Zhi-Ling Hou

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.

Facile fabrication of ultrathin graphene papers for effective electromagnetic shielding

Ultrathin electromagnetic interference (EMI) shielding materials promise great application potential in portable electronic devices and communication instruments. Lightweight graphene-based materials have been pursued for their exclusive microstructures and unique shielding mechanism. However, the large thickness of the current low-density graphene-based composites still limits their application potential in ultrathin devices. In this work, a novel approach has been taken to use conductive graphene paper (GP) in the fabrication of ultrathin EMI shielding materials. The as-prepared flexible GPs exhibit highly effective shielding capabilities, reaching ∼19.0 dB at ∼0.1 mm in thickness and ∼46.3 dB at ∼0.3 mm in thickness, thus the thinnest GPs having the best shielding performance among graphene-based shielding materials. Double-layered shielding attenuators have been designed and fabricated for a high shielding performance of up to ∼47.7 dB at a GP thickness of ∼0.1 mm. Mechanistically, the high performance should be due to Fabry–Perot resonance, which is unusual in carbon-based shielding materials. The preparation of conductive GPs of superior shielding performance is relatively simple, amenable to large-scale production of ultrathin materials for EMI shielding and electromagnetic attenuators, with broad applications in lightweight portable electronic devices.

Alignment of graphene sheets in wax composites for electromagnetic interference shielding improvement

Rapid advancements in carbon-based fillers have enabled a new and more promising platform in the development of electromagnetic attenuation composites. Alignment of fillers in composites with specific structures and morphologies has been widely pursued to achieve high performance based on taking advantage of unique filler characteristics. In this work, few-layer graphene (FLG), obtained from direct exfoliation of graphite, was fabricated into paraffin wax to prepare FLG/wax composites and investigate their electromagnetic interference (EMI) shielding performance. The as-exfoliated FLG/wax samples have shown much improved EMI performance compared to the commercial graphite/wax ones. For further improvement of EMI shielding performance, split-press-merge approaches were applied to align the FLG fillers to achieve anisotropic characteristics in the plane perpendicular to the pressing direction. Much enhanced EMI shielding performance coupled with an improvement in absorption and reflection was observed in the post-alignment FLG/wax composites. An average interparticle distance model associated with improved electrically conducting interconnection and enlarged effective reflection regions with respect to enhanced reflection efficiency were discussed. The results suggest a platform and promising opportunities for preparing high-performance EMI shielding composites.

Boron Nitride Nanomaterials for Thermal Management Applications

Hexagonal boron nitride nanosheets (BNNs) are analogous to their two-dimensional carbon counterparts in many materials properties, in particular, ultrahigh thermal conductivity, but also offer some unique attributes, including being electrically insulating, high thermal stability, chemical and oxidation resistance, low color, and high mechanical strength. Significant recent advances in the production of BNNs, understanding of their properties, and the development of polymeric nanocomposites with BNNs for thermally conductive yet electrically insulating materials and systems are highlighted herein. Major opportunities and challenges for further studies in this rapidly advancing field are also discussed.

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...

Low dielectric loss and non-Debye relaxation of gamma-Y2Si2O7 ceramic at elevated temperature in X-band

Bulk single-phase gamma-Y2Si2O7 ceramic has been synthesized from a mixture of Y2O3 powder and SiO2 nanopowder at 1400 °C. The dielectric properties are reported at the temperature ranging from room temperature to 1400 °C in X-band. The results show that gamma-Y2Si2O7 exhibits low dielectric loss and non–Debye relaxation behavior different from that of SiO2. The peculiar relaxation peak is attributed to the structural relaxation polarization caused by thermal-excitation structural defects, which implies that no ionic conductance exists in this material. Such low dielectric loss will draw much attention for potential dielectric applications at high temperature.

A wearable microwave absorption cloth

Wearable functional materials and textiles have attracted overwhelming attention in a broad range of industries owing to their exclusive merits for developing smart electronic and energy devices. As they are massively utilized in the telecommunication and aerospace communities, microwave absorption materials also require fascinating properties that enable them to exhibit excellent performance ranging from mechanical features to functionalities. Unfortunately, conventionally developed microwave absorbing fillers are generally limited in practice for the undesirable performance in terms of stability and poor durability, which is out of the scope for exploiting wearable and long-term microwave absorption materials. To overcome such limitations, a wearable microwave absorption cloth was fabricated via in situ employing carbon materials into a nonwoven matrix, showing a range of advantages that meet the criteria of high-performance wearable electromagnetic functional materials. According to the best performance curve as well as radar cross section values from a CST simulation, the as-fabricated cloths can deliver ideal microwave absorption performance based on the unique structural configuration. Practical applications indicate that the effective absorption bandwidth of 8.2–14.5 GHz at a thickness of 4 mm has been achieved in a wearable fashion, manifesting a novel platform for developing advanced wearable functional cloth.

Electronic scattering leads to anomalous thermal conductivity of n-type cubic silicon carbide in the high-temperature region.

This study simulates thermal conductivity via a carrier scattering mechanism and the related parameters are obtained based on first principles for intrinsic and doped silicon carbide (SiC) over a temperature range of 300-1450 K. The theoretical analysis results show that the thermal conductivity decreases with increasing temperature along each orientation for both cubic SiC (3C-SiC) and doped SiC. Compared with traditional calculations, the thermal conductivity of doped SiC is larger than that of intrinsic SiC in the high-temperature region. In particular, the n-type thermal conductivity is higher than the p-type thermal conductivity because of the scattering probability between electrons and the ionization impurity increasing with the temperature. Our studies are important to a further understanding of thermal transportation.