KR100302231B1 - Method for producing silica radome using sol-gel 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, artificially removing impurities from the mixture, and subjecting the mixture to the sol-gel 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 acid to the mixture to adjust pH to 5-2.5, so as to form silica sol; and (iv) gelating the silica sol, and drying, calcinating and sintering the formed silica product to produce silica radome.

Description

Method for preparing silica radome using sol-gel method

The present invention relates to a method for producing a silica radome using the wet hydrostatic molding method, and more specifically, a silica radome which can be used for a high-speed missile is added to a molten silica with a silicon-based alkoxide hydrolyzate and artificially removes impurities and then sol. The present invention relates to a method of manufacturing using a gel (Sol-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 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 product depends on the surface of the mold to be employed, the smooth surface of the molded product has the same level of smoothness, so that post-molding is hardly required after molding, and the thick molded product 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 powder, 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.

The present invention for solving the above problems by adding the silicon alkoxide having its own gelling properties to the molten silica aqueous solution by filling the pores between the particles of the molten silica by the ultra-fine silica particles generated in the hydrolysis of the silicon alkoxide. The purpose of the present invention is to provide a method for producing silica radome by improving the gelation reaction by using hydrolysis and polycondensation of silicon-based alkoxide.

In order to achieve the above object, the present invention provides a molten silica powder and a silicon-based alkoxide hydrolyzate of which a specific surface area is increased by ball milling and metal and alkali ions are removed by pickling and washing with a molten silica: silicone alkoxide. The mass ratio of the hydrolyzate dried was in the range of 70:30 to 99: 1, and the content of distilled water in the total silica was in the range of 5-80%, mixed with distilled water and adjusted to pH1.5-2.5 by adding acid. The silica sol is prepared by dispersing, gelling the prepared silica sol, and then drying, calcining and sintering the formed silica molded body to produce a silica radome.

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.

The obtained molten silica powder is mixed with distilled water, stirred using a homogeneous mixer to give a first dispersion, and silicon alkoxide is added to the obtained molten silica slurry to carry out secondary stirring.

Examples of the silicon-based alkoxide that may be used in the present invention include TEOS [tetraethylorthosilicate, Si (OC ₂ H ₅ ) ₄ ], TMOS [tetramethylorthosilicate, Si (OCH ₃ ) ₄ )], and the like.

In this case, the molten silica powder: silicone alkoxide is mixed in a mass ratio of 70:30 to 99: 1, and the total silica: distilled water is in the mass ratio of 30:70 to 70:30.

If the amount of the silicon-based alkoxide is greater than the amount filling the voids between the molten silica powder, the gel may break during drying due to the nature of the silica hydrolyzate. If the amount of the silicon-alkoxide is too small, the gelation ability of the silicon-based alkoxide may be reduced, resulting in poor gelation or gelation. There is a weak strength.

Therefore, the molten silica powder: silicone alkoxide is preferably mixed in a mass ratio of 70:30 to 99: 1.

When the amount of the mixture of the molten silica and the silicon alkoxide is too small with respect to the distilled water, the strength of the gel may be too weak after the gelation reaction, and when the silicon alkoxide is added with a sudden increase in viscosity The gelation occurs so rapidly that there is no time to pour the slurry into the mold.

Therefore, it is desirable that the total silica: distilled water falls within a mass ratio of 30:70 to 70:30.

Although the silicon alkoxide to be added may induce gelation reaction, the gelation reaction is mediated through hydrolysis and polycondensation of silicon alkoxide, so that several microunits of ultra-fine silica precursors are obtained in the intermediate process. After drying and calcining, it turns into silica and improves sinterability due to the fineness of particles during sintering.

While stirring the mixture slurry mechanically, the acid solution is added to adjust the pH to 2, and after adjusting the pH, degassing is performed by a vacuum pump 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 slurry is gelled within 4 hours in the mold. At this time, increasing the temperature of the slurry increases the gelation rate, the gelation time is shortened to less than 1 hour if maintained at 80 ℃.

After the gelation is completed and maintained for 24 hours, the silica gel former is removed from the mold.

Silica gel formed product is different depending on the thickness of the gel, but dried for about two weeks in a constant temperature and humidity of 90% humidity, 30 ℃ temperature removes 98% of the moisture. The resulting dried gel was lowered to 70% of humidity, and then heated up to 110 ° C. for 24 hours and maintained at 110 ° C. for at least 24 hours to completely remove the adsorbed water.

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)

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 molten silica powder and distilled water are mixed at a mass ratio of 4: 5, and then vigorously stirred in a homogeneous mixer, followed by primary dispersion, followed by mixing with TEOS at a mass ratio of 9: 1 for secondary stirring.

While stirring the mixture slurry by mechanical method, pH is adjusted to 2.0 by adding hydrochloric acid solution, and degassing is performed in a vacuum vessel to remove bubbles, and then injected into a polypropylene mold of cone type. After injection of the silica sol, the mold is sealed to prevent evaporation of moisture.

Gelled in the mold for 3 hours and then maintained for 24 hours.

The prepared silica gel was dried for 2 weeks in a constant temperature and humidity chamber at a humidity of 90% and a temperature of 30 ° C., and the humidity was lowered to 70% and the temperature was raised to 110 ° C. for 24 hours and maintained for 24 hours.

The dried gel is calcined while maintaining the temperature at 900 ° C. for 20 hours by raising the temperature to 100 ° C. per hour up to 900 ° C.

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%.

In the present invention as described above, by reducing the amount of impurities in the silica to suppress the crystallization of the silica by the impurities, high temperature firing is possible, so that the silica has a high-density microstructure and the sinterability of the molded body by the addition of the hydrolyzate of silicon-based alkoxide. As a result, the relative density of the molded article is increased by gelling the sol, thereby increasing 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 (2)

The mass ratio of molten silica: silicone alkoxide hydrolyzate of molten silica powder and silicon-based alkoxide was increased at 70:30 by increasing the specific surface area by pulverization by ball milling and removing the metal and alkali ions through pickling and washing. Silica sol was prepared by mixing in distilled water so as to fall within the range of 99: 1 and the content of distilled water of the total silica in the range of 5-80%, adding acid and adjusting to pH 1.5-2.5 to prepare silica sol A method for producing silica radome using the sol-gel method, characterized in that after the sol is gelled, the formed silica molded product 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 acid is hydrochloric acid.