Jialin Gu

Carbon nanotubes filled with ferromagnetic alloy nanowires: Lightweight and wide-band microwave absorber

Thin-walled carbon nanotubes (CNTs) filled with different ferromagnetic alloy (FeCo, FeNi, and FeCoNi) nanowires were prepared by using trichlorobenzene as carbon precursor. They were dispersed into epoxy resin and then coated onto 180×180 mm2 aluminum substrates to form microwave-absorption coatings with 2.0 mm thickness. Reflection loss exceeding −5 dB was obtained between 5 and 18 GHz for coating containing 1.3 wt % FeCo-filled CNTs. A minimum reflection loss value of −28.2 dB was achieved at 15.2 GHz in FeCoNi-filled CNTs/epoxy coating. The areal densities of coatings are only 2.35 kg/m2, which is favorable for the applications requiring low density.

Influences of density and flake size on the mechanical properties of flexible graphite

Abstract The mechanical properties of flexible graphite sheets made by exfoliated graphite flakes have been investigated in term of the density changes and the flake size variation. The fracture toughness of flexible graphite sheets has been characterized using the concept of essential fracture work (we). The experimental results indicate that both tensile strength and the fracture toughness monotonically increase, but the sheet flexibility decreases with the density of flexible graphite sheets and the size of pristine graphite flakes. A compromise combination of compression and recovery can be achieved in the density range of 1.0–1.4 g cm−3. The flake size has little effects on both compression and recovery. A micro-disc model has been developed based on friction between the interlocked micro-discs in which air is encapsulated. This model interprets the effects of the density and flake size on certain mechanical properties of the flexible graphite sheets.

Optimization of electromagnetic matching of Fe-filled carbon nanotubes/ferrite composites for microwave absorption

Composite microwave absorbers have been fabricated by Fe-filled carbon nanotubes (CNTs) and a W-type hexagonal ferrite. The microwave absorption of the absorbers can be tuned by matching the dielectric loss and the magnetic loss by changing the ratio of ferrite in the composites. The microwave absorbing properties of the absorber are evidently improved as compared with the absorber containing only CNTs or ferrite. The reflection loss of the composites with 40 wt% ferrite and 6.2 vol% CNTs at 2.0 mm thickness reached 21.9 dB at 8.5 GHz. The maximum absorption capability of 50.5 dB GHz was obtained at a ferrite concentration of 60 wt%. With the increase in the weight percentage of ferrite in the composites the resonance absorption peak shifted towards the lower frequency region.

High-yield bamboo-shaped carbon nanotubes from cresol for electrochemical application

High-yield bamboo-shaped carbon nanotubes (BCNTs) have been produced by using cresol as the precursor, for the first time and there are almost no straight CNTs or amorphous carbon found in the product: the role of cresol in promoting the growth of BCNTs is discussed; improved cycle stability and electric conductivity of the BCNTs as an anode additive in a lithium ion battery are achieved.

Synthesis and Enhanced Field-Emission of Thin-Walled, Open-Ended, and Well-Aligned N-Doped Carbon Nanotubes

Thin-walled, open-ended, and well-aligned N-doped carbon nanotubes (CNTs) on the quartz slides were synthesized by using acetonitrile as carbon sources. As-obtained products possess large thin-walled index (TWI, defined as the ratio of inner diameter and wall thickness of a CNT). The effect of temperature on the growth of CNTs using acetonitrile as the carbon source was also investigated. It is found that the diameter, the TWI of CNTs increase and the Fe encapsulation in CNTs decreases as the growth temperature rises in the range of 780–860°C. When the growth temperature is kept at 860°C, CNTs with TWI = 6.2 can be obtained. It was found that the filed-emission properties became better as CNT growth temperatures increased from 780 to 860°C. The lowest turn-on and threshold field was 0.27 and 0.49 V/μm, respectively. And the best field-enhancement factors reached 1.09 × 105, which is significantly improved about an order of magnitude compared with previous reports. In this study, about 30 × 50 mm2 free-standing film of thin-walled open-ended well-aligned N-doped carbon nanotubes was also prepared. The free-standing film can be transferred easily to other substrates, which would promote their applications in different fields.

New origin of spirals and new growth process of carbon whiskers

Abstract A new kind of carbon whisker, different from others, has been found by means of high resolution electron microscopy and field emission scanning electron microscopy. Carbon layers are almost perpendicular to the whisker axis and a spiral structure is formed around it. Structure analysis indicates the growth process consists of two stages; firstly, carbon layers stacked in turbostratic manner, and then gradually graphitized. Accordingly, the final structure of whiskers is perfect graphitized texture. The spirals are not similar to the so-called ‘growth spirals’ formed by the screw dislocation mechanism, and a more reasonable growth mechanism of whiskers is suggested in accordance with the analysis.

Fracture mechanism of flexible graphite sheets

The fracture behavior of flexible graphite sheets (FGSs) was investigated by loading along and perpendicular to the sheet surface. The vertical strength is lower than the tensile strength by two orders of magnitude, at only 0.03 MPa. The fracture processes of a notched specimen of FGS and compressed graphite worms were also studied directly and by in-situ observation by SEM. From observations of the fractured surfaces, structural units for the fracture of FGS are assumed to be interlocked micro-discs composed of orientated graphite layers, which originate from the worm-like particles of the original exfoliated graphite. During the formation of FGS by compression and rolling, micro-discs in the different worms interlock and become structural units. When a static tensile load was applied to FGS, some structural units rotated, cleaved and produced microcracks. When the load was increased, slip between the structural units of FGS started and the structural units slipped away from each other and the FGS was broken. Therefore, The tensile strength of FGS originates from the frictional force between the structural units.

Three-dimensional reduced graphene oxide powder for efficient microwave absorption in the S-band (2–4 GHz)

Efficient absorption in the S-band (2–4 GHz) has been a very challenging task for developing high-performance microwave absorption materials. Three-dimensional reduced graphene oxide (3D-rGO) powders were prepared by using a hydrothermal method and subsequent thermal treatment. It is found that as-prepared 3D-rGO could significantly enhance the electromagnetic wave attenuation in 2–4 GHz. When the content in the paraffin matrix is 4%, the 3D-rGO shows the strongest absorption in S-band, and the absorption will get stronger with the increase of coating thickness. When the thickness is 5 mm, the bandwidth of reflection loss less than −5 dB is in the range of 2.3 to 4.1 GHz, that is, it can almost cover the whole S-band. The excellent microwave absorption could be attributed to the honeycomb-like structures and the strong polarization of 3D-rGO powders. Considering the low density and good corrosion resistance, 3D-rGO powders may serve as an excellent component for the design of lightweight electromagnetic wave absorption coatings.