Boron nitride system including an hBN starting material with a catalyst and a sintered cNB body having a high heat conductivity based on the catalyst

A boron nitride system starts with an hBN material and yields a directly converted sintered cBN body having a high heat conductivity within the range of at least 4 W/cm..degree.C. to about 6.2 W/cm..degree.C. For this purpose the hBN starting material of the system has diffused therein an additive of an alkaline earth metal or alkali metal in an amount of from 0.6 mol % to 1.3 mol %. This starting material is directly converted into the cBN at a sintering temperature of at least 1350.degree. C. under a thermodynamically stabilized condition for the cBN, which contains cBN within the range of 99.9 to 99.3 wt. % of the sintered body and a metal remainder from the additive of the starting material within the range of 0.1 to 0.7 wt. % of the sintered body, except for minute naturally occurring components.

FIELD OF THE INVENTION 
The invention relates to a boron nitride system including an hBN starting 
material with a catalyst and a sintered cBN body having a high heat 
conductivity based on the catalyst. Such a sintered body is suitable for 
use as a heat sink material, for example in a semiconductor laser, a 
microwave device or an IC device. The invention also relates to a method 
for manufacturing a cubic boron nitride sintered body to provide said 
sintered body with a heat conductivity of at least 4 W/cm..degree.C. 
BACKGROUND INFORMATION 
Cubic system boron nitride, hereinafter referred to simply as cBN, has a 
hardness next to that of diamond and it has excellent thermal and chemical 
stability characteristics. Therefore, cBN has atracted special interest as 
a material for working tools. In addition, cBN has a high heat 
conductivity also next to that of diamond and accordingly, it is expected 
to be used for purposes such as the material of a heat-radiation 
substrate. 
As the material of a heat-radiation substrate, various materials having 
characteristics as shown in Table 1 have been used conventionally. 
TABLE 1 
______________________________________ 
Characteristics 
Thermal dielectric 
Heat Expansion constant 
Conductivity 
Coefficient 
(1 MHz Resistivity 
(at room (at room at room 
(at room 
temperature) 
temperature 
temper- 
temperature) 
Material 
W/cm .multidot. .degree.C. 
to 400.degree. C. 
ature) .OMEGA. .multidot. cm 
______________________________________ 
SiC 2.7 3.7 45 10.sup.13 
BeO 2.6 7.6 6.about.8 
10.sup.14 
AlN 0.6.about.1.6 
4.0 8 10.sup.12 
Al.sub.2 O.sub.3 
0.2 6.7 8.about.10 
10.sup.14 
Si 1.3 3.6 12 10.sup.-3.about.3 
Diamond 
20 2.3 5.7 10.sup.16 
______________________________________ 
Table 1 shows that diamond has a very high heat-conductivity compared with 
the other materials. 
On the other hand, Slack predicted in J. Phys. Chem. Solids, Vol. 34 
(1972)pages 321 to 334 that pure single crystal cBN would have a 
heat-conductivity as high as approximately 13 W/cm.degree.C. at room 
temperature and suggested the possibility of using it as the material for 
a radiation substrate. 
However, a large-size single crystal of cBN has not yet been produced as 
far as we know, and accordingly, the heat conductivity of 13 
W/cm.degree.C. has not been confirmed for cBN. 
In addition, the largest value reported up to the present as the heat 
conductivity of a cBN sintered body containing a binding phase is only 2 
W/cm..degree.C. The reason for this is supposed to be that the binding 
phase acts as an important factor of phonon scattering, causing the heat 
conductivity to be lowered excessively. Heat conductivity in non-metallic 
electrically insulating crystals is directly proportional to the phonon 
mean free path, see U.S. Pat. No. 4,188,194 (Corrigan), issued Feb. 12, 
1980, col. 19. 
Corrigan discloses a method for manufacturing a high heat-conductivity cBN 
sintered body without a binding phase, wherein a high density cBN sintered 
body having a high heat conductivity within the range of about 3 
W/cm.degree.C. to about 9 W/cm.degree.C. is manufactured by a direct 
conversion process using pyrolytic boron nitride (pBN) as the starting raw 
material, please see FIG. 16 of Corrigan. Where Corrigan starts with 
hexagonal boron nitride in his Examples 29 and 30 the obtained heat 
conductivity is only about 1.33 W/cm..degree.C. or 1.07 W/cm.degree.C., 
respectively. In col. 22, Corrigan states: "The room temperature thermal 
conductivity of the best U-PBN compacts is higher by a factor of 6-8 
compared to the directly converted HBN powder compact (Example 29) and by 
a factor of about 10 compared to the composite compact (Example 30).". 
This result does not suggest, nor does it motivate the use of hBN as a 
starting material if one wants to obtain cBN with a high thermal 
conductivity. 
Further, in the method of the U.S. Pat. No. 4,188,194 (Corrigan), a very 
high pressure of about 7 GPa and high temperatures of 2000.degree. C. or 
more are required to manufacture a sintered body having a 
heat-conductivity of 4 W/cm..degree.C. or more. There is also a problem in 
that the results disclosed by Corrigan are not consistently reproducible. 
In addition, pyrolytic BN is a very expensive material. 
On the other hand, a method for manufacturing a cBN sintered body not 
containing a binding phase under relatively mild conditions and at low 
cost is disclosed for example in a paper by Wakatsuki et al. in "Mat Res 
Bull" Vol 7 (1972) page 999, in which a cBN sintered body is obtained by a 
direct conversion process using hexagonal system boron nitride having a 
low degree of crystallization. However, the hBN of low degree of 
crystallization used as the starting raw material by Wakatsuki et al. 
lacks chemical stability and is liable to react with the oxygen in the 
air, which makes it difficult to obtain a homogenous body uniformly and 
sufficiently sintered overall. 
The inventors of the present invention have conducted experiments on 
synthetic materials using various methods in order to manufacture a 
sintered cBN body having a high thermal conductivity of at least 4 
W/cm.degree.C. and at a low cost while permitting a good reproductiveness 
with consistent results. As a result, they found it most suitable to use 
methods as disclosed in U.S. Pat. No. 4,469,802 (Endo et al.), issued on 
Sep. 4, 1984, where boron nitride of an alkaline earth metal or alkali 
metal is mixed or diffused into a hexagonal boron nitride (hBN) and then 
the material is subjected to a high temperature of 1350.degree. C. or 
above under a thermodynamically stabilized pressure condition of cBN. Endo 
et al. intend to provide a sintered cBN body having a good 
light-transmitting property and are not concerned with obtaining a 
sintered cBN body having a good heat conductivity. Part or all of the 
added hBN is diffused and removed out of the system in the above stated 
Endo et al. method at the time of sintering under a high pressure, whereby 
a sintered body comprised of substantially 100% cBN can be obtained. 
As a result of measuring the heat conductivity of a sintered body obtained 
as described above according to Endo et al., it was found that the heat 
conductivity of such a sintered body had a relatively high value of 2 to 3 
W/cm..degree.C. on the average compared with other sintered bodies using 
binding materials. However, the heat conductivity measurements also showed 
that in some cases the heat conductivity of such a sintered body was as 
low as 1.7 W/cm..degree.C. and thus the measured values were scattered in 
a rather wide range. 
French Patent Publication 2,344,642 (Lalaurie et al.), published Oct. 14, 
1977 discloses a process for realizing metal deposits on supports of boron 
nitride. Lalaurie et al. use the type of boron nitride disclosed in an 
article entitled "Chemical Vapor Deposited Materials for Electron Tubes" 
by S. R. Steele et al., published by "Clearinghouse for Federal Scientific 
and Technical Information" Springfield, VA, 22151, Apr. 1969, No: AD 
686,342, pages 29 and 43 to 48. According to page 29 of the Steele et al. 
disclosure, the isotropic CVD boron nitride (standard grade) has a thermal 
conductivity of 0.188 W/cm..degree.C. (=0.045 cal/cm.sup.2 
/cm/sec/.degree.C. at 300.degree. C.). Lalaurie et al. do not mention 
anything regarding the thermal conductivity their boron nitride substrate 
should have. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a sintered body of cBN 
having a heat conductivity of at least 4 W/cm..degree.C. which can be 
manufactured stably at low cost. Results must be reproducible 
consistently. 
According to the present invention it has been discovered that the heat 
conductivity of a sintered body depends not on the conditions of sintering 
but considerably on the composition of the material before it undergoes 
the sintering treatment, that is, the amount of addition of a catalyst or 
additive specifically of an alkaline earth metal or an alkali metal boron 
nitride to the starting material of hBN. In addition, it was found that it 
is necessary to closely control the amount of the added alkaline earth 
metal or alkali metal boron nitride within a range of from 0.6 mol % to 
1.3 mol % in order to achieve the desired thermal conductivity of at least 
4 W/cm..degree.C. by the presence of a remainder metal from the catalyst 
in the resulting cBN sintered body. 
The present invention achieves the cBN body having a high thermal 
conductivity of at least 4 W/cm..degree.C. in the form of a sintered cubic 
boron nitride body which consists of 99.3% wt. to 99.9% wt. of cBN and 
about 0.7 to about 0.1 wt.% of the remainder metal stemming from the 
alkaline earth metal boron nitride or from the alkali metal boron nitride 
diffused into the hBN starting material as a catalyst for reducing the 
sintering temperature and sintering pressure and for leaving the metal 
remainder in the sintered cBN body. The invention is thus a system of 
boron nitrides which starts out with a hexagonal boron nitride having 
uniformly diffused therein, as a catalyst and source for the metal 
remainder, boron nitride of an alkaline earth metal or of alkali metal in 
an amount of from 0.6 to 1.3 mol %, and which ends up as a cBN sintered 
body with the above metal remainder and with the above high thermal 
conductivity. The starting material is directly converted by sintering the 
hBN starting material at a temperature of 1350.degree. C. and above under 
a high pressure sufficient to maintain a thermodynamically stabilized 
condition for the cubic system boron nitride.

DETAILED DESCRIPTION OF PREFERRED EXAMPLE EMBODIMENTS AND OF THE BEST MODE 
OF THE INVENTION 
The present invention will now be specifically described in connection with 
a case in which a hot-pressed hBN body is used as the raw material and 
magnesium boron nitride (Mg.sub.3 B.sub.2 N.sub.4) is used as an additive. 
First, an amount of Mg.sub.3 B.sub.2 N.sub.4 was added to the hBN with the 
temperature condition being changed and thus, a relation as shown in FIG. 
1 was established between the treatment temperature and the added amount 
to Mg.sub.3 B.sub.2 N.sub.4. 
Then, based on the result shown in FIG. 1, several materials containing 
different amounts of added Mg.sub.3 B.sub.2 N.sub.4 were prepared and were 
subjected respectively to a treatment under prescribed temperature and 
pressure conditions, whereby sintered bodies were obtained respectively. 
The thermal conductivity of each of the thus obtained sintered cBN bodies 
was measured and the relationship between the heat conductivity and the 
added amount of Mg.sub.3 B.sub.2 N.sub.4 was examined. FIG. 2 shows the 
results. FIG. 2 illustrates the discovery of the invention that the 
diffusion of very small amounts of the additive in the form of an alkaline 
earth metal boron nitride e.g. magnesium boron nitride yields a distinct 
peak in the thermal conductivity. About 0.6 mol % of additive, measured as 
a mol percentage of the hBN starting material, must be uniformly diffused 
into the starting hBN material in order to assure a thermal conductivity 
of about 4 W/cm..degree.C. in the final cBN sintered body. A thermal 
conductivity peak occurs at about 0.85 to 0.9 mol % of the additive. The 
thermal conductivity is back to about 4W/cm..degree. C. when the additive 
is about 1.3 mol %. 
From FIG. 2 it can be seen that adding an amount of Mg.sub.3 B.sub.2 
N.sub.4 of approximately 0.85 to 0.9 mol %, yields the best thermal 
conductivity. This feature of the invention is believed to be due to the 
fact that, as the amount of added Mg.sub.3 B.sub.2 N.sub.4 first 
increases, the unit particle diameter of cBN becomes larger, whereby 
phonon scattering in the particle boundary is decreased, which causes the 
heat conductivity to be high. FIG. 3 shows the relationship between the 
added amount of Mg.sub.3 B.sub.2 N.sub.4 and an average unit particle 
diameter of an obtained sintered body. From FIG. 3 it can be understood 
that the particle diameter becomes larger according to the increase of the 
added amount of Mg.sub.3 B.sub.2 N.sub.4. 
However, from FIG. 2 it can also be seen that adding more than the optimal 
amount of Mg.sub.3 B.sub.2 N.sub.4, namely more than 0.9 mol %, the 
resulting cBN sintered body contains Mg of more than 0.5 % by weight, as 
shown in FIG. 14. This remainder of Mg is considered to act as a 
determining cause of phonon scattering, causing the thermal conductivity 
to decrease again as is evident from FIG. 14, wherein, as in FIG. 2, the 
thermal conductivity is within the range of about 4 W/cm..degree.C. to 
about 6.2 W/cm..degree.C., whereby the lower limit is obtained when the 
metal remainder, here the Mg-remainder is either about 0.1 wt. % or about 
0.7 wt. %, while the peak of about 6.2 W/cm..degree.C. is obtained for a 
metal remainder of about 0.3 wt. % of the cBN sintered body. 
FIG. 4 shows the relationship between the heat conductivity of the 
respective sintered bodies with the added amount of Mg.sub.3 B.sub.2 
N.sub.4 being changed and the Vickers hardness thereof. It can be seen 
from FIG. 4 that a sintered body having a heat conductivity in a range 
from 2 to 3 W/cm..degree.C. has a Vickers hardness partially lowered and 
the sintered state thereof is not uniform. It is assumed that in the 
portion where the sintered state is not good, that is, in the portion 
where the hardness is lowered, phonon scattering in the particle boundary 
develops conspicuously due to an insufficient bonding of particles and as 
a result, the sintered body as a whole has a relatively low value of heat 
conductivity. On the other hand, a sintered body having a uniform hardness 
of 5000 kg/mm.sup.2 or more has a heat conductivity 4 W/cm..degree.C. or 
above. From the relationship between the added amount of Mg.sub.3 B.sub.2 
N.sub.4 and the heat conductivity shown in FIGS. 2, 14, it is understood 
that a sintered body having a high heat conductivity of at least 4 
W/cm..degree.C. or above can be stably obtained with the added amount of 
Mg.sub.3 B.sub.2 N.sub.4 being in a range from 0.6 mol % to 1.3mol %. 
Furthermore, it has also been discovered according to the invention that in 
a cBN sintered body, each of the following features contribute to an 
improved heat conductivity, namely: (a) the weight ratio of the cubic 
system boron nitride to the metal remainder in the sintered cBN body, (b) 
the lattice constant of the cubic boron nitride in the sintered body, and 
(c) the proportion of a substantial continuity between adjoining particles 
of the cubic system boron nitride particle surface. More specifically, as 
a result of measuring the heat conductivity while the weight ratio of the 
cubic system boron nitride to the metal remainder in the sintered cBN body 
is being increased to at least 99.3 wt. %, it was found that the heat 
conductivity of a sintered body becomes remarkably high, namely at least 4 
W/cm..degree.C. or higher. 
In addition, the sintering conditions were selected so that the lattice 
constant of the cubic system boron nitride may be changed from 3.608 .ANG. 
to 3.630 .ANG., and under these conditions, the heat conductivity was 
measured. As a result, the maximum value of the heat conductivity was 
found to be in the vicinity of 3.615 .ANG. and high values of at least 
4.0 W/cm..degree.C. were found in the range from 3.610 .ANG. to 3.625 
.ANG.. Accordingly, the lattice constant of the cBN in the cBN sintered 
body is preferably in a range from 3.610 .ANG. to 3.625 .ANG.. This is 
because a deviation from the stoichiometrical BN would increase lattice 
defects to cause phonon scattering. In such a case, if the amount of 
nitrogen is small, the lattice constant would be increased. 
According to the invention sintered bodies were produced while changing the 
sintering conditions and the binding state of the cubic boron nitride 
particles was observed by wrapping the sintered bodies thus obtained. 
Then, the heat conductivity values of the sintered bodies were measured 
and it was found that a sintered body having a substantial continuity of 
60% or more between adjoining particles has a high value heat conductivity 
of 4.0 W/cm..degree.C. or above. The reason for this is believed to be 
that the influence of phonon scattering in the particle boundary 
contributes to the heat conductivity. 
According to the present invention, a cBN sintered body obtained as 
described above exhibits a heat conductivity as high as 4.0 
W/cm..degree.C. and above and consequently, it becomes possible to provide 
a heat sink of an excellent quality for use in an electronic device. 
A sintered body having a heat conductivity of 4 W/cm..degree.C. or above 
has a particle diameter of 5/.mu.m or more, a Vickers hardness of 5000 
kg/mm.sup.2 or more and a content of Mg of not more than 0.7% by weight or 
less. This means that from the hardness and the impurity content, the heat 
conductivity of the sintered body can be known. 
Embodiments 
EXAMPLE 1 
Bodies of hBN were used as starting materials. These hBN bodies were 
obtained by forming hBN powder with a hot-press method or a cold isostatic 
pressurizing method. The hBN bodies so obtained were held in N.sub.2 -gas 
at 2000.about.2100.degree. C. for 4 hours to highly purify them. 
Impurities contained in the hBN bodies were not more than 0.05% by weight 
of O.sub.2, not more than 0.02% by weight of carbon and not more than 200 
ppm of metal impurities such as Fe, Ca or the like, and the density of the 
hBN bodies was in a range of 1.75.about.1.85 g/cm.sup.3. 
Under normal pressure in a nitrogen atmosphere at various temperatures in a 
range from 1160.degree. to 1175.degree. C., Mg3N.sub.2 was diffused into 
high purity hBN bodies described above and eight specimens having 
different amounts of Mg.sub.3 B.sub.2 N.sub.4 in a range from 0.4 to 1.3 
mol % were obtained as a result of reaction in the hBN body. These 
specimens were respectively put in recipients and subjected to a pressure 
of 5.5 GPa and a temperature of 1450.degree. C. for 30 minutes using a 
belt-type apparatus for a direct conversion of the hBN to cBN. This 
pressure and temperature are lower than in conventional methods not using 
the system with the additive as taught herein. 
The obtained cBN sintered bodies were monophase high density sintered 
bodies each having a diameter of 30 nun and a thickness of approximately 
1.5 nun. In the sintered body of specimen 1, the not converted hBN 
remained partial, while the other specimens appeared to be in a 
homogenously and strongly sintered state over the whole surface. In 
consequence, it is understood that for a 100% conversion of the hBN to 
cBN, the amount of Mg.sub.3 B.sub.2 N.sub.4 added to the raw material hBN 
body is required to be approximately 0.45 mol % or more. 
From the sintered bodies of specimens 2 to 8, pieces of 
2.5.times.2.5.times.1.5 mm were cut respectively and the heat conductivity 
of each piece was measured at room temperature by a direct measuring 
method using an InSb infrared radiation microscope. The results obtained 
are shown in Table 2 below. 
TABLE 2 
__________________________________________________________________________ 
Specimen No. 
Added Amount 
Appearance of Sintered Body 
Heat conductivity 
__________________________________________________________________________ 
1 0.439 mol % 
gray and not transparent 
not measured 
(partially white) 
2 0.498 mol % 
gray and not transparent 
1.7 W/cm .multidot. .degree.C. 
3 0.544 mol % 
Gray and not transparent 
2.1 W/cm .multidot. .degree.C. 
4 0.639 mol % 
green and semi-transparent 
3.8 W/cm .multidot. .degree.C. 
5 0.665 mol % 
bluish green and 
4.8 W/cm .multidot. .degree.C. 
semi-transparent 
6 0.909 mol % 
bluish green and 
6.2 W/cm .multidot. .degree.C. 
semi-transparent 
7 0.995 mol % 
brown and semi-transparent 
5.1 W/cm .multidot. .degree.C. 
8 1.291 mol % 
brown and semi-transparent 
4.0 W/cm .multidot. .degree.C. 
__________________________________________________________________________ 
It is understood from Table 2 that a sintered body obtained with an amount 
of Mg.sub.3 B.sub.2 N.sub.4 added to the material hBN body in a range from 
0.6 mol % to 1.3 mol %, has a heat conductivity of approximately 4 
W/cm..degree.C. or above, providing a maximum heat conductivity of 6.2 
W/cm..degree.C. for 0.909 mol % of the additive. 
EXAMPLE 2 
For the purpose of examining the characteristics required for a sintered 
body for the desired heat conductivity, the particle diameter, the 
hardness and the impurity content of each of the specimens 2 to 8 obtained 
in example 1 were measured. 
The particle diameter was measured by a scanning type electron microscope 
(SEM) after the surface of each specimen was etched with KOH to make clear 
the particle boundaries. 
The hardness was examined by using a Vickers indentor under the load of 10 
kg for 15 seconds. 
The content of impurity, that is the content of Mg was measured by using an 
ion microanalyzer. 
The results are shown in Table 3. 
TABLE 3 
______________________________________ 
Particle 
Diameter Mg Content 
Specimen No. 
(.mu.m) Vickers Hardness 
(% by weight) 
______________________________________ 
2 2.about.10 
2900.about.6300 
&lt;0.1 
3 2.about.12 
4100.about.6100 
&lt;0.1 
4 5.about.12 
5300.about.6100 
0.1 
5 7.about.15 
6100.about.7100 
0.1 
6 10.about.17 
5300.about.6100 
0.3 
7 12.about.20 
5000.about.6100 
0.5 
8 12.about.27 
6100.about.6900 
0.7 
______________________________________ 
Impurities other than Mg were analyzed by using an ion microanalyzer for 
each specimen and C, Al, Ca and Si were detected, the amount of each of 
them being less than 200 ppm. 
From Example 1 and the results shown in Table 3, it is understood that a 
sintered body having a heat conductivity of 4W/cm..degree.C. or above, has 
a unit particle diameter of 5 .mu.m or more, a Vickers hardness of 5000 
Kg/mm.sup.2 or more, and an Mg content of 0.7 % by weight or less. 
EXAMPLE 3 
Instead of the Mg.sub.3 B.sub.2 N.sub.4 used in Example 1, Li.sub.3 
BN.sub.2, Ca.sub.3 B.sub.2 N.sub.4 and Sr.sub.3 B.sub.2 N.sub.4 were 
respectively diffused and contained in an amount of from 0.6 to 1.2 mol % 
in the hBN and sintered bodies were obtained in the same manner as in 
Example 1. In all of the sintered bodies thus obtained, the heat, 
conductivity was 4W/cm..degree.C. or above and the particle diameter, the 
hardness and the impurity content were almost the same as the results of 
Example 2. 
EXAMPLE 4 
hBN bodies containing 0.8.about.0.9 mol % of Mg.sub.3 B.sub.2 N.sub.4 were 
obtained in the same manner as in Example 1. Thereafter, the body was put 
in a recipient and using a belt-type apparatus, six sintered bodies were 
formed under the conditions of 5.5 GPa and 1400.degree. C., with the 
holding time being changed. The proportions of transformation to the cubic 
system boron nitride in the obtained sintered bodies were respectively 
60%, 80%, 90%, 95%, 98% and 100%. The remaining portions were compounds of 
the not transformed hexagonal system boron nitride and the catalyst or 
additive. 
The heat conductivity of each of the above stated sintered bodies was 
measured and the results are shown in Table 4 below. 
From Table 4, it is understood that a sintered body having a volume ratio 
of cubic system boron nitride of 98% or more has a heat conductivity as 
high as 4.0 W/cm..degree.C. or above. 
TABLE 4 
__________________________________________________________________________ 
No. 
Items 1 2 3 4 5 6 
__________________________________________________________________________ 
Sintering Pressure 
55 kb .fwdarw. 
.fwdarw. 
.fwdarw. 
.fwdarw. 
.fwdarw. 
Sintering Temperature 
1400.degree. C. 
.fwdarw. 
.fwdarw. 
.fwdarw. 
.fwdarw. 
.fwdarw. 
Holding Time 
2 min. 
5 min. 
10 min. 
15 min. 
20 min. 
30 min. 
Transformation Ratio 
60% 80% 90% 95% 98% 100% 
Heat Conductivity 
0.2 0.4 0.9 1.8 4.1 5.5 
__________________________________________________________________________ 
EXAMPLE 5 
As a raw material, a hexagonal system boron nitride body containing 
uniformly dispersed 0.8 to 0.9 mol % of Mg.sub.3 B.sub.2 N.sub.4 was 
prepared, and the material thus prepared was put in a recipient of Mo. 
Then, the material was subjected to a treatment under the conditions of 
5.7 GPa and 1450.degree. C. with the temperature being raised by two steps 
and sintered bodies were obtained. By changing the holding time in the 
first step, the lattice constants of the cubic system boron nitride of the 
sintered bodies were made different. 
Table 5 shows the measured heat conductivity values of these sintered 
bodies. 
From Table 5 it is understood that a high value of heat conductivity of 4.0 
W/cm..degree.C. or above was obtained if the lattice constant of the cubic 
system boron nitride is in a range from 3.610 .ANG. to 3.625 .ANG.. 
It is to be noted that the sintering time shown in Table 5 was 20 minutes 
in each case. 
TABLE 5 
______________________________________ 
Specimen No. 
Items 1 2 3 4 5 
______________________________________ 
Sintering Pressure 
57 kb .fwdarw. 
.fwdarw. 
.fwdarw. 
.fwdarw. 
Sintering 1450.degree. C. 
.fwdarw. 
.fwdarw. 
.fwdarw. 
.fwdarw. 
Temperature in the 
Second Step 
Raised Temperature 
1250.degree. C. 
.fwdarw. 
.fwdarw. 
.fwdarw. 
.fwdarw. 
in the First Step 
Holding Time in the 
1 min. 10 min. 20 min. 
40 min. 
60 min. 
First Step 
Lattice Constant .ANG. 
3.630 3.624 3.617 3.613 3.608 
Heat Conductivity 
2.0 4.3 5.9 6.0 3.4 
W/cm .multidot. .degree.C. 
______________________________________ 
EXAMPLE 6 
As a starting material a hexagonal system boron nitride body containing 
uniformly dispersed 0.8 0.9 mol % of Mg.sub.3 B.sub.2 N.sub.4 was prepared 
and this material was put in a recipient. Then, using a belt-type 
apparatus, the temperture was raised by two steps under the conditions of 
5.5 GPa and 1450.degree. C., whereby sintered bodies were formed with the 
holding time in the second step being changed in this example. The 
sintered bodies thus obtained were polished with fine diamond powder and 
then the particle boundary was checked by using a scanning type electron 
microscope. As a result it was found that the cubic system boron nitride 
particles were substantially continuous with the surrounding adjacent 
particles and no particle boundary was observed. 
In addition, the relationship between an average value of the surface 
proportion of continuity and the heat conductivity was examined and the 
result is shown in Table 6 below. 
TABLE 6 
______________________________________ 
Specimen No. 
Items 1 2 3 4 5 
______________________________________ 
Proportion of Continuity 
40% 50% 60% 80% 85% 
Heat Conductivity 
2.3 2.9 4.0 4.5 6.1 
W/cm .multidot. .degree.C. 
______________________________________ 
As described above, U.S. Pat. No. 4,188,194 discloses a metallization 
technique in which the surface of cubic boron nitride is coated with Ni by 
sputtering and further coated with silver. However, such coating films 
have the disadvantage that the adhesion to the cubic system boron nitride 
sintered body is insufficient and accordingly, they are liable to be 
detached at the time of inserting the heat sink in a substrate. By 
contrast, in a heat sink using a cBN sintered body of the present 
invention, the above stated disadvantages have been avoided and coating 
films having an excellent adhesion property can be provided. More 
specifically, a sintered body of the present invention may be provided on 
its surface with coating layers of one or more transition metals of the 
groups IVa, Va and VIa of the periodic table or the alloys thereof. In 
this case, boride, nitride or boron nitride of such metals is formed by a 
reaction of the metal with the cBN in the interface, said reaction 
resulting in a chemical reaction bond providing a strong adherence of the 
coating films to the cBN body. Copper or a copper alloy is useful for 
forming such a coating film, especially in a heat sink. 
In addition, in the present invention, one or more coating layers of metals 
not oxidized at a temperature of 300.degree. C. or alloys thereof may be 
provided for preventing oxidation of the metal of the coating films. As 
the metals which are not oxidized at a temperature of 300.degree. C., Au, 
Ag, Cu, Pt, Al, Ni, Co and the like are preferred. The temperature of 
300.degree. C. is based on the fact that the maximum temperature necessary 
for adhesion between a semiconductor device and lead wires, between the 
semiconductor device and the sink, and between the heat sink and the 
substrate is approximately 300.degree. C. 
EXAMPLE 7 
The sintered body of specimen 4 in Example 5 was cut into 20 pieces each 
having a size of 1 mm.sup.2 and 0.4 mm thickness. After that, the pieces 
were heated to a high temperature and Ti was coated to a thickness of 500 
.ANG. on the surface of each piece. Then, each piece was further coated to 
a thickness of 1 .mu.m of the Ti coating layer. Then, a semiconductor 
laser device was soldered on each sintered body thus formed and leads were 
connected. 
For comparison, a BeO body was cut into 20 pieces each having the same size 
as described above and in the same manner a coating was applied and 
semiconductor laser devices of the same kind were placed on the specimens. 
When an electric current of 150 mA was caused to flow through the device, 
the surface temperature of each device was measured. As a result, it was 
observed that the temperature of the devices each provided with a heat 
sink comprising a cubic system boron nitride sintered body in accordance 
with the present invention was lowered by approximately 15.degree. C. 
In addition, in a scratching test using a sapphire needle, some of the 
coating films of the heat sinks of BeO were detached, while the coating 
films on the sintered bodies in accordance with the present invention were 
not detached. 
Furthermore, the interface between the cBN sintered body and a coating film 
was examined by a TEM and there was found titanium boride. 
EXAMPLE 8 
The sintered body of specimen 5 in Example 5 was cut into four pieces each 
having a size of 100 mm.sup.2 and 0.5 mm in thickness. 
Then, the pieces were heated and Cr was coated on the surface of each piece 
and Ni and Ag were further coated thereon. Each of the sintered bodies 
containing the coating films thus formed was attached to an IC substrate. 
As a result, it was observed that the IC operated more stably and the 
lifetime of the IC became remarkably long. 
Now, in the following, a concrete structure of a heat sink for an IC using 
a sintered body in accordance with the present invention will be 
described. 
A sintered body of the invention is shaped such that the area of the outer 
surface of the sintered body may be relatively larger than the surface 
thereof facing the semiconductor device for improving the heat radiation 
property. For the shaping there are the following three possibilities: 
(a) The surface area of the heat sink on the side of an envelope is made 
larger than the area of the portion of the heat sink contacting the IC 
device so that the heat content per unit area is lowered on the side of 
the envelope. Thus, the temperature of the heat sink material can be made 
close to the temperature of the external atmosphere and accordingly, the 
temperature of the IC device can be lowered effectively. 
(b) The surface area of the heat sink in the portion contacting the 
external gas or liquid is made large so that the heat conduction to the 
exterior is improved. 
(c) Radiation fins are provided in the portion of the heat sink contacting 
the external gas or liquid so that the heat radiation property is further 
improved. 
The above described heat sink uses a cBN sintered body having an excellent 
heat conductivity and being shaped such that the area of the outer surface 
thereof is relatively large compared with the surface thereof on the side 
facing the semiconductor device, whereby heat can be emitted efficiently 
from the inside of the device to the exterior not only because of the good 
heat conductivity of the material itself but also because of its 
structure. 
FIGS. 5A and 5B are respectively plane and side sectional views of the 
first example of a heat sink in accordance with the present invention. 
Referring to FIGS. 5A and 5B, a Si device or a compound semiconductor 2 as 
an IC device is disposed in an envelope 1 and input and output pins 4 are 
provided through leads 3 connected to the Si device 2. 
On the other hand, in the envelope 1 on the side of the Si device 2, a 
tapered opening is formed, the diameter thereof being small on the inner 
side and increasing toward the outer surface of the envelope 1. In 
accordance with an embodiment of the present invention, a heat sink 5 for 
an IC device is placed in the opening. As is shown in FIG. 5B, the heat 
sink 5 has a form corresponding to the tapered opening and accordingly the 
surface area thereof on the side of the Si device 2 as the semiconductor 
device is small and the surface area thereof on the outer side of the 
envelope 1, that is on the side contacting the external atmosphere, is 
increased. Thus, as described above, the heat content per unit area of the 
heat sink is smaller on the outer side than on the side of the Si device 2 
and radiation of heat becomes more efficient. 
FIGS. 6A and 6B are respectively plane and side sectional views of an IC 
device where the second example of a heat sink in accordance with the 
present invention is applied. In this example, the envelope 1 and the heat 
sink 5 are formed in the direction in which the input and output pins 4 
extend. The other portions are the same as in the example shown in FIGS. 
5A and 5B. Accordingly, it is understood that heat is radiated efficiently 
also in the heat sink 5 of this example. 
FIGS. 7A and 7B are respectively plane and side sectional views of an IC 
device where the third example of a heat sink in accordance with the 
present invention is applied. In this third example, a through hole not 
tapered is formed in the envelope and the heat sink 5 for the IC is placed 
in the through hole. However, the heat sink 5 in this example extends on 
the outer side of the envelope 1 over a range wider than the diameter of 
the through hole. More specifically, the heat sink 5 is provided over an 
area larger than the opening area so as to be in contact with the outer 
surface of the envelope 1. Accordingly, also in this example, in the same 
manner as in the example shown in FIGS. 5A and 5B, the surface area of the 
heat sink 5 on the external side is made larger than that on the side of 
the Si device 2 and the same improved heat radiation effect can be 
obtained. 
FIGS. 8A and 8B are respectively plane and side sectional views of a device 
where the fourth example of a heat sink in accordance with the present 
invention is applied. In this fourth example, in the same manner as in the 
heat sink 5 of the example shown in FIGS. 6A and 6B, the envelope 1 and 
the heat sink 5 for the IC are provided in the direction in which the 
input and output pins 4 extend. The structure of the envelope 1 and heat 
sink 5 are the same as those in the third example shown in FIGS. 7A and 7B 
and therefore, the same effect can be obtained. 
FIGS. 9A and 9B are plane and side sectional views of a device where the 
fifth example of a heat sink in accordance with the present invention is 
applied. In this fifth example, radiation fins 6 are formed on the 
external side of the heat sink 5. The other portions are the same as in 
the example shown in FIGS. 7A and 7B. It is understood that the example 
shown in FIGS. 9A and 9B has a further improved radiation characteristic 
compared with the third example since the radiation fins 6 are provided in 
this fifth example. 
The form of the radiation fins 6 is not limited to that shown in FIGS. 9A 
and 9B. They may have any suitable form as shown in the sixth to eighth 
examples in FIGS. 10A-10B, 11A-11B and 12A-12B. In any of these forms, the 
area of the fins 6 can be made larger than the outer surface area of the 
heat sink 5 and accordingly, the heat radiation characteristic can be 
further improved. 
Now, an example of a method of manufacturing a heat sink for an IC in 
accordance with the present invention will be described with reference to 
FIG. 13 shown partially in section. As shown in FIG. 13, in a girdle-type 
extra-high pressure generating apparatus, cBN bodies 11 containing 
diffused magnesium boron nitride are disposed respectively between the 
ultra-hard alloy plates 12 and the Mo plates 13 of 0.1 mm in thickness 
shaped according to the form of the radiation fins so that a multilayer 
structure is formed. The thus formed multilayer structure is placed in a 
recipient 14 of Mo. Then, under a pressure of 5.5 GPa, the Mo recipient 14 
is heated at a temperature of 1450.degree. C. for 30 minutes. After that, 
the sintered bodies 11 are taken out from the Mo recipient 14. In advance, 
the respective surfaces of the Mo plates 13 disposed between the 
respective sintered bodies 11 have been coated with a peeling agent so 
that the multilayer sintered bodies 11 may be easily separated from one 
another. The sintered bodies 11 taken out of the recipient are separated 
with the Mo plates 13 being attached thereto. The Mo plate 13 attached to 
the surface of each sintered body 11 is removed for example by a treatment 
using an acid such as nitric acid, whereby the same form as that of the 
radiation fins 6 of the heat sink 5 in the example shown in FIGS. 12A and 
12B can be obtained. 
On the face portion of contact between the thus obtained sintered body 11 
and the IC device, a multilayer coating film of Ti and Ni and Au is formed 
and this coating film is soldered by Au-Sn to the IC logic device, so that 
the structure thus formed is fixed to the envelope whereby the IC device 
shown in FIGS. 12A and 12B can be obtained. For comparison, an IC device 
as shown in FIGS. 12A and 12B was manufactured using a heat sink of BeO 
and the performances of the heat sinks of both IC devices were tested. In 
this test, the same electric power and the same electric current were 
applied and the temperatures on the surface of the envelopes of both IC 
devices were measured. As a result, it was ascertained that the device 
using the heat sink of the present invention had a lower temperature and 
exhibited improved characteristics when it was operated. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.