U.S. patent application number 12/871009 was filed with the patent office on 2011-03-03 for manufacture of sintered silicon alloy.
This patent application is currently assigned to ISMAN J CORPORATION. Invention is credited to YUMIKO KUBOTA, AKIKO MATSUSHITA, MASAFUMI MATSUSHITA, YOKO MATSUSHITA, OSAMU MATSUZONO, TAKAYOSHI MISAKI, TAKASHI MIZUSHIMA, KUNIO SAITO, TOSHITAKA SAKURAI, KAZUYA SATO, KOUKI SHIMIZU, AYUMI SHINDO, SETSUKO SHINDO, TAKUMI SHITARA, TOSHIYUKI WATANABE, FUTOSHI YANAGINO, TAKASHI YOSHIDA.
Application Number | 20110052440 12/871009 |
Document ID | / |
Family ID | 43039121 |
Filed Date | 2011-03-03 |
United States Patent
Application |
20110052440 |
Kind Code |
A1 |
WATANABE; TOSHIYUKI ; et
al. |
March 3, 2011 |
MANUFACTURE OF SINTERED SILICON ALLOY
Abstract
Dehydration and drying of a silicon alloy argil which uses water
as a principal binder are carried out by a freeze-drying process, a
microwave irradiation process, or a combination thereof. In the
freeze-drying process, the shaped compact is put into a cooling
medium within 5 minutes after completion of shape forming, retained
therein for at least 5 minutes to quick-freeze water within the
compact while the water is still in a finely-dispersed condition.
The compact is exposed to a pressure below the triple point
pressure of water. In the microwave irradiation process, the shaped
compact is put into a container exposed to continuous microwave
irradiation at 2.450 GHz for at least 5 minutes while under a
reduced pressure below atmospheric pressure.
Inventors: |
WATANABE; TOSHIYUKI;
(SHIBUYA-KU, JP) ; MATSUSHITA; MASAFUMI;
(YOKOHAMA-SHI, JP) ; SAKURAI; TOSHITAKA;
(YOKOHAMA-SHI, JP) ; SATO; KAZUYA; (YOKOHAMA-SHI,
JP) ; MATSUSHITA; YOKO; (YOKOHAMA-SHI, JP) ;
MISAKI; TAKAYOSHI; (HIRATSUKA-SHI, JP) ; SHINDO;
SETSUKO; (KAWASAKI-SHI, JP) ; SHINDO; AYUMI;
(YOKOHAMA-SHI, JP) ; KUBOTA; YUMIKO;
(KAMAKURA-SHI, JP) ; MATSUSHITA; AKIKO;
(YOKOHAMA-SHI, JP) ; SAITO; KUNIO; (YOKOHAMA-SHI,
JP) ; SHITARA; TAKUMI; (YOKOHAMA-SHI, JP) ;
YANAGINO; FUTOSHI; (KAWASAKI-SHI, JP) ; YOSHIDA;
TAKASHI; (KAWASAKI-SHI, JP) ; MIZUSHIMA; TAKASHI;
(SAITAMA-SHI, JP) ; MATSUZONO; OSAMU;
(SAITAMA-SHI, JP) ; SHIMIZU; KOUKI; (ADACHI-KU,
JP) |
Assignee: |
ISMAN J CORPORATION
KAWASAKI-SHI
JP
|
Family ID: |
43039121 |
Appl. No.: |
12/871009 |
Filed: |
August 30, 2010 |
Current U.S.
Class: |
419/32 |
Current CPC
Class: |
C04B 35/591 20130101;
C04B 35/581 20130101; C04B 35/6263 20130101; C04B 2235/96 20130101;
C04B 2235/6022 20130101; C04B 2235/428 20130101; C04B 2235/3418
20130101; C04B 2235/402 20130101; C04B 2235/606 20130101; C04B
2235/9692 20130101; C04B 2235/3895 20130101; C04B 2235/602
20130101 |
Class at
Publication: |
419/32 |
International
Class: |
B22F 1/00 20060101
B22F001/00; B22F 3/12 20060101 B22F003/12 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 2, 2009 |
JP |
202440/2009 |
Claims
1. A method for manufacturing a sintered silicon alloy comprising
the steps of: obtaining a raw material comprising a silicon alloy
powder including 30-70 weight % silicon, 10-45 weight % nitrogen,
1-40 weight % aluminum, and 1-40 weight % oxygen; kneading the raw
material with the addition of 10-40 weight % water as a binder to
form a kneaded silicon alloy argil; shaping the kneaded argil to
form a silicon alloy compact having a three-dimensional shape;
putting said compact in a cooling medium within 5 minutes after
completion of the shaping step, and retaining the compact in said
cooling medium for at least 5 minutes, thereby quick-freezing water
contained within said compact in a finely-dispersed condition;
retaining the compact in a pressure-regulated container in which
the pressure is maintained at a level below the triple point
pressure of water; and thereafter, sintering said compact.
2. A method for manufacturing a sintered silicon alloy according to
claim 1, in which at least one additive is added to the silicon
alloy powder to form a mixture, the additive being from the group
consisting of an inorganic binder and a sintering additive, the
inorganic binder being composed mainly of silicon dioxide and
aluminum, and, if present, constituting 0.5-10 weight % of the
mixture, and the sintering additive, if present, constituting up to
5 weight % of the mixture.
3. A method for manufacturing a sintered silicon alloy according to
claim 1, in which at least one of the kneading step and the shape
forming step are conducted in a reduced-pressure environment below
atmospheric pressure.
4. A method for manufacturing a sintered silicon alloy according to
claim 3, in which at least one additive is added to the silicon
alloy powder to form a mixture, the additive being from the group
consisting of an inorganic binder and a sintering additive, the
inorganic binder being composed mainly of silicon dioxide and
aluminum, and, if present, constituting 0.5-10 weight % of the
mixture, and the sintering additive, if present, constituting up to
5 weight % of the mixture.
5. A method for manufacturing a sintered silicon alloy comprising
the steps of: obtaining a raw material comprising a silicon alloy
powder including 30-70 weight % silicon, 10-45 weight % nitrogen,
1-40 weight % aluminum, and 1-40 weight % oxygen; kneading the raw
material with the addition of 10-40 weight % water as a binder to
form a kneaded silicon alloy argil; shaping the kneaded argil to
form a silicon alloy compact having a three-dimensional shape;
exposing said compact to continuous microwave radiation for at
least 5 minutes; and sintering said compact.
6. A method for manufacturing a sintered silicon alloy according to
claim 5, including the step of putting said compact in a container
and exposing the compact to a reduced pressure below atmospheric
pressure within 5 minutes following completion of the shaping step,
wherein the step of exposing the compact to continuous microwave
radiation is commenced within 5 minutes following completion of the
shaping step, and wherein, over at least part of the time during
which the compact is exposed to continuous microwave radiation, the
compact is in said container and the pressure within said container
is held at a pressure below 1 atmosphere.
7. A method for manufacturing a sintered silicon alloy according to
claim 5, including the step of putting said compact in a container
and exposing the compact to a reduced pressure below atmospheric
pressure within 5 minutes following completion of the shaping step,
wherein the step of exposing the compact to continuous microwave
radiation is commenced within 5 minutes following completion of the
shaping step, and wherein, over substantially the entire time
during which the compact is exposed to continuous microwave
radiation, the compact is in said container and the pressure within
said container is held at a pressure below 1 atmosphere.
8. A method for manufacturing a sintered silicon alloy according to
claim 5, wherein the frequency of said microwave radiation is
within the range from about 0.9 GHz to 6 GHz.
9. A method for manufacturing a sintered silicon alloy according to
claim 5, wherein the frequency of said microwave radiation is 2.450
GHz.
10. A method for manufacturing a sintered silicon alloy according
to claim 5, in which at least one additive is added to the silicon
alloy powder to form a mixture, the additive being from the group
consisting of an inorganic binder and a sintering additive, the
inorganic binder being composed mainly of silicon dioxide and
aluminum, and, if present, constituting 0.5-10 weight % of the
mixture, and the sintering additive, if present, constituting up to
5 weight % of the mixture.
11. A method for manufacturing a sintered silicon alloy according
to claim 5, in which at least one of the kneading step and the
shape forming step are conducted in a reduced-pressure environment
below atmospheric pressure.
12. A method for manufacturing a sintered silicon alloy according
to claim 11, in which at least one additive is added to the silicon
alloy powder to form a mixture, the additive being from the group
consisting of an inorganic binder and a sintering additive, the
inorganic binder being composed mainly of silicon dioxide and
aluminum, and, if present, constituting 0.5-10 weight % of the
mixture, and the sintering additive, if present, constituting up to
5 weight % of the mixture.
13. A method for manufacturing a sintered silicon alloy comprising
the steps of: obtaining a raw material comprising a silicon alloy
powder including 30-70 weight % silicon, 10-45 weight % nitrogen,
1-40 weight % aluminum, and 1-40 weight % oxygen; kneading the raw
material with the addition of 10-40 weight % water as a binder to
form a kneaded silicon alloy argil; shaping the kneaded argil to
form a silicon alloy compact having a three-dimensional shape;
putting said compact in a cooling medium within 5 minutes after
completion of the shaping step, and retaining the compact in said
cooling medium for at least 5 minutes, thereby quick-freezing water
contained within said compact in a finely-dispersed condition;
exposing said compact to continuous microwave radiation for at
least 5 minutes; and sintering said compact.
14. A method for manufacturing a sintered silicon alloy according
to claim 13, wherein said compact is exposed to a reduced pressure
below atmospheric pressure following retention of the compact in
the cooling medium, and wherein, over at least part of the time
during which the compact is exposed to continuous microwave
radiation, the compact is exposed to a pressure below 1
atmosphere.
15. A method for manufacturing a sintered silicon alloy according
to claim 13, wherein said compact is exposed to a reduced pressure
below atmospheric pressure following retention of the compact in
the cooling medium, and wherein, over substantially the entire time
during which the compact is exposed to continuous microwave
radiation, the compact is exposed to a pressure below 1
atmosphere.
16. A method for manufacturing a sintered silicon alloy according
to claim 13, wherein the frequency of said microwave radiation is
within the range from about 0.9 GHz to 6 GHz.
17. A method for manufacturing a sintered silicon alloy according
to claim 13, wherein the frequency of said microwave radiation is
2.450 GHz.
18. A method for manufacturing a sintered silicon alloy according
to claim 13, in which at least one additive is added to the silicon
alloy powder to form a mixture, the additive being from the group
consisting of an inorganic binder and a sintering additive, the
inorganic binder being composed mainly of silicon dioxide and
aluminum, and, if present, constituting 0.5-10 weight % of the
mixture, and the sintering additive, if present, constituting up to
5 weight % of the mixture.
19. A method for manufacturing a sintered silicon alloy according
to claim 13, in which at least one of the kneading step and the
shape forming step are conducted in a reduced-pressure environment
below atmospheric pressure.
20. A method for manufacturing a sintered silicon alloy according
to claim 19, in which at least one additive is added to the silicon
alloy powder to form a mixture, the additive being from the group
consisting of an inorganic binder and a sintering additive, the
inorganic binder being composed mainly of silicon dioxide and
aluminum, and, if present, constituting 0.5-10 weight % of the
mixture, and the sintering additive, if present, constituting up to
5 weight % of the mixture.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority on the basis of Japanese
patent application 202440/2009, filed Sep. 2, 2009. The disclosure
of Japanese application 202440/2009 is hereby incorporated by
reference.
FIELD OF THE INVENTION
[0002] This invention relates generally to the manufacture of a
sintered silicon alloy using a silicon-based silicon alloy, and
specifically to a dehydration and drying method in the
manufacturing process.
BACKGROUND OF THE INVENTION
[0003] Conventional methods for manufacturing industrial ceramics
products have generally used organic solvents as a binder. A
forming process that dispenses with the use of organic solvents and
utilizes water instead as the principal binder is desirable.
However, water has not been employed as a binder because it has
been extremely difficult to ensure stable product quality when
carrying out dehydration and drying after forming.
[0004] We have carried out investigations into a technique for
synthesizing a silicon alloy from silicon, specifically inexpensive
silicon, which can include impurities consisting of metallic
elements such as oxygen and iron, with the objective of utilizing
inexpensive silicon as an industrial structural material. As a
result, we have succeeded in synthesizing a silicon alloy which
includes 30-70 weight % silicon, 10-45 weight % nitrogen, 1-40
weight % aluminum and 1-40 weight % oxygen from above-mentioned
inexpensive silicon by way of controlled combustion synthesis. This
silicon alloy manufacturing process and the apparatus used to carry
out the process are described in Japanese Patent Application No.
60453/2009, which is a divisional application based on Japanese
Patent Application No. 354835/2006.
[0005] We have also developed a new manufacturing technique for a
sintered silicon alloy product with the combined use of a wet
compound method and sintering methods. The wet compound process is
characterized in that it utilizes a silicon alloy powder the
average particle diameter of which is kept under a specified value,
water, and a shape-forming binder. This manufacturing technique is
described in Japanese Patent No. 4339352.
[0006] Silicon alloy products are manufactured by adding a binder
and water to the silicon alloy powder, kneading the composition in
a kneading machine, forming the composition into a shape
corresponding to that of the final product, and then carrying out
drying and sintering. In this process, however, inner cracks such
as shown in FIGS. 6 and 7 are generated after sintering a dried
silicon alloy argil shaped with the use of water as the principal
binder.
[0007] Our research has revealed the following cause. During the
formation of the argil, water within the compacted material clumps
together to a plurality of points over time. The water turns into
steam and evaporates, passing out of the compact through minute
spaces between particles. Air then passes into holes which are left
within the compact after evaporation of the water. During sintering
of the dried compact, expansion of the air due to heat causes
cracks to be generated within the sintered compact.
[0008] Prior art relating to sintering of silicon alloys is also
found in unexamined Japanese Patent Publication No. 162851/2008,
and in various published documents including "Automotive Technology
9," p. 97, Nikkei Business Publications, Inc., 2008; Toshiyuki
Watanabe et al., "FC Report 26," p. 68, Japan Fine Ceramics
Association, Tokyo, 2008; Teiroku Sueno and Syuichi Iwao, "Clay and
its Application," by Asakura Publishing Co., Ltd., Tokyo, 1962;
Japan Society for the Promotion of Science, ed., "Production and
Application of Advanced Ceramics," Nikkan Kogyo Shimbun Ltd.,
Tokyo, 2005; and Masaaki Nakamura and Yuji Tatemoto, "Learning
Basics of Drying Technique," Kogyo Chosakai Publishing Co., Ltd.,
Tokyo, 2008.
SUMMARY OF THE INVENTION
[0009] As already mentioned, a conventional process of
manufacturing industrial ceramic products generally includes a
forming step using an organic binder, followed by removal of the
binder and sintering. Organic resin is used as a binder, because it
can be easily removed by heating after the forming step.
[0010] When heated at a high temperature, the organic binder turns
into soot, which causes product performance degradation, and
therefore the organic binder needs to be removed before sintering.
The binder can be removed by slow-paced heating at a low
temperature below the point at which the binder turns into
soot.
[0011] Since organic binders are environment-unfriendly, inorganic
binders have come under study. However, the above-mentioned problem
of inner crack generation after sintering, caused by problems
associated with the dehydration and drying process, has not
heretofore been solved.
[0012] This invention is aimed at providing an effective method of
dehydration and drying of silicon alloy argil in which water is
used as a principal binder, so that silicon alloy industrial
products without inner cracks can be manufactured effectively.
[0013] In each of several variations of the method according to the
invention, a raw material is obtained comprising a silicon alloy
powder including 30-70 weight % silicon, 10-45 weight % nitrogen,
1-40 weight % aluminum, and 1-40 weight % oxygen. The raw material
is kneaded with the addition of 10-40 weight % water as a binder to
form a kneaded silicon alloy argil. Then, the kneaded argil is
shaped to form a silicon alloy compact having a three-dimensional
shape.
[0014] In accordance with a first aspect of the invention, the
compact is placed in a cooling medium within 5 minutes after
completion of the shaping step, and retained in the cooling medium
for at least 5 minutes, thereby quick-freezing water contained
within the compact while the water is in a finely-dispersed
condition. The compact is then retained in a pressure-regulated
container in which the pressure is maintained at a level below the
triple point pressure of water. Thereafter the compact is
sintered.
[0015] In accordance with another aspect of the invention, the
compact is placed in a container, and exposed to a reduced-pressure
environment below 1 atmosphere within 5 minutes after completion of
the shaping step. Within 5 minutes after completion of the shaping
step, exposure of the compact to continuous microwave radiation is
commenced, the radiation being preferably in the frequency range
from 0.9 to 6 GHz, e.g., at 2.450 GHz. Exposure of the compact to
microwave radiation is continued for at least 5 minutes.
Preferably, the compact is exposed to reduced pressure throughout
substantially the entire time during which it is exposed to
microwave radiation. The compact is then sintered.
[0016] In accordance with still another aspect of the invention,
the compact is placed in a cooling medium within 5 minutes after
completion of the shaping step, and retained in the cooling medium
for at least 5 minutes, thereby quick-freezing water contained
within the compact while the water is in a finely-dispersed
condition. The compact is than placed in a container, and exposed
to a reduced-pressure environment below 1 atmosphere. The compact
is exposed to continuous microwave radiation at 2.450 GHz for at
least 5 minutes, and then sintered. Here again, the compact is
preferably exposed to reduced pressure throughout substantially the
entire time during which it is exposed to microwave radiation.
[0017] In each of the above variations at least one additive may be
optionally added to the silicon alloy powder to form a mixture, the
additive being from the group consisting of an inorganic binder and
a sintering additive. The inorganic binder is composed mainly of
silicon dioxide and aluminum, and, if present, constitutes 0.5-10
weight % of the mixture. The sintering additive, if present,
constitutes up to 5 weight % of the mixture.
[0018] In each variation, optionally the kneading step the shape
forming step, or both, are conducted in a reduced-pressure
environment below atmospheric pressure.
[0019] The invention enables water within a compact to be removed
while still in a finely dispersed condition before aggregation. The
invention thereby prevents the generation of voids that result in
the formation of cracks after sintering. The process of the
invention allows shape forming of ceramic products to be carried
out at high speed, and thereby dramatically improves productivity
in comparison to productivity achieved by the use of conventional
methods. Improved productivity results in a reduction in the price
of industrial ceramic products, and increases opportunities for
their application.
[0020] The method according to the invention also enables stable
production of high quality ceramics products using water as a
principal binder, and without using harmful organic solvents. The
application of this technique, which enables water to be used as a
principal binder, can be expected to spread throughout the ceramics
industry, which would bring about beneficial effects in terms of
environmental conservation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a schematic flow chart illustrating the process of
manufacturing a sintered silicon alloy in accordance with the
invention;
[0022] FIG. 2 is a pressure-temperature phase diagram for water,
illustrating the mechanism of the quick freeze-drying method;
[0023] FIG. 3 is a cross-sectional photograph showing the inside of
a silicon alloy ball after drying and before sintering;
[0024] FIG. 4 is a cross-sectional photograph of silicon alloy
balls after sintering;
[0025] FIG. 5 is a pressure-temperature phase diagram for water,
illustrating the mechanism of microwave irradiation;
[0026] FIG. 6 is a photograph showing the inner cracks in a
sintered silicon alloy made by the conventional dehydration and
drying method; and
[0027] FIG. 7 is a magnified photograph showing the inner cracks in
a sintered silicon alloy made by the conventional dehydration and
drying method.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] As already mentioned, cracks are generated because water
within a silicon alloy argil tends to aggregate in clumps at a
plurality of points over time. The water in these clumps turns into
steam and evaporates out of the compact, leaving within the dried
compact holes which fill with air.
[0029] We have found two effective ways to minimize the aggregation
of water within the compact in order to prevent generation of
cracks. A first method is the quick freeze-drying method
(hereinafter referred to as the "FD method"). The second method is
the microwave irradiation method (hereinafter referred to as the
"MW method". Each method will be described in detail below.
[0030] According to the FD method, a silicon alloy argil is formed
into shapes corresponding to final products, and placed in a
cooling medium for quick freezing, whereby finely dispersed water
is turned into fine ice particles within the compact. Examples of
preferable cooling media include liquid nitrogen, and soluble
alcohol cooled by dry ice or by refrigerating or freezing
machinery.
[0031] Finely-dispersed ice particles are evaporated directly and
dispersed out of the compact without returning to the water, i.e.,
the liquid phase. The pressure-temperature phase diagram of water
as shown in FIG. 2 was utilized to achieve this goal. The diagram
shows that solid phase (ice), liquid phase (water), gas phase
(vapor) are dependent on temperature and pressure, and that the
borders comprise two phases and the triple point T comprises three
phases.
[0032] As detailed below and shown in FIG. 2, in accordance with
the method of the invention, water is rapidly turned into ice
particles and the ice particles evaporate directly out from the
ceramic material without first turning into water.
[0033] In FIG. 2, T1 represents the temperature of the water
contained in the compact immediately after shape-forming of the
silicon alloy argil. Since the water is under a pressure of 1
atmosphere, or about 1013 hPa, point a represents the state of the
water within the shaped compact.
[0034] Immediately after the shape forming, the water contained in
the compact is dispersed in fine particles. When the compact is
cooled down quickly to the temperature T2 of the cooling medium
such as liquid nitrogen, the water within the compact is frozen
while still in a finely-dispersed condition and is transferred to
the state represented by point b.
[0035] The frozen compact is put in a container which is then
evacuated to reduce the pressure within the container to a low
level, below about 6.1 hPa, the pressure at the triple point T in
the pressure-temperature phase diagram. While the compact is
maintained under this reduced pressure, the frozen water within the
compact is transferred to the state represented by point c.
[0036] The temperature is then raised to T3 (point d), while the
pressure is maintained at a level below the pressure at the triple
point T. When the temperature is raised to T3, the ice within the
compact turns directly into vapor and rapidly sublimates out from
the compact. Provided that the vacuum is maintained so that the
pressure is kept below the level at the triple point T, sublimation
of ice to vapor occurs regardless of how the temperature is raised.
Subsequent drying can be made more efficient by warming the compact
using a heating means.
[0037] It was possible to decrease water content ratio in the
compact from an initial value of 23% to 1% or less without external
heating, by retaining the compact for 30 hours in a container at a
pressure below the pressure at the triple point T.
[0038] As the container is depressurized, gas such as air needs to
enter the container to return it to atmospheric pressure before the
compact can be removed. However, it is preferable to introduce
nitrogen instead of air in the depressurization step, because, if
air is introduced, oxygen in the air goes into the dried compact
and reacts with carbon in the wall material of the sintering
furnace, generating carbon dioxide, which will cause damage to the
wall.
[0039] FIG. 3 is a cross-sectional photograph showing the inside of
a ball after drying and before sintering. The photograph shows a
condition after the ice particles turn into vapor, which passes out
of the compact through minute spaces between ceramic particles. The
maximum dimension of the spaces left between ceramics particles
after water evaporation is 0.1 micrometers or less, and the dried
compact had no holes of the kind that would have been created by
the conventional method of shape-forming and drying.
[0040] FIG. 4 is a cross-sectional photograph showing the inside of
the balls after sintering. The minute spaces as seen in FIG. 3 are
completely sintered, the inside of the ball is densified, and there
are no inner cracks. A comparison of FIG. 4 with FIGS. 6 and 7,
which show the inner cracks associated with the conventional
method, shows the effects that distinguish the invention.
[0041] The microwave irradiation (MW) method will now be described
with reference to the pressure temperature phase diagram of FIG. 5.
In the MW method, the formation of inner cracks after sintering is
prevented by using microwave heating to evaporate and disperse
water within the compact.
[0042] When the temperature of the water contained in the silicon
alloy argil immediately after its shape-forming is at an ambient
level, T1, with the water under a pressure of 1 atmosphere, point a
represents the state of the water in the shaped ceramic, as in FIG.
2.
[0043] When the pressure inside a container in which the compact is
retained is reduced to P1, i.e., to a pressure below 1 atmosphere,
the vaporization temperature is lowered from 100.degree. C. to T4,
which is recommended because it makes dehydration more effective.
Point e represents the state of water within the compact under a
reduced pressure.
[0044] Microwave radiation at a frequency of 2.450 GHz is directed
into the compact in which water is in the condition represented by
point e, and is still in a finely dispersed condition because it
has not yet started to aggregate. Microwave generators that
generate microwave radiation at 2.450 GHz are readily available,
and microwave radiation at that frequency is absorbed well by water
and able to heat water easily. Other frequencies, preferably in the
range from about 0.9 GHz to 6 GHz, can be used. When heated from
inside the compact, water is evaporated at temperature T4 (point
f). Maintaining the inside of the container under a reduced
pressure during microwave irradiation makes the process more
effective.
[0045] As in the FD method, it has been confirmed that the MW
method is effective in preventing the inner cracks in the sintered
compacts as seen in FIGS. 6 and 7.
EXAMPLE 1
[0046] Specific examples of the invention will be described with
reference to FIG. 1.
[0047] Two kinds of a silicon alloy, having compositions as shown
in Table 1, were used as testing materials. Testing material A is a
silicon alloy powder which has been commercially marketed under the
trademark "MERAMIX" as "MERAMIX S1" by ISMAN J Corporation at Think
Maraikobo, 8 Minami-Watarida-cho 1-chome, Kawasaki-ku,
Kawasaki-shi, Kanagawa 210-0855, Japan. Testing material B is a
silicon alloy powder which can be sintered at a lower temperature
as compared to testing material A.
TABLE-US-00001 TABLE 1 Testing Material Si N Al O A 47.2 33.0 10.3
7.0 B 50.2 29.8 7.2 12.8
[0048] Ordinary tap water or ion-exchanged water may be used as the
main binder, but either inorganic binders or sintering additives,
or both, may be added. The preferable weight % ratios are 10-40 w %
water, 0.5-10 w % for the inorganic binders, which are mostly
composed of silicon dioxide and aluminum, and 5 w % or less for the
sintering additives.
[0049] These materials are kneaded to make a silicon alloy argil
which is shaped to a form corresponding to the final product.
[0050] Conducting the kneading process, the shaping process, or
both, in a reduced-pressure environment below atmospheric pressure
enables the sizes of the microscopic pores that are inevitably
included within the argil or compacts to be reduced as much as
possible. The term "microscopic pores" refers to minute pores which
can be detected under a microscope.
[0051] Accordingly, conducting the kneading process, the forming
process, or both in a reduced-pressure environment below normal
pressure is recommended for the manufacture of silicon alloy
sintered bodies for applications in which high strength is a
special requirement.
[0052] Experiments were conducted by varying following conditions
to determine optimum conditions for dehydration and drying by the
FD method and its effect: [0053] (1) amount of time (in minutes)
from the completion of shape forming as shown in FIG. 2 (point a)
to the placing of the compact in liquid nitrogen or other cooling
medium (point b). [0054] (2) the cooling medium and its temperature
(.degree. C.) [0055] (3) the pressure (hPa) inside the
reduced-pressure container for retaining frozen compacts (point c)
[0056] (4) the holding temperature (.degree. C.) and holding time
(in hours) inside the reduced-pressure container (point c) until
the water content ratio of the compact becomes 1 weight % or
less.
[0057] The compacts, after the drying process, were sintered in a
continuous sintering furnace designed for mass production and run
under normal pressure in a nitrogen environment with 400V output.
The sintering temperature was set at 1750.degree. C. for the
material A and at 1600.degree. C. for the material B. Both
materials were sintered for 2 hours.
[0058] The compacts were cut through their centers after sintering
to measure cross-sectional hardness. The "cracks", the problem
addressed by the invention, were examined by visual observation of
the polished surface of the center section and by microscopic
observation using a microscope magnification power of 100 times.
The effects of the inorganic binder and the sintering additive were
also examined.
[0059] The results are shown in Table 2.
TABLE-US-00002 TABLE 2 FD Conditions Conditions at "d"
Characteristics Binder Sintering a.fwdarw.b Cooling Pressure
Retention Sintering after sintering Testing Water Inorganic
Additive Time Media at "c" time Temperature Hardness Inner Examples
Material (%) Binder Added None (minutes) .degree. C. hPa .degree.
C. (hours) (C. .degree.) (HV) Cracks 1 A 25 Added .largecircle. 1
-196 6.0 0 30 1750 1520 None 2 A 25 Added .largecircle. 3 -196 6.0
20 10 1750 1515 None 3 A 25 Added .largecircle. 5 -196 6.0 30 5
1750 1535 None 4 A 27 Added .largecircle. 5 -80 6.0 30 5 1750 1530
None 5 A 25 Added .largecircle. 5 -196 7.0 0 30 1750 1525 Observed
6 A 25 Added .largecircle. 10 -196 6.0 0 30 1750 1520 Observed 7 A
25 None .largecircle. 5 -196 6.0 30 5 1750 1530 None 8 A 25 Added
.largecircle. 5 -196 6.0 30 5 1750 1500 None 9 B 25 Added
.largecircle. 5 -196 6.0 0 30 1600 1495 None 10 B 25 Added
.largecircle. 5 -196 6.0 0 30 1600 1485 None
[0060] As is clear from Table 2, inner cracks were generated in
Example 6, which was put into liquid nitrogen or other cooling
media 10 minutes after shape forming (the time from point a to
point b), and in Example 5, in which the pressure inside the
reduced-pressure container retaining the compact was set at 7.0
hPa.
[0061] These results revealed that inner cracks in sintered
compacts can be prevented by putting the silicon alloy argil into
liquid nitrogen or other cooling media within 5 minutes after shape
forming, and by setting the pressure inside the reduced-pressure
container to retain the compact below the triple point pressure
shown in FIG. 2.
[0062] It was demonstrated that, if these ED method conditions are
applied, the problem of inner cracks of sintered compacts can be
resolved regardless of the addition of inorganic binders or
sintering additives, and regardless of difference in sintering
temperatures.
[0063] In addition, it was also determined that the testing
material B can be sintered at a lower temperature as compared to
the material A, and that, even without sintering additives, it can
be sintered without generating cracks.
[0064] It should be noted that the optimal retaining time in the
reduced-pressure container varies depending on the size of the
reduced-pressure container, the capacity of the vacuum pump, and
the size or quantity of the compacts to be put in the container.
Under the conditions of this example, the shortest retention time
required was 5 hours (see Table 2).
EXAMPLE 2
[0065] Experiments were also conducted by varying the following
conditions to determine the optimum conditions for dehydration and
drying by the MW method and its effects: [0066] (1) the amount of
time (in minutes) from the completion of shape forming as shown in
FIG. 5 (point a) to the placing of the compact into a
reduced-pressure container (point e) to which microwave radiation
is applied [0067] (2) the pressure (P1 in hPa) inside the
reduced-pressure container for retaining compacts (point c) [0068]
(3) the evaporation temperature determined by P1 (T4 in .degree.
C.) [0069] (4) the holding time (minutes) at T4 (point f).
[0070] The microwave generator was operated on three-phase AC, and
had a power output of 0.1-3 kW, and a frequency of 2.450 GHz.
[0071] The testing materials A and B were sintered after drying for
2 hours at 1750.degree. C. and 1600.degree. C. respectively.
[0072] The cross-sectional hardness and inner cracks were measured
by the same method as in Example 1. The results are shown in Table
3.
TABLE-US-00003 TABLE 3 MW Conditions Conditions at "f"
Characteristics Binder Sintering a.fwdarw.e e.fwdarw.f Pressure
Retention Sintering after sintering Testing Water Inorganic
Additive Time Time at P.sub.1 time Temperature Hardness Inner
Examples Material (%) Binder Added None (minutes) (minutes) hPa
.degree. C. (minutes) (C. .degree.) (HV) Cracks 11 A 25 Added
.largecircle. 3 5 950 90 5 1750 1525 None 12 A 25 Added
.largecircle. 3 5 950 90 7 1750 1510 None 13 A 25 Added
.largecircle. 5 3 950 90 3 1750 1540 Observed 14 A 27 Added
.largecircle. 5 5 950 90 5 1750 1525 None 15 A 25 Added
.largecircle. 10 5 950 90 5 1750 1530 Observed 16 A 25 Added
.largecircle. 10 5 950 90 10 1750 1510 Observed 17 A 25 None
.largecircle. 5 7 900 80 2 1750 1520 Observed 18 A 25 Added
.largecircle. 5 7 900 80 2 1750 1510 Observed 19 B 25 Added
.largecircle. 5 5 950 90 5 1600 1495 None 20 B 25 Added
.largecircle. 5 5 950 90 5 1600 1490 None
[0073] As is clear from Table 3, inner cracks were generated in
Examples 15 and 16, in which the compacts were put into the
reduced-pressure container to which microwave radiation was applied
10 minutes after shape forming (the time from point a to point e).
Inner cracks were also generated in Examples 13, 17, and 18, in
which the compacts were retained at T4 (point f) for 2 and 3
minutes.
[0074] These results revealed that no inner cracks are generated if
silicon alloy argil is put into the reduced-pressure container and
receives microwave irradiation within 5 minutes after shape
forming, and is retained at temperature T4 (point f) for at least 5
minutes.
INDUSTRIAL APPLICABILITY
[0075] Silicon alloy products manufactured by means of the methods
of the invention have various characteristics as shown in FIG.
4.
TABLE-US-00004 TABLE 4 Specialty Steel Item Silicon Alloy (Metal
Ceramics) (Ferrous Alloy) Weight (ratio) 3.2 (1/3 weight of 8
specialty steel) Hardness HV1500 HV700 Rigidity Equal to specialty
steel 210 GPa Fatigue life L.sub.10Life .gtoreq. 4.0E+07
L.sub.10Life .gtoreq. 1.0E+07 (at least equal to specialty steel)
Lubricity Lubricant unnecessary Lubricant necessary Heatproof at
least 1250.degree. C. 800.degree. C. Temperature Resistance to
Strongly resistant especially Poor Chemicals to acid Magnetism
Nonmagnetic Paramagnetic Conductivity Nonconductive Highly
conductive Conductivity Low thermal conductance Highly conductive (
1/10 of steel) Provides extra thermal insulation Thermal Low ( 1/10
of steel) High Expansion Electromagnetic Not to absorb up to 20 GHz
To absorb wave reaction Affinity High Low with resin Recyclability
Not to generate metallic oxide Generates metallic oxide (harmful to
humans)
[0076] As indicated above, having distinguishing characteristics
such as lightness, high mechanical strength, enhanced fatigue
strength, nonmagnetism, heat resistance, or resistance to
chemicals, ceramics products manufactured by the invention are
applicable to a variety of purposes. Examples include bearing
balls, races for bearing balls, parts for linear motion bearings,
automobile components for power trains, power transmission shafts,
turbochargers, exhaust manifolds, fuel injection systems such as
common rail fuel injectors, components for turbines and landing
gear of aircrafts, component parts of artificial skeletons, and
parts for semiconductor manufacturing machines.
[0077] Further, due to its complete nonmagnetism, a silicon alloy
can be advantageously used for bearing members for inverter-type
power generators, electric motors. The silicon alloy ceramic
material does not need to be treated by coating with nonmagnetic
materials such as those that have been applied to ferrous bearing
members for the purpose of preventing electric corrosion or iron
loss caused by alternating electromagnetic fields.
[0078] Other specific applications in which the nonmagnetic
character of the silicon alloy ceramic material is advantageous
include bearing balls, roll axes, taper roll axes, inner and outer
races holding these members, for wind power stations operated in an
alternating electromagnetic field environment, and bearing balls,
roll axes, taper roll axes, inner and outer races holding these
members used for electric motors of hybrid vehicles and electric
vehicles driven in an alternating electromagnetic field
environment.
[0079] As indicated in Table 4, the products are distinguished by
the fact that they are able to block electromagnetic waves from
electric motors of hybrid vehicles or electric vehicles, utilizing
the characteristic that they reflect electromagnetic waves up to 20
GHz.
[0080] The dehydration and drying method of this invention can be
utilized to manufacture quality compacts by being applied to the
doctor blade method or the slip cast method, specifically in the
process of shape forming of compacts from water slurry without
organic binders.
[0081] Furthermore, the dehydration and drying method of this
invention is also applicable to CIM (ceramics injection molding),
an injection molding method conventionally carried out with the
addition of significant quantities of organic solvents or waxes.
CIM-shaped sintered compacts can be manufactured without a
binder-removal step by using only water as a binder in place of
conventional organic solvents or waxes, followed by injection
molding, and then dehydration using the method of the
invention.
[0082] Thus, the invention is capable of removing water within the
compacts effectively and preventing cracks after sintering, even
when water is used as a principal binder in manufacturing ceramics
products. Accordingly, the invention allows high-speed
shape-forming of ceramic products and therefore improves
productivity, which reduces the price of industrial ceramics
products and thereby increases opportunities for their
application.
[0083] As mentioned previously, the method according to the
invention also enables stable production of high quality ceramics
products using water as a principal binder, without using harmful
organic solvents. The application of this technique, can be
expected to spread throughout the ceramics industry, which would
bring about beneficial effects especially in view of the growing
importance of environmental conservation, and thus would be able to
meet the needs of various industries.
* * * * *