JP2895147B2 - Aluminum nitride composite sintered body - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a novel aluminum nitride composite sintered body. Specifically, the present invention relates to an aluminum nitride composite sintered body having high strength.

[Prior Art] Aluminum nitride sintered bodies are expected to be used as materials for heat sinks and circuit boards of semiconductors, utilizing their high thermal conductivity and electrical insulation. In addition, because of its excellent heat resistance, utilization as a structural material is also being studied. However, conventionally, only efforts have been made to improve the thermal conductivity of aluminum nitride, and little consideration has been given to strength. For example, Japanese Patent Application Laid-Open No. 63-303863 discusses the thermal conductivity and crystal grain size of aluminum nitride, and it is important that the crystal grain size of aluminum nitride is 7 μm or more to achieve high thermal conductivity. Is disclosed, but there is no description regarding strength.

[Problems to be Solved by the Invention] When aluminum nitride is used as a structural material, it is necessary to stably obtain high strength. Nevertheless, heretofore, there is no known method for controlling the strength of the aluminum nitride sintered body, and in some cases, unexpected breakage may occur during use. I was On the other hand, JP-A-53-45313 discloses that
There is also disclosed a method of adding 5 to 60% of silicon oxide to aluminum nitride to obtain a high-strength sintered body substantially consisting of an Al—Si—ON phase. Properties such as conductivity and corrosion resistance are lost.
Japanese Patent Publication No. 46-25850 also discloses a method of increasing the strength by combining aluminum nitride and beryllium oxide. In this case, however, a [(Al.Be) N] solid solution is formed. The original properties of aluminum are impaired. Further, a method of strengthening the grain boundaries of the aluminum nitride sintered body by adding a specific sintering aid to improve the strength has been attempted, but in this method, an auxiliary component remaining in the sintered body is used. As a result, problems such as a decrease in high-temperature strength and contamination occur, so that it was inevitable that the application fields were limited.

[Means for Solving the Problems] On the other hand, the present inventors have conducted intensive studies on the improvement of the strength of the aluminum nitride sintered body, and as a result, obtained a composite of aluminum nitride and hexagonal boron nitride. By controlling the size of the crystal grains of the aluminum nitride crystal phase in the specified range, it has been found that a high-strength aluminum nitride composite sintered body can be obtained, and the present invention has been completed. That is, the present invention relates to aluminum nitride 60-95.
% By weight and 40 to 5% by weight of hexagonal boron nitride, and the average particle size of the crystal grains of the aluminum nitride crystal phase is 4.0 to 5.2.
It is an aluminum nitride composite sintered body in the range of μm.

Although the method for producing the aluminum nitride composite sintered body in the present invention is not particularly limited, it is generally produced by adding and mixing a sintering aid to aluminum nitride powder and boron nitride powder, and sintering the mixture. However, when the sinterability of the raw material aluminum nitride powder is sufficiently excellent, the addition of a sintering aid is not essential.

Although the raw material aluminum nitride powder is not particularly limited,
For example, the aluminum nitride content is 95% by weight or more, the oxygen content is 3% by weight or less, and the content of a metal compound as an impurity is 0.3% by weight or less as a metal, and the average particle diameter is 2 μm or less. 3 μm or less 70
Aluminum nitride powder containing at a rate of at least volume% is preferably used. Generally, among the properties of aluminum nitride powder, it is important that the grain size is small and uniform, in order to make the grain size of the aluminum nitride crystal in the obtained sintered body uniform, and the average grain size is 1.5 μm. Aluminum nitride powder containing 3 μm or less at a rate of 85% by volume or more is more preferably used.

Here, the hexagonal boron nitride powder suitably used as a raw material is not particularly limited, and any powder can be used. As an example of a typical example generally used, the purity of boron nitride is 95.0% by weight.
Or more, preferably 98.0% by weight or more, more preferably 99.0% by weight.
% By weight or more and an average particle diameter of 5 μm or less, preferably 3 μm or less.
μm or less, more preferably 1 μm or less. In general, when the purity of boron nitride is low, the impurities may form a solid solution in the aluminum nitride crystal to lower the thermal conductivity or lower the heat resistance of the sintered body. Also, when the particle diameter of the boron nitride powder is large, it is difficult to mix uniformly with the aluminum nitride powder, and as a result, the aluminum nitride crystal particles in the sintered body may grow non-uniformly. Note that, in the aluminum nitride composite sintered body of the present invention, boron nitride exists in a hexagonal crystal phase. However, the boron nitride powder used as a raw material is not necessarily required to be hexagonal, and may be changed to hexagonal during the sintering process, for example, may be amorphous or have a turbostratic structure.

In the aluminum nitride composite sintered body of the present invention, the presence of hexagonal boron nitride is extremely important. The content of such hexagonal boron nitride is preferably in the range of 5 to 40%,
A range of 30% is more preferred. If the content of hexagonal boron nitride is too high, it is not preferable because characteristics such as aluminum nitride's original high thermal conductivity cannot be sufficiently exhibited.
Hexagonal boron nitride inhibits sintering of aluminum nitride and makes densification difficult. As a result, the high-strength sintered body intended by the present invention may not be obtained. Conversely, if the content of hexagonal boron nitride is too low, the effect of suppressing the grain growth of the aluminum nitride detection phase described below will not be sufficiently exhibited, and the particle size of the aluminum nitride crystal phase, which is the most important element of the present invention, will be described. Control becomes difficult.

Examples of the sintering aid include Group IIa of the periodic table and / or
Alternatively, known compounds such as a compound of a Group IIIa element can be used as appropriate. As the Group IIa element, beryllium, magnesium, calcium, strontium, barium, and the like, particularly calcium, strontium, and barium, are preferably used. As the IIIa element, yttrium or a lanthanum element is generally preferably used, and more specifically, yttrium, lanthanum, cerium, praseosium, neodymium, promesium, samarium, europium, gadolinium, terbium, dysprosium,
Holmium, erbium, thulium, ytterbium, lutetium and the like, especially yttrium, lanthanum, cerium, neodymium and the like are suitable. When actually added as a sintering aid, it is generally used as an oxide of these elements, but it can also be used as a compound that becomes an oxide under the conditions under which aluminum nitride powder is sintered. By way of example, nitrates, carbonates, chlorides and the like can be used. Further, it can be added as a fluoride, carbide, aluminate or the like of the element. For example, yttrium fluoride, calcium carbide, calcium aluminate (3CaO.Al ₂ O ₃ )
Etc. are preferably used. Since it is important that these sintering aids are uniformly mixed with the aluminum nitride powder, it is preferable that the particle diameter of the sintering aids be as small as possible. Also, a dispersant for improving the dispersibility of the sintering aid is appropriately used as needed. Furthermore, the so-called sol
Known methods for uniformly mixing the sintering aid, such as a gel method,
It can be adopted as appropriate. The amount of the sintering additive is not particularly limited, but is generally 0.01 to 15 parts by weight, preferably 0.1 to 10 parts by weight, based on 100 parts by weight of the total of the aluminum nitride powder and the boron nitride powder. It is suitable. The sintering method is also not particularly limited, and pressureless sintering, pressure sintering, or the like may be selected as necessary.

The crystal grain size of the aluminum nitride composite sintered body of the present invention is generally controlled by changing the sintering temperature and the sintering time (holding time at the sintering temperature). That is, when starting from the same raw material composition, to increase the crystal grain size, the sintering temperature is increased and the sintering time is lengthened. To make the crystal grain size smaller, the reverse is true. Generally, the sintering temperature is selected from the range of 1550-2050 ° C, preferably 1700-2000 ° C, and the sintering time is selected from the range of 1-50 hours, preferably 3-24 hours. However, the average grain size of the crystal grains of the aluminum nitride crystal phase in the sintered body may be 4.0, depending on the grain shape and grain size distribution of the starting material, the pressure of hexagonal boron nitride, and the like.
Since the sintering conditions for setting the range to about 5.2 μm vary, the average particle size may be controlled in consideration of these conditions. As one of the specific control methods, sintering experiments are performed by changing the temperature and time using a starting material having a desired composition, and the average particle size of the crystal grains of the aluminum nitride crystal phase in each sintered body is determined. There is a method in which sintering conditions for controlling the particle size to a preferable range by measuring are determined by a statistical method. The present invention has been completed by finding that it is extremely effective to set the crystal grain size of the aluminum nitride phase in the sintered body to a specific range in order to obtain a high-strength aluminum nitride composite sintered body. The sintering conditions for obtaining such a sintered body are not limited at all.

The use of hexagonal boron nitride as a composite component is an important requirement of the present invention, for the following reasons. First, hexagonal boron nitride does not directly react with aluminum nitride, and thus does not impair the excellent properties inherent in aluminum nitride. Further, hexagonal boron nitride intervenes as a layer crystal at the grain boundaries of crystal grains of the aluminum nitride crystal phase in the sintered body, and thus hinders the growth of aluminum nitride crystal grains. This effect of suppressing grain growth facilitates controlling the size of the crystal grains of the aluminum nitride phase in the sintered body to a specific range. This is extremely effective when the aluminum nitride composite sintered body of the present invention is manufactured industrially. In addition, since the dielectric constant of the aluminum nitride composite sintered body can be lowered according to the content of hexagonal boron nitride, it is useful for high-frequency related applications such as radar domes and isolators.

Further, Japanese Unexamined Patent Publication No.
97% by weight, 3 to 50% by weight of hexagonal boron nitride, according to the periodic table
A metal compound of Group IIa or Group IIIa, wherein the metal compound is converted to the highest valence oxide of the metal and is 0.01 to 0.01% based on the total amount of the aluminum nitride and hexagonal boron nitride.
Aluminum nitride composite sintered body such as 10% by weight,
It shows that it is a so-called machinable ceramic that can be machined with a carbide tool or the like. According to the conventional common sense, it is considered that the aluminum nitride composite sintered body containing hexagonal boron nitride as in this example has a lower mechanical strength than the case where hexagonal boron nitride is not contained. Also in the example described in JP-A-60-195059,
The results show that the higher the content of hexagonal boron nitride, the lower the bending strength of the sintered body. However, by controlling the grain size of aluminum nitride crystals in the sintered body, the present inventors can not only minimize such a decrease in strength, but also reduce the content of hexagonal boron nitride to 12%.
On the contrary, the present inventors have found a surprising new fact that strength can be improved in the range of 1818% by weight. Although it is not clear why the strength is improved in this way, the present inventors have set the average particle size of the aluminum nitride crystal particles in the sintered body in the range of 4.0 to 5.2 μm, and added hexagonal boron nitride. It is thought that the mechanism of strengthening of the particle dispersion worked by intervening in the grain boundaries of the aluminum nitride crystal particles in an appropriate amount.

Since it is difficult to directly measure the crystal grain size of the sintered body, various indirect measurement methods have been proposed. However, the value of the obtained particle size changes depending on the measurement method, and therefore, it is necessary to unify the measurement methods when comparing the crystal particle sizes. Therefore, in the present invention, the average value of the particle size of the aluminum nitride crystal in the sintered body was measured by the following method. [Reference: Journal of the American Ceramic Society, 52 [8], 443-446 (196
9)] Take a scanning electron micrograph of the fracture surface (such as the fracture surface of a bending strength test piece) of the sintered body whose particle size is to be measured. The magnification of the photograph is 1500-20 when the particle size is several μm.
00 times is appropriate. If the grain boundaries are not clear in the photograph of the fractured surface, polishing, etching, etc.
It must be observable. In some cases, backscattered electron image observation is effective.

Draw straight lines at equal intervals (for example, 10 mm intervals) on the photograph.
This interval is preferably substantially the same as the particle size observed on a photograph. Even if the interval is extremely reduced, the number of crystal grains sampled does not increase, and the measurement accuracy does not improve.

The intersection between the straight line and the grain boundary is determined, and the average of the length of the line segment (intercept length) between the intersections is defined as L. In order to increase the measurement accuracy, it is desirable to count about 200 line segments.

From the average value L of the intercept length, the average value D of the actual crystal grain size is obtained by the following equation.

D = 1.570L The present inventors manufactured a number of aluminum nitride composite sintered bodies having different aluminum nitride crystal grain sizes in the sintered bodies,
The relationship with these bending strengths was investigated. In addition, the measurement of the bending strength was performed according to the three-point bending strength test method (span 30 mm) specified in JIS-C2141 as described in Examples.

FIG. 1 is a graph showing the relationship between the bending strength of an aluminum nitride composite sintered body and the grain size of aluminum nitride in the sintered body. From FIG. 1, it was found that the bending strength of the aluminum nitride composite sintered body was maximized when the aluminum nitride crystal grain size in the sintered body was in the range of 4.0 to 5.2 μm. Heretofore, there has been no report on the composite sintered body, and much less the relationship with the bending strength.
Therefore, the present inventors have found out that the bending strength is maximized when the aluminum nitride crystal grain size is in a certain range as a result of accumulation of a large amount of experimental data.

In addition, FIG. 3 is a diagram in which the maximum value of the bending strength for each composition in FIG. 1 is extracted and plotted. From FIG. 3, when the ratio of hexagonal boron nitride in the aluminum nitride composite sintered body is in the range of 12 to 18% by weight, it has higher bending strength than the sintered body not containing hexagonal boron nitride, Fifteen
It can be seen that the bending strength is maximized when the weight% is used.
This fact is also surprising, given the conventional common sense that the addition of hexagonal boron nitride only lowers the strength of the aluminum nitride sintered body. In other words, the facts clarified by the present inventors have enabled the realization of a machinable ceramic having a higher strength than a sintered body of aluminum nitride alone.

[Effect] The aluminum nitride composite sintered body of the present invention exhibits high mechanical strength stably without impairing the properties inherent in aluminum nitride, such as high thermal conductivity, corrosion resistance, and heat resistance. Therefore, it is suitably used in a wide range of fields such as high-temperature furnace members, crucibles, insulating heat sinks, temperature sensor protection tubes, engine parts, vacuum equipment parts, and the like. In particular, the present invention is useful in application fields in which sufficient reliability cannot be obtained with a conventional aluminum nitride sintered body, such as a portion subjected to a load and a portion exposed to vibration. Further, the aluminum nitride composite sintered body of the present invention has high utility value as a high-strength machinable ceramics, and in particular, hexagonal boron nitride is
Since those containing aluminum by weight have higher strength than the sintered body of aluminum nitride alone, it can be widely used as engineering ceramics beyond the conventional concept of machinable ceramics. As described above, the aluminum nitride composite sintered body of the present invention broadens the application range of aluminum nitride, while increasing the choice of ceramics for structural material applications from the user's side. The contribution is enormous.

Hereinafter, the present invention will be described specifically with reference to Examples.
The present invention is not limited to this.

[Example 1] 90% by volume of particles having an average particle diameter of 1.31 µm and 3 µm or less
Of aluminum nitride powder having the composition shown in Table 1 and particles having an average particle diameter of 0.98 μm and 3 μm or less.
A hexagonal boron nitride powder having a volume percentage of 99.5% by weight and a calcium aluminate (3CaO.Al ₂ O ₃ ) powder were mixed in the ratio shown in Table 2. Mixing was performed using a nylon pot and a ball coated with nylon, using ethanol as a dispersion medium. Here, the particle size is a value measured by an automatic particle size distribution analyzer CAPA-500 manufactured by Horiba, Ltd.

After drying the obtained mixed powder, about 40 g was placed in a graphite mold having an inner diameter of 40 mm coated with hexagonal boron nitride powder and uniaxially pressurized at a pressure of 200 kg / cm ² while nitrogen gas of about 1 atm was applied. Among them, hot press sintering was performed while changing various mixing and firing conditions as shown in Table 4 to produce an aluminum nitride composite sintered body having different average particle diameters of aluminum nitride crystal particles in the sintered body ( Specific hot press sintering conditions are described in Table 3). The heating rate is from room temperature to 1400
5 ° C / min to 1 ° C, 3 ° C / min from 1400 ° C to each firing temperature
Min, and the temperature drop rate was 5 ° C./min. After cooling to room temperature, the sintered body taken out of the graphite mold had a different shrinkage ratio depending on each composition and thus different dimensions, but was approximately φ40 mm × 10 mm. Further, when the densities of the respective sintered bodies were obtained by the Archimedes method, it was found that the densities were 98% or more of the theoretical densities (Table 2).

From each of the sintered bodies, a 3 mm × 4 mm × 36 mm column-shaped test piece was cut out and subjected to No. 1500 sand paper finishing, and a three-point bending strength at a crosshead speed of 0.5 mm / min and a span of 30 mm was measured. Also, the same sintered body has a diameter of 10 mm and a thickness of 3 mm
Was cut out, and the thermal conductivity at room temperature in a direction parallel to the hot press axis was measured by a one-dimensional laser flash method. Further, the average particle size of the crystal grains of the aluminum nitride phase in each sintered body was determined by the method described above. Table 4 also shows the physical property values of each sintered body thus determined.

FIG. 1 is a graph showing the relationship between the bending strength of the sintered body and the grain size of the aluminum nitride crystal in the sintered body for each composition (mixing No.). FIG. 1 shows that the bending strength of the aluminum nitride composite sintered body shows the same tendency regardless of the composition. That is, in each composition, the graph is convex upward, and the average particle size of the aluminum nitride crystal particles when the bending strength is maximized is almost constant.

FIG. 2 is a graph that makes this easier to understand. FIG. 2 shows the average particle size of the aluminum nitride crystal particles at the time when the bending strength is maximized in each composition from the graph shown in FIG.
From FIG. 2, it can be seen that the average particle size of the aluminum nitride crystal particles when the bending strength of the sintered body is maximized is distributed around 4.6 μm regardless of the composition of the sintered body.
Considering the bending strength and the measurement error of the crystal grain size, the average grain size of the aluminum nitride crystal grains in the sintered body is 4.0 ± 0.6μ.
It can be seen that the bending strength of the sintered body is highest when m, that is, in the range of 4.0 to 5.2 μm.

In addition, FIG. 3 is a diagram in which the maximum value of the bending strength for each composition in FIG. 1 is extracted and plotted. From FIG. 3, when the ratio of hexagonal boron nitride in the aluminum nitride composite sintered body is in the range of 12 to 18% by weight, it has higher bending strength than the sintered body not containing hexagonal boron nitride, Fifteen
It can be seen that the bending strength is maximized when the weight% is used.
Conventionally, it was thought that the addition of hexagonal boron nitride only reduced the strength of the aluminum nitride composite sintered body, but by controlling the composition in an extremely narrow range, it was possible to increase the strength. It is. Although this mechanism is unknown, it is thought that when hexagonal boron nitride exists in the grain boundaries of the aluminum nitride crystal without excess or shortage, the function of strengthening the particle dispersion may be produced.

Example 2 85 parts by weight of the aluminum nitride powder used in Example 1, 15 parts by weight of hexagonal boron nitride also used in Example 1, and 3 parts by weight of yttrium oxide powder having an average particle diameter of 0.9 μm were prepared. 1 and mixed in the same manner. After the obtained mixed powder was dried, about 290 g was filled in a rubber mold and subjected to isostatic pressing at a pressure of 2000 kg / cm ² to obtain a cylindrical molded body having an outer diameter of 56 mm and a length of 60 mm.

The compact was placed in a crucible made of aluminum nitride having a purity of 99% and fired in a nitrogen gas at about 1 atm at a temperature of 1850 ° C. for 3 hours to obtain a sintered body. The temperature was raised at a rate of 5 ° C./min. The outer dimensions of the obtained sintered body were 50 mmφ × 50 mm. The density measured in the same manner as in Example 1 was 3.03 g / cm ³
Met. The average particle size of the aluminum nitride crystal particles in the sintered body determined in the same manner as in Example 1 was 4.85 μm. When physical properties of this sintered body were measured in the same manner as in Example 1, the bending strength was 49 kg / mm ² , and the thermal conductivity was 165 W / m ·
At K, no anisotropy was observed in any case, and it was found that the strength was higher than that of the sintered body containing no hexagonal boron nitride obtained in Example 1. Further, when the workability of this sintered body was examined, lathing with a carbide tool, drilling with a carbide drill, and thread cutting with a carbide hand tap could all be easily performed.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the average particle size of AlN crystal particles in a sintered body and the bending strength of the sintered body. Figure 2 shows the BN
4 is a graph showing the relationship between the content and the average grain size of AlN crystal grain size when the sintered body exhibits the maximum bending strength. FIG.
4 is a graph showing the relationship between the BN content and the maximum bending strength.

Claims (1)

(57) [Claims] 1. An aluminum nitride containing 60 to 95% by weight of aluminum nitride and 40 to 5% by weight of hexagonal boron nitride and having an average particle size of crystal grains of an aluminum nitride crystal phase in a range of 4.0 to 5.2 μm. Composite sintered body.