U.S. patent application number 11/318341 was filed with the patent office on 2007-05-03 for small particle compositions and associated methods.
This patent application is currently assigned to Primet Precision Materials, Inc.. Invention is credited to Robert J. Dobbs, Archit Lal.
Application Number | 20070098803 11/318341 |
Document ID | / |
Family ID | 37996654 |
Filed Date | 2007-05-03 |
United States Patent
Application |
20070098803 |
Kind Code |
A1 |
Dobbs; Robert J. ; et
al. |
May 3, 2007 |
Small particle compositions and associated methods
Abstract
Small particle compositions are provided. The particles may have
one or more desired features related to particle morphology,
topology and crystallographic orientation. The small particle size
coupled with such feature(s) can lead to significant property
advantages in a variety of different applications including
catalytic applications.
Inventors: |
Dobbs; Robert J.; (Newfield,
NY) ; Lal; Archit; (Ithaca, NY) |
Correspondence
Address: |
WOLF GREENFIELD & SACKS, PC
FEDERAL RESERVE PLAZA
600 ATLANTIC AVENUE
BOSTON
MA
02210-2206
US
|
Assignee: |
Primet Precision Materials,
Inc.
Ithaca
NY
|
Family ID: |
37996654 |
Appl. No.: |
11/318341 |
Filed: |
December 23, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60731307 |
Oct 27, 2005 |
|
|
|
Current U.S.
Class: |
424/489 ;
977/906 |
Current CPC
Class: |
C01P 2004/54 20130101;
C01P 2004/20 20130101; B01J 35/0013 20130101; B02C 17/20 20130101;
B01J 37/0036 20130101; C01P 2004/04 20130101; C09C 1/407 20130101;
C01G 23/047 20130101; C01P 2004/62 20130101; C01F 7/023 20130101;
C09C 1/36 20130101; B01J 21/063 20130101; B01J 35/0046 20130101;
H01M 4/9016 20130101; H01M 4/921 20130101; C09C 1/3623 20130101;
A61K 9/14 20130101; B01J 23/6447 20130101; C09K 3/1436 20130101;
B82Y 30/00 20130101; Y02E 60/50 20130101; C01F 7/02 20130101; C01P
2004/30 20130101; C01P 2004/64 20130101 |
Class at
Publication: |
424/489 ;
977/906 |
International
Class: |
A61K 9/14 20060101
A61K009/14 |
Claims
1. A nanoparticle composition comprising: nanoparticles having a
lenticular cross-section including a thickness that decreases from
a center portion to edge portions of the nanoparticles, wherein the
nanoparticles have an average particle size of less than 150
nm.
2. The composition of claim 1, wherein the average thickness at the
center portion is less than about 50 nm.
3. The composition of claim 1, wherein the nanoparticles comprise
an intermetallic compound.
4. The composition of claim 1, wherein the nanoparticles comprise
titania.
5. The composition of claim 1, wherein the nanoparticles comprise
Al.sub.2O.sub.3.
6. The composition of claim 1, wherein the nanoparticles are
milled.
7. The composition of claim 1, wherein the nanoparticles have an
anisotropic crystal structure.
8. The composition of claim 1, wherein the average particle size is
less than 50 nm.
9. The composition of claim 1, wherein greater than 75% of the
nanoparticles have the same crystallographic orientation.
10. The composition of claim 1, wherein the nanoparticles have a
stepped surface.
11. The composition of claim 1, wherein the nanoparticles comprises
a catalytic material.
12. The composition of claim 11, wherein the catalytic material is
consumed, in part, when catalyzing a desired reaction.
13. The composition of claim 11, wherein the catalytic material
catalyzes a desired reaction without being consumed.
14. A nanoparticle composition comprising: nanoparticles having an
average particle size of less than 150 nm, wherein the
nanoparticles have a stepped surface.
15. The composition of claim 14, wherein the nanoparticles are
milled.
16. The composition of claim 14, wherein the nanoparticles have an
anisotropic crystal structure.
17. The composition of claim 14, wherein the average particle size
is less than 50 nm.
18. The composition of claim 14, wherein greater than 75% of the
nanoparticles have the same crystallographic orientation.
19. The composition of claim 14, wherein the nanoparticles comprise
an intermetallic compound.
20. The composition of claim 14, wherein the nanoparticles include
one or more steps having a height of less than about 5 nm.
21. The composition of claim 14, wherein the nanoparticles comprise
a catalytic material.
22. A nanoparticle composition comprising: nanoparticles having an
average particle size of less than 150 nm and a crystallographic
orientation, wherein a majority of the nanoparticles have the same
crystallographic orientation.
23. The composition of claim 22, wherein greater than 75% of the
nanoparticles have the same crystallographic orientation.
24. The composition of claim 22, wherein the nanoparticles are
milled.
25. The composition of claim 22, wherein the nanoparticles have an
anisotropic crystal structure.
26. The composition of claim 22, wherein the average particle size
is less than 50 nm.
27. A nanoparticle composition comprising: nanoparticles having an
average particle size of less than 150 nm and a crystallographic
orientation, wherein the nanoparticles are substantially free of
carbon-based surface contamination.
28. The composition of claim 27, wherein the nanoparticles are
formed of a metal.
29. A method of producing nanoparticles comprising: milling feed
particles to form a milled nanoparticle composition including
nanoparticles having a lenticular cross-section including a
thickness that decreases from a center portion to edge portions of
the nanoparticles, wherein the nanoparticles have an average
particle size of less than 150 nm.
30. A method of producing nanoparticles comprising: milling feed
particles to form a milled nanoparticle composition including
nanoparticles having an average particle size of less than 150 nm,
wherein the nanoparticles have a stepped surface.
31. A method of producing nanoparticles comprising: milling feed
particles to form a milled nanoparticle composition including
nanoparticles having an average particle size of less than 150 nm
and a crystallographic orientation, wherein a majority of the
nanoparticles have the same crystallographic orientation.
32. A method of producing nanoparticles comprising: milling feed
nanoparticles having an average particle size of less than 150 nm
to produce surface features on the nanoparticles; and recovering
the nanoparticles including surface features having an average
particle size within about 10% of the average particle size of the
feed particles.
33. A method of producing nanoparticles comprising: milling
particles having a first composition and particles having a second
composition in a milling apparatus; and forming a region comprising
the second composition on respective surfaces of particles having
the first composition.
Description
RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application Ser. No. 60/731,307, filed Oct. 27, 2005, which is
incorporated herein by reference.
FIELD OF INVENTION
[0002] The invention relates generally to small particle
compositions, as well as methods (e.g., milling methods) and
components associated with such particle compositions.
BACKGROUND OF INVENTION
[0003] Particle reduction (also known as comminution) is a very old
technology, practiced, for example, by the ancients to produce
flour from grain by stone wheel grinding. More refined techniques,
such as milling, were developed to produce smaller and more regular
powders for use in a variety of industrial applications. Milling
processes typically use grinding media to crush, or beat, a product
material to smaller dimensions. For example, the product material
may be provided in the form of a powder having relatively large
particles and the milling process may be used to reduce the size of
the particles. Mills typically operate by distributing product
material around grinding media and rotating to cause collisions
between grinding media that fracture product material particles
into smaller dimensions.
[0004] Particle compositions having extremely small particle sizes
(e.g., nanometer-sized and lower) are proving to be useful for many
new applications. However, certain conventional milling methods may
be limited in their ability to produce such compositions at very
small particle sizes and/or with other features related to particle
morphology, topology and/or crystallography. Other processes for
producing small particles, such as chemical precipitation and
sol-gel, have also been utilized. However, precipitation processes
may also have some of the above-noted limitations. In addition,
precipitation processes may use relatively long processing times
and result in high costs.
SUMMARY OF INVENTION
[0005] Small particle compositions, as well as methods and
components associated with the same, are described.
[0006] In one aspect, a nanoparticle composition is provided. The
nanoparticle composition comprises nanoparticles having a
lenticular cross-section including a thickness that decreases from
a center portion to edge portions of the nanoparticles. The
nanoparticles have an average particle size of less than 150
nm.
[0007] In another aspect, a nanoparticle composition is provided.
The nanoparticle composition comprises nanoparticles having an
average particle size of less than 150 nm, wherein the
nanoparticles have a stepped surface.
[0008] In another aspect, a nanoparticle composition is provided.
The nanoparticle composition comprises nanoparticles having an
average particle size of less than 150 nm and a crystallographic
orientation. A majority of the nanoparticles have the same
crystallographic orientation.
[0009] In another aspect, a nanoparticle composition is provided.
The nanoparticle composition comprises nanoparticles having an
average particle size of less than 150 nm and a crystallographic
orientation. The nanoparticles are substantially free of
carbon-based surface residue.
[0010] In another aspect, a method of producing nanoparticles is
provided. The method comprises milling feed particles to form a
milled nanoparticle composition including nanoparticles having a
lenticular cross-section including a thickness that decreases from
a center portion to edge portions of the nanoparticles. The
nanoparticles have an average particle size of less than 150
nm.
[0011] In another aspect, a method of producing nanoparticles is
provided. The method comprises milling feed particles to form a
milled nanoparticle composition including nanoparticles having an
average particle size of less than 150 nm. The nanoparticles have a
stepped surface.
[0012] In another aspect, a method of producing nanoparticles is
provided. The method comprises milling feed particles to form a
milled nanoparticle composition including nanoparticles having an
average particle size of less than 150 nm and a crystallographic
orientation. A majority of the nanoparticles have the same
crystallographic orientation.
[0013] In another aspect, a method is provided. The method
comprises milling feed nanoparticles having an average particle
size of less than 150 nm to produce surface features on the
nanoparticles. The method further comprises recovering the
nanoparticles including surface features having an average particle
size within about 25% of the average particle size of the feed
particles.
[0014] In another aspect, a method is provided. The method
comprises milling particles having a first composition and
particles having a second composition in a milling apparatus. The
method further comprises forming a region comprising the second
composition on respective surfaces of particles having the first
composition.
[0015] Other aspects, embodiments and features of the invention
will become apparent from the following detailed description when
considered in conjunction with the accompanying drawings. The
accompanying figures are schematic and are not intended to be drawn
to scale. For purposes of clarity, not every component is labeled
in every figure. Nor is every component of each embodiment of the
invention shown where illustration is not necessary to allow those
of ordinary skill in the art to understand the invention. All
patent applications and patents incorporated herein by reference
are incorporated by reference in their entirety. In case of
conflict, the present specification, including definitions, will
control.
BRIEF DESCRIPTION OF DRAWINGS
[0016] FIG. 1 shows a representative cross-section of a lenticular
particle according to an embodiment of the invention.
[0017] FIG. 2 shows the results of the cyclic voltammetry (CV)
measurements made in Example 2.
[0018] FIG. 3A is a copy of an AFM image of a representative region
of the particle composition described in Example 3.
[0019] FIG. 3B is a height vs. distance plot of the line on the
image in FIG. 3A.
[0020] FIG. 4 is a copy of a TEM image of the alumina particles
described in Example 3.
[0021] FIGS. 5-7, 8A, 9A and 10 are copies of AFM images of the
silicon particles described in Example 4.
[0022] FIGS. 8B and 9B are respective height vs. distance plots of
the lines on FIGS. 8A and 9A.
[0023] FIG. 11 is a copy of an FETEM image of the silicon particles
described in Example 4.
DETAILED DESCRIPTION
[0024] The invention provides small particle compositions. The
particle compositions, in some cases, are characterized by having
an extremely small average particle size (e.g., 1 micron or less).
As described further below, the particle compositions may be
produced in a milling process that uses preferred types of grinding
media and/or preferred milling conditions. The process may be
controlled to produce small particle compositions having one or
more desired features related to particle morphology (e.g.,
lenticular-shaped), topology (e.g., stepped surfaces) and
crystallographic orientation. The small particle size coupled with
such feature(s) can lead to significant property advantages in a
variety of different applications including catalytic
applications.
[0025] The particle compositions may be produced at very small
particle sizes. In some embodiments, the average particle size of
the composition is less than 1 micron. In certain embodiments, the
average particle size may be even smaller. For example, the average
particle size may be less than 600 nm, less than 250 nm, or less
than 100 nm. In some cases, it is even possible to produce particle
compositions having an average particle size of less than 50 nm, or
less than 10 nm. Such particle sizes may be obtained, in part, by
using grinding media having certain preferred characteristics, as
described further below.
[0026] The preferred average particle size of the composition
typically depends on the intended application. In certain
applications, it may be desired for the average particle size to be
extremely small (e.g., less than 100 nm); while, in other
applications, it may be desired for the average particle size to be
slightly larger (e.g., between 100 nm and 1 micron). In general,
milling parameters may be controlled to provide a desired particle
size, though in certain cases it may be preferable for the average
particle size to be greater than 1 nm to facilitate milling. For
example, the average particle size of the milled material may be
controlled by a number of factors including grinding media
characteristics (e.g., density, size, hardness, toughness), as well
as milling conditions (e.g., specific energy input).
[0027] It should be understood that the average particle size of a
particle composition may be determined by measuring an average
cross-sectional dimension (e.g., diameter for substantially
spherical particles) of a representative number of particles. For
example, the average cross-sectional dimension of a substantially
spherical particle is its diameter; and, the average
cross-sectional dimension of a non-spherical particle is the
average of its three cross-sectional dimensions (e.g., length,
width, thickness), as described further below. The particle size
may be measured using a laser particle measurement instrument, a
scanning electron microscope or other conventional techniques.
[0028] It should also be understood that particle compositions
having average particle sizes outside the above-described ranges
(e.g., greater than 1 micron) may be useful in certain embodiments
of the invention.
[0029] The particle compositions may also be relatively free of
large particles. That is, the particle compositions may include
only a small concentration of larger particles. For example, the
D.sub.90 values for the compositions may be any of the
above-described average particle sizes. Though, it should be
understood that the invention is not limited to such D.sub.90
values.
[0030] The particle compositions may also have a very high average
surface area. The high surface area is, in part, due to the very
small particle sizes noted above. The average surface area of the
particle compositions may be greater than 1 m.sup.2/g; in other
cases, greater than 5 m.sup.2/g; and, in other cases, greater than
50 m.sup.2/g. In some cases, the particles may have extremely high
average surface areas of greater than 100 m.sup.2/g; or, even
greater than 500 m.sup.2/g. It should be understood that these high
average surface areas are even achievable in particles that are
substantially non-porous, though other particles may have surface
pores. Such high surface areas may be obtained, in part, by using
grinding media having certain preferred characteristics, as
described further below.
[0031] Similar to particle size, the preferred average surface area
of the particle composition typically depends on the intended
application. In certain applications, it may be desired for the
average surface area to be extremely large (e.g., greater than 50
m.sup.2/g); while, in other applications, it may be desired for the
average surface area to be slightly smaller (e.g., between 50
m.sup.2/g and 1 m.sup.2/g). In general, milling parameters may be
controlled to provide a desired surface area, though in certain
cases it may be preferable for the average surface area to be less
than 3,000 m.sup.2/g (e.g., for substantially non-porous
particles). For example, the average surface area of the milled
particle compositions may be controlled by a number of factors
including grinding media characteristics (e.g., density, size,
hardness, toughness), as well as milling conditions (e.g., energy,
time).
[0032] As described further below, the particles of the present
invention can be produced in a milling process. Thus, these
particle compositions may be described as having a characteristic
"milled" morphology/topology. Those of ordinary skill in the art
can identify "milled particles" as particles that include one or
more of the following microscopic features: multiple sharp edges,
faceted surfaces, and being free of smooth rounded "corners" such
as those typically observed in chemically-precipitated particles.
It should be understood that the milled particles described herein
may have one or more of the above-described microscopic features,
while having other shapes (e.g., lenticular, spherical) when viewed
at lower magnifications.
[0033] In some embodiments, the particles have a lenticular
morphology. The lenticular particles may have a shape defined
around an elliptical plane and bounded by curved (e.g.,
lens-shaped) surfaces. FIG. 1 shows a representative cross-section
of a lenticular particle 10. As shown, a thickness of the center
portion (t.sub.c) of the particle is greater than a thickness of
the edge portion (t.sub.e) of the particle. In some embodiments,
(t.sub.c) is about 5 times greater than (t.sub.e) (e.g., between
about 5 and 10 times greater); and, in some embodiments, (t.sub.c)
is about 10 times greater than (t.sub.e).
[0034] Lenticular particles also may be characterized by a
dimension (d) perpendicular to the respective thicknesses. In
embodiments in which lenticular particles have a substantially
circular cross-section defining a plane perpendicular to the
lens-shaped cross-section, dimension (d) is the diameter of the
circular cross-section. However, it should be understood that the
cross-section of a lenticular particle perpendicular to the
lens-shaped cross-section may not be spherical and, thus, dimension
(d) does not refer to a diameter. For example, the cross-section of
a lenticular particle perpendicular to the lens-shaped
cross-section may be elliptical.
[0035] In certain embodiments, (d) is greater than or equal to
about 10 times (t.sub.c); and, in some embodiments, (d) is greater
than or equal to about 7 times (t.sub.c).
[0036] It should be understood that the average particle size of a
composition including lenticular particles may be determined by
measuring the average cross-sectional dimension of a representative
number of particles. The average cross-sectional dimension is the
average of the three cross-sectional dimensions of the particle.
For a lenticular particle having a circular cross-section
perpendicular to its lens-shaped cross-section the average
cross-sectional dimension is (2.times.(d)+(t.sub.e))/3.
[0037] Lenticular particles may be particularly desired when the
compositions are used in catalytic applications. Without being
bound by any theory, it is believed that such a shape can increase
the number of catalytic sites on particle surfaces, as described
further below.
[0038] In other embodiments, the particles may not be lenticular.
For example, the particles may have a substantially spherical, or
oblate spheroid shape, amongst others. The particles may be
platelets. It should be understood that a particle composition may
include particles having one or more of the above-described
shapes.
[0039] In some embodiments, the particles have a stepped-surface
topography. That is, the particle surface defines a series of
steps. In some cases, it may be preferable for the particles to
include a high density of steps; that is, a high number of steps
per unit area. In some cases, the length-to-height ratio of the
steps may be small. In some embodiments, the particles include one
or more steps having a height of greater than about 5 nm (e.g.,
between about 5 nm and about 20 nm); in some embodiments, the step
height is less than about 10 nm, or less than about 5 nm. In some
cases, where the step height is less than about 5 nm, it should be
understood that the step height may be below the resolution of
instrument (e.g., AFM) used to examine the step height. In such
cases, other considerations, may be used to determine the presence
of such step heights. In some embodiments, the particles include
one or more steps having a width of greater than about 5 nm (e.g.,
between about 5 nm and about 20 nm); in some embodiments, the step
width is less than about 10 nm or less than about 5 nm.
[0040] The step density on milled particles of the invention may be
significantly higher than that of similarly shaped particles
produced using conventional chemical precipitation techniques.
Without being bound by any theory, it is believed the steps on
milled particles of the invention are produced as a result of
mechanical forces (e.g., particle-grinding media contact). In
contrast, the production of steps on chemically-precipitated
particles is dictated by free-energy considerations. It is believed
that it is possible to produce many more steps by properly
selecting milling conditions than those that generally arise from
free-energy considerations during chemical precipitation.
[0041] The surface topography of a particle may be characterized by
atomic force microscopy (AFM). For example, AFM may be used to
generate a surface map that can be used to observe the steps. Also,
high-resolution TEM may be used to examine surface topography.
[0042] A lenticular particle shape is particularly conducive to
having stepped surfaces. However, it should be understood that
particles having other morphologies may also have stepped
surfaces.
[0043] Particles having a high density of surface steps may be
particularly desired when the compositions are used in catalytic
applications. Without being bound by any theory, it is believed
that each step may function as a catalytic site and, thus,
increasing the number of steps on a particle surface can increase
the number of catalytic sites and surface area.
[0044] In some embodiments, the compositions of the invention may
comprise particles having a preferred crystallographic orientation.
In these embodiments, a majority (i.e., greater than 50%) of the
particles in a composition may have the same crystallographic
orientation. In other embodiments, greater than 75% of the
particles, or even greater than 95%, or even substantially all, of
the particles in a composition may have the same crystallographic
orientation.
[0045] The preferred crystallographic orientation of the particles
may depend, in part, on the crystal structure (e.g., hexagonal,
tetragonal) of the material that forms the particles.
[0046] Crystals generally preferentially fracture along specific
planes with characteristic amounts of energy being required to
induce fracture along such planes. During milling, such energy
results from particle/grinding media collisions. It is observed
that, by controlling the energy of such collisions via milling
parameters (e.g., specific energy input), it is possible to
preferentially fracture particles along certain crystallographic
planes which creates a particle composition having a preferred
crystallographic orientation.
[0047] In some embodiments, the preferred crystallographic
orientation is defined by a basal plane (i.e., the plane which is
perpendicular to the principal axis (c axis) in a tetragonal or
hexagonal structure). For example, the basal plane, and
crystallographic orientation, may be the (0001) or (001) plane.
[0048] Crystallographic orientation of particles may be measured
using known techniques. A suitable technique is x-ray diffraction
(XRD). It may be possible to assess the relative percentage of
particles having the same preferred crystallographic orientation
using XRD.
[0049] Particle compositions having a preferred crystallographic
orientation may be particularly desired when the compositions are
used in catalytic applications. Because certain planes can exhibit
enhanced catalytically activity for certain reactions, it may be
possible to significantly increase the overall catalytic activity
of a particle composition by exposing such planes. As noted above,
it is believed that milling conditions may be controlled to
preferentially fracture particles along certain crystallographic
planes which may be especially catalytically active for certain
reactions, thus, creating a particle composition that has a
dramatically high catalytic activity for such reactions.
[0050] In some embodiments, the material composition along the
exposed crystallographic orientation may also affect catalytic
activity. For example, the material composition of the exposed
plane in an ordered intermetallic compound particle composition
(which are described further below) may be important in determining
catalytic activity. It may be desired to expose planes that have
high catalytic activity. For example, for a Pt--Bi ordered
intermetallic compound particle composition, the preferred
crystallographic orientation may be the (001) Pt plane (i.e., the
Pt--Pt plane) which is highly catalytic for a number of reactions
(e.g., fuel oxidation).
[0051] In some embodiments, particles may be substantially free
(and/or, entirely free) of surface contamination that may result
from certain chemical precipitation processes because particles of
the invention may be produced by milling. The surface contamination
(often in the form of a layer or a portion of a layer) that results
from certain chemical precipitation typically processes comprises
element(s) from precursors used to form the particles. These
precursors may comprise organic compounds (and, thus, also comprise
carbon). Thus, in some embodiments, particles of the invention may
be substantially free (and/or, entirely free) of surface
contamination comprising organic compounds such as carbon-based
surface contamination. In this context, "substantially free of
organic compounds" refers to carbon concentration levels of less
than 200 ppm. "Surface contamination", in this context, refers to
any species (e.g., compound, element, etc.) that is present on the
particle surface and has a different composition than the
composition of the particle. Such low surface contamination
concentrations are even achievable when the particles are metallic
(e.g., pure metals, alloys, intermetallic compounds).
[0052] In some embodiments, the particles may be substantially-free
of halogen (e.g., chlorine)-based surface contamination (i.e.,
halogen concentration levels of less than 200 ppm). Precursors used
in chemical processes also may comprise halogens (e.g.,
chlorine).
[0053] Surface contamination concentrations may be determined by
known techniques. For example, thermal gravimetric analysis (TGA),
Raman Spectroscopy, or GDMS mass spectroscopy may be suitable to
determine such concentrations.
[0054] It should be understood that in these embodiments in which
particles are substantially free of the above-described types of
surface contamination, other types of surface contamination may be
present. For example, oxide-based surface contamination may be
formed on a portion, or the entire, particle surface as a result of
oxidation processes (e.g., when particles are exposed to air).
However, in other embodiments, the particles may be substantially
free of all types of surface contamination. Even particles of the
invention that would otherwise oxidize if exposed to air may be
substantially free of all types of surface contamination by
dispersing such particles in a suitable liquid (e.g., water-based,
organic-based).
[0055] Minimization of surface contamination (e.g., carbon-based
surface contamination) is preferable in certain applications in
which particle compositions of the invention are used. For example,
in catalytic applications, it may be preferable to minimize surface
contamination which would otherwise inhibit catalytic activity.
Thus, particles being relatively free of the above-noted types of
surface contamination may have enhanced catalytic activity.
[0056] An advantage of certain embodiments of the invention is that
the above-noted particle sizes can be achieved at very low
contamination levels. The grinding media properties and/or
compositions noted above may enable the low contamination levels
because such characteristics lead to very low wear rates. For
example, the contamination levels may be less than 900 ppm, less
than 500 ppm, less than 200 ppm, or even less than 100 ppm. In some
processes, virtually no contamination may be detected which is
generally representative of contamination levels of less than 10
ppm. As used herein, a "contaminant" is grinding media material
introduced into the product material composition during milling. It
should be understood that typical commercially available product
materials may include a certain impurity concentration (prior to
milling) and that such impurities are not includes in the
definition of contaminant as used herein. Also, other sources of
impurities introduced in to the product material, such as material
from the milling equipment, are not included in the definition of
contaminant as used herein. The "contamination level" refers to the
weight concentration of the contaminant relative to the weight
concentration of the milled material. Typical units for the
contamination level are ppm. Standard techniques for measuring
contamination levels are known to those of skill in the art
including chemical composition analysis techniques.
[0057] It should be understood that methods of the invention may
produce compositions having any of the above-described particle
size values (including values of relative size between particles
before and after milling) combined with any of the above-described
contamination levels. For example, one method of the invention
involves milling feed particles having an average initial particle
size to form a milled particle composition having an average final
particle size of less than 100 nm, wherein the initial particle
size is greater than 100 times the final particle size and the
milled particle composition has a contamination level of less than
500 ppm.
[0058] As noted above, particle compositions of the invention may
have enhanced catalytic activity. Catalytic activity may be
characterized by chemisorption techniques as described further
below in the Example 1.
[0059] Particle compositions having enhanced catalytic activity are
suitable for use as catalysts in a number of applications including
fuel cells. When used in fuel cells, such particle compositions may
be incorporated into one or more of the electrodes (e.g., anodes,
cathodes). The particle compositions catalyze reactions that occur
at the electrode(s) (e.g., oxidation of fuel) and, thus, enhance
fuel cell performance (e.g., onset potential and peak current).
Catalytic performance of the particle compositions are further
described in the examples below.
[0060] The particle compositions may be of a variety of material
compositions. Suitable material compositions include metals (such
as cobalt, molybdenum, titanium, tungsten), metal compounds (such
as intermetallic compounds, metal hydrides or metal nitrides),
metal alloys, ceramics (including oxides, such as titanium oxide
(titania), aluminum oxide (Al.sub.2O.sub.3), and carbides such as
silicon carbide) and diamond, amongst many others. As described
further below, in some embodiments, the particles are formed of a
catalytic material. In some cases, the catalytic material may be
consumed, in part, when catalyzing the desired reactions; in other
cases, the catalytic material catalyzes the reaction without being
consumed;
[0061] In some embodiments, it may be preferred for the material
composition to be formed of a crystalline material or
semi-crystalline material. In some cases, it is preferred that the
crystalline material have an anisotropic crystal structure (e.g.,
hexagonal, tetragonal, rhombohedral and the like). It may be easier
to produce particles having one or more of the above-described
characteristics (e.g., lenticular morphology, stepped-surface
topography, preferred crystallographic orientation) using such
material compositions. Without being bound by any theory, it is
believed that such structures may fracture in ways that lead to
such characteristics. For example, aluminum oxide (Al.sub.2O.sub.3)
particles have a rhombohedral structure and may be used to produce
particle compositions having a lenticular morphology,
stepped-surface topography and preferred crystallographic
orientation.
[0062] In some embodiments, it may be preferred for the particle
composition to be formed of a material that is relatively brittle.
In general, brittle materials fracture with lower energies and less
deformation than ductile materials. Such fracture behavior may
facilitate production of particles having one of more of the
above-described features. Those of ordinary skill in the art can
determine whether a material is brittle using standard measurement
techniques such as a Charpy Impact test.
[0063] In certain embodiments, particularly when used in catalytic
applications, it may be preferred that the particles are formed of
an ordered intermetallic compound composition. As used herein, the
term "ordered intermetallic compound" refers to compounds that
comprise more than one metal and have an ordered atomic structure.
In an ordered intermetallic compound, substantially all unit cells
include the same arrangement of metal atoms. Suitable ordered
intermetallic compounds are described in U.S. Patent Application
Publication No. 20040126267, based on U.S. patent application Ser.
No. 10/630,237, entitled "Intermetallic Compounds for use as
Catalysts and Catalytic Systems", filed Jul. 29, 2003, which is
incorporated herein by reference. Ordered intermetallic compounds
are to be distinguished from metal alloys and metal solid
solutions. Metal alloys and metal solid solutions do not have an
ordered atomic structure, as described above.
[0064] Examples of suitable intermetallic compounds that may be
used in accordance with the invention include, but are not limited
to, PtBi, PtBi.sub.2, PtIn, PtPb, PtGe, PtIn.sub.2, PtIn.sub.3,
Pt.sub.3In.sub.7, PdGe, PdSb, IrBi, NiBi, PtSn, PtSn.sub.2,
Pt.sub.3Sn, Pt.sub.2Sn.sub.3, PtSn.sub.4, PtSb, PtSb.sub.2, RhBi,
PtGa, PtCd.sub.2, PtMn and BiPd. PtBi may be particularly preferred
in certain applications. Other ordered intermetallic compounds also
may be used in accordance with the invention including other
intermetallic compounds that comprise the aforementioned elements
at different stoichiometries.
[0065] One aspect of the invention is the discovery that particle
compositions having the features described above can be produced in
a milling process. The features are achievable by using grinding
media having selected characteristics (e.g., density, mechanical
properties, size) and controlling milling conditions, as described
further below.
[0066] In some embodiments, the grinding media is formed of a
material having a density of greater than 6 grams/cm.sup.3; in some
embodiments, greater than 8 grams/cm.sup.3; in some embodiments,
the density is greater than 10 grams/cm.sup.3; or greater than 15
grams/cm.sup.3; or, even, greater than 18 grams/cm.sup.3. Though,
in certain embodiments, the density of the grinding media may be
less than 22 grams/cm.sup.3, in part, due to difficulties in
producing suitable grinding materials having greater densities. It
should be understood that conventional techniques may be used to
measure grinding media material density.
[0067] In certain embodiments, it also may be preferable for the
grinding media to be formed of a material having a high fracture
toughness. For example, in some cases, the grinding media is formed
of a material having a fracture toughness of greater than 6
MPa/m.sup.1/2; and in some cases, the fracture toughness is greater
than 9 MPa/m.sup.1/2. The fracture toughness may be greater than 12
MPa/m.sup.1/2 in certain embodiments. Conventional techniques may
be used to measure fracture toughness. Suitable techniques may
depend, in part, on the type of material being tested and are known
to those of ordinary skill in the art. For example, an indentation
fracture toughness test may be used. Also, a Palmqvist fracture
toughness technique may be suitable, for example, when testing hard
metals.
[0068] It should be understood that the fracture toughness values
disclosed herein refer to fracture toughness values measured on
bulk samples of the material. In some cases, for example, when the
grinding media are in the form of very small particles (e.g., less
than 150 micron), it may be difficult to measure fracture toughness
and the actual fracture toughness may be different than that
measured on the bulk samples.
[0069] In certain embodiments, it also may be preferable for the
grinding media to be formed of a material having a high hardness.
It has been found that media having a high hardness can lead to
increased energy transfer per collision with product material
which, in turn, can increase milling efficiency. In some
embodiments, the grinding media is formed a material having a
hardness of greater than 75 kgf/mm.sup.2; and, in some cases, the
hardness is greater than 200 kgf/mm.sup.2. The hardness may even be
greater than 900 kgf/mm.sup.2 in certain embodiments. Conventional
techniques may be used to measure hardness. Suitable techniques
depend, in part, on the type of material being tested and are known
to those of ordinary skill in the art. For example, suitable
techniques may include Rockwell hardness tests or Vickers hardness
tests (following ASTM 1327). It should be understood that the
hardness values disclosed herein refer to hardness values measured
on bulk samples of the material. In some cases, for example, when
the grinding media are in the form of very small particles (e.g.,
less than 150 micron), it may be difficult to measure hardness and
the actual hardness may be greater than that measured on the bulk
samples.
[0070] It should be understood that not all milling processes of
the present invention use grinding media having each of the
above-described characteristics.
[0071] The grinding media may have a wide range of dimensions. In
general, the average size of the grinding media is between about
0.5 micron and 10 cm. The preferred size of the grinding media used
depends of a number of factors including the size of the feed
particles, desired size of the milled particle composition,
grinding media composition, and grinding media density, amongst
others.
[0072] In certain embodiments, it may be advantageous to use
grinding media that are very small. It may be preferred to use
grinding media having an average size of less than about 250
microns; or, less than about 150 microns (e.g., between about 75
and 125 microns). In some cases, the grinding media may have an
average size of less than about 100 microns; or even less than
about 10 microns. Grinding media having a small size have been
shown to be particularly effective in producing particle
compositions having very small particle sizes (e.g., less than 1
micron). In some cases, the grinding media may have an average size
of greater than 0.5 micron.
[0073] It should be understood that the average size of grinding
media used in a process may be determined by measuring the average
cross-sectional dimension (e.g., diameter for substantially
spherical grinding media) of a representative number of grinding
media particles. The grinding media size may be measured using
conventional techniques such as suitable microscopy techniques or
standard sieve size screening techniques.
[0074] The grinding media may also have a variety of shapes. In
general, the grinding media may have any suitable shape known in
the art. In some embodiments, it is preferred that the grinding
media be substantially spherical (which may be used herein
interchangeably with "spherical"). Substantially spherical grinding
media have been found to be particularly effective in obtaining
desired milling performance.
[0075] It should also be understood that any of the grinding media
used in methods of the invention may have any of the
characteristics (e.g., properties, size, shape, composition)
described herein in combination with one another. For example,
grinding media used in methods of the invention may have any of the
above-noted densities and above-noted average sizes (e.g., grinding
media may have a density of greater than about 6 grams/cm.sup.3 and
an average size of less than about 250 micron).
[0076] The above-described grinding media characteristics (e.g.,
density, hardness, toughness) are dictated, in part, by the
composition of the grinding media. In certain embodiments, the
grinding media may be formed of a metallic material including metal
alloys or metal compounds. In one set of embodiments, it may be
preferred that the grinding media are formed of ferro-tungsten
material (i.e., Fe--W). In some cases, the compositions may
comprise between 75 and 80 weight percent iron and between 20 and
25 weight percent tungsten. In some cases, ferro-tungsten grinding
media may be carburized to improve wear resistance.
[0077] In other embodiments, the grinding media may be formed of a
ceramic material such as a carbide material. In some embodiments,
the grinding media to be formed of a single carbide material (e.g.,
iron carbide (Fe.sub.3C), chromium carbide (Cr.sub.7C.sub.3),
molybdenum carbide (Mo.sub.2C), tungsten carbide (WC, W.sub.2C),
niobium carbide (NbC), vanadium carbide (VC), and titanium carbide
(TiC)). In some cases, it may be preferred for the grinding media
to be formed of a multi-carbide material. A multi-carbide material
comprises at least two carbide forming elements (e.g., metal
elements) and carbon.
[0078] A multi-carbide material may comprise a multi-carbide
compound (i.e., a carbide compound having a specific stoichiometry;
or, a blend of single carbide compounds (e.g., blend of WC and
TiC); or, both a multi-carbide compound and a blend of single
carbide compounds. It should be understood that multi-carbide
materials may also include other components such as nitrogen,
carbide-forming elements that are in elemental form (e.g., that
were not converted to a carbide during processing of the
multi-carbide material), amongst others including those present as
impurities. Typically, but not always, these other components are
present in relatively minor amounts (e.g., less than 10 atomic
percent).
[0079] Suitable carbide forming elements in multi-carbide grinding
media of the invention include iron, chromium, hafnium, molybdenum,
niobium, rhenium, tantalum, titanium, tungsten, vanadium,
zirconium, though other elements may also be suitable. In some
cases, the multi-carbide material comprises at least two of these
elements. For example, in some embodiments, the multi-carbide
material comprises tungsten, rhenium and carbon; in other cases,
tungsten, hafnium and carbon; in other cases, molybdenum, titanium
and carbon.
[0080] Suitable grinding media compositions have been described,
for example, in U.S. patent application Ser. No. 11/193,688, filed
Jul. 29, 2005, entitled "Grinding Media and Methods Associated With
the Same," by Robert Dobbs, which is incorporated herein by
reference.
[0081] In some embodiments, it may be preferred for the
multi-carbide material to comprise at least tungsten, titanium and
carbon. In some of these cases, the multi-carbide material may
consist essentially of tungsten, titanium and carbon, and is free
of additional elements in amounts that materially affect
properties. Though in other cases, the multi-carbide material may
include additional metal carbide forming elements in amounts that
materially affect properties. For example, in these embodiments,
tungsten may be present in the multi-carbide material in amounts
between 10 and 90 atomic %; and, in some embodiments, in amounts
between 30 and 50 atomic %. The amount of titanium in the
multi-carbide material may be between 1 and 97 atomic %; and, in
some embodiments, between 2 and 50 atomic %. In these embodiments
that utilize tungsten-titanium carbide multi-carbide material, the
balance may be carbon. For example, carbon may be present in
amounts between 10 and 40 atomic %. As noted above, it should also
be understood that any other suitable carbide forming elements can
also be present in the multi-carbide material in these embodiments
in addition to tungsten, titanium and carbon. In some cases, one or
more suitable carbide forming elements may substitute for titanium
at certain sites in the multi-carbide crystal structure. Hafnium,
niobium, tantalum and zirconium may be particularly preferred as
elements that can substitute for titanium. Carbide forming elements
that substitute for titanium may be present, for example, in
amounts of up to 30 atomic % (based on the multi-carbide material).
In some cases, suitable multi-carbide elements may substitute for
tungsten at certain sites in the multi-carbide crystal structure.
Chromium, molybdenum, vanadium, tantalum, and niobium may be
particularly preferred as elements that can substitute for
tungsten. Carbide forming elements that substitute for tungsten may
be present, for example, in amounts of up to 30 atomic % (based on
the multi-carbide material).
[0082] It should also be understood that the substituting carbide
forming elements noted above may completely substitute for titanium
and/or tungsten to form a multi-carbide material free of tungsten
and/or titanium.
[0083] It should be understood that grinding media compositions
that are not disclosed herein but have certain above-noted
characteristics (e.g., high density) may be used in embodiments of
the invention. Milling processes of the present invention are not
limited to the grinding media compositions described herein.
[0084] In general, any suitable process for forming grinding media
compositions having the desired characteristics may be used. In
some cases, the processes involve heating the components of the
composition to temperatures higher than the respective melting
temperatures of the components followed by a cooling step to form
the grinding media. A variety of different heating techniques may
be used including a thermal plasma torch, melt atomization, and arc
melting, amongst others. For example, one suitable process involves
admixing fine particles of the elements intended to comprise the
grinding media in appropriate ratios. The stability of the mixture
may be enhanced by introduction of an inert binding agent (e.g.,
which burns off and does not form a component of the grinding
material). The mixture may be subdivided into a plurality of
aggregates (e.g., each having a mass approximately equal to that of
the desired media particle to be formed). The aggregates may be
heated to fuse (e.g., to 90% of theoretical density) and,
eventually, melt individual aggregates to form droplets that are
cooled to form the grinding media.
[0085] In some embodiments, the grinding media may be formed of two
different materials. For example, the grinding media may be formed
of a blend of two different ceramic materials (e.g., a blend of
high density ceramic particles in a ceramic matrix); or a blend of
a ceramic material and a metal (e.g., a blend of high density
ceramic materials in a metal matrix).
[0086] In some embodiments in which the grinding media comprises
more than one material component, the grinding media may comprise
coated particles. The particles may have a core material and a
coating formed on the core material. The coating typically
completely covers the core material, but not in all cases. The
composition of the core and coating materials may be selected to
provide the grinding media with desired properties such as a high
density. For example, the core material may be formed of a high
density material (e.g., greater than 8 grams/cm.sup.3). The core,
for example, may be formed of a metal such as steel or depleted
uranium; or a ceramic such as a metal carbide.
[0087] As noted above, particle compositions may be produced in a
milling process that use grinding media as described herein. The
processes may utilize a wide range of conventional mills having a
variety of different designs and capacities. Suitable types of
mills include, but are not limited to, ball mills, rod mills,
attritor mills, stirred media mills, pebble mills and vibratory
mills, among others.
[0088] In some cases, conventional milling conditions (e.g.,
energy, time) may be used to process the particle compositions
using the grinding media described herein. In other cases, the
grinding media described herein may enable use of milling
conditions that are significantly less burdensome (e.g., less
energy, less time) than those of typical conventional milling
processes, while achieving a superior milling performance (e.g.,
very small average particle sizes).
[0089] One aspect of the invention is that small particle
compositions may be produced using very low specific energy input
(i.e., energy consumed in milling process per weight of feed
material). The preferred grinding media described above may enable
advantageous milling conditions. For example, lower milling times
and specific energy inputs can be utilized because of the high
milling efficiency of the grinding media. As used herein, the
"specific energy input" is the milling energy consumed per weight
product material. Even milled particle compositions having the
above-noted particle sizes and contamination levels can be produced
at low milling input energies and/or low milling times. For
example, the specific energy input may be less than 125,000 kJ/kg;
or less than 90,000 kJ/kg. In some cases, the specific energy input
may be even lower such as less than 50,000 kJ/kg or less than
25,000 kJ/kg. The actual specific energy input and milling time
depends strongly on the composition of the product material and the
desired reduction in particle size, amongst other factors. For
example, grinding media of the invention may be used to produce a
titania milled particle composition at a specific energy input of
less than about 25,000 kJ/kg (e.g., about 20,000 kJ/kg), an average
particle size of less than about 100 nm (e.g., about 80 nm) and a
contamination level of less than 500 ppm, wherein the titania feed
particles have an average particle size (e.g., about 600 nm) of
greater than 50 times the average particle size of the milled
titania particle composition.
[0090] Milling processes of the invention typically involve the
introduction of a slurry of feed material and a milling fluid
(e.g., water or non-aqueous fluids) into a processing space in a
mill in which the grinding media are confined. The viscosity of the
slurry may be controlled, for example, by adding additives to the
slurry such as dispersants. The mill is rotated at a desired speed
and material particles mix with the grinding media. Collisions
between the particles and the grinding media can reduce the size of
the particles. In certain processes, it is believed that the
mechanism for particle size reduction is dominated by wearing of
particle surfaces; while, in other processes, it is believed the
mechanism for particle size reduction is dominated by particle
fracture. The particular mechanism may affect the final particle
characteristics (e.g., morphology, topography). The particles are
typically exposed to the grinding media for a certain mill time
after which the milled material is separated from the grinding
media using conventional techniques, such as washing and filtering,
screening or gravitation separation.
[0091] It should be understood that, in certain methods, the goal
of the milling process may be to produce surface features (e.g.,
morphology, topography, crystallographic orientation) on the
particles rather than to reduce particle size. In these methods,
particle size also may be reduced, though the particle size
reduction may be negligible in some cases. For example, these
methods may involve recovering milled nanoparticles having one or
more desired surface features and an average particle size within
about 25%, or within about 10%, of the average particle size of the
feed particles. In some embodiments, the milled nanoparticles may
even have substantially the same average particle size as the feed
particles.
[0092] To produce desired surface features on the nanoparticles,
milling conditions are appropriately selected. For example, the
specific energy input may be selected to be relatively low when
providing surface features without significantly reducing particle
size. It should be understood that the specific milling conditions
strongly depend on the particle material composition.
[0093] In some embodiments, the milling process may be used to form
a region (or regions) of a second material composition on surfaces
of particles having a first material composition. In these cases,
particles having the first and the second composition are milled
simultaneously. The milling process may fracture portions of
particles of the second composition and such portions may be
deposited on surfaces of particles of the first composition.
[0094] The particle compositions of the invention may be used in a
wide variety of applications. In general, any application that uses
small particles may be suitable. As noted above, catalytic
applications (such as in fuel cells) may be particularly
preferred.
[0095] The following examples are not intended to be limiting in
any way.
EXAMPLE 1
[0096] This example compares the catalytic activity of a particle
composition produced according to an embodiment of the invention to
a commercially-available particle composition.
[0097] A titania (rutile) particle composition having an average
particle size of less than 150 nm was prepared using a milling
process of the present invention. A commercially available titania
(rutile) particle composition was obtained. A chemisorption
technique was used to assess catalytic activity of the two
compositions by evaluating quantitatively the number of surface
active sites which are likely to promote (catalyze) chemical
reactions. The chemisorption technique used ammonia isotherms that
give a quantitative measure of the chemisorbed and physisorbed
ammonia on the samples. The milled titania composition of the
invention showed an increase in ammonia uptake of greater than 50%
than the commercially available titania composition.
[0098] This result shows that the milled titania composition of the
invention is significantly more catalytically active than the
commercially available titania composition.
EXAMPLE 2
[0099] This example compares the catalytic activity of fuel cell
catalyst particle compositions produced according to an embodiment
of the invention to a fuel cell catalyst particle composition
produced according to a conventional process.
[0100] PtBi particle compositions having an average particle size
of 15 nm (A1), an average particle of 30 nm (A2), and an average
particle size of 10 nm (A3) were prepared using a milling process
of the present invention. PtBi particle compositions having an
average particle size of 19 nm (C1), 4-5 nm (C2) and 4-5 nm (C3)
produced using a conventional chemical precipitation process were
obtained.
[0101] Following reductive pre-treatment cleaning, cyclic
voltammetry (CV) was performed on each particle composition. A
potential range of -0.2 V to 0.2 V at 10 mV/s sweep rate was
probed. Typically three cycles were executed.
[0102] All electrochemical evaluations were performed in a standard
3-electrode electrochemical cell. The volume of solution in the
cell was approx. 50 mL. Deoxygenated 10M formic acid with no
supporting electrolyte was used in the electrochemical evaluations.
A counter electrode of Pt wire was used. A reference electrode of
Ag/AgCl (sat'd NaCl) was used. Experiments were conducted at room
temperature. All electrochemical data and procedures were recorded
using a Potentiostat/Galvanostat Model 283 (EG&G Instruments,
Princeton Applied Research) controlled with the CorrWare 2 software
package (Solartron Analytical).
[0103] FIG. 2 shows the current obtained for each sample. The
results show that fuel cell catalyst particle compositions produced
according to an embodiment of the invention exhibit significantly
higher catalytic behavior compared to the fuel cell catalyst
particle composition produced according to the conventional
process.
EXAMPLE 3
[0104] This example illustrates characterization of an alumina
particle composition produced according to an embodiment of the
invention.
[0105] An alumina particle composition having an average particle
size of less than 150 nm was prepared using a milling process of
the present invention. Atomic Force Microscopy (AFM) was used to
characterize the particles. The analysis showed that the particles
had a lenticular morphology. The particles generally had a
thickness of about 10 nm and a length of about 60 nm in length.
FIG. 3A is a copy of an AFM image of a representative region of the
particle composition. FIG. 3B is a height vs. distance plot of the
line on the image in FIG. 3A. FIG. 3B shows a few atomic steps
having a height of about 6 nm with the remaining steps being below
the resolution of the AFM.
[0106] FIG. 4 is a TEM image performed on these nanoparticles and
TEM confirmed a platelet (e.g., lenticular) structure with a
preferred orientation. XRD was also performed showing alpha alumina
phase.
EXAMPLE 4
[0107] This example illustrates characterization of a silicon
particle composition produced according to an embodiment of the
invention.
[0108] A silicon particle composition having an average particle
size of less than 150 nm was prepared using a milling process of
the present invention. The particle composition was characterized
using a number of techniques including AFM. Copies of AFM images of
the composition are shown in FIGS. 5-10. FIGS. 8B and 9B are height
vs. distance plots of the lines on FIGS. 8A and 9A, respectively.
The particles were about 50 nm in length and 6 nm in thickness.
[0109] The particle size calculated by using BET equivalent
spherical diameter indicates that the particle size was smaller
than the size measured by using an acoustic instrument (e.g.,
DT-1200 acoustic spectrometer) and TEM. This is an additional
indication that the particles were not spherical, but platelet.
Additionally, the AFM images show the particles appeared to be
substantially flat. A majority of the particles appear to have been
fractured on the (111) planes to produce "flakes" that may be later
reduced to small flat particles as the milling process-proceeds.
The milling process produced nanoscale, flat particles even on a
silicon material including a cubic crystal structure.
[0110] Having thus described several aspects of at least one
embodiment of this invention, it is to be appreciated various
alterations, modifications, and improvements will readily occur to
those skilled in the art. Such alterations, modifications, and
improvements are intended to be part of this disclosure, and are
intended to be within the spirit and scope of the invention.
Accordingly, the foregoing description and drawings are by way of
example only.
* * * * *