KR100302230B1 - Method for producing silica radome using wet hydrostatic pressure molding process - Google Patents

Abstract

PURPOSE: Provided is a method for producing silica radome which can be used in high speed missile, by adding silicone based alkoxide hydrolysate to fused silica, artificially removing impurities from the mixture, and subjecting the mixture to the wet hydrostatic pressure molding process. CONSTITUTION: The method comprises the steps of (i) crushing fused silica powder and silicone based alkoxide hydrolysate by ball milling and washing the fused silica powder and silicone based alkoxide hydrolysate with acid and water to remove metal and alkali ion; (ii) mixing the fused silica powder and silicone based alkoxide hydrolysate into distilled water so that mass ratio of the fused silica to alkoxide hydrolysate is the range of 70:30 - 99:1 and content of total silica based on distilled water is 5-80%; (iii) adding dispersing agent, plasticizer and binder to the mixture, so as to form silica sol; (iv) granulating the silica sol by spray drying, shaping the silica sol in rubber mold, and subjecting the silica sol to hydrostatic pressure shaping process; and (v) drying, calcinating and sintering the formed silica product to produce silica radome.

Description

Method for preparing silica radome using wet hydrostatic molding method

The present invention relates to a method for producing a silica radome using the wet hydrostatic molding method, and more specifically, after adding a hydrolyzate of silicon-based alkoxide to the molten silica and adding impurities to the silica radome which can be used for a missile flying at high speed. It relates to a method of manufacturing by 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 water with ceramic powder is poured into a mold of a plaster material coated with a release agent, and the moisture of the slip is absorbed into the gypsum. The mold is removed and dried. 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. 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, as the thickness of the layer of the molded body increases, the diffusion path of the moisture is longer, and the process time is longer when a thicker body is desired.

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 a separate 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 to solve 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 adding a silicon-based alkoxide hydrolyzate It is an object of the present invention to provide a method for producing silica radome which improves sinterability.

In order to achieve the above object, the present invention provides a molten silica powder and a silicon alkoxide hydrolyzate in which a specific surface area is increased by grinding by ball milling and metals and alkali ions are removed through pickling and washing with a molten silica: silicone alkoxide hydrolyzate. Silica sol was prepared by mixing in distilled water so that the mass ratio was in the range of 70:30 to 99: 1 and the content of distilled water of the total silica was in the range of 5-80%, followed by dispersing by adding a dispersant, a plasticizer and a binder. The silica sol is spray-dried to granulate and then molded in a rubber mold, isostatically formed using hydrostatic pressure molding, and the wet silica is formed by drying, calcining and sintering the formed silica molded body. Silica radome production method using.

Hereinafter, the present invention will be described in detail.

Molten silica powder, which is a material of silica radome, is mixed with distilled water at a mass ratio of 1: 100 to 200: 100 and ball milled to pulverize the particles so that the average particle size of the molten silica 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 powder is added to improve the sintering property of the molded body, and as the ultrafine silica powder, a hydrolyzate of silicon alkoxide is used.

Silicon alkoxides usable 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 to be added to the molten silica powder is mixed with distilled water in which the silicon-alkoxide is added with an acidic aqueous solution to adjust the pH to 1.0-2.5, followed by vigorous stirring at room temperature, followed by hydrolysis. It is prepared by the process of gelling by maintaining the temperature at 80 ℃ over 4 hours. 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 silica hydrate particles of several to several tens of kilowatts are connected.

When the hydrolyzate of the silicon-based alkoxide and the molten silica powder are mixed and then 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 molten silica powder: silicone-based alkoxide hydrolyzate is mixed in a mass ratio of 70:30 to 99: 1, and it is advantageous to increase the density of the molded body so that the total amount of silica is within the range of 5-80% with respect to distilled water.

The mixed silica powder sol is heated to 110 ° C. and maintained for at least 48 hours to form a completely dried silica mass. The silica mass is coated with ultra-fine silica hydrate particles around the coarse molten silica powder.

These silica masses are made into granules of 0.5mm size through dry grinding process and 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 the primary dry gel.

The heat-treated powder is mixed with secondary distilled water and mixed in a high speed blender to have some fluidity, and ball milling to increase the degree of mixing 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.

In addition, after the addition of the binder and plasticizer, ball milling is performed to increase the dispersion of the mixture and to grind the coarse granule, thereby making a slurry having a very low viscosity.

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%.

The prepared slurry is a silica sol uniformly mixed with ultrafine particles of Å unit and molten silica powder having an average particle size of 0.5 μm obtained through hydrolysis of silicon-based alkoxide.

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, the air in the mold is sufficiently removed using a vacuum pump, and the mold is sealed to prevent water from entering the mold.

The mold filled with silica powder isostatically molded by applying a pressure of 0.1-3 ton / cm ² using a hydrostatic pressure molding apparatus, and the silica-form after the molding process is removed from the mold, and then heated up by 100 ° C. per hour at 900 ° C. Hold for 20 hours or more to 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 76% is obtained, which is on average 4% higher than that 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 is sufficiently removed with water, washed with 10% by volume of hydrochloric acid solution, washed with secondary distilled water, and the washed silica mass is washed with a dryer at 110 ° C for 24 hours.

The hydrolyzate of the silicon alkoxide to be added to the molten silica powder is mixed with distilled water whose pH is adjusted to 2 by adding hydrochloric acid (TEOS), which is a silicon alkoxide, in a mass ratio of 1: 4, followed by vigorous stirring at room temperature for 24 hours. After the temperature was again raised to 80 ° C. and maintained at 80 ° C. for 4 hours to prepare a gel. At this time, the container in which the reaction is progressing is sealed.

Molten silica powder and TEOS hydrolyzate are mixed in a mass ratio of 1: 5, and then mixed in distilled water in a mass ratio of 6: 5, and then mixed in a high speed blender for 10 minutes, followed by ball milling for 8 hours using quartz jars and balls.

After ball milling, the mixed silica sol is heated up to 110 ° C and dried for 48 hours to make silica lumps.The silica lumps are made into granules of about 0.5mm by dry grinding and then calcined at 600 ° C for 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 ball milled for 8 hours using a quartz jar and balls to obtain a slurry having a very low viscosity.

0.3% of dispersant and 1% of PVA are added to the prepared slurry, and 0.3% of glycerin is added. 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 180 ° 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 isothermally molded 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 up 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 94%.

The density of the sintered compact increased by more than 6% 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. It is easy and there is an effect that the sintering property of a molded object is improved by addition of the hydrolyzate of silicon type alkoxide.

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 (9)

The mass ratio of molten silica: silicone alkoxide hydrolyzate is 70:30 to 99: 1. Silica sol was prepared by mixing in distilled water so that the content of distilled water of the total silica was in the range of 5-80%, followed by dispersing by adding a dispersant, plasticizer and binder, and granulating the silica sol by spray drying. After molding in a rubber mold and subjected to isostatic pressing using a hydrostatic pressure molding method, the method for producing a silica radome using a wet hydrostatic molding method characterized in that the silica formed by drying, calcining and sintering the formed silica molded body. 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 of claim 1, wherein the hydrostatic pressure forming pressure is 0.1-3 ton / cm ² . 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 wet hydrostatic pressure molding method, wherein the temperature is increased to 0 ° C. and maintained at 80 ° C. for 4 hours to produce gelation.