U.S. patent application number 12/859457 was filed with the patent office on 2011-05-12 for method of manufacturing porous sintered reaction-bonded silicon nitride ceramics from granular si mixture powder and porous sintered reaction-bonded silicon nitride ceramics manufactured thereby.
This patent application is currently assigned to KOREA INSTITUTE OF MACHINERY & MATERIALS. Invention is credited to Boo Won Park, Young Jo PARK, In Hyuck Song.
Application Number | 20110111205 12/859457 |
Document ID | / |
Family ID | 43974384 |
Filed Date | 2011-05-12 |
United States Patent
Application |
20110111205 |
Kind Code |
A1 |
PARK; Young Jo ; et
al. |
May 12, 2011 |
METHOD OF MANUFACTURING POROUS SINTERED REACTION-BONDED SILICON
NITRIDE CERAMICS FROM GRANULAR Si MIXTURE POWDER AND POROUS
SINTERED REACTION-BONDED SILICON NITRIDE CERAMICS MANUFACTURED
THEREBY
Abstract
Disclosed is a porous sintered reaction-bonded silicon nitride
ceramic, which includes an array of sintered granules having fine
pore channels in the sintered granules and coarse pore channels
formed between the sintered granules, and in which the pore channel
size is controlled so that both coarse pores and fine pores are
formed together in the ceramic, thus simultaneously enhancing air
permeability and capturing efficiency. A method of manufacturing
the porous sintered reaction-bonded silicon nitride ceramic is also
provided.
Inventors: |
PARK; Young Jo; (Changwon,
KR) ; Park; Boo Won; (Busan, KR) ; Song; In
Hyuck; (Changwon, KR) |
Assignee: |
KOREA INSTITUTE OF MACHINERY &
MATERIALS
Daejeon
KR
|
Family ID: |
43974384 |
Appl. No.: |
12/859457 |
Filed: |
August 19, 2010 |
Current U.S.
Class: |
428/313.9 ;
264/628; 501/97.2 |
Current CPC
Class: |
C04B 2235/5427 20130101;
C04B 2235/428 20130101; C04B 2235/3217 20130101; C04B 2235/3225
20130101; C04B 35/62695 20130101; C04B 2235/5436 20130101; C04B
35/591 20130101; C04B 35/62655 20130101; C04B 38/0022 20130101;
C04B 2235/656 20130101; C04B 2235/528 20130101; C04B 2235/662
20130101; C04B 35/65 20130101; C04B 2235/3222 20130101; C04B
2235/767 20130101; C04B 2111/00793 20130101; C04B 2235/81 20130101;
C04B 2235/3852 20130101; C04B 35/591 20130101; C04B 38/0064
20130101; C04B 38/0054 20130101; C04B 2235/3205 20130101; C04B
2235/3208 20130101; Y10T 428/249974 20150401; C04B 38/0022
20130101; C04B 2235/661 20130101; C04B 35/63416 20130101; C04B
2235/80 20130101 |
Class at
Publication: |
428/313.9 ;
264/628; 501/97.2 |
International
Class: |
B32B 3/26 20060101
B32B003/26; C04B 35/65 20060101 C04B035/65; C04B 35/584 20060101
C04B035/584 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 9, 2009 |
KR |
10-2009-0107392 |
Nov 24, 2009 |
KR |
10-2009-0114185 |
Jan 13, 2010 |
KR |
10-2010-0003000 |
Claims
1. A porous sintered reaction-bonded silicon nitride ceramic,
comprising an array of sintered granules having fine pore channels
in the sintered granules and coarse pore channels formed between
the sintered granules.
2. The porous sintered reaction-bonded silicon nitride ceramic
according to claim 1, which exhibits a bimodal pore distribution
having a first peak and a second peak having a pore size larger
than that of the first peak, and the first peak is based on the
fine pore channels and the second peak is based on the coarse pore
channels.
3. The porous sintered reaction-bonded silicon nitride ceramic
according to claim 1, wherein the pore size of the first peak falls
in a range of less than 1 .mu.m, and the pore size of the second
peak falls in a range of 1 .mu.m or more.
4. The porous sintered reaction-bonded silicon nitride ceramic
according to claim 1, wherein the pore size of the first peak falls
in a range of less than 1 .mu.m, and the pore size of the second
peak falls in a range of 5.about.20 .mu.m.
5. The porous sintered reaction-bonded silicon nitride ceramic
according to claim 1, wherein an average diameter of the sintered
granules falls in a range of 30.about.150 .mu.m.
6. The porous sintered reaction-bonded silicon nitride ceramic
according to claim 1, wherein a maximum frequency diameter of the
sintered granules falls in a range of 50.about.150 .mu.m.
7. A method of manufacturing a porous sintered reaction-bonded
silicon nitride ceramic, comprising: granulating a material
comprising silicon and a sintering additive for preparing a
sintered silicon nitride from the silicon, thus obtaining material
granules; subjecting the material granules to pressureless
compacting in a mold, thus producing a material compact; and
subjecting the material compact to reaction-bonding in a nitriding
gas atmosphere and post-sintering in a nitrogen atmosphere.
8. The method according to claim 7, wherein the sintering additive
comprises yttria and alumina.
9. The method according to claim 7, wherein the sintering additive
is used in an amount of 2.about.10 wt % based on complete
nitridation of the silicon.
10. The method according to claim 7, wherein a maximum weight
frequency of the granules falls in a range of 30.about.150
.mu.m.
11. A method of manufacturing a porous sintered reaction-bonded
silicon nitride ceramic, comprising: granulating a material
comprising silicon and a sintering additive for preparing a
sintered silicon nitride from the W silicon, thus obtaining
material granules; pre-sintering the material granules in an inert
atmosphere, thus obtaining pre-sintered granules; subjecting the
pre-sintered granules to pressing, thus producing a material
compact; and subjecting the material compact to reaction-bonding in
a nitriding gas atmosphere and post-sintering in a nitrogen
atmosphere.
12. The method according to claim 11, wherein the sintering
additive comprises at least one of alkali earth metal oxides.
13. The method according to claim 11, wherein the post-sintering is
performed at 1700.about.1900.degree. C.
14. The method according to claim 11, wherein the sintering
additive is used in an amount of 2.about.10 wt % based on complete
nitridation of the silicon.
15. The method according to claim 11, wherein the pressing is
performed at a pressure of 1.about.20 MPa.
16. A pre-sintered granular powder for sintered reaction-bonded
silicon nitride ceramics, which is a spherical porous granular
powder having open pores and comprising silicon and a sintering
additive for high-temperature liquid phase sintering conducted
after nitridation of the silicon, the sintering additive comprising
yttria, alumina and a compound thereof, the granular powder having
a yield strength of 1.about.20 MPa.
17. The pre-sintered granular powder according to claim 16, wherein
the yield strength is 5 MPa or more.
18. The pre-sintered granular powder according to claim 16, wherein
the granular powder has a flowability of 0.2.about.0.5 g/sec.
19. A compact for sintered reaction-bonded silicon nitride
ceramics, which is a compact of spherical porous granules having
fine pore channels and comprising silicon and a sintering additive
for high-temperature liquid phase sintering conducted after
nitridation of the silicon, the sintering additive comprising
yttria, alumina and a compound thereof, the porous granules having
a maximum weight frequency in a range of 30.about.150 .mu.m, the
compact having a pore structure including fine pore channels in the
porous granules and coarse pore channels between the porous
granules.
20. The compact according to claim 19, wherein the coarse pore
channels comprise coarse pores of 1 .mu.m or more.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Technical Field
[0002] The present invention relates to a method of manufacturing
porous sintered reaction-bonded silicon nitride ceramics and porous
sintered reaction-bonded silicon nitride ceramics manufactured
thereby. More particularly, the present invention relates to a
method of manufacturing a porous sintered reaction-bonded silicon
nitride ceramics, which controls a pore structure so that the
specific surface area of pores is increased to improve capturing
performance and coarse pores are formed to enhance air
permeability, and to a porous sintered reaction-bonded silicon
nitride ceramics manufactured using the same.
[0003] 2. Description of the Related Art
[0004] Silicon nitride based materials are superior in terms of
strength, toughness, impact resistance, heat resistance and
corrosion resistance despite being lightweight, and thus have been
widely used in fields requiring good thermo-mechanical properties
and chemical resistance.
[0005] Conventionally, silicon carbide based porous materials have
been mainly utilized in fields requiring thermo-mechanical
properties and chemical resistance, but are problematic because
silicon carbide has low thermal shock resistance and high hardness,
and thus wears a mold upon extrusion, undesirably considerably
shortening the lifetime thereof, and also because silicon carbide
is sintered at a high temperature equal to or higher than
2000.degree. C., undesirably increasing the cost of
preparation.
[0006] The porous silicon nitride based materials which have
superior heat resistance, mechanical properties and corrosion
resistance as mentioned above are considered to be promising for
use in filters, catalyst supports, heat insulating materials,
filters for high-temperature and high-pressure gas, and diesel
particulate filters, in lieu of silicon carbide based
materials.
[0007] However, research into silicon nitride based materials is
mainly focused on making the microstructure thereof dense so as to
enhance thermo-mechanical properties, and thus methods of
manufacturing the porous silicon nitride based materials have not
yet been sufficiently studied to date.
[0008] As one example of the techniques for manufacturing porous
silicon nitride ceramics, there is Korean Unexamined Patent
Publication No. 1995.about.702510 which discloses a method of
manufacturing porous silicon nitride ceramics composed of silicon
nitride (Si.sub.3N.sub.4) and a rare earth element compound and/or
a transition metal compound so as to be used as a filter for
removing impurities or a catalyst support. According to this
method, a compact of mixture powder is thermally treated at
temperature equal to or higher than 1500.degree. C., thus
manufacturing the porous ceramic having a porosity of 30% or
more.
[0009] In addition, Korean Patent No. 10-0311694 discloses a method
of manufacturing porous sintered silicon oxynitride ceramics
adapted for the refractory tiles of space shuttles. This method
includes agglomerating a low-melting-point powder composition
composed of 11.about.16 wt % of Si.sub.3N.sub.4, 3.about.5 wt % of
AlN, 35.about.45 wt % of Al.sub.2O.sub.3 and 35.about.45 wt % of
Y.sub.2O.sub.3, adding 10.about.25 wt % of the agglomerated
low-melting-point powder to .beta.--Sialon silicon oxynitride
powder composed of 57.about.100 wt % of Si.sub.3N.sub.4, 0.about.9
wt % of Al.sub.2O.sub.3 and 0.about.33 wt % of AlN, compacting this
powder mixture, and sintering the compact at
1600.about.1700.degree. C. for 1.about.8 hours, thus obtaining the
porous sintered silicon oxynitride ceramics.
[0010] In addition, Japanese Unexamined Patent Publication No. Hei.
9-100179 discloses a method of manufacturing a porous silicon
nitride ceramics usable as a filter or a catalyst support. This
method includes bringing the porous ceramic composed mainly of
silicon nitride into contact with an acid and/or an alkali so that
part or all of the components other than silicon nitride are
dissolved, thus manufacturing the porous ceramic.
[0011] However, because all the above methods use the expensive
silicon nitride, the actual use thereof is basically limited, and
also, methods used to form pores are not practical. For example, in
the case of Korean Patent No. 10-311694, in order to form pores in
the sintered ceramic, the low-melting-point powder composition is
compacted into agglomerates, after which the compact thus obtained
is mixed with the high-melting-point powder composition, so that
the pores are ensured depending on the size of the agglomerated
compact. Upon mixing, however, it is difficult for the compact to
maintain its shape. If the shape of the compact aims to be
maintained, sufficient mixing is impossible. Moreover, it is
difficult to consistently control the manufacturing process, and
the manufacturing cost cannot but increase. Also, as in Japanese
Unexamined Patent Publication No. Hei. 9-100179, the pore formation
method which includes chemically treating the manufactured porous
ceramic requires additional chemical treatment, and furthermore, if
the components between the silicon nitride particles are dissolved,
whether the structure of the ceramic is maintained by the silicon
nitride backbone cannot be ensured.
SUMMARY OF THE INVENTION
[0012] With the goal of solving the conventional problems, the
present inventors proposed a method of manufacturing a silicon
nitride filter for automobiles by mixing silicon (Si) with a
sintering additive, namely, a rare earth metal oxide, a rare earth
metal oxide/alumina or a rare earth metal oxide/magnesia,
compacting the mixture, burning the compact in a medium temperature
range in a nitrogen atmosphere thus obtaining reaction-bonded
silicon nitride which is then sintered in a high temperature range
(Patent Application No. 10-2008-0040395).
[0013] This method is advantageous because silicon which is
inexpensive is used as the starting material, and thus the
resultant filter is superior in mechanical properties including
thermal shock resistance and strength, and thermal stability, and
is thus able to be actually utilized in diesel particulate filters.
Furthermore, the particles are made acicular and the aspect ratio
thereof is optimized, and thus fine dust that cannot be filtered by
means of conventional diesel particulate filters is able to be
filtered, and the sintering process is possible at lower
temperature, effectively decreasing the manufacturing cost.
[0014] However, the above method is problematic because the pore
size is limited due to the gas-solid nitridation mechanism and the
particle size of post-sintered silicon nitride, making it
impossible to form pore channels having the desired size.
[0015] Accordingly, the present invention has been made keeping in
mind the problems encountered in the related art and the present
invention is intended to provide a porous sintered reaction-bonded
silicon nitride ceramic which ensures that pore channels have a
sufficient size, and a method of manufacturing the same.
[0016] On the other hand, according to the prior invention, fine
pore channels which are relatively uniform are formed. In the case
where a porous ceramic having such fine pore channels having a
uniform size is applied to a diesel particulate filter, particulate
capturing efficiency is high but air permeability is not ensured
attributable to the size of fine pore channels, undesirably causing
large back pressure when the filter is operated, and consequently
performance of the system to which the corresponding filter is
mounted may deteriorate.
[0017] Accordingly, the present invention is intended to provide a
porous sintered reaction-bonded silicon nitride ceramic in which
the pore channel size is controlled so that both coarse pores and
fine pores are formed together in the ceramic thus simultaneously
increasing air permeability and capturing performance, and a method
of manufacturing the same.
[0018] Also the present invention is intended to provide granular
powder for sintered reaction-bonded silicon nitride ceramics,
having strength adapted for compacting, and a compact of the
granule powder.
[0019] An aspect of the present invention provides a porous
sintered reaction-bonded silicon nitride ceramic, including an
array of sintered granules having fine pore channels in the
sintered granules and coarse pore channels formed between the
sintered granules.
[0020] In this aspect, the porous sintered reaction-bonded silicon
nitride ceramic may exhibit a bimodal pore distribution having a
first peak and a second peak having a pore size larger than that of
the first peak, and the first peak is based on the fine pore
channels and the second peak is based on the coarse pore
channels.
[0021] In this aspect, the pore size of the first peak may fall in
a range of less than 1 .mu.m, and the pore size of the second peak
preferably falls in a range of 1 .mu.m or more, and more preferably
falls in a range of 5.about.20 .mu.m.
[0022] In this aspect, the average diameter of the sintered
granules may fall in a range of 30.about.150 .mu.m.
[0023] In this aspect, the maximum frequency diameter of the
sintered granules may fall in a range of 50.about.150 .mu.m.
[0024] Another aspect of the present invention provides a method of
manufacturing a porous sintered reaction-bonded silicon nitride
ceramic, including granulating a material composed of silicon and a
sintering additive for preparing a sintered reaction-bonded silicon
nitride from the silicon, thus obtaining material granules;
subjecting the material granules to pressureless compacting in a
mold, thus producing a material compact; and subjecting the
material compact to reaction-bonding in a nitriding gas atmosphere
and post-sintering in a nitrogen atmosphere.
[0025] In this aspect, the sintering additive may include yttria
and alumina, and may be used in an amount of 2.about.10 wt % based
on complete nitridation of the silicon.
[0026] In this aspect, the maximum weight frequency of the granules
may fall in a range of 30.about.150 .mu.m.
[0027] A further aspect of the present invention provides a method
of manufacturing a porous sintered reaction-bonded silicon nitride
ceramic, including granulating a material comprising silicon and a
sintering additive for preparing a sintered reaction-bonded silicon
nitride from the silicon, thus obtaining material granules;
pre-sintering the material granules in an inert atmosphere, thus
obtaining pre-sintered granules; subjecting the pre-sintered
granules to pressing, thus producing a material compact; and
subjecting the material compact to reaction-bonding in a nitriding
gas atmosphere and post-sintering in a nitrogen atmosphere.
[0028] In this aspect, the sintering additive may include at least
one of alkali earth metal oxides.
[0029] In this aspect, post-sintering may be performed at
1700.about.1900.degree. C.
[0030] In this aspect, the sintering additive may be used in an
amount of 2.about.10 wt % based on complete nitridation of the
silicon.
[0031] In this aspect, pressing may be performed at a pressure of
1.about.20 MPa.
[0032] Still another aspect of the present invention provides
pre-sintered granular powder for sintered reaction-bonded silicon
nitride ceramics, which is spherical porous granules having open
pores and composed of silicon and a sintering additive for
high-temperature liquid sintering conducted after nitridation of
the silicon, the sintering additive including yttria, alumina and a
compound thereof, the granules having a yield strength of
1.about.20 MPa. As such, the yield strength of the granular powder
may be 5 MPa or more.
[0033] In this aspect, the granular powder may have flowability of
0.2.about.0.5 g/sec.
[0034] Yet another aspect of the present invention provides a
compact for sintered reaction-bonded silicon nitride ceramics,
which is a compact of spherical porous granules having fine pore
channels and composed of silicon and a sintering additive for
high-temperature liquid sintering conducted after nitridation of
the silicon, the sintering additive including yttria, alumina and a
compound thereof, the porous granules having a maximum weight
frequency in a range of 30.about.150 .mu.m, the compact having a
pore structure including fine pore channels in the porous granules
and coarse pore channels between the porous granules.
[0035] In this aspect, the coarse pore channels may include coarse
pores of 1 .mu.m or more.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] The features and advantages of the present invention will be
more clearly understood from the following detailed description
taken in conjunction with the accompanying drawings, in which:
[0037] FIGS. 1A, 1B and 1C are photographs showing raw granules
having various sizes sorted using sieving according to an
embodiment of the present invention;
[0038] FIG. 2 is an enlarged photograph of the raw granules
according to the embodiment of the present invention;
[0039] FIG. 3 is a graph showing the weight distribution of samples
sorted using sieving according to an embodiment of the present
invention;
[0040] FIG. 4 is a graph showing the nitridation rate of the
nitrided specimens according to an embodiment of the present
invention;
[0041] FIG. 5A is a graph showing the porosity of SD5-RBSN and
SD5-SRBSN specimens according to an embodiment of the present
invention, and FIG. 5B is a graph showing the shrinkage and weight
change of SD5-SRBSN;
[0042] FIG. 6A is a graph showing the porosity of SD51-RBSN and
SD51-SRBSN specimens, and FIG. 6B is a graph showing the shrinkage
and weight change of SD51-SRBSN;
[0043] FIG. 7 is of scanning electron micrographs showing
SD51-SRBSN according to an embodiment of the present invention;
[0044] FIG. 8 is of scanning electron micrographs showing the
fracture surface of SD5-SRBSN at different granule sizes according
to an embodiment of the present invention;
[0045] FIG. 9 is of optical micrographs showing the polished
surface of SD5-SRBSN after resin impregnation, according to an
embodiment of the present invention;
[0046] FIG. 10 is a graph showing results of analysis of X-ray
diffraction (XRD) of SD5-SRBSN according to an embodiment of the
present invention;
[0047] FIGS. 11A and 11B are graphs showing the size distribution
of pore channels of SD5-SRBSN using mercury porosimetry according
to an embodiment of the present invention;
[0048] FIG. 12 is a graph showing cumulative specific surface area
of SD5-SRBSN depending on the particle size according to an
embodiment of the present invention;
[0049] FIGS. 13A and 13B are graphs showing the results of mercury
porosimetry of SD5-SRBSN and SD51-SRBSN according to an embodiment
of the present invention;
[0050] FIG. 14 is a graph showing the shrinkage and porosity
behavior of SD51-SRBSN depending on the sintering temperature
according to an embodiment of the present invention;
[0051] FIGS. 15A and 15B are graphs showing the cumulative specific
surface area change of SD51-SRBSN depending on the pore
distribution and the pore size at different sintering temperatures
according to an embodiment of the present invention;
[0052] FIG. 16 is a schematic view showing the pore structure of
the porous sintered ceramic according to the present invention;
[0053] FIG. 17A is a photograph showing the shape of raw granules
before pre-sintering, FIG. 17B is a photograph showing the shape of
granules after pre-sintering and grinding, and FIG. 17C is a
photograph showing the polished cross-section of granules having
been subjected to isostatic pressing at a pressure of 100 MPa after
pre-sintering and grinding;
[0054] FIG. 18 is a graph showing the load-displacement relation of
granules according to the present invention;
[0055] FIG. 19 is a graph showing the pressing density-pressing
pressure relation of the granules according to the present
invention;
[0056] FIGS. 20A and 20B are photographs showing the cross-section
of raw granules uniaxially pressed at 3.7 MPa and 18.6 MPa
according to the present invention;
[0057] FIGS. 21A to 21D are photographs showing the cross-section
of pre-sintered granules uniaxially pressed at 3.7 MPa, 7.5 MPa,
18.6 MPa and 46.6 MPa according to the present invention;
[0058] FIGS. 22A to 22E are scanning electron micrographs showing
raw granules and pre-sintered granules according to an embodiment
of the present invention;
[0059] FIGS. 23A and 23B are graphs showing the porosity, shrinkage
and weight loss of a uniaxially pressed compact of m76.5
pre-sintered granules depending on the post-sintering temperature
according to the present invention;
[0060] FIGS. 24A to 24C are low-magnification (.times.300)
photographs showing the fracture surface of the post-sintered
specimens of m76.5 pre-sintered granules at 1700.degree. C.,
1800.degree. C. and 1900.degree. C. according to the present
invention;
[0061] FIGS. 25A to 25C are high-magnification (.times.10 k)
photographs showing the inside of the granules of respective
specimens of FIGS. 24A to 24C;
[0062] FIGS. 26A and 26B are graphs showing the pore distribution
and the specific pore surface area of the post-sintered specimens
of FIGS. 24A to 24C using mercury porosimetry;
[0063] FIG. 27 is a graph showing air permeability of the sintered
specimens at 1700.degree. C. and 1800.degree. C., among the
post-sintered specimens of FIGS. 24A to 24C
[0064] FIGS. 28A and 28B are graphs showing the porosity and
shrinkage (weight loss) of the post-sintered specimens at different
granule sizes using sorting according to the present invention;
[0065] FIG. 29 is of photographs showing the fracture surface of
the post-sintered specimens of FIGS. 28A and 28B at different
granule sizes;
[0066] FIG. 30 is of photographs showing the polished surface of
the post-sintered specimens of FIGS. 28A and 28B after resin
impregnation;
[0067] FIGS. 31A and 31B are graphs showing the results of mercury
porosimetry of the post-sintered specimens of FIGS. 28A and 28B;
and
[0068] FIG. 32 is a graph showing the air permeability of the
post-sintered specimens of FIGS. 28A and 28B.
DESCRIPTION OF SPECIFIC EMBODIMENTS
[0069] Hereinafter, a detailed description will be given of the
present invention with reference to the appended drawings.
[0070] In the present invention, the term "pressureless compacting"
is used to include the meaning of a mold being packed with powder
by tapping using for example vibrations or shaking of the mold, as
well as including the typical meaning of a mold being packed using
the own weight of the powder instead of applying pressure when
compacting the powder.
[0071] Also in the present invention, the term "reaction-bonding"
indicates a process of simultaneously generating a chemical
reaction and the sintering of a material into a desired target
compound using thermal treatment. The term "reaction-bonding"
originally represents the features of the process but is currently
used to specify a sintered ceramic, for example, sintered
reaction-bonded silicon nitride (SRBSN) resulting from nitridation
and sintering of a silicon precursor.
[0072] A. Granulation of Material Powder Including Si+Sintering
Additive
[0073] In the present invention, granular powder was manufactured
using a spray drying process. The granular powder includes silicon
(Si) and a sintering additive for accelerating the nitridation and
sintering of Si. Examples of the sintering additive for sintering
silicon nitride may include a typical binary high-melting-point
sintering additive composed of yttria and alumina, and ternary or
more low-melting-point sintering additives further including an
alkali earth metal oxide such as MgO, CaO, SrO, BaO or the like in
addition to the binary sintering additive composition. Furthermore,
a SiO.sub.2 film is typically formed on the surface of Si, and this
SiO.sub.2 film may support the sintering in a subsequent sintering
process along with the sintering additive.
[0074] The sintering additive may be used in an amount of
2.about.10 wt % based on Si.sub.3N.sub.4 resulting from the
complete nitridation of Si. In the present invention, as the amount
of sintering additive increases, the strength of the sintered
ceramic may be enhanced.
[0075] In an embodiment according to the present invention, as
shown in Table 1 below, Si powder was mixed with a sintering
additive including high-melting-point YA
(Y.sub.2O.sub.3--Al.sub.2O.sub.3, T.sub.eu=1370.degree. C.) or
low-melting-point YAC(Y.sub.2O.sub.3-Al.sub.2O.sub.3--CaO,
T.sub.eu=1170.degree. C.) and then granulated at different atomizer
rotation speeds (1,000 and 10,000 rpm).
[0076] Also, a spray slurry was prepared using ball milling. As
such, the ratio of solid and water was 1:1, and the sintering
additive was added in an amount of 3 wt % based on Si.sub.3N.sub.4
resulting from the complete nitridation of Si. Furthermore, a
dispersant was used in the amount of 0.1.about.0.8 wt % based on
solid content (Si+sintering additive), and a binder was used in the
amount of 2.about.5 wt % based on solid content (Si+sintering
additive). Upon spray drying, the rotation speed of the stirrer was
100 rpm, and the inlet and outlet temperatures thereof were
maintained at 150.about.30010 and 80.about.120.degree. C.,
respectively.
TABLE-US-00001 TABLE 1 Pre- Strength Sintering Sinter- of Molding
additive Temp. ing Granules Process YA
Y.sub.2O.sub.3--Al.sub.2O.sub.3-- 1370.degree. C. x Weak Tapping,
(SiO.sub.2) Pressureless Compacting YAC
Y.sub.2O.sub.3--Al.sub.2O.sub.3-- 1170.degree. C. .smallcircle.
Strong Uniaxial CaO--(SiO.sub.2) Pressing, Extrusion
[0077] Because of the spray drying, coarse granules (50.about.250
.mu.m) were formed under conditions of the low-speed atomizer
(1,000 rpm), and the yield of granules was low on the order of 1%
or less for 100 g of Si loaded. Also, the yield was 40% or more at
10,000 rpm. The granule size and the yield were increased in
proportion to an increase in the loaded amount and the amount of
added binder. The granule size was similar but the yield was
greater when using the YAC type sintering additive than when using
the YA type sintering additive (Table 2).
TABLE-US-00002 TABLE 2 Yield for Sintering PVA Rotation Granule 100
g of Si Sample additive (wt %) Speed (rpm) Size (.mu.m) Loaded (%)
SD4 YA 2 10,000 30~150 43.7 SD5 YA 5 10,000 30~150 57.1 SD6 YAC 2
10,000 30~250 69.8
[0078] Subsequently, SD4 (Y.sub.2O.sub.3:Al.sub.2O.sub.3=2:1; PVA 2
wt %) granules were sorted using sieving and the outer appearance
thereof was then observed using a scanning electron microscope
(SEM). FIG. 1A shows granules having a size of 45.about.63 .mu.m
(which is referred to as "m54" corresponding to the mean granule
size), FIG. 1B shows granules having a size of 90.about.125 .mu.m
(m107.5), and FIG. 10 shows granules having a size of 125.about.150
.mu.m (m137.5). From these photographs, small granules can be seen
to have maintained their spherical shape and suffered only slight
damage (FIGS. 1A and 1B), whereas large granules are observed to be
partially damaged due to the impacts received upon sieving (the
arrows in FIG. 1C).
[0079] FIG. 2 is an enlarged photograph of the surface of the
granules composed of particles smaller than the starting Si having
an average particle size of 2 .mu.m. This is considered to be
because the particle size of the added sintering additive is
smaller than that of Si and also because the Si particles which are
a main material are pulverized in the course of milling, thus
reducing the average particle size.
[0080] Also, the weight distribution in relation to the size of
sieved granules was measured, and this showed that the size
distribution is similar regardless of the sintering additive
composition and the amount of added binder under conditions of the
same atomizer rotation speed (FIG. 3). Almost all particles of
respective samples as a result of sieving can be seen to have a
size of 30.about.150 .mu.m, and about 50 wt % of the granules are
present in the size range of 90.about.106 .mu.m.
[0081] B. Manufacture of Porous Sintered Reaction-Bonded Silicon
Nitride Ceramic Using Pressureless Packing
[0082] (1) Pressureless Packing
[0083] Pressureless compacting was performed using a granular
powder composition including a high-melting-point YA type sintering
additive as shown in Table 3 below. In Table 3, the amounts of
respective components are given based on 100 g of Si loaded, and
SD5 granules including a sintering additive are represented by 97
wt % Si.sub.3N.sub.4-2 wt % Y.sub.2O.sub.3-1 wt % Al.sub.2O.sub.3
under conditions of the complete nitridation of Si. In Table 3
below, SD51 is a composition in which the amount of sintering
additive is doubled (94 wt % Si.sub.3N.sub.4-4 wt %
Y.sub.2O.sub.3-2 wt % Al.sub.2O.sub.3) compared to SD5.
[0084] In order to evaluate the properties and phase of a
microstructure in terms of for example porosity, coarse pore
channel size and specific surface area of pores depending on the
granule size, the granules were sieved and sorted into granules
having the mean granule size of 38.5 .mu.m (in the range of
32.about.45 .mu.m), granules having the mean granule size of 54
.mu.m (in the range of 45.about.63 .mu.m), granules having the mean
granule size of 76.5 .mu.m (in the range of 65.about.90 .mu.m) and
granules having the mean granule size of 107.5 .mu.m (in the range
of 90-125 .mu.m). For comparison, unsieved granules (as-SD) were
also prepared. Below, respective specimens were expressed by m38.5,
m54, m76.5, m107.5 and as-SD. In the case of sorted granules using
sieving, coarse granules having a size of 125 an or more were not
tested. The unsieved as-SD includes granules smaller than 32 .mu.m
and coarse granules greater than 125 .mu.m.
TABLE-US-00003 TABLE 3 Sample Si Y.sub.2O.sub.3 Al.sub.2O.sub.3 PAA
PVA SD5 100 3.43 1.72 0.53 5.26 SD51 100 7.09 3.54 0.55 5.53
[0085] The pressureless compacting was performed through tapping so
as to pack a mold with powder.
[0086] The mold used was a cylindrical graphite mold having an
inner diameter of 30 mm, and a plurality of holes was formed in the
upper plate and another in the lower plate of the mold so as to
enable inflow and outflow of the nitriding gas. The thickness of a
disk specimen was maintained uniform by the two plates. About 2 g
of granules were charged into the mold, and then tapped so as to
obtain a compact about 4 mm thick. The density of the tapped
compact was about 0.71 g/cm.sup.3 and the packing ratio was about
30%.
[0087] (2) Nitridation and Sintering of Granular Powder Compact
[0088] The granular powder compacts (SD5, SD51) were nitrided. The
nitridation was carried out using a tube furnace at 1450.degree. C.
Subsequently, the nitrided specimens (respectively referred to as
"SD5-RBSN" and "SD51-RBSN") were post-sintered at 1700.degree. C.
in a nitrogen atmosphere of 0.1 MPa for 2 hours, thus manufacturing
post-sintered specimens (respectively referred to as "SD5-SRBSN"
and "SD51-SRBSN").
[0089] FIG. 4 is a graph showing the nitridation rate of SD5-RBSN.
When the nitridation rate of 90% or more is obtained based on loss
due to volatilization of a material during nitridation, the
nitridation is typically regarded as being completed. In the
present test, when the specimen denoting the lowest nitridation
rate was subjected to XRD, residual Si was not detected, and thus
the nitridation was confirmed to be completed in all the
specimens.
[0090] FIG. 5A is a graph showing the porosity of SD5-RBSN and
SD5-SRBSN, FIG. 5B is a graph showing the shrinkage and weight
change of SD5-SRBSN, and FIG. 6A is a graph showing the porosity of
SD51-RBSN and SD51-SRBSN, and FIG. 6B is a graph showing the
shrinkage and weight change of SD51-SRBSN.
[0091] With reference to FIGS. 5A and 5B, all the specimens show a
porosity of about 65.about.70% regardless of the granule size. The
reason why the porosity of SD5-SRBSN is smaller than that of
SD5-RBSN despite the shrinkage of SD5-SRBSN being approximately
zero is considered to be because the packing powder is charged in
the porous specimen in the course of post-sintering, thus
increasing the weight.
[0092] On the other hand, in the case of SD51-RBSN in which the
amount of sintering additive is doubled, the porosity thereof can
be seen to be slightly larger than that of SD5-RBSN (FIGS. 6A and
6B). This is considered to be because low-density k-phase
(YSiO.sub.2N, p=0.714 g/cm.sup.3) is deposited due to the use of
the sintering additive the amount of which has increased. In the
case of SD51-SRBSN, the weight seldom changes and the shrinkage is
determined to be 4.about.6%, thus reducing the porosity.
Furthermore, the porosity can be seen to be about 65% at different
granule sizes.
[0093] FIG. 7 is of SEM photographs showing SD51-SRBSN, from which
intrinsic acicular morphology of silicon nitride particles can be
seen to be developed through post-sintering. Such a microstructure
enlarges the specific surface area of the pores and also forms a
complicated pore channel structure, and is thus expected to
increase the efficiency with which fine dust is captured.
[0094] FIG. 8 is of SEM photographs showing the fracture surface of
SD5-SRBSN at different granule sizes. From this, raw granules
having low strength can be seen to be manufactured into a porous
ceramic in which a spherical shape of granules is maintained using
tapping, reaction-bonding and post-sintering. Moreover, silicon
nitride in a whisker phase which is observed with white contrast in
the void space between the granules plays a role in increasing the
specific surface area of a filter and thus has a favorable
influence on capturing nano particles.
[0095] In order to directly observe the coarse pore channels formed
between spherical granules of SD5-SRBSN, SRBSN was impregnated with
resin and the polished surface thereof was observed using an
optical microscope. The results are shown in FIG. 9. From FIG. 9,
almost all of the granules can be seen to maintain the spherical
shape obtained shortly after spray drying, in which dot contacts
are prevalently formed between the granules and thus continuous
pore channels are developed in proportion to the granule size. The
white contrast in the granules indicates the resin portion
incorporated into the pores in the granules. According to the
present invention, because of the formation of not only coarse pore
channels between the granules but also fine pore channels in the
granules, the specific surface area of the pores may be enlarged,
thereby increasing the performance of capturing nano particles.
[0096] FIG. 10 is a graph showing results of XRD of SD5-SRBSN, from
which only the .beta.-Si.sub.3N.sub.4 peaks (represented by
.quadrature.) can be observed due to complete phase conversion,
without detection of .alpha.-Si.sub.3N.sub.4 and other second
phases.
[0097] FIGS. 11A and 118 are graphs showing the pore channel size
distributions of SD5-SRBSN using mercury porosimetry, and FIG. 12
is a graph showing the specific surface area. In the present test,
the mercury porosimeter used was Autopore IV 9510 available from
Micromeritics, and measurement conditions were an equilibration
time 10 sec, and the stem volume used 25% or more.
[0098] With reference to FIGS. 11A and 11B, the size of the pore
channels of all the specimens can be seen to show a bimodal
distribution including peaks based on fine pores equal to or
smaller than 1 .mu.m and peaks based on coarse pores ranging from 1
.mu.m to about 10 .mu.m. More specifically, in the case of fine
pores, the maximum peak is observed in the range of 0.1.about.1
.mu.m, and the maximum peak of the coarse pores is observed in the
range of 1.about.20 .mu.m.
[0099] Because fine pores are formed in the granules, they are not
dependant on the granule size and have a similar size in all the
specimens. However, the size of coarse pore channels has a tendency
to depend on the granule size, and unsorted raw granules (as-SD)
show the second largest pore size.
[0100] Whereas comparatively coarse particles are captured by an
impact mechanism depending on the size and weight, fine nano
particles are captured by a diffusion mechanism due to Brownian
movement. Thus, as the specific surface area of the pores of the
porous ceramic increases, the efficiency of capturing nano
particles is raised. As shown in FIG. 12, the specific surface area
of the porous sintered reaction-bonded silicon nitride ceramic made
from granular Si mixture powder according to the present invention
is about 1.9.about.2.3 m.sup.2/g which is about 10 times larger
than about 0.2 m.sup.2/g which is the specific surface area of a
SiC filter mounted to a commercially available diesel particulate
filter (DPF), and is thus expected to exhibit high efficiency of
capturing nano particles. On the other hand, the specific surface
area of the coarse pores between the granules is 0.5 m.sup.2/g or
less in all the specimens as shown in the graph, from which the
specific surface area of the fine pores in the granules can be seen
to be extremely large. As is apparent from the graph, the granule
size-specific surface area relation has a tendency of being the
opposite of the granule size-coarse pore channel size relation.
Specifically, the smallest specific surface area was measured in
the specimen composed of the largest granules (m107.5). In
particular, the largest specific surface area was measured in the
specimen composed of the unsorted raw granules (as-SD). This is
because the raw granules without being subjected to sorting using
sieving include a plurality of granules smaller than the smallest
m38.5 granules used in the present test.
[0101] FIGS. 13A and 13B are graphs showing the results of mercury
porosimetry of SD5-SRBSN and SD51-SRBSN. Whereas SD51-SRBSN
including a large amount of sintering additive has a large pore
channel size, the specific surface area of pores is seen to be
greater in SD5-SRBSN including a small amount of sintering
additive. Thereby, it appears that the pore structure of a liquid
sintered system is dominated by the absolute amount of produced
liquid. When the graph is strictly analyzed, both the coarse pore
channels (-10 .mu.m) between the granules and the fine pore
channels (<1 .mu.m) in the granules are larger for SD51 than for
SD5. Compared to SD5 composed of isotropic particles, SD51 composed
of acicular particles has a smaller specific surface area of pores.
This is because the morphology of the particles is changed and the
particles actively grow owing to the increased amount of liquid.
Even in this case, however, the specific surface area of pores is
measured to be about 1.0 m.sup.2/g and is regarded as larger
compared to a commercially available SiC filter composed of coarse
isotropic particles.
[0102] In order to evaluate the optimal sintering conditions of
SD51 granules depending on the sintering temperature, specimens
were manufactured at different sintering temperatures of
1700.degree. C., 1800.degree. C. and 1900.degree. C. and the
properties thereof were measured. The results are shown in FIG. 14.
Although shrinkage and porosity upon sintering at 1700.degree. C.
were similar to shrinkage and porosity upon sintering at
1800.degree. C., the porosity was remarkably decreased due to
drastic shrinkage upon sintering at 1900.degree. C. In particular,
although the weight loss upon sintering at 1800.degree. C. or lower
is approximately zero, sintering at 1900.degree. C. is accompanied
by the weight loss of about 10% due to the decomposition of silicon
nitride despite a nitrogen pressure atmosphere of 0.9 MPa,
undesirably weakening the strength of the sintered ceramic.
[0103] Among SD51 specimens, pore structures of the porous sintered
ceramics at 1700.degree. C. and 1800.degree. C., but not for the
ceramic sintered at 1900.degree. C. which has a strength and
shrinkage unsuitable for a filter, were analyzed. The results are
FIGS. 15A and 15B. As shown in FIGS. 15A and 15B, the coarse pore
channel sizes are similar in the above two sintered ceramics having
similar porosity and shrinkage, but the fine pore channels have
grown to 1 .mu.m or less in the sintered ceramic at 1700.degree. C.
but have grown to 1 .mu.m or more in the sintered ceramic at
1800.degree. C. The specific surface area of pores was measured to
be large in the sintered ceramic at 1700.degree. C. the fine pore
size of which is comparatively small.
[0104] As is apparent from the above embodiment, the sintered
ceramic having a pore structure in which both coarse pores and fine
pores are formed can be manufactured from the granular powder
according to the present invention.
[0105] FIG. 16 schematically depicts the pore structure of the
porous sintered ceramic according to the present invention. As
shown in the left drawing, when the granular powder according to
the present invention is subjected to pressureless compacting,
coarse pores having a predetermined size are formed between the
pre-sintered powder particles. These pores are connected between
the layered powder particles, thus forming pore channels.
[0106] Then, when the granular Si powder compact is nitrided and
post-sintered, the sintering additive contained in the material
powder forms a liquid phase in the course of heating and the formed
liquid phase remains in the fine pores in the granular powder
because of the capillary principle, and thus aids the intragranular
sintering but does not pact the coarse pores. As a result, as shown
in the right drawing, the microstructure in which the granules
having fine pores therein are arranged through dot contacts may be
obtained. This microstructure is advantageous because the sintered
granules constitute almost the same shape as the shape of the
compact and coarse pore channels are formed between the
granules.
[0107] The size of such coarse pores is dependent on the size of
the granular powder. For example, assuming that pre-sintered
granules have the same size and are layered in a very dense
structure, the minimum size of coarse pores is theoretically
determined to be about 0.077*D (D is the diameter of a powder), and
the pore diameter calculated from an equivalent area is about
0.23*D. As mentioned in the embodiment, in the case where the
granules have a size of 30.about.150 .mu.m, coarse pore channels of
at least 1 .mu.m or at least 10 .mu.m may be ensured, and, in the
granules, as silicon nitride is produced through nitridation and
then sintered, fine pore channels of less than 1 .mu.m may be
formed.
[0108] Therefore, the Si granular powder according to the present
invention may provide porous sintered reaction-bonded silicon
nitride ceramics having a microstructure in which both coarse pores
and fine pores are formed through nitridation and
post-sintering.
[0109] C. Manufacture of Porous Sintered Reaction-Bonded Silicon
Nitride Ceramics from Pre-sintered Granules
[0110] (1) Preparation of Pre-Sintered Granular Powder
[0111] Typically, the strength of granules resulting from a spray
drying process is weak on the order of 0.5 MPa or less. Hence, when
a pressing process is applied, granules are broken in a typical
pressing pressure range, thus making it impossible to maintain the
spherical shape.
[0112] In the present invention, the Si granular powder obtained
per the above was pre-sintered, thus manufacturing pre-sintered
granular powder able to ensure compacting strength.
[0113] The pre-sintering process of enhancing the strength of Si
mixture powder granules may take into consideration the following
two items. First, heating is performed at a temperature equal to or
higher than a eutectic liquid temperature in an inert (albeit
non-nitrogen) atmosphere able to prevent the oxidation of material
powder, thereby inducing the intragranular sintering through liquid
phase sintering. Second, thermal treatment is performed in a
nitrogen atmosphere thereby inducing the nitridation, so that the
strength in the granules may be enhanced. In the above two
pre-sintering treatments, undesired intergranular sintering may
take place along with desired intragranular sintering, and thus
there may occur a case where it is difficult to maintain the
spherical shape of granules after grinding. Hence, the process
conditions such as temperature, time and atmosphere should be
controlled.
[0114] The granular powder used in the present embodiment includes
sorted granules having the mean granule size of 38.5 .mu.m
(32.about.45 .mu.m, referred to as "m38.5"), 54 .mu.m (m54), 76.5
.mu.m (m76.5) and 107.5 .mu.m (m107.5) through sieving, and
unsieved granules (as-SD) of all sizes.
[0115] (a) Pre-Sintering Via Nitridation
[0116] The granules including high-melting-point YA (SD4;
Y.sub.2O.sub.3:Al.sub.2O.sub.3=2:1) were pre-sintered through
nitridation under conditions of 1300.degree. C.-6 h. Because the
eutectic liquid temperature of the same type is 1370.degree. C.,
heating to 1370.degree. C. or higher is required to induce
pre-sintering via liquid phase sintering. However, because the
above high temperature is very approximate to the melting point
(1412.degree. C.) of the main material Si, stable process control
is considered to be difficult, and thus, a nitridation mechanism
performed at a temperature equal to or lower than the eutectic
liquid temperature was adopted. The nitridation conditions are set
to induce the nitridation at a medium level so as to facilitate the
separation of granules by grinding after pre-sintering. Taking
measurements showed that the nitridation rate of 65.6% was
obtained.
[0117] However, not only intragranular sintering but also
intergranular sintering were carried out by virtue of the
pre-sintering process using nitridation, and most spherical
granules were observed to be broken into the angular shape by
grinding. FIG. 17A shows the granules before pre-sintering, and
FIG. 17B shows the granules after pre-sintering and grinding. Also,
the ground and pre-sintered granules were subjected to isostatic
pressing at a pressure of 100 MPa, and the cross-section thereof
was polished and observed. The results are shown in FIG. 17C. As
can be seen in FIG. 17C, granules having a spherical shape are
partially observed, from which the shape of the granules resulting
from such pre-sintering can be confirmed to be maintained despite
the subsequent pressing at high pressure.
[0118] (b) Pre-Sintering in Inert Atmosphere
[0119] In the present invention, pre-sintering was performed in an
inert atmosphere so that intergranular sintering did not occur via
nitridation upon pre-sintering of granular powder. The
pre-sintering temperature was set to be equal to or lower than the
melting point of Si.
[0120] The low-melting-point type composition may be pre-sintered
via liquid phase sintering at a comparatively low temperature, thus
facilitating the separation of granules. Hence, even after
pre-sintering, the pre-sintered granular powder the spherical shape
of which is maintained is considered to be obtained. For example,
because the eutectic liquid temperature of the YAC type composition
is 1170.degree. C. which is different from the melting point
(1412.degree. C.) of Si, when thermal treatment is carried out in
an inert gas atmosphere such as Ar at a temperature equal to or
higher than the eutectic liquid temperature, the nitridation is
excluded, so that the intergranular sintering insignificantly
occurs and liquid phase sintering is carried out in the granules by
virtue of the sintering additive.
[0121] The YAC type sintering additive used in the present
embodiment is shown in Table 4 below, in which
Y.sub.2O.sub.3:Al.sub.2O.sub.3=2:1 by weight ratio is maintained
and the amount of CaO is set to maintain the proportional relation
of Al.sub.2O.sub.3 and CaO corresponding to the process liquid
composition in the ternary Al.sub.2O.sub.3--SiO.sub.2--CaO phase
system (Table 4).
TABLE-US-00004 TABLE 4 Unit: g Si Y.sub.2O.sub.3 Al.sub.2O.sub.3
CaO PAA PVA SD6 100 2.67 1.33 1.15 0.53 2.12
[0122] To find suitable pre-sintering conditions, thermal treatment
was performed for 10 min using a tube furnace in an Ar atmosphere
at 1200.degree. C., 1300.degree. C. and 1350.degree. C. equal to or
higher than the eutectic liquid temperature (PG1, PG3, PG4,
respectively). Furthermore, 60 min treatment at 1200.degree. C. was
performed (PG2).
[0123] FIG. 22 shows the microstructures of surfaces of raw
granules and pre-sintered granules. In the case of the granules
pre-sintered at 1200.degree. C. (for 10 min in FIG. 22B, and for 60
min in FIG. 22C), the pores between the particles are observed to
be filled with the amorphous liquid produced between the particles,
thus decreasing the surface roughness. Furthermore, when the
pre-sintering temperature was increased to 1300.degree. C. (FIG.
22D) and 1350.degree. C. (FIG. 22E), the particles were actively
integrated by the progress of liquid phase sintering, so that
particle agglomerates are clearly discerned from each other. As
results of phase analysis using XRD, h-phase
(Y.sub.5Si.sub.3O.sub.12N) was detected under all the pre-sintering
temperature conditions, which indicates that liquid is
interposed.
[0124] The powder flowability before and after pre-sintering was
measured according to JIS Z 2502-1979. The flowability was compared
by drying about 5 g of granules in an oven at 105.degree. C. for 1
hour to be dewatered, cooling the dried granules to room
temperature in a desiccator, and measuring the time required to
pass such granules through an orifice having a diameter of 0.1''
(2.54 mm). As such, the flowability of the pre-sintered granules
was measured in a state of any grinding process not being performed
after pre-sintering.
[0125] In the case of sorted m107.5, the flowability values of raw
granules (as-SD), 1200.degree. C.-10 min pre-sintered granules
(PG1) and 1350.degree. C.-10 min pre-sintered granules (PG4) were
measured to be 0.4136 g/sec, 0.4068 g/sec, and 0.3180 g/sec,
respectively. From this, the flowability of the pre-sintered
granules can be seen to be similar to that of the raw granules.
This is because intergranular sintering is inhibited in the course
of pre-sintering, thus obtaining powder for which the separation of
granules is easy. The reason why the flowability of PG4 is slightly
reduced is considered to be due to an increase in surface roughness
by the agglomeration of particles as shown in FIG. 22E.
[0126] According to the method of the present invention,
intergranular sintering is inhibited and thus powder for which the
separation of the granules is easy may be obtained.
[0127] The properties of the resultant pre-sintered granular powder
compact are described below.
[0128] (2) Pressing Behavior of Pre-sintered Granular Powder
[0129] In order to evaluate the strength of raw granules and
pre-sintered granules, a cylinder mold having a diameter of 10 mm
was packed with the granules under their own weight, and the
load-displacement relation was determined under
displacement-controlled loading (0.5 mm/min) (FIG. 18), and the
weight of the granules used in the test was substituted thereto,
thus determining the pressing density-pressing pressure relation
(FIG. 19).
[0130] The drastic increase in the load at a predetermined
displacement or more is based on the increase in the pressing
density after breaking the granules, and thus the gradual load
increase and the large displacement observed in the previous step
are known to be caused by the flow and deformation of granules. In
FIG. 18, the displacement size of raw granules observed under a
weak load until before the load is drastically increased is
remarkably larger than that of the pre-sintered granules, which
coincides with typically known pressing behavior. FIG. 18 shows the
load-displacement curves depending on the pre-sintering conditions.
Pre-sintering for 10 min at 1200.degree. C., 1300.degree. C. and
1350.degree. C. resulted in steeper load-displacement curves
because the degree of liquid phase sintering increased in
proportion to the increase in the sintering temperature.
Furthermore, similar behavior is manifested at different sintering
time periods at 1200.degree. C., which indicates that the
load-displacement curve is more greatly affected by the sintering
temperature than by the sintering time.
[0131] In FIG. 19, the pressure at the inflection point indicates
the yield strength at which the granules begin to break, and this
inflection point starts to drastically increase the density. In
FIG. 19, the yield strength of raw granules is 0.2 MPa or less, the
yield strength of 1200.degree. C. pre-sintered granules is about
5.about.6 MPa, the yield strength of 1300.degree. C. pre-sintered
granules is about 10 MPa, and the yield strength of 1350.degree. C.
pre-sintered granules is about 20 MPa. The strength is estimated to
be enhanced by about 25.about.100 times of as-spray dried weak
granules by pre-sintering. Though additionally shown in the present
embodiment, when a sample having 3 wt % or more of a sintering
additive is used, the yield strength is estimated to be higher than
the above values.
[0132] In order to compare the sphere stability of the pre-sintered
granules after pressing, the raw granules and the pre-sintered
granules were uniaxially pressed and then impregnated with resin,
after the polished surface thereof was observed using an SEM. The
1200.degree. C.-10 min pre-sintered granules (PG1) were taken.
Because the yield strength of the granules was measured to be about
5.about.6 MPa as shown in FIG. 19, four uniaxial pressing pressure
values, for example, 3.7 MPa and 7.5 MPa at which the spherical
shape of granules is considered to be stable, and 18.6 MPa and 46.6
MPa at which the granules may be deformed or broken, were
selected.
[0133] Observation of the polished surfaces revealed that the
spherical shape of the raw granules were completely broken at 3.7
MPa (FIG. 20A) and 18.6 MPa (FIG. 20B), whereas the spherical shape
of the pre-sintered granules was completely maintained at a
pressure equal to or lower than 7.5 MPa (FIGS. 21A and 21B) and was
deformed or broken at a pressure equal to or higher than 18.6 MPa
(FIGS. 21C and 21D). Typically, because uniaxial pressing is
carried out at about 5 MPa and extrusion is performed at about 8
MPa, the pre-sintered granules according to the present invention
are considered to have enough strength to be applied to a
compacting process useful on industrial sites.
[0134] (3) Production of Compact of Pre-sintered Granules for
Reaction Bonding
[0135] As described in the above embodiment, a compact having a
pore structure in which both coarse pores and fine pores are formed
together may be manufactured from the granular powder according to
the present invention.
[0136] When the pre-sintered granular powder according to the
present invention is subjected to uniaxial pressing, extrusion or
injection molding, coarse pores having a predetermined size are
formed between the pre-sintered powder particles. These pores are
connected between the layered powder particles, thus forming pore
channels (FIG. 16).
[0137] When such a Si granular powder compact is subjected to
nitridation and post-sintering, the sintering additive contained in
the material powder forms a liquid phase during the heating process
and the produced liquid phase remains in the fine pores in the
granules because of the capillary principle thus aiding the
intragranular sintering but not packing the coarse pores. As a
result, as shown in the right drawing of FIG. 16, the
microstructure having coarse pores of almost the same size as the
shape of the compact may be obtained.
[0138] Ultimately, the size of the coarse pores is dependent on the
size of the pre-sintered granules. For example, assuming that
pre-sintered granules have the same size and are layered in a very
dense structure, the minimum size of the coarse pores is
theoretically determined to be about 0.077*D (D is the diameter of
a powder), and the pore diameter calculated from an equivalent area
is about 0.23*D. However, because the pore size is larger at
typical low pressing pressures, in the case where the size of the
granules is in the range of 30.about.150 .mu.m, pore channels
having coarse pores equal to or more than 1.about.10 .mu.m may be
ensured. On the other hand, in the granules, as silicon nitride is
produced through nitridation and post-sintering, fine pore channels
of less than 1 .mu.m may be formed.
[0139] Thus, the Si granular powder according to the present
invention may be subjected to reaction-bonding and post-sintering,
thereby manufacturing porous sintered reaction-bonded silicon
nitride ceramics having a microstructure in which both coarse pores
and fine pores are formed together.
[0140] (4) Post-Sintering of Compact
[0141] Among the pre-sintered granules of SD6 including YAC, PG1
(1200.degree. C.-10 min pre-sintered) was added with an appropriate
amount of 5% PVA solution, uniaxially pressed at a pressure of 3.7
MPa equal to or lower than the yield strength of the granules, and
then dried in an oven at 105.degree. C. for 24 hours so as to be
dewatered.
[0142] The dried specimen was nitrided in a flowing nitrogen
atmosphere including hydrogen and post-sintered in a static
nitrogen atmosphere of 0.1.about.0.9 MPa, thus manufacturing a
porous ceramic.
[0143] In order to determine the optimal post-sintering
temperature, the porosity, shrinkage and weight loss of the
uniaxially pressed compact of m76.5 pre-sintered granules were
measured depending on the post-sintering temperature. The results
are shown in FIGS. 23A and 23B. Because of the increase in the
shrinkage in proportion to the sintering temperature, the porosity
was reduced, and the weight loss of about 7% was measured upon
sintering at 1800.degree. C. or higher despite the nitrogen
pressure atmosphere.
[0144] FIGS. 24A, 24B and 24C are low-magnification (.times.300)
photographs showing the fracture surface of the post-sintered
specimens at 1700.degree. C., 1800.degree. C. and 1900.degree. C.
using m76.5 pre-sintered granules. When sintering is performed at
1800.degree. C. or higher (FIGS. 24B and 24C), the silicon nitride
particles grown in a whisker phase are obviously observed in the
void space between the granules compared to when sintering is
performed at 1700.degree. C. (FIG. 24A).
[0145] FIGS. 25A, 25B and 25C are high-magnification (.times.10 k)
photographs showing the inside of the granules of respective
specimens of FIGS. 24A, 24B and 24C. The microstructure in which
intrinsic acicular morphology of silicon nitride particles has been
considerably developed is seen in all of the specimens.
[0146] FIGS. 26A and 26B are graphs showing the pore distribution
and the pore specific surface area of the above porous
post-sintered ceramics as measured using mercury porosimetry. As
the sintering temperature increases, the frequency of fine pores is
reduced and the size thereof increases, whereas the size of the
coarse pores is maintained at almost the same level. Typically,
because liquid phase sintering increases the amount of liquid and
decreases the viscosity thereof at higher temperature, it is known
to facilitate the densification and growth of particles. Also in
the present invention, the integration and growth of the particles
and the growth of the pores are simultaneously generated in the
granules, and thus the size of fine pores is estimated to be
increased.
[0147] As shown in FIG. 26B, the results of measuring the specific
surface area of pores coincide with those of FIG. 14A.
Specifically, in FIG. 26B, the specific surface areas defined by
the coarse pores between the granules are similar regardless of the
sintering temperature, but the specific surface area defined by
fine pores in the granules is drastically reduced due to the
integration of the pores in proportion to the increase in the
sintering temperature.
[0148] Furthermore, the specific surface area of the post-sintered
specimen at 1900.degree. C. is remarkably reduced due to excessive
shrinkage.
[0149] FIG. 27 is a graph showing the air permeability of the above
specimens sintered at 1700.degree. C. and 1800.degree. C. The
measurement device was CFP-1200-AEL available from Porous Materials
Inc. Measuring showed that two specimens resulting from different
sintering temperatures were measured to have similar permeability
constants (.kappa.=0.28.about.0.30.times.10.sup.12 m.sup.2). This
is thought to be because the size of coarse pore channels
dominating the air permeability is constant regardless of the
sintering temperature.
[0150] FIGS. 28A and 28B are graphs showing porosity and shrinkage
(weight loss) of post-sintered specimens having different granule
sizes, in which as-SD is unsorted pre-sintered granules, and
post-sintered specimens, for example, m38.5, m54, m76.5 and m107.5
indicate the sorted granules having the mean size. The sintering
temperature of these specimens was 1700.degree. C. The porosity was
measured to be similar regardless of the granule size, and was
about 50% which was slightly reduced compared to that of the
reaction-bonded silicon nitride (RBSN). This is estimated to be due
to the shrinkage more than the weight loss effect.
[0151] FIG. 29 is of photographs showing the fracture surfaces of
respective post-sintered specimens of FIGS. 28A and 28B at
different granule sizes. The m38.5 and m54 specimens having the
small granule size show that the pore channels between the granules
are blocked by the acicular silicon nitride particles. Thus, the
air permeability of the uniaxially pressed porous ceramic having a
small granule size is estimated to decrease.
[0152] FIG. 30 is of photographs showing the polished surfaces of
respective post-sintered specimens of FIGS. 28A and 28B after resin
impregnation. In the m38.5 specimen, unclear portions of boundaries
between the granules are observed. This is considered to be because
the channels between the granules are blocked by means of silicon
nitride particles formed between the granules and the deformation
of the granules due to uniaxial pressing pressure. When the granule
size is increased as in the m76.5 and m107.5 specimens, such
portions are not observed.
[0153] FIGS. 31A and 31B are graphs showing the results of mercury
porosimetry of respective post-sintered specimens of FIGS. 28A and
28B. The graphs show a bimodal distribution in which peaks
corresponding to fine pores of less than 1 .mu.m and peaks
corresponding to coarse pores of 1 .mu.m or more are formed
together. In the case of the m38.5 specimen having the smallest
granule size, the peak corresponding to the coarse pore channels is
not clearly observed, from which the channels are considered to be
blocked by the reaction product and the deformation of granules.
Not in the m107.5 specimen but in the m76.5 specimen, the maximum
coarse pore channel size and volume fraction were measured. This is
estimated to be because part of the coarse granules is broken upon
uniaxial pressing of the m107.5 specimen, thus partially blocking
the pores between the granules.
[0154] FIG. 32 is a graph showing the air permeability of
respective specimens. The m76.5 specimen exhibits the greatest air
permeability. As the granule size decreases, air permeability can
be seen to deteriorate.
[0155] The post-sintered reaction-bonded silicon nitride ceramic
according to the present invention has the microstructure phase
features described in FIG. 16. This is because the pre-sintered
granular powder according to the present invention has sufficient
yield strength despite having been pressed. Therefore, coarse pores
having a predetermined size are formed between the granules
constituting the sintered ceramic, and the shape thereof may be
maintained even in the post-sintering process. Although the size of
pore channels varies depending on the pressing pressure and the
size distribution of granules, in the present invention, it is
possible to manufacture sintered ceramics having coarse pore
channels of 10 .mu.m or more.
[0156] Furthermore, the fine pore channels are formed in the
granules of the sintered ceramic according to the present
invention, resulting in porous sintered reaction-bonded silicon
nitride ceramics having a microstructure in which both coarse pores
and fine pores are formed together.
[0157] As described hereinbefore, the present invention provides a
method of manufacturing a Porous sintered reaction-bonded silicon
nitride ceramic and a porous sintered reaction-bonded silicon
nitride ceramic manufactured using the same. According to the
present invention, the sintered reaction-bonded silicon nitride
ceramic has a controlled pore channel size so that both coarse
pores and fine pores are formed therein, simultaneously increasing
air permeability and capturing efficiency.
[0158] Also, according to the present invention, pre-sintered
granular powder is not deformed even under high compacting
pressure, and thus the spherical morphology of the granules can be
maintained unchanged regardless of typical pressing methods, for
example, uniaxial pressing, extrusion and injection molding, thus
easily applying it to a filter for high-temperature/high-pressure
gas or a diesel particulate filter.
[0159] Although the embodiments of the present invention have been
disclosed for illustrative purposes, those skilled in the art will
appreciate that a variety of different modifications, additions and
substitutions are possible, without departing from the scope and
spirit of the invention as disclosed in the accompanying claims.
Accordingly, such modifications, additions and substitutions should
also be understood as falling within the scope of the present
invention.
* * * * *