KR100328917B1 - Method for preparing silica radome by wet isostatic process - Google Patents

Abstract

PURPOSE: A method for preparing a silica radome using the wet hydrostatic pressure molding is provided, to prevent the input of the impurities at the interface with the molded one by employing a rubber mold, to enhance the relative density and strength of the molded one by filling up the pores between the fused silica particles with the fumed silica, and to improve the mechanical strength of the molded one. CONSTITUTION: The method comprises the steps of mixing the fused silica powder whose specific surface area is increased by ball milling and from which metals and alkali ions are removed by acid washing and water washing, and the fumed silica with a specific surface area of 20-300 m2/g in the ratio of 70:30 to 99:1 by weight; mixing them with distilled water to make the ratio of the total silica to distilled water be 5:95 to 70:30 by weight; adding a dispersant, a binder and a plasticizer to the solution and dispersing them to prepare a silica sol; spray drying the silica sol to granulate it; molding the granulated one at a rubber mold and isotactic pressure molding the molded one by hydrostatic pressure molding; and drying, calcining and sintering the obtained one to prepare a silica radome. Preferably the binder is a poly(vinyl alcohol) with a molecular weight of 5,000-500,000; the plasticizer is a polyalcohol; the spray drying is carried out at 100-250 deg.C; and the hydrostatic pressure is maintained to be 0.1-3 ton/cm2.

Description

Method for preparing silica radome by wet hydrostatic molding method {Method for preparing silica radome by wet isostatic process}

The present invention relates to a method for producing silica radome using wet hydrostatic molding, and more particularly, fumed silica is added to molten silica and impurities are added to a silica radome which can be used for a missile flying at high speed. After removal, the present invention relates to a wet hydrostatic molding in a rubber mold.

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.

Conventionally, a slip casting method has been used to prepare silica radome.

In the slip casting method, a slip of a liquid suspended in ceramic powder or the like is poured into a mold of a plaster material coated with a release agent to absorb the moisture of the slip into the gypsum, the mold is removed and dried, and then the product is completely dried and then sintered. How to produce.

When manufacturing the molten silica by the slip casting method as described above has the following problems.

First, the direct contact between the surface of the gypsum, which is a mold, and the molded body occurs, in which alkali-based impurities are adsorbed from the gypsum to the surface of the molded body, and unlike other ceramics, when silica is used as a raw material, the process is calcined at a high temperature of 1100 ° C. or higher. In this case, the alkali component adsorbed on the surface of the molded body causes the crystallization of the silica molded body, resulting in the destruction of the sintered body.

Therefore, the alkali component adsorbed on the surface of the molded body should be removed through mechanical processing.

Second, when manufacturing the molded body by the gypsum mold, the thickness of the layer of the molded body increases, the longer the diffusion path of water, the longer the process time if you want a thicker body.

Third, 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.Because mechanical processing after firing requires a separate process and takes additional time. Economical low.

Fourth, the molded article produced by the slip casting method is generally weak in mechanical strength, so there is a high risk of breakage during processing.

The present invention for solving the above problems is to increase the mechanical strength of the molded body by using the wet hydrostatic molding method, by using a rubber mold instead of the gypsum mold to prevent the inflow of impurities from the contact surface with the molded body, and added fumed silica powder It is an object of the present invention to provide a method for producing silica radome which improves sinterability by filling pores between molten silica particles having relatively large particles.

The present invention relates to a molten silica powder in which a specific surface area is increased by grinding by ball milling and metal and alkali ions are removed through pickling and washing, and a fumed silica having a specific surface area of 20-300 m 2 / g is melted silica: fumed silica. The silica sol was prepared by mixing in distilled water so that the mass ratio of 70:30 to 99: 1 and the mass ratio of total silica: distilled water to 5:95 to 70:30 and then dispersing by adding a dispersant, a plasticizer and a binder. Next, the silica sol is spray-dried to granulate and then molded in a rubber mold and subjected to isostatic pressure molding using hydrostatic pressure molding method, followed by drying, calcining and sintering the formed silica molded body to produce a silica radome. Silica radome is manufactured by hydrostatic pressing method.

Hereinafter, the present invention will be described in detail.

Molten silica powder, which is a material of silica radome, is mixed with distilled water in a mass ratio of 1: 100 to 200: 100, followed by ball milling to pulverize the particles so that the average particle size of the molten silica aqueous solution is 1 µm or less and more preferably 0.5 µm or less. do. The particle size distribution of the pulverized product is preferably a radian distribution.

Ball milling reduces the size of the molten silica particles and exposes a new surface, so that the specific surface area of the molten silica is relatively increased, thereby increasing the effect of removing impurities during washing. Therefore, as the molten silica is pulverized, the area in contact with water increases in the washing process, thereby increasing the effect of removing impurities.

The molten silica slurry obtained by ball milling removes water and then pickles. The molten silica slurry is dissolved in an acid solution to remove metal components, followed by filtration to remove an acid solution in which a liquid metal component is dissolved.

After filtration, a water washing process using secondary distilled water is continuously performed. The washing operation is continued until no acid is detected in the aqueous solution passing through the molten silica layer.

After the pickling and washing, the molten silica lump is completely removed using a dryer.

After the pickling and washing process, the molten silica mass decreases the amount of impurities. In particular, the amount of alkali elements such as sodium, potassium, and calcium in the first molten silica powder is reduced to less than half. This is because iron is eluted during pickling and alkali ions are removed during washing.

In particular, the ball milling increases the specific surface area of the molten silica, which increases the area of contact with distilled water, thereby increasing the effect of removing alkali ions.

Metals and alkali ions cause crystallization of silica severely in the process of calcining molten silica at high temperature, and thus removing these elements may ultimately suppress the crystallization of silica, thereby increasing the calcining temperature of molten silica.

The molten silica that has undergone the above process is difficult to drop the average particle size to 0.1㎛ or less because it is difficult to grind the melted silica to a particle size of 0.1㎛ or less by the ball mill process.

Therefore, only by raising the firing temperature by removing impurities, there is a limit to the densification of the molten silica.

Therefore, in the present invention, to increase the density of the molten silica, ultrafine silica is added to improve the sinterability of the molded body, and the ultrafine silica powder has a specific surface area of 20-300? SP>? / SP> / g. Use desilica.

The 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 of 40 nm. Conventional products, hereinafter referred to as 'Aerosil OX-50').

Silica slurry is prepared by mixing and dispersing the molten silica and fumed silica which have undergone pickling and washing with distilled water.

At this time, the mass ratio of molten silica: fumed silica is in the range of 70:30 to 99: 1, and the mass ratio of total silica: distilled water is in the range of 5:95 to 70:30.

When the amount of fumed silica is too high relative to the molten silica, the sinterability is relatively excellent, but there is a problem in that the molding density is lowered during isostatic pressing. This results in higher molding density by filling the voids between molten silica particles having a relatively larger average particle size in the structure of the molded body with fumed silica having a smaller average particle size. This is because the molding density cancels out the effect of increasing the sinterability due to the particles.

Therefore, it is preferable that the optimum ratio of molten silica: fumed silica belongs to 70:30 to 99: 1 by mass ratio.

The prepared mixture of molten silica and fumed silica is mixed with distilled water and an organic dispersant is added to enhance dispersibility. The addition amount of the organic substance dispersant is preferably 0.3%.

In addition, after adding the binder and the plasticizer, the dispersion is finally completed using a homogenizer.

As a binder, when the molecular weight is 5,000-500,000, polyvinyl alcohol (hereinafter referred to as "PVA") and the like are used, and PVA is preferably added in an amount of 0.05-2%. As a plasticizer, polyalcohol etc. are used, and when polyalcohol is used, the addition amount is preferably 0.1-0.5%.

When the mixture is mixed in a blender, a slurry having considerable fluidity is obtained, and the obtained silica slurry has a very low viscosity and a homogeneous distribution of molten silica powder and fumed silica.

The prepared slurry is granulated by spray drying at 100-250 ° C. using a spray dryer to form granules suitable for wet isostatic molding, and a certain amount of the granulated composite silica powder is filled in a rubber mold. At this time, it is more effective to be evenly filled in the mold using a vibrator.

After the powder is filled in the rubber mold, a vacuum pump is used to sufficiently remove the air in the mold and seal the mold so that water does not enter the mold.

The mold filled with silica powder isostatically formed by applying a pressure of 0.1-3 ton / cm2 using a hydrostatic pressure molding apparatus, and the silica-form after the molding process is removed from the mold and heated up by 100 ° C per hour to 20 ° C at 900 ° C. Maintain over time and calcinate.

The calcination process at 900 ° C. completely removes the structure water of the silica, since the residual hydroxide (OH) group acts as a crystallization factor of the silica when calcined at a high temperature.

The relative density of the molded product after the calcination process varies depending on the mixing ratio, but a relative density of up to 75% is obtained, which is 5% higher than the average of the slip casted product.

The high density of such shaped bodies facilitates relatively mechanical workability and there is no possibility of crystallization of the surface of the shaped bodies upon firing since there is no process of introducing impurities into the surface of the silica shaped bodies.

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

Distilled water is weighed 1: 1 by mass ratio to molten silica powder with a purity of 99.5% or more, and is ball milled using a jar made of SUS and a ball.

The slurry obtained by ball milling was sufficiently removed with water, washed with 10% by volume aqueous hydrochloric acid solution, washed with secondary distilled water, and washed with silica. The molten silica mass was dried at 110 ° C. for more than 24 hours.

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

Aerosil OX-50 and distilled water are mixed at a mass ratio of 1:10 and dispersed using a blender to prepare a silica solution, and the previously prepared molten silica is added to the silica solution at a mass ratio of 11: 9.

In order to improve the dispersibility of the silica slurry in which molten silica and aerosil OX-50 are mixed in distilled water, 0.3% of organic matter dispersant is added, 1% of PVA is added as a binder, and 0.3% of glycerin is added as a plasticizer.

The mixture is first put into a high speed blender and mixed for 10 minutes, and then dispersed in a homogeneous mixer to make a very low and homogeneously dispersed silica slurry.

The prepared silica slurry is granulated by spray drying at 150 ° C. using a spray dryer, and then the granulated silica is filled into a rubber mold, and the air in the mold is sufficiently removed using a vacuum pump to seal the mold.

The sealed mold is subjected to isostatic molding by applying a pressure of 1 ton / cm 2 using a hydrostatic pressure molding apparatus, and then the silica molded body is removed from the mold, heated to 100 ° C per hour to 900 ° C, and calcined while maintaining at 900 ° C 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 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 93%.

The density of the sintered compact increased by more than 5% compared with that of the sintered compact produced by various firing conditions using only molten silica powder of 88%.

As described above, the present invention prevents the introduction of impurities on the surface of the molded body by using a rubber mold instead of the gypsum mold, thereby preventing the crystallization of the surface of the molded body during firing, and by manufacturing the molded body by a wet hydrostatic molding method, the mechanical strength of the molded body is increased, thereby processing the molded body. This facilitates the fumed silica filling the intergranular pores of the molten silica, thereby improving the relative density and strength of the molded body and promoting sintering.

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 (8)

Molten silica powder with a specific surface area increased by grinding by ball milling and metal and alkali ions removed through pickling and washing with fumed silica having a specific surface area of 20-300 m 2 / g Silica sol was prepared by mixing in distilled water so that the mass ratio of the silica was in the range of 70:30 to 99: 1 and the mass ratio of the total silica: distilled water was in the range of 5:95 to 70:30, and then added by dispersing agent, plasticizer and binder. Next, the silica sol is spray-dried to granulate, molded in a rubber mold and subjected to isostatic molding using hydrostatic pressure molding, and then drying, calcining, and sintering the formed silica molded body to produce a silica radome. Method for preparing silica radome using wet hydrostatic molding method. The method of claim 1, wherein the binder is a polyvinyl alcohol having a molecular weight of 5,000 to 500,000. 3. The method of claim 2, wherein the polyvinyl alcohol is added by 0.05-2%. The method of claim 1, wherein the plasticizer uses polyalcohol. 5. The method of claim 4, wherein the polyalcohol is added 0.1-0.5%. [5] The method of claim 4, wherein the polyalcohol uses glycerin. The method for producing silica radome using the wet hydrostatic molding method according to claim 1, wherein the spray drying temperature is 100-250 ° C when granulating by spray drying. The method for producing silica radome using the wet hydrostatic pressing method according to claim 1, wherein the hydrostatic pressure is 0.1-3 ton / cm2.