Preparation And Applications Of High Performance Carbon Nanotube Film

A single carbon nanotube?CNT?consisting of only one element of carbon can be regarded as a molecule or crystal with a seamless tubular structure.Ideally,three sp²hybrid orbitals for each atom are bonded with adjacent atoms through head-on overlapping?bonds,while a big unlocalized?bond is formed by overlapping p orbitals over the whole CNT.The combination of this distinctive bonding configuration and the one-dimensional tubular structure gives individual CNTs superior physical and chemical properties,and this has motivated scientists both in industrial and academic industries to make efforts to assemble individual CNTs into macroscopic forms,such as fiber,ribbon,film etc.,for practical applications.However,the performances of these assemblies are not as good as expected.For example,the tensile strength is only one-tenth or even one percent of those of individual CNTs.In this dissertation,methods based on a hollow cylindrical CNT assembly are exploited for preparing high performance CNT films.Their performances and applications in the mechanical,electrochemical,and microwave absorbing fields are studied in detail.Using a high temperature tube furnace,a hollow cylindrical CNT assembly was continuously prepared.The formation of the assembly benefited both from the confinement effect of the tubular reactor and from the continuous feeding of the carbon source and the catalyst precursor.As the catalyst precursor decomposed in the hot zone of the tube reactor,iron atoms were released and clustered into particles.The carbon source was pyrolyzed on the surfaces of the particles and CNTs grew therein simultaneously.CNTs were continuously connected into a large network by self-assembling of individual CNTs through the Van der Waals force and rebinding of the dangling bonds on the CNT walls,and blown out from the tube reactor to air atmosphere by the carrier gas.The CNT alignment in the assembly was conveniently tailored by varying the blowing velocity.The faster the velocity,the higher the alignment.The assembly mainly consisted of few-walled CNTs with a diameter of 2-6nm and Fe particles coated with a few layers of turbostratic graphite.Most CNTs presented double walls and had good graphitic structures.As measured from the Raman spectrum of the assembly,the ratio of G and D band intensities?I_G/I_D?was as high as4.65.Organic contamination and amorphous carbon were rarely detected.Under the spray of alcohol,a film with aligned CNTs was prepared by drawing,winding,and depositing the CNT assemblies onto the surface of a winding drum that was wrapped with an aluminum foil.With the assistance of the uniform winding,the reciprocating movement of the drum along its axial direction,and pressurized rolling,the prepared film had a uniform thickness and demonstrated excellent mechanical and electrical properties.All values,such as the specific strength and modulus,the tensile strength,and the Young's modulus,were very high,based on the measurements by direct tensile testing and sound velocity approach.The tensile strength was in the range of 10.0-17.0 GPa,while the modulus and ductility in the ranges of 150-300 GPa and6.0-8.5%,respectively.It is the first time that the tensile strength has exceeded 10 GPa for macroscopic CNT structures.The electrical conductivity of the film was in the range of 1.53-2.02×10⁴ S cm^?1.By electrochemical deposition,various porous Pt aggregates were successfully deposited on the CNT film.The size and morphology of Pt particles could be tailored by changing the electrolyte,deposition potential,deposition time,and specific electrochemical deposition technique.We achieved spherical,flower-like and cubic aggregates stacked with polyhedron-,pyramid-and flake-shaped crystals with grain sizes of 50-500 nm.Since all the constituting crystals of the aggregates had faceted structures,the aggregates were porous with interconnected fine crystals.Accelerated degradation testing showed that the prepared catalysts had a high catalytic activity toward oxygen reduction reaction and degraded little even after 8000 potential cycles between 0.6 and 1.24 V_RHE in a 0.5 M H₂SO₄ electrolyte.In contrast,the state of art commercial catalyst of Pt/C tested under identical conditions lost a large portion of activity in the first 1000 cycles and almost all after 3000 cycles.The outstanding durability of the Pt catalyst deposited on CNT film was attributed to the faceted nanocrystals and their interconnected porous network,both of which limited the pathways for Ostwald ripening,Pt migration,and agglomeration processes that adversely affect the catalyst stability.An ultra-thin free-standing film with an excellent microwave absorbing property was prepared under the same conditions as the CNT film mentioned above,except for the incorporation of additional Fe particles.A film with a thickness of only⁶?m was attached on a standard aluminum plate and tested at the frequency range of 2-18 GHz.Results showed that the reflectivity was less than-10 dB over the frequencies from 5.5to 18 GHz.Two maximum absorbing peaks were located at 16.3 and 8.15 GHz with a reflectivity of-39.5 and-37 dB,respectively.Thanks to the excellent load transfer efficiency of the hollow cylindrical CNT assembly,this film also had a good mechanical property.Its tensile strength was in the range of 203-273 MPa,and ductility7.8-10.8%,reaching the levels of general aerospace aluminum alloys.Since the iron content was only about 24 wt.%,the prepared thin film could also be regarded as a kind of light materials.Compared with the previous reported microwave absorbing materials,including those based on CNTs as the main absorber,this film has the potential to meet the requirements of high absorption,wide absorption band,light weight,strong strength,and thin thickness,simultaneously.