Patent Application
20090078914
The present disclosure relates to the production of carbon nanotube-matrix particle complexes and the electrophoretic deposition of the complexes to form carbon nanotube composite films with improved surface uniformity and adhesion between carbon nanotubes and matrix materials.
Nanostructure materials, such as carbon nanotubes (CNT), possess promising properties, such as electron field emission characteristics which appear to be far superior to that of conventional field emitting materials. In particular, CNT materials exhibit low emission threshold fields as well as large emission current densities. Such properties make them attractive for a variety of microelectronic applications, such as lighting elements, field emission flat panel displays, gas discharge tubes for over voltage protection and x-ray generating devices. However, the effective incorporation of such materials into these devices has been hindered by difficulties encountered in the processing of such materials.
Electrophoretic deposition (EPD) is known to be a highly efficient and versatile technique for the deposition of nanostructure materials, including CNT, on a variety of substrates to produce novel coatings and films. An increasing interest in EPD techniques is driven by the availability of a variety of suitable materials, the technique's simplicity and low cost setup, the ability to control deposition, and the potential to scale-up to large dimensions.
The method of EPD can comprise the use of nanostructure materials, matrix materials and a substrate upon which the nanostructure materials are deposited. The matrix material is generally used to enhance the bonding of the nanostructure material with the substrate. For example, nanostructure materials and matrix materials can be co-deposited onto the substrate using EPD to form a composite film. However, composite films produced using this method can have less than desirable characteristics, particularly regarding uniformity of deposition, surface topography and adhesion strength. Thus, there is a need for a method of deposing nanostructure materials and matrix materials to form a composite film with improved surface uniformity, nanostructure material dispersion and adhesion between nanostructure materials and matrix materials.
It is an object of the presently disclosed subject matter to provide methods and devices related to depositing nanostructure materials onto a substrate to form uniform nanostructure composite films with improved adhesion and emission characteristics.
An object of the presently disclosed subject matter having been stated hereinabove, and which is achieved in whole or in part by the presently disclosed subject matter, other objects will become evident as the description proceeds when taken in connection with the accompanying drawings as best described hereinbelow.
Following long-standing patent law convention, the terms “a” and “an” mean “one or more” when used in this application, including the claims.
In accordance with the present disclosure, methods of depositing carbon nanotubes (CNT) onto a surface of a substrate are provided. Carbon-nanotubes can be electrophoretically deposited onto a substrate according the present disclosure. The CNT can be linked to a matrix material prior to EPD to provide CNT-matrix particle complexes. In some embodiments, CNT-matrix particle complexes can be formed by dispersing CNT, matrix particles and linker molecules together in a suspension. In some embodiments, CNT-matrix particle complexes can be formed by linking CNT to functionalized matrix particles, wherein the functionalized matrix particles are formed by reacting a linker molecule and a matrix particle.
In accordance with the present disclosure, methods of fabricating CNT-electron field emission cathodes are provided. A CNT-electron field emission cathode according to the present disclosure can include an electron field emission cathode comprising a composite film adhered to a substrate surface. Further, the composite film can comprise an intimate mixture of CNT and matrix materials. The substrate can comprise a conducting surface.
Electrophoretically depositing CNT linked to a matrix material onto a substrate according to the present disclosure provides for improved adhesion of CNT to the substrate. Further, fabricating electron field emission cathodes by electrophoretically depositing CNT-matrix particle complexes provides a smooth and homogenous composite film of CNT and matrix material adhered to the substrate with improved emission characteristics.
The method illustrated in
The terms “matrix particles”, “matrix materials” or “adhesion promoting materials” are used interchangeable herein, and are meant to refer to any particle or material that enhances the attachment or adhesion of nanostructure materials to a substrate. As illustrated in
In addition to CNT, any nanostructure material, nanostructure-containing material or CNT-containing material are provided in accordance with the presently disclosed subject matter. Nanostructure material can be fabricated according to a number of different techniques familiar to those in the art. For example, raw CNT-containing material can be fabricated by laser ablation techniques (see, for example, U.S. Pat. No. 6,280,697), chemical vapor deposition techniques (see, for example, Bower et al., 2000, Appl. Phys. Lett. Vol. 77(6):830-32), or arc-discharge techniques (see, for example, C. Journet et al., 1997, Nature Vol. 388:756).
The term “nanostructure material” is meant to refer to material including nanoparticles such as C60 fullerenes, fullerene-type concentric graphitic particles, metal, compound semiconductors such as CdSe, InP, nanowires/nanorods such as Si, Ge, SiOx, GeOx, or nanotubes composed of either single or multiple elements such as carbon, BxNy, BxCyNz, (B=boron, C=carbon, and N=nitrogen), MoS2 and WS2. One of the common features of nanostructure materials is their basic building blocks. A single nanoparticle or a CNT has a dimension that is less than 500 nm in at least one direction. The terms “nanostructure-containing” or “CNT-containing” are intended to encompass materials which are composed entirely, or almost entirely of nanostructure materials or CNT, such as a material composed of nanostructure materials and a minor amount of impurities. The nanostructure-containing material may also include purposefully added materials and/or agents.
Carbon nanotubes can be purified or as formed single wall nanotubes (SWNTs), multi-wall nanotubes (MWNTs), or double wall carbon nanotubes (DWNTs), or mixtures thereof. Carbon nanotubes can be synthesized by laser-ablation, arc-discharge, chemical vapor deposition (CVD), plasma enhanced chemical vapor deposition (PECVD), or other suitable methods. Carbon nanotubes can be 200 nm to 20 microns in length. Carbon nanotubes can have a diameter of 0.4 nm to 100 nm. As-grown CNT may contain a significant amount of impurities, such as amorphous carbon, metal catalyst (Ni, Co, Fe, Mo, Pd, Rh, and Au, etc.), and catalyst supporting materials, which include various kinds of oxides, such as Al2O3, SiO2, MgO, and CaO.
Also provided within the scope of the presently disclosed subject matter are raw nanostructure materials in the form of nanowires, comprising at least one of the following: elemental metal, Si, Ge, oxide, carbide, nitride, chalcogenide. In addition, the raw nanostructure materials can be in the form of nanoparticles of elemental metal, metal oxide and elemental and compound semiconducting materials.
Raw nanostructure materials can be purified by any appropriate technique known in the art. For example, raw nanostructure material can be purified by reflux in a suitable solvent, such as a combination of peroxide (H2O2) and water, with subsequent rinsing in CS2 and then in methanol, followed by filtration. According to an exemplary technique, approximately 10-100 ml of peroxide is introduced into the medium for every 1-10 mg of CNT in the medium, and the reflux reaction is carried out at a temperature of 20-100° C. See, for example, U.S. Pat. No. 6,553,096 to Zhou et al.
Continuing with
In some embodiments, the linker molecule comprises an aminosilane molecule. As used herein the term “aminosilane” refers to a molecule having at least one aminoalkyl group and at least one good leaving group, such as an alkoxy, aryloxy, or halo (i.e., Cl, F, I, or Br) group. As used herein the term “aminoalkyl” refers to an alkyl substituent comprising one or more amino functionality. The term “amino” can refer to a primary, secondary, tertiary or quaternary amine. In some embodiments, the aminoalkyl group can have the structure —R—NH2 or —R—NH3+, wherein R is alkylene (i.e., —(CH2)n—, wherein n is an integer from 1 to 20). In some embodiments, one or more of the CH2 groups of the alkylene chain can be replaced by NH, such that the alkylene group comprises a secondary amino group. In some embodiments the linker molecules can be aminosilane molecules selected from the group consisting of (3-aminopropyl) triethoxysilane (APS), (3-aminopropyl) trimethoxysilane, (3-aminopropyl) methyldiethoxysilane, (3-aminopropyl) methyldimethoxysilane, (N-(2-aminoethyl)-3-aminopropyl) triethoxysilane, (N-(2-aminoethyl)-3-minopropyl) trimethoxysilane, and (N-(2-aminoethyl)-3-aminopropyl) methyldimethoxysilane.
Using
The methods of
The term “functionalized matrix particle” refers to any matrix particle bound to a linker molecule having functional groups capable of binding to nanostructure materials, particularly CNT. In some embodiments a functionalized matrix particle can comprise a glass particle having a plurality of APS linker molecules bound thereto. This functionalized matrix particle can be encapsulated with an APS shell, wherein the APS shell can be positively charged due to the positively charged amine groups of the APS molecule. Such a functionalized matrix particle can consequently bind to CNT through an electrostatic interaction.
The functionalized matrix particles or APS-modified matrix particles can be further processed prior to their dispersion into the liquid medium with CNT. This further processing can include filtering the functionalized matrix particles through a filter membrane to remove the excessive linking molecules (APS). For example, the filter membrane can have a pore size of about 5 μm. After repeated rinsing with an alcohol (e.g., ethanol), the functionalized matrix particles can be exposed to heat, such as in an oven at 120° C. for 3 h. The heating of the functionalized matrix particles cross-links the linking molecule and matrix particle such that the chemical bond is strengthened. For example, in some embodiments, treating a matrix particle comprising hydroxyl groups with an aminosilane linker results in non-covalent association between the linker and the particle or the formation of only a few out of a plurality of possible covalent linkages to form. Heating can catalyze the completion of the condensation reactions between hydroxyl groups on the matrix material and all of the potential leaving groups on the linker molecule. In some embodiments, a single linking molecule is attached to a single matrix particle via a plurality of covalent linkages. In some embodiments, a single linking molecule is attached to more than one matrix particle.
Alternatively, the excessive linking molecules do not need to be removed from the functionalized matrix particle suspension prior to cross-linking.
Although any suitable concentration of matrix particles and linker molecules can be added to the liquid medium in the methods described above, a non-limiting example consists of adding APS and glass particles at a ratio of 1:800 (ml/mg) as well as a small amount of water. With the help of sonication for 15 min, a white glass suspension is formed.
An optional step of adding a dispersant to the liquid medium is provided for both methods illustrated in
For the methods described above, a suitable liquid medium is selected which will permit the formation of a stable suspension of the CNT, matrix particles and linker molecules. The liquid medium can comprise water, an alcohol (e.g., methanol, ethanol, isopropanol, butanol, etc.), dimethylformamide, DMSO, or a combination thereof. In some embodiments, the liquid medium can comprise at least one of water, methanol, ethanol, alcohol, and dimethylformamide.(DMF). Upon adding the CNT, matrix particles and linker molecules to the liquid medium, the mixture can optionally be subjected to ultrasonic energy or stirring using, for example, a magnetic stirrer bar, in order to facilitate the formation of a stable suspension. The amount of time that the ultrasonic energy or stirring is applied can vary, but it has been found that approximately two hours at room temperature is sufficient.
As an optional step to the methods described above and illustrated in
Also optional is a final step of rinsing the CNT-matrix particle complexes. The rinsing step can comprise filtering the CNT-matrix particle complexes through a filter membrane (e.g., with 5 μm pore size) to remove the excessive linker molecules.
In accordance with the presently disclosed subject matter, linking the CNT to matrix particles to produce CNT-matrix particle complexes substantially reduces agglomeration of CNT and matrix particles in the suspension. Further, the formation of CNT-matrix particle complexes strengthens the adhesion between CNT and matrix particles. Therefore, when deposited on a substrate via EPD, CNT in the form of CNT-matrix particle complexes form a substantially more uniform composite film than depositing CNT alone or co-depositing CNT and matrix particles without linker molecules.
Accordingly, methods are also provided for the fabrication of a uniform carbon nanotube composite film by electrophoretically depositing CNT-matrix particle complexes. The method comprises forming CNT-matrix particle complexes, according to the methods described above and illustrated in
An illustrative EPD technique is shown in
An optional “charger” can be added to liquid suspension LS in order to facilitate electrophoretic deposition. Chargers can be selected from the group consisting of MgCl2, Mg(NO3)2, La(NO3)3, Y(NO3)3, AlCl3, and sodium hydroxide. Any suitable amount can be utilized. Amounts ranging from less than 1% up to 50%, by weight, as measured relative to to the amount of nanostructure-containing material, are feasible.
Any suitable substrate material is within the scope of the presently disclosed subject matter, so long as it possesses the requisite degree of electrical conductivity. Exemplary conductive substrates include, but are not limited to, metal, metal alloy, graphite and doped silicon. Alternatively, the substrate can be formed by a non-conductive material coated with a conductive layer, such as indium-tin oxide glasses, or glass or silicon wafer with a deposited metal layer.
The electrical current applied to the electrodes to produce an electrical field between the electrodes can comprise a direct or alternating current. According to one embodiment, the electrical field applied between the electrodes is 0.1-1000 V/cm, and a direct current of 0.1-200 mA/cm2 is applied for between about 1 second to about 1 hour.
This electrical current causes the CNT-matrix particle complexes in the suspension to migrate toward and attach to the substrate electrode. The particular electrode (i.e.—anode or the cathode) to which the nanostructure material migrates can be controlled through the selection of the charger material. For example, the use of a “negative” charger, such as sodium hydroxide (NaOH) imparts a negative charge to the nanostructure material, thereby creating a tendency for the nanostructure material to migrate towards the positive electrode (cathode). Conversely, when a “positive” charger material is used, such as MgCl2, a positive charge is imparted to the nanostructure material, thereby creating a tendency for the nanostructure material to migrate toward the negative electrode (anode).
An optional step following the EPD process comprises annealing the substrate to form a uniform CNT composite film or layer, wherein the CNT and matrix particles of the CNT-matrix particle complexes are intimately mixed. In one non-limiting example, the coated substrate is removed from the EPD suspension upon completion of the EPD process, dried at room temperature and finally annealed at 700° C. at 5×10−7 torr vacuum for 12 h. After annealing, a smooth and uniform composite film is formed on the substrate surface with the carbon nanotubes dispersed uniformly in the matrix material.
The composite film or composite layer comprises CNT and a matrix layer, wherein the matrix layer is adhered to the surface of the substrate. The CNT protrude out from the matrix layer with their longitudinal axes substantially perpendicular to the substrate surface and the one end embedded in the matrix. The embedded portion of the CNT is linked to the matrix particles in the matrix layer through the linker molecules.
In one embodiment, the CNT protrude 1 to 10 microns from the substrate surface. Additionally, the protruding CNT are spaced apart with a separation spacing of 0.5 to 10 microns.
If the annealed CNT composite film is to be included in an electron field emission cathode or similar application, the annealed substrate can optionally be subjected to a taping process to activate the CNT tips for effective electron emission. As can be appreciated by one of skill in the art upon review of the present disclosure, the taping process can comprise any suitable taping method known in the art. See, for example, U.S. Pat. No. 6,436,221, incorporated herein in its entirety.
Thus, the above-described EPD process is advantageously well-adapted for high output and automation. This process and similar EPD processes are very versatile and can be used to form uniform coatings of various thicknesses (e.g.—tens of nanometers to a few micrometers thick), to form coatings on complex shapes, and to form complicated structures such as composites and “gated” electrodes. The methods of the presently disclosed subject matter are useful in producing nanostructure materials which have properties that make them beneficial for use in a number of different applications. The methods of the presently disclosed subject matter are especially beneficial in producing nanostructure materials for incorporation into electron field emission cathodes for devices such as x-ray generating devices, gas discharge tubes, lighting devices, microwave power amplifiers, ion guns, electron beam lithography devices, high energy accelerators, free electron lasers, electron microscopes and microprobes, and flat panel displays.
As previously discussed,
Accordingly, the formation of CNT-matrix particle complexes prior to electrophoretic deposition provides for a suspension of well-dispersed CNT strongly adhered to matrix particles. Therefore, when deposited on a substrate via EPD, the CNT-matrix particle complexes provide for the homogenous deposition of CNT and matrix particles to form a composite film with improved uniformity, stronger adhesion and reduced cracking. The field emission properties from a homogeneous CNT composite film produced by the methods of the presently disclosed subject matter are greatly improved in emission uniformity and current density. Further, electron field emission cathodes produced by the methods of the presently disclosed subject matter have enhanced field emission properties, particularly improved emission uniformity and current density.
The following examples have been included to illustrate representative and exemplary modes of the presently disclosed subject matter. In light of the present disclosure and the general level of skill in the art, those of skill will appreciate that the following examples are intended to be exemplary only and that numerous changes, modifications, and alterations can be employed without departing from the spirit and scope of the presently disclosed subject matter.
Commercially available glass particles were processed via planetary ball milling to produce glass particles with a diameter of 300 nm˜3 μm. The ball-milled glass particles were dispersed in a liquid medium comprising ethanol and a small amount of water. The liquid medium further contained 3-aminopropyl)triethoxysilane (APS) linker molecules at a ratio of APS to glass of 1:800 (ml/mg). The liquid medium containing the APS linker molecules and matrix particles was sonicated for 15 min, whereby a white glass suspension formed. Although not a required step, the suspension is filtered through a filter membrane (5 μm) to remove the excessive APS. After repeated rinsing with ethanol, the functionalized matrix particles, i.e. the APS-modified glass particles, were dried in an oven at 120° C. for 3 hours to cross-link the adsorbed APS to the glass.
Pre-formed CNT were prepared by chemical vapor deposition followed by an acid treatment to remove the catalyst. The acid treatment can also introduce some oxygen-containing groups onto the CNT. The CNT were then dispersed in a liquid medium comprising ethanol, polyvinyl pyrrolidone (PVP) as a dispersing agent, and MgCl2 as a charger. The CNT were included at 0.05 g/L and the PVP at 0.1 g/L. The suspension was sonicated for 30 min to ensure that the CNT were well dispersed and possible agglomerates were separated. The functionalized matrix particles were then added to the suspension, followed by another 30 minute sonication to form a stable suspension. The functionalized matrix particles were added to the suspension at 400 wt % of the total CNT. Electrostatic attraction between the CNT and functionalized matrix particles resulted in the formation of CNT-matrix particle complexes, as illustrated in
Electrophoretic deposition was carried out at room temperature under constant DC voltage (20 V). Stainless steel electrodes, one of which comprised the substrate, were used with an electrode separation of 1 cm. As an optional first step, the stainless steel substrate was immersed into a binder solution containing glass particles and CNT for 25 seconds. Then, CNT-matrix particle complexes were electrophoretically deposited onto the substrate electrode by immersing both the substrate electrode and counter electrode into a suspension of CNT-matrix particle complexes for 25 sec under the applied electrical current. The CNT-matrix particle complex coated substrate was then removed from the EPD suspension, dried at room temperature, and annealed at 700° C. at 5×10−7 torr vacuum for 12 hours.
It will be understood that various details of the presently disclosed subject matter may be changed without departing from the scope of the presently disclosed subject matter. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation.
