U.S. patent application number 09/899736 was filed with the patent office on 2002-01-03 for si3n4 ceramic, si-base composition for production thereof and processes for producing these.
This patent application is currently assigned to Sumitomo Electric Industries, Ltd.. Invention is credited to Nakahata, Seiji, Yamakawa, Akira.
Application Number | 20020002107 09/899736 |
Document ID | / |
Family ID | 17853885 |
Filed Date | 2002-01-03 |
United States Patent
Application |
20020002107 |
Kind Code |
A1 |
Nakahata, Seiji ; et
al. |
January 3, 2002 |
Si3N4 ceramic, Si-base composition for production thereof and
processes for producing these
Abstract
A slurry Si-base composition comprising an Si powder having a
thickness of a surface oxide film ranging from 1.5 to 15 nm, 50 to
90% by weight of water, 0.2 to 7.5% by weight, in terms of oxide,
of a sintering aid and 0.05 to 3% by weight of a dispersant, the
Si-base composition having a pH value adjusted to 8-12. This slurry
Si-base composition is produced by a process which comprises
subjecting Si powder to oxidation treatment at 200 to 800.degree.
C. in air, adding 50 to 90% by weight of water, 0.2 to 7.5% by
weight, in terms of oxide, of a sintering aid and 0.05 to 3% by
weight of a dispersant to the oxidized Si powder and performing
such a pH adjustment that the resultant mixture has a pH value of 8
to 12. The slurry Si-base composition not only enables producing a
ceramic of Si.sub.3N.sub.4 at a lowered cost without the need to
install explosionproof facilities but also allows the obtained
Si.sub.3N.sub.4 ceramic having a relative density of at least 96%
and a flexural strength of at least 800 MPa can be obtained.
Inventors: |
Nakahata, Seiji; (Itami-shi,
JP) ; Yamakawa, Akira; (Itami-shi, JP) |
Correspondence
Address: |
Charles A. Muserlian
c/o Bierman, Muserlian and Lucas
600 Third Avenue
New York
NY
10016
US
|
Assignee: |
Sumitomo Electric Industries,
Ltd.
|
Family ID: |
17853885 |
Appl. No.: |
09/899736 |
Filed: |
July 5, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09899736 |
Jul 5, 2001 |
|
|
|
08708933 |
Sep 6, 1996 |
|
|
|
Current U.S.
Class: |
501/97.1 |
Current CPC
Class: |
C04B 35/591
20130101 |
Class at
Publication: |
501/97.1 |
International
Class: |
C04B 035/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 16, 1995 |
JP |
7-298004 |
Claims
What is claimed is:
1. A slurry Si-base composition comprising Si powder having a
thickness of a surface oxide film ranging from 1.5 to 15 nm, 50 to
90% by weight of water, 0.2 to 7.5% by weight, in terms of oxide,
of a sintering aid and 0.05 to 3% by weight of a dispersant, said
Si-base composition having a pH value adjusted to 8-12.
2. The Si-base composition according to claim 1, wherein the
sintering aid is at least one selected from among compounds of
Group IIa, Group IIIa and rare earth elements.
3. The Si-base composition according to claim 2, wherein the Group
IIa, Group IIIa and rare earth elements are Ca, Sr, Mg, Al, Y, La,
Sm, Nd and Yb.
4. The Si-base composition according to claim 2, wherein the
compounds of the Group IIa, Group IIIa and rare earth elements are
oxides, nitrides and oxynitrides or sintering aid precursors
convertible by heating to the oxides, nitrides and oxynitrides.
5. The Si-base composition according to claim 3, wherein the
compounds of the Group IIa, Group IIIa and rare earth elements are
oxides, nitrides and oxynitrides or sintering aid precursors
convertible by heating to the oxides, nitrides and oxynitrides.
6. The Si-base composition according to claim 1, which further
comprises an organic binder.
7. The Si-base composition according to claim 6, wherein the
organic binder is contained in an amount of 0.05 to 3% by
weight.
8. A process for producing a slurry Si-base composition, which
comprises subjecting an Si powder to oxidation treatment at 200 to
800.degree. C. in air, adding 50 to 90% by weight of water, 0.2 to
7.5% by weight, in terms of oxide, of a sintering aid and 0.05 to
3% by weight of a dispersant to the Si powder and performing such a
pH adjustment that the resultant mixture has a pH value of 8 to
12.
9. The process according to claim 8, wherein the sintering aid is
at least one selected from among compounds of Group IIa, Group IIIa
and rare earth elements.
10. The process according to claim 9, wherein the Group IIa, Group
IIIa and rare earth elements are Ca, Sr, Mg, Al, Y, La, Sm, Nd and
Yb.
11. The process according to claim 9, wherein the compounds of the
Group IIa, Group IIIa and rare earth elements are powder of oxides,
nitrides and oxynitrides or sintering aid precursors convertible by
heating to the oxides, nitrides and oxynitrides.
12. The process according to claim 10, wherein the compounds of the
Group IIa, Group IIIa and rare earth elements are powder of oxides,
nitrides and oxynitrides or sintering aid precursors convertible by
heating to the oxides, nitrides and oxynitrides.
13. The process according to claim 8, which further comprises
adding an organic binder to the slurry Si-base composition.
14. The process according to claim 13, wherein the organic binder
is added in an amount of 0.05 to 3% by weight.
15. An Si.sub.3N.sub.4 ceramic produced from a slurry Si-base
composition, said ceramic having a relative density of at least 96%
and a flexural strength of at least 800 MPa, wherein said slurry
Si-base composition comprises Si powder having a thickness of a
surface oxide film ranging from 1.5 to 15 nm, 50 to 90% by weight
of water, 0.2 to 7.5% by weight, in terms of oxide, of a sintering
aid and 0.05 to 3% by weight of a dispersant and has a pH value
adjusted to 8-12.
16. A process for producing an Si.sub.3N.sub.4 ceramic, which
comprises molding a slurry Si-base composition to thereby obtain a
molded body and heating the molded body in an atmosphere of
nitrogen to thereby sinter the same, wherein said slurry Si-base
composition comprises Si powder having a thickness of a surface
oxide film ranging from 1.5 to 15 nm, 50 to 90% by weight of water,
0.2 to 7.5% by weight, in terms of oxide, of a sintering aid and
0.05 to 3% by weight of a dispersant and has a pH value adjusted to
8-12.
17. The process according to claim 16, wherein the slurry Si-base
composition contains an organic binder and the Si-base composition
is molded while dehydrating the same under pressure.
18. The process according to claim 16, wherein the slurry Si-base
composition is dried to thereby obtain dry powder, an organic
binder is sprayed over the obtained dry powder and the molding is
performed.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a slurry Si-base
composition for use in producing an Si.sub.3N.sub.4 ceramic, which
composition contains water as a dispersion medium. Also, the
present invention relates to a dense Si.sub.3N.sub.4 ceramic of a
high strength produced from the above slurry Si-base composition
and processes for producing these.
[0003] 2. Description of the Prior Art
[0004] Alcohols have been employed as a dispersion medium for
ensuring an effective dispersion of a silicon (Si) powder in the
mixing/molding step when a ceramic is produced from the silicon
powder as a starting material, as described in Mat. Res. Soc. Symp.
Proc., vol. 60, pp. 51-61 (1986). However, from the industrial
point of view, the process in which use is made of an alcohol must
be provided with expensive explosionproof facilities, which
constitute a cause of the increase of product cost.
[0005] On the other hand, Japanese Patent Laid-Open Nos.
261662/1991 and 212279/1989 proposed processes in which water not
requiring explosionproof facilities is used as a dispersion medium.
These processes produce a sintered body of Si.sub.3N.sub.4 through
dispersion of an Si powder in water, addition of an organic binder,
a nitriding agent, etc. so as to obtain a slurry, dehydration of
the slurry, molding and heating for sintering the molding.
[0006] However, the above processes described in Japanese Patent
Laid-Open Nos. 261662/1991 and 212279/1989 fail to add a sintering
aid required for densification of the resulting sintered body.
Therefore, for example, as clearly set forth in Japanese Patent
Laid-Open No. 261662/1991, the obtained sintered body of
Si.sub.3N.sub.4 cannot help having a multiplicity of minute pores
dispersed therein. Consequently, the flexural strength of the
sintered body of Si.sub.3N.sub.4 is anticipated to be very low.
Actually, only a flexural strength as low as 600 MPa or less has
been realized.
[0007] The reason for the failure to add a sintering aid to the
water-base slurry in the above conventional processes would be that
the dispersion of Si powder and sintering aid is poor in water as
different from that in an alcohol, thereby causing the resultant
specimen to have a nonuniform density therein.
SUMMARY OF THE INVENTION
[0008] In view of the above circumstances to now, objects of the
present invention are to provide a slurry Si-base composition for
production of a ceramic of Si.sub.3N.sub.4 which uses water as a
dispersion medium and contains a sintering aid by improving the
dispersion of powder in water and further to provide an inexpensive
dense Si.sub.3N.sub.4 ceramic of a high strength which is produced
from the above Si-base composition.
[0009] The slurry Si-base composition provided by the present
invention for attaining the above objects comprises an Si powder
having a thickness of a surface oxide film ranging from 1.5 to 15
nm, 50 to 90% by weight of water, 0.2 to 7.5% by weight, in terms
of oxide, of a sintering aid and 0.05 to 3% by weight of a
dispersant, the Si-base composition having a pH value adjusted to
8-12.
[0010] This slurry Si-base composition is produced by a process
which comprises subjecting Si powder to oxidation treatment at 200
to 800.degree. C. in air, adding 50 to 90% by weight of water, 0.2
to 7.5% by weight, in terms of oxide, of a sintering aid and 0.05
to 3% by weight of a dispersant to the Si powder and performing
such a pH adjustment that the resultant mixture has a pH value of 8
to 12.
[0011] Further, the use of the above slurry Si-base composition not
only enables producing a ceramic of Si.sub.3N.sub.4 at a lowered
cost without the need to install explosionproof facilities but also
allows the obtained Si.sub.3N.sub.4 ceramic to be dense, uniform
and highly strong. Specifically, an Si.sub.3N.sub.4 ceramic having
a relative density of at least 96% and a flexural strength of at
least 800 MPa can be obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a constitutional diagram of the texture of an
Si.sub.3N.sub.4 ceramic of the present invention as observed by
EPMA, in which Y and Sm as the sintering aid are dotted.
[0013] FIG. 2 is a constitutional diagram of the texture of an
Si.sub.3N.sub.4 ceramic produced from a slurry of commercially
available Si powder of which no oxidation treatment was effected as
observed by EPMA, in which also Y and Sm as the sintering aid are
dotted.
[0014] FIG. 3 is a constitutional diagram of the texture of an
Si.sub.3N.sub.4 ceramic produced from a prior art slurry in which
use was made of an alcohol as observed by EPMA, in which also Y and
Sm as the sintering aid are dotted.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] In the present invention, the dispersion of Si powder and
sintering aid in water has been improved by treating the surface of
the Si powder, regulating the amount of water as a dispersion
medium, adjusting the pH value of the slurry and applying a
dispersant, preferably, adding an organic binder, thereby enabling
the step of water mixing and molding. Accordingly, the slurry
Si-base composition of the present invention can be handled without
the need to install explosionproof facilities, so that an
inexpensive Si.sub.3N.sub.4 ceramic can be provided.
[0016] The Si powder for use in the present invention is obtained
by subjecting a commercially available Si powder to oxidation
treatment in air at 200 to 800.degree. C. to thereby form an oxide
film having a thickness as large as 1.5 to 15 nm at the surface of
the Si powder. Although the thickness of the oxide film present at
the surface of the commercially available Si powder generally
ranges from 0.3 to 1.0 nm, the oxidation treatment thereof in air
at 200 to 800.degree. C. enables regulating the thickness of the
oxide film so as to range from 1.5 to 15 nm. The above thickness of
the oxide film is a value estimated from the results of measurement
of the oxygen content and specific surface area of the powder.
[0017] This oxidation treatment enables enhancing the
dispersibility of Si powder. Although the reason for the
dispersibility enhancement has not yet been elucidated, it can be
presumed as follows. The surface of customary commercially
available Si powder is composed of SiO and has a high content of Si
of poor water wettability. The foregoing oxidation treatment
increases the content of oxygen, which is a component of water, so
that SiO is converted to SiO.sub.2. As a result, the water
wettability of the Si powder is enhanced to thereby cause each Si
powder aggregate to loosen into separate particles with the result
that the Si powder dispersibility is enhanced. Further, the
dispersibility enhancement can partly be attributed to that, even
if the surface oxide film is partially peeled by mutual powder
contacts at the time of mixing, the thickness of the oxide film is
so large that Si of poor water wettability is scarcely exposed at
the surface of the Si powder.
[0018] When the treating temperature is lower than 200.degree. C.
in the oxidation treatment of the Si powder, satisfactory surface
oxidation treatment cannot be performed, so that powder particles
aggregate with each other to thereby disenable uniform mixing
thereof with a subsequently added sintering aid. Thus, after
sintering, the sintered body suffers from density dispersion, so
that the obtained Si.sub.3N.sub.4 ceramic has poor strength without
exception. On the other hand, when the oxidation treatment is
conducted at temperatures higher than 800.degree. C., SiO.sub.2 is
formed in high proportion at the surface of the silicon powder, so
that a large amount of the SiO.sub.2 remains in the ceramic as
impurities in the grain boundary phase. Thus, also, the obtained
Si.sub.3N.sub.4 ceramic has poor strength without exception.
[0019] It is preferred that the average particle size of the Si
powder ranges from 0.1 to 10 .mu.m. The reason is that a powder
having an average particle size of less than 0.1 .mu.m has such a
high bulk density that handling thereof is difficult and that, on
the other hand, it is difficult to bring powder having an average
particle size exceeding 10 .mu.m into a uniform slurry because of
the occurrence of aggregation and precipitation.
[0020] Although the sintering aid for use in the present invention
can be any of those having commonly been employed in the sintering
of Si.sub.3N.sub.4, it is preferred to use at least one selected
from among compounds of Group IIa elements, Group IIIa elements and
rare earth elements of the periodic table and especially preferred
to use at least one of compounds of Ca, Sr, Mg, Al, Y, La, Sm, Nd
and Yb. In particular, it is most especially preferred to use at
least one member selected from among oxides, nitrides and
oxynitrides of Ca, Mg, Al, Y, La and Sm, which are available at
relatively low costs, and composite compounds thereof. Powder of at
least one of the above compounds can be added as the sintering aid.
Alternatively, use can be made of precursors which are converted to
the above oxides, nitrides and oxynitrides at up to 1500.degree. C.
by heating, for example, calcium laurate and yttrium stearate.
[0021] The sintering aid is added in an amount of 0.2 to 7.5% by
weight in term of oxide based on the entirety of the slurry
composition (0.5 to 17% by weight in terms of oxide based on Si
powder). When the amount of sintering aid is smaller than 0.2% by
weight based on the entirety of the composition, the function of
the sintering aid cannot be fulfilled and the relative density of
the obtained sintered body is lower than 96% with a multiplicity of
pores present therein with the result that only a low flexural
strength can be realized. On the other hand, when it exceeds 7.5%
by weight, the proportion of the grain boundary phase of a low
strength is unfavorably increased in the sintered body to thereby
cause the strength of the obtained Si.sub.3N.sub.4 ceramic to be
low.
[0022] Especially preferred dispersants are those of ammonium
polycarboxylate. The dispersant is added in an amount of 0.05 to 3%
by weight (0.5 to 5% by weight based on Si powder), preferably,
0.05 to 2.5% by weight based on the entirety of the slurry
composition. The addition of the dispersant improves the dispersion
of powder and eliminates powder aggregation, so that, upon molding,
the density dispersion is less in the molded body. Thus, the
obtained molded body has a high relative density and ensures an
excellent handleability. When the amount of dispersant is smaller
than 0.05% by weight, the function of the dispersant cannot be
fulfilled. Although the molded body has no density change even when
the addition of the dispersant exceeds 2.5% by weight, the addition
in an amount of more than 3% by weight unfavorably causes the
dispersant to remain in the sintered body obtained by heating as
impurities, so that the strength of the sintered body is
lowered.
[0023] An extremely uniform slurry Si-base composition in which the
dispersion of powder has markedly been improved can be obtained by
mixing together the above Si powder, sintering aid, dispersant and
water, performing such a regulation that the proportion of water in
the slurry becomes 50 to 90% by weight and adjusting the pH value
of the slurry to 8-12 with the use of an alkaline solution such as
ammonia.
[0024] When the proportion of water is less than 50% by weight in
the above regulation, the viscosity of the slurry is increased to
such an extent that the Si powder and the sintering aid powder
cannot be uniformly mixed with each other. On the other hand, when
the proportion of water exceeds 90% by weight, only a molded body
of a low density can be obtained in the step of preparing a molded
body from the slurry, so that the handleability of the molded body
is deteriorated. Moreover, when the pH value of the slurry is
smaller than 8 or larger than 12, powder aggregation would
occur.
[0025] The process for producing an Si.sub.3N.sub.4 ceramic from
the thus obtained slurry Si-base composition comprises
satisfactorily mixing the slurry by means of, for example, a ball
mill, molding and heating the molded body in an atmosphere of
nitrogen to thereby sinter the same.
[0026] When in advance an organic binder, for example, an acrylic
one is added to the slurry and mixed, molding can be conducted
while removing water from the slurry under pressure. The organic
binder is added in the same amount as the dispersant, namely,
ranging from 0.05 to 3% by weight, preferably, from 0.05 to 2.5% by
weight. The reason is the same as in the dispersant. On the other
hand, when no organic binder is added to the slurry, the slurry is
dried by the use of, for example, a freeze dryer or a spray dryer,
an organic binder is sprayed over the resultant dry powder and a
dry press molding thereof is performed.
[0027] The organic binder is removed from the molded body obtained
through the above process and the resultant molded body is heated
to thereby sinter the same. Thus, an Si.sub.3N.sub.4 ceramic is
obtained. An EPMA (Electron Probe Micro Analysis) observation of
the obtained Si.sub.3N.sub.4 ceramic showed that a texture was
realized in which the sintering aid (dotted in the figure) was
uniformly dispersed as shown in FIG. 1.
[0028] For reference, FIG. 2 shows the texture of the
Si.sub.3N.sub.4 ceramic produced from a commercially available Si
powder of which no oxidation treatment was carried out and FIG. 3
shows the texture of the Si.sub.3N.sub.4 ceramic obtained by the
conventional process using an alcohol. It is found by comparison of
the dispersions of sintering aid that the sintering aid is
dispersed in FIG. 1 according to the present invention with the
same uniformity as in FIG. 3 according to the prior art using an
alcohol while the sintering aid particles are aggregated with each
other in FIG. 2 showing the dispersion exhibited when use is made
of the commercially available Si powder.
[0029] The thus obtained Si.sub.3N.sub.4 ceramic of the present
invention is uniform as mentioned above and rendered dense so as to
have a relative density of at least 96%. Further, a three-point
flexural strength test conducted in accordance with Japanese
Industrial Standard R-1601 showed that the Si.sub.3N.sub.4 ceramic
had a strength as high as at least 800 MPa. It is seen that this
flexural strength is very high because the highest flexural
strength of the ceramic obtained by the conventional water mixing
process described in, for example, the above Japanese Patent
Laid-Open No. 261662/1991 is only about 600 MPa.
[0030] Moreover, the dispersion medium is water and no alcohol is
used in the process of the present invention. Therefore, the
process of the present invention enables using an inexpensive
production line requiring no explosionproof facilities and hence
providing an inexpensive Si.sub.3N.sub.4 ceramic product of a high
quality.
EXAMPLE 1
[0031] A commercially available Si powder of 2.8 .mu.m in an
average particle size was subjected to oxidation treatment in air
at each of the temperatures specified in Table 1 for 5 hr. The
thickness of the oxide film formed at the surface of each of the
obtained sample Si powders is also given in Table 1. Water,
Y.sub.2O.sub.3 powder and Sm.sub.2O.sub.3 powder as a sintering aid
and an aluminum polycarboxylate dispersant were added to each Si
powder so that their respective amounts were 70% by weight, 2% by
weight, 3% by weight and 0.5% by weight based on the entirety of
the resultant slurry.
[0032] Subsequently, the pH value of each of the above mixtures was
adjusted to 8.5 with ammonia and the mixtures were individually
mixed by means of a ball mill for 48 hr, thereby obtaining a
slurry. With respect to sample 2, an acrylic organic binder was
added to the slurry in an amount of 0.5% by weight and a molded
body was prepared while removing water from the slurry under
pressure. With respect to the other samples, an acrylic organic
binder was sprayed while each slurry obtained in the above manner
was dried by means of a spray dryer to thereby effect granulation
and a molded body was obtained.
[0033] Thereafter, each of the above molded bodies was heated at
800.degree. C. in an atmosphere of nitrogen to thereby remove the
organic binder and the temperature was raised to 1800.degree. C. in
the nitrogen atmosphere. At that temperature, the heating was
continued for 4 hr, thereby obtaining each sintered body of
Si.sub.3N.sub.4. With respect to the elements of the sintering aid
which were present in each of the sintered bodies of
Si.sub.3N.sub.4, Y was 4.4% by weight and Sm 6.7% by weight.
[0034] The relative density of each of the obtained sintered bodies
of Si.sub.3N.sub.4 and each of the molded bodies obtained in the
same manner as described above was measured by the method of
Archimedes. Further, the flexural strength of each of the sintered
bodies was measured by the three-point flexural test complying with
Japanese Industrial Standard R-1601. The obtained results are
collectively given in Table 1.
1 TABLE 1 Oxidation Thickness of Relative density (%) Flexural
treatment oxide film molded sintered strength Sample (.degree. C.
.times. hr) (nm) body body (MPa) 1 200 .times. 5 1.5 53 96 960 2
600 .times. 5 5.3 56 98 960 3 800 .times. 5 14.6 53 96 820 4* none
0.5 50 92 430 5* 900 .times. 5 15.5 53 95 480 (Note) Samples
asterisked in the table are comparative ones.
[0035] As apparent from the above results, with respect to each of
the samples of the present invention, the treatment of Si powder at
200 to 800.degree. C. enabled regulating the thickness of the oxide
film at the powder surface to 1.5-15 nm, so that the molded body
and sintered body were uniform and had desirable relative densities
and that the sintered body of Si.sub.3N.sub.4 having a flexural
strength of at least 800 MPa was obtained.
[0036] In contrast, with respect to sample 4 from the Si powder of
which no oxidation treatment was carried out, powder particles
aggregated with each other to thereby disenable obtaining a uniform
molded body and to render its relative density as low as 50%.
Although sintering was conducted under the same conditions, the
obtained sintered body had relative density and flexural strength
which were both low. Further, with respect to sample 5 from the Si
powder subjected to oxidation treatment at 900.degree. C., a large
amount of oxide originating in the oxide film was present at grain
boundaries of the sintered body, so that its flexural strength was
poor.
EXAMPLE 2
[0037] Sintered bodies of Si.sub.3N.sub.4 were obtained in the same
manner as in Example 1, except that the proportion of water as a
dispersion medium of the slurry was varied as specified in Table 2
below. The employed Si powder was the same as sample 2 of Example
1, which was obtained by subjecting commercially available Si
powder to oxidation treatment at 600.degree. C. in air for 5 hr and
had an oxide film of 5.3 nm in thickness.
[0038] The type and amount of employed sintering aid were the same
as in Example 1. Thus, with respect to the elements of the
sintering aid which were present in each of the obtained sintered
bodies of Si.sub.3N.sub.4, Y was 4.4% by weight and Sm 6.7% by
weight. Further, the pH value of the slurry and the dispersant for
use therein were also the same as in Example 1. With respect to
sample 8, 0.5% by weight of the organic binder was added to the
slurry and, with respect to the other samples, the organic binder
was sprayed over dry powder and molding was conducted.
[0039] The relative density and flexural strength of each of the
obtained sintered bodies of Si.sub.3N.sub.4 were measured in the
same manner as in Example 1 and given in Table 2 along with the
relative density of each of the molded bodies which was measured in
the same manner.
2 TABLE 2 Amount of Relative density (%) Flexural water added
molded sintered strength Sample (wt. %) body body (MPa) 6 50 53 96
850 7 80 54 97 950 8 90 56 98 980 9* 45 49 87 350 10* 95 48 87 380
(Note) Samples asterisked in the table are comparative ones.
[0040] As apparent from the above results, the regulation of the
proportion of water to the slurry to 50-90% by weight enables
obtaining a molded body of desired density and the sintering
thereof enables producing a sintered body having a flexural
strength of at least 800 MPa.
EXAMPLE 3
[0041] Sintered bodies of Si.sub.3N.sub.4 were obtained in the same
manner as in Example 1, except that the type and amount of added
sintering aid were varied as specified in Table 3 below. The
employed Si powder was the same as sample 2 of Example 1, which was
obtained by subjecting the commercially available Si powder to
oxidation treatment at 600.degree. C. in air for 5 hr and had an
oxide film of 5.3 nm in thickness.
[0042] The sintering aid was added in the form of oxides in sample
Nos. 11-13, 18, 19 and 21, a nitride in sample No. 14, oxynitrides
in sample Nos. 15 and 20, lauric acid in sample No. 16 and stearic
acid in sample No. 17. The amount of sintering aid added was
expressed in terms of oxide without exception. The proportion of
water, slurry pH and dispersant were the same as in Example 1. With
respect to sample Nos. 16 to 19, 0.5% by weight of organic binder
was added to the slurry and, with respect to the other samples, the
organic binder was sprayed over dry powder and molding was
performed.
[0043] Not only the relative density and flexural strength of each
of the obtained sintered bodies of Si.sub.3N.sub.4 but also the
relative density of each of the molded bodies and the contents of
sintering aid elements in each of the sintered bodies were measured
in the same manner as in Example 1 and given in Table 3.
3 TABLE 3 Sintering aid Relative density element in (%) Flexural
Sintering aid sintered body molded sintered strength Sample (wt. %)
(wt. %) body body (MPa) 11 Sm (0.15) Ca (0.1) Sm (0.33) Ca (0.22)
54 96 820 12 Sm (0.3) Y (0.2) Sm (0.67) Y (0.44) 54 96 880 13 Sm
(0.5) La (0.5) Sm (1.1) La (1.1) 53 96 920 14 Sm (1.5) Nd (1.5) Sm
(3.3) Nd (3.3) 54 97 840 15 Sm (2.0) Yb (2.0) Sm (4.4) Yb (4.4) 54
97 1080 16 Yb (4.0) Al (2.0) Yb (8.9) Al (4.4) 57 98 980 17 La
(5.0) Sr (2.0) La (11.1) Sr (4.4) 57 98 1020 18 Sm (5.0) Mg (2.0)
Sm (11.1) Mg (4.4) 56 98 1250 19 Sm (5.0) Mg (2.3) Sm (11.1) Mg
(5.8) 55 97 1200 20* Sm (0.1) Ca (0.05) Sm (0.2) Ca (0.1) 53 92 410
21* Sm (5.0) Y (3.0) Sm (11.1) Y (6.7) 54 94 520 (Note) Samples
asterisked in the table are comparative ones.
[0044] As apparent from the above results, the addition of the
Group IIa, Group IIIa and/or rare earth elements as the sintering
aid in an amount of 0.2 to 7.5% by weight in terms of oxide enables
obtaining a sintered body of Si.sub.3N.sub.4 having a relative
density of at least 96% and a flexural strength of at least 800
MPa.
EXAMPLE 4
[0045] Sintered bodies of Si.sub.3N.sub.4 were obtained in the same
manner as in Example 1, except that the pH value of slurry was
varied as specified in Table 4 below. The employed Si powder was
the same as sample 2 of Example 1, which was obtained by subjecting
commercially available Si powder to oxidation treatment at
600.degree. C. in air for 5 hr and had an oxide film of 5.3 nm in
thickness.
[0046] The employed sintering aid consisted of 2% by weight of
Y.sub.2O.sub.3 and 3% by weight of Sm.sub.2O.sub.3 as in Example 1.
Thus, with respect to the contents of sintering aid elements in
each of the obtained sintered bodies, Y was 4.4% by weight and Sm
6.7% by weight. The proportion of water and the dispersant were
also the same as in Example 1. Further, the organic binder was
added by spraying the same over dry powder without exception.
[0047] The relative density and flexural strength of each of the
obtained sintered bodies of Si.sub.3N.sub.4 were measured in the
same manner as in Example 1 and given in Table 4 along with the
relative density of each of the molded bodies which was measured in
the same manner.
4 TABLE 4 Relative density (%) Flexural molded sintered strength
Sample pH body body (MPa) 22 8 53 96 820 23 10 53 96 850 24 12 53
96 825 25* 7.5 48 92 390 26* 13 49 91 410 (Note) Samples asterisked
in the table are comparative ones.
[0048] As apparent from the above results, maintaining the pH value
of the slurry at 8 to 12 enables obtaining a molded body of desired
density and the sintering of the molded body enables obtaining a
dense sintered body of Si.sub.3N.sub.4 having an flexural strength
of at least 800 MPa.
EXAMPLE 5
[0049] Sintered bodies of Si.sub.3N.sub.4 were obtained in the same
manner as in Example 1, except that the amounts of dispersant and
organic binder were as specified in Table 5 below. The employed Si
powder was the same as sample 2 of Example 1, which was obtained by
subjecting commercially available Si powder to oxidation treatment
at 600.degree. C. in air for 5 hr and had an oxide film of 5.3 nm
in thickness.
[0050] The organic binder was added by spraying the organic binder
over powder obtained by drying the slurry with respect to sample
Nos. 27 to 31 and by charging the organic binder into the slurry
with respect to the other sample Nos. 32 to 37. The type and amount
of added sintering aid were the same as in Example 1. Thus, with
respect to the contents of sintering aid elements in each of the
obtained sintered bodies of Si.sub.3N.sub.4, Y was 4.4% by weight
and Sm 6.7% by weight. The proportion of water and the slurry pH
were also the same as in Example 1.
[0051] The relative density and flexural strength of each of the
obtained sintered bodies of Si.sub.3N.sub.4 were measured in the
same manner as in Example 1 and given in Table 5 along with the
relative density of each of the molded bodies which was measured in
the same manner.
5 TABLE 5 Relative density (%) Flexural Dispersant Binder molded
sintered strength Sample (wt. %) (wt. %) body body (MPa) 27 0.05
sprayed 53 96 850 28 2.5 sprayed 53 96 870 29 3.0 sprayed 52 96 810
30* 0.03 sprayed 52 93 480 31* 3.5 sprayed 52 93 470 32 0.5 0.05 55
97 1100 33 0.5 1.5 57 98 1210 34 0.5 2.5 57 98 1250 35 0.5 3.0 53
97 820 36* 0.5 0.04 43 86 320 37* 0.5 3.2 53 91 450 (Note) Samples
asterisked in the table are comparative ones.
[0052] As apparent from the above results, maintaining each of the
respective proportions of dispersant and organic binder to the
slurry at 0.05 to 3% by weight enables directly obtaining a molded
body of desired density from the slurry and the sintering of the
molded body enables obtaining a dense sintered body of
Si.sub.3N.sub.4 having a flexural strength of at least 800 MPa.
[0053] The slurry Si-base composition for use in the production of
the Si.sub.3N.sub.4 ceramic which contains water as the dispersion
medium and the sintering aid can be obtained by the present
invention. The use of this silicon-base composition enables the
powder mixing and molding without the need to install
explosionproof facilities, so that inexpensive product can be
provided. Moreover, the dense sintered body of a high strength can
be produced through the molded body of a uniform density from the
silicon-base composition adjusted to appropriate conditions. Thus,
the high-quality Si.sub.3N.sub.4 ceramic whose relative density is
at least 96% and whose flexural strength is at least 800 MPa can be
provided by the present invention.
* * * * *