Silicon nitride sinter having high thermal conductivity and process for preparing the same

A high thermal conductive silicon nitride base sintered body which comprises a phase comprising crystal grains of silicon nitride and a grain boundary phase containing a compound of at least one element selected from the group consisting of yttrium and the lanthanide elements in an amount of 1 to 20% by weight in terms of oxide amount, and contains free silicon dispersed therein in an amount of 0.01 to 10% by weight based on the whole. This high thermal conductive silicon nitride base sintered body has high strength coupled with high thermal conductivity and thus is useful not only as various parts for semiconductor devices, such as radiating insulating substrates, but as various structural parts for machines, OA apparatuses, etc.

TECHNICAL FIELD 
This invention relates to an Si.sub.3 N.sub.4 base sintered body which is 
useful not only as various parts for use in semiconductor devices, 
including insulating substrates and various radiating plates, but also as 
various structural parts for motor vehicles, machines, OA apparatuses, 
etc. and is excellent in productivity and especially in mechanical 
strength and radiating properties. This invention also relates to a 
process for producing the sintered body. 
BACKGROUND ART 
Ceramics comprising silicon nitride as the main component are superior in 
heat resistance, mechanical strength, and toughness to other ceramic 
materials, and are materials suitable for various structural parts such as 
automotive parts and OA apparatus parts. Attempts are being made to use 
them as insulating radiating substrates for semiconductor devices, etc. so 
as to take advantage of their high insulating properties. 
Alumina and the like have conventionally been used extensively as ceramic 
substrates for semiconductors. However, with the trend toward higher 
speeds, higher degrees of integration, and higher outputs in semiconductor 
devices, materials having higher thermal conductivity and excellent 
radiating properties have come to be desired and the application of AlN 
and SiC has progressed. However, no high thermal conductive substrate has 
been obtained so far which is made of AlN or the like and is sufficient in 
strength and toughness, and the current substrates have drawbacks in 
product handling and shape because of breakage caused by external force, 
etc. There is hence a desire for the development of a ceramic material 
combining high-strength properties which enable the material to withstand 
external force with excellent radiating properties. 
Silicon nitride (Si.sub.3 N.sub.4), which intrinsically has high strength, 
is expected to be used as insulating radiating substrates if its thermal 
conductivity can be improved. However, since the conventionally known 
silicon nitride sintered bodies have lower thermal conductivities than AlN 
and SiC, they have not been put to practical use as an insulating 
radiating substrate. 
The thermal conductivity of insulating ceramics such as silicon nitride is 
mainly attributable to the transmission of phonons. Since phonons are 
scattered by phases having different impedances, such as lattice defects 
and impurities, present in the sintered body, the thermal conductivity 
.kappa. is defined by the following numerical formula 1: 
EQU .kappa.=c.times.V.times.l/3 (Numerical 
formula 1) 
(wherein c is specific heat capacity; V is average velocity of phonons; and 
l is the mean free path of phonons). 
The specific heat capacity c and the group velocity V in numerical formula 
1 each is a number which varies from material to material and can be 
regarded as almost the same in the same material. Consequently, the 
thermal conductivity of silicon nitride crystal grains is governed 
substantially by the mean free path of phonons. For example, when AlN or 
Al.sub.2 O.sub.3, which have conventionally been used generally, is added 
as a sintering aid, then aluminum ions or oxygen ions form a solid 
solution in Si.sub.3 N.sub.4 crystal grains and thus scatter phonons, 
resulting in a reduced thermal conductivity. Because of this, general 
silicon nitride base sintered bodies to which Al.sub.2 O.sub.3, AlN, 
Y.sub.2 O.sub.3, or the like has been added have a thermal conductivity as 
low as about 15 W/m.multidot.k. 
Various investigations have hence been made in order to obtain a silicon 
nitride base sintered body having a high thermal conductivity. For 
example, the thermal conductivity of a silicon nitride base sintered body 
obtained through an HIP treatment after the addition of Y.sub.2 O.sub.3 
and Al.sub.2 O.sub.3 in combination as a sintering aid is discussed in 
"Paper Journal of Ceramics Society of Japan)," Vol.97 (1989), No.1, 
pp.56-62. The result given therein is that the thermal conductivity of the 
sintered body becomes higher as the proportion of .beta.-form crystal 
grains increases or as the proportions of Y.sub.2 O.sub.3 and Al.sub.2 
O.sub.3 in the sintering aid increases and decreases, respectively. There 
is a description in the paper, section 4.2 to the effect that high thermal 
conductivity is obtained by .beta.-form crystal grains because .beta.-form 
crystal grains have a larger mean free path of phonons than .alpha.-form 
crystal grains. 
It is therefore important for heightening the thermal conductivity of a 
silicon nitride base ceramic to accelerate the formation of .beta.-form 
Si.sub.3 N.sub.4 crystal grains, to use a rare earth element compound such 
as Y.sub.2 O.sub.3, which is regarded as less apt to form a solid solution 
in the crystal grains, and to diminish the addition of an aluminum 
compound containing aluminum ions, which are apt to form a solid solution 
in the crystal grains. 
For example, Japanese Patent Laid-Open Nos. 175268/1992 and 219371/1992 
show a case in which a dense silicon nitride base sintered body having a 
thermal conductivity of 40 W/m.multidot.k or higher and consisting of 
.beta.-form Si.sub.3 N.sub.4 crystal grains was obtained by using a 
.beta.-form Si.sub.3 N.sub.4 powder reduced in the contents of oxygen and 
cationic impurities so as to diminish the amounts of cationic impurities 
such as aluminum and oxygen, which form a solid solution in Si.sub.3 
N.sub.4 crystal grains, and by additionally adding a compound of, e.g., a 
Group 4A element when a colored sintered body was to be obtained. 
Japanese Patent Laid-Open No. 30866/1997 discloses a dense silicon nitride 
base sintered body having a thermal conductivity of 80 W/m.multidot.k or 
higher and a flexural strength of 600 MPa or higher which is obtained by 
adding a compound of an alkaline earth/rare earth element and conducting 
sintering in high-pressure nitrogen gas at a relatively high temperature 
around 2,000.degree. C. to thereby heighten the proportion of large 
.beta.-form crystal grains having a minor diameter of 5 .mu.m or larger. 
Japanese Patent Laid-Open Nos. 135771/1994 and 48174/1995 disclose a method 
for obtaining a dense silicon nitride base sintered body consisting of 
.beta.-form crystal grains which comprises adding an appropriate amount of 
aluminum ions together with a rare earth element compound and a Group 4A 
element compound, without limiting the amount of aluminum ions, and 
gradually cooling the shape after sintering to thereby accelerate 
crystallization in the grain boundary phase. There is a description in 
these patent documents to the effect that an Si.sub.3 N.sub.4 base 
sintered body having a flexural strength of 800 MPa or higher and a 
thermal conductivity of 60 W/m.multidot.k or higher is obtained. 
On the other hand, Japanese Patent Laid-Open Nos. 149588/1995, 319187/1996, 
and 64235/1997 disclose: a metallized substrate comprising a silicon 
nitride base and formed thereon a high-melting metallizing layer made of 
tungsten or molybdenum; and a semiconductor module comprising the 
substrate and a conductor circuit bonded thereto. Japanese Patent 
Laid-Open No. 187793/1995 discloses various semiconductor devices 
containing a similar metallized substrate and various structural members 
comprising the same silicon nitride base sintered body. The above 
high-melting metallizing layer is one formed on the base through an oxide 
film made of SiO.sub.2, a layer of one or more Group 4A metals or of a 
brazing material containing these, or through a Cu--Cu.sub.2 O eutectic 
layer, and has a peel strength of 3 kgf/mm.sup.2 or higher. 
As described above, it is important in the conventional methods to use a 
high-purity Si.sub.3 N.sub.4 powder reduced in the contents of oxygen and 
cationic impurities and to add an appropriate kind of sintering aid in an 
appropriate amount in order to inhibit oxygen ions and cationic impurities 
from forming a solid solution in crystal grains. Namely, in order to 
obtain crystal grains in which impurities or defects have been diminished, 
the purity of the grains should be increased by using expensive 
high-purity powder feedstocks as the main and minor ingredients and 
causing grain growth at a high temperature and a high pressure. For 
example, as described in Japanese Patent Laid-Open No. 30866/1997, cited 
above, it is necessary to employ a method in which high-purity .beta.-form 
silicon nitride is used as a feedstock powder and grain growth is caused 
at a high temperature and a high pressure (2,000.degree. C., 300 atm). 
DISCLOSURE OF INVENTION 
As described above, improvements for obtaining a silicon nitride base 
sintered body having high strength and high thermal conductivity have 
hitherto been made by properly controlling a feedstock powder and a 
sintering aid. However, merely selecting a proper feedstock powder and a 
proper sintering aid not only results in increased feedstock and process 
costs but also is limited in further improving the thermal conductivity of 
a silicon nitride base sintered body. 
There is hence a desire for another means for further reducing the amount 
of impurities, especially the amount of oxygen, contained in Si.sub.3 
N.sub.4 crystal grains. However, it is not easy to inexpensively obtain a 
silicon nitride powder reduced in oxygen content. In addition, since 
general silicon nitride powders on the market have an oxygen content of at 
least 0.7 to 1.0% by weight, the thermal conductivities of the silicon 
nitride base sintered bodies obtained from such inexpensive commercial 
powder feedstocks have been limited to about 70 W/m.multidot.k. 
Even when a high-purity .beta.-form silicon nitride powder having a 
relatively low oxygen content is used, high-temperature high-pressure 
sintering is necessary for obtaining high thermal conductivity. This 
method is therefore inferior in productivity because of exceedingly high 
feedstock and production costs, etc., and is disadvantageous in that the 
sintered body is apt to have a reduced strength and poor suitability for 
practical use since the treatment conducted at a temperature as high as 
around 2,000.degree. C. is accompanied with considerable grain growth. 
In view of such prior art circumstances, an object of this invention is to 
provide a silicon nitride base sintered body having excellent productivity 
and high strength and simultaneously having high thermal conductivity not 
possessed by any conventional silicon nitride base sintered body and to 
provide a process for producing the sintered body. 
The silicon nitride base sintered body provided by this invention in order 
to accomplish the above object is characterized by comprising a phase 
comprising crystal grains of .beta.-form silicon nitride and a grain 
boundary phase containing a compound of at least one element selected from 
the group consisting of yttrium and the lanthanide elements in an amount 
of 1 to 20% by weight in terms of oxide amount, and by containing free 
silicon dispersed in the crystal grains of silicon nitride in an amount of 
0.01 to 10% by weight based on the whole. The sintered body combines high 
strength and high thermal conductivity. As used herein throughout the 
specification and claims, the expression "lanthanide elements" is intended 
to include elements of atomic numbers 57 through 71. 
The high thermal conductive silicon nitride base sintered body of this 
invention can contain a compound of at least one element selected among 
the Group 4A elements in an amount of 0.01 to 3% by weight in terms of 
element amount and/or contain a compound of at least one element selected 
from the group consisting of calcium and lithium in an amount of 0.1 to 5% 
by weight in terms of oxide amount. In this high thermal conductive 
silicon nitride base sintered body, the amount of oxygen contained in the 
crystal grains of silicon nitride is preferably 0.6% by weight or smaller. 
The process for producing the above-described high thermal conductive 
silicon nitride base sintered body of this invention is characterized by 
comprising: a mixing step in which a silicon powder in an amount of 99 to 
80% by weight in terms of Si.sub.3 N.sub.4 is mixed with 1 to 20% by 
weight powder of a compound of at least one element selected from the 
group consisting of yttrium and the lanthanide elements; a molding step in 
which the powder mixture is molded; a nitriding step in which the 
resultant compact is heated in an atmosphere containing nitrogen at 1,200 
to 1,400.degree. C. to nitride the same until the amount of free silicon 
is reduced to 0.01 to 10% by weight based on the whole; and a sintering 
step in which the nitrided body is sintered by heating in an atmosphere 
containing nitrogen at 1,600 to 2,000.degree. C. 
In the nitriding step in the process of this invention, the compact is 
preferably heated at a rate of 0.3 to 0.5.degree. C./min in the 
temperature range of from 1,200 to 1,300.degree. C. and then heated in the 
temperature range of from 1,300 to 1,400.degree. C. In the mixing step, it 
is preferred to add a powder of a compound of at least one element 
selected among the Group 4A elements or use a feedstock powder containing 
the Group 4A element so that the amount of the Group 4A element is 0.01 to 
3% by weight based on the whole. Furthermore, a powder of a compound of at 
least one element selected between lithium and calcium can be added in an 
amount of 1 to 5% by weight in terms of oxide amount based on the whole. 
BEST MODE FOR CARRYING OUT THE INVENTION 
In this invention, it has become possible to obtain defect-free high-purity 
Si.sub.3 N.sub.4 crystal grains by a new method in which a silicon powder 
which can be easily available with a high-purity is used as the main 
feedstock powder and nitrided into Si.sub.3 N.sub.4 while leaving an 
adequate amount of free silicon. Hence, an inexpensive silicon nitride 
base sintered body combining high thermal conductivity with mechanical 
strength can be obtained. 
Namely, in the process of this invention, a silicon feedstock powder is 
mixed with 1 to 20% by weight compound of at least one rare earth element 
selected from the group consisting of yttrium and the lanthanide elements, 
and a compact of the resultant mixture is nitrided at 1,200 to 
1,400.degree. C. to obtain a nitrided body comprising high-purity Si.sub.3 
N.sub.4 crystal grains containing 0.05 to 10% by weight free silicon. 
Thereafter, this nitrided body is sintered at 1,600 to 2,000 C. to thereby 
obtain a silicon nitride base sintered body having high strength and high 
thermal conductivity. 
Compared especially to the case where nitrided bodies are produced from 
Si.sub.3 N.sub.4, powders obtained through a pulverization step, such as 
commercial ready-made nitrided Si.sub.3 N.sub.4 powders, the process of 
this invention can yield a nitrided body comprising Si.sub.3 N.sub.4 
crystal grains reduced in defects such as dislocations. Furthermore, due 
to the nitriding method in which free silicon is left in the nitrided 
body, an increase in the purity. of the Si.sub.3 N.sub.4 crystal grains 
(diminution of oxygen and defects) in the subsequent sintering step can be 
easily attained. As a result, a silicon nitride base sintered body having 
a greatly improved thermal conductivity and a high strength is obtained. 
The high thermal conductive silicon nitride base sintered body of this 
invention is constituted substantially of .beta.-form silicon nitride and 
0.01 to 10% by weight free silicon finely dispersed in crystal grains of 
the silicon nitride, and contains as a grain boundary phase at least one 
rare earth element in an amount of 1 to 20% by weight in terms of oxide 
amount. 
The amount of the free silicon dispersed in the silicon nitride crystal 
grains is 0.01 to 10% by weight, preferably 0.01 to 5% by weight, based on 
the whole sintered body. The dispersed silicon particles are desirably 
fine particles specifically having a maximum diameter of 3 .mu.m or 
smaller. If the amount of the dispersed silicon particles is below 0.01% 
by weight, the sintered body has reduced thermal conductivity. If the 
amount thereof exceeds 10% by weight, the sintered body has reduced 
flexural strength and reduced heat resistance. The reason why the silicon 
nitride grains should be .beta.-form is that the .beta.-form is reduced in 
crystal strain and in phonon scattering as compared with the .alpha.-form 
and hence has excellent thermal conductivity. 
The grain boundary phase contains a compound of at least one rare earth 
element selected from the group consisting of yttrium and the lanthanide 
elements, and its content is 1 to 20% by weight in terms of oxide amount 
based on the whole sintered body. If the content of the compound is below 
1% by weight, the nitriding reaction proceeds insufficiently, making it 
difficult to regulate the amount of free silicon to a target value. If the 
content thereof exceeds 20% by weight, a liquid phase is present in excess 
during sintering, resulting in a sintered body reduced in both thermal 
conductivity and flexural strength. Either case is hence undesirable. 
Especially preferred rare earth elements are those having an ionic field 
strength [(valence/(ionic radius).sup.2 ] of 0.54 or higher, e.g., 
samarium, yttrium, ytterbium, gadolinium, dysprosium, and erbium. 
The grain boundary phase can contain a compound of at least one element 
selected among the Group 4A elements, besides the rare earth element 
compound. Due to the addition of at least one Group 4A element compound, 
the thermal conductivity of the sintered body can be further improved. 
This is because the Group 4A element compound is thought to function to 
enable the yielded Si.sub.3 N.sub.4 crystal grains to be considerably 
reduced in impurity amount and in the amount of crystal strain 
attributable to impurities. The content of Group 4A element compounds is 
preferably 0.01 to 3% by weight in terms of element amount based on the 
whole. If the content thereof is below 0.01% by weight, the effect of 
further heightening thermal conductivity cannot be obtained. If the 
content thereof exceeds 3% by weight, there are cases where a mechanical 
strength sufficient for practical use cannot be obtained. 
The grain boundary phase may contain at least one element selected from the 
group consisting of calcium and lithium in an amount of 0.1 to 5% by 
weight in terms of its oxide. These elements improve suitability for 
sintering and contribute to densification in low-temperature sintering. 
The reason for the above amount range is that calcium or lithium contents 
below 0.1% by weight are ineffective in improving suitability for 
sintering, while contents thereof exceeding 5% by weight result in a 
sintered body reduced in mechanical strength. 
The amount of oxygen contained in the Si.sub.3 N.sub.4 crystal grains in 
the silicon nitride base sintered body is desirably 0.6% by weight or 
smaller, preferably 0.3% by weight or smaller. By reducing the oxygen 
amount to such a low value, even higher thermal conductivity can be stably 
obtained. 
The process for producing a silicon nitride base sintered body of this 
invention is explained next. First, in the mixing step in the process of 
this invention, a weighed amount of a silicon powder as the main 
ingredient is mixed with a weighed amount of a powder of a compound of at 
least one rare earth element selected from the group consisting of yttrium 
and the lanthanide elements as a minor ingredient. The amount of the 
silicon powder is 99 to 80% by weight in terms of Si.sub.3 N.sub.4, while 
that of the minor ingredient powder is 1 to 20% by weight in terms of 
oxide amount. The mixing may be conducted by a known method. 
The silicon powder for use as the main ingredient has an intraparticulate 
oxygen content(oxygen content in silicon particles) of desirably 0.6% by 
weight or lower, preferably 0.3% by weight or lower. This is because if a 
silicon powder having an intraparticulate oxygen content exceeding 0.6% by 
weight is used, the oxygen content in the Si.sub.3 N.sub.4 crystal grains 
obtained through the later nitriding step increases and such an increased 
oxygen content becomes an obstacle to higher thermal conductivity. The 
average particle diameter of the silicon powder is desirably 20 .mu.m or 
smaller, preferably 5 .mu.m or smaller. This is because if the average 
particle diameter thereof exceeds 20 .mu.m, there is a possibility that 
nitriding might proceed insufficiently in the nitriding step. 
The kind and proportion of the minor ingredients which respectively are 
within the ranges specified above are intended to enable the target 
Si.sub.3 N.sub.4 base sintered body to have a structure containing free 
silicon finely dispersed therein in an amount of 0.01 to 10% by weight and 
to thereby have an improved strength and improved thermal conductivity. In 
particular, when a compound of an element having a high ionic field 
strength (0.54 or higher), e.g., yttrium, samarium, or ytterbium, is 
added, the element combines with free oxygen ions in the SiO.sub.2 film 
present on the silicon powder surface to inhibit the oxygen from forming a 
solid solution in the Si.sub.3 N.sub.4 crystal grains. The addition of 
such a compound is hence preferred for enhancing thermal conductivity. If 
the total amount of the minor ingredient is below 1% by weight, the 
nitriding reaction does not proceed sufficiently and unnitrided silicon 
remains in excess. The resultant silicon agglomerates serve as sites from 
which breakage occurs, resulting in considerably reduced strength 
properties. If the amount thereof exceeds 10% by weight, a grain boundary 
phase is formed in an excess amount, resulting in a reduced strength and 
reduced thermal conductivity. 
Besides these powders of the main and minor ingredients, a compound of at 
least one Group 4A element, e.g., titanium, zirconium, or hafnium, may be 
optionally incorporated in an amount of 0.01 to 3% by weight in terms of 
element amount based on the whole, by adding a powder of the compound or 
using a feedstock containing the compound as an impurity. These Group 4A 
elements are effective in improving thermal conductivity as long as they 
are used in an amount within the above range. 
It is also possible to add a powder of a compound of lithium and/or 
calcium, especially of the oxide(s), in an amount of 0.1 to 5% by weight 
based on the whole. This addition can improve the sintering properties 
without influencing thermal conductivity, and is especially effective in 
strength enhancement through low-temperature sintering. Lithium does not 
form a solid solution in Si.sub.3 N.sub.4 crystal grains because it 
volatilizes during sintering, while calcium also is less apt to form a 
solid solution in the crystal grains because it has a large ionic radius. 
Consequently, the sintered body can retain the excellent thermal 
conductivity. 
In the molding step as the second step, the mixed feedstock powder obtained 
is molded to obtain a compact in a given shape. An ordinary molding method 
can be used, such as the generally used mold pressing method or sheet 
forming method. 
The nitriding step as the third step in this invention is conducted in a 
nitrogen atmosphere at 1,200 to 1,400.degree. C. Nitriding temperatures 
below 1,200.degree. C. are undesirable in that the reaction rate is 
considerably low, resulting in a sintered body having reduced mechanical 
properties. In contrast, nitriding temperatures exceeding 1,400.degree. C. 
are undesirable in that since the compact is partly heated to or above the 
melting point of silicon, silicon melting occurs and the melted silicon 
remains as coarse unnitrided agglomerates, resulting in a sintered body 
having reduced mechanical properties. 
In this nitriding step, the compact is preferably heated at a rate of 0.3 
to 0.5.degree. C./min in the temperature range of especially from 1,200 to 
1,300.degree. C. and then heat-treated in the temperature range of from 
1,300 to 1,400.degree. C. This is because thus controlling the heating 
rate is suitable for regulating the free silicon remaining unnitrided so 
as to be present in a desirable dispersed state and in a desirable amount. 
If too high a heating rate is used, free silicon forms coarse agglomerates 
due to the heat generated by the reaction and is hence less apt to come 
into the desired, evenly and finely dispersed state. If the heating rate 
is lower than the lower limit, impurities are apt to form a solid solution 
in the Si.sub.3 N.sub.4 crystal grains being yielded. 
In the sintering step as the final step, the nitrided compact which has 
undergone the nitriding step is sintered in a nitrogen atmosphere at 1,600 
to 2,000.degree. C. If the sintering temperature is below 1,600.degree. 
C., the resultant sintered body has an increased porosity and hence a 
reduced thermal conductivity. Conversely, sintering temperatures exceeding 
2,000.degree. C. are undesirable in that the Si.sub.3 N.sub.4 is apt to 
decompose. Especially in the case where sintering is conducted at ordinary 
pressure, a temperature of 1,800.degree. C. or lower is desirably used. It 
is also preferred to place the nitrided body in a vessel made of carbon or 
to use a furnace whose inner wall is made of carbon, in order to prevent 
external oxygen inclusion. The nitrogen atmospheres for use in the 
nitriding step and sintering step may contain ammonia gas or other inert 
gases, besides nitrogen. 
In this sintering step, the Si.sub.3 N.sub.4 grains in the nitrided body 
change from the .alpha.-form to the .beta.-form to thereby form a network 
structure comprising densified columnar crystal grains. In the process of 
this invention, a dense, high thermal conductive silicon nitride base 
sintered body can be obtained usually through sintering at 1,700 to 
1,900.degree. C. in a nitrogen atmosphere having a pressure of about 1 to 
5 atm. Unlike conventional sintering steps, the sintering in this process 
need not be conducted, for example, at a temperature as high as about 
2,000.degree. C. and a pressure as high as 100 atm or higher. 
The silicon nitride base sintered body obtained by the above-described 
process of this invention has high thermal conductivity together with 
excellent mechanical strength. Specifically, a silicon nitride base 
sintered body having a relative density of 95% or higher, a thermal 
conductivity of 50 W/m.multidot.k or higher, and a flexural strength of 
600 MPa or higher can be provided by the inexpensive production process.

EXAMPLES 
Example 1 
Various silicon powders having the intraparticulate oxygen contents and 
average particle diameters shown in the following Table 1 and an Sm.sub.2 
O.sub.3 power having an average particle diameter of 0.5 .mu.m were 
prepared. Each silicon powder and the Sm.sub.2 O.sub.3 powder were weighed 
out in such respective amounts that the silicon powder amount in terms of 
Si.sub.3 N.sub.4 is shown in Table 1 and the Sm.sub.2 O.sub.3 powder 
accounted for the remainder. The two powders were mixed with each other in 
ethyl alcohol by means of a ball mill. The resultant slurry was dried and 
granulated with a spray dryer to obtain a granular powder mixture having 
an average particle diameter of about 100 .mu.m. 
TABLE 1 
______________________________________ 
Silicon powder Amount 
Particle Oxygen Of Sm.sub.2 O.sub.3 
diameter content Amount 
Powder 
Sample (.mu.m) (wt %) (wt %) 
(wt %) 
______________________________________ 
1* 0.05 1.0 90 10 
2 1 0.6 90 10 
3 5 0.4 90 10 
4 10 0.3 90 10 
5 20 0.3 90 10 
6 25 0.3 90 10 
7* 25 1.0 90 10 
8* 5 0.4 78 22 
9 5 0.4 60 20 
10 5 0.4 90 10 
11 5 0.4 99 1 
12* 5 0.4 99.5 0.5 
______________________________________ 
(Note) 
The asterisked samples in the table are comparative examples. 
Each granular powder mixture was molded by dry pressing into test pieces in 
two forms, i.e., test pieces having a length of 45 mm, a width of 8 mm, 
and a thickness of 5 mm (for flexural strength measurement) and ones 
having an diameter of 12.5 mm and a thickness of 5 mm (for thermal 
conductivity measurement). Thereafter, these compacts were placed in a 
refractory case made of carbon and lined with BN, nitrided in 1-atm 
nitrogen gas at 1,300.degree. C. for 3 hours, and successively heated to 
1,850.degree. C. to conduct 3-hour sintering in 4-atom nitrogen gas. 
As a result of X-ray diffractometry, all Si.sub.3 N.sub.4 crystal grains in 
each sintered body obtained were ascertained to be .beta.-form. The two 
kinds of test pieces for each sintered body were examined for relative 
density (proportion of the found density value measured by the Archimedes 
method to the theoretical density) and for three-point flexural strength 
and thermal conductivity (laser flash method). Furthermore, the amount of 
oxygen contained in the Si.sub.3 N.sub.4 crystal grains was ascertained by 
Auger electron spectroscopy or the XPS method. With respect to the 
nitrided compacts which had undergone the nitriding step and the sintered 
bodies obtained after the sintering step, the amount of free silicon was 
ascertained by EPMA or Auger electron spectroscopy. In each sample, the 
maximum diameter of the silicon particles was about 0.8 .mu.m. The results 
of the above examinations are shown in the following Table 2. 
TABLE 2 
______________________________________ 
Silicon 
amount Si.sub.3 N.sub.4 sintered body 
in Thermal 
nitrided Flexural 
Conduc- 
Silicon 
Oxygen 
body Density strength 
tivity Amount amount 
Sample 
(wt %) (%) (MPa) (W/mK) (wt %) (wt %) 
______________________________________ 
1* &lt;0.01 99 1000 40 &lt;0.01 1.0 
2 0.5 99 1000 85 0.5 0.4 
3 2.0 99 900 90 2.0 0.3 
4 4.0 97 800 92 4.0 0.2 
5 7.0 90 700 80 8.0 0.3 
6 8.0 89 500 70 8.0 0.3 
7* 15 80 400 40 15 1.0 
8* &lt;0.01 80 500 45 &lt;0.01 0.7 
9 0.1 99 850 80 0.1 0.3 
10 1.0 99 900 90 1.0 0.3 
11 10 80 600 70 10 0.4 
12* 20 70 400 40 20 0.8 
______________________________________ 
(Note) 
The asterisked samples in the table are comparative examples. 
The above results show the following. The content of silicon in .beta.-form 
Si.sub.3 N.sub.4 crystal grains obtained after sintering can be regulated 
within the range of 0.01 to 10% by weight and a high thermal conductive 
Si.sub.3 N.sub.4 base sintered body having a three-point flexural strength 
of 600 MPa or higher and a thermal conductivity of 50 W/m.multidot.k or 
higher is obtained by using a silicon feedstock power having an oxygen 
content of 1% by weight or lower adding a rare earth oxide (Sm.sub.2 
O.sub.3) powder as a minor ingredient in an amount of 1 to 20% by weight, 
and nitriding the mixture of both in a nitrogen atmosphere at 1,200 to 
1,400.degree. C. 
Example 2 
The same feedstock powders as those used for sample 3 in Example 1 were 
used in the same proportion to likewise prepare compacts in the given 
shapes. The compacts were nitrided for 3 hours in 1-atm nitrogen gas at 
each of the nitriding temperatures shown in the following Table 3. In this 
nitriding, some of the samples were regulated with respect to heating rate 
in the temperature range of from 1,200 to 1,300.degree. C. as shown in 
Table 3. Thereafter, the nitrided bodies were sintered for 3 hours in 
4-atm nitrogen gas at each of the sintering temperatures shown in Table 3. 
The Si.sub.3 N.sub.4 base sintered body samples obtained were evaluated in 
the same manner as in Example 1. The results of the evaluations are shown 
in the following Table 4. 
TABLE 3 
______________________________________ 
Silicon 
Nitriding Conditions 
amount in 
Heating Nitriding nitrided 
Sintering 
rate Treatment body conditions 
Sample (.degree. C./min) 
(.degree. C. xhr) 
(wt %) (.degree. C. xhr) 
______________________________________ 
13 0.3 1300 .times. 3 
0.5 1850 .times. 3 
14 0.5 1300 .times. 3 
1.2 1850 .times. 3 
15 0.7 1300 .times. 3 
1.5 1850 .times. 3 
16* -- 1180 .times. 3 
30 1850 .times. 3 
17* -- 1200 .times. 3 
20 1850 .times. 3 
18 0.4 1300 .times. 3 
1.0 1850 .times. 3 
19 0.4 1400 .times. 3 
0.3 1850 .times. 3 
20* 0.4 1420 .times. 3 
5.0 1850 .times. 3 
21* 0.4 1300 .times. 3 
1.0 1580 .times. 3 
22 0.4 1300 .times. 3 
1.0 1600 .times. 3 
23 0.4 1300 .times. 3 
1.0 1700 .times. 3 
24 0.4 1300 .times. 3 
1.0 1800 .times. 3 
25 0.4 1300 .times. 3 
1.0 1900 .times. 3 
26 0.4 1300 .times. 3 
1.0 2000 .times. 3 
27* 0.4 1300 .times. 3 
1.0 2200 .times. 3 
______________________________________ 
(Note) 
The asterisked samples in the table are comparative examples. 
TABLE 4 
______________________________________ 
Si.sub.3 N.sub.4 sintered body 
Thermal 
Flexural 
conduc- Silicon 
Oxygen 
Density Strength 
tivity amount 
amount 
Sample 
(%) (MPa) (W/mK) (wt %) 
(wt %) 
______________________________________ 
13 99 950 110 0.5 0.2 
14 99 900 110 1.2 0.2 
15 99 900 92 1.5 0.3 
16* 75 350 50 20 0.7 
17* 80 500 55 10 0.7 
18 99 1000 120 1.0 0.15 
19 99 1100 110 0.3 0.2 
20* 90 400 70 5 0.6 
21* 70 550 50 1.0 0.3 
22 90 850 90 1.0 0.2 
23 98 900 100 1.0 0.2 
24 99 1000 115 1.0 0.15 
25 99 1100 115 1.0 0.15 
26 95 900 95 1.0 0.2 
27* 85 500 80 1.0 0.4 
______________________________________ 
(Note) 
The asterisked samples in the table are comparative examples. 
The above results show that by using a nitriding temperature of 1,200 to 
1,400.degree. C., the Si.sub.3 N.sub.4 crystal grains in the resultant 
nitrided body include therein silicon in a content range of from 0.01 to 
10% by weight. The results further show that by sintering the nitrided 
body at 1,600 to 2,000.degree. C., a high thermal conductive Si.sub.3 
N.sub.4 base sintered body can be obtained in which the silicon content in 
the Si.sub.3 N.sub.4 crystal grains is in the range of 0.01 to 10% by 
weight and which has a three-point flexural strength of 600 MPa or higher 
and a thermal conductivity of 50 W/m.multidot.k or higher. The results 
furthermore show that by regulating the heating rate in the temperature 
range of from 1,200 to 1,300.degree. C. during nitriding to 0.3 to 
0.5.degree. C./min, sintered bodies even more improved in three-point 
flexural strength and thermal conductivity can be obtained. 
Example 3 
Powder mixtures were prepared using the same silicon powder and Sm.sub.2 
O.sub.3 powder as those for sample 4 in Example 1 in such a manner that 
the silicon powder was used in an amount of 90% by weight and part of the 
10% by weight Sm.sub.2 O.sub.3 powder as the remainder was replaced with 
each of the substitute compound powders shown in the following Table 5. 
Using each of the powder mixtures, compacts were prepared in the same 
manner as Example 1. The compacts were nitrided under the same conditions 
as those for sample 15 in Example 2 and then sintered for 3 hours in 5-atm 
nitrogen gas respectively at the sintering temperatures shown in Table 5. 
The sintered bodies obtained were evaluated in the same manner as in 
Example 1. The results of the evaluations are shown in the following Table 
6. 
TABLE 5 
______________________________________ 
Substitute compound Nitrided body 
Powder Silicon Sintering 
Kind of the 
Amount Amount 
Temperature 
Sample compound (wt %) (wt %) 
(.degree. C.) 
______________________________________ 
28 TiN 0.005 1.5 1800 
29 TiN 0.01 1.5 1800 
30 TiN 0.1 1.5 1800 
31 TiN 1 2.0 1800 
32 TiN 3 4.0 1800 
33 TiN 4 12.0 1800 
34 ZrC 1 2.0 1800 
35 CaO 0.08 1.5 1800 
36 CaO 0.1 1.5 1750 
37 CaO 2 1.5 1650 
38 CaO 5 2.0 1620 
39 CaO 6 10.0 1600 
40 Li.sub.2 O 
0.08 1.5 1750 
41 Li.sub.2 O 
0.1 1.5 1700 
42 Li.sub.2 O 
2 1.5 1650 
43 Li.sub.2 O 
6 6.0 1630 
______________________________________ 
(Note) 
The asterisked samples in the table are comparative examples. 
TABLE 6 
______________________________________ 
Si.sub.3 N.sub.4 sintered body 
Thermal 
Flexural 
conduc- Silicon 
Oxygen 
Density Strength 
tivity amount 
amount 
Sample 
(%) (MPa) (W/mK) (wt %) 
(wt %) 
______________________________________ 
28 99 900 92 1.5 0.2 
29 99 900 105 1.5 0.15 
30 99 900 110 1.5 0.15 
31 99 900 120 2.0 0.15 
32 98 800 105 4.0 0.15 
33 90 500 90 10.0 0.2 
34 99 800 105 2.0 0.2 
35 99 900 90 1.5 0.2 
36 99 1100 90 1.5 0.2 
37 99 1000 90 1.5 0.2 
38 98 900 85 2.0 0.3 
39 85 600 60 8.0 0.6 
40 99 800 90 1.5 0.3 
41 99 950 90 1.5 0.3 
42 99 920 90 1.5. 0.3 
43 80 450 60 6.0 0.3 
______________________________________ 
The above results show that the addition of a Group 4A element compound in 
an amount of 0.01 to 3% by weight in terms of element amount is effective 
in obtaining a sintered body having an even more improved thermal 
conductivity while retaining the high flexural strength. The results 
further show that the addition of a calcium or lithium compound in an 
amount of 0.1 to 5% weight in terms of oxide amount is effective in 
improving the sinterability and in thus enabling low-temperature 
densification. The results furthermore show that although the addition of 
these ingredients is effective, it leads to a decrease in the strength of 
a sintered body when the amount of the Group 4A element exceeds 3% by 
weight or that of the calcium or lithium compound exceeds 5% by weight. 
Example 4 
A silicon powder having an average particle diameter of 1 .mu.m and an 
intraparticulate oxygen content of 0.5% by weight and the rare earth 
element compound powders described in the following Table 7 which each had 
an average particle diameter of 0.5 .mu.m were prepared. The silicon 
powder and each rare earth element compound powder were weighed out so 
that the silicon powder amount in terms of Si.sub.3 N.sub.4 was 90% by 
weight and the other powder accounted for the remainder, i.e., 10% by 
weight. Powder mixtures were prepared in the same manner as in Example 1. 
Thereafter, each powder mixture was molded into compacts in the two forms 
in the same manner as in Example 1. The resultant shapes were placed in a 
refractory case made of carbon and lined with Si.sub.3 N.sub.4, and then 
nitrided and sintered under the same conditions as in Example 1. 
X-ray diffractometry revealed that all the Si.sub.3 N.sub.4 crystal grains 
in each of the sintered bodies obtained were .beta.-form. The sintered 
bodies each had a relative density of 98 to 99%. Each sintered body was 
evaluated for silicon amount and oxygen amount in the crystal grains and 
for three-point flexural strength and thermal conductivity by the same 
methods as in Example 1. The results of the evaluations are shown in Table 
7. The results show that the addition of a rare earth element having a 
field strength [valence/(ionic radius).sup.2 ] of 0.54 or higher is 
effective in obtaining an even higher thermal conductivity. 
TABLE 7 
______________________________________ 
Rare earth compound 
Si.sub.3 N.sub.4 sintered body 
powder Thermal 
Kind of Field Flexural 
Conduc- 
Silicon 
Oxygen 
Sam- the Com- strength of 
strength 
tivity amount 
amount 
ple pound the element 
(MPa) (W/mk) (wt %) 
(wt %) 
______________________________________ 
44 La.sub.2 O.sub.3 
0.507 700 60 1.8 0.6 
45 CeO.sub.2 
0.517 750 65 2.2 0.6 
46 Nd.sub.2 O.sub.3 
0.528 800 60 2.1 0.6 
47 Sm.sub.2 O.sub.3 
0.540 1000 90 0.5 0.4 
48 Y.sub.2 O.sub.3 
0.567 800 85 2.0 0.3 
49 Yb.sub.2 O.sub.3 
0.583 1000 105 1.0 0.2 
______________________________________ 
Example 5 
Silicon powders respectively having the intraparticulate oxygen contents 
shown in the following Table 8 and powders of the rare earth element 
compounds shown in Table 8 which each had an average particle diameter of 
0.5 .mu.m were prepared. Each silicon powder each rare earth element 
compound powder were weighed out so that the silicon powder amount in 
terms of Si.sub.3 N.sub.4 was 90% by weight and the other powder accounted 
for the remainder, i.e., 10% by weight. Granular powder mixtures were 
prepared in the same manner as in Example 1. Thereafter, each granular 
powder mixture was nitrided and sintered under the same conditions as 
those for sample 18 in Example 2. The sintered bodies obtained were 
evaluated in the same manner as in Example 1. The results of the 
evaluations are shown in Table 8. The results show that the lower the 
intraparticulate oxygen content of the starting silicon powder, the higher 
the thermal conductivity of the Si.sub.3 N.sub.4 sintered body obtained. 
TABLE 8 
______________________________________ 
Oxygen 
content Si.sub.3 N.sub.4 sintered body 
of Thermal 
Silicon Rare Flexural 
conduct 
Silicon 
Oxygen 
powder earth strength 
ivity Amount amount 
Sample 
(wt %) compound (MPa) (W/mk) (wt %) (wt %) 
______________________________________ 
50 0.2 La.sub.2 O.sub.3 
700 90 1.5 0.3 
51 0.2 Sm.sub.2 O.sub.3 
900 130 1.0 0.1 
52 0.3 Yb.sub.2 O.sub.3 
1100 120 0.5 0.15 
53 0.1 Yb.sub.2 O.sub.3 
1000 130 0.5 0.1 
______________________________________ 
INDUSTRIAL APPLICABILITY 
According to this invention, a silicon nitride base sintered body combining 
high strength with high thermal conductivity not possessed by any 
conventional silicon nitride base sintered body can be provided by a novel 
process having excellent productivity by removing or diminishing 
impurities which can form a solid solution in silicon nitride crystal 
grains. This high thermal conductive silicon nitride base sintered body is 
extremely useful not only as various parts for semiconductor devices, such 
as radiating insulating substrates, but as various structural parts for 
machines, OA apparatuses, etc.