JP2920970B2 - Manufacturing method of ceramic composite material - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a ceramic composite material, and more particularly to a method for producing a ceramic composite material used as a traveling wave tube support material for electronics.

[Prior art] As a support (support rod) of a metal spiral of W or Mo of a traveling wave tube, which is used as a structure in electronics, quartz, steatite,
Sapphire and beryllia have been studied (Journal of Telecommunications Research Institute, Nippon Telegraph and Telephone Public Corporation, published by Maruzen Co., Ltd., edited by Jiro Koyama, “Traveling Wave Tube”, p.

On the other hand, in recent years, in a traveling wave tube for a communication satellite or a broadcasting satellite, a low dielectric constant as a high-frequency characteristic superior to a conventional material has become important for a support in order to increase the frequency. Further, in order to prevent high-frequency loss due to inflow or heating of the electron beam, the support is required to have high thermal conductivity for heat dissipation. Conventional support materials such as quartz glass, quartz, steatite, sapphire, and beryllia have a dielectric constant of 3.6, 4.3, 6.0, 9.6, 6.9 at room temperature, respectively, and a thermal conductivity of 2, 7, 3,40,260 W / m · K, and it was difficult to maintain high thermal conductivity while realizing a low dielectric constant. On the other hand, recently, hexagonal boron nitride (hBN) has been attracting attention as a material having both low dielectric constant and high thermal conductivity.

[Problems to be Solved by the Invention] Hexagonal boron nitride (hBN) has a laminated structure of a hexagonal mesh plane like graphite, the a-axis direction in the layer is covalently bonded, and c is perpendicular to the laminated plane. The axial direction shows remarkable anisotropy in the dielectric constant and thermal conductivity due to the coupling structure by Van der Waals coupling. That is, the permittivity and thermal conductivity in the a-axis direction of hBN are 5.1 and 62 W / m · K, respectively, and in the c-axis direction, they are 3.5 and 2 W / m · K.
There is a report of mk.

Currently, as a traveling wave tube support material, for example, BORALLOY, a trade name of Union Carbaide, which is pyrolytic boron nitride (PBN) formed by a vapor phase growth method, has been studied and partially used. . However BORALL
OY has a high degree of orientation because it is deposited on a substrate such as graphite by vapor phase epitaxy, and the anisotropy and thermal conductivity of the material differ significantly in the in-plane direction and thickness direction of the material as described above. Therefore, a low dielectric constant and a high thermal conductivity cannot be simultaneously exhibited.
In other words, when using the support in the c-axis direction to use a low dielectric constant (3.5), the thermal conductivity is about 2 W / m ·
K is considerably smaller than conventional quartz, steatite, sapphire, and beryllia, and has a problem in heat dissipation. When the support is used in the a-axis direction (62 W / m · K), which has good thermal conductivity for heat dissipation, the dielectric constant is 5.1, which is a sufficient requirement for a low dielectric constant for higher frequencies. Was not. In addition, BORALLOY is an oriented structure with anisotropy of a-axis and c-axis based on the laminated structure, which is an essential property of hexagonal boron nitride. There were many problems with gender. In addition, pyrolytic boron nitride is manufactured by a vapor growth method, so large and thick products cannot be mass-produced.
There were also industrial problems such as high costs.

The present inventor has worked diligently to deal with such a point, and as a result, the ceramic composite material composed of hexagonal boron nitride and aluminum nitride has both low dielectric constant and high thermal conductivity, as well as structural reliability. The inventors have found that they are excellent and are most suitable as supports for traveling wave tubes, and have completed the present invention.

[Means for Solving the Problems] The present invention provides a method for impregnating a porous hexagonal boron nitride sintered body with a mixed solution containing an aluminum alkoxide as an aluminum nitride precursor and carbon after hydrolysis, or impregnating a mixed solution containing aluminum alkoxide and carbon. A method for producing a ceramic composite material, which comprises decomposing and then drying the sintered body, followed by heat treatment in an inert gas atmosphere containing nitrogen or ammonium.

The ceramic composite material obtained by the method of the present invention has a structure in which aluminum nitride is present on the surface of the boron nitride particles of the porous hexagonal boron nitride sintered body and / or the grain boundary part of the sintered body.

 Hereinafter, the present invention will be described in more detail.

The ceramic composite material according to the method of the present invention contains hexagonal boron nitride (hBN) and aluminum nitride (AlN) as main components, and AlN is obtained by firing a mixed solution containing aluminum alkoxide and carbon as precursors in a porous hBN. A characteristic in the manufacturing process of impregnating the inside of the sintered body after hydrolysis or hydrolyzing after impregnation, then drying, and then heat-treating in an inert gas atmosphere containing nitrogen or ammonia to produce an AlN phase. Having. As a result, porous hB
Inside the N sintered body, it has a composite structure in which an AlN phase is formed on the surface and grain boundary of hBN particles. FIG. 1 is a sectional view schematically showing the structure of the ceramic composite material 1 of the present invention.
AlN3 exists on the surface of hBN2 and its grain boundaries.

Furthermore, in the ceramic composite material 1 of the present invention, hBN2 or
In addition to AlN3, as shown in FIG.
Is effective for lowering the dielectric constant. However, if the porosity exceeds 50%, there is a problem that the mechanical strength becomes insufficient in structural use of a traveling wave tube support or the like.

The content of boron nitride in the ceramic composite material of the present invention is not particularly limited.
In this case, there is an advantage that cutting can be performed with a normal tool.

Next, a method for producing the ceramic composite material of the present invention will be described.

Porous hexagonal boron nitride (hBN) sintered body is produced by normal pressure sintering method or hot press method, and its purity is
Although 98% or more is preferable, about 95-98% can also be used. The porosity must be at least 5% in order for the mixed solution containing aluminum alkoxide and carbon, which is the precursor of AlN, or its hydrolysis solution to uniformly penetrate into the hBN sintered body during the impregnation process. However, in order to obtain sufficient mechanical strength, the content is desirably 50% or less.

The aluminum alkoxide, aluminum ethylate _{(Al [OC 2 H 5]} 3), aluminum isopropylate _{(Al [OC 3 H 7]} 3), aluminum butyrate (A
Numerous types such as l [OC ₄ H ₉ ] ₃ ), butoxyaluminum diisopropylate, ethyl acetoacetate aluminum diisopropylate, aluminum tris (ethyl acetoacetate), alkyl acetoacetate aluminum diisopropylate are available.

On the other hand, as the carbon, a carbon source substance such as a phenol resin, a styrene resin, a nylon resin, and an acrylic resin, which is a substance that generates carbon during the heat treatment process, is also effective in addition to general carbon black and graphite powder.

Dissolve or disperse and mix these aluminum alkoxides and carbon in a suitable solvent.
Hydrolyze while adjusting, then put the porous hBN sintered body into the solution after hydrolysis and impregnate, or first impregnate the mixed solution into the porous hBN sintered body, and then hydrolyze Do. In any of the methods, in order to improve the permeability of the mixed solution during the impregnation, and to uniformly disperse and add the solution, it is effective to inject under a reduced pressure atmosphere.

Then, after drying this porous hBN sintered body, under an inert gas atmosphere containing nitrogen or ammonia 1400 ~ 1600
After heating to a temperature of 1 to 20 ° C./min to 1 ° C., a heat treatment is performed for 1 to 10 hours. As a result, a ceramic composite material having a structure in which aluminum nitride is present on the surface of the boron nitride particles inside the porous hBN sintered body and / or the grain boundaries of the sintered body by the reduction and nitridation reaction of the aluminum compound is obtained.

The content of the AlN phase in the ceramic composite material of the present invention can be increased by repeating the steps of impregnation and heat treatment. In addition, AlN generated in the heat treatment at 1400 to 1600 ° C
Is in a fine powder state, so that the thermal conductivity is not significantly improved compared to the first porous hBN sintered body. Therefore, by subjecting this to further heat treatment in an inert gas atmosphere such as nitrogen at 1600 to 2000 ° C, sintering of AlN powder generated by thermal decomposition of hBN particles on the surface and at the grain boundary occurs. Increase significantly.

EXAMPLES The present invention will now be described more specifically with reference to examples, but the present invention is not limited to these examples.

[Example] Example 1 Purity 99.5% by hot pressing (impurity oxygen 0.4
%), A hexagonal boron nitride sintered body having an average particle size of 5 μm and a porosity of 20% was formed into 110 × 110 × 5 mm by machining. This boron nitride sintered body had a dielectric constant of 3.4 at room temperature and a thermal conductivity of 55 W / m · K.

Aluminum isopropylate _{(Al [O-C 3 H} 7] 3)
80 g of 100 g and 9 g of carbon black (average particle size 200 mm)
The solution was added to a beaker containing isobutyl alcohol 1 at a temperature of 0 ° C. and heated for 3 hours to form a uniform solution. Next, after cooling to room temperature, 300 g of distilled water was added dropwise, and 80 g of 20% ammonia water was further added, and the mixture was kept at room temperature for 10 hours. Thereafter, the mixture was kept at 80 ° C. for 5 hours to complete the hydrolysis. The above-mentioned boron nitride sintered body was put into this solution, and aluminum sol and carbon black were uniformly vacuum impregnated inside the boron nitride sintered body under a reduced pressure atmosphere.

After that, the impregnated boron nitride sintered body was dried at 50 ° C., heated to 1500 ° C. in a nitrogen gas atmosphere at a rate of 5 ° C./min, and kept at 1500 ° C. for 5 hours. In order to sinter the powdered AlN produced in this process, this ceramic composite material consisting of hexagonal boron nitride and aluminum nitride was heated to 1900 ° C at a temperature increase rate of 20 ° C / min in a nitrogen atmosphere. Hold for hours. As a result, a porosity was reduced to 15%, and a ceramic composite material composed of hexagonal boron nitride and aluminum nitride having a dielectric constant of 3.6 at room temperature and a thermal conductivity of 120 W / m · K was obtained.

After cutting this ceramic composite material, 0.25 × 0.5
A long rectangular parallelepiped traveling wave tube support of 100 mm was prepared and mounted on the traveling wave tube.

FIG. 2 (a) is a cross-sectional view thereof, and FIG. 2 (b) is a cross-sectional view taken along line AA 'in FIG. 2 (a). The tungsten coil 6 is compressed and held by a stainless steel protection tube 9 from three directions by three supports 5. 7 is a cathode and 8 is a collector. The support 5 was subjected to compressive and shear stresses from three points of the tungsten coil 6 and the inner wall of the stainless protective tube 9, but no peeling or cracking caused by pyrolytic boron nitride occurred.

Example 2 The impregnation and drying of the solution containing aluminum sol and carbon black into the boron nitride sintered body in Example 1 were repeated three times, and heated to 1500 ° C. for 5 hours in a nitrogen atmosphere in the same manner as in Example 1. Kept in. Then 19
Heat treatment was performed in a nitrogen atmosphere at 00 ° C. for 4 hours. as a result,
The porosity was reduced to 10%, and a ceramic composite material composed of hexagonal boron nitride and aluminum nitride having a dielectric constant of 3.7 at room temperature and a thermal conductivity of 150 W / m · K was obtained.

[Effects of the Invention] As described above, the ceramic composite material composed of hexagonal boron nitride and aluminum nitride manufactured by the method of the present invention can be used as a traveling wave tube support, such as quartz, which is a conventional support material. A material that has both a low dielectric constant and a high thermal conductivity that are superior to steatite, sapphire, beryllia, and pyrolytic boron nitride. No problem. In addition, it is possible to produce large and thick products in large quantities at low cost, which is difficult with pyrolytic boron nitride.
It has many industrial advantages.

Further, the ceramic composite material according to the method of the present invention may be used for electronic components, insulating substrates,
There is also an effect that it can be used for high-temperature furnace jigs.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view schematically showing the structure of a ceramic composite material obtained by the method of the present invention, and FIG. 2 is a cross-sectional view of the traveling-wave tube (a) and a cross-sectional view taken along line AA 'of (a). (B). DESCRIPTION OF SYMBOLS 1 ... Ceramic composite material 2 ... Hexagonal boron nitride 3 ... Aluminum nitride 4 ... Pores 5 ... Support 6 ... Tungsten coil 7 ... Cathode 8 ... Collector 9 ... Stainless steel protection tube

Claims (1)

(57) [Claims] 1. A porous hexagonal boron nitride sintered body is hydrolyzed and impregnated with a mixed solution containing aluminum alkoxide and carbon, which are aluminum nitride precursors, or hydrolyzed after impregnation, and then the sintered body is sintered. A method for producing a ceramics composite material, comprising drying and heating in an inert gas atmosphere containing nitrogen or ammonia.