Abstract
A method encapsulates nanoscale material by producing a suspension of the nanostructure material in a first solvent using a micelle surrounding the nanostructure material. The micelle surrounding the suspended nanostructure material is swollen by adding to and mixing with the suspension an immiscible phase second solvent containing a precursor. The precursor is then reduced by adding a reducing reactant selectively soluble in the first solvent that reacts to the precursor containing reactant selectively solvated in the second solvent to encapsulate the nanostructure material. A metal-nanostructure composite can be provided by collecting and mixing the metal-shell encapsulated nanostructure product produced by the aforementioned method into a metal matrix.
Claims
1. A method for encapsulating nanoscale material, comprising: (a) producing a suspension of the nanostructure material in a first solvent using a micelle surrounding the nanostructure material, (b) swelling the micelle surrounding the suspended nanostructure material by adding to and mixing with the suspension an immiscible phase second solvent containing a precursor, and (c) reducing the precursor by consisting of the step of adding a reducing reactant selectively soluble in the first solvent that reacts to the precursor containing reactant selectively solvated in the second solvent to encapsulate the nanostructure material; wherein the reducing reactant being used is hydrazine.
2. The method of claim 1, further comprising: (d) collecting and mixing the encapsulated nanostructure material into a matrix.
3. The method of claim 2, wherein the matrix is a metal matrix.
4. The method of claim 1, wherein the nanostructure material is comprised of individual carbon nanotubes suspended in the first solvent.
5. The method of claim 1, wherein the second solvent comprises an oil phase with the precursor.
6. The method of claim 1, wherein the precursor is an organometallic precursor that is reduced to its base metal in step (c).
7. The method of claim 6, wherein the second solvent comprises an oil phase with the precursor.
8. The method of claim 1, wherein the surfactant is selected to provide a hydrophilic head and a hydrophobic head for producing an oleophilic micellar region around the suspended nanostructure material.
9. The method of claim 8, wherein the nanostructure material is comprised of individual carbon nanotubes.
10. The method of claim 9, wherein the nanostructure material is comprised of single-wall carbon nanotubes and the metal salt is iron (III) acetylacetonate (Fe(acac).sub.3) or copper (II) acetylacetonate (Cu(acac).sub.2) and with chloroform as the immiscible phase being used in step (b).
11. The method of claim 1, wherein the precursor contains dissolved metal salt.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIGS. 1A-1E are schematic step-by-step diagrams illustrating the CNT encapsulation process of the present invention.
(2) FIGS. 2A-2B are, respectively, a tunneling electron micrograph with electron energy loss spectroscopy which show bright edges indicative of metal presence on the edges of the CNTs and a TEM image of a coated CNT bundle with electron energy loss elemental scan showing copper on the CNT surface;
(3) FIGS. 3A-3C are Raman spectra of CNTs, dissolved Cu salt, and Cu-encapsulated CNTs showing the CNT structure remains stable after encapsulation according to the method of the present invention;
(4) FIG. 4 is a table indicating the stability limits for the concentrations of each reactant in the process according to the present invention;
(5) FIG. 5 is a graph showing the improvement in specific conductivity of a copper metal by adding Cu-encapsulated CNTs made in accordance with the present invention; and
(6) FIG. 6 is a chart showing improvement cyclic fatigue life by adding Fe-encapsulated CNT made according to the present invention over adding untreated CNTs to an iron matrix.
DETAILED DESCRIPTION OF THE DRAWINGS
(7) Referring to FIGS. 1A-1E, our method is comprised of a series of mixing and reacting steps which ultimately lead to encapsulated CNTs. In FIG. 1A, the CNT 101 is suspended in water using a surfactant composed of a hydrophilic head 102 and a hydrophobic head 103 which creates an immiscible oleophilic micellar region 104 around the CNT. In FIG. 1B, the aqueous CNT suspension is then mixed with an oil phase containing dissolved metal salts 105 which remain inside the micelle, near the CNT and cause the micelle to swell. In FIG. 1C, an aqueous reducing agent 106 is then added to the swelled suspension and thoroughly mixed. In FIG. 1D at the water-oil interface, the reducing agent in the aqueous phase reacts with the dissolved metal salt in the oil phase reducing the metal salt and leading the metal to collapse around the CNT 107. In FIG. 1E, the surfactant is rinsed away causing the suspension to collapse and the metal-coated CNTs 108 are collected. The two solution phases, i.e., the water and oleophilic micellar region 104 of FIGS. 1A and 1B must be immiscible and the two reactants 105, 106 of FIGS. 1B and 1C must be respectively soluble only in the opposing phases such that reaction and reduction of the metal only occurs at the interface between the phases. For example, the metal precursor, being only soluble in the immiscible phase immediately adjacent to the nanostructure 104, reacts with the reducing reactant, being only soluble in the bulk solution, react and form the metal coating at the interface between the bulk solution and immiscible phase 104.
(8) FIG. 2A is a dark field tunneling electron micrograph of CNTs that have undergone the encapsulation process. The bright spots around the edges are indicative of heavier elements, namely a metal, at these sites. The metal layer measures 0.49 nm. FIG. 2B is a TEM image of a CNT bundle with EELS elemental mapping showing Cu on the surface. There is also oxygen observed on the surface likely due to oxidation of the copper layer during storage under air.
(9) FIGS. 3A-3C are, respectively, Raman spectra of the suspended CNTs, Cu in an organic solvent, and suspended Cu-coated CNTs. In FIG. 3A, the peak at 1650 cm.sup.−1 is indicative of highly graphitic CNTs while the wide peaks between 1700 and 2500 cm.sup.−1 are artifacts of the sodium dodecyl beneze sulfonate (SDBS) surfactant micelle structure used to suspend the CNTs. FIG. 3B shows the Raman spectra of copper acetyleacetonate (Cu(acac).sub.2) in chloroform where the peaks at 150, 350, 600, 700, and 1250 cm.sup.−1 are from the chloroform. FIG. 3C shows the Raman spectra of the suspended CNTs after encapsulating with copper and rinsing to remove the SDBS. There are no chloroform peaks present in the Raman signal indicating that the oil-phase and any of the remaining and unreacted dissolved metal-containing salts have been removed.
(10) FIG. 4 is a table showing the stability of a CNT suspension with swelled surfactant micelles containing Cu(acac).sub.2 in chloroform at various Cu(acac).sub.2 concentrations as the ratio of the reducing agent to metal precursor concentration is varied. For this specific case of Cu(acac).sub.2 in chloroform with hydrazine as the reducing agent, Cu(acac).sub.2 concentrations below 0.10 M and a molar ratio of hydrazine to Cu(acac).sub.2 concentrations below 10:1. Working within a stable reaction zone is critical to completing the CNT metal encapsulation process and forming completely encapsulated CNTs.
(11) FIG. 5 is a graph showing the effect of Cu-encapsulated CNT loading on the specific conductivity of a Cu-CNT composite. Suspended single-wall CNTs were encapsulated using Cu(acac).sub.2 as the metal salt selectively soluble in the oil, chloroform as the oil phase, and hydrazine as the reducing agent that is selectively soluble in the water phase. As the loading of Cu-encapsulated CNTs was increased from 0-1% m/m the specific conductivity of the material increased from 4.5×10.sup.9 to 6.1×10.sup.9 S-cm.sup.2/g based on four-point-probe measurements.
(12) FIG. 6 is a chart showing the effectiveness of our Fe-encapsulation method on the fatigue life performance of an iron-CNT composite. Suspended, single-wall CNTs were coated with Fe using iron (III) acetylacetonate (Fe(acac).sub.3) as the metal salt selectively soluble in the oil, chloroform as the oil phase, and hydrazine as the reducing agent selectively soluble in the water phase. The Fe-encapsulated CNTs were dry mixed with pure iron powder at loadings up to 0.09% v/v. Non-encapsulated CNTs were also mixed with pure Fe powder at 0.5% and 1% v/v. The powders were first spark-plasma sintered (SPS) to form solids, then milled using electrical discharge machining (EDM) to form dog-bone shapes for fatigue life testing. Adding untreated CNTs to the metal powder reduced the fatigue life likely by providing more sources of crack propagation and weak points where the CNTs aggregate. As can be seen in FIG. 6, the sample containing Fe-encapsulated CNTs increased the fatigue life of the pure iron by nearly 100%
(13) While we have shown and described several embodiments in accordance with our invention, it should be understood that the same is susceptible to further changes and modifications without departing from the scope of our invention. Therefore, we do not want to be limited to the details shown and described herein but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.
