Xiao-Yong Fang

Reduced graphene oxides: light-weight and high-efficiency electromagnetic interference shielding at elevated temperatures.

Chemical graphitized r-GOs, as the thinnest and lightest material in the carbon family, exhibit high-efficiency electromagnetic interference (EMI) shielding at elevated temperature, attributed to the cooperation of dipole polarization and hopping conductivity. The r-GO composites show different temperature-dependent imaginary permittivities and EMI shielding performances with changing mass ratio.

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.

Multi-wall carbon nanotubes decorated with ZnO nanocrystals: mild solution-process synthesis and highly efficient microwave absorption properties at elevated temperature

Light weight and high efficiency are two key factors for microwave absorption materials. In particular, it is extremely important that absorption materials meet the harsh requirements of thermal environments. In this work, multi-wall carbon nanotubes decorated with ZnO nanocrystals (ZnO@MWCNTs) were synthesized by a mild solution-process synthesis. The high-temperature dielectric and microwave absorption properties of SiO2-based composites loaded with ZnO@MWCNTs (ZnO@MWCNTs/SiO2) are investigated in 8.2–12.4 GHz and in the 373–673 K temperature range. The imaginary permittivity e′′ of the composite with 5 wt% loading presents a weak downward trend, while those of the composites with 10 and 15 wt% loading show an upward trend with increasing temperature, which reveals different temperature dependences of e′′. The e′′ for 15 wt% loading is about 10 times that for 5 wt% loading. The maximum loss tangent tan δ values of the composites with 10 and 15 wt% loading exceed 0.8, while that of the composites with 5 wt% loading is less than 0.3. High tan δ is mainly attributed to the conductivity of ZnO@MWCNTs, which is dominated by the hopping of electrons in the ZnO@MWCNT network, which increases with elevated temperature. The addition of ZnO properly adjusts the complex permittivity to endow the ZnO@MWCNT/SiO2 composites with highly efficient and thermally stable microwave absorption coupled with a broad attenuation bandwidth, which almost covers the full X-band for RL ≤ −10 dB. A series of outstanding properties of ZnO@MWCNTs imply that it is a promising functional material in the world of microwave absorption.

Synthesis, magnetic and electromagnetic wave absorption properties of porous Fe3O4/Fe/SiO2 core/shell nanorods

Porous Fe3O4/Fe/SiO2 core/shell nanorods were fabricated, in which the diameter of the pores was 5–30 nm. The magnetic and electromagnetic properties were investigated. The temperature dependent magnetic measurements showed that these nanorods were ferromagnetic with a Verwey temperature of 129 K. The electromagnetic data indicated that effective complementarities between the dielectric loss and the magnetic loss were realized, suggesting that they have excellent electromagnetic wave absorption properties. Thus the porous core/shell nanorods could be used as a kind of candidate absorber.

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-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.

The enhanced polarization relaxation and excellent high-temperature dielectric properties of N-doped SiC

The dielectric properties and microwave attenuation performance of N-doped SiC have been evaluated in 8.2–12.4 GHz in the temperature range of 293–673 K. The N doping dramatically improves the microwave absorption capability of SiC. The minimum reflection loss of N-doped SiC is enhanced to nearly −30 dB with the effective absorption bandwidth [RL(dB) ≤ −10 dB] up to 3 GHz at 673 K. The excellent high-temperature dielectric properties are attributed to multi-relaxations, originated from the polarization relaxations of dipoles induced by the N doping and vacancy defects.

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...

Thermal frequency shift and tunable microwave absorption in BiFeO3 family

Tunable frequency is highly sought-after task of researcher, because of the potential for applications in selecting frequency, absorber, imaging and biomedical diagnosis. Here, we report the original observation of thermal frequency shift of dielectric relaxation in La/Nd doped BiFeO3 (BFO) in X-band from 300 to 673 K. It exhibits an unexpected result: the relaxation shifts to lower frequency with increasing temperature. The relaxation maximally shifts about a quarter of X-band. The nonlinear term of lattice vibration plays an important role in the frequency shift. The frequency shift leads to tuning microwave absorption, which almost covers the whole X-band by changing temperature. Meanwhile, the great increase of dielectric loss of La/Nd doped BFO due to thermal excited electron hopping enhances microwave absorption above ~460 and ~480 K, respectively. The microwave absorption of La/Nd doped BFO surpasses −20 dB at 673 K, and the minimum reflection loss of La doped BFO reaches −39 dB. These results open a new pathway to develop BFO-based materials in electromagnetic functional materials and devices for tunable frequency, stealth and thermal imaging at long wavelength.

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.