Patent Application
20090321364
1. Technical Field
This description generally relates to the field of nanowire manufacturing, and more particularly to filtering solutions containing nanowires.
2. Description of the Related Art
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.
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.
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.
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.
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.
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.
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.
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.
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.
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 μm.
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.
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.
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.”
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.
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.
The headings and Abstract of the Disclosure provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.
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.
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® 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® FSN, ZONYL® FSO, ZONYL® FSH, TRITON® (x100, x114, x45), DYNOL™ (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° C., no more than 150° C., or no more than 100° C.
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.
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.
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.
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.
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.
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.
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° 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
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.
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.
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° 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° 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.
The average length of the resulting silver nanowires was 24 μm with a standard deviation of 15 μm. 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 %.
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.
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+, NO3−) 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.
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.
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.
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.
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
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.
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
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
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 μm because the expected filtrate may have a diameter up to approximately 5 μm. In another embodiment, the openings 306 may have an average diameter greater than 10 μm. 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.
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.
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.
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.
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).
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.
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.
In one embodiment, the openings 604 may have an average diameter of approximately 5 μm, 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.
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.
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.
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.
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 μm because the expected filtrate may have a diameter up to approximately 5 μm. In another embodiment, the filter paper may have a porosity of greater than 10 μm. The porosity of the filter paper may be varied, as described above to achieve particular filtering characteristics.
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.
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.
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.
A schematic view of the microscopic filtering process is illustrated in
As may be understood with reference to
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.
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.
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 μm because the expected filtrate may have a diameter up to approximately 5 μm. In another embodiment, the filter paper may have a porosity of greater than 10 μm. 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.
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.
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.
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
As described above with reference to
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.
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.
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.
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.
The parallel ridges may also be separated by a distance greater than 5 μm because the expected filtrate may have a diameter up to approximately 5 μm. In another embodiment, the parallel ridges may be separated by a distance greater than 10 μm. A cross-section of the valleys formed by the ridges may be approximately square, such that the valleys are deeper than 5 μm or 10 μm 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.
Turning to
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.
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.
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.
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 μm, in order to filter out particles having a diameter greater than 2 μm. In another embodiment, the aperture 1708 may have a width W less than 1 μm, or less than 0.5 μm, 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.
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.
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.
A schematic view of the nanowire filter 1700 in operation is illustrated in
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.
As illustrated in
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.
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.
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.
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.
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
In operation, as best illustrated in
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.
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 μm and the exit 2512 has a width of 1 μm, then 5 μm particles may be captured somewhere near the middle of the passage 2506, and 1.1 μm particles may be captured very close to the exit 2512.
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.
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.
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 μm).
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 μm). 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.
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 μm). 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.
In other embodiments, more or fewer apertures of various sizes may be used to filter out particular sets of contaminant particles.
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 μm at the first aperture 2908, other contaminant particles having diameters between 1 and 2 μm at the second aperture 2928 and still more contaminant particles having diameters between 0.5 and 1 μm at the third aperture 2938.
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.
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.
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
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.
The micro-structured surface may comprise any of a variety of microstructures. As illustrated in
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
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.
In another embodiment, as illustrated in
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.
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
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.
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.
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.
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.
The passage and the aperture defined thereby may comprise any of a variety of shapes and configurations. In one embodiment, as illustrated in
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.
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.
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 μm.
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 μm), 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 μm) (as illustrated in
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.
This application claims the benefit under 35 U.S.C. § 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.
| Number | Date | Country | |
|---|---|---|---|
| 60913231 | Apr 2007 | US |
