Biao Zhao

Morphology-Control Synthesis of a Core–Shell Structured NiCu Alloy with Tunable Electromagnetic-Wave Absorption Capabilities

In this work, dendritelike and rodlike NiCu alloys were prepared by a one-pot hydrothermal process at various reaction temperatures (120, 140, and 160 °C). The structure and morphology were analyzed by scanning electron microscopy, energy-dispersive spectrometry, X-ray diffraction, and transmission electron microscopy, which that demonstrate NiCu alloys have core-shell heterostructures with Ni as the shell and Cu as the core. The formation mechanism of the core-shell structures was also discussed. The uniform and perfect dendritelike NiCu alloy obtained at 140 °C shows outstanding electromagnetic-wave absorption properties. The lowest reflection loss (RL) of -31.13 dB was observed at 14.3 GHz, and the effective absorption (below -10 dB, 90% attenuation) bandwidth can be adjusted between 4.4 and 18 GHz with a thin absorber thickness in the range of 1.2-4.0 mm. The outstanding electromagnetic-wave-absorbing properties are ascribed to space-charge polarization arising from the heterogeneous structure of the NiCu alloy, interfacial polarization between the alloy and paraffin, and continuous micronetworks and vibrating microcurrent dissipation originating from the uniform and perfect dendritelike shape of NiCu prepared at 140 °C.

Yolk–Shell Ni@SnO2 Composites with a Designable Interspace To Improve the Electromagnetic Wave Absorption Properties

In this study, yolk-shell Ni@SnO2 composites with a designable interspace were successfully prepared by the simple acid etching hydrothermal method. The Ni@void@SnO2 composites were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The results indicate that interspaces exist between the Ni cores and SnO2 shells. Moreover, the void can be adjusted by controlling the hydrothermal reaction time. The unique yolk-shell Ni@void@SnO2 composites show outstanding electromagnetic wave absorption properties. A minimum reflection loss (RLmin) of -50.2 dB was obtained at 17.4 GHz with absorber thickness of 1.5 mm. In addition, considering the absorber thickness, minimal reflection loss, and effective bandwidth, a novel method to judge the effective microwave absorption properties is proposed. On the basis of this method, the best microwave absorption properties were obtained with a 1.7 mm thick absorber layer (RLmin= -29.7 dB, bandwidth of 4.8 GHz). The outstanding electromagnetic wave absorption properties stem from the unique yolk-shell structure. These yolk-shell structures can tune the dielectric properties of the Ni@air@SnO2 composite to achieve good impedance matching. Moreover, the designable interspace can induce interfacial polarization, multiple reflections, and microwave plasma.

Facile Synthesis of Novel Heterostructure Based on SnO2 Nanorods Grown on Submicron Ni Walnut with Tunable Electromagnetic Wave Absorption Capabilities.

In this work, the magnetic-dielectric core-shell heterostructure composites with the core of Ni submicron spheres and the shell of SnO2 nanorods were prepared by a facile two-step route. The crystal structure and morphology were investigated by X-ray diffraction analysis, transmission electron microscopy (TEM), and field emission scanning electron microscopy (FESEM). FESEM and TEM measurements present that SnO2 nanorods were perpendicularly grown on the surfaces of Ni spheres and the density of the SnO2 nanorods could be tuned by simply varying the addition amount of Sn(2+) in this process. The morphology of Ni/SnO2 composites were also determined by the concentration of hydrochloric acid and a plausible formation mechanism of SnO2 nanorods-coated Ni spheres was proposed based on hydrochloric acid concentration dependent experiments. Ni/SnO2 composites exhibit better thermal stability than pristine Ni spheres based on thermalgravimetric analysis (TGA). The measurement on the electromagnetic (EM) parameters indicates that SnO2 nanorods can improve the impedance matching condition, which is beneficial for the improvement of electromagnetic wave absorption. When the coverage density of SnO2 nanorod is in an optimum state (diameter of 10 nm and length of about 40-50 nm), the optimal reflection loss (RL) of electromagnetic wave is -45.0 dB at 13.9 GHz and the effective bandwidth (RL below -10 dB) could reach to 3.8 GHz (12.3-16.1 GHz) with the absorber thickness of only 1.8 mm. By changing the loading density of SnO2 nanorods, the best microwave absorption state could be tuned at 1-18 GHz band. These results pave an efficient way for designing new types of high-performance electromagnetic wave absorbing materials.

Preparation of Honeycomb SnO2 Foams and Configuration-Dependent Microwave Absorption Features

Ordered honeycomb-like SnO2 foams were successfully synthesized by means of a template method. The honeycomb SnO2 foams were analyzed by X-ray diffraction (XRD), thermogravimetric and differential scanning calorimetry (TG-DSC), laser Raman spectra, scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR). It can be found that the SnO2 foam configurations were determined by the size of polystyrene templates. The electromagnetic properties of ordered SnO2 foams were also investigated by a network analyzer. The results reveal that the microwave absorption properties of SnO2 foams were dependent on their configuration. The microwave absorption capabilities of SnO2 foams were increased by increasing the size of pores in the foam configuration. Furthermore, the electromagnetic wave absorption was also correlated with the pore contents in SnO2 foams. The large and high amounts pores can bring about more interfacial polarization and corresponding relaxation. Thus, the perfect ordered honeycomb-like SnO2 foams obtained in the existence of large amounts of 322 nm polystyrene spheres showed the outstanding electromagnetic wave absorption properties. The minimal reflection loss (RL) is -37.6 dB at 17.1 GHz, and RL less than -10 dB reaches 5.6 GHz (12.4-18.0 GHz) with thin thickness of 2.0 mm. The bandwidth (<-10 dB, 90% microwave dissipation) can be monitored in the frequency regime of 4.0-18.0 GHz with absorber thickness of 2.0-5.0 mm. The results indicate that these ordered honeycomb SnO2 foams show the superiorities of wide-band, high-efficiency absorption, multiple reflection and scatting, high antioxidation, lightweight, and thin thickness.

Poly(vinylidene fluoride) foams: a promising low-k dielectric and heat-insulating material

In this study, we used a batch-foaming method to prepare closed-cell poly(vinylidene fluoride) (PVDF) foams with tailored microcellular structures. This is a simple, cost-effective, and environmentally-friendly method, and it offers a wide range of tunable microcellular structures. The cell size and porosity volume of PVDF foams can be effectively monitored at their saturation temperatures. The PVDF foam prepared at 169.5 °C possesses an ultra-low dielectric constant of k ∼ 1.1 and a thermal conductivity of 0.027 W m−1 K−1. We used numerous theoretical models to predict the effective dielectric permittivity of the microcellular PVDF foams, and found that the Bruggeman model was very close to the measured results. The ultra-low thermal conductivities of the PVDF foams were the result of a large number of air pockets in the microcellular structure. Based on these results, we concluded that this PVDF foam would be a promising low-k dielectric and heat insulating material.

Fluffy microrods to heighten the microwave absorption properties through tuning the electronic state of Co/CoO

In this work, we fabricated unique fluffy Co/CoO micro-rod composites with various weight ratios between Co and CoO. The microwave absorption properties of the Co/CoO micro-rod composites were significantly affected by the Co content. With increasing Co content, due to the high conductive loss, the microwave absorption properties were improved, correspondingly. The fluffy Co/CoO micro-rod composite (Co57) prepared at 700 °C under nitrogen flow after its precursor was annealed at 500 °C exhibited outstanding microwave absorption properties. The optimal reflection loss (RL) was −21.7 dB with a thickness of 2.3 mm and the effective absorption bandwidth (RL is lower than −10 dB) can reach more than 6.1 GHz. In addition, the microwave absorption properties of paraffin/Co57 composites with different Co57 amounts were also investigated, and the paraffin-based composite containing 40 wt% Co57 shows excellent microwave absorption properties. Furthermore, density functional theory (DFT) calculations were used to evaluate the electronic structure of different Co/CoO composites to describe the microwave absorption variation. When the Co amount is gradually increased, the electrical conductivity is enhanced, affecting the dielectric properties of the products. Thus, the difference of the microwave absorbing properties of the experimentally obtained sample was explained from the viewpoint of the electronic structure. We paved a new avenue to use the electronic structure of absorbers to forecast the microwave absorption performance.

Novel two-dimensional Ti3C2Tx MXenes/nano-carbon sphere hybrids for high-performance microwave absorption

Ti3C2Tx MXenes are highly desirable as a microwave absorption material because of their unique two-dimensional (2D) laminated structure, native defects and surface functional groups (–OH and –F). However, the conductivity of Ti3C2Tx MXenes is too high to meet the requirement of impedance matching, which results in strong reflection and weak absorption. In order to enhance the microwave absorption of Ti3C2Tx MXenes, a novel 2D laminated structure of Ti3C2Tx MXenes/nano-carbon-sphere hybrids is designed and successfully prepared for the first time and the formation mechanism of the Ti3C2Tx MXenes/nano-carbon-sphere hybrids is discussed. In addition, the obtained 2D Ti3C2Tx MXenes/nano-carbon-sphere hybrids exhibit excellent microwave absorption performance, which is mainly attributed to the unique microstructure of the Ti3C2Tx MXenes/nano-carbon-sphere hybrids, and the formation of a heterogeneous interface structure, which plays an important role in microwave absorption. The Ti3C2Tx MXenes/nano-carbon-sphere hybrids exhibit an optimal reflection loss (RL) of −54.67 dB at 3.97 GHz. Thus, the Ti3C2Tx MXenes/nano-carbon-sphere hybrids are expected to be a candidate material for microwave absorption applications.

Incorporating a microcellular structure into PVDF/graphene-nanoplatelet composites to tune their electrical conductivity and electromagnetic interference shielding properties

In this study, we found a simple and effective method to fabricate lightweight poly(vinylidene fluoride) (PVDF)/10 wt% graphene nanoplatelet (GnP) nanocomposite foams with excellent electromagnetic interference (EMI) shielding effectiveness. To this end, solvent blending, film casting, and hot compression procedures were used. The PVDF/10 wt%-GnP nanocomposite foams, which had different microcellular structures, were obtained by adjusting the foaming parameter. Notably, the electrical conductivity and the EMI shielding properties decreased linearly with elevated foaming degree (i.e., the void fraction). Furthermore, they quickly decreased, having a large slope with an increasing void fraction, when the void fraction was below the critical foaming degree of 55% void fraction. When the void fraction was above this critical foaming degree, the electrical conductivity and EMI shielding values decreased slowly with a smaller slope. The EMI shielding properties were critically determined by the foam thickness. The EMI shielding properties of the PVDF/10 wt%-GnP foam with a void fraction of 48.7% increased from 12.4 to 32.2 dB at 26.5 GHz and from 15.2 to 37.4 dB at 40 GHz when the sample thickness increased from 1.5 to 3.0 mm. We concluded that the PVDF/GnP composite foams with tunable electrical conductivity and light weight offered much promise for use as excellent EMI shielding materials. Moreover, this study adopted a novel approach toward the design of conductive lightweight polymer/carbon composite foams for use in a wide range of electronic applications.

A versatile foaming platform to fabricate polymer/carbon composites with high dielectric permittivity and ultra-low dielectric loss

There is an urgent need for dielectric-based capacitors to manage the increase in storage systems related to renewable energy production. Such capacitors must have superior qualities that include light weight, a high dielectric constant, and ultra-low dielectric loss. Poly(vinylidene fluoride) (PVDF)/carbon (carbon nanotube (CNT) or graphene nanoplatelet (GnP)) nanocomposite foams are considered promising alternatives to solid PVDF/carbon nanocomposites. This is because they have excellent dielectric properties, which are due to the preferred orientation of their carbon materials occurring in the foaming process. In the PVDF/carbon foams, their microcellular structure significantly influenced their electrical conductivity and dielectric properties. In the PVDF/CNT composite foams, the electrical conductivity was increased by an increased degree of foaming that was below a critical foaming degree. The CNTs even formed conductive networks and this caused current leakage. Thus, in the PVDF/CNT foam sample with an expansion ratio of 4.0 where a high dielectric constant of 80.6 was obtained, a relatively high dielectric loss of 3.51 was observed at the same time. In the PVDF/GnP composite foams, the presence of a microcellular structure forcefully increased the distance between GnPs. This induced and produced the insulating quality of the PVDF/GnP foams. In addition, the parallel graphene nanoplatelets that accompanied this process were close together, and they isolated the polymer layer, or air, as a medium between themselves. An unprecedentedly high dielectric constant of 112.1 and an ultra-low dielectric loss of 0.032 at 100 Hz were obtained from the PVDF/GnP composite foam with a high expansion ratio of 4.4 due to charge accumulation at the aligned conductive filler/insulating polymer (or air bubble) interface.