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JULY 2019         TAP TIMES          -14-         compound’s solution, coating the entire
                                                                surface in sodium. Finally, they placed the
                                                                material in a furnace and carried out the
             Nanotube ingredients (from 13)                     typical steps involved in the CVD process

                                                                to grow CNTs.

        Li and his colleagues were experimenting
        with ways to grow CNTs on various sur-                  Short, dense forests
        faces by coating them with different solu-              In general, they found that, while iron
        tions of iron-containing compounds, when                catalysts form carbon nanotubes at around
        the team noticed the resulting carbon                   800 degrees Celsius, the sodium catalysts
        nanotubes looked different from what they               were able to form short, dense forests
        expected.                                               of CNTs at much lower temperatures, of
                                                                around 480 C.

        “A little funny”                                        What’s more, after surfaces spent about
        “The tubes looked a little funny, and Rich              15 to 30 minutes in the furnace, the sodi-
        and the team carefully peeled the onion                 um simply vaporized away, leaving behind
        back, as it were, and it turns out a small              hollow carbon nanotubes.
        quantity of sodium, which we suspected
        was inactive, was actually causing all the              “A large part of CNT research is not on
        growth,” Wardle said.                                   growing them, but on cleaning them —
                                                                getting the different metals used to grow
        For the most part, iron has been the tra-               them out of the product,” Wardle said.
        ditional catalyst for growing CNTs. Wardle
        said this is the first time that researchers            “The neat thing with sodium is, we can
        have seen sodium have a similar effect.                 just heat it and get rid of it, and get pure
                                                                CNT as product, which you can’t do with
        “Sodium and other alkali metals have not                traditional catalysts.”
        been explored for CNT catalysis,” Wardle
        said. “This work has led us to a different
        part of the periodic table.”                            Quality improvement is the goal
                                                                Li says future work may focus on improv-
                                                                ing the quality of CNTs that are grown us-
        Testing their theory                                    ing sodium catalysts.
        To make sure their initial observation
        wasn’t just a fluke, the team tested a                  The researchers observed that while sodi-
        range of sodium-containing compounds.                   um was able to generate forests of carbon
        They initially experimented with commer-                nanotubes, the walls of the tubes were not
        cial-grade sodium, in the form of baking                perfectly aligned in perfectly hexagonal
        soda, table salt, and detergent pellets,                patterns—crystal-like configurations that
        which they obtained from the campus con-                give CNTs their characteristic strength.
        venience store.
                                                                Li plans to “tune various knobs” in the CVD
        Eventually, however, they upgraded to pu-               process, changing the timing, tempera-
        rified versions of those compounds, which               ture, and environmental conditions, to im-
        they dissolved in water.                                prove the quality of sodium-grown CNTs.
                                                                                                          mit.edu
        They then immersed a carbon fiber in each
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