KR100328921B1 - Method for preparing silica radom by sol-gel process - Google Patents

Abstract

PURPOSE: A method for preparing a silica radome using the sol-gel method is provided, to reduce the amount of the impurities and to improve the sintering efficiency by employing the granulated fumed silica of high purity instead of the fused silica and to improve the relative density of the molded one by gelling the sol, thereby enhancing the mechanical strength of the silica radome. CONSTITUTION: The method comprises the steps of dispersing the fumed silica with a specific surface area of 20-300 m2/g into distilled water in the ratio of 1:99 to 40:60 by weight and homogenizing it to prepare a primary silica slurry; drying and calcining the primary silica slurry and pulverizing it mechanically to prepare a granular secondary silica slurry; re-dispersing the secondary silica slurry into distilled water in the ratio of 20:80 to 70:30 by weight and ball milling the solution to prepare the final slurry; adding a gelation accelerator to the final silica slurry to gel it; and drying, calcining and sintering the obtained silica molded one to prepare a silica radome. Preferably the gelation accelerator is a salt containing a fluoride ion.

Description

Method for preparing silica radom by sol-gel method

The present invention relates to a method for producing silica radome using wet hydrostatic molding, and more particularly, a sol-gel (Sol-) using a fumed silica obtained by granulating silica radome which can be used for a missile flying at high speed. Gel) method.

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 body increases, so that the diffusion path of moisture is long, and thus, the process time is long when a thick fleshed 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 the workability is excellent.However, the filling of the molten silica powder charged into the mold during the hydrostatic molding is carried out in a dry state, so that there is a possibility of uneven distribution of pores in the molded body after molding. 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 there is a problem in that many toxic substances are released during the organic matter removal.

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 product depends on the surface of the mold to be employed, the surface of the molded product has the same smoothness as the surface of the molded product, which requires almost no post-molding after molding, and also has the advantage of easily manufacturing thick materials. .

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.

However, in terms of using molten silica powder, impurities inevitably contained in the molten silica powder remain in a small amount even after the pickling and washing process, which causes crystallization during high temperature sintering.

In order to improve the above problems of raw materials, attempts have been made to obtain silica molded bodies by making silica sol using high purity fumed silica powder and then gelling them, but the fine pores present in the gel due to fine fumed silica particles Therefore, due to the increase in capillary pressure during drying, the larger the gel body, the easier it is destroyed, and succeeded in producing a dry gel body of several tens of g, but there is a problem that can not produce a large molded body more than 1kg.

In order to solve the above problems, the present invention uses granules of high-purity fumed silica instead of molten silica, which has limitations in removing impurities, and uses them as raw materials. It is an object of the present invention to provide a method for producing a silica radome having excellent mechanical strength by reducing the amount of impurities to increase the firing temperature to increase the firing temperature and gelation by adding a gelling accelerator to the silica sol.

In order to achieve the above object, the present invention disperses fumed silica having a specific surface area of 20-300 m ² / g in distilled water at a mass ratio of 1:99 to 40:60, and then homogenizes to prepare a primary silica slurry. After drying and calcining the primary silica slurry, mechanically pulverized to prepare granular secondary silica slurry, the secondary silica slurry is redispersed in distilled water at a mass ratio of 20:80 to 70:30, and then ball milling. To prepare a final slurry, followed by gelation by adding a gelling accelerator to the final slurry, and drying, calcining, and sintering the formed silica molded body to prepare a silica radome, wherein the method for producing a silica radome using the sol-gel method to be.

Hereinafter, the present invention will be described in detail.

Since molten silica used in the prior art has a lot of impurities and there is a limit to removing impurities, in the present invention, low-cost and high-purity fumed silica is used as a raw material for silica radome instead of molten silica.

Fumed silica having a specific surface area of 20-300 m 2 / g is used as a raw material for producing silica radome, and fumed silica that can be used in the present invention is made of silicon dioxide and has a specific surface area of 50 m 2 / g and an average particle size. 40-nm-size Aerosil OX-50 (Aero-sil OX-50 from Degu, Germany).

However, when the slurry is prepared by dispersing fine fumed silica directly in water, there is no problem in dispersing a small amount of silica, but when the amount of the powder is 40% or more, the viscosity increases rapidly and dispersion becomes extremely difficult. There is this.

To compensate for this disadvantage, first, the silica powder is first dispersed in a mass ratio of 1:99 to 40:60 with respect to distilled water to obtain homogeneously dispersed primary silicaazole.

Primary silicasol is heated to 110 ℃ and maintained at 110 ℃ for more than 48 hours to form a completely dried silica mass.

The dried silica mass is pulverized by dry grinding process to make granules of 0.5mm size through sieving and calcined for 1-5 hours at 400-1200 ℃ in air to make solid 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. The pH of the silica sol is about 4.5.

A salt containing fluorine (F) ion is added to the prepared sol with 0.1% of distilled water, followed by ball milling to obtain silica slurry having a low viscosity and good dispersion.

This decrease in viscosity and increase in dispersibility are due to the effect of salts containing fluorine added in addition to mechanical dispersion by ball milling. The added salt is dissolved in distilled water in the slurry to change the pH of the slurry to about 5.5, so that the pH is easily distributed to the slurry of 5.5.

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. In addition, the higher the calcination temperature, the harder the granules, so the ball milling time depends on the temperature during the calcination process.

The prepared silica sol is defoamed in a vacuum container to remove bubbles and then injected into a cone-shaped mold. After injection, the mold is sealed to prevent evaporation of moisture.

The mold uses a hard and water-prone material. Examples of molds that can be used include polypropylene molds, polystyrene molds, Teflon molds, wax molds, and the like.

The silica sol injected into the mold is gelled between 5 minutes and 24 hours. The completion time of the gelation reaction is generally within 3 hours, depending on the amount of solids and the amount of gelling accelerator. The gelation time is shortened when the temperature is increased.

After gelation is completed, the mixture is maintained for 24 hours, and then the silica gel is removed from the mold and dried at room temperature.

These wet gels are not destroyed even by drying in the air at normal temperature, which is achieved by strong calcination of the primary slurry dry matter of fine fumed silica particles at high temperature and a relatively coarse 2, which is a combination of primary particles. This is because relatively large pores exist between the indwelling particles, so that the pressure due to capillary pressure during drying is not so large.

Drying of the molded article is different depending on the thickness of the molded article, but if it is maintained 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 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.

The obtained sintered compact has a smooth surface corresponding to the processing state of the mold surface, and when the shrinkage ratio of the sintered compact is controlled in advance, a precise sintered compact which requires little post processing of the molded body or the sintered compact is obtained.

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

EXAMPLE

As fumed silica powder, commercially available low-cost aerosil OX-50 (Aero-sil OX-50 manufactured by Degus Co., Germany) is used.

The silica powder was first dispersed in distilled water at a mass ratio of 3:10 to obtain homogeneously dispersed primary silica sol. The primary silica sol was heated to 110 ° C. for 48 hours, dried to form a silica mass.

Silica mass is made into granules of about 0.5mm by dry grinding process and calcined at 900-1100 ℃ for 1-5 hours in air to make solid silica granules.

The heat-treated powder is mixed with secondary distilled water in a mass ratio of 1: 1, mixed in a high speed blender for 10 minutes, and 0.1% ammonium fluoride is added to distilled water, followed by ball milling for 8 hours using quartz jars and balls. .

After ball milling, defoaming is done in a vacuum container and injected into a cone-shaped polypropylene mold. After injection of the silica sol, the mold is sealed to prevent evaporation of moisture.

The injected silica sol is gelled for 60 minutes, held for 24 hours, and then the silica gel is removed from the mold.

The silica gel was kept at room temperature for 14 days and dried, and then again heated to a temperature increase rate of 5 ° C. per hour to 110 ° C., and then dried at 110 ° C. for 48 hours. It heats and calcines, maintaining at 900 degreeC for 20 hours.

After calcination, the product was sintered 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%.

In the present invention as described above, the high purity fumed silica is used as a raw material instead of molten silica, which is limited in removing impurities, thereby reducing the amount of impurities, thereby improving sintering properties, and adding a gelation accelerator to gel the sol to form a relative body of the molded body. The higher the density, the higher the mechanical strength of the manufactured silica radome.

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 (4)

Fumed silica having a specific surface area of 20-300 m 2 / g was dispersed in distilled water at a mass ratio of 1:99 to 40:60, and then homogenized to prepare a primary silica slurry, and then the primary silica slurry was prepared. Drying, calcining, and then mechanically pulverized to prepare granular secondary silica slurry, the secondary silica slurry was redispersed in distilled water at a mass ratio of 20:80 to 70:30, and then ball milled to prepare a final slurry. Next, after the gelling accelerator is added to the final slurry and gelled, the formed silica molded body is dried, calcined, and sintered to produce a silica radome. The method for producing silica radome using the sol-gel method according to claim 1, wherein the gelling accelerator uses a salt containing fluorine ion. The method for producing silica radome using the sol-gel method according to claim 2, wherein ammonium fluoride is used in a salt containing fluorine ions. The method for producing silica radome using the sol-gel method according to claim 1, wherein the amount of the gelling accelerator added is 0.01-10%.