Conventional products, hereinafter referred to as 'Aerosil OX-50').

When the fumed silica-soluble slurry and the hydrolyzate of the silicon-based alkoxide are mixed in a blender, the mixture has some fluidity. Ball milling is used to increase the mixing and dispersing properties of the mixture, 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.

If the amount of silicon-based alkoxide hydrolyzate to fumed silica is too high, the sintering property is relatively better, but the size of pores is reduced by increasing the amount of fine powder, so that the removal rate of water by the gypsum mold is slow, so slip casting There is a drawback that the slowing down time is too slow.

Therefore, the mass ratio of fumed silica powder: silicone alkoxide hydrolyzate is preferably in the range of 99: 1 to 70:30.

In the present invention, the optimum mass ratio of silicon alkoxide hydrolyzate to fumed silica is 80:20 to 95: 5 in terms of solid phase ratio.

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 by 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 as compared with 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.

In order to enhance the dispersibility of the slurry, an organic dispersant is added and mixed in the blender. The addition amount of the organic dispersant is preferably 0.3%, and the mixture to which the organic dispersant is added has considerable fluidity.

At this time, when the amount of the mixed silica with respect to the distilled water is too small, the amount of moisture is large, the slip casting efficiency is lowered.

Therefore, the mass ratio of the total silica and distilled water is preferably 30:70 to 70:30, the optimum solid content in the sol is 35-55%.

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.

When the prepared slurry is poured into the gypsum mold, the silica contact layer is gradually formed on the gypsum wall. When the silica contact layer of the desired thickness is obtained, the remaining silica slurry is poured out and the gypsum mold is maintained at room temperature for 24 hours. Remove from

The obtained molded product is primarily dried, but there is a difference depending on the thickness of the molded product, but 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, and the temperature is raised to 110 ° C. at a rate of 5 ° C. per hour 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 ℃ per hour and maintained at 900 ℃ 20 or more. 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.

As fumed silica, a commercially available low-cost aerosil OX-50 (Aero-sil OX-50, manufactured by Degussa, Germany) is used, and the aerosil OX-50 and distilled water are mixed in a mass ratio of 1: 5.

The hydrolyzate of TEOS is added to the fumed silica-soluble slurry 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. After ball milling, dispersant is added to increase the dispersity.

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.

The silica slurry prepared by the above process was poured into a corn-shaped gypsum mold to obtain a silica contact layer. Then, the remaining silica slurry was poured out, the gypsum mold was kept at room temperature for 24 hours, and then the silica molded body was removed from the mold. Position it above.

The obtained molded product is kept at room temperature for 14 days and dried, and then heated again at a temperature increase rate of 5 ° C. per hour to 110 ° C. and then dried at 110 ° C. for 48 hours.

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

As a result of analyzing the sintered body 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 body is 94%.

The density of the sintered compact increased by more than 5% compared to 89% of the maximum density of the sintered compact manufactured by various sintering conditions using only molten silica powder.

As described above, the present invention uses ultra-fine silica particles, which are hydrolyzates of silicon-based alkoxides, and high-purity fumed silica, which improves the growth rate through granulation by aqueous dispersion and secondary heat treatment, to reduce the amount of impurities. By suppressing the crystallization caused by impurities, the firing temperature is improved and the mechanical strength of the silica radome is increased.

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 (2)

The mass ratio of fumed silica and silicon alkoxide hydrolyzate having a specific surface area of 20-300 m2 / g is between 99: 1 and 70:30, and the mass ratio of total silica and distilled water Silica slurry was prepared by mixing and dispersing in distilled water so as to fall in the range of 30:70 to 70:30, and then, casting the slurry on a gypsum mold to form a ground layer, and then drying, calcining, and sintering the formed silica molded silica. Silica radome manufacturing method using a slip casting method, characterized in that for producing a radome. The hydrolyzate of the silicon-based alkoxide is prepared by mixing the silicon alkoxide with distilled water, hydrolyzing with vigorous stirring at room temperature, and gelling it by maintaining the temperature at 80 ° C. for 4 hours or more when the hydrolysis is completed. Silica radome manufacturing method using a slip casting method characterized in that.