KR101311731B1 - Manufacturing method of zirconium diboride-silicon carbide composite with high thermal conductivity - Google Patents

Abstract

The present invention is a mixture of zirconium diboride, nano silicon carbide having an average particle diameter of 1/10 or less of the zirconium diboride and an average particle diameter of 10 to 200nm, a dispersant for inhibiting aggregation of the zirconium diboride and the nano silicon carbide And wet pulverizing, drying the wet pulverized product, charging the dried product to a high-temperature pressurizing furnace, heating the sintering temperature of 1800 to 2200 ° C., and sintering under pressure at the sintering temperature. It relates to a method for producing a zirconium diboride-silicon carbide composite material comprising. According to the present invention, the zirconium diboride-silicon carbide can be used as a thermal protection system, and the process is simple, which is advantageous for mass production, and the grain growth of zirconium diboride is suppressed and the contactivity (contiguity) and thermal conductivity between silicon carbides are improved. Composite materials can be prepared.

Description

Manufacturing method of zirconium diboride-silicon carbide composite material with excellent thermal conductivity {Manufacturing method of zirconium diboride-silicon carbide composite with high thermal conductivity}

The present invention relates to a method for producing a zirconium diboride-silicon carbide composite material, and more particularly, the zirconium diboride-silicon is improved in the contact (contiguity) and improved thermal conductivity of silicon carbide while suppressing the grain growth of zirconium diboride It relates to a method of manufacturing a carbide composite material.

Non-oxide ceramics, such as borides, carbides and nitrides, which have characteristics such as high oxidation resistance in extreme environments such as melting point above 3000 ° C, chemical stability, and reentry into the atmosphere, are classified as ultra-high temperature ceramics.

A prominent application of these ultra-high temperature ceramics is a thermal protection system for protecting the leading edge of aerospace aircraft such as supersonic and space shuttles.

Figures 1A and 1B are schematic diagrams showing two examples of aerospace tip.

1A and 1B, conventionally used aerospace vehicles have a blunt distal end (see FIG. 1A), which is a structure or structure that disperses heat concentration by high-speed flight to overcome material limitations. Limited to maneuver or stability. In order to overcome these shortcomings, the recently developed aerospace vehicle has a sharp tip (see FIG. 1b), and thus, the thermal protection system becomes more important as it improves the maneuvering or safety of the aircraft.

In the sharp tip, the heat generated during the high-speed flight is concentrated in a narrow area, so the material of the thermal protection system applied to the tip should have a high melting point like ultra-high temperature ceramics, and the higher the thermal conductivity, the more concentrated the heat can be dissipated. It can reduce the fracture by melting or thermal stress.

From this point of view, improvement of thermal conductivity of ultrahigh temperature ceramics is a very important factor. The thermal conductivity of zirconium diboride is about 60 W / m * K, and the thermal conductivity of silicon carbide is about 100-300 W / m * K. The thermal conductivity of composite materials is determined by percolation rather than the rule of mixture.

2 is a typical schematic diagram of a percolation phenomenon showing the conduction characteristic of a composite material.

Referring to Figure 2, percolation is a term used to describe the flow of fluid through a porous material is used to describe the conductive properties of a composite material composed of two materials having different conductive properties. A typical example of such percolation is a case in which a dispersed phase having conductive properties is dispersed on a matrix having insulating properties, thereby obtaining a composite material having conductive properties only above a threshold of the volume fraction of the dispersed phase. The volume fraction of this threshold varies with shape, size ratio, etc. between the known phase / dispersed phase.

The problem to be solved by the present invention can be used as a thermal protection system, the process is simple and advantageous to mass production, while the growth of zirconium diboride suppressed grain growth of the silicon carbide increases (contiguity) and improved thermal conductivity zirconium dibo It is to provide a method of manufacturing a ride-silicon carbide composite material.

The present invention, (a) zirconium diboride, nano silicon carbide less than or equal to 1/10 of the average particle diameter of the zirconium diboride, the average particle diameter of 10 to 200nm, suppresses the aggregation of the zirconium diboride and the nano silicon carbide Mixing and wet milling the dispersant for drying, (b) drying the wet milled product, and (c) charging the dried product to a high-temperature pressurizing furnace and raising the temperature to a sintering temperature of 1800 to 2200 ° C. And (d) provides a method for producing a zirconium diboride-silicon carbide composite material comprising the step of sintering while pressing at the sintering temperature.

In the step (a), the zirconium diboride and the nano silicon carbide is mixed so that 50 to 95% by volume of zirconium diboride and 5 to 50% by volume of the nano silicon carbide, and the dispersing agent of the zirconium diboride and nano silicon carbide It is preferable to mix 0.1-10 weight part with respect to 100 weight part of total contents.

In the method for preparing the zirconium diboride-silicon carbide composite material, the solvent in which polycarbosilane is dissolved may be further mixed in step (a), and the polycarbosilane is thermally decomposed before the temperature is raised to the sintering temperature. In order to be converted to carbide may further comprise the step of heat treatment at a temperature of 1100 ~ 1500 ℃ lower than the sintering temperature.

In the step (a), the polycarbosilane, the nano silicon carbide and the zirconium diboride are mixed so that 5 to 50% by volume of polycarbosilane and nano silicon carbide and 50 to 95% by volume of zirconium diboride are mixed. In addition, the dispersant is preferably mixed 0.1 to 10 parts by weight based on 100 parts by weight of the total content of zirconium diboride and nano silicon carbide.

The method of manufacturing the zirconium diboride-silicon carbide composite material is lower than the heat treatment temperature in order to allow the gas generated in the process of pyrolyzing the polycarbosilane to escape from the heat treatment step before the step of raising the temperature to the sintering temperature. The method may further include cooling to a temperature.

The solvent may be hexane or toluene.

The heat treatment is preferably performed in a non-oxidizing atmosphere of vacuum or argon gas.

The sintering is preferably carried out for 30 minutes to 48 hours at a temperature of 1800 ~ 2200 ℃ while pressing at a pressure of 1 ~ 60MPa.

The sintering is performed in a non-oxidizing atmosphere, and the non-oxidizing atmosphere is preferably a vacuum or argon gas atmosphere.

The drying is preferably performed for 1 to 100 hours while stirring at a rate of 5 ~ 200rpm to prevent segregation due to evaporation of the solvent.

In addition, the present invention, the zirconium is produced by the above method, the zirconium diboride has an average particle diameter of 1 ~ 3㎛, the zirconium diboride forms a matrix phase and a structure in which silicon carbide is distributed in the matrix phase It provides a diboride-silicon carbide composite.

According to the present invention, the silicon carbide is uniformly distributed in the zirconium diboride-silicon carbide composite material and the grain size of the zirconium diboride particles by using the nano silicon carbide alone or by using the silicon carbide precursor polycarbosilane together with the nano silicon carbide. The field can be suppressed to increase the contact between silicon carbide and improve the thermal conductivity, thereby increasing the application temperature of the thermal protection system.

Further, according to the present invention, the zirconium diboride-silicon carbide (ZrB ₂ -SiC) composite material produced by the present invention is advantageous for mass production because of its simple process and provides improved thermal conductivity. Therefore, It can be used as waste material.

Figures 1A and 1B are schematic diagrams showing two examples of aerospace tip.
2 is a typical schematic diagram of a percolation phenomenon showing the conduction characteristic of a composite material.
3 is a view illustrating a heat treatment process for obtaining a zirconium diboride-silicon carbide composite material according to Experimental Example 1.
4a is an X-ray diffraction (XRD) pattern of a zirconium diboride-silicon carbide composite material synthesized according to Experimental Example 2 and Comparative Experimental Example.
4B is an X-ray diffraction (XRD) pattern of a zirconium diboride-silicon carbide composite material synthesized according to Experimental Example 1. FIG.
Figure 4c is an X-ray diffraction (XRD) pattern for the starting material before the heat treatment after ball milling the starting material of zirconium boride and polycarbosilane according to Experimental Example 1.
5A and 5B are scanning electron microscope (SEM) photographs of a zirconium diboride-silicon carbide composite material synthesized by heat-treating zirconium diboride and polycarbosilane and sintering in a high-temperature pressurization sintering furnace according to Experimental Example 1 admit.
6A and 6B are transmission electron microscopes of a zirconium diboride-silicon carbide composite material synthesized by sequentially performing heat treatment and sintering of zirconium diboride and polycarbosilane in a high-temperature pressurization sintering furnace according to Experimental Example 1 ) Is a bright field image and a dark field image.
7 is a scanning electron microscope (SEM) photograph of a zirconium dichloride-silicon carbide composite material synthesized according to Experimental Example 2. FIG.
8 is a scanning electron microscope (SEM) photograph of a zirconium diboride-silicon carbide composite material synthesized according to a comparative example.
Figure 9a is a graph showing the specific heat measured for the zirconium diboride-silicon carbide composite materials synthesized according to Experimental Example 1, Experimental Example 2 and Comparative Experimental Example.
Figure 9b is a graph showing the thermal diffusivity measured for the zirconium diboride-silicon carbide composite material synthesized according to Experimental Example 1, Experimental Example 2 and Comparative Experimental Example.
Figure 9c is a graph showing the thermal conductivity measured for the zirconium diboride-silicon carbide composite material synthesized according to Experimental Example 1, Experimental Example 2 and Comparative Experimental Example.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, it should be understood that the following embodiments are provided so that those skilled in the art will be able to fully understand the present invention, and that various modifications may be made without departing from the scope of the present invention. It is not.

In the following description, nano is used to mean 1 to 1000 nm in size, and nano silicon carbide is used to mean silicon carbide in which the average particle size is nano.

Method for producing a zirconium diboride-silicon carbide composite material according to a preferred embodiment of the present invention, (a) zirconium diboride, the average diameter of the zirconium diboride 1/10 or less and the average particle diameter of 10 to 200nm Mixing and wet grinding a carbide, the zirconium diboride and a dispersant for inhibiting agglomeration of the nano silicon carbide, (b) drying the wet milled product, and (c) hot sintering the dried product. Charging to a furnace and heating to a sintering temperature of 1800-2200 ° C. and (d) sintering under pressure at the sintering temperature.

In the step (a), the zirconium diboride and the nano silicon carbide is mixed so that 50 to 95% by volume of zirconium diboride and 5 to 50% by volume of the nano silicon carbide, and the dispersing agent of the zirconium diboride and nano silicon carbide It is preferable to mix 0.1-10 weight part with respect to 100 weight part of total contents.

In the method for preparing the zirconium diboride-silicon carbide composite material, the solvent in which polycarbosilane is dissolved may be further mixed in step (a), and the polycarbosilane is thermally decomposed before the temperature is raised to the sintering temperature. In order to be converted to carbide may further comprise the step of heat treatment at a temperature of 1100 ~ 1500 ℃ lower than the sintering temperature.

In the step (a), the polycarbosilane, the nano silicon carbide and the zirconium diboride are mixed so that 5 to 50% by volume of polycarbosilane and nano silicon carbide and 50 to 95% by volume of zirconium diboride are mixed. In addition, the dispersant is preferably mixed 0.1 to 10 parts by weight based on 100 parts by weight of the total content of zirconium diboride and nano silicon carbide.

The method of manufacturing the zirconium diboride-silicon carbide composite material is lower than the heat treatment temperature in order to allow the gas generated in the process of pyrolyzing the polycarbosilane to escape from the heat treatment step before the step of raising the temperature to the sintering temperature. The method may further include cooling to a temperature.

The solvent may be hexane or toluene.

The heat treatment is preferably performed in a non-oxidizing atmosphere of vacuum or argon gas.

The sintering is preferably carried out for 30 minutes to 48 hours at a temperature of 1800 ~ 2200 ℃ while pressing at a pressure of 1 ~ 60MPa.

The sintering is performed in a non-oxidizing atmosphere, and the non-oxidizing atmosphere is preferably a vacuum or argon gas atmosphere.

The drying is preferably performed for 1 to 100 hours while stirring at a rate of 5 ~ 200rpm to prevent segregation due to evaporation of the solvent.

The zirconium diboride-silicon carbide composite material prepared by the above method has an average particle diameter of 1 to 3 µm (1.5 times or less of the average particle diameter of the zirconium diboride powder of starting material), and the zirconium diboride is known forming a matrix phase and forming a structure in which silicon carbide is distributed in the matrix phase.

Hereinafter, embodiments according to the present invention will be described in more detail, and the present invention is not limited to the following examples.

≪ Example 1 >

Zirconium diboride, nano silicon carbide having an average particle diameter of 1/10 or less of the zirconium diboride and an average particle diameter of 10 to 200 nm, a dispersant for inhibiting the aggregation of the zirconium diboride and the nano silicon carbide and wet grinding .

In this embodiment, zirconium diboride and nano silicon carbide and dispersant having an average particle diameter of 1/10 or less of the zirconium diboride and an average particle diameter of 10 to 200 nm are used as starting materials.

The zirconium diboride and the nano silicon carbide are mixed so that 50 to 95% by volume of zirconium diboride and 5 to 50% by volume of nano silicon carbide are mixed, and the dispersant is 100 parts by weight of the total content of zirconium diboride and nano silicon carbide. It is preferable to mix 0.1-10 weight part.

The dispersant serves to suppress aggregation of zirconium diboride and nano silicon carbide and to improve dispersibility. As the dispersant, a commercially available brand name RE610 may be used.

The wet milling may be performed by a wet ball milling process or the like. The wet ball milling process will be described below. The starting material is charged into a ball mill, mixed by milling by ball milling, and the ball used for ball milling is made of ceramic materials such as WC-Co, alumina (Al ₂ O ₃ ), and zirconia (ZrO ₂ ). Balls may be used, and the balls may be all the same size, or two or more balls may be used together. Adjust the size of the ball, milling time, rotation speed per minute of the ball milling machine, for example, the size of the ball is set in the range of about 1 ~ 30㎜, the rotation speed of the ball milling machine in the range of 50 ~ 500rpm The ball milling can be performed for 1 to 50 hours. By the ball milling, the starting materials are homogeneously mixed and pulverized.

The wet milled starting material is dried at a temperature of 60-200 ° C. The drying is preferably carried out while stirring for 1 to 100 hours to prevent segregation of starting materials due to evaporation of the solvent. The stirring is preferably made at a speed of about 5 ~ 200rpm.

The dried starting material is charged into a mold of a high-temperature sintering furnace. The sintering is performed by hot pressing sintering, and there is an advantage that a zirconium diboride-silicon carbide composite material can be produced without adding a sintering aid. Hot pressing sintering is a sintering method with high pressure applied. It can be sintered at a lower temperature than pressureless sintering, and sintering time is also shortened. The zirconium diboride-silicon carbide composite material sintered by atmospheric pressure sintering has a low relative density and has a problem in that its mechanical properties are lower than those obtained by high temperature sintering. In addition, when using atmospheric pressure sintering, since the temperature must be raised to near the melting point of the starting material, sintering must be performed, resulting in high energy consumption and excessive thermal stress. Therefore, in the present invention, by using the high-temperature sintering method, a high density zirconium dichloride-silicon carbide composite material in which the intervals between the particles become very dense and pores are hardly formed can be produced.

The mold may be provided in the form of a cylinder or a prism. After the starting material is charged into the mold, the mold may be vertically bi-directionally compressed or unidirectionally compressed in the upper and lower portions of the mold to form a desired shape.

The sintering is preferably performed in a non-oxidizing atmosphere, and the non-oxidizing atmosphere is preferably a vacuum or argon (Ar) gas atmosphere. In the case of performing the sintering in a vacuum atmosphere, to remove the impurities present in the gas to the hot-pressing, and by operating a rotary pump (not shown) to create a vacuum, a vacuum state (for example, 1 × 10 ^-1 ~1 × 10 ^-3 Torr). At this time, when power is supplied to the heating means (not shown) surrounding the periphery of the high-temperature sintering furnace and the high-temperature sintering furnace is heated, the impurity gas remaining in the high-temperature sintering furnace can be efficiently exhausted. In the case of supplying argon (Ar) gas, the gas atmosphere of the high-temperature sintering furnace is sufficiently supplied in an argon (Ar) gas atmosphere. For example, it is preferable to supply the gas atmosphere at a flow rate of about 200 to 2000 sccm.

The temperature of the high-temperature pressurizing furnace is raised to a sintering temperature of 1800 ~ 2200 ℃. The sintering temperature is preferably about 1800 ~ 2100 ℃ in consideration of the diffusion of particles, the necking (necking) between the particles, etc. If the sintering temperature is too high, mechanical properties may decrease due to excessive growth of particles, If the sintering temperature is too low, it is preferable to sinter at the sintering temperature in the above range because the characteristics of the sintered body may be poor due to incomplete sintering. At this time, it is preferable that the temperature rising temperature of the high temperature sintering furnace is about 1 to 50 ° C./min. If the temperature rising rate of the high temperature sintering furnace is too slow, it takes a long time and the productivity decreases. In a rapid case, it is preferable to raise the temperature of the high-temperature pressurization furnace at a temperature rising rate in the above range because it may adversely affect the properties of the zirconium diboride-silicon carbide composite material by the rapid temperature rise.

When the temperature of the high-temperature pressurizing furnace rises to the target sintering temperature, it is sintered while maintaining a predetermined time (for example, 30 minutes to 48 hours) and pressurizing. Depending on the sintering temperature, the zirconium diboride-silicon carbide composite material has a fine structure and a particle size. The surface diffusion is dominant at the low sintering temperature, but the lattice diffusion and grain boundary diffusion are proceeded at the high sintering temperature. If the sintering temperature is less than 1800 ℃ may not be densified due to poor sintering, if the temperature exceeds 2200 ℃ the size of the particles may be large due to the characteristics of the zirconium diboride-silicon carbide composite material and energy High consumption, not economical. It is preferable that the sintering time is about 30 minutes to 48 hours. If the sintering time is too long, energy consumption is high, so it is not economical, and it is difficult to expect any further sintering effect, and the size of the particles is increased. In this small case, the characteristics of the sintered body may be poor due to incomplete sintering.

The pressure applied to the starting material during the sintering (the pressure to be compressed by the mold) is preferably about 1 to 60 MPa. If the pressure is too low, a large amount of voids are formed between the starting materials. Therefore, the desired high density zirconium diboride- If the silicon carbide composite material is difficult to obtain and the pressing pressure is too high, the stress due to the stress may appear on the microstructure, so that the characteristics of the sintered body may be rather poor.

Zirconium diboride is surrounded by nano silicon carbide so that grain growth is suppressed during the sintering so that coarsening does not occur. As the grain boundary of zirconium diboride is suppressed, the contact between silicon carbide increases and the thermal conductivity can be improved. Nano silicon carbide is 1/10 or less of the average particle diameter of zirconium diboride, has a particle size of about 10 to 200 nm, and nano silicon carbide is used when using the same volume fraction as that of silicon carbide having a large particle size. In this case, the contact between silicon carbide increases, thereby improving thermal conductivity of the zirconium diboride-silicon carbide composite material.

After performing the sintering process, the temperature of the high-temperature sintering furnace is lowered to unload the zirconium diboride-silicon carbide composite material. The high-temperature sintering furnace may be cooled by shutting off the power supply to the high-temperature sintering furnace to cool the sintering furnace in a natural state or by setting a temperature lowering rate (for example, 10 ° C / min). It is preferable to keep the pressure inside the high-temperature sintering furnace constant even while the temperature of the high-temperature sintering furnace is lowered.

After the sintering process, a zirconium diboride-silicon carbide composite material is finally obtained. The zirconium diboride-silicon carbide composite material thus obtained has a uniform distribution of silicon carbide and suppresses grain growth of zirconium diboride particles, thereby increasing the contact between silicon carbide and improving the thermal conductivity, thereby increasing the application temperature of the thermal protection system. There are advantages to it.

<Example 2>

Zirconium diboride, nano silicon carbide having an average particle diameter of 1/10 or less of the zirconium diboride and an average particle diameter of 10 to 200 nm, a solvent in which polycarbosilane is dissolved, and inhibiting aggregation of the zirconium diboride and the nano silicon carbide Dispersant for mixing and wet grinding.

In this embodiment, zirconium diboride and nano silicon carbide, polycarbosilane, and dispersant having an average particle diameter of 1/10 or less of the zirconium diboride and an average particle diameter of 10 to 200 nm are used as starting materials. Polycabosilane (PCS), a silicon carbide precursor, is dissolved in a solvent and used as a starting material.

The polycarbosilane, the nano silicon carbide and the zirconium diboride are mixed to mix 5 to 50% by volume of polycarbosilane and nano silicon carbide and 50 to 95% by volume of zirconium diboride, and the dispersing agent is zirconium diboride. It is preferable to mix 0.1-10 weight part with respect to 100 weight part of total contents of a ride and a nano silicon carbide.

Polycarbonate Silane _{(SiH (CH 3) -CH 2} -) n is a precursor of silicon carbide. Polycarbosilanes are generally made of polymeric materials. The cross-linking of the hydrocarbon group (CH) between Si and Si is a general structure of polycarbosilane. These polycarbosilanes are used by dissolving them in solvents such as hexane and toluene.

The dispersant serves to suppress aggregation of zirconium diboride and nano silicon carbide and to improve dispersibility. As the dispersant, a commercially available brand name RE610 may be used.

The wet milling may be performed by a wet ball milling process or the like. The wet ball milling process will be described below. The starting material is charged into a ball mill, mixed by milling by ball milling, and the ball used for ball milling is made of ceramic materials such as WC-Co, alumina (Al ₂ O ₃ ), and zirconia (ZrO ₂ ). Balls may be used, and the balls may be all the same size, or two or more balls may be used together. Adjust the size of the ball, milling time, rotation speed per minute of the ball milling machine, for example, the size of the ball is set in the range of about 1 ~ 30㎜, the rotation speed of the ball milling machine in the range of 50 ~ 500rpm The ball milling can be performed for 1 to 50 hours. By the ball milling, the starting materials are homogeneously mixed and pulverized.

The wet milled starting material is dried at a temperature of 60-200 ° C. The drying is preferably carried out while stirring for 1 to 100 hours to prevent segregation of starting materials due to evaporation of the solvent. The stirring is preferably made at a speed of about 5 ~ 200rpm. When dried, zirconium diboride has a structure coated with polycarbosilane.

The dried starting material is charged into a mold of a high-temperature sintering furnace. The mold may be provided in the form of a cylinder or a prism. After the starting material is charged into the mold, the mold may be vertically bi-directionally compressed or unidirectionally compressed in the upper and lower portions of the mold to form a desired shape.

In order to thermally decompose the polycarbosilane into silicon carbide, heat treatment is performed at a temperature of 1100 to 1500 ° C. lower than the sintering temperature.

After the heat treatment, the polycarbosilane undergoes a thermal decomposition process whereby both O and H in the polycarbosilane disappear and are converted to silicon carbide. Volatile hydrogen (H ₂ ) and methane (CH ₄ ) are pyrolyzed and evaporated during the heat treatment process.

By the heat treatment, the polycarbosilane coating the zirconium diboride is converted to silicon carbide, and the silicon carbide converted from the polycarbosilane surrounds the particles of the zirconium diboride.

Hereinafter, the heat treatment process will be described in more detail. The temperature of the high-temperature sintering furnace is raised to a temperature at which the polycarbosilane is pyrolyzed (for example, 1100 to 1500 ° C). The heat treatment is preferably performed in a non-oxidizing atmosphere, and the non-oxidizing atmosphere is preferably a vacuum or argon (Ar) gas atmosphere. In the case of performing the heat treatment in a vacuum atmosphere, to remove the impurities present in the gas to the hot-pressing, and by operating a rotary pump (not shown) to create a vacuum, a vacuum state (for example, 1 × 10 ^-1 ~1 × 10 ^-3 Torr). At this time, when power is supplied to the heating means (not shown) surrounding the periphery of the high-temperature sintering furnace and the high-temperature sintering furnace is heated, the impurity gas remaining in the high-temperature sintering furnace can be efficiently exhausted. In the case of supplying argon (Ar) gas, the gas atmosphere of the high-temperature sintering furnace is sufficiently supplied in an argon (Ar) gas atmosphere. For example, it is preferable to supply the gas atmosphere at a flow rate of about 200 to 2000 sccm. The rising temperature of the high-temperature sintering furnace is preferably about 1 to 50 占 폚 / min. If the heating rate of the sintering furnace is too slow, the productivity tends to be long and the temperature of the sintering furnace is too high The polycarbosilane becomes unstable due to the rapid temperature rise. Therefore, it is preferable to increase the temperature of the high-temperature sintering furnace with the temperature rise in the above range. Generally, the polycarbosilane gradually starts to change to silicon carbide at a temperature of 1100 ° C or higher.

When the temperature of the high-temperature sintering furnace rises to a heat treatment temperature (for example, 1100 to 1500 占 폚), the polycarbonate is heat-treated for a predetermined time (for example, 10 minutes to 100 hours) so that the polycarbosilane can be sufficiently converted to silicon carbide. Polycarbosilane generally begins to pyrolyze at temperatures above 1100 ° C. Therefore, if the temperature of the high-temperature sintering furnace is maintained at a temperature of 1100 ° C or higher for a certain period of time, the polycarbosilane will be completely pyrolyzed. If the pyrolysis temperature is too low, a residual phase is formed, and silicon carbide can not be completely replaced. Also, if the thermal decomposition temperature is too high, oxidation behavior of zirconium diboride may be accompanied and it is preferably carried out at a temperature of 1500 ° C or lower. The polycarbosilane surrounding the zirconium diboride is converted into silicon carbide by the heat treatment.

Since the polymeric polycarbosilane is in a thermally unstable state, it is converted into silicon carbide, which is a thermally more stable structure by the heat treatment.

The heat treatment is preferably performed in a non-oxidizing atmosphere. If the heat treatment is performed in an oxidizing atmosphere, the cross-linking between Si and Si produced during pyrolysis may not be broken. Therefore, ). &Lt; / RTI &gt;

After the heat treatment is performed, before the temperature is raised to the sintering temperature, a process of cooling to a temperature lower than the heat treatment temperature may be performed so that the gas generated in the process of pyrolyzing the polycarbosilane is released.

After the heat treatment, the temperature of the high-temperature pressurizing furnace is raised to a sintering temperature of 1800-2200 ° C.

The sintering is performed by hot pressing sintering, and there is an advantage that a zirconium diboride-silicon carbide composite material can be produced without adding a sintering aid. Hot pressing sintering is a sintering method with high pressure applied. It can be sintered at a lower temperature than pressureless sintering, and sintering time is also shortened. The zirconium diboride-silicon carbide composite material sintered by atmospheric pressure sintering has a low relative density and has a problem in that its mechanical properties are lower than those obtained by high temperature sintering. In addition, when using atmospheric pressure sintering, since the temperature must be raised to near the melting point of the starting material, sintering must be performed, resulting in high energy consumption and excessive thermal stress. Therefore, in the present invention, by using the high-temperature sintering method, a high density zirconium dichloride-silicon carbide composite material in which the intervals between the particles become very dense and pores are hardly formed can be produced.

The sintering temperature is preferably about 1800 ~ 2100 ℃ in consideration of the diffusion of particles, the necking (necking) between the particles, etc. If the sintering temperature is too high, mechanical properties may decrease due to excessive growth of particles, If the sintering temperature is too low, it is preferable to sinter at the sintering temperature in the above range because the characteristics of the sintered body may be poor due to incomplete sintering. At this time, it is preferable that the temperature rising temperature of the high temperature sintering furnace is about 1 to 50 ° C./min. If the temperature rising rate of the high temperature sintering furnace is too slow, it takes a long time and the productivity decreases. In a rapid case, it is preferable to raise the temperature of the high-temperature pressurization furnace at a temperature rising rate in the above range because it may adversely affect the properties of the zirconium diboride-silicon carbide composite material by the rapid temperature rise.

When the temperature of the high-temperature pressurizing furnace rises to the target sintering temperature, it is sintered while maintaining a predetermined time (for example, 30 minutes to 48 hours) and pressurizing. Depending on the sintering temperature, the zirconium diboride-silicon carbide composite material has a fine structure and a particle size. The surface diffusion is dominant at the low sintering temperature, but the lattice diffusion and grain boundary diffusion are proceeded at the high sintering temperature. If the sintering temperature is less than 1800 ℃ may not be densified due to poor sintering, if the temperature exceeds 2200 ℃ the size of the particles may be large due to the characteristics of the zirconium diboride-silicon carbide composite material and energy High consumption, not economical. It is preferable that the sintering time is about 30 minutes to 48 hours. If the sintering time is too long, energy consumption is high, so it is not economical, and it is difficult to expect any further sintering effect, and the size of the particles is increased. In this small case, the characteristics of the sintered body may be poor due to incomplete sintering.

The pressure applied to the starting material during the sintering (the pressure to be compressed by the mold) is preferably about 1 to 60 MPa. If the pressure is too low, a large amount of voids are formed between the starting materials. Therefore, the desired high density zirconium diboride- If the silicon carbide composite material is difficult to obtain and the pressing pressure is too high, the stress due to the stress may appear on the microstructure, so that the characteristics of the sintered body may be rather poor.

The zirconium diboride is surrounded by nano silicon carbide to prevent oxidation of the zirconium diboride during the sintering and the grain growth is suppressed so that coarsening does not occur. As the grain boundary of zirconium diboride is suppressed, the contact between silicon carbide increases and the thermal conductivity can be improved. Nano silicon carbide is 1/10 or less of the average particle diameter of zirconium diboride, has a particle size of about 10 to 200 nm, and nano silicon carbide is used when using the same volume fraction as that of silicon carbide having a large particle size. In this case, the contact between silicon carbide increases, thereby improving thermal conductivity of the zirconium diboride-silicon carbide composite material.

After performing the sintering process, the temperature of the high-temperature sintering furnace is lowered to unload the zirconium diboride-silicon carbide composite material. The high-temperature sintering furnace may be cooled by shutting off the power supply to the high-temperature sintering furnace to cool the sintering furnace in a natural state or by setting a temperature lowering rate (for example, 10 ° C / min). It is preferable to keep the pressure inside the high-temperature sintering furnace constant even while the temperature of the high-temperature sintering furnace is lowered.

After the sintering process, a zirconium diboride-silicon carbide composite material is finally obtained. The zirconium diboride-silicon carbide composite material thus obtained has a uniform distribution of silicon carbide and suppresses grain growth of zirconium diboride particles, thereby increasing the contact between silicon carbide and improving the thermal conductivity, thereby increasing the application temperature of the thermal protection system. There are advantages to it.

Hereinafter, experimental examples according to the present invention will be presented, and the present invention is not limited to the following experimental examples.

<Experimental Example 1>

Zirconium diboride (average particle size 1.88 占 퐉, Japan New Metals Co. Ltd., Japan) was 80% by volume, and polycarbosilane (Type A, NIPUSI, Japan) was weighed to achieve 20% by volume. The thus weighed starting materials were charged into a ball mill and hexane capable of dissolving the polycarbosilane was added thereto to perform ball milling for 24 hours using a cemented carbide ball (WC-Co) while completely dissolving the polycarbosilane .

When ball milling, the polycarbosilane dissolved in hexane surrounds the zirconium diboride which does not dissolve in hexane. When the mixed slurry of zirconium diboride and polycarbosilane is dried, zirconium diboride powder coated with polycarbosilane is obtained. In the drying process, drying was carried out with stirring to suppress segregation.

The dried starting materials were sieved through a 120 mesh sieve and then heat treated and sintered in a hot pressing machine. 3 is a view illustrating a heat treatment process for obtaining a zirconium diboride-silicon carbide composite material according to Experimental Example 1.

Referring to FIG. 3, a zirconium diboride coated with polycarbosilane was charged in a high-temperature pressurization sintering furnace, and the temperature was increased at a rate of 10 ° C. per minute up to a temperature (T 1) of 1400 ° C. And maintained at a temperature (T1) of 1400 DEG C for 1 hour (t2) to perform heat treatment so that the polycarbosilane was thermally decomposed and converted to silicon carbide. Since the polycarbosilane is pyrolyzed in the heat treatment step, the silicon carbide is prevented from being oxidized by proceeding in an argon (Ar) gas atmosphere. Through this process, a silicon carbide layer is formed on the surface layer of the zirconium diboride to prevent the oxidation of the zirconium diboride and to prevent the growth of the zirconium diboride. Since silicon carbide is in an unstable state at a high temperature, it is preferably oxidized in an oxidizing atmosphere and becomes silicon oxide (silica), so that it is preferable to carry out heat treatment in a non-oxidizing atmosphere.

The high-temperature sintering furnace was heated at a temperature of 1900 占 폚 (T2) at a rate of 10 占 폚 / min and sintered at a pressure of 30 MPa and maintained at 1900 占 폚 for 2 hours (t4).

<Experimental Example 2>

Zirconium diboride (average particle size 1.88 mu m, Japan New Metals Co. Ltd, Japan) was weighed to make 80% by volume, and nano silicon carbide (average particle size is 100nm or less, SIGMA-ALDRICH, Germany) to 20% by volume. . In order to suppress aggregation of zirconium diboride and nano silicon carbide, a dispersant (RE610) was added to 0.5 parts by weight based on 100 parts by weight of the total content of zirconium diboride and nano silicon carbide.

Weighed starting material was charged to a ball mill, wet mixed and ground for 24 hours using carbide balls (WC-Co) and isopropyl alcohol as solvents.

The mixed and pulverized starting materials were dried with stirring to avoid sedimentation of the slurry, and the solvent was evaporated. The dried starting material was sieved through a 120 mesh sieve.

Starting materials of the zirconium diboride and silicon carbide sieved in a high-temperature pressurizing furnace were charged and heated up at a rate of 10 ° C. per minute to a temperature of 1900 ° C. Then, the sintered body was maintained at a temperature of 1900 캜 for 2 hours under a pressure of 30 MPa in an Ar gas atmosphere.

In the following, comparative experiments are presented to comparatively evaluate the characteristics of Experimental Example 1 and Experimental Example 2. However, the comparative experimental examples presented below are merely presented for comparison with the experimental examples presented above, and it is clear that they are not prior art of the present invention.

&Lt; Comparative Experimental Example &

Zirconium diboride (average particle size 1.88 mu m, Japan New Metals Co. Ltd., Japan) was 80% by volume, and silicon carbide (average particle diameter 0.5 mu m, SIKA Tech, Germany) was weighed to achieve 20 volume%. Weighed powders were charged to a ball mill, wet mixed and ground for 24 hours using carbide balls (WC-Co) and isopropyl alcohol as solvents.

The mixed and pulverized starting materials were dried to evaporate the solvent. The dried starting material was sieved through a 120 mesh sieve.

Starting materials of the zirconium diboride and silicon carbide sieved in a high-temperature pressurizing furnace were charged and heated up at a rate of 10 ° C. per minute to a temperature of 1900 ° C. Then, the sintered body was maintained at a temperature of 1900 캜 for 2 hours under a pressure of 30 MPa in an Ar gas atmosphere.

4a is an X-ray diffraction (XRD) pattern of a zirconium diboride-silicon carbide composite material synthesized according to Experimental Example 2 and Comparative Experimental Example.

Referring to FIG. 4A, peaks corresponding to zirconium diboride and silicon carbide included in starting materials were observed.

4B is an X-ray diffraction (XRD) pattern of a zirconium diboride-silicon carbide composite material synthesized according to Experimental Example 1. FIG.

Referring to FIG. 4B, it is determined that the polycarbosilane is converted to silicon carbide to form a silicon carbide phase.

Figure 4c is an X-ray diffraction (XRD) pattern for the starting material before the heat treatment after ball milling the starting material of zirconium boride and polycarbosilane according to Experimental Example 1.

Referring to FIG. 4C, polycarbosilane was not phase-transformed with silicon carbide, and no X-ray diffraction peak corresponding to silicon carbide was observed. From this, it can be seen that the transition of polycarbosilane to silicon carbide occurs during the heat treatment process.

By heat treatment, polycarbosilane will be converted to silicon carbide through pyrolysis process, and since polycarbosilane is completely converted into silicon carbide by heat treatment and encapsulates zirconium diboride, the sintering process is performed at a temperature of 1800 ~ 2200 ℃ Does not thermally oxidize zirconium diboride.

5A and 5B are scanning electron microscope (SEM) photographs of a zirconium diboride-silicon carbide composite material synthesized by heat-treating zirconium diboride and polycarbosilane and sintering in a high-temperature pressurization sintering furnace according to Experimental Example 1 admit. 5A is a photograph taken at 3000 times magnification, and FIG. 5B is a photograph taken at 5000 times magnification.

5A and 5B, the particle size of the zirconium diboride in the zirconium diboride-silicon carbide composite was about 2 μm. Although zirconium diboride with an average particle size of 1.88 ㎛ was used as a starting material, the particle size of zirconium diboride in the zirconium diboride-silicon carbide composite material made up to sintering indicates that the particle size of zirconium diboride was increased during heat treatment and sintering. It means that it is rarely done.

6A and 6B are transmission electron microscopes of a zirconium diboride-silicon carbide composite material synthesized by sequentially performing heat treatment and sintering of zirconium diboride and polycarbosilane in a high-temperature pressurization sintering furnace according to Experimental Example 1 ) Is a bright field image and a dark field image.

6A and 6B, it can be seen that zirconium diboride exists in the center, and silicon carbide, carbon, and pores exist around the center. The carbon particles are in an amorphous state. Silicon carbide and carbon particles are believed to be derived from polycarbosilane. The carbon particles and silicon carbide surround the zirconium diboride, thereby inhibiting contact between the zirconium diboride and preventing grain growth.

When the grain boundary of the zirconium diboride, the ultrahigh temperature curing material, is suppressed, the overall particle size depends on the original particle size of the zirconium diboride. In addition, since silicon carbide particles surround the zirconium diboride and obtain driving force for densification through high-temperature pressurization sintering, the contact between silicon carbides with high thermal conductivity increases when the same silicon carbide volume fraction is used, resulting in high thermal conductivity. It is possible to obtain a composite material having a.

7 is a scanning electron microscope (SEM) photograph of a zirconium dichloride-silicon carbide composite material synthesized according to Experimental Example 2. FIG.

Referring to Fig. 7, the average particle size of zirconium diboride in the zirconium diboride-silicon carbide composite material was about 2 탆 (1.5 times or less of the average particle size of the starting zirconium diboride powder). Considering that the average particle size of the starting material zirconium diboride is 1.88 μm and the average particle diameter of the nano silicon carbide is 100 nm or less, the use of nano silicon carbide having a size of 100 nm or less suppresses the grain growth of the zirconium diboride to some extent. The pinning effect appears and the contact between silicon carbides having high thermal conductivity is considered to increase.

8 is a scanning electron microscope (SEM) photograph of a zirconium diboride-silicon carbide composite material synthesized according to a comparative example.

Referring to FIG. 8, the average particle size of zirconium diboride in the zirconium diboride-silicon carbide composite material was about 5 μm (more than twice the average particle diameter of the zirconium diboride powder as the starting material). Considering that the average particle size of the starting material of zirconium diboride is 1.88 μm and that the average particle diameter of silicon carbide is 0.5 μm, it can be seen that zirconium diboride-silicon carbide has a significant grain growth during the heat treatment process.

9A, 9B, and 9C are graphs showing specific heat, thermal diffusivity, and thermal conductivity of zirconium diboride-silicon carbide composites synthesized according to Experimental Example 1, Experimental Example 2, and Comparative Experimental Example.

In order to measure the thermal conductivity, it is calculated from the following equation (1).

[Equation 1]

K = ρC _p λ

Where p is the bulk density of the material, C _p is the specific heat of static pressure, and λ is the thermal diffusion coefficient. The apparent density of an object is measured using Archimedes' principle, and laser flash analysis (Netzsch, Germany) is used to measure the thermal diffusivity (λ) at high temperature. The measurement temperature is measured according to the temperature of the specimen heated to 1000 ° C.

9A to 9C, the zirconium diboride-silicon carbide composite material synthesized using 0.5 μm silicon carbide according to the comparative example shows a low thermal conductivity value of 30 to 40 W / m · K. gave. This approximates the thermal conductivity of zirconium diboride and is the result of grain growth.

The thermal conductivity of the zirconium diboride-silicon carbide composite material synthesized according to Experimental Example 1 and Experimental Example 2 showed a relatively high thermal conductivity value of 80 ~ 110 W / m · K. This suppresses the grain growth of zirconium diboride during sintering in the case of using silicon carbide or nano silicon carbide derived from polycarbosilane, thereby increasing the contact between silicon carbide with high thermal conductivity, thereby increasing the similar volume fraction of silicon carbide. We believe that the percolation threshold is also implemented. From this, it is considered that the thermal properties to be used as a material for a thermal protection system can be satisfied.

As mentioned above, although preferred embodiment of this invention was described in detail, this invention is not limited to the said embodiment, A various deformation | transformation by a person of ordinary skill in the art within the scope of the technical idea of this invention is carried out. This is possible.

Claims (11)

(a) zirconium diboride, nano silicon carbide less than or equal to 1/10 of the average particle diameter of the zirconium diboride and an average particle diameter of 10 to 200nm, a dispersant for inhibiting aggregation of the zirconium diboride and the nano silicon carbide Mixing and wet milling;
(b) drying the wet milled product;
(c) charging the dried resultant to a high-temperature sintering furnace and heating to a sintering temperature of 1800 ~ 2200 ℃; And
(d) sintering under pressure at the sintering temperature,
In the step (a) further mixed with a solvent in which polycarbosilane is dissolved,
Before the temperature is raised to the sintering temperature, further comprising the step of heat treatment at a temperature of 1100 ~ 1500 ℃ lower than the sintering temperature in order to thermally decompose the polycarbosilane is converted to silicon carbide,
After the heat treatment step and before the step of raising the temperature to the sintering temperature,
And cooling the polycarbosilane to a temperature lower than the heat treatment temperature so as to allow gas generated during pyrolysis of the polycarbosilane to escape.
The method according to claim 1, wherein, in the step (a)
The zirconium diboride and the nano silicon carbide are mixed so that 50 to 95% by volume of zirconium diboride and 5 to 50% by volume of nano silicon carbide are mixed, and the dispersant is 100 parts by weight of the total content of zirconium diboride and nano silicon carbide. A method for producing a zirconium diboride-silicon carbide composite material, characterized in that mixing 0.1 to 10 parts by weight.
delete The method according to claim 1, wherein, in the step (a)
The polycarbosilane, the nano silicon carbide and the zirconium diboride are mixed to mix 5 to 50% by volume of polycarbosilane and nano silicon carbide and 50 to 95% by volume of zirconium diboride, and the dispersing agent is zirconium diboride. A method for producing a zirconium diboride-silicon carbide composite material, characterized in that mixing 0.1 to 10 parts by weight based on 100 parts by weight of the total amount of the ride and nano silicon carbide.
delete According to claim 1, wherein the solvent is a method of producing a zirconium diboride-silicon carbide composite, characterized in that hexane or toluene.
The method of claim 1, wherein the heat treatment is performed in a non-oxidizing atmosphere of vacuum or argon gas.
The method according to claim 1,
Method of producing a zirconium diboride-silicon carbide composite material, characterized in that carried out for 30 minutes to 48 hours at a temperature of 1800 ~ 2200 ℃ while pressing at a pressure of 1 ~ 60MPa.
The method of claim 1, wherein the sintering is performed in a non-oxidizing atmosphere, and the non-oxidizing atmosphere is a vacuum or argon gas atmosphere.
The method of claim 1, wherein the drying is performed for 1 to 100 hours while stirring at a rate of 5 to 200 rpm to prevent segregation due to evaporation of the solvent.
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