Wen-Qiang Cao

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.

Ultrathin graphene: electrical properties and highly efficient electromagnetic interference shielding

Ultrathin graphene, a wonder material, exhibits great promise in various fields with its unique electronic structure and excellent physical, chemical, electrochemical, thermal and mechanical properties. Graphene presents great progress in electromagnetic interference (EMI) shielding. Herein, we review the advance in graphene-based EMI shielding materials. Towards graphene composites, we intensively evaluate EMI shielding efficiency and meaningfully describe the mechanism, such as polarization, hopping conduction and interface scattering. Moreover, we highlight an important direction for enhancing EMI shielding, the architectures, including alignment, paper, film and foam. Following that, the problems are summarized and the prospect is also highlighted for significant applications of ultrathin graphene in the field of EMI shielding.

NiO Hierarchical Nanorings on SiC: Enhancing Relaxation to Tune Microwave Absorption at Elevated Temperature

We fabricated NiO nanorings on SiC, a novel hierarchical architecture, by a facile two-step method. The dielectric properties depend on temperature and frequency in the range from 373 to 773 K and X band. The imaginary part and loss tangent increase more than four times and three times with increasing temperature, respectively. The architecture demonstrates multirelaxation and possesses high-efficient absorption. The reflection loss exceeds -40 dB and the bandwidth covers 85% of X band (approximately -20 dB). The synergistic effect between multirelaxation and conductance is beneficial to the microwave absorption. Our findings provide a novel and feasible strategy to tune microwave absorption.

Temperature dependent microwave absorption of ultrathin graphene composites

Ultrathin, lightweight graphene composites exhibit high-efficiency microwave absorption at elevated temperatures as well as thermal-stability permittivity. The minimum reflection loss reaches −42 dB and the widest bandwidth covers the entire X-band (−10 dB). More significantly, the composites possess one high-efficiency absorption belt with a value ≤−15 dB, as well as two ‘islands’ of reflection loss of ≤−17 dB and −30 dB. These excellent properties arise from the synergistic effect of polarization and conductivity. Our finding demonstrates that ultrathin graphene is a promising microwave absorber for microwave attenuation devices, information security and electromagnetic pollution defense.

Enhanced permittivity and multi-region microwave absorption of nanoneedle-like ZnO in the X-band at elevated temperature

We fabricated nanoneedle-like ZnO (ZnOn) by a facile combustion synthesis route, demonstrating the original observations on the complex permittivity of ZnOn ranging from 298 to 573 K and in the X band. Both real and imaginary permittivities (e′ and e′′) of ZnOn depend on temperature and frequency. The e′ increases while the e′′ possesses a maximum value with increasing temperature. The maximum value of e′′ appears at 473 K, and the corresponding loss tangent increases by 100% than that at 298 K. The relaxations in e′′ mainly arise from dipole polarization and interfacial polarization in ZnOn. Multi-region microwave absorption of ZnOn appears at the investigated temperature and frequency as well as thickness. The minimum reflection loss value reaches −44 dB, which is superior to the previous reports. These findings provide a novel and feasible strategy to tune microwave absorption towards a structural absorber at elevated temperature.

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.

Multiscale Assembly of Grape-Like Ferroferric Oxide and Carbon Nanotubes: A Smart Absorber Prototype Varying Temperature to Tune Intensities

Ideal electromagnetic attenuation material should not only shield the electromagnetic interference but also need strong absorption. Lightweight microwave absorber with thermal stability and high efficiency is a highly sought-after goal of researchers. Tuning microwave absorption to meet the harsh requirements of thermal environments has been a great challenge. Here, grape-like Fe3O4-multiwalled carbon nanotubes (MWCNTs) are synthesized, which have unique multiscale-assembled morphology, relatively uniform size, good crystallinity, high magnetization, and favorable superparamagnetism. The Fe3O4-MWCNTs is proven to be a smart microwave-absorber prototype with tunable high intensities in double belts in the temperature range of 323-473 K and X band. Maximum absorption in two absorbing belts can be simultaneously tuned from ∼-10 to ∼-15 dB and from ∼-16 to ∼-25 dB by varying temperature, respectively. The belt for reflection loss ≤-20 dB can almost cover the X band at 323 K. The tunable microwave absorption is attributed to effective impedance matching, benefiting from abundant interfacial polarizations and increased magnetic loss resulting from the grape-like Fe3O4 nanocrystals. Temperature adjusts the impedance matching by changing both the dielectric and magnetic loss. The special assembly of MWCNTs and magnetic loss nanocrystals provides an effective pathway to realize excellent absorbers at elevated temperature.

3D Fe3O4 nanocrystals decorating carbon nanotubes to tune electromagnetic properties and enhance microwave absorption capacity

We fabricated a novel dielectric–magnetic nanostructure by hybridizing 3D Fe3O4 nanocrystals and multi-walled carbon nanotubes through a simple co-precipitation route. The 3D Fe3O4-MWCNTs composites demonstrate enhanced microwave absorption with tunable strong-absorption wavebands in the frequency range of 2–18 GHz. Double-band microwave absorption appears in the investigated frequency range and at various thicknesses. This depends on the loading concentration of 3D Fe3O4-MWCNTs. Minimum reflection loss values at 20 wt% loading of −23.0 dB and −52.8 dB are observed at 4.1 GHz and 12.8 GHz, respectively, which are superior to those of pure MWCNTs as well as other hybrids of Fe3O4. The improved absorption capacity arises from the synergy of dielectric loss and magnetic loss, as well as the enhancement of multiple interfaces among 3D Fe3O4 nanocrystals. All of these factors increase the flexibility of tuning microwave absorption. These results provide a new strategy to tune electromagnetic properties and enhance the capacity of high-efficient microwave absorbers.

Nd doping of bismuth ferrite to tune electromagnetic properties and increase microwave absorption by magnetic–dielectric synergy

Simultaneously achieving tunable electromagnetic parameters and strong absorption capacity in a single material is still a great challenge. Here, we present Nd doped BiFeO3 with electromagnetic matching, which exhibits tunable electromagnetic properties and high-performance microwave absorption. The experimental and calculated results demonstrate that Nd doping generates the ordered domain structure and changes the coupling states of electrons, which induce difficult polarization rotation and strong natural ferromagnetic resonance, leading to the decrease of dielectric loss and the increase of magnetic loss with increasing Nd concentration. The electromagnetic parameters are tuned from mismatching to matching, and the microwave absorption is improved. Bi0.8Nd0.2FeO3 exhibits a remarkable reflection loss (RL) of −42 dB and bandwidth (RL ≤ −10 dB) which covers nearly three quarters of the X-band at a thickness from 1.9 to 2.1 mm. This work highlights the applications of BFO as a high-performance microwave absorber and opens up a promising feasible route to the development of microwave absorbers in imaging, healthcare, information safety and military fields.

Hierarchical three-dimensional flower-like Co 3 O 4 architectures with a mesocrystal structure as high capacity anode materials for long-lived lithium-ion batteries

In this work, we rationally design a high-capacity electrode based on three-dimensional (3D) hierarchical Co3O4 flower-like architectures with a mesocrystal nanostructure. The specific combination of the micro-sized 3D hierarchical architecture and the mesocrystal structure with a high porosity and single crystal-like nature can address the capacity fading and cycling stability as presented in many conversion electrodes for lithium-ion batteries. The hierarchical 3D flower-like Co3O4 architecture accommodates the volume change and mitigates mechanical stress during the lithiation–delithiation processes, and the mesocrystal structure provides extra lithium-ion storage and electron/ion transport paths. The achieved hierarchical 3D Co3O4 flower-like architectures with a mesocrystal nanostructure exhibit a high reversible capacity of 920 mA·h·g−1 after 800 cycles at 1.12 C (1 C = 890 mA·h·g−1), improved rate performance, and cycling stability. The finding in this work offers a new perspective for designing advanced and long-lived lithium-ion batteries.