Xiao-Ling Shi

Dual nonlinear dielectric resonance and nesting microwave absorption peaks of hollow cobalt nanochains composites with negative permeability

The permittivity and permeability behaviors of the hollow cobalt nanochains composites have been investigated in 2–18 GHz. The permittivity presents two dielectric resonance peaks at about 12.3 and 14.5 GHz, respectively, which mainly results from the cooperative consequence of the hollow structure and the one-dimensional structure of the as-synthesized Co nanochains. The negative permeability behavior within 12.3–18 GHz is attributed to radiation of the magnetic energy according to the as-established equivalent circuit model. Two strong absorption peaks of the composites nest at the resonance frequencies due to the effect of the dual nonlinear dielectric resonance and the negative permeability behavior.

Synthesis and enhanced ethanol sensing characteristics of α-Fe2O3/SnO2 core–shell nanorods

Alpha-Fe(2)O(3)/SnO(2) core-shell nanorods are synthesized via a three-step process. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses reveal that their diameters and lengths are respectively in the ranges 35-120 nm and 0.35-1.2 microm, and the thickness of the shell composed of 3.5 nm SnO(2) nanoparticles is about 10 nm. The core-shell nanostructures exhibit a dramatic improvement in ethanol sensing characteristics compared to pure alpha-Fe(2)O(3) nanorods. The sensor response is up to 19.6 under 10 ppm ethanol exposure at 220 degrees C. Both the response time and the recovery time of the core-shell structures are less than 30 s. Based on the space-charge layer model and semiconductor heterojunction theory, the small thickness of the SnO(2) shell and the formation of heterojunctions contribute to the enhanced ethanol sensing characteristics. Our results demonstrate that one-dimensional metal oxide core-shell nanostructures whose shell thickness is smaller than the Debye length are very promising materials for fabricating gas sensors with good performances.

High dielectric loss and its monotonic dependence of conducting-dominated multiwalled carbon nanotubes/silica nanocomposite on temperature ranging from 373 to 873 K in X-band

The dielectric properties of multiwalled carbon nanotubes/silica (MWNTs/SiO2) nanocomposite with 10 wt % MWNTs are investigated in the temperature range of 373–873 K at frequencies between 8.2 and 12.4 GHz (X-band). MWNTs/SiO2 exhibits a high dielectric loss and a positive temperature coefficient (PTC) of dielectric effect that complex permittivity increases monotonically with increasing temperature. The PTC effect on the dielectric constant is ascribed to the decreased relaxation time of interface charge polarization, and the PTC effect on the dielectric loss is mainly attributed to the increasing electrical conductivity. The loss tangent strongly supports the dominating contribution of conductance to the dielectric loss.

Nonlinear resonant and high dielectric loss behavior of CdS/α-Fe2O3 heterostructure nanocomposites

CdS∕α-Fe2O3 heterostructures, where the CdS nanorods grow irregularly on the side surface of α-Fe2O3 nanorods, were synthesized via a three-step process. The dielectric properties of the CdS∕α-Fe2O3 heterostructure nanocomposites have been investigated. The equivalent circuit model of the CdS∕α-Fe2O3 heterostructures was established, which reasonably explained the nonlinear dielectric resonant behavior of the CdS∕α-Fe2O3 heterostructure nanocomposites in the range of 5–15GHz. The high dielectric loss is mainly attributed to the conductance loss and the dipole relaxation loss in the CdS∕α-Fe2O3 heterostructures.

High capacity and excellent cycling stability of single-walled carbon nanotube/SnO2 core-shell structures as Li-insertion materials

The single-walled carbon nanotube (SWNT)/SnO2 core-shell structures with small diameters as Li-insertion materials are investigated. The initial discharge capacity is up to 1339mAh∕g, and the reversible capacity retention is 89.8% after 100cycles, which is comparable to the performance of commercial graphite anodes. Such good electrochemical properties are attributed to large surface-to-volume ratio of SnO2 and good electrical conductivity of SWNT. Our results demonstrate that the SWNT/core-shell structures are very promising for active Li-insertion materials for Li-ion batteries.

High-temperature dielectric properties and enhanced temperature-response attenuation of β-MnO2 nanorods

Large-scale β-MnO2 nanorods were synthesized by the hydrothermal method. In X band, the microwave attenuation of the β-MnO2 nanorods is evidently enhanced with increasing temperature from 293 to 773 K. The enhanced temperature-response attenuation of β-MnO2 nanorods is mainly attributed to the decrease in the real permittivity and the increase in the imaginary permittivity at high temperature. The decrease in real permittivity would be mainly ascribed to the increase in the disorder degree of orientational alignment of the intrinsic polar moment in the β-MnO2 nanorods with temperature increasing. The increase in imaginary permittivity may result from the lower resistivity with rising temperature.

Dynamic response and reinforcement mechanism of composites embedded with tetraneedlelike ZnO nanowhiskers

A type of glass-fiber reinforced composites was prepared by dispersing tetraneedlelike ZnO nanowhiskers in epoxy matrix. The as-prepared composites exhibited excellent dynamic mechanical properties after the effective dispersion of ZnO nanowhiskers in epoxy. Patulous and fractured reinforcement modes for the composites were proposed. The high strength of composites was attributed to the three dimensional structure of tetraneedlelike ZnO nanowhiskers and the corresponding stress transfer.

Microwave responses and general model of nanotetraneedle ZnO: Integration of interface scattering, microcurrent, dielectric relaxation, and microantenna

Based on the unique geometrical structure of nanotetra-ZnO needle (T-ZnON), we investigate the microwave responses of T-ZnON, including interface scattering, microcurrent attenuation, microantenna radiation, and dielectric relaxation, and build an energy attenuation model. The associated quantitative formula is deduced for calculating the microwave absorption properties of T-ZnON/SiO2 nanocomposite (T-ZnON/SiO2) in the range 8–14 GHz according to the present energy attenuation model. Very good agreement between the calculated and experimental results is obtained in a wide frequency range. The maximum deviation less than 0.5 dB in the range 8–14 GHz is obtained. Using the aforementioned model, we analyze the contribution of microwave responses to the energy attenuation in the frequency range 2–18 GHz, and the results reveal that interface scattering and microcurrent attenuation make the contribution most important. In addition, we calculate the effects of the volume fraction, conductivity, permittivity, ne...

High-temperature conductance loss dominated defect level in h-BN: Experiments and first principles calculations

The dielectric properties of hexagonal boron nitride are investigated in detail. The permittivities hold extremely low values ranging from room temperature to 1500 °C, however, the dielectric loss tangents increase rapidly above 1000 °C. At 1500 °C, the dielectric loss tangent is 20 times more than that at room temperature. The first principles calculations show that the boron vacancy (VB) that gives an acceptor energy level near the valence band presents the lowest ionization energy in the investigated defects, and the calculated VB ionization energy agrees with the experimental value. It indicates that the rapid increase in dielectric loss tangents at high temperature is contributed by electrical conductivity produced by VB ionization under thermal excitation.

Micro-current attenuation modeling and numerical simulation for cage-like ZnO/SiO2 nanocomposite

Based on the microwave absorption properties and the micro-current attenuation mechanism for the cage-like ZnO/SiO2 nanocomposite reported in our previous paper, we established a micro-current attenuation model and the associated quantitative formula for the calculation of microwave absorption properties. Very good correlation between the calculated and the experimental results has been obtained for a broad range of frequencies. The maximum deviation less than 3 dB in X-band was obtained. The model provides useful information for understanding the microwave absorption mechanism.