U.S. patent application number 13/534890 was filed with the patent office on 2012-10-25 for surface charge enabled nanoporous semi-permeable membrane for desalination.
This patent application is currently assigned to INTERNATIONAL BUSINESS MACHINES CORPORATION. Invention is credited to JOHN M. COTTE, Christopher V. Jahnes, Hongbo Peng, Stephen M. Rossnagel.
Application Number | 20120267249 13/534890 |
Document ID | / |
Family ID | 43897469 |
Filed Date | 2012-10-25 |
United States Patent
Application |
20120267249 |
Kind Code |
A1 |
COTTE; JOHN M. ; et
al. |
October 25, 2012 |
SURFACE CHARGE ENABLED NANOPOROUS SEMI-PERMEABLE MEMBRANE FOR
DESALINATION
Abstract
A filter includes a membrane having a plurality of nanochannels
formed therein. A first surface charge material is deposited on an
end portion of the nanochannels. The first surface charge material
includes a surface charge to electrostatically influence ions in an
electrolytic solution such that the nanochannels reflect ions back
into the electrolytic solution while passing a fluid of the
electrolytic solution. Methods for making and using the filter are
also provided.
Inventors: |
COTTE; JOHN M.; (Yorktown
Heights, NY) ; Jahnes; Christopher V.; (Yorktown
Heights, NY) ; Peng; Hongbo; (Yorktown Heights,
NY) ; Rossnagel; Stephen M.; (Yorktown Heights,
NY) |
Assignee: |
INTERNATIONAL BUSINESS MACHINES
CORPORATION
ARMONK
NY
|
Family ID: |
43897469 |
Appl. No.: |
13/534890 |
Filed: |
June 27, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12607258 |
Oct 28, 2009 |
|
|
|
13534890 |
|
|
|
|
Current U.S.
Class: |
204/665 ;
205/131; 977/781; 977/890; 977/902 |
Current CPC
Class: |
B01D 2323/283 20130101;
B01D 67/0065 20130101; B01D 71/022 20130101; Y10T 428/249953
20150401; B01D 2325/021 20130101 |
Class at
Publication: |
204/665 ;
205/131; 977/781; 977/902; 977/890 |
International
Class: |
B03C 5/02 20060101
B03C005/02; C25D 5/02 20060101 C25D005/02 |
Claims
1. A filter, comprising: a membrane having a plurality of parallel
nanochannels formed in an aluminum substrate by anodic aluminum
oxidation; and a first surface charge material deposited on at
least one end portion of the nanochannels, the first surface charge
material including a surface charge to electrostatically influence
ions in an electrolytic solution such that the nanochannels reflect
ions back into the electrolytic solution while passing a fluid of
the electrolytic solution.
2. The filter as recited in claim 1, wherein the nanochannels
include a diameter of between about 3 nm and 200 nm.
3. The filter as recited in claim 1, wherein the first surface
charge material includes a material having a negative surface
charge to repel negative ions.
4. The filter as recited in claim 3, wherein the first surface
charge material includes at least one of titanium oxide and silicon
oxide.
5. The filter as recited in claim 1, wherein the first surface
charge material includes a material having a positive surface
charge to repel positive ions.
6. The filter as recited in claim 5, wherein the first surface
charge material includes silicon nitride.
7. The filter as recited in claim 1, further comprising a second
surface charge material deposited on a second end portion of the
nanochannels, the second surface charge material including a
surface charge to electrostatically influence ions in an
electrolytic solution such that the nanochannels reflect ions back
into the electrolytic solution while passing the fluid of the
electrolytic solution.
8. The filter as recited in claim 1, wherein the nanochannels
include an aperture dimension narrowed by a deposited material.
9. The filter as recited in claim 8, wherein the aperture dimension
is controlled by controlling a deposition rate of the deposited
material.
10. The filter as recited in claim 1, wherein the electrolytic
solution includes sea water and the nanochannels include an
aperture of between about 1 nm to about 3 nm.
11. The filter as recited in claim 1, wherein the nanochannels
include a diameter of about double a size of an electrical double
layer formed by the electrostatically influenced ions in an
electrolytic solution.
12. The filter as recited in claim 1, wherein the first surface
charge material is deposited on a filler layer formed over the at
least one end portion of the nanochannels, the filler material
adjusting an initial diameter of the nanochannels.
13. A filter system, comprising: a first volume configured to
receive an electrolytic solution at a pressure; and a second volume
separated from the first volume by a membrane having a plurality of
parallel nanochannels formed in an aluminum substrate by anodic
aluminum oxidation; and the nanochannels including a first surface
charge material deposited on at least one end portion of the
nanochannels, the first surface charge material including a surface
charge to electrostatically influence ions in an electrolytic
solution such that the nanochannels reflect ions back into the
electrolytic solution while passing a fluid of the electrolytic
solution.
14. The filter system as recited in claim 13, further comprising a
pressure regulator configured to regulate the pressure of the first
volume.
15. The filter system as recited in claim 13, further comprising a
support structure configured to support the membrane against the
pressure.
16. The filter system as recited in claim 13, further comprising a
mixer configured to mix fluid in the first volume.
17. The filter system as recited in claim 13, further comprising a
plurality of membranes sequentially arranged to filter the
electrolytic fluid and filtered electrolytic fluid.
18. The filter system as recited in claim 13, wherein the first
surface charge material includes a material having a negative
surface charge to repel negative ions.
19. The filter system as recited in claim 13, wherein the first
surface charge material includes a material having a positive
surface charge to repel positive ions.
20. The filter system as recited in claim 13, further comprising a
second surface charge material deposited on a second end portion of
the nanochannels, the second surface charge material including a
surface charge to electrostatically influence ions in an
electrolytic solution such that the nanochannels reflect ions back
into the electrolytic solution while passing the fluid of the
electrolytic solution.
21. The filter system as recited in claim 13, wherein the
nanochannels include an aperture dimension narrowed by a deposited
material.
22. The filter system as recited in claim 21, wherein the aperture
dimension is controlled by controlling a deposition rate of the
deposited material.
23. The filter system as recited in claim 13, wherein the
nanochannels include a diameter of about double a size of an
electrical double layer formed by the electrostatically influenced
ions in an electrolytic solution.
24. The filter system as recited in claim 13, wherein the first
surface charge material is deposited on a filler layer formed over
the at least one end portion of the nanochannels, the filler
material adjusting an initial diameter of the nanochannels.
Description
RELATED APPLICATION INFORMATION
[0001] This application is a Continuation application of co-pending
U.S. patent application Ser. No. 12/607,258 filed on Oct. 28, 2009,
incorporated herein by reference in its entirety.
BACKGROUND
[0002] 1. Technical Field
[0003] The present invention relates to semi-permeable membranes
and more particularly to a semi-permeable nanoporous membrane and
methods for making and using the same for desalination and other
processes.
[0004] 2. Description of the Related Art
[0005] Water desalination may be thought of in terms of two
approaches. The two basic approaches for water desalination include
reverse osmosis and distillation. The distillation approach
requires converting fluid water to the vapor phase and condensing
water from the vapor. This approach is fairly high cost and
requires significant energy usage. The reverse osmosis approach
uses pressure on a salinated liquid to force water molecules
through a semi-permeable membrane. This approach has a relativity
low rate of energy consumption.
[0006] The specific (per unit of produced potable water) energy of
desalination using reverse osmosis has been reduced from over 10
kWh/m.sup.3 in the 1980s to below 4 kWh/m.sup.3, approaching the
theoretical minimum required energy of 0.7 kWh/m.sup.3. To improve
the state of art of the reverse osmosis approach, new membranes,
with a uniform pore distribution and a more permeable separation
layer can potentially maintain or improve salt rejection while
increasing the flux in the reverse osmosis method. Such
improvements have not yet been developed in the conventional
art.
SUMMARY
[0007] A filter includes a membrane having a plurality of
nanochannels formed therein. A first surface charge material is
deposited on an end portion of the nanochannels. The first surface
charge material includes a surface charge to electrostatically
influence ions in an electrolytic solution such that the
nanochannels reflect ions back into the electrolytic solution while
passing a fluid of the electrolytic solution. Methods for making
and using the filter are also provided.
[0008] A filter system includes a first volume configured to
receive an electrolytic solution at a pressure, and a second volume
separated from the first volume by a membrane having a plurality of
nanochannels formed therein. The nanochannels include a first
surface charge material deposited on at least one end portion of
the nanochannels. The first surface charge material includes a
surface charge to electrostatically influence ions in an
electrolytic solution such that the nanochannels reflect ions back
into the electrolytic solution while passing a fluid of the
electrolytic solution.
[0009] A method for making a filter includes forming a plurality of
nanochannels in a membrane; and depositing a first surface charge
material on at least one end portion of the nanochannels. The first
surface charge material includes a surface charge to
electrostatically influence ions in an electrolytic solution such
that the nanochannels reflect ions back into the electrolytic
solution while passing a fluid of the electrolytic solution.
[0010] A method for filtering an electrolytic solution includes
filling a first volume with an electrolytic solution, applying a
pressure below a threshold value to the electrolytic solution in
the first volume, and passing a fluid of the electrolytic solution
into a second volume separated from the first volume by a membrane.
The membrane has a plurality of nanochannels formed therein. The
nanochannels include a first surface charge material deposited on
at least one end portion of the nanochannels. The first surface
charge material includes a surface charge to electrostatically
influence ions in an electrolytic solution such that the
nanochannels reflect ions back into the electrolytic solution while
passing the fluid of the electrolytic solution.
[0011] These and other features and advantages will become apparent
from the following detailed description of illustrative embodiments
thereof, which is to be read in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0012] The disclosure will provide details in the following
description of preferred embodiments with reference to the
following figures wherein:
[0013] FIG. 1 is a cross-sectional view taken along a longitudinal
axis of nanotubes or nanochannels through a membrane showing the
nanochannels forming a double electrical layer in accordance with
one illustrative embodiment;
[0014] FIG. 2 is a top view of a membrane showing nanochannels and
locating a section A-A in accordance with one illustrative
embodiment;
[0015] FIG. 3 is a cross-sectional view taken along section A-A of
FIG. 2 showing the formation of nanotubes or nanochannels through
the membrane in accordance with one illustrative embodiment;
[0016] FIG. 4 is a cross-sectional view showing a surface charge
layer formed over one end portion of the nanotubes or nanochannels
in accordance with one illustrative embodiment;
[0017] FIG. 5 is a cross-sectional view showing two surface charge
layers formed over two end portions of the nanotubes or
nanochannels in accordance with another illustrative
embodiment;
[0018] FIG. 6 is a block diagram illustrative depicting a
desalination system in accordance with an illustrative
embodiment;
[0019] FIG. 7 is a flow diagram showing a method for making a
filter in accordance with the present principles; and
[0020] FIG. 8 is a flow diagram showing a method for using a filter
in accordance with the present principles.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0021] In accordance with the present principles, a new membrane is
described, which utilizes a surface charge of nanopores and/or
nanochannels. In one embodiment, for reverse osmosis, a high salt
rejection is achieved while simultaneously maintaining high flux.
In one embodiment, a nanoscale filter includes arrays of parallel
nanopores or channels which are formed into a membrane material.
The surface of the nanopore or channel is configured with a
material, which has a high negative (or positive) surface charge
when exposed to an electrolyte. This effect blocks the transport of
ions through the channel, and is effectively an ion filter.
[0022] It is to be understood that the present invention will be
described in terms of a non-limiting semi-permeable membrane formed
from a material including aluminum; however, other structures,
membrane materials, coating materials, process features and steps
may be varied within the scope of the present invention. The
membrane may be formed in sheets and cut to size or may be formed
in or included with pre-sized panels.
[0023] In particularly useful embodiments, the membrane is employed
for desalination of water. However, other physical or chemical
processes may employ the present principles.
[0024] Referring now to the drawings in which like numerals
represent the same or similar elements and initially to FIG. 1,
across-sectional view of a nanoscale filter 10 in accordance with
the present principles is illustratively shown. Filter 10 includes
arrays 12 of parallel nanopores or channels 14 which are formed
into a membrane material 16. A surface of the nanopore or channel
14 is configured with a material 18, which has a high negative
surface charge (or high positive surface charge depending on the
application) when exposed to an electrolyte 20, such as salt water.
In one embodiment, a deposition material 17 may be deposited to
narrow the openings in the nanotubes 14. The material 17 is
preferably deposited using, e.g., a chemical vapor deposition (CVD)
or a physical vapor deposition (PVD). Material 17 is needed only to
adjust the size of the channels 14 and may be omitted if channels
14 are of sufficient size using a layer of material 18.
[0025] In one embodiment, the material 18 may include, for example,
titanium dioxide or silicon dioxide. In the electrolyte 20, a
negative surface charge on surfaces 22 will attract positive ions
(counter-ions) in the electrolyte 20, which forms an electrical
double layer 26 (in a vicinity of layer 18). A positive surface
charge on surface 22 will attract negative ions (anions) in the
electrolyte 20, which forms the electrical double layer 26. The
double layer 26 includes the surface charge of the nanochannels 14
and the electrolytically responsive ions thereto.
[0026] In one embodiment, a thickness t of the electrical double
layer will depend on a charge density of the electrolyte 20, and is
around 1 nm when electrolyte densities are in the range of 1.0
molarity (M). As an example, if the thickness of this electrical
double layer is about 1/2 of a pore or channel diameter, the
counter-ion regions from one side of the pore or channel will merge
with the region from the opposite site, forming a region 30 across
the pore or channel diameter which contains only positive charge,
since the negative charge is repelled by the negative surface
charge 22 of the nanopore or nanochannel surface. This effect
blocks the transport of negative ions through the channel, and is
effectively a negative ion filter. The opposite polarity effect
should occur for a surface which is positively charged, such as,
with a silicon nitride surface.
[0027] Referring again to FIG. 1, for sea water, the charge density
of ions is such that the thickness of the electrical double layer
26 is on the order of 1 nm, so a nanopore or nanochannel of
diameter less than 3 nm would be needed. Lower concentration salt
water would allow the usage of a larger diameter nanopore or
channel. One criteria being a ratio of approximately 2:1 for the
diameter of the pore or channel compared to the thickness of
electrical double layer in the salt-water (electrolyte).
[0028] Referring to FIG. 2, a top view looking into an array 12 of
nanopores or nanochannels 14 is illustratively shown. A device or
membrane 10 can be fabricated with parallel nanopores or
nanochannels 14 on the scale of 3 nm-20 nm in diameter. Other sizes
are also possible and may be employed depending on the application.
One approach to accomplish this would be to form nanochannels 14 in
aluminum foils (16) using anodic aluminum oxide (AAO) (See e.g., O.
Jessensky et al., "Self-organized formation of hexagonal pore
arrays in anodic alumina" Appl. Phys. Lett, 72, (1998) p 1173, also
G. Sklar et al, "Pulsed deposition into AAO templates for CVD
growth of carbon nanotube arrays", Nanotechnology, 16 (2005)
1265-1271). This process forms high aspect ratio, parallel channels
into A1 by an anodic oxidation process.
[0029] Referring to FIG. 3, a cross-sectional view taken at section
A-A of FIG. 2 is illustratively shown. Nanotubes or channels 14 are
formed in a parallel manner through material 16.
[0030] Referring to FIG. 4, once the array 12 of nanochannels 14 is
formed, the surface can be conditioned by depositing material 17,
such as, SiO.sub.2 or other suitable material preferably using a
chemical vapor deposition (CVD) technology or sputter deposition
(known as Physical Vapor Deposition or PVD). The goal here is to
close off the top aperture of the nanochannel 14. Both PVD and CVD
are not very conformal deposition technologies; they tend to clog
the channel at the opening of the hole. By controlling the
thickness of this deposition, the opening of the nanochannel can be
controllably shrunk to any dimension. The material used for this
can have an impact on the surface charge, or else it can be covered
over with a thin layer of TiO.sub.2 or other charge material 18
(FIG. 1 or FIG. 5), using, e.g., atomic layer deposition (ALD),
which forms a surface with a high negative surface charge.
Alternately, material 18 may include a material for a positively
charged surface. The material 18 may be deposited on one end of the
nanochannel 14.
[0031] Referring to FIG. 5, if desired, a second layer 52 of
material could be deposited on the far end of the nanochannel 14.
For example, a first end portion 55 includes a negatively charged
surface while a second end portion 56 includes a positively charged
surface. In one embodiment, silicon nitride may be deposited by a
plasma-enhanced CVD or a reactive sputter deposition process to
form a surface with a positive surface charge. The opening of each
end of the nanochannel 14 can be adjusted to the 3-10 nm range by
controlling both the initial diameter of the nanochannel 14 in AAO
(e.g., 20 to 200 nm) and the subsequent deposition of a filler
material 17, or the surface coating material 18 (and/or 52). It is
only necessary for this coating 18 (or 52) to occur at the very
ends of the channel, since that is where the electrostatic
filtering of ions will take place.
[0032] Referring to FIG. 6, a desalination device 100 includes a
membrane or filter 102 having a large array of parallel
nanochannels. The membrane 102 may be configured on a grid, mesh or
other structural member 110 for strength. The nanochannel arrays of
membrane 102 separate a fluid volume in a first reservoir or
container 104, e.g., containing a salt water solution from a second
volume 106 in which the sodium and chlorine ions do not penetrate,
hence forming desalinated water. The application of pressure P to
the salinated side would increase the permeation of water molecules
through the nanopore/nanochannel array of the membrane 102 up to a
point. At a high enough pressure P, the flow of water through the
apertures would exceed the ability of the surface charges to
reflect the ions, and hence the ionic filtering capability would
break down. The volume 104 in FIG. 6 could have flow into and out
of the volume 104. It should be noted that the flow of water into
volume 104 should be well mixed to prevent a significant build up
of ionic charge in the volume 104. Salinity of water in volume 104
should be regulated over time because as the desalinated water
leaves, the salt stays behind. This will increase the concentration
until eventually the surface charge can no longer block the
nanochannels of membrane 102. It is therefore preferably to have an
open system where water is replenished to counter build up in ionic
concentration. A mixer or other perturbation device 120 may be
useful to stir the water in volume 104.
[0033] A critical flow could be calculated in a manner using
Child's Law, in which the charges within the aperture shield the
upstream charges from the applied fields, and hence at that point
(and flow), ionic filtering would cease. A pressure regulator
device 112 may be employed to maintain the pressure P at or below
this critical pressure value to ensure proper functioning of the
desalination system 100. Alternately, a container may be configured
to provide a working pressure P using the height of a water column
in the fluid volume or container 104 or by other means.
[0034] In one embodiment, the arrays of parallel nanopores or
channels in the membrane 102 are coated on one end with a material
to create a negative surface charge (e.g., titanium dioxide or
silicon dioxide). In another embodiment, the other end may be
coated with a material to create a positive surface charge (e.g.,
silicon nitride). Note coating one end, for example, with a
negative (or positive) surface charge material will work for both
types of ions. The high surface charges in an electrolyte attract
or repel ions and form an electrical double layer at one or more
end to repel ions. The thickness of this electrical double layer
may be about 1 nm for electrolyte densities in the 1.0 M range
(e.g., sea water). For a thickness of this electrical double layer
of half the pore diameter, the transport of ions through the
channel is blocked and an ion filter is formed.
[0035] The embodiment depicted in FIG. 6 may be extended to include
a plurality of membranes in series to further refine the
filtration. In one embodiment, different filtration stages may be
employed wherein at each stage pressure is controlled to ensure
that each stage is performing efficiently. This may include
increasing or decreasing the intermediate pressures of the fluid at
each stage.
[0036] A sequential embodiment may include additional membranes
102' and stages 116 for filtering at different dimensions. For
example, sea water in the first volume 104 could go through an
intermediate filter (102) first, which would block some but not all
of the ions. A second-stage filter 102' could then be used with has
different diameters for nanochannels and hence blocks a different
concentration.
[0037] While desalination of water has been described as an
illustrative example, other fluids may be filtered in accordance
with the present principles. In addition, different materials and
combinations of materials may be employed to provide electrostatic
filtering. Advantageously, a desalination system can be provided
that is passive (does not require a power source) and may be
employed as an inexpensive desalination system, an emergency
desalination system (e.g., on life rafts) etc.
[0038] Referring to FIG. 7, a flow diagram is shown for an
illustrative method for making a filter in accordance with the
present principles. In block 202, a plurality of nanochannels is
formed in a membrane. The forming may include using anodic aluminum
oxide to form the nanochannels in an aluminum membrane. The
nanochannels may include a diameter of between about 3 nm and 200
nm.
[0039] In block 203, a material may be deposited to adjust the
apertures of the nanochannels. The material is preferably deposited
using a CVD or PVD process to incrementally narrow the channel
openings to provide an appropriate aperture dimension.
[0040] In block 204, a surface charge material is deposited on at
least one end portion of the nanochannels. The surface charge
material includes a surface charge to electrostatically influence
ions in an electrolytic solution such that the nanochannels reflect
ions back into the electrolytic solution while passing a fluid of
the electrolytic solution. In block 206, a second surface charge
material may be deposited on a second end portion of the
nanochannels. The second material includes a surface charge to
electrostatically influence ions in an electrolytic solution such
that the nanochannels reflect ions back into the electrolytic
solution while passing the fluid of the electrolytic solution.
[0041] In an alternative embodiment, depositing the first surface
charge material may include depositing the first surface charge
material on first locations on a first end portion of the
nanochannels, and depositing a second surface charge material on
second locations on the first end portion of the nanochannels, such
that the first and second surface charge materials provide opposite
polarities for the surface charge. Different polarities may exist
on a same side of the membrane by using resist masks or other large
scale integration techniques. In addition, one configuration may
include different surface polarities on a same side of the membrane
and different polarities on the opposite side of the membrane.
Different patterns and different configurations may be provided.
For example, the first surface charge material may include a
material having a negative surface charge to repel negative ions
and a positive surface charge to repel positive ions on an opposite
side of the membrane or on the same side of the membrane, etc. It
should be understood that the illustrative examples as described
here may be combined in any manner and provide many useful
configurations in accordance with the present principles.
[0042] Referring to FIG. 8, a flow diagram is shown for an
illustrative method for using a filter in accordance with the
present principles. In block 250, a first volume is filled with an
electrolytic solution. This volume is preferably mixed or
constantly replenished. In block 252, a pressure is applied to the
electrolytic solution in the first volume that is below a pressure
threshold value. In block 254, a fluid of the electrolytic solution
is passed into a second volume separated from the first volume by a
membrane. The membrane has a plurality of nanochannels formed
therein as described above. The fluid in the second volume is
desalinated or partially desalinated. Additional stage may be
added.
[0043] Having described preferred embodiments of a surface charge
enabled nanoporous semi-permeable membrane for desalination (which
are intended to be illustrative and not limiting), it is noted that
modifications and variations can be made by persons skilled in the
art in light of the above teachings. It is therefore to be
understood that changes may be made in the particular embodiments
disclosed which are within the scope of the invention as outlined
by the appended claims. Having thus described aspects of the
invention, with the details and particularity required by the
patent laws, what is claimed and desired protected by Letters
Patent is set forth in the appended claims.
* * * * *