U.S. patent application number 12/105525 was filed with the patent office on 2009-12-31 for systems and methods for filtering nanowires.
This patent application is currently assigned to CAMBRIOS TECHNOLOGIES CORPORATION. Invention is credited to Pierre-Marc Allemand, Manfred Heidecker, Michael A. Spaid, Frank Wallace.
Application Number | 20090321364 12/105525 |
Document ID | / |
Family ID | 39620244 |
Filed Date | 2009-12-31 |
United States Patent
Application |
20090321364 |
Kind Code |
A1 |
Spaid; Michael A. ; et
al. |
December 31, 2009 |
SYSTEMS AND METHODS FOR FILTERING NANOWIRES
Abstract
In order to filter a solution containing nanowires, a flow of
the solution is generated and directed through a passage defining
an aperture having a narrow width. Alternatively, a flow of the
solution may be generated and directed over a micro-structured
surface configured to filter the solution.
Inventors: |
Spaid; Michael A.; (Mountain
View, CA) ; Heidecker; Manfred; (Mountain View,
CA) ; Allemand; Pierre-Marc; (San Jose, CA) ;
Wallace; Frank; (San Francisco, CA) |
Correspondence
Address: |
SEED INTELLECTUAL PROPERTY LAW GROUP PLLC
701 FIFTH AVE, SUITE 5400
SEATTLE
WA
98104
US
|
Assignee: |
CAMBRIOS TECHNOLOGIES
CORPORATION
Mountain View
CA
|
Family ID: |
39620244 |
Appl. No.: |
12/105525 |
Filed: |
April 18, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60913231 |
Apr 20, 2007 |
|
|
|
Current U.S.
Class: |
210/741 ;
210/252; 210/258; 210/323.2; 210/767; 210/798 |
Current CPC
Class: |
H01L 31/1884 20130101;
H05K 1/097 20130101; B82Y 10/00 20130101; Y02E 10/50 20130101; B82Y
30/00 20130101; H05K 3/245 20130101; H01L 51/0048 20130101; H05K
3/249 20130101; H01L 51/5206 20130101; H05K 2201/0108 20130101;
H05K 2201/026 20130101; G02F 1/13439 20130101; H01L 29/0669
20130101; H01L 29/413 20130101 |
Class at
Publication: |
210/741 ;
210/767; 210/798; 210/323.2; 210/252; 210/258 |
International
Class: |
B01D 29/60 20060101
B01D029/60; B01D 29/66 20060101 B01D029/66; B01D 29/56 20060101
B01D029/56 |
Claims
1. A method, comprising: providing a solution containing nanowires
and a first set of contaminant particles; generating a flow of the
solution; and filtering the solution by directing the flow through
a passage defining an aperture having a width less than at least
one dimension of the first set of contaminant particles.
2. The method of claim 1, wherein the passage is formed at least in
part by two substantially parallel plates separated by less than
the at least one dimension of the first set of contaminant
particles.
3. The method of claim 2, wherein at least one of the plates
includes an opening therethrough, through which the flow is
directed.
4. The method of claim 3, wherein the solution flows radially
outward between the pair of plates from the opening.
5. The method of claim 1, further comprising: stopping the flow of
the solution; generating a reverse flow of a liquid; and directing
the reverse flow through the passage in a direction opposite to the
flow of the solution.
6. The method of claim 5, wherein the reverse flow is generated in
response to a reduction in a flow rate of the flow of the solution
through the passage.
7. The method of claim 5, wherein the reverse flow is generated
periodically.
8. The method of claim 1, wherein the solution further contains a
second set of contaminant particles, the second set of contaminant
particles having at least one dimension smaller than the at least
one dimension of the first set of contaminant particles, the method
further comprising: directing the flow through a second passage
defining a second aperture having a width less than the at least
one dimension of the second set of contaminant particles.
9. The method of claim 8, wherein the solution further contains a
third set of contaminant particles, the third set of contaminant
particles having at least one dimension smaller than the at least
one dimension of the second set of contaminant particles, the
method further comprising: directing the flow through a third
passage defining a third aperture having a width less than the at
least one dimension of the third set of contaminant particles.
10. The method of claim 9, wherein the flow is directed through the
passage, then the second passage, and then the third passage.
11. The method of claim 1, wherein the passage is formed at least
in part by two converging plates, and wherein the passage narrows
from an entrance of the passage to an exit of the passage.
12. The method of claim 11, wherein the aperture comprises the exit
of the passage.
13. The method of claim 1, further comprising: generating a reverse
flow of the solution; and directing the reverse flow through the
passage in a direction opposite to the flow of the solution;
wherein a flow rate of the flow and a flow rate of the reverse flow
are chosen such that there is a net flow of the solution towards an
exit of the passage.
14. A nanowire filtering system comprising: a source container for
holding a solution containing nanowires and a first set of
contaminant particles; and a nanowire filter passage
communicatively coupled to the source container for receiving the
solution, the nanowire filter passage defined at least in part by:
a first plate; and a second plate disposed adjacent the first plate
with a minimum separation distance between the first plate and the
second plate of less than at least one dimension of the first set
of contaminant particles.
15. The nanowire filtering system of claim 14, wherein the first
plate defines an opening therethrough, the opening disposed
opposite the second plate.
16. The nanowire filtering system of claim 15, wherein the opening
is substantially circular.
17. The nanowire filtering system of claim 14, wherein the first
plate and the second plate are substantially parallel.
18. The nanowire filtering system of claim 14, wherein the solution
further contains a second set of contaminant particles, the second
set of contaminant particles having at least one dimension smaller
than the at least one dimension of the first set of contaminant
particles, the nanowire filtering system further comprising: a
second nanowire filter passage communicatively coupled to the
nanowire filter passage, the second nanowire filter passage defined
at least in part by: a third plate; and a fourth plate disposed
adjacent the third plate with a minimum separation distance between
the third plate and the fourth plate of less than the at least one
dimension of the second set of contaminant particles.
19. The nanowire filtering system of claim 18, wherein the solution
further contains a third set of contaminant particles, the third
set of contaminant particles having at least one dimension smaller
than the at least one dimension of the second set of contaminant
particles, the nanowire filtering system further comprising: a
third nanowire filter passage communicatively coupled to the second
nanowire filter passage, the third nanowire filter passage defined
at least in part by: a fifth plate; and a sixth plate disposed
adjacent the fifth plate with a minimum separation distance between
the fifth plate and the sixth plate of less than the at least one
dimension of the third set of contaminant particles.
20. The nanowire filtering system of claim 14, wherein the first
plate and the second plate converge, and wherein the nanowire
filter passage narrows.
21. The nanowire filtering system of claim 14, further comprising a
pump for generating a flow of the solution from the source
container through the nanowire filter passage.
22. A method, comprising: providing a solution containing
nanowires; generating a primary flow of the solution; and filtering
the solution by directing the primary flow over a micro-structured
surface configured to filter the solution.
23. The method of claim 22, wherein the micro-structured surface
includes a plurality of openings through the surface.
24. The method of claim 22, wherein the plurality of openings have
an average diameter of greater than 5 .mu.m.
25. A nanowire filtering system comprising: a source container for
holding a solution containing nanowires; and a nanowire filter
communicatively coupled to the source container for receiving the
solution, the nanowire filter including: a rotatable tube defining
a passage for the solution; a micro-structured surface lining an
inside of the rotatable tube; a substantially helical element
adjacent the micro-structured surface and extending at least
partially into the passage; and a drive member adapted to turn the
rotatable tube.
26. A nanowire filtering system comprising: a source container for
holding a solution containing nanowires; and a nanowire filter
communicatively coupled to the source container for receiving the
solution, the nanowire filter including: an elongate channel
defining a passage for the solution flowing along a long axis, the
elongate channel having a lower surface including a plurality of
parallel ridges disposed at an angle to the long axis; wherein the
plurality of parallel ridges at least partially define a plurality
of openings from the elongate channel.
27. A nanowire filtering system comprising: a source container for
holding a solution containing nanowires; and a nanowire filter
communicatively coupled to the source container for receiving the
solution, the nanowire filter including: an elongate channel
defining a passage for the solution; and a collection chamber
defined in part by an outer surface of the elongate channel, the
collection chamber communicatively coupled to the elongate channel
via a plurality of openings having an average diameter of greater
than 5 .mu.m.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit under 35 U.S.C. .sctn.
119(e) of U.S. provisional patent application Ser. No. 60/913,231,
filed Apr. 20, 2007, the content of which application is herein
incorporated by reference in its entirety.
BACKGROUND
[0002] 1. Technical Field
[0003] This description generally relates to the field of nanowire
manufacturing, and more particularly to filtering solutions
containing nanowires.
[0004] 2. Description of the Related Art
[0005] Conductive and non-conductive nanowires may be used in a
variety of applications. These high aspect ratio nano-structures
may be used to form transparent conductors, similar to those
manufactured currently using indium tin oxide (ITO). They may prove
useful in quantum computing, sensing applications, flexible
electronics and integration with biotechnology. In addition, they
may someday be used to create high speed, high density
microprocessors.
[0006] Current methods of manufacturing such nanowires often result
in polydisperse solutions containing a mixture of structures of
various shapes and sizes. These structures may include reaction
byproducts, unreacted precursors, synthesis catalysts, etc., in
addition to nanowires having the desired dimensions. In many
applications, a more uniform solution of high aspect ratio
nanowires is desirable. For example, depending on the size and
amount, low aspect ratio nano-structures may tend to worsen the
optical properties (e.g., higher haze, lower contrast ratio and
lower transmission) in transparent conductors without improving
conductivity. In addition, the solvent used in the manufacturing
process may be unsuitable for later applications of the nanowires.
For example, a solvent useful in nanowire synthesis may need to be
exchanged before applying the nanowires in a surface coating.
[0007] Unfortunately, many conventional methods of
separating/filtering particles and solvents (e.g., tortuous path
filtration, conventional filtration, chromatography, sedimentation,
centrifugation, etc.) are inefficient for or incapable of
separating high aspect ratio nanowires from other structures in a
solution.
[0008] Accordingly, there remains a need to effectively filter
nanowires from a solution containing both nanowires and other
structures. There is also a need to effectively exchange the
solvent in a solution containing nanowires.
BRIEF SUMMARY OF THE INVENTION
[0009] In one embodiment, a method of filtering a solution
containing nanowires and a first set of contaminant particles
comprises: providing the solution; generating a flow of the
solution; and filtering the solution by directing the flow through
a passage defining an aperture having a width less than at least
one dimension of the first set of contaminant particles.
[0010] In another embodiment, a nanowire filtering system
comprises: a source container for holding a solution containing
nanowires and a first set of contaminant particles; and a nanowire
filter passage communicatively coupled to the source container for
receiving the solution, the nanowire filter passage defined at
least in part by: a first plate; and a second plate disposed
adjacent the first plate with a minimum separation distance between
the first plate and the second plate of less than at least one
dimension of the first set of contaminant particles.
[0011] In yet another embodiment, a method of filtering a solution
containing nanowires comprises: providing the solution; generating
a primary flow of the solution; and filtering the solution by
directing the primary flow over a micro-structured surface
configured to filter the solution.
[0012] In another embodiment, a nanowire filtering system
comprises: a source container for holding a solution containing
nanowires; and a nanowire filter communicatively coupled to the
source container for receiving the solution, the nanowire filter
including: a rotatable tube defining a passage for the solution; a
micro-structured surface lining an inside of the rotatable tube; a
substantially helical surface adjacent the micro-structured surface
and extending at least partially into the passage; and a drive
member adapted to turn the rotatable tube.
[0013] In yet another embodiment, a nanowire filtering system
comprises: a source container for holding a solution containing
nanowires; and a nanowire filter communicatively coupled to the
source container for receiving the solution, the nanowire filter
including: an elongate channel defining a passage for the solution
flowing along a long axis, the elongate channel having a lower
surface including a plurality of parallel ridges disposed at an
angle to the long axis; wherein the plurality of parallel ridges at
least partially define a plurality of openings from the elongate
channel.
[0014] In yet another embodiment, a nanowire filtering system
comprises: a source container for holding a solution containing
nanowires; and a nanowire filter communicatively coupled to the
source container for receiving the solution, the nanowire filter
including: an elongate channel defining a passage for the solution;
and a collection chamber defined in part by an outer surface of the
elongate channel, the collection chamber communicatively coupled to
the elongate channel via a plurality of openings having an average
diameter of greater than 5 .mu.m.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0015] In the drawings, identical reference numbers identify
similar elements or acts. The sizes and relative positions of
elements in the drawings are not necessarily drawn to scale. For
example, the shapes of various elements and angles are not drawn to
scale, and some of these elements are arbitrarily enlarged and
positioned to improve drawing legibility. Further, the particular
shapes of the elements as drawn are not intended to convey any
information regarding the actual shape of the particular elements,
and have been selected solely for ease of recognition in the
drawings.
[0016] FIG. 1 is a schematic diagram of a nanowire filtering
system, according to one illustrated embodiment.
[0017] FIG. 2 is a schematic diagram of another nanowire filtering
system, according to another illustrated embodiment.
[0018] FIG. 3 is a perspective view of an example micro-structured
nanowire filter, according to one illustrated embodiment.
[0019] FIG. 4 is a longitudinal cross-section of the nanowire
filter of FIG. 3.
[0020] FIG. 5 is radial cross-section of the nanowire filter of
FIG. 3.
[0021] FIG. 6 is a perspective view of another example
micro-structured nanowire filter, according to one illustrated
embodiment.
[0022] FIG. 7 is a bottom view of the nanowire filter of FIG.
6.
[0023] FIG. 8 is a perspective view of another example
micro-structured nanowire filter, according to one illustrated
embodiment.
[0024] FIG. 9 is a front view of the nanowire filter of FIG. 8.
[0025] FIG. 10 illustrates schematically nanowires and other
nano-particles flowing in a solution over the nanowire filter of
FIG. 8.
[0026] FIG. 11 is a perspective view of another example
micro-structured nanowire filter, according to one illustrated
embodiment, with inner portions of the nanowire filter shown in
dashed lines.
[0027] FIG. 12 is a radial cross-section of the nanowire filter of
FIG. 11.
[0028] FIG. 13 is a longitudinal cross-section of the nanowire
filter of FIG. 11.
[0029] FIG. 14 is a perspective view of another example
micro-structured nanowire filter, according to one illustrated
embodiment.
[0030] FIG. 15 is a top view of the nanowire filter of FIG. 14.
[0031] FIG. 16 is an enlarged, schematic view of a bottom surface
of the nanowire filter of FIG. 14 in operation.
[0032] FIG. 17 is a perspective view of an example nanowire filter
having a narrow aperture, according to one illustrated
embodiment.
[0033] FIG. 18 is a cross-section of the nanowire filter of FIG.
17.
[0034] FIG. 19 illustrates schematically nanowires and other
particles flowing in a solution through the nanowire filter of FIG.
17.
[0035] FIG. 20 is a perspective view of an example micro-structured
nanowire filter having a narrow aperture, according to one
illustrated embodiment.
[0036] FIG. 21 is a bottom view of the nanowire filter of FIG.
20.
[0037] FIG. 22 is a perspective view of another example nanowire
filter having a narrow aperture, according to one illustrated
embodiment.
[0038] FIG. 23 is a cross-sectional, schematic view of the nanowire
filter of FIG. 22 in operation.
[0039] FIG. 24 is a top view of the nanowire filter of FIG. 22.
[0040] FIG. 25 is a perspective view of another example nanowire
filter having a narrow aperture, according to one illustrated
embodiment.
[0041] FIG. 26 is a side view of the nanowire filter of FIG.
25.
[0042] FIG. 27 is a perspective view of another example nanowire
filter having a narrow aperture, according to one illustrated
embodiment.
[0043] FIG. 28 is a side view of the nanowire filter of FIG.
27.
[0044] FIG. 29 is a perspective view of another example nanowire
filter having a plurality of narrow apertures, according to one
illustrated embodiment.
[0045] FIG. 30 is a side view of the nanowire filter of FIG.
29.
[0046] FIG. 31 is a flow diagram illustrating a method of filtering
a solution containing nanowires using a micro-structured nanowire
filter, according to one illustrated embodiment
[0047] FIG. 32 is a flow diagram illustrating another method of
filtering a solution containing nanowires using a nanowire filter
having a narrow aperture, according to another illustrated
embodiment.
DETAILED DESCRIPTION OF THE INVENTION
[0048] In the following description, certain specific details are
set forth in order to provide a thorough understanding of various
disclosed embodiments. However, one skilled in the relevant art
will recognize that embodiments may be practiced without one or
more of these specific details, or with other methods, components,
materials, etc. In other instances, well-known structures and
methodologies associated with nanowires, filters, pumps, and fluid
dynamics have not been shown or described in detail to avoid
unnecessarily obscuring descriptions of the embodiments.
[0049] Unless the context requires otherwise, throughout the
specification and claims which follow, the word "comprise" and
variations thereof, such as, "comprises" and "comprising" are to be
construed in an open, inclusive sense, that is, as "including, but
not limited to."
[0050] Reference throughout this specification to "one embodiment"
or "an embodiment" means that a particular feature, structure or
characteristic described in connection with the embodiment is
included in at least one embodiment. Thus, the appearances of the
phrases "in one embodiment" or "in an embodiment" in various places
throughout this specification are not necessarily all referring to
the same embodiment. Furthermore, the particular features,
structures, or characteristics may be combined in any suitable
manner in one or more embodiments.
[0051] As used in this specification and the appended claims, the
singular forms "a," "an," and "the" include plural referents unless
the context clearly dictates otherwise. It should also be noted
that the term "or" is generally employed in its sense including
"and/or" unless the context clearly dictates otherwise.
[0052] The headings and Abstract of the Disclosure provided herein
are for convenience only and do not interpret the scope or meaning
of the embodiments.
Description of an Exemplary Nanowire Filtering System
[0053] FIG. 1 illustrates an exemplary nanowire filtering system
10. As illustrated, the nanowire filtering system 10 comprises a
source container 12, a pump 14 and a nanowire filter 16. In one
embodiment, the components of the nanowire filtering system 10
function together to filter a solution containing nanowires,
removing undesirable contaminant particles and/or solvent from the
solution to achieve a more uniform solution of high aspect ratio
nanowires.
[0054] The source container 12 may comprise any of a variety of
containers for holding a solution containing nanowires. For
example, the source container 12 may comprise a stainless steel or
glass vessel, within which the nanowires were formed. In another
embodiment, the source container 12 may simply comprise tubing
through which the solution containing nanowires may travel.
[0055] The solution containing nanowires within the source
container 12 may comprise any liquid carrying nanowires. In one
example, the solution containing the nanowires may come directly
from a synthesis reaction prior to any formulation. The solution
containing nanowires may include, by weight, from 0.0025% to 0.1%
surfactant (e.g., a preferred range is from 0.0025% to 0.05% of
ZONYL.RTM. FSO-100), from 0.02% to 4% viscosity modifier (e.g., a
preferred range is 0.02% to 0.5% of hydroxypropyl methyl cellulose
("HPMC")), from 94.5% to 99.0% solvent and from 0.05% to 1.4%
nanowires. Representative examples of suitable surfactants include
ZONYL.RTM. FSN, ZONYL.RTM. FSO, ZONYL.RTM. FSH, TRITON.RTM. (x100,
x114, x45), DYNOL.TM. (604, 607), n-Dodecyl b-D-maltoside and
Novek. Examples of suitable viscosity modifiers include HPMC,
methyl cellulose, xanthan gum, polyvinyl alcohol, carboxy methyl
cellulose, and hydroxy ethyl cellulose. Examples of suitable
solvents include water, alcohol (e.g., isopropanol), ketones,
ether, or hydrocarbon or aromatic solvents (e.g., benzene, toluene
or xylene). In addition, the solvent may be volatile, having a
boiling point of no more than 200.degree. C., no more than
150.degree. C., or no more than 100.degree. C.
[0056] The amount of solvent can be adjusted to provide a desired
viscosity and concentration of nanowires in the solution. For
example, different pumps 14 and different nanowire filters 16 may
function optimally on different concentration solutions. In one
embodiment, however, the relative ratios of the other ingredients
may remain the same. In particular, the ratio of the surfactant to
the viscosity modifier may be kept in the range of about 80 to
about 0.01; the ratio of the viscosity modifier to the nanowires
may remain in the range of about 5 to about 0.000625; and the ratio
of the nanowires to the surfactant may be in the range of about 560
to about 5. In one embodiment, the viscosity range for the nanowire
solution may be from 1 to 100 cP.
[0057] A number of contaminant particles and other structures may
also be present in the solution, including low aspect ratio
nano-particles (e.g., short rods, discs or spheres) made from the
same material as the nanowires, as well as synthesis catalysts,
reaction byproducts and unreacted precursors. For many
applications, the presence of such contaminant particles may be
undesirable.
[0058] As used herein, a "nanowire" refers generally to a
nano-structure having a high aspect ratio (e.g., higher than 10).
Examples of non-metallic nanowires include, but are not limited to,
carbon nanotubes (CNTs), metal oxide nanowires, conductive polymer
fibers and the like. Metallic nanowires may comprise elemental
metals, metal alloys or metal compounds. Suitable metal nanowires
can be based on any metal or combinations and/or alloys of metals,
including without limitation, silver, gold, copper, nickel,
gold-plated silver, gold-silver alloys, platinum, and
palladium.
[0059] In one embodiment, at least one cross-sectional dimension of
a nanowire is less than 500 nm. In another embodiment, at least one
cross-sectional dimension of a nanowire is less than 200 nm, and in
yet another embodiment, at least one cross-sectional dimension is
less than 100 nm. As noted above, the nanowire may have an aspect
ratio (length:diameter) of greater than 10. In another embodiment,
the aspect ratio may be greater than 50. In yet another embodiment,
the aspect ratio may be greater than 100. Nanowires may have aspect
ratios anywhere in the range of 10 to 100,000.
[0060] The nanowires can be prepared by any of a number of methods.
In one embodiment, large-scale production of silver nanowires of
uniform size may be carried out according to the methods described
in, e.g., Xia, Y. et al., Chem. Mater. (2002), vol. 14, 4736-4745,
and Xia, Y. et al., Nanoletters (2003) vol. 3(7), 955-960, the
contents of which are hereby incorporated herein by reference in
their entirety.
[0061] In another embodiment, silver nanowires may be synthesized
in a batch process by the reduction of silver nitrate in propylene
glycol. The chemistry of such a process is described in co-pending
U.S. patent application Ser. No. 11/766,552, titled METHODS OF
CONTROLLING NANOSTRUCTURE FORMATIONS AND SHAPES, filed Jun. 21,
2007, the contents of which are hereby incorporated herein by
reference in their entirely.
[0062] Nanowire formation may be accomplished by the use of a
surface active polymer (e.g., polyvinylpyrrolidone ("PVP")) and
chloride (e.g., added in the form of tetra-n-butylammonium chloride
("TBAC")). The process may be carried out in an agitated, jacketed
glass reactor including glass impellers, an automated temperature
controller, a small glass feed vessel (which may also be agitated),
and a precision metering pump. Propylene glycol, PVP, and TBAC may
first be added to the reactor and heated to a target temperature
(e.g., 100.degree. C.) under agitation. Meanwhile, a solution of
silver nitrate and propylene glycol may be prepared in the small
glass feed vessel. Once the silver nitrate is fully dissolved, and
the reactor has stabilized at the target temperature, the silver
nitrate mixture may be added to the reactor at a controlled
rate
[0063] The solution may then react under agitation at atmospheric
pressure. As the reaction progresses, nano-particles may form
first, followed by nanowires that grow to the desired length and
width. Nano-particles may be indicated by an orange-brown or
brown-green color, and, as nanowires form, the mixture may become
increasingly grey and metallic in appearance. Once the target
nanowire morphology is achieved (e.g., as determined by dark field
optical microscopy), the reaction may be quenched by the rapid
addition of water, which both cools the reaction mixture and
inhibits further reaction. Reaction temperature, reaction time, and
silver nitrate addition rate may be varied to control the
dimensions of the resulting nanowires.
[0064] Following reaction, the reactor may be cleaned using a
clean-in-place system consisting of a spray ball and a persistaltic
pump. Residue from previous reactions may have adverse effects on
the synthesis process.
Example
30 kg Nanowire Synthesis
TABLE-US-00001 [0065] Raw Material Weight % Quantity Propylene
Glycol 79.0% 23700 g PVP 0.5% 150 g TBAC 0.01% 3.0 g AgNO.sub.3
0.83% 250 g Propylene Glycol 3.0% 900 g (added with AgNO.sub.3)
Deionized Water 16.7% 5000 g
[0066] Propylene glycol was first added to a 30 L glass reactor.
PVP and TBAC were also added to the glass reactor. The agitator for
the glass reactor was turned to 100 rpm, and the solution in the
glass reactor was heated to 100.degree. C. While the solution was
heating, propylene glycol and silver nitrate were premixed in a 4 L
glass feed vessel until all of the solids were dissolved. Once the
solution in the reactor reached a stable 100.degree. C., the
propylene glycol/silver nitrate solution were added to the reactor
via a metering pump. 900 mL of propylene glycol and silver nitrate
were added to the reactor at an addition rate of 45 mL/min for 20
minutes. Starting a timer at the start of the silver nitrate
addition, the solution was mixed for 4 hours in the reactor before
the heating was turned off and the reaction quenched with deionized
water.
[0067] The average length of the resulting silver nanowires was 24
.mu.m with a standard deviation of 15 .mu.m. The average width of
the resulting silver nanowires was 65 nm with a standard deviation
of 14 nm. The estimated yield of silver converted into silver
nanowires was 50 wt %.
[0068] Alternatively, nanowires may be prepared using biological
templates (or biological scaffolds) that can be mineralized. For
example, biological materials such as viruses and phages can
function as templates to create metal nanowires. In certain
embodiments, the biological templates can be engineered to exhibit
selective affinity for a particular type of material, such as a
metal or a metal oxide. More detailed descriptions of
biofabrication of nanowires can be found in, e.g., Mao, C. B. et
al., "Virus-Based Toolkit for the Directed Synthesis of Magnetic
and Semiconducting Nanowires," (2004) Science, 303, 213-217; Mao,
C. B. et al., "Viral Assembly of Oriented Quantum Dot Nanowires,"
(2003) PNAS, vol. 100, no. 12, 6946-6951; U.S. patent application
Ser. No. 10/976,179 and U.S. provisional patent application Ser.
No. 60/680,491, all of which are hereby incorporated herein by
reference in their entireties.
[0069] Regardless of the exact methodology used for nanowire
synthesis, the resulting solution may be a polydisperse solution
containing a mixture of contaminant particles and nanowires of
various shapes and sizes. For many applications, purification may
be desirable in order to achieve a more uniform solution of high
aspect ratio nanowires. In some embodiments, solubilized ion
contaminants (e.g., Cl-, Ag+, NO.sub.3-) that might lead to
nanowire degradation should also be removed. In addition, exchange
of the solvent may be desirable based on the particular application
for the nanowire solution.
[0070] In one embodiment, the source container 12 may serve as the
reactor within which the nanowires are formed. However, in other
embodiments, a solution containing nanowires may be generated in
another container/reactor and be subsequently transferred to the
source container 12. In yet another embodiment, the solution
containing nanowires need not comprise the solution within which
the nanowires were originally formed. Thus, the nanowire filtering
system 10 may be used to filter any solution containing
nanowires.
[0071] As illustrated, the nanowire filtering system 10 may include
a pump 14 to generate a flow of the solution containing nanowires
from the source container 12 to the nanowire filter 16. The pump 14
may comprise any of a variety of liquid pumps. For example, the
pump 14 may comprise a bellows pump, a centrifugal pump, a
diaphragm pump, a drum pump, a flexible liner/impeller pump, a gear
pump, a peristaltic pump, a piston pump, a progressing cavity pump,
a rotary lobe pump, a rotary vane pump, etc.
[0072] In another embodiment, the nanowire filtering system 10 may
not include a pump. For example, in one embodiment, a flow of the
solution containing nanowires may be generated by gravity. In
another embodiment, the pump 14 may be incorporated into the
nanowire filter 16.
[0073] The nanowire filter 16 may comprise any of a variety of
filters configured to separate nanowires from contaminant particles
and other nano-structures. The nanowire filter 16 may be further
configured to separate the nanowires from a solvent in order to
facilitate a solvent exchange. In one embodiment, the nanowire
filter 16 may be configured to yield a retentate 18, which
comprises a more uniform solution containing nanowires, and a
filtrate (not shown), which may comprise solvent and/or the
contaminant particles filtered from the solution. The retentate 18
may have a higher weight percentage of nanowires than the flow of
solution 20 entering the nanowire filter 16. As discussed below
with reference to FIGS. 3-30, the nanowire filter 16 may include a
plurality of micro-structures and/or may include one or more narrow
apertures configured to filter the solution. The nanowire filter 16
may also, in some embodiments, comprise a plurality of nanowire
filters arranged in parallel or in series to filter the solution
containing nanowires.
[0074] In one embodiment, the nanowire filter 16 may filter out
nanowires having aspect ratios below a certain threshold. For
example, in one embodiment, the nanowire filter 16 may generally
filter out nanowires having aspect ratios lower than 100. The
aspect ratio targeted by a particular nanowire filter 16 may be
selected based upon an application for the solution.
[0075] In one embodiment, the retentate 18 may be collected in a
container (not shown) for subsequent processing or use. For
example, in one embodiment, the retentate 18 may be added to a
solvent useful in coating formulations. In another embodiment, as
illustrated in FIG. 2, a nanowire filtering system 22 may
recirculate the retentate 18 from the nanowire filter 16 back to
the source container 12 for further filtering. In such an
embodiment, the filtering and subsequent recirculating of the
solution containing nanowires may continue for a predetermined time
period, or until the solution containing nanowires has reached a
desired purity. In order to maintain a viscosity of the solution or
in order to effect a solvent exchange, solvent (not shown) may also
be added to the nanowire filtering system 22 (e.g., at the source
container 12) as the retentate 18 is recirculated. In one
embodiment, the filtering, recirculating, and addition of a new
solvent may continue until the solution containing nanowires
achieves a predetermined concentration of the new solvent.
Description of an Exemplary Micro-Structured Nanowire Filter
[0076] FIG. 3 is a perspective view of a micro-structured nanowire
filter 300, which may be used in the nanowire filtering system 10
or the nanowire filtering system 22. FIGS. 4 and 5 present
longitudinal and radial cross-sections, respectively, of the
nanowire filter 300 to facilitate an understanding of its inner
structure. As illustrated, the nanowire filter 300 comprises an
elongate channel 302 having an entrance 308 and an exit 310 and
defining a passage for a primary flow (designated by the arrow 301)
of the solution containing nanowires. The elongate channel 302 may
include a micro-structured surface between the entrance 308 and
exit 310 having a plurality of openings 306 defined therethrough.
In one embodiment, the elongate channel 302 is surrounded by a
plurality of collection chambers 304 communicatively coupled to the
elongate channel 302 by the plurality of openings 306. The nanowire
filter 300 may, of course, be formed from a variety of different
materials, including metallic and non-metallic materials, and may
be coupled to the rest of the nanowire filtering system 10 by any
of a variety of fluid connectors, tubes and/or conduits.
[0077] The plurality of openings 306 through the surface of the
elongate channel 302 are micro-structures configured to filter the
solution. The terms micro-structures and micro-structured may
reference any small structures formed in, on or through a surface
that may interfere with a fluid flow. For example, micro-structures
may refer to structures having at least one dimension less than 1
cm. In the illustrated embodiment, the micro-structures comprise
the plurality of openings 306. However, in other embodiments,
micro-structures may comprise a plurality of niches, valleys,
detents, peaks, protrusions, etc. Other examples of
micro-structures and micro-structured surfaces are presented with
reference to FIGS. 6-16.
[0078] The size, arrangement and configuration of the openings 306
may be varied to filter different contaminant particles. In one
embodiment, the size of the openings 306 may be chosen based at
least in part on the desired length/diameter/aspect ratio of the
nanowires, the size/aspect ratio of the contaminant particles that
should be filtered from the solution as well as a viscosity and
flow rate of the solvent. For example, the openings 306 may have an
average diameter greater than 5 .mu.m because the expected filtrate
may have a diameter up to approximately 5 .mu.m. In another
embodiment, the openings 306 may have an average diameter greater
than 10 .mu.m. As the diameter of the openings 306 increases, a
greater secondary flow may be generated through the openings 306,
and the nanowire filter 300 may filter out more contaminant
particles and solvent on each pass. However, with larger openings
306, the nanowire filter 300 may also become less selective, and
more nanowires may be lost in the filtrate.
[0079] In one embodiment, the elongate channel 302 may be
approximately 3 cm in diameter, and approximately 50 cm long. In
other embodiments, the length and diameter of the elongate channel
302 may be varied. As the elongate channel 302 is lengthened or its
diameter made smaller, a greater amount of filtrate may be
separated from the primary flow of solution as the solution passes
through the nanowire filter 300. However, a greater quantity of
nanowires may also be lost in the filtrate. The length, diameter
and geometry of the elongate channel 302 may therefore be varied to
achieve desired characteristics for the nanowire filter 300.
[0080] In one embodiment, as illustrated, the elongate channel 302
may comprise a cylindrical passage, and the openings 306 may extend
along the entire surface of this cylindrical passage. Of course, in
other embodiments, other configurations are possible. The elongate
channel 302 may have a variety of shapes, and the openings 306 may
be formed on only a portion of the channel's surface. For example,
in one embodiment, the openings 306 may be formed only along a
bottom half of the surface of the elongate channel 302, as the
filtrate may preferentially flow through these openings 306 by
gravity. In another embodiment, the openings 306 may be formed
along only a portion of the entire length of the elongate channel
302.
[0081] As illustrated, eight collection chambers 304 are defined at
least in part by an outer surface of the elongate channel 302. The
eight collection chambers 304 may be separated by radially
extending fins extending from the outer surface of the elongate
channel 302 to an outer wall 312 of the nanowire filter 300. Of
course, in other embodiments, the collection chambers 304 may be
configured differently. In one embodiment, more or fewer collection
chambers 304 may be formed around the elongate channel 302, and
they may have different geometries. In another embodiment, the
collection chambers 304 need not be integrally formed with the
elongate channel 302. For example, the elongate channel 302 may be
suspended over one or more collection chambers, and, in operation,
the filtrate emerging from the openings 306 of the elongate channel
302 may fall into the collection chambers.
[0082] During operation, a primary flow 301 of the solution may
pass through the entrance 308, through the elongate channel 302 and
emerge from the exit 310 as retentate 18. Meanwhile, the plurality
of openings 306 may create a secondary flow of at least a portion
of the solution, i.e., the filtrate, through the plurality of
openings 306 and into the collection chambers 304. In one
embodiment, the collection chambers 304 may transfer the secondary
flow to a filtrate container (not shown).
[0083] Although the diameter of the nanowires may be equal to or
smaller than the diameter of the filtered contaminant particles,
the nanowires (due to their high aspect ratio) may substantially
align with the primary flow 301 passing through the elongate
channel 302, and this alignment may inhibit or effectively prevent
the nanowires from passing through the plurality of openings 306.
In one embodiment, the primary flow 301 of the solution through the
elongate channel 302 may be greater than the secondary flow through
the plurality of openings 306 into the collection chambers 304 to
take advantage of this alignment. For example, in one embodiment,
the primary flow 301 may be at least 100 times greater than the
secondary flow of the solution. This relatively high flow rate
through the elongate channel 302 may help to align the nanowires
with the primary flow 301 and prevent the nanowires from
inadvertently passing through the plurality of openings 306.
[0084] In one embodiment, if the diameter of the openings 306 is
increased, the primary flow rate may be correspondingly increased
to help prevent nanowires from slipping through the enlarged
openings 306. Thus, the size of the openings 306 and the primary
flow rate through the elongate channel 302 may be varied in
different embodiments of the nanowire filter 300 in order to change
its filtering characteristics.
Description of Another Exemplary Micro-Structured Nanowire
Filter
[0085] FIG. 6 is a perspective view of another micro-structured
nanowire filter 600 that operates similarly to the nanowire filter
300 of FIGS. 3-5. FIG. 7 is a bottom view of the nanowire filter
600. In one embodiment, the nanowire filter 600 comprises an
elongate channel 606 having an entrance 608 and an exit 610 and
defining a passage for a primary flow (designated by the arrow 601)
of the solution containing nanowires. The elongate channel 606 may,
in turn, be defined at least in part by a micro-structured surface
602 comprising a plurality of openings 604.
[0086] In one embodiment, the openings 604 may have an average
diameter of approximately 5 .mu.m, and the elongate channel 606 may
be approximately 50 cm in length. Of course, as described above
with respect to the nanowire filter 300, the size and shape of the
openings 604, the size and shape of the elongate channel 606, and
the primary flow rate of the solution may be varied to achieve
desired filtering characteristics. In addition, an average height
of the solution passing over the micro-structured surface 602 may
also be varied to achieve the desired filtering
characteristics.
[0087] In operation, a primary flow 601 of the solution may pass
through the entrance 608, through the elongate chamber 606 and
emerge from the exit 610 as retentate 18. Meanwhile, the plurality
of openings 604 may create a secondary flow of filtrate out from
the elongate chamber 606. The nanowires in the solution may
substantially align with the primary flow 601 passing through the
elongate chamber 606, and this alignment may inhibit or effectively
prevent the nanowires from passing through the plurality of
openings 604.
[0088] In one embodiment, a trough or another type of collection
chamber (not shown) may be disposed beneath the micro-structured
surface 602 to collect the filtrate. In another embodiment, the
elongate chamber 606 may be coupled to at least one collection
chamber in an arrangement similar to that of the nanowire filter
300.
Description of Yet Another Exemplary Micro-Structured Nanowire
Filter
[0089] FIG. 8 is a perspective view, and FIG. 9 is a front view of
another example micro-structured nanowire filter 800. As
illustrated, the nanowire filter 800 comprises a frame 802 defining
a generally V-shaped trough between an entrance 804 and an exit 806
that may direct a primary flow (designated by the arrow 801) of the
solution containing nanowires over a micro-structured surface 808
supported by the frame 802. The micro-structured surface 808 may,
in one embodiment, comprise a plurality of surface protrusions and
pores.
[0090] In one embodiment, the frame 802 may comprise a metallic
plate bent into the desired V-shape. In other embodiments, the
frame 802 may comprise other materials, such as plastics. The frame
802 may also have other shapes for directing the primary flow 801
of the solution. For example, the frame 802 may define a
cylindrical or a U shape.
[0091] In one embodiment, the micro-structured surface 808 may be
defined by filter paper. The filter paper may be any type of filter
paper configured to filter the solution containing nanowires. For
example, the filter paper may have a porosity of greater than 5
.mu.m because the expected filtrate may have a diameter up to
approximately 5 .mu.m. In another embodiment, the filter paper may
have a porosity of greater than 10 .mu.m. The porosity of the
filter paper may be varied, as described above to achieve
particular filtering characteristics.
[0092] In other embodiments, the micro-structured surface 808 may
be defined by a more permanent filtering substrate. For example, an
inner surface of the frame 802 itself may have small protrusions
defined thereon.
[0093] In operation, a primary flow 801 of the solution may pass
through the entrance 804, over the micro-structured surface 808 and
emerge from the exit 806 as retentate 18. More compact contaminant
particles, which may tend to have lower drag in a flowing solution,
may be pulled by gravity towards the micro-structured surface 808,
where they may be trapped by the micro-structures. Of course, more
massive contaminant particles may sediment more quickly out of the
solution, while smaller contaminant particles may sediment more
slowly. The dimensions and arrangement of the nanowire filter 800
may be configured to filter different sizes of the contaminant
particles as desired. Meanwhile, the nanowires in the solution may
substantially align with the primary flow 801, and this alignment
may inhibit or effectively prevent the nanowires from being trapped
by the micro-structured surface 808.
[0094] In one embodiment, a flow rate of the primary flow 801 of
the solution may be monitored and controlled to ensure that the
nanowire filter 800 is, indeed, preferentially filtering out the
more compact, low aspect ratio particles. If the flow rate is too
high, even the low aspect ratio contaminant particles may emerge as
retentate 18. However, if the flow rate is too low, high aspect
ratio nanowires may settle out of the solution onto the bottom of
the nanowire filter 800.
[0095] A schematic view of the microscopic filtering process is
illustrated in FIG. 10. As shown, the nanowires 1002 may be
generally aligned with the primary flow 801 of the solution while
low aspect ratio contaminant particles 1006 are trapped by the
micro-structures 1008.
[0096] As may be understood with reference to FIG. 10, the nanowire
filter 800 may trap filtrate within the micro-structures 1008. As a
result, it may be desirable to occasionally clean the
micro-structured surface 808 to maintain the filtering efficiency
of the nanowire filter 800. In one embodiment, the primary flow 801
of the solution may be stopped, and a separate cleaning solution
passed over the micro-structured surface 808 to eliminate the
filtrate. In another embodiment, the micro-structured surface 808
may be occasionally replaced. For example, new filter paper may
replace the old filter paper. Other methods of cleaning the
micro-structured surface 808 may be used in other embodiments.
[0097] The micro-structured surface 808 may be cleaned
periodically, according to some time interval, or may be cleaned
after a certain amount of solution has been filtered. In another
embodiment, the micro-structured surface 808 may be cleaned when
the performance of the nanowire filter 800 has degraded by a
certain amount.
Description of Yet Another Exemplary Micro-Structured Nanowire
Filter
[0098] FIG. 11 is a perspective view of another example
micro-structured nanowire filter 1100, with interior portions of
the nanowire filter 1100 illustrated in dashed lines. FIGS. 12 and
13 present radial and longitudinal cross-sections, respectively, of
the nanowire filter 1100 to facilitate a greater understanding of
its inner structure. As illustrated, the nanowire filter 1100
comprises a rotatable tube 1102 having an entrance 1110 and an exit
1112 and defining a passage for a primary flow (designated by the
arrow 1101) of the solution containing nanowires. A
micro-structured surface 1108 lines an inside of the rotatable tube
1102. The rotatable tube 1102 may also have disposed therein a
substantially helical element 1104 and may be coupled to a drive
member 1106 for rotating the rotatable tube 1102 about a
longitudinal axis.
[0099] The rotatable tube 1102 may be formed from any metallic or
non-metallic materials. The size and shape of the rotatable tube
1102 may also be varied to achieve desired filtering
characteristics.
[0100] In one embodiment, the micro-structured surface 1108 lining
the rotatable tube 1102 may comprise filter paper. The filter paper
may be any type of filter paper configured to filter the solution.
For example, the filter paper may have a porosity of greater than 5
.mu.m because the expected filtrate may have a diameter up to
approximately 5 .mu.m. In another embodiment, the filter paper may
have a porosity of greater than 10 .mu.m. The porosity of the
filter paper may be varied, as described above. In another
embodiment, the micro-structured surface 1108 may be defined by an
inner surface of the rotatable tube 1102 itself. For example, the
rotatable tube 1102 may include a plurality of openings (not shown)
that comprise the micro-structures.
[0101] In one embodiment, the substantially helical element 1104
may be arranged adjacent the micro-structured surface 1108 and may
comprise a strip of fluid impermeable material wound around an
interior of the rotatable tube 1102. The substantially helical
element 1104 may be formed integrally with or may be separate from
the rotatable tube 1102. The substantially helical element 1104 is
illustrated as extending only a short way into the passage defined
by the rotatable tube 1102. However, in other embodiments, the
substantially helical element 1104 may extend much further. For
example, in some embodiments, the substantially helical element
1104 may have a height approximately equal to a radius of the
rotatable tube 1102.
[0102] The drive member 1106 may comprise any appropriate
combination of a motor and fittings adapted to turn the rotatable
tube 1102. In one embodiment, the drive member 1106 may be
configured to turn the rotatable tube 1102 at a variable angular
velocity.
[0103] In operation, in order to drive a primary flow 1101 of the
solution containing nanowires through the entrance 1110 and out the
exit 1112 of the rotatable tube 1102, the drive member 1106 may
turn the rotatable tube 1102 in a counter-clockwise direction (from
the vantage point of FIG. 12). The primary flow 1101 of the
solution may be maintained at a level lower than a height of the
substantially helical element 1104, such that the solution cannot
pass over the barrier represented by the substantially helical
element 1104. As the rotatable tube 1102 turns in a
counter-clockwise direction, the solution may be driven through the
rotatable tube 1102 by the substantially helical element 1104, and
thus, a flow rate of the solution may be controlled by the drive
member 1106.
[0104] As described above with reference to FIG. 10, low aspect
ratio contaminant particles, which may tend to have lower drag in a
flowing solution, may be pulled by gravity towards the
micro-structured surface 1108, where they may be trapped by
micro-structures. Meanwhile, nanowires in the solution may
substantially align with the primary flow 1101, and this alignment
may inhibit or effectively prevent the nanowires from being trapped
by the micro-structured surface 1108.
[0105] It may be desirable to occasionally clean the
micro-structured surface 1108 to maintain the filtering efficiency
of the nanowire filter 1100. In one embodiment, the primary flow of
the solution may be stopped, and a separate cleaning solution
passed over the micro-structured surface 1108 to eliminate the
filtrate. Alternatively, the micro-structured surface 1108 may be
occasionally replaced. For example, new filter paper may replace
the old filter paper. Other methods of cleaning the
micro-structured surface 1108 may be used in other embodiments.
[0106] The micro-structured surface 1108 may be cleaned
periodically, according to some time interval, or after a certain
amount of solution has been filtered. In another embodiment, the
micro-structured surface 1108 may be cleaned when the performance
of the nanowire filter 1100 has degraded by a certain amount.
Description of Another Exemplary Micro-Structured Nanowire
Filter
[0107] FIG. 14 is a perspective view, and FIG. 15 is a top view of
another micro-structured nanowire filter 1400. As illustrated, the
nanowire filter 1400 may include an elongate channel 1402 having an
entrance 1410 and an exit 1412 and defining a passage for a primary
flow (designated by the arrow 1401) of the solution containing
nanowires along a long axis 1404. The elongate channel 1402 may
further include a micro-structured, bottom surface 1406 having a
plurality of parallel ridges oriented at an angle to the long axis
1404.
[0108] The elongate channel 1402 may be integral with or may be
formed separately from the micro-structured surface 1406. In one
embodiment, walls 1414, 1416 of the elongate channel 1402 as well
as the micro-structured surface 1406 may be formed from any of a
variety of metallic or non-metallic materials. Although illustrated
as generally U-shaped, the elongate channel 1402 may have any of a
number of other shapes and configurations. In one embodiment, the
elongate channel 1402 may be fully enclosed, forming a generally
rectangular cross-sectional shape.
[0109] The micro-structures of the bottom surface 1406 may comprise
a plurality of parallel ridges (and corresponding valleys) that
form a non-right angle with the long axis 1404. In one embodiment,
the ridges may at least partially define a plurality of fluid
passages ending at a plurality of secondary openings 1408 from the
elongate channel 1402. The plurality of secondary openings 1408
may, in one embodiment, allow filtrate to exit the elongate channel
1402. Of course, in other embodiments, the ridges may be configured
differently. For example, they need not be parallel, and, in one
embodiment, the ridges may be oriented at a right angle to the long
axis 1404.
[0110] The parallel ridges may also be separated by a distance
greater than 5 .mu.m because the expected filtrate may have a
diameter up to approximately 5 .mu.m. In another embodiment, the
parallel ridges may be separated by a distance greater than 10
.mu.m. A cross-section of the valleys formed by the ridges may be
approximately square, such that the valleys are deeper than 5 .mu.m
or 10 .mu.m in respective embodiments. The size and shape of the
ridges, the size and shape of the elongate channel 1402, and the
primary flow rate of the solution may be varied to achieve desired
filtering characteristics.
[0111] Turning to FIG. 16, an enlarged, schematic view of the
micro-structured surface 1406 of the nanowire filter 1400 is
illustrated in operation. As shown, a primary flow 1401 of the
solution may flow across the micro-structured surface 1406, and
thereby across the plurality of parallel ridges. The parallel
ridges may then create a plurality of secondary flows 1604, as
filtrate from the solution is diverted by the parallel ridges
through the secondary openings 1408. These secondary flows 1604
containing filtrate may or may not be collected in collection
chambers (not shown). Since the filtrate may thus be diverted away
from the nanowire filter 1400, the nanowire filter 1400 may remain
relatively clear of the filtrate. Thus, there may be a reduced need
to clean the nanowire filter 1400.
[0112] As discussed above, the plurality of parallel ridges may
filter low aspect ratio contaminant particles from the nanowires
due to the different drag characteristics of these particles in a
fluid flow.
Description of an Exemplary Nanowire Filter Having a Narrow
Aperture
[0113] FIG. 17 is a perspective view, and FIG. 18 is a
cross-section of a nanowire filter 1700 having a narrow aperture
1708, which filter may be used in the nanowire filtering system 10
or the nanowire filtering system 22. The nanowire filter 1700 may
comprise a first plate 1702 and a second plate 1704 disposed
adjacent the first plate 1702. The first and second plates 1702,
1704 may at least partially define a passage 1706 extending through
the filter, the passage 1706 having an entrance 1710 and an exit
1712. In one embodiment, the passage 1706 defines an aperture 1708
having a width W less than at least one dimension of a set of
contaminant particles.
[0114] The nanowire filter 1700 may be formed from a variety of
different materials. In one embodiment, the nanowire filter 1700
may comprise a molded plastic. In another embodiment, the nanowire
filter 1700 may be formed from stainless steel. In yet another
embodiment, the nanowire filter 1700 may comprise stainless steel
first and second plates 1702, 1704 separated by relatively hard
micro- or nano-particles (e.g., silica). In one embodiment, a
plurality of such plates may be stacked one upon the other in order
to achieve a high flow rate through the nanowire filter 1700.
[0115] In one embodiment, the first plate 1702 and the second plate
1704 are substantially parallel and define a separation distance
between them of less than at least one dimension of a set of
contaminant particles. Since the separation distance between the
two plates 1702, 1704 is substantially invariant, the aperture 1708
may coincide with the entrance 1710 to the nanowire filter
1700.
[0116] The aperture 1708 may have a width W selected to filter out
the set of contaminant particles having at least one dimension
greater than the width. For example, in one embodiment, the
aperture 1708 may have a width W less than 2 .mu.m, in order to
filter out particles having a diameter greater than 2 .mu.m. In
another embodiment, the aperture 1708 may have a width W less than
1 .mu.m, or less than 0.5 .mu.m, in order to filter out contaminant
particles having greater dimensions. As the width W of the aperture
1708 is decreased, the flow through the nanowire filter 1700 may
also decrease, and the nanowire filter 1700 may filter out more
contaminant particles. The width W of the aperture 1708 may be
varied in different embodiments to filter out different sets of
contaminant particles, while allowing nanowires to pass through the
filter 1700 unimpeded.
[0117] The length L of the aperture 1708 may also be varied to pass
more or less solution. In one embodiment, a very long aperture 1708
may be used to enable a greater flow of solution through the
passage 1706 of the nanowire filter 1700.
[0118] In general, as with the micro-structured nanowire filters
described above, nanowires in the solution may substantially align
with the flow through the passage 1706 of the nanowire filter 1700.
Thus, as the nanowires approach the aperture 1708, they may present
a relatively small cross-section. For example, in one embodiment,
the nanowires may have an average diameter ranging from 20 to 200
nm. Although, the nanowires may be as long as, or longer than, the
width W, the narrow cross-section of the nanowires may enable the
nanowires to align with the flow and pass through the nanowire
filter 1700.
[0119] A schematic view of the nanowire filter 1700 in operation is
illustrated in FIG. 19. The first plate 1702 is illustrated
transparently, in order to schematically show the nanowires 1902 in
the solution aligned with a flow 1906 through the nanowire filter
1700. Meanwhile, low aspect ratio contaminant particles 1904 (which
may, for example, have a diameter approximately equal to a length
of the nanowires) may be "captured" at the aperture 1708, unable to
pass through the nanowire filter 1700 with the rest of the
retentate 18.
[0120] Although the nanowire filter 1700 is illustrated as
comprising two substantially parallel plates forming an aperture
1708 sized to prevent large diameter contaminant particles from
passing therethrough, other configurations are, of course,
possible. In one embodiment, the nanowire filter 1700 may include
any other aperture shape (e.g., circular, elliptical, triangular)
having at least one width less than at least one dimension of a set
of contaminant particles. In another embodiment, the nanowire
filter 1700 may comprise a plurality of cylindrical passages, each
of the passages having a diameter less than the at least one
dimension of the set of contaminant particles.
[0121] As illustrated in FIG. 19, the nanowire filter 1700 may
build up filtrate at the aperture 1708, which may eventually become
clogged by these large contaminant particles. As a result, it may
be desirable to "de-clog" the filter 1700 by occasionally removing
these particles from the aperture 1708 in order to maintain the
filtering efficiency of the nanowire filter 1700. In one
embodiment, the primary flow of the solution (designated by the
arrow 1906) may be occasionally stopped and the nanowire filter
1700 removed for cleaning. In another embodiment, the primary flow
1906 of the solution may be stopped, and a reverse flow (not shown)
of a liquid generated through the passage 1706 in order to dislodge
the larger particles from the aperture 1708. Indeed, in one
embodiment, a reverse flow of the solution itself may be
periodically generated through the passage 1706 in order to
dislodge the larger particles from the aperture 1708. This reverse
flow may also be coupled with an external cleaning, ultrasonic
energy, or another mechanism to ensure that the filtered
contaminant particles are well-separated from the aperture 1708 and
do not immediately re-clog the nanowire filter 1700. Although the
solution may flow through the nanowire filter 1700 in both
directions, a net flow may be directed from the entrance 1710 to
the exit 1712 of the nanowire filter 1700.
[0122] In one embodiment, the nanowire filter 1700 may be
de-clogged periodically, according to some time interval. In
another embodiment, the nanowire filter 1700 may be de-clogged
after a certain amount of solution has been filtered. In yet
another embodiment, the nanowire filter 1700 may be de-clogged when
the performance of the nanowire filter 1700 (as measured, for
example, by a flow rate of the primary flow 1906 through the
nanowire filter 1700) has degraded by a certain amount.
Description of an Exemplary Micro-Structured Nanowire Filter Having
a Narrow Aperture
[0123] FIG. 20 is a perspective view, and FIG. 21 is a bottom view
of another nanowire filter 2000 having a narrow aperture 2008
defined at least in part by a top plate 2002 and a bottom plate
2004. The nanowire filter 2000 may be configured similarly to the
nanowire filter 1700, except that the bottom plate 2004 may further
include a plurality of openings 2010. As described above with
reference to the other micro-structured nanowire filters, the
plurality of openings 2010 may be considered micro-structures. In
other embodiments, different micro-structures may be used in
conjunction with a narrow aperture to form other nanowire
filters.
[0124] In operation, the nanowire filter 2000 may filter out larger
contaminant particles at the aperture 2008 and may filter out
smaller contaminant particles via the openings 2010 in the bottom
plate 2004. Thus, the nanowire filter 2000 may effectively combine
the filtering capabilities of the nanowire filter 1700 with the
filtering capabilities of, for example, the nanowire filter 600.
The flow rate, solution composition and dimensions of the
components of the nanowire filter 2000 may be varied to optimize
one or both of these filtering capabilities.
Description of Another Exemplary Nanowire Filter Having a Narrow
Aperture
[0125] FIG. 22 is a perspective view of another nanowire filter
2200 having a narrow aperture 2208. FIGS. 23 and 24 illustrate a
cross-sectional view and a top view of the nanowire filter 2200,
respectively. The nanowire filter 2200 may comprise a top plate
2202 and a bottom plate 2204 disposed adjacent the top plate 2202.
The top plate 2202 and the bottom plate 2204 may at least partially
define a passage 2216 extending through the nanowire filter 2200.
In one embodiment, the passage 2216 defines at least one aperture
2208 having a width less than at least one dimension of a set of
contaminant particles.
[0126] The top plate 2202 may further include an entrance 2212
therethrough. The entrance 2212 may define an opening through which
a primary flow (designated by the arrows 2201) of the solution may
be directed. A conduit 2214 for the solution may be coupled to the
entrance 2212 in order to guide a primary flow 2201 of the solution
from the source container 12 into the nanowire filter 2200.
[0127] The nanowire filter 2200, like the nanowire filter 1700, may
be formed from a variety of different materials. In one embodiment,
the nanowire filter 2200 may comprise a molded plastic. In another
embodiment, the nanowire filter 2200 may be formed from stainless
steel.
[0128] In the illustrated embodiment, the top plate 2202 and the
bottom plate 2204 are substantially parallel and define a
separation distance between them of less than at least one
dimension of a set of contaminant particles. The aperture 2208
having a width W may coincide with the entrance 2212 of the
nanowire filter 2200 and may have a generally cylindrical shape, as
illustrated by the dashed lines of FIG. 23. As described above, the
size and configuration of the aperture 2208 and the position of the
plates 2202, 2204 may be varied to filter out particular
contaminant particles from the solution.
[0129] In operation, as best illustrated in FIG. 23, the solution
containing nanowires may flow outwards from the entrance 2212
between the two plates 2202, 2204. In a manner similar to that
described above with reference to FIG. 17, nanowires in the
solution may align with the primary flow 2201 through the nanowire
filter 2200, while large, low aspect ratio, contaminant particles
may be prevented from passing radially outwards between the top and
bottom plates 2202, 2204. Thus, the nanowire filter 2200 may build
up filtrate at the aperture 2208. As described above with reference
to FIG. 17, the nanowire filter 2200 may be occasionally de-clogged
to maintain its filtering efficiency.
Description of Another Exemplary Nanowire Filter Having a Narrow
Aperture
[0130] FIG. 25 is a perspective view, and FIG. 26 is a side view of
another nanowire filter 2500 having a narrow aperture 2508. The
nanowire filter 2500 may comprise a first plate 2502 and a second
plate 2504 disposed adjacent the first plate 2502. The first and
second plates 2502, 2504 may converge, such that a passage 2506
extending through the nanowire filter 2500 may narrow between an
entrance 2510 and an exit 2512. In one embodiment, the aperture
2508 may be defined at the exit 2512 and may have a width less than
at least one dimension of a set of contaminant particles.
[0131] The nanowire filter 2500 may be configured and may function
similarly to the nanowire filter 1700. In addition, the size and
configuration of the components of the nanowire filter 2500 may be
varied depending on the desired filtering characteristics.
[0132] In operation, as large contaminant particles travel along
the passage 2506 between the entrance 2510 and the exit 2512, each
particle may be captured at that portion of the passage 2506 having
a width approximately equal to that particle's diameter. Thus, for
example, if the entrance 2510 of the nanowire filter 2500 has a
width of 10 .mu.m and the exit 2512 has a width of 1 .mu.m, then 5
.mu.m particles may be captured somewhere near the middle of the
passage 2506, and 1.1 .mu.m particles may be captured very close to
the exit 2512.
[0133] As a result, unlike the nanowire filter 1700, which may
capture all filtered particles at the entrance 1710, the nanowire
filter 2500 may filter out contaminant particles along its entire
length. Thus, it may take longer for the nanowire filter 2500 to
become clogged.
Description of Another Exemplary Nanowire Filter Having a Narrow
Aperture
[0134] FIG. 27 is a perspective view, and FIG. 28 is a side view of
another nanowire filter 2700 having a narrow aperture 2708. The
nanowire filter 2700 may comprise a first plate 2702, a second
plate 2704 disposed adjacent the first plate 2702, and a passage
2706 defined between the two plates 2702, 2704. The passage 2706
may define at least one aperture 2708 approximately halfway through
having a width less than at least one dimension of a set of
contaminant particles.
[0135] The nanowire filter 2700 may have an aperture 2708 arranged
substantially anywhere along the passage 2706 defined between the
two plates 2702, 2704, and the plates 2702, 2704 may have a variety
of different shapes and configurations. The nanowire filter 2700
may function generally similarly to the nanowire filter 2500
described above.
Description of an Exemplary Nanowire Filter Having Narrow
Apertures
[0136] FIG. 29 is a perspective view, and FIG. 30 is a side view of
another nanowire filter 2900 having a plurality of narrow apertures
2908, 2928 and 2938.
[0137] In one embodiment, the nanowire filter 2900 may comprise a
first plate 2902 and a second plate 2904 disposed adjacent the
first plate 2902. The two plates 2902, 2904 may at least partially
define a passage having an entrance 2910 and an exit 2912, and may
at least partially define an aperture 2908 having a width less than
at least one dimension of a first set of contaminant particles
(e.g., 2 .mu.m).
[0138] The nanowire filter 2900 may further comprise a third plate
2922 and a fourth plate 2924 disposed adjacent the third plate
2922. The two plates 2922, 2924 may at least partially define a
second passage having a second entrance 2926 and a second exit
2927, and may at least partially define a second aperture 2928
having a width less than at least one dimension of a second set of
contaminant particles (e.g., 1 .mu.m). As illustrated, the second
set of contaminant particles may have at least one dimension
smaller than the at least one dimension of the first set of
contaminant particles.
[0139] Finally, the nanowire filter 2900 may comprise a fifth plate
2932 and a sixth plate 2934 disposed adjacent the fifth plate 2932.
The two plates 2932, 2934 may at least partially define a third
passage having a third entrance 2936 and a third exit 2937, and may
at least partially define a third aperture 2938 having a width less
than at least one dimension of a third set of contaminant particles
(e.g., 0.5 .mu.m). As illustrated, the third set of contaminant
particles may have at least one dimension smaller than the at least
one dimension of the second set of contaminant particles.
[0140] In other embodiments, more or fewer apertures of various
sizes may be used to filter out particular sets of contaminant
particles.
[0141] In operation, the nanowire filter 2900 may function
generally similarly to the nanowire filter 2500 described above.
For example, the nanowire filter 2900 may filter out contaminant
particles having diameters larger than 2 .mu.m at the first
aperture 2908, other contaminant particles having diameters between
1 and 2 .mu.m at the second aperture 2928 and still more
contaminant particles having diameters between 0.5 and 1 .mu.m at
the third aperture 2938.
Description of an Exemplary Method of Filtering a Solution
Containing Nanowires
[0142] FIG. 31 illustrates a flow diagram for a method 3100 of
filtering a solution containing nanowires using a micro-structured
nanowire filter, according to one embodiment. This method 3100 will
be discussed primarily in the context of the nanowire filter 300
incorporated into the nanowire filtering system 10. However, it may
be understood that the acts disclosed herein may also be executed
using a variety of other micro-structured nanowire filters (e.g.,
nanowire filters 600, 800, 1100, 1400, and 2000), in accordance
with the described method.
[0143] The method begins at 3102, when a solution containing
nanowires is provided. As discussed above, in one embodiment, the
solution containing nanowires may comprise the solution within
which the nanowires were formed. In other embodiments, the solution
within which the nanowires were formed may have already undergone a
variety of processing and/or filtering acts.
[0144] The solution containing nanowires may comprise a
polydisperse solution including a variety of particles and
nano-structures in addition to the desired nanowires. A variety of
different solutions may be filtered in different embodiments,
including different percentages of nanowires, different solvents
and additives, different shapes and types of low aspect ratio
particles, etc. In one embodiment, based on these variable
characteristics of the solution, the nanowire filtering system 10
and, in particular, the nanowire filter 300 may be configured
differently.
[0145] At 3104, a primary flow of the solution is generated. The
primary flow of the solution may be generated by any of a variety
of mechanisms. In one embodiment, a pump 14, as illustrated in FIG.
1, may be used to generate the primary flow of the solution. In
another embodiment, the primary flow of the solution may be
generated by gravity from the source container 12. In yet another
embodiment, a pressure differential (e.g., a source pressurized
tank) may be used to generate the primary flow of the solution. A
flow rate of this primary flow may also be varied in different
embodiments, depending on the configuration of the nanowire filter
300, the source container 12, a pump 14, tubes and conduits
connecting these components, a target filtration rate, etc.
[0146] At 3106, the solution is filtered by directing the primary
flow over a micro-structured surface configured to filter the
solution. The primary flow may be directed over the
micro-structured surface in a variety of ways. In one embodiment, a
plurality of tubes, connectors, valves and other fluid conduits may
direct the primary flow towards, and subsequently over the
micro-structured surface. In one embodiment, the primary flow may
be directed over the micro-structured surface, at least in part, by
structures (such as the interior walls of the elongate channel 302)
within the nanowire filter 300 itself. The flow rate of the primary
flow may also be varied in order to control an average height of
the solution above the micro-structured surface.
[0147] The micro-structured surface may comprise any of a variety
of microstructures. As illustrated in FIG. 3, the nanowire filter
300 may include a plurality of openings 306. As illustrated in FIG.
8, the nanowire filter 800 may include a micro-structured surface
808 having a plurality of microscopic protrusions and pores. As
illustrated in FIG. 14, the nanowire filter 1400 may comprise a
plurality of parallel ridges. As described above, micro-structures
may include any small structures formed in, on or through a surface
that may interfere with a fluid flow. The micro-structures are
preferably configured to filter the solution by removing
undesirable contaminant particles. Examples of suitable
configurations are described above in greater detail with reference
to the exemplary micro-structured nanowire filters.
[0148] In one embodiment, after passing over the micro-structured
surface, the retentate 18 emerging from the nanowire filter 300 may
comprise a more uniform solution of nanowires. Meanwhile, the
filtrate from the solution may flow away from the micro-structured
surface and thereby away from the nanowire filter 300. In other
embodiment, the filtrate may be captured and held by the
micro-structured surface (e.g., as illustrated in FIGS. 8-10).
[0149] In one embodiment, directing the primary flow over the
micro-structured surface may further comprise creating a secondary
flow through the plurality of openings 306. As described in greater
detail above, as the primary flow travels over the plurality of
openings 306, at least a portion of that primary flow may be
diverted as a secondary flow through the plurality of openings 306.
In one embodiment, the secondary flow through the plurality of
openings 306 may include both solvent and low aspect ratio
contaminant particles. A flow rate of the primary flow may be
selected to be at least 10 times greater than a flow rate of the
secondary flow. In another embodiment, a flow rate of the primary
flow may be at least 100 times greater than a flow rate of the
secondary flow. By increasing the ratio of the primary flow rate to
the secondary flow rate, it may become less likely that the
nanowires (which may align with a flow of the solution due to their
higher aspect ratios) will be diverted through the plurality of
openings 306 with the filtrate.
[0150] In another embodiment, as illustrated in FIGS. 14-16,
directing the primary flow over the micro-structured surface may
further comprise creating a secondary flow directed away from the
primary flow of the solution via a plurality of fluid passages
defined by a plurality of parallel ridges. A flow rate of the
primary flow may be selected to be at least 10 times greater than a
flow rate of the secondary flow. In another embodiment, a flow rate
of the primary flow may be at least 100 times greater than a flow
rate of the secondary flow.
[0151] The primary flow of the solution may also be occasionally
stopped, and a cleaning solution may be passed over the
micro-structured surface. For example, when micro-structures are
implemented that capture and hold filtrate, this act of passing the
cleaning solution over the micro-structured surface may be
desirable to mitigate or prevent the build-up of filtrate and any
resulting degradation in filtering efficiency. In one embodiment,
the primary flow may be stopped and the cleaning solution applied
periodically, according to some time interval. In another
embodiment, these acts may be performed after a certain amount of
solution has been filtered. In yet another embodiment, these acts
may be performed when the performance of the nanowire filter has
degraded by a certain amount.
[0152] In another embodiment, the retentate 18 may be collected,
liquid may be added, and the retentate 18 may be recirculated over
the micro-structured surface. An exemplary nanowire filtering
system 22 for performing such acts is illustrated in FIG. 2. The
retentate 18 may be collected in a variety of ways. In one
embodiment, a second pump (not illustrated) may generate a flow of
the retentate 18 from the nanowire filter 16 back to the source
container 12, where it may be collected. At any stage in the
recirculation of the retentate 18, replacement solvent may be
added. In one embodiment, for example, an inlet (not shown) may
combine additional solvent with the retentate 18 before the
retentate 18 is collected at the source container 12. In another
embodiment, the additional solvent may be added directly to the
source container 12 (e.g., at a rate generally corresponding to the
loss of filtrate from the solution).
[0153] The retentate 18 may be recirculated over the
micro-structured surface a number of times. In one embodiment, for
example, the retentate 18 may be recirculated a pre-determined
number of times calibrated to approximately filter the solution to
a desired purity. In another embodiment, a purity of the retentate
18 (corresponding, for example, to the percentage weight of
nanowires in the retentate 18 or to a percentage concentration of
replacement solvent) may be tested periodically or continuously, in
order to determine whether or not to continue recirculating the
retentate 18 over the micro-structured surface. Once a desired
purity is reached, the recirculation of the retentate 18 may be
stopped, and the solution collected in the source container 12.
Description of Another Exemplary Method of Filtering a Solution
Containing Nanowires
[0154] FIG. 32 illustrates a flow diagram for an alternative method
3200 of filtering a solution containing nanowires using a nanowire
filter having a narrow aperture, according to one embodiment. This
method 3200 will be discussed in the context of the nanowire filter
1700 incorporated into the nanowire filtering system 10. However,
it may be understood that the acts disclosed herein may also be
executed using a variety of other nanowire filters having narrow
apertures (e.g., nanowire filters 2000, 2200, 2500, 2700, and
2900), in accordance with the described method.
[0155] The method begins at 3202, when a solution containing
nanowires and a first set of contaminant particles is provided. As
discussed above, in one embodiment, the solution containing
nanowires may comprise the solution within which the nanowires were
formed. In other embodiments, the solution within which the
nanowires were formed may have already undergone a variety of
processing and/or filtering acts.
[0156] At 3204, a flow of the solution is generated. The flow of
the solution may be generated by any of a variety of mechanisms, as
described above with respect to act 3104.
[0157] At 3206, the solution is filtered by directing the flow
through a passage defining an aperture having a width less than at
least one dimension of the first set of contaminant particles. The
flow may be directed through the passage in any of a variety of
ways. In one embodiment, a plurality of tubes, connectors, valves
and other fluid conduits may direct the flow towards and through
the passage.
[0158] The passage and the aperture defined thereby may comprise
any of a variety of shapes and configurations. In one embodiment,
as illustrated in FIG. 17, a pair of parallel plates 1702, 1704 may
at least partially define a passage having a generally rectangular
cross-section. In other embodiments, the passage may define
circular, elliptical, triangular or irregularly shaped
apertures.
[0159] In one embodiment, as described in detail above, the
nanowire filter 1700 may eventually become clogged by filtrate
collecting at the entrance 1710 to the passage. The flow of the
solution may therefore occasionally be stopped, a reverse flow of a
liquid generated, and the reverse flow directed through the passage
in a direction opposite to the flow of the solution. In one
embodiment, a cleaning solution (e.g., water) may be periodically
passed through the nanowire filter 1700 from the exit 1712 to the
entrance 1720 in order to keep the nanowire filter 1700 running
efficiently. In another embodiment, a reverse flow of the solution
itself may occasionally be generated. For example, the pump 14 may
be configured to pump in both a forward and reverse direction and
may periodically switch direction in order to drive the solution
back and forth through the nanowire filter 1700. A flow rate of the
forward flow of the solution, and a flow rate of the reverse flow
may be chosen such that there is a net flow of the solution towards
the exit 1712 of the passage (i.e., in the forward direction).
Thus, potential clogging of the nanowire filter 1700, as described
above, may be avoided or at least delayed by the periodic flushing
of the entrance 1710.
[0160] The reverse flow may be generated periodically, according to
some time interval, or may be generated after a certain amount of
solution has been filtered. In another embodiment, the reverse flow
may be generated when the performance of the nanowire filter 1700
has degraded by a certain amount. For example, the reverse flow may
be generated based on a reduction in a forward flow rate of the
solution.
[0161] In another embodiment, the nanowire filter 16 may further
include a tortuous path filter (not illustrated) located upstream
from the aperture 1708. The tortuous path filter may comprise any
type of tortuous path filter. In one embodiment, for example, the
tortuous path filter may be configured similarly to a beta pure
depth filter, manufactured by 3M, with a nominal pore size of 125
.mu.m.
[0162] The flow of the solution may be further directed to a second
passage defining a second aperture having a width less than at
least one dimension of a second set of contaminant particles (e.g.,
1 .mu.m), and may then be directed to a third passage defining a
third aperture having a width less than at least one dimension of a
third set of contaminant particles (e.g., 0.5 .mu.m) (as
illustrated in FIG. 29). In one embodiment, the flow of the
solution may be directed first through the first passage, then
through the second passage, and then through the third passage.
[0163] Various embodiments described above can be combined to
provide further embodiments. From the foregoing it will be
appreciated that, although specific embodiments have been described
herein for purposes of illustration, various modifications may be
made without deviating from the spirit and scope of the teachings.
Accordingly, the claims are not limited by the disclosed
embodiments.
* * * * *