U.S. patent application number 10/224691 was filed with the patent office on 2003-01-02 for sintered metal oxide articles and methods of making.
Invention is credited to Buslaev, Yuri, Chernyavsky, Andrei, Montano, Richard, Morgunov, Vyacheslav, Myasoedov, Sergei, Shustorovich, Alexander, Shustorovich, Eugene, Solntsev, Konstantin.
Application Number | 20030001320 10/224691 |
Document ID | / |
Family ID | 22951565 |
Filed Date | 2003-01-02 |
United States Patent
Application |
20030001320 |
Kind Code |
A1 |
Solntsev, Konstantin ; et
al. |
January 2, 2003 |
Sintered metal oxide articles and methods of making
Abstract
Methods of making metal oxide articles, preferably iron oxide
articles, and articles thereby produced. The method comprises the
steps of slightly pressing powder to a compact, the powder
consisting essentially of a first oxide of the metal; and
subjecting the compact to a heat treatment that causes the powder
to sinter into a unitary body and results in the transformation of
at least a portion of the first oxide to a second oxide by
oxidation or deoxidation during the heat treatment. In disclosed
embodiments, the heat treatment is conducted either in air at
atmospheric pressure or at a subatmospheric pressure. The method
optionally includes more heating/cooling steps resulting in
additional oxidation/deoxidation cycles. Sintered iron oxide
articles of the invention have high mechanical strengths and
interconnected pore structures, providing for efficient filtering
of liquids and gases.
Inventors: |
Solntsev, Konstantin;
(Moscow, RU) ; Shustorovich, Eugene; (Pittsford,
NY) ; Myasoedov, Sergei; (Moscow, RU) ;
Morgunov, Vyacheslav; (Moscow, RU) ; Chernyavsky,
Andrei; (Dubna, RU) ; Buslaev, Yuri; (Moscow,
RU) ; Montano, Richard; (Falls Church, VA) ;
Shustorovich, Alexander; (Pittsford, NY) |
Correspondence
Address: |
KENYON & KENYON
ONE BROADWAY
NEW YORK
NY
10004
US
|
Family ID: |
22951565 |
Appl. No.: |
10/224691 |
Filed: |
August 21, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10224691 |
Aug 21, 2002 |
|
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09251348 |
Feb 17, 1999 |
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6461562 |
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Current U.S.
Class: |
264/611 ;
264/658; 501/127; 55/523 |
Current CPC
Class: |
C04B 35/26 20130101 |
Class at
Publication: |
264/611 ;
264/658; 55/523; 501/127 |
International
Class: |
C04B 035/26 |
Claims
We claim:
1. A method of making a metal oxide article, comprising the steps
of: pressing powder to a compact, said powder consisting
essentially of a first oxide of the metal; and subjecting said
compact to a heat treatment, said heat treatment causing said
powder to sinter into a unitary body and resulting in the
transformation of at least a portion of said first oxide to a
second oxide of the metal.
2. The method of claim 1, wherein said step of subjecting said
compact to said heat treatment comprises the steps of: subjecting
said compact to a first temperature such that at least a portion of
said first oxide transforms to said second oxide; and subjecting
said compact to a second temperature after said step of subjecting
said compact to said first temperature, said step of subjecting
said compact to said second temperature causing at least a portion
of said second oxide to transform to said first oxide.
3. The method of claim 2, wherein said second temperature is
greater than said first temperature.
4. The method of claim 2, wherein said second temperature is less
than said first temperature.
5. The method of claim 2, wherein said heat treatment further
comprises the step of subjecting said compact to a third
temperature after said step of subjecting said compact to said
second temperature, thus causing at least a portion of said first
oxide to transform to said second oxide.
6. The method of claim 5, wherein said third temperature is greater
than said second temperature.
7. The method of claim 5, wherein said second temperature is less
than said second temperature.
8. The method of claim 1, wherein at least a portion of said heat
treatment is conducted at a subatmospheric pressure.
9. The method of claim 1, wherein at least a portion of said heat
treatment is conducted in air at atmospheric pressure.
10. A method of making an iron oxide article, comprising the steps
of: pressing powder to a compact, said powder consisting
essentially of a first iron oxide; and subjecting said compact to a
heat treatment, said heat treatment causing said powder to sinter
into a unitary body and resulting in the transformation of at least
a portion of said first iron oxide to a second iron oxide.
11. The method of claim 10, wherein said first iron oxide is
hematite; said second iron oxide is magnetite; said heat treatment
includes the step of heating said compact to a temperature up to
about 1250.degree. C.; and said heat treatment is conducted at a
subatmospheric pressure.
12. The method of claim 11, wherein said subatmospheric pressure is
within the range of about 10.sup.-4 torr to about 10.sup.-5
torr.
13. The method of claim 10, wherein said step of subjecting said
compact to said heat treatment comprises the steps of: subjecting
said compact to a first temperature such that at least a portion of
said first iron oxide transforms to said second iron oxide;
subjecting said compact to a second temperature after said step of
subjecting said compact to said first temperature, said step of
subjecting said compact to said second temperature causing at least
a portion of said second iron oxide to transform to said first iron
oxide; and subjecting said compact to a third temperature after
said step of subjecting said compact to said second temperature,
said step of subjecting said compact to said third temperature
causing at least a portion of said first iron oxide to transform to
said second iron oxide.
14. The method of claim 13, wherein said first iron oxide is
magnetite; said second iron oxide is hematite; said first
temperature is up to about 1300.degree. C.; said second temperature
is up to about 1450.degree. C.; said third temperature is less than
about 1300.degree. C.; and said heat treatment is conducted in air
at atmospheric pressure.
15. The method of claim 13, further comprising the step of
subjecting said compact to a fourth temperature after said step of
subjecting said compact to said third temperature, said step of
subjecting said compact to said fourth temperature causing at least
a portion of said second iron oxide to transform to said first iron
oxide.
16. The method of claim 15, wherein said first iron oxide is
hematite; said second iron oxide is magnetite; said first
temperature is up to about 1450.degree. C.; said second temperature
is up to about 1250.degree. C.; said third temperature is up to
about 1450.degree. C.; said fourth temperature is less than about
1300.degree. C.; and said heat treatment is conducted in air at
atmospheric pressure.
17. A sintered metal oxide article made by the process of claim
1.
18. A sintered metal oxide article made by the process of claim
2.
19. A sintered metal oxide article made by the process of claim
5.
20. A sintered iron oxide article made by the process of claim
10.
21. A sintered iron oxide article made by the process of claim
11.
22. The iron oxide article of claim 21, wherein said article has an
interconnected pore structure having a pore size of up to about 15
microns and a water filter productivity of at least about 150
cm.sup.3/cm.sup.2 min.
23. The iron oxide article of claim 21, wherein said article has an
interconnected pore structure having a pore size of up to about 40
microns and a water filter productivity of at least about 150
cm.sup.3/cm.sup.2 min.
24. The iron oxide article of claim 21, wherein said article has an
interconnected pore structure and a porosity of at least about 35
percent.
25. A sintered iron oxide article made by the process of claim
13.
26. A sintered iron oxide article made by the process of claim
14.
27. The iron oxide article of claim 26, wherein said article has an
interconnected pore structure having a pore size of up to about 10
microns and a water filter productivity of at least about 50
cm.sup.3/cm.sup.2 min.
28. The iron oxide article of claim 27, wherein said article has a
crush strength of at least about 260 atmospheres.
29. The iron oxide article of claim 26, wherein said article has an
interconnected pore structure having a pore size of up to about 15
microns and a water filter productivity of at least about 175
cm.sup.3/cm.sup.2 min.
30. The iron oxide article of claim 29, wherein said article has a
crush strength of at least about 200 atmospheres.
31. The iron oxide article of claim 26, wherein said article has an
interconnected pore structure and a porosity of at least about 35
percent.
32. The iron oxide article of claim 26, wherein said article has an
interconnected pore structure having a pore size of up to about 40
microns and a water filter productivity of at least about 800
cm.sup.3/cm.sup.2 min.
33. The iron oxide article of claim 32, wherein said article has a
crush strength of at least about 30 atmospheres.
34. A sintered iron oxide article made by the process of claim
15.
35. A sintered iron oxide article made by the process of claim
16.
36. The iron oxide article of claim 35, wherein said article has an
interconnected pore structure having a pore size of up to about 15
microns and a water filter productivity of at least about 40
cm.sup.3/cm.sup.2 min.
37. The iron oxide article of claim 35, wherein said article has an
interconnected pore structure having a pore size of up to about 40
microns and a water filter productivity of at least about 50
cm.sup.3/cm.sup.2 min.
38. The iron oxide article of claim 35, wherein said article has an
interconnected pore structure having a pore size of up to about 100
microns and a water filter productivity of at least about 175
cm.sup.3/cm.sup.2 min.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to methods of making sintered
metal oxide articles having desired mechanical properties and
interconnected pore structures, and oxide articles thereby
produced.
BACKGROUND
[0002] Sintering of inorganic powder compacts into useful solid
products is a common and efficient way of fabricating metals,
ceramics, and cermets. The general pattern of ceramic sintering
includes three stages--initial, intermediate, and final. In the
initial stage, the pore shape may vary greatly depending on the
size and geometry of particle contacts, and the pore structure is
open and fully interconnected. In the intermediate stage, where the
porosity typically shrinks to 40-10%, the pores become smoother and
typically have a cylindrical structure that remains interconnected.
The open pore network becomes unstable when the porosity is reduced
to about 8%; then, the cylindrical pores collapse into spherical
pores, which become closed and isolated. The appearance of isolated
pores manifests the beginning of the final stage of sintering,
leading to the densest products.
[0003] Major efforts in ceramic sintering have been made to obtain
advanced materials such as electronic ceramics, structural
ceramics, and high toughness composites where desired properties
are sought to be reached at maximal densification (minimal
porosity). The use of ceramic materials that have been sintered
through only the intermediate sintering stage, however, has been
more limited. One such use of these materials is in the filtration
of gases and liquids. Among ceramic metal oxides, filter materials
which have been obtained are commonly made of alumina
(Al.sub.2O.sub.3), zirconia (ZrO.sub.2), and
aluminum-silicates.
[0004] The intrinsic properties of iron oxides, hematite
(.alpha.-Fe.sub.2O.sub.3) and magnetite (Fe.sub.3O.sub.4), make
them well-suited for diverse applications. These oxides are among
the least expensive and naturally abundant substances. They are
refractory ceramic materials that are chemically stable in various
gas and liquid media, hematite being particularly appropriate for
use in corrosive and oxidative environments. Furthermore, hematite
and magnetite are environmentally benign, which make them suitable
for water filtration and various applications in food, wine,
pharmaceutical and other industries where environmental and health
requirements are paramount. Moreover, hematite is electrically
non-conductive and non-magnetic, and magnetite is highly conductive
and magnetic, so the two iron oxides cover a wide spectrum of
desirable electric and magnetic properties.
[0005] There exist numerous methods to prepare hematite and
magnetite powders to be used as powders in various applications.
However, there is a need in efficient and practical (economical)
processes of making mechanically strong hematite and magnetite
articles by sintering the respective powders, particularly into
filter materials. U.S. Pat. No. 3,984,229 discusses attempts to
briquette iron oxide raw materials at elevated temperatures of
800.degree. C. to 1100.degree. C. and concludes that it has been
impossible to find a sufficiently strong material for the briquette
molds (col. 1, lines 60-68). U.S. Pat. No. 5,512,195 describes
efficient transformation of hematite powder into a magnetite single
phase by mixing hematite powder with various organic substances,
serving as a binder and reducing agent, and sintering at
1200.degree. C. to 1450.degree. C. in an inert gas. The strength of
the sintered magnetite phase and its pore structure have not been
characterized.
[0006] To obtain strong sintered articles, high pressure is
conventionally employed. For example, U.S. Pat. No. 4,019,239
describes manufacturing magnetite articles by sintering and hot
compacting magnetite powder in air at 900.degree. C. to
1300.degree. C. and a pressure of 100 to 600 MPa (1000 to 6000
atm), leading to a dense body with a porosity less than 3%.
[0007] In addition to high pressure requirements, conventional
sintering of metal oxide powders usually requires binders and other
extraneous agents to shape a powder preform and obtain the
desirable composition. For example, in U.S. Pat. No. 5,512,195,
sintering of hematite powder to a magnetite single phase requires
mixing hematite powder with various organic substances that serve
as binders and reducing agents. By contrast, the sintering of
hematite powder without incorporation of any organic substance at
1200.degree. C. to 1450.degree. C. in an inert gas makes the
hematite-magnetite conversion so low that the process is unfit for
industrialization. It would be highly desirable to develop an
effective and economical sintering process of iron oxides without
the use of any additives or high pressures.
SUMMARY OF THE INVENTION
[0008] In one aspect, the present invention includes a method of
making metal oxide articles, and preferably iron oxide articles.
The method comprises the steps of slightly pressing powder to a
compact, the powder consisting essentially of a first oxide of the
metal; and subjecting the compact to a heat treatment that causes
the powder to sinter into a unitary body and results in the
transformation of at least a portion of the first oxide to a second
oxide of the metal. The powder comprises a first oxide that is
substantially free from additives, at least a portion of which is
transformed to a second oxide by oxidation or deoxidation during
the method of the present invention. The method optionally includes
one or more heating/cooling steps during the heat treatment
process, resulting in additional oxidation/deoxidation cycles.
[0009] In another aspect, the invention includes sintered metal
oxide articles, and preferably iron oxide articles, made by the
method of the invention.
[0010] One advantage of the present invention is that it provides
sintered metal oxide articles, and preferably iron oxide articles,
of high mechanical strength and other desired mechanical
properties.
[0011] Another advantage of the invention is that it provides
sintered metal oxide articles, and preferably iron oxide articles,
having interconnected pore structures capable of efficient
filtering gases and liquids.
[0012] Yet another advantage of the invention is that it provides
efficient and economical processes of making sintered metal oxide
articles, and preferably iron oxide articles, without the need for
sintering additives of any kind and/or high pressures.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0013] The present invention provides sintered metal oxide
articles, and preferably iron oxide articles, of desired mechanical
properties, such as high strength, and an interconnected pore
structure capable of efficient filtering of gases and liquids. In
accordance with the invention, metal oxide powder is subjected to a
heat treatment to transform at least a portion of the oxide into a
different oxide. The heat treatment is conducted at temperatures
less than the melting points of the oxides and for suitable holding
times to sinter the powder into a unitary oxide article. The
powders used in the present invention are said to consist
essentially of metal oxide in that such powders are substantially
free from other compounds and additives such as binders, reducing
agents, and the like.
[0014] Heating regimes for sintering are chosen to cause the
oxidation and/or deoxidation of the oxide such that it is
transformed to a different oxide, with several
oxidation/deoxidation cycles possible. Although not wishing to be
bound by theory, it is believed that oxygen transport during
deoxidation and/or oxidation contributes to effective sintering and
the resulting desired mechanical properties and uniformity in
appearance and interconnected pore structure of the sintered
article body. The invention thus obviates the need for sintering
additives and high sintering pressures.
[0015] The present invention is described with specific reference
to iron oxide articles, and specifically iron oxide filters, that
are made by sintering iron oxide powders. The scope of the
invention, however, includes articles of other metal oxide
materials, of any form and intended use, that are made by sintering
metal oxide powders that undergo oxidation and/or deoxidation
during the sintering process.
[0016] In cited embodiments, sintered iron oxide filters are
produced in accordance with the present invention. For example, in
one embodiment, hematite (.alpha.-Fe.sub.2O.sub.3) filters are made
from magnetite (Fe.sub.3O.sub.4) powder. In another embodiment,
hematite filters are made from hematite powder, which transforms to
magnetite and back to hematite during sintering. In yet another
embodiment, magnetite filters are made from hematite powder.
[0017] The filters are made in a sintering process wherein metal
oxide powder is placed into a mold and hand-pressed into a compact
and subjected to a suitable heat treatment to cause sintering into
a unitary body and oxidative/deoxidative transformation of the
powder. Preferred molds are alumina rings typically having an inner
diameter of from about 10 to about 70 mm, and a height of from
about 3 to about 60 mm. The powder particles are of any suitable
size for sintering such as, for example, about 50 to about 200
microns. Such powders are readily available.
[0018] The heat treatment of the invention is selected based on the
thermal properties of iron oxides. In air at atmospheric pressure,
hematite is stable at elevated temperatures up to about
1350.degree. C. but decomposes to magnetite at higher temperatures
up to about 1450.degree. C. Because magnetite begins to decompose
to wustite FeO at temperatures above 1450.degree. C., this
represents an upper limit of sintering temperatures in atmospheric
air. For subatmospheric pressures, suitable sintering temperatures
are lower according to the pressure within a vacuum furnace. In
cited embodiments of the present invention, vacuum sintering
typically occurs at a pressure within the range of about 10.sup.-4
to about 10.sup.-5 torr, wherein hematite begins to decompose to
magnetite at about 750.degree. C. and magnetite begins to melt at
about 1300.degree. C. The process is optimized on the premise that
higher sintering temperatures allow for shorter sintering times. At
pressures of about 10.sup.-4 to about 10.sup.-5 torr, efficient
sintering occurs at 950.degree. C. to 1250.degree. C., preferably
at 1000.degree. C. to 1250.degree. C., and more preferably at
1150.degree. C. to 1200.degree. C.
[0019] Embodiments of the present invention are further described
with reference to the following non-limiting examples. In all
examples, the iron oxide materials, namely hematite and magnetite,
are distinguished by stoichiometry and magnetic properties
EXAMPLE 1
Production of Sintered Hematite Filters from Magnetite Powder
[0020] Hematite filters were made from magnetite powder according
to an embodiment of the present invention.
[0021] Magnetite powder was obtained my milling thin-walled
magnetite structures produced in accordance with U.S. Pat. Nos.
5,786,296 and 5,814,164, which are incorporated herein by
reference. The powder was separated on commercial sieves into
fractions according to the following particle size ranges (in
microns): 160 to 100, 100 to 80, 80 to 50 and <50. Portions of
each powder fraction were poured into closed-ended molds to form
multiple (e.g., at least three) samples of each powder fraction.
The molds were in the form of alumina rings having an internal
diameter of about 11 mm and a height of about 8 mm, placed on a
platinum plate serving as the mold bottom. Each sample was
compacted by hand with a metal rod to a density of from about 2.3
to about 3.5 g/cm.sup.3, and placed at room temperature into an
electrically heated and unsealed furnace for subsequent heat
treatment in atmospheric air.
[0022] Groups of samples were subjected to the following separate
heat treatments (all heating rates were about 2.degree. C. per
minute), based on the inventors' finding that the sintering process
in air was inefficient below around 1350.degree. C. but efficient
for temperatures up to around 1450.degree. C.:
[0023] (a) Samples of all powder fractions were heated to about
1300.degree. C. and held for about three hours (thus causing a
transformation from magnetite to hematite), then heated to about
1450.degree. C. and held for about 15 minutes (thus causing a
transformation from hematite to magnetite), and then furnace cooled
(thus causing a transformation from magnetite to hematite) (as used
herein, "furnace cooled" refers to cooling by leaving samples in
the furnace after sintering and turning off the furnace power),
[0024] (b) Samples of powder fractions 160 to 100 microns, 100 to
80 microns, and 80 to 50 microns were heated to about 1450.degree.
C. and held for about 15 minutes (during which heating, the
magnetite transforms to hematite and back to magnetite), then
cooled to about 1300.degree. C. and held for about three hours
(thus causing a transformation from magnetite to hematite), and
then furnace cooled.
[0025] (c) Samples of powder fractions 160 to 100 microns were
heated to about 1200.degree. C. and held for about three hours
(thus causing a transformation from magnetite to hematite), then
heated to about 1450.degree. C. and held for about 15 minutes (thus
causing a transformation from hematite to magnetite), and then
furnace cooled (thus causing a transformation from magnetite to
hematite).
[0026] As described, each of the heat treatments resulted in more
than one transformation between magnetite and hematite. After the
samples were cooled to about room temperature, they were removed
from the molds. The resulting hematite samples had substantially
the same appearance and properties regardless of variations in the
heat treatments employed. The sample densities were within the
range of about 2.3 to about 3.4 g/cm.sup.3, which is about 45 to
about 65 percent of hematite bulk density (i.e., corresponding to a
porosity of about 55 to about 35 percent, respectively). The
samples were mechanically strong enough to be ground by common
abrasives, and were characterized by an open interconnected pore
structure capable of effective filtration of liquids. This Example
thus demonstrates a relatively simple method for making strong,
uniform iron oxide filters, particularly hematite filters, in
accordance with the present invention.
EXAMPLE 2
Production of Sintered Hematite Filters from Hematite Powder
[0027] Hematite filters were made from hematite powder according to
an embodiment of the present invention.
[0028] Hematite powder was obtained my milling thin-walled hematite
structures produced in accordance with U.S. Pat. Nos. 5,786,296 and
5,814,164, which are incorporated herein by reference. The powder
was separated according to size and placed into molds according to
Example 1.
[0029] Samples (e.g., at least three) were heated at a rate of
about 2.degree. C. per minute to about 1450.degree. C., held for
about three hours (thus causing a transformation from hematite to
magnetite), and furnace cooled (thus causing a transformation from
magnetite to hematite). The resulting sintered hematite samples
could be removed from their molds but were mechanically weak such
that they were crushed by slight hand pressure.
[0030] In an effort to improve mechanical strength, additional
powder samples were heated to about 1300.degree. C. and held for
about three hours, then heated to about 1450.degree. C. to about
1500.degree. C., and held for about one hour (thus causing a
transformation from hematite to magnetite), and then furnace cooled
(thus causing a transformation from magnetite to hematite). The
sintered samples showed only a marginal increase in strength. As
such, substantially monotonic heating followed by monotonic cooling
was found to be inefficient for producing hematite filters from
hematite powder.
[0031] However, when more cooling/heating steps were added to the
heat treatment to provide more oxidation/deoxidation cycles, the
resulting hematite samples showed a significant increase in
strength. For example, powder samples were heated to about
1250.degree. C. and held for about three hours, then heated to
about 1450.degree. C. and held for about 15 minutes (thus causing a
transformation from hematite to magnetite), then cooled to about
1250.degree. C. and held for about 15 minutes (thus causing a
transformation from magnetite to hematite), then heated again to
about 1450.degree. C. and held for about 15 minutes (thus causing a
transformation from hematite to magnetite), then cooled again to
about 1250.degree. C. and held for about three hours (thus causing
a transformation from magnetite to hematite), and then furnace
cooled. The resulting sintered hematite samples were strong enough
to be ground by common abrasives. Moreover, the resulting samples
were uniform in appearance and were characterized by an open
interconnected pore structure.
[0032] Although not wishing to be bound by theory, the significant
increase in filter strength resulting from the several
oxidation/deoxidation cycles may be due to oxygen transport within
the sintered body contributing to effective sintering, and to the
resulting mechanical properties and uniformity of the sintered
article body having an interconnecting pore structure.
EXAMPLE 3
Production of Sintered Magnetite Filters from Hematite Powder
[0033] Magnetite filters were made from hematite powder according
to an embodiment of the present invention.
[0034] Hematite powder was made and separated according to size and
placed into molds according to Example 2. Samples (e.g., at least
three) were placed in a vacuum furnace at a pressure of about
10.sup.-4 to about 10.sup.-5 torr, heated at a rate of about
8-9.degree. C. per minute to about 1210.degree. C. to about
1250.degree. C. and held for about 5 to about 30 minutes (thus
causing a transformation from hematite to magnetite), and then
furnace cooled while maintaining vacuum (thus preventing a
transformation from magnetite to hematite, which would occur in
air).
[0035] The sintered magnetite filters were easily removed from
their molds, and were mechanically strong enough to be ground by
common abrasives. The sample densities were within the range of
about 2.3 to about 3.4 g/cm.sup.3, which is about 45 to about 65
percent of magnetite bulk density (i.e., corresponding to a
porosity of about 55 to about 35 percent, respectively), typically
increasing with a decrease in the initial hematite particle size.
The sintered samples were uniform and were characterized by an open
interconnected pore structure.
EXAMPLE 4
Production of Sintered Magnetite Filters from Magnetite Powder
[0036] Magnetite filters were made from magnetite powder. Magnetite
power was made, separated according to size and placed into molds
according to Example 1. Samples (e.g., at least three) were placed
in a vacuum furnace at a pressure of about 10.sup.-4 to about
10.sup.-5 torr, heated at a rate of about 8.degree. C. per minute
to about 1250.degree. C. and held for about 30 minutes, and then
furnace cooled.
[0037] In this example, the iron oxide (magnetite) powder did not
undergo any transformation to any other iron oxide during
sintering. The resulting sintered magnetite filters were
significantly weaker and much less uniform than the magnetic
filters made from hematite powder as described in Example 3.
Notably, the weaker magnetite samples in Example 4 had, on average,
higher densities (up to about 4 g/cm.sup.3) than the magnetite
samples produced in Example 3. While not wishing to be bound by
theory, this unusual inverse relation between strength and density
indicates that in producing samples of high strength the
oxidation/deoxidation cycles are more important than simple
densification.
EXAMPLE 5
Evaluation of Sintered Hematite and Magnetite Filters
[0038] The hematite and magnetite filters formed according to
Examples 1 to 3 were evaluated against standard glass filters with
known pore sizes. The pore size of each hematite and magnetite
filter was estimated by determining their ability to filter freshly
prepared suspensions of Fe(OH).sub.3, CaCO.sub.3 and Al(OH).sub.3.
Filtration efficiency for all filters was evaluated by measuring
the water filter productivity ("WFP"), which is the volume of water
filtrated per filter unit surface area per unit time, for a given
pressure. The results of the filtration testing are shown in Table
I.
[0039] The filtration efficiencies for the filters produced in
accordance with Examples 1 to 3 were found to be much greater than
efficiencies for glass filters of comparable pore sizes. For
example, for a hematite filter made in accordance with Example 1
from magnetite powder and having a pore size up to about 40
microns, the WFP was found to be 829 cm.sup.3/cm.sup.2 min at a
pressure of about 10 torr. By comparison, a glass filter having a
similar pore size has a WPF of about 100 cm.sup.3/cm.sup.2 min at
the same pressure. As another example, for a hematite filter made
in accordance with Example 1 from magnetite powder and having a
pore size up to about 15 microns, the WFP was found to be 186
cm.sup.3/cm.sup.2 min at a pressure of about 10 torr. By
comparison, a glass filter having a similar pore size has a WPF of
about 3 cm.sup.3/cm.sup.2 min at the same pressure.
[0040] Inspection of Table I reveals several structure-property
relationships for the sintered filters of the present invention.
For example, for a given sintering process, a decrease in powder
particle size results in a decrease in filter pore size and an
increase in filter density. Also, a decrease in powder particle
size results in a decrease in WFP.
1TABLE I Results of filtration testing for sintered iron oxide
filters tested under a pressure of about 10 torr. WFP Fil- Sintered
Powder (cm.sup.3/ ter Powder Filter fraction Density Pore size
cm.sup.2 no. Material Material (microns) (g/cm.sup.3) (microns)
min) 1 magnetite hematite 160 - 2.4 40 - 15 829 100 2 magnetite
hematite 100 - 83 2.7 15 - 10 186 3 magnetite hematite 83 - 50 3.1
<10 56 4 hematite magnetite 100 - 83 2.5 15 - 10 160 5 hematite
magnetite 100 - 83 2.6 40 - 15 159 6 hematite hematite 160 - 2.6
100 - 40 179 100 7 hematite hematite 100 - 83 2.7 40 - 15 58 8
hematite hematite 100 - 83 2.9 15 - 10 46
[0041] The mechanical strength of filters 1 to 3, as listed in
Table I, was evaluated on the basis of crush strength. Crush
strength was measured by polishing cylindrical filter samples,
having diameters of about 10 to about 11 millimeters and heights of
about 5 to about 6 millimeters, to obtain smooth, parallel top and
bottom surfaces. The samples were wrapped by a polyethylene film,
placed in a press (compressive force about 39 kN), and compressed
at a rate of about 0.4 atm/sec. The moment of sample crush was
distinctly seen on a press manometer. These filters were found to
have crush strengths of about 30 atm, about 200 atm and about 260
atm, respectively, showing a strong inverse correlation with powder
particle size. This expected inverse correlation is an additional
indication that the filters of the present invention possess a
normal interconnected pore structure.
[0042] The results of the filtration demonstrate that the methods
of the present invention result in the production of strong iron
oxide articles having an interconnected pore structure suitable for
efficient filtering.
[0043] The present invention provides a novel method of making
sintered metal oxide articles. The sintered articles of the
invention are characterized by desired mechanical properties, such
as high strength, and an interconnected pore structure. Those with
skill in the art may recognize various modifications to the
embodiments of the invention described and illustrated herein. Such
modifications are meant to be covered by the spirit and scope of
the appended claims.
* * * * *