Synthesis And Applications Of Thin-walled Carbon Nanotubes Filled With Ferromagnetic Nanowires

Ferromagnetic metal (e.g. Fe, Co, Ni or their alloy) nanowires are usually not resistant to oxidation. If they could be filled into the inner cores of carbon nanotubes (CNTs), the antioxidant ability of ferromagnetic metals will be remarkably enhanced due to the effective protection of CNTs. Furthermore, CNTs filled with ferromagnetic metals (M@CNTs) can combine the magnetic property of metal nanowires and electric property of CNTs together, and show potential applications in many fields, such as high-density magnetic data storage, probes for magnetic force microscopy, human tumor therapy, microwave absorption and field emission display.Despite of the great progresses made in the synthesis of M@CNTs, the filling efficiency of ferromagnetic metals into CNTs of previous reports seems very low. This can be seen from the following two facts: 1) the sidewalls of CNTs are very thick (8～40 nm) and 2) most of the encapsulated metals exist as particles or short rods (lengths <500 nm). Such low filling efficiency will limit their practical applications. Therefore, it is crucial to develop an easy, high-efficiency and well-controlled method to prepare thin-walled CNTs filled with long continuous ferromagnetic nanowires.Aiming at the above-mentioned problems, water-assisted pyrolysis method was proposed for the synthesis of CNTs filled with iron nanowires (Fe@CNTs). When the flow of water is 0.04 mL/min, Fe@CNTs with thinner sidewalls (～4 nm) and better soft magnetic performance can be obtained.Effect of using different chlorine-containing precursors (C₆H_6-xCl_x (x=0～3)) in the synthesis of FeNi-filled CNTs (FeNi@CNTs) was also investigated. A ratio din/W is used to evaluate quantitatively the hollow degree of CNTs, where din is the inner diameter and W is the wall thickness of a nanotube. The larger this ratio is, the wider inner cavity of the CNTs will be. Experimental results show that both the hollow degrees of CNTs and FeNi encapsulation increase with the increase of Cl content in carbon precursors. A general strategy for the in-situ solution-feeding synthesis of thin-walled CNTs filled with long continuous ferromagnetic (FeNi, FeCo, FeCoNi, etc.) nanowires was proposed, by using Cl-contained hydrocarbons (e.g. trichlorobenzene) as solvent and combination of different metalocenes (e.g. ferrocene, nickelocene, cobaltocene) as solute. Furthermore, theoretical calculations were carried out to prove that Cl radicals have size-dependent etching effect on the sidewalls of CNTs and are promotional to the growth of thin-walled CNTs. On the basis of experimental results, an open-ended growth model for the in-situ growth of thin-walled CNTs filled with long continuous ferromagnetic nanowires was proposed. The field-emission property of thin-walled FeNi@CNTs was investigated.As-synthesized thin-walled FeNi@CNTs exhibit remarkably enhanced field electron emission performance with a low turn-on field of 0.3 V/μm and large enhancement factors over 1～2 order of magnitude than those reported in recent literatures. The field-emission properties of as-synthesized thin-walled FeNi@CNTs are better than both semiconductor nanomaterials (e.g. ZnO nanowires, ZnS nanobelts) and unfilled CNTs (e.g. aligned CNTs, single-walled CNTs).Microwave-absorption coatings were made by dispersing as-prepared M@CNTs into epoxy resin matrix. It is found that the reflection losses in S-band (2～4 GHz), C-band (4～8 GHz) and X-band (8～12 GHz) are enhanced in the order of FeCoNi@CNTs < FeNi@CNTs< FeCo@CNTs. The areal density of as-prepared coatings is only 2.35 kg/m² when the coating thickness is 2.0 mm. Therefore, the ferromagnetic alloy-filled CNTs are promising to be used as lightweight and wide-band microwave absorbers in the aircrafts.