Project description:Microwave-absorbing materials have attracted increased research interest in recent years because of their core roles in the fields of electromagnetic (EM) pollution precaution and information security. In this paper, microwave-absorbing material NiFe-layered double hydroxide (NiFe-LDH) was synthesized by a simple co-precipitation method and calcined for the fabrication of NiFe-mixed metal oxide (NiFe-MMO). The phase structure and micromorphology of the NiFe-LDH and NiFe-MMO were analyzed, and their microwave-absorbing properties were investigated with a vector network analyzer in 2-18 GHz. Both NiFe-LDH and NiFe-MMO possessed abundant interfaces and a low dielectric constant, which were beneficial to electromagnetic wave absorption, owing to the synergistic effect of multi-relaxation and impedance matching. The optimum reflection loss (RL) of NiFe-LDH and NiFe-MMO was -58.8 dB and -64.4 dB, respectively, with the thickness of 4.0 mm in the C band. This work demonstrates that LDH-based materials have a potential application in electromagnetic wave absorption.
Project description:In this study, we report a simple and efficient two-step method consisting of water-in-oil (W/O) emulsion technique and subsequent annealing process for synthesizing the hollow reduced graphene oxide microspheres embedded with Co nanoparticles (Air@rGO€Co). The microspheres showed good electromagnetic properties because of the coexistence of magnetic loss and dielectric loss to microwaves. The minimum reflection loss (RLmin) value of S1.5 reaches -68.1 dB at 13.8 GHz with a thickness of 2.2 mm, and the absorption bandwidth (lower than -10 dB) is 7.1 GHz covering from 10.9 GHz to 18.0 GHz. More interestingly, we can easily controll the microwave absorbing properties of the microspheres by changing the ratio of the two components in the composites. The excellent electromagnetic match at the corresponding resonance peaks for dielectric and magnetic loss play an important role in improving microwave absorption property. Our study provides a good potential method for preparation of lightweight microwave absorbing materials.
Project description:Amorphous structures may play important roles in achieving highly efficient microwave absorption performance due to the polarization losses induced by the disorders, vacancies and other functional groups existed in them. Herein, a kind of amorphous TiO2/rGO composite (a-TiO2/rGO) was successfully fabricated via a facile one-step solvothermal method. The complex permittivity of the composites can be regulated by adjusting the addition of precursor solution. The minimum reflection loss of a-TiO2/rGO composites reached -42.8 dB at 8.72 GHz with a thickness of 3.25 mm, and the widest efficient absorption bandwidth (EAB) was up to 6.2 GHz (11.8 to 18 GHz) with a thickness of only 2.15 mm, which achieved the full absorption in Ku band (12 to 18 GHz). Furthermore, the EAB was achieved ranging from 3.97 to 18 GHz by adjusting the thickness of the absorber, covering 87.7% of the whole radar frequency band. It is considered that the well-matched impedance, various polarization processes, capacitor-like structure and conductive networks all contributed to the excellent microwave absorption of a-TiO2/rGO. This study provides reference on constructing amorphous structures for future microwave absorber researches and the as-prepared a-TiO2/rGO composites also have great potential owing to its facile synthesis and highly efficient microwave absorption.
Project description:Herein we report a microwave assisted, fungal (Ganoderma lucidum) extract mediated synthesis of noble metal decorated reduced graphene oxide(r-GO). The carbon to oxygen ratio increased from 1.46 in GO to 2.72 in r-GO. The electron rate transfer capabilities of Pt, Pd, Ru, Pt-Pd and Pt- Ru decorated r- GO were tested in ferri-ferro coupling reaction. Excellent electrochemical behaviour were observed in their ability to oxidize hydrazine, reduce H2O2, as well as oxygen reduction in alkaline medium and hydrogen evolution in acidic medium. These reactions represent diverse applications in energy production, storage, electrochemical sensing and electro catalysis, thus the method presented here can be quite useful in diverse applications.
Project description:High performance carbonized bamboo fibers were synthesized for a wide range of temperature dependent energy storage applications. The structural and electrochemical properties of the carbonized bamboo fibers were studied for flexible supercapacitor applications. The galvanostatic charge-discharge studies on carbonized fibers exhibited specific capacity of ~510F/g at 0.4 A/g with energy density of 54 Wh/kg. Interestingly, the carbonized bamboo fibers displayed excellent charge storage stability without any appreciable degradation in charge storage capacity over 5,000 charge-discharge cycles. The symmetrical supercapacitor device fabricated using these carbonized bamboo fibers exhibited an areal capacitance of ~1.55 F/cm(2) at room temperature. In addition to high charge storage capacity and cyclic stability, the device showed excellent flexibility without any degradation to charge storage capacity on bending the electrode. The performance of the supercapacitor device exhibited ~65% improvement at 70 °C compare to that at 10 °C. Our studies suggest that carbonized bamboo fibers are promising candidates for stable, high performance and flexible supercapacitor devices.
Project description:Carbon allotropes and their derivatives have exhibited superior performances for microwave absorption ascribed to their dielectric attenuation capacity raised from the multiple dipolar configurations. Such characteristics could be achieved by constructing diverse micro/nanoscale architectures, incorporating vacancies and heteroatoms, and composing with heterogeneous components. Alternatively, we herein present a facile approach for the synthesis of carbonized hydrochars, which are composed of dispersed ultrafine nanocrystallines graphite and amorphous matrix. Such a isomeric construction has a high-density interfaces, accompanied with significant interfacial polarizations, leading to the improvement of microwave absorbing capabilities. For carbonized hydrochars, the RLmin value can reach -40.36 dB, and corresponding effective bandwidth is 2 GHz. This work provides a concept for designing microwave absorption materials based on isomeric heterogeneous interfacial polarizations in single-element systems.
Project description:High-surface-area carbon-based capacitors exhibit significant advantages relative to conventional graphite-based systems, such as high power density, low weight, and mechanical flexibility. In this work, novel porous carbon-based electrodes were obtained from commercial cotton fibers (CFs) impregnated with graphene oxide (GO) at different dipping times. A subsequent thermal treatment under inert atmosphere conditions enables the synthesis of electrodes based on reduced GO (RGO) supported on carbon fibers. Those synthetized with 15 min and 30 min of dipping time displayed high specific capacitance given their optimal micro-/ mesoporosity ratio. Particularly, the RGO/CCF15A supercapacitor reports a remarkable specific capacitance of 74.1 F g-1 at 0.2 A g-1 and a high cycling stability with a 97.7% capacitive retention, making this electrode a promising candidate for supercapacitor design. Finally, we conducted a density functional theory study to obtain deeper information about the driving forces leading to the GO/CF structures.
Project description:This work presents lightweight epoxy foams loaded with very low weight percentages (≤0.5 wt.%) of carbon fibers (CFs) with different lengths (3 mm, 6 mm, and 12 mm) as broadband microwave absorbing materials for anechoic chamber application. The effect of CF length on microwave absorption, especially on the absorption frequency band, is investigated for frequencies between 1 and 15 GHz. For the elaboration of composites, three different methods-spatula, shear mixing, and ultrasounds-are used for the dispersion of CFs. The observation of these CFs, after the dispersion step, shows a high fiber breakage rate when shear mixing is used, unlike when spatula or ultrasounds methods are used. On the other hand, the characterization of the elaborated composites highlights a correlation between the mixing methods, hence the fiber brakeage, and the measured reflection coefficient (reflection loss) of the composites. As a result, the minimum value of the reflection coefficient is shifted toward the high frequencies when the fiber breakage is observed, suggesting that short CFs absorb at high frequencies while long CFs absorb at low frequencies. Dielectric properties, extracted from the measurement in free space, of composites elaborated with different fiber lengths (3 mm, 6 mm, and 12 mm) confirm that short CFs (3 mm) show maximum losses at high frequencies (around 15 GHz) while long CFs (12 mm) show maximum dielectric losses at low frequencies (below 4 GHz). However, no significant variation is observed on the real part of the relative permittivity, as a function of fiber length, for these porous composites loaded with very low CF rates. A hybrid composite, with a mix of different CF lengths, is prepared and characterized. The simulation of the absorption performance of a pyramidal absorber, based on this hybrid composite, is compared to the one of pyramidal absorber based on composites loaded with a single length of carbon fibers. The pyramidal absorber-based hybrid composite predicts the best absorption performance, especially at the low frequency band. The simulated reflection coefficient of this absorber is less than -12 dB in all the studied frequency range, and less than -40 dB for frequencies higher than 3 GHz. This result confirms the interest of using a mix of carbon fiber lengths to achieve a broadband microwave absorber.
Project description:Composites obtained by incorporating magnetic nanoparticles into porous carbon materials are promising in serving as microwave absorbing materials. In this study, Ni/C porous fibers were successfully synthesized through a simple in situ template method by using low-cost jute fibers as carbon source and template. The results showed that the Ni nanoparticles were uniformly loaded on the surface and hollow porous structure of the Ni/C porous fibers. Meanwhile, the content and size of the Ni nanoparticles on the Ni/C porous fibers can be controlled. Due to a suitable filling content, the synergistic effect of dielectric loss, interface polarization loss, magnetic loss and porous structure of the Ni/C porous fibers, an excellent microwave absorption performance was achieved. The minimum reflection loss value reached -43.0 dB, and a reflection loss value less than -10 dB was in the frequency range of 11.2-16.1 GHz with 2.0 mm thickness. In particular, under matching thickness (1.5-3.5 mm), the values of all the reflection loss peaks were below -20.0 dB. It is believed that this work can not only provide a new way to design excellent carbon-based microwave absorbing materials, but also offer an effective design strategy to synthesize biomass nanocomposites.
Project description:Excellent mechanical, electrical, and thermal properties of graphene have been achieved at the macroscale by assembling individual graphene or graphene oxide (GO) particles. Wet-spinning is an efficient and well-established process that can provide GO assemblies in fiber form. The coagulation bath in the wet-spinning process has rarely been considered for the design of mechanically robust GO fibers (GOFs). In this study, locating the amidation reaction in the coagulation bath yielded mechanically improved GOFs. The imides 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinimide were used to form covalent amide bonds between GO flakes and chitosan, thereby reinforcing the GOFs. Evidence and effects of the amidation reaction were systematically examined. The tensile strength and breaking strain of the GOFs improved by 41.6% and 75.2%, respectively, and the toughness almost doubled because of the optimized crosslinking reaction. Our work demonstrated that using a coagulation bath is a facile way to enhance the mechanical properties of GOFs.