JP3018354B2 - Ceramic composite material and method for producing the same - Google Patents

Description

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

[Prior Art] Quartz, aatite, sapphire, and beryllia have been considered as conventional techniques as a support (support rod) for a metal spiral of W or Mo in a traveling wave tube which is used as a structure in electronics. (Journal of Telecommunications Research Institute, Nippon Telegraph and Telephone Public Corporation, published by Maruzen Co., Ltd., Jiro Koyama, “Traveling Wave Tube,” p. 207).

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, making it 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 crystal structure by Van der Waals coupling. That is, the permittivity and thermal conductivity of hBN in the a-axis direction are 5.1 and 62 W / mk, respectively, and in the c-axis direction, they are 3.5 and 2 W / mk.
There is a report of mk.

Currently, Union Wave, which is pyrolytic boron nitride (PBN) by vapor deposition, is used as a traveling wave tube support material.
BORALLOY, a trade name of Union Carbide, has been studied and some have been put to practical use. However BORALLOY
Has a high degree of orientation because it is deposited on a substrate such as graphite by vapor phase epitaxy, and has a high anisotropy in which the dielectric constant and thermal conductivity differ significantly in the in-plane direction and thickness direction of the material as described above. Therefore, low dielectric constant and high thermal conductivity cannot be exhibited simultaneously. In other words, when the direction of use of the support is set to the c-axis direction in order to use a low dielectric constant (3.5), the thermal conductivity is about 2 W / m · k.
And it was considerably smaller than conventional quartz, steatite, sapphire, and beryllia, and had a problem in heat dissipation. Also, when the a-axis direction (62 W / m · k) with good thermal conductivity for heat dissipation is used as the support, the dielectric constant is 5.1, which is sufficient for the requirement of low dielectric constant for high frequency. Was not. Also
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 also many problems. Furthermore, since pyrolytic boron nitride is produced by a vapor phase growth method, large and thick products cannot be mass-produced, and there are also industrial problems such as high cost.

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, and has a high structural reliability. As a result, the present invention was found to be most suitable as a support for a traveling wave tube, and the present invention was completed.

[Means for Solving the Problems] The present invention is characterized in that the porous hexagonal boron nitride sintered body has a structure in which aluminum nitride is present on the surface of the boron nitride particles and / or at the grain boundaries of the sintered body. After impregnating a ceramic composite material for a traveling-wave tube support and a porous hexagonal boron nitride sintered body with an organometallic compound solution containing aluminum and nitrogen, which is an aluminum nitride precursor, a non-nitrogen or ammonia containing A method for producing a ceramic composite material for a traveling wave tube support, which is characterized by performing a heat treatment in an active gas atmosphere.

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

The ceramic composite material of the present invention contains hexagonal boron nitride (hBN) and aluminum nitride (AlN) as main components, and AlN impregnates a precursor of an organometallic compound containing Al and N into porous hBN. After that, it has a feature in the manufacturing process that heat treatment is performed to generate an AlN phase. as a result,
Inside the porous hBN sintered body, it has a composite structure in which AlN phase is generated on the surface and grain boundary of hBN particles. FIG. 1 is a cross-sectional view schematically showing the structure of the ceramic composite material 1 of the present invention, in which AlN3 is present on the surface of hBN2 and at the grain boundary thereof.

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 organometallic compound solution as the AlN precursor to uniformly penetrate into the hBN sintered body during the impregnation process, and in order to obtain sufficient mechanical strength. Is desirably 50%.

As an organometallic compound which is an aluminum nitride precursor containing aluminum and nitrogen, an organoaluminum polymer C ₂ H ₅ ) ₂ AlNH _{2 m} prepared from triethylaluminum (Al (C ₂ H ₅ ) ₃ ) and ammonia (NH ₃ ) , C ₂
H ₅₎ AlNH _n and the like are possible. In the step of dissolving the organometallic compound, which is an AlN precursor, in a solvent if necessary, and impregnating the porous hBN sintered body, the permeability of the organometallic compound is improved, and a reduced-pressure atmosphere is used to uniformly disperse and add the organometallic compound. It is effective to perform the injection below.

Then, after heating to 300 to 1000 ° C. at a rate of 1 to 10 ° C./min in an inert gas atmosphere containing nitrogen or ammonia, heat treatment is performed at a set temperature for 1 to 5 hours. Organometallic polymer C ₂ H ₅ ) ₂ AlNH which is an AlN precursor described above
_{2 m,} the C ₂ H ₅₎ AlNH _n, heat treatment in an inert gas atmosphere containing ammonia at 300 to 800 ° C. is Al
It is effective to generate N phase. The content of the AlN phase in the ceramic composite material of the present invention can be increased by repeating the steps of impregnating the AlN precursor and heating. 300-1000
In the heat treatment of the AlN precursor at ° C., since the generated AlN is in a fine powder state, the thermal conductivity of the porous hexagonal boron nitride sintered body is not significantly improved. Therefore, by subjecting this to further heat treatment in an inert gas atmosphere such as nitrogen at 1600 to 2000 ° C, sintering of the AlN powder generated by thermal decomposition occurs on the surface and grain boundary of the hBN particles. Significantly increase.

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. The dielectric constant of this boron nitride sintered body at room temperature was 3.4, and the thermal conductivity was 55 W / mk.

An organic metal compound solution C ₂ H _5, which is a precursor of aluminum nitride placed in a beaker, is added to the boron nitride sintered body.
_{2 m 2} NH ₂ was vacuum impregnated at room temperature under reduced pressure atmosphere. Thereafter, the boron nitride sintered body impregnated with the organometallic compound was heated to 500 ° C. in a nitrogen gas atmosphere containing 10% of ammonia at a rate of 1 ° C./min and kept at 500 ° C. for 2 hours. In order to sinter the powdery AlN produced in this process, this ceramic composite material consisting of hexagonal boron nitride and aluminum nitride is heated to 1800 ° C at a temperature increase rate of 20 ° C / min in a nitrogen gas atmosphere. For one hour. As a result, the porosity became 15%, the dielectric constant at room temperature was 3.6, and the thermal conductivity was 1
A ceramic composite material composed of hexagonal boron nitride and aluminum nitride of 00 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 the 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.

[Effects of the Invention] As described above, the ceramic composite material composed of hexagonal boron nitride and aluminum nitride according to the present invention can be used as a conventional support material for a traveling wave tube support, such as quartz, asbestite, sapphire, Beryllia is a material having both a low dielectric constant and a high thermal conductivity superior to pyrolytic boron nitride, and has little anisotropy of properties in pyrolytic boronization, and has no problem of peeling or cracking. In addition, it has many industrial advantages, such as the ability to produce large and thick products in large quantities at low cost, which is difficult with pyrolytic boron nitride.

Further, the ceramic composite material of the present invention has an effect that it can be used for electronic parts, insulating substrates, high-temperature furnace jigs, and the like, which are applications other than the support of the traveling wave tube.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view schematically showing the structure of the ceramic composite material of the present invention, and FIG. 2 is a cross-sectional view of the traveling-wave tube (a) and a cross-sectional view (b) taken along the line AA 'of (a). is there. 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 (2)

(57) [Claims] 1. A traveling wave tube support, characterized in that the porous hexagonal boron nitride sintered body has a structure in which aluminum nitride is present on the surface of boron nitride particles and / or grain boundaries of the sintered body. Ceramic composite materials. 2. A porous hexagonal boron nitride sintered body is impregnated with an organometallic compound solution containing aluminum and nitrogen, which is a precursor of aluminum nitride, and then heat-treated in an inert gas atmosphere containing nitrogen or ammonia. A method for producing a ceramic composite material for a traveling wave tube support, comprising: