U.S. patent application number 10/468058 was filed with the patent office on 2004-06-10 for titanium powder sintered compact.
Invention is credited to Kato, Masamichi, Ogasawara, Tadashi, Onishi, Takashi, Watanabe, Munetoshi.
Application Number | 20040110059 10/468058 |
Document ID | / |
Family ID | 27482049 |
Filed Date | 2004-06-10 |
United States Patent
Application |
20040110059 |
Kind Code |
A1 |
Onishi, Takashi ; et
al. |
June 10, 2004 |
Titanium powder sintered compact
Abstract
Provided are a porous sintered compact suitable for a filter, a
power feeder in a polymer electrolyte membrane type water
electrolyzer, a current collector in a solid polymer fuel cell and
in addition a liquid dispersion plate, especially an ink dispersion
plate for an ink jet printer ink and the like. A titanium powder
sintered compact made of a plate-like porous compact is obtained by
sintering spherical powder made of titanium or a titanium alloy
produced by means of a gas atomization method. A void ratio in the
range of from 35 to 55% is realized by filling without applying a
pressure and sintering without applying a pressure.
Inventors: |
Onishi, Takashi; (Hyogo,
JP) ; Ogasawara, Tadashi; (Hyogo, JP) ;
Watanabe, Munetoshi; (Hyogo, JP) ; Kato,
Masamichi; (Hyogo, JP) |
Correspondence
Address: |
MORRISON & FOERSTER LLP
1650 TYSONS BOULEVARD
SUITE 300
MCLEAN
VA
22102
US
|
Family ID: |
27482049 |
Appl. No.: |
10/468058 |
Filed: |
February 2, 2004 |
PCT Filed: |
February 15, 2002 |
PCT NO: |
PCT/JP02/01332 |
Current U.S.
Class: |
210/510.1 ;
428/546; 429/410; 429/533 |
Current CPC
Class: |
B22F 2999/00 20130101;
B01D 39/2034 20130101; B22F 2998/10 20130101; B22F 3/1109 20130101;
H01M 8/0247 20130101; C25B 11/037 20210101; B22F 3/1103 20130101;
B22F 2998/00 20130101; Y02E 60/50 20130101; B01D 2201/188 20130101;
C25B 11/031 20210101; B22F 3/10 20130101; Y10T 428/12021 20150115;
B22F 3/004 20130101; B22F 1/065 20220101; B01D 39/2079 20130101;
C25B 11/042 20210101; H01M 8/0232 20130101; B22F 3/11 20130101;
Y10T 428/12014 20150115; B22F 2998/00 20130101; B22F 1/065
20220101; B22F 2998/00 20130101; B22F 9/082 20130101; B22F 1/065
20220101; B22F 2998/10 20130101; B22F 9/082 20130101; B22F 3/004
20130101; B22F 3/10 20130101; B22F 2998/10 20130101; B22F 9/082
20130101; B22F 1/065 20220101; B22F 3/10 20130101; B22F 2999/00
20130101; B22F 3/1109 20130101; B22F 2207/11 20130101; B22F 2998/10
20130101; B22F 9/082 20130101; B22F 3/02 20130101; B22F 3/11
20130101; B22F 2998/00 20130101; B22F 1/065 20220101; B22F 2998/00
20130101; B22F 1/065 20220101; B22F 9/082 20130101; B22F 2998/10
20130101; B22F 1/065 20220101; B22F 3/10 20130101; B22F 9/082
20130101 |
Class at
Publication: |
429/044 ;
428/546 |
International
Class: |
H01M 004/86; C22C
014/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 16, 2001 |
JP |
2001-40161 |
Feb 23, 2001 |
JP |
2001-48564 |
Apr 19, 2001 |
JP |
2001-121356 |
Apr 20, 2001 |
JP |
2001-122645 |
Claims
What is claimed is:
1. A titanium powders intered compact made of a plate-like porous
compact, obtained by sintering spherical gas atomized titanium
powder, and having a void ratio thereof in the range of from 35 to
55%.
2. The titanium powder sintered compact according to claim 1,
fabricated by filling without applying a pressure and sintering
without applying a pressure.
3. The titanium powder sintered compact according to claim 1,
wherein an average of particle diameters of said gas atomized
titanium powder is in the range of from 10 to 150 .mu.m.
4. A sintered titanium filter made of said titanium powder sintered
compact according to claim 1.
5. The sintered titanium filter according claim 4, wherein a
maximum pore diameter is in the range of from 3 to 70 .mu.m.
6. The titanium powder sintered compact according to claim 1,
wherein a thickness of said plate-like porous compact is 500 .mu.m
or less.
7. A cylindrical filter obtained by bending said titanium powder
sintered compact according to claim 6.
8. A cylindrical porous compact made of said titanium powder
sintered compact according to claim 1, and obtained by sintering
said spherical gas atomized titanium powder into a cylinder.
9. The cylindrical porous compact according to claim 8 serving as a
powder sintered filter.
10. A highly corrosion resistant metal filter made of said titanium
powder sintered compact according to claim 1, wherein a cavity
diameter is stepwise increased from one surface of said plate-like
porous compact to the other surface thereof.
11. The highly corrosion resistant metal filter according to claim
10, wherein said cavity diameter is in the range of from 3 to 70
.mu.m.
12. The highly corrosion resistant metal filter according to claim
10, wherein a void ratio is almost constant between one surface of
said plate-like porous compact and the other surface and
thereof.
13. A porous conductive plate made of said titanium powder sintered
compact according to claim 1, and used as a power feeder in a
polymer electrolyte membrane type water electrolyzer or a current
collector in a solid polymer fuel cell.
14. A highly corrosion resistant porous plate made of said titanium
powder sintered compact according to claim 1, wherein a ratio of a
plate thickness T of said plate-like porous compact (in mm) to an
area S (in mm.sup.2) thereof is 1/10000 or less.
15. The highly corrosion resistant porous plate according to claim
14, wherein a variation in a void ratio in a surface of said
plate-like porous compact is 3% or less in standard deviation.
16. A dispersion plate using said highly corrosion resistant porous
plate according to claim 14.
Description
TECHNICAL FIELD
[0001] The present invention relates to a porous titanium powder
sintered compact employed as a filter, a power feeder in a polymer
electrolyte membrane type water electrolyzer, a current collector
in a solid polymer fuel cell and in addition a liquid dispersion
plate, especially an ink dispersion plate for an ink jet printer
and the like.
BACKGROUND ART
[0002] A metal powder sintered compact has been adopted as one of
filters employed in the chemical industry, the polymer industry,
the chemicals industry and others. As metals herein, there have
been used generally brass, stainless steel and, recently,
titanium.
[0003] Titanium is greatly excellent in corrosion resistance, acid
proofness and the like as compared with stainless steel, but on the
other hand, extremely poor in moldability. Hence, generally, a
titanium sintered filter has been fabricated according to a method
in which hydrogenation/dehydrogenation titanium powder accepted as
having a comparatively good moldability is molded with a die press,
followed by sintering, and furthermore, a method disclosed in JP A
1995(H7)-238302 uses titanium sponge powder comparatively good in
moldability similarly to hydrogenation/dehydrogenation powder.
[0004] Such titanium sintered filters have started finding
applications thereof to, for example, highly corrosion resistant
filters for a carrier gas inlet section of a gas chromatography
apparatus, production of food such as a liquid condiment, and a
liquid pigment.
[0005] Filters well used in various application fields have been
faced a demand for maximum pore diameters adapted for respective
purposes of usage. The term, "a maximum pore diameter" is used as
an index expressing the size of a particle removable by a filter,
wherein with the same value of a maximum pore diameter, it may be
considered that filters having respective pore shapes different
from each other can remove particles having at least the same
diameter. A filter with the smaller pressure drop is more requested
among filters with the same maximum pore diameter. For example, as
the carrier gas inlet section filter for a gas chromatography
apparatus, a filter has been desired that is not only excellent in
corrosion resistance but also especially has a maximum pore
diameter of 70 .mu.m or less and a smaller pressure drop.
[0006] In a titanium sintered filter using
hydrogenation/dehydrogenation powder or titanium sponge powder,
however, there exists a constraint to disable a pressure drop to be
reduced to a small value in a case where a maximum pore diameter is
adjusted to 70 .mu.m or less.
[0007] A titanium sintered filter using
hydrogenation/dehydrogenation powder or titanium sponge powder has
another problem that the filter is very hard and fragile without
flexibility; therefore it is easy to be broken if being thin and
difficult to fabricate the filter large in area. In addition, since
bending is difficult at room temperature, a product cannot be
fabricated by bending, causing a problem of a high fabrication cost
except for a plate-like shape.
[0008] For example, a case arises where there is requested a
titanium sintered filter of the shape of a cylinder, of the order
of 40 mm in diameter (with a radius of curvature of 20 mm), whereas
since it is impossible to bend a titanium sintered compact in the
form of a flat plate into the form of a cylinder at room
temperature, a necessity arises for working by cold isostatic
pressing called CIP for short as described in JP No. 2791737,
thereby increasing in fabrication cost cannot be avoided.
[0009] Even with hydrogenation/dehydrogenation titanium powder and
titanium sponge powder, moldability is inferior to that of
stainless steel. Hence, it is difficult to mold the titanium
powders into shapes except for a thin flat plate. Therefore, it is
also difficult to directly mold a filter in the shape of a cylinder
without resorting to bending process.
[0010] That is, in a case where a sintered compact in the shape of
a cylinder is fabricated by press molding using
hydrogenation/dehydrogenati- on titanium powder or titanium sponge
powder, a press force in a direction of height does not effectively
act, which results in difficulty in molding a middle portion in the
direction of height; therefore a cylinder with a large height
cannot be produced though a low-profile ring can be produced. While
a cylinder with a large height can be fabricated by cold isostatic
pressing called CIP instead of a press, a high cost is encountered,
making CIP improper as a fabrication method for a sintered filter.
Therefore, while it is imagined to stack rings along the central
axis direction to weld the rings, needless to say that thus
fabricated sintered filter is much higher in cost as compared with
a sintered filter fabricated by press molding stainless steel
powder.
[0011] Incidentally, a method is described in JP No. 2791737 in
which stainless steel powder is subjected to cold isostatic
pressing to fabricate a sintered filter in the shape of a
cylinder.
[0012] As a different problem of a titanium sintered filter using
hydrogenation/dehydrogenation titanium powder or titanium sponge
powder, low reverse-washing property arises. That is, sizes and
shapes of cavities are randomly distributed in titanium sintered
filters made of each powder. While a filter of this kind is used
over a long term repeating reverse-washing, if sizes and shapes of
cavities are random, solid matter trapped therein are not
sufficiently removed even with reverse-washing. Hence, the problem
of low reverse-washing reproducibility has remained.
[0013] It is described in JP No. 2791737 that, as for a stainless
steel filter, a diameter of a cavity is increased in a direction
from the front surface to the rear surface of a stainless steel
sintered filter in order to enhance reverse-washing reproducibility
of a metal powder sintered filter. To be concrete, a slurry
obtained by dispersing fine powder into a binder resin solution is
applied on the surface of a porous compact obtained by presintering
and thereafter, sintering of the porous compact is conducted,
thereby forming a skin layer with fine pores on the surface of the
porous compact.
[0014] In such a multilayer structure, since almost all solid
matter in a treated liquid is trapped in the skin layer with fine
pores formed therein and no foreign matter is trapped in cavities
inside the base layer, the solid matter trapped and accumulated in
the skin layer is easy to be removed by reverse-washing. On the
other hand, the following problem arises however.
[0015] Stainless steel is inferior to titanium in corrosion
resistance. Furthermore, stainless steel powder used here is
composed of irregularly shaped particles produced by a water
atomization technique; therefore, sizes and shapes of cavities in a
sintered compact are random in not only the base layer but also the
skin layer. Besides, since the skin layer does not receive the
action of press molding though the base layer receives the action
of press molding, sizes and shapes of fine pores in the skin layer
are more random than in the base layer. For this reason, solid
matter remains in the skin layer after reverse-washing, thereby
disabling reverse-washing reproducibility as high as expectation.
In addition, since a void ratio of the skin layer receiving no
press molding is largely different from that of the base layer
receiving press molding, there also arises a risk that permeability
of a treated liquid is lowered.
[0016] A sintering compact is also used as a power feeder in a
water electrolytic cell producing hydrogen and oxygen using a
polymer electrolytic film. Concrete description will be given of
the water electrolytic cell; a construction is generally employed
in which a unit is formed by placing power feeders on both surfaces
of a film electrode laminates formed by laminating catalyst layers
onto both surfaces of the polymer electrolyte film, multiple of
units are stacked and electrodes are provided on both sides
thereof.
[0017] The power feeders herein are made each of a porous
conductive plate and placed in close contact with an adjacent film
electrode laminates. Why a porous conductive plate is used as a
power feeder is that a current is required to flow through, that
water is required to be supplied for a water electrolytic reaction
and that gas generated in the water electrolytic reaction is
quickly expelled out.
[0018] A structure of a fuel cell using a polymer electrolytic film
is also the same as that of the water electrolyzer and porous
conductive plates are placed on both surfaces of a film electrode
laminates. In the case of a fuel cell, since an electric power is
obtained with hydrogen as fuel, the porous conductive plates are
called current collectors.
[0019] As to a porous conductive plate such as a power feeder in
such a polymer electrolyte membrane type water electrolyzer or a
current collector in such a solid polymer fuel cell, titanium have
been studied because of a necessity for characteristics enabling a
material to be used in an oxidizing atmosphere, and among titanium
with actual natures and conditions, especially a sintered compact
has drawn attention since a surface is smooth, it is difficult to
damage an adjacent film electrode laminates and a proper void ratio
can be attained with ease.
[0020] As porous conductive plates made of a titanium sintered
compact, there are exemplified: a titanium powder sintered plate
obtained by sintering powder obtained by crushing titanium sponge
or powder produced by pulverizing titanium sponge by hydrogenation
and dehydrogenation thereof; a titanium fiber sintered plate
obtained by compression molding titanium fibers to sinter the
preform; and a titanium fiber sintered plate on a surface of which
a plasma sprayed layer of metallic titanium is formed, the last of
which is disclosed in JP A 1999(H11)-302891.
[0021] A porous conductive plate made of a prior art titanium
sintered compact described above, however, has the following
problems.
[0022] Though a titanium powder sintered compact has an advantage
that it is smooth on surfaces thereof and gives no damage on an
adjacent film electrode laminates, the sintered compact has a fatal
restraint that it is poor in press moldability and easily broken;
therefore, it cannot be fabricated with a small thickness and a
large area. On the other hand, though a titanium fiber sintered
plate is good in moldability and can be produced with a small
thickness and a large area, it has acute angled protrusions and
depressions on surfaces thereof with large spacings between fibers.
Therefore, if titanium fiber sintered plates are brought into press
contact with an adjacent film electrode laminates, there is a high
risk to damage the film electrode laminates. Furthermore, there
remains a problem to increase a contact resistance between the
titanium fiber sintered plate and the film electrode laminates.
[0023] In contrast to the sintered compacts described above, a
titanium fiber sintered plate disclosed in JP A 1999 (H11) -302891
is a sintered plate in which a plasma sprayed layer of metallic
titanium is formed on a surface of the titanium fiber sintered
plate to thereby cancel acute angled protrusions and depressions,
and large spacings between fibers, and can be said to be excellent
in moldability thereof and contactability between the sintered
plate and the film electrode laminates.
[0024] Since in addition to requirement of an extra cost due to
plasma spraying, avoid ratio and surface profiles of a titanium
fiber sintered plate are different from those of a plasma sprayed
titanium layer on the surface of the plate, an electric resistance
increases at a bonding interface therebetween, leading to an
electric resistance of a porous conductive plate higher than to be
expected from an apparent void ratio. As a result, in a water
electrolytic cell used at a high current density, for example, in
the range of from 1 to 3 A/cm.sup.2 , a large voltage drop results.
Needless to say that such a voltage drop is not at all allowed in a
fuel cell.
[0025] Moreover, a large change in void ratio at a bonding
interface leads to a worry to adversely influence on permeability
of a gas and a liquid.
[0026] On the other hand, as an ink dispersion plate for a large
ink jet printer, there has been requested a porous plate of, for
example, a thickness of as thin as 2 mm or less and an area of as
large as 200 mm .times.100 mm or more. This porous plate requires a
small variation in void ratio from a nature of this kind. As such
an ink dispersion plate, there has been used a sintered plate made
of irregular shaped powder of stainless steel.
[0027] As the recent trend, a demand has started to be generated on
a porous plate more excellent in corrosion resistance than a
sintered plate of stainless steel powder, for which it is
considered to use titanium powder more excellent in corrosion
resistance than stainless steel.
[0028] Though titanium in greatly excellent in corrosion resistance
and acid proofness as compared with stainless steel, it is
extremely poor in moldability to the contrary. Hence, a general
fabrication method for a titanium sintered plate has been thought
to be such that hydrogenation/dehydrogenation titanium powder,
which has been accepted to be comparatively good in moldability, is
molded with a die press, followed by sintering the preform and
furthermore, another fabrication method is also described in JP A
1995 (H7) -238302 in which there is used titanium sponge powder,
which is comparatively good in moldability, similar to the case of
hydrogenation/dehydrogenation titanium powder.
[0029] Moreover, a different method is described in JP A 1996 (H8)
-170107 in which a metal powder sintered plate uniform in void
ratio is fabricated by HIP.
[0030] The present inventors tried a procedure in which
hydrogenation/dehydrogenation titanium powder or titanium sponge
powder is molded with a die press to sinter the preform for the
purpose to fabricate a dispersion plate uniform in void ratio with
a thickness as thin as 2 mm or less and an area as large as 200 mm
.times.100 mm or more, and since the dispersion plate was
excessively thin, it was broken after press, having disabled
fabrication thereof.
[0031] The present inventors tried fabrication of the dispersion
plate described above by HIP only to find difficulty. The reason
for the difficulty is that a porous plate after sintering was not
able to be separated from a capsule maintaining a shape of a
sintered compact during HIP. Moreover, it is also difficult to
select a material of which the capsule is made, which together with
the above reason, causes a fabrication cost to be raised to a very
high value.
[0032] The present invention has been made in light of such
circumstances and it is a first object to provide a titanium powder
sintered compact excellent in corrosion resistance, having a small
maximum pore diameter, and showing a performance of a small
pressure drop during usage as a sintered titanium filter.
[0033] It is a second object of the present invention to provide a
titanium powder sintered compact excellent in bending.
[0034] It is a third subject of the present invention to provide a
cylindrical porous compact low in fabrication cost despite using
titanium powder, and excellent in reverse-washing reproducibility
while being used as a powder sintered filter.
[0035] It is a fourth object of the present invention to provide a
metal sintered filter excellent in corrosion resistance and
reverse-washing reproducibility.
[0036] It is a fifth object of the present invention to provide a
porous conductive plate excellent not only naturally in
moldability, but also in surface smoothness even without coating
like plasma spraying, and in addition, easy in production and
excellent in economy.
[0037] It is a sixth object of the present invention to provide a
highly corrosion resistant porous plate capable of economically
satisfying a condition to realize a uniform void ratio and a small
thickness as required by an ink dispersion plate for use in a large
ink jet printer.
DISCLOSURE OF THE INVENTION
[0038] A prior art titanium powder sintered filter was fabricated
with hydrogenation/dehydrogenation titanium powder or titanium
sponge powder. This is mainly because particles included in powder
are in irregular shapes; therefore the powder is excellent in press
moldability. In a case where particles are irregular in the shapes,
a cavity diameter is varied only if a mold is filled with powder;
therefore, a necessity arises for making a cavity diameter uniform
by press molding, which also makes press molding indispensable.
[0039] Such a titanium powder sintered compact is, however, very
poor in bendability as described above. Furthermore, since
hydrogenation/dehydrogenation titanium powder or titanium sponge
powder is composed of particles in irregular shapes, a cavity
diameter is made uniform at a comparatively small level by press
molding, thereby causing press moldability to be comparatively
good. It is difficult to mold the powder into a cylinder with a
large height, however, and reverse-washing reproducibility is also
poor when the titanium sintered compact is used as a filter. Even
with press molding applied, uniformity of a cavity diameter is
still insufficient and a skin layer receiving no press molding is
conspicuously non-uniform in cavity diameter as described
above.
[0040] In order to solve these problems, the present inventors
focused attention on spherical gas atomized titanium powder.
Spherical gas atomized titanium powder is powder of titanium or a
titanium alloy produced by means of a gas atomization method and
individual particles are of a sphere with a smooth surface since
the individual particles are formed by solidification during the
time when melt spray of titanium or a titanium alloy is flying.
Furthermore, particle diameters can be very much reduced down to as
small as 100 .mu.m or less on average and screening is easily
applied for classification by particle diameter.
[0041] Such spherical gas atomized titanium powder is excellent in
fluidity and has a good contactability between particles;
therefore, a uniform and sufficient packing density can be attained
in the powder filling a sintering vessel without applying a
pressure thereto. Then, by sintering the powder in the vessel, a
porous compact with a high mechanical strength is fabricated
without press molding and in thus fabricated porous compact,
adjacent spherical particles are fused to each other at contact
points and the fused points are uniformly distributed in the bulk
thereof; therefore, it was found that in a case where a porous
compact was comparatively small in thickness, excellency in bending
characteristic was assured. Furthermore, a sintered compact in any
shape and any size including a cylinder is fabricated without press
molding and thus fabricated sintered compact has not only a
sufficient strength but a uniform cavity therein with certainty and
furthermore, a shape of each cavity is of a smooth spherical
surface. In addition, by changing diameters of particles in raw
material powder, that is by adjusting diameters of particles in
use, diameters of cavities are controlled in a wide range with a
constant void ratio. A void ratio in a porous compact thus obtained
is in the range of from 35 to 55% without applying a pressure to
powder in a sintering vessel.
[0042] A titanium powder sintered compact of the present invention
has reached its completion based on findings described above, and
is characterized by that the sintered compact is a plate-like
porous compact obtained by sintering spherical gas atomized
titanium powder and a void ratio of the porous compact is in the
range of from 35 to 55%.
[0043] In a sintered titanium filter made of the titanium sintered
compact, a maximum pore diameter can be controlled in the range of
from 3 to 70 .mu.m and a pressure drop can be restricted to a small
value. Note that a void ratio of the porous compact, though
detailed later, is not limited in the range of from 35 to 55% in
actual fabrication thereof. This is because a void ratio in the
range of from 35 to 55% is especially suited for a variety of
applications and, at the same time, obtained with ease in
fabrication thereof.
[0044] A titanium powder sintered compact of the present invention
can acquire so excellent a bending characteristic that the sintered
compact can be bent into a cylinder by restricting a thickness of
the porous compact to 500 .mu.m or less. If a thickness of the
porous compact is larger than 500 .mu.m, bending at room
temperature is impossible. In a case where powder with particles in
irregular shapes such as hydrogenation/dehydrogenation titanium
powder, titanium sponge powder or the like is used instead of
spherical gas atomized titanium powder, uniformity of a cavity
diameter cannot be achieved in molding without applying a pressure
to the powder even if the plate thickness is 500 .mu.m or less.
What's worse, since fused points between particles are distributed
in non-uniformity, there locally arise portions with shortage of
strength, thereby disabling bending at room temperature.
[0045] The plate thickness is especially preferably 100 .mu.m or
less from the viewpoint of bendability at room temperature. It is
preferable that the thinner the lower limit of the plate thickness
is, the better it is from the viewpoint of bendability at room
temperature, while in a case where a ratio of a particle diameter/a
plate thickness is excessively large, for example in a
mono-particulate layer, a void ratio is larger than the range of
from 35 to 55%, which is preferably applied to a metal powder
sintered compact. Therefore, the plate thickness is preferably
three times an average diameter of particles in powder in use.
[0046] While a shape of a titanium powder sintered compact is
basically a flat plate, the sintered compact may assume one of
other shapes, for example a curved plate or the like and it is
naturally possible to bend a flat plate into a plate with a
semicircular shape in section, or a U letter in section, to work a
flat plate into a corrugated plate, or to bend a flat plate into a
cylinder according to a kind of application, without imposing any
limitation on a particular shape in a molding stage or a usage
stage.
[0047] A cylindrical porous compact of the present invention is a
titanium powder sintered compact described above and formed by
sintering spherical gas atomized powder directly into a cylinder,
and for example, a cylindrical titanium powder sintered filter can
be provided as a product large in height and good reverse-washing
reproducibility and fabricated at a low cost without using a
press.
[0048] A metal sintered filter of the present invention is of a
titanium powder sintered compact, in a titanium porous structure of
which, a cavity diameter is stepwise increased from one surface
thereof to the other surface, and which not only is excellent in
reverse-washing reproducibility, but can also have a uniform void
ratio independently of increase in cavity diameter.
[0049] That is, particle diameters of spherical gas atomized
titanium powder affect cavity diameters. By increasing, stepwise, a
particle diameter of used powder from one surface of a filter to
the other surface, a cavity diameter can be stepwise increased,
thus enabling a layered structure in which plural porous layers are
stacked in ascending order of cavity diameter increasing stepwise.
Since even in a case where particle diameters of spherical powder
changes to another ones, a void ratio in a sintered compact is
fundamentally constant without a pressure applied to the powder in
a sintering vessel, cavity diameters can be changed without
changing a void ratio. If a sintering temperature is changed,
contact areas between particles also become different, thereby
controlling cavity diameters and in turn, controlling a void ratio
as well.
[0050] FIGS. 1(a) and 1(b) are image views showing a difference in
structure between a prior art example metal sintered filter and an
example metal sintered filter of the present invention,
respectively.
[0051] In the prior art example shown in FIG. 1(a), there is used
powder including irregular shaped titanium particles 1 such as
hydrogenation/dehydrogenation titanium powder, titanium sponge
powder or the like as titanium powder, wherein finer particles are
used in a layer closer to the surface thereof and a cavity diameter
is smaller in a layer closer to the surface thereof. In this case,
press moldability is comparatively good and a void ratio is made
uniform by press molding, while a degree of the uniformity in void
ratio is insufficient. Since shapes of cavities 2 formed between
titanium particles 1 are not made of smooth curved surfaces, it is
difficult to remove solid matter.
[0052] In contrast to this, in the example of the present invention
shown in FIG. 1(b), there is used powder including spherical
titanium particles 1 produced by means of a gas atomization method
as titanium powder, wherein finer particles are used in a layer
closer to the surface thereof and a cavity diameter is smaller in a
layer closer to the surface thereof, whereas a void ratio is
constant even without applying a pressure. Since shapes of cavities
2 formed between titanium particles 1 are course of fabrication nor
applied with surface coating after the fabrication, but shows a
very excellent aptitude shown in terms of both performance and
economy as a power feeder in a polymer electrolyte membrane type
water electrolyzer or a current collector in a solid polymer fuel
cell.
[0053] A porous conductive plate of the present invention has been
developed based on such findings and is a titanium powder sintered
compact and used as a power feeder in a polymer electrolyte
membrane type water electrolyzer or a current collector in a solid
polymer fuel cell.
[0054] In contrast, with a prior art titanium sintered plate, it
was difficult to fabricate a thin and large area sintered plate as
requested in application to an ink jet dispersion plate for use in
an ink jet printer as described above. Moreover, the present
inventors conducted a trial in which hydrogenation/dehydrogenation
titanium powder was not applied with die pressing prior to
sintering and sintered without a pressure applied thereto, but with
the result of obtaining no uniformity in void ratio required in a
dispersion plate.
[0055] In order to solve this problem, the present inventors
focused attention again on spherical gas atomized titanium powder.
Since spherical gas atomized titanium powder is very excellent in
fluidity and good in contactability between particles, a uniform
and sufficient packing density can be made of smooth curved
surfaces each of a spherical surface, it is easy to remove solid
matter from the cavities 2.
[0056] The present inventors, furthermore, fabricated, on a trial
base, sintered plates imagined as a power feeder in a polymer
electrolyte membrane type water electrolyzer or a current collector
in a solid polymer fuel cell using spherical gas atomized titanium
powder and features, characteristics thereof and the like were
evaluated. As a result, the following facts were made clear.
[0057] Spherical gas atomized titanium powder is excellent in
fluidity and the powder in a sintering vessel fills the vessel at a
sufficient density even without applying a pressure. If sintering
the powder, (1) a sufficient mechanical strength is ensured in a
case of a shape even with a thin and large area, (2) void ratios
preferable as a power feeder or a current collector can be obtained
with simplicity without any special operation applied, and (3) a
surface is high in smoothness and no fear arises of being brought
into close contact with an adjacent film electrode laminates to
damage it even without coating by plasma spraying or the like.
Therefore, there are avoided a voltage drop due to increase in
resistance at a bonding interface and an adverse influence on
permeabilities of a gas and a liquid.
[0058] That is, a sintered compact using spherical gas atomized
titanium powder is not applied even with a pressure in the attained
by filling a sintering vessel with the powder without a pressure
applied to the powder in the sintering vessel. By sintering the
powder in the sintering vessel, a porous compact in the shape of a
thin plate with a high mechanical strength was fabricated without
press molding and in addition, in thus fabricated thin plate,
adjacent spherical particles were fused to each other in point
contacts and the fused points were distributed uniformly;
therefore, it was found that a variation in void ratio in a plate
surface was also small.
[0059] A highly corrosion resistant porous plate of the present
invention has been completed based on such findings and made of a
titanium powder sintered compact described above and a ratio T/S of
a plate thickness T (in mm) of the porous compact to an area S of
the porous compact (in mm.sup.2) is controlled to be 1/10000 or
less.
[0060] If the ratio T/S is in excess of 1/10000, a porous plate
excellent in uniformity in void ratio can be fabricated by HIP but
without using spherical gas atomized titanium powder. A production
cost thereof is highly expensive, however. A highly corrosion
resistant porous plate of the present invention is significant
because of being provided at a very low cost with uniformity in
void ratio, but without using even press molding, let alone HIP
naturally not used.
[0061] A variation in void ratio in a surface of the plate is
preferably 3% or less in standard deviation. A porous plate with
the uniformity inferior thereto can be fabricated by a combination
of powder including particles in irregular shapes and press
molding. The lower limit is especially not defined since a smaller
variation in void ratio is better. In the present invention, the
variation can be 3% or less and can also be 1% or less.
[0062] As spherical gas atomized titanium powder used in a titanium
powder sintered compact of the present invention, three kinds, for
example, classified by a range of particle diameters are sold on
the market. That is, the three kinds include fine particles of 45
.mu.m or less in diameter, coarse particles in the range of from 45
to 150 .mu.m in diameter and coarser particles 150 .mu.m or more in
diameter, and the fine particles has an average particle diameter
of about 25 .mu.m and the coarse particles has an average particle
diameter of about 80 .mu.m.
[0063] An average particle diameter of spherical gas atomized
titanium power is preferably selected in the range of 150 .mu.m or
less. If the average particle diameter exceeds 150 .mu.m, spacings
between fused points between particles are excessively wide;
therefore, a possibility of breakage in bending is high. A void
ratio, for example, in a case where a titanium powder sintered
compact of a plate thickness in the vicinity of 500 .mu.m is larger
than the range of from 35 to 55% preferably applied to a metal
powder sintered filter. A relationship between a plate thickness
and fused points between particles are desirably such that two or
more fused points are present within a plate thickness range. The
lower limit is not specifically defined since there is a tendency
that workability is improved with decrease in particle
diameter.
[0064] A plate thickness of a porous compact, that is a titanium
powder sintered compact of the present invention is 500 .mu.m or
less from the viewpoint of bendability described above and it is
preferable for the thickness to be 100 .mu.m or less in
consideration of bendability at room temperature.
[0065] A void ratio can be attained in the range of from 35 to 55%
in a case where spherical gas atomized titanium powder sold on the
market is used even without applying a pressure to the powder in
filling it in a vessel or sintering. According to an investigation
having been conducted by the present inventors, the void ratios in
the range are preferable for use in a metal powder sintered
filter.
[0066] Particle diameters of spherical gas atomized titanium powder
for used in a cylindrical porous compact of the present invention
is not specifically limited to those in any particular diameter
range and no problem arises with a level of commercial powder of
this kind, while it is difficult to produce extremely fine
particles industrially with a good yield even according to a gas
atomization method. In a case where coarse particles are used to
fabricate a thin porous compact, a contact area between particles
in titanium powder relative to a thickness thereof is smaller, so
there arises a worry of shortage of strength. This is because in a
case where coarse particles are used in fabrication of a thin
porous compact, the number of contact points between particles in
titanium powder is small. On the other hand, if a contact area
between particles in titanium powder is increased so as to
supplement a decrease in the number of contact points to thereby
improve the strength, a void ratio cannot fall inside the range of
from 35 to 55%. Therefore, particle diameters are preferably in the
range of from 10 to 150 .mu.m on average.
[0067] A void ratio of a cylindrical porous compact can be attained
in the range of from 35 to 55% using spherical gas atomized
titanium powder sold on the market even without a pressure applied
in filling and sintering. According to an investigation having been
conducted by the present inventors, void ratios in the range are
preferable for use in a metal powder sintered filter.
[0068] A void ratio can be strictly controllable by adjustment of a
sintering temperature, selection of particle diameters, adjustment
of a pressure and the like. In a general tendency, with a higher
sintering temperature, a contact area between particles increases
and a cavity diameter decreases, resulting in reduction in void
ratio. Likewise, as a particle diameter becomes smaller, a
sinterability is improved with the same sintering temperature held
and as a result, a cavity diameter gets smaller, leading to a
tendency of decreasing a void ratio. If a pressure is applied in
filling powder into a vessel and sintering of the powder, a void
ratio is reduced.
[0069] A cavity diameter can be controlled, similarly to the case
of a void ratio, by adjustment of a sintering temperature,
selection of particle diameters and the like. In a cylindrical
porous compact of the present invention, a cavity diameter is made
uniform without applying a press because of excellency in fluidity
of spherical gas atomized titanium powder. With particle diameters
made more uniform, uniformity in cavity diameter is further
promoted. Anyway, specifications of a product is substantially
determined by specifications of raw material powder, which makes
fabrication of the cylindrical porous compact simple.
[0070] A shape and size of a porous cylinder are properly
determined by a shape and size of a product such as a filter to be
fabricated and in a case of natural filling without pressing, a
shape and size of a product is governed by an inner shape and size
of a sintering vessel.
[0071] Note that in JP No. 2791737 described above, the use of
spherical gas atomized powder is described, while the spherical
powder is used not in formation of a base section in the shape of a
cylinder but in formation of a fine powder layer coated on the
surface of the base section, wherein the base section is fabricated
by sintering powder including particles of irregular shapes into a
cylinder by cold isostatic pressing.
[0072] Cavity diameters are important in a metal sintered filter of
the present invention. Cavity diameters are preferably selected in
the range of from 3 to 70 .mu.m. That is, while in a highly
corrosion resistant metal sintered filter, cavity diameters are
desirably 70 .mu.m or less in consideration of filterabilty,
spherical gas atomized powder of an average particle diameter of 10
.mu.m or less is required to be used in order to obtain cavity
diameters of 3 .mu.m or less, leading to a high fabrication
cost.
[0073] A void ratio in the range of from 35 to 55% can be attained
by using spherical gas atomized titanium powder sold on the market
without applying a pressure to the powder in filling and sintering.
According to an investigation having been conducted by the present
inventors, the void ratios in the range are preferable in a metal
powder sintered filter.
[0074] No specific limitation is placed on a range of particle
diameters of spherical gas atomized titanium powder, and the powder
of this kind at an off the shelf level described above can be
non-problematically used, whereas extremely fine powder is
difficult in industrial production even according to a gas
atomization method with a good yield. On the other hand, a shortage
of strength is worried in a thin porous compact using coarse
particles since a contact area of particles of titanium powder is
small relative to a thickness of the thin porous compact.
Therefore, particle diameters are preferably selected in the range
of from 10 to 150 .mu.m on average so as to adapt for required
cavity diameters.
[0075] No specific limitation is placed on a range of particle
diameters of spherical gas atomized titanium powder for use in a
porous conductive plate of the present invention and the powder of
this kind at an off the shelf level is non-problematically used,
whereas extremely fine powder is difficult in industrial production
even according to a gas atomization method with a good yield. On
the other hand, a shortage of strength is worried in a thin porous
compact using coarse particles since a contact area of particles of
titanium powder is small relative to a thickness of the thin porous
compact. Therefore, particle diameters are preferably selected in
the range of from 10 to 150 .mu.m on average.
[0076] The range of from 35 to 55% in void ratio of a porous
conductive plate can be attained by using spherical gas atomized
titanium powder sold on the market without applying a pressure in
filling and sintering. According to an investigation having been
conducted by the present inventors, the range of the void ratio is
preferable in consideration of electrical and mechanical properties
of a porous conductive plate made of a titanium powder sintered
compact. Note that adjustment of a void ratio so as to be 35% or
less can also be realized by applying a pressure in filling and
sintering and selecting other sintering conditions.
[0077] A void ratio can be controllable by adjustment of a
sintering temperature, selection of particle diameters, adjustment
of a pressure and the like. In a general tendency, with a higher
sintering temperature, a contact area between particles increases,
resulting in reduction in void ratio. Likewise, as a particle
diameter becomes smaller, a contact area between particles
increases, leading to a tendency of decreasing a void ratio. If a
pressure is applied in filling and sintering, a void ratio
decreases. As particle diameters are larger relative to a thickness
of a porous conductive plate, there arises a tendency of increasing
a void ratio.
[0078] With combinations of parameters or conditions described
above adopted, a void ratio is controlled arbitrarily in a
comparatively wide range. Note that an increase and decrease in
void ratio to extremes become causes for degradation of
reception/supply efficiency of water and gas in a reaction and
shortage of a strength of a porous conductive plate.
[0079] A size of a porous conductive plate is properly selected
depending on a size of a power feeder or a current collector to be
fabricated.
[0080] An average particle diameter D of spherical gas atomized
titanium powder for use in a highly corrosion resistant porous
plate of the present invention is preferably 150 .mu.m or less. If
the average particle diameter exceeds 150 .mu.m, cavity diameters
grow large, so a dispersion effect is difficult to be attained. No
specific limitation is imposed on the lower limit of an average
particle diameter D since the smaller an average particle diameter
is, the better it is.
[0081] A thickness T of a porous plate is preferably 2 mm or less
and more preferably 1 mm or less in order to reduce a pressure
drop.
[0082] A void ratio is preferably in the range of from 35 to 55%.
This is because if a void ratio is less than 35%, a problem arises
that a dispersibility is degraded and a pressure drop is increased.
The upper limit is reasonably 55% in consideration of geometry in a
case where spherical particles are used as powder.
[0083] A highly corrosion resistant porous plate of the present
invention is especially preferable as an ink dispersion plate for
an ink jet printer small in thickness and large in area, requiring
a uniform void ratio and a high corrosion resistance, and greatly
contributes to reduction in fabrication cost for the dispersion
plate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0084] FIGS. 1(a) and 1(b) are image views showing a difference in
structure between a prior art example metal sintered filter and an
example metal sintered filter of the present invention,
respectively.
[0085] FIG. 2 is an electron microscopic photograph of a titanium
sintered filter obtained by sintering without applying a pressure
using spherical powder particles produced by means of a gas
atomization method from titanium sponge as a raw material in an
embodiment of the present invention.
[0086] FIG. 3 is an electron microscopic photograph of a titanium
sintered filter obtained by sintering without applying a pressure
using particles in irregular shapes obtained by pulverizing
titanium sponge as a raw material with a
hydrogenation/dehydrogenation method
[0087] FIG. 4 is a graph showing, by comparison, relationships
between a flow rate of a passing fluid and a pressure drop in
Example 3 of the present invention and Comparative examples 4 to
6.
[0088] FIG. 5 is an image view of a titanium powder sintered
compact showing a second embodiment of the present invention.
[0089] FIG. 6 is a descriptive view for a fabrication method for a
cylindrical porous compact showing a third embodiment of the
present invention and a sectional view showing a filling state of
spherical gas atomized titanium powder.
[0090] FIG. 7 is a model sectional view of a metal sintered filter
showing a fourth embodiment of the present invention.
[0091] FIG. 8 is a descriptive view for a fabrication method for a
porous conductive plate showing a fifth embodiment of the present
invention and a sectional view showing an example filling state of
spherical gas atomized powder.
[0092] FIG. 9 is a sectional view showing another example filling
state of spherical gas atomized powder.
[0093] FIG. 10 is a sectional view showing still another example
filling state of spherical gas atomized powder.
[0094] FIG. 11 is an image view of a highly corrosion resistant
porous plate showing a sixth embodiment of the present
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0095] Description will be given of embodiments of the present
invention below with reference to the accompanying drawings.
[0096] Raw material powder of titanium or a titanium alloy used in
embodiments of the present invention is spherical particles of 150
.mu.m or less (hereinafter referred to as spherical titanium powder
for short) produced by means of a gas atomization method from
titanium sponge. Since the spherical particles obtained by means of
the gas atomization method is powder solidified during flying of
melt spray of titanium, surfaces of particles of particles included
in the powder is extremely smooth as compared with particles in
irregular shapes in powder obtained by pulverizing titanium sponge
or in powder obtained by hydrogenation/dehydrogenation.
[0097] In a case where a filter is fabricated using the spherical
titanium powder described above, diameters of particles of the
powder are desirably made uniform using a screen in order to
achieve a desired performance. Then, a sintering vessel is filled
with the spherical titanium powder uniform in particle diameter
without applying a pressure to the powder therein. A void ratio of
a sintering raw material filling the vessel without applying a
pressure is adjustable in the range of from 35 to 55% by adjusting
a particle size distribution of the sintering raw material. By
applying vibration to the spherical titanium powder prior to the
sintering, a void ratio is reduced so as to fall in the range of
from 35 to 55%. There arises no chance of being 35% or less,
however. Note that in a case where a pressure is applied in
filling, a void ratio is reduced generally 35% or less.
[0098] While by sintering the spherical powder filling the
sintering vessel without applying a pressure as described above,
only contact points between spherical particles are fused to each
other to bond, a mechanical strength required by a filter is
sufficiently ensured. Since by sintering the spherical powder at a
temperature range much lower than the melting point of titanium,
the spherical powder is sintered while sustaining shapes of
spherical particles prior to the sintering, a void ratio of the
sintered compact does not change and the void ratio after the
sintering remains in the range of from 35 to 55% unchanged from
that prior to the sintering. Note that as far as the sintering is
performed at the low temperature range, a sintered compact can be
obtained in the range of 35 to 55% in void ratio.
[0099] Since spherical titanium powder can be industrially produced
so as to be as small as in the range of from 10 to 150 .mu.m in
average particle diameter by means of a gas atomization method, a
spherical titanium filter can be fabricated with a maximum pore
diameter in the range of from 3 to 70 .mu.m with the spherical
titanium powder. That is, a titanium filter with fine pores and a
small pressure drop can be fabricated with a high productivity.
Note that if the spherical titanium powder falls outside and
exceeds the range of from 10 to 150 .mu.m in average particle
diameter, it is impossible to obtain a sintered compact with a
maximum pore diameter in the range of 3 to 70 .mu.m
[0100] On the other hand, while spherical powder can be produced
according to a rotating electrode method, an obtained average
particle diameter of spherical powder is generally 400 .mu.m or
more and it is difficult to industrially produce spherical powder
with an average particle diameter of 150 .mu.m or less and
therefore, much harder to industrially produce spherical powder
with an average particle diameter of 30 .mu.m or less with a good
yield.
[0101] A maximum pore diameter described above is measured with a
mercury porosimeter method. The mercury porosimeter method gets
started with immersing a specimen into mercury and then gradually
raises a pressure of mercury. During the increase in pressure, as a
pressure is raised, mercury intrudes into a pore with a smaller
diameter; thereby obtaining a value discriminating between pore
sizes of a porous compact. That is, a porous compact with a smaller
maximum pore diameter has smaller pores and makes it possible to
realize a filter so excellent in performance as to remove foreign
matter with a smaller size.
[0102] In implementing the present invention, it is desirable to
sinter raw material spherical titanium powder filling a cylindrical
vessel at a temperature in the range of from 650 to 1200.degree. C.
much lower than the melting point of titanium without applying a
pressure in order to equally maintain a void ratio of the raw
material spherical titanium powder in the sintered compact without
reduction in the void ratio of the raw material spherical titanium
powder in the course of the sintering. If a sintering temperature
is lower than 650.degree. C., sintering is performed
insufficiently, while if exceeding 1200.degree. C., sintered
portions are not limited to contact points between particles, but
the bodies of particles are molten together, with the result that
original shapes of spherical particles cannot be maintained to
deform and contract, decreasing a void ratio and in turn,
increasing a pressure drop.
[0103] Implementation of the present invention features no adoption
of molding with a press which causes deformation of particles in
powder; therefore, a sintered titanium filter can also be
fabricated in a procedure in which a green preform is obtained by
mixing spherical titanium powder with a proper binder as in a
doctor plate method or an extrusion method and then, thus obtained
green preform is degreased to remove the binder and vacuum
sintered.
[0104] Example 1
[0105] Billets were obtained from raw material titanium sponge and
a melt thereof produced by electromagnetic induction heating was
gas atomized in an Ar gas atmosphere. Obtained titanium powder was
classified by vibration screening to obtain spherical powder with
an average particle diameter of 10 .mu.m. A high density alumina
vessel in the shape of a square having one inner side of 100 mm and
a depth of 3 mm was filled with the powder without applying a
pressure thereto and then the powder was sintered keeping it at a
vacuum degree of 7.times.10.sup.-3 Pa at 1000.degree. C. for 15
minutes without applying pressure to the powder.
[0106] Example 2
[0107] A titanium sintered filter was fabricated in the same method
and conditions as in Example 1 with the exception that the gas
atomized powder was classified by vibration screening to obtain
spherical powder with an average particle diameter of 29 .mu.m.
[0108] Example 3
[0109] A titanium sintered filter was fabricated in the same method
and conditions as in Example 1 with the exception that the gas
atomized powder was classified by vibration screening to obtain
spherical powder with an average particle diameter of 124 .mu.m. In
FIG. 2, there is shown an electron microscopic photograph of the
titanium sintered filter. It is found that each of particles of the
titanium sintered filter is maintained in the shape of an unchanged
sphere with many of voids.
[0110] Example 4
[0111] A titanium sintered filter was fabricated in the same method
and conditions as in Example 1 with the exception that the gas
atomized powder was classified by vibration screening to obtain
spherical powder with an average particle diameter of 140 .mu.m.
Furthermore, the same vessel as in Example 1 was filled with the
powder without applying pressure, but followed by vibrations of 100
cycles imposed on the vessel with a vibration machine. On this
occasion, also different from Example 1, the vessel was filled with
the powder higher than 3 mm in height prior to the vibrations so
that 3 mm in height resulted after the vibrations.
[0112] Example 5
[0113] A titanium sintered filter was fabricated in the same method
and conditions as in Example 1 with the exception that the gas
atomized powder was classified by vibration screening to obtain
spherical powder with an average particle diameter of 148 .mu.m.
Furthermore, the same vessel as in Example 1 was filled with the
powder without applying pressure, but followed by vibrations of 100
cycles imposed on the vessel with a vibration machine. On this
occasion, also different from Example 1, the vessel was filled with
the powder higher than 3 mm in height prior to the vibrations so
that 3 mm in height resulted after the vibrations.
[0114] In Examples 3, 4 and 5, an average particle diameter of the
raw material powder was adjusted so that a maximum pore diameter of
a sintered filter obtained in each of the examples was in the range
of from 47 to 68 .mu.m, wherein if a pressure is applied, the
pressure can be adjusted for the adjustment. The reason why a
maximum pore diameter was adjusted in the range of from 47 to 68
.mu.m is that a maximum pore diameter of a sintered filter used in
a gas chromatography apparatus satisfies a condition requiring the
maximum pore diameter of 70 .mu.m or less. Since a sintered filter
with the same maximum pore diameter is desirably more excellent in
corrosion resistance and smaller in pressure drop, filters in the
same shape were fabricated in Comparative examples 1, 2, and 4 to
6, described below, and pressure drops were compared with each
other in a condition of a flow rate of 1 litter/min/cm.sup.2.
[0115] Comparative example 1
[0116] Billets were obtained from raw material titanium sponge and
a melt thereof produced by electromagnetic induction heating was
gas atomized in an Ar gas atmosphere. Obtained titanium powder was
classified by vibration screening to obtain spherical powder with
an average particle diameter of 212 .mu.m. A high density graphite
vessel in the shape of a square having one inner side of 100 mm was
filled with the powder and then the powder was sintered keeping it
at a vacuum degree of 7.times.10.sup.-3 Pa at 1660.degree. C. for
15 minutes under a pressure of 800 kg/cm.sup.2 thereto to obtain a
titanium sintered filter with a thickness of 3 mm.
[0117] Comparative example 2
[0118] A titanium sintered filter was fabricated in the same method
and conditions as in Comparative example 1 with the exception that
the gas atomized powder was classified by vibration screening to
obtain spherical powder with an average particle diameter of 246
.mu.m. Furthermore, a high density graphite vessel in the shape of
a square having one inner side of 100 mm was filled with the powder
and then the powder was sintered keeping it at a vacuum degree of
7.times.10.sup.-3 Pa at 1660.degree. C. for 15 minutes under a
pressure of 1200 kg/cm.sup.2, which pressure was different from the
temperature adopted in Comparative example 1, thereto to obtain a
titanium sintered filter with a thickness of 3 mm.
[0119] Comparative example 3
[0120] A cylindrical titanium ingot is pulverized by means of a
plasma rotating electrode method and the powder was classified by
vibration screening to obtain spherical powder with an average
diameter of 450 .mu.m. A high density alumina vessel in the shape
of a square having one inner side of 100 mm and a depth of 3 mm was
filled with the classified powder without applying a pressure and
then the classified powder was sintered keeping it at a vacuum
degree of 7.times.10.sup.-3 Pa at 1000.degree. C. for 15 minutes
without applying a pressure thereto to obtain a titanium sintered
filter.
[0121] Comparative example 4
[0122] Stainless steel powder sold on the market produced by means
of a water atomization method was classified by vibration screening
to obtain powder including particles in irregular shapes with an
average particle diameter of 147 .mu.m. The classified powder was
sintered in the same condition as in Comparative example 3 to
obtain a titanium sintered filter.
[0123] Comparative example 5
[0124] Powder obtained by pulverizing titanium sponge with a
hydrogenation/dehydrogenation method was classified by vibration
screening to obtain powder including particles in irregular shapes
with an average particle diameter of 102 .mu.m. The classified
powder was sintered in the same condition as in Comparative example
3 to obtain a titanium sintered filter. In FIG. 3, there is shown
an electron microscopic photograph of the titanium sintered filter.
The sintered compact is composed of particles of irregular
shapes.
[0125] Comparative example 6
[0126] Powder obtained by pulverizing titanium sponge with a
mechanical crushing method was classified by vibration screening to
obtain powder including particles in irregular shapes with an
average particle diameter of 103 .mu.m. The classified powder was
sintered in the same condition as in Comparative example 3 to
obtain a titanium sintered filter.
[0127] Physical parameters of raw material powder used in Examples
1 to 5 and Comparative examples 1 to 6 were compared and shown in
table 1. Furthermore, in Table 2, there are shown physical
parameters (void ratios, maximum pore diameters, particle diameters
and pressure drops) of thus obtained sintered filters. Note that
there are shown measured particle diameters of sintered filters
only in Examples 1 to 4 of the present invention and Comparative
example 3, where shapes of spherical particles in raw material
powder are maintained after the sintering. Furthermore, pressure
drops of a fluid are shown at a flow rate of 1 litter/min/cm.sup.2
by comparison.
1 TABLE 1 Raw material powder Raw Average Material Production
Diameter of Powder Method Shape (.mu.m) Example 1 Titanium gas
atomization Spherical 10 sponge method Example 2 " " " 29 Example 3
" " " 124 Example 4 " " " 140 Example 5 " " " 148 Comparative " " "
212 Example 1 Comparative " " " 246 Example 2 Comparative Titanium
plasma rotating " 450 Example 3 ingot electrode method Comparative
Stainless Water Irregular 147 Example 4 steel atomization ingot
method Comparative Titanium Hydrogenation/ " 102 dehydrogenation
Example 5 sponge method Comparative " Mechanical " 103 example 6
crushing method
[0128]
2 TABLE 2 sintered filter maximum void pore Particle Pressure
Sintering ratio diameter diameter drop method (%) (.mu.m) (.mu.m)
(kgf/cm.sup.2) Example 1 Without 41 3 10 -- pressure Example 2 " 42
12 32 -- Example 3 " 44 47 156 0.12 Example 4 " 37 48 190 0.16
Example 5 " 44 68 146 0.11 Comparative Under 34 49 -- 0.42 Example
1 pressure Comparative " 30 48 -- 1.2 Example 2 Comparative Without
47 140 460 -- Example 3 pressure Comparative " 51 48 -- 0.55
Example 4 Comparative " 56 47 -- 0.40 Example 5 Comparative " 61 49
-- 0.38 example 6
[0129] Note that an average particle diameter of spherical
particles included in a titanium sintered filter is measured in the
following way. A diagonal is drawn between opposed vertices in a
field of view in the shape of a rectangle when observing with a
microscope and measurement is performed on diameters of all of
selected particles, 50% or more of the contour of each of which is
viewed, among particles on the diagonal. Then, first 10 measured
values are selected in descending order of a diameter to calculate
an average thereof. The measurement is repeated 10 times at
different sites and 10 calculated average values are further
averaged to eventually obtain an average particle diameter of the
spherical particles. It is found from Tables 1 and 2 that an
average spherical particle diameter of a titanium sintered filter
obtained according to this procedure is almost the same as that of
corresponding raw material powder.
[0130] While in the above examples, titanium sponge was raw
material, there can be used as raw material: titanium scrap and
titanium ingot. Furthermore, in a case where a sintered filter of a
titanium alloy is fabricated, a desired titanium alloy ingot is
used for producing raw material powder.
[0131] In Examples 3 and 4 and Comparative examples 1, 2, 4, 5 and
6 described above shown in Tables 1 and 2, particle diameters of
raw material and a sintering pressure are adjusted and sintered so
that in each example, a maximum pore diameter of a sintered filter
took 48 .+-.1 .mu.m. It is found from the results of the comparison
tests that there is a great difference in pressure drop between
Examples 2 and 3 in which sintering was performed without applying
a pressure using powder with an average particle diameter of 181
.mu.m or less, and Comparative examples 1 and 2 in which sintering
was performed under a pressure using powder with an average
particle diameter of 200 .mu.m or more, though the powder produced
according to the same gas atomization method was used as raw
material in both example groups, and it is further found that a
sintered filter fabricated by execution of the present invention
has a smaller pressure drop.
[0132] It is found that a pressure drop is large in any of sintered
filters of Comparative examples 4 to 6 fabricated by sintering
powder including particles in irregular shapes produced according
to a water atomization method, a hydrogenation/dehydrogenation
method, a mechanical crushing method except for a gas atomization
method without applying a pressure. What's worse, the stainless
steel sintered filter of Comparative example 4 has a problem of
poor corrosion resistance. Note that in FIG. 4, there are shown
relationships between a flow rate of a passing fluid and a fluid
pressure drop in Example 3 and Comparative examples 4, 5, and 6.
While in any case, a pressure drop is larger with an increased flow
rate, a pressure drop in Example 3 of the present invention is the
smallest.
[0133] FIG. 5 is a model sectional view of a titanium powder
sintered compact showing a second embodiment of the present
invention.
[0134] A sintering vessel in the shape of a dish made of high
density alumina is filled with spherical gas atomized titanium
powder 11 having a prescribed average diameter and thereafter, the
spherical gas atomized titanium powder 11 is vacuum sintered
without applying a pressure, thereby fabricating a porous sintered
compact 10 in the form of a thin plate.
[0135] A plate thickness T of the sintered compact 10 is 500 .mu.m
or less. Adjacent spherical particles are fused to each other in
point contact and the sintered compact 10 of a plate thickness of
500 .mu.m or less exerts an excellent bending characteristic. That
is, portions where adjacent particles are fused to each other in
point contact are uniformly distributed throughout all of a
titanium powder sintered compact using spherical gas atomized
powder to thereby cause no local concentration of a bending stress,
leading to excellency in a bending characteristic of the sintered
compact.
[0136] A sintering temperature is preferably selected in the range
of from 650 to 1200.degree. C. much lower than the melting point of
titanium. If a sintering temperature is lower than 650.degree. C.,
sintering is not sufficiently performed. If exceeding 1200.degree.
C., sintering portions are not limited to contact points between
individual particles but the bodies of particles are molten
together, so there arises a risk that a void ratio and cavity
diameters at proper levels cannot be ensured. A sintering
temperature is changed in the range of temperatures, thereby
controlling a void ratio and cavity diameters. Furthermore, a
bending characteristic is also controlled.
[0137] As examples of the present invention and comparative
examples thereof, there were fabricated titanium powder sintered
compacts each in the shape of a thin plate with kinds of plate
thickness and commonly in the shape of a square having one side of
150 mm using spherical gas atomized titanium powder sold on the
market described above, that is fine particles in the range of 45
.mu.m or less (an average particle diameter of 25 .mu.m) and coarse
particles in the range of from 45 to 150 .mu.m (an average particle
diameter of 80 .mu.m).
[0138] Furthermore, a titanium powder sintered compact in the shape
of a thin plate with similar dimensions was fabricated, as a prior
art example, by a press in molding using
hydrogenation/dehydrogenation titanium powder (an average particle
diameter of 25 .mu.m) sold on the market.
[0139] Breakage states of thus fabricated titanium powder sintered
compact in the shape of a thin plate wound around a cylinder with
an outer diameter of 40 mm (a radius of curvature of 20 mm) were
investigated to thereby compare between bending characteristics.
Results are shown in Table 3.
3TABLE 3 average presence or Thickness particle absence of T .mu.m
diameter D .mu.m Void ratio % breakage Example 1 100 80 67
.smallcircle. Example 2 400 80 50 .smallcircle. Example 3 500 80 47
.smallcircle. Comparative 600 80 44 x Example 1 Example 4 100 25 55
.smallcircle. Example 5 400 25 48 .smallcircle. Example 6 500 25 45
.smallcircle. Comparative 600 25 42 x Example 2 Prior Art 100 25 62
x Example 1 Prior Art 400 25 56 x Example 2 Prior Art 500 25 53 x
Example 3 Prior Art 600 25 51 x Example 4
[0140] As understood from Table 3, in a case where spherical gas
atomized titanium powder was used as titanium powder and a plate
thickness is 500 .mu.m or less, an excellent characteristic can be
attained independently of particle diameters in the plate (in a
case of either fine particles or coarse particles).
[0141] FIG. 6 is a descriptive view for a fabrication method for a
cylindrical porous compact showing a third embodiment of the
present invention and a sectional view showing a filling state of
spherical gas atomized titanium powder.
[0142] A sintering vessel 20 made of high density alumina is
constructed of: an inner mold part 21 in the shape of a cylinder;
an outer mold part 22 in the shape of a cylinder arranged at the
outside of the inner mold part 21 with a prescribed clearance
therebetween; a fixing mold part 23 arranged at the outside of the
outer mold part 22 to fix the outer mold part 22; a spacer 24 in
the shape of a ring arranged at the lowest position of the
sintering vessel 20 between the inner mold part 21 and the outer
mold part 22 in order to form an annular space 25 therebetween.
[0143] The inner mold part 21 is divided into two pieces obliquely
to the height direction for removal and, together with the spacer
24 in the shape of a ring, inserted inside the outer mold part 22.
The outer mold part 22 is also divided into two pieces in a
circumferential direction for removal and firmly held by the fixing
mold part 23 outside the outer mold part 22 into a coalesced
state.
[0144] The sintering vessel 20 is assembled to form a clearance 25
of an annular shape in section on and above the spacer 24 between
the inner mold part 21 and the outer mold part 22. The clearance 25
in an annular shape in section is filled with spherical gas
atomized titanium powder 30 without applying a pressure. Then, the
spherical gas atomized titanium powder 30 in the sintering vessel
20 was vacuum sintered without applying a pressure.
[0145] In such a way, a cylindrical titanium powder sintered filter
is fabricated. In the filter, contact states between particles are
good and sizes of cavities formed between particles are made
uniform; therefore, a sufficient strength and uniform cavity
diameters can be ensured. As a result, a tall filter can be
fabricated at a low cost. With decrease in particle diameters,
cavity diameters can be reduced without changing a packing density.
By making particle diameters uniform, uniformity in cavity
diameters can be further improved. Furthermore, since each of
shapes of cavities is enclosed by a smooth curved surface, there is
a small chance for plugging pores and the filter is excellent in
reverse-washing reproducibility.
[0146] A sintering temperature is preferably in the range of from
650 to 1200.degree. C. much lower than the melting point of
titanium. If a sintering temperature is lower than 650.degree. C.,
sintering is not sufficiently performed. If exceeding 1200.degree.
C., sintering portions are not limited to contact points between
individual particles but the bodies of particles are molten
together, even without applying a pressure, so there arises a risk
that a void ratio and cavity diameters at proper levels cannot be
ensured.
[0147] By changing a sintering temperature, a void ratio and cavity
diameters are controlled as described above. Hence, an optimal
sintering temperature differs according to particle diameters in
the powder to be sintered. For example, it is desirable that a
sintering temperature is especially in the range of from 850 to
1200.degree. C. for coarse particles in the range of from 45 to 150
.mu.m in particle diameter. If a sintering temperature is lower
than 850.degree. C., there is a risk that sintering is not
sufficiently performed. On the other hand, in a case where fine
particles of 45 .mu.m or less are used, it is desirable that a
sintering temperature is especially in the range of 650 to
850.degree. C. since sufficient sinterability is ensured even in a
comparatively low temperature range.
[0148] As an example of the present invention, fabricated in the
method described above was a cylindrical titanium sintered filter
with a height of 250 mm, an outer diameter of 60 mm, an inner
diameter of 56 mm and a wall thickness of 2 mm. Used spherical gas
atomized titanium powder was in the range of from 45 to 150 .mu.m
in particle diameter, an atmosphere in the sintering furnace was in
a vacuum state, a sintering temperature was 1100.degree. C. and a
sintering time was 30 minutes. Investigation was performed about a
void ratio and cavity diameters of a fabricated filter at 5 points
in the height direction.
[0149] As a comparative example, a titanium sintered filter in the
shape of the same cylinder was fabricated using
hydrogenation/dehydrogenation titanium powder sold on the market
(in the range of from 45 to 150 .mu.m in particle diameter) while
additionally using press molding. Investigation was performed about
a void ratio and cavity diameters of a fabricated filter at 5
points in the height direction.
[0150] Results of the investigations are shown in Tables 4 and 5.
In the example, the void ratios and cavity diameters are made
uniform in the height direction despite no use of a press, whereas
in the comparative example, filling of titanium powder was not
uniformly performed despite application of press molding;
therefore, variations were large in void ratios and cavity
diameters.
4 TABLE 4 measuring Void ratio (%) points 1 2 3 4 5 averages
Example 41 42 42 42 43 42 Comparative 63 57 56 65 54 59 Example
[0151]
5 TABLE 5 measuring Average Cavity Diameter (.mu.m) points 1 2 3 4
5 averages Example 26 27 26 25 23 25 Comparative 41 36 41 44 33 39
Example
[0152] Comparison was performed between both cases in
reverse-washing reproducibility. That is, a solution obtained by
mixing silica beads having an average diameter of 10 .mu.m into
water at a concentration of 10 mg/litter was filtered through
filters so that an increase in weight of a filter after drying is
constant, thereafter, reverse-washing was performed on the filters
for a prescribed time at an air pressure of 5 kgf/cm.sup.2 and
after drying, the filters were weighed to obtain a change between
weights before and after the procedure described above, thereby
having evaluated reverse-washing reproducibility. In the example,
94% of the increase in weight was removed by reverse-washing, while
in the comparative example, only 78% were removed.
[0153] Note that while in the embodiments described above,
cylindrical products were fabricated directly form the powder, it
is possible that two parts of the product in the shape of a
semi-cylinder are separately fabricated and thereafter, the two
parts are welded to complete a cylinder. Besides, a shape of a
sintered product is not limited to a cylinder, but may be a long,
hollow body with a straight side and a section in the shape of a
polygon or the like.
[0154] FIG. 7 is a model sectional view of a metal sintered filter
showing a fourth embodiment of the present invention.
[0155] A sintering vessel made of high density alumina is filled
with spherical gas atomized titanium powder with a prescribed
average particle diameter without applying a pressure and
thereafter, the spherical gas atomized titanium powder is vacuum
sintered without applying a pressure, thereby fabricating a first
plate-like porous compact 41.
[0156] A second plate-like porous compact 42 is fabricated in a
similar way with the exception that spherical gas atomized titanium
powder with an average particle diameter larger than the spherical
gas atomized titanium powder used in the first plate-like porous
compact 41. In this process, a sintering temperature is adjusted so
that the same void ratio as in the first plate-like porous compact
41 is realized.
[0157] A third plate-like porous compact 43 is fabricated in a
similar way with the exception that spherical gas atomized titanium
powder with an average diameter larger than the spherical gas
atomized titanium powder used in the second plate-like porous
compact 42. In this process, a sintering temperature is adjusted so
that the same void ratio as in the first plate-like porous compact
41 and the second plate-like porous compact 42 is realized.
[0158] The fabricated three plate-like porous compacts 41, 42 and
43 are superposed one on another and sintered to thereby fabricate
a sintered filter 40 of a three-layer structure. Since the
fabricated sintered filter 40 is constructed with three kinds of
powder each different in diameters of particles used therein from
the other, cavity diameters in a porous compact are increased in
order of the plate-like porous compacts 41, 42 and 43. A void ratio
is almost constant in plate-like porous compacts because of filling
without applying a pressure. A variation among cavity diameters in
each porous compact is small and shapes thereof each are enclosed
with a smooth curved surface and uniform.
[0159] A performance of the sintered filter 40 as a filter is
excellent by adopting a design in which a treated liquid is caused
to pass through the plate-like porous compacts 41, 42 and 43 in the
order to thereby trap almost all of solid matter in the treated
liquid with the plate-like porous compact 41 having cavity
diameters smallest in a diameter range, thereby obtaining excellent
reverse-washing reproducibility. That is, since in this design, the
solid matter is not filtered out on the plate-like porous compacts
42 and 43 in a distributed state and in addition, shapes of
cavities in the plate-like porous compact 41 are smooth and
uniform, the solid matter trapped in the cavities is removed
smoothly in reverse-washing.
[0160] Sintering temperatures in respective sintering are
preferably selected in the range of from 650 to 1200.degree. C.
much lower than the melting point of titanium. If a sintering
temperature is lower than 650.degree. C., sintering is not
sufficiently performed. If exceeding 1200.degree. C., sintering
portions are not limited to contact points between individual
particles but the bodies of particles are molten together even
without applying a pressure, so there arises a risk that void
ratios and cavity diameters at proper levels cannot be ensured. By
changing sintering temperature in this range, void ratios and
cavity diameters are controlled as described above.
[0161] As an example of the present invention, a titanium sintered
filter of a three-layer structure was fabricated according to the
method described above. A thickness of each layer was 1 mm, (3 mm
in total). Average diameters of particles included in spherical gas
atomized titanium powder used in layers were 20 .mu.m, 60 .mu.m and
100 .mu.m, respectively, and maximum cavity diameters of the layers
were 6 .mu.m, 22 .mu.m and 37 .mu.m, respectively. The void ratios
of the layers were all 45%.
[0162] As a comparative example, a titanium sintered filter of a
similar structure was fabricated using
hydrogenation/dehydrogenation titanium powder sold on the market.
In fabrication of three plate-like porous compacts, a press was
necessary for molding and making cavity diameters uniform.
Variations were observed in void ratios of respective layers and
55%, 48% and 37%.
[0163] Comparison was performed between both cases in
reverse-washing reproducibility. That is, a solution obtained by
mixing silica beads having an average diameter of 10 .mu.m into
water at a concentration of 10 mg/litter was filtered through the
filters so that an increase in weight of a filter after drying is
constant, thereafter, reverse-washing was performed on the filters
for a prescribed time at an air pressure of 5 kgf/cm.sup.2 and
after drying, the filters were weighed to obtain a change between
weights before and after the procedure described above, thereby
having evaluated reverse-washing reproducibility. In the example,
97% of the increase in weight was removed by reverse-washing, while
in the comparative example, only 83% was removed.
[0164] In the embodiment described above, the plate-like porous
compacts each different in cavity diameters therein from another
were individually fabricated in advance, while a similar layered
structure can also be obtained in a procedure in which titanium
particle layers each different in diameters of particles therein
from another are sequentially stacked and sintered. Incidentally, a
layered structure shown in FIG. 1(b) is fabricated by mans of the
latter method.
[0165] FIGS. 8 to 10 are descriptive views of fabrication methods
for porous conductive plates showing a fifth embodiment of the
present invention and sectional views showing filling states of
particles included in spherical gas atomized powder.
[0166] As shown in FIG. 8, first of all, a sintering vessel 60 made
of high density alumina is filled with spherical gas atomized
titanium powder 50 having prescribed particle diameters without
applying a pressure. A shape of an inner space of the sintering
vessel 60 is of the shape of a thin plate corresponding to a shape
of a porous conductive plate to be fabricated. Then, the spherical
gas atomized titanium powder 50 filling the sintering vessel 60 is
vacuum sintered without applying a pressure.
[0167] A sintering temperature is preferably selected in the range
of from 650 to 1200.degree. C. much lower than the melting point of
titanium. If a sintering temperature is lower than 650.degree. C.,
sintering is not sufficiently performed. If exceeding 1200.degree.
C., sintering portions are not limited to contact points between
individual particles but the bodies of particles are molten
together even without applying a pressure, so there arises a risk
that a void ratio at a proper level cannot be ensured.
[0168] By means of such a method, there were fabricated three kinds
of porous conductive plates, commonly in the shape of a square
having one side of 50 mm each, and respective thickness of 1 mm,
0.5 mm and 0.2 mm as an example of the present invention.
[0169] Spherical gas atomized titanium powder was powder sold on
the market as described above and powder of coarse particles (in
the range of from 45 to 150 .mu.m) was used for fabricating the
porous conductive plates of 1 mm and 0.5 mm in thickness,
respectively, while powder of fine particles (of 45 .mu.m or less)
was used for fabricating the porous conductive plate of 0.2 mm in
thickness. A degree of a vacuum was 7.times.10.sup.-3 Pa and a
sintering temperature was about 1000.degree. C. for the coarse
particles while being about 800.degree. C. for the fine particles.
Furthermore, a temperature holding time was a constant value of
about 15 minutes for both of the coarse particles and the fine
particles. The void ratios of the fabricated porous conductive
plates were all about 45%.
[0170] Electrical resistance of thus fabricated porous conductive
plates were measured with a four-probe method with the results that
the porous conductive plate of 1 mm in thickness had 10 m.OMEGA.,
the porous conductive plate of 0.5 mm in thickness had 15 m.OMEGA.
and the porous conductive plate of 0.2 mm in thickness had 12
m.OMEGA. because of the use of the powder of fine particles in this
last case. As to physical conditions of one surfaces of the plates,
the one surfaces were planarized since particles included in
spherical gas atomized titanium powder were arranged in conformity
with the upper surface profile of the bottom of the sintering
vessel. Since spherical gas atomized titanium powder is good in
fluidity, a void ratio is comparatively uniform throughout all of a
porous conductive plate.
[0171] For the purpose of comparison, hydrogenation/dehydrogenation
titanium powder sold on the market (in the range of from 50 to 150
.mu.m in particle diameter with an average particle diameter of 100
.mu.m) was sintered to fabricate porous conductive plates that were
each in the shape of a square of 50 mm in one side, had thickness
values of 1 mm and 0.5 mm, respectively, and commonly had a void
ratio of 45%. Molding with a press was necessary to attain a void
ratio of 45%. Electrical resistance values were equal to those in
the example, while strength values were insufficient. This is
inferred because powder of particles in irregular shapes are used,
therefore titanium particles are not uniformly bonded therebetween.
This non-uniformity in bonding between particles is resulted in
variations in void ratios in all of a porous conductive plate.
[0172] On the other hand, a titanium fiber sintered plate sold on
the market (with a thickness of 0.8 mm) has a void ratio as large
as 60% and electrical resistance was as high as 30 m.OMEGA.. Though
a strength was sufficient, on a surface thereof were so much of
fine protrusions that the sintered plate cannot be brought into
press contact with a film electrode laminates. Spherical gas
atomized titanium powder sold on the market described above was
plasma sprayed on one surface of the titanium fiber sintered
compact to a thickness of 0.2 mm, totaling 1 mm as a whole. Though
a void ratio of all of the sintered compact assumed 45% and the one
surface was planarized, an electrical resistance was still as large
as 20 m.OMEGA., which was twice as high as in the example.
[0173] In the example described above, while a sintering
temperature in the case where the coarse particles were used was
about 1000.degree. C., a void ratio of a porous conductive plate at
a sintering temperature of 1100.degree. C. was reduced to about
40%. A void ratio of a porous conductive plate in the example at a
sintering temperature of 900.degree. C. increased to about 50%. Any
of the porous conductive plates was high in strength, excellent in
surface smoothness and low in resistance.
[0174] As a method for raising a surface smoothness to a higher
level, there is exemplified a method in which a sintering vessel
with a necessary size is filled with spherical gas atomized
titanium powder while giving vibrations to the powder. With a
vibration filling adopted, as shown in FIG. 9, a surface smoothness
can be improved not only on a surface in contact with the upper
surface of the bottom of the sintering vessel 60, but also on the
surface of the opening side and in addition, a void ratio is made
more uniform. As shown in FIG. 10, it is effective to use a
sintering vessel 60 constructed so that a plate-shaped space formed
inside the vessel is vertically long. With an inner plate like
space extended vertically adopted, spherical gas atomized titanium
powder 50 filling the space receives loads in the plate thickness
direction caused by a weight of itself to improve surface
smoothness on both surfaces. In any of the methods, increase in
packing density accompanies reduction in void ratio and both can be
used in parallel.
[0175] As molding methods, there may be exemplified in addition to
combined natural filling and vacuum sintering: a doctor blade
method, an injection molding method, an extrusion method and the
like, with any of which it is allowed that a green preform is
prepared using a mixture of spherical gas atomized titanium powder
with a binder, followed by sequentially removing a binder from the
green preform and sintering. Furthermore, it is possible to roll a
porous sintered conductive plate after sintering, or alternatively,
to roll a green preform, thereby enabling more of surface
smoothness and adjustment of a void ratio in the plate. Still
furthermore, it is also effective for surface smoothness to narrow
a range of a particle diameter distribution of spherical gas
atomized titanium powder.
[0176] FIG. 11 is a model sectional view of a highly corrosion
resistant porous plate showing a sixth embodiment of the present
invention.
[0177] A sintering vessel in the shape of a dish made of high
density alumina is filled with spherical gas atomized titanium
powder 71 having a prescribed average particle diameter and
thereafter, the spherical gas atomized titanium powder 71 is vacuum
sintered without applying a pressure, thereby fabricating a thin,
large area porous plate 70 with a high corrosion resistance.
[0178] Here, a plate thickness T of the porous plate 70 is 1/10000
times or less of a numerical value of the area S. That is, T/S
<1/10000. Since adjacent spherical particles are in point
contact and fused to each other and sizes of cavity 72 formed
between particles are uniform, uniformity in void ratio in a porous
plate is high and the uniformity increases as particle diameters
are made more uniform, satisfying a requirement for a standard
deviation being 3% or less.
[0179] A sintering temperature is preferably selected in the range
of from 650 to 1200.degree. C. much lower than the melting point of
titanium. If a sintering temperature is lower than 650.degree. C.,
sintering is not sufficiently performed. If exceeding 1200.degree.
C., sintering portions are not limited to contact points between
individual particles but the bodies of particles are molten
together even without applying a pressure, so there arises a risk
that void ratios and cavity diameters at proper levels cannot be
ensured. A void ratio is controlled by changing a sintering
temperature within the temperature range.
[0180] As an example of the present invention, a porous plate in
the shape of a square with one side of 200 mm and having a
thickness of 2 mm was fabricated using the above described
spherical gas atomized titanium powder sold on the market, that is
fine particles in the range of 45 .mu.m or less in particle
diameter (with an average particle diameter of 25 .mu.m) and coarse
particles in the range of from 45 to 150 .mu.m in particle diameter
(with an average particle diameter of 80 .mu.m). It is T/S
=1/20000. A high density alumina vessel was used as a sintering
vessel and the vessel was filled with the spherical gas atomized
titanium powder without applying a pressure, followed by vacuum
sintering without applying a pressure. Conditions for sintering
were at 800.degree. C. for 1 hour or the case where the fine
particles and at 1000.degree. C. for 1 hour for the case where the
coarse particles.
[0181] As Comparative example 1, hydrogenation/dehydrogenation
powder sold on the market (with an average particle diameter of 25
.mu.m) was used and conditions for sintering were the same as in
the example described above with the exception of a sintering
temperature of 800.degree. C.) The void ratios were measured at 5
points (A to E) on a surface of fabricated porous plate. The
measuring points were 5 points on a diagonal drawn between opposed
vertices of a square with one side of 200 mm with which points
together with both vertices the diagonal is divided into 6 segments
equal in length to one another. The void ratios were obtained in a
procedure in which thickness values, areas and mass values were
measured on five square samples, each in the shape of a square with
one side of 20 mm, and having measuring points at respective
centers thereof to thereby obtain apparent densities and to further
calculate the void ratios according to the following expression. In
Table 6, there are shown the void ratios at respective measuring
points, average values and standard deviations thereof.
[0182] Void ratio (%) =(1-- an apparent density/a true density of
titanium) .times.100
6 TABLE 6 Void ratios (%) Standard A B C D E Averages deviations
Example 1 42 41 43 40 41 41 1 Example 2 43 39 46 42 46 43 3
Comparative 41 53 39 60 36 46 10 Example 1
[0183] As is understood from Table 6, spherical gas atomized
titanium powder is used, thereby enabling fabrication of a porous
plate even with a thickness as thin as 2 mm or less in plate
thickness and with uniform void ratios.
[0184] A similar shape was tried to obtain by means of HIP using
the powder used in Comparative example 1. Since when HIP was used,
a porous plate was not able to be separated from a capsule made of
tantalum without breaking the plate, it was impossible to fabricate
a porous plate in the shape of a square with one side of 200 mm and
a thickness of 2 mm. The minimum thickness of a porous plate that
can be fabricated by means of HIP was a thickness of 5 mm in a
porous plate in the shape of a square with one side of 200 mm
(wherein T/S =1/8000).
[0185] While a sintered compact in a similar shape was tried to
fabricate by sintering a preform after molding the powder used in
Comparative example 1 with a die press, the preform is excessively
thin, so it is broken after pressing and the process was not able
to enter even into a sintering step. The minimum thickness that can
be fabricated according to the die press is a thickness of 5 mm in
a porous plate in the shape of a square with one side of 200 mm
(wherein T/S =1/8000).
[0186] Industrial Applicability
[0187] As described above, a titanium powder sintered compact of
the present invention can be provided as a titanium sintered filter
with a maximum pore diameter of 70 .mu.m or less, small in pressure
drop and excellent in filtration performance while maintaining an
average particle diameter and a void ratio of a preform of raw
material spherical powder.
[0188] A titanium powders intered compact of the present invention
can be provided with a high bending characteristic as far as
spherical gas atomized titanium powder is used and a plate
thickness is restricted to 500 .mu.m or less; therefore, there can
be fabricated with the titanium powder sintered compact: for
example, filter elements in a three-dimensional shape, such as a
cylinder and a corrugated plate, a dispersion element and the like
at a low cost without using CIP.
[0189] A cylindrical porous compact even with a large height of the
present invention can also be fabricated without applying a
pressure and uniformity in quality in the height direction is
excellent despite no application of a pressure. Therefore, high
quality cylindrical titanium powder sintered filters with various
sizes can be economically fabricated. Furthermore, excellent
reverse-washing reproducibility can be imparted to the filters.
[0190] Since a highly corrosion resistant metal sintered filter of
the present invention is fabricated with titanium powder, the
filter is very excellent in corrosion resistance. Furthermore,
since in a highly corrosion resistant metal sintered filter of the
present invention, a cavity diameter stepwise increases from one
surface to the other surface, sizes of cavities in each layers are
uniform and shapes of cavities each are formed with a smooth curved
surface; therefore, the sintered filter is excellent in
reverse-washing reproducibility. In addition, since a void ratio
can be made constant between one surface and the other surface, an
adverse influence on a liquid permeability can be avoided. Since a
fabrication process is simple, a fabrication cost can be restricted
to a low value.
[0191] Moreover, since a porous conductive plate of the present
invention is made of a sintered compact of spherical gas atomized
titanium powder, which makes itself excellent in moldability, a
thin, large area product can be fabricated with simplicity.
Besides, since a surface smoothness is excellent even without
coating such as plasma spraying, a protectability for and
contactability with a thin film electrode laminates can be improved
without accompanying increase in electrical resistance, causing
economy to be also excellent. With such advantages, there can be
provided a powder feeder and a current collector, both with a high
performance at a low cost.
[0192] Besides, a highly corrosion resistant porous plate of the
present invention can be economically fabricated as a plate too
thin to be fabricated with HIP, even without pressing and,
furthermore, uniformity in void ratio can be even higher than a
product by means of pressing. Therefore, thin porous plates uniform
in void ratio can be fabricated at a very low cost to therefore,
economically provide a high quality product applicable to, for
example, an ink dispersion plate for use in an ink jet printer.
* * * * *