Wenchuan Lai

Towards efficient microwave absorption: intrinsic heterostructure of fluorinated SWCNTs

The construction of heterostructures is always effective to achieve efficient microwave absorption (MA) properties. Herein, different from conventional hybrid techniques, a novel microwave absorber with an intrinsic heterostructure is fabricated via direct heating fluorination of SWCNTs (F-SWCNTs) utilizing F2/N2. The as-prepared inhomogeneous F-SWCNTs are confirmed to contain both fluorinated domains and aromatic domains at the nanoscale. The evolution of the fluorinated domains is closely related to the development of the C–F groups, whereas the –CF2 groups have no effect. For MA, it is proposed that the aromatic domains function as attenuation regions, whereas the fluorinated domains serve as microwave transparent regions. When the area of aromatic domains is almost equal to that of fluorinated domains, the complementarity between attenuation ability and impedance match endows F-SWCNTs (fluorine content equals of 6.8%) with a good MA performance. Specifically, with only 4.8 wt% loading, the minimum reflection loss (RL) reaches −64.3 dB at the thickness of 1.61 mm, and the effective absorption region (RL < −10 dB) covers a range of 14.1–18 GHz at the thickness of 1.15 mm. The MA performances rely on the evolution of the physical heterostructure, instead of the introduced chemical bonds or fluorine-containing functional groups. The simplicity and feasibility of our design concept indicate the potential application of F-SWCNTs industrially.

Skin–core structured fluorinated MWCNTs: a nanofiller towards a broadband dielectric material with a high dielectric constant and low dielectric loss

This paper presents a novel skin–core structured multiwalled carbon nanotube (MWCNTs) nanofiller to enhance the dielectric performance of epoxy resin through selective fluorination of the outer shell of the MWCNTs. The outer fluorinated shells not only guarantee good dispersibility of the fluorinated MWCNTs (F-MWCNTs) in epoxy, but also prevent the direct contact of the conductive inner tubes. As a result, the inner tubes in adjacent F-MWCNTs can form abundant microcapacitors, leading to an obvious improvement of the dielectric constant of the epoxy resin with low dielectric loss. When the filler content is 8 wt%, the dielectric constant of the F-MWCNTs/epoxy composite varies from 57 at 100 Hz to 44.9 at 1 MHz, about 11.9 times and 10.8 times that of neat epoxy resin, respectively. Simultaneously, the dielectric loss maintains low values of 0.043 at 100 Hz and 0.036 at 1 MHz, respectively. The broadband high-κ and low dielectric loss demonstrate the frequency-stable dielectric behavior of the F-MWCNTs/epoxy composites, which is attributed to the suppression of the space charge polarization induced by the grafting of the strongly electronegative element fluorine onto the outer shell. We believe that the convenient preparation process and special structure will provide a new design for fabricating broadband high-κ dielectric materials with low dielectric loss.

Low temperature preparation of highly fluorinated multiwalled carbon nanotubes activated by Fe3O4 to enhance microwave absorbing property

Conventional approaches to preparing highly fluorinated multiwalled carbon nanotubes (MWCNTs) always require a high temperature. This paper presents a catalytic approach to realizing the effective fluorination of MWNCTs at room temperature (RT). Fe3O4/MWCNTs composites with Fe3O4 loaded on MWCNTs were first prepared using the solvothermal method, followed by fluorination treatment at RT. The attachment of Fe3O4 changes the charge distribution and dramatically improves the fluorination activity of MWCNTs. Consequently, the fluorine content of fluorinated Fe3O4/MWCNTs (F-Fe3O4/MWCNTs) can reach up to 17.13 at% (almost six times that of the unloaded sample) only after fluorination at room temperature, which leads to an obvious decrease in permittivity. Besides, the partial fluorination of Fe3O4 brings about abnormally enhanced permeability due to strengthened exchange resonance. Benefiting from the lower permittivity and higher permeability, F-Fe3O4/CNTs composite exhibits increased impedance matching and thus an enhanced microwave absorption property with a minimal reflection loss of -45 dB at 2.61 mm when the filler content is 13 wt%. The efficient absorption bandwidth (<-10 dB) reaches 4.1 GHz when the thickness is 2.5 mm. This work illustrates a novel catalytic approach to preparing highly fluorinated MWCNTs as promising microwave absorbers, and the design concept can also be extended to the fluorination of other carbon materials.

Aligned fluorinated single-walled carbon nanotubes as a transmission channel towards attenuation of broadband electromagnetic waves

Ideal microwave absorbing materials need high impedance matching and high attenuation capacity. Towards this goal, in this paper, we present a novel lightweight and broadband electromagnetic wave absorber prepared through facile hybridization of fluorinated single-walled carbon nanotubes (SWCNTs) and pristine SWCNTs. Fluorinated SWCNTs are conveniently oriented under the action of stress due to the strong electrostatic interaction. Thus, the fluorinated SWCNT arrays with low permittivity and high impedance matching can function as a transmission channel to allow the directional propagation of electromagnetic waves. The pristine SWCNT network can guarantee high attenuation capacity for electromagnetic waves. Benefiting from the combination of high impedance matching and high attenuation ability, the hybrids exhibit enhanced microwave absorption performance with a minimal reflection loss of −65.6 dB with only 4.8 wt% loading. The hierarchical structures also endow the hybrid with multiple reflection loss peaks and thus a wide efficient absorption region (5.1 GHz, RL < −10 dB) at a thickness of only 1.45 mm. Besides, the hybrids display favorable low-frequency microwave absorption properties. We believe that the idea of constructing loss area and wave-transparent area can provide a guide to the design of other lightweight and broadband microwave absorbers.

Towards enhanced tribological performance as water-based lubricant additive: Selective fluorination of graphene oxide at mild temperature

HYPOTHESIS It is still difficult to prepare hydrophilic fluorinated graphene with abundant oxygen functional groups and high F/C ratio. EXPERIMENTS Herein, selective fluorination for graphene oxide (GO) was realized to prepare highly oxygen/fluorine dual functionalized graphene (OFG) under a mild temperature without damaging the sheet structure. Its O/C and F/C ratios came up to 0.58 and 0.19 respectively, which contained the intactly reserved oxygen functional groups from GO during fluorination and C-F bonds from addition reaction between aromatic region and F2, as demonstrated via X-ray photoelectron spectroscopy (XPS), thermo gravimetric analysis (TGA), Raman measurements and so on. FINDINGS Experiments and density functional theory (DFT) calculations revealed that oxygen functional groups in-situ promoted addition reaction between aromatic region and F2, and the critical point was confirmed that oxygen functional groups began to react with F2 by substitution reaction. As-prepared OFG presented a 47% and 31% decrease of wear rate compared with that of pure water and GO, respectively. During friction process, the abundant oxygen functional groups of OFG contributed to hydrophilic property and introduced fluorine played an important role in enhancing the tribological performance by self-lubricating behavior. Such mild and simple method is achieved to expand the application of fluorinated graphene in aqueous environment.

Towards Excellent Tribological Performance as Oil-Based Lubricant Additive: Particular Tribological Behavior of Fluorinated Graphene

The poor dispersibility, strong interlayer interaction, and inferior crack resistance ability restrict the employment of graphene as a lubricant additive. Herein, we prepared fluorinated graphene with different F/C ratios by direct fluorination of multilayer graphene utilizing F2. Among them, highly fluorinated graphene (HFG) with an F/C ratio of about 1.0 presented prominent thermal stability and excellent tribological performance as an oil-based lubricant additive, whose friction coefficient and wear rate had a 51.4 and 90.9% decrease compared to that of pristine graphene, respectively. It was confirmed that C-F bonds perpendicular to the graphene plane contributed to increasing the interlayer distance and tribological performance of fluorinated graphene, while the randomly oriented CF2 and CF3 groups did not count as influential, as demonstrated via X-ray diffraction, X-ray photoelectron spectroscopy, and polarized attenuated total reflection-Fourier transform infrared spectroscopy. Meanwhile, Raman measurements traced the formation process of integrated and stable HFG tribofilm during friction process, and the corresponding stability was attributed to the physical and chemical interactions between HFG and friction pairs. More interestingly, the outstanding crack resistance ability of HFG preserved the sheet structure from destruction due to decreased in-plane stiffness and out-plane stress, thus constructing the tough tribofilm. The simple and feasible preparation makes HFG a promising candidate as advanced lubricant in industrial fabrication.